Abstract A100: Entinostat alters the M1/M2 ratio in ascites compared to tumors derived from an ID8 murine model of ovarian cancer

Abstract Poly ADP ribose inhibitors (PARPi) are most effective in ovarian cancer tumors with homologous recombination (HR) deficiency. Our group has shown that histone deacetylase inhibitors (HDACi) sensitize HR proficient ovarian cancer cells to PARPi. Our current efforts are directed towards understanding how this therapeutic regimen alters tumor-associated macrophages (TAMs) in murine models of high grade serous ovarian cancer. To investigate the effects of Ola combined with Ent, we used HR-proficient ID8 P53 wild-type and ID8 P53−/− syngeneic murine models. Mice were randomized into 4 groups: control, Ola, Ent and Ola+Ent. Mice (Ent and Ola+Ent) were pre-treated with 15 mg/kg entinostat or vehicle (Ola and Ola+Ent) for one week via oral gavage and then treated with vehicle/Ent/Ola (100 mg/kg)/Ola+Ent for two weeks. Mice were sacrificed 24 h after the last dose to harvest tumors and ascites. Tumors were processed for histology to determine cell proliferation (Ki67) and immune cell markers (CCL2, M1 and M2-like macrophages). Ascites fluid was processed for flow cytometric analysis of immune cells. Tumors from parental ID8 P53 wild type mice showed significantly lower cell proliferation marker Ki67 (P<0.05), higher anti-tumorigenic M1-like CCL2 (P<0.05), and lower pro-tumorigenic M2-like mannose receptor (P<0.05) in Ent and Ola+Ent groups compared to vehicle and Ola. Ascites showed no significant change in anti-tumorigenic M1-like macrophages, but significantly increased pro-tumorigenic M2-like macrophages (P<0.005). Arginase-1, another marker of M1 macrophage, assessed by IF also confirmed significant decrease in pro-tumorigenic M2-like macrophage in Ent and Ola+Ent treated mice tumors. Additional immune cell markers identified in ascites are being analyzed. To summarize, in ID8 syngeneic mouse models, Ola and Ent treatment exerted anti-tumorigenic effects in tumors but potentially pro-tumorigenic effects in ascites. In conclusion, concomitant targeting of tumor TAMs and ascites TAMs may be a therapeutic regimen to investigate in the future. Citation Format: Vijayalaxmi Gupta, Katherine Roby, SImona Miceska, Andrew Wilson, Fiona Yull, Marta Crispens, Wendy Zhang, Dineo Khabele. Entinostat alters the M1/M2 ratio in ascites compared to tumors derived from an ID8 murine model of ovarian cancer [abstract]. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_2):Abstract nr A100.