This open-label, phase 1/2 study assessed palbo 125 mg (21 days on, 7 off) + amcenestrant in dose escalation (Part C; n = 15; 200 and 400 mg QD in 28-day cycles) and dose expansion (Part D; n = 30; 200 mg recommended phase 2 dose) in ER+/HER2– aBC patients (pts) with ≥ 6 months of prior endocrine therapy....CB was observed for 7/8 pts with Y537S, D538G, E380Q, or Y537N baseline ESR1 mutations...