Radiodefluorination of 3-Fluoro-5-(2-(2-[18F](fluoromethyl)-thiazol-4-yl)ethynyl)benzonitrile ([18F]SP203), a Radioligand for Imaging Brain Metabotropic Glutamate Subtype-5 Receptors with Positron Emission Tomography, Occurs by Glutathionylation in Rat Brain

Metabotropic glutamate subtype-5 receptors (mGluR5) are implicated in several neuropsychiatric disorders. Positron emission tomography (PET) with a suitable radioligand may enable monitoring of regional brain mGluR5 density before and during treatments. We have developed a new radioligand, 3-fluoro-5-(2-(2[ 18 F](fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile ([ 18 F]SP203), for imaging brain mGluR5 in monkey and human. In monkey, radioactivity was observed in bone, showing release of [ 18 F]fluoride ion from [ 18 F]SP203. This defluorination was not inhibited by disulfiram, a potent inhibitor of CYP2E1. PET confirmed bone uptake of radioactivity and therefore defluorination of [ 18 F]SP203 in rats. To understand the biochemical basis for defluorination, we administered [ 18 F]SP203 plus SP203 in rats for ex vivo analysis of metabolites. Radio-high-performance liquid chromatography detected [ 18 F]fluoride ion as a major radiometabolite in both brain extract and urine. Incubation of [ 18 F]SP203 with brain homogenate also generated this radiometabolite, whereas no metabolism was detected in whole blood in vitro. Liquid chromatography-mass spectrometry analysis of the brain extract detected m/z 548 and 404 ions, assignable to the [M H] of S-glutathione (SP203Glu) and N-acetyl-S-Lcysteine (SP203Nac) conjugates of SP203, respectively. In urine, only the [M H] of SP203Nac was detected. Mass spectrometry/mass spectrometry and multi-stage mass spectrometry analyses of each metabolite yielded product ions consistent with its proposed structure, including the former fluoromethyl group as the site of conjugation. Metabolite structures were confirmed by similar analyses of SP203Glu and SP203Nac, prepared by glutathione S-transferase reaction and chemical synthesis, respectively. Thus, glutathionylation at the 2-fluoromethyl group is responsible for the radiodefluorination of [ 18 F]SP203 in rat. This study provides the first demonstration of glutathione-promoted radiodefluorination of a PET radioligand.