Abstract 1987: PARP inhibitor (PARPi) rechallenge shows differential sensitivity in PARPi olaparib-resistant BRCA1-mutated (BRCA1m) high-grade serous ovarian cancer (HGSC)

Abstract Background: PARPis have changed the treatment paradigm in HGSC, particularly for women with BRCA mutations. PARPi, olaparib (O), is a standard of care maintenance therapy following platinum-based chemotherapy in newly diagnosed BRCA-mutated (BRCAm) HGSC. But patients ultimately develop resistance to O. We investigated whether rechallenge with different PARPi monotherapy could cause cytotoxicity in BRCAm HGSC after becoming O-resistant. We also aimed to identify possible mechanisms of actions of PARPi’s differential sensitivity. Methods: We developed a PARPi-resistant line (UWB-OlaJR) from BRCA1m PARPi-sensitive cells (UWB1. 289) by gradient exposure of O with initial lower doses to a final concentration of 20 μM over 10 months. Four PARPis including O, niraparib (N), talazoparib (T), and veliparib (V) were tested. Cell growth was assessed using 5-day XTT and 10-day clonogenic assays. DNA damage and homologous recombination (HR) repair activity were evaluated using immunofluorescence (IF) staining of γH2AX and RAD51 foci, respectively. Replication fork (RF) dynamics was measured by DNA fiber assay. All data were repeated in triplicate and analyzed using t-test or one-way ANOVA. Data were shown as mean ± SD and P 0. 05 was considered statistically significant. Results: 5-day XTT assay shows a 53. 7-fold increase in resistance to O in UWB-OlaJR relative to UWB1. 289 (IC50 2. 2 μM vs. 118. 1 μM). Cross-resistance was observed with other 3 PARPis, showing increased IC50 values compared to its parental line (V: 23. 4-fold, N: 160. 5-fold, T: 229. 6-fold). However, 10-day clonogenic assay showed no resistance against N (1. 09-fold) and T (0. 51-fold) in UWB-OlaJR, indicating differential sensitivity to different PARPis by a long-term cytotoxicity measure. Surprisingly, γH2AX foci did not increase in response to PARPis in UWB-OlaJR compared to UWB1. 289 (O: 2. 58-fold, V: 2. 33-fold; N: 2. 89-fold, T: 2. 58-fold; all P 0. 05), suggesting possible restored DNA repair function. All four PARPis induced replication stress in both O-sensitive and -resistant cell lines as evidenced by decreased RF speed compared to untreated group (O: 0. 57- and 0. 7-fold, V: 0. 68- and 0. 74-fold, N: 0. 47- and 0. 47-fold, T: 0. 47- and 0. 52-fold; all P 0. 001). Notably, N and T treatments reduced fork speed to a greater extent than O and V in UWB-OlaJR. However, RF protection was not affected when UWB-OlaJR treated with different PARPis compared to UWB1. 289 (O: 0. 89-fold, V: 0. 87-fold, N: 0. 71-fold, T: 0. 83-fold; all P 0. 05), suggesting that differential sensitivity to different PARPis was unlikely due to modulation of RF stabilization. Conclusions: Our findings demonstrate different PARPis may induce varied levels of cytotoxicity and replication stress in O-resistant BRCA1m HGSC, possibly via modulating alternative RF dynamics besides RF protection. Citation Format: Chi-Ting Shih, Tzu-Ting Huang, Jayakumar R. Nair, Jung-Min Lee. PARP inhibitor (PARPi) rechallenge shows differential sensitivity in PARPi olaparib-resistant BRCA1-mutated (BRCA1m) high-grade serous ovarian cancer (HGSC) abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 1987.