Overall survival with abemaciclib in early breast cancer☆

Background: Adjuvant abemaciclib combined with endocrine therapy (ET) significantly improved invasive disease-free survival (IDFS) in patients with hormone receptor (HR)-positive, human epidermal growth factor 2 (HER2)-negative, node-positive, high-risk early breast cancer (EBC). The impact on overall survival (OS) remained unknown. Patients and methods: In the phase III monarchE trial (NCT03155997), patients received ET for at least 5 years with or without abemaciclib for 2 years. In this article, we report the primary OS results, a key secondary endpoint, and updated estimates of IDFS and distant relapse-free survival (DRFS). Results: Overall, 5637 patients underwent randomization, with 2808 assigned to abemaciclib–ET, and 2829 to ET. In the intent-to-treat population, with a median follow-up of 76.2 months, abemaciclib–ET resulted in a 15.8% lower risk of death than ET [661 deaths; hazard ratio 0.842, 95% confidence interval (CI) 0.722-0.981, P = 0.027], meeting the prespecified boundary for significance. The 7-year OS was 86.8% with abemaciclib–ET and 85.0% with ET (absolute difference, 1.8%). OS benefit was consistent across prespecified subgroups. In addition to patients who had already died of metastatic disease, fewer patients in the abemaciclib–ET arm were living with metastatic disease compared with the ET arm (6.4% versus 9.4%). Sustained improvement was demonstrated in IDFS and DRFS (hazard ratio 0.734, 95% CI 0.657-0.820 and hazard ratio 0.746, 95% CI 0.662-0.840, respectively). Seven-year IDFS was 77.4% with abemaciclib–ET and 70.9% with ET (absolute difference, 6.5%) and 7-year DRFS were 80.0% and 74.9% (absolute difference, 5.1%). The long-term safety data compiled did not support any concerns of delayed toxicities. Conclusions: Adjuvant abemaciclib–ET resulted in a statistically significant and clinically meaningful improvement in OS compared with ET in patients with HR-positive, HER2-negative, node-positive, high-risk EBC. At 7 years, abemaciclib–ET continued to demonstrate a sustained IDFS and DRFS benefit.