Platform Study for Prostate Researching Translational Endpoints Correlated to Response to Inform Use of Novel Combinations

Key Inclusion Criteria:

1. Metastatic castration resistant prostate cancer with castrate-level testosterone (\< 50 ng/dL) at screening.
2. Disease progression per Prostate Cancer Working Group 3 (PCWG3) criteria.
3. Provide fresh pre-treatment core needle or incisional biopsy of a metastatic tumor lesion not previously irradiated. Fine needle aspiration is not acceptable.

   1. Additionally, if a pre-treatment biopsy is not medically feasible for participants with bone only disease, formalin-fixed paraffin-embedded (FFPE) tumor specimen in a paraffin block (preferred) or at least 10 slides containing unstained, freshly cut, serial sections must be provided.
   2. For all participants, in addition to fresh pre-treatment biopsy, consent for archival tissue is required.
4. Must be willing to undergo tumor biopsy(ies) on treatment, if medically feasible.
5. Have received and progressed on prior secondary androgen receptor signaling inhibitor therapy (eg, abiraterone, enzalutamide, apalutamide).
6. Participants must discontinue antiandrogen therapy (ie, bicalutamide, flutamide, nilutamide) at least 4-6 weeks prior to registration with no evidence of prostate-specific antigen (PSA) decline after washout.

   1. Bicalutamide: Washout period at least 6 weeks
   2. Flutamide and nilutamide: Washout period at least 4 weeks
7. Participants must discontinue therapies for mCRPC for 5 half-lives or 28 days, whichever is shorter.

   1. Participants will remain on gonadotropin-releasing hormone (GnRH) agents throughout this study.
   2. Prior chemotherapy is allowed if no progression of disease on chemotherapy as defined by PCWG3-modified RECIST 1.1.
   3. Prior treatment with sipuleucel-T, radium-223, or poly ADP ribose polymerase (PARP) inhibitor (eg, olaparib) is allowed.
   4. Tissue biopsy may be performed during washout period.

Key Exclusion Criteria:

1. Has a diagnosis of immunodeficiency or conditions that need systemic corticosteroid replacement therapy \> 10 mg/day prednisone (or equivalent) or other immunosuppressive medications within 28 days prior to the first dose of study intervention. Inhaled steroids are permitted if necessary.
2. Has any active known or suspected autoimmune disease. Participants with vitiligo, type I diabetes mellitus, controlled autoimmune hypothyroidism, psoriasis not requiring systemic treatment, or other conditions under control are permitted to enroll.
3. Has a known history of active TB (Bacillus Tuberculosis).
4. Has known history of, or any evidence of active, non-infectious pneumonitis.
5. Known history of testing positive for human immunodeficiency virus (HIV), known acquired immunodeficiency syndrome (AIDS), or any positive test for hepatitis B or hepatitis C virus representing acute or chronic disease.
6. Has received a live vaccine within 30 days of planned start of study intervention.

   Note: Seasonal influenza vaccines for injection are generally inactivated flu vaccines and are allowed; however intranasal influenza vaccines (eg, Flu-Mist®) are live attenuated vaccines, and are not allowed.
7. Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis. Participants with previously treated brain metastases may participate provided they are stable (without evidence of progression by imaging for at least 4 weeks prior to the first dose of study intervention and any neurologic symptoms have returned to baseline), have no evidence of new or enlarging brain metastases, and are not using steroids for at least 7 days prior to study intervention. This exception does not include carcinomatous meningitis which is excluded regardless of clinical stability.