Use of Isolated Hepatocyte Preparations for Cytochrome P450 Inhibition Studies: Comparison with Microsomes forKiDetermination

Predicting drug-drug interactions requires an assessment of the drug concentration available to the enzyme active site, both in vivo, and within an in vitro incubation. These predictions are confounded when the inhibitor accumulates within the liver, either as a result of active transport processes or intracellular binding (including lysosomal trapping). In theory, hepatocytes should provide a more accurate estimation of inhibitory potency compared with microsomes for those compounds that undergo hepatic accumulation. However, they are not routinely used for Ki determination and there is limited comparative information available. Therefore, the aims of this study were to compare Ki values determined in rat microsomes and freshly isolated hepatocytes using six cytochrome P450 inhibitors (miconazole, fluconazole, ketoconazole, quinine, fluoxetine, and fluvoxamine) with a range of uptake properties (cell-to-medium concentration ratios 4.2-6000). Inhibition studies were performed using four probe substrates for CYP2C, CYP2D, and CYP3A enzymes (tolbutamide and phenytoin, dextromethorphan and midazolam, respectively). Comparison of unbound Ki values (range 0.05-30 microM) showed good agreement between microsomes and hepatocytes for inhibition of 18 pathways of metabolism. In addition to this, there was no relationship between the cell-to-medium concentration ratios (covering over 3 orders of magnitude) and the microsomal to hepatocyte Ki ratio of these inhibitors. These data suggest that the hepatic accumulation of these inhibitors results from intracellular binding rather than the involvement of uptake transporters and indicate that microsomes and hepatocytes appear to be equivalent for determining the inhibitory potency of the six inhibitors investigated in the present study.

Formulation of Spray-Dried Phenytoin Loaded Poly(ε-Caprolactone) Microcarrier Intended for Brain Delivery to Treat Epilepsy

This study evaluates the efficacy of the spray-drying technique in the bioengineering of phenytoin (PHT) containing poly(epsilon-caprolactone) (PCL) microcarrier intended for brain delivery for long-term treatment of epilepsy. Through orthogonally designed experiments, the optimal formulation and process variables for the preparation of PCL-microcarriers containing PHT were obtained. The produced microcarriers were characterized by coulter counter, scanning electron, scanning transmission electron microscopies, differential scanning calorimetry, powder X-ray diffraction, and in vitro release. The results showed that the produced microcarriers have a spherical structure, uniform size distribution, and a particle mean diameter of about 4.0 microm, which is suitable for brain delivery. The PHT was loaded as dispersed microcrystals within the PCL-microcarriers. From this system, PHT was released slowly into a buffer solution for approximately 14 days without any burst effect. These data suggested that PHT containing spray-dried PCL-microcarrier may be a promising drug delivery system for local brain delivery and long-term treatment of pharmacoresistant epilepsy.

Lack of Support for a Role for RLIP76 (RALBP1) in Response to Treatment or Predisposition to Epilepsy

BACKGROUND

Multidrug transporters are postulated to contribute to antiepileptic drug (AED) resistance. The transporter best studied is P-glycoprotein, an ATP-Binding Cassette (ABC) transporter superfamily member. RLIP76 is suggested to be an energy-dependent non-ABC transporter, reducing AED blood-brain barrier penetration, with a more important role than P-glycoprotein. Knowledge of which transporters may be critical in drug resistance is important for design of potential therapies. We tested the hypothesis that RLIP76 mediates AED resistance using methods complementary to those in the original report.

METHODS

Double-labeling fluorescent immunohistochemistry localized RLIP76 expression. Population genetics was used to explore association of variation in the RLIP76-encoding gene with drug-response and epilepsy phenotypes. Comparative protein structure modeling and bioinformatic annotation were used to predict RLIP76 structure and features.

RESULTS

In normal and epileptogenic brain tissue, immunoreactivity for RLIP76 was cytoplasmic, with colocalization with a neuronal, but not an endothelial, marker. Genotyping of six tagging SNPs, representing common genetic variation in RLIP76, in patients with epilepsy responsive (n = 262) or resistant (n = 107) to AEDs showed no association with phenotype at any level. RLIP76 genotypic and haplotypic frequencies in 783 patients with epilepsy and 359 healthy controls showed no association with epilepsy susceptibility. RLIP76 is not predicted to have transmembrane localization or ATPase activity.

CONCLUSIONS

No support for RLIP76 itself in directly mediating resistance to AEDs nor in increasing susceptibility to epilepsy was found. More evidence is required before either a role for RLIP76 in drug resistance can be accepted or focus directed away from other transporters, such as P-glycoprotein.

Development of a Humanized In Vitro Blood?Brain Barrier Model to Screen for Brain Penetration of Antiepileptic Drugs

PURPOSE

A biotechnologic breakthrough for the study of drug permeability across the blood-brain barrier (BBB) would be the use of a reproducible in vitro model that recapitulates the functional, structural, and pathologic properties of the BBB in situ. We developed a humanized dynamic in vitro BBB model (DIV-BBB) based on cocultures of human microvascular endothelial cells (HBMECs) from "normal" and drug-resistant epileptic brain tissue with human brain astrocytes (HAs) from epilepsy patients or controls.

METHODS

HBMECs and HAs were cocultured for 28 days in polypropylene capillaries. HBMECs were exposed to physiologic levels of shear stress generated by intraluminal flow. Permeability to [3H]sucrose, [14C]phenytoin, and [14C]diazepam was measured in control and drug-resistant DIV-BBB with and without pretreatment with the MDR1 inhibitor XR9576. BBB integrity was monitored by transendothelial electrical resistance measurements (TEERs). Cell growth and viability were assessed by measurement of glucose consumption and lactate production.

RESULTS

PSucrose and TEER values did not depend on the origin of the endothelium used (epileptic or normal). PPhenytoin was 10-fold less (1.54 x 10(-6) cm/s) in drug-resistant BBB models than in controls (1.74 x 10(-5) cm/s). MDR1 blockade with XR9576 was effective (3.5-fold increase) only in drug-resistant cultures. PDiazepam in control and drug-resistant DIV-BBB was not affected by XR9576 and did not depend on the epileptic or control origin of endothelia. The overall contribution of epileptic glia to pharmacoresistance was negligible.

CONCLUSIONS

These results show that, for the substances used, the humanized DIV-BBB recapitulates the physiologic permeability properties of the BBB in vivo and is also capable of mimicking a drug-resistant BBB phenotype.

Population Estimation of the Effects of Cytochrome P450 2C9 and 2C19 Polymorphisms on Phenobarbital Clearance in Japanese

A nonlinear mixed-effect modeling (NONMEM) program was used to evaluate the effects of cytochrome P450 (CYP) 2C9 and CYP2C19 polymorphisms on the phenobarbital (PB) population clearance for Japanese epileptics. The pharmacokinetics of the 260 PB concentrations at a steady-state obtained from 79 patients was described with a one-compartment open pharmacokinetic model with first-order elimination. The covariates screened included the total body weight (BW), age, gender, PB daily dose, CYP2C9 and CYP2C19 genotypes, the coadministered antiepileptic drugs (AEDs), and complications. The final model of PB apparent clearance was as follows: CL = 0.23 x (BW/40)0.21 x 0.52CYP2C9*1/*3 x 0.68VPA x 0.85PHT x 0.85SMID x (1 + etaCL) where CL = the clearance of PB; CYP2C9*1/*3 = 1, otherwise 0; VPA = 1 if valproic acid is coadministered, otherwise 0; PHT = 1 if phenytoin is coadministered, otherwise 0; SMID = 1 if complications of severe or profound mental retardation with a significant behavior impairment are presented, otherwise 0; and etaCL = the independent random error distributed normally with the mean zero and variance equal to omegaP2. The total clearance of PB decreased by 48% in patients with CYP2C9*1/*3 genotype in comparison with those with CYP2C9*1/*1 genotype (P < 0.001). An effect of CYP2C19 polymorphisms was not detected. To our knowledge, this is the first report to demonstrate that the CYP2C9 genotype affects the PB metabolism in routine care, but the results should be further verified in other ethnic populations.

CYP2C9 Inhibition: Impact of Probe Selection and Pharmacogenetics on in Vitro Inhibition Profiles

Drug-drug interactions may cause serious adverse events in the clinical setting, and the cytochromes P450 are the enzyme system most often implicated in these interactions. Cytochrome P450 2C is the second most abundant subfamily of cytochrome P450 enzymes and is responsible for metabolism of almost 20% of currently marketed drugs. The most abundant isoform of this subfamily is CYP2C9, which is the major clearance pathway for the low therapeutic index drugs warfarin and phenytoin. Considering the importance of CYP2C9 to drug-drug interactions, the in vitro-in vivo extrapolation of drug-drug interactions for CYP2C9 may be confounded by the presence of polymorphic variants and the possibility of multiple binding regions within the CYP2C9 active site, leading to the potential for genotype- and substrate-dependent inhibition. To address the issues of genotype-dependent enzyme inhibition as well as probe substrate correlations, the inhibitory potency (Ki) of 28 effector molecules was assessed with five commonly used probes of CYP2C9 in both the CYP2C9.1 and CYP2C9.3 proteins. The inhibition of CYP2C9.1 and CYP2C9.3 by the battery of inhibitors with five substrate probes demonstrated differential inhibition potency not only between the two genotypes but also across substrate probes. Furthermore, the substrate probes fell into three distinct classes depending on genotype, suggesting that multiple probes may be needed to fully assess inhibition of CYP2C9 in vitro. Thus, both genotype and choice of probe substrate must be considered when attempting to predict potential CYP2C9 drug-drug interactions from in vitro data.

Side by side comparison between dynamic versus static models of blood–brain barrier in vitro: A permeability study

Endothelial cells in vivo are continuously exposed to shear stress, a tangential force generated by the flow of blood across their apical surfaces that affects endothelial cell structure and function. By contrast, the Transwell apparatus cannot reproduce the presence of intraluminal blood flow that is essential for the formation and differentiation of the BBB. In contrast, the dynamic in vitro model of the BBB (DIV-BBB) mimics both functionally and anatomically the brain microvasculature, creating quasi-physiological conditions for co-culturing human and non-human endothelial cells and astrocytes in a capillary-like structure. We used intraluminal bovine aortic endothelial cells (BAEC) co-cultured with extraluminal glial cells (C6) to obtain elevated trans-endothelial electrical resistance (TEER) and selective permeability to sucrose and phenytoin. The experiments were performed in parallel using Transwell systems DIV-BBB models and data were then cross compared. By contrast with Transwell, C6 and BAEC co-cultured in the DIV-BBB demonstrated predominantly aerobic metabolism evidenced by a robust increase in glucose consumption that was paralleled by a similar change in lactate production. BAEC exposed to glia under dynamic conditions grow in a monolayer fashion and developed a more stringent barrier as demonstrated by high TEER values and a selective permeability to [14C] phenytoin and the well-known paracellular marker [3H] sucrose. In conclusion, these data demonstrate that the exposure to intraluminal flow plays an essential role in promoting endothelial cell differentiation and increasing BBB tightness, thus making the use of the DIV-BBB well suited for pharmacological studies.

Paradoxical urinary phenytoin metabolite (S)/(R) ratios in CYP2C19*1/*2 patients

Phenytoin (PHT) is primarily metabolized to 5-(4'-hydroxyphenyl)-5-phenylhydantoin (p-HPPH), accounting for 67-88% of an administered dose in humans. p-HPPH is formed by the cytochrome (CYP) 450 enzymes CYP2C9 and CYP2C19, then glucuronidated and excreted into the urine. CYP2C9 catalyses the prochiral formation of (R) and (S)-p-HPPH, and is approximately 40 times more stereoselective towards the formation of the (S) isomer whereas CYP2C19 is not stereoselective. Because of differential stereoselectivity, polymorphisms in the genes can alter the (S)/(R)-p-HPPH ratios. Genotyping for CYP2C9 and CYP2C19 was accomplished by a Taqman based assay. Twelve and twenty-four hour urine samples were collected from 45 epilepsy patients taking PHT under steady-state conditions and (S)/(R) ratios of p-HPPH were determined by chiral HPLC separation. The mean urinary (S)/(R) ratio in the 12-24h urine collection in subjects homozygous for CYP2C9*1/*1, CYP2C19*1/*1 was 24.2+/-3.1(n=21), whereas ratios in CYP2C9*1/*2 and CYP2C9*1/*3 subjects, were 11.1+/-3.3(n=7) and 2.7+/-0.6(n=2), respectively. One CYP2C9*2/*3 patient had a ratio of 2.1. Unexpectedly, CYP2C9*1/*1, CYP2C19*1/*2 subjects had a mean (S)/(R) ratio as low as 12.9+/-1.7(n=12). Our results are generally consistent with single dose PHT studies. However, the (S)/(R)-p-HPPH ratios for the CYP2C9*1/*1, CYP2C19*1/*2 subjects, expected to be in the range of 30-40, were only 12.9, suggesting some undetected linkage disequilibrium between CYP2C9 and CYP2C19 genes that could affect PHT elimination. Furthermore, our study suggests that measurement of urine ratios cannot be used as a marker for genotype determination.

Influence of the CYP2C9 AND CYP2C19 polymorphisms on phenytoin hydroxylation in healthy individuals from south India

BACKGROUND AND OBJECTIVES

Phenytoin, a widely used anti-epileptic drug, is metabolized mainly by CYP2C9 (90%) and partly by CYP2C19 (10%) to its major metabolite 5-(para-hydroxyphenyl)-5- phenylhydantoin (p-HPPH). The CYP2C9 and CYP2C19 genes encoding these enzymes are polymorphically expressed and most of the variants result in decreased metabolism of the respective substrates. The present study was undertaken to investigate the influence of the CYP2C9*2 and *3 as well as CYP2C19*2 and *3 variant genotypes on phenytoin hydroxylation in healthy subjects from south India.

METHODS

A total of 27 healthy, unrelated, subjects were administered a single oral dose of 300 mg phenytoin. Four hours later, 5 ml of blood was collected and genotyped for CYP2C9*1, *2, *3, CYP2C19*1, *2 and *3 by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Phenytoin and the major metabolite p-HPPH were estimated by reverse phase HPLC. The metabolic ratio was calculated as concentration of phenytoin/p-HPPH.

RESULTS

A significant correlation was observed between the CYP2C9 genotype and metabolic ratio of phenytoin/p-HPPH (r = 0.472, 95% CI 0.100 to 0.728; P = 0.01). While no association was found with CYP2C19 alone, a significant correlation was observed between the combined CYP2C9 and CYP2C19 genotypes and phenytoin metabolic ratio (r = 0.507, 95% CI 0.146 to 0.749; P< 0.01).

INTERPRETATION AND CONCLUSION

CYP2C9*2 and *3 mutant alleles caused decreased hydroxylation of phenytoin in vivo, whereas the mutant alleles of CYP2C19 played only a minor role in the metabolism of phenytoin in subjects of our study. The results of present preliminary study needs to be confirmed with a larger sample.

Studies by biointeraction chromatography of binding by phenytoin metabolites to human serum albumin

Biointeraction studies based on high performance affinity chromatography were used to investigate the binding of human serum albumin (HSA) to two major phenytoin metabolites: 5-(3-hydroxyphenyl)-5-phenylhydantoin (m-HPPH) and 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH). This was initially examined by conducting self-competition zonal elution experiments in which m-HPPH or p-HPPH were placed in both the mobile phase and injected sample. It was found that each metabolite had a single major binding site on HSA. Competitive zonal elution experiments using l-tryptophan, warfarin, digitoxin, and cis-clomiphene as site-selective probes indicated that m-HPPH and p-HPPH were interacting with the indole-benzodiazepine site of HSA. The estimated association equilibrium constants for m-HPPH and p-HPPH at this site were 3.2 (+/-1.2)x10(3) and 5.7 (+/-0.7)x10(3)M(-1), respectively, at pH 7.4 and 37 degrees C. Use of these metabolites as competing agents for injections of phenytoin demonstrated that m-HPPH and p-HPPH had direct competition with this drug at the indole-benzodiazepine site. However, the use of phenytoin as a competing agent indicated that this drug had additional negative allosteric interactions on the binding of these metabolites to HSA. These results agreed with previous studies on the binding of phenytoin to HSA and its effects on the interactions of HSA with site-selective probes for the indole-benzodiazepine site.

P450 2C18 catalyzes the metabolic bioactivation of phenytoin

The safe clinical use of phenytoin (PHT) is compromised by a drug hypersensitivity reaction, hypothesized to be due to bioactivation of the drug to a protein-reactive metabolite. Previous studies have shown PHT is metabolized to the primary phenol metabolite, HPPH, then converted to a catechol which then autoxidizes to produce reactive quinone. PHT is known to be metabolized to HPPH by cytochromes P450 (P450s) 2C9 and 2C19 and then to the catechol by P450s 2C9, 2C19, 3A4, 3A5, and 3A7. However, the role of many poorly expressed or extrahepatic P450s in the metabolism and/or bioactivation of PHT is not known. The aim of this study was to assess the ability of other human P450s to catalyze PHT metabolism. P450 2C18 catalyzed the primary hydroxylation of PHT with a kcat (2.46 +/- 0.09 min-1) more than an order of magnitude higher than that of P450 2C9 (0.051 +/- 0.004 min-1) and P450 2C19 (0.054 +/- 0.002 min-1) and Km (45 +/- 5 microM) slightly greater than those of P450 2C9 (12 +/- 4 microM) and P450 2C19 (29 +/- 4 microM). P450 2C18 also efficiently catalyzed the secondary hydroxylation of PHT as well as covalent drug-protein adduct formation from both PHT and HPPH in vitro. While P450 2C18 is expressed poorly in the liver, significant expression has been reported in the skin. Thus, P450 2C18 may be important for the extrahepatic tissue-specific bioactivation of PHT in vivo.

Inhibition of the Multidrug Transporter P-Glycoprotein Improves Seizure Control in Phenytoin-treated Chronic Epileptic Rats

PURPOSE

Overexpression of multidrug transporters such as P-glycoprotein (P-gp) may play a significant role in pharmacoresistance, by preventing antiepileptic drugs (AEDs) from reaching their targets in the brain. Until now, many studies have described increased P-gp expression in epileptic tissue or have shown that several AEDs act as substrates for P-gp. However, definitive proof showing the functional involvement of P-gp in pharmacoresistance is still lacking. Here we tested whether P-gp contributes to pharmacoresistance to phenytoin (PHT) by using a specific P-gp inhibitor in a model of spontaneous seizures in rats.

METHODS

The effects of PHT on spontaneous seizure activity were investigated in the electrical post-status epilepticus rat model for temporal lobe epilepsy, before and after administration of tariquidar (TQD), a selective inhibitor of P-gp.

RESULTS

A 7-day treatment with therapeutic doses of PHT suppressed spontaneous seizure activity in rats, but only partially. However, an almost complete control of seizures by PHT (93 +/- 7%) was obtained in all rats when PHT was coadministered with TQD. This specific P-gp inhibitor was effective in improving the anticonvulsive action of PHT during the first 3-4 days of the treatment. Western blot analysis confirmed P-gp upregulation in epileptic brains (140-200% of control levels), along with approximately 20% reduced PHT brain levels. Inhibition of P-gp by TQD significantly increased PHT brain levels in chronic epileptic rats.

CONCLUSIONS

These findings show that TQD significantly improves the anticonvulsive action of PHT, thus establishing a proof-of-concept that the administration of AEDs in combination with P-gp inhibitors may be a promising therapeutic strategy in pharmacoresistant patients.

CYP2C9, CYP2C19, ABCB1 (MDR1) genetic polymorphisms and phenytoin metabolism in a Black Beninese population

The genetically polymorphic cytochrome P450 2C9 (CYP2C9) metabolizes many important drugs. Among them, phenytoin has been used as a probe to determine CYP2C9 phenotype by measuring the urinary excretion of its major metabolite, S-enantiomer of 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH). Phenytoin pharmacokinetic is also dependent on the activity of CYP2C19 and p-glycoprotein (ABCB1). To determine the influence of CYP2C9, CYP2C19 and ABCB1 genetic polymorphisms on phenytoin metabolism in a Black population, 109 healthy Beninese subjects received a single 300 mg oral dose of phenytoin. Blood was drawn 4 h after drug intake and urine was collected during the first 8 h. Plasma phenytoin and urine S- and R-enantiomers of p-HPPH were determined by high-performance liquid chromatography. Urinary excretion of (S)-p-HPPH [defined as urinary volumex(S)-p-HPPH urinary concentration] and PMR (defined as the ratio of p-HPPH in urine to 4 h phenytoin plasma concentration), both markers of CYP2C9 activity, were used to determine the functional relevance of new variants of CYP2C9 (*5, *6, *8, *9 and *11) in this population. Plasma phenytoin concentration was significantly associated with ABCB1 haplotype/genotype (P=0.05, Kruskal-Wallis test) and levels increased significantly in the genotype order: wild-type, T3421A and Block-2 genotypes (P=0.015, Jonckheere-Terpstra test). Urinary excretion of (S)-p-HPPH and PMR were significantly associated with the CYP2C9 genotype (P=0.001, analysis of variance (ANOVA) and P<0.0001, Kruskal-Wallis test, respectively) and decreased in the order: CYP2C9*1/*1, CYP2C9*1/*9, CYP2C9*9/*9, CYP2C9*1/*8, CYP2C9*8/*9, CYP2C9*9/*11, CYP2C9*1/*5, CYP2C9*6/*9, CYP2C9*1/*6, CYP2C9*8/*11, CYP2C9*5/*8 and CYP2C9*5/*6 (P<0.001, Jonckheere-Terpstra test). A combined analysis of CYP2C9, 2C19 and ABCB1 revealed that only ABCB1 predicted phenytoin concentration at 4 h and explained 8% of the variability (r=0.08, P=0.04). On the other hand, only CYP2C9 was predictive for the urinary excretion of (S)-p-HPPH and PMR (r=0.21, P=0.001 and r=0.25, P<0.001, respectively). Furthermore, significant relation was found between urinary excretion of (R)-p-HPPH and CYP2C9 genotype (P=0.035) and levels significantly increased in the genotype order: CYP2C9*1/*9, CYP2C9*1/*1, CYP2C9*9/*11, CYP2C9*1/*8 and CYP2C9*1/*5 (P<0.001, Jonckheere-Terpstra test). In summary, the present study demonstrates that, in a Black population, CYP2C9*5, *6, *8 and *11 variants, but not CYP2C9*9, are associated with a decreased phenytoin metabolism. The data also confirm the limited contribution of MDR1 gene to inter-individual phenytoin pharmacokinetic variation.

Expression of multidrug transporters MRP1, MRP2, and BCRP shortly after status epilepticus, during the latent period, and in chronic epileptic rats

PURPOSE

Overexpression of multidrug transporters may play a role in the development of pharmacoresistance by decreasing extracellular drug levels in the brain. However, it is not known whether overexpression is due to an initial insult or evolves more gradually because of recurrent spontaneous seizures. In the present study, we investigated the expression of different multidrug transporters during epileptogenesis in the rat. In addition, we determined whether these transporters affected phenytoin (PHT) distribution in the brain.

METHODS

Expression of multidrug resistance-associated proteins MRP1 and MRP2 and breast cancer-resistance protein (BCRP) was examined after electrically induced status epilepticus (SE) by immunocytochemistry and Western blot analysis. Brain/blood PHT levels were determined by high-performance liquid chromatography (HPLC) analysis in the presence and absence of the MRP inhibitor probenecid.

RESULTS

Shortly after SE, MRP1, MRP2, and BCRP were upregulated in astrocytes within several limbic structures, including hippocampus. In chronic epileptic rats, these proteins were overexpressed in the parahippocampal cortex, specifically in blood vessels and astrocytes surrounding these vessels. Overexpression was related to the occurrence of SE and was present mainly in rats with a high seizure frequency. Brain PHT levels were significantly lower in epileptic rats compared with control rats, but pharmacologic inhibition of MRPs increased the PHT levels.

CONCLUSIONS

Overexpression of MRP and BCRP was induced by SE as well as recurrent seizures. Moreover, overexpression was associated with lower PHT levels in the brain, which was reversed through inhibition of MRPs. These data suggest that administration of antiepileptic drugs in combination with specific inhibitors for multidrug transporters may be a promising therapeutic strategy in pharmacoresistant patients.

Evaluation of the role of P-glycoprotein in the uptake of paroxetine, clozapine, phenytoin and carbamazapine by bovine retinal endothelial cells

Expression of the drug transport proteins, including P-glycoprotein (Pgp), in the brain vascular endothelium represents a challenge for the effective delivery of drugs for the treatment of several central nervous system (CNS) disorders including depression, schizophrenia and epilepsy. It has been hypothesized that Pgp plays a major role in drug efflux at the blood-brain barrier, and may be an underlying factor in the variable responses of patients to CNS drugs. However, the role of Pgp in the transport of many CNS drugs has not been directly demonstrated. To explore the role of Pgp in drug transport across an endothelial cell barrier derived from the central nervous system, the expression and activity of Pgp in bovine retinal endothelial cells (BRECs) and the effects of representative CNS drugs on Pgp activity were examined. Significant Pgp expression in BRECs was demonstrated by western analyses, and expression was increased by treatment of the cells with hydrocortisone. Intracellular accumulation of the well-characterized Pgp-substrate Taxol was markedly increased by the non-selective transporter inhibitor verapamil and the Pgp-selective antagonist PGP-4008, demonstrating that Pgp is active in these endothelial cells. In contrast, neither verapamil nor PGP-4008 affected the intracellular accumulation of [3H]paroxetine, [14C]phenytoin, [3H]clozapine or [14C]carbamazapine, indicating that these drugs are not substrates for Pgp. Paroxetine, clozapine and phenytoin were shown to be Pgp inhibitors, while carbamazapine did not inhibit Pgp at any concentration tested. These results indicate that Pgp is not likely to modulate patient responses to these drugs.

RLIP76, a non-ABC transporter, and drug resistance in epilepsy

Background

Permeability of the blood-brain barrier is one of the factors determining the bioavailability of therapeutic drugs and resistance to chemically different antiepileptic drugs is a consequence of decreased intracerebral accumulation. The ABC transporters, particularly P-glycoprotein, are known to play a role in antiepileptic drug extrusion, but are not by themselves sufficient to fully explain the phenomenon of drug-resistant epilepsy. Proteomic analyses of membrane protein differentially expressed in epileptic foci brain tissue revealed the frequently increased expression of RLIP76/RALBP1, a recently described non-ABC multi-specific transporter. Because of a significant overlap in substrates between P-glycoprotein and RLIP76, present studies were carried out to determine the potential role of RLIP76 in AED transport in the brain.

Results

RLIP76 was expressed in brain tissue, preferentially in the lumenal surface of endothelial cell membranes. The expression was most prominent in blood brain barrier tissue from excised epileptic foci. Saturable, energy-dependent, anti-gradient transport of both phenytoin and carbamazepine were demonstrated using recombinant RLIP76 reconstituted into artificial membrane liposomes. Immunotitration studies of transport activity in crude membrane vesicles prepared from whole-brain tissue endothelium showed that RLIP76 represented the dominant transport mechanism for both drugs. RLIP76^-/- knockout mice exhibited dramatic toxicity upon phenytoin administration due to decreased drug extrusion mechanisms at the blood-brain barrier.

Conclusion

We conclude that RLIP76 is the predominant transporter of AED in the blood brain barrier, and that it may be a transporter involved in mechanisms of drug-resistant epilepsy.

Studies of phenytoin binding to human serum albumin by high-performance affinity chromatography

High-performance affinity chromatography was used to study the binding of phenytoin to an immobilized human serum albumin (HSA) column. This was accomplished through frontal analysis and competitive binding zonal elution experiments, the latter of which used four probe compounds for the major and minor binding sites of HSA injected into the presence of mobile phases containing known concentrations of phenytoin. It was found that phenytoin can interact with HSA at the warfarin-azapropazone, indole-benzodiazepine, tamoxifen, and digitoxin sites of this protein. The association constants for phenytoin at the indole-benzodiazepine and digitoxin sites were determined to be 1.04 (+/-0.05) x 10(4)M(-1) and 6.5 (+/-0.6) x 10(3)M(-1), respectively, at pH 7.4 and 37 degrees C. Both allosteric interactions and direct binding for phenytoin appear to take place at the warfarin-azapropazone and tamoxifen sites. This rather complex binding system indicates the importance of identifying the binding regions on HSA for specific drugs as a means for understanding the transport of such substances in blood and in characterizing their potential for drug-drug interactions.

The effects of phenytoin and its metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin on cellular glucose transport

Glucose is the principal fuel for brain metabolism and its movement across the blood-brain barrier depends on Glut1. Impaired glucose transport to the brain may have deleterious consequences. For example, Glut1 deficiency syndrome (Glut1DS) is the result of heterozygous loss of function Glut1 mutation leading to energy failure of the brain and subsequently, epileptic encephalopathy. To preserve the integrity of the energy supply to the brain in patients with compromised glucose transport function, consumption of compounds with glucose transport inhibiting properties should be avoided. Phenytoin is a widely used anticonvulsant that affects carbohydrate metabolism. In this study, the hypothesis that phenytoin and its metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH) affect cellular glucose transport was tested. With a focus on Glut1, the effects of phenytoin and HPPH on cellular glucose transport were studied. Glucose uptake assay measuring the zero-trans influx of radioactive-labeled glucose analogues showed that phenytoin and HPPH did not exert immediate effects on erythrocyte Glut1 activity or glucose transport in Hs68 control fibroblasts, Glut1DS primary fibroblasts isolated from two patients, or in rat primary astrocytes. Prolonged exposure to the two compounds could stimulate glucose transport by up to 30-60% over the control level (p <0.05) in Hs68 and Glut1DS fibroblasts as well as in rat astrocytes. The stimulation of glucose transport by HPPH was dose-dependent and accompanied by an up-regulation of GLUT1 mRNA expression (p <0.05). In conclusion, phenytoin and HPPH do not compromise cellular glucose transport. Prolonged exposure to these compounds can modify carbohydrate homeostasis by up-regulating glucose transport in both normal and Glut1DS conditions in vitro.

Differences in the allosteric modulation by phenytoin of the binding properties of the σ1 ligands [3H](+)-pentazocine and [3H]NE-100

The present study evaluated the effects of phenytoin (DPH) on the binding to synaptosomal fraction membranes from guinea pig brain of the prototypic sigma1 (sigma1) receptor agonist [3H](+)-pentazocine and the putative sigma1 antagonist [3H]NE-100. Equilibrium and binding kinetics studies were done. The order of affinity of 12 sigma1 ligands for binding sites labeled with [3H](+)-pentazocine correlated well with their order of affinity for sites labeled with [3H]NE-100, suggesting that both radioligands label the same receptor. Phenytoin increased the binding of [3H](+)-pentazocine, enhancing its affinity (K(D) value) for sigma1 receptors and decreasing its dissociation rate from these receptors. The maximal number of receptors (B(max) value) labeled with [3H](+)-pentazocine was not changed. In contrast, phenytoin decreased the specific binding and maximal number of receptors labeled with [3H]NE-100, and increased its dissociation rate from sigma1 receptors. The affinity of this radioligand for sigma1 receptors was not modified. In conclusion, phenytoin behaved as a positive allosteric modulator on the binding of [3H](+)-pentazocine, whereas it negatively modulated the binding of [3H]NE-100. These results add evidence in favor of the use of phenytoin in vitro to distinguish between agonists and antagonists of sigma1 receptors.

Mechanisms regulating toxicant disposition to the embryo during early pregnancy: An interspecies comparison

The dose of toxicant reaching the embryo is a critical determinant of developmental toxicity, and is likely to be a key factor responsible for interspecies variability in response to many test agents. This review compares the mechanisms regulating disposition of toxicants from the maternal circulation to the embryo during organogenesis in humans and the two species used predominantly in regulatory developmental toxicity testing, rats and rabbits. These three species utilize fundamentally different strategies for maternal-embryonic exchange during early pregnancy. Early postimplantation rat embryos rely on the inverted visceral yolk sac placenta, which is in intimate contact with the uterine epithelium and is equipped with an extensive repertoire of transport mechanisms, such as pinocytosis, endocytosis, and specific transporter proteins. Also, the rat yolk sac completely surrounds the embryo, such that the fluid-filled exocoelom survives through most of the period of organogenesis, and can concentrate compounds such as certain weak acids due to pH differences between maternal blood and exocelomic fluid. The early postimplantation rabbit conceptus differs from the rat in that the yolk sac is not closely apposed to the uterus during early organogenesis and does not completely enclose the embryo until relatively later in development (approximately GD13). This suggests that the early rabbit yolk sac might be a relatively inefficient transporter, a conclusion supported by limited data with ethylene glycol and one of its predominant metabolites, glycolic acid, given to GD9 rabbits. In humans, maternal-embryo exchange is thought to occur via the chorioallantoic placenta, although it has recently been conjectured that a supplemental route of transfer could occur via absorption into the yolk sac. Knowledge of the mechanisms underlying species-specific embryonic disposition, factored together with other pharmacokinetic characteristics of the test compound and knowledge of critical periods of susceptibility, can be used on a case-by-case basis to make more accurate extrapolations of test animal data to the human.

Non-invasive monitoring of phenytoin by reverse iontophoresis

Transdermal iontophoresis offers a non-invasive sampling method for therapeutic drug monitoring. This study examined whether iontophoretic extraction (a) is concentration dependent, (b) reflects the subdermal level of unbound drug, (c) follows protein binding changes, and (d) becomes truly non-invasive when a co-extracted compound is used as an internal standard for calibration. Iontophoresis was conducted in vitro using dermatomed pig-ear skin. The subdermal solution was a buffer containing phenytoin at therapeutic concentrations, an internal standard at fixed level, human albumin and/or valproic acid. The ionized form of phenytoin was recovered at the anode by electro-migration, while the neutral form was extracted to the cathode by electroosmosis. A satisfactory correlation between the reverse iontophoretic extracted amount of phenytoin and the subdermal concentration was observed. Iontophoresis extracted only the free fraction of phenytoin. At steady state, reverse iontophoresis monitored changes in free drug concentration provoked in the subdermal compartment. Acetate was introduced at a fixed concentration into the subdermal compartment to act as an 'internal standard'. Subsequently, acetate and the ionized form of phenytoin were co-extracted to the anode. The ratio of the extracted amounts was proportional to the subdermal concentration ratio demonstrating a means by which the method may become truly non-invasive.