BRCA Status Does Not Predict Synergism of a Carboplatin and Olaparib Combination in High-Grade Serous Ovarian Cancer Cell Lines

Over 50% of epithelial ovarian cancers express the BRCAness profile that leads to a dysfunctional homologous recombination repair system. The combination of a dysfunctional homologous recombination repair system and a poly(ADP-ribose) polymerase (PARP) inhibitor results in a synthetic lethal phenotype. The PARP inhibitor olaparib, approved as a monotherapy for patients with a germline BRCA mutation, has shown promising results in preclinical studies when combined with DNA damaging agents, such as carboplatin. However, dose-limiting toxicities have hindered the use of a combination therapy with olaparib in the clinical setting. By concurrent administration of carboplatin and olaparib at various molar ratios of drugs, the aim of this study was to explore the optimal dosing ratio of carboplatin-olaparib combinations in a comprehensive panel of eight BRCA-proficient and -deficient high-grade serous ovarian cancer (HGSOC) cell lines. Overall, synergy was observed in the BRCA1/2-mutated or defective cell lines when olaparib was combined at lower molar ratios of olaparib to carboplatin. Immunostaining of gamma H2AX foci revealed increased DNA damage as a result of this synergistic drug combination in the UTATB1.289 paired cell lines. In vitro activity of the individual agents, carboplatin and olaparib, did not correlate with PARP1 expression in each cell line. Importantly, synergism was also observed in a subset of BRCA wild-type cell lines (OV90 and PEO4) suggesting therapeutic benefits of this combination beyond BRCA-dependent synthetic lethality. The administration of drugs at synergistic ratios has the potential to increase efficacy and reduce toxicity.