Epacadostat and Pembrolizumab in Treating Patients With Metastatic or Unresectable Gastroesophageal Junction or Gastric Cancer

INCLUSION CRITERIA * ≥ 18 years of age on day of consent * Histologically-or cytologically-confirmed adenocarcinoma of the distal esophagus \[within 5 centimeters of the gastroesophageal junction (GEJ)\], gastroesophageal junction or stomach, including HER2+ disease * Metastatic or unresectable disease, including those with HER2+ disease * Progressed on at least 1 line of prior therapy for metastatic disease, or intolerant to that therapy if not progressed * If HER2+ disease, should have received prior trastuzumab * Life expectancy ≥ 12 weeks * Eastern Cooperative Oncology (ECOG) Performance Status 0 or 1 * Measurable disease per RECIST v1.1, assessed within 4 weeks prior to study entry * Tumor deemed amenable to biopsy by core for metastatic site or endoscopic biopsy for primary tumor (for both before and on-treatment biopsies) * Able to swallow pills * Female subject of childbearing potential should have a negative urine or serum pregnancy within 3 days 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 must be willing to use an adequate method of contraception starting with the date of consent 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 * Male subjects of childbearing potential must agree to use an adeq…