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