Interaction of human organic anion transporters 2 and 4 with organic anion transport inhibitors

The putative transmembrane segment 7 of human organic anion transporter hOAT1 dictates transporter substrate binding and stability

Human organic anion transporter hOAT1 plays a critical role in the body disposition of clinically important drugs. We examined the role of the putative transmembrane segment (TM) 7 in the function of hOAT1. Each residue within putative TM7 was replaced by alanine, and the uptake of para-aminohippurate was studied in cells expressing the mutants. We discovered four critical amino acid residues: Trp-346, Thr-349, Tyr-353, and Tyr-354. Substitution of Tyr-353 and Tyr-354 with alanine led to the loss of transport activity without affecting the surface expression of the transporter, whereas substitution of Trp-346 and Thr-349 with alanine lead to the loss of the total expression of the transporter. The effect of side chains of Tyr-353 and Tyr-354 on transporter functions were further evaluated by replacing these residues with Phe or Trp. Among all the mutants studied (Y353W, Y353F, Y354W, and Y354F), only mutant Y353F regained 30% transport activity, which was lost from replacement of Tyr-353 with alanine, suggesting that both the -OH group and the size of the side chain at positions 353 and 354 are critical for maintaining the full transport activity. To investigate the mechanisms underlying the loss of total protein expression when Trp-346 and Thr-349 were replaced with alanine, mutant-expressing cells were treated with lysosomal or proteasomal inhibitors. Our results showed that only proteasomal inhibitors resulted in the accumulation of mutant proteins, indicating that proteasome is involved in the degradation of the mutant transporters. Therefore, Trp-346 and Thr-349 are critically involved in the stability of the transporter.

Molecular insights into the structure-function relationship of organic anion transporters OATs

The organic anion transporter (OAT) family encoded by SLC22A mediates the absorption, distribution, and excretion of a diverse array of environmental toxins, and clinically important drugs, including anti-HIV therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories, and therefore is critical for the survival of mammalian species. Several OATs have been identified: OAT1 (SLC22A6), OAT2 (SLC22A7), OAT3 (SLC22A8), OAT4 (SLC22A11), OAT5 (SLC22A19) OAT6 (SLC22A20) and URAT1 (SLC22A12). The expressions of these OATs have been detected in key organs such as kidney, liver, brain and placenta. OAT dysfunction in these organs may contribute to the renal, hepatic, neurological and fetal toxicity and diseases. In this review, we summarize, according to the work done by our laboratory as well as by others, the most updated molecular studies on these OAT members, especially on the aspect of their structure-function relationships. The functional roles of N-glycosylation, transmembrane domains and individual amino acids, cell surface assembly, as well as associating proteins will be discussed. In addition, we will show the recent analyses of coding region polymorphisms of OATs, which give us information on the genetic variants of OATs and their potential effects on OAT functions.

Drug-drug interactions involving membrane transporters in the human kidney

The kidneys play a critical role in the elimination of xenobiotics. Factors affecting the ability of the kidney to eliminate drugs may result in marked changes in the pharmacokinetics of a given compound. Drug-drug interactions due to competitive inhibition of renal organic anion or cation secretion systems have been noticed clinically for a long time. However, our understanding of the physical sites of interactions, that is, the specific transport proteins that the interacting drugs act on, has just begun very recently. This review summarises the latest progress in molecular identification and functional characterisation of major drug transporters in the human kidney. In particular, the review focuses on relating cloned renal drug transporters to clinically observed drug-drug interactions. The authors' opinion on the current status and future directions of research in these areas is also offered.

Renal expression of organic anion transporter OAT2 in rats and mice is regulated by sex hormones

The renal reabsorption and/or excretion of various organic anions is mediated by specific organic anion transporters (OATs). OAT2 (Slc22a7) has been identified in rat kidney, where its mRNA expression exhibits gender differences [females (F) > males (M)]. The exact localization of OAT2 protein in the mammalian kidney has not been reported. Here we studied the expression of OAT2 mRNA by RT-PCR and its protein by Western blotting (WB) and immunocytochemistry (IC) in kidneys of adult intact and gonadectomized M and F, sex hormone-treated castrated M, and prepubertal M and F rats, and the protein in adult M and F mice. In adult rats, the expression of OAT2 mRNA was predominant in the outer stripe (OS) tissue, exhibiting 1) gender dependency (F > M), 2) upregulation by castration and downregulation by ovariectomy, and 3) strong downregulation by testosterone and weak upregulation by estradiol and progesterone treatment. A polyclonal antibody against rat OAT2 on WB of isolated renal membranes labeled a approximately 66-kDa protein band that was stronger in F. By IC, the antibody exclusively stained brush border (BB) of the proximal tubule S3 segment (S3) in the OS and medullary rays (F > M). In variously treated rats, the pattern of 66-kDa band density in the OS membranes and the staining intensity of BB in S3 matched the mRNA expression. The expression of OAT2 protein in prepubertal rats was low and gender independent. In mice, the expression pattern largely resembled that in rats. Therefore, OAT2 in rat (and mouse) kidney is localized to the BB of S3, exhibiting gender differences (F > M) that appear in puberty and are caused by strong androgen inhibition and weak estrogen and progesterone stimulation.

Human renal organic anion transporters: Characteristics and contributions to drug and drug metabolite excretion

The kidney is a key organ for promoting the excretion of drugs and drug metabolites. One of the mechanisms by which the kidney promotes excretion is via active secretion. Secretion of drugs and their metabolites from blood to luminal fluid in the nephron is a protein-mediated process that normally involves either the direct or indirect expenditure of energy. Renal transporters for organic anions are located in the proximal tubule segment of the nephron. The primary transporters of organic anions on the basolateral membrane (BLM) of proximal tubule cells are members of the organic anion transporter (OAT) family (mainly OAT1 and OAT3). The sulfate-anion antiporter 1 (SAT-1; hsat-1) may also contribute to organic anion transport at the basolateral membrane. On the apical membrane, the multi-drug resistance-associated protein 2 (MRP2) is an important transport protein to complete the secretion process. However, there are several transport proteins on the basolateral and apical membranes of proximal tubule cells in human kidneys that have not been fully characterized and whose role in the secretion of organic anions remains to be determined. This review will primarily focus on the human renal basolateral and apical membrane transporters for organic anions that may play a role in the excretion of drugs and drug metabolites.

Transport of estrone sulfate by the novel organic anion transporter Oat6 (Slc22a20)

Recently, a novel Slc22 gene family member expressed in murine olfactory mucosa was identified and based on sequence homology proposed to be an organic anion transporter [Oat6 (Slc22a20); J. C. Monte, M. A. Nagle, S. A. Eraly, and S. K. Nigam. Biochem Biophys Res Commun 323: 429-436, 2004]. However, no functional data for Oat6 was reported. In the present study, we demonstrate that murine Oat6 mediates the inhibitable transport of estrone sulfate using both Xenopus oocyte expression assay and Chinese hamster ovary (CHO) cells stably transfected with mOat6 (CHO-mOat6). Uptake was virtually eliminated by probenecid and the anionic herbicide 2,4-dichlorophenoxyacetate. The organic anions ochratoxin A, salicylate, penicillin G, p-aminohippurate, and urate inhibited mOat6-mediated accumulation to varying degrees. Transport of estrone sulfate by mOat6 was demonstrated to be saturable, and K(m) estimates of 109.8 +/- 22.6 microM in oocytes and 44.8 +/- 7.3 microM in CHO-mOat6 cells were obtained. Inhibitory constants for 2,4-dichlorophenoxyacetate (15.7 +/- 2.0 microM), salicylate (49.0 +/- 4.4 microM), probenecid (8.3 +/- 2.5 microM), and penicillin G (1,450 +/- 480 microM) were also determined. Accumulation of estrone sulfate mediated by mOat6 was significantly trans-stimulated by glutarate, indicating that mOat6 functions as an organic anion/dicarboxylate exchanger. These data demonstrate for the first time that the novel murine gene Oat6 (Slc22a20) encodes a functional organic anion transporter and mOat6 is indeed the newest member of the OAT gene family.

A Beneficial Interaction between Imipenem and Piperacillin Possibly through Their Renal Excretory Process

In order to assess the beneficial mechanism of the concomitant use of imipenem (IPM) with piperacillin (PIPC) for the treatment of serious infectious diseases such as sepsis, the effects of PIPC on the uptake of IPM by rat renal cortical slices and on the plasma concentrations of IPM after intravenous infusion to rabbits were studied. The uptake of IPM by the rat renal cortical slices was significantly inhibited by p-aminohippurate, probenecid and PIPC whereas the uptake of PIPC by the slices was slightly decreased in the presence of IPM. When IPM was administered together with PIPC by 1-h infusion, the plasma concentrations of IPM were significantly increased during the infusion. These results imply that PIPC possibly interferes with the renal transport of IPM mediated by an organic anion transporter across the renal basolateral membranes, which leads to a longer period above the minimum inhibitory concentrations of IPM.

Quantitative analysis of the effect of probenecid on pharmacokinetics of 99mTc-mercaptoacetyltriglycine in dogs

Effect of probenecid on pharmacokinetics of 99mTc-mercaptoacetylytriglycine (99mTc-MAG3) in dogs was investigated before (control), and after 15 min and 24 h of i.v. injection of probenecid (20 mg/kg). Plasma concentration-time profiles of 99mTc-MAG3 were described with a two-compartment open model. Plasma 99mTc-MAG3 clearances (Clp, ml/min/kg) were 7.9 +/- 0.5, 3.3 +/- 0.5 and 4.8 +/- 1.3 in control, 15 min and 24 h after probenecid administration respectively. Similarly, the biological half-lives at elimination phase (t(1/2), h) were 0.61 +/- 0.09, 0.79 +/- 0.11 and 0.74 +/- 0.12, and volumes of distribution at steady state (Vdss, L/kg) were 0.29 +/- 0.04, 0.20 +/- 0.05 and 0.25 +/- 0.06 respectively. The prolonged biological half-life and decreased Vdss decreased Clp significantly. Clp was a function of plasma probenecid concentration based on Michaelis-Menten kinetics. The maximum Clp inhibition (Imax) by probenecid and the plasma probenecid concentration that induced 50% of Imax (I50) were estimated to be 72 +/- 12% and 13 +/- 8 microg/ml respectively. This means that the rest (about 28%) of the Clp is not blocked by probenecid alone, suggesting the possibility of another route(s) of elimination or renal transporters which are independent from probenecid. Moreover, inter-species correlation between Clp of 99mTc-MAG3 and body weight are discussed.

Roles of organic anion transporters (OATs) and a urate transporter (URAT1) in the pathophysiology of human disease

Renal proximal and distal tubules are highly polarized epithelial cells that carry out the specialized directional transport of various solutes. This renal function, which is essential for homeostasis in the body, is achieved through the close pairing of apical and basolateral carriers expressed in the renal epithelial cells. The family of organic anion transporters (OATs), which belong to the major facilitator superfamily (SLC22A), are expressed in the renal epithelial cells to regulate the excretion and reabsorption of endogenous and exogenous organic anions. We now understand that these OATs are crucial components in the renal handling of drugs and their metabolites, and they are implicated in various clinically important drug interactions, and their adverse reactions. In recent years, the molecular entities of these transporters have been identified, and their function and regulatory mechanisms have been partially clarified. Workers in this field have identified URAT1 (urate transporter 1), a novel member of the OAT family that displays unique and selective substrate specificity compared with other multispecific OATs. In the OAT family, URAT1 is the main transporster responsible for human genetic diseases. In this review, we introduce and discuss some novel aspects of OATs, with special emphasis on URAT1, in the context of their biological significance, functional regulation, and roles in human disease.

Decreased renal organic anion secretion and plasma accumulation of endogenous organic anions in OAT1 knock-out mice

The "classical" organic anion secretory pathway of the renal proximal tubule is critical for the renal excretion of the prototypic organic anion, para-aminohippurate, as well as of a large number of commonly prescribed drugs among other significant substrates. Organic anion transporter 1 (OAT1), originally identified as NKT (Lopez-Nieto, C. E., You, G., Bush, K. T., Barros, E. J. G., Beier, D. R., and Nigam, S. K. (1997) J. Biol. Chem. 272, 6471-6478), has physiological properties consistent with a role in this pathway. However, several other transporters (e.g. OAT2, OAT3, and MRP1) have also been proposed as important PAH transporters on the basis of in vitro studies; therefore, the relative contribution of OAT1 has remained unclear. We have now generated a colony of OAT1 knock-out mice, permitting elucidation of the role of OAT1 in the context of these other potentially functionally redundant transporters. We find that the knock-out mice manifest a profound loss of organic anion transport (e.g. para-aminohippurate) both ex vivo (in isolated renal slices) as well as in vivo (as indicated by loss of renal secretion). In the case of the organic anion, furosemide, loss of renal secretion in the knock-out results in impaired diuretic responsiveness to this drug. These results indicate a critical role for OAT1 in the functioning of the classical pathway. In addition, we have determined the levels of approximately 60 endogenous organic anions in the plasma and urine of wild-type and knock-out mice. This has led to identification of several compounds with significantly higher plasma concentrations and/or lower urinary concentrations in knock-out mice, suggesting the involvement of OAT1 in their renal secretion. We have also demonstrated in xenopus oocytes that some of these compounds interact with OAT1 in vitro. Thus, these latter compounds might represent physiological substrates of OAT1.

Apical/Basolateral Surface Expression of Drug Transporters and its Role in Vectorial Drug Transport

It is well known that transporter proteins play a key role in governing drug absorption, distribution, and elimination in the body, and, accordingly, they are now considered as causes of drug-drug interactions and interindividual differences in pharmacokinetic profiles. Polarized tissues directly involved in drug disposition (intestine, kidney, and liver) and restricted distribution to naive sanctuaries (blood-tissue barriers) asymmetrically express a variety of drug transporters on the apical and basolateral sides, resulting in vectorial drug transport. For example, the organic anion transporting polypeptide (OATP) family on the sinusoidal (basolateral) membrane and multidrug resistance-associated protein 2 (MRP2/ABCC2) on the apical bile canalicular membrane of hepatocytes take up and excrete organic anionic compounds from blood to bile. Such vectorial transcellular transport is fundamentally attributable to the asymmetrical distribution of transporter molecules in polarized cells. Besides the apical/basolateral sorting direction, distribution of the transporter protein between the membrane surface (active site) and the intracellular fraction (inactive site) is of practical importance for the quantitative evaluation of drug transport processes. The most characterized drug transporter associated with this issue is MRP2 on the hepatocyte canalicular (apical) membrane, and it is linked to a genetic disease. Dubin-Johnson syndrome is sometimes caused by impaired canalicular surface expression of MRP2 by a single amino acid substitution. Moreover, single nucleotide polymorphisms in OATP-C/SLC21A6 (SLCO1B1) also affect membrane surface expression, and actually lead to the altered pharmacokinetic profile of pravastatin in healthy subjects. In this review article, the asymmetrical transporter distribution and altered surface expression in polarized tissues are discussed.

Transport mechanism and substrate specificity of human organic anion transporter 2 (hOat2 [SLC22A7])

Human organic anion transporter 2 (hOat2[SLC22A7]) is highly expressed in the human liver. Although localization, gene expression, substrate specificity and transport mechanisms of other human Oat isoforms such as human Oat1 (hOat1), human Oat3 (hOat3) and human Oat4 (hOat4) have been elucidated, information concerning human Oat2 (hOat2) is less defined. The objective of this study was to provide further information on the transport mechanism and substrate specificity of hOat2. When expressed in Xenopus laevis oocytes, the transport of organic compounds mediated by hOat2 was not affected by the replacement of extracellular sodium with lithium, choline and mannitol. The uptake of estrone sulfate (ES) in hOat2-expressing oocytes was significantly trans-stimulated by preloading the oocytes with fumarate and succinate, but not glutarate. Moreover, we observed that hOat2 mediates the transport of bumetanide, ES, glutarate, dehydroepiandrosterone sulfate, allopurinol, prostaglandin E2, 5-fluorouracil, paclitaxel and L-ascorbic acid. These compounds are identified for the first time as hOat2 substrates. A wide range of structurally unrelated organic compounds inhibited the hOat2-mediated uptake of tetracycline, except for sulfobromophthalein. All of these findings indicate that hOat2 is a sodium-independent multi-specific organic anion/dimethyldicarboxylate exchanger. Our present findings thus provide further insights into the role of hOat2 in hepatic drug transport.

Interactions of Stevioside and Steviol with Renal Organic Anion Transporters in S2 Cells and Mouse Renal Cortical Slices

PURPOSE

Our previous studies have shown that both stevioside and steviol inhibited transepithelial transport of para-aminohippurate (PAH) in isolated rabbit renal proximal tubules by interfering with organic anion transport system. The current study examined the direct interactions of stevioside and steviol with specific organic anion transporters.

METHODS

S2 cells expressing human organic anion transporters (hOAT1, hOAT2, hOAT3, and hOAT4) and an intact renal epithelium were used to determine the inhibitory effect of stevioside and steviol on organic anion transport.

RESULTS

Stevioside at 0.5-1 mM showed no interaction with any OAT. In contrast, steviol markedly inhibited substrate uptake in all S2hOAT cells. Steviol had low IC50 for hOAT1 (11.4 microM) and hOAT3 (36.5 microM) similar to that of probenecid, whereas IC50 for hOAT2 (1000 microM) and hOAT4 (285 microM) was much higher. Results obtained in mouse renal cortical slices were very similar; that is, stevioside was without inhibitory effect and steviol was a potent inhibitor of PAH and estrone sulfate (ES) transport.

CONCLUSIONS

Stevioside has no interaction with human or mouse OATs. In contrast, steviol interacts directly with human OATs, in particular, hOAT1 and hOAT3, with a potency approximating probenecid, suggesting that the inhibition of OAT-mediated transport by steviol could alter renal drug clearance.

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.

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.

Organic anion transporter (Slc22a) family members as mediators of toxicity

Exposure of the body to toxic organic anions is unavoidable and occurs from both intentional and unintentional sources. Many hormones, neurotransmitters, and waste products of cellular metabolism, or their metabolites, are organic anions. The same is true for a wide variety of medications, herbicides, pesticides, plant and animal toxins, and industrial chemicals and solvents. Rapid and efficient elimination of these substances is often the body's best defense for limiting both systemic exposure and the duration of their pharmacological or toxicological effects. For organic anions, active transepithelial transport across the renal proximal tubule followed by elimination via the urine is a major pathway in this detoxification process. Accordingly, a large number of organic anion transport proteins belonging to several different gene families have been identified and found to be expressed in the proximal nephron. The function of these transporters, in combination with the high volume of renal blood flow, predisposes the kidney to increased toxic susceptibility. Understanding how the kidney mediates the transport of organic anions is integral to achieving desired therapeutic outcomes in response to drug interactions and chemical exposures, to understanding the progression of some disease states, and to predicting the influence of genetic variation upon these processes. This review will focus on the organic anion transporter (OAT) family and discuss the known members, their mechanisms of action, subcellular localization, and current evidence implicating their function as a determinant of the toxicity of certain endogenous and xenobiotic agents.

Molecular Cloning and Functional Analyses of OAT1 and OAT3 from Cynomolgus Monkey Kidney

PURPOSE

The functional characterization of monkey OAT1 (SLC22A6) and OAT3 (SLC22A8) was carried out to elucidate species differences in the OAT1- and OAT3-mediated transport between monkey and human.

METHODS

The cDNAs of monkey OAT1 and OAT3 were isolated from monkey kidney, and their stable transfectants were established in HEK293 cells (mkOAT1- and mkOAT3-HEK). Transport studies were performed using cDNA transfectants, and kinetic parameters were compared among rat, monkey and human.

RESULTS

The amino acid sequences of mkOAT1 and mkOAT3 exhibit 97% and 96% identity to their corresponding human orthologues. For OAT1, there was no obvious species difference in the K(m) values and the relative transport activities of 11 substrates with regard to p-aminohippurate transport. For OAT3, there was no species difference in the K(m) values and in the relative transport activities of nine substrates with regard to benzylpenicillin transport between monkey and human. However, the relative transport activities of indoxyl sulfate, 3-carboxy-4-methyl-5-propyl-2-furanpropionate, and estrone-3-sulfate showed a difference between primates and rat and gave a poor correlation.

CONCLUSIONS

These results suggest that monkey is a good predictor of the renal uptake of organic anions in the human.

Molecular and cellular physiology of renal organic cation and anion transport

Organic cations and anions (OCs and OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport reflects the concerted activity of a suite of separate transport processes arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney now allows the development of models describing the molecular basis of the renal secretion of OCs and OAs. This review examines recent work on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.

Inhibition of the human organic anion transporter 1 by the caffeine metabolite 1-methylxanthine

Caffeine (1,3,7-trimethylxanthine) is daily and widely consumed in beverages and food and is mainly metabolized to 1,7-dimethylxanthine and 1-methylxanthine. Indirect clinical evidence suggests that 1-methylxanthine interacts with the organic anion transport system in the human kidney. In this study the effect of caffeine and its main metabolites on the human organic anion transporter 1 (hOAT1) was investigated using CHO cells overexpressing hOAT1. The uptake of 6-carboxyfluorescein into CHO(hOAT) cells was significantly inhibited by > or = 100 microM of 1-methylxanthine. Five hundred micromolar 1-methylxanthine was equieffective to 100 microM probenecid. In contrast, caffeine and 1,7-dimethylxanthine did not inhibit the transport of 6-carboxyfluorescein at concentrations up to 500 microM. In conclusion, the caffeine metabolite 1-methylxanthine inhibits the transport activity of hOAT1 in vitro. The central involvement of hOAT1 in the renal excretion of numerous drugs suggests that this inhibition may alter the pharmacokinetics of a series of clinically important drugs in humans.

Identification and functional assessment of the novel murine organic anion transporter Oat5 (Slc22a19) expressed in kidney

An uncharacterized murine cDNA clone was identified and, through sequence, phylogenetic, and functional analysis, determined to encode the newest member of the organic anion transporter family, organic anion transporter 5 (Oat5; Slc22a19). The Oat5 cDNA clone contained an insert 1,964 bp in length with a predicted open reading frame (from bp 84 to bp 1,739) coding for a peptide 551 amino acids long. Slc22a19 was localized to mouse chromosome 19 near the genes encoding Oat1 (Slc22a6) and Oat3 (Slc22a8). Northern blot analysis revealed Oat5 is highly expressed in the kidney of adult mice and rats. No sexual dimorphism in renal or hepatic expression of Oat5 was observed. Unlike Oat1-3, Oat5 expression was not detected in the choroid plexus of either mice or rats. Murine Oat5-expressing Xenopus laevis oocytes supported increased accumulation of the mycotoxin ochratoxin A, compared with water-injected control oocytes. This uptake was significantly inhibited by probenecid and the organic anions 2,4-dichlorophenoxyacetic acid, salicylate, and estrone sulfate but not by para-aminohippurate or urate. Transport of ochratoxin A by murine Oat5 was saturable, with an estimated K(m) of 2.0 +/- 0.45 microM. Oat5-mediated transport was neither cis-inhibited nor trans-stimulated by the dicarboxylate glutarate. Uptake was also completely unaffected by short-circuiting of the membrane potential. Thus the motive forces behind Oat5 function, which provide insight into its membrane localization, need to be further resolved. These data demonstrate for the first time that this newly identified gene encodes a protein that functions as an organic anion transporter.

Pharmacogenomics of the OATP and OAT families

Drug disposition is highly dependent on the interplay between drug metabolism and transport in organs such as the intestine, kidney, and liver. Genetically determined variation in drug transporter function or expression is now increasingly recognized to have a significant role as a determinant of intersubject variability in drug response. Similar to the discoveries of functional genetic variations in drug efflux transporters, such as multi-drug resistance proteins 1 and 2, there have been considerable advances in the identification of single nucleotide polymorphisms in transporters that facilitate cellular drug uptake. Among the uptake transporters, members of the organic anion-transporting polypeptides and organic anion transporters can mediate the cellular uptake of a large number of structurally divergent compounds. Accordingly, functionally relevant polymorphisms in these transporters may contribute to interindividual and interethnic variability in drug disposition and response. In this review, recent progress relating to pharmacogenomics of organic anion transporters will be outlined along with a compilation of currently known genetic polymorphisms.

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.

The SLC22 drug transporter family

The SLC22 family comprises organic cation transporters (OCTs), zwitterion/cation transporters (OCTNs), and organic anion transporters (OATs). These transporters contain 12 predicted alpha-helical transmembrane domains (TMDs) and one large extracellular loop between TMDs 1 and 2. Transporters of the SLC22 family function in different ways: (1) as uniporters that mediate facilitated diffusion in either direction (OCTs), (2) as anion exchangers (OAT1, OAT3 and URAT1), and (3) as Na(+)/ l-carnitine cotransporter (OCTN2). They participate in the absorption and/or excretion of drugs, xenobiotics, and endogenous compounds in intestine, liver and/or kidney, and perform homeostatic functions in brain and heart. The endogenous substrates include monoamine neurotransmitters, choline, l-carnitine, alpha-ketoglutarate, cAMP, cGMP, prostaglandins, and urate. Defect mutations of transporters of the SLC22 family may cause specific diseases such as "primary systemic carnitine deficiency" or "idiopathic renal hypouricemia" or change drug absorption or excretion.

Evidence for a role of human organic anion transporters in the muscular side effects of HMG-CoA reductase inhibitors

The purpose of this study was to elucidate the role of human organic anion transporters (human OATs) in the induction of drug-induced skeletal muscle abnormalities. 3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors have been clinically used for lowering plasma cholesterol levels, and are known to induce various forms of skeletal muscle abnormalities including myopathy and rhabdomyolysis. Immunohistochemical analysis revealed that human OAT1 and human OAT3 are localized in the cytoplasmic membrane of the human skeletal muscles. The activities of human OATs were measured using mouse cell lines from renal proximal tubules stably expressing human OATs. Human OAT3, but not human OAT1, mediates the transport of pravastatin. Fluvastatin inhibited organic anion uptake mediated by human OAT1 in a mixture of competitive and noncompetitive manner, whereas simvastatin and fluvastatin noncompetitively inhibited the organic anion uptake mediated by human OAT3. In conclusion, the organic anion transporters OAT1 and OAT3 are localized in the cytoplasmic membrane of human skeletal muscles. Pravastatin, simvasatin, and fluvasatin inhibit human OATs activity. These results suggest that muscle organic anion transporters play a role in the muscular side effects of HMG-CoA reductase inhibitors.

Interaction of the metal chelator DMPS with OAT1 and OAT3 in intact isolated rabbit renal proximal tubules

2,3-Dimercapto-1-propanesulfonic acid (DMPS) is used clinically to increase urinary excretion of heavy metals, including mercury and arsenic. We used single S2 segments and suspensions of rabbit renal proximal tubules (RPT) to test the interaction of this anionic heavy metal chelator with basolateral transporters OAT1 and OAT3. RTPCR revealed expression of both transporters in single S2 segments. [3H]PAH and 3H-labeled estrone sulfate ([3H]ES) were used as specific substrates for rbOAT1 and rbOAT3, respectively. PAH and ES were transported into nonperfused single RPT segments with Kt values of 67 +/- 20 and 3.4 +/- 1.2 microM, respectively, and into tubule suspensions with Kt values of 58 +/- 17 and 7.7 +/- 2.1 microM, respectively. Reduced DMPS (DMPSH) inhibited uptake of both substrates into single tubule segments with Kapp values of 405 +/- 49 microM (for [3H]PAH) and 320 +/- 66 microM (for [3H]ES). Oxidized DMPS (DMPSS), the prevalent form in the blood, also inhibited uptakes of [3H]PAH (Kapp of 766 +/- 190 microM) and [3H]ES (696 +/- 166 microM). Inward gradients of ES, DMPSH, and DMPSS trans-stimulated the 30-s efflux of preloaded [3H]ES across the basolateral membrane of RPT. Similarly, DMPSH, and PAH itself, trans-stimulated the 15-s efflux of [3H]PAH. In contrast, efflux of [3H]PAH was inhibited by the presence of DMPSS in the bathing medium. These data suggest that, whereas both OAT1 and OAT3 probably transport DMPSH, DMPSS transport may be limited to OAT3. This is the first evidence showing that both OAT1 and OAT3 can transport DMPS across the basolateral membrane of RPT.

Interactions of Human- and Rat-Organic Anion Transporters With Pravastatin and Cimetidine

We have elucidated the interactions of human and rat organic anion transporters (hOATs and rOATs) with pravastatin and cimetidine. Pravastatin inhibited hOAT1/rOAT1, hOAT2/rOAT2, hOAT3/rOAT3, and hOAT4. The mode of inhibition was noncompetitive for hOAT1 and hOAT2, whereas it was competitive for hOAT3 and hOAT4. Cimetidine also inhibited hOAT1/rOAT1, hOAT3/rOAT3, and hOAT4. The mode of inhibition was a combination of competitive and noncompetitive manners for hOAT1, whereas it was competitive for hOAT3. The effects of OAT inhibitors on OAT1, OAT2, and OAT3 exhibited some but not so remarkable interspecies differences between humans and rats. In conclusion, we have characterized pravastatin and cimetidine as OAT inhibitors.

Interactions of human organic anion transporters with diuretics

The tubular secretion of diuretics in the proximal tubule has been shown to be critical for the action of drugs. To elucidate the molecular mechanisms for the tubular excretion of diuretics, we have elucidated the interactions of human organic anion transporters (hOATs) with diuretics using cells stably expressing hOATs. Diuretics tested were thiazides, including chlorothiazide, cyclothiazide, hydrochlorothiazide, and trichlormethiazide; loop diuretics, including bumetanide, ethacrynic acid, and furosemide; and carbonic anhydrase inhibitors, including acetazolamide and methazolamide. These diuretics inhibited organic anion uptake mediated by hOAT1, hOAT2, hOAT3, and hOAT4 in a competitive manner. hOAT1 exhibited the highest affinity interactions for thiazides, whereas hOAT3 did those for loop diuretics. hOAT1, hOAT3, and hOAT4 but not hOAT2, mediated the uptake of bumetanide. hOAT3 and hOAT4, but not hOAT1 mediated the efflux of bumetanide. hOAT1 and hOAT3, but not hOAT2 and hOAT4 mediated the uptake of furosemide. In conclusion, it was suggested that hOAT1 may play an important role in the basolateral uptake of thiazides, and hOAT3 in the uptake of loop diuretics. In addition, it was also suggested that bumetanide taken up by hOAT3 and/or hOAT1 is excreted into the urine by hOAT4.

Pharmacogenomics of diuretic drugs: data on rare monogenic disorders and on polymorphisms and requirements for further research

This review summarizes the current status of our knowledge about the role of pharmacogenetic variation in response to diuretics and suggests future research topics for the field. Genes with a role in the pharmacokinetics of most diuretics are renal drug transporters, especially OAT1, OAT3 and OCT2 (genes SLC22A6, SLC22A8 and SLC22A2) whereas variants in carbonic anhydrase (CA), cytochrome P450 enzymes and sulfotransferases are relevant only for specific substances. Genes on the pharmacodynamic side include the primary targets of thiazide, loop, K+-sparing and aldosterone antagonistic diuretics: NCC, NKCC2, ENaC and the mineralocorticoid receptor (genes SLC12A3, SLC12A1, SCNN1A, B, G and NR3C2). Rare variants of these proteins cause Gitelman’s syndrome, Bartter’s syndrome, Liddle’s syndrome or pregnancy-induced hypertension. Polymorphisms in these and in associated proteins such as GNB3, α-adducin and angiotensin-converting enzyme (ACE) seem to be clinically relevant. In conclusion, first knowledge has evolved that efficacy of diuretic drugs may be determined by genetic polymorphisms in genes determining pharmacokinetics and pharmacodynamics of this drug class. In the future, the selection of a diuretic drug or the dosing schedules may be individually chosen based on pharmacogenetic parameters, however, many questions remain to be answered before this fantasy becomes reality.

The human multidrug resistance protein MRP4 functions as a prostaglandin efflux transporter and is inhibited by nonsteroidal antiinflammatory drugs

Prostaglandins are involved in a wide variety of physiological and pathophysiological processes, but the mechanism of prostaglandin release from cells is not completely understood. Although poorly membrane permeable, prostaglandins are believed to exit cells by passive diffusion. We have investigated the interaction between prostaglandins and members of the ATP-binding cassette (ABC) transporter ABCC [multidrug resistance protein (MRP)] family of membrane export pumps. In inside-out membrane vesicles derived from insect cells or HEK293 cells, MRP4 catalyzed the time- and ATP-dependent uptake of prostaglandin E1 (PGE1) and PGE2. In contrast, MRP1, MRP2, MRP3, and MRP5 did not transport PGE1 or PGE2. The MRP4-mediated transport of PGE1 and PGE2 displayed saturation kinetics, with Km values of 2.1 and 3.4 microM, respectively. Further studies showed that PGF1alpha, PGF2alpha, PGA1, and thromboxane B2 were high-affinity inhibitors (and therefore presumably substrates) of MRP4. Furthermore, several nonsteroidal antiinflammatory drugs were potent inhibitors of MRP4 at concentrations that did not inhibit MRP1. In cells expressing the prostaglandin transporter PGT, the steady-state accumulation of PGE1 and PGE2 was reduced proportional to MRP4 expression. Inhibition of MRP4 by an MRP4-specific RNA interference construct or by indomethacin reversed this accumulation deficit. Together, these data suggest that MRP4 can release prostaglandins from cells, and that, in addition to inhibiting prostaglandin synthesis, some nonsteroidal antiinflammatory drugs might also act by inhibiting this release.

Panipenem/betamipron

Panipenem is a parenteral carbapenem antibacterial agent with a broad spectrum of in vitro activity covering a wide range of Gram-negative and Gram-positive aerobic and anaerobic bacteria, including Streptococcus pneumoniae and species producing beta-lactamases. Panipenem is coadministered with betamipron to inhibit panipenem uptake into the renal tubule and prevent nephrotoxicity. In large, randomised clinical trials, panipenem/betamipron demonstrated good clinical and bacteriological efficacy (similar to that of imipenem/cilastatin) in adults with respiratory tract or urinary tract infections. Panipenem/betamipron was also effective in adults with surgical or gynaecological infections, and in paediatric patients with respiratory tract and urinary tract infections in noncomparative trials. In small trials in elderly patients reported as abstracts, panipenem/betamipron demonstrated clinical efficacy similar to intravenous piperacillin and greater than oral ofloxacin in urinary tract infections. Elderly patients with respiratory tract infections also responded to therapy. Panipenem/betamipron is well tolerated with few adverse events reported in clinical trials, most commonly elevated serum levels of hepatic transaminases and eosinophils, rash and diarrhoea.

Transport of organic anions across the basolateral membrane of proximal tubule cells

Renal proximal tubules secrete diverse organic anions (OA) including widely prescribed anionic drugs. Here, we review the molecular properties of cloned transporters involved in uptake of OA from blood into proximal tubule cells and provide extensive lists of substrates handled by these transport systems. Where tested, transporters have been immunolocalized to the basolateral cell membrane. The sulfate anion transporter 1 (sat-1) cloned from human, rat and mouse, transported oxalate and sulfate. Drugs found earlier to interact with sulfate transport in vivo have not yet been tested with sat-1. The Na(+)-dicarboxylate cotransporter 3 (NaDC-3) was cloned from human, rat, mouse and flounder, and transported three Na(+) with one divalent di- or tricarboxylate, such as citric acid cycle intermediates and the heavy metal chelator 2,3-dimercaptosuccinate (succimer). The organic anion transporter 1 (OAT1) cloned from several species was shown to exchange extracellular OA against intracellular alpha-ketoglutarate. OAT1 translocated, e.g., anti-inflammatory drugs, antiviral drugs, beta-lactam antibiotics, loop diuretics, ochratoxin A, and p-aminohippurate. Several OA, including probenecid, inhibited OAT1. Human, rat and mouse OAT2 transported selected anti-inflammatory and antiviral drugs, methotrexate, ochratoxin A, and, with high affinities, prostaglandins E(2) and F(2alpha). OAT3 cloned from human, rat and mouse showed a substrate specificity overlapping with that of OAT1. In addition, OAT3 interacted with sulfated steroid hormones such as estrone-3-sulfate. The driving forces for OAT2 and OAT3, the relative contributions of all OA transporters to, and the impact of transporter regulation by protein kinases on renal drug excretion in vivo must be determined in future experiments.

Contribution of organic anion transporters to the renal uptake of anionic compounds and nucleoside derivatives in rat

Our previous kinetic analyses have shown that rat organic anion transporter 1 (rOat1; Slc22a6) and rOat3 (Slc22a8) are responsible for the renal uptake of p-aminohippurate and pravastatin, respectively. In this study, their contribution to the renal uptake of organic anions and nucleoside derivatives was examined by investigating the uptake by rOat1- and rOat3-expressing cells and kidney slices. Transfection of rOat1 resulted in an increase of the uptake of temocaprilat (Km = 0.56 microM), 2,4-dichlorophenoxyacetate (2,4-D; Km = 10 microM), and 3'-azido-3'-deoxythymidine (AZT; Km = 43 microM). rOat3-expressing cells showed significant uptake of temocaprilat (Km = 1.4 microM), estrone sulfate (Km = 5.3 microM), dehydroepiandrosterone sulfate (DHEAS; Km = 12 microM), and benzylpenicillin (PCG; Km = 85 microM). All the test compounds were accumulated in kidney slices in a carrier-mediated manner, although the saturable components of AZT and acyclovir were small. The Km of 2,4-D uptake by kidney slices was comparable with that of rOat1, and the corresponding values of DHEAS and PCG were similar to those of rOat3. The uptake of estrone sulfate and temocaprilat by kidney slices consisted of two saturable components, with the Km values of their high-affinity components being similar to those for rOat3 (estrone sulfate), and rOat1 and rOat3 (temocaprilat), respectively. These results suggest that the renal uptake of 2,4-D is mainly accounted for by rOat1 and the uptake of PCG and DHEAS by rOat3, and rOat3 is partly involved in the renal uptake of temocaprilat and estrone sulfate.

Probenecid-induced changes in the clearance of pranoprofen enantiomers

Probenecid is known to inhibit the elimination of several acidic drugs. Its influence on the pharmacokinetics of pranoprofen was investigated in rabbit after a single intravenous injection of racemic mixture (5 mg/kg). Levels of (-)-(R)- and (+)-(S)-pranoprofen and their glucuronide (after hydrolysis with sodium hydroxide) were determined in plasma, urine, and several tissues. The plasma concentration of the (+)-(S)-isomer was higher than that of the (-)-(R)-form. Oral coadministered probenecid (100 mg/kg) resulted in an increased plasma concentration of both enantiomers. Probenecid reduced the apparent total clearance and excretion of pranoprofen enantiomers in urine. It had a slight effect on the tissue distribution of pranoprofen at the dose used, but significantly reduced the formation of glucuronide for both enantiomers to the same extent in kidney microsomes. The differences caused by probenecid were significant with respect to its ability to inhibit glucuronidation in the kidney and subsequent excretion into urine, but enantioselective effects were negligible.

Interactions of human organic anion as well as cation transporters with indoxyl sulfate

Various uremic toxicants including indoxyl sulfate exert a number of biological effects on uremic patients. In order to elucidate the molecular mechanisms for the pharmacokinetics of indoxyl sulfate in human, we examined the interactions of human organic anion transporters (human-OATs) and human organic cation transporters (human-OCTs) with indoxyl sulfate using stable transfectants. Indoxyl sulfate inhibited human-OAT1, human-OAT3 and human-OAT4, but not human-OAT2, human-OCT1 and human-OCT2. Kinetic analysis revealed that the K(i) values for human-OAT1, human-OAT3 and human-OAT4 were 22.7, 168.7 and 181.3 microM, respectively. Human-OAT1 and human-OAT3 mediated the uptake of indoxyl sulfate and human-OAT4 mediated not only the uptake but also the efflux of indoxyl sulfate. In conclusion, by comparing the K(i) values with the plasma concentration of unbound indoxyl sulfate, it was predicted that human-OAT1 and human-OAT3 mediate the transport of indoxyl sulfate in vivo. In addition, it was suggested that human-OAT1 and human-OAT3 are involved in the urinary excretion of indoxyl sulfate, the exacerbation of renal dysfunction and the induction of uremic encephalopathy by indoxyl sulfate.

Organic anion transporter 3 (Slc22a8) is a dicarboxylate exchanger indirectly coupled to the Na+ gradient

Basolateral uptake of organic anions in renal proximal tubule cells is indirectly coupled to the Na(+) gradient through Na(+)-dicarboxylate cotransport and organic anion/dicarboxylate exchange. One member of the organic anion transporter (OAT) family, Oat1, is expressed in the proximal tubule and is an organic anion/dicarboxylate exchanger. However, a second organic anion carrier, Oat3, is also highly expressed in the renal proximal tubule, but its mechanism is unclear. Thus we have assessed Oat3 function in Xenopus laevis oocytes and rat renal cortical slices. Probenecid-sensitive uptake of p-aminohippurate (PAH, an Oat1 and Oat3 substrate) and estrone sulfate (ES, an Oat3 substrate) in rat Oat3-expressing oocytes was significantly trans-stimulated by preloading the oocytes with the dicarboxylate glutarate (GA). GA stimulation of ES transport by oocytes coexpressing rabbit Na(+)-dicarboxylate cotransporter 1 and rat Oat3 was significantly inhibited when the preloading medium contained Li(+) or methylsuccinate (MS) or when Na(+) was absent. All these treatments inhibit the Na(+)-dicarboxylate cotransporter, but not rat Oat3. Li(+), MS, and Na(+) removal had no effect when applied during the ES uptake step, rather than during the GA preloading step. Concentrative ES uptake in rat renal cortical slices was also demonstrated to be probenecid and Na(+) sensitive. Accumulation of ES was stimulated by GA, and this stimulation was completely blocked by probenecid, Li(+), MS, taurocholate, and removal of Na(+). Thus Oat3 functions as an organic anion/dicarboxylate exchanger that couples organic anion uptake indirectly to the Na(+) gradient.

Interaction of human and rat organic anion transporter 2 with various cephalosporin antibiotics

Cephalosporin antibiotics are thought to be excreted into the urine via organic anion transporters (OATs) and OAT can mediate nephrotoxicity by cephalosporins, particularly by cephaloridine. The purpose of this study was to elucidate the interaction of human-OAT2 and rat-OAT2 with cephalosporin antibiotics using proximal tubule cells stably expressing human-OAT2 and rat-OAT2. Human-OAT2 is localized to the basolateral side of the proximal tubule, whereas rat-OAT2 is localized to the apical side of the proximal tubule. Cephalosporins tested were cephalothin, cefoperazone, cefazolin, ceftriaxone, cephaloridine, cefotaxime, cefadroxil and cefamandole. These cephalosporins dose-dependently inhibited organic anion uptake mediated by human-OAT2 and rat-OAT2. There was no species difference observed for the effects of OAT2 with cephalosporins between human and rat transporters. Kinetic analysis revealed that the inhibitory effects for human-OAT2 were competitive. Cephaloridine significantly decreased the viability of cells stably expressing human-OAT2, human-OAT1, human-OAT3 and human-OAT4. The decreased viability of cells stably expressing human-OAT1, human-OAT3 and human-OAT4 but not human-OAT2 was reversed by probenecid. In conclusion, human-OAT2 interacts with cephalosporins, and thus, human-OAT2 may mediate the uptake of cephalosporins on the basolateral side of the proximal tubule. The interaction of human-OAT2 with cephalosporins was the weakest among the basolateral human-OATs tested. In addition, it is suggested that human-OATs mediate cephaloridine-induced nephrotoxicity.

Expression of Human Organic Anion Transporters in the Choroid Plexus and Their Interactions With Neurotransmitter Metabolites

The purpose of the present study was to elucidate the expression of human organic anion transporter 1 (hOAT1) and hOAT3 in the choroid plexus of the human brain and their interactions with neurotransmitter metabolites using stable cell lines. Immunohistochemical analysis revealed that hOAT1 and hOAT3 are expressed in the cytoplasmic membrane and cytoplasm of human choroid plexus. Neurotransmitter metabolites, namely, 5-methoxyindole-3-acetic acid (5-MI-3-AA), homovanillic acid (HVA), vanilmandelic acid (VMA), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindole-3-acetic acid (5-HI-3-AA), N-acetyl-5-hydroxytryptamine (NA-5-HTT), melatonin, 5-methoxytryptamine (5-MTT), 3,4-dihidroxymandelic acid (DHMA), 5-hydroxytryptophol, and 5-methoxytryptophol (5-MTP), but not methanephrine (MN), normethanephrine (NMN), and 3-methyltyramine (3-MT), at 2 mM, inhibited para-aminohippuric acid uptake mediated by hOAT1. On the other hand, melatonin, 5-MI-3-AA, NA-5-HTT, 5-MTT, 5-MTP, HVA, 5-HI-3-AA, VMA, DOPAC, 5-hydroxytryptophol, and MN, but not 3-MT, DHMA, and NMN, at 2 mM, inhibited estrone sulfate uptake mediated by hOAT3. Differences in the IC(50) values between hOAT1 and hOAT3 were observed for DHMA, DOPAC, HVA, 5-HI-3-AA, melatonin, 5-MI-3-AA, 5-MTP, 5-MTT, and VMA. HOAT1 and hOAT3 mediated the transport of VMA but not HVA and melatonin. These results suggest that hOAT1 and hOAT3 are involved in the efflux of various neurotransmitter metabolites from the cerebrospinal fluid to the blood across the choroid plexus.

Interactions of human organic anion transporters and human organic cation transporters with nonsteroidal anti-inflammatory drugs

The purpose of this study was to elucidate the interactions of human organic anion transporters (hOATs) and human organic cation transporters (hOCTs) with nonsteroidal anti-inflammatory drugs (NSAIDs) using cells stably expressing hOATs and hOCTs. NSAIDs tested were acetaminophen, acetylsalicylate, salicylate, diclofenac, ibuprofen, indomethacin, ketoprofen, mefenamic acid, naproxen, piroxicam, phenacetin, and sulindac. These NSAIDs inhibited organic anion uptake mediated by hOAT1, hOAT2, hOAT3, and hOAT4. By comparing the IC(50) values of NSAIDs for hOATs, it was found that hOAT1 and hOAT3 exhibited higher affinity interactions with NSAIDs than did hOAT2 and hOAT4. HOAT1, hOAT2, hOAT3, and hOAT4 mediated the uptake of either ibuprofen, indomethacin, ketoprofen, or salicylate, but not acetylsalicylate. Although organic cation uptake mediated by hOCT1 and hOCT2 was also inhibited by some NSAIDs, hOCT1 and hOCT2 did not mediate the uptake of NSAIDs. In conclusion, hOATs and hOCTs interacted with various NSAIDs, whereas hOATs but not hOCTs mediated the transport of some of these NSAIDs. Considering the localization of hOATs, it was suggested that the interactions of hOATs with NSAIDs are associated with the pharmacokinetics and the induction of adverse reactions of NSAIDs.

Prostaglandin transport

Newly synthesized prostaglandins (PGs) efflux from cells by simple diffusion, driven by pH and the membrane potential. Metabolic clearance requires energy-dependent uptake across the plasma membrane, followed by cytoplasmic oxidation. Several PG carriers have been cloned and characterized. PGT is broadly expressed in cyclooxygenase (COX)-positive cells, appears to be a lactate/PG exchanger, and is coordinately regulated with COX. By analogy with neurotransmitter release and re-uptake, PGT may regulate pericellular PG levels via re-uptake. PGT may also direct PGs towards and/or away from specific sets of PG receptors. Other members of the OATP transporter family also catalyze PG uptake; these are variably expressed and have variable affinities for PGs. The OATs are alpha-ketoglutarate/organic anion exchangers that accept PGs; these probably represent the uptake step in renal and hepatic PG degradation and excretion. Finally, certain glutathione-conjugated leukotrienes and PGs are actively extruded from cells by the MRPs; these may also play a role in metabolic clearance of PGs.