Читать книгу Canine and Feline Epilepsy - Luisa De Risio - Страница 59

A drug can only enter the brain via two routes, either traversing the BBB or via the ventricular system and cerebrospinal fluid (CSF). The BBB restricts the entrance of any drug or other xenobiotic substance in order to protect the CNS from toxicity. The endothelial cells in the BBB are connected by tight junctions and surrounded by a basement membrane with astrocytic foot processes covering 95% of the endothelial lining. The BBB lacks transendothelial pathways such as transcellular channels or fenestrations (Löscher and Potschka, 2002). The functional significance is that the BBB resembles continuous phospholipid membranes, which stops the diffusion of hydrophilic, large or protein-bound drugs. Lipid-soluble drugs were thought on the other hand to diffuse easily through the BBB. Apart from the passive transport mechanism of lipophilic compounds, the BBB hosts a carrier-mediated transport system (Pardridge, 1999). In the last decade, multiple multidrug transporters of the ATP-binding cassette superfamily, especially P-gp and multidrug resistance-associated protein (MRP), have been shown to be expressed physiologically on the luminal side of the endothelial cells of the BBB. P-gp and MRP are also expressed in the choroid plexus epithelial cells limiting brain entrance even further (Rao et al., 1999). These transporters appear to act as an active defence mechanism against the penetration of potential CNS toxic lipophilic compounds, therefore limiting the penetration of lipophilic drugs, such as AEDs (Fromm, 2000; Spector, 2000). AEDs which are transported by multidrug transporters include valproate, gabapentin, topiramate, phenytoin, carbamazepine, phenobarbitone, felbamate and lamotrigine (Löscher and Potschka, 2002).

In contrast to the aforementioned hypotheses of drug-refractoriness, the multidrug transporter hypothesis is based on the assumption that it is not the brain target itself but the reduced AED concentration at the target site that causes pharmacoresistance. Indeed, an increased expression of multidrug transporters, such as P-gp and MRP in brain capillary endothelial cells comprising the BBB has been demonstrated both in epileptogenic brain tissue of human pharmacoresistant patients and in animal models of pharmacoresistant epilepsy (Tishler et al., 1995; Sisodiya et al., 2002; Aronica et al., 2003; Potschka et al., 2004; Volk et al., 2004a, b; Volk and Löscher, 2005; Hoffmann et al., 2006). In pharmacoresistant epileptic tissue, P-gp is also over-expressed in astrocytic foot processes and neurons limiting the AED concentration at the target even further (Aronica et al., 2003; Volk et al., 2004a). Over-expression of the multidrug transporter P-gp was demonstrated in dogs after increased seizure activity, status epilepticus and cluster seizures (Pekcec et al., 2009b). This is an interesting finding as seizure frequency and severity are, as aforementioned, clinical risk factors for pharmacoresistant epilepsy.

More than 50 SNPs and insertion/deletion polymorphisms have been reported for ABCB1, with some of them resulting in a change of P-gp expression and/or function (Löscher and Potschka, 2005). An aforementioned study has also described a polymorphism in the promoter region of ABCB1 in Border collies associated with poor controlled epilepsy (Alves et al., 2011). This polymorphism could also have resulted in a change of function or over-expression of P-gp. However, no functional study was performed. The authors also found three other SNPs in the coding region of ABCB1, which were not associated with drug response or epilepsy. Apart from genetic influences, drug-induced mechanisms will interfere with multidrug transporter expression at the BBB. The BBB efflux multidrug transporter expression adapts continuously to ensure protection and detoxification of the CNS from xenobiotic substances. Efflux transporter expression is regulated by pregnane X receptor (PXR), which reacts to xenobiotic (foreign toxic) compound exposure (Masuyama et al., 2005; Miller, 2010; Shukla et al., 2011). This results in a clearing of these compounds from the brain and/or body. In addition to the up-regulation of multidrug transporter expression, PXR co-regulates drug metabolizing CytP450 enzymes (Potschka, 2012). The binding domain of PXR and P-gp have many similarities and will interact with similar compounds, resulting in a dynamic process of regulating the efflux of xenobiotic compounds. AED have been reported to cause an up-regulation of P-gp expression via PXR activation. However, a clear interaction of PXR with standard AEDs remains contentious (Potschka, 2012). It appears that the main driving force for P-gp over-expression is seizure activity (Potschka, 2010). Over-expression is transient and includes brain regions involved in seizure initiation and propagation (Kwan et al., 2002). A high seizure frequency therefore results in an accumulation of efflux transporters at the BBB.

Each epileptic seizure results in a glutamate release, which activates an intracellular signalling cascade in BBB endothelial cells (Bankstahl et al., 2008; Bauer et al., 2008). The glutamate binds to endothelial N-methyl-Daspartate (NMDA) receptors, which starts arachidonic acid signalling. Cyclooxygenase-2 (COX-2) processes the arachidonic acid and produces prostaglandin E2 (PGE2). PGE2 binds on the prostaglandin E receptor (EP1) resulting in P-gp expression (Pekcec et al., 2009a).

Should P-gp over-expression be one of the major reasons for drug refractoriness, blocking or reducing the expression of P-gp could reverse the lack of drug response. Several case reports in human medicine and studies in animal models of refractory epilepsy have shown an improved seizure control when P-gp inhibitors were used (Brandt et al., 2006; Potschka, 2012). However, a recent study performed in dogs using verapamil as a P-gp inhibitor did not show a significant reduction of the seizure frequency. Verapamil is not a very specific P-gp inhibitor, which could explain the negative results. Furthermore, verapamil dosage was limited due to its cardiovascular side effects. The other problem with using a P-gp inhibitor is the lack of specificity for the BBB, such that the rest of the excretory body system will be limited in function, therefore the long-term safety of this treatment approach needs to be questioned. A more promising route might be the use of COX-2 inhibitors as they have been shown to decrease P-gp in rodent epilepsy studies that also resulted in a reversal of the drug refractoriness (Potschka, 2012).