Organic cation transporter 1 (OCT1/mOct1) is localized in the apical membrane of Caco-2 cell monolayers and enterocytes

Expression, regulation and function of intestinal drug transporters: an update

Although oral drug administration is currently the favorable route of administration, intestinal drug absorption is challenged by several highly variable and poorly predictable processes such as gastrointestinal motility, intestinal drug solubility and intestinal metabolism. One further determinant identified and characterized during the last two decades is the intestinal drug transport that is mediated by several transmembrane proteins such as P-gp, BCRP, PEPT1 and OATP2B1. It is well-established that intestinal transporters can affect oral absorption of many drugs in a significant manner either by facilitating their cellular uptake or by pumping them back to gut lumen, which limits their oral bioavailability. Their functional relevance becomes even more apparent in cases of unwanted drug-drug interactions when concomitantly given drugs that cause transporter induction or inhibition, which in turn leads to increased or decreased drug exposure. The longitudinal expression of several intestinal transporters is not homogeneous along the human intestine, which may have functional implications on the preferable site of intestinal drug absorption. Besides the knowledge about the expression of pharmacologically relevant transporters in human intestinal tissue, their exact localization on the apical or basolateral membrane of enterocytes is also of interest but in several cases debatable. Finally, there is obviously a coordinative interplay of intestinal transporters (apical-basolateral), intestinal enzymes and transporters as well as intestinal and hepatic transporters. This review aims to give an updated overview about the expression, localization, regulation and function of clinically relevant transporter proteins in the human intestine.

The effects of dietary and herbal phytochemicals on drug transporters

Membrane transporter proteins (the ABC transporters and SLC transporters) play pivotal roles in drug absorption and disposition, and thus determine their efficacy and safety. Accumulating evidence suggests that the expression and activity of these transporters may be modulated by various phytochemicals (PCs) found in diets rich in plants and herbs. PC absorption and disposition are also subject to the function of membrane transporter and drug metabolizing enzymes. PC-drug interactions may involve multiple major drug transporters (and metabolizing enzymes) in the body, leading to alterations in the pharmacokinetics of substrate drugs, and thus their efficacy and toxicity. This review summarizes the reported in vitro and in vivo interactions between common dietary PCs and the major drug transporters. The oral absorption, distribution into pharmacological sanctuaries and excretion of substrate drugs and PCs are considered, along with their possible interactions with the ABC and SLC transporters which influence these processes.

The Ussing Chamber Assay to Study Drug Metabolism and Transport in the Human Intestine

The Ussing chamber is an old but still powerful technique originally designed to study the vectorial transport of ions through frog skin. This technique is also used to investigate the transport of chemical agents through the intestinal barrier as well as drug metabolism in enterocytes, both of which are key determinants for the bioavailability of orally administered drugs. More contemporary model systems, such as Caco-2 cell monolayers or stably transfected cells, are more limited in their use compared to the Ussing chamber because of differences in expression rates of transporter proteins and/or metabolizing enzymes. While there are limitations to the Ussing chamber assay, the use of human intestinal tissue remains the best laboratory test for characterizing the transport and metabolism of compounds following oral administration. Detailed in this unit is a step-by-step protocol for preparing human intestinal tissue, for designing Ussing chamber experiments, and for analyzing and interpreting the findings. © 2017 by John Wiley & Sons, Inc.

Cellular Models and In Vitro Assays for the Screening of modulators of P-gp, MRP1 and BCRP

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are highly expressed in tumor cells, as well as in organs involved in absorption and secretion processes, mediating the ATP-dependent efflux of compounds, both endogenous substances and xenobiotics, including drugs. Their expression and activity levels are modulated by the presence of inhibitors, inducers and/or activators. In vitro, ex vivo and in vivo studies with both known and newly synthesized P-glycoprotein (P-gp) inducers and/or activators have shown the usefulness of these transport mechanisms in reducing the systemic exposure and specific tissue access of potentially harmful compounds. This article focuses on the main ABC transporters involved in multidrug resistance [P-gp, multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP)] expressed in tissues of toxicological relevance, such as the blood-brain barrier, cardiovascular system, liver, kidney and intestine. Moreover, it provides a review of the available cellular models, in vitro and ex vivo assays for the screening and selection of safe and specific inducers and activators of these membrane transporters. The available cellular models and in vitro assays have been proposed as high throughput and low-cost alternatives to excessive animal testing, allowing the evaluation of a large number of compounds.

Models for drug absorption from the small intestine: where are we and where are we going?

The small intestine is a complex organ with movements, flora, mucus and flows. Despite this, the most widely used absorption models consider the organ a cylindrical monoepithelial tube. This review presents the recent evolution of models to take into consideration the complex nature of gut physiology. The most commonly encountered issues are ethical (in vivo models) and differences in drug transport as a result of a modified expression of drug transporters or metabolic enzymes compared with human (in vitro and in vivo models). Finally, this review discusses the way forward to reach an ideal equilibrium between reproducibility, predictability and efficiency for predicting permeability. The features of an ideal model are listed as a guideline for future development.

Functional Identification of Plasma Membrane Monoamine Transporter (PMAT/SLC29A4) as an Atenolol Transporter Sensitive to Flavonoids Contained in Apple Juice

The intestinal absorption of atenolol has recently been reported to be reduced by simultaneous ingestion of fruit juices, such as apple juice. This finding implies a possibility that an unidentified carrier-mediated transport system, which could be interfered by some components of those juices, might be involved in atenolol absorption. In an attempt to explore that possibility, we successfully identified plasma membrane monoamine transporter (PMAT/SLC29A4) as a transporter that can operate for cellular atenolol uptake in the intestine, using Madin-Darby canine kidney II cells stably expressing PMAT. The specific uptake of atenolol by PMAT was greatest at around pH 6.0 and decreased with an increase in pH. At pH 6.0, the PMAT-specific uptake of atenolol was saturable with a Michaelis constant of 0.907 mM. Moreover, PMAT-specific atenolol uptake was extensively inhibited by phloretin and quercetin, which are the major flavonoids contained in apple juice, with the half maximal inhibitory concentrations of 33.3 and 116.3 μM, respectively. PMAT-specific atenolol uptake was also inhibited by several ß-blockers, suggesting that they may also be recognized and transported by PMAT. These results suggest that PMAT is an atenolol transporter that may be involved in intestinal atenolol absorption and sensitive to flavonoids contained in apple juice.

Effect of Serotonin Transporter 5-HTTLPR Polymorphism on Gastrointestinal Intolerance to Metformin: A GoDARTS Study

OBJECTIVE

The mechanism causing gastrointestinal intolerance to metformin treatment is unknown. We have previously shown that reduced-function alleles of organic cation transporter 1 (OCT1) are associated with increased intolerance to metformin. Considering recent findings that serotonin reuptake transporter (SERT) might also be involved in metformin intestinal absorption, and the role of serotonin in gastrointestinal physiology, in this study we investigated the association between a common polymorphism in the SERT gene and metformin gastrointestinal intolerance.

RESEARCH DESIGN AND METHODS

We explored the effect of composite SERT 5-HTTLPR/rs25531 genotypes, L*L* (LALA), L*S* (LALG, LAS), and S*S* (SS, SLG, LGLG), in 1,356 fully tolerant and 164 extreme metformin-intolerant patients by using a logistic regression model, adjusted for age, sex, weight, OCT1 genotype, and concomitant use of medications known to inhibit OCT1 activity.

RESULTS

The number of low-expressing SERT S* alleles increased the odds of metformin intolerance (odds ratio [OR] 1.31 [95% CI 1.02-1.67], P = 0.031). Moreover, a multiplicative interaction between the OCT1 and SERT genotypes was observed (P = 0.003). In the analyses stratified by SERT genotype, the presence of two deficient OCT1 alleles was associated with more than a ninefold higher odds of metformin intolerance in patients carrying the L*L* genotype (OR 9.25 [95% CI 3.18-27.0], P < 10-4); however, it showed a much smaller effect in L*S* carriers and no effect in S*S* carriers.

CONCLUSIONS

Our results indicate that the interaction between OCT1 and SERT genes might play an important role in metformin intolerance. Further studies are needed to replicate these findings and to substantiate the hypothesis that metformin gastrointestinal side effects could be related to the reduced intestinal serotonin uptake.

Trospium chloride is absorbed from two intestinal "absorption windows" with different permeability in healthy subjects

Intestinal P-glycoprotein is regio-selectively expressed and is a high affinity, low capacity efflux carrier for the cationic, poorly permeable trospium. Organic cation transporter 1 (OCT1) provides lower affinity but higher capacity for trospium uptake. To evaluate regional intestinal permeability, absorption profiles after gastric infusion of trospium chloride (30mg/250ml=[I]₂) for 6h and after swallowing 30mg immediate-release tablets in fasted and fed healthy subjects, were evaluated using an inverse Gaussian density function to model input rate and mean absorption time (MAT). Trospium chloride was slowly absorbed (MAT ∼10h) after gastric infusion involving two processes with different input rates, peaking at about 3h and 7h. Input rates and MAT were influenced by dosage form and meal. In conclusion, trospium is absorbed from two "windows" located in the jejunum and cecum/ascending colon, whose uptake capacity might result from local abundance and functional interplay of P-glycoprotein and OCT1.

Role of Human Organic Cation Transporter 1 (hOCT1) Polymorphisms in Lamivudine (3TC) Uptake and Drug-Drug Interactions

Lamivudine (3TC), a drug used in the treatment of HIV infection, needs to cross the plasma membrane to exert its therapeutic action. Human Organic cation transporter 1 (hOCT1), encoded by the SLC22A1 gene, is the transporter responsible for its uptake into target cells. As SLC22A1 is a highly polymorphic gene, the aim of this study was to determine how SNPs in the OCT1-encoding gene affected 3TC internalization and its interaction with other co-administered drugs. HEK293 cells stably transfected with either the wild type form or the polymorphic variants of hOCT1 were used to perform kinetic and drug-drug interaction studies. Protein co-immunoprecipitation was used to assess the impact of selected polymorphic cysteines on the oligomerization of the transporter. Results showed that 3TC transport efficiency was reduced in all polymorphic variants tested (R61C, C88R, S189L, M420del, and G465R). This was not caused by lack of oligomerization in case of variants located at the transporter extracellular loop (R61C and C88R). Drug-drug interaction measurements showed that co-administered drugs [abacavir (ABC), zidovudine (AZT), emtricitabine (FTC), tenofovir diproxil fumarate (TDF), efavirenz (EFV) and raltegravir (RAL)], differently inhibited 3TC uptake depending upon the polymorphic variant analyzed. These data highlight the need for accurate analysis of drug transporter polymorphic variants of clinical relevance, because polymorphisms can impact on substrate (3TC) translocation but even more importantly they can differentially affect drug-drug interactions at the transporter level.

Involvement of organic cation transporter 3 (Oct3/Slc22a3) in the bioavailability and pharmacokinetics of antidiabetic metformin in mice

Metformin is widely used for the treatment of type II diabetes mellitus. It was reported to be substrate of OCT3/Oct3, which is expressed in the brush boarder membrane of the enterocytes. However, the role of OCT3/Oct3 in the intestinal absorption process of metformin remains obscure. In the present study, we aimed to clarify the impact of Oct3 on the oral bioavailability and pharmacokinetics of metformin in mice, by means of in vivo pharmacokinetic study using wild-type (Oct3^+/+) and Oct3-knockout (Oct3^-/-) mice. When metformin (8.0 mg/kg) was intravenously administered to male Oct3^+/+ and Oct3^-/- mice, AUC_0-∞ of metformin was evaluated to be 659 ± 133 μg h/mL and 734 ± 213 μg h/mL, respectively. In the case of orally administered metformin (15 mg/kg), AUC_0-∞ was 578 ± 158 μg h/mL and 449 ± 101 μg h/mL in Oct3^+/+ and Oct3^-/- mice, respectively. Based on these pharmacokinetic parameters, absolute bioavailability (F) of metformin in Oct3^+/+ mice was evaluated as 46.8%, and it was significantly decreased to 32.6% in Oct3^-/- mice. Taking into account the fact that metformin undergoes negligible metabolism, these results imply that intestinal absorption of metformin is mediated at least in part by Oct3 in mice.

Early Discontinuation of Metformin in Individuals Treated with Inhibitors of Transporters of Metformin

The aim of this study was to examine the risk of early discontinuation of metformin as a proxy for intolerance, associated with use of drugs known to inhibit transporters involved in metformin distribution. We analysed all incident users of metformin in Denmark between 2000 and 2012 (n = 132,221) and in a cohort of US patients (n = 296,903). Risk of early discontinuation of metformin was assessed using adjusted logistic regression for 28 drugs putatively inhibiting metformin transporters and four negative controls. Increased odds ratio of early discontinuation of metformin was only associated with codeine, an inhibitor of organic cation transporter 1 in both cohorts [adjusted odds ratio (OR) in Danish cohort (95% CI): 1.13 (1.02-1.26), adjusted OR in American cohort (95% CI): 1.32 (1.19-1.47)]. The remaining drugs were not associated with increased odds ratio of early discontinuation and, surprisingly, four drugs were associated with a decreased risk. These findings indicate that codeine use may be associated with risk of early discontinuation of metformin and could be used as a basis for further investigation.

Pharmacogenetics in type 2 diabetes: influence on response to oral hypoglycemic agents

Type 2 diabetes is one of the leading causes of morbidity and mortality, consuming a significant proportion of public health spending. Oral hypoglycemic agents (OHAs) are the frontline treatment approaches after lifestyle changes. However, huge interindividual variation in response to OHAs results in unnecessary treatment failure. In addition to nongenetic factors, genetic factors are thought to contribute to much of such variability, highlighting the importance of the potential of pharmacogenetics to improve therapeutic outcome. Despite the presence of conflicting results, significant progress has been made in an effort to identify the genetic markers associated with pharmacokinetics, pharmacodynamics, and ultimately therapeutic response and/or adverse outcomes to OHAs. As such, this article presents a comprehensive review of current knowledge on pharmacogenetics of OHAs and provides insights into knowledge gaps and future directions.

Organic cation transporter 1 variants and gastrointestinal side effects of metformin in patients with Type 2 diabetes

AIMS

Metformin is the most widely used oral anti-diabetes agent and has considerable benefits over other therapies, yet 20-30% of people develop gastrointestinal side effects, and 5% are unable to tolerate metformin due to the severity of these side effects. The mechanism for gastrointestinal side effects and their considerable inter-individual variability is unclear. We have recently shown the association between organic cation transporter 1 (OCT1) variants and severe intolerance to metformin in people with Type 2 diabetes. The aim of this study was to explore the association of OCT1 reduced-function polymorphisms with common metformin-induced gastrointestinal side effects in Type 2 diabetes.

METHODS

This prospective observational cohort study included 92 patients with newly diagnosed Type 2 diabetes, incident users of metformin. Patients were genotyped for two common loss-of-function variants in the OCT1 gene (SLC22A1): R61C (rs12208357) and M420del (rs72552763). The association of OCT1 reduced-function alleles with gastrointestinal side effects was analysed using logistic regression.

RESULTS

Forty-three patients (47%) experienced gastrointestinal adverse effects in the first 6 months of metformin treatment. Interestingly, the number of OCT1 reduced-function alleles was significantly associated with over two-fold higher odds of the common metformin-induced gastrointestinal side effects (odds ratio = 2.31, 95% confidence interval 1.07-5.01, P = 0.034).

CONCLUSIONS

In conclusion, we showed for the first time the association between OCT1 variants and common metformin-induced gastrointestinal side effects. These results confirm recent findings related to the role of OCT1 in severe metformin intolerance, and suggest that high inter-individual variability in mild/moderate and severe gastrointestinal intolerance share a common underlying mechanism. These data could contribute to more personalized and safer metformin treatment.

Metformin and the gastrointestinal tract

Metformin is an effective agent with a good safety profile that is widely used as a first-line treatment for type 2 diabetes, yet its mechanisms of action and variability in terms of efficacy and side effects remain poorly understood. Although the liver is recognised as a major site of metformin pharmacodynamics, recent evidence also implicates the gut as an important site of action. Metformin has a number of actions within the gut. It increases intestinal glucose uptake and lactate production, increases GLP-1 concentrations and the bile acid pool within the intestine, and alters the microbiome. A novel delayed-release preparation of metformin has recently been shown to improve glycaemic control to a similar extent to immediate-release metformin, but with less systemic exposure. We believe that metformin response and tolerance is intrinsically linked with the gut. This review examines the passage of metformin through the gut, and how this can affect the efficacy of metformin treatment in the individual, and contribute to the side effects associated with metformin intolerance.

Assessment of the Relation between the Expression of Oxaliplatin Transporters in Colorectal Cancer and Response to FOLFOX-4 Adjuvant Chemotherapy: A Case Control Study

Background

Adjuvant chemotherapy for colorectal cancer is mainly based on the combination of 5-fluorouracil, folinic acid and oxaliplatin (FOLFOX-4). The pharmacological target of oxaliplatin remains intracellular and therefore dependent on its entry into cells. The intracellular distribution of oxaliplatin is mediated by organic cation transporters 1, 2 and 3 (OCT1, 2 and 3), copper transporter 1 (CTR1) and ATPase Cu²⁺ transporting beta polypeptide (ATP7B) and may modulate the efficacy of oxaliplatin-based chemotherapy. The aim of this study was to perform a retrospective study to assess the relation between the expression of oxaliplatin transporters in colorectal cancer before chemotherapy and the response to FOLFOX-4 adjuvant chemotherapy in responder and non-responder patients.

Methods

This retrospective study was conducted at a single center (University Hospital of Clermont-Ferrand, France). The target population was patients with resectable colorectal cancer operated between 2006 and 2013. Inclusion criteria were defined for the responder patients as no cancer recurrence 3 years after the end of chemotherapy, and for the non-responder patients as cancer recurrence within 1 year. Other inclusion criteria were stages IIb–IV cancers, first-line adjuvant FOLFOX-4 chemotherapy, and the availability of resected primary tumor samples. Exclusion criteria were preoperative chemotherapy and/or radiotherapy, a targeted therapy, other anticancer drugs, cancer recurrence between the first and the third year after the end of chemotherapy and follow-up < 3 years. Immunostaining of oxaliplatin transporters (OCT1, 2, 3, CTR1 and ATP7B) and Ki-67 was assessed in tumor samples.

Results

Retrospectively, 31 patients have been selected according to inclusion and exclusion criteria (15 responders and 16 non-responders). Before FOLFOX-4 regimen, OCT3 expression was significantly lower in responder patients compared to non-responders (p<0.001). According to multivariate analysis, OCT3 remains an independent criterion for adjuvant FOLFOX chemotherapy response (p = 0.039). No significant relation is reported between chemotherapy response and the expression of OCT1 (p = 0.49), OCT2 (p = 0.09), CTR1 (p = 0.45), ATP7B (p = 0.94) and Ki-67 (p = 0.34) in tumors.

Conclusions

High expression of OCT3 could be an independent factor related to resistance to FOLFOX-4 chemotherapy.

Role of SLC22A1 polymorphic variants in drug disposition, therapeutic responses, and drug-drug interactions

The SCL22A1 gene encodes the broad selectivity transporter hOCT1. hOCT1 is expressed in most epithelial barriers thereby contributing to drug pharmacokinetics. It is also expressed in different drug target cells, including immune system cells and others. Thus, this membrane protein might also contribute to drug pharmacodynamics. Up to 1000 hOCT1 polymorphisms have been identified so far, although only a small fraction of those have been mechanistically studied. A paradigm in the field of drug transporter pharmacogenetics is the impact of hOCT1 gene variability on metformin clinical parameters, affecting area under the concentration-time curve, Cmax and responsiveness. However, hOCT1 also mediates the translocation of a variety of drugs used as anticancer, antiviral, anti-inflammatory, antiemetic agents as well as drugs used in the treatment of neurological diseases among. This review focuses exclusively on those drugs for which some pharmacogenetic data are available, and aims at highlighting the need for further clinical research in this area.

Prediction and validation of enzyme and transporter off-targets for metformin

Metformin, an established first-line treatment for patients with type 2 diabetes, has been associated with gastrointestinal (GI) adverse effects that limit its use. Histamine and serotonin have potent effects on the GI tract. The effects of metformin on histamine and serotonin uptake were evaluated in cell lines overexpressing several amine transporters (OCT1, OCT3 and SERT). Metformin inhibited histamine and serotonin uptake by OCT1, OCT3 and SERT in a dose-dependent manner, with OCT1-mediated amine uptake being most potently inhibited (IC50 = 1.5 mM). A chemoinformatics-based method known as Similarity Ensemble Approach predicted diamine oxidase (DAO) as an additional intestinal target of metformin, with an E-value of 7.4 × 10(-5). Inhibition of DAO was experimentally validated using a spectrophotometric assay with putrescine as the substrate. The Ki of metformin for DAO was measured to be 8.6 ± 3.1 mM. In this study, we found that metformin inhibited intestinal amine transporters and DAO at concentrations that may be achieved in the intestine after therapeutic doses. Further studies are warranted to determine the relevance of these interactions to the adverse effects of metformin on the gastrointestinal tract.

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.

Functional identification of organic cation transporter 1 as an atenolol transporter sensitive to flavonoids

Atenolol, a β1-adrenergic receptor blocker, is administered orally and its intestinal absorption has recently been indicated to be mediated by carrier protein and reduced markedly by ingestion of some fruit juices, such as apple and orange juices. This could be postulated to be a problem arising from the interaction of some components of fruit juices with atenolol at a transporter involved in its intestinal uptake, but the responsible transporter and its interacting components have not been identified yet. In an attempt to examine that possibility, we could successfully find that human organic cation transporter 1 (OCT1/SLC22A1), which is suggested to be expressed at the brush border membrane of enterocytes, is highly capable of transporting atenolol. In this attempt, OCT1 was stably expressed in Madin-Darby canine kidney II cells and the specific uptake of atenolol by the transporter was found to be saturable, conforming to the Michaelis-Menten kinetics with the maximum transport rate (V_max) of 4.00 nmol/min/mg protein and the Michaelis constant (K_m) of 3.08 mM. Furthermore, the OCT1-specific uptake was found to be inhibited by various flavonoids, including those contained in fruit juices that have been suggested to interfere with intestinal atenolol absorption. Particularly, phloretin and quercetin, which are major components of apple juice, were potent in inhibiting OCT1-mediated atenolol transport with the inhibition constants of 38.0 and 48.0 µM, respectively. It is also notable that the inhibition by these flavonoids was of the noncompetitive type. These results indicate that OCT1 is an atenolol transporter that may be involved in intestinal atenolol uptake and sensitive to fruit juices, although its physiological and clinical relevance remains to be further examined.

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We explored an atenolol transporter from among the cation or anion transporters.

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OCT1 expressed on apical side in enterocytes has transport activity of atenolol.

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Transport of atenolol by OCT1 is inhibited by flavonoids.

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Phloretin and quercetin noncompetitively inhibit OCT1-mediated atenolol transport.

Association of Organic Cation Transporter 1 With Intolerance to Metformin in Type 2 Diabetes: A GoDARTS Study

Metformin is the most widely prescribed medication for the treatment of type 2 diabetes (T2D). However, gastrointestinal (GI) side effects develop in ~25% of patients treated with metformin, leading to the discontinuation of therapy in ~5% of cases. We hypothesized that reduced transport of metformin via organic cation transporter 1 (OCT1) could increase metformin concentration in the intestine, leading to increased risk of severe GI side effects and drug discontinuation. We compared the phenotype, carriage of reduced-function OCT1 variants, and concomitant prescribing of drugs known to inhibit OCT1 transport in 251 intolerant and 1,915 fully metformin-tolerant T2D patients. We showed that women and older people were more likely to be intolerant to metformin. Concomitant use of medications, known to inhibit OCT1 activity, was associated with intolerance (odds ratio [OR] 1.63 [95% CI 1.22-2.17], P = 0.001) as was carriage of two reduced-function OCT1 alleles compared with carriage of one or no deficient allele (OR 2.41 [95% CI 1.48-3.93], P < 0.001). Intolerance was over four times more likely to develop (OR 4.13 [95% CI 2.09-8.16], P < 0.001) in individuals with two reduced-function OCT1 alleles who were treated with OCT1 inhibitors. Our results suggest that reduced OCT1 transport is an important determinant of metformin intolerance.

Four cation-selective transporters contribute to apical uptake and accumulation of metformin in Caco-2 cell monolayers

Metformin is the frontline therapy for type II diabetes mellitus. The oral bioavailability of metformin is unexpectedly high, between 40 and 60%, given its hydrophilicity and positive charge at all physiologic pH values. Previous studies in Caco-2 cell monolayers, a cellular model of the human intestinal epithelium, showed that during absorptive transport metformin is taken up into the cells via transporters in the apical (AP) membrane; however, predominant transport to the basolateral (BL) side occurs via the paracellular route because intracellular metformin cannot egress across the BL membrane. Furthermore, these studies have suggested that the AP transporters can contribute to intestinal accumulation and absorption of metformin. Transporter-specific inhibitors as well as a novel approach involving a cocktail of transporter inhibitors with overlapping selectivity were used to identify the AP transporters that mediate metformin uptake in Caco-2 cell monolayers; furthermore, the relative contributions of these transporters in metformin AP uptake were also determined. The organic cation transporter 1, plasma membrane monoamine transporter (PMAT), serotonin reuptake transporter, and choline high-affinity transporter contributed to approximately 25%, 20%, 20%, and 15%, respectively, of the AP uptake of metformin. PMAT-knockdown Caco-2 cells were constructed to confirm the contribution of PMAT in metformin AP uptake because a PMAT-selective inhibitor is not available. The identification of four intestinal transporters that contribute to AP uptake and potentially intestinal absorption of metformin is a significant novel finding that can influence our understanding of metformin pharmacology and intestinal drug-drug interactions involving this highly prescribed drug.

The poorly membrane permeable antipsychotic drugs amisulpride and sulpiride are substrates of the organic cation transporters from the SLC22 family

Variations in influx transport at the blood-brain barrier might affect the concentration of psychotropic drugs at their site of action and as a consequence might alter therapy response. Furthermore, influx transporters in organs such as the gut, liver and kidney may influence absorption, distribution, and elimination. Here, we analyzed 30 commonly used psychotropic drugs using a parallel artificial membrane permeability assay. Amisulpride and sulpiride showed the lowest membrane permeability (P e < 1.5 × 10(-6) cm/s) and will require influx transport to penetrate the blood-brain barrier and other physiological barriers. We then studied the uptake of amisulpride and sulpiride by the organic cation transporters of the SLC22 family OCT1, OCT2, OCT3, OCTN1, and OCTN2 Amisulpride was found to be transported by all five transporters studied. In contrast, sulpiride was only transported by OCT1 and OCT2. OCT1 showed the highest transport ability both for amisulpride (CLint = 1.9 ml/min/mg protein) and sulpiride (CLint = 4.2 ml/min/mg protein) and polymorphisms in OCT1 significantly reduced the uptake of both drugs. Furthermore, we observed carrier-mediated uptake that was inhibitable by known OCT inhibitors in the immortalized human brain microvascular endothelial cell line hCMEC/D3. In conclusion, this study demonstrates that amisulpride and sulpiride are substrates of organic cation transporters of the SLC22 family. SLC22 transporters may play an important role in the distribution of amisulpride and sulpiride, including their ability to penetrate the blood-brain barrier.

SLC22, SLC44, and SLC47 transporters--organic anion and cation transporters: molecular and cellular properties

Transporters within the SLC22, SLC44, and SLC47 families of solute carriers mediate transport of a structurally diverse array of organic electrolytes, that is, molecules that are generally charged (cationic, anionic, or zwitterionic) at physiological pH. Transporters in the SLC22 family--all of which are members of the major facilitator superfamily (MFS) of transporters--represent a mechanistically diverse set of processes, including the organic anion transporters (OATs and URAT1) that physiologically operate as organic anion (OA) exchangers, the organic cation transporters (OCTs) that operate as electrogenic uniporters of organic cations (OCs), and the so-called "novel" organic cation transporters (OCTNs) that support Na-cotransport of selected zwitterions. Whereas the OCTNs display a high degree of substrate selectivity, the physiological hallmark of the OATs and OCTs is their multiselectivity--consistent with a principal role in renal and hepatic clearance of a wide array of both endogenous and xenobiotic compounds. SLC47 consists of members of the multidrug and toxin extruder (MATE) family, which are carriers that are obligatory exchangers and that physiologically support electroneutral H⁺ exchange. The MATEs also display a characteristic multiselectivity and are frequently paired with OCTs to mediate transepithelial OC secretion, with the OCTs typically supporting basolateral OC entry and the MATEs supporting apical OC efflux. The SLC44 family contains the choline transporter-like (CTL) transporters. Largely restricted to choline and a limited set of structural congeners, the CTLs appear to support the Na-independent, electrogenic uniport of choline, thereby providing choline for membrane biogenesis. The solution of X-ray crystal structures of representative prokaryotic MFS and MATE transporters has led to the development of homology models of mammalian OAT, OCT, and MATE transporters that, in turn, have supplemented studies of the molecular basis of the complex interactions of ligands with these multiselective proteins.

The inhibitory effects of the bioactive components isolated from Scutellaria baicalensis on the cellular uptake mediated by the essential solute carrier transporters

Solute carrier transporters (SLCs), in particular the organic anion transporters (OATs), OAT polypeptides (OATPs), and organic cation transporters (OCTs/OCTNs), are the important membrane proteins responsible for the cellular influx of various drugs. Baicalein (BA), baicalin (BG), and wogonin (WG) are the three major bioactive components of Scutellaria baicalensis. In this study, we evaluated the inhibitory effects of BA, BG, and WG on the cellular uptake of specific substrates mediated by the essential SLCs in human embryonic kidney-293 cells. Our data demonstrated that BA and WG significantly inhibit the OAT1-, OAT3-, and OATP1B3-mediated uptake; BG effectively reduces the influx of substrates of OAT3, OAT4, OATP1B3, and OATP2B1; WG is a potent inhibitor of OCT3. Our further kinetic analysis derived the IC50 values of these compounds with pronounced inhibitory effects on SLCs, particularly the inhibitions of WG on OAT1 and OCT3 and that of BA and WG on OAT3. Our study comprehensively evaluated the inhibitory effects of three bioactive components of Scutellaria baicalensis on the uptake of specific substrates mediated by the essential SLC transporters, which suggested that precautions will be needed when coadministrating drugs with Scutellaria baicalensis so as to prevent the unfavorable drug-drug/herb interactions in human.

Genetically modified mouse models for oral drug absorption and disposition

Intestinal absorption is an essential step in the therapeutic use of most orally administered drugs and often mediated by enterocyte transmembrane transporters. Here we discuss several of these drug transport systems and knockout mouse models to study them. These studies showed that Multidrug resistance-associated protein 2 (Mrp2) can limit intestinal drug absorption. Organic cation transporter n1 (Octn1) and Octn2 might also facilitate intestinal drug absorption, although direct in vivo evidence is lacking. On the other hand, intestinal uptake of drugs is facilitated by the Equilibrative nucleoside transporter 1 (Ent1), Mrp3 and possibly Mrp4. No significant role in intestinal absorption for Oct1 and Oct2 or for Organic anion-transporting polypeptides (Oatp) 1a and 1b was found so far.