Real-world experience with CDK4/6 inhibitors in the first-line palliative setting for HR+/HER2- advanced breast cancer.

e13066 Background: CDK4/6 inhibitors paired with aromatase inhibitors (AIs) have redefined the first-line treatment for metastatic breast cancer, as evidenced by several pivotal trials. Although progression-free survival (PFS) has been consistent across these studies, notable differences in overall survival (OS) have been observed. Our study evaluates the real-world efficacy of palbociclib and ribociclib, in combination with AIs, for hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancer at our institution. Methods: We retrospectively reviewed charts of adult patients with advanced metastatic breast cancer at a single academic center from January 1, 2015, to December 1, 2022. We extracted relevant clinical data to estimate the median PFS and OS associated with palbociclib and ribociclib. Results: Among 75 patients analyzed, the majority were female (98.7%) and postmenopausal (77.3%). Palbociclib was prescribed to 74.7%, while 52.0% presented with de novo stage IV disease. Those on ribociclib were significantly younger (median age 57.6 vs. 67.5 years; p = 0.013) and more likely premenopausal (42.1% vs. 5.4%; p < 0.001). Median PFS for palbociclib was 20.3 months (95% CI: 14.8-46.0), with an OS of 37.2 months (20.3 months to not reached). Median PFS and OS for ribociclib were not reached. The differences in PFS and OS between the drugs were not statistically significant (PFS log-rank p = 0.1; OS log-rank p = 0.2). Conclusions: Our real-world analysis suggests a non-significant trend towards improved PFS and OS for ribociclib over palbociclib when used with AIs for treating HR+/HER2- advanced breast cancer, potentially influenced by the younger age of patients on ribociclib. Further research with larger cohorts and extended follow-up is warranted to substantiate these observations.