Phase II Trial of Pembrolizumab With Trastuzumab and Chemotherapy in Advanced HER2 Positive Esophagogastric (EG) Cancer

Inclusion Criteria

• Patient must have pathologically or cytologically MSKCC confirmed esophageal, gastric or gastroesophageal junction (GEJ) adenocarcinoma by the enrolling institution.
• Patients must have esophageal, gastric or gastroesophageal adenocarcinoma with HER2 overexpression and/or amplification as determined by next generation sequencing assay, immunohistochemistry (IHC 3+) or fluorescent in situ hybridization (FISH+ is defined as HER2:CEP17 ratio ≥ 2.0). MSKCC or enrolling institution confirmation of HER2 status is not mandatory prior to enrollment and treatment on study. For patients with outside HER2 testing, if sufficient tissue is available HER2 testing will be repeated at MSKCC or enrolling institution for purpose of analysis and will not impact the patient's eligibility.
• Additional available archival tumor tissue in the form of 15-20 unstained slides should be submitted to MSKCC for future correlative analysis, but will not be required prior registration. Note: if tissue is depleted, patient will still be eligible after discussion with the MSK PI.
• Patients may have received no prior chemotherapy for Stage IV disease. Patients may have received prior adjuvant therapy (chemotherapy and/or chemoradiation) if more than 6 months have elapsed between the end of adjuvant therapy and registration
• Patients must have disease that can be evaluated radiographically. This may be measurable disease or non-measurable disease per RECIST 1.1.
• Patient must have a normal LVEF (>/= 53%). If a patient has a borderline LVEF (40-52%) they may be considered after consultation with cardiology and study PI and treated per the guidelines in section 11.2.2.
• Age 18 years or older.
• ECOG performance status 0-2.
• Demonstrate adequate organ function as defined below. All screening labs should be performed within 10 days of treatment initiation.

Hematological Absolute neutrophil count (ANC) ≥1,500 /mcL Platelets ≥100,000 / mcL Hemoglobin ≥9 g/dL or ≥5.6 mmol/L Renal Serum creatinine ≤1.5 X upper limit of normal (ULN) OR Measured or calculated creatinine clearance (measured via 24-hour urine collection) ≥60 mL/min for subject with creatinine levels > 1.5 X institutional ULN (GFR can also be used in place of creatinine or CrCl) Hepatic Serum total bilirubin ≤ 1.5 X ULN (1.5 mg/dL or 25.65 μmol/L) OR Direct bilirubin ≤ ULN for subjects with total bilirubin levels > 1.5 ULN. Except patients with Gilbert's disease (≤3x ULN) AST (SGOT) and ALT (SGPT) ≤ 2.5 X ULN OR AST (SGOT) and ALT (SGPT) ≤ 5 X ULN for subjects with liver metastases Albumin >2.5 mg/dL Coagulation Prothrombin Time (PT) ≤1.5 X ULN unless subject is receiving anticoagulant therapy as long as PT or PTT is within therapeutic range of intended use of anticoagulants Activated Partial Thromboplastin Time (aPTT) ≤1.5 X ULN unless subject is receiving anticoagulant therapy as long as PT or PTT is within therapeutic range of intended use of anticoagulants

• Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours 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 should be willing to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the study through 120 days after the last dose of study medication. Subjects of childbearing potential are those who have not been surgically sterilized or have not been free from menses for > 1 year.
• Male subjects should agree to use an adequate method of contraception starting with the first dose of study therapy through 120 days after the last dose of study therapy.
• Peripheral neuropathy ≤grade 1

Exclusion Criteria

• Is currently participating and receiving study therapy or has participated in a study of an investigational agent and received study therapy or used an invest