The modified dipeptide, enalapril, an angiotensin-converting enzyme inhibitor, is transported by the rat liver organic anion transport protein

Scleroderma: An insight into causes, pathogenesis and treatment strategies

Scleroderma is an autoimmune disorder, characterized by morphological changes in skin followed by visceral organs. The pathogenesis of scleroderma involves immune imbalance and generation of auto antibodies. The major causes of scleroderma include multitude of factors such as immune imbalance, oxidative stress, genetics and environment factors. A constant effort has been made to treat scleroderma through different approaches and necessitates life time administration of drugs for maintenance of a good quality life. It has been reported more in women compared to men. Traditional treatment strategies are restricted by limited therapeutic capability due to associated side effects. Advancement in development of novel drug delivery approaches has opened a newer avenue for efficient therapy. Current review is an effort to reflect scleroderma in provisions of its pathogenesis, causative factors, and therapeutic approaches, with concern to mode of action, pharmacokinetics, marketed products, and side effects of drugs.

Metabolomic Profiling of Portal Blood and Bile Reveals Metabolic Signatures of Primary Sclerosing Cholangitis

Primary sclerosing cholangitis (PSC) is a pathogenically complex, chronic, fibroinflammatory disorder of the bile ducts without known etiology or effective pharmacotherapy. Emerging in vitro and in vivo evidence support fundamental pathophysiologic mechanisms in PSC centered on enterohepatic circulation. To date, no studies have specifically interrogated the chemical footprint of enterohepatic circulation in PSC. Herein, we evaluated the metabolome and lipidome of portal venous blood and bile obtained at the time of liver transplantation in patients with PSC (n = 7) as compared to individuals with noncholestatic, end-stage liver disease (viral, metabolic, etc. (disease control, DC, n = 19)) and to nondisease controls (NC, living donors, n = 12). Global metabolomic and lipidomic profiling was performed on serum derived from portal venous blood (portal serum) and bile using ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) and differential mobility spectroscopy-mass spectroscopy (DMS-MS; complex lipid platform). The Mann–Whitney U test was used to identify metabolites that significantly differed between groups. Principal-component analysis (PCA) showed significant separation of both PSC and DC from NC for both portal serum and bile. Metabolite set enrichment analysis of portal serum and bile demonstrated that the liver-disease cohorts (PSC and DC) exhibited similar enrichment in several metabolite categories compared to NC. Interestingly, the bile in PSC was uniquely enriched for dipeptide and polyamine metabolites. Finally, analysis of patient-matched portal serum and biliary metabolome revealed that these biological fluids were more homogeneous in PSC than in DC or NC, suggesting aberrant bile formation and enterohepatic circulation. In summary, PSC and DC patients exhibited alterations in several metabolites in portal serum and bile, while PSC patients exhibited a unique bile metabolome. These specific alterations in PSC are amenable to hypothesis testing and, potentially, therapeutic pharmacologic manipulation.

How drugs get into cells: tested and testable predictions to help discriminate between transporter-mediated uptake and lipoidal bilayer diffusion

One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest. We use this strategy to compare the intellectual status and available evidence for two models or views of mechanisms of transmembrane drug transport into intact biological cells. One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose “natural” biological roles, and substrates are based in intermediary metabolism. Despite a recent review elsewhere, we can find no evidence able to support BDII as we can find no experiments in intact cells in which phospholipid bilayer diffusion was either varied independently or measured directly (although there are many papers where it was inferred by seeing a covariation of other dependent variables). By contrast, we find an abundance of evidence showing cases in which changes in the activities of named and genetically identified transporters led to measurable changes in the rate or extent of drug uptake. PBIN also has considerable predictive power, and accounts readily for the large differences in drug uptake between tissues, cells and species, in accounting for the metabolite-likeness of marketed drugs, in pharmacogenomics, and in providing a straightforward explanation for the late-stage appearance of toxicity and of lack of efficacy during drug discovery programmes despite macroscopically adequate pharmacokinetics. Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive.

Role of multidrug resistance-associated protein 4 in the basolateral efflux of hepatically derived enalaprilat

Hepatic uptake and efflux transporters govern the systemic and hepatic exposure of many drugs and metabolites. Enalapril is a pharmacologically inactive prodrug of enalaprilat. Following oral administration, enalapril is converted to enalaprilat in hepatocytes and undergoes translocation into the systemic circulation to exert its pharmacologic effect by inhibiting angiotensin-converting enzyme. Although the transport proteins governing hepatic uptake of enalapril and the biliary excretion of enalapril and enalaprilat are well established, it remains unknown how hepatically derived enalaprilat translocates across the basolateral membrane into the systemic circulation. In this study, the role of ATP-binding cassette transporters in the hepatic basolateral efflux of enalaprilat was investigated using membrane vesicles. ATP-dependent uptake of enalaprilat into vesicles expressing multidrug resistance-associated protein (MRP) 4 was significantly greater (∼3.8-fold) than in control vesicles. In contrast, enalaprilat was not transported to a significant extent by MRP3, and enalapril was not transported by either MRP3 or MRP4. The functional importance of MRP4 in the basolateral excretion of derived enalaprilat was evaluated using a novel basolateral efflux protocol developed in human sandwich-cultured hepatocytes. Under normal culture conditions, the mean intrinsic basolateral efflux clearance (CLint ,basolateral) of enalaprilat was 0.026 ± 0.012 µl/min; enalaprilat CLint,basolateral was significantly reduced to 0.009 ± 0.009 µl/min by pretreatment with the pan-MRP inhibitor MK-571. Results suggest that hepatically derived enalaprilat is excreted across the hepatic basolateral membrane by MRP4. Changes in MRP4-mediated basolateral efflux may alter the systemic concentrations of this active metabolite, and potentially the efficacy of enalapril.

Formed and preformed metabolites: facts and comparisons

The administration of metabolites arising from new drug entities is often employed in drug discovery to investigate their associated toxicity. It is expected that administration of metabolites can predict the exposure of metabolites originating from the administration of precursor drug. Whether exact and meaningful information can be obtained from this has been a topic of debate. This communication summarizes observations and theoretical relationships based on physiological modelling for the liver, kidney and intestine, three major eliminating organs/tissues. Theoretical solutions based on physiological modelling of organs were solved, and the results suggest that deviations are expected. Here, examples of metabolite kinetics observed mostly in perfused organs that did not match predictions are provided. For the liver, discrepancies in fate between formed and preformed metabolites may be explained by the heterogeneity of enzymes, the presence of membrane barriers and whether transporters are involved. For the kidney, differences have been attributed to glomerular filtration of the preformed but not the formed metabolite. For the intestine, the complexity of segregated flows to the enterocyte and serosal layers and differences in metabolism due to the route of administration are addressed. Administration of the metabolite may or may not directly reflect the toxicity associated with drug use. However, kinetic data on the preformed metabolite will be extremely useful to develop a sound model for modelling and simulations; in-vitro evidence on metabolite handling at the target organ is also paramount. Subsequent modelling and simulation of metabolite data arising from a combined model based on both drug and preformed metabolite data are needed to improve predictions on the behaviours of formed metabolites.

Safety testing of metabolites: Expectations and outcomes

Metabolites arising from chemical entities, old or new, are often mediators of toxicity. Frequently, metabolites are investigated in test animals, with the expectation that the resultant toxicity or activity will mimic the exposure of their formed counterparts. This communication described observations that showed discrepant kinetics between formed and preformed metabolites in the liver, intestine, and kidney, major drug removal organs. Differences in the observed areas under the curve (AUCs) or the extraction ratios (Es) of formed and preformed metabolites in the liver had been attributed to zonal, enzyme heterogeneity, membrane barriers, or transporters. Preformed and formed metabolite also differed in their handling by the kidney; only the preformed and not the formed metabolite would be filtered. In the intestine, differences in the absorption of the precursor and the metabolite and the flow pattern in the intestine would bring about discrepancy in the time-courses of the formed vs. preformed metabolites. Analytical solutions of the AUCs of the metabolites and extraction ratios, based on physiological modeling of the liver, kidney, and intestine, showed that the AUC of the preformed, administered metabolite was dependent only on metabolite parameters, whereas the AUC of the formed metabolite was modulated additionally by the metabolic, secretory and intestinal absorptive intrinsic clearances of the precursor drug. Hence, administration of the synthetic metabolite would not reflect the toxicity associated with the metabolite formed via bioactivation. However, data on preformed metabolite may be used for simultaneous fitting by a combined model of drug and metabolite. Such a strategy is shown to be successful in risk assessment of environmental chemicals. Upon refinement of the resultant model with data on metabolite transport and handling by modeling and simulations, the resultant model would be more robust to provide improved predictions on metabolite toxicity pursuant to drug administration.

Transport of angiotensin-converting enzyme inhibitors by H+/peptide transporters revisited

Angiotensin-converting enzyme (ACE) inhibitors are often regarded as substrates for the H+/peptide transporters (PEPT)1 and PEPT2. Even though the conclusions drawn from published data are quite inconsistent, in most review articles PEPT1 is claimed to mediate the intestinal absorption of ACE inhibitors and thus to determine their oral availability. We systematically investigated the interaction of a series of ACE inhibitors with PEPT1 and PEPT2. First, we studied the effect of 14 ACE inhibitors including new drugs on the uptake of the dipeptide [14C]glycylsarcosine into human intestinal Caco-2 cells constitutively expressing PEPT1 and rat renal SKPT cells expressing PEPT2. In a second approach, the interaction of ACE inhibitors with heterologously expressed human PEPT1 and PEPT2 was determined. In both assay systems, zofenopril and fosinopril were found to have very high affinity for binding to peptide transporters. Medium to low affinity for transporter interaction was found for benazepril, quinapril, trandolapril, spirapril, cilazapril, ramipril, moexipril, quinaprilat, and perindopril. For enalapril, lisinopril, and captopril, very weak affinity or lack of interaction was found. Transport currents of PEPT1 and PEPT2 expressed in Xenopus laevis oocytes were recorded by the two-electrode voltage-clamp technique. Statistically significant, but very low currents were only observed for lisinopril, enalapril, quinapril, and benazepril at PEPT1 and for spirapril at PEPT2. For the other ACE inhibitors, electrogenic transport activity was extremely low or not measurable at all. The present results suggest that peptide transporters do not control intestinal absorption and renal reabsorption of ACE inhibitors.

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.

Transporters, enzymes, and enalapril removal in a rat (CC531-induced) liver metastatic model

Temporal changes in physiological spaces, protein expression of transporters and enzymes, and enalapril removal were appraised in the metastatic liver tumor model developed from male Wag/Rij rats after the intraportal injection of CC531 colon adenocarcinoma cells; sham-operated preparations received PBS. Liver tissue spaces, investigated with multiple indicator dilution technique in liver perfusion studies, were unchanged at week 3 after tumor induction. At week 4, however, the sinusoidal blood volume and albumin Disse space in tumor-bearing livers were slightly lower compared with those of shams. Increased levels of the canalicular ATP transporters, P-glycoprotein, multidrug resistance-associated protein 2 (Mrp2), and bile salt export pump (Bsep) at week 2 (P < 0.05), unchanged levels of Ntcp, Oatp1a1, Oatp1a4, and Mct2, but decreased levels of cytochrome P450 3a2 (Cyp3a2) and glutathione S-transferase (Gst4-4) at week 4 (P < 0.05) were observed in peritumor vs. sham-operated liver tissues with Western blotting. The steady-state extraction ratio of enalapril, a substrate that enters the liver rapidly via Oatp1a1 and primarily undergoes metabolism by the carboxylesterases, was unaffected by liver metastasis at week 4 regardless of its delivery via the portal vein or hepatic artery into the perfused liver preparations.

Down-regulation of messenger ribonucleic acid encoding an importer of sulfoconjugated steroids during human chorionic gonadotropin-induced follicular luteinization in vivo

Members of the organic anion transporting polypeptide (SLCO/OATP) superfamily are capable of importing anionic compounds across the lipid bilayer in a sodium-independent manner. Member 2B1 has been shown to transport few substrates, two of which are dihydroepiandrosterone-3-sulfate (DHEA-S) and estrone-3-sulfate. Steroid sulfatase (STS) catalyses the hydrolysis of these steroids into their unconjugated counterparts. The objective of this study was to investigate the regulation of SLCO2B1 and STS mRNAs during human chorionic gonadotropin (hCG)-induced ovulation/luteinization. The equine SLCO2B1 cDNA was cloned and shown to encode a 709-amino acid protein (OATP2B1) that is highly conserved when compared to mammalian orthologs. RT-PCR/Southern blot analyses were performed to study the regulation of SLCO2B1 and STS transcripts in equine preovulatory follicles isolated between 0 and 39h after hCG treatment. Results showed high levels of SLCO2B1 mRNA expression before hCG, with a marked decrease observed in follicles obtained 24-39h post-hCG (P<0.05). Analyses of isolated granulosa and theca interna cells identified high mRNA expression in both cell types prior to hCG treatment, with granulosa cells showing a more rapid SLCO2B1 mRNA down-regulation. No significant change in STS mRNA was observed in intact follicle walls. However, when both cell types were isolated, a significant decrease in STS mRNA was observed in granulosa cells 24-39h post-hCG. Collectively, these results demonstrate that the hCG-dependent induction of follicular luteinization is accompanied by the down-regulation of SLCO2B1 and STS transcripts. Considering that OATP2B1 can import sulfoconjugated DHEA and estrogens, and that STS can remove the sulfonate moiety from these steroids, their down-regulation in luteinizing preovulatory follicles may provide an additional biochemical basis for the decrease in ovarian 17beta-estradiol biosynthesis after the LH surge.

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.

An integrated approach to model hepatic drug clearance

It has been well accepted that hepatic drug extraction depends on the blood flow, vascular binding, transmembrane barriers, transporters, enzymes and cosubstrate and their zonal heterogeneity. Models of hepatic drug clearances have been appraised with respect to their utility in predicting drug removal by the liver. Among these models, the "well-stirred" model is the simplest since it assumes venous equilibration, with drug emerging from the outflow being in equilibrium with drug within the liver, and the concentration is the same throughout. The "parallel tube" and dispersion models, and distributed model of Goresky and co-workers have been used to account for the observed sinusoidal concentration gradient from the inlet and outlet. Departure from these models exists to include heterogeneity in flow, enzymes, and transporters. This article utilized the physiologically based pharmacokinetic (PBPK) liver model and its extension that include heterogeneity in enzymes and transporters to illustrate how in vitro uptake and metabolic data from zonal hepatocytes on transport and enzymes may be used to predict the kinetics of removal in the intact liver; binding data were also necessary. In doing so, an integrative platform was provided to examine determinants of hepatic drug clearance. We used enalapril and digoxin as examples, and described a simple liver PBPK model that included transmembrane transport and metabolism occurring behind the membrane, and a zonal model in which the PBPK model was expanded three sets of sub-compartments that are arranged sequentially to represent zones 1, 2, and 3 along the flow path. The latter model readily accommodated the heterogeneous distribution of hepatic enzymes and transporters. Transport and metabolic data, piecewise information that served as initial estimates, allowed for the unknown efflux and other intrinsic clearances to be estimated. The simple or zonal PBPK model provides predictive views on the hepatic removal of drugs and metabolites.

Rat organic anion transporting protein 1A1 (Oatp1a1): purification and phosphopeptide assignment

Rat organic anion transporting protein 1a1 (oatp1a1), a hepatocyte basolateral plasma membrane protein, mediates transport of various amphipathic compounds. Our previous studies indicated that serine phosphorylation of a single tryptic peptide inhibits its transport activity without changing its cell surface content. The site of phosphorylation is unknown and was the subject of the present study. Following immunoaffinity chromatographic purification from rat liver, oatp1a1 was subjected to trypsin digestion and MALDI-TOF. Except for predicted N-glycosylated peptides, 97% of oatp1a1 tryptic peptides were observed. A single tryptic phosphopeptide was found in the C-terminus (aa 626-647), existing in unphosphorylated or singly or doubly phosphorylated forms and sensitive to alkaline phosphatase treatment. The beta-elimination reaction resulted in a mass loss of 98 or 196 Da from this peptide, and subsequent Michael addition with cysteamine increased masses by the predicated 77 and 154 Da, indicating that oatp1a1 can be singly or doubly phosphorylated at serine or threonine residues in the C-terminal sequence SSATDHT (aa 634-640). Subsequent tandem MS/MS analysis revealed that phosphorylation at S634 accounted for all singly phosphorylated peptide, while phosphorylation at S634 and S635 accounted for all doubly phosphorylated peptide. These findings identify the site of oatp1a1 phosphorylation and demonstrate that it is an ordered process, in which phosphorylation at S634 precedes that at S635. The mechanism by which phosphorylation results in loss of transport activity in hepatocytes remains to be established. Whether phosphorylation near the C-terminus inhibits C-terminal oligomerization of oatp1a1, required for normal transport function, can be speculated upon but is as yet unknown.

Rapid identification of three functionally relevant polymorphisms in the OATP1B1 transporter gene using Pyrosequencing

INTRODUCTION

Clinical evidence suggests there are three single nucleotide polymorphisms (SNPs) of the solute carrier organic anion transporter family member B1 (SLCO1B1) gene for which in vivo evidence for a functional relevance for organic anion transporter polypeptides subgroup C (OATP1B1, formerly OATP-C) has been provided. These genetic variants have been shown to lead to altered pharmacokinetics of OATP1B1 substrates, mainly pravastatin, but also the irinotecan metabolite SN-38, estrone-3-sulfate, and estradiol-17beta-glucuronide. The authors therefore developed reliable and quick screening assays to identify the SLCO1B1 SNPs -11187G>A, 388A>G and 521T>C, in order to facilitate the judgment of their clinical role and to identify allelic frequencies of SNPs and haplotypes in a Caucasian random sample.

METHODS

Three simplex Pyrosequencing assays were developed and the three selected SLCO1B1 SNPs were screened for in 250 DNA samples from healthy young female and male unrelated volunteers of Caucasian ethnicity. SLCO1B1 haplotypes involving DNA positions -11187, 388 and 521 were identified by in silico haplotyping.

RESULTS

A clear identification of the three single nucleotide polymorphisms in the 250 DNA samples was possible and was verified by routine implementation of 40 control samples obtained by conventional sequencing. The frequencies of the variant alleles -11187A, 388G and 521C were 0.09, 0.47 and 0.12, respectively. All observed frequencies of heterozygous of homozygous carriers of SLCO1B1 alleles were in agreement with the Hardy-Weinberg equilibrium. SLCO1B1 haplotypes reportedly associated with altered substrate pharmacokinetics, i.e., SLCO1B1*15B (-11187G/388G/521C) and *17 (-11187A/388G/521C), were found at allelic frequencies of 0.09 and 0.02, respectively.

CONCLUSION

The presently developed Pyrosequencing assays allowed for quick and reliable identification of those SLCO1B1 SNPs that had been proposed to cause functional alternations in OATP1B1 with shown consequences for the pharmacokinetics of drugs that are OATP1B1 substrates.

Vectorial transport of enalapril by Oatp1a1/Mrp2 and OATP1B1 and OATP1B3/MRP2 in rat and human livers

Although Oatp1a1 (rat organic anion-transporting polypeptide 1a1) was the transporter found responsible for the hepatocellular entry of enalapril (EN) into the rat liver, the canalicular transporter involved for excretion of EN and the metabolite, enalaprilat (ENA), was unknown. The Eisai hyperbilirubinemic rat (EHBR) that lacks Mrp2 (multidrug resistance-associated protein 2) was used to appraise the role of Mrp2 in the excretion of [3H]EN and its metabolite [3H]ENA in single-pass rat liver preparations. Although the total and metabolic clearances and hepatic extraction ratios at steady-state were virtually unaltered for EN in EHBR compared with published values of Sprague-Dawley rats, the biliary clearances of EN and ENA were significantly reduced almost to zero (P<0.05). Involvement of human OATP1B1, OATP1B3, and MRP2 in EN transport was further assessed in single- or double-transfected mammalian cells. Human embryonic kidney 293 cells that expressed OATP1B1 or OATP1B3 showed that OATP1B3 transport of EN (20-500 microM) was of low affinity, whereas transport of EN by OATP1B1 was associated with the Km of 262+/-35 microM, a value similar to that for Oatp1a1 (214 microM). The transcellular transport of EN via human OATP1B1 and MRP2, investigated with the double-transfected Madin-Darby canine kidney (MDCK) II cells in the Transwell system, showed that the sinusoidal to canalicular flux of EN in the OATP1B1/MRP2/MDCK cells was significantly higher (P<0.05) than that of mock/MDCK and OATP1B1/MDCK cells. EN was transported by Oatp1a1 and Mrp2 in rats and OATP1B1/OATP1B3 and MRP2 in humans.

Transporters as a determinant of drug clearance and tissue distribution

Transporters play an important role in the processes of drug absorption, distribution and excretion. In this review, we have focused on the involvement of transporters in drug excretion in the liver and kidney. The rate of transporter-mediated uptake and efflux determines the rate of renal and hepatobiliary elimination. Transporters are thus important as a determinant of the clearance in the body. Even when drugs ultimately undergo metabolism, their elimination rate is sometimes determined by the uptake rate mediated by transporters. Transporters regulate the pharmacological and/or toxicological effect of drugs because they limit their distribution to tissues responsible for their effect and/or toxicity. For example, the liver-specific distribution of some statins via organic anion transporters helps them to produce their high pharmacological effect. On the other hand, as in the case of metformin taken up by organic cation transporter 1, drug distribution to the tissue(s) may enhance its toxicity. As transporter-mediated uptake is a determinant of the drug elimination rate, drug-drug interactions involving the process of transporter-mediated uptake can occur. In this review, we have introduced some examples and described their mechanisms. More recently, some methods to analyze such transporter-mediated transport have been reported. The estimation of the contributions of transporters to the net clearance of a drug makes it possible to predict the net clearance from data involving drug transport in transporter-expressing cells. Double transfected cells, where both uptake and efflux transporters are expressed on the same polarized cells, are also helpful for the analysis of the rate of transporter-mediated transcellular transport.

Vascular binding, blood flow, transporter, and enzyme interactions on the processing of digoxin in rat liver

The roles of vascular binding, flow, transporters, and enzymes as determinants of the clearance of digoxin were examined in the rat liver. Digoxin is metabolized by Cyp3a and utilizes the organic anion transporting polypeptide 2 (Oatp2) and P-glycoprotein (Pgp) for influx and excretion, respectively. Uptake of digoxin was found to be similar among rat periportal (PP) and perivenous (PV) hepatocytes isolated by the digitonin-collagenase method. The Km values for uptake were 180 +/- 112 and 390 +/- 406 nM, Vmax values were 13 +/- 8 and 18 +/- 4.9 pmol/min/mg protein, and nonsaturable components were 9.2 +/- 1.3 and 10.7 +/- 2.5 microl/min/mg for PP and PV, respectively. The evenness of distribution of Oatp2 and Pgp was confirmed by Western blotting and confocal immunofluorescent microscopy. When digoxin was recirculated to the rat liver preparation in Krebs-Henseleit bicarbonate (KHB) for 3 h in absence or presence of 1% bovine serum albumin (BSA) and 20% red blood cell (rbc) at flow rates of 40 and 10 ml/min, respectively, biexponential decays were observed. Fitted results based on compartmental analyses revealed a higher clearance (0.244 +/- 0.082 ml/min/g) for KHB-perfused livers over the rbc-albumin-perfused livers (0.114 +/- 0.057 ml/min/g) (P < 0.05). We further found that binding of digoxin to 1% BSA was modest (unbound fraction = 0.64), whereas binding to rbc was associated with slow on (0.468 +/- 0.021 min(-1)) and off (1.81 +/- 0.12 min(-1)) rate constants. We then used a zonal, physiologically based pharmacokinetic model to show that the difference in digoxin clearance was attributed to binding to BSA and rbc and not to the difference in flow rate and that clearance was unaffected by transporter or enzyme heterogeneity.

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.

Comparative Pharmacophore Modeling of Organic Anion Transporting Polypeptides: A Meta-Analysis of Rat Oatp1a1 and Human OATP1B1

The organic anion transporting polypeptides OATPs are key membrane transporters for which crystal structures are not currently available. They transport a diverse array of xenobiotics and are expressed at the interface of hepatocytes, renal tubular cells, enterocytes, and the choroid plexus. To aid the understanding of the key molecular features for substrate-transporter interactions, pharmacophore models were produced for the two OATPs that have been most extensively studied, namely rat Oatp1a1 and human OATP1B1. Literature data from Chinese hamster ovary, HeLa, human embryonic kidney 293 cells, and Xenopus laevis oocytes were used to construct pharmacophores for each individual transporter which were later merged to show similarities across cell lines for the same transporter. Additionally, meta-pharmacophores were generated from the combined datasets of each cell system used with the same transporter. The pharmacophores for each transporter consisted of hydrogen bond acceptor and hydrophobic features. There was good agreement between the merged and meta-pharmacophores containing two hydrogen bond acceptors and two or three hydrophobic features for Oatp1a1 and OATP1B1. External test sets were used to validate the individual pharmacophores. The meta-pharmacophores were also used to make predictions for molecules not included in the models and provided new molecular insight into the key features for these OATP transporters. This approach can be extended to other transporters for which limited data are available.

Functional characteristics and pharmacokinetic significance of kidney-specific organic anion transporters, OAT-K1 and OAT-K2, in the urinary excretion of anionic drugs

During the last decade, cDNA cloning has identified various gene families of drug transporters, and pharmacokinetic studies of drugs based on the molecular characteristics of transporters have advanced. We cloned and characterized two organic anion transporters OAT-K1 and OAT-K2 from the rat kidney. The expression of both transporters was limited to the kidney, especially the brush-border membranes of proximal tubules, with an apparent molecular mass of 40 kDa. Using MDCK or LLC-PK1 cells stably expressing OAT-K1, posttranslational cleavage was suggested to affect the membrane localization and functional characteristics; 50 kDa with multispecificity in the apical membrane of MDCK cells and 70 kDa with methotrexate specific transport in the basolateral membrane of LLC-PK1 cells. A wide variety of anionic compounds including methotrexate are bidirectionally transported via OAT-K1 and OAT-K2 across the apical membrane in the MDCK-transfectants. The urinary secretion of methotrexate was depressed in 5/6 nephrectomized rats in association with the selective loss of OAT-K1 and OAT-K2 expression, and both transporters were suggested to be target molecules for methotrexate-folinic acid rescue. In this review, recent advances in the study of OAT-K1 and OAT-K2 were summarized in comparison with other transporters.

Pharmacogenetics‐Based New Therapeutic Concepts

Pharmacogenetics, one of the fields of clinical pharmacology, studies how genetic factors influence drug response. If hereditary traits are taken into account appropriately before starting drug treatment, the type of drug and its dosage can be tailored to the individual patient's needs. Pharmacogenetics adds a considerable amount of stringency to the doctor's therapeutic approach. Today, it is the relationship between dosage requirements and genetic variations in drug metabolizing enzymes like cytochrome P450 (CYP) 2D6 and CYP2C19, or in drug transporters like p-glycoprotein, that is substantiated best. A standard dose will bring about more adverse effects than usual if enzymatic activity is lacking or feeble. Sometimes, however, therapeutic response might be better due to higher concentrations: proton pump inhibitors for eradication of Helicobacter pylori are more efficacious in carriers of a deficient CYP2C19 variant. The drug's interaction with its target (e.g. receptor) also depends on genetic factors. In some cases genetic tests can help distinguish between responders and non-responders of a specific drug treatment. The first pharmacogenetic tests are already on the market.

THE ROLES OF TRANSPORTERS AND ENZYMES IN HEPATIC DRUG PROCESSING

A simple, physiological model was used to illustrate the competing nature of transporters and metabolic enzymes in hepatic drug processing. Enalapril, a drug whose basolateral influx and canalicular efflux are mediated by rat organic anion-transporting polypeptide 1 (Oatp1) and rat multidrug resistance-associated protein 2 (Mrp2), respectively, and metabolism by the carboxylesterases, was enlisted as the example to illustrate how the transport and intrinsic clearances are inter-related in the estimation of the hepatic and metabolic, and excretion clearances. Moreover, simulations were performed to explore the effects of inhibitors or inducers of transporters/enzymes to unravel the compensatory changes of alternate pathways. Generally speaking, inhibition of one pathway led to an apparent increase in the alternate (competing) pathway and total hepatic clearance was decreased; induction would lead to an apparent decrease in the alternate pathway and an increase in total hepatic clearance. A reduction in influx clearance brought about parallel decreases in the biliary and metabolic clearances, whereas a reduction in efflux basolateral clearance evoked similar increases in biliary and metabolic clearances. However, the steady-state tissue concentration (C(L,ss)) or area under the tissue concentration-time curve (AUC(L)) was reliant only on the unbound fraction in liver, and the secretory and metabolic intrinsic clearances and not the influx and efflux clearances. Variations in the influx and efflux intrinsic clearances evoked temporal changes in the tissue concentration-time profile but not the AUC(L) or C(L,ss). The pharmacokinetic theory developed offers data interpretation from literature reports on P-glycoprotein and cytochrome P450 substrates with mdr1a/1b knockout versus wild-type mice, and rat liver perfusion studies, with and without the use of inhibitors. In some cases, critiques on data interpretation were made.

Molecular cloning and functional characterization of the bovine (Bos taurus) organic anion transporting polypeptide Oatp1a2 (Slco1a2)

We describe the cloning, functional characterization and tissue localization of a novel membrane transporter of the OATP/Oatp-gene family obtained from liver and kidney of cattle (Bos taurus). The carrier protein exhibits highest sequence identity to the human OATP1A2 (previously called OATP-A) and is, therefore, named bovine Oatp1a2. Bovine Oatp1a2 received the gene symbol Slco1a2 that is identical to the SLC classification of human OATP1A2 (SLCO1A2, previously called SLC21A3) and is likely an orthologue of the human gene. Two different full-length bOatp1a2 cDNAs of 2316-bp and 3504-bp were obtained and encoded for a 666 amino acid membrane protein, which contains twelve putative transmembrane spanning domains. Bovine Oatp1a2 expression was detected in liver, kidney, brain and adrenal gland. Uptake studies in cRNA-injected oocytes demonstrated that bOatp1a2 transports estrone-3-sulfate and taurocholate, with K(m) values of 9.6 microM and 51 microM, respectively, and estradiol-17beta-glucuronide. However, the structurally-related heart glycosides ouabain (1 microM) and digoxin (1 microM) are neither transported by bovine Oatp1a2 nor by human OATP1A2. We conclude that based on the tested substrates bovine Oatp1a2 shows functional homology to human OATP1A2.

TRANSPORTERS ANDRENALDRUGELIMINATION

Carrier-mediated processes, often referred to as transporters, play key roles in the reabsorption and secretion of many endogenous and xenobiotic compounds by the kidney. The renal proximal tubule is the primary site of active transport for a wide variety of substrates, including organic anions/cations, peptides, and nucleosides. During the past decade, significant advances in molecular identification and characterization of transporter proteins have been made. Although it is generally noted that these transporters significantly contribute to renal drug handling and variability in drug disposition, the extent of our knowledge regarding the specific roles of such transporters in drug disposition and drug-drug interactions remains, for the most part, limited. In this review, we summarize recent progress in terms of molecular and functional characterization of renal transporters and their clinical relevance to drug therapy.

Substrate specificities of rat oatp1 and ntcp: implications for hepatic organic anion uptake

Transport of a series of 3H-radiolabeled C23, C24, and C27 bile acid derivatives was compared and contrasted in HeLa cell lines stably transfected with rat Na+/taurocholate cotransporting polypeptide (ntcp) or organic anion transporting polypeptide 1 (oatp1) in which expression was under regulation of a zinc-inducible promoter. Similar uptake patterns were observed for both ntcp and oatp1, except that unconjugated hyodeoxycholate was a substrate of oatp1 but not ntcp. Conjugated bile acids were transported better than nonconjugated bile acids, and the configuration of the hydroxyl groups (alpha or beta) had little influence on uptake. Although cholic and 23 norcholic acids were transported by ntcp and oatp1, other unconjugated bile acids (chenodeoxycholic, ursodeoxycholic) were not. In contrast to ntcp, oatp1-mediated uptake of the trihydroxy bile acids taurocholate and glycocholate was four- to eightfold below that of the corresponding dihydroxy conjugates. Ntcp mediated high affinity, sodium-dependent transport of [35S]sulfobromophthalein with a Km similar to that of oatp1-mediated transport of [35S]sulfobromophthalein (Km = 3.7 vs. 3.3 muM, respectively). In addition, for both transporters, uptake of sulfobromophthalein and taurocholic acid showed mutual competitive inhibition. These results indicate that the substrate specificity of ntcp is considerably broader than previously suspected and caution the extrapolation of transport data obtained in vitro to physiological function in vivo.

Drug efflux transporters in the CNS

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

The superfamily of organic anion transporting polypeptides

Organic anion transporting polypeptides (Oatps/OATPs) form a growing gene superfamily and mediate transport of a wide spectrum of amphipathic organic solutes. Different Oatps/OATPs have partially overlapping and partially distinct substrate preferences for organic solutes such as bile salts, steroid conjugates, thyroid hormones, anionic oligopeptides, drugs, toxins and other xenobiotics. While some Oatps/OATPs are preferentially or even selectively expressed in one tissue such as the liver, others are expressed in multiple organs including the blood-brain barrier (BBB), choroid plexus, lung, heart, intestine, kidney, placenta and testis. This review summarizes the actual state of the rapidly expanding OATP superfamily and covers the structural properties, the genomic classification, the phylogenetic relationships and the functional transport characteristics. In addition, we propose a new species independent and open ended nomenclature and classification system, which is based on divergent evolution and agrees with the guidelines of the Human Genome Nomenclature Committee.

Sinusoidal efflux of taurocholate correlates with the hepatic expression level of Mrp3

Multidrug resistance-associated protein 3 (Mrp3/ABCC3), which can mediate the cellular extrusion of bile acids, is induced on the hepatic sinusoidal membrane of Mrp2/ABCC2-deficient rats (Eisai hyperbilirubinemic rats; EHBRs) and phenobarbital-treated Sprague-Dawley rats. In the present study, the correlation between the sinusoidal efflux clearance (PS(eff)) of [3H]taurocholate (TC) and the hepatic expression of Mrp3 was investigated using perfused liver from these rats. A significant correlation was observed between the PS(eff) and the hepatic expression level of Mrp3, suggesting a contribution by Mrp3 to the sinusoidal efflux of TC. The results of the kinetic analysis also suggested that other transporter(s) on the sinusoidal plasma membrane may participate in the efflux of TC under physiological conditions. The contribution of Mrp3 to the sinusoidal efflux of TC in EHBRs and phenobarbital (80 and 40 mg/kg)-treated rats was revealed to be 58%, 48%, and 31%, respectively.

Mammalian peptide transporters as targets for drug delivery

Peptide transporters are integral plasma membrane proteins that mediate the cellular uptake of dipeptides and tripeptides in addition to a variety of peptidomimetics. The carriers, which occur predominantly in the brush-border membranes of epithelial cells of the small intestine, lung, choroid plexus and kidney, contribute to absorption, distribution and elimination of their substrates. The cellular uptake of peptides and peptidomimetics involves the cotransport of protons down an inwardly directed, electrochemical proton gradient that provides the driving force and causes the electrogenicity of the translocation step. Peptide transporters represent excellent targets for the delivery of pharmacologically active compounds because their substrate-binding site can accommodate a wide range of molecules of differing size, hydrophobicity and charge.

Molecular aspects of renal anionic drug transport

Multiple organic anion transporters in the proximal tubule of the kidney are involved in the secretion of drugs, toxic compounds, and their metabolites. Many of these compounds are potentially hazardous on accumulation, and it is therefore not surprising that the proximal tubule is also an important target for toxicity. In the past few years, considerable progress has been made in the cloning of these transporters and their functional characterization following heterologous expression. Members of the organic anion transporter (OAT), organic anion transporting polypeptide (OATP), multidrug resistance protein (MRP), sodium-phosphate transporter (NPT), and peptide transporter (PEPT) families have been identified in the kidney. In this review, we summarize our current knowledge on their localization, molecular and functional characteristics, and substrate and inhibitor specificity. A major challenge for the future will be to understand how these transporters work in concert to accomplish the renal secretion of specific anionic substrates.

Transport of the sulfated, amidated bile acid, sulfolithocholyltaurine, into rat hepatocytes is mediated by Oatp1 and Oatp2

The uptake of the sulfated bile acid sulfolithocholyltaurine (SLCT) was investigated in isolated rat hepatocytes and in HeLa cells transfected with complementary DNAs (cDNAs) of organic anion transporting polypeptides (Oatps) 1 and 2 cloned from rat liver. In hepatocytes, transport of SLCT was greatly reduced by bromosulfophthalein (BSP), estrone sulfate, the precursor bile acids cholyltaurine and lithocholyltaurine, and 4,4'-diisothiocyanostilbene-2-2'-disulfonic acid (DIDS). However, SLCT transport was insensitive to 4-methylumbelliferyl sulfate, harmol sulfate, digoxin, fexofenadine, and lack of sodium ion. Because the estimation of kinetic constants was enhanced with use of inhibitors, BSP (1-50 micromol/L) was added to isolated rat hepatocytes to assess the various transport components for SLCT uptake. The resulting data showed a nonsaturable pathway and at least 2 pathways of different Michaelis-Menten constants (K(m)) (70 and 6 micromol/L) and similar maximum velocities (V(max)) (1.73 and 1.2 nmol/min/mg protein) and inhibition constants of 0.63 and 10.3 micromol/L for BSP. In expression systems, SLCT was taken up by Oatp1 and Oatp2 expressed in HeLa cells with similar K(m) values (12.6 +/- 6.2 and 14.6 +/- 1.9 micromol/L). These K(m) values were comparable to that observed for the high-affinity pathway in rat hepatocytes. In conclusion, the results suggest that transport of SLCT into rat liver is mediated in part by Oatp1 and Oatp2, high-affinity pathways, a lower-affinity pathway of unknown origin, and a nonsaturable pathway that is compatible with a transport system of high K(m) and/or passive diffusion.

Mechanisms and clinical implications of renal drug excretion

The body defends itself against potentially harmful compounds like drugs, toxic compounds, and their metabolites by elimination, in which the kidney plays an important role. Renal clearance is used to determine renal elimination mechanisms of a drug, which is the result of glomerular filtration, active tubular secretion and reabsorption. The renal proximal tubule is the primary site of carrier-mediated transport from blood to urine. Renal secretory mechanisms exists for, anionic compounds and organic cations. Both systems comprises several transport proteins, and knowledge of the molecular identity of these transporters and their substrate specificity has increased considerably in the past decade. Due to overlapping specificities of the transport proteins, drug interactions at the level of tubular secretion is an event that may occur in clinical situation. This review describes the different processes that determine renal drug handling, the techniques that have been developed to attain more insight in the various aspects of drug excretion, the functional characteristics of the individual transport proteins, and finally the implications of drug interactions in a clinical perspective.

Transporters involved in the elimination of drugs in the kidney: organic anion transporters and organic cation transporters

Transporters in the kidney mediate the secretion or reabsorption of many compounds and thereby influence the plasma levels of their substrates. Organic anion transporters and organic cation transporters are two major classes of secretory transporters in the mammalian kidney. During the past decade, significant progress has been made in the cloning, functional expression, and initial characterization of these transporters. To date, five organic cation transporters and nine organic anion transporters have been cloned. In this review, we summarize the available data on organic anion and organic cation transporters, focusing in particular on their molecular characteristics, tissue distribution, and inhibitor and substrate selectivities. Currently we have a good understanding of the inhibitor selectivities for most of these transporters, and with the development of more robust assays, we will soon have a better understanding of their substrate selectivities. Based on the available data, summarized in this review, it appears that many compounds interact with multiple transporters. Furthermore, there appears to be substantial overlap in the selectivities of organic cation transporters, and the same appears true for organic anion transporters. At the present time, it is unclear what the roles of these multiple transporters are in renal drug elimination. With the development of new assays, reagents, and experimental methods, we will soon have a better understanding of the roles of each transporter isoform in the renal elimination of drugs.

Commentary: using the convection-dispersion model and transit time density functions in the analysis of organ distribution kinetics

The convection-dispersion model and its extended form have been used to describe solute disposition in organs and to predict hepatic availabilities. A range of empirical transit-time density functions has also been used for a similar purpose. The use of the dispersion model with mixed boundary conditions and transit-time density functions has been queried recently by Hisaka and Sugiyama in this journal. We suggest that, consistent with soil science and chemical engineering literature, the mixed boundary conditions are appropriate providing concentrations are defined in terms of flux to ensure continuity at the boundaries and mass balance. It is suggested that the use of the inverse Gaussian or other functions as empirical transit-time densities is independent of any boundary condition consideration. The mixed boundary condition solutions of the convection-dispersion model are the easiest to use when linear kinetics applies. In contrast, the closed conditions are easier to apply in a numerical analysis of nonlinear disposition of solutes in organs. We therefore argue that the use of hepatic elimination models should be based on pragmatic considerations, giving emphasis to using the simplest or easiest solution that will give a sufficiently accurate prediction of hepatic pharmacokinetics for a particular application.

Molecular pharmacology of renal organic anion transporters

Renal organic anion transport systems play an important role in the elimination of drugs, toxic compounds, and their metabolites, many of which are potentially harmful to the body. The renal proximal tubule is the primary site of carrier-mediated transport from blood to urine of a wide variety of anionic substrates. Recent studies have shown that organic anion secretion in renal proximal tubule is mediated by distinct sodium-dependent and sodium-independent transport systems. Knowledge of the molecular identity of these transporters and their substrate specificity has increased considerably in the past few years by cloning of various carrier proteins. However, a number of fundamental questions still have to be answered to elucidate the participation of the cloned transporters in the overall tubular secretion of anionic xenobiotics. This review summarizes the latest knowledge on molecular and pharmacological properties of renal organic anion transporters and homologs, with special reference to their nephron and plasma membrane localization, transport characteristics, and substrate and inhibitor specificity. A number of the recently cloned transporters, such as the p-aminohippurate/dicarboxylate exchanger OAT1, the anion/sulfate exchanger SAT1, the peptide transporters PEPT1 and PEPT2, and the nucleoside transporters CNT1 and CNT2, are key proteins in organic anion handling that possess the same characteristics as has been predicted from previous physiological studies. The role of other cloned transporters, such as MRP1, MRP2, OATP1, OAT-K1, and OAT-K2, is still poorly characterized, whereas the only information that is available on the homologs OAT2, OAT3, OATP3, and MRP3-6 is that they are expressed in the kidney, but their localization, not to mention their function, remains to be elucidated.

Molecular cloning and functional characterization of the mouse organic-anion-transporting polypeptide 1 (Oatp1) and mapping of the gene to chromosome X

We have cloned a murine member of the organic-anion-transporting polypeptide (Oatp) family of membrane-transport proteins from mouse liver. The cloned cDNA insert of 2783 bp with an open reading frame of 2011 bp codes for a 12-transmembrane 670-amino-acid protein with highest amino acid identity with the rat Oatp1. When expressed in Xenopus laevis oocytes, the mouse Oatp exhibited the same substrate specificity as the rat Oatp1. Besides the common Oatp substrates bromosulphophthalein, taurocholate, oestrone 3-sulphate and ouabain, the new mouse Oatp also mediates transport of the Oatp1-specific magnetic-resonance-imaging agent gadoxetate. The Oatp2-specific cardiac glycoside digoxin, however, is not transported. Kinetic analyses performed for taurocholate and oestrone 3-sulphate revealed apparent K(m) values of 12 microM and 5 microM respectively. Northern-blot analysis demonstrated a predominant expression in the liver with an additional moderate expression in the kidney. Taken together, the amino acid identity, the functional characteristics and the tissue distribution suggest that we have isolated the murine orthologue of the rat Oatp1, and consequently the identified protein will be called Oatp1. Using fluorescence in situ hybridization, the murine Oatp1 gene was mapped to chromosome XA3-A5.

Localization and function of the organic anion-transporting polypeptide Oatp2 in rat liver

BACKGROUND & AIMS

Multispecific organic anion-transporting polypeptides (Oatps) are involved in the transcellular movement of amphipathic compounds in many tissues including the liver, kidney, and blood-brain barrier. Recently, a high-affinity digoxin transporter (Oatp2) was cloned from rat brain and shown to be also expressed in the liver.

METHODS

We investigated the cellular and subcellular distribution of Oatp2 in rat liver by in situ hybridization technology and immunofluorescence microscopy and compared its substrate specificity with that of Oatp1 in complementary RNA-injected Xenopus laevis oocytes.

RESULTS

The results show a selective basolateral (sinusoidal) expression of Oatp2 in midzonal to perivenous hepatocytes, but not in periportal or the innermost layer of perivenous hepatocytes. Common substrates of both Oatp1 and Oatp2 include bile salts, steroid conjugates, thyroid hormones (T3, T4), ouabain, and the endothelin receptor antagonist BQ-123 (Michaelis constants: Oatp1, approximately 600 micromol/L; Oatp2, approximately 30 micromol/L). Other organic anions including sulfolithotaurocholate, bilirubin monoglucuronide, and sulfobromophthalein were transported only by Oatp1.

CONCLUSIONS

These results provide definite evidence for the partially overlapping and partially selective substrate specificities of Oatp1 and Oatp2. The unique acinar distribution of Oatp2 might indicate that it represents a high-affinity "backup" system for complete hepatocellular removal of certain cholephilic substances from portal blood plasma.

Polyspecific substrate uptake by the hepatic organic anion transporter Oatp1 in stably transfected CHO cells

The rat liver organic anion transporting polypeptide (Oatp1) has been extensively characterized mainly in the Xenopus laevis expression system as a polyspecific carrier transporting organic anions (bile salts), neutral compounds, and even organic cations. In this study, we extended this characterization using a mammalian expression system and confirm the basolateral hepatic expression of Oatp1 with a new antibody. Besides sulfobromophthalein [Michaelis-Menten constant (Km) of approximately 3 microM], taurocholate (Km of approximately 32 microM), and estradiol- 17beta-glucuronide (Km of approximately 4 microM), substrates previously shown to be transported by Oatp1 in transfected HeLa cells, we determined the kinetic parameters for cholate (Km of approximately 54 microM), glycocholate (Km of approximately 54 microM), estrone-3-sulfate (Km of approximately 11 microM), CRC-220 (Km of approximately 57 microM), ouabain (Km of approximately 3,000 microM), and ochratoxin A (Km of approximately 29 microM) in stably transfected Chinese hamster ovary (CHO) cells. In addition, three new substrates, taurochenodeoxycholate (Km of approximately 7 microM), tauroursodeoxycholate (Km of approximately 13 microM), and dehydroepiandrosterone sulfate (Km of approximately 5 microM), were also investigated. The results establish the polyspecific nature of Oatp1 in a mammalian expression system and definitely identify conjugated dihydroxy bile salts and steroid conjugates as high-affinity endogenous substrates of Oatp1.