Sapacitabine is an orally bioavailable prodrug of the deoxycytidine analog, CNDAC. Sapacitabine 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 AML and ALL (NCT01702155). Sapacitabine/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, including ATM, Rad51, Xrcc3, Brca2 and Brca1, confer sensitivity to CNDAC. Brca1 and Brca2 function is frequently compromised in ovarian cancer. To determine the role of Brca1 in DNA damage repair after sapacitabine, we used a Brca1-null ovarian carcinoma cell line, UWB1.289 and its complemented line, UWB1.289+Brca1 in this study. First, the clonogenic sensitivities of the two lines to therapeutic agents were compared. The deficient cells were 3-4 fold more sensitive to CNDAC than the repleted cells. In contrast, Brca1 repletion did not confer resistance to cytarabine, fludarabine or gemcitabine. These results confirm the unique action mechanism of CNDAC among nucleoside analogs. Second, a cytogenetic approach was taken to compare CNDAC-induced chromosome damage in both lines (N = 50 metaphases scored for each sample). We found Brca1-null cells bearing more chromosomal structural abnormalities (∼50% metaphases) than Brca1-complemented cells (∼30%), apparently due to genetic instability when lacking Brca1. UWB1.289 cells exposed to 15 nM CNDAC for 27 hr (1 cell cycle) and 54 hr (2 cell cycles) manifested massive chromosomal aberrations (>60% and >90% metaphases, respectively), nearly 40% and 70% of which could not be scored. In contrast, UWB1.289+Brca1 cells showed significantly fewer chromosome aberrations (42% and 48% metaphases, respectively), the majority of which were scorable, upon extended incubation with CNDAC under the same conditions. These results provided cytogenetic evidence for Brca1 involvement in DNA damage repair after CNDAC. Third, interaction between sapacitabine and other classes of therapeutic agents was explored using clonogenic assays. For example, CO-338, a camsylate salt of the PARP inhibitor, rucaparib, greatly sensitized Brca1-deficient cells. Despite the distinctive sensitivities of the two lines, the CO-338 - CNDAC combination showed synergistic cell killing by median-effect analysis (combination index <1). Since PARP inhibitors have promising efficacy in Brca1-deficient ovarian cancer patients, the synergistic combination of rucaparib and sapacitabine could be a novel therapeutic strategy to be tested in clinic. Together, this study provided mechanistic insight into the function of Brca1 in repairing sapacitabine/CNDAC-induced DNA damage and rationale for sapacitabine-PARP inhibitor combination to target Brca1-deficient ovarian cancer.

Citation Format: Xiaojun Liu, Yingjun Jiang, Billie Nowak, Dariya Tikhomirova, William Plunkett. Brca1-deficient ovarian cancer cells are sensitized to the DNA-strand-breaking nucleoside analog sapacitabine that synergizes with PARP inhibition. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2551. doi:10.1158/1538-7445.AM2015-2551