The phase 3 NATALEE and monarchE trials demonstrated the efficacy of ribociclib and abemaciclib, respectively, in reducing recurrence risk in patients with hormone receptor (HR)-positive/HER2-negative early breast cancer (EBC) at high risk of recurrence. This article reports real-world incidence, characteristics, treatments, and outcomes of NATALEE- and monarchE-eligible HR-positive/HER2-negative EBC patients using hospital records from the Helsinki and Uusimaa Hospital District in Finland.

This retrospective analysis included adult patients with HR-positive/HER2-negative EBC diagnosed between 2014 and 2023 who met NATALEE or monarchE risk criteria. Patients were stratified by trial eligibility criteria and characterized at diagnosis. Patients were followed from initial diagnosis until June 2024 or death. Outcomes included invasive disease-free survival (IDFS), distant disease-free survival (DDFS), and overall survival (OS).

The study included 3596 NATALEE-eligible and 1336 monarchE-eligible patients, with a median age of 64 years in both groups. Among NATALEE-eligible patients, 69.6% were postmenopausal compared with 64.0% in the monarchE-eligible group. A higher proportion of monarchE-eligible patients had Ki-67 scores ≥20% (79.6% vs 64.5%) and grade 3 tumors (68.3% vs 34.7%), reflecting the stricter eligibility criteria of monarchE. 79.1% of NATALEE-eligible patients were diagnosed with stage II disease, whereas stage III disease was most common in monarchE patients (55.8%). Among NATALEE-eligible patients, 17.1% were N0, 66.2% were N1, and 16.7% were N2-3, whereas among monarchE-eligible patients, 54.8% were N1 and 45.2% were N2-3. Among NATALEE- and monarchE-eligible patients, 97.3% and 97.8% received endocrine therapy, and 45.4% and 56.0% received chemotherapy, respectively. At 5 years, NATALEE-eligible patients had IDFS, DDFS, and OS rates of 82.5% (95% confidence interval [CI], 81.0-83.8), 88.3% (95% CI, 87.1-89.5), and 90.9% (95% CI, 89.8-91.9), respectively. Patients eligible for monarchE had corresponding rates of 76.8% (95% CI, 74.2-79.1), 82.3% (95% CI, 79.9-84.4), and 86.8% (95% CI, 84.6-88.6). At 10 years, NATALEE-eligible patients had IDFS, DDFS, and OS rates of 66.4% (95% CI, 63.7-69.0), 74.0% (95% CI, 71.2-76.6), and 76.3% (95% CI, 73.5-78.8), respectively. Patients eligible for monarchE had corresponding rates of 59.8% (95% CI, 55.7-63.6), 66.9% (95% CI, 62.9-70.6), and 70.7% (95% CI, 66.5-74.5). Median follow-up durations were 5.4 years for NATALEE-eligible patients and 5.9 years for monarchE-eligible patients.

Despite receiving standard-of-care therapies, patients meeting NATALEE and monarchE risk criteria in Finland demonstrated substantial risk of recurrence, highlighting the unmet need for more effective treatment strategies. These findings reinforce the importance of optimizing adjuvant therapy to improve long-term outcomes in high-risk, HR-positive/HER2-negative EBC populations.

Source: Voutilainen S, Tuominen S, Hernesniemi S, et al. Real-world characteristics, treatments and outcomes of NATALEE and monarchE-eligible HR+/HER2- early breast cancer patients in the hospital district of Helsinki and Uusimaa (HUS), Finland. Presented at: ESMO Congress 2025. October 20, 2025; Berlin, Germany. Abstract 360P.