Abstract 1707: Characterization of egfr ectodomain mutation in acquired resistance to cetuximab in colorectal cancer

Abstract Colorectal cancer (CRC) remains the second leading cause of cancer-related deaths in the United States (US), and individuals with surgically unresectable stage four CRC have a five-year overall survival of 13%. Targeting of the receptor tyrosine kinase (RTK), EGF receptor (EGFR) with therapeutic monoclonal antibodies (cetuximab, panitumumab) is approved for use in wild-type KRAS/NRAS/BRAF (RAS/RAF) advanced CRC. However, these antibodies as a single agent have response rates of 12-17% and expected progression-free survival of only 3-4 months. Accordingly, resistance may be present at the outset (de novo resistance) or develop during treatment (acquired resistance). To model and understand drug resistance, we cultured CRC lines in a defined 3D matrix (type 1 collagen) with anti-epidermal growth factor receptor (EGFR) antibody cetuximab. Here, we report the identification of an EGFR ectodomain missense mutation acquired in vitro in response to cetuximab treatment. Using 2D and 3D drug sensitivity assays, we demonstrate that this EGFR ectodomain mutation, in addition to being resistant to cetuximab and panitumumab, also could not be targeted by the oligoclonal anti-EGFR antibody cocktail MM-151. To determine whether this EGFR ectodomain mutation is necessary and sufficient to impart cetuximab resistance, (i) we engineered stable cell lines that inducibly overexpress mutant EGFR and (ii) demonstrated the sufficiency of mutant EGFR overexpression in conferring cetuximab resistance under both in vitro and in nude mouse xenografts. Further, our structural, functional, and biochemical studies implicate reduced binding of EGFR mutant to cetuximab and sustained downstream signaling activity as a cause of cetuximab resistance. Collectively, this mutation necessitates additional treatment strategies for a subset of subjects with advanced CRC who do not respond to treatment with current anti-EGFR biologics. Citation Format: Neeraj Joshi, Hidenori Tanaka, Galina Bogatcheva, Ramona Graves Deal, Claudia Wahoski, Sarah Harmych, Claiborne Tydings, Marisol Ramirez, James Higginbotham, Maria J. Irudayam, Jeffrey L. Franklin, Qi Liu, Jens Meiler, Robert J. Coffey, Eben L. Rosenthal, Bhuminder Singh. Characterization of egfr ectodomain mutation in acquired resistance to cetuximab in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1707.