Introduction
The addition of CDK4/6 inhibitors (CDK4/6is) to endocrine therapy (ET) has greatly improved outcomes for patients with hormone receptor–positive (HR+)/human epidermal growth factor receptor 2–negative (HER2−) advanced breast cancer (ABC) [First-line palbociclib in combination with the nonsteroidal aromatase inhibitor (NSAI) letrozole failed to demonstrate a statistically significant improvement in OS over letrozole alone in postmenopausal patients in the PALOMA-2 trial (median OS [mOS], 53.9 vs. 51.2 months [hazard ratio (HR) 0.96; 95% CI 0.78–1.18; Ribociclib has demonstrated a significant PFS and OS benefit in all three of its pivotal phase III clinical trials in patients with HR+/HER2− ABC. In MONALEESA-2, first-line ribociclib + letrozole in postmenopausal women demonstrated a significant 12.5-month improvement over letrozole alone, with an mOS of 63.9 versus 51.4 months (HR 0.76; 95% CI 0.63–0.93; As the combination of a CDK4/6i + ET is the recommended first-line option for patients with HR+/HER2− ABC, it is highly clinically relevant to understand the OS benefits of ribociclib + fulvestrant in this population [