- PIK3CA wildtype/preserved PTEN expression. This trial will explore, if the combination of BKM120 and tamoxifen can overcome resistance to antihormonal therapies. BKM120 is selective for class I PI3K enzymes with no mTOR inhibitory activity that has entered Phase II and III clinical trials. The tumor suppressor PTEN is the most important negative regulator of the PI3K signaling pathway. Therefore, in addition the trial will prospectively evaluate PIK3CA mutations and/or loss of PTEN expression as predictive biomarkers for clinical benefit from combined treatment with BKM120 and tamoxifen.