CAPOX, Bevacizumab and Trastuzumab for Patients With HER2-Positive Metastatic Esophagogastric Cancer

Inclusion Criteria: * Confirmed HER2-positive esophageal, GE junction or gastric adenocarcinoma that is metastatic or unresectable. * All patients must have available tumor sample (either paraffin block or 15 freshly cut, unstained slides) prior to study entry. Part II: Patient must have primary esophagogastric tumor in place or other tumor that is accessible for mandatory biopsy. * Measurable disease, defined in RECIST 1.1 * 18 years of age or older * Life expectancy of greater than 12 weeks * ECOG performance status of 0 or 1 * Organ and marrow function as outlined in the protocol * Women of child-bearing potential and men must agree to use adequate contraception during study participation and for 30 days from the date of the last study drug administration. * Part II only: Participant agrees to undergo mandatory pre and post loading dose of trastuzumab biopsy for correlative science. Exclusion Criteria: * Prior therapy with any of the following; capecitabine, oxaliplatin, bevacizumab or trastuzumab is not allowed. May have received and completed adjuvant therapy at least 6 months prior to study entry or one prior therapy for metastatic disease as long as it did not include any of the above agents. * Chemotherapy or radiotherapy to greater then 25% of bone marrow within 4 weeks prior to entering the study. * Palliative radiation therapy to isolated bone metastasis within 2 weeks of initiating therapy. * Major surgery, open biopsy, significant traumatic injury within 4 w…