Abstract Labeled with carbon‐11, N‐(2‐chloro‐5‐thiomethylphenyl)‐N′‐(3‐methoxyphenyl)‐N′‐methylguanidine ([11C]GMOM) is currently the only positron emission tomography (PET) tracer that has shown selectivity for the ion‐channel site of N‐methyl‐D‐aspartate (NMDA) receptors in human imaging studies. The present study reports on the selectivity profile and in vitro binding properties of GMOM. The compound was screened on a panel of 80 targets, and labeled with tritium ([3H]GMOM). The binding properties of [3H]GMOM were compared to those of the reference ion‐channel ligand [3H](+)‐dizocilpine maleate ([3H]MK‐801), in a set of concentration‐response, homologous and heterologous inhibition, and association kinetics assays, performed with repeatedly washed rat forebrain preparations. GMOM was at least 70‐fold more selective for NMDA receptors compared to all other targets examined. In homologous inhibition and concentration‐response assays, the binding of [3H]GMOM was regulated by NMDA receptor agonists, albeit in a less prominent manner compared to [3H]MK‐801. Scatchard transformation of homologous inhibition data produced concave upward curves for [3H]GMOM and [3H]MK‐801. The radioligands showed bi‐exponential association kinetics in the presence of 100 μmol L−1 l‐glutamate/30 μmol L−1 glycine. [3H]GMOM (3 nmol L−1 and 10 nmol L−1) was inhibited with dual affinity by (+)‐MK‐801, (R,S)‐ketamine and memantine, in both presence and absence of agonists. [3H]MK‐801 (2 nmol L−1) was inhibited in a monophasic manner by GMOM under baseline and combined agonist conditions, with an IC50 value of ~19 nmol L−1. The non‐linear Scatchard plots, biphasic inhibition by open channel blockers, and bi‐exponential kinetics of [3H]GMOM indicate a complex mechanism of interaction with the NMDA receptor ionophore. The implications for quantifying the PET signal of [11C]GMOM are discussed.

Introduction: The present study was designed to assess whether [18F1PK-209 (3-(2-chloro-5-(methylthio)pheny1)-1-(3-([189fluoromethoxy)pheny1)-1-methylguanidine) is a suitable ligand for imaging the ion-channel site of N-methyl-o-aspartate receptors (NMDArs) using positron emission tomography (PET). Methods: Dynamic nu scans were acquired from male rhesus monkeys over 120 min, at baseline and after the acute administration of dizocilpine (MK-801, 0.3 mg/1<g; n = 3/condition). Continuous and discrete arterial blood samples were manually obtained, to generate metabolite-corrected input functions. Parametric volumeof-distribution (VT) images were obtained using Logan analysis. The selectivity profile of PK-209 was assessed in vitro, on a broad screen of 79 targets. Results: PK-209 was at least 50-fold more selective for NMDArs over all other targets examined. At baseline, prolonged retention of radioactivity was observed in NMDAr-rich cortical regions relative to the cerebellum. Pretreatment with MK-801 reduced the VT of [18F]PK-209 compared with baseline in two of three subjects. The rate of radioligand metabolism was high, both at baseline and after MK-801 administration. Conclusions: PK-209 targets the intrachannel site with high selectivity. Imaging of the NMDAr is feasible with [18F]PK-209, despite its fast metabolism. Further in vivo evaluation in humans is warranted. 2014 Elsevier Inc. All rights reserved.

(−)-OSU6162 is a dopamine stabilizer that can counteract both hyperdopaminergic and hypodopaminergic states. In this study, D2/D3 receptor occupancy of (−)-OSU6162 in the human brain was investigated using positron emission tomography (PET). Twelve male healthy volunteers underwent [11C]raclopride PET scanning before and 1 h after a single oral dose of (−)-OSU6162 (15–90 mg). Blood samples for determination of (−)-OSU6162 and prolactin plasma levels were collected at Tmax. Parametric images of [11C]raclopride binding potential relative to nondisplaceable tissue (cerebellar grey matter) uptake (BPND) at baseline and after (−)-OSU6162 administration were generated using the simplified reference tissue model. MRI-based regions of interest were defined for the striatum, composed of caudate nucleus and putamen, and projected onto the co-registered parametric [11C]raclopride BPND image. Furthermore, three striatal subregions, ie, anterior dorsal caudate, anterior dorsal putamen, and ventral striatum, were defined manually and additionally analyzed. Plasma concentrations of (−)-OSU6162, ranging from 0.01 to 0.9 μM, showed a linear relationship with prolactin levels, reflecting blockade of pituitary D2 receptors. A concentration-dependent increase in striatal D2/D3 receptor occupancy was observed, reaching a value of about 20% at an (−)-OSU6162 plasma level of 0.2 μM, and which for higher concentrations leveled off to a maximal occupancy of about 40%. Findings were similar in the striatal subregions. The present data corroborate the notion that (−)-OSU6162 binds preferentially to a subpopulation of D2/D3 receptors, possibly predominantly extrasynaptic, and this may form the basis for the dopamine-stabilizing properties of (−)-OSU6162.

Introduction: The N-methyl-D-Aspartate (NMDA) receptor plays an important role in learning and memory. Overactivation is thought to play an important role in neurodegenerative disorders such as Alzheimer's disease. Currently, it is not possible to assess N-methyl-D-aspartate receptor (NMDAr) bio-availability in vivo. The purpose of this study was to develop a positron emission tomography (PET) ligand for the NR2B binding site of the NMDA receptor.Methods: N-((5-(4-fluoro-2-methoxyphenyl)pyridin-3-yl)methyl)cyclopentanamine was radiolabelled with carbon-11 in the phenyl moiety. Biodistribution and blocking studies were carried out in anaesthetized mice and in non-anaesthetized rats.Results: N-((5-(4-fluoro-2-[C-11]methoxyphenyl)pyridin-3-yl)methyl)cyclopentanamine was prepared in 49 +/- 3% (decay-corrected) yield, affording 4.1 +/- 0.3 GBq of formulated product at the end of synthesis with a radiochemical purity of >99% and with a specific activity of 78 +/- 10 GBq/mu mol.Conclusion: A new NR2B PET ligand was developed in high yield. [C-11]4 readily enters the brain and binds to the NR2B subunit-containing NMDAr in the rodent brain. High sigma-1 receptor binding may, however, limit its future application as a PET probe for imaging the NR2B subunit-containing NMDAr. Anaesthesia has an effect on NMDAr function and therefore can complicate interpretation of preclinical in vivo results. In addition, effects of endogenous compounds cannot be excluded. Despite these potential limitations, further studies are warranted to investigate the values of [C-11]4 as an NR2B PET ligand. (C) 2014 Elsevier Inc. All rightrs reserved.

(−)-OSU6162 is a dopamine stabilizer that can counteract both hyperdopaminergic and hypodopaminergic states. In this study, D2/D3 receptor occupancy of (−)-OSU6162 in the human brain was investigated using positron emission tomography (PET). Twelve male healthy volunteers underwent [11C]raclopride PET scanning before and 1 h after a single oral dose of (−)-OSU6162 (15–90 mg). Blood samples for determination of (−)-OSU6162 and prolactin plasma levels were collected at Tmax. Parametric images of [11C]raclopride binding potential relative to nondisplaceable tissue (cerebellar grey matter) uptake (BPND) at baseline and after (−)-OSU6162 administration were generated using the simplified reference tissue model. MRI-based regions of interest were defined for the striatum, composed of caudate nucleus and putamen, and projected onto the co-registered parametric [11C]raclopride BPND image. Furthermore, three striatal subregions, ie, anterior dorsal caudate, anterior dorsal putamen, and ventral striatum, were defined manually and additionally analyzed. Plasma concentrations of (−)-OSU6162, ranging from 0.01 to 0.9 μM, showed a linear relationship with prolactin levels, reflecting blockade of pituitary D2 receptors. A concentration-dependent increase in striatal D2/D3 receptor occupancy was observed, reaching a value of about 20% at an (−)-OSU6162 plasma level of 0.2 μM, and which for higher concentrations leveled off to a maximal occupancy of about 40%. Findings were similar in the striatal subregions. The present data corroborate the notion that (−)-OSU6162 binds preferentially to a subpopulation of D2/D3 receptors, possibly predominantly extrasynaptic, and this may form the basis for the dopamine-stabilizing properties of (−)-OSU6162.

Serotonin 5-HT4 receptor (5-HT4-R) agonists are potential therapeutic agents for enterokinetic and cognitive disorders and are marketed for treatment of constipation. The aim of this study was to develop an agonist positron emission tomography (PET) ligand in order to label the active G-protein coupled 5-HT4-R in peripheral and central tissues. For this purpose prucalopride, a high-affinity selective 5-HT4-R agonist, was selected.

PurposeM1 muscarinic acetylcholine receptor (M1ACh‐R) agonists are potential therapeutics for cognitive disorders. The aim of this study was to develop an orthosteric agonist positron emission tomography (PET) ligand for the M1ACh‐R that would provide a tool to explore the active G‐protein coupled receptor. To that extend, the radiosynthesis, receptor binding in vitro, and brain uptake of [11C]AF150(S), a selective M1ACh‐R agonist, were investigated.Procedures[11C]AF150(S) was synthesized, its binding properties were investigated using autoradiography on rat brain sections, and brain uptake was determined in biodistribution studies ex vivo in rats. Metabolites in brain and blood were measured using high‐performance liquid chromatography.ResultsYield of the [11C]AF150(S) radiosynthesis was 69 ± 9% with radiochemical purity >99%. The logDoct,7.4 was 0.05. Autoradiography showed binding in M1ACh‐R rich brain areas and selective inhibition by muscarinic agents in conditions promoting agonist binding. Biodistribution studies revealed rapid and high brain uptake, to levels exceeding five times the blood level. In M1ACh‐R rich areas, specific uptake versus cerebellum was apparent, remaining constant over 60 min in spite of rapid decline of radioactivity from the brain and rapid peripheral metabolism. Brain uptake of [11C]AF150(S) may involve facilitated transport.Conclusion[11C]AF150(S) was successfully synthesized, showed M1ACh‐R agonist binding properties, and high brain uptake. It provides an interesting lead for the development of an M1ACh‐R agonist PET ligand meant to explore the active receptor state. Copyright © 2012 John Wiley & Sons, Ltd.

N-methyl-d-aspartate receptors (NMDA-R) are a subtype of glutamatergic ligand-gated ion channels with key roles in physiological brain function, most notably in learning and memory. Perturbations of NMDA-R mediated neuronal activity are thought to be involved in the pathophysiology of various brain disorders, including Alzheimer's disease (AD). We have examined the regional activation state of NMDA-R in a mouse model of AD, using in vitro and ex vivo autoradiography of [3 H]MK-801 binding sites. For in vitro studies, coronal brain sections of drug naive, male APPswe-PS1dE9 and control mice were either prewashed and incubated for 4 hr with 20 nM [3 H]MK-801 in the presence of 10 μM glutamate and 30 μM glycine (conventional protocol), or were directly labeled with [3 H]MK-801 (in situ binding protocol). Non-specific binding was determined in adjacent brain sections in the presence of GMOM, an experimental NMDA-R antagonist. For ex vivo autoradiography, non-anaesthetized transgenic and control mice were injected with either saline (10 ml/kg, i.p) or ketamine (50 mg/kg, i.p), followed 15 min later by a tail vein injection of [3 H]MK-801 (200 μ Ci/kg). Total and non-specific binding was determined in brains of saline and ketamine-treated mice, respectively. The distribution pattern of [3 H]MK-801 binding sites was similar between control and transgenic animals. There was a significant, age-dependent genotype effect on specific NMDA-R binding, both in situ and ex vivo. [3 H]MK-801 binding levels were region-specifically increased in the hippocampus of 13 month-old APPswe-PS1dE9 mice compared to age-matched controls. NMDA-R may become over-active in the APPswe-PS1dE9 brain, in an age-dependent manner. Our findings hold relevance both for identifying new functional targets for molecular imaging in AD, and for the development of novel drugs targeting NMDA-R.

Background. An important step in the analysis of positron emission tomography (PET) studies of the brain is the definition of regions of interest (ROI). Image coregistration, ROI analysis, and quantification of brain PET data in small animals can be observer dependent. The purpose of this study was to investigate the feasibility of ROI analysis based on a standard MR template and an additional [(18)F]NaF scan. Methods. [(18)F]NaF scans of 10 Wistar rats were coregistered with a standard MR template by 3 observers and derived transformation matrices were applied to corresponding [(11)C]AF150(S) images. Uptake measures were derived for several brain regions delineated using the MR template. Overall agreement between the 3 observers was assessed by interclass correlation coefficients (ICC) of uptake data. In addition, [(11)C]AF150(S) ROI data were compared with ex vivo biodistribution data. Results. For all brain regions, ICC analysis showed excellent agreement between observers. Reproducibility, estimated by calculation of standard deviation of the between-observer differences, was demonstrated by an average of 17% expressed as coefficient of variation. Uptake of [(11)C]AF150(S) derived from ROI analysis closely matched ex vivo biodistribution data. Conclusions. The proposed method provides a reproducible and tracer-independent method for ROI analysis of rat brain PET data.

Here we describe the design, synthesis, and pharmacological profile of 5-HT(1A) receptor ligands related to 1 (WAY-100635). The cyclohexyl moiety in 1 and its O-desmethylated analogue 3 were replaced by the bridgehead iodinated bridge-fused rings: adamantyl, cubyl, bicyclo[2.2.2]octyl, or bicyclo[2.2.1]heptyl. All analogues displayed a (sub)nanomolar affinity for the 5-HT(1A) receptor in vitro. Compounds 6b and 7b appeared to be selective for this receptor over other relevant receptors and could easily be iodinated with radioactive iodine-123. In humane hepatocytes, [(123)I]6b showed a low propensity for amide hydrolysis and a stable carbon-iodine bond. The biodistribution of [(123)I]6b and [(123)I]7b in rats revealed that the carbon-iodine bond was also stable in vivo. Unfortunately, the brain uptake and the specificity for both radioligands were significantly lower than those of the parent molecule 1. In conclusion, the designed tracers are not suitable for SPECT imaging.

Purpose Neurokinin 1 (NK1) receptors have been implicated in depression, anxiety, and pain perception. Recently, it was shown that, in the human brain, a specific NK1 receptor-related signal was obtained with the novel radioligand, [C-11]R116301, using positron emission tomography. The purpose of this study was to evaluate various methods for quantifying specific [C-11]R116301 binding.Methods Two dynamic 90-min [C-11]R116301 scans, separated by 5 h, were performed in 11 healthy volunteers. In three patients, the second scan was performed after an oral blocking dose of 125 mg of aprepitant, whereas in the other eight, no intervention was performed (test-retest). Whole striatum was used as the tissue of interest, as it has the highest density of NK1 receptors. Cerebellum was used as the reference tissue.Results Reference tissue models were stable with the simplified reference tissue model (SRTM) performing best. Average (+/- standard deviation) SRTM-derived mean nondisplaceable binding potential (BPND) of all (first) baseline scans was 0.64 +/- 0.31 (n=11), which reduced to -0.01 +/- 0.03 (n=3) after aprepitant administration. Test-retest results showed low variability (14.0 +/- 10.7%) and excellent reliability, as indicated by the intraclass correlation coefficient (0.93). The ratio of standardized uptake values of striatum and cerebellum minus 1, an approximation of BPND, showed very low variability (6.2 +/- 3.1%) with excellent reliability (intraclass correlation coefficient=0.98), and correlated well with SRTM-derived BPND (R-2 = 0.96).Conclusion SRTM is the model of choice for quantifying [C-11]R116301 binding. Semiquantitative tissue ratios hold promise for routine clinical applications. Nucl Med Commun 32:896-902 (C) 2011 Wolters Kluwer Health vertical bar I Lippincott Williams & Wilkins. Nuclear Medicine Communications 2011, 32:896-902

At present, P-glycoprotein (P-gp) function can be studied using positron emission tomography (PET) together with a labelled P-gp substrate such as R-[11C]verapamil. Such a tracer is, however, less suitable for investigating P-gp (over)expression. Laniquidar is a third-generation P-gp inhibitor, which has been used in clinic trials for modulating multidrug resistance transporters. The purpose of the present study was to develop the radiosynthesis of [11C]laniquidar and to assess its suitability as a tracer of P-gp expression. The radiosynthesis of [11C]laniquidar was performed by methylation of the carboxylic acid precursor with [11C]CH3I. The product was purified by HPLC and reformulated over a tC18 Seppak, yielding a sterile solution of [11C]laniquidar in saline. For evaluating [11C]laniquidar, rats were injected with 20 MBq [11C]laniquidar via a tail vein and sacrificed at 5, 15, 30 and 60 min after injection. Several tissues and distinct brain regions were dissected and counted for radioactivity. In addition, uptake of [11C]laniquidar in rats pretreated with cyclosporine A and valspodar (PSC 833) was determined at 30 min after injection. Finally, the metabolic profile of [11C]laniquidar in plasma was determined. [11C]Laniquidar could be synthesized in moderate yields with high specific activity. Uptake in brain was low, but significantly increased after administration of cyclosporine A. Valspodar did not have any effect on cerebral uptake of [11C]laniquidar. In vivo rate of metabolism was relatively low. Further kinetic studies are needed to investigate the antagonistic behaviour of [11C]laniquidar at tracer level.