Drug Resistance in Epilepsy: Putative Neurobiologic and Clinical Mechanisms

Epilepsy in patients with a brain tumour: focal epilepsy requires focused treatment

Brain tumours frequently cause epileptic seizures. Medical antiepileptic treatment is often met with limited success. Pharmacoresistance, drug interactions and adverse events are common problems during treatment with antiepileptic drugs. The unpredictability of epileptic seizures and the treatment-related problems deeply affect the quality of life of patients with a brain tumour. In this review, we focus on both clinical and basic aspects of possible mechanisms in epileptogenesis in patients with a brain tumour. We provide an overview of the factors that are involved in epileptogenesis, starting focally at the tumour and the peritumoral tissue and eventually extending to alterations in functional connectivity throughout the brain. We correlate this knowledge to the known mechanisms of antiepileptic drugs. We conclude that the underlying mechanisms of epileptogenesis in patients with a brain tumour are poorly understood. The currently available antiepileptic drugs have little to no influence on the known epileptogenic mechanisms that could contribute to the poor efficacy. Better understanding of focal changes that are involved in epileptogenesis may provide new tools for optimal treatment of both the seizures and the underlying tumour. In our opinion, therapy for every patient with a brain tumour suffering from epilepsy should first and foremost aim at eliminating the tumour as well as the epileptic focus through resection combined with postoperative treatment, and only if this strategy does not result in adequate seizure control should medical antiepileptic treatment be intensified. If this strategy, however, results in sustained seizure freedom, tapering of antiepileptic drugs should be considered in the long term.

Systems biology impact on antiepileptic drug discovery

Systems biology (SB), a recent trend in bioscience research to consider the complex interactions in biological systems from a holistic perspective, sees the disease as a disturbed network of interactions, rather than alteration of single molecular component(s). SB-relying network pharmacology replaces the prevailing focus on specific drug-receptor interaction and the corollary of rational drug design of "magic bullets", by the search for multi-target drugs that would act on biological networks as "magic shotguns". Epilepsy being a multi-factorial, polygenic and dynamic pathology, SB approach appears particularly fit and promising for antiepileptic drug (AED) discovery. In fact, long before the advent of SB, AED discovery already involved some SB-like elements. A reported SB project aimed to find out new drug targets in epilepsy relies on a relational database that integrates clinical information, recordings from deep electrodes and 3D-brain imagery with histology and molecular biology data on modified expression of specific genes in the brain regions displaying spontaneous epileptic activity. Since hitting a single target does not treat complex diseases, a proper pharmacological promiscuity might impart on an AED the merit of being multi-potent. However, multi-target drug discovery entails the complicated task of optimizing multiple activities of compounds, while having to balance drug-like properties and to control unwanted effects. Specific design tools for this new approach in drug discovery barely emerge, but computational methods making reliable in silico predictions of poly-pharmacology did appear, and their progress might be quite rapid. The current move away from reductionism into network pharmacology allows expecting that a proper integration of the intrinsic complexity of epileptic pathology in AED discovery might result in literally anti-epileptic drugs.

Antiepileptic drug combinations--have newer agents altered clinical outcomes?

In 2000, 332 (20.5%) of 1617 patients registered with the Western Infirmary Epilepsy Unit required antiepileptic drug (AED) polytherapy to remain seizure-free for at least 1 year. The analysis was repeated 10 years later. Of 2379 seizure-free patients, 20.4% (n=486 - 254 women, 232 men, aged 18-95 years [median age 49 years]) were receiving combination therapy. Two AEDs were taken by 395 (81.3%) patients in 2010, and by 287 (86.4%) in 2000. Sodium valproate with lamotrigine was the commonest of 64 successful pairings. As a combination, mean daily doses of both AEDs were lower (n=96; sodium valproate 1200 mg, lamotrigine 155 mg) than when sodium valproate was taken with carbamazepine or levetiracetam (n=42; 1621 mg; p<0.001), and lamotrigine was combined with topiramate or levetiracetam (n=33; 430 mg; p<0.001), suggesting possible synergism. In 2010, a higher percentage of patients (n=85) remained seizure-free on 3 AEDs (17.5% in 2010, 12.7% in 2000) in 57 separate regimens. Only 0.9% (n=3) of patients in 2000, and 1.2% (n=6) in 2010 responded to 4 AEDs. Levetiracetam (n=109; 10.2%) and topiramate (n=81; 7.6%) were the newer agents most commonly represented in successful combinations. These data tend to imply that drug substitution rather than addition has largely led to these marginally improved results. In the last decade, when used as adjunctive therapies, newer agents appear not to have impacted substantially on the likelihood of producing seizure freedom. An alternative approach to AED development may be required to change this disappointing scenario.

Up-regulation of apelin in brain tissue of patients with epilepsy and an epileptic rat model

Prolonged epileptic seizures or SE can cause neuronal cell death. However, the exact role of neuroprotectant against brain injury during epileptic seizure needs to be further elucidated. The aim of this study was to investigate the expression of the apelin, a novel neuroprotective peptide, in brain tissues of the patients with temporal lobe epilepsy (TLE) and experimental rats using immunohistochemistry, immunofluorescence and Western blotting analysis and to discuss the possible role of apelin in TLE. Thirty temporal neocortical tissue samples from the patients with drug-refractory TLE underwent surgical therapy and nine histologically normal temporal lobes tissues as controls were used in our study. Fifty-six Sprague-Dawley rats were randomly divided into seven groups, including one control group and six groups with epilepsy induced by lithium-pilocarpine. Hippocampus and adjacent cortex were taken from the controls and epileptic rats at 1, 3, 7, 14, 30, and 60 days after onset of seizures. Apelin was mainly expressed in the neurons of TLE patients and controls, and was significantly increased in TLE patients compared with the controls. Apelin was also expressed in the neurons of experimental and control rats, it was gradually increased in the experimental rat post-seizure and reached a stable high level in chronic epileptic phase. Our results demonstrated that the increased expression of apelin in the brain may be involved in human TLE.

Antiepileptic drug therapy: does mechanism of action matter?

This article represents a synthesis of presentations made by the authors during a scientific meeting held in London on 7 June 2010 and organized by GlaxoSmithKline. Each speaker produced a short précis of his lecture to answer a specific question, resulting in an overview of what we know about the relevance of the mechanisms of action of antiepileptic drugs in determining appropriate combination therapies for the treatment of drug-resistant epilepsy.

Coffee and its consumption: benefits and risks

Coffee is the leading worldwide beverage after water and its trade exceeds US $10 billion worldwide. Controversies regarding its benefits and risks still exist as reliable evidence is becoming available supporting its health promoting potential; however, some researchers have argued about the association of coffee consumption with cardiovascular complications and cancer insurgence. The health-promoting properties of coffee are often attributed to its rich phytochemistry, including caffeine, chlorogenic acid, caffeic acid, hydroxyhydroquinone (HHQ), etc. Many research investigations, epidemiological studies, and meta-analyses regarding coffee consumption revealed its inverse correlation with that of diabetes mellitus, various cancer lines, Parkinsonism, and Alzheimer's disease. Moreover, it ameliorates oxidative stress because of its ability to induce mRNA and protein expression, and mediates Nrf2-ARE pathway stimulation. Furthermore, caffeine and its metabolites help in proper cognitive functionality. Coffee lipid fraction containing cafestol and kahweol act as a safeguard against some malignant cells by modulating the detoxifying enzymes. On the other hand, their higher levels raise serum cholesterol, posing a possible threat to coronary health, for example, myocardial and cerebral infarction, insomnia, and cardiovascular complications. Caffeine also affects adenosine receptors and its withdrawal is accompanied with muscle fatigue and allied problems in those addicted to coffee. An array of evidence showed that pregnant women or those with postmenopausal problems should avoid excessive consumption of coffee because of its interference with oral contraceptives or postmenopausal hormones. This review article is an attempt to disseminate general information, health claims, and obviously the risk factors associated with coffee consumption to scientists, allied stakeholders, and certainly readers.

Drug resistance in epilepsy and the ABCB1 gene: The clinical perspective

Multidrug resistance is one of the most serious problems in the treatment of epilepsy that is likely to have a complex genetic and acquired basis. Various experimental data support the hypothesis that over-expression of antiepileptic drug (AED) transporters may play a pivotal role in drug resistance. Hyyt 6however, key questions concerning their functionality remain unanswered. The idea that P-glycoprotein, encoded by the ABCB1 gene, might mediate at least part of the drug resistance was met with both enthusiasm and skepticism. As in oncology, initial optimism has been clouded subsequently by conflicting results. The first study reporting a positive association between genetic variation in the P-glycoprotein and multidrug-resistant epilepsy was published in 2003. Since then, several other genetic association studies have attempted to verify this result. However, taken overall, the role of P-glycoprotein in drug resistance in epilepsy still remains uncertain. We intend to critically review the inherent problems associated with epilepsy pharmacogenetic studies in general and with ABCB1 polymorphisms studies in particular. The lessons learnt from the ABCB1 studies can help us to guide future association genetics studies to investigate AED resistance, and thereby taking us closer to the cherished dream of personalized AED therapy.

Efficacy of new antiepileptic drugs

Regarding efficacy of new antiepileptic drugs (AEDs) for seizure control, there are three important clinical questions. How effective are new AEDs when corrected for the efficacy of placebo? And even more important: How do new AEDs fare in terms of seizure remission compared with established agents? And finally: Have patients seizure-free on new AEDs a better chance for lasting remission after withdrawal versus those withdrawing from older agents? The answers raise concerns. Although add-on therapy with marketed new AEDs is more effective than placebo, as expected, the treatment difference for becoming seizure-free is disappointingly small (6%; 95% CI: 4-8%; z = 6.47; p < 0.001). Although many, but not all, new AEDs have comparable efficacy to old standard drugs in well-controlled trials, none of the new AEDs is superior to old drugs in terms of seizure remission. So far, we have no antiepileptogenic treatments that prevent the development of epilepsy or modify its detrimental course. The sobering results suggest the need for novel experimental and clinical strategies for the development of more effective new AEDs that interrupt ictogenesis more effectively and prevent or abort epileptogenesis. Ideally, we need new drugs that block both ictogenesis and epileptogenesis, resulting in complete cure of epilepsy.

ASIC1a polymorphism is associated with temporal lobe epilepsy

Recent in vitro and in vivo data show that acid-sensing ion channel 1a (ASIC1a) activation enhances neuronal excitability in the hippocampus and neocortex, indicating that ASIC1a might play a role in the generation and maintenance of epileptic seizures. The aim of this study was to investigate association of the ASIC1a gene with temporal lobe epilepsy (TLE) for the first time. Six tag single-nucleotide polymorphisms (SNPs) of the ASIC1a gene were selected and genotyped using polymerase chain reaction-restriction fragment length polymorphism in 560 TLE patients and 401 healthy controls. There was a significant allelic and genotypic association between rs844347:A>C and TLE compared with controls. The rs844347-A allele frequency was 88.1% in the patients and 83.0% in control subjects (OR=1.516, 95% CI 1.142-2.013, p=0.004). Furthermore, the haplotype analysis revealed a significant association with TLE. The results of this study demonstrate for the first time an association between an ASC1a variant allele and TLE in a Han Chinese population.

Prevention or modification of epileptogenesis after brain insults: experimental approaches and translational research

Diverse brain insults, including traumatic brain injury, stroke, infections, tumors, neurodegenerative diseases, and prolonged acute symptomatic seizures, such as complex febrile seizures or status epilepticus (SE), can induce "epileptogenesis," a process by which normal brain tissue is transformed into tissue capable of generating spontaneous recurrent seizures. Furthermore, epileptogenesis operates in cryptogenic causes of epilepsy. In view of the accumulating information about cellular and molecular mechanisms of epileptogenesis, it should be possible to intervene in this process before the onset of seizures and thereby either prevent the development of epilepsy in patients at risk or increase the potential for better long-term outcome, which constitutes a major clinical need. For identifying pharmacological interventions that prevent, interrupt or reverse the epileptogenic process in people at risk, two groups of animal models, kindling and SE-induced recurrent seizures, have been recommended as potentially useful tools. Furthermore, genetic rodent models of epileptogenesis are increasingly used in assessing antiepileptogenic treatments. Two approaches have been used in these different model categories: screening of clinically established antiepileptic drugs (AEDs) for antiepileptogenic or disease-modifying potential, and targeting the key causal mechanisms that underlie epileptogenesis. The first approach indicated that among various AEDs, topiramate, levetiracetam, carisbamate, and valproate may be the most promising. On the basis of these experimental findings, two ongoing clinical trials will address the antiepileptogenic potential of topiramate and levetiracetam in patients with traumatic brain injury, hopefully translating laboratory discoveries into successful therapies. The second approach has highlighted neurodegeneration, inflammation and up-regulation of immune responses, and neuronal hyperexcitability as potential targets for antiepileptogenesis or disease modification. This article reviews these areas of progress and discusses the challenges associated with discovery of antiepileptogenic therapies.

Lack of association between ABCB1 gene polymorphisms and pharmacoresistant epilepsy: an analysis in a western Chinese pediatric population

OBJECTIVE

The genetic polymorphisms of the ABCB1 (ATP-binding cassette B1) gene encoding P-glycoprotein have been proposed to be associated with pharmacoresistance phenotype in epilepsy patients. P-glycoprotein, a transmembrane transporter, works as an efflux pump by limiting antiepileptic drugs across the blood brain barrier, with correspondingly lowering drug concentrations in epileptogenic loci. In this study, we analyzed whether the three single nucleotide polymorphisms (C1236T, G2677T/A, and C3435T) in the ABCB1 gene were associated with pharmacoresistant epilepsy in a western Chinese pediatric population.

METHODS

A total of 350 children with epilepsy who had been prescribed antiepileptic drugs for at least 1year were included. Of this patient group 193 were drug responsive and 157 were drug resistant according to the presence of seizures. Genotypes of the three loci of ABCB1 gene were detected in 368 age- and sex-matched normal children and 350 epileptic children using the polymerase chain reaction (PCR)-restriction fragment length polymorphism technique. Normal population sample populace from the same ethnicity and territory was genotyped to check for population stratification. The allele, genotype, haplotype, and diplotype frequencies of ABCB1 polymorphisms were compared between drug-resistant and drug-responsive subjects.

RESULTS

No significant differences were observed in the frequencies of genotype, allele, haplotype, or diplotype of ABCB1 polymorphisms between patients with drug-resistant and drug-responsive epilepsy (p>0.05).

CONCLUSION

The above three polymorphisms in the ABCB1 gene were not found to be significantly associated with drug resistant epilepsy in a western Chinese pediatric population.

Association of ABCB1 gene polymorphisms and their haplotypes with response to antiepileptic drugs: a systematic review and meta-analysis

AIMS

Several studies demonstrated a link between ABCB1 gene variants and the response to treatment in epilepsy, but the results have been inconclusive. Here, we performed the first haplotype meta-analysis to examine the association of haplotypes of ABCB1 common variants with the response to treatment in epilepsy.

MATERIALS & METHODS

We meta-analyzed the studies that evaluated the role of ABCB1 C1236T, G2677T/A and C3435T polymorphisms and their haplotypes in the response to treatment.

RESULTS

Meta-analysis of 23 studies (7067 patients) showed no significant association of ABCB1 alleles, genotypes and haplotypes with the response to treatment in the overall population or in each ethnicity subgroup.

CONCLUSION

Our data suggest that the haplotypes of these loci may not be involved in the response to treatment.

Characterization of the gene expression profile of human hippocampus in mesial temporal lobe epilepsy with hippocampal sclerosis

One of the main putative causes of therapy refractory epilepsy in mesial temporal lobe epilepsy (MTLE) with hippocampal sclerosis is the overexpression of multidrug transporters (MDTs) at the blood-brain barrier (BBB). It steps up the removal of antiepileptic drugs (AEDs) out of the brain cells across the BBB resulting in a low concentration of AEDs within the target cells. Some of the mechanisms of AED resistance are most likely to be genetically determined. To obtain more information about the underlying pathophysiology of intractability in epilepsy, we compared the global gene expression profile of human hippocampus and hippocampal-derived microvascular endothelial cells from MTLE with HS patients and controls. At the level of MDT, a significant up-regulation was found for ABCB1 (P-gp), ABCB2, ABCB3, and ABCB4, which was mainly related to endothelial cells. The data on the MDT were validated and extended by quantitative RT-PCR. Surprisingly, inflammatory factors such as interleukins (IL-1α, IL-1β, IL-6, and IL-18) and cytokines (TNF-α and TGF-β1) were found to be up-regulated in hippocampal parenchyma. The overexpression of P-gp, IL-1β, and IL-6 was also confirmed by immunohistochemistry (IHC). Our results suggest that complex expression changes of ABC-transporters may play a decisive role in pharmacoresistance in MTLE. Further studies on the new and unexpected overexpression of inflammatory cytokines may unlock hitherto undiscovered pathways of the underlying pathophysiology of human MTLE.

Anticonvulsant neuropeptides as drug leads for neurological diseases

Anticonvulsant neuropeptides are best known for their ability to suppress seizures and modulate pain pathways. Galanin, neuropeptide Y, somatostatin, neurotensin, dynorphin, among others, have been validated as potential first-in-class anti-epileptic or/and analgesic compounds in animal models of epilepsy and pain, but their therapeutic potential extends to other neurological indications, including neurodegenerative and psychatric disorders. Disease-modifying properties of neuropeptides make them even more attractive templates for developing new-generation neurotherapeutics. Arguably, efforts to transform this class of neuropeptides into drugs have been limited compared to those for other bioactive peptides. Key challenges in developing neuropeptide-based anticonvulsants are: to engineer optimal receptor-subtype selectivity, to improve metabolic stability and to enhance their bioavailability, including penetration across the blood–brain barrier (BBB). Here, we summarize advances toward developing systemically active and CNS-penetrant neuropeptide analogs. Two main objectives of this review are: (1) to provide an overview of structural and pharmacological properties for selected anticonvulsant neuropeptides and their analogs and (2) to encourage broader efforts to convert these endogenous natural products into drug leads for pain, epilepsy and other neurological diseases.

Epileptic hypersynchrony revisited

Synchronization of neuronal responses, which allows coordination of distributed activity patterns, is instrumental in brain functioning, as altered neuronal synchronization is involved in a variety of brain pathologies. Epileptic hypersynchrony chiefly relies on brain wiring, which, in a broader sense, means including astrocytic release of gliotransmitters and electrotonic coupling through gap junctions, beyond classical synaptic connections. Epileptic hypersynchrony also relies on electrical field effects and ion concentration changes in the extracellular space, and it relates to intracellular mechanisms underlying neuronal hyperexcitability. The current lack of a specific impact of hypersynchrony on antiepileptic drug development might be next surpassed, as hypersynchrony seems to be a worthy and approachable, though challenging target of antiepileptic pharmacology.

(R)-[11C]verapamil PET studies to assess changes in P-glycoprotein expression and functionality in rat blood-brain barrier after exposure to kainate-induced status epilepticus

BACKGROUND

Increased functionality of efflux transporters at the blood-brain barrier may contribute to decreased drug concentrations at the target site in CNS diseases like epilepsy. In the rat, pharmacoresistant epilepsy can be mimicked by inducing status epilepticus by intraperitoneal injection of kainate, which leads to development of spontaneous seizures after 3 weeks to 3 months. The aim of this study was to investigate potential changes in P-glycoprotein (P-gp) expression and functionality at an early stage after induction of status epilepticus by kainate.

METHODS

(R)-[11C]verapamil, which is currently the most frequently used positron emission tomography (PET) ligand for determining P-gp functionality at the blood-brain barrier, was used in kainate and saline (control) treated rats, at 7 days after treatment. To investigate the effect of P-gp on (R)-[11C]verapamil brain distribution, both groups were studied without or with co-administration of the P-gp inhibitor tariquidar. P-gp expression was determined using immunohistochemistry in post mortem brains. (R)-[11C]verapamil kinetics were analyzed with approaches common in PET research (Logan analysis, and compartmental modelling of individual profiles) as well as by population mixed effects modelling (NONMEM).

RESULTS

All data analysis approaches indicated only modest differences in brain distribution of (R)-[11C]verapamil between saline and kainate treated rats, while tariquidar treatment in both groups resulted in a more than 10-fold increase. NONMEM provided most precise parameter estimates. P-gp expression was found to be similar for kainate and saline treated rats.

CONCLUSIONS

P-gp expression and functionality does not seem to change at early stage after induction of anticipated pharmacoresistant epilepsy by kainate.

Functional, metabolic, and synaptic changes after seizures as potential targets for antiepileptic therapy

Little is known about how the brain limits seizure duration and terminates seizures. Depending on severity and duration, a single seizure is followed by various functional, metabolic, and synaptic changes that may form targets for novel therapeutic strategies. It is long known that most seizures are followed by a period of postictal refractoriness during which the threshold for induction of additional seizures is increased. The endogenous anticonvulsant mechanisms involved in this phenomenon may be relevant for both spontaneous seizure arrest and increase of seizure threshold after seizure arrest. Postictal refractoriness has been extensively studied in various seizure and epilepsy models, including electrically and chemically induced seizures, kindling, and genetic animal models of epilepsy. During kindling development, two antagonistic processes occur simultaneously, one responsible for kindling-like events and the other for terminating ictus and postictal refractoriness. Frequently occurring seizures may lead to an accumulation of postictal refractoriness that may last weeks. The mechanisms involved in seizure termination and postictal refractoriness include changes in ionic microenvironment, in pH, and in various endogenous neuromodulators such as adenosine and neuropeptides. In animal models, the anticonvulsant efficacy of several antiepileptic drugs (AEDs) is increased during postictal refractoriness, which is a logical consequence of the interaction between endogenous anticonvulsant processes and the mechanism of AEDs. As discussed in this review, enhanced understanding of these endogenous processes may lead to novel targets for AED development.

Increased insulin receptor expression in anterior temporal neocortex of patients with intractable epilepsy

The insulin receptor (IR) is a tyrosine kinase receptor that binds to insulin and plays pivotal roles in energy homeostasis, neuronal growth, neuronal survival, synaptic plasticity and cognitive function. The biological mechanisms of intractable epilepsy involve energy metabolism, neuron loss, neurogenesis and abnormal neural networks. Here, we evaluated the expression of the IR in the anterior temporal neocortex of patients with intractable epilepsy (IE) by immunohistochemistry, double-label immunofluorescence and immunoblotting. We compared these tissues against histologically normal anterior temporal lobes from individuals treated for post-trauma intracranial hypertension. We found that the IR was coexpressed with neuron-specific enolase (NSE) and that IR expression increased in the anterior temporal neocortex of epileptic patients. On the basis of the potential physiological effects of IR, our findings suggest that increased expression of the IR is a consequence of epileptic seizures and a cause of IE.

Difficulties in Treatment and Management of Epilepsy and Challenges in New Drug Development

Epilepsy is a serious neurological disorder that affects around 50 million people worldwide. Almost 30% of epileptic patients suffer from pharmacoresistance, which is associated with social isolation, dependent behaviour, low marriage rates, unemployment, psychological issues and reduced quality of life. Currently available antiepileptic drugs have a limited efficacy, and their negative properties limit their use and cause difficulties in patient management. Antiepileptic drugs can provide only symptomatic relief as these drugs suppress seizures but do not have ability to cure epileptogenesis. The long term use of antiepileptic drugs is limited due to their adverse effects, withdrawal symptoms, deleterious interactions with other drugs and economic burden, especially in developing countries. Furthermore, some of the available antiepileptic drugs may even potentiate certain type of seizures. Several in vivo and in vitro animal models have been proposed and many new antiepileptic drugs have been marketed recently, but large numbers of patients are still pharmacoresistant. This review will highlight the difficulties in treatment and management of epilepsy and the limitations of available antiepileptic drugs and animal seizure models.

New developments in antiepileptic drug resistance: an integrative view

Current theories on drug resistance in epilepsy include the drug transporter hypothesis, the drug target hypothesis, and a novel approach called the inherent severity model of epilepsy, which posits that the severity of the disease determines its relative response to medication. Valuable as each of these hypotheses is, none is currently a stand-alone theory that is able to convincingly explain drug resistance in human epilepsy. As a consequence, it may be of interest to update and integrate the various hypotheses of drug resistance and to explore possible links to the severity of epilepsy. The observation that a high frequency of seizures prior to onset of treatment is a prognostic signal of increased severity and future drug failure suggests that common neurobiological factors may underlie both disease severity and pharmacoresistance. Such a link has been proposed for depression; however, the evidence for a direct mechanistic link, genetic or otherwise, between drug response and disease severity of human epilepsy is still elusive. Although emerging data from experimental studies suggest that alterations in GABA(A) receptors may present one example of a mechanistic link, clearly more work is needed to explore whether common neurobiological factors may underlie both epilepsy severity and drug failure.

Intrinsic severity as a determinant of antiepileptic drug refractoriness

For the most part, resistance to medications in epilepsy is independent of the choice of antiepileptic drug. This simple clinical observation constrains the possible biological mechanisms for drug refractory epilepsy by imposing a requirement to explain resistance for a diverse set of chemical structures that act on an even more varied group of molecular targets. To date, research on antiepileptic drug refractoriness has been guided by the "drug transporter overexpression" and the "reduced drug-target sensitivity" hypotheses. These concepts posit that drug refractoriness is a condition separate from the underlying epilepsy. Inadequacies in both hypotheses mandate a fresh approach to the problem. In this article, we propose a novel approach that considers epilepsy pharmacoresistance in terms of intrinsic disease severity. We suggest that neurobiological factors that confer increased disease severity lead to drug intractability. The occurrence of frequent seizures at disease onset is an important factor that signals increased severity.

Is epilepsy intractability predetermined or acquired?

How Long Does it Take for Epilepsy to Become Intractable? A Prospective Investigation Berg AT, Vickrey BG, Testa FM, Levy SR, Shinnar S, DiMario F, Smith S. Ann Neurol 2006;60:73–79.

Objective

To determine prospectively when in the course of epilepsy intractability becomes apparent.

Methods

Data are from a prospective cohort of 613 children followed for a median of 9.7 years. Epilepsy syndromes were grouped as focal, idiopathic, catastrophic, and other. Intractability was defined in two ways: (a) two drugs failed, 1 seizure/month, on average, for 18 months (stringent), and (b) failure of two drugs. Delayed intractability was defined as 3 or more years after epilepsy diagnosis.

Results

Eighty-three children (13.8%) met the stringent and 142 (23.2%) met the two-drug definition. Intractability depended on syndrome ( p < 0.0001): 26 (31.3%) children meeting stringent and 39 (27.5%) meeting the two-drug definition had delayed intractability. Intractability was delayed more often in focal than catastrophic epilepsy (stringent: 46.2 vs 14.3%, p = 0.003; two-drug: 40.3 vs 2.2%, p = 0.0001). Early remission periods preceded delayed intractability in 65.4–74.3% of cases. After becoming intractable, 20.5% subsequently entered remission and 13.3% were seizure free at last contact.

Interpretation

Intractable epilepsy may be delayed, especially in focal epilepsy. It often is preceded by a quiescent period, followed by further remissions. These findings help explain why surgically treatable epilepsies may take 20 years or longer before referral to surgery.

Means, motive, and opportunity: establishing culpability in pharmacoresistant epilepsy

Inhibition of the Multidrug Transporter P-Glycoprotein Improves Seizure Control in Phenytoin-Treated Chronic Epileptic Rats. van Vliet EA, van Schaik R, Edelbroek PM, Redeker S, Aronica E, Wadman WJ, Marchi N, Vezzani A, Gorter JA. Epilepsia 2006;47:672–680.

Purpose

Overexpression of multidrug transporters such as P-glycoprotein (P-gp) may play a significant role in pharmacoresistance, by preventing antiepileptic drugs (AEDs) from reaching their targets in the brain. Until now, many studies have described increased P-gp expression in epileptic tissue or have shown that several AEDs act as substrates for P-gp. However, definitive proof showing the functional involvement of P-gp in pharmacoresistance is still lacking. Here we tested whether P-gp contributes to pharmacoresistance to phenytoin (PHT) by using a specific P-gp inhibitor in a model of spontaneous seizures in rats.

Methods

The effects of PHT on spontaneous seizure activity were investigated in the electrical post-status epilepticus rat model for temporal lobe epilepsy, before and after administration of tariquidar (TQD), a selective inhibitor of P-gp.

Results

A 7-day treatment with therapeutic doses of PHT suppressed spontaneous seizure activity in rats, but only partially. However, an almost complete control of seizures by PHT (93% μ 7%) was obtained in all rats when PHT was coadministered with TQD. This specific P-gp inhibitor was effective in improving the anticonvulsive action of PHT during the first 3–4 days of the treatment. Western blot analysis confirmed P-gp upregulation in epileptic brains (140–200% of control levels), along with approximately 20% reduced PHT brain levels. Inhibition of P-gp by TQD significantly increased PHT brain levels in chronic epileptic rats.

Conclusions

These findings show that TQD significantly improves the anticonvulsive action of PHT, thus establishing a proof-of-concept that the administration of AEDs in combination with P-gp inhibitors may be a promising therapeutic strategy in pharmacoresistant patients.

ABCB1 C3435T polymorphism and the risk of resistance to antiepileptic drugs in epilepsy: a systematic review and meta-analysis

OBJECTIVE

The C3435T, a major allelic variant of the ABCB1 gene, is proposed to play a crucial role in drug-resistance in epilepsy. The C/C genotype carriers reportedly are at higher risk of pharmacoresistance to AEDs, but only in some studies. The hypothesis of the C-variant associated risk and resistance to antiepileptic drugs (AEDs) has been hampered by conflicting results from inadequate power in case-control studies. To assess the role of C3435T polymorphism in drug-resistance in epilepsy, a systematic review and meta-analysis was conducted.

METHODS

Databases were obtained from the Cochrane Library, MEDLINE, EMBASE, major American and European conference abstracts, and www.google.my for genetic association studies up to February 2010. All the case-control association studies evaluating the role of ABCB1 C3435T in pharmacoresistance to AEDs were identified. The new definition of treatment outcome from International League Against Epilepsy (ILAE) was used for including studies for sub-analysis. To measure the strength of genetic association for the gene variant, the odds ratios (ORs) with 95% confidence intervals (CIs) were calculated using models of both fixed- and random-effects for comparisons of the alleles and genotypes with co-dominant (C/C vs. T/T, C/T vs. T/T), dominant (C/C+C/T vs. T/T), and recessive (C/C vs. C/T+T/T) models in overall and in ethnicity subgroups. The 19 studies were selected for the next sub-analysis based on the new definition of drug-responsiveness and drug-resistance from ILAE. The same analysis was also performed for treatment outcome and ethnicity subgroups.

RESULTS

A total of 22 association studies including 3231 (47.8%) drug-resistant patients and 3524 (52.2%) drug-responsive patients or healthy controls (genotyped for C3435T) were pooled in this meta-analysis. The allelic association of ABCB1 C3435T with risk of drug-resistance was not significant under fixed-effects model, 1.06 (95% CI 0.98-1.14, p=0.12) and random-effects model, 1.10 (0.93-1.30, p=0.28) in overall and in the subgroup analysis by ethnicity. Similar results were also obtained for all genetic models in the stratified analyses by new definition of drug-resistance by ILAE and ethnicity subgroups. There was no publication bias.

CONCLUSION

We failed to show an association between the ABCB1 C3435T polymorphism and the risk of drug-resistance suggesting a revision in contribution of this polymorphism in the multi-drug transporters hypothesis of pharmacoresistance to AEDs in epilepsy.

Modulating P-glycoprotein regulation: future perspectives for pharmacoresistant epilepsies?

Enhanced brain efflux of antiepileptic drugs by the blood-brain barrier transporter P-glycoprotein is discussed as one mechanism contributing to pharmacoresistance of epilepsies. P-glycoprotein overexpression has been proven to occur as a consequence of seizure activity. Therefore, blocking respective signaling events should help to improve brain penetration and efficacy of P-glycoprotein substrates. A series of recent studies revealed key signaling factors involved in seizure-associated transcriptional activation of P-glycoprotein. These data suggested several interesting targets, including the N-methyl-d-aspartate (NMDA) receptor, the inflammatory enzyme cyclooxygenase-2, and the prostaglandin E2 EP1 receptor. These targets have been further evaluated in rodent models, demonstrating that targeting these factors can control P-glycoprotein expression, improve antiepileptic drug brain penetration, and help to overcome pharmacoresistance. In general, the approach offers particular advantages over transporter inhibition as it preserves basal transporter function. In this review the different strategies for blocking P-glycoprotein upregulation, including their therapeutic promise and drawbacks are discussed. Moreover, pros and cons of the approach are compared to those of alternative strategies to overcome transporter-associated resistance. Regarding future perspectives of the novel approach, there is an obvious need to more clearly define the clinical relevance of transporter overexpression. In this context current efforts are discussed, including the development of imaging tools that allow an evaluation of P-glycoprotein function in individual patients.

The antiepileptic drug topiramate is a substrate for human P-glycoprotein but not multidrug resistance proteins

PURPOSE

Resistance to antiepileptic drugs (AEDs) is the major problem in the treatment of epilepsy. One of the candidate mechanisms of pharmacoresistance is the limitation of AED access to the seizure focus by overexpression of efflux transporters, including P-glycoprotein (Pgp) and multidrug resistance proteins (MRPs).In this respect, it is important to know which AEDs are substrates for such drug transporters in humans.

METHODS

In the present study, we used polarized kidney cell lines (LLC, MDCK) transfected with human drug transporters (Pgp, MRP1, MRP2 or MRP5) to evaluate whether the AED topiramate is a substrate for any of these transporters. Known Pgp and MRP substrates were used for comparison.

RESULTS

Basolateral-to-apical transport of topiramate, which could be counteracted with the Pgp inhibitor, tariquidar, was determined in Pgp overexpressing LLC cells, whereas topiramate was not transported by any of the MRPs. A comparison with previous experiments in the same transport assay showed that topiramate exhibited the most pronounced Pgp-mediated efflux transport among the AEDS that have been studied as yet.

CONCLUSIONS

Thus, these data indicate that brain levels of topiramate may be affected by overexpression of Pgp as determined in patients with intractable epilepsy.

Association between temporal lobe P-glycoprotein expression and seizure recurrence after surgery for pharmacoresistant temporal lobe epilepsy

Surgery is recommended for pharmacoresistant temporal lobe epilepsy (TLE), but seizures recur in approximately one third of patients postsurgery. P-glycoprotein is an efflux multidrug transporter that is overexpressed in a range of epileptogenic pathologies. We hypothesized that increased expression of P-glycoprotein in the epileptogenic temporal lobe might be a marker for recurrence of pharmacoresistant seizures postsurgery. We performed immunohistochemistry on temporal lobe tissues resected from 69 patients who underwent anterior temporal lobectomy for pharmacoresistant TLE with histopathological proven hippocampal sclerosis. P-glycoprotein expression was rated by three pathologists independently. Patients with seizure recurrence (n=22) had greater number of positively stained capillaries (p=0.001) and higher P-glycoprotein immunoreactive score in capillaries (p=0.002) in the white matter of resected temporal lobe. The differences remained significant in multivariate analysis (p=0.002 and 0.006, respectively). The results suggest that P-glycoprotein expression in temporal lobe may be associated with seizure recurrence after surgery for pharmacoresistant TLE.

The COX-2 inhibitor parecoxib is neuroprotective but not antiepileptogenic in the pilocarpine model of temporal lobe epilepsy

The enzyme cyclooxygenase-2 (COX-2), which catalyzes the production of pro-inflammatory prostaglandins, is induced in the brain after various insults, thus contributing to brain inflammatory processes involved in the long-term consequences of such insults. Mounting evidence supports that inflammation may contribute to epileptogenesis and neuronal injury developing after brain insults. Anti-inflammatory treatments, such as selective COX-2 inhibitors, may thus constitute a novel approach for anti-epileptogenesis or disease-modification after brain injuries such as head trauma, cerebral ischemia or status epilepticus (SE). However, recent rat experiments with prophylactic administration of two different COX-2 inhibitors after SE resulted in conflicting results. In the present study, we evaluated whether treatment with parecoxib, a pro-drug of the highly potent and selective COX-2 inhibitor valdecoxib, alters the long-term consequences of a pilocarpine-induced SE in rats. Parecoxib was administered twice daily at 10 mg/kg for 18 days following SE. Five weeks after termination of treatment, spontaneous recurrent seizures were recorded by continuous video/EEG monitoring. Prophylactic treatment with parecoxib prevented the SE-induced increase in prostaglandin E(2) and reduced neuronal damage in the hippocampus and piriform cortex. However, the incidence, frequency or duration of spontaneous seizures developing after SE or the behavioral and cognitive alterations associated with epilepsy were not affected by parecoxib. Only the severity of spontaneous seizures was reduced, indicating a disease-modifying effect. These results substantiate that COX-2 contributes to neuronal injury developing after SE, but inhibition of COX-2 is no effective means to modify epileptogenesis.

Network dynamics during development of pharmacologically induced epileptic seizures in rats in vivo

In epilepsy, the cortical network fluctuates between the asymptomatic interictal state and the symptomatic ictal state of seizures. Despite their importance, the network dynamics responsible for the transition between the interictal and ictal states are largely unknown. Here we used multielectrode single-unit recordings from the hippocampus to investigate the network dynamics during the development of seizures evoked by various chemoconvulsants in vivo. In these experiments, we detected a typical network dynamics signature that preceded seizure initiation. The preictal state preceding pilocarpine-, kainate-, and picrotoxin-induced seizures was characterized by biphasic network dynamics composed of an early desynchronization phase in which the tendency of neurons to fire correlated action potentials decreased, followed by a late resynchronization phase in which the activity and synchronization of the network gradually increased. This biphasic network dynamics preceded the initiation both of the initial seizure and of recurrent spontaneous seizures that followed. During seizures, firing of individual neurons and interneuronal synchronization further increased. These findings advance our understanding of the network dynamics leading to seizure initiation and may in future help in the development of novel seizure prediction algorithms.

New and investigational antiepileptic drugs

In the last fifteen years, new antiepileptic medications have been offered for the treatment of patients with epilepsy. Nevertheless, despite optimal medical treatment, up to 30% of patients still experience recurrent seizures and the challenge for new, more efficacious and better-tolerated drugs continues. New antiepileptic drugs include the evolution of pre-existing drugs and new compounds identified through the investigation of additional molecular targets, such as SV2A synaptic vesicle protein, voltage-gated potassium channels, ionotropic and metabotropic glutamate receptors, and gap junctions. This paper reviews the available information on various classes of molecules that are in the pipeline as well as on the innovative approaches to the treatment of epilepsy.

Synthesis and anticonvulsant activity of a new 6-alkoxy-[1,2,4]triazolo[4,3-b]pyridazine

A series of 6-alkoxy-[1,2,4]triazolo[4,3-b]pyridazine derivatives were synthesized. In initial screening and quantitative evaluation, compound 2r was among the most active agents, exhibiting in the same time the lowest toxicity. In the anti-maximal electroshock test, it showed median effective dose (ED50) of 17.3 mg/kg and median toxicity dose (TD50) of 380.3 mg/kg, and the protective index (PI) of 22.0, which is much better than PI of the reference drugs. In a subsequent test, compound 2r had median hypnotic dose (HD50) of 746.6 mg/kg, thus demonstrating much better margin of safety compared to reference drugs. Compound 2r also showed oral activity against MES-induced seizures and lower oral neurotoxicity. For explanation of the putative mechanism of action, compound 2r was tested in chemical induced models.

Derivatives of benzimidazole pharmacophore: synthesis, anticonvulsant, antidiabetic and DNA cleavage studies

In seeking broad spectrum pharmacological activities of benzimidazole derivatives, a group of 4-thiazolidinones 5(a-j) and 1,3,4-oxadiazoles 6(a-j) containing 2-mercapto benzimidazole moiety were synthesized and screened for in vivo anticonvulsant activity by Maximal Electroshock (MES) model and antidiabetic activity using Oral Glucose Tolerance Test (OGTT). Compounds (5c), (5d), (5g) and (5i) exhibited potent anticonvulsant results and (6c), (6d), (6h) and (6i) showed excellent antidiabetic activities and also pharmacophore derived from active molecules suggested that presence of -OH group was a common feature in all active compounds. In DNA cleavage studies, compound (5d) cleaved DNA completely as no trace of DNA was found. On the other hand, a sharp streak was found for compounds (5c), (6a) and (6d).

Six months of postmarketing experience with adjunctive lacosamide in patients with pharmacoresistant focal epilepsy at a tertiary epilepsy center in Germany

We report our 6 months of experience with adjunctive lacosamide in 25 patients with pharmacoresistant focal epilepsy. Baseline characteristics of our patients were similar to those of the populations in the three clinical trials that evaluated lacosamide for refractory focal epilepsy. One patient experienced sustained seizure freedom for 5 months; two more patients had nonsustained periods of seizure freedom of 1 and 4 months. A total of eight patients (32%) reported a greater than 50% reduction in seizure frequency. Thirteen patients (52%) reported side effects during the titration, mostly dizziness, fatigue, nausea, and gait instability. In five patients (20%), these disappeared during the maintenance phase and/or with dose reduction. Two patients lost more than 10% of their body weight. Otherwise, in terms of efficacy and adverse effects, our data mirror the profile of lacosamide described in the three clinical trials. Substantial weight loss may occur in individual patients.

Delayed time to first remission identifies poor long-term drug response of childhood-onset epilepsy: a prospective population-based study

We determined if time to first remission predicts long-term antiepileptic drug response. We assessed time to first 1-year remission (1YR) as a determinant of entering future terminal 5-year remission (5YTR) in a population-based cohort of 144 children prospectively followed-up since their first unprovoked seizure before the age of 16 years up to the mean age of 48 years. The proportion of patients entering 5YTR was highly dependent on the length of time to first 1YR after starting adequate treatment. For 144 patients, the overall 5YTR rate decreased from 32% for those in remission at year 1 to 24% at year 2, to 5% after 3 and 4 years, to 2% after 5 years or longer. Patients who entered 1YR within the first 5 years of treatment had an 11-fold better chance to enter 5YTR (odds ratio=11.4, 95% CI=2.9-45.3, P=0.0005) and a 9-fold chance for uninterrupted 5YTR off medications (OR=9.0, 95% CI=1.171.9, P=0.0383) compared with those who did not enter 1YR within the first 5 years of treatment. Three additional independent prognostic factors for predicting terminal 5YTR were confirmed: etiology, seizure frequency prior to treatment, and seizure frequency during treatment. We conclude that delayed efficacy after starting drug treatment gradually diminishes chances for long-term seizure remission, whether on medication or not. Not entering remission within 5 years of starting treatment predicts failure to achieve long-term seizure freedom in the future for the vast majority of patients.

[Drug-resistant epilepsy]

BACKGROUND

Despite the introduction of many new antiepileptic drugs during recent years, about one third of the epilepsy population continue to have drug-resistant seizures. We present two hypotheses on mechanisms of drug-resistant epileptic seizures.

MATERIAL AND METHODS

The article is based on our own clinical experience and literature identified through a PubMed search using the words: refractory, intractable, pharmacoresistant, drug resistant epilepsy/seizures.

RESULTS

There is no international consensus on the definition of drug-resistant epilepsy. Some epilepsy types appear to respond to drug treatment, while others are more resistant. Animal models and refractory patients have been studied to find mechanisms that can explain the lack of drug response in some types of epilepsy. Currently, two main hypotheses prevail. The transporter hypothesis implies an over-expression of the proteins (e.g. P-glycoprotein) capable of transporting drugs out of the epileptic focus. The target hypothesis implies changes in receptors or ion channels in the epileptic cellular network, which render this area less susceptible to antiepileptic drugs.

INTERPRETATION

Both hypotheses have weaknesses, and alternative theories have been presented. Much remains before the causes of drug-resistant epilepsy are understood. If patients do not achieve seizure control after having tried 2-3 antiepileptic drugs, they should be referred to the national epilepsy centre for a renewed diagnostic evaluation and for consideration of non-pharmacological treatment.