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 additional malignancy that is progressing or has required active treatment within the past 5 years
* Has clinically active central nervous system (CNS) metastases and/or carcinomatous meningitis
* Has an active autoimmune disease that has required systemic treatment in the past 2 years with use of disease modifying agents, corticosteroids, or immunosuppressive drugs
* Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment
* Has a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis
* Has any active infection requiring systemic therapy
* Will be on flucytosine at the time of enrollment
* Has grade ≥ 2 peripheral sensory neuropathy
* Has poorly controlled diarrhea (e.g., watery stool, uncontrollable bowel movement with drugs, grade ≥ 2 and number of defecations, ≥ 5/day)
* Has accumulation of pleural, ascitic, or pericardial fluid requiring drainage within 2 weeks prior to enrollment
* Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with participation for the full duration of the trial, or is not in the best interest of the participant, in the opinion of the treating investigator
* Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial
* Is pregnant or breastfeeding or expecting to conceive or father children within the projected duration of the study, starting with the screening visit through 120 days after the last dose of study treatment
* Has received prior therapy with an anti-programmed death (PD)-1, anti-PD-L1, or anti PD L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor
* Has known history of human immunodeficiency virus (HIV) \[HIV1/2 antibodies\]
* Has a known history of Hepatitis B
* Has received live vaccine within 30 days of the planned start of study therapy
* Is currently participating in and receiving study therapy or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks prior to the first dose of trial treatment