Abstract 5507: Mechanisms of resistance to RAS-GTP inhibition in pancreatic cancer

Abstract Targeting oncogenic RAS signaling has the potential to be effective for many RAS-addicted cancers, including pancreatic ductal adenocarcinoma (PDAC). RMC-7977 is a RAS(ON) multi-selective tri-complex inhibitor exhibiting broad-spectrum preclinical activity against both wild-type and oncogenic RAS variants and has shown encouraging anti-tumor activity in preclinical models of PDAC. A related compound, RMC-6236, is under clinical investigation for both RAS-mutant solid tumors and metastatic PDAC. Tumor progression due to acquired resistance is frequently observed for targeted agents, but potential mechanisms of resistance specific to RAS(ON) multi-selective inhibitors have not been characterized. Here, we investigate both on-target and off-target mechanisms of resistance to RMC-7977 in pre-clinical models of PDAC. Using deep mutational scanning and structure-function analysis, we define the landscape of secondary KRAS mutations that confer resistance to RMC-7977. Leveraging insights from ctDNA analysis of patients, we validated putative genetic mechanisms of resistance in PDAC cell line and xenograft models, finding that PI3K pathway activation can drive proliferation under RAS blockade. Furthermore, we performed a multi-omic characterization of human PDAC cell lines and organoids, as well as murine PDAC tumors, to identify factors contributing to RAS independence. Epithelial-to-mesenchymal transition emerged as highly associated with both innate and acquired resistance. To further understand the adaptive signaling that promotes survival in mesenchymal, resistant PDAC cells, we applied phosphoproteomics followed by kinase motif enrichment analysis in resistant cells after RAS inhibition. We determined multiple kinases involved in ECM and cytoskeletal signaling that are activated upon MAPK suppression uniquely in mesenchymal PDAC cells. Targeting these kinases in combination with RMC-7977 reverted resistance in models of innate and acquired resistance, suggesting a potential approach to investigate preventing or overcoming resistance in patients. Collectively, our pre-clinical data suggest that resistance to RAS inhibition with RMC-7977 may arise via multiple mechanisms, among them secondary RAS mutations, PI3K pathway-activating mutations, or cell-state changes that drive adaptive signaling through ECM-signaling pathways. Citation Format: Joshua H. Choe, Julien Dilly, Eejung Kim, Lick P. Lai, Nicole Sindoni, Fan Yi Kong, Dennis Gong, Jaclyn Varga-Wiles, Brendan Parent, Junning Wang, Kyle Seamon, Jennifer A. Chan, Clint A. Stalnecker, Channing J. Der, William L. Hwang, Brian M. Wolpin, David Wildes, Joseph D. Mancias, Lewis C. Cantley, Andrew J. Aguirre. Mechanisms of resistance to RAS-GTP inhibition in pancreatic 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 5507.