Study of Pembrolizumab (MK-3475) as First-Line Monotherapy and Combination Therapy for Treatment of Advanced Gastric or Gastroesophageal Junction Adenocarcinoma (MK-3475-062/KEYNOTE-062)

Inclusion Criteria:

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

- Have histologically- or cytologically-confirmed diagnosis of locally advanced
unresectable or metastatic gastric or GEJ adenocarcinoma

- HER2/neu protein negative and programmed cell death ligand 1 (PD-L1)-positive

- Have measurable disease

- Female participants of childbearing potential must be willing to use adequate
contraception 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

- Male participants of childbearing potential should agree to use an adequate method of
contraception starting with the first dose of study medication through 120 days after
the last dose of study medication

- Adequate organ function

Exclusion Criteria:

- Squamous cell or undifferentiated gastric cancer

- Previous therapy for locally advanced, unresectable or metastatic gastric/GEJ cancer.
Participant may have received prior neoadjuvant or adjuvant therapy as long as it was
completed at least 6 months prior to randomization

- Major surgery, open biopsy or significant traumatic injury within 28 days prior to
randomization, or anticipation of the need for major surgery during the course of
study treatment.

- Radiotherapy within 14 days of randomization

- Known additional malignancy that is progressing or requires active treatment with the
exception of basal cell carcinoma of the skin, 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

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

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

- History of non-infectious pneumonitis that required steroids or current pneumonitis

- Active infection requiring systemic therapy

- Pregnant, breastfeeding, or expecting to conceive or father children within the
projected duration of the trial, starting with the screening visit through 180 days
after the last dose of study medication

- Prior therapy with an anti-programmed cell death (PD)-1, anti-PD-L1, or anti-PD-L2
agent

- Known history of human immunodeficiency virus (HIV)

- Known active Hepatitis B or C

- 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 study medication

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