Abstract PS1-11-18: Prescription Patterns and Treatment Persistence in Patients with Metastatic Breast Cancer on CDK4/6 Inhibitors

Abstract Background: Since the FDA’s approval of palbociclib, the first cyclin dependent kinase 4/6 inhibitor (CD4/6i), in 2015, these agents have transformed the treatment landscape for breast cancer (BC). In particular, the combination of CDK4/6i with endocrine therapy forms the cornerstone of the first-line treatment of hormone receptor-positive (HR+)/HER2-negative (HER2-) metastatic BC (mBC). Choosing among the 3 approved CDK4/6i (palbociclib, ribociclib, and abemaciclib) remains challenging, as each has a distinct side-effect profile and there are no head-to-head trial data comparing their therapeutic efficacy. This study examines real-world prescription patterns and usage trends of CDK4/6i among patients with mBC, providing insight into evolving prescription preferences and the tolerability of these treatments. Methods: Utilizing the MarketScan health care claims database, we identified patients prescribed palbociclib, ribociclib, and/or abemaciclib from 2/2015 (post-FDA approval of palbociclib) until 12/2022 (prior to FDA approval of CDK4/6i for adjuvant use). We assessed changes in prescription rates for specific CDK4/6i over time and the frequency of switches between these agents. To evaluate treatment duration, we analyzed cohorts of patients who received their first CDK4/6i prescription in 2015, 2016, and 2017, tracking ongoing prescriptions over subsequent years. Descriptive statistics, including frequencies, percentages, means, and medians, were employed to summarize our findings. Results: A total of 13,514 patients were prescribed a CDK4/6i between 2/2015 and 12/2022 (98.2% female; 23.1% 70 yo; 18.6% on Medicare, 19.9% on Medicaid). The percentage of patients < 50 yo and patients covered by Medicaid prescribed CDK4/6i consistently increased over time, from 17.9% and 11.5% in 2015, to 31.8% and 22.6% in 2022, respectively (p 70 yo and patients covered by Medicare decreased over time, from 16.9% and 24.9% in 2015, to 9.8% and 12.5% in 2022, respectively (p