ESR1 and breast cancer: Significant antitumor activity was detected only at doses where there was a sustained and significant reduction of these sites on Rb.21 Palbociclib has recently been FDA approved for the treatment of postmenopausal women with estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer as initial therapy in metastatic disease in combination with letrozole.