Synthesis of N-(piperidin-1-yl)-5-(4-methoxyphenyl)-1-(2-chlorophenyl)-4-[18F]fluoro-1H-pyrazole-3-carboxamide by nucleophilic [18F] fluorination:: a PET radiotracer for studying CB1 cannabinoid receptors

The feasibility of nucleophilic displacement of bromide in the 4-bromopyrazole ring with [F-18]fluoride has been demonstrated by the synthesis of two radiolabeled compounds: N-(piperidin-1-yl)-5-(4-methoxyphenyl)-1-(2-chlorophenyl)-4-[F-18]fluoro-1H-pyrazole-3-carboxamide, ([F-18] NIDA-42033) 1b and 1-(2-chlorophenyl)-4-[F-18]fluoro-5-(4-methoxyphenyl)-1H-pyrazole-3-carboxylic acid, ethyl ester 4. The radiochemical yields were in the range of 1-6%. [F-18]NIDA-42033, a potential radiotracer for the study of CB, cannabinoid receptors in the animal brain by positron emission tomography, has been synthesized in sufficient quantities with specific radioactivity greater than 2500 mCi/mumol and radiochemical purity > 95%. Copyright (C) 2002 John Wiley Sons, Ltd.