Study of Pembrolizumab in Subjects With Advanced Gastric or Gastroesophageal Junction Adenocarcinoma Who Progressed After First-Line Therapy With Platinum and Fluoropyrimidine: Integration of Molecular Subtypes Through Integrative Genomic Analysis

Inclusion Criteria

• Be willing and able to provide written informed consent/assent for the trial. The subject may also provide consent for Biomedical Research. However, the subject may participate in the main trial without participating in Biomedical Research.
• Be 19 years of age on day of signing informed consent
• Have histologically or cytologically-confirmed diagnosis of gastric or Gastroesophageal Junction Adenocarcinoma
• Have metastatic disease or locally advanced, unresectable disease.
• Has experienced documented objective radiographic or clinical disease progression during or after first-line therapy containing any platinum,fluoropyrimidine doublet.
• Have measurable disease based on RECIST as determined by investigator. Tumor lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions.
• Be willing to provide fresh tissue for biomarker analysis, and, based on the adequacy of the tissue sample quality for assessment of biomarker status. Repeat samples may be required if adequate tissue is not provided. Newly obtained endoscopic biopsy specimens are preferred to archived samples and formalin-fixed, paraffin-embedded block specimens are preferred to slides.
• Have a performance status of 0 or 1 on the ECOG Performance Scale.
• Demonstrate adequate organ function, all screening labs should be performed within 10 days of treatment initiation.
• Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours 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.
• Female subjects of childbearing potential should 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. Subjects of childbearing potential are those who have not been surgically sterilized or have not been free from menses for > 1 year.
• Male subjects should 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.
• (Only Cohort B) confirmed as MSI-H via the PCR or IHC staining by institutional standard.

Exclusion Criteria

• Is currently participating 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 of the first dose of treatment.
• Has squamous cell or undifferentiated gastric cancer.
• 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 known history of active Bacillus Tuberculosis
• Hypersensitivity to pembrolizumab or any of its excipients.
• Has had a prior anti-cancer monoclonal antibody within 4 weeks prior to study Day 1 or who has not recovered from adverse events due to agents administered more than 4 weeks earlier.
• Has had 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.
• Has a known additional malignancy that is progressing or requires active treatment. Exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer.
• Has known active central nervous system metastases and/or carcinomatous meningitis. Subjects with previously treated brain metastases may participate provided they are stable (without evidence of progression by imaging for at least four weeks prior to the first dose of trial treatment and any neurologic symptoms have returned to baseline), have no evidence of new or en