Switching to camizestrant at ESR1 mutation emergence before disease progression during first-line treatment of hormone receptor-positive advanced breast cancer (SERENA-6): extended analysis of a double-blind, placebo-controlled, randomised, phase 3 trial

Background SERENA-6 is, to our knowledge, the first global registrational study to use prospective circulating tumour DNA (ctDNA) monitoring to identify the emergence of an acquired resistance mutation before clinical progression and then direct a change in therapy in patients with hormone receptor-positive advanced breast cancer. Switching to camizestrant from aromatase inhibitor with continued cyclin-dependent kinase (CDK) 4/6 inhibitor at ESR1 mutation emergence during first-line therapy significantly improved progression-free survival at interim analysis. We report comprehensive results from ESR1 mutation surveillance in SERENA-6, updated progression-free survival, final analysis of second progression-free survival with longer follow-up, and an exploratory analysis of ESR1 mutation ctDNA dynamics on treatment to give a comprehensive report of this new treatment strategy. Methods This double-blind, placebo-controlled, randomised, phase 3 trial was conducted at 264 hospitals and cancer centres in 23 countries and enrolled women with any menopausal status, or men, aged 18 years or older who were receiving first-line treatment with aromatase inhibitor plus CDK4/6 inhibitor for at least 6 months for oestrogen receptor-positive, HER2-negative locally advanced or metastatic breast cancer. Patients were required to have an Eastern Cooperative Oncology Group performance status score of 0 or 1. Eligible patients were enrolled and had ctDNA tested for ESR1 mutation every 2-3 months, coinciding with routine clinical assessments, using the Guardant360 CDx assay. Patients with an ESR1 mutation in ctDNA and without radiological progression were randomly assigned (1:1) using block randomisation (stratified by disease site, time of ESR1 mutation detection, time from initiation of aromatase inhibitor plus CDK4/6 inhibitor to randomisation, and CDK4/6 inhibitor) to switch to camizestrant (75 mg orally once daily) with continued CDK4/6 inhibitor (orally at the same dose) or to continue receiving aromatase inhibitor (anastrozole 1 mg or letrozole 2 & centerdot;5 mg orally once daily) plus CDK4/6 inhibitor (orally at the same dose as was received during ESR1 mutation surveillance phase). Palbociclib and ribociclib were dosed, orally, once daily for 21 days and then 7 days with no treatment in 28-day cycles, while abemaciclib was dosed, orally, twice daily every day in 28-day cycles. The SERENA-6 sample size was determined to ensure sufficient power for both the primary endpoint (investigator-assessed according to RECIST 1.1 progression-free survival) and the key secondary endpoint of investigator-assessed second progression-free survival (time from randomisation to disease progression after first subsequent therapy or death). For patients who had experienced a first progression, scans to assess second progression-free survival were conducted every 8-12 weeks. Updated progression-free survival analysis at this data cutoff was descriptive. Efficacy analyses included all randomly assigned patients (intention to treat). This study is registered with ClinicalTrials.gov, NCT04964934, and is ongoing. Findings From June 30, 2021, to June 14, 2024, 3325 patients were screened and 3256 patients received at least one ESR1 mutation test during first-line therapy and 548 patients had a positive ESR1 mutation test by the time of screening closure.315 patients (312 [99%] female; 199 [63%] White, 73 [23%] Asian, six [2%] Black or African American, 37 [12%] other, not reported, or with missing race data) were randomly assigned: 157 to camizestrant plus CDK4/6 inhibitor and 158 to aromatase inhibitor plus CDK4/6 inhibitor. After a median follow-up of 23 & centerdot;5 months (IQR 17 & centerdot;9-32 & centerdot;1; data cutoff Jan 2, 2026), median prog