The main purpose of this study is to determine the safety, tolerability (how the body reacts to the drug[s]) and effectiveness (ability to treat the cancer) of REGN5678 (Nezastomig) 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

Key Inclusion Criteria:

mCRPC cohorts (men):

1. Men with histologically or cytologically confirmed adenocarcinoma of the prostate without pure small cell carcinoma.

2. 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. 177Lu-PSMA-617 radiotherapy, or another lutetium-based PSMA targeted radioligand, as described in the protocol

ccRCC cohorts (men and women):

1. 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 a PSMA targeting radioligand (eg. 177Lu-PSMA-617) in mCRPC
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 described 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, as described in the protocol
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

All individual patient data (IPD) that underlie publicly available results will be considered for sharing

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf