Abstract 7595: PRMT5 inhibitor synergizes with PARP inhibitors in triple-negative breast cancer cells

Abstract Background: Treating triple-negative breast cancer (TNBC) patients, an aggressive subtype of breast cancer, is an ongoing clinical challenge. Small-molecule inhibitors targeting the poly ADP-ribose polymerase (PARP) DNA repair enzymes, have been approved for treating only a subset of TNBC patients with a specific genetic landscape (BRCA1 mutation). However, TNBC patients with wildtype BRCA1 (BRCA1WT) are still lacking targeted therapy and chemotherapy remains their main treatment strategy. High expression of protein arginine methyltransferase 5 (PRMT5) is linked to poor prognosis in breast cancer and PRMT5 alone or via its interacting proteins plays an oncogenic role in breast cancer. Recent evidence suggests that PRMT5 is a key player in regulating the repair of DNA double strand breaks (DSBs) either through homologous recombination (HR) or non-homologous end joining (NHEJ) depending on the PRMT5 expression level within the cell. This dependency of cancer cells on PRMT5 expression levels in deciding the repair pathway of choice can be exploited for developing therapies. We hypothesize that in BRCA1 wildtype TNBC cells, inhibiting PRMT5 activity will force the cells to rely on NHEJ rather than HR to repair DNA DSBs, and introducing PARP inhibitors in this setting will inactivate NHEJ as well, causing synthetic lethality and cell death. In this study, we evaluated the pre-clinical benefit of combining PARP inhibitors with a PRMT5 inhibitor in TNBC cells. Methods: Human TNBC cell lines with wildtype BRCA1 (MDA-MB-468, HCC1806) and mutant BRCA1 (MDA-MB-436, SUM149-PT) were treated with varying doses of two PARP inhibitors (Olaparib, Talazoparib) and a PRMT5 inhibitor (GSK3326595) as single agents or in combination. MTT-based cell viability assay was used to determine cytotoxicity of the treatments. mRNA expression was determined by RT-qPCR after drug treatment. Results and Conclusion: We report that PRMT5 inhibition synergizes with both Olaparib and Talazoparib, to potentiate cell death in two BRCA1WT but not in BRCA1mut TNBC cell lines. Furthermore, we show that inhibiting PRMT5 in BRCA1WT cells upregulates the mRNA expression of RAD50, a DNA damage sensing gene in the HR pathway. Our ongoing study is to delineate the molecular mechanism of action for the observed synergy and whether PRMT5 inhibition can overcome resistance to PARP inhibitors. Through this study, we will identify a novel treatment strategy for the majority of TNBC patients (BRCA1WT) who currently lack targeted therapies. Citation Format: Samyuktha Suresh, Lisa A. McPherson, James M. Ford. PRMT5 inhibitor synergizes with PARP inhibitors in triple-negative breast cancer cells abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 7595.