Abstract 377: Mechanistic and phenotypic insights into temporal and dosage-dependent responses to PARP inhibition in advanced prostate cancer

Abstract Background: Poly ADP-ribose polymerase (PARP) inhibition effectively treats prostate cancer (PCa) by inducing replication stress and cell death through synthetic lethality. However, PARP inhibitor (PARPi) resistance inevitably develops, necessitating the study of short- and long-term effects. Responses to PARP inhibition in PCa vary from cytostasis to cell death, with the impact of different dosages and durations underexplored. This study examines how temporal and dosage variations of Olaparib (Ola) influence PCa cell phenotypes and identifies mechanisms underlying treatment response and putative therapeutic strategies to address vulnerabilities following PARP inhibition. Methods: C4-2B cells were treated with 0, 1, and 5 µM Olaparib for 1 and 5 days. RNA sequencing (RNA-seq), gene set enrichment analysis (GSEA), cell growth assays, microscopy, and western blotting were used to identify pathway alterations and putative vulnerabilities. Results: C4-2B cells respond well to clinically relevant doses of olaparib. Morphological assessment at 1 and 5 days suggests significant differences in response based on duration and intensity of treatment. After 1 day, RNA-seq and western blots support a dominant p53-associated cell cycle checkpoint response which may be driven by ATM. By day 5, surviving cells exhibited significant morphology changes indicating epithelial-mesenchymal transition (EMT) and suggesting acquisition of a stress-induced phenotype. RNA-seq and GSEA support phenotypic shifts including EMT and significant changes in fatty acid metabolism. These findings suggest that prolonged PARP inhibition leads to adaptations in surviving cells. Strategies to either co-target cell cycle checkpoint activation or putative survival adaptations show promise in combination with olaparib. Conclusion: PARP inhibition induces distinct cellular and metabolic changes based on dosage and duration. Acute exposure may trigger p53-mediated cell cycle arrest, while prolonged exposure results in phenotypic shifts which may promote cell survival. Our study underscores the need for the design of rational combination therapies to address both immediate and long-term effects of treatment. Citation Format: Love A. Moore, Akshaya Karthikeyan, Bryan Correa Gonzalez, Anamitra Bhaumik, Ethan Sandoval, Alan Paul Lombard. Mechanistic and phenotypic insights into temporal and dosage-dependent responses to PARP inhibition in advanced prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 377.