Palbociclib is an oral selective inhibitor of cyclin-dependent kinase( CDK) 4 and 6,which can restore the control of cell cycle and block the proliferation of tumor cells. In February 2015,palbociclib was approved by the accelerated approval process of the US FDA,and indicated in combination with letrozole for the treatment of postmenopausal women with estrogen receptor( ER)-positive,human epidermal growth factor receptor 2( HER2)-negative advanced breast cancer as initial endocrine-based therapy for their ...