AZD6738 overcomes acquired olaparib resistance in BRCA1 mutation triple-negative breast cancer through down-regulation of BRCA2 and RAD51

Although olaparib, a PARP inhibitor, has been approved for the treatment with triple-negative breast cancer (TNBC) harboring BRCA1/2 mutations, resistance inevitably emerges in TNBC patients. Ataxia telangiectasia and Rad3-related (ATR) protein play a role in the chemotherapy and radiotherapy of various malignancies. However, the role of ATR in olaparib resistance to TNBC remains unclear. This study aimed to explore the synergistic cytotoxic effects of dual inhibition of ATR and PARP in olaparib-resistant TNBC cell lines and to elucidate the underlying molecular mechanism. Herein, olaparib-resistant and parental HCC1937 and MDA-MB-436 cell lines were treated with the ATR inhibitor AZD6738 and the PARP inhibitor olaparib at various concentrations. Cell viability was assessed using MTT. DNA damage was evaluated by alkaline comet assay, while apoptosis and cell cycle were analyzed using flow cytometry. The expression levels of BRCA2, RAD51, and γH2AX were examined via immunofluorescence, western blot, and immunohistochemistry analysis. Additionally, an olaparib-resistant HCC1937 xenograft model was employed to evaluate the in vivo antitumor efficacy of AZD6738, olaparib, either alone or in combination. Our results demonstrate that AZD6738 and olaparib exerted synergistic cytotoxic effects in olaparib-resistant TNBC cell lines by accelerating cell cycle progression, inducing DNA damage and apoptosis. Furthermore, in vivo experiments revealed that the combination of olaparib and AZD6738 significantly inhibited the growth of olaparib-resistant HCC1937 xenografts compared with the monotherapy groups, while also promoting increased γH2AX expression without inducing significant body weight loss. Importantly, AZD6738 and olaparib significantly downregulated the protein expression of BRCA2, and RAD51, thereby reversing olaparib-induced activation of the DNA homologous recombination (HR) repair signaling pathways. In conclusion, dual inhibition of ATR and PARP represents a highly promising therapeutic strategy for TNBC with acquired resistance to olaparib.