Abstract PS1-01-23: Dose Adjustments and Adherence to Adjuvant Abemaciclib in a Brazilian Real-World Scenario

Abstract Background: The use of abemaciclib in combination with endocrine therapy (ET) is an established adjuvant option for patients with hormone receptor-positive (HR+), HER2-negative early-stage breast cancer at high risk of recurrence. However, the prolonged treatment duration (up to two years) in the curative setting presents unique challenges related to tolerability and adherence, particularly given the known toxicity profile of CDK4/6 inhibitors. Diarrhea, fatigue, and hematologic adverse events are common and may lead to dose interruptions or permanent discontinuation. Real-world data are essential to better understand patient tolerance, frequency of dose adjustments, and their impact on treatment persistence and outcomes. We aimed to evaluate adjuvant abemaciclib use in a Brazilian real-world setting, focusing on adherence and the need for dose modifications. Methods: We retrospectively reviewed records of patients with high-risk, early-stage HR+ breast cancer who received adjuvant abemaciclib between 2019 and 2025. The endpoints evaluated included: initial dose, treatment interruptions, dose reductions, final maintenance dose, permanent drug discontinuation, and reasons for discontinuation. Results: A total of 189 patients were included, with a median age of 47 years (range 24-79); 58.7% were premenopausal and 37.0% postmenopausal. Most tumors were ductal (70.4%) or lobular (15.9%) in histology, stage II (45.0%) or III (38.1%), grade 2 (51.8%) or grade 3 (33.3%), and had a Ki-67 index ≥ 20% (77.2%). Nearly half of the patients (52.9%) had received neoadjuvant chemotherapy. For adjuvant ET, most patients (86.8%) initiated treatment with an aromatase inhibitor, and 53.9% received ovarian suppression.The majority (93.3%) initiated abemaciclib at the label dose of 300 mg daily (150 mg twice daily). At the time of analysis, 35 patients (18.5%) had completed the planned two years of treatment. In the overall cohort, dose interruptions occurred in 27.6% of cases and dose reductions in 43.1%. Maintenance dosing after adjustment was: 300 mg daily in 58.2%, 200-250 mg in 26.8%, 150 mg in 3.9%, and 150 mg in 11.1%. Permanent discontinuation due to adverse events was required in only 6.3% of patients, and all but one had undergone prior dose reductions. Grade 3 or higher diarrhea and neutropenia were registered in 6.3% and 7.4% of the cases, respectively. Among 152 patients with available adherence data, 91.5% reported good adherence (defined as taking ≥70% of prescribed doses). Conclusions: Our findings align with other real-world studies showing frequent need for dose adjustments during adjuvant abemaciclib therapy. Nevertheless, permanent discontinuation due to toxicity was infrequent, suggesting that proactive management through dose reductions can effectively support treatment persistence. Given that reduced doses do not appear to compromise efficacy, close monitoring of adverse events and timely dose adjustments are essential to maintain adherence and maximize benefit in the adjuvant setting. Citation Format: R. Colombo Bonadio, L. Holland, C. A. Cavalcanti, J. Bessa, K. Cayres, P. do Amor Divino, R. Naves, J. Bines, L. Testa. Dose Adjustments and Adherence to Adjuvant Abemaciclib in a Brazilian Real-World Scenario 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 PS1-01-23.