Emerging Clinical Importance of Hepatic Organic Cation Transporter 1 (OCT1) in Drug Pharmacokinetics, Dynamics, Pharmacogenetic Variability, and Drug Interactions

Effect of changes in metabolic enzymes and transporters on drug metabolism in the context of liver disease: Impact on pharmacokinetics and drug-drug interactions

Changes in the pharmacokinetic and resulting pharmacodynamic properties of drugs are common in many chronic liver diseases, leading to adverse effects, drug interactions and increased risk of over- or underdosing of medications. Structural and functional hepatic impairment can have major effects on drug metabolism and transport. This review summarizes research on the functional changes in phase I and II metabolic enzymes and in transport proteins in patients with metabolic diseases such as type 2 diabetes, metabolic dysfunction-associated steatotic liver disease, metabolic dysfunction-associated steatohepatitis and cirrhosis, providing a clinical perspective on how these changes affect drug uptake and metabolism. Generally, a decrease in expression and/or activity of many enzymes of the cytochrome P450 family (e.g. CYP2E1 and CYP3A4), and of influx and efflux transporters (e.g. organic anion-transporting polypeptide [OATP]1B1, OATP2B1, OAT2 and bile salt export pump), has been recently documented in patients with liver disease. Decreased enzyme levels often correlate with increased severity of chronic liver disease. In subjects with hepatic impairment, there is potential for strong alterations of drug pharmacokinetics due to reduced absorption, increased volume of distribution, metabolism and extraction. Due to the altered pharmacokinetics, specific drug-drug interactions are also a potential issue to consider in patients with liver disease. Given the huge burden of liver disease in western societies, there is a need to improve awareness among all healthcare professionals and patients with liver disease to ensure appropriate drug prescriptions.

Preincubation Time-Dependent, Long-Lasting Inhibition of Drug Transporters and Impact on the Prediction of Drug-Drug Interactions

Transporter-mediated drug-drug interaction (DDI) is of clinical concern, and the quantitative prediction of DDIs is an indispensable part of drug development. Cell-based inhibition assays, in which a representative probe substrate and a potential inhibitor are coincubated, are routinely performed to assess the inhibitory potential of new molecular entities on drug transporters. However, the inhibitory effect of cyclosporine A (CsA) on organic anion transporting polypeptide (OATP) 1B1 is substantially potentiated with CsA preincubation, and this effect is both long-lasting and dependent on the preincubation time. This phenomenon has also been reported with transporters other than OATP1Bs, but it is considered more prevalent among OATP1Bs and organic cation transporters. Regulatory agencies have also noted this preincubation effect and have recommended that pharmaceutical companies consider inhibitor preincubation when performing in vitro OATP1B1 and OATP1B3 inhibition studies. Although the underlying mechanisms responsible for the preincubation effect are not fully understood, a trans-inhibition mechanism was recently demonstrated for OATP1B1 inhibition by CsA, in which CsA inhibited OATP1B1 not only extracellularly (cis-inhibition) but also intracellularly (trans-inhibition). Furthermore, the trans-inhibition potency of CsA was much greater than that of cis-inhibition, suggesting that trans-inhibition might be a key driver of clinical DDIs of CsA with OATP1B substrate drugs. Although confidence in transporter-mediated DDI prediction is generally considered to be low, the predictability might be further improved by incorporating the trans-inhibition mechanism into static and dynamic models for preincubation-dependent inhibitors of OATP1Bs and perhaps other transporters. SIGNIFICANCE STATEMENT: Preincubation time-dependent, long-lasting inhibition has been observed for OATP1B1 and other solute carrier transporters in vitro. Recently, a trans-inhibition mechanism for the preincubation effect of CsA on OATP1B1 inhibition was identified, with the trans-inhibition potency being greater than that of cis-inhibition. The concept of trans-inhibition may allow us to further understand the mechanism of transporter-mediated DDIs not only for OATP1B1 but also for other transporters and to improve the accuracy and confidence of DDI predictions.

Studying the right transporter at the right time: an in vitro strategy for assessing drug-drug interaction risk during drug discovery and development

INTRODUCTION

Transporters are significant in dictating drug pharmacokinetics, thus inhibition of transporter function can alter drug concentrations resulting in drug-drug interactions (DDIs). Because they can impact drug toxicity, transporter DDIs are a regulatory concern for which prediction of clinical effect from in vitro data is critical to understanding risk.

AREA COVERED

The authors propose in vitro strategies to assist mitigating/removing transporter DDI risk during development by frontloading specific studies, or managing patient risk in the clinic. An overview of clinically relevant drug transporters and observed DDIs are provided, alongside presentation of key considerations/recommendations for in vitro study design evaluating drugs as inhibitors or substrates. Guidance on identifying critical co-medications, clinically relevant disposition pathways and using mechanistic static equations for quantitative prediction of DDI is compiled.

EXPERT OPINION

The strategies provided will facilitate project teams to study the right transporter at the right time to minimise development risks associated with DDIs. To truly alleviate or manage clinical risk, the industry will benefit from moving away from current qualitative basic static equation approaches to transporter DDI hazard assessment towards adopting the use of mechanistic models to enable quantitative DDI prediction, thereby contextualising risk to ascertain whether a transporter DDI is simply pharmacokinetic or clinically significant requiring intervention.

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.

Recent advances in the translation of drug metabolism and pharmacokinetics science for drug discovery and development

Drug metabolism and pharmacokinetics (DMPK) is an important branch of pharmaceutical sciences. The nature of ADME (absorption, distribution, metabolism, excretion) and PK (pharmacokinetics) inquiries during drug discovery and development has evolved in recent years from being largely descriptive to seeking a more quantitative and mechanistic understanding of the fate of drug candidates in biological systems. Tremendous progress has been made in the past decade, not only in the characterization of physiochemical properties of drugs that influence their ADME, target organ exposure, and toxicity, but also in the identification of design principles that can minimize drug-drug interaction (DDI) potentials and reduce the attritions. The importance of membrane transporters in drug disposition, efficacy, and safety, as well as the interplay with metabolic processes, has been increasingly recognized. Dramatic increases in investments on new modalities beyond traditional small and large molecule drugs, such as peptides, oligonucleotides, and antibody-drug conjugates, necessitated further innovations in bioanalytical and experimental tools for the characterization of their ADME properties. In this review, we highlight some of the most notable advances in the last decade, and provide future perspectives on potential major breakthroughs and innovations in the translation of DMPK science in various stages of drug discovery and development.

Cloning and Functional Characterization of Dog OCT1 and OCT2: Another Step in Exploring Species Differences in Organic Cation Transporters

OCT1 and OCT2 are polyspecific membrane transporters that are involved in hepatic and renal drug clearance in humans and mice. In this study, we cloned dog OCT1 and OCT2 and compared their function to the human and mouse orthologs. We used liver and kidney RNA to clone dog OCT1 and OCT2. The cloned and the publicly available RNA-Seq sequences differed from the annotated exon-intron structure of OCT1 in the dog genome CanFam3.1. An additional exon between exons 2 and 3 was identified and confirmed by sequencing in six additional dog breeds. Next, dog OCT1 and OCT2 were stably overexpressed in HEK293 cells and the transport kinetics of five drugs were analyzed. We observed strong differences in the transport kinetics between dog and human orthologs. Dog OCT1 transported fenoterol with 12.9-fold higher capacity but 14.3-fold lower affinity (higher K_M) than human OCT1. Human OCT1 transported ipratropium with 5.2-fold higher capacity but 8.4-fold lower affinity than dog OCT1. Compared to human OCT2, dog OCT2 showed 10-fold lower transport of fenoterol and butylscopolamine. In conclusion, the functional characterization of dog OCT1 and OCT2 reported here may have implications when using dogs as pre-clinical models as well as for drug therapy in dogs.

Interaction Profiles of Central Nervous System Active Drugs at Human Organic Cation Transporters 1-3 and Human Plasma Membrane Monoamine Transporter

Many psychoactive compounds have been shown to primarily interact with high-affinity and low-capacity solute carrier 6 (SLC6) monoamine transporters for norepinephrine (NET; norepinephrine transporter), dopamine (DAT; dopamine transporter) and serotonin (SERT; serotonin transporter). Previous studies indicate an overlap between the inhibitory capacities of substances at SLC6 and SLC22 human organic cation transporters (SLC22A1-3; hOCT1-3) and the human plasma membrane monoamine transporter (SLC29A4; hPMAT), which can be classified as high-capacity, low-affinity monoamine transporters. However, interactions between central nervous system active substances, the OCTs, and the functionally-related PMAT have largely been understudied. Herein, we report data from 17 psychoactive substances interacting with the SLC6 monoamine transporters, concerning their potential to interact with the human OCT isoforms and hPMAT by utilizing radiotracer-based in vitro uptake inhibition assays at stably expressing human embryonic kidney 293 cells (HEK293) cells. Many compounds inhibit substrate uptake by hOCT1 and hOCT2 in the low micromolar range, whereas only a few substances interact with hOCT3 and hPMAT. Interestingly, methylphenidate and ketamine selectively interact with hOCT1 or hOCT2, respectively. Additionally, 3,4-methylenedioxymethamphetamine (MDMA) is a potent inhibitor of hOCT1 and 2 and hPMAT. Enantiospecific differences of R- and S-α-pyrrolidinovalerophenone (R- and S-α-PVP) and R- and S-citalopram and the effects of aromatic substituents are explored. Our results highlight the significance of investigating drug interactions with hOCTs and hPMAT, due to their role in regulating monoamine concentrations and xenobiotic clearance.

The Transporter-Mediated Cellular Uptake and Efflux of Pharmaceutical Drugs and Biotechnology Products: How and Why Phospholipid Bilayer Transport Is Negligible in Real Biomembranes

Over the years, my colleagues and I have come to realise that the likelihood of pharmaceutical drugs being able to diffuse through whatever unhindered phospholipid bilayer may exist in intact biological membranes in vivo is vanishingly low. This is because (i) most real biomembranes are mostly protein, not lipid, (ii) unlike purely lipid bilayers that can form transient aqueous channels, the high concentrations of proteins serve to stop such activity, (iii) natural evolution long ago selected against transport methods that just let any undesirable products enter a cell, (iv) transporters have now been identified for all kinds of molecules (even water) that were once thought not to require them, (v) many experiments show a massive variation in the uptake of drugs between different cells, tissues, and organisms, that cannot be explained if lipid bilayer transport is significant or if efflux were the only differentiator, and (vi) many experiments that manipulate the expression level of individual transporters as an independent variable demonstrate their role in drug and nutrient uptake (including in cytotoxicity or adverse drug reactions). This makes such transporters valuable both as a means of targeting drugs (not least anti-infectives) to selected cells or tissues and also as drug targets. The same considerations apply to the exploitation of substrate uptake and product efflux transporters in biotechnology. We are also beginning to recognise that transporters are more promiscuous, and antiporter activity is much more widespread, than had been realised, and that such processes are adaptive (i.e., were selected by natural evolution). The purpose of the present review is to summarise the above, and to rehearse and update readers on recent developments. These developments lead us to retain and indeed to strengthen our contention that for transmembrane pharmaceutical drug transport "phospholipid bilayer transport is negligible".

Transport of Drugs and Endogenous Compounds Mediated by Human OCT1: Studies in Single- and Double-Transfected Cell Models

Organic Cation Transporter 1 (OCT1, gene symbol: SLC22A1) is predominately expressed in human liver, localized in the basolateral membrane of hepatocytes and facilitates the uptake of endogenous compounds (e.g. serotonin, acetylcholine, thiamine), and widely prescribed drugs (e.g. metformin, fenoterol, morphine). Furthermore, exogenous compounds such as MPP⁺, ASP⁺ and Tetraethylammonium can be used as prototypic substrates to study the OCT1-mediated transport in vitro. Single-transfected cell lines recombinantly overexpressing OCT1 (e.g., HEK-OCT1) were established to study OCT1-mediated uptake and to evaluate transporter-mediated drug-drug interactions in vitro. Furthermore, double-transfected cell models simultaneously overexpressing basolaterally localized OCT1 together with an apically localized export protein have been established. Most of these cell models are based on polarized grown MDCK cells and can be used to analyze transcellular transport, mimicking the transport processes e.g. during the hepatobiliary elimination of drugs. Multidrug and toxin extrusion protein 1 (MATE1, gene symbol: SLC47A1) and the ATP-driven efflux pump P-glycoprotein (P-gp, gene symbol: ABCB1) are both expressed in the canalicular membrane of human hepatocytes and are described as transporters of organic cations. OCT1 and MATE1 have an overlapping substrate spectrum, indicating an important interplay of both transport proteins during the hepatobiliary elimination of drugs. Due to the important role of OCT1 for the transport of endogenous compounds and drugs, in vitro cell systems are important for the determination of the substrate spectrum of OCT1, the understanding of the molecular mechanisms of polarized transport, and the investigation of potential drug-drug interactions. Therefore, the aim of this review article is to summarize the current knowledge on cell systems recombinantly overexpressing human OCT1.

Interaction of Remdesivir with Clinically Relevant Hepatic Drug Uptake Transporters

Remdesivir has been approved for treatment of COVID-19 and shortens the time to recovery in hospitalized patients. Drug transporters removing remdesivir from the circulation may reduce efficacy of treatment by lowering its plasma levels. Information on the interaction of remdesivir with drug transporters is limited. We therefore assessed remdesivir as substrate and inhibitor of the clinically relevant hepatic drug uptake transporters organic anion transporting poly-peptide (OATP)-1B1 (SLCO1B1), its common genetic variants OATP1B1*1b, OATP1B1*5, OATP1B1*15, as well as OATP1B3 (SLCO1B3), OATP2B1 (SLCO2B1) and organic cation transporter (OCT)-1 (SLC22A1). Previously established transporter-overexpressing cells were used to measure (i) cellular remdesivir uptake and (ii) cellular uptake of transporter probe substrates in the presence of remdesivir. There was a high remdesivir uptake into vector-transfected control cells. Moderate, but statistically significant higher uptake was detected only for OATP1B1-, OATP1B1*1b and OATP1B1*15-expressing cells when compared with control cells at 5 µM. Remdesivir inhibited all investigated transporters at 10 µM and above. In conclusion, the low uptake rates suggest that OATP1B1 and its genetic variants, OATP1B3, OATP2B1 and OCT1 are not relevant for hepatocellular uptake of remdesivir in humans. Due to the rapid clearance of remdesivir, no clinically relevant transporter-mediated drug-drug interactions are expected.

Hepatic transporter-mediated pharmacokinetic drug-drug interactions: Recent studies and regulatory recommendations

Transporter-mediated drug-drug interactions are one of the major mechanisms in pharmacokinetic-based drug interactions and correspondingly affecting drugs' safety and efficacy. Regulatory bodies underlined the importance of the evaluation of transporter-mediated interactions as a part of the drug development process. The liver is responsible for the elimination of a wide range of endogenous and exogenous compounds via metabolism and biliary excretion. Therefore, hepatic uptake transporters, expressed on the sinusoidal membranes of hepatocytes, and efflux transporters mediating the transport from hepatocytes to the bile are determinant factors for pharmacokinetics of drugs, and hence, drug-drug interactions. In parallel with the growing research interest in this area, regulatory guidances have been updated with detailed assay models and criteria. According to well-established preclinical results, observed or expected hepatic transporter-mediated drug-drug interactions can be taken into account for clinical studies. In this paper, various methods including in vitro, in situ, in vivo, in silico approaches, and combinational concepts and several clinical studies on the assessment of transporter-mediated drug-drug interactions were reviewed. Informative and effective evaluation by preclinical tools together with the integration of pharmacokinetic modeling and simulation can reduce unexpected clinical outcomes and enhance the success rate in drug development.

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.

Role of SLC transporters in toxicity induced by anticancer drugs

INTRODUCTION

. Membrane transporters are integral to the maintenance of cellular integrity of all tissue and cell types. While transporters play an established role in the systemic pharmacokinetics of therapeutic drugs, tissue specific expression of uptake transporters can serve as an initiating mechanism that governs the accumulation and impact of cytotoxic drugs.

AREAS COVERED

. This review provides an overview of organic cation transporters as determinants of chemotherapy-induced toxicities. We also provide insights into the recently updated FDA guidelines for in vitro drug interaction studies, with a particular focus on the class of tyrosine kinase inhibitors as perpetrators of transporter-mediated drug interactions.

EXPERT OPINION

. Studies performed over the last few decades have highlighted the important role of basolateral uptake and apical efflux transporters in the pathophysiology of drug-induced organ damage. Increased understanding of the mechanisms that govern the accumulation of cytotoxic drugs has provided insights into the development of novel strategies to prevent debilitating toxicities. Furthermore, we argue that current regulatory guidelines provide inadequate recommendations for in vitro studies to identify substrates or inhibitors of drug transporters. Therefore, the translational and predictive power of FDA-approved drugs as modulators of transport function remains ambiguous and warrants further revision of the current guidelines.

High affinity of 4-(4-(dimethylamino)styryl)-N-methylpyridinium transport for assessing organic cation drugs in hepatocellular carcinoma cells

Human organic cation transporter 1 (hOCT1) and human organic cation transporter 3 (hOCT3) are highly expressed in hepatocytes and play important roles in cationic drug absorption, distribution, and elimination. A previous study demonstrated that downregulation of hOCT1 and hOCT3 mRNA was related to hepatocellular carcinoma (HepG2) prognosis and severity. Whether these transporters expressed in HepG2 cells serve for cationic drug delivery has not been investigated. Besides radioactive transport, options for assessing hOCTs in hepatocytes are limited. This study clarified the significant roles of hOCTs in HepG2 by comparing cationic fluorescent 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP⁺ ) with traditional [³ H]-1-methyl-4-phenylpyridinium (MPP⁺ ). The results showed ASP⁺ was preferably transported into HepG2 compared to [³ H]-MPP⁺ with high affinity and a high maximal transport rate. Selective transport of ASP⁺ mediated by hOCTs was influenced by extracellular pH, temperature, and membrane depolarization, corresponding to hOCT1 and hOCT3 expressions. Furthermore, transport of cationic drugs, metformin, and paclitaxel in HepG2 cells was blunted by OCT inhibitors, suggesting that hOCT1 and hOCT3 expressed in HepG2 cells exhibit notable impacts on cationic drug actions. The fluorescent ASP⁺ -based in vitro model may also provide a rapid and powerful analytical tool for further screening of cationic drug actions and interactions with hOCTs, particularly hOCT1 and hOCT3 in hepatocellular carcinoma.

Physiologically-Based Pharmacokinetic Models for Evaluating Membrane Transporter Mediated Drug-Drug Interactions: Current Capabilities, Case Studies, Future Opportunities, and Recommendations

Physiologically-based pharmacokinetic (PBPK) modeling has been extensively used to quantitatively translate in vitro data and evaluate temporal effects from drug-drug interactions (DDIs), arising due to reversible enzyme and transporter inhibition, irreversible time-dependent inhibition, enzyme induction, and/or suppression. PBPK modeling has now gained reasonable acceptance with the regulatory authorities for the cytochrome-P450-mediated DDIs and is routinely used. However, the application of PBPK for transporter-mediated DDIs (tDDI) in drug development is relatively uncommon. Because the predictive performance of PBPK models for tDDI is not well established, here, we represent and discuss examples of PBPK analyses included in regulatory submission (the US Food and Drug Administration (FDA), the European Medicines Agency (EMA), and the Pharmaceuticals and Medical Devices Agency (PMDA)) across various tDDIs. The goal of this collaborative effort (involving scientists representing 17 pharmaceutical companies in the Consortium and from academia) is to reflect on the use of current databases and models to address tDDIs. This challenges the common perceptions on applications of PBPK for tDDIs and further delves into the requirements to improve such PBPK predictions. This review provides a reflection on the current trends in PBPK modeling for tDDIs and provides a framework to promote continuous use, verification, and improvement in industrialization of the transporter PBPK modeling.

Pharmacokinetics of Organic Cation Transporter 1 (OCT1) Substrates in Oct1/2 Knockout Mice and Species Difference in Hepatic OCT1-Mediated Uptake

Organic cation transporter 1 (OCT1) plays a role in hepatic uptake of drugs, affecting in vivo exposure, distinguished primarily through pharmacogenetics of the SLC22A1 gene. The role of OCT1 in vivo has not been confirmed, however, via drug-drug interactions that similarly affect exposure. In the current research, we used Oct1/2 knockout mice to assess the role of Oct1 in hepatic clearance and liver partitioning of clinical substrates and assess the model for predicting an effect of OCT1 function on pharmacokinetics in humans. Four OCT1 substrates (sumatriptan, fenoterol, ondansetron, and tropisetron) were administered to wild-type and knockout mice, and plasma, tissue, and urine were collected. Tissue transporter expression was evaluated using liquid chromatography-mass spectrometry. In vitro, uptake of all compounds in human and mouse hepatocytes and human OCT1- and OCT2-expressing cells was evaluated. The largest effect of knockout was on hepatic clearance and liver partitioning of sumatriptan (2- to 5-fold change), followed by fenoterol, whereas minimal changes in the pharmacokinetics of ondansetron and tropisetron were observed. This aligned with uptake in mouse hepatocytes, in which inhibition of uptake of sumatriptan and fenoterol into mouse hepatocytes by an OCT1 inhibitor was much greater compared with ondansetron and tropisetron. Conversely, inhibition of all four substrates was evident in human hepatocytes, in line with reported clinical pharmacogenetic data. These data confirm the role of Oct1 in the hepatic uptake of the four OCT1 substrates and elucidate species differences in OCT1-mediated hepatocyte uptake that should be considered when utilizing the model to predict effects in humans. SIGNIFICANCE STATEMENT: Studies in carriers of SLC22A1 null variants indicate a role of organic cation transporter 1 (OCT1) in the hepatic uptake of therapeutic agents, although OCT1-mediated drug-drug interactions have not been reported. This work used Oct1/2 knockout mice to confirm the role of Oct1 in the hepatic clearance and liver partitioning in mice for OCT1 substrates with reported pharmacogenetic effects. Species differences observed in mouse and human hepatocyte uptake clarify limitations of the knockout model for predicting exposure changes in humans for some OCT1 substrates.

Multiple binding sites in organic cation transporters require sophisticated procedures to identify interactions of novel drugs

In vitro evaluation of drugs for interaction with transporters is essential during drug development. As polyspecific organic cation transporters (OCTs) are critical for pharmacokinetics of many cationic drugs, in vitro testing of human OCT1 and human OCT2 is recommended. In the currently applied tests it is determined whether uptake of one model cation in stably transfected epithelial cells is inhibited using a substrate concentration in the micromolar range. In this review experimental evidence for the existence of low- and high-affinity cation binding sites in OCTs that may interact with drugs is compiled. Most data were obtained from studies performed with rat Oct1. Whereas overlapping low-affinity cation binding sites are directly involved in transport, the high-affinity cation binding sites may induce allosteric inhibition of transport. Remarkably, high-affinity inhibition is only observed when uptake is measured using nanomolar substrate concentrations far below the respective Km values. Affinities of inhibitors are dependent on molecular structure and concentration of the employed substrate. Because the currently applied in vitro tests for identification of interaction of novel drugs with OCTs do not consider the influence of substrate structure and are not capable of identifying high-affinity inhibition, more sophisticated testing protocols are proposed.

Enhanced and Persistent Inhibition of Organic Cation Transporter 1 Activity by Preincubation of Cyclosporine A

Recent pharmacogenetic evidence indicates that hepatic organic cation transporter (OCT) 1 can serve as the locus of drug-drug interactions (DDIs) with significant pharmacokinetic and pharmacodynamic consequences. We examined the impact of preincubation on the extent of OCT1 inhibition in transfected human embryonic kidney 293 (HEK293) cells. Following 30-minute preincubation with an inhibitor, approximately 50-fold higher inhibition potency was observed for cyclosporine A (CsA) against OCT1-mediated uptake of metformin compared with coincubation, with IC₅₀ values of 0.43 ± 0.12 and 21.6 ± 4.5 µM, respectively. By comparison, only small shifts (≤2-fold) in preincubation IC₅₀ versus coincubation were observed for quinidine, pyrimethamine, ritonavir, and trimethoprim. The shift in CsA OCT1 IC₅₀ was substrate dependent since it ranged from >1.2- to 50.2-fold using different experimental substrates. The inhibition potential of CsA toward OCT1 was confirmed by fenoterol hepatocyte uptake experiment. Furthermore, no shift in CsA IC₅₀ was observed with HEK293 cells transfected with OCT2 and organic anion transporter (OAT) 1 and OAT3. Short exposure (30 minutes) to 10 µM CsA produced long-lasting inhibition (at least 120 minutes) of the OCT1-mediated uptake of metformin in OCT1-HEK293 cells, which was likely attributable to the retention of CsA in the cells, as shown by the fact that inhibitory cellular concentrations of CsA were maintained long after the removal of the compound from the incubation buffer. The potent and persistent inhibitory effect after exposure to CsA warrants careful consideration in the design and interpretation of clinical OCT1 DDI studies. SIGNIFICANCE STATEMENT: Preincubation of OATP1B1 and OATP1B3 with their inhibitor may result in the enhancement of the inhibitory potency in a cell-based assay. However, limited data are available on potentiation of OCT1 inhibition by preincubation, which is a clinically relevant drug transporter. For the first time, we observed a 50-fold increase in CsA inhibitory potency against OCT1-mediated transport of metformin following a preincubation step. The CsA preincubation effect on OCT1 inhibition is substrate dependent. Moreover, the inhibition potential of CsA toward OCT1 is confirmed by hepatocyte uptake experiment. This study delivers clear evidences about the potent and persistent inhibitory effect on OCT1 after exposure to CsA. Further studies are needed to assess the effect of CsA on OCT1 drug substrates in vivo.

The influence of SLC22A1 rs622342 and ABCC8 rs757110 genetic variants on the efficacy of metformin and glimepiride combination therapy in Egyptian patients with type 2 diabetes

Background: The incidence of Type 2 Diabetes Mellitus (T2DM) in Egypt is considered one of the highest in the world. Metformin and Sulfonylureas are usually prescribed together due to their efficacy and their relatively low cost. Organic cation transport 1, encoded by SLC22A1 gene, is the main transporter of metformin into hepatocytes, which is considered metformin site of action. Sulfonylureas enhance insulin release from pancreatic B-cells through binding to sulfonylurea receptor 1, encoded by ABCC8 gene. Single nucleotide polymorphisms in the SLC22A1 and ABCC8 genes might affect the response of each drug.

Aims: To investigate the influence of SLC22A1 rs622342 (A>C) and ABCC8 rs757110 (A>C) genetic variants on the efficacy of metformin and glimepiride combination therapy in Egyptian T2DM patients.

Methods: Observational cross-sectional study in which patients receiving metformin and glimepiride combination therapy for at least 6 months were included for genotyping and classified into either responders or non-responders, based on their HbA1C level.

Results: A total of 127 patients were included and genotyped. They were divided into 93 responders (HbA1C<7%) and 34 non-responders (HbA1C≥7%). Minor allele frequencies for rs622342 and rs757110 were 0.189 and 0.271, respectively. Only SLC22A1 rs622342 variant was found to be associated with the response of combination therapy, in which AA alleles carriers were 2.7-times more responsive to metformin than C allele carriers (Recessive model, odds ratio = 2.718, p = 0.025, 95% CI = 1.112–6.385).

Conclusion: Genotyping of rs622342 can be useful in predicting the response to metformin in combination therapy in Egyptian T2DM patients.

Clinical Aspects of Transporter-Mediated Drug-Drug Interactions

Drug transporters play an essential role in disposition and effects of multiple drugs. Plasma concentrations of the victim drug can be modified by drug-drug interactions occurring in enterocytes (e.g., P-glycoprotein), hepatocytes (e.g., organic anion-transporting polypeptide 1B1 (OATP1B1)), and/or renal proximal tubular cells (e.g., organic cation transporter 2 (OCT2)/multidrug and toxin extrusion 1 and 2-K (MATE1/MATE2-K)). In addition, transporter-mediated drug-drug interactions can cause altered local tissue concentrations and possibly altered effects/toxicity (e.g., in liver and kidneys). During drug development, there is now an intensive in vitro screening of new molecular entities as transporter substrates and inhibitors, followed if necessary by drug-drug interaction studies in healthy volunteers. Nevertheless, there are still unresolved issues, which will also be discussed in this review article (e.g., the clinical significance of transporter-mediated drug-drug interactions of particular relevance to the elderly who are prescribed multiple drugs, with additional impaired liver or kidney function, and the extent to which medication safety in real life could be improved by a reduction of those interactions).

Impact of Promoter Polymorphisms on the Transcriptional Regulation of the Organic Cation Transporter OCT1 (SLC22A1)

The organic cation transporter 1 (OCT1, SLC22A1) is strongly expressed in the human liver and facilitates the hepatic uptake of drugs such as morphine, metformin, tropisetron, sumatriptan and fenoterol and of endogenous substances such as thiamine. OCT1 expression is inter-individually highly variable. Here, we analyzed SNPs in the OCT1 promoter concerning their potential contribution to the variability in OCT1 expression. Using electrophoretic mobility shift and luciferase reporter gene assays in HepG2, Hep3B, and Huh7 cell lines, we identified the SNPs −1795G>A (rs6935207) and −201C>G (rs58812592) as having effects on transcription factor binding and/or promoter activity. The A-allele of the −1795G>A SNP showed allele-specific binding of the transcription factor NF-Y leading to 2.5-fold increased enhancer activity of the artificial SV40 promoter. However, the −1795G>A SNP showed no significant effects on the native OCT1 promoter activity. Furthermore, the −1795G>A SNP was not associated with the pharmacokinetics of metformin, fenoterol, sumatriptan and proguanil in healthy individuals or tropisetron efficacy in patients undergoing chemotherapy. Allele-dependent differences in USF1/2 binding and nearly total loss in OCT1 promoter activity were detected for the G-allele of −201C>G, but the SNP is apparently very rare. In conclusion, common OCT1 promoter SNPs have only minor effects on OCT1 expression.

Prenatal metformin exposure or organic cation transporter 3 knock-out curbs social interaction preference in male mice

Poorly managed gestational diabetes can lead to severe complications for mother and child including fetal overgrowth, neonatal hypoglycemia and increased autism risk. Use of metformin to control it is relatively new and promising. Yet safety concerns regarding gestational metformin use remain, as its long-term effects in offspring are unclear. In light of beneficial findings with metformin for adult mouse social behavior, we hypothesized gestational metformin treatment might also promote offspring sociability. To test this, metformin was administered to non-diabetic, lean C57BL/6 J female mice at mating, with treatment discontinued at birth or wean. Male offspring exposed to metformin through birth lost social interaction preference relative to controls by time in chambers, but not by sniffing measures. Further, prenatal metformin exposure appeared to enhance social novelty preference only in females. However due to unbalanced litters and lack of statistical power, firm establishment of any sex-dependency of metformin's effects on sociability was not possible. Since organic cation transporter 3 (OCT3) transports metformin and is dense in placenta, social preferences of OCT3 knock-out males were measured. Relative to wild-type, OCT3 knock-outs had reduced interaction preference. Our data indicate gestational metformin exposure under non-diabetic conditions, or lack of OCT3, can impair social behavior in male C57BL6/J mice. Since OCT3 transports serotonin and tryptophan, impaired placental OCT3 function is one common mechanism that could persistently impact central serotonin systems and social behavior. Yet no gross alterations in serotonergic function were evident by measure of serotonin transporter density in OCT3, or serotonin turnover in metformin-exposed offspring brains. Mechanisms underlying the behavioral outcomes, and if with gestational diabetes the same would occur, remain unclear. Metformin's impacts on placental transporters and serotonin metabolism or AMPK activity in fetal brain need further investigation to clarify benefits and risks to offspring sociability from use of metformin to treat gestational diabetes.

Co-administration of nuciferine reduces the concentration of metformin in liver via differential inhibition of hepatic drug transporter OCT1 and MATE1

Nuciferine (NF), one of the main and effective components in Nelumbo nucifera Gaertn. leaf extracts, is a promising drug candidate for the treatment of obesity-related diseases, while metformin is a first line therapeutic drug for type 2 diabetes mellitus. Since nuciferine and metformin are likely to be co-administered, the aim of the present study was to evaluate whether co-administration of nuciferine would influence the liver (target tissue) distribution and the anti-diabetic effect of metformin by inhibiting hepatic organic cation transporter 1 (OCT1) and multidrug and toxin extrusion 1 (MATE1). The data demonstrated that nuciferine significantly reduced metformin accumulation in MDCK cells stably expressing human OCT1 (MDCK-hOCT1) or hMATE1 (MDCK-hMATE1), and primary cultured mouse hepatocytes. Furthermore, the presence of nuciferine in the basal compartment caused a concentration-dependent reduction of intracellular metformin accumulation in MDCK-hOCT1/hMATE1 cell monolayers. Compared with the metformin treatment-alone group, co-administration of nuciferine (40 mg/kg) markedly reduced the metformin concentration in mouse livers at 30 and 60 min after a single oral dose of metformin (200 mg/kg), and subsequently impaired the glucose-lowering effect of metformin (200 mg/kg), but the glucose-lowering effect became no different at 90 and 120 min. Therefore, nuciferine influenced the liver concentration and glucose-lowering effect of metformin only for a period of time after dose, administration of nuciferine and metformin with an interval might prevent the drug-drug interaction mediated by OCT1 and MATE1.

Transporters in Drug Development: 2018 ITC Recommendations for Transporters of Emerging Clinical Importance

This white paper provides updated International Transporter Consortium (ITC) recommendations on transporters that are important in drug development following the 3^rd ITC workshop. New additions include prospective evaluation of organic cation transporter 1 (OCT1) and retrospective evaluation of organic anion transporting polypeptide (OATP)2B1 because of their important roles in drug absorption, disposition, and effects. For the first time, the ITC underscores the importance of transporters involved in drug-induced vitamin deficiency (THTR2) and those involved in the disposition of biomarkers of organ function (OAT2 and bile acid transporters).

OCT1 Deficiency Affects Hepatocellular Concentrations and Pharmacokinetics of Cycloguanil, the Active Metabolite of the Antimalarial Drug Proguanil

Cycloguanil, the active metabolite of proguanil, acts on malaria schizonts in erythrocytes and hepatocytes. We analyzed the impact of the organic cation transporter OCT1 on hepatocellular uptake and pharmacokinetics of proguanil and cycloguanil. OCT1 transported both proguanil and cycloguanil. Common variants OCT1*3 and OCT1*4 caused a substantial decrease and OCT1*5 and OCT1*6 complete abolishment of proguanil uptake. In 39 healthy subjects, low-activity variants OCT1*3 and OCT1*4 had only minor effects on proguanil pharmacokinetics. However, both, cycloguanil area under the time-concentration curve and the cycloguanil-to-proguanil ratio were significantly dependent on number of these low-functional alleles (P = 0.02 for both). Together, CYP2C19, CYP3A5, OCT1 polymorphisms, and sex accounted for 61% of the variation in the cycloguanil-to-proguanil ratio. Most importantly, in vitro OCT1 inhibition caused a fivefold decrease of intracellular cycloguanil concentrations in primary human hepatocytes. In conclusion, OCT1-mediated uptake is a limiting step in bioactivation of proguanil, and OCT1 polymorphisms may affect proguanil efficacy against hepatic malaria schizonts.