Fluoroestradiol F18 CAS# 94153-53-4

Fluoroestradiol F18 (16α-[18F]-fluoro-17β-estradiol, FES) is a radiolabeled estrogen analog developed as a positron emission tomography (PET) tracer for imaging estrogen receptor (ER) expression, primarily ERα, in tumors. In preclinical studies, FES demonstrated high affinity binding to ERα with a dissociation constant (Kd) of ~0.13–0.15 nM, comparable to estradiol, and showed >90% specific uptake in ER-positive human breast cancer xenografts (e.g., MCF-7) compared to ER-negative controls. In rodent models, FES displayed rapid blood clearance with a tumor-to-muscle ratio exceeding 5:1 at 60 min post-injection and strong correlation between tumor uptake and ER density (r > 0.9). Biodistribution studies in mice and rats revealed primary hepatic metabolism and predominantly biliary excretion, with minimal nonspecific binding in ER-negative tissues. Blocking experiments with tamoxifen or unlabeled estradiol reduced tumor uptake by >80%, confirming ER-mediated specificity. These data established FES as a sensitive, high-affinity tracer for noninvasive quantification of ER expression in vivo.