in updated results of a trial.
This was based on data collected over a median follow-up of 54 months. Previously reported data from this phase III study, known as monarchE, showed the same outcomes but over a 2-year treatment period, the researchers said.
Risk of cancer recurrence may be as much as 30% at 5 years in these high-risk patients, who will likely need more intense treatment, wrote Priya Rastogi, MD, of the University of Pittsburgh Medical Center, and colleagues.
In the new study published in the Journal of Clinical Oncology (2023 Jan 9. doi: 10.1200/JCO.23.019), the researchers reported 5-year efficacy results from an interim analysis of overall survival in the monarchE trial.
The intent-to-treat population included 2808 individuals randomized to abemaciclib plus ET and 2814 to ET alone; the median age was 51 years, and approximately 70% of the participants were White.
The addition of abemaciclib significantly reduced the risk of IDFS and DRFS over a median follow-up period of 54 months with hazard ratios of 0.680 and 0.675, respectively. Adjuvant abemaciclib also significantly improved DRFS over ET alone (HR 0.675).
The findings were limited by the lack of statistical significance for overall survival with abemaciclib. However, the increased benefits for IDFS and DRFS with abemaciclib plus ET vs. ET alone were consistent across all subgroups, and the benefit of abemaciclib was consistent regardless of the number of nodes involved, the researchers wrote.
“Prior reports from this trial with shorter follow-up demonstrated benefit of abemaciclib. However, with longer follow-up of a median 54 months, we see that the benefit of the drug is not only sustained (32% reduction in the risk of a disease event), but that there is further separation of the curves with an absolute difference in IDFS and DRFS rates of 7.6% and 6.7, comparing the ET alone vs. ET plus abemaciclib arms,” study coauthor Matthew P. Goetz, MD, said in an interview.
Although statistical significance was not reached for overall survival, fewer deaths occurred in the abemaciclib-plus-ET group compared with the ET-only group, said Dr. Goetz, of the Mayo Clinic, Rochester, Minnesota. However, patients with the worst prognosis (Ki-67–high subgroup) tended to have higher overall survival.
A total of 208 deaths occurred in the combination group vs. 234 in the ET-only group, and no new safety signals were observed. The occurrence of serious adverse events of any cause was similar in the abemaciclib group and the ET-only group (6.5% vs. 7.3%).
“These data are a pleasant surprise, as there were concerns that the benefit of the drug seen with shorter follow-up would wane over time,” Dr. Goetz said. “However, the opposite has occurred; with increasing length of follow-up, the curves continue to separate.”
Based on the new results, “we have high confidence that for patients with ER+/HER2- breast cancer at high risk of recurrence, the addition of 2 years of adjuvant abemaciclib to ET results in clinically significant improvements in IDFS,” he said.
Looking ahead, “we need additional follow-up to determine whether the benefit we now see in terms of IDFS will eventually translate into improvements in overall survival,” Dr. Goetz said. “We need to identify biomarkers that can identify patients at risk for early recurrence despite administration of adjuvant abemaciclib and further, biomarkers that will allow us to select patients that can be safely treated with ET alone.”