Comparison of antitumor efficacy of first-line palbociclib, ribociclib, or abemaciclib in patients with HR+/HER2- aBC: Results of the multicenter, real-world, Italian study PALMARES-2.

1014 Background: The Cyclin Dependent Kinase 4/6 inhibitors (CDK4/6i) Palbociclib, Ribociclib and Abemaciclib in combination with Endocrine Therapy (ET) represent the standard-of-care, 1 st line treatment for patients with Hormone Receptor-positive, Human Epidermal growth factor Receptor 2-negative, advanced Breast Cancer (HR+/HER2- aBC). So far, no large real-world studies have compared the efficacy of the three CDK4/6i in this clinical setting. Methods: The multicenter, population-based study PALMARES-2 evaluated the antitumor efficacy of 1 st line Palbociclib, Ribociclib or Abemaciclib in combination with ET in consecutive HR+/HER2- aBC patients treated in 18 Italian cancer centers between 1 st January 2016 and 1 st September 2023. The primary study endpoint was real-world Progression-Free Survival (rwPFS), as defined as the time interval between ET plus CDK4/6i initiation and disease progression. Multivariate Cox regression model was used to adjust the association between individual CDK4/6i and rwPFS for clinically relevant variables. Results: With a data cut-off date of January 1 st , 2024, we enrolled 1850 patients, 750 (40.6%), 676 (36.5%) and 424 (22.9%) of whom received Palbociclib, Ribociclib and Abemaciclib, respectively. Among 1226 (66.3%) patients with endocrine-sensitive disease, 1087 (89%) received concomitant Aromatase Inhibitors, whereas 441 (70.7%) out of 624 (33.7%) patients with endocrine-resistant disease received concomitant Fulvestrant. Patients treated with Abemaciclib were more likely to have endocrine-resistant disease, liver metastases, lobular tumor histology and lower PgR tumor expression, and less likely to have de novometastatic disease (p<0.001). Median rwPFS in the whole study cohort was 34.9 months (95% CI 32.0-37.4). Abemaciclib and Ribociclib were independently associated with better rwPFS when compared to Palbociclib (Table), whereas Abemaciclib and Ribociclib showed no significantly different efficacy (aHR 0.91, 95% CI 0.70-1.19; p=0.505). Other covariates independently associated with rwPFS are shown (Table). Conclusions: The real-world study PALMARES-2 revealed different antitumor efficacy of individual CDK4/6i in HR+/HER2- aBC patients. Longer follow-up is required to study if Palbociclib, Ribociclib and Abemaciclib are associated with different overall survival. Table: see text