Some 5% to 10% of patients with colorectal cancer harbor the BRAF mutation, placing them at risk for poor treatment response and worse outcomes. The ASCO Post interviewed S. Gail Eckhardt, MD, an expert in this area who is Professor and Head of the Division of Medical Oncology at the University of Colorado Cancer Center and holds the Stapp-Harlow Endowed Chair in Cancer Research. Dr. Eckhardt also directs the Program for the Evaluation of Targeted Therapy, a multidisciplinary effort focused on finding alternative treatments for colorectal cancer and melanoma.
Role of BRAF in Cancer
What is the BRAF mutation?
BRAF is a protein kinase downstream of RAS in the RAS/RAF/MEK/ERK pathway and has been a promising target in several cancers. The predominant mutation is similar to what we see in melanoma, the classic V600E mutation. The BRAF gene was discovered in the 1980s, but it was 10 years later before we identified the downstream effectors of this pathway. Mutations in BRAF lead to constitutive activation of downstream signaling through the MAPK pathway.
How does the BRAF mutation impact patients clinically?
The BRAF mutant genotype impacts the molecular and phenotypic characteristics of colorectal cancer. The mutation occurs fairly early in the progression of colorectal cancer and is associated with CpG island methylation. It leads to serrated adenoma-type tissue that has defective mismatch repair. In the end, you get a distinct type of tumor that is diploid and microsatellite-unstable.
The clinical characteristics associated with this genotype are female gender, older age, right-sided tumors, high-grade features, and microsatellite instability–high status (but not hereditary nonpolyposis colon cancer). Metastatic BRAF-mutated tumors carry a poor prognosis. In a cohort of 524 patients, overall survival for patients with BRAF-mutant colorectal cancer was 10.4 months compared with 34.7 months for BRAF wild-type patients.1 In early retrospective studies of both cetuximab (Erbitux) and panitumumab (Vectibix), we clearly observed worse outcomes—shorter progression-free and overall survival—in BRAF-mutant patients. Studies have been consistent in this regard.
Inhibition of BRAF
How is BRAF inhibition affected by tumor type?
The BRAF mutation activates the MEK/ERK pathway through its downstream effectors, where we see production and promotion of the malignant phenotype through gene expression and proliferation. However, the clinical results with BRAF inhibitors in colorectal tumors have been quite disappointing. Unlike what we observe in melanoma—an 81% response rate in patients with advanced disease—the response rate in patients with metastatic colorectal cancer is only about 5%. There is evidence that BRAF inhibition does work in some patients with colorectal cancer, as we have seen dramatic effects on metastatic disease in the liver. But in general, BRAF inhibition yields disparate effects between these two tumor types.
This may reflect a more heterogeneous pattern of BRAF activation in patients with colorectal cancer, but we also observe tumor heterogeneity in melanoma as well—mixed clinical responses to vemurafenib and rapidly acquired resistance. Even when different malignancies exhibit similar mutations, the mutations may not always signify similar tumor biology. Clearly, we need to obtain tissue from responding and nonresponding sites of disease to get a better sense of the resistance mechanisms.
Response and Resistance Pathways
Can BRAF be considered a targetable driver mutation in colorectal cancer?
As we discuss new findings regarding subsets of colorectal cancer, it is important to go back to some of our large genetic databases, such as The Cancer Genome Atlas (TCGA), which has performed an integrative analysis of genomic changes in 195 colorectal tumors. We see that within the subset of tumors that are hypermutated, 46% are mutant for BRAF. Interestingly, these tumors have mutations in genes involving the WNT and TGF-beta pathways, among others.
Preclinical studies have been conducted to help us model response and resistance patterns that might translate into clinical scenarios. In colorectal cancer cell line–derived xenograft models, there was a nice dose-dependent reduction in tumor growth with BRAF inhibition by vemurafenib (Zelboraf) and no response in BRAF wild-type tumors. These results were positive but potentially misleading in colorectal cancer.
For example, in work reported by Yang et al in BRAF-mutant colorectal cell lines, a diversity of responses to vemurafenib was observed.2 Interestingly, the KRAS-mutant cells were fairly uniformly resistant, indicating the existence of resistance pathways that may operate through upstream receptor tyrosine kinases that signal via CRAF and MEK. These data provide evidence that BRAF mutation is necessary but not sufficient for the activity of vemurafenib and that colorectal cancer cells can escape though receptor tyrosine kinase bypass and activation of the PTEN/PI3K/AKT pathway.
It has also been shown that incomplete suppression of phosphorylated ERK in BRAF-mutant colorectal cancer is associated with decreased sensitivity to vemurafenib. This suggests that phosphorylated ERK could be a pharmacodynamic marker of effective BRAF inhibition.
Multiple Pathway Inhibition
Why doesn’t targeting BRAF in BRAF-mutant colorectal cancer work?
In a study published in 2012, investigators conducted a synthetic lethal screen of BRAF-mutant colorectal cancer cells that were resistant to vemurafenib and ultimately revealed that BRAF inhibition by vemurafenib caused rapid feedback activation of epidermal growth factor receptor (EGFR), which supported continued proliferation in the presence of BRAF inhibition.3 The researchers showed that if you suppressed EGFR with cetuximab, erlotinib (Tarceva), or gefitinib (Iressa), added to vemurafanib, BRAF-mutant colorectal cancer was inhibited in both in vitro and in vivo models, suggesting that combination therapy with BRAF and EGFR inhibitors may be more effective in this patient population. This was corroborated by another group, which showed that the combination of vemurafenib and erlotinib led to tumor regression in colorectal cancer cell line xenografts and reduction of the proliferative marker Ki67.4
Data are becoming consistent, therefore, that EGFR or other receptor tyrosine kinases may mediate resistance to vemurafenib, and that combinations with an EGFR or MEK inhibitor may produce greater suppression of tumor growth.
Dr. Scott Kopetz’s laboratory has demonstrated that vemurafenib-resistant colorectal cancer cell lines exhibit PI3K/AKT activation. Enhanced tumor growth inhibition was observed when vemurafenib was combined with an inhibitor of this pathway.5 They concluded that activation of the PI3K/AKT pathway is a mechanism of both innate and acquired resistance to BRAF inhibitors and suggest combinatorial approaches to improve outcomes in this poor-prognosis subset. Thus, we should regard the PI3K/AKT pathway as another potential target for leveraging the activity of BRAF inhibition in BRAF-mutant tumors.
This research is telling us what we already know: In colorectal cancer, mutations have to be viewed within the context of other resistance pathways. “Mutation targeting” in colorectal cancer is quite distinct from what we see, for example, in non–small cell lung cancer with EGFR or ALK mutations. We are just not observing the successes seen in non–small cell lung cancer by mutation targeting alone.
Future Studies
What other questions must be addressed?
Do we need to segregate patients into different bins of resistance (inherent or acquired) and develop combination strategies that are tailored to those? Should combination therapy approaches be used up front or at the time of progression, in the case of acquired resistance? In a xenograft model, the addition of vemurafenib to irinotecan, cetuximab, bevacizumab (Avastin), or capecitabine (Xeloda) in metastatic disease resulted in increased antitumor activity and improved survival.2 How should we prioritize testing these regimens and patient selection strategies for this small subset in colorectal cancer?
We also need to learn more about the immunomodulatory effects of BRAF inhibition, which may complement some of the newer approaches augmenting T-cell activation.
Closing Thoughts
Any final thoughts about BRAF-mutated colorectal cancer?
What we know is that BRAF-mutated colorectal cancer is a distinct molecular, phenotypic, and clinical subset that is in dire need of new treatment strategies. The existence of the mutation does not confer sufficient sensitivity that a single agent targeting this mutation will be effective. This is probably related to both inherent and acquired resistance mechanisms.
In this area, preclinical models have been used to characterize resistance mechanisms and to develop actionable strategies with the result being that we now know that resistance is heterogeneous. Finally, we must obtain tissue from nonresponding patients and conduct sophisticated bench-to-bedside studies, as is being done in melanoma, to support preclinical rationales.
A proposed Intergroup trial will enroll 63 patients with BRAF-mutated colorectal cancer, who will receive one of three treatments: cetuximab plus irinotecan, cetuximab and irinotecan plus vemurafenib, or vemurafenib plus cetuximab. The endpoint is progression-free survival. ■
Disclosure: Dr. Eckhardt has received research funding from Roche/Genetech.
References
1. Tran B, Kopetz S, Tie J, et al: Impact of BRAF mutation and microsatellite instability on the pattern of metastatic spread and prognosis in metastatic colorectal cancer. Cancer 117:4623-4632, 2011.
2. Yang H, Higgins B, Kolinsky, K et al: Antitumor activity of BRAF inhibitor vemurafenib in preclinical models of BRAF-mutant colorectal cancer. Cancer Res 72:779-789, 2012.
3. Prahallad A, Sun C, Huang S, et al: Unresponsiveness of colon cancer to BRAF(V600E) inhibition through feedback activation of EGFR. Nature 483:100-103, 2012.
4. Corcoran RB, Ebi H, Turke AB, et al: EGFR-mediated re-activation of MAPK signaling contributes to insensitivity of BRAF mutant colorectal cancers to RAF inhibition with vemurafenib. Cancer Discovery 3:227-235, 2012.
5. Mao M, Tian F, Mariadason JM, et al: Resistance to BRAF inhibition in BRAF-mutant colon cancer can be overcome with PI3K inhibition or demethylating agents. Clin Cancer Res 19:657-667, 2012.
