5-HT1A Receptors In Human Brain Research Articles

Disulfiram Inhibits Defluorination of 18F-FCWAY, Reduces Bone Radioactivity, and Enhances Visualization of Radioligand Binding to Serotonin 5-HT1A Receptors in Human Brain

(18)F-trans-4-Fluoro-N-2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-(2-pyridyl)cyclohexanecarboxamide ((18)F-FCWAY) is a PET radioligand for imaging serotonin 5-hydroxytryptamine-1A receptors in brain. (18)F-FCWAY undergoes significant defluorination, with high uptake of radioactivity in the skull and resulting spillover contamination in the underlying neocortex. The cytochrome P450 enzyme CYP2E1 defluorinates many drugs. We previously showed that miconazole, an inhibitor of CYP2E1, blocks defluorination of FCWAY in rats. Here, we used (18)F-FCWAY to test the ability of the less toxic agent disulfiram to inhibit defluorination in humans. Eight healthy volunteers underwent a PET scan before and after administration of 500 mg of disulfiram (n = 6) or 2,000 mg of cimetidine (n = 2). Seven of the subjects had arterial blood sampling during both scans. Although cimetidine had relatively small and variable effects on 2 subjects, disulfiram reduced skull radioactivity by about 70% and increased peak brain uptake by about 50% (n = 5). Disulfiram decreased plasma-free (18)F-fluoride ion (from peak levels of 340% +/- 62% standardized uptake value (SUV) to 62% +/- 43% SUV; P < 0.01) and increased the concentration of the parent (18)F-FCWAY (with a corresponding decrease of clearance from 14.8 +/- 7.8 L x h(-1) at baseline to 7.9 +/- 2.8 L x h(-1) after drug treatment (P < 0.05). Using compartmental modeling with input of both (18)F-FCWAY and the radiometabolite (18)F-FC (trans-4-fluorocyclohexanecarboxylic acid), distribution volumes attributed to the parent radioligand unexpectedly decreased about 40%-60% after disulfiram, but the accuracy of the radiometabolite correction is uncertain. Disulfiram changed the shape of the brain time-activity curves in a manner that could occur with inhibition of the efflux transporter P-glycoprotein (P-gp). However, disulfiram showed no in vivo efficacy in monkeys to enhance the uptake of the known P-gp substrate (11)C-loperamide, suggesting that the effects of disulfiram in humans were mediated entirely by inhibition of CYP2E1. A single oral dose of disulfiram inhibited about 70% of the defluorination of (18)F-FCWAY, increased the plasma concentration of (18)F-FCWAY, increased brain uptake of activity, and resulted in better visualization of 5-HT(1A) receptor in the brain. Disulfiram is a safe and well-tolerated drug that may be useful for other radioligands that undergo defluorination via CYP2E1.

Carbonyl-11C]Desmethyl-WAY-100635 (DWAY) is a potent and selective radioligand for central 5-HT1A receptors in vitro and in vivo.

[carbonyl-11C]Desmethyl-WAY-100635 (DWAY) is possibly a low-level metabolite appearing in plasma after intravenous administration of [carbonyl-11C]WAY-100635 to human subjects for positron emission tomographic (PET) imaging of brain 5-HT1A receptors. In this study we set out to assess the ability of DWAY to enter brain in vivo and to elucidate its possible interaction with 5-HT1A receptors. Desmethyl-WAY-100635 was labelled efficiently with carbon-11 (t1/2 = 20.4 min) in high specific radioactivity by reaction of its descyclohexanecarbonyl analogue with [carbonyl-11C]cyclohexanecarbonyl chloride. The product was separated in high radiochemical purity by high-performance liquid chromatography (HPLC) and formulated for intravenous injection. Rats were injected intravenously with DWAY, sacrificed at known times and dissected to establish radioactivity content in brain tissues. At 60 min after injection, the ratios of radioactivity concentration in each brain region to that in cerebellum correlated with previous in vitro and in vivo measures of 5-HT1A receptor density. The highest ratio was about 22 in hippocampus. Radioactivity cleared rapidly from plasma; HPLC analysis revealed that DWAY represented 55% of the radioactivity in plasma at 5 min and 33% at 30 min. Only polar radioactive metabolites were detected. Subsequently, a cynomolgus monkey was injected intravenously with DWAY and examined by PET. Maximal whole brain uptake of radioactivity was 5.7% of the administered dose at 5 min after injection. The image acquired between 9 and 90 min showed high radioactivity uptake in brain regions rich in 5-HT1A receptors (e.g. frontal cortex and neocortex), moderate uptake in raphe nuclei and low uptake in cerebellum. A transient equilibrium was achieved in cortical regions at about 60 min, when the ratio of radioactivity concentration in frontal cortex to that in cerebellum reached 6. The corresponding ratio for raphe nuclei was about 3. Radioactive metabolites appeared rapidly in plasma, but these were all more polar than DWAY, which represented 52% of the radioactivity in plasma at 4 min and 20% at 55 min. In a second PET experiment, in which a cynomolgus monkey was pretreated with the selective 5-HT1A receptor antagonist, WAY-100635, at 25 min before DWAY injection, radioactivity in all brain regions was reduced to that in cerebellum. Autoradiography of post mortem human brain cryosections after incubation with DWAY successfully delineated 5-HT1A receptor distribution. Receptor-specific binding was eliminated in the presence of the selective 5-HT1A receptor agonist, 8-OH-DPAT [(+/-)-8-hydroxy-2-dipropylaminotetralin]. These findings show that: (a) intravenously administered DWAY is well able to penetrate brain in rat and monkey, (b) DWAY is a highly effective radioligand for brain 5-HT1A receptors in rat and monkey in vivo and for human brain in vitro, and (c) the metabolism and kinetics of DWAY appear favourable to successful biomathematical modelling of acquired PET data. Thus, DWAY warrants further evaluation as a radioligand for PET studies of 5-HT1A receptors in human brain.

Remotely-controlled production of the 5-HT1A receptor radioligand, [carbonyl-11C]WAY-100635, via 11C-carboxylation of an immobilized Grignard reagent

WAY-100635 [N-(2-(1-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl))-N-(2-pyridyl)-cyclohexanecarboxamide], when labelled in its carbonyl position with carbon-11 (t1/2 = 20.4 min), is an effective radioligand for the study of 5-HT1A receptors in human brain with positron emission tomography (PET). A simple remotely-controlled procedure was developed for the routine synthesis of [carbonyl-11C]WAY-100635. Thus, cyclohexylmagnesium chloride is coated onto the inner surface of a narrow polypropylene tube. Cyclotron-produced [11C]carbon dioxide is passed into the tube in a nitrogen stream. A solution of thionyl chloride in tetrahydrofuran is then passed through the tube to convert the trapped radioactive adduct into [carbonyl-11C]cyclohexanecarbonyl chloride and to release this labelling agent into a vial containing 1-(2-methoxyphenyl)-4-(2-(2-pyridylamino)ethyl)piperazine plus triethylamine. The vial is sealed and heated to 70°C for 5 min. [carbonyl-11C]WAY-100635 is isolated by sample-enrichment and reverse phase HPLC and formulated for human intravenous injection by evaporation of solvent and dissolution in ‘saline for injection’. The novel use of the immobilized Grignard reagent has the advantages that only small quantities of all reagents are required so simplifying product purification. Moreover, the procedure was readily adapted for operation in a shielded hot-cell with remote control for radiation safety. The remotely-controlled radiosynthesis takes 45 min and gives high radioactivities (2.96–5.92 GBq) of formulated [carbonyl-11C]WAY-100635 in > 99% radiochemical purity and high specific radioactivity (average, 192 GBq/μmol).