Drug Therapy

Ezetimibe (Zetia) was licensed by the US Food and Drug Administration in 2002 on the basis of its ability to reduce low-density lipoprotein cholesterol (LDL-C) levels. The reductions are mild, approximately 15%,¹ which is comparable to the effects of a stringent diet and exercise or of a statin in titrated doses.

See related commentary

However, there was no evidence that ezetimbe, which has a unique mechanism of action, delivers a benefit in terms of clinical outcomes. Despite this, the use of ezetimibe (alone or in fixed-dose combination with simvastatin, a preparation sold as Vytorin) grew rapidly, generating annual sales of $5.2 billion. Clinicians and the manufacturer (Merck/Schering-Plough) broadly assumed that LDL-C reduction would carry ezetimibe’s day as clinical trials emerged.

The assumption seemed reasonable, since evidence from the past 3 decades has established a clear link between lowering LDL-C levels via diverse mechanisms and positive clinical outcomes, particularly lower rates of cardiovascular disease and death. Indeed, LDL-C measurement is now a focus of cardiovascular risk assessment and management, as reflected in national treatment guidelines.

THE ENHANCE TRIAL: EZETIMIBE FAILS A KEY TEST

Unexpectedly, ezetimibe failed its first step in clinical trial validation, the Ezetimibe and Simvastatin in Hypercholesterolemia Enhances Atherosclerosis Regression (ENHANCE) trial.² Apart from the scientifically irrelevant political regulatory intrigue generated by the sponsor’s conduct in this trial, ENHANCE’s findings challenge us to confront issues of what we assume vs what we really know, and how to interpret the complex results of clinical trials.

To be fair to the trial’s investigators, ENHANCE achieved its objective of enrolling a population with a very high LDL-C level, which is ezetimibe’s target and has been widely used in the study of atherosclerosis progression as a marker of potential drug benefit. Nevertheless, and even though the LDL-C level 2 years later was 52 mg/dL lower in the group receiving ezetimibe/simvastatin than in the group receiving simvastatin alone (Zocor), at LDL-C levels that are typically associated with atherosclerosis progression (140–190 mg/dL), ezetimibe failed to reduce the progression of atherosclerosis.

Supplementary appendix to Kastelein JJ, et al. Simvastatin with or without ezetimibe in familial hypercholesterolemia. N Engl J Med 2008; 358:1431–1443. doi:10.1056/NEJMoa0800742. Copyright 2008, Massachusetts Medical Society.

These trends are particularly worrisome, given that the ezetimibe/simvastatin group achieved a greater reduction in C-reactive protein levels, which typically has resulted in superior outcomes in atherosclerosis³ and clinical effects⁴ in combination with LDL-C reduction.

In view of these findings, should clinicians stand firm and continue to use ezetimibe? Or should we reevaluate our position and await more data about this unique, first-in-class compound?

WISHFUL POST HOC HYPOTHESES

In this issue of the Cleveland Clinic Journal of Medicine, Dr. Michael Davidson,⁵ a respected lipid expert but one invested in ezetimibe’s development, assures us that all is in order and that the results of ENHANCE can be explained away by several arguments, most notably that most of the trial’s participants had previously received lipid-lowering treatment, which obscured the effects of ezetimibe. Moreover, he argues that ezetimibe’s mechanism of action is well understood and that the drug is safe and well tolerated and thus should remain a first-line treatment for hyperlipidemia.

These arguments may eventually prove to be correct, but as of now they are merely wishful post hoc hypotheses awaiting more data apart from ENHANCE. Negative clinical trials do occur as a matter of chance, but we should be cautious in any attempts to explain away a trial that was designed, executed, and reported as conceived simply because the results do not match our expectations.

Confronted with ENHANCE, the astute clinician should ask three questions: Do we really understand ezetimibe’s mechanism of action? Do other lines of evidence indicate the drug is beneficial? And how reliable is the arterial thickness as a surrogate end point?

DO WE UNDERSTAND EZETIMIBE’S MECHANISM OF ACTION?

Do we understand ezetimibe’s full mechanism of action? Not really.

True, ezetimibe inhibits cholesterol transport, a process that is integral both to cholesterol’s enteric absorption and to its systemic clearance. But although Dr. Davidson asserts that ezetimibe has cellular effects similar to those of statins, in fact it has the opposite effect on HMG-coA reductase, and no effects on LDL receptors.⁶

Furthermore, although initial studies suggested that ezetimibe inhibits enteric cholesterol absorption by inhibiting the Niemann-Pick C1L1 (NPC1L1) receptor, more recent investigations call this into serious question and point more definitively at a receptor known as scavenger receptor-B1 (SR-B1). As stated in a recent editorial, “SR-B1 in the apical site of enterocytes is the primary high-affinity site of cholesterol uptake and ezetimibe can inhibit this process. Moreover, the [possibility is ruled out] of NPC1L1 being a major player in this cholesterol uptake. This is at variance with the view of the colleagues from Schering-Plough who claim the same for NPC1L1.”⁷

SR-B1 is also a high-affinity receptor for high-density lipoprotein⁸ and thus is active in the antiatherosclerotic process of reverse cholesterol transport, inhibition of which significantly accelerates the development of atherosclerosis.⁹

Additionally, in vitro and thus unrelated to the effects of changing cholesterol concentration, ezetimibe down-regulates SR-B1 and another key cholesterol transporter protein called ABCA1.¹⁰ Further, ezetimibe induces down-regulation of raft protein domains, including CD36,¹¹ another effect opposite to that of statins.

These little-recognized effects of ezetimibe are among many that are completely unrelated to enteric cholesterol absorption. Yet, they are likely to be active within the liver and systemically where these proteins reside, and they are putatively proatherosclerotic. Contrary to often-cited opinion, ezetimibe is systemically absorbed, with 11% of the compound excreted in the urine.¹² Thus, the compound is systemically available to exert these same actions in the liver and elsewhere. Moreover, the absorbed drug is glucuronidated and is extensively recirculated in the liver in a form (its glucuronide) that is more potent than the parent compound.

In sum, present opinion is that ezetimibe inhibits lipid transport and interacts with a variety of receptors, not only in the gut but also systemically at the cell membrane and also inside the cell, focally disrupting several tightly regulated biologic processes.⁷ Thus, although ezetimibe reduces serum LDL-C levels via its effect in the gut, this effect may well be offset or even overridden systemically by other, unmeasurable effects, leading to counterintuitive results in terms of atherosclerosis or clinical events.

This would not be the first time a lipid-lowering drug has disappointed us: torcetrapib, another transport inhibitor, dramatically raises serum high-density lipoprotein cholesterol levels and reduces LDL-C but was found not only to have no effect on atherosclerosis, but also to potentiate adverse clinical outcomes.

The net impact of these other actions of ezetimibe is not known. We will discover its true clinical effects only through studies of endothelial function, atherosclerosis, and clinical cardiovascular outcomes. ENHANCE, which looked at atherosclerosis, is thus our strongest signal to date on the net effect of ezetimibe.

DO OTHER LINES OF EVIDENCE INDICATE EZETIMIBE IS BENEFICIAL?

Can we be reassured that ENHANCE’s results are spurious on the basis of other lines of evidence? Again, not really.

Experiments in animals, particularly in mice,¹³ have shown that ezetimibe may be antiatherosclerotic, although mice are considered the “worst model”⁷ for the study of ezetimibe, and notably, LDL-C levels were lowered far more in these experiments than they are clinically. Enthusiasm for these animal models should be tempered by interspecies variability in ezetimibe’s “off-target” effects and in the recent failure of other lipid transport drugs in human trials (torcetrapib and ACAT inhibitors) that had shown initial success in animals. No animal model is established for evaluating drugs of ezetimibe’s class, given its complex mechanism of action.

In human studies, the only other surrogate of the net effect of ezetimibe is endothelial function. Among several randomized clinical trials of ezetimibe,^14–18 only one was designed to compare the effects of ezetimibe alone, ezetimibe plus a statin, and a statin by itself in titrated or in maximum doses.¹⁵ After 4 weeks of therapy, all groups had lower LDL-C levels. However, ezetimibe monotherapy and ezetimibe/simvastatin combination therapy had no detectable effect on the arterial response to acetylcholine, but atorvastatin (Lipitor) monotherapy did. To be fair, the other (very small) trials showed mixed results, thus keeping the hypothesis of ezetimibe’s benefit alive, but with nothing close to a clear signal of benefit.

IS ARTERIAL THICKNESS RELIABLE AS A SURROGATE END POINT?

Was the principal problem in ENHANCE the use of carotid intima-media thickness as the primary end point? No.

This issue has received a lot of attention, much of which I believe is misinformed. No trial end point is infallible, including carotid intima-media thickness, and one must remain open to the possibility of chance findings. However, it has been a relatively reasonable end point in trials of diverse cardiovascular preventive strategies, including lipid-lowering, blood-pressure-lowering, and lifestyle interventions and as a directional biomarker of clinical atherosclerotic events.

We should be cautious about comparing data on carotid intima-media thickness from different trials, as Dr. Davidson attempts to do, in view of methodologic and population differences: each trial must be considered independently. Of greatest concern in ENHANCE is the consistency among intima-media thickness end points, including strong trends toward adverse effects in the most diseased carotid and femoral segments.

Moreover, ENHANCE’s detractors contend that the carotid intima-media thickness of the studied population was normal, citing this as evidence of delipidation from prior treatment. Although not impossible (as shown by the work of Zhao and colleagues in the setting of prolonged, intense lipid-lowering therapy¹⁹), at the moment this hypothesis is a matter of conjecture in the ENHANCE participants, particularly because their LDL-C levels were still quite elevated during the trial and conceivably even before randomization.

But these patients were not normal: they were typical patients with familial hypercholesterolemia with extremely elevated LDL-C levels and abnormally thick arteries for their age. Population screening estimates show that, for age and sex, the carotid intima-media thickness values in ENHANCE would lie in the upper quartile of those in the general population.²⁰ Moreover, their mean value is consistent with that in similar-aged groups of patients with familial hypercholesterolemia, even with lower rates of prior statin pretreatment.²¹

The most convincing evidence for the validity of the ENHANCE findings comes from the published subgroup data (Figure 1). In participants whose baseline carotid intima-media thickness was above the median at baseline, the thickness increased more with ezetimibe/simvastatin than with simvastatin alone. The same was true in the subgroup with above-average LDL-C levels at baseline. The subgroups with no prior statin treatment, low-dose prior statin treatment, and high-dose prior statin showed no heterogeneity of response: their carotid intima-media thickness increased more with ezetimibe/simvastatin than with simvastatin alone. None of these differences was statistically significant; however, these prespecified subgroup data seemingly invalidate arguments against the ENHANCE results based on carotid intima-media thickness findings.

In this context, ENHANCE can only be interpreted as a strong initial negative signal, a “red flag” about ezetimibe’s net health benefits.

WHAT NEXT?

The proper present focus of this debate is not on LDL-C but rather on ezetimibe, its unique mechanism of action, and on the need for more evidence about this complex compound.

At present, ezetimibe’s mechanism of action is not fully understood, and its benefit—for now, only mild LDL-C reduction—is too uncertain for us to be spending $5.2 billion a year for it. Its manufacturer is fortunate that the drug is even licensed, given the current and seemingly appropriate regulatory changes under which drugs introducing new therapeutic classes are scrutinized more closely for benefits and risks. “Safe and well tolerated,” as contended by Dr. Davidson, is not nearly enough: drugs must show clinically important benefits. We still know too little about this drug, the manufacturer of which has invested far more in marketing than in science, a point on which Dr. Davidson and I agree.

In 2008, ezetimibe is an appropriate candidate for testing in clinical trials, and in years to come it may be worthy of clinical attention—if rigorous and objectively conducted clinical trials prove its worth. At present, clinical equipoise dictates that ezetimibe is not an appropriate alternative to a statin in titrated doses, to the addition of other lipid-lowering drugs to a statin, to greater attention to drug adherence, or to lifestyle modification.

For the moment, given the ENHANCE results, the clinical usefulness of ezetimibe still remains to be proven. Much more evidence is needed before we can confidently reembrace the clinical use of ezetimibe.

Ezetimibe (Zetia) was licensed by the US Food and Drug Administration in 2002 on the basis of its ability to reduce low-density lipoprotein cholesterol (LDL-C) levels. The reductions are mild, approximately 15%,¹ which is comparable to the effects of a stringent diet and exercise or of a statin in titrated doses.

See related commentary

However, there was no evidence that ezetimbe, which has a unique mechanism of action, delivers a benefit in terms of clinical outcomes. Despite this, the use of ezetimibe (alone or in fixed-dose combination with simvastatin, a preparation sold as Vytorin) grew rapidly, generating annual sales of $5.2 billion. Clinicians and the manufacturer (Merck/Schering-Plough) broadly assumed that LDL-C reduction would carry ezetimibe’s day as clinical trials emerged.

The assumption seemed reasonable, since evidence from the past 3 decades has established a clear link between lowering LDL-C levels via diverse mechanisms and positive clinical outcomes, particularly lower rates of cardiovascular disease and death. Indeed, LDL-C measurement is now a focus of cardiovascular risk assessment and management, as reflected in national treatment guidelines.

THE ENHANCE TRIAL: EZETIMIBE FAILS A KEY TEST

Unexpectedly, ezetimibe failed its first step in clinical trial validation, the Ezetimibe and Simvastatin in Hypercholesterolemia Enhances Atherosclerosis Regression (ENHANCE) trial.² Apart from the scientifically irrelevant political regulatory intrigue generated by the sponsor’s conduct in this trial, ENHANCE’s findings challenge us to confront issues of what we assume vs what we really know, and how to interpret the complex results of clinical trials.

To be fair to the trial’s investigators, ENHANCE achieved its objective of enrolling a population with a very high LDL-C level, which is ezetimibe’s target and has been widely used in the study of atherosclerosis progression as a marker of potential drug benefit. Nevertheless, and even though the LDL-C level 2 years later was 52 mg/dL lower in the group receiving ezetimibe/simvastatin than in the group receiving simvastatin alone (Zocor), at LDL-C levels that are typically associated with atherosclerosis progression (140–190 mg/dL), ezetimibe failed to reduce the progression of atherosclerosis.

Supplementary appendix to Kastelein JJ, et al. Simvastatin with or without ezetimibe in familial hypercholesterolemia. N Engl J Med 2008; 358:1431–1443. doi:10.1056/NEJMoa0800742. Copyright 2008, Massachusetts Medical Society.

These trends are particularly worrisome, given that the ezetimibe/simvastatin group achieved a greater reduction in C-reactive protein levels, which typically has resulted in superior outcomes in atherosclerosis³ and clinical effects⁴ in combination with LDL-C reduction.

In view of these findings, should clinicians stand firm and continue to use ezetimibe? Or should we reevaluate our position and await more data about this unique, first-in-class compound?

WISHFUL POST HOC HYPOTHESES

In this issue of the Cleveland Clinic Journal of Medicine, Dr. Michael Davidson,⁵ a respected lipid expert but one invested in ezetimibe’s development, assures us that all is in order and that the results of ENHANCE can be explained away by several arguments, most notably that most of the trial’s participants had previously received lipid-lowering treatment, which obscured the effects of ezetimibe. Moreover, he argues that ezetimibe’s mechanism of action is well understood and that the drug is safe and well tolerated and thus should remain a first-line treatment for hyperlipidemia.

These arguments may eventually prove to be correct, but as of now they are merely wishful post hoc hypotheses awaiting more data apart from ENHANCE. Negative clinical trials do occur as a matter of chance, but we should be cautious in any attempts to explain away a trial that was designed, executed, and reported as conceived simply because the results do not match our expectations.

Confronted with ENHANCE, the astute clinician should ask three questions: Do we really understand ezetimibe’s mechanism of action? Do other lines of evidence indicate the drug is beneficial? And how reliable is the arterial thickness as a surrogate end point?

DO WE UNDERSTAND EZETIMIBE’S MECHANISM OF ACTION?

Do we understand ezetimibe’s full mechanism of action? Not really.

True, ezetimibe inhibits cholesterol transport, a process that is integral both to cholesterol’s enteric absorption and to its systemic clearance. But although Dr. Davidson asserts that ezetimibe has cellular effects similar to those of statins, in fact it has the opposite effect on HMG-coA reductase, and no effects on LDL receptors.⁶

Furthermore, although initial studies suggested that ezetimibe inhibits enteric cholesterol absorption by inhibiting the Niemann-Pick C1L1 (NPC1L1) receptor, more recent investigations call this into serious question and point more definitively at a receptor known as scavenger receptor-B1 (SR-B1). As stated in a recent editorial, “SR-B1 in the apical site of enterocytes is the primary high-affinity site of cholesterol uptake and ezetimibe can inhibit this process. Moreover, the [possibility is ruled out] of NPC1L1 being a major player in this cholesterol uptake. This is at variance with the view of the colleagues from Schering-Plough who claim the same for NPC1L1.”⁷

SR-B1 is also a high-affinity receptor for high-density lipoprotein⁸ and thus is active in the antiatherosclerotic process of reverse cholesterol transport, inhibition of which significantly accelerates the development of atherosclerosis.⁹

Additionally, in vitro and thus unrelated to the effects of changing cholesterol concentration, ezetimibe down-regulates SR-B1 and another key cholesterol transporter protein called ABCA1.¹⁰ Further, ezetimibe induces down-regulation of raft protein domains, including CD36,¹¹ another effect opposite to that of statins.

These little-recognized effects of ezetimibe are among many that are completely unrelated to enteric cholesterol absorption. Yet, they are likely to be active within the liver and systemically where these proteins reside, and they are putatively proatherosclerotic. Contrary to often-cited opinion, ezetimibe is systemically absorbed, with 11% of the compound excreted in the urine.¹² Thus, the compound is systemically available to exert these same actions in the liver and elsewhere. Moreover, the absorbed drug is glucuronidated and is extensively recirculated in the liver in a form (its glucuronide) that is more potent than the parent compound.

In sum, present opinion is that ezetimibe inhibits lipid transport and interacts with a variety of receptors, not only in the gut but also systemically at the cell membrane and also inside the cell, focally disrupting several tightly regulated biologic processes.⁷ Thus, although ezetimibe reduces serum LDL-C levels via its effect in the gut, this effect may well be offset or even overridden systemically by other, unmeasurable effects, leading to counterintuitive results in terms of atherosclerosis or clinical events.

This would not be the first time a lipid-lowering drug has disappointed us: torcetrapib, another transport inhibitor, dramatically raises serum high-density lipoprotein cholesterol levels and reduces LDL-C but was found not only to have no effect on atherosclerosis, but also to potentiate adverse clinical outcomes.

The net impact of these other actions of ezetimibe is not known. We will discover its true clinical effects only through studies of endothelial function, atherosclerosis, and clinical cardiovascular outcomes. ENHANCE, which looked at atherosclerosis, is thus our strongest signal to date on the net effect of ezetimibe.

DO OTHER LINES OF EVIDENCE INDICATE EZETIMIBE IS BENEFICIAL?

Can we be reassured that ENHANCE’s results are spurious on the basis of other lines of evidence? Again, not really.

Experiments in animals, particularly in mice,¹³ have shown that ezetimibe may be antiatherosclerotic, although mice are considered the “worst model”⁷ for the study of ezetimibe, and notably, LDL-C levels were lowered far more in these experiments than they are clinically. Enthusiasm for these animal models should be tempered by interspecies variability in ezetimibe’s “off-target” effects and in the recent failure of other lipid transport drugs in human trials (torcetrapib and ACAT inhibitors) that had shown initial success in animals. No animal model is established for evaluating drugs of ezetimibe’s class, given its complex mechanism of action.

In human studies, the only other surrogate of the net effect of ezetimibe is endothelial function. Among several randomized clinical trials of ezetimibe,^14–18 only one was designed to compare the effects of ezetimibe alone, ezetimibe plus a statin, and a statin by itself in titrated or in maximum doses.¹⁵ After 4 weeks of therapy, all groups had lower LDL-C levels. However, ezetimibe monotherapy and ezetimibe/simvastatin combination therapy had no detectable effect on the arterial response to acetylcholine, but atorvastatin (Lipitor) monotherapy did. To be fair, the other (very small) trials showed mixed results, thus keeping the hypothesis of ezetimibe’s benefit alive, but with nothing close to a clear signal of benefit.

IS ARTERIAL THICKNESS RELIABLE AS A SURROGATE END POINT?

Was the principal problem in ENHANCE the use of carotid intima-media thickness as the primary end point? No.

This issue has received a lot of attention, much of which I believe is misinformed. No trial end point is infallible, including carotid intima-media thickness, and one must remain open to the possibility of chance findings. However, it has been a relatively reasonable end point in trials of diverse cardiovascular preventive strategies, including lipid-lowering, blood-pressure-lowering, and lifestyle interventions and as a directional biomarker of clinical atherosclerotic events.

We should be cautious about comparing data on carotid intima-media thickness from different trials, as Dr. Davidson attempts to do, in view of methodologic and population differences: each trial must be considered independently. Of greatest concern in ENHANCE is the consistency among intima-media thickness end points, including strong trends toward adverse effects in the most diseased carotid and femoral segments.

Moreover, ENHANCE’s detractors contend that the carotid intima-media thickness of the studied population was normal, citing this as evidence of delipidation from prior treatment. Although not impossible (as shown by the work of Zhao and colleagues in the setting of prolonged, intense lipid-lowering therapy¹⁹), at the moment this hypothesis is a matter of conjecture in the ENHANCE participants, particularly because their LDL-C levels were still quite elevated during the trial and conceivably even before randomization.

But these patients were not normal: they were typical patients with familial hypercholesterolemia with extremely elevated LDL-C levels and abnormally thick arteries for their age. Population screening estimates show that, for age and sex, the carotid intima-media thickness values in ENHANCE would lie in the upper quartile of those in the general population.²⁰ Moreover, their mean value is consistent with that in similar-aged groups of patients with familial hypercholesterolemia, even with lower rates of prior statin pretreatment.²¹

The most convincing evidence for the validity of the ENHANCE findings comes from the published subgroup data (Figure 1). In participants whose baseline carotid intima-media thickness was above the median at baseline, the thickness increased more with ezetimibe/simvastatin than with simvastatin alone. The same was true in the subgroup with above-average LDL-C levels at baseline. The subgroups with no prior statin treatment, low-dose prior statin treatment, and high-dose prior statin showed no heterogeneity of response: their carotid intima-media thickness increased more with ezetimibe/simvastatin than with simvastatin alone. None of these differences was statistically significant; however, these prespecified subgroup data seemingly invalidate arguments against the ENHANCE results based on carotid intima-media thickness findings.

In this context, ENHANCE can only be interpreted as a strong initial negative signal, a “red flag” about ezetimibe’s net health benefits.

WHAT NEXT?

The proper present focus of this debate is not on LDL-C but rather on ezetimibe, its unique mechanism of action, and on the need for more evidence about this complex compound.

At present, ezetimibe’s mechanism of action is not fully understood, and its benefit—for now, only mild LDL-C reduction—is too uncertain for us to be spending $5.2 billion a year for it. Its manufacturer is fortunate that the drug is even licensed, given the current and seemingly appropriate regulatory changes under which drugs introducing new therapeutic classes are scrutinized more closely for benefits and risks. “Safe and well tolerated,” as contended by Dr. Davidson, is not nearly enough: drugs must show clinically important benefits. We still know too little about this drug, the manufacturer of which has invested far more in marketing than in science, a point on which Dr. Davidson and I agree.

In 2008, ezetimibe is an appropriate candidate for testing in clinical trials, and in years to come it may be worthy of clinical attention—if rigorous and objectively conducted clinical trials prove its worth. At present, clinical equipoise dictates that ezetimibe is not an appropriate alternative to a statin in titrated doses, to the addition of other lipid-lowering drugs to a statin, to greater attention to drug adherence, or to lifestyle modification.

For the moment, given the ENHANCE results, the clinical usefulness of ezetimibe still remains to be proven. Much more evidence is needed before we can confidently reembrace the clinical use of ezetimibe.

Ezetimibe (Zetia) was licensed by the US Food and Drug Administration in 2002 on the basis of its ability to reduce low-density lipoprotein cholesterol (LDL-C) levels. The reductions are mild, approximately 15%,¹ which is comparable to the effects of a stringent diet and exercise or of a statin in titrated doses.

See related commentary

However, there was no evidence that ezetimbe, which has a unique mechanism of action, delivers a benefit in terms of clinical outcomes. Despite this, the use of ezetimibe (alone or in fixed-dose combination with simvastatin, a preparation sold as Vytorin) grew rapidly, generating annual sales of $5.2 billion. Clinicians and the manufacturer (Merck/Schering-Plough) broadly assumed that LDL-C reduction would carry ezetimibe’s day as clinical trials emerged.

The assumption seemed reasonable, since evidence from the past 3 decades has established a clear link between lowering LDL-C levels via diverse mechanisms and positive clinical outcomes, particularly lower rates of cardiovascular disease and death. Indeed, LDL-C measurement is now a focus of cardiovascular risk assessment and management, as reflected in national treatment guidelines.

THE ENHANCE TRIAL: EZETIMIBE FAILS A KEY TEST

Unexpectedly, ezetimibe failed its first step in clinical trial validation, the Ezetimibe and Simvastatin in Hypercholesterolemia Enhances Atherosclerosis Regression (ENHANCE) trial.² Apart from the scientifically irrelevant political regulatory intrigue generated by the sponsor’s conduct in this trial, ENHANCE’s findings challenge us to confront issues of what we assume vs what we really know, and how to interpret the complex results of clinical trials.

To be fair to the trial’s investigators, ENHANCE achieved its objective of enrolling a population with a very high LDL-C level, which is ezetimibe’s target and has been widely used in the study of atherosclerosis progression as a marker of potential drug benefit. Nevertheless, and even though the LDL-C level 2 years later was 52 mg/dL lower in the group receiving ezetimibe/simvastatin than in the group receiving simvastatin alone (Zocor), at LDL-C levels that are typically associated with atherosclerosis progression (140–190 mg/dL), ezetimibe failed to reduce the progression of atherosclerosis.

Supplementary appendix to Kastelein JJ, et al. Simvastatin with or without ezetimibe in familial hypercholesterolemia. N Engl J Med 2008; 358:1431–1443. doi:10.1056/NEJMoa0800742. Copyright 2008, Massachusetts Medical Society.

These trends are particularly worrisome, given that the ezetimibe/simvastatin group achieved a greater reduction in C-reactive protein levels, which typically has resulted in superior outcomes in atherosclerosis³ and clinical effects⁴ in combination with LDL-C reduction.

In view of these findings, should clinicians stand firm and continue to use ezetimibe? Or should we reevaluate our position and await more data about this unique, first-in-class compound?

WISHFUL POST HOC HYPOTHESES

In this issue of the Cleveland Clinic Journal of Medicine, Dr. Michael Davidson,⁵ a respected lipid expert but one invested in ezetimibe’s development, assures us that all is in order and that the results of ENHANCE can be explained away by several arguments, most notably that most of the trial’s participants had previously received lipid-lowering treatment, which obscured the effects of ezetimibe. Moreover, he argues that ezetimibe’s mechanism of action is well understood and that the drug is safe and well tolerated and thus should remain a first-line treatment for hyperlipidemia.

These arguments may eventually prove to be correct, but as of now they are merely wishful post hoc hypotheses awaiting more data apart from ENHANCE. Negative clinical trials do occur as a matter of chance, but we should be cautious in any attempts to explain away a trial that was designed, executed, and reported as conceived simply because the results do not match our expectations.

Confronted with ENHANCE, the astute clinician should ask three questions: Do we really understand ezetimibe’s mechanism of action? Do other lines of evidence indicate the drug is beneficial? And how reliable is the arterial thickness as a surrogate end point?

DO WE UNDERSTAND EZETIMIBE’S MECHANISM OF ACTION?

Do we understand ezetimibe’s full mechanism of action? Not really.

True, ezetimibe inhibits cholesterol transport, a process that is integral both to cholesterol’s enteric absorption and to its systemic clearance. But although Dr. Davidson asserts that ezetimibe has cellular effects similar to those of statins, in fact it has the opposite effect on HMG-coA reductase, and no effects on LDL receptors.⁶

Furthermore, although initial studies suggested that ezetimibe inhibits enteric cholesterol absorption by inhibiting the Niemann-Pick C1L1 (NPC1L1) receptor, more recent investigations call this into serious question and point more definitively at a receptor known as scavenger receptor-B1 (SR-B1). As stated in a recent editorial, “SR-B1 in the apical site of enterocytes is the primary high-affinity site of cholesterol uptake and ezetimibe can inhibit this process. Moreover, the [possibility is ruled out] of NPC1L1 being a major player in this cholesterol uptake. This is at variance with the view of the colleagues from Schering-Plough who claim the same for NPC1L1.”⁷

SR-B1 is also a high-affinity receptor for high-density lipoprotein⁸ and thus is active in the antiatherosclerotic process of reverse cholesterol transport, inhibition of which significantly accelerates the development of atherosclerosis.⁹

Additionally, in vitro and thus unrelated to the effects of changing cholesterol concentration, ezetimibe down-regulates SR-B1 and another key cholesterol transporter protein called ABCA1.¹⁰ Further, ezetimibe induces down-regulation of raft protein domains, including CD36,¹¹ another effect opposite to that of statins.

These little-recognized effects of ezetimibe are among many that are completely unrelated to enteric cholesterol absorption. Yet, they are likely to be active within the liver and systemically where these proteins reside, and they are putatively proatherosclerotic. Contrary to often-cited opinion, ezetimibe is systemically absorbed, with 11% of the compound excreted in the urine.¹² Thus, the compound is systemically available to exert these same actions in the liver and elsewhere. Moreover, the absorbed drug is glucuronidated and is extensively recirculated in the liver in a form (its glucuronide) that is more potent than the parent compound.

In sum, present opinion is that ezetimibe inhibits lipid transport and interacts with a variety of receptors, not only in the gut but also systemically at the cell membrane and also inside the cell, focally disrupting several tightly regulated biologic processes.⁷ Thus, although ezetimibe reduces serum LDL-C levels via its effect in the gut, this effect may well be offset or even overridden systemically by other, unmeasurable effects, leading to counterintuitive results in terms of atherosclerosis or clinical events.

This would not be the first time a lipid-lowering drug has disappointed us: torcetrapib, another transport inhibitor, dramatically raises serum high-density lipoprotein cholesterol levels and reduces LDL-C but was found not only to have no effect on atherosclerosis, but also to potentiate adverse clinical outcomes.

The net impact of these other actions of ezetimibe is not known. We will discover its true clinical effects only through studies of endothelial function, atherosclerosis, and clinical cardiovascular outcomes. ENHANCE, which looked at atherosclerosis, is thus our strongest signal to date on the net effect of ezetimibe.

DO OTHER LINES OF EVIDENCE INDICATE EZETIMIBE IS BENEFICIAL?

Can we be reassured that ENHANCE’s results are spurious on the basis of other lines of evidence? Again, not really.

Experiments in animals, particularly in mice,¹³ have shown that ezetimibe may be antiatherosclerotic, although mice are considered the “worst model”⁷ for the study of ezetimibe, and notably, LDL-C levels were lowered far more in these experiments than they are clinically. Enthusiasm for these animal models should be tempered by interspecies variability in ezetimibe’s “off-target” effects and in the recent failure of other lipid transport drugs in human trials (torcetrapib and ACAT inhibitors) that had shown initial success in animals. No animal model is established for evaluating drugs of ezetimibe’s class, given its complex mechanism of action.

In human studies, the only other surrogate of the net effect of ezetimibe is endothelial function. Among several randomized clinical trials of ezetimibe,^14–18 only one was designed to compare the effects of ezetimibe alone, ezetimibe plus a statin, and a statin by itself in titrated or in maximum doses.¹⁵ After 4 weeks of therapy, all groups had lower LDL-C levels. However, ezetimibe monotherapy and ezetimibe/simvastatin combination therapy had no detectable effect on the arterial response to acetylcholine, but atorvastatin (Lipitor) monotherapy did. To be fair, the other (very small) trials showed mixed results, thus keeping the hypothesis of ezetimibe’s benefit alive, but with nothing close to a clear signal of benefit.

IS ARTERIAL THICKNESS RELIABLE AS A SURROGATE END POINT?

Was the principal problem in ENHANCE the use of carotid intima-media thickness as the primary end point? No.

This issue has received a lot of attention, much of which I believe is misinformed. No trial end point is infallible, including carotid intima-media thickness, and one must remain open to the possibility of chance findings. However, it has been a relatively reasonable end point in trials of diverse cardiovascular preventive strategies, including lipid-lowering, blood-pressure-lowering, and lifestyle interventions and as a directional biomarker of clinical atherosclerotic events.

We should be cautious about comparing data on carotid intima-media thickness from different trials, as Dr. Davidson attempts to do, in view of methodologic and population differences: each trial must be considered independently. Of greatest concern in ENHANCE is the consistency among intima-media thickness end points, including strong trends toward adverse effects in the most diseased carotid and femoral segments.

Moreover, ENHANCE’s detractors contend that the carotid intima-media thickness of the studied population was normal, citing this as evidence of delipidation from prior treatment. Although not impossible (as shown by the work of Zhao and colleagues in the setting of prolonged, intense lipid-lowering therapy¹⁹), at the moment this hypothesis is a matter of conjecture in the ENHANCE participants, particularly because their LDL-C levels were still quite elevated during the trial and conceivably even before randomization.

But these patients were not normal: they were typical patients with familial hypercholesterolemia with extremely elevated LDL-C levels and abnormally thick arteries for their age. Population screening estimates show that, for age and sex, the carotid intima-media thickness values in ENHANCE would lie in the upper quartile of those in the general population.²⁰ Moreover, their mean value is consistent with that in similar-aged groups of patients with familial hypercholesterolemia, even with lower rates of prior statin pretreatment.²¹

The most convincing evidence for the validity of the ENHANCE findings comes from the published subgroup data (Figure 1). In participants whose baseline carotid intima-media thickness was above the median at baseline, the thickness increased more with ezetimibe/simvastatin than with simvastatin alone. The same was true in the subgroup with above-average LDL-C levels at baseline. The subgroups with no prior statin treatment, low-dose prior statin treatment, and high-dose prior statin showed no heterogeneity of response: their carotid intima-media thickness increased more with ezetimibe/simvastatin than with simvastatin alone. None of these differences was statistically significant; however, these prespecified subgroup data seemingly invalidate arguments against the ENHANCE results based on carotid intima-media thickness findings.

In this context, ENHANCE can only be interpreted as a strong initial negative signal, a “red flag” about ezetimibe’s net health benefits.

WHAT NEXT?

The proper present focus of this debate is not on LDL-C but rather on ezetimibe, its unique mechanism of action, and on the need for more evidence about this complex compound.

At present, ezetimibe’s mechanism of action is not fully understood, and its benefit—for now, only mild LDL-C reduction—is too uncertain for us to be spending $5.2 billion a year for it. Its manufacturer is fortunate that the drug is even licensed, given the current and seemingly appropriate regulatory changes under which drugs introducing new therapeutic classes are scrutinized more closely for benefits and risks. “Safe and well tolerated,” as contended by Dr. Davidson, is not nearly enough: drugs must show clinically important benefits. We still know too little about this drug, the manufacturer of which has invested far more in marketing than in science, a point on which Dr. Davidson and I agree.

In 2008, ezetimibe is an appropriate candidate for testing in clinical trials, and in years to come it may be worthy of clinical attention—if rigorous and objectively conducted clinical trials prove its worth. At present, clinical equipoise dictates that ezetimibe is not an appropriate alternative to a statin in titrated doses, to the addition of other lipid-lowering drugs to a statin, to greater attention to drug adherence, or to lifestyle modification.

For the moment, given the ENHANCE results, the clinical usefulness of ezetimibe still remains to be proven. Much more evidence is needed before we can confidently reembrace the clinical use of ezetimibe.

The Ezetimibe and Simvastatin in Hypercholesterolemia Enhances Atherosclerosis Regression (ENHANCE) trial¹ was probably the most widely publicized clinical study of the past decade. How did a 720-patient imaging trial with a neutral result in patients with severe hypercholesterolemia rise to a level warranting massive media attention, a congressional investigation, and a recommendation to curtail the use of a drug widely used to reduce levels of low-density-lipoprotein cholesterol (LDL-C)?

See related editorial

The reaction to the ENHANCE trial reveals more about the political climate and the relationship between the pharmaceutical industry and the American public than it does about the effects of ezetimibe (available combined with simvastatin as Vytorin and by itself as Zetia) on the progression of atherosclerosis.

SOME SELF-DISCLOSURE

Before I discuss the clinical implications of the ENHANCE trial, I must describe both my financial conflicts and intellectual biases. I am a paid consultant, speaker, and researcher on behalf of Merck/Schering-Plough, the sponsor of the ENHANCE trial. I was a principal investigator in the first phase II trial of ezetimibe and have conducted more than 10 clinical trials of either ezetimibe or ezetimibe/simvastatin. I also have been a strong advocate for imaging trials to assist in the clinical development of novel therapeutic agents and to support regulatory approval.

Therefore, I believe that the thickness of the intima and media layers of the carotid arteries is a useful surrogate to evaluate the potential antiatherosclerotic effects of drugs (more on this topic below). Also, I believe that the LDL-C-lowering hypothesis has been proven: ie, that all drugs that lower LDL-C safely, without off-target adverse effects, should reduce cardiovascular events. I support the goal levels of LDL-C and non-high-density-lipoprotein cholesterol set by the National Cholesterol Education Program’s third Adult Treatment Panel (ATP III) guidelines,^2,3 which specify LDL-C targets rather than the use of specific drugs. In spite of these conflicts and potential biases, I believe I have always served the best interests of patient care.

HISTORY OF THE ENHANCE TRIAL

The end point defined as the mean of six measurements

The primary end point was the change in the thickness of the intima and media layers of the carotid arteries over a 2-year period, measured by ultrasonography. A composite measure was used: the mean of the thicknesses in the far walls of the right and left common carotid arteries, the right and left carotid bulbs, and the right and left internal carotid arteries. Secondary end points included the change in the mean maximal carotid artery intima-media thickness (ie, the thickest of the six baseline measurements), the proportion of participants who developed new carotid artery plaque (defined arbitrarily as an intima-media thickness > 1.3 mm), and changes in the mean of the intima-media thickness of the six carotid sites plus the common femoral arteries.

The last participant completed the trial in April 2006. Reading of the almost 30,000 scans was not started until the last participant was finished, so that all scans for each participant could be read in a blinded, randomized order by five separate readers. A significant proportion of the images that the protocol called for could not be obtained or analyzed, particularly in the internal carotid artery and the carotid bulb, which are often difficult to visualize. As a result, 17% of the internal carotid or carotid bulb measurements were discarded.

To change the end point post hoc, or not to change the end point?

The sponsor of the trial was concerned about the missing data points and convened a special advisory board to review the blinded data. This group suggested a solution: changing the primary end point from the six-site composite value to the mean value in just the common carotid arteries. They based this suggestion on the greater success rate in measuring the common carotids (97%) than in measuring all six sites (88%), as well as on recent trials that indicated that the common carotid artery measurement correlates better with clinical outcomes (because the internal carotid and the bulb measurements vary more). On November 26, 2007, Merck/Schering-Plough announced the primary end point would be changed to the mean change in the common carotid arteries.

However, during a separate meeting on November 30, 2007, some members of the Merck/Schering-Plough advisory board objected to the change. On December 11, 2007, the company announced that the original primary end point would not be changed.

On December 31, 2007, the ENHANCE study was unblinded, and on January 14, 2008, Merck/Schering-Plough issued a press release announcing the results. The press release stated that there were no statistically significant differences between the treatment groups in the primary end point or in any of the secondary end points, despite a 16.5% greater reduction in LDL-C (about 50 mg/dL) in the group receiving the ezetimibe/simvastatin combination. The composite intima-media thickness had increased by an average of 0.0111 mm in the combined-therapy group vs 0.0058 mm in the simvastatin-only group (P = .29) over the 24-month treatment period.⁵

The press release received unprecedented international media attention. One leading cardiologist commented to the media that ENHANCE showed “millions of patients may be taking a drug [ezetimibe] that does not benefit them, raising their risk of heart attacks and exposing them to potential side effects.”⁶ The perceived message that ezetimibe/simvastatin is harmful resulted in thousands of phone calls from concerned patients to their physicians throughout the United States. The American Heart Association (AHA) and the American College of Cardiology (ACC) issued a joint statement the next day saying that ezetimibe/simvastatin does not appear to be unsafe and that patients should not stop taking the drug on their own. In the following days, Merck/Schering-Plough placed advertisements in newspapers reaffirming the safety of ezetimibe and quoting the AHA/ACC statement.

But the full results of the study were not available at that point. In fact, Senator Charles Grassley (R-Iowa) had launched a congressional investigation into the delays in releasing the results of the ENHANCE trial in December 2007. A focus of the investigation was whether the sponsor was delaying the release either because the data reflected negatively on its product or because it was legitimately concerned about the quality of the measurements of the carotid intima-media thickness. After Merck/Schering-Plough placed the advertisements quoting the AHA/ACC statement, these organizations were criticized for touting the safety of ezetimibe while receiving educational grants and other funds from Merck/Schering-Plough. Senator Grassley sent a letter to the ACC in late March requesting information about the amount of funds the ACC had received.

Full results are published, and the ACC is misquoted

The ENHANCE study was selected for a special presentation at the ACC annual scientific session on March 30, 2008. The full ENHANCE results were presented by Dr. Kastelein, after which an expert panel led by Harlan M. Krumholz, MD, discussed the trial’s implications. The ENHANCE results were simultaneously published in the New England Journal of Medicine,¹ accompanied by an editorial by B. Greg Brown, MD, and Allen J. Taylor, MD,⁷ and another editorial by the editors of that journal, Jeffrey M. Drazen, MD, and colleagues.⁸ The expert panel and the editorialists concluded that the ENHANCE trial data raised concerns about the cardiovascular benefits of ezetimibe; that statins should be used as initial therapy for hyperlipidemia and titrated to the goal LDL-C level or to the maximally tolerated dose; and that other drugs such as bile acid sequestrants, fibrates, and niacin should be used in combination with statins before considering ezetimibe.⁹

The next day, stories appeared in the media mistakenly stating that the ACC had recommended that ezetimibe/simvastatin be discontinued. This view was fueled by an article in the ACC’s Scientific Session News, penned by a contract writer and editor, with the headline, “ACC on Vytorin: Go Back to Statins” that said, “After waiting for 18 months for the results of the ENHANCE study, an ACC panel on Sunday encouraged physicians to use statins as a first line and prescribe Vytorin only as a last resort for patients unable to tolerate other cholesterol-lowering agents.”¹⁰

The ACC later clarified that this was the opinion of the panelists and not that of the ACC, and they reiterated statements from the AHA/ACC Secondary Prevention Guidelines¹¹ recommending statins in maximally tolerated doses or titrated to a goal LDL-C level for first-line drug treatment of coronary artery disease, and recommending that patients speak with their physicians before discontinuing any therapy.

WHY WERE THE ENHANCE STUDY RESULTS NEUTRAL?

The ACC expert panel concluded that the most likely reason for the neutral ENHANCE results was that ezetimibe lowers LDL-C but does not confer a cardiovascular benefit. In the words of Dr. Krumholz (as quoted by Shannon Pettypiece and Michelle Fay Cortez on bloomberg.com), ezetimibe is “just an expensive placebo.”¹²

There are at least three potential explanations for the lack of benefit with ezetimibe in the ENHANCE trial. I list them below in order of lowest to highest probability, in my opinion:

Theory 1: Ezetimibe lowers LDL-C but is not antiatherogenic

Since almost all experts agree that lowering LDL-C confers cardiovascular benefits, if ezetimibe does not inhibit atherosclerosis it must have some “off-target” effect that negates its LDL-C-lowering benefit. Critics of ezetimibe point out that oral estrogen and torcetrapib also lower LDL-C but do not improve cardiovascular outcomes.^13,14

The lack of benefit with these two other agents can be explained. Oral estrogen does not lower apolipoprotein B (an indication of the number of atherogenic particles), but rather it increases the levels of both triglycerides and C-reactive protein, and it is prothrombotic in some people.¹⁵ Torcetrapib increases aldosterone production and substantially raises blood pressure.¹⁶ Therefore, both drugs have true off-target effects that could explain their failure to reduce cardiovascular risk despite reductions in LDL-C. (Interestingly, though, oral estrogen has been shown to slow the progression of carotid intima-media thickness in newly postmenopausal women.¹⁷

Ezetimibe, however, lowers LDL-C by an ultimate mechanism similar to that of statins and bile acid sequestrants, ie, by up-regulating LDL receptors, although these drugs reach this mechanism via different pathways. Statins inhibit cholesterol synthesis, thereby lowering hepatic intracellular cholesterol and thus up-regulating LDL-receptors and enhancing LDL-C clearance from the plasma. Bile acid sequestrants interrupt bile acid reabsorption in the ileum, thereby decreasing intracellular hepatic cholesterol and up-regulating LDL receptors. Ezetimibe, like bile acid sequestrants, also decreases cholesterol return to the liver, lowering hepatic intracellular levels and thus up-regulating LDL receptors.¹⁸

Ezetimibe is unlikely to have an off-target effect because it is only fractionally absorbed systemically, and a recent animal study showed that it enhances macrophage efflux of cholesterol, thereby potentially increasing reverse cholesterol transport.¹⁹ Ezetimibe has also been shown to reduce atherosclerosis in animal models.²⁰

In their editorial, Drs. Brown and Taylor⁷ noted that ezetimibe reduces the expression of adenosine triphosphate binding cassette A1 (ABCA1) in Caco-2 (an intestinal cell line), and this may be an example of an off-target effect. However, statins also reduce ABCA1 expression in macrophages.²¹ ABCA1 is sensitive to intracellular cholesterol, and when cholesterol levels are decreased, whether by statins or by ezetimibe, ABCA1 expression is down-regulated.²²

Theory 2: Intima-media thickness does not reflect the true benefits of lowering LDL-C

The carotid intima-media thickness is a surrogate end point that predicts coronary events and the rate of progression of coronary atherosclerosis.²³ In trials of lovastatin (Mevacor),²⁴ pravastatin (Pravachol),²⁵ and rosuvastatin (Crestor),²⁶ the carotid intima-media was thinner at 24 months with the active drug than with placebo. In two relatively small trials—ARBITER 1 (n = 161),²⁷ which was open-label, and ASAP (n = 325)^28,29—aggressive lipid-lowering reduced the progression of intima-media thickness better than less-aggressive therapy. However, this measure has been used to evaluate the effects of differing degrees of LDL-C reduction between active treatments in fewer than 500 research participants.

Furthermore, what part or parts of the carotid system are we talking about? In recent trials led by Dr. Kastelein, the intima-media thickness of the common carotid arteries increased with pactimibe (an acyl-coenzyme A:cholesterol O-acyltransferase, or ACAT, inhibitor)³⁰ and torcetrapib,³¹ but the six-site composite measure (which was the primary end point in these trials, as in ENHANCE) did not increase more than in the control groups. Pactimibe was also shown to increase atheroma volume as measured by intravascular ultrasonography in the ACTIVATE trial.³² Therefore, the thickness of the common carotid arteries has been shown to be a better predictor of harm from a therapy than the composite measurement.

The advantage of measuring the common carotid artery is that it is easier to visualize and measure, and therefore the measurements vary less. In the METEOR trial,²⁶ the six-site measurement increased significantly less with rosuvastatin than with placebo, but the common carotid measurement alone was more strongly associated with a difference in progression. In the ENHANCE trial, the thickness of the common carotid arteries increased by 0.0024 mm with simvastatin alone vs 0.0019 mm with simvastatin/ezetimibe, a difference of 0.005 mm that was not statistically significant (P = .93).¹

Although the six-site measurement appears to be good for predicting coronary events and evaluating therapies, the measurement in the common carotid arteries appears to be a more reliable surrogate end point for predicting both benefit and harm from antiatherogenic agents. However, trials of statins and other lipid-lowering therapies that assessed clinical events have shown that the reduction in risk associated with a given reduction in cholesterol is similar regardless of the mechanism by which cholesterol is lowered.³³ Therefore, the LDL-C level is far superior as a marker of clinical benefit.

Theory 3: Previous statin treatment affected the ENHANCE results

By far the most likely explanation for the neutral findings in ENHANCE is that the patients were so well treated before entry that it was impossible to detect a difference between the two treatment groups in carotid intima-media thickness at the end of the study. Eighty percent of the patients had received statins previously, and at baseline the mean intima-media thickness of the common carotid arteries was only 0.68 mm.¹ In contrast, most other trials required a thickness greater than 0.7 mm for entry.

The two main reasons for selecting a population with familial hypercholesterolemia were the assumptions that these participants would have a greater-than-average carotid intima-media thickness at baseline and that they would show an above-average progression rate, even on high-dose statin therapy.⁴ Both of these assumptions were incorrect: the baseline thickness was normal and the progression rate was negligible in both groups.

Accordingly, the high prevalence of statin pretreatment and the near-normal carotid intima-media thickness at baseline may have prevented the 16.5% greater reduction in LDL-C due to ezetimibe from producing a difference in progression over 24 months of treatment. This conclusion is supported by the long-term follow-up results from ASAP, RADIANCE 1, and CAPTIVATE, all of which showed that in patients with familial hypercholesterolemia well treated with statins, progression of carotid intima-media thickness is negligible.^30,31

Further supporting this view, in a previous trial by Dr. Kastelein’s group in patients with familial hypercholesterolemia,³⁴ giving simvastatin 80 mg for 2 years decreased the intima-medial thickness by .081 mm (P < .001), compared with 0.0058 mm in ENHANCE (a 14-fold difference). In the previous trial, the baseline measurement was 1.07 mm (vs 0.68 mm in ENHANCE), and the extent of the change was significantly associated with the baseline measurement (r = .53, P < .001) but not with the change in LDL-C levels.

This is powerful evidence that, in two similar studies that used the same methodology and the same drug, the thinner arteries in the ENHANCE trial are by far the most likely explanation for the lack of change with the addition of ezetimibe to high-dose simvastatin. The METEOR trial enrolled only patients who had never received statins and whose carotid intima-media was thicker than 1.2 mm. In retrospect, a similar design would have been preferable for ENHANCE.³⁵

LESSONS LEARNED AND CLINICAL IMPLICATIONS

For Merck/Schering-Plough, missed opportunities

Although Dr. Krumholz (the spokesman for the ACC panel discussion) and I disagree on the clinical implications of the ENHANCE trial, we do agree on an important point. Dr. Krumholz posed the question that if the LDL-C-lowering hypothesis was already proven for ezetimibe, why was the ENHANCE trial conducted? After 6 years on the market, the efficacy of ezetimibe on cardiovascular outcomes should already have been established. It should not take this long to determine the clinical outcome benefit for a drug.

Merck/Schering-Plough’s outcome program for ezetimibe was inadequately designed to demonstrate the clinical value of this novel compound. Rather than assuming the LDL-C-lowering hypothesis was already established, they conducted another “lower-is-better” trial with the carotid intima-media thickness as the end point, and they succeeded only in raising doubt about the benefits of ezetimibe rather than showing that dual therapy is at least equivalent to high-dose statin therapy.

A preferable approach would have been to compare the effects of a statin in low doses plus ezetimibe vs high-dose statin monotherapy on either surrogate or hard outcomes. If the low-dose statin/ezetimibe combination, which should lower the LDL-C level as much as high-dose statin monotherapy, could provide similar or better outcomes with fewer side effects, this trial would change our practice.

One had hoped that dual therapy, by reducing both intestinal cholesterol absorption and hepatic synthesis of cholesterol, would improve outcomes by modifying postprandial chylomicron composition or by reducing plant sterol absorption.³⁶ Unfortunately, other outcome trials of ezetimibe/simvastatin will not provide an answer regarding the potential advantages of dual therapy. The SEAS study is comparing the number of clinical events in patients with aortic stenosis who receive ezetimibe/simvastatin or placebo; SHARP is being conducted in patients with chronic kidney disease. Although both groups of patients have high rates of coronary events, these trials will not address whether adding ezetimibe provides additional benefits. In fact, if the results of these trials turn out neutral, as in ENHANCE, then ezetimibe will be blamed for potentially offsetting the benefits of simvastatin, and if the trials show a benefit, the simvastatin component of ezetimibe/simvastatin will be given the credit.

The answer may come in 3 to 4 years with the results of IMPROVE-IT, a study of 18,000 patients with acute coronary syndrome treated with ezetimibe/simvastatin or simvastatin. The simvastatin monotherapy group will have a target LDL-C level of less than 80 mg/dL and the ezetimibe/simvastatin group will have an LDL-C target about 15% less. Although this trial is testing the lower-is-better hypothesis with ezetimibe, if the study does not show a benefit, it may not be because ezetimibe lacks clinical efficacy but rather because the LDL-C effect is curvilinear, and there is minimal further benefit of lowering the LDL-C level past 70 mg/dL. If the results of the IMPROVE-IT trial are negative, it may mean the end of ezetimibe as an LDL-C-lowering drug.

Merck/Schering-Plough has lost valuable time in not demonstrating the benefits of ezetimibe on clinical events. In contrast, consider rosuvastatin, an AstraZeneca product. Rosuvastatin was approved about the same time as ezetimibe/simvastatin, and 6 years later it has already received a label change for the reduction of progression of atherosclerosis, based on positive outcomes in the METEOR trial,³⁵ the ASTEROID intravascular ultrasonography trial,³⁷ and the CORONA trial (an important trial that examined hard clinical end points).³⁸ More importantly, the JUPITER trial was recently stopped early owing to a reduction in cardiovascular deaths. Initially, rosuvastatin received an unfair media portrayal as an unsafe drug. Now, because of its proven benefits in outcome trials, it will receive more widespread consideration for clinical use.

For preventive cardiologists, a painful reminder to focus on LDL-C

For the preventive cardiologist or lipidologist, the ENHANCE trial has been a painful reminder that despite overwhelming evidence, the mantra of “the lower the LDL-C the better” is still not universally accepted. We acknowledge the great benefits of statins, but the lure of “pleiotropic effects” distracts many of us from the necessity of more aggressive LDL-C reduction.

The pleiotropic benefits of statins were first raised as a means of supporting increased clinical use of pravastatin vis-a-vis other, more efficacious statins. It was not until the PROVE-IT study that pravastatin’s pleiotropic effects were found not to translate into a benefit equivalent to that of the more efficacious statin, atorvastatin.³⁹

The success of ezetimibe was its ability to safely and easily lower LDL-C in combination with statins to achieve treatment goals. For many patients, a lower-dose statin and ezetimibe together provide a well-tolerated and efficacious approach to treating hyperlipidemia. The fallout from the ENHANCE trial is that many patients who were well treated or who could be better treated with ezetimibe in combination with a statin will not receive the best tolerated regimen. In fact, preliminary prescription data after the release of the ENHANCE study support our worse fear, ie, that patients at high risk will receive less aggressive LDL-C reduction. Since the ENHANCE data were released, more than 300,000 patients have stopped taking either ezetimibe/simvastatin or ezetimibe, and nearly all have continued on generic simvastatin or on a dose of statin with less overall efficacy.

An example is Senator John McCain, who, according to his recently released medical records, has a Framingham 10-year risk of more than 20% and was on ezetimibe/simvastatin to treat an elevated cholesterol level. After release of the ENHANCE trial, he was switched to generic simvastatin, and his LDL-C increased from 82 mg/dL to 122 mg/dL. He most likely has an LDL-C goal of less than 100 mg/dL according to the ATP III guidelines, and he is therefore no longer at his target.

For the physicians who have received hundreds of phone calls and e-mails from concerned patients, the ENHANCE trial results must have been both discouraging and confusing. At present, I think we should remember the following:

• Ezetimibe’s mechanism of action is well understood
• It is safe and well-tolerated
• It still has a role as an add-on to statin therapy (or as monotherapy or combined with other agents in those who cannot tolerate statins) for patients who have not yet achieved their LDL-C target.

For the pharmaceutical industry, enormous challenges

The neutral ENHANCE trial results created an uncomfortable situation for the trial sponsor. A heavily marketed drug failed to achieve its expected result after the study results were delayed for a few months. The pharmaceutical industry ranks 14th out of 17 industries in public trust among the American public, and this study provided an opportunity for its critics to attack what is, in their opinion, an overly marketed drug.

Enormous challenges are on the horizon for the pharmaceutical industry, with a shrinking pipeline of potential new drugs, increasing regulatory hurdles, greater liability risk, political pressure for price controls, enhanced scrutiny of sales practices, and a growing media bias. As a cardiologist and clinical researcher whose father died at age 47 of a myocardial infarction, I am concerned that, unless change occurs, a vibrant pharmaceutical industry with the financial and intellectual capital to find and develop new, more effective treatments will cease to exist.

The Ezetimibe and Simvastatin in Hypercholesterolemia Enhances Atherosclerosis Regression (ENHANCE) trial¹ was probably the most widely publicized clinical study of the past decade. How did a 720-patient imaging trial with a neutral result in patients with severe hypercholesterolemia rise to a level warranting massive media attention, a congressional investigation, and a recommendation to curtail the use of a drug widely used to reduce levels of low-density-lipoprotein cholesterol (LDL-C)?

See related editorial

The reaction to the ENHANCE trial reveals more about the political climate and the relationship between the pharmaceutical industry and the American public than it does about the effects of ezetimibe (available combined with simvastatin as Vytorin and by itself as Zetia) on the progression of atherosclerosis.

SOME SELF-DISCLOSURE

Before I discuss the clinical implications of the ENHANCE trial, I must describe both my financial conflicts and intellectual biases. I am a paid consultant, speaker, and researcher on behalf of Merck/Schering-Plough, the sponsor of the ENHANCE trial. I was a principal investigator in the first phase II trial of ezetimibe and have conducted more than 10 clinical trials of either ezetimibe or ezetimibe/simvastatin. I also have been a strong advocate for imaging trials to assist in the clinical development of novel therapeutic agents and to support regulatory approval.

Therefore, I believe that the thickness of the intima and media layers of the carotid arteries is a useful surrogate to evaluate the potential antiatherosclerotic effects of drugs (more on this topic below). Also, I believe that the LDL-C-lowering hypothesis has been proven: ie, that all drugs that lower LDL-C safely, without off-target adverse effects, should reduce cardiovascular events. I support the goal levels of LDL-C and non-high-density-lipoprotein cholesterol set by the National Cholesterol Education Program’s third Adult Treatment Panel (ATP III) guidelines,^2,3 which specify LDL-C targets rather than the use of specific drugs. In spite of these conflicts and potential biases, I believe I have always served the best interests of patient care.

HISTORY OF THE ENHANCE TRIAL

The end point defined as the mean of six measurements

The primary end point was the change in the thickness of the intima and media layers of the carotid arteries over a 2-year period, measured by ultrasonography. A composite measure was used: the mean of the thicknesses in the far walls of the right and left common carotid arteries, the right and left carotid bulbs, and the right and left internal carotid arteries. Secondary end points included the change in the mean maximal carotid artery intima-media thickness (ie, the thickest of the six baseline measurements), the proportion of participants who developed new carotid artery plaque (defined arbitrarily as an intima-media thickness > 1.3 mm), and changes in the mean of the intima-media thickness of the six carotid sites plus the common femoral arteries.

The last participant completed the trial in April 2006. Reading of the almost 30,000 scans was not started until the last participant was finished, so that all scans for each participant could be read in a blinded, randomized order by five separate readers. A significant proportion of the images that the protocol called for could not be obtained or analyzed, particularly in the internal carotid artery and the carotid bulb, which are often difficult to visualize. As a result, 17% of the internal carotid or carotid bulb measurements were discarded.

To change the end point post hoc, or not to change the end point?

The sponsor of the trial was concerned about the missing data points and convened a special advisory board to review the blinded data. This group suggested a solution: changing the primary end point from the six-site composite value to the mean value in just the common carotid arteries. They based this suggestion on the greater success rate in measuring the common carotids (97%) than in measuring all six sites (88%), as well as on recent trials that indicated that the common carotid artery measurement correlates better with clinical outcomes (because the internal carotid and the bulb measurements vary more). On November 26, 2007, Merck/Schering-Plough announced the primary end point would be changed to the mean change in the common carotid arteries.

However, during a separate meeting on November 30, 2007, some members of the Merck/Schering-Plough advisory board objected to the change. On December 11, 2007, the company announced that the original primary end point would not be changed.

On December 31, 2007, the ENHANCE study was unblinded, and on January 14, 2008, Merck/Schering-Plough issued a press release announcing the results. The press release stated that there were no statistically significant differences between the treatment groups in the primary end point or in any of the secondary end points, despite a 16.5% greater reduction in LDL-C (about 50 mg/dL) in the group receiving the ezetimibe/simvastatin combination. The composite intima-media thickness had increased by an average of 0.0111 mm in the combined-therapy group vs 0.0058 mm in the simvastatin-only group (P = .29) over the 24-month treatment period.⁵

The press release received unprecedented international media attention. One leading cardiologist commented to the media that ENHANCE showed “millions of patients may be taking a drug [ezetimibe] that does not benefit them, raising their risk of heart attacks and exposing them to potential side effects.”⁶ The perceived message that ezetimibe/simvastatin is harmful resulted in thousands of phone calls from concerned patients to their physicians throughout the United States. The American Heart Association (AHA) and the American College of Cardiology (ACC) issued a joint statement the next day saying that ezetimibe/simvastatin does not appear to be unsafe and that patients should not stop taking the drug on their own. In the following days, Merck/Schering-Plough placed advertisements in newspapers reaffirming the safety of ezetimibe and quoting the AHA/ACC statement.

But the full results of the study were not available at that point. In fact, Senator Charles Grassley (R-Iowa) had launched a congressional investigation into the delays in releasing the results of the ENHANCE trial in December 2007. A focus of the investigation was whether the sponsor was delaying the release either because the data reflected negatively on its product or because it was legitimately concerned about the quality of the measurements of the carotid intima-media thickness. After Merck/Schering-Plough placed the advertisements quoting the AHA/ACC statement, these organizations were criticized for touting the safety of ezetimibe while receiving educational grants and other funds from Merck/Schering-Plough. Senator Grassley sent a letter to the ACC in late March requesting information about the amount of funds the ACC had received.

Full results are published, and the ACC is misquoted

The ENHANCE study was selected for a special presentation at the ACC annual scientific session on March 30, 2008. The full ENHANCE results were presented by Dr. Kastelein, after which an expert panel led by Harlan M. Krumholz, MD, discussed the trial’s implications. The ENHANCE results were simultaneously published in the New England Journal of Medicine,¹ accompanied by an editorial by B. Greg Brown, MD, and Allen J. Taylor, MD,⁷ and another editorial by the editors of that journal, Jeffrey M. Drazen, MD, and colleagues.⁸ The expert panel and the editorialists concluded that the ENHANCE trial data raised concerns about the cardiovascular benefits of ezetimibe; that statins should be used as initial therapy for hyperlipidemia and titrated to the goal LDL-C level or to the maximally tolerated dose; and that other drugs such as bile acid sequestrants, fibrates, and niacin should be used in combination with statins before considering ezetimibe.⁹

The next day, stories appeared in the media mistakenly stating that the ACC had recommended that ezetimibe/simvastatin be discontinued. This view was fueled by an article in the ACC’s Scientific Session News, penned by a contract writer and editor, with the headline, “ACC on Vytorin: Go Back to Statins” that said, “After waiting for 18 months for the results of the ENHANCE study, an ACC panel on Sunday encouraged physicians to use statins as a first line and prescribe Vytorin only as a last resort for patients unable to tolerate other cholesterol-lowering agents.”¹⁰

The ACC later clarified that this was the opinion of the panelists and not that of the ACC, and they reiterated statements from the AHA/ACC Secondary Prevention Guidelines¹¹ recommending statins in maximally tolerated doses or titrated to a goal LDL-C level for first-line drug treatment of coronary artery disease, and recommending that patients speak with their physicians before discontinuing any therapy.

WHY WERE THE ENHANCE STUDY RESULTS NEUTRAL?

The ACC expert panel concluded that the most likely reason for the neutral ENHANCE results was that ezetimibe lowers LDL-C but does not confer a cardiovascular benefit. In the words of Dr. Krumholz (as quoted by Shannon Pettypiece and Michelle Fay Cortez on bloomberg.com), ezetimibe is “just an expensive placebo.”¹²

There are at least three potential explanations for the lack of benefit with ezetimibe in the ENHANCE trial. I list them below in order of lowest to highest probability, in my opinion:

Theory 1: Ezetimibe lowers LDL-C but is not antiatherogenic

Since almost all experts agree that lowering LDL-C confers cardiovascular benefits, if ezetimibe does not inhibit atherosclerosis it must have some “off-target” effect that negates its LDL-C-lowering benefit. Critics of ezetimibe point out that oral estrogen and torcetrapib also lower LDL-C but do not improve cardiovascular outcomes.^13,14

The lack of benefit with these two other agents can be explained. Oral estrogen does not lower apolipoprotein B (an indication of the number of atherogenic particles), but rather it increases the levels of both triglycerides and C-reactive protein, and it is prothrombotic in some people.¹⁵ Torcetrapib increases aldosterone production and substantially raises blood pressure.¹⁶ Therefore, both drugs have true off-target effects that could explain their failure to reduce cardiovascular risk despite reductions in LDL-C. (Interestingly, though, oral estrogen has been shown to slow the progression of carotid intima-media thickness in newly postmenopausal women.¹⁷

Ezetimibe, however, lowers LDL-C by an ultimate mechanism similar to that of statins and bile acid sequestrants, ie, by up-regulating LDL receptors, although these drugs reach this mechanism via different pathways. Statins inhibit cholesterol synthesis, thereby lowering hepatic intracellular cholesterol and thus up-regulating LDL-receptors and enhancing LDL-C clearance from the plasma. Bile acid sequestrants interrupt bile acid reabsorption in the ileum, thereby decreasing intracellular hepatic cholesterol and up-regulating LDL receptors. Ezetimibe, like bile acid sequestrants, also decreases cholesterol return to the liver, lowering hepatic intracellular levels and thus up-regulating LDL receptors.¹⁸

Ezetimibe is unlikely to have an off-target effect because it is only fractionally absorbed systemically, and a recent animal study showed that it enhances macrophage efflux of cholesterol, thereby potentially increasing reverse cholesterol transport.¹⁹ Ezetimibe has also been shown to reduce atherosclerosis in animal models.²⁰

In their editorial, Drs. Brown and Taylor⁷ noted that ezetimibe reduces the expression of adenosine triphosphate binding cassette A1 (ABCA1) in Caco-2 (an intestinal cell line), and this may be an example of an off-target effect. However, statins also reduce ABCA1 expression in macrophages.²¹ ABCA1 is sensitive to intracellular cholesterol, and when cholesterol levels are decreased, whether by statins or by ezetimibe, ABCA1 expression is down-regulated.²²

Theory 2: Intima-media thickness does not reflect the true benefits of lowering LDL-C

The carotid intima-media thickness is a surrogate end point that predicts coronary events and the rate of progression of coronary atherosclerosis.²³ In trials of lovastatin (Mevacor),²⁴ pravastatin (Pravachol),²⁵ and rosuvastatin (Crestor),²⁶ the carotid intima-media was thinner at 24 months with the active drug than with placebo. In two relatively small trials—ARBITER 1 (n = 161),²⁷ which was open-label, and ASAP (n = 325)^28,29—aggressive lipid-lowering reduced the progression of intima-media thickness better than less-aggressive therapy. However, this measure has been used to evaluate the effects of differing degrees of LDL-C reduction between active treatments in fewer than 500 research participants.

Furthermore, what part or parts of the carotid system are we talking about? In recent trials led by Dr. Kastelein, the intima-media thickness of the common carotid arteries increased with pactimibe (an acyl-coenzyme A:cholesterol O-acyltransferase, or ACAT, inhibitor)³⁰ and torcetrapib,³¹ but the six-site composite measure (which was the primary end point in these trials, as in ENHANCE) did not increase more than in the control groups. Pactimibe was also shown to increase atheroma volume as measured by intravascular ultrasonography in the ACTIVATE trial.³² Therefore, the thickness of the common carotid arteries has been shown to be a better predictor of harm from a therapy than the composite measurement.

The advantage of measuring the common carotid artery is that it is easier to visualize and measure, and therefore the measurements vary less. In the METEOR trial,²⁶ the six-site measurement increased significantly less with rosuvastatin than with placebo, but the common carotid measurement alone was more strongly associated with a difference in progression. In the ENHANCE trial, the thickness of the common carotid arteries increased by 0.0024 mm with simvastatin alone vs 0.0019 mm with simvastatin/ezetimibe, a difference of 0.005 mm that was not statistically significant (P = .93).¹

Although the six-site measurement appears to be good for predicting coronary events and evaluating therapies, the measurement in the common carotid arteries appears to be a more reliable surrogate end point for predicting both benefit and harm from antiatherogenic agents. However, trials of statins and other lipid-lowering therapies that assessed clinical events have shown that the reduction in risk associated with a given reduction in cholesterol is similar regardless of the mechanism by which cholesterol is lowered.³³ Therefore, the LDL-C level is far superior as a marker of clinical benefit.

Theory 3: Previous statin treatment affected the ENHANCE results

By far the most likely explanation for the neutral findings in ENHANCE is that the patients were so well treated before entry that it was impossible to detect a difference between the two treatment groups in carotid intima-media thickness at the end of the study. Eighty percent of the patients had received statins previously, and at baseline the mean intima-media thickness of the common carotid arteries was only 0.68 mm.¹ In contrast, most other trials required a thickness greater than 0.7 mm for entry.

The two main reasons for selecting a population with familial hypercholesterolemia were the assumptions that these participants would have a greater-than-average carotid intima-media thickness at baseline and that they would show an above-average progression rate, even on high-dose statin therapy.⁴ Both of these assumptions were incorrect: the baseline thickness was normal and the progression rate was negligible in both groups.

Accordingly, the high prevalence of statin pretreatment and the near-normal carotid intima-media thickness at baseline may have prevented the 16.5% greater reduction in LDL-C due to ezetimibe from producing a difference in progression over 24 months of treatment. This conclusion is supported by the long-term follow-up results from ASAP, RADIANCE 1, and CAPTIVATE, all of which showed that in patients with familial hypercholesterolemia well treated with statins, progression of carotid intima-media thickness is negligible.^30,31

Further supporting this view, in a previous trial by Dr. Kastelein’s group in patients with familial hypercholesterolemia,³⁴ giving simvastatin 80 mg for 2 years decreased the intima-medial thickness by .081 mm (P < .001), compared with 0.0058 mm in ENHANCE (a 14-fold difference). In the previous trial, the baseline measurement was 1.07 mm (vs 0.68 mm in ENHANCE), and the extent of the change was significantly associated with the baseline measurement (r = .53, P < .001) but not with the change in LDL-C levels.

This is powerful evidence that, in two similar studies that used the same methodology and the same drug, the thinner arteries in the ENHANCE trial are by far the most likely explanation for the lack of change with the addition of ezetimibe to high-dose simvastatin. The METEOR trial enrolled only patients who had never received statins and whose carotid intima-media was thicker than 1.2 mm. In retrospect, a similar design would have been preferable for ENHANCE.³⁵

LESSONS LEARNED AND CLINICAL IMPLICATIONS

For Merck/Schering-Plough, missed opportunities

Although Dr. Krumholz (the spokesman for the ACC panel discussion) and I disagree on the clinical implications of the ENHANCE trial, we do agree on an important point. Dr. Krumholz posed the question that if the LDL-C-lowering hypothesis was already proven for ezetimibe, why was the ENHANCE trial conducted? After 6 years on the market, the efficacy of ezetimibe on cardiovascular outcomes should already have been established. It should not take this long to determine the clinical outcome benefit for a drug.

Merck/Schering-Plough’s outcome program for ezetimibe was inadequately designed to demonstrate the clinical value of this novel compound. Rather than assuming the LDL-C-lowering hypothesis was already established, they conducted another “lower-is-better” trial with the carotid intima-media thickness as the end point, and they succeeded only in raising doubt about the benefits of ezetimibe rather than showing that dual therapy is at least equivalent to high-dose statin therapy.

A preferable approach would have been to compare the effects of a statin in low doses plus ezetimibe vs high-dose statin monotherapy on either surrogate or hard outcomes. If the low-dose statin/ezetimibe combination, which should lower the LDL-C level as much as high-dose statin monotherapy, could provide similar or better outcomes with fewer side effects, this trial would change our practice.

One had hoped that dual therapy, by reducing both intestinal cholesterol absorption and hepatic synthesis of cholesterol, would improve outcomes by modifying postprandial chylomicron composition or by reducing plant sterol absorption.³⁶ Unfortunately, other outcome trials of ezetimibe/simvastatin will not provide an answer regarding the potential advantages of dual therapy. The SEAS study is comparing the number of clinical events in patients with aortic stenosis who receive ezetimibe/simvastatin or placebo; SHARP is being conducted in patients with chronic kidney disease. Although both groups of patients have high rates of coronary events, these trials will not address whether adding ezetimibe provides additional benefits. In fact, if the results of these trials turn out neutral, as in ENHANCE, then ezetimibe will be blamed for potentially offsetting the benefits of simvastatin, and if the trials show a benefit, the simvastatin component of ezetimibe/simvastatin will be given the credit.

The answer may come in 3 to 4 years with the results of IMPROVE-IT, a study of 18,000 patients with acute coronary syndrome treated with ezetimibe/simvastatin or simvastatin. The simvastatin monotherapy group will have a target LDL-C level of less than 80 mg/dL and the ezetimibe/simvastatin group will have an LDL-C target about 15% less. Although this trial is testing the lower-is-better hypothesis with ezetimibe, if the study does not show a benefit, it may not be because ezetimibe lacks clinical efficacy but rather because the LDL-C effect is curvilinear, and there is minimal further benefit of lowering the LDL-C level past 70 mg/dL. If the results of the IMPROVE-IT trial are negative, it may mean the end of ezetimibe as an LDL-C-lowering drug.

Merck/Schering-Plough has lost valuable time in not demonstrating the benefits of ezetimibe on clinical events. In contrast, consider rosuvastatin, an AstraZeneca product. Rosuvastatin was approved about the same time as ezetimibe/simvastatin, and 6 years later it has already received a label change for the reduction of progression of atherosclerosis, based on positive outcomes in the METEOR trial,³⁵ the ASTEROID intravascular ultrasonography trial,³⁷ and the CORONA trial (an important trial that examined hard clinical end points).³⁸ More importantly, the JUPITER trial was recently stopped early owing to a reduction in cardiovascular deaths. Initially, rosuvastatin received an unfair media portrayal as an unsafe drug. Now, because of its proven benefits in outcome trials, it will receive more widespread consideration for clinical use.

For preventive cardiologists, a painful reminder to focus on LDL-C

For the preventive cardiologist or lipidologist, the ENHANCE trial has been a painful reminder that despite overwhelming evidence, the mantra of “the lower the LDL-C the better” is still not universally accepted. We acknowledge the great benefits of statins, but the lure of “pleiotropic effects” distracts many of us from the necessity of more aggressive LDL-C reduction.

The pleiotropic benefits of statins were first raised as a means of supporting increased clinical use of pravastatin vis-a-vis other, more efficacious statins. It was not until the PROVE-IT study that pravastatin’s pleiotropic effects were found not to translate into a benefit equivalent to that of the more efficacious statin, atorvastatin.³⁹

The success of ezetimibe was its ability to safely and easily lower LDL-C in combination with statins to achieve treatment goals. For many patients, a lower-dose statin and ezetimibe together provide a well-tolerated and efficacious approach to treating hyperlipidemia. The fallout from the ENHANCE trial is that many patients who were well treated or who could be better treated with ezetimibe in combination with a statin will not receive the best tolerated regimen. In fact, preliminary prescription data after the release of the ENHANCE study support our worse fear, ie, that patients at high risk will receive less aggressive LDL-C reduction. Since the ENHANCE data were released, more than 300,000 patients have stopped taking either ezetimibe/simvastatin or ezetimibe, and nearly all have continued on generic simvastatin or on a dose of statin with less overall efficacy.

An example is Senator John McCain, who, according to his recently released medical records, has a Framingham 10-year risk of more than 20% and was on ezetimibe/simvastatin to treat an elevated cholesterol level. After release of the ENHANCE trial, he was switched to generic simvastatin, and his LDL-C increased from 82 mg/dL to 122 mg/dL. He most likely has an LDL-C goal of less than 100 mg/dL according to the ATP III guidelines, and he is therefore no longer at his target.

For the physicians who have received hundreds of phone calls and e-mails from concerned patients, the ENHANCE trial results must have been both discouraging and confusing. At present, I think we should remember the following:

• Ezetimibe’s mechanism of action is well understood
• It is safe and well-tolerated
• It still has a role as an add-on to statin therapy (or as monotherapy or combined with other agents in those who cannot tolerate statins) for patients who have not yet achieved their LDL-C target.

For the pharmaceutical industry, enormous challenges

The neutral ENHANCE trial results created an uncomfortable situation for the trial sponsor. A heavily marketed drug failed to achieve its expected result after the study results were delayed for a few months. The pharmaceutical industry ranks 14th out of 17 industries in public trust among the American public, and this study provided an opportunity for its critics to attack what is, in their opinion, an overly marketed drug.

Enormous challenges are on the horizon for the pharmaceutical industry, with a shrinking pipeline of potential new drugs, increasing regulatory hurdles, greater liability risk, political pressure for price controls, enhanced scrutiny of sales practices, and a growing media bias. As a cardiologist and clinical researcher whose father died at age 47 of a myocardial infarction, I am concerned that, unless change occurs, a vibrant pharmaceutical industry with the financial and intellectual capital to find and develop new, more effective treatments will cease to exist.

The Ezetimibe and Simvastatin in Hypercholesterolemia Enhances Atherosclerosis Regression (ENHANCE) trial¹ was probably the most widely publicized clinical study of the past decade. How did a 720-patient imaging trial with a neutral result in patients with severe hypercholesterolemia rise to a level warranting massive media attention, a congressional investigation, and a recommendation to curtail the use of a drug widely used to reduce levels of low-density-lipoprotein cholesterol (LDL-C)?

See related editorial

The reaction to the ENHANCE trial reveals more about the political climate and the relationship between the pharmaceutical industry and the American public than it does about the effects of ezetimibe (available combined with simvastatin as Vytorin and by itself as Zetia) on the progression of atherosclerosis.

SOME SELF-DISCLOSURE

Before I discuss the clinical implications of the ENHANCE trial, I must describe both my financial conflicts and intellectual biases. I am a paid consultant, speaker, and researcher on behalf of Merck/Schering-Plough, the sponsor of the ENHANCE trial. I was a principal investigator in the first phase II trial of ezetimibe and have conducted more than 10 clinical trials of either ezetimibe or ezetimibe/simvastatin. I also have been a strong advocate for imaging trials to assist in the clinical development of novel therapeutic agents and to support regulatory approval.

Therefore, I believe that the thickness of the intima and media layers of the carotid arteries is a useful surrogate to evaluate the potential antiatherosclerotic effects of drugs (more on this topic below). Also, I believe that the LDL-C-lowering hypothesis has been proven: ie, that all drugs that lower LDL-C safely, without off-target adverse effects, should reduce cardiovascular events. I support the goal levels of LDL-C and non-high-density-lipoprotein cholesterol set by the National Cholesterol Education Program’s third Adult Treatment Panel (ATP III) guidelines,^2,3 which specify LDL-C targets rather than the use of specific drugs. In spite of these conflicts and potential biases, I believe I have always served the best interests of patient care.

HISTORY OF THE ENHANCE TRIAL

The end point defined as the mean of six measurements

The primary end point was the change in the thickness of the intima and media layers of the carotid arteries over a 2-year period, measured by ultrasonography. A composite measure was used: the mean of the thicknesses in the far walls of the right and left common carotid arteries, the right and left carotid bulbs, and the right and left internal carotid arteries. Secondary end points included the change in the mean maximal carotid artery intima-media thickness (ie, the thickest of the six baseline measurements), the proportion of participants who developed new carotid artery plaque (defined arbitrarily as an intima-media thickness > 1.3 mm), and changes in the mean of the intima-media thickness of the six carotid sites plus the common femoral arteries.

The last participant completed the trial in April 2006. Reading of the almost 30,000 scans was not started until the last participant was finished, so that all scans for each participant could be read in a blinded, randomized order by five separate readers. A significant proportion of the images that the protocol called for could not be obtained or analyzed, particularly in the internal carotid artery and the carotid bulb, which are often difficult to visualize. As a result, 17% of the internal carotid or carotid bulb measurements were discarded.

To change the end point post hoc, or not to change the end point?

The sponsor of the trial was concerned about the missing data points and convened a special advisory board to review the blinded data. This group suggested a solution: changing the primary end point from the six-site composite value to the mean value in just the common carotid arteries. They based this suggestion on the greater success rate in measuring the common carotids (97%) than in measuring all six sites (88%), as well as on recent trials that indicated that the common carotid artery measurement correlates better with clinical outcomes (because the internal carotid and the bulb measurements vary more). On November 26, 2007, Merck/Schering-Plough announced the primary end point would be changed to the mean change in the common carotid arteries.

However, during a separate meeting on November 30, 2007, some members of the Merck/Schering-Plough advisory board objected to the change. On December 11, 2007, the company announced that the original primary end point would not be changed.

On December 31, 2007, the ENHANCE study was unblinded, and on January 14, 2008, Merck/Schering-Plough issued a press release announcing the results. The press release stated that there were no statistically significant differences between the treatment groups in the primary end point or in any of the secondary end points, despite a 16.5% greater reduction in LDL-C (about 50 mg/dL) in the group receiving the ezetimibe/simvastatin combination. The composite intima-media thickness had increased by an average of 0.0111 mm in the combined-therapy group vs 0.0058 mm in the simvastatin-only group (P = .29) over the 24-month treatment period.⁵

The press release received unprecedented international media attention. One leading cardiologist commented to the media that ENHANCE showed “millions of patients may be taking a drug [ezetimibe] that does not benefit them, raising their risk of heart attacks and exposing them to potential side effects.”⁶ The perceived message that ezetimibe/simvastatin is harmful resulted in thousands of phone calls from concerned patients to their physicians throughout the United States. The American Heart Association (AHA) and the American College of Cardiology (ACC) issued a joint statement the next day saying that ezetimibe/simvastatin does not appear to be unsafe and that patients should not stop taking the drug on their own. In the following days, Merck/Schering-Plough placed advertisements in newspapers reaffirming the safety of ezetimibe and quoting the AHA/ACC statement.

But the full results of the study were not available at that point. In fact, Senator Charles Grassley (R-Iowa) had launched a congressional investigation into the delays in releasing the results of the ENHANCE trial in December 2007. A focus of the investigation was whether the sponsor was delaying the release either because the data reflected negatively on its product or because it was legitimately concerned about the quality of the measurements of the carotid intima-media thickness. After Merck/Schering-Plough placed the advertisements quoting the AHA/ACC statement, these organizations were criticized for touting the safety of ezetimibe while receiving educational grants and other funds from Merck/Schering-Plough. Senator Grassley sent a letter to the ACC in late March requesting information about the amount of funds the ACC had received.

Full results are published, and the ACC is misquoted

The ENHANCE study was selected for a special presentation at the ACC annual scientific session on March 30, 2008. The full ENHANCE results were presented by Dr. Kastelein, after which an expert panel led by Harlan M. Krumholz, MD, discussed the trial’s implications. The ENHANCE results were simultaneously published in the New England Journal of Medicine,¹ accompanied by an editorial by B. Greg Brown, MD, and Allen J. Taylor, MD,⁷ and another editorial by the editors of that journal, Jeffrey M. Drazen, MD, and colleagues.⁸ The expert panel and the editorialists concluded that the ENHANCE trial data raised concerns about the cardiovascular benefits of ezetimibe; that statins should be used as initial therapy for hyperlipidemia and titrated to the goal LDL-C level or to the maximally tolerated dose; and that other drugs such as bile acid sequestrants, fibrates, and niacin should be used in combination with statins before considering ezetimibe.⁹

The next day, stories appeared in the media mistakenly stating that the ACC had recommended that ezetimibe/simvastatin be discontinued. This view was fueled by an article in the ACC’s Scientific Session News, penned by a contract writer and editor, with the headline, “ACC on Vytorin: Go Back to Statins” that said, “After waiting for 18 months for the results of the ENHANCE study, an ACC panel on Sunday encouraged physicians to use statins as a first line and prescribe Vytorin only as a last resort for patients unable to tolerate other cholesterol-lowering agents.”¹⁰

The ACC later clarified that this was the opinion of the panelists and not that of the ACC, and they reiterated statements from the AHA/ACC Secondary Prevention Guidelines¹¹ recommending statins in maximally tolerated doses or titrated to a goal LDL-C level for first-line drug treatment of coronary artery disease, and recommending that patients speak with their physicians before discontinuing any therapy.

WHY WERE THE ENHANCE STUDY RESULTS NEUTRAL?

The ACC expert panel concluded that the most likely reason for the neutral ENHANCE results was that ezetimibe lowers LDL-C but does not confer a cardiovascular benefit. In the words of Dr. Krumholz (as quoted by Shannon Pettypiece and Michelle Fay Cortez on bloomberg.com), ezetimibe is “just an expensive placebo.”¹²

There are at least three potential explanations for the lack of benefit with ezetimibe in the ENHANCE trial. I list them below in order of lowest to highest probability, in my opinion:

Theory 1: Ezetimibe lowers LDL-C but is not antiatherogenic

Since almost all experts agree that lowering LDL-C confers cardiovascular benefits, if ezetimibe does not inhibit atherosclerosis it must have some “off-target” effect that negates its LDL-C-lowering benefit. Critics of ezetimibe point out that oral estrogen and torcetrapib also lower LDL-C but do not improve cardiovascular outcomes.^13,14

The lack of benefit with these two other agents can be explained. Oral estrogen does not lower apolipoprotein B (an indication of the number of atherogenic particles), but rather it increases the levels of both triglycerides and C-reactive protein, and it is prothrombotic in some people.¹⁵ Torcetrapib increases aldosterone production and substantially raises blood pressure.¹⁶ Therefore, both drugs have true off-target effects that could explain their failure to reduce cardiovascular risk despite reductions in LDL-C. (Interestingly, though, oral estrogen has been shown to slow the progression of carotid intima-media thickness in newly postmenopausal women.¹⁷

Ezetimibe, however, lowers LDL-C by an ultimate mechanism similar to that of statins and bile acid sequestrants, ie, by up-regulating LDL receptors, although these drugs reach this mechanism via different pathways. Statins inhibit cholesterol synthesis, thereby lowering hepatic intracellular cholesterol and thus up-regulating LDL-receptors and enhancing LDL-C clearance from the plasma. Bile acid sequestrants interrupt bile acid reabsorption in the ileum, thereby decreasing intracellular hepatic cholesterol and up-regulating LDL receptors. Ezetimibe, like bile acid sequestrants, also decreases cholesterol return to the liver, lowering hepatic intracellular levels and thus up-regulating LDL receptors.¹⁸

Ezetimibe is unlikely to have an off-target effect because it is only fractionally absorbed systemically, and a recent animal study showed that it enhances macrophage efflux of cholesterol, thereby potentially increasing reverse cholesterol transport.¹⁹ Ezetimibe has also been shown to reduce atherosclerosis in animal models.²⁰

In their editorial, Drs. Brown and Taylor⁷ noted that ezetimibe reduces the expression of adenosine triphosphate binding cassette A1 (ABCA1) in Caco-2 (an intestinal cell line), and this may be an example of an off-target effect. However, statins also reduce ABCA1 expression in macrophages.²¹ ABCA1 is sensitive to intracellular cholesterol, and when cholesterol levels are decreased, whether by statins or by ezetimibe, ABCA1 expression is down-regulated.²²

Theory 2: Intima-media thickness does not reflect the true benefits of lowering LDL-C

The carotid intima-media thickness is a surrogate end point that predicts coronary events and the rate of progression of coronary atherosclerosis.²³ In trials of lovastatin (Mevacor),²⁴ pravastatin (Pravachol),²⁵ and rosuvastatin (Crestor),²⁶ the carotid intima-media was thinner at 24 months with the active drug than with placebo. In two relatively small trials—ARBITER 1 (n = 161),²⁷ which was open-label, and ASAP (n = 325)^28,29—aggressive lipid-lowering reduced the progression of intima-media thickness better than less-aggressive therapy. However, this measure has been used to evaluate the effects of differing degrees of LDL-C reduction between active treatments in fewer than 500 research participants.

Furthermore, what part or parts of the carotid system are we talking about? In recent trials led by Dr. Kastelein, the intima-media thickness of the common carotid arteries increased with pactimibe (an acyl-coenzyme A:cholesterol O-acyltransferase, or ACAT, inhibitor)³⁰ and torcetrapib,³¹ but the six-site composite measure (which was the primary end point in these trials, as in ENHANCE) did not increase more than in the control groups. Pactimibe was also shown to increase atheroma volume as measured by intravascular ultrasonography in the ACTIVATE trial.³² Therefore, the thickness of the common carotid arteries has been shown to be a better predictor of harm from a therapy than the composite measurement.

The advantage of measuring the common carotid artery is that it is easier to visualize and measure, and therefore the measurements vary less. In the METEOR trial,²⁶ the six-site measurement increased significantly less with rosuvastatin than with placebo, but the common carotid measurement alone was more strongly associated with a difference in progression. In the ENHANCE trial, the thickness of the common carotid arteries increased by 0.0024 mm with simvastatin alone vs 0.0019 mm with simvastatin/ezetimibe, a difference of 0.005 mm that was not statistically significant (P = .93).¹

Although the six-site measurement appears to be good for predicting coronary events and evaluating therapies, the measurement in the common carotid arteries appears to be a more reliable surrogate end point for predicting both benefit and harm from antiatherogenic agents. However, trials of statins and other lipid-lowering therapies that assessed clinical events have shown that the reduction in risk associated with a given reduction in cholesterol is similar regardless of the mechanism by which cholesterol is lowered.³³ Therefore, the LDL-C level is far superior as a marker of clinical benefit.

Theory 3: Previous statin treatment affected the ENHANCE results

By far the most likely explanation for the neutral findings in ENHANCE is that the patients were so well treated before entry that it was impossible to detect a difference between the two treatment groups in carotid intima-media thickness at the end of the study. Eighty percent of the patients had received statins previously, and at baseline the mean intima-media thickness of the common carotid arteries was only 0.68 mm.¹ In contrast, most other trials required a thickness greater than 0.7 mm for entry.

The two main reasons for selecting a population with familial hypercholesterolemia were the assumptions that these participants would have a greater-than-average carotid intima-media thickness at baseline and that they would show an above-average progression rate, even on high-dose statin therapy.⁴ Both of these assumptions were incorrect: the baseline thickness was normal and the progression rate was negligible in both groups.

Accordingly, the high prevalence of statin pretreatment and the near-normal carotid intima-media thickness at baseline may have prevented the 16.5% greater reduction in LDL-C due to ezetimibe from producing a difference in progression over 24 months of treatment. This conclusion is supported by the long-term follow-up results from ASAP, RADIANCE 1, and CAPTIVATE, all of which showed that in patients with familial hypercholesterolemia well treated with statins, progression of carotid intima-media thickness is negligible.^30,31

Further supporting this view, in a previous trial by Dr. Kastelein’s group in patients with familial hypercholesterolemia,³⁴ giving simvastatin 80 mg for 2 years decreased the intima-medial thickness by .081 mm (P < .001), compared with 0.0058 mm in ENHANCE (a 14-fold difference). In the previous trial, the baseline measurement was 1.07 mm (vs 0.68 mm in ENHANCE), and the extent of the change was significantly associated with the baseline measurement (r = .53, P < .001) but not with the change in LDL-C levels.

This is powerful evidence that, in two similar studies that used the same methodology and the same drug, the thinner arteries in the ENHANCE trial are by far the most likely explanation for the lack of change with the addition of ezetimibe to high-dose simvastatin. The METEOR trial enrolled only patients who had never received statins and whose carotid intima-media was thicker than 1.2 mm. In retrospect, a similar design would have been preferable for ENHANCE.³⁵

LESSONS LEARNED AND CLINICAL IMPLICATIONS

For Merck/Schering-Plough, missed opportunities

Although Dr. Krumholz (the spokesman for the ACC panel discussion) and I disagree on the clinical implications of the ENHANCE trial, we do agree on an important point. Dr. Krumholz posed the question that if the LDL-C-lowering hypothesis was already proven for ezetimibe, why was the ENHANCE trial conducted? After 6 years on the market, the efficacy of ezetimibe on cardiovascular outcomes should already have been established. It should not take this long to determine the clinical outcome benefit for a drug.

Merck/Schering-Plough’s outcome program for ezetimibe was inadequately designed to demonstrate the clinical value of this novel compound. Rather than assuming the LDL-C-lowering hypothesis was already established, they conducted another “lower-is-better” trial with the carotid intima-media thickness as the end point, and they succeeded only in raising doubt about the benefits of ezetimibe rather than showing that dual therapy is at least equivalent to high-dose statin therapy.

A preferable approach would have been to compare the effects of a statin in low doses plus ezetimibe vs high-dose statin monotherapy on either surrogate or hard outcomes. If the low-dose statin/ezetimibe combination, which should lower the LDL-C level as much as high-dose statin monotherapy, could provide similar or better outcomes with fewer side effects, this trial would change our practice.

One had hoped that dual therapy, by reducing both intestinal cholesterol absorption and hepatic synthesis of cholesterol, would improve outcomes by modifying postprandial chylomicron composition or by reducing plant sterol absorption.³⁶ Unfortunately, other outcome trials of ezetimibe/simvastatin will not provide an answer regarding the potential advantages of dual therapy. The SEAS study is comparing the number of clinical events in patients with aortic stenosis who receive ezetimibe/simvastatin or placebo; SHARP is being conducted in patients with chronic kidney disease. Although both groups of patients have high rates of coronary events, these trials will not address whether adding ezetimibe provides additional benefits. In fact, if the results of these trials turn out neutral, as in ENHANCE, then ezetimibe will be blamed for potentially offsetting the benefits of simvastatin, and if the trials show a benefit, the simvastatin component of ezetimibe/simvastatin will be given the credit.

The answer may come in 3 to 4 years with the results of IMPROVE-IT, a study of 18,000 patients with acute coronary syndrome treated with ezetimibe/simvastatin or simvastatin. The simvastatin monotherapy group will have a target LDL-C level of less than 80 mg/dL and the ezetimibe/simvastatin group will have an LDL-C target about 15% less. Although this trial is testing the lower-is-better hypothesis with ezetimibe, if the study does not show a benefit, it may not be because ezetimibe lacks clinical efficacy but rather because the LDL-C effect is curvilinear, and there is minimal further benefit of lowering the LDL-C level past 70 mg/dL. If the results of the IMPROVE-IT trial are negative, it may mean the end of ezetimibe as an LDL-C-lowering drug.

Merck/Schering-Plough has lost valuable time in not demonstrating the benefits of ezetimibe on clinical events. In contrast, consider rosuvastatin, an AstraZeneca product. Rosuvastatin was approved about the same time as ezetimibe/simvastatin, and 6 years later it has already received a label change for the reduction of progression of atherosclerosis, based on positive outcomes in the METEOR trial,³⁵ the ASTEROID intravascular ultrasonography trial,³⁷ and the CORONA trial (an important trial that examined hard clinical end points).³⁸ More importantly, the JUPITER trial was recently stopped early owing to a reduction in cardiovascular deaths. Initially, rosuvastatin received an unfair media portrayal as an unsafe drug. Now, because of its proven benefits in outcome trials, it will receive more widespread consideration for clinical use.

For preventive cardiologists, a painful reminder to focus on LDL-C

For the preventive cardiologist or lipidologist, the ENHANCE trial has been a painful reminder that despite overwhelming evidence, the mantra of “the lower the LDL-C the better” is still not universally accepted. We acknowledge the great benefits of statins, but the lure of “pleiotropic effects” distracts many of us from the necessity of more aggressive LDL-C reduction.

The pleiotropic benefits of statins were first raised as a means of supporting increased clinical use of pravastatin vis-a-vis other, more efficacious statins. It was not until the PROVE-IT study that pravastatin’s pleiotropic effects were found not to translate into a benefit equivalent to that of the more efficacious statin, atorvastatin.³⁹

The success of ezetimibe was its ability to safely and easily lower LDL-C in combination with statins to achieve treatment goals. For many patients, a lower-dose statin and ezetimibe together provide a well-tolerated and efficacious approach to treating hyperlipidemia. The fallout from the ENHANCE trial is that many patients who were well treated or who could be better treated with ezetimibe in combination with a statin will not receive the best tolerated regimen. In fact, preliminary prescription data after the release of the ENHANCE study support our worse fear, ie, that patients at high risk will receive less aggressive LDL-C reduction. Since the ENHANCE data were released, more than 300,000 patients have stopped taking either ezetimibe/simvastatin or ezetimibe, and nearly all have continued on generic simvastatin or on a dose of statin with less overall efficacy.

An example is Senator John McCain, who, according to his recently released medical records, has a Framingham 10-year risk of more than 20% and was on ezetimibe/simvastatin to treat an elevated cholesterol level. After release of the ENHANCE trial, he was switched to generic simvastatin, and his LDL-C increased from 82 mg/dL to 122 mg/dL. He most likely has an LDL-C goal of less than 100 mg/dL according to the ATP III guidelines, and he is therefore no longer at his target.

For the physicians who have received hundreds of phone calls and e-mails from concerned patients, the ENHANCE trial results must have been both discouraging and confusing. At present, I think we should remember the following:

• Ezetimibe’s mechanism of action is well understood
• It is safe and well-tolerated
• It still has a role as an add-on to statin therapy (or as monotherapy or combined with other agents in those who cannot tolerate statins) for patients who have not yet achieved their LDL-C target.

For the pharmaceutical industry, enormous challenges

The neutral ENHANCE trial results created an uncomfortable situation for the trial sponsor. A heavily marketed drug failed to achieve its expected result after the study results were delayed for a few months. The pharmaceutical industry ranks 14th out of 17 industries in public trust among the American public, and this study provided an opportunity for its critics to attack what is, in their opinion, an overly marketed drug.

Enormous challenges are on the horizon for the pharmaceutical industry, with a shrinking pipeline of potential new drugs, increasing regulatory hurdles, greater liability risk, political pressure for price controls, enhanced scrutiny of sales practices, and a growing media bias. As a cardiologist and clinical researcher whose father died at age 47 of a myocardial infarction, I am concerned that, unless change occurs, a vibrant pharmaceutical industry with the financial and intellectual capital to find and develop new, more effective treatments will cease to exist.

More than 2 years have passed since we published the results of the Women’s Health Initiative (WHI), which caused a storm of information—and misinformation—about the effect of long-term dietary intervention on disease outcomes in postmenopausal women. Now that the dust has long settled, what have we learned from this landmark study?

The WHI results led to numerous additional analyses of all aspects of the study.^1–7 What are the implications of all the analyses to clinical practice?

In this article, we summarize key aspects of the clinical trial, including study design, interventions, main results, and future plans. We also discuss potential clinical applications and practical considerations for public health efforts.

WHO WAS ELIGIBLE, WHO WAS NOT

A total of 48,835 postmenopausal women were randomly assigned to either no dietary intervention (n = 29,294) or a dietary intervention (n = 19,541) (see below).⁷ Participants were followed at 40 clinical centers between 1993 and 2005.⁴ Their mean age was 62.3 years; 18.6% were members of minorities.

Women were eligible if they were post-menopausal and had a daily dietary fat intake of at least 32% of total calories, based on assessment via a food-frequency questionnaire. They were excluded from the study if they had any of the following: a history of breast cancer, colorectal cancer, or other cancer except skin cancer during the past 10 years; type 1 diabetes; a medical condition in which the predicted survival was less than 3 years; and a potential barrier to adherence to the study regimen, including alcoholism or a lifestyle that involved often eating meals away from home.

THE WHI DIET: LESS FAT, BUT MORE FRUITS, VEGETABLES, GRAINS

The WHI dietary intervention was designed to prevent breast cancer, based on the evidence available when the study was planned. The targets included a total fat intake of less than 20% of energy (in kilocalories), increasing the intake of fruits and vegetables to at least five servings per day, and increasing the intake of grains to at least six servings per day.

Although reduction in saturated fat intake per se was not part of the WHI protocol, we assumed from previous pilot studies⁸ that the reduction of total fat intake would simultaneously produce a reduction in saturated fat intake to 7% of total calories.

A simpler dietary intervention

Unlike the 2006 American Heart Association guidelines and the US Department of Agriculture’s Dietary Guidelines for Americans 2005, the WHI dietary intervention had no specifications for dietary fiber, specific fatty acids (trans-fatty acids, omega-3 fatty acids, conjugated linoleic acid), complex carbohydrates, whole grains, vegetable protein, or other factors that have emerged as potential risk factors for cardiovascular and other chronic diseases since the study began. The WHI intervention also included no specific recommendation for total calorie intake, nor were patients in the intervention group encouraged to lose weight, as this could have confounded the results of the dietary intervention.

Education and encouragement

Those in the intervention group were each assigned a fat-gram goal, calculated on the basis of height. They were taught how to monitor their intake of total fat, fruits, vegetables, and grains. They attended intensive behavioral modification sessions to encourage them to keep to the dietary program: 18 group sessions in the first year and quarterly maintenance sessions thereafter, touching on a wide variety of nutrition- and behavior-related topics.^7,9 Specially trained and certified nutritionists supervised the dietary intervention and the behavioral modification sessions according to the WHI study protocol.

Control-group participants received a copy of the US Department of Agriculture’s Dietary Guidelines for Americans¹⁰ and other health-related materials. They had no contact with the study nutritionists.

Other arms of the study

The WHI trial design included several arms,^4,11–13 and many participants joined more than one arm: 20,592 postmenopausal women (42.2% of the total enrollment) chose dietary modification only, 8,050 (16.5%) chose diet plus hormone replacement therapy, 25,210 (51.6%) chose diet plus calcium and vitamin D supplementation, and 5,017 (10.3%) enrolled in all three.

Length of follow-up

Participants were followed from enrollment until they died, were lost to follow-up, or requested no further contact, or until the trial’s planned completion date, regardless of adherence to the dietary intervention, according to intention-to-treat analysis. All participants were contacted by clinic staff at 6-month intervals to provide updates on their health outcomes.

Factors assessed

Height, weight, waist circumference, and blood pressure were measured at annual visits using standardized procedures. Fasting blood samples were collected at baseline and at year 1 from all participants and from a subsample of 2,816 women (5.8% of the study population) at years 3 and 6. This subsample was randomly chosen with oversampling of minority women, for whom the odds for selection were six times higher than for white women.

Physical activity was assessed at baseline and at years 1, 3, 6, and 9. Walking and participation in sports and hours of activity per week were calculated for each participant. Physical activity was expressed as metabolic equivalent tasks per week for the analyses.

A food-frequency questionnaire⁶ to assess average dietary intake in the past 3 months was given at baseline and at year 1 for all participants. A third of all participants completed the questionnaire each year in a rotating sample. Completion rates were 100% at baseline and 81% thereafter. Follow-up data were collected from years 5 through 7. Also, 4-day food records were provided by all women before randomization.

HOW OUTCOMES WERE ASSESSED

The primary assessments of clinical outcome^1–3 were mammographic screening, a self-reported medical history documented by a review of medical records, and electrocardiograms digitally obtained every 3 years. Mammograms and electrocardiograms were centrally adjudicated. The diagnosis of acute myocardial infarction was based on an algorithm that included cardiac pain, enzyme levels, and electrocardiographic readings.

OVERALL RESULTS

At 8.1 years, the incidence of breast cancer was 9% lower in the intervention group than in the comparison group (95% confidence interval [CI] = 0.83–1.01; P = .07, P = .09 weighted for length of follow-up).³ Subgroup analysis further showed that women who reported higher intakes of total dietary fat at baseline reduced their risk of breast cancer by 22% (95% CI = 0.64–0.96). Whether extended follow-up will show a significant association has yet to be determined.

Colon cancer rates did not differ between groups, but the number of polyps and adenomas reported was significantly lower in the dietary intervention group.¹ The rate of colon cancer will also be included in the extended follow-up study of the WHI.

Risk factors for coronary heart disease in both groups—including levels of serum total cholesterol and serum low-density lipoprotein cholesterol, body weight, body mass index, diastolic blood pressure, and factor VIIc—improved slightly, but at year 3 of the trial, differences in overall rates of coronary heart disease and stroke in the two groups were not statistically significant.² In addition, the low-fat diet intervention was associated with a reduction in blood estradiol concentrations between baseline and year 1.³ At the end of the study, however, differences in rates of breast cancer, colorectal cancer, and heart disease between the two groups were not statistically significant.

RESULTS OF DIETARY MODIFICATIONS

Fat as a percentage of total calories

At the beginning of the WHI, all participants reported consuming an average of 35% of their caloric intake from fat (Table 1). At 1 year from baseline, the fat intake decreased to 24.3% in the intervention group (short of the study goal of 20%); this level had risen again to 26.7% by year 3 and to 28.8% at the end of the study. Stratified by quartile, women who achieved the greatest reductions in saturated and trans-fatty acids or the largest increases in their intake of fruits and vegetables appeared to have a moderate reduction in the risk of coronary heart disease.² Women in the comparison group also decreased their fat intake initially, but to a lesser degree, and gradually increased it again thereafter. The mean net difference in self-reported total fat intake between the intervention group and the comparison group at 6 years was 8.2% (P < .001) (study goal, 13%).^1–3

Intake of fruits, vegetables, and grains

At baseline, fruit and vegetable intake averaged 3.6 servings per day (Table 1). In the intervention group, this increased to 5.1 servings per day at year 1, and to 5.2 servings at year 3, but at the end of the study it had decreased to 4.9 servings.

Women in the intervention group were eating 4.7 servings of grains per day at baseline. This increased to 5.1 servings at year 1 and then decreased to 4.6 servings at year 3 and to 4.3 servings at the end of the study. It seems that as the women grew older their determination to increase servings of these foods diminished.

Proponents of some currently popular diets blame weight gain on a higher intake of carbohydrates, but the women following the WHI low-fat diet did not gain weight.²

Total fat vs saturated fat

Intake of total fat and saturated fat decreased in the intervention group during the study, but the difference between fat intake in the intervention group and that in the comparison group did not reach the degree expected.

At year 1, total fat as a percentage of total caloric intake was 10.8 percentage points below that of the comparison group, whereas the study expected difference was 13.0. At the end of the trial, the difference was only 8.2 percentage points, whereas the expected difference was 11.0.

Intake of all fatty acids (saturated and unsaturated) decreased at year 1, but then went back up slightly by the end of the trial but did not exceed baseline levels, and saturated fatty acids remained well below baseline levels: 9.5% vs 12.5% of caloric intake at baseline.⁴

INTERPRETING THE RESULTS

It might be tempting to dismiss the results of the WHI dietary intervention trial as not significant and therefore not meaningful. This would be unfortunate. The trial had some remarkable accomplishments and offers important lessons for future investigations.

The initial reductions in total fat intake were impressive, and women who had the highest total fat intake at baseline achieved the greatest reduction of total fat (to less than 22% of total calories).³ Nonetheless, the dietary intervention goal of less than 20% of calories from fat was not achieved despite intensive dietary counseling and a highly motivated study population. Thus, this dietary fat target may not be reasonable in the general population.

Also, despite the absence of targeted intervention on specific fatty acids, the observed blood cholesterol levels were as expected based on the well-known formula of Mensink and Katan,¹⁴ which incorporates information on changes in saturated fat, polyunsaturated fat, and dietary cholesterol intake. The predicted reduction in low-density lipoprotein cholesterol was 2.7 mg/dL; the observed reduction was 2.3 mg/dL.² This illustrates that with greater modifications in specific known dietary risk factors for cardiovascular disease, such as saturated fatty acids, cholesterol, and unsaturated fatty acids, blood cholesterol levels respond in a predictable fashion. This was presumably not observed in WHI precisely because no goals and objectives were provided to participants for intake of saturated or polyunsaturated fatty acids.

Recent findings from the Optimal Macronutrient Intake Trial to Prevent Heart Disease (OmniHeart)¹⁵ further highlight differences in the total cholesterol response to diets of varying macronutrient (carbohydrate, protein, fat) content compared with the WHI dietary intervention.¹⁵ Participants in OmniHeart had reductions in levels of low-density lipoprotein cholesterol that were predictable from the changes reported in intake of saturated fatty acids. Presumably, the results of the WHI intervention would have been similar if the study had included this level of detail.

QUESTIONS REMAIN

Questions from the WHI that need consideration for future clinical applications include whether the study population may have already been “too old” to achieve a benefit from dietary modification, and whether the best timing for dietary intervention might be earlier adulthood with sustained changes in saturated fat, cholesterol, and unsaturated fat intake throughout life. Future subgroup analyses based on age at baseline will need to address these questions. Likewise, a longer follow-up period may be needed for a definitive evaluation of the impact of a regular low-fat diet on different health outcomes.

As reported by Patterson et al,¹⁶ the major contributors to total dietary fat intake at baseline were “added fats” such as sauces, gravies, butter, and margarines (25.1% of fat intake), followed by meats (20.9% of fat intake), and desserts (12.8% of fat intake). These findings highlight target areas for future interventions in women of this age group.

Another issue is how to standardize the dietary intervention from one clinical center to another—ie, to minimize differences in how each clinical center manages the study patients. Such differences were noted in WHI and other studies.¹⁷ Despite standardized training in delivering the dietary intervention, nutritionists encountered regional and cultural differences that required tailoring the dietary intervention to their patients’ needs. Staff turnover, an unavoidable phenomenon in long-term studies, has previously been reported to negatively influence dietary adherence.¹⁸

LIMITATIONS

A major limitation of diet modification research in general is the self-reporting of dietary intake, primarily by a food-frequency questionnaire. Although the use of a questionnaire is the most practical way to obtain dietary data for large studies, systematic biases may exist that obscure true nutrient-outcome relationships.¹⁹ Biomarker studies of energy balance suggest that people who are overweight or obese may under-report energy intake to a greater degree than people who are not overweight.²⁰ Also, we still do not know how to get people to follow a healthy diet, although theories and models abound, such as social learning and cognitive-behavioral theory, and a lack of data limits our understanding of factors related to dietary adherence.^21,22

FUTURE DIRECTIONS IN WHI

The WHI Extension Study is under way and has been funded through the year 2010. Outcomes ascertainment is the primary focus with no ongoing intervention, although the intervention group participants continue to receive a WHI newsletter that simply reiterates the importance of the study and encourages ongoing participation. As of 2006, an estimated 84% of the cohort, including both observational study and clinical trial participants, are involved. Efforts continue to recruit the remaining 16%, but many of these participants now consider themselves too old or too feeble to respond reliably.

In regard to breast cancer, the results published in 2006 are promising, albeit not statistically significant, and definitive statements cannot yet be made. However, postmenopausal women who are eating the diets highest in fat may have the greatest benefit from reductions in total fat.

Other considerations regarding the lack of statistically significant differences between groups may include the possibility that women in the intervention group may have been at lower risk for breast cancer at baseline. Likewise, although the results of the WHI dietary intervention do not include a statistically significant impact on colorectal cancer outcomes, the significant reduction in polyps and adenomas may later translate into a reduction in invasive cancer risk.

Finally, although no significant reduction was seen in the rate of death due to cardiovascular causes, greater reductions in saturated and trans-fatty acid intake were associated with greater reductions in blood cholesterol and cardiovascular risk.

Numerous subgroup analyses and ongoing assessments of the long-term impact of the diet modification are planned. Further associations are expected to emerge. The current and future results will continue to provide new insights that may lead to new clinical and public health recommendations in the future.

The WHI has raised additional issues that warrant further investigation:

• Will earlier dietary intervention, eg, during premenopausal years or even childhood, alter these results?
• Does the low-fat, high-carbohydrate diet used in WHI facilitate weight maintenance or even weight loss, as proposed by Howard et al²³?
• Do quantitative changes in physical activity and weight control attenuate morbidity and mortality rates beyond changes in diet alone?
• Do vitamin and mineral supplements or hormone therapy alter disease outcomes or quality of life?
• Which behavioral approaches are best suited to the recruitment of patients for dietary intervention trials?

More than 2 years have passed since we published the results of the Women’s Health Initiative (WHI), which caused a storm of information—and misinformation—about the effect of long-term dietary intervention on disease outcomes in postmenopausal women. Now that the dust has long settled, what have we learned from this landmark study?

The WHI results led to numerous additional analyses of all aspects of the study.^1–7 What are the implications of all the analyses to clinical practice?

In this article, we summarize key aspects of the clinical trial, including study design, interventions, main results, and future plans. We also discuss potential clinical applications and practical considerations for public health efforts.

WHO WAS ELIGIBLE, WHO WAS NOT

A total of 48,835 postmenopausal women were randomly assigned to either no dietary intervention (n = 29,294) or a dietary intervention (n = 19,541) (see below).⁷ Participants were followed at 40 clinical centers between 1993 and 2005.⁴ Their mean age was 62.3 years; 18.6% were members of minorities.

Women were eligible if they were post-menopausal and had a daily dietary fat intake of at least 32% of total calories, based on assessment via a food-frequency questionnaire. They were excluded from the study if they had any of the following: a history of breast cancer, colorectal cancer, or other cancer except skin cancer during the past 10 years; type 1 diabetes; a medical condition in which the predicted survival was less than 3 years; and a potential barrier to adherence to the study regimen, including alcoholism or a lifestyle that involved often eating meals away from home.

THE WHI DIET: LESS FAT, BUT MORE FRUITS, VEGETABLES, GRAINS

The WHI dietary intervention was designed to prevent breast cancer, based on the evidence available when the study was planned. The targets included a total fat intake of less than 20% of energy (in kilocalories), increasing the intake of fruits and vegetables to at least five servings per day, and increasing the intake of grains to at least six servings per day.

Although reduction in saturated fat intake per se was not part of the WHI protocol, we assumed from previous pilot studies⁸ that the reduction of total fat intake would simultaneously produce a reduction in saturated fat intake to 7% of total calories.

A simpler dietary intervention

Unlike the 2006 American Heart Association guidelines and the US Department of Agriculture’s Dietary Guidelines for Americans 2005, the WHI dietary intervention had no specifications for dietary fiber, specific fatty acids (trans-fatty acids, omega-3 fatty acids, conjugated linoleic acid), complex carbohydrates, whole grains, vegetable protein, or other factors that have emerged as potential risk factors for cardiovascular and other chronic diseases since the study began. The WHI intervention also included no specific recommendation for total calorie intake, nor were patients in the intervention group encouraged to lose weight, as this could have confounded the results of the dietary intervention.

Education and encouragement

Those in the intervention group were each assigned a fat-gram goal, calculated on the basis of height. They were taught how to monitor their intake of total fat, fruits, vegetables, and grains. They attended intensive behavioral modification sessions to encourage them to keep to the dietary program: 18 group sessions in the first year and quarterly maintenance sessions thereafter, touching on a wide variety of nutrition- and behavior-related topics.^7,9 Specially trained and certified nutritionists supervised the dietary intervention and the behavioral modification sessions according to the WHI study protocol.

Control-group participants received a copy of the US Department of Agriculture’s Dietary Guidelines for Americans¹⁰ and other health-related materials. They had no contact with the study nutritionists.

Other arms of the study

The WHI trial design included several arms,^4,11–13 and many participants joined more than one arm: 20,592 postmenopausal women (42.2% of the total enrollment) chose dietary modification only, 8,050 (16.5%) chose diet plus hormone replacement therapy, 25,210 (51.6%) chose diet plus calcium and vitamin D supplementation, and 5,017 (10.3%) enrolled in all three.

Length of follow-up

Participants were followed from enrollment until they died, were lost to follow-up, or requested no further contact, or until the trial’s planned completion date, regardless of adherence to the dietary intervention, according to intention-to-treat analysis. All participants were contacted by clinic staff at 6-month intervals to provide updates on their health outcomes.

Factors assessed

Height, weight, waist circumference, and blood pressure were measured at annual visits using standardized procedures. Fasting blood samples were collected at baseline and at year 1 from all participants and from a subsample of 2,816 women (5.8% of the study population) at years 3 and 6. This subsample was randomly chosen with oversampling of minority women, for whom the odds for selection were six times higher than for white women.

Physical activity was assessed at baseline and at years 1, 3, 6, and 9. Walking and participation in sports and hours of activity per week were calculated for each participant. Physical activity was expressed as metabolic equivalent tasks per week for the analyses.

A food-frequency questionnaire⁶ to assess average dietary intake in the past 3 months was given at baseline and at year 1 for all participants. A third of all participants completed the questionnaire each year in a rotating sample. Completion rates were 100% at baseline and 81% thereafter. Follow-up data were collected from years 5 through 7. Also, 4-day food records were provided by all women before randomization.

HOW OUTCOMES WERE ASSESSED

The primary assessments of clinical outcome^1–3 were mammographic screening, a self-reported medical history documented by a review of medical records, and electrocardiograms digitally obtained every 3 years. Mammograms and electrocardiograms were centrally adjudicated. The diagnosis of acute myocardial infarction was based on an algorithm that included cardiac pain, enzyme levels, and electrocardiographic readings.

OVERALL RESULTS

At 8.1 years, the incidence of breast cancer was 9% lower in the intervention group than in the comparison group (95% confidence interval [CI] = 0.83–1.01; P = .07, P = .09 weighted for length of follow-up).³ Subgroup analysis further showed that women who reported higher intakes of total dietary fat at baseline reduced their risk of breast cancer by 22% (95% CI = 0.64–0.96). Whether extended follow-up will show a significant association has yet to be determined.

Colon cancer rates did not differ between groups, but the number of polyps and adenomas reported was significantly lower in the dietary intervention group.¹ The rate of colon cancer will also be included in the extended follow-up study of the WHI.

Risk factors for coronary heart disease in both groups—including levels of serum total cholesterol and serum low-density lipoprotein cholesterol, body weight, body mass index, diastolic blood pressure, and factor VIIc—improved slightly, but at year 3 of the trial, differences in overall rates of coronary heart disease and stroke in the two groups were not statistically significant.² In addition, the low-fat diet intervention was associated with a reduction in blood estradiol concentrations between baseline and year 1.³ At the end of the study, however, differences in rates of breast cancer, colorectal cancer, and heart disease between the two groups were not statistically significant.

RESULTS OF DIETARY MODIFICATIONS

Fat as a percentage of total calories

At the beginning of the WHI, all participants reported consuming an average of 35% of their caloric intake from fat (Table 1). At 1 year from baseline, the fat intake decreased to 24.3% in the intervention group (short of the study goal of 20%); this level had risen again to 26.7% by year 3 and to 28.8% at the end of the study. Stratified by quartile, women who achieved the greatest reductions in saturated and trans-fatty acids or the largest increases in their intake of fruits and vegetables appeared to have a moderate reduction in the risk of coronary heart disease.² Women in the comparison group also decreased their fat intake initially, but to a lesser degree, and gradually increased it again thereafter. The mean net difference in self-reported total fat intake between the intervention group and the comparison group at 6 years was 8.2% (P < .001) (study goal, 13%).^1–3

Intake of fruits, vegetables, and grains

At baseline, fruit and vegetable intake averaged 3.6 servings per day (Table 1). In the intervention group, this increased to 5.1 servings per day at year 1, and to 5.2 servings at year 3, but at the end of the study it had decreased to 4.9 servings.

Women in the intervention group were eating 4.7 servings of grains per day at baseline. This increased to 5.1 servings at year 1 and then decreased to 4.6 servings at year 3 and to 4.3 servings at the end of the study. It seems that as the women grew older their determination to increase servings of these foods diminished.

Proponents of some currently popular diets blame weight gain on a higher intake of carbohydrates, but the women following the WHI low-fat diet did not gain weight.²

Total fat vs saturated fat

Intake of total fat and saturated fat decreased in the intervention group during the study, but the difference between fat intake in the intervention group and that in the comparison group did not reach the degree expected.

At year 1, total fat as a percentage of total caloric intake was 10.8 percentage points below that of the comparison group, whereas the study expected difference was 13.0. At the end of the trial, the difference was only 8.2 percentage points, whereas the expected difference was 11.0.

Intake of all fatty acids (saturated and unsaturated) decreased at year 1, but then went back up slightly by the end of the trial but did not exceed baseline levels, and saturated fatty acids remained well below baseline levels: 9.5% vs 12.5% of caloric intake at baseline.⁴

INTERPRETING THE RESULTS

It might be tempting to dismiss the results of the WHI dietary intervention trial as not significant and therefore not meaningful. This would be unfortunate. The trial had some remarkable accomplishments and offers important lessons for future investigations.

The initial reductions in total fat intake were impressive, and women who had the highest total fat intake at baseline achieved the greatest reduction of total fat (to less than 22% of total calories).³ Nonetheless, the dietary intervention goal of less than 20% of calories from fat was not achieved despite intensive dietary counseling and a highly motivated study population. Thus, this dietary fat target may not be reasonable in the general population.

Also, despite the absence of targeted intervention on specific fatty acids, the observed blood cholesterol levels were as expected based on the well-known formula of Mensink and Katan,¹⁴ which incorporates information on changes in saturated fat, polyunsaturated fat, and dietary cholesterol intake. The predicted reduction in low-density lipoprotein cholesterol was 2.7 mg/dL; the observed reduction was 2.3 mg/dL.² This illustrates that with greater modifications in specific known dietary risk factors for cardiovascular disease, such as saturated fatty acids, cholesterol, and unsaturated fatty acids, blood cholesterol levels respond in a predictable fashion. This was presumably not observed in WHI precisely because no goals and objectives were provided to participants for intake of saturated or polyunsaturated fatty acids.

Recent findings from the Optimal Macronutrient Intake Trial to Prevent Heart Disease (OmniHeart)¹⁵ further highlight differences in the total cholesterol response to diets of varying macronutrient (carbohydrate, protein, fat) content compared with the WHI dietary intervention.¹⁵ Participants in OmniHeart had reductions in levels of low-density lipoprotein cholesterol that were predictable from the changes reported in intake of saturated fatty acids. Presumably, the results of the WHI intervention would have been similar if the study had included this level of detail.

QUESTIONS REMAIN

Questions from the WHI that need consideration for future clinical applications include whether the study population may have already been “too old” to achieve a benefit from dietary modification, and whether the best timing for dietary intervention might be earlier adulthood with sustained changes in saturated fat, cholesterol, and unsaturated fat intake throughout life. Future subgroup analyses based on age at baseline will need to address these questions. Likewise, a longer follow-up period may be needed for a definitive evaluation of the impact of a regular low-fat diet on different health outcomes.

As reported by Patterson et al,¹⁶ the major contributors to total dietary fat intake at baseline were “added fats” such as sauces, gravies, butter, and margarines (25.1% of fat intake), followed by meats (20.9% of fat intake), and desserts (12.8% of fat intake). These findings highlight target areas for future interventions in women of this age group.

Another issue is how to standardize the dietary intervention from one clinical center to another—ie, to minimize differences in how each clinical center manages the study patients. Such differences were noted in WHI and other studies.¹⁷ Despite standardized training in delivering the dietary intervention, nutritionists encountered regional and cultural differences that required tailoring the dietary intervention to their patients’ needs. Staff turnover, an unavoidable phenomenon in long-term studies, has previously been reported to negatively influence dietary adherence.¹⁸

LIMITATIONS

A major limitation of diet modification research in general is the self-reporting of dietary intake, primarily by a food-frequency questionnaire. Although the use of a questionnaire is the most practical way to obtain dietary data for large studies, systematic biases may exist that obscure true nutrient-outcome relationships.¹⁹ Biomarker studies of energy balance suggest that people who are overweight or obese may under-report energy intake to a greater degree than people who are not overweight.²⁰ Also, we still do not know how to get people to follow a healthy diet, although theories and models abound, such as social learning and cognitive-behavioral theory, and a lack of data limits our understanding of factors related to dietary adherence.^21,22

FUTURE DIRECTIONS IN WHI

The WHI Extension Study is under way and has been funded through the year 2010. Outcomes ascertainment is the primary focus with no ongoing intervention, although the intervention group participants continue to receive a WHI newsletter that simply reiterates the importance of the study and encourages ongoing participation. As of 2006, an estimated 84% of the cohort, including both observational study and clinical trial participants, are involved. Efforts continue to recruit the remaining 16%, but many of these participants now consider themselves too old or too feeble to respond reliably.

In regard to breast cancer, the results published in 2006 are promising, albeit not statistically significant, and definitive statements cannot yet be made. However, postmenopausal women who are eating the diets highest in fat may have the greatest benefit from reductions in total fat.

Other considerations regarding the lack of statistically significant differences between groups may include the possibility that women in the intervention group may have been at lower risk for breast cancer at baseline. Likewise, although the results of the WHI dietary intervention do not include a statistically significant impact on colorectal cancer outcomes, the significant reduction in polyps and adenomas may later translate into a reduction in invasive cancer risk.

Finally, although no significant reduction was seen in the rate of death due to cardiovascular causes, greater reductions in saturated and trans-fatty acid intake were associated with greater reductions in blood cholesterol and cardiovascular risk.

Numerous subgroup analyses and ongoing assessments of the long-term impact of the diet modification are planned. Further associations are expected to emerge. The current and future results will continue to provide new insights that may lead to new clinical and public health recommendations in the future.

The WHI has raised additional issues that warrant further investigation:

• Will earlier dietary intervention, eg, during premenopausal years or even childhood, alter these results?
• Does the low-fat, high-carbohydrate diet used in WHI facilitate weight maintenance or even weight loss, as proposed by Howard et al²³?
• Do quantitative changes in physical activity and weight control attenuate morbidity and mortality rates beyond changes in diet alone?
• Do vitamin and mineral supplements or hormone therapy alter disease outcomes or quality of life?
• Which behavioral approaches are best suited to the recruitment of patients for dietary intervention trials?

More than 2 years have passed since we published the results of the Women’s Health Initiative (WHI), which caused a storm of information—and misinformation—about the effect of long-term dietary intervention on disease outcomes in postmenopausal women. Now that the dust has long settled, what have we learned from this landmark study?

The WHI results led to numerous additional analyses of all aspects of the study.^1–7 What are the implications of all the analyses to clinical practice?

In this article, we summarize key aspects of the clinical trial, including study design, interventions, main results, and future plans. We also discuss potential clinical applications and practical considerations for public health efforts.

WHO WAS ELIGIBLE, WHO WAS NOT

A total of 48,835 postmenopausal women were randomly assigned to either no dietary intervention (n = 29,294) or a dietary intervention (n = 19,541) (see below).⁷ Participants were followed at 40 clinical centers between 1993 and 2005.⁴ Their mean age was 62.3 years; 18.6% were members of minorities.

Women were eligible if they were post-menopausal and had a daily dietary fat intake of at least 32% of total calories, based on assessment via a food-frequency questionnaire. They were excluded from the study if they had any of the following: a history of breast cancer, colorectal cancer, or other cancer except skin cancer during the past 10 years; type 1 diabetes; a medical condition in which the predicted survival was less than 3 years; and a potential barrier to adherence to the study regimen, including alcoholism or a lifestyle that involved often eating meals away from home.

THE WHI DIET: LESS FAT, BUT MORE FRUITS, VEGETABLES, GRAINS

The WHI dietary intervention was designed to prevent breast cancer, based on the evidence available when the study was planned. The targets included a total fat intake of less than 20% of energy (in kilocalories), increasing the intake of fruits and vegetables to at least five servings per day, and increasing the intake of grains to at least six servings per day.

Although reduction in saturated fat intake per se was not part of the WHI protocol, we assumed from previous pilot studies⁸ that the reduction of total fat intake would simultaneously produce a reduction in saturated fat intake to 7% of total calories.

A simpler dietary intervention

Unlike the 2006 American Heart Association guidelines and the US Department of Agriculture’s Dietary Guidelines for Americans 2005, the WHI dietary intervention had no specifications for dietary fiber, specific fatty acids (trans-fatty acids, omega-3 fatty acids, conjugated linoleic acid), complex carbohydrates, whole grains, vegetable protein, or other factors that have emerged as potential risk factors for cardiovascular and other chronic diseases since the study began. The WHI intervention also included no specific recommendation for total calorie intake, nor were patients in the intervention group encouraged to lose weight, as this could have confounded the results of the dietary intervention.

Education and encouragement

Those in the intervention group were each assigned a fat-gram goal, calculated on the basis of height. They were taught how to monitor their intake of total fat, fruits, vegetables, and grains. They attended intensive behavioral modification sessions to encourage them to keep to the dietary program: 18 group sessions in the first year and quarterly maintenance sessions thereafter, touching on a wide variety of nutrition- and behavior-related topics.^7,9 Specially trained and certified nutritionists supervised the dietary intervention and the behavioral modification sessions according to the WHI study protocol.

Control-group participants received a copy of the US Department of Agriculture’s Dietary Guidelines for Americans¹⁰ and other health-related materials. They had no contact with the study nutritionists.

Other arms of the study

The WHI trial design included several arms,^4,11–13 and many participants joined more than one arm: 20,592 postmenopausal women (42.2% of the total enrollment) chose dietary modification only, 8,050 (16.5%) chose diet plus hormone replacement therapy, 25,210 (51.6%) chose diet plus calcium and vitamin D supplementation, and 5,017 (10.3%) enrolled in all three.

Length of follow-up

Participants were followed from enrollment until they died, were lost to follow-up, or requested no further contact, or until the trial’s planned completion date, regardless of adherence to the dietary intervention, according to intention-to-treat analysis. All participants were contacted by clinic staff at 6-month intervals to provide updates on their health outcomes.

Factors assessed

Height, weight, waist circumference, and blood pressure were measured at annual visits using standardized procedures. Fasting blood samples were collected at baseline and at year 1 from all participants and from a subsample of 2,816 women (5.8% of the study population) at years 3 and 6. This subsample was randomly chosen with oversampling of minority women, for whom the odds for selection were six times higher than for white women.

Physical activity was assessed at baseline and at years 1, 3, 6, and 9. Walking and participation in sports and hours of activity per week were calculated for each participant. Physical activity was expressed as metabolic equivalent tasks per week for the analyses.

A food-frequency questionnaire⁶ to assess average dietary intake in the past 3 months was given at baseline and at year 1 for all participants. A third of all participants completed the questionnaire each year in a rotating sample. Completion rates were 100% at baseline and 81% thereafter. Follow-up data were collected from years 5 through 7. Also, 4-day food records were provided by all women before randomization.

HOW OUTCOMES WERE ASSESSED

The primary assessments of clinical outcome^1–3 were mammographic screening, a self-reported medical history documented by a review of medical records, and electrocardiograms digitally obtained every 3 years. Mammograms and electrocardiograms were centrally adjudicated. The diagnosis of acute myocardial infarction was based on an algorithm that included cardiac pain, enzyme levels, and electrocardiographic readings.

OVERALL RESULTS

At 8.1 years, the incidence of breast cancer was 9% lower in the intervention group than in the comparison group (95% confidence interval [CI] = 0.83–1.01; P = .07, P = .09 weighted for length of follow-up).³ Subgroup analysis further showed that women who reported higher intakes of total dietary fat at baseline reduced their risk of breast cancer by 22% (95% CI = 0.64–0.96). Whether extended follow-up will show a significant association has yet to be determined.

Colon cancer rates did not differ between groups, but the number of polyps and adenomas reported was significantly lower in the dietary intervention group.¹ The rate of colon cancer will also be included in the extended follow-up study of the WHI.

Risk factors for coronary heart disease in both groups—including levels of serum total cholesterol and serum low-density lipoprotein cholesterol, body weight, body mass index, diastolic blood pressure, and factor VIIc—improved slightly, but at year 3 of the trial, differences in overall rates of coronary heart disease and stroke in the two groups were not statistically significant.² In addition, the low-fat diet intervention was associated with a reduction in blood estradiol concentrations between baseline and year 1.³ At the end of the study, however, differences in rates of breast cancer, colorectal cancer, and heart disease between the two groups were not statistically significant.

RESULTS OF DIETARY MODIFICATIONS

Fat as a percentage of total calories

At the beginning of the WHI, all participants reported consuming an average of 35% of their caloric intake from fat (Table 1). At 1 year from baseline, the fat intake decreased to 24.3% in the intervention group (short of the study goal of 20%); this level had risen again to 26.7% by year 3 and to 28.8% at the end of the study. Stratified by quartile, women who achieved the greatest reductions in saturated and trans-fatty acids or the largest increases in their intake of fruits and vegetables appeared to have a moderate reduction in the risk of coronary heart disease.² Women in the comparison group also decreased their fat intake initially, but to a lesser degree, and gradually increased it again thereafter. The mean net difference in self-reported total fat intake between the intervention group and the comparison group at 6 years was 8.2% (P < .001) (study goal, 13%).^1–3

Intake of fruits, vegetables, and grains

At baseline, fruit and vegetable intake averaged 3.6 servings per day (Table 1). In the intervention group, this increased to 5.1 servings per day at year 1, and to 5.2 servings at year 3, but at the end of the study it had decreased to 4.9 servings.

Women in the intervention group were eating 4.7 servings of grains per day at baseline. This increased to 5.1 servings at year 1 and then decreased to 4.6 servings at year 3 and to 4.3 servings at the end of the study. It seems that as the women grew older their determination to increase servings of these foods diminished.

Proponents of some currently popular diets blame weight gain on a higher intake of carbohydrates, but the women following the WHI low-fat diet did not gain weight.²

Total fat vs saturated fat

Intake of total fat and saturated fat decreased in the intervention group during the study, but the difference between fat intake in the intervention group and that in the comparison group did not reach the degree expected.

At year 1, total fat as a percentage of total caloric intake was 10.8 percentage points below that of the comparison group, whereas the study expected difference was 13.0. At the end of the trial, the difference was only 8.2 percentage points, whereas the expected difference was 11.0.

Intake of all fatty acids (saturated and unsaturated) decreased at year 1, but then went back up slightly by the end of the trial but did not exceed baseline levels, and saturated fatty acids remained well below baseline levels: 9.5% vs 12.5% of caloric intake at baseline.⁴

INTERPRETING THE RESULTS

It might be tempting to dismiss the results of the WHI dietary intervention trial as not significant and therefore not meaningful. This would be unfortunate. The trial had some remarkable accomplishments and offers important lessons for future investigations.

The initial reductions in total fat intake were impressive, and women who had the highest total fat intake at baseline achieved the greatest reduction of total fat (to less than 22% of total calories).³ Nonetheless, the dietary intervention goal of less than 20% of calories from fat was not achieved despite intensive dietary counseling and a highly motivated study population. Thus, this dietary fat target may not be reasonable in the general population.

Also, despite the absence of targeted intervention on specific fatty acids, the observed blood cholesterol levels were as expected based on the well-known formula of Mensink and Katan,¹⁴ which incorporates information on changes in saturated fat, polyunsaturated fat, and dietary cholesterol intake. The predicted reduction in low-density lipoprotein cholesterol was 2.7 mg/dL; the observed reduction was 2.3 mg/dL.² This illustrates that with greater modifications in specific known dietary risk factors for cardiovascular disease, such as saturated fatty acids, cholesterol, and unsaturated fatty acids, blood cholesterol levels respond in a predictable fashion. This was presumably not observed in WHI precisely because no goals and objectives were provided to participants for intake of saturated or polyunsaturated fatty acids.

Recent findings from the Optimal Macronutrient Intake Trial to Prevent Heart Disease (OmniHeart)¹⁵ further highlight differences in the total cholesterol response to diets of varying macronutrient (carbohydrate, protein, fat) content compared with the WHI dietary intervention.¹⁵ Participants in OmniHeart had reductions in levels of low-density lipoprotein cholesterol that were predictable from the changes reported in intake of saturated fatty acids. Presumably, the results of the WHI intervention would have been similar if the study had included this level of detail.

QUESTIONS REMAIN

Questions from the WHI that need consideration for future clinical applications include whether the study population may have already been “too old” to achieve a benefit from dietary modification, and whether the best timing for dietary intervention might be earlier adulthood with sustained changes in saturated fat, cholesterol, and unsaturated fat intake throughout life. Future subgroup analyses based on age at baseline will need to address these questions. Likewise, a longer follow-up period may be needed for a definitive evaluation of the impact of a regular low-fat diet on different health outcomes.

As reported by Patterson et al,¹⁶ the major contributors to total dietary fat intake at baseline were “added fats” such as sauces, gravies, butter, and margarines (25.1% of fat intake), followed by meats (20.9% of fat intake), and desserts (12.8% of fat intake). These findings highlight target areas for future interventions in women of this age group.

Another issue is how to standardize the dietary intervention from one clinical center to another—ie, to minimize differences in how each clinical center manages the study patients. Such differences were noted in WHI and other studies.¹⁷ Despite standardized training in delivering the dietary intervention, nutritionists encountered regional and cultural differences that required tailoring the dietary intervention to their patients’ needs. Staff turnover, an unavoidable phenomenon in long-term studies, has previously been reported to negatively influence dietary adherence.¹⁸

LIMITATIONS

A major limitation of diet modification research in general is the self-reporting of dietary intake, primarily by a food-frequency questionnaire. Although the use of a questionnaire is the most practical way to obtain dietary data for large studies, systematic biases may exist that obscure true nutrient-outcome relationships.¹⁹ Biomarker studies of energy balance suggest that people who are overweight or obese may under-report energy intake to a greater degree than people who are not overweight.²⁰ Also, we still do not know how to get people to follow a healthy diet, although theories and models abound, such as social learning and cognitive-behavioral theory, and a lack of data limits our understanding of factors related to dietary adherence.^21,22

FUTURE DIRECTIONS IN WHI

The WHI Extension Study is under way and has been funded through the year 2010. Outcomes ascertainment is the primary focus with no ongoing intervention, although the intervention group participants continue to receive a WHI newsletter that simply reiterates the importance of the study and encourages ongoing participation. As of 2006, an estimated 84% of the cohort, including both observational study and clinical trial participants, are involved. Efforts continue to recruit the remaining 16%, but many of these participants now consider themselves too old or too feeble to respond reliably.

In regard to breast cancer, the results published in 2006 are promising, albeit not statistically significant, and definitive statements cannot yet be made. However, postmenopausal women who are eating the diets highest in fat may have the greatest benefit from reductions in total fat.

Other considerations regarding the lack of statistically significant differences between groups may include the possibility that women in the intervention group may have been at lower risk for breast cancer at baseline. Likewise, although the results of the WHI dietary intervention do not include a statistically significant impact on colorectal cancer outcomes, the significant reduction in polyps and adenomas may later translate into a reduction in invasive cancer risk.

Finally, although no significant reduction was seen in the rate of death due to cardiovascular causes, greater reductions in saturated and trans-fatty acid intake were associated with greater reductions in blood cholesterol and cardiovascular risk.

Numerous subgroup analyses and ongoing assessments of the long-term impact of the diet modification are planned. Further associations are expected to emerge. The current and future results will continue to provide new insights that may lead to new clinical and public health recommendations in the future.

The WHI has raised additional issues that warrant further investigation:

• Will earlier dietary intervention, eg, during premenopausal years or even childhood, alter these results?
• Does the low-fat, high-carbohydrate diet used in WHI facilitate weight maintenance or even weight loss, as proposed by Howard et al²³?
• Do quantitative changes in physical activity and weight control attenuate morbidity and mortality rates beyond changes in diet alone?
• Do vitamin and mineral supplements or hormone therapy alter disease outcomes or quality of life?
• Which behavioral approaches are best suited to the recruitment of patients for dietary intervention trials?

The “gliptins”—the nickname for dipeptidyl peptidase 4 (DPP-4) inhibitors—are one of the newest classes of drugs for the treatment of type 2 diabetes mellitus.

These drugs work by prolonging the action of gut hormones called incretins, which boost insulin levels. The greatest advantage of the gliptins appears to be their ability to stimulate insulin production with little risk of corresponding hypoglycemia.

Sitagliptin (Januvia), the first commercially available DPP-4 inhibitor, has been approved by the US Food and Drug Administration (FDA) and is currently in clinical use, and vildagliptin (Galvus) awaits FDA approval at the time of this writing. Other drugs of this class are in development.

However, because these drugs are so new, a number of questions remain about their use. In this article, we discuss the rationale behind gliptin drugs, the evidence to date on their use alone or in combination with current oral hypoglycemic drugs (and even with insulin), and when and how to use them in daily practice.

THE NEED FOR MORE EFFECTIVE DIABETES TREATMENT

As the number of patients with type 2 diabetes continues its steep and steady rise,^1,2 much work has gone into studying treatment goals and how to achieve them. Although experts generally agree on glycemic goals,³ we currently fail to achieve those goals in close to two-thirds of patients: only 37% have a hemo-globin A_1c (HbA_1c) value at or below the goal of 7%, and the same number have levels exceeding 8%.⁴

Part of the problem is that treatment regimens are not adjusted in a timely fashion. In a prescribing database of almost 4,000 patients with type 2 diabetes,⁵ the mean time from the first HbA_1c reading above 8% to an actual change in therapy was about 15 months for those taking metformin (Glucophage) alone, and 21 months for those taking a sulfonylurea alone. Another part of the problem is that, on average, patients with an HbA_1c of 8.0% to 8.9% can expect only a 0.6% lowering with the addition of one agent.⁶ Clearly, we need new pharmacologic approaches and new management paradigms. One new approach is the use of gliptins.

HOW GLIPTINS WORK

Incretins promote insulin secretion

We have known for more than 20 years that insulin levels rise considerably higher in response to an oral glucose load than to an intravenous glucose infusion, even though the plasma glucose concentrations may be similar.⁷ This phenomenon involves a myriad of neural and nutritional factors, but the gut hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) appear to be key.

These peptides—called incretins—have a high degree of homology, and both promote insulin secretion. However, GLP-1, produced by the L cells of the ileum and colon, inhibits glucagon secretion and slows gastric emptying, whereas GIP, secreted from the K cells of the duodenum, has no effect on glucagon and little effect on gastric emptying. Both peptides appear to promote pancreatic beta cell growth and survival,^8,9 an effect that in theory might allow us to slow the progressive loss of insulin secretory capacity in type 2 diabetes.

Furthermore, the effect of GLP-1 on insulin secretion depends on the plasma glucose concentration, with a greater insulin secretory effect at higher glucose levels and minimal effect at euglycemic levels.¹⁰ This phenomenon suggests that drugs that boost GLP-1 activity should not cause the troublesome hypoglycemia typically seen in patients taking insulin, insulin secretagogues, sulfonyl-ureas, or the meglitinides repaglinide (Prandin) or nateglinide (Starlix). Studies of combination treatment with metformin and the GLP-1 receptor agonist exenatide (Byetta) have shown little risk of hypoglycemia,¹¹ offering evidence favoring this conjecture.

Inhibition of DPP-4 boosts incretin action

The challenge for creating treatments that take advantage of the beneficial effects of GLP-1 and GIP is that they have very short physiologic half-lives, ie, less than 10 minutes. GLP-1 and GIP both have two N-terminal amino acids that are quickly cleaved by DPP-4,¹² an enzyme present in the circulation¹³ and on endothelial cells.¹⁴

Currently, there are two classes of drugs based on incretins. One class, the incretin mimetics or GLP-1 receptor agonists, includes drugs that mimic the effect of GLP-1 but are not so quickly degraded by DPP-4. Examples of these drugs are exenatide, which is currently FDA-approved, and liraglutide, which is not yet approved.

On the other hand, by inhibiting the cleaving action of DPP-4, the gliptins can prolong the half-life of endogenous GLP-1, increasing its physiologic effects.

Studies comparing gliptins with GLP-1 receptor agonists are only at the preclinical phase. Liraglutide showed an antiglycemic effect similar to that of vildagliptin in an animal model of glucose intolerance.¹⁵ This and other^16,17 preclinical studies have shown evidence of improved beta cell growth and survival with DPP-4 inhibitor treatment, to an extent similar to that reported with thiazo-lidinediones, whereas sulfonylureas show no evidence either of increase in beta cells or of improved intrinsic beta cell secretory function in these models. Of course, animal studies can only be cautiously extrapolated to potential effects in humans, and it is uncertain whether such benefits will occur with the therapeutic use of DPP-4 inhibitors.

RANDOMIZED CLINICAL TRIALS OF SITAGLIPTIN

Sitagliptin is effective when used by itself,reducing a baseline HbA1c level of about 8% by 0.6% to 0.8%,^19,20,24 and is similarly effective when combined with metformin^21,22,25 or pioglitazone (Actos, a thiazolidinedione).²³ It also decreases fasting blood glucose levels and improves other measures of glucose control.

A study comparing sitagliptin and the sul-fonylurea glipizide (Glucotrol) showed identical glucose-lowering over a 1-year period, with less hypoglycemia and weight gain with sitagliptin.²⁵ Hypoglycemic episodes occurred in 32% of patients taking glipizide but in only 5% of those taking sitagliptin.

Studies noted several trends in laboratory values, though none was associated with clinical evidence of adverse outcome:

• White blood cell counts were noted to increase in three of the studies by 4.7% to 10%, owing to increases in neutro-phils^19,20,22
• Alkaline phosphatase concentrations decreased in four studies^19,20,22,23
• Uric acid levels increased in four studies.^19,20,22,23

RENAL INSUFFICIENCY SLOWS SITAGLIPTIN CLEARANCE

Lower doses and periodic monitoring of renal function are recommended in patients taking sitagliptin who have some degree of renal insufficiency. Clearance of sitagliptin is delayed in patients with renal insufficiency (creatinine clearance < 50 mL/minute).

In a placebo-controlled study of sitagliptin safety, Scott et al²⁶ found that the area under the sitagliptin concentration-time curve was 2.3 times greater in patients with moderate renal insufficiency (creatinine clearance rate 30–49.9 mL/minute), 3.8 times greater in those with severe renal insufficiency (15–29.9 mL/minute), and 4.5 times greater in those with end-stage renal disease (< 15 mL/minute).

The Januvia package insert²⁷ recommends that the daily dose be decreased to 50 mg in patients with creatinine clearance rates of 30 to 49.9 mL/minute (serum creatinine > 1.7 mg/dL in men, > 1.5 mg/dL in women), and that the dose be decreased to 25 mg per day in those with creatinine clearance rates below 30 mL/minute (creatinine > 3.0/2.5 mg/dL).

CLINICAL TRIALS OF VILDAGLIPTIN BEGIN

A study comparing vildagliptin against metformin³⁴ showed less glucose-lowering over a 1-year period with vildagliptin, albeit with fewer gastrointestinal side effects, while comparisons with rosiglitazone (Avandia)³⁵ and with pioglitazone³⁶ showed similar glucose-lowering ability.

In a 24-week study,³³ 256 patients with type 2 diabetes who had a mean body mass index of 33 kg/m² and who were taking more than 30 units of insulin daily (an average of 82 units) were randomized to additionally receive either vildagliptin 50 mg twice daily or placebo. The HbA_1c decreased by 0.5% with vildagliptin and by 0.2% with placebo, from a baseline level of 8.5%. Of interest, 33 patients receiving vildagliptin had a hypo-glycemic episode (a total of 113 events), compared with 45 patients in the placebo group (185 events). None of the episodes in the vildagliptin group was classified as severe, whereas six episodes in the placebo group were classified as severe. This suggests that adding vildagliptin in patients taking insulin can improve glycemia without causing excessive hypoglycemia.

A weakness of the design of this study is that it did not include patients who were receiving an insulin sensitizer, an approach that is typically taken. Given this, it is understandable that overall glycemic control was relatively poor. More effort is needed to explore the use of gliptins with insulin.

WHAT ROLE FOR GLIPTINS?

The evidence from the studies reviewed in this article suggests that gliptins can play an important role in the treatment of type 2 diabetes. In certain patient groups such as the elderly, who cannot take either metformin or a thiazolidinedione and in whom concerns about hypoglycemia are greatest, thus precluding sulfonylurea therapy, gliptins may be the agents of choice. The trials reviewed here suggest that gliptins have glucose-lowering efficacy similar to that of these classes of agents. Gliptins are also effective when combined with metformin or a thiazolidinedione and, as discussed above, may prove to be useful in combination with insulin.

The eventual role of gliptins in the treatment of type 2 diabetes will depend on the answers to several questions. For example, do they preserve beta cell function and reverse the progression of diabetes? Do they affect insulin resistance? Do they have cardiovascular benefits beyond glucose-lowering? Also, since DPP-4 is widely distributed in the body, and since we do not yet know the effects of all the proteins cleaved by this enzyme, will this affect the long-term safety of these drugs?

For now, we can state with reasonable certainty that gliptins lower blood sugar levels to a degree similar to that of other oral hypo-glycemic therapies, with minimal risk of hypo-glycemia, with few immediate adverse effects, and without requiring dose titration. These characteristics suggest that gliptins should be considered useful agents in monotherapy and combination therapy for the treatment of type 2 diabetes.