Abstract PS3-08-21: Real-world survival outcomes in patients with HR+/HER2− early breast cancer meeting Monarche and Natalee criteria: analysis from a Brazilian cohort

Abstract Background: While adjuvant CDK4/6 inhibitors improve outcomes in selected patients with high-risk HR+/HER2− early breast cancer (EBC), real-world evidence from Latin America remains limited. We aimed to evaluate clinical characteristics and survival outcomes of patients treated in a private oncology facility in Brazil, stratified by eligibility for MonarchE and NATALEE criteria. Methods: We analyzed HR+/HER2− stage I-III EBC patients treated from 2012 to 2024. Patients who received adjuvant abemaciclib were excluded from the analysis. Eligibility followed clinical-pathological criteria from MonarchE and NATALEE (excluding Ki-67). Kaplan-Meier and Cox models were used to estimate overall survival (OS) and disease-free survival (DFS) across groups: MonarchE and NATALEE-eligible, NATALEE-low-risk (eligible for NATALEE but not MonarchE), and non-eligible. Results: Among 2,609 patients, 18% met MonarchE criteria, 56% NATALEE criteria, and 44% were non-eligible. The NATALEE-low risk subgroup accounted for 38% of the cohort. Most patients had stage I or II disease (84%) and breast-conserving surgery (64%). 67% of the population were post-menopausal, with a mean age of 59 years old. 24% of patients are black or mixed race. Patients in the MonarchE- and NATALEE-eligible groups were more frequently premenopausal (42% and 39%, respectively, vs. 25% in the ineligible group), more likely to have received chemotherapy (89% and 77%, respectively, vs. 21%), and more often underwent ovarian function suppression (8.3% and 7.5% vs. 1.5%). After a median follow-up of 37 months, the multivariable-adjusted hazard ratios (HR) for recurrence were 2.31 (95%CI 1.67-3.18; p < 0.001) for MonarchE and 2.23 (1.49-3.34; p < 0.001) for NATALEE. At 36 months, DFS rates were 86.0% in MonarchE-eligible patients, 90.2% in NATALEE-eligible patients, 92.0% in NATALEE-low-risk patients, and 96.0% in non-eligible patients; at 60 months, the estimated DFS was 80.0%, 85.1%, 87.5%, and 93.0%, respectively. OS was significantly worse in MonarchE- and NATALEE-eligible patients compared to non-eligible patients, with an HR of 4.16 (95% CI, 2.54-6.82; p < 0.001). In the NATALEE-low-risk subgroup, OS was also inferior compared to the non-eligible group, with an HR of 2.36 (95% CI, 1.47-3.80; p < 0.001). At 36 months, OS rates were 89.0% (MonarchE-eligible), 93.6% (NATALEE-eligible), 95.0% (NATALEE low-risk-only), and 97.0% (non-eligible); 60-month estimates were 84.5%, 89.2%, 91.3%, and 95.4%, respectively. Independent predictors of poorer OS included ≥3 positive lymph nodes, T3/T4 tumors, use of chemotherapy, and absence of endocrine therapy. Conclusions: A substantial proportion of HR+/HER2− EBC patients treated in Brazilian private practice met eligibility for adjuvant CDK4/6 inhibitors, especially under the broader NATALEE criteria. These patients experienced significantly worse DFS and OS compared to non-eligible patients. These findings support expanding access to adjuvant CDK4/6 inhibitors and underscore the need to address treatment disparities in Latin America. Citation Format: N. C. Nunes, M. Monteiro, G. Carvalho, J. Pecoraro, T. Ferreira, D. Azevedo, P. Santos, R. Barroso-Sousa. Real-world survival outcomes in patients with HR+/HER2− early breast cancer meeting Monarche and Natalee criteria: analysis from a Brazilian cohort [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 PS3-08-21.