Abstract 5357: Total abdominal ultra-rapid FLASH irradiation enhances the efficacy of PD-1 inhibition in preclinical models of ovarian cancer

Objective: Ovarian cancer often presents with advanced stage disease. Despite surgery and chemotherapy, recurrence remains inevitable and prognosis poor. Advances in immunotherapy including programmed cell death-1 (PD-1) inhibition show promise, however clinical trials show modest response rates as a single agent therapy in ovarian cancer patients. Radiation therapy is an attractive strategy to enhance the efficacy of immune checkpoint blockade in ovarian cancer due to its ability to increase tumor-infiltrating lymphocytes and enhance immune activation. While ovarian cancer is known to be radiosensitive, total abdominal radiation is not routinely utilized due to high abdominopelvic toxicity. We have recently shown that ultra-high dose rate FLASH irradiation reduces intestinal radiation-induced toxicity. Importantly, we demonstrated efficient tumor control in preclinical models of ovarian cancer peritoneal metastasis. Here we sought to determine the efficacy and safety of FLASH irradiation combined with PD-1 inhibition in preclinical models of ovarian cancer. Methods: Female C57BL/6 mice were intraperitoneally inoculated with ID8-Asc or UPK10 ovarian cancer cells. Total abdominal FLASH irradiation at 14 Gy was administered 10 days post-inoculation. Anti-PD-1 and isotype control antibodies were injected on days 7, 10, and 13 post-inoculation. Mice were sacrificed 22-27 days post-inoculation for tumor burden and immune cell analysis. Stool count, stool weight, complete blood count, and survival were evaluated to compare acute and chronic toxicity as well as efficacy in the treatment arms. Results: Four cohorts of mice were analyzed: Isotype control antibody (IgG), anti-PD-1 (aPD-1), FLASH + IgG, and FLASH + aPD-1. In the ID8-Asc model, irradiated mice showed a 44% decrease in solid stool production with subsequent recovery to baseline by 14 days post-treatment. There was no evidence of hematopoietic toxicity in any of the treatment arms. Survival analysis is ongoing. Exploratory necropsy demonstrated significant tumor burden control in the FLASH cohorts, and in particular, a 25-fold decrease in tumor number in the FLASH + aPD-1 cohort compared to control (mean tumor weight 1.82 mg vs. 357.2 mg, respectively). FLASH + aPD-1 increased intratumoral CD4+ and CD8+ T cells and correlated with enhanced tumor control compared to FLASH or aPD-1 single agent treatments. Similar results were observed in the UPK10 model. Conclusion: Combination FLASH and PD-1 inhibition enhanced tumor control and recruitment of tumor infiltrating lymphocytes compared to IgG, aPD-1, and FLASH treatments alone, while demonstrating safety and tolerability. Our data identify a new opportunity for FLASH and checkpoint inhibition in the treatment of ovarian cancer. Citation Format: Stephanie Chow, Joshua T. Eggold, Karen Levy, Jinghui Wang, Rakesh Manjappa, Dylan Y. Breitkreutz, Amy S. Yu, Karl Bush, Oliver Dorigo, Billy W. Loo, Erinn B. Rankin. Total abdominal ultra-rapid FLASH irradiation enhances the efficacy of PD-1 inhibition in preclinical models of ovarian cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5357.