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% 50 yo, 14.9% 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 (p0.0001). Conversely, the proportion of patients 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 (p0.0001). In 2015-2016, all patients on CDK4/6i were prescribed palbocilcib. From 2017 to 2022, the proportion of patients on CDK4/6i prescribed ribociclib and abemaciclib consistently increased each year. By 2022, among 2,682 patients with CDK4/6i claims, 1,726 (64.4%) were prescribed abemaciclib, 508 (18.9%) were prescribed ribociclib, and 448 (16.7%) were prescribed palbociclib. Switching between CDK4/6i was infrequent, with only 854 patients (6.3%) having claims for multiple distinct CDK4/6i. The most common switch was from palbociclib to abemaciclib (62.8% of patients with claims for at least 2 CDK4/6i), followed by palbociclib to ribociclib (13.2%). Among patients with a first claim for CDK4/6i in 2015-2017, long-term persistence was noted, with 11.1% of patients starting treatment in 2015 continuing into 2018, 12.4% of those starting in 2016 continuing into 2019, and 10.8% of those starting in 2017 continuing into 2020. Conclusions: This real-world study of CDK4/6i prescription patterns following their FDA approval for mBC reveals an increasing trend in younger and lower-income patients on Medicaid receiving these agents, likely reflecting improved access and coverage. Oncologists showed a preference for newer agents such as ribociclib and abemaciclib post-approval, even prior to overall survival data becoming available. The rarity of switching between CDK4/6i suggests good tolerability. Over 10% of patients maintained long-term use of CDK4/6i into the third year after the year of their initial prescription. Citation Format: C. Sathe, R. Raghunathan, M. K. Accordino, M. M. Caplan, J. D. Wright, D. L. Hershman. Prescription Patterns and Treatment Persistence in Patients with Metastatic Breast Cancer on CDK4/6 Inhibitors abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-11-18.