[Show abstract][Hide abstract] ABSTRACT: The anticonvulsant activity and safety of imepitoin, a novel antiepileptic drug licensed in the European Union, were evaluated in a multicentre field efficacy study as well as in a safety study under laboratory conditions. Efficacy of imepitoin was compared with phenobarbital in 226 client-owned dogs in a blinded parallel group design. The administration of imepitoin twice daily in incremental doses of 10, 20 or 30 mg/kg demonstrated comparable efficacy to phenobarbital in controlling seizures in dogs. The frequency of adverse events including somnolence/sedation, polydipsia and increased appetite was significantly higher in the phenobarbital group. In phenobarbital-treated dogs, significantly increased levels of alkaline phosphatase, gamma-glutamyl-transferase and other liver enzymes occurred, while no such effect was observed in the imepitoin group. In a safety study under laboratory conditions, healthy beagle dogs were administered 0, 30, 90 or 150 mg/kg imepitoin twice daily for 26 weeks. A complete safety evaluation including histopathology was included in the study. A no-observed-adverse-event level of 90 mg/kg twice daily was determined. These results indicate that imepitoin is a potent and safe antiepileptic drug for dogs.
Journal of Veterinary Pharmacology and Therapeutics 07/2014; · 1.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The expression of P-glycoprotein (Pgp) is increased in brain capillary endothelial cells (BCECs) of patients with pharmacoresistant epilepsy. This may restrict the penetration of antiepileptic drugs (AEDs) into the brain. However, the mechanisms underlying increased Pgp expression in epilepsy patients are not known. One possibility is that AEDs induce the expression and functionality of Pgp in BCECs. Several older AEDs that induce human cytochrome P450 enzymes also induce Pgp in hepatocytes and enterocytes, but whether this extends to Pgp at the human BBB and to newer AEDs is not known.
This prompted us to study the effects of various old and new AEDs on Pgp functionality in the human BCEC line, hCMEC/D3, using the rhodamine 123 (Rho123) efflux assay. For comparison, experiments were performed in two rat BCEC lines, RBE4 and GPNT, and primary cultures of rat and pig BCECs. Furthermore, known Pgp inducers, such as dexamethasone and several cytostatic drugs, were included in our experiments.
Under control conditions, GPNT cells exhibited the highest and RBE4 the lowest Pgp expression and Rho123 efflux, while intermediate values were determined in hCMEC/D3. Known Pgp inducers increased Rho123 efflux in all cell lines, but marked inter-cell line differences in effect size were observed. Of the various AEDs examined, only carbamazepine (100 μM) moderately increased Pgp functionality in hCMEC/D3, while valproate (300 μM) inhibited Pgp.
These data do not indicate that treatment with AEDs causes a clinically relevant induction in Pgp functionality in BCECs that form the BBB.
Pharmaceutical Research 01/2014; · 4.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A major unmet medical need is the lack of treatments to prevent (or modify) epilepsy in patients at risk, for example, after epileptogenic brain insults such as traumatic brain injury, stroke, or prolonged acute symptomatic seizures like complex febrile seizures or status epilepticus. Typically, following such brain insults there is a seizure-free interval ("latent period"), lasting months to years before the onset of spontaneous recurrent epileptic seizures. The latent period after a brain insult offers a window of opportunity in which an appropriate treatment may prevent or modify the epileptogenic process induced by a brain insult. A similar latent period occurs in patients with epileptogenic gene mutations. Studies using animal models of epilepsy have led to a greater understanding of the factors underlying epileptogenesis and have provided significant insight into potential targets by which the development of epilepsy may be prevented or modified. This review focuses largely on some of the most common animal models of epileptogenesis and their potential utility for evaluating proposed antiepileptogenic therapies and identifying useful biomarkers. The authors also describe some of the limitations of using animal models in the search for therapies that move beyond the symptomatic treatment of epilepsy. Promising results of previous studies designed to evaluate antiepileptogenesis and the role of monotherapy versus polytherapy approaches are also discussed. Recent data from both models of genetic and acquired epilepsies strongly indicate that it is possible to prevent or modify epileptogenesis, and, hopefully, such promising results can ultimately be translated into the clinic.
Journal of the American Society for Experimental NeuroTherapeutics 01/2014; · 5.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Chronically implanted intracranial depth electrodes are widely used for studying electroencephalographic activities in deep cerebral locations and for electrical stimulation of such locations. We have previously reported that prolonged implantation of an electrode in the basolateral amygdala (BLA) of rats facilitates subsequent kindling from this site, indicating a pro-kindling or pro-epileptogenic effect. To further characterize this phenomenon, we analyzed data from experiments in which we induced a self-sustaining status epilepticus (SSSE) by BLA stimulation following different periods of post-surgical delay. In a total of 183 Sprague-Dawley rats, three groups with different periods of postsurgical delay to onset of electrical stimulation were compared: group 1 (16 days on average), group 2 (28 days) and group 3 (48 days). Three types of SSSE were observed after BLA stimulation: type 1 (nonconvulsive), type 2 (nonconvulsive occasionally interrupted by generalized convulsive activity), and type 3 (generalized convulsive). While groups 1 and 2 did not differ in the frequency of these SSSE types, the group with the longest interval between electrode implantation and stimulation (group 3) showed significantly more severe SSSE than the two other groups. The data indicate that intracranial electrode implantation may increase the sensitivity of the implanted area to seizure induction, extending previous findings in the kindling model. Potential mechanisms of these findings include the functional consequences of local microhemorrhages and blood-brain barrier destruction.
[Show abstract][Hide abstract] ABSTRACT: P-glycoprotein (Pgp; ABCB1/MDR1) is a major efflux transporter at the blood-brain barrier (BBB), restricting the penetration of various compounds. In other tissues, trafficking of Pgp from subcellular stores to the cell surface has been demonstrated and may constitute a rapid way of the cell to respond to toxic compounds by functional membrane insertion of the transporter. It is not known whether drug-induced Pgp trafficking also occurs in brain capillary endothelial cells that form the BBB. In this study, trafficking of Pgp was investigated in human brain capillary endothelial cells (hCMEC/D3) that were stably transfected with a doxycycline-inducible MDR1-EGFP fusion plasmid. In the presence of doxycycline, these cells exhibited a 15-fold increase in Pgp-EGFP fusion protein expression, which was associated with an increased efflux of the Pgp substrate rhodamine 123 (Rho123). The chemotherapeutic agent mitomycin C (MMC) was used to study drug-induced trafficking of Pgp. Confocal fluorescence microscopy of single hCMEC/D3-MDR1-EGFP cells revealed that Pgp redistribution from intracellular pools to the cell surface occurred within 2 h of MMC exposure. Pgp-EGFP exhibited a punctuate pattern at the cell surface compatible with concentrated regions of the fusion protein in membrane microdomains, i.e., lipid rafts, which was confirmed by Western blot analysis of biotinylated cell surface proteins in Lubrol-resistant membranes. MMC exposure also increased the functionality of Pgp as assessed in three functional assays with Pgp substrates (Rho123, eFluxx-ID Gold, calcein-AM). However, this increase occurred with some delay after the increased Pgp expression and coincided with the release of Pgp from the Lubrol-resistant membrane complexes. Disrupting rafts by depleting the membrane of cholesterol increased the functionality of Pgp. Our data present the first direct evidence of drug-induced Pgp trafficking at the human BBB and indicate that Pgp has to be released from lipid rafts to gain its full functionality.
PLoS ONE 01/2014; 9(2):e88154. · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Electroconvulsive therapy is the most effective therapy for major depressive disorder (MDD). The remission rate is above 50% in previously pharmacoresistant patients but the mechanisms of action are not fully understood. Electroconvulsive stimulation (ECS) in rodents mimics antidepressant electroconvulsive therapy (ECT) in humans and is widely used to investigate the underlying mechanisms of ECT. For the translational value of findings in animal models it is essential to establish models with the highest construct, face and predictive validity possible. The commonly used model for ECT in rodents does not meet the demand for high construct validity. For ECT, cortical surface electrodes are used to induce therapeutic seizures whereas ECS in rodents is exclusively performed by auricular or corneal electrodes. However, the stimulation site has a major impact on the type and spread of the induced seizure activity and its antidepressant effect. We propose a method in which ECS is performed by screw electrodes placed above the motor cortex of rats to closely simulate the clinical situation and thereby increase the construct validity of the model. Cortical ECS in rats induced reliably seizures comparable to human ECT. Cortical ECS was more effective than auricular ECS to reduce immobility in the forced swim test. Importantly, auricular stimulation had a negative influence on the general health condition of the rats with signs of fear during the stimulation sessions. These results suggest that auricular ECS in rats is not a suitable ECT model. Cortical ECS in rats promises to be a valid method to mimic ECT.
Journal of psychiatric research 01/2014; · 3.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: About 25% of patients with epilepsy are refractory to treatment, so that new, more effective antiepileptic drugs (AEDs) are urgently needed. Animal models that simulate the clinical situation with individuals responding and not responding to treatment are important to determine mechanisms of AED resistance and develop novel more effective treatments. We have previously developed and characterized such a model in which spontaneous recurrent seizures (SRS) develop after a status epilepticus induced by sustained electrical stimulation of the basolateral amygdala. In this model, prolonged treatment of epileptic rats with phenobarbital (PB) results in two subgroups, PB responders and PB nonresponders. When PB nonresponders were treated in previous experiments with phenytoin (PHT), 83% of the PB-resistant rats were also resistant to PHT. In the present study we examined if rats with PB resistant seizures are also resistant to lamotrigine (LTG), using continuous EEG/video recording of spontaneous seizures over 10 consecutive weeks. For this purpose, a new group of epileptic rats was produced and selected by treatment with PB into responders and nonresponders. As in previous studies, PB nonresponders had a significantly higher seizure frequency before onset of treatment. During subsequent treatment with LTG, all PB nonresponders and 60% of the PB responders exhibited >75% reduction of seizure frequency and were therefore considered LTG responders. Plasma levels of LTG did not differ significantly between responders and nonresponders. The data of this pilot study indicate that LTG is more effective than PHT to suppress seizures in PB nonresponders in this model, but that not all PB responders also respond to LTG. Overall, our data provide further evidence that AED studies in post-SE TLE models are useful in determining and comparing AED efficacy and investigating predictors and mechanisms of pharmacoresistance.
[Show abstract][Hide abstract] ABSTRACT: Although benzodiazepines (BZDs) offer a wide spectrum of antiepileptic activity against diverse types of epileptic seizures, their use in the treatment of epilepsy is limited because of adverse effects, loss of efficacy (tolerance), and development of physical and psychological dependence. BZDs act as positive allosteric modulators of the inhibitory neurotransmitter GABA by binding to the BZD recognition site ("BZD receptor") of the GABAA receptor. Traditional BZDs such as diazepam or clonazepam act as full agonists at this site, so that one strategy to resolve the disadvantages of these compounds would be the development of partial agonists with lower intrinsic efficacy at the BZD site of the GABAA receptor. Several BZD site partial or subtype selective compounds, including bretazenil, abecarnil, or alpidem, have been developed as anxioselective anxiolytic drugs, but epilepsy was not a target indication for such compounds. More recently, the imidazolone derivatives imepitoin (ELB138) and ELB139 were shown to act as low-affinity partial agonists at the BZD site of the GABAA receptor, and imepitoin was developed for the treatment of epilepsy. Imepitoin displayed a broad spectrum of anticonvulsant activity in diverse seizure and epilepsy models at tolerable doses, and, as expected from its mechanism of action, lacked tolerance and abuse liability in rodent and primate models. The more favorable pharmacokinetic profile of imepitoin in dogs versus humans led to the decision to develop imepitoin for the treatment of canine epilepsy. Based on randomized controlled trials that demonstrated antiepileptic efficacy and high tolerability and safety in epileptic dogs, the drug was recently approved for this indication in Europe. Hopefully, the favorable profile of imepitoin for the treatment of epilepsy in dogs will reactivate the interest in partial BZD site agonists as new treatments for human epilepsy.
[Show abstract][Hide abstract] ABSTRACT: The diuretic bumetanide, which acts by blocking the Na-K-Cl cotransporter (NKCC), is widely used to inhibit neuronal NKCC1, particularly when NKCC1 expression is abnormally increased in brain diseases such as epilepsy. However, bumetanide poorly penetrates into the brain and, in rodents, is rapidly eliminated because of extensive oxidation of its N-butyl sidechain, reducing the translational value of rodent experiments. Inhibition of oxidation by piperonyl butoxide (PBO) has previously been reported to increase the half-life and diuretic activity of bumetanide in rats. Here we studied whether inhibition of bumetanide metabolism by PBO also increases brain levels of bumetanide in rats, and whether this alters pharmacodynamic effects in the kindling model of epilepsy. Furthermore, we studied the effects of PBO in mice. Mice eliminated bumetanide less rapidly than rats (elimination half-life 47 min vs. 13 min). Pretreatment with PBO increased the half-life in mice to average values (70 min) previously determined in humans, and markedly elevated brain levels of bumetanide. In rats, the increase in plasma and brain levels of bumetanide by PBO was less marked than in mice. PBO significantly increased the diuretic activity of bumetanide in rats and, less effectively, in mice. In epileptic mice, bumetanide (with PBO) did not suppress spontaneous seizures. In the rat kindling model, bumetanide (with or without PBO) did not exert anticonvulsant effects on fully kindled seizures, but dose-dependently altered kindling development. These data indicate that PBO offers a simple means to enhance the translational properties of rodent experiments with bumetanide, particularly when using mice.
European Journal of Neuroscience 11/2013; · 3.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In human medicine, adverse outcomes associated with switching between bioequivalent brand name and generic antiepileptic drug products is a subject of concern among clinicians. In veterinary medicine, epilepsy in dogs is usually treated with phenobarbital, either with the standard brand name formulation Luminal(R) or the veterinary products Luminal(R) vet and the generic formulation Phenoleptil(R). Luminal(R) and Luminal(R) vet are identical 100 mg tablet formulations, while Phenoleptil(R) is available in the form of 12.5 and 50 mg tablets. Following approval of Phenoleptil(R) for treatment of canine epilepsy, it was repeatedly reported by clinicians and dog owners that switching from Luminal(R) (human tablets) to Phenoleptil(R) in epileptic dogs, which were controlled by treatment with Luminal(R), induced recurrence of seizures. In the present study, we compared bioavailability of phenobarbital after single dose administration of Luminal(R) vet vs. Phenoleptil(R) with a crossover design in 8 healthy Beagle dogs. Both drugs were administered at a dose of 100 mg/dog, resulting in 8 mg/kg phenobarbital on average.
Peak plasma concentrations (Cmax) following Luminal(R) vet vs. Phenoleptil(R) were about the same in most dogs (10.9 +/- 0.92 vs. 10.5 +/- 0.77 mug/ml), and only one dog showed noticeable lower concentrations after Phenoleptil(R) vs. Luminal(R) vet. Elimination half-life was about 50 h (50.3 +/- 3.1 vs. 52.9 +/- 2.8 h) without differences between the formulations. The relative bioavailability of the two products (Phenoleptil(R) vs. Luminal(R) vet.) was 0.98 +/- 0.031, indicating that both formulations resulted in about the same bioavailability.
Overall, the two formulations did not differ significantly with respect to pharmacokinetic parameters when mean group parameters were compared. Thus, the reasons for the anecdotal reports, if true, that switching from the brand to the generic formulation of phenobarbital may lead to recurrence of seizures are obviously not related to a generally lower bioavailability of the generic formulation, although single dogs may exhibit lower plasma levels after the generic formulation that could be clinically meaningful.
BMC Veterinary Research 10/2013; 9(1):202. · 1.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The 6-Hz psychomotor seizure model in mice is increasingly been used as a model for differentiation of anticonvulsant activity during development of new antiepileptic drugs (AEDs). It was previously proposed as a useful model of AED-resistant seizures, but more recent data have cast doubt on this proposal. The aim of the present study was to determine whether performing the 6-Hz test not in normal but epileptic mice renders the 6-Hz test more resistant to AEDs. Furthermore, thresholds for induction of 6-Hz seizures, maximal electroshock seizures (MES) and pentylenetetrazole (PTZ) seizures were compared in normal and epileptic mice, using the pilocarpine model. Epileptic mice had a significantly lower threshold for induction of 6-Hz seizures and were more susceptible to PTZ, whereas the MES threshold was not altered. Unexpectedly, 6-Hz seizures were not more resistant to AEDs in epileptic vs. nonepileptic mice, but instead showed an increased sensitivity to the anticonvulsant effects of some AEDs, particularly levetiracetam. The anticonvulsant ED50 of levetiracetam in the 6-Hz test (using a 32mA stimulus) was 17.6mg/kg in nonepileptic mice, but only 1.5mg/kg in epileptic mice. The data indicate that the complex pathophysiological and functional alterations associated with epilepsy in mice may strikingly alter the sensitivity of acutely induced seizures to an AED.
[Show abstract][Hide abstract] ABSTRACT: Despite the introduction of over 15 third-generation anti-epileptic drugs, current medications fail to control seizures in 20-30% of patients. However, our understanding of the mechanisms mediating the development of epilepsy and the causes of drug resistance has grown substantially over the past decade, providing opportunities for the discovery and development of more efficacious anti-epileptic and anti-epileptogenic drugs. In this Review we discuss how previous preclinical models and clinical trial designs may have hampered the discovery of better treatments. We propose that future anti-epileptic drug development may be improved through a new joint endeavour between academia and the industry, through the identification and application of tools for new target-driven approaches, and through comparative preclinical proof-of-concept studies and innovative clinical trials designs.
dressNature Reviews Drug Discovery 09/2013; · 33.08 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, the imidazolinone derivative imepitoin has been approved for treatment of canine epilepsy. Imepitoin acts as a low-affinity partial agonist at the benzodiazepine (BZD) site of the GABAA receptor and is the first compound with such mechanism that has been developed as an antiepileptic drug (AED). This mechanism offers several advantages compared to full agonists, including less severe adverse effects and a lack of tolerance and dependence liability, which has been demonstrated in rodents, dogs, and nonhuman primates. In clinical trials in epileptic dogs, imepitoin was shown to be an effective and safe AED. Recently, seizures in dogs have been proposed as a translational platform for human therapeutic trials on new epilepsy treatments. In the present study, we compared the anticonvulsant efficacy of imepitoin, phenobarbital and the high-affinity partial BZD agonist abecarnil in the timed i.v. pentylenetetrazole (PTZ) seizure threshold test in dogs and, for comparison, in mice. Furthermore, adverse effects of treatments were compared in both species. All drugs dose-dependently increased the PTZ threshold in both species, but anticonvulsant efficacy was higher in dogs than mice. At the doses selected for this study, imepitoin was slightly less potent than phenobarbital in increasing seizure threshold, but markedly more tolerable in both species. Effective doses of imepitoin in the PTZ seizure model were in the same range as those suppressing spontaneous recurrent seizures in epileptic dogs. The study demonstrates that low-affinity partial agonists at the benzodiazepine site of the GABAA receptor, such as imepitoin, offer advantages as a new category of AEDs.
Pharmacological Research 09/2013; · 4.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied whether pharmacological blockade of the IL-1β-mediated signaling, rapidly activated in forebrain by epileptogenic injuries, affords neuroprotection in two different rat models of status epilepticus (SE). As secondary outcome, we measured treatment's effect on SE-induced epileptogenesis. IL-1β signaling was blocked by systemic administration of two antiinflammatory drugs, namely human recombinant IL-1 receptor antagonist (anakinra), the naturally occurring and clinically used competitive IL-1 receptor type 1 antagonist, and VX-765 a specific non-peptide inhibitor of IL-1β cleavage and release. Antiinflammatory drugs were given 60min after antiepileptic (AED) drug-controlled SE induced by pilocarpine, or 180min after unrestrained electrical SE, for 7days using a protocol yielding therapeutic drug levels in brain. This drug combination significantly decreased both IL-1β expression in astrocytes and cell loss in rat forebrain. Neuroprotection and the antiinflammatory effect were more pronounced in the electrical SE model. Onset of epilepsy, and frequency and duration of seizures 3months after electrical SE were not significantly modified. Transcriptomic analysis in the hippocampus showed that the combined treatment did not affect the broad inflammatory response induced by SE during epileptogenesis. In particular, the treatment did not prevent the induction of the complement system and Toll-like receptors, both contributing to cell loss and seizure generation. We conclude that the IL-1β signaling represents an important target for reducing cell loss after SE. The data highlight a new class of clinically tested agents affording neuroprotection after a delayed post-injury intervention. Earlier blockade of this rapid onset inflammatory pathway during SE, or concomitant treatment with antiinflammatory drugs targeting additional components of the broad inflammatory response to SE, or co-treatment with AEDs, is likely to be required for optimizing beneficial outcomes.
Neurobiology of Disease 08/2013; · 5.62 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Several preclinical proof-of-concept studies have provided evidence for positive treatment effects on epileptogenesis. However, none of these hypothetical treatments has advanced to the clinic. The experience in other fields of neurology such as stroke, Alzheimer's disease, or amyotrophic lateral sclerosis has indicated several problems in the design of preclinical studies, which likely contribute to failures in translating the positive preclinical data to the clinic. The Working Group on "Issues related to development of antiepileptogenic therapies" of the International League Against Epilepsy (ILAE) and the American Epilepsy Society (AES) has considered the possible problems that arise when moving from proof-of-concept antiepileptogenesis (AEG) studies to preclinical AEG trials, and eventually to clinical AEG trials. This article summarizes the discussions and provides recommendations on how to design a preclinical AEG monotherapy trial in adult animals. We specifically address study design, animal and model selection, number of studies needed, issues related to administration of the treatment, outcome measures, statistics, and reporting. In addition, we give recommendations for future actions to advance the preclinical AEG testing.
[Show abstract][Hide abstract] ABSTRACT: Positron emission tomography (PET) with [(11)C]verapamil, either in racemic form or in form of the (R)-enantiomer, has been used to measure the functional activity of the adenosine triphosphate-binding cassette (ABC) transporter P-glycoprotein (Pgp) at the blood-brain barrier (BBB). There is some evidence in literature that verapamil inhibits two other ABC transporters expressed at the BBB, i.e. multidrug resistance protein 1 (MRP1) and breast cancer resistance protein (BCRP). However, previous data were obtained with micromolar concentrations of verapamil and do not necessarily reflect the transporter selectivity of verapamil at nanomolar concentrations, which are relevant for PET experiments. The aim of this study was to assess the selectivity of verapamil, in nanomolar concentrations, for Pgp over MRP1 and BCRP.
Concentration equilibrium transport assays were performed with [(3)H]verapamil (5nM) in cell lines expressing murine or human Pgp, human MRP1, and murine Bcrp1 or human BCRP. Paired PET scans were performed with (R)-[(11)C]verapamil in female FVB/N (wild-type), Mrp1((-/-)), Mdr1a/b((-/-)), Bcrp1((-/-)) and Mdr1a/b((-/-))Bcrp1((-/-)) mice, before and after Pgp inhibition with 15mg/kg tariquidar.
In vitro transport experiments exclusively showed directed transport of [(3)H]verapamil in Mdr1a- and MDR1-overexpressing cells which could be inhibited by tariquidar (0.5μM). In PET scans acquired before tariquidar administration, brain-to-blood ratio (Kb,brain) of (R)-[(11)C]verapamil was low in wild-type (1.3±0.1), Mrp1((-/-)) (1.4±0.1) and Bcrp1((-/-)) mice (1.8±0.1) and high in Mdr1a/b((-/-)) (6.9±0.8) and Mdr1a/b((-/-))Bcrp1((-/-)) mice (7.9±0.5). In PET scans after tariquidar administration, Kb,brain was significantly increased in Pgp-expressing mice (wild-type: 5.0±0.3-fold, Mrp1((-/-)): 3.2±0.6-fold, Bcrp1((-/-)): 4.3±0.1-fold) but not in Pgp knockout mice (Mdr1a/b((-/-)) and Mdr1a/b((-/-))Bcrp1((-/-))).
Our combined in vitro and in vivo data demonstrate that verapamil, in nanomolar concentrations, is selectively transported by Pgp and not by MRP1 and BCRP at the BBB, which supports the use of (R)-[(11)C]verapamil or racemic [(11)C]verapamil as PET tracers of cerebral Pgp function.
Nuclear Medicine and Biology 07/2013; · 2.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: INTRODUCTION: The adenosine triphosphate-binding cassette (ABC) transporter P-glycoprotein (Pgp) protects the brain from accumulation of lipophilic compounds by active efflux transport across the blood-brain barrier. Changes in Pgp function/expression may occur in neurological disorders, such as epilepsy, Alzheimer's or Parkinson's disease. In this work we investigated the suitability of the radiolabeled Pgp inhibitors [(11)C]elacridar and [(11)C]tariquidar to visualize Pgp density in rat brain with PET. METHODS: Rats underwent a first PET scan with [(11)C]elacridar (n=5) or [(11)C]tariquidar (n=6) followed by a second scan with the Pgp substrate (R)-[(11)C]verapamil after administration of unlabeled tariquidar at a dose which half-maximally inhibits cerebral Pgp (3mg/kg). Compartmental modeling using an arterial input function and Logan graphical analysis were used to estimate rate constants and volumes of distribution (VT) of radiotracers in different brain regions. RESULTS: Brain PET signals of [(11)C]elacridar and [(11)C]tariquidar were very low (~0.5 standardized uptake value, SUV). There was a significant negative correlation between VT and K1 (i.e. influx rate constant from plasma into brain) values of [(11)C]elacridar or [(11)C]tariquidar and VT and K1 values of (R)-[(11)C]verapamil in different brain regions which was consistent with binding of [(11)C]inhibitors to Pgp and efflux of (R)-[(11)C]verapamil by Pgp. CONCLUSION: The small Pgp binding signals obtained with [(11)C]elacridar and [(11)C]tariquidar limit the applicability of these tracers to measure cerebral Pgp density. PET tracers with higher (i.e. subnanomolar) binding affinities will be needed to visualize the low density of Pgp in brain.
Nuclear Medicine and Biology 06/2013; · 2.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: PURPOSE: In humans, traumatic brain injury (TBI) is one of the most common causes of acquired (symptomatic) epilepsy, but as yet there is no treatment to prevent the development of epilepsy after TBI. Animal models of posttraumatic epilepsy (PTE) are important to characterize epileptogenic mechanisms of TBI and to identify clinically effective antiepileptogenic treatments. The prevalence and phenomenology of naturally occurring canine epilepsy are similar to those in human epilepsy. However, the risk of epilepsy after TBI has not been systemically studied in dogs. We therefore performed a large retrospective study in 1,000 dogs referred to our clinical department over a period of 11.5 years with the aim to determine the incidence of early and late seizures after head trauma in this species. METHODS: Two strategies were used: in group I (n = 392), we evaluated whether dogs referred for the treatment of a head trauma (group Ia) or other trauma (group Ib) developed seizures after the trauma, whereas in group II (n = 608) we evaluated whether dogs referred for the treatment of recurrent epileptic seizures had a history of head trauma. Data for this study were obtained from our clinical database, questionnaires sent to the dogs' owners, and owner interviews. KEY FINDINGS: In group Ia, 6.6% of the dogs developed PTE, which was significantly different from group Ib (1.9%), indicating that head trauma increased the risk of developing epilepsy by a factor of 3.4. The risk of PTE increased with severity of TBI; 14.3% of the dogs with skull fracture developed PTE. In group II, 15.5% of the dogs with epilepsy had a history of head injury, which was significantly higher than the incidence of PTE determined for group Ia. SIGNIFICANCE: Our study indicates that head trauma in dogs is associated with a significant risk of developing epilepsy. Therefore, dogs with severe TBI are an interesting natural model of PTE that provides a novel translational platform for studies on human PTE.
[Show abstract][Hide abstract] ABSTRACT: The barbiturate phenobarbital has been in use in the treatment of epilepsy for 100 years. It has long been recognized that barbiturates act by prolonging and potentiating the action of γ-aminobutyric acid (GABA) on GABAA receptors and at higher concentrations directly activating the receptors. A large body of data supports the concept that GABAA receptors are the primary central nervous system target for barbiturates, including the finding that transgenic mice with a point mutation in the β3 GABAA-receptor subunit exhibit diminished sensitivity to the sedative and immobilizing actions of the anesthetic barbiturate pentobarbital. Although phenobarbital is only modestly less potent as a GABAA-receptor modulator than pentobarbital, phenobarbital is minimally sedating at effective anticonvulsant doses. Possible explanations for the reduced sedative effect of phenobarbital include more regionally restricted action; partial agonist activity; reduced propensity to directly activate GABAA receptors (possibly including extrasynaptic receptors containing δ subunits); and reduced activity at other ion channel targets, including voltage-gated calcium channels. In recent years, substantial progress has been made in defining the structural features of GABAA receptors responsible for gating and allosteric modulation by drugs. Although the precise sites of action of barbiturates have not yet been defined, the second and third transmembrane domains of the β subunit appear to be critical; binding may involve a pocket formed by β-subunit methionine 286 as well as α-subunit methionine 236. In addition to effects on GABAA receptors, barbiturates block AMPA/kainate receptors, and they inhibit glutamate release through an effect on P/Q-type high-voltage activated calcium channels. The combination of these various actions likely accounts for their diverse clinical activities. Despite the remarkable progress of the last century, there is still much to learn about the actions of barbiturates that can be applied to the discovery of new, more therapeutically useful agents.