Safety and Efficacy of Pembrolizumab (MK-3475) in Combination With TS-1+Cisplatin or TS-1+Oxaliplatin as First Line Chemotherapy in Gastric Cancer (MK-3475-659/KEYNOTE-659)

Inclusion Criteria: * Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Has histologically- or cytologically-confirmed diagnosis of locally advanced unresectable or metastatic gastric or GEJ adenocarcinoma * Has a PD-L1 positive tumor as determined by immunohistochemistry (IHC) at a central laboratory * Has measurable disease as defined by RECIST 1.1 as determined by investigator assessment. Tumor lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions * Women of childbearing potential (WOCBP) must have a negative urine or serum pregnancy test at the timing of enrollment * Participants of childbearing potential must be willing to use an adequate method of contraception for the course of the study through 120 days after the last dose of study medication * Has adequate organ function Exclusion Criteria: * Has squamous cell or undifferentiated gastric cancer * HER2-positive status * Has had previous therapy for locally advanced, unresectable or metastatic gastric/GEJ cancer * A WOCBP who has a positive urine pregnancy test within 72 hours prior to allocation * Has received prior therapy with a platinum-based anti-cancer drug * Has had major surgery, open biopsy or significant traumatic injury within 28 days prior to enrollment, or anticipation of the need for major surgery during the course of study treatment * Has had radiotherapy within 14 days of enrollment * Has a known a…