91 Real-World Survival Outcomes by Risk Features in Patients With HR+/HER2− Early Breast Cancer in the United States

Background

Patients with node-positive (N+) high-risk clinicopathologic features in the monarchE trial (N+ high-risk: N1 with grade 3 or tumor ≥ 5 cm; N2; N3) have a real-world 5-year recurrence risk of 30% on endocrine therapy (ET). In monarchE, 2-year abemaciclib plus ET improved invasive disease-free survival (iDFS; HR, 0.68, 5-year rates: 84% abemaciclib plus ET vs 76% ET; 5-year absolute benefit: 8%). This study describes real-world overall survival (OS)/iDFS in patients with early breast cancer with monarchE high-risk features and in patients with early triple-negative breast cancer (TNBC) for context.

Methods

This retrospective study used the US nationwide Flatiron Health electronic health records-derived deidentified database from January 2011 to June 2024. Eligible patients with surgically resected hormone receptor–positive (HR+)/HER2-negative (HER2−) early breast cancer received ET, but no targeted adjuvant agents. Patient groups were N+ high-risk vs non–high-risk (N0; N1 < grade 3, tumor < 5 cm, and Ki-67 < 20%; NX). Nodal subgroups included N1 high-risk (N1 with grade 3 or tumor ≥ 5 cm) and N1 non–high-risk (N1 with < grade 3, tumor < 5 cm, and Ki-67 < 20%). OS/iDFS for HR+/HER2− early breast cancer and early TNBC were estimated from adjuvant ET and chemotherapy initiation, respectively. Comparisons of high-risk vs non–high-risk were estimated with Cox regression models.

Results

For 6350 eligible patients with HR+/HER2− early breast cancer and for 515 patients with early TNBC, median follow-up was 56 months (IQR: 28, 84) and 48 months (22, 79), respectively. Risk of death was greater in N+ high-risk vs non–high-risk (adjusted HR, 2.3; 95% CI, 2.0-2.8) and in N1 high-risk vs N1 non–high-risk (HR, 1.6; 95% CI, 1.1-2.3). OS/iDFS 5-year rates were similar for N+ high-risk (81.6%/70.9%) and early TNBC (79.2%/74.3%). For nodal subgroups, 5-year OS/iDFS were 86.5%/75.7% (N1 high-risk group), 78.9%/68.3% (N2 group), and 70.6%/59.6% (N3 group), respectively. Non–high-risk groups exhibited higher survival rates, with an overall 5-year OS of 93.9% and 5-year iDFS of 91.2%, and the N1 non–high-risk subgroup showing 92.8% and 90.0%, respectively.

Conclusion

Real-world patients with N+ high-risk monarchE features receiving ET have an increased risk of death vs patients with lower risk features, and a 5-year mortality risk similar to early TNBC. To mitigate the high risk of incurable metastatic recurrence, these patients should be considered for 2-years of adjuvant abemaciclib plus ET.

Previously presented at ESMO Breast Cancer 2025.

Study is sponsored by Eli Lilly and Company.