• Signed and dated Institutional Review Board (IRB)/Independent Ethics Committee (IEC)-approved informed consent form prior to beginning study and undergoing procedures.

• Diagnosis of mCRPC with (a) any histology and (b) currently on or previously on abiraterone/prednisone (or abiraterone/dexamethasone) with documented progression in either the mCRPC or mHSPC settings, and with:

‣ rising PSA (a rising PSA requires at least 3 measurements obtained at least 1 week apart showing increase from nadir with the last level above 2 ng/mL by local testing); or

⁃ progression of new or existing bone or soft tissue metastatic lesions by CT, MRI or bone scan; no abiraterone washout necessary.

• Availability of archival tumor tissue for pathologic review and correlative studies. Tumor tissue (localized or metastatic) does not need to be received but rather identified and available (slides and blocks) upon later request for future pathologic review and possible correlative studies.

• Castrate levels of serum total testosterone (\<50 ng/dl) OR ongoing documented ADT.

• Karnofsky performance status of 70 or higher.

• ≥ 18 years of age

• Life expectancy of ≥ 6 months

• Recovered to ≤ Grade 2 toxicity from prior therapy (per CTCAE Version 5.0)

• Adequate bone marrow function:

‣ Absolute neutrophil count (ANC) ≥ 1.2 × 10\^9/L without any growth factors in prior 7 days

⁃ Hemoglobin ≥ 9.0 g/dL with no blood transfusion in the prior 14 days

⁃ Platelet count ≥ 75 × 10\^9/L with no platelet transfusion in the prior 7 days

• Adequate hepatic function:

⁃ Total bilirubin ≤ 1.5 × ULN (≤ 3 × ULN for Gilbert's syndrome)

⁃ AST (serum glutamic oxaloacetic transaminase \[SGOT\]) / ALT (serum glutamic pyruvate transaminase \[SGPT\]) ≤ 3 × institutional ULN

• Adequate renal function:

• \- Creatinine clearance per Cockcroft-Gault equation (or institutional equivalent) of ≥ 50 mL/min

⁃ Willingness of patients who are not surgically sterile or with partners who are not postmenopausal to use medically acceptable methods of birth control for the duration of the study treatment, including 90 days after the last dose of study drug.

⁃ Willing and able to provide written informed consent and HIPAA authorization for the release of personal health information.