Tracking emerging KRAS, BRAF and EGFR mutations through ccfDNA in colorectal cancers treated with EGFR blockade.

547 Background: Epidermal growth factor receptor (EGFR) blockade can achieve considerable tumor shrinkage in patients with metastatic colorectal cancer (CRC). However, most patients who benefit from EGFR blockade acquire resistance within a year through emerging KRAS, BRAF or EGFR ( S492R) mutations. We have shown that patients with KRAS or BRAF mutations in circulating cell-free DNA (ccfDNA) do not respond favorably to EGFR blockade. In this study, we aimed to detect acquired resistance to EGFR blockade early by using ccfDNA to track emerging KRAS, BRAF and S492R mutations during chemotherapy. Methods: We enrolled 24 patients with metastatic CRC and no KRAS mutations in their primary tumors who were to be treated with systemic chemotherapy that included EGFR blockade. We purified ccfDNA from 1 mL of serum from each patient before they started chemotherapy. We also extracted ccfDNA from these patients every 2–3 months until disease progression. We detected nine KRAS, BRAF (V600E) and S492R mutations using digital polymerase chain reaction. Results: We detected KRAS mutations in ccfDNA of three patients before chemotherapy (12.5%; 3/24). The response rate was 83% (20/24); the four non-responders comprised the three with KRAS mutations and the other with BRAF mutations in their ccfDNA before chemotherapy. All patients with no KRAS or BRAF mutations in their ccfDNA before chemotherapy responded to the chemotherapy (20/20). Of these 20 initially responsive patients, 12 (60%) acquired resistance. We detected emerging KRAS mutations in the ccfDNA of eight of these 12 patients (67%) prior to relapse, six of them having multiple mutations. Five of these patients had mutations in codon 61 and one had solo codon 61 mutations. We also detected BRAF mutations in five patients; none had solo BRAF mutations. Four patients had S492R mutations; none had solo S492R mutations. Conclusions: If KRAS or BRAF mutation do not detected in primary tumor, EGFR blockade has no beneficial effect in patients with KRAS or BRAF mutations in their ccfDNA prior to commencing chemotherapy. Emerging KRAS, BRAF or S492R mutations that were undetectable before starting chemotherapy are associated with acquired resistance to EGFR blockade.