Sapacitabine, an orally bioavailable prodrug of the deoxycytidine analog, CNDAC, is currently in a Phase III registration trial for elderly AML patients (NCT01303796). CNDAC (as DFP-10917) is in a Phase I/II trial for relapsed or refractory acute leukemia (NCT01702155) with very limited toxicity. CNDAC-induced DNA damage, double-strand breaks converted from initial single-strand breaks, is repaired mainly by the homologous recombination (HR) pathway. Deficiency in HR components (e.g. ATM, Rad51, Xrcc3, Brca1 and Brca2) confers sensitivity to CNDAC. Brca1/2 function is frequently compromised in ovarian cancer (OC). To determine the role of Brca2 in DNA damage repair after CNDAC, we used in this study a Brca2-mutant ovarian adenocarcinoma cell line, PEO1 and its revertant line, PEO1 C4-2, in which Brca2 function is restored due to a secondary mutation. First, the clonogenic sensitivities of the two lines to therapeutic agents were compared. The mutant was 6-10 fold more sensitive to CNDAC than the revertant. In contrast, Brca2 restoration did not confer resistance to cytarabine or gemcitabine, confirming the unique action mechanism of CNDAC among nucleoside analogs. Second, CNDAC-induced chromosome damage was compared in both lines. We found Brca2- mutant cells bearing more chromosomal structural abnormalities (67% metaphases with scorable breaks or fusions, N = 51) than Brca2-restored cells (14%, N = 50), apparently due to genetic instability when lacking Brca2. The mutant cells exposed to 25 nM CNDAC for 3d manifested massive chromosomal aberrations (100% metaphases unscorable, N = 51). In contrast, the revertant cells showed significantly fewer chromosome aberrations (30% scorable, N = 50). These results provided cytogenetic evidence for Brca2 involvement in DNA damage repair after CNDAC. Third, the combination effect of CNDAC with two classes of front-line anti-OC drugs, platinum compounds and taxanes, was explored using clonogenic assays and median effect analyses. Both cisplatin and oxaliplatin each had additive cell killing effects with CNDAC (combination index, CI ∼1) independent of Brca2 status. Similarly, CNDAC was additive with either paclitaxel or docetaxel in both lines (CI ∼1). Finally, two PARP inhibitors (PARPis), rucaparib and talazoparib, greatly sensitized Brca2-mutant cells. Despite the distinctive sensitivities of the two lines, both rucaparib-CNDAC and talazoparib-CNDAC combinations showed synergistic killing effect (CI <1). Since PARPis have promising efficacy in Brca2-deficient OC patients, the synergistic combination of PARPi and sapacitabine could be a novel therapeutic strategy to be tested in clinic. This study provides mechanistic insight into the function of Brca2 in repairing sapacitabine/CNDAC-induced DNA damage, and rationale for sapacitabine-PARPi combination to target Brca2-deficient OC.

Citation Format: Xiaojun Liu, Yingjun Jiang, Billie Nowak, Bethany Qiang, Nancy Cheng, William Plunkett. The DNA strand breaking nucleoside analogue sapacitabine sensitizes Brca2-deficient ovarian cancer cells and synergizes with PARP inhibitors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4633.