Phase II Trial of Pembrolizumab in Combination With Trastuzumab, Fluoropyrimidine, and Platinum Chemotherapy in First Line Stage IV HER2-positive Metastatic Esophagogastric (EG) Cancer

Oxaliplatin may be administered instead, at a dose of 130 mg/m2/day as an I.V.over approximately 2 hours on Day 1 every 21-day cycles, up to a maximum of 8 cycles in the absence of disease progression or until other withdrawl criteria are met. Patients may begin with reduced dose of oxaliplatin 104 mg/m2 as starting dose if deemed necessary per the treating physician discretion. Oxaliplatin will be administered after the completion of the pembrolizumab infusion.

Only for patients unable to take oral medications (because of certain circumstances such as malabsorption, difficulty swallowing, or other conditions that could affect intake of oral capecitabine medication), 5-FU may be administered instead, at a dose of 800 mg/m2/day as a continuous infusion over 5 days (Day 1 to Day 5 of each cycle), every 21 days. This decision should be made prior to registration by the treating physician; switching from capecitabine to 5-FU, or vice versa, will not be allowed during the study. 5-FU will be administered after completion of the cisplatin infusion.