FPI-2265 (225Ac-PSMA-I&T) for Patients With PSMA-Positive Metastatic Castration-Resistant Prostate Cancer (mCRPC)

Key Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1 * Diagnosis of adenocarcinoma of prostate proven by histopathology. * Must have had prior orchiectomy and/or ongoing androgen-deprivation therapy and a castrate level of serum/plasma testosterone * Progressive mCRPC at time of study entry. * Must have been previously treated with lutetium-PSMA therapy (lutetium-177 vipivotide tetraxetan or other lutetium-177-PSMA RLT). Treatment must have been completed \>6 weeks prior to the first dose of study drug. * Participants with known BRCA mutations should have received FDA-approved therapies such as PARP inhibitors, per Investigator discretion. * Positive PSMA PET/CT scan * Adequate organ function * For participants who have partners of childbearing potential: Partner and/or participant must not be planning to conceive and must use a method of birth control with adequate barrier protection deemed acceptable by the Principal Investigator during the study treatment and for six months after last study drug administration. Key Exclusion Criteria: * Participants who received more than two prior lines of cytotoxic chemotherapy for CRPC. * Participants who progress prior to administration of the 3rd cycle of prior treatment with 177Lu-PSMA therapy * All prior treatment-related adverse events must have resolved to Grade ≤1 (CTCAE v5.0). Alopecia and stable persistent Grade 2 peripheral neuropathy may be allowed at the discretion of the Investigator. * Participants with known, unresolved, urinary tract obstruction are excluded. * Administration of any systemic cytotoxic or investigational therapy ≤30 days of the first dose of study treatment or five half-lives, whichever is shorter. Completion of large-field external beam radiotherapy ≤four weeks of the first dose of study treatment. * Participants with a history of central nervous system (CNS) metastases are excluded except those who have received therapy * Participants with any liver metastases will be excluded * Participants with skeletal metastases presented as a superscan on a ⁹⁹ᵐTc bone scan. * Previous or concurrent cancer that is distinct from the cancer under investigation in primary site or histology, except treated cutaneous basal cell carcinoma or squamous cell carcinoma and superficial bladder tumors. Any cancer curatively treated \>two years prior to the first dose of treatment is permitted. * Concurrent serious (as determined by the investigator) medical conditions * Major surgery ≤30 days prior to the first dose of study treatment.