A prodrug of NMDA/glycine site antagonist, L-703,717, with improved BBB permeability: 4-acetoxy derivative and its positron-emitter labeled analog

Positron Emission Tomography (PET) Ligand Development for Ionotropic Glutamate Receptors: Challenges and Opportunities for Radiotracer Targeting N-Methyl-d-aspartate (NMDA), α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA), and Kainate Receptors

Ionotropic glutamate receptors (iGluRs) mediate excitatory neurotransmission within the mammalian central nervous system. iGluRs exist as three main groups: N-methyl-d-aspartate receptors (NMDARs), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and kainate receptors. The past decades have witnessed a remarkable development of PET tracers targeting different iGluRs including NMDARs and AMPARs, and several of the tracers have advanced to clinical imaging studies. Here, we assess the recent development of iGluR PET probes, focusing on tracer design, brain kinetics, and performance in PET imaging studies. Furthermore, this review will not only present challenges in the tracer development but also provide novel approaches in conjunction with most recent drug discovery efforts on these iGluRs, including subtype-selective NMDAR and transmembrane AMPAR regulatory protein modulators and positive allosteric modulators (PAMs) of AMPARs. These approaches, if successful as PET tracers, may provide fundamental knowledge to understand the roles of iGluR receptors under physiological and pathological conditions.

Selected PET Radioligands for Ion Channel Linked Neuroreceptor Imaging: Focus on GABA, NMDA and nACh Receptors

Positron emission tomography (PET) neuroimaging of ion channel linked receptors is a developing area of preclinical and clinical research. The present review focuses on recent advances with radiochemistry, preclinical and clinical PET imaging studies of three receptors that are actively pursued in neuropsychiatric drug discovery: namely the γ-aminobutyric acid-benzodiazapine (GABA) receptor, nicotinic acetylcholine receptor (nAChR), and N-methyl-D-aspartate (NMDA) receptor. Recent efforts to develop new PET radioligands for these targets with improved brain uptake, selectivity, stability and pharmacokinetics are highlighted.

Development of PET and SPECT probes for glutamate receptors

l-Glutamate and its receptors (GluRs) play a key role in excitatory neurotransmission within the mammalian central nervous system (CNS). Impaired regulation of GluRs has also been implicated in various neurological disorders. GluRs are classified into two major groups: ionotropic GluRs (iGluRs), which are ligand-gated ion channels, and metabotropic GluRs (mGluRs), which are coupled to heterotrimeric guanosine nucleotide binding proteins (G-proteins). Positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging of GluRs could provide a novel view of CNS function and of a range of brain disorders, potentially leading to the development of new drug therapies. Although no satisfactory imaging agents have yet been developed for iGluRs, several PET ligands for mGluRs have been successfully employed in clinical studies. This paper reviews current progress towards the development of PET and SPECT probes for GluRs.

Synthesis, spectral features and biological activity of some novel hetarylazo dyes derived from 8-chloro-4-hydroxyl-2-quinolone

In this study, 8-chloro-4-hydroxyl-2-quinolone was synthesized from cyclocondensation of corresponding dianilide and subsequently used as a potent coupling component with some diazotized heterocyclic amines. These compounds were characterized by UV-vis, FT-IR, (1)H NMR spectroscopic techniques and elemental analysis. Absorption spectra of these dyes were measured in six polar solvents and discussed with respect to the nature of solvents and substituted groups. The effects of acid, base, temperature and concentration on the visible absorption spectra of the dyes were reported. In addition, the antimicrobial activity of the dyes was explored in detail.

In-vivo visualization of key molecular processes involved in Alzheimer's disease pathogenesis: Insights from neuroimaging research in humans and rodent models

Diverse age-associated neurodegenerative disorders are featured at a molecular level by depositions of self-aggregating molecules, as represented by amyloid beta peptides (Abeta) and tau proteins in Alzheimer's disease, and cascade-type chain reactions are supposedly commenced with biochemical aberrancies of these amyloidogenic components. Mutagenesis and multiplication of the genes encoding Abeta, tau and other pathogenic initiators may accelerate the incipient process at the cascade top, rationalizing generations of transgenic and knock-in animal models of these illnesses. Meanwhile, these genetic manipulations do not necessarily compress the timelines of crucial intermediate events linking amyloidogenesis and neuronal lethality, resulting in an incomplete recapitulation of the diseases. Requirements for modeling the entire cascade can be illustrated by a side-by-side comparison of humans and animal models with the aid of imaging-based biomarkers commonly applicable to different species. Notably, key components in a highly reactive state are assayable by probe-assisted neuroimaging techniques exemplified by positron emission tomography (PET), providing critical information on the in-vivo accessibility of these target molecules. In fact, multispecies PET studies in conjunction with biochemical, electrophysiological and neuropathological tests have revealed putative neurotoxic subspecies of Abeta assemblies, translocator proteins accumulating in aggressive but not neuroprotective microglia, and functionally active neuroreceptors available to endogenous neurotransmitters and exogenous agonistic ligands. Bidirectional translational studies between human cases and model strains based on this experimental paradigm are presently aimed at clarifying the tau pathogenesis, and would be expanded to analyses of disrupted calcium homeostasis and mitochondrial impairments. Since reciprocal causalities among the key processes have indicated an architectural interchangeability between cascade and network connections as an etiological representation, longitudinal imaging assays with manifold probes covering the cascade from top to bottom virtually delineate the network dynamics continuously altering in the course of the disease and its treatment, and therefore expedite the evaluation and optimization of therapeutic strategies intended for suppressing the neurodegenerative pathway over its full length.

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.

Measurement of glycine binding site ofN-methyl-d-asparate receptors in living human brain using 4-acetoxy derivative of L-703,717, 4-acetoxy-7-chloro-3-[3-(4-[11c] methoxybenzyl) phenyl]-2(1H)-quinolone (AcL703) with positron emission tomography

N-methyl-D-aspartate (NMDA) receptors are of major interest in brain functions and neuropsychiatric disorders. However, at present there are few suitable radioligands for in vivo imaging of NMDA receptors. 7-choloro-4-hydroxy-3-[3-(4-methoxybenzyl) phenyl]-2(1H)-quinolone (L-703,717) is one of the potent ligands for the glycine-binding site of NMDA receptors. 4-Acetoxy derivative of L-703,717 (AcL703) is a candidate, as a positron emission tomography (PET) ligand for NMDA receptors, because of its better permeability at the blood-brain barrier compared with L-703,717. After intravenous injection of 624-851 MBq of [11C]AcL703, dynamic PET scan was performed on six healthy males for 90 min. Regions-of-interest were located on the cerebral cortices, cerebellar cortex, and cerebral white matter. The binding potential (BP) was calculated from the ratio of the area under the curve (AUC) of radioactivities from 40 to 90 min in the target region to that in white matter. Regional radioactivities reached close to equilibrium in all regions after about 40 min postinjection. Regional brain uptake of [11C]AcL703 at 40 min after injection was 0.00028-0.00065% of the injected dose/milliliter. Radioactivity concentration of [11C]AcL703 was highest in the cerebellar cortex and lowest in white matter. AUC in the cerebellar cortex was higher than those of cerebral cortices, thalamus, striatum, and white matter. BP in the cerebellar cortex was twofold higher than in the cerebral cortices (cerebellar cortex: BP=2.20+/-0.72; cerebral cortices: BP=1.05+/-0.45). Despite the low brain uptake of [11C]AcL703, regional distributions were in good agreement with our previous studies of rodents. This indicates the possibility of in vivo evaluation of NMDA receptors using PET with [11C]AcL703 in living human brain.

Radiosynthesis and pharmacological evaluation of [11C]EMD-95885: a high affinity ligand for NR2B-containing NMDA receptors

EMD-95885, 6-[3-[4-(4-fluorobenzyl)piperidino]propionyl]-3H-benzoxazol-2-one (1) has been described as a selective antagonist for the NMDA receptors containing NR2B subunits, displaying an IC50 of 3.9 nM for this subtype. EMD-95885 (1) has been synthesized in good overall yield and labelled with carbon-11 ( T1/2 : 20.4 min) at its benzoxazolinone moiety using [11C]phosgene. The pharmacological profile of [11C]EMD-95885 ([11C]-1) was evaluated in vivo in rats with biodistribution studies and brain radioactivity monitored with intracerebral radiosensitive beta-microprobes. The brain uptake of [11C]-1 was homogeneous (0.4-0.6%ID/mL) across the different brain structures studied. This in vivo brain regional distribution of [11C]-1 was not consistent with the known distribution of NR2B subunits. Also as a measure of specificity the hippocampus/cerebellum ratio reached 0.8 throughout the time course of the experiment supporting the lack of specificity. Competition studies with the NR2B prototypic ligand ifenprodil and EMD-95885 (1), 30 min before the radioligand injection, displayed homogeneous reduction of [11C]-1 uptake of 40-60%. Pre-treatment of rats with DTG (sigma ligand), MDL105519 (glycine site antagonist) and MK801 (ion channel blocker) had no inhibitory effect on [11C]-1 uptake. Use of haloperidol as a blocking drug also resulted in a homogeneous inhibition of [11C]-1 uptake by 66-60%, which does not reflect binding to dopamine or sigma receptors. Due to the homogeneous radioligand uptake and inhibition and no measure of cerebral blood flow effects during these blocking studies it is uncertain whether any specific binding is observed. In view of these results, [11C]EMD-95885 ([11C]-1) does not have the required properties for imaging NR2B containing NMDA receptors using positron emission tomography.

Synthesis, radiosynthesis and In vivo evaluation of 5-[3-(4-Benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-[11C]one, as a potent NR1A/2B subtype selective NMDA PET radiotracer

Recently, a new series of potent and highly subtype-selective 1-(heteroarylalkynyl)-4-benzylpiperidine antagonists of the NMDA receptors has been described by Pfizer Laboratories. In this series, 5-[3-(4-benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-one (1) was identified as a selective antagonist for the NR1(A)/2B subtype, displaying IC(50) values for inhibition of the NMDA responses of 5.3 nM for this subtype (compared to NR1(A)/2A: 35 microM and NR1(A)/2C>100 microM) and was active in rat at a relatively low dosage (10mg/kg po). Derivative 1 has been synthesized in four chemical steps in good overall yield and labelled with carbon-11 at its benzoimidazolone ring using [(11)C]phosgene. The pharmacological profile of [(11)C]-1 was evaluated in vivo in rats with biodistribution studies and brain radioactivity monitored with intracerebral radiosensitive beta-microprobes. The brain uptake of [(11)C]-1 was extremely low (0.07% I.D./mL on average at 30 min) and rather uniform across the different brain structures. This in vivo brain regional distribution of [(11)C]-1 did not match with autoradiographic or binding data obtained with other NR2B subtype-selective NMDA ligands. Competition studies with ifenprodil (20 mg/kg, ip, 30 min before the radiotracer injection) failed to demonstrate specific binding of the radiotracer in the brain. In view of these results, and especially considering the low brain penetration of the radiotracer, [(11)C]-1 does not have the required properties for imaging NMDA receptors using positron emission tomography.

In vivo evaluation of [11C]-3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ([11C]3MPICA) as a PET radiotracer for the glycine site of the NMDA ion channel

Alterations in normal NMDA receptor composition, densities and function have been implicated in the pathophysiology of certain neurological and neuropsychiatric disorders such as Parkinson's Disease, Huntington's Chorea, schizophrenia, alcoholism and stroke. In our first effort to provide PET ligands for the NMDA/glycine site, we reported the synthesis of a novel high affinity glycine site ligand, 3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ((3MPICA), Ki = 4.8 +/- 0.9 nM) and the corresponding carbon-11 labeled PET ligand, [11C]3MPICA. We report here the in vivo evaluation of [11C]3MPICA in rats. Biodistribution analysis revealed that [11C]3MPICA exhibited low degree of brain penetration and high blood concentration. The average uptake at two minutes was highest in the cerebellum (0.19 +/- 0.04 %ID/g) and thalamus (0.18 +/- 0.05 %ID/g) and lower in the hippocampus (0.13 +/- 0.03) and frontal cortex (0.11 +/- 0.04 %ID/g). The radioactivity cleared quickly from all brain regions examined. Administration of unlabeled 3MPICA (1 mg/kg, i.v.) revealed at 60 minutes a small general reduction in regional brain radioactivity concentrations in treated animals versus controls, however, the blood radioactivity concentration was also lowered, confounding the assessment of the degree of saturable binding. Warfarin co-administration (100 mg/kg, i.v.) significantly lowered blood activity at 5 minutes post-injection (-27%, P < 0.01) but failed to significantly increase the brain uptake of the radiotracer. In view of these results, and especially considering the low brain penetration of this tracer, [11C]3MPICA does not appear to be a promising PET radiotracer for in vivo use.

A positron-emitter labeled glycine(B) site antagonist, [(11)C]L-703,717, preferentially binds to a cerebellar NMDA receptor subtype consisting of GluR epsilon3 subunit in vivo, but not in vitro

In previous studies, we have found that [(11)C]L-703,717, a positron-emitter labeled antagonist for the glycine-binding site of NMDA receptors, only localizes in rodent cerebellum under in vivo conditions. In order to understand the unusual cerebellar localization, we have examined the binding of [(11)C]L-703,717 to a cerebellar-specific NMDA receptor subtype consisting of GLuRepsilon3 subunit, by comparing its autoradiographic distributions between GluRepsilon3-deficient and wild-type mice. Ex vivo [(11)C]L-703,717 binding to wild-type mice showed a highly specific localization of radioactivity in the cerebellum, whereas that to the GluRepsilon3-deficient mice showed no specific localization of radioactivity in any of the brain regions. In contrast to the ex vivo binding, in vitro [(11)C]L-703,717 binding displayed a similar binding characteristic between GluRepsilon3-deficient and wild-type mice with highly specific localizations in the hippocampus and cerebral cortex. Therefore, the present study clearly demonstrated that [(11)C]L-703,717 preferentially binds to a cerebellar NMDA receptor subtype consisting of GluRepsilon3 subunit in vivo, but not in vitro.