Abstract B008: Pan-RAS-GTP inhibition disrupts the KRAS-SHH axis to reprogram cancer-associated fibroblast subtypes and reduce stiffness in pancreatic cancer

Abstract

Oncogenic KRAS mutations are present in ∼95% of pancreatic ductal adenocarcinoma (PDAC) cases and have long been considered undruggable. The recent clinical emergence of RAS inhibitors marks a transformative moment in PDAC therapy, yet their impact on the tumor microenvironment (TME) remains largely unknown. This is a critical gap: the desmoplastic stroma can comprise up to 90% of PDAC tumor volume, and its dominant cellular component, cancer-associated fibroblasts (CAFs), actively shape tumor progression, drug delivery, and immune exclusion. KRAS-dependent Sonic Hedgehog (SHH) signaling is a key driver of myofibroblastic CAF (myCAF) dominance, but whether RAS inhibition can reprogram this fibroblast landscape has never been addressed. Using the autochthonous KPC mouse model and patient-derived PDAC explants harboring four distinct oncogenic KRAS mutations, we dissected stromal remodeling by RMC-7977, a multi-selective RAS(ON) inhibitor, through scRNA-seq of 29 KPC tumors, co-immunofluorescence, RNAscope, SHH rescue experiments, curated ECM gene set analyses, and nanoindentation. RMC-7977 dramatically rebalanced the PDAC stroma: the myCAF-dominant TME shifted toward iCAF dominance after just a single dose, an effect that deepened and consolidated with prolonged treatment. This shift was driven by suppression of epithelial SHH secretion downstream of KRAS, with exogenous SHH administration fully reversing the myCAF depletion, directly implicating the KRAS-SHH axis as a master regulator of CAF state in PDAC. Decreased myCAF proliferation alongside increased non-myCAF proliferation, together scRNA-seq data revealing progressive confinement of CAFs to an iCAF identity subsequent to RMC-7977 treatment indicate that active phenotypic plasticity, and not clonal selection alone, underlies this stromal remodeling. SMO inhibition phenocopied RAS inhibition, thus confirming that abrogation of paracrine SHH signaling is necessary and sufficient to drive the myCAF-to-iCAF transition. At the ECM level, myCAFs and iCAFs play complementary but distinct roles: myCAFs dominate crosslinking and receptor expression while iCAFs drive matrix remodeling. RAS inhibition exploits this division of labor by shifting the balance from a crosslinker-rich, stiffness-promoting matrix toward a degradation-permissive ECM. Strikingly, one week of RMC-7977 significantly reduced tumor stiffness by nanoindentation in KPC mice, an effect recapitulated in KRAS-mutant but entirely absent in KRAS wild-type human PDAC explants ex vivo, thus establishing KRAS dependency as a prerequisite for stromal softening. For the first time, we demonstrate that pharmacological RAS inhibition fundamentally reprograms the pancreatic stroma by dismantling the KRAS-SHH-myCAF axis, remodeling the ECM, and softening the biomechanical barrier that characterizes PDAC. These findings recast RAS inhibitors not merely as epithelial-targeting agents, but as powerful stromal reprogrammers, thus opening new avenues for combination strategies that exploit a less rigid, less immune-excluded TME.