A Randomized, Active-Controlled, Partially Blinded, Biomarker Select, Phase III Clinical Trial of Pembrolizumab as Monotherapy and in Combination with Cisplatin+5-Fluorouracil versus Placebo+Cisplatin+5-Fluorouracil as First-Line Treatment in Subjects with Advanced Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma;

1. Be willing and able to provide documented informed consent/assent for the
trial. The subject may also provide consent/assent for Future Biomedical
Research. However, the subject may participate in the main trial without
participating in Future Biomedical Research., 2. Be * 18 years of age on day of
providing documented informed consent (or acceptable age according to local
regulations, whichever is older)., 3. Have a performance status of 0 or 1 on
the Eastern Cooperative Oncology Group (ECOG) Performance Scale within 10 days
prior to the first dose of trial treatment., 4. Have histologically or
cytologically confirmed diagnosis of locally advanced unresectable or
metastatic gastric or GEJ adenocarcinoma. , 5. Be HER2/neu negative and PD-L1
positive., 6. Have 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.
, 7. Have provided tumor tissue sample deemed adequate for PD-L1 biomarker
analysis. , a. Notification of eligibility must be received prior to
randomization., b. Additional samples may be required if adequate tissue is not
provided., 8. Female subjects of childbearing potential must have a highly
sensitive negative urine or serum pregnancy test as required by local
regulation within 24 hours (urine) and within 72 hours (serum) prior to
receiving the first dose of study medication. If the urine test is positive or
cannot be confirmed as negative, a serum pregnancy test will be required. , 9.
Female subjects 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. Note: Abstinence is acceptable if this is the
usual lifestyle and preferred contraception for the subject. , 10. Male
subjects of childbearing potential must agree to use an adequate method of
contraception starting with the first dose of study therapy through 120 days
after the last dose of study therapy. Note: Abstinence is acceptable if this is
the usual lifestyle and preferred contraception for the subject. , 11.
Demonstrate adequate organ function. All screening labs should be performed
within 10 days of treatment initiation.

1. Has squamous cell or undifferentiated gastric cancer., 2. Has had previous
therapy for locally advanced, unresectable or metastatic gastric/GEJ cancer.
Subjects may have received prior neoadjuvant or adjuvant therapy as long as it
was completed at least 6 months prior to randomization., 3. Has had 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., 4. Has had radiotherapy within 14 days of randomization.
Subjects who received radiotherapy >14 days prior to randomization must have
completely recovered from any radiotherapy related AEs/toxicities., 5. Has a
known additional malignancy that is progressing or requires active treatment.
Exceptions include basal cell carcinoma of the skin, squamous cell carcinoma of
the skin that has undergone potentially curative therapy or in situ cervical
cancer., 6. Has known active central nervous system (CNS) metastases and/or
carcinomatous meningitis. Subjects with previously treated brain metastases
may participate provided they are stable (without evidence of progression by
imaging at least four weeks prior to the first dose of trial treatment and
neurologic symptoms have returned to baseline), have no evidence of new or
enlarging brain metastases, and are not using steroids for at least 7 days
prior to trial treatment. This exception does not include carcinomatous
meningitis which is excluded regardless of clinical stability., 7. Has an
active autoimmune disease that has required systemic treatment in past 2 years
(i.e. with use of disease modifying agents, corticosteroids or
immunosuppressive drugs). Replacement therapy (e.g., thyroxine, insulin, or
physiologic corticosteroid replacement therapy for adrenal or pituitary
insufficiency, etc.) is not considered a form of systemic treatment. , 8. Has
a diagnosis of immunodeficiency or is receiving chronic systemic steroid
therapy (in dosin exceeding 10 mg daily of prednisone equivalent) or any other
form of immunosuppressive therapy within 7 days prior to the first dose of
trial drug. , 9. Has a history of (non-infectious) pneumonitis/interstitial
lung disease that required steroids or current pneumonitis/interstitial lung
disease. , 10. Has an active infection requiring systemic therapy., 11. Has a
history or current evidence of any condition (e.g. known deficiency of the
enzyme dihydropyrimidine dehydrogenase [DPD]), therapy, or laboratory
abnormality that might confound the results of the trial, interfere with the
subject*s participation for the full duration of the trial, or is not in the
best interest of the subject to participate, in the opinion of the treating
investigator., 12. Has known psychiatric or substance abuse disorders that
would interfere with cooperation with the requirements of the trial., 13. Is
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 trial treatment., 14. Has received prior
therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent., 15. Has a known
history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies)., 16. Has
current active Hepatitis B (e.g., HBsAg reactive, positive and/or a detectable
HBV DNA) or Hepatitis C virus (e.g., anti-HCV Ab positive and detectable HCV
RNA) infection., 17. Is currently participating in and receiving study therapy
or has participated in a trial of an investigational agent or has used an
investigational device within 4 weeks prior to the first dose of trial
treatment., 18. Has received a live vaccine within 30 days of planned start of
study therapy., Note: Examples of live vaccines include, but are not limited
to, the following: measles, mumps, rubella, varicella/zoster (chicken pox),
yellow fever, rabies, Bacillus Calmetta-Guérin (BCG), and typhoid vaccine.
Seasonal influenza vaccines for injection are generally killed virus vaccines
and are allowed; however, intranasal influenza vaccines (e.g., FluMist®) are
live attenuated vaccines, and are not allowed.