Abstract PS3-07-12: The CONCISE Study : A Pan-UK Real-World Audit of Adjuvant Abemaciclib Use in High-Risk Early Breast Cancer

Abstract Background Adjuvant Abemaciclib (AA) combined with endocrine therapy (ET) is an established treatment for hormone receptor-positive (HR+), HER2-negative (HER2-), node-positive early breast cancer (EBC) at high risk of recurrence, as demonstrated in the monarchE trial. However, data on its tolerability and use in routine UK clinical practice are limited. This study presents multicentre real-world data on the use of adjuvant AA across the UK, focusing on treatment duration, dose modifications, discontinuations and toxicity. Methods CONCISE is a retrospective, multicentre observational study across 21 NHS Trusts including patients aged ≥18 years with HR+/HER2-, node-positive, high-risk EBC treated with AA between July 2022 and April 2025. High-risk was defined as ≥4 positive axillary lymph nodes or 1-3 nodes with tumour size ≥5 cm or histological grade 3. Clinical data were obtained from patients electronic records and a data registry was established on Ledidi. Primary endpoint: real-world treatment duration of AA. Secondary endpoints: rates of dose reduction, discontinuation, and grade ≥2 toxicities; correlation with age, endocrine therapy type and prior chemotherapy. Results A total of 1,028 patients were included; 98.5% were female and 82.8% Caucasian, with a median age of 54 years. Majority were post-menopausal (56.5%). Most patients had stage II (57.5%) and grade 2 (52.6%) tumours. Lymph node involvement was present in 95% (median = 5 nodes). Chemotherapy was administered in 88.4% (neoadjuvant 16.3%, adjuvant 72.1%). Endocrine backbone (AI, 86.2% vs Tamoxifen 13.8%).The median duration of AA was 15.5 months. At the time of analysis, 25.3% had completed the full 2-year course with 74.7% remaining on treatment. AA was discontinued in 15.9% due to toxicity and 3.8% due to disease progression (median discontinuation time: 10.3 months). Single dose level reduction occurred in 64.6%, with further reduction in 25.4%.Common toxicities included diarrhoea (76.4%, ≥Grade 2 in 40%), neutropenia (43.4%, ≥Grade 2 in 61.1%), anaemia (33.2%, ≥Grade 2 in 11.7%), thrombocytopenia (10%, ≥Grade 2 in 6.8%) and transaminitis (17.2%, ≥Grade 2 in 25%). There were 21 deaths (2.0%) of which 16 were cancer-related and one attributed to treatment-related renal failure. Conclusion This large UK real-world cohort confirms the feasibility of AA use in routine practice, though dose modifications and early discontinuations were more frequent than reported in clinical trials. Toxicities remain consistent with known safety profiles, but adherence may be impacted by age, comorbidities, and supportive care access. Optimising management of side effects is critical to maintaining long-term adherence and maximising clinical benefit in real-world settings. Citation Format: B. Baraka, A. Ghose, H. Abdallah, A. Maniam, Y. Owoseni, A. Konstantis, S. Horne, A. Nazir, R. Kussaibati, L. Mooney, F. Nazeer, N. Atsumi, E. Daniels, L. McAvan, M. Abraham, D. Morgan, G. Langford, E. Bean, E. Morris, R. Muhammad, S. Khan, R. Pravinkumar, S. Mohammed, S. A. Raza, C. Blair, R. Khan, M. Tsalic, R. Douglas, K. Panagiotis, S. Waters, J. Smith, E. Papadimitraki, C. Michie, C. Wilson, O. Ayodele. The CONCISE Study : A Pan-UK Real-World Audit of Adjuvant Abemaciclib Use in High-Risk Early Breast Cancer 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 PS3-07-12.