Characterization of phenytoin-resistant kindled rats, a new model of drug-resistant partial epilepsy: comparison of inbred strains

Animal Models of Drug-Resistant Epilepsy as Tools for Deciphering the Cellular and Molecular Mechanisms of Pharmacoresistance and Discovering More Effective Treatments

In the last 30 years, over 20 new anti-seizure medicines (ASMs) have been introduced into the market for the treatment of epilepsy using well-established preclinical seizure and epilepsy models. Despite this success, approximately 20-30% of patients with epilepsy have drug-resistant epilepsy (DRE). The current approach to ASM discovery for DRE relies largely on drug testing in various preclinical model systems that display varying degrees of ASM drug resistance. In recent years, attempts have been made to include more etiologically relevant models in the preclinical evaluation of a new investigational drug. Such models have played an important role in advancing a greater understanding of DRE at a mechanistic level and for hypothesis testing as new experimental evidence becomes available. This review provides a critical discussion of the pharmacology of models of adult focal epilepsy that allow for the selection of ASM responders and nonresponders and those models that display a pharmacoresistance per se to two or more ASMs. In addition, the pharmacology of animal models of major genetic epilepsies is discussed. Importantly, in addition to testing chemical compounds, several of the models discussed here can be used to evaluate other potential therapies for epilepsy such as neurostimulation, dietary treatments, gene therapy, or cell transplantation. This review also discusses the challenges associated with identifying novel therapies in the absence of a greater understanding of the mechanisms that contribute to DRE. Finally, this review discusses the lessons learned from the profile of the recently approved highly efficacious and broad-spectrum ASM cenobamate.

Evaluation of antiseizure drug efficacy and tolerability in the rat lamotrigine-resistant amygdala kindling model

OBJECTIVE

The lamotrigine-resistant amygdala kindling model uses repeated administration of a low dose of lamotrigine during the kindling process to produce resistance to lamotrigine, which also extends to some other antiseizure drugs (ASDs). This model of pharmacoresistant epilepsy has been incorporated into the testing scheme utilized by the Epilepsy Therapy Screening Program (ETSP). Although some ASDs have been evaluated in this model, a comprehensive evaluation of ASD prototypes has not been reported.

METHODS

Following depth electrode implantation and recovery, rats were exposed to lamotrigine (5 mg/kg, i.p.) prior to each stimulation during the kindling development process (~3 weeks). A test dose of lamotrigine was used to confirm that fully kindled rats were lamotrigine-resistant. Efficacy (unambiguous protection against electrically elicited convulsive seizures) was defined as a Racine score < 3 in the absence of overt compound-induced side effects. Various ASDs, comprising several mechanistic classes, were administered to fully kindled, lamotrigine-resistant rats. Where possible, multiple doses of each drug were administered in order to obtain median effective dose (ED50) values.

RESULTS

Five sodium channel blockers tested (eslicarbazepine, lacosamide, lamotrigine, phenytoin, and rufinamide) were either not efficacious or effective only at doses that were not well-tolerated in this model. In contrast, compounds targeting either GABA receptors (clobazam, clonazepam, phenobarbital) or GABA-uptake proteins (tiagabine) produced dose-dependent efficacy against convulsive seizures. Compounds acting to modulate Ca2+ channels show differential activity: Ethosuximide was not effective, whereas gabapentin was moderately efficacious. Ezogabine and valproate were also highly effective, whereas topiramate and levetiracetam were not effective at the doses tested.

SIGNIFICANCE

These results strengthen the conclusion that the lamotrigine-resistant amygdala kindling model demonstrates pharmacoresistance to certain ASDs, including, but not limited to, sodium channel blockers, and supports the utility of the model for helping to identify compounds with potential efficacy against pharmacoresistant seizures.

In vitro and in vivo experimental models employed in the discovery and development of antiepileptic drugs for pharmacoresistant epilepsy

Epilepsy is one of the most common chronic, recurrent and progressive neurological diseases. In spite of the large number of antiepileptic drugs currently available for the suppression of seizures, about one-third of patients develop drug-resistant epilepsy, even when they are administered the most appropriate treatment available. Thus, nonclinical models can be valuable tools for the elucidation of the mechanisms underlying the development of pharmacoresistance and also for the development of new therapeutic agents that may be promising therapeutic approaches for this unmet medical need. Up today, several epilepsy and seizure models have been developed, exhibiting similar physiopathological features of human drug-resistant epilepsy; moreover, pharmacological response to antiepileptic drugs clinically available tends to be similar in animal models and humans. Therefore, they should be more intensively used in the preclinical discovery and development of new candidates to antiepileptic drugs. Although useful, in vitro models cannot completely replicate the complexity of a living being and their potential for a systematic use in antiepileptic drug screening is limited. The whole-animal models are the most commonly employed and they can be classified as per se drug-resistant due to an inherent poor drug response or be based on the selection of subgroups of epileptic animals that respond or not to a specific antiepileptic drug. Although more expensive and time-consuming, the latter are chronic models of epilepsy that better exhibit the disease-associated alterations found in human epilepsy. Several antiepileptic drugs in development or already marketed have been already tested and shown to be effective in these models of drug-resistant epilepsy, constituting a new hope for the treatment of drug-resistant epilepsy. This review will provide epilepsy researchers with detailed information on the in vitro and in vivo nonclinical models of interest in drug-resistant epilepsy, which may enable a refined selection of most relevant models for understanding the mechanisms of the disease and developing novel antiepileptic drugs.

The relevance of inter- and intrastrain differences in mice and rats and their implications for models of seizures and epilepsy

It is becoming increasingly clear that the genetic background of mice and rats, even in inbred strains, can have a profound influence on measures of seizure susceptibility and epilepsy. These differences can be capitalized upon through genetic mapping studies to reveal genes important for seizures and epilepsy. However, strain background and particularly mixed genetic backgrounds of transgenic animals need careful consideration in both the selection of strains and in the interpretation of results and conclusions. For instance, mice with targeted deletions of genes involved in epilepsy can have profoundly disparate phenotypes depending on the background strain. In this review, we discuss findings related to how this genetic heterogeneity has and can be utilized in the epilepsy field to reveal novel insights into seizures and epilepsy. Moreover, we discuss how caution is needed in regards to rodent strain or even animal vendor choice, and how this can significantly influence seizure and epilepsy parameters in unexpected ways. This is particularly critical in decisions regarding the strain of choice used in generating mice with targeted deletions of genes. Finally, we discuss the role of environment (at vendor and/or laboratory) and epigenetic factors for inter- and intrastrain differences and how such differences can affect the expression of seizures and the animals' performance in behavioral tests that often accompany acute and chronic seizure testing.

Issues related to development of new antiseizure treatments

This report represents a summary of the discussions led by the antiseizure treatment working group of the International League Against Epilepsy (ILAE)/American Epilepsy Society (AES) Working Groups joint meeting in London (London Meeting). We review here what is currently known about the pharmacologic characteristics of current models of refractory seizures, both for adult and pediatric epilepsy. In addition, we address how the National Institute of Neurological Disorders and Stroke (NINDS)-funded Anticonvulsant Screening Program (ASP) is evolving to incorporate appropriate animal models in the search for molecules that might be sufficiently novel to warrant further pharmacologic development. We also briefly address what we believe is necessary, going forward, to achieve the goal of stopping seizures in all patients, with a call to arms for funding agencies, the pharmaceutical industry, and basic researchers.

Design of innovative therapeutics for pharmacoresistant epilepsy: challenges and needs

Effective therapy for pharmacoresistant epilepsy is an unmet clinical need. Pharmacologically, there are two logical approaches: super-antiictogenic drugs (SAIDs) or antiepileptogenic agents. However, can either of these agents be successfully designed and developed? Designing SAIDs or antiepileptogenics will require applying the techniques of twenty-first century drug design to this ancient "sacred disease." Before this task can be effectively realized, five fundamental needs will first have to be addressed: need for antiepileptogenic drug targets, need for druggable targets for SAIDs, need for new drug molecule platforms, need for new and relevant animal models, and need for innovative funding strategies.

Effects of low-frequency hippocampal stimulation on gamma-amino butyric acid type B receptor expression in pharmacoresistant amygdaloid kindling epileptic rats

OBJECTIVE

To observe the effect of low-frequency hippocampal stimulation on gamma-amino butyric acid type B (GABA-B) receptor expression in hippocampus pharmacoresistant epileptic rats.

MATERIALS AND METHODS

Sixteen pharmacoresistant epileptic rats were selected by testing their seizure response to phenytoin and phenobarbital, and they were randomly divided into a pharmacoresistant control group (PRC group, eight rats) and a pharmacoresistant stimulation group (PRS group, eight rats). Another 16 pharmacosensitive epileptic rats were served as control, also divided randomly into a pharmacosensitive control group (PSC group) and a pharmacosensitive stimulation group (PSS group). A stimulation electrode was implanted into the rats' hippocampus in the four groups. Low-frequency hippocampal stimulation was administered twice per day for two weeks. Following these weeks of stimulation, GABA-B receptor-positive neurons were counted and the gray values of GABA-B receptor expression in the rats' hippocampal tissues were measured.

RESULTS

The amygdale stimulus-induced epileptic seizures were decreased significantly in the PRS group compared with the PRC group. The parameters of the amygdale after discharge also were improved after hippocampal stimulation. Simultaneously, the GABA-B receptor-positive neurons increased and the GABA-B expression gray values decreased markedly in the PRS group compared with the PRC group. The same phenomenon also was observed between the PSS group and the PSC group. However, no significant difference was found in the GABA-B receptor-positive neurons and the gray values of GABA-B between the PRS group and the PSC group.

CONCLUSIONS

The low-frequency hippocampal stimulation may inhibit the amygdale stimulus-induced epileptic seizures and the after discharges. The antiepileptic effects of the hippocampal stimulation may be achieved partly by increasing the expression of the GABA-B receptor.

Finding a better drug for epilepsy: preclinical screening strategies and experimental trial design

The antiepileptic drugs (AEDs) introduced during the past two decades have provided several benefits: they offered new treatment options for symptomatic treatment of seizures, improved ease of use and tolerability, and lowered risk for hypersensitivity reactions and detrimental drug-drug interactions. These drugs, however, neither attenuated the problem of drug-refractory epilepsy nor proved capable of preventing or curing the disease. Therefore, new preclinical screening strategies are needed to identify AEDs that target these unmet medical needs. New therapies may derive from novel targets identified on the basis of existing hypotheses for drug-refractory epilepsy and the biology of epileptogenesis; from research on genetics, transcriptomics, and epigenetics; and from mechanisms relevant for other therapy areas. Novel targets should be explored using new preclinical screening strategies, and new technologies should be used to develop medium- to high-throughput screening models. In vivo testing of novel drugs should be performed in models mimicking relevant aspects of drug refractory epilepsy and/or epileptogenesis. To minimize the high attrition rate associated with drug development, which arises mainly from a failure to demonstrate sufficient clinical efficacy of new treatments, it is important to define integrated strategies for preclinical screening and experimental trial design. An important tool will be the discovery and implementation of relevant biomarkers that will facilitate a continuum of proof-of-concept approaches during early clinical testing to rapidly confirm or reject preclinical findings, and thereby lower the risk of the overall development effort. In this review, we overview some of the issues related to these topics and provide examples of new approaches that we hope will be more successful than those used in the past.

Effects of high frequency electrical stimulation and R-verapamil on seizure susceptibility and glutamate and GABA release in a model of phenytoin-resistant seizures

The present study was focused to characterize the effects of intrahippocampal application of R-verapamil, a P-glycoprotein blocker, and High Frequency Electrical Stimulation (HFS) at 130 Hz, on seizure susceptibility and extracellular concentrations of glutamate and γ-aminobutyric acid (GABA) in hippocampus of kindled rats with drug-resistant seizures. Fully kindled rats classified in responsive and non-responsive to phenytoin were used for this purpose. In contrast with responsive animals, non-responsive rats showed lower afterdischarge threshold (ADT) values in pre-kindling conditions and required less number of kindling trials to achieve the kindled state. Once the animals attained the kindled state, both epileptic groups presented high glutamate and low GABA interictal release, effect more evident in non-responsive rats. In hippocampus of responsive animals, GABA levels demonstrated two increases at 120 and 240 min after the ictal event, a situation no detected for non-responsive rats. Kindled animals receiving hippocampal HFS showed augmented ADT, an effect associated with enhanced GABA release in responsive rats. Intrahippocampal perfusion of R-verapamil (5 mM) decreased the seizure susceptibility (high ADT values), enhanced the interictal GABA release and the postictal levels of glutamate and GABA in responsive and non-responsive rats. It is conclude that alterations of glutamate and GABA release in the epileptic hippocampus of non-responsive animals resemble those found in hippocampus of patients with refractory TLE. In addition, intrahippocampal application of HFS and R-verapamil modifies the amino acid release and reduces the seizure susceptibility of both, responsive and non-responsive rats.

Animal models for vagus nerve stimulation in epilepsy

Vagus nerve stimulation (VNS) is a moderately effective adjunctive treatment for patients suffering from medically refractory epilepsy and is explored as a treatment option for several other disorders. The present review provides a critical appraisal of the studies on VNS in animal models of seizures and epilepsy. So far, these studies mostly applied short-term VNS in seizure models, demonstrating that VNS can suppress and prevent seizures and affect epileptogenesis. However, the mechanism of action is still largely unknown. Moreover, studies with a clinically more relevant setup where VNS is chronically applied in epilepsy models are scarce. Future directions for research and the application of this technology in animal models of epilepsy are discussed.

Preliminary explorations of the role of mitochondrial proteins in refractory epilepsy: some findings from comparative proteomics

Approximately 20-30% of patients with epilepsy continue to have seizures despite carefully monitored treatment with antiepileptic drugs. The mechanisms that underlie why some patients are responsive and others prove resistant to antiepileptic drugs are poorly understood. Increasing evidence supports a role for altered mitochondrial function in the pathogenesis of epilepsy. To gain greater molecular insight in the pathogenesis of intractable epilepsy, we undertook a global analysis of protein expressions in a pharmacoresistant epileptic model selected by phenytoin in electrical amygdala-kindled rats by using two-dimensional gel electrophoresis coupled with matrix-assisted laser desorption/ionization time of flight (MALDI-TOF-TOF). We identified five increased proteins and 14 decreased proteins including voltage-dependent anion channel 1 (VDAC1) with a 2.82-fold increased level (P < 0.05) and voltage-dependent anion channel 2 (VDAC2) with a 3.97-fold decreased level (P < 0.05) in hippocampus of pharmacoresistant rats. The increased VDAC1 and decreased VDAC2 were confirmed by Western blot analysis and immunohistochemistry. Vascular mitochondria and apoptosis neurons were observed through electron microscopy. Energy contents, the adenine nucleotides, were measured by high-performance liquid chromatography (HPLC). The correlation analyses were carried out between VDAC and the energy charge. These findings indicate that the increase of VDAC1 and the decrease of VDAC2 play an important role during the process and provide new molecular evidence in understanding mechanism of refractory epilepsy.

Polymorphic variants of the multidrug resistance gene Mdr1a and response to antiepileptic drug treatment in the kindling model of epilepsy

Allelic variants of the human P-glycoprotein encoding gene MDR1 (ABCB1) are discussed to be associated with different clinical conditions including pharmacoresistance of epilepsy. However, conflicting data have been reported with regard to the functional relevance of MDR1 allelic variants for the response to antiepileptic drugs. To our knowledge, it is not known whether functionally relevant genetic polymorphisms also occur in the two genes (Mdr1a/Abcb1a, Mdr1b/Abcb1b) coding for P-glycoprotein in the brain of rodents. Therefore, we have started to search for polymorphisms in the Mdr1a gene, which governs the expression of P-glycoprotein in brain capillary endothelial cells in rats. In the kindling model of temporal lobe epilepsy, subgroups of phenytoin-sensitive and phenytoin-resistant rats were selected in repeated drug trials. Sequencing of the Mdr1a gene coding sequence in the subgroups revealed no general differences between drug-resistant and drug-sensitive rats of the Wistar outbred strain. A comparison between different inbred and outbred rat strains also gave no evidence for polymorphisms in the Mdr1a coding sequence. However, in exon-flanking intron sequences, four genetic variants were identified by comparison between these rats strains. In conclusion, the finding that Wistar rats vary in their response to phenytoin, while having the same genetic background, argues against a major impact of Mdr1a genetics on pharmacosensitivity to antiepileptic drugs in the amygdala kindling model.

Update on pharmacogenetics in epilepsy: a brief review

Recent developments in the pharmacogenetics of antiepileptic drugs provide new prospects for predicting the efficacy of treatment and potential side-effects. Epilepsy is a common, serious, and treatable neurological disorder, yet current treatment is limited by high rates of adverse drug reactions and lack of complete seizure control in a significant proportion of patients. The disorder is especially suitable for pharmacogenetic investigation because treatment response can be quantified and side-effects can be assessed with validated measures. Additionally, there is substantial knowledge of the pharmacodynamics and kinetics of antiepileptic drugs, and some candidate genes implicated in the disorder have been identified. However, recent studies of the association of particular genes and their genetic variants with seizure control and adverse drug reactions have not provided unifying conclusions. This article reviews the published work and summarises the state of research in this area. Future directions for research and the application of this technology to the clinical practice of individualising treatment for epilepsy are discussed.

Animal models of limbic epilepsies: what can they tell us?

In this review, we have provided an overview of the implementation and characteristics of some of the most prevalent models of temporal lobe epilepsy in use in laboratories around the world today. These include spontaneously seizing models with status epilepticus as the initial precipitating injury (including the kainate, pilocarpine, and electrical stimulation models), kindling, and models of drug refractoriness. These models share various features with one another, and also differ in many aspects, providing a broader representation of the full spectrum of clinical limbic epilepsies. We have also provided a brief introduction into how animal models of temporal lobe epilepsy facilitate use of modern state-of-the-art techniques in neurobiology to address critical questions in the pathogenesis of epilepsy.

Effect of phenytoin on sodium and calcium currents in hippocampal CA1 neurons of phenytoin-resistant kindled rats

About 20-30% of patients with epilepsy continue to have seizures despite carefully monitored treatment with antiepileptic drugs. The mechanisms explaining why some patients' respond and others prove resistant to antiepileptic drugs are poorly understood. It has been proposed that pharmacoresistance is related to reduced sensitivity of sodium channels in hippocampal neurons to antiepileptic drugs such as carbamazepine or phenytoin. In line with this proposal, a reduced effect of carbamazepine on sodium currents in hippocampal CA1 neurons was found in the rat kindling model of temporal lobe epilepsy (TLE), i.e. a form of epilepsy with the poorest prognosis of all epilepsy types in adult patients. To address directly the possibility that neuronal sodium currents in the hippocampus play a crucial role in the pharmacoresistance of TLE, we selected amygdala-kindled rats with respect to their in vivo anticonvulsant response to phenytoin into responders and nonresponders and then compared phenytoin's effect on voltage-activated sodium currents in CA1 neurons. Furthermore, in view of the potential role of calcium current modulation in the anticonvulsant action of phenytoin, the effect of phenytoin on high-voltage-activated calcium currents was studied in CA1 neurons. Electrode-implanted but not kindled rats were used as sham controls for comparison with the kindled rats. In all experiments, the interval between last kindled seizure and ion channel measurements was at least 5 weeks. In kindled rats with in vivo resistance to the anticonvulsant effect of phenytoin (phenytoin nonresponders), in vitro modulation of sodium and calcium currents by phenytoin in hippocampal CA1 neurons did not significantly differ from respective data obtained in phenytoin responders, i.e. phenytoin resistance was not associated with a changed modulation of the sodium or calcium currents by this drug. Compared to sham controls, phenytoin's inhibitory effect on sodium currents was significantly reduced by kindling without difference between the responder and nonresponder subgroups. Further studies in phenytoin-resistant kindled rats may help to elucidate the mechanisms that can explain therapy resistance.

Is the mineralocorticoid receptor in Brown Norway rats constitutively active?

In a previous study using corticosterone treatment of adrenalectomized rats, we hypothesized that mineralocorticoid receptor (MR)-related mechanisms are constitutively active and that glucocorticoid receptor (GR)-mediated mechanisms are more efficient in Brown Norway rats compared to Fischer 344 (F344) rats. In order to discriminate the mineralocorticoid from the glucocorticoid actions exerted by corticosterone, F344 and Brown Norway adrenalectomized rats were treated with increasing doses (1, 5 and 25 microg/ml of drinking water) of deoxycorticosterone (DOC, MR-specific ligand) or RU 28362 (GR-specific ligand). These rats were compared with long-term adrenalectomized (ADX) untreated rats and sham-ADX rats. This study confirms our previous results, notably the lack of effect of ADX on body weight and fluid intake in Brown Norway rats. Moreover, DOC treatment had no effect in Brown Norway rats whereas the higher dose restored fluid intake of the F344 ADX group to sham values. These results support the hypothesis of a constitutive activation of the MR and therefore the insensitivity of this receptor to its ligand in Brown Norway rats. Alternatively, RU 28362 treatment induced greater weight loss, decrease in food intake, anxiolysis, thymus involution, and decrease in plasma transcortin concentration and pituitary corticosteroid receptor densities in Brown Norway rats than in F344 rats, which is consistent with greater efficiency of GR mechanisms in Brown Norway rats than in F344 rats. Therefore, these strains are of great utility to disentangle MR and GR effects on complex phenotypes.

Kindling alters the anticonvulsant efficacy of phenytoin in Wistar rats

We have previously shown that subgroups can be selected from large groups of amygdala kindled Wistar rats which either respond consistently or do not respond to the anticonvulsant effect of phenytoin. Phenytoin nonresponders were proposed as a model for pharmaco-resistant temporal lobe epilepsy. In the present study we examined whether the differences of individual rats in response to phenytoin are already present before kindling or are a consequence of kindling. For this purpose, 52 rats were once tested with phenytoin, then kindled, and then repeatedly tested with phenytoin for selection of subgroups. For subgroup selection after kindling, the phenytoin prodrug fosphenytoin was used because of its water solubility and its improved tolerability and absorption after i.p. administration in rats. Before kindling, phenytoin significantly increased the afterdischarge threshold (ADT), i.e. a sensitive measure of focal seizure activity, but there was large individual variation with only 32 of the 52 rats reacting with an ADT increase, while the remaining rats showed either no effect or ADT decreases. After kindling, the selection resulted in 16 rats with consistent ADT increases in response to phenytoin and ten nonresponders (the remaining 26 rats showed variable responses). Unexpectedly, in rats which were responders after kindling, phenytoin exerted no significant anticonvulsant effect before kindling, while kindled nonresponders were very sensitive to phenytoin before kindling, indicating that the kindling process was responsible for the loss of anticonvulsant efficacy in kindled nonresponders and the development of phenytoin's efficacy in kindled responders. The present results substantiate that kindled subgroups of Wistar rats with different response to phenytoin are a valuable source for studying the mechanisms underlying the development of pharmaco-resistant limbic seizures.