Abstract P068: Different treatment schemes cause distinct PARP inhibitor resistance mechanisms in BRCA2-mutant ovarian cancer cells
Authors: Huang Tzu-Ting , Nair Jayakumar R. , Gupta Nitasha , Yamamoto Tomomi M. , Bitler Benjamin G. , Lee Jung-Min 
Published: December 2021

Abstract
Poly (ADP-ribose) polymerase inhibitors (PARPis) represent a major advance in ovarian cancer management, now as treatment for recurrent ovarian cancer and as a maintenance therapy following platinum-based chemotherapy in the upfront and platinum-sensitive recurrent settings. Yet, a growing number of patients progress through PARPis, creating a need for further understanding on resistance mechanisms. We hypothesized that different treatment dosing/timing schemes would cause distinct mechanisms of acquired resistance to PARPis. To test this hypothesis, we exposed BRCA2-mutant PEO1 ovarian cancer cells to 0.5 μM olaparib, doubling its concentration up to 40 μM over 3–4 months (PEO1/OlaJR). The second PARPi-resistant PEO1 cell line (PEO1/OlaR) was developed using a relatively low initial dose of 5 nM olaparib, and gradually increasing its concentration up to 20 μM over 6 months. To confirm PARPi resistance, cell proliferation and viability were measured by MTT and colony formation assay, respectively. Both cell lines were resistant to two different PARPis beyond clinically achievable concentrations (olaparib IC50 141.7–372.8 μM and rucaparib IC50 37.1–51.2 μM) and demonstrated restored homologous recombination (HR) repair as evidenced by increased HR-reporter activity (1.32 to 1.45-fold compared to PEO1, p &lt; 0.05) and RAD51 foci formation (5.2 to 6.1-fold compared to PEO1, p &lt; 0.001). For PEO1/OlaJR, distinct resistance mechanisms to PARPi were noted: 1) promoting drug efflux confirmed by increased MDR efflux activity (1.92-fold compared to PEO1, p &lt; 0.001) and increased protein levels of an ABCB1 transporter (7.2-fold compared to PEO1); and 2) enhancing HR repair restoration, likely via reduced REV7 protein expression (0.43-fold compared to PEO1). On the other hand, PEO1/OlaR exhibited a mesenchymal cell phenotype with greater invasion ability as measured by transwell invasion assay (2.25-fold compared to PEO1, p &lt; 0.001) and restored HR repair with upregulation of TRIP13 protein (1.85-fold compared to PEO1). PEO1/OlaR also showed increased cell survival, possibly via activation of PI3K/AKT signaling, as evidenced by increased p-AKT (2.12-fold compared to PEO1). No loss of 53BP1 was observed in either PEO1/OlaJR or PEO1/OlaR based on whole exome sequencing and immunoblotting of 53BP1. BRCA2 reversion mutation status is currently under investigation. Altogether, our findings suggest that mechanisms of resistance to PARPis arise differently based on the duration and concentrations of drug treatment, highlighting the importance of appreciating interplay between specific treatment schemes and the downstream consequences. Thus, developing tailored therapeutic strategies should consider various mechanisms of PARPi resistance.
Citation Format: Tzu-Ting Huang, Jayakumar R. Nair, Nitasha Gupta, Tomomi M. Yamamoto, Benjamin G. Bitler, Jung-Min Lee. Different treatment schemes cause distinct PARP inhibitor resistance mechanisms in BRCA2-mutant ovarian cancer cells [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P068.