334PClinical outcome and toxicity data in patients with advanced breast cancer treated with cyclin-dependent kinase 4/6 (CDK4/6) inhibitors combined with endocrine therapy in a real-world clinical setting

Abstract Background Clinical trials evaluating treatment with cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) in combination with aromatase inhibitors (AI) or fulvestrant have shown promising clinical benefit in patients with advanced breast cancer. We evaluated real-world clinical outcomes and toxicity rates in patients with advanced breast cancer who received treatment with CDK4/6i in routine clinical practice. Methods We retrospectively reviewed medical records of patients with advanced hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancer treated at Departments of Oncology-affiliated with the Hellenic Cooperative Oncology Group (HeCOG). All patients received hormonal therapy in combination with CDK4/6i as first-, second- or third-line of treatment and beyond. The primary end point was progression-free survival (PFS). Results From 07/2015 to 03/2019, 191 women were treated with CDK4/6i combined with endocrine therapy (median age: 52.2 years, postmenopausal 87, 45.5%); 86 (45%) patients received CDK4/6i in combination with AI and 105 (55%) with fulvestrant. CDK4/6i were administered as first-line treatment in 65 (34%) patients, second-line treatment in 39 (20.4%) and third-line and beyond in 83 (43.5%) patients. The median follow-up was 54.3 months (95%CI 43.0-61.2). The median PFS for patients who received first-line treatment was 18.7 months, second-line treatment 11.5 months and ≥3 lines of treatment 7.5 months. The median overall survival since the initiation of CDK4-6i treatment was 23.2 months (95% CI 20.4-30.3). The most common adverse events were neutropenia (100 patients, 52.4%) and fatigue (24, 12.6%). Grade 3-4 adverse events occurred in 55 (28.8%) patients, while 2 (1.0%) patients permanently discontinued treatment due to toxicity. Conclusions Among patients with HR-positive advanced breast cancer treated with CDK4/6i combined with endocrine, the estimated PFS was comparable to previously reported data. Treatment was well tolerated, with a low drug discontinuation rate. Legal entity responsible for the study Hellenic Cooperative Oncology Group. Funding Hellenic Cooperative Oncology Group (HeCOG). Disclosure E. Fountzilas: Travel / Accommodation / Expenses: K.A.M ONCOLOGY / HEMATOLOGY; Travel / Accommodation / Expenses: Merck; Shareholder / Stockholder / Stock options: Deciphera. G. Aravantinos: Advisory / Consultancy: Novartis; Advisory / Consultancy: Bristol-Myers Squibb; Advisory / Consultan