Abstract

e13095 Background: Multiple clinical trials have demonstrated the efficacy of the commercially available CDK4/6 inhibitors (CDK4/6i) combined with endocrine therapy (ET) as upfront therapy in patients (pts) with (HR+)/HER2-negative (HER2-) metastatic breast cancer (MBC). Recently, differences in overall survival (OS) benefit in MBC and invasive disease-free survival benefit in early-stage BC suggest greater potency of abemaciclib and ribociclib compared to palbociclib. Here we report a comprehensive OS analysis of a cohort of pts with HR+/HER2- MBC treated with ribociclib, palbociclib or abemaciclib and ET in 1 st line setting. Methods: We retrieved medical records of pts with HR+/HER2- MBC receiving CDK4/6i as 1st line therapy between December 2013 and October 2023 at Memorial Sloan Kettering Cancer Center. We investigated the impact of clinico-pathological characteristics and treatment type as categorical variables on OS using uni- and multivariate Cox (proportional hazard) regression model. Any p-value (p) <0.05 was deemed to be statistically significant. Results: 854 pts were included in the analysis, 838 females (98%) and 16 males at birth. The median age was 56 (IQR: 48-65). Overall, 722 (84%) pts received palbociclib, 88 (11%) abemaciclib, and 44 (5%) ribociclib. Most pts received aromatase inhibitor (AI) as ET backbone (n=634, 74%); 214 received fulvestrant (35%), and 6 received tamoxifen (1%). After a median follow-up of 3.14 years (range: 1.7-4.6), the median OS of the full cohort was 54.7 months (95%CI: 50-63.1). Clinico-pathological characteristics that resulted independently associated with worse OS were: male gender at birth (HR:2, 95%CI:1.4-4), disease-free interval ≤ 2 years (HR:1.64, 95%CI:1.11-2.44), ER-low status (HR: 2.27, 95%CI:1.42-3.63), high grade(HR: 1.28, 95%CI:1.01-1.62), liver involvement (HR: 1.48, 95%CI:1.08-2.03), and number of disease sites >3 (HR:1.35, 95%CI:1.03-1.78). HER2-low status was associated with better OS (HR: 0.75, 95%CI:0.59-0.96). In terms of type of treatment, there was a trend toward improved OS in patients receiving AI rather than fulvestrant or tamoxifen (HR:0.76, 95%: 0.58-1), while the type of CDK4/6i did not affect the outcome. Conclusions: Clinical parameters of early relapse, high disease burden, and intrinsic tumor aggressiveness seem to be the main drivers of poor prognosis in patients with HR+/HER2- MBC in 1 st line setting. In this analysis, type of CDK4/6i did not have a significant impact on OS, although pts treated with abemaciclib and ribociclib were underrepresented in the cohort due to the earlier and prevalent use of palbociclib as the first-in-class FDA-approved CDK4/6i. These data are reassuring about the use of palbociclib suggesting that a switch of CKD4/6i is not needed in pts with ongoing treatment and demonstrated benefit.