Real-World Safety Profile of Adjuvant Abemaciclib in HR+/HER2− Early Breast Cancer

Background/Objectives: Treatment with adjuvant abemaciclib plus endocrine therapy (ET) has been demonstrated to improve outcomes in patients with HR+/HER2- early breast cancer at high risk of recurrence. However, real-world data regarding abemaciclib tolerability and treatment maintenance remain limited. In this study we describe the occurrence of adverse events (AEs) and their management through dose adjustments—dose reductions and temporary treatment interruptions—reflecting strategies that maximize compliance. Methods: We conducted a retrospective cohort study at the Oncology Department of ULS São João, in Porto, Portugal. Inclusion criteria comprised patients with HR+/HER2- early breast cancer treated, for at least three months, with abemaciclib, between January 2022 and January 2026. Clinical data were collected from electronic medical records and were analyzed using descriptive statistics, with continuous variables reported as median (IQR) and categorical variables as frequencies and percentages. Results: Fifty-six patients were included in this cohort. With the median follow-up of 14.5 months, 98.2% of patients experienced at least one AE. Most common AEs were gastrointestinal and hematological and occurred within the first 3 months of treatment. Dose reductions occurred in 46.4% of patients and temporary treatment interruptions in 25% of patients. However, no permanent treatment discontinuations were reported. Conclusions: In this cohort, AEs associated with adjuvant abemaciclib were common but generally low-grade and manageable. Early recognition of AEs allows appropriate dose adjustments and supportive measures, which support treatment maintenance. These findings highlight the importance of close clinical monitoring and patient counseling to enhance treatment compliance.