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

Albumin-bound kynurenic acid is an appropriate endogenous biomarker for assessment of the renal tubular OATs-MRP4 channel

Renal tubular secretion mediated by organic anion transporters (OATs) and the multidrug resistance-associated protein 4 (MRP4) is an important means of drug and toxin excretion. Unfortunately, there are no biomarkers to evaluate their function. The aim of this study was to identify and characterize an endogenous biomarker of the renal tubular OATs-MRP4 channel. Twenty-six uremic toxins were selected as candidate compounds, of which kynurenic acid was identified as a potential biomarker by assessing the protein-binding ratio and the uptake in OAT1-, OAT3-, and MRP4-overexpressing cell lines. OAT1/3 and MRP4 mediated the transcellular vectorial transport of kynurenic acid in vitro. Serum kynurenic acid concentration was dramatically increased in rats treated with a rat OAT1/3 (rOAT1/3) inhibitor and in rOAT1/3 double knockout (rOAT1/3-/-) rats, and the renal concentrations were markedly elevated by the rat MRP4 (rMRP4) inhibitor. Kynurenic acid was not filtered at the glomerulus (99% of albumin binding), and was specifically secreted in renal tubules through the OAT1/3-MRP4 channel with an appropriate affinity (Km) (496.7 μM and 382.2 μM for OAT1 and OAT3, respectively) and renal clearance half-life (t1/2) in vivo (3.7 ± 0.7 h). There is a strong correlation in area under the plasma drug concentration-time curve (AUC0-t) between cefmetazole and kynurenic acid, but not with creatinine, after inhibition of rOATs. In addition, the phase of increased kynurenic acid level is earlier than that of creatinine in acute kidney injury process. These results suggest that albumin-bound kynurenic acid is an appropriate endogenous biomarker for adjusting the dosage of drugs secreted by this channel or predicting kidney injury.

Choroid Plexus and Drug Removal Mechanisms

Timely and efficient removal of xenobiotics and metabolites from the brain is crucial in maintaining the homeostasis and normal function of the brain. The choroid plexus (CP) forms the blood-cerebrospinal fluid barrier and vitally removes drugs and wastes from the brain through several co-existing clearance mechanisms. The CP epithelial (CPE) cells synthesize and secrete the cerebrospinal fluid (CSF). As the CSF passes through the ventricular and subarachnoid spaces and eventually drains into the general circulation, it collects and removes drugs, toxins, and metabolic wastes from the brain. This bulk flow of the CSF serves as a default and non-selective pathway for the removal of solutes and macromolecules from the brain interstitium. Besides clearance by CSF bulk flow, the CPE cells express several multispecific membrane transporters to actively transport substrates from the CSF side into the blood side. In addition, several phase I and II drug-metabolizing enzymes are expressed in the CPE cells, which enzymatically inactivate a broad spectrum of reactive or toxic substances. This review summarizes our current knowledge of the functional characteristics and key contributors to the various clearance pathways in the CP-CSF system, overviewing recent developments in our understanding of CSF flow dynamics and the functional roles of CP uptake and efflux transporters in influencing CSF drug concentrations.

Investigation of the arcane inhibition of human organic anion transporter 3 by benzofuran antiarrhythmic agents

The combination of antiarrhythmic agents, amiodarone or dronedarone, with the anticoagulant rivaroxaban is used clinically in the management of atrial fibrillation for rhythm control and secondary stroke prevention respectively. Renal drug-drug interactions (DDIs) between amiodarone or dronedarone and rivaroxaban were previously ascribed to inhibition of rivaroxaban secretion by P-glycoprotein at the apical membrane of renal proximal tubular epithelial cells. Benzbromarone, a known inhibitor of organic anion transporter 3 (OAT3), shares a benzofuran scaffold with amiodarone and dronedarone. However, inhibitory activity of amiodarone and dronedarone against OAT3 remains arcane. Here, we conducted in vitro transporter inhibition assays in OAT3-transfected HEK293 cells which revealed amiodarone, dronedarone and their respective major pharmacologically-active metabolites N-desethylamiodarone and N-desbutyldronedarone possess inhibitory activity against OAT3, with corrected K_i values of 0.042, 0.019, 0.028 and 0.0046 μM respectively. Protein binding effects and probe substrate dependency were accounted for in our assays. Static modelling predicted 1.29-, 1.01-, 1.29- and 1.16-fold increase in rivaroxaban exposure, culminating in a predicted 1.29-, 1.01-, 1.28- and 1.15-fold increase in major bleeding risk respectively, suggesting potential OAT3-mediated DDI between amiodarone and rivaroxaban. Future work involving physiologically-based pharmacokinetic modelling is crucial in holistically predicting the complex DDIs between the benzofuran antiarrhythmic agents and rivaroxaban.

Acamprosate Is a Substrate of the Human Organic Anion Transporter (OAT) 1 without OAT3 Inhibitory Properties: Implications for Renal Acamprosate Secretion and Drug-Drug Interactions

Acamprosate is an anionic drug substance widely used in treating symptoms of alcohol withdrawal. It was recently shown that oral acamprosate absorption is likely due to paracellular transport. In contrast, little is known about the eliminating mechanism clearing acamprosate from the blood in the kidneys, despite the fact that studies have shown renal secretion of acamprosate. The hypothesis of the present study was therefore that renal organic anion transporters (OATs) facilitate the renal excretion of acamprosate in humans. The aim of the present study was to establish and apply OAT1 (gene product of SLC22A6) and OAT3 (gene product of SLC22A8) expressing cell lines to investigate whether acamprosate is a substrate or inhibitor of OAT1 and/or OAT3. The studies were performed in HEK293-Flp-In cells stably transfected with SLC22A6 or SLC22A8. Protein and functional data showed that the established cell lines are useful for studying OAT1- and OAT3-mediated transport in bi-laboratory studies. Acamprosate inhibited OAT1-mediated p-aminohippuric acid (PAH) uptake but did not inhibit substrate uptake via OAT3 expressing cells, neither when applied concomitantly nor after a 3 h preincubation with acamprosate. The uptake of PAH via OAT1 was inhibited in a competitive manner by acamprosate and cellular uptake studies showed that acamprosate is a substrate for OAT1 with a K_m-value of approximately 700 µM. Probenecid inhibited OAT1-mediated acamprosate uptake with a K_i-value of approximately 13 µM, which may translate into an estimated clinically significant DDI index. In conclusion, acamprosate was identified as a substrate of OAT1 but not OAT3.

Differential interactions of the β-lactam cloxacillin with human renal organic anion transporters (OATs)

The β-lactam penicillin antibiotic cloxacillin (CLX) presents wide inter-individual pharmacokinetics variability. To better understand its molecular basis, the precise identification of the detoxifying actors involved in CLX disposition and elimination would be useful, notably with respect to renal secretion known to play a notable role in CLX elimination. The present study was consequently designed to analyze the interactions of CLX with the solute carrier transporters organic anion transporter (OAT) 1 and OAT3, implicated in tubular secretion through mediating drug entry at the basolateral pole of renal proximal cells. CLX was first shown to block OAT1 and OAT3 activity in cultured OAT-overexpressing HEK293 cells. Half maximal inhibitory concentration (IC₅₀ ) value for OAT3 (13 µm) was however much lower than that for OAT1 (560 µm); clinical inhibition of OAT activity and drug-drug interactions may consequently be predicted for OAT3, but not OAT1. OAT3, unlike OAT1, was next shown to mediate CLX uptake in OAT-overexpressing HEK293 cells. Kinetic parameters for this OAT3-mediated transport of CLX (K_m  = 10.7 µm) were consistent with a possible in vivo saturation of this process for high CLX plasma concentrations. OAT3 is consequently likely to play a pivotal role in renal CLX secretion and consequently in total renal CLX elimination, owing to the low plasma unbound fraction of the antibiotic. OAT3 genetic polymorphisms as well as co-administered drugs inhibiting in vivo OAT3 activity may therefore be considered as potential sources of CLX pharmacokinetics variability.

Drug Transporters in Xenobiotic Disposition and Pharmacokinetic Prediction

Drug transporters are widely expressed in organs and tissue barriers throughout human and animal bodies. Studies over the last two decades have identified various ATP-binding cassette and solute carrier transporters that play critical roles in the absorption, distribution, metabolism, and elimination of drugs and xenobiotics. This special section contains more than 20 original manuscripts and reviews that cover the most recent advances in the areas of drug transporter research, including the basic biology and function of transporters, expression of drug transporters in organ and tissue barriers, the mechanisms underlying regulation of transporter expression, transporter-mediated drug disposition in animal models, and the development and utilization of new technologies in drug transporter study, as well as pharmacokinetic modeling and simulation to assess transporter involvement in drug disposition and drug-drug interactions. We believe that the topics covered in this special section will advance our understanding of the roles of transporters in drug disposition, efficacy, and safety.

Molecular Mechanisms for Species Differences in Organic Anion Transporter 1, OAT1: Implications for Renal Drug Toxicity

Species differences in renal drug transporters continue to plague drug development with animal models failing to adequately predict renal drug toxicity. For example, adefovir, a renally excreted antiviral drug, failed clinical studies for human immunodeficiency virus due to pronounced nephrotoxicity in humans. In this study, we demonstrated that there are large species differences in the kinetics of interactions of a key class of antiviral drugs, acyclic nucleoside phosphonates (ANPs), with organic anion transporter 1 [(OAT1) SLC22A6] and identified a key amino acid residue responsible for these differences. In OAT1 stably transfected human embryonic kidney 293 cells, the K_m value of tenofovir for human OAT1 (hOAT1) was significantly lower than for OAT1 orthologs from common preclinical animals, including cynomolgus monkey, mouse, rat, and dog. Chimeric and site-directed mutagenesis studies along with comparative structure modeling identified serine at position 203 (S203) in hOAT1 as a determinant of its lower K_m value. Furthermore, S203 is conserved in apes, and in contrast alanine at the equivalent position is conserved in preclinical animals and Old World monkeys, the most related primates to apes. Intriguingly, transport efficiencies are significantly higher for OAT1 orthologs from apes with high serum uric acid (SUA) levels than for the orthologs from species with low serum uric acid levels. In conclusion, our data provide a molecular mechanism underlying species differences in renal accumulation of nephrotoxic ANPs and a novel insight into OAT1 transport function in primate evolution.

Discovery and Validation of Pyridoxic Acid and Homovanillic Acid as Novel Endogenous Plasma Biomarkers of Organic Anion Transporter (OAT) 1 and OAT3 in Cynomolgus Monkeys

Perturbation of organic anion transporter (OAT) 1- and OAT3-mediated transport can alter the exposure, efficacy, and safety of drugs. Although there have been reports of the endogenous biomarkers for OAT1/3, none of these have all of the characteristics required for a clinical useful biomarker. Cynomolgus monkeys were treated with intravenous probenecid (PROB) at a dose of 40 mg/kg in this study. As expected, PROB increased the area under the plasma concentration-time curve (AUC) of coadministered furosemide, a known substrate of OAT1 and OAT3, by 4.1-fold, consistent with the values reported in humans (3.1- to 3.7-fold). Of the 233 plasma metabolites analyzed using a liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based metabolomics method, 29 metabolites, including pyridoxic acid (PDA) and homovanillic acid (HVA), were significantly increased after either 1 or 3 hours in plasma from the monkeys pretreated with PROB compared with the treated animals. The plasma of animals was then subjected to targeted LC-MS/MS analysis, which confirmed that the PDA and HVA AUCs increased by approximately 2- to 3-fold by PROB pretreatments. PROB also increased the plasma concentrations of hexadecanedioic acid (HDA) and tetradecanedioic acid (TDA), although the increases were not statistically significant. Moreover, transporter profiling assessed using stable cell lines constitutively expressing transporters demonstrated that PDA and HVA are substrates for human OAT1, OAT3, OAT2 (HVA), and OAT4 (PDA), but not OCT2, MATE1, MATE2K, OATP1B1, OATP1B3, and sodium taurocholate cotransporting polypeptide. Collectively, these findings suggest that PDA and HVA might serve as blood-based endogenous probes of cynomolgus monkey OAT1 and OAT3, and investigation of PDA and HVA as circulating endogenous biomarkers of human OAT1 and OAT3 function is warranted.

Prediction of renal transporter-mediated drug-drug interactions for a drug which is an OAT substrate and inhibitor using PBPK modelling

A PBPK modelling approach was used to predict organic anion transporter (OAT) mediated drug-drug interactions involving S44121, a substrate and an inhibitor of OAT1 and OAT3. Model predictions were then compared to the results of a clinical DDI study which was carried out to investigate the interaction of S44121 with probenecid, tenofovir and ciprofloxacin. PBPK models were developed and qualified using existing clinical data, and inhibition constants were determined in vitro. The model predictions for S44121 as an OAT inhibitor were similar to the results obtained from the clinical DDI study, with no interaction observed for tenofovir or ciprofloxacin in the presence of S44121. An observed AUC ratio of 2.2 was obtained for S44121 in the presence of probenecid, which was slightly higher than the model predicted AUC ratio of 1.6. A DDI study in the monkey was also carried out for the interaction between S44121 and probenecid, since the monkey has previously been reported to be a good preclinical model for OAT-mediated DDI. However, this study highlighted a species difference in the major route of S44121 elimination between monkey (mainly hepatic metabolism) and human (mainly renal excretion of unchanged drug), rendering a comparison between the two DDI studies difficult. Overall, for S44121 the PBPK modelling approach gave a better prediction of the extent of DDI than the static predictions based on inhibitor C_max and IC₅₀, therefore this can be considered a potentially valuable tool within drug development.

The organic anion transporter (OAT) family: a systems biology perspective

The organic anion transporter (OAT) subfamily, which constitutes roughly half of the SLC22 (solute carrier 22) transporter family, has received a great deal of attention because of its role in handling of common drugs (antibiotics, antivirals, diuretics, nonsteroidal anti-inflammatory drugs), toxins (mercury, aristolochic acid), and nutrients (vitamins, flavonoids). Oats are expressed in many tissues, including kidney, liver, choroid plexus, olfactory mucosa, brain, retina, and placenta. Recent metabolomics and microarray data from Oat1 [Slc22a6, originally identified as NKT (novel kidney transporter)] and Oat3 (Slc22a8) knockouts, as well as systems biology studies, indicate that this pathway plays a central role in the metabolism and handling of gut microbiome metabolites as well as putative uremic toxins of kidney disease. Nuclear receptors and other transcription factors, such as Hnf4α and Hnf1α, appear to regulate the expression of certain Oats in conjunction with phase I and phase II drug metabolizing enzymes. Some Oats have a strong selectivity for particular signaling molecules, including cyclic nucleotides, conjugated sex steroids, odorants, uric acid, and prostaglandins and/or their metabolites. According to the "Remote Sensing and Signaling Hypothesis," which is elaborated in detail here, Oats may function in remote interorgan communication by regulating levels of signaling molecules and key metabolites in tissues and body fluids. Oats may also play a major role in interorganismal communication (via movement of small molecules across the intestine, placental barrier, into breast milk, and volatile odorants into the urine). The role of various Oat isoforms in systems physiology appears quite complex, and their ramifications are discussed in the context of remote sensing and signaling.

Comparison of minipig, dog, monkey and human drug metabolism and disposition

INTRODUCTION

This article gives an overview of the drug metabolism and disposition (ADME) characteristics of the most common non-rodent species used in toxicity testing of drugs (minipigs, dogs, and monkeys) and compares these to human characteristics with regard to enzymes mediating the metabolism of drugs and the transport proteins which contribute to the absorption, distribution and excretion of drugs.

METHODS

Literature on ADME and regulatory guidelines of relevance in drug development of small molecules has been gathered.

RESULTS

Non-human primates (monkeys) are the species that is closest to humans in terms of genetic homology. Dogs have an advantage due to the ready availability of comprehensive background data for toxicological safety assessment and dogs are easy to handle. Pigs have been used less than dogs and monkeys as a model in safety assessment of drug candidates. However, when a drug candidate is metabolised by aldehyde oxidase (AOX1), N-acetyltransferases (NAT1 and NAT2) or cytochrome (CYP2C9-like) enzymes which are not expressed in dogs, but are present in pigs, this species may be a better choice than dogs, provided that adequate exposure can be obtained in pigs. Conversely, pigs might not be the right choice if sulfation, involving 3-phospho-adenosyl-5-phosphosulphate sulphotransferase (PAPS) is an important pathway in the human metabolism of a drug candidate.

DISCUSSION

In general, the species selection should be based on comparison between in vitro studies with human cell-based systems and animal-cell-based systems. Results from pharmacokinetic studies are also important for decision-making by establishing the obtainable exposure level in the species. Access to genetically humanized mouse models and highly sensitive analytical methods (accelerator mass spectrometry) makes it possible to improve the chance of finding all metabolites relevant for humans before clinical trials have been initiated and, if necessary, to include another animal species before long term toxicity studies are initiated. In conclusion, safety testing can be optimized by applying knowledge about species ADME differences and utilising advanced analytical techniques.

Novel pre-clinical methodologies for pharmacokinetic drug-drug interaction studies: spotlight on "humanized" animal models

Poly-therapy is common due to co-occurrence of several ailments in patients, leading to the elevated possibility of drug-drug interactions (DDI). Pharmacokinetic DDI often accounts for severe adverse drug reactions in patients resulting in withdrawal of drug from the market. Hence, the prediction of DDI is necessary at pre-clinical stage of drug development. Several human tissue and cell line-based in vitro systems are routinely used for screening metabolic and transporter pathways of investigational drugs and for predicting their clinical DDI potentials. However, ample constraints are associated with the in vitro systems and sometimes in vitro-in vivo extrapolation (IVIVE) fail to assess the risk of DDI in clinic. In vitro-in vivo correlation model in animals combined with human in vitro studies may be helpful in better prediction of clinical outcome. Native animal models vary remarkably from humans in drug metabolizing enzymes and transporters, hence, the interpretation of results from animal DDI studies is difficult. With the advent of modern molecular biology and engineering tools, novel pre-clinical animal models, namely, knockout rat/mouse, transgenic rat/mouse with humanized drug metabolizing enzymes and/or transporters and chimeric rat/mouse with humanized liver are developed. These models nearly simulate human-like drug metabolism and help to validate the in vivo relevance of the in vitro human DDI data. This review briefly discusses the application of such novel pre-clinical models for screening various type of DDI along with their advantages and limitations.

Kinetic Interpretation of the Importance of OATP1B3 and MRP2 in Docetaxel-Induced Hematopoietic Toxicity

Neutropenia is a lethal dose-limiting toxicity of docetaxel. Our previous report indicated that the prevalence of severe docetaxel-induced neutropenia is significantly associated with genetic polymorphisms in solute carrier organic anion transporter 1B3 (SLCO1B3) (encoding organic anion–transporting polypeptide 1B3 (OATP1B3)) and ATP-binding cassette subfamily C2 (ABCC2) (encoding multidrug-resistant–associated protein 2 (MRP2)). Therefore, we investigated their significance in docetaxel-induced neutropenia. In vitro experiments suggested their possible involvement in the hepatic uptake of docetaxel and its efflux from bone marrow cells. To further characterize a quantitative impact of OATP1B3 and MRP2 on neutropenia, we used an in silico simulation of the neutrophil count in docetaxel-treated subjects with functional changes in OATP1B3 and MRP2 in a pharmacokinetic/pharmacodynamic model. The clinically reported odds ratios for docetaxel-induced neutropenia risk were explained by the decreased function of OATP1B3 and MRP2 to 41 and 32%, respectively. These results suggest that reduced activities of OATP1B3 and MRP2 associated with systemic exposure and local accumulation in bone marrow cells, respectively, account for the docetaxel-induced neutropenia observed clinically.

Role of solute carriers in response to anticancer drugs

Membrane transporters play critical roles in moving a variety of anticancer drugs across cancer cell membrane, thereby determining chemotherapy efficacy and/or toxicity. The retention of anticancer drugs in cancer cells is the result of net function of efflux and influx transporters. The ATP-binding cassette (ABC) transporters are mainly the efflux transporters expressing at cancer cells, conferring the chemo-resistance in various malignant tumors, which has been well documented over the past decades. However, the function of influx transporters, in particular the solute carriers (SLC) in cancer cells, has only been recently well recognized to have significant impact on cancer therapy. The SLC transporters not only directly bring anticancer agents into cancer cells but also serve as the uptake mediators of essential nutrients for tumor growth and survival. In this review, we concentrate on the interaction of SLC transporters with anticancer drugs and nutrients, and their impact on chemo-sensitivity or -resistance of cancer cells. The differential expression patterns of SLC transporters between normal and tumor tissues may be well utilized to achieve specific delivery of chemotherapeutic agents.

Inhibitory effects of p-aminohippurate and probenecid on the renal clearance of adefovir and benzylpenicillin as probe drugs for organic anion transporter (OAT) 1 and OAT3 in humans

Probe substrates for, and inhibitors of, specific transporters are desired to evaluate quantitatively the in vivo functions of transporters in humans. Based on published data, adefovir and benzylpenicillin were selected as organic anion transporter (OAT) 1- and OAT3-selective probe substrates, respectively. In human kidney slices, probenecid potently inhibited the uptake of both adefovir and benzylpenicillin with inhibition constant (Ki) values of 18.6±5.1 and 12.6±4.2μM, respectively, whereas p-aminohippurate (PAH) preferentially inhibited adefovir uptake. A clinical drug-interaction study involving healthy subjects was performed to investigate the dose-dependent inhibition potencies of probenecid and PAH on the renal clearance of the probe substrates. Adefovir or benzylpenicillin was coadministered with different oral doses of probenecid (500, 750, or 1500mg) or intravenous PAH infusion rates (70, 120, or 210mg/min/person) to the same subject using a crossover design. The renal clearance of adefovir was reduced by 45% and 46% in the subjects treated with the maximum dose of probenecid and PAH, respectively, which was in accordance with the results of in vitro inhibition study. On the other hand, renal clearance of benzylpenicillin was reduced by 78% in the subjects treated with the maximum dose of probenecid (1500mg), which could be explained by its in vitro Ki values. However, PAH unexpectedly increased the renal clearance of benzylpenicillin by 47%. These results suggest that adefovir and benzylpenicillin can be used as probe drugs for OAT1 and OAT3, respectively, and that PAH can be used to investigate the role of OAT1 in the urinary excretion of drugs in humans, whereas it may modulate other transport processes in the kidney.

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.

A perspective on the prediction of drug pharmacokinetics and disposition in drug research and development

Prediction of human pharmacokinetics of new drugs, as well as other disposition attributes, has become a routine practice in drug research and development. Prior to the 1990s, drug disposition science was used in a mostly descriptive manner in the drug development phase. With the advent of in vitro methods and availability of human-derived reagents for in vitro studies, drug-disposition scientists became engaged in the compound design phase of drug discovery to optimize and predict human disposition properties prior to nomination of candidate compounds into the drug development phase. This has reaped benefits in that the attrition rate of new drug candidates in drug development for reasons of unacceptable pharmacokinetics has greatly decreased. Attributes that are predicted include clearance, volume of distribution, half-life, absorption, and drug-drug interactions. In this article, we offer our experience-based perspectives on the tools and methods of predicting human drug disposition using in vitro and animal data.

Species differences of organic anion transporters involved in the renal uptake of 4-amino-3-chlorophenyl hydrogen sulfate, a metabolite of resatorvid, between rats and dogs

Previous studies on the metabolic fate of resatorvid (TAK-242) have shown that species differences in the pharmacokinetics of 4-amino-3-chlorophenyl hydrogen sulfate (M-III), a metabolite of TAK-242, between rats and dogs are mainly attributable to the urinary excretion process. In the present study, the renal uptake mechanism of M-III was investigated using kidney slices and Xenopus laevis oocytes expressing rat organic anion transporter 1 (rOat1; Slc22a6) and rOat3 (Slc22a8). The uptake of p-aminohippuric acid (PAH), a substrate for Oats, by kidney slices from rats and dogs increased at 37 °C and M-III inhibited the uptake. The initial uptake clearance of M-III by rat kidney slices was 0.295 and 0.0114 ml/min/g at 37 °C and 4 °C, respectively. The Eadie-Hofstee plot of M-III uptake at 37 °C revealed two-component transport processes with K(m) values being 6.48 and 724 µmol/l. The uptake was inhibited by probenecid (PBC), PAH and benzylpenicillin (PCG). In contrast, in dog kidney slices, the initial uptake clearance of M-III was 8.70 × 10(-3) and 9.00 × 10(-3) ml/min/g at 37 °C and 4 °C, respectively, and the uptake was not inhibited by PBC. Furthermore, rOat1- and rOat3-expressing oocytes mediated M-III uptake and the uptake was inhibited by PAH and PCG, respectively. These results suggest that rOat1 and rOat3 are responsible for the renal uptake of M-III in rats. Moreover, it is speculated that Oat(s) is unable to transport M-III in dogs and that the difference in the substrate recognition of Oat(s) contributes to the species difference in the pharmacokinetics of M-III between rats and dogs.

Cynomolgus monkey as a potential model to assess drug interactions involving hepatic organic anion transporting polypeptides: in vitro, in vivo, and in vitro-to-in vivo extrapolation

Organic anion-transporting polypeptides (OATP) 1B1, 1B3, and 2B1 can serve as the loci of drug-drug interactions (DDIs). In the present work, the cynomolgus monkey was evaluated as a potential model for studying OATP-mediated DDIs. Three cynomolgus monkey OATPs (cOATPs), with a high degree of amino acid sequence identity (91.9, 93.5, and 96.6% for OATP1B1, 1B3, and 2B1, respectively) to their human counterparts, were cloned, expressed, and characterized. The cOATPs were stably transfected in human embryonic kidney cells and were functionally similar to the corresponding human OATPs (hOATPs), as evident from the similar uptake rate of typical substrates (estradiol-17β-d-glucuronide, cholecystokinin octapeptide, and estrone-3-sulfate). Moreover, six known hOATP inhibitors exhibited similar IC(50) values against cOATPs. To further evaluate the appropriateness of the cynomolgus monkey as a model, a known hOATP substrate [rosuvastatin (RSV)]-inhibitor [rifampicin (RIF)] pair was examined in vitro; the monkey-derived parameters (RSV K(m) and RIF IC(50)) were similar (within 3.5-fold) to those obtained with hOATPs and human primary hepatocytes. In vivo, the area under the plasma concentration-time curve of RSV (3 mg/kg, oral) given 1 hour after a single RIF dose (15 mg/kg, oral) was increased 2.9-fold in cynomolgus monkeys, consistent with the value (3.0-fold) reported in humans. A number of in vitro-in vivo extrapolation approaches, considering the fraction of the pathways affected and free versus total inhibitor concentrations, were also explored. It is concluded that the cynomolgus monkey has the potential to serve as a useful model for the assessment of OATP-mediated DDIs in a nonclinical setting.

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.

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.

Prediction of human renal clearance from preclinical species for a diverse set of drugs that exhibit both active secretion and net reabsorption

Identifying any extrahepatic excretion phenomenon in preclinical species is crucial for an accurate prediction of the pharmacokinetics in man. This understanding is particularly key for drugs with a small volume of distribution, because they require an especially low total clearance to be suitable for a once-a-day dosing regimen in man. In this study, three animal scaling techniques were applied for the prediction of the human renal clearance of 36 diverse drugs that show active secretion or net reabsorption: 1) direct correlations between renal clearance in man and each of the two main preclinical species (rat and dog); 2) simple allometry; and 3) Mahmood's renal clearance scaling method. The results show clearly that the predictions to man for the methods are improved significantly when corrections are made for species differences in plasma protein binding. Overall, the most accurate predictions were obtained by using a direct correlation with the dog renal clearance after correcting for differences in plasma protein binding and kidney blood flow (r² = 0.84), where predictions, on average, were within 2-fold of the observed renal clearance values in human.

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.

Analysis of three-dimensional systems for developing and mature kidneys clarifies the role of OAT1 and OAT3 in antiviral handling

The organic anion transporters OAT1 (SLC22A6, originally identified by us as NKT) and OAT3 (SLC22A8) are critical for handling many toxins, metabolites, and drugs, including antivirals (Truong, D. M., Kaler, G., Khandelwal, A., Swaan, P. W., and Nigam, S. K. (2008) J. Biol. Chem. 283, 8654-8663). Although microinjected Xenopus oocytes and/or transfected cells indicate overlapping specificities, the individual contributions of these transporters in the three-dimensional context of the tissues in which they normally function remain unclear. Here, handling of HIV antivirals (stavudine, tenofovir, lamivudine, acyclovir, and zidovudine) was analyzed with three-dimensional ex vivo functional assays using knock-out tissue. To investigate the contribution of OAT1 and OAT3 in various nephron segments, the OAT-selective fluorescent tracer substrates 5-carboxyfluorescein and 6-carboxyfluorescein were used. Although OAT1 function (uptake in oat3(-/-) tissue) was confined to portions of the cortex, consistent with a proximal tubular localization, OAT3 function (uptake in oat1(-/-) tissue) was apparent throughout the cortex, indicating localization in the distal as well as proximal nephron. This functional localization indicates a complex three-dimensional context, which needs to be considered for metabolites, toxins, and drugs (e.g. antivirals) handled by both transporters. These results also raise the possibility of functional differences in the relative importance of OAT1 and OAT3 in antiviral handling in developing and mature tissue. Because the HIV antivirals are used in pregnant women, the results may also help in understanding how these drugs are handled by developing organs.

Use of in vivo animal models to assess pharmacokinetic drug-drug interactions

Animal models are used commonly in various stages of drug discovery and development to aid in the prospective assessment of drug-drug interaction (DDI) potential and the understanding of the underlying mechanism for DDI of a drug candidate. In vivo assessments in an appropriate animal model can be very valuable, when used in combination with in vitro systems, to help verify in vivo relevance of the in vitro animal-based results, and thus substantiate the extrapolation of in vitro human data to clinical outcomes. From a pharmacokinetic standpoint, a key consideration for rational selection of an animal model is based on broad similarities to humans in important physiological and biochemical parameters governing drug absorption, distribution, metabolism or excretion (ADME) processes in question for both the perpetrator and victim drugs. Equally critical are specific in vitro and/or in vivo experiments to demonstrate those similarities, usually both qualitative and quantitative, in the ADME properties/processes under investigation. In this review, theoretical basis and specific examples are presented to illustrate the utility of the animal models in assessing the potential and understanding the mechanisms of DDIs.

Organic anion transporters: discovery, pharmacology, regulation and roles in pathophysiology

Our understanding of the mechanisms behind inter- and intra-patient variability in drug response is inadequate. Advances in the cytochrome P450 drug metabolizing enzyme field have been remarkable, but those in the drug transporter field have trailed behind. Currently, however, interest in carrier-mediated disposition of pharmacotherapeutics is on a substantial uprise. This is exemplified by the 2006 FDA guidance statement directed to the pharmaceutical industry. The guidance recommended that industry ascertain whether novel drug entities interact with transporters. This suggestion likely stems from the observation that several novel cloned transporters contribute significantly to the disposition of various approved drugs. Many drugs bear anionic functional groups, and thus interact with organic anion transporters (OATs). Collectively, these transporters are nearly ubiquitously expressed in barrier epithelia. Moreover, several reports indicate that OATs are subject to diverse forms of regulation, much like drug metabolizing enzymes and receptors. Thus, critical to furthering our understanding of patient- and condition-specific responses to pharmacotherapy is the complete characterization of OAT interactions with drugs and regulatory factors. This review provides the reader with a comprehensive account of the function and substrate profile of cloned OATs. In addition, a major focus of this review is on the regulation of OATs including the impact of transcriptional and epigenetic factors, phosphorylation, hormones and gender.

Evidence of carrier-mediated transport in the penetration of donepezil into the rat brain

The objective of this study was to characterize the mechanism that controls the transport of donepezil into the brain. The apparent brain uptake clearance (CL(app,br)) was decreased as a function of the dose of donepezil, suggesting an involvement of a saturable transport process via transporter(s) in the penetration across the blood-brain barrier (BBB). Consistent with in vivo results, the uptake of substrates for organic cation transporters was significantly reduced by donepezil in both MBEC4 cells (i.e., a model for BBB) and HEK 293 cells expressing the transporters found in the brain, indicative of the involvement of organic cation transporters in the transport of the drug. Furthermore, donepezil transport was enhanced (p < 0.01) in HEK 293 cells expressing rOCNT1, rOCTN2, or rCHT1. The CL(app,br) was reduced up to 52.8% of the control in rats that had been pretreated with choline, while the CL(app,br) was unaffected with pretreatments with organic cations other than choline, suggesting that choline and donepezil share a common transport mechanism in the penetration across the BBB in vivo. Taken together, these observations suggest that the transport of donepezil across the BBB is mediated by organic cation transporters such as choline transport system(s).

Inhibition of flucloxacillin tubular renal secretion by piperacillin

AIMS

To explore the extent, time course, site(s), mechanism and possible clinical relevance of the pharmacokinetic (PK) interaction between piperacillin and flucloxacillin.

METHODS

A single-dose, randomized, six-way crossover study in 10 healthy volunteers where all subjects received all of the following as 5-min intravenous infusions: (i) 1.5 g piperacillin, (ii) 0.5 g flucloxacillin, (iii) 1.5 g piperacillin + 0.5 g flucloxacillin, (iv) 3 g piperacillin, (v) 1 g flucloxacillin, and (vi) 3 g piperacillin + 1 g flucloxacillin. Drug concentrations in plasma and urine were determined by high-performance liquid chromatography. WinNonlin was used for PK modelling and statistics.

RESULTS

Piperacillin significantly decreased the renal clearance of flucloxacillin from 5.44 to 2.29 l h(-1) (medians, P < 0.01) and the nonrenal clearance of flucloxacillin from 2.67 to 1.80 l h(-1) (P < 0.01). The renal clearance of flucloxacillin was reduced to 45% (point estimate, 90% confidence interval 40 to 50%) and the nonrenal clearance to 66% (59, 73). The extent of interaction was larger at the higher doses. Competitive inhibition of tubular secretion by piperacillin was identified as the most likely mechanism for the decreased renal clearance of flucloxacillin. Piperacillin had a 15-times higher affinity for the renal transporter than flucloxacillin based on the molar ratio. Piperacillin PK was only slightly affected by flucloxacillin.

CONCLUSIONS

Piperacillin inhibits renal and nonrenal elimination of flucloxacillin. This interaction seems clinically significant, as total clearance was reduced by a factor of 1.5 for the lower and 2.1 for the higher doses. PK interactions, especially with piperacillin, are likely to occur also with other beta-lactam combinations and might be useful to improve the effectiveness of antibacterial treatment.

Physiology, structure, and regulation of the cloned organic anion transporters

1. The transport of negatively charged drugs, xenobiotics, and metabolites by epithelial tissues, particularly the kidney, plays critical roles in controlling their distribution, concentration, and retention in the body. Thus, organic anion transporters (OATs) impact both their therapeutic efficacy and potential toxicity. 2. This review summarizes current knowledge of the properties and functional roles of the cloned OATs, the relationships between transporter structure and function, and those factors that determine the efficacy of transport. Such factors include plasma protein binding of substrates, genetic polymorphisms among the transporters, and regulation of transporter expression. 3. Clearly, much progress has been made in the decade since the first OAT was cloned. However, unresolved questions remain. Several of these issues--drug-drug interactions, functional characterization of newly cloned OATs, tissue differences in expression and function, and details of the nature and consequences of transporter regulation at genomic and intracellular sites--are discussed in the concluding Perspectives section.

Inhibition of human organic anion transporter 3 mediated pravastatin transport by gemfibrozil and the metabolites in humans

Coadministration of gemfibrozil (600 mg, b.i.d., 3 days) with pravastatin (40 mg/day) decreased the renal clearance of pravastatin by approximately 40% in healthy volunteers. To investigate the mechanism of this drug-drug interaction in the renal excretion process, we undertook an uptake study of pravastatin using human organic anion transporters (hOATs)-expressing S2 cells. hOAT3 and hOAT4 transported pravastatin in a saturatable manner with Michaelis--Menten constants of 27.7 microM and 257 microM respectively. On the other hand, hOAT1 and hOAT2 did not transport pravastatin. Gemfibrozil and its glucuronide and carboxylic metabolite forms inhibited the uptake of pravastatin by hOAT3 with IC(50) values of 6.8 microM, 19.7 microM and 5.4 microM, respectively. Considering the plasma concentrations of gemfibrozil and its metabolites in humans, the inhibition of hOAT3-mediated pravastatin transport by gemfibrozil and its metabolites would lead to a decrease in the renal clearance of pravastatin in clinical settings.

Pharmacokinetics and metabolism of KW-4490, a selective phosphodiesterase 4 inhibitor: difference in excretion of KW-4490 and acylglucuronide metabolites between rats and cynomolgus monkeys

1. The pharmacokinetics and metabolism of KW-4490, a selective phosphodiesterase 4 inhibitor, were investigated in rats and monkeys. After oral administration, KW-4490 was rapidly absorbed, and then its plasma concentrations apparently declined with half-lives of approximately 5 h in rats and 3.5 h in cynomolgus monkeys; however, a number of secondary peaks were apparent in the profiles for both species. The plasma pharmacokinetics of KW-4490 were comparable between rats and monkeys. 2. After oral administration, KW-4490 was mainly eliminated by metabolism to acylglucuronides and renal excretion in the unchanged form. KW-4490 acylglucuronides were found in monkey but not rat urine. In rats, KW-4490 acylglucuronides were excreted only in bile. Although the pathway of excretion of acylglucuronides differed between rats and monkeys, cumulative excretion in the two animals was very similar, as expected from comparable hepatic clearance for glucuronidation in rat and monkey liver microsomes. 3. The glomerular filtration rate of unbound KW-4490 indicated that renal tubular secretion was significant in monkeys, whereas reabsorption was significant in rats. These species differences in urinary excretion of KW-4490 and its acylglucuronide metabolites are most likely due to substrate specificity of active transporters in rat and monkey kidney.