The purpose of this study is to see if a combination of drugs can help to treat breast cancer. This is a Phase II study which will test whether this combination of drugs is effective and will provide further information on side effects and safety. A standard chemotherapy, gemcitabine, will be combined with 2 other drugs that target the HER2 receptor. The HER2 receptor is a growth protein on the surface of some breast cancer cells that provides messages telling the breast cancer cell to grow. The standard chemotherapy will be gemcitabine.. The other two drugs work against HER2. One is called trastuzumab (Herceptin) and it is commonly given to women with advanced and early HER2 positive breast cancer. The other drug, pertuzumab (Perjeta), is also given to women with advanced HER2 positive breast cancer. The drugs in this study are each individually approved for the treatment of metastatic breast cancer. However, this study is looking at the effectiveness of gemcitabine with trastuzumab and pertuzumab when given to women who have advanced HER2 positive breast cancer who have had 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