Elsevier

Neurobiology of Disease

Volume 24, Issue 1, October 2006, Pages 202-211
Neurobiology of Disease

The multidrug transporter hypothesis of drug resistance in epilepsy: Proof-of-principle in a rat model of temporal lobe epilepsy

https://doi.org/10.1016/j.nbd.2006.06.014Get rights and content

Abstract

Resistance to drug treatment is an important hurdle in the therapy of many diseases, including cancer, infectious diseases and brain disorders such as epilepsy. A phenotype that is referred to as multidrug resistance was first described for chemotherapy-resistant cancer cells that overexpressed the drug efflux transporter P-glycoprotein (P-gp). More recently, overexpression of P-gp has been found in capillary endothelial cells of epileptogenic brain tissue from patients with medically intractable epilepsy. Such regionally restricted P-gp overexpression in the blood–brain barrier is likely to reduce the concentration of antiepileptic drugs at epileptic neurons, which would be a plausible explanation for multidrug resistance in epilepsy. However, a definite proof-of-principle for this hypothesis is lacking. In the present study, we used a rat model of temporal lobe epilepsy that allows selecting drug-resistant and drug-responsive subgroups of epileptic rats by prolonged treatment with the antiepileptic drug phenobarbital at maximum tolerated doses. We have shown recently that drug-resistant rats selected from this model exhibit a marked overexpression of P-gp in the hippocampus and other limbic brain regions. This model is thus ideally suited to prove the multidrug transporter hypothesis of drug resistance. For this purpose, we selected a group of phenobarbital-resistant rats, which was subsequently treated by combinations of phenobarbital with the selective P-gp inhibitor tariquidar. Coadministration of tariquidar (15–20 mg/kg) fully restored the anticonvulsant activity of phenobarbital without altering plasma pharmacokinetics or neurotoxicity of the antiepileptic drug. These data demonstrate that inhibiting P-gp in epileptic rats with proven drug resistance counteracts resistance, providing the first proof-of-principle of the multidrug transporter hypothesis of medically refractory epilepsy.

Introduction

The effectiveness of epilepsy therapy is limited by poor response or frank resistance to drug treatment in at least 30% of all patients, resulting in persistent seizures despite trials of multiple antiepileptic medications (Kwan and Brodie, 2000). Resistance to antiepileptic drugs (AEDs) is one of the most serious clinical problems in epilepsy, resulting in shortened lifespan, excessive bodily injury, neuropsychological and psychiatric impairment and social disability (Sperling, 2004). Enhanced understanding of the underlying mechanisms of drug resistance is a prerequisite for improving drug therapy by preventing or reversing resistance (Löscher and Potschka, 2005a).

Based on experimental and clinical data, recently two major hypotheses have been put forward to explain medical refractoriness of epilepsy, the target hypothesis and the multidrug transporter hypothesis (Schmidt and Löscher, 2005, Remy and Beck, 2006). Based on the target hypothesis, intrinsic or acquired changes in AED targets in the brain underlie drug-resistant epilepsy, while the multidrug transporter hypothesis claims that the target is never reached because intrinsic or acquired overexpression of multidrug efflux transporters at the blood–brain barrier (BBB) restrict brain uptake of AEDs. However, although these hypotheses of drug resistance are biologically plausible, any direct proof-of-principle is lacking as yet (Sisodiya, 2003, Schmidt and Löscher, 2005). The multidrug transporter hypothesis has the theoretical advantage that it can be proven by combining AEDs with inhibitors of multidrug transporters such as P-glycoprotein (P-gp or MDR1). Anecdotal clinical observations from coadministration of the non-selective P-gp inhibitor verapamil in two patients with drug-resistant epilepsy seem to support the hypothesis (Summers et al., 2004, Iannetti et al., 2005), but these data are difficult to interpret because verapamil also blocks calcium channels and inhibits the metabolism of AEDs (Thomas and Coley, 2003). Highly selective P-gp inhibitors such as tariquidar (XR9576) are available and currently evaluated in clinical trials of chemotherapy-resistant cancers (Thomas and Coley, 2003, Pusztai et al., 2005), but such drugs have not been tested yet for their potential to reverse drug resistance in epilepsy.

In the present study, we evaluated whether AED resistance can be counteracted by coadministration of tariquidar in a rat model of temporal lobe epilepsy (TLE). In this model, spontaneous recurrent seizures (SRS) develop after a latency period following a status epilepticus (SE) which is induced by prolonged electrical stimulation of the basolateral amygdala (BLA) (Brandt et al., 2003). We have recently shown that daily administration of the AED phenobarbital (PB) at maximum tolerable doses in epileptic rats of this model results in two subgroups, i.e., a responder subgroup with suppression of seizures and a nonresponder subgroup without any significant reduction in seizure frequency (Brandt et al., 2004). The severity or duration of the initial brain insult (the SE) did not differ between responders and nonresponders, indicating that the different AED response in the two subgroups is genetically determined (Brandt et al., 2004). In line with the multidrug transporter hypothesis (Löscher and Potschka, 2005a), PB-resistant rats exhibited a marked overexpression of P-gp in brain capillary endothelial cells that form the BBB, whereas PB responsive rats had a significantly lower P-gp expression (Volk and Löscher, 2005). Because PB is a substrate of P-gp (Schuetz et al., 1996, Potschka et al., 2002), overexpression of P-gp at the BBB is likely to reduce penetration of this AED into affected brain regions, thereby restricting its anticonvulsant effect. Inhibition of P-gp has been shown to increase brain penetration of PB in normal rats (Potschka et al., 2002), but it is not known whether this would counteract drug resistance in epileptic rats with overexpression of P-gp at the BBB. This prompted us to perform a proof-of-principle experiment in PB-resistant epileptic rats. To our knowledge, the present study is the first to directly address whether coadministration of a selective P-gp inhibitor is a useful therapeutic strategy for epileptic individuals with proven AED resistance associated with up-regulated P-gp expression in the BBB. A preliminary report of the present findings has been previously presented in abstract form (Brandt et al., 2005).

Section snippets

Animals

As in our previous experiments in rats with SRS developing after SE induced by prolonged electrical stimulation of the BLA (Brandt et al., 2004, Volk and Löscher, 2005, Volk et al., 2006), adult female Sprague–Dawley rats (Harlan-Winkelmann, Borchen, Germany) were used for this study. All animal experiments were carried out in accordance with the European Communities Council Directive of 24 November 1986 (86/609/EEC) and were formally approved by the animal subjects review board of our

Selection of responders and nonresponders

With the dosing protocol used for selection of responders and nonresponders, PB induced marked sedation, ataxia and rotarod failures, indicating that maximum tolerated doses were used. Analysis of plasma drug concentrations showed that drug concentrations within or above the therapeutic range (10–40 μg/ml) known from patients with epilepsy were maintained in all rats throughout the period of treatment. In the 15 rats used in this experiment, average concentrations 10 h after drug application in

Discussion

The present experiments were based on a number of prerequisites. (1) We knew that PB nonresponders selected from the post-SE model of TLE used in our study exhibit a marked overexpression of the drug efflux transporter P-gp in brain capillary endothelial cells in limbic brain regions, including the hippocampus (Volk and Löscher, 2005). Because PB is a known substrate of P-gp (Schuetz et al., 1996, Potschka et al., 2002), the increased P-gp expression is likely to result in sub-therapeutic

Acknowledgments

We thank Prof. Heidrun Potschka for discussions during the experiments and preparation of the manuscript and Mrs. Nicole Ernst and Ms. Nadja Thonig for skilful technical assistance. The study was supported by a grant (Lo 274/9) from the Deutsche Forschungsgemeinschaft (Bonn, Germany) and a grant (1 R21 NS049592-01) from the National Institutes of Health (NIH; Bethesda, MD, USA). Tariquidar was kindly provided by Xenova Ltd. (Slough, Berkshire, U.K.). We thank Dr. John F. Waterfall (Xenova

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