Fluorine-18 Labeled Fluorofuranylnorprogesterone ([ 18 F]FFNP) and Dihydrotestosterone ([ 18 F]FDHT) Prepared by "Fluorination on Sep-Pak" Method.

To further explore the scope of our recently developed "fluorination on Sep-Pak" method, we prepared two well-known positron emission tomography (PET) tracers 21-[ ¹⁸ F]fluoro-16α,17α-[(R)-(1'-α-furylmethylidene)dioxy]-19-norpregn-4-ene-3,20-dione furanyl norprogesterone ([ ¹⁸ F]FFNP) and 16β-[ ¹⁸ F]fluoro-5α-dihydrotestosterone ([ ¹⁸ F]FDHT). Following the "fluorination on Sep-Pak" method, over 70% elution efficiency was observed with 3 mg of triflate precursor of [ ¹⁸ F]FFNP. The overall yield of [ ¹⁸ F]FFNP was 64-72% (decay corrected) in 40 min synthesis time with a molar activity of 37-81 GBq/µmol (1000-2200 Ci/mmol). Slightly lower elution efficiency (~55%) was observed with the triflate precursor of [ ¹⁸ F]FDHT. Fluorine-18 labeling, reduction, and deprotection to prepare [ ¹⁸ F]FDHT were performed on Sep-Pak cartridges (PS-HCO ₃ and Sep-Pak plus C-18). The overall yield of [ ¹⁸ F]FDHT was 25-32% (decay corrected) in 70 min. The molar activity determined by using mass spectrometry was 63-148 GBq/µmol (1700-4000 Ci/mmol). Applying this quantitative measure of molar activity to in vitro assays [ ¹⁸ F]FDHT exhibited high-affinity binding to androgen receptors (K _d ~2.5 nM) providing biological validation of this method.