Clinical activity of camizestrant, a next-generation SERD, versus fulvestrant in patients with a detectable ESR1 mutation: Exploratory analysis of the SERENA-2 phase 2 trial.

Abstract

1066 Background: Camizestrant, a next-generation oral selective estrogen receptor antagonist and degrader (ngSERD), was compared at two dose levels to fulvestrant 500 mg (F) in post-menopausal women with advanced ER+, HER2˗ breast cancer with disease recurrence or progression after ≤1 endocrine therapy in the advanced setting in the Phase 2 randomized SERENA-2 study (NCT04214288). Camizestrant demonstrated statistically significant and clinically meaningful benefit vs F in progression-free survival (PFS) in the overall study population (Oliveira M et al. SABCS 2022 Annual Meeting. Abstract GS3-02.). Methods: Baseline circulating tumour DNA was collected at screening and/or Cycle 1 Day 1 and analysed by next-generation sequencing. A post hoc exploratory analysis compared investigator assessed PFS with camizestrant (data from 75 and 150 mg combined) vs F in patients with detectable baseline ESR1 (the gene that encodes ERα) hotspot mutations ( ESR1m). Patients were divided into subgroups based on whether 1 or >1 ESR1m variants were detected, the specific ESR1m, and whether mutations were detected in BRCA1/2 or the MAPK pathway. The most prevalent mutations are presented. A Cox proportional hazards model was used to compare PFS. Results: 48/147 (32.7%) patients treated with camizestrant and 35/73 (47.9%) treated with F had a detectable ESR1m in at least 1 baseline sample. ESR1m were detected in 6/9 (67%) patients with a BRCA1/2 mutation and 5/26 (19%) with a MAPK pathway alteration; however, the numbers were too small to analyse efficacy in these subgroups. Conclusions: Camizestrant showed improved outcome vs F in patients with a detectable ESR1m at baseline and in the subgroups tested. The greatest median PFS improvement was seen in patients where a single ESR1m variant was detected, suggesting that early intervention upon detection of an ESR1m may provide the maximum patient benefit for camizestrant, a hypothesis that is being tested in the SERENA-6 clinical trial (NCT04964934). Clinical trial information: NCT04214288 . [Table: see text]