Abstract PS2-02-22: Impact of Body Mass Index on the Efficacy and Safety of Adjuvant Abemaciclib in a Real-World Brazilian Cohort of Patients With High-Risk Early Breast Cancer

Abstract Background: Obesity is an increasing concern in breast cancer, influencing both prognosis and treatment tolerance. While abemaciclib has demonstrated efficacy in high-risk HR (hormone receptor) +/HER2- patients, real-world evidence regarding its safety and efficacy across BMI (body mass index) categories remains limited. Methods: We conducted a retrospective and prospective, observational study involving 146 patients who received adjuvant abemaciclib combined with endocrine therapy between January 2021 to June 2025 at a comprehensive cancer center. Patients were stratified by BMI (kg/m2) as obese (≥30), overweight (25 ≤ BMI 30), and non-overweight (25). We assessed treatment-related toxicities, dose reductions, pathological response to neoadjuvant chemotherapy (RCB), recurrence, and disease-free survival. Statistical analyses included chi-square tests, Spearman correlation, Kaplan-Meier survival curves, and Cox regression. Results: Obese patients experienced higher rates of dose reduction (72%) compared to non-obese (47%) and overweight patients (29%), although the difference did not reach statistical significance (p = 0.075). Regarding neoadjuvant response, non-overweight patients achieved more RCB-0 (complete response), while 30.8% of obese patients had RCB-III (high residual disease). Although no significant association was found between BMI and RCB (p = 0.135), a clinical trend was observed. Larger tumors (5 cm) were more frequent in obese patients (23.1%) compared to non-obese (14.3%). Anemia showed a positive correlation with BMI (Spearman p = 0.043), with a higher frequency in obese patients (27.3%) versus non-overweight (15.2%). No statistically significant associations were observed between BMI and other toxicities (diarrhea, neutropenia, transaminase elevation, arthralgia). Four recurrences were documented: three in obese and one in a non-overweight patient. Kaplan-Meier curves suggested a trend toward longer disease-free survival among obese patients (median: 641 days) compared to non-overweight patients (237 days). However, this finding did not reach statistical significance (Cox regression HR = 2.6; p = 0.235). ANOVA for time to recurrence showed a significant difference between BMI groups (p = 0.003), favoring delayed recurrence in obese patients. Conclusion: In this real-world Brazilian cohort, obesity was associated with higher rates of dose reduction due to toxicity, lower pathological response to neoadjuvant chemotherapy, and larger tumors. Paradoxically, obese patients demonstrated longer disease-free survival, a finding consistent with the "obesity paradox" described in the literature. These results highlight the need for further research into the complex role of body composition in HR+/HER2- breast cancer treated with CDK4/6 inhibitors. Citation Format: C. Pereira dos Santos, M. Celeleste Tavares, C. Campanholo Marques, S. Moraes Sanches. Impact of Body Mass Index on the Efficacy and Safety of Adjuvant Abemaciclib in a Real-World Brazilian Cohort of Patients With 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 PS2-02-22.