Abstract 3229: Distinct types of cetuximab resistant KRAS mutant of colorectal cancer revealed by an integrated sensitivity assay

Abstract

Abstract Purpose: Cetuximab, an anti-EGFR antibody, is clinically used in colorectal cancer (CRC) patients. Several clinical trials revealed that KRAS mutant is resistant to cetuximab. As KRAS is one of the downstream molecules in EGFR pathway, KRAS mutants are thought to be resistant to cetuximab due to the constitutive activation of downstream signaling, although mechanism of resistance in KRAS-mutant CRC remains to be elusive. We developed a primary culture system, CTOS method, by which cancer cells are prepared and cultured from patient tumors, maintaining histological and molecular characteristics of original tumors (Kondo 2011 PNAS). CTOS method can be applied to patient tumors as well as mouse xenografts. In this study, we performed integrated sensitivity assays by in vivo/in vitro CTOS sensitivity assay, and CTOS signaling assay in order to investigate characteristics of KRAS mutant CRC cells. Methods: Nine CTOS lines (3 KRAS-wild type, and 6 KRAS-mutant) were analyzed. For in vivo sensitivity assay, growth suppression by cetuximab was evaluated in mouse xenografts derived from CTOSs. CTOSs were classified into three groups according to the response (Regression: tumors became smaller than initial size; Intermediate; tumors became smaller than untreated control but bigger than initial size; Progression: no effect). For in vitro sensitivity assay, growth suppression by cetuximab was evaluated in CTOSs. For signaling analysis, CTOSs were treated with EGF and/or cetuximab, and phosphorylation of EGFR, AKT, and ERK were evaluated. Results: Regression in vivo was found in the tumors originated only from KRAS-wt (2/3). KRAS-mutant CTOSs were able to be clearly divided into two groups. Type I (2 CTOSs) showed Progression in vivo, no response in vitro, and no response in signaling assay. Even the phosphorylation of EGFR was not inhibited by cetuximab treatment. On the other hand, Type II (4 COTSs) showed Intermediate in vivo, moderate response in vitro, and variable responses in signaling assay. ERK phosphorylation was inhibited by cetuximab treatment in all 4 CTOSs. Conclusions: Using integrated in vivo and in vitro assays, KRAS mutant was able to be divided into two distinct groups according to the response to cetuximab. Resistant mechanism of KRAS mutant rather than MAPK pathway activation can be studied in this integrated assay system. Citation Format: Takahiro Tashiro, Hiroaki Okuyama, Hiroko Endo, Kenji Kawada, Masayuki Ohue, Yoshiharu Sakai, Masahiro Inoue. Distinct types of cetuximab resistant KRAS mutant of colorectal cancer revealed by an integrated sensitivity assay. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3229. doi:10.1158/1538-7445.AM2015-3229