Tuberous sclerosis associated with MDR1 gene expression and drug-resistant epilepsy

Transcranial Focal Electric Stimulation Avoids P-Glycoprotein Over-Expression during Electrical Amygdala Kindling and Delays Epileptogenesis in Rats

Recent evidence suggests that P-glycoprotein (P-gp) overexpression mediates hyperexcitability and is associated with epileptogenesis. Transcranial focal electrical stimulation (TFS) delays epileptogenesis and inhibits P-gp overexpression after a generalized seizure. Here, first we measured P-gp expression during epileptogenesis and second, we assessed if TFS antiepileptogenic effect was related with P-gp overexpression avoidance. Male Wistar rats were implanted in right basolateral amygdala and stimulated daily for electrical amygdala kindling (EAK), P-gp expression was assessed during epileptogenesis in relevant brain areas. Stage I group showed 85% increase in P-gp in ipsilateral hippocampus (p < 0.001). Stage III group presented 58% and 57% increase in P-gp in both hippocampi (p < 0.05). Kindled group had 92% and 90% increase in P-gp in both hippocampi (p < 0.01), and 93% and 143% increase in both neocortices (p < 0.01). For the second experiment, TFS was administrated daily after each EAK stimulation for 20 days and P-gp concentration was assessed. No changes were found in the TFS group (p > 0.05). Kindled group showed 132% and 138% increase in P-gp in both hippocampi (p < 0.001) and 51% and 92% increase in both cortices (p < 0.001). Kindled + TFS group presented no changes (p > 0.05). Our experiments revealed that progression of EAK is associated with increased P-gp expression. These changes are structure-specific and dependent on seizure severity. EAK-induced P-gp overexpression would be associated with neuronal hyperexcitability and thus, epileptogenesis. P-gp could be a novel therapeutical target to avoid epileptogenesis. In accordance with this, TFS inhibited P-gp overexpression and interfered with EAK. An important limitation of the present study is that P-gp neuronal expression was not evaluated under the different experimental conditions. Future studies should be carried out to determine P-gp neuronal overexpression in hyperexcitable networks during epileptogenesis. The TFS-induced lessening of P-gp overexpression could be a novel therapeutical strategy to avoid epileptogenesis in high-risk patients.

Factors not considered in the study of drug-resistant epilepsy: Drug-resistant epilepsy: assessment of neuroinflammation

More than one‐third of people with epilepsy develop drug‐resistant epilepsy (DRE). Different hypotheses have been proposed to explain the origin of DRE. Accumulating evidence suggests the contribution of neuroinflammation, modifications in the integrity of the blood‐brain barrier (BBB), and altered immune responses in the pathophysiology of DRE. The inflammatory response is mainly due to the increase of cytokines and related molecules; these molecules have neuromodulatory effects that contribute to hyperexcitability in neural networks that cause seizure generation. Some patients with DRE display the presence of autoantibodies in the serum and mainly cerebrospinal fluid. These patients are refractory to the different treatments with standard antiseizure medications (ASMs), and they could be responding well to immunomodulatory therapies. This observation emphasizes that the etiopathogenesis of DRE is involved with immunology responses and associated long‐term events and chronic inflammation processes. Furthermore, multiple studies have shown that functional polymorphisms as risk factors are involved in inflammation processes. Several relevant polymorphisms could be considered risk factors involved in inflammation‐related DRE such as receptor for advanced glycation end products (RAGE) and interleukin 1β (IL‐1β). All these evidences sustained the hypothesis that the chronic inflammation process is associated with the DRE. However, the effect of the chronic inflammation process should be investigated in further clinical studies to promote the development of novel therapeutics useful in treatment of DRE.

Transporter hypothesis in pharmacoresistant epilepsies Is it at the central or peripheral level?

The multidrug‐resistance (MDR) phenotype is typically observed in patients with refractory epilepsy (RE) whose seizures are not controlled despite receiving several combinations of more than two antiseizure medications (ASMs) directed against different ion channels or neurotransmitter receptors. Since the use of bromide in 1860, more than 20 ASMs have been developed; however, historically ~30% of cases of RE with MDR phenotype remains unchanged. Irrespective of metabolic biotransformation, the biodistribution of ASMs and their metabolites depends on the functional expression of some ATP‐binding cassette transporters (ABC‐t) in different organs, such as the blood‐brain barrier (BBB), bowel, liver, and kidney, among others. ABC‐t, such as P‐glycoprotein (P‐gp), multidrug resistance–associated protein (MRP‐1), and breast cancer–resistance protein (BCRP), are mainly expressed in excretory organs and play a critical role in the pharmacokinetics (PK) of all drugs. The transporter hypothesis can explain pharmacoresistance to a broad spectrum of ASMs, even when administered simultaneously. Since ABC‐t expression can be induced by hypoxia, inflammation, or seizures, a high frequency of uncontrolled seizures increases the risk of RE. These stimuli can induce ABC‐t expression in excretory organs and in previously non‐expressing (electrically responsive) cells, such as neurons or cardiomyocytes. In this regard, an alternative mechanism to the classical pumping function of P‐gp indicates that P‐gp activity can also produce a significant reduction in resting membrane potential (ΔΨ0 = −60 to −10 mV). P‐gp expression in neurons and cardiomyocytes can produce membrane depolarization and participate in epileptogenesis, heart failure, and sudden unexpected death in epilepsy. On this basis, ABC‐t play a peripheral role in controlling the PK of ASMs and their access to the brain and act at a central level, favoring neuronal depolarization by mechanisms independent of ion channels or neurotransmitters that current ASMs cannot control.

The role of P-glycoprotein (P-gp) and inwardly rectifying potassium (Kir) channels in sudden unexpected death in epilepsy (SUDEP)

Sudden unexpected death in epilepsy (SUDEP) is the major cause of death that affects patients with epilepsy. The risk of SUDEP increases according to the frequency and severity of uncontrolled seizures; therefore, SUDEP risk is higher in patients with refractory epilepsy (RE), in whom most antiepileptic drugs (AEDs) are ineffective for both seizure control and SUDEP prevention. Consequently, RE and SUDEP share a multidrug resistance (MDR) phenotype, which is mainly associated with brain overexpression of ABC-transporters such as P-glycoprotein (P-gp). The activity of P-gp can also contribute to membrane depolarization and affect the normal function of neurons and cardiomyocytes. Other molecular regulators of membrane potential are the inwardly rectifying potassium channels (Kir), whose genetic variants have been related to both epilepsy and heart dysfunctions. Although it has been suggested that dysfunctions of the cardiac, respiratory, and brainstem arousal systems are the causes of SUDEP, the molecular basis for explaining its dysfunctions remain unknown. In rats, repetitive seizures or status epilepticus induced high expression of P-gp and loss Kir expression in the brain and heart, and promoted membrane depolarization, malignant bradycardia, and the high rate of mortality. Here we reviewed clinical and experimental evidences suggesting that abnormal expression of depolarizing/repolarizing factors as P-gp and Kir could favor persistent depolarization of membranes without any rapid functional recovery capacity. This condition induced by convulsive stress could be the molecular mechanism leading to acquired severe bradycardia, as an ineffective heart response generating the appropriate scenario for SUDEP development. This article is part of the Special Issue "NEWroscience 2018".

Drug-resistant epilepsy: From multiple hypotheses to an integral explanation using preclinical resources

Drug-resistant epilepsy affects approximately one-third of the patients with epilepsy. The pharmacoresistant condition in epilepsy is mainly explained by six hypotheses. In addition, several experimental models have been used to understand the mechanisms involved in pharmacoresistant epilepsy and to identify novel therapies to control this condition. However, the global prevalence of this disease persists without changes. Several factors can explain this situation. First of all, the pharmacoresistant epilepsy is explained by different and independent hypotheses. Each hypothesis indicates specific mechanisms to explain the drug-resistant condition in epilepsy. However, there are different findings suggesting common mechanisms between the different hypotheses. Other important situation is that the experimental models designed for the screening of drugs with potential anticonvulsant effect do not consider factors such as age, gender, type of epilepsy, and comorbid disorders. The present review focuses on indicating the limitations for each hypothesis and the relationships among them. The relevance to consider central and peripheral phenomena associated with the drug-resistant condition in different types of epilepsy is also indicated. The necessity to establish a global hypothesis that integrates all the phenomena associated with the pharmacoresistant epilepsy is proposed. This article is part of the Special Issue "NEWroscience 2018".

EPO and EPO-Receptor System as Potential Actionable Mechanism for the Protection of Brain and Heart in Refractory Epilepsy and SUDEP

The most important activity of erythropoietin (EPO) is the regulation of erythrocyte production by activation of the erythropoietin receptor (EPO-R), which triggers the activation of anti-apoptotic and proliferative responses of erythroid progenitor cells. Additionally, to erythropoietic EPO activity, an antiapoptotic effect has been described in a wide spectrum of tissues. EPO low levels are found in the central nervous system (CNS), while EPO-R is expressed in most CNS cell types. In spite of EPO-R high levels expressed during the hypoxicischemic brain, insufficient production of endogenous cerebral EPO could be the cause of determined circuit alterations that lead to the loss of specific neuronal populations. In the heart, high EPO-R expression in cardiac progenitor cells appears to contribute to myocardial regeneration under EPO stimulation. Several lines of evidence have linked EPO to an antiapoptotic role in CNS and in heart tissue. In this review, an antiapoptotic role of EPO/EPO-R system in both brain and heart under hypoxic conditions, such as epilepsy and sudden death (SUDEP) has been resumed. Additionally, their protective effects could be a new field of research and a novel therapeutic strategy for the early treatment of these conditions and avoid SUDEP.

Cannabidiol (CBD) Inhibited Rhodamine-123 Efflux in Cultured Vascular Endothelial Cells and Astrocytes Under Hypoxic Conditions

Despite the constant development of new antiepileptic drugs (AEDs), more than 30% of patients develop refractory epilepsy (RE) characterized by a multidrug-resistant (MDR) phenotype. The “transporters hypothesis” indicates that the mechanism of this MDR phenotype is the overexpression of ABC transporters such as P-glycoprotein (P-gp) in the neurovascular unit cells, limiting access of the AEDs to the brain. Recent clinical trials and basic studies have shown encouraging results for the use of cannabinoids in RE, although its mechanisms of action are still not fully understood. Here, we have employed astrocytes and vascular endothelial cell cultures subjected to hypoxia, to test the effect of cannabidiol (CBD) on the P-gp-dependent Rhodamine-123 (Rho-123) efflux. Results show that during hypoxia, intracellular Rho-123 accumulation after CBD treatment is similar to that induced by the P-gp inhibitor Tariquidar (Tq). Noteworthy, this inhibition is like that registered in non-hypoxia conditions. Additionally, docking studies predicted that CBD could behave as a P-gp substrate by the interaction with several residues in the α-helix of the P-gp transmembrane domain. Overall, these findings suggest a direct effect of CBD on the Rho-123 P-gp-dependent efflux activity, which might explain why the CBD add-on treatment regimen in RE patients results in a significant reduction in seizure frequency.

Convulsive Stress Mimics Brain Hypoxia and Promotes the P-Glycoprotein (P-gp) and Erythropoietin Receptor Overexpression. Recombinant Human Erythropoietin Effect on P-gp Activity

Erythropoietin (EPO) is not only a hormone that promotes erythropoiesis but also has a neuroprotective effect on neurons attributed to its known anti-apoptotic action. Previously, our group has demonstrated that recombinant-human EPO (rHu-EPO) can protect neurons and recovery motor activity in a chemical focal brain hypoxia model (Merelli et al., 2011). We and others also have reported that repetitive seizures can mimic a hypoxic- like condition by HIF-1α nuclear translocation and high neuronal expression P-gp. Here, we report that a single 20-min status epilepticus (SE) induces P-gp and EPO-R expression in cortical pyramidal neurons and only P-gp expression in astrocytes. In vitro, excitotoxic stress (300 μM glutamate, 5 min), can also induce the expression of EPO-R and P-gp simultaneously with both HIF-1α and NFkB nuclear translocation in primary cortical neurons. Primary astrocytes exposed to chemical hypoxia with CoCl₂ (0.3 mM, 6 h) increased P-gp expression as well as an increased efflux of Rhodamine 123 (Rho123) that is a P-gp substrate. Tariquidar, a specific 3^er generation P-gp-blocker was used as an efflux inhibitor control. Astrocytes treated with rHu-EPO showed a significant recovery of the Rho123 retention in a similar way as seen by Tariquidar, demonstrating for first time that rHu-EPO can inhibit the P-gp-dependent efflux activity. Taking together, these data suggest that stimulation of EPO depending signaling system could not only play a central role in brain cell protection, but this system could be a new tool for reverse the pharmacoresistant phenotype in refractory epilepsy as well as in other pharmacoresistant hypoxic brain diseases expressing P-gp.

ABC transporters in drug-resistant epilepsy: mechanisms of upregulation and therapeutic approaches

Drug-resistant epilepsy (DRE) affects approximately one third of epileptic patients. Among various theories that try to explain multidrug resistance, the transporter hypothesis is the most extensively studied. Accordingly, the overexpression of efflux transporters in the blood-brain barrier (BBB), mainly from the ATP binding cassette (ABC) superfamily, may be responsible for hampering the access of antiepileptic drugs into the brain. P-glycoprotein and other efflux transporters are known to be upregulated in endothelial cells, astrocytes and neurons of the neurovascular unit, a functional barrier critically involved in the brain penetration of drugs. Inflammation and oxidative stress involved in the pathophysiology of epilepsy together with uncontrolled recurrent seizures, drug-associated induction and genetic polymorphisms are among the possible causes of ABC transporters overexpression in DRE. The aforementioned pathological mechanisms will be herein discussed together with the multiple strategies to overcome the activity of efflux transporters in the BBB - from direct transporters inhibition to down-regulation of gene expression resorting to RNA interference (RNAi), or by targeting key modulators of inflammation and seizure-mediated signalling.

Understanding the Role of Hypoxia Inducible Factor During Neurodegeneration for New Therapeutics Opportunities

Neurodegeneration (NDG) is linked with the progressive loss of neural function with intellectual and/or motor impairment. Several diseases affecting older individuals, including Alzheimer's disease, Amyotrophic Lateral Sclerosis, Huntington's disease, Parkinson's disease, stroke, Multiple Sclerosis and many others, are the most relevant disorders associated with NDG. Since other pathologies such as refractory epilepsy, brain infections, or hereditary diseases such as "neurodegeneration with brain iron accumulation", also lead to chronic brain inflammation with loss of neural cells, NDG can be said to affect all ages. Owing to an energy and/or oxygen supply imbalance, different signaling mechanisms including MAPK/PI3K-Akt signaling pathways, glutamatergic synapse formation, and/or translocation of phosphatidylserine, might activate some central executing mechanism common to all these pathologies and also related to oxidative stress. Hypoxia inducible factor 1-α (HIF-1α) plays a twofold role through gene activation, in the sense that this factor has to "choose" whether to protect or to kill the affected cells. Most of the afore-mentioned processes follow a protracted course and are accompanied by progressive iron accumulation in the brain. We hypothesize that the neuroprotective effects of iron chelators are acting against the generation of free radicals derived from iron, and also induce sufficient -but not excessive- activation of HIF-1α, so that only the hypoxia-rescue genes will be activated. In this regard, the expression of the erythropoietin receptor in hypoxic/inflammatory neurons could be the cellular "sign" to act upon by the nasal administration of pharmacological doses of Neuro-EPO, inducing not only neuroprotection, but eventually, neurorepair as well.

In Vitro Assessment of the Effect of Antiepileptic Drugs on Expression and Function of ABC Transporters and Their Interactions with ABCC2

ABC transporters have a significant role in drug disposition and response and various studies have implicated their involvement in epilepsy pharmacoresistance. Since genetic studies till now are inconclusive, we thought of investigating the role of xenobiotics as transcriptional modulators of ABC transporters. Here, we investigated the effect of six antiepileptic drugs (AEDs) viz. phenytoin, carbamazepine, valproate, lamotrigine, topiramate and levetiracetam, on the expression and function of ABCB1, ABCC1, ABCC2 and ABCG2 in Caco2 and HepG2 cell lines through real time PCR, western blot and functional activity assays. Further, the interaction of AEDs with maximally induced ABCC2 was studied. Carbamazepine caused a significant induction in expression of ABCB1 and ABCC2 in HepG2 and Caco2 cells, both at the transcript and protein level, together with increased functional activity. Valproate caused a significant increase in the expression and functional activity of ABCB1 in HepG2 only. No significant effect of phenytoin, lamotrigine, topiramate and levetiracetam on the transporters under study was observed in either of the cell lines. We demonstrated the interaction of carbamazepine and valproate with ABCC2 with ATPase and 5,6-carboxyfluorescein inhibition assays. Thus, altered functionality of ABCB1 and ABCC2 can affect the disposition and bioavailability of administered drugs, interfering with AED therapy.

Drug-Resistant Epilepsy: Multiple Hypotheses, Few Answers

Epilepsy is a common neurological disorder that affects over 70 million people worldwide. Despite the recent introduction of new antiseizure drugs (ASDs), about one-third of patients with epilepsy have seizures refractory to pharmacotherapy. Early identification of patients who will become refractory to ASDs could help direct such patients to appropriate non-pharmacological treatment, but the complexity in the temporal patterns of epilepsy could make such identification difficult. The target hypothesis and transporter hypothesis are the most cited theories trying to explain refractory epilepsy, but neither theory alone fully explains the neurobiological basis of pharmacoresistance. This review summarizes evidence for and against several major theories, including the pharmacokinetic hypothesis, neural network hypothesis, intrinsic severity hypothesis, gene variant hypothesis, target hypothesis, and transporter hypothesis. The discussion is mainly focused on the transporter hypothesis, where clinical and experimental data are discussed on multidrug transporter overexpression, substrate profiles of ASDs, mechanism of transporter upregulation, polymorphisms of transporters, and the use of transporter inhibitors. Finally, future perspectives are presented for the improvement of current hypotheses and the development of treatment strategies as guided by the current understanding of refractory epilepsy.

Research Progress on the Role of ABC Transporters in the Drug Resistance Mechanism of Intractable Epilepsy

The pathogenesis of intractable epilepsy is not fully clear. In recent years, both animal and clinical trials have shown that the expression of ATP-binding cassette (ABC) transporters is increased in patients with intractable epilepsy; additionally, epileptic seizures can lead to an increase in the number of sites that express ABC transporters. These findings suggest that ABC transporters play an important role in the drug resistance mechanism of epilepsy. ABC transporters can perform the funcions of a drug efflux pump, which can reduce the effective drug concentration at epilepsy lesions by reducing the permeability of the blood brain barrier to antiepileptic drugs, thus causing resistance to antiepileptic drugs. Given the important role of ABC transporters in refractory epilepsy drug resistance, antiepileptic drugs that are not substrates of ABC transporters were used to obtain ABC transporter inhibitors with strong specificity, high safety, and few side effects, making them suitable for long-term use; therefore, these drugs can be used for future clinical treatment of intractable epilepsy.

Intractable epilepsy and the P-glycoprotein hypothesis

Epilepsy is a serious neurological disorder that affects more than 60 million people worldwide. Intractable epilepsy (IE) refers to approximately 20%-30% of epileptic patients who fail to achieve seizure control with antiepileptic drug (AED) treatment. Although the mechanisms underlying IE are not well understood, it has been hypothesized that multidrug transporters such as P-glycoprotein (P-gp) play a major role in drug efflux at the blood-brain barrier, and may be the underlying factor in the variable responses of patients to AEDs. The main goal of the present review is to show evidence from different areas that support the idea that the overexpression of P-gp is associated with IE. We discuss here evidence from animal studies, pharmacology, clinical cases and genetic studies.

Drug resistance in cortical and hippocampal slices from resected tissue of epilepsy patients: no significant impact of p-glycoprotein and multidrug resistance-associated proteins

Drug resistant patients undergoing epilepsy surgery have a good chance to become sensitive to anticonvulsant medication, suggesting that the resected brain tissue is responsible for drug resistance. Here, we address the question whether P-glycoprotein (Pgp) and multidrug resistance-associated proteins (MRPs) expressed in the resected tissue contribute to drug resistance in vitro. Effects of anti-epileptic drugs [carbamazepine (CBZ), sodium valproate, phenytoin] and two unspecific inhibitors of Pgp and MRPs [verapamil (VPM) and probenecid (PBN)] on seizure-like events (SLEs) induced in slices from 35 hippocampal and 35 temporal cortex specimens of altogether 51 patients (161 slices) were studied. Although in slice preparations the blood brain barrier is not functional, we found that SLEs predominantly persisted in the presence of anticonvulsant drugs (90%) and also in the presence of VPM and PBN (86%). Following subsequent co-administration of anti-epileptic drugs and drug transport inhibitors, SLEs continued in 63% of 143 slices. Drug sensitivity in slices was recognized either as transition to recurrent epileptiform transients (30%) or as suppression (7%), particularly by perfusion with CBZ in PBN containing solutions (43, 9%). Summarizing responses to co-administration from more than one slice per patient revealed that suppression of seizure-like activity in all slices was only observed in 7% of patients. Patients whose tissue was completely or partially sensitive (65%) presented with higher seizure frequencies than those with resistant tissue (35%). However, corresponding subgroups of patients do not differ with respect to expression rates of drug transporters. Our results imply that parenchymal MRPs and Pgp are not responsible for drug resistance in resected tissue.

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.

The transport of antiepileptic drugs by P-glycoprotein

Epilepsy is the most common serious chronic neurological disorder. Current data show that one-third of patients do not respond to anti-epileptic drugs (AEDs). Most non-responsive epilepsy patients are resistant to several, often all, AEDs, even though the drugs differ from each other in pharmacokinetics, mechanisms of action, and interaction potential. The mechanisms underlying drug resistance of epilepsy patients are still not clear. In recent years, one of the potential mechanisms interesting researchers is over-expression of P-glycoprotein (P-gp, also known as ABCB1 or MDR1) in endothelial cells of the blood-brain barrier (BBB) in epilepsy patients. P-gp plays a central role in drug absorption and distribution in many organisms. The expression of P-gp is greater in drug-resistant than in drug-responsive patients. Some studies also indicate that several AEDs are substrates or inhibitors of P-gp, implying that P-gp may play an important role in drug resistance in refractory epilepsy. In this article, we review the clinical and laboratory evidence that P-gp expression is increased in epileptic brain tissues and that AEDs are substrates of P-gp in vitro and in vivo. We discuss criteria for identifying the substrate status of AEDs and use structure-activity relationship (SAR) models to predict which AEDs act as P-gp substrates.

A novel positron emission tomography imaging protocol identifies seizure-induced regional overactivity of P-glycoprotein at the blood-brain barrier

Approximately one-third of epilepsy patients are pharmacoresistant. Overexpression of P-glycoprotein and other multidrug transporters at the blood-brain barrier is thought to play an important role in drug-refractory epilepsy. Thus, quantification of regionally different P-glycoprotein activity in the brain in vivo is essential to identify P-glycoprotein overactivity as the relevant mechanism for drug resistance in an individual patient. Using the radiolabeled P-glycoprotein substrate (R)-[(11)C]verapamil and different doses of coadministered tariquidar, which is an inhibitor of P-glycoprotein, we evaluated whether small-animal positron emission tomography can quantify regional changes in transporter function in the rat brain at baseline and 48 h after a pilocarpine-induced status epilepticus. P-glycoprotein expression was additionally quantified by immunohistochemistry. To reveal putative seizure-induced changes in blood-brain barrier integrity, we performed gadolinium-enhanced magnetic resonance scans on a 7.0 tesla small-animal scanner. Before P-glycoprotein modulation, brain uptake of (R)-[(11)C]verapamil was low in all regions investigated in control and post-status epilepticus rats. After administration of 3 mg/kg tariquidar, which inhibits P-glycoprotein only partially, we observed increased regional differentiation in brain activity uptake in post-status epilepticus versus control rats, which diminished after maximal P-glycoprotein inhibition. Regional increases in the efflux rate constant k(2), but not in distribution volume V(T) or influx rate constant K(1), correlated significantly with increases in P-glycoprotein expression measured by immunohistochemistry. This imaging protocol proves to be suitable to detect seizure-induced regional changes in P-glycoprotein activity and is readily applicable to humans, with the aim to detect relevant mechanisms of pharmacoresistance in epilepsy in vivo.

Novel and emerging strategies in drug delivery for overcoming the blood-brain barrier

Two decades of molecular research have revealed the presence of transporters and receptors expressed in the brain vascular endothelium that provide potential novel targets for the rational design of blood-brain barrier-penetrating drugs. In this review, we briefly introduce the reader to the molecular characteristics of the blood-brain barrier that make this one of the most important obstacles towards the development of efficacious CNS drugs. We highlight recent attempts to rationally target influx and bidirectional transport systems expressed on the brain endothelial cell and avoid the important obstacle presented in the form of efflux transporters. Many of these approaches are highly innovative and show promise for future human application. Some of these approaches, however, have revealed significant limitations and are critiqued in this review. Nonetheless, these combined efforts have left the field of CNS drug delivery better positioned for developing novel approaches towards the rational design of CNS-penetrating drugs.

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.

One of the hottest topics in epileptology: ABC proteins. Their inhibition may be the future for patients with intractable seizures

One of the new topics in epileptology is the ABC proteins, which seem to control whether or not anti-epileptic drugs (AEDs) can come in contact with and affect the epileptogenic areas that cause seizures. The goal of this report is to simplify the concepts involved in these proteins and then to review the progress made in the field, especially of one protein called P-glycoprotein (P-gp). First, the ABC proteins are reviewed, mainly P-gp, which appears to alter drug permeability (like an extra blood-brain barrier). The possibility is discussed that changes in P-gp are the result of many seizures; are caused by the AEDs, or truly reflect pharmacoresistance. The different locations where these changes can be seen include the endothelial cells, glia and also neurons. The polymorphism of P-gp, called C3435T, probably has little functional significance and finally the importance of inhibitors of P-g to reverse pharmacoresistance is emphasized. Tariquidar (XR9576) is likely to be a good candidate that appears to inhibit these proteins and therefore to allow the AEDs to control the intractable seizures that may account for nearly 40% of our patients.

Clinicopathological and immunohistochemical findings in an autopsy case of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant, multisystem disorder caused by mutations in either the TSC1 or TSC2 genes and characterized by developmental brain abnormalities. In the present study we discuss the neuropathological findings of a 32-year-old patient with a germ-line mutation in the TSC2 gene. Post mortem MRI combined with histology and immunocytochemical analysis was applied to demonstrate widespread anatomical abnormalities of gray and white matter structure. TSC brain lesions were analyzed for loss of heterozygosity (LOH) on chromosome 16p13. The neuropathological supratentorial abnormalities were represented by multiple subependymal nodules (SENs) and cortical tubers. In addition to cerebral cortical lesions, cerebellar lesions and hippocampal sclerosis were also observed. LOH was not found in the cortical tubers and SENs of this patient. Immunocytochemical analysis of the TSC brain lesions confirmed the cell-specific activation of the mTOR pathway in cortical tubers, SENs and cerebellum, as well as differential cellular localization of hamartin and tuberin, the TSC1 and TSC2 gene products. Examination of the pathological brain regions revealed activated microglial cells and disruption of blood-brain barrier permeability. Predominant intralesional cell-specific distribution was also detected for the multidrug transporter protein P-gp, possibly explaining the mechanisms underlying the pharmacoresistance to antiepileptic drugs. Autopsy findings confirm the complexity of the brain abnormalities encountered in TSC patients and proved useful in clarifying certain aspects of the pathogenesis, epileptogenesis and pharmacoresistance of TSC lesions.

The influence of C3435T polymorphism of ABCB1 gene on penetration of phenobarbital across the blood-brain barrier in patients with generalized epilepsy

BACKGROUND

Epilepsy is refractory to medical treatment in about one-third of the patients. The exact pathological mechanism of epilepsy pharmacoresistance is still unclear, but a decreased antiepileptic drug (AED) uptake into the brain is suspected to play a role. P-glycoprotein (Pgp), a transmembrane transporter encoded by ABCB1 gene and located at the endothelial cells of the blood-brain barrier (BBB), has been associated with epilepsy pharmacoresistance.

OBJECTIVE

To analyze the effect of two ABCB1 gene polymorphisms, C3435T and G2677T/A, on phenobarbital (PB) concentrations in the cerebrospinal fluid (CSF) and serum (S) and to assess the relationship of ABCB1 polymorphisms to phenobarbital penetration across BBB in vivo and seizure frequency.

METHODS

CSF PB and S PB concentrations were measured in 60 patients with idiopathic primary generalized epilepsy receiving phenobarbital monotherapy. CSF/S PB concentration ratio was calculated as an index of phenobarbital penetration across BBB. The patients were genotyped for the ABCB1 gene C3435T and G2677T/A polymorphisms. Seizure frequency was recorded during the 6-month phenobarbital monotherapy.

RESULTS

Patients with different C3435T polymorphism had significantly different CSF PB concentrations and CSF/S PB concentration ratio. In comparison with CT heterozygotes and TT homozygotes, CC homozygotes had a significantly lower CSF PB concentration (p=0.006) and CSF/PB concentration ratio (p<0.001). G2677T/A polymorphism showed no such effect (p=0.466). CC genotype and low CSF/S PB concentration ratio correlated with increased seizure frequency.

CONCLUSIONS

C3435T polymorphism of ABCB1 gene was demonstrated in vivo to significantly influence the CSF/S PB concentration ratio and seizure frequency.

Pathophysiologic characteristics of balloon cells in cortical dysplasia

OBJECTS

Balloon cells are histopathological hallmarks of cortical malformations, i.e., focal cortical dysplasia (FCD) of the Taylor type or the cortical tubers of tuberous sclerosis, and they are believed to be the epileptogenic substrate and cause therapeutic drug resistant epilepsy in man. This study was carried out to investigate the developmental histogenesis and epileptogenesis of balloon cells in FCD.

MATERIALS AND METHODS

We used an immunohistochemical approach to examine the expressions of primitive neuroepithelial cell antigens (CD34, nestin, and vimentin), ionotrophic glutamate receptor subunits (NR1, NR2A/B, GluR1, GluR2, GluR3, GluR4, and GluR5/6/7), and P-glycoprotein in balloon cells from FCD and normal cerebral cortex epileptogenic lesions.

CONCLUSION

Balloon cells presented in clusters or as scattered cells throughout FCD lesions involving the gray and white matter. We found the balloon cells to be classifiable into three subtypes based on glial fibrillary acidic protein (GFAP) and neurofilament protein (NF-L) immunohistochemistry, i.e., as neuronal, astrocytic, and uncommitted. Immunopositivity for nestin, CD34, and vimentin in balloon cells of FCD suggests that they may be derived from the abnormal development and differentiation of neural stem cells. Moreover, it appears that epileptogenesis in cortical dysplasia is partly caused by the upregulations of some glutamate receptor subunit proteins (NR1, NR2A/B, GluR1, and GluR3) in balloon cells and dysplastic neurons. We speculate that the presence of the drug resistance protein P-glycoprotein in balloon cells might explain medically refractory epilepsy in FCD.

Enaminones: Exploring Additional Therapeutic Activities

Enaminones, enamines of beta-dicarbonyl compounds, have been known for many years. Their early use has been relegated to serving as synthetic intermediates in organic synthesis and of late, in pharmaceutical development. Recently, the therapeutic potential of these entities has been realized. This review provides the background and current research in this area with emphasis of these agents as potential anticonvulsants, their proposed mechanisms of action, and as potential modulators of multidrug resistance (MDR).

ABC Transporters during Epilepsy and Mechanisms Underlying Multidrug Resistance in Refractory Epilepsy

It is estimated 20-25% of the epileptic patients fails to achieve good control with the different antiepileptic drugs (AEDs) treatments, developing refractory epilepsy (RE). Discovered first in cancer, the activity of P-glycoprotein (P-gp) and others ABC transporters as multidrug-resistance-associated proteins (MRPs) and breast cancer resistant protein (BCRP) are directly related with the refractoriness. We have observed the overexpression of these all transporters in the brain of patients with RE, and according with other authors, all these data suggests an active drug efflux from brain. Both constitutive and seizure induced brain P-gp overexpression was also suggested. As confirmation of these clinical evidences, different models of experimental epilepsy have demonstrated P-gp overexpression on blood brain barrier (BBB) and brain parenchyma cells, as astrocytes and neurons. In our model, early P-pg detection in vessel-related cells and later additional P-gp detection in neurons, correlated with the gradual loss of protective effect of phenytoin. The progressive neuronal P-gp expression, depending on intensity and time-constancy of seizure-injury, was in agreement with the development of "P-gp-positive seizure-axis" proposed by Kwan & Brodie, who also showed that the development of RE directly correlated with the number and frequency of seizures before initiation of drug therapy. P-gp expression in excretory organs suggests that P-gp have a central role in drug elimination. Persistent low levels of AEDs in plasma and P-gp brain overexpression in several RE pediatric patients were reported. We also observed in adult RE patients, an increased liver clearance of 99mTc-hexakis-2-methoxyisobutylisonitrile (99mTc-MIBI) (a P-gp substrate), and the surgically treated cases showed P-gp brain overexpression. These results suggest the systemic hyperactivity of P-gp in RE patients, including brain P-gp over-expression should be suspected when persistent subtherapeutic levels of AEDs in plasma are detected. P-gp neuronal expression described in both clinical and experimental reports indicates that additional mechanisms could be operative from seizure-affected P-gp-positive neurons, due to AEDs targets are expressed at membrane level. An alternative mechanism was demonstrated in P-gp-expressed cells that exhibit lower membrane potential (Deltapsi(0)=-10 to -20) compared to normal physiological Deltapsi(0) of -60 mV. Under this situation and irrespective to the P-gp pharmacoresistant property or type of drug treatment selected, P-gp-expressed neurons could increase their sensitivity to new seizures perhaps as an epileptogenic mechanism. The understanding of properties of these ABC transporters can offer new tools for better selection of more effective preventive or therapeutic strategies and avoid the invasive surgical treatments for RE.

Therapeutic drug monitoring and clinical outcomes in epileptic Egyptian patients: a gene polymorphism perspective study

This work was performed to explore the effect of polymorphism in multidrug resistant genes on plasma phenytoin levels and patient outcome to evaluate its involvement in drug resistance and toxicity, which is usually associated with antiepileptic drugs. Therefore, we genotyped the adenosine triphosphate-binding cassette subfamily B member 1 (ABCB1) in 100 patients suffering from partial or generalized tonic-clonic seizures and receiving phenytoin and 50 healthy control subjects. Steady state plasma phenytoin levels were also determined in the epileptic patients. Patients were evaluated after 3 and 6 months and were classified either as drug resistant patients or responsive patients. Results revealed 37 patients with drug responsive epilepsy and 63 patients with drug resistant epilepsy. Genotyping of our patients and control subjects revealed a genotype distribution of CC, CT, TT: 55.50%, 38.00%, 6.50% for drug resistant patients, CC, CT, TT: 13.50%, 46.00%, 40.50% for drug responsive patients, and CC, CT, TT: 24.00%, 48.00%, 28.00% for the control subjects. Patients with drug-resistant epilepsy were more likely to have the CC than the TT genotype compared with either responsive patients (P < 0.0001) or control subjects (P < 0.0001). The C polymorphism was over-represented among patients with drug-resistant epilepsy as compared with either those with drug-responsive epilepsy (P < 0.001) or control subjects (P < 0.001). Of the total 100 epileptic patients, 13 patients had their plasma phenytoin levels exceeding the maximum safe concentration. These 13 patients were more likely to have TT genotype than the CC genotype compared with the remainder of patients who had their plasma phenytoin levels at 20 microg/mL or less. Responsive patients showed no deviation from the control group regarding the genotype (P > 0.05) or allele frequency (P > 0.05). In conclusion, because most of the antiepileptic drugs are multidrug resistant gene substrates, the ABCB1 is thus an important candidate gene for potentially influencing the response to antiepileptic drugs. Our findings suggest that using genotype data may make it possible to safely reduce the time required to reach an effective dose. Therefore, it is a priority to assess the utility of dose adjustment on the basis of genotype for these medicines that are substrates for this gene.

Expression and cellular distribution of multidrug resistance-related proteins in patients with focal cortical dysplasia

Recent arouse of interest indicated that drug resistant proteins are markedly over-expressed in the epileptogenic tissue and they may be responsible for the one-third of the epileptic patients who were refractory to anti-epileptic drugs (AEDs). Since several AEDs may act as substrates for these drug resistant proteins, the enhanced function of such proteins may increase drug extrusion, resulting in inadequate response to drug therapy in patients with epilepsy. We studied expression of the multidrug resistance protein 1 (MDR1) and multidrug resistance-associated protein 1 (MRP1) in the epileptic tissues resected surgically in 28 patients with focal cortical dysplasia (FCD) by immunohistochemistry. The results were compared with 10 normal necropsy brain tissues. Normal brain showed no MDR1 expression in neurons and astrocytes, while MRP1 expression was very weak, which were encountered in a few samples. MDR1 expression was mainly localized on the vascular endothelial cells. In contrast to normal brain, we found intense MDR1 and MRP1 expression in both neurons and reactive astrocytes in the vast majority of dysplastic tissues. The majority of the dysplastic neurons demonstrated moderate to strong MRP1 immunoreactivity. Endothelial cells showed both MDR1 and MRP1 expression in the majority of the specimens studied. Multidrug transporters are over-expressed in the epileptogenic zone in patients with FCD. These results are concordant with previous studies, in which over-expression of multidrug proteins were shown in epileptogenic brain tissue in patients with FCD, that the over-expression of drug transport proteins in tissue from patients with refractory epilepsy may explain one possible mechanism for drug resistant in these pathologies.

Genetic association studies in epilepsy pharmacogenomics: lessons learnt and potential applications

Although epilepsy is one of the most common neurological disorders and genetic factors are well known to play a role in response to antiepileptic drug (AED) treatment, the study of the pharmacogenetics of epilepsy has received relatively little attention and has not resulted in clinical applications to date. Our improved understanding of the pathogenesis of epilepsy and the mechanism of action of AEDs, together with recent advances in genetics and decreasing genotyping costs, have now paved the way for a more systematic application of pharmacogenetics in the field of epilepsy. It is hoped that the resulting knowledge will lead to a more rational treatment of epilepsy, development of more efficacious AEDs, and facilitation of clinical trials of new AEDs. However, there are formidable practical, methodological and theoretical hurdles to overcome before pharmacogenomic information will have any major utility in the clinical setting. Here, we discuss the evidence for a genetic contribution to AED response, review current knowledge in epilepsy pharmacogenetics and discuss potential future avenues with their implications, both for the clinical treatment of epilepsy and new AED development.

Methodological approaches to exploring epileptic disorders in the human brain in vitro

Brain surgery, and in particular epilepsy surgery, offers the unique opportunity to study viable human central nervous tissue in vitro. This does not only open a window to address the basic mechanisms underlying human disease, such as epilepsy, but it allows to venture into investigating neurophysiological functions per se. In the present paper, we describe the most commonly used methods in the electrophysiological (and, at least to some extent, also histochemical and molecular) analysis of human tissue in vitro. In addition, we consider the pitfalls and limitations of such studies, in particular regarding the issue of tissue sampling procedures and control experiments.

Cellular and molecular mechanisms of epilepsy in the human brain

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

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.

Influence of Lamotrigine and Topiramate on MDR1 Expression in Difficult-to-Treat Temporal Lobe Epilepsy

PURPOSE

Overexpression of the multiple drug resistance gene 1 (MDR1) was quantified in brain tissue from Coriaria lactone (CL)-kindled Sprague-Dawley (SD) rats after treatment with lamotrigine (LTG) or topiramate (TPM) and compared with that found in rats treated with carbamazepine (CBZ) and valproate (VPA).

METHODS

Twenty-five CL-kindled SD rats were randomized into five groups (n = 5 for each group) to receive once-daily feeding of CBZ, VPA, TPM, and LTG as the monotherapy equivalent of maximum human adult dosage, or normal saline (NS control) for 1 month. The expression of P-gp in brain tissues of all rats was quantified by using an image analysis and measuring system (Image Pro-plus 4.0). Mean area and mean integrated optical density (mean IOD) of P-gp expression were calculated. In addition, the changes in seizure severity were analyzed via video-camera monitoring.

RESULTS

A significant decrease in the number and duration of seizures with antiepileptic drug (AED) treatment was observed in the TPM and LTG groups. The mean area and mean IOD of P-gp expression were highest in the CBZ group and next highest in the VPA group; much lower values were measured in the TPM and LTG groups, and the lowest in the NS control group (p < 0.05).

CONCLUSIONS

TPM and LTG significantly inhibited seizures in this CL model. The expression of P-gp was not significantly increased by TPM or LTG treatment in this study.

New proteins configure a brain drug resistance map in tuberous sclerosis

Epileptogenic cortical tubers, characterized by dysplastic neurons and balloon cells, is a frequent feature of tuberous sclerosis. In severe tuberous sclerosis-affected individuals, seizures are refractory to medication. Multidrug resistance proteins (multidrug resistance protein-1 [MDR-1] and multidrug resistance-associated protein-1 [MRP-1]) have been found to be highly expressed in epileptogenic cortical tubers. However, two new proteins related to refractoriness in cancer (breast cancer resistance protein and major vault protein) have not been investigated in tuberous sclerosis and refractory epilepsy. On the same brain specimens previously describing the MDR-1 and MRP-1 expression, we investigated retrospectively breast cancer resistance protein and major vault protein by specific monoclonal antibodies and routine immunohistochemistry methods. Breast cancer resistance protein was present in vascular endothelial cells from all the vessels examined in 3 of 3 cases. Major vault protein was detected in only one case, and selectively expressed in several but not all ballooned cells. In epileptogenic cortical tubers, the additional expression of breast cancer resistance protein on vessels and major vault protein in some ballooned cells to the previously demonstrated expression of MDR-1 and MRP-1 (in vessels, astroglia, microglia, neurons, and ballooned cells) configures a brain protein pharmacoresistance map from patients with tuberous sclerosis and refractory epilepsy.

Molecular and cellular mechanisms of pharmacoresistance in epilepsy

Epilepsy is a common and devastating neurological disorder. In many patients with epilepsy, seizures are well-controlled with currently available anti-epileptic drugs (AEDs), but a substantial (approximately 30%) proportion of patients continue to have seizures despite carefully optimized drug treatment. Two concepts have been put forward to explain the development of pharmacoresistance. The transporter hypothesis contends that the expression or function of multidrug transporters in the brain is augmented, leading to impaired access of AEDs to CNS targets. The target hypothesis holds that epilepsy-related changes in the properties of the drug targets themselves may result in reduced drug sensitivity. Recent studies have started to dissect the molecular underpinnings of both transporter- and target-mediated mechanisms of pharmacoresistance in human and experimental epilepsy. An emerging understanding of these underlying molecular and cellular mechanisms is likely to provide important impetus for the development of new pharmacological treatment strategies.

A pilot study on brain-to-plasma partition of 10,11-dyhydro-10-hydroxy-5H-dibenzo(b,f)azepine-5-carboxamide and MDR1 brain expression in epilepsy patients not responding to oxcarbazepine

PURPOSE

We measured the brain-to-plasma partition of 10,11-dihydro-10-hydroxy-5H-dibenzo(b,f)azepine-5-carboxamide (10-OHCBZ) in epilepsy patients undergoing surgery to alleviate drug-resistant seizures and administered with different oral doses of oxcarbazepine (OXC). We addressed the possible contribution of the multidrug transporter P-glycoprotein (P-gp or MDR1) in determining 10-OHCBZ brain levels by measuring whether this active metabolite is a substrate of P-gp and the relation between the level of expression of MDR1 and the drug concentration in the same brain tissue specimens.

METHODS

Steady-state plasma and brain concentrations (C(ss)) of 10-OHCBZ were determined intraoperatively in 11 patients by high-performance liquid chromatography (HPLC) with UV detection. The level of expression of MDR1 mRNA was measured in surgically resected brain tissue by reverse transcriptase polymerase chain reaction (RT-PCR). The ability of 10-OHCBZ to act as substate of P-gp was evaluated by measuring its uptake in cell lines expressing different levels of P-gp, in the presence or absence of a selective P-gp inhibitor.

RESULTS

OXC was converted to 10-OHCBZ and to Di-OHCBZ, the two main metabolites measured in plasma. The brain concentrations of the active metabolite 10-OHCBZ did not reflect plasma C(ss). A significant inverse linear correlation was found between 10-OHCBZ brain-to-plasma concentration ratio and the level of brain expression of MDR1 mRNA. In vitro uptake studies demonstrated lower intracellular 10-OHCBZ levels in cells with higher P-gp expression. Intracellular drug concentration was increased by XR9576, a specific P-gp blocker.

CONCLUSIONS

Pharmacologic failure of OXC in pharmacoresistant epilepsy is unlikely to be due to alterations in drug metabolism. 10-OHCBZ does not appear to cross the blood-brain barrier by simple diffusion, and it acts as a substrate of P-gp. The level of expression of MDR1 is inversely correlated with 10-OHCBZ concentration in the epileptic tissue. P-gp may play a role in the pharmacoresistance to OXC by determining the attainment of insufficient concentrations of its active metabolite at neuronal targets.

The role of common variation in drug transporter genes in refractory epilepsy

Resistance to antiepileptic drugs (AEDs) is one of the most serious clinical problems in epilepsy, and along with AED teratogenicity, perhaps the major concern of epilepsy pharmacogenetics. Studying the genetics of drug resistance in epilepsy is important, as it may identify or confirm key mechanisms underlying this phenomenon that have real clinical importance; it might also offer insights into its prediction and management. Drug resistance in epilepsy is likely to be multifactorial: overactivity of multi-drug transporters provides one likely underlying mechanism through lowering of AED concentration in the epileptogenic focus. Genetic association studies may provide a tool to assess this 'transporter' hypothesis by determining whether differences between individuals contribute to resistance phenotypes. Most of these studies have investigated one variant in the ABCB1 gene, and have provided, thus far, inconclusive evidence. This review also considers current knowledge of the role of genetic polymorphisms in multi-drug transporters in pharmacoresistant epilepsy, to highlight possible confounding factors affecting the implementation and interpretation of association studies in this field.

Localization of breast cancer resistance protein (BCRP) in microvessel endothelium of human control and epileptic brain

PURPOSE

Breast cancer resistance protein (BCRP) is a half adenosine triphosphate (ATP)-binding cassette (ABC) transporter expressed on cellular membranes and included in the group of multidrug resistant (MDR)-related proteins. Recently, upregulation of different MDR proteins has been shown in human epilepsy-associated conditions. This study investigated the expression and cellular distribution of BCRP in human control and epileptic brain, including a large number of both neoplastic and nonneoplastic specimens from patients with chronic pharmacoresistant epilepsy.

METHODS

Several epileptogenic pathologies, such as hippocampal sclerosis (HS), focal cortical dysplasia (FCD), dysembryoplastic neuroepithelial tumor, oligodendroglioma astrocytoma, and glioblastoma multiforme were studied by using Western blot and immunocytochemistry.

RESULTS

With Western blot, we could demonstrate the presence of BCRP in both normal and epileptic human brain tissue. In contrast to P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP) 2, BCRP expression levels did not change in tissue from patients with HS, compared with control hippocampus. No BCRP immunoreactivity was observed in glial or neuronal cells, including reactive astrocytes and dysplastic neurons in FCD. BCRP expression was, however, increased in tumor brain tissue. Immunocytochemistry demonstrated that BCRP was exclusively located in blood vessels and was highly expressed at the luminal cell surface and in newly formed tumor capillaries. This localization closely resembles that of P-gp. The higher expression observed in astrocytomas by Western blot analysis was related to the higher vascular density within the tumor tissue.

CONCLUSIONS

These results indicate a constitutive expression of BCRP in human endothelial cells, representing an important barrier against drug access to the brain. In particular, the strong BCRP expression in the microvasculature of epileptogenic brain tumors could critically influence the bioavailability of drugs within the tumor and contribute to pharmacoresistance.