A preoperative window-of-opportunity study of oral SERD, imlunestrant, in newly diagnosed ER-positive, HER2-negative early breast cancer: Results from EMBER-2 Study.

Abstract

Imlunestrant is an oral SERD with favorable safety and preliminary efficacy in patients with ABC. PD biomarker data can optimize drug dosing; herein is the PD data from the EMBER-2 study. Postmenopausal women with untreated, operable ER-positive, HER2-negative EBC were randomized to 400mg vs 800mg of imlunestrant daily for ~2 weeks before surgery. A single arm testing 200mg daily was later accrued. PD biomarker changes (ER, PR, Ki-67 by IHC, and mRNA expression of ER-related genes) were evaluated in paired tumor samples (pre-/ post-treatment). Safety and PK were also assessed. Among evaluable paired samples (n=75), PD profiles demonstrated consistent ER targeting between 400 and 800mg doses with less toxicity at the 400mg dose. Although inducing the lowest rate of CCCA, PD and PK results were similar for the 200mg dose. EMBER-2 combined with existing phase 1 data has identified 400mg as the optimal imlunestrant dose.