Tucatinib With Chemotherapy and Trastuzumab in Advanced Her-2-neu Overexpressing, Previously Treated Breast Cancer.

This phase I/II study will assess the recommended dosing of tucatinib in combination with trastuzumab and either vinorelbine or gemcitabine in patients with advanced, HER2+ breast cancer. The study will be conducted as a parallel cohort study looking at optimal dose and safety and efficacy. Arm 1 Gemcitabine + Tucatinib + Trastuzumab: Gemcitabine (1000 mg/m2) will be administered intravenously on Days 1 and 8. The investigational study drug (tucatinib) will be administered as 300mg by mouth taken twice a day of every day in each cycle. Trastuzumab will be administered per package insert on Day 1 of each cycle. Arm 2 Vinorelbine + Tucatinib + Trastuzumab: Vinorelbine (25 mg/m2) will be administered intravenously on Days 1 and 8 of each cycle. The investigational study drug (tucatinib) will be administered as 300mg by mouth twice a day of every day in each cycle. Trastuzumab will be administered per package insert on Day 1 of each 2 cycle. Note: Cycle length is 21 days.