MONARCH 2 was a phase III, randomized, double-blind, placebo-controlled study of fulvestrant with or without abemaciclib in women with HR+/HER2− ABC whose disease had progressed while receiving prior ET. Abemaciclib at 150 mg twice daily plus fulvestrant was effective, significantly improving PFS, ORR and OS. The OS benefit was consistent across subgroups, with a numerically greater effect observed among patients with poorer prognoses. The results also demonstrated the safety and tolerability of abemaciclib.