Trastuzumab is an anticancer drug used to treat breast cancers that produce the HER2 protein. However, in some women, the cancer becomes resistant to trastuzumab and continues to grow. This resistance may develop because two proteins become activated: PI3 kinase (PI3K) and HER3. Combining trastuzumab with drugs that block these two proteins may be an effective way to treat HER2-positive breast cancer.
LJM716 is a drug that blocks some functions of HER3. It is given intravenously (by vein). BYL719 works by blocking some functions of PI3K. It is taken orally (by mouth). Both LJM716 and BYL719 are investigational drugs. In this study, researchers want to find the highest dose of BYL719 that can be given safely with LJM716 and trastuzumab (which is also given intravenously) in women with advanced HER2-positive breast cancer.