Abstract PS3-09-13: Adjuvant Abemaciclib in Male Breast Cancer: A UK Multicentre Case Series from the CONCISE Study

Abstract Background Male breast cancer (MBC) is rare, comprising approximately 1% of all breast cancer diagnoses. Due to its low incidence, treatment strategies are largely extrapolated from studies in female patients, and prospective data in men are limited. The monarchE trial, which established the benefit of Adjuvant Abemaciclib (AA) in high-risk hormone receptor-positive (HR+), HER2-negative early breast cancer (EBC), did not provide sex-specific outcomes. To our knowledge, this is the first report of real-world data on the use of AA in male breast cancer patients. Methods This case series is a subset of the CONCISE study, a retrospective, multicentre UK audit evaluating AA use across 21 centres between July 2022 and April 2025. Male patients with HR+/HER2- EBC who received AA were included. Data were collected using the Ledidi platform and analysed descriptively using Jamovi. Variables assessed included disease characteristics, systemic therapies received, abemaciclib duration, dose modifications, and adverse events (AEs). Results Of 1,026 patients in the CONCISE dataset, 13 (1.3%) were male. Median age at diagnosis was 72 years (range 28-81), and most patients (69%) were White. Staging at diagnosis included: Stage IB (n = 6), IIA (n = 1), IIB (n = 2), IIIA (n = 2), and IIIB (n = 1). All tumours were ER-positive, HER2-negative (IHC 0-2+). Tumour grade distribution: G1 (n = 1), G2 (n = 6), and G3 (n = 5).Neoadjuvant chemotherapy was administered in 2 patients; neoadjuvant endocrine therapy (tamoxifen) in 1. Breast-conserving surgery was performed in 1 case; the remainder underwent mastectomy. Adjuvant chemotherapy was delivered in 8/13 patients (62%), with 75% completing ≥6 cycles. Adjuvant ET included tamoxifen (n = 8), aromatase inhibitors with ovarian function suppression (OFS) (n = 5).Median AA duration was 12.3 months (range 1.9-24). At the time of analysis, 4 patients (31%) remained on therapy. Among those who discontinued, 5 (56%) stopped prematurely due to toxicity, 1 due to disease progression, and 3 completed the full 24-month course. At least one dose reduction occurred in 6 patients (46%), with a second reduction in 1.AEs included diarrhoea in 7 patients (54%, ≥G2 in 1), anaemia in 4 (31%, ≥G2 in 2), neutropenia in 6 (46%, ≥G2 in 5; including G3 in 3), thrombocytopenia in 3 (23%, ≥G2 in 2), and transaminitis in 2 (15%, G1 only). No treatment-related deaths were reported. Conclusion This is the first UK real-world cohort reporting on AA use in male breast cancer, an underrepresented population. Compared to the monarchE AA+ET cohort, male patients were older (median age 72 vs. 51 years), had slightly shorter treatment duration (12.3 vs. 14 months), and similar rates of dose reduction (46% vs. 41.2%). However, discontinuation due to toxicity was higher in this cohort (56% vs. 16.6%), highlighting the need of larger, prospective cohorts for tailored toxicity monitoring and management strategies in men receiving AA. While these findings should be generalised with caution due to limited sample size, treatment overall appears manageable and associated with toxicity profiles seen in other populations. Citation Format: A. Ghose, H. Abdallah, Y. Owoseni, A. Maniam, B. Baraka, S. Horne, A. Nazir, L. Mooney, F. Nazeer, N. Atsumi, L. McAvan, M. Abraham, D. Morgan, G. Langford, E. Bean, 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, O. Ayodele. Adjuvant Abemaciclib in Male Breast Cancer: A UK Multicentre Case Series 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 PS3-09-13.