This study is researching a new drug called REGN15505 when used alone or in combination with cemiplimab or in combination with REGN4336 in adult patients with mCRPC and ccRCC. The goal is to explore new ways to treat these cancers by helping immune cells target and destroy cancer cells.

The study will evaluate the use of REGN15505 when administered alone, in combination with cemiplimab, or in combination with REGN4336 for:

• Any side effects of study drugs
• How well the study drugs work
• How much REGN15505, cemiplimab, and REGN4336 are in the blood at different times
• If the body makes antibodies to REGN15505 or REGN4336, which may mean the study drugs will not work as well as expected
• What is the best dose of REGN15505 when administered alone and with cemiplimab and the best dose of REGN15505 and REGN4336 when used in combination

Key Inclusion Criteria:

Participants with mCRPC:

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

2. mCRPC with PSA value at screening ≥4 ng/mL and that has progressed within 6 months prior to screening as described in the protocol

3. Has received ≥2 lines of prior systemic therapy approved in the metastatic and/or castration-resistant setting as described in the protocol

4. Have had either Orchiectomy or be on Luteinizing Hormone-Releasing Hormone (LHRH) agonist or antagonist therapy with serum testosterone <50 ng/dL AND agree to stay on LHRH agonist or antagonist therapy during the study

   Participants with ccRCC:

5. Men and women with histologically or cytologically confirmed Renal Cell Carcinoma (RCC) with a clear-cell component

6. Diagnosis of metastatic ccRCC with at least 1 measurable lesion via Response evaluation criteria in solid tumors (RECIST) 1.1 criteria

7. Has progressed on or after ≥1 line of prior systemic therapy approved in the metastatic setting. Prior treatment must include an Anti Program Cell Death 1 (PD-1)/ Program Death Ligand 1 (PD-L1) therapy and either ipilimumab and/or a Tyrosine Kinase Inhibitor (TKI)

Key Exclusion Criteria:

For Both mCRPC and ccRCC Cohorts:

1. Has received treatment with an approved systemic therapy (including sipuleucel-T for mCRPC patients) within 3 weeks of dosing or has not yet recovered (ie, grade ≤1 or baseline) from any acute toxicities except for laboratory changes as described in the protocol

   mCRPC Cohort Only:

2. Has received prior Prostate Specific Membrane Antigen (PSMA)-targeting therapy with the exception of a PSMA-targeting radioligand (eg, 177Lu-PSMA-617)

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