SEER registry analysis of HR-positive metastatic breast cancer survival before and after CDK4/6 inhibitor introduction in the United States

CDK4/6 inhibitors have improved patient survival in HR+/HER2- metastatic breast cancer (mBC) in clinical trials and real-world studies. A retrospective study using SEER registry data evaluated breast cancer-specific survival (BCSS) of patients with de novo HR+ mBC before and after introduction of CDK4/6 inhibitors in the United States in 2015. After baseline characteristic adjustment, patients with HR+/HER2- mBC diagnosed post-2015 had an approximately 10% reduction in risk of BC-specific death compared with patients diagnosed pre-2015. No significant change was observed in HR+/HER2+ BCCS post-2015 versus pre-2015. The authors conclude that coinciding with the introduction of CDK4/6 inhibitors in the USA, significant improvements in BCSS were noted among patients with HR+/HER2- mBC. No such improvement was seen among HR+/HER2+ mBC patients, likely due to the availability of HER2-directed therapies before and after 2015.