Investigation of Endogenous Compounds for Assessing the Drug Interactions in the Urinary Excretion Involving Multidrug and Toxin Extrusion Proteins

Unlocking mitochondrial drug targets: The importance of mitochondrial transport proteins

A disturbed mitochondrial function contributes to the pathology of many common diseases. These organelles are therefore important therapeutic targets. On the contrary, many adverse effects of drugs can be explained by a mitochondrial off-target effect, in particular, due to an interaction with carrier proteins in the inner membrane. Yet this class of transport proteins remains underappreciated and understudied. The aim of this review is to provide a deeper understanding of the role of mitochondrial carriers in health and disease and their significance as drug targets. We present literature-based evidence that mitochondrial carrier proteins are associated with prevalent diseases and emphasize their potential as drug (off-)target sites by summarizing known mitochondrial drug-transporter interactions. Studying these carriers will enhance our knowledge of mitochondrial drug on- and off-targets and provide opportunities to further improve the efficacy and safety of drugs.

A Metabolomic Analysis of Sensitivity and Specificity of 23 Previously Proposed Biomarkers for Renal Transporter-Mediated Drug-Drug Interactions

Endogenous biomarkers are discussed as tools for detection of drug-drug interactions (DDIs) mediated by renal transport proteins, such as organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATE1 and MATE2-K) and organic anion transporters (OAT1 and OAT3). Whereas sensitivity of some endogenous biomarkers against at least one clinical transporter inhibitor has frequently been shown, intra-study comparisons of the extent of effects of inhibitors on different biomarkers are frequently lacking. Moreover, in vivo specificity of such discussed biomarkers has frequently not been studied. We therefore investigated changes of 10 previously described putative biomarkers for inhibition of OCT2/MATEs, as well as 15 previously described putative biomarkers for OATs in human plasma and urine samples of healthy volunteers in response to treatment with 4 inhibitors of transport proteins [verapamil (P-glycoprotein), rifampin (organic anion transporting polypeptides), cimetidine (OCT2/MATEs), and probenecid (OATs)]. Two of the putative biomarkers had been suggested for both OCT2/MATEs and OATs. All substances were unequivocally identified in an untargeted metabolomics assay. The OCT2/MATE biomarkers choline and trimethylamine N-oxide were both sensitive and specific (median log2-fold changes -1.18 in estimated renal elimination and -0.85 in urinary excretion, respectively). For renal OATs, indoleacetyl glutamine and indoleacetic acid (median log2-fold changes -3.77 and -2.85 in estimated renal elimination, respectively) were the candidates for sensitive and specific biomarkers with the most extensive change, followed by taurine, indolelactic acid, and hypoxanthine. This comprehensive study adds further knowledge on sensitivity and specificity of 23 previously described biomarkers of renal OCT2/MATE- and OAT-mediated DDIs.

Novel drug transporter substrates identification: An innovative approach based on metabolomic profiling, in silico ligand screening and biological validation

Solute carrier (SLC) transport proteins are fundamental for the translocation of endogenous compounds and drugs across membranes, thus playing a critical role in disease susceptibility and drug response. Because only a limited number of transporter substrates are currently known, the function of a large number of SLC transporters is elusive. Here, we describe the proof-of-concept of a novel strategy to identify SLC transporter substrates exemplarily for the proton-coupled peptide transporter (PEPT) 2 (SLC15A2) and multidrug and toxin extrusion (MATE) 1 transporter (SLC47A1), which are important renal transporters of drug reabsorption and excretion, respectively. By combining metabolomic profiling of mice with genetically-disrupted transporters, in silico ligand screening and in vitro transport studies for experimental validation, we identified nucleobases and nucleoside-derived anticancer and antiviral agents (flucytosine, cytarabine, gemcitabine, capecitabine) as novel drug substrates of the MATE1 transporter. Our data confirms the successful applicability of this new approach for the identification of transporter substrates in general, which may prove particularly relevant in drug research.

Progress in the Quantitative Assessment of Transporter-Mediated Drug-Drug Interactions Using Endogenous Substrates in Clinical Studies

Variations in drug transporter activities, caused by genetic polymorphism and drug-drug interactions (DDIs), alter the systemic exposure of substrate drugs, leading to differences in drug responses. Recently, some endogenous substrates of drug transporters, particularly the solute carrier family transporters such as OATP1B1, OATP1B3, OAT1, OAT3, OCT1, OCT2, MATE1, and MATE2-K, have been identified to investigate variations in drug transporters in humans. Clinical data obtained support their performance as surrogate probes in terms of specificity and reproducibility. Pharmacokinetic parameters of the endogenous biomarkers depend on the genotypes of drug transporters and the systemic exposure to perpetrator drugs. Furthermore, the development of physiologically based pharmacokinetic models for the endogenous biomarkers has enabled a top-down approach to obtain insights into the effect of perpetrators on drug transporters and to more precisely simulate the DDI with victim drugs, including probe drugs. The endogenous biomarkers can address the uncertainty in the DDI prediction in the preclinical and early phases of clinical development and have the potential to fulfill regulatory requirements. Therefore, the endogenous biomarkers should be able to predict disease effects on the variations in drug transporter activities observed in patients. This mini-review focuses on recent progress in the identification and use of the endogenous drug transporter substrate biomarkers and their application in drug development. SIGNIFICANCE STATEMENT: Advances in analytical methods have enabled the identification of endogenous substrates of drug transporters. Changes in the pharmacokinetic parameters (Cmax, AUC, or CLR) of these endogenous biomarkers relative to baseline values can serve as a quantitative index to assess variations in drug transporter activities during clinical studies and thereby provide more precise DDI predictions.

The Impacts of Slc19a3 Deletion and Intestinal SLC19A3 Insertion on Thiamine Distribution and Brain Metabolism in the Mouse

The Thiamine Transporter 2 (THTR2) encoded by SLC19A3 plays an ill-defined role in the maintenance of tissue thiamine, thiamine monophosphate, and thiamine diphosphate (TDP) levels. To evaluate the impact of THTR2 on tissue thiamine status and metabolism, we expressed the human SLC19A3 transgene in the intestine of total body Slc19a3 knockout (KO) mice. Male and female wildtype (WT) and transgenic (TG) mice were fed either 17 mg/kg (1×) or 85 mg/kg (5×) thiamine hydrochloride diet, while KOs were only fed the 5× diet. Thiamine vitamers in plasma, red blood cells, duodenum, brain, liver, kidney, heart, and adipose tissue were measured. Untargeted metabolomics were performed on the brain tissues of groups with equivalent plasma thiamine. KO mice had ~two- and ~three-fold lower plasma and brain thiamine levels than WT on the 5× diet. Circulating vitamers were sensitive to diet and equivalent in TG and WT mice. However, TG had 60% lower thiamine but normal brain TDP levels regardless of diet, with subtle differences in the heart and liver. The loss of THTR2 reduced levels of nucleic acid and amino acid derivatives in the brain. Therefore, mutation or inhibition of THTR2 may alter the brain metabolome and reduce the thiamine reservoir for TDP biosynthesis.

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.

Bile duct ligation differently regulates protein expressions of organic cation transporters in intestine, liver and kidney of rats through activation of farnesoid X receptor by cholate and bilirubin

Body is equipped with organic cation transporters (OCTs). These OCTs mediate drug transport and are also involved in some disease process. We aimed to investigate whether liver failure alters intestinal, hepatic and renal Oct expressions using bile duct ligation (BDL) rats. Pharmacokinetic analysis demonstrates that BDL decreases plasma metformin exposure, associated with decreased intestinal absorption and increased urinary excretion. Western blot shows that BDL significantly downregulates intestinal Oct2 and hepatic Oct1 but upregulates renal and hepatic Oct2. In vitro cell experiments show that chenodeoxycholic acid (CDCA), bilirubin and farnesoid X receptor (FXR) agonist GW4064 increase OCT2/Oct2 but decrease OCT1/Oct1, which are remarkably attenuated by glycine-β-muricholic acid and silencing FXR. Significantly lowered intestinal CDCA and increased plasma bilirubin levels contribute to different Octs regulation by BDL, which are confirmed using CDCA-treated and bilirubin-treated rats. A disease-based physiologically based pharmacokinetic model characterizing intestinal, hepatic and renal Octs was successfully developed to predict metformin pharmacokinetics in rats. In conclusion, BDL remarkably downregulates expressions of intestinal Oct2 and hepatic Oct1 protein while upregulates expressions of renal and hepatic Oct2 protein in rats, finally, decreasing plasma exposure and impairing hypoglycemic effects of metformin. BDL differently regulates Oct expressions via Fxr activation by CDCA and bilirubin.

Reimagining the Framework Supporting the Static Analysis of Transporter Drug Interaction Risk; Integrated Use of Biomarkers to Generate Pan‐Transporter Inhibition Signatures

Solute carrier (SLC) transporters present as the loci of important drug-drug interactions (DDIs). Therefore, sponsors generate in vitro half-maximal inhibitory concentration (IC₅₀ ) data and apply regulatory agency-guided "static" methods to assess DDI risk and the need for a formal clinical DDI study. Because such methods are conservative and high false-positive rates are likely (e.g., DDI study triggered when liver SLC R value ≥ 1.04 and renal SLC maximal unbound plasma (C_max,u )/IC₅₀ ratio ≥ 0.02), investigators have attempted to deploy plasma- and urine-based SLC biomarkers in phase I studies to de-risk DDI and obviate the need for drug probe-based studies. In this regard, it was possible to generate in-house in vitro SLC IC₅₀ data for various clinically (biomarker)-qualified perpetrator drugs, under standard assay conditions, and then estimate "% inhibition" for each SLC and relate it empirically to published clinical biomarker data (area under the plasma concentration vs. time curve (AUC) ratio (AUCR, AUC_inhibitor /AUC_reference ) and % decrease in renal clearance (ΔCL_renal )). After such a "calibration" exercise, it was determined that only compounds with high R values (> 1.5) and C_max,u /IC₅₀ ratios (> 0.5) are likely to significantly modulate liver (AUCR > 1.25) and renal (ΔCL_renal  > 25%) biomarkers and evoke DDI risk. The % inhibition approach supports integration of liver and renal SLC data and allows one to generate pan-SLC inhibition signatures for different test perpetrators (e.g., SLC % inhibition ranking). In turn, such signatures can guide the selection of the most appropriate individual (or combinations of) biomarkers for testing in phase I studies.

Endogenous Biomarkers for SLC Transporter-Mediated Drug-Drug Interaction Evaluation

Membrane transporters play an important role in the absorption, distribution, metabolism, and excretion of xenobiotic substrates, as well as endogenous compounds. The evaluation of transporter-mediated drug-drug interactions (DDIs) is an important consideration during the drug development process and can guide the safe use of polypharmacy regimens in clinical practice. In recent years, several endogenous substrates of drug transporters have been identified as potential biomarkers for predicting changes in drug transport function and the potential for DDIs associated with drug candidates in early phases of drug development. These biomarker-driven investigations have been applied in both preclinical and clinical studies and proposed as a predictive strategy that can be supplanted in order to conduct prospective DDIs trials. Here we provide an overview of this rapidly emerging field, with particular emphasis on endogenous biomarkers recently proposed for clinically relevant uptake transporters.

Transporter-mediated drug-drug interactions: advancement in models, analytical tools, and regulatory perspective

Drug-drug interactions mediated by transporters are a serious clinical concern hence a tremendous amount of work has been done on the characterization of the transporter-mediated proteins in humans and animals. The underlying mechanism for the transporter-mediated drug-drug interaction is the induction or inhibition of the transporter which is involved in the cellular uptake and efflux of drugs. Transporter of the brain, liver, kidney, and intestine are major determinants that alter the absorption, distribution, metabolism, excretion profile of drugs, and considerably influence the pharmacokinetic profile of drugs. As a consequence, transporter proteins may affect the therapeutic activity and safety of drugs. However, mounting evidence suggests that many drugs change the activity and/or expression of the transporter protein. Accordingly, evaluation of drug interaction during the drug development process is an integral part of risk assessment and regulatory requirements. Therefore, this review will highlight the clinical significance of the transporter, their role in disease, possible cause underlying the drug-drug interactions using analytical tools, and update on the regulatory requirement. The recent in-silico approaches which emphasize the advancement in the discovery of drug-drug interactions are also highlighted in this review. Besides, we discuss several endogenous biomarkers that have shown to act as substrates for many transporters, which could be potent determinants to find the drug-drug interactions mediated by transporters. Transporter-mediated drug-drug interactions are taken into consideration in the drug approval process therefore we also provided the extrapolated decision trees from in-vitro to in-vivo, which may trigger the follow-up to clinical studies.

Wernicke's encephalopathy - from basic science to clinical practice. Part 1: Understanding the role of thiamine

Wernicke's encephalopathy (WE) is an acute neuropsychiatric state. Untreated, WE can lead to coma or death, or progress to Korsakoff syndrome (KS) - a dementia characterized by irreversible loss of anterograde memory. Thiamine (vitamin B1) deficiency lies at the heart of this condition. Yet, our understanding of thiamine regarding prophylaxis and treatment of WE remains limited. This may contribute to the current undertreatment of WE in clinical practice. The overall aim of this review is to identify the best strategies for prophylaxis and treatment of WE in regard to (a) dose of thiamine, (b) mode of administration, (c) timing of switch from one mode of administration to another, (d) duration of administration, and (e) use of magnesium along thiamine as an essential cofactor. Evidence from randomized controlled trials and other intervention studies is virtually absent. Therefore, we have to resort to basic science for proof of principle instead. Here, we present the first part of our clinical review, in which we explore the physiology of thiamine and the pathophysiology of thiamine deficiency. We first explore both of these in their historical context. We then review the pharmacodynamics and pharmacokinetics of thiamine, exploring the roles of the six currently known thiamine compounds, their transporters, and target enzymes. We also explore the significance of magnesium as a cofactor in thiamine-facilitated enzymatic reactions and thiamine transport. In the second (forthcoming) part of this review, we will use the findings of the current review to make evidence-based inferences about strategies for prophylaxis and treatment of WE.

Current progress in identifying endogenous biomarker candidates for drug transporter phenotyping and their potential application to drug development

Drug transporters play important roles in the elimination of various compounds from the blood. Genetic variation and drug-drug interactions underlie the pharmacokinetic differences for the substrates of drug transporters. Some endogenous substrates of drug transporters have emerged as biomarkers to assess differences in drug transporter activity-not only in animals, but also in humans. Metabolomic analysis is a promising approach for identifying such endogenous substrates through their metabolites. The appropriateness of metabolites is supported by studies in vitro and in vivo, both in animals and through pharmacogenomic or drug-drug interaction studies in humans. This review summarizes current progress in identifying such endogenous biomarkers and applying them to drug transporter phenotyping.

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.

Recent progress in in vivo phenotyping technologies for better prediction of transporter-mediated drug-drug interactions

Clinical reports on transporter-mediated drug-drug interactions (TP-DDIs) have rapidly accumulated and regulatory guidance/guidelines recommend that sponsors consider performing quantitative prediction of TP-DDI risks in the process of drug development. In vitro experiments for characterizing the function of drug transporters have been established and various parameters such as the inhibition constant (K_i) of drugs and the intrinsic uptake/efflux clearance for a certain transporter can be obtained. However, many reports have indicated large discrepancies between the parameters estimated from in vitro experiments and those rationally explaining drug pharmacokinetics. Thus, it is essential to evaluate directly the function of each transporter isoform in vivo in humans. At present, several transporter substrate drugs and endogenous compounds have been recognized as probe substrates for a specific transporter and transporter function was evaluated by monitoring the plasma and urine concentration of those probes; however, few compounds specifically transported via a single transporter isoform have been found. For monitoring the intraorgan concentration of drugs, positron emission tomography can be a powerful tool and clinical examples for quantification of in vivo transporter function have been published. In this review, novel methodologies for in vivo phenotyping of transporter function are summarized.

Mechanisms of Non-coenzyme Action of Thiamine: Protein Targets and Medical Significance

Thiamine (vitamin B1) is a precursor of the well-known coenzyme of central metabolic pathways thiamine diphosphate (ThDP). Highly intense glucose oxidation in the brain requires ThDP-dependent enzymes, which determines the critical significance of thiamine for neuronal functions. However, thiamine can also act through the non-coenzyme mechanisms. The well-known facilitation of acetylcholinergic neurotransmission upon the thiamine and acetylcholine co-release into the synaptic cleft has been supported by the discovery of thiamine triphosphate (ThTP)-dependent phosphorylation of the acetylcholine receptor-associated protein rapsyn, and thiamine interaction with the TAS2R1 receptor, resulting in the activation of synaptic ion currents. The non-coenzyme regulatory binding of thiamine compounds has been demonstrated for the transcriptional regulator p53, poly(ADP-ribose) polymerase, prion protein PRNP, and a number of key metabolic enzymes that do not use ThDP as a coenzyme. The accumulated data indicate that the molecular mechanisms of the neurotropic action of thiamine are far broader than it has been originally believed, and closely linked to the metabolism of thiamine and its derivatives in animals. The significance of this topic has been illustrated by the recently established competition between thiamine and the antidiabetic drug metformin for common transporters, which can be the reason for the thiamine deficiency underlying metformin side effects. Here, we also discuss the medical implications of the research on thiamine, including the role of thiaminases in thiamine reutilization and biosynthesis of thiamine antagonists; molecular mechanisms of action of natural and synthetic thiamine antagonists, and biotransformation of pharmacological forms of thiamine. Given the wide medical application of thiamine and its synthetic forms, these aspects are of high importance for medicine and pharmacology, including the therapy of neurodegenerative diseases.

Biomarkers for In Vivo Assessment of Transporter Function

Drug-drug interactions are a major concern not only during clinical practice, but also in drug development. Due to limitations of in vitro-in vivo predictions of transporter-mediated drug-drug interactions, multiple clinical Phase I drug-drug interaction studies may become necessary for a new molecular entity to assess potential drug interaction liabilities. This is a resource-intensive process and exposes study participants, who frequently are healthy volunteers without benefit from study treatment, to the potential risks of a new drug in development. Therefore, there is currently a major interest in new approaches for better prediction of transporter-mediated drug-drug interactions. In particular, researchers in the field attempt to identify endogenous compounds as biomarkers for transporter function, such as hexadecanedioate, tetradecanedioate, coproporphyrins I and III, or glycochenodeoxycholate sulfate for hepatic uptake via organic anion transporting polypeptide 1B or N¹-methylnicotinamide for multidrug and toxin extrusion protein-mediated renal secretion. We summarize in this review the currently proposed biomarkers and potential limitations of the substances identified to date. Moreover, we suggest criteria based on current experiences, which may be used to assess the suitability of a biomarker for transporter function. Finally, further alternatives and supplemental approaches to classic drug-drug interaction studies are discussed.

Influence of Transporter Polymorphisms on Drug Disposition and Response: A Perspective From the International Transporter Consortium

Advances in genomic technologies have led to a wealth of information identifying genetic polymorphisms in membrane transporters, specifically how these polymorphisms affect drug disposition and response. This review describes the current perspective of the International Transporter Consortium (ITC) on clinically important polymorphisms in membrane transporters. ITC suggests that, in addition to previously recommended polymorphisms in ABCG2 (BCRP) and SLCO1B1 (OATP1B1), polymorphisms in the emerging transporter, SLC22A1 (OCT1), be considered during drug development. Collectively, polymorphisms in these transporters are important determinants of interindividual differences in the levels, toxicities, and response to many drugs.

Beyond drug-drug interactions: effects of transporter inhibition on endobiotics, nutrients and toxins

INTRODUCTION

Membrane transport proteins play a central role in regulating the disposition of endobiotics, dietary nutrients and environmental toxins. The inhibition of transporters by drugs has potential physiologic consequences. The full extent of the effect of drugs on the function of transporters is poorly understood because only a small subset of the hundreds of transporters expressed in humans - primarily those mediating the rate-determining step in the elimination of specific drugs - are assessed during clinical development. Areas covered: We provide a comprehensive overview of literature reports implicating the inhibition of transporters as the mechanism for off-target effects of drugs. Expert opinion: Transporter inhibition, the mechanism of action of many marketed drugs, appears to play an underappreciated role in a number of side effects including vitamin deficiency, edema, dyslipidemia, cholestasis and gout. Cell systems more broadly expressing transporter networks and methods like unbiased metabolomics should be incorporated into the screening paradigm to expand our understanding of the impact of drugs on the physiologic function of transporters and to allow for these effects to be taken into account in drug discovery and clinical practice.

Renal Transporter-Mediated Drug-Drug Interactions: Are They Clinically Relevant?

The kidney, through the distinct processes of passive glomerular filtration and active tubular secretion, plays an important role in the elimination of numerous endobiotics (eg, hormones, metabolites), toxins, nutrients, and drugs. Renal transport pathways mediating active tubular secretion and reabsorption in the proximal tubule are complex, involving apical and basolateral transporters acting in concert. Detailed studies of the molecular mechanisms of net active tubular secretion have established the involvement of multiple transporters with overlapping substrate specificity mediating competing secretion and reabsorption pathways. Although drug interactions arising from inhibition of renal transporters are rare relative to other mechanisms, they can involve commonly administered drugs (eg, cimetidine, metformin), may be underappreciated due to muted effects on plasma pharmacokinetics relative to tissue levels, can affect narrow-therapeutic-index medications (eg, antiarrhythmic, oncology medications), and may disproportionately affect sensitive populations where polypharmacy is common (eg, the elderly, diabetics). In particular, there is the potential for larger-magnitude interactions in subjects with reduced glomerular filtration rates due to the increased relative contribution of tubular secretion. The assessment of additional endpoints in drug-drug interaction studies including pharmacodynamics, positron emission tomography imaging, and metabolomics promises to expand our understanding of the clinical relevance of renal drug interactions.

Evaluation and Quantitative Prediction of Renal Transporter-Mediated Drug-Drug Interactions

With numerous drugs cleared renally, inhibition of uptake transporters localized on the basolateral membrane of renal proximal tubule cells, eg, organic anion transporters (OATs) and organic cation transporters (OCTs), may lead to clinically meaningful drug-drug interactions (DDIs). Additionally, clinical evidence for the possible involvement of efflux transporters, such as P-glycoprotein (P-gp) and multidrug and toxin extrusion protein 1/2-K (MATE1/2-K), in the renal DDIs is emerging. Herein, we review recent progress regarding mechanistic understanding of transporter-mediated renal DDIs as well as the quantitative predictability of renal DDIs using static and physiologically based pharmacokinetic (PBPK) models. Generally, clinical DDI data suggest that the magnitude of plasma exposure changes attributable to renal DDIs is less than 2-fold, unlike the DDIs associated with inhibition of cytochrome P-450s and/or hepatic uptake transporters. It is concluded that although there is a need for risk assessment early in drug development, current available data imply that safety concerns related to the renal DDIs are generally low. Nevertheless, consideration must be given to the therapeutic index of the victim drug and potential risk in a specific patient population (eg, renal impairment). Finally, in vitro transporter data and clinical pharmacokinetic parameters obtained from the first-in-human studies have proven useful in support of quantitative prediction of DDIs associated with inhibition of renal secretory transporters, OATs or OCTs.

The Role of Transporters in the Toxicity of Chemotherapeutic Drugs: Focus on Transporters for Organic Cations

The introduction of chemotherapy in the treatment of cancer is one of the most important achievements of modern medicine, even allowing the cure of some lethal diseases such as testicular cancer and other malignant neoplasms. The number and type of chemotherapeutic agents available have steadily increased and have developed until the introduction of targeted tumor therapy. It is now evident that transporters play an important role for determining toxicity of chemotherapeutic drugs not only against target but also against nontarget cells. This is of special importance for intracellularly active hydrophilic drugs, which cannot freely penetrate the plasma membrane. Because many important chemotherapeutic agents are substrates of transporters for organic cations, this review discusses the known interaction of these substances with these transporters. A particular focus is given to the role of transporters for organic cations in the development of side effects of chemotherapy with platinum derivatives and in the efficacy of recently developed tyrosine kinase inhibitors to specifically target cancer cells. It is evident that specific inhibition of uptake transporters may be a possible strategy to protect against undesired side effects of platinum derivatives without compromising their antitumor efficacy. These transporters are also important for efficient targeting of tyrosine kinase inhibitors to cancer cells. However, in order to achieve the aims of protecting from undesired toxicities and improving the specificity of uptake by tumor cells, an exact knowledge of transporter expression, function, regulation under normal and pathologic conditions, and of genetically and epigenetically regulation is mandatory.

High-dose thiamine prevents brain lesions and prolongs survival of Slc19a3-deficient mice

SLC19A3 deficiency, also called thiamine metabolism dysfunction syndrome-2 (THMD2; OMIM 607483), is an autosomal recessive neurodegenerative disorder caused by mutations in SLC19A3, the gene encoding thiamine transporter 2. To investigate the molecular mechanisms of neurodegeneration in SLC19A3 deficiency and whether administration of high-dose thiamine prevents neurodegeneration, we generated homozygous Slc19a3 E314Q knock-in (KI) mice harboring the mutation corresponding to the human SLC19A3 E320Q, which is associated with the severe form of THMD2. Homozygous KI mice and previously reported homozygous Slc19a3 knock-out (KO) mice fed a thiamine-restricted diet (thiamine: 0.60 mg/100 g food) died within 30 and 12 days, respectively, with dramatically decreased thiamine concentration in the blood and brain, acute neurodegeneration, and astrogliosis in the submedial nucleus of the thalamus and ventral anterior-lateral complex of the thalamus. These findings may bear some features of thiamine-deficient mice generated by pyrithiamine injection and a thiamine-deficient diet, suggesting that the primary cause of THMD2 could be thiamine pyrophosphate (TPP) deficiency. Next, we analyzed the therapeutic effects of high-dose thiamine treatment. When the diet was reverted to a conventional diet (thiamine: 1.71 mg/100 g food) after thiamine restriction, all homozygous KO mice died. In contrast, when the diet was changed to a high-thiamine diet (thiamine: 8.50 mg/100 g food) after thiamine restriction, more than half of homozygous KO mice survived, without progression of brain lesions. Unexpectedly, when the high-thiamine diet of recovered mice was reverted to a conventional diet, some homozygous KO mice died. These results showed that acute neurodegeneration caused by thiamine deficiency is preventable in most parts, and prompt high-dose thiamine administration is critical for the treatment of THMD2. However, reduction of thiamine should be performed carefully to prevent recurrence after recovery of the disease.

Investigation of Glycochenodeoxycholate Sulfate and Chenodeoxycholate Glucuronide as Surrogate Endogenous Probes for Drug Interaction Studies of OATP1B1 and OATP1B3 in Healthy Japanese Volunteers

PURPOSE

To assess the use of glycochenodeoxycholate-3-sulfate (GCDCA-S) and chenodeoxycholate 3- or 24-glucuronide (CDCA-3G or -24G) as surrogate endogenous substrates in the investigation of drug interactions involving OATP1B1 and OATP1B3.

METHODS

Uptake of GCDCA-S and CDCA-24G was examined in HEK293 cells transfected with cDNA for OATP1B1, OATP1B3, and NTCP and in cryopreserved human hepatocytes. Plasma concentrations of bile acids and their metabolites (GCDCA-S, CDCA-3G, and CDCA-24G) were determined by LC-MS/MS in eight healthy volunteers with or without administration of rifampicin (600 mg, po).

RESULTS

GCDCA-S and CDCA-24G were substrates for OATP1B1, OATP1B3, and NTCP. The uptake of [³H]atorvastatin, GCDCA-S, and CDCA-24G by human hepatocytes was significantly inhibited by both rifampicin and pioglitazone, whereas that of taurocholate was inhibited only by pioglitazone. Rifampicin elevated plasma concentrations of GCDCA-S more than those of other bile acids. The area under the plasma concentration-time curve for GCDCA-S was 20.3 times higher in rifampicin-treated samples. CDCA-24G could be detected only in plasma from the rifampicin-treatment phase, and CDCA-3G was undetectable in both phases.

CONCLUSIONS

We identified GCDCA-S and CDCA-24G as substrates of NTCP, OATP1B1, and OATP1B3. GCDCA-S is a surrogate endogenous probe for the assessment of drug interactions involving hepatic OATP1B1 and OATP1B3.

Evaluation of Renal Transporter Inhibition Using Creatinine as a Substrate In Vitro to Assess the Clinical Risk of Elevated Serum Creatinine

Creatinine is a widely accepted biomarker for renal toxicity, but its renal clearance via transporter-mediated active secretion is significant. For a given new chemical entity, therefore, elevations in serum creatinine (SCr) can be caused by the inhibition of renal transporter(s) without renal toxicity. In the present study, an effort was made to assess the correlation between the inhibition of renal transporters in vitro and elevations in SCr. A total of 15 compounds were chosen based on their known effect on SCr and minimal impact on glomerular filtration rate. Their inhibition potencies against the major creatinine renal transporters, including organic cation transporter 2, organic anion transporter 2, and 2 forms of multidrug and toxin extrusion (MATE1 and MATE2K), were assessed in transporter-transfected cell lines using creatinine as a probe substrate. Collectively, the data suggest that the observed elevations in SCr can be attributed to the inhibition of renal transporter(s), but inhibition of renal transporters does not necessarily lead to elevated SCr. Thus, renal transporter inhibition data can be used to rationalize SCr changes. Additionally, differing renal transporter inhibition potencies using creatinine and metformin as probe substrates suggest that substrate-dependent inhibition exists for some compounds.

Metabolomic and Genome-wide Association Studies Reveal Potential Endogenous Biomarkers for OATP1B1

Transporter-mediated drug-drug interactions (DDIs) are a major cause of drug toxicities. Using published genome-wide association studies (GWAS) of the human metabolome, we identified 20 metabolites associated with genetic variants in organic anion transporter, OATP1B1 (P < 5 × 10-8 ). Of these, 12 metabolites were significantly higher in plasma samples from volunteers dosed with the OATP1B1 inhibitor, cyclosporine (CSA) vs. placebo (q-value < 0.2). Conjugated bile acids and fatty acid dicarboxylates were among the metabolites discovered using both GWAS and CSA administration. In vitro studies confirmed tetradecanedioate (TDA) and hexadecanedioate (HDA) were novel substrates of OATP1B1 as well as OAT1 and OAT3. This study highlights the use of multiple datasets for the discovery of endogenous metabolites that represent potential in vivo biomarkers for transporter-mediated DDIs. Future studies are needed to determine whether these metabolites can serve as qualified biomarkers for organic anion transporters. Quantitative relationships between metabolite levels and modulation of transporters should be established.

Interaction of 2,4-Diaminopyrimidine-Containing Drugs Including Fedratinib and Trimethoprim with Thiamine Transporters

Inhibition of thiamine transporters has been proposed as a putative mechanism for the observation of Wernicke's encephalopathy and subsequent termination of clinical development of fedratinib, a Janus kinase inhibitor (JAKi). This study aimed to determine the potential for other JAKi to inhibit thiamine transport using human epithelial colorectal adenocarcinoma (Caco-2) and thiamine transporter (THTR) overexpressing cells and to better elucidate the structural basis for interacting with THTR. Only JAKi containing a 2,4-diaminopyrimidine were observed to inhibit thiamine transporters. Fedratinib inhibited thiamine uptake into Caco-2 cells (IC₅₀ = 0.940 µM) and THTR-2 (IC₅₀ = 1.36 µM) and, to a lesser extent, THTR-1 (IC₅₀ = 7.10 µM) overexpressing cells. Two other JAKi containing this moiety, AZD1480 and cerdulatinib, were weaker inhibitors of the thiamine transporters. Other JAKi-including monoaminopyrimidines, such as momelotinib, and nonaminopyrimidines, such as filgotinib-did not have any inhibitory effects on thiamine transport. A pharmacophore model derived from the minimized structure of thiamine suggests that 2,4-diaminopyrimidine-containing compounds can adopt a conformation matching several key features of thiamine. Further studies with drugs containing a 2,4-diaminopyrimidine resulted in the discovery that the antibiotic trimethoprim also potently inhibits thiamine uptake mediated by THTR-1 (IC₅₀ = 6.84 µM) and THTR-2 (IC₅₀ = 5.56 µM). Fedratinib and trimethoprim were also found to be substrates for THTR, a finding with important implications for their disposition in the body. In summary, our results show that not all JAKi have the potential to inhibit thiamine transport and further establish the interaction of these transporters with xenobiotics.

Urinary Dopamine as a Potential Index of the Transport Activity of Multidrug and Toxin Extrusion in the Kidney

Dopamine is a cationic natriuretic catecholamine synthesized in proximal tubular cells (PTCs) of the kidney before secretion into the lumen, a key site of its action. However, the molecular mechanisms underlying dopamine secretion into the lumen remain unclear. Multidrug and toxin extrusion (MATE) is a H⁺/organic cation antiporter that is highly expressed in the brush border membrane of PTCs and mediates the efflux of organic cations, including metformin and cisplatin, from the epithelial cells into the urine. Therefore, we hypothesized that MATE mediates dopamine secretion, a cationic catecholamine, into the tubule lumen, thereby regulating natriuresis. Here, we show that [³H]dopamine uptake in human (h) MATE1-, hMATE-2K- and mouse (m) MATE-expressing cells exhibited saturable kinetics. Fluid retention and decreased urinary excretion of dopamine and Na⁺ were observed in Mate1-knockout mice compared to that in wild-type mice. Imatinib, a MATE inhibitor, inhibited [³H]dopamine uptake by hMATE1-, hMATE2-K- and mMATE1-expressing cells in a concentration-dependent manner. At clinically-relevant concentrations, imatinib inhibited [³H]dopamine uptake by hMATE1- and hMATE2-K-expressing cells. The urinary excretion of dopamine and Na⁺ decreased and fluid retention occurred in imatinib-treated mice. In conclusion, MATE transporters secrete renally-synthesized dopamine, and therefore, urinary dopamine has the potential to be an index of the MATE transporter activity.

The first characterization of multidrug and toxin extrusion (MATE/SLC47) proteins in zebrafish (Danio rerio)

Multidrug and toxin extrusion (MATE) proteins are involved in the extrusion of endogenous compounds and xenobiotics across the plasma membrane. They are conserved from bacteria to mammals, with different numbers of genes within groups. Here, we present the first data on identification and functional characterization of Mate proteins in zebrafish (Danio rerio). Phylogenetic analysis revealed six Mates in teleost fish, annotated as Mate3-8, which form a distinct cluster separated from the tetrapod MATEs/Mates. Synteny analysis showed that zebrafish mate genes are orthologous to human MATEs. Gene expression analysis revealed that all the mate transcripts were constitutively and differentially expressed during embryonic development, followed by pronounced and tissue-specific expression in adults. Functional analyses were performed using transport activity assays with model substrates after heterologous overexpression of five zebrafish Mates in HEK293T cells. The results showed that zebrafish Mates interact with both physiological and xenobiotic substances but also substantially differ with respect to the interacting compounds and interaction strength in comparison to mammalian MATEs/Mates. Taken together, our data clearly indicate a potentially important role for zebrafish Mate transporters in zebrafish embryos and adults and provide a basis for detailed functional characterizations of single zebrafish Mate transporters.

Impact of Experimental Conditions on the Evaluation of Interactions between Multidrug and Toxin Extrusion Proteins and Candidate Drugs

Multidrug and toxin extrusion transporters (MATEs) have a determining influence on the pharmacokinetic profiles of many drugs and are involved in several clinical drug-drug interactions (DDIs). Cellular uptake assays with recombinant cells expressing human MATE1 or MATE2-K are widely used to investigate MATE-mediated transport for DDI assessment; however, the experimental conditions and used test substrates vary among laboratories. We therefore initially examined the impact of three assay conditions that have been applied for MATE substrate and inhibitor profiling in the literature. One of the tested conditions resulted in significantly higher uptake rates of the three test substrates, [(14)C]metformin, [(3)H]thiamine, and [(3)H]1-methyl-4-phenylpyridinium (MPP(+)), but IC50 values of four tested MATE inhibitors varied only slightly among the three conditions (<2.5-fold difference). Subsequently, we investigated the uptake characteristics of the five MATE substrates: [(14)C]metformin, [(3)H]thiamine, [(3)H]MPP(+), [(3)H]estrone-3-sulfate (E3S), and rhodamine 123, as well as the impact of the used test substrate on the inhibition profiles of 10 MATE inhibitors at one selected assay condition. [(3)H]E3S showed atypical uptake characteristics compared with those observed with the other four substrates. IC50 values of the tested inhibitors were in a similar range (<4-fold difference) when [(14)C]metformin, [(3)H]thiamine, [(3)H]MPP(+), or [(3)H]E3S were used as substrates but were considerably higher with rhodamine 123 (9.8-fold and 4.1-fold differences compared with [(14)C]metformin with MATE1 and MATE2-K, respectively). This study demonstrated for the first time that the impact of assay conditions on IC50 determination is negligible, that kinetic characteristics differ among used test substrates, and that substrate-dependent inhibition exists for MATE1 and MATE2-K, giving valuable insight into the assessment of clinically relevant MATE-mediated DDIs in vitro.

Structure and function of multidrug and toxin extrusion proteins (MATEs) and their relevance to drug therapy and personalized medicine

Multidrug and toxin extrusion (MATE; SLC47A) proteins are membrane transporters mediating the excretion of organic cations and zwitterions into bile and urine and thereby contributing to the hepatic and renal elimination of many xenobiotics. Transported substrates include creatinine as endogenous substrate, the vitamin thiamine and a number of drug agents with in part chemically different structures such as the antidiabetic metformin, the antiviral agents acyclovir and ganciclovir as well as the antibiotics cephalexin and cephradine. This review summarizes current knowledge on the structural and molecular features of human MATE transporters including data on expression and localization in different tissues, important aspects on regulation and their functional role in drug transport. The role of genetic variation of MATE proteins for drug pharmacokinetics and drug response will be discussed with consequences for personalized medicine.

The Complexities of Interpreting Reversible Elevated Serum Creatinine Levels in Drug Development: Does a Correlation with Inhibition of Renal Transporters Exist?

In humans, creatinine is formed by a multistep process in liver and muscle and eliminated via the kidney by a combination of glomerular filtration and active transport. Based on current evidence, creatinine can be taken up into renal proximal tubule cells by the basolaterally localized organic cation transporter 2 (OCT2) and the organic anion transporter 2, and effluxed into the urine by the apically localized multidrug and toxin extrusion protein 1 (MATE1) and MATE2K. Drug-induced elevation of serum creatinine (SCr) and/or reduced creatinine renal clearance is routinely used as a marker for acute kidney injury. Interpretation of elevated SCr can be complex, because such increases can be reversible and explained by inhibition of renal transporters involved in active secretion of creatinine or other secondary factors, such as diet and disease state. Distinction between these possibilities is important from a drug development perspective, as increases in SCr can result in the termination of otherwise efficacious drug candidates. In this review, we discuss the challenges associated with using creatinine as a marker for kidney damage. Furthermore, to evaluate whether reversible changes in SCr can be predicted prospectively based on in vitro transporter inhibition data, an in-depth in vitro-in vivo correlation (IVIVC) analysis was conducted for 16 drugs with in-house and literature in vitro transporter inhibition data for OCT2, MATE1, and MATE2K, as well as total and unbound maximum plasma concentration (Cmax and Cmax,u) data measured in the clinic.

Metformin Is a Substrate and Inhibitor of the Human Thiamine Transporter, THTR-2 (SLC19A3)

The biguanide metformin is widely used as first-line therapy for the treatment of type 2 diabetes. Predominately a cation at physiological pH's, metformin is transported by membrane transporters, which play major roles in its absorption and disposition. Recently, our laboratory demonstrated that organic cation transporter 1, OCT1, the major hepatic uptake transporter for metformin, was also the primary hepatic uptake transporter for thiamine, vitamin B1. In this study, we tested the reverse, i.e., that metformin is a substrate of thiamine transporters (THTR-1, SLC19A2, and THTR-2, SLC19A3). Our study demonstrated that human THTR-2 (hTHTR-2), SLC19A3, which is highly expressed in the small intestine, but not hTHTR-1, transports metformin (Km = 1.15 ± 0.2 mM) and other cationic compounds (MPP(+) and famotidine). The uptake mechanism for hTHTR-2 was pH and electrochemical gradient sensitive. Furthermore, metformin as well as other drugs including phenformin, chloroquine, verapamil, famotidine, and amprolium inhibited hTHTR-2 mediated uptake of both thiamine and metformin. Species differences in the substrate specificity of THTR-2 between human and mouse orthologues were observed. Taken together, our data suggest that hTHTR-2 may play a role in the intestinal absorption and tissue distribution of metformin and other organic cations and that the transporter may be a target for drug-drug and drug-nutrient interactions.

Role of organic cation transporters in drug-drug interaction

INTRODUCTION

Organic cation transporters OCT1, OCT2 and OCT3 expressed in the small intestine, liver, brain and other organs play important roles in absorption, excretion and distribution of cationic drugs. Drug-drug interactions at OCTs may change pharmacokinetics, pharmacodynamics and drug toxicity. Knowledge about physiological and biomedical functions of OCTs and the molecular mechanisms of transport and inhibition is required to anticipate drug-drug interactions and their potential biomedical impact.

AREAS COVERED

Current knowledge about structure, polyspecific cation binding and transport of OCTs is summarized. Tissue distributions of OCT1-3 and their presumed physiological roles in the small intestine, liver, kidney and brain are reported, and drugs that are transported by human OCT1-3 are listed. The impact of human OCTs for pharmacokinetics and pharmacodynamics of the antidiabetic metformin and antineoplastic platinum derivatives are discussed. In addition, interactions of drugs that are transported by OCTs observed in the kidney and liver are reported. Procedures to test novel drugs for drug-drug interactions at OCTs in vitro and in clinical studies are recommended.

EXPERT OPINION

When performing in vitro testing for drug-drug interactions, it must be considered that one inhibitory drug may inhibit different transported drugs with different affinities. After positive in vitro testing for drug-drug interaction, clinical tests are obligatory.

Evaluation of cynomolgus monkeys for the identification of endogenous biomarkers for hepatic transporter inhibition and as a translatable model to predict pharmacokinetic interactions with statins in humans

Inhibition of hepatic transporters such as organic anion transporting polypeptides (OATPs) 1B can cause drug-drug interactions (DDIs). Determining the impact of perpetrator drugs on the plasma exposure of endogenous substrates for OATP1B could be valuable to assess the risk for DDIs early in drug development. As OATP1B orthologs are well conserved between human and monkey, we assessed in cynomolgus monkeys the endogenous OATP1B substrates that are potentially suitable to assess DDI risk in humans. The effect of rifampin (RIF), a potent inhibitor for OATP1B, on plasma exposure of endogenous substrates of hepatic transporters was measured. From the 18 biomarkers tested, RIF (18 mg/kg, oral) caused significant elevation of plasma unconjugated and conjugated bilirubin, which may be attributed to inhibition of cOATP1B1 and cOATP1B3 based on in vitro to in vivo extrapolation analysis. To further evaluate whether cynomolgus monkeys are a suitable translational model to study OATP1B-mediated DDIs, we determined the inhibitory effect of RIF on in vitro transport and pharmacokinetics of rosuvastatin (RSV) and atorvastatin (ATV). RIF strongly inhibited the uptake of RSV and ATV by cOATP1B1 and cOATP1B3 in vitro. In agreement with clinical observations, RIF (18 mg/kg, oral) significantly decreased plasma clearance and increased the area under the plasma concentration curve (AUC) of intravenously administered RSV by 2.8- and 2.7-fold, and increased the AUC and maximum plasma concentration of orally administered RSV by 6- and 10.3-fold, respectively. In contrast to clinical findings, RIF did not significantly increase plasma exposure of either intravenous or orally administered ATV, indicating species differences in the rate-limiting elimination pathways.

The physiological role of drug transporters

Transporters comprise the largest family of membrane proteins in human organism, including members of solute carrier transporter and ATP-binding cassette transporter families. They play pivotal roles in the absorption, distribution and excretion of xenobiotic and endogenous molecules. Transporters are widely expressed in various human tissues and are routinely evaluated during the process of drug development and approval. Over the past decade, increasing evidence shows that drug transporters are important in both normal physiology and disease. Currently, transporters are utilized as therapeutic targets to treat numerous diseases such as diabetes, major depression, hypertension and constipation. Despite the steady growth of the field of transporter biology, more than half of the members in transporter superfamily have little information available about their endogenous substrate(s) or physiological functions. This review outlines current research methods in transporter studies, and summarizes the drug-transporter interactions including drug-drug and drug-endogenous substrate interactions. In the end, we also discuss the therapeutic perspective of transporters based on their physiological and pathophysiological roles.