Beyond Standard Endocrine Therapy: A New Adjuvant Treatment in High-Risk Early Breast Cancer

The standard adjuvant treatment of estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative early breast cancer (EBC) is endocrine therapy (ET). Despite this treatment, 20% of patients will have their disease recur. Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors with ET have shown overall survival (OS) benefit in ER-positive, HER2-negative breast cancer in the metastatic setting. Clinical trials are studying the role of oral CDK4/6 inhibitors in the adjuvant treatment of ER-positive, HER2-negative EBC patients who are clinically and pathologically at high risk for recurrence while on standard ET. The monarchE phase III, randomized, controlled trial, looked at one arm of high-risk ER-positive, HER2-negative EBC patients receiving standard ET and the second arm receiving standard ET with a CDK4/6 inhibitor, abemaciclib. Primary endpoint data showed improvement in invasive disease-free survival of 92.2% in the ET and abemaciclib arm vs. 88.7% in the ET arm at 2 years. At 5 years, a preplanned interim analysis showed continued absolute improvement in invasive disease-free survival. Secondary endpoint data for OS have not yet matured. Abemaciclib is approved for use with ET in patients with high-risk, ER-positive, HER2-negative EBC. This article aims to review a case study and the rationale for using oral CDK4/6 inhibitors as adjuvant treatment for this high-risk subset of patients.