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