Gemcitabine, Trastuzumab, and Pertuzumab in the Treatment of Metastatic HER2-Positive Breast Cancer After Prior Trastuzumab/Pertuzumab, or Pertuzumab Based Therapy

Inclusion Criteria: * Age ≥ to 18 * Stage IV HER2 (+) breast cancer * Histologically documented HER2 (+) breast cancer as defined as IHC 3+ or FISH amplification of ≥ 2.0 of primary or metastatic site; results from the local lab are acceptable. * lECOG performance status 0 -1 * Prior treatment with trastuzumab + pertuzumab (HP)-based therapy or pertuzumab-based in the neoadjuvantadjuvant, unresectable, locally advanced, or metastatic setting. * ≤ 3 prior chemotherapies in the metastatic setting. Prior anthracycline, taxane, gemcitabine, and anti-HER2 agents (i.e. trastuzumab, pertuzumab, lapatinib, neratinib, TDM-1, etc.) are allowed. If patients received prior gemcitabine, it could not have been combined with pertuzumab. Patients should have progression of disease on current therapy. * Measurable or non-measurable disease. * LVEF ≥ 50% * Hematologic parameters: white blood cell (WBC) count of ≥ 3000/ul, absolute neutrophil count (ANC) ≥ 1000/ul, platelets ≥ 100,000/ul, hemoglobin ≥10.0 g/dl * Non-hematologic parameters: bilirubin ≤ 1.5 mg/dl, AST/ALT≤ 2.5 x upper limit of normal (ULN), alkaline phosphatase ≤ 5 x ULN. * Creatinine ≤ 1.5 mg/dl * Patients with "treated and stable" brain lesions of a duration of ≥ 2 months may be enrolled. Exclusion Criteria: * History of prior unstable angina, myocardial infarction, CHF, uncontrolled ventricular arrhythmias within 12 months * History of prior ≥ G 3 hypersensitivity (HSR) or any toxicity to trastuzumab or pertuzumab that warranted permanent cessation of this agent * History of hepatitis B or C * Pregnant patients