PROJECT SUMMARY/ABSTRACT Oncogenic KRASG12C (KG12C) mutations underpin the development of ~14% of non-squamous non-small cell lung cancer (NSCLC) and account for ~10,000 deaths annually in the U.S. The development of potent, selective and clinically active covalent inhibitors of the KG12C oncoprotein represents one of the most exciting recent advances in the field of targeted cancer therapy, yet strategies to circumvent the development of adaptive resistance and improve the durability of individual responses to KG12C inhibitors are urgently needed in order to transform clinical outcomes for patients. The role of crosstalk between tumor cells and the tumor microenvironment (TME) in the development of acquired KG12C inhibitor resistance in NSCLC has not been systematically examined to date, despite evidence that in more than 50% of cases no genomic resistance mechanisms can be identified at the time of radiological progression. Furthermore, key mediators of TME remodeling and immune escape in response to KG12C inhibitor therapy remain poorly defined and therapeutic strategies that target the adverse TME in order to prevent, delay or overcome KG12C inhibitor adaptation/acquired resistance have not been established. Finally, the impact of major co-occurring genomic alterations in STK11/LKB1 and TP53 and that shape the immune contexture of KRAS-mutant NSCLC– on non-genetic mechanisms of acquired KG12C inhibitor resistance is not known. Based on our preliminary findings and previous work we hypothesize that: 1. Remodeling of the tumor microenvironment and immune escape can promote non-tumor cell autonomous adaptation/acquired resistance to KG12C inhibitors; 2. Targeting STAT3 signaling with TTI-101 can forestall and possibly overcome non-genetic acquired resistance to KG12C inhibitors through effects on tumor cells and/or the TME. In Aim 1, we will determine the contribution of TME remodeling and immune escape to acquired KG12C inhibitor resistance in NSCLC, using immune competent models of KG12C NSCLC that recapitulate its co-mutational complexity. We will further interrogate the role of master mediators of TME adaptive remodeling with initial focus on STAT3 and we will validate key findings using paired biopsies from patients with metastatic KG12C-mutant NSCLC that were treated with sotorasib as part of standard of care. In Aim 2, we will evaluate the anti-tumor efficacy and TME- modifying effects of STAT3 inhibition with TTI-101 in combination with direct KG12C inhibitors in immune- competent models of KG12C NSCLC. Clinical significance: This work will yield fresh insights into non-genetic mechanisms of acquired resistance to KG12C inhibitors that rely on tumor-TME crosstalk and will facilitate the development of novel therapeutic strategies that tackle the adverse TME in order to maximize long-term clinical benefit from KG12C inhibitors.