Abemaciclib for the Treatment of HR+HER2- Metastatic Breast Cancer: An Institutional Experience

(1) Background: Abemaciclib combined with endocrine therapy is a standard first- or later-line of treatment for HR+/HER2- metastatic breast cancer (MBC). The aim of this retrospective cohort study was to describe the outcomes of patients treated in a real-world setting, with particular focus on elderly patients. (2) Patients and methods: Patients treated with abemaciclib between November 2019 and February 2022 were included in the study. Data were collected from electronic medical records. The primary objective was to determine real-world progression-free survival (rwPFS), and secondary objectives included median overall survival (mOS) and safety. (3) Results: Analysis included 134 patients, with a median follow-up of 42 months. Median age was 62 years, with 29.9% aged 70+ years. A total of 51.5% of patients received abemaciclib in first-line, predominantly with aromatase inhibitor (68.1%). Median rwPFS was 21 in first-line and 20 months in the second-line, with no significant difference between treatment lines (HR 0.96; = 0.88). Patients treated in the third- or later-line had a significantly shorter rwPFS, at 7 months (HR 1.48, = 0.003). mOS was not reached in the first-line setting. For second- and third- or later-lines, mOS was 29 and 19 months, respectively. There was no significant difference in mOS between first- or second-line (HR 1.37, = 0.36). In the 70+ group, median rwPFS was 15 months and mOS was 25 months with no significant difference compared to younger patients (rwPFS HR 1.1; = 0.65; OS HR 1.4; = 0.21). Most common adverse events (AEs) were diarrhoea (68.7%), anaemia (64.9%), and increased serum creatinine (63.4%). Grade 3/4 AEs were reported in 21.6% of patients. Dose reductions occurred in 30.6% of patients and were more frequent in patients 70+ (40%) compared to younger patients (28%); the difference was not significant ( = 0.22). At study cut-off, 64.9% of patients discontinued abemaciclib, primarily due to disease progression (73.5%). (4) Conclusions: Our study provides valuable insights into the effectiveness and safety of abemaciclib for the treatment of MBC. We observed comparable outcomes in terms of rwPFS and OS between the first two lines, suggesting consistent effectiveness across treatment lines. In addition, our findings suggest that older age (70+) does not significantly impact the effectiveness and tolerability of abemaciclib, although the careful monitoring and management of AEs are warranted.