Venous and Arterial Thrombosis Following Abemaciclib Therapy for Metastatic Breast Cancer

BackgroundThe CDK4/6 inhibitor abemaciclib is a mainstay of treatment for hormone receptor–positive breast cancer. However, increased venous thromboembolism (VTE) rates in multiple clinical trials resulted in a black‐box warning for this agent. Thrombosis rates in unselected real‐world populations receiving abemaciclib remain ill defined.MethodsA multicenter observational cohort study was conducted of patients with metastatic breast cancer receiving abemaciclib. The primary end point was thrombosis during treatment or within 30 days of discontinuation. Multivariable logistic models assessed predictors of VTE, and a multivariable Cox proportional hazards model assessed mortality.ResultsA total of 364 patients were included, with a median treatment duration of 5.5 months. Twenty‐six patients developed 27 (7.4%) thrombotic events (17 VTE, nine arterial thrombosis, and one with both events). No baseline characteristics were associated with increased VTE risk in multivariable modeling. Patients developing VTE during therapy had a higher risk of death than those who did not (hazard ratio, 2.09; 95% CI, 1.07–4.13). Median survival in patients who developed VTE compared with those who did not was 9.6 vs 25.8 months, respectively. The rate of VTE and any thrombosis during abemaciclib therapy was 9.1 and 13.7 events per 100 person‐years, respectively, which is notably higher than rates observed in clinical trials.ConclusionsIn a real‐world setting, abemaciclib was associated with a VTE rate approximately two‐fold greater than the already elevated rates reported in the MONARCH trials. Patients developing thrombosis on abemaciclib had a significantly higher risk of death. Given these findings, studies evaluating the role of thromboprophylaxis in patients receiving abemaciclib are needed.