Abstract 6154: Diffusing alpha-emitters radiation therapy in combination with anti-CTLA-4, as well as combination with both ATR inhibitor, and anti-CTLA-4 achieves long-lasting tumor regression in murine pancreatic cancer

Pancreatic cancers (PC) are characterized by their aggressiveness, radioresistance, and immunologically "cold" nature, making them the third leading cause of cancer deaths in the US. Specifically, conventional radiotherapy approaches are limited by collateral damage to adjacent organs. Given the lack of effective treatments, this study investigates a novel therapy combining α-particle radiation through Diffusing Alpha-emitters Radiation Therapy (Alpha DaRT), an intratumoral source that releases alpha emitting atoms into the tumor microenvironment, with ATR inhibition (ATRi) to sensitize immunologically cold PC to anti-CTLA-4 (aCTLA-4). While conventional low linear energy transfer (LET) radiation (0.2 keV/μm) has lower capacity to induce double-strand breaks (DSBs), higher LET α-particles (70-200 keV/μm) generate more complex DSBs and micronuclei, activating an immune response through the cGAS-STING pathway. By combining Alpha DaRT with ATRi, we aim to impair radiation-induced DSB repair and checkpoint arrest, forcing mitotic progression despite unrepaired DNA damage, overstimulating this pathway. We hypothesize that the triple treatment (Alpha DaRT+ATRi+aCTLA-4) sensitizes immunologically cold tumors to aCTLA-4, delaying tumor growth, inducing tumor regression, and extending survival. Mice were implanted with 5 × 105 KPC cells subcutaneously in the left leg. After 7 days of tumor inoculation (day 0), Alpha DaRT sources were implanted. ATRi (or DMSO) was administered via oral gavage on days 0, 1, 2, 7, 8, and 9, at 75 mg/kg. Anti-CTLA-4 (or IgG) was delivered intraperitoneally on days 3, 6, 9, and 12. Tumor volumes were measured three times/week and mice were randomized into eight experimental groups, ranging from triple control to triple treatment. Tumor development was assessed according to tumor volume fold change, relative to Alpha DaRT insertion day. Both Alpha DaRT +ATRi or Alpha DaRT+aCTLA4 significantly reduced tumor development vs. monotherapies. Remarkably, more than 40% achieved complete tumor regression in the Alpha DaRT+DMSO+aCTLA-4 or Alpha DaRT+ATRi+aCTLA-4. Mice in which tumors regressed were re-challenged on day 130 with 5 × 105 KPC cells. Both the double treatment (Alpha DART + aCTLA4) or the triple treatment exhibited significantly smaller tumor development relative to naïve mice. These novel results suggest that the dual combination Alpha DaRT and aCTLA4 or the triple combination Alpha DaRT, ATRi, and aCTLA-4 may be a strategy to induce a robust and long-lasting immune activation in immunologically cold PC. Citation Format: Marco Reis, Poliana Marinello, Alexandre Rubinstein, Mark Wasley, Walison Brito, Ronen Sega, Vered Bachar, Gabriel Sawakuchi. Diffusing alpha-emitters radiation therapy in combination with anti-CTLA-4, as well as combination with both ATR inhibitor, and anti-CTLA-4 achieves long-lasting tumor regression in murine pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6154.