This study is an open-label, multicenter study designed to investigate the efficacy, safety and tolerability of FPI-2265 (225Ac-PSMA-I\&T) in combination with Olaparib in participants with mCRPC. The dose optimization Phase 2 part will be investigating the safety, tolerability, and anti-tumor activity of novel dosing regimens of FPI-2265 and Olaparib in participants with metastatic castration-resistant prostate cancer.