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
- H. Umesha Shetty,
- Sami S. Zoghbi,
- Fabrice G. Siméon,
- Jeih-San Liow,
- Amira K. Brown,
- Pavitra Kannan,
- Robert B. Innis and
- Victor W. Pike
- Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland
- Address correspondence to:
Dr. H. Umesha Shetty, PET Radiopharmaceutical Sciences, Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Bldg. 10, Rm B3 C351, 10 Center Dr., MSC 1003, Bethesda, MD 20892-1003. E-mail: shettyu{at}mail.nih.gov
Abstract
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-[18F](fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile ([18F]SP203), for imaging brain mGluR5 in monkey and human. In monkey, radioactivity was observed in bone, showing release of [18F]-fluoride ion from [18F]SP203. This defluorination was not inhibited by disulfiram, a potent inhibitor of CYP2E1. PET confirmed bone uptake of radioactivity and therefore defluorination of [18F]SP203 in rats. To understand the biochemical basis for defluorination, we administered [18F]SP203 plus SP203 in rats for ex vivo analysis of metabolites. Radio-high-performance liquid chromatography detected [18F]fluoride ion as a major radiometabolite in both brain extract and urine. Incubation of [18F]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-l-cysteine (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 [18F]SP203 in rat. This study provides the first demonstration of glutathione-promoted radiodefluorination of a PET radioligand.
Footnotes
-
This work was supported by the Intramural Research Program of National Institutes of Health (National Institute of Mental Health project Z01-MH-002795-04). A portion of this work was presented at the 55th American Society for Mass Spectrometry Conference on Mass Spectrometry and Allied Topics, 2007 Jun 3–7, Indianapolis, IN.
-
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
-
doi:10.1124/jpet.108.143347.
-
ABBREVIATIONS: mGluR5, metabotropic glutamate subtype-5 receptor(s); PET, positron emission tomography; SP203, 3-fluoro-5-(2-(2-(fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile; [18F]FCWAY, 18F-trans-4-fluoro-N-(2-[4-(2-methoxyphenyl)piperazin-1-yl)ethyl]-N-(2-pyridyl)cyclohexanecarboxamide; [18F]SP203, 3-fluoro-5-(2-(2-[18F](fluoromethyl)thiazol-4-yl)ethynyl)benzonitrile; HPLC, high-performance liquid chromatography; LC-MS, liquid chromatography-mass spectrometry; MS/MS, mass spectrometry/mass spectrometry; amu, atomic mass unit(s); MS3, multi-stage mass spectrometry; SUV, standardized uptake value; CID, collision-induced dissociation.
-
- Received July 9, 2008.
- Accepted September 18, 2008.
- The American Society for Pharmacology and Experimental Therapeutics
