Compound Library Screening Identified Cladribine as a Novel Radiosensitizer for Prostate Cancer

Radiation therapy effectively treats prostate cancer, but tumor recurrence remains a clinical challenge, highlighting the need for strategies to enhance radiosensitivity. In this study, we developed a simple, high-throughput drug repurposing screening platform to find radiosensitizers from compounds approved by the U.S. Food and Drug Administration. A library of 1134 compounds was systematically screened at two concentrations (0.2 and 2 μM) using LNCaP cells stably expressing Metridia luciferase, allowing highly sensitive and quantitative assessment of cell viability through luciferase activity in culture supernatants. In the primary screening, 8 and 12 candidate radiosensitizers were identified at 0.2 μM and 2 μM, respectively. In the secondary screening, 19 compounds were evaluated at two radiation doses (4 Gy and 6 Gy) and six drug concentrations, identifying 5 radiation-sensitizing candidate compounds. Through this stepwise screening approach, cladribine was identified as the most potent radiosensitizer. Cladribine increased radiation-induced cytotoxicity in multiple prostate cancer cell lines (22Rv1, DU145, and PC3), with dose-modifying factors of 1.46, 1.55, and 1.43, respectively, based on the radiation dose needed to achieve 90% cell death. Mechanistically, cladribine prevented the repair of radiation-induced DNA double-strand breaks, shown by increased γH2AX levels. Importantly, its radiosensitizing effect was further confirmed in vivo using 22Rv1 and DU145 xenograft models. This study demonstrates that a luciferase-based high-throughput drug repurposing platform is useful for identifying clinically relevant radiosensitizers, revealing that cladribine is a promising candidate for further translational research in prostate cancer radiotherapy.