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

Population Pharmacokinetic Model of Platinum Disposition in Cancer Patients Receiving Cisplatin and Randomized to 5-HT3 Antagonist Antiemetic Drugs

Cisplatin is a platinum-based chemotherapeutic drug used to treat many types of cancer. The aim of this study was to develop a population pharmacokinetic model that incorporates plasma unbound and bound platinum levels. Cancer patients undergoing their first or second cycle of cisplatin-containing chemotherapy (n = 33) were prospectively randomized to receive a 5-hydroxytryptamine (5-HT3) antagonist (5-HT3A) antiemetic (ondansetron, granisetron, or palonosetron) followed by blood collection over 10 days. Total and unbound platinum levels were quantified using inductively coupled plasma mass spectrometry. Plasma concentrations of bound and unbound platinum were used to develop a nonlinear mixed-effect pharmacokinetic model in Phoenix NLME (v8.3, Certara Inc.). A stepwise search was used to screen covariates that influenced pharmacokinetic parameters. A compartment for bound platinum was added to a two-compartment unbound platinum model to create a combined platinum model. The volume of the central compartment for unbound platinum (V1_u) was significantly impacted by previous cisplatin exposure and the intercompartmental clearance of unbound platinum (CL2_u) was significantly influenced by concomitant lorazepam use. The models also suggested ondansetron- and granisetron-treated subjects had a 331% and 114% increase, respectively, in circulating exposures to unbound platinum than palonosetron-treated subjects. The results suggest platinum pharmacokinetics are altered by concomitant 5-HT3A antiemetic use, concomitant lorazepam use, and previous exposure to cisplatin. Ondansetron and granisetron co-treatment increased unbound platinum exposure compared to palonosetron co-treatment, suggesting that palonosetron may be a preferred 5-HT3A to reduce the risk of cisplatin-induced kidney injury.

Substrate-specific inhibition of organic cation transporter 1 revealed using a multisubstrate drug cocktail

Transporters of the SLC22 family, such as organic cation transporter 1 (OCT1), possess very broad substrate specificity. It is unclear to what extent the inhibitory potencies of OCT1 depend on the substrate used. Here, we describe a multisubstrate drug cocktail that allows for the simultaneous testing of drug-drug interactions using 8 different victim drugs: fenoterol, salbutamol, sumatriptan, zolmitriptan, ipratropium, trospium, methylnaltrexone, and metformin. There were no significant differences in Michaelis constant (KM) and vmax of the OCT1-mediated uptake of the substrates alone or in the cocktail. Depending on the victim drug analyzed, we observed 6.7-fold differences in the inhibitory potency of fenoterol (IC50 of 0.75 μM for metformin and 5.1 μM for sumatriptan). Similarly, the inhibitory potency of verapamil varied 6.7-fold (IC50 of 1.3 μM for zolmitriptan and 8.7 μM for ipratropium). Two groups of inhibitors showed strong correlations in their victim-dependent inhibitory potencies. Group 1 comprised verapamil, quinidine, fenoterol, and ipratropium, and group 2 comprised metformin, sumatriptan, and trimethoprim. By comparing OCT1 paralogs and orthologs, the broadest substrate spectra were observed for OCT1 and multidrug and toxin extrusion 1, followed by OCT2, multidrug and toxin extrusion 2-K, and OCT3. In contrast, organic cation transporters novel 1 and organic cation transporters novel 2 exhibited very narrow substrate specificity, transporting only L-carnitine and L-ergothioneine, respectively. In conclusion, OCT1 demonstrates substantial differences in inhibitory potencies, depending on the victim drug used. We developed a cocktail approach that enables rapid screening for such differences, facilitating the identification of drug-drug interactions at the early stages of drug development. This approach can be extended to other transporters with broad substrate specificity. SIGNIFICANCE STATEMENT: Polyspecific transporters have a broad substrate-binding cavity with no defined single binding position. Consequently, inhibitors may exhibit different inhibitory potencies depending on the victim drug used for testing. Here, we demonstrate this for organic cation transporter 1 (OCT1, SLC22A1) and presents a drug cocktail designed to identify varying inhibitory potencies in vitro and prevent false-negative drug-drug interaction results during early drug development. This approach can be extended to other polyspecific drug transporters.

The population-specific Thr44Met OCT3 coding variant affects metformin pharmacokinetics with subsequent effects on insulin sensitivity in C57Bl/6J mice

AIMS/HYPOTHESIS

Metformin is an important first-line treatment for type 2 diabetes and acts by increasing the body's ability to dispose of glucose. Metformin's efficacy can be affected by genetic variants in the transporters that regulate its uptake into cells. The SLC22A3 gene (also known as EMT; EMTH; OCT3) codes for organic cation transporter 3 (OCT3), which is a broad-specificity cation transporter that also transports metformin. Most SLC22A3 variants reduce the rate of metformin transport but the rs8187715 variant (p.Thr44Met) is reported to increase uptake of metformin in vitro. However, the impact of this on in vivo metformin transport and efficacy is unknown. Very few carriers of this variant have been reported globally, but, notably, all were of Pacific Island descent. Therefore, this study aims to understand the prevalence of this variant in Polynesian peoples (Māori and Pacific peoples) and to understand its impact on metformin transport and efficacy in vivo.

METHODS

rs8187715 was genotyped in 310 individuals with Māori and Pacific ancestry recruited in Aotearoa New Zealand. To study this variant in a physiological context, an orthologous knockin mouse model with C57BL/6J background was used. Pharmacokinetic analysis compared uptake rate of metformin into tissues. Plasma growth/differentiation factor 15 (GDF-15) was also measured as a marker of metformin efficacy. Glucose and insulin tolerance was assessed after acute or sustained metformin treatment in knockin and wild-type control mice to examine the impact of the variant on metformin's glycaemic control.

RESULTS

The minor allele frequency of this variant in the Māori and Pacific participants was 15.4%. There was no association of the variant with common metabolic parameters including diabetes status, BMI, blood pressure, lipids, or blood glucose and HbA1c. However, in the orthologous knockin mouse model, the rate of metformin uptake into the blood and tissues was increased. Acute metformin dosing increased insulin sensitivity in variant knockin mice but this effect was lost after longer-term metformin treatment. Metformin's effects on GDF-15 levels were also lost in variant knockin mice with longer-term metformin treatment.

CONCLUSIONS/INTERPRETATION

These data provide evidence that the SLC22A3 rs8187715 variant accelerates metformin uptake rate in vivo. While this acutely improves insulin sensitivity, there was no increased effect of metformin with longer-term dosing. Thus, our finding of a high prevalence of this variant specifically in Māori and Pacific peoples identifies it as a potential population-specific pharmacogenetic marker with potential to guide metformin therapy in these peoples.

Structural basis of promiscuous substrate transport by Organic Cation Transporter 1

Organic Cation Transporter 1 (OCT1) plays a crucial role in hepatic metabolism by mediating the uptake of a range of metabolites and drugs. Genetic variations can alter the efficacy and safety of compounds transported by OCT1, such as those used for cardiovascular, oncological, and psychological indications. Despite its importance in drug pharmacokinetics, the substrate selectivity and underlying structural mechanisms of OCT1 remain poorly understood. Here, we present cryo-EM structures of full-length human OCT1 in the inward-open conformation, both ligand-free and drug-bound, indicating the basis for its broad substrate recognition. Comparison of our structures with those of outward-open OCTs provides molecular insight into the alternating access mechanism of OCTs. We observe that hydrophobic gates stabilize the inward-facing conformation, whereas charge neutralization in the binding pocket facilitates the release of cationic substrates. These findings provide a framework for understanding the structural basis of the promiscuity of drug binding and substrate translocation in OCT1.

Combined and independent effects of OCT1 and CYP2D6 on the cellular disposition of drugs

The organic cation transporter 1 (OCT1) mediates the cell uptake and cytochrome P450 2D6 (CYP2D6) the metabolism of many cationic substrates. Activities of OCT1 and CYP2D6 are affected by enormous genetic variation and frequent drug-drug interactions. Single or combined deficiency of OCT1 and CYP2D6 might result in dramatic differences in systemic exposure, adverse drug reactions, and efficacy. Thus, one should know what drugs are affected to what extent by OCT1, CYP2D6 or both. Here, we compiled all data on CYP2D6 and OCT1 drug substrates. Among 246 CYP2D6 substrates and 132 OCT1 substrates, we identified 31 shared substrates. In OCT1 and CYP2D6 single and double-transfected cells, we studied which, OCT1 or CYP2D6, is more critical for a given drug and whether there are additive, antagonistic or synergistic effects. In general, OCT1 substrates were more hydrophilic than CYP2D6 substrates and smaller in size. Inhibition studies showed unexpectedly pronounced inhibition of substrate depletion by shared OCT1/CYP2D6 inhibitors. In conclusion, there is a distinct overlap in the OCT1/CYP2D6 substrate and inhibitor spectra, so in vivo pharmacokinetics and -dynamics of shared substrates may be significantly affected by frequent OCT1- and CYP2D6-polymorphisms and by comedication with shared inhibitors.

SPK1/S1P axis confers gastrointestinal stromal tumors (GISTs) resistance of imatinib

BACKGROUND

Imatinib mesylate (IM) is highly effective in the treatment of gastrointestinal stromal tumors (GISTs). However, the most of GISTs patients develop secondary drug resistance after 1-3 years of IM treatment. The aim of this study was to explore the IM-resistance mechanism via the multi-scope combined with plasma concentration of IM, genetic polymorphisms and plasma sensitive metabolites.

METHODS

This study included a total of 40 GISTs patients who had been regularly treated and not treated with IM. The plasma samples were divided into three experiments, containing therapeutic drug monitoring (TDM), OCT1 genetic polymorphisms and non-targeted metabolomics. According to the data of above three experiments, the IM-resistant cell line, GIST-T1/IMR cells, was constructed for verification the IM-resistance mechanism.

RESULTS

The results of non-targeted metabolomics analysis suggested that the sphingophospholipid metabolic pathway including the SPK1/S1P axis was inferred in IM-insensitive patients with GISTs. A GIST cell line (GIST-T1) was immediately induced as an IM resistance cell model (GIST-T1/IMR) and we found that blocking the signal pathway of SPK1/S1P in the GIST-T1/IMR could sensitize treatment of IM and reverse the IM-resistance.

CONCLUSIONS

Our findings suggest that IM secondary resistance is associated with the elevation of S1P, and blockage the signaling pathway of SPK1/S1P warrants evaluation as a potential therapeutic strategy in IM-resistant GISTs. The design of this study from blood management, group information collection, IM plasma concentration with different elements, identification of sphingolipid metabolism and lastly verification the function of SPK1/S1P in the IM-resistance GISTs cells.

Human Hepatic Transporter Signature Peptides for Quantitative Targeted Absolute Proteomics: Selection, Digestion Efficiency, and Peptide Stability

PURPOSE

Quantitative targeted absolute proteomics (QTAP) quantifies proteins by measuring the signature peptides produced from target proteins by trypsin digestion. The selection of signature peptides is critical for reliable peptide quantification. The purpose of this study was to comprehensively assess the digestion efficiency and stability of tryptic peptides and to identify optimal signature peptides for human hepatic transporters and membrane marker proteins.

METHODS

The plasma membrane fraction of the human liver was digested at different time points and the peptides were comprehensively quantified using quantitative proteomics. Transporters and membrane markers were quantified using the signature peptides by QTAP.

RESULTS

Tryptic peptides were classified into clusters with low digestion efficiency, low stability, and high digestion efficiency and stability. Using the cluster information, we found that a proline residue next to the digestion site or the peptide position in or close to the transmembrane domains lowers digestion efficiency. A peptide containing cysteine at the N-terminus or arginine-glycine lowers peptide stability. Based on this information and the time course of peptide quantification, optimal signature peptides were identified for human hepatic transporters and membrane markers. The quantification of transporters with multiple signature peptides yielded consistent absolute values with less than 30% of coefficient variants in human liver microsomes and homogenates.

CONCLUSIONS

The signature peptides selected in the present study enabled the reliable quantification of human hepatic transporters. The QTAP protocol using these optimal signature peptides provides quantitative data on hepatic transporters usable for integrated pharmacokinetic studies.

The interaction of alkaloids in Coptis chinensis Franch -Tetradium ruticarpum (A. Juss.) T.G. Hartley with hOCT1 and hOCT2

ETHNOPHARMACOLOGICAL RELEVANCE

Zuojin Pill, a traditional poly-herbal drug, comprises Coptis chinensis Franch - Tetradium ruticarpum (A. Juss.) T.G. Hartley (6:1). The significant quantity of alkaloids found in the participating herbs is a key aspect of the Zuojin Pill. According to traditional Chinese medicine (TCM), these numerous alkaloidal compounds within Zuojin Pill have various essential therapeutic effects.

AIM OF THE STUDY

The alkaloids in Tetradium are mainly indole alkaloids, while the alkaloids in Coptis are mostly isoquinoline alkaloids with low bioavailability. Alkaloids and their metabolites are nitrogen-containing compounds or weakly alkaline substances that can be partially ionized under physiological pH conditions. Fortunately, organic cation transporters (OCTs) play a crucial role in the cellular uptake of weakly alkaline compounds. Therefore, we speculated that the alkaloidal compounds might interact with liver cation transporters hOCT1 and kidney cation transporters hOCT2 to alter cell drug disposal. In order to clarify our hypothesis, a series of alkaloids-OCTs interaction experiments were conducted.

MATERIALS AND METHODS

HEK293 cells stably expressing hOCT1 and hOCT2 were modeled and evaluated. Afterward, high-content screening (HCS) was conducted to analyze whether the main alkaloids and their metabolites of Coptis - Tetradium were inhibitors of hOCT1 and hOCT2 transporters. Meanwhile, LC-MS/MS was used to investigate whether the alkaloidal compounds were substrates of hOCT1 and hOCT2 transporters. Finally, drug interactions at the cellular level were assessed by LC-MS/MS after co-administration of berberine and rutacorine.

RESULTS

Berberine, jateorhizine, coptisine, epiberberine, columbamine, demethyleneberberine, and berberrubine could significantly inhibit hOCT1 and hOCT2 activity. Isoquinoline alkaloids, including berberine, jateorhizine, coptisine, epiberberine, columbamine, and palmatine, were substrates of hOCT1 and hOCT2, but not the indole alkaloids evodiamine and rutaecarpine. Furthermore, evodiamine at a concentration of 20 μmol/L had a trivial effect on berberine accumulation in HEK293-hOCT2 cells.

CONCLUSIONS

These results support the idea that alkaloidal compounds within Coptis and Tetradium have hOCT1 and hOCT2 inhibitory activity or be their substrates, and the increased oral bioavailability of berberine in vivo was closely related to the potential interactions of small molecules in Coptis- Tetradium. Overall, our study provides a framework for investigating the potential interactions of small molecules in Coptis- Tetradium.

Guide to development of compound files for PBPK modeling in the Simcyp population-based simulator

The Simcyp Simulator is a software platform for population physiologically‐based pharmacokinetic (PBPK) modeling and simulation. It links in vitro data to in vivo absorption, distribution, metabolism, excretion and pharmacokinetic/pharmacodynamic outcomes to explore clinical scenarios and support drug development decisions, including regulatory submissions and drug labels. This tutorial describes the different input parameters required, as well as the considerations needed when developing a PBPK model within the Simulator, for a small molecule intended for oral administration. A case study showing the development and application of a PBPK model for ondansetron is herein used to aid the understanding of different PBPK model development concepts.

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.

Amino acids in transmembrane helix 1 confer major functional differences between human and mouse orthologs of the polyspecific membrane transporter OCT1

Organic cation transporter 1 (OCT1) is a membrane transporter that affects hepatic uptake of cationic and weakly basic drugs. OCT1 transports structurally highly diverse substrates. The mechanisms conferring this polyspecificity are unknown. Here, we analyzed differences in transport kinetics between human and mouse OCT1 orthologs to identify amino acids that contribute to the polyspecificity of OCT1. Following stable transfection of HEK293 cells, we observed more than twofold differences in the transport kinetics of 22 out of 28 tested substrates. We found that the β2-adrenergic drug fenoterol was transported with eightfold higher affinity but at ninefold lower capacity by human OCT1. In contrast, the anticholinergic drug trospium was transported with 11-fold higher affinity but at ninefold lower capacity by mouse Oct1. Using human–mouse chimeric constructs and site-directed mutagenesis, we identified nonconserved amino acids Cys36 and Phe32 as responsible for the species-specific differences in fenoterol and trospium uptake. Substitution of Cys36 (human) to Tyr36 (mouse) caused a reversal of the affinity and capacity of fenoterol but not trospium uptake. Substitution of Phe32 to Leu32 caused reversal of trospium but not fenoterol uptake kinetics. Comparison of the uptake of structurally similar β2-adrenergics and molecular docking analyses indicated the second phenol ring, 3.3 to 4.8 Å from the protonated amino group, as essential for the affinity for fenoterol conferred by Cys36. This is the first study to report single amino acids as determinants of OCT1 polyspecificity. Our findings suggest that structure–function data of OCT1 is not directly transferrable between substrates or species.

Relationships between Inhibition, Transport and Enhanced Transport via the Organic Cation Transporter 1

The organic cation transporter 1 (OCT1, SLC22A1) transports a large number of structurally diverse endogenous and exogenous substrates. There are numerous known competitive and non-competitive inhibitors of OCT1, but there are no studies systematically analyzing the relationship between transport, stimulation, and inhibition. Here, we tested in vitro OCT1 inhibition by OCT1 substrates and transport of OCT1 inhibitors under uniform analytical conditions. Beyond inhibition testing with two model substrates, we tested nine additional OCT1 substrates for their mutual inhibition. Inhibition of ASP⁺ uptake by most OCT1 substrates was weak. The model substrate sumatriptan, with its moderately stronger inhibitability, was used to confirm this. Interestingly, OCT1 substrates exhibiting stronger OCT1 inhibition were mainly biaromatic β-agonistic drugs, such as dobutamine, fenoterol, ractopamine and ritodrine. Biaromatic organic cations were both, strong inhibitors and good substrates, but many OCT1 substrates showed little pairwise inhibition. Surprisingly, sumatriptan did significantly enhance dobutamine uptake. This effect was concentration dependent and additional experiments indicated that efflux inhibition may be one of the underlying mechanisms. Our data suggests, that OCT1 substrates are mainly weak OCT1 inhibitors and among those inhibiting well, noncompetitive inhibition could be responsible. Weak competitive inhibition confirms that OCT1 inhibition screenings poorly predict OCT1 substrates. Additionally, we showed that the OCT1 substrate sumatriptan can enhance uptake of some other OCT1 substrates. OCT1 transport stimulation was already observed earlier but is still poorly understood. Low OCT1 uptake inhibition and strong OCT1 efflux inhibition could be mechanisms exploitable for enhancing transport.

In Vitro Inhibition of Renal OCT2 and MATE1 Secretion by Antiemetic Drugs

The organic cation transporter 2 (OCT2) and multidrug and toxin extrusion protein 1 (MATE1) mediate the renal secretion of drugs. Recent studies suggest that ondansetron, a 5-HT3 antagonist drug used to prevent nausea and vomiting, can inhibit OCT2- and MATE1-mediated transport. The purpose of this study was to test the ability of five 5-HT3 antagonist drugs to inhibit the OCT2 and MATE1 transporters. The transport of the OCT2/MATE1 probe substrate ASP+ was assessed using two models: (1) HEK293 kidney cells overexpressing human OCT2 or MATE1, and (2) MDCK cells transfected with human OCT2 and MATE1. In HEK293 cells, the inhibition of ASP+ uptake by OCT2 listed in order of potency was palonosetron (IC50: 2.6 μM) > ondansetron > granisetron > tropisetron > dolasetron (IC50: 85.4 μM) and the inhibition of ASP+ uptake by MATE1 in order of potency was ondansetron (IC50: 0.1 μM) > palonosetron = tropisetron > granisetron > dolasetron (IC50: 27.4 μM). Ondansetron (0.5-20 μM) inhibited the basolateral-to-apical transcellular transport of ASP+ up to 64%. Higher concentrations (10 and 20 μM) of palonosetron, tropisetron, and dolasetron similarly reduced the transcellular transport of ASP+. In double-transfected OCT2-MATE1 MDCK cells, ondansetron at concentrations of 0.5 and 2.5 μM caused significant intracellular accumulation of ASP+. Taken together, these data suggest that 5-HT3 antagonist drugs may inhibit the renal secretion of cationic drugs by interfering with OCT2 and/or MATE1 function.

Comparison of Hepatic Transporter Tissue Expression in Rodents and Interspecies Hepatic OCT1 Activity

Hepatic clearance may be uptake rate limited by organic anion transporting polypeptides (OATPs) and organic cation transporter 1 (OCT1). While comparison of OATP activity has been investigated across species, little has been reported for OCT1. Additionally, while data on interspecies transporter expression in the liver exist, quantitative comparison of these transporters in multiple tissues is lacking. In the current research, the pharmacokinetics of OCT1 substrates (sumatriptan and metformin) were assessed in Oct knockout rats for comparison with previous Oct1/2-/- mice data and OCT1 pharmacogenetics in humans. Effect of OCT1 inhibitors verapamil and erlotinib on OCT1 substrate liver partitioning was also evaluated in rats. Expression of 18 transporters, including Oatps and Octs, in 9 tissues from mice and rats was quantitated using nanoLC/MS-MS, along with uptake transporters in hepatocytes from 5 species. Interspecies differences in OCT1 activity were further evaluated via uptake of OCT1 substrates in hepatocytes with corresponding in vivo liver partitioning in rodents and monkey. In Oct1-/- rats, sumatriptan hepatic clearance and liver partitioning decreased; however, metformin pharmacokinetics were unaffected. OCT1 inhibitor coadministration decreased sumatriptan liver partitioning. In rodents, Oatp expression was highest in the liver, although comparable expression of Oatps in other tissues was determined. Expression of Octs was highest in the kidney, with liver Oct1 expression comparably lower than Oatps. Liver partitioning of OCT1 substrates was lower in rodents than in monkey, in agreement with the highest OCT1 expression and uptake of OCT1 substrates in monkey hepatocytes. Species-dependent OCT1 activity requires consideration when translating preclinical data to the clinic.

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.

Organic Cation Transporter 1 an Intestinal Uptake Transporter: Fact or Fiction?

Intestinal transporter proteins are known to affect the pharmacokinetics and in turn the efficacy and safety of many orally administered drugs in a clinically relevant manner. This knowledge is especially well-established for intestinal ATP-binding cassette transporters such as P-gp and BCRP. In contrast to this, information about intestinal uptake carriers is much more limited although many hydrophilic or ionic drugs are not expected to undergo passive diffusion but probably require specific uptake transporters. A transporter which is controversially discussed with respect to its expression, localization and function in the human intestine is the organic cation transporter 1 (OCT1). This review article provides an up-to-date summary on the available data from expression analysis as well as functional studies in vitro, animal findings and clinical observations. The current evidence suggests that OCT1 is expressed in the human intestine in small amounts (on gene and protein levels), while its cellular localization in the apical or basolateral membrane of the enterocytes remains to be finally defined, but functional data point to a secretory function of the transporter at the basolateral membrane. Thus, OCT1 should not be considered as a classical uptake transporter in the intestine but rather as an intestinal elimination pathway for cationic compounds from the systemic circulation.

Effects of Metformin on Hepatic Steatosis in Adults with Nonalcoholic Fatty Liver Disease and Diabetes: Insights from the Cellular to Patient Levels

Nonalcoholic fatty liver disease (NAFLD) patients with diabetes constitute a subgroup of patients with a high rate of liver-related complications. Currently, there are no specific drug recommendations for these patients. Metformin, a conventional insulin sensitizer agent, has been widely prescribed in patients with diabetes. Metformin treatment has been shown to be effective at alleviating hepatic lipogenesis in animal models of NAFLD, with a variety of mechanisms being deemed responsible. To date, most studies have enrolled diabetic patients who are treated with metformin, with the drug being taken continuously throughout the study. Although evidence exists regarding the benefits of metformin for NAFLD in preclinical studies, reports on the efficacy of metformin in adult NAFLD patients have had some discrepancies regarding changes in liver biochemistry and hepatic fat content. Evidence has also suggested possible effects of metformin as regards the prevention of hepatocellular carcinoma tumorigenesis. This review was performed to comprehensively summarize the available in vitro, in vivo and clinical studies regarding the effects of metformin on liver steatosis for the treatment of adult NAFLD patients with diabetes. Consistent reports as well as controversial findings are included in this review, and the mechanistic insights are also provided. In addition, this review focuses on the efficacy of metformin as a monotherapy and as a combined therapy with other antidiabetic medications.

Expansion of Knowledge on OCT1 Variant Activity In Vitro and In Vivo Using Oct1/2-/- Mice

The role of organic cation transporter 1 (OCT1) in humans is gaining attention as data emerges regarding its role in physiology, drug exposure, and drug response. OCT1 variants with decreased in vitro function correlate well with altered exposure of multiple OCT1 substrates in variant carriers. In the current research, we investigate mechanisms behind activity of OCT1 variants in vitro by generating cell lines expressing known OCT1 variants and quantifying membrane OCT1 protein expression with corresponding OCT1 activity and kinetics. Oct knockout mice have provided additional insight into the role of Oct1 in the liver and have reproduced effects of altered OCT1 activity observed in the clinic. To assess the complex effect of Oct1 depletion on pharmacokinetics of prodrug proguanil and its active moiety cycloguanil, both of which are OCT1 substrates, Oct1/2-/- mice were used. Decreased membrane expression of OCT1 was demonstrated for all variant cell lines, although activity was substrate-dependent, as reported previously. Lack of change in activity for OCT1*2 resulted in increased intrinsic activity per pmol of OCT1 protein, particularly for sumatriptan but also for proguanil and cycloguanil. Similar to that reported in humans with decreased OCT1 function, systemic exposure of proguanil was minimally affected in Oct1/2-/- mice. However, proguanil liver partitioning and exposure decreased. Cycloguanil exposure decreased following proguanil administration in Oct1/2-/- mice, as did the systemic metabolite:parent ratio. When administered directly, systemic exposure of cycloguanil decreased slightly; however liver partitioning and exposure were decreased in Oct1/2-/- mice. Unexpectedly, following proguanil administration, the metabolite ratio in the liver changed only minimally, and liver partitioning of cycloguanil was affected in Oct1/2-/- mice to a lesser extent following proguanil administration than direct administration of cycloguanil. In conclusion, these in vitro and in vivo data offer additional complexity in understanding mechanisms of OCT1 variant activity as well as the effects of these variants in vivo. From cell lines, it is apparent that intrinsic activity is not directly related to OCT1 membrane expression. Additionally, in situations with a more complicated role of OCT1 in drug pharmacokinetics there is difficulty translating in vivo impact simply from intrinsic activity from cellular data.

Differences in Metformin and Thiamine Uptake between Human and Mouse Organic Cation Transporter 1: Structural Determinants and Potential Consequences for Intrahepatic Concentrations

The most commonly used oral antidiabetic drug, metformin, is a substrate of the hepatic uptake transporter OCT1 (gene name SLC22A1). However, OCT1 deficiency leads to more pronounced reductions of metformin concentrations in mouse than in human liver. Similarly, the effects of OCT1 deficiency on the pharmacokinetics of thiamine were reported to differ between human and mouse. Here, we compared the uptake characteristics of metformin and thiamine between human and mouse OCT1 using stably transfected human embryonic kidney 293 cells. The affinity for metformin was 4.9-fold lower in human than in mouse OCT1, resulting in a 6.5-fold lower intrinsic clearance. Therefore, the estimated liver-to-blood partition coefficient is only 3.34 in human compared with 14.4 in mouse and may contribute to higher intrahepatic concentrations in mice. Similarly, the affinity for thiamine was 9.5-fold lower in human than in mouse OCT1. Using human-mouse chimeric OCT1, we showed that simultaneous substitution of transmembrane helices TMH2 and TMH3 resulted in the reversal of affinity for metformin. Using homology modeling, we suggest several explanations, of which a different interaction of Leu155 (human TMH2) compared with Val156 (mouse TMH2) with residues in TMH3 had the strongest experimental support. In conclusion, the contribution of human OCT1 to the cellular uptake of thiamine and especially of metformin may be much lower than that of mouse OCT1. This may lead to an overestimation of the effects of OCT1 on hepatic concentrations in humans when using mouse as a model. In addition, comparative analyses of human and mouse orthologs may help reveal mechanisms of OCT1 transport. SIGNIFICANCE STATEMENT: OCT1 is a major hepatic uptake transporter of metformin and thiamine, but this study reports strong differences in the affinity for both compounds between human and mouse OCT1. Consequently, intrahepatic metformin concentrations could be much higher in mice than in humans, impacting metformin actions and representing a strong limitation of using rodent animal models for predictions of OCT1-related pharmacokinetics and efficacy in humans. Furthermore, OCT1 transmembrane helices TMH2 and TMH3 were identified to confer the observed species-specific differences in metformin affinity.

Role of Organic Solute Transporter Alpha/Beta in Hepatotoxic Bile Acid Transport and Drug Interactions

Organic solute transporter (OST) α/β is a key bile acid transporter expressed in various organs, including the liver under cholestatic conditions. However, little is known about the involvement of OSTα/β in bile acid-mediated drug-induced liver injury (DILI), a major safety concern in drug development. This study investigated whether OSTα/β preferentially transports more hepatotoxic, conjugated, primary bile acids and to what extent xenobiotics inhibit this transport. Kinetic studies with OSTα/β-overexpressing cells revealed that OSTα/β preferentially transported bile acids in the following order: taurochenodeoxycholate > glycochenodeoxycholate > taurocholate > glycocholate. The apparent half-maximal inhibitory concentrations for OSTα/β-mediated bile acid (5 µM) transport inhibition by fidaxomicin, troglitazone sulfate, and ethinyl estradiol were: 210, 334, and 1050 µM, respectively, for taurochenodeoxycholate; 97.6, 333, and 337 µM, respectively, for glycochenodeoxycholate; 140, 265, and 527 µM, respectively, for taurocholate; 59.8, 102, and 117 µM, respectively, for glycocholate. The potential role of OSTα/β in hepatocellular glycine-conjugated bile acid accumulation and cholestatic DILI was evaluated using sandwich-cultured human hepatocytes (SCHH). Treatment of SCHH with the farnesoid X receptor agonist chenodeoxycholate (100 µM) resulted in substantial OSTα/β induction, among other proteomic alterations, reducing glycochenodeoxycholate and glycocholate accumulation in cells+bile 4.0- and 4.5-fold, respectively. Treatment of SCHH with troglitazone and fidaxomicin together under cholestatic conditions resulted in increased hepatocellular toxicity compared with either compound alone, suggesting that OSTα/β inhibition may accentuate DILI. In conclusion, this study provides insights into the role of OSTα/β in preferential disposition of bile acids associated with hepatotoxicity, the impact of xenobiotics on OSTα/β-mediated bile acid transport, and the role of this transporter in SCHH and cholestatic DILI.

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.