This is the first comparative analysis of real-world tumor response in patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative metastatic breast cancer (HR+/HER2– MBC) treated with first-line palbociclib plus letrozole versus letrozole alone in the real-world clinical setting.

Patients with HR+/HER2‒ MBC were more likely to respond to palbociclib plus letrozole compared with letrozole alone.

These data complement the clinical benefit of palbociclib plus endocrine therapy observed in randomized clinical trials and support palbociclib plus letrozole as a standard of care for patients with HR+/HER2‒ MBC.