265 - Combining Proton Radiation Therapy and Mesothelin-Targeting Chimeric Antigen Receptor T-cell Therapy to Improve Outcomes in Pancreatic Cancer

U. Amit Sr^1,2, U. Uslu³, I. I. Verginadis¹, M. M. Kim¹, S. A. O. Motlagh¹, E. S. Diffenderfer¹, C. A. Assenmacher⁴, S. Bicher¹, E. Ben-Josef¹, R. Young³, C. H. June³, and C. Koumenis¹; ¹Department of Radiation Oncology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, ²Department of Radiation Oncology, Tel Aviv Medical Center, Tel Aviv, Israel, ³Center for Cellular Immunotherapies, Department of Pathology and Laboratory Medicine, University of Pennsylvania Perelman School of Medicine, Philadelphia, PA, ⁴Comparative Pathology Core, Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA

Purpose/Objective(s): Pancreatic ductal adenocarcinoma (PDAC) represents a challenge in oncology, with limited treatment options for advanced-stage patients. Chimeric antigen receptor T (CAR T) cell therapy targeting mesothelin (MSLN) shows promise, but challenges such as the hostile immunosuppressive tumor microenvironment (TME) hinder its efficacy. Proton radiation therapy (RT) allows targeted delivery to the tumor site with reduced exposure to surrounding organs and immune cells. We hypothesized that combining proton RT with MSLN-targeting CAR T therapy would increase tumor cells’ MSLN expression, increase CAR T trafficking, and improve outcomes in a PDAC mouse model. Materials/

Methods: Syngeneic mouse PDAC models were used by inoculating the murine PDAC cell line, PDA7940b, into the flanks and orthotopically in the pancreatic tail of C57BL/6J mice. Proton RT was delivered to the tumors when they reached an average volume of 50 mm³, using a single fraction of 12 Gy. MSLN-targeting CAR T were injected through the tail vein two days after RT and a week later. Flow cytometry analysis was performed to assess CAR T trafficking and activation markers. ELISA was used to measure cytokines in serum. Tumor growth kinetics were evaluated by caliper measurements.
Results: Proton RT caused a 9.5-fold increase in MSLN expression in tumor cells and a significant increase in CAR T cell infiltration into tumors. The combination therapy reshaped the immunosuppressive TME, promoting antitumorigenic M1 polarized macrophages and reducing myeloid-derived suppressor cells (MDSC). In a flank PDAC model, the combination therapy demonstrated superior attenuation of tumor growth and improved survival compared to individual treatments alone. In an orthotopic PDAC model treated with image-guided proton RT, tumor growth was significantly reduced in the combination group compared to the RT treatment alone. Further, the combination therapy induced an abscopal effect in a dual-flank tumor model, with increased serum interferon-? and enhanced proliferation of extratumoral CAR T cells.
Conclusion: Combining proton RT with MSLN-targeting CAR T therapy proves effective in modulating the TME, enhancing CAR T cell trafficking, and exerting systemic antitumor effects. This approach presents a promising strategy for improving outcomes in unresectable PDAC.