PROJECT SUMMARY/ABSTRACT: Radiation therapy has a definitive role in curing high risk, very high risk, and even oligometastatic or low- volume metastatic disease as phase III trials in these settings have demonstrated overall survival benefits. However, radiation therapy also involves incidental radiation dose to rectum, small bowel and urethra, which cause radiation dose-limiting toxicity. Locoregional control, particularly within pelvic lymph node regions where currently possible radiation doses are non-curative, can be a limiting factor in overall prostate curability in these settings. In this proposal, we seek to create state-of-the-art PSMA-targeting therapeutics, taking advantage of this relatively prostate-specific surface antigen. We will create two molecules consisting of the PSMA ligand linked to a lysosomal cleavable linker which is turn linked to radiation sensitizing small molecule inhibitors of the DNA damaging response. One molecule will involve a potent inhibitor of ATM and the second will involve an inhibitor of DNA-PK, two targets integral to radiation efficacy. In SA1 we will test these novel molecules with in vitro assessments of inhibition of the ATM and DNA-PK kinases in PSMA expressing prostate cancer cell lines compared to PSMA non-expressing cells. In SA2, we will test these molecules for in vivo efficacy in prostate cancer xenografts and determine target engagement and preliminary normal tissue toxicity. The overarching goal addressed by the proposed research project is to develop treatments that could improve survival for men with lethal prostate cancer by improving locoregional control, which does impact overall survival in these men. In addition, if these molecules successfully radiosensitize, it is also possible they will allow for external beam radiation dose de-escalation and enable less normal tissue toxicity in order to improve quality of life outcomes. Finally, in downstream work the molecules could be combined with Radium-223 to systemically radiosensitize a therapy proven to extend survival. In this pilot grant, we will determine the initial feasibility of this approach, laying the groundwork for further therapeutic development.