KeyLargo: Pembrolizumab + Oxaliplatin + Capecitabine in Gastric Cancer

This study will be conducted in two stages: 1) safety validation and 2) dose expansion 1. Safety Validation Cohort: The first portion of the study will preliminarily establish the tolerability of the combination of pembrolizumab, oxaliplatin and capecitabine. Five (5) subjects will be enrolled and their safety data after 21 days of treatment will be reviewed before additional subjects are enrolled. Subjects on this portion of the study will only be enrolled at the Duke Cancer Institute. 2. Dose Expansion Cohort: The second portion of the study (ie. phase II) will enroll 30 subjects. In the dose expansion cohort, the first cycle will be modified to allow one week of pembrolizumab monotherapy before starting capecitabine and oxaliplatin (XELOX) chemotherapy, which will allow analysis of biomarkers related to pembrolizumab. Subjects on this portion of the study will be enrolled at the Duke Cancer institute and select external collaborating institutions. The primary objective of this trial is to describe the progression free survival (PFS) associated with the combination of pembrolizumab, oxaliplatin and capecitabine (pembrolizumab +XELOX) in all patients with previously untreated metastatic esophagogastric adenocarcinoma.