Thromboembolic Events in Patients with HER2-Negative, Hormone Receptor-Positive, Metastatic Breast Cancer Treated with Ribociclib Combined with Letrozole or Fulvestrant: A Real-World Data

PURPOSE: Cyclin dependent kinase (CDK) 4/6 inhibitors (palbociclib, ribociclib and abemaciclib) modulate endocrine resistance and are integral treatment for patients with advanced hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancer. Since their approval, CDK4/6 inhibitors are widely used in clinical practice. Thromboembolic events (TEE) were not a major issue in patients treated on clinical trials utilizing these agents. However, conflicting data started to emerge describing higher than expected rates of both arterial and venous thrombosis in patients treated with CDK4/6 inhibitors. In this study, we report our experience on TEE in patients treated with one of these agents (ribociclib) in real-world settings. PATIENTS AND METHODS: All consecutive patients with metastatic breast cancer (mBC) treated with ribociclib combined with letrozole or fulvestrant were retrospectively reviewed. All episodes of radiologically confirmed arterial or venous thrombosis were recorded. TEE was considered ribociclib-related if diagnosed while patients are on the drug, or within 4 weeks after the last dose. RESULTS: A total of 305 patients, median age (range), 49 (22–87) years were enrolled. All patients had metastatic disease, and most (n=241, 79.0%) were with visceral metastasis. Ribociclib was used for a median duration of 7 months (range: 1–45) and was used beyond the first-line setting in 110 (35.9%) patients. TEE were confirmed on 6 (1.97%) patients; 3 were pulmonary embolism, 2 cerebral venous sinus thrombosis (CVST), and one case of limb ischemia and all were symptomatic. Similar rates of TEE were noted prior to initiation, and after stopping ribociclib. CONCLUSION: In real-world settings, breast cancer patients treated with ribociclib, combined with aromatase inhibitors or fulvestrant, may not be at higher risk for thromboembolic events. However, unusual sites of thrombosis, like CVST, may raise some concerns.