Real-world patterns of dose reduction and early discontinuation of abemaciclib for early-stage, hormone-receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) breast cancer in insured United States adults.

e12718 Background: The MonarchE clinical trial of adjuvant abemaciclib 150 mg twice daily in women with early-stage HR+/HER2- breast cancer showed a high adverse event burden leading to frequent dose reductions over two years of treatment. There is limited real-world data characterizing dose of abemaciclib used over time for early-stage HR+/HER2- breast cancer. We sought to describe variation in dose used at initiation, frequency of dose reductions over time, and rates of early discontinuation using a large national claims database. Methods: We conducted an observational, retrospective analysis of 2021-23 MerativeTM Marketscan Commercial and Medicare claims (Copyright ©2026 Merative; All Rights Reserved). The protocol was reviewed and deemed exempt by the Institutional Review Board. Adult females who initiated abemaciclib (150, 100, or 50 mg) in 2022-23 were included if, prior to the first abemaciclib claim, they were continuously enrolled in a plan with drug coverage, had ≥2 breast cancer diagnosis codes, used an aromatase inhibitor or tamoxifen, had no HER2+ breast cancer medications, and had no claims for secondary or non-breast neoplasms. Patients were followed until disenrollment, diagnosis with a secondary or non-breast neoplasm, or end of available data (Dec. 31, 2023). We categorized dose at initiation (150, 100, or 50 mg) and estimated cumulative incidence of dose reduction and treatment discontinuation, conservatively defined as a gap in days’ supply of ≥12 weeks to avoid misclassification of temporary stoppages due to adverse events or other reasons. Results: Among 874 females initiating abemaciclib for early-stage breast cancer, 25% were ages 18-44 years, 36% were 45-54 years, and 40% were ≥55 years; 97% used an aromatase inhibitor and 3% tamoxifen. Most (83%) started abemaciclib at 150 mg; 13% started at 100 mg and 4% at 50 mg. Patients were followed for a median of 249 days (IQR 117-417) before censoring due to disenrollment (18%), diagnosis with secondary/non-breast neoplasm (5%), and end of data (77%). The overall one-year cumulative incidence of discontinuation in the full sample was 25% (95% CI 21%,29%), and was 23% (95% CI 20%,27%) for those initiating at the 150 mg dose vs. 33% (95% CI 24%,44%) in those initiating at 100 or 50 mg. Among the subset initiating at doses of 100 mg or higher, the one-year cumulative incidence of dose reduction was 44% (95% CI 40%,49%), and was higher for patients who initiated at 150 mg (48% 95% CI 44%,53%) vs. 100 mg (23% 95% CI 17%,33%). Conclusions: The one-year incidences of dose reduction and discontinuation observed in this study were higher than those reported in MonarchE. More research is needed to understand effects of dose reductions and early discontinuation on effectiveness and safety outcomes in real-world populations.