First SPET images of glutamate (NMDA) receptor activation in vivo in cerebral ischaemia

Glutamate's Effects on the N-Methyl-D-Aspartate (NMDA) Receptor Ion Channel in Alzheimer's Disease Brain: Challenges for PET Radiotracer Development for Imaging the NMDA Ion Channel

In an effort to further understand the challenges facing in vivo imaging probe development for the N-methyl-D-aspartate (NMDA) receptor ion channel, we have evaluated the effect of glutamate on the Alzheimer's disease (AD) brain. Human post-mortem AD brain slices of the frontal cortex and anterior cingulate were incubated with [3H]MK-801 and adjacent sections were tested for Aβ and Tau. The binding of [3H]MK-801 was measured in the absence and presence of glutamate and glycine. Increased [3H]MK-801 binding in AD brains was observed at baseline and in the presence of glutamate, indicating a significant increase (>100%) in glutamate-induced NMDA ion channel activity in AD brains compared to cognitively normal brains. The glycine effect was lower, suggesting a decrease of the co-agonist effect of glutamate and glycine in the AD brain. Our preliminary findings suggest that the targeting of the NMDA ion channel as well as the glutamate site may be appropriate in the diagnosis and treatment of AD. However, the low baseline levels of [3H]MK-801 binding in the frontal cortex and anterior cingulate in the absence of glutamate and glycine indicate significant hurdles for in vivo imaging probe development and validation.

Automated production of a N-methyl-D-aspartate receptor radioligand [18F]GE179 for clinical use

N-Methyl-d-aspartate (NMDA) receptors are ligand and voltage-gated heteromeric ion channel receptors. Excessive activation of NMDA receptors is implicated in many neurological and psychiatric disorders, including ischemic stroke, neuropathic pain, epilepsy, drug addition, Alzheimer's disease, and schizophrenia. [¹⁸F]GE179 is a promising PET probe for imaging functional NMDA receptor alterations (activated or 'open' channel) with a high binding affinity (K_d = 2.4 nM). Here, we report the production of the NMDA receptor radioligand [¹⁸F]GE179 in a current Good Manufacturing Practice (cGMP) facility through a one-pot two-step strategy. [¹⁸F]GE179 was produced in approximately 110 min with a radiochemical yield of 12 ± 6% (n = 4, decay corrected), radiochemical purity >95%, molar activity of 146 ± 32 GBq/μmol (at the end of synthesis), an average mass of GE179 at 2.2 μg/batch, and total impurities less than 0.5 μg/batch (n = 4). The radiopharmaceutical dose meets all quality control (QC) criteria for human use, and is suitable for clinical PET studies of activated NMDA receptor ion channels.

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.

Peroxisome proliferator-activated receptor γ (PPARγ): A master gatekeeper in CNS injury and repair

Peroxisome proliferator-activated receptor γ (PPARγ) is a widely expressed ligand-modulated transcription factor that governs the expression of genes involved in inflammation, redox equilibrium, trophic factor production, insulin sensitivity, and the metabolism of lipids and glucose. Synthetic PPARγ agonists (e.g. thiazolidinediones) are used to treat Type II diabetes and have the potential to limit the risk of developing brain injuries such as stroke by mitigating the influence of comorbidities. If brain injury develops, PPARγ serves as a master gatekeeper of cytoprotective stress responses, improving the chances of cellular survival and recovery of homeostatic equilibrium. In the acute injury phase, PPARγ directly restricts tissue damage by inhibiting the NFκB pathway to mitigate inflammation and stimulating the Nrf2/ARE axis to neutralize oxidative stress. During the chronic phase of acute brain injuries, PPARγ activation in injured cells culminates in the repair of gray and white matter, preservation of the blood-brain barrier, reconstruction of the neurovascular unit, resolution of inflammation, and long-term functional recovery. Thus, PPARγ lies at the apex of cell fate decisions and exerts profound effects on the chronic progression of acute injury conditions. Here, we review the therapeutic potential of PPARγ in stroke and brain trauma and highlight the novel role of PPARγ in long-term tissue repair. We describe its structure and function and identify the genes that it targets. PPARγ regulation of inflammation, metabolism, cell fate (proliferation/differentiation/maturation/survival), and many other processes also has relevance to other neurological diseases. Therefore, PPARγ is an attractive target for therapies against a number of progressive neurological disorders.

To look beyond vasospasm in aneurysmal subarachnoid haemorrhage

Delayed cerebral vasospasm has classically been considered the most important and treatable cause of mortality and morbidity in patients with aneurysmal subarachnoid hemorrhage (aSAH). Secondary ischemia (or delayed ischemic neurological deficit, DIND) has been shown to be the leading determinant of poor clinical outcome in patients with aSAH surviving the early phase and cerebral vasospasm has been attributed to being primarily responsible. Recently, various clinical trials aimed at treating vasospasm have produced disappointing results. DIND seems to have a multifactorial etiology and vasospasm may simply represent one contributing factor and not the major determinant. Increasing evidence shows that a series of early secondary cerebral insults may occur following aneurysm rupture (the so-called early brain injury). This further aggravates the initial insult and actually determines the functional outcome. A better understanding of these mechanisms and their prevention in the very early phase is needed to improve the prognosis. The aim of this review is to summarize the existing literature on this topic and so to illustrate how the presence of cerebral vasospasm may not necessarily be a prerequisite for DIND development. The various factors determining DIND that worsen functional outcome and prognosis are then discussed.

Controversies and evolving new mechanisms in subarachnoid hemorrhage

Despite decades of study, subarachnoid hemorrhage (SAH) continues to be a serious and significant health problem in the United States and worldwide. The mechanisms contributing to brain injury after SAH remain unclear. Traditionally, most in vivo research has heavily emphasized the basic mechanisms of SAH over the pathophysiological or morphological changes of delayed cerebral vasospasm after SAH. Unfortunately, the results of clinical trials based on this premise have mostly been disappointing, implicating some other pathophysiological factors, independent of vasospasm, as contributors to poor clinical outcomes. Delayed cerebral vasospasm is no longer the only culprit. In this review, we summarize recent data from both experimental and clinical studies of SAH and discuss the vast array of physiological dysfunctions following SAH that ultimately lead to cell death. Based on the progress in neurobiological understanding of SAH, the terms "early brain injury" and "delayed brain injury" are used according to the temporal progression of SAH-induced brain injury. Additionally, a new concept of the vasculo-neuronal-glia triad model for SAH study is highlighted and presents the challenges and opportunities of this model for future SAH applications.

Initial evaluation of 18F-GE-179, a putative PET Tracer for activated N-methyl D-aspartate receptors

N-methyl D-aspartate (NMDA) ion channels play a key role in a wide range of physiologic (e.g., memory and learning tasks) and pathologic processes (e.g., excitotoxicity). To date, suitable PET markers of NMDA ion channel activity have not been available. (18)F-GE-179 is a novel radioligand that selectively binds to the open/active state of the NMDA receptor ion channel, displacing the binding of (3)H-tenocyclidine from the intrachannel binding site with an affinity of 2.4 nM. No significant binding was observed with 10 nM GE-179 at 60 other neuroreceptors, channels, or transporters. We describe the kinetic behavior of the radioligand in vivo in humans.

METHODS

Nine healthy participants (6 men, 3 women; median age, 37 y) each underwent a 90-min PET scan after an intravenous injection of (18)F-GE-179. Continuous arterial blood sampling over the first 15 min was followed by discrete blood sampling over the duration of the scan. Brain radioactivity (KBq/mL) was measured in summation images created from the attenuation- and motion-corrected dynamic images. Metabolite-corrected parent plasma input functions were generated. We assessed the abilities of 1-, 2-, and 3-compartment models to kinetically describe cerebral time-activity curves using 6 bilateral regions of interest. Parametric volume-of-distribution (V(T)) images were generated by voxelwise rank-shaping regularization of exponential spectral analysis (RS-ESA).

RESULTS

A 2-brain-compartment, 4-rate-constant model best described the radioligand's kinetics in normal gray matter of subjects at rest. At 30 min after injection, 37% of plasma radioactivity represented unmetabolized (18)F-GE-179. The highest mean levels of gray matter radioactivity were seen in the putamina and peaked at 7.5 min. A significant positive correlation was observed between K1 and V(T) (Spearman ρ = 0.398; P = 0.003). Between-subject coefficients of variation of V(T) ranged between 12% and 16%. Voxelwise RS-ESA yielded similar V(T)s and coefficients of variation.

CONCLUSION

(18)F-GE-179 exhibits high and rapid brain extraction, with a relatively homogeneous distribution in gray matter and acceptable between-subject variability. Despite its rapid peripheral metabolism, quantification of (18)F-GE-179 VT is feasible both within regions of interest and at the voxel level. The specificity of (18)F-GE-179 binding, however, requires further characterization with in vivo studies using activation and disease models.

Preliminary studies of (99m)Tc-memantine derivatives for NMDA receptor imaging

INTRODUCTION

Novel technetium-labeled ligands, (99m)Tc-NCAM and (99m)Tc-NHAM were developed from the N-methyl-d-aspartate (NMDA) receptor agonist memantine as a lead compound by coupling with N(2)S(2). This study evaluated the binding affinity and specificity of the ligands for the NMDA receptor.

METHODS

Ligand biodistribution and uptake specificity in the brain were investigated in mice. Binding affinity and specificity were determined by radioligand receptor binding assay. Three antagonists were used for competitive binding analysis. In addition, uptake of the complexes into SH-SY5Y nerve cells was evaluated.

RESULTS

The radiochemical purity of (99m)Tc-labeled ligands was more than 95%. Analysis of brain regional uptake showed higher concentration in the frontal lobe and specific uptake in the hippocampus. (99m)Tc-NCAM reached a higher target to nontarget ratio than (99m)Tc-NHAM. The results indicated that (99m)Tc-NCAM bound to a single site on the NMDA receptor with a K(d) of 701.21 nmol/l and a B(max) of 62.47 nmol/mg. Specific inhibitors of the NMDA receptor, ketamine and dizocilpine, but not the dopamine D(2) and 5HT(1A) receptor partial agonist aripiprazole, inhibited specific binding of (99m)Tc-NCAM to the NMDA receptor. Cell physiology experiments showed that NCAM can increase the viability of SH-SY5Y cells after glutamate-induced injury.

CONCLUSIONS

The new radioligand (99m)Tc-NCAM has good affinity for and specific binding to the NMDA receptor, and easily crosses the blood-brain barrier; suggesting that it might be a potentially useful tracer for NMDA receptor expression.

The importance of early brain injury after subarachnoid hemorrhage

Aneurysmal subarachnoid hemorrhage (aSAH) is a medical emergency that accounts for 5% of all stroke cases. Individuals affected are typically in the prime of their lives (mean age 50 years). Approximately 12% of patients die before receiving medical attention, 33% within 48 h and 50% within 30 days of aSAH. Of the survivors 50% suffer from permanent disability with an estimated lifetime cost more than double that of an ischemic stroke. Traditionally, spasm that develops in large cerebral arteries 3-7 days after aneurysm rupture is considered the most important determinant of brain injury and outcome after aSAH. However, recent studies show that prevention of delayed vasospasm does not improve outcome in aSAH patients. This finding has finally brought in focus the influence of early brain injury on outcome of aSAH. A substantial amount of evidence indicates that brain injury begins at the aneurysm rupture, evolves with time and plays an important role in patients' outcome. In this manuscript we review early brain injury after aSAH. Due to the early nature, most of the information on this injury comes from animals and few only from autopsy of patients who died within days after aSAH. Consequently, we began with a review of animal models of early brain injury, next we review the mechanisms of brain injury according to the sequence of their temporal appearance and finally we discuss the failure of clinical translation of therapies successful in animal models of aSAH.

In vivo [(123)I]CNS-1261 binding to D-serine-activated and MK801-blocked NMDA receptors: A storage phosphor imaging study in rats

Disturbances of activity of the glutamatergic neurotransmitter system in the brain are present in many neuropsychiatric disorders. The N-methyl-D-aspartate (NMDA) receptor is the most abundant receptor of the glutamatergic system. In the neurodegenerative events of Alzheimer's disease, excessive activation of NMDA receptors may contribute to neuronal death. Inhibition of NMDA receptor activation may have neuroprotective effects and (semi)quantitative imaging of the activated system may help in the selection of patients for such inhibition therapies. In this study we evaluated [(123)I]CNS-1261 binding in the rat brain. This radiotracer binds in vivo to the MK801 binding site of activated NMDA receptors. To determine the optimal time point for ex vivo assessments after bolus injection [(123)I]CNS-1261 binding in rats, we performed a time course biodistribution study using dissection techniques. [(123)I]CNS-1261 binding was also studied in the rat brain using autoradiography by means of storage phosphor imaging, with prior facilitation of NMDA receptor activation by injection of the potent coagonist D-serine and after blocking of the NMDA receptor binding site by MK801 injection in D-serine pretreated rats. Measurements of [(123)I]CNS-1261 uptake matched the distribution of similar tracers for the MK801 binding site of the NMDA receptor and revealed an optimal time point of 2 h post injection for the assessment of tracer distribution in the rat brain. The blocking experiments indicated specific binding of [(123)I]CNS-1261 to NMDA receptors but also a considerable amount of nonspecific binding. Facilitation of NMDA receptor activation by D-serine did not result in an enhancement of binding of the radiotracer in the NMDA receptor-rich rat hippocampus compared to the untreated group, as measured by autoradiography. In conclusion, our study has shown that [(123)I]CNS-1261 binding is influenced by NMDA receptor availability. However, high nonspecific binding limits quantification and small changes in receptor availability are unlikely to be detected.

In vitro and in vivo characterization of [3H]CNS-5161—A use-dependent ligand for theN-methyl-d-aspartate receptor in rat brain

Glutamate is the major excitatory neurotransmitter in the brain. Glutamate activation of the N-methyl-D-aspartate (NMDA) receptor subtype is thought to mediate important physiological and pathological processes, including memory formation and excitotoxicity. The goal of the present work was to characterize and validate a candidate agent for noninvasive positron emission tomography (PET) imaging of this receptor. [(3)H]-labeled N-[3-(3)H]-methyl-3-(thiomethylphenyl)cyanamide (CNS-5161) was incubated with rat brain homogenates at increasing concentrations, temperatures, and times to establish the binding kinetics and affinity of the ligand in vitro. Nonspecific binding was measured with 100 microM MK-801. The compound was also injected i.v. in rats pretreated with saline, NMDA, MK801, or a combination, and organ and brain regional uptake was assessed at various times after injection by autoradiography or dissection. Blood and brain samples were assayed for metabolites by high-performance liquid chromatography. CNS-5161 binds brain membranes with high affinity (K(d) < 4 nM) and fast association and dissociation kinetics. Specific binding increased in the presence of glutamate and glycine. Intravenous administration in control rats resulted in a heterogeneous brain distribution with hippocampus and cortex > thalamus > striatum > cerebellum, and a cortex/cerebellum ratio of 1.4. Pretreatment with NMDA increased the hippocampus-to-cerebellum ratio to 1.6-1.9 while MK801 abolished this increase, resulting in ratios close to 1. Thus, CNS-5161 binds preferentially to the activated state of the NMDA receptor channel in vitro and in vivo. The high affinity and fast kinetics make it compatible with PET imaging of a carbon-11 labeled CNS-5161.

Radiotracer development in psychiatry

Over the last 20 years a number of radiotracers that target various neurotransmitter systems have been developed for use in imaging studies in psychiatry, but there are many more targets still to be investigated. The development of a radiotracer for clinical positron emission tomography (PET) or single photon emission computed tomography (SPECT) neuroimaging studies can be a complex and lengthy process with few imaging agents successfully progressing into clinical human studies. One of the most challenging aspects in the procedure is the development of a rapid and simple radiosynthesis protocol to obtain the potential radiotracer with adequate specific activity, isolated radiochemical yield and radiochemical purity for human imaging. Once a candidate has been radiolabelled, full characterization of the radiotracer is required before it can be used in clinical human studies. Pre-clinical studies include investigation into the binding distribution, pharmacokinetics, metabolism, toxicology and dosimetry of a radiotracer. There are many points during the development procedure where a potential radiotracer can be rejected. Due to interspecies differences the development of a radiotracer can either go too far with an unsuccessful candidate or can potentially lead to rejection of a candidate too soon. It is only when the radiotracer has been used in humans can we be certain that a radiotracer is a useful imaging agent for clinical research studies. The development of new technologies, such as micro-PET or SPECT can only improve our ability to predict the success of a radiotracer.

Imaging the PCP site of the NMDA ion channel

The N-methyl-D-aspartate (NMDA) ion channel plays a role in neuroprotection, neurodegeneration, long-term potentiation, memory, and cognition. It is implicated in the pathophysiology of several neurological and neuropsychiatric disorders including Parkinson's Disease, Huntington's Chorea, schizophrenia, alcoholism and stroke. The development of effective radiotracers for the study of NMDA receptors is critical for our understanding of their function, and their modulation by endogenous substances or therapeutic drugs. Since the NMDA/PCP receptor lies within the channel, it is a unique target and is theoretically accessible only when the channel is in the active and "open" state, but not when it is in the inactive or "closed" state. The physical location of the NMDA/PCP receptor not only makes it an important imaging target but also complicates the development of suitable PET and SPECT radiotracers for this site. An intimate understanding of the biochemical, pharmacological, physiological and behavioral processes associated with the NMDA ion channel is essential to develop improved imaging agents. This review outlines progress made towards the development of radiolabeled agents for PCP sites of the NMDA ion channel. In addition, the animal and pharmacological models used for in vitro and in vivo assessment of NMDA receptor targeted agents are discussed.

Kinetic modelling of [123I]CNS 1261--a potential SPET tracer for the NMDA receptor

N-(1-napthyl)-N'-(3-[(123)I]-iodophenyl)-N-methylguanidine ([(123)I]CNS 1261) is a novel SPET ligand developed for imaging the NMDA receptor intra-channel MK 801/PCP/ketamine site. Data was acquired in 7 healthy volunteers after bolus injection of [(123)I]CNS 1261. Kinetic modeling showed reversible tracer binding. Arterial and venous time-activity curves overlapped after 90 min. The rank order of binding was: Thalamus > striatum > cortical regions > white matter. This distribution concurs with [(11)C]-ketamine and [(18)F]-memantine PET studies. These data provide a methodological basis for further direct in vivo challenge studies.

Hypotheses from functional neuroimaging studies

Functional neuroimaging, especially positron emission tomography (PET) using various tracers, provided new insights into the pathophysiology of West syndrome in the past decade. Glucose PET studies revealed a unique corticosubcortical circuitry assumed to be involved in the age-dependent generalization of seizure activity leading to symmetric spasms. The findings strongly suggested that cortical abnormalities, mostly consistent with dysplastic lesions or diffuse cortical dysfunction due to an underlying systemic disorder, trigger brain stem nuclei and activate basal ganglia bilaterally. PET is also able to investigate developmental abnormalities of serotonergic and GABAergic neurotransmitter systems in vivo. Involvement of these systems in the pathophysiology of infantile spasms is strongly supported by animal data and can be further elucidated by future PET studies. In addition, the development of new PET tracers (such as neurotracers for imaging NMDA receptors) could help further clarify the role of altered neurotransmission in generation of spasms. This review of the most important functional neuroimaging findings illustrates how human PET and single photon emission computed tomography data help answer basic questions regarding the pathomechanisms involved in this often devastating condition and how these findings might facilitate development of a useful animal model of West syndrome.

Synthesis and in vitro and in vivo evaluation of [11C]methyl-BIII277CL for imaging the PCP-binding site of the NMDA receptor by pet

A new benzomorphane derivative, [11C]methyl-BIII277CL, was evaluated as a potential radiotracer for visualizing the PCP-binding site of the N-methyl-D-aspartate (NMDA) receptor by positron emission tomography (PET). Methyl-BIII277CL was prepared by reacting the desmethyl compound (BIII277CL) with dimethylsulfate. The pharmacological profile of methyl-BIII277CL was determined by in vitro receptor-screening assays. At a concentration of 100 nM, methyl-BIII277CL showed a significant interaction with the PCP-binding site of the NMDA receptor (79% inhibition of specific binding) and the sigma-binding site (46% inhibition). In displacement assays using mice cortical membranes, methyl-BIII277CL displayed a high affinity at the PCP-binding site of the NMDA receptor (Ki = 49 +/- 14 nmol/L) and a 130-fold lower interaction with the sigma1-binding site (Ki = 6.35 +/- 0.26 micromol/L). For saturation experiments and in vivo studies, methyl-BIII277CL was radiolabeled with 11C at the O-position of the desmethyl precursor (BIII277CL) using [11C]methyliodide with a specific activity of 35-70 GBq/micromol at the end of synthesis (EOS). In saturation assays using rat whole brain membranes [11C]methyl-BIII277CL showed a Kd of 6 +/- 1 nmol/L and a Bmax of 670 +/- 154 fmol/mg protein. Biodistribution and PET studies in rats and pigs, however, indicated a lack of specific binding and unfavorable pharmacokinetics. Kinetic modeling using the 1-tissue compartment model demonstrated for [11C]methyl-BIII277CL a low distribution volume (Dv = 0.98 mL/mL(tissue)) and very high values for the kinetic parameters K1 and k2 (K1 = 0.36 mL/mL(tissue)/min and k2 = 0.37min(-1)) in pig cortex. [11C]methyl-BIII277CL, due to the lack of specificity in vivo, may not be a candidate for imaging the PCP-binding site of the NMDA receptor.

PET studies of 18F-memantine in healthy volunteers

Previous studies in mice and PET investigations in a Rhesus monkey showed that the regional uptake of 18F-memantine could be blocked by pharmacological doses of memantine and (+)-MK-801. In the present study, the binding characteristics of 18F-memantine was examined in five healthy volunteers. In humans, 18F-memantine was homogeneously distributed in gray matter i.e. cortex and basal ganglia regions, as well as the cerebellum. No radioactive metabolites were detected in plasma during the time-frame of the PET studies. The uptake of 18F-memantine in receptor-rich regions such as striatum and frontal cortex could be well described by a 1-tissue compartment model. The DV" values of all gray matter regions were similar and ranged from 15 to 20 ml/ml. The white matter showed lower DV" values of 15 +/- 1.4 ml/ml. These results suggest that 18F-memantine distribution in human brain does not reflect the regional NMDA receptor concentration, and therefore, this radioligand is not suitable for the PET imaging of the NMDA receptors.

Synthesis and binding characteristics of N-(1-naphthyl)-N'-(3-[(125)I]-iodophenyl)-N'-methylguanidine ([(125)I]-CNS 1261): a potential SPECT agent for imaging NMDA receptor activation

N-(1-Naphthyl)-N'-(3-[(125)I]-iodophenyl)-N'-methylguanidine ([(125)I]-CNS 1261) was synthesized as a potential radioligand to image N-methyl-D-aspartate (NMDA) receptor activation. [(125)I]-CNS 1261 was prepared by radioiodination of N-(1-naphthyl)-N'-(3-tributylstannylphenyl)-N'-methylguanidine using Na(125)I and peracetic acid. [(125)I]-CNS 1261 uptake in vivo reflected NMDA receptor distribution in normal rat brain, whereas in ischemic rat brain, uptake was markedly increased in areas of NMDA receptor activation. Radiolabeled CNS 1261 appears to be a good candidate for further development as a single photon emission computed tomography tracer in the investigation of NMDA receptor activation in cerebral ischemia.

Evidence for coupling between glucose metabolism and glutamate cycling using FDG PET and 1H magnetic resonance spectroscopy in patients with epilepsy

The purpose of this study was to examine the relation between glucose metabolism and glutamate concentration in the human brain, in both the normal and diseased state. Regional values of glucose metabolism measured with 2-deoxy-2[F-18]fluoro-D-glucose positron emission tomography (FDG PET) studies and single-voxel proton magnetic resonance spectroscopy (1H MRS) measurements of the glutamate/ glutamine/gamma-aminobutyric acid (Glx) tissue concentration were determined in multiple brain regions in 11 patients (5 girls and 6 boys, mean age 7.5 years) with medically intractable partial epilepsy. FDG PET and 1H MRS studies were performed in the interictal state in seven patients and in the ictal/periictal state in four patients. Regions of interest were identified in epileptic cortex (determined by intracranial and/or scalp electroencephalography) and in contralateral normal brain regions. Lower glucose metabolism and lower Glx concentrations were found in the epileptic focus than in the contralateral normal cortex in all seven patients examined in the interictal state, whereas higher glucose metabolism and higher Glx concentrations were observed in the epileptic focus in the four patients who had ictal/periictal studies. Significant correlations were found between the values of cerebral glucose utilization and Glx concentration in epileptic brain region, in nonepileptic brain regions, and in epileptic and nonepileptic regions combined. These results demonstrate a significant relation between glucose metabolism and glutamate/glutamine concentration in normal and epileptic cerebral cortex. This relation is maintained in both the interictal and ictal states.

Nuclear medicine in neurology and psychiatry

Progress in nuclear medicine has always been a function of technological advances, and applications in neurology and psychiatry illustrate the point. Improvements in radiation detectors now allow for three-dimensional and quantitative mapping of the distribution of a labelled compound in the human brain. New ligands permit the study of specific functioning signals of the blood/brain barrier, blood flow, metabolism (oxygen, glucose, aminoacids), and neurotransmission (dopamine, benzodiazepine, serotonin receptors). The picomolar sensitivity of nuclear medicine can now be coupled to a wide group of ligands which offer specific information that can be obtained in no other way in the living patient.

NMDA-receptor activity visualized with (S)-[N-methyl-11C]ketamine and positron emission tomography in patients with medial temporal lobe epilepsy

PURPOSE

To determine whether neurochemical activation of the N-methyl-D-aspartate (NMDA) receptor-gated ion channel shows quantitative changes, measured as binding of 11C-labeled (S)-[N-methyl]ketamine, in patients with medial temporal lobe epilepsy (MTLE).

METHODS

Eight patients with MTLE who were evaluated regarding epilepsy surgery underwent positron emission tomography (PET) with (S)-[N-methyl-11C]ketamine. The presurgical investigations included magnetic resonance imaging (MRI), PET with 18F-fluoro-deoxyglucose (18FDG), and seizure monitoring by using video-EEG. The uptake of (S)-[N-methyl-11C]ketamine in the temporal lobe of ictal onset was compared with the contralateral side and correlated to changes in regional glucose metabolism measured by PET with 18FDG.

RESULTS

(S)-[N-methyl-11C]ketamine rapidly reached the brain, and high radioactivities were measured in the striatum, thalamic nuclei, and cortical regions. Overall the brain uptake and regional binding potentials of (S)-[N-methyl-11C]ketamine were similar to measurements observed previously in healthy controls. However, 20 min after administration, when blood flow influence was negligible, a side-to-side comparison revealed a 9-34% reduction of tracer radioactivity in the temporal lobes of ictal onset. At earlier times, the differences in binding potentials were less pronounced, 9-21%. The magnitude and distribution of the reduction were similar to the metabolic pattern seen on PET scans with 18FDG.

CONCLUSIONS

Radioactivity uptake of intravenously administered (S)-[N-methyl-11C]ketamine was reduced in temporal lobes of ictal in patients with TLE. This may reflect reduced NMDA-receptor density, reduced perfusion, focal atrophy, or other factors.

123Iodo-MK-801: a spect agent for imaging the pattern and extent of glutamate (NMDA) receptor activation in Alzheimer's disease

Glutamate, and the NMDA glutamate receptor, may be involved in Alzheimer's Disease (AD). Reductions in NMDA receptors are found in AD, possibly contributing to memory deficits. However the NMDA receptor is involved in excitotoxicity, which may play a role in cell death and the production of neurofibrillary tangles in AD; although with lower levels of glutamate than occur in cerebral ischaemia. Therefore reductions in the NMDA receptor may worsen memory deficit in AD, but increased stimulation of the receptor may contribute to the progress of the disease. MK-801 has been used to image excessive glutamate activation following ischaemia in rats. However, it is unclear how effective MK-801 is in conditions with lower levels of glutamate release. This study attempts to gain insight into the utility of the tracer in these conditions, exploring glutamatergic mechanisms in AD. It describes the retention and elimination of 123iodo-MK-801 in five AD and five control subjects, comparing this to regional cerebral blood flow (rCBF). The initial uptake of 123I-MK-801 is dominated by delivery of the ligand. However, despite significant reductions in rCBF in the AD patients, there is no significant difference in the uptake of 123I-MK-801. This suggests increased retention of 123I-MK-801 in the AD patients. In addition the washout of 123I-MK-801 was less in the AD patients, again suggesting increased retention, although this only reached significance in one region. Theses data hint at possible increases in NMDA activation in AD but ultimately 123I-MK-801 does not provide a sufficiently accurate measurement to demonstrate this conclusively. Further NMDA ligands are now at a late stage of development and may provide more conclusive answers to the role of glutamate in AD.