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

The main purpose of this study is to determine the safety, tolerability (how your body reacts to the drug) and effectiveness (ability to treat your cancer) of REGN5678 alone, or in combination with cemiplimab.

The study has 2 parts. The goal of Part 1 (dose escalation) is to determine a safe dose(s) of REGN5678 when it is given alone or in combination with cemiplimab. The goal of Part 2 (dose expansion) is to use the REGN5678 drug dose(s) found in Part 1 to see how well REGN5678 alone or in combination with cemiplimab works to shrink tumors.

This study is looking at several other research questions, including:

1. Side effects that may be experienced by taking REGN5678 alone or in combination with cemiplimab
2. How REGN5678 alone or in combination with cemiplimab works in the body
3. How much REGN5678 and/or cemiplimab are present in the blood
4. To see if REGN5678 alone or in combination with cemiplimab works to reduce the size of the tumor by helping the immune system destroy the tumor


Eligibility Criteria:
Key Inclusion Criteria:

mCRPC cohorts:

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.

ccRCC cohorts:

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

Key Exclusion Criteria:

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

NOTE: Other protocol defined Inclusion/Exclusion Criteria apply