Phase I/II Study of [225Ac]Ac-PSMA-R2 in PSMA-positive Prostate Cancer, With/Without Prior 177Lu-PSMA RLT

Key Inclusion Criteria:

* Evidence of PSMA-positive disease by 68Ga-PSMA-11 PET/CT and eligible as determined by central reading
* Documented progressive mCRPC or mHSPC
* Adequate organ function
* Prior orchiectomy or ongoing ADT and should have received prior 177Lu-PSMA-RLT (Group1 dose escalation \& expansion) or never received 177Lu-PSMA-RLT (Group 2 and Group 3 dose escalation \& expansion).

Key Exclusion Criteria:

* Any other investigational agents within 28 days of the anticipated C1D1 of 225Ac-PSMA-R2 therapy
* Any systemic anti-cancer therapy within 28 days of the anticipated C1D1 of 225Ac-PSMA-R2 therapy
* Uncontrolled pain or incompatibility that may result in participant's lack of ability to comply with imaging procedures
* History of CNS metastases and symptomatic cord compression, or clinical or radiologic findings indicative of impending cord compression
* History of myocardial infarction, angina pectoris, or coronary artery bypass graft within 6 months prior to ICF signature and/or clinically active significant cardiac disease
* Diagnosis of other malignancies in the past three years expected to alter life expectancy or may interfere with disease assessment

Other protocol-defined inclusion/exclusion criteria may apply.