A Study to Evaluate Concurrent VRP-HER2 Vaccination and Pembrolizumab for Patients With Breast Cancer

The primary objective of this phase II study is to determine whether pembrolizumab increases the tumor infiltrating and peripheral blood immune response to the VRP-HER2 vaccine. The investigators hypothesize that HER2 specific T cell responses and anti-tumor immunity induced with HER2 vaccination will be augmented by concurrent anti-PD-1 antibody therapy. The investigators will additionally determine whether the administration of pembrolizumab is safe in patients with recurrent or metastatic HER2+ cancers who are receiving the anti-HER2 vaccine VRP-HER2. Participants will be randomized 1:1:1 into 3 arms (n=12 per arm). Subjects with metastatic HER2 overexpressing breast cancer receiving trastuzumab and pertuzumab will continue these antibodies. They will undergo a biopsy of their tumor and peripheral blood draw for immune cell analyses and be assigned to the applicable arm of the study. Arm A will consist of the the VRP-HER2 immunizations; Arm B will consist of pembrolizumab; Arm C will consist of the VRP-HER2 immunizations plus pembrolizumab. Tumor biopsies and peripheral blood draws will be performed following the course of immunizations.