BYL719 + T-DM1 in HER2(+) Metastatic Breast Cancer Pts Who Progress on Prior Trastuzumab & Taxane Tx

The purpose of this study is to see whether a combination of two different drugs - trastuzumab-MCC-DM1 (T-DM1) and BYL719 is safe, and if it might be effective in treating metastatic breast cancer. T-DM1 is a type of drug that contains an antibody (trastuzumab) linked to chemotherapy. The antibody in T-DM1 targets a marker on breast cancer cells called HER2, which allows the drug to go directly to the cancer cells. The use of T-DM1 in this study is considered standard treatment for the type of cancer in this study. Participants in this study have already been treated with trastuzumab and chemotherapy in the past, and their cancer has gotten worse in spite of those treatments. BYL719 is an oral drug (taken by mouth) that the researchers think may help T-DM1 to work better.