Background: Although treatment options have improved, mCRPC remains highly lethal. Immunotherapy holds potential for durable remissions but not yet in mCRPC. CAR-T cell therapy has yielded cure for patients with refractory hematologic malignancies, and shows promise in solid tumors. Identifying and targeting the optimal antigen will be key to successful translation in solid tumors. Prostate stem cell antigen (PSCA) is highly expressed on prostate cancer cells, especially metastatic foci, and has limited expression on normal tissues. City of Hope has developed a second-generation PSCA CAR-T cell therapy containing an intracellular 4-1BB co-stimulatory domain (PSCA-BBζ) for first-in-human testing in men with mCRPC refractory to standard therapy (NCT03873805). Methods: The toxicity equivalence range (TEQR) design of Blanchard and Longmate will be used to evaluate select doses of PSCA-BBζ cells and determine the maximum tolerated dose (MTD). Doses include Cohort 1 = 100 million (M) CAR-T x 1 alone; Cohort 1b = 100M CAR-T x1 after lymphodepletion; Cohort 2 = 300M after lymphodepletion; Cohort 3 = 600M after lymphodepletion. 12 subjects will be accrued at the MTD. Eligibility: mCRPC treated with either abiraterone, enzalutamide or both; prior chemotherapy and/or radium223 allowed but not mandated. Tumor tissue, primary or metastatic, must stain positive for PSCA by IHC (Abcam 15168) and biopsy is repeated 28 days post infusion. Endpoints: Primary: all toxicities and dose-limiting toxicities defined as grade 3+ toxicity with attribution of possibly related or above, except CRS grade 3 resolved to < grade 2 within 72 hours or grade 3 encephalopathy resolved to baseline within 28 days. Secondary: persistence/expansion of CAR-T, response (soft tissue by RECIST, bone by PCWG3 criteria). Correlative studies include blood (immunophenotyping, cytokines, circulating tumor cell enumeration and PSCA expression), MRI (enhancement/diffusion changes in target lesions) and metastatic core biopsies (PSCA expression and local immune phenotype changes). Progress: 4 subjects have successfully manufactured PSCA-BBζ T cells; 1 completed treatment. Clinical trial information: NCT03873805