Oncogenic KRAS mutations can predict lack of response by colorectal cancer (CRC) to epidermal growth factor receptor (EGFR) blockade. Although KRAS status is usually determined through primary tumor biopsies, genomic profiles of their metastases may differ because of intrinsic molecular heterogeneity. We used circulating cell-free DNA (ccfDNA) to genotype CRC, and then to track clonal evolution during treatment with EGFR blockade, to evaluate the utility of ccfDNA to detect KRAS and BRAF mutations before and during chemotherapy.
We enrolled 55 patients with metastatic CRC, with no KRAS mutations in their primary tumors. Before starting chemotherapy, ccfDNA was purified from 1 mL serum from each patient. We detected 9 KRAS (G12A, G12R, G12D, G12C, G12S, G12V, G13D, Q61H, and Q61R) and BRAF (V600E) mutations, using the Invader method and digital PCR. Of the 55 patients, 24 were treated with systemic chemotherapy that included EGFR blockade. We extracted ccfDNA from these patients every 2–3 months until disease progression.
KRAS mutations in ccfDNA were detected in 5 patients before chemotherapy (9% [5/55], codon 12: 3 patients; codon 13: 2 patients). The response rate was 83% (20/24); 3 of the 4 non-responders were patients whose KRAS mutations were detected by ccfDNA before chemotherapy. The response rate of patients with no KRAS mutations in ccfDNA before chemotherapy was 95% (20/21). Of these 20 initially responsive patients, 10 (50%) acquired resistance. In 5 of these 10 patients (50%) ccfDNA detected emerging KRAS mutations. Three of these 5 patients had multiple mutations, including codon 12. Mutations in codon 61 were detected in 3 patients, but no solo codon-61 mutations were detected. BRAF mutation were also detected in 3 patients, but no solo BRAF mutations were detected.
EGFR blockade had no beneficial effect for patients in whom ccfDNA detected KRAS mutations in the primary tumor. Emerging KRAS or BRAF mutations that were undetected before starting chemotherapy can lead to acquired resistance to EGFR blockade. However, emerging KRAS or BRAF mutations were detected in only 50% of patients who acquired resistance, which implies the presence of another mechanism of acquiring resistance.
Clinical trial identification
Legal entity responsible for the study
Nippon Medical School
research funding of Department of Digestive Surgery, Nippon Medical School
All authors have declared no conflicts of interest.