Abstract PS3-08-29: Retrospective study evaluating eligibility, treatment patterns, and clinicopathologic factors associated with adjuvant abemaciclib use in patients with high-risk estrogen receptor-positive (ER+) HER2-negative (HER2-) early breast cancer (EBC)

Abstract Background: 2 years of adjuvant (adj) abemaciclib (abema) and endocrine therapy (ET) is standard for high-risk ER+HER2- EBC. Although eligibility rates are known, real-world use remains unclear. We examined the gap between eligible and treated patients (pts) and their clinicopathologic differences. Methods: Pts with ER+HER2- EBC eligible for adj abema per monarchE cohort 1 criteria (≥4 positive nodes, or 1-3 nodes plus G3 or tumor ≥5 cm), who had surgery between the date of FDA approval (10/12/2021) and the data lock (04/15/2024), were retrospectively identified from a single institution database. Pts were classified as EAT (eligible and treated) if an abema start date was recorded, or ENT (eligible, not treated) if no abema initiation was documented. Clinicopathologic features were compared using chi-square or Wilcoxon tests (p