Abstract 3244: Exploring the mechanisms of olaparib resistance in resistant-BRCA1 deficient murine ovarian cells

Abstract Background: High-grade serous ovarian carcinoma (HGSOC) is the most frequent histotype of ovarian cancer. More than half of HGSOC is characterized by homologous recombination deficiency (HRD) due to mutations in genes involved in this pathway, including BRCA1/2. Olaparib is a poly(ADP-ribose) polymerase inhibitor (PARPi) recently approved in front line and maintenance therapy in platinum-sensitive BRCA mutated ovarian cancer patients. Despite olaparib clinical benefits, this treatment is associated with the development of resistance. A better understanding of the mechanisms at its basis could help in finding strategies to delay or overcome/counteract it, possibly translatable in the clinical setting. Methods: We generated an olaparib resistant cell line (BRCA1-/- OlaR), starting from the BRCA1 and TP53 deleted murine ID8 cells (BRCA1-/-) by step wise increasing drug concentrations. Sensitive and resistant cell lines were biologically and pharmacologically characterized and the molecular mechanisms at the basis of olaparib resistance investigated. Cellular viability was assessed by MTS assay, for molecular studies real-time PCR, western blot and RAD51-foci immunofluorescence were performed. Results: BRCA1-/- OlaR cells were 24 fold time more resistant than BRCA1-/- cells and displayed a similar in vitro cell growth compared to the parental cells. Olaparib treatment at IC50 dose caused a longer and stronger G2-M block of the cell cycle in sensitive as compared to resistant cells. BRCA1-/- OlaR cells were cross-resistant to the other PARPis tested, suggesting a common mechanism of resistance. Platinum compounds showed similar cytotoxic activity in sensitive and resistant cells, while a partial cross-resistance was observed with doxorubicin. Interestingly, resistant cells were less responsive to a panel of DNA damage response (DDR) inhibitors (i.e. ATR, WEE1 and CHK1 inhibitors) compared to parental ones; however their combinations with olaparib were synergic in both the sensitive and resistant lines. A partial restoration of the HR pathway (increase in RAD51 foci formation after olaparib and IR treatment), a downregulation of PARP1 protein levels without mutation in PARP1 gene and an upregulation of the MDR transcript could be found in BRCA1-/- resistant cells, strongly supporting multiple heterogeneous mechanisms of resistance. Conclusions: The HR deficient cell line resistant to olaparib we generated showed multiple mechanisms of resistance. The combinations of olaparib with different DDR inhibitors were equally active in sensitive and resistant cells. Citation Format: Michela Chiappa, Martina Anselmi, Luca Russo, Monica Lupi, Nicolò Panini, Federica Guffanti, Giovanna Damia. Exploring the mechanisms of olaparib resistance in resistant-BRCA1 deficient murine ovarian cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3244.