Fluorine-18 radiolabelling, biodistribution studies and preliminary PET evaluation of a new memantine derivative for imaging the NMDA receptor

Targeting N-Methyl-d-Aspartate Receptors in Neurodegenerative Diseases

N-methyl-d-aspartate receptors (NMDARs) are the main class of ionotropic receptors for the excitatory neurotransmitter glutamate. They play a crucial role in the permeability of Ca2+ ions and excitatory neurotransmission in the brain. Being heteromeric receptors, they are composed of several subunits, including two obligatory GluN1 subunits (eight splice variants) and regulatory GluN2 (GluN2A~D) or GluN3 (GluN3A~B) subunits. Widely distributed in the brain, they regulate other neurotransmission systems and are therefore involved in essential functions such as synaptic transmission, learning and memory, plasticity, and excitotoxicity. The present review will detail the structure, composition, and localization of NMDARs, their role and regulation at the glutamatergic synapse, and their impact on cognitive processes and in neurodegenerative diseases (Alzheimer's, Huntington's, and Parkinson's disease). The pharmacology of different NMDAR antagonists and their therapeutic potentialities will be presented. In particular, a focus will be given on fluoroethylnormemantine (FENM), an investigational drug with very promising development as a neuroprotective agent in Alzheimer's disease, in complement to its reported efficacy as a tomography radiotracer for NMDARs and an anxiolytic drug in post-traumatic stress disorder.

Fluoroethylnormemantine (FENM) shows synergistic protection in combination with a sigma-1 receptor agonist in a mouse model of Alzheimer's disease

Fluoroethylnormemantine (FENM) is a Memantine derivative with anti-amnesic and neuroprotective activities showed in the Aβ25-35 pharmacological mouse model of Alzheimer's disease (AD). As AD is a complex multi-factorial neurodegenerative pathology, combination therapies relying on drugs acting through different pathways, have been suggested to more adequately address neuroprotection. As several agonists of the sigma-1 receptor (S1R), an intracellular chaperone, are presently in phase 2 or 3 clinical trials in neurodegenetrative diseases including AD, we examined the potentialities of S1R drug-based combinations with FENM, or Memantine. Aβ25-35-treated mice were treated with S1R agonists (PRE-084, Igmesine, Cutamesine) and/or FENM, or Memantine, during 7 days after intracerebroventricular administration of oligomerized Aβ25-35. Mice were then tested for spatial short-term memory on day 8 and non-spatial long-term memory on days 9-10, using the spontaneous alternation or passive avoidance tests, respectively. The FENM or Memantine combination with Donepezil, that non-selectively inhibits acetylcholinesterase and activates S1R, was also tested. The efficacy of combinations using maximal non-active or minimal active doses of S1R agonist or FENM was analyzed using calculations of the combination index, based on simple isobologram representation. Data showed that most of the FENM-based combinations led to synergistic protection against Aβ25-35-induced learning deficits, for both long- and short-term memory responses, with a higher efficiency on the latter. Memantine led to synergistic combination in short-term memory but poorly in long-term memory responses, with either PRE-084 or Donepezil. These study showed that drug combinations based on FENM and S1R agonists may lead to highly effective and synergistic protection in AD, particularly on short-term memory.

A Review of Molecular Imaging of Glutamate Receptors

Molecular imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) is a well-established and important in vivo technique to evaluate fundamental biological processes and unravel the role of neurotransmitter receptors in various neuropsychiatric disorders. Specific ligands are available for PET/SPECT studies of dopamine, serotonin, and opiate receptors, but corresponding development of radiotracers for receptors of glutamate, the main excitatory neurotransmitter in mammalian brain, has lagged behind. This state of affairs has persisted despite the central importance of glutamate neurotransmission in brain physiology and in disorders such as stroke, epilepsy, schizophrenia, and neurodegenerative diseases. Recent years have seen extensive efforts to develop useful ligands for molecular imaging of subtypes of the ionotropic (N-methyl-D-aspartate (NMDA), kainate, and AMPA/quisqualate receptors) and metabotropic glutamate receptors (types I, II, and III mGluRs). We now review the state of development of radioligands for glutamate receptor imaging, placing main emphasis on the suitability of available ligands for reliable in vivo applications. We give a brief account of the radiosynthetic approach for selected molecules. In general, with the exception of ligands for the GluN2B subunit of NMDA receptors, there has been little success in developing radiotracers for imaging ionotropic glutamate receptors; failure of ligands for the PCP/MK801 binding site in vivo doubtless relates their dependence on the open, unblocked state of the ion channel. Many AMPA and kainite receptor ligands with good binding properties in vitro have failed to give measurable specific binding in the living brain. This may reflect the challenge of developing brain-penetrating ligands for amino acid receptors, compounded by conformational differences in vivo. The situation is better with respect to mGluR imaging, particularly for the mGluR5 subtype. Several successful PET ligands serve for investigations of mGluRs in conditions such as schizophrenia, depression, substance abuse and aging. Considering the centrality and diversity of glutamatergic signaling in brain function, we have relatively few selective and sensitive tools for molecular imaging of ionotropic and metabotropic glutamate receptors. Further radiopharmaceutical research targeting specific subtypes and subunits of the glutamate receptors may yet open up new investigational vistas with broad applications in basic and clinical research.

Anti-Amnesic and Neuroprotective Effects of Fluoroethylnormemantine in a Pharmacological Mouse Model of Alzheimer's Disease

BACKGROUND

Current therapies in Alzheimer's disease (AD), including Memantine, have proven to be only symptomatic but not curative or disease modifying. Fluoroethylnormemantine (FENM) is a structural analogue of Memantine, functionalized with a fluorine group that allowed its use as a positron emission tomography tracer. We here analyzed FENM neuroprotective potential in a pharmacological model of AD compared with Memantine.

METHODS

Swiss mice were treated intracerebroventricularly with aggregated Aβ 25-35 peptide and examined after 1 week in a battery of memory tests (spontaneous alternation, passive avoidance, object recognition, place learning in the water-maze, topographic memory in the Hamlet). Toxicity induced in the mouse hippocampus or cortex was analyzed biochemically or morphologically.

RESULTS

Both Memantine and FENM showed symptomatic anti-amnesic effects in Aβ 25-35-treated mice. Interestingly, FENM was not amnesic when tested alone at 10 mg/kg, contrarily to Memantine. Drugs injected once per day prevented Aβ 25-35-induced memory deficits, oxidative stress (lipid peroxidation, cytochrome c release), inflammation (interleukin-6, tumor necrosis factor-α increases; glial fibrillary acidic protein and Iba1 immunoreactivity in the hippocampus and cortex), and apoptosis and cell loss (Bcl-2-associated X/B-cell lymphoma 2 ratio; cell loss in the hippocampus CA1 area). However, FENM effects were more robust than observed with Memantine, with significant attenuations vs the Aβ 25-35-treated group.

CONCLUSIONS

FENM therefore appeared as a potent neuroprotective drug in an AD model, with a superior efficacy compared with Memantine and an absence of direct amnesic effect at higher doses. These results open the possibility to use the compound at more relevant dosages than those actually proposed in Memantine treatment for AD.

NMDA Receptor Antagonists: Repositioning of Memantine as a Multitargeting Agent for Alzheimer's Therapy

Alzheimer's disease (AD) is a progressive neurodegenerative disease that causes problems with memory, thinking, and behavior. Currently, there is no drug that can reduce the pathological events of this degenerative disease but symptomatic relief is possible that can abate the disease condition. N-methyl-D-aspartate (NMDA) receptors exert a critical role for synaptic plasticity as well as transmission. Overstimulation of glutamate receptors, predominantly NMDA type, may cause excitotoxic effects on neurons and is recommended as a mechanism for neurodegeneration. Atypical activation of the NMDA receptor has been suggested for AD by synaptic dysfunction. NMDA receptor antagonists especially memantine block the NMDA receptor and can reduce the influx of calcium (Ca2+) ions into neuron, thus, toxic intracellular events are not activated. This review represents the role of NMDA receptors antagonists as potential therapeutic agents to reduce AD. Moreover, this review highlights the repositioning of memantine as a potential novel therapeutic multitargeting agent for AD.

Innovative Molecular Imaging for Clinical Research, Therapeutic Stratification, and Nosography in Neuroscience

Over the past few decades, several radiotracers have been developed for neuroimaging applications, especially in PET. Because of their low steric hindrance, PET radionuclides can be used to label molecules that are small enough to cross the blood brain barrier, without modifying their biological properties. As the use of 11C is limited by its short physical half-life (20 min), there has been an increasing focus on developing tracers labeled with 18F for clinical use. The first such tracers allowed cerebral blood flow and glucose metabolism to be measured, and the development of molecular imaging has since enabled to focus more closely on specific targets such as receptors, neurotransmitter transporters, and other proteins. Hence, PET and SPECT biomarkers have become indispensable for innovative clinical research. Currently, the treatment options for a number of pathologies, notably neurodegenerative diseases, remain only supportive and symptomatic. Treatments that slow down or reverse disease progression are therefore the subject of numerous studies, in which molecular imaging is proving to be a powerful tool. PET and SPECT biomarkers already make it possible to diagnose several neurological diseases in vivo and at preclinical stages, yielding topographic, and quantitative data about the target. As a result, they can be used for assessing patients' eligibility for new treatments, or for treatment follow-up. The aim of the present review was to map major innovative radiotracers used in neuroscience, and explain their contribution to clinical research. We categorized them according to their target: dopaminergic, cholinergic or serotoninergic systems, β-amyloid plaques, tau protein, neuroinflammation, glutamate or GABA receptors, or α-synuclein. Most neurological disorders, and indeed mental disorders, involve the dysfunction of one or more of these targets. Combinations of molecular imaging biomarkers can afford us a better understanding of the mechanisms underlying disease development over time, and contribute to early detection/screening, diagnosis, therapy delivery/monitoring, and treatment follow-up in both research and clinical settings.

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.

Radiolabeling of [18F]-fluoroethylnormemantine and initial in vivo evaluation of this innovative PET tracer for imaging the PCP sites of NMDA receptors

INTRODUCTION

The N-methyl-D-aspartate receptor (NMDAr) is an ionotropic receptor that mediates excitatory transmission. NMDAr overexcitation is thought to be involved in neurological and neuropsychiatric disorders such as Alzheimer disease and schizophrenia. We synthesized [(18)F]-fluoroethylnormemantine ([(18)F]-FNM), a memantine derivative that binds to phencyclidine (PCP) sites within the NMDA channel pore. These sites are primarily accessible when the channel is in the active and open state.

METHODS

Radiosynthesis was carried out using the Raytest® SynChrom R&D fluorination module. Affinity of this new compound was determined by competition assay. We ran a kinetic study in rats and computed a time-activity curve based on a volume-of-interest analysis, using CARIMAS® software. We performed an ex vivo autoradiography, exposing frozen rat brain sections to a phosphorscreen. Adjacent sections were used to detect NMDAr by immunohistochemistry with an anti-NR1 antibody. As a control of the specificity of our compound for NMDAr, we used a rat anesthetized with ketamine. Correlation analysis was performed with ImageJ software between signal of autoradiography and immunostaining.

RESULTS

Fluorination yield was 10.5% (end of synthesis), with a mean activity of 3145 MBq and a specific activity above 355 GBq/μmol. Affinity assessment allowed us to determine [(19)F]-FNM IC50 at 6.1 10(-6)M. [(18)F]-FMN concentration gradually increased in the brain, stabilizing at 40 minutes post injection. The brain-to-blood ratio was 6, and 0.4% of the injected dose was found in the brain. Combined ex vivo autoradiography and immunohistochemical staining demonstrated colocalization of NMDAr and [(18)F]-FNM (r=0.622, p<0.0001). The highest intensity was found in the cortex and cerebellum, and the lowest in white matter. A low and homogeneous signal corresponding to unspecific binding was observed when PCP sites were blocked with ketamine.

CONCLUSIONS

[(18)F]-FNM appears to be a promising tracer for imaging NMDAr activity for undertaking preclinical studies in perspective of clinical detection of neurological or neuropsychological disorders.

PET and SPECT tracers for glutamate receptors

Radioligands for PET imaging of glutamate receptors will have the potential for studying neurological and neuropsychiatric disorders and their diagnosis and therapeutic intervention. Glutamate is the major excitatory neurotransmitter in the brain and is implicated in the pathophysiology of many neurodegenerative and neuropsychiatric disorders. Glutamate and its receptors are potential targets in the treatment of these disorders. Glutamate signaling is mediated through ionotropic and metabotropic receptors. The abundant concentration of these receptors can facilitate their in vivo quantification using positron emission tomography (PET). Glutamate receptors are a potentially important set of targets for monitoring disease progression, for evaluating the effect of therapy and for new treatment development based on the quantification of receptor occupancy. Here, we review the PET and single-photon emission computed tomography (SPECT) radioligands that have been developed for imaging glutamate receptors in living brain.

Metabotropic glutamate receptor modulation, translational methods, and biomarkers: relationships with anxiety

RATIONALE

The increasing awareness of the need to align clinical and preclinical research to facilitate rapid development of new drug therapies is reflected in the recent introduction of the term "translational medicine". This review examines the implications of translational medicine for psychiatric disorders, focusing on metabotropic glutamate (mGlu) receptor biology in anxiety disorders and on anxiety-related biomarkers.

OBJECTIVES

This review aims to (1) examine recent progress in translational medicine, emphasizing the role that translational research has played in understanding of the potential of mGlu receptor agonists and antagonists as anxiolytics, (2) identify lacunas where animal and human research have yet to be connected, and (3) suggest areas where translational research can be further developed.

RESULTS

Current data show that animal and human mGlu(5) binding can be directly compared in experiments using the PET ligand (11)C-ABP688. Testing of the mGlu(2/3) receptor agonist LY354740 in the fear-potentiated startle paradigm allows direct functional comparisons between animals and humans. LY354740 has been tested in panic models, but in different models in rats and humans, hindering efforts at translation. Other potentially translatable methods, such as stress-induced hyperthermia and HPA-axis measures, either have been underexploited or are associated with technical difficulties. New techniques such as quantitative trait loci (QTL) analysis may be useful for generating novel biomarkers of anxiety.

CONCLUSIONS

Translational medicine approaches can be valuable to the development of anxiolytics, but the amount of cross-fertilization between clinical and pre-clinical departments will need to be expanded to realize the full potential of these approaches.

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.

In vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine as a potential radiotracer for PET assessment of CNS sigma-1 receptors

INTRODUCTION

Sigma-1 receptors are expressed throughout the mammalian central nervous system (CNS) and are implicated in several psychiatric disorders, including schizophrenia and depression. We have recently evaluated the high-affinity (K(D)=0.5+/-0.2 nM, log P=2.9) sigma-1 receptor radiotracer [(18)F]1-(3-fluoropropyl)-4-(4-cyanophenoxymethyl)piperidine, [(18)F]FPS, in humans. In contrast to appropriate kinetics exhibited in baboon brain, in the human CNS, [(18)F]FPS does not reach pseudoequilibrium by 4 h, supporting the development of a lower-affinity tracer [Waterhouse RN, Nobler MS, Chang RC, Zhou Y, Morales O, Kuwabara H, et al. First evaluation of the sigma-1 receptor radioligand [(18)F]1-3-fluoropropyl-4-((4-cyanophenoxy)-methyl)piperidine ([(18)F]FPS) in healthy humans. Neuroreceptor Mapping 2004, July 15-18th, Vancouver, BC Canada 2004]. We describe herein the in vivo evaluation in rats of [(18)F]1-(2-fluoroethyl)-4-[(4-cyanophenoxy)methyl]piperidine ([(18)F]SFE) (K(D)=5 nM, log P=2.4), a structurally similar, lower-affinity sigma-1 receptor radioligand.

METHODS

[(18)F]SFE was synthesized (n=4) as previously described in good yield (54+/-6% EOB), high specific activity (2.1+/-0.6 Ci/micromol EOS) and radiochemical purity (98+/-1%) and evaluated in awake adult male rats.

RESULTS

Similar to [(18)F]FPS, regional brain radioactivity concentrations [percentage of injected dose per gram of tissue (%ID/g), 15 min] for [(18)F]SFE were highest in occipital cortex (1.86+/-0.06 %ID/g) and frontal cortex (1.76+/-0.38 %ID/g), and lowest in the hippocampus (1.01+/-0.02%ID/g). Unlike [(18)F]FPS, [(18)F]SFE cleared from the brain with approximately 40% reduction in peak activity over a 90-min period. Metabolite analysis (1 h) revealed that [(18)F]SFE was largely intact in the brain. Blocking studies showed a large degree (>80%) of saturable binding for [(18)F]SFE in discrete brain regions.

CONCLUSIONS

We conclude that [(18)F]SFE exhibits excellent characteristics in vivo and may provide a superior PET radiotracer for human studies due to its faster CNS clearance compared to [(18)F]FPS.

What to learn from in vivo opioidergic brain imaging?

Ligand-PET studies are attracting increasing interest in experimental and clinical research. As the most elaborated of PET techniques, ligand-PET allows the demonstration of receptor distributions, and thus, the delineation of neurochemical pathologies in the disease state. Recent developments are promising that ligand-PET will even allow to characterize dynamic and short-term changes in neurotransmission and will tremendously add to the understanding of neurophysiology on the receptor level. In pain studies, mainly the mu-opioidergic agonist [(11)C]-carfentanil and the unspecific opioid receptor antagonist [(11)C]-diprenorphine are applied. Utilizing these ligands the thalamus, prefrontal and cingulate cortex, basal ganglia and midbrain structures have been shown to possess high amounts of opioidergic receptors in vivo and it is well accepted, that the receptor density is higher in projections of the medial than those of the lateral pain system. Changes in receptor availability were observed in patients suffering from chronic pain. Rheumatoid arthritis, trigeminal neuralgia and central poststroke pain (CPSP) all lead to decreased ligand binding in pain processing regions during the painful period in comparison to pain free intervals or healthy subjects. These decreases may either be the consequence of increased endogenous release or indicate receptor internalization/down-regulation or loss of neurons carrying these receptors. Recent studies also evidenced [(11)C]-carfentanil binding changes due to acute experimental pain. One possible interpretation of these changes is that the PET-ligand might be displaced by endogenous opioidergic ligands. One major region, where this "ligand displacement" was observed, was the thalamus. These findings highlight the importance of the opioidergic system in pain processing and the power of ligand-PET to advance the understanding of pain.

Dementia of Alzheimer's disease and other neurodegenerative disorders--memantine, a new hope

Alzheimer's disease is the fourth largest cause of death for people over 65 years of age. Dementia of Alzheimer's type is the commonest form of dementia, the other two forms being vascular dementia and mixed dementia. At present, the therapy of Alzheimer's disease is aimed at improving both, cognitive and behavioural symptoms and thereby, quality of life for the patients. Since the discovery of Alzheimer's disease by Alois Alzheimer, many pathological mechanisms have been proposed which led to the testing of various new treatments. Until recently the available drugs for the treatment of Alzheimer's disease are cholinesterase inhibitors, which have limited success because these drugs improve cognitive functions only in mild dementia and cannot stop the process of neurodegeneration. Moreover, drugs of this category show gastrointestinal side effects. As the cells of central and peripheral nervous system cannot regenerate, newer strategies are aimed at preserving the surviving neurons by preventing their degeneration. NMDA-receptor-mediated glutamate excitotoxicity plays a major role in Abeta-induced neuronal death. Hence, it was thought that NMDA receptors could be a promising target for preventing the progression of Alzheimer's disease. All the compounds synthesized initially in this category showed toxicity mainly because of their high affinity for NMDA receptors. Memantine (1-amino adamantane derivative), NMDA-receptor antagonist was reported to be effective therapeutically in Alzheimer's disease. It was available in Germany as well as European Union and has been approved for moderate to severe dementia in United States of America recently. It is an uncompetitive, moderate affinity antagonist of NMDA receptors that inhibits the pathological functions of NMDA receptors while physiological processes in learning and memory are unaffected. Memantine is also reported to have beneficial effects in other CNS disorders viz., Parkinson's disease (PD), stroke, epilepsy, CNS trauma, amyotrophic lateral sclerosis (ALS), drug dependence and chronic pain. Mechanisms of neuroprotection, preclinical and clinical evidence for effectiveness of memantine have been provided. Pharmacology and pharmacokinetics of memantine and other NMDA-receptor antagonists in comparison with currently approved drugs for dementia treatment have been discussed. The focus is on 'glutamate excitotoxicity' and glutamate receptors as drug target. Various other novel strategies for the treatment of dementia of neurodegenerative disorders have also been discussed.

In vivo evaluation of [11C]N-(2-chloro-5-thiomethylphenyl)-N′- (3-methoxy-phenyl)-N′-methylguanidine ([11C]GMOM) as a potential PET radiotracer for the PCP/NMDA receptor

The development of imaging methods to measure changes in NMDA ion channel activation would provide a powerful means to probe the mechanisms of drugs and device based treatments (e.g., ECT) thought to alter glutamate neurotransmission. To provide a potential NMDA/PCP receptor PET tracer, we synthesized the radioligand [11C]GMOM (ki = 5.2 +/-0.3 nM; log P = 2.34) and evaluated this ligand in vivo in awake male rats and isoflurane anesthetized baboons. In rats, the regional brain uptake of [11C]GMOM ranged from 0.75+/-0.13% ID/g in the medulla and pons to 1.15+/-0.17% ID/g in the occipital cortex. MK801 (1 mg/kg i.v.) significantly reduced (24-28%) [11C]GMOM uptake in all regions. D-serine (10 mg/kg i.v.) increased [11C]GMOM %ID/g values in all regions (10-24%) reaching significance in the frontal cortex and cerebellum only. The NR2B ligand RO 25-6981 (10 mg/kg i.v.) reduced [11C]GMOM uptake significantly (24-38%) in all regions except for the cerebellum and striatum. Blood activity was 0.11+/-0.03 %ID/g in the controls group and did not vary significantly across groups. PET imaging in isoflurane-anesthetized baboons with high specific activity [11C]GMOM provided fairly uniform regional brain distribution volume (VT) values (12.8-17.1 ml g(-1)). MK801 (0.5 mg/kg, i.v., n = 1, and 1.0 mg/kg, i.v., n = 1) did not significantly alter regional VT values, indicating a lack of saturable binding. However, the potential confounding effects associated with ketamine induction of anesthesia along with isoflurane maintenance must be considered because both agents are known to reduce NMDA ion channel activation. Future and carefully designed studies, presumably utilizing an optimized NMDA/PCP site tracer, will be carried out to further explore these hypotheses. We conclude that, even though [11C]GMOM is not an optimized PCP site radiotracer, its binding is altered in vivo in awake rats as expected by modulation of NMDA ion channel activity by MK801, D-serine or RO 25-6981. The development of higher affinity NMDA/PCP site radioligands is in progress.

Emerging experimental therapeutics for bipolar disorder: clues from the molecular pathophysiology

Bipolar affective disorder (manic-depressive illness) is a common, severe, chronic, and often life-threatening illness, associated with significant comorbidity. The recognition of the significant morbidity and mortality of patients with bipolar disorder, as well as the growing appreciation that a high percentage of patients respond poorly to existing treatments, has made the task of discovering new therapeutic agents, that are both efficacious and have few side effects increasingly more important. Most recent agents introduced into the pharmacopeia for the treatment of bipolar disorder have been anticonvulsants and atypical antipsychotics. We propose that novel treatments developed specifically for bipolar disorder will arise from (1) understanding more precisely the molecular mechanisms of treatments that are clearly efficacious or (2) developing medications based on the knowledge obtained of the underlying pathophysiology of bipolar disorder. Knowledge with regard to the underlying pathophysiology of bipolar disorder is increasing at a rapid pace, including alterations in intracellular signaling cascades as well as impairments of cellular plasticity and resilience in critical neuronal circuits. We propose that therapeutics designed to enhance cellular plasticity and resilience and that counter maladaptive stress-responsive systems may have considerable utility for the treatment of bipolar disorder. Therapeutic strategies designed to address cellular resilience and plasticity include the regulation of neurotrophic pathways, glucocorticoid signaling, phosphodiesterase activity, and glutamatergic throughput and mitochondrial function. While the task of developing novel medications for bipolar disorder is truly daunting, these and similar approaches will ultimately lead to better medications for the millions who suffer from this devastating illness.

Regulation of cellular plasticity cascades in the pathophysiology and treatment of mood disorders: role of the glutamatergic system

There is increasing evidence from a variety of sources that mood disorders are associated with regional reductions in brain volume, as well as reductions in the number, size, and density of glia and neurons in discrete brain areas. Although the precise pathophysiology underlying these morphometric changes remains to be fully elucidated, the data suggest that severe mood disorders are associated with impairments of structural plasticity and cellular resilience. In this context, it is noteworthy that a growing body of data suggests that the glutamatergic system--which is known to play a major role in neuronal plasticity and cellular resilience--may be involved in the pathophysiology and treatment of mood disorders. Preclinical studies have shown that the glutamatergic system represents targets (often indirect) for the actions of antidepressants and mood stabilizers. There are a number of glutamatergic "plasticity enhancing" strategies that may be of considerable utility in the treatment of mood disorders. Among the most immediate ones are NMDA antagonists, inhibitors of glutamate-release agents, and AMPA potentiators; this research progress holds much promise for the development of novel therapeutics for the treatment of severe, refractory mood disorders.

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.

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 evaluation of N,N'-diphenyl and N-naphthyl-N'-phenylguanidines as N-methyl-D-aspartate receptor ion-channel ligands

A series of N,N'-diphenyl and N-naphthyl-N'-phenyl guanidine derivatives was synthesized as potential N-methyl-D-aspartate (NMDA) receptor positron emission tomography (PET) ligands. The affinity of the different compounds was determined using in vitro receptor binding assays, and their log P values were estimated using HPLC analysis. The effect of N'-3 and N'-3,5 substitution on affinity and lipophilicity was examined. The K(i) values ranged from 1.87 to 839nM, while log P values between 1.22 and 2.88 were observed.

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.

PET in psychopharmacology

Emission tomography techniques and, in particular, positron emission tomography (PET) enable the in vivo study of several physiological and neurochemical variables in human subjects using methods originally developed for quantitative autoradiography. In particular, PET allows one to evaluate in human subjects: (a) the effect of specific neurochemical challenges on regional brain function at rest or under activation; (b) the activity of neurotransmitters and the regional expression of specific molecular targets during pathology including their modulation by drug treatment; (c) the kinetics of drug disposition and activity directly in the target organ. This is of primary interest in the field of biological psychiatry and in psychoactive drugs development, where it is particularly difficult to reproduce human diseases using animal models in view of the peculiarity of this field and the large heterogeneity of each psychiatric illness also inside the same clinical definition. The aim of this paper is to review the principal strategies and the main results of the use of PET in psychopharmacology.

Neuroimaging in bipolar disorder: what have we learned?

New technologies are offering increasingly powerful means to obtain structural, chemical, and functional images of the brain during life, often without the use of ionizing radiation. Bipolar disorder, with its clear physiologic features, would appear to be a prime candidate for the application of current brain imaging; however, only a modest number of studies have been reported to date, and most studies have small sample sizes and heterogeneous subject groups. Nonetheless, there are a few consistent findings among these studies, including the following: 1) Structural imaging studies suggest an increased number of white matter hyperintensities in patients with bipolar disorder. These may be lesions unique to bipolar disorder and its treatment, or related to cardiovascular risk factors, which are more common in bipolar patients. Decreased cerebellar size and anomalies of cerebellar blood volume have also been reported. Increased sulcal prominence and enlargement of the lateral and third ventricles are less consistently observed findings. 2) Spectroscopic imaging suggests abnormalities of metabolism of choline-containing compounds in symptomatically ill bipolar patients and, possibly, treatment-induced changes in choline- and myoinositol-containing compounds. Each of these groups of metabolites serves as a component of membrane phospholipids and cellular second-messenger cycles. 3) Metabolic and blood flow studies provide evidence for decreased activity of the prefrontal cortex (PFC) in bipolar patients during depression. It is not clear if these changes are restricted to particular subregions of the PFC, nor if they are reversed with mania. No single pathophysiologic mechanism yet explains these findings, although all might be due to regional alterations in cellular activity and metabolism or changes in cell membrane composition and turnover. The development of imaging technologies has far outpaced their use in bipolar disorder. The promise of future studies is great, with more powerful magnetic resonance scanners, additional ligands for positron emission tomography and single photon emission computed tomography imaging, and improved image generation and processing already available.