Quantitative Cell-Free DNA, KRAS , and BRAF Mutations in Plasma from Patients with Metastatic Colorectal Cancer during Treatment with Cetuximab and Irinotecan

PURPOSE: The present study investigated the levels of circulating cell-free DNA (cfDNA) in plasma from patients with metastatic colorectal cancer (mCRC) in relation to third-line treatment with cetuximab and irinotecan and the quantitative relationship of cfDNA with tumor-specific mutations in plasma. EXPERIMENTAL DESIGN: Inclusion criteria were histopathologically verified chemotherapy-resistant mCRC, adequate Eastern Cooperative Oncology Group performance status, and organ function. Treatment consisted of irinotecan being administered at 350 mg/m(2) for 3 weeks and weekly administration of 250 mg/m(2) cetuximab until progression or unacceptable toxicity. A quantitative PCR method was developed to assess the number of cfDNA alleles and KRAS and BRAF mutation alleles in plasma at baseline. RESULTS: The study included 108 patients. Only three patients were positive for BRAF mutations. The majority of KRAS mutations detected in tumors were also found in the plasma 32 of 41 (78%). Plasma cfDNA and plasma mutant KRAS levels (pmKRAS) were strongly correlated (r = 0.85, P 75% quartile (P = 0.009)]. Patients with pmKRAS levels higher than 75% had a disease control rate of 0% compared with 42% in patients with lower pmKRAS (P = 0.048). Cox analysis confirmed the prognostic importance of both cfDNA and pmKRAS. High levels were clear indicators of a poor outcome. CONCLUSIONS: KRAS analysis in plasma is a viable alternative to tissue analysis. Quantitative levels of cfDNA and pmKRAS are strongly correlated and hold promise of clinical application.