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.