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Molecular Profiles Guide Colorectal Cancer Treatment
Colorectal cancer (CRC) is the third leading cause of cancer-related death in veterans, despite significant advances in treatment options.1,2 Over the past 20 years, the median survival of patients with metastatic CRC (mCRC), has improved with the most recent clinical trials demonstrating a median overall survival (OS) of up to 29 months.3
In addition to standard chemotherapeutic regimens using 5-fluorouracil, oxaliplatin, and irinotecan, biologic therapies have resulted in improved OS for patients with mCRC. These therapies include the human vascular endothelial growth factor (VEGF) monoclonal antibody bevacizumab and the epidermal growth factor receptor (EGFR) monoclonal antibodies cetuximab and panitumumab. Additional agents, including aflibercept, ramucirumab, regorafenib, and TAS-102, also have been FDA approved for mCRC, though the OS benefit for these agents as part of the series of standard-of-care treatments is less clear.
Investigators continue to determine subtypes of CRC to further advance treatment options. The histologic classification of colon cancers is actually a collection of multiple cancer subtypes. Each subtype possesses a unique biology largely dependent on the mutations present within the cancer. Recent data, reviewed below, indicate predictive and prognostic benefits to understanding the unique mutational profile of mCRC. Here, the authors present a brief updated summary of these biomarkers and a discussion of treatment strategies.
Resistance to Anti-EGFR Therapies
KRAS and NRAS are members of the RAS family of oncogenes. Activating mutations in these genes results in the propagation of growth factor signals independent of EGFR. The most common KRAS mutations are found in exon 2 (codon 12 or 13). Numerous studies over the past 10 years have confirmed that KRAS mutations at exon 2 predict resistance to cetuximab and panitumumab.4-11 Since at least 2009, restricting use of cetuximab and panitumumab has been the standard of care for patients with KRAS exon 2 wild-type cancers.12
Recent investigations have indicated a predictive role for extended-spectrum KRAS and NRAS mutations (KRAS mutations at exons 2, 3, and 4 and NRAS mutations at exons 2, 3, and 4). In the OPUS clinical trial, patients whose cancers possessed extended-spectrum RAS mutations received no benefit with the addition of cetuximab to standard chemotherapy in response rate (RR), progression-free survival (PFS), or OS compared with standard chemotherapy alone.13 Interestingly, median OS was shorter for those treated with cetuximab when a RAS mutation was present, though the difference was not statistically significant. Additional studies also have confirmed similar benefits in different settings.8,14-18
The CALGB/SWOG 80405 phase 3 clinical trial investigated the first-line use of biologic therapies in combination with standard chemotherapy. The extendedspectrum RAS testing from this study now has been presented.3,19 In the RAS wild-type population, the median OS was 31.2 months in the chemotherapy plus bevacizumab arm and 32.0 months in the chemotherapy plus cetuximab arm (no significant difference). No difference in PFS was observed. A significant improvement in the RR was seen in the cetuximab arm for the RAS wild-type population.
Predictive Biomarkers
BRAF is an oncogene in the RAF gene family that encodes a serine-threonine protein kinase found in the Ras-Raf-MAPK cascade. About 10% of CRC harbor a BRAF mutation.20,21 The most significant and prevalent mutation occurs at the kinase domain from the single substitution V600E. Numerous clinical studies have suggested the presence of this mutation as a predictor of resistance to anti-EGFR therapies and a marker of poor prognosis.6,17,22-25 In a retrospective analysis of RAS and BRAF mutation status of PRIME study data, patients without RAS and BRAF mutations showed significantly better OS and PFS when treated with FOLFOX4 (5-fluorouracil, oxaliplatin, and leucovorin) plus panitumumab, compared with FOLFOX4 alone.8 The presence of BRAF mutations in RAS wild-type patients resulted in a worse outcome. Treatment with anti-EGFR therapy did not significantly improve median PFS or OS.
PIK3CA mutations. Phosphoinositide 3-kinase (PI3K) is a lipid kinase important for multiple cellular processes including cell growth, proliferation, survival, and apoptosis. PIK3CA encodes the catalytic subunit and is mutant in about 20% of mCRC.26 The PI3K is downstream of EGFR signaling; activation of this pathway in the setting of an oncogenic mutation might lead to resistance to anti-EGFR therapies. Sartore-Bianchi and colleagues examined 110 patients with mCRC treated with either panitumumab or cetuximab.27 Of these, 15 patient cancers featured PIK3CA mutations, and none of these responded to anti-EGFR therapies. In addition, preclinical studies have demonstrated that targeting CRC downstream of PI3K might result in significant treatment benefit.28,29
Human epidermal growth factor receptor 2 (HER2) amplification. A subpopulation of CRC with amplification of HER2, a growth factor receptor commonly used in selecting treatment options in breast cancer, has recently been described. The HERACLES phase 2 study evaluated dual HER2 targeting with lapatinib and trastuzumab in therapy-refractory mCRC with HER2 amplification.30 A RR of 35% was observed in this treatment-refractory population.
BRAF mutations. In addition to predicting a poor prognosis and resistance to EGFR-directed therapies, BRAF mutations might be predictive of treatment response using combination regimens containing RAF inhibitors. A recent phase 1B study of a combination therapy using the BRAF inhibitor vemurafenib with irinotecan and cetuximab observed a partial RR of 35%.31 This is being investigated further in the Southwest Oncology Group 1406 phase 2 trial.
Mismatch repair deficiency. Detection of microsatellite instability or the presence of mismatch repair deficiency has become standard-of-care testing for CRC. This is important for the detection of Lynch syndrome and predicting potential resistance to adjuvant 5-fluorouracil in the adjuvant setting.32,33 A recent clinical trial has demonstrated benefits for the use of programmed death 1 (PD-1) inhibitors in the setting of mismatch repair deficiency, including a RR of 40% and PFS of 5.4 months.34
Discussion
Metastatic CRC is now better understood as a collection of multiple cancer subtypes based on mutational profile. This improved understanding of the biology of CRC is altering treatment strategies to a precision medicine-based approach. It is now the standard of care for all patients with mCRC to have the cancers assayed for mutations in KRAS (exons 2, 3, and 4), NRAS (exons 2, 3, and 4), and BRAF. Anti-EGFR therapies should not be used for patients with RAS or BRAF mutations outside of a clinical trial because of a demonstrated lack of benefit in all lines of therapy. Currently, there is no evidence that these mutations significantly alter the response to the approved anti-angiogenic agents bevacizumab, aflibercept, ramucirumab, and regorafenib.
The timing of EGFR-directed therapies for patients with wild-type KRAS, NRAS, and BRAF is still being debated. According to the available data, first-line treatment with anti-EGFR agents in combination with FOLFOX or FOLFIRI (5-fluorouracil, irinotecan, and leucovorin) should be considered for all patients with KRAS, NRAS, and BRAF wild-type mCRCs. The toxicities of anti-EGFR therapies also should be considered for this setting, as some patients find that the acneiform rash, fatigue, nausea, and diarrhea that occur with these agents can have a negative impact on quality of life. As there is no improvement in OS with first-line anti-EGFR therapies for these patients, the increased toxicity from these agents limits their use. In addition, patients with mCRC with known PIK3CA mutation should consider use of EGFR-directed therapies only in the later line setting.
Research is focused on how to best use the mutational profile of the tumor to tailor therapies for mCRC. High-quality, large-volume data sets will become more important as molecular subtypes of cancer become more narrowly defined and less common. Further investigations are needed to look for other markers of resistance and to identify biomarkers predictive of treatment sensitivity.
This is an exciting time in the treatment of many cancers, especially mCRC, which significantly impacts the veteran population, because routine DNA sequencing of patient samples has allowed for rapid advances in the realization of precision medicine. This allows for improved patient selection to reduce costs and toxicities while increasing the benefit for veterans.
Acknowledgments
This work was supported by the National Institutes of Health (P30 CA014520, Core Grant, University of Wisconsin Carbone Cancer Center).
Author disclosures
The authors report no actual or potential conflicts of interest with regard to this article.
Disclaimer
The opinions expressed herein are those of the authors and do not necessarily reflect those of Federal Practitioner, Frontline Medical Communications Inc., the U.S. Government, or any of its agencies.
Click here to read the digital edition.
1. Landrum MB, Keating NL, Lamont EB, et al. Survival of older patients with cancer in the Veterans Health Administration versus fee-for-service Medicare. J Clin Oncol. 2012;30(10):1072-1079.
2. Hynes DM, Tarlov E, Durazo-Arvizu R, et al. Surgery and adjuvant chemotherapy use among veterans with colon cancer: insights from a California study. J Clin Oncol. 2010;28(15):2571-2576.
3. Venook AP, Niedzwiecki D, Lenz H-J, et al; Cancer and Leukemia Group B (Alliance), SWOG, and ECOG. CALGB/SWOG 80405: Phase III trial of irinotecan/5-FU/leucovorin (FOLFIRI) or oxaliplatin/5-FU/leucovorin (mFOLFOX6) with bevacizumab (BV) or cetuximab (CET) for patients (pts) with KRAS wild-type (wt) untreated metastatic adenocarcinoma of the colon or rectum (MCRC). J Clin Oncol. 2014;32(suppl 18):Abstract LBA3.
4. Benvenuti S, Sartore-Bianchi A, Di Nicolantonio F, et al. Oncogenic activation of the RAS/RAF signaling pathway impairs the response of metastatic colorectal cancers to anti-epidermal growth factor receptor antibody therapies. Cancer Res. 2007;67(6):2643-2648.
5. Bokemeyer C, Bondarenko I, Hartmann JT, et al. Efficacy according to biomarker status of cetuximab plus FOLFOX-4 as first-line treatment for metastatic colorectal cancer: the OPUS study. Ann Oncol. 2011;22(7):1535-1546.
6. De Roock W, Claes B, Bernasconi D, et al. Effects of KRAS, BRAF, NRAS, and PIK3CA mutations on the efficacy of cetuximab plus chemotherapy in chemotherapy-refractory metastatic colorectal cancer: a retrospective consortium analysis. Lancet Oncol. 2010;11(8):753-7562.
7. Di Fiore F, Blanchard F, Charbonnier F, et al. Clinical relevance of KRAS mutation detection in metastatic colorectal cancer treated by cetuximab plus chemotherapy. Br J Cancer. 2007;96(8):1166-1169.
8. Douillard JY, Oliner KS, Siena S, et al. Panitumumab-FOLFOX4 treatment and RAS mutations in colorectal cancer. N Engl J Med. 2013;369(11):1023-1034.
9. Lièvre A, Bachet JB, Boige V, et al. KRAS mutations as an independent prognostic factor in patients with advanced colorectal cancer treated with cetuximab. J Clin Oncol. 2008;26(3):374-379.
10. Lièvre A, Bachet JB, Le Corre D, et al. KRAS mutation status is predictive of response to cetuximab therapy in colorectal cancer. Cancer Res. 2006;66(8):3992-3995.
11. Van Cutsem E, Köhne CH, Hitre E, et al. Cetuximab and chemotherapy as initial treatment for metastatic colorectal cancer. N Engl J Med. 2009;360(14):1408-1417.
12. Allegra CJ, Jessup JM, Somerfield MR, et al. American Society of Clinical Oncology provisional clinical opinion: testing for KRAS gene mutations in patients with metastatic colorectal carcinoma to predict response to anti-epidermal growth factor receptor monoclonal antibody therapy. J Clin Oncol. 2009;27(12):2091-2096.
13. Tejpar S, Lenz HJ, Kohne CH, et al. Effect of KRAS and NRAS mutations on treatment outcomes in patients with metastatic colorectal cancer (mCRC) treated first-line with cetuximab plus FOLFOX4: new results from the OPUS study. J Clin Oncol. 2014;32(suppl 3):LBA444.
14. Abad A, Massuti B, Gravalos C, et al. Panitumumab plus FOLFOX4 or panitumumab plus FOLFIRI in subjects with wild-type KRAS (exon 2) colorectal cancer and multiple or unresectable liver-limited metastases: data from the randomized, phase II planet study. Ann Oncol. 2014;25(suppl 2):ii7-ii18.
15. Schwartzberg LS, Rivera F, Karthaus M, et al. PEAK: a randomized, multicenter phase II study of panitumumab plus modified fluorouracil, leucovorin, and oxaliplatin (mFOLFOX6) or bevacizumab plus mFOLFOX6 in patients with previously untreated, unresectable, wild-type KRAS exon 2 metastatic colorectal cancer. J Clin Oncol. 2014;32(21):2240-2247.
16. Peeters M, Oliner K, Price T, et al. KRAS/NRAS and BRAF mutations in the 20050181 study of panitumumab plus FOLFIRI for the 2nd-line treatment of metastatic colorectal cancer: updated analysis. Ann Oncol. 2014;25(suppl 2):ii5.
17. Bokemeyer C, Van Cutsem E, Rougier P, et al. Addition of cetuximab to chemotherapy as first-line treatment for KRAS wild-type metastatic colorectal cancer: pooled analysis of the CRYSTAL and OPUS randomised clinical trials. Eur J Cancer. 2012;48(10):1466-1475.
18. Van Cutsem E, Lenz HJ, Köhne CH, et al. Fluorouracil, leucovorin, and irinotecan plus cetuximab treatment and RAS mutations in colorectal cancer. J Clin Oncol. 2015;33(7):692-700.
19. ESMO. ESMO 2014: results from the CALGB/SWOG 80405 and FIRE-3 (AIO KRK-0306) studies in all RAS wild type population. ESMO website. http://www.esmo.org/Conferences/Past-Conferences/ESMO-2014-Congress/News-Articles/Results-From-the-CALGB-SWOG-80405-and-FIRE-3-AIO-KRK -0306-Studies-In-All-RAS-Wild-Type-Population. Updated September 29, 2014. Accessed April 6, 2016.
20. Davies H, Bignell GR, Cox C, et al. Mutations of the BRAF gene in human cancer. Nature. 2002;417(6892):949-954.
21. Samowitz WS, Sweeney C, Herrick J, Albertsen H, Levin TR, Murtaugh MA, Wolff RK, Slattery ML. Poor survival associated with the BRAF V600E mutation in microsatellite-stable colon cancers. Cancer Res. 2005;65(14):6063-6069.
22. Di Nicolantonio F, Martini M, Molinari F, et al. Wild-type BRAF is required for response to panitumumab or cetuximab in metastatic colorectal cancer. J Clin Oncol. 2008;26(35):5705-5712.
23. Laurent-Puig P, Cayre A, Manceau G, et al. Analysis of PTEN, BRAF, and EGFR status in determining benefit from cetuximab therapy in wild-type KRAS metastatic colon cancer. J Clin Oncol. 2009;27(35):5924-5930.
24. Richman SD, Seymour MT, Chambers P, et al. KRAS and BRAF mutations in advanced colorectal cancer are associated with poor prognosis but do not preclude benefit from oxaliplatin or irinotecan: results from the MRC FOCUS trial. J Clin Oncol. 2009;27(35):5931-5937.
25. Tveit KM, Guren T, et al. Phase III trial of cetuximab with continuous or intermittent fluorouracil, leucovorin, and oxaliplatin (Nordic FLOX) versus FLOX alone in first-line treatment of metastatic colorectal cancer: The NORDIC-VII study. J Clin Oncol. 2012;30(15):1755-1762.
26. Samuels Y, Wang Z, Bardelli A, et al. High frequency of mutations of the PIK3CA gene in human cancers. Science. 2004;304(5670):554.
27. Sartore-Bianchi A, Martini M, Molinari F, et al. PIK3CA mutations in colorectal cancer are associated with clinical resistance to EGFR-targeted monoclonal antibodies. Cancer Res. 2009;69(5):1851-1857.
28. Deming DA, Leystra AA, Farhoud M, et al. mTOR inhibition elicits a dramatic response in PI3K-dependent colon cancers. PLoS One. 2013;8(4):e60709.
29. Yueh AE, Payne SN, Leystra AA, et al. Colon cancer tumorigenesis initiated by the H1047R mutant PI3K. PLoS One. 2016;11(2):e0148730.
30. Siena S, Sartore-Bianchi A, Lonardi S, et al. Trastuzumab and lapatinib in HER2-amplified metastatic colorectal cancer patients (mCRC): the HERACLES trial. J Clin Oncol. 2015;33(suppl 15):3508.
31. Hong DS, Morris VK, El Osta BE, et al. Phase Ib study of vemurafenib in combination with irinotecan and cetuximab in patients with BRAF-mutated metastatic colorectal cancer and advanced cancers. J Clin Oncol. 2015;33(suppl 15):3511.
32. Benatti P, Gafà R, Barana D, et al. Microsatellite instability and colorectal cancer prognosis. Clin Cancer Res. 2005;11(23):8332-8340.
33. Funkhouser WK, Jr, Lubin IM, Monzon FA, et al. Relevance, pathogenesis, and testing algorithm for mismatch repair-defective colorectal carcinomas: a report of the association for molecular pathology. J Mol Diagn. 2012;14(2):91-103.
34. Le DT, Uram JN, Wang H, et al. PD-1 blockade in tumors with mismatch-repair
deficiency. N Engl J Med. 2015;372(26):2509-2520.
Colorectal cancer (CRC) is the third leading cause of cancer-related death in veterans, despite significant advances in treatment options.1,2 Over the past 20 years, the median survival of patients with metastatic CRC (mCRC), has improved with the most recent clinical trials demonstrating a median overall survival (OS) of up to 29 months.3
In addition to standard chemotherapeutic regimens using 5-fluorouracil, oxaliplatin, and irinotecan, biologic therapies have resulted in improved OS for patients with mCRC. These therapies include the human vascular endothelial growth factor (VEGF) monoclonal antibody bevacizumab and the epidermal growth factor receptor (EGFR) monoclonal antibodies cetuximab and panitumumab. Additional agents, including aflibercept, ramucirumab, regorafenib, and TAS-102, also have been FDA approved for mCRC, though the OS benefit for these agents as part of the series of standard-of-care treatments is less clear.
Investigators continue to determine subtypes of CRC to further advance treatment options. The histologic classification of colon cancers is actually a collection of multiple cancer subtypes. Each subtype possesses a unique biology largely dependent on the mutations present within the cancer. Recent data, reviewed below, indicate predictive and prognostic benefits to understanding the unique mutational profile of mCRC. Here, the authors present a brief updated summary of these biomarkers and a discussion of treatment strategies.
Resistance to Anti-EGFR Therapies
KRAS and NRAS are members of the RAS family of oncogenes. Activating mutations in these genes results in the propagation of growth factor signals independent of EGFR. The most common KRAS mutations are found in exon 2 (codon 12 or 13). Numerous studies over the past 10 years have confirmed that KRAS mutations at exon 2 predict resistance to cetuximab and panitumumab.4-11 Since at least 2009, restricting use of cetuximab and panitumumab has been the standard of care for patients with KRAS exon 2 wild-type cancers.12
Recent investigations have indicated a predictive role for extended-spectrum KRAS and NRAS mutations (KRAS mutations at exons 2, 3, and 4 and NRAS mutations at exons 2, 3, and 4). In the OPUS clinical trial, patients whose cancers possessed extended-spectrum RAS mutations received no benefit with the addition of cetuximab to standard chemotherapy in response rate (RR), progression-free survival (PFS), or OS compared with standard chemotherapy alone.13 Interestingly, median OS was shorter for those treated with cetuximab when a RAS mutation was present, though the difference was not statistically significant. Additional studies also have confirmed similar benefits in different settings.8,14-18
The CALGB/SWOG 80405 phase 3 clinical trial investigated the first-line use of biologic therapies in combination with standard chemotherapy. The extendedspectrum RAS testing from this study now has been presented.3,19 In the RAS wild-type population, the median OS was 31.2 months in the chemotherapy plus bevacizumab arm and 32.0 months in the chemotherapy plus cetuximab arm (no significant difference). No difference in PFS was observed. A significant improvement in the RR was seen in the cetuximab arm for the RAS wild-type population.
Predictive Biomarkers
BRAF is an oncogene in the RAF gene family that encodes a serine-threonine protein kinase found in the Ras-Raf-MAPK cascade. About 10% of CRC harbor a BRAF mutation.20,21 The most significant and prevalent mutation occurs at the kinase domain from the single substitution V600E. Numerous clinical studies have suggested the presence of this mutation as a predictor of resistance to anti-EGFR therapies and a marker of poor prognosis.6,17,22-25 In a retrospective analysis of RAS and BRAF mutation status of PRIME study data, patients without RAS and BRAF mutations showed significantly better OS and PFS when treated with FOLFOX4 (5-fluorouracil, oxaliplatin, and leucovorin) plus panitumumab, compared with FOLFOX4 alone.8 The presence of BRAF mutations in RAS wild-type patients resulted in a worse outcome. Treatment with anti-EGFR therapy did not significantly improve median PFS or OS.
PIK3CA mutations. Phosphoinositide 3-kinase (PI3K) is a lipid kinase important for multiple cellular processes including cell growth, proliferation, survival, and apoptosis. PIK3CA encodes the catalytic subunit and is mutant in about 20% of mCRC.26 The PI3K is downstream of EGFR signaling; activation of this pathway in the setting of an oncogenic mutation might lead to resistance to anti-EGFR therapies. Sartore-Bianchi and colleagues examined 110 patients with mCRC treated with either panitumumab or cetuximab.27 Of these, 15 patient cancers featured PIK3CA mutations, and none of these responded to anti-EGFR therapies. In addition, preclinical studies have demonstrated that targeting CRC downstream of PI3K might result in significant treatment benefit.28,29
Human epidermal growth factor receptor 2 (HER2) amplification. A subpopulation of CRC with amplification of HER2, a growth factor receptor commonly used in selecting treatment options in breast cancer, has recently been described. The HERACLES phase 2 study evaluated dual HER2 targeting with lapatinib and trastuzumab in therapy-refractory mCRC with HER2 amplification.30 A RR of 35% was observed in this treatment-refractory population.
BRAF mutations. In addition to predicting a poor prognosis and resistance to EGFR-directed therapies, BRAF mutations might be predictive of treatment response using combination regimens containing RAF inhibitors. A recent phase 1B study of a combination therapy using the BRAF inhibitor vemurafenib with irinotecan and cetuximab observed a partial RR of 35%.31 This is being investigated further in the Southwest Oncology Group 1406 phase 2 trial.
Mismatch repair deficiency. Detection of microsatellite instability or the presence of mismatch repair deficiency has become standard-of-care testing for CRC. This is important for the detection of Lynch syndrome and predicting potential resistance to adjuvant 5-fluorouracil in the adjuvant setting.32,33 A recent clinical trial has demonstrated benefits for the use of programmed death 1 (PD-1) inhibitors in the setting of mismatch repair deficiency, including a RR of 40% and PFS of 5.4 months.34
Discussion
Metastatic CRC is now better understood as a collection of multiple cancer subtypes based on mutational profile. This improved understanding of the biology of CRC is altering treatment strategies to a precision medicine-based approach. It is now the standard of care for all patients with mCRC to have the cancers assayed for mutations in KRAS (exons 2, 3, and 4), NRAS (exons 2, 3, and 4), and BRAF. Anti-EGFR therapies should not be used for patients with RAS or BRAF mutations outside of a clinical trial because of a demonstrated lack of benefit in all lines of therapy. Currently, there is no evidence that these mutations significantly alter the response to the approved anti-angiogenic agents bevacizumab, aflibercept, ramucirumab, and regorafenib.
The timing of EGFR-directed therapies for patients with wild-type KRAS, NRAS, and BRAF is still being debated. According to the available data, first-line treatment with anti-EGFR agents in combination with FOLFOX or FOLFIRI (5-fluorouracil, irinotecan, and leucovorin) should be considered for all patients with KRAS, NRAS, and BRAF wild-type mCRCs. The toxicities of anti-EGFR therapies also should be considered for this setting, as some patients find that the acneiform rash, fatigue, nausea, and diarrhea that occur with these agents can have a negative impact on quality of life. As there is no improvement in OS with first-line anti-EGFR therapies for these patients, the increased toxicity from these agents limits their use. In addition, patients with mCRC with known PIK3CA mutation should consider use of EGFR-directed therapies only in the later line setting.
Research is focused on how to best use the mutational profile of the tumor to tailor therapies for mCRC. High-quality, large-volume data sets will become more important as molecular subtypes of cancer become more narrowly defined and less common. Further investigations are needed to look for other markers of resistance and to identify biomarkers predictive of treatment sensitivity.
This is an exciting time in the treatment of many cancers, especially mCRC, which significantly impacts the veteran population, because routine DNA sequencing of patient samples has allowed for rapid advances in the realization of precision medicine. This allows for improved patient selection to reduce costs and toxicities while increasing the benefit for veterans.
Acknowledgments
This work was supported by the National Institutes of Health (P30 CA014520, Core Grant, University of Wisconsin Carbone Cancer Center).
Author disclosures
The authors report no actual or potential conflicts of interest with regard to this article.
Disclaimer
The opinions expressed herein are those of the authors and do not necessarily reflect those of Federal Practitioner, Frontline Medical Communications Inc., the U.S. Government, or any of its agencies.
Click here to read the digital edition.
Colorectal cancer (CRC) is the third leading cause of cancer-related death in veterans, despite significant advances in treatment options.1,2 Over the past 20 years, the median survival of patients with metastatic CRC (mCRC), has improved with the most recent clinical trials demonstrating a median overall survival (OS) of up to 29 months.3
In addition to standard chemotherapeutic regimens using 5-fluorouracil, oxaliplatin, and irinotecan, biologic therapies have resulted in improved OS for patients with mCRC. These therapies include the human vascular endothelial growth factor (VEGF) monoclonal antibody bevacizumab and the epidermal growth factor receptor (EGFR) monoclonal antibodies cetuximab and panitumumab. Additional agents, including aflibercept, ramucirumab, regorafenib, and TAS-102, also have been FDA approved for mCRC, though the OS benefit for these agents as part of the series of standard-of-care treatments is less clear.
Investigators continue to determine subtypes of CRC to further advance treatment options. The histologic classification of colon cancers is actually a collection of multiple cancer subtypes. Each subtype possesses a unique biology largely dependent on the mutations present within the cancer. Recent data, reviewed below, indicate predictive and prognostic benefits to understanding the unique mutational profile of mCRC. Here, the authors present a brief updated summary of these biomarkers and a discussion of treatment strategies.
Resistance to Anti-EGFR Therapies
KRAS and NRAS are members of the RAS family of oncogenes. Activating mutations in these genes results in the propagation of growth factor signals independent of EGFR. The most common KRAS mutations are found in exon 2 (codon 12 or 13). Numerous studies over the past 10 years have confirmed that KRAS mutations at exon 2 predict resistance to cetuximab and panitumumab.4-11 Since at least 2009, restricting use of cetuximab and panitumumab has been the standard of care for patients with KRAS exon 2 wild-type cancers.12
Recent investigations have indicated a predictive role for extended-spectrum KRAS and NRAS mutations (KRAS mutations at exons 2, 3, and 4 and NRAS mutations at exons 2, 3, and 4). In the OPUS clinical trial, patients whose cancers possessed extended-spectrum RAS mutations received no benefit with the addition of cetuximab to standard chemotherapy in response rate (RR), progression-free survival (PFS), or OS compared with standard chemotherapy alone.13 Interestingly, median OS was shorter for those treated with cetuximab when a RAS mutation was present, though the difference was not statistically significant. Additional studies also have confirmed similar benefits in different settings.8,14-18
The CALGB/SWOG 80405 phase 3 clinical trial investigated the first-line use of biologic therapies in combination with standard chemotherapy. The extendedspectrum RAS testing from this study now has been presented.3,19 In the RAS wild-type population, the median OS was 31.2 months in the chemotherapy plus bevacizumab arm and 32.0 months in the chemotherapy plus cetuximab arm (no significant difference). No difference in PFS was observed. A significant improvement in the RR was seen in the cetuximab arm for the RAS wild-type population.
Predictive Biomarkers
BRAF is an oncogene in the RAF gene family that encodes a serine-threonine protein kinase found in the Ras-Raf-MAPK cascade. About 10% of CRC harbor a BRAF mutation.20,21 The most significant and prevalent mutation occurs at the kinase domain from the single substitution V600E. Numerous clinical studies have suggested the presence of this mutation as a predictor of resistance to anti-EGFR therapies and a marker of poor prognosis.6,17,22-25 In a retrospective analysis of RAS and BRAF mutation status of PRIME study data, patients without RAS and BRAF mutations showed significantly better OS and PFS when treated with FOLFOX4 (5-fluorouracil, oxaliplatin, and leucovorin) plus panitumumab, compared with FOLFOX4 alone.8 The presence of BRAF mutations in RAS wild-type patients resulted in a worse outcome. Treatment with anti-EGFR therapy did not significantly improve median PFS or OS.
PIK3CA mutations. Phosphoinositide 3-kinase (PI3K) is a lipid kinase important for multiple cellular processes including cell growth, proliferation, survival, and apoptosis. PIK3CA encodes the catalytic subunit and is mutant in about 20% of mCRC.26 The PI3K is downstream of EGFR signaling; activation of this pathway in the setting of an oncogenic mutation might lead to resistance to anti-EGFR therapies. Sartore-Bianchi and colleagues examined 110 patients with mCRC treated with either panitumumab or cetuximab.27 Of these, 15 patient cancers featured PIK3CA mutations, and none of these responded to anti-EGFR therapies. In addition, preclinical studies have demonstrated that targeting CRC downstream of PI3K might result in significant treatment benefit.28,29
Human epidermal growth factor receptor 2 (HER2) amplification. A subpopulation of CRC with amplification of HER2, a growth factor receptor commonly used in selecting treatment options in breast cancer, has recently been described. The HERACLES phase 2 study evaluated dual HER2 targeting with lapatinib and trastuzumab in therapy-refractory mCRC with HER2 amplification.30 A RR of 35% was observed in this treatment-refractory population.
BRAF mutations. In addition to predicting a poor prognosis and resistance to EGFR-directed therapies, BRAF mutations might be predictive of treatment response using combination regimens containing RAF inhibitors. A recent phase 1B study of a combination therapy using the BRAF inhibitor vemurafenib with irinotecan and cetuximab observed a partial RR of 35%.31 This is being investigated further in the Southwest Oncology Group 1406 phase 2 trial.
Mismatch repair deficiency. Detection of microsatellite instability or the presence of mismatch repair deficiency has become standard-of-care testing for CRC. This is important for the detection of Lynch syndrome and predicting potential resistance to adjuvant 5-fluorouracil in the adjuvant setting.32,33 A recent clinical trial has demonstrated benefits for the use of programmed death 1 (PD-1) inhibitors in the setting of mismatch repair deficiency, including a RR of 40% and PFS of 5.4 months.34
Discussion
Metastatic CRC is now better understood as a collection of multiple cancer subtypes based on mutational profile. This improved understanding of the biology of CRC is altering treatment strategies to a precision medicine-based approach. It is now the standard of care for all patients with mCRC to have the cancers assayed for mutations in KRAS (exons 2, 3, and 4), NRAS (exons 2, 3, and 4), and BRAF. Anti-EGFR therapies should not be used for patients with RAS or BRAF mutations outside of a clinical trial because of a demonstrated lack of benefit in all lines of therapy. Currently, there is no evidence that these mutations significantly alter the response to the approved anti-angiogenic agents bevacizumab, aflibercept, ramucirumab, and regorafenib.
The timing of EGFR-directed therapies for patients with wild-type KRAS, NRAS, and BRAF is still being debated. According to the available data, first-line treatment with anti-EGFR agents in combination with FOLFOX or FOLFIRI (5-fluorouracil, irinotecan, and leucovorin) should be considered for all patients with KRAS, NRAS, and BRAF wild-type mCRCs. The toxicities of anti-EGFR therapies also should be considered for this setting, as some patients find that the acneiform rash, fatigue, nausea, and diarrhea that occur with these agents can have a negative impact on quality of life. As there is no improvement in OS with first-line anti-EGFR therapies for these patients, the increased toxicity from these agents limits their use. In addition, patients with mCRC with known PIK3CA mutation should consider use of EGFR-directed therapies only in the later line setting.
Research is focused on how to best use the mutational profile of the tumor to tailor therapies for mCRC. High-quality, large-volume data sets will become more important as molecular subtypes of cancer become more narrowly defined and less common. Further investigations are needed to look for other markers of resistance and to identify biomarkers predictive of treatment sensitivity.
This is an exciting time in the treatment of many cancers, especially mCRC, which significantly impacts the veteran population, because routine DNA sequencing of patient samples has allowed for rapid advances in the realization of precision medicine. This allows for improved patient selection to reduce costs and toxicities while increasing the benefit for veterans.
Acknowledgments
This work was supported by the National Institutes of Health (P30 CA014520, Core Grant, University of Wisconsin Carbone Cancer Center).
Author disclosures
The authors report no actual or potential conflicts of interest with regard to this article.
Disclaimer
The opinions expressed herein are those of the authors and do not necessarily reflect those of Federal Practitioner, Frontline Medical Communications Inc., the U.S. Government, or any of its agencies.
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1. Landrum MB, Keating NL, Lamont EB, et al. Survival of older patients with cancer in the Veterans Health Administration versus fee-for-service Medicare. J Clin Oncol. 2012;30(10):1072-1079.
2. Hynes DM, Tarlov E, Durazo-Arvizu R, et al. Surgery and adjuvant chemotherapy use among veterans with colon cancer: insights from a California study. J Clin Oncol. 2010;28(15):2571-2576.
3. Venook AP, Niedzwiecki D, Lenz H-J, et al; Cancer and Leukemia Group B (Alliance), SWOG, and ECOG. CALGB/SWOG 80405: Phase III trial of irinotecan/5-FU/leucovorin (FOLFIRI) or oxaliplatin/5-FU/leucovorin (mFOLFOX6) with bevacizumab (BV) or cetuximab (CET) for patients (pts) with KRAS wild-type (wt) untreated metastatic adenocarcinoma of the colon or rectum (MCRC). J Clin Oncol. 2014;32(suppl 18):Abstract LBA3.
4. Benvenuti S, Sartore-Bianchi A, Di Nicolantonio F, et al. Oncogenic activation of the RAS/RAF signaling pathway impairs the response of metastatic colorectal cancers to anti-epidermal growth factor receptor antibody therapies. Cancer Res. 2007;67(6):2643-2648.
5. Bokemeyer C, Bondarenko I, Hartmann JT, et al. Efficacy according to biomarker status of cetuximab plus FOLFOX-4 as first-line treatment for metastatic colorectal cancer: the OPUS study. Ann Oncol. 2011;22(7):1535-1546.
6. De Roock W, Claes B, Bernasconi D, et al. Effects of KRAS, BRAF, NRAS, and PIK3CA mutations on the efficacy of cetuximab plus chemotherapy in chemotherapy-refractory metastatic colorectal cancer: a retrospective consortium analysis. Lancet Oncol. 2010;11(8):753-7562.
7. Di Fiore F, Blanchard F, Charbonnier F, et al. Clinical relevance of KRAS mutation detection in metastatic colorectal cancer treated by cetuximab plus chemotherapy. Br J Cancer. 2007;96(8):1166-1169.
8. Douillard JY, Oliner KS, Siena S, et al. Panitumumab-FOLFOX4 treatment and RAS mutations in colorectal cancer. N Engl J Med. 2013;369(11):1023-1034.
9. Lièvre A, Bachet JB, Boige V, et al. KRAS mutations as an independent prognostic factor in patients with advanced colorectal cancer treated with cetuximab. J Clin Oncol. 2008;26(3):374-379.
10. Lièvre A, Bachet JB, Le Corre D, et al. KRAS mutation status is predictive of response to cetuximab therapy in colorectal cancer. Cancer Res. 2006;66(8):3992-3995.
11. Van Cutsem E, Köhne CH, Hitre E, et al. Cetuximab and chemotherapy as initial treatment for metastatic colorectal cancer. N Engl J Med. 2009;360(14):1408-1417.
12. Allegra CJ, Jessup JM, Somerfield MR, et al. American Society of Clinical Oncology provisional clinical opinion: testing for KRAS gene mutations in patients with metastatic colorectal carcinoma to predict response to anti-epidermal growth factor receptor monoclonal antibody therapy. J Clin Oncol. 2009;27(12):2091-2096.
13. Tejpar S, Lenz HJ, Kohne CH, et al. Effect of KRAS and NRAS mutations on treatment outcomes in patients with metastatic colorectal cancer (mCRC) treated first-line with cetuximab plus FOLFOX4: new results from the OPUS study. J Clin Oncol. 2014;32(suppl 3):LBA444.
14. Abad A, Massuti B, Gravalos C, et al. Panitumumab plus FOLFOX4 or panitumumab plus FOLFIRI in subjects with wild-type KRAS (exon 2) colorectal cancer and multiple or unresectable liver-limited metastases: data from the randomized, phase II planet study. Ann Oncol. 2014;25(suppl 2):ii7-ii18.
15. Schwartzberg LS, Rivera F, Karthaus M, et al. PEAK: a randomized, multicenter phase II study of panitumumab plus modified fluorouracil, leucovorin, and oxaliplatin (mFOLFOX6) or bevacizumab plus mFOLFOX6 in patients with previously untreated, unresectable, wild-type KRAS exon 2 metastatic colorectal cancer. J Clin Oncol. 2014;32(21):2240-2247.
16. Peeters M, Oliner K, Price T, et al. KRAS/NRAS and BRAF mutations in the 20050181 study of panitumumab plus FOLFIRI for the 2nd-line treatment of metastatic colorectal cancer: updated analysis. Ann Oncol. 2014;25(suppl 2):ii5.
17. Bokemeyer C, Van Cutsem E, Rougier P, et al. Addition of cetuximab to chemotherapy as first-line treatment for KRAS wild-type metastatic colorectal cancer: pooled analysis of the CRYSTAL and OPUS randomised clinical trials. Eur J Cancer. 2012;48(10):1466-1475.
18. Van Cutsem E, Lenz HJ, Köhne CH, et al. Fluorouracil, leucovorin, and irinotecan plus cetuximab treatment and RAS mutations in colorectal cancer. J Clin Oncol. 2015;33(7):692-700.
19. ESMO. ESMO 2014: results from the CALGB/SWOG 80405 and FIRE-3 (AIO KRK-0306) studies in all RAS wild type population. ESMO website. http://www.esmo.org/Conferences/Past-Conferences/ESMO-2014-Congress/News-Articles/Results-From-the-CALGB-SWOG-80405-and-FIRE-3-AIO-KRK -0306-Studies-In-All-RAS-Wild-Type-Population. Updated September 29, 2014. Accessed April 6, 2016.
20. Davies H, Bignell GR, Cox C, et al. Mutations of the BRAF gene in human cancer. Nature. 2002;417(6892):949-954.
21. Samowitz WS, Sweeney C, Herrick J, Albertsen H, Levin TR, Murtaugh MA, Wolff RK, Slattery ML. Poor survival associated with the BRAF V600E mutation in microsatellite-stable colon cancers. Cancer Res. 2005;65(14):6063-6069.
22. Di Nicolantonio F, Martini M, Molinari F, et al. Wild-type BRAF is required for response to panitumumab or cetuximab in metastatic colorectal cancer. J Clin Oncol. 2008;26(35):5705-5712.
23. Laurent-Puig P, Cayre A, Manceau G, et al. Analysis of PTEN, BRAF, and EGFR status in determining benefit from cetuximab therapy in wild-type KRAS metastatic colon cancer. J Clin Oncol. 2009;27(35):5924-5930.
24. Richman SD, Seymour MT, Chambers P, et al. KRAS and BRAF mutations in advanced colorectal cancer are associated with poor prognosis but do not preclude benefit from oxaliplatin or irinotecan: results from the MRC FOCUS trial. J Clin Oncol. 2009;27(35):5931-5937.
25. Tveit KM, Guren T, et al. Phase III trial of cetuximab with continuous or intermittent fluorouracil, leucovorin, and oxaliplatin (Nordic FLOX) versus FLOX alone in first-line treatment of metastatic colorectal cancer: The NORDIC-VII study. J Clin Oncol. 2012;30(15):1755-1762.
26. Samuels Y, Wang Z, Bardelli A, et al. High frequency of mutations of the PIK3CA gene in human cancers. Science. 2004;304(5670):554.
27. Sartore-Bianchi A, Martini M, Molinari F, et al. PIK3CA mutations in colorectal cancer are associated with clinical resistance to EGFR-targeted monoclonal antibodies. Cancer Res. 2009;69(5):1851-1857.
28. Deming DA, Leystra AA, Farhoud M, et al. mTOR inhibition elicits a dramatic response in PI3K-dependent colon cancers. PLoS One. 2013;8(4):e60709.
29. Yueh AE, Payne SN, Leystra AA, et al. Colon cancer tumorigenesis initiated by the H1047R mutant PI3K. PLoS One. 2016;11(2):e0148730.
30. Siena S, Sartore-Bianchi A, Lonardi S, et al. Trastuzumab and lapatinib in HER2-amplified metastatic colorectal cancer patients (mCRC): the HERACLES trial. J Clin Oncol. 2015;33(suppl 15):3508.
31. Hong DS, Morris VK, El Osta BE, et al. Phase Ib study of vemurafenib in combination with irinotecan and cetuximab in patients with BRAF-mutated metastatic colorectal cancer and advanced cancers. J Clin Oncol. 2015;33(suppl 15):3511.
32. Benatti P, Gafà R, Barana D, et al. Microsatellite instability and colorectal cancer prognosis. Clin Cancer Res. 2005;11(23):8332-8340.
33. Funkhouser WK, Jr, Lubin IM, Monzon FA, et al. Relevance, pathogenesis, and testing algorithm for mismatch repair-defective colorectal carcinomas: a report of the association for molecular pathology. J Mol Diagn. 2012;14(2):91-103.
34. Le DT, Uram JN, Wang H, et al. PD-1 blockade in tumors with mismatch-repair
deficiency. N Engl J Med. 2015;372(26):2509-2520.
1. Landrum MB, Keating NL, Lamont EB, et al. Survival of older patients with cancer in the Veterans Health Administration versus fee-for-service Medicare. J Clin Oncol. 2012;30(10):1072-1079.
2. Hynes DM, Tarlov E, Durazo-Arvizu R, et al. Surgery and adjuvant chemotherapy use among veterans with colon cancer: insights from a California study. J Clin Oncol. 2010;28(15):2571-2576.
3. Venook AP, Niedzwiecki D, Lenz H-J, et al; Cancer and Leukemia Group B (Alliance), SWOG, and ECOG. CALGB/SWOG 80405: Phase III trial of irinotecan/5-FU/leucovorin (FOLFIRI) or oxaliplatin/5-FU/leucovorin (mFOLFOX6) with bevacizumab (BV) or cetuximab (CET) for patients (pts) with KRAS wild-type (wt) untreated metastatic adenocarcinoma of the colon or rectum (MCRC). J Clin Oncol. 2014;32(suppl 18):Abstract LBA3.
4. Benvenuti S, Sartore-Bianchi A, Di Nicolantonio F, et al. Oncogenic activation of the RAS/RAF signaling pathway impairs the response of metastatic colorectal cancers to anti-epidermal growth factor receptor antibody therapies. Cancer Res. 2007;67(6):2643-2648.
5. Bokemeyer C, Bondarenko I, Hartmann JT, et al. Efficacy according to biomarker status of cetuximab plus FOLFOX-4 as first-line treatment for metastatic colorectal cancer: the OPUS study. Ann Oncol. 2011;22(7):1535-1546.
6. De Roock W, Claes B, Bernasconi D, et al. Effects of KRAS, BRAF, NRAS, and PIK3CA mutations on the efficacy of cetuximab plus chemotherapy in chemotherapy-refractory metastatic colorectal cancer: a retrospective consortium analysis. Lancet Oncol. 2010;11(8):753-7562.
7. Di Fiore F, Blanchard F, Charbonnier F, et al. Clinical relevance of KRAS mutation detection in metastatic colorectal cancer treated by cetuximab plus chemotherapy. Br J Cancer. 2007;96(8):1166-1169.
8. Douillard JY, Oliner KS, Siena S, et al. Panitumumab-FOLFOX4 treatment and RAS mutations in colorectal cancer. N Engl J Med. 2013;369(11):1023-1034.
9. Lièvre A, Bachet JB, Boige V, et al. KRAS mutations as an independent prognostic factor in patients with advanced colorectal cancer treated with cetuximab. J Clin Oncol. 2008;26(3):374-379.
10. Lièvre A, Bachet JB, Le Corre D, et al. KRAS mutation status is predictive of response to cetuximab therapy in colorectal cancer. Cancer Res. 2006;66(8):3992-3995.
11. Van Cutsem E, Köhne CH, Hitre E, et al. Cetuximab and chemotherapy as initial treatment for metastatic colorectal cancer. N Engl J Med. 2009;360(14):1408-1417.
12. Allegra CJ, Jessup JM, Somerfield MR, et al. American Society of Clinical Oncology provisional clinical opinion: testing for KRAS gene mutations in patients with metastatic colorectal carcinoma to predict response to anti-epidermal growth factor receptor monoclonal antibody therapy. J Clin Oncol. 2009;27(12):2091-2096.
13. Tejpar S, Lenz HJ, Kohne CH, et al. Effect of KRAS and NRAS mutations on treatment outcomes in patients with metastatic colorectal cancer (mCRC) treated first-line with cetuximab plus FOLFOX4: new results from the OPUS study. J Clin Oncol. 2014;32(suppl 3):LBA444.
14. Abad A, Massuti B, Gravalos C, et al. Panitumumab plus FOLFOX4 or panitumumab plus FOLFIRI in subjects with wild-type KRAS (exon 2) colorectal cancer and multiple or unresectable liver-limited metastases: data from the randomized, phase II planet study. Ann Oncol. 2014;25(suppl 2):ii7-ii18.
15. Schwartzberg LS, Rivera F, Karthaus M, et al. PEAK: a randomized, multicenter phase II study of panitumumab plus modified fluorouracil, leucovorin, and oxaliplatin (mFOLFOX6) or bevacizumab plus mFOLFOX6 in patients with previously untreated, unresectable, wild-type KRAS exon 2 metastatic colorectal cancer. J Clin Oncol. 2014;32(21):2240-2247.
16. Peeters M, Oliner K, Price T, et al. KRAS/NRAS and BRAF mutations in the 20050181 study of panitumumab plus FOLFIRI for the 2nd-line treatment of metastatic colorectal cancer: updated analysis. Ann Oncol. 2014;25(suppl 2):ii5.
17. Bokemeyer C, Van Cutsem E, Rougier P, et al. Addition of cetuximab to chemotherapy as first-line treatment for KRAS wild-type metastatic colorectal cancer: pooled analysis of the CRYSTAL and OPUS randomised clinical trials. Eur J Cancer. 2012;48(10):1466-1475.
18. Van Cutsem E, Lenz HJ, Köhne CH, et al. Fluorouracil, leucovorin, and irinotecan plus cetuximab treatment and RAS mutations in colorectal cancer. J Clin Oncol. 2015;33(7):692-700.
19. ESMO. ESMO 2014: results from the CALGB/SWOG 80405 and FIRE-3 (AIO KRK-0306) studies in all RAS wild type population. ESMO website. http://www.esmo.org/Conferences/Past-Conferences/ESMO-2014-Congress/News-Articles/Results-From-the-CALGB-SWOG-80405-and-FIRE-3-AIO-KRK -0306-Studies-In-All-RAS-Wild-Type-Population. Updated September 29, 2014. Accessed April 6, 2016.
20. Davies H, Bignell GR, Cox C, et al. Mutations of the BRAF gene in human cancer. Nature. 2002;417(6892):949-954.
21. Samowitz WS, Sweeney C, Herrick J, Albertsen H, Levin TR, Murtaugh MA, Wolff RK, Slattery ML. Poor survival associated with the BRAF V600E mutation in microsatellite-stable colon cancers. Cancer Res. 2005;65(14):6063-6069.
22. Di Nicolantonio F, Martini M, Molinari F, et al. Wild-type BRAF is required for response to panitumumab or cetuximab in metastatic colorectal cancer. J Clin Oncol. 2008;26(35):5705-5712.
23. Laurent-Puig P, Cayre A, Manceau G, et al. Analysis of PTEN, BRAF, and EGFR status in determining benefit from cetuximab therapy in wild-type KRAS metastatic colon cancer. J Clin Oncol. 2009;27(35):5924-5930.
24. Richman SD, Seymour MT, Chambers P, et al. KRAS and BRAF mutations in advanced colorectal cancer are associated with poor prognosis but do not preclude benefit from oxaliplatin or irinotecan: results from the MRC FOCUS trial. J Clin Oncol. 2009;27(35):5931-5937.
25. Tveit KM, Guren T, et al. Phase III trial of cetuximab with continuous or intermittent fluorouracil, leucovorin, and oxaliplatin (Nordic FLOX) versus FLOX alone in first-line treatment of metastatic colorectal cancer: The NORDIC-VII study. J Clin Oncol. 2012;30(15):1755-1762.
26. Samuels Y, Wang Z, Bardelli A, et al. High frequency of mutations of the PIK3CA gene in human cancers. Science. 2004;304(5670):554.
27. Sartore-Bianchi A, Martini M, Molinari F, et al. PIK3CA mutations in colorectal cancer are associated with clinical resistance to EGFR-targeted monoclonal antibodies. Cancer Res. 2009;69(5):1851-1857.
28. Deming DA, Leystra AA, Farhoud M, et al. mTOR inhibition elicits a dramatic response in PI3K-dependent colon cancers. PLoS One. 2013;8(4):e60709.
29. Yueh AE, Payne SN, Leystra AA, et al. Colon cancer tumorigenesis initiated by the H1047R mutant PI3K. PLoS One. 2016;11(2):e0148730.
30. Siena S, Sartore-Bianchi A, Lonardi S, et al. Trastuzumab and lapatinib in HER2-amplified metastatic colorectal cancer patients (mCRC): the HERACLES trial. J Clin Oncol. 2015;33(suppl 15):3508.
31. Hong DS, Morris VK, El Osta BE, et al. Phase Ib study of vemurafenib in combination with irinotecan and cetuximab in patients with BRAF-mutated metastatic colorectal cancer and advanced cancers. J Clin Oncol. 2015;33(suppl 15):3511.
32. Benatti P, Gafà R, Barana D, et al. Microsatellite instability and colorectal cancer prognosis. Clin Cancer Res. 2005;11(23):8332-8340.
33. Funkhouser WK, Jr, Lubin IM, Monzon FA, et al. Relevance, pathogenesis, and testing algorithm for mismatch repair-defective colorectal carcinomas: a report of the association for molecular pathology. J Mol Diagn. 2012;14(2):91-103.
34. Le DT, Uram JN, Wang H, et al. PD-1 blockade in tumors with mismatch-repair
deficiency. N Engl J Med. 2015;372(26):2509-2520.