Structural determinants of inhibitor interaction with the human organic cation transporter OCT2 (SLC22A2)

Renal transport of organic anions and cations

Organic anions and cations (OAs and OCs, respectively) comprise an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. The kidney, primarily the renal proximal tubule, plays a critical role in regulating the plasma concentrations of these organic electrolytes and in clearing the body of potentially toxic xenobiotics agents, a process that involves active, transepithelial secretion. This transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. Basolateral and luminal OA and OC transport reflects the concerted activity of a suite of separate proteins 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 OAs and OCs. New information on naturally occurring genetic variation of many of these processes provides insight into the basis of observed variability of drug efficacy and unwanted drug-drug interactions in human populations. The present review examines recent work on these issues.

Combinatorial pharmacophore modeling of organic cation transporter 2 (OCT2) inhibitors: insights into multiple inhibitory mechanisms

Organic cation transporter 2 (OCT2) is responsible for the entry step of many drugs in renal elimination, of which the changing activity may cause unwanted drug-drug interactions (DDIs). To develop drugs with favorable safety profile and provide instruction for rational clinical drug administration, it is of great interest to investigate the multiple mechanisms of OCT2 inhibition. In this study, we designed a combinatorial scheme to screen the optimum combination of pharmacophores from a pool of hypotheses established based on 162 OCT2 inhibitors. Among them, one single pharmacophore hypothesis represents a potential binding mode that may account for one unique inhibitory mechanism, and the obtained pharmacophore combination describes the multimechanisms of OCT2 inhibition. The final model consists of four individual pharmacophores, i.e., DHPR18, APR2, PRR5 and HHR4. Given a query ligand, it is considered as an inhibitor if it matches at least one of the hypotheses, or a noninhibitor if it fails to match any of four hypotheses. Our combinatorial pharmacophore model performs reasonably well to discriminate inhibitors and noninhibitors, yielding an overall accuracy around 0.70 for a test set containing 81 OCT2 inhibitors and 218 noninhibitors. Intriguingly, we found that the number of matched hypotheses was positively correlated with inhibition rate, which coincides with the pharmacophore modeling result of P-gp substrate binding. Further analysis suggested that the hypothesis PRR5 was responsible for competitive inhibition of OCT2, and other hypotheses were important for interaction between the inhibitor and OCT2. In light of the results, a hypothetical model for inhibiting transporting mediated by OCT2 was proposed.

Prediction of clinical drug-drug interactions of veliparib (ABT-888) with human renal transporters (OAT1, OAT3, OCT2, MATE1, and MATE2K)

Veliparib (ABT-888) is largely eliminated as parent drug in human urine (70% of the dose). Renal unbound clearance exceeds glomerular filtration rate, suggesting the involvement of transporter-mediated active secretion. Clinically relevant pharmacokinetic interactions in the kidney have been associated with OAT1, OAT3, OCT2, MATE1, and MATE2K. In the present study, interactions of veliparib with these transporters were investigated. Veliparib inhibited OAT1, OAT3, OCT2, MATE1, and MATE2K with IC50 values of 1371, 505, 3913, 69.9, and 69.5 μM, respectively. The clinical unbound maximum plasma concentration of veliparib after single oral dose of 50 mg (0.45 μM) is manyfold lower than IC50 values for OAT1, OAT3, OCT2, MATE1, or MATE2K. These results indicate a low potential for drug-drug interaction (DDI) with OAT1/3, OCT2, or MATE1/2K. Additional studies demonstrated that veliparib is a substrate of OCT2. In Oct1/Oct2 double-knockout mice, the plasma exposure of veliparib was increased by 1.5-fold, and the renal clearance was decreased by 1.8-fold as compared with wild-type mice, demonstrating that organic cation transporters contribute to the renal elimination in vivo. In summary, the in vitro transporter data for veliparib predicts minimal potential for an OAT1/3-, OCT2-, and MATE1/2K-mediated DDI given the clinical exposure after single oral dose of 50 mg.

Substrate-dependent inhibition of organic anion transporting polypeptide 1B1: comparative analysis with prototypical probe substrates estradiol-17β-glucuronide, estrone-3-sulfate, and sulfobromophthalein

Organic anion transporting polypeptide (OATP) 1B1 plays an important role in the hepatic uptake of many drugs, and the evaluation of OATP1B1-mediated drug-drug interactions (DDIs) is emphasized in the latest DDI (draft) guidance documents from U.S. and E.U. regulatory agencies. It has been suggested that some OATP1B1 inhibitors show a discrepancy in their inhibitory potential, depending on the substrates used in the cell-based assay. In this study, inhibitory effects of 14 compounds on the OATP1B1-mediated uptake of the prototypical substrates [³H]estradiol-17β-glucuronide (E₂G), [³H]estrone-3-sulfate (E₁S), and [³H]sulfobromophthalein (BSP) were studied in OATP1B1-transfected cells. Inhibitory potencies of tested compounds varied depending on the substrates. Ritonavir, gemfibrozil, and erythromycin caused remarkable substrate-dependent inhibition with up to 117-, 14-, and 13-fold difference in their IC₅₀ values, respectively. Also, the clinically relevant OATP inhibitors rifampin and cyclosporin A exhibited up to 12- and 6-fold variation in their IC₅₀ values, respectively. Regardless of the inhibitors tested, the most potent OATP1B1 inhibition was observed when [³H]E₂G was used as a substrate. Mutual inhibition studies of OATP1B1 indicated that E₂G and E₁S competitively inhibited each other, whereas BSP noncompetitively inhibited E₂G uptake. In addition, BSP inhibited E₁S in a competitive manner, but E₁S caused an atypical kinetics on BSP uptake. This study showed substrate-dependent inhibition of OATP1B1 and demonstrated that E₂G was the most sensitive in vitro OATP1B1 probe substrate among three substrates tested. This will give us an insight into the assessment of clinically relevant OATP1B1-mediated DDI in vitro with minimum potential of false-negative prediction.

Role of organic cation transporter OCT2 and multidrug and toxin extrusion proteins MATE1 and MATE2-K for transport and drug interactions of the antiviral lamivudine

The antiviral lamivudine is cleared predominantly by the kidney with a relevant contribution of renal tubular secretion. It is not clear which drug transporters mediate lamivudine renal secretion. Our aim was to investigate lamivudine as substrate of the renal drug transporters organic cation transporter 2 (OCT2) and multidrug and toxin extrusion proteins MATE1 and MATE2-K. Uptake experiments were performed in OCT2, MATE1, or MATE2-K single-transfected human embryonic kidney 293 (HEK) cells. Transcellular transport experiments were performed in OCT2 and/or MATE1 single- or double-transfected Madin-Darby canine kidney II (MDCK) cells grown on transwell filters. Lamivudine uptake was significantly increased in HEK-OCT2, HEK-MATE1, and HEK-MATE2-K cells compared to control cells. In transcellular experiments, OCT2 located in the basolateral membrane had no effect on transcellular lamivudine transport. MATE1 located in the apical membrane decreased intracellular concentrations and increased transcellular transport of lamivudine from the basal to the apical compartment. MATE1- or MATE2-K-mediated transport was increased by an oppositely directed pH gradient. Several simultaneously administered drugs inhibited OCT2- or MATE2-K-mediated lamivudine uptake. The strongest inhibitors were carvedilol for OCT2 and trimethoprim for MATE2-K (inhibition by 96.3 and 83.7% at 15 μM, respectively, p<0.001). Trimethoprim inhibited OCT2- and MATE2-K-mediated lamivudine uptake with IC₅₀ values of 13.2 and 0.66 μM, respectively. Transcellular lamivudine transport in OCT2-MATE1 double-transfected cells was inhibited by trimethoprim with an IC₅₀ value of 6.9 μM. Lamivudine is a substrate of renal drug transporters OCT2, MATE1, and MATE2-K. Concomitant administration of drugs that inhibit these transporters could decrease renal clearance of lamivudine.

Transport of asymmetric dimethylarginine (ADMA) by cationic amino acid transporter 2 (CAT2), organic cation transporter 2 (OCT2) and multidrug and toxin extrusion protein 1 (MATE1)

Asymmetric dimethylarginine (ADMA), inhibiting the nitric oxide (NO) synthesis from L-arginine, is a known cardiovascular risk factor. Our aim was to investigate if ADMA and/or L-arginine are substrates of the human cationic amino acid transporters 2A (CAT2A, SLC7A2A) and 2B (CAT2B, SLC7A2B), the organic cation transporter 2 (OCT2, SLC22A2), and the multidrug and toxin extrusion protein 1 (MATE1, SLC47A1). We systematically investigated the kinetics of ADMA and L-arginine transport in human embryonic kidney (HEK293) cells stably overexpressing CAT2A, CAT2B, OCT2, or MATE1. Vector-only transfected HEK293 cells served as controls. Compared to vector control cells, uptake of ADMA and L-arginine was significantly higher (p < 0.05) in cells expressing CAT2B and OCT2 at almost all investigated concentrations, while cells expressing CAT2A only showed a significant uptake at concentrations above 300 μM. Uptake of MATE1 overexpressing cells was significantly (p < 0.05) higher at pH 7.8 and 8.2 than controls. Apparent V max values (nmol mg protein(-1) min(-1)) for cellular uptake of ADMA and L-arginine were ≈11.8 ± 1.2 and 19.5 ± 0.7 for CAT2A, ≈14.3 ± 1.0 and 15.3 ± 0.4 for CAT2B, and 6.3 ± 0.3 and >50 for OCT2, respectively. Apparent K m values (μmol/l) for cellular uptake of ADMA and L-arginine were ≈3,033 ± 675 and 3,510 ± 419 for CAT2A, ≈4,021 ± 532 and 952 ± 92 for CAT2B, and 967 ± 143 and >10,000 for OCT2, respectively. ADMA and L-arginine are substrates of human CAT2A, CAT2B, OCT2 and MATE1. Transport kinetics of CAT2A, CAT2B, and OCT2 indicate a low affinity, high capacity transport, which may be relevant for renal and hepatic elimination of ADMA or L-arginine.

Substrate-dependent inhibition of human MATE1 by cationic ionic liquids

The multidrug and toxin extruders 1- and 2-K (MATE1 and MATE2-K) are expressed in the luminal membrane of renal proximal tubule cells and provide the active step in the secretion of molecules that carry a net positive charge at physiologic pH, so-called organic cations. The present study tested whether structurally distinct MATE substrates can display different quantitative profiles of inhibition when interacting with structurally distinct ligands. The tested ligands were three structurally similar cationic ionic liquids (ILs, salts in the liquid state: N-butylpyridinium, NBuPy; 1-methyl-3-butylimidazolium, Bmim; and N-butyl-N-methylpyrrolidinium, BmPy). Uptake was measured using Chinese hamster ovary cells that stably expressed MATE1 or MATE2-K. By trans-stimulation, all three ILs were transported by both MATE transporters. The three ILs also inhibited uptake of three structurally distinct MATE substrates: 1-methyl-4-phenylpyridinium (MPP), triethylmethylammonium (TEMA), and N,N,N-trimethyl-2-[methyl(7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)amino]ethanaminium (NBD-MTMA). MATE1 displayed a higher affinity for the pyridinium-based NBuPy (IC50 values, 2-4 µM) than for either the pyrrolidinium- (BmPy; 20-70 µM) or imidazolium-based ILs (Bmim; 15-60 µM). Inhibition of MPP, TEMA, and NBD-MTMA transport by NBuPy was competitive, with comparable Ki values against all substrates. Bmim also competitively blocked the three substrates but with Ki values that differed significantly (20 µM against MPP and 30 µM against NBD-MTMA versus 60 µM against TEMA). Together, these data indicate that renal secretion of ILs by the human kidney involves MATE transporters and suggest that the mechanism of transport inhibition is ligand-dependent, supporting the hypothesis that the binding of substrates to MATE transporters involves interaction with a binding surface with multiple binding sites.

Ischemia/Reperfusion-inducible protein modulates the function of organic cation transporter 1 and multidrug and toxin extrusion 1

The recently identified ischemia/reperfusion-inducible protein (IRIP) has been reported to negatively modulate the activities of several transporters in cell culture systems. The goal of this study is to determine whether IRIP regulates the activities of OCT1 and MATE1, and hence the disposition in vivo of their substrate metformin, a therapeutic drug for diabetes and other obesity-related syndromes. In the uptake studies in the human embryonic kidney 293 cells overexpressing IRIP with and without OCT1 or MATE1, IRIP overexpression was found to significantly inhibit the uptake of 1-methyl-4-phenylpyridinium mediated by OCT1 or MATE1. In contrast, knockdown of IRIP by small hairpin RNA (shRNA) increased the transporter activities in vitro. IRIP overexpression decreased the membrane localization of transporter proteins without any changes in transcript levels in cells. By overexpressing IRIP in mouse liver via hydrodynamic tail vein injection, we demonstrated that increased IRIP expression could cause a significant reduction in hepatic accumulation of metformin (P < 0.01). In addition, we observed that the expression of IRIP was approximately half (P < 0.01) in ob/ob mice when compared to their lean littermates, with significant increases in hepatic Oct1 protein expression and metformin accumulation. In conclusion, IRIP negatively modulates the function of OCT1 and MATE1 in cells. Importantly, we provide in vivo evidence for such modulation that may cause an alteration in drug disposition. The regulation by IRIP on transporter activities likely occurs at a post-transcriptional level, and future studies are needed to characterize the exact mechanism.

Substrate-dependent ligand inhibition of the human organic cation transporter OCT2

Organic cation transporter 2 (OCT2) mediates the initial step in renal secretion of organic cations: uptake from the blood, across the basolateral membrane, and into the renal proximal tubule cells. Because of its potential as a target for unwanted drug-drug interactions (DDIs), considerable attention has been directed toward understanding the basis of OCT2 selectivity. These studies typically assess selectivity based on ligand inhibition profiles for OCT2-mediated transport of a probe substrate. However, little attention has been given to the potential influence of the substrate on the profile of ligand inhibition. Here we compared the IC50 values obtained for a set of structurally distinct inhibitors against OCT2-mediated transport of three structurally distinct substrates: 1-methyl-4-phenylpyridinium (MPP); metformin; and a novel fluorescent substrate, N,N,N-trimethyl-2-[methyl(7-nitrobenzo[c][l,2,5]oxadiazol-4-yl)amino]ethanaminium iodide (NBD-MTMA). The median IC50 value for inhibition of MPP transport was 9-fold higher than that for inhibition of metformin transport. Similarly, the median IC50 value for inhibition of MPP transport was 5-fold higher than that for NBD-MTMA transport. However, this was not a systematic difference in inhibitory efficacy; the ratio of IC50 values, MPP versus NBD-MTMA, ranged from 88-fold (ipratropium) to 0.3-fold (metformin). These data show that 1) the choice of OCT2 substrate significantly influences both quantitative and qualitative inhibitory interactions with cationic drugs; and 2) ligand interactions with OCT2 are not restricted to competition for a common ligand binding site, consistent with a binding surface characterized by multiple, possibly overlapping interaction sites. Development of predictive models of DDIs with OCT2 must take into account the substrate dependence of ligand interaction with this protein.

Polyamine transport by the polyspecific organic cation transporters OCT1, OCT2, and OCT3

Polyamines are ubiquitous organic cations implicated in many physiological processes. Because they are positively charged at physiological pH, carrier-mediated systems are necessary for effective membrane permeation, but the identity of specific polyamine transporter proteins in eukaryotic cells remains unclear. Polyspecific organic cation transporters (OCTs) interact with many natural and xenobiotic monovalent cations and have been reported to transport dicationic compounds, including the short polyamine putrescine. In this study, we used Xenopus oocytes expressing mammalian OCT1 (SLC22A1), OCT2 (SLC22A2), or OCT3 (SLC22A3) to assess binding and transport of longer-chain polyvalent polyamines. In OCT-expressing oocytes, [(3)H]MPP(+) uptake rates were 15- to 35-fold higher than in noninjected oocytes, whereas those for [(3)H]spermidine increased more modestly above the background, up to 3-fold. This reflected up to 20-fold lower affinity for spermidine than for MPP(+); thus, K(0.5) for MPP(+) was ~50 μM in OCT1, ~170 μM in OCT2, and ~60 μM in OCT3, whereas for spermidine, K(0.5) was ~1 mM in OCT1, OCT2, and OCT3. J(max) values for MPP(+) and spermidine were within the same range, suggesting that both compounds are transported at a similar turnover rate. To gain further insight into OCT substrate specificity, we screened a selection of structural polyamine analogues for effect on [(3)H]MPP(+) uptake. In general, blocking potency increased with overall hydrophobic character, which indicates that, as for monovalent cations, hydrophobicity is a major requirement for recognition in polyvalent OCT substrates and inhibitors. Our results demonstrate that the natural polyamines are low affinity, but relatively high turnover, substrates for OCTs. The identification of OCTs as polyamine transport systems may contribute to further understanding of the mechanisms involved in polyamine homeostasis and aid in the design of polyamine-like OCT-targeted drugs.

Multiple mechanisms of ligand interaction with the human organic cation transporter, OCT2

OCT2 is the entry step for organic cation (OC) secretion by renal proximal tubules. Although many drugs inhibit OCT2 activity, neither the mechanistic basis of their inhibition nor their transport status is generally known. Using representatives of several structural classes of OCT2-inhibitory ligands described recently (Kido Y, Matsson P, Giacomini KM. J Med Chem 54: 4548-4558, 2011), we determined the kinetic basis of their inhibition of 1-methyl-4-phenylpyridinium (MPP) transport into Chinese hamster ovary cells that stably expressed hOCT2. The "cluster II" inhibitors (which contain known OCT2 substrates) metformin and cimetidine interacted competitively with MPP. However, other cluster II compounds, including tetraethylammonium (TEA), diphenidol and phenyltoloxamine, were mixed-type inhibitors of MPP transport (i.e., decreasing J(max) and increasing K(t)). A cluster III (neutral steroid) representative, adrenosterone, and a cluster I (large, flexible cation) representative, carvedilol, displayed noncompetitive inhibitory profiles. Competitive counterflow (CCF) was used to determine whether the inhibitory ligands served as substrates of hOCT2. Carvedilol (cluster I) and adrenosterone (cluster III) did not support CCF, consistent with the prediction that members of these structural classes are likely to be nontransported inhibitors of OCT2. The cluster II representatives MPP, metformin, cimetidine, and TEA all supported CCF, consistent with independent assessments of their OCT2-mediated transport. However, the other cluster II representatives, diphenidol and phenyltoloxamine, failed to support CCF, suggesting that neither compound is transported by OCT2. An independent assessment of diphenidol transport (using liquid chromatography with tandem mass spectroscopy) confirmed this observation. The results underscore the caution required for development of predictive models of ligand interaction with multidrug transporters.

Potent inhibitors of human organic anion transporters 1 and 3 from clinical drug libraries: discovery and molecular characterization

Transporter-mediated drug-drug interactions in the kidney dramatically influence the pharmacokinetics and other clinical effects of drugs. Human organic anion transporters 1 (hOAT1) and 3 (hOAT3) are the major transporters in the basolateral membrane of kidney proximal tubules, mediating the rate-limiting step in the elimination of a broad spectrum of drugs. In the present study, we screened two clinical drug libraries against hOAT1 and hOAT3. Of the 727 compounds screened, 92 compounds inhibited hOAT1 and 262 compounds inhibited hOAT3. When prioritized based on the peak unbound plasma concentrations of these compounds, three inhibitors for hOAT1 and seven inhibitors for hOAT3 were subsequently identified with high inhibitory potency (>95%). Computational analyses revealed that inhibitors and noninhibitors can be differentiated from each other on the basis of several physicochemical features, including number of hydrogen-bond donors, number of rotatable bonds, and topological polar surface area (TPSA) for hOAT1; and molecular weight, number of hydrogen-bond donors and acceptors, TPSA, partition coefficient (log P(7.4)), and polarizability for hOAT3. Pharmacophore modeling identified two common structural features associated with inhibitors for hOAT1 and hOAT3, viz., an anionic hydrogen-bond acceptor atom, and an aromatic center separated by ∼5.7 Å. Such model provides mechanistic insights for predicting new OAT inhibitors.

Molecular determinants of ligand selectivity for the human multidrug and toxin extruder proteins MATE1 and MATE2-K

The present study compared the selectivity of two homologous transport proteins, multidrug and toxin extruders 1 and 2-K (MATE1 and MATE2-K), and developed three-dimensional pharmacophores for inhibitory ligand interaction with human MATE1 (hMATE1). The human orthologs of MATE1 and MATE2-K were stably expressed in Chinese hamster ovary cells, and transport function was determined by measuring uptake of the prototypic organic cation (OC) substrate 1-methyl-4-phenylpyridinium (MPP). Both MATEs had similar apparent affinities for MPP, with K(tapp) values of 4.4 and 3.7 μM for MATE1 and MATE2-K, respectively. Selectivity was assessed for both transporters from IC(50) values for 59 structurally diverse compounds. Whereas the two transporters discriminated markedly between a few of the test compounds, the IC(50) values for MATE1 and MATE2-K were within a factor of 3 for most of them. For hMATE1 there was little or no correlation between IC(50) values and the individual molecular descriptors LogP, total polar surface area, or pK(a). The IC(50) values were used to generate a common-features pharmacophore, quantitative pharmacophores for hMATE1, and a bayesian model suggesting molecular features favoring and not favoring the interaction of ligands with hMATE1. The models identified hydrophobic regions, hydrogen bond donor and hydrogen bond acceptor sites, and an ionizable (cationic) feature as key determinants for ligand binding to MATE1. In summary, using a combined in vitro and computational approach, MATE1 and MATE2-K were found to have markedly overlapping selectivities for a broad range of cationic compounds, including representatives from seven novel drug classes of Food and Drug Administration-approved drugs.

Disposition of metformin: variability due to polymorphisms of organic cation transporters

Considerable interindividual variability in clinical efficacy is recognized in the treatment of type 2 diabetes mellitus with the biguanide metformin. Metformin is a substrate of organic cation transporters, which play important roles in gastrointestinal absorption, renal and biliary elimination, and distribution to target sites of substrate drugs. This raises the question of whether genetic variations in these transporters affect efficacy and risk of adverse events associated with metformin use. In this review, the pharmacogenetics of metformin is discussed in the light of the most recent literature. Overall, results from healthy volunteers support the notion that metformin pharmacokinetics can be affected by polymorphisms in genes encoding organic cation transporters. When considering the glycemic response to metformin in patients, however, the likely multifactorial nature of metformin response masks the effects of transporter polymorphisms observed in some clinical studies.

Improving the prediction of the brain disposition for orally administered drugs using BDDCS

In modeling blood-brain barrier (BBB) passage, in silico models have yielded ~80% prediction accuracy, and are currently used in early drug discovery. Being derived from molecular structural information only, these models do not take into account the biological factors responsible for the in vivo outcome. Passive permeability and P-glycoprotein (Pgp, ABCB1) efflux have been successfully recognized to impact xenobiotic extrusion from the brain, as Pgp is known to play a role in limiting the BBB penetration of oral drugs in humans. However, these two properties alone fail to explain the BBB penetration for a significant number of marketed central nervous system (CNS) agents. The Biopharmaceutics Drug Disposition Classification System (BDDCS) has proved useful in predicting drug disposition in the human body, particularly in the liver and intestine. Here we discuss the value of using BDDCS to improve BBB predictions of oral drugs. BDDCS class membership was integrated with in vitro Pgp efflux and in silico permeability data to create a simple 3-step classification tree that accurately predicted CNS disposition for more than 90% of 153 drugs in our data set. About 98% of BDDCS class 1 drugs were found to markedly distribute throughout the brain; this includes a number of BDDCS class 1 drugs shown to be Pgp substrates. This new perspective provides a further interpretation of how Pgp influences the sedative effects of H1-histamine receptor antagonists.

Membrane transporters in drug development

Membrane transporters have wide, but specific tissue distributions. They can impact on multiple endogenous and xenobiotic processes. Knowledge and awareness within the pharmaceutical industry of their impact on drug absorption, distribution, metabolism and elimination (ADME) and drug safety is growing rapidly. Clinically important transporter-mediated drug-drug interactions (DDIs) have been observed. Up to nine diverse transporters are implicated in the DDIs of a number of widely prescribed drugs, posing a significant challenge to the pharmaceutical industry. There is a complex interplay between multiple transporters and/or enzymes in the ADME and pharmacogenomics of drugs. Integrating these different mechanisms to understand their relative contributions to ADME is a key challenge. Many different factors complicate the study of membrane transporters in drug development. These include a lack of specific substrates and inhibitors, non-standard in vitro tools, and competing/complementary mechanisms (e.g. passive permeability and metabolism). Discovering and contextualizing the contribution of membrane transporters to drug toxicity is a significant new challenge. Drug interactions with key membrane transporters are routinely assessed for central nervous system (CNS) drug discovery therapies, but are not generally considered across the wider drug discovery. But, there is interest in utilizing membrane transporters as drug delivery agents. Computational modeling approaches, notably physiology-based/pharmacokinetic (PB/PK) modeling are increasingly applied to transporter interactions, and permit integration of multiple ADME mechanisms. Because of the range of tissues and transporters of interest, robust transporter, in vitro to in vivo, scaling factors are required. Empirical factors have been applied, but absolute protein quantitation will probably be required.

Transporter-mediated drug-drug interactions

Drug-drug interactions are a serious clinical issue. An important mechanism underlying drug-drug interactions is induction or inhibition of drug transporters that mediate the cellular uptake and efflux of xenobiotics. Especially drug transporters of the small intestine, liver and kidney are major determinants of the pharmacokinetic profile of drugs. Transporter-mediated drug-drug interactions in these three organs can considerably influence the pharmacokinetics and clinical effects of drugs. In this article, we focus on probe drugs lacking significant metabolism to highlight mechanisms of interactions of selected intestinal, hepatic and renal drug transporters (e.g., organic anion transporting polypeptide [OATP] 1A2, OATP2B1, OATP1B1, OATP1B3, P-gp, organic anion transporter [OAT] 1, OAT3, breast cancer resistance protein [BCRP], organic cation transporter [OCT] 2 and multidrug and toxin extrusion protein [MATE] 1). Genotype-dependent drug-drug interactions are also discussed.

Transporter-mediated drug-drug interactions with oral antidiabetic drugs

Uptake transporters (e.g., members of the SLC superfamily of solute carriers) and export proteins (e.g., members of the ABC transporter superfamily) are important determinants for the pharmacokinetics of drugs. Alterations of drug transport due to concomitantly administered drugs that interfere with drug transport may alter the kinetics of drug substrates. In vitro and in vivo studies indicate that many drugs used for the treatment of metabolic disorders and cardiovascular diseases (e.g., oral antidiabetic drugs, statins) are substrates for uptake transporters and export proteins expressed in the intestine, the liver and the kidney. Since most patients with type 2 diabetes receive more than one drug, transporter-mediated drug-drug interactions are important molecular mechanisms leading to alterations in oral antidiabetic drug pharmacokinetics with the risk of adverse drug reactions. This review focuses on uptake transporters of the SLCO/SLC21 (OATP) and SLC22 (OCT/OAT) family of solute carriers and export pumps of the ABC (ATP-binding cassette) transporter superfamily (especially P-glycoprotein) as well as the export proteins of the SLC47 (MATE) family and their role for transporter-mediated drug-drug interactions with oral antidiabetic drugs.

Pharmacogenetic analyses of cisplatin-induced nephrotoxicity indicate a renoprotective effect of ERCC1 polymorphisms

AIM

We investigated whether genetic polymorphisms may contribute to the interpatient variability of cisplatin-induced nephrotoxicity.

PATIENTS & METHODS

Polymorphisms in the candidate genes GSTM1, GSTT1, OCT1, OCT2, LARP2, ERCC1, XRCC1 and EPO were analyzed for associations with nephrotoxicity in 79 cancer patients receiving cisplatin-containing chemotherapy.

RESULTS

Higher cisplatin dose was associated with strongly decreased estimated glomerular filtration rates (eGFR) (r(2) = 0.205). Two highly genetically linked polymorphisms in the ERCC1 gene, 8092C>A and Asn118Asn, were significantly associated with change in eGFR, accounting for an additional 13% of interindividual variability. Homozygous carriers of the 8092A allele in ERCC1 showed no reduction in eGFR, compared with the 11.5% mean eGFR decrease in C allele carriers (p = 0.004). Homozygous carriers of the C allele of Asn118Asn showed no reduction in eGFR, compared with the 12.8% mean eGFR decrease seen in T allele carriers (p = 0.047). Polymorphisms in the other candidate genes were not associated with cisplatin-induced nephrotoxicity.

CONCLUSION

Genetic polymorphisms in ERCC1 may be valuable predictors of cisplatin-induced nephrotoxicity.

Molecular analysis and structure-activity relationship modeling of the substrate/inhibitor interaction site of plasma membrane monoamine transporter

Plasma membrane monoamine transporter (PMAT) is a new polyspecific transporter that interacts with a wide range of structurally diverse organic cations. To map the physicochemical descriptors of cationic compounds that allow interaction with PMAT, we systematically analyzed the interactions between PMAT and three series of structural analogs of known organic cation substrates including phenylalkylamines, n-tetraalkylammonium (n-TAA) compounds, and β-carbolines. Our results showed that phenylalkylamines with a distance between the aromatic ring and the positively charged amine nitrogen atom of ∼6.4 Å confer optimal interactions with PMAT, whereas studies with n-TAA compounds revealed an excellent correlation between IC(50) values and hydrophobicity. The five β-carbolines that we tested, which possess a pyridinium-like structure and are structurally related to the neurotoxin 1-methyl-4-phenylpyridinium, inhibited PMAT with high affinity (IC(50) values of 39.1-65.5 μM). Cytotoxicity analysis further showed that cells expressing PMAT are 14- to 15-fold more sensitive to harmalan and norharmanium, suggesting that these two β-carbolines are also transportable substrates of PMAT. We then used computer-aided modeling to generate qualitative and quantitative three-dimensional pharmacophore models on the basis of 23 previously reported and currently identified PMAT inhibitors and noninhibitors. These models are characterized by a hydrogen bond donor and two to three hydrophobic features with distances between the hydrogen bond donor and hydrophobic features ranging between 5.20 and 7.02 Å. The consistency between the mapping results and observed PMAT affinity of a set of test compounds indicates that the models performed well in inhibitor prediction and could be useful for future virtual screening of new PMAT inhibitors.

Clinical pharmacokinetics of metformin

Metformin is widely used for the treatment of type 2 diabetes mellitus. It is a biguanide developed from galegine, a guanidine derivative found in Galega officinalis (French lilac). Chemically, it is a hydrophilic base which exists at physiological pH as the cationic species (>99.9%). Consequently, its passive diffusion through cell membranes should be very limited. The mean ± SD fractional oral bioavailability (F) of metformin is 55 ± 16%. It is absorbed predominately from the small intestine. Metformin is excreted unchanged in urine. The elimination half-life (t(&frac12;)) of metformin during multiple dosages in patients with good renal function is approximately 5 hours. From published data on the pharmacokinetics of metformin, the population mean of its clearances were calculated. The population mean renal clearance (CL(R)) and apparent total clearance after oral administration (CL/F) of metformin were estimated to be 510 ± 130 mL/min and 1140 ± 330 mL/min, respectively, in healthy subjects and diabetic patients with good renal function. Over a range of renal function, the population mean values of CL(R) and CL/F of metformin are 4.3 ± 1.5 and 10.7 ± 3.5 times as great, respectively, as the clearance of creatinine (CL(CR)). As the CL(R) and CL/F decrease approximately in proportion to CL(CR), the dosage of metformin should be reduced in patients with renal impairment in proportion to the reduced CL(CR). The oral absorption, hepatic uptake and renal excretion of metformin are mediated very largely by organic cation transporters (OCTs). An intron variant of OCT1 (single nucleotide polymorphism [SNP] rs622342) has been associated with a decreased effect on blood glucose in heterozygotes and a lack of effect of metformin on plasma glucose in homozygotes. An intron variant of multidrug and toxin extrusion transporter [MATE1] (G>A, SNP rs2289669) has also been associated with a small increase in antihyperglycaemic effect of metformin. Overall, the effect of structural variants of OCTs and other cation transporters on the pharmacokinetics of metformin appears small and the subsequent effects on clinical response are also limited. However, intersubject differences in the levels of expression of OCT1 and OCT3 in the liver are very large and may contribute more to the variations in the hepatic uptake and clinical effect of metformin. Lactic acidosis is the feared adverse effect of the biguanide drugs but its incidence is very low in patients treated with metformin. We suggest that the mean plasma concentrations of metformin over a dosage interval be maintained below 2.5 mg/L in order to minimize the development of this adverse effect.

Proton pump inhibitors inhibit metformin uptake by organic cation transporters (OCTs)

Metformin, an oral insulin-sensitizing drug, is actively transported into cells by organic cation transporters (OCT) 1, 2, and 3 (encoded by SLC22A1, SLC22A2, or SLC22A3), which are tissue specifically expressed at significant levels in various organs such as liver, muscle, and kidney. Because metformin does not undergo hepatic metabolism, drug-drug interaction by inhibition of OCT transporters may be important. So far, comprehensive data on the interaction of proton pump inhibitors (PPIs) with OCTs are missing although PPIs are frequently used in metformin-treated patients. Using in silico modeling and computational analyses, we derived pharmacophore models indicating that PPIs (i.e. omeprazole, pantoprazole, lansoprazole, rabeprazole, and tenatoprazole) are potent OCT inhibitors. We then established stably transfected cell lines expressing the human uptake transporters OCT1, OCT2, or OCT3 and tested whether these PPIs inhibit OCT-mediated metformin uptake in vitro. All tested PPIs significantly inhibited metformin uptake by OCT1, OCT2, and OCT3 in a concentration-dependent manner. Half-maximal inhibitory concentration values (IC(50)) were in the low micromolar range (3-36 µM) and thereby in the range of IC(50) values of other potent OCT drug inhibitors. Finally, we tested whether the PPIs are also transported by OCTs, but did not identify PPIs as OCT substrates. In conclusion, PPIs are potent inhibitors of the OCT-mediated metformin transport in vitro. Further studies are needed to elucidate the clinical relevance of this drug-drug interaction with potential consequences on metformin disposition and/or efficacy.

Molecular mechanism of renal tubular secretion of the antimalarial drug chloroquine

The antimalarial drug chloroquine is eliminated to a significant extent by renal tubular secretion. The molecular mechanism of renal chloroquine secretion remains unknown. We hypothesized that organic cation transporter 2 (OCT2) and multidrug and toxin extrusion protein 1 (MATE1), localized in the basolateral and luminal membranes of proximal tubule cells, respectively, are involved in chloroquine transport. The interaction of chloroquine with both transporters was investigated using single-transfected human embryonic kidney 293 (HEK293)-MATE1 cells in uptake experiments and single-transfected Madin-Darby canine kidney II (MDCK)-OCT2 and MDCK-MATE1 cells as well as double-transfected MDCK-OCT2-MATE1 cells grown as polarized monolayers on transwell filters. In HEK293-MATE1 cells, chloroquine competitively inhibited MATE1-mediated metformin uptake (K(i) = 2.8 μM). Cellular accumulation of chloroquine was significantly lower (P < 0.001) and transcellular chloroquine transport was significantly increased (P < 0.001) in MDCK-MATE1 and MDCK-OCT2-MATE1 cells compared to vector control cells after basal addition of chloroquine (0.1 to 10 μM). In contrast, no difference in cellular accumulation or transcellular transport of chloroquine was observed between MDCK-OCT2 and vector control cells. In line with an oppositely directed proton gradient acting as a driving force for MATE1, basal-to-apical transport of chloroquine by MDCK-OCT2-MATE1 cells increased with decreasing apical pH from 7.8 to 6.0. Transcellular transport of chloroquine by MDCK-OCT2-MATE1 cells was inhibited by cimetidine, trimethoprim, and amitriptyline. Our data demonstrate that chloroquine is a substrate and potent competitive inhibitor of MATE1, whereas OCT2 seems to play no role in chloroquine uptake. Concomitantly administered MATE1 inhibitors are likely to modify the renal secretion of chloroquine.

Organic cation transporters (OCTs, MATEs), in vitro and in vivo evidence for the importance in drug therapy

Organic cation transporters (OCTs) of the solute carrier family (SLC) 22 and multidrug and toxin extrusion (MATE) transporters of the SLC47 family have been identified as uptake and efflux transporters, respectively, for xenobiotics including several clinically used drugs such as the antidiabetic agent metformin, the antiviral agent lamivudine, and the anticancer drug oxaliplatin. Expression of human OCT1 (SLC22A1) and OCT2 (SLC22A2) is highly restricted to the liver and kidney, respectively. By contrast, OCT3 (SLC22A3) is more widely distributed. MATEs (SLC47A1, SLC47A2) are predominantly expressed in human kidney. Data on in vitro studies reporting a large number of substrates and inhibitors of OCTs and MATEs are systematically summarized. Several genetic variants of human OCTs and in part of MATE1 have been reported, and some of them result in reduced in vitro transport activity corroborating data from studies with knockout mice. A comprehensive overview is given on currently known genotype-phenotype correlations for variants in OCTs and MATE1 related to protein expression, pharmacokinetics/-dynamics of transporter substrates, treatment outcome, and disease susceptibility.

Drug transporters of platinum-based anticancer agents and their clinical significance

Platinum-based drugs are among the most active anticancer agents and are successfully used in a wide variety of human malignancies. However, acquired and/or intrinsic resistance still represent a major limitation. Lately, in particular mechanisms leading to impaired uptake and/or decreased cellular accumulation of platinum compounds have attracted attention. In this review, we focus on the role of active platinum uptake and efflux systems as determinants of platinum sensitivity and -resistance and their contribution to platinum pharmacokinetics (PK) and pharmacodynamics (PD). First, the three mostly used platinum-based anticancer agents as well as the most promising novel platinum compounds in development are put into clinical perspective. Next, we describe the presently known potential platinum transporters--with special emphasis on organic cation transporters (OCTs)--and discuss their role on clinical outcome (i.e. efficacy and adverse events) of platinum-based chemotherapy. In addition, transporter-mediated tumour resistance, the impact of potential platinum transporter-mediated drug-drug interactions, and the role of drug transporters in the renal elimination of platinum compounds are discussed.

Oxybutynin and trospium are substrates of the human organic cation transporters

The muscarinic antagonists oxybutynin and trospium are used as spasmolytic agents for the treatment of overactive urinary bladder disease. Recently, it has been shown that trospium, but not oxybutynin, is a substrate of the multidrug efflux carrier P-glycoprotein, but carrier-mediated drug uptake has not been directly analysed for both drugs. However, trospium has been previously shown to exhibit inhibitory potency for the organic cation transporters (OCTs). The aim of the present study was to examine whether trospium and oxybutynin are substrates, i.e. are transported by the human OCTs (hOCT(1), hOCT(2) and hOCT(3)). Therefore, we measured total and specific (decynium-22-sensitive) uptake, and saturation kinetics of the uptake for [(3)H]oxybutynin and [(3)H]trospium in human embryonic kidney (HEK293) cells transiently transfected with the cDNA of hOCT(1), hOCT(2) or hOCT(3). In addition, we determined IC(50) values for inhibition of hOCT-mediated [(3)H]MPP(+) uptake by unlabelled trospium and oxybutynin. Total uptake of [(3)H]oxybutynin was very high in all transfected HEK293 cells and only a small portion was due to specific, decynium-22-sensitive hOCT-mediated uptake. Oxybutynin inhibited [(3)H]MPP(+) uptake by the three hOCTs with IC(50) values between 20 and 130 μM. Direct determination of transport kinetics was measurable only at hOCT(1) with K (m) of 8 μM and V (max) of 484 pmol/mg protein/min. The rank order of affinity (1/IC(50) or 1/K (m)) of specific oxybutynin uptake was hOCT(1) > hOCT(2) = hOCT(3). The observed high non-specific uptake is obviously a consequence of the high lipophilicity of this uncharged drug. Thus, hOCTs may not play a significant role for the overall pharmacokinetics and tissue distribution of oxybutynin. However, and in contrast to oxybutynin, uptake of [(3)H]trospium, an organic cation, was mainly due to carrier-mediated uptake by the three hOCTs. With IC(50) values of 18, 1.4 and 710 μM (at hOCT(1), hOCT(2) and hOCT(3), respectively) and K (m) values of 17 and 8 μM and about identical V (max) values of about 90 pmol/mg protein/min at hOCT(1) and hOCT(2), respectively; the rank order of affinity (1/IC(50) or 1/K (m)) of specific uptake of trospium was hOCT(2) > hOCT(1) > > hOCT(3). Thus, hOCTs very probably contribute to the active tubular and hepatobiliary secretion of trospium. Furthermore, hOCT(1) and hOCT(3) may be involved in the tissue uptake of this drug in the urinary bladder.

Effect of phospholipidosis on the cellular pharmacokinetics of chloroquine

In vivo, the weakly basic, lipophilic drug chloroquine (CQ) accumulates in the kidney to concentrations more than a thousand-fold greater than those in plasma. To study the cellular pharmacokinetics of chloroquine in cells derived from the distal tubule, Madin-Darby canine kidney cells were incubated with CQ under various conditions. CQ progressively accumulated without exhibiting steady-state behavior. Experiments failed to yield evidence that known active transport mechanisms mediated CQ uptake at the plasma membrane. CQ induced a phospholipidosis-like phenotype, characterized by the appearance of numerous multivesicular and multilamellar bodies (MLBs/MVBs) within the lumen of expanded cytoplasmic vesicles. Other induced phenotypic changes including changes in the volume and pH of acidic organelles were measured, and the integrated effects of all these changes were computationally modeled to establish their impact on intracellular CQ mass accumulation. Based on the passive transport behavior of CQ, the measured phenotypic changes fully accounted for the continuous, nonsteady-state CQ accumulation kinetics. Consistent with the simulation results, Raman confocal microscopy of live cells confirmed that CQ became highly concentrated within induced, expanded cytoplasmic vesicles that contained multiple MLBs/MVBs. Progressive CQ accumulation was increased by sucrose, a compound that stimulated the phospholipidosis-like phenotype, and was decreased by bafilomycin A1, a compound that inhibited this phenotype. Thus, phospholipidosis-associated changes in organelle structure and intracellular membrane content can exert a major influence on the local bioaccumulation and biodistribution of drugs.

Genotype-dependent effects of inhibitors of the organic cation transporter, OCT1: predictions of metformin interactions

Common genetic variants of the liver-specific human organic cation transporter 1 (OCT1; SLC22A1) have reduced transport capacity for substrates such as the antidiabetic drug metformin. The effect of the reduced OCT1 function on drug interactions associated with OCT1 has not been investigated and was, therefore, the focus of the study presented here. HEK293 cells expressing human OCT1-reference or the variants R61C, V408M, M420del and G465R were first used to study the kinetics and inhibition pattern of the OCT1 substrate 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP(+)). In the second part OCT1-mediated (14)C-metformin uptake was studied in the presence of drugs administered concomitantly with metformin. Transport studies using ASP(+) showed that the function of the variants decreased in the following order: OCT1-reference=V408M=M420del >R61C >>G465R. Variants M420del and R61C were more sensitive to drug inhibition, with IC(50) values up to 23 times lower than those of the OCT1-reference. Uptake studies using (14)C-metformin were in qualitative agreement with those using ASP(+), with the exception that a larger reduction in transport capacity was observed for M420del. Concomitantly administered drugs, such as verapamil and amitriptyline, revealed potential drug-drug interactions at clinical plasma concentrations of metformin for OCT1-M420del.

Transport of ipratropium, an anti-chronic obstructive pulmonary disease drug, is mediated by organic cation/carnitine transporters in human bronchial epithelial cells: implications for carrier-mediated pulmonary absorption

Ipratropium bromide, an anticholinergic drug used for the treatment of asthma and chronic obstructive pulmonary disease, has low oral bioavailability, but systemic exposure, superior to oral administration, can be achieved by inhalation. Therefore, we investigated the pulmonary absorption mechanism of ipratropium using human bronchial epithelial BEAS-2B cells. [3H]Ipratropium uptake by BEAS-2B cells was temperature-dependent and saturable, with a K(m) value of 78.0 microM, suggesting involvement of carrier-mediated uptake. An RT-PCR study showed that organic cation/carnitine transporters OCTN1 and OCTN2 are expressed in BEAS-2B cells, but organic cation transporters (OCTs) are not. Uptake of [3H]ipratropium by HEK293 cells expressing OCTN1 (HEK293/OCTN1) and OCTN2 (HEK293/OCTN2) was significantly increased, compared with mock-transfected cells, and the estimated K(m) values were 444 microM and 53.0 microM, respectively. Finally, the contributions of OCTN1 and OCTN2 to ipratropium uptake were evaluated by measuring [3H]ipratropium uptake by BEAS-2B cells in which OCTN1 or OCTN2 gene expression had been silenced. Knock-down of OCTN1 or OCTN2 suppressed the uptake of [3H]ipratropium to 78.2% and 14.8% of that by control BEAS-2B cells, respectively. In addition, another anticholinergic, tiotropium, was also taken up by both HEK293/OCTN1 and HEK293/OCTN2 cells. Therefore, ipratropium and tiotropium are taken up primarily by OCTN2, and to a lesser extent by OCTN1, in bronchial epithelial cells. These findings are consistent with the pharmacological activity of the drugs after administration via inhalation.

Differential transport of platinum compounds by the human organic cation transporter hOCT2 (hSLC22A2)

BACKGROUND

Solute carriers (SLCs), in particular organic cation transporters (OCTs), have been implicated in the cellular uptake of platinum-containing anticancer compounds. The activity of these carriers may determine the pharmacokinetics and the severity of side effects, including neuro- and nephrotoxicity of platinum-based chemotherapy. As decreased drug accumulation is a key mechanism of platinum resistance, SLCs may also contribute to the development of resistance. Here, we define the role of hSLC22A2 (OCT2) in the cellular uptake of platinum compounds.

EXPERIMENTAL APPROACH

Human embryonic kidney (HEK) 293 cells stably expressing the hSLC22A2 gene (HEK293/hSLC22A2) were used in platinum accumulation studies. Following a 2 h exposure to various platinum compounds (100 microM), intracellular platinum levels were determined by flameless atomic absorption spectrometry.

KEY RESULTS

HEK293/hSLC22A2 cells, compared with HEK293/Neo control cells, displayed significant increases in oxaliplatin (28.6-fold), Pt[DACH]Cl(2) (20.6-fold), ormaplatin (8.1-fold), tetraplatin (4.5-fold), transplatin (3.7-fold) and cisplatin (1.3-fold), but not carboplatin. SLC22A2-mediated transport could be inhibited by 1-methyl-4-phenylpyridinium. Furthermore, hSLC22A2-mediated oxaliplatin and cisplatin accumulation was time- and concentration-dependent, but non-saturable. Expression of hSLC22A2 in HEK293 cells resulted in enhanced sensitivity to oxaliplatin (12-fold) and cisplatin (1.8-fold). Although, hSLC22A2 mRNA expression was frequently found in ovarian cancer cell lines, its expression in clinical ovarian cancer specimens (n= 80) was low and did not correlate with the treatment outcome of platinum-based regimens.

CONCLUSIONS AND IMPLICATIONS

The hSLC22A2 drug transporter is a critical determinant in the uptake and cytotoxicity of various platinum compounds, particularly oxaliplatin.

Human multidrug and toxin extrusion 1 (MATE1/SLC47A1) transporter: functional characterization, interaction with OCT2 (SLC22A2), and single nucleotide polymorphisms

Renal elimination of a number of cationic compounds is thought to be mediated by the organic cation transporter 2 (OCT2, SLC22A2), a drug uptake transporter expressed at the basolateral domain of renal tubular cells. Recently, the key efflux transporter for the secretion organic cations was identified as an electroneutral H(+)/organic cation exchanger termed the multidrug and toxin extrusion (MATE)-type transporter 1 (MATE1, SLC47A1). The key goals of this study were to assess the interplay between the renal cationic transporters OCT2 and MATE1 and the functional assessment of genetic variation in human MATE1. First, the ability of various agents to interact with OCT2- or MATE1-mediated transport was determined using a recombinant vaccinia expression system. We were able to identify several drugs in clinical use with a divergent inhibitory capacity for these transporters. Subsequently, we further assessed the effect of those compounds on the cellular accumulation of shared substrates using OCT2 and MATE1 double-transfected cells. Consistent with data obtained using single transporter transfected cells, compounds that exhibited preferential inhibition of MATE1 such as rapamycin and mitoxantrone induced significant cellular accumulation of cationic substrates. We next assessed the functional relevance of MATE1 genetic polymorphisms. Significant loss of transport activity for metformin and tetraethylammonium was noted for two nonsynonymous single nucleotide polymorphisms (SNPs), c.404T>C (p.159T>M) and c.1012G>A (p.338V>A). The c.404T>C was only seen in Asian subjects with an allele frequency of 1%, and the c.1012G>A SNP was much more common, especially among those of African descent. In conclusion, we show that coordinate function of MATE1 with OCT2 likely contributes to the vectorial renal elimination of organic cationic drugs and that altered activity of MATE1, whether by drugs or polymorphisms, should be considered as an important determinant of renal cationic drug elimination.

Interaction of beta-blockers with the renal uptake transporter OCT2

AIM

The uptake of drugs from the blood into the renal tubular cells is a key determinant for renal secretion and may influence their systemic plasma concentrations and extrarenal effects. Metformin, used for treatment of type 2 diabetes, is taken up into renal tubular cells by the organic cation transporter 2 (OCT2). Because many patients with type 2 diabetes receiving metformin are concomitantly treated with beta-blockers, we tested whether beta-blockers can inhibit OCT2-mediated drug transport.

METHOD

Using Madin-Darby canine kidney II cells stably expressing the uptake transporter OCT2, we analysed whether the beta-blockers bisoprolol, carvedilol, metoprolol and propranolol inhibit the transport of OCT2 substrates 1-methyl-4-phenylpyridinium (MPP(+)) and metformin.

RESULTS

Neither bisoprolol nor metoprolol significantly inhibited the uptake of MPP(+), whereas a significant inhibition was observed for carvedilol und propranolol (half maximal inhibitory concentration IC(50): 26.3 and 67.5 microM) respectively. Moreover, all beta-blockers significantly inhibited OCT2-mediated metformin uptake (IC(50) for bisoprolol: 2.4 microM, IC(50) for carvedilol: 2.3 microM, IC(50) for metoprolol: 50.2 microM and IC(50) for propranolol: 8.3 microM).

CONCLUSION

These in vitro results demonstrate that alterations of uptake transporter function by beta-blockers have to be considered as potential mechanisms underlying drug-drug interactions in the kidney.

Endogenous gene and protein expression of drug-transporting proteins in cell lines routinely used in drug discovery programs

The aim of this study was to investigate the gene and protein expression profiles of important drug-transporting proteins in human cell lines commonly used for studies of drug transport mechanisms. Human cell lines used to transiently or stably express single transporters [HeLa, human embryonic kidney (HEK) 293] and leukemia cell lines used to study drug resistance by ATP-binding cassette transporters (HL-60, K562) were investigated and compared with organotypic cell lines (HepG2, Saos-2, Caco-2, and Caco-2 TC7). For gene expression studies, real-time polymerase chain reaction was used, whereas monospecific polyclonal antibodies were generated and used to investigate protein expression by immunohistochemistry. Thirty-six transporters were studied for gene expression, and nine were studied for protein expression. The antibodies were validated using expression patterns in human tissues. Finally, the function of one ubiquitously expressed transporter, MCT1/SLC16A1, was investigated using [(14)C]lactic acid as a substrate. In general, the adherent cell lines (HeLa, HEK293) displayed low transporter expression, and the expression patterns were barely affected by transfection. The leukemia cell lines (K562, HL-60) and Saos-2 also had low endogenous transporter expression, whereas the organotypic cell lines (HepG2 and Caco-2) showed higher expression of some transporters. Comparison of gene and protein expression profiles gave poor correlations, but better agreement was obtained for antibodies with a good validation score, indicating that antibody quality was a significant variable. It is noteworthy that the monocarboxylic acid-transporting protein MCT1 was significantly expressed in all and was functional in most of the cell lines, indicating that MCT1 may be a confounding factor when the transport of small anionic drugs is investigated.

In vitro evidence for the role of OATP and OCT uptake transporters in drug-drug interactions

BACKGROUND

Transport proteins, for example the drug export pump P-glycoprotein, are important for the absorption, distribution and excretion of drugs. Inhibition and induction of P-glycoprotein efflux function is a well-established mechanism of drug-drug interactions. Alteration of transporter-mediated drug uptake by concomitantly administered drugs may also result in a change in drug pharmacokinetics. These uptake transporter-mediated drug-drug interactions are the focus of this review.

OBJECTIVE

To examine the current in vitro evidence on interactions mediated by OATPs (organic anion transporting polypeptides) and OCTs (organic cation transporters).

METHODS

Comparing data of in vivo observed drug-drug interactions with in vitro analysed alterations in drug transport mediated by the hepatic expressed uptake transporters OATP1B1, OATP1B3 and OCT1 and by the renal expressed OCT2 protein.

RESULTS/CONCLUSIONS

Some of the previously in vivo described drug-drug interactions could be explained by alteration in uptake transporter function demonstrating that inhibition or induction of uptake transporters is a newly recognised mechanism of potential drug-drug interactions.

Functional characterization of the human organic cation transporter 2 variant p.270Ala>Ser

The organic cation transporter 2 (OCT2, SLC22A2) plays an important role for renal drug elimination. Recent clinical studies indicate an impact of the frequent nonsynonymous c.808G>T (p.270Ala>Ser) polymorphism on renal clearance of metformin and the extent of the metformin-cimetidine interaction. The role of this polymorphism for renal disposition of endogenous compounds and drugs other than metformin has not been investigated. In addition, it is unclear whether the observed genotype dependence of an OCT2-mediated drug-drug interaction might occur also with other OCT inhibitors. To address these issues, we generated human embryonic kidney cells stably expressing wild-type OCT2 or the p.270Ala>Ser variant. No differences in protein expression levels and membrane incorporation pattern were observed between the two cell lines. The p.270Ala>Ser variant significantly impaired uptake kinetics of 1-methyl-4-phenylpyridinium, dopamine, norepinephrine, and propranolol. V(max) values were significantly reduced for uptake of all four compounds mediated by the p.270Ala>Ser variant compared with wild-type OCT2. In addition, a significant difference in the affinity to wild-type and mutant OCT2 was observed for dopamine (K(m) dopamine: 932 +/- 77 versus 1285 +/- 132 microM). Moreover, out of a set of 27 compounds p.270Ala>Ser OCT2 was significantly less sensitive to inhibition by cimetidine, flurazepam, metformin, mexiletine, propranolol, and verapamil than wild-type OCT2 (e.g., for propranolol: IC(50) wild type versus p.270Ala>Ser 189 versus 895 microM, P < 0.001). Our results indicate that the common OCT2 c.808G>T single nucleotide polymorphism significantly alters uptake of endogenous compounds and drugs. Moreover, for selected compounds the extent of OCT2-mediated drug interactions could depend on OCT2 c.808G>T genotype.