Abstract 376: Characterization of PARP inhibitor response in prostate tumor cells reveals drug tolerant persistent phenotype

Abstract Background: PARP inhibitors (PARPi) have improved management of prostate cancer, but acquired resistance and progression remain inevitable. Understanding treatment failure will improve outcomes. Drug tolerant persistence (DTP) is characterized by a small population of tumor cells which survive treatment through transient acquisition of insensitivity. DTP cells are not intrinsically resistant but may slowly proliferate, allowing for mutation accumulation, development of resistance, and progression. DTP cells may also increase mutability, further driving this process. Characterization of drug tolerant persistence and mechanisms enabling a DTP state under PARP inhibition remain poorly understood in prostate cancer. Methods: Viability assays determined efficacious doses of olaparib and rucaparib in PARPi sensitive C4-2B metastatic castration-resistant prostate cancer cells and the C4-2B abiraterone-resistant derivative cells, AbiR. Phase contrast microscopy, western blots, beta-galactosidase activity assays, and flow cytometry characterized response to olaparib and rucaparib. RNA-seq determined the transcriptomic profile of PARPi DTP cells. Results: C4-2B and AbiR response to olaparib and rucaparib is heterogeneous, characterized by cell death and cytostasis. Despite prolonged exposure to high dose PARPi, we observe a small minority of cells which persist, display varying altered morphologies, and altered cell cycle dynamics. While most of these cells appear arrested, we consistently note the presence of a subset which form colonies. C4-2B and AbiR cells exposed to high PARPi dosing for 9 days followed by drug holiday regain normal, parental cell morphology and become re-sensitized to treatment in line with acquisition of a DTP phenotype. RNA-seq reveals transcriptomic characteristics of drug tolerant persistence including evidence for increased mutability, altered metabolism including fatty acid metabolism and lysosomal activity, altered inflammatory signaling, and cell identity changes. Conclusions: Our data suggest that transient, drug tolerant persistence may mediate survival of a minority of tumor cells via modulating metabolic and immune-associated pathways. Overtime, these persisters may acquire resistance and fuel progression due to increased mutability via down-regulation of DNA repair mechanisms. Future efforts will characterize this phenotype further and investigate vulnerabilities that target survivors to improve treatment efficacy. Citation Format: Akshaya Karthikeyan, Love A. Moore, Bryan Correa Gonzalez, Anamitra Bhaumik, Ethan Sandoval, Alan P. Lombard. Characterization of PARP inhibitor response in prostate tumor cells reveals drug tolerant persistent phenotype [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 376.