Synthesis and evaluation of fluorine-18 21-fluoroprednisone as a potential ligand for neuro-PET studies.

No-carrier-added fluorine-18-labeled fluoroprednisone ([18F]21-fluoroprednisone) was synthesized by tosylate displacement in 2%-8% radiochemical yield in 80 min end of cyclotron bombardment (EOB), and its metabolism and distribution were investigated. After intravenous administration to rats, [18F]21-fluoroprednisone was rapidly cleared from the blood and biotransformed into [18F]20-dihydro-21-fluoroprednisone. The suitability of [18F]21-fluorocorticoids for receptor imaging in humans with positron emitting tomography will depend on the synthesis of compounds with high binding affinity and low rate of carbonyl reduction at C-20.