A Study of Pembrolizumab (MK-3475) in Participants With Recurrent or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma (MK-3475-059/KEYNOTE-059)

This study will have 3 cohorts. Cohorts 1 and 2 will run simultaneously; Cohort 3 will start
when biomarker assay validation is complete. In Cohort 1, participants who have received at
least two prior therapies for their advanced disease will receive monotherapy with
pembrolizumab. In Cohort 2, participants who have not received any previous therapy for
their disease will receive pembrolizumab in combination with cisplatin and 5-FU or (Japan
only) capecitabine. In Cohort 3, participants who have not received any previous therapy and
who have programmed cell death ligand 1 (PD-L1)-positive tumors will receive monotherapy
with pembrolizumab.

Inclusion Criteria Cohort 1:

- Received and progressed on at least 2 prior chemotherapy regimens for their advanced
disease; prior regimen must have included a cisplatin and a fluoropyridine

- Human epidermal growth factor receptor 2 (HER-2/neu) negative, or, if HER2/neu
positive, must have previously received treatment with trastuzumab

Inclusion Criteria Cohort 2 or 3:

- HER2/neu negative

- Has not received prior systemic anti-cancer therapy for their advanced carcinoma
(systemic therapy received in the neoadjuvant and adjuvant setting does not count)

Inclusion Criteria All Participants:

- Histologically- or cytologically-confirmed recurrent or metastatic gastric or
gastroesophageal junction adenocarcinoma that is considered incurable by local
therapies

- Willing to provide tissue for PD-L1 biomarker analysis from newly-obtained and/or
archival tissue

- Measurable disease based on Response Evaluation Criteria in Solid Tumors version 1.1
(RECIST 1.1)

- Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 3 days
prior to first dose of study medication

- Life expectancy of >= 3months

- Female participants of childbearing potential should have a negative pregnancy test
and be willing to use 2 methods of birth control or be surgically sterile, or abstain
from heterosexual activity for the course of the study through 120 days after the
last dose of study medication (180 days for participants receiving cisplatin + 5FU)

- Male participants should agree to use an adequate method of contraception starting
with the first dose through 120 days after the last dose of study medication (180
days for participants receiving cisplatin + 5FU)

- Adequate organ function

Exclusion Criteria:

- Currently participating and receiving study therapy or participated in a study of an
investigational agent and received study therapy or used an investigation device
within 4 weeks of the first dose of study medication

- Active autoimmune disease that has required systemic treatment in past 2 years

- Immunodeficiency or receiving systemic steroid therapy or any other form of
immunosuppressive therapy within 7 days prior to the first dose of study medication

- Weight loss >10% over 2 months prior to first dose of study medication

- Clinical evidence of ascites by physical exam

- Prior anti-cancer monoclonal antibody (mAb) within 4 weeks prior to study Day 1 or
not recovered from adverse events due to agents administered more than 4 weeks
earlier

- Prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2
weeks prior to study Day 1 or who has not recovered from adverse events due to a
previously administered agent

- Known additional malignancy that is progressing or requires active treatment
excepting basal cell carcinoma of the skin or squamous cell carcinoma of the skin
that has undergone potentially curative therapy or in situ cervical cancer

- Known active central nervous system (CNS) metastases and/or carcinomatous meningitis

- Known history of, or any evidence of active, non-infectious pneumonitis

- Active infection requiring systemic therapy

- Psychiatric or substance abuse disorders that would interfere with cooperation with
the requirements of the study

- Pregnant or breastfeeding, or expecting to conceive or father children within the
projected duration of the trial, starting with the screening visit through 120 days
after the last dose of study medication (180 days for participants receiving
cisplatin + 5FU)

- Prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent

- Human immunodeficiency virus (HIV)

- Hepatitis B or C

- Received live vaccine within 30 days of planned start of study medication