Letrozole and exemestane - drugs called “aromatase inhibitors” that block estrogen production - are used to treat metastatic breast cancer in postmenopausal women whose tumors contain hormone receptors. However, these drugs often stop working. The purpose of this study is to find the highest dose of an investigational drug called BYL719 that can be given safely in women with locally advanced inoperable or metastatic breast cancer who are already taking letrozole or exemestane.
BYL719 is an inhibitor of PI3 kinase, an enzyme which fuels the growth of several types of cancer, including many breast cancers. It is taken orally (by mouth). In this study, women who are taking either letrozole or exemestane will continue taking those drugs and will also receive BYL719.