Role of transporters in the tissue-selective distribution and elimination of drugs: transporters in the liver, small intestine, brain and kidney

Effect of culture time on the basal expression levels of drug transporters in sandwich-cultured primary rat hepatocytes

Sandwich-cultured rat hepatocytes are used in drug discovery for pharmacological and toxicological assessment of drug candidates, yet their utility as a functional model for drug transporters has not been fully characterized. To evaluate the system as an in vitro model for drug transport, expression changes of hepatic transporters relative to whole liver and freshly isolated hepatocytes (day 0) were examined by real-time quantitative reverse transcription-polymerase chain reaction for 4 consecutive days of culture. No significant differences in transporter expression levels were observed between freshly isolated hepatocytes and whole liver. Two distinct mRNA profiles were detected over time showing 1) a more than 5-fold decline in levels of uptake transporters such as Na(+)-taurocholate cotransporting polypeptide (Ntcp), organic anion transporter (Oat) 2, organic anion-transporting polypeptide (Oatp) 1a1, Oatp1a4, and Oatp1b2 and 2) a greater than 5-fold increase of efflux transporters P-glycoprotein (P-gp), breast cancer resistance protein (Bcrp), and multidrug resistance-related proteins (Mrp) 1, 2, 3, and 4. In addition, protein levels and functional activities for selected transporters were also determined. Protein levels for Mrp2, Bcrp, P-gp, Ntcp, and Oatp1a4 corresponded to changes in mRNA. Functional activities of Oatps and Oct1 exhibited a 3- and 4-fold decrease on day 2 and day 4, respectively, relative to that on day 0, whereas a more than 10-fold reduction in Oat2 activity was observed. These results indicate that the cell culture conditions used herein did not provide an optimal environment for expression of all hepatic transporters. Significant time-dependent alterations in basal gene expression patterns of transporters were detected compared with those in liver or freshly isolated hepatocytes. Further work and new strategies are required to improve the validity of this model as an in vitro tool for in vivo drug transport or biliary clearance prediction.

Peptide cotransporter 1 in intestine and organic anion transporters in kidney are targets of interaction between JBP485 and lisinopril in rats

The purpose of this study was to clarify the pharmacokinetic mechanism of interaction between JBP485 (cyclo-trans-4-L-hydroxyprolyl-L-serine, a dipeptide with antihepatitis activity) and lisinopril (an angiotensin-converting enzyme inhibitor) in vitro and in vivo. When JBP485 and lisinopril were administered orally simultaneously, the plasma concentrations of the two drugs were decreased significantly, but few changes were observed after simultaneous intravenous administration of the two drugs. The uptake of JBP485 and lisinopril in everted intestinal sacs and in HeLa cells transfected with human peptide cotransporter 1 (PEPT1), as well as absorption of JBP485 and lisinopril after jejunal perfusion were reduced after simultaneous drug administration, which suggested that the first target of drug interaction was PEPT1 in the intestine during the absorption process. The cumulative urinary excretions and renal clearance of the two drugs were decreased after intravenous co-administration, while uptakes of the two drugs in kidney slices and hOAT1/hOAT3-transfected HEK293 cells were decreased. These results indicated that the second target of drug-drug interaction was located in the kidney. These findings confirmed that the pharmacokinetic mechanism of interaction between JBP485 and lisinopril could be explained by their inhibition of the same transporters in the intestinal mucosa (PEPT1) and kidneys (OATs).

Functional analysis of nonsynonymous single nucleotide polymorphisms of multidrug resistance-associated protein 2 (ABCC2)

BACKGROUND

Multidrug resistance-associated protein 2 (MRP2; ABCC2) mediates the biliary excretion of glutathione, glucuronide, and sulfate conjugates of endobiotics and xenobiotics. Single nucleotide polymorphisms (SNPs) of MRP2 contribute to interindividual variability in drug disposition and ultimately in drug response.

OBJECTIVES

To characterize the transport function of human wild-type (WT) MRP2 and four SNP variants, S789F, A1450T, V417I, and T1477M.

METHODS

The four SNP variants were expressed in Sf9 cells using recombinant baculovirus infection. The kinetic parameters [Km, (μmol/l); V(max), (pmol/mg/min); the Hill coefficient] of ATP-dependent transport of leukotriene C(4) (LTC(4)), estradiol-3-glucuronide (E(2)3G), estradiol-17β-glucuronide (E(2)17G), and tauroursodeoxycholic acid (TUDC) were determined in Sf9-derived plasma membrane vesicles. Transport activity was normalized for expression level.

RESULTS

The V(max) for transport activity was decreased for all substrates for S789F, and for all substrates except E(2)17G for A1450T. V417I showed decreased apparent affinity for LTC(4), E(2)3G, and E(2)17G, whereas transport was similar between wild-type (WT) and T1477M, except for a modest increase in TUDC transport. Examination of substrate-stimulated MRP2-dependent ATPase activity of S789F and A1450T, SNPs located in MRP2 nucleotide-binding domains (NBDs), demonstrated significantly decreased ATPase activity and only modestly decreased affinity for ATP compared with WT.

CONCLUSION

SNPs in the NBDs (S789F in the D-loop of NBD1, or A1450T near the ABC signature motif of NBD2) variably decreased the transport of all substrates. V417I in membrane spanning domain 1 selectively decreased the apparent affinity for the glutathione and glucuronide conjugated substrates, whereas the T1477M SNP in the carboxyl terminus altered only TUDC transport.

Enhancement of oral bioavailability of 20(S)-ginsenoside Rh2 through improved understanding of its absorption and efflux mechanisms

The development of 20(S)-ginsenoside Rh2 (Rh2s) as a chemoprevention agent is limited by its low oral bioavailability. The goals of this study were to determine the mechanisms responsible for its poor oral absorption and to improve its bioavailability by overcoming the barrier to its absorption. Comprehensive studies were conducted using the following models: 1) monolayers of Caco-2, parental, and multidrug resistance gene (MDR1)-overexpressing Madin-Darby canine kidney II (MDCKII) cells; 2) pharmacokinetics in wild-type (WT) FVB, MDR1a/b knockout [MDR1a/b⁻/⁻] FVB, and A/J mice; and 3) intestinal perfusion in WT, MDR1a/b⁻/⁻ FVB, and A/J mice. Two P-glycoprotein (P-gp) inhibitors, verapamil and cyclosporine A, substantially decreased the efflux ratio of Rh2s from 28.5 to 1.0 and 1.2, respectively, in Caco-2 cells. The intracellular concentrations of Rh2s were also significantly increased (2.3- and 3.9-fold) in the presence of inhibitors. Similar results were obtained when transcellular transport of Rh2s were determined using MDR1-overexpressing MDCKII cells in the absence or presence of cyclosporine A. Compared with WT mice, the plasma C(max) and AUC₀-∞ of Rh2s were substantially increased by 17- and 23-fold in MDR1a/b⁻/⁻ FVB mice, respectively. In the A/J mice, the oral bioavailability of Rh2s (0.94% at 5 mg/kg and 0.52% at 20 mg/kg) was substantially increased by P-gp inhibitor to 33.18 and 27.14%, respectively. As expected, deletion or inhibition of P-gp significantly increased absorption and steady-state plasma concentration of Rh2s in a mouse intestinal perfusion model. In conclusion, Rh2s is a good substrate of P-gp, and inhibition of P-gp can significantly enhance its oral bioavailability.

Hepatic OATP and OCT uptake transporters: their role for drug-drug interactions and pharmacogenetic aspects

Uptake transporters in the basolateral membrane of hepatocytes are important for the hepatobiliary elimination of drugs. Further, since drug-metabolizing enzymes are located intracellularly, uptake into hepatocytes is a prerequisite for their subsequent metabolism. Therefore, alteration of uptake transporter function (e.g., by concomitantly administered drugs or due to functional consequences of genetic variations, leading to reduced transport function) may result in a change in drug pharmacokinetics. In this review, we focus on the hepatocellularly expressed members of the OATP and OCT family, their impact on transport-mediated drug-drug interactions, and on the functional consequences of variations in genes encoding these transporters.

Genome-wide analysis of epigenetic signatures for kidney-specific transporters

DNA methylation-dependent gene silencing is one of the most characterized mechanisms in epigenetic regulation of gene expression. This process is thought to influence the ability of hepatocyte nuclear factor 1 (HNF1) to transactivate organic anion transporter expression in the liver and kidney. To evaluate this further we profiled 282 mouse solute carrier transporters by examining regions near their transcription start sites for tissue-dependent differentially methylated regions (T-DMR) using restriction tag-mediated amplification to determine T-DMR disparity between the liver and kidney. Forty-two of these were associated with T-DMR tags hypomethylated in the kidney but hypermethylated in the liver. Computational analysis found a canonical HNF1-binding motif within 1 kbp of the promoter region of 13 carriers including the amino acid transporters Slc6a19, Slc6a20, Slc7a8 and Slc7a9; all expressed predominantly in the kidney. Bisulfite genomic sequencing found that CpG dinucleotides neighboring the T-DMR tags were hypomethylated in the kidney compared with the liver. The Hnf1alpha promoter region itself contained a T-DMR hypomethylated in the liver and kidney but hypermethylated in the cerebrum, consistent with the tissue distribution of Hnf1alpha. Taken together, our results show a central role of DNA methylation in the kidney-specific expression of amino acid transporters thus determining both the tissue distribution of their master regulator, Hnf1alpha, and its interaction with downstream genes.

Pharmacokinetic interaction between JBP485 and cephalexin in rats

The purpose of this study was to investigate the pharmacokinetic mechanism of interaction between JBP485 (cyclo-trans-4-L-hydroxyprolyl-L-serine, a dipeptide) and cephalexin when they were coadministered in rats. The plasma concentrations of JBP485 and cephalexin were both decreased significantly after oral combination, but little difference was observed after simultaneous intravenous administration of the two agents, suggesting that the interaction target localized in the intestine during the absorption process. The uptake in everted intestinal sacs and absorption in jejunal perfusions of JBP485 and cephalexin were dramatically reduced after drug combination. When JBP485 and cephalexin were coadministered, both the decrease in accumulative renal excretion (81.9-68.1% of JBP485 and 91.8-74.5% of cephalexin) and in renal clearance (2.89-1.87 ml/min/kg JBP485 and 2.23-1.58 ml/min/kg cephalexin) indicated that transporter(s) other than H(+)/peptide transporter (PEPT) 2 are involved in the process of excretion. Probenecid could reduce renal excretion of JBP485 and cephalexin. Moreover, the decreased uptake of JBP485 with probenecid, p-aminohippuate, or benzylpenicillin in kidney slices could be explained by an inhibition in the kidney via organic anion transporters (OATs), at least in part. The accumulation of JBP485 in human (h) OAT1- or hOAT3-human embryonic kidney (HEK) 293 cells was greater than that in vector-HEK293 cells, and the uptake could be inhibited by probenecid. These findings further confirmed that the pharmacokinetic mechanism of the drug-drug interaction between JBP485 and cephalexin could be explained by their inhibition of the same transporters in the intestinal mucosa (PEPT1) and kidneys (PEPT2 and OATs). We provide the first evidence that JBP485 is not only a substrate of PEPTs but also is excreted through OATs.

Berry anthocyanins and anthocyanidins exhibit distinct affinities for the efflux transporters BCRP and MDR1

BACKGROUND AND PURPOSE

Dietary anthocyanins hold great promise in the prevention of chronic disease but factors affecting their bioavailability remain poorly defined. Specifically, the role played by transport mechanisms at the intestinal and blood-brain barriers (BBB) is currently unknown.

EXPERIMENTAL APPROACH

In the present study, 16 anthocyanins and anthocyanidins were exposed to the human efflux transporters multidrug resistance protein 1 (MDR1) and breast cancer resistance protein (BCRP), using dye efflux, ATPase and, for BCRP, vesicular transport assays.

KEY RESULTS

All test compounds interacted with the BCRP transporter in vitro. Of these, seven emerged as potential BCRP substrates (malvidin, petunidin, malvidin-3-galactoside, malvidin-3,5-diglucoside, cyanidin-3-galactoside, peonidin-3-glucoside, cyanidin-3-glucoside) and 12 as potential inhibitors of BCRP (cyanidin, peonidin, cyanidin-3,5-diglucoside, malvidin, pelargonidin, delphinidin, petunidin, delphinidin-3-glucoside, cyanidin-3-rutinoside, malvidin-3-glucoside, pelargonidin-3,5-diglucoside, malvidin-3-galactoside). Malvidin, malvidin-3-galactoside and petunidin exhibited bimodal activities serving as BCRP substrates at low concentrations and, at higher concentrations, as BCRP inhibitors. Effects on MDR1, in contrast, were weak. Only aglycones exerted mild inhibitory activity.

CONCLUSIONS AND IMPLICATIONS

Although the anthocyanidins under study may alter pharmacokinetics of drugs that are BCRP substrates, they are less likely to interfere with activities of MDR1 substrates. The present data suggest that several anthocyanins and anthocyanidins may be actively transported out of intestinal tissues and endothelia, limiting their bioavailability in plasma and brain.

Colonic treatments and targets: issues and opportunities

The colon provides a plethora of therapeutic opportunities. There are multiple disease targets, drug molecules, and colon-specific delivery systems to be explored. Clinical studies highlight the potential for systemic delivery via the colon, and the emerging data on the levels of cell membrane transporters and metabolic enzymes along the gut could prove advantageous for this. Often efflux transporters and metabolic enzyme levels are lower in the colon, suggesting a potential for improved bioavailability of drug substrates at this site. The locoregional distribution of multiple metabolic enzymes (including cytochromes), efflux transporters (including P-glycoprotein and breast cancer resistance proteins), and influx transporters (including the solute carrier family) along the intestine is summarized. Local delivery to the colonic mucosa remains a valuable therapeutic option. New therapies that target inflammatory mediators could improve the treatment of inflammatory bowel disease, and old and new anticancer molecules could, when delivered topically, prove to be beneficial adjuncts to the current systemic or surgical treatments. New issues such as pharmacogenomics, chronotherapeutics, and the delivery of prebiotics and probiotics are also discussed in this review. Targeting drugs to the colon utilizes various strategies, each with their advantages and flaws. The most promising systems are considered in the light of the physiological data which influence their in vivo behavior.

Advancement of structure-activity relationship of multidrug resistance-associated protein 2 interactions

Multidrug resistance-associated protein 2 (MRP2/ABCC2) is mainly expressed in the apical phase of barrier membranes. It functions as a critical efflux pump in the biliary excretion of endogenous substances, such as conjugated bilirubin and bile salts, as well as many structurally diverse xenobiotics and their metabolites. Due to its important role in defining ADME/Tox properties, efforts have emerged to build the structure-activity relationship (SAR) for MRP2/ABCC2 at early stages of drug discovery process. MRP2/ABCC2 is a member of the integral membrane protein family whose high-resolution crystal structure has not been described. To overcome the obstacle of lacking detailed structural depiction, various molecular modeling approaches have been applied to derive the structural requirements for binding interactions with MRP2/ABCC2 protein, including two-dimensional (2D) and three-dimensional (3D) quantitative SAR (QSAR) analysis, pharmacophore models, and homology modeling of the transporter. Here we summarize recent progresses in understanding the SAR of MRP2/ABCC2 recognition of substrates and/or inhibitors, and describe some of the useful in vitro tools for characterizing the interactions with the transporter.

Lipophilic but not hydrophilic statins selectively induce cell death in gynaecological cancers expressing high levels of HMGCoA reductase

Recent reports have suggested that statins induce cell death in certain epithelial cancers and that patients taking statins to reduce cholesterol levels possess lower cancer incidence. However, little is known about the mechanisms of action of different statins or the effects of these statins in gynaecological malignancies. The apoptotic potential of two lipophilic statins (lovastatin and simvastatin) and one hydrophilic statin (pravastatin) was assessed in cancer cell lines (ovarian, endometrial and cervical) and primary cultured cancerous and normal tissues. Cell viability was studied by MTS assays and apoptosis was confirmed by Western blotting of PARP and flow cytometry. The expressions of key apoptotic cascade proteins were analysed. Our results demonstrate that both lovastatin and simvastatin, but not pravastatin, selectively induced cell death in dose- and time-dependent manner in ovarian, endometrial and cervical cancers. Little or no toxicity was observed with any statin on normal cells. Lipophilic statins induced activation of caspase-8 and -9; BID cleavage, cytochrome C release and PARP cleavage. Statin-sensitive cancers expressed high levels of HMG-CoA reductase compared with resistant cultures. The effect of lipophilic statins was dependent on inhibition of enzymatic activity of HMG-CoA reductase since mevalonate pre-incubation almost completely abrogated the apoptotic effect. Moreover, the apoptotic effect involved the inhibition of synthesis of geranylgeranyl pyrophosphate rather than farnesyl pyrophosphate. In conclusion, lipophilic but not hydrophilic statins induce cell death through activation of extrinsic and intrinsic apoptotic cascades in cancerous cells from the human female genital tract, which express high levels of HMG-CoA reductase. These results promote further investigation in the use of lipophilic statins as anticancer agents in gynaecological malignancies.

Esterified trityl radicals as intracellular oxygen probes

Triarylmethyl (trityl) radicals exhibit high stability and narrow linewidth under physiological conditions which provide high sensitivity and resolution for the measurement of O2 concentrations, making them attractive as EPR oximetry probes. However, the application of previously available compounds has been limited by their poor intracellular permeability. We recently reported the synthesis and characterization of esterified trityl radicals as potential intracellular EPR probes and their oxygen sensitivity, redox properties, and enzyme-mediated hydrolysis were investigated. In this paper, we report the cellular permeability and stability of these trityls in the presence of bovine aortic endothelial cells. Results show that the acetoxymethoxycarbonyl-containing trityl AMT-02 exhibits high stability in the presence of cells and can be effectively internalized. The intracellular hydrolysis of AMT-02 to the carboxylate form of the trityl (CT-03) was also observed. In addition, this internalized trityl probe was applied to measure intracellular O2 concentrations and the effects of menadione and KCN on the rates of O2 consumption in endothelial cells. This study demonstrates that these esterified trityl radicals can function as effective EPR oximetry probes measuring intracellular O2 concentration and consumption.

Analysis of DNA methylation and histone modification profiles of liver-specific transporters

Tissue-specific expression of transporters is tightly linked with their physiological functions through the regulation of the membrane transport of their substrates. We hypothesized that epigenetic regulation underlies the tissue-specific expression of mouse liver-specific transporters (Oatp1b2/Slco1b2, Ntcp/Slc10a1, Bsep/Abcb11, and Abcg5/g8). We examined their DNA methylation and histone modification profiles near the transcriptional start site (TSS) in the liver, kidney, and cerebrum. Genome-wide DNA methylation profiling with tissue-dependent differentially methylated region profiling with restriction tag-mediated amplification and subsequent bisulfite genomic sequencing demonstrated that the CpG dinucleotides around the TSS of Oatp1b2 (from -515 to +149 CpGs), Ntcp (from -481 to +495 CpGs), Bsep (from -339 to +282 CpGs), and Abcg5/g8 (from -161 to +5 CpGs for Abcg5, i.e., from -213 to -48 CpGs for Abcg8) were hypomethylated in the liver and hypermethylated in the kidney and cerebrum. The opposite pattern was observed for Pept2/Slc15a2 (from -638 to +4 CpGs), which was expressed in the kidney and cerebrum but not in the liver. These DNA methylation profiles are consistent with the tissue distribution of these transporters. A chromatin immunoprecipitation assay demonstrated that the histone H3 associated with Oatp1b2, Ntcp, Bsep, and Abcg5/g8 promoters was hyperacetylated in the liver but was acetylated very little in the kidney and cerebrum, whereas the upstream region of Pept2 was hyperacetylated only in the kidney and cerebrum. These results suggest the involvement of epigenetic systems in the tissue-specific expression of mouse transporters Oatp1b2, Ntcp, Bsep, Abcg5/g8, and Pept2.

Pharmacokinetics and metabolism of TR-14035, a novel antagonist of α4β1/α4β7 integrin mediated cell adhesion, in rat and dog

The pharmacokinetics and disposition of N-(2,6-dichlorobenzoyl)-4-(2,6-dimethoxyphenyl)-L-phenylalanine (TR-14035), a novel a4ss1/a4ss7 antagonist, were investigated in the rat and dog. Results indicate extensive clearance of TR-14035 and low oral bioavailability, 17% and 13% in the rat and dog, respectively, at an oral dose of 10 mg/kg. At least 63% of the oral dose was absorbed from the gastrointestinal tract in the rat, and about one-third of the intravenous dose was excreted into bile as unchanged drug in the rat and dog. These data indicate that the oral bioavailability of TR-14035 was limited due to significant first-pass metabolism and biliary excretion in the liver. A species-dependent difference in metabolism was observed. The principal metabolite, O-desmethyl TR-14035, observed in rat, dog and probably human, was further conjugated with sulfate in the rat, but never in dog and human, based on in vitro metabolism and in vivo metabolite profile studies. Urinary excretion was a minor elimination route, but an interesting species difference was recognized. TR-14035 was reabsorbed from the rat renal proximal tubules, and by contrast, secreted into the tubules in the dog, probably via active transport systems.

Carrier-mediated cellular uptake of pharmaceutical drugs: an exception or the rule?

It is generally thought that many drug molecules are transported across biological membranes via passive diffusion at a rate related to their lipophilicity. However, the types of biophysical forces involved in the interaction of drugs with lipid membranes are no different from those involved in their interaction with proteins, and so arguments based on lipophilicity could also be applied to drug uptake by membrane transporters or carriers. In this article, we discuss the evidence supporting the idea that rather than being an exception, carrier-mediated and active uptake of drugs may be more common than is usually assumed - including a summary of specific cases in which drugs are known to be taken up into cells via defined carriers - and consider the implications for drug discovery and development.

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.

Structure and function of the MRP2 (ABCC2) protein and its role in drug disposition

The multi-drug resistance protein 2 (MRP2; ABCC2) is an ATP-binding cassette transporter playing an important role in detoxification and chemoprotection by transporting a wide range of compounds, especially conjugates of lipophilic substances with glutathione, glucuronate and sulfate, which are collectively known as phase II products of biotransformation. In addition, MRP2 can also transport uncharged compounds in cotransport with glutathione, and thus can modulate the pharmacokinetics of many drugs. The other way around, its expression and activity are also altered by certain drugs and disease states. Unlike other members of the MRP/ABCC family, MRP2 is specifically expressed on the apical membrane domain of polarised cells as hepatocytes, renal proximal tubular cells, enterocytes and syncytiotrophoblasts of the placenta. Several naturally occurring mutations leading to the absence of functional MRP2 protein from the apical membrane have been described causing the human Dubin-Johnson syndrome associated with conjugated hyperbilirubinaemia. Experimental mutation studies have revealed critical amino acids for substrate binding in the MRP2 molecule. This review is, therefore, focused on the structure and function of MRP2, the substrates transported and the clinical relevance of MRP2.

Human organic anion transporters 1 (hOAT1/SLC22A6) and 3 (hOAT3/SLC22A8) transport edaravone (MCI-186; 3-methyl-1-phenyl-2-pyrazolin-5-one) and its sulfate conjugate

3-Methyl-1-phenyl-2-pyrazolin-5-one (MCI-186; edaravone), a novel free radical scavenger, is used for the treatment of acute cerebral infarction. After marketing, a few cases of acute renal failure were reported in patients following treatment with this drug. Because edaravone is mainly excreted into the urine following conjugation to glucuronide or sulfate, the renal excretion mechanisms of edaravone should help provide important information when considering the clinical cases. We examined the transport of edaravone and its sulfate and glucuronide conjugates via human organic anion transporter 1 (hOAT1) and 3 (hOAT3), expressed on the basolateral membranes of proximal tubules. The hOAT1- and hOAT3-transfected human embryonic kidney (HEK)-293 cells exhibited a markedly higher uptake of edaravone sulfate and a slightly higher uptake of edaravone than vector-transfected cells. The K(m) values of edaravone sulfate uptake by hOAT1 and hOAT3 were 11 and 15 microM, respectively. Estimation of the relative contribution of hOAT1 and hOAT3 using reference compounds suggested that hOAT1 and hOAT3 might contribute to the renal uptake of edaravone sulfate to the same extent. However, edaravone and its sulfate showed no cytotoxicity toward both hOAT1-HEK and control cells, suggesting that higher uptake in hOAT1-HEK did not associate with cytotoxicity of these compounds. In conclusion, our results suggest that both hOAT1 and hOAT3 are responsible for the basolateral uptake of edaravone sulfate in the kidney.

Effect of dronedarone on renal function in healthy subjects

AIMS

To assess the effects of dronedarone on renal function and tubular cation handling.

METHODS

Twelve healthy males were enrolled in a randomized, cross-over, placebo-controlled, double-blind study. They received 400 mg dronedarone or placebo twice daily for 7 days. Baseline and on-treatment renal function tests were performed under strict standardization of intakes, by assessing creatinine, sinistrin, para-amino-hippurate (PAH) and N-methylnicotinamide (NMN) CLs, and electrolyte excretion.

RESULTS

Compared with placebo, dronedarone significantly decreased renal creatinine CL (mean 138-119 ml min(-1) after dronedarone vs. 142-149 ml min(-1) after placebo) and NMN CL (448-368 ml min(-1)vs. 435-430 ml min(-1)), but did not alter renal sinistrin CL, PAH CL and other renal parameters.

CONCLUSIONS

Dronedarone reduces renal creatinine and NMN clearance by about 18%, without evidence of an effect on GFR, renal plasma flow or electrolyte exchanges. This suggests a specific partial inhibition of tubular organic cation transporters (OCT). A limited increase in serum creatinine is therefore expected with dronedarone treatment, but does not mean there is a decline in renal function.

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.

Comparative gene expression profiles of intestinal transporters in mice, rats and humans

We have studied gene expression profiles of intestinal transporters in model animals and humans. Total RNA was isolated from duodenum and the mRNA expression was measured using Affymetrix GeneChip oligonucleotide arrays. Detected genes from the intestine of mice, rats, and humans were about 60% of 22,690 sequences, 40% of 8739, and 47% of 12,559, respectively. A total of 86 genes involving transporters expressed in mice, 50 genes in rats, and 61 genes in humans were detected. Mice exhibited abundant mRNA expressions for peptide transporter HPT1, amino acid transporters CSNU3, CT1 and ASC1, nucleoside transporter CNT2, organic cation transporter SFXN1, organic anion transporter NBC3, glucose transporter SGLT1, and fatty acid transporters FABP1 and FABP2. Rats showed high expression profiles of peptide transporter PEPT1, amino acid transporters CSNU1 and 4F2HC, nucleoside transporter CNT2, organic cation transporter OCT5, organic anion transporter SDCT1, glucose transporter GLUT2 and GLUT5, and folate carrier FOLT. In humans, the highly expressed genes were peptide transporter HPT1, amino acid transporters LAT3, 4F2HC and PROT, nucleoside transporter CNT2, organic cation transporter OCTN2, organic anion transporters NADC1, NBC1 and SBC2, glucose transporters SGLT1 and GLUT5, multidrug resistance-associated protein RHO12, fatty acid transporters FABP1 and FABP2, and phosphate carrier PHC. Overall these data reveal diverse transcriptomic profiles for intestinal transporters among these species. Therefore, this transcriptional data may lead to more effective use of the laboratory animals as a model for oral drug development.

Structure-activity relationships for interaction with multidrug resistance protein 2 (ABCC2/MRP2): the role of torsion angle for a series of biphenyl-substituted heterocycles

Multidrug resistance protein 2 (ABCC2/MRP2) is an ATP-binding cassette transporter involved in the absorption, distribution, and excretion of drugs and xenobiotics. Identifying compounds that are ABCC2/MRP2 substrates and/or inhibitors and understanding their structure-activity relationships (SARs) are important considerations in the selection and optimization of drug candidates. In the present study, the interactions between ABCC2/MRP2 and a series of biphenyl-substituted heterocycles were evaluated using Caco-2 cells and human ABCC2/MRP2 gene-transfected Madin-Darby canine kidney cells. It was observed that ABCC2/MRP2 transport and/or inhibition profile, both in nature and in magnitude, depends strongly on the substitution patterns of the biphenyl system. In particular, different ortho-substitutions cause various degrees of twisting between the two-phenyl rings, resulting in changing interactions between the ligands and ABCC2/MRP2. The compounds with small ortho functions (hydrogen, fluorine, and oxygen) and, thus, the ones displaying the smallest torsion angles of biphenyl (37-45 degrees) are neither substrates nor inhibitors of human ABCC2/MRP2. The transporter interactions increase as the steric bulkiness of the ortho-substitutions increase. When the tested compounds are 2-methyl substituted biphenyls, they exhibit moderate torsion angles (54-65 degrees) and behave as ABCC2/MRP2 substrates as well as mild inhibitors [10-40% compared with 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethyl-sulfanyl)methylsulfanyl] propionic acid (MK571)]. For the 2,2'-dimethyl substituted biphenyls, the torsions are enhanced (78-87 degrees) and so is the inhibition of ABCC2/MRP2. This class of compounds behaves as strong inhibitors of ABCC2/MRP2. These results can be used to define the three-dimensional structural requirements of ABCC2/MRP2 interaction with their substrates and inhibitors, as well as to provide SAR guidance to support drug discovery.

LXR alpha transactivates mouse organic solute transporter alpha and beta via IR-1 elements shared with FXR

PURPOSE

Recently identified organic solute transporter (Ost) alpha and beta are located on the basolateral membrane of enterocytes and may be responsible for the intestinal absorption of many substrates including bile acids. In the present study, the mechanism governing the transcriptional regulation of their expression was investigated.

METHODS AND RESULTS

To clarify the transcriptional regulation of Osts, reporter gene assays were performed using mouse Ostalpha/beta promoter-luciferase reporter constructs. Co-transfection of the constructs with farnesoid X receptor (FXR) and retinoid X receptor alpha (RXRalpha) or liver X receptor alpha (LXRalpha) and RXRalpha into Caco-2 cells induced the transcriptional activities of both Ost alpha and beta and further increases were observed following treatment with each agonist. Sequence analyses indicated the presence of IR-1 regions in Ostalpha and Ostbeta promoters, which was confirmed by the finding that the deletion of IR-1 sequences abolished the response to FXR and LXRalpha. Furthermore, mutations in IR-1 reduced the FXR- and LXRalpha-dependent transactivation of Ostalpha/beta. Together with the detection of direct binding of FXR/RXRalpha and LXRalpha/RXRalpha to the IR-1 elements, the presence of functional FXRE/LXRE was revealed in the promoter region of both Ostalpha and Ostbeta. In addition, the stimulatory effect of FXR/RXRalpha and LXRalpha/RXRalpha on Ostalpha, but not on Ostbeta, was further enhanced by HNF-4alpha.

CONCLUSIONS

It was concluded that LXRalpha/RXRalpha transcriptionally regulate mouse Ostalpha/beta via IR-1 elements shared with FXR/RXRalpha. Exposure to FXR/LXRalpha modulators may affect the disposition of Ostalpha/beta substrates.

Application of the isolated perfused rat kidney model to assess gender effects on drug excretion

PURPOSE

To study the effect of gender on the renal disposition of two organic anions, p-aminohippuric acid (PAH) and furosemide (FSM) in the isolated perfused rat kidney (IPK).

METHODS

IPK experiments (3-4 per treatment group) were conducted using kidneys from male and female Sprague Dawley rats. PAH was administered as a continuous infusion (with loading dose, targeted steady-state concentration 10 ug/mL). FSM was added as a bolus dose (2.65 mg, targeted concentration 33 ug/mL). Urine was collected in 10-min. intervals and perfusate was sampled at the midpoint of each collection period. Control (drug naïve) perfusions were performed for both genders. PAH and FSM were measured by HPLC. Kidney viability (GFR [estimated using inulin clearance], sodium reabsorption, glucose reabsorption) was monitored continuously during each perfusion experiment (2-h duration).

RESULTS

Good kidney function was maintained across all study groups, and lower GFR estimates in female kidneys were due to differences in kidney weight. For PAH, kidney weight corrected renal clearance (0.88 +/- 0.37 mL/min/g vs. 0.59 +/- 0.19 mL/min/g) and excretion ratio (3.8 +/- 1.7 vs. 2.2 +/- 0.72) were significantly higher in male kidneys. For FSM, renal clearance was significantly lower in female (0.10 +/- 0.05 mL/min/g) compared to male kidneys (0.15 +/- 0.07 mL/min/g). Mass balance analysis showed that FSM cumulative urinary excretion was significantly higher and kidney accumulation was significantly lower in experiments with male kidneys.

CONCLUSIONS

The study demonstrates that the IPK is a useful model to assess gender effects on renal drug disposition. The renal excretion of organic anions is reduced in female rats, possibly due to gender differences in expression and/or activity of membrane transporters (both basolateral and luminal) in the kidney.

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.

Complicating factors in safety testing of drug metabolites: Kinetic differences between generated and preformed metabolites

This paper aims to provide a scientifically based perspective on issues surrounding the proposed toxicology testing of synthetic drug metabolites as a means of ensuring adequate nonclinical safety evaluation of drug candidates that generate metabolites considered either to be unique to humans or are present at much higher levels in humans than in preclinical species. We put forward a number of theoretical considerations and present several specific examples where the kinetic behavior of a preformed metabolite given to animals or humans differs from that of the corresponding metabolite generated endogenously from its parent. The potential ramifications of this phenomenon are that the results of toxicity testing of the preformed metabolite may be misleading and fail to characterize the true toxicological contribution of the metabolite when formed from the parent. It is anticipated that such complications would be evident in situations where (a) differences exist in the accumulation of the preformed versus generated metabolites in specific tissues, and (b) the metabolite undergoes sequential metabolism to a downstream product that is toxic, leading to differences in tissue-specific toxicity. Owing to the complex nature of this subject, there is a need to treat drug metabolite issues in safety assessment on a case-by-case basis, in which a knowledge of metabolite kinetics is employed to validate experimental paradigms that entail administration of preformed metabolites to animal models.

Pharmacogenetic differences and drug-drug interactions in immunosuppressive therapy

With the advent of new immunosuppressants and formulations, the elucidation of molecular targets and the evolution of therapeutic drug monitoring, the field of organ transplantation has witnessed significant reductions in acute rejection rates, prolonged graft survival and improved patient outcome. Nonetheless, challenges persist in the use of immunosuppressive medications. Marked interindividual variability remains in drug concentrations and drug response. As medications with narrow therapeutic indices, variations in immunosuppressant concentrations can result in acute toxicity or transplant rejection. Recent studies have begun to identify factors that contribute to this variability with the promise of tailoring immunosuppressive regimens to the individual patient. These advances have uncovered differences in genetic composition in drug-metabolising enzymes, drug transporters and drug targets. This review focuses on commonly used maintenance immunosuppressants (including cyclosporin, mycophenolate mofetil, tacrolimus, sirolimus, everolimus, azathioprine and corticosteroids), examines current studies on pharmacogenetic differences in drug-metabolising enzymes, drug transporters and drug targets and addresses common drug-drug interactions with immunosuppressant therapies. The potential role of drug-metabolising enzymes in contributing to these drug-drug interactions is briefly considered.

Hepatobiliary Disposition in Primary Cultures of Dog and Monkey Hepatocytes

Hepatobiliary transporters are a major route for elimination of xenobiotics and endogenous products. In vitro hepatobiliary models have been reported for human and rat, but not for the other preclinical species used in safety evaluation. We have established methodologies for culturing dog and monkey hepatocytes with optimal bile canalicular formation and function, using a sandwich culture comprising rigid collagen substratum and gelled collagen overlay. Hepatic uptake utilizing sinusoidal transporters and biliary excretion through canalicular transporters were assessed using the bile salt taurocholate, salicylate (negative control), and the Bsep inhibitors cyclosporin A (CsA) and glyburide. There was significant taurocholate and salicylate canalicular efflux in dog and monkey hepatocytes, although the amount of salicylate transported was one thousandth that of taurocholate. Species differences were observed, as glyburide significantly inhibited taurocholate uptake in monkey (64% at 10 microM) but not dog hepatocytes, and inhibited taurocholate efflux in dog (100% at 10 microM) but not monkey hepatocytes. CsA did not inhibit bile salt uptake and significantly inhibited canalicular efflux in dog (at 0.1 microM) and monkey (at 1 and 10 microM) hepatocyte cultures. These results suggest that glyburide is a bile salt uptake inhibitor in monkey but not in dog hepatocytes and that CsA inhibits bile salt canalicular efflux but not basolateral uptake in these species. We have established dog and monkey hepatocytes in sandwich culture with intact bile canalicular formation and function. The differences observed in taurocholate transport between dog and monkey hepatocytes may be indicative of in vivo species differences.

Comparison of drug efflux transport kinetics in various blood-brain barrier models

The present study quantitatively compared the drug efflux transport kinetics of 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM) and its fluorescent metabolite 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) in various blood-brain barrier (BBB) models. BCECF-AM was exposed to freshly isolated bovine brain microvessels (BBM), primary cultured bovine brain microvessel endothelial cells (BBMEC), and MDCK-MDR1 cells for 30 min in the presence or absence of the P-glycoprotein (P-gp) inhibitor 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). P-gp transport kinetics were determined indirectly by calculating the difference in BCECF accumulation when P-gp was functional and completely inhibited by GF120918 (3.2 microM). Multidrug resistance-associated protein (MRP) transport kinetics were determined by measuring the amount of BCECF transported out of the cell over time. For P-gp-related transport, Km values for BCECF-AM were approximately the same in all three models (around 2 microM), whereas the Vmax was 4-fold greater in the BBM than in the BBMEC or MDCKII-MDR1 cells. For MRP-related transport, Km values for BCECF varied widely among the three BBB models with a rank order of MDCKII-MDR1 < BBMEC < BBM. Like P-gp, the Vmax of BCECF for MRP-related transport was overwhelmingly higher in the BBM compared with the cultured cells. Because differences in the expression of P-gp, MRP5, and MRP6 were observed in the various BBB models using reverse transcription-polymerase chain reaction techniques, the disparity in transport kinetics between the BBB models may be linked to variations in the amount or type of drug efflux transporters expressed in each model. The present study introduces a method of quantitatively evaluating drug efflux transport kinetics in the BBB.

Metabolism and transporter-mediated drug–drug interactions of the endothelin-A receptor antagonist CI-1034

CI-1034, an endothelin-A receptor antagonist was being developed for pulmonary hypertension. Drug-drug interaction studies using human hepatic microsomes were conducted to assess CYP1A2, CYP2C9, CYP2C19, CYP3A4 and CYP2D6 inhibition potential; CYP3A4 induction potential was evaluated using primary human hepatocytes. CI-1034 moderately inhibited CYP2C9 (IC(50) 39.6 microM) and CYP3A4 activity (IC(50) 21.6 microM); CYP3A4 inhibition was metabolism-dependent. In human hepatocytes, no increase in CYP3A4 activity was observed in vitro, while mRNA was induced 15-fold, similar to rifampin, indicating that CI-1034 is both an inhibitor and inducer of CYP3A4. A 2-week clinical study was conducted to assess pharmacokinetics, pharmacodynamics and safety. No significant changes were observed in [formula: see text] between days 1 and 14. However, reversible elevations of serum liver enzymes were observed with a 50mg BID dose and the program was terminated. To further understand the interactions of CI-1034 in the liver and possible mechanisms of the observed hepatotoxicity, we evaluated the effect of CI-1034 on bile acid transport and previously reported that CI-1034 inhibited biliary efflux of taurocholate by 60%, in vitro. This indicated that inhibition of major hepatic transporters could be involved in the observed hepatotoxicity. We next evaluated the in vitro inhibition potential of CI-1034 with the major hepatic transporters OATP1B1, OATP1B3, OATP2B1, MDR1, MRP2 and OCT. CI-1034 inhibited OATP1B1 (K(i) 2 microM), OATP1B3 (K(i) 1.8 microM) and OATP2B1 activity (K(i) 3.3 microM) but not OCT, MDR1 or MRP2 mediated transport. Our data indicates that CI-1034 is an inhibitor of major hepatic transporters and inhibition of bile efflux may have contributed to the observed clinical hepatotoxicity. We recommend that in vitro drug-drug interaction panels include inhibition and induction studies with transporters and drug metabolizing enzymes, to more completely assess potential in vivo interactions or toxicity.

Carrier-mediated hepatic uptake of a novel nonrenal excretion type uric acid generation inhibitor, Y-700

Y-700, a novel xanthine oxidase inhibitor, was recently developed for the treatment of hyperuricemia and gout. Since the major elimination route of this compound is hepatic metabolism and excretion, the aim of the present study was to characterize the uptake mechanism of Y-700 in the liver, which is also the pharmacological target of Y-700. Efficient uptake of Y-700 was observed both in the liver in vivo and in isolated rat hepatocytes. The uptake was Na(+)-dependent, saturable and inhibited both by ATP-depressants and various organic anions. Indomethacin competitively inhibited Y-700 uptake, whereas the inhibitory effect of organic cations and nucleosides was not so remarkable. Saturable and Na(+)-dependent uptake of Y-700 was also observed in freshly isolated human hepatocytes. Uptake of Y-700 by sinusoidal membrane transporters, such as organic anion transporter (Oat) 2 and organic anion transporting polypeptide (OATP)-B, OATP-C, OATP-8, and Oatp1, could not be detected although uptake of Y-700 in the oocytes expressing sodium/taurocholate cotransporting polypeptide (NTCP) was slightly observed. In conclusion, active transport system(s), which specifically recognize certain types of anionic compounds, are involved in the hepatic uptake of Y-700 and, at least partially, relevant to its elimination from the circulation as well as delivery to pharmacological target.

Functional assessment of multiple P-glycoprotein (P-gp) probe substrates: influence of cell line and modulator concentration on P-gp activity

Compounds known to modulate P-glycoprotein (P-gp) activity were evaluated in cell monolayers expressing P-gp for their effects on the secretory transport of P-gp substrates paclitaxel, vinblastine, and digoxin. Paclitaxel has been proposed to selectively interact with a binding site on P-gp that is distinct from the vinblastine and digoxin-binding site. Using Madin-Darby canine kidney (MDCK)-multidrug resistance-1 (MDR1), MDCK-wild-type (WT), and Caco-2 cell monolayers, the basal-to-apical (BL-AP) apparent permeability (Papp) of [3H]paclitaxel, [3H]vinblastine, and [3H]digoxin in the presence of various concentrations of a series of structurally diverse P-gp substrates and modulators of P-gp function were determined. MDCK-WT cell monolayers demonstrated active secretory transport of all P-gp substrate probes, although the sensitivity to inhibition by verapamil was lower than that demonstrated in MDCK-MDR1 cell monolayers. When evaluated as competitive inhibitors, several known P-gp substrates had no effect or only a slight modulatory effect on the BL-AP Papp of all probe substrates in MDCK-MDR1 cells. The secretory transport of P-gp substrates in MDCK-WT cells was more sensitive to inhibition by known P-gp modulators compared with MDCK-MDR1 cells. Low concentrations of ketoconazole (1-3 microM) activated the BL-AP Papp of [3H]vinblastine and [3H]digoxin in MDCK-MDR1 cells but not in MDCK-WT or Caco-2 cells. Determination of secretory transport in P-gp expressing cell monolayers, such as MDCK-MDR1 and Caco-2, may be complicated by substrate cooperativity and allosteric binding, which may result in the activation of P-gp. In addition, expression of other efflux transporters in these cell lines introduces additional complexity in distinguishing which transporter is responsible for substrate recognition and transport.

Functional characterization of SLCO1B1 (OATP-C) variants, SLCO1B1*5, SLCO1B1*15 and SLCO1B1*15+C1007G, by using transient expression systems of HeLa and HEK293 cells

OBJECTIVES

SLCO1B1*5 and SLCO1B1*15 have been reported to reduce the clearance of pravastatin in healthy volunteers. However, there remains controversy in the effects of SLCO1B1*5 on the activity of OATP1B1 in vitro. In addition, the effect of SLCO1B1*15 on the function of OATP1B1 has not been studied using cDNA-expression systems. Object of the present study was to study the influence of SLCO1B1*5, *15 and *15+C1007G, a novel haplotype found in a patient with pravastatin-induced myopathy, on the functional properties of OATP1B1 by transient expression systems of HEK293 and HeLa cells using endogenous conjugates and statins as substrates.

METHODS

Transporting assays for endogenous substrates were performed using tritium labeled estradiol-17beta-D-glucuronide and estrone-3-sulfate. Quantitation of pravastatin, atorvastatin, cerivastatin and simvastatin were carried out using HPLC tandem mass spectrometry.

RESULTS

The transporting activities of cells expressing SLCO1B1*5, *15 and *15+C1007G decreased significantly but those of SLCO1B1*1b, *1a+C1007G and *1b+C1007G were not altered for all of the substrates tested except for simvastatin. Kinetic analysis of pravastatin and atorvastatin showed that Km values were not altered but Vmax values decreased significantly in cells expressing SLCO1B1*5, *15 and *15+C1007G. Immunocytochemical study showed that SLCO1B1*5, *15 and *15+C1007G proteins are localized not only at the plasma membrane but also in the intracellular space.

CONCLUSIONS

These findings suggest that 521T>C, existing commonly in SLCO1B1*5, *15 and *15+C1007G, is the key single nucleotide polymorphism (SNP) that determines the functional properties of SLCO1B1*5, *15 and *15+C1007G allelic proteins and that decreased activities of these variant proteins are mainly caused by a sorting error produced by this SNP.

Gender-specific expression of liver organic anion transporters in rat

BACKGROUND

Sex differences in drug pharmacokinetics have been well recognized and gender has been considered a risk factor for adverse events to medications. The aim of this study was to investigate the effect of gender on the expression of hepatocellular transport proteins involved in uptake and secretion of organic anions in rat.

MATERIALS AND METHODS

Expression of the rat liver organic anion transporting polypeptides (Oatps) and multidrug resistance proteins (Mrps) was analysed by reverse transcription polymerase chain reaction (RT-PCR), immunoblot analysis and immunofluorescence microscopy in male and female rats. Regulation of these transport proteins in response to the steroid dehydroepiandrosterone (DHEA) was investigated.

RESULTS

In untreated rats, protein expression significantly differed between genders being higher (Mrp2, Mrp3), comparable [Oatp1a1 (Oatp1); Oatp1b2 (Oatp4)] or lower [Oatp1a4 (Oatp2)] in female than in male rat. DHEA treatment over 3 days (100 mg d(-1)) led to a further increase in Mrp3 expression only in female rats. Mrp2 expression was not influenced by DHEA treatment. Oatp1a1 and Oatp1b2 were significantly down-regulated after DHEA treatment in both male and female rats. In contrast, Oatp1a4 was down-regulated in male rats only.

CONCLUSIONS

In rat, liver transport proteins of the Oatp and Mrp family are expressed and regulated in a gender-specific manner according to sexual differences in the hepatic metabolism of steroids and drugs. These findings may partly explain the well-known sex differences in hepatic handling of organic anions.

Use ofin vitrotransporter assays to understand hepatic and renal disposition of new drug candidates

Hepatic and renal transporters contribute to the uptake, secretion and reabsorption of endogenous compounds, xenobiotics and their metabolites and have been implicated in drug-drug interactions and toxicities. Characterising the renal and hepatic disposition of drug candidates early in development would lead to more rational drug design, as chemotypes with 'ideal' pharmacokinetic characteristics could be identified and further refined. Because transporters are often organ specific, 'custom' transporter panels need to be identified for each major organ and chemotype to be evaluated, and appropriate studies planned. This review outlines the major renal and hepatic transporters and some of the in vitro transporter reagents, assays and processes that can be used to evaluate the renal and hepatic disposition of new chemical entities during drug discovery and development.

Comparison of P-glycoprotein-mediated drug-digoxin interactions in Caco-2 with human and rodent intestine: relevance to in vivo prediction

Inhibition of P-glycoprotein (PGP) resulting from the co-administration of substrate drugs represents a potential source of drug-drug interactions. Although in vitro screens can readily identify such interactions, the accuracy with which they mimic interactions in tissues or their value in predicting interactions in vivo is unresolved. This was addressed for the model PGP substrate digoxin by comparing the modulation of its permeability across Caco-2 cells and ex vivo human and rodent intestine by drugs for which pharmacokinetic data on interactions with digoxin in man is available. All five compounds (talinolol, omeprazole, verapamil, quinidine, cyclosporin) dose-dependently increased absorptive (A-B) digoxin permeability with maximal increases of 2.2-4.5-fold across Caco-2. Quantitatively similar increases were observed in ex vivo human and mouse intestine and studies in mdr1a(-/-) intestine confirmed that these interactions are mediated solely by PGP. In vitro changes in digoxin permeability were qualitative indicators of the increase in digoxin C(max) for these compounds in man, although accounting for the luminal drug concentrations expected for a given oral dose was a critical consideration. Based on a limited dataset these data suggest that Caco-2 accurately mimics intestinal digoxin interactions and may be useful in predicting the threshold dose at which interactions become clinically significant. Further studies across a wider range of drugs are needed to determine the broader applicability of in vitro data for quantitative prediction of clinical drug interactions.

Identification of esterases expressed in Caco-2 cells and effects of their hydrolyzing activity in predicting human intestinal absorption

The absorption characteristics of temocapril were investigated using Caco-2 cells, and the esterases expressed in Caco-2 cells were identified. Temocapril was almost completely hydrolyzed to temocaprilat during transport across Caco-2 cells. Hydrolysis experiments of temocapril in Caco-2 cell 9000g supernatant (S9) and brush-border membrane vesicles showed that temocapril was mainly hydrolyzed within the cells after uptake, after which the temocaprilat formed was transported to both the apical and basolateral surfaces. In native polyacrylamide gel electrophoresis by detection of hydrolase activity for 1-naphthylbutyrate, Caco-2 cell S9 showed a band with high esterase activity and another band with extremely low activity. The proteins in the major and minor bands were identified as carboxylesterase-1 (hCE-1) and carboxylesterase-2 (hCE-2). The abundant expression of hCE-1 in Caco-2 cells was supported by reverse transcription-polymerase chain reaction. In the normal human small intestine, hCE-2 is abundantly present, although the human liver expresses much higher levels of hCE-1 and lower levels of hCE-2. The expression pattern of carboxylesterases in Caco-2 cells is completely different from that in human small intestine but very similar to that in human liver. Since the substrate specificity of hCE-1 differs from that of hCE-2, it is suggested that the prediction of human intestinal absorption using Caco-2 cell monolayers should be performed carefully in the case of ester- and amide-containing drugs such as prodrugs.

Renal clearance of endogenous hippurate correlates with expression levels of renal organic anion transporters in uremic rats

Hippurate (HA) is a harmful uremic toxin that accumulates during chronic renal failure, and failure of the excretion system for uremic toxins is thought to be responsible. Recently, we reported that rat organic anion transporter 1 (rOat1) is the primary mediator of HA uptake in the kidney, and so now we have studied the pharmacokinetics and tissue distribution of HA after a single i.v. dose of HA to normal and 5/6 nephrectomized rats (5/6Nx rats). In control rats, the renal and biliary clearances of HA were 18.1 and 0.1 ml/min/kg, respectively. Plasma clearance decreased as dosage increased from 0.1 to 5 mg/kg, which suggests that renal tubular secretion is the primary route for elimination of HA. The plasma clearance of HA was significantly decreased in 5/6 Nx rats compared with normal rats. In 5/6 Nx rats, renal clearance of endogenous HA correlated more closely with clearance of p-aminohippurate than with that of creatinine. Protein expression of rOat1 and rOat3, assessed by Western blot analysis, was decreased in 5/6 Nx rats. Furthermore, in 5/6 Nx rats, the renal secretory clearance of endogenous HA correlated closely with protein expression of renal rOats. Thus, HA is primarily eliminated from the plasma via the kidney by active tubular secretion. The renal clearance of endogenous HA seems to be a useful indicator of changes in renal secretion that accompany the reduced levels of OAT protein in chronic renal failure.

Comparison of human and monkey peptide transporters: PEPT1 and PEPT2

Human proton-dependent peptide transporters, PEPT1 and PEPT2, mediate the cellular uptake of di- and tripeptides as well as a variety of drug molecules. Although PEPT1 and PEPT2 have been cloned from many species, there are no data available for monkey, an important pharmacological and preclinical species in drug development. In this study, it was first verified that monkey intestine transports a model dipeptide, Gly-Sar, in a proton-dependent manner (0.30 +/- 0.05 pmol cm(-2) s(-1) at pH 6.0 and 0.10 +/- 0.03 pmol cm(-2) s(-1) at pH 7.4) in the absorptive direction, presumably by monkey PEPT1. RT-PCR and rapid amplification of cDNA ends (RACE) were then used to clone monkey PEPT1 and PEPT2. Monkey PEPT1 (2127 bp and 708 amino acids) was found to be >94 and > 92% identical to human PEPT1 at the cDNA and amino acid level, respectively. Monkey PEPT2 (2190 bp and 729 amino acids) was found to be > 97% identical to human PEPT2 at both the cDNA and amino acid levels. Functional comparison of human and monkey peptide transporters expressed in HeLa cells suggested that functionalities of PEPT1 and PEPT2 were largely conserved in terms of Gly-Sar uptake kinetics and inhibitor specificity (for most tested substrates). Finally, Northern and RT-PCR analyses revealed some differences in tissue mRNA levels of peptide transporters between human and monkey.

Evaluation of drug-drug interaction in the hepatobiliary and renal transport of drugs

Recent studies have revealed the import role played by transporters in the renal and hepatobiliary excretion of many drugs. These transporters exhibit a broad substrate specificity with a degree of overlap, suggesting the possibility of transporter-mediated drug-drug interactions with other substrates. This review is an overview of the roles of transporters and the possibility of transporter-mediated drug-drug interactions. Among the large number of transporters, we compare the Ki values of inhibitors for organic anion transporting polypeptides (OATPs) and organic anion transporters (OATs) and their therapeutic unbound concentrations. Among them, cephalosporins and probenecid have the potential to produce clinically relevant OAT-mediated drug-drug interactions, whereas cyclosporin A and rifampicin may trigger OATP-mediated ones. These drugs have been reported to cause drug-drug interactions in vivo with OATs or OATP substrates, suggesting the possibility of transporter-mediated drug-drug interactions. To avoid adverse consequences of such transporter-mediated drug-drug interactions, we need to be more aware of the role played by drug transporters as well as those caused by drug metabolizing enzymes.

Strategies to improve oral drug bioavailability

Efforts to improve oral drug bioavailability have grown in parallel with the pharmaceutical industry. As the number and chemical diversity of drugs has increased, new strategies have been required to develop orally active therapeutics. The past two decades have been characterised by an increased understanding of the causes of low bioavailability and a great deal of innovation in oral drug delivery technologies, marked by an unprecedented growth of the drug delivery industry. The advent of biotechnology and consequent proliferation of biopharmaceuticals have brought new challenges to the drug delivery field. In spite of the difficulties associated with developing oral forms of this type of therapeutics, significant progress has been made in the past few years, with some oral proteins, peptides and other macromolecules currently advancing through clinical trials. This article reviews the approaches that have been successfully applied to improve oral drug bioavailability, primarily, prodrug strategies, lead optimisation through medicinal chemistry and formulation design. Specific strategies to improve the oral bioavailability of biopharmaceuticals are also discussed.

Differential contributions of rOat1 (Slc22a6) and rOat3 (Slc22a8) to the in vivo renal uptake of uremic toxins in rats

PURPOSE

Evidence suggests that uremic toxins such as hippurate (HA), indoleacetate (IA), indoxyl sulfate (IS), and 3-carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF) promote the progression of renal failure by damaging tubular cells via rat organic anion transporter 1 (rOat1) and rOat3 on the basolateral membrane of the proximal tubules. The purpose of the current study is to evaluate the in vivo transport mechanism responsible for their renal uptake.

METHODS

We investigated the uremic toxins transport mechanism using the abdominal aorta injection technique [i.e., kidney uptake index (KUI) method], assuming minimal mixing of the bolus with serum protein from circulating serum.

RESULTS

Maximum mixing was estimated to be 5.8% of rat serum by measuring estrone sulfate extraction after addition of 0-90% rat serum to the arterial injection solution. Saturable renal uptake of p-aminohippurate (PAH, K(m) = 408 microM) and benzylpenicillin (PCG, K(m) = 346 microM) was observed, respectively. The uptake of PAH and PCG was inhibited in a dose-dependent manner by unlabeled PCG (IC(50) = 47.3 mM) and PAH (IC(50) = 512 microM), respectively, suggesting that different transporters are responsible for their uptake. A number of uremic toxins inhibited the renal uptake of PAH and PCG. Excess PAH, which could inhibit rOat1 and rOat3, completely inhibited the saturable uptake of IA, IS, and CMPF by the kidney, and by 85% for HA uptake. PCG inhibited the total saturable uptake of HA, IA, IS, and CMPF by 10%, 10%, 45%, and 65%, respectively, at the concentration selective for rOat3.

CONCLUSIONS

rOat1 could be the primary mediator of the renal uptake of HA and IA, accounting for approximately 75% and 90% of their transport, respectively. rOat1 and rOat3 contributed equally to the renal uptake of IS. rOat3 could account for about 65% of the uptake of CMPF under in vivo physiologic conditions. These results suggest that rOat1 and rOat3 play an important role in the renal uptake of uremic toxins and the induction of their nephrotoxicity.

Physiological, pharmacological and clinical features of the multidrug resistance protein 2

Multidrug resistance protein 2 (MRP2, ABCC2) is a drug efflux pump belonging to the ATP-binding cassette (ABC) transporter superfamily. MRP2 is present predominantly at the biliary pole of hepatocytes and is also expressed in the kidney and intestine. It plays a major role in hepato-biliary elimination of many structurally diverse xenobiotics, including organic anions and drug conjugates, and therefore most likely contributes to pharmacokinetic parameters of these compounds. MRP2 also handles endogenous molecules such as bilirubin, and its overexpression has been shown to confer a multidrug resistance phenotype to tumoral cells. MRP2 expression can be regulated by endogenous substances such as inflammatory cytokines and biliary acids. The MRP2 levels and activity can also be affected by a large panel of xenobiotics, including chemopreventive agents and ligands of the pregnane X receptor, which may be a potential source of drug-drug interactions and drug adverse effects. MRP2 appears therefore as one of the major drug efflux pumps of the organism, whose functional and regulatory features are important to consider, notably for drug disposition.