Interaction of drugs with P-glycoprotein in brain capillaries

Tailoring of P-glycoprotein for effective transportation of actives across blood-brain-barrier

P-Glycoprotein serves as a gatekeeper in the blood-brain-barrier wherein, it shows a vital part in the elimination of xenobiotics, drugs, foreign molecules etc. and guards the central nervous system from infections and external toxic molecules by functioning as an efflux transporter. It plays an essential role in various brain-related conditions like Parkinsonism, Alzheimer's disease, depression, cancer, etc. and terminates the entry of therapeutic agents across blood-brain-barrier which remains a significant challenge serving as major hindrance in pharmacotherapy of disease. The physiological structure and topology of P-glycoprotein and its relation with blood-brain-barrier and central nervous system gives an idea for targeting nanocarriers across the barrier into brain. This review article provides an overview of current understanding of the nanoformulations-based P-gp trafficking strategies like nanocarriers, stem cell therapy, drugs, substrates, polymeric materials, chemical compounds as well as naturally occurring active constituents for improving drug transport in brain across blood-brain-barrier and contributing in effective nanotherapeutic development for treatment of CNS disorders.

Molecular Physiology and Pathophysiology of Bilirubin Handling by the Blood, Liver, Intestine, and Brain in the Newborn

Bilirubin is the end product of heme catabolism formed during a process that involves oxidation-reduction reactions and conserves iron body stores. Unconjugated hyperbilirubinemia is common in newborn infants, but rare later in life. The basic physiology of bilirubin metabolism, such as production, transport, and excretion, has been well described. However, in the neonate, numerous variables related to nutrition, ethnicity, and genetic variants at several metabolic steps may be superimposed on the normal physiological hyperbilirubinemia that occurs in the first week of life and results in bilirubin levels that may be toxic to the brain. Bilirubin exists in several isomeric forms that differ in their polarities and is considered a physiologically important antioxidant. Here we review the chemistry of the bilirubin molecule and its metabolism in the body with a particular focus on the processes that impact the newborn infant, and how differences relative to older children and adults contribute to the risk of developing both acute and long-term neurological sequelae in the newborn infant. The final section deals with the interplay between the brain and bilirubin and its entry, clearance, and accumulation. We conclude with a discussion of the current state of knowledge regarding the mechanism(s) of bilirubin neurotoxicity.

Modulation of bilirubin neurotoxicity by the Abcb1 transporter in the Ugt1-/- lethal mouse model of neonatal hyperbilirubinemia

Moderate neonatal jaundice is the most common clinical condition during newborn life. However, a combination of factors may result in acute hyperbilirubinemia, placing infants at risk of developing bilirubin encephalopathy and death by kernicterus. While most risk factors are known, the mechanisms acting to reduce susceptibility to bilirubin neurotoxicity remain unclear. The presence of modifier genes modulating the risk of developing bilirubin-induced brain damage is increasingly being recognised. The Abcb1 and Abcc1 members of the ABC family of transporters have been suggested to have an active role in exporting unconjugated bilirubin from the central nervous system into plasma. However, their role in reducing the risk of developing neurological damage and death during neonatal development is still unknown.To this end, we mated Abcb1a/b-/- and Abcc1-/- strains with Ugt1-/- mice, which develop severe neonatal hyperbilirubinemia. While about 60% of Ugt1-/- mice survived after temporary phototherapy, all Abcb1a/b-/-/Ugt1-/- mice died before postnatal day 21, showing higher cerebellar levels of unconjugated bilirubin. Interestingly, Abcc1 role appeared to be less important.In the cerebellum of Ugt1-/- mice, hyperbilirubinemia induced the expression of Car and Pxr nuclear receptors, known regulators of genes involved in the genotoxic response.We demonstrated a critical role of Abcb1 in protecting the cerebellum from bilirubin toxicity during neonatal development, the most clinically relevant phase for human babies, providing further understanding of the mechanisms regulating bilirubin neurotoxicity in vivo. Pharmacological treatments aimed to increase Abcb1 and Abcc1 expression, could represent a therapeutic option to reduce the risk of bilirubin neurotoxicity.

The ontogeny of P-glycoprotein in the developing human blood-brain barrier: implication for opioid toxicity in neonates

BACKGROUND

Neonates have been shown to have a heightened sensitivity to the central depressive effects of opioids compared to older infants and adults. The limited development of P-glycoprotein (P-gp) may limit the ability of the neonate to efflux morphine from the brain back to the systemic circulation. The objective of the study was to determine the ontogeny of P-gp in the human brain.

METHODS

Postmortem cortex samples from gestational age (GA) 20-26 wk, GA 36-40 wk, postnatal age (PNA) 0-3 mo, PNA 3-6 mo, and adults were immunostained for P-gp.

RESULTS

The intensity of P-gp staining in adults was significantly higher compared to at GA 20-26 wk (P < 0.05), GA 36-40 wk (P < 0.05), and PNA 0-3 mo (P < 0.05). P-gp intensity at GA 20-26 wk (P < 0.05), GA 36-40 wk (P < 0.05), and PNA 0-3 mo (P < 0.05) was significantly lower compared to at PNA 3-6 mo.

CONCLUSION

P-gp expression in the brain is limited at birth, increases with postnatal maturation, and reaches adult levels at ~3-6 mo of age. Given the immaturity of blood-brain barrier (BBB) P-gp after birth, morphine may concentrate in the brain. This provides mechanistic support to life threatening opioid toxicity seen with maternal codeine use during breastfeeding.

Blockade of endothelin B receptor improves the efficacy of levetiracetam in chronic epileptic rats

PURPOSE

To elucidate the mechanisms that regulate p-glycoprotein (PGP) expression and function in pharmacoresistant epilepsy, we investigated the effect of an ETB receptor antagonist (BQ788) and a p38 mitogen-activated protein kinase (p38MAPK) inhibitor (SB202190) on intractable seizures in chronic epileptic rats.

METHODS

Lithium-pilocarpine-induced chronic epileptic rats were used in the present study. Animals were given levetiracetam (LEV), LEV + SB202190, LEV + BQ788, SB202190 or BQ788 over a 3-day period using an osmotic pump. Seizure activity was recorded by video-EEG monitoring with 2h of recording per day at the same time of day. We also performed western blot after EEG analysis.

RESULTS

Compared to control animals, PGP, ETB receptor and p38MAPK expression was increased in the hippocampus of epileptic animals. Neither SB202190 nor BQ788 affected the spontaneous seizure activity in epileptic rats. Three of ten rats were responders and achieved complete seizure control or significant reduction in seizure activity by LEV. In four of ten rats, seizure frequency was unaltered by LEV (non-responders). LEV + SB202190 reduced seizure duration, but not seizure frequency, in both responders and non-responders. LEV + BQ788 alleviated seizure frequency and seizure duration in both responders and non-responders. Compared to responders, PGP and ETB receptor expression was enhanced in the hippocampus of non-responders.

CONCLUSION

To the best of our knowledge, these findings are the first indications of the role of ETB receptor in pharmacoresistant epilepsy. Therefore, the present data suggest that the regulation of the ETB receptor-mediated signaling pathway may be important for identification of new therapeutic strategies for improving antiepileptic drug efficacy.

Cytotoxicity of the indole alkaloid reserpine from Rauwolfia serpentina against drug-resistant tumor cells

BACKGROUND

The antihypertensive reserpine is an indole alkaloid from Rauwolfia serpentina and exerts also profound activity against cancer cells in vitro and in vivo. The present investigation was undertaken to investigate possible modes of action to explain its activity toward drug-resistant tumor cells.

MATERIAL AND METHODS

Sensitive and drug-resistant tumor cell lines overexpressing P-glycoprotein (ABCB1/MDR1), breast cancer resistance protein (ABCG2/BCRP), mutation-activated epidermal growth factor receptor (EGFR), wild-type and p53-knockout cells as well as the NCI panel of cell lines from different tumor origin were analyzed. Reserpine's cytotoxicity was investigated by resazurin and sulforhodamine assays, flow cytometry, and COMPARE and hierarchical cluster analyses of transcriptome-wide microarray-based RNA expressions.

RESULTS

P-glycoprotein- or BCRP overexpressing tumor cells did not reveal cross-resistance to reserpine. EGFR-overexpressing cells were collateral sensitive and p53- Knockout cells cross-resistant to this drug compared to their wild-type parental cell lines. Reserpine increased the uptake of doxorubicin in P-glycoprotein-overexpressing cells, indicating that reserpine inhibited the efflux function of P-glycoprotein. Using molecular docking, we found that reserpine bound with even higher binding energy to P-glycoprotein and EGFR than the control drugs verapamil (P-glycoprotein inhibitor) and erlotinib (EGFR inhibitor). COMPARE and cluster analyses of microarray data showed that the mRNA expression of a panel of genes predicted the sensitivity or resistance of the NCI tumor cell line panel with statistical significance. The genes belonged to diverse pathways and biological functions, e.g. cell survival and apoptosis, EGFR activation, regulation of angiogenesis, cell mobility, cell adhesion, immunological functions, mTOR signaling, and Wnt signaling.

CONCLUSION

The lack of cross-resistance to most resistance mechanisms and the collateral sensitivity in EGFR-transfectants compared to wild-type cells speak for a promising role of reserpine in cancer chemotherapy. Reserpine deserves further consideration for cancer therapy in the clinical setting.

Preclinical impact of bevacizumab on brain and tumor distribution of irinotecan and temozolomide

Glioblastoma (GBM) is the most common primary malignant brain tumour in adults. Prognosis of GBM patients is poor with median overall survival around 15 months. Temozolomide is the chemotherapeutic agent used in the standard of care of newly diagnosed GBM patients relying on radiotherapy with concurrent chemotherapy followed by chemotherapy alone. Irinotecan has shown some efficacy in recurrent malignant gliomas. Bevacizumab has been combined with irinotecan in the treatment of recurrent GBM and with temozolomide in newly diagnosed GBM. As the efficacy of GBM treatments relies on their brain distribution through the blood brain barrier, the aim of the present preclinical work was to study, in in vivo models, the impact of bevacizumab on brain and tumor distribution of temozolomide and irinotecan. Our results show that bevacizumab pre-treatment was associated with a reduced temozolomide brain distribution in tumor-free mice. In tumor bearing mice, bevacizumab increased temozolomide tumor distribution, although not statistically significant. In both tumor-free and tumor-bearing mice, bevacizumab does not modify brain distribution of irinotecan and its metabolite SN-38. Bevacizumab impacts brain distribution of some anti-tumor drugs and potentially their efficacy in GBM. Further studies are warranted to investigate other therapeutic combination.

Irinotecan and temozolomide brain distribution: a focus on ABCB1

Glioblastoma (GBM), the most common primary brain tumor in adults, is usually rapidly fatal with median survival duration of only 15 months and a 3-year survival rate of <7 %. Temozolomide (TMZ) is the only anticancer drug that has improved survival in GBM when administered with concomitant radiotherapy. Irinotecan (CPT-11) has also shown efficacy in recurrent gliomas monotherapy with moderate response. As the efficacy of GBM treatments relies on their brain distribution through the blood-brain barrier (BBB), the aim of the present work was to study, on an in vivo model, the brain distribution of TMZ, CPT-11 and its active metabolite, SN-38. We have focussed on the role of ABCB1, the main efflux transporter at the BBB level, through pharmacokinetics studies in CF1 mdr1a(+/+) and mdr1a(-/-) mice. Our results show that TMZ, CPT-11 and SN-38 are transported by ABCB1 at the BBB level with brain/plasma ratios of 1.1, 2.1 and 2.3, respectively.

The role of canalicular ABC transporters in cholestasis

Cholestasis, a hallmark feature of hepatobiliary disease, is characterized by the retention of biliary constituents. Some of these constituents, such as bile acids, inflict damage to hepatocytes and bile duct cells. This damage may lead to inflammation, fibrosis, cirrhosis, and eventually carcinogenesis, sequelae that aggravate the underlying disease and deteriorate clinical outcome. Canalicular ATP-binding cassette (ABC) transporters, which mediate the excretion of individual bile constituents, play a key role in bile formation and cholestasis. The study of these transporters and their regulatory nuclear receptors has revolutionized our understanding of cholestatic disease. This knowledge has served as a template to develop novel treatment strategies, some of which are currently already undergoing phase III clinical trials. In this review we aim to provide an overview of the structure, function, and regulation of canalicular ABC transporters. In addition, we will focus on the role of these transporters in the pathogenesis and treatment of cholestatic bile duct and liver diseases.

Marine natural products with P-glycoprotein inhibitor properties

P-glycoprotein (P-gp) is a protein belonging to the ATP-binding cassette (ABC) transporters superfamily that has clinical relevance due to its role in drug metabolism and multi-drug resistance (MDR) in several human pathogens and diseases. P-gp is a major cause of drug resistance in cancer, parasitic diseases, epilepsy and other disorders. This review article aims to summarize the research findings on the marine natural products with P-glycoprotein inhibitor properties. Natural compounds that modulate P-gp offer great possibilities for semi-synthetic modification to create new drugs and are valuable research tools to understand the function of complex ABC transporters.

[11C]quinidine and [11C]laniquidar PET imaging in a chronic rodent epilepsy model: impact of epilepsy and drug-responsiveness

INTRODUCTION

To analyse the impact of both epilepsy and pharmacological modulation of P-glycoprotein on brain uptake and kinetics of positron emission tomography (PET) radiotracers [(11)C]quinidine and [(11)C]laniquidar.

METHODS

Metabolism and brain kinetics of both [(11)C]quinidine and [(11)C]laniquidar were assessed in naive rats, electrode-implanted control rats, and rats with spontaneous recurrent seizures. The latter group was further classified according to their response to the antiepileptic drug phenobarbital into "responders" and "non-responders". Additional experiments were performed following pre-treatment with the P-glycoprotein modulator tariquidar.

RESULTS

[(11)C]quinidine was metabolized rapidly, whereas [(11)C]laniquidar was more stable. Brain concentrations of both radiotracers remained at relatively low levels at baseline conditions. Tariquidar pre-treatment resulted in significant increases of [(11)C]quinidine and [(11)C]laniquidar brain concentrations. In the epileptic subgroup "non-responders", brain uptake of [(11)C]quinidine in selected brain regions reached higher levels than in electrode-implanted control rats. However, the relative response to tariquidar did not differ between groups with full blockade of P-glycoprotein by 15 mg/kg of tariquidar. For [(11)C]laniquidar differences between epileptic and control animals were only evident at baseline conditions but not after tariquidar pretreatment.

CONCLUSIONS

We confirmed that both [(11)C]quinidine and [(11)C]laniquidar are P-glycoprotein substrates. At full P-gp blockade, tariquidar pre-treatment only demonstrated slight differences for [(11)C]quinidine between drug-resistant and drug-sensitive animals.

[Dysfunction of the blood-brain barrier during ischaemia: a therapeutic concern]

Since it was discovered and its brain-protective role characterized, the blood-brain barrier (BBB), through the permeability-restricting action of the brain capillary endothelial cells, has been representing a hurdle for 95% of new medical compounds targeting the central nervous system. Recently, a BBB dysfunction is being found in an increasing number of pathologies such as brain ischaemic stroke, whose only therapy consists in a pharmacological thrombolysis limited to a small percentage of the admitted patients, because of the toxical effects of thrombolytics. And since the clinical failure of promising neuroprotectants, numerous studies of brain ischaemia were carried out, with physiopathological or pharmacological approaches refocused on the BBB, whose structural complexity is now expanded to perivascular cells, all forming a functional unit named the neurovascular unit (NVU). Nevertheless, in spite of the numerous molecular mechanisms identified, the process of BBB dysfunction in the ischaemia/reperfusion cascade remains insufficiently established to explain the pleiotropic action exerted by new pharmacological compounds, possibly protecting the entire NVU and representing potential treatments.

Modulation of Mrp1 (ABCc1) and Pgp (ABCb1) by bilirubin at the blood-CSF and blood-brain barriers in the Gunn rat

Accumulation of unconjugated bilirubin (UCB) in the brain causes bilirubin encephalopathy. Pgp (ABCb1) and Mrp1 (ABCc1), highly expressed in the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) respectively, may modulate the accumulation of UCB in brain. We examined the effect of prolonged exposure to elevated concentrations of UCB on expression of the two transporters in homozygous, jaundiced (jj) Gunn rats compared to heterozygous, not jaundiced (Jj) littermates at different developmental stages (2, 9, 17 and 60 days after birth). BBB Pgp protein expression was low in both jj and Jj pups at 9 days (about 16-27% of adult values), despite the up-regulation in jj animals (2 and 1.3 fold higher than age matched Jj animals at P9 and P17-P60, respectively); Mrp1 protein expression was barely detectable. Conversely, at the BCSFB Mrp1 protein expression was rather high (60-70% of the adult values) in both jj and Jj at P2, but was markedly (50%) down-regulated in jj pups starting at P9, particularly in the 4(th) ventricle choroid plexuses: Pgp was almost undetectable. The Mrp1 protein down regulation was accompanied by a modest up-regulation of mRNA, suggesting a translational rather than a transcriptional inhibition. In vitro exposure of choroid plexus epithelial cells obtained from normal rats to UCB, also resulted in a down-regulation of Mrp1 protein. These data suggest that down-regulation of Mrp1 protein at the BSCFB, resulting from a direct effect of UCB on epithelial cells, may impact the Mrp1-mediated neuroprotective functions of the blood-cerebrospinal fluid barrier and actually potentiate UCB neurotoxicity.

Global changes in gene regulation demonstrate that unconjugated bilirubin is able to upregulate and activate select components of the endoplasmic reticulum stress response pathway

Elevated concentrations of unconjugated bilirubin (UCB) are responsible for neonatal jaundice and can eventually lead to kernicterus or death. The molecular mechanism of UCB toxicity is incompletely elucidated. The purpose of this study was to analyze changes in gene regulation mediated by UCB to determine novel pathways that contribute to UCB-mediated toxicity. We employed microarray analysis to determine changes in gene regulation mediated by UCB at both pro- (50 microM) and antioxidant (70 nM) concentrations in Hepa 1c1c7 cells at 1 and 6 h. The changes observed in select genes were validated with qPCR. Using immunoblot analysis, we validated these changes at the protein level for select genes and documented the activation of two proteins involved in the endoplasmic reticulum (ER) stress pathway, eIF2 alpha and PERK. Following treatment with 50 microM UCB, microarray analysis revealed the upregulation of many genes involved in ER stress (ATF3, BiP, CHOP, Dnajb1, and Herp). We demonstrate that upregulation of the proapoptotic transcription factor CHOP results in increased intracellular protein content. It was determined that activation of proteins involved in ER stress was an early event in UCB toxicity as eIF2 alpha and PERK were both phosphorylated and activated by 1 h posttreatment. We also demonstrate that procaspase-12 content, a proposed initiator caspase in ER stress-mediated apoptosis, is decreased by 4 h posttreatment. In conclusion, this study demonstrates that elevated concentrations of UCB (50 microM) are able to activate select components of the ER stress pathway in Hepa 1c1c7 cells, which may contribute to UCB-mediated apoptosis.

The cytotoxic effect of unconjugated bilirubin in human neuroblastoma SH-SY5Y cells is modulated by the expression level of MRP1 but not MDR1

In vitro and in vivo studies have demonstrated that UCB (unconjugated bilirubin) is neurotoxic. Although previous studies suggested that both MRP1 (multidrug resistance-associated protein 1) and MDR1 (multidrug resistance protein 1) may protect cells against accumulation of UCB, direct comparison of their role in UCB transport was never performed. To this end, we used an inducible siRNA (small interfering RNA) expression system to silence the expression of MRP1 and MDR1 in human neuroblastoma SH-SY5Y cells. The effects of in vitro exposure to clinically-relevant levels of unbound UCB were compared between unsilenced (control) cells and cells with similar reductions in the expression of MRP1 or MDR1, documented by RT–PCR (reverse transcription–PCR) (mRNA), immunoblotting (protein), and for MDR1, the enhanced net uptake of a specific fluorescent substrate. Cytotoxicity was assessed by the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide] test. MRP1-deficient cells accumulated significantly more UCB and suffered greater cytotoxicity than controls. By contrast, MDR1-deficient cells exhibited UCB uptake and cytotoxicity comparable with controls. At intermediate levels of silencing, the increased susceptibility to UCB toxicity closely correlated with the decrease in the expression of MRP1, but not of MDR1. These data support the concept that limitation of cellular UCB accumulation, due to UCB export mediated by MRP1, but not MDR1, plays an important role in preventing bilirubin encephalopathy in the newborn.

Positron emission tomography studies on binding of central nervous system drugs and P-glycoprotein function in the rodent brain

The permeability of the blood-brain barrier (BBB) is one of the factors determining the bioavailability of drugs in the brain. The BBB only allows passage of lipophilic drugs by passive diffusion. However, some lipophilic drugs hardly enter the brain. The transmembrane protein P-glycoprotein (P-gp) is one of the carrier systems that is responsible for transportation of drugs out of the brain. P-Glycoprotein affects the pharmacokinetics of many drugs and can be inhibited by administration of modulators or competitive substrates. Identification and classification of central nervous system (CNS) drugs as P-gp substrates or inhibitors are of crucial importance in drug development. Positron emission tomography (PET) studies can play an important role in the screening process as a follow-up of high-throughput in vitro assays. Several rodent studies have shown the potential value of PET to measure the effect of P-gp on the pharmacokinetics and brain uptake of radiolabeled compounds. P-Glycoprotein-mediated effects were observed for two 5-HT(1a) receptor ligands, [(18)F]MPPF vs. [carbonyl-(11)C]WAY100635. Under control conditions, the specific brain uptake of [(18)F]MPPF is five- to eightfold lower than that of [(11)C]WAY100635. After cyclosporin A (CsA) modulation, [(18)F]MPPF uptake in the rat brain increased five- to tenfold. Cerebral uptake of [carbonyl-(11)C]WAY100635 was also increased by modulation, but in general the increase was lower than that observed for [(18)F]MPPF (two- to threefold). Brain uptake of the beta-adrenergic receptor ligands [(11)C]carazolol and [(18)F]fluorocarazolol was increased in P-gp knockout mice and CsA-treated rats. Both the specific and nonspecific binding of [(18)F]fluorocarazolol were doubled by CsA. Cerebral uptake of [(11)C]carazolol in rats was much lower than that of [(18)F]fluorocarazolol and no specific binding was measured. After CsA modulation, the uptake of [(11)C]carazolol increased five- to sixfold, but this uptake was not receptor-mediated. Quantitative PET studies in rodents on P-gp functionality demonstrated a dose-dependent increase of radioligands after administration of CsA. Studies with [(11)C]verapamil and [(11)C]carvedilol showed that complete modulation was achieved at 50 mg/kg CsA. The distribution volume of [(11)C]carvedilol increased from 0.25 in the control study to 1.0 after full modulation with CsA. By quantitative PET measurement of P-gp function, the dose of modulators required to increase the concentration of CNS drugs may be determined, which may result in improved drug therapy.

Fexofenadine hydrochloride in the treatment of allergic disease: a review

Fexofenadine is a selective, non-sedating H1 receptor antagonist, marketed in the United States since 2000. The FDA approved an oral suspension in 2006, for the treatment of seasonal allergic rhinitis and chronic idiopathic urticaria in children. The tablet, capsule, and oral suspension are bioequivalent. Although fexofenadine does not use P450 CYP 3A4 it does interact with a number of drugs at P-glycoprotein and organic anion transporter polypeptides. The risk of toxicity from other drugs may increase with the administration of fexofenadine. Orange and grapefruit juices reduce the bioavailability of fexofenadine. Fexofenadine has been shown to have an impact on inflammatory mediators, other than histamine, such as decreasing the production of LTC₄, LTD₄, LTE₄, PGE₂, and PGF_2α; inhibiting cyclo-oxygenase 2, thromboxane; limiting iNOS generation of NO; decreasing cytokine levels (ICAM-1, ELAM-1, VCAM-1, RANTES, I-TAC, MDC, TARC, MMP-2, MMP-9, tryptase); and diminishing eosinophil adherence, chemotaxis, and opsonization of particles. These effects may provide benefit to some of the inflammatory responses of an acute allergic reaction and provide a basis for future development of H1 antagonists with stronger anti-inflammatory effects. These studies also support the contention that fexofenadine is effective for the treatment of allergic rhinits and chronic idiopathic urticaria.

Increased uptake of quinine into the brain by inhibition of P-glycoprotein

The impact of P-glycoprotein (P-gp) on the distribution of quinine between brain and plasma was studied experimentally in mice. Administration of quinine (20mg/kg, i.v.) to mdrla knockout mice resulted in enhanced brain concentrations of quinine as compared to the wild-type mice (7.9+/-1.4 microg/g versus 1.6+/-0.8 microg/g, respectively). Quinine concentrations and quinine-to-3-hydroxyquinine ratio in plasma were similar in normal and P-gp-deficient mice. The effect of intravenously administered drugs before quinine (20mg/kg, i.v.) was evaluated on brain uptake and biotransformation of quinine in mice. Cyclosporine A (50 mg/kg), erythromycine (40 mg/kg), verapamil (5mg/kg) or mefloquine (20 mg/kg) increased the brain-to-plasma quinine concentration ratio (by factors of 3.8-, 1.8-, 1.9- and 2.5-fold, respectively) and the quinine-to-3-hydroxyquinine ratio in plasma (by factors 2.1-, 3.7-, 1.8- and 2.0-fold, respectively). After cinchonine (40 mg/kg) and halofantrine (40 mg/kg) pre-treatment, the brain-to-plasma ratio for quinine increased by factor of 2.3 and 1.8, respectively without changes of quinine or metabolite concentrations in plasma. Doxycycline (20 mg/kg), artesunate (50 mg/kg) or artemether (50 mg/kg) did not alter quinine disposition. These results confirm in vivo that quinine is a substrate for mdr1a P-gp. Drug associations led not only to metabolic interactions but also increased quinine uptake by tissues protected by P-gp. Such interactions may have implications for the improvement of chemotherapy but should be also taken into account for potential enhancement of adverse effects.

A functional in vitro model of rat blood-brain barrier for molecular analysis of efflux transporters

Physiological studies of the blood-brain barrier (BBB) are often performed in rats. We describe the functional characterization of a reproducible in vitro model of the rat BBB and its validation for investigating mechanisms involved in BBB regulation. Puromycin-purified primary cultures of brain endothelial cells, co-cultured with astrocytes in the presence of hydrocortisone (HC) and cAMP, presented low sucrose permeability (< or =0.1 x 10(-3) cm/min) and high transendothelial electrical resistance (> or =270 Omega cm(2)). Expression of specific BBB markers and their transcripts was detected by immunostaining and RT-PCR, respectively: tight junction proteins (claudin-3 and -5, ZO-1 and occludin) and transporters (P-gp, Bcrp and Oatp-2). RT-PCR experiments demonstrated a role of treatment by astrocytes, HC and cAMP in regulation of the transcript level of tight junction proteins (claudin-5 and ZO-1) as well as transporters (Mdr1a, Mrp3, Mrp4, Bcrp, Glut-1), while transcript level of Mdr1b was significantly decreased. The functionality of efflux pumps (P-gp, Mrps and Bcrp) was demonstrated in the presence of specific inhibitors (PSC833, MK571 or Ko143, respectively) by (i) assessing the uptake of the common substrates rhodamine 123 and daunorubicin and (ii) evaluating apical to basolateral and basolateral to apical polarized transport of daunorubicin. In addition, a good correlation (R=0.94) was obtained between the permeability coefficients of a series of compounds of various lipophilicity and their corresponding in vivo rodent blood-brain transfer coefficients. Taken together, our results provide compelling evidence that puromycin-purified rat brain endothelial cells constitute a reliable model of the rat BBB for physiological and pharmacological characterization of BBB transporters.

CONTRARY TO ADULT, NEONATAL RATS SHOW PRONOUNCED BRAIN UPTAKE OF CORTICOSTEROIDS

Neurotoxic adverse effects after systemic corticosteroid administration are elevated in preterm infants. To test whether this might be related to an immature blood-brain barrier (BBB) that permits corticosteroids to enter the brain and induce neurotoxic effects, this study assessed the differences in brain permeability of triamcinolone acetonide after intratracheal administration to neonatal (10- to 11-day-old) and adult rats. Triamcinolone acetonide (or the phosphate prodrug in the case of neonatal rats) was administered intratracheally to neonatal rats at doses of 2.5, 25, or 50 microg/kg and to adult rats at 100 microg/kg. An ex vivo receptor binding assay was used to monitor the cumulative brain and liver glucocorticoid receptor occupancies over 6 h. Brain and liver receptor occupancies in neonates were similar for the 25 and 50 microg/kg triamcinolone acetonide phosphate (brain/liver receptor occupancy ratio, 1.10 +/- 0.14 and 0.87 +/- 0.13, respectively), whereas some reduction in the brain permeability was seen at the lower dose. After intratracheal administration of 100 microg/kg triamcinolone acetonide to adult rats, receptor occupancies in the brain were significantly lower (brain/liver ratio, 0.21 +/- 0.14; p < 0.001). The study demonstrated that glucocorticoids enter the brain of neonatal rats because of an immature BBB. The results of this study support the hypothesis that neurotoxic adverse effects in preterm infants after systemic corticosteroid administration might be related to an immature BBB.

Predictive model of blood-brain barrier penetration of organic compounds

AIM

To build up a theoretical model of organic compounds for the prediction of the activity of small molecules through the blood-brain barrier (BBB) in drug design.

METHODS

A training set of 37 structurally diverse compounds was used to construct quantitative structure-activity relationship (QSAR) models. Intermolecular and intramolecular solute descriptors were calculated using molecular mechanics, molecular dynamics simulations, quantum chemistry and so on. The QSAR models were optimized using multidimensional linear regression fitting and stepwise method. A test set of 8 compounds was evaluated using the models as part of a validation process.

RESULTS

Significant QSAR models (R=0.955, s=0.232) of the BBB penetration of organic compounds were constructed. BBB penetration was found to depend upon the polar surface area, the octanol/water partition coefficient, Balaban Index, the strength of a small molecule to combine with the membrane-water complex, and the changeability of the structure of a solute-membrane-water complex.

CONCLUSION

The QSAR models indicate that the distribution of organic molecules through BBB is not only influenced by organic solutes themselves, but also relates to the properties of the solute-membrane-water complex, that is, interactions of the molecule with the phospholipid-rich regions of cellular membranes.

Does inhibition of P-glycoprotein lead to drug–drug interactions?

Permeability-glycoprotein (Pgp) is a drug transporter responsible for the efflux of xenobiotics out of cells that influence the pharmacokinetics of numerous drugs. However, the role of this transporter in drug-drug interactions is still poorly studied even though a lot of P-glycoprotein substrates and P-glycoprotein inhibitors are identified among drugs of standard usage. On one hand, Pgp is distributed within a lot of organs and tissues implicated in the absorption or excretion of xenobiotics, and drug-drug interactions with this protein may increase the bioavailability of simultaneously administered active drugs. On the other hand, Pgp is linked to the integrity of blood-tissue barriers, such as the blood-brain barrier or placenta, and a partial blockage of Pgp could be responsible for a new drug distribution in the organism with possible increase of drug rates in organs behind these barriers. Therefore, concomitant administration of substrates and Pgp inhibitors would modify drug pharmacokinetics by increasing bioavailability and organ uptake, leading to more adverse drug reactions and toxicities. Consequently, the identification and comprehension of these drug-drug interactions remain important keys to risk assessment.

Molecular basis of bilirubin-induced neurotoxicity

Unconjugated bilirubin (UCB), at slightly elevated unbound concentrations, is toxic to astrocytes and neurons, damaging mitochondria (causing impaired energy metabolism and apoptosis) and plasma membranes (causing oxidative damage and disrupting transport of neurotransmitters). Accumulation of UCB in the CSF and CNS is limited by its active export, probably mediated by MRP1/Mrp1 present in choroid plexus epithelia, capillary endothelia, astrocytes and neurons. Upregulation of MRP1/Mrp1 protein levels by UCB might represent an important adaptive mechanism that protects the CNS from UCB toxicity. These concepts could explain the varied susceptibility of newborns to bilirubin neurotoxicity and the occurrence of neurological damage at plasma UCB concentrations well below therapeutic guidelines, and are relevant to the increasing prevalence of bilirubin encephalopathy in newborns.

P-glycoprotein in blood-brain barrier endothelial cells: interaction and oligomerization with caveolins

P-glycoprotein (P-gp), an adenosine triphosphate (ATP)-binding cassette transporter which acts as a drug efflux pump, is highly expressed at the blood-brain barrier (BBB) where it plays an important role in brain protection. Recently, P-gp has been reported to be located in the caveolae of multidrug-resistant cells. In this study, we investigated the localization and the activity of P-gp in the caveolae of endothelial cells of the BBB. We used an in vitro model of the BBB which is formed by co-culture of bovine brain capillary endothelial cells (BBCEC) with astrocytes. Caveolar microdomains isolated from BBCEC are enriched in P-gp, cholesterol, caveolin-1, and caveolin-2. Moreover, P-gp interacts with caveolin-1 and caveolin-2; together, they form a high molecular mass complex. P-gp in isolated caveolae is able to bind its substrates, and the caveolae-disrupting agents filipin III and nystatin decrease P-gp transport activity. In addition, mutations in the caveolin-binding motif present in P-gp reduced the interaction of P-gp with caveolin-1 and increased the transport activity of P-gp. Thus, P-gp expressed at the BBB is mainly localized in caveolae and its activity may be modulated by interaction with caveolin-1.

Puromycin selectively increases mdr1a expression in immortalized rat brain endothelial cell lines

The blood-brain barrier (BBB) plays an important role in controlling the passage of molecules from blood to brain extracellular fluid. The multidrug efflux pump P-glycoprotein (P-gp) is highly expressed in the luminal membrane of brain endothelium and contributes to the formation of a functional barrier to lipid-soluble drugs such as anticancer agents. The mdr1a P-gp-encoding gene is exclusively expressed in the rodent BBB. Primary cultures of rat brain endothelial cells and GP8.3 cells showed a dramatic decrease in mdr1a mRNA level and some expression of mdr1b mRNA. GPNT cells, derived from GP8.3 cells after transfection with a puromycin resistance gene, were chronically treated with 5 microg/mL puromycin, a P-gp substrate. Compared with rat brain endothelial cells and GP8.3 cells, GPNT cells exhibited a very high level of expression of mdr1a mRNA together with a moderate level of mdr1b mRNA expression. Accordingly, P-gp expression and activity were strongly increased. When GP8.3 and puromycin-starved GPNT cells were treated with puromycin, mdr1a expression was selectively increased. High expression of mdr1a mRNA in GPNT cells may thus be related to the chronic treatment with puromycin. We conclude that GPNT cells may be used as a valuable rat in vitro model for studying the regulation of mdr1a expression at the BBB level.

Disposition of 1,2,3,4,-tetrahydroisoquinoline in the brain of male Wistar and Dark Agouti rats

Direct evidence for accumulation of 1,2,3,4-tetrahydroisoquinoline (TIQ), an endo- and exogenous substance suspected of producing Parkinsonism in humans, has not yet been shown. This study aimed to examine TIQ disposition in the whole rat brain and in the striatum and substantia nigra (SN). TIQ was administered to male Wistar and Dark Agouti rats (20, 40 and 100 mg/kg i.p.) alone or jointly with specific CYP2D inhibitor quinine (20, 40, 80 mg/kg i.p.), acutely or chronically. TIQ concentration in brain of both strains was several-fold higher than in plasma. The level of its metabolite, 4-OH-TIQ, was very low in the brain and plasma of TIQ-treated Wistar while in those receiving additionally quinine or in Dark Agouti rats, 4-OH-TIQ was absent or negligible. Inhibition of CYP2D catalyzing TIQ 4-hydroxylation in the liver had no influence on TIQ accumulation in the brain. Exogenous TIQ was actively transported from periphery into the brain by the organic cation transporter system, mainly OCT3, and quickly eliminated from it by P-glycoprotein. TIQ accumulation after chronic injection to Wistar rats was short-lasting and limited to SN. High concentration of TIQ in SN induces while in the liver inhibits the nigral and hepatic activity CYP2D, respectively.

Administration of drugs known to inhibit P-glycoprotein increases brain bilirubin and alters the regional distribution of bilirubin in rat brain

P-glycoprotein (P-gp) is an ATP-dependent integral plasma membrane efflux pump, expressed in abundance in brain capillary endothelial cells and astrocytes. P-gp contributes to the blood-brain barrier in limiting the influx and retention of a variety of lipophilic compounds, including unconjugated bilirubin. Several drugs block P-gp function and thereby enhance intracellular accumulation of P-gp substrates. In this study we proposed that pretreatment with drugs known to inhibit P-gp function in clinically relevant doses would alter the uptake of bilirubin in the brain of 32- to 36-d-old rats. In the first arm of the study, the animals received pretreatment with an i.v. infusion of either propanolol, erythromycin, verapamil, ceftriaxone, rifampin, or saline, 10 min before an i.v. bolus of 50 mg/kg bilirubin was given. Except for the erythromycin-treated rats, all treatment groups had significantly higher brain-to-serum bilirubin ratios than control animals (p < 0.05, Welch's t test). In the second arm of the study, treatment with either ceftriaxone or rifampin or saline i.v. preceded a 50 mg/kg i.v. bolus of radioactive bilirubin. Analysis of seven different brain regions by scintillation counting showed that the distribution patterns differed significantly between the study groups (p < 0.001, ANOVA), however, not in accordance with a kernicteric staining pattern. Because of limited knowledge of expression and function of P-gp and other membrane transport proteins in the newborn, the implications of this study remain to be seen. We speculate that drugs known to inhibit P-gp function may increase the risk of bilirubin encephalopathy in the hyperbilirubinemic infant.

Genetic polymorphisms of the human MDR1 drug transporter

P-glycoprotein is an ATP-dependent efflux pump that contributes to the protection of the body from environmental toxins. It transports a huge variety of structurally diverse compounds. P-glycoprotein is involved in limiting absorption of xenobiotics from the gut lumen, in protection of sensitive tissues (brain, fetus, testis), and in biliary and urinary excretion of its substrates. P-glycoprotein can be inhibited or induced by xenobiotics, thereby contributing to variable drug disposition and drug interactions. Recently, several SNPs have been identified in the MDR1 gene, some of which can affect P-glycoprotein expression and function. Potential implications of MDR1 polymorphisms for drug disposition, drug effects, and disease risk are discussed.

Drug efflux transporters in the CNS

The central nervous system (CNS) contains important cellular barriers that maintain homeostasis by protecting the brain from circulating toxins and through the elimination of toxic metabolites generated in the brain. The barriers that limit the concentration of toxins and xenobiotics in the interstitial fluids of the CNS are the capillary endothelial cells of the blood-brain barrier (BBB) and the epithelial cells of the blood-cerebrospinal fluid barrier (BCSFB). Both of these barriers have cellular tight junctions and express transport systems which serve to actively transport nutrients into the brain, and actively efflux toxic metabolites and xenobiotics out of the brain. This review will focus on the expression and function of selected drug efflux transporters in these two barriers, specifically the multidrug resistance transporter, p-glycoprotein, and various organic anion transporters, such as multidrug resistance-associated proteins, organic anion transporter polypeptides, and organic anion transporters. These transport systems are increasingly recognized as important determinants of drug distribution to, and elimination from, different compartments of the CNS. Consequences of drug efflux transporters in barriers of the CNS include limiting the distribution of substrates that are beneficial to treat CNS diseases, and increasing the possibility of drug-drug interactions that may lead to untoward toxicities. Therefore, the study of these transporters is important in examining the various determinants of drug delivery to the CNS.

The influence of MDR1 polymorphisms on P-glycoprotein expression and function in humans

The MDR1 (ABCB1) gene product P-glycoprotein is a membrane protein, which functions as an ATP-dependent exporter of xenobiotics from cells. Its importance was first recognized because of its role in the development of multidrug resistance (MDR) of cultured tumor cells against various anticancer agents. It is now, however, well established that this transporter is not only expressed in tumor cells, but also in normal tissues with excretory function (intestine, liver, kidney). Since P-glycoprotein has a very broad substrate specificity, it determines disposition of a broad variety of drugs. Moreover, induction and inhibition of P-glycoprotein are new mechanisms for drug interactions in humans. Very recently, systematic screens of the MDR1 gene have identified multiple single nucleotide polymorphisms. Some of those appear to be associated with altered transporter function and expression. This review discusses the currently available data on the influence of MDR1 polymorphisms on P-glycoprotein tissue expression, drug disposition, treatment outcome and disease risk.

Selective electrical silencing of mammalian neurons in vitro by the use of invertebrate ligand-gated chloride channels

Selectively reducing the excitability of specific neurons will (1) allow for the creation of animal models of human neurological disorders and (2) provide insight into the global function of specific sets of neurons. We focus on a combined genetic and pharmacological approach to silence neurons electrically. We express invertebrate ivermectin (IVM)-sensitive chloride channels (Caenorhabditis elegans GluCl alpha and beta) with a Sindbis virus and then activate these channels with IVM to produce inhibition via a Cl- conductance. We constructed a three-cistron Sindbis virus that expresses the alpha and beta subunits of a glutamate-gated chloride channel (GluCl) along with the green fluorescent protein (EGFP) marker. Expression of the C. elegans channel does not affect the normal spike activity or GABA/glutamate postsynaptic currents of cultured embryonic day 18 hippocampal neurons. At concentrations as low as 5 nm, IVM activates a Cl- current large enough to silence infected neurons effectively. This conductance reverses in 8 hr. These low concentrations of IVM do not potentiate GABA responses. Comparable results are observed with plasmid transfection of yellow fluorescent protein-tagged (EYFP) GluCl alpha and cyan fluorescent protein-tagged (ECFP) GluCl beta. The present study provides an in vitro model mimicking conditions that can be obtained in transgenic mice and in viral-mediated gene therapy. These experiments demonstrate the feasibility of using invertebrate ligand-activated Cl- channels as an approach to modulate excitability.

Drug transport to the brain: key roles for the efflux pump P-glycoprotein in the blood-brain barrier

1. The blood-brain barrier (BBB) contributes to brain homeostastis and fulfills a protective function by controlling the access of solutes and toxic substances to the central nervous system (CNS). The efflux transporter P-glycoprotein (P-gp) is a key element of the molecular machinery that confers special permeability properties to the BBB. 2. P-gp, which was initially recognized for its ability to expel anticancer drugs from multidrug-resistant cancer cells, is strongly expressed in brain capillaries. Its expression in the BBB limits the accumulation of many hydrophobic molecules and potentially toxic substances in the brain. 3. The purpose of this review is to summarize the current state of knowledge about the expression of P-gp, its cellular localization as well as its possible functions in the BBB.

Role of multidrug transporters in pharmacoresistance to antiepileptic drugs

Epilepsy, one of the most common neurologic disorders, is a major public health issue. Despite more than 20 approved antiepileptic drugs (AEDs), about 30% of patients are refractory to treatment. An important characteristic of pharmacoresistant epilepsy is that most patients with refractory epilepsy are resistant to several, if not all, AEDs, even though these drugs act by different mechanisms. This argues against epilepsy-induced alterations in specific drug targets as a major cause of pharmacoresistant epilepsy, but rather points to nonspecific and possibly adaptive mechanisms, such as decreased drug uptake into the brain by intrinsic or acquired over-expression of multidrug transporters in the blood-brain barrier (BBB). There is accumulating evidence demonstrating that multidrug transporters such as P-glycoprotein (PGP) and members of the multidrug resistance-associated protein (MRP) family are over-expressed in capillary endothelial cells and astrocytes in epileptogenic brain tissue surgically resected from patients with medically intractable epilepsy. PGP and MRPs in the BBB are thought to act as an active defense mechanism, restricting the penetration of lipophilic substances into the brain. A large variety of compounds, including many lipophilic drugs, are substrates for either PGP or MRPs or both. It is thus not astonishing that several AEDs, which have been made lipophilic to penetrate into the brain, seem to be substrates for multidrug transporters in the BBB. Over-expression of such transporters in epileptogenic tissue is thus likely to reduce the amount of drug that reaches the epileptic neurons, which would be a likely explanation for pharmacoresistance. PGP and MRPs can be blocked by specific inhibitors, which raises the option to use such inhibitors as adjunctive treatment for medically refractory epilepsy. However, although over-expression of multidrug transporters is a novel and reasonable hypothesis to explain multidrug resistance in epilepsy, further studies are needed to establish this concept. Furthermore, there are certainly other mechanisms of pharmacoresistance that need to be identified.

Inhibition of the multidrug resistance P-glycoprotein activity by green tea polyphenols

Many beneficial proprieties have been associated with polyphenols from green tea, such as chemopreventive, anticarcinogenic, antiatherogenic and antioxidant actions. In this study, we investigated the effects of green tea polyphenols (GTPs) and their principal catechins on the function of P-glycoprotein (P-gp), which is involved in the multidrug resistance phenotype of cancer cells. GTPs (30 microg/ml) inhibit the photolabeling of P-gp by 75% and increase the accumulation of rhodamine-123 (R-123) 3-fold in the multidrug-resistant cell line CH(R)C5, indicating that GTPs interact with P-gp and inhibit its transport activity. Moreover, the modulation of P-gp transport by GTPs was a reversible process. Among the catechins present in GTPs, EGCG, ECG and CG are responsible for inhibiting P-gp. In addition, EGCG potentiates the cytotoxicity of vinblastine (VBL) in CH(R)C5 cells. The inhibitory effect of EGCG on P-gp was also observed in human Caco-2 cells, which form an intestinal epithelial-like monolayer. Our results indicate that, in addition to their anti-cancer properties, GTPs and more particularly EGCG inhibit the binding and efflux of drugs by P-gp. Thus, GTPs or EGCG might be potential agents for modulating the bioavailability of P-gp substrates at the intestine and the multidrug resistance phenotype associated with expression of this transporter in cancer cells.

P-glycoprotein expression in mouse brain increases with maturation

The mdr1a isoform of P-glycoprotein (Pgp) is an integral plasma membrane efflux pump expressed in adult brain capillary endothelial cells and astrocytes of the blood-brain barrier. We determined the developmental pattern of Pgp expression in brain tissue at embryonic day 16 (E16), day of life 0 (D0), day of life 7 (D7), day of life 21 (D21), and adults (Ad). The relative expression of Pgp mRNA and protein was indexed as a percent (mean +/- SEM) of D0 levels. Pgp mRNA levels increased significantly (p < 0.01) with maturation (E16: 75 +/- 8%; D21: 303 +/- 37%, and Ad: 1,160 +/- 120%). Similarly, Pgp protein expression was observed at E16 and increased significantly (p < 0.01) during development (E16: 52 +/- 8%; D7: 187 +/- 23%; D21: 440 +/- 48%, and Ad: 441 +/- 56%). This developmental pattern of enhanced blood-brain barrier Pgp expression with maturation was confirmed by immunohistochemistry. We conclude that (i) Pgp expression in mouse brain is limited during late embryogenesis and the newborn period; (ii) Pgp expression increases markedly with postnatal maturation, and (iii) by D21 brain Pgp protein expression approximates adult levels.

Rapid induction of long-lasting drug efflux activity in brain vascular endothelial cells but not malignant glioma following irradiation

The influence of radiotherapy on malignant glioma multidrug resistance to chemotherapy was evaluated because patients with glioma often are treated with a combination of radiotherapy and chemotherapy. Multidrug resistance gene (MDR1, mdr1a, and mdr1b) transcripts were found in human and rat glioma cell lines. P-Glycoprotein (Pgp) was immunohistochemically detected in glioma cell lines and in the rat brain vascular endothelial cell line (RBE4). A multidrug resistance pump efflux activity assay demonstrated increased calcein efflux of RBE4 endothelial cells, but not glioma cells, 2 h after irradiation and still increased 14 d after irradiation. The increased efflux was equally inhibited by verapamil with or without irradiation. In the rat intracranial glioma model (BT4C), Pgp was demonstrated in capillary endothelial cells of the tumor tissue and surrounding normal brain, but not in tumor cells. The expression of gene transcripts or Pgp was not affected by irradiation. The results indicate that long-lasting verapamil-resistant drug efflux mechanisms are activated in brain endothelial cells after irradiation. The results might explain the poor efficacy of chemotherapy following radiotherapy and contribute to consideration of new treatment strategies in the management of malignant glioma.

P-glycoprotein and bilirubin disposition

P-glycoprotein (Pgp), an ATP-dependent plasma membrane efflux pump, is expressed in abundance on the luminal aspect of brain capillary endothelial cells and astrocytes of the blood-brain barrier where it limits the passage of a variety of lipophilic substrates into the central nervous system. This review summarizes current evidence characterizing (1) unconjugated bilirubin as a potential substrate for Pgp and (2) the ontogeny of Pgp expression at the blood-brain barrier and apical brush border epithelium of the gastrointestinal tract, findings that may provide insights regarding the disposition of bilirubin in immature subjects.

Overexpression of multiple drug resistance genes in endothelial cells from patients with refractory epilepsy

PURPOSE

It has been suggested that altered drug permeability across the blood-brain barrier (BBB) may be involved in pharmacoresistance to antiepileptic drugs (AEDs). To test this hypothesis further, we measured multiple drug resistance (MDR) gene expression in endothelial cells (ECs) isolated from temporal lobe blood vessels of patients with refractory epilepsy. ECs from umbilical cord or temporal lobe vessels obtained from aneurysm surgeries were used as comparison tissue.

METHODS

cDNA arrays were used to determine MDR expression. MDR protein (MRP1) immunocytochemistry and Western blot analysis were used to confirm cDNA array data.

RESULTS

We found overexpression of selected MDR and significantly higher P-glycoprotein levels in "epileptic" versus "control" ECs. Specifically, MDR1, cMRP/MRP2, and MRP5 were upregulated in epileptic tissue, whereas Pgp3/MDR3 levels were comparable to those measured in comparison tissue. The gene encoding cisplatin resistance--associated protein (hCRA-alpha) also was overexpressed in epileptic tissue. Immunocytochemical analysis revealed that MDR1 immunoreactivity was localized primarily in ECs; MRP1 protein levels also were significantly higher in epileptic tissue.

CONCLUSIONS

Complex MDR expression changes may play a role in AEDs pharmacoresistance by altering the permeability of AEDs across the BBB.

Ritonavir induces P-glycoprotein expression, multidrug resistance-associated protein (MRP1) expression, and drug transporter-mediated activity in a human intestinal cell line

The present study characterized the response of P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP1) to chronic ritonavir (RIT) exposure by assessing increases in P-gp and MRP1 protein expression and activity. LS-180V intestinal carcinoma cells were exposed for 3 days to 1-100 microM RIT concurrently with controls. P-gp and MRP1 protein was quantified by Western blot analysis. Cell accumulation assays, using the P-gp substrate rhodamine 123 (RH123), the P-gp/MRP1 substrate doxorubicin (DOX), and the MRP substrate carboxyfluorescein (CBF), were performed as a measure of transporter activity. RIT strongly induced P-gp and MRP1 expression (maximum 6-fold and 3-fold increases, respectively) in a concentration-dependent fashion. Following extended exposure to RIT (> 10 microM), cells accumulated < 50% of the RH123 and DOX compared with controls, whereas accumulation of CBF was decreased by 30% at 30 microM. Differences in cell accumulation of RH123 could be eliminated with verapamil (100 microM; a P-gp inhibitor), whereas decreased DOX cell accumulation was only partially reversed by verapamil. Indomethacin (100 microM; an MRP1 inhibitor) had no significant effect on RH123 or DOX accumulation, suggesting limited MRP1-mediated activity. Thus, RIT induced protein expression of P-gp and MRP1 and increased cellular drug exclusion of RH123, DOX, and CBF. Similar in vivo phenomena may occur during anti-HIV drug therapy, explaining potential decrements in therapeutic efficacy due to decreases in bioavailability or alterations in drug distribution.

In vivo evidence for P-glycoprotein-mediated transport of phenytoin at the blood-brain barrier of rats

PURPOSE

The multidrug transporter P-glycoprotein (P-gp) is expressed at high levels in a variety of tissues such as the endothelial cells of the blood-brain barrier (BBB) capillaries, where it is thought to be involved in the exclusion of various drugs from the capillary endothelial cells, blocking their entry into brain. It was previously shown that pharmacoresistant partial epilepsy is associated with an increased expression of P-gp in brain capillary endothelium and astrocytes, leading to the hypothesis that increased P-gp expression may be involved in medically intractable epilepsy. However, it is not known whether the distribution of antiepileptic drugs (AEDs) into the brain is limited by P-gp. We used in vivo microdialysis in freely moving rats to study whether the concentration of the major AED phenytoin (PHT) in the extra-cellular fluid (ECF) of the cerebral cortex can be enhanced by inhibition of P-gp.

METHODS

Three different P-gp inhibitors, sodium cyanide, verapamil, and PSC 833, were used. These drugs were given via the microdialysis probe in the right frontal cortex, while a probe in the left cortex served as vehicle control side. Perfusion with the inhibitor started 15-60 min before systemic (i.p.) administration of PHT, 50 mg/kg.

RESULTS

PHT rapidly entered the brain ECF compartment, but ECF plasma ratios at time of maximal ECF levels were only approximately 0.04. All P-gp inhibitors significantly increased the ECF concentrations of PHT after local administration, indicating that P-gp in the BBB normally limits the distribution of PHT into the brain parenchyma. Cremorphor EL, the vehicle used to administer PSC, also was able to increase ECF PHT, which is explained by the previously reported inhibitory effect of cremophor on P-gp.

CONCLUSIONS

Provided that multidrug transporters such as P-gp also are involved in the BBB outward transport of other AEDs, increased expression of multidrug transporters, leading to inadequate accumulation of AEDs in the brain, would be a likely explanation for pharmacoresistant epilepsy.

Identification and localization of three photobinding sites of iodoarylazidoprazosin in hamster P-glycoprotein

P-glycoprotein is an ATP-dependent drug-efflux pump which can transport a diverse range of structurally and functionally unrelated substrates across the plasma membrane. Overexpression of this protein may result in multidrug resistance and is a major cause of the failure of cancer chemotherapy. The most commonly used photoreactive substrate is iodoarylazidoprazosin. Its binding domains within the P-glycoprotein have so far been inferred from indirect methods such as epitope mapping. In this study, the binding sites were refined and relocalized by direct analysis of photolabeled peptides. P-glycoprotein-containing plasma membrane vesicles of Chinese hamster ovary B30 cells were photoaffinity-labeled with iodoarylazidoprazosin. After chemical cleavage behind tryptophan residues or enzymatic cleavage behind lysine residues, the resulting 125I-labeled peptides were separated by tricine/PAGE and HPLC and subjected to Edman sequencing. The major photoaffinity binding sites of iodoarylazidoprazosin were localized in the amino-acid regions 248-312 [transmembrane segment (TM)4 to TM5], 758-800 (beyond TM7 to beyond TM8) and 1160-1218 (after the Walker A motif of the second nucleotide-binding domain). Therefore the binding pocket of iodoarylazidoprazosin is made up of at least three binding epitopes.

Pharmacokinetics of pefloxacin and its interaction with cyclosporin A, a P-glycoprotein modulator, in rat blood, brain and bile, using simultaneous microdialysis

1. In vivo microdialysis with HPLC was used to investigate the pharmacokinetics of pefloxacin and its interaction with cyclosporin A. Microdialysis probes were inserted into the jugular vein/right atrium, the striatum and the bile duct of male Sprague-Dawley rats. Biological fluid sampling thereby allowed the simultaneous determination of pefloxacin levels in blood, brain and bile. 2. Following pefloxacin administration, the brain-to-blood coefficient of distribution was 0.036. This was calculated by dividing the area under the concentration curve (AUC) of pefloxacin in brain by its AUC in blood (k=AUC(brain)/AUC(blood)). 3. When the P-glycoprotein cyclosporin A (10 mg kg(-1)) was co-administered with pefloxacin (10 mg kg(-1)), the AUC and the mean residence time in rat blood did not differ significantly (P>0.05). Similarly, the pharmacokinetics of pefloxacin in rat brain was not affected by the presence of cyclosporin A. 4. The AUC of unbound pefloxacin in bile was significantly greater than that in blood. The disposition of pefloxacin in rat bile shows a slow elimination phase following a peak concentration 30 min after pefloxacin administration (10 mg kg(-1), i.v.). The bile-to-blood coefficient of distribution (k=AUC(bile)/AUC(blood)) was 1.53. 5. The results indicated that pefloxacin was able to penetrate the blood-brain barrier and that the concentration in bile was greater than that in the blood, suggesting active biliary excretion of pefloxacin. Current data obtained from rats show no significant impact of cyclosporin A on the pharmacokinetics of pefloxacin in rat blood and brain when administered by concomitant i.v. bolus.

Screening of multidrug-resistance sensitive drugs by in situ brain perfusion in P-glycoprotein-deficient mice

PURPOSE

This study was conducted to assess the influence of P-glycoprotein (P-gp) on brain uptake of multidrug resistance sensitive drugs using an in situ brain perfusion technique in P-gp-deficient (mdr1a[-/-]) and wild-type mice.

METHODS

The blood-brain transport of radiolabeled vinblastine, vincristine, doxorubicin, colchicine, and morphine was evaluated in mdr1a(-/-) and wild-type CF-1 mice with the in situ brain perfusion technique. Brain uptake of drugs after intravenous pretreatment with P-gp reversal agents, (PSC 833, GF 120918, or (+/-)-verapamil), or vehicle also was studied in wild-type mice. In all experiments, cerebral vascular volume was determined by co-perfusion of sucrose.

RESULTS

Cerebral vascular volume was preserved during perfusion, indicating maintenance of blood-brain barrier integrity in both types of mice within the concentration range of substrates in the perfusate. The apparent brain transport of colchicine. vinblastine, doxorubicin, and morphine was increased 3.0, 2.7, 1.5, and 1.4-fold, respectively, in mdr1a(-/-) mice compared with the wild-type: the brain uptake of vincristine was not affected by P-gp. Preadministration of PSC 833 or GF 120918 in wild-type mice led to a -3-fold increase in the brain transport of colchicine and vinblastine, but no effect was observed for the other compounds. Intravenous verapamil enhanced colchicine brain transport (1.8-fold), but failed to increase the brain uptake of vinblastine and morphine.

CONCLUSION

The in situ brain perfusion technique appears to be a sensitive and powerful tool for medium throughput screening of the brain uptake of multidrug resistance sensitive drugs. The effect of P-gp is characterized more efficiently with mdr1a(-/-) mice than by using modulators of P-gp in wild-type mice.

The blood-brain barrier and oncology: new insights into function and modulation

The efficacy of chemotherapy for malignant primary or metastatic brain tumours is still poor. This is at least partly due to the presence of the blood-brain barrier (BBB). The functionality of the BBB can be explained by physicochemical features and efflux pump mechanisms. An overview of the literature is presented with emphasis on oncology. The BBB consists of capillary endothelial cells that lack fenestrations and are connected together with continuous tight junctions, with a high electrical resistance. Permeability of tight junctions can be increased in vitro by contraction of the cytoskeleton, caused by bradykinin agonists. Different efflux pumps are present in the BBB. Examples are P-glycoprotein (P-gp), organic anion transporters, (OAT) and multidrug-resistance-associated proteins (MRP)(1 and 3). These pumps act as a multi-specific efflux pump for various chemotherapeutic drugs. Experiments have shown that P-gp can be inhibited by different non-chemotherapeutic substrates such as cyclosporin A. The functionality in vivo of P-gp can be measured with positron emission tomography and [(11)C]-verapamil or with single photon emission computer tomography and(99m)Tc-sestamibi. MRP(1)and MRP(3)act as organic anion transporters that in vitro act as efflux pumps for substances that are conjugated or co-transported with glutathione and glucuronide, respectively. Methotrexate has been recently demonstrated to be transported by MRP(1)and MRP(3). Results of studies which demonstrate the clinical relevance and applicability of BBB modulators are eagerly awaited.