Substudy 02C: Safety and Efficacy of Pembrolizumab in Combination With Investigational Agents or Pembrolizumab Alone in Participants With Stage III Melanoma Who Are Candidates for Neoadjuvant Therapy (MK-3475-02C/KEYMAKER-U02)

Substudy 02C is part of a larger research study that is testing experimental treatments for melanoma, a type of skin cancer. The larger study is the umbrella study. The goal of substudy 02C is to evaluate the safety and efficacy of investigational treatment arms in participants with Stage III melanoma who are candidates for neoadjuvant therapy to identify the investigational agent(s) that, when used in combination, are superior to the current treatment options/historical control available. Arm 1: Pembrolizumab + Vibostolimab, Arm 2: Pembrolizumab + Gebasaxturev, and Arm 3: Pembrolizumab were added in the base protocol on 13-Nov-2019, and enrollment into those arms has been completed. Arm 4: Pembrolizumab + MK-4830 was added in Amendment 04 on 20-Dec-2021, and enrollment into that arm has been completed. Arm 5: Favezelimab + Pembrolizumab and Arm 6: Pembrolizumab + all-trans retinoic acid (ATRA) were added in Amendment 06 on 25-Jun-2022, and enrollment is ongoing.

Inclusion Criteria: * Has histologically or cytologically confirmed melanoma * Has clinically detectable and resectable Stage IIIB or IIIC or IIID melanoma amenable to surgery * Has been untreated for Stage IIIB, IIIC or IIID melanoma * Surgical resection and re-resection of primary melanoma is allowed * Prior radiotherapy to the primary melanoma is allowed * Has provided a baseline tumor biopsy * Male participants who receive gebasaxturev are abstinent from heterosexual intercourse or agree to use contraception during the intervention period and for at least 120 days after the last dose of gebasaxturev * Male participants who receive all-trans retinoic acid (ATRA) are abstinent from heterosexual intercourse or agree to use contraception during the intervention period and for at least 7 days after the last dose of ATRA * Female participants are not pregnant or breastfeeding and are either not a woman of child-bearing potential (WOCBP) OR use a contraceptive method that is highly effective or are abstinent from heterosexual intercourse during the intervention period and for at least 120 days after the last dose of pembrolizumab, vibostolimab, gebasaxturev, or MK-4830, favezelimab + pembrolizumab, or 30 days after the last dose of ATRA, whichever occurs last * Has adequate organ function * Has resolution of toxic effect(s) of the most recent prior therapy to Grade 1 or less (except alopecia) Exclusion Criteria: * Has a diagnosis of immunodeficiency or is receiving immunosuppressive therapy within 7 days before the first dose of study intervention * Has a known additional malignancy that is progressing or requires active treatment within the past 2 years * Has known central nervous system (CNS) metastases and/or carcinomatous meningitis * Has ocular or mucosal melanoma * Has known hypersensitivity including previous clinically significant hypersensitivity reaction to treatment with another monoclonal antibody (mAb) * Has an active autoimmune disease that has required systemic treatment in the past 2 years * Has an active infection requiring systemic therapy * Has known history of human immunodeficiency virus (HIV) * Has known history of hepatitis B * Has a history of (noninfectious) pneumonitis * Has a history of active tuberculosis (TB) * Has received prior systemic anticancer therapy within 4 weeks prior to randomization * Has received prior radiotherapy within 2 weeks of first dose of study intervention * Has had major surgery \