CDK4 and breast cancer: However, the efficacy of three CDK4/6 inhibitors, palbociclib, abemaciclib and ribociclib, for improvement of PFS in HR-positive HER2-negative metastatic/recurrent breast cancer has been demonstrated in all Phase III clinical studies of postmenopausal first-line ET and pre-/postmenopausal second-line ET [9, 26–35].