This study is researching an experimental drug called fianlimab (also known as REGN3767), combined with another medication called cemiplimab (also known as REGN2810), called "study drugs". The study is focused on patients with a type of skin cancer known as melanoma. The aim of the study is to see how safe and effective the combination of fianlimab and cemiplimab is in treating melanoma, in comparison with the combination of two medications, relatlimab and nivolumab, commercialized under the brand name Opdualag™ and approved for the treatment of melanoma in adults and children.

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

• What side effects may happen from taking the study drugs.
• How much study drug is in the blood at different times.
• Whether the body makes antibodies against the study drugs (which could make the drug less effective or could lead to side effects)

Key Inclusion Criteria:

1. Participants with histologically confirmed unresectable stage III and stage IV (metastatic) melanoma per American Joint Committee on Cancer (AJCC), eighth revised edition.
2. Participants must not have received prior systemic therapy for unresectable or metastatic melanoma as described in the protocol.
3. Measurable disease per RECIST version 1.1.
4. Eastern Cooperative Oncology Group (ECOG) performance status (PS) ≤1
5. Adequate bone marrow, hepatic, and kidney function
6. Known B-Rapidly Accelerated Fibrosarcoma protein (BRAF) V600 mutation status or submitted sample for BRAF V600 mutation assessment as described in the protocol

Key Exclusion Criteria:

Medical Conditions:

1. Uveal, acral or mucosal melanoma.

2. Ongoing or recent (within 2 years) evidence of an autoimmune disease that required systemic treatment with immunosuppressive agents as described in the protocol.

3. Uncontrolled infection with human immunodeficiency virus (HIV), hepatitis B (HBV), or hepatitis C virus (HCV) infection; or diagnosis of immunodeficiency that is related to, or results in chronic infection. Mild cancer-related immunodeficiency (such as immunodeficiency treated with gamma globulin and without chronic or recurrent infection) is allowed.

   Prior/Concomitant Therapy:

4. Prior immune checkpoint inhibitor therapy other than anti-PD1/PD-L1 as described in the protocol

5. Systemic immune suppression as described in the protocol.

   Other Comorbidities:

6. Participants with a history of myocarditis.

7. Troponin T (TnT) or troponin I (TnI) >2x institutional upper limit of normal (ULN).

8. Active or untreated brain metastases or spinal cord compression as described in the protocol.

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.