Biomarker-guided therapy for colorectal cancer: strength in complexity

The number of molecularly stratified treatment options available to patients with colorectal cancer (CRC) is increasing, with a parallel rise in the use of biomarkers to guide prognostication and treatment decision-making. The increase in both the number of biomarkers and their use has resulted in a progressively complex situation, evident both from the extensive interactions between biomarkers and from their sometimes complex associations with patient prognosis and treatment benefit. Current and emerging biomarkers also reflect the genomic complexity of CRC, and include a wide range of aberrations such as point mutations, amplifications, fusions and hypermutator phenotypes, in addition to global gene expression subtypes. In this Review, we provide an overview of current and emerging clinically relevant biomarkers and their role in the management of patients with CRC, illustrating the intricacies of biomarker interactions and the growing treatment opportunities created by the availability of comprehensive molecular profiling.

Atypical, Non-V600 BRAF Mutations as a Potential Mechanism of Resistance to EGFR Inhibition in Metastatic Colorectal Cancer

PURPOSE

Atypical, non-V600 BRAF (aBRAF) mutations represent a rare molecular subtype of metastatic colorectal cancer (mCRC). Preclinical data are used to categorize aBRAF mutations into class II (intermediate to high levels of kinase activity, RAS independent) and III (low kinase activity level, RAS dependent). The clinical impact of these mutations on anti-EGFR treatment efficacy is unknown.

PATIENTS AND METHODS

Data from 2,084 patients with mCRC at a single institution and from an external cohort of 5,257 circulating tumor DNA (ctDNA) samples were retrospectively analyzed. Overall survival (OS) was calculated using Kaplan-Meier and log-rank tests. Statistical tests were two-sided.

RESULTS

BRAF mutations were harbored by 257 patients, including 36 with aBRAF mutations: 22 class III, 10 class II, four unclassified. For patients with aBRAF mCRC, median OS was 36.1 months, without a difference between classes, and median OS was 21.0 months for patients with BRAF^V600E mCRC. In contrast to right-sided predominance of tumors with BRAF^V600E mutation, 53% of patients with aBRAF mCRC had left-sided primary tumors. Concurrent RAS mutations were noted in 33% of patients with aBRAF mCRC, and 67% of patients had microsatellite stable disease. Among patients with aBRAF RAS wild-type mCRC who received anti-EGFR antibodies (monotherapy, n = 1; combination therapy, n = 10), no responses to anti-EGFR therapy were reported, and six patients (four with class III aBRAF mutations, one with class II, and one unclassified) achieved stable disease as best response. Median time receiving therapy was 4 months (range, 1 to 16). In the ctDNA cohort, there was an increased prevalence of aBRAF mutations and subclonal aBRAF mutations (P < .001 for both) among predicted anti-EGFR exposed compared with nonexposed patients.

CONCLUSION

Efficacy of anti-EGFR therapy is limited in class II and III aBRAF mCRC. Detection of aBRAF mutations in ctDNA after EGFR inhibition may represent a novel mechanism of resistance.

Targeted Therapy for Colorectal Cancers With Non-V600 BRAF Mutations: Perspectives for Precision Oncology

BRAF mutations are found in up to 10% of colorectal cancers (CRC). Whereas the majority of BRAF mutant CRCs harbor V600 mutations, up to 25% express non-V600 BRAF mutations. It has been established that BRAF V600E mutations in CRC predict unresponsiveness to epidermal growth factor receptor (EGFR) inhibition—cetuximab and/or panitumumab—as a result of the constitutive activation of the mitogen-activated protein kinase pathway downstream of EGFR signaling. As more centers begin using next-generation sequencing assays to detect BRAF mutations, oncologists are more frequently confronted with treating patients with non-V600 BRAF mutations. In many instances, clinicians may be hesitant to use EGFR inhibitors for these patients, as it is largely assumed that tumors with non-V600 BRAF mutations activate the mitogen-activated protein kinase pathway in a similar manner to RAS or BRAF V600E mutations and would therefore be equally refractory to EGFR inhibition; however, the evidence that currently exists to substantiate this claim is mixed and incomplete. Recent data demonstrate that non-V600 BRAF mutant CRC is a distinct clinical entity with a favorable prognosis compared with CRC with V600E mutations. Preclinical data and several case reports suggest that a subset of BRAF non-V600 mutations that impair the protein's kinase activity may in fact confer heightened sensitivity to EGFR inhibition because of dependency on upstream receptor tyrosine kinase signaling. This review summarizes the clinical characteristics and targeted therapy approaches for non-V600 BRAF mutant CRCs, speculates on the value of non-V600 BRAF mutations as predictive biomarkers of responsiveness to EGFR inhibitors, and highlights outstanding questions in this emerging area of precision oncology.