Eligibility for Adjuvant Cyclin-Dependent Kinase 4/6 Inhibitors in Endocrine Receptor-Positive and HER2-Negative Early Breast Cancer by Age and Type of Surgery

<b>Background:</b> Despite early diagnosis, approximately 20% of patients with ER-positive and HER2-negative breast cancer (BC) will experience disease recurrence. Improved survival has been reported with adjuvant treatment combining cyclin-dependent kinase 4/6 inhibitors with endocrine therapy, in high-risk patients with ER-positive and HER2-negative BC, regardless of age. Older patients have higher rates of ER-positive/HER2-negative BC than younger patients. <b>Methods:</b> In this real-world data analysis, MonarchE and NataLEE high-risk patients accounted for 9.5% and 33% of patients undergoing upfront surgery, respectively. Significantly higher eligibility rates were observed in patients who underwent a mastectomy, >70 years and ≤40 years for adjuvant abemaciclib and ribociclib, and in patients >80 years for ribociclib. <b>Results:</b> Eligibility rates in patients ≤40 years and >80 years who underwent mastectomy were 27.8% and 24.7% for abemaciclib, respectively, and 56.6% and 65.2% for ribociclib, respectively. A higher discontinuation rate for abemaciclib was reported in patients aged ≥65 years and it can be assumed that discontinuation rates may increase in even older patients. <b>Conclusions:</b> If the results of the NataLEE trial translate into clinical practice, the number of patients potentially eligible for adjuvant CDK4/6 inhibitors may increase, especially in the elderly population.