Lapatinib With Trastuzumab in Treating Patients With HER2-Negative/HER2 Mutant Metastatic Breast Cancer

This phase II trial studies the effectiveness of lapatinib ditosylate (lapatinib) together with trastuzumab in treating patients with HER2-negative breast cancer that carries HER2 gene mutations. Lapatinib may kill tumor cells by blocking some of the enzymes needed for cell division and growth. Trastuzumab, a monoclonal antibody, may block the ability of tumor cells to grow and spread. Giving lapatinib together with trastuzumab may provide a more effective treatment for patients with this type of cancer.