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

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<.05 significant). Results: 129 pts were identified; 78 (60.5%) received adj abema, 51 (39.5%) did not. Selected clinicopathologic characteristics are shown in the Table. No differences in race and ethnicity were found. ENT pts were older (median 65.5 vs 50.8 years; p<.001), more often pre-operatively node-negative (cN0, 78.4% vs 51.3%, p=.004) and had lower nodal burden postoperatively (pN1, 78.4% vs 44.9%, p<.001). Lobular histology was more frequent in EAT (26.9% vs 11.8%; p=.040). Clinical stage, pT category, tumor size ≥5 cm, multifocality, and ER status were similar. ET regimens varied (p<.001), with more EAT pts receiving AI+OFS (38.5% vs 11.8%). We explored the reasons why ENT pts did not receive abema. Among 51 ENT pts, 16 (31.4%) were not treated per provider clinical judgment. Of these, 12 (75.0%) had surgery before Ki67-based FDA indication language was removed, with factors including Ki67 unavailable (n=2), N1mi+Ki67 unavailable (n=5), low Oncotype+Ki67 unavailable (n=3), advanced age+low Oncotype+Ki67 unavailable (n=1), surgical ER-low+Ki67 unavailable (n=1). Four (25%) had surgery after the FDA removed Ki67-based indication language (surgical ER-low n=1, N1mi n=2, N1mi+age+comorbidities n=1). 13 (25.5%) pts were offered but declined abema, 4 (7.8%) were lost to follow-up, 3 (5.9%) started ribociclib. Other reasons: comorbidity+age (n=4, 7.8%), ≥3 comorbidities (n=1, 2%), severe comorbidity (n=2, 3.9% [Crohn’s disease and psychiatric disorder]), poor ET tolerance (n=2, 3.9%), ongoing olaparib (n=1, 2%), metastatic disease at post-surgical staging (n=1, 2%), pt hesitation despite abema prescription (n=2, 3.9%). The reason was unknown in 2 (3.9%) cases. Conclusions: In this cohort, a gap between EAT and ENT pts was seen. Older age and lower nodal burden were associated with not receiving adj abema. Pt choice and comorbidities also contributed to non-initiation. Citation Format: C. Corti, N. Zhou, A. R. Martin, C. Stever, T. Parker, C. Snow, G. Curigliano, T. A. King, E. A. Mittendorf, N. U. Lin, N. Tayob, S. M. Tolaney. 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]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-08-29.