UCSF

PURPOSE: To conduct an updated exploratory analysis of overall survival (OS) with a longer median follow-up of 73.3 months and evaluate the prognostic value of molecular analysis by circulating tumor DNA (ctDNA). PATIENTS AND METHODS: Patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2-) advanced breast cancer (ABC) were randomized 2:1 to receive palbociclib (125 mg orally/day; 3/1 week schedule) and fulvestrant (500 mg intramuscularly) or placebo and fulvestrant. This OS analysis was performed when 75% of enrolled patients died (393 events in 521 randomized patients). ctDNA analysis was performed among patients who provided consent. RESULTS: At the data cutoff (August 17, 2020), 258 and 135 deaths occurred in the palbociclib and placebo groups, respectively. The median OS [95% confidence interval (CI)] was 34.8 months (28.8-39.9) in the palbociclib group and 28.0 months (23.5-33.8) in the placebo group (stratified hazard ratio, 0.81; 95% CI, 0.65-0.99). The 6-year OS rate (95% CI) was 19.1% (14.9-23.7) and 12.9% (8.0-19.1) in the palbociclib and placebo groups, respectively. Favorable OS with palbociclib plus fulvestrant compared with placebo plus fulvestrant was observed in most subgroups, particularly in patients with endocrine-sensitive disease, no prior chemotherapy for ABC and low circulating tumor fraction and regardless of ESR1, PIK3CA, or TP53 mutation status. No new safety signals were identified. CONCLUSIONS: The clinically meaningful improvement in OS associated with palbociclib plus fulvestrant was maintained with >6 years of follow-up in patients with HR+/HER2- ABC, supporting palbociclib plus fulvestrant as a standard of care in these patients.