Role of transport proteins in drug absorption, distribution and excretion

The association of ABCB1 polymorphisms and elevated serum digitoxin concentrations in geriatric patients

OBJECTIVE

Digitoxin is a known substrate of the efflux pump P-glycoprotein (gene name: ABCB1). P-glycoprotein expression was shown to be modulated by single nucleotide polymorphisms in the ABCB1 gene, but it remains unclear whether these polymorphisms influence digitoxin blood levels. Our objective was to examine the association of ABCB1 C3435T genotype and elevated serum digitoxin concentrations (SDC) in a cohort of 77 geriatric patients consecutively admitted to a geriatric department over a 12-month period.

METHODS

The impact of ABCB1 3435 CC, CT, and TT genotypes on SDC and SDC normalized for daily digitoxin dosage and body weight was assessed by multivariate regression analysis.

RESULTS

Among participants, 18 (23%) had the CC, 36 (47%) the CT, and 23 (30%) the TT genotype. Adjusting for relevant covariates, no significant association of ABCB1 C3435T genotype and SDC or normalized SDC was detected. Mean SDC was 22.4 ng/ml (95% CI 18.9-25.9) for the TT, 21.8 ng/ml (95% CI 18.1-25.5) for the CT, and 25.7 ng/ml (95% CI 20.6-30.8) for the CC genotype. The means for normalized SDC were 5.2 kg.l(-1) (95% CI 4.3-6.1) for the TT, 6.1 kg.l(-1) (95% CI 4.7-7.5) for the CT, and 6.2 kg.l(-1) (95% CI 4.6-7.7) for the CC genotype.

CONCLUSION

In this sample of frail geriatric patients, the impact of ABCB1 C3435T genotype on serum digitoxin concentration was not of major relevance. Regular monitoring of digitoxin blood levels and surveillance of appropriate drug use remain the best ways to prevent digitoxin intoxications in the elderly.

How do predators cope with chemically defended foods?

Many prey species (including plants) deter predators with defensive chemicals. These defensive chemicals act by rendering the prey's tissues noxious, toxic, or both. Here, I explore how predators cope with the presence of these chemicals in their diet. First, I describe the chemosensory mechanisms by which predators (including herbivores) detect defensive chemicals. Second, I review the mechanisms by which predators either avoid or tolerate defensive chemicals in prey. Third, I examine how effectively free-ranging predators can overcome the chemical defenses of prey. The available evidence indicates that predators have mixed success overcoming these defenses. This conclusion is based on reports of free-ranging predators rejecting unpalatable but harmless prey, or voluntarily ingesting toxic prey.

Functional analysis of phenolsulfonphthalein transport system in Long-Evans Cinnamon rats

It has been reported that the transport function for organic anions on the kidney is maintained in multidrug resistance-associated protein 2 (Mrp2)-deficient rats. Different from Mrp2-deficient rats, Long-Evans Cinnamon (LEC) rats have impaired urinary excretion of Mrp2-substrate, phenolsulfonphthalein (PSP). PSP is transported by the potential-sensitive urate transport system in rat brush-border membranes. We analyzed the function of PSP transport system in LEC rats. Unlike Long-Evans Agouti (LEA) rats, the initial uptake of PSP and urate into the renal brush-border membrane vesicles of LEC rats were not significantly enhanced in the presence of positive intravesicular potential, suggesting that the potential-sensitive urate transport system is impaired in LEC rats. LEC rats should be useful for elucidating the potential-sensitive urate transport system in rats at the molecular level.

Induction of hepatobiliary efflux transporters in acetaminophen-induced acute liver failure cases

Alterations in transporter expression may represent a compensatory mechanism of damaged hepatocytes to reduce accumulation of potentially toxic compounds. The present study was conducted to investigate the expression of hepatobiliary efflux transporters in livers from patients after toxic acetaminophen (APAP) ingestion, with livers from patients with primary biliary cirrhosis (PBC) serving as positive controls. mRNA and protein expression of multidrug resistance-associated protein (MRP) 1-6, multidrug resistance protein (MDR) 1-3/P-glycoprotein (P-gp), and breast cancer resistance protein (BCRP) in normal (n = 6), APAP overdose (n = 5), and PBC (n = 6) human liver samples were determined by branched DNA and Western blot analysis, respectively. Double immunohistochemical staining of P-gp and proliferating cell nuclear antigen (PCNA), a marker of proliferation, was performed on paraffin-embedded tissue sections. Compared with normal liver specimens, MRP1 and MRP4 mRNA levels were elevated after APAP overdose and in PBC. Up-regulation of MRP5, MDR1, and BCRP mRNA occurred in PBC livers. Protein levels of MRP4, MRP5, BCRP, and P-gp were increased in both disease states, with MRP1 and MRP3 protein also being induced in PBC. Increased P-gp protein was confirmed immunohistochemically and was found to localize to areas of PCNA-positive hepatocytes, which were detected in APAP overdose and PBC livers. The findings from this study demonstrate that hepatic efflux transporter expression is up-regulated in cases of APAP-induced liver failure and PBC. This adaptation may aid in reducing retention of byproducts of cellular injury and bile constituents within hepatocytes. The close proximity of P-gp and PCNA-positive hepatocytes during liver injury suggests that along with cell regeneration, increased efflux transporter expression is a critical response to hepatic damage to protect the liver from additional insult.

Small P-gp modulating molecules: SAR studies on tetrahydroisoquinoline derivatives

The development of small molecules as P-gp modulating agents and SAR studies on these ligands represented the aim of the present work. A series of 6,7-dimethoxytetrahydroisoquinoline derivatives was prepared and their ability to inhibit P-gp activity has been evaluated. The basic nucleus of these compounds, common to the best P-gp inhibitors such as Tariquidar and Elacridar, has been functionalized with no-basic moiety from our studied sigma receptor ligands displaying potent P-gp inhibition. The best results were obtained for compounds 3c and 3a (EC(50)=1.64 and 4.86 microM, respectively) and these results were remarkable because Elacridar showed in the same biological evaluation similar inhibitory activity (EC(50)=2 microM). SAR studies displayed that the removal of double bond on the spacer or its shifting into tetraline ring decreased the P-gp inhibiting activity. Moreover, the P-gp inhibition mechanism of tested compounds was investigated by three selected biological experiments. The results displayed that only compound 3c was P-gp inhibitor as Elacridar, while compound 3a and reference compounds Cyclosporin A and Verapamil modulated P-gp activity saturating the efflux pump as substrates. Flow cytometry studies carried out in Doxorubicin resistant breast cancer cell line (MCF7/Adr) confirmed that compound 3c increased Doxorubicin cell accumulation 5.7-fold. In addition, in MCF7/Adr, antiproliferative effect of 5 microM Doxorubicin shifted from 5% to 95% when co-administered with compound 3c (20 microM). The present study suggested a new class of small molecules displaying P-gp inhibitor activity differing from reference compounds Elacridar and Tariquidar for a simplified, and in the meantime, efficacious no-basic moiety.

Hesperetin attenuates the highly reducing sugar-triggered inhibition of osteoblast differentiation

Diabetic bone disease is associated with increased oxidative damage and 2-deoxy-D: -ribose (dRib) is used to induce oxidative damage similar to that observed in diabetics. To determine if hesperetin (3',5,7-trihydroxy-4-methoxyflavanone) could influence osteoblast dysfunction induced by dRib, osteoblastic MC3T3-E1 cells were treated with dRib and hesperetin. Then, markers of osteoblast function and oxidative damage were examined. Hesperetin (10(-7)-10(-5) M) caused a significant elevation of alkaline phosphatase (ALP) activity, collagen content, and total antioxidant potential of MC3T3-E1 cells in the presence of 20 mM dRib (p < 0.05). Moreover, hesperetin (10(-7) M) decreased cellular protein carbonyl (PCO), advanced oxidation protein products (AOPP), and malondialdehyde (MDA) contents of osteoblastic MC3T3-E1 cells in the presence of 20 mM dRib. These results demonstrate that hesperetin attenuates dRib-induced damage, suggesting that hesperetin may be a useful dietary supplement for minimizing oxidative injury in diabetes related bone diseases.

Impact of physicochemical and structural properties on the pharmacokinetics of a series of alpha1L-adrenoceptor antagonists

A rational drug discovery process was initiated to design a potent and prostate-selective alpha1(L)-adrenoceptor antagonist with pharmacokinetic properties suitable for once a day administration after oral dosing, for the treatment of benign prostatic hyperplasia. Two series of compounds based on a quinoline or quinazoline template were identified with appropriate pharmacology. A series of high molecular weight cations with high hydrogen-bonding potential had extensive in vivo clearance, despite demonstrating metabolic stability. Studies in the isolated perfused rat liver and fresh rat hepatocytes indicated that active transport protein-mediated hepatobiliary elimination is an efficient clearance process for these compounds. A reduction in molecular weight and hydrogen-bonding potential resulted in a second series of compounds with in vivo hepatic clearance predictable from in vitro metabolic clearance. Initially, lipophilicity was reduced within this second series to reduce metabolic clearance and increase elimination half-life. However, this strategy also resulted in a concomitant reduction in volume of distribution and a negligible effect on prolonging half-life. An alternative strategy was to increase the intrinsic metabolic stability of the molecule by careful structural modifications while maintaining lipophilicity. Replacement of the metabolically vulnerable morpholine side chain resulted in identification of UK-338,003, (N-[2-(4-amino-6,7-dimethoxy-5-pyridin-2-yl-quinazolin-2-yl)-1,2,3,4-tetrahydro-isoquinolin-5-yl]-methanesulfonamide), which fulfilled the objectives of the discovery program with suitable pharmacology (human prostate alpha1(L) pA(2) of 9.2 with 25-fold selectivity over rat aorta alpha1(D)) and sufficiently long elimination half-life in human volunteers (11-17 h) for once a day administration.

Effect of itraconazole on pharmacokinetics of paroxetine: the role of gut transporters

A recent in vitro study has shown that paroxetine is a substrate of P-glycoprotein. However, there was no in vivo information indicating the involvement of P-glycoprotein on the pharmacokinetics of paroxetine. The aim of this study was to examine the effects of itraconazole, a P-glycoprotein inhibitor, on the pharmacokinetics of paroxetine. Two 6 day courses of either 200 mg itraconazole daily or placebo with at least a 4 week washout period were conducted. Thirteen volunteers took a single oral 20 mg dose of paroxetine on day 6 of both courses. Plasma concentrations of paroxetine were monitored up to 48 hours after the dosing. Compared with placebo, itraconazole treatment significantly increased the peak plasma concentration (Cmax) of paroxetine by 1.3 fold (6.7 +/- 2.5 versus 9.0 +/- 3.3 ng/mL, P < 0.05) and the area under the plasma concentration-time curve from zero to 48 hours [AUC (0-48)] of paroxetine by 1.5 fold (137 +/- 73 versus 199 +/- 91 ng*h/mL, P < 0.01). Although elimination half-life differed significantly (16.1 +/- 3.4 versus 18.8 +/- 5.9 hours, P < 0.05), the alteration was small (1.1 fold). The present study demonstrated that the bioavailability of paroxetine was increased by itraconazole, suggesting a possible involvement of P-glycoprotein in the pharmacokinetics of paroxetine.

Scintillation proximity assay for measuring uptake by the human drug transporters hOCT1, hOAT3, and hOATP1B1

Increasing evidence suggests a key role of transport proteins in the pharmacokinetics of drugs. Within the solute carrier (SLC) family, various organic cation transporters (OCTs), organic anion transporters (OATs), and organic anion transporting polypeptides (OATPs) that interact with drug molecules have been identified. Traditionally, cellular uptake assays require multiple steps and provide low experimental throughput. We here demonstrate the use of a scintillation proximity approach to detect substrate uptake by human drug transporters in real time. HEK293 cells stably transfected with hOCT1, hOATP1B1, or hOAT3 were grown directly in Cytostar-T scintillating microplates. Confluent cell monolayers were incubated with 14C- or 3H-labeled transporter substrates. Cellular uptake brings the radioisotopes into proximity with the scintillation plate base. The resulting light emission signals were recorded on-line in a microplate scintillation counter. Results show time- and concentration-dependent uptake of 14C-tetraethylammonium, 3H-methylphenylpyridinium (HEK-hOCT1), 3H-estradiol-17beta-D-glucuronide (HEK-hOATP1B1), and 3H-estrone-3-sulfate (HEK-hOAT3), while no respective uptake was detected in empty vector-transfected cells. Km of 14C-tetraethylammonium and 3H-estrone-3-sulfate uptake and hOAT3 inhibition by ibuprofen and furosemide were similar to conventional dish uptake studies. The scintillation proximity approach is high throughput, amenable to automation and allows for identification of SLC transporter substrates and inhibitors in a convenient and reliable fashion, suggesting its broad applicability in drug discovery.

Treatment heterogeneity in asthma: genetics of response to leukotriene modifiers

Despite advances in treatment, asthma continues to be a significant health and economic burden. Although asthma cannot be cured, several drugs, including beta2 agonists, corticosteroids, and leukotriene (LT) modifiers, are well tolerated and effective in minimizing symptoms, improving lung function, and preventing exacerbations. However, inter-patient variability in response to asthma drugs limits their effectiveness. It has been estimated that 60-80% of this inter-patient variability may be attributable to genetic variation. LT modifiers, in particular, have been associated with heterogeneity in response. These drugs exert their action by inhibiting the activity of cysteinyl leukotrienes (CysLTs), which are potent bronchoconstrictors and pro-inflammatory agents. Two classes of LT modifiers are 5-lipoxygenase (ALOX5) inhibitors (zileuton) and leukotriene receptor antagonists (LTRAs) [montelukast, pranlukast, and zarfirlukast]. LT modifiers can be used as alternatives to low-dose inhaled corticosteroids (ICS) in mild persistent asthma, as add-on therapy to low- to medium-dose ICS in moderate persistent asthma, and as add-on to high-dose ICS and a long-acting ss2 agonist in severe persistent asthma. At least six genes encode key proteins in the LT pathway: arachidonate 5-lipoxygenase (ALOX5), ALOX5 activating protein (ALOX5AP [FLAP]), leukotriene A4 hydrolase (LTA4H), LTC4 synthase (LTC4S), the ATP-binding cassette family member ABCC1 (multidrug resistance protein 1 [MRP1]), and cysteinyl leukotriene receptor 1 (CYSLTR1). Studies have reported that genetic variation in ALOX5, LTA4H, LTC4S, and ABCC1 influences response to LT modifiers. Plasma concentrations of LTRAs vary considerably among patients. Physio-chemical characteristics make it likely that membrane efflux and uptake transporters mediate the absorption of LTRAs into the systemic circulation following oral administration. Genes that encode efflux and uptake transport proteins harbor many variants that could influence the pharmacokinetics, and particularly the bioavailability, of LTRAs, and could contribute to heterogeneity in response. In the future, large, well designed clinical trials studying the pharmacogenetics of LT modifiers in diverse populations are warranted to determine whether a genetic signature can be developed that will accurately predict which patients will respond.

Expression, localisation and activity of ATP binding cassette (ABC) family of drug transporters in human amnion membranes

Placental ATP-binding cassette (ABC) transporters limit fetal exposure to xenobiotics by regulating transplacental passage into the fetal circulation; their expression and function in fetal membranes, however, has not been studied. In the present study the expression, localisation and function of ABC transporters in human amnion was examined to explore their potential role in modulating amniotic fluid drug disposition in pregnancy. Single-assay oligo-microarrays were used to profile amnion gene expression, and drug transporters expressed at significant levels were identified and selected for further studies. The expression of ABCG2/breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins (MRP) 1 (ABCC1), 2 (ABCC2) and 5 (ABCC5) was detected on the arrays, and verified by RT-PCR and immunoblotting. On confocal microscopy of fetal membrane cryosections, MRP1 and MRP5 were immunolocalised to both apical and basolateral surfaces of the amniotic epithelium, while MRP2 was expressed at low levels only in the apical membrane. BCRP in contrast showed cytoplasmic staining throughout the amniotic epithelium. In addition to the amnion, MRP1 and BCRP immunostaining was observed in the chorion and the decidua. Cell accumulation studies using selective MRP and BCRP inhibitors showed the transporters to be functionally active in amnion epithelial monolayer cultures. In contrast, transwell transport studies using intact amnion membranes did not show significant vectorial transport. These findings identify the amnion as a novel site of ABC drug transporter expression. Functional studies indicate that they may act primarily to prevent cellular xenobiotic accumulation, rather than to confer fetal protection through reduced accumulation in amniotic fluid.

Upregulation of Brain Expression of P-Glycoprotein in MRP2-deficient TR-Rats Resembles Seizure-induced Up-regulation of This Drug Efflux Transporter in Normal Rats

PURPOSE

The multidrug resistance protein 2 (MRP2) is a drug efflux transporter that is expressed predominantly at the apical domain of hepatocytes but seems also to be expressed at the apical membrane of brain capillary endothelial cells that form the blood-brain barrier (BBB). MRP2 is absent in the transport-deficient (TR(-)) Wistar rat mutant, so that this rat strain was very helpful in defining substrates of MRP2 by comparing tissue concentrations or functional activities of compounds in MRP2-deficient rats with those in transport-competent Wistar rats. By using this strategy to study the involvement of MRP2 in brain access of antiepileptic drugs (AEDs), we recently reported that phenytoin is a substrate for MRP2 in the BBB. However, one drawback of such studies in genetically deficient rats is the fact that compensatory changes with upregulation of other transporters can occur. This prompted us to study the brain expression of P-glycoprotein (Pgp), a major drug efflux transporter in many tissues, including the BBB, in TR(-) rats compared with nonmutant (wild-type) Wistar rats.

METHODS

The expression of MRP2 and Pgp in brain and liver sections of TR(-) rats and normal Wistar rats was determined with immunohistochemistry, by using a novel, highly selective monoclonal MRP2 antibody and the monoclonal Pgp antibody C219, respectively.

RESULTS

Immunofluorescence staining with the MRP2 antibody was found to label a high number of microvessels throughout the brain in normal Wistar rats, whereas such labeling was absent in TR(-) rats. TR(-) rats exhibited a significant up-regulation of Pgp in brain capillary endothelial cells compared with wild-type controls. No such obvious upregulation of Pgp was observed in liver sections. A comparable overexpression of Pgp in the BBB was obtained after pilocarpine-induced seizures in wild-type Wistar rats. Experiments with systemic administration of the Pgp substrate phenobarbital and the selective Pgp inhibitor tariquidar in TR(-) rats substantiated that Pgp is functional and compensates for the lack of MRP2 in the BBB.

CONCLUSIONS

The data on TR(-) rats indicate that Pgp plays an important role in the compensation of MRP2 deficiency in the BBB. Because such a compensatory mechanism most likely occurs to reduce injury to the brain from cytotoxic compounds, the present data substantiate the concept that MRP2 performs a protective role in the BBB. Furthermore, our data suggest that TR(-) rats are an interesting tool to study consequences of overexpression of Pgp in the BBB on access of drugs in the brain, without the need of inducing seizures or other Pgp-enhancing events for this purpose.

Risperidone and paliperidone inhibit p-glycoprotein activity in vitro

Risperidone (RSP) and its major active metabolite, 9-hydroxy-risperidone (paliperidone, PALI), are substrates of the drug transporter P-glycoprotein (P-gp). The goal of this study was to examine the in vitro effects of RSP and PALI on P-gp-mediated transport. The intracellular accumulation of rhodamine123 (Rh123) and doxorubicin (DOX) were examined in LLC-PK1/MDR1 cells to evaluate P-gp inhibition by RSP and PALI. Both compounds significantly increased the intracellular accumulation of Rh123 and DOX in a concentration-dependent manner. The IC(50) values of RSP for inhibiting P-gp-mediated transport of Rh123 and DOX were 63.26 and 15.78 microM, respectively, whereas the IC(50) values of PALI were >100 microM, indicating that PALI is a less potent P-gp inhibitor. Caco-2 and primary cultured rat brain microvessel endothelial cells (RBMECs) were utilized to investigate the possible influence of RSP on intestinal absorption and blood-brain barrier (BBB) transport of coadministered drugs that are P-gp substrates. RSP, 1-50 microM, significantly enhanced the intracellular accumulation of Rh123 in Caco-2 cells by inhibiting P-gp activity with an IC(50) value of 5.87 microM. Following exposure to 10 microM RSP, the apparent permeability coefficient of Rh123 across Caco-2 and RBMECs monolayers was increased to 2.02 and 2.63-fold in the apical to basolateral direction, but decreased to 0.37 and 0.21-fold in the basolateral to apical direction, respectively. These data suggest that RSP and PALI, to a lesser extent, have a potential to influence the pharmacokinetics and hence the pharmacodynamics of coadministered drugs via inhibition of P-gp-mediated transport. However, no human data exist that address this issue. In particular, RSP may interact with its own active metabolite PALI by promoting its brain concentration through inhibiting P-gp-mediated efflux of PALI across endothelial cells of the BBB.

Effect of cell differentiation and passage number on the expression of efflux proteins in wild type and vinblastine-induced Caco-2 cell lines

The mRNA level expression of MDR1, MRP1-6, BCRP and CYP3A4 was determined by quantitative PCR in wild type (Caco-2WT) and vinblastine-treated (Caco-2VBL) Caco-2 cells at different passage levels (32-53). Differentiation increased the mRNA levels of MDR1, BCRP and all the MRPs except MRP4. Corresponding mRNA levels were observed in Caco-2WT and Caco-2VBL, except that the expression of MRD1 was higher in Caco-2VBL than in Caco-2WT cells. CYP3A4 was barely detected in either cell line. MDR1 functionality was studied using rhodamine123 and verapamil as a substrate-inhibitor pair. Corresponding to the observed differences in mRNA levels, MDR1 activity was higher in the Caco-2VBL cells. In Caco-2WT, MDR1 functionality was elevated at low passage numbers (32-35) compared to higher ones (49-53). Verapamil inhibited MDR1 efflux except at higher passage Caco-2WT cells, where no MDR1 activity could be observed. The results support the use of Caco-2VBL cells in MDR1 screening. The functional expression is higher than in Caco-2WT and remains consistent across the studied passages without major differences in mRNA levels of other efflux proteins. As both the passage number and the level of cell differentiation affect the expression profile of efflux proteins, short-term cell growth protocols should be evaluated accordingly.

Modeling methadone pharmacokinetics in rats in presence of P-glycoprotein inhibitor valspodar

PURPOSE

To quantify the in vivo role of P-glycoprotein (P-gp) in the pharmacokinetics of methadone after intravenous and oral administration, using valspodar as a P-gp inhibitor.

MATERIALS AND METHODS

Methadone plasma concentrations after intravenous (0.35 mg/kg) and oral (6 mg/kg) administration were analyzed, in absence and presence of valspodar, using nonlinear mixed effects modeling (NONMEM V). Non-parametric bootstrap analysis and posterior predictive check were employed as model evaluation techniques.

RESULTS

The pharmacokinetics of methadone in the rat was successfully modeled using a two-compartmental model with a linear elimination from the central compartment and a first-order absorption process with lag time. Valspodar increased methadone F by 122% (95%CI: 34-269%) and decreased the V ( c ) and V ( p ) by 35% (95%CI: 16-49%) and 81% (95%CI: 63-93%), respectively. No effect of valspodar on other pharmacokinetic parameters was discernible. The non-parametric bootstrap analysis confirmed the absence of bias on the parameter estimates, and visual predictive check evidence the adequacy of the model to reproduce the observed time course of methadone plasma concentrations.

CONCLUSION

Valspodar increased methadone's bioavailability as consequence of P-gp inhibition, which resulted in an increased analgesic effect of methadone.

P-glycoprotein-mediated transport of oxytetracycline in the Caco-2 cell model

ATP-dependent drug transporters such as P-glycoprotein (P-gp), multi-drug resistance associated protein (MRP2) and breast cancer resistant protein (BCRP) are expressed at the brush border membrane of enterocytes. These efflux transporters excrete their substrates, among other various classes of antibiotics, into the lumen thus reducing net absorption as indicated by a low bioavailability after oral administration. Oxytetracycline (OTC) has been used for decennia in veterinary medicine for its extensive spectrum of antimicrobial activity. A major limitation has been, and still remains, its low bioavailability following oral administration. The present study aimed to investigate to what extent this low bioavailability is attributable to the fact that OTC is a substrate for one or more efflux transporters. As an experimental model to study the transmembrane transport of OTC, differentiated Caco-2 cells grown as monolayers on permeable supports were used. With this model it was shown that the secretion of OTC is slightly higher than its absorption. PSC833, a potent inhibitor of P-gp, decreased the secretion of OTC without affecting its absorption, while the MRP-inhibitor MK571 did not exert any effect. These data indicate that OTC is a substrate for P-gp. The affinity of OTC to these transporters seems to be rather low, as suggested by the low efflux ratio of 1:1.3. In competition experiments, OTC decreased the effluxes of other P-gp substrates such as Rhodamine123 and ivermectin. These findings are of clinical relevance, as they clearly indicate potential drug-drug interactions at the level of P-gp-mediated drug transport.

Flavonoids alter P-gp expression in intestinal epithelial cells in vitro and in vivo

Flavonoids are secondary plant metabolites included in our diet but are also provided in a growing number of supplements. They are suggested to interact with intestinal transport systems including phospho-glycoprotein (P-gp) which mediates the efflux of a variety of xenobiotics back into the gut lumen. In human intestinal Caco-2 cells, we tested the effects of 14 different flavonoids on P-gp expression in vitro. Protein expression levels were quantified by Western blotting, flow cytometry, and real-time PCR. Except apigenin, all flavonoids at concentrations of 10 microM increased P-gp expression in Western blotting experiments when cells were exposed to the compounds over 4 wk. Flavone was one of the most effective P-gp inducers in Caco-2 cells and its effects were, therefore, also assessed for changes in P-gp in vivo in the gastrointestinal tract of C57BL/6 mice. P-gp expression was significantly increased by flavone (400 mg/kg body weight x day over 4 wk) in the small intestine but not in the colon which displayed intrinsically the highest expression level. In conclusion, the increase in P-gp expression caused by flavonoids in intestinal epithelial cells in vitro and also in vivo may serve as an adaptation and defense mechanism limiting the entry of lipophilic xenobiotics into the organism.

Functional studies on the activity of efflux transporters in an ex vivo model with chicken splenocytes and evaluation of selected fluoroquinolones in this model

The efflux proteins P-glycoprotein (P-gp), BCRP and members of the MRP-family (MRPs) are increasingly recognized as determinants of the absorption, tissue distribution and excretion of numerous drugs. A widely applied in vitro screening method, to assess the effect of these efflux transporters in transmembrane transport of drugs is based on the use of peripheral blood mononuclear cells (PBMC), in which the efflux of fluorescent dye Rhodamine 123 (Rh-123) can be easily measured. In avian species, the isolation of PBMCs is compromised by the presence of thrombocytes having approximately the same size. As an alternative, we validated the use of isolated splenocytes to assess Rhodamine 123 transport in the presence and absence of specific inhibitors for P-gp, MRPs and BCRP. Rh-123 efflux was concentration-dependent with the percentage of efflux that decreased with increasing concentrations. P-gp inhibitors, PSC833 and GF120918, significantly inhibit Rh-123 efflux, whereas inhibitors for MRPs and BCRP, MK571 and Ko-143, respectively, have a limited inhibitory effect. However, the effect of GF120918 was more pronounced as compared to PSC833, suggesting an additional role for BCRP next to P-gp in Rh-123 efflux. Moreover, fluoroquinolones were selected to test the applicability of the described model. None of these fluoroquinolones significantly inhibit P-gp function at concentrations up to 50 microM, with exception of danofloxacin and danofloxacin mesylate that were found to reduce Rh-123 efflux by approximately 15%.

Efflux transporters in ulcerative colitis: decreased expression of BCRP (ABCG2) and Pgp (ABCB1)

BACKGROUND

Efflux transport proteins are important components of the intestinal barrier against bacterial toxins, carcinogens, and drugs. This investigation was conducted to determine the expression of Breast Cancer Resistance Protein (BCRP/ABCG2), P-glycoprotein (Pgp/MDR1/ABCB1), and Multidrug Resistance Protein 2 (MRP2/ABCC2) in the gut mucosa of patients with ulcerative colitis (UC).

METHODS

Patients were thoroughly diagnosed according to well-established clinical, endoscopic, and histologic criteria to be included in the group of patients with active UC (n = 16) or UC in remission (n = 17). Colonic and rectal mucosa from patients with UC were compared with tissues from control subjects (n = 15). The mRNA expression (TaqMan) of the efflux transporters and the proinflammatory cytokines interleukin (IL)-1beta and IL-6 was determined. Western blot was used in the analysis of protein expression and the tissue localization of BCRP was determined with confocal microscopy.

RESULTS

BCRP and Pgp expression was strongly reduced in individuals with active inflammation compared with controls and was negatively correlated with the levels of IL-6 mRNA. The BCRP staining of colonic epithelium seen in healthy mucosa was diminished in inflamed tissues, with concurrent disruption of epithelial F-actin structure.

CONCLUSIONS

Two of the efflux transporters of importance for the barrier function of the gut mucosa, Pgp and BCRP, are expressed at strongly reduced levels during active inflammation in patients with UC. Investigations are warranted to determine whether the low levels of efflux transporters during active UC contribute to altered transport and tissue exposure of carcinogens, bacterial toxins, and drugs.

Evidence of Oatp and Mdr1 in cryopreserved rat hepatocytes

Transport proteins control uptake of drugs into the liver (e.g., organic anion transporting polypeptide (Oatp)) and excretion of drugs from the liver (e.g., multidrug resistance protein 1 (Mdr1)). In this study, cryopreserved rat hepatocytes were used to investigate the effect of different culture conditions (suspension, conventional culture and sandwich culture) on the uptake of [(3)H]-taurocholate+/-probenecid and the efflux of [(3)H]-vinblastine+/-ketoconazole; mRNA levels of Oatp1a1, Oatp1a4, Mdr1a and Mdr1b were determined using real-time reverse transcription polymerase chain reaction (RT-PCR) and protein expression of Mdr was assessed by immunocytochemistry. The uptake of [(3)H]-taurocholate was higher in cryopreserved rat hepatocytes maintained in suspension as compared to hepatocytes in culture. A significant time dependent decline in the uptake of [(3)H]-taurocholate was noticed from day 2 to day 4 in conventional and sandwich cultures. [(3)H]-taurocholate uptake was significantly reduced using the inhibitor probenecid. Oatp mRNA expression in hepatocytes in suspension was similar to that of liver, whereas much lower levels were detected in the cultures; this was in accordance with [(3)H]-taurocholate uptake results. Mdr1 activity was assessed by accumulation of the Mdr1 selective substrate, [(3)H]-vinblastine, in hepatocytes using ketoconazole as an inhibitor. The results showed Mdr1 activity in cryopreserved rat hepatocytes in conventional and sandwich cultures. A time dependent increase in Mdr1 activity was noticed from day 2 to day 4. Mdr1 activity was not found using hepatocytes in suspension. Mdr1 mRNA expression was high in cryopreserved hepatocytes from both culture systems. Immunocytochemistry showed the Mdr protein in membranes of hepatocytes in culture as well as in that of hepatocytes in liver sections. In conclusion, the present study showed that cryopreserved rat hepatocytes maintained canalicular transport activity (Mdr1) and basolateral transport activity. Hepatocytes in suspension had a higher uptake of taurocholate with a high Oatp (1a1 and 1a4) mRNA expression as compared to hepatocytes in culture. The presence of Mdr1 in both conventional and sandwich culture was confirmed at mRNA level, by protein expression as well as transport activity.

Membrane transporter proteins: a challenge for CNS drug development

Drug transporters are membrane proteins present in various tissues such as the lymphocytes, intestine, liver, kidney, testis, placenta, and central nervous system. These transporters play a significant role in drug absorption and distribution to organic systems, particularly if the organs are protected by blood-organ barriers, such as the blood-brain barrier or the maternal-fetal barrier. In contrast to neurotransmitters and receptor-coupled transporters or other modes of interneuronal transmission, drug transporters are not directly involved in specific neuronal functions, but provide global protection to the central nervous system. The lack of capillary fenestration, the low pinocytic activity, and the tight junctions between brain capillary and choroid plexus endothelial cells represent further gatekeepers limiting the entrance of endogenous and exogenous compounds into the central nervous system. Drug transport is a result of the concerted action of efflux and influx pumps (transporters) located both in the basolateral and apical membranes of brain capillary and choroid plexus endothelial cells. By regulating efflux and influx of endogenous or exogenous substances, the blood-brain barrier and, to a lesser extent, the blood-cerebrospinal barrier in the ventricles, represents the main interface between the central nervous system and the blood, ie, the rest of the body. As drug distribution to organs is dependent on the affinity of a substrate for a specific transport system, membrane transporter proteins are increasingly recognized as a key determinant of drug disposition. Many drug transporters are members of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter superfamily or the solute-linked carrier (SLC) class. The multidrug resistance protein MDR1 (ABCB1), also called P-glycoprotein, the multidrug resistance-associated proteins MRP1 (ABCC1) and MRP2 (ABCC2), and the breast cancer-resistance protein BCRP (ABCG2) are ATP-dependent efflux transporters expressed in the blood-brain barrier. They belong to the superfamily of ABC transporters, which export drugs from the intracellular to the extracellular milieu. Members of the SLC class of solute carriers include, for example, organic ion transporting peptides, organic cation transporters, and organic ion transporters. They are ATP-independent polypeptides principally expressed at the basolateral membrane of brain capillary and choroid plexus endothelial cells that also mediate drug transport through central nervous system barriers.

Human pharmacogenomic variations and their implications for antifungal efficacy

Pharmacogenomics is defined as the study of the impacts of heritable traits on pharmacology and toxicology. Candidate genes with potential pharmacogenomic importance include drug transporters involved in absorption and excretion, phase I enzymes (e.g., cytochrome P450-dependent mixed-function oxidases) and phase II enzymes (e.g., glucuronosyltransferases) contributing to metabolism, and those molecules (e.g., albumin, A1-acid glycoprotein, and lipoproteins) involved in the distribution of antifungal compounds. By using the tools of population genetics to define interindividual differences in drug absorption, distribution, metabolism, and excretion, pharmacogenomic models for genetic variations in antifungal pharmacokinetics can be derived. Pharmacogenomic factors may become especially important in the treatment of immunocompromised patients or those with persistent or refractory mycoses that cannot be explained by elevated MICs and where rational dosage optimization of the antifungal agent may be particularly critical. Pharmacogenomics has the potential to shift the paradigm of therapy and to improve the selection of antifungal compounds and adjustment of dosage based upon individual variations in drug absorption, metabolism, and excretion.

The pivotal role of hepatocytes in drug discovery

This review promotes the value of isolated hepatocytes in modern Drug Discovery programmes and outlines how increased understanding, particularly in the area of in vitro-in vivo extrapolation (IVIVE), has led to more widespread use. The importance of in vitro metabolic intrinsic clearance data for predicting in vivo clearance has been acknowledged for several years and the greater utility of hepatocytes, compared with hepatic microsomes and liver slices, for this application is discussed. The application of hepatocytes in predicting drug-drug interactions (DDIs) resulting from reversible and irreversible (time-dependent) inhibition is relatively novel but affords the potential to study both phase I and phase II processes together with any impact of drug efflux and/or uptake (cellular accumulation). Progress in this area is reviewed along with current opinions on the comparative use of primary hepatocytes and higher throughput reporter gene-based systems for studying cytochrome P450 (CYP) induction. The appreciation of the role of transporter proteins in drug disposition continues to evolve. The study of hepatic uptake using isolated hepatocytes and the interplay between drug transport and metabolism with respect to both clearance and DDIs and subsequent IVIVE is also considered.

Effects of controlled-release on the pharmacokinetics and absorption characteristics of a compound undergoing intestinal efflux in humans

OBJECTIVE

The number of active pharmaceutical ingredients (API) undergoing inhibitable and saturable intestinal efflux is considerable. As a consequence, absorption and bioavailability may depend on the intestinal concentration profile of the drug and may vary as a function of dose and release rate of the drug from the dosage form. The impact of controlled versus immediate-release on the absorption of P-glycoprotein substrates is currently unknown. Thus, the main focus of the present study was a comparison of the pharmacokinetics of the P-gp model substrate talinolol following administration of immediate-release (IR) and controlled-release (CR) tablets to healthy human volunteers with a particular focus on the absorption characteristics of the active pharmaceutical ingredients.

METHODS

Talinolol immediate-release (Cordanum), 100mg), one controlled-release (100mg) and two controlled-release tablets (200mg) were administered as single doses to fasting healthy volunteers in a crossover design with a 1 week washout period between treatments. Sufficient blood and urine samples were drawn and analysed using a specific HPLC method with UV detection to describe the resulting plasma and urinary excretion versus time profiles.

RESULTS

The bioavailability of talinolol in term of AUC(0-->infinity) for IR talinolol was approximately twice as high as compared to the administration of the same dose in a controlled-release dosage form. After administration of talinolol IR tablets, the drug was rapidly absorbed and reached maximum concentrations C(max) of 204.5 ng/ml+/-121.8 (means+/-S.D.) 2h after dosing. The terminal half-life of the drug averaged 19.8h following IR administration in comparison to 32 h under CR dosing conditions. Following administration of the IR dosage form, significant secondary peaks were observed in one healthy subject. Secondary peaks were not clearly apparent in the CR plasma profiles.

CONCLUSION

The present study demonstrates a considerable loss of bioavailability of drugs that are substrates of intestinal secretory transporters upon their administration in controlled-release dosage forms.

Effects of itraconazole and diltiazem on the pharmacokinetics of fexofenadine, a substrate of P-glycoprotein

AIMS

Fexofenadine is a substrate of several drug transporters including P-glycoprotein. Our objective was to evaluate the possible effects of two P-glycoprotein inhibitors, itraconazole and diltiazem, on the pharmacokinetics of fexofenadine, a putative probe of P-glycoprotein activity in vivo, and compare the inhibitory effect between the two in healthy volunteers.

METHODS

In a randomized three-phase crossover study, eight healthy volunteers were given oral doses of 100 mg itraconazole twice daily, 100 mg diltiazem twice daily or a placebo capsule twice daily (control) for 5 days. On the morning of day 5 each subject was given 120 mg fexofenadine, and plasma concentrations and urinary excretion of fexofenadine were measured up to 48 h after dosing.

RESULTS

Itraconazole pretreatment significantly increased mean (+/-SD) peak plasma concentration (Cmax) of fexofenadine from 699 (+/-366) ng ml-1 to 1346 (+/-561) ng ml-1 (95% CI of differences 253, 1040; P<0.005) and the area under the plasma concentration-time curve [AUC0,infinity] from 4133 (+/-1776) ng ml-1 h to 11287 (+/-4552) ng ml-1 h (95% CI 3731, 10575; P<0.0001). Elimination half-life and renal clearance in the itraconazole phase were not altered significantly compared with those in the control phase. In contrast, diltiazem pretreatment did not affect Cmax (704+/-316 ng ml-1, 95% CI -145, 155), AUC0, infinity (4433+/-1565 ng ml-1 h, 95% CI -1353, 754), or other pharmacokinetic parameters of fexofenadine.

CONCLUSIONS

Although some drug transporters other than P-glycoprotein are thought to play an important role in fexofenadine pharmacokinetics, itraconazole pretreatment increased fexofenadine exposure, probably due to the reduced first-pass effect by inhibiting the P-glycoprotein activity. As diltiazem pretreatment did not alter fexofenadine pharmacokinetics, therapeutic doses of diltiazem are unlikely to affect the P-glycoprotein activity in vivo.

Evaluation of antipsychotic drugs as inhibitors of multidrug resistance transporter P-glycoprotein

RATIONALE

The multidrug resistance transporter, P-glycoprotein (P-gp), is involved in efflux transport of several antipsychotics in the blood-brain barrier (BBB).

OBJECTIVES

In the present study, we evaluated the inhibitory effect of the antipsychotics, i.e., risperidone, olanzapine, quetiapine, clozapine, haloperidol, chlorpromazine, a major metabolite of risperidone, 9-OH-risperidone, and a positive control inhibitor, PSC833, on the cellular uptake of a prototypic substrate of P-gp, rhodamine (Rhd) 123, in LLC-PK1 and L-MDR1 cells.

MATERIALS AND METHODS

After incubation of the antipsychotics (1-100 microM) and the positive (10 microM PSC833) or negative (1% dimethyl sulfoxide) controls with 5 microM Rhd 123 for 1 h, the effects of the antipsychotics on the intracellular accumulation of Rhd 123 were examined using a flow cytometric method.

RESULTS

All the antipsychotics showed various degrees of inhibitory effects on P-gp activity. The rank order of the concentration of inhibitor to cause 50% of the maximal increment of intracellular Rhd 123 fluorescence (EC(50)) was: PSC833 (0.5 microM) < olanzapine (3.9 microM) < chlorpromazine (5.8 microM) < risperidone (6.6 microM) < haloperidol (9.1 microM) < quetiapine (9.8 microM) < 9-OH-risperidone (12.5 microM) < clozapine (30 microM). Considering that the antipsychotics' plasma concentrations are generally lower than 1 microM, the present results suggest that olanzapine and risperidone are the only agents that may inhibit P-gp activity in the BBB. However, most of the antipsychotics are extensively accumulated in tissues. In addition, when given orally, the drug concentrations in the gastrointestinal tract are likely to be high.

CONCLUSIONS

Pharmacokinetic interactions due to inhibition of P-gp activity by the antipsychotics appear possible and warrant further investigation.

Role of P glycoprotein in absorption of novel antimalarial drugs

Bidirectional transport of four novel antimalarial compounds was determined using Caco-2 cell monolayers. P glycoprotein-mediated efflux was greatest for pyronaridine (5 to 20 microM) and low for naphthoquine (5 microM). With 20 microM naphthoquine, net efflux was blocked, suggesting saturation of the transporter. Piperaquine and dihydroartemisinin were not transported by the system.

Apical sodium dependent bile acid transporter (ASBT, SLC10A2): a potential prodrug target

A major hurdle impeding the successful clinical development of drug candidates can be poor intestinal permeability. Low intestinal permeability may be enhanced by a prodrug approach targeting membrane transporters in the small intestine. Transporter specificity, affinity, and capacity are three factors in targeted prodrug design. The human apical sodium dependent bile acid transporter (SLC10A2) belongs to the solute carrier family (SLC) of transporters and is an important carrier protein expressed in the small intestine. In spite of its appearing to be an excellent target for prodrug design, few studies have targeted human apical sodium dependent bile acid transporter (hASBT) to improve oral bioavailability. This review discusses bile acids including their chemistry and their absorptive disposition. Additionally, hASBT-mediated prodrug targeting is discussed, including QSAR, in vitro models for hASBT assay, and the current progress in utilizing hASBT as a drug delivery target.

The potential inhibitory effect of antiparasitic drugs and natural products on P-glycoprotein mediated efflux

The potential inhibitory effect on P-glycoprotein (Pgp) by antiparasitic drugs and natural compounds was investigated. Compounds were screened for Pgp interaction based on inhibition of Pgp mediated [3H]-taxol transport in Caco-2 cells. Bidirectional transport of selected inhibitors was further evaluated to identify potential Pgp substrates using the Caco-2 cells. Of 21 antiparasitics tested, 14 were found to inhibit Pgp mediated [3H]-taxol with K(iapp) values in the range 4-2000 microM. The antimalarial quinine was the most potent inhibitor with a K(iapp) of 4 microM. Of the 12 natural compounds tested, 3 inhibited [3H]-taxol transport with K(iapp) values in the range 50-400 microM. Quinine, amodiaquine, chloroquine, flavone, genistein, praziquantel, quercetin and thiabendazole were further investigated in bidirectional transport assays to determine whether they were substrates for Pgp. Transport of quinine in the secretory direction exceeded that in the absorptive direction and was saturable, suggesting quinine being a Pgp substrate. The rest of the compounds inhibiting Pgp showed no evidence of being Pgp substrates. In conclusion, we have demonstrated that a substantial number of antiparasitic and natural compounds, in a range of concentrations, are capable of inhibiting Pgp mediated [3H]-taxol efflux in Caco-2 cells, without being substrates and this may have implications for drug interactions with Pgp.

BacMam recombinant baculovirus in transporter expression: A study of BCRP and OATP1B1

Human BCRP and OATP1B1 have recently been identified as important transporters in the absorption, distribution, and elimination of clinically significant drugs. In this report, we illustrate the use of modified baculoviruses, termed BacMam viruses for the expression of functional BCRP and OATP1B1 in mammalian cells. We show a variety of host cells efficiently transduced to express BCRP including HEK 293, LLC-PK, and U-2 OS, where protein levels on the cell-surface were modulated by titrating different amounts of viral inoculum. In addition, using the BODIPY-prazosin efflux assay and the BacMam reagent we illustrate inhibition of BCRP activity with GF120918 or Fumitremorgin C. Furthermore, we present data demonstrating simultaneous expression of BCRP and OATP1B1 in BacMam transduced mammalian cells by simply adding viral inoculum of each transporter. Thus these results indicate that BacMam mediated gene delivery provides a novel and efficient research tool for the investigation of single or multiple transporters in vitro.

Physiologically based pharmacokinetic modelling 2: Predicting the tissue distribution of acids, very weak bases, neutrals and zwitterions

A key component of whole body physiologically based pharmacokinetic (WBPBPK) models is the tissue-to-plasma water partition coefficients (Kpu's). The predictability of Kpu values using mechanistically derived equations has been investigated for 7 very weak bases, 20 acids, 4 neutral drugs and 8 zwitterions in rat adipose, bone, brain, gut, heart, kidney, liver, lung, muscle, pancreas, skin, spleen and thymus. These equations incorporate expressions for dissolution in tissue water and, partitioning into neutral lipids and neutral phospholipids. Additionally, associations with acidic phospholipids were incorporated for zwitterions with a highly basic functionality, or extracellular proteins for the other compound classes. The affinity for these cellular constituents was determined from blood cell data or plasma protein binding, respectively. These equations assume drugs are passively distributed and that processes are nonsaturating. Resultant Kpu predictions were more accurate when compared to published equations, with 84% as opposed to 61% of the predicted values agreeing with experimental values to within a factor of 3. This improvement was largely due to the incorporation of distribution processes related to drug ionisation, an issue that is not addressed in earlier equations. Such advancements in parameter prediction will assist WBPBPK modelling, where time, cost and labour requirements greatly deter its application.

Scientific perspectives on drug transporters and their role in drug interactions

Recently, increased interest in drug transporters and research in this area has revealed that drug transporters play an important role in modulating drug absorption, distribution, and elimination. Acting alone or in concert with drug metabolizing enzymes they can affect the pharmacokinetics and pharmacodynamics of a drug. This commentary will focus on the potential role that drug transporters may play in drug-drug interactions and what information may be needed during drug development and new drug application (NDA) submissions to address potential drug interactions mediated by transporters.

Pharmacological Perspectives on the Detoxification of Plant Secondary Metabolites: Implications for Ingestive Behavior of Herbivores

Plant secondary metabolites (PSMs) are a major constraint to the ingestion of food by folivorous and browsing herbivores. Understanding the way in which mammalian detoxification pathways are adapted to deal with PSMs is crucial to understanding how PSMs influence ingestive behavior of herbivores and hence their fitness and the impact that they have on vegetation. Pharmacological concepts can provide insights into the relationship between the absorption and metabolic fate of PSMs and ingestive behavior. Lipophilic PSMs will be absorbed into the bloodstream and must be removed fast enough to prevent their accumulation to toxic levels. Elimination depends on their metabolism, usually by cytochrome P450 enzymes, to more polar metabolites that can be excreted by the kidney. The concentration of PSM in blood (C) is a better measure of exposure to a toxin compared to the amount ingested because there can be great variability in the rate and degree of absorption from the gut. C rises and falls depending on the relative rates of absorption and elimination. These rates depend in part on metabolic and transport processes that are saturable and liable to inhibition and induction by PSMs, indicating that complex interactions are likely. Herbivores can use diet choice and the rate and amount of PSM consumption to prevent C from reaching a critical level that produces significant adverse effects.

Application of Pharmacological Approaches to Plant–Mammal Interactions

The dominant theory in the field of mammalian herbivore-plant interactions is that intake, and therefore tolerance, of plant secondary metabolites (PSMs) is regulated by mechanisms that reduce absorption and increase detoxification of PSMs. Methods designed by pharmacologists to measure detoxification enzyme activity, metabolite excretion, and most recently, drug absorption, have been successfully applied by ecologists to study PSM intake in a variety of mammalian study systems. Here, we describe several pharmacological and molecular techniques used to investigate the fate of drugs in human that have potential to further advance knowledge of mammalian herbivore-plant interactions.

Efflux transporters as a novel herbivore countermechanism to plant chemical defenses

The recent discovery of efflux transporters in the gut has revolutionized our understanding of the absorption and bioavailability of pharmaceuticals and other xenobiotics in humans. Despite the celebrity of efflux transporters in the areas of pharmacology and medicine, their significance is only beginning to be realized in the area of plant-herbivore interactions. This review integrates reports on the importance of gut efflux transporters to diet selection by herbivores. The diets of herbivores are laden with toxic plant secondary metabolites (PSMs) that until recently were thought to be processed almost exclusively by detoxification enzymes in the liver. We describe how efflux transporters in the gut may play a critical role in regulating the absorption of PSMs in herbivores and dictating diet selection. Recent studies suggest that the role of efflux transporters in mediating diet selection in herbivores may be as critical as detoxification enzymes. In addition to diet selection, gut efflux transporters have implications for other aspects of plant-animal interactions. They may be significant components of the evolutionary arms race that influences chemical diversity in plants. Furthermore, in agricultural systems, gut efflux transporters may play an important role in the effectiveness of pesticides. This synthesis paper introduces a new direction in plant-herbivore interactions by providing a complementary mechanism, regulated absorption, to detoxification that may define tolerance to PSMs by herbivores.

In vitro p-glycoprotein inhibition assays for assessment of clinical drug interaction potential of new drug candidates: a recommendation for probe substrates

Because modulation of P-glycoprotein (Pgp) through inhibition or induction can lead to drug-drug interactions by altering intestinal, central nervous system, renal, or biliary efflux, it is anticipated that information regarding the potential interaction of drug candidates with Pgp will be a future regulatory expectation. Therefore, to be able to utilize in vitro Pgp inhibition findings to guide clinical drug interaction studies, the utility of five probe substrates (calcein-AM, colchicine, digoxin, prazosin, and vinblastine) was evaluated by inhibiting their Pgp-mediated transport across multidrug resistance-1-transfected Madin-Darby canine kidney cell type II monolayers with 20 diverse drugs having various degrees of Pgp interaction (e.g., efflux ratio, ATPase, and calcein-AM inhibition). Overall, the rank order of inhibition was generally similar with IC(50) values typically within 3- to 5-fold of each other. However, several notable differences in the IC(50) values were observed. Digoxin and prazosin were the most sensitive probes (e.g., lowest IC(50) values), followed by colchicine, vinblastine, and calcein-AM. Inclusion of other considerations such as a large dynamic range, commercially available radiolabel, and a clinically meaningful probe makes digoxin an attractive probe substrate. Therefore, it is recommended that digoxin be considered as the standard in vitro probe to investigate the inhibition profiles of new drug candidates. Furthermore, this study shows that it may not be necessary to generate IC(50) values with multiple probe substrates for Pgp as is currently done for cytochrome P450 3A4. Finally, a strategy integrating results from in vitro assays (efflux, inhibition, and ATPase) is provided to further guide clinical interaction studies.

Eletriptan in migraine

Migraine is a highly prevalent, chronic and disabling illness in which the gap between practice guideline recommendations and actual clinical practice remains wide. Eletriptan, similar to other triptans, is a potent 5-HT(1B/1D) receptor agonist with a high selectivity for cranial versus coronary artery constriction and favorable pharmacokinetic profile. An extensive program of double-blind, placebo-controlled, head-to-head comparator trials has demonstrated the superior efficacy of eletriptan compared with the combination of ergotamine and caffeine, and selected oral triptans for the acute treatment of migraine. Eletriptans tolerability profile makes it a good choice as a first-line treatment of migraine. An early treatment study suggests that treatment of mild headache is associated with unusually high sustained pain-free rates and a tolerability profile that is equivalent to placebo.

Drug transporters in pharmacokinetics

This review deals with the drug transporters allowing drugs to enter and leave cells by carrier-mediated pathways. Emphasis is put on liver transporters but systems in gut, kidney, and blood-brain barrier are mentioned as well. Drug-drug interactions on carriers may provoke significant modification in pharmacokinetics as do carrier gene polymorphisms yielding functional carrier protein mutations. An integrated phase concept should reflect the interplay between drug metabolism and drug transport.

Multiple dose pharmacokinetics of risperidone and 9-hydroxyrisperidone in Chinese female patients with schizophrenia

AIM

To study the multiple dose clinical pharmacokinetics of risperidone and its main active metabolite, 9-hydroxyrisperidone, in Chinese female patients with schizophrenia.

METHODS

The subjects were 23 Chinese female inpatients aged 18-65 years who met the CCMD-III (third revision of the Chinese Criteria of Mental Disorders) criteria for schizophrenia. Subjects were tested after 17 d of treatment with 2 mg risperidone twice daily. Plasma concentrations of risperidone and 9-hydroxy-risperidone were assayed by using validated high performance liquid chromatography-mass spectrometry (HPLC-MS) methods.

RESULTS

Risperidone was rapidly absorbed (Tmax was 1.6 h) and its T1/2 in plasma was short (3.2 h). 9-hydroxy-risperidone was quickly metabolized from the parent drug with a mean Tmax of 2.5 h. It had a long half-life of 24.7 h. The C(ss)(av) of risperidone and 9-hydroxy-risperidone were 36.9+/-33.1 and 110.6+/-30.5 microg x h x L(-1), respectively, and the AUC(ss)0-12 were 443.2+/- 397.4 and 1327.2+/- 402.3 microg x h x L(-1), respectively. CL/F and V/F of risperidone were 8.7+/- 6.2 L/h and 34.1+/- 24.3 L, respectively. Interindividual variations for pharmacokinetic parameters were quite large for risperidone. All 23 subjects experienced high prolactin levels when treated with risperidone. However there was no correlation between prolactin level and the concentration of risperidone, 9-hydroxy-risperidone, or the active moiety.

CONCLUSION

Risperidone showed large interindividual variations in pharmacokinetics. Administration of risperidone resulted in high serum prolactin levels. The results indicate that systemic exposure to risperidone and 9-hydroxy-risperidone in female Chinese schizophrenic patients is higher relative to published data for white Caucasian patients. Larger studies regarding the PK/PD relationship may be required to develop a reasonable clinical dosage regimen for Chinese female patients.

MDR- and CYP3A4-mediated drug–herbal interactions

According to recent epidemiological reports, almost 40% of American population use complimentary and alternative medicine (CAM) during their lifetime. Patients detected with HIV or cancer often consume herbal products especially St. John's wort (SJW) for antidepressants in combination with prescription medicines. Such self-administered herbal products along with prescribed medicines raise concerns of therapeutic activity due to possible drug-herbal interactions. P-glycoprotein (P-gp) and cytochrome P450 3A4 (CYP3A4) together constitute a highly efficient barrier for many orally absorbed drugs. Available literature, clinical reports and in vitro studies from our laboratory indicate that many drugs and herbal active constituents are substrates for both P-gp and CYP3A4. Results from clinical studies and case reports indicate that self-administered SJW reduce steady state plasma concentrations of amitriptyline, cyclosporine, digoxin, fexofenadine, amprenavir, indonavir, lopinavir, ritonavir, saquinavir, benzodiazepines, theophyline, irinotecan, midazolan and warfarin. This herbal agent has been also reported to cause bleeding and unwanted pregnancies when concomitantly administered with oral contraceptives. Most of these medicinal agents and SJW are substrates for P-gp and/or CYP3A4. In vitro studies from our laboratory suggest that short-term exposure with pure herbal agents such as hypericin, kaempferol and quercetin or extract of SJW resulted in higher uptake or influx of ritonavir and erythromycin. Hypericin, kaempferol and quercetin also caused a remarkable inhibition of cortisol metabolism with the percent intact cortisol values of 64.58%, 89.6% and 90.1%, respectively, during short-term in vitro experiments. Conversely, long-term exposure of herbal agents (hyperforin, kaempferol and quercetin) showed enhanced expression of CYP3A4 mRNA in Caco-2 cells. In another study, we observed that long-term exposure of hypericin, kaempferol, quercetin and silibinin resulted in higher MDR-1 mRNA expression in Caco-2 cells. Therefore, herbs can pharmacokinetically act as inhibitors or inducers. Medicinal agents that are substrates P-gp-mediated efflux and/or CYP-mediated metabolism are likely to be potential candidates for drug-herbal interactions. The duration of exposure of cells/healthy volunteers/animals to herbals appears to be critical for drug-herbal interaction. An increase in plasma drug concentration is possible during concomitant administration of SJW and prescribed drugs. In contrast, prolonged intake of herbal supplement followed by drug administration may result in subtherapeutic concentrations. Therefore, clinical implications of such drug herbal interactions depend on a variety of factors such as dose, frequency and timing of herbal intake, dosing regimen, route of drug administration and therapeutic range. In vitro screening techniques will play a major role in identifying possible herb-drug interactions and thus create a platform for clinical studies to emerge. Mechanisms of drug-herbal interaction have been discussed in this review article.

Implications of genetic polymorphisms in drug transporters for pharmacotherapy

Drug transporters are increasingly recognized as a key determinant of drug disposition and response. It is now widely appreciated that expression of the ATP-dependent efflux transporter, MDR1 (ABCB1, P-glycoprotein), in organs such as the gastrointestinal tract, liver and kidney significantly alters the extent of drug absorption and excretion. Moreover, expression of MDR1 at the level of the blood-brain barrier limits the entry of many drugs into the central nervous system. Given such an important role of MDR1 in the drug disposition process, it is not surprising to see increasing focus on the role of single nucleotide polymorphisms (SNPs) in this transporter as a potential determinant of interindividual variability in drug disposition and pharmacological response. However, drug transport is often the result of the concerted action of efflux and uptake pumps located both in the basolateral and apical membranes of epithelial cells. A growing list of membrane-spanning proteins involved in the in- or outward transport of a large variety of drugs has been recognized and characterized over the past few years in almost all tissues, including organic anion and cation transporters (OAT, OCT, solute carrier family SLC22A), organic anion transport proteins (OATP, solute carrier family SLCO, formerly SLC21A), and MRPs (ABCCs), other members of the ATP-binding cassette family. We are just beginning to appreciate their role for drug delivery and disposition and the contribution of genetic polymorphisms in these transport proteins to interindividual variability in the efficacy and safety for pharmacotherapy. This review summarizes the consequences of inherited differences in drug transport for pharmacotherapy. With the main focus on ABCB1, an update of recent advances is given and clinically relevant examples are used to illustrate how heritable differential drug transport can help to explain individual variability in drug response. The pharmacogenetics of other transporters is briefly introduced.

Knocking down transport: applications of RNA interference in the study of drug transport proteins

RNA interference (RNAi) is a gene silencing process mediated by double-stranded RNA (dsRNA). The silencing process is comprised of an initiation step, in which small interfering RNA (siRNA) is introduced to the cell, and an effector step, which involves degrading mRNA molecules of the target gene. RNA interference has been observed in most organisms from plants to vertebrates. As a gene silencing approach, RNAi has proven to be extremely useful in characterizing gene function and developing new tools in cancer therapy and drug delivery. The development of RNAi-related technologies is an emerging area in biomedical research. In this review, recent progress in the application of RNAi to the study of transport proteins is summarized and evaluated; the advantages, disadvantages and future directions of RNAi technology are discussed.

Utility of human/human-derived reagents in drug discovery and development: An industrial perspective

The shift to combinatorial chemistry and parallel synthesis in drug discovery has resulted in large numbers of compounds entering the lead seeking and lead development phases of the process. To support this, higher throughput computational (in silico) and in vitro approaches have become the forefront of the drug metabolism and pharmacokinetic (DMPK) input into drug discovery. This has been accompanied by a shift in focus from animal-derived data to human based studies, reflecting the realisation that extrapolation from animals to human has its limitations. In silico approaches may be regarded as human derived tools for DMPK, since models (template/pharmacophore and protein homology modelling), for example, for the human CYP enzymes, are widely used for identifying qualitatively enzyme/substrate interactions. Quantitative assessment of drug metabolism using human hepatocytes or sub-cellular fractions provide a valuable tool both for the screening out of high metabolic lability and in estimations of human intrinsic clearance. In terms of drug absorption, the human colon adenocarcinoma cell line, Caco-2, offers a versatile human derived system for measuring drug permeability, despite over expression of the efflux transporter P-glycoprotein (P-gp). The importance of P-gp can then be further assessed in recombinant systems expressing the human P-gp, where substrate affinity and inhibition potency can be measured, important factors when considering transporter mediated drug-drug interactions. The primary cause of pharmacokinetic-based drug-drug interactions (DDIs) is through enzyme inhibition or induction, with the CYP enzymes being of major importance. Human liver microsomes and hepatocytes are invaluable tools in assessment of DDI vulnerability of new chemical entities, having the capacity to identify enzymes responsible for specific routes of metabolism, and hence areas of vulnerability for a DDI. In addition, human-based screening tools can be used to identify the perpetrator of a DDI through enzyme inhibition/induction. Large differences in the nature of enzymes induced and the extent of induction when comparing animals to man are known. Thus, in vitro models allowing assessment of induction potential in human tissue, establishes some relevance to the clinical situation.

Relationship between Excretion Clearance of Rhodamine 123 and P-Glycoprotein (Pgp) Expression Induced by Representative Pgp Inducers

P-Glycoprotein (Pgp) locates in several tissues in the living body and acts as an efflux pump for many drugs. In this study, the usefulness of intravenous rhodamine 123 (Rho123) administration as a marker for detecting the inducing effect of Pgp by drugs was identified, and the relationship between excretion clearances of Rho123 via Pgp and its expression during treatment with the representative Pgp inducers rifampicin (RFP), dexamethasone (DEX) and St. John's Wort (SJW) were examined in rat liver, intestine and kidney. After pretreatment with RFP (10 mg/kg/d) for 4 d, DEX (50 mg/kg/d) for 4 d or SJW (15 mg/kg/d) for 7 d orally, the biliary excretion of Rho123 after intravenous administration (0.2 mg/kg) increased significantly by 40%, 55% and 14%, respectively, and the intestinal excretion increased significantly by 24%, 50% and 27%, respectively, as compared with the controls. In contrast, there were no notable changes in the urinary excretion of Rho123 among rats that received these inducers. Western blot analysis with a monoclonal antibody for Pgp (C219) showed that Pgp levels in the small intestine and liver in the inducer-treated rats increased markedly as compared with the controls. In addition, there was a significant correlation between the induction levels of Pgp in the liver or small intestine and their clearance ratios (r2=0.7583, p<0.05), but not in the kidney. These observations suggest that the excretion clearances of Rho123 from blood circulation to the small intestine or to the bile after its intravenous administration are useful indicators to assess the Pgp function in the presence of Pgp inducers.

P-Glycoprotein Recognition of Substrates and Circumvention through Rational Drug Design

It is now well recognized that membrane efflux transporters, especially P-glycoprotein (P-gp; ABCB1), play a role in determining the absorption, distribution, metabolism, excretion, and toxicology behaviors of some drugs and molecules in development. An investment in screening structure-activity relationship (SAR) is warranted in early discovery when exposure and/or target activity in an in vivo efficacy model is not achieved and P-gp efflux is identified as a rate-limiting factor. However, the amount of investment in SAR must be placed into perspective by assessing the risks associated with the intended therapeutic target, the potency and margin of safety of the compound, the intended patient population(s), and the market competition. The task of rationally designing a chemistry strategy for circumventing a limiting P-gp interaction can be daunting. The necessity of retaining biological potency and metabolic stability places restrictions on what can be done, and the factors for P-gp recognition of substrates are complicated and poorly understood. The parameters within the assays that affect overall pump efficiency or net efflux, such as passive diffusion, membrane partitioning, and molecular interaction between pump and substrate, should be understood when interpreting data sets associated with chemistry around a scaffold. No single, functional group alone is often the cause, but one group can accentuate the recognition points existing within a scaffold. This can be likened to a rheostat, rather than an on/off switch, where addition or removal of a key group can increase or decrease the pumping efficiency. The most practical approach to de-emphasize the limiting effects of P-gp on a particular scaffold is to increase passive diffusion. Efflux pumping efficiency may be overcome when passive diffusion is fast enough. Eliminating, or substituting with fewer, groups that solvate in water, or decreasing their hydrogen bonding capacity, and adding halogen groups can increase passive diffusion. Reducing molecular size, replacing electronegative atoms, blocking or masking H-bond donors with N-alkylation or bulky flanking groups, introducing constrained conformation, or by promoting intramolecular hydrogen bonds are all examples of steps to take. This review discusses our understanding of how P-gp recognizes and pumps compounds as substrates and describes cases where structural changes were made in a chemical scaffold to circumvent the effects of P-gp interactions.