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

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.

Research Methods and New Advances in Drug-Drug Interactions Mediated by Renal Transporters

The kidney is critical in the human body's excretion of drugs and their metabolites. Renal transporters participate in actively secreting substances from the proximal tubular cells and reabsorbing them in the distal renal tubules. They can affect the clearance rates (CLr) of drugs and their metabolites, eventually influence the clinical efficiency and side effects of drugs, and may produce drug-drug interactions (DDIs) of clinical significance. Renal transporters and renal transporter-mediated DDIs have also been studied by many researchers. In this article, the main types of in vitro research models used for the study of renal transporter-mediated DDIs are membrane-based assays, cell-based assays, and the renal slice uptake model. In vivo research models include animal experiments, gene knockout animal models, positron emission tomography (PET) technology, and studies on human beings. In addition, in vitro-in vivo extrapolation (IVIVE), ex vivo kidney perfusion (EVKP) models, and, more recently, biomarker methods and in silico models are included. This article reviews the traditional research methods of renal transporter-mediated DDIs, updates the recent progress in the development of the methods, and then classifies and summarizes the advantages and disadvantages of each method. Through the sorting work conducted in this paper, it will be convenient for researchers at different learning stages to choose the best method for their own research based on their own subject's situation when they are going to study DDIs mediated by renal transporters.

Detection of potential drug-drug interactions for risk of acute kidney injury: a population-based case-control study using interpretable machine-learning models

Background: Acute kidney injury (AKI), with an increase in serum creatinine, is a common adverse drug event. Although various clinical studies have investigated whether a combination of two nephrotoxic drugs has an increased risk of AKI using traditional statistical models such as multivariable logistic regression (MLR), the evaluation metrics have not been evaluated despite the fact that traditional statistical models may over-fit the data. The aim of the present study was to detect drug-drug interactions with an increased risk of AKI by interpreting machine-learning models to avoid overfitting. Methods: We developed six machine-learning models trained using electronic medical records: MLR, logistic least absolute shrinkage and selection operator regression (LLR), random forest, extreme gradient boosting (XGB) tree, and two support vector machine models (kernel = linear function and radial basis function). In order to detect drug-drug interactions, the XGB and LLR models that showed good predictive performance were interpreted by SHapley Additive exPlanations (SHAP) and relative excess risk due to interaction (RERI), respectively. Results: Among approximately 2.5 million patients, 65,667 patients were extracted from the electronic medical records, and assigned to case (N = 5,319) and control (N = 60,348) groups. In the XGB model, a combination of loop diuretic and histamine H₂ blocker [mean (|SHAP|) = 0.011] was identified as a relatively important risk factor for AKI. The combination of loop diuretic and H₂ blocker showed a significant synergistic interaction on an additive scale (RERI 1.289, 95% confidence interval 0.226-5.591) also in the LLR model. Conclusion: The present population-based case-control study using interpretable machine-learning models suggested that although the relative importance of the individual and combined effects of loop diuretics and H₂ blockers is lower than that of well-known risk factors such as older age and sex, concomitant use of a loop diuretic and histamine H₂ blocker is associated with increased risk of AKI.

Predicting disruptions to drug pharmacokinetics and the risk of adverse drug reactions in non-alcoholic steatohepatitis patients

The liver plays a central role in the pharmacokinetics of drugs through drug metabolizing enzymes and transporters. Non-alcoholic steatohepatitis (NASH) causes disease-specific alterations to the absorption, distribution, metabolism, and excretion (ADME) processes, including a decrease in protein expression of basolateral uptake transporters, an increase in efflux transporters, and modifications to enzyme activity. This can result in increased drug exposure and adverse drug reactions (ADRs). Our goal was to predict drugs that pose increased risks for ADRs in NASH patients. Bibliographic research identified 71 drugs with reported ADRs in patients with liver disease, mainly non-alcoholic fatty liver disease (NAFLD), 54 of which are known substrates of transporters and/or metabolizing enzymes. Since NASH is the progressive form of NAFLD but is most frequently undiagnosed, we identified other drugs at risk based on NASH-specific alterations to ADME processes. Here, we present another list of 71 drugs at risk of pharmacokinetic disruption in NASH, based on their transport and/or metabolism processes. It encompasses drugs from various pharmacological classes for which ADRs may occur when used in NASH patients, especially when eliminated through multiple pathways altered by the disease. Therefore, these results may inform clinicians regarding the selection of drugs for use in NASH patients.

Investigations with Drugs and Pesticides Revealed New Species- and Substrate-Dependent Inhibition by Elacridar and Imazalil in Human and Mouse Organic Cation Transporter OCT2

Multiple drugs are used to treat various indications as well as pesticides that are ingested unintentionally and enter the bloodstream. The residence time or bioavailability of these substances in circulation depends on several mechanisms, such as drug−drug interaction (DDI), drug−pesticide interaction, metabolizing enzymes and the hepatic and renal transport systems, involved in the elimination of the compounds from the body. One of these transporters is the Organic Cation Transporter 2 (OCT2) member of the solute carrier (SLC22) transporter family. OCT2 is highly expressed in the proximal tubule epithelial cells in human and mouse kidney, where it mediates the uptake of endogenous organic cations as well as numerous drugs and xenobiotics, and contributes to the first step of renal clearance. In this study, we examined OCT2 on two subjects: First, the transferability of data from mouse to human, since mice are initially examined in the development of new drugs to assess the renal excretion of organic cations. Second, to what extent the choice of substrate affects the properties of an inhibitor. For this purpose, the functional properties of hOCT2 and mOct2 were validated under the same experimental conditions with the known substrates metformin and 1-Methyl-4-phenylpyridinium iodide (MPP). While hOCT2 and mOct2 showed very low affinities for metformin with Km values of 3.9 mM and 3.5 mM, the affinity of hOCT2 and mOct2 for MPP (62 and 40 µM) was 64- and 89-fold higher, respectively. For our positive control inhibitor decynium22, we determined the following IC50 values for hOCT2 and mOct2: 2.2 and 2.6 µM for metformin uptake, and 16 and 6.9 µM for MPP uptake. A correlation analysis of the inhibitory effects of 13 drugs and 9 pesticides on hOCT2- and mOct2-mediated transport of metformin showed a correlation coefficient R2 of 0.88, indicating good interspecies correlation. Nevertheless, the bioenhancer elacridar and the fungicide imazalil showed species-dependent inhibitory potentials. Concentration-dependent inhibition of hOCT2- and mOct2-mediated metformin uptake by elacridar showed IC50 values of 20 µM and 1.9 µM and by imazalil 4.7 µM and 0.58 µM, respectively. In conclusion, although our data show comparable species-independent interactions for most compounds, there can be large species−specific differences in the interactions of individual compounds, which should be considered when extrapolating data from mice to humans. Furthermore, a comparison of the inhibitory potential of elacridar and imazalil on metformin uptake with that on MPP uptake reveals substrate-dependent differences in hOCT2 and mOct2 for both inhibitors. Therefore, it might be useful to test two different substrates in inhibition studies.

Substrate-Dependent Trans-Stimulation of Organic Cation Transporter 2 Activity

The search of substrates for solute carriers (SLCs) constitutes a major issue, owing notably to the role played by some SLCs, such as the renal electrogenic organic cation transporter (OCT) 2 (SLC22A2), in pharmacokinetics, drug-drug interactions and drug toxicity. For this purpose, substrates have been proposed to be identified by their cis-inhibition and trans-stimulation properties towards transporter activity. To get insights on the sensitivity of this approach for identifying SLC substrates, 15 various exogenous and endogenous OCT2 substrates were analysed in the present study, using 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (DiASP) as a fluorescent OCT2 tracer substrate. All OCT2 substrates cis-inhibited DiASP uptake in OCT2-overexpressing HEK293 cells, with IC₅₀ values ranging from 0.24 µM (for ipratropium) to 2.39 mM (for dopamine). By contrast, only 4/15 substrates, i.e., acetylcholine, agmatine, choline and metformin, trans-stimulated DiASP uptake, with a full suppression of the trans-stimulating effect of metformin by the reference OCT2 inhibitor amitriptyline. An analysis of molecular descriptors next indicated that trans-stimulating OCT2 substrates exhibit lower molecular weight, volume, polarizability and lipophilicity than non-trans-stimulating counterparts. Overall, these data indicated a rather low sensitivity (26.7%) of the trans-stimulation assay for identifying OCT2 substrates, and caution with respect to the use of such assay may therefore be considered.

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.

Multifaceted Factors Causing Conflicting Outcomes in Herb-Drug Interactions

Metabolic enzyme and/or transporter-mediated pharmacokinetic (PK) changes in a drug caused by concomitant herbal products have been a primary issue of herb and drug interactions (HDIs), because PK changes of a drug may result in the alternation of efficacy and toxicity. Studies on HDIs have been carried out by predictive in vitro and in vivo preclinical studies, and clinical trials. Nevertheless, the discrepancies between predictive data and the clinical significance on HDIs still exist, and different reports of HDIs add to rather than clarify the confusion regarding the use of herbal products and drug combinations. Here, we briefly review the underlying mechanisms causing PK-based HDIs, and more importantly summarize challenging issues, such as dose and treatment period effects, to be considered in study designs and interpretations of HDI evaluations.

Organic Cation Transporters in Health and Disease

The organic cation transporters (OCTs) OCT1, OCT2, OCT3, novel OCT (OCTN)1, OCTN2, multidrug and toxin exclusion (MATE)1, and MATE kidney-specific 2 are polyspecific transporters exhibiting broadly overlapping substrate selectivities. They transport organic cations, zwitterions, and some uncharged compounds and operate as facilitated diffusion systems and/or antiporters. OCTs are critically involved in intestinal absorption, hepatic uptake, and renal excretion of hydrophilic drugs. They modulate the distribution of endogenous compounds such as thiamine, L-carnitine, and neurotransmitters. Sites of expression and functions of OCTs have important impact on energy metabolism, pharmacokinetics, and toxicity of drugs, and on drug-drug interactions. In this work, an overview about the human OCTs is presented. Functional properties of human OCTs, including identified substrates and inhibitors of the individual transporters, are described. Sites of expression are compiled, and data on regulation of OCTs are presented. In addition, genetic variations of OCTs are listed, and data on their impact on transport, drug treatment, and diseases are reported. Moreover, recent data are summarized that indicate complex drug-drug interaction at OCTs, such as allosteric high-affinity inhibition of transport and substrate dependence of inhibitor efficacies. A hypothesis about the molecular mechanism of polyspecific substrate recognition by OCTs is presented that is based on functional studies and mutagenesis experiments in OCT1 and OCT2. This hypothesis provides a framework to imagine how observed complex drug-drug interactions at OCTs arise. Finally, preclinical in vitro tests that are performed by pharmaceutical companies to identify interaction of novel drugs with OCTs are discussed. Optimized experimental procedures are proposed that allow a gapless detection of inhibitory and transported drugs.

Concomitant lansoprazole ameliorates cisplatin-induced nephrotoxicity by inhibiting renal organic cation transporter 2 in rats

Cisplatin is used widely for the treatment of multiple solid tumors. Cisplatin-induced nephrotoxicity is caused by renal accumulation of cisplatin via human organic cation transporter 2 (hOCT2). As lansoprazole, a proton pump inhibitor, is known to inhibit hOCT2 activity, lansoprazole might ameliorate cisplatin-induced nephrotoxicity. A previous study showed that concomitant lansoprazole administration ameliorated nephrotoxicity in patients receiving cisplatin. However, the detailed mechanism remains to be clarified. In the present study, the drug-drug interaction between lansoprazole and cisplatin was examined using hOCT2-expressing cultured cells and rat renal slices. Moreover, the effect of lansoprazole on cisplatin-induced nephrotoxicity and the pharmacokinetics of cisplatin in rats was investigated. In the uptake study, lansoprazole potently inhibited the uptake of cisplatin in hOCT2-expressing cultured cells and rat renal slices. The in vivo rat study showed that concomitant lansoprazole significantly ameliorated cisplatin-induced nephrotoxicity and reduced the renal accumulation of platinum up to approximately 60% of cisplatin alone at 72 h after cisplatin intraperitoneal administration. Furthermore, the renal uptake of platinum at 3 min after intravenous cisplatin administration in rats with cisplatin and lansoprazole decreased to 78% of rats with cisplatin alone. In addition, there was no significant difference in the plasma platinum concentration between rats treated with and without lansoprazole at 3 min after cisplatin intravenous administration. These findings suggested that concomitant lansoprazole ameliorated cisplatin-induced nephrotoxicity by inhibiting rOCT2-mediated cisplatin uptake in rats, thus decreasing cisplatin accumulation in the kidney. The present findings provided important information for the establishment of novel protective approaches to minimize cisplatin-induced nephrotoxicity.

Influence of Several Compounds and Drugs on the Renal Uptake of Radiolabeled Affibody Molecules

Affibody molecules are the most studied class of engineered scaffold proteins (ESPs) in radionuclide molecular imaging. Attempts to use affibody molecules directly labelled with radiometals for targeted radionuclide therapy were hampered by the high uptake and retention of radioactivity in kidneys. Several promising strategies have been implemented to circumvent this problem. Here, we investigated whether a pharmacological approach targeting different components of the reabsorption system could be used to lower the uptake of [^99mTc]Tc-Z_HER:2395 affibody molecule in kidneys. Pre-injection of probenecid, furosemide, mannitol or colchicine had no influence on activity uptake in kidneys compared to the control group. Mice pre-injected with maleate and fructose had 33% and 51% reduction in the kidney-associated activity, respectively, compared to the control group. Autoradiography images showed that the accumulation of activity after [^99mTc]Tc-Z_HER2:2395 injection was in the renal cortex and that both maleate and fructose could significantly reduce it. Results from this study demonstrate that pharmacological intervention with maleate and fructose was effective in reducing the kidney uptake of affibody molecules. A presumable mechanism is the disruption of ATP-mediated cellular uptake and endocytosis processes of affibody molecules by tubular cells.

Lactational Transfer of Long-Chain Perfluorinated Carboxylic Acids in Mice: A Method to Directly Collect Milk and Evaluate Chemical Transferability

Perfluoroalkyl carboxylic acids (PFCAs), such as perfluorooctanoic acid (PFOA, C8), are a group of industrial chemicals that are detected in the serum of people throughout the world. Long-chain PFCAs (C9 to C13) have high lipophilicity, therefore they may have a high transfer rate to breast milk. This study investigated the lactational transfer of PFCAs with carbon chain lengths of 8 to 13 in mice. Lactating dams were given a single intravenous administration of PFCAs (C8 to C13) during the postnatal period (8–13 days after delivery). Milk was collected from the dam 24 h after administration using a milking device built in-house. Plasma was obtained from the dam at the same time as milk collection. The observed milk/plasma (M/P) concentration ratios were 0.32 for C8, 0.30 for C9, 0.17 for C10, 0.21 for C11, 0.32 for C12, and 0.49 for C13. These results indicate that the M/P concentration ratio is not related to the lipophilicity of PFCAs. However, estimated relative daily intake, an indicator of how much PFCA is transferred from dams to pups per body weight, increased with chain length: 4.16 for C8, 8.98 for C9, 9.35 for C10, 9.51 for C11, 10.20 for C12, and 10.49 for C13, which may be related to the lower clearance of long-chain PFCAs. These results indicate the importance of future risk assessment of long-chain PFCAs.

Overview of organic anion transporters and organic anion transporter polypeptides and their roles in the liver

Organic anion transporters (OATs) and organic anion transporter polypeptides (OATPs) are classified within two SLC superfamilies, namely, the SLC22A superfamily and the SLCO superfamily (formerly the SLC21A family), respectively. They are expressed in many tissues, such as the liver and kidney, and mediate the absorption and excretion of many endogenous and exogenous substances, including various drugs. Most are composed of 12 transmembrane polypeptide chains with the C-terminus and the N-terminus located in the cell cytoplasm. OATs and OATPs are abundantly expressed in the liver, where they mainly promote the uptake of various endogenous substrates such as bile acids and various exogenous drugs such as antifibrotic and anticancer drugs. However, differences in the locations of glycosylation sites, phosphorylation sites, and amino acids in the OAT and OATP structures lead to different substrates being transported to the liver, which ultimately results in their different roles in the liver. To date, few articles have addressed these aspects of OAT and OATP structures, and we study further the similarities and differences in their structures, tissue distribution, substrates, and roles in liver diseases.

Effective reconstruction of functional organotypic kidney spheroid for in vitro nephrotoxicity studies

Stable and reproducible kidney cellular models could accelerate our understanding of diseases, help therapeutics development, and improve nephrotoxicity screenings. Generation of a reproducible in vitro kidney models has been challenging owing to the cellular heterogeneity and structural complexity of the kidney. We generated mixed immortalized cell lines that stably maintained their characteristic expression of renal epithelial progenitor markers for the different lineages of kidney cellular compartments via the BMP7 signaling pathway from a mouse and a human whole kidney. These cells were used to generate functional and matured kidney spheroids containing multiple renal lineages, such as the proximal tubule, loop of Henle, distal tubules, and podocytes, using extracellular matrix and physiological force, named spheroid-forming unit (SFU). They expressed all apical and basolateral transporters that are important for drug metabolism and displayed key functional aspects of the proximal tubule, including protein endocytosis and increased gamma-glutamyltransferase activity, and cyclic AMP responded to external cues, such as parathyroid hormone. Following exposure, cells fluxed and took up drugs via proximal tubule-specific apical or basolateral transporters, and displayed increased cell death and expression of renal injury marker. Here, we developed a new differentiation method to generate kidney spheroids that structurally recapitulate important features of the kidney effectively and reproducibly using mixed immortalized renal cells, and showed their application for renal toxicity studies.

Transporter-Mediated Drug-Drug Interactions and Their Significance

Drug transporters are considered to be determinants of drug disposition and effects/toxicities by affecting the absorption, distribution, and excretion of drugs. Drug transporters are generally divided into solute carrier (SLC) family and ATP binding cassette (ABC) family. Widely studied ABC family transporters include P-glycoprotein (P-GP), breast cancer resistance protein (BCRP), and multidrug resistance proteins (MRPs). SLC family transporters related to drug transport mainly include organic anion-transporting polypeptides (OATPs), organic anion transporters (OATs), organic cation transporters (OCTs), organic cation/carnitine transporters (OCTNs), peptide transporters (PEPTs), and multidrug/toxin extrusions (MATEs). These transporters are often expressed in tissues related to drug disposition, such as the small intestine, liver, and kidney, implicating intestinal absorption of drugs, uptake of drugs into hepatocytes, and renal/bile excretion of drugs. Most of therapeutic drugs are their substrates or inhibitors. When they are comedicated, serious drug-drug interactions (DDIs) may occur due to alterations in intestinal absorption, hepatic uptake, or renal/bile secretion of drugs, leading to enhancement of their activities or toxicities or therapeutic failure. This chapter will illustrate transporter-mediated DDIs (including food drug interaction) in human and their clinical significances.

Pharmacokinetic Drug-Drug Interactions Between Trospium Chloride and Ranitidine Substrates of Organic Cation Transporters in Healthy Human Subjects

Trospium chloride, a muscarinic receptor blocker, is poorly absorbed with different rates from areas in the jejunum and the cecum/ascending colon. To evaluate whether organic cation transporter (OCT) 1, OCT2 and multidrug and toxin extrusion (MATE) 1 and MATE2-K are involved in pharmacokinetics, competitions with ranitidine, a probe inhibitor of the cation transporters, were evaluated in transfected HEK293 cells. Furthermore, a drug interaction study with trospium chloride after intravenous (2 mg) and oral dosing (30 mg) plus ranitidine (300 mg) was performed in 12 healthy subjects and evaluated by noncompartmental analysis and population pharmacokinetic modeling. Ranitidine inhibited OCT1, OCT2, MATE1, and MATE2-K with half maximal inhibitory concentration values of 186 ± 25 µM, 482 ± 105 µM, 134 ± 37 µM, and 35 ± 11 µM, respectively. In contrast to our hypothesis, coadministration of ranitidine did not significantly decrease oral absorption of trospium. Instead, renal clearance was lowered by ∼15% (530 ± 99 vs 460 ± 120 mL/min; P < .05). It is possible that ranitidine was not available in competitive concentrations at the major colonic absorption site, as the inhibitor is absorbed in the small intestine and undergoes degradation by microbiota. The renal effects apparently result from inhibition of MATE1 and/or MATE2-K by ranitidine as predicted by in vitro to in vivo extrapolation. However, all pharmacokinetic changes were not of clinical relevance for the drug with highly variable pharmacokinetics. Intravenous trospium significantly lowered mean absorption time and relative bioavailability of ranitidine, which was most likely caused by muscarinic receptor blocking effects on intestinal motility and water turnover.

Renal secretion of hydrochlorothiazide involves organic anion transporter 1/3, organic cation transporter 2, and multidrug and toxin extrusion protein 2-K

Hydrochlorothiazide (HCTZ) is the most widely used thiazide diuretic for the treatment of hypertension either alone or in combination with other antihypertensives. HCTZ is mainly cleared by the kidney via tubular secretion, but the underlying molecular mechanisms are unclear. Using cells stably expressing major renal organic anion and cation transporters [human organic anion transporter 1 (hOAT1), human organic anion transporter 3 (hOAT3), human organic cation transporter 2 (hOCT2), human multidrug and toxin extrusion 1 (hMATE1), and human multidrug and toxin extrusion 2-K (hMATE2-K)], we found that HCTZ interacted with both organic cation and anion transporters. Uptake experiments further showed that HCTZ is transported by hOAT1, hOAT3, hOCT2, and hMATE2-K but not by hMATE1. Detailed kinetic analysis coupled with quantification of membrane transporter proteins by targeted proteomics revealed that HCTZ is an excellent substrate for hOAT1 and hOAT3. The apparent affinities (K_m) for hOAT1 and hOAT3 were 112 ± 8 and 134 ± 13 μM, respectively, and the calculated turnover numbers (k_cat) were 2.48 and 0.79 s^-1, respectively. On the other hand, hOCT2 and hMATE2-K showed much lower affinity for HCTZ. The calculated transport efficiency (k_cat/K_m) at the single transporter level followed the rank order of hOAT1> hOAT3 > hOCT2 and hMATE2-K, suggesting a major role of organic anion transporters in tubular secretion of HCTZ. In vitro inhibition experiments further suggested that HCTZ is not a clinically relevant inhibitor for hOAT1 or hOAT3. However, strong in vivo inhibitors of hOAT1/3 may alter renal secretion of HCTZ. Together, our study elucidated the molecular mechanisms underlying renal handling of HCTZ and revealed potential pathways involved in the disposition and drug-drug interactions for this important antihypertensive drug in the kidney.

Physiologically-Based Pharmacokinetic Modeling for Drug-Drug Interactions of Procainamide and N-Acetylprocainamide with Cimetidine, an Inhibitor of rOCT2 and rMATE1, in Rats

Previous observations demonstrated that cimetidine decreased the clearance of procainamide (PA) and/or N-acetylprocainamide (NAPA; the primary metabolite of PA) resulting in the increased systemic exposure and the decrease of urinary excretion. Despite an abundance of in vitro and in vivo data regarding pharmacokinetic interactions between PA/NAPA and cimetidine, however, a mechanistic approach to elucidate these interactions has not been reported yet. The primary objective of this study was to construct a physiological model that describes pharmacokinetic interactions between PA/NAPA and cimetidine, an inhibitor of rat organic cation transporter 2 (rOCT2) and rat multidrug and toxin extrusion proteins (rMATE1), by performing extensive in vivo and in vitro pharmacokinetic studies for PA and NAPA performed in the absence or presence of cimetidine in rats. When a single intravenous injection of PA HCl (10 mg/kg) was administered to rats, co-administration of cimetidine (100 mg/kg) significantly increased systemic exposure and decreased the systemic (CL) and renal (CL_R) clearance of PA, and reduced its tissue distribution. Similarly, cimetidine significantly decreased the CL_R of NAPA formed by the metabolism of PA and increased the AUC of NAPA. Considering that these drugs could share similar renal secretory pathways (e.g., via rOCT2 and rMATE1), a physiologically-based pharmacokinetic (PBPK) model incorporating semi-mechanistic kidney compartments was devised to predict drug-drug interactions (DDIs). Using our proposed PBPK model, DDIs between PA/NAPA and cimetidine were successfully predicted for the plasma concentrations and urinary excretion profiles of PA and NAPA observed in rats. Moreover, sensitivity analyses of the pharmacokinetics of PA and NAPA showed the inhibitory effects of cimetidine via rMATE1 were probably important for the renal elimination of PA and NAPA in rats. The proposed PBPK model may be useful for understanding the mechanisms of interactions between PA/NAPA and cimetidine in vivo.

Interplay of the Organic Cation Transporters OCT1 and OCT2 with the Apically Localized Export Protein MATE1 for the Polarized Transport of Trospium

The anticholinergic drug trospium is secreted into urine and, to a smaller extent, into bile. Chemically, it is an organic cation, and it is a substrate of the uptake transporters OCT1 and OCT2 as well as for the export proteins MATE1 and MATE2-K as determined in uptake studies using HEK293 cells. So far, neither MATE-mediated export nor the interplay of OCT-mediated uptake and MATE-mediated export have been investigated. Therefore, we used polarized monolayers of single- and double-transfected MDCKII cells (MDCK-OCT1, MDCK-OCT2, MDCK-MATE1, MDCK-OCT1-MATE1, and MDCK-OCT2-MATE1) and the respective control cells (MDCK-Co) for transcellular transport assays. We demonstrate that the transcellular, basal-to-apical transport of trospium is significantly higher in all cell lines compared to control cells over nearly the complete concentration range tested. The transcellular transport mediated by double-transfected MDCK-OCT1-MATE1 and MDCK-OCT2-MATE1 exceeded that in the single-transfected cells (MDCK-OCT1-MATE1 vs MDCK-OCT1: 2.2-fold; MDCK-OCT1-MATE1 vs MDCK-MATE1: 1.7-fold; MDCK-OCT2-MATE1 vs MDCK-OCT2: 6.1-fold; MDCK-OCT2-MATE1 vs MDCK-MATE1: 1.8-fold at a trospium concentration of 1.0 μM; p < 0.001 each). Thus, we show that MATE1 does not only mediate the uptake of trospium into HEK293 cells but also the efflux of trospium out of polarized MDCKII-cells. Furthermore, our results indicate that OCT1 or OCT2 as uptake transporters and MATE1 as an export protein contribute to the transcellular transport of trospium at concentrations normally reached during trospium therapy. These data suggest that both, OCT-mediated uptake as well as MATE1-mediated efflux may contribute to trospium renal and biliary elimination.

Roles of Renal Drug Transporter in Drug Disposition and Renal Toxicity

The kidney plays an important role in maintaining total body homeostasis and eliminating toxic xenobiotics and metabolites. Numerous drugs and their metabolites are ultimately eliminated in the urine. The reabsorption and secretion functions of the nephron are mediated by a variety of transporters located in the basolateral and luminal membranes of the tubular cells. In the past decade, many studies indicated that transporters play important roles in drug pharmacokinetics and demonstrated the impact of renal transporters on the disposition of drugs, drug-drug interactions, and nephrotoxicities. Here, we focus on several important renal transporters and their roles in drug elimination and disposition, drug-induced nephrotoxicities and potential clinical solutions.

The role of various transporters in the placental uptake of ofloxacin in an in vitro model of human villous trophoblasts

Introduction

Six years after the US Food and Drug Administration approval of the broad-spectrum antibiotic ofloxacin (OFLX), the chiral switching of this racemic mixture resulted in a drug composed of the L-optical isomer levofloxacin (LVFX). Since both fluoroquinolones (FQs) were introduced to the pharmaceutical market, they have been widely prescribed by physicians, with careful administration during pregnancy and breastfeeding. Therefore, the role of the influx and efflux placental transporters in the concentrations of these drugs that permeate through human placental barrier model was investigated in this study.

Methods

The contribution of major carriers on the transplacental flux of OFLX and LVFX uptake into choriocarcinoma BeWo cells was evaluated in the presence vs absence of well-known inhibitors.

Results

Our results reveal that neither the influx transporters such as organic cation transporters, organic anion transporters, and monocarboxylate transporters nor the efflux transporters such as P-glycoprotein or breast cancer resistance protein significantly affected the transport of OFLX. In contrast, multiple transporters revealed pronounced involvement in the transfer of the levorotatory enantiomer in and out of the in vitro placental barrier. These data suggest a non-carrier-mediated mechanism of transport of the racemic mixture, while LVFX is subjected to major influx and efflux passage through the placental brush border membranes.

Conclusion

This study provides underlying insights to elucidate the governing factors that influence the flux of these FQs through organ barriers, in view of the controversial safety profile of these drugs in pregnant population.

Importance of OCT2 and MATE1 for the Cimetidine-Metformin Interaction: Insights from Investigations of Polarized Transport in Single- And Double-Transfected MDCK Cells with a Focus on Perpetrator Disposition

Cimetidine decreases the renal clearance of metformin by inhibition of renal tubular cation transport, and the underlying molecular mechanisms are still not fully understood. We investigated polarized metformin transport without and with the addition of cimetidine as well as polarized cimetidine transport in double-transfected MDCK-OCT2-MATE1 cells that mimic organic cation transport processes in proximal renal tubule cells and in MDCK vector control and single-transfected MDCK-OCT2 and MDCK-MATE1 cells. At all tested concentrations (1, 10, 100 μM), the intracellular accumulation of cimetidine after administration to the basal compartment was considerably higher in MDCK-OCT2 cells compared to that in all other cells ( p < 0.001). Whereas cimetidine transcellular, basal-to-apical transport was only slightly higher in MDCK-OCT2 cells, the presence of MATE1 in the apical membrane caused a pronounced translocation of cimetidine in both single- and double-transfected cells ( p < 0.001). Transcellular, basal-to-apical metformin net transport was reduced by 89.1, 74.5, and 91.0% in MDCK-OCT2-MATE1 cells after the addition of cimetidine (100 μM) to the basal, the apical, or both compartments ( p < 0.001). In MDCK-MATE1 and MDCK-OCT2-MATE1 cells, transcellular net transport of metformin was inhibited by cimetidine with IC₅₀ values of 8.0 and 6.6 μM, respectively. Our data confirm the relevance of MATE1 and suggest the relevance of OCT2 for the cimetidine-metformin interaction, primarily because OCT2 mediates uptake of the perpetrator cimetidine into renal proximal tubular cells and thereby to the site of the metformin exporter MATE1. This work supports the notion that a thorough understanding of transporter-mediated drug-drug interactions may require investigations on the impact of transporters on cellular uptake and transcellular transport of victim as well as perpetrator drugs.

Quantification of Hepatic Organic Anion Transport Proteins OAT2 and OAT7 in Human Liver Tissue and Primary Hepatocytes

Organic anion transporter (OAT) 2 and OAT7 were recently shown to be involved in the hepatic uptake of drugs; however, there is limited understanding of the population variability in the expression of these transporters in liver. There is also a need to derive relative expression-based scaling factors (REFs) that can be used to bridge in vitro functional data to the in vivo drug disposition. To this end, we quantified OAT2 and OAT7 surrogate peptide abundance in a large number of human liver tissue samples ( n = 52), as well as several single-donor cryopreserved human hepatocyte lots ( n = 30) by a novel, validated liquid chromatography tandem mass spectrometry (LC-MS/MS) method. The average surrogate peptide expression of OAT2 and OAT7 in the liver samples was 1.52 ± 0.57 and 4.63 ± 1.58 fmol/μg membrane protein, respectively. While we noted statistically significant differences ( p < 0.05) in hepatocyte and liver tissue abundances for both OAT2 and OAT7, the differences were relatively small (1.8- and 1.5-fold difference in median values, respectively). Large interindividual variability was noted in the hepatic expression of OAT2 (16-fold in liver tissue and 23-fold in hepatocytes). OAT7, on the other hand, showed less interindividual variability (4-fold) in the livers, but high variability for the hepatocyte lots (27-fold). A significant positive correlation in OAT2 and OAT7 expression was observed, but expression levels were neither associated with age nor sex. In conclusion, our data suggest marked interindividual variability in the hepatic expression of OAT2/7, which may contribute to the pharmacokinetic variability of their substrates. Because both transporters were less abundant in hepatocytes than livers, a REF-based approach is recommended when scaling in vitro hepatocyte transport data to predict hepatic drug clearance and liver exposure of OAT2/7 substrates.

Genetic Heterogeneity of SLC22 Family of Transporters in Drug Disposition

An important aspect of modern medicine is its orientation to achieve more personalized pharmacological treatments. In this context, transporters involved in drug disposition have gained well-justified attention. Owing to its broad spectrum of substrate specificity, including endogenous compounds and xenobiotics, and its strategical expression in organs accounting for drug disposition, such as intestine, liver and kidney, the SLC22 family of transporters plays an important role in physiology, pharmacology and toxicology. Among these carriers are plasma membrane transporters for organic cations (OCTs) and anions (OATs) with a marked overlap in substrate specificity. These two major clades of SLC22 proteins share a similar membrane topology but differ in their degree of genetic variability. Members of the OCT subfamily are highly polymorphic, whereas OATs have a lower number of genetic variants. Regarding drug disposition, changes in the activity of these variants affect intestinal absorption and target tissue uptake, but more frequently they modify plasma levels due to enhanced or reduced clearance by the liver and secretion by the kidney. The consequences of these changes in transport-associated function markedly affect the effectiveness and toxicity of the treatment in patients carrying the mutation. In solid tumors, changes in the expression of these transporters and the existence of genetic variants substantially determine the response to anticancer drugs. Moreover, chemoresistance usually evolves in response to pharmacological and radiological treatment. Future personalized medicine will require monitoring these changes in a dynamic way to adapt the treatment to the weaknesses shown by each tumor at each stage in each patient.

Effects of genetic polymorphisms on the OCT1 and OCT2-mediated uptake of ranitidine

Background

Ranitidine (Zantac^®) is a H₂-receptor antagonist commonly used for the treatment of acid-related gastrointestinal diseases. Ranitidine was reported to be a substrate of the organic cation transporters OCT1 and OCT2. The hepatic transporter OCT1 is highly genetically variable. Twelve major alleles confer partial or complete loss of OCT1 activity. The effects of these polymorphisms are highly substrate-specific and therefore difficult to predict. The renal transporter OCT2 has a common polymorphism, Ala270Ser, which was reported to affect OCT2 activity.

Aim

In this study we analyzed the effects of genetic polymorphisms in OCT1 and OCT2 on the uptake of ranitidine and on its potency to inhibit uptake of other drugs.

Methods and results

We characterized ranitidine uptake using HEK293 and CHO cells stably transfected to overexpress wild type OCT1, OCT2, or their naturally occurring allelic variants. Ranitidine was transported by wild-type OCT1 with a K_m of 62.9 μM and a v_max of 1125 pmol/min/mg protein. Alleles OCT1*5, *6, *12, and *13 completely lacked ranitidine uptake. Alleles OCT1*2, *3, *4, and *10 had v_max values decreased by more than 50%. In contrast, OCT1*8 showed an increase of v_max by 25%. The effects of OCT1 alleles on ranitidine uptake strongly correlated with the effects on morphine uptake suggesting common interaction mechanisms of both drugs with OCT1. Ranitidine inhibited the OCT1-mediated uptake of metformin and morphine at clinically relevant concentrations. The inhibitory potency for morphine uptake was affected by the OCT1*2 allele. OCT2 showed only a limited uptake of ranitidine that was not significantly affected by the Ala270Ser polymorphism.

Conclusions

We confirmed ranitidine as an OCT1 substrate and demonstrated that common genetic polymorphisms in OCT1 strongly affect ranitidine uptake and modulate ranitidine’s potential to cause drug-drug interactions. The effects of the frequent OCT1 polymorphisms on ranitidine pharmacokinetics in humans remain to be analyzed.

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.

In vitro characterization of zebrafish (Danio rerio) organic anion transporters Oat2a-e

OATS/Oats are transmembrane proteins that transport a variety of drugs, environmental toxins and endogenous metabolites into the cell. Zebrafish (Danio rerio) has seven OAT orthologs: Oat1, Oat2a-e and Oat3. In this study we specifically address Oat2 (Slc22a7) family. Conserved synteny analysis showed localization of zebrafish oat2 genes on two chromosomes, 11 and 17. All five zebrafish Oats were localized by live cell imaging in membranes of transiently transfected HEK293-T cells, and Oat2a, b, d, and e were confirmed using western blot analysis. Functional studies using the HEK293T cells overexpressing zebrafish Oats revealed two model fluorescent substrates of three Oats: Lucifer yellow for Oat2a and Oat2d (Km 122, and 49.7μM), and 6-carboxyfluorescein for Oat2b and Oat2d (Km 199.7, and 266.9μM). The initial screening of a series of diverse endo- and xenobiotics showed interaction with a number of compounds, including cGMP and diclofenac (IC50 27.74, and 19.14μM) with Oat2a; estrone-3-sulfate and diclofenac (IC50 30.96, and 12.6μM) with Oat2b; and fumarate and indomethacin (IC50 68.24, and 20.41μM) with Oat2d. This study provides the first comprehensive data set on Oat2 in zebrafish and offers an important basis for more detailed molecular and (eco)toxicological characterizations of these transporters.

The conditional stimulation of rat organic cation transporter 2, but not its human ortholog, by mesoridazine: the possibility of the involvement of the high-affinity binding site of the transporter in the stimulation

Objectives

To study the functional consequences of the human and rat forms of OCT2 in the presence of phenothiazines.

Methods

MDCK cells expressing human or rat OCT2 were established, and MPP+ transport was determined by uptake assays. Concentration dependency was studied for the stimulatory/inhibitory effects of phenothiazines on MPP+ transport.

Key findings

Among the 11 phenothiazines examined, the majority were found to have comparable effects on transporter function between the orthologous forms, while three phenothiazines, particularly mesoridazine, had complex impacts on transporter function. For rOCT2, mesoridazine stimulated transport at 0.1 and 1 μmMPP+ with the mesoridazine concentration–uptake curve becoming bell-shaped. This conditional effect became less pronounced at 30 μmMPP+, resulting in an inhibition curve with a typical profile. For hOCT2, mesoridazine behaved as a typical inhibitor of transporter function at all MPP+ concentrations, although the kinetics of inhibition were still affected by the substrate concentration.

Conclusions

The conditional stimulation by mesoridazine in rOCT2, and the lack thereof in hOCT2, may be a manifestation of the interaction of phenothiazine with substrate binding at the high-affinity site of the OCT2. As OCT2 was previously indicated in some drug–drug interactions, the conditional stimulation of OCT2 and its potential species-differences may be of practical relevance.

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.

Urinary Excretion of Tetrodotoxin Modeled in a Porcine Renal Proximal Tubule Epithelial Cell Line, LLC-PK₁

This study examined the urinary excretion of tetrodotoxin (TTX) modeled in a porcine renal proximal tubule epithelial cell line, LLC-PK₁. Time course profiles of TTX excretion and reabsorption across the cell monolayers at 37 °C showed that the amount of TTX transported increased linearly for 60 min. However, at 4 °C, the amount of TTX transported was approximately 20% of the value at 37 °C. These results indicate that TTX transport is both a transcellular and carrier-mediated process. Using a transport inhibition assay in which cell monolayers were incubated with 50 µM TTX and 5 mM of a transport inhibitor at 37 °C for 30 min, urinary excretion was significantly reduced by probenecid, tetraethylammonium (TEA), l-carnitine, and cimetidine, slightly reduced by p-aminohippuric acid (PAH), and unaffected by 1-methyl-4-phenylpyridinium (MPP+), oxaliplatin, and cefalexin. Renal reabsorption was significantly reduced by PAH, but was unaffected by probenecid, TEA and l-carnitine. These findings indicate that TTX is primarily excreted by organic cation transporters (OCTs) and organic cation/carnitine transporters (OCTNs), partially transported by organic anion transporters (OATs) and multidrug resistance-associated proteins (MRPs), and negligibly transported by multidrug and toxic compound extrusion transporters (MATEs).

Evaluation of transporters in drug development: Current status and contemporary issues

Transporters govern the access of molecules to cells or their exit from cells, thereby controlling the overall distribution of drugs to their intracellular site of action. Clinically relevant drug-drug interactions mediated by transporters are of increasing interest in drug development. Drug transporters, acting alone or in concert with drug metabolizing enzymes, can play an important role in modulating drug absorption, distribution, metabolism and excretion, thus affecting the pharmacokinetics and/or pharmacodynamics of a drug. The drug interaction guidance documents from regulatory agencies include various decision criteria that may be used to predict the need for in vivo assessment of transporter-mediated drug-drug interactions. Regulatory science research continues to assess the prediction performances of various criteria as well as to examine the strength and limitations of each prediction criterion to foster discussions related to harmonized decision criteria that may be used to facilitate global drug development. This review discusses the role of transporters in drug development with a focus on methodologies in assessing transporter-mediated drug-drug interactions, challenges in both in vitro and in vivo assessments of transporters, and emerging transporter research areas including biomarkers, assessment of tissue concentrations, and effect of diseases on transporters.

Differential Interaction of Dantrolene, Glafenine, Nalidixic Acid, and Prazosin with Human Organic Anion Transporters 1 and 3

In renal proximal tubule cells, the organic anion transporters 1 and 3 (OAT1 and OAT3) in the basolateral membrane and the multidrug resistance-associated protein 4 (MRP4) in the apical membrane share substrates and co-operate in renal drug secretion. We hypothesized that recently identified MRP4 inhibitors dantrolene, glafenine, nalidixic acid, and prazosin also interact with human OAT1 and/or OAT3 stably transfected in human embryonic kidney 293 cells. These four drugs were tested as possible inhibitors of p-[³H]aminohippurate (PAH) and [¹⁴C]glutarate uptake by OAT1, and of [³H]estrone-3-sulfate (ES) uptake by OAT3. In addition, we explored whether these drugs decrease the equilibrium distribution of radiolabeled PAH, glutarate, or ES, an approach intended to indirectly suggest drug/substrate exchange through OAT1 and OAT3. With OAT3, a dose-dependent inhibition of [³H]ES uptake and a downward shift in [³H]ES equilibrium were observed, indicating that all four drugs bind to OAT3 and may possibly be translocated. In contrast, the interaction with OAT1 was more complex. With [¹⁴C]glutarate as substrate, all four drugs inhibited uptake but only glafenine and nalidixic acid shifted glutarate equilibrium. Using [³H]PAH as a substrate of OAT1, nalidixic acid inhibited but dantrolene, glafenine, and prazosin stimulated uptake. Nalidixic acid decreased equilibrium content of [³H]PAH, suggesting that it may possibly be exchanged by OAT1. Taken together, OAT1 and OAT3 interact with the MRP4 inhibitors dantrolene, glafenine, nalidixic acid, and prazosin, indicating overlapping specificities. At OAT1, more than one binding site must be assumed to explain substrate and drug-dependent stimulation and inhibition of transport activity.

Acute effects of focused ultrasound-induced increases in blood-brain barrier permeability on rat microvascular transcriptome

Therapeutic treatment options for central nervous system diseases are greatly limited by the blood-brain barrier (BBB). Focused ultrasound (FUS), in conjunction with circulating microbubbles, can be used to induce a targeted and transient increase in BBB permeability, providing a unique approach for the delivery of drugs from the systemic circulation into the brain. While preclinical research has demonstrated the utility of FUS, there remains a large gap in our knowledge regarding the impact of sonication on BBB gene expression. This work is focused on investigating the transcriptional changes in dorsal hippocampal rat microvessels in the acute stages following sonication. Microarray analysis of microvessels was performed at 6 and 24 hrs post-FUS. Expression changes in individual genes and bioinformatic analysis suggests that FUS may induce a transient inflammatory response in microvessels. Increased transcription of proinflammatory cytokine genes appears to be short-lived, largely returning to baseline by 24 hrs. This observation may help to explain some previously observed bioeffects of FUS and may also be a driving force for the angiogenic processes and reduced drug efflux suggested by this work. While further studies are necessary, these results open up intriguing possibilities for novel FUS applications and suggest possible routes for pharmacologically modifying the technique.

Organic Anion Transporter 2: An Enigmatic Human Solute Carrier

As a member of the solute carrier 22A (SLC22A) family, organic anion transporter 2 (OAT2; SLC22A7) is emerging as an important drug transporter because of its expression in both the liver and kidney, two major eliminating organs, and its ability to transport not only a wide variety of xenobiotics but also numerous physiologically important endogenous compounds, like creatinine and cGMP. However, OAT2 has received relatively little attention compared with other OATs and solute carriers (SLCs), like organic cation transporters, sodium-dependent taurocholate cotransporting polypeptide, multidrug and toxin extrusion proteins, and organic anion-transporting polypeptides. Overall, the literature describing OAT2 is rapidly evolving, with numerous publications contradicting each other regarding the transport mechanism, tissue distribution, and transport of creatinine and cGMP, two important endogenous OAT2 substrates. Despite its status as a liver and kidney SLC, tools for assessing its activity and inhibition are lacking, and its role in drug disposition and elimination remains to be defined. The current review focuses on the available and emerging literature describing OAT2. We envision that OAT2 will gain more prominence as its expression, substrate, and inhibitor profile is investigated further and compared with other SLCs.

Impact of Substrate-Dependent Inhibition on Renal Organic Cation Transporters hOCT2 and hMATE1/2-K-Mediated Drug Transport and Intracellular Accumulation

Renal transporter-mediated drug-drug interactions (DDIs) are of significant clinical concern, as they can adversely impact drug disposition, efficacy, and toxicity. Emerging evidence suggests that human renal organic cation transporter 2 (hOCT2) and multidrug and toxin extrusion proteins 1 and 2-K (hMATE1/2-K) exhibit substrate-dependent inhibition, but their impact on renal drug secretion and intracellular accumulation is unknown. Using metformin and atenolol as the probe substrates, we found that the classic inhibitors (e.g., cimetidine) of renal organic cation secretion were approximately 10-fold more potent for hOCT2 when atenolol was used, suggesting that atenolol is a more sensitive in vitro substrate for hOCT2 than metformin. In contrast, inhibition of hMATE1/2-K was influenced much less by the choice of substrate. Cimetidine is a much more potent inhibitor for hMATE1/2-K when metformin is the substrate but acts as an equally potent inhibitor of hOCT2 and hMATE1/2-K when atenolol is the substrate. Using hOCT2/hMATE1 double-transfected Madin-Darby canine kidney cells, we evaluated the impact of substrate-dependent inhibition on hOCT2/hMATE1-mediated transepithelial flux and intracellular drug accumulation. At clinically relevant concentrations, cimetidine dose dependently inhibited basal-to-apical flux of atenolol and metformin but impacted their intracellular accumulation differently, indicating that substrate-dependent inhibition may shift the major substrate-inhibitor interaction site between apical and basolateral transporters. Cimetidine is effective only when applied to the basal compartment. Our findings revealed the complex and dynamic nature of substrate-dependent inhibition of renal organic cation drug transporters and highlighted the importance of considering substrate-dependent inhibition in predicting transporter-mediated renal drug interaction, accumulation, and toxicity.

Gene expression study of phase I and II metabolizing enzymes in RPTEC/TERT1 cell line: application in in vitro nephrotoxicity prediction

1. The phase I and II metabolizing enzymes of kidneys play an important role in the metabolism of xenobiotic as well as endogenous compounds and proximal tubules of kidney constitute high concentration of these metabolizing enzymes compared with the other parts. 2. It has been shown previously that differential enzyme expression among human and rodent/non-rodent species can be a roadblock in drug discovery and development process. Currently, proximal tubule cell lines of human origin such as RPTEC/TERT1 and HK-2 are used to understand the pathophysiology of kidney diseases, therapeutic efficacy of drugs, and nephrotoxicity of compounds. 3. The purpose of the present study is to understand the metabolic enzymes present in RPTEC/TERT1 and HK-2 cell lines that would help to interpret and predict probable in vitro behavior of the molecule being tested. 4. We analyzed the expression of phase I and II metabolizing enzymes of RPTEC/TERT1 and HK-2 cell lines. We found equal expression of CYP1B1, 2J2, 3A4, 3A5, UGT1A9, SULT2A1 and GSTA, higher expression of 2B6, 2D6, 4A11, 4F2, 4F8, 4F11, UGT2B7, SULT1E1 in RPTEC/TERT1 and absence of GSTT in RPTEC/TERT1 compared to HK-2 at mRNA level. Such differences can affect the outcome of in vitro nephrotoxicity prediction.

Renal drug transporters and their significance in drug-drug interactions

The kidney is a vital organ for the elimination of therapeutic drugs and their metabolites. Renal drug transporters, which are primarily located in the renal proximal tubules, play an important role in tubular secretion and reabsorption of drug molecules in the kidney. Tubular secretion is characterized by high clearance capacities, broad substrate specificities, and distinct charge selectivity for organic cations and anions. In the past two decades, substantial progress has been made in understanding the roles of transporters in drug disposition, efficacy, toxicity and drug-drug interactions (DDIs). In the kidney, several transporters are involved in renal handling of organic cation (OC) and organic anion (OA) drugs. These transporters are increasingly recognized as the target for clinically significant DDIs. This review focuses on the functional characteristics of major human renal drug transporters and their involvement in clinically significant DDIs.

Species specificity profiling of rat and human organic cation/carnitine transporter Slc22a5/SLC22A5 (Octn2/OCTN2)

The purpose of this study was to characterize the uptake of carnitine, the physiological substrate, and the uptake of 3-(2,2,2-trimethylhydrazinium)propionate, a consensus substrate by rat Octn2 and human OCTN2 transporters as well as to characterize drug-mediated inhibition of l-carnitine uptake by the rat and human orthologs overexpressed in CHO-K1 cells. l-carnitine and 3-(2,2,2-trimethylhydrazinium)propionate were found to be a lower affinity substrate for rat Octn2 (K_M = 32.66 ± 5.11 μM and 23.62 ± 4.99 μM respectively) than for human OCTN2 (K_M = 3.08 ± 0.74 μM and 7.98 ± 0.63 μM). The intrinsic clearance (CL_int) value for carnitine was higher for the human than for the rat transporter (22.82 ± 5.57 ml/min*mg vs 4.008 ± 0.675 ml/min*mg). For 3-(2,2,2-trimethylhydrazinium)propionate, in contrast, the CL_int value for rat Octn2 was higher than for human OCTN2 (323.9 ± 72.8 ml/min*mg vs 65.11 ± 5.33 ml/min*mg). Furthermore, many pharmacologically important drugs were shown to affect l-carnitine transport by Octn2/OCTN2. The correlation between the IC₅₀ datasets for the rat and human transporter resulted in an r value of 0.47 (p > 0.05). However, the greatest difference was less than seven-fold and 13 of 15 compounds yielded a difference less than 3-fold. Thus, the transporters from these two species showed an overlapping but somewhat different substrate and inhibitor specificity.

Identification and Quantitative Assessment of Uremic Solutes as Inhibitors of Renal Organic Anion Transporters, OAT1 and OAT3

One of the characteristics of chronic kidney disease (CKD) is the accumulation of uremic solutes in the plasma. Less is known about the effects of uremic solutes on transporters that may play critical roles in pharmacokinetics. We evaluated the effect of 72 uremic solutes on organic anion transporter 1 and 3 (OAT1 and OAT3) using a fluorescent probe substrate, 6-carboxyfluorescein. A total of 12 and 13 solutes were identified as inhibitors of OAT1 and OAT3, respectively. Several of them inhibited OAT1 or OAT3 at clinically relevant concentrations and reduced the transport of other OAT1/3 substrates in vitro. Review of clinical studies showed that the active secretion of most drugs that are known substrates of OAT1/3 deteriorated faster than the renal filtration in CKD. Collectively, these data suggest that through inhibition of OAT1 and OAT3, uremic solutes contribute to the decline in renal drug clearance in patients with CKD.

Upscaling of a living membrane for bioartificial kidney device

The limited removal of metabolic waste products in dialyzed kidney patients leads to high morbidity and mortality. One powerful solution for a more complete removal of those metabolites might be offered by a bioartificial kidney device (BAK), which contains a hybrid "living membrane" with functional proximal tubule epithelial cells (PTEC). These cells are supported by an artificial functionalized hollow fiber membrane (HFM) and are able to actively remove the waste products. In our earlier studies, conditionally immortalized human PTEC (ciPTEC) showed to express functional organic cationic transporter 2 (OCT2) when seeded on small size flat or hollow fiber polyethersulfone (PES) membranes. Here, an upscaled "living membrane" is presented. We developed and assessed the functionality of modules containing three commercially available MicroPES HFM supporting ciPTEC. The HFM were optimally coated with L-Dopa and collagen IV to support a uniform and tight monolayer formation of matured ciPTEC under static culturing conditions. Both abundant expression of zonula occludens-1 (ZO-1) protein and limited diffusion of FITC-inulin confirm a clear barrier function of the monolayer. Furthermore, the uptake of 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP⁺), a fluorescent OCT2 substrate, was studied in absence and presence of known OCT inhibitors, such as cimetidine and a cationic uremic solutes mixture. The ASP⁺ uptake by the living upscaled membrane was decreased by 60% in the presence of either inhibitor, proving the active function of OCT2. In conclusion, this study presents a successful upscaling of a living membrane with active organic cation transport as a support for BAK device.

Metformin and cimetidine: Physiologically based pharmacokinetic modelling to investigate transporter mediated drug-drug interactions

Metformin is used as a probe for OCT2 mediated transport when investigating possible DDIs with new chemical entities. The aim of the current study was to investigate the ability of physiologically-based pharmacokinetic (PBPK) models to simulate the effects of OCT and MATE inhibition by cimetidine on metformin kinetics. PBPK models were developed, incorporating mechanistic kidney and liver sub-models for metformin (OCT and MATE substrate) and a mechanistic kidney sub-model for cimetidine. The models were used to simulate inhibition of the MATE1, MATE2-K, OCT1 and OCT2 mediated transport of metformin by cimetidine. Assuming competitive inhibition and using cimetidine Ki values determined in vitro, the predicted metformin AUC ratio was 1.0 compared to an observed value of 1.46. The observed AUC ratio could only be recovered with this model when the cimetidine Ki for OCT2 was decreased 1000-fold or the Ki's for both OCT1 and OCT2 were decreased 500-fold. An alternative description of metformin renal transport by OCT1 and OCT2, incorporating electrochemical modulation of the rate of metformin uptake together with 8-18-fold decreases in cimetidine Ki's for OCTs and MATEs, allowed recovery of the extent of the observed effect of cimetidine on metformin AUC. While the final PBPK model has limitations, it demonstrates the benefit of allowing for the complexities of passive permeability combined with active cellular uptake modulated by an electrochemical gradient and active efflux.