Cohort A is allocated for the patients with Advanced Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma who confirmed with HER2 negative, and approximately 78 subjects will be enrollment to evaluate the efficacy and safety of pembrolizumab + Capecitabine/oxaliplatin.~Cohort B is allocated for the patients with Advanced Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma who confirmed with HER2 positive, and approximately 15 subjects will be enrollment to evaluate the efficacy and safety of pembrolizumab + Trastuzumab + Capecitabine/Cisplatin~(1 Cycle: Day 1 through Day 21)~Planned Biopsy Analysis immediate pre-treatment biopsy before chemotherapy; Newly-obtained specimens are defined as FFPE-preserved blocks or fresh tissue collected up to 12 weeks prior to C1D1.~2.on treatment primary tumor biopsy (C2D1 - 3 days window; before administration of C2D1) to explore whether tumor has change in characters (i.e., immune desert to immune infiltration) 3.on treatment primary tumor biopsy (C7D1 - 3 days window; before administration of C7D1) to explore whether tumor has change in characters (i.e., immune desert to immune infiltration) 4.At progression biopsy will be optional whenever the tumor biopsy is feasible.
This study is an open-label, single arm, phase II trial the efficacy and safety of pembrolizumab + CapeOx (HER2 negative ARM) or pembrolizumab + Trastuzumab + Capecitabine/Cisplatin (HER2 positive ARM) as first line therapy in advanced gastric or gastroesophageal junction (GEJ) adenocarcinoma.