A Trial to Find Out if REGN5678 is Safe and How Well it Works Alone or in Combination With Cemiplimab for Adult Participants With Metastatic Castration-Resistant Prostate Cancer and Other Tumors

Eligibility Criteria

• 1. Men with histologically or cytologically confirmed adenocarcinoma of the prostate without pure small cell carcinoma.
• 2. Prostate specific antigen (PSA) value at screening ≥4 ng/mL that has progressed within 6 months prior to screening as defined in the protocol.
• 3. Has received ≥2 lines prior systemic therapy approved in the metastatic and/or castration-resistant setting (in addition to androgen deprivation therapy \[ADT\]) including at least:
• 1. one second-generation anti-androgen therapy (eg, abiraterone, enzalutamide, apalutamide, or darolutamide)
• 2. post-177Lu-PSMA-617 radiotherapy expansion cohort only. Must have received at least 2 doses of 177Lu-PSMA-617.
• 1. Men and women with histologically or cytologically confirmed RCC with a clear-cell component.
• 2. Diagnosis of metastatic ccRCC with at least one measurable lesion via RECIST 1.1 criteria
• 3. Has progressed on or after ≥1 line prior systemic therapy approved in the metastatic setting. Prior treatment must include an anti-programmed death-1 (receptor) \[PD-1\]/programmed death-ligand 1 (PD-L1) therapy and either ipilimumab and/or a tyrosine kinase inhibitor
• 1. Has received treatment with an approved systemic therapy within 3 weeks of dosing or has not yet recovered (ie, grade ≤1 or baseline) from any acute toxicities, as described in the protocol
• 2. Has received any previous systemic biologic therapy within 5 half-lives of first dose of study therapy, as described in the protocol
• 3. Has received prior PSMA-targeting therapy with the exception of approved radiopharmaceutical therapy (eg. 177Lu-PSMA-617) in mCRPC patients
• 4. Dose Escalation: Has had prior anti-cancer immunotherapy (other than sipuleucel-T) within 5 half-lives prior to study therapy.
• 5. Dose Expansion (mCRPC only): Has had prior anti-cancer immunotherapy, as describe in the protocol
• 6. Any condition that requires ongoing/continuous corticosteroid therapy (\>10 mg prednisone/day or anti-inflammatory equivalent) within 1 week prior to the first dose of study therapy
• 7. Ongoing or recent (within 5 years) evidence of significant autoimmune disease that required treatment with systemic immunosuppressive treatments
• 8. Encephalitis, meningitis, neurodegenerative disease (with the exception of mild dementia that does not interfere with activities of daily living \[ADLs\]) or uncontrolled seizures in the year prior to first dose of study therapy
• 9. Uncontrolled infection with human immunodeficiency virus (HIV), hepatitis B or hepatitis C infection; or diagnosis of immunodeficiency