This study is researching an experimental drug called REGN3767, also known as fianlimab (R3767), when combined with another medication called cemiplimab (each individually called a "study drug" or called "study drugs" when combined) compared with cemiplimab alone. These types of immunotherapy study drugs are collectively known as immune checkpoint inhibitors. Immunotherapies are treatments that use the immune system to recognize and kill cancer cells. The study is focused on participants with a type of skin cancer known as melanoma.

The objective of this study is to see if the combination of fianlimab and cemiplimab is an effective treatment compared to cemiplimab in participants with high-risk, resectable melanoma. Participants will receive treatment before surgery, undergo resection, and then will have the option to continue treatment after resection.

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

• What side effects may happen from receiving the study drug(s).
• How much study drug(s) is in the blood at different times.
• Whether the body makes antibodies against the study drug(s) (which could make the drug less effective or could lead to side effects). Antibodies are proteins that are naturally found in the blood stream that fight infections.
• How administering the study drugs might improve quality of life.

Key Inclusion Criteria:

1. All patients must be either stage III (IIIB, IIIC, IIID) or stage IV (M1a, M1b, M1c) per American Joint Committee on Cancer (AJCC) 8th edition (Amin 2017) and have histologically confirmed cutaneous melanoma that is deemed completely surgically resectable in order to be eligible as described in the protocol.
2. Patients with stage III melanoma must have clinically detectable disease that is confirmed as malignant on the pathology report. The pathology report must be reviewed, signed and dated by the investigator; this process will be confirmed during the interactive voice response system (IVRS) process as described in the protocol.
3. Patients must be candidates for full resection with curative intent and must be able to be surgically rendered free of disease with negative margins on resected specimens at surgery. The treatment plan including date of surgery must be documented by the investigator prior to randomization.
4. All patients must undergo full disease staging through a complete physical examination and imaging studies within 4 weeks prior to randomization. Imaging must include a computer tomography (CT) scan of the chest, abdomen, pelvis (if the primary tumor is on the head/neck then include a CT scan of head/neck), and all known sites of previously resected disease (if applicable) and brain magnetic resonance imaging (MRI) (or brain CT with contrast allowed if MRI is contraindicated).
5. Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0 or 1

Key Exclusion Criteria:

Medical conditions:

1. Primary uveal melanoma

2. Ongoing or recent (within 2 years) evidence of an autoimmune disease that required systemic treatment with immunosuppressive agents. The following are non-exclusionary: vitiligo, childhood asthma that has resolved, residual hypothyroidism that requires only hormone replacement, psoriasis not requiring systemic treatment.

3. Patients must not have received any prior systemic anti-cancer therapy for melanoma. Prior radiotherapy for melanoma is allowed if not given to a target lesion or, if given to a target lesion, there is pathological evidence of disease progression in the same lesion.

4. 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 as described in the protocol.

   Prior/concomitant therapy:

5. Use of immunosuppressive doses of corticosteroids (≥10mg of prednisone per day or equivalent) within 14 days of the first dose of study medication as described in the protocol.

6. Treatment with any anti-cancer therapy for malignancies other than melanoma, including immuno- therapy, chemotherapy, radiotherapy, or biological therapy in the 5 years prior to randomization as described in the protocol.

   Other comorbidities:

7. Participants with a history of myocarditis.

8. History or current evidence of significant (CTCAE grade ≥2) local or systemic infection (e. g., cellulitis, pneumonia, septicemia) requiring systemic antibiotic treatment within 2 weeks prior to the first dose of trial medication.

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