ESR1 and breast cancer: In conclusion, amcenestrant, an oral SERD, at RP2D of 400 mg QD for monotherapy showed no DLTs and a favorable safety profile with no clinically significant cardiac or eye safety findings, and demonstrated preliminary antitumor activity irrespective of baseline ESR1 mutation status among postmenopausal women with metastatic ER+/HER2− breast cancer, most of whom were heavily pretreated with targeted therapies and/or fulvestrant.