Tan, Na

Abstract Background Endocrine therapy has been the mainstay treatment for metastatic hormone receptor-positive / HER2-negative (HR+/HER2-) breast cancer. Given that cell cycle dysregulation and consequent uncontrolled cellular proliferation is a key mechanism behind cancer growth, agents targeting this pathway have been of major therapeutic interest.[1] The cyclin-dependent kinase 4/6 (CDK 4/6) inhibitors palbociclib, ribociclib, and abemaciclib have shown a synergistic effect in combination with endocrine therapy.[2-3] Despite favorable results, post-approval studies have revealed potential concerns for cardiotoxicity. [4] Given this, we sought to characterize the incidence and outcomes of atrial arrhythmia among metastatic HR+/HER2- breast cancer patients who were treated with CDK 4/6 inhibitors at our institution. Purpose To investigate the incidence and risk factors of atrial arrhythmia among hormone receptor-positive/HER2-negative breast cancer patients treated with CDK4/6 inhibitors at our institution. Methods A retrospective cohort study was conducted among 2754 patients ≥18 years of age who received treatment with CDK4/6 inhibitors (palbociclib, ribociclib, or abemaciclib) for an established diagnosis of hormone receptor-positive, HER-2 negative breast cancer between 2015-2024. Patients with new onset atrial fibrillation and/or flutter were identified through chart review. The cumulative incidence of new-onset atrial arrhythmia was estimated using the Kaplan-Meier method. Univariable and multivariable Cox regression analyses were performed to identify predictors of atrial arrhythmia. Results A total of 2754 patients was included in the study; 97.3% were female and mean age was 61 years (13). Among these, 101 (3.6%) had a prior history of atrial arrhythmia. The 1-year and 5-year cumulative incidence of new-onset AA were 1.1% and 2.8% respectively (Figure 1). When stratified by specific CDK4/6 inhibitor agents, there was no statistically significant difference. On univariable analysis, age at treatment, hypertension, coronary artery disease, and peripheral vascular disease were associated with new-onset atrial arrhythmia. However, following multivariable adjustment, only age at treatment was statistically significant. Conclusions New-onset atrial arrhythmias may occur during or after CDK4/6 inhibitor use. Older patients appeared to have a higher risk. There were no clear differences in atrial arrhythmia risk between the three CDK4/6 inhibitors.