Ribociclib plus letrozole in subgroups of special clinical interest with hormone receptor–positive, human epidermal growth factor receptor 2–negative advanced breast cancer: Subgroup analysis of the phase IIIb CompLEEment-1 trial

BACKGROUND: The phase IIIb CompLEEment-1 study evaluated ribociclib plus letrozole in patients with hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2–) advanced breast cancer (ABC). Outcomes were investigated in the following subgroups: central nervous system (CNS) metastases, prior chemotherapy for advanced disease, Eastern Cooperative Oncology Group (ECOG) performance status (PS) 2, and visceral metastases plus prior chemotherapy for advanced disease or ECOG PS 2. PATIENTS AND METHODS: Patients with HR+, HER2– ABC without prior hormonal treatment for advanced disease received oral ribociclib (600 mg once daily, 3 weeks on/1 week off) plus letrozole (2.5 mg once daily, continuous). Primary endpoint was safety/tolerability, assessed via occurrence of adverse events (AEs); key secondary endpoints included time to progression (TTP), overall response rate, and clinical benefit rate. RESULTS: 51 patients had CNS metastases, 194 received prior chemotherapy for advanced disease, 112 had ECOG PS 2, 146 had visceral metastases plus prior chemotherapy, and 77 had visceral metastases plus ECOG PS 2. Safety results were consistent with those in the overall CompLEEment-1 population; no new safety concerns were identified. The AE profile was manageable with low rates of discontinuations due to AEs. TTP in patients with CNS metastases was consistent with the overall study population and shorter for other patient subgroups. Each patient subgroup achieved meaningful clinical benefit from treatment, consistent with the overall population. CONCLUSION: These findings confirm the clinical benefit of ribociclib plus endocrine therapy in high-risk patient subgroups of clinical interest commonly underrepresented in clinical trials.