PROJECT SUMMARY Pancreatic Ductal Adenocarcinoma (PDAC) is one the most lethal cancers with a 5-year survival rate of 8%. Current therapeutic options that successfully treat other cancers, including radiation therapy (RT), are only minimally effective in PDAC. This recalcitrant nature of PDAC has been linked in part to its unique tumor microenvironment (TME), which is mainly composed of collagen-rich extracellular matrix and abundant cancer associated fibroblasts. Combined chemoradiotherapy strategies currently used against PDAC are ineffective at generating sufficient tumor regression needed to achieve resectability. This lack of effective treatment options highlights an emergent need to understand how the fibrotic nature of PDAC determines RT efficacy and RT- induced tumor immunity. Our group recently demonstrated that Focal Adhesion Kinase (FAK), a cytoplasmic protein tyrosine kinase hyperactivated in many cancers, plays a role in regulating tumor stromal fibrosis. FAK inhibition was shown to reduce fibrosis and alter the desmoplastic stroma in PDAC, rendering it more responsive to immunotherapies. My preliminary data shows that modulating the PDAC fibrotic stroma using FAK inhibitor can increase the sensitivity of PDAC to radiotherapy both in vitro and in vivo. Thus, I hypothesize that the fibrotic stroma contributes to PDAC resistance to RT and RT-induced tumor immunity. To test this hypothesis, we aim to determine: 1) how RT shapes the PDAC TME, 2) the mechanism(s) by which inhibition of FAK signaling improves RT efficacy, and 3) whether stromal disruption by FAK inhibition enhances RT-induced anti-tumor immunity and response to checkpoint immunotherapy. The proposed study will not only help us understand how different RT regimens shape the unique PDAC fibrotic stroma and how this fibrosis impacts RT efficacy, but also inform us on how to best integrate fibrosis-depleting agents, such as FAK inhibitor, to improve RT efficacy in PDAC. We hope that this study will eventually translate into clinical trials with the potential to directly benefit PDAC patients.