Abstract PS2-02-12: Tolerability of Adjuvant Abemaciclib in Older Adults with High-Risk Early Breast Cancer: Multicentre UK Real-World Data from the CONCISE Study

Abstract Background: Adjuvant Abemaciclib (AA) combined with endocrine therapy (ET) significantly improves invasive disease-free survival (IDFS) in patients with hormone receptor-positive (HR+), HER2-negative (HER2 -), node-positive early breast cancer (EBC), as demonstrated in the monarchE trial. However, only 15.6% of patients in monarchE were aged ≥65 and no dedicated geriatric safety analysis was performed. Managing older adults with EBC is complex due to comorbidities, reduced physiological reserve and polypharmacy. There is limited real-world evidence on the safety and tolerability of AA in this population. Methods: The CONCISE study is a retrospective, multicentre audit of UK practice across 21 centres, including patients treated with AA from July 2022 to April 2025. This analysis focuses on patients aged ≥65 years. Data were collected using the Ledidi platform and analysed using descriptive statistics in Jamovi. Key outcomes include treatment duration, dose modifications and adverse events (AEs), stratified by two subgroups: Cohort 1 (age 65-75) and Cohort 2 (75). Group comparisons were performed using chi-squared and t-test. Results: Of 1,026 patients, 215 (21%) were aged ≥65: 152 in Cohort 1 (median age 69) and 63 in Cohort 2 (median age 78). Most were female (96.3%) and Caucasian (88.4%). Stage II disease was most common (56.7%), with grade 2 tumours (57.8%) and nodal involvement (72.8%). Chemotherapy use differed by age: in Cohort 1, 68% received adjuvant chemotherapy and 11% neoadjuvant chemotherapy; in Cohort 2, only 31% received adjuvant chemotherapy and one patient had neoadjuvant chemotherapy. Median AA duration was 12.6 months in Cohort 1 vs. 8.3 months in Cohort 2 (p=0.028). Discontinuation of the treatment due to toxicity was significantly more common in Cohort 2 (79%) than in Cohort 1 (49%) p=0.002. Treatment completion (24 months) was achieved in 47% (Cohort 1) and 21% (Cohort 2). At data cut-off, 53% and 40% of patients remained on treatment in Cohorts 1 and 2, respectively. Disease progression occurred in 4% (Cohort 1) and 0% (Cohort 2). One-level dose reductions were required in 70% (Cohort 1) and 76% (Cohort 2) p=0.29, while further reductions were needed in 34% and 38% p=0.55, respectively. The most frequent AEs were diarrhoea (75% vs. 71.4%), anaemia (40% in both), and neutropenia (34.2% vs. 30.2%), with grade ≥2 rates comparable between groups. Conclusion: In this large UK real-world cohort, older adults with high-risk EBC experienced similar AE profiles to younger counterparts, but with numerically higher discontinuation rates, particularly in those aged 75. Dose reductions were common across both age groups. These findings underscore the importance of careful patient selection, supportive care, and geriatric assessment when offering AA in routine clinical practice. Citation Format: H. Abdallah, O. Ayodele, A. Ghose, Y. Owoseni, A. Maniam, B. Baraka, S. Horne, A. Nazir, L. Mooney, F. Nazeer, N. Atsumi, L. McAvan, M. Abraham, E. Bean, D. Morgan, G. Langford, E. Morris, R. Muhammad, E. Daniels, R. Pravinkumar, S. Mohammed, S. Raza, C. Blair, R. Khan, M. Tsalic, R. Kussaibati, R. Douglas, K. Panagiotis, S. Waters, J. Smith, E. Papadimitraki, C. Michie, C. Wilson, A. Konstantis. Tolerability of Adjuvant Abemaciclib in Older Adults with High-Risk Early Breast Cancer: Multicentre UK Real-World Data from the CONCISE Study 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 PS2-02-12.