Importance of P-glycoprotein at blood-tissue barriers

Vitamin E TPGS P-glycoprotein inhibition mechanism: influence on conformational flexibility, intracellular ATP levels, and role of time and site of access

Previous work conducted in our laboratories established the notion that TPGS 1000 (d-alpha-tocopheryl polyethylene glycol 1000 succinate), a nonionic surfactant, modulates P-glycoprotein (P-gp) efflux transport via P-gp ATPase inhibition. The current in vitro research using Caco-2 cells was conducted to further explore the P-gp ATPase inhibition mechanism. Using a monoclonal CD243 P-gp antibody shift assay (UIC2), we probed P-gp conformational changes induced via TPGS 1000. In the presence of TPGS 1000, UIC2 binding was slightly decreased. TPGS 1000 does not appear to be a P-gp substrate, nor does it function as a competitive inhibitor in P-gp substrate efflux transport. The reduction in UIC2 binding with TPGS 1000 was markedly weaker than with orthovanadate, data ruling out trapping P-gp in a transition state by direct interaction with one or both of the P-gp ATP nucleotide binding domains. An intracellular ATP depletion mechanism could be ruled out in the UIC2 assay, and by monitoring intracellular ATP levels in the presence of TPGS 1000. Indicating slow distribution of TPGS 1000 into the membrane, and in agreement with an intramembranal or intracellular side of action, Caco-2 cell monolayer experiments preincubated with TPGS 1000 produce stronger substrate inhibitory activity than those conducted by direct substrate and surfactant coapplication.

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.

P-glycoprotein function at the blood-brain barrier in humans can be quantified with the substrate radiotracer 11C-N-desmethyl-loperamide

Permeability-glycoprotein (P-gp), an efflux transporter in several organs, acts at the blood-brain barrier to protect the brain from exogenous toxins. P-gp almost completely blocks brain entry of the PET radiotracer (11)C-N-desmethyl-loperamide ((11)C-dLop). We examined the ability of (11)C-dLop to quantify P-gp function in humans after increasing doses of tariquidar, an inhibitor of P-gp.

METHODS

Seventeen healthy volunteers had a total of 23 PET scans with (11)C-dLop at baseline and after increasing doses of tariquidar (2, 4, and 6 mg/kg intravenously). A subset of subjects received PET with (15)O-H(2)O to measure cerebral blood flow. Brain uptake of (11)C-dLop was quantified in 2 ways. Without blood data, uptake was measured as area under the time-activity curve in the brain from 10 to 30 min (AUC(10-30)). With arterial blood data, brain uptake was quantified with compartmental modeling to estimate the rates of entry into (K(1)) and efflux from (k(2)) the brain.

RESULTS

Brain uptake of radioactivity was negligible at baseline and increased only slightly (approximately 30%) after 2 mg of tariquidar per kilogram. In contrast, 4 and 6 mg of tariquidar per kilogram increased brain uptake 2- and 4-fold, respectively. Greater brain uptake reflected greater brain entry (K(1)), because efflux (k(2)) and cerebral blood flow did not differ between tariquidar-treated and untreated subjects. In the subjects who received the highest dose of tariquidar (and had the highest brain uptake), regional values of K(1) correlated linearly with absolute cerebral blood flow, consistent with high single-pass extraction of (11)C-dLop. AUC(10-30) correlated linearly with K(1).

CONCLUSION

P-gp function at the blood-brain barrier in humans can be quantified using PET and (11)C-dLop. A simple measure of brain uptake (AUC(10-30)) may be used as a surrogate of the fully quantified rate constant for brain entry (K(1)) and thereby avoid arterial sampling. However, to dissect the function of P-gp itself, both brain uptake and the influx rate constant must be corrected for radiotracer delivery (blood flow).

No significant effect of ABCB1 haplotypes on the pharmacokinetics of fluvastatin, pravastatin, lovastatin, and rosuvastatin

AIMS

This study aimed to investigate possible effects of ABCB1 genotype on fluvastatin, pravastatin, lovastatin, and rosuvastatin pharmacokinetics.

METHODS

In a fixed-order crossover study, 10 healthy volunteers with the ABCB1 c.1236C/C-c.2677G/G-c.3435C/C (CGC/CGC) genotype and 10 with the c.1236T/T-c.2677T/T-c.3435T/T (TTT/TTT) genotype ingested a single 20-mg dose of fluvastatin, pravastatin, lovastatin, and rosuvastatin. Plasma fluvastatin, pravastatin, and lovastatin concentrations were measured up to 12 h and plasma and urine rosuvastatin concentrations up to 48 and 24 h, respectively.

RESULTS

The ABCB1 genotype had no significant effect on the pharmacokinetics of any of the investigated statins. The geometric mean ratio (95% confidence interval) of the area under the plasma concentration-time curve from 0 h to infinity (AUC(0-infinity)) in participants with the TTT/TTT genotype to that in those with the CGC/CGC genotype was 0.96 (0.77, 1.20; P= 0.737) for fluvastatin, 0.92 (0.53, 1.62; P= 0.772) for pravastatin, 0.83 (0.36, 1.90; P= 0.644) for lovastatin, 1.25 (0.72, 2.17; P= 0.400) for lovastatin acid, and 1.10 (0.73, 1.65; P= 0.626) for rosuvastatin. The AUC(0-infinity) of lovastatin acid correlated significantly with that of rosuvastatin (r= 0.570, P= 0.009), but none of the other AUC(0-infinity) pairs showed a significant correlation.

CONCLUSIONS

These data suggest that the ABCB1 c.1236C-c.2677G-c.3435C and c.1236T-c.2677T-c.3435T haplotypes play no significant role in the interindividual variability in the pharmacokinetics of fluvastatin, pravastatin, lovastatin, and rosuvastatin.

Model analysis of the concentration-dependent permeability of P-gp substrates

PURPOSE

Recently, it was reported that the apparent Michaelis-Menten constant (Km(app)) of a P-glycoprotein (P-gp) substrate, defined for the extracellular substrate concentration, increases as the P-gp expression level in the cell increases. By its nature, the Km value should not depend on the level of P-gp expression. The purpose of this study is to establish a model which can estimate the Km value independent of the P-gp expression level in cells.

METHODS

The previously reported concentration-dependent permeability of verapamil, quinidine, and vinblastine in MDR1-MDCKII, P-gp-highly induced Caco-2, P-gp-induced Caco-2, normal Caco-2, and MDR1-knockdown Caco-2 cells data were analyzed using a model in which the Km value was defined for the intracellular substrate concentration.

RESULTS

The estimated Km values defined for the substrate concentration inside the cells were almost the same among various cells with different levels of P-gp expression. The estimated Vmax values were approximately proportional to the P-gp expression level.

CONCLUSION

The established kinetic model was found to be rational based on the results that the Km values of P-gp substrates were about the same for cells expressing various levels of P-gp, while the Vmax values were proportional to the expression levels of P-gp.

Pharmacogenetics of membrane transporters: an update on current approaches

This review provides an overview of the pharmacogenetics of membrane transporters including selected ABC transporters (ABCB1, ABCC1, ABCC2, and ABCG2) and OATPs (OATP1B1 and OATP1B3). Membrane transporters are heavily involved in drug clearance and alters drug disposition by actively transporting substrate drugs between organs and tissues. As such, polymorphisms in the genes encoding these proteins may have significant effects on the absorption, distribution, metabolism and excretion of compounds, and may alter pharmacodynamics of many agents. This review discusses the techniques used to identify substrates and inhibitors of these proteins and subsequently to assess the effect of genetic mutation on transport, both in vitro and in vivo. A comprehensive list of substrates for the major drug transporters is included. Finally, studies linking transporter genotype with clinical outcomes are discussed.

Expression and function of p-glycoprotein in normal tissues: effect on pharmacokinetics

ATP-binding cassette (ABC) drug efflux transporters limit intracellular concentration of their substrates by pumping them out of cell through an active, energy dependent mechanism. Several of these proteins have been originally associated with the phenomenon of multidrug resistance; however, later on, they have also been shown to control body disposition of their substrates. P-glycoprotein (Pgp) is the first detected and the best characterized of ABC drug efflux transporters. Apart from tumor cells, its constitutive expression has been reported in a variety of other tissues, such as the intestine, brain, liver, placenta, kidney, and others. Being located on the apical site of the plasma membrane, Pgp can remove a variety of structurally unrelated compounds, including clinically relevant drugs, their metabolites, and conjugates from cells. Driven by energy from ATP, it affects many pharmacokinetic events such as intestinal absorption, brain penetration, transplacental passage, and hepatobiliary excretion of drugs and their metabolites. It is widely believed that Pgp, together with other ABC drug efflux transporters, plays a crucial role in the host detoxication and protection against xenobiotic substances. On the other hand, the presence of these transporters in normal tissues may prevent pharmacotherapeutic agents from reaching their site of action, thus limiting their therapeutic potential. This chapter focuses on P-glycoprotein, its expression, localization, and function in nontumor tissues and the pharmacological consequences hereof.

Functional characterization of mouse organic anion transporting peptide 1a4 in the uptake and efflux of drugs across the blood-brain barrier

This study investigated the role of a multispecific organic anion transporter, Oatp1a4/Slco1a4, in drug transport across the blood-brain barrier. In vitro transport studies using human embryonic kidney 293 cells expressing mouse Oatp1a4 identified the following compounds as Oatp1a4 substrates: pitavastatin (K(m) = 8.3 microM), rosuvastatin (K(m) = 12 microM), pravastatin, taurocholate (K(m) = 40 microM), digoxin, ochratoxin A, and [d-penicillamine(2,5)]-enkephalin. Double immunohistochemical staining of Oatp1a4 with P-glycoprotein (P-gp) or glial fibrillary acidic protein demonstrated that Oatp1a4 signals colocalized with P-gp signals partly but not with glial fibrillary acidic protein, suggesting that Oatp1a4 is expressed in both the luminal and the abluminal membranes of mouse brain capillary endothelial cells. The brain-to-blood transport of pitavastatin, rosuvastatin, pravastatin, and taurocholate after microinjection into the cerebral cortex was significantly decreased in Oatp1a4(-/-) mice compared with that in wild-type mice. The blood-to-brain transport of pitavastatin, rosuvastatin, taurocholate, and ochratoxin A, determined by in situ brain perfusion, was significantly lower in Oatp1a4(-/-) mice than in wild-type mice, whereas transport of pravastatin and [D-penicillamine(2,5)]-enkephalin was unchanged. The blood-to-brain transport of digoxin was significantly lower in Oatp1a4(-/-) mice than in wild-type mice only when P-gp was inhibited by N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918). Taken together, these results show that Oatp1a4 can mediate the brain-to-blood and blood-to-brain transport of its substrate drugs across the blood-brain barrier. The brain-to-plasma ratio of taurocholate, pitavastatin, and rosuvastatin was close to the capillary volume in wild-type mice, and it was not affected by Oatp1a4 dysfunction. Whether Oatp1a4 can deliver drugs from the blood to the brain remains controversial.

Pharmacokinetic and pharmacodynamic implications of P-glycoprotein modulation

Modulation of P-glycoprotein (Pgp)-mediated transport has significant pharmacokinetic implications for Pgp substrates. Pharmacokinetic alterations may be at the systemic (blood concentrations), regional (organ or tissue concentrations), or local (intracellular concentrations) level. Regardless of the particular location of Pgp modulation, changes in substrate pharmacokinetics will have the potential to alter the magnitude and duration of pharmacologic effect (pharmacodynamics). It is important to understand each of the aspects of Pgp modulation for a given Pgp substrate in order to predict the degree to which Pgp modulation may affect that substrate, to minimize untoward effects associated with that modulation, or to exploit that modulation for specific therapeutic advantage.

Evaluation of [C]S14506 and [F]S14506 in rat and monkey as agonist PET radioligands for brain 5-HT(1A) receptors

In vitro and ex vivo measurements have shown that the binding of the selective high-affinity agonist, S14506 (1-[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxy-naphthyl)piperazine), to 5-HT(1A) receptors, is similar in affinity (K(d) = 0.79 nM) and extent (B(max)) to that of the antagonist, WAY 100635. We aimed to test whether S14506, labeled with a positron-emitter, might serve as a radioligand for imaging brain 5-HT(1A) receptors in vivo with positron emission tomography (PET). Here we evaluated [(11)C]S14506 and [(18)F]S14506 in rat and rhesus monkey in vivo. After intravenous administration of [(11)C]S14506 into rat, radioactivity entered brain, reaching 210% SUV at 2 min. Radioactivity uptake into brain was higher (~ 350% SUV) in rats pre-treated with the P-glycoprotein (P-gp) inhibitor, cyclosporin A. In rhesus monkey, peak brain uptake of radioactivity after administration of [(11)C]S14506 or [(18)F]S14506 was also moderate and for [(11)C]S14506 increased from ~ 170% SUV after 7 min, to 240% SUV in a monkey pre-treated with the P-gp inhibitor, tariquidar. The ratios of radioactivity in 5-HT(1A) receptor-rich regions, such as cingulate or hippocampus to that in receptor-poor cerebellum reached between 1.35 and 1.5 at 60 min for both [(11)C]S14506 and [(18)F]S14506. [(11)C]S14506 gave one major polar radiometabolite in monkey plasma, and [(18)F]S14506 gave three and two more polar radiometabolites in rat and monkey plasma, respectively. The rat radiometabolites of [(18)F]S14506 did not accumulate in brain. [(18)F]S14506 was not radiodefluorinated in monkey. Thus, despite high-affinity and lack of troublesome brain radiometabolites, both [(11)C]S14506 and [(18)F]S14506 were ineffective for imaging rat or monkey brain 5-HT(1A) receptors in vivo, even under P-gp inhibited conditions. Explanations for the failure of these radioligands are offered.

Mechanisms of multidrug resistance in cancer

The development of multidrug resistance (MDR) to chemotherapy remains a major challenge in the treatment of cancer. Resistance exists against every effective anticancer drug and can develop by numerous mechanisms including decreased drug uptake, increased drug efflux, activation of detoxifying systems, activation of DNA repair mechanisms, evasion of drug-induced apoptosis, etc. In the first part of this chapter, we briefly summarize the current knowledge on individual cellular mechanisms responsible for MDR, with a special emphasis on ATP-binding cassette transporters, perhaps the main theme of this textbook. Although extensive work has been done to characterize MDR mechanisms in vitro, the translation of this knowledge to the clinic has not been crowned with success. Therefore, identifying genes and mechanisms critical to the development of MDR in vivo and establishing a reliable method for analyzing clinical samples could help to predict the development of resistance and lead to treatments designed to circumvent it. Our thoughts about translational research needed to achieve significant progress in the understanding of this complex phenomenon are therefore discussed in a third section. The pleotropic response of cancer cells to chemotherapy is summarized in a concluding diagram.

Drug transporter, P-glycoprotein (MDR1), is an integrated component of the mammalian blood-testis barrier

Throughout spermatogenesis, leptotene spermatocytes traverse the blood-testis barrier (BTB) to enter the adluminal compartment of the seminiferous epithelium for continued development. At the same time, the integrity of the BTB, which is constituted by co-existing tight junctions (TJ), basal ectoplasmic specializations (basal ES) and desmosome-like junctions, must be maintained since a breach in barrier function can result in spermatogenic arrest and even infertility. There is evidence to suggest that drug transporters may function at the BTB, but little is known about how they contribute to spermatogenesis. In this study, we investigate the role of P-glycoprotein (P-gp), a drug efflux pump, in BTB dynamics. A survey by RT-PCR revealed several transporter genes to be expressed by the testis, including Mdr1 (gene symbol for P-gp), Mrp1, Abcc5 and Slc15a1. It was also demonstrated that P-gp localizes to the BTB in all stages of the seminiferous epithelial cycle in the adult rat testis, as well as to the Sertoli cell-elongated spermatid interface in stages VII and VIII. We continued our study by examining the levels of several transporters in the testis following oral administration of Adjudin, a compound known to affect Sertoli-germ cell adhesion. In this experiment, the steady-state levels of P-gp, MRP1, ABCG1 and SLC15A1 were all found to increase by several-fold within hours of Adjudin treatment during junction restructuring. More importantly, an increase in P-gp association with TJ proteins (e.g., occludin, claudin-11 and JAM-A) was noted when testis lysates from Adjudin-treated rats were used for co-immunoprecipitation experiments, suggesting that P-gp may enhance BTB function during Sertoli-germ cell junction restructuring.

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.

Age-dependent decline of blood-brain barrier P-glycoprotein expression in the canine brain

The efflux transporter P-glycoprotein serves as a major molecular gatekeeper at the blood-brain barrier. It has been suggested that a reduction of P-glycoprotein activity with aging might enhance exposure of brain tissue to exogenous and endogenous compounds thereby contributing to the development of neurodegenerative diseases. Brain tissue from owner-kept dogs renders an excellent tool to study the impact of aging on the background of variable environmental and genetic influencing factors. Therefore, we determined expression rates of P-glycoprotein in canine post-mortem tissue from 23 non-laboratory dogs. P-glycoprotein expression in the parahippocampal cortex exhibited a negative correlation with age. Analysis of the area labeled for P-glycoprotein in dogs aged >100 months revealed a 72% drop in P-glycoprotein expression as compared to young adults aged 23-36 months. Respective data from the dentate hilus and dentate gyrus indicated an earlier drop with a reduction by 77 and 80% in dogs aged 37-99 months in comparison with younger individuals. In contrast to the decline observed with aging in dogs without plaques, P-glycoprotein expression rates rather tended to increase with further aging in dogs with plaque formation. In conclusion, the thorough analysis of P-glycoprotein expression rates in non-laboratory dogs revealed a significant decline with aging. The data strongly support the concept that age-dependent changes might predispose to neurodegenerative diseases. In the early pathogenesis of Alzheimer's disease which is modelled by diffuse plaques in the canine brain, an up-regulation of P-glycoprotein might act as a compensatory mechanism to enhance Abeta efflux from the brain. Future studies are necessary to further evaluate the correlation between Abeta deposits and P-glycoprotein expression in different phases of the disease.

Active transport across the human placenta: impact on drug efficacy and toxicity

The human placenta expresses a large number of transport proteins. The ATP-binding cassette (ABC) family of active efflux pumps, predominantly localised to the maternal-facing syncytial membrane of placental microvilli, comprise the major placental drug efflux transporters. A variety of other transporters are also expressed in the placenta that can facilitate xenobiotic transfer in both the maternal and fetal directions. Many drugs administered in pregnancy are ABC transporter substrates, and many are either teratogenic or fetotoxic. The in vitro, in vivo and clinical evidence reviewed in this article argues that active efflux of drugs by placental transporters helps to maintain its barrier function, reducing the incidence of adverse fetal effects. ABC transporter polymorphisms may explain the wide variability observed in fetal drug concentrations, incidence of teratogenesis or drug failure in pregnancies exposed to therapeutic agents. Although our understanding of the molecular mechanics and dynamics of placental drug transfer is advancing, much work is needed to fully appreciate the significance of placental drug transporters in the face of increasing drug administration in pregnancy.

Effect of cyclosporine on drug transport and pharmacokinetics of nifedipine

Nifedipine (NFP) is an anti-hypersensitive drug and a well-known substrate of cytochrome P450 3A4 (CYP3A4), while cyclosporine (CSP) is a potent p-glycoprotein (P-gp) inhibitor. P-gp is a drug transporter, which determines the absorption and bioavailability of many drugs that are substrates for P-gp. Drugs that induce or inhibit P-gp may have a profound effect on the absorption and pharmacokinetics (PK) of drugs transported by P-gp within the body, possibly compromising their bioavailability. But the role of P-gp in the NFP efflux and its impact on PK profile is not known. Hence in our present study we attempted to investigate the effect of CSP on oral absorption and PK of NFP. Rhodamine 123 (Rho 123), a known P-gp substrate was used as a positive control. Male Wistar rats (350-400 g) were used for the study. Rats were divided into 4 groups (n=6 each); one group was treated with vehicle (cremophor) followed by NFP (0.2 mg/kg; i.v. bolus) and the other group with CSP (10 mg/kg; i.v.) followed by NFP. Group 3 and 4 were treated with vehicle (cremophor) followed by Rho 123 (0.2 mg/kg, i.v.) and CSP (10 mg/kg; i.v.) followed by Rho 123 (0.2 mg/kg, i.v.) respectively. The blood samples were collected at 0, 5, 10, 15, 30, 60, 90, 120, 180 and 240 min after NFP administration. NFP concentrations in plasma were analyzed by LC-MS-MS and Rho 123 was analyzed by fluorimetric detector. NFP efflux was significantly decreased in CSP treated rats (49.1% decrease, P<0.05), while NFP concentration in plasma were not changed. However the decrease in NFP efflux did not show any significant changes in NFP PK parameters (T(max); 2.0 vs. 2.5 min, C(max); 0.084 vs. 0.076 microg/ml, T(1/2); 84.0 vs. 91.4 min, AUC(0-t); 4.183 vs. 3.467 microg h/ml, AUC(infinity); 5.915 vs. 4.769 microg h/ml, AUMC(0-t); 224.073 vs. 173.063 microg h/ml, AUMC(infinity); 776.871 vs. 575.038 microg h/ml, MRT(0-t); 53.608 vs. 49.538 microg h/ml, MRT(infinity); 118.194 vs. 115.246 microg h/ml, CL(tot); 0.0375 vs. 0.0433 l/h, Vd(ss); 3.999 vs. 4.641 l in NFP alone vs. CSP+NFP groups respectively). Thus the results indicate that NFP would belong to a group of P-gp substrate. The decrease in efflux of NFP by CSP, through inhibition of P-gp, into the intestinal lumen did not show any impact on PK. This could be due to the activity of other transporters and/or CYP3A4 may have more limiting role than P-gp on NFP metabolism and disposition that is why inhibiting P-gp did not lead to increase the bioavailability and PK alterations.

Targeting prostaglandin E2 EP1 receptors prevents seizure-associated P-glycoprotein up-regulation

Up-regulation of the blood-brain barrier efflux transporter P-glycoprotein in central nervous system disorders results in restricted brain access and limited efficacy of therapeutic drugs. In epilepsies, seizure activity strongly triggers expression of P-glycoprotein. Here, we identified the prostaglandin E2 receptor, EP1, as a key factor in the signaling pathway that mediates seizure-induced up-regulation of P-glycoprotein at the blood-brain barrier. In the rat pilocarpine model, status epilepticus significantly increased P-glycoprotein expression by 92 to 197% in the hippocampal hilus and granule cell layer as well as the piriform cortex. The EP1 receptor antagonist 8-chlorodibenz[b,f][1,4]oxazepine-10(11H)-carboxylic acid, 2-[1-oxo-3-(4-pyridinyl)propyl]hydrazide hydrochloride (SC-51089) abolished seizure-induced P-glycoprotein up-regulation and retained its expression at the control level. The control of P-glycoprotein expression despite prolonged seizure activity suggests that EP1 receptor antagonism will also improve antiepileptic drug efficacy. Preliminary evidence for this concept has been obtained using a massive kindling paradigm during which animals received a subchronic SC-51089 treatment. After withdrawal of the EP1 receptor antagonist, a low dose of the P-glycoprotein substrate phenobarbital resulted in an anticonvulsant effect in this pretreated group, whereas the same dosage of phenobarbital did not exert a significant effect in the respective control group. In conclusion, our data demonstrate that EP1 is a key signaling factor in the regulatory pathway that drives P-glycoprotein up-regulation during seizures. These findings suggest new intriguing possibilities to prevent and interrupt P-glycoprotein overexpression in epilepsy. Future studies are necessary to further evaluate the appropriateness of the strategy to enhance the efficacy of antiepileptic drugs.

An intracellular trafficking pathway in the seminiferous epithelium regulating spermatogenesis: a biochemical and molecular perspective

During spermatogenesis in adult rat testes, fully developed spermatids (i.e. spermatozoa) at the luminal edge of the seminiferous epithelium undergo "spermiation" at stage VIII of the seminiferous epithelial cycle. This is manifested by the disruption of the apical ectoplasmic specialization (apical ES) so that spermatozoa can enter the tubule lumen and to complete their maturation in the epididymis. At the same time, the blood-testis barrier (BTB) located near the basement membrane undergoes extensive restructuring to allow transit of preleptotene spermatocytes so that post-meiotic germ cells complete their development behind the BTB. While spermiation and BTB restructuring take place concurrently at opposite ends of the Sertoli cell epithelium, the biochemical mechanism(s) by which they are coordinated were not known until recently. Studies have shown that fragments of laminin chains are generated from the laminin/integrin protein complex at the apical ES via the action of MMP-2 (matrix metalloprotease-2) at spermiation. These peptides serve as the local autocrine factors to destabilize the BTB. These laminin peptides also exert their effects on hemidesmosome which, in turn, further potentiates BTB restructuring. Thus, a novel apical ES-BTB-hemidesmosome regulatory loop is operating in the seminiferous epithelium to coordinate these two crucial cellular events of spermatogenesis. This functional loop is further assisted by the Par3/Par6-based polarity protein complex in coordination with cytokines and testosterone at the BTB. Herein, we provide a critical review based on the latest findings in the field regarding the regulation of these cellular events. These recent findings also open up a new window for investigators studying blood-tissue barriers.

In vivo down-regulation of mouse brain capillary P-glycoprotein: a preliminary investigation

Over-expression of blood-brain barrier P-glycoprotein is considered as a major hurdle in the treatment of various CNS disorders. A down-regulation strategy is considered as one means to counteract disease- or therapy-associated induction of P-glycoprotein. Here, we evaluated whether a targeting of P-glycoprotein can be achieved in mouse brain capillary endothelial cells using siRNA. A 4-day treatment paradigm with once daily hydrodynamic intravenous injections of siRNA resulted in a significant reduction of the P-glycoprotein-labeled area in the hippocampal hilus and parietal cortex. P-glycoprotein expression proved to be down-regulated in these brain regions by 31 and 16%, respectively. An impact of siRNA administration on density of brain capillaries was excluded by quantification of the endothelial cell marker GLUT-1. In conclusion, the study provides first preliminary evidence that a down-regulation of P-glycoprotein can be achieved in brain capillary endothelial cells by administration of siRNA in vivo.

Involvement of Vitamin D receptor in the intestinal induction of human ABCB1

ABCB1 (P-glycoprotein) is an efflux transporter that limits the cellular uptake levels of various drugs in intestine, brain, and other tissues. The expression of human ABCB1 has recently been reported to be under the control of nuclear receptor NR1I subfamily members, pregnane X receptor (PXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3). Here, we have investigated the involvement of another NR1I member, vitamin D receptor (VDR, NR1I1), in ABCB1 expression. In the human colorectal adenocarcinoma cell line LS174T, which abundantly expresses VDR, both 1alpha,25-dihydroxyvitamin D(3) (1,25-VD3) and lithocholic acid (LCA) increased ABCB1 mRNA levels. Reporter gene assays in LS174T cells with constructs containing various lengths of the ABCB1 regulatory region revealed that the region containing multiple nuclear receptor binding motifs located at -7.8 kilobases [termed nuclear receptor-responsive module (NURREM)], to which PXR and CAR also bind, is essential for the VDR-mediated ABCB1 transactivation. Further reporter assays with constructs containing truncated NURREM and gel shift assays suggested simultaneous binding of multiple VDR/retinoid X receptor alpha heterodimers to NURREM. Furthermore, knockdown of VDR expression in LS174T cells blocked the LCA- and the 1,25-VD3-induced transcription of ABCB1 reporter genes. In human hepatoma HepG2 cells, in contrast with LS174T cells, 1,25-VD3 activated the ABCB1 transcription only in the presence of ectopically expressed VDR. These results suggest that the NR1I subfamily members regulate the ABCB1 expression sharing the binding sites within NURREM and that the physiologically produced LCA and 1,25-VD3 may modulate the ABCB1 expression in human intestines, possibly associated with interindividual variations of ABCB1 expression.

Vitreal kinetics of quinidine in rabbits in the presence of topically coadministered P-glycoprotein substrates/modulators

The purpose of this study was to investigate whether topically administered P-glycoprotein (P-gp) substrates/modulators can alter vitreal kinetics of intravitreally administered quinidine. Male New Zealand rabbits were used under anesthesia. Vitreal kinetics of intravitreally administered quinidine (0.75-microg dose) was determined alone and in the presence of verapamil (coadministered topically/intravitreally) or prednisolone hemisuccinate sodium (PHS) (coadministered topically). In the presence of topically instilled verapamil (1% w/v), elimination half-life (t(1/2)) (176 +/- 7 min), apparent elimination rate constant (lambda(z)) (0.0039 +/- 0.0001 min(-1)), and mean retention time (MRT) (143 +/- 30 min) of intravitreally administered quinidine were significantly different from those of the control (105 +/- 11 min, 0.0066 +/- 0.0007 min(-1), and 83 +/- 13 min, respectively). A 2-fold increase in the t(1/2) with a corresponding decrease in lambda(z) and a 1.5-fold increase in the MRT of quinidine were observed in the presence of topically coadministered 2% w/v PHS. Intravitreal coadministration of quinidine and verapamil resulted in a significant increase in t(1/2) (159 +/- 9 min) and a decrease in lambda(z) (0.0043 +/- 0.0002 min(-1)) of quinidine. The vitreal pharmacokinetic parameters of sodium fluorescein, alone or in the presence of topically instilled verapamil, did not show any statistically significant difference, indicating that ocular barrier integrity was not affected by topical verapamil administration. Results from this study suggest that topically applied P-gp substrates/modulators can alter vitreal pharmacokinetics of intravitreally administered P-gp substrates, possibly through the inhibition of P-gp expressed on the basolateral membrane of the retinal pigmented epithelium.

Evaluation of current methods used to analyze the expression profiles of ATP-binding cassette transporters yields an improved drug-discovery database

The development of multidrug resistance (MDR) to chemotherapy remains a major challenge in the treatment of cancer. Resistance exists against every effective anticancer drug and can develop by multiple mechanisms. These mechanisms can act individually or synergistically, leading to MDR, in which the cell becomes resistant to a variety of structurally and mechanistically unrelated drugs in addition to the drug initially administered. Although extensive work has been done to characterize MDR mechanisms in vitro, the translation of this knowledge to the clinic has not been successful. Therefore, identifying genes and mechanisms critical to the development of MDR in vivo and establishing a reliable method for analyzing highly homologous genes from small amounts of tissue is fundamental to achieving any significant enhancement in our understanding of MDR mechanisms and could lead to treatments designed to circumvent it. In this study, we use a previously established database that allows the identification of lead compounds in the early stages of drug discovery that are not ATP-binding cassette (ABC) transporter substrates. We believe this can serve as a model for appraising the accuracy and sensitivity of current methods used to analyze the expression profiles of ABC transporters. We found two platforms to be superior methods for the analysis of expression profiles of highly homologous gene superfamilies. This study also led to an improved database by revealing previously unidentified substrates for ABCB1, ABCC1, and ABCG2, transporters that contribute to MDR.

The role of p-glycoprotein in limiting brain penetration of the peripherally acting anticholinergic overactive bladder drug trospium chloride

The aim of the present study was to characterize the role of the drug-efflux transporter P-glycoprotein (P-gp) for the disposition of trospium chloride, a widely used anticholinergic drug for the treatment of overactive bladder. P-gp-deficient mdr1a,b(-/-) knockout mice were given either 1 mg/kg trospium chloride orally or 1 mg/kg intravenously to analyze brain penetration, intestinal secretion, and hepatobiliary excretion of the drug. The concentrations of trospium chloride in the brain were up to 7 times higher in the mdr1a,b(-/-) knockout mice compared with wild-type mice (p < 0.05), making P-gp a limiting factor for the blood-brain barrier penetration of this drug. Moreover, the residence time of the drug in the central nervous system was significantly prolonged in mdr1a,b(-/-) knockout mice. Apart from the blood-brain barrier, P-gp also had significant effects on the overall pharmacokinetics of trospium chloride. In the mdr1a,b(-/-) knockout mice, hepatobiliary excretion and intestinal secretion were significantly reduced compared with the wild-type mice. Our study indicates that the multidrug resistance transporter P-gp is a major determinant for the distribution of trospium chloride in the body and highly restricts its entry into the brain.

ABCG2 polymorphism markedly affects the pharmacokinetics of atorvastatin and rosuvastatin

The ABCG2 c.421C>A single-nucleotide polymorphism (SNP) was determined in 660 healthy Finnish volunteers, of whom 32 participated in a pharmacokinetic crossover study involving the administration of 20 mg atorvastatin and rosuvastatin. The frequency of the c.421A variant allele was 9.5% (95% confidence interval 8.1-11.3%). Subjects with the c.421AA genotype (n = 4) had a 72% larger mean area under the plasma atorvastatin concentration-time curve from time 0 to infinity (AUC(0-infinity)) than individuals with the c.421CC genotype had (n = 16; P = 0.049). In participants with the c.421AA genotype, the rosuvastatin AUC(0-infinity) was 100% greater than in those with c.421CA (n = 12) and 144% greater than in those with the c.421CC genotype. Also, those with the c.421AA genotype showed peak plasma rosuvastatin concentrations 108% higher than those in the c.421CA genotype group and 131% higher than those in the c.421CC genotype group (P < or = 0.01). In MDCKII-ABCG2 cells, atorvastatin transport was increased in the apical direction as compared with vector control cells (transport ratio 1.9 +/- 0.1 vs. 1.1 +/- 0.1). These results indicate that the ABCG2 polymorphism markedly affects the pharmacokinetics of atorvastatin and, even more so, of rosuvastatin-potentially affecting the efficacy and toxicity of statin therapy.

The effect of probenecid and MK-571 on the feto-maternal transfer of saquinavir in dually perfused human term placenta

Human placenta, particularly the blood-placenta barrier, with various transporters has crucial role to protect the fetus and, on the other hand, to facilitate movement of compounds towards the fetal circulation. This study aimed to characterize the role of basal transporters of the syncytiotrophoblast, which appear to be yet less studied, in the fetal-to-maternal transfer of saquinavir by use of dually perfused human placentas. A dual perfusion of human placenta was performed to study effect of MK-571 and probenecid, inhibitors of the MRP1 and OATP transporters, expressed in the basal trophoblast membrane, on the transfer of saquinavir. The fetal-to-maternal placental transfer of saquinavir in the control group as measured by TPT(AUC)% (absolute fraction of the dose crossing placenta) was 14.0%, which is 73% less than the transfer of the freely diffusible antipyrine. The two inhibitors, MK-571 and probenecid caused a non-significant (P = 0.34 for ANOVA) reduction of 43% and 24%, respectively, in the mean amount of saquinavir transferred from the fetal to the maternal side. MK-571 also somewhat (by 31%) reduced the TPT(AUC)% of antipyrine, but this finding did not reach statistical significance (P = 0.25). Neither of the employed inhibitors had an effect on the placental transfer index of saquinavir transfer (P = 0.77). The present results indicated lack of significant effect by MK-571 and probenecid on the fetal-to-maternal transfer of saquinavir and suggest that MRP1 and, possibly, OATP2B1 do not play a significant role in the fetal-to-maternal transfer of saquinavir.

Human brain imaging and radiation dosimetry of 11C-N-desmethyl-loperamide, a PET radiotracer to measure the function of P-glycoprotein

P-glycoprotein (P-gp) is a membrane-bound efflux pump that limits the distribution of drugs to several organs of the body. At the blood-brain barrier, P-gp blocks the entry of both loperamide and its metabolite, N-desmethyl-loperamide (N-dLop), and thereby prevents central opiate effects. Animal studies have shown that (11)C-dLop, compared with (11)C-loperamide, is an especially promising radiotracer because it generates negligible radiometabolites that enter the brain. The purposes of this study were to determine whether (11)C-dLop is a substrate for P-gp at the blood-brain barrier in humans and to measure the distribution of radioactivity in the entire body to estimate radiation exposure.

METHODS

Brain PET scans were acquired in 4 healthy subjects for 90 min and included concurrent measurements of the plasma concentration of unchanged radiotracer. Time-activity data from the whole brain were quantified using a 1-tissue-compartment model to estimate the rate of entry (K(1)) of radiotracer into the brain. Whole-body PET scans were acquired in 8 healthy subjects for 120 min.

RESULTS

For brain imaging, after the injection of (11)C-dLop the concentration of radioactivity in the brain was low (standardized uptake value, approximately 15%) and stable after approximately 20 min. In contrast, uptake of radioactivity in the pituitary was about 50-fold higher than that in the brain. The plasma concentration of (11)C-dLop declined rapidly, but the percentage composition of plasma was unusually stable, with the parent radiotracer constituting 85% of total radioactivity after approximately 5 min. The rate of brain entry was low (K(1) = 0.009 +/- 0.002 mL.cm(-3).min(-1); n = 4). For whole-body imaging, as a measure of radiation exposure to the entire body the effective dose of (11)C-dLop was 7.8 +/- 0.6 muSv/MBq (n = 8).

CONCLUSION

The low brain uptake of radioactivity is consistent with (11)C-dLop being a substrate for P-gp in humans and confirms that this radiotracer generates negligible quantities of brain-penetrant radiometabolites. In addition, the low rate of K(1) is consistent with P-gp rapidly effluxing substrates while they transit through the lipid bilayer. The radiation exposure of (11)C-dLop is similar to that of many other (11)C-radiotracers. Thus, (11)C-dLop is a promising radiotracer to study the function of P-gp at the blood-brain barrier, at which impaired function would allow increased uptake into the brain.

The H2 receptor antagonist nizatidine is a P-glycoprotein substrate: characterization of its intestinal epithelial cell efflux transport

The aim of this study was to elucidate the intestinal epithelial cell efflux transport processes that are involved in the intestinal transport of the H(2) receptor antagonist nizatidine. The intestinal epithelial efflux transport mechanisms of nizatidine were investigated and characterized across Caco-2 cell monolayers, in the concentration range 0.05-10 mM in both apical-basolateral (AP-BL) and BL-AP directions, and the transport constants of P-glycoprotein (P-gp) efflux activity were calculated. The concentration-dependent effects of various P-gp (verapamil, quinidine, erythromycin, ketoconazole, and cyclosporine A), multidrug resistant-associated protein 2 (MRP2; MK-571, probenecid, indomethacin, and p-aminohipuric acid), and breast cancer resistance protein (BCRP; Fumitremorgin C) inhibitors on nizatidine bidirectional transport were examined. Nizatidine exhibited 7.7-fold higher BL-AP than AP-BL Caco-2 permeability, indicative of net mucosal secretion. All P-gp inhibitors investigated displayed concentration-dependent inhibition on nizatidine secretion in both directions. The IC(50) of verapamil on nizatidine P-gp secretion was 1.2 x 10(-2) mM. In the absence of inhibitors, nizatidine displayed concentration-dependent secretion, with one saturable (J(max) = 5.7 x 10(-3) nmol cm(-2) s(-1) and K(m) = 2.2 mM) and one nonsaturable component (K(d) = 7 x 10(-4) microL cm(-2) s(-1)). Under complete P-gp inhibition, nizatidine exhibited linear secretory flux, with a slope similar to the nonsaturable component. V(max) and K(m) estimated for nizatidine P-gp-mediated secretion were 4 x 10(-3) nmol cm(-2) s(-1) and 1.2 mM, respectively. No effect was obtained with the MRP2 or the BCRP inhibitors. Being a drug commonly used in pediatrics, adults, and elderly, nizatidine susceptibility to efflux transport by P-gp revealed in this paper may be of significance in its absorption, distribution, and clearance, as well as possible drug-drug interactions.

Segmental dependent transport of low permeability compounds along the small intestine due to P-glycoprotein: the role of efflux transport in the oral absorption of BCS class III drugs

The purpose of this study was to investigate the role of P-gp efflux in the in vivo intestinal absorption process of BCS class III P-gp substrates, i.e. high-solubility low-permeability drugs. The in vivo permeability of two H (2)-antagonists, cimetidine and famotidine, was determined by the single-pass intestinal perfusion model in different regions of the rat small intestine, in the presence or absence of the P-gp inhibitor verapamil. The apical to basolateral (AP-BL) and the BL-AP transport of the compounds in the presence or absence of various efflux transporters inhibitors (verapamil, erythromycin, quinidine, MK-571 and fumitremorgin C) was investigated across Caco-2 cell monolayers. P-gp expression levels in the different intestinal segments were confirmed by immunoblotting. Cimetidine and famotidine exhibited segmental dependent permeability through the gut wall, with decreased P(eff) in the distal ileum in comparison to the proximal regions of the intestine. Coperfusion of verapamil with the drugs significantly increased the permeability in the ileum, while no significant change in the jejunal permeability was observed. Both drugs exhibited significantly greater BL-AP than AP-BL Caco-2 permeability, indicative of net mucosal secretion. Concentration dependent decrease of this secretion was obtained by the P-gp inhibitors verapamil, erythromycin and quinidine, while no effect was evident by the MRP2 inhibitor MK-571 and the BCRP inhibitor FTC, indicating that P-gp is the transporter mediates the intestinal efflux of cimetidine and famotidine. P-gp levels throughout the intestine were inversely related to the in vivo permeability of the drugs from the different segments. The data demonstrate that for these high-solubility low-permeability P-gp substrates, P-gp limits in vivo intestinal absorption in the distal segments of the small intestine; however P-gp plays a minimal role in the proximal intestinal segments due to significant lower P-gp expression levels in this region.

Excretion of biliary compounds during intrauterine life

In adults, the hepatobiliary system, together with the kidney, constitute the main routes for the elimination of several endogenous and xenobiotic compounds into bile and urine, respectively. However, during intrauterine life the biliary route of excretion for cholephilic compounds, such as bile acids and biliary pigments, is very poor. Although very early in pregnancy the fetal liver produces bile acids, bilirubin and biliverdin, these compounds cannot be efficiently eliminated by the fetal hepatobiliary system, owing to the immaturity of the excretory machinery in the fetal liver. Therefore, the potentially harmful accumulation of cholephilic compounds in the fetus is prevented by their elimination across the placenta. Owing to the presence of detoxifying enzymes and specific transport systems at different locations of the placental barrier, such as the endothelial cells of chorionic vessels and trophoblast cells, this organ plays an important role in the hepatobiliary-like function during intrauterine life. The relevance of this excretory function in normal fetal physiology is evident in situations where high concentrations of biliary compounds are accumulated in the mother. This may result in oxidative stress and apoptosis, mainly in the placenta and fetal liver, which might affect normal fetal development and challenge the fate of the pregnancy. The present article reviews current knowledge of the mechanisms underlying the hepatobiliary function of the fetal-placental unit and the repercussions of several pathological conditions on this tandem.

Delivery of peptide and protein drugs over the blood-brain barrier

Peptide and protein (P/P) drugs have been identified as showing great promises for the treatment of various neurodegenerative diseases. A major challenge in this regard, however, is the delivery of P/P drugs over the blood-brain barrier (BBB). Intense research over the last 25 years has enabled a better understanding of the cellular and molecular transport mechanisms at the BBB, and several strategies for enhanced P/P drug delivery over the BBB have been developed and tested in preclinical and clinical-experimental research. Among them, technology-based approaches (comprising functionalized nanocarriers and liposomes) and pharmacological strategies (such as the use of carrier systems and chimeric peptide technology) appear to be the most promising ones. This review combines a comprehensive overview on the current understanding of the transport mechanisms at the BBB with promising selected strategies published so far that can be applied to facilitate enhanced P/P drug delivery over the BBB.

Cloning and pharmacological characterization of a novel gene encoding human nucleoside transporter 1 (hNT1) from a human breast cancer cDNA library

AIMS

We isolated a novel gene encoding human nucleoside transporter 1 (hNT1), from a human breast cancer cDNA library.

MAIN METHODS

A nondirectional cDNA library was screened by an EST clone (GenBanktrade mark/EMBL/DDBJ: BU944345). A Xenopus laevis oocyte expression system was used for functional characterization. Membrane localization in the human breast was determined by immunohistochemistry.

KEY FINDINGS

Isolated hNT1 cDNA consisted of 246 base pairs that encoded an 82-amino acid protein. By RT-PCR analysis, hNT1 mRNA was strongly detected in the breast cancer tissues. When expressed in X. oocytes, hNT1 mediated the high affinity transport of [(3)H]5-fluorouracil (5-FU) with a K(m) value of 69.2+/-24.5 nM in time- and pH-dependent, and Na(+)-independent manners. A cis-inhibition experiment revealed that hNT1 mediated transport of [(3)H]5-FU is strongly inhibited by various nucleosides such as pyrimidine, uracil, uridine, guanosine, inosine, thymidine, adenosine, cytidine and purine suggesting that hNT1 may be involved in the trans epithelial transport of these endogenous substrates. Immunohistochemical analysis revealed that the hNT1 protein is localized in the lactiferous duct epithelium.

SIGNIFICANCE

Our present results indicate that a newly isolated cDNA clone, hNT1, is a key molecule for the breast handling of 5-FU in humans.

Polymorphism of the MDR1/ABCB1 C3435T drug-transporter and resistance to anticonvulsant drugs: a meta-analysis

BACKGROUND

Approximately one-third of patients with epilepsy patients have recurrent seizures despite therapy. It has been suggested that therapeutic failure is associated with high expression of the multidrug efflux ABCB1 (MDR1) drug-transporter; specifically, that patients with the 3435CC genotype have higher efflux of anticonvulsants out of brain tissue, with correspondingly lower concentrations in the central nervous system.

METHODS

We conducted a meta-analysis to examine the association between MDR1 polymorphisms and the response to anticonvulsants. We included all published studies until September 2007, in which patients with responsive and unresponsive seizure disorders underwent genotyping for ABCB1 C3435T. Individual and summary odds ratios were calculated using a random effects model. A secondary analysis was also performed, stratifying the studies by their ethnic distribution to account for genetic heterogeneity. We also performed a cumulative analysis by date of publication for the included studies using a random effects model.

RESULTS

We identified 11 case-control studies involving 3,371 patients (1,646 patients with drug-resistant epilepsy and 1,725 controls). We identified no significant association between anticonvulsant drug resistance and MDR1 polymorphism [odds ratio 1.15; 95% confidence interval (CI) 0.78-1.70; p = 0.48). Subanalysis of studies according to ethnicity yielded similar findings [European cohort: OR = 1.31; 95% CI 0.89-1.94, p = 0.18; Asian cohort: OR = 0.99; 95% CI 0.51-1.89, p = 0.96).

CONCLUSIONS

We found no association between ABCB1 genotype and response to anticonvulsant drugs. At the present time, genetic typing for MDR1 polymorphism is not warranted for patients with drug-resistant epilepsy.

Evaluation of the selected barrier properties of retinal pigment epithelial cell line ARPE-19 for an in-vitro blood-brain barrier model

In-vitro models that maintain complex transport mechanisms and structural properties associated with the blood-brain barrier in vivo would be useful in drug permeability and neurotoxicological studies. To evaluate the suitability of a human retinal pigment epithelial cell line for a blood-brain barrier model, we have compared the barrier properties of the human retinal pigment epithelial cell line ARPE-19, the human colonic adenocarcinoma cell line Caco-2, and primary porcine microvessel endothelial cells. The tight junction proteins occludin and ZO-1 were stained immunocytochemically. The paracellular ionic permeability was evaluated by measuring the trans-epithelial or trans-endothelial electric resistance. To evaluate the active transport mechanisms, the existence and the activity of the efflux transporters, P-glycoprotein and multidrug resistance-associated proteins, were studied. All the cell types in this study stained positively for occludin and ZO-1. However, the trans-endothelial electric resistance of ARPE-19 cells was low compared with that of primary porcine microvessel endothelial cell and Caco-2 cells. In addition, both the P-glycoprotein expression and its activity in ARPE-19 cells were low. In conclusion, the barrier properties of the human ARPE-19 cell line were not satisfactory for a blood-brain barrier model. For future studies, it is important to develop a human brain endothelial cell line with expression of the complex in-vivo properties of the blood-brain barrier.

Role of ATP-binding cassette drug transporters in the intestinal absorption of tanshinone IIB, one of the major active diterpenoids from the root ofSalvia miltiorrhiza

There is an increasing use of herbal medicines worldwide, and the extracts from the root of Salvia miltiorrhiza are widely used in the treatment of angina and stroke. In this study, we investigated the mechanism for the intestinal absorption of tanshinone IIB (TSB), a major constituent of S. miltiorrhiza. The oral bioavailability of TSB was about 3% in rats with less proportional increase in its maximum plasma concentration (C(max)) and area under the plasma concentration-time curve (AUC) with increasing dosage. The time to C(max) (T(max)) was prolonged at higher oral dosage. In a single pass rat intestinal perfusion model, the permeability coefficients (P(app)) based on TSB disappearance from the lumen (P(lumen)) were 6.2- to 7.2-fold higher (p < 0.01) than those based on drug appearance in mesenteric venous blood (P(blood)). The uptake and efflux of TSB in Caco-2 cells were also significantly altered in the presence of an inhibitor for P-glycoprotein (PgP) or for multi-drug resistance associated protein (MRP1/2). TSB transport from the apical (AP) to basolateral (BL) side in Caco-2 monolayers was 3.3- to 5.7-fold lower than that from BL to AP side, but this polarized transport was attenuated by co-incubation of PgP or MRP1/2 inhibitors. The P(app) values of TSB in the BL-AP direction were significantly higher in MDCKII cells over-expressing MDR1 or MRP1, but not in cells over-expressing MRP2-5, as compared with the wild-type cells. The plasma AUC(0-24hr) in mdr1a and mrp1 gene-deficient mice was 10.2- to 1.7-fold higher than that in the wild-type mice. Furthermore, TSB significantly inhibited the uptake of digoxin and vinblastine in membrane vesicles containing PgP or MRP1. TSB also moderately stimulated PgP ATPase activity. Taken collectively, our findings indicate that TSB is a substrate for PgP and MRP1 and that drug resistance to TSB therapy and drug interactions may occur through PgP and MRP1 modulation.

A novel screening strategy to identify ABCB1 substrates and inhibitors

We tested the hypothesis whether data on ABCB1 ATPase activity and passive permeability can be used in combination to identify ABCB1 substrates and inhibitors. We determined passive permeability using an artificial membrane permeability assay (HDM-PAMPA) and ABCB1 function, i.e., vanadate-sensitive ATPase activity for a training set (40 INN drugs) and a validation set (26 development compounds). In parallel experiments, we determined ABCB1 function, i.e., vectorial transport in a Caco-2 cell monolayer, and ABCB1 inhibition, i.e., calcein AM extrusion out of K562-MDR cells, to cross-validate the results with cellular assays. We found that compounds that did not modulate ABCB1-ATPase did also not affect calcein AM extrusion and were not actively transported by ABCB1 in Caco-2 cell monolayers. The results corroborated the effect of passive permeability as an important covariate of active transport: active transport in Caco-2 monolayer was only apparent for compounds showing low passive permeability (<5.0 cmx10(-6)/s) in the HDM-PAMPA assay whereas compounds with high passive permeability (>50 cmx10(-6)/s) were shown to inhibit calcein AM efflux with IC50 values close to their respective Km value obtained for ABCB1-ATPase. The use of HDM-PAMPA in combination with ABCB1-ATPase offers a simple, inexpensive experimental approach capable of identifying ABCB1 inhibitors as well as transported substrates.

MDR1 genotypes do not influence the absorption of a single oral dose of 600 mg valacyclovir in healthy Chinese Han ethnic males

AIMS

To investigate the influence of three single nucleotide polymorphisms (SNPs) in exon 12 (C1236T), exon 21 (G2677T/A) and exon 26 (C3435T) of MDR1 gene on the absorption of valacyclovir after a single oral administration in the Chinese Han ethnic population.

METHODS

Two hundred healthy Chinese subjects were genotyped for the SNPs of C1236T, G2677T/A and C3435T in the MDR1 gene using allele-specific polymerase chain reaction. Linkage disequilibrium (LD) was analysed. Twenty-four subjects derived from a large random sample (n = 200) received a single oral dose of 600 mg valacyclovir. Plasma concentrations of acyclovir were determined up to 14 h after administration to obtain a pharmacokinetic profile.

RESULTS

LD existed between G2677T/A in exon 21 and C3435T in exon 26 (P < 0.001), between C1236T in exon 12 and C3435T (P < 0.001), but not between C1236T and G2677T/A (P > 0.05). C(max), AUC(0-1.5 h) and AUC(0-infinity) were used as indices of valacyclovir absorption. AUC(0-infinity) for the 2677TA genotype was 17.45 +/- 2.40 microg x h/ml, which was much higher compared with the 2677GG, GA and TT genotypes of 10.44 +/- 1.00, 11.84 +/- 2.83, 11.34 +/- 2.32 microg x h/ml, respectively (P < 0.05). Similarly, a statistically significant difference of AUC(0-infinity) was also observed for different linked genotypes at position 2677 vs. 3435, and 1236 vs. 3435 (P < 0.05). However, there was no significant difference in valacyclovir absorptive pharmacokinetics between carriers and noncarriers of different haplotypes (P > 0.05).

CONCLUSIONS

Three SNPs of MDR1 gene did not influence the absorption of a single oral dose of 600 mg valacyclovir in healthy Chinese Han ethnic subjects.

MDR1 gene polymorphism: therapeutic response to paroxetine among patients with major depression

The multidrug resistance transporter, P-glycoprotein (P-gp), encoded by polymorphic MDR1 (ABCB1) gene, is involved in efflux transport of several antidepressants and acts as a barrier to different exogenous noxa in the blood-brain barrier. MDR1 gene belongs to the best understood mediators of drug resistance. Different polymorphisms in MDR1 have been found to be connected with P-gp expression and function. The aims of the study were to investigate the potential influence of MDR1 polymorphisms, exon 26 C3435T and exon 21 G2677T/A, on treatment response to paroxetine (20 mg/day) in patients with major depression. To assess and evaluate therapeutic response to paroxetine, all patients were rated weekly using the HAMD-17 scale. Responders were defined as subjects with a decrease in HAMD scale by >or=50% at week 6 of treatment. The study population included 127 patients with major depression (diagnosed by Structured Clinical Interview for DSM-IV disorders). Our results indicated that MDR1 variants G2677T and C3435T are not associated with therapeutic response to paroxetine in patients with major depressive disorder. The associations between paroxetine and P-glycoprotein still need to be clarified.