ACTInium-J591 Radionuclide Therapy in PSMA-Detected Metastatic HOrmone-Sensitive Recurrent Prostate CaNcer

Inclusion Criteria: * Pathologically (histologically or cytologically) proven diagnosis of prostate adenocarcinoma at any anatomical location (for example, prostate, metastatic site), including intraductal or ductal carcinoma, at any time before registration. * Age ≥ 18 years * ECOG Performance Status 0-2. * Prior curative-intent treatment to the prostate, by either: * External beam and/or brachytherapy to: Prostate alone, prostate and seminal vesicles, prostate and pelvic nodes, or radiation to all three sites. * Radical prostatectomy alone, radical prostatectomy plus postoperative radiotherapy to the prostate bed, or radical prostatectomy plus postoperative radiotherapy to the pelvic nodes. * Must meet study entry criteria based on the following diagnostic workup within 120 days prior to enrollment: • History and physical examination * 99mTc bone scan (Must be negative or equivocal); * Either CT or MRI of pelvis +/- abdomen (Must be negative or equivocal); * PSMA PET scan (Must be positive with exception of local disease); Note: All 3 scans are mandatory (bone scan; CT/MR; PET) * Serum total testosterone \>100 ng/dL within 120 days prior to enrollment. * Be willing to use effective contraception during the entire study period. * To have adequate organ and marrow function, as defined below: * a. Absolute neutrophil count (ANC) of ≥2,000/mm3 * b. Hemoglobin ≥9 g/dL without need for transfusion * c. Platelet count ≥150,000/mm3 and absent history or primary quantitative or qualitative platelet defect * d. Serum creatinine of ≤1.5 x ULN or calculated creatinine clearance of ≥60 mL/min/1.73 m2 by Cockcroft-Gault (or determined by 24 hour urine collection) * e. Serum total bilirubin ≤1.5 x ULN (unless due to Gilbert's syndrome, in which case direct bilirubin must be normal) * f. Serum AST and ALT ≤1.5 x ULN. Exclusion Criteria: * 1\. Currently on androgen deprivation or anti-androgen therapy. * Definitive radiologic evidence of metastatic disease on conventional imaging, defined by one of the following: * a. osseous metastasis on 99mTc radionuclide bone scan, or * b. extra pelvic nodal/soft tissue disease (\> 1.5cm in short axis) on CT or MRI pelvis +/- abdomen * Spinal cord compression, or spinal intramedullary, brain, and/or visceral (for example liver, lung, etc.) metastasis. Note: Spinal metastases (PSMA PET- detected) with epidural extension are eligible if there is \>0.3cm spatial separation between the gross tumor volume and spinal cord. * Biopsy-proven prostatic carcinoma with signet-ring, sarcomatoid, or neuroendocrine features (for example, small cell). * Prior metastatic or non-metastatic, invasive malignancy (except non metastatic, non-melanomatous skin cancer) unless continuously disease free for \> 3 years. * Prior chemotherapy for prostate cancer or bilateral orchiectomy. Note: Prior chemotherapy for a different cancer is allowed if continuously disease-free for \> 3 years. * Intrapelvic lymph nodes as only site of prostate cancer recurrence. * Received a platelet transfusion within 4 weeks of treatment * Received growth factors for white blood cells or platelets within 4 weeks of treatment * Prior treatment with unsealed radiation sources such as 89Strontium or 153Samarium-containing compounds (e.g. Metastron, Quadramet) * History of prior PSMA-TRT * Known history of myelodysplastic syndrome * Other serious illness(es) involving the cardiac, respiratory, central nervous, renal, hepatic, or hematological systems which might preclude completion of this study or interfere with determination of causality of any adverse effects experienced in this study.