Non-adherence of cyclin-dependent kinases 4 and 6 inhibitors reduces overall and progression-free survival in patients with hormone receptor-positive breast cancer.

Purpose: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) combined with endocrine therapy improve survival in patients with hormone receptor-positive, HER2-negative (HR + /HER2-) breast cancer (BC). This retrospective study aimed to evaluate CDK4/6i adherence, factors associated with non-adherence, the impact of health-system specialty pharmacy on adherence, and effects of non-adherence on survival outcomes in HR + /HER2- BC patients.

Methods: Data was collected from Houston Methodist Hospital System (HM) from HR + /HER2- BC patients with medication fill history utilizing electronic medical records. CDK4/6i adherence was calculated using the mean possession ratio ≥ 80%. Multivariable logistic regression model and Kaplan-Meier analysis were utilized to evaluate factors associated with non-adherence and its impact on survival, respectively.

Results: A total of 121 patients were assessed and analyzed; 55% patients received abemaciclib, 40% were on palbociclib, and 4% were on ribociclib. More patients were on aromatase inhibitors and tamoxifen (79%) than fulvestrant (21%). Most of the patients were Caucasian (64%), non-Hispanic or Latino (84%), and postmenopausal (66%). Overall, 52 patients (43%) were non-adherent. Patients ≥ 65 years of age (OR: 0.304, [95% CI: 0.110-0.840], P-value: 0.022) and those of Hispanic or Latino ethnicity (OR: 0.291, [95% CI: 0.086-0.985], P-value: 0.047) were more likely to be non-adherent to CDK4/6i. Non-adherence to CDK4/6i was associated with worse overall survival and progression-free survival.

Conclusion: 43% patients were non-adherent to CDK4/6i. Older patients and those of Hispanic ethnicity were more likely to be non-adherent. Non-adherent patients had worse survival outcomes, highlighting the unmet need to implement interventions to improve CDK4/6i adherence in these patients.