Risk of early progression according to circulating ESR1 mutation, CA-15.3 and cfDNA increases under first-line anti-aromatase treatment in metastatic breast cancer

Endocrine therapy is recommended as a first-line treatment for hormone receptor-positive metastatic breast cancer (HR+MBC) patients. No biomarker has been validated to predict tumor progression in that setting. We aimed to prospectively compare the risk of early progression according to circulating ESR1 mutations, CA-15.3, and circulating cell-free DNA in MBC patients treated with a first-line aromatase inhibitor (AI). Patients with MBC treated with a first-line AI were prospectively included. Circulating biomarker assessment was performed every 3 months. The primary objective was to determine the risk of progression or death at the next follow-up visit (after 3 months) in case of circulating ESR1 mutation detection among patients treated with a first-line AI for HR+MBC. Overall, 103 patients were included, and 70 (68%) had progressive disease (PD). Circulating ESR1 mutations were detected in 22/70 patients with PD and in 0/33 patients without progression (p