[carbonyl-11C]4-Fluoro-N-methyl-N-(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide ([11C]FIMX) is an effective radioligand for PET imaging of metabotropic glutamate receptor 1 (mGluR1) in monkey brain.

INTRODUCTION: Metabotropic glutamate subtype receptor 1 (mGluR1) is implicated in several neuropsychiatric disorders and is a target for drug development. [(18)F]FIMX ([(18)F]4-fluoro--N-methyl-N--(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide) is an effective radioligand for imaging brain mGluR1 with PET. A similarly effective radioligand with a shorter half-life would usefully allow PET studies of mGluR1 at baseline and after pharmacological or other challenge on the same day. Here we describe the preparation of [(11)C]FIMX for evaluation in monkey with PET. METHODS: [(11)C]FIMX was prepared via Pd-promoted carbonylation of 1-fluoro-4-iodobenzene with [(11)C]carbon monoxide, aminolysis of the [(11)C]acyl-palladium complex with the requisite Boc-protected amine, and deprotection with HCl in THF. PET scans of [(11)C]FIMX injected into a monkey were performed at baseline and after preblock of mGluR1 with measurement of the arterial input function. RESULTS: The radiosynthesis required 42 min and gave [(11)C]FIMX in about 5% overall decay-corrected radiochemical yield and with a specific activity of about 100 GBq/µmol. PET in rhesus monkey at baseline showed that radioactivity peaked high in receptor-rich cerebellum and much lower in receptor-poor occipital cortex. Radioactivity in cerebellum declined to 32% of peak at 85 min. VT at baseline appeared stable in all brain regions after 60 min. Under mGluR1 pre-blocked condition, radioactivity uptake in all regions declined more rapidly to a low level. Receptor pre-block reduced VT from 13.0 to 1.5 in cerebellum and from 2.9 to 1.4 in occipital cortex. CONCLUSION: [(11)C]FIMX is an effective radioligand for imaging mGluR1 in monkey with PET.

Synthesis and evaluation of translocator 18 kDa protein (TSPO) positron emission tomography (PET) radioligands with low binding sensitivity to human single nucleotide polymorphism rs6971.

The imaging of translocator 18 kDa protein (TSPO) in living human brain with radioligands by positron emission tomography (PET) has become an important means for the study of neuroinflammatory conditions occurring in several neuropsychiatric disorders. The widely used prototypical PET radioligand [(11)C](R)-PK 11195 ([(11)C](R)-1; [N-methyl-(11)C](R)-N-sec-butyl-1-(2-chlorophenyl)-N-methylisoquinoline-3-carboxamide) gives a low PET signal and is difficult to quantify, whereas later generation radioligands have binding sensitivity to a human single nucleotide polymorphism (SNP) rs6971, which imposes limitations on their utility for comparative quantitative PET studies of normal and diseased subjects. Recently, azaisosteres of 1 have been developed with improved drug-like properties, including enhanced TSPO affinity accompanied by moderated lipophilicity. Here we selected three of these new ligands (7-9) for labeling with carbon-11 and for evaluation in monkey as candidate PET radioligands for imaging brain TSPO. Each radioligand was readily prepared by (11)C-methylation of an N-desmethyl precursor and was found to give a high proportion of TSPO-specific binding in monkey brain. One of these radioligands, [(11)C]7, the direct 4-azaisostere of 1, presents many radioligand properties that are superior to those reported for [(11)C]1, including higher affinity, lower lipophilicity, and stable quantifiable PET signal. Importantly, 7 was also found to show very low sensitivity to the human SNP rs6971 in vitro. Therefore, [(11)C]7 now warrants evaluation in human subjects with PET to assess its utility for imaging TSPO in human brain, irrespective of subject genotype.

Synthesis and evaluation in monkey of [(18)F]4-fluoro-N-methyl-N-(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide ([(18)F]FIMX): a promising radioligand for PET imaging of brain metabotropic glutamate receptor 1 (mGluR1).

We sought to develop a PET radioligand that would be useful for imaging human brain metabotropic subtype 1 receptors (mGluR1) in neuropsychiatric disorders and in drug development. 4-Fluoro-N-methyl-N-(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide (FIMX, 11) was identified as having favorable properties for development as a PET radioligand. We developed a method for preparing [(18)F]11 in useful radiochemical yield and in high specific activity from [(18)F]fluoride ion and an N-Boc-protected (phenyl)aryliodonium salt precursor (15). In baseline experiments in rhesus monkey, [(18)F]11 gave high brain radioactivity uptake, reflecting the expected distribution of mGluR1 with notably high uptake in cerebellum, which became 47% lower by 120 min after radioligand injection. Pharmacological challenges demonstrated that a very high proportion of the radioactivity in monkey brain was bound specifically and reversibly to mGluR1. [(18)F]11 is concluded to be an effective PET radioligand for imaging mGluR1 in monkey brain and therefore merits further evaluation in human subjects.

Investigation of the metabolites of (S,S)-[(11)C]MeNER in humans, monkeys and rats.

INTRODUCTION: (S,S)-[(11)C]MeNER ((S,S)-2-(alpha-(2-[(11)C]methoxyphenoxy)benzyl)morpholine) is a positron emission tomography (PET) radioligand recently applied in clinical studies of norepinephrine transporters (NETs) in the human brain in vivo. In view of further assessment of the suitability of (S,S)-[(11)C]MeNER as a NET radioligand, its metabolism and the identity of the in vivo radiometabolites of (S,S)-[(11)C]MeNER are of great interest. MATERIALS AND METHODS: Thus, PET studies were used to measure brain dynamics of (S,S)-[(11)C]MeNER, and plasma reverse-phase radiochromatographic analysis was performed to monitor and quantify its rate of metabolism. Eighteen healthy human volunteers, five cynomolgus monkeys, and five rats were studied. RESULTS AND DISCUSSION: In human subjects, the plasma radioactivity representing (S,S)-[(11)C]MeNER decreased from 88 +/- 5% at 4 min after injection to 82 +/- 7% at 40 min, while a polar radiometabolite increased from 3 +/- 3% to 16 +/- 7% at the same time-points, respectively. A more lipophilic radiometabolite than (S,S)-[(11)C]MeNER decreased from 9 +/- 5% at 4 min to 1 +/- 2% at 40 min. In monkeys, plasma radioactivity representing (S,S)-[(11)C]MeNER decreased from 97 +/- 2% at 4 min to 74 +/- 7% at 45 min, with a polar fraction as the major radiometabolite. A more lipophilic radiometabolite than (S,S)-[(11)C]MeNER, constituted 3 +/- 2% of radioactivity at 4 min and was not detectable later on. In rats, 17 +/- 4% of plasma radioactivity was parent radioligand at 30 min with the remainder comprising mainly a polar radiometabolite. (S,S)-[(11)C]MeNER in rat brain and urine at 30 min after injection were 90% and 4%, respectively. On a brain regional level, parent radioligand ranged from 87.5 +/- 3.9% (57.2 +/- 14.2% SUV [standard uptake values, %injected radioactivity per mL multiplied with animal weight (in g)]; cerebellum) to 92.9 +/- 1.8% (36.1 +/- 4.7% SUV; striatum), with differential distribution of the radiometabolite in the cerebellum (6.7 +/- 0.3% SUV) and the striatum (2.5 +/- 0.3% SUV). Liquid chromatography-mass spectrometry analysis of rat urine identified a hydroxylation product of the methoxyphenoxy ring of (S,S)-MeNER as the main metabolite. In the brain, the corresponding main metabolite was the product from O-de-methylation of (S,S)-MeNER. PET measurements were performed in rats as well as in wild-type and P-gp-knock-out mice. In rats, the brain peak level of radioactivity was found to be very low (65%SUV). In mice, there was only a small difference in peak brain accumulation between P-gp knock-out and wild-type mice (145 vs. 125%SUV) with the following rank order of regional brain radioactivity: cerebellum x thalamus > cortical regions > striatum. CONCLUSION: It can be concluded that radiometabolites of (S,S)-[(11)C]MeNER are of minor importance in rat and monkey brain imaging. The presence of a transient lipophilic radiometabolite in peripheral human plasma may induce complications with brain imaging, but its kinetics appear favorable in relation to the slow kinetics of (S,S)-[(11)C]MeNER in humans.

Evaluation of [11C]PipISB and [18F]PipISB in monkey as candidate radioligands for imaging brain cannabinoid type-1 receptors in vivo.

N-(4-Fluorobenzyl)-4-[3-(piperidin-1-yl)indole-1-sulfonyl]benzamide] (PipISB, 3) is a selective and high-potency cannabinoid subtype-1 (CB1) receptor inverse agonist. We have previously reported radiosyntheses of [11C]3 and [18F]3. Here, we aimed to evaluate the uptake and CB(1) receptor-specific binding of each radioligand in monkey brain in vivo with positron emission tomography (PET). [11C]3 or [18F]3 was injected intravenously into rhesus or cynomolgus monkey, respectively, and examined with PET at baseline or after pretreatment with a receptor-saturating dose of CB1 receptor-selective ligand (3 for [11C]3 or 8 for [18F]3). In one PET experiment, the dose of 3 was administered at 100 min after [11C]3. Relative plasma concentrations of radioligand and radiometabolites were concurrently measured in baseline experiments with high-performance liquid chromatography. Brain radioactivity uptake was highest in striatum and cerebellum, and it reached 170-270% standardized uptake value (SUV) at 120 min after injection of [11C]3 and 180% SUV at 240 min after injection of [18F]3. Radioactivity was well retained in all CB1 receptor-rich regions. No reference region could be identified for nonspecifically bound radioligand. Under CB1 receptor pretreatment and displacement conditions, initial brain uptakes of radioactivity were similar to those at baseline. Regional brain radioactivity concentrations then became homogeneous and diminished to between 70 and 80% SUV at 120 min after injection of [11C]3 and to 25% SUV at 240 min after injection of [18F]3. [18F]3 was not defluorinated but was metabolized to less lipophilic radiometabolites, as was [11C]3. Hence, [11C]3 and [18F]3 showed high CB1 receptor-specific binding in monkey brain in vivo and merit further investigation as prospective PET radioligands in humans.

Synthesis and initial evaluation of [11C](R)-RWAY in monkey-a new, simply labeled antagonist radioligand for imaging brain 5-HT1A receptors with PET.

PURPOSE: We aimed to fulfill a need for a radioligand that may be simply labeled with carbon-11 for effective positron emission tomography (PET) imaging of brain 5-HT(1A) receptors. METHODS: Racemic RWAY (2,3,4,5,6,7-hexahydro-1-[4-[1-[4-(2-methoxyphenyl)piperazinyl]]-2-phenylbutyryl]-1H-azepine) has high affinity for 5-HT(1A) receptors. The enantiomers of RWAY and O-desmethyl-RWAY, synthesized from commercially available materials, were each labeled with carbon-11 by treating the respective O-desmethyl precursor with [(11)C]iodomethane, and injected into rhesus monkey for measurement of regional brain uptake. The 5-HT(1A) selectivity of (R)-[(11)C]RWAY was checked by administering WAY-100635, before and after radioligand administration. Radiometabolites of (R)-[(11)C]RWAY in blood and urine were analyzed by HPLC with partial elucidation of their structures by LC-MS-MS. RESULTS: (R)-[(11)C]RWAY was a 5-HT(1A) receptor antagonist exhibiting high brain uptake with regional distribution consistent with specific binding to 5-HT(1A) receptors. The similar affinity, (S)-[(11)C]RWAY was a weak partial agonist at 5-HT(1A) receptors exhibiting similar brain peak uptake with much less 5-HT(1A) receptor-specific binding. The maximal ratio in receptor-rich cingulate gyrus to receptor-devoid cerebellum reached 6.4 at 87.5 min after injection of (R)-[(11)C]RWAY. After treatment with WAY-100635 before or after (R)-[(11)C]RWAY administration, radioactivity levels in 5-HT(1A) receptor-rich regions were reduced almost to that in cerebellum. Blood and urine radiometabolites were less lipophilic than parent and were not due to hydrolysis but to ring hydroxylations, oxidation, and dephenylation. CONCLUSION: (R)-[(11)C]RWAY is simply prepared and an effective antagonist for imaging brain 5-HT(1A) receptors. This radioligand resists hydrolysis in vivo, gives less lipophilic radiometabolites, and warrants further PET studies in human subjects.