Abstract A034: Combination of ATR inhibitor (ATRi) and AKT inhibitor (AKTi) reduces tumor growth and prolongs survival in BRCA2 mutant (BRCA2m) PARP inhibitor (PARPi)-resistant high-grade serous ovarian cancer (HGSC)

Abstract Background: PARPis are widely used to treat BRCA mutant HGSC. However, acquired resistance to PARPis remains a significant challenge and effective treatment strategies for recurrent HGSC are limited. To address this unmet need, we previously conducted high-throughput drug combination screens using an ATRi in PARPi-resistant HGSC cell lines. Through the screens, we identified inhibitors targeting the PI3K/AKT pathway that exhibited synergistic cytotoxic effects with ATRi in BRCA2m PARPi-resistant HGSC cell lines. We hypothesized that AKTi could enhance ATRi-induced aberrant R-loops and reduce tumor growth. Methods: BRCA2m PARPi-sensitive (PEO1) and its resistant lines (PEO1/OlaR, PEO1/OlaJR) were used to evaluate the efficacy of ATRi ceralasertib and AKTi capivasertib in vivo. Tumor growth was assessed in the subcutaneous xenograft models, while overall survival was monitored in the intraperitoneal xenograft models. Mice received either a vehicle or 50 mg/kg ceralasertib orally once daily for 5 days, followed by 2 days of no treatment, for 4 weeks. For combination studies, two study arms were added, and the mice received 130 mg/kg capivasertib orally twice daily for 4 days, followed by 3 days of no treatment, with or without 50 mg/kg ceralasertib, for 4 weeks. A positive control group received 100 mg/kg olaparib orally once daily for 5 days, followed by 2 days of no treatment. R-loop accumulation and DNA damage were evaluated by immunohistochemistry (IHC) of DNA: RNA hybrid and γH2AX, respectively. Data were analyzed by one-way ANOVA test and presented as mean ± SEM. Overall survival of mice was assessed using Kaplan-Meier curves, and statistical significance was determined using the log-rank (Mantel-Cox) test. P 0. 05 was considered statistically significant. Results: The therapeutic efficacy of the ATRi and AKTi combination was evaluated in in vivo HGSC models. The combination treatment led to nearly complete tumor regression (P 0. 001) without significant weight loss compared to monotherapy, in two BRCA2m PARPi-resistant models. While olaparib alone transiently suppressed tumor growth in PARPi-sensitive PEO1 models after 1 week of treatment, this effect was not sustained after a few weeks. Notably, the combination of ATRi and AKTi resulted in a more than 3-fold increase in median overall survival compared to the control group (24-29. 5 days vs. 82. 5-102. 5 days, P 0. 001), ATRi alone (31. 5-34 days vs. 82. 5-102. 5 days, P 0. 001), and AKTi alone (32-32. 5 days vs. 82. 5-102. 5 days, P 0. 001) in PARPi-resistant HGSC models, and an approximately 2-fold increase in median overall survival in PEO1 models (P 0. 001). Furthermore, IHC showed that the levels of γH2AX (3. 5-4. 1-fold increased, P 0. 05) and R-loops (1. 5-3. 2-fold increased, P 0. 01) were significantly elevated in tumors treated with the combination of ATRi and AKTi compared to monotherapy. Conclusions: Our findings demonstrate the therapeutic potential of the combination treatment of ATRi and AKTi, possibly via modulation of R-loop dynamics and replication stress in vivo. Citation Format: Tzu-Ting Huang, Jayakumar R. Nair, Chi-Ting Shih, Jung-Min Lee. Combination of ATR inhibitor (ATRi) and AKT inhibitor (AKTi) reduces tumor growth and prolongs survival in BRCA2 mutant (BRCA2m) PARP inhibitor (PARPi) -resistant high-grade serous ovarian cancer (HGSC) 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 Suppl₂): Abstract nr A034.