Combination Margetuximab and Pembrolizumab for Advanced, Metastatic HER2(+) Gastric or Gastroesophageal Junction Cancer

Detailed Description: Both margetuximab and pembrolizumab are monoclonal antibodies used in combination to treat HER2+ gastric and gastroesophageal junction cancer. This study has two parts: Dose Escalation and Dose Expansion. The Dose Escalation phase of the study will evaluate safety of escalating doses of the combination treatment. The Dose Expansion phase will evaluate safety and activity of the combination in patients with gastric or gastroesophageal cancer once the final dose and schedule are defined. In addition, a cohort of patients with HER2+ 3+ gastric cancer patients will be enrolled in the Dose Expansion Phase.