History and Perspectives of A2A Adenosine Receptor Antagonists as Potential Therapeutic Agents

Growing evidence emphasizes that the purine nucleoside adenosine plays an active role as a local regulator in different pathologies. Adenosine is a ubiquitous nucleoside involved in various physiological and pathological functions by stimulating A₁, A_2A, A_2B, and A₃ adenosine receptors (ARs). At the present time, the role of A_2AARs is well known in physiological conditions and in a variety of pathologies, including inflammatory tissue damage and neurodegenerative disorders. In particular, the use of selective A_2A antagonists has been reported to be potentially useful in the treatment of Parkinson's disease (PD). In this review, A_2AAR signal transduction pathways, together with an analysis of the structure–activity relationships of A_2A antagonists, and their corresponding pharmacological roles and therapeutic potential have been presented. The initial results from an emerging polypharmacological approach are also analyzed. This approach is based on the optimization of the affinity and/or functional activity of the examined compounds toward multiple targets, such as A₁/A_2AARs and monoamine oxidase‐B (MAO‐B), both closely implicated in the pathogenesis of PD.     

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A_2A receptors (A_2AR) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A_2A receptors in the basal ganglia. At present it is believed that A_2AR antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A_2AR antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A_2A adenosine receptors. Thus, the development of heteromer-specific A_2A receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.     

Suitability of [18F]Altanserin and PET to Determine 5-HT2A Receptor Availability in the Rat Brain: In Vivo and In Vitro Validation of Invasive and Non-Invasive Kinetic Models

Purpose

While the selective 5-hydroxytryptamine type 2a receptor (5-HT_2AR) radiotracer [¹⁸F]altanserin is well established in humans, the present study evaluated its suitability for quantifying cerebral 5-HT_2ARs with positron emission tomography (PET) in albino rats.

Procedures

Ten Sprague Dawley rats underwent 180 min PET scans with arterial blood sampling. Reference tissue methods were evaluated on the basis of invasive kinetic models with metabolite-corrected arterial input functions. In vivo 5-HT_2AR quantification with PET was validated by in vitro autoradiographic saturation experiments in the same animals.

Result

Overall brain uptake of [¹⁸F]altanserin was reliably quantified by invasive and non-invasive models with the cerebellum as reference region shown by linear correlation of outcome parameters. Unlike in humans, no lipophilic metabolites occurred so that brain activity derived solely from parent compound. PET data correlated very well with in vitro autoradiographic data of the same animals.

Conclusion

[¹⁸F]Altanserin PET is a reliable tool for in vivo quantification of 5-HT_2AR availability in albino rats. Models based on both blood input and reference tissue describe radiotracer kinetics adequately. Low cerebral tracer uptake might, however, cause restrictions in experimental usage.     

5‐HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5‐HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5‐HT) receptors, the 5‐HT transporter (SERT), and 5‐HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5‐HT system in humans include antagonists for the 5‐HT_1A, 5‐HT_1B, 5‐HT_2A, and 5‐HT₄ receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging. © 2011 Wiley Periodicals, Inc.     

Cardiac‐Restricted Overexpression of the A2A‐Adenosine Receptor in FVB Mice Transiently Increases Contractile Performance and Rescues the Heart Failure Phenotype in Mice Overexpressing the A1‐Adenosine Receptor

In the heart, adenosine binds to pharmacologically distinct G‐protein‐coupled receptors (A₁‐R, A_2A‐R, and A₃‐R). While the role of A₁‐and A₃‐Rs in the heart has been clarifed, the effect of genetically manipulating the A_2A‐R has not been defned. Thus, we created mice overexpressing a cardiac‐restricted A_2A‐R transgene. Mice with both low (Lo) and high (Hi) levels of A_2A‐R overexpression demonstrated an increase in cardiac contractility at 12 weeks. These changes were associated with a signifcantly higher systolic but not diastolic [Ca²⁺]ⁱ, higher maximal contraction amplitudes, and a signifcantly enhanced sarcoplasmic reticulum Ca²⁺ uptake activity. At 20 weeks, the effects of A_2A‐R overexpression on cardiac contractility diminished. The positive effects elicited by A_2A‐R overexpression differ from the heart failure phenotype we observed with A₁‐R overexpresson. Interestingly, coexpression of A_2A‐R TG_Hi, but not A_2A‐R TGLo, enhanced survival, prevented the development of left ventricular dysfunction and heart failure, and improved Ca²⁺ handling in mice overexpressing the A₁‐R. These results suggest that adenosine‐mediated signaling in the heart requires a balance between A₁‐ and A_2A‐Rs—a fnding that may have important implications for the ongoing clinical evaluation of adenosine receptor subtype‐specifc agonists and antagonists for the treatment of cardiovascular diseases.     

Sustained increase in α5GABAA receptor function impairs memory after anesthesia

Many patients who undergo general anesthesia and surgery experience cognitive dysfunction, particularly memory deficits that can persist for days to months. The mechanisms underlying this postoperative cognitive dysfunction in the adult brain remain poorly understood. Depression of brain function during anesthesia is attributed primarily to increased activity of γ-aminobutyric acid type A receptors (GABA_ARs), and it is assumed that once the anesthetic drug is eliminated, the activity of GABA_ARs rapidly returns to baseline and these receptors no longer impair memory. Here, using a murine model, we found that a single in vivo treatment with the injectable anesthetic etomidate increased a tonic inhibitory current generated by α5 subunit–containing GABA_ARs (α5GABA_ARs) and cell-surface expression of α5GABA_ARs for at least 1 week. The sustained increase in α5GABA_AR activity impaired memory performance and synaptic plasticity in the hippocampus. Inhibition of α5GABA_ARs completely reversed the memory deficits after anesthesia. Similarly, the inhaled anesthetic isoflurane triggered a persistent increase in tonic current and cell-surface expression of α5GABA_ARs. Thus, α5GABA_AR function does not return to baseline after the anesthetic is eliminated, suggesting a mechanism to account for persistent memory deficits after general anesthesia.     

Preclinical Safety Assessment of the 5-HT2A Receptor Agonist PET Radioligand [11C]Cimbi-36

Purpose

[¹¹C]Cimbi-36 was recently developed as an agonist radioligand for brain imaging of serotonin 2A receptors (5-HT_2A) with positron emission tomography (PET). This may be used to quantify the high-affinity state of 5-HT_2A receptors and may have the potential to quantify changes in cerebral 5-HT levels in vivo. We here investigated safety aspects related to clinical use of [¹¹C]Cimbi-36, including radiation dosimetry and in vivo pharmacology.

Procedures

[¹¹C]Cimbi-36 was injected in rats or pigs, and radiation dosimetry was examined by ex vivo dissection or with PET scanning, respectively. Based on animal data, the Organ Level INternal Dose Assessment software was used to estimate extrapolated human dosimetry for [¹¹C]Cimbi-36. The 5-HT_2A receptor agonist actions of [¹¹C]Cimbi-36 in vivo pharmacological effects in mice elicited by increasing doses of Cimbi-36 were assessed with the head-twitch response (HTR).

Results

The effective dose as extrapolated from both rat and pig data was low, 7.67 and 4.88 μSv/MBq, respectively. In addition, the estimated absorbed radiation dose to human target organs did not exceed safety levels. Administration of 0.5 mg/kg Cimbi-36 leads to significant HTR compared to saline, whereas 0.05 mg/kg Cimbi-36 (doses much larger than those given in conjunction with a PET scan) did not elicit a significant HTR.

Conclusions

Administration of tracer doses of [¹¹C]Cimbi-36 does not seem to be associated with unusual radiation burden or adverse clinical effects.     

Involvement of Pro‐Nociceptive 5‐HT2A Receptor in the Pathogenesis of Medication‐Overuse Headache

(Headache 2010;50:185‐197) Objectives.— To determine the involvement of 5‐HT_2A (5‐HT_2A) receptor in the process of trigeminal plasticity induced by chronic analgesic exposure and in the process of inflammatory‐induced thermal hyperalgesia. Background.— Derangement in 5‐HT_2A serotonin receptor has been reported to implicate in pathogenesis of medication‐overuse headache. No clear explanation concerning the precise roles of these receptors in the process. Methods.— Wistar rats were daily administered with paracetamol (200 mg/kg) for 30 days. On the next day, ketanserin, a 5‐HT_2A antagonist, or saline was given prior to cortical spreading depression (CSD) induction. Electrocorticogram, cortical blood flow, Fos and 5‐HT_2A‐immunoreactivity in cortex and trigeminal pathway were studied. In the other experiment, complete Freund's adjuvant was injected into the rat hind paw to induce tissue inflammation. Three days later, ketanserin was given and noxious heat was applied to both inflamed and noninflamed paws. The response between 2 sides was compared by measuring paw withdrawal latency. Results.— Chronic paracetamol exposure led to an increase in CSD frequency and CSD‐evoked Fos expression in cerebral cortex indicating the increase in neuronal excitability. Prolonged medication exposure also facilitated trigeminal nociception as evident by an increase in CSD‐evoked Fos expression in trigeminal nucleus caudalis. The expression of 5‐HT_2A receptor in cerebral cortex and trigeminal ganglia was enhanced by chronic paracetamol administration. Pretreatment with ketanserin significantly attenuated these effects. The second experiment showed that ketanserin was able to lengthen the paw withdrawal latency in the inflamed side but did not alter nociceptive response in the noninflamed side. Conclusion.— These findings suggest that up‐regulation of pro‐nociceptive 5‐HT_2A receptor is an important step in the process of cortical hyper‐excitation and nociceptive facilitation induced by chronic analgesic exposure.     

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology,medicinal chemistry,and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A₁, A_2A, A_2B, and A₃. Among these G protein‐coupled receptors, the A₃ subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A₃ adenosine receptor (AR), together with an overview on the progress of hA₃AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure–activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA₃AR and drug design. © 2011 Wiley Periodicals, Inc. Med Res Rev     

Direct blockade of inflammatory hypernociception by peripheral A1 adenosine receptors: Involvement of the NO/cGMP/PKG/KATP signaling pathway

Through activation of the A1 adenosine receptors (A1Rs) at both the central and peripheral level, adenosine produces antinociception in a wide range of tests. However, the mechanisms involved in the peripheral effect are still not fully understood. Therefore, the mechanisms by which peripheral activation of A1Rs reduces inflammatory hypernociception (a decrease in the nociceptive threshold) were addressed in the present study. Immunofluorescence of rat dorsal root ganglion revealed significant expression of A1Rs in primary sensory neurons associated with nociceptive pathways. Functionally, peripheral activation of A1Rs reduced inflammatory hypernociception because intraplantar (i.pl.) administration of an A1R antagonist (DPCPX) enhanced carrageenan-induced hypernociception. On the other hand, local (paw) administration of CPA (a selective A1R agonist) reversed mechanical hypernociception induced by carrageenan or by the directly acting hypernociceptive mediator prostaglandin E₂ (PGE₂). Down-regulation of A1Rs expression in primary nociceptive neurons by intrathecal treatment with antisense oligodeoxinucleotides significantly reduced peripheral antinociceptive action of CPA. Direct blockade of PGE₂ inflammatory hypernociception by the activation of A1Rs depends on the nitric oxide/cGMP/Protein Kinase G/KATP signaling pathway because the peripheral antinociceptive effect of CPA was prevented by pretreatment with inhibitors of neuronal nitric oxide synthase (N-propyl-l-arginine), guanylyl cyclase (ODQ), and Protein Kinase G (KT5823) as well as with a KATP blocker (glibenclamide). However, this effect of CPA was not reduced by naloxone, excluding the participation of endogenous opioids. These results suggest that the peripheral activation of A1R plays a role in the regulation of inflammatory hypernociception by a mechanism that involves the NO/cGMP/PKG/KATP intracellular signaling pathway.     

The purinergic P2X7 receptor as a potential drug target to combat neuroinflammation in neurodegenerative diseases

Neurodegenerative diseases (NDDs) represent a huge social burden, particularly in Alzheimer's disease (AD) in which all proposed treatments investigated in murine models have failed during clinical trials (CTs). Thus, novel therapeutic strategies remain crucial. Neuroinflammation is a common pathogenic feature of NDDs. As purinergic P2X7 receptors (P2X7Rs) are gatekeepers of inflammation, they could be developed as drug targets for NDDs. Herein, we review this challenging hypothesis and comment on the numerous studies that have investigated P2X7Rs, emphasizing their molecular structure and functions, as well as their role in inflammation. Then, we elaborate on research undertaken in the field of medicinal chemistry to determine potential P2X7R antagonists. Subsequently, we review the state of neuroinflammation and P2X7R expression in the brain, in animal models and patients suffering from AD, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, multiple sclerosis, and retinal degeneration. Next, we summarize the in vivo studies testing the hypothesis that by mitigating neuroinflammation, P2X7R blockers afford neuroprotection, increasing neuroplasticity and neuronal repair in animal models of NDDs. Finally, we reviewed previous and ongoing CTs investigating compounds directed toward targets associated with NDDs; we propose that CTs with P2X7R antagonists should be initiated. Despite the high expectations for putative P2X7Rs antagonists in various central nervous system diseases, the field is moving forward at a relatively slow pace, presumably due to the complexity of P2X7Rs. A better pharmacological approach to combat NDDs would be a dual strategy, combining P2X7R antagonism with drugs targeting a selective pathway in a given NDD.     

Reproducibility of Non-Invasive A1 Adenosine Receptor Quantification in the Rat Brain Using [18F]CPFPX and Positron Emission Tomography

Purpose

The A₁AR antagonist 8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([¹⁸F]CPFPX) has recently been shown to be a suitable radiotracer for quantitative in vivo imaging of the A₁ adenosine receptor (A₁AR) in rats. The present study evaluates the reproducibility of non-invasive longitudinal A₁AR studies with [¹⁸F]CPFPX and a dedicated small animal positron emission tomography (PET) scanner.

Procedures

Twelve male Sprague Dawley rats underwent four repeated dynamic PET scans with a bolus injection of [¹⁸F]CPFPX. A₁AR availability was determined by different non-invasive approaches including simplified and multilinear reference tissue (olfactory bulb)-based models and graphical methods. The outcome parameter binding potential (BP) was evaluated in terms of variability and reproducibility.

Results

Repeated estimations of [¹⁸F]CPFPX BP _ND gave reliable results with acceptable variability (mean 12 %) and reproducibility (intraclass correlation coefficients raging from 0.57 to 0.68) in cortical and subcortical regions of the rat brain. With regard to kinetic models, test-retest stability of the simplified reference-tissue model (SRTM) was superior to multilinear and graphical approaches.

Conclusions

Non-invasive quantification of A₁AR density in the rat brain is reproducible and reliable with [¹⁸F]CPFPX PET and allows longitudinal designs of in vivo imaging studies in rodents.     

Dendritic cells tolerized with adenosine A2AR agonist attenuate acute kidney injury

DC-mediated NKT cell activation is critical in initiating the immune response following kidney ischemia/reperfusion injury (IRI), which mimics human acute kidney injury (AKI). Adenosine is an important antiinflammatory molecule in tissue inflammation, and adenosine 2A receptor (A_2AR) agonists protect kidneys from IRI through their actions on leukocytes. In this study, we showed that mice with A_2AR-deficient DCs are more susceptible to kidney IRI and are not protected from injury by A_2AR agonists. In addition, administration of DCs treated ex vivo with an A_2AR agonist protected the kidneys of WT mice from IRI by suppressing NKT production of IFN-γ and by regulating DC costimulatory molecules that are important for NKT cell activation. A_2AR agonists had no effect on DC antigen presentation or on Tregs. We conclude that ex vivo A_2AR–induced tolerized DCs suppress NKT cell activation in vivo and provide a unique and potent cell-based strategy to attenuate organ IRI.     

Inhibitors of Microsomal Prostaglandin E2 Synthase‐1 Enzyme as Emerging Anti‐Inflammatory Candidates

Cyclooxygenases (COX‐1 and COX‐2) catalyze the conversion of arachidonic acid (AA) into PGH₂ that is further metabolized by terminal prostaglandin (PG) synthases into biologically active PGs, for example, prostaglandin E₂ (PGE₂), prostacyclin I₂ (PGI₂), thromboxane A₂ (TXA₂), prostaglandin D₂ (PGD₂), and prostaglandin F₂ alpha (PGF_2α). Among them, PGE₂ is a widely distributed PG in the human body, and an important mediator of inflammatory processes. The successful modulation of this PG provides a beneficial strategy for the potential anti‐inflammatory therapy. For instance, nonsteroidal anti‐inflammatory agents (NSAIDs), both classical nonselective (cNSAIDs) and the selective COX‐2 inhibitors (coxibs) attenuate the generation of PGH₂ from AA that in turn reduces the synthesis of PGE₂ and modifies the inflammatory conditions. However, the long‐term use of these agents causes severe side effects due to the nonselective inhibition of other PGs, such as PGI₂ and TXA₂, etc. Microsomal prostaglandin E₂ synthase‐1 (mPGES‐1), a downstream PG synthase, specifically catalyzes the biosynthesis of COX‐2‐derived PGE₂ from PGH₂, and describes itself as a valuable therapeutic target for the treatment of acute and chronic inflammatory disease conditions. Therefore, the small molecule inhibitors of mPGES‐1 would serve as a beneficial anti‐inflammatory therapy, with reduced side effects that are usually associated with the nonselective inhibition of PG biosynthesis.     

A-Type GABA Receptor as a Central Target of TRPM8 Agonist Menthol

Menthol is a widely-used cooling and flavoring agent derived from mint leaves. In the peripheral nervous system, menthol regulates sensory transduction by activating TRPM8 channels residing specifically in primary sensory neurons. Although behavioral studies have implicated menthol actions in the brain, no direct central target of menthol has been identified. Here we show that menthol reduces the excitation of rat hippocampal neurons in culture and suppresses the epileptic activity induced by pentylenetetrazole injection and electrical kindling in vivo. We found menthol not only enhanced the currents induced by low concentrations of GABA but also directly activated GABA_A receptor (GABA_AR) in hippocampal neurons in culture. Furthermore, in the CA1 region of rat hippocampal slices, menthol enhanced tonic GABAergic inhibition although phasic GABAergic inhibition was unaffected. Finally, the structure-effect relationship of menthol indicated that hydroxyl plays a critical role in menthol enhancement of tonic GABA_AR. Our results thus reveal a novel cellular mechanism that may underlie the ambivalent perception and psychophysical effects of menthol and underscore the importance of tonic inhibition by GABA_ARs in regulating neuronal activity.     

Extended characterisation of the serotonin 2A (5-HT2A) receptor-selective PET radiotracer 11C-MDL100907 in humans: quantitative analysis, test-retest reproducibility, and vulnerability to endogenous 5-HT tone

Introduction

Scanning properties and analytic methodology of the 5-HT_2A receptor-selective positron emission tomography (PET) tracer ¹¹C-MDL100907 have been partially characterised in previous reports. We present an extended characterisation in healthy human subjects.

Methods

64 ¹¹C-MDL100907 PET scans with metabolite-corrected arterial input function were performed in 39 healthy adults (18-55 years). 12 subjects were scanned twice (duration 150 min) to provide data on plasma analysis, model order estimation, and stability and test-retest characteristics of outcome measures. All other scans were 90 min duration. 3 subjects completed scanning at baseline and following 5-HT_2A receptor antagonist medication (risperidone or ciproheptadine) to provide definitive data on the suitability of the cerebellum as reference region. 10 subjects were scanned under reduced 5-HT and control conditions using rapid tryptophan depletion to investigate vulnerability to competition with endogenous 5-HT. 13 subjects were scanned as controls in clinical protocols. Pooled data were used to analyse the relationship between tracer injected mass and receptor occupancy, and age-related decline in 5-HT_2A receptors.

Results

Optimum analytic method was a 2-tissue compartment model with arterial input function. However, basis function implementation of SRTM may be suitable for measuring between-group differences non-invasively and warrants further investigation. Scan duration of 90 min achieved stable outcome measures in all cortical regions except orbitofrontal which required 120 min. Binding potential (BP_P and BP_ND) test-retest variability was very good (7-11%) in neocortical regions other than orbitofrontal, and moderately good (14-20%) in orbitofrontal cortex and medial temporal lobe. Saturation occupancy of 5-HT_2A receptors by risperidone validates the use of the cerebellum as a region devoid of specific binding for the purposes of PET. We advocate a mass limit of 4.6 μg to remain below 5% receptor occupancy. ¹¹C-MDL100907 specific binding is not vulnerable to competition with endogenous 5-HT in humans. Paradoxical decreases in BP_ND were found in right prefrontal cortex following reduced 5-HT, possibly representing receptor internalisation. Mean age-related decline in brain 5-HT_2A receptors was 14.0 ± 5.0% per decade, and higher in prefrontal regions.

Conclusions

Our data confirm and extend support for ¹¹C-MDL100907 as a PET tracer with very favourable properties for quantifying 5-HT_2A receptors in the human brain.     

Disrupting 5-HT2A Receptor/PDZ Protein Interactions Reduces Hyperalgesia and Enhances SSRI Efficacy in Neuropathic Pain

Antidepressants are one of the first-line treatments for neuropathic pain. Despite the influence of serotonin (5-hydroxytryptamine, 5-HT) in pain modulation, selective serotonin reuptake inhibitors (SSRIs) are less effective than tricyclic antidepressants. Here, we show, in diabetic neuropathic rats, an alteration of the antihyperalgesic effect induced by stimulation of 5-HT_2A receptors, which are known to mediate SSRI-induced analgesia. 5-HT_2A receptor density was not changed in the spinal cord of diabetic rats, whereas postsynaptic density protein-95 (PSD-95), one of the PSD-95/disc large suppressor/zonula occludens-1 (PDZ) domain containing proteins interacting with these receptors, was upregulated. Intrathecal injection of a cell-penetrating peptidyl mimetic of the 5-HT_2A receptor C-terminus, which disrupts 5-HT_2A receptor–PDZ protein interactions, induced an antihyperalgesic effect in diabetic rats, which results from activation of 5-HT_2A receptors by endogenous 5-HT. The peptide also enhanced antihyperalgesia induced by the SSRI fluoxetine. Its effects likely resulted from an increase in receptor responsiveness, because it revealed functional 5-HT_2A receptor-operated Ca²⁺ responses in neurons, an effect mimicked by knockdown of PSD-95. Hence, 5-HT_2A receptor/PDZ protein interactions might contribute to the resistance to SSRI-induced analgesia in painful diabetic neuropathy. Disruption of these interactions might be a valuable strategy to design novel treatments for neuropathic pain and to increase the effectiveness of SSRIs.     

Whole-Body Biodistribution and Radiation Dosimetry of the Cannabinoid Type 2 Receptor Ligand [11C]-NE40 in Healthy Subjects

Purpose

The type 2 cannabinoid receptor (CB2R) is part of the human endocannabinoid system and is involved in central and peripheral inflammatory processes. In vivo imaging of the CB2R would allow study of several (neuro)inflammatory disorders. In this study we have investigated the safety and tolerability of [¹¹C]-NE40, a CB2R positron emission tomography (PET) ligand, in healthy human male subjects and determined its biodistribution and radiation dosimetry.

Procedure

Six healthy male subjects (age 20–65 years) underwent a dynamic series of nine whole-body PET/CT scans for up to 140 min, after injection of an average bolus of 286 MBq of [¹¹C]-NE40. Organ absorbed and total effective doses were calculated through OLINDA.

Results

[¹¹C]-NE40 showed high initial uptake in the spleen and a predominant hepatobiliary excretion. In the brain, rapid uptake and swift washout were seen. Organ absorbed doses were largest for the small intestine and liver, with 15.6 and 11.5 μGy/MBq, respectively. The mean effective dose was 3.64?±?0.81 μSv/MBq. There were no changes with aging observed. No adverse events were encountered.

Conclusions

This first-in-man study of [¹¹C]-NE40 showed an expected biodistribution compatible with lymphoid tissue uptake and appropriate fast brain kinetics in the healthy human brain, underscoring the potential of this tracer for further application in central and peripheral inflammation imaging. The effective dose is within the typical expected range for ¹¹C ligands.     

Pharmacologic Stress Using Selective A2A Adenosine Receptor Agonists

Adenosine and dipyridamole are nonselective A_2A adenosine receptor agonists that provide reproducible coronary vasodilation thereby inducing flow disparities in the setting of physiologically important coronary stenosis. Although myocardial perfusion imaging in conjunction with pharmacologic stress with both agents has yielded high diagnostic accuracy and overall safety, limitations include concerns in selected populations, such as those with bronchospastic lung disease and heart block, frequent side effects, and a complicated method of delivery (sustained intravenous infusion). Regadenoson, binodenoson, and apadenoson are selective A_2A adenosine receptor agonists that have been shown to provide more selective coronary vasodilatation, with improved safety profiles and patient tolerability. Additionally, these new agents may be delivered by an intravenous bolus eliminating the requirement for an infusion pump. Regadenoson is already in widespread clinical use in the United States. Binodenoson completed phase 3 clinical trials but has not yet been approved for clinical use. Apadenoson is currently in pivotal phase 3 clinical trials.