Fulvestrant-Palbociclib vs Letrozole-Palbociclib as Initial Therapy for Endocrine-Sensitive, Hormone Receptor–Positive, ERBB2-Negative Advanced Breast CancerA Randomized Clinical Trial

Question  Which is the optimal endocrine partner (fulvestrant vs letrozole) for cyclin-dependent kinase 4 and 6 inhibitor palbociclib in previously untreated, endocrine-sensitive, hormone receptor–positive, ERBB2-negative advanced breast cancer?

Findings  In this randomized, open-label, phase 2 trial, 486 patients were assigned in equal numbers to receive palbociclib plus fulvestrant or letrozole. Median investigator-assessed progression-free survival was 27.9 months for fulvestrant-palbociclib vs 32.8 months for letrozole-palbociclib, a difference that was not statistically significant.

Meaning  Fulvestrant-palbociclib demonstrated no improvement in progression-free survival over letrozole-palbociclib, confirming letrozole as the preferred palbociclib partner in this patient population.

Importance  The cyclin-dependent kinase 4 and 6 inhibitor palbociclib in combination with letrozole has become a standard first-line treatment for patients with endocrine-sensitive, hormone receptor–positive, ERBB2-negative advanced breast cancer. Meanwhile, the antiestrogen fulvestrant was shown to be superior to anastrozole in the absence of cyclin-dependent kinase 4 and 6 inhibition for this patient population.

Objective  To assess whether fulvestrant is superior to letrozole when combined with palbociclib in the first-line scenario.

Design, Setting, and Participants  In this international, randomized, open-label, phase 2 clinical study conducted from July 30, 2015, to January 8, 2018, patients with hormone receptor–positive, ERBB2-negative advanced breast cancer with no prior therapy in the metastatic setting and endocrine-sensitive criteria were recruited from 47 centers in 7 countries. Data were analyzed from February 11 to May 15, 2020.

Interventions  Patients were randomly assigned (1:1 ratio) to receive palbociclib with either fulvestrant or letrozole. Stratification factors were type of disease presentation (de novo vs recurrent) and the presence of visceral involvement (yes vs no).

Main Outcomes and Measures  The primary end point was investigator-assessed progression-free survival determined by Response Evaluation Criteria in Solid Tumors, version 1.1.

Results  A total of 486 women (median age, 63 years [range, 25-90 years]; 3 Asian women [0.6%]; 4 Black women [0.8%]; 461 White women [94.9%]; 18 women of unknown race [3.7%]) were randomized (243 to fulvestrant-palbociclib and 243 to letrozole-palbociclib). Median investigator-assessed progression-free survival was 27.9 months (95% CI, 24.2-33.1 months) in the fulvestrant-palbociclib group vs 32.8 months (95% CI, 25.8-35.9 months) in the letrozole-palbociclib group (hazard ratio, 1.13; 95% CI, 0.89-1.45; P = .32). This result was consistent across the stratification factors. No significant differences were observed in objective response rate (46.5% vs 50.2%) and 3-year overall survival rate (79.4% vs 77.1%) for fulvestrant-palbociclib and letrozole-palbociclib, respectively. Grade 3-4 adverse events were comparable among treatment groups, and no new safety signals were identified. No treatment-related deaths were reported.

Conclusions and Relevance  Although fulvestrant-palbociclib demonstrated significant antitumor activity, this randomized clinical trial failed to identify an improvement in progression-free survival with this regimen over letrozole-palbociclib in patients with endocrine-sensitive, hormone receptor–positive, ERBB2-negative advanced breast cancer.

Trial Registration  ClinicalTrials.gov Identifier: NCT02491983