Adherence to Aromatase Inhibitors ± Abemaciclib Treatment in Patients With Early-stage HER2-negative Breast Cancer

Around 90% of breast cancer patients are diagnosed at an early stage and approximately 70% are hormone receptor-positive and HER2-negative (HR+/HER2-). Despite advancements in adjuvant endocrine therapy, 20-30% of early-stage breast cancer patients relapse within the first decade post-surgery. A recent clinically meaningful therapeutic option for these patients has been cyclin-dependent kinases 4/6 inhibitors (CDK4/6 inhibitors). Abemaciclib and ribociclib were assessed in the adjuvant setting, both showing improvement in invasive disease-free survival (IDFS). Abemaciclib has been approved by the FDA and EMA for HR+/HER2- early breast cancer at high risk of disease recurrence and is the first addition to the Slovenian treatment regimen in routine clinical practice. Poor medication adherence can directly affect the effectiveness of treatment for early HR+/HER2- breast cancer. While adherence data in patients treated with aromatase inhibitors are available, the adherence rate in patients with early HR+/HER2- breast cancer taking abemaciclib remains unclear. In this study, investigators hypothesize that patients receiving abemaciclib in combination with aromatase inhibitors will have lower medication adherence and higher discontinuation rates compared to those receiving aromatase inhibitors alone. It is expected that patients with better quality of life, better cognitive functioning, and a more positive attitude toward their therapy will demonstrate higher medication adherence rates. Adherence may also be influenced by additional factors, such as age and prior treatments.