Abstract 3499: Inhibition of SOS1 by MRTX0902 augments the anti-tumor response of the targeted EGFR inhibitor osimertinib in NSCLC

Abstract Osimertinib is a third generation, irreversible epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor recommended as a first line therapy in patients with advanced non-small lung cancer (NSCLC) characterized by EGFR activating mutations (exon 19 deletions and exon 21 L858R) and the resistance associated exon 20 T790M mutation, which is present in approximately 40-55% of patients following treatment with first generation EGFR-targeting inhibitors. Unfortunately, osimertinib monotherapy inevitably leads to both EGFR-dependent and EGFR-independent resistance mechanisms that reactivate RTK/MAPK pathway signaling. Thus, exploration of new combination strategies that target RTK/MAPK-mediated bypass of EGFR dependence may circumvent or delay therapeutic resistance are a key research focus. Preclinically, it has been shown that EGFR-mutant NSCLC cell lines are sensitive to inhibition of an upstream node of the MAPK pathway, the Son of Sevenless homolog 1 (SOS1) protein, which functions as a guanine nucleotide exchange factor (GEF) for the RAS subfamily of small GTPases. MRTX0902 is a selective and potent SOS1 inhibitor currently in Phase 1 clinical trials and functions by disrupting the KRAS/SOS1 protein-protein interaction to prevent SOS1-mediated nucleotide exchange on KRAS, ultimately leading to downregulation of KRAS-MAPK pathway signaling. Here we demonstrate that dual inhibition of SOS1 and mutant EGFR by osimertinib results in greater anchorage-independent (3D) cell death and further downregulation of MAPK and PI3K/AKT pathway signaling compared to single agent osimertinib treatment. In our current study, we characterized the anti-tumor effects of SOS1 inhibition by MRTX0902 as a monotherapy and in combination with osimertinib in EGFR mutant models of NSCLC. While single agent MRTX0902 treatment effectively inhibited EGFR mutant cell survival, combination treatment of MRTX0902 with osimertinib resulted in (1) deeper and sustained inhibition of MAPK and PI3K/AKT pathway signaling, (2) enhanced inhibition of 3D cell viability and increased apoptosis, and (3) increased anti-tumor efficacy compared to osimertinib monotherapy in EGFR mutant CDX models in vivo. These studies uncover the potential clinical application of combined vertical inhibition of RTK/MAPK pathway signaling by osimertinib and MRTX0902 and ultimately aide in the understanding of EGFR and RAS biology in targeted cancer therapy. Citation Format: Shilpi Khare, Niranjan Sudhakar, Jade Laguer, David M. Briere, Larry Yan, Allan Hebbert, Andrew Calinisan, Lars D. Engstrom, Fadia Qiryaqos, Peter Olson, James G. Christensen, Jacob R. Haling. Inhibition of SOS1 by MRTX0902 augments the anti-tumor response of the targeted EGFR inhibitor osimertinib in NSCLC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3499.