Results from a Phase 1 clinical study of AC699, Accutar Biotechnology’s investigational oral estrogen receptor (ER) degrader, were published in Nature Communications on December 17, 2025. The study assessed safety and preliminary antitumor activity in 37 patients with heavily pretreated ER-positive/HER2-negative advanced or metastatic breast cancer.
All adverse events considered related to AC699 were mild or moderate (Grade 1 or 2), with no dose reductions or treatment discontinuations due to these events. Among 26 patients evaluable for efficacy, four achieved a confirmed partial response (15%). In an exploratory subgroup of 10 patients with ESR1 mutations, four achieved a partial response (40%).
The findings support further clinical evaluation of AC699, particularly in ESR1-mutated disease. These are preliminary results from a small study without a comparison group; larger studies are needed to better assess clinical benefit.
