Immunologic Determinants of Response to Pembrolizumab (MK-3475) in Advanced Melanoma (MK-3475-161/KEYNOTE-161)

Inclusion Criteria:

* Has a histologically confirmed diagnosis of unresectable stage III or metastatic melanoma not amenable to local therapy
* Has testing for a BRAF mutation prior to study entry
* Has measurable disease per RECIST 1.1 as assessed by the investigator/radiology
* Has resolution of toxic effect(s) of the most recent prior chemotherapy to Grade 1 or less (except alopecia). If the participant received major surgery or radiation therapy of \>30 Gray units, they must have recovered from the toxicity and/or complications from the intervention
* Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 10 days of study start
* A female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies: is not a woman of childbearing potential (WOCBP) OR is a WOCBP and agrees to use a contraceptive method, consistent with local regulations regarding the methods of contraception for those participating in clinical studies, during the treatment period and for at least 120 days after the last dose of study treatment
* A male participant is eligible to participate if he agrees not to donate sperm PLUS to be abstinent from heterosexual intercourse as their preferred and usual lifestyle OR agree to use contraception, consistent with local regulations regarding the methods of contraception for those participating in clinical studies, during the treatment period and for at least 120 days after the last dose of study treatment

Exclusion Criteria:

* Has disease that is suitable for local therapy administered with curative intent
* Has a history of interstitial lung disease
* Has a positive pregnancy test within 72 hours before the first dose of study therapy
* Has received prior therapy with an anti-Programmed Cell Death Protein 1 (PD-1), anti-Programmed Cell Death-Ligand 1 (PD-L1), anti-Programmed Cell Death-Ligand 2 (PD-L2) agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (eg, cytotoxic T-lymphocyte-associated protein 4 \[CTLA-4\], OX-40, CD137)
* Has received prior systemic anti-cancer therapy including investigational agents within 4 weeks prior to study start
* Has received prior radiotherapy within 2 weeks of start of study therapy
* Has received a live vaccine within 30 days prior to the first dose of study therapy
* Has received transfusion of blood products (including platelets or red blood cells) or administration of colony stimulating factors (including granulocyte colony stimulating factor \[G-CSF\], granulocyte macrophage colony stimulating factor \[GMCSF\] or recombinant erythropoietin) within 4 weeks prior to study start
* Is currently participating in or has participated in a study of an investigational agent or has used an investigational device within 4 weeks prior to the first dose of study therapy
* Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior the first dose of study therapy
* Has a known additional malignancy that is progressing or has required active treatment within the past 3 years with the exception of basal cell and squamous cell carcinoma of the skin, transitional cell carcinoma of urothelial cancer, or carcinoma in situ (eg, breast carcinoma, cervical cancer in situ) that have undergone potentially curative therapy
* Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis
* Has severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients
* Has an active autoimmune disease that has required systemic treatment in past 2 years
* Has a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis
* Has an active infection requiring systemic therapy
* Has a known history of human immunodeficiency virus (HIV) infection
* Has a known history of Hepatitis B (defined as Hepatitis B surface antigen \[HBsAg\] reactive) or known active Hepatitis C virus (defined as HCV RNA \[qualitative\] is detected) infection
* Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the participant's participation for the full duration of the study, or is not in the best interest of the participant to participate, in the opinion of the treating investigator
* Has known psychiatric or substance abuse disorders that would interfere with cooperating with the requirements of the study
* Is pregnant or breastfeeding or expecting to conceive or father children starting with the screening visit through 120 days after the last dose of study therapy