 - Participants must have documented disease progression while receiving dual anti-HER2 targeted therapy for metastatic breast cancer, as per investigator assessment. Any combination of biologic therapies is acceptable. Prior chemotherapy is acceptable, but patients must have been off cytotoxic chemotherapy for at least 1 month. Patients with ER-HER2 disease have must be failed at least 1 line of chemotherapy in the metastatic setting. Patients with ERHER2 disease who progressed on dual anti-HER2 therapy plus endocrine therapy are eligible. Concomitant endocrine therapy is acceptable and may be continued at the discretion of the treating physician. 