Phase I/IIa Trial of Gemcitabine Plus Trastuzumab and Pertuzumab in Previously Treated Metastatic HER2+ Breast Cancer

Inclusion Criteria:

Adult males or females (aged 18 or older) with histologically confirmed, metastatic human epidermal growth factor receptor 2 (HER2)+ (by immunohistochemistry (IHC) 3+ or fluorescence in situ hydridization (FISH) ratio ≥ 2.0) breast cancer
Have progressed on at least one prior line of chemotherapy plus HER2 directed therapy such as trastuzumab and/or pertuzumab in the metastatic setting. T-DM1 would count as a line of therapy and patients previously treated with T-DM1 are eligible.
Have not been treated with gemcitabine in the metastatic setting
Measurable disease per Response Evaluation in Solid Tumors (RECIST) 1.1 criteria
Eastern Cooperative Oncology Group (ECOG) performance status 2≤
Left Ventricular Ejection Fraction (LVEF) ≥ 50% at baseline as determined by either echocardiogram (ECHO) or multiple gated acquisition scan (MUGA)
Adequate bone marrow function as indicated by the following: absolute neutrophil count (ANC) >1500/µL; Platelets ≥100,000/µL; Hemoglobin >10 g/dL
Adequate renal function, as indicated by creatinine ≤1.5x upper limit of normal (ULN)
Adequate liver function, as indicated by bilirubin ≤1.5x ULN, aspartic transaminase (AST) or alanine transaminase (ALT) <2x ULN unless related to metastatic breast cancer to the liver (in which case AST/ALT < 5x ULN is allowed).
Signed informed consent
Adequate birth control in sexually active women of childbearing potential

Exclusion Criteria:

Active uncontrolled infection or major concurrent illness which in the opinion of the investigator would render the participant unsafe to proceed with the study
Uncontrolled central nervous system (CNS) metastases. Treated, non-progressing CNS disease (documented by brain magnetic resonance imaging [MRI]) off corticosteroids for at least 1 month potential participants are eligible.
Women who are pregnant or lactating
Prior chemotherapy within the last 3 weeks (last 6 weeks for nitrosureas/mitomycin)
Prior radiation therapy within the last 4 weeks; prior radiation therapy to indicator lesion (unless objective disease recurrence or progression within the radiation portal has been documented since completion of radiation)
Other concomitant active malignancies
History of significant cardiac disease, cardiac risk factors or uncontrolled arrhythmias
Ejection fraction <50% or below the lower limit of the institutional normal range, whichever is lower
Hypersensitivity to any of the study medications
Untreated psychiatric conditions preventing informed consent