Induction of nuclear receptors and drug resistance in the brain microvascular endothelial cells treated with antiepileptic drugs

Advancements in the application of nanotechnology for the management of epileptic seizures

Epilepsy is a common yet complex neurological disorder. Historically, antiseizure medications (ASMs) have faced challenges in crossing the blood-brain barrier (BBB) and targeting the epileptogenic zone, creating a bottleneck in seizure management. Certain nanomaterials can facilitate drug penetration through the BBB and enable stimulus-responsive drug release, thereby enhancing targeted and efficient drug utilization while reducing adverse reactions in other brain tissues and peripherally. This article reviews the current researches on stimulus-responsive nanosystems applicable in antiepileptic therapy, as well as nanotechnology applications that improve the brain delivery of ASMs.

Interactions between antiepileptic drugs and direct oral anticoagulants for primary and secondary stroke prevention

INTRODUCTION

Direct oral anticoagulants (DOAC) are the guideline-recommended therapy for prevention of stroke in atrial fibrillation (AF) and venous thromboembolism. Since approximately 10% of patients using antiepileptic drugs (AED) also receive DOAC, aim of this review is to summarize data about drug-drug interactions (DDI) of DOAC with AED by using data from PubMed until December 2023.

AREAS COVERED

Of 49 AED, only 16 have been investigated regarding DDI with DOAC by case reports or observational studies. No increased risk for stroke was reported only for topiramate, zonisamide, pregabalin, and gabapentin, whereas for the remaining 12 AED conflicting results regarding the risk for stroke and bleeding were found. Further 16 AED have the potential for pharmacodynamic or pharmacokinetic DDI, but no data regarding DOAC are available. For the remaining 17 AED it is unknown if they have DDI with DOAC.

EXPERT OPINION

Knowledge about pharmacokinetic and pharmacodynamic DDI of AED and DOAC is limited and frequently restricted to in vitro and in vivo findings. Since no data about DDI with DOAC are available for 67% of AED and an increasing number of patients have a combined medication of DOAC and AED, there is an urgent need for research on this topic.

The Effect of Levetiracetam Compared with Enzyme-Inducing Antiseizure Medications on Apixaban and Rivaroxaban Peak Plasma Concentrations

BACKGROUND AND OBJECTIVE

Post-stroke epilepsy represents an important clinical challenge as it often requires both treatment with direct oral anticoagulants (DOACs) and antiseizure medications (ASMs). Levetiracetam (LEV), an ASM not known to induce metabolizing enzymes, has been suggested as a safer alternative to enzyme-inducing (EI)-ASMs in patients treated with DOACs; however, current clinical guidelines suggest caution when LEV is used with DOACs because of possible P-glycoprotein induction and competition (based on preclinical studies). We investigated whether LEV affects apixaban and rivaroxaban concentrations compared with two control groups: (a) patients treated with EI-ASMs and (b) patients not treated with any ASM.

METHODS

In this retrospective observational study, we monitored apixaban and rivaroxaban peak plasma concentrations (Cmax) in 203 patients treated with LEV (n = 28) and with EI-ASM (n = 33), and in patients not treated with any ASM (n = 142). Enzyme-inducing ASMs included carbamazepine, phenytoin, phenobarbital, primidone, and oxcarbazepine. We collected clinical and laboratory data for analysis, and DOAC Cmax of patients taking LEV were compared with the other two groups.

RESULTS

In 203 patients, 55% were female and the mean age was 78 ± 0.8 years. One hundred and eighty-six patients received apixaban and 17 patients received rivaroxaban. The proportion of patients with DOAC Cmax below their therapeutic range was 7.1% in the LEV group, 10.6% in the non-ASM group, and 36.4% in the EI-ASM group (p < 0.001). The odds of having DOAC Cmax below the therapeutic range (compared with control groups) was not significantly different in patients taking LEV (adjusted odds ratio 0.70, 95% confidence interval 0.19-2.67, p = 0.61), but it was 12.7-fold higher in patients taking EI-ASM (p < 0.001). In an analysis in patients treated with apixaban, there was no difference in apixaban Cmax between patients treated with LEV and non-ASM controls, and LEV clinical use was not associated with variability in apixaban Cmax in a multivariate linear regression.

CONCLUSIONS

In this study, we show that unlike EI-ASMs, LEV clinical use was not significantly associated with lower apixaban Cmax and was similar to that in patients not treated with any ASM. Our findings suggest that the combination of LEV with apixaban and rivaroxaban may not be associated with decreased apixaban and rivaroxaban Cmax. Therefore, prospective controlled studies are required to examine the possible non-pharmacokinetic mechanism of the effect of the LEV-apixaban or LEV-rivaroxaban combination on patients' outcomes.

Hyperoside ameliorates cisplatin-induced acute kidney injury by regulating the expression and function of Oat1

Cisplatin is a widely used chemotherapeutic agent to treat solid tumours in clinics. However, cisplatin-induced acute kidney injury (AKI) limits its clinical application. This study investigated the effect of hyperoside (a flavonol glycoside compound) on regulating AKI.The model of cisplatin-induced AKI was established, and hyperoside was preadministered to investigate its effect on improving kidney injury.Hyperoside ameliorated renal pathological damage, reduced the accumulation of SCr, BUN, Kim-1 and indoxyl sulphate in vivo, increased the excretion of indoxyl sulphate into the urine, and upregulated the expression of renal organic anion transporter 1 (Oat1). Moreover, evaluation of rat kidney slices demonstrated that hyperoside promoted the uptake of PAH (p-aminohippurate, the Oat1 substrate), which was confirmed by transient over-expression of OAT1 in HEK-293T cells. Additionally, hyperoside upregulated the mRNA expression of Oat1 upstream regulators hepatocyte nuclear factor-1α (HNF-1α) and pregnane X receptor (PXR).These findings indicated hyperoside could protect against cisplatin-induced AKI by promoting indoxyl sulphate excretion through regulating the expression and function of Oat1, suggesting hyperoside may offer a potential tactic for cisplatin-induced AKI treatment.

AhR, PXR and CAR: From Xenobiotic Receptors to Metabolic Sensors

Traditionally, xenobiotic receptors are known for their role in chemical sensing and detoxification, as receptor activation regulates the expression of various key enzymes and receptors. However, recent studies have highlighted that xenobiotic receptors also play a key role in the regulation of lipid metabolism and therefore function also as metabolic sensors. Since dyslipidemia is a major risk factor for various cardiometabolic diseases, like atherosclerosis and non-alcoholic fatty liver disease, it is of major importance to understand the molecular mechanisms that are regulated by xenobiotic receptors. In this review, three major xenobiotic receptors will be discussed, being the aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Specifically, this review will focus on recent insights into the metabolic functions of these receptors, especially in the field of lipid metabolism and the associated dyslipidemia.

Valproic Acid-Induced Upregulation of Multidrug Efflux Transporter ABCG2/BCRP via PPARα-Dependent Mechanism in Human Brain Endothelial Cells

Despite the progress made in the development of new antiepileptic drugs (AEDs), poor response to them is a rising concern in epilepsy treatment. Of several hypotheses explaining AED treatment failure, the most promising theory is the overexpression of multidrug transporters belonging to ATP-binding cassette (ABC) transporter family at blood-brain barrier. Previous data show that AEDs themselves can induce these transporters, in turn affecting their own brain bioavailability. Presently, this induction and the underlying regulatory mechanism involved at human blood-brain barrier is not well elucidated. Herein, we sought to explore the effect of most prescribed first- and second-line AEDs on multidrug transporters in human cerebral microvascular endothelial cells, hCMEC/D3. Our work demonstrated that exposure of these cells to valproic acid (VPA) induced mRNA, protein, and functional activity of breast cancer resistance protein (BCRP/ABCG2). On examining the substrate interaction status of AEDs with BCRP, VPA, phenytoin, and lamotrigine were found to be potential BCRP substrates. Furthermore, we observed that siRNA-mediated knockdown of peroxisome proliferator-activated receptor alpha (PPARα) or use of PPARα antagonist, resulted in attenuation of VPA-induced BCRP expression and transporter activity. VPA was found to increase PPARα expression and trigger its translocation from cytoplasm to nucleus. Findings from chromatin immunoprecipitation and luciferase assays showed that VPA enhances the binding of PPARα to its response element in the ABCG2 promoter, resulting in elevated ABCG2 transcriptional activity. Taken together, these in vitro findings highlight PPARα as the potential molecular target to prevent VPA-mediated BCRP induction, which may have important implications in VPA pharmacoresistance. SIGNIFICANCE STATEMENT: Induction of multidrug transporters at blood-brain barrier can largely affect the bioavailability of the substrate antiepileptic drugs in the brains of patients with epilepsy, thus affecting their therapeutic efficacy. The present study reports a mechanistic pathway of breast cancer resistance protein (BCRP/ABCG2) upregulation by valproic acid in human brain endothelial cells via peroxisome proliferator-activated receptor alpha involvement, thereby providing a potential strategy to prevent valproic acid pharmacoresistance in epilepsy.

Pharmacokinetic Interactions Between Antiseizure and Psychiatric Medications

Antiseizure medications and drugs for psychiatric diseases are frequently used in combination. In this context, pharmacokinetic interactions between these drugs may occur. The vast majority of these interactions are primarily observed at a metabolic level and result from changes in the activity of the cytochrome P450 (CYP). Carbamazepine, phenytoin, and barbiturates induce the oxidative biotransformation and can consequently reduce the plasma concentrations of tricyclic antidepressants, many typical and atypical antipsychotics and some benzodiazepines. Newer antiseizure medications show a lower potential for clinically relevant interactions with drugs for psychiatric disease. The pharmacokinetics of many antiseizure medications is not influenced by antipsychotics and anxiolytics, while some newer antidepressants, namely fluoxetine, fluvoxamine and viloxazine, may inhibit CYP enzymes leading to increased serum concentrations of some antiseizure medications, including phenytoin and carbamazepine. Clinically relevant pharmacokinetic interactions may be anticipated by knowledge of CYP enzymes involved in the biotransformation of individual medications and of the influence of the specific comedication on the activity of these CYP enzymes. As a general rule, these interactions can be managed by careful evaluation of clinical response and, when indicated, individualized dosage adjustments guided by measurement of drugs serum concentrations, especially if pharmacokinetic interactions may cause any change in seizure control or signs of toxicity. Further studies are required to improve predictions of pharmacokinetic interactions between antiseizure medications and drugs for psychiatric diseases providing practical helps for clinicians in the clinical setting.

Association Between Use of Pharmacokinetic-Interacting Drugs and Effectiveness and Safety of Direct Acting Oral Anticoagulants: Nested Case-Control Study

Concomitant use of direct oral anticoagulants (DOACs) and medications with inhibition/induction effect on P-gp/CYP3A might increase risk of bleeding/treatment failure, respectively. We designed a nested case-control study within a Clalit cohort of patients with atrial fibrillation (AF) and a cohort of patients with venous thromboembolism, new users of a DOAC (January 1, 2010 to August 24, 2020). Propensity scores were constructed from demographic/clinical characteristics, and medications at cohort entry. Each case of: (i) serious bleeding event; (ii) stroke/systemic emboli (SE) in patients with AF; (iii) recurrent thromboembolism in patients with thromboembolism, was matched by age, sex, length of follow-up, year of cohort entry, DOAC type, and DOAC indication, to up to 20 controls. Within 89,284 patients with AF and venous thromboembolism and 126,302 patient-years of follow-up, there were 1,587 serious bleeding events. Risk of serious bleeding increased in association with concurrent prescription of P-gp/CYP3A4 inhibitors. Specifically, higher bleeding risk was associated with dabigatran-verapamil, rivaroxaban-verapamil, and rivaroxaban-amiodarone concurrent prescriptions: adjusted odds ratios (ORs) 2.29 (1.13-4.60), 2.18 (1.07-4.40), and 1.68 (1.14-2.49), respectively. There were 1,116 events of stroke/SE, in 79,302 DOAC-treated patients with AF and 118,124 patient-years of follow-up. Concomitant use of phenytoin, carbamazepine, valproic acid, or levetiracetam was associated with risk for stroke/SE: adjusted OR 2.18 (1.55-3.10). Risk of recurrent venous thromboembolism could not be assessed due to the low number of cases. Concurrent prescriptions of dabigatran or rivaroxaban with verapamil, and of rivaroxaban with amiodarone, are associated with increased risk for serious bleeding. Higher risk for stroke/SE in patients with AF is associated with concurrent prescriptions of DOACs with phenytoin, carbamazepine, valproic acid, or levetiracetam.

Neuroprotective and Angiogenesis Effects of Levetiracetam Following Ischemic Stroke in Rats

Objective: The present study explored whether levetiracetam (LEV) could protect against experimental brain ischemia and enhance angiogenesis in rats, and investigated the potential mechanisms in vivo and in vitro.

Methods: The middle cerebral artery was occluded for 60 min to induce middle cerebral artery occlusion (MCAO). The Morris water maze was used to measure cognitive ability. The rotation test was used to assess locomotor function. T2-weighted MRI was used to assess infarct volume. The neuronal cells in the cortex area were stained with cresyl purple. The anti-inflammatory effects of LEV on microglia were observed by immunohistochemistry. Enzyme-linked immunosorbent assays (ELISA) were used to measure the production of pro-inflammatory cytokines. Western blotting was used to detect the levels of heat shock protein 70 (HSP70), vascular endothelial growth factor (VEGF), and hypoxia-inducible factor-1α (HIF-1α) in extracts from the ischemic cortex. Flow cytometry was used to observe the effect of LEV on neuronal cell apoptosis.

Results: LEV treatment significantly increased the density of the surviving neurons in the cerebral cortex and reduced the infarct size (17.8 ± 3.3% vs. 12.9 ± 1.4%, p < 0.01) after MCAO. Concurrently, the time required to reach the platform for LEV-treated rats was shorter than that in the saline group on day 11 after MCAO (p < 0.01). LEV treatment prolonged the rotarod retention time on day 14 after MCAO (84.5 ± 6.7 s vs. 59.1 ± 6.2 s on day 14 compared with the saline-treated groups, p < 0.01). It also suppressed the activation of microglia and inhibited TNF-α and Il-1β in the ischemic brain (135.6 ± 5.2 pg/ml vs. 255.3 ± 12.5 pg/ml, 18.5 ± 1.3 pg/ml vs. 38.9 ± 2.3 pg/ml on day 14 compared with the saline-treated groups, p < 0.01). LEV treatment resulted in a significant increase in HIF-1α, VEGF, and HSP70 levels in extracts from the ischemic cerebral cortex. At the same time, LEV reduced neuronal cell cytotoxicity and apoptosis induced by an ischemic stroke (p < 0.01).

Conclusion: LEV treatment promoted angiogenesis and functional recovery after cerebral ischemia in rats. These effects seem to be mediated through anti-inflammatory and antiapoptotic activities, as well as inducing the expression of HSP70, VEGF, and HIF-1α.

Association of xenobiotic receptor polymorphisms with carbamazepine response in epilepsy patients

PURPOSE

Drug-resistant epilepsy is a problem worldwide. Xenobiotic receptors may play a significant role in the establishment of resistance to antiepileptic agents. Previous studies have confirmed that the metabolism and efficacy of carbamazepine (CBZ) can be influenced by xenobiotic receptors, especially pregnane X receptor (PXR), constitutive androstane receptor (CAR), and aryl hydrocarbon receptor (AHR). Therefore, this study intends to elucidate the pharmacogenomic associations of polymorphisms of these xenobiotic receptors with the CBZ response in epilepsy patients, and these genetic data may be useful for the treatment of clinical prophylaxis and individualized treatment of intractable epilepsy.

METHODS

Adult patients with epilepsy who were on CBZ-based monotherapy and combination therapy (n = 257) were genotyped, and the patients were divided into drug-responsive and drug-resistant groups according to the International League Against Epilepsy criteria. We sought to tag single-nucleotide polymorphisms (SNPs) of PXR, CAR and AHR that principally represent alleles associated with drug resistance risk; in addition, a gene interaction analysis reference panel was constructed for SNP-based imputation.

RESULTS

No significant effects of PXR or AHR polymorphisms were observed. However, an interaction between the CAR rs2502815 variant and CBZ response was observed: in CBZ-based monotherapy and combination therapy patients, the GG genotype of the CAR rs2502815 variant (vs. wild-type homozygous) was independently associated with CBZ response after adjusting for variables [odds ratio (OR) = 0.389, 95% confidence interval (CI) 0.203-0.743, p = 0.004]. The results of the haplotype and gene interaction case-control analyses of the CBZ response were negative. Our results provide clinical data regarding the genetic possibilities of drug responses related to CAR variation in epilepsy patients.

CONCLUSION

This study is the first to indicate a potentially relevant interaction between the CAR rs2502815 polymorphism and the CBZ response in epilepsy patients.

Neurovascular Drug Biotransformation Machinery in Focal Human Epilepsies: Brain CYP3A4 Correlates with Seizure Frequency and Antiepileptic Drug Therapy

Pharmacoresistance is a major clinical challenge for approximately 30% of patients with epilepsy. Previous studies indicate nuclear receptors (NRs), drug efflux transporters, and cytochrome P450 enzymes (CYPs) control drug passage across the blood-brain barrier (BBB) in drug-resistant epilepsy. Here, we (1) evaluate BBB changes, neurovascular nuclear receptors, and drug transporters in lesional/epileptic (EPI) and non-lesional/non-epileptic (NON-EPI) regions of the same brain, (2) examine regional CYP expression and activity, and (3) investigate the association among CYP brain expression, seizure frequency, duration of epilepsy, and antiepileptic drug (AED) combination. We used surgically resected brain specimens from patients with medically intractable epilepsy (n = 22) where the epileptogenic loci were well-characterized by invasive and non-invasive methods; histology confirmed distinction of small NON-EPI regions from EPI tissues. NRs, transporters, CYPs, and tight-junction proteins were assessed by western blots/immunohistochemistry, and CYP metabolic activity was determined and compared. The relationship of CYP expression with seizure frequency, duration of epilepsy, and prescribed AEDs was evaluated. Decreased BBB tight-junction proteins accompanied IgG leakage in EPI regions and correlated with upregulated NR and efflux transporter levels. CYP expression and activity significantly increased in EPI compared to NON-EPI tissues. Change in EPI and NON-EPI CYP3A4 expression increased in patients taking AEDs that were CYP substrates, was downregulated when CYP- and non-CYP-substrate AEDs were given together, and correlated with seizure frequency. Our studies suggest focal neurovascular CYP-NR-transporter alterations, as demonstrated by the relationship of seizure frequency and AED combination to brain CYP3A4, might together impact biotransformation machinery of human pharmacoresistant epilepsy.

ABC Transporters in Extrahepatic Tissues: Pharmacological Regulation in Heart and Intestine

ATP binding cassette (ABC) transporters are transmembrane proteins expressed in secretory epithelia like the liver, kidneys and intestine, in the epithelia exhibiting barrier function such as the blood-brain barrier and placenta, and to a much lesser extent, in tissues like reproductive organs, lungs, heart and pancreas, among others. They regulate internal distribution of endogenous metabolites and xenobiotics including drugs of therapeutic use and also participate in their elimination from the body. We here describe the function and regulation of ABC transporters in the heart and small intestine, as examples of extrahepatic tissues, in which ABC proteins play clearly different roles. In the heart, they are involved in tissue pathogenesis as well as in protecting this organ against toxic compounds and druginduced oxidative stress. The small intestine is highly exposed to therapeutic drugs taken orally and, consequently, ABC transporters localized on its surface strongly influence drug absorption and pharmacokinetics. Examples of the ABC proteins currently described are Multidrug Resistance-associated Proteins 1 and 2 (MRP1 and 2) for heart and small intestine, respectively, and P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) for both organs.

Regulation of brain drug metabolizing enzymes and transporters by nuclear receptors

Nuclear receptors (NRs) belong to a family of ligand-dependent transcription factors. The target genes of NRs include many drug metabolizing enzymes and transporters. The central nervous system (CNS) bears the expression of NRs, drug metabolizing enzymes and transporters. NRs that express in the brain can be divided into three groups according to their characteristics of ligand binding: steroid hormone receptors, non-steroid hormone receptors, and orphan receptors. The NR-mediated regulation of drug metabolizing enzymes and transporters plays important roles in the metabolism and disposition of drugs in the CNS and the penetration of endogenous and exogenous substances through the blood-brain barrier (BBB). NR-mediated regulation of drug metabolizing enzymes and transporters can cause the toxicological effects of xenobiotics in the CNS and also lead to drug resistance in the centrum. The regulatory pathways of drug metabolizing enzymes and transporters can provide new strategies for selective regulation of the BBB permeability and drug metabolism in the brain. This review focuses on the importance of NR-mediated regulation of drug metabolizing enzymes and transporters in the CNS and the implications of this regulation in the therapeutic effect of CNS drugs and CNS side effects of drugs and other xenotoxicants.

Modulation of Opioid Transport at the Blood-Brain Barrier by Altered ATP-Binding Cassette (ABC) Transporter Expression and Activity

Opioids are highly effective analgesics that have a serious potential for adverse drug reactions and for development of addiction and tolerance. Since the use of opioids has escalated in recent years, it is increasingly important to understand biological mechanisms that can increase the probability of opioid-associated adverse events occurring in patient populations. This is emphasized by the current opioid epidemic in the United States where opioid analgesics are frequently abused and misused. It has been established that the effectiveness of opioids is maximized when these drugs readily access opioid receptors in the central nervous system (CNS). Indeed, opioid delivery to the brain is significantly influenced by the blood-brain barrier (BBB). In particular, ATP-binding cassette (ABC) transporters that are endogenously expressed at the BBB are critical determinants of CNS opioid penetration. In this review, we will discuss current knowledge on the transport of opioid analgesic drugs by ABC transporters at the BBB. We will also examine how expression and trafficking of ABC transporters can be modified by pain and/or opioid pharmacotherapy, a novel mechanism that can promote opioid-associated adverse drug events and development of addiction and tolerance.

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.

Pregnane X Receptor Not Nuclear Factor-kappa B Up-regulates P-glycoprotein Expression in the Brain of Chronic Epileptic Rats Induced by Kainic Acid

Drug-resistance epilepsy (DRE) is attributed to the brain P-glycoprotein (P-gp) overexpression. We previously reported that nuclear factor-kappa B (NF-κB) played a critical role in regulating P-gp expression at the brain of the acute seizure rats. This study was extended further to investigate the interaction effect of NF-κB and pregnane X receptor (PXR) on P-gp expression at the brain of chronic epileptic rats treated with carbamazepine (CBZ). The chronic epileptic models were induced by the micro-injection of kainic acid (KA) into rats' hippocampus. Subsequently, the successful models were treated with different intervention agents of CBZ; PMA(a non-specific PXR activity inhibitor) or PDTC(a specific NF-κB activity inhibitor) respectively. The expression levels of P-gp and its encoded gene mdr1a/b were significantly up-regulated on the brain of KA-induced chronic epilepsy rats or the epilepsy rats treated with CBZ for 1 week, meanwhile with a high expression of PXR. The treatment of PMA dramatically reduced both PXR and P-gp expressions at the protein and mRNA levels in the chronic epilepsy brain. By compared to the epilepsy model group, the P-gp expression was not markedly attenuated by the inhibition of NF-κB activity with PDTC treatment, nevertheless with a decrease of NF-κB expression in this intervention group. Higher levels of proinflammatory cytokines(IL-1β, IL-6, TNF-α) were found both in the brain tissue and the serum in the epilepsy rats of each group. There was a declined trend of the pro-inflammatory cytokines expression of the PDTC treatment group but with no statistical significance. This study demonstrates for the first time that P-gp up-regulation is due to increase PXR expression in the chronic phase of epilepsy, differently from that NF-κB signaling may induce the P-gp expression in the acute seizure phase. Our results offer insights into the mechanism underlying the development of DRE using or not using CBZ treatment.

A concentration addition model to assess activation of the pregnane X receptor (PXR) by pesticide mixtures found in the French diet

French consumers are exposed to mixtures of pesticide residues in part through food consumption. As a xenosensor, the pregnane X receptor (hPXR) is activated by numerous pesticides, the combined effect of which is currently unknown. We examined the activation of hPXR by seven pesticide mixtures most likely found in the French diet and their individual components. The mixture's effect was estimated using the concentration addition (CA) model. PXR transactivation was measured by monitoring luciferase activity in hPXR/HepG2 cells and CYP3A4 expression in human hepatocytes. The three mixtures with the highest potency were evaluated using the CA model, at equimolar concentrations and at their relative proportion in the diet. The seven mixtures significantly activated hPXR and induced the expression of CYP3A4 in human hepatocytes. Of the 14 pesticides which constitute the three most active mixtures, four were found to be strong hPXR agonists, four medium, and six weak. Depending on the mixture and pesticide proportions, additive, greater than additive or less than additive effects between compounds were demonstrated. Predictions of the combined effects were obtained with both real-life and equimolar proportions at low concentrations. Pesticides act mostly additively to activate hPXR, when present in a mixture. Modulation of hPXR activation and its target genes induction may represent a risk factor contributing to exacerbate the physiological response of the hPXR signaling pathways and to explain some adverse effects in humans.

Marked differences in the effect of antiepileptic and cytostatic drugs on the functionality of P-glycoprotein in human and rat brain capillary endothelial cell lines

PURPOSE

The expression of P-glycoprotein (Pgp) is increased in brain capillary endothelial cells (BCECs) of patients with pharmacoresistant epilepsy. This may restrict the penetration of antiepileptic drugs (AEDs) into the brain. However, the mechanisms underlying increased Pgp expression in epilepsy patients are not known. One possibility is that AEDs induce the expression and functionality of Pgp in BCECs. Several older AEDs that induce human cytochrome P450 enzymes also induce Pgp in hepatocytes and enterocytes, but whether this extends to Pgp at the human BBB and to newer AEDs is not known.

METHODS

This prompted us to study the effects of various old and new AEDs on Pgp functionality in the human BCEC line, hCMEC/D3, using the rhodamine 123 (Rho123) efflux assay. For comparison, experiments were performed in two rat BCEC lines, RBE4 and GPNT, and primary cultures of rat and pig BCECs. Furthermore, known Pgp inducers, such as dexamethasone and several cytostatic drugs, were included in our experiments.

RESULTS

Under control conditions, GPNT cells exhibited the highest and RBE4 the lowest Pgp expression and Rho123 efflux, while intermediate values were determined in hCMEC/D3. Known Pgp inducers increased Rho123 efflux in all cell lines, but marked inter-cell line differences in effect size were observed. Of the various AEDs examined, only carbamazepine (100 μM) moderately increased Pgp functionality in hCMEC/D3, while valproate (300 μM) inhibited Pgp.

CONCLUSIONS

These data do not indicate that treatment with AEDs causes a clinically relevant induction in Pgp functionality in BCECs that form the BBB.

P-glycoprotein expression and function in patients with temporal lobe epilepsy: a case-control study

BACKGROUND

Studies in rodent models of epilepsy suggest that multidrug efflux transporters at the blood-brain barrier, such as P-glycoprotein, might contribute to pharmacoresistance by reducing target-site concentrations of antiepileptic drugs. We assessed P-glycoprotein activity in vivo in patients with temporal lobe epilepsy.

METHODS

We selected 16 patients with pharmacoresistant temporal lobe epilepsy who had seizures despite treatment with at least two antiepileptic drugs, eight patients who had been seizure-free on antiepileptic drugs for at least a year after 3 or more years of active temporal lobe epilepsy, and 17 healthy controls. All participants had a baseline PET scan with the P-glycoprotein substrate (R)-[(11)C]verapamil. Pharmacoresistant patients and healthy controls then received a 30-min infusion of the P-glycoprotein-inhibitor tariquidar followed by another (R)-[(11)C]verapamil PET scan 60 min later. Seizure-free patients had a second scan on the same day, but without tariquidar infusion. Voxel-by-voxel, we calculated the (R)-[(11)C]verapamil plasma-to-brain transport rate constant, K1 (mL/min/cm(3)). Low baseline K1 and attenuated K1 increases after tariquidar correspond to high P-glycoprotein activity.

FINDINGS

Between October, 2008, and November, 2011, we completed (R)-[(11)C]verapamil PET studies in 14 pharmacoresistant patients, eight seizure-free patients, and 13 healthy controls. Voxel-based analysis revealed that pharmacoresistant patients had lower baseline K1, corresponding to higher baseline P-glycoprotein activity, than seizure-free patients in ipsilateral amygdala (0·031 vs 0·036 mL/min/cm(3); p=0·014), bilateral parahippocampus (0·032 vs 0·037; p<0·0001), fusiform gyrus (0·036 vs 0·041; p<0·0001), inferior temporal gyrus (0·035 vs 0·041; p<0·0001), and middle temporal gyrus (0·038 vs 0·044; p<0·0001). Higher P-glycoprotein activity was associated with higher seizure frequency in whole-brain grey matter (p=0·016) and the hippocampus (p=0·029). In healthy controls, we noted a 56·8% increase of whole-brain K1 after 2 mg/kg tariquidar, and 57·9% for 3 mg/kg; in patients with pharmacoresistant temporal lobe epilepsy, whole-brain K1 increased by only 21·9% for 2 mg/kg and 42·6% after 3 mg/kg. This difference in tariquidar response was most pronounced in the sclerotic hippocampus (mean 24·5% increase in patients vs mean 65% increase in healthy controls, p<0·0001).

INTERPRETATION

Our results support the hypothesis that there is an association between P-glycoprotein overactivity in some regions of the brain and pharmacoresistance in temporal lobe epilepsy. If this relation is confirmed, and P-glycoprotein can be identified as a contributor to pharmacoresistance, overcoming P-glycoprotein overactivity could be investigated as a potential treatment strategy.

FUNDING

EU-FP7 programme (EURIPIDES number 201380).

Role of nuclear receptors in the regulation of drug transporters in the brain

ATP-binding cassette membrane-associated drug efflux transporters and solute carrier influx transporters, expressed at the blood-brain barrier, blood-cerebrospinal fluid barrier, and in brain parenchyma, are important determinants of drug disposition in the central nervous system. Targeting the regulatory pathways that govern the expression of these transporters could provide novel approaches to selectively alter drug permeability into the brain. Nuclear receptors are ligand-activated transcription factors which regulate the gene expression of several metabolic enzymes and drug efflux/influx transporters. Although efforts have primarily been focused on investigating these regulatory pathways in peripheral organs (i.e., liver and intestine), recent findings demonstrate their significance in the brain. This review addresses the role of nuclear receptors in the regulation of drug transporter functional expression in the brain. An in-depth understanding of these pathways could guide the development of novel pharmacotherapy with either enhanced efficacy in the central nervous system or minimal associated neurotoxicity.

Psychotropic drug-drug interactions involving P-glycoprotein

Multidrug resistance P-glycoprotein (P-gp; also known as MDR1 and ABCB1) is expressed in the luminal membrane of the small intestine and blood-brain barrier, and the apical membranes of excretory cells such as hepatocytes and kidney proximal tubule epithelia. P-gp regulates the absorption and elimination of a wide range of compounds, such as digoxin, paclitaxel, HIV protease inhibitors and psychotropic drugs. Its substrate specificity is as broad as that of cytochrome P450 (CYP) 3A4, which encompasses up to 50 % of the currently marketed drugs. There has been considerable interest in variations in the ABCB1 gene as predictors of the pharmacokinetics and/or treatment outcomes of several drug classes, including antidepressants and antipsychotics. Moreover, P-gp-mediated transport activity is saturable, and is subject to modulation by inhibition and induction, which can affect the pharmacokinetics, efficacy or safety of P-gp substrates. In addition, many of the P-gp substrates overlap with CYP3A4 substrates, and several psychotropic drugs that are P-gp substrates are also CYP3A4 substrates. Therefore, psychotropic drugs that are P-gp substrates may cause a drug interaction when P-gp inhibitors and inducers are coadministered, or when psychotropic drugs or other medicines that are P-gp substrates are added to a prescription. Hence, it is clinically important to accumulate data about drug interactions through studies on P-gp, in addition to CYP3A4, to assist in the selection of appropriate psychotropic medications and in avoiding inappropriate combinations of therapeutic agents. There is currently insufficient information available on the psychotropic drug interactions related to P-gp, and therefore we summarize the recent clinical data in this review.

Role of CNS efflux drug transporters in antiepileptic drug delivery: overcoming CNS efflux drug transport

Experimental support for the transporter hypothesis of drug resistance in epilepsies has triggered efforts developing and validating approaches to overcome enhanced blood-brain barrier efflux transport. Testing in rodent models has rendered proof-of-concept for an add-on therapy with antiepileptic drugs. However, further development of the approach would require tolerability considerations as efflux transporters serve an important protective function throughout the body limiting distribution of harmful xenobiotics. Relevant progress has been made in the elucidation of mechanisms driving up-regulation of the multidrug transporter P-glycoprotein in response to seizure activity. Based on this knowledge, novel strategies have been evaluated targeting the signaling cascade that regulates P-glycoprotein in the epileptic brain. Further concepts might include by-passing blood-brain barrier transporters by intracerebral administration or by encapsulation of antiepileptic drugs in nano-sized carrier systems. It is important to note that the future perspectives of respective approaches are still questionable based on the limited evidence for a clinical relevance of transporter expression. Thus, techniques are urgently needed for non-invasive assessment of blood-brain barrier transporter function. Respective techniques would allow testing for a clinical correlation between pharmacosensitivity and transporter function, validating therapeutic strategies targeting efflux transporters and selecting patients with transporter over-expression for respective clinical trials. Provided that further clinical data render support for the transporter hypothesis, the main question remains whether patients exist in which transporter over-expression is the predominant mechanism of drug resistance and in which overcoming drug efflux is equivalent with overcoming drug resistance. Imaging techniques might provide a tool to address these questions in clinical epileptology. However, the complex pharmacological interactions between antiepileptic drugs, radiotracers, and transporter modulators used in these approaches as well as interindividual differences in the brain pathology might hamper clear-cut conclusions and limit the diagnostic significance.

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.

Transporter hypothesis of drug-resistant epilepsy: challenges for pharmacogenetic approaches

Drug resistance in epilepsy is considered a complex and multifactorial problem. Overexpression of efflux transporters at the blood-brain barrier is discussed as one factor that might limit brain penetration and efficacy of antiepileptic drugs. Whereas experimental data render support for this hypothesis, there is still a lack of sufficient clinical evidence indicating a functional role of efflux transporters. Pharmacogenetic analysis has been considered as one approach in the evaluation of a putative link between transporters and drug-resistant epilepsy. However, the likelihood of a multifactorial nature of drug resistance and the complexity of the events regulating transporters pose a major challenge to any attempt at linking selected genetic polymorphisms to the outcome of drug therapy. In this article, the evidence for an impact of efflux transporters on the response to antiepileptic drugs is discussed, focusing in particular on the different issues presenting a challenge for pharmacogenetic approaches in this field.

Influence of efflux transporters on drug metabolism: theoretical approach for bioavailability and clearance prediction

Cytochrome P450 enzymes and efflux transporters, expressed in the intestine and/or in the liver, play important roles in drug clearance and oral bioavailability. The relative contribution of transporters and enzymes in drug metabolism is still controversial. Some antiepileptic drugs, such as carbamazepine, phenytoin and phenobarbital (phenobarbitone), show time-dependent and dose-dependent pharmacokinetics due to their inductive effect on both efflux transporters and enzymes. However, steady-state plasma drug concentrations for each antiepileptic drug do not relate to oral daily dose in the same way, with decreased or increased apparent clearance according to the drug. A multicompartment pharmacokinetic model was developed in order to explain these different behaviours using a single mechanism of inductive action. The key for solving these apparent dissimilarities was to consider in the model the unique physiological connection that intestine, liver and bloodstream have. Efflux transporters not only enhance enzymatic competition in relation to first-order processes, but also change the predominance of some elimination routes. For instance, the carbamazepine-10,11-epoxide formation increases at the expense of other carbamazepine metabolites, enhancing both the systemic and presystemic elimination of parent drug. Conversely, the major hepatic metabolism of phenytoin diminishes in favour of its minor intestinal elimination, decreasing the total drug clearance.

Targeting blood-brain barrier changes during inflammatory pain: an opportunity for optimizing CNS drug delivery

The blood-brain barrier (BBB) is the most significant obstacle to effective CNS drug delivery. It possesses structural and biochemical features (i.e., tight-junction protein complexes and, influx and efflux transporters) that restrict xenobiotic permeation. Pathophysiological stressors (i.e., peripheral inflammatory pain) can alter BBB tight junctions and transporters, which leads to drug-permeation changes. This is especially critical for opioids, which require precise CNS concentrations to be safe and effective analgesics. Recent studies have identified molecular targets (i.e., endogenous transporters and intracellular signaling systems) that can be exploited for optimization of CNS drug delivery. This article summarizes current knowledge in this area and emphasizes those targets that present the greatest opportunity for controlling drug permeation and/or drug transport across the BBB in an effort to achieve optimal CNS opioid delivery.

Expression of multidrug resistance proteins is localized principally to the Malpighian tubules in larvae of the cabbage looper moth, Trichoplusia ni

The multidrug resistance proteins (MRPs) serve a number of important roles in development, physiological homeostasis and metabolic resistance. In insects, they may also contribute to resistance against xenobiotics including insecticides and plant secondary metabolites. To investigate their contribution to xenobiotic resistance, we have examined the tissue distribution of gene and protein expression of the multidrug resistance proteins TrnMRP1 and TrnMRP4 of the lepidopteran insect, Trichoplusia ni. Using quantitative PCR and immunohistochemistry, we have identified high expression levels of both transporters in the Malpighian tubules relative to levels in other major tissues of the body, where they probably contribute to excretion of metabolic wastes or ingested xenobiotics. We have specifically located TrnMRP protein expression in a subpopulation of Malpighian tubule secondary cells. Expression of TrnMRP1 was also detected both at a high level in specific cortical neurons of larval ganglia and at a lower level throughout the cortex, where it may act in signaling or protective functions, respectively. In contrast, expression of TrnMRP4 was low to absent in larval ganglia, with the exception of single cells in the central connective. We discuss the potential implications of this TrnMRP activity on insect development and metabolic resistance.

Constitutive androstane receptor-mediated up-regulation of ATP-driven xenobiotic efflux transporters at the blood-brain barrier

ATP-driven efflux transporters at the blood-brain barrier both protect against neurotoxicants and limit drug delivery to the brain. In other barrier and excretory tissues, efflux transporter expression is regulated by certain ligand-activated nuclear receptors. Here we identified constitutive androstane receptor (CAR) as a positive regulator of P-glycoprotein, multidrug resistance-associated protein 2 (Mrp2), and breast cancer resistance protein (BCRP) expression in rat and mouse brain capillaries. Exposing rat brain capillaries to the CAR activator, phenobarbital (PB), increased the transport activity and protein expression (Western blots) of P-glycoprotein, Mrp2, and BCRP. Induction of transport was abolished by the protein phosphatase 2A inhibitor, OA. Similar effects on transporter activity and expression were found when mouse brain capillaries were exposed to the mouse-specific CAR ligand, 1,4-bis-[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP). In brain capillaries from CAR-null mice, TCPOBOP did not increase transporter activity. Finally, treating mice with 0.33 mg/kg TCPOBOP or rats with 80 mg/kg PB increased P-glycoprotein-, Mrp2-, and BCRP-mediated transport and protein expression in brain capillaries assayed ex vivo. Thus, CAR activation selectively tightens the blood-brain barrier by increasing transport activity and protein expression of three xenobiotic efflux pumps.

ABC transporters in human lymphocytes: expression, activity and role, modulating factors and consequences for antiretroviral therapies

IMPORTANCE OF THE FIELD

ATP-binding cassette (ABC) transporters are a superfamily of efflux pumps that transport numerous compounds across cell membranes. These transporters are located in various human tissues including peripheral blood cells, in particular lymphocytes, and present a high variability of expression and activity. This variability may affect the intracellular concentrations and efficacy of drugs acting within lymphocytes, such as antiretroviral drugs.

AREAS COVERED IN THIS REVIEW

This review focuses on the current knowledge about the expression, activity, roles and variability of ABC drug transporters in human lymphocytes. The identified modulating factors and their impact on the intracellular pharmacokinetics and efficacy of antiretroviral drugs are also detailed.

WHAT THE READER WILL GAIN

Controversial data regarding the expression, activity and sources of variability of ABC transporters in lymphocytes are discussed. The modulating factors and their pharmacological consequences regarding antiretroviral therapies are also provided.

TAKE HOME MESSAGE

Numerous studies have reported conflicting results regarding the expression and activity of ABC drug transporters in lymphocytes. Despite these discrepancies, which may partly result from heterogeneous analytical methods, ABCC1 appears to have the highest expression in lymphocytes and may thus play a predominant role in the resistance to antiretroviral drugs, particularly to protease inhibitors.

Modulating P-glycoprotein regulation: future perspectives for pharmacoresistant epilepsies?

Enhanced brain efflux of antiepileptic drugs by the blood-brain barrier transporter P-glycoprotein is discussed as one mechanism contributing to pharmacoresistance of epilepsies. P-glycoprotein overexpression has been proven to occur as a consequence of seizure activity. Therefore, blocking respective signaling events should help to improve brain penetration and efficacy of P-glycoprotein substrates. A series of recent studies revealed key signaling factors involved in seizure-associated transcriptional activation of P-glycoprotein. These data suggested several interesting targets, including the N-methyl-d-aspartate (NMDA) receptor, the inflammatory enzyme cyclooxygenase-2, and the prostaglandin E2 EP1 receptor. These targets have been further evaluated in rodent models, demonstrating that targeting these factors can control P-glycoprotein expression, improve antiepileptic drug brain penetration, and help to overcome pharmacoresistance. In general, the approach offers particular advantages over transporter inhibition as it preserves basal transporter function. In this review the different strategies for blocking P-glycoprotein upregulation, including their therapeutic promise and drawbacks are discussed. Moreover, pros and cons of the approach are compared to those of alternative strategies to overcome transporter-associated resistance. Regarding future perspectives of the novel approach, there is an obvious need to more clearly define the clinical relevance of transporter overexpression. In this context current efforts are discussed, including the development of imaging tools that allow an evaluation of P-glycoprotein function in individual patients.

Gefitinib-phenytoin interaction is not correlated with the C-erythromycin breath test in healthy male volunteers

AIMS

We aimed to describe the pharmacokinetic interaction between phenytoin, a potent CYP3A4 and P-glycoprotein (P-gp) (ABCB1) inducer, and gefitinib, a CYP3A4, CYP2D6 and P-gp substrate.

METHODS

An open-label, randomized, two-phase crossover study was conducted. Eighteen healthy male volunteers (nine homozygous CC and nine homozygous TT as determined by their ABCB1 C3435T polymorphism in exon 26) received a single oral dose of 250 mg gefitinib alone or after 5 days treatment with phenytoin (5 mg kg(-1) daily). Gefitinib plasma concentrations were determined by high-performance liquid chromatography. Hepatic CYP3A4 activity was evaluated by the (14)C-erythromycin breath test (ERMBT) and the ABCB1 and CYP2D6 genetic polymorphisms were determined by the TaqMan allelic discrimination assay and long polymerase chain reaction, respectively.

RESULTS

Following treatment with phenytoin, mean gefitinib C(max) and AUC(0-infinity) decreased by 26 +/- 44% [95% confidence interval (CI) for the difference 5-48%, P= 0.005] and 47 +/- 26% (95% CI for the difference 34-60%, P= 0.001), respectively, and apparent oral clearance increased by 126 +/- 93% (95% CI for the difference 80-172%, P= 0.004). Concomitantly, phenytoin increased the mean ERMBT by 91 +/- 44% (95% CI 75-105%, P < 0.001) from baseline, but the extent of liver CYP3A4 induction was not correlated to the extent of interaction. Furthermore, this interaction was independent of ABCB1 genetic polymorphism. The CYP2D6 genotype was slightly but significantly related to gefitinib clearance (P= 0.04) during the control phase.

CONCLUSIONS

The significant interaction between gefitinib and phenytoin was not correlated with the erythromycin breath test and was independent of ABCB1 polymorphism, but may involve presystemic CYP3A-mediated intestinal first-pass.

Effects of the P-glycoprotein inducer carbamazepine on fexofenadine pharmacokinetics

The aim of this study was to evaluate the possible effects of carbamazepine, a P-glycoprotein inducer, on fexofenadine pharmacokinetics. Twelve healthy Japanese volunteers (nine males and three females) were enrolled in this study after giving written informed consent. This randomized open-label study consisted of two phases (control and 7-day treatment) with a 2-week washout period. In the control phase, volunteers received 60 mg fexofenadine hydrochloride after an overnight fast. In the treatment phase, carbamazepine was dosed 100 mg three times daily (for a total daily dose of 300 mg) for 7 days, and on Day 7, a single 60-mg dose of fexofenadine was coadministered with a 100-mg dose of carbamazepine. The plasma concentrations and urinary excretion of fexofenadine were measured for 24 hours after dosing. Carbamazepine pretreatment significantly altered fexofenadine pharmacokinetics, decreasing the mean (+/- standard deviation) peak plasma concentration from 176.6 (+/- 82.1) ng/mL to 103.2 (+/- 33.6) ng/mL (P < 0.01) and the area under the plasma concentration-time curve from 1058.4 (+/- 528.7) ng/h/mL to 604.8 (+/- 255.9) ng/h/mL (P < 0.01) without changing the elimination half-life. Relatively, carbamazepine significantly reduced the amount of fexofenadine excreted into the urine from 8.1 (+/- 2.1) mg to 4.5 (+/- 1.4) mg (P < 0.001), although the renal clearance of fexofenadine remained constant between the two study phases. Thus, this study indicates that carbamazepine significantly decreases fexofenadine plasma concentrations, probably as a result of P-glycoprotein induction in the small intestine. Carbamazepine treatment, therefore, is of moderate clinical significance for patients receiving fexofenadine.

Targeting regulation of ABC efflux transporters in brain diseases: a novel therapeutic approach

Blood-brain barrier efflux transporters limit the brain penetration and efficacy of various central nervous system drugs. In several CNS diseases, therapy- or pathophysiology-associated transcriptional activation of efflux transporters further strengthens the barrier function. Targeting the regulatory pathways that drive efflux transporter expression in different diseases represents an intriguing approach for prevention of these events thereby promoting delivery to the brain and enhancing or restoring drug efficacy. In particular, the approach holds the promise to preserve basal transporter expression and activity, which is of specific relevance in view of the protective function of efflux transport. The elucidation of the signaling cascades involved in transporter regulation is a major presupposition for the development of preventive strategies. Orphan nuclear receptors as well as the Wnt/beta-catenin signaling pathway have been implicated in drug-induced changes in transporter expression. Targeting these xenobiotic sensors is therefore discussed as a means to optimize brain delivery and therapeutic outcome. Relevant progress has also been made with the identification of key signaling events that drive P-glycoprotein expression in response to pathophysiological mechanisms. In the epileptic brain, complex signaling events involving cyclooxygenase-2 activity trigger P-glycoprotein expression in response to glutamate release and activation of endothelial NMDA receptors. Moreover, reactive oxygen species and inflammatory cytokines have been identified as regulatory factors which might affect P-glycoprotein in several CNS diseases. Recent data substantiated several interesting targets in the respective signaling cascades thereby rendering a basis for the ongoing development of innovative approaches to optimize central nervous system drug brain penetration and efficacy.

Elucidating the 'Jekyll and Hyde' nature of PXR: the case for discovering antagonists or allosteric antagonists

The pregnane X receptor belongs to the nuclear hormone receptor superfamily and is involved in the transcriptional control of numerous genes. It was originally thought that it was a xenobiotic sensor controlling detoxification pathways. Recent studies have shown an increasingly important role in inflammation and cancer, supporting its function in abrogating tissue damage. PXR orthologs and PXR-like pathways have been identified in several non-mammalian species which corroborate a conserved role for PXR in cellular detoxification. In summary, PXR has a multiplicity of roles in vivo and is being revealed as behaving like a "Jekyll and Hyde" nuclear hormone receptor. The importance of this review is to elucidate the need for discovery of antagonists of PXR to further probe its biology and therapeutic applications. Although several PXR agonists are already reported, virtually nothing is known about PXR antagonists. Here, we propose the development of PXR antagonists through chemical, genetic and molecular modeling approaches. Based on this review it will be clear that antagonists of PXR and PXR-like pathways will have widespread utility in PXR biology and therapeutics.

Drug interactions at the blood-brain barrier: fact or fantasy?

There is considerable interest in the therapeutic and adverse outcomes of drug interactions at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). These include altered efficacy of drugs used in the treatment of CNS disorders, such as AIDS dementia and malignant tumors, and enhanced neurotoxicity of drugs that normally penetrate poorly into the brain. BBB- and BCSFB-mediated interactions are possible because these interfaces are not only passive anatomical barriers, but are also dynamic in that they express a variety of influx and efflux transporters and drug metabolizing enzymes. Based on studies in rodents, it has been widely postulated that efflux transporters play an important role at the human BBB in terms of drug delivery. Furthermore, it is assumed that chemical inhibition of transporters or their genetic ablation in rodents is predictive of the magnitude of interaction to be expected at the human BBB. However, studies in humans challenge this well-established paradigm and claim that such drug interactions will be lesser in magnitude but yet may be clinically significant. This review focuses on current known mechanisms of drug interactions at the blood-brain and blood-CSF barriers and the potential impact of such interactions in humans. We also explore whether such drug interactions can be predicted from preclinical studies. Defining the mechanisms and the impact of drug-drug interactions at the BBB is important for improving efficacy of drugs used in the treatment of CNS disorders while minimizing their toxicity as well as minimizing neurotoxicity of non-CNS drugs.

Up-regulation of P-glycoprotein by HIV protease inhibitors in a human brain microvessel endothelial cell line

A major concern regarding the chronic administration of antiretroviral drugs is the potential for induction of drug efflux transporter expression (i.e., P-glycoprotein, P-gp) at tissue sites that can significantly affect drug distribution and treatment efficacy. Previous data have shown that the inductive effect of human immunodeficiency virus protease inhibitors (PIs) is mediated through the human orphan nuclear receptor, steroid xenobiotic receptor (SXR or hPXR). The objectives of this study were to investigate transport and inductive properties on efflux drug transporters of two PIs, atazanavir and ritonavir, at the blood-brain barrier by using a human brain microvessel endothelial cell line, hCMEC/D3. Transport properties of PIs by the drug efflux transporters P-gp and multidrug resistance protein 1 (MRP1) were assessed by measuring the cellular uptake of (3)H-atazanavir or (3)H-ritonavir in P-gp and MRP1 overexpressing cells as well as hCMEC/D3. Whereas the P-gp inhibitor, PSC833, increased atazanavir and ritonavir accumulation in hCMEC/D3 cells by 2-fold, the MRP inhibitor MK571 had no effect. P-gp, MRP1, and hPXR expression and localization were examined by Western blot analysis and immunogold cytochemistry at the electron microscope level. Treatment of hCMEC/D3 cells for 72 hr with rifampin or SR12813 (two well-established hPXR ligands) or PIs (atazanavir or ritonavir) resulted in an increase in P-gp expression by 1.8-, 6-, and 2-fold, respectively, with no effect observed for MRP1 expression. In hCMEC/D3 cells, cellular accumulation of these PIs appears to be primarily limited by P-gp efflux activity. Long-term exposure of atazanavir or ritonavir to brain microvessel endothelium may result in further limitations in brain drug permeability as a result of the up-regulation of P-gp expression and function.