Abstract LB164: Entinostat restores sensitivity to olaparib in two in vitro models of PARPi resistant ovarian cancer cells
Authors: Fashemi Bisiayo E. , van Biljon1 Lilian N. , Woodard Tyler , Gupta Vijayalaxmi , Mullen Mary M. , Bitler Benjamin , Khabele Dineo 
Published: April 2023

The use of Poly ADP ribose polymerase inhibitors (PARPi) has revolutionized the treatment of homologous recombination (HR) deficient ovarian cancer tumors. A subset of these tumors exhibit genetic and acquired resistance to PARPi treatment. We have previously shown histone deacetylase inhibitors (HDACi) resensitizes ovarian cancer cells to PARPi. The objective of this study is to examine the effectiveness of combination HDACi and PARPi treatment in two in vitro models of mouse ovarian cancer cells. To investigate these effects, we used the following three ID8 murine ovarian cancer epithelial cell lines: TP53− / − (HR deficient), TP53− / −/BRCA2− / − (HR proficient), and an olaparib resistant line TP53− / −/BRCA2− / −-OR (ID8_OR). We also created ovarian cancer organoids from these three lines. In short, 8-week old female C57BL/6 mice were injected with 7 million untreated cells in PBS. Following the formation of ascites, mice were sacrificed and organoids were derived from both ascites and tumors. For viability assays, cells were pretreated with entinostat (Ent, 0.25uM) for 24h, followed by 72h (2D cells) or 7 days (organoids) of Ent (0-2uM), olaparib (Ola, 0-40uM), or in combination. Cell proliferation and viability was assessed using MTS and ATP-based assays. For immunofluorescence and western blotting analysis, cells were treated with 0.5uM Ent, 10uM Ola or the combination.TP53 adherent cells and organoids treated with 0.5uM Ent + 10uM of Ola significantly reduced cell proliferation when compared to Ola alone (p= 0.0028, 0.0021 Two-Way ANOVA). When compared to Ola and Ent alone, TP53/BRCA2 adherent cells treated with 0.125uM of Ent + 2.5uM of Ola also reduced cell proliferation (p=0.0011 and p=0.0072, 0.0845). Lastly, ID8_OR adherent cells and organoids treated with 0.125uM of Ent + 2.5μM of Ola significantly reduced cell proliferation when compared to Ola alone (p=0.0018, 0.0264).Immunofluorescences analysis revealed, when treated with Ola and Ent combination, adherent TP53 (p= 0.002, n.s), TP53/BRCA2 (p=0.0033, 0.0315), and ID8_OR (n.s) cells displayed increased expression of DNA damage marker gH2AX when compared to Control and Ola alone. We also observed decreased expression of RAD51 in cells treated with combination when compared to control and Ola alone, TP53 (p= &lt;0.0001, 0.0023) and ID8_OR (n.s). In our organoids, we found an increase in gH2AX across all three cell lines when treated with Ola+Ent compared to control and Ola alone: TP53 (p= &lt;0.0001, 0.0057), TP53/BRCA2 (n.s), and ID8_OR (p= 0.0116, 0.0151). Similarly, we found a decrease in RAD51 when cells were treated with Ola+Ent compared to control and Ola alone: TP53 (p= n.s, 0.0418) and ID8_OR (n.s). Lastly, preliminary western blot analysis of the organoids, revealed a decreases in proliferation marker PCNA in both adherent cells and organoids when comparing the control to combination treatment. In conclusion, the combination of Ola and Ent treatment restores responsiveness of two in vitro models of genetic and acquired PARPi resistance through increasing DNA damage and reducing DNA repair and proliferation.

Citation Format: Bisiayo E. Fashemi, Lilian N. van Biljon1, Tyler Woodard, Vijayalaxmi Gupta, Mary M. Mullen, Benjamin Bitler, Dineo Khabele. Entinostat restores sensitivity to olaparib in two in vitro models of PARPi resistant ovarian cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB164.