ERBB2 and breast cancer: In the MONARCH 3 study, abemaciclib plus a NSAI (letrozole or anastrozole) exhibited efficacy as initial therapy for HR+, HER2- advanced breast cancer, significantly improving PFS (median 28.2 vs. 14.8 months) and ORR (61.0% vs. 45.5%) compared to placebo plus NSAI, while maintaining a tolerable safety profile (7, 18).