Clinical Review

Hidradenitis suppurativa (HS)—a chronic, relapsing, inflammatory disorder involving terminal hair follicles in apocrine gland–rich skin—manifests as tender inflamed nodules that transform into abscesses, sinus tracts, and scarring.^1,2 The etiology of HS is multifactorial, encompassing lifestyle, microbiota, hormonal status, and genetic and environmental factors. These factors activate the immune system around the terminal hair follicles and lead to hyperkeratosis of the infundibulum of the hair follicles in intertriginous regions. This progresses to follicular occlusion, stasis, and eventual rupture. Bacterial multiplication within the plugged pilosebaceous units further boosts immune activation. Resident and migrated cells of the innate and adaptive immune system then release proinflammatory cytokines such as tumor necrosis factor, IL-1β, and IL-17, which further enhance immune cell influx and inflammation.^3,4 This aberrant immune response propagates the production of deep-seated inflammatory nodules and abscesses.^3-8

The estimated prevalence of HS is 1% worldwide.⁹ It is more prevalent in female and Black patients (0.30%) than White patients (0.09%) and is intermediate in prevalence in the biracial population (0.22%).¹⁰ Hidradenitis suppurativa is thought to be associated with lower socioeconomic status (SES). In a retrospective analysis of HS patients (N=375), approximately one-third of patients were Black, had advanced disease, and had a notably lower SES.¹¹ Furthermore, HS has been reported to be associated with systemic inflammation and comorbidities such as morbid obesity (38.3%) and hypertension (39.6%) as well as other metabolic syndrome–related disorders and depression (48.1%).¹

Hidradenitis suppurativa may contribute to the risk for depression through its substantial impact on health-related quality of life, which culminates in social withdrawal, unemployment, and suicidal thoughts.¹² The high prevalence of depression in individuals with HS¹ and its association with systemic inflammation¹³ increases the likelihood that a common genetic predisposition also may exist between both conditions. Because depression frequently has been discovered as a concomitant diagnosis in patients with HS, we hypothesize that a shared genetic susceptibility also may exist between the 2 disorders. Our study sought to explore data on the co-occurrence of depression with HS, including its demographics and racial data.

Methods

We conducted a PubMed search of articles indexed for MEDLINE as well as Google Scholar using the terms depression and hidradenitis suppurativa to obtain all research articles published from 2000 to 2022. Articles were selected based on relevance to the topic of exploration. English-language articles that directly addressed the epidemiology, etiology, pathophysiology, and co-occurrence of both depression and HS with numerical data were included. Articles were excluded if they did not explore the information of interest on these 2 disorders or did not contain clear statistical data of patients with the 2 concurrent medical conditions.

Results

Twenty-two cross-sectional, prospective, and retrospective studies that fit the search criteria were identified and included in the analysis (eTable).^1,14-34 Sixteen (72.7%) studies were cross-sectional, 5 (22.7%) were retrospective, and only 1 (4.5%) was a prospective study. Only 6 of the studies provided racial data,^{1,14,17,26,28,32} and of them, 4 had predominately White patients,^1,14,26,32 whereas the other 2 had predominantly Black patients.^17,28

Hidradenitis suppurativa was found to coexist with depression in all the studies, with a prevalence of 1.2% to 48.1%. There also was a higher prevalence of depression in HS patients than in the control patients without HS. Furthermore, a recent study by Wright and colleagues¹⁴ stratified the depression prevalence data by age and found a higher prevalence of depression in adults vs children with HS (30% vs 12%).

Comment

Major depression—a chronic and debilitating illness—is the chief cause of disability globally and in the United States alone and has a global lifetime prevalence of 17%.³⁵ In a study of 388 patients diagnosed with depression and 404 community-matched controls who were observed for 10 years, depressed patients had a two-thirds higher likelihood of developing a serious physical illness than controls. The depression-associated elevated risk for serious physical illness persisted after controlling for confounding variables such as alcohol abuse, smoking, and level of physical activity.³⁶ Studies also have demonstrated that HS is more prevalent in Black individuals¹⁰ and in individuals of low SES,³⁷ who are mostly the Black and Hispanic populations that experience the highest burden of racial microaggression³⁸ and disparities in health access and outcomes.^39,40 The severity and chronicity of major depressive disorder also is higher in Black patients compared with White patients (57% vs 39%).⁴¹ Because major depression and HS are most common among Black patients who experience the highest-burden negative financial and health disparities, there may be a shared genetic disposition to both medical conditions.

Moreover, the common detrimental lifestyle choices associated with patients with depression and HS also suggest the possibility of a collective genetic susceptibility. Patients with depression also report increased consumption of alcohol, tobacco, and illicit substances; sedentary lifestyle leading to obesity; and poor compliance with prescribed medical treatment.⁴² Smoking and obesity are known contributors to the pathogenesis of HS, and their modification also is known to positively impact the disease course. In a retrospective single-cohort study, 50% of obese HS patients (n=35) reported a substantial decrease in disease severity after a reduction of more than 15% in body mass index over 2 years following bariatric surgery (n=35).⁴³ Patients with HS also have reported disease remission following extensive weight loss.⁴⁴ In addition, evidence has supported smoking cessation in improving the disease course of HS.⁴³ Because these detrimental lifestyle choices are prevalent in both patients with HS and those with depression, a co-genetic susceptibility also may exist.

Furthermore, depression is characterized by a persistent inflammatory state,^13,45 similar to HS.⁴⁶ Elevated levels of a variety of inflammatory markers, such as C-reactive protein (CRP), IL-6, and soluble intercellular adhesion molecule 1, have been reported in patients with depression compared with healthy controls.^13,45 Further analysis found a positive correlation and a strong association between depression and these inflammatory markers.⁴⁷ Moreover, adipokines regulate inflammatory responses, and adipokines play a role in the pathogenesis of HS. Adipokine levels such as elevated omentin-1 (a recently identified adipokine) were found to be altered in patients with HS compared with controls.⁴⁸ Results from clinical studies and meta-analyses of patients with depression also have demonstrated that adipokines are dysregulated in this population,^49,50 which may be another potential genetic link between depression and HS.

In addition, genetic susceptibility to depression and HS may be shared because the inflammatory markers that have a strong association with depression also have been found to play an important role in HS treatment and disease severity prediction. In a retrospective cohort study of 404 patients, CRP or IL-6 levels were found to be reliable predictors of HS disease severity, which may explain why anti–tumor necrosis factor antibody regimens such as adalimumab and infliximab have clinically ameliorated disease activity in several cases of HS.⁵¹ In a study evaluating these drugs, high baseline levels of high-sensitivity CRP and IL-6 were predictive of patient response to infliximab.⁵² In a meta-analysis evaluating 20,791 participants, an association was found between concurrent depression and CRP. Furthermore, inflammation measured by high levels of CRP or IL-6 was observed to predict future depression.⁵³ If the same inflammatory markers—CRP and IL-6—both play a major role in the disease activity of depression and HS, then a concurrent genetic predisposition may exist.

Conclusion

Understanding the comorbidities, etiologies, and risk factors for the development and progression of HS is an important step toward improved disease management. Available studies on comorbid depression in HS largely involve White patients, and more studies are needed in patients with skin of color, particularly the Black population, who have the highest prevalence of HS.¹⁰ Given the evidence for an association between depression and HS, we suggest a large-scale investigation of this patient population that includes a complete medical history, onset of HS in comparison to the onset of depression, and specific measures of disease progress and lifetime management of depression, which may help to increase knowledge about the role of depression in HS and encourage more research in this area. If shared genetic susceptibility is established, aggressive management of depression in patients at risk for HS may reduce disease incidence and severity as well as the psychological burden on patients.

Hidradenitis suppurativa (HS)—a chronic, relapsing, inflammatory disorder involving terminal hair follicles in apocrine gland–rich skin—manifests as tender inflamed nodules that transform into abscesses, sinus tracts, and scarring.^1,2 The etiology of HS is multifactorial, encompassing lifestyle, microbiota, hormonal status, and genetic and environmental factors. These factors activate the immune system around the terminal hair follicles and lead to hyperkeratosis of the infundibulum of the hair follicles in intertriginous regions. This progresses to follicular occlusion, stasis, and eventual rupture. Bacterial multiplication within the plugged pilosebaceous units further boosts immune activation. Resident and migrated cells of the innate and adaptive immune system then release proinflammatory cytokines such as tumor necrosis factor, IL-1β, and IL-17, which further enhance immune cell influx and inflammation.^3,4 This aberrant immune response propagates the production of deep-seated inflammatory nodules and abscesses.^3-8

The estimated prevalence of HS is 1% worldwide.⁹ It is more prevalent in female and Black patients (0.30%) than White patients (0.09%) and is intermediate in prevalence in the biracial population (0.22%).¹⁰ Hidradenitis suppurativa is thought to be associated with lower socioeconomic status (SES). In a retrospective analysis of HS patients (N=375), approximately one-third of patients were Black, had advanced disease, and had a notably lower SES.¹¹ Furthermore, HS has been reported to be associated with systemic inflammation and comorbidities such as morbid obesity (38.3%) and hypertension (39.6%) as well as other metabolic syndrome–related disorders and depression (48.1%).¹

Hidradenitis suppurativa may contribute to the risk for depression through its substantial impact on health-related quality of life, which culminates in social withdrawal, unemployment, and suicidal thoughts.¹² The high prevalence of depression in individuals with HS¹ and its association with systemic inflammation¹³ increases the likelihood that a common genetic predisposition also may exist between both conditions. Because depression frequently has been discovered as a concomitant diagnosis in patients with HS, we hypothesize that a shared genetic susceptibility also may exist between the 2 disorders. Our study sought to explore data on the co-occurrence of depression with HS, including its demographics and racial data.

Methods

We conducted a PubMed search of articles indexed for MEDLINE as well as Google Scholar using the terms depression and hidradenitis suppurativa to obtain all research articles published from 2000 to 2022. Articles were selected based on relevance to the topic of exploration. English-language articles that directly addressed the epidemiology, etiology, pathophysiology, and co-occurrence of both depression and HS with numerical data were included. Articles were excluded if they did not explore the information of interest on these 2 disorders or did not contain clear statistical data of patients with the 2 concurrent medical conditions.

Results

Twenty-two cross-sectional, prospective, and retrospective studies that fit the search criteria were identified and included in the analysis (eTable).^1,14-34 Sixteen (72.7%) studies were cross-sectional, 5 (22.7%) were retrospective, and only 1 (4.5%) was a prospective study. Only 6 of the studies provided racial data,^{1,14,17,26,28,32} and of them, 4 had predominately White patients,^1,14,26,32 whereas the other 2 had predominantly Black patients.^17,28

Hidradenitis suppurativa was found to coexist with depression in all the studies, with a prevalence of 1.2% to 48.1%. There also was a higher prevalence of depression in HS patients than in the control patients without HS. Furthermore, a recent study by Wright and colleagues¹⁴ stratified the depression prevalence data by age and found a higher prevalence of depression in adults vs children with HS (30% vs 12%).

Comment

Major depression—a chronic and debilitating illness—is the chief cause of disability globally and in the United States alone and has a global lifetime prevalence of 17%.³⁵ In a study of 388 patients diagnosed with depression and 404 community-matched controls who were observed for 10 years, depressed patients had a two-thirds higher likelihood of developing a serious physical illness than controls. The depression-associated elevated risk for serious physical illness persisted after controlling for confounding variables such as alcohol abuse, smoking, and level of physical activity.³⁶ Studies also have demonstrated that HS is more prevalent in Black individuals¹⁰ and in individuals of low SES,³⁷ who are mostly the Black and Hispanic populations that experience the highest burden of racial microaggression³⁸ and disparities in health access and outcomes.^39,40 The severity and chronicity of major depressive disorder also is higher in Black patients compared with White patients (57% vs 39%).⁴¹ Because major depression and HS are most common among Black patients who experience the highest-burden negative financial and health disparities, there may be a shared genetic disposition to both medical conditions.

Moreover, the common detrimental lifestyle choices associated with patients with depression and HS also suggest the possibility of a collective genetic susceptibility. Patients with depression also report increased consumption of alcohol, tobacco, and illicit substances; sedentary lifestyle leading to obesity; and poor compliance with prescribed medical treatment.⁴² Smoking and obesity are known contributors to the pathogenesis of HS, and their modification also is known to positively impact the disease course. In a retrospective single-cohort study, 50% of obese HS patients (n=35) reported a substantial decrease in disease severity after a reduction of more than 15% in body mass index over 2 years following bariatric surgery (n=35).⁴³ Patients with HS also have reported disease remission following extensive weight loss.⁴⁴ In addition, evidence has supported smoking cessation in improving the disease course of HS.⁴³ Because these detrimental lifestyle choices are prevalent in both patients with HS and those with depression, a co-genetic susceptibility also may exist.

Furthermore, depression is characterized by a persistent inflammatory state,^13,45 similar to HS.⁴⁶ Elevated levels of a variety of inflammatory markers, such as C-reactive protein (CRP), IL-6, and soluble intercellular adhesion molecule 1, have been reported in patients with depression compared with healthy controls.^13,45 Further analysis found a positive correlation and a strong association between depression and these inflammatory markers.⁴⁷ Moreover, adipokines regulate inflammatory responses, and adipokines play a role in the pathogenesis of HS. Adipokine levels such as elevated omentin-1 (a recently identified adipokine) were found to be altered in patients with HS compared with controls.⁴⁸ Results from clinical studies and meta-analyses of patients with depression also have demonstrated that adipokines are dysregulated in this population,^49,50 which may be another potential genetic link between depression and HS.

In addition, genetic susceptibility to depression and HS may be shared because the inflammatory markers that have a strong association with depression also have been found to play an important role in HS treatment and disease severity prediction. In a retrospective cohort study of 404 patients, CRP or IL-6 levels were found to be reliable predictors of HS disease severity, which may explain why anti–tumor necrosis factor antibody regimens such as adalimumab and infliximab have clinically ameliorated disease activity in several cases of HS.⁵¹ In a study evaluating these drugs, high baseline levels of high-sensitivity CRP and IL-6 were predictive of patient response to infliximab.⁵² In a meta-analysis evaluating 20,791 participants, an association was found between concurrent depression and CRP. Furthermore, inflammation measured by high levels of CRP or IL-6 was observed to predict future depression.⁵³ If the same inflammatory markers—CRP and IL-6—both play a major role in the disease activity of depression and HS, then a concurrent genetic predisposition may exist.

Conclusion

Understanding the comorbidities, etiologies, and risk factors for the development and progression of HS is an important step toward improved disease management. Available studies on comorbid depression in HS largely involve White patients, and more studies are needed in patients with skin of color, particularly the Black population, who have the highest prevalence of HS.¹⁰ Given the evidence for an association between depression and HS, we suggest a large-scale investigation of this patient population that includes a complete medical history, onset of HS in comparison to the onset of depression, and specific measures of disease progress and lifetime management of depression, which may help to increase knowledge about the role of depression in HS and encourage more research in this area. If shared genetic susceptibility is established, aggressive management of depression in patients at risk for HS may reduce disease incidence and severity as well as the psychological burden on patients.

Hidradenitis suppurativa (HS)—a chronic, relapsing, inflammatory disorder involving terminal hair follicles in apocrine gland–rich skin—manifests as tender inflamed nodules that transform into abscesses, sinus tracts, and scarring.^1,2 The etiology of HS is multifactorial, encompassing lifestyle, microbiota, hormonal status, and genetic and environmental factors. These factors activate the immune system around the terminal hair follicles and lead to hyperkeratosis of the infundibulum of the hair follicles in intertriginous regions. This progresses to follicular occlusion, stasis, and eventual rupture. Bacterial multiplication within the plugged pilosebaceous units further boosts immune activation. Resident and migrated cells of the innate and adaptive immune system then release proinflammatory cytokines such as tumor necrosis factor, IL-1β, and IL-17, which further enhance immune cell influx and inflammation.^3,4 This aberrant immune response propagates the production of deep-seated inflammatory nodules and abscesses.^3-8

The estimated prevalence of HS is 1% worldwide.⁹ It is more prevalent in female and Black patients (0.30%) than White patients (0.09%) and is intermediate in prevalence in the biracial population (0.22%).¹⁰ Hidradenitis suppurativa is thought to be associated with lower socioeconomic status (SES). In a retrospective analysis of HS patients (N=375), approximately one-third of patients were Black, had advanced disease, and had a notably lower SES.¹¹ Furthermore, HS has been reported to be associated with systemic inflammation and comorbidities such as morbid obesity (38.3%) and hypertension (39.6%) as well as other metabolic syndrome–related disorders and depression (48.1%).¹

Hidradenitis suppurativa may contribute to the risk for depression through its substantial impact on health-related quality of life, which culminates in social withdrawal, unemployment, and suicidal thoughts.¹² The high prevalence of depression in individuals with HS¹ and its association with systemic inflammation¹³ increases the likelihood that a common genetic predisposition also may exist between both conditions. Because depression frequently has been discovered as a concomitant diagnosis in patients with HS, we hypothesize that a shared genetic susceptibility also may exist between the 2 disorders. Our study sought to explore data on the co-occurrence of depression with HS, including its demographics and racial data.

Methods

We conducted a PubMed search of articles indexed for MEDLINE as well as Google Scholar using the terms depression and hidradenitis suppurativa to obtain all research articles published from 2000 to 2022. Articles were selected based on relevance to the topic of exploration. English-language articles that directly addressed the epidemiology, etiology, pathophysiology, and co-occurrence of both depression and HS with numerical data were included. Articles were excluded if they did not explore the information of interest on these 2 disorders or did not contain clear statistical data of patients with the 2 concurrent medical conditions.

Results

Twenty-two cross-sectional, prospective, and retrospective studies that fit the search criteria were identified and included in the analysis (eTable).^1,14-34 Sixteen (72.7%) studies were cross-sectional, 5 (22.7%) were retrospective, and only 1 (4.5%) was a prospective study. Only 6 of the studies provided racial data,^{1,14,17,26,28,32} and of them, 4 had predominately White patients,^1,14,26,32 whereas the other 2 had predominantly Black patients.^17,28

Hidradenitis suppurativa was found to coexist with depression in all the studies, with a prevalence of 1.2% to 48.1%. There also was a higher prevalence of depression in HS patients than in the control patients without HS. Furthermore, a recent study by Wright and colleagues¹⁴ stratified the depression prevalence data by age and found a higher prevalence of depression in adults vs children with HS (30% vs 12%).

Comment

Major depression—a chronic and debilitating illness—is the chief cause of disability globally and in the United States alone and has a global lifetime prevalence of 17%.³⁵ In a study of 388 patients diagnosed with depression and 404 community-matched controls who were observed for 10 years, depressed patients had a two-thirds higher likelihood of developing a serious physical illness than controls. The depression-associated elevated risk for serious physical illness persisted after controlling for confounding variables such as alcohol abuse, smoking, and level of physical activity.³⁶ Studies also have demonstrated that HS is more prevalent in Black individuals¹⁰ and in individuals of low SES,³⁷ who are mostly the Black and Hispanic populations that experience the highest burden of racial microaggression³⁸ and disparities in health access and outcomes.^39,40 The severity and chronicity of major depressive disorder also is higher in Black patients compared with White patients (57% vs 39%).⁴¹ Because major depression and HS are most common among Black patients who experience the highest-burden negative financial and health disparities, there may be a shared genetic disposition to both medical conditions.

Moreover, the common detrimental lifestyle choices associated with patients with depression and HS also suggest the possibility of a collective genetic susceptibility. Patients with depression also report increased consumption of alcohol, tobacco, and illicit substances; sedentary lifestyle leading to obesity; and poor compliance with prescribed medical treatment.⁴² Smoking and obesity are known contributors to the pathogenesis of HS, and their modification also is known to positively impact the disease course. In a retrospective single-cohort study, 50% of obese HS patients (n=35) reported a substantial decrease in disease severity after a reduction of more than 15% in body mass index over 2 years following bariatric surgery (n=35).⁴³ Patients with HS also have reported disease remission following extensive weight loss.⁴⁴ In addition, evidence has supported smoking cessation in improving the disease course of HS.⁴³ Because these detrimental lifestyle choices are prevalent in both patients with HS and those with depression, a co-genetic susceptibility also may exist.

Furthermore, depression is characterized by a persistent inflammatory state,^13,45 similar to HS.⁴⁶ Elevated levels of a variety of inflammatory markers, such as C-reactive protein (CRP), IL-6, and soluble intercellular adhesion molecule 1, have been reported in patients with depression compared with healthy controls.^13,45 Further analysis found a positive correlation and a strong association between depression and these inflammatory markers.⁴⁷ Moreover, adipokines regulate inflammatory responses, and adipokines play a role in the pathogenesis of HS. Adipokine levels such as elevated omentin-1 (a recently identified adipokine) were found to be altered in patients with HS compared with controls.⁴⁸ Results from clinical studies and meta-analyses of patients with depression also have demonstrated that adipokines are dysregulated in this population,^49,50 which may be another potential genetic link between depression and HS.

In addition, genetic susceptibility to depression and HS may be shared because the inflammatory markers that have a strong association with depression also have been found to play an important role in HS treatment and disease severity prediction. In a retrospective cohort study of 404 patients, CRP or IL-6 levels were found to be reliable predictors of HS disease severity, which may explain why anti–tumor necrosis factor antibody regimens such as adalimumab and infliximab have clinically ameliorated disease activity in several cases of HS.⁵¹ In a study evaluating these drugs, high baseline levels of high-sensitivity CRP and IL-6 were predictive of patient response to infliximab.⁵² In a meta-analysis evaluating 20,791 participants, an association was found between concurrent depression and CRP. Furthermore, inflammation measured by high levels of CRP or IL-6 was observed to predict future depression.⁵³ If the same inflammatory markers—CRP and IL-6—both play a major role in the disease activity of depression and HS, then a concurrent genetic predisposition may exist.

Conclusion

Understanding the comorbidities, etiologies, and risk factors for the development and progression of HS is an important step toward improved disease management. Available studies on comorbid depression in HS largely involve White patients, and more studies are needed in patients with skin of color, particularly the Black population, who have the highest prevalence of HS.¹⁰ Given the evidence for an association between depression and HS, we suggest a large-scale investigation of this patient population that includes a complete medical history, onset of HS in comparison to the onset of depression, and specific measures of disease progress and lifetime management of depression, which may help to increase knowledge about the role of depression in HS and encourage more research in this area. If shared genetic susceptibility is established, aggressive management of depression in patients at risk for HS may reduce disease incidence and severity as well as the psychological burden on patients.

Inflammatory skin conditions often have a relapsing and remitting course and represent a large proportion of chronic skin diseases. Common inflammatory skin disorders include acne, psoriasis, hidradenitis suppurativa (HS), atopic dermatitis (AD), and seborrheic dermatitis (SD).¹ Although each of these conditions has a unique pathogenesis, they all are driven by a background of chronic inflammation. It has been reported that diets with high levels of refined carbohydrates and saturated or trans-fatty acids may exacerbate existing inflammation.² Consequently, dietary interventions, such as the ketogenic and low-glycemic diets, have potential anti-inflammatory and metabolic effects that are being assessed as stand-alone or adjunctive therapies for dermatologic diseases.

Diet may partially influence systemic inflammation through its effect on weight. Higher body mass index and obesity are linked to a low-grade inflammatory state and higher levels of circulating inflammatory markers. Therefore, weight loss leads to decreases in inflammatory cytokines, including C-reactive protein, tumor necrosis factor α, and IL-6.³ These cytokines and metabolic effects overlap with inflammatory skin condition pathways. It also is posited that decreased insulin release associated with weight loss results in decreased sebaceous lipogenesis and androgens, which drive keratinocyte proliferation and acne development.^4,5 For instance, in a 2015 meta-analysis of 5 randomized controlled trials on psoriasis, patients in the weight loss intervention group had more substantial reductions in psoriasis area and severity index (PASI) scores compared with controls receiving usual care (P=.004).⁶ However, in a systematic review of 35 studies on acne vulgaris, overweight and obese patients (defined by a body mass index of ≥23 kg/m²) had similar odds of having acne compared with normal-weight individuals (P=.671).⁷

Similar to weight loss, ketogenesis acts as a negative feedback mechanism to reduce insulin release, leading to decreased inflammation and androgens that often exacerbate inflammatory skin diseases.⁸ Ketogenesis ensues when daily carbohydrate intake is limited to less than 50 g, and long-term adherence to a ketogenic diet results in metabolic reliance on ketone bodies such as acetoacetate, β-hydroxybutyrate, and acetone.⁹ These metabolites may decrease free radical damage and consequently improve signs and symptoms of acne, psoriasis, and other inflammatory skin diseases.^10-12 Similarly, increased ketones also may decrease activation of the NLRP3 (NOD-, LRR-, and Pyrin domain-containing protein 3) inflammasome and therefore reduce inflammatory markers such as IL-1β and IL-1.^4,13 Several proposed mechanisms are outlined in the Table.

Collectively, low-glycemic and ketogenic diets have been proposed as potential interventions for reducing inflammatory skin conditions. These dietary approaches are hypothesized to exert their effects by facilitating weight loss, elevating ketone levels, and reducing systemic inflammation. The current review summarizes the existing evidence on ketogenic and low-glycemic diets as treatments for inflammatory skin conditions and evaluates the potential benefits of these dietary interventions in managing and improving outcomes for individuals with inflammatory skin conditions.

Methods

Using PubMed for articles indexed for MEDLINE and Google Scholar, a review of the literature was conducted with a combination of the following search terms: low-glycemic diet, inflammatory, dermatologic, ketogenic diet, inflammation, dermatology, acne, psoriasis, eczema, seborrheic dermatitis, and hidradenitis suppurativa. Reference citations in identified works also were reviewed. Interventional (experimental studies or clinical trials), survey-based, and observational studies that investigated the effects of low-glycemic or ketogenic diets for the treatment of inflammatory skin conditions were included. Inclusion criteria were studies assessing acne, psoriasis, SD, AD, and HS. Exclusion criteria were studies published before 1965; those written in languages other than English; and those analyzing other diets, such as the Mediterranean or low-fat diets. The search yielded a total of 11 observational studies and 4 controlled studies published between 1966 and January 2023. Because this analysis utilized publicly available data and did not qualify as human subject research, institutional review board approval was not required.

Results

Acne Vulgaris—Acne vulgaris is a disease of chronic pilosebaceous inflammation and follicular epithelial proliferation associated with Propionibacterium acnes. The association between acne and low-glycemic diets has been examined in several studies. Diet quality is measured and assessed using the glycemic index (GI), which is the effect of a single food on postprandial blood glucose, and the glycemic load, which is the GI adjusted for carbohydrates per serving.¹⁴ High levels of GI and glycemic load are associated with hyperinsulinemia and an increase in insulinlike growth factor 1 concentration that promotes mechanistic target of rapamycin (mTOR) complex 1–mediated follicular lipogenesis, sebum fatty acid production, and androgen synthesis.¹⁵Propionibacterium acnes directly activates toll-like receptor 2 on monocytes through damage-associated molecular patterns and indirectly through products of triglyceride catalysis, causing release of IL-12, IL-6, tumor necrosis factor α, and other proinflammatory cytokines.¹⁶ Therefore, lifestyle modifications focused on strict glucose control have been postulated to reduce acne severity via modulation of lipogenesis, androgen concentration, and inflammation.

Six survey-based studies evaluated sugar intake in patients with acne compared to healthy matched controls (eTable). Among these studies, 5 reported higher glycemic loads or daily sugar intake in acne patients compared to individuals without acne.^{12,19,20,26,28} The remaining study was conducted in 1967 and enrolled 16 acne patients and 32 matched controls. It reported no significant difference in sugar intake between the groups (P>.05).¹⁷

Smith et al¹⁸ randomized 43 male patients aged 15 to 25 years with facial acne into 2 cohorts for 12 weeks, each consuming either a low-glycemic diet (25% protein, 45% low-glycemic food [fruits, whole grains], and 30% fat) or a carbohydrate-dense diet of foods with medium to high GI based on prior documentation of the original diet. Patients were instructed to use a noncomedogenic cleanser as their only acne treatment. At 12 weeks, patients consuming the low-glycemic diet had an average of 23.5 fewer inflammatory lesions, while those in the intervention group had 12.0 fewer lesions (P=.03).¹⁸

In another controlled study by Kwon et al,²¹ 32 male and female acne patients were randomized to a low-glycemic diet (25% protein, 45% low-glycemic food, and 30% fat) or a standard diet for 10 weeks. Patients on the low-glycemic diet experienced a 70.9% reduction in inflammatory lesions (P<.05). Hematoxylin and eosin staining and image analysis were performed to measure sebaceous gland surface area in the low-glycemic diet group, which decreased from 0.32 to 0.24 mm² (P=.03). The sebaceous gland surface area in the control group was not reported. Moreover, patients on the low-glycemic diet had reduced IL-8 immunohistochemical staining (decreasing from 2.9 to 1.7 [P=.03]) and sterol regulatory element-binding protein 1 levels (decreasing from 2.6 to 1.3 [P=.03]), suggesting suppression of ongoing inflammation. Patients on the low-glycemic diet had no significant difference in transforming growth factor β1(P=.83). In the control group, there was no difference in IL-8, sterol regulatory element binding protein 1, or transforming growth factor β1 (P>.05) on immunohistochemical staining.²¹

Psoriasis—Psoriasis is a systemic inflammatory disease characterized by hyperproliferation and aberrant keratinocyte plaque formation. The innate immune response of keratinocytes in response to epidermal damage or infection begins with neutrophil recruitment and dendritic cell activation. Dendritic cell secretion of IL-23 promotes T-cell differentiation into helper T cells (T_H1) that subsequently secrete IL-17 and IL-22, thereby stimulating keratinocyte proliferation and eventual plaque formation. The relationship between diet and psoriasis is poorly understood; however, hyperinsulinemia is associated with greater severity of psoriasis.³¹ 

Four observational studies examined sugar intake in psoriasis patients. Barrea et al²³ conducted a survey-based study of 82 male participants (41 with psoriasis and 41 healthy controls), reporting that PASI score was correlated with intake of simple carbohydrates (percentage of total kilocalorie)(r=0.564, P<.001). Another study by Yamashita et al²⁷ found higher sugar intake in psoriasis patients than controls (P=.003) based on surveys from 70 patients with psoriasis and 70 matched healthy controls.

These findings contrast with 2 survey-based studies by Johnson et al²² and Afifi et al²⁵ of sugar intake in psoriasis patients using the National Health and Nutrition Examination Survey. Johnson et al²² reported reduced sugar intake among 156 psoriasis patients compared with 6104 unmatched controls (odds ratio, 0.998; CI, 0.996-1 [P=.04]) from 2003 to 2006. Similarly, Afifi et al²⁵ reported decreased sugar intake in 1206 psoriasis patients compared with sex- and age-matched controls (P<.0001) in 2009 and 2010. When patients were asked about dietary triggers, 13.8% of psoriasis patients reported sugar as the most common trigger, which was more frequent than alcohol (13.6%), gluten (7.2%), and dairy (6%).²⁵

Castaldo et al^29,30 published 2 nonrandomized clinical intervention studies in 2020 and 2021 evaluating the impact of the ketogenic diet on psoriasis. In the first study, 37 psoriasis patients followed a 10-week diet consisting of 4 weeks on a ketogenic diet (500 kcal/d) followed by 6 weeks on a low-caloric Mediterranean diet.²⁹ At the end of the intervention, there was a 17.4% reduction in PASI score, a 33.2-point reduction in itch severity score, and a 13.4-point reduction in the dermatology life quality index score; however, this study did not include a control diet group for comparison.²⁹ The second study included 30 psoriasis patients on a ketogenic diet and 30 control patients without psoriasis on a regular diet.³⁰ The ketogenic diet consisted of 400 to 500 g of vegetables, 20 to 30 g of fat, and a proportion of protein based on body weight with at least 12 g of whey protein and various amino acids. Patients on the ketogenic diet had significant reduction in PASI scores (value relative to clinical features, 1.4916 [P=.007]). Furthermore, concentrations of cytokines IL-2 (P=.04) and IL-1β (P=.006) decreased following the ketogenic diet but were not measured in the control group.³⁰

Seborrheic Dermatitis—Seborrheic dermatitis is associated with overcolonization of Malassezia species near lipid-rich sebaceous glands. Malassezia hydrolyzes free fatty acids, yielding oleic acids and leading to T-cell release of IL-8 and IL-17.³² Literature is sparse regarding how dietary modifications may play a role in disease severity. In a survey study, Bett et al¹⁷ compared 16 SD patients to 1:2 matched controls (N=29) to investigate the relationship between sugar consumption and presence of disease. Two control cohorts were selected, 1 from clinic patients diagnosed with verruca and 1 matched by age and sex from a survey-based study at a facility in London, England. Sugar intake was measured both in total grams per day and in “beverage sugar” per day, defined as sugar taken in tea and coffee. There was higher total sugar and higher beverage sugar intake among the SD group compared with both control groups (P<.05).¹⁷

Atopic Dermatitis—Atopic dermatitis is a disease of epidermal barrier dysfunction and IgE-mediated allergic sensitization.³³ There are several mechanisms by which skin structure may be disrupted. It is well established that filaggrin mutations inhibit stratum corneum maturation and lamellar matrix deposition.³⁴ Upregulation of IL-4–, IL-13–, and IL-17–secreting T_H2 cells also is associated with disruption of tight junctions and reduction of filaggrin.^35,36 Given that a T cell–mediated inflammatory response is involved in disease pathogenesis, glycemic control is hypothesized to have therapeutic potential.

Nosrati et al²⁴ surveyed 169 AD patients about their perceived dietary triggers through a 61-question survey based on the National Health and Nutrition Examination Survey. Respondents were queried about their perceptions and dietary changes, such as removal or addition of specific food groups and trial of specific diets. Overall, 16.5% of patients reported sugar being a trigger, making it the fourth most common among those surveyed and less common than dairy (24.8%), gluten (18.3%), and alcohol (17.1%).²⁴

Hidradenitis Suppurativa—Hidradenitis suppurativa is driven by hyperkeratosis, dilatation, and occlusion of pilosebaceous follicular ducts, whose eventual rupture evokes a local acute inflammatory response.³⁷ The inciting event for both acne and HS involves mTOR complex–mediated follicular hyperproliferation andinsulinlike growth factor 1 stimulation of androgen receptors in pilosebaceous glands. Given the similarities between the pathogenesis of acne and HS, it is hypothesized that lifestyle changes, including diet modification, may have a beneficial effect on HS.^38-40

Comment

Acne—Overall, there is strong evidence supporting the efficacy of a low-glycemic diet in the treatment of acne. Notably, among the 6 observational studies identified, there was 1 conflicting study by Bett et al¹⁷ that did not find a statistically significant difference in glucose intake between acne and control patients. However, this study included only 16 acne patients, whereas the other 5 observational studies included 32 to 2255 patients.¹⁷ The strongest evidence supporting low-glycemic dietary interventions in acne treatment is from 2 rigorous randomized clinical trials by Kwon et al²¹ and Smith et al.¹⁸ These trials used intention-to-treat models and maintained consistency in gender, age, and acne treatment protocols across both control and treatment groups. To ensure compliance with dietary interventions, daily telephone calls, food logs, and 24-hour urea sampling were utilized. Acne outcomes were assessed by a dermatologist who remained blinded with well-defined outcome measures. An important limitation of these studies is the difficulty in attributing the observed results solely to reduced glucose intake, as low-glycemic diets often lead to other dietary changes, including reduced fat intake and increased nutrient consumption.^18,21

A 2022 systematic review of acne by Meixiong et al⁴¹ further reinforced the beneficial effects of low-glycemic diets in the management of acne patients. The group reviewed 6 interventional studies and 28 observational studies to investigate the relationship among acne, dairy, and glycemic content and found an association between decreased glucose and dairy on reduction of acne.⁴¹

It is likely that the ketogenic diet, which limits glucose, would be beneficial for acne patients. There may be added benefit through elevated ketone bodies and substantially reduced insulin secretion. However, because there are no observational or interventional studies, further research is needed to draw firm conclusions regarding diet for acne treatment. A randomized clinical trial investigating the effects of the ketogenic diet compared to the low-glycemic diet compared to a regular diet would be valuable.

Psoriasis—Among psoriasis studies, there was a lack of consensus regarding glucose intake and correlation with disease. Among the 4 observational studies, 2 reported increased glucose intake among psoriasis patients and 2 reported decreased glucose intake. It is plausible that the variability in studies is due to differences in sample size and diet heterogeneity among study populations. More specifically, Johnson et al²² and Afifi et al²⁵ analyzed large sample sizes of 6260 and 2412 US participants, respectively, and found decreased sugar intake among psoriasis patients compared to controls. In comparison, Barrea et al²³ and Yamashita et al²⁷ analyzed substantially smaller and more specific populations consisting of 82 Italian and 140 Japanese participants, respectively; both reported increased glucose intake among psoriasis patients compared to controls. These seemingly antithetical results may be explained by regional dietary differences, with varying proportions of meats, vegetables, antioxidants, and vitamins.

Moreover, the variation among studies may be further explained by the high prevalence of comorbidities among psoriasis patients. In the study by Barrea et al,²³ psoriasis patients had higher fasting glucose (P=.004) and insulin (P=.022) levels than healthy patients. After adjusting for body mass index and metabolic syndrome, the correlation coefficient measuring the relationship between the PASI score and intake of simple carbohydrates changed from r=0.564 (P<.001) to r=0.352 (P=.028). The confounding impact of these comorbidities was further highlighted by Yamashita et al,²⁷ who found statistically significant differences in glucose intake between psoriasis and healthy patients (P=.003). However, they reported diminished significance on additional subgroup analysis accounting for potential comorbidities (P=.994).²⁷ Johnson et al²² and Afifi et al²⁵ did not account for comorbidities; therefore, the 4 observational study results must be interpreted cautiously.

The 2 randomized clinical trials by Castaldo et al^29,30 weakly suggest that a ketogenic diet may be beneficial for psoriasis patients. The studies have several notable limitations, including insufficient sample sizes and control groups. Thus, the decreased PASI scores reported in psoriasis patients on the ketogenic diets are challenging to interpret. Additionally, both studies placed patients on highly restrictive diets of 500 kcal/d for 4 weeks. The feasibility of recommending such a diet to patients in clinical practice is questionable. Diets of less than 500 kcal/d may be dangerous for patients with underlying comorbidities and are unlikely to serve as long-term solutions.²³ To contextualize our findings, a 2022 review by Chung et al⁴² examined the impact of various diets—low-caloric, gluten-free, Mediterranean, Western, and ketogenic—on psoriasis and reported insufficient evidence to suggest a benefit to the ketogenic diet for psoriasis patients, though the Mediterranean diet may be well suited for psoriasis patients because of improved cardiovascular health and reduced mortality.

Seborrheic Dermatitis—Sanders et al⁴³ found that patients with a high-fruit diet had lower odds of having SD, while those on a Western diet had higher odds of having SD. Although the study did not measure glycemic load, it is conceivable that the high glycemic load characteristic of the Western diet contributed to these findings.⁴³ However, no studies have investigated the direct link between low-glycemic or ketogenic diets and SD, leaving this area open for further study.

Atopic Dermatitis—It has been hypothesized that mitigating T cell–mediated inflammation via glucose control may contribute to the improvement in AD.^35,36 However, in one study, 16.5% of AD patients self-identified sugar as a dietary trigger, ranking fourth among other dietary triggers.²⁴ Thus, the connection between glucose levels and AD warrants further exploration.

Hidradenitis Suppurativa—Given the role of metabolic and hormonal influence in HS as well as the overlapping pathophysiology with acne, it is possible that low-glycemic and ketogenic diets may have a role in improving HS.^38-40 However, there is a gap in observation and controlled studies investigating the link between low-glycemic or ketogenic diets and HS.

Conclusion

Our analysis focused on interventional and observational research exploring the effects of low-glycemic and ketogenic diets on associations and treatment of inflammatory skin conditions. There is sufficient evidence to counsel acne patients on the benefits of a low-glycemic diet as an adjunctive treatment for acne. Currently, there is insufficient evidence to recommend a low-glycemic or ketogenic diet as a treatment for patients with any other inflammatory skin disease. Prospective and controlled clinical trials are needed to clarify the utility of dietary interventions for treating inflammatory skin conditions.

Inflammatory skin conditions often have a relapsing and remitting course and represent a large proportion of chronic skin diseases. Common inflammatory skin disorders include acne, psoriasis, hidradenitis suppurativa (HS), atopic dermatitis (AD), and seborrheic dermatitis (SD).¹ Although each of these conditions has a unique pathogenesis, they all are driven by a background of chronic inflammation. It has been reported that diets with high levels of refined carbohydrates and saturated or trans-fatty acids may exacerbate existing inflammation.² Consequently, dietary interventions, such as the ketogenic and low-glycemic diets, have potential anti-inflammatory and metabolic effects that are being assessed as stand-alone or adjunctive therapies for dermatologic diseases.

Diet may partially influence systemic inflammation through its effect on weight. Higher body mass index and obesity are linked to a low-grade inflammatory state and higher levels of circulating inflammatory markers. Therefore, weight loss leads to decreases in inflammatory cytokines, including C-reactive protein, tumor necrosis factor α, and IL-6.³ These cytokines and metabolic effects overlap with inflammatory skin condition pathways. It also is posited that decreased insulin release associated with weight loss results in decreased sebaceous lipogenesis and androgens, which drive keratinocyte proliferation and acne development.^4,5 For instance, in a 2015 meta-analysis of 5 randomized controlled trials on psoriasis, patients in the weight loss intervention group had more substantial reductions in psoriasis area and severity index (PASI) scores compared with controls receiving usual care (P=.004).⁶ However, in a systematic review of 35 studies on acne vulgaris, overweight and obese patients (defined by a body mass index of ≥23 kg/m²) had similar odds of having acne compared with normal-weight individuals (P=.671).⁷

Similar to weight loss, ketogenesis acts as a negative feedback mechanism to reduce insulin release, leading to decreased inflammation and androgens that often exacerbate inflammatory skin diseases.⁸ Ketogenesis ensues when daily carbohydrate intake is limited to less than 50 g, and long-term adherence to a ketogenic diet results in metabolic reliance on ketone bodies such as acetoacetate, β-hydroxybutyrate, and acetone.⁹ These metabolites may decrease free radical damage and consequently improve signs and symptoms of acne, psoriasis, and other inflammatory skin diseases.^10-12 Similarly, increased ketones also may decrease activation of the NLRP3 (NOD-, LRR-, and Pyrin domain-containing protein 3) inflammasome and therefore reduce inflammatory markers such as IL-1β and IL-1.^4,13 Several proposed mechanisms are outlined in the Table.

Collectively, low-glycemic and ketogenic diets have been proposed as potential interventions for reducing inflammatory skin conditions. These dietary approaches are hypothesized to exert their effects by facilitating weight loss, elevating ketone levels, and reducing systemic inflammation. The current review summarizes the existing evidence on ketogenic and low-glycemic diets as treatments for inflammatory skin conditions and evaluates the potential benefits of these dietary interventions in managing and improving outcomes for individuals with inflammatory skin conditions.

Methods

Using PubMed for articles indexed for MEDLINE and Google Scholar, a review of the literature was conducted with a combination of the following search terms: low-glycemic diet, inflammatory, dermatologic, ketogenic diet, inflammation, dermatology, acne, psoriasis, eczema, seborrheic dermatitis, and hidradenitis suppurativa. Reference citations in identified works also were reviewed. Interventional (experimental studies or clinical trials), survey-based, and observational studies that investigated the effects of low-glycemic or ketogenic diets for the treatment of inflammatory skin conditions were included. Inclusion criteria were studies assessing acne, psoriasis, SD, AD, and HS. Exclusion criteria were studies published before 1965; those written in languages other than English; and those analyzing other diets, such as the Mediterranean or low-fat diets. The search yielded a total of 11 observational studies and 4 controlled studies published between 1966 and January 2023. Because this analysis utilized publicly available data and did not qualify as human subject research, institutional review board approval was not required.

Results

Acne Vulgaris—Acne vulgaris is a disease of chronic pilosebaceous inflammation and follicular epithelial proliferation associated with Propionibacterium acnes. The association between acne and low-glycemic diets has been examined in several studies. Diet quality is measured and assessed using the glycemic index (GI), which is the effect of a single food on postprandial blood glucose, and the glycemic load, which is the GI adjusted for carbohydrates per serving.¹⁴ High levels of GI and glycemic load are associated with hyperinsulinemia and an increase in insulinlike growth factor 1 concentration that promotes mechanistic target of rapamycin (mTOR) complex 1–mediated follicular lipogenesis, sebum fatty acid production, and androgen synthesis.¹⁵Propionibacterium acnes directly activates toll-like receptor 2 on monocytes through damage-associated molecular patterns and indirectly through products of triglyceride catalysis, causing release of IL-12, IL-6, tumor necrosis factor α, and other proinflammatory cytokines.¹⁶ Therefore, lifestyle modifications focused on strict glucose control have been postulated to reduce acne severity via modulation of lipogenesis, androgen concentration, and inflammation.

Six survey-based studies evaluated sugar intake in patients with acne compared to healthy matched controls (eTable). Among these studies, 5 reported higher glycemic loads or daily sugar intake in acne patients compared to individuals without acne.^{12,19,20,26,28} The remaining study was conducted in 1967 and enrolled 16 acne patients and 32 matched controls. It reported no significant difference in sugar intake between the groups (P>.05).¹⁷

Smith et al¹⁸ randomized 43 male patients aged 15 to 25 years with facial acne into 2 cohorts for 12 weeks, each consuming either a low-glycemic diet (25% protein, 45% low-glycemic food [fruits, whole grains], and 30% fat) or a carbohydrate-dense diet of foods with medium to high GI based on prior documentation of the original diet. Patients were instructed to use a noncomedogenic cleanser as their only acne treatment. At 12 weeks, patients consuming the low-glycemic diet had an average of 23.5 fewer inflammatory lesions, while those in the intervention group had 12.0 fewer lesions (P=.03).¹⁸

In another controlled study by Kwon et al,²¹ 32 male and female acne patients were randomized to a low-glycemic diet (25% protein, 45% low-glycemic food, and 30% fat) or a standard diet for 10 weeks. Patients on the low-glycemic diet experienced a 70.9% reduction in inflammatory lesions (P<.05). Hematoxylin and eosin staining and image analysis were performed to measure sebaceous gland surface area in the low-glycemic diet group, which decreased from 0.32 to 0.24 mm² (P=.03). The sebaceous gland surface area in the control group was not reported. Moreover, patients on the low-glycemic diet had reduced IL-8 immunohistochemical staining (decreasing from 2.9 to 1.7 [P=.03]) and sterol regulatory element-binding protein 1 levels (decreasing from 2.6 to 1.3 [P=.03]), suggesting suppression of ongoing inflammation. Patients on the low-glycemic diet had no significant difference in transforming growth factor β1(P=.83). In the control group, there was no difference in IL-8, sterol regulatory element binding protein 1, or transforming growth factor β1 (P>.05) on immunohistochemical staining.²¹

Psoriasis—Psoriasis is a systemic inflammatory disease characterized by hyperproliferation and aberrant keratinocyte plaque formation. The innate immune response of keratinocytes in response to epidermal damage or infection begins with neutrophil recruitment and dendritic cell activation. Dendritic cell secretion of IL-23 promotes T-cell differentiation into helper T cells (T_H1) that subsequently secrete IL-17 and IL-22, thereby stimulating keratinocyte proliferation and eventual plaque formation. The relationship between diet and psoriasis is poorly understood; however, hyperinsulinemia is associated with greater severity of psoriasis.³¹ 

Four observational studies examined sugar intake in psoriasis patients. Barrea et al²³ conducted a survey-based study of 82 male participants (41 with psoriasis and 41 healthy controls), reporting that PASI score was correlated with intake of simple carbohydrates (percentage of total kilocalorie)(r=0.564, P<.001). Another study by Yamashita et al²⁷ found higher sugar intake in psoriasis patients than controls (P=.003) based on surveys from 70 patients with psoriasis and 70 matched healthy controls.

These findings contrast with 2 survey-based studies by Johnson et al²² and Afifi et al²⁵ of sugar intake in psoriasis patients using the National Health and Nutrition Examination Survey. Johnson et al²² reported reduced sugar intake among 156 psoriasis patients compared with 6104 unmatched controls (odds ratio, 0.998; CI, 0.996-1 [P=.04]) from 2003 to 2006. Similarly, Afifi et al²⁵ reported decreased sugar intake in 1206 psoriasis patients compared with sex- and age-matched controls (P<.0001) in 2009 and 2010. When patients were asked about dietary triggers, 13.8% of psoriasis patients reported sugar as the most common trigger, which was more frequent than alcohol (13.6%), gluten (7.2%), and dairy (6%).²⁵

Castaldo et al^29,30 published 2 nonrandomized clinical intervention studies in 2020 and 2021 evaluating the impact of the ketogenic diet on psoriasis. In the first study, 37 psoriasis patients followed a 10-week diet consisting of 4 weeks on a ketogenic diet (500 kcal/d) followed by 6 weeks on a low-caloric Mediterranean diet.²⁹ At the end of the intervention, there was a 17.4% reduction in PASI score, a 33.2-point reduction in itch severity score, and a 13.4-point reduction in the dermatology life quality index score; however, this study did not include a control diet group for comparison.²⁹ The second study included 30 psoriasis patients on a ketogenic diet and 30 control patients without psoriasis on a regular diet.³⁰ The ketogenic diet consisted of 400 to 500 g of vegetables, 20 to 30 g of fat, and a proportion of protein based on body weight with at least 12 g of whey protein and various amino acids. Patients on the ketogenic diet had significant reduction in PASI scores (value relative to clinical features, 1.4916 [P=.007]). Furthermore, concentrations of cytokines IL-2 (P=.04) and IL-1β (P=.006) decreased following the ketogenic diet but were not measured in the control group.³⁰

Seborrheic Dermatitis—Seborrheic dermatitis is associated with overcolonization of Malassezia species near lipid-rich sebaceous glands. Malassezia hydrolyzes free fatty acids, yielding oleic acids and leading to T-cell release of IL-8 and IL-17.³² Literature is sparse regarding how dietary modifications may play a role in disease severity. In a survey study, Bett et al¹⁷ compared 16 SD patients to 1:2 matched controls (N=29) to investigate the relationship between sugar consumption and presence of disease. Two control cohorts were selected, 1 from clinic patients diagnosed with verruca and 1 matched by age and sex from a survey-based study at a facility in London, England. Sugar intake was measured both in total grams per day and in “beverage sugar” per day, defined as sugar taken in tea and coffee. There was higher total sugar and higher beverage sugar intake among the SD group compared with both control groups (P<.05).¹⁷

Atopic Dermatitis—Atopic dermatitis is a disease of epidermal barrier dysfunction and IgE-mediated allergic sensitization.³³ There are several mechanisms by which skin structure may be disrupted. It is well established that filaggrin mutations inhibit stratum corneum maturation and lamellar matrix deposition.³⁴ Upregulation of IL-4–, IL-13–, and IL-17–secreting T_H2 cells also is associated with disruption of tight junctions and reduction of filaggrin.^35,36 Given that a T cell–mediated inflammatory response is involved in disease pathogenesis, glycemic control is hypothesized to have therapeutic potential.

Nosrati et al²⁴ surveyed 169 AD patients about their perceived dietary triggers through a 61-question survey based on the National Health and Nutrition Examination Survey. Respondents were queried about their perceptions and dietary changes, such as removal or addition of specific food groups and trial of specific diets. Overall, 16.5% of patients reported sugar being a trigger, making it the fourth most common among those surveyed and less common than dairy (24.8%), gluten (18.3%), and alcohol (17.1%).²⁴

Hidradenitis Suppurativa—Hidradenitis suppurativa is driven by hyperkeratosis, dilatation, and occlusion of pilosebaceous follicular ducts, whose eventual rupture evokes a local acute inflammatory response.³⁷ The inciting event for both acne and HS involves mTOR complex–mediated follicular hyperproliferation andinsulinlike growth factor 1 stimulation of androgen receptors in pilosebaceous glands. Given the similarities between the pathogenesis of acne and HS, it is hypothesized that lifestyle changes, including diet modification, may have a beneficial effect on HS.^38-40

Comment

Acne—Overall, there is strong evidence supporting the efficacy of a low-glycemic diet in the treatment of acne. Notably, among the 6 observational studies identified, there was 1 conflicting study by Bett et al¹⁷ that did not find a statistically significant difference in glucose intake between acne and control patients. However, this study included only 16 acne patients, whereas the other 5 observational studies included 32 to 2255 patients.¹⁷ The strongest evidence supporting low-glycemic dietary interventions in acne treatment is from 2 rigorous randomized clinical trials by Kwon et al²¹ and Smith et al.¹⁸ These trials used intention-to-treat models and maintained consistency in gender, age, and acne treatment protocols across both control and treatment groups. To ensure compliance with dietary interventions, daily telephone calls, food logs, and 24-hour urea sampling were utilized. Acne outcomes were assessed by a dermatologist who remained blinded with well-defined outcome measures. An important limitation of these studies is the difficulty in attributing the observed results solely to reduced glucose intake, as low-glycemic diets often lead to other dietary changes, including reduced fat intake and increased nutrient consumption.^18,21

A 2022 systematic review of acne by Meixiong et al⁴¹ further reinforced the beneficial effects of low-glycemic diets in the management of acne patients. The group reviewed 6 interventional studies and 28 observational studies to investigate the relationship among acne, dairy, and glycemic content and found an association between decreased glucose and dairy on reduction of acne.⁴¹

It is likely that the ketogenic diet, which limits glucose, would be beneficial for acne patients. There may be added benefit through elevated ketone bodies and substantially reduced insulin secretion. However, because there are no observational or interventional studies, further research is needed to draw firm conclusions regarding diet for acne treatment. A randomized clinical trial investigating the effects of the ketogenic diet compared to the low-glycemic diet compared to a regular diet would be valuable.

Psoriasis—Among psoriasis studies, there was a lack of consensus regarding glucose intake and correlation with disease. Among the 4 observational studies, 2 reported increased glucose intake among psoriasis patients and 2 reported decreased glucose intake. It is plausible that the variability in studies is due to differences in sample size and diet heterogeneity among study populations. More specifically, Johnson et al²² and Afifi et al²⁵ analyzed large sample sizes of 6260 and 2412 US participants, respectively, and found decreased sugar intake among psoriasis patients compared to controls. In comparison, Barrea et al²³ and Yamashita et al²⁷ analyzed substantially smaller and more specific populations consisting of 82 Italian and 140 Japanese participants, respectively; both reported increased glucose intake among psoriasis patients compared to controls. These seemingly antithetical results may be explained by regional dietary differences, with varying proportions of meats, vegetables, antioxidants, and vitamins.

Moreover, the variation among studies may be further explained by the high prevalence of comorbidities among psoriasis patients. In the study by Barrea et al,²³ psoriasis patients had higher fasting glucose (P=.004) and insulin (P=.022) levels than healthy patients. After adjusting for body mass index and metabolic syndrome, the correlation coefficient measuring the relationship between the PASI score and intake of simple carbohydrates changed from r=0.564 (P<.001) to r=0.352 (P=.028). The confounding impact of these comorbidities was further highlighted by Yamashita et al,²⁷ who found statistically significant differences in glucose intake between psoriasis and healthy patients (P=.003). However, they reported diminished significance on additional subgroup analysis accounting for potential comorbidities (P=.994).²⁷ Johnson et al²² and Afifi et al²⁵ did not account for comorbidities; therefore, the 4 observational study results must be interpreted cautiously.

The 2 randomized clinical trials by Castaldo et al^29,30 weakly suggest that a ketogenic diet may be beneficial for psoriasis patients. The studies have several notable limitations, including insufficient sample sizes and control groups. Thus, the decreased PASI scores reported in psoriasis patients on the ketogenic diets are challenging to interpret. Additionally, both studies placed patients on highly restrictive diets of 500 kcal/d for 4 weeks. The feasibility of recommending such a diet to patients in clinical practice is questionable. Diets of less than 500 kcal/d may be dangerous for patients with underlying comorbidities and are unlikely to serve as long-term solutions.²³ To contextualize our findings, a 2022 review by Chung et al⁴² examined the impact of various diets—low-caloric, gluten-free, Mediterranean, Western, and ketogenic—on psoriasis and reported insufficient evidence to suggest a benefit to the ketogenic diet for psoriasis patients, though the Mediterranean diet may be well suited for psoriasis patients because of improved cardiovascular health and reduced mortality.

Seborrheic Dermatitis—Sanders et al⁴³ found that patients with a high-fruit diet had lower odds of having SD, while those on a Western diet had higher odds of having SD. Although the study did not measure glycemic load, it is conceivable that the high glycemic load characteristic of the Western diet contributed to these findings.⁴³ However, no studies have investigated the direct link between low-glycemic or ketogenic diets and SD, leaving this area open for further study.

Atopic Dermatitis—It has been hypothesized that mitigating T cell–mediated inflammation via glucose control may contribute to the improvement in AD.^35,36 However, in one study, 16.5% of AD patients self-identified sugar as a dietary trigger, ranking fourth among other dietary triggers.²⁴ Thus, the connection between glucose levels and AD warrants further exploration.

Hidradenitis Suppurativa—Given the role of metabolic and hormonal influence in HS as well as the overlapping pathophysiology with acne, it is possible that low-glycemic and ketogenic diets may have a role in improving HS.^38-40 However, there is a gap in observation and controlled studies investigating the link between low-glycemic or ketogenic diets and HS.

Conclusion

Our analysis focused on interventional and observational research exploring the effects of low-glycemic and ketogenic diets on associations and treatment of inflammatory skin conditions. There is sufficient evidence to counsel acne patients on the benefits of a low-glycemic diet as an adjunctive treatment for acne. Currently, there is insufficient evidence to recommend a low-glycemic or ketogenic diet as a treatment for patients with any other inflammatory skin disease. Prospective and controlled clinical trials are needed to clarify the utility of dietary interventions for treating inflammatory skin conditions.

Inflammatory skin conditions often have a relapsing and remitting course and represent a large proportion of chronic skin diseases. Common inflammatory skin disorders include acne, psoriasis, hidradenitis suppurativa (HS), atopic dermatitis (AD), and seborrheic dermatitis (SD).¹ Although each of these conditions has a unique pathogenesis, they all are driven by a background of chronic inflammation. It has been reported that diets with high levels of refined carbohydrates and saturated or trans-fatty acids may exacerbate existing inflammation.² Consequently, dietary interventions, such as the ketogenic and low-glycemic diets, have potential anti-inflammatory and metabolic effects that are being assessed as stand-alone or adjunctive therapies for dermatologic diseases.

Diet may partially influence systemic inflammation through its effect on weight. Higher body mass index and obesity are linked to a low-grade inflammatory state and higher levels of circulating inflammatory markers. Therefore, weight loss leads to decreases in inflammatory cytokines, including C-reactive protein, tumor necrosis factor α, and IL-6.³ These cytokines and metabolic effects overlap with inflammatory skin condition pathways. It also is posited that decreased insulin release associated with weight loss results in decreased sebaceous lipogenesis and androgens, which drive keratinocyte proliferation and acne development.^4,5 For instance, in a 2015 meta-analysis of 5 randomized controlled trials on psoriasis, patients in the weight loss intervention group had more substantial reductions in psoriasis area and severity index (PASI) scores compared with controls receiving usual care (P=.004).⁶ However, in a systematic review of 35 studies on acne vulgaris, overweight and obese patients (defined by a body mass index of ≥23 kg/m²) had similar odds of having acne compared with normal-weight individuals (P=.671).⁷

Similar to weight loss, ketogenesis acts as a negative feedback mechanism to reduce insulin release, leading to decreased inflammation and androgens that often exacerbate inflammatory skin diseases.⁸ Ketogenesis ensues when daily carbohydrate intake is limited to less than 50 g, and long-term adherence to a ketogenic diet results in metabolic reliance on ketone bodies such as acetoacetate, β-hydroxybutyrate, and acetone.⁹ These metabolites may decrease free radical damage and consequently improve signs and symptoms of acne, psoriasis, and other inflammatory skin diseases.^10-12 Similarly, increased ketones also may decrease activation of the NLRP3 (NOD-, LRR-, and Pyrin domain-containing protein 3) inflammasome and therefore reduce inflammatory markers such as IL-1β and IL-1.^4,13 Several proposed mechanisms are outlined in the Table.

Collectively, low-glycemic and ketogenic diets have been proposed as potential interventions for reducing inflammatory skin conditions. These dietary approaches are hypothesized to exert their effects by facilitating weight loss, elevating ketone levels, and reducing systemic inflammation. The current review summarizes the existing evidence on ketogenic and low-glycemic diets as treatments for inflammatory skin conditions and evaluates the potential benefits of these dietary interventions in managing and improving outcomes for individuals with inflammatory skin conditions.

Methods

Using PubMed for articles indexed for MEDLINE and Google Scholar, a review of the literature was conducted with a combination of the following search terms: low-glycemic diet, inflammatory, dermatologic, ketogenic diet, inflammation, dermatology, acne, psoriasis, eczema, seborrheic dermatitis, and hidradenitis suppurativa. Reference citations in identified works also were reviewed. Interventional (experimental studies or clinical trials), survey-based, and observational studies that investigated the effects of low-glycemic or ketogenic diets for the treatment of inflammatory skin conditions were included. Inclusion criteria were studies assessing acne, psoriasis, SD, AD, and HS. Exclusion criteria were studies published before 1965; those written in languages other than English; and those analyzing other diets, such as the Mediterranean or low-fat diets. The search yielded a total of 11 observational studies and 4 controlled studies published between 1966 and January 2023. Because this analysis utilized publicly available data and did not qualify as human subject research, institutional review board approval was not required.

Results

Acne Vulgaris—Acne vulgaris is a disease of chronic pilosebaceous inflammation and follicular epithelial proliferation associated with Propionibacterium acnes. The association between acne and low-glycemic diets has been examined in several studies. Diet quality is measured and assessed using the glycemic index (GI), which is the effect of a single food on postprandial blood glucose, and the glycemic load, which is the GI adjusted for carbohydrates per serving.¹⁴ High levels of GI and glycemic load are associated with hyperinsulinemia and an increase in insulinlike growth factor 1 concentration that promotes mechanistic target of rapamycin (mTOR) complex 1–mediated follicular lipogenesis, sebum fatty acid production, and androgen synthesis.¹⁵Propionibacterium acnes directly activates toll-like receptor 2 on monocytes through damage-associated molecular patterns and indirectly through products of triglyceride catalysis, causing release of IL-12, IL-6, tumor necrosis factor α, and other proinflammatory cytokines.¹⁶ Therefore, lifestyle modifications focused on strict glucose control have been postulated to reduce acne severity via modulation of lipogenesis, androgen concentration, and inflammation.

Six survey-based studies evaluated sugar intake in patients with acne compared to healthy matched controls (eTable). Among these studies, 5 reported higher glycemic loads or daily sugar intake in acne patients compared to individuals without acne.^{12,19,20,26,28} The remaining study was conducted in 1967 and enrolled 16 acne patients and 32 matched controls. It reported no significant difference in sugar intake between the groups (P>.05).¹⁷

Smith et al¹⁸ randomized 43 male patients aged 15 to 25 years with facial acne into 2 cohorts for 12 weeks, each consuming either a low-glycemic diet (25% protein, 45% low-glycemic food [fruits, whole grains], and 30% fat) or a carbohydrate-dense diet of foods with medium to high GI based on prior documentation of the original diet. Patients were instructed to use a noncomedogenic cleanser as their only acne treatment. At 12 weeks, patients consuming the low-glycemic diet had an average of 23.5 fewer inflammatory lesions, while those in the intervention group had 12.0 fewer lesions (P=.03).¹⁸

In another controlled study by Kwon et al,²¹ 32 male and female acne patients were randomized to a low-glycemic diet (25% protein, 45% low-glycemic food, and 30% fat) or a standard diet for 10 weeks. Patients on the low-glycemic diet experienced a 70.9% reduction in inflammatory lesions (P<.05). Hematoxylin and eosin staining and image analysis were performed to measure sebaceous gland surface area in the low-glycemic diet group, which decreased from 0.32 to 0.24 mm² (P=.03). The sebaceous gland surface area in the control group was not reported. Moreover, patients on the low-glycemic diet had reduced IL-8 immunohistochemical staining (decreasing from 2.9 to 1.7 [P=.03]) and sterol regulatory element-binding protein 1 levels (decreasing from 2.6 to 1.3 [P=.03]), suggesting suppression of ongoing inflammation. Patients on the low-glycemic diet had no significant difference in transforming growth factor β1(P=.83). In the control group, there was no difference in IL-8, sterol regulatory element binding protein 1, or transforming growth factor β1 (P>.05) on immunohistochemical staining.²¹

Psoriasis—Psoriasis is a systemic inflammatory disease characterized by hyperproliferation and aberrant keratinocyte plaque formation. The innate immune response of keratinocytes in response to epidermal damage or infection begins with neutrophil recruitment and dendritic cell activation. Dendritic cell secretion of IL-23 promotes T-cell differentiation into helper T cells (T_H1) that subsequently secrete IL-17 and IL-22, thereby stimulating keratinocyte proliferation and eventual plaque formation. The relationship between diet and psoriasis is poorly understood; however, hyperinsulinemia is associated with greater severity of psoriasis.³¹ 

Four observational studies examined sugar intake in psoriasis patients. Barrea et al²³ conducted a survey-based study of 82 male participants (41 with psoriasis and 41 healthy controls), reporting that PASI score was correlated with intake of simple carbohydrates (percentage of total kilocalorie)(r=0.564, P<.001). Another study by Yamashita et al²⁷ found higher sugar intake in psoriasis patients than controls (P=.003) based on surveys from 70 patients with psoriasis and 70 matched healthy controls.

These findings contrast with 2 survey-based studies by Johnson et al²² and Afifi et al²⁵ of sugar intake in psoriasis patients using the National Health and Nutrition Examination Survey. Johnson et al²² reported reduced sugar intake among 156 psoriasis patients compared with 6104 unmatched controls (odds ratio, 0.998; CI, 0.996-1 [P=.04]) from 2003 to 2006. Similarly, Afifi et al²⁵ reported decreased sugar intake in 1206 psoriasis patients compared with sex- and age-matched controls (P<.0001) in 2009 and 2010. When patients were asked about dietary triggers, 13.8% of psoriasis patients reported sugar as the most common trigger, which was more frequent than alcohol (13.6%), gluten (7.2%), and dairy (6%).²⁵

Castaldo et al^29,30 published 2 nonrandomized clinical intervention studies in 2020 and 2021 evaluating the impact of the ketogenic diet on psoriasis. In the first study, 37 psoriasis patients followed a 10-week diet consisting of 4 weeks on a ketogenic diet (500 kcal/d) followed by 6 weeks on a low-caloric Mediterranean diet.²⁹ At the end of the intervention, there was a 17.4% reduction in PASI score, a 33.2-point reduction in itch severity score, and a 13.4-point reduction in the dermatology life quality index score; however, this study did not include a control diet group for comparison.²⁹ The second study included 30 psoriasis patients on a ketogenic diet and 30 control patients without psoriasis on a regular diet.³⁰ The ketogenic diet consisted of 400 to 500 g of vegetables, 20 to 30 g of fat, and a proportion of protein based on body weight with at least 12 g of whey protein and various amino acids. Patients on the ketogenic diet had significant reduction in PASI scores (value relative to clinical features, 1.4916 [P=.007]). Furthermore, concentrations of cytokines IL-2 (P=.04) and IL-1β (P=.006) decreased following the ketogenic diet but were not measured in the control group.³⁰

Seborrheic Dermatitis—Seborrheic dermatitis is associated with overcolonization of Malassezia species near lipid-rich sebaceous glands. Malassezia hydrolyzes free fatty acids, yielding oleic acids and leading to T-cell release of IL-8 and IL-17.³² Literature is sparse regarding how dietary modifications may play a role in disease severity. In a survey study, Bett et al¹⁷ compared 16 SD patients to 1:2 matched controls (N=29) to investigate the relationship between sugar consumption and presence of disease. Two control cohorts were selected, 1 from clinic patients diagnosed with verruca and 1 matched by age and sex from a survey-based study at a facility in London, England. Sugar intake was measured both in total grams per day and in “beverage sugar” per day, defined as sugar taken in tea and coffee. There was higher total sugar and higher beverage sugar intake among the SD group compared with both control groups (P<.05).¹⁷

Atopic Dermatitis—Atopic dermatitis is a disease of epidermal barrier dysfunction and IgE-mediated allergic sensitization.³³ There are several mechanisms by which skin structure may be disrupted. It is well established that filaggrin mutations inhibit stratum corneum maturation and lamellar matrix deposition.³⁴ Upregulation of IL-4–, IL-13–, and IL-17–secreting T_H2 cells also is associated with disruption of tight junctions and reduction of filaggrin.^35,36 Given that a T cell–mediated inflammatory response is involved in disease pathogenesis, glycemic control is hypothesized to have therapeutic potential.

Nosrati et al²⁴ surveyed 169 AD patients about their perceived dietary triggers through a 61-question survey based on the National Health and Nutrition Examination Survey. Respondents were queried about their perceptions and dietary changes, such as removal or addition of specific food groups and trial of specific diets. Overall, 16.5% of patients reported sugar being a trigger, making it the fourth most common among those surveyed and less common than dairy (24.8%), gluten (18.3%), and alcohol (17.1%).²⁴

Hidradenitis Suppurativa—Hidradenitis suppurativa is driven by hyperkeratosis, dilatation, and occlusion of pilosebaceous follicular ducts, whose eventual rupture evokes a local acute inflammatory response.³⁷ The inciting event for both acne and HS involves mTOR complex–mediated follicular hyperproliferation andinsulinlike growth factor 1 stimulation of androgen receptors in pilosebaceous glands. Given the similarities between the pathogenesis of acne and HS, it is hypothesized that lifestyle changes, including diet modification, may have a beneficial effect on HS.^38-40

Comment

Acne—Overall, there is strong evidence supporting the efficacy of a low-glycemic diet in the treatment of acne. Notably, among the 6 observational studies identified, there was 1 conflicting study by Bett et al¹⁷ that did not find a statistically significant difference in glucose intake between acne and control patients. However, this study included only 16 acne patients, whereas the other 5 observational studies included 32 to 2255 patients.¹⁷ The strongest evidence supporting low-glycemic dietary interventions in acne treatment is from 2 rigorous randomized clinical trials by Kwon et al²¹ and Smith et al.¹⁸ These trials used intention-to-treat models and maintained consistency in gender, age, and acne treatment protocols across both control and treatment groups. To ensure compliance with dietary interventions, daily telephone calls, food logs, and 24-hour urea sampling were utilized. Acne outcomes were assessed by a dermatologist who remained blinded with well-defined outcome measures. An important limitation of these studies is the difficulty in attributing the observed results solely to reduced glucose intake, as low-glycemic diets often lead to other dietary changes, including reduced fat intake and increased nutrient consumption.^18,21

A 2022 systematic review of acne by Meixiong et al⁴¹ further reinforced the beneficial effects of low-glycemic diets in the management of acne patients. The group reviewed 6 interventional studies and 28 observational studies to investigate the relationship among acne, dairy, and glycemic content and found an association between decreased glucose and dairy on reduction of acne.⁴¹

It is likely that the ketogenic diet, which limits glucose, would be beneficial for acne patients. There may be added benefit through elevated ketone bodies and substantially reduced insulin secretion. However, because there are no observational or interventional studies, further research is needed to draw firm conclusions regarding diet for acne treatment. A randomized clinical trial investigating the effects of the ketogenic diet compared to the low-glycemic diet compared to a regular diet would be valuable.

Psoriasis—Among psoriasis studies, there was a lack of consensus regarding glucose intake and correlation with disease. Among the 4 observational studies, 2 reported increased glucose intake among psoriasis patients and 2 reported decreased glucose intake. It is plausible that the variability in studies is due to differences in sample size and diet heterogeneity among study populations. More specifically, Johnson et al²² and Afifi et al²⁵ analyzed large sample sizes of 6260 and 2412 US participants, respectively, and found decreased sugar intake among psoriasis patients compared to controls. In comparison, Barrea et al²³ and Yamashita et al²⁷ analyzed substantially smaller and more specific populations consisting of 82 Italian and 140 Japanese participants, respectively; both reported increased glucose intake among psoriasis patients compared to controls. These seemingly antithetical results may be explained by regional dietary differences, with varying proportions of meats, vegetables, antioxidants, and vitamins.

Moreover, the variation among studies may be further explained by the high prevalence of comorbidities among psoriasis patients. In the study by Barrea et al,²³ psoriasis patients had higher fasting glucose (P=.004) and insulin (P=.022) levels than healthy patients. After adjusting for body mass index and metabolic syndrome, the correlation coefficient measuring the relationship between the PASI score and intake of simple carbohydrates changed from r=0.564 (P<.001) to r=0.352 (P=.028). The confounding impact of these comorbidities was further highlighted by Yamashita et al,²⁷ who found statistically significant differences in glucose intake between psoriasis and healthy patients (P=.003). However, they reported diminished significance on additional subgroup analysis accounting for potential comorbidities (P=.994).²⁷ Johnson et al²² and Afifi et al²⁵ did not account for comorbidities; therefore, the 4 observational study results must be interpreted cautiously.

The 2 randomized clinical trials by Castaldo et al^29,30 weakly suggest that a ketogenic diet may be beneficial for psoriasis patients. The studies have several notable limitations, including insufficient sample sizes and control groups. Thus, the decreased PASI scores reported in psoriasis patients on the ketogenic diets are challenging to interpret. Additionally, both studies placed patients on highly restrictive diets of 500 kcal/d for 4 weeks. The feasibility of recommending such a diet to patients in clinical practice is questionable. Diets of less than 500 kcal/d may be dangerous for patients with underlying comorbidities and are unlikely to serve as long-term solutions.²³ To contextualize our findings, a 2022 review by Chung et al⁴² examined the impact of various diets—low-caloric, gluten-free, Mediterranean, Western, and ketogenic—on psoriasis and reported insufficient evidence to suggest a benefit to the ketogenic diet for psoriasis patients, though the Mediterranean diet may be well suited for psoriasis patients because of improved cardiovascular health and reduced mortality.

Seborrheic Dermatitis—Sanders et al⁴³ found that patients with a high-fruit diet had lower odds of having SD, while those on a Western diet had higher odds of having SD. Although the study did not measure glycemic load, it is conceivable that the high glycemic load characteristic of the Western diet contributed to these findings.⁴³ However, no studies have investigated the direct link between low-glycemic or ketogenic diets and SD, leaving this area open for further study.

Atopic Dermatitis—It has been hypothesized that mitigating T cell–mediated inflammation via glucose control may contribute to the improvement in AD.^35,36 However, in one study, 16.5% of AD patients self-identified sugar as a dietary trigger, ranking fourth among other dietary triggers.²⁴ Thus, the connection between glucose levels and AD warrants further exploration.

Hidradenitis Suppurativa—Given the role of metabolic and hormonal influence in HS as well as the overlapping pathophysiology with acne, it is possible that low-glycemic and ketogenic diets may have a role in improving HS.^38-40 However, there is a gap in observation and controlled studies investigating the link between low-glycemic or ketogenic diets and HS.

Conclusion

Our analysis focused on interventional and observational research exploring the effects of low-glycemic and ketogenic diets on associations and treatment of inflammatory skin conditions. There is sufficient evidence to counsel acne patients on the benefits of a low-glycemic diet as an adjunctive treatment for acne. Currently, there is insufficient evidence to recommend a low-glycemic or ketogenic diet as a treatment for patients with any other inflammatory skin disease. Prospective and controlled clinical trials are needed to clarify the utility of dietary interventions for treating inflammatory skin conditions.

Psoriasis is a chronic inflammatory disease that affects approximately 3% of the US population.¹ Plaque psoriasis comprises 80% to 90% of cases, while pustular, erythrodermic, guttate, inverse, and palmoplantar disease are less common variants (Figure 1). Psoriatic skin manifestations range from localized to widespread or generalized disease with recurrent flares. Body surface area or psoriasis area and severity index (PASI) measurements primarily focus on skin manifestations and are important for evaluating disease activity and response to treatment, but they have inherent limitations: they do not capture extracutaneous disease activity, systemic inflammation, comorbid conditions, quality of life impact, or the economic burden of psoriasis.

A common manifestation of psoriasis is psoriatic arthritis (PsA), which can involve the nails, joints, ligaments, or tendons in 30% to 41% of affected individuals (Figure 2).^2,3 A growing number of psoriasis-associated comorbidities also have been reported including metabolic syndrome⁴; hyperlipidemia⁵; cardiovascular disease⁶; stroke⁷; hypertension⁸; obesity⁹; sleep disorders¹⁰; malignancy¹¹; infections¹²; inflammatory bowel disease¹³; and mental health disorders such as depression,¹⁴ anxiety,¹⁵ and suicidal ideation.¹⁵ Psoriatic disease also interferes with daily life activities and a patient’s overall quality of life, including interpersonal relationships, intimacy, employment, and work productivity.¹⁶ Finally, the total estimated cost of psoriasis-related health care is more than $35 billion annually,¹⁷ representing a substantial economic burden to our health care system and individual patients.

The overall burden of psoriatic disease has declined markedly in the last 2 decades due to revolutionary advances in our understanding of the immunopathogenesis of psoriasis and the subsequent development of improved therapies that predominantly interrupt IL-23/IL-17 cytokine signaling; however, critical knowledge and treatment gaps persist, underscoring the importance of ongoing clinical and research efforts in psoriatic disease. We review the working immune model of psoriasis, summarize related immune discoveries, and highlight recent therapeutic innovations that are shaping psoriatic disease management.

Current Immune Model of Psoriatic Disease

Psoriasis is an autoinflammatory T cell–mediated disease with negligible contributions from the humoral immune response. Early clinical observations reported increased inflammatory infiltrates in psoriatic skin lesions primarily consisting of both CD4⁺ and CD8⁺ T-cell populations.^18,19 Additionally, patients treated with broad-acting, systemic immunosuppressive medications (eg, cyclosporine, oral corticosteroids) experienced improvement of psoriatic lesions and normalization of the immune infiltrates observed in skin biopsy specimens.^20,21 These early clinical findings led to more sophisticated experimentation in xenotransplant models of psoriasis,^22,23 which explored the clinical efficacy of several less immunosuppressive (eg, methotrexate, anti–tumor necrosis factor [TNF] biologics)²⁴ or T cell–specific agents (eg, alefacept, abatacept, efalizumab).^25-27 The results of these translational studies provided indisputable evidence for the role of the dysregulated immune response as the primary pathogenic process driving plaque formation; they also led to a paradigm shift in how the immunopathogenesis of psoriatic disease was viewed and paved the way for the identification and targeting of other specific proinflammatory signals produced by activated dendritic cell (DC) and T-lymphocyte populations. Among the psoriasis-associated cytokines subsequently identified and studied, elevated IL-23 and IL-17 cytokine levels in psoriatic skin were most closely associated with disease activity, and rapid normalization of IL-23/IL-17 signaling in response to effective oral or injectable antipsoriatic treatments was the hallmark of skin clearance.²⁸ The predominant role of IL-23/IL-17 signaling in the development and maintenance of psoriatic disease is the central feature of all working immune models for this disease (Figure 3).

Psoriasis-Associated Genetic and Environmental Risk Factors

The exact sequence of events that lead to the initiation and formation of plaque psoriasis in susceptible individuals is still poorly understood; however, several important risk factors and key immune events have been identified. First, decades of genetic research have reported more than 80 known psoriasis-associated susceptibility loci,²⁹ which explains approximately 50% of psoriasis heritability. The major genetic determinant of psoriasis, HLA-C*06:02 (formerly HLA-Cw6), resides in the major histocompatibility complex class I region on chromosome 6p21.3 (psoriasis susceptibility gene 1, PSORS1) and is most strongly associated with psoriatic disease.³⁰ Less common psoriasis-associated susceptibility genes also are known to directly or indirectly impact innate and adaptive immune functions that contribute to the pathogenesis of psoriasis.

Second, several nongenetic environmental risk factors for psoriasis have been reported across diverse patient populations, including skin trauma/injury, infections, alcohol/tobacco use, obesity, medication exposure (eg, lithium, antimalarials, beta-blockers), and stress.³¹ These genetic and/or environmental risk factors can trigger the onset of psoriatic disease at any stage of life, though most patients develop disease in early adulthood or later (age range, 50–60 years). Some patients never develop psoriasis despite exposure to environmental risk factors and/or a genetic makeup that is similar to affected first-degree relatives, which requires further study.

Prepsoriatic Skin and Initiation of Plaque Development

In response to environmental stimuli and/or other triggers of the immune system, DC and resident IL-17–producing T-cell (T17) populations become activated in predisposed individuals. Dendritic cell activation leads to the upregulation and increase of several proinflammatory cytokines, including TNF, interferon (IFN) α, IFN-γ, IL-12, and IL-23. Tumor necrosis factor and IL-23 play a vital role in psoriasis by helping to regulate the polarization and expansion of T22 and T17 cells in the skin, whereas IL-12 promotes a corresponding type 1 inflammatory response.³² Increased IL-17 and IL-22 result in alteration of the terminal differentiation and proliferative potential of epidermal keratinocytes, leading to the early clinical hallmarks of psoriatic plaques. The potential contribution of overexpressed psoriasis-related autoantigens, such as LL-37/cathelicidin, ADAMTSL5, and PLA2G4D,³³ in the initiation of psoriatic plaques has been suggested but is poorly characterized.³⁴ Whether these specific autoantigens or others presented by HLA-C variants found on antigen-presenting cells are required for the breakdown of immune tolerance and psoriatic disease initiation is highly relevant but requires further investigation and validation.

In response to the upstream production of IL-23 by dermal DCs, high levels of IL-17 cytokines can be found in mature psoriatic plaques. The IL-17 family consists of 6 dimeric cytokines (IL-17A through IL-17F) that provide innate cutaneous protection against bacterial, viral, and fungal infectious agents, such as Candida albicans. Unlike other IL-17 isoforms, IL-17A and IL-17F share the same receptor complex and have the highest structural homology of any pair (approximately 50% similar).³⁵ The relative expression of IL-17F is higher than IL-17A in psoriasis,³⁶ though IL-17A has been considered as the predominant IL-17 cytokine found in psoriatic skin lesions due to its higher potency.

Binding of IL-17A/F with the IL-17 receptor (IL-17R) on keratinocytes contributes to the development of psoriatic plaques by inducing epidermal hyperplasia via activation of CCAAT/enhancer-binding proteins β and δ, nuclear factor κB, and signal transducer and activator of transcription 1 gene (STAT1).^37,38 This also increases the expression of other keratinocyte-derived proteins (eg, human β-defensins, S-100 proteins, LL-37, other antimicrobial peptides, IL-19, IL-36, IL-17C) that act as reinforcing proinflammatory signals or chemotactic factors (eg, chemokine [C-C motif] ligand 20 [CCL20], chemokine [C-C motif] ligand 1/2/3/5 [CXCL1/2/3/5], CXCL8, IL-8) that facilitate the recruitment of additional immune cells to the skin including polymorphonuclear neutrophils (PMNs), macrophages, and DCs.^39-41 Routine immunohistochemical staining for these keratinocyte-derived proteins reveals a striking epidermal gene expression gradient wherein levels of IL-17–induced proteins are most highly expressed in the uppermost layers of keratinocytes and facilitate the recruitment of immune cells into the epidermis. Activated T17 cells also stimulate the production of keratinocyte-derived chemokines (eg, CXCL9/10/11), which recruit type 1 inflammatory T-cell populations into developing psoriatic plaques.^42,43 Finally, TNF, IL-36, and IL-17C cytokines act synergistically with IL-17A/F to amplify the proinflammatory effects of IL-17 signaling and further stimulate their production from T17 cell populations.⁴⁰ This inflammatory circuit in the skin creates and supports a self-amplifying or positive feedback loop between the skin and immune system that commonly is referred to as feed-forward inflammation (Figure 3).³⁴ The feed-forward inflammatory loop in psoriasis—predominantly driven by increased IL-23/IL-17 signaling—best characterizes the mature psoriatic plaque.

Several findings suggest that the influx of persistent, long-lived resident memory T cells (Trms) may contribute to the mature psoriatic plaque. It is believed that CD8⁺CD103⁺CD49a⁻ Trm cell populations may be responsible for the sharply demarcated borders of untreated psoriasis plaques or their recurrence at specific body sites such as the scalp, buttocks, extremity extensor surfaces, umbilicus, or acral skin following specific stimuli or trauma (Koebner phenomenon or isomorphic response).^44,45 It is not known if repeated stimuli or trauma induce disease formation via the activation of Trm cell populations; further study in large patient cohorts is needed, but this remains an intriguing area of study for durable treatment responses and potential cures for psoriasis.

Recent Discoveries in Psoriatic Disease

Remarkable treatment outcomes for psoriasis have been achieved with multiple selective IL-17 and IL-23 inhibitors (eTable). As demonstrated in several pivotal phase 3 clinical trials for members of these classes of medications, the majority of treated psoriasis patients achieved PASI90 clearance.⁴⁶ Due to their more favorable dosing schedule (ie, fewer injections) and ability to induce a durable remissionlike treatment response, IL-23 inhibitors have become the preferred treatment class for cutaneous disease, while IL-17 inhibitors may be preferred when treating patients with both plaque psoriasis and PsA.^47,48 Nevertheless, the complexity of this disease is punctuated by treated patients who do not adequately respond to selective IL-23/IL-17 blockade.⁴⁹ Recent and emerging treatments may shed light on these recalcitrant cases and will add to the rapidly growing arsenal of available psoriasis therapies.

The Role of IL-17F in Psoriasis and Other Inflammatory Skin Diseases

Dysregulation of IL-17A and IL-17F is associated with several chronic inflammatory conditions, such as psoriasis and PsA.^35,50 Both cytokines, either as homodimers or heterodimers, can selectively bind to the heterodimeric IL-17R formed by the IL-17RA and IL-17RC subunits.³⁵ IL-17F and IL-17C also can synergize with TNF and other cytokines to promote and support the self-sustaining inflammatory circuits in mature psoriatic plaques, though their inflammatory effects in the skin are more limited than IL-17A.^51,52 Therefore, incomplete blockade of IL-17 signaling (ie, unopposed IL-17F and IL-17C) represents a potential mechanism to explain the persistence of psoriasis in patients treated with selective IL-17A inhibitors. This hypothesis is supported by reports of psoriasis patients who have inadequate clinical responses to selective IL-17A inhibition but subsequently improve with IL-17R blockade, which results in disruption of IL-17A as well as IL-17C/E/F cytokine signaling. This formed the basis for further study into the specific role of IL-17F in psoriatic disease and any potential therapeutic benefits associated with its inhibition.

Recently approved in the European Union, Canada, Australia, Japan, the United Kingdom, and the United States for moderate to severe psoriasis, bimekizumab is a novel humanized IgG antibody that selectively inhibits both IL-17A and IL-17F cytokines.⁵³ Specifically, bimekizumab simultaneously prevents binding of IL-17A/A, IL-17A/F, and IL-17F/F dimers with the IL-17R. Compared to other IL-17 and IL-23 biologic therapies, bimekizumab (320 mg) achieved relatively higher response rates for PASI75, PASI90, and PASI100.⁴⁹ Neutralization of IL-17A and IL-17F by bimekizumab also resulted in more complete suppression of cytokine responses and PMN chemotaxis than either cytokine alone in treated PsA patients,⁵⁴ which is notable because of the incremental benefits of recent IL-23 and IL-17 inhibitors on inflammatory arthritis symptoms in contrast to the substantial improvements observed for cutaneous disease with those same agents.

The primary disadvantage of bimekizumab and its more complete blockade of the IL-17 signaling pathway is that treated patients have a substantially increased risk for oral candidiasis (>10%).⁵⁵ However, the precise link between candidiasis and IL-17 blockade is not yet fully understood because other targeted agents that also broadly suppress IL-17 signaling (ie, IL-17R, IL-23 inhibitors) are associated with much lower rates of candidiasis.^56-58 Bimekizumab also is being investigated as a novel therapy for hidradenitis suppurativa and will provide important reference information regarding the role for bispecific biologic agents in the treatment of chronic inflammatory skin diseases.⁵⁹

Recent genetic and clinical studies have expanded our understanding of the role of IL-36 signaling in the immunopathogenesis of pustular psoriasis variants. Generalized pustular psoriasis (GPP) is a rare distinct psoriasis subtype characterized by the recurrent development of widespread erythema, superficial sterile pustules, and desquamation. Systemic symptoms such as fever, malaise, itching, and skin pain accompany acute GPP flares.⁶⁰ Generalized pustular psoriasis is more common in female patients (in contrast with plaque psoriasis), and acute flares may be caused by multiple stimuli including infections, hypocalcemia, initiation or discontinuation of medications (eg, oral corticosteroids), pregnancy, or stress.^61,62 Flares of GPP often require emergency or in-patient care, as untreated symptoms increase the risk for severe health complications such as secondary infections, sepsis, or multisystem organ failure.⁶³ The prevalence of GPP is estimated to be approximately 1 in 10,000 individuals in the United States,^64-67 with mortality rates ranging from 0 to 3.3 deaths per 100 patient-years.⁶⁷

In contrast to plaque psoriasis, aberrant IL-36 signaling is the predominant driver of GPP. IL-36 is a member of the IL-1 cytokine family that includes three IL-36 agonists (IL-36α, IL-36β, IL-36γ) and 1 endogenous antagonist (IL-36Ra, encoded by IL36RN).⁶⁸ The immunopathogenesis of GPP involves dysregulation of the IL-36–chemokine–PMN axis, resulting in unopposed IL-36 signaling and the subsequent recruitment and influx of PMNs into the epidermis. IL36RN mutations are strongly associated with GPP and result in impaired function of the IL-36Ra protein, leading to unopposed IL-36 signaling.⁶⁹ However, approximately two-thirds of GPP patients lack identifiable gene mutations, suggesting other immune mechanisms or triggers causing upregulated IL-36 signaling.⁷⁰ In response to these triggers, increased IL-36 cytokines released by keratinocytes bind to the IL-36R, resulting in substantial keratinocyte hyperproliferation, increased IL-36 levels, and the expression of hundreds of additional inflammatory signals (eg, IL-17C, antimicrobial peptides, TNF, IL-6).⁷¹ Increased IL-36 levels also drive the production of PMN chemotactic proteins (eg, CXCL1/2/3/5/6/8 and CXCR1/2) and act synergistically with IL-17 cytokines to create an autoamplifying circuit that is analogous to the feed-forward inflammatory loop in plaque psoriasis.⁷² Biopsies of involved GPP skin reveal increased expression of IL-36 in the uppermost layers of the epidermis, which creates a gene expression gradient that acts as a strong attractant for PMNs and forms the basis for the hallmark pustular lesions observed in GPP patients.

Until recently, treatment strategies for GPP involved the off-label use of topical, oral, or biologic therapies approved for plaque psoriasis, which often was associated with variable or incomplete disease control. In September 2022, the US Food and Drug Administration (FDA) approved intravenous spesolimab as a first-in-class humanized monoclonal IgG1 antibody for the treatment of GPP flares in adults. Spesolimab binds to IL-36R and prevents its activation by its endogenous agonists. A phase 2, randomized, 12-week clinical trial (Effisayil-1) evaluated the efficacy and safety of a single 900-mg intravenous dose of spesolimab followed by an optional second dose 1 week later for inadequate treatment responses in 53 enrolled GPP patients (2:1 treatment to placebo randomization).⁷³ Remarkably, more than half (19/35 [54%]) of GPP patients experienced complete resolution of pustules (GPP physician global assessment subscore of 0 [range, 0–4]) and showed sustained efficacy out to week 12 after just 1 or 2 doses of spesolimab. Overall, the safety profile of spesolimab was good; asthenia, fatigue, nausea, vomiting, headache, pruritus, infusion-related reaction and symptoms, and mild infections (eg, urinary tract infection) were the most common adverse events reported.⁷³

Imsidolimab, a high-affinity humanized IgG4 monoclonal antibody that binds and blocks activation of IL-36R, also has completed phase 2 testing,⁷⁴ with phase 3 study results expected in early 2024. The rapid onset of action and overall safety of imsidolimab was in line with and similar to spesolimab. Future approval of imsidolimab would add to the limited treatment options available for GPP and has the additional convenience of being administered to patients subcutaneously. Overall, the development of selective IL-36R inhibitors offers a much-needed therapeutic option for GPP and illustrates the importance of translational research.

Role of Tyrosine Kinase in Psoriatic Disease

The Janus kinase (JAK) enzyme family consists of 4 enzymes—tyrosine kinase 2 (TYK2), JAK1, JAK2, and JAK3—that function as intracellular transduction signals that mediate the biologic response of most extracellular cytokines and growth factors.⁷⁵ Critical psoriasis-related cytokines are dependent on intact JAK-STAT signaling, including IL-23, IL-12, and type I IFNs. In 2010, a genome-wide association identified TYK2 as a psoriasis susceptibility locus,⁷⁶ and loss-of-function TYK2 mutations confer a reduced risk for psoriasis.⁷⁷ Unlike other JAK isoforms, TYK2 mediates biologic functions that are highly restricted to the immune responses associated with IL-23, IL-12, and type I IFN signaling.^78,79 For these reasons, blockade of TYK2 signaling is an attractive therapeutic target for the potential treatment of psoriatic disease.

In September 2022, the FDA approved deucravacitinib as a first-in-class, oral, selective TYK2 inhibitor for the treatment of adult patients with moderate to severe plaque psoriasis. It was the first FDA approval of an oral small-molecule treatment for plaque psoriasis in nearly a decade. Deucravacitinib inhibits TYK2 signaling via selective binding of its unique regulatory domain, resulting in a conformational (allosteric) change that interferes with its active domain.⁸⁰ This novel mechanism of action limits the unwanted blockade of other broad biologic processes mediated by JAK1/2/3. Of note, the FDA did not issue any boxed warnings for deucravacitinib as it did for other FDA-approved JAK inhibitors.

In a head-to-head, 52-week, double-blind, prospective, randomized, phase 3 study, deucravacitinib showed clear superiority over apremilast for PASI75 at week 16 (53.0% [271/511] vs 39.8% [101/254]) and week 24 (58.7% [296/504] vs 37.8% [96/254]).⁸¹ Clinical responses were sustained through week 52 and showed efficacy for difficult-to-treat areas such as the scalp, acral sites, and nails. Other advantages of deucravacitinib include once-daily dosing with no need for dose titration or adjustments for renal insufficiency as well as the absence of statistically significant differences in gastrointestinal tract symptoms compared to placebo. The most common adverse effects included nasopharyngitis, upper respiratory tract infections, headache, diarrhea, and herpes infections.⁸¹ The potential benefit of deucravacitinib for PsA and psoriasis comorbidities remains to be seen, but it is promising due to its simultaneous disruption of multiple psoriasis-related cytokine networks. Several other TYK2 inhibitors are being developed for psoriatic disease and related inflammatory conditions, underscoring the promise of targeting this intracellular pathway.

Topical steroids are the mainstay treatment option for localized or limited plaque psoriasis due to their potent immunosuppressive effect on the skin and relatively low cost. Combined with vitamin D analogs, topical steroids result in marked improvements in disease severity and improved tolerability.⁸² However, chronic use of topical steroids is limited by the need for twice-daily application, resulting in poor treatment compliance; loss of efficacy over time; risk for steroid-induced skin atrophy on special body sites; and patient concerns of potential systemic effects. The discovery of novel drug targets amenable to topical inhibition is needed.

Dysregulated aryl hydrocarbon receptor (AHR) levels have been reported in atopic dermatitis and psoriasis.⁸³ Aryl hydrocarbon receptors are ubiquitously expressed in many cell types and play an integral role in immune homeostasis within the skin, skin barrier function, protection against oxidative stressors, and regulation of proliferating melanocytes and keratinocytes.^84,85 They are widely expressed in multiple immune cell types (eg, antigen-presenting cells, T lymphocytes, fibroblasts) and modulate the differentiation of T17 and T22 cells as well as their balance with regulatory T-cell populations.⁸⁶ In keratinocytes, AHR helps to regulate terminal differentiation, enhance skin barrier integrity via AHR-dependent filaggrin (FLG) expression, and prevent transepidermal water loss.^87,88 The mechanisms by which AHR ligands lead to the upregulation or downregulation of specific genes is intricate and highly context dependent, such as the specific ligand and cell type involved. In preclinical studies, AHR-deficient mice develop psoriasiform skin inflammation, increased IL-17 and IL-22 expression, and abnormal skin barrier function.⁸⁹ Keratinocytes treated with AHR ligands in vitro modulated psoriasis-associated inflammatory cytokines, such as IL-6, IL-8, and type I and II IFNs.^89,90 The use of coal tar, one of the earliest historical treatments for psoriasis, is thought to activate AHRs in the skin via organic compound mixtures containing polyaromatic hydrocarbons that help normalize the proinflammatory environment in psoriatic skin.⁹¹

In June 2022, the FDA approved tapinarof as a first-in-class, topical, nonsteroidal AHR agonist for the treatment of plaque psoriasis in adults. Although the exact mechanism of action for tapinarof has not been fully elucidated, early studies suggest that its primary function is the activation of AHR, leading to reduced T-cell expansion and T17 cell differentiation. In the imiquimod mouse model, cytokine expression of IL-17A, IL-17F, IL-19, IL-22, IL-23A, and IL-lβ in psoriasiform skin lesions were downregulated following tapinarof treatment.⁹² In humans, tapinarof treatment is associated with a remittive effect, in which the average time for tapinarof-treated psoriasis lesions to remain clear was approximately 4 months.⁹³ Preliminary research investigating the mechanism by which tapinarof induces this remittive effect is ongoing and may involve the reduced activation and influx of T17 and Trm populations into the skin.⁹⁴ However, these preclinical studies were performed on healthy dermatome-derived skin tissue cultured in T17-skewing conditions and needs to be replicated in larger samples sizes using human-derived psoriatic tissue. Alternatively, a strong inhibitory effect on IL-23 cytokine signaling may, in part, explain the remittive effect of tapinarof, as an analogous response is observed in patients who start and discontinue treatment with selective IL-23 antagonists. Regardless, the once-daily dosing of tapinarof and sustained treatment response is appealing to psoriasis patients. Tapinarof generally is well tolerated with mild folliculitis (>20% of patients) and contact dermatitis (5% of patients) reported as the most common skin-related adverse events.

New Roles for Phosphodiesterase 4 Inhibition

Phosphodiesterases (PDEs) are enzymes that hydrolyze cyclic nucleotides (eg, cyclic adenosine monophosphate) to regulate intracellular secondary messengers involved in the inflammatory response. One of several enzymes in the PDE family, PDE4, has been shown to have greater activity in psoriatic skin compared to healthy skin.⁹⁵ Phosphodiesterase inhibitors decrease the degradation of cyclic adenosine monophosphate, which triggers protein kinase A to downregulate proinflammatory (eg, TNF-α, IL-6, IL-17, IL-12, IL-23) cytokines and increased expression of anti-inflammatory signals such as IL-10.^96,97 Apremilast, the first oral PDE4 inhibitor approved by the FDA for psoriasis, offered a safe alternative to traditional oral immunosuppressive agents that had extensive risks and potential end-organ adverse effects. Unfortunately, apremilast demonstrated modest efficacy for psoriatic disease (better efficacy in the skin vs joint manifestations) and was supplanted easily by next-generation targeted biologic agents that were more efficacious and lacked the troublesome gastrointestinal tract adverse effects of PDE4 inhibition.⁹⁸

Crisaborole became the first topical PDE4 inhibitor approved in the United States in December 2016 for twice-daily treatment of atopic dermatitis. Although phase 2 trial results were reported in psoriasis, this indication was never pursued, presumably due to similar improvements in primary outcome measures at week 12, compared to placebo (ClinicalTrials.gov Identifier NCT01300052).

In July 2022, the first topical PDE4 inhibitor indicated for plaque psoriasis was approved by the FDA—­roflumilast cream 0.3% for once-daily use in individuals 12 years and older. Roflumilast was found to be clinically efficacious as early as 2 weeks after its use in an early-phase clinical trial.⁹⁹ In 2 phase 3 clinical trials (DERMIS-1 and DERMIS-2), roflumilast significantly increased the proportion of patients achieving PASI75 at week 8 compared to vehicle (39%–41.6% vs 5.3%–7.6%, respectively)(P<.001).¹⁰⁰ Overall, this nonsteroidal topical therapy was found to be well tolerated, with infrequent reports of application site pain or irritation as adverse events. Similar to tapinarof, patients can apply roflumilast on all body surface areas including the face, external genitalia, and other intertriginous areas.¹⁰⁰ Importantly, the broad immune impact of PDE4 inhibition suggests that topical roflumilast likely will be an effective treatment for several additional inflammatory conditions, including seborrheic dermatitis and atopic dermatitis, which would expand the clinical utility of this specific medication.

Conclusion

In the last 2 decades, we have witnessed a translational revolution in our understanding of the underlying genetics and immunology of psoriatic disease. Psoriasis is widely considered one of the best-managed inflammatory conditions in all of medicine due to the development and availability of highly targeted, effective topical and systemic therapies that predominantly disrupt IL-23/IL-17 cytokine signaling in affected tissues. However, future clinical studies and laboratory research are necessary to elucidate the precise cause of psoriasis as well as the underlying genetic and immune signaling pathways driving less common clinical variants and recalcitrant disease.

Psoriasis is a chronic inflammatory disease that affects approximately 3% of the US population.¹ Plaque psoriasis comprises 80% to 90% of cases, while pustular, erythrodermic, guttate, inverse, and palmoplantar disease are less common variants (Figure 1). Psoriatic skin manifestations range from localized to widespread or generalized disease with recurrent flares. Body surface area or psoriasis area and severity index (PASI) measurements primarily focus on skin manifestations and are important for evaluating disease activity and response to treatment, but they have inherent limitations: they do not capture extracutaneous disease activity, systemic inflammation, comorbid conditions, quality of life impact, or the economic burden of psoriasis.

A common manifestation of psoriasis is psoriatic arthritis (PsA), which can involve the nails, joints, ligaments, or tendons in 30% to 41% of affected individuals (Figure 2).^2,3 A growing number of psoriasis-associated comorbidities also have been reported including metabolic syndrome⁴; hyperlipidemia⁵; cardiovascular disease⁶; stroke⁷; hypertension⁸; obesity⁹; sleep disorders¹⁰; malignancy¹¹; infections¹²; inflammatory bowel disease¹³; and mental health disorders such as depression,¹⁴ anxiety,¹⁵ and suicidal ideation.¹⁵ Psoriatic disease also interferes with daily life activities and a patient’s overall quality of life, including interpersonal relationships, intimacy, employment, and work productivity.¹⁶ Finally, the total estimated cost of psoriasis-related health care is more than $35 billion annually,¹⁷ representing a substantial economic burden to our health care system and individual patients.

The overall burden of psoriatic disease has declined markedly in the last 2 decades due to revolutionary advances in our understanding of the immunopathogenesis of psoriasis and the subsequent development of improved therapies that predominantly interrupt IL-23/IL-17 cytokine signaling; however, critical knowledge and treatment gaps persist, underscoring the importance of ongoing clinical and research efforts in psoriatic disease. We review the working immune model of psoriasis, summarize related immune discoveries, and highlight recent therapeutic innovations that are shaping psoriatic disease management.

Current Immune Model of Psoriatic Disease

Psoriasis is an autoinflammatory T cell–mediated disease with negligible contributions from the humoral immune response. Early clinical observations reported increased inflammatory infiltrates in psoriatic skin lesions primarily consisting of both CD4⁺ and CD8⁺ T-cell populations.^18,19 Additionally, patients treated with broad-acting, systemic immunosuppressive medications (eg, cyclosporine, oral corticosteroids) experienced improvement of psoriatic lesions and normalization of the immune infiltrates observed in skin biopsy specimens.^20,21 These early clinical findings led to more sophisticated experimentation in xenotransplant models of psoriasis,^22,23 which explored the clinical efficacy of several less immunosuppressive (eg, methotrexate, anti–tumor necrosis factor [TNF] biologics)²⁴ or T cell–specific agents (eg, alefacept, abatacept, efalizumab).^25-27 The results of these translational studies provided indisputable evidence for the role of the dysregulated immune response as the primary pathogenic process driving plaque formation; they also led to a paradigm shift in how the immunopathogenesis of psoriatic disease was viewed and paved the way for the identification and targeting of other specific proinflammatory signals produced by activated dendritic cell (DC) and T-lymphocyte populations. Among the psoriasis-associated cytokines subsequently identified and studied, elevated IL-23 and IL-17 cytokine levels in psoriatic skin were most closely associated with disease activity, and rapid normalization of IL-23/IL-17 signaling in response to effective oral or injectable antipsoriatic treatments was the hallmark of skin clearance.²⁸ The predominant role of IL-23/IL-17 signaling in the development and maintenance of psoriatic disease is the central feature of all working immune models for this disease (Figure 3).

Psoriasis-Associated Genetic and Environmental Risk Factors

The exact sequence of events that lead to the initiation and formation of plaque psoriasis in susceptible individuals is still poorly understood; however, several important risk factors and key immune events have been identified. First, decades of genetic research have reported more than 80 known psoriasis-associated susceptibility loci,²⁹ which explains approximately 50% of psoriasis heritability. The major genetic determinant of psoriasis, HLA-C*06:02 (formerly HLA-Cw6), resides in the major histocompatibility complex class I region on chromosome 6p21.3 (psoriasis susceptibility gene 1, PSORS1) and is most strongly associated with psoriatic disease.³⁰ Less common psoriasis-associated susceptibility genes also are known to directly or indirectly impact innate and adaptive immune functions that contribute to the pathogenesis of psoriasis.

Second, several nongenetic environmental risk factors for psoriasis have been reported across diverse patient populations, including skin trauma/injury, infections, alcohol/tobacco use, obesity, medication exposure (eg, lithium, antimalarials, beta-blockers), and stress.³¹ These genetic and/or environmental risk factors can trigger the onset of psoriatic disease at any stage of life, though most patients develop disease in early adulthood or later (age range, 50–60 years). Some patients never develop psoriasis despite exposure to environmental risk factors and/or a genetic makeup that is similar to affected first-degree relatives, which requires further study.

Prepsoriatic Skin and Initiation of Plaque Development

In response to environmental stimuli and/or other triggers of the immune system, DC and resident IL-17–producing T-cell (T17) populations become activated in predisposed individuals. Dendritic cell activation leads to the upregulation and increase of several proinflammatory cytokines, including TNF, interferon (IFN) α, IFN-γ, IL-12, and IL-23. Tumor necrosis factor and IL-23 play a vital role in psoriasis by helping to regulate the polarization and expansion of T22 and T17 cells in the skin, whereas IL-12 promotes a corresponding type 1 inflammatory response.³² Increased IL-17 and IL-22 result in alteration of the terminal differentiation and proliferative potential of epidermal keratinocytes, leading to the early clinical hallmarks of psoriatic plaques. The potential contribution of overexpressed psoriasis-related autoantigens, such as LL-37/cathelicidin, ADAMTSL5, and PLA2G4D,³³ in the initiation of psoriatic plaques has been suggested but is poorly characterized.³⁴ Whether these specific autoantigens or others presented by HLA-C variants found on antigen-presenting cells are required for the breakdown of immune tolerance and psoriatic disease initiation is highly relevant but requires further investigation and validation.

In response to the upstream production of IL-23 by dermal DCs, high levels of IL-17 cytokines can be found in mature psoriatic plaques. The IL-17 family consists of 6 dimeric cytokines (IL-17A through IL-17F) that provide innate cutaneous protection against bacterial, viral, and fungal infectious agents, such as Candida albicans. Unlike other IL-17 isoforms, IL-17A and IL-17F share the same receptor complex and have the highest structural homology of any pair (approximately 50% similar).³⁵ The relative expression of IL-17F is higher than IL-17A in psoriasis,³⁶ though IL-17A has been considered as the predominant IL-17 cytokine found in psoriatic skin lesions due to its higher potency.

Binding of IL-17A/F with the IL-17 receptor (IL-17R) on keratinocytes contributes to the development of psoriatic plaques by inducing epidermal hyperplasia via activation of CCAAT/enhancer-binding proteins β and δ, nuclear factor κB, and signal transducer and activator of transcription 1 gene (STAT1).^37,38 This also increases the expression of other keratinocyte-derived proteins (eg, human β-defensins, S-100 proteins, LL-37, other antimicrobial peptides, IL-19, IL-36, IL-17C) that act as reinforcing proinflammatory signals or chemotactic factors (eg, chemokine [C-C motif] ligand 20 [CCL20], chemokine [C-C motif] ligand 1/2/3/5 [CXCL1/2/3/5], CXCL8, IL-8) that facilitate the recruitment of additional immune cells to the skin including polymorphonuclear neutrophils (PMNs), macrophages, and DCs.^39-41 Routine immunohistochemical staining for these keratinocyte-derived proteins reveals a striking epidermal gene expression gradient wherein levels of IL-17–induced proteins are most highly expressed in the uppermost layers of keratinocytes and facilitate the recruitment of immune cells into the epidermis. Activated T17 cells also stimulate the production of keratinocyte-derived chemokines (eg, CXCL9/10/11), which recruit type 1 inflammatory T-cell populations into developing psoriatic plaques.^42,43 Finally, TNF, IL-36, and IL-17C cytokines act synergistically with IL-17A/F to amplify the proinflammatory effects of IL-17 signaling and further stimulate their production from T17 cell populations.⁴⁰ This inflammatory circuit in the skin creates and supports a self-amplifying or positive feedback loop between the skin and immune system that commonly is referred to as feed-forward inflammation (Figure 3).³⁴ The feed-forward inflammatory loop in psoriasis—predominantly driven by increased IL-23/IL-17 signaling—best characterizes the mature psoriatic plaque.

Several findings suggest that the influx of persistent, long-lived resident memory T cells (Trms) may contribute to the mature psoriatic plaque. It is believed that CD8⁺CD103⁺CD49a⁻ Trm cell populations may be responsible for the sharply demarcated borders of untreated psoriasis plaques or their recurrence at specific body sites such as the scalp, buttocks, extremity extensor surfaces, umbilicus, or acral skin following specific stimuli or trauma (Koebner phenomenon or isomorphic response).^44,45 It is not known if repeated stimuli or trauma induce disease formation via the activation of Trm cell populations; further study in large patient cohorts is needed, but this remains an intriguing area of study for durable treatment responses and potential cures for psoriasis.

Recent Discoveries in Psoriatic Disease

Remarkable treatment outcomes for psoriasis have been achieved with multiple selective IL-17 and IL-23 inhibitors (eTable). As demonstrated in several pivotal phase 3 clinical trials for members of these classes of medications, the majority of treated psoriasis patients achieved PASI90 clearance.⁴⁶ Due to their more favorable dosing schedule (ie, fewer injections) and ability to induce a durable remissionlike treatment response, IL-23 inhibitors have become the preferred treatment class for cutaneous disease, while IL-17 inhibitors may be preferred when treating patients with both plaque psoriasis and PsA.^47,48 Nevertheless, the complexity of this disease is punctuated by treated patients who do not adequately respond to selective IL-23/IL-17 blockade.⁴⁹ Recent and emerging treatments may shed light on these recalcitrant cases and will add to the rapidly growing arsenal of available psoriasis therapies.

The Role of IL-17F in Psoriasis and Other Inflammatory Skin Diseases

Dysregulation of IL-17A and IL-17F is associated with several chronic inflammatory conditions, such as psoriasis and PsA.^35,50 Both cytokines, either as homodimers or heterodimers, can selectively bind to the heterodimeric IL-17R formed by the IL-17RA and IL-17RC subunits.³⁵ IL-17F and IL-17C also can synergize with TNF and other cytokines to promote and support the self-sustaining inflammatory circuits in mature psoriatic plaques, though their inflammatory effects in the skin are more limited than IL-17A.^51,52 Therefore, incomplete blockade of IL-17 signaling (ie, unopposed IL-17F and IL-17C) represents a potential mechanism to explain the persistence of psoriasis in patients treated with selective IL-17A inhibitors. This hypothesis is supported by reports of psoriasis patients who have inadequate clinical responses to selective IL-17A inhibition but subsequently improve with IL-17R blockade, which results in disruption of IL-17A as well as IL-17C/E/F cytokine signaling. This formed the basis for further study into the specific role of IL-17F in psoriatic disease and any potential therapeutic benefits associated with its inhibition.

Recently approved in the European Union, Canada, Australia, Japan, the United Kingdom, and the United States for moderate to severe psoriasis, bimekizumab is a novel humanized IgG antibody that selectively inhibits both IL-17A and IL-17F cytokines.⁵³ Specifically, bimekizumab simultaneously prevents binding of IL-17A/A, IL-17A/F, and IL-17F/F dimers with the IL-17R. Compared to other IL-17 and IL-23 biologic therapies, bimekizumab (320 mg) achieved relatively higher response rates for PASI75, PASI90, and PASI100.⁴⁹ Neutralization of IL-17A and IL-17F by bimekizumab also resulted in more complete suppression of cytokine responses and PMN chemotaxis than either cytokine alone in treated PsA patients,⁵⁴ which is notable because of the incremental benefits of recent IL-23 and IL-17 inhibitors on inflammatory arthritis symptoms in contrast to the substantial improvements observed for cutaneous disease with those same agents.

The primary disadvantage of bimekizumab and its more complete blockade of the IL-17 signaling pathway is that treated patients have a substantially increased risk for oral candidiasis (>10%).⁵⁵ However, the precise link between candidiasis and IL-17 blockade is not yet fully understood because other targeted agents that also broadly suppress IL-17 signaling (ie, IL-17R, IL-23 inhibitors) are associated with much lower rates of candidiasis.^56-58 Bimekizumab also is being investigated as a novel therapy for hidradenitis suppurativa and will provide important reference information regarding the role for bispecific biologic agents in the treatment of chronic inflammatory skin diseases.⁵⁹

Recent genetic and clinical studies have expanded our understanding of the role of IL-36 signaling in the immunopathogenesis of pustular psoriasis variants. Generalized pustular psoriasis (GPP) is a rare distinct psoriasis subtype characterized by the recurrent development of widespread erythema, superficial sterile pustules, and desquamation. Systemic symptoms such as fever, malaise, itching, and skin pain accompany acute GPP flares.⁶⁰ Generalized pustular psoriasis is more common in female patients (in contrast with plaque psoriasis), and acute flares may be caused by multiple stimuli including infections, hypocalcemia, initiation or discontinuation of medications (eg, oral corticosteroids), pregnancy, or stress.^61,62 Flares of GPP often require emergency or in-patient care, as untreated symptoms increase the risk for severe health complications such as secondary infections, sepsis, or multisystem organ failure.⁶³ The prevalence of GPP is estimated to be approximately 1 in 10,000 individuals in the United States,^64-67 with mortality rates ranging from 0 to 3.3 deaths per 100 patient-years.⁶⁷

In contrast to plaque psoriasis, aberrant IL-36 signaling is the predominant driver of GPP. IL-36 is a member of the IL-1 cytokine family that includes three IL-36 agonists (IL-36α, IL-36β, IL-36γ) and 1 endogenous antagonist (IL-36Ra, encoded by IL36RN).⁶⁸ The immunopathogenesis of GPP involves dysregulation of the IL-36–chemokine–PMN axis, resulting in unopposed IL-36 signaling and the subsequent recruitment and influx of PMNs into the epidermis. IL36RN mutations are strongly associated with GPP and result in impaired function of the IL-36Ra protein, leading to unopposed IL-36 signaling.⁶⁹ However, approximately two-thirds of GPP patients lack identifiable gene mutations, suggesting other immune mechanisms or triggers causing upregulated IL-36 signaling.⁷⁰ In response to these triggers, increased IL-36 cytokines released by keratinocytes bind to the IL-36R, resulting in substantial keratinocyte hyperproliferation, increased IL-36 levels, and the expression of hundreds of additional inflammatory signals (eg, IL-17C, antimicrobial peptides, TNF, IL-6).⁷¹ Increased IL-36 levels also drive the production of PMN chemotactic proteins (eg, CXCL1/2/3/5/6/8 and CXCR1/2) and act synergistically with IL-17 cytokines to create an autoamplifying circuit that is analogous to the feed-forward inflammatory loop in plaque psoriasis.⁷² Biopsies of involved GPP skin reveal increased expression of IL-36 in the uppermost layers of the epidermis, which creates a gene expression gradient that acts as a strong attractant for PMNs and forms the basis for the hallmark pustular lesions observed in GPP patients.

Until recently, treatment strategies for GPP involved the off-label use of topical, oral, or biologic therapies approved for plaque psoriasis, which often was associated with variable or incomplete disease control. In September 2022, the US Food and Drug Administration (FDA) approved intravenous spesolimab as a first-in-class humanized monoclonal IgG1 antibody for the treatment of GPP flares in adults. Spesolimab binds to IL-36R and prevents its activation by its endogenous agonists. A phase 2, randomized, 12-week clinical trial (Effisayil-1) evaluated the efficacy and safety of a single 900-mg intravenous dose of spesolimab followed by an optional second dose 1 week later for inadequate treatment responses in 53 enrolled GPP patients (2:1 treatment to placebo randomization).⁷³ Remarkably, more than half (19/35 [54%]) of GPP patients experienced complete resolution of pustules (GPP physician global assessment subscore of 0 [range, 0–4]) and showed sustained efficacy out to week 12 after just 1 or 2 doses of spesolimab. Overall, the safety profile of spesolimab was good; asthenia, fatigue, nausea, vomiting, headache, pruritus, infusion-related reaction and symptoms, and mild infections (eg, urinary tract infection) were the most common adverse events reported.⁷³

Imsidolimab, a high-affinity humanized IgG4 monoclonal antibody that binds and blocks activation of IL-36R, also has completed phase 2 testing,⁷⁴ with phase 3 study results expected in early 2024. The rapid onset of action and overall safety of imsidolimab was in line with and similar to spesolimab. Future approval of imsidolimab would add to the limited treatment options available for GPP and has the additional convenience of being administered to patients subcutaneously. Overall, the development of selective IL-36R inhibitors offers a much-needed therapeutic option for GPP and illustrates the importance of translational research.

Role of Tyrosine Kinase in Psoriatic Disease

The Janus kinase (JAK) enzyme family consists of 4 enzymes—tyrosine kinase 2 (TYK2), JAK1, JAK2, and JAK3—that function as intracellular transduction signals that mediate the biologic response of most extracellular cytokines and growth factors.⁷⁵ Critical psoriasis-related cytokines are dependent on intact JAK-STAT signaling, including IL-23, IL-12, and type I IFNs. In 2010, a genome-wide association identified TYK2 as a psoriasis susceptibility locus,⁷⁶ and loss-of-function TYK2 mutations confer a reduced risk for psoriasis.⁷⁷ Unlike other JAK isoforms, TYK2 mediates biologic functions that are highly restricted to the immune responses associated with IL-23, IL-12, and type I IFN signaling.^78,79 For these reasons, blockade of TYK2 signaling is an attractive therapeutic target for the potential treatment of psoriatic disease.

In September 2022, the FDA approved deucravacitinib as a first-in-class, oral, selective TYK2 inhibitor for the treatment of adult patients with moderate to severe plaque psoriasis. It was the first FDA approval of an oral small-molecule treatment for plaque psoriasis in nearly a decade. Deucravacitinib inhibits TYK2 signaling via selective binding of its unique regulatory domain, resulting in a conformational (allosteric) change that interferes with its active domain.⁸⁰ This novel mechanism of action limits the unwanted blockade of other broad biologic processes mediated by JAK1/2/3. Of note, the FDA did not issue any boxed warnings for deucravacitinib as it did for other FDA-approved JAK inhibitors.

In a head-to-head, 52-week, double-blind, prospective, randomized, phase 3 study, deucravacitinib showed clear superiority over apremilast for PASI75 at week 16 (53.0% [271/511] vs 39.8% [101/254]) and week 24 (58.7% [296/504] vs 37.8% [96/254]).⁸¹ Clinical responses were sustained through week 52 and showed efficacy for difficult-to-treat areas such as the scalp, acral sites, and nails. Other advantages of deucravacitinib include once-daily dosing with no need for dose titration or adjustments for renal insufficiency as well as the absence of statistically significant differences in gastrointestinal tract symptoms compared to placebo. The most common adverse effects included nasopharyngitis, upper respiratory tract infections, headache, diarrhea, and herpes infections.⁸¹ The potential benefit of deucravacitinib for PsA and psoriasis comorbidities remains to be seen, but it is promising due to its simultaneous disruption of multiple psoriasis-related cytokine networks. Several other TYK2 inhibitors are being developed for psoriatic disease and related inflammatory conditions, underscoring the promise of targeting this intracellular pathway.

Topical steroids are the mainstay treatment option for localized or limited plaque psoriasis due to their potent immunosuppressive effect on the skin and relatively low cost. Combined with vitamin D analogs, topical steroids result in marked improvements in disease severity and improved tolerability.⁸² However, chronic use of topical steroids is limited by the need for twice-daily application, resulting in poor treatment compliance; loss of efficacy over time; risk for steroid-induced skin atrophy on special body sites; and patient concerns of potential systemic effects. The discovery of novel drug targets amenable to topical inhibition is needed.

Dysregulated aryl hydrocarbon receptor (AHR) levels have been reported in atopic dermatitis and psoriasis.⁸³ Aryl hydrocarbon receptors are ubiquitously expressed in many cell types and play an integral role in immune homeostasis within the skin, skin barrier function, protection against oxidative stressors, and regulation of proliferating melanocytes and keratinocytes.^84,85 They are widely expressed in multiple immune cell types (eg, antigen-presenting cells, T lymphocytes, fibroblasts) and modulate the differentiation of T17 and T22 cells as well as their balance with regulatory T-cell populations.⁸⁶ In keratinocytes, AHR helps to regulate terminal differentiation, enhance skin barrier integrity via AHR-dependent filaggrin (FLG) expression, and prevent transepidermal water loss.^87,88 The mechanisms by which AHR ligands lead to the upregulation or downregulation of specific genes is intricate and highly context dependent, such as the specific ligand and cell type involved. In preclinical studies, AHR-deficient mice develop psoriasiform skin inflammation, increased IL-17 and IL-22 expression, and abnormal skin barrier function.⁸⁹ Keratinocytes treated with AHR ligands in vitro modulated psoriasis-associated inflammatory cytokines, such as IL-6, IL-8, and type I and II IFNs.^89,90 The use of coal tar, one of the earliest historical treatments for psoriasis, is thought to activate AHRs in the skin via organic compound mixtures containing polyaromatic hydrocarbons that help normalize the proinflammatory environment in psoriatic skin.⁹¹

In June 2022, the FDA approved tapinarof as a first-in-class, topical, nonsteroidal AHR agonist for the treatment of plaque psoriasis in adults. Although the exact mechanism of action for tapinarof has not been fully elucidated, early studies suggest that its primary function is the activation of AHR, leading to reduced T-cell expansion and T17 cell differentiation. In the imiquimod mouse model, cytokine expression of IL-17A, IL-17F, IL-19, IL-22, IL-23A, and IL-lβ in psoriasiform skin lesions were downregulated following tapinarof treatment.⁹² In humans, tapinarof treatment is associated with a remittive effect, in which the average time for tapinarof-treated psoriasis lesions to remain clear was approximately 4 months.⁹³ Preliminary research investigating the mechanism by which tapinarof induces this remittive effect is ongoing and may involve the reduced activation and influx of T17 and Trm populations into the skin.⁹⁴ However, these preclinical studies were performed on healthy dermatome-derived skin tissue cultured in T17-skewing conditions and needs to be replicated in larger samples sizes using human-derived psoriatic tissue. Alternatively, a strong inhibitory effect on IL-23 cytokine signaling may, in part, explain the remittive effect of tapinarof, as an analogous response is observed in patients who start and discontinue treatment with selective IL-23 antagonists. Regardless, the once-daily dosing of tapinarof and sustained treatment response is appealing to psoriasis patients. Tapinarof generally is well tolerated with mild folliculitis (>20% of patients) and contact dermatitis (5% of patients) reported as the most common skin-related adverse events.

New Roles for Phosphodiesterase 4 Inhibition

Phosphodiesterases (PDEs) are enzymes that hydrolyze cyclic nucleotides (eg, cyclic adenosine monophosphate) to regulate intracellular secondary messengers involved in the inflammatory response. One of several enzymes in the PDE family, PDE4, has been shown to have greater activity in psoriatic skin compared to healthy skin.⁹⁵ Phosphodiesterase inhibitors decrease the degradation of cyclic adenosine monophosphate, which triggers protein kinase A to downregulate proinflammatory (eg, TNF-α, IL-6, IL-17, IL-12, IL-23) cytokines and increased expression of anti-inflammatory signals such as IL-10.^96,97 Apremilast, the first oral PDE4 inhibitor approved by the FDA for psoriasis, offered a safe alternative to traditional oral immunosuppressive agents that had extensive risks and potential end-organ adverse effects. Unfortunately, apremilast demonstrated modest efficacy for psoriatic disease (better efficacy in the skin vs joint manifestations) and was supplanted easily by next-generation targeted biologic agents that were more efficacious and lacked the troublesome gastrointestinal tract adverse effects of PDE4 inhibition.⁹⁸

Crisaborole became the first topical PDE4 inhibitor approved in the United States in December 2016 for twice-daily treatment of atopic dermatitis. Although phase 2 trial results were reported in psoriasis, this indication was never pursued, presumably due to similar improvements in primary outcome measures at week 12, compared to placebo (ClinicalTrials.gov Identifier NCT01300052).

In July 2022, the first topical PDE4 inhibitor indicated for plaque psoriasis was approved by the FDA—­roflumilast cream 0.3% for once-daily use in individuals 12 years and older. Roflumilast was found to be clinically efficacious as early as 2 weeks after its use in an early-phase clinical trial.⁹⁹ In 2 phase 3 clinical trials (DERMIS-1 and DERMIS-2), roflumilast significantly increased the proportion of patients achieving PASI75 at week 8 compared to vehicle (39%–41.6% vs 5.3%–7.6%, respectively)(P<.001).¹⁰⁰ Overall, this nonsteroidal topical therapy was found to be well tolerated, with infrequent reports of application site pain or irritation as adverse events. Similar to tapinarof, patients can apply roflumilast on all body surface areas including the face, external genitalia, and other intertriginous areas.¹⁰⁰ Importantly, the broad immune impact of PDE4 inhibition suggests that topical roflumilast likely will be an effective treatment for several additional inflammatory conditions, including seborrheic dermatitis and atopic dermatitis, which would expand the clinical utility of this specific medication.

Conclusion

In the last 2 decades, we have witnessed a translational revolution in our understanding of the underlying genetics and immunology of psoriatic disease. Psoriasis is widely considered one of the best-managed inflammatory conditions in all of medicine due to the development and availability of highly targeted, effective topical and systemic therapies that predominantly disrupt IL-23/IL-17 cytokine signaling in affected tissues. However, future clinical studies and laboratory research are necessary to elucidate the precise cause of psoriasis as well as the underlying genetic and immune signaling pathways driving less common clinical variants and recalcitrant disease.

Psoriasis is a chronic inflammatory disease that affects approximately 3% of the US population.¹ Plaque psoriasis comprises 80% to 90% of cases, while pustular, erythrodermic, guttate, inverse, and palmoplantar disease are less common variants (Figure 1). Psoriatic skin manifestations range from localized to widespread or generalized disease with recurrent flares. Body surface area or psoriasis area and severity index (PASI) measurements primarily focus on skin manifestations and are important for evaluating disease activity and response to treatment, but they have inherent limitations: they do not capture extracutaneous disease activity, systemic inflammation, comorbid conditions, quality of life impact, or the economic burden of psoriasis.

A common manifestation of psoriasis is psoriatic arthritis (PsA), which can involve the nails, joints, ligaments, or tendons in 30% to 41% of affected individuals (Figure 2).^2,3 A growing number of psoriasis-associated comorbidities also have been reported including metabolic syndrome⁴; hyperlipidemia⁵; cardiovascular disease⁶; stroke⁷; hypertension⁸; obesity⁹; sleep disorders¹⁰; malignancy¹¹; infections¹²; inflammatory bowel disease¹³; and mental health disorders such as depression,¹⁴ anxiety,¹⁵ and suicidal ideation.¹⁵ Psoriatic disease also interferes with daily life activities and a patient’s overall quality of life, including interpersonal relationships, intimacy, employment, and work productivity.¹⁶ Finally, the total estimated cost of psoriasis-related health care is more than $35 billion annually,¹⁷ representing a substantial economic burden to our health care system and individual patients.

The overall burden of psoriatic disease has declined markedly in the last 2 decades due to revolutionary advances in our understanding of the immunopathogenesis of psoriasis and the subsequent development of improved therapies that predominantly interrupt IL-23/IL-17 cytokine signaling; however, critical knowledge and treatment gaps persist, underscoring the importance of ongoing clinical and research efforts in psoriatic disease. We review the working immune model of psoriasis, summarize related immune discoveries, and highlight recent therapeutic innovations that are shaping psoriatic disease management.

Current Immune Model of Psoriatic Disease

Psoriasis is an autoinflammatory T cell–mediated disease with negligible contributions from the humoral immune response. Early clinical observations reported increased inflammatory infiltrates in psoriatic skin lesions primarily consisting of both CD4⁺ and CD8⁺ T-cell populations.^18,19 Additionally, patients treated with broad-acting, systemic immunosuppressive medications (eg, cyclosporine, oral corticosteroids) experienced improvement of psoriatic lesions and normalization of the immune infiltrates observed in skin biopsy specimens.^20,21 These early clinical findings led to more sophisticated experimentation in xenotransplant models of psoriasis,^22,23 which explored the clinical efficacy of several less immunosuppressive (eg, methotrexate, anti–tumor necrosis factor [TNF] biologics)²⁴ or T cell–specific agents (eg, alefacept, abatacept, efalizumab).^25-27 The results of these translational studies provided indisputable evidence for the role of the dysregulated immune response as the primary pathogenic process driving plaque formation; they also led to a paradigm shift in how the immunopathogenesis of psoriatic disease was viewed and paved the way for the identification and targeting of other specific proinflammatory signals produced by activated dendritic cell (DC) and T-lymphocyte populations. Among the psoriasis-associated cytokines subsequently identified and studied, elevated IL-23 and IL-17 cytokine levels in psoriatic skin were most closely associated with disease activity, and rapid normalization of IL-23/IL-17 signaling in response to effective oral or injectable antipsoriatic treatments was the hallmark of skin clearance.²⁸ The predominant role of IL-23/IL-17 signaling in the development and maintenance of psoriatic disease is the central feature of all working immune models for this disease (Figure 3).

Psoriasis-Associated Genetic and Environmental Risk Factors

The exact sequence of events that lead to the initiation and formation of plaque psoriasis in susceptible individuals is still poorly understood; however, several important risk factors and key immune events have been identified. First, decades of genetic research have reported more than 80 known psoriasis-associated susceptibility loci,²⁹ which explains approximately 50% of psoriasis heritability. The major genetic determinant of psoriasis, HLA-C*06:02 (formerly HLA-Cw6), resides in the major histocompatibility complex class I region on chromosome 6p21.3 (psoriasis susceptibility gene 1, PSORS1) and is most strongly associated with psoriatic disease.³⁰ Less common psoriasis-associated susceptibility genes also are known to directly or indirectly impact innate and adaptive immune functions that contribute to the pathogenesis of psoriasis.

Second, several nongenetic environmental risk factors for psoriasis have been reported across diverse patient populations, including skin trauma/injury, infections, alcohol/tobacco use, obesity, medication exposure (eg, lithium, antimalarials, beta-blockers), and stress.³¹ These genetic and/or environmental risk factors can trigger the onset of psoriatic disease at any stage of life, though most patients develop disease in early adulthood or later (age range, 50–60 years). Some patients never develop psoriasis despite exposure to environmental risk factors and/or a genetic makeup that is similar to affected first-degree relatives, which requires further study.

Prepsoriatic Skin and Initiation of Plaque Development

In response to environmental stimuli and/or other triggers of the immune system, DC and resident IL-17–producing T-cell (T17) populations become activated in predisposed individuals. Dendritic cell activation leads to the upregulation and increase of several proinflammatory cytokines, including TNF, interferon (IFN) α, IFN-γ, IL-12, and IL-23. Tumor necrosis factor and IL-23 play a vital role in psoriasis by helping to regulate the polarization and expansion of T22 and T17 cells in the skin, whereas IL-12 promotes a corresponding type 1 inflammatory response.³² Increased IL-17 and IL-22 result in alteration of the terminal differentiation and proliferative potential of epidermal keratinocytes, leading to the early clinical hallmarks of psoriatic plaques. The potential contribution of overexpressed psoriasis-related autoantigens, such as LL-37/cathelicidin, ADAMTSL5, and PLA2G4D,³³ in the initiation of psoriatic plaques has been suggested but is poorly characterized.³⁴ Whether these specific autoantigens or others presented by HLA-C variants found on antigen-presenting cells are required for the breakdown of immune tolerance and psoriatic disease initiation is highly relevant but requires further investigation and validation.

In response to the upstream production of IL-23 by dermal DCs, high levels of IL-17 cytokines can be found in mature psoriatic plaques. The IL-17 family consists of 6 dimeric cytokines (IL-17A through IL-17F) that provide innate cutaneous protection against bacterial, viral, and fungal infectious agents, such as Candida albicans. Unlike other IL-17 isoforms, IL-17A and IL-17F share the same receptor complex and have the highest structural homology of any pair (approximately 50% similar).³⁵ The relative expression of IL-17F is higher than IL-17A in psoriasis,³⁶ though IL-17A has been considered as the predominant IL-17 cytokine found in psoriatic skin lesions due to its higher potency.

Binding of IL-17A/F with the IL-17 receptor (IL-17R) on keratinocytes contributes to the development of psoriatic plaques by inducing epidermal hyperplasia via activation of CCAAT/enhancer-binding proteins β and δ, nuclear factor κB, and signal transducer and activator of transcription 1 gene (STAT1).^37,38 This also increases the expression of other keratinocyte-derived proteins (eg, human β-defensins, S-100 proteins, LL-37, other antimicrobial peptides, IL-19, IL-36, IL-17C) that act as reinforcing proinflammatory signals or chemotactic factors (eg, chemokine [C-C motif] ligand 20 [CCL20], chemokine [C-C motif] ligand 1/2/3/5 [CXCL1/2/3/5], CXCL8, IL-8) that facilitate the recruitment of additional immune cells to the skin including polymorphonuclear neutrophils (PMNs), macrophages, and DCs.^39-41 Routine immunohistochemical staining for these keratinocyte-derived proteins reveals a striking epidermal gene expression gradient wherein levels of IL-17–induced proteins are most highly expressed in the uppermost layers of keratinocytes and facilitate the recruitment of immune cells into the epidermis. Activated T17 cells also stimulate the production of keratinocyte-derived chemokines (eg, CXCL9/10/11), which recruit type 1 inflammatory T-cell populations into developing psoriatic plaques.^42,43 Finally, TNF, IL-36, and IL-17C cytokines act synergistically with IL-17A/F to amplify the proinflammatory effects of IL-17 signaling and further stimulate their production from T17 cell populations.⁴⁰ This inflammatory circuit in the skin creates and supports a self-amplifying or positive feedback loop between the skin and immune system that commonly is referred to as feed-forward inflammation (Figure 3).³⁴ The feed-forward inflammatory loop in psoriasis—predominantly driven by increased IL-23/IL-17 signaling—best characterizes the mature psoriatic plaque.

Several findings suggest that the influx of persistent, long-lived resident memory T cells (Trms) may contribute to the mature psoriatic plaque. It is believed that CD8⁺CD103⁺CD49a⁻ Trm cell populations may be responsible for the sharply demarcated borders of untreated psoriasis plaques or their recurrence at specific body sites such as the scalp, buttocks, extremity extensor surfaces, umbilicus, or acral skin following specific stimuli or trauma (Koebner phenomenon or isomorphic response).^44,45 It is not known if repeated stimuli or trauma induce disease formation via the activation of Trm cell populations; further study in large patient cohorts is needed, but this remains an intriguing area of study for durable treatment responses and potential cures for psoriasis.

Recent Discoveries in Psoriatic Disease

Remarkable treatment outcomes for psoriasis have been achieved with multiple selective IL-17 and IL-23 inhibitors (eTable). As demonstrated in several pivotal phase 3 clinical trials for members of these classes of medications, the majority of treated psoriasis patients achieved PASI90 clearance.⁴⁶ Due to their more favorable dosing schedule (ie, fewer injections) and ability to induce a durable remissionlike treatment response, IL-23 inhibitors have become the preferred treatment class for cutaneous disease, while IL-17 inhibitors may be preferred when treating patients with both plaque psoriasis and PsA.^47,48 Nevertheless, the complexity of this disease is punctuated by treated patients who do not adequately respond to selective IL-23/IL-17 blockade.⁴⁹ Recent and emerging treatments may shed light on these recalcitrant cases and will add to the rapidly growing arsenal of available psoriasis therapies.

The Role of IL-17F in Psoriasis and Other Inflammatory Skin Diseases

Dysregulation of IL-17A and IL-17F is associated with several chronic inflammatory conditions, such as psoriasis and PsA.^35,50 Both cytokines, either as homodimers or heterodimers, can selectively bind to the heterodimeric IL-17R formed by the IL-17RA and IL-17RC subunits.³⁵ IL-17F and IL-17C also can synergize with TNF and other cytokines to promote and support the self-sustaining inflammatory circuits in mature psoriatic plaques, though their inflammatory effects in the skin are more limited than IL-17A.^51,52 Therefore, incomplete blockade of IL-17 signaling (ie, unopposed IL-17F and IL-17C) represents a potential mechanism to explain the persistence of psoriasis in patients treated with selective IL-17A inhibitors. This hypothesis is supported by reports of psoriasis patients who have inadequate clinical responses to selective IL-17A inhibition but subsequently improve with IL-17R blockade, which results in disruption of IL-17A as well as IL-17C/E/F cytokine signaling. This formed the basis for further study into the specific role of IL-17F in psoriatic disease and any potential therapeutic benefits associated with its inhibition.

Recently approved in the European Union, Canada, Australia, Japan, the United Kingdom, and the United States for moderate to severe psoriasis, bimekizumab is a novel humanized IgG antibody that selectively inhibits both IL-17A and IL-17F cytokines.⁵³ Specifically, bimekizumab simultaneously prevents binding of IL-17A/A, IL-17A/F, and IL-17F/F dimers with the IL-17R. Compared to other IL-17 and IL-23 biologic therapies, bimekizumab (320 mg) achieved relatively higher response rates for PASI75, PASI90, and PASI100.⁴⁹ Neutralization of IL-17A and IL-17F by bimekizumab also resulted in more complete suppression of cytokine responses and PMN chemotaxis than either cytokine alone in treated PsA patients,⁵⁴ which is notable because of the incremental benefits of recent IL-23 and IL-17 inhibitors on inflammatory arthritis symptoms in contrast to the substantial improvements observed for cutaneous disease with those same agents.

The primary disadvantage of bimekizumab and its more complete blockade of the IL-17 signaling pathway is that treated patients have a substantially increased risk for oral candidiasis (>10%).⁵⁵ However, the precise link between candidiasis and IL-17 blockade is not yet fully understood because other targeted agents that also broadly suppress IL-17 signaling (ie, IL-17R, IL-23 inhibitors) are associated with much lower rates of candidiasis.^56-58 Bimekizumab also is being investigated as a novel therapy for hidradenitis suppurativa and will provide important reference information regarding the role for bispecific biologic agents in the treatment of chronic inflammatory skin diseases.⁵⁹

Recent genetic and clinical studies have expanded our understanding of the role of IL-36 signaling in the immunopathogenesis of pustular psoriasis variants. Generalized pustular psoriasis (GPP) is a rare distinct psoriasis subtype characterized by the recurrent development of widespread erythema, superficial sterile pustules, and desquamation. Systemic symptoms such as fever, malaise, itching, and skin pain accompany acute GPP flares.⁶⁰ Generalized pustular psoriasis is more common in female patients (in contrast with plaque psoriasis), and acute flares may be caused by multiple stimuli including infections, hypocalcemia, initiation or discontinuation of medications (eg, oral corticosteroids), pregnancy, or stress.^61,62 Flares of GPP often require emergency or in-patient care, as untreated symptoms increase the risk for severe health complications such as secondary infections, sepsis, or multisystem organ failure.⁶³ The prevalence of GPP is estimated to be approximately 1 in 10,000 individuals in the United States,^64-67 with mortality rates ranging from 0 to 3.3 deaths per 100 patient-years.⁶⁷

In contrast to plaque psoriasis, aberrant IL-36 signaling is the predominant driver of GPP. IL-36 is a member of the IL-1 cytokine family that includes three IL-36 agonists (IL-36α, IL-36β, IL-36γ) and 1 endogenous antagonist (IL-36Ra, encoded by IL36RN).⁶⁸ The immunopathogenesis of GPP involves dysregulation of the IL-36–chemokine–PMN axis, resulting in unopposed IL-36 signaling and the subsequent recruitment and influx of PMNs into the epidermis. IL36RN mutations are strongly associated with GPP and result in impaired function of the IL-36Ra protein, leading to unopposed IL-36 signaling.⁶⁹ However, approximately two-thirds of GPP patients lack identifiable gene mutations, suggesting other immune mechanisms or triggers causing upregulated IL-36 signaling.⁷⁰ In response to these triggers, increased IL-36 cytokines released by keratinocytes bind to the IL-36R, resulting in substantial keratinocyte hyperproliferation, increased IL-36 levels, and the expression of hundreds of additional inflammatory signals (eg, IL-17C, antimicrobial peptides, TNF, IL-6).⁷¹ Increased IL-36 levels also drive the production of PMN chemotactic proteins (eg, CXCL1/2/3/5/6/8 and CXCR1/2) and act synergistically with IL-17 cytokines to create an autoamplifying circuit that is analogous to the feed-forward inflammatory loop in plaque psoriasis.⁷² Biopsies of involved GPP skin reveal increased expression of IL-36 in the uppermost layers of the epidermis, which creates a gene expression gradient that acts as a strong attractant for PMNs and forms the basis for the hallmark pustular lesions observed in GPP patients.

Until recently, treatment strategies for GPP involved the off-label use of topical, oral, or biologic therapies approved for plaque psoriasis, which often was associated with variable or incomplete disease control. In September 2022, the US Food and Drug Administration (FDA) approved intravenous spesolimab as a first-in-class humanized monoclonal IgG1 antibody for the treatment of GPP flares in adults. Spesolimab binds to IL-36R and prevents its activation by its endogenous agonists. A phase 2, randomized, 12-week clinical trial (Effisayil-1) evaluated the efficacy and safety of a single 900-mg intravenous dose of spesolimab followed by an optional second dose 1 week later for inadequate treatment responses in 53 enrolled GPP patients (2:1 treatment to placebo randomization).⁷³ Remarkably, more than half (19/35 [54%]) of GPP patients experienced complete resolution of pustules (GPP physician global assessment subscore of 0 [range, 0–4]) and showed sustained efficacy out to week 12 after just 1 or 2 doses of spesolimab. Overall, the safety profile of spesolimab was good; asthenia, fatigue, nausea, vomiting, headache, pruritus, infusion-related reaction and symptoms, and mild infections (eg, urinary tract infection) were the most common adverse events reported.⁷³

Imsidolimab, a high-affinity humanized IgG4 monoclonal antibody that binds and blocks activation of IL-36R, also has completed phase 2 testing,⁷⁴ with phase 3 study results expected in early 2024. The rapid onset of action and overall safety of imsidolimab was in line with and similar to spesolimab. Future approval of imsidolimab would add to the limited treatment options available for GPP and has the additional convenience of being administered to patients subcutaneously. Overall, the development of selective IL-36R inhibitors offers a much-needed therapeutic option for GPP and illustrates the importance of translational research.

Role of Tyrosine Kinase in Psoriatic Disease

The Janus kinase (JAK) enzyme family consists of 4 enzymes—tyrosine kinase 2 (TYK2), JAK1, JAK2, and JAK3—that function as intracellular transduction signals that mediate the biologic response of most extracellular cytokines and growth factors.⁷⁵ Critical psoriasis-related cytokines are dependent on intact JAK-STAT signaling, including IL-23, IL-12, and type I IFNs. In 2010, a genome-wide association identified TYK2 as a psoriasis susceptibility locus,⁷⁶ and loss-of-function TYK2 mutations confer a reduced risk for psoriasis.⁷⁷ Unlike other JAK isoforms, TYK2 mediates biologic functions that are highly restricted to the immune responses associated with IL-23, IL-12, and type I IFN signaling.^78,79 For these reasons, blockade of TYK2 signaling is an attractive therapeutic target for the potential treatment of psoriatic disease.

In September 2022, the FDA approved deucravacitinib as a first-in-class, oral, selective TYK2 inhibitor for the treatment of adult patients with moderate to severe plaque psoriasis. It was the first FDA approval of an oral small-molecule treatment for plaque psoriasis in nearly a decade. Deucravacitinib inhibits TYK2 signaling via selective binding of its unique regulatory domain, resulting in a conformational (allosteric) change that interferes with its active domain.⁸⁰ This novel mechanism of action limits the unwanted blockade of other broad biologic processes mediated by JAK1/2/3. Of note, the FDA did not issue any boxed warnings for deucravacitinib as it did for other FDA-approved JAK inhibitors.

In a head-to-head, 52-week, double-blind, prospective, randomized, phase 3 study, deucravacitinib showed clear superiority over apremilast for PASI75 at week 16 (53.0% [271/511] vs 39.8% [101/254]) and week 24 (58.7% [296/504] vs 37.8% [96/254]).⁸¹ Clinical responses were sustained through week 52 and showed efficacy for difficult-to-treat areas such as the scalp, acral sites, and nails. Other advantages of deucravacitinib include once-daily dosing with no need for dose titration or adjustments for renal insufficiency as well as the absence of statistically significant differences in gastrointestinal tract symptoms compared to placebo. The most common adverse effects included nasopharyngitis, upper respiratory tract infections, headache, diarrhea, and herpes infections.⁸¹ The potential benefit of deucravacitinib for PsA and psoriasis comorbidities remains to be seen, but it is promising due to its simultaneous disruption of multiple psoriasis-related cytokine networks. Several other TYK2 inhibitors are being developed for psoriatic disease and related inflammatory conditions, underscoring the promise of targeting this intracellular pathway.

Topical steroids are the mainstay treatment option for localized or limited plaque psoriasis due to their potent immunosuppressive effect on the skin and relatively low cost. Combined with vitamin D analogs, topical steroids result in marked improvements in disease severity and improved tolerability.⁸² However, chronic use of topical steroids is limited by the need for twice-daily application, resulting in poor treatment compliance; loss of efficacy over time; risk for steroid-induced skin atrophy on special body sites; and patient concerns of potential systemic effects. The discovery of novel drug targets amenable to topical inhibition is needed.

Dysregulated aryl hydrocarbon receptor (AHR) levels have been reported in atopic dermatitis and psoriasis.⁸³ Aryl hydrocarbon receptors are ubiquitously expressed in many cell types and play an integral role in immune homeostasis within the skin, skin barrier function, protection against oxidative stressors, and regulation of proliferating melanocytes and keratinocytes.^84,85 They are widely expressed in multiple immune cell types (eg, antigen-presenting cells, T lymphocytes, fibroblasts) and modulate the differentiation of T17 and T22 cells as well as their balance with regulatory T-cell populations.⁸⁶ In keratinocytes, AHR helps to regulate terminal differentiation, enhance skin barrier integrity via AHR-dependent filaggrin (FLG) expression, and prevent transepidermal water loss.^87,88 The mechanisms by which AHR ligands lead to the upregulation or downregulation of specific genes is intricate and highly context dependent, such as the specific ligand and cell type involved. In preclinical studies, AHR-deficient mice develop psoriasiform skin inflammation, increased IL-17 and IL-22 expression, and abnormal skin barrier function.⁸⁹ Keratinocytes treated with AHR ligands in vitro modulated psoriasis-associated inflammatory cytokines, such as IL-6, IL-8, and type I and II IFNs.^89,90 The use of coal tar, one of the earliest historical treatments for psoriasis, is thought to activate AHRs in the skin via organic compound mixtures containing polyaromatic hydrocarbons that help normalize the proinflammatory environment in psoriatic skin.⁹¹

In June 2022, the FDA approved tapinarof as a first-in-class, topical, nonsteroidal AHR agonist for the treatment of plaque psoriasis in adults. Although the exact mechanism of action for tapinarof has not been fully elucidated, early studies suggest that its primary function is the activation of AHR, leading to reduced T-cell expansion and T17 cell differentiation. In the imiquimod mouse model, cytokine expression of IL-17A, IL-17F, IL-19, IL-22, IL-23A, and IL-lβ in psoriasiform skin lesions were downregulated following tapinarof treatment.⁹² In humans, tapinarof treatment is associated with a remittive effect, in which the average time for tapinarof-treated psoriasis lesions to remain clear was approximately 4 months.⁹³ Preliminary research investigating the mechanism by which tapinarof induces this remittive effect is ongoing and may involve the reduced activation and influx of T17 and Trm populations into the skin.⁹⁴ However, these preclinical studies were performed on healthy dermatome-derived skin tissue cultured in T17-skewing conditions and needs to be replicated in larger samples sizes using human-derived psoriatic tissue. Alternatively, a strong inhibitory effect on IL-23 cytokine signaling may, in part, explain the remittive effect of tapinarof, as an analogous response is observed in patients who start and discontinue treatment with selective IL-23 antagonists. Regardless, the once-daily dosing of tapinarof and sustained treatment response is appealing to psoriasis patients. Tapinarof generally is well tolerated with mild folliculitis (>20% of patients) and contact dermatitis (5% of patients) reported as the most common skin-related adverse events.

New Roles for Phosphodiesterase 4 Inhibition

Phosphodiesterases (PDEs) are enzymes that hydrolyze cyclic nucleotides (eg, cyclic adenosine monophosphate) to regulate intracellular secondary messengers involved in the inflammatory response. One of several enzymes in the PDE family, PDE4, has been shown to have greater activity in psoriatic skin compared to healthy skin.⁹⁵ Phosphodiesterase inhibitors decrease the degradation of cyclic adenosine monophosphate, which triggers protein kinase A to downregulate proinflammatory (eg, TNF-α, IL-6, IL-17, IL-12, IL-23) cytokines and increased expression of anti-inflammatory signals such as IL-10.^96,97 Apremilast, the first oral PDE4 inhibitor approved by the FDA for psoriasis, offered a safe alternative to traditional oral immunosuppressive agents that had extensive risks and potential end-organ adverse effects. Unfortunately, apremilast demonstrated modest efficacy for psoriatic disease (better efficacy in the skin vs joint manifestations) and was supplanted easily by next-generation targeted biologic agents that were more efficacious and lacked the troublesome gastrointestinal tract adverse effects of PDE4 inhibition.⁹⁸

Crisaborole became the first topical PDE4 inhibitor approved in the United States in December 2016 for twice-daily treatment of atopic dermatitis. Although phase 2 trial results were reported in psoriasis, this indication was never pursued, presumably due to similar improvements in primary outcome measures at week 12, compared to placebo (ClinicalTrials.gov Identifier NCT01300052).

In July 2022, the first topical PDE4 inhibitor indicated for plaque psoriasis was approved by the FDA—­roflumilast cream 0.3% for once-daily use in individuals 12 years and older. Roflumilast was found to be clinically efficacious as early as 2 weeks after its use in an early-phase clinical trial.⁹⁹ In 2 phase 3 clinical trials (DERMIS-1 and DERMIS-2), roflumilast significantly increased the proportion of patients achieving PASI75 at week 8 compared to vehicle (39%–41.6% vs 5.3%–7.6%, respectively)(P<.001).¹⁰⁰ Overall, this nonsteroidal topical therapy was found to be well tolerated, with infrequent reports of application site pain or irritation as adverse events. Similar to tapinarof, patients can apply roflumilast on all body surface areas including the face, external genitalia, and other intertriginous areas.¹⁰⁰ Importantly, the broad immune impact of PDE4 inhibition suggests that topical roflumilast likely will be an effective treatment for several additional inflammatory conditions, including seborrheic dermatitis and atopic dermatitis, which would expand the clinical utility of this specific medication.

Conclusion

In the last 2 decades, we have witnessed a translational revolution in our understanding of the underlying genetics and immunology of psoriatic disease. Psoriasis is widely considered one of the best-managed inflammatory conditions in all of medicine due to the development and availability of highly targeted, effective topical and systemic therapies that predominantly disrupt IL-23/IL-17 cytokine signaling in affected tissues. However, future clinical studies and laboratory research are necessary to elucidate the precise cause of psoriasis as well as the underlying genetic and immune signaling pathways driving less common clinical variants and recalcitrant disease.

Acne vulgaris, or acne, is a highly common inflammatory skin disorder affecting up to 85% of the population, and it constitutes the most commonly presenting chief concern in routine dermatology practice.¹ Older teenagers and young adults are most often affected by acne.² Although acne generally is more common in males, adult-onset acne occurs more frequently in women.^2,3 Black and Hispanic women are at higher risk for acne compared to those of Asian, White, or Continental Indian descent.⁴ As such, acne is a common concern in all women of childbearing age.

Concerns for maternal and fetal safety are important therapeutic considerations, especially because hormonal and physiologic changes in pregnancy can lead to onset of inflammatory acne lesions, particularly during the second and third trimesters.⁵ Female patients younger than 25 years; with a higher body mass index, prior irregular menstruation, or polycystic ovary syndrome; or those experiencing their first pregnancy are thought to be more commonly affected.^5-7 In fact, acne affects up to 43% of pregnant women, and lesions typically extend beyond the face to involve the trunk.^6,8-10 Importantly, one-third of women with a history of acne experience symptom relapse after disease-free periods, while two-thirds of those with ongoing disease experience symptom deterioration during pregnancy.¹⁰ Although acne is not a life-threatening condition, it has a well-documented, detrimental impact on social, emotional, and psychological well-being, namely self-perception, social interactions, quality-of-life scores, depression, and anxiety.¹¹

Therefore, safe and effective treatment of pregnant women is of paramount importance. Because pregnant women are not included in clinical trials, there is a paucity of medication safety data, further augmented by inefficient access to available information. The US Food and Drug Administration (FDA) pregnancy safety categories were updated in 2015, letting go of the traditional A, B, C, D, and X categories.¹² The Table reviews the current pregnancy classification system. In this narrative review, we summarize the most recent available data and recommendations on the safety and efficacy of acne treatment during pregnancy.

Topical Treatments for Acne

Benzoyl Peroxide—Benzoyl peroxide commonly is used as first-line therapy alone or in combination with other agents for the treatment of mild to moderate acne.¹³ It is safe for use during pregnancy.¹⁴ Although the medication is systemically absorbed, it undergoes complete metabolism to benzoic acid, a commonly used food additive.^15,16 Benzoic acid has low bioavailability, as it gets rapidly metabolized by the kidneys; therefore, benzoyl peroxide is unlikely to reach clinically significant levels in the maternal circulation and consequently the fetal circulation. Additionally, it has a low risk for causing congenital malformations.¹⁷

Salicylic Acid—For mild to moderate acne, salicylic acid is a second-line agent that likely is safe for use by pregnant women at low concentrations and over limited body surface areas.^14,18,19 There is minimal systemic absorption of the drug.²⁰ Additionally, aspirin, which is broken down in the body into salicylic acid, is used in low doses for the treatment of pre-eclampsia during pregnancy.²¹

Dapsone—The use of dapsone gel 5% as a second-line agent has shown efficacy for mild to moderate acne.²² The oral formulation, commonly used for malaria and leprosy prophylaxis, has failed to show associated fetal toxicity or congenital anomalies.^23,24 It also has been used as a first-line treatment for dermatitis herpetiformis in pregnancy.²⁵ Although the medication likely is safe, it is better to minimize its use during the third trimester to reduce the theoretical risk for hyperbilirubinemia in the neonate.^17,26-29

Azelaic Acid—Azelaic acid effectively targets noninflammatory and inflammatory acne and generally is well tolerated, harboring a good safety profile.³⁰ Topical 20% azelaic acid has localized antibacterial and comedolytic effects and is safe for use during pregnancy.^31,32

Glycolic Acid—Limited data exist on the safety of glycolic acid during pregnancy. In vitro studies have shown up to 27% systemic absorption depending on pH, concentration, and duration of application.³³ Animal reproductive studies involving rats have shown fetal multisystem malformations and developmental abnormalities with oral administration of glycolic acid at doses far exceeding those used in humans.³⁴ Although no human reproductive studies exist, topical glycolic acid is unlikely to reach the developing fetus in notable amounts, and the medication is likely safe for use.^17,35

Clindamycin—Topical clindamycin phosphate is an effective and well-tolerated agent for the treatment of mild to moderate acne.³⁶ Its systemic absorption is minimal, and it is considered safe for use during all trimesters of pregnancy.^{14,17,26,27,35,37}

Erythromycin—Topical erythromycin is another commonly prescribed topical antibiotic used to target mild to moderate acne. However, its use recently has been associated with a decrease in efficacy secondary to the rise of antibacterial resistance in the community.^38-40 Nevertheless, it remains a safe treatment for use during all trimesters of pregnancy.^{14,17,26,27,35,37}

Topical Retinoids—Vitamin A derivatives (also known as retinoids) are the mainstay for the treatment of mild to moderate acne. Limited data exist regarding pregnancy outcomes after in utero exposure.⁴¹ A rare case report suggested topical tretinoin has been associated with fetal otocerebral anomalies.⁴² For tazarotene, teratogenic effects were seen in animal reproductive studies at doses exceeding maximum recommended human doses.^41,43 However, a large meta-analysis failed to find a clear risk for increased congenital malformations, spontaneous abortions, stillbirth, elective termination of pregnancy, low birthweight, or prematurity following first-trimester exposure to topical retinoids.⁴⁴ As the level of exposure that could lead to teratogenicity in humans is unknown, avoidance of both tretinoin and tazarotene is recommended in pregnant women.^41,45 Nevertheless, women inadvertently exposed should be reassured.⁴⁴

Conversely, adapalene has been associated with 1 case of anophthalmia and agenesis of the optic chiasma in a fetus following exposure until 13 weeks’ gestation.⁴⁶ However, a large, open-label trial prior to the patient transitioning from adapalene to over-the-counter treatment showed that the drug harbors a large and reassuring margin of safety and no risk for teratogenicity in a maximal usage trial and Pregnancy Safety Review.⁴⁷ Therefore, adapalene gel 0.1% is a safe and effective medication for the treatment of acne in a nonprescription environment and does not pose harm to the fetus.

Clascoterone—Clascoterone is a novel topical antiandrogenic drug approved for the treatment of hormonal and inflammatory moderate to severe acne.^48-51 Human reproductive data are limited to 1 case of pregnancy that occurred during phase 3 trial investigations, and no adverse outcomes were reported.⁵¹ Minimal systemic absorption follows topical use.⁵² Nonetheless, dose-independent malformations were reported in animal reproductive studies.⁵³ As such, it remains better to avoid the use of clascoterone during pregnancy pending further safety data.

Minocycline Foam—Minocycline foam 4% is approved to treat inflammatory lesions of nonnodular moderate to severe acne in patients 9 years and older.⁵⁴ Systemic absorption is minimal, and the drug has limited bioavailability with minimal systemic accumulation in the patient’s serum.⁵⁵ Given this information, it is unlikely that topical minocycline will reach notable levels in the fetal serum or harbor teratogenic effects, as seen with the oral formulation.⁵⁶ However, it may be best to avoid its use during the second and third trimesters given the potential risk for tooth discoloration in the fetus.^57,58

Systemic Treatments for Acne

Isotretinoin—Isotretinoin is the most effective treatment for moderate to severe acne with a well-documented potential for long-term clearance.⁵⁹ Its use during pregnancy is absolutely contraindicated, as the medication is a well-known teratogen. Associated congenital malformations include numerous craniofacial defects, cardiovascular and neurologic malformations, or thymic disorders that are estimated to affect 20% to 35% of infants exposed in utero.⁶⁰ Furthermore, strict contraception use during treatment is mandated for patients who can become pregnant. It is recommended to wait at least 1 month and 1 menstrual cycle after medication discontinuation before attempting to conceive.¹⁷ Pregnancy termination is recommended if conception occurs during treatment with isotretinoin.

Spironolactone—Spironolactone is an androgen-receptor antagonist commonly prescribed off label for mild to severe acne in females.^61,62 Spironolactone promotes the feminization of male fetuses and should be avoided in pregnancy.⁶³

Doxycycline/Minocycline—Tetracyclines are the most commonly prescribed oral antibiotics for moderate to severe acne.⁶⁴ Although highly effective at treating acne, tetracyclines generally should be avoided in pregnancy. First-trimester use of doxycycline is not absolutely contraindicated but should be reserved for severe illness and not employed for the treatment of acne. However, accidental exposure to doxycycline has not been associated with congenital malformations.⁶⁵ Nevertheless, after the 15th week of gestation, permanent tooth discoloration and bone growth inhibition in the fetus are serious and well-documented risks.^14,17 Additional adverse events following in utero exposure include infantile inguinal hernia, hypospadias, and limb hypoplasia.⁶³

Sarecycline—Sarecycline is a novel tetracycline-class antibiotic for the treatment of moderate to severe inflammatory acne. It has a narrower spectrum of activity compared to its counterparts within its class, which translates to an improved safety profile, namely when it comes to gastrointestinal tract microbiome disruption and potentially decreased likelihood of developing bacterial resistance.⁶⁶ Data on human reproductive studies are limited, but it is advisable to avoid sarecycline in pregnancy, as it may cause adverse developmental effects in the fetus, such as reduced bone growth, in addition to the well-known tetracycline-associated risk for permanent discoloration of the teeth if used during the second and third trimesters.^67,68

Erythromycin—Oral erythromycin targets moderate to severe inflammatory acne and is considered safe for use during pregnancy.^69,70 There has been 1 study reporting an increased risk for atrial and ventricular septal defects (1.8%) and pyloric stenosis (0.2%), but these risks are still uncertain, and erythromycin is considered compatible with pregnancy.⁷¹ However, erythromycin estolate formulations should be avoided given the associated 10% to 15% risk for reversible cholestatic liver injury.⁷² Erythromycin base or erythromycin ethylsuccinate formulations should be favored.

Systemic Steroids—Prednisone is indicated for severe acne with scarring and should only be used during pregnancy after clearance from the patient’s obstetrician. Doses of 0.5 mg/kg or less should be prescribed in combination with systemic antibiotics as well as agents for bone and gastrointestinal tract prophylaxis.²⁹

Zinc—The exact mechanism by which zinc exerts its effects to improve acne remains largely obscure. It has been found effective against inflammatory lesions of mild to moderate acne.⁷³ Generally recommended dosages range from 30 to 200 mg/d but may be associated with gastrointestinal tract disturbances. Dosages of 75 mg/d have shown no harm to the fetus.⁷⁴ When taking this supplement, patients should not exceed the recommended doses given the risk for hypocupremia associated with high-dose zinc supplementation.

Light-Based Therapies

Phototherapy—Narrowband UVB phototherapy is effective for the treatment of mild to moderate acne.⁷⁵ It has been proven to be a safe treatment option during pregnancy, but its use has been associated with decreased folic acid levels.^76-79 Therefore, in addition to attaining baseline folic acid serum levels, supplementation with folic acid prior to treatment, as per routine prenatal guidelines, should be sought.⁸⁰

AviClear—The AviClear (Cutera) laser is the first device cleared by the FDA for mild to severe acne in March 2022.⁸¹ The FDA clearance for the Accure (Accure Acne Inc) laser, also targeting mild to severe acne, followed soon after (November 2022). Both lasers harbor a wavelength of 1726 nm and target sebaceous glands with electrothermolysis.^82,83 Further research and long-term safety data are required before using them in pregnancy.

Other Therapies

Cosmetic Peels—Glycolic acid peels induce epidermolysis and desquamation.⁸⁴ Although data on use during pregnancy are limited, these peels have limited dermal penetration and are considered safe for use in pregnancy.^33,85,86 Similarly, keratolytic lactic acid peels harbor limited dermal penetration and can be safely used in pregnant women.^87-89 Salicylic acid peels also work through epidermolysis and desquamation⁸⁴; however, they tend to penetrate deeper into the skin, reaching down to the basal layer, if large areas are treated or when applied under occlusion.^86,90 Although their use is not contraindicated in pregnancy, they should be limited to small areas of coverage.⁹¹

Intralesional Triamcinolone—Acne cysts and inflammatory papules can be treated with intralesional triamcinolone injections to relieve acute symptoms such as pain.⁹² Low doses at concentrations of 2.5 mg/mL are considered compatible with pregnancy when indicated.²⁹

Approaching the Patient Clinical Encounter

In patients seeking treatment prior to conception, a few recommendations can be made to minimize the risk for acne recurrence or flares during pregnancy. For instance, because data show an association between increased acne severity in those with a higher body mass index and in pregnancy, weight loss may be recommended prior to pregnancy to help mitigate symptoms after conception.⁷ The Figure summarizes our recommendations for approaching and treating acne in pregnancy.

In all patients, grading the severity of the patient’s acne as mild, moderate, or severe is the first step. The presence of scarring is an additional consideration during the physical examination and should be documented. A careful discussion of treatment expectations and prognosis should be the focus before treatment initiation. Meticulous documentation of the physical examination and discussion with the patient should be prioritized.

To minimize toxicity and risks to the developing fetus, monotherapy is favored. Topical therapy should be considered first line. Safe regimens include mild nonabrasive washes, such as those containing benzoyl peroxide or glycolic acid, or topical azelaic acid or clindamycin phosphate for mild to moderate acne. More severe cases warrant the consideration of systemic medications as second line, as more severe acne is better treated with oral antibiotics such as the macrolides erythromycin or clindamycin or systemic corticosteroids when concern exists for severe scarring. The additional use of physical sunscreen also is recommended.

An important topic to address during the clinical encounter is cautious intake of oral supplements for acne during pregnancy, as they may contain harmful and teratogenic ingredients. A recent search focusing on acne supplements available online between March and May 2020 uncovered 49 different supplements, 26 (53%) of which contained vitamin A.⁹³ Importantly, 3 (6%) of these 49 supplements were likely teratogenic, 4 (8%) contained vitamin A doses exceeding the recommended daily nutritional intake level, and 15 (31%) harbored an unknown teratogenic risk. Furthermore, among the 6 (12%) supplements with vitamin A levels exceeding 10,000 IU, 2 lacked any mention of pregnancy warning, including the supplement with the highest vitamin A dose found in this study.⁹³ Because dietary supplements are not subject to the same stringent regulations by the FDA as drugs, inadvertent use by unaware patients ought to be prevented by careful counseling and education.

Finally, patients should be counseled to seek care following delivery for potentially updated medication management of acne, especially if they are breastfeeding. Co-management with a pediatrician may be indicated during lactation, particularly when newborns are born preterm or with other health conditions that may warrant additional caution with the use of certain agents.

Acne vulgaris, or acne, is a highly common inflammatory skin disorder affecting up to 85% of the population, and it constitutes the most commonly presenting chief concern in routine dermatology practice.¹ Older teenagers and young adults are most often affected by acne.² Although acne generally is more common in males, adult-onset acne occurs more frequently in women.^2,3 Black and Hispanic women are at higher risk for acne compared to those of Asian, White, or Continental Indian descent.⁴ As such, acne is a common concern in all women of childbearing age.

Concerns for maternal and fetal safety are important therapeutic considerations, especially because hormonal and physiologic changes in pregnancy can lead to onset of inflammatory acne lesions, particularly during the second and third trimesters.⁵ Female patients younger than 25 years; with a higher body mass index, prior irregular menstruation, or polycystic ovary syndrome; or those experiencing their first pregnancy are thought to be more commonly affected.^5-7 In fact, acne affects up to 43% of pregnant women, and lesions typically extend beyond the face to involve the trunk.^6,8-10 Importantly, one-third of women with a history of acne experience symptom relapse after disease-free periods, while two-thirds of those with ongoing disease experience symptom deterioration during pregnancy.¹⁰ Although acne is not a life-threatening condition, it has a well-documented, detrimental impact on social, emotional, and psychological well-being, namely self-perception, social interactions, quality-of-life scores, depression, and anxiety.¹¹

Therefore, safe and effective treatment of pregnant women is of paramount importance. Because pregnant women are not included in clinical trials, there is a paucity of medication safety data, further augmented by inefficient access to available information. The US Food and Drug Administration (FDA) pregnancy safety categories were updated in 2015, letting go of the traditional A, B, C, D, and X categories.¹² The Table reviews the current pregnancy classification system. In this narrative review, we summarize the most recent available data and recommendations on the safety and efficacy of acne treatment during pregnancy.

Topical Treatments for Acne

Benzoyl Peroxide—Benzoyl peroxide commonly is used as first-line therapy alone or in combination with other agents for the treatment of mild to moderate acne.¹³ It is safe for use during pregnancy.¹⁴ Although the medication is systemically absorbed, it undergoes complete metabolism to benzoic acid, a commonly used food additive.^15,16 Benzoic acid has low bioavailability, as it gets rapidly metabolized by the kidneys; therefore, benzoyl peroxide is unlikely to reach clinically significant levels in the maternal circulation and consequently the fetal circulation. Additionally, it has a low risk for causing congenital malformations.¹⁷

Salicylic Acid—For mild to moderate acne, salicylic acid is a second-line agent that likely is safe for use by pregnant women at low concentrations and over limited body surface areas.^14,18,19 There is minimal systemic absorption of the drug.²⁰ Additionally, aspirin, which is broken down in the body into salicylic acid, is used in low doses for the treatment of pre-eclampsia during pregnancy.²¹

Dapsone—The use of dapsone gel 5% as a second-line agent has shown efficacy for mild to moderate acne.²² The oral formulation, commonly used for malaria and leprosy prophylaxis, has failed to show associated fetal toxicity or congenital anomalies.^23,24 It also has been used as a first-line treatment for dermatitis herpetiformis in pregnancy.²⁵ Although the medication likely is safe, it is better to minimize its use during the third trimester to reduce the theoretical risk for hyperbilirubinemia in the neonate.^17,26-29

Azelaic Acid—Azelaic acid effectively targets noninflammatory and inflammatory acne and generally is well tolerated, harboring a good safety profile.³⁰ Topical 20% azelaic acid has localized antibacterial and comedolytic effects and is safe for use during pregnancy.^31,32

Glycolic Acid—Limited data exist on the safety of glycolic acid during pregnancy. In vitro studies have shown up to 27% systemic absorption depending on pH, concentration, and duration of application.³³ Animal reproductive studies involving rats have shown fetal multisystem malformations and developmental abnormalities with oral administration of glycolic acid at doses far exceeding those used in humans.³⁴ Although no human reproductive studies exist, topical glycolic acid is unlikely to reach the developing fetus in notable amounts, and the medication is likely safe for use.^17,35

Clindamycin—Topical clindamycin phosphate is an effective and well-tolerated agent for the treatment of mild to moderate acne.³⁶ Its systemic absorption is minimal, and it is considered safe for use during all trimesters of pregnancy.^{14,17,26,27,35,37}

Erythromycin—Topical erythromycin is another commonly prescribed topical antibiotic used to target mild to moderate acne. However, its use recently has been associated with a decrease in efficacy secondary to the rise of antibacterial resistance in the community.^38-40 Nevertheless, it remains a safe treatment for use during all trimesters of pregnancy.^{14,17,26,27,35,37}

Topical Retinoids—Vitamin A derivatives (also known as retinoids) are the mainstay for the treatment of mild to moderate acne. Limited data exist regarding pregnancy outcomes after in utero exposure.⁴¹ A rare case report suggested topical tretinoin has been associated with fetal otocerebral anomalies.⁴² For tazarotene, teratogenic effects were seen in animal reproductive studies at doses exceeding maximum recommended human doses.^41,43 However, a large meta-analysis failed to find a clear risk for increased congenital malformations, spontaneous abortions, stillbirth, elective termination of pregnancy, low birthweight, or prematurity following first-trimester exposure to topical retinoids.⁴⁴ As the level of exposure that could lead to teratogenicity in humans is unknown, avoidance of both tretinoin and tazarotene is recommended in pregnant women.^41,45 Nevertheless, women inadvertently exposed should be reassured.⁴⁴

Conversely, adapalene has been associated with 1 case of anophthalmia and agenesis of the optic chiasma in a fetus following exposure until 13 weeks’ gestation.⁴⁶ However, a large, open-label trial prior to the patient transitioning from adapalene to over-the-counter treatment showed that the drug harbors a large and reassuring margin of safety and no risk for teratogenicity in a maximal usage trial and Pregnancy Safety Review.⁴⁷ Therefore, adapalene gel 0.1% is a safe and effective medication for the treatment of acne in a nonprescription environment and does not pose harm to the fetus.

Clascoterone—Clascoterone is a novel topical antiandrogenic drug approved for the treatment of hormonal and inflammatory moderate to severe acne.^48-51 Human reproductive data are limited to 1 case of pregnancy that occurred during phase 3 trial investigations, and no adverse outcomes were reported.⁵¹ Minimal systemic absorption follows topical use.⁵² Nonetheless, dose-independent malformations were reported in animal reproductive studies.⁵³ As such, it remains better to avoid the use of clascoterone during pregnancy pending further safety data.

Minocycline Foam—Minocycline foam 4% is approved to treat inflammatory lesions of nonnodular moderate to severe acne in patients 9 years and older.⁵⁴ Systemic absorption is minimal, and the drug has limited bioavailability with minimal systemic accumulation in the patient’s serum.⁵⁵ Given this information, it is unlikely that topical minocycline will reach notable levels in the fetal serum or harbor teratogenic effects, as seen with the oral formulation.⁵⁶ However, it may be best to avoid its use during the second and third trimesters given the potential risk for tooth discoloration in the fetus.^57,58

Systemic Treatments for Acne

Isotretinoin—Isotretinoin is the most effective treatment for moderate to severe acne with a well-documented potential for long-term clearance.⁵⁹ Its use during pregnancy is absolutely contraindicated, as the medication is a well-known teratogen. Associated congenital malformations include numerous craniofacial defects, cardiovascular and neurologic malformations, or thymic disorders that are estimated to affect 20% to 35% of infants exposed in utero.⁶⁰ Furthermore, strict contraception use during treatment is mandated for patients who can become pregnant. It is recommended to wait at least 1 month and 1 menstrual cycle after medication discontinuation before attempting to conceive.¹⁷ Pregnancy termination is recommended if conception occurs during treatment with isotretinoin.

Spironolactone—Spironolactone is an androgen-receptor antagonist commonly prescribed off label for mild to severe acne in females.^61,62 Spironolactone promotes the feminization of male fetuses and should be avoided in pregnancy.⁶³

Doxycycline/Minocycline—Tetracyclines are the most commonly prescribed oral antibiotics for moderate to severe acne.⁶⁴ Although highly effective at treating acne, tetracyclines generally should be avoided in pregnancy. First-trimester use of doxycycline is not absolutely contraindicated but should be reserved for severe illness and not employed for the treatment of acne. However, accidental exposure to doxycycline has not been associated with congenital malformations.⁶⁵ Nevertheless, after the 15th week of gestation, permanent tooth discoloration and bone growth inhibition in the fetus are serious and well-documented risks.^14,17 Additional adverse events following in utero exposure include infantile inguinal hernia, hypospadias, and limb hypoplasia.⁶³

Sarecycline—Sarecycline is a novel tetracycline-class antibiotic for the treatment of moderate to severe inflammatory acne. It has a narrower spectrum of activity compared to its counterparts within its class, which translates to an improved safety profile, namely when it comes to gastrointestinal tract microbiome disruption and potentially decreased likelihood of developing bacterial resistance.⁶⁶ Data on human reproductive studies are limited, but it is advisable to avoid sarecycline in pregnancy, as it may cause adverse developmental effects in the fetus, such as reduced bone growth, in addition to the well-known tetracycline-associated risk for permanent discoloration of the teeth if used during the second and third trimesters.^67,68

Erythromycin—Oral erythromycin targets moderate to severe inflammatory acne and is considered safe for use during pregnancy.^69,70 There has been 1 study reporting an increased risk for atrial and ventricular septal defects (1.8%) and pyloric stenosis (0.2%), but these risks are still uncertain, and erythromycin is considered compatible with pregnancy.⁷¹ However, erythromycin estolate formulations should be avoided given the associated 10% to 15% risk for reversible cholestatic liver injury.⁷² Erythromycin base or erythromycin ethylsuccinate formulations should be favored.

Systemic Steroids—Prednisone is indicated for severe acne with scarring and should only be used during pregnancy after clearance from the patient’s obstetrician. Doses of 0.5 mg/kg or less should be prescribed in combination with systemic antibiotics as well as agents for bone and gastrointestinal tract prophylaxis.²⁹

Zinc—The exact mechanism by which zinc exerts its effects to improve acne remains largely obscure. It has been found effective against inflammatory lesions of mild to moderate acne.⁷³ Generally recommended dosages range from 30 to 200 mg/d but may be associated with gastrointestinal tract disturbances. Dosages of 75 mg/d have shown no harm to the fetus.⁷⁴ When taking this supplement, patients should not exceed the recommended doses given the risk for hypocupremia associated with high-dose zinc supplementation.

Light-Based Therapies

Phototherapy—Narrowband UVB phototherapy is effective for the treatment of mild to moderate acne.⁷⁵ It has been proven to be a safe treatment option during pregnancy, but its use has been associated with decreased folic acid levels.^76-79 Therefore, in addition to attaining baseline folic acid serum levels, supplementation with folic acid prior to treatment, as per routine prenatal guidelines, should be sought.⁸⁰

AviClear—The AviClear (Cutera) laser is the first device cleared by the FDA for mild to severe acne in March 2022.⁸¹ The FDA clearance for the Accure (Accure Acne Inc) laser, also targeting mild to severe acne, followed soon after (November 2022). Both lasers harbor a wavelength of 1726 nm and target sebaceous glands with electrothermolysis.^82,83 Further research and long-term safety data are required before using them in pregnancy.

Other Therapies

Cosmetic Peels—Glycolic acid peels induce epidermolysis and desquamation.⁸⁴ Although data on use during pregnancy are limited, these peels have limited dermal penetration and are considered safe for use in pregnancy.^33,85,86 Similarly, keratolytic lactic acid peels harbor limited dermal penetration and can be safely used in pregnant women.^87-89 Salicylic acid peels also work through epidermolysis and desquamation⁸⁴; however, they tend to penetrate deeper into the skin, reaching down to the basal layer, if large areas are treated or when applied under occlusion.^86,90 Although their use is not contraindicated in pregnancy, they should be limited to small areas of coverage.⁹¹

Intralesional Triamcinolone—Acne cysts and inflammatory papules can be treated with intralesional triamcinolone injections to relieve acute symptoms such as pain.⁹² Low doses at concentrations of 2.5 mg/mL are considered compatible with pregnancy when indicated.²⁹

Approaching the Patient Clinical Encounter

In patients seeking treatment prior to conception, a few recommendations can be made to minimize the risk for acne recurrence or flares during pregnancy. For instance, because data show an association between increased acne severity in those with a higher body mass index and in pregnancy, weight loss may be recommended prior to pregnancy to help mitigate symptoms after conception.⁷ The Figure summarizes our recommendations for approaching and treating acne in pregnancy.

In all patients, grading the severity of the patient’s acne as mild, moderate, or severe is the first step. The presence of scarring is an additional consideration during the physical examination and should be documented. A careful discussion of treatment expectations and prognosis should be the focus before treatment initiation. Meticulous documentation of the physical examination and discussion with the patient should be prioritized.

To minimize toxicity and risks to the developing fetus, monotherapy is favored. Topical therapy should be considered first line. Safe regimens include mild nonabrasive washes, such as those containing benzoyl peroxide or glycolic acid, or topical azelaic acid or clindamycin phosphate for mild to moderate acne. More severe cases warrant the consideration of systemic medications as second line, as more severe acne is better treated with oral antibiotics such as the macrolides erythromycin or clindamycin or systemic corticosteroids when concern exists for severe scarring. The additional use of physical sunscreen also is recommended.

An important topic to address during the clinical encounter is cautious intake of oral supplements for acne during pregnancy, as they may contain harmful and teratogenic ingredients. A recent search focusing on acne supplements available online between March and May 2020 uncovered 49 different supplements, 26 (53%) of which contained vitamin A.⁹³ Importantly, 3 (6%) of these 49 supplements were likely teratogenic, 4 (8%) contained vitamin A doses exceeding the recommended daily nutritional intake level, and 15 (31%) harbored an unknown teratogenic risk. Furthermore, among the 6 (12%) supplements with vitamin A levels exceeding 10,000 IU, 2 lacked any mention of pregnancy warning, including the supplement with the highest vitamin A dose found in this study.⁹³ Because dietary supplements are not subject to the same stringent regulations by the FDA as drugs, inadvertent use by unaware patients ought to be prevented by careful counseling and education.

Finally, patients should be counseled to seek care following delivery for potentially updated medication management of acne, especially if they are breastfeeding. Co-management with a pediatrician may be indicated during lactation, particularly when newborns are born preterm or with other health conditions that may warrant additional caution with the use of certain agents.

Acne vulgaris, or acne, is a highly common inflammatory skin disorder affecting up to 85% of the population, and it constitutes the most commonly presenting chief concern in routine dermatology practice.¹ Older teenagers and young adults are most often affected by acne.² Although acne generally is more common in males, adult-onset acne occurs more frequently in women.^2,3 Black and Hispanic women are at higher risk for acne compared to those of Asian, White, or Continental Indian descent.⁴ As such, acne is a common concern in all women of childbearing age.

Concerns for maternal and fetal safety are important therapeutic considerations, especially because hormonal and physiologic changes in pregnancy can lead to onset of inflammatory acne lesions, particularly during the second and third trimesters.⁵ Female patients younger than 25 years; with a higher body mass index, prior irregular menstruation, or polycystic ovary syndrome; or those experiencing their first pregnancy are thought to be more commonly affected.^5-7 In fact, acne affects up to 43% of pregnant women, and lesions typically extend beyond the face to involve the trunk.^6,8-10 Importantly, one-third of women with a history of acne experience symptom relapse after disease-free periods, while two-thirds of those with ongoing disease experience symptom deterioration during pregnancy.¹⁰ Although acne is not a life-threatening condition, it has a well-documented, detrimental impact on social, emotional, and psychological well-being, namely self-perception, social interactions, quality-of-life scores, depression, and anxiety.¹¹

Therefore, safe and effective treatment of pregnant women is of paramount importance. Because pregnant women are not included in clinical trials, there is a paucity of medication safety data, further augmented by inefficient access to available information. The US Food and Drug Administration (FDA) pregnancy safety categories were updated in 2015, letting go of the traditional A, B, C, D, and X categories.¹² The Table reviews the current pregnancy classification system. In this narrative review, we summarize the most recent available data and recommendations on the safety and efficacy of acne treatment during pregnancy.

Topical Treatments for Acne

Benzoyl Peroxide—Benzoyl peroxide commonly is used as first-line therapy alone or in combination with other agents for the treatment of mild to moderate acne.¹³ It is safe for use during pregnancy.¹⁴ Although the medication is systemically absorbed, it undergoes complete metabolism to benzoic acid, a commonly used food additive.^15,16 Benzoic acid has low bioavailability, as it gets rapidly metabolized by the kidneys; therefore, benzoyl peroxide is unlikely to reach clinically significant levels in the maternal circulation and consequently the fetal circulation. Additionally, it has a low risk for causing congenital malformations.¹⁷

Salicylic Acid—For mild to moderate acne, salicylic acid is a second-line agent that likely is safe for use by pregnant women at low concentrations and over limited body surface areas.^14,18,19 There is minimal systemic absorption of the drug.²⁰ Additionally, aspirin, which is broken down in the body into salicylic acid, is used in low doses for the treatment of pre-eclampsia during pregnancy.²¹

Dapsone—The use of dapsone gel 5% as a second-line agent has shown efficacy for mild to moderate acne.²² The oral formulation, commonly used for malaria and leprosy prophylaxis, has failed to show associated fetal toxicity or congenital anomalies.^23,24 It also has been used as a first-line treatment for dermatitis herpetiformis in pregnancy.²⁵ Although the medication likely is safe, it is better to minimize its use during the third trimester to reduce the theoretical risk for hyperbilirubinemia in the neonate.^17,26-29

Azelaic Acid—Azelaic acid effectively targets noninflammatory and inflammatory acne and generally is well tolerated, harboring a good safety profile.³⁰ Topical 20% azelaic acid has localized antibacterial and comedolytic effects and is safe for use during pregnancy.^31,32

Glycolic Acid—Limited data exist on the safety of glycolic acid during pregnancy. In vitro studies have shown up to 27% systemic absorption depending on pH, concentration, and duration of application.³³ Animal reproductive studies involving rats have shown fetal multisystem malformations and developmental abnormalities with oral administration of glycolic acid at doses far exceeding those used in humans.³⁴ Although no human reproductive studies exist, topical glycolic acid is unlikely to reach the developing fetus in notable amounts, and the medication is likely safe for use.^17,35

Clindamycin—Topical clindamycin phosphate is an effective and well-tolerated agent for the treatment of mild to moderate acne.³⁶ Its systemic absorption is minimal, and it is considered safe for use during all trimesters of pregnancy.^{14,17,26,27,35,37}

Erythromycin—Topical erythromycin is another commonly prescribed topical antibiotic used to target mild to moderate acne. However, its use recently has been associated with a decrease in efficacy secondary to the rise of antibacterial resistance in the community.^38-40 Nevertheless, it remains a safe treatment for use during all trimesters of pregnancy.^{14,17,26,27,35,37}

Topical Retinoids—Vitamin A derivatives (also known as retinoids) are the mainstay for the treatment of mild to moderate acne. Limited data exist regarding pregnancy outcomes after in utero exposure.⁴¹ A rare case report suggested topical tretinoin has been associated with fetal otocerebral anomalies.⁴² For tazarotene, teratogenic effects were seen in animal reproductive studies at doses exceeding maximum recommended human doses.^41,43 However, a large meta-analysis failed to find a clear risk for increased congenital malformations, spontaneous abortions, stillbirth, elective termination of pregnancy, low birthweight, or prematurity following first-trimester exposure to topical retinoids.⁴⁴ As the level of exposure that could lead to teratogenicity in humans is unknown, avoidance of both tretinoin and tazarotene is recommended in pregnant women.^41,45 Nevertheless, women inadvertently exposed should be reassured.⁴⁴

Conversely, adapalene has been associated with 1 case of anophthalmia and agenesis of the optic chiasma in a fetus following exposure until 13 weeks’ gestation.⁴⁶ However, a large, open-label trial prior to the patient transitioning from adapalene to over-the-counter treatment showed that the drug harbors a large and reassuring margin of safety and no risk for teratogenicity in a maximal usage trial and Pregnancy Safety Review.⁴⁷ Therefore, adapalene gel 0.1% is a safe and effective medication for the treatment of acne in a nonprescription environment and does not pose harm to the fetus.

Clascoterone—Clascoterone is a novel topical antiandrogenic drug approved for the treatment of hormonal and inflammatory moderate to severe acne.^48-51 Human reproductive data are limited to 1 case of pregnancy that occurred during phase 3 trial investigations, and no adverse outcomes were reported.⁵¹ Minimal systemic absorption follows topical use.⁵² Nonetheless, dose-independent malformations were reported in animal reproductive studies.⁵³ As such, it remains better to avoid the use of clascoterone during pregnancy pending further safety data.

Minocycline Foam—Minocycline foam 4% is approved to treat inflammatory lesions of nonnodular moderate to severe acne in patients 9 years and older.⁵⁴ Systemic absorption is minimal, and the drug has limited bioavailability with minimal systemic accumulation in the patient’s serum.⁵⁵ Given this information, it is unlikely that topical minocycline will reach notable levels in the fetal serum or harbor teratogenic effects, as seen with the oral formulation.⁵⁶ However, it may be best to avoid its use during the second and third trimesters given the potential risk for tooth discoloration in the fetus.^57,58

Systemic Treatments for Acne

Isotretinoin—Isotretinoin is the most effective treatment for moderate to severe acne with a well-documented potential for long-term clearance.⁵⁹ Its use during pregnancy is absolutely contraindicated, as the medication is a well-known teratogen. Associated congenital malformations include numerous craniofacial defects, cardiovascular and neurologic malformations, or thymic disorders that are estimated to affect 20% to 35% of infants exposed in utero.⁶⁰ Furthermore, strict contraception use during treatment is mandated for patients who can become pregnant. It is recommended to wait at least 1 month and 1 menstrual cycle after medication discontinuation before attempting to conceive.¹⁷ Pregnancy termination is recommended if conception occurs during treatment with isotretinoin.

Spironolactone—Spironolactone is an androgen-receptor antagonist commonly prescribed off label for mild to severe acne in females.^61,62 Spironolactone promotes the feminization of male fetuses and should be avoided in pregnancy.⁶³

Doxycycline/Minocycline—Tetracyclines are the most commonly prescribed oral antibiotics for moderate to severe acne.⁶⁴ Although highly effective at treating acne, tetracyclines generally should be avoided in pregnancy. First-trimester use of doxycycline is not absolutely contraindicated but should be reserved for severe illness and not employed for the treatment of acne. However, accidental exposure to doxycycline has not been associated with congenital malformations.⁶⁵ Nevertheless, after the 15th week of gestation, permanent tooth discoloration and bone growth inhibition in the fetus are serious and well-documented risks.^14,17 Additional adverse events following in utero exposure include infantile inguinal hernia, hypospadias, and limb hypoplasia.⁶³

Sarecycline—Sarecycline is a novel tetracycline-class antibiotic for the treatment of moderate to severe inflammatory acne. It has a narrower spectrum of activity compared to its counterparts within its class, which translates to an improved safety profile, namely when it comes to gastrointestinal tract microbiome disruption and potentially decreased likelihood of developing bacterial resistance.⁶⁶ Data on human reproductive studies are limited, but it is advisable to avoid sarecycline in pregnancy, as it may cause adverse developmental effects in the fetus, such as reduced bone growth, in addition to the well-known tetracycline-associated risk for permanent discoloration of the teeth if used during the second and third trimesters.^67,68

Erythromycin—Oral erythromycin targets moderate to severe inflammatory acne and is considered safe for use during pregnancy.^69,70 There has been 1 study reporting an increased risk for atrial and ventricular septal defects (1.8%) and pyloric stenosis (0.2%), but these risks are still uncertain, and erythromycin is considered compatible with pregnancy.⁷¹ However, erythromycin estolate formulations should be avoided given the associated 10% to 15% risk for reversible cholestatic liver injury.⁷² Erythromycin base or erythromycin ethylsuccinate formulations should be favored.

Systemic Steroids—Prednisone is indicated for severe acne with scarring and should only be used during pregnancy after clearance from the patient’s obstetrician. Doses of 0.5 mg/kg or less should be prescribed in combination with systemic antibiotics as well as agents for bone and gastrointestinal tract prophylaxis.²⁹

Zinc—The exact mechanism by which zinc exerts its effects to improve acne remains largely obscure. It has been found effective against inflammatory lesions of mild to moderate acne.⁷³ Generally recommended dosages range from 30 to 200 mg/d but may be associated with gastrointestinal tract disturbances. Dosages of 75 mg/d have shown no harm to the fetus.⁷⁴ When taking this supplement, patients should not exceed the recommended doses given the risk for hypocupremia associated with high-dose zinc supplementation.

Light-Based Therapies

Phototherapy—Narrowband UVB phototherapy is effective for the treatment of mild to moderate acne.⁷⁵ It has been proven to be a safe treatment option during pregnancy, but its use has been associated with decreased folic acid levels.^76-79 Therefore, in addition to attaining baseline folic acid serum levels, supplementation with folic acid prior to treatment, as per routine prenatal guidelines, should be sought.⁸⁰

AviClear—The AviClear (Cutera) laser is the first device cleared by the FDA for mild to severe acne in March 2022.⁸¹ The FDA clearance for the Accure (Accure Acne Inc) laser, also targeting mild to severe acne, followed soon after (November 2022). Both lasers harbor a wavelength of 1726 nm and target sebaceous glands with electrothermolysis.^82,83 Further research and long-term safety data are required before using them in pregnancy.

Other Therapies

Cosmetic Peels—Glycolic acid peels induce epidermolysis and desquamation.⁸⁴ Although data on use during pregnancy are limited, these peels have limited dermal penetration and are considered safe for use in pregnancy.^33,85,86 Similarly, keratolytic lactic acid peels harbor limited dermal penetration and can be safely used in pregnant women.^87-89 Salicylic acid peels also work through epidermolysis and desquamation⁸⁴; however, they tend to penetrate deeper into the skin, reaching down to the basal layer, if large areas are treated or when applied under occlusion.^86,90 Although their use is not contraindicated in pregnancy, they should be limited to small areas of coverage.⁹¹

Intralesional Triamcinolone—Acne cysts and inflammatory papules can be treated with intralesional triamcinolone injections to relieve acute symptoms such as pain.⁹² Low doses at concentrations of 2.5 mg/mL are considered compatible with pregnancy when indicated.²⁹

Approaching the Patient Clinical Encounter

In patients seeking treatment prior to conception, a few recommendations can be made to minimize the risk for acne recurrence or flares during pregnancy. For instance, because data show an association between increased acne severity in those with a higher body mass index and in pregnancy, weight loss may be recommended prior to pregnancy to help mitigate symptoms after conception.⁷ The Figure summarizes our recommendations for approaching and treating acne in pregnancy.

In all patients, grading the severity of the patient’s acne as mild, moderate, or severe is the first step. The presence of scarring is an additional consideration during the physical examination and should be documented. A careful discussion of treatment expectations and prognosis should be the focus before treatment initiation. Meticulous documentation of the physical examination and discussion with the patient should be prioritized.

To minimize toxicity and risks to the developing fetus, monotherapy is favored. Topical therapy should be considered first line. Safe regimens include mild nonabrasive washes, such as those containing benzoyl peroxide or glycolic acid, or topical azelaic acid or clindamycin phosphate for mild to moderate acne. More severe cases warrant the consideration of systemic medications as second line, as more severe acne is better treated with oral antibiotics such as the macrolides erythromycin or clindamycin or systemic corticosteroids when concern exists for severe scarring. The additional use of physical sunscreen also is recommended.

An important topic to address during the clinical encounter is cautious intake of oral supplements for acne during pregnancy, as they may contain harmful and teratogenic ingredients. A recent search focusing on acne supplements available online between March and May 2020 uncovered 49 different supplements, 26 (53%) of which contained vitamin A.⁹³ Importantly, 3 (6%) of these 49 supplements were likely teratogenic, 4 (8%) contained vitamin A doses exceeding the recommended daily nutritional intake level, and 15 (31%) harbored an unknown teratogenic risk. Furthermore, among the 6 (12%) supplements with vitamin A levels exceeding 10,000 IU, 2 lacked any mention of pregnancy warning, including the supplement with the highest vitamin A dose found in this study.⁹³ Because dietary supplements are not subject to the same stringent regulations by the FDA as drugs, inadvertent use by unaware patients ought to be prevented by careful counseling and education.

Finally, patients should be counseled to seek care following delivery for potentially updated medication management of acne, especially if they are breastfeeding. Co-management with a pediatrician may be indicated during lactation, particularly when newborns are born preterm or with other health conditions that may warrant additional caution with the use of certain agents.

Approximately 20% of the general population has a contact allergy.¹ Allergic contact dermatitis (ACD) is a delayed type IV hypersensitivity reaction mediated by T lymphocytes.² Axillary ACD presentation is variable but typically includes an eczematous eruption with erythematous scaly patches or plaques. Common products in contact with the axillae include deodorants, antiperspirants, razors, bodywash, and clothing.

Axillary skin is distinct from skin elsewhere on the body due to both anatomical characteristics and unique human self-care practices. Axillary skin has reduced barrier function, faster stratum corneum turnover, and altered lipid levels.^3-5 Moreover, the axillae often are subject to shaving or other hair removal practices that alter the local environment, as layers of stratum corneum and hair are mechanically removed, which causes irritation and predisposes the skin to enhanced sensitivity to topical exposures.^6,7 With the abundance of apocrine and eccrine glands, the axillae are prone to sweat, which also can accentuate contact allergy.^2,3 Other factors, such as occlusion and friction, contribute to axillary contact allergy.^8,9

Patch testing is the gold standard for the diagnosis of ACD and aids in identification of culprit allergens. A thorough patient history and examination of the rash distribution may provide further clues; for example, dermatitis due to a deodorant typically affects the vault, whereas textile dye dermatitis tends to spare the vault.^10,11 Baseline-limited patch testing detects up to two-thirds of clinically relevant allergens.¹² Therefore, patients may require subsequent testing with supplemental allergens.

The differential diagnosis for axillary lesions is broad—including inflammatory diseases such as irritant contact dermatitis and hidradenitis suppurativa, genetic disorders such as Hailey-Hailey disease, and infectious causes such as erythrasma—but may be narrowed with a thorough physical examination and patient history, histopathology, bedside diagnostic techniques (eg, scrapings and Wood lamp examination), and patch testing. Systemic contact dermatitis (SCD) or symmetrical drug-related intertriginous and flexural exanthema (SDRIFE) also may be suspected in cases of intertriginous dermatoses.

We review the potential allergens in products used on the axillae as well as the management of axillary ACD. We also discuss axillary dermatitis as a manifestation of SCD and SDRIFE.

Top Allergens in Products Used on the Axillae

Fragrance—A 1982 North American Contact Dermatitis Group study on cosmetic products identified fragrances as the most common cause of ACD,¹³ and this trend continues to hold true with more recent data.¹⁴ The incidence of fragrance allergy may be increasing, with positive patch tests to a fragrance chemical in 10% of patch test clinic populations.¹⁵ Fragrances are a ubiquitous ingredient in deodorants and antiperspirants, which are important sources implicated in the development and elicitation of fragrance ACD.¹⁶ One study found that fragrance was present in 97 of 107 (90%) deodorants available at Walgreens pharmacies.¹¹

In a study of patients with a history of an axillary rash caused by a deodorant spray, Johansen et al¹⁷ reported that the likelihood of fragrance allergy is increased by a factor of 2.4. This risk of developing a fragrance allergy may be exacerbated in those who shave; Edman¹⁸ reported that the odds ratio of developing a fragrance allergy among men who shave their beards was 2.9. Although there are no specific data on the effects of shaving on ACD, shaving in general can induce localized irritation and increase percutaneous absorption.¹⁹

The individual fragrance components in deodorants most likely to cause ACD include hydroxycitronellal, eugenol, and geraniol—all constituent ingredients in patch test formulations of fragrance mixture I.^11,20 Other common fragrance allergens associated with ACD include hydroxymethylpentylcyclohexenecarboxaldehyde, farnesol, and balsam of Peru.^21-27 Hydroperoxides of limonene and linalool, common fragrances in detergents and personal care products, are increasingly recognized as contact allergens and have been reported to cause axillary ACD from deodorants.^28-30

Dermatitis involving the bilateral axillary vaults wherever deodorant or antiperspirant was directly applied is the most common presentation of ACD due to fragrance (Figure 1).¹⁷ An eczematous eruption is common, though scale may be less apparent than in nonflexural regions. Axillary ACD secondary to fragrances also may result from use of fragranced laundry detergents, fabric softeners, soaps, and perfumes, and may spare the vaults.^10,29,31,32 Less common presentations of axillary ACD due to fragrance include pigmented dermatoses; for example, ACD from an antiperspirant containing hydroperoxide of limonene presented as hyperpigmented patches with minimal erythema and scaling in the edges of the axillary folds.^33,34

Diagnosis of a fragrance ACD typically is made with a standard patch test series including fragrance mixture I and balsam of Peru, which may detect 75% and 50% of fragrance sensitivities, respectively.³⁵ Patch testing may be followed with a repeated open application test of the product in question.³⁶ Additionally, it may be appropriate to test for other fragrance allergens including balsam of Tolu, fragrance mixture II, lichen acid mix, and hydroxyperoxides of linalool and limonene (among other botanicals) if standard patch testing is negative and suspicion of fragrance ACD remains elevated.¹¹

Propylene Glycol—Propylene glycol (PG)—a versatile substance that functions as a solvent, humectant, emulsifier, stabilizer, and antimicrobial—is the second most common contact allergen present in deodorants.¹¹ It is prevalent in both personal care and household products, including deodorants, cosmetics, foods, toothpaste, cleaning agents, and detergents.^11,37 Propylene glycol is both an allergen and an irritant. Among deodorants/antiperspirants, PG is both a common irritant and allergen, as its concentration may be particularly high (as much as 73%).³⁸ One commonly reported example of PG contact dermatitis is from the topical medicament minoxidil.^39,40

Patch testing data have demonstrated a positivity rate for PG ranging between 0.1% to 3.8%. The variability in these findings likely is due to differences in the tested concentrations of PG, as higher concentrations sometimes required to elicit an allergic reaction also may create a stronger irritation effect.⁴¹ Propylene glycol irritancy and the occlusive nature of the axillae may enhance sensitization to other allergens, as demonstrated by Agren-Jonsson and Magnusson,⁴² who reported sensitization to propantheline bromide and trichlorocarbanilide in patients who used a lotion with 90% PG. Many PG-containing products beyond deodorants/antiperspirants may be applied to the axillae, including steroid creams, lotions, shaving creams, and bodywashes.^38,43

The diagnosis of PG allergy via patch testing is challenging and at times controversial given its irritant nature. False-positive irritant reactions have been documented, characterized by a weak reaction at 48 hours that is absent by 96 hours (decrescendo reaction). A reaction may not appear until 96 hours (crescendo reaction), which typically indicates a true contact allergy but in the case of PG also may be the substance acting as a “late irritant.”⁴⁴ Fast (<24 hours) and well-demarcated reactions suggest irritation.⁴⁵ Regardless, reactions to PG on patch testing, even those regarded as weak, may be considered relevant in consideration of the clinical context.³⁷

Aluminum—Aluminum is the active ingredient in most antiperspirants, typically in the form of aluminum chloride, aluminum chlorohydrate, aluminum zirconium trichlorohydrex gly, or aluminum zirconium tetrachlorohydrex gly.⁴⁶ Aluminum mechanically obstructs the eccrine glands to reduce sweat.⁴⁷ Although aluminum is an uncommon allergen, a possible presentation of aluminum allergy is axillary vault dermatitis secondary to antiperspirant use.⁴⁶ Another potential manifestation is a ringlike reaction to the Finn Chambers (SmartPractice) used in patch testing.⁴⁶ In one case of aluminum-induced axillary dermatitis, a 28-year-old woman presented with eczema of the axillae, and subsequent patch testing revealed an allergy to aluminum chloride. The rash resolved upon cessation of use of an aluminum-containing deodorant.⁴⁸

Aluminum has been reported to cause granulomatous dermatitis in the axillae. This reaction typically presents as red-brown, pruritic papules limited to the area in which deodorant was applied, with histopathology revealing epithelioid granulomas.^49-51

Alum deodorants—considered a natural alternative—contain aluminum bound to potassium or ammonium in the form of a crystal or powder. Alum crystal deodorants have been reported to cause both a typical erythematous pruritic dermatitis as well as a granulomatous dermatitis with red-brown papules.^52,53 The granulomatous dermatitis caused by either form of aluminum resolves with avoidance and use of topical steroids or topical tacrolimus.^49,50,52,53

The diagnosis of aluminum ACD via patch testing may be identified with empty Finn Chambers, which are metallic aluminum, or with patch placement of aluminum chloride hexahydrate, though the former is only positive in patients with a strong allergy.^54,55 In 2022, aluminum was named Allergen of the Year by the American Contact Dermatitis Society, with recommendations to conduct patch testing with aluminum chloride hexahydrate 10% rather than the traditional 2% to increase diagnostic yield.⁵⁵ Additionally, it is recommended that aluminum be included in baseline patch testing for children due to the high prevalence of aluminum allergy in children and early exposure via childhood vaccines.^54-56 In patients with aluminum allergy, providers may suggest purchasing aluminum-free deodorants or provide recipes for homemade deodorant that includes ingredients such as arrowroot powder, cornstarch, and diatomaceous earth.⁴⁶

Nickel—Nickel is the most commonly identified contact allergen on patch testing yet an infrequent cause of axillary dermatitis. A case report from 2014 described axillary dermatitis in a woman that worsened during a positive patch test to nickel. Improvement was noted when the patient switched to titanium shaving razors.⁵⁷ Nickel allergy also may present in the form of SCD. In one report, a woman developed dermatitis of the flexural areas, including the axillae, 3 months after undergoing a sterilization procedure in which nickel-containing tubal implants were placed.⁵⁸ Patch testing revealed a positive reaction to nickel. The patient experienced complete resolution of the steroid-resistant dermatitis following removal of the implants via salpingectomy.⁵⁸

Textile Dye—In contrast to dermatitis caused by deodorants/antiperspirants, contact allergy to textile dyes presents as dermatitis involving the axillary borders but sparing the axillary vaults (Figures 2 and 3).¹⁰ Other potential presentations of textile dye dermatitis include erythema multiforme–like eruptions and erythematous wheal–type reactions.⁵⁹ Textile dyes are classified as disperse vs nondisperse, with the majority of contact dermatoses caused by disperse dyes, specifically Disperse Orange 1, blue 106, and blue 124.^60-62 Ryberg et al⁶¹ found that the axilla is one of the more common locations to be affected by textile dye allergy, particularly in women, which was further supported by Seidenari et al,⁶³ who found that skin folds were affected in 27% of study participants allergic to textile dyes (N=437), a finding that is likely due to friction, sweat, and occlusion.⁶² In one case report of a patient with dermatitis caused by reactive dyes, the garment required 3 washes before the patient experienced resolution of dermatitis.⁶⁴ For patients with textile dye dermatitis, mitigation strategies include washing clothing before wearing, especially for darkly dyed items; avoiding tight clothing; wearing garments made of cotton, wool, silk, or linen; and choosing light-colored garments.^9,64,65

Axillary Dermatitis as a Manifestation of SCD and SDRIFE

Systemic contact dermatitis occurs when an individual who was previously sensitized to a particular allergen develops ACD of the skin with systemic exposure to that allergen or immunochemically related allergens. Exposure may occur via ingestion, inhalation, intravenous, intramuscular, and transepidermal routes.⁶⁶ Systemic contact dermatitis manifests in a variety of ways, including focal flares at sites of prior contact dermatitis (recall reaction), vesicular hand dermatitis, intertriginous eruptions including axillary dermatitis, and generalized eruptions.⁶⁷

Systemic contact dermatitis rarely involves systemic symptoms, and onset typically is within days of exposure. The 3 most common groups of allergens causing SCD are metals, medications, and plants and herbals.⁶⁸ These allergens have all been reported to cause axillary dermatitis via SCD.^58,69,70 Foods containing balsam of Peru that may lead to SCD include citrus, chocolate, tomato, and certain alcohols.^70,71 Patients with a positive patch test to balsam of Peru may experience improvement of their dermatitis after reduction of balsam of Peru–rich foods from their diet.⁷⁰ Metals implicated in SCD include mercury, nickel, and gold.^72-74 Finally, PG ingestion also has been implicated in cases of SCD.³⁷

Symmetrical drug-related intertriginous and flexural exanthema is another condition that presents as intertriginous dermatitis and differs from SCD in that the eruption does not require presensitization; there may be no known prior exposure to the agent causing dermatitis. Historically, SDRIFE was described as baboon syndrome because of its frequent involvement of the buttocks with diffuse, well-demarcated, erythematous dermatitis resembling that of a baboon. This term is no longer used due to its insensitive nature and incomplete depiction of SDRIFE, which can affect body sites other than the buttocks.^68,75,76 Specific criteria to make this diagnosis include sharply demarcated and/or V-shaped erythema of the gluteal/perianal area, involvement of at least 1 other intertriginous or flexural region, symmetry of affected areas, and an absence of systemic symptoms.⁷⁶ There also may be papules, pustules, and vesicles present in affected areas. Symmetrical drug-related intertriginous and flexural exanthema most often is caused by β-lactam antibiotics, but other associated drugs include chemotherapeutic agents, such as mitomycin C.⁷⁶

Histopathology of both SCD and SDRIFE is variable and typically nonspecific, often revealing epidermal spongiosis and a perivascular mononuclear cell infiltrate with occasional neutrophils and eosinophils.⁷⁶ A case of SCD to mercury presenting as intertriginous dermatitis demonstrated a leukocytoclastic vasculitis pattern on biopsy.⁷⁷

Systemic contact dermatitis is diagnosed via a patch test, while SDRIFE typically has a negative patch test result and requires oral rechallenge testing, which reproduces the rash within hours.^78,79

Additional Allergens Causing Axillary ACD

Although fragrance is the most common allergen in deodorants, other ingredients have been shown to cause axillary ACD (Table).^80-90 In addition to these ingredients, allergens not previously mentioned that may be present in deodorants include lanolin, essential oils, and parabens.¹¹ Methylisothiazolinone in laundry detergent also has been found to instigate ACD.⁹¹ Fragrances and preservatives in laundry detergents also may contribute to dermatitis.⁹²

Other products that have caused axillary contact dermatitis include topical exposure to medicaments including clindamycin,⁹³ ethylenediamine in nystatin cream,⁹⁴ methylprednisolone acetate⁹⁵ and dipropylene glycol in a hydrocortisone lotion,⁹⁶ wood dusts from tropical hardwoods,⁹⁷ and tobacco.⁹⁸

Management of ACD

The most effective strategy in the management of patients with contact dermatitis is avoidance of the offending agent. Additionally, clinicians may recommend the use of topical steroids and/or calcineurin inhibitors to hasten resolution.²

For patients with contact dermatitis, a clinician may recommend product substitutions with few potential allergens to use prior to patch testing. Patients with a fragrance allergy should look for products specifically labeled as “fragrance free” rather than “hypoallergenic” or “unscented,” as the latter two may still contain minimal amounts of fragrance.³⁵ Patients should be educated on the functions of the allergens to which they are allergic so they may adequately avoid potential sources of contact.⁹⁹ For suspected textile dye dermatitis, instructing patients to wash clothing before wearing and to avoid synthetic fabrics, dark dyes, and tightly fitted clothing may help.^9,64,65

The differential diagnosis for axillary lesions is broad, including infectious, inflammatory, and autoimmune etiologies. Irritant contact dermatitis (ICD) presents similar to ACD, though it is more immediate in onsetand typically demonstrates symptoms of burning and stinging rather than pruritus. Although histopathology is not reliable in differentiating ICD and ACD, it has been shown that focal parakeratosis is associated with ACD, whereas necrotic epidermal keratinocytes are found in ICD.¹⁰⁰

Intertrigo presents as large, erythematous, opposing patches or plaques confined to inguinal, submammary, axillary, and/or abdominal folds. Findings of beefy red erythema and peripheral satellite pustules may implicate presence of Candida, which can be identified with potassium hydroxide preparations.

Inverse psoriasis presents as sharply demarcated, erythematous, moist, smooth plaques or patches with minimal scale. The most common area of involvement is the inguinal folds, followed by the axillae, inframammary folds, perianal area, umbilicus, and retroauricular areas. Involvement of the elbows and knees or a positive family history of psoriasis may be useful knowledge in establishing the diagnosis. A biopsy may show dermal eosinophils, epidermal spongiosis, and focal serum in the scale, in addition to features of typical psoriasis plaques.¹⁰¹

Seborrheic dermatitis typically is an erythematous eruption, often with yellowish greasy scale. Simultaneous involvement of the face and scalp may be noted. Although typically a clinical diagnosis, biopsy demonstrates shoulder parakeratosis with follicular plugging and lymphocytic exocytosis.

Hailey-Hailey disease (also called benign familial pemphigus) is an autosomal-dominant genetic condition presenting as moist, malodorous, painful, vegetative plaques, patches, or scaly pustules in flexural areas, frequently with flaccid blisters. Lesions are provoked by traumatic stimuli. Onset occurs in the second to fourth decades and may improve with age. The diagnosis is confirmed by biopsy, which demonstrates acantholysis of the epidermis. The moist superficial patches of Hailey-Hailey disease help distinguish it from comparably drier Darier disease, the other acantholytic disease of the axillae.

Granular parakeratosis (also called hyperkeratotic flexural erythema) is an uncommon dermatosis most often observed in middle-aged women. It presents as red-brown keratotic papules coalescing into plaques, often with overlying scale in intertriginous areas. This disorder may be related to exposure to aluminum, a key component of antiperspirants.¹⁰² Diagnosis with a skin biopsy demonstrates granular parakeratosis.

Infections most commonly include erythrasma, tinea, and candidiasis. Erythrasma caused by Corynebacterium minutissimum may present in the axillae and/or groin with sharply demarcated, red-brown patches. Wood lamp examination reveals coral red fluorescence. Tinea corporis, a dermatophyte infection, may present as scaly erythematous plaques with advancing borders and central clearing. Fungal cultures and potassium hydroxide preparations are useful to confirm the diagnosis.

Pseudofolliculitis barbae most often is thought of as a condition affecting the beard in Black men, but it also may present in individuals of all races who shave the axillary and inguinal regions. Typical features include pruritic inflammatory papules and pustules with surrounding erythema and hyperpigmentation.

Fox-Fordyce disease is a disorder of the apocrine sweat glands that presents as several flesh-colored, perifollicular, monomorphic papules in the axillae. It typically is a disease of young females and also can involve the areola and vulva. Histopathology may show hyperkeratosis, irregular acanthosis, and dilated sweat glands.

Hidradenitis suppurativa is a chronic inflammatory condition that presents with multiple cysts; nodules; abscesses; sinus tract formation; and suppuration of the axillary, anogenital, and sometimes inframammary areas, typically at the onset of puberty. The diagnosis is best supported by history and physical examination, which may be notable for recurrent abscesses, draining tracts, double comedones, and ropelike scarring.

Extramammary Paget disease is a rare malignancy affecting apocrine gland–bearing areas, including axillary and genital regions. It most commonly presents as a unilateral or asymmetric, scaly, erythematous plaque. Histopathology demonstrates Paget cells with abundant clear cytoplasm and pleomorphic nuclei, typically grouped in the lower portion of the epidermis.

Final Thoughts

Axillary dermatoses often can be challenging to diagnose given the range of pathologies that can present in intertriginous areas. Allergic contact dermatitis is a common culprit due to unique anatomical considerations and self-care practices, including shaving/hair removal; use of deodorants, antiperspirants, bodywashes, and clothing; and frictional and moisture influences. The most likely offender among contact allergens is fragrance, but other possibilities to consider include PG, preservatives, aluminum, nickel, and textile dyes. Albeit less common, systemic exposure to allergens may result in SCD and SDRIFE with a rash in intertriginous zones, including the axillae. Additionally, other infectious, inflammatory, and autoimmune etiologies should be considered and ruled out.

Patch testing is the most reliable method to diagnose suspected ACD. Once confirmed, management includes the use of topical steroids and avoidance of the causative agent. Additionally, patients should be informed of the American Contact Dermatitis Society Contact Allergen Management Program (https://www.contactderm.org/patient-support/camp-access), which provides patients with useful information on products that are safe to use based on their patch testing results.

Approximately 20% of the general population has a contact allergy.¹ Allergic contact dermatitis (ACD) is a delayed type IV hypersensitivity reaction mediated by T lymphocytes.² Axillary ACD presentation is variable but typically includes an eczematous eruption with erythematous scaly patches or plaques. Common products in contact with the axillae include deodorants, antiperspirants, razors, bodywash, and clothing.

Axillary skin is distinct from skin elsewhere on the body due to both anatomical characteristics and unique human self-care practices. Axillary skin has reduced barrier function, faster stratum corneum turnover, and altered lipid levels.^3-5 Moreover, the axillae often are subject to shaving or other hair removal practices that alter the local environment, as layers of stratum corneum and hair are mechanically removed, which causes irritation and predisposes the skin to enhanced sensitivity to topical exposures.^6,7 With the abundance of apocrine and eccrine glands, the axillae are prone to sweat, which also can accentuate contact allergy.^2,3 Other factors, such as occlusion and friction, contribute to axillary contact allergy.^8,9

Patch testing is the gold standard for the diagnosis of ACD and aids in identification of culprit allergens. A thorough patient history and examination of the rash distribution may provide further clues; for example, dermatitis due to a deodorant typically affects the vault, whereas textile dye dermatitis tends to spare the vault.^10,11 Baseline-limited patch testing detects up to two-thirds of clinically relevant allergens.¹² Therefore, patients may require subsequent testing with supplemental allergens.

The differential diagnosis for axillary lesions is broad—including inflammatory diseases such as irritant contact dermatitis and hidradenitis suppurativa, genetic disorders such as Hailey-Hailey disease, and infectious causes such as erythrasma—but may be narrowed with a thorough physical examination and patient history, histopathology, bedside diagnostic techniques (eg, scrapings and Wood lamp examination), and patch testing. Systemic contact dermatitis (SCD) or symmetrical drug-related intertriginous and flexural exanthema (SDRIFE) also may be suspected in cases of intertriginous dermatoses.

We review the potential allergens in products used on the axillae as well as the management of axillary ACD. We also discuss axillary dermatitis as a manifestation of SCD and SDRIFE.

Top Allergens in Products Used on the Axillae

Fragrance—A 1982 North American Contact Dermatitis Group study on cosmetic products identified fragrances as the most common cause of ACD,¹³ and this trend continues to hold true with more recent data.¹⁴ The incidence of fragrance allergy may be increasing, with positive patch tests to a fragrance chemical in 10% of patch test clinic populations.¹⁵ Fragrances are a ubiquitous ingredient in deodorants and antiperspirants, which are important sources implicated in the development and elicitation of fragrance ACD.¹⁶ One study found that fragrance was present in 97 of 107 (90%) deodorants available at Walgreens pharmacies.¹¹

In a study of patients with a history of an axillary rash caused by a deodorant spray, Johansen et al¹⁷ reported that the likelihood of fragrance allergy is increased by a factor of 2.4. This risk of developing a fragrance allergy may be exacerbated in those who shave; Edman¹⁸ reported that the odds ratio of developing a fragrance allergy among men who shave their beards was 2.9. Although there are no specific data on the effects of shaving on ACD, shaving in general can induce localized irritation and increase percutaneous absorption.¹⁹

The individual fragrance components in deodorants most likely to cause ACD include hydroxycitronellal, eugenol, and geraniol—all constituent ingredients in patch test formulations of fragrance mixture I.^11,20 Other common fragrance allergens associated with ACD include hydroxymethylpentylcyclohexenecarboxaldehyde, farnesol, and balsam of Peru.^21-27 Hydroperoxides of limonene and linalool, common fragrances in detergents and personal care products, are increasingly recognized as contact allergens and have been reported to cause axillary ACD from deodorants.^28-30

Dermatitis involving the bilateral axillary vaults wherever deodorant or antiperspirant was directly applied is the most common presentation of ACD due to fragrance (Figure 1).¹⁷ An eczematous eruption is common, though scale may be less apparent than in nonflexural regions. Axillary ACD secondary to fragrances also may result from use of fragranced laundry detergents, fabric softeners, soaps, and perfumes, and may spare the vaults.^10,29,31,32 Less common presentations of axillary ACD due to fragrance include pigmented dermatoses; for example, ACD from an antiperspirant containing hydroperoxide of limonene presented as hyperpigmented patches with minimal erythema and scaling in the edges of the axillary folds.^33,34

Diagnosis of a fragrance ACD typically is made with a standard patch test series including fragrance mixture I and balsam of Peru, which may detect 75% and 50% of fragrance sensitivities, respectively.³⁵ Patch testing may be followed with a repeated open application test of the product in question.³⁶ Additionally, it may be appropriate to test for other fragrance allergens including balsam of Tolu, fragrance mixture II, lichen acid mix, and hydroxyperoxides of linalool and limonene (among other botanicals) if standard patch testing is negative and suspicion of fragrance ACD remains elevated.¹¹

Propylene Glycol—Propylene glycol (PG)—a versatile substance that functions as a solvent, humectant, emulsifier, stabilizer, and antimicrobial—is the second most common contact allergen present in deodorants.¹¹ It is prevalent in both personal care and household products, including deodorants, cosmetics, foods, toothpaste, cleaning agents, and detergents.^11,37 Propylene glycol is both an allergen and an irritant. Among deodorants/antiperspirants, PG is both a common irritant and allergen, as its concentration may be particularly high (as much as 73%).³⁸ One commonly reported example of PG contact dermatitis is from the topical medicament minoxidil.^39,40

Patch testing data have demonstrated a positivity rate for PG ranging between 0.1% to 3.8%. The variability in these findings likely is due to differences in the tested concentrations of PG, as higher concentrations sometimes required to elicit an allergic reaction also may create a stronger irritation effect.⁴¹ Propylene glycol irritancy and the occlusive nature of the axillae may enhance sensitization to other allergens, as demonstrated by Agren-Jonsson and Magnusson,⁴² who reported sensitization to propantheline bromide and trichlorocarbanilide in patients who used a lotion with 90% PG. Many PG-containing products beyond deodorants/antiperspirants may be applied to the axillae, including steroid creams, lotions, shaving creams, and bodywashes.^38,43

The diagnosis of PG allergy via patch testing is challenging and at times controversial given its irritant nature. False-positive irritant reactions have been documented, characterized by a weak reaction at 48 hours that is absent by 96 hours (decrescendo reaction). A reaction may not appear until 96 hours (crescendo reaction), which typically indicates a true contact allergy but in the case of PG also may be the substance acting as a “late irritant.”⁴⁴ Fast (<24 hours) and well-demarcated reactions suggest irritation.⁴⁵ Regardless, reactions to PG on patch testing, even those regarded as weak, may be considered relevant in consideration of the clinical context.³⁷

Aluminum—Aluminum is the active ingredient in most antiperspirants, typically in the form of aluminum chloride, aluminum chlorohydrate, aluminum zirconium trichlorohydrex gly, or aluminum zirconium tetrachlorohydrex gly.⁴⁶ Aluminum mechanically obstructs the eccrine glands to reduce sweat.⁴⁷ Although aluminum is an uncommon allergen, a possible presentation of aluminum allergy is axillary vault dermatitis secondary to antiperspirant use.⁴⁶ Another potential manifestation is a ringlike reaction to the Finn Chambers (SmartPractice) used in patch testing.⁴⁶ In one case of aluminum-induced axillary dermatitis, a 28-year-old woman presented with eczema of the axillae, and subsequent patch testing revealed an allergy to aluminum chloride. The rash resolved upon cessation of use of an aluminum-containing deodorant.⁴⁸

Aluminum has been reported to cause granulomatous dermatitis in the axillae. This reaction typically presents as red-brown, pruritic papules limited to the area in which deodorant was applied, with histopathology revealing epithelioid granulomas.^49-51

Alum deodorants—considered a natural alternative—contain aluminum bound to potassium or ammonium in the form of a crystal or powder. Alum crystal deodorants have been reported to cause both a typical erythematous pruritic dermatitis as well as a granulomatous dermatitis with red-brown papules.^52,53 The granulomatous dermatitis caused by either form of aluminum resolves with avoidance and use of topical steroids or topical tacrolimus.^49,50,52,53

The diagnosis of aluminum ACD via patch testing may be identified with empty Finn Chambers, which are metallic aluminum, or with patch placement of aluminum chloride hexahydrate, though the former is only positive in patients with a strong allergy.^54,55 In 2022, aluminum was named Allergen of the Year by the American Contact Dermatitis Society, with recommendations to conduct patch testing with aluminum chloride hexahydrate 10% rather than the traditional 2% to increase diagnostic yield.⁵⁵ Additionally, it is recommended that aluminum be included in baseline patch testing for children due to the high prevalence of aluminum allergy in children and early exposure via childhood vaccines.^54-56 In patients with aluminum allergy, providers may suggest purchasing aluminum-free deodorants or provide recipes for homemade deodorant that includes ingredients such as arrowroot powder, cornstarch, and diatomaceous earth.⁴⁶

Nickel—Nickel is the most commonly identified contact allergen on patch testing yet an infrequent cause of axillary dermatitis. A case report from 2014 described axillary dermatitis in a woman that worsened during a positive patch test to nickel. Improvement was noted when the patient switched to titanium shaving razors.⁵⁷ Nickel allergy also may present in the form of SCD. In one report, a woman developed dermatitis of the flexural areas, including the axillae, 3 months after undergoing a sterilization procedure in which nickel-containing tubal implants were placed.⁵⁸ Patch testing revealed a positive reaction to nickel. The patient experienced complete resolution of the steroid-resistant dermatitis following removal of the implants via salpingectomy.⁵⁸

Textile Dye—In contrast to dermatitis caused by deodorants/antiperspirants, contact allergy to textile dyes presents as dermatitis involving the axillary borders but sparing the axillary vaults (Figures 2 and 3).¹⁰ Other potential presentations of textile dye dermatitis include erythema multiforme–like eruptions and erythematous wheal–type reactions.⁵⁹ Textile dyes are classified as disperse vs nondisperse, with the majority of contact dermatoses caused by disperse dyes, specifically Disperse Orange 1, blue 106, and blue 124.^60-62 Ryberg et al⁶¹ found that the axilla is one of the more common locations to be affected by textile dye allergy, particularly in women, which was further supported by Seidenari et al,⁶³ who found that skin folds were affected in 27% of study participants allergic to textile dyes (N=437), a finding that is likely due to friction, sweat, and occlusion.⁶² In one case report of a patient with dermatitis caused by reactive dyes, the garment required 3 washes before the patient experienced resolution of dermatitis.⁶⁴ For patients with textile dye dermatitis, mitigation strategies include washing clothing before wearing, especially for darkly dyed items; avoiding tight clothing; wearing garments made of cotton, wool, silk, or linen; and choosing light-colored garments.^9,64,65

Axillary Dermatitis as a Manifestation of SCD and SDRIFE

Systemic contact dermatitis occurs when an individual who was previously sensitized to a particular allergen develops ACD of the skin with systemic exposure to that allergen or immunochemically related allergens. Exposure may occur via ingestion, inhalation, intravenous, intramuscular, and transepidermal routes.⁶⁶ Systemic contact dermatitis manifests in a variety of ways, including focal flares at sites of prior contact dermatitis (recall reaction), vesicular hand dermatitis, intertriginous eruptions including axillary dermatitis, and generalized eruptions.⁶⁷

Systemic contact dermatitis rarely involves systemic symptoms, and onset typically is within days of exposure. The 3 most common groups of allergens causing SCD are metals, medications, and plants and herbals.⁶⁸ These allergens have all been reported to cause axillary dermatitis via SCD.^58,69,70 Foods containing balsam of Peru that may lead to SCD include citrus, chocolate, tomato, and certain alcohols.^70,71 Patients with a positive patch test to balsam of Peru may experience improvement of their dermatitis after reduction of balsam of Peru–rich foods from their diet.⁷⁰ Metals implicated in SCD include mercury, nickel, and gold.^72-74 Finally, PG ingestion also has been implicated in cases of SCD.³⁷

Symmetrical drug-related intertriginous and flexural exanthema is another condition that presents as intertriginous dermatitis and differs from SCD in that the eruption does not require presensitization; there may be no known prior exposure to the agent causing dermatitis. Historically, SDRIFE was described as baboon syndrome because of its frequent involvement of the buttocks with diffuse, well-demarcated, erythematous dermatitis resembling that of a baboon. This term is no longer used due to its insensitive nature and incomplete depiction of SDRIFE, which can affect body sites other than the buttocks.^68,75,76 Specific criteria to make this diagnosis include sharply demarcated and/or V-shaped erythema of the gluteal/perianal area, involvement of at least 1 other intertriginous or flexural region, symmetry of affected areas, and an absence of systemic symptoms.⁷⁶ There also may be papules, pustules, and vesicles present in affected areas. Symmetrical drug-related intertriginous and flexural exanthema most often is caused by β-lactam antibiotics, but other associated drugs include chemotherapeutic agents, such as mitomycin C.⁷⁶

Histopathology of both SCD and SDRIFE is variable and typically nonspecific, often revealing epidermal spongiosis and a perivascular mononuclear cell infiltrate with occasional neutrophils and eosinophils.⁷⁶ A case of SCD to mercury presenting as intertriginous dermatitis demonstrated a leukocytoclastic vasculitis pattern on biopsy.⁷⁷

Systemic contact dermatitis is diagnosed via a patch test, while SDRIFE typically has a negative patch test result and requires oral rechallenge testing, which reproduces the rash within hours.^78,79

Additional Allergens Causing Axillary ACD

Although fragrance is the most common allergen in deodorants, other ingredients have been shown to cause axillary ACD (Table).^80-90 In addition to these ingredients, allergens not previously mentioned that may be present in deodorants include lanolin, essential oils, and parabens.¹¹ Methylisothiazolinone in laundry detergent also has been found to instigate ACD.⁹¹ Fragrances and preservatives in laundry detergents also may contribute to dermatitis.⁹²

Other products that have caused axillary contact dermatitis include topical exposure to medicaments including clindamycin,⁹³ ethylenediamine in nystatin cream,⁹⁴ methylprednisolone acetate⁹⁵ and dipropylene glycol in a hydrocortisone lotion,⁹⁶ wood dusts from tropical hardwoods,⁹⁷ and tobacco.⁹⁸

Management of ACD

The most effective strategy in the management of patients with contact dermatitis is avoidance of the offending agent. Additionally, clinicians may recommend the use of topical steroids and/or calcineurin inhibitors to hasten resolution.²

For patients with contact dermatitis, a clinician may recommend product substitutions with few potential allergens to use prior to patch testing. Patients with a fragrance allergy should look for products specifically labeled as “fragrance free” rather than “hypoallergenic” or “unscented,” as the latter two may still contain minimal amounts of fragrance.³⁵ Patients should be educated on the functions of the allergens to which they are allergic so they may adequately avoid potential sources of contact.⁹⁹ For suspected textile dye dermatitis, instructing patients to wash clothing before wearing and to avoid synthetic fabrics, dark dyes, and tightly fitted clothing may help.^9,64,65

The differential diagnosis for axillary lesions is broad, including infectious, inflammatory, and autoimmune etiologies. Irritant contact dermatitis (ICD) presents similar to ACD, though it is more immediate in onsetand typically demonstrates symptoms of burning and stinging rather than pruritus. Although histopathology is not reliable in differentiating ICD and ACD, it has been shown that focal parakeratosis is associated with ACD, whereas necrotic epidermal keratinocytes are found in ICD.¹⁰⁰

Intertrigo presents as large, erythematous, opposing patches or plaques confined to inguinal, submammary, axillary, and/or abdominal folds. Findings of beefy red erythema and peripheral satellite pustules may implicate presence of Candida, which can be identified with potassium hydroxide preparations.

Inverse psoriasis presents as sharply demarcated, erythematous, moist, smooth plaques or patches with minimal scale. The most common area of involvement is the inguinal folds, followed by the axillae, inframammary folds, perianal area, umbilicus, and retroauricular areas. Involvement of the elbows and knees or a positive family history of psoriasis may be useful knowledge in establishing the diagnosis. A biopsy may show dermal eosinophils, epidermal spongiosis, and focal serum in the scale, in addition to features of typical psoriasis plaques.¹⁰¹

Seborrheic dermatitis typically is an erythematous eruption, often with yellowish greasy scale. Simultaneous involvement of the face and scalp may be noted. Although typically a clinical diagnosis, biopsy demonstrates shoulder parakeratosis with follicular plugging and lymphocytic exocytosis.

Hailey-Hailey disease (also called benign familial pemphigus) is an autosomal-dominant genetic condition presenting as moist, malodorous, painful, vegetative plaques, patches, or scaly pustules in flexural areas, frequently with flaccid blisters. Lesions are provoked by traumatic stimuli. Onset occurs in the second to fourth decades and may improve with age. The diagnosis is confirmed by biopsy, which demonstrates acantholysis of the epidermis. The moist superficial patches of Hailey-Hailey disease help distinguish it from comparably drier Darier disease, the other acantholytic disease of the axillae.

Granular parakeratosis (also called hyperkeratotic flexural erythema) is an uncommon dermatosis most often observed in middle-aged women. It presents as red-brown keratotic papules coalescing into plaques, often with overlying scale in intertriginous areas. This disorder may be related to exposure to aluminum, a key component of antiperspirants.¹⁰² Diagnosis with a skin biopsy demonstrates granular parakeratosis.

Infections most commonly include erythrasma, tinea, and candidiasis. Erythrasma caused by Corynebacterium minutissimum may present in the axillae and/or groin with sharply demarcated, red-brown patches. Wood lamp examination reveals coral red fluorescence. Tinea corporis, a dermatophyte infection, may present as scaly erythematous plaques with advancing borders and central clearing. Fungal cultures and potassium hydroxide preparations are useful to confirm the diagnosis.

Pseudofolliculitis barbae most often is thought of as a condition affecting the beard in Black men, but it also may present in individuals of all races who shave the axillary and inguinal regions. Typical features include pruritic inflammatory papules and pustules with surrounding erythema and hyperpigmentation.

Fox-Fordyce disease is a disorder of the apocrine sweat glands that presents as several flesh-colored, perifollicular, monomorphic papules in the axillae. It typically is a disease of young females and also can involve the areola and vulva. Histopathology may show hyperkeratosis, irregular acanthosis, and dilated sweat glands.

Hidradenitis suppurativa is a chronic inflammatory condition that presents with multiple cysts; nodules; abscesses; sinus tract formation; and suppuration of the axillary, anogenital, and sometimes inframammary areas, typically at the onset of puberty. The diagnosis is best supported by history and physical examination, which may be notable for recurrent abscesses, draining tracts, double comedones, and ropelike scarring.

Extramammary Paget disease is a rare malignancy affecting apocrine gland–bearing areas, including axillary and genital regions. It most commonly presents as a unilateral or asymmetric, scaly, erythematous plaque. Histopathology demonstrates Paget cells with abundant clear cytoplasm and pleomorphic nuclei, typically grouped in the lower portion of the epidermis.

Final Thoughts

Axillary dermatoses often can be challenging to diagnose given the range of pathologies that can present in intertriginous areas. Allergic contact dermatitis is a common culprit due to unique anatomical considerations and self-care practices, including shaving/hair removal; use of deodorants, antiperspirants, bodywashes, and clothing; and frictional and moisture influences. The most likely offender among contact allergens is fragrance, but other possibilities to consider include PG, preservatives, aluminum, nickel, and textile dyes. Albeit less common, systemic exposure to allergens may result in SCD and SDRIFE with a rash in intertriginous zones, including the axillae. Additionally, other infectious, inflammatory, and autoimmune etiologies should be considered and ruled out.

Patch testing is the most reliable method to diagnose suspected ACD. Once confirmed, management includes the use of topical steroids and avoidance of the causative agent. Additionally, patients should be informed of the American Contact Dermatitis Society Contact Allergen Management Program (https://www.contactderm.org/patient-support/camp-access), which provides patients with useful information on products that are safe to use based on their patch testing results.

Approximately 20% of the general population has a contact allergy.¹ Allergic contact dermatitis (ACD) is a delayed type IV hypersensitivity reaction mediated by T lymphocytes.² Axillary ACD presentation is variable but typically includes an eczematous eruption with erythematous scaly patches or plaques. Common products in contact with the axillae include deodorants, antiperspirants, razors, bodywash, and clothing.

Axillary skin is distinct from skin elsewhere on the body due to both anatomical characteristics and unique human self-care practices. Axillary skin has reduced barrier function, faster stratum corneum turnover, and altered lipid levels.^3-5 Moreover, the axillae often are subject to shaving or other hair removal practices that alter the local environment, as layers of stratum corneum and hair are mechanically removed, which causes irritation and predisposes the skin to enhanced sensitivity to topical exposures.^6,7 With the abundance of apocrine and eccrine glands, the axillae are prone to sweat, which also can accentuate contact allergy.^2,3 Other factors, such as occlusion and friction, contribute to axillary contact allergy.^8,9

Patch testing is the gold standard for the diagnosis of ACD and aids in identification of culprit allergens. A thorough patient history and examination of the rash distribution may provide further clues; for example, dermatitis due to a deodorant typically affects the vault, whereas textile dye dermatitis tends to spare the vault.^10,11 Baseline-limited patch testing detects up to two-thirds of clinically relevant allergens.¹² Therefore, patients may require subsequent testing with supplemental allergens.

The differential diagnosis for axillary lesions is broad—including inflammatory diseases such as irritant contact dermatitis and hidradenitis suppurativa, genetic disorders such as Hailey-Hailey disease, and infectious causes such as erythrasma—but may be narrowed with a thorough physical examination and patient history, histopathology, bedside diagnostic techniques (eg, scrapings and Wood lamp examination), and patch testing. Systemic contact dermatitis (SCD) or symmetrical drug-related intertriginous and flexural exanthema (SDRIFE) also may be suspected in cases of intertriginous dermatoses.

We review the potential allergens in products used on the axillae as well as the management of axillary ACD. We also discuss axillary dermatitis as a manifestation of SCD and SDRIFE.

Top Allergens in Products Used on the Axillae

Fragrance—A 1982 North American Contact Dermatitis Group study on cosmetic products identified fragrances as the most common cause of ACD,¹³ and this trend continues to hold true with more recent data.¹⁴ The incidence of fragrance allergy may be increasing, with positive patch tests to a fragrance chemical in 10% of patch test clinic populations.¹⁵ Fragrances are a ubiquitous ingredient in deodorants and antiperspirants, which are important sources implicated in the development and elicitation of fragrance ACD.¹⁶ One study found that fragrance was present in 97 of 107 (90%) deodorants available at Walgreens pharmacies.¹¹

In a study of patients with a history of an axillary rash caused by a deodorant spray, Johansen et al¹⁷ reported that the likelihood of fragrance allergy is increased by a factor of 2.4. This risk of developing a fragrance allergy may be exacerbated in those who shave; Edman¹⁸ reported that the odds ratio of developing a fragrance allergy among men who shave their beards was 2.9. Although there are no specific data on the effects of shaving on ACD, shaving in general can induce localized irritation and increase percutaneous absorption.¹⁹

The individual fragrance components in deodorants most likely to cause ACD include hydroxycitronellal, eugenol, and geraniol—all constituent ingredients in patch test formulations of fragrance mixture I.^11,20 Other common fragrance allergens associated with ACD include hydroxymethylpentylcyclohexenecarboxaldehyde, farnesol, and balsam of Peru.^21-27 Hydroperoxides of limonene and linalool, common fragrances in detergents and personal care products, are increasingly recognized as contact allergens and have been reported to cause axillary ACD from deodorants.^28-30

Dermatitis involving the bilateral axillary vaults wherever deodorant or antiperspirant was directly applied is the most common presentation of ACD due to fragrance (Figure 1).¹⁷ An eczematous eruption is common, though scale may be less apparent than in nonflexural regions. Axillary ACD secondary to fragrances also may result from use of fragranced laundry detergents, fabric softeners, soaps, and perfumes, and may spare the vaults.^10,29,31,32 Less common presentations of axillary ACD due to fragrance include pigmented dermatoses; for example, ACD from an antiperspirant containing hydroperoxide of limonene presented as hyperpigmented patches with minimal erythema and scaling in the edges of the axillary folds.^33,34

Diagnosis of a fragrance ACD typically is made with a standard patch test series including fragrance mixture I and balsam of Peru, which may detect 75% and 50% of fragrance sensitivities, respectively.³⁵ Patch testing may be followed with a repeated open application test of the product in question.³⁶ Additionally, it may be appropriate to test for other fragrance allergens including balsam of Tolu, fragrance mixture II, lichen acid mix, and hydroxyperoxides of linalool and limonene (among other botanicals) if standard patch testing is negative and suspicion of fragrance ACD remains elevated.¹¹

Propylene Glycol—Propylene glycol (PG)—a versatile substance that functions as a solvent, humectant, emulsifier, stabilizer, and antimicrobial—is the second most common contact allergen present in deodorants.¹¹ It is prevalent in both personal care and household products, including deodorants, cosmetics, foods, toothpaste, cleaning agents, and detergents.^11,37 Propylene glycol is both an allergen and an irritant. Among deodorants/antiperspirants, PG is both a common irritant and allergen, as its concentration may be particularly high (as much as 73%).³⁸ One commonly reported example of PG contact dermatitis is from the topical medicament minoxidil.^39,40

Patch testing data have demonstrated a positivity rate for PG ranging between 0.1% to 3.8%. The variability in these findings likely is due to differences in the tested concentrations of PG, as higher concentrations sometimes required to elicit an allergic reaction also may create a stronger irritation effect.⁴¹ Propylene glycol irritancy and the occlusive nature of the axillae may enhance sensitization to other allergens, as demonstrated by Agren-Jonsson and Magnusson,⁴² who reported sensitization to propantheline bromide and trichlorocarbanilide in patients who used a lotion with 90% PG. Many PG-containing products beyond deodorants/antiperspirants may be applied to the axillae, including steroid creams, lotions, shaving creams, and bodywashes.^38,43

The diagnosis of PG allergy via patch testing is challenging and at times controversial given its irritant nature. False-positive irritant reactions have been documented, characterized by a weak reaction at 48 hours that is absent by 96 hours (decrescendo reaction). A reaction may not appear until 96 hours (crescendo reaction), which typically indicates a true contact allergy but in the case of PG also may be the substance acting as a “late irritant.”⁴⁴ Fast (<24 hours) and well-demarcated reactions suggest irritation.⁴⁵ Regardless, reactions to PG on patch testing, even those regarded as weak, may be considered relevant in consideration of the clinical context.³⁷

Aluminum—Aluminum is the active ingredient in most antiperspirants, typically in the form of aluminum chloride, aluminum chlorohydrate, aluminum zirconium trichlorohydrex gly, or aluminum zirconium tetrachlorohydrex gly.⁴⁶ Aluminum mechanically obstructs the eccrine glands to reduce sweat.⁴⁷ Although aluminum is an uncommon allergen, a possible presentation of aluminum allergy is axillary vault dermatitis secondary to antiperspirant use.⁴⁶ Another potential manifestation is a ringlike reaction to the Finn Chambers (SmartPractice) used in patch testing.⁴⁶ In one case of aluminum-induced axillary dermatitis, a 28-year-old woman presented with eczema of the axillae, and subsequent patch testing revealed an allergy to aluminum chloride. The rash resolved upon cessation of use of an aluminum-containing deodorant.⁴⁸

Aluminum has been reported to cause granulomatous dermatitis in the axillae. This reaction typically presents as red-brown, pruritic papules limited to the area in which deodorant was applied, with histopathology revealing epithelioid granulomas.^49-51

Alum deodorants—considered a natural alternative—contain aluminum bound to potassium or ammonium in the form of a crystal or powder. Alum crystal deodorants have been reported to cause both a typical erythematous pruritic dermatitis as well as a granulomatous dermatitis with red-brown papules.^52,53 The granulomatous dermatitis caused by either form of aluminum resolves with avoidance and use of topical steroids or topical tacrolimus.^49,50,52,53

The diagnosis of aluminum ACD via patch testing may be identified with empty Finn Chambers, which are metallic aluminum, or with patch placement of aluminum chloride hexahydrate, though the former is only positive in patients with a strong allergy.^54,55 In 2022, aluminum was named Allergen of the Year by the American Contact Dermatitis Society, with recommendations to conduct patch testing with aluminum chloride hexahydrate 10% rather than the traditional 2% to increase diagnostic yield.⁵⁵ Additionally, it is recommended that aluminum be included in baseline patch testing for children due to the high prevalence of aluminum allergy in children and early exposure via childhood vaccines.^54-56 In patients with aluminum allergy, providers may suggest purchasing aluminum-free deodorants or provide recipes for homemade deodorant that includes ingredients such as arrowroot powder, cornstarch, and diatomaceous earth.⁴⁶

Nickel—Nickel is the most commonly identified contact allergen on patch testing yet an infrequent cause of axillary dermatitis. A case report from 2014 described axillary dermatitis in a woman that worsened during a positive patch test to nickel. Improvement was noted when the patient switched to titanium shaving razors.⁵⁷ Nickel allergy also may present in the form of SCD. In one report, a woman developed dermatitis of the flexural areas, including the axillae, 3 months after undergoing a sterilization procedure in which nickel-containing tubal implants were placed.⁵⁸ Patch testing revealed a positive reaction to nickel. The patient experienced complete resolution of the steroid-resistant dermatitis following removal of the implants via salpingectomy.⁵⁸

Textile Dye—In contrast to dermatitis caused by deodorants/antiperspirants, contact allergy to textile dyes presents as dermatitis involving the axillary borders but sparing the axillary vaults (Figures 2 and 3).¹⁰ Other potential presentations of textile dye dermatitis include erythema multiforme–like eruptions and erythematous wheal–type reactions.⁵⁹ Textile dyes are classified as disperse vs nondisperse, with the majority of contact dermatoses caused by disperse dyes, specifically Disperse Orange 1, blue 106, and blue 124.^60-62 Ryberg et al⁶¹ found that the axilla is one of the more common locations to be affected by textile dye allergy, particularly in women, which was further supported by Seidenari et al,⁶³ who found that skin folds were affected in 27% of study participants allergic to textile dyes (N=437), a finding that is likely due to friction, sweat, and occlusion.⁶² In one case report of a patient with dermatitis caused by reactive dyes, the garment required 3 washes before the patient experienced resolution of dermatitis.⁶⁴ For patients with textile dye dermatitis, mitigation strategies include washing clothing before wearing, especially for darkly dyed items; avoiding tight clothing; wearing garments made of cotton, wool, silk, or linen; and choosing light-colored garments.^9,64,65

Axillary Dermatitis as a Manifestation of SCD and SDRIFE

Systemic contact dermatitis occurs when an individual who was previously sensitized to a particular allergen develops ACD of the skin with systemic exposure to that allergen or immunochemically related allergens. Exposure may occur via ingestion, inhalation, intravenous, intramuscular, and transepidermal routes.⁶⁶ Systemic contact dermatitis manifests in a variety of ways, including focal flares at sites of prior contact dermatitis (recall reaction), vesicular hand dermatitis, intertriginous eruptions including axillary dermatitis, and generalized eruptions.⁶⁷

Systemic contact dermatitis rarely involves systemic symptoms, and onset typically is within days of exposure. The 3 most common groups of allergens causing SCD are metals, medications, and plants and herbals.⁶⁸ These allergens have all been reported to cause axillary dermatitis via SCD.^58,69,70 Foods containing balsam of Peru that may lead to SCD include citrus, chocolate, tomato, and certain alcohols.^70,71 Patients with a positive patch test to balsam of Peru may experience improvement of their dermatitis after reduction of balsam of Peru–rich foods from their diet.⁷⁰ Metals implicated in SCD include mercury, nickel, and gold.^72-74 Finally, PG ingestion also has been implicated in cases of SCD.³⁷

Symmetrical drug-related intertriginous and flexural exanthema is another condition that presents as intertriginous dermatitis and differs from SCD in that the eruption does not require presensitization; there may be no known prior exposure to the agent causing dermatitis. Historically, SDRIFE was described as baboon syndrome because of its frequent involvement of the buttocks with diffuse, well-demarcated, erythematous dermatitis resembling that of a baboon. This term is no longer used due to its insensitive nature and incomplete depiction of SDRIFE, which can affect body sites other than the buttocks.^68,75,76 Specific criteria to make this diagnosis include sharply demarcated and/or V-shaped erythema of the gluteal/perianal area, involvement of at least 1 other intertriginous or flexural region, symmetry of affected areas, and an absence of systemic symptoms.⁷⁶ There also may be papules, pustules, and vesicles present in affected areas. Symmetrical drug-related intertriginous and flexural exanthema most often is caused by β-lactam antibiotics, but other associated drugs include chemotherapeutic agents, such as mitomycin C.⁷⁶

Histopathology of both SCD and SDRIFE is variable and typically nonspecific, often revealing epidermal spongiosis and a perivascular mononuclear cell infiltrate with occasional neutrophils and eosinophils.⁷⁶ A case of SCD to mercury presenting as intertriginous dermatitis demonstrated a leukocytoclastic vasculitis pattern on biopsy.⁷⁷

Systemic contact dermatitis is diagnosed via a patch test, while SDRIFE typically has a negative patch test result and requires oral rechallenge testing, which reproduces the rash within hours.^78,79

Additional Allergens Causing Axillary ACD

Although fragrance is the most common allergen in deodorants, other ingredients have been shown to cause axillary ACD (Table).^80-90 In addition to these ingredients, allergens not previously mentioned that may be present in deodorants include lanolin, essential oils, and parabens.¹¹ Methylisothiazolinone in laundry detergent also has been found to instigate ACD.⁹¹ Fragrances and preservatives in laundry detergents also may contribute to dermatitis.⁹²

Other products that have caused axillary contact dermatitis include topical exposure to medicaments including clindamycin,⁹³ ethylenediamine in nystatin cream,⁹⁴ methylprednisolone acetate⁹⁵ and dipropylene glycol in a hydrocortisone lotion,⁹⁶ wood dusts from tropical hardwoods,⁹⁷ and tobacco.⁹⁸

Management of ACD

The most effective strategy in the management of patients with contact dermatitis is avoidance of the offending agent. Additionally, clinicians may recommend the use of topical steroids and/or calcineurin inhibitors to hasten resolution.²

For patients with contact dermatitis, a clinician may recommend product substitutions with few potential allergens to use prior to patch testing. Patients with a fragrance allergy should look for products specifically labeled as “fragrance free” rather than “hypoallergenic” or “unscented,” as the latter two may still contain minimal amounts of fragrance.³⁵ Patients should be educated on the functions of the allergens to which they are allergic so they may adequately avoid potential sources of contact.⁹⁹ For suspected textile dye dermatitis, instructing patients to wash clothing before wearing and to avoid synthetic fabrics, dark dyes, and tightly fitted clothing may help.^9,64,65

The differential diagnosis for axillary lesions is broad, including infectious, inflammatory, and autoimmune etiologies. Irritant contact dermatitis (ICD) presents similar to ACD, though it is more immediate in onsetand typically demonstrates symptoms of burning and stinging rather than pruritus. Although histopathology is not reliable in differentiating ICD and ACD, it has been shown that focal parakeratosis is associated with ACD, whereas necrotic epidermal keratinocytes are found in ICD.¹⁰⁰

Intertrigo presents as large, erythematous, opposing patches or plaques confined to inguinal, submammary, axillary, and/or abdominal folds. Findings of beefy red erythema and peripheral satellite pustules may implicate presence of Candida, which can be identified with potassium hydroxide preparations.

Inverse psoriasis presents as sharply demarcated, erythematous, moist, smooth plaques or patches with minimal scale. The most common area of involvement is the inguinal folds, followed by the axillae, inframammary folds, perianal area, umbilicus, and retroauricular areas. Involvement of the elbows and knees or a positive family history of psoriasis may be useful knowledge in establishing the diagnosis. A biopsy may show dermal eosinophils, epidermal spongiosis, and focal serum in the scale, in addition to features of typical psoriasis plaques.¹⁰¹

Seborrheic dermatitis typically is an erythematous eruption, often with yellowish greasy scale. Simultaneous involvement of the face and scalp may be noted. Although typically a clinical diagnosis, biopsy demonstrates shoulder parakeratosis with follicular plugging and lymphocytic exocytosis.

Hailey-Hailey disease (also called benign familial pemphigus) is an autosomal-dominant genetic condition presenting as moist, malodorous, painful, vegetative plaques, patches, or scaly pustules in flexural areas, frequently with flaccid blisters. Lesions are provoked by traumatic stimuli. Onset occurs in the second to fourth decades and may improve with age. The diagnosis is confirmed by biopsy, which demonstrates acantholysis of the epidermis. The moist superficial patches of Hailey-Hailey disease help distinguish it from comparably drier Darier disease, the other acantholytic disease of the axillae.

Granular parakeratosis (also called hyperkeratotic flexural erythema) is an uncommon dermatosis most often observed in middle-aged women. It presents as red-brown keratotic papules coalescing into plaques, often with overlying scale in intertriginous areas. This disorder may be related to exposure to aluminum, a key component of antiperspirants.¹⁰² Diagnosis with a skin biopsy demonstrates granular parakeratosis.

Infections most commonly include erythrasma, tinea, and candidiasis. Erythrasma caused by Corynebacterium minutissimum may present in the axillae and/or groin with sharply demarcated, red-brown patches. Wood lamp examination reveals coral red fluorescence. Tinea corporis, a dermatophyte infection, may present as scaly erythematous plaques with advancing borders and central clearing. Fungal cultures and potassium hydroxide preparations are useful to confirm the diagnosis.

Pseudofolliculitis barbae most often is thought of as a condition affecting the beard in Black men, but it also may present in individuals of all races who shave the axillary and inguinal regions. Typical features include pruritic inflammatory papules and pustules with surrounding erythema and hyperpigmentation.

Fox-Fordyce disease is a disorder of the apocrine sweat glands that presents as several flesh-colored, perifollicular, monomorphic papules in the axillae. It typically is a disease of young females and also can involve the areola and vulva. Histopathology may show hyperkeratosis, irregular acanthosis, and dilated sweat glands.

Hidradenitis suppurativa is a chronic inflammatory condition that presents with multiple cysts; nodules; abscesses; sinus tract formation; and suppuration of the axillary, anogenital, and sometimes inframammary areas, typically at the onset of puberty. The diagnosis is best supported by history and physical examination, which may be notable for recurrent abscesses, draining tracts, double comedones, and ropelike scarring.

Extramammary Paget disease is a rare malignancy affecting apocrine gland–bearing areas, including axillary and genital regions. It most commonly presents as a unilateral or asymmetric, scaly, erythematous plaque. Histopathology demonstrates Paget cells with abundant clear cytoplasm and pleomorphic nuclei, typically grouped in the lower portion of the epidermis.

Final Thoughts

Axillary dermatoses often can be challenging to diagnose given the range of pathologies that can present in intertriginous areas. Allergic contact dermatitis is a common culprit due to unique anatomical considerations and self-care practices, including shaving/hair removal; use of deodorants, antiperspirants, bodywashes, and clothing; and frictional and moisture influences. The most likely offender among contact allergens is fragrance, but other possibilities to consider include PG, preservatives, aluminum, nickel, and textile dyes. Albeit less common, systemic exposure to allergens may result in SCD and SDRIFE with a rash in intertriginous zones, including the axillae. Additionally, other infectious, inflammatory, and autoimmune etiologies should be considered and ruled out.

Patch testing is the most reliable method to diagnose suspected ACD. Once confirmed, management includes the use of topical steroids and avoidance of the causative agent. Additionally, patients should be informed of the American Contact Dermatitis Society Contact Allergen Management Program (https://www.contactderm.org/patient-support/camp-access), which provides patients with useful information on products that are safe to use based on their patch testing results.

As many as one-quarter of human cancers are related to chronic inflammation, chronic infection, or both.¹ Extrinsic inflammation leads to generation of proinflammatory cytokines that in turn recruit other inflammatory cells, which is thought to generate a positive amplification loop.² Intrinsic stimuli from proto-oncogenes and mutations in tumor suppressor genes lead to transformed cancer cells that also secrete proinflammatory cytokines, thus propagating the cycle.

Numerous factors have been observed in association with tumor growth, progression, invasion, and metastasis.³ One factor for the development of squamous cell carcinoma (SCC) may be chronic inflammatory dermatoses. To date, reviews of chronic inflammation–associated malignancy have focused on solid organ cancers. We sought to provide an up-to-date review of SCC arising within chronic dermatoses, with an emphasis on the anatomic location of dermatoses involved in the transformation of cancer cells, the lag time from onset of dermatosis to diagnosis of SCC, and the distinctive mechanisms thought to be involved in the tumorigenesis in particular dermatoses.

Discoid Lupus Erythematosus

Discoid lupus erythematosus (DLE) is a chronic cutaneous lupus erythematosus variant with a female to male predominance of 3:1,⁴ and DLE lesions are prone to malignant transformation. Retrospective cohort studies have attempted to characterize who is at risk for SCC and how SCCs behave depending on their location. Cohorts from China,⁵ India,⁶ and Japan⁷ have noted a higher rate of SCC within DLE lesions in men (female to male ratios of 1:2.2, 1:1.6, and 1:2, respectively) and shorter lag times for SCC onset within DLE lesions of the lips (13, 5, and 10 years, respectively) compared to SCC arising in DLE elsewhere (19.2, 11.2, and 26 years, respectively). Studies have noted that DLE lesions of the lips may be prone to more rapid SCC tumorigenesis compared to DLE on cutaneous sites. One study reported SCC in DLE recurrence, metastasis, and death rates of 29%, 16.1%, and 19.4%, respectively,⁵ which exceeds reported rates in non-DLE SCCs (20%, 0.5% to 6%, and 1%, respectively).^5,8

Because SCC arising within DLE is most common on the lips (Figure 1), it has been hypothesized that the high rate of transformation of DLE lesions on the lips may be due to constant exposure to irritation and tobacco, which may accelerate carcinogenesis.⁵ It also has been hypothesized that atrophic discoid lesions have lost sun protection and are more prone to mutagenic UV radiation,⁹ as SCCs arising in DLE lesions virtually always display prominent solar elastosis⁶; however, SCC has been observed to arise in non–sun-exposed DLE lesions in both White and Black patients.¹⁰

Photograph courtesy of Andrea Murina, MD.

Additionally, use of immunosuppressant medications may accelerate the emergence of malignancy or more aggressive forms of malignancy; however, patients with autoimmune disease have a greater risk for malignancy at baseline,¹¹ thus making it difficult to determine the excess risk from medications. There also may be a role for human papillomavirus (HPV) accelerating SCC development in DLE lesions, as demonstrated in a case of SCC arising in DLE lesions of the ears, with viral staining evident within the tumors.¹² However, testing for HPV is not routinely performed in these cases.

Dermatologists need to be aware of the relatively rapid tumorigenesis and aggressive behavior of transformation and aggression seen with SCC arising within orolabial DLE lesions compared to cutaneous lesions, especially those on the lips.

Lichen Planus

Although patients with typical cutaneous lichen planus lesions do not have an increased risk for SCC,¹³ variants of lichen planus may predispose patients to SCC.

Oral Lichen Planus—Oral lichen planus (OLP) lesions are prone to malignant transformation. A systematic review of 16 studies evaluating the risk for OLP-associated SCC revealed an overall transformation rate of 1.09%, with a mean lag time of 4.3 years,¹⁴ compared to a reference rate of 0.2% for oral SCC.¹⁵ A meta-analysis of 19,676 patients with OLP and other oral lichenoid lesions revealed an oral SCC rate of 1.1%, with higher rates of transformation seen in cigarette smokers, alcoholics, and patients with hepatitis C virus infection.¹⁶ The ulcerative subtype of OLP appears to present a greater risk for malignant transformation.¹⁵ Dermatologists also should be cognizant that treatments for OLP such as topical calcineurin inhibitors may support the development of malignancy within inflammatory lesions.¹⁷

Hypertrophic Lichen Planus—The hypertrophic variant of lichen planus (HLP) also is prone to malignant transformation. A 1991 epidemiologic study from Sweden of malignancy arising in lichen planus revealed a disproportionate number of cases arising in verrucous or hypertrophic lesions, with a mean of 12.2 years from onset of the dermatosis to malignancy diagnosis.¹³ A subsequent 2015 retrospective study of 38 patients revealed that SCC had a propensity for the lower limb, favoring the pretibial region and the calf over the foot and the ankle with a reported lag time of 11 years.¹⁸

Although metastatic SCC arising in HLP is rare, 2 cases have been reported. A 24-year-old woman presented with an HLP plaque on the lower leg that developed during childhood and rapidly enlarged 2 months prior to presentation; she eventually died from metastatic disease.¹⁹ In another case, a 34-year-old man presented with an HLP lesion of approximately 10 years’ duration. A well-differentiated SCC was excised, and he developed lymph node metastases 5 months later.²⁰

It is important to note that HLP on the legs often is misdiagnosed as SCC, as pseudoepitheliomatous hyperplasia and squamous metaplasia can be difficult to differentiate clinically and histologically.^21,22 In the case of multiple eruptive SCCs of the lower leg, clinical correlation is essential to avoid unnecessary and ineffective surgical treatment.

Patients with HLP may exhibit Wickham striae, follicular accentuation, and mucocutaneous lichen planus at other sites, or a correlative initiation of possible culprit medications.²³ Because true SCC arising within HLP is relatively rare, its malignant potential is not as clear as those arising within DLE; however, the lower limb appears to be the most common location for SCC within HLP.Nail Lichen Planus—Squamous cell carcinoma arising in nail lichen planus is rare. A report of 2 patients were diagnosed with lichen planus approximately 15 years prior to diagnosis of ungual SCC.²⁴ Given the rarity of this presentation, it is difficult to ascertain the approximate lag time and other risk factors. Furthermore, the role of HPV in these cases was not ruled out. Oncogenic HPV strains have been reported in patients with periungual SCC.^25,26

Lichen Sclerosus

Lichen sclerosus (LS) is a chronic inflammatory dermatosis that favors the anogenital area in a female to male ratio of 10:1.²⁷ It is considered a premalignant condition for SCC tumorigenesis and may be a strong predictor of vulvar SCC (Figure 2), as 62% of vulvar SCC cases (N=78) may have adjacent LS.²⁸

Photograph courtesy of Laura C. Williams, MD (New Orleans, Louisiana).

In a Dutch cohort of 3038 women with LS, 2.6% of patients developed vulvar SCC at a median of 3.3 years after LS diagnosis.²⁹ Other studies have estimated a lag time of 4 years until SCC presentation.³⁰ An Italian cohort of 976 women similarly observed that 2.7% of patients developed premalignancy or SCC.³¹ It was previously estimated that 3% to 5% of patients with LS developed SCC; however, prior studies may have included cases of vulvar intraepithelial neoplasia with low risk for invasive SCC, which might have overestimated true risk of SCC.³² Another confounding factor for elucidating SCC on a background of LS may be the presence of HPV.³³ Extragenital LS does not appear to have similar potential for malignant transformation.³⁴

In a prospective Australian cohort of 507 women with LS (mean age, 55.4 years), remission was induced with potent topical corticosteroids.³⁵ Patients who were adherent to a topical regimen did not develop SCC during follow-up. Those who were nonadherent or partially adherent had a 4.7% risk for SCC.³⁵ In a similar prospective study of 83 women in France, the SCC rate was 9.6% in lesions that were untreated or irregularly treated.³⁶ These studies provide essential evidence that appropriately treating LS can prevent SCC at a later date, though longer-term data are lacking.

The rate of SCC arising in male genital LS may approach 8.4%,³⁷ with a lag time of 17 years from onset of LS to SCC diagnosis.³⁸ Although circumcision often is considered curative for male genital LS, patients have been observed to develop penile SCC at least 5 years after circumcision.³⁹ Male penile SCC in a background of LS may not necessarily be HPV associated.⁴⁰

Marjolin Ulcer

Chronic ulcers or scars, typically postburn scars, may undergo malignant transformation, with SCC being the most common carcinoma.⁴¹ Squamous cell carcinoma in the context of a chronic ulcer or wound is known as a Marjolin ulcer (MU). Up to 2% of burn scars have been observed to undergo malignant transformation.⁴² Marjolin ulcers tend to behave aggressively once they form, and it has been proposed that removal of scar tissue may be a preventive therapeutic strategy.⁴³ Cohort studies of MU on the lower extremities have observed lag times of 26.4⁴⁴ and 37.9⁴⁵ years, with both studies also noting relatively high rates of local recurrence.

The pathogenesis of MU appears to be multifactorial. Chronic inflammation and scar formation have been implicated. Chronic inflammation and irritation of lesions at natural creases are thought to increase mitotic activity,⁴¹ and local accumulation of toxin may promote mutagenesis.⁴⁶ Scar formation may create a locally immunoprivileged site, allowing for developing tumors to evade the immune system⁴⁷ and become even more aggressive as the tumor accumulates.⁴⁸ Scar formation also may prevent the ability of immune cells to penetrate the tumor microenvironment and access lymphatic channels.⁴⁹

Hidradenitis Suppurativa

As many as 3.2% of patients with chronic hidradenitis suppurativa (HS) experience malignant transformation to SCC.⁵⁰ Early HS displays subclinical lymphedema in affected sites, which can progress to chronic fibrosis, stasis, and accumulation of protein-rich fluid.⁵¹ Stasis changes have been associated with altered local inflammatory proteins, such as toll-like receptors, β-defensins, and interleukins.⁵²

A retrospective cohort study of 12 patients revealed a lag time of 28.5 years from HS diagnosis to the manifestation of malignancy.⁵³ After local excision, 7 patients developed recurrence, with 100% mortality. Squamous cell carcinomas were well differentiated and moderately differentiated.⁵³ A 2017 literature review of 62 case reports calculated a mean lag time of 27 years. Despite 85% of SCCs being well differentiated and moderately differentiated, nearly half of patients died within 2 years.⁵⁴ As seen in other inflammatory conditions, HPV can complicate perineal HS and promote SCC tumorigenesis.⁵⁵

Squamous cell carcinomas arising within HS lesions are more prevalent in males (6.75:1 ratio),^54,56 despite HS being more prevalent in females (2:1 ratio).⁵⁷ Similar to DLE, SCCs arising in HS are aggressive and are seen more in males, despite both conditions being female predominant. Incidence and mortality rates for primary cutaneous SCC are higher for men vs women⁵⁸; however, the discordance in aggressive behavior seen more commonly in SCC arising from HS or DLE in male patients has yet to be explained.

Malignancy arising within necrobiosis lipoidica diabeticorum (NLD) is rare. A review of 14 published cases noted that 13 were SCC and 1 was leiomyosarcoma.⁵⁹ The lag time was 21.5 years; 31% of cases (N=14) presented with regional lymph node metastasis. Although chronic ulceration is a risk factor for SCC and occurs in as many as one-third of NLD cases, its correlation with ulceration and malignant transformation has not been characterized.

Epidermolysis Bullosa

Recessive dystrophic epidermolysis bullosa (RDEB) is a noninflammatory inherited blistering disease, and patients have an inherently high risk for aggressive SCC.⁶⁰ Other forms of epidermolysis bullosa can lead to SCC, but the rarer RDEB accounts for 69% of SCC cases, with a median age of 36 years at presentation.⁶¹ Although SCCs tend to be well differentiated in RDEB (73.9%),⁶¹ they also exhibit highly aggressive behavior.⁶² In the most severe variant—RDEB-generalized severe—the cumulative risk for SCC-related death in an Australian population was 84.4% at 34 years of age.⁶³

As RDEB is an inherited disorder with potential for malignancy at a young age, the pathogenesis is plausibly different from the previously discussed inflammatory dermatoses. This disease is characterized by a mutation in the collagen VII gene, leading to loss of anchoring fibrils and a basement membrane zone split.⁶⁴ There also can be inherent fibroblast alterations; RDEB fibroblasts create an environment for tumor growth by supporting malignant-cell adhesion and invasion.⁶⁵ Mutations in p53,⁶⁶ local alterations in transforming growth factor β activity,⁶⁷ and downstream matrix metalloproteinase activity⁶⁸ have been implicated.

Additionally, keratinocytes may retain the N-terminal noncollagenous (NC1) domain of truncated collagen VII while losing the anchoring NC2 domain in mutated collagen VII RDEB, thereby supporting anchorless keratinocyte survival and higher metastatic potential.⁶⁹ Retention of this truncated NC1 domain has shown conversion of RDEB keratinocytes to tumor in a xenotransplant mouse model.⁷⁰ A high level of type VII collagen itself may inherently be protumorigenic for keratinocytes.⁷¹

There does not appear to be evidence for HPV involvement in RDEB-associated SCC.⁷² Squamous cell carcinoma development in RDEB appears to be multifactorial,⁷³ but validated tumor models are lacking. Other than conventional oncologic therapy, future directions in the management of RDEB may include gene-, protein- and cell-targeted therapies.⁷³

Conclusion

Squamous cell carcinomas are known to arise within chronic cutaneous inflammatory dermatoses. Tumorigenesis peaks relatively early in new orolabial DLE, LS, and OLP cases, and can occur over many decades in cutaneous DLE, HLP, HS, NLD, and chronic wounds or scars, summarized in the Table. Frequent SCCs are observed in high-risk subtypes of epidermolysis bullosa. Dermatologists must examine areas affected by these diseases at regular intervals, being mindful of the possibility of SCC development. Furthermore, dermatologists should adopt a lower threshold to biopsy suspicious lesions, especially those that develop within relatively new orolabial DLE, chronic HS, or chronic wound cases, as SCC in these settings is particularly aggressive and displays mortality and metastasis rates that exceed those of common cutaneous SCC.