New developments in antiepileptic drug resistance: an integrative view

Cerebral expression of drug transporters in epilepsy

Over-expression of drug efflux transporters at the level of the blood-brain barrier (BBB) has been proposed as a mechanism responsible for multidrug resistance. Drug transporters in epileptogenic tissue are not only expressed in endothelial cells at the BBB, but also in other brain parenchymal cells, such as astrocytes, microglia and neurons, suggesting a complex cell type-specific regulation under pathological conditions associated with epilepsy. This review focuses on the cerebral expression patterns of several classes of well-known membrane drug transporters such as P-glycoprotein (Pgp), and multidrug resistance-associated proteins (MRPs) in the epileptogenic brain. Both experimental and clinical evidence of epilepsy-associated cerebral drug transporter regulation and the possible mechanisms underlying drug transporter regulation are discussed. Knowledge of the cerebral expression patterns of drug transporters in normal and epileptogenic brain will provide relevant information to guide strategies attempting to overcome drug resistance by targeting specific transporters.

Neuroimaging for the prediction of response to medical and surgical treatment in epilepsy

INTRODUCTION

Approximately 30% of patients with epilepsy do not respond to adequate medication and are candidates for surgical treatment. Outcome predictors can improve the selection of more suitable treatment options for each patient. Therefore, the authors aimed to review the role of neuroimaging studies in predicting outcomes for both clinical and surgical treatment of epilepsy.

AREAS COVERED

This review analyzes studies that investigated different neuroimaging techniques as predictors of clinical and surgical treatment outcome in epilepsy. Studies involving both structural (i.e., T1-weighted images and diffusion tensor images) and functional MRI (fMRI) were identified, as well as other modalities such as spectroscopy, PET, SPECT and MEG. The authors also evaluated the importance of fMRI in predicting memory outcome after surgical resections in temporal lobe epilepsy.

EXPERT OPINION

The identification of reliable biomarkers to predict response to medical and surgical treatments are much needed in order to provide more adequate patient counseling about prognosis and treatment options individually. Different neuroimaging techniques may provide combined measurements that potentially may become these biomarkers in the near future.

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.

Exploring the genomic basis of pharmacoresistance in epilepsy: an integrative analysis of large-scale gene expression profiling studies on brain tissue from epilepsy surgery

Some patients with pharmacoresistant epilepsy undergo therapeutic resection of the epileptic focus. At least 12 large-scale microarray studies on brain tissue from epilepsy surgery have been published over the last 10 years, but they have failed to make a significant impact upon our understanding of pharmacoresistance, because (1) doubts have been raised about their reproducibility, (2) only a small number of the gene expression changes found in each microarray study have been independently validated and (3) the results of different studies have not been integrated to give a coherent picture of the genetic changes involved in epilepsy pharmacoresistance. To overcome these limitations, we (1) assessed the reproducibility of the microarray studies by calculating the overlap between lists of differentially regulated genes from pairs of microarray studies and determining if this was greater than would be expected by chance alone, (2) used an inter-study cross-validation technique to simultaneously verify the expression changes of large numbers of genes and (3) used the combined results of the different microarray studies to perform an integrative analysis based on enriched gene ontology terms, networks and pathways. Using this approach, we respectively (1) demonstrate that there are statistically significant overlaps between the gene expression changes in different publications, (2) verify the differential expression of 233 genes and (3) identify the biological processes, networks and genes likely to be most important in the development of pharmacoresistant epilepsy. Our analysis provides novel biologically plausible candidate genes and pathways which warrant further investigation to assess their causal relevance.

Antiepileptic drug discovery: does mechanism of action matter?

This commentary discusses briefly the role of the mechanism of antiepileptic action in discovery of drugs for the treatment of epilepsy. More specifically, two questions are addressed. (1) Has mechanism-driven antiepileptic drug discovery brought us better epilepsy treatment? Although this question is difficult to answer, the short answer is "not yet." Modern antiepileptic drugs with new or modified mechanisms of action do not seem to have substantially improved the efficacy or the safety of epilepsy treatment. In fact, some modern antiepileptic drugs such as progabide, tiagabine, and vigabatrin have been associated with a number of safety issues. (2) Why do drugs with new mechanisms seem to have failed to deliver better treatment? Although it is always difficult to know why something did not occur, one putative explanation may be worthwhile to consider. The past development of new antiepileptic drugs targeted putative mechanisms of seizure generation. As seizures are only symptoms of the underlying epilepsy, blocking seizure generation can provide at best only symptomatic treatment. It may be that the failure in treating drug-resistant seizures is related, at least in part, to the failure of current drugs in targeting the mechanisms underlying epilepsy. In conclusion, continuing to develop new antiepileptic drugs for drug-resistant epilepsy by targeting seizure generation may be futile and one possible explanation of why we do not seem to make substantial progress in the treatment of drug-resistant epilepsy. Developing antiepileptic drugs with antiepileptogenic activity may be a clue to better treatment of presently drug-resistant epilepsy.

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.

Antiepileptic drugs modulate P-glycoproteins in the brain: a mice study with (11)C-desmethylloperamide

P-glycoprotein transporters (P-gp) located at the blood-brain barrier (BBB) are likely to play a role in refractory epilepsy. In vitro studies already pointed out that several antiepileptic drugs (AEDs) are substrate of P-gp. This study proposes a new in vivo approach to investigate the interaction between some AEDs and P-gp located at the BBB. (11)C-desmethylloperamide ((11)C-dLop), a radiolabelled substrate of P-gp, was intravenously administrated after pretreatment with saline or AEDs (sodium valproate, levetiracetam, topiramate and phenytoin) at their human therapeutic and four times their therapeutic dose. The effect of the different pretreatment on the intracerebral concentration of (11)C-dLop was determined to indirectly investigate possible in vivo interactions between AEDs and P-gp. Pretreatment with levetiracetam, topiramate and phenytoin at therapeutic doses significantly decreased intracerebral concentration of (11)C-dLop. Pretreatment with a therapeutic dose of sodium valproate did not influence brain uptake of (11)C-dLop. In case of pretreatment with supratherapeutic doses of AED, (11)C-dLop brain uptake was not different compared to pretreatment with saline. The metabolisation rate of (11)C-dLop in plasma was unaltered, indicating that observed differences in brain uptake of the tracer were not due to pharmacokinetic changes. The following conclusion can be made: levetiracetam, topiramate and phenytoin demonstrate biphasic modulation of the BBB P-gp. At therapeutic doses they act as inducers of efflux, at supratherapeutic doses they have no effect on the efflux rate. Sodium valproate does not interact with P-gp at therapeutic nor at higher doses.

ECoG studies of valproate, carbamazepine and halothane in frontal-lobe epilepsy induced by head injury in the rat

The use of electrocorticography (ECoG) with etiologically realistic epilepsy models promises to facilitate the discovery of better anti-epileptic drugs (AEDs). However, this novel approach is labor intensive, and must be optimized. To this end, we employed rostral parasagittal fluid percussion injury (rpFPI) in the adolescent rat, which closely replicates human contusive closed head injury and results in posttraumatic epilepsy (PTE). We systematically examined variables affecting the power to detect anti-epileptic effects by ECoG and used a non-parametric bootstrap strategy to test several different statistics, study designs, statistical tests, and impact of non-responders. We found that logarithmically transformed data acquired in repeated-measures experiments provided the greatest statistical power to detect decreases in seizure frequencies of preclinical interest with just 8 subjects and with up to approximately 40% non-responders. We then used this optimized design to study the anti-epileptic effects of acute exposure to halothane, and chronic (1 week) exposures to carbamazepine (CBZ) and valproate (VPA) 1 month post-injury. While CBZ was ineffective in all animals, VPA induced, during treatment, a progressive decrease in seizure frequency in animals primarily suffering from non-spreading neocortical seizures, but was ineffective in animals with a high frequency of spreading seizures. Halothane powerfully blocked all seizure activity. The data show that rpFPI and chronic ECoG can conveniently be employed for the evaluation of AEDs, suggest that VPA may be more effective than CBZ to treat some forms of PTE, and support the theory that pharmacoresistance may depend on the severity of epilepsy. The data also demonstrate the utility of chronic exposures to experimental drugs in preclinical studies and highlight the need for greater attention to etiology in clinical studies of AEDs.

Combined effects of epileptic seizure and phenobarbital induced overexpression of P-glycoprotein in brain of chemically kindled rats

BACKGROUND AND PURPOSE

The multidrug resistance of epilepsy may result from the overexpression of P-glycoprotein, but the mechanisms are unclear. We investigated whether the overexpression of P-glycoprotein in the brains of subjects with pharmacoresistant epilepsy resulted from both drug effects and seizure activity.

EXPERIMENTAL APPROACH

Kindled rats were developed by injecting a subconvulsive dose of pentylenetetrazole (33 mg.kg(-1).day(-1), i.p.) for 28 days. Groups were then treated with an oral dose of phenobarbital (45 mg x kg(-1) x day(-1)) for 40 days. In accord with behavioural observations, P-glycoprotein activity in brain was assessed using brain-to-plasma concentration ratios of rhodamine 123. P-glycoprotein levels in the brain regions were further evaluated using RT-PCR and Western blot analysis. The distribution of phenobarbital in the brain was assessed by measuring phenobarbital concentrations 1 h following its oral administration.

KEY RESULTS

The kindling significantly increased P-glycoprotein activity and expression. Good associations were found among P-glycoprotein activity, expression and phenobarbital concentration in the hippocampus. Short-term treatment with phenobarbital showed good anti-epileptic effect; the maximum effect occurred on day 14 when overexpression of P-glycoprotein was reversed. Continuous treatment with phenobarbital had a gradually reduced anti-epileptic effect and on day 40, phenobarbital exhibited no anti-epileptic effect; this was accompanied by both a re-enhancement of P-glycoprotein expression and decreased phenobarbital concentration in the hippocampus. P-glycoprotein function and expression were also increased in age-matched normal rats treated with phenobarbital.

CONCLUSIONS AND IMPLICATIONS

The overexpression of P-glycoprotein in the brain of subjects with pharmacoresistant epilepsy is due to a combination of drug effects and epileptic seizures.

Evaluation of transport of common antiepileptic drugs by human multidrug resistance-associated proteins (MRP1, 2 and 5) that are overexpressed in pharmacoresistant epilepsy

Resistance to antiepileptic drugs (AEDs) is one of the most serious problems in the treatment of epilepsy. Accumulating experimental evidence suggests that increased expression of the drug efflux transporter P-glycoprotein (Pgp) at the blood-brain barrier may be involved in the mechanisms leading to AED resistance. In addition to Pgp, increased expression of several multidrug resistance-associated proteins (MRPs) has been determined in epileptogenic brain regions of patients with pharmacoresistant epilepsy. However, it is not known whether AEDs are substrates for MRPs. In the present experiments, we evaluated whether common AEDs are transported by human MRPs (MRP1, 2 and 5) that are overexpressed in AED resistant epilepsy. For this purpose, we used a highly sensitive assay (concentration equilibrium transport assay; CETA) in polarized kidney cell lines (LLC, MDCKII) transfected with human MRPs. The assay was validated by known MRP substrates, including calcein-AM (MRP1), vinblastine (MRP2) and chloromethylfluorescein diacetate (CMFDA; MRP5). The directional transport determined with these drugs in MRP-transfected cell lines could be blocked with the MRP inhibitor MK571. However, in contrast to transport of known MRP substrates, none of the common AEDs (carbamazepine, valproate, levetiracetam, phenytoin, lamotrigine and phenobarbital) used in this study was transported by MRP1, MRP2 or MRP5. A basolateral-to-apical transport of valproate, which could be inhibited by MK571 and probenecid, was determined in LLC cells (both wildtype and transfected), but the specific transporter involved was not identified. The data indicate that common AEDs are not substrates for human MRP1, MRP2 or MRP5, at least in the in vitro models used in this study.

Imaging of P-glycoprotein function and expression to elucidate mechanisms of pharmacoresistance in epilepsy

The issue of pharmacoresistance in epilepsy has received considerable attention in recent years, and a number of plausible hypotheses have been proposed. Of these, the so-called transporter hypothesis is the most extensively researched and documented. This hypothesis assumes that refractory epilepsy is associated with a localised over-expression of drug transporter proteins such as P-glycoprotein (Pgp) in the region of the epileptic focus, which actively extrudes antiepileptic drugs (AEDs) from their intended site of action. However, although this hypothesis has biological plausibility, there is no clinical evidence to support the assertion that AEDs are sufficiently strong substrates for transporter-mediated extrusion from the brain. The use of modern brain imaging techniques to determine Pgp function in patients with refractory epilepsy has started only recently, and may ultimately determine whether increased expression and function of Pgp or other efflux transporters are involved in AED resistance.

Management of the patient with medically refractory epilepsy

Epilepsy imposes a significant clinical, epidemiologic and economic burden on societies throughout the world. Despite the development of more than ten new antiepileptic drugs over the past 15 years, approximately a third of patients with epilepsy remain resistant to pharmacotherapy. Individuals who fail to respond, or respond only partially, continue to have incapacitating seizures. Managing patients with medically refractory epilepsy is challenging and requires a structured multidisciplinary approach in specialized clinics. If the problems related to drug resistance could be resolved, even in part, by improving the pharmacokinetic profile of existing drugs, the economic savings would be remarkable and the time required to design drugs that achieve seizure control would be shorter than the discovery of new targets and molecules was required. A promising approach is the use of corticosteroids that may have a dual beneficial effect. Resective brain surgery remains the ultimate and highly successful approach to multiple drug resistance in epileptic patients.

Significance of the EEG in the decision to initiate antiepileptic treatment in patients with epilepsy: a perspective on recent evidence

The significance of electroencephalography in the prediction of seizure recurrence after a first unprovoked seizure remains a topic of debate. Opinion on the initiation of antiepileptic treatment after a first seizure also remains divided. However, in view of recent evidence, this article is intended to highlight the significance of a properly performed EEG in the decision to initiate antiepileptic drug treatment as early as possible to prevent further morbidity and other consequences.

High seizure frequency prior to antiepileptic treatment is a predictor of pharmacoresistant epilepsy in a rat model of temporal lobe epilepsy

PURPOSE

Progress in the management of patients with medically intractable epilepsy is impeded because we do not fully understand why pharmacoresistance happens and how it can be predicted. The presence of multiple seizures prior to medical treatment has been suggested as a potential predictor of poor outcome. In the present study, we used an animal model of temporal lobe epilepsy to investigate whether pharmacoresistant rats differ in seizure frequency from pharmacoresponsive animals.

METHODS

Epilepsy with spontaneous recurrent seizures (SRS) was induced by status epilepticus. Frequency of SRS was determined by video/EEG (electroencephalography) monitoring in a total of 33 epileptic rats before onset of treatment with phenobarbital (PB).

RESULTS

Thirteen (39%) rats did not respond to treatment with PB. Before treatment with PB, average seizure frequency in PB nonresponders was significantly higher than seizure frequency in responders, which, however, was due to six nonresponders that exhibited > 3 seizures per day. Such high seizure frequency was not observed in responders, demonstrating that high seizure frequency predicts pharmacoresistance in this model, but does not occur in all nonresponders.

DISCUSSION

The data from this study are in line with clinical experience that the frequency of seizures in the early phase of epilepsy is a dominant risk factor that predicts refractoriness. However, resistance to treatment also occurred in rats that did not differ in seizure frequency from responders, indicating that disease severity alone is not sufficient to explain antiepileptic drug (AED) resistance. These data provide further evidence that epilepsy models are useful in the search for predictors and mechanisms of pharmacoresistance.

Drug treatment of epilepsy: options and limitations

The modern antiepileptic drug (AED) era--spanning a period of more than 150 years from the first use of bromide in 1857 to 2008--has seen the introduction into clinical practice of a diverse group of effective and safe drugs. These AEDs have provided considerable benefits for those afflicted with epilepsy of all kinds. In as many as 60-70% of newly treated patients, current AEDs lead to satisfactory control of seizures and a favorable risk-benefit balance for the great majority of patients, albeit with considerable differences in response depending on the type of seizure and epilepsy syndrome and rare serious adverse events. Unfortunately, in 20-30% of patients, epilepsy cannot be controlled. Patients with drug-resistant epilepsy often have serious comorbidity, including injury, depression, anxiety, and increased mortality. The aim of antiepileptic treatment should be to control seizures as quickly as possible with no or minimal side effects and with no negative impact on the quality of life. Improved seizure control is likely to reduce the morbidity and increased mortality associated with uncontrolled epilepsy. In this short overview, the options and the limitations of treating patients with epilepsy are briefly summarized.