Abstract P5-21-27: Efficacy and safety of ribociclib plus letrozole in US patients enrolled in the MONALEESA-2 study

Abstract Background: Endocrine-based therapy is the standard of care for patients with hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2−) advanced breast cancer (ABC), in whom delaying disease progression is the primary goal. In the randomized Phase 3 MONALEESA-2 study (ClinicalTrials.gov identifier, NCT01958021), first-line ribociclib (a selective inhibitor of cyclin-dependent kinases 4 and 6) + letrozole significantly prolonged progression-free survival (PFS) compared with placebo + letrozole in postmenopausal women with HR+, HER2− ABC (hazard ratio, 0.556; 95% confidence interval [CI], 0.429–0.720) at the interim analysis cutoff date. Here, we present safety and efficacy data from US patients who were enrolled in the MONALEESA-2 study. Methods: Postmenopausal women (N=668) with HR+, HER2− ABC who did not receive prior systemic treatment for ABC and had an Eastern Cooperative Oncology Group performance status score of ≤1, adequate bone marrow and organ function, and no history of active cardiac dysfunction were randomized 1:1 to receive either ribociclib (600 mg/d, 3 weeks on/1 week off) + letrozole (2.5 mg/d, continuous) or placebo + letrozole. The primary end point was locally assessed PFS. The data cutoff for this analysis was January 29, 2016. Results: Baseline demographics, patient disposition, and prior therapy were well balanced across treatment groups in the US subset (N=213; ribociclib group, n=100; placebo group, n=113). Median treatment duration was 16.3 months. In US patients, median PFS was significantly prolonged in the ribociclib group (median PFS, not reached [NR]; 95% CI, 17.1 months to NR) versus the placebo group (median PFS, 14.4 months [95% CI, 11.1–18.4 months]; hazard ratio, 0.451 [95% CI, 0.281–0.724]; P=0.000363), with a median follow-up of 11.0 months. In a subsequent preplanned analysis of overall survival, median PFS was 27.6 months in the ribociclib group versus 15.0 months in the placebo group (hazard ratio, 0.527; 95% CI, 0.351–0.793). The overall response rate (ORR) was significantly greater in the ribociclib group (ORR, 38.0%; 95% CI, 28.5–47.5%) versus the placebo group (ORR, 26.5%; 95% CI, 18.4–34.7%; P=0.040). A trend toward benefit was observed with ribociclib in all subgroups (including age, disease burden, prior therapy, and biomarker status), which was consistent with the overall population. No on-treatment deaths occurred. The most common all-grade adverse events, irrespective of causality, were neutropenia (ribociclib, 69.0%; placebo, 3.7%), nausea (ribociclib, 65.0%; placebo, 39.4%), fatigue (ribociclib, 56.0%; placebo, 46.8%), and diarrhea (ribociclib, 41.0%; placebo, 32.1%). The most common grade 3 (≥25% in either group) and grade 4 (≥10% in either group) AE was neutropenia (grade 3: ribociclib, 39.0%; placebo, 0%; grade 4: ribociclib, 14.0%; placebo, 0%). Two patients (2.0%) in the ribociclib group and none in the placebo group experienced febrile neutropenia. Conclusions: The US patients in the MONALEESA-2 study derived significant PFS benefit from ribociclib + letrozole compared with placebo + letrozole, with a 55% reduction in the relative risk of disease progression. The safety profile was consistent with and comparable to the global study population. Citation Format: Yardley DA, Hart L, Favret A, Blau S, Diab S, Richards D, Sparano J, Beck JT, Richards P, Ward P, Ramaswamy B, Tsai M, Pluard T, Tolaney S, Esteva F, Small T, Purkayastha D, Miller M, Hortobagyi G. Efficacy and safety of ribociclib plus letrozole in US patients enrolled in the MONALEESA-2 study [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-21-27.