Background: KRAS glycine-to-cysteine amino acid substitutions at codon 12 (KRASG12C) occur in ~13% of non-small cell lung cancers (NSCLC), ~3% of colorectal cancers (CRC), and less commonly in other cancer types. In early phase clinical trials of patients with KRASG12C-mutant cancers, promising antitumor activity has been reported with drugs such as adagrasib (MRTX849) and sotorasib (AMG510) which are direct inhibitors of KRASG12C. These small molecule irreversible inhibitors bind covalently to cysteine 12 within the switch II pocket which is formed, in part, by residues H95, Y96, and Q99. Mechanisms of acquired resistance to these therapies are currently unknown. Methods: Patients with KRASG12C-mutant NSCLC and CRC who were enrolled on adagrasib clinical trials and developed subsequent disease progression were included in this study if they were also consented to institutional review board-approved correlative studies at participating institutions. Tissue biopsies were analyzed histologically; tumor and/or circulating tumor DNA samples underwent next generation sequencing at the time of disease progression which was compared to that from pre-treatment samples when available. Results: A total of 30 patients were included in this study, 23 with NSCLC and 7 with CRC (25 with ctDNA, 7 with tissue sequencing, 2 with both). At the time of acquired resistance to adagrasib, we observed multiple on-target acquired KRAS alterations: mutations of the covalent-binding C12 residue, including C12W, C12F, C12V; a KRAS G13D mutation; high-level amplification of the KRASG12C allele; and mutation of the switch II binding pocket residues R68S, H95D, H95R, and Y96C. Furthermore, we detected several acquired off-target bypass mechanisms of resistance such as EGFR or MET amplification; activating mutations in NRAS (Q61K), BRAF (V600E), MAP2K1 (K57N, I99_K104del, E102_I103del), and RET (M918T); and oncogenic fusions involving RET, BRAF, RAF1, and FGFR. In two NSCLC cases with repeat tissue biopsies, histologic transformation from lung adenocarcinoma to squamous cell carcinoma was observed with no identifiable genomic mechanism of acquired resistance. In several cases, multiple coincident resistance mechanisms were identified in the same patient. Deep scanning mutagenesis studies were performed in parallel and identified the landscape of resistance mutations to adagrasib and sotorasib. While most resistance mutations confer high-level resistance to both therapies, some second-site mutations display differential sensitivity to distinct KRASG12C inhibitors, suggesting potential therapeutic strategies for overcoming drug resistance to specific mutations. Conclusion: Diverse genomic and histologic mechanisms impart resistance to covalent KRASG12C inhibitors in patients with cancer. Acquired genomic mutations, amplifications, and rearrangements may be potentially targetable by combining KRASG12C inhibition with available kinase inhibitors or SHP2 inhibitors.

Citation Format: Mark Awad, Shengwu Liu, Kathryn Arbour, Viola Zhu, Melissa Johnson, Rebecca Heist, Tejas Patil, Gregory Riely, Joseph Jacobson, Julien Dilly, Xiaoping Yang, Nicole Persky, David Root, Lynette Sholl, Lee Lim, Kavita Garg, Mark Li, Lars Engstrom, Laura Waters, J. David Lawson, Peter Olson, James Christensen, Piro Lito, Sai-Hong Inatius Ou, Pasi Janne, Andrew Aguirre. Mechanisms of acquired resistance to KRAS G12C inhibition in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB002.