Abstract P1-10-10: Outcomes of Abemaciclib Dose Escalation Strategy in High-Risk Early Breast Cancer

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Abstract Background: Abemaciclib is recommended as adjuvant therapy in node-positive, HR+, HER2-, high-risk early breast cancer, based on improved invasive disease-free survival (iDFS) in the monarchE trial. However, there are challenges with tolerability of standard dose abemaciclib. In monarchE, 16.6% discontinued abemaciclib due to adverse events (AEs), 56.9% experienced dose interruptions, and 41.2% had dose reductions. At our institution, we have implemented a dose escalation (DE) strategy where patients initiate abemaciclib at 100 mg BID (50 mg BID in select patients) and increase to the full dose over 1-2 months, based on tolerance and provider discretion. This study compares the tolerability of abemaciclib using a DE versus standard dosing (SD) strategy. Methods: This real-world analysis includes 164 HR+, HER2-, node-positive, high-risk EBC patients receiving adjuvant abemaciclib and endocrine therapy (ET) from October 12, 2021, to June 9, 2024. Patients were prescribed either a DE (n=83) or SD (n=81) per provider discretion. Primary outcomes included discontinuation, dose reduction, and treatment interruption rates, which were compared between the DE and SD groups using chi-square. Results: Median ages were 55 years (DE) and 54 years (SD); 70% (DE) and 62% (SD) were postmenopausal. After a median follow-up time of 7.3 months in the DE group, 48.2% were titrated to full dose. Median time to full dose was 46 days (range 21-165 days). Abemaciclib discontinuation rates were 18% (DE) vs. 27% (SD) (P=0.139); median times to discontinuation were 66 days (DE) vs. 58 days (SD). Interruptions occurred in 25.3% (DE) vs. 65.4% (SD) (P=0.001); dose reductions in 14.4% (DE) vs. 43.2% (SD) (P<0.005). The most common reasons for discontinuation were diarrhea (SD 14.6%, DE 6.1%) and fatigue (SD 7.3%, DE 6.1%). Leading AE-related dose modifications were diarrhea (SD 37.8%, DE 9.8%, P=0.001), nausea (SD 12.2%, DE 1.2%, P=0.005), and fatigue (SD 12.2%, DE 4.9%, P=0.094). Conclusion: This real-world study demonstrates that compared to the monarchE trial, the SD of abemaciclib is associated with a higher discontinuation rate (27% vs 16.6%), underscoring the importance of strategies to improve tolerability. Early discontinuations suggest that enhanced initial tolerability may improve adherence. An abemaciclib DE strategy may improve tolerability, minimizing interruptions, dose reductions, and AEs vs SD. However, formal protocols are needed to ensure full dose escalation. Future studies should assess long-term outcomes (iDFS) with DE vs. SD abemaciclib schedules. Citation Format: Noor Lad, Priyanka Sharma, Qamar Khan, Lauren Nye, Anne O'Dea. Outcomes of Abemaciclib Dose Escalation Strategy in High-Risk Early Breast Cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P1-10-10.

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