The multidrug transporter P-glycoprotein in pharmacoresistance to antiepileptic drugs

This review provides an overview of the knowledge on P-glycoprotein (P-gp) and its role as a membrane transporter in drug resistance in epilepsy and drug interactions. Overexpression of P-gp, encoded by the ABCB1 gene, is involved in resistance to antiepileptic drugs (AEDs), limits gastrointestinal absorption and brain access of AEDs. Although several association studies on ABCB1 gene with drug disposition and disease susceptibility are completed to date, the data remain unclear and incongruous. Although the literature describes other multidrug resistance transporters, P-gp is the main extensively studied drug efflux transporter in epilepsy.     

Celecoxib treatment restores pharmacosensitivity in a rat model of pharmacoresistant epilepsy

Background and purpose:

A functional link between seizure-induced P-glycoprotein overexpression at the blood–brain barrier and therapeutic failure has been suggested by several studies using rodent epilepsy models and human epileptic tissue. Recently, we reported that interference with the mechanisms that up-regulate P-glycoprotein in response to seizure activity might provide a novel approach to control its expression in the epileptic brain. Based on these data, we hypothesized that blocking the appropriate signalling cascade by cyclooxygenase-2 inhibition should improve brain penetration of antiepileptic drugs and help to overcome drug resistance.

Experimental approach:

Effects of the selective cyclooxygenase-2 inhibitor celecoxib on the response to the P-glycoprotein substrate, phenobarbital, was evaluated in a chronic model of drug-resistant temporal lobe epilepsy in rats. Drug-resistant rats selected from this model exhibit a marked overexpression of P-glycoprotein in the hippocampus and other limbic brain regions.

Key results:

Responders and non-responders were selected from a group of rats with spontaneous recurrent seizures after prolonged treatment with phenobarbital at maximum tolerated doses. The efficacy of phenobarbital was re-evaluated following a 6 day treatment with celecoxib and the frequency of spontaneous recurrent seizures was significantly reduced in both groups of rats, phenobarbital responders or non-responders selected from the previous drug trial.

Conclusions and implications:

Pretreatment with the cyclooxygenase-2 inhibitor restored the anticonvulsant activity of phenobarbital in rats that failed to exhibit a relevant response before celecoxib treatment. Our data provide further support for a novel therapeutic approach to overcome transporter-mediated drug resistance in epilepsies.     

A 2017 review of pharmacotherapy for treating focal epilepsy: where are we now and how will treatment develop?

Introduction: Focal epilepsy is the most common type of epilepsy with approximately 30 million patients affected worldwide. There is a major challenge to develop new antiepileptic treatments as currently approximately one third of patients remain uncontrolled under our best standards of care.

Areas covered: An overview is given on first- and second generation antiepileptic drugs and their mechanisms of action, and on recent new strategies for antiepileptic targets, including drugs aiming at disease modification.

Expert opinion: Newer antiepileptic drugs have enabled a better tolerated and individualized treatment for many patients. Despite the successful history of antiepileptic drug development programs, second and third generation antiepileptic drugs targeting synaptic transmission have, however, failed to solve the problem of pharmacoresistance. New directions in pharmacological development include chronic models of epilepsy in drug screening and address primary and secondary epileptogenesis rather than focusing on the suppression of the symptoms, acute seizures. There is hope that the new approaches will allow for patient stratification for targeted therapy and will prove efficacy particularly in the patient group so far drug resistant.     

Padaczka oporna na leczenie – epidemiologia i aktualny stan badań

In the last years interest in epileptology has focused on determining the cause and pathomechanism of farmacoresistance in epilepsy. According to epidemiology, the problem of pharmacoresistance concerns about 30% of patients suffering from epilepsy, mainly temporal epilepsy coexisting with hippocampal sclerosis. Drug-resistant epilepsy is frequently based on static or dynamic organic brain damage. Furthermore, those morphological changes and related clinical/psychological dysfunctions lead to intellectual and social consequences as well as to an increase of the mortality rate.Pharmacoresistant epilepsy afflicts various patients with the same form of epilepsy and concerns many antiepileptic drugs with different mechanisms of action. Thus, the genetic conditioning of this state was considered. So far, the multidirectional investigations have proved the importance of interleukin IL-1β gene polymorphism, prodynorphin, GABA receptors and the APOE∑4 allele in pharmaco-resistant epilepsy. Moreover, further factors such as transport protein gene polymorphism (ABCB₁, RLIP76/RALBP₁), genetically modifying receptor changes within the activating/inhibiting system (SCN1A – sodium channels, KV₇ – potassium channels) and drug metabolizing enzyme gene polymorphism (UG T1 A1*28) are also considered important.     

Personalized medicine approaches in epilepsy

Epilepsy affects 50 million persons worldwide, a third of whom continue to experience debilitating seizures despite optimum anti‐epileptic drug (AED) treatment. Twelve‐month remission from seizures is less likely in female patients, individuals aged 11–36 years and those with neurological insults and shorter time between first seizure and starting treatment. It has been found that the presence of multiple seizures prior to diagnosis is a risk factor for pharmacoresistance and is correlated with epilepsy type as well as intrinsic severity. The key role of neuroinflammation in the pathophysiology of resistant epilepsy is becoming clear. Our work in this area suggests that high‐mobility group box 1 isoforms may be candidate biomarkers for treatment stratification and novel drug targets in epilepsy. Furthermore, transporter polymorphisms contributing to the intrinsic severity of epilepsy are providing robust neurobiological evidence on an emerging theory of drug resistance, which may also provide new insights into disease stratification. Some of the rare genetic epilepsies enable treatment stratification through testing for the causal mutation, for example SCN1A mutations in patients with Dravet's syndrome. Up to 50% of patients develop adverse reactions to AEDs which in turn affects tolerability and compliance. Immune‐mediated hypersensitivity reactions to AED therapy, such as toxic epidermal necrolysis, are the most serious adverse reactions and have been associated with polymorphisms in the human leucocyte antigen (HLA) complex. Pharmacogenetic screening for HLA‐B*15:02 in Asian populations can prevent carbamazepine‐induced Stevens–Johnson syndrome. We have identified HLA‐A*31:01 as a potential risk marker for all phenotypes of carbamazepine‐induced hypersensitivity with applicability in European and other populations. In this review, we explore the currently available key stratification approaches to address the therapeutic challenges in epilepsy.     

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

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

Ontogeny of ABC and SLC transporters in the microvessels of developing rat brain

The blood–brain barrier (BBB) is responsible for the control of solutes’ concentration in the brain. Tight junctions and multiple ATP‐binding cassette (ABC) and SoLute Carrier (SLC) efflux transporters protect brain cells from xenobiotics, therefore reducing brain exposure to intentionally administered drugs. In epilepsy, polymorphisms and overexpression of efflux transporters genes could be associated with pharmacoresistance. The ontogeny of these efflux transporters should also be addressed because their expression during development may be related to different brain exposure to antiepileptic drugs in the immature brain. We detected statistically significant higher expression of Abcb1b and Slc16a1 genes, and lower expression of Abcb1a and Abcg2 genes between the post‐natal day 14 (P14) and the adult rat microvessels. P‐gP efflux activity was also shown to be lower in P14 rats when compared with the adults. The P‐gP proteins coded by rodent genes Abcb1a and Abcb1b are known to have different substrate affinities. The role of the Abcg2 gene is less clear in pharmacoresistance in epilepsy, nonetheless the coded protein Bcrp is frequently associated with drug resistance. Finally, we observed a higher expression of the Mct1 transporter gene in the P14 rat brain microvessels. Accordingly to our results, we suppose that age may be another factor influencing brain exposure to antiepileptics as a consequence of different expression patterns of efflux transporters between the adult and immature BBB.