Molecularly profiling colorectal cancer has opened many potential opportunities for the use of this information in therapeutic decision-making. However, at present, only RAS testing in the metastatic setting has a definitive place in the decision-making paradigm.
On February 12, 2004, cetuximab was approved in the United States for the treatment of advanced colorectal cancer (CRC), both alone and in combination with irinotecan for irinotecan-refractory disease. As Sridharan and colleagues excellently review in the accompanying article, many subsequent studies have since been reported that have confirmed the relative benefit of anti–epidermal growth factor receptor (EGFR) antibodies-cetuximab and panitumumab-both in late-stage and early metastatic disease. Five years later, Karapetis et al and Amando et al independently reported that the benefit of these anti-EGFR antibodies was not seen in KRAS-mutant tumors (exon 2), and the subsequent re-analysis of each major anti-EGFR antibody trial has confirmed this finding.[2,3] Now, another 5 years later, two studies reported in 2013 extended RAS testing to KRAS exon 3 as well as to NRAS exons 2 through 4 and BRAF exon 5. Mutation in these regions were present in ~50% of patients and also predicted for lack of benefit.
These results have changed clinical practice, but have they really improved survival for our patients over the past 10 years? Clearly it is now standard of care to test patients for an extended list of RAS/RAF mutations-but with this testing, we are simply finding those patients who have always been the drivers of the observed benefits in these trials. In truth, we are merely selecting our patients better-a very important development-but those receiving therapy would always have received the benefit, testing or no. Of course, some of the retrospective analyses of the first EGFR-targeted antibody trials showed a detrimental effect of the therapy in patients with RAS-mutated tumors; we are definitely sparing those patients toxicity as well as possibly a negative therapeutic impact. More compelling are the recently reported data in the FIRE-3 trial (powered for response rate, not survival) and the preliminary results of the phase II PEAK trial, which showed that patients with RAS/RAF wild-type tumors treated with standard therapy and EGFR blockade in the first-line setting enjoyed a survival advantage of 7 months or greater compared with bevacizumab-based therapy.[5,6] We eagerly await the results of the Cancer and Leukemia Group B (CALGB)/Southwest Oncology Group (SWOG) 80405 study, which was powered for an overall survival endpoint. If the results of that study are positive, RAS/RAF testing will be pivotal in decision making for first-line therapy and may significantly change clinical practice with regard to the sequencing of first-line treatment options.
Beyond RAS testing, there have been major developments in molecular science for CRC over the past 10 years. Independent genomics groups as well as The Cancer Genome Atlas (TCGA) have reported a comprehensive molecular characterization of the mutations observed across hundreds of CRC patient samples. And concurrently, multiple agents targeting aberrant molecular signaling pathways have been tested in patients, although with predominantly disappointing results. BRAF mutation–positive patients have been unsuccessfully treated with BRAF small molecules. MEK and ERK inhibitors have been tested broadly in CRC and in KRAS/BRAF-mutated tumors with negative results. TCGA has revealed that ~2% of CRC may be human epidermal growth factor receptor 2 (HER2)-amplified, and testing anti-HER2 agents in that small subset is ongoing, although understandably challenging in terms of accrual. PIK3CA mutations have been the most compelling in terms of predicting benefit, but not from Akt or phosphoinositide 3 kinase (PI3K) inhibitors; rather, a survival benefit seen now in multiple studies has come from treatment with aspirin or nonsteroidal anti-inflammatory drugs (NSAIDS), the former with a hazard ratio of 0.5, which is rarely seen in solid tumor therapeutics.
There has been little progress in our ability to determine which patients will receive benefit from the other drugs approved for the treatment of CRC. A robust predictive biomarker or biomarker panel for anti-angiogenic drugs such as bevacizumab, aflibercept, and possibly regorafenib has long eluded the oncology research community. It has been difficult to identify molecular selection markers for cytotoxic chemotherapies across tumor types, and none exist presently for fluorouracil (5-FU)/capecitabine, irinotecan, or oxaliplatin-the approved cytotoxics in CRC. Epigenetic dysregulation of CRC may be a useful therapeutic selection tool in the future, and the methylation of various genes in other tumor types has been associated with chemosensitivity or chemoresistance to certain agents (eg, MGMT methylation predicts alkylating agent sensitivitiy, CHFR methylation predicts taxane sensitivity). A recent high-impact paper reported that TFAP2E hypermethylation predicted chemoresistance in four separate cohorts of CRC patients; however, how to integrate these findings into clinical practice is uncertain.
There are some signals that molecular alterations in CRC may be prognostic and predictive. A recent report of a 12-gene recurrence score in stage II and stage III CRC found that patients with a low recurrence score had a better prognosis and no benefit from the addition of oxalipatin to 5-FU–based chemotherapy, and the reverse was true for patients with a high recurrence score. Whether this gene panel can be used in patients with metastatic CRC is an open question. BRAF mutation–positive patients have a poorer prognosis, and some studies have shown that triplet cytotoxic treatment may be better in that setting (leucovorin, 5-FU, irinotecan, and oxaliplatin [FOLFIRINOX]). These studies have also shown a possible improvement in survival for CRC patients with wild-type tumors; it may be that upfront intensification of therapy is better for all patients, not just those with BRAF mutation–positive tumors. An appropriately powered phase III study of triplet vs doublet chemotherapy will be necessary to answer this question.
The herculean accomplishment of molecularly profiling CRC has opened many potential opportunities for the use of this information in therapeutic decision-making. However, we agree with Sridharan and colleagues that at present, only RAS testing in the metastatic setting has a definitive place in the decision-making paradigm. We continue to hope that, as new therapeutic agents are brought forward by the drug development community, we will be able to make larger strides in selecting potentially small subsets of patients who may benefit from novel therapies, and achieve further progress in understanding the agents we have in our CRC armamentarium.
Financial Disclosure:Dr. Azad serves on advisory boards for Amgen and Genentech. Dr. Diaz serves as a paid consultant to Amgen.
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2. Karapetis CS, Khambata-Ford S, Jonker DJ, et al. K-ras mutations and benefit from cetuximab in advanced colorectal cancer. N Engl J Med. 2008;359:1757-65.
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10. Yothers G, O’Connell MJ, Lee M, et al. Validation of the 12-gene colon cancer recurrence score in NSABP C-07 as a predictor of recurrence in patients with stage II and III colon cancer treated with fluorouracil and leucovorin (FU/LV) and FU/LV plus oxaliplatin. J Clin Oncol. 2013;31:4512-9.