18F-labeled difluoroestradiols: preparation and preclinical evaluation as estrogen receptor-binding radiopharmaceuticals

A-ring fluorination of estradiol (ES) at position 2 or 4 decreases the rate of metabolism by blocking the formation of catechol estrogens, one of the major metabolic pathways of ES. We postulate that adding a 2- or 4-fluoro substituent to 16alpha-18F]fluoroestradiol (FES), a positron emission tomography (PET) radiopharmaceutical used for estrogen receptor (ER) imaging, should prolong its blood circulation time, and thus, improve its localization in ER-rich target tissues. On such account, we prepared a series of FES derivatives substituted with a fluorine atom at C2 or C4, with or without an 11beta-OMe group, and we tested their binding affinities for the ER and different serum proteins including rat alphafetoprotein (AFP) and human sex hormone-binding globulin (SHBG). Labeling at the 16alpha-position was accomplished via nucleophilic substitution with 18F]F(-) on the reactive 16beta,17beta-cyclic sulfate intermediates. Decay corrected yields varied between 30 and 50% for a total synthesis time of 120 min, providing final products with specific activities >3000 Ci/mmol. The 18F-labeled analogs were evaluated for their biodistribution in immature female rats. Substitutions with the 4-F have little effect on binding affinities. Addition of the 2-F diminishes ER and AFP-binding affinities while augmenting the affinity for the SHBG. Addition of the 11beta-OMe decreases all binding affinities, particularly to AFP and SHBG. In contrast, biodistribution of the corresponding 16alpha-18F]fluoro analogs in immature female rats revealed that the presence of the 11beta-OMe group improves ER-mediated uterus uptake, with the 4,16alpha-16alpha-18F]difluoro-11beta-methoxyestradiol showing the highest uptake values (15% ID at 1-h post-injection). These data suggest that the addition of both a 4-F and 11beta-OMe group onto FES may provide an improved radiopharmaceutical for PET imaging of ER densities in breast cancer patients.