Study to Evaluate the Safety and Efficacy of Two Different Dosing Schedules of Pembrolizumab (MK-3475) Compared to Ipilimumab in Participants With Advanced Melanoma (MK-3475-006/KEYNOTE-006)

This study will evaluate the safety and efficacy of 2 different dosing schedules of pembrolizumab (MK-3475), every 2 weeks (Q2W) and every 3 weeks (Q3W), and compare the 2 schedules to treatment with ipilimumab in ipilimumab-naïve participants with unresectable or metastatic melanoma. The primary hypotheses are that pembrolizumab is superior to ipilimumab with respect to progression-free survival (PFS) and overall survival (OS).

Participants assigned to a primary course of pembrolizumab can receive up to 24 months of treatment. Participants with Stable Disease (SD) or better will then proceed to Post Treatment Follow-up. All efficacy and safety analyses will be based on the primary pembrolizumab course.

Participants who experience disease progression during the Post Treatment Follow-up will be eligible for a Second Course of pembrolizumab treatment for up to 1 additional year. With Amendment 05, all Second Course participants will be treated with a fixed dose of pembrolizumab 200 mg Q3W. With Amendment 06, after the study has achieved its key objectives or the study has ended, participants will be discontinued from this study and enrolled in an extension study to continue protocol-defined assessments and treatment.

Eligibility Criteria:
Inclusion Criteria:

* Histologically-confirmed diagnosis of unresectable Stage III or metastatic melanoma not amenable to local therapy (excluding uveal or ocular melanoma)
* At least one measurable lesion
* No prior systemic treatment (excluding adjuvant or neoadjuvant therapy) for melanoma (first line) or one prior systemic treatment (excluding adjuvant or neoadjuvant therapy) for melanoma (second line)
* Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1
* Archived tissue sample or new biopsy sample
* Female participants of childbearing potential must agree to use effective contraception from Visit 1 to 120 days after the last dose of study drug; male participants must agree to use an adequate method of contraception starting with the first dose of study drug through 120 days after the last dose of study drug

Exclusion criteria:

* Prior treatment with ipilimumab or other anti-cytotoxic T-Lymphocyte Antigen 4 (CTLA-4) agent or any anti-programmed cell death (PD-1 or PD-L2) agent
* Chemotherapy, radioactive, or biological cancer therapy within four weeks prior to the first dose of study drug, or not recovered from adverse events caused by cancer therapeutics administered more than four weeks earlier
* Currently participating or has participated in a study of an investigational agent or using an investigational device within 30 days of the first dose of study drug
* Expected to require any other form of systemic or localized antineoplastic therapy while on study
* On any systemic steroid therapy within one week before the planned date for first dose of randomized treatment or on any other form of immunosuppressive medication
* History of a malignancy (other than the disease under treatment in the study) within 5 years prior to first study drug administration, excluding adequately treated Stage 1 or Stage 2 basal/squamous cell carcinoma of the skin, carcinoma in situ of the cervix or breast, or other in situ cancers.
* Known active central nervous system (CNS) metastases and/or carcinomatous meningitis; participants with previously treated brain metastases are eligible
* Severe hypersensitivity reaction to treatment with another monoclonal antibody
* Active autoimmune disease or a documented history of autoimmune disease or syndrome that requires systemic steroids or immunosuppressive agents
* Active infection requiring systemic therapy
* Known history of Human Immunodeficiency Virus (HIV)
* Known history of or positive for Hepatitis B or C
* Known psychiatric or substance abuse disorder
* Regular user (including recreational use) of illicit drugs or had a recent history (within the last year) of substance abuse (including alcohol)
* Pregnant or breastfeeding, or expecting to conceive, or father children within the projected duration of the study
* Received a live vaccine within 30 days prior to first dose of study drug