600- vs 400-mg First-Line Ribociclib in Hormone Receptor–Positive/ ERBB2 -Negative Advanced Breast Cancer

Importance Ribociclib, 600 mg showed substantial survival benefits in patients with hormone receptor–positive (HR + )/ ERRB2 –negative ( ERBB2 − ; formerly HER2 ) advanced breast cancer (ABC) in the phase 3 MONALEESA trials but was associated with dose-dependent adverse events (AEs) that were manageable with dose reductions. Objective To investigate whether a 400-mg ribociclib starting dose could reduce the incidence of AEs while maintaining efficacy in ABC. Design, Setting, and Participants The AMALEE phase 2, multicenter, randomized, open-label, interventional noninferiority study was conducted between June 18, 2019, and December 8, 2020, and included pre- and postmenopausal women with newly diagnosed HR + / ERBB2 − ABC. The study was conducted across 107 sites in 23 countries (across Europe and Australia, Latin America, North America, and Asia). The data were analyzed at the final data cutoff (August 30, 2024). Interventions Randomization 1:1 to ribociclib, 400 mg + a nonsteroidal aromatase inhibitor or ribociclib, 600 mg + a nonsteroidal aromatase inhibitor (premenopausal patients also received goserelin). Main Outcomes and Measures Overall response rate (ORR; primary end point); ΔFridericia-corrected QT interval (QTcF) from baseline to cycle 1 day 15, 2 hours postdose (ΔQTcF; secondary end point); duration of response (DOR); time to response (TTR); progression-free survival (PFS); pharmacokinetics; and safety. Final analysis results are reported. Results Baseline characteristics and prior anticancer therapy were balanced among the 376 patients (median [range] age, 58.0 [27-96] years). Median (range) follow-up from randomization was 53.5 (36.0-64.0) months (final data cutoff: August 30, 2024). The absolute ORR difference between ribociclib, 400 mg and ribociclib, 600 mg was −7.2% (ORR ratio, 0.87; 90% CI, 0.74-1.03). With ribociclib, 400 mg vs ribociclib, 600 mg, median PFS (26.9 vs 25.1 months) and DOR (26.5 vs 28.8 months) were similar; TTR was longer (13.1 vs 9.0 months). The maximal plasma concentration after dose and the 24-hour area under the curve (measured at the primary data cutoff) were 28.0% and 42.7% lower, respectively, with ribociclib, 400 mg than ribociclib, 600 mg. Ribociclib, 400 mg had a shorter ΔQTcF (12.5 vs 19.7 milliseconds at cycle 1 day 15, 2 hours postdose), lower grade 3 or4 neutropenia rate (41.0% vs 58.5%), and fewer patients who required dose reduction due to AEs (29 patients [15.4%] vs 69 patients [36.9%]). Liver-related AEs, kidney toxic effects, interstitial lung disease or pneumonitis, and AE-prompted discontinuation rates were similar between arms. Conclusions and Relevance The AMALEE randomized clinical trial did not demonstrate ORR noninferiority of ribociclib, 400 mg vs ribociclib, 600 mg, with comparable DOR and PFS between doses. Ribociclib, 400 mg had longer TTR, lower pharmacokinetic exposure, and lower rates of QTcF prolongation and neutropenia. The final results confirmed the standard ribociclib, 600 mg starting dose in HR + / ERBB2 − ABC while supporting dose reduction to manage dose-dependent AEs. Trial Registration ClinicalTrials.gov Identifier: NCT03822468