Abstract 3135: Prediction of acquired resistance in colorectal cancer patients treated with EGFR blockade by detection of a new KRAS mutation in ccfDNA

Abstract

Abstract Background: Oncogenic KRAS mutations can be used to predict a lack of response to epidermal growth factor receptor (EGFR) blockade. KRAS status is usually determined by biopsy from the primary site of colorectal cancer (CRC). However, the genomic profiles of primary tumors and metastases are not always concordant because of intrinsic molecular heterogeneity. Furthermore, chemotherapeutic agents and targeted drugs can alter the tumor molecular landscape. We exploited circulating tumor DNA (ctDNA) to genotype colorectal tumors and track clonal evolution during treatment involving EGFR blockade. To account for these spatial and temporal changes, the genomic profiles of patients with CRC can be repeatedly evaluated during the course of therapy. In this study, we evaluated the utility of KRAS mutation detection using ctDNA before and during chemotherapy. Method: Experiment 1: We enrolled 46 metastatic colorectal cancer patients. Before starting chemotherapy, ctDNA was purified from 1 mL serum using the QIAamp circulating nucleic acid kit. We detected nine KRAS (G12A, G12R, G12D, G12C, G12S, G12V, G13D, Q61H, and Q61R) mutations using the Invader method and digital PCR. Experiment 2: Fifteen patients were treated with systemic chemotherapy including EGFR blockade. ctDNA was extracted from these patients every 2 months until disease progression, and the KRAS mutation was detected in nine. Results: Experiment 1: KRAS mutations in circulating tumor DNA were detected in 88% (14/16) of patients with KRAS mutations in their primary tumor, but in 10% (10/30) of patients without KRAS mutations in their primary tumors. Experiment 2: The response rate was 87% (13/15). In two non-responders, KRAS mutations in ctDNA were detected before chemotherapy. Disease progression was identified in five patients and KRAS mutations in ctDNA were detected in all patients; the five patients with disease progression included the two non-responders in whom KRAS mutations in ctDNA were detected before chemotherapy. However, the genotypes detected after disease progression were different from those detected before chemotherapy (G13D to G12C and Q61R to Q61H). New KRAS mutations occurred in codon 12 in the other three patients in whom no KRAS mutations in ctDNA were detected before chemotherapy. New KRAS mutations in codon 61 disappeared during chemotherapy and disease progression was not detected at that time in two patients. Discussion: It has been reported in a retrospective study that new KRAS mutations were detected in patients who acquired resistance to EFGR blockade, and many of them were in codon 61. However, new KRAS mutations in codon61 can disappear during treatment involving EGFR blockade. This phenomenon may indicate that new KRAS mutations in ctDNA do not always indicate acquired resistance to chemotherapy involving EGFR blockade. Citation Format: Takeshi Yamada, Hayato Kan, Takuma Iwai, Goro Takahashi, Michihiro Koizumi, Akihisa Matsuda, Seiichi Shinji, Yasuyuki Yokoyama, Atsushi Tatsguchi, Tetsuro Kawagoe, Shiro Kitano, Masato Nakayama, Satoshi Matsumoto, Keiichiro Ohta, Eiji Uchida. Prediction of acquired resistance in colorectal cancer patients treated with EGFR blockade by detection of a new KRAS mutation in ccfDNA. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3135.