Different transport properties between famotidine and cimetidine by human renal organic ion transporters (SLC22A)

Use of a Double-Transfected System to Predict hOCT2/hMATE1-Mediated Renal Drug-Drug Interactions

Accurate predictions of renal drug-drug interactions (DDIs) mediated by the human organic cation transporter 2 (hOCT2) and multidrug and toxin extrusion proteins (hMATEs) remain challenging. Current DDI evaluation using plasma maximal unbound inhibitor concentrations (Imax,u) and IC50 values determined in single transporter-transfected cells frequently leads to false or overprediction especially for hMATE1. Emerging evidence suggests intracellular unbound inhibitor concentration may be more relevant for hMATE1 inhibition in vivo. However, determination of intrarenal inhibitor concentrations is impractical. Here, we explored the use of hOCT2/hMATE1 double-transfected Madin-Darby canine kidney (MDCK) cells as a new in vitro tool for DDI risk assessment. Our results showed that potent in vitro hMATE1 inhibitors (hydroxychloroquine, brigatinib, and famotidine) failed to inhibit metformin B-to-A flux in the double-transfected system. On the other side, the classic hOCT2/hMATE1 inhibitors, pyrimethamine and cimetidine, dose-dependently inhibited metformin apparent B-to-A permeability (Papp). The different behaviors of these hMATE1 inhibitors in the double-transfected system can be explained by their different ability to gain intracellular access either via passive diffusion or transporter-mediated uptake. A new parameter (IC50,flux) was proposed reflecting the inhibitor's potency on overall hOCT2/hMATE1-mediated tubular secretion. The IC50,flux values significantly differ from the IC50 values determined in single transporter-transfected cells. Importantly, the IC50,flux accurately predicted in vivo DDIs (within 2-fold) when used in a static model. Our data demonstrated that the IC50,flux approach circumvents the need to measure intracellular inhibitor concentrations and more accurately predicted hOCT2/hMATE1-mediated renal DDIs. This system represents a new approach that could be used for improved DDI assessment during drug development. SIGNIFICANCE STATEMENT: This study demonstrated that flux studies in double-transfected MDCK cells and the IC50,flux represents a better approach to assess in vivo DDI potential for the renal organic cation secretion system. This study highlights the importance of inhibitor intracellular accessibility for accurate prediction of hMATE1-mediated renal DDIs. This approach has the potential to identify in vitro hMATE1 inhibitors that are unlikely to result in in vivo DDIs, thus reducing the burden of unnecessary and costly clinical DDI investigations.

In vitro validation of an in vivo phenotyping drug cocktail for major drug transporters in humans

PURPOSE

Cocktails of transporter probe drugs are used in vivo to assess transporter activity and respective drug-drug interactions. An inhibitory effect of components on transporter activities should be ruled out. Here, for a clinically tested cocktail consisting of adefovir, digoxin, metformin, sitagliptin, and pitavastatin, inhibition of major transporters by individual probe substrates was investigated in vitro.

METHODS

Transporter transfected HEK293 cells were used in all evaluations. Cell-based assays were applied for uptake by human organic cation transporters 1/2 (hOCT1/2), organic anion transporters 1/3 (hOAT1/3), multidrug and toxin extrusion proteins 1/2K (hMATE1/2K), and organic anion transporter polypeptide 1B1 (hOATP1B1). For P-glycoprotein (hMDR1) a cell-based efflux assay was used whereas an inside-out vesicle-based assay was used for the bile salt export pump (hBSEP). All assays used standard substrates and established inhibitors (as positive controls). Inhibition experiments using clinically achievable concentrations of potential perpetrators at the relevant transporter expression site were carried out initially. If there was a significant effect, the inhibition potency (K_i) was studied in detail.

RESULTS

In the inhibition tests, only sitagliptin had an effect and reduced hOCT1- and hOCT2- mediated metformin uptake and hMATE2K mediated MPP⁺ uptake by more than 70%, 80%, and 30%, respectively. The ratios of unbound C_max (observed clinically) to K_i of sitagliptin were low with 0.009, 0.03, and 0.001 for hOCT1, hOCT2, and hMATE2K, respectively.

CONCLUSION

The inhibition of hOCT2 in vitro by sitagliptin is in agreement with the borderline inhibition of renal metformin elimination observed clinically, supporting a dose reduction of sitagliptin in the cocktail.

Substrates and Inhibitors of Organic Cation Transporters (OCTs) and Plasma Membrane Monoamine Transporter (PMAT) and Therapeutic Implications

The gene products of the SLC22A gene family (hOCT1, hOCT2, and hOCT3) and of the SLC29A4 gene (hPMAT or hENT4) are all polyspecific organic cation transporters. Human OCTs (including hPMAT) are expressed in peripheral tissues such as small intestine, liver, and kidney involved in the pharmacokinetics of drugs. In the human brain, all four transporters are expressed at the blood-brain barrier (BBB), hOCT2 is additionally expressed in neurons, and hOCT3 and hPMAT in glia. More than 40% of the presently used drugs are organic cations. This chapter lists and discusses all known drugs acting as substrates or inhibitors of these four organic cation transporters, independently of whether the transporter is expressed in the central nervous system (CNS) or in peripheral tissues. Of interest is their involvement in drug absorption, distribution, and excretion as well as potential OCT-associated drug-drug interactions (DDIs), with a focus on drugs that act in the CNS.

Solute Carrier Transportome in Chemotherapy-Induced Adverse Drug Reactions

Members of the solute carrier (SLC) family of transporters are responsible for the cellular influx of a broad range of endogenous compounds and xenobiotics. These proteins are highly expressed in the gastrointestinal tract and eliminating organs such as the liver and kidney, and are considered to be of particular importance in governing drug absorption and elimination. Many of the same transporters are also expressed in a wide variety of organs targeted by clinically important anticancer drugs, directly affect cellular sensitivity to these agents, and indirectly influence treatment-related side effects. Furthermore, targeted intervention strategies involving the use of transport inhibitors have been recently developed, and have provided promising lead candidates for combinatorial therapies associated with decreased toxicity. Gaining a better understanding of the complex interplay between transporter-mediated on-target and off-target drug disposition will help guide the further development of these novel treatment strategies to prevent drug accumulation in toxicity-associated organs, and improve the safety of currently available treatment modalities. In this report, we provide an update on this rapidly emerging field with particular emphasis on anticancer drugs belonging to the classes of taxanes, platinum derivatives, nucleoside analogs, and anthracyclines.

A Comprehensive Analysis of Ontogeny of Renal Drug Transporters: mRNA Analyses, Quantitative Proteomics, and Localization

Human renal membrane transporters play key roles in the disposition of renally cleared drugs and endogenous substrates, but their ontogeny is largely unknown. Using 184 human postmortem frozen renal cortical tissues (preterm newborns to adults) and a subset of 62 tissue samples, we measured the mRNA levels of 11 renal transporters and the transcription factor pregnane X receptor (PXR) with quantitative real‐time polymerase chain reaction, and protein abundance of nine transporters using liquid chromatography tandem mass spectrometry selective reaction monitoring, respectively. Expression levels of p‐glycoprotein, urate transporter 1, organic anion transporter 1, organic anion transporter 3, and organic cation transporter 2 increased with age. Protein levels of multidrug and toxin extrusion transporter 2‐K and breast cancer resistance protein showed no difference from newborns to adults, despite age‐related changes in mRNA expression. Multidrug and toxin extrusion transporter 1, glucose transporter 2, multidrug resistance‐associated protein 2, multidrug resistance‐associated protein 4 (MRP4), and PXR expression levels were stable. Using immunohistochemistry, we found that MRP4 localization in pediatric samples was similar to that in adult samples. Collectively, our study revealed that renal drug transporters exhibited different rates and patterns of maturation, suggesting that renal handling of substrates may change with age.

Effects of proton pump inhibitors and famotidine on elimination of plasma methotrexate: Evaluation of drug-drug interactions mediated by organic anion transporter 3

Methotrexate (MTX) is an antifolate agent used in the treatment of numerous types of cancer, and eliminated by active tubular secretion via organic anion transporter 3 (OAT3). Gastric antisecretory drugs, such as proton pump inhibitors (PPIs) and histamine H₂ receptor antagonists, are widely used among patients with cancer in clinical practice. The aim of the present study was to analyse the potential drug-drug interactions between MTX and gastric antisecretory drugs in high-dose MTX (HD-MTX) therapy. The impact of PPIs on the plasma MTX concentration on 73 cycles of HD-MTX therapy was analysed retrospectively in 43 patients. Also investigated was the involvement of OAT3 in PPI-MTX drug interaction in an in vitro study using human OAT3 expressing HEK293 cells. In a retrospective study, patients who received a PPI had significantly higher MTX levels at 48 h (0.38 vs. 0.15 μmol l^-1 , respectively, p = 0.000018) and 72 h (0.13 vs. 0.05 μmol l^-1 , respectively, p = 0.0002) compared with patients who did not receive a PPI (but received famotidine). Moreover, in vitro experiments demonstrated that PPIs (esomeprazole, lansoprazole, omeprazole and rabeprazole) inhibited hOAT3-mediated uptake of MTX in a concentration-dependent manner (IC₅₀ values of 0.40-5.5 μ m), with a rank order of lansoprazole > esomeprazole > rabeprazole > omeprazole. In contrast to PPIs, famotidine showed little inhibitory effect on hOAT3-mediated MTX uptake. These results demonstrated that co-administration of PPI, but not famotidine, could result in a pharmacokinetic interaction that increases the plasma MTX levels, at least in part, via hOAT3 inhibition.

The Effect of Famotidine, a MATE1-Selective Inhibitor, on the Pharmacokinetics and Pharmacodynamics of Metformin

INTRODUCTION

Pharmacokinetic outcomes of transporter-mediated drug-drug interactions (TMDDIs) are increasingly being evaluated clinically. The goal of our study was to determine the effects of selective inhibition of multidrug and toxin extrusion protein 1 (MATE1), using famotidine, on the pharmacokinetics and pharmacodynamics of metformin in healthy volunteers.

METHODS

Volunteers received metformin alone or with famotidine in a crossover design. As a positive control, the longitudinal effects of famotidine on the plasma levels of creatinine (an endogenous substrate of MATE1) were quantified in parallel. Famotidine unbound concentrations in plasma reached 1 µM, thus exceeding the in vitro concentrations that inhibit MATE1 [concentration of drug producing 50 % inhibition (IC50) 0.25 µM]. Based on current regulatory guidance, these concentrations are expected to inhibit MATE1 clinically [i.e. maximum unbound plasma drug concentration (C max,u)/IC50 >0.1].

RESULTS

Consistent with MATE1 inhibition, famotidine administration significantly altered creatinine plasma and urine levels in opposing directions (p < 0.005). Interestingly, famotidine increased the estimated bioavailability of metformin [cumulative amount of unchanged drug excreted in urine from time zero to infinity (A e∞)/dose; p < 0.005] without affecting its systemic exposure [area under the plasma concentration-time curve (AUC) or maximum concentration in plasma (C max)] as a result of a counteracting increase in metformin renal clearance. Moreover, metformin-famotidine co-therapy caused a transient effect on oral glucose tolerance tests [area under the glucose plasma concentration-time curve between time zero and 0.5 h (AUCglu,0.5); p < 0.005].

CONCLUSIONS

These results suggest that famotidine may improve the bioavailability and enhance the renal clearance of metformin.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Trafficking and other regulatory mechanisms for organic anion transporting polypeptides and organic anion transporters that modulate cellular drug and xenobiotic influx and that are dysregulated in disease

Organic anion transporters (OATs) and organic anion-transporting polypeptides (OATPs), encoded by a number of solute carrier (SLC)22A and SLC organic anion (SLCO) genes, mediate the absorption and distribution of drugs and other xenobiotics. The regulation of OATs and OATPs is complex, comprising both transcriptional and post-translational mechanisms. Plasma membrane expression is required for cellular substrate influx by OATs/OATPs. Thus, interest in post-translational regulatory processes, including membrane targeting, endocytosis, recycling and degradation of transporter proteins, is increasing because these are critical for plasma membrane expression. After being synthesized, transporters undergo N-glycosylation in the endoplasmic reticulum and Golgi apparatus and are delivered to the plasma membrane by vesicular transport. Their expression at the cell surface is maintained by de novo synthesis and recycling, which occurs after clathrin- and/or caveolin-dependent endocytosis of existing protein. Several studies have shown that phosphorylation by signalling kinases is important for the internalization and recycling processes, although the transporter protein does not appear to be directly phosphorylated. After internalization, transporters that are targeted for degradation undergo ubiquitination, most likely on intracellular loop residues. Epigenetic mechanisms, including methylation of gene regulatory regions and transcription from alternate promoters, are also significant in the regulation of certain SLC22A/SLCO genes. The membrane expression of OATs/OATPs is dysregulated in disease, which affects drug efficacy and detoxification. Several transporters are expressed in the cytoplasmic subcompartment in disease states, which suggests that membrane targeting/internalization/recycling may be impaired. This article focuses on recent developments in OAT and OATP regulation, their dysregulation in disease and the significance for drug therapy.

A Human Renal Proximal Tubule Cell Line with Stable Organic Anion Transporter 1 and 3 Expression Predictive for Antiviral-Induced Toxicity

Drug-induced nephrotoxicity still hampers drug development, because current translation from in vitro or animal studies to human lacks high predictivity. Often, renal adverse effects are recognized only during clinical stages of drug development. The current study aimed to establish a robust and a more complete human cell model suitable for screening of drug-related interactions and nephrotoxicity. In addition to endogenously expressed renal organic cation transporters and efflux transporters, conditionally immortalized proximal tubule epithelial cells (ciPTEC) were completed by transduction of cells with the organic anion transporter (OAT) 1 or OAT3. Fluorescence-activated cell sorting upon exposure to the OAT substrate fluorescein successfully enriched transduced cells. A panel of organic anions was screened for drug-interactions in ciPTEC-OAT1 and ciPTEC-OAT3. The cytotoxic response to the drug-interactions with antivirals was further examined by cell viability assays. Upon subcloning, concentration-dependent fluorescein uptake was found with a higher affinity for ciPTEC-OAT1 (K_m = 0.8 ± 0.1 μM) than ciPTEC-OAT3 (K_m = 3.7 ± 0.5 μM). Co-exposure to known OAT1 and/or OAT3 substrates (viz. para-aminohippurate, estrone sulfate, probenecid, furosemide, diclofenac, and cimetidine) in cultures spanning 29 passage numbers revealed relevant inhibitory potencies, confirming the robustness of our model for drug-drug interactions studies. Functional OAT1 was directly responsible for cytotoxicity of adefovir, cidofovir, and tenofovir, while a drug interaction with zidovudine was not associated with decreased cell viability. Our data demonstrate that human-derived ciPTEC-OAT1 and ciPTEC-OAT3 are promising platforms for highly predictive drug screening during early phases of drug development.

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

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

The promiscuous binding of pharmaceutical drugs and their transporter-mediated uptake into cells: what we (need to) know and how we can do so

A recent paper in this journal sought to counter evidence for the role of transport proteins in effecting drug uptake into cells, and questions that transporters can recognize drug molecules in addition to their endogenous substrates. However, there is abundant evidence that both drugs and proteins are highly promiscuous. Most proteins bind to many drugs and most drugs bind to multiple proteins (on average more than six), including transporters (mutations in these can determine resistance); most drugs are known to recognise at least one transporter. In this response, we alert readers to the relevant evidence that exists or is required. This needs to be acquired in cells that contain the relevant proteins, and we highlight an experimental system for simultaneous genome-wide assessment of carrier-mediated uptake in a eukaryotic cell (yeast).

Drug transport by Organic Anion Transporters (OATs)

Common to all so far functionally characterized Organic Anion Transporters (OATs) is their broad substrate specificity and their ability to exchange extracellular against intracellular organic anions. Many OATs occur in renal proximal tubules, the site of active drug secretion. Exceptions are murine Oat6 (nasal epithelium), human OAT7 (liver), and rat Oat8 (renal collecting ducts). In human kidneys, OAT1, OAT2, and OAT3 are localized in the basolateral membrane, and OAT4, OAT10, and URAT1 in the apical cell membrane of proximal tubule cells, respectively. In rats and mice, Oat1 and Oat3 are located basolaterally, and Oat2, Oat5, Oat9, Oat10, and Urat1 apically. Several classes of drugs interact with human OAT1-3, including ACE inhibitors, angiotensin II receptor antagonists, diuretics, HMG CoA reductase inhibitors, β-lactam antibiotics, antineoplastic and antiviral drugs, and uricosuric drugs. For most drugs, interaction was demonstrated in vitro by inhibition of OAT-mediated transport of model substrates; for some drugs, transport by OATs was directly proven. Based on IC₅₀ values reported in the literature, OAT1 and OAT3 show comparable affinities for diuretics, cephalosporins, and nonsteroidal anti-inflammatory drugs whereas OAT2 has a lower affinity to most of these compounds. Drug-drug interactions at OAT1 and OAT3 may retard renal drug secretion and cause untoward effects. OAT4, OAT10, and URAT1 in the apical membrane contribute to proximal tubular urate absorption, and OAT10 to nicotinate absorption. OAT4 is in addition able to release drugs, e.g. diuretics, into the tubule lumen.

In vitro and in vivo evidence of the importance of organic anion transporters (OATs) in drug therapy

Organic anion transporters 1-10 (OAT1-10) and the urate transporter 1 (URAT1) belong to the SLC22A gene family and accept a huge variety of chemically unrelated endogenous and exogenous organic anions including many frequently described drugs. OAT1 and OAT3 are located in the basolateral membrane of renal proximal tubule cells and are responsible for drug uptake from the blood into the cells. OAT4 in the apical membrane of human proximal tubule cells is related to drug exit into the lumen and to uptake of estrone sulfate and urate from the lumen into the cell. URAT1 is the major urate-absorbing transporter in the apical membrane and is a target for uricosuric drugs. OAT10, also located in the luminal membrane, transports nicotinate with high affinity and interacts with drugs. Major extrarenal locations of OATs include the blood-brain barrier for OAT3, the placenta for OAT4, the nasal epithelium for OAT6, and the liver for OAT2 and OAT7. For all transporters we provide information on cloning, tissue distribution, factors influencing OAT abundance, interaction with endogenous compounds and different drug classes, drug/drug interactions and, if known, single nucleotide polymorphisms.

The inhibition of human multidrug and toxin extrusion 1 is involved in the drug-drug interaction caused by cimetidine

Cimetidine is known to cause drug-drug interactions (DDIs) with organic cations in the kidney, and a previous clinical study showed that coadministration of cimetidine or probenecid with fexofenadine (FEX) decreased its renal clearance. FEX was taken up into human kidney by human organic anion transporter (hOAT) 3 (SLC22A8), but the mechanism of its luminal efflux has not been clarified. The present study examined the molecular mechanism of these DDIs. Saturable uptake of FEX was observed in human kidney slices, with K(m) and V(max) values of 157+/-7 microM and 418+/-16 nmol/15 min/g kidney, respectively. Cimetidine only slightly inhibited its uptake even at 100 microM, far greater than its clinically relevant concentration, whereas 10 microM probenecid markedly inhibited its uptake. As candidate transporters for the luminal efflux of FEX, we focused on human multidrug and toxin extrusions MATE1 (SLC47A1) and MATE2-K (SLC47A2). Saturable uptake of FEX could be observed in human embryonic kidney 293 cells expressing human MATE1 (hMATE1), whereas hMATE2-K-specific uptake of FEX was too small to conduct its further kinetic analysis. The hMATE1-mediated uptake clearance of FEX was inhibited by cimetidine in a concentration-dependent manner, and it was decreased to 60% of the control value in the presence of 3 microM cimetidine. Taken together, our results suggest that the DDI of FEX with probenecid can be explained by the inhibition of renal uptake mediated by hOAT3, whereas the DDI with cimetidine is mainly caused by the inhibition of hMATE1-mediated efflux of FEX rather than the inhibition of its renal uptake process.

Carrier-mediated transport of metformin across the human placenta determined by using the ex vivo perfusion of the placental cotyledon model

OBJECTIVE

Metformin is a polar positively charged compound. The aim of the study was to characterize its permeability across the human placenta using the ex vivo placental perfusion model.

METHODS

Selected cotyledons from term placentas were cannulated and dually perfused. Metformin (10 mg/L, 1000 mg/L) and a permeability reference, antipyrine (50 mg/L), were added to the maternal circulation. Samples from maternal and fetal compartments were analyzed for metformin and antipyrine by high performance liquid chromatography (HPLC). The permeation of metformin was also studied using the parallel artificial membrane permeation assay (PAMPA) that was designed to predict passive transcellular permeability of drugs.

RESULTS

In this study, 15 complete placental perfusion experimental set-ups were performed. The mean percent transport increased as metformin concentrations were raised and it was 11 +/- 1.32 and 16.92 +/- 0.98 for 10 mg/L and 1000 mg/L, respectively. The transport rate of metformin across the placenta was asymmetric yet, an active efflux against the gradient concentration could not be observed. Using the PAMPA assay, we confirmed that metformin does not cross by passive diffusion.

CONCLUSION

The data suggest that metformin permeability across the placenta is mediated by a carrier that transport cationic compounds bi-directionally, with a higher transfer rate from the fetal to the maternal side.

Possible multiple transporters were involved in hepatobiliary excretion of antofloxacin in rats

1. The aim of the current study was to investigate the characteristics of biliary excretion of antofloxacin (ATFX) in rats. Rats received a bolus intravenous injection followed by a constant-rate infusion of ATFX. When plasma concentrations of ATFX reached steady state, cyclosporin A, erythromycin, probenecid, cimetidine and diclofenac were administered intravenously to the rats. Samples of blood and bile were collected and the concentrations of ATFX were measured and the corresponding pharmacokinetic parameters were estimated. 2. Biliary excretion of ATFX was observed in rats subjected to CCl(4)-induced experimental hepatic injury for 24 h (CCl(4)-EHI(24h)). Steady state concentrations of ATFX were attained at 60 min following infusion. 3. A slight increase in concentration of ATFX in plasma was observed after cyclosporin A, erythromycin, probenecid and cimetidine treatment. Significant increases in ATFX plasma levels were found in rats treated with diclofenac. Cyclosporin A, erythromycin, probenecid, cimetidine and diclofenac treatment significantly decreased the steady state biliary clearance of ATFX to 55, 68, 54, 53 and 56% of control values, respectively. 4. Cyclosprin A, probenecid, erythromycin and cimetidine also inhibited the biliary excretion of ATFX glucuronide. Significant decrease in the steady state biliary clearance of ATFX and its glucuronide was observed in CCl(4)-EHI(24h) rats. 5. These results indicate that multiple transporters are possibly involved in the biliary excretion of ATFX.

Comparison of the kinetic characteristics of inhibitory effects exerted by biguanides and H2-blockers on human and rat organic cation transporter-mediated transport: Insight into the development of drug candidates

In this study, the comparison of the transport of substrates (1-methyl-4-phenylpydinium (MPP) and tetraethyl ammonium (TEA)) and the inhibition potency of the inhibitors (biguanides and H(2)-blockers) for human and rat organic cation transporters (hOCTs and rOcts), and the inhibition type of inhibitors for these transporters were investigated using HEK293 cells that stably express hOCT/rOct. The concentration-dependent uptake of [(3)H]-MPP and [(14)C]-TEA by hOCT1-3/rOct1-3 had K(m) values similar to those in the literature. It was also deduced that MPP and TEA are competitive inhibitors for hOCT1-2/rOct1-2. The K(i) values for phenformin inhibition of [(3)H]-MPP and [(14)C]-TEA uptake by hOCT1-3/rOct1-3 were lower than that for metformin. The [(3)H]-MPP uptake by hOCT1/rOct1 and hOCT3/rOct3 was inhibited by famotidine and ranitidine whereas that by hOCT2/rOct2 was not. The inhibitory potency of cimetidine for hOCT1-2 was very weak. In most cases, the differences in the V(max)/K(m) values of substrates and the K(i) values of inhibitors between hOCT and rOct were minor. The acquisition of information on OCT/Oct mediated-transport and/or inhibition such as that presented in this report is very useful for further understanding of certain aspects of uptake, distribution, and excretion for drug candidates.

Functional Involvement of the Organic Cation Transporter 2 (rOct2) in the Renal Uptake of Organic Cations in Rats

This study examined the contribution made by organic cation transporters (hOCT/rOct) to the saturable component of the renal uptake of 1-methyl-4-phenylpyridinium, tetraethylammonium (TEA), cimetidine and metformin into rOct2-expressing HEK293 cells and rat kidney slices. All the test compounds accumulated in the rat kidney slices in a carrier-mediated manner. The Michaelis–Menten constant ( Km) values for saturable uptake of TEA, cimetidine and metformin into rat kidney slices were relatively comparable with those for the rOct2-expressing HEK293 cells. In addition, the relative uptake activity values of TEA, cimetidine and metformin in rat kidney slices were similar to those in rOct2-expressing HEK293 cells. This suggests that the saturable components involved in the renal uptake of TEA, cimetidine and metformin are mediated mainly by rOct2. The saturable uptake profile of cationic compounds into rat kidney can be evaluated in both cDNA-expressing cells and rat kidney slices, as well as the transporter expression pattern. This approach can also be used to estimate the saturable uptake mechanism of cationic compounds into the human kidney when human kidney slices and hOCT2-expressing cells are used.

Effect of human renal cationic transporter inhibition on the pharmacokinetics of varenicline, a new therapy for smoking cessation: an in vitro-in vivo study

Varenicline is predominantly eliminated unchanged in urine, and active tubular secretion partially contributes to its renal elimination. Transporter inhibition assays using human embryonic kidney 293 cells transfected with human renal transporters demonstrated that high concentrations of varenicline inhibited substrate uptake by hOCT2 (IC(50)=890 microM), with very weak or no measurable interactions with the other transporters hOAT1, hOAT3, hOCTN1, and hOCTN2. Varenicline was characterized as a moderate-affinity substrate for hOCT2 (K(m)=370 microM) and its hOCT2-mediated uptake was partially inhibited by cimetidine. Co-administration of cimetidine (1,200 mg/day) reduced the renal clearance of varenicline in 12 smokers, resulting in a 29.0% (90% CI: 21.5%-36.9%) increase in systemic exposure. This increase is not considered clinically relevant, as it should not give rise to safety concerns. Consequently, it can be reasonably expected that other inhibitors of hOCT2 would not cause greater renal interactions with varenicline than that seen with the efficient hOCT2 inhibitor cimetidine.

Functional involvement of organic cation transporter1 (OCT1/Oct1) in the hepatic uptake of organic cations in humans and rats

The contribution of organic cation transporters to the saturable component in the hepatic uptake of 1-methyl-4-phenylpyridinium (MPP), tetraethylammonium (TEA), cimetidine, and metformin was examined by the use of human/rat organic cation transporter (hOCT1/rOct1)-expressing cells and human/rat hepatocytes. Transfection of rOct1 resulted in a considerable increase in the uptake of metformin, whereas that of hOCT1 resulted in only a slight increase. All test compounds (MPP, TEA, cimetidine, and metformin) accumulated in human and rat hepatocytes in a carrier-mediated manner. The Km values for the uptake of MPP, TEA, cimetidine, and metformin into human and rat hepatocytes were comparable with those into hOCT1 and rOct1-expressing cells, respectively. In addition, the relative uptake activities, which were obtained by normalizing the intrinsic uptake clearances of TEA, cimetidine, and metformin against those values of MPP in human and rat hepatocytes, were similar with the uptake activities in hOCT1 and rOct1, respectively. These results suggest that the saturable component in the hepatic uptake of these cationic compounds may be mediated mainly by hOCT1/rOct1; therefore, it is meaningful to evaluate the saturable uptake profile of cationic compounds by the liver using both hOCT1/rOct1-expressing cells and human/rat hepatocytes.

Interaction and transport characteristics of mycophenolic acid and its glucuronide via human organic anion transporters hOAT1 and hOAT3

The immunosuppressant mycophenolate mofetil (MMF) is frequently administered with calcineurin inhibitors and corticosteroids to recipients of organ transplantations. However, the renal handling of the active metabolite mycophenolic acid (MPA) and 7-O-MPA-glucuronide (MPAG) has been unclear. The purpose of the present study was to assess the interaction of MPA and MPAG with the human renal organic anion transporters hOAT1 (SLC22A6) and hOAT3 (SLC22A8), by conducting uptake experiments using HEK293 cells stably expressing these transporters. MPA and MPAG inhibited the time-dependent uptake of p-[(14)C]aminohippurate by hOAT1 and that of [(3)H]estrone sulfate by hOAT3. The apparent 50% inhibitory concentration (IC(50)) of MPA for hOAT1 and hOAT3 was estimated at 10.7 and 1.5 microM, respectively. In the case of MPAG, the IC(50) values were calculated at 512.3 microM for hOAT1 and 69.1 microM for hOAT3. Eadie-Hofstee plot analyses showed that they inhibited hOAT1 noncompetitively and hOAT3 competitively. No inhibitory effects of tacrolimus, cyclosporin A and azathioprine on transport of p-[(14)C]aminohippurate by hOAT1 and of [(3)H]estrone sulfate by hOAT3 were observed. No transport of MPA by these transporters was observed. On the other hand, the uptake of MPAG into cells was stimulated by the expression of hOAT3, but not hOAT1. These findings propose the possibility that the administration of MMF decreases the renal clearance of drugs which are substrates of hOAT1 and hOAT3. Present data suggest that hOAT3 contributes to the renal tubular secretion of MPAG.

Lack of effect of DX-619, a novel des-fluoro(6)-quinolone, on glomerular filtration rate measured by serum clearance of cold iohexol

DX-619 is a novel des-fluoro(6)-quinolone with activity against a broad range of bacterial strains, including methicillin-resistant Staphylococcus aureus. The effects of DX-619 on the glomerular filtration rate (GFR) were evaluated because drug-related increases in serum creatinine levels were observed in studies with healthy volunteers. Forty-one healthy subjects were randomized to receive intravenous DX-619 at 800 mg or placebo once daily for 4 days, and the GFR was directly measured by determination of the clearance of a bolus iohexol injection in 33 subjects who completed the study per protocol. DX-619 was noninferior to placebo for the GFR on the basis of a criterion for a clinically significant difference of -12 ml/min/1.73 m(2). The mean GFRs on day 4 were 101.1 +/- 14.2 ml/min/1.73 m(2) and 100.2 +/- 15.6 ml/min/1.73 m(2) for the volunteers receiving placebo and DX-619, respectively. On day 4 the mean serum creatinine concentration for volunteers receiving DX-619 increased by 30 to 40%, with a corresponding decrease in mean creatinine clearance. Both parameters normalized within 7 days after the cessation of DX-619 treatment. Nonclinical studies suggest that DX-619 increases the serum creatinine concentration by inhibiting excretory tubular transporters. In conclusion, DX-619 administered intravenously at 800 mg once a day for 4 days did not affect the GFR in healthy volunteers. Glomerular toxicity is not expected to present a risk to patients receiving DX-619 in clinical trials, but monitoring of the renal function, with an emphasis on the serum creatinine concentration, is still warranted.

Organic anion transporters of the SLC22 family: biopharmaceutical, physiological, and pathological roles

The human organic anion transporters OAT1, OAT2, OAT3, OAT4 and URAT1 belong to a family of poly-specific transporters mainly located in kidneys. Selected OATs occur also in liver, placenta, and brain. OATs interact with endogenous metabolic end products such as urate and acidic neutrotransmitter metabolites, as well as with a multitude of widely used drugs, including antibiotics, antihypertensives, antivirals, anti-inflammatory drugs, diuretics and uricosurics. Thereby, OATs play an important role in renal drug elimination and have an impact on pharmacokinetics. In this review we focus on the interaction of human OATs with drugs. We report the affinities of human OATs for drug classes and compare the putative importance of individual OATs for renal drug excretion. The role of OATs as sites of drug-drug interaction and mediators cell toxicity, their gender-dependent regulation in health and diseased states, and the possible impact of single nucleotide polymorphisms are also dealt with.

Renal Transport of Adefovir, Cidofovir, and Tenofovir by SLC22A Family Members (hOAT1, hOAT3, and hOCT2)

PURPOSE

The nephrotoxicity of the nucleotide antivirals adefovir, cidofovir and tenofovir is considered to depend on the renal tubular transport of them. Although it is known that the antivirals are substrates of the human renal organic anion transporter hOAT1 (SLC22A6), there is no information available on other organic ion transporters. The aim of the present study was to investigate whether the other renal organic anion transporter hOAT3 (SLC22A8) and organic cation transporter hOCT2 (SLC22A2) transport the antivirals.

MATERIALS AND METHODS

Uptake experiments were performed using HEK293 cells transfected with cDNA of the organic ion transporters.

RESULTS

The uptake of adefovir, cidofovir and tenofovir in monolayers stably expressing hOAT3 increased time-dependently, compared with control. Probenecid, a typical inhibitor of organic anion transporters, completely inhibited their transport. The amounts of the antivirals taken up by hOAT3 were much lower than those by hOAT1. The transient expression of hOCT2 did not increase uptake of the antivirals.

CONCLUSION

These results indicate that adefovir, cidofovir and tenofovir are substrates of hOAT3 as well as hOAT1, but that quantitatively hOAT1 is the major renal transporter for these drugs.

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.

Saturable Absorptive Transport of the Hydrophilic Organic Cation Ranitidine in Caco-2 Cells: Role of pH-Dependent Organic Cation Uptake System and P-Glycoprotein

PURPOSE

The purpose of this work was to investigate the involvement of carrier-mediated apical (AP) uptake and efflux mechanisms in the absorptive intestinal transport of the hydrophilic cationic drug ranitidine in Caco-2 cells.

METHODS

Absorptive transport and AP uptake of ranitidine were determined in Caco-2 cells as a function of concentration. Permeability of ranitidine in the absorptive and secretory directions was assessed in the absence or presence of the P-glycoprotein (P-gp) inhibitor, GW918. Characterization of the uptake mechanism was performed with respect to inhibitor specificity, pH, energy, membrane potential, and Na+ dependence. Efflux from preloaded monolayers was evaluated over a range of concentrations and in the absence or presence of high extracellular ranitidine concentrations.

RESULTS

Saturable absorptive transport and AP uptake of ranitidine were observed with Km values of 0.27 and 0.45 mM, respectively. The ranitidine absorptive permeability increased and secretory permeability decreased upon inhibition of P-gp. AP ranitidine uptake was inhibited in a concentration-dependent fashion by a diverse set of organic cations including tetraethylammonium, 1-methyl-4-phenylpyridinium, famotidine, and quinidine. AP ranitidine uptake was pH and membrane potential dependent and reduced under conditions that deplete metabolic energy. Efflux of [3H]ranitidine across the basolateral membrane was neither saturable as a function of concentration nor trans stimulated by unlabeled ranitidine.

CONCLUSIONS

Saturable absorptive transport of ranitidine in Caco-2 cells is partially mediated via a pH-dependent uptake transporter for organic cations and is subject to attenuation by P-gp. Inhibition and driving force studies suggest the uptake carrier exhibits similar properties to cloned human organic cation transporters. The results also imply ranitidine transport is not solely restricted to the paracellular space.

Interactions of Fluoroquinolone Antibacterials, DX-619 and Levofloxacin, with Creatinine Transport by Renal Organic Cation Transporter hOCT2

Interactions of DX-619, a novel fluoroquinolone antibacterial, and levofloxacin (LVFX) with the human renal organic cation transporter hOCT2 were studied. The intracellular accumulation of [(14)C]creatinine in stable transfectants of HEK293 cells expressing hOCT2 (hOCT2-HEK293) as well as vector-transfected HEK293 cells (VEC-HEK293) was evaluated in the presence of DX-619 and LVFX at various concentrations. When added extracellularly, both DX-619 and LVFX inhibited the uptake of [(14)C]creatinine (5 microM) by hOCT2-HEK293 cells in a dose-dependent manner. Unlike in hOCT2-HEK293 cells, the uptake in VEC-HEK293 cells was not inhibited by either fluoroquinolone suggesting that hOCT2 was specifically involved in the inhibition. The apparent IC(50) value for the inhibition of [(14)C]creatinine uptake in hOCT2-HEK293 cells was 1.29+/-0.23 microM for DX-619 and 127+/-27 microM for LVFX, indicating DX-619 to be approximately 100-fold more potent than LVFX at inhibiting the transport of [(14)C]creatinine by hOCT2. A Dixon plot revealed that the inhibition by DX-619 of the hOCT2-mediated transport of [(14)C]creatinine was competitive. Fluoroquinolone antibacterials have the ability to inhibit the transport of creatinine by hOCT2, with DX-619 being much more effective than LVFX.

Pharmacokinetic Significance of Renal OAT3 (SLC22A8) for Anionic Drug Elimination in Patients with Mesangial Proliferative Glomerulonephritis

PURPOSE

Our previous studies showed that the mRNA level of human organic anion transporter (hOAT) 3 in the kidney was correlated with the rate of elimination of an anionic antibiotic cefazolin. However, the correlation coefficient was not so high. In the present study, therefore, we enrolled more patients to examine whether additional factors were responsible for the correlation.

METHODS

hOAT mRNA levels in renal biopsy specimens were quantified using the real-time polymerase chain reaction method. The elimination rates for the free fraction of cefazolin were determined in patients with various renal diseases.

RESULTS

In the present study, the coefficient of correlation between the hOAT3 mRNA level and the elimination rates for the free fraction of cefazolin was not so high in the patients overall as in our previous study (r = 0.536). However, following the classification of renal diseases, a better correlation was obtained in patients with mesangial proliferative glomerulonephritis (r = 0.723). In contrast, multiple regression analyses including gender, age, and liver function did not result in any improvements in the correlation coefficients.

CONCLUSIONS

These results suggest that the hOAT3 mRNA level is a significant marker of pharmacokinetics with which to predict the rate of elimination of cefazolin in patients with mesangial proliferative glomerulonephritis.

Differential substrate and inhibitory activities of ranitidine and famotidine toward human organic cation transporter 1 (hOCT1; SLC22A1), hOCT2 (SLC22A2), and hOCT3 (SLC22A3)

Human organic cation transporters (hOCTs) are expressed in organs of drug absorption and elimination and play an important role in the uptake and elimination of xenobiotics. The purpose of this study was to evaluate the substrate and inhibitory activity of the H2-receptor antagonists ranitidine and famotidine toward hOCTs and to determine the hOCT isoforms involved in the absorption and elimination of these compounds in humans. Inhibition and substrate specificity of hOCT1, hOCT2, and hOCT3 for ranitidine and famotidine were elucidated in cRNA-injected Xenopus laevis oocytes. Ranitidine and famotidine exhibited similarly potent inhibition of [3H]1-methyl-4-phenyl pyridinium uptake into hOCT1-expressing (IC50= 33 and 28 microM, respectively) and hOCT2-expressing oocytes (IC50= 76 and 114 microM, respectively). Famotidine exhibited potent inhibition of hOCT3; in contrast, ranitidine was a moderately weak inhibitor (IC50= 6.7 and 290 microM, respectively). [3H]Ranitidine uptake was stimulated by hOCT1 (Km= 70 +/- 9 microM) and to a much smaller extent by hOCT2. No stimulation of [3H]ranitidine uptake was observed in hOCT3-expressing oocytes. trans-Stimulation and electrophysiology studies suggested that famotidine also is an hOCT1 substrate and exhibits poor or no substrate activity toward hOCT2 and hOCT3. Thus, hOCT1, which is expressed in the intestine and liver, is likely to play a major role in the intestinal absorption and hepatic disposition of ranitidine and famotidine in humans, whereas hOCT2, the major isoform present in the kidney, may play only a minor role in their renal elimination. Famotidine seems to be one of the most potent inhibitors of hOCT3 yet identified.

A species difference in the transport activities of H2 receptor antagonists by rat and human renal organic anion and cation transporters

A clinical drug-drug interaction between famotidine (a H2 receptor antagonist) and probenecid has not been reproduced in rats. The present study hypothesized that the species-dependent probenecid sensitivity is due to a species difference in the contribution of renal organic anion and cation transporters. The transport activities of the H2 receptor antagonists (cimetidine, famotidine, and ranitidine) by rat and human basolateral organic anion and cation transporters [human organic anion transporter (hOAT) 1, hOAT2, r/hOAT3, rat organic cation transporter (rOct) 1, and r/hOCT2] were compared using their cDNA transfectants. The transport activities (Vmax/Km) of famotidine (Km, 345 microM) by rOat3 were 8- and 15-fold lower than those of cimetidine (Km, 91 microM) and ranitidine (Km, 155 microM), respectively, whereas the activity by hOAT3 (Km, 124 microM) was 3-fold lower than that of cimetidine (Km, 149 microM) but similar to that of ranitidine (Km, 234 microM). Comparison of the relative transport activity with regard to that of cimetidine suggests that famotidine was more efficiently transported by hOAT3 than rOat3, and vice versa, for ranitidine. Only ranitidine was efficiently transported by hOAT2 (Km, 396 microM). rOct1 accepts all of the H2 receptor antagonists with a similar activity, whereas the transport activities of ranitidine and famotidine (Km, 61/56 microM) by r/hOCT2 were markedly lower than that of cimetidine (Km, 69/73 microM). Probenecid was a potent inhibitor of r/OAT3 (Ki, 2.6-5.8 microM), whereas it did not interact with OCTs. These results suggest that, in addition to the absence of OCT1 in human kidney, a species difference in the transport activity by hOAT3 and rOat3 accounts, at least in part, for the species difference in the drug-drug interaction between famotidine and probenecid.

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.