A novel human organic anion transporting polypeptide localized to the basolateral hepatocyte membrane

OATPs, OATs and OCTs: the organic anion and cation transporters of the SLCO and SLC22A gene superfamilies

The human organic anion and cation transporters are classified within two SLC superfamilies. Superfamily SLCO (formerly SLC21A) consists of organic anion transporting polypeptides (OATPs), while the organic anion transporters (OATs) and the organic cation transporters (OCTs) are classified in the SLC22A superfamily. Individual members of each superfamily are expressed in essentially every epithelium throughout the body, where they play a significant role in drug absorption, distribution and elimination. Substrates of OATPs are mainly large hydrophobic organic anions, while OATs transport smaller and more hydrophilic organic anions and OCTs transport organic cations. In addition to endogenous substrates, such as steroids, hormones and neurotransmitters, numerous drugs and other xenobiotics are transported by these proteins, including statins, antivirals, antibiotics and anticancer drugs. Expression of OATPs, OATs and OCTs can be regulated at the protein or transcriptional level and appears to vary within each family by both protein and tissue type. All three superfamilies consist of 12 transmembrane domain proteins that have intracellular termini. Although no crystal structures have yet been determined, combinations of homology modelling and mutation experiments have been used to explore the mechanism of substrate recognition and transport. Several polymorphisms identified in members of these superfamilies have been shown to affect pharmacokinetics of their drug substrates, confirming the importance of these drug transporters for efficient pharmacological therapy. This review, unlike other reviews that focus on a single transporter family, briefly summarizes the current knowledge of all the functionally characterized human organic anion and cation drug uptake transporters of the SLCO and the SLC22A superfamilies.

Interaction of the antiviral drug telaprevir with renal and hepatic drug transporters

Telaprevir is a new, direct-acting antiviral drug that has been approved for the treatment of chronic hepatitis C viral infection. First data on drug-drug interactions with co-medications such as cyclosporine, tacrolimus and atorvastatin have been reported recently. Drug transporting proteins have been shown to play an important role in clinically observed drug-drug interactions. The aim of this study was therefore to systematically investigate the potential of telaprevir to inhibit drug transporting proteins. The effect of telaprevir on substrate uptake mediated by drug transporters located in human kidney and liver was investigated on a functional level in HEK293 cell lines that over-express single transporter. Telaprevir was shown to exhibit significant inhibition of the human renal drug transporters OCT2 and MATE1 with IC(50) values of 6.4 μM and 23.0 μM, respectively, whereas no inhibitory effect on OAT1 and OAT3 mediated transport by telaprevir was demonstrated. Liver drug transporters were inhibited with an IC(50) of 2.2 μM for OATP1B1, 6.8 μM for OATP1B3 and 20.7 μM for OCT1. Our data show that telaprevir exhibited significant potential to inhibit human drug transporters. In view of the inhibitory potential of telaprevir, clinical co-administration of telaprevir together with drugs that are substrates of renal or hepatic transporters should be carefully monitored.

Inhibition of the organic anion-transporting polypeptide 1B1 by quercetin: an in vitro and in vivo assessment

AIM

To investigate the effect of quercetin on organic anion transporting polypeptide 1B1 (OATP1B1) activities in vitro and on the pharmacokinetics of pravastatin, a typical substrate for OATP1B1 in healthy Chinese-Han male subjects.

METHODS

Using human embryonic kidney 293 (HEK293) cells stably expressing OATP1B1, we observed the effect of quercetin on OATP1B1-mediated uptake of estrone-3-sulphate (E3S) and pravastatin. The influence of quercetin on the pharmacokinetics of pravastatin was measured in 16 healthy Chinese-Han male volunteers receiving a single dose of pravastatin (40 mg orally) after co-administration of placebo or 500 mg quercetin capsules (once daily orally for 14 days).

RESULTS

Quercetin competitively inhibited OATP1B1-mediated E3S uptake with a K(i) value of 17.9 ± 4.6 µm and also inhibited OATP1B1-mediated pravastatin uptake in a concentration dependent manner (IC(50) , 15.9 ± 1.4 µm). In healthy Chinese-Han male subjects, quercetin increased the pravastatin area under the plasma concentration - time curve (AUC(0,10 h) and the peak plasma drug concentration (C(max)) to 24% (95% CI 15, 32%, P < 0.001) and 31% (95% CI 20, 42%, P < 0.001), respectively. After administration of quercetin, the elimination half-life (t(1/2) ) of pravastatin was prolonged by 14% (95% CI 4, 24%, P = 0.027), with no change in the time to reach C(max) (t(max) ). Moreover, quercetin decreased the apparent clearance (CL/F) of pravastatin by 18% (95% CI 75, 89%, P < 0.001).

CONCLUSIONS

These findings suggest that quercetin inhibits the OATP1B1-mediated transport of E3S and pravastatin in vitro and also has a modest inhibitory influence on the pharmacokinetics of pravastatin in healthy Chinese-Han male volunteers. The effects of quercetin on other OATP1B1 substrate drugs deserve further investigation.

Identification of drugs and drug metabolites as substrates of multidrug resistance protein 2 (MRP2) using triple-transfected MDCK-OATP1B1-UGT1A1-MRP2 cells

BACKGROUND AND PURPOSE

The coordinate activity of hepatic uptake transporters [e.g. organic anion transporting polypeptide 1B1 (OATP1B1)], drug-metabolizing enzymes [e.g. UDP-glucuronosyltransferase 1A1 (UGT1A1)] and efflux pumps (e.g. MRP2) is a crucial determinant of drug disposition. However, limited data are available on transport of drugs (e.g. ezetimibe, etoposide) and their glucuronidated metabolites by human MRP2 in intact cell systems.

EXPERIMENTAL APPROACH

Using monolayers of newly established triple-transfected MDCK-OATP1B1-UGT1A1-MRP2 cells as well as MDCK control cells, single- (OATP1B1) and double-transfected (OATP1B1-UGT1A1, OATP1B1-MRP2) MDCK cells, we therefore studied intracellular concentrations and transcellular transport after administration of ezetimibe or etoposide to the basal compartment.

KEY RESULTS

Intracellular accumulation of ezetimibe was significantly lower in MDCK-OATP1B1-UGT1A1-MRP2 triple-transfected cells compared with all other cell lines. Considerably higher amounts of ezetimibe glucuronide were found in the apical compartment of MDCK-OATP1B1-UGT1A1-MRP2 monolayers compared with all other cell lines. Using HEK cells, etoposide was identified as a substrate of OATP1B1. Intracellular concentrations of etoposide equivalents (i.e. parent compound plus metabolites) were affected only to a minor extent by the absence or presence of OATP1B1/UGT1A1/MRP2. In contrast, apical accumulation of etoposide equivalents was significantly higher in monolayers of both cell lines expressing MRP2 (MDCK-OATP1B1-MRP2, MDCK-OATP1B1-UGT1A1-MRP2) compared with the single-transfected (OATP1B1) and the control cell line.

CONCLUSIONS AND IMPLICATIONS

Ezetimibe glucuronide is a substrate of human MRP2. Moreover, etoposide and possibly also its glucuronide are substrates of MRP2. These data demonstrate the functional interplay between transporter-mediated uptake, phase II metabolism and export by hepatic proteins involved in drug disposition.

The dual role of pharmacogenetics in HIV treatment: mutations and polymorphisms regulating antiretroviral drug resistance and disposition

Significant intra- and interindividual variability has been observed in response to use of pharmacological agents in treatment of HIV infection. Treatment of HIV infection is limited by high rates of adverse drug reactions and development of resistance in a significant proportion of patients as a result of suboptimal drug concentrations. The efficacy of antiretroviral therapy is challenged by the emergence of resistant HIV-1 mutants with reduced susceptibility to antiretroviral drugs. Moreover, pharmacotherapy of patients infected with HIV is challenging because a great number of comorbidities increase polypharmacy and the risk for drug-drug interactions. Drug-metabolizing enzymes and drug transporters regulate drug access to the systemic circulation, target cells, and sanctuary sites. These factors, which determine drug exposure, along with the emergence of mutations conferring resistance to HIV medications, could explain variability in efficacy and adverse drug reactions associated with antiretroviral drugs. In this review, the major factors affecting the disposition of antiretroviral drugs, including key drug-metabolizing enzymes and membrane drug transporters, are outlined. Genetic polymorphisms affecting the activity and/or the expression of cytochromes P450 or UGT isozymes and membrane drug transport proteins are highlighted and include such examples as the association of neurotoxicity with efavirenz, nephrotoxicity with tenofovir, hepatotoxicity with nevirapine, and hyperbilirubinemia with indinavir and atazanavir. Mechanisms of drug resistance conferred by specific viral mutations are also reviewed, with particular attention to replicative viral fitness and transmitted HIV drug resistance with the objectives of providing a better understanding of mechanisms involved in HIV drug resistance and helping health care providers to better manage interpatient variability in drug efficacy and toxicity.

Classification of inhibitors of hepatic organic anion transporting polypeptides (OATPs): influence of protein expression on drug-drug interactions

The hepatic organic anion transporting polypeptides (OATPs) influence the pharmacokinetics of several drug classes and are involved in many clinical drug–drug interactions. Predicting potential interactions with OATPs is, therefore, of value. Here, we developed in vitro and in silico models for identification and prediction of specific and general inhibitors of OATP1B1, OATP1B3, and OATP2B1. The maximal transport activity (MTA) of each OATP in human liver was predicted from transport kinetics and protein quantification. We then used MTA to predict the effects of a subset of inhibitors on atorvastatin uptake in vivo. Using a data set of 225 drug-like compounds, 91 OATP inhibitors were identified. In silico models indicated that lipophilicity and polar surface area are key molecular features of OATP inhibition. MTA predictions identified OATP1B1 and OATP1B3 as major determinants of atorvastatin uptake in vivo. The relative contributions to overall hepatic uptake varied with isoform specificities of the inhibitors.

Pharmacogenetics of drug transporters in the enterohepatic circulation

This article summarizes the impact of the pharmacogenetics of drug transporters expressed in the enterohepatic circulation on the pharmacokinetics and pharmacodynamics of drugs. The role of pharmacogenetics in the function of drug transporter proteins in vitro is now well established and evidence is rapidly accumulating from in vivo pharmacokinetic studies, which suggests that genetic variants of drug transporter proteins can translate into clinically relevant phenotypes. However, a large amount of conflicting information on the clinical relevance of drug transporter proteins has so far precluded the emergence of a clear picture regarding the role of drug transporter pharmacogenetics in medical practice. This is very well exemplified by the case of P-glycoprotein (MDR1, ABCB1). The challenge is now to develop pharmacogenetic models with sufficient predictive power to allow for translation into drug therapy. This will require a combination of pharmacogenetics of drug transporters, drug metabolism and pharmacodynamics of the respective drugs.

Prediction of the in vivo OATP1B1-mediated drug-drug interaction potential of an investigational drug against a range of statins

To support drug development, the drug-drug interaction potential (DDI) of an investigational drug (AZX) was assessed against the probe estradiol 17β-glucuronide as well as against simvastatin acid, atorvastatin, pravastatin, pitavastatin, fluvastatin, rosuvastatin and estrone 3-sulfate. The inhibitory potentials of the OATP1B1 inhibitors rifamycin SV and gemfibrozil were assessed in parallel. Monolayer cellular uptake assays were used to determine inhibition of human OATP1B1. Apparent K(m) values for the OATP1B1-mediated transport of [(3)H] substrates were determined prior to their use as probes in inhibition studies, and ranged from 0.6 to 29 μM for statins. The K(m) of lipophilic simvastatin acid could not be determined due to its high passive permeability that masked OATP1B1 transport, and therefore this statin could not be used as a probe. Estrone 3-sulfate exhibited biphasic kinetics, whereas estradiol 17β-glucuronide demonstrated simple Michaelis-Menton kinetics. AZX moderately inhibited OATP1B1-mediated transport of all statins (IC(50)=4.6-9.7 μM), except fluvastatin, of estradiol 17β-glucuronide (IC(50)=5.3 μM), and weakly inhibited estrone 3-sulfate (IC(50)=79 μM). Rifamycin SV strongly, and gemfibrozil weakly, inhibited the OATP1B1-mediated transport of substrates. Estradiol 17β-glucuronide was identified as a good surrogate probe for statins when assessing OATP1B1 inhibitory potential using this test system. Inhibition data was used to predict the likelihood of a clinical DDI, using current draft US FDA guidance and recommendations of the International Transporter Consortium. Predictions for AZX indicated the potential for an OATP1B1-mediated DDI in vivo and that a clinical interaction study is warranted to confirm whether AZX is an OATP1B1 inhibitor in the clinic.

Interaction of three regiospecific amino acid residues is required for OATP1B1 gain of OATP1B3 substrate specificity

The human organic anion-transporting polypeptides OATP1B1 (SLCO1B1) and OATP1B3 (SLCO1B3) are liver-enriched membrane transporters of major importance to hepatic uptake of numerous endogenous compounds, including bile acids, steroid conjugates, hormones, and drugs, including the 3-hydroxy-3-methylglutaryl Co-A reductase inhibitor (statin) family of cholesterol-lowering compounds. Despite their remarkable substrate overlap, there are notable exceptions: in particular, the gastrointestinal peptide hormone cholecystokinin-8 (CCK-8) is a high affinity substrate for OATP1B3 but not OATP1B1. We utilized homologous recombination of linear DNA by E. coli to generate a library of cDNA containing monomer size chimeric OATP1B1-1B3 and OATP1B3-1B1 transporters with randomly distributed chimeric junctions to identify three discrete regions of the transporter involved in conferring CCK-8 transport activity. Site-directed mutagenesis of three key residues in OATP1B1 transmembrane helices 1 and 10, and extracellular loop 6, to the corresponding residues in OATP1B3, resulted in a gain of CCK-8 transport by OATP1B1. The residues appear specific to CCK-8, as the mutations did not affect transport of the shared OATP1B substrate atorvastatin or the OATP1B1-specific substrate estrone sulfate. Regions involved in gain of CCK-8 transport by OATP1B1, when mapped to the crystal structures of bacterial transporters from the major facilitator superfamily, are positioned to suggest these regions could readily interact with drug substrates. Accordingly, our data provide new insight into the molecular determinants of the substrate specificity of these hepatic uptake transporters with relevance to targeted drug design and prediction of drug-drug interactions.

Inhibition of hepatic uptake transporters by flavonoids

Members of the human SLC superfamily such as organic anion transporting polypeptide 1B1 (OATP1B1), OATP1B3, and organic cation transporter 1 (OCT1) are drug uptake transporters that are localised on the basolateral membrane of hepatocytes mediating the uptake of drugs such as atorvastatin and metformin into hepatocytes. Ingredients of food such as flavonoids influence the effects of drugs, e.g. by inhibition of drug transporters. Therefore, we investigated the impact of the Ginkgo biloba flavonoids apigenin, kaempferol, and quercetin, and the grapefruit flavonoids naringenin, naringin, and rutin on the OATP1B1, OATP1B3, and OCT1 transport activity. Transporter expressing HEK293 cell lines were used with [3H]sulfobromophthalein ([3H]BSP) as substrate for OATP1B1 and OATP1B3, [3H]atorvastatin as substrate for OATP1B1, and [3H]1-methyl-4-phenylpyridinium ([3H]MPP(+)) as substrate for OCT1. The G. biloba flavonoids showed a competitive inhibition of the OATP1B1- and OATP1B3-mediated [3H]BSP and the OATP1B1-mediated [3H]atorvastatin uptake. Quercetin was the most potent inhibitor of the OATP1B1- and OATP1B3-mediated [3H]BSP transport with K(i)-values of 8.8±0.8μM and 7.8±1.7μM, respectively. For the inhibition of the OATP1B1-mediated [3H]atorvastatin transport, apigenin was the most potent inhibitor with a K(i) value of 0.6±0.2μM. Among the grapefruit flavonoids, naringenin was the most potent inhibitor of the OATP1B1- and OATP1B3-mediated [3H]BSP transport with IC(50)-values of 81.6±1.1μM and 101.1±1.1μM, respectively. All investigated flavonoids showed no significant inhibition of the OCT1-mediated [3H]MPP(+) uptake. Taken together, these in vitro studies showed that the investigated flavonoids inhibit the OATP1B1- and OATP1B3-mediated drug transport, which could be a mechanism for food-drug interactions in humans.

Complete OATP1B1 and OATP1B3 deficiency causes human Rotor syndrome by interrupting conjugated bilirubin reuptake into the liver

Bilirubin, a breakdown product of heme, is normally glucuronidated and excreted by the liver into bile. Failure of this system can lead to a buildup of conjugated bilirubin in the blood, resulting in jaundice. The mechanistic basis of bilirubin excretion and hyperbilirubinemia syndromes is largely understood, but that of Rotor syndrome, an autosomal recessive disorder characterized by conjugated hyperbilirubinemia, coproporphyrinuria, and near-absent hepatic uptake of anionic diagnostics, has remained enigmatic. Here, we analyzed 8 Rotor-syndrome families and found that Rotor syndrome was linked to mutations predicted to cause complete and simultaneous deficiencies of the organic anion transporting polypeptides OATP1B1 and OATP1B3. These important detoxification-limiting proteins mediate uptake and clearance of countless drugs and drug conjugates across the sinusoidal hepatocyte membrane. OATP1B1 polymorphisms have previously been linked to drug hypersensitivities. Using mice deficient in Oatp1a/1b and in the multispecific sinusoidal export pump Abcc3, we found that Abcc3 secretes bilirubin conjugates into the blood, while Oatp1a/1b transporters mediate their hepatic reuptake. Transgenic expression of human OATP1B1 or OATP1B3 restored the function of this detoxification-enhancing liver-blood shuttle in Oatp1a/1b-deficient mice. Within liver lobules, this shuttle may allow flexible transfer of bilirubin conjugates (and probably also drug conjugates) formed in upstream hepatocytes to downstream hepatocytes, thereby preventing local saturation of further detoxification processes and hepatocyte toxic injury. Thus, disruption of hepatic reuptake of bilirubin glucuronide due to coexisting OATP1B1 and OATP1B3 deficiencies explains Rotor-type hyperbilirubinemia. Moreover, OATP1B1 and OATP1B3 null mutations may confer substantial drug toxicity risks.

Pharmacological actions of statins: a critical appraisal in the management of cancer

Statins, among the most commonly prescribed drugs worldwide, are cholesterol-lowering agents used to manage and prevent cardiovascular and coronary heart diseases. Recently, a multifaceted action in different physiological and pathological conditions has been also proposed for statins, beyond anti-inflammation and neuroprotection. Statins have been shown to act through cholesterol-dependent and -independent mechanisms and are able to affect several tissue functions and modulate specific signal transduction pathways that could account for statin pleiotropic effects. Typically, statins are prescribed in middle-aged or elderly patients in a therapeutic regimen covering a long life span during which metabolic processes, aging, and concomitant novel diseases, including cancer, could occur. In this context, safety, toxicity, interaction with other drugs, and the state of health have to be taken into account in subjects treated with statins. Some evidence has shown a dichotomous effect of statins with either cancer-inhibiting or -promoting effects. To date, clinical trials failed to demonstrate a reduced cancer occurrence in statin users and no sufficient data are available to define the long-term effects of statin use over a period of 10 years. Moreover, results from clinical trials performed to evaluate the therapeutic efficacy of statins in cancer did not suggest statin use as chemotherapeutic or adjuvant agents. Here, we reviewed the pharmacology of the statins, providing a comprehensive update of the current knowledge of their effects on tissues, biological processes, and pathological conditions, and we dissected the disappointing evidence on the possible future use of statin-based drugs in cancer therapy.

Differential effects of pravastatin and simvastatin on the growth of tumor cells from different organ sites

3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) inhibitors, commonly known as statins, may possess cancer preventive and therapeutic properties. Statins are effective suppressors of cholesterol synthesis with a well-established risk-benefit ratio in cardiovascular disease prevention. Mechanistically, targeting HMGCR activity primarily influences cholesterol biosynthesis and prenylation of signaling proteins. Pravastatin is a hydrophilic statin that is selectively taken up by a sodium-independent organic anion transporter protein-1B1 (OATP1B1) exclusively expressed in liver. Simvastatin is a hydrophobic statin that enters cells by other mechanisms. Poorly-differentiated and well-differentiated cancer cell lines were selected from various tissues and examined for their response to these two statins. Simvastatin inhibited the growth of most tumor cell lines more effectively than pravastatin in a dose dependent manner. Poorly-differentiated cancer cells were generally more responsive to simvastatin than well-differentiated cancer cells, and the levels of HMGCR expression did not consistently correlate with response to statin treatment. Pravastatin had a significant effect on normal hepatocytes due to facilitated uptake and a lesser effect on prostate PC3 and colon Caco-2 cancer cells since the OATP1B1 mRNA and protein were only found in the normal liver and hepatocytes. The inhibition of cell growth was accompanied by distinct alterations in mitochondrial networks and dramatic changes in cellular morphology related to cofilin regulation and loss of p-caveolin. Both statins, hydrophilic pravastatin and hypdrophobic simvastatin caused redistribution of OATP1B1 and HMGCR to perinuclear sites. In conclusion, the specific chemical properties of different classes of statins dictate mechanistic properties which may be relevant when evaluating biological responses to statins.

Genetic polymorphisms of OATP transporters and their impact on intestinal absorption and hepatic disposition of drugs

There is convincing evidence that many organic anion transporting polypeptide (OATP) transporters influence the pharmacokinetics and pharmacological efficacy of their substrate drugs. Each OATP family member has a unique combination of tissue distribution, substrate specificity and mechanisms of gene expression. Among them, OATP1B1, OATP1B3 and OATP2B1 have been considered as critical molecular determinants of the pharmacokinetics of a variety of clinically important drugs. Liver-specific expression of OATP1B1 and OATP1B3 contributes to the hepatic uptake of drugs from the portal vein, and OATP2B1 may alter their intestinal absorption as well as hepatic extraction. Accordingly, changes in function and expression of these three OATPs owing to genetic polymorphisms may lead to altered pharmacological effects, including decreased drug efficacy and increased risk of adverse effects. Association of genetic polymorphisms in OATP genes with alterations in the pharmacokinetic properties of their substrate drugs has been reported; however, there still exists a degree of discordance between the reported outcomes in different clinical settings. For better understanding of the clinical relevance of genetic polymorphisms of OATP1B1, OATP1B3 and OATP2B1, the present review focuses on the association of the genotypes of these OATPs with in vitro activity changes and in vivo clinical outcomes of substrate drugs.

Influence of SLCO1B1 polymorphisms on the drug-drug interaction between darunavir/ritonavir and pravastatin

The authors investigated whether SLCO1B1 polymorphisms contribute to variability in pravastatin pharmacokinetics when pravastatin is administered alone versus with darunavir/ritonavir. HIV-negative healthy participants were prospectively enrolled on the basis of SLCO1B1 diplotype: group 1 (*1A/*1A, n = 9); group 2 (*1A/*1B, n = 10; or *1B/*1B, n = 2); and group 3 (*1A/*15, n = 1; *1B/*15, n = 5; or *1B/*17, n = 1). Participants received pravastatin (40 mg) daily on days 1 through 4, washout on days 5 through 11, darunavir/ritonavir (600/100 mg) twice daily on days 12 through 18, with pravastatin 40 mg added back on days 15 through 18. Pharmacokinetic studies were conducted on day 4 (pravastatin alone) and day 18 (pravastatin + darunavir/ritonavir). Pravastatin area under the plasma concentration-time curve (AUC(tau)) was 21% higher during administration with darunavir/ritonavir compared with pravastatin alone; however, this difference was not statistically significant (P = .11). Group 3 variants had 96% higher pravastatin AUC(tau) on day 4 and 113% higher pravastatin AUC(tau) on day 18 compared with group 1. The relative change in pravastatin pharmacokinetics was largest in group 3 but did not differ significantly between diplotype groups. In sum, the influence of SLCO1B1*15 and *17 haplotypes on pravastatin pharmacokinetics was maintained in the presence of darunavir/ritonavir. Because OATP1B1 inhibition would be expected to be greater in carriers of normal or high-functioning SLCO1B1 haplotypes, these findings suggest that darunavir/ritonavir is not a potent inhibitor of OATP1B1-mediated pravastatin transport in vivo.

Rare versus common variants in pharmacogenetics: SLCO1B1 variation and methotrexate disposition

Methotrexate is used to treat autoimmune diseases and malignancies, including acute lymphoblastic leukemia (ALL). Inter-individual variation in clearance of methotrexate results in heterogeneous systemic exposure, clinical efficacy, and toxicity. In a genome-wide association study of children with ALL, we identified SLCO1B1 as harboring multiple common polymorphisms associated with methotrexate clearance. The extent of influence of rare versus common variants on pharmacogenomic phenotypes remains largely unexplored. We tested the hypothesis that rare variants in SLCO1B1 could affect methotrexate clearance and compared the influence of common versus rare variants in addition to clinical covariates on clearance. From deep resequencing of SLCO1B1 exons in 699 children, we identified 93 SNPs, 15 of which were non-synonymous (NS). Three of these NS SNPs were common, with a minor allele frequency (MAF) >5%, one had low frequency (MAF 1%-5%), and 11 were rare (MAF <1%). NS SNPs (common or rare) predicted to be functionally damaging were more likely to be found among patients with the lowest methotrexate clearance than patients with high clearance. We verified lower function in vitro of four SLCO1B1 haplotypes that were associated with reduced methotrexate clearance. In a multivariate stepwise regression analysis adjusting for other genetic and non-genetic covariates, SLCO1B1 variants accounted for 10.7% of the population variability in clearance. Of that variability, common NS variants accounted for the majority, but rare damaging NS variants constituted 17.8% of SLCO1B1's effects (1.9% of total variation) and had larger effect sizes than common NS variants. Our results show that rare variants are likely to have an important effect on pharmacogenetic phenotypes.

The emerging role of transport systems in liver function tests

Liver function tests are of critical importance for the management of patients with severe or terminal liver disease. They are also used as prognostic tools for planning liver resections. In recent years many transport systems have been identified that also transport substances employed in liver function tests. Such substances include endogenous bilirubin or exogenously administered indocyanine green, agents for magnetic resonance imaging, agents for single photon emission computed tomography or agents for breath tests. The increasing functional and molecular information on the respective transport systems should improve the management and as a result the outcome of patients scheduled for liver surgery or transplantation. To achieve the latter goal, clinical studies that assess individual patients' liver function over the course of their disease with liver function tests are needed to firmly establish and validate recently introduced and novel liver function markers.

Functional consequences of genetic variations in the human organic anion transporting polypeptide 1B3 (OATP1B3) in the Korean population

The objectives of this study were to investigate the allele frequencies and linkage disequilibrium (LD) in the organic anion transporting polypeptide 1B3 (OATP1B3) in the Korean population and to examine the functional consequences. Using samples from 48 Koreans, direct sequencing was carried out to determine the allele frequencies and LD of OATP1B3 in a representative Korean population. Thirty-six genetic variations in the transporter were found in Koreans; among them, five undocumented variations (i.e.,-6436G>C in the 5'-upstream region, 26A>C and 586A>G in the protein coding region, and IVS6-72A>T and IVS12-80A>T in intron regions) were identified. In the upstream region, -5035G>A was found to have lowered gene expression, as determined by a reporter gene assay, suggesting that this variation reduces the expression of OATP1B3 in humans. The functional relevance of the genetic variations in the protein coding region was determined by an uptake study involving representative substrates in human embryonic kidney 293 cells expressing wild type or variant forms. Variations involving 699G>A showed a reduced uptake activity for testosterone, but not for estradiol 17β-d-glucuronide or methotrexate, indicating that the functional impact of the variations is substrate specific. Considering the kinetic relevance of OATP1B3, the functionally affected variations may be therapeutically important.

Hepatic uptake in the dog: comparison of uptake in hepatocytes and human embryonic kidney cells expressing dog organic anion-transporting polypeptide 1B4

Although the dog is frequently used in pharmacological, pharmacokinetic, and drug safety studies, little is known about canine drug transporters. Dog organic anion-transporting polypeptide (Oatp1b4) has recently been cloned (Comp Biochem Physiol C Toxicol Pharmacol 151:393-399, 2010), but the contribution of Oatp1b4 to hepatic uptake has yet to be clarified. This study compares the transport characteristics of dog Oatp1b4 with those of human OATP1B1/1B3 and demonstrates the importance of Oatp1b4 in the uptake of anionic compounds in dog hepatocytes. Oatp1b4 is the predominant Oatp in dog liver with expression levels double and 30 times those of Oatp2b1 and Oatp1a2, respectively. Uptake of a range of typical OATP substrates by Oatp1b4-expressing HEK293 cells was compared with that in fresh dog hepatocytes. All compounds tested were transported by Oatp1b4 and uptake intrinsic clearance (CL(int, uptake)) in dog hepatocytes in sodium-free buffer was correlated significantly with CL(int, uptake) in Oatp1b4-expressing cells. Dog in vivo clearance for five substrates was predicted more accurately from CL(int, uptake) than from metabolic intrinsic clearance (CL(int, met)), indicating that uptake governs the overall in vivo hepatic clearance of these anionic compounds in dog. The substrate specificities of dog Oatp1b4 appear to be similar to those of human OATP1B1/OATP1B3, whereas the relative uptake clearance of substrates for Oatp1b4 correlate better with OATP1B3 than with the more abundant hepatic analog OATP1B1.

Transporter-mediated drug-drug interactions

Drug-drug interactions are a serious clinical issue. An important mechanism underlying drug-drug interactions is induction or inhibition of drug transporters that mediate the cellular uptake and efflux of xenobiotics. Especially drug transporters of the small intestine, liver and kidney are major determinants of the pharmacokinetic profile of drugs. Transporter-mediated drug-drug interactions in these three organs can considerably influence the pharmacokinetics and clinical effects of drugs. In this article, we focus on probe drugs lacking significant metabolism to highlight mechanisms of interactions of selected intestinal, hepatic and renal drug transporters (e.g., organic anion transporting polypeptide [OATP] 1A2, OATP2B1, OATP1B1, OATP1B3, P-gp, organic anion transporter [OAT] 1, OAT3, breast cancer resistance protein [BCRP], organic cation transporter [OCT] 2 and multidrug and toxin extrusion protein [MATE] 1). Genotype-dependent drug-drug interactions are also discussed.

Mechanisms of membrane transport of folates into cells and across epithelia

Until recently, the transport of folates into cells and across epithelia has been interpreted primarily within the context of two transporters with high affinity and specificity for folates, the reduced folate carrier and the folate receptors. However, there were discrepancies between the properties of these transporters and characteristics of folate transport in many tissues, most notably the intestinal absorption of folates, in terms of pH dependency and substrate specificity. With the recent cloning of the proton-coupled folate transporter (PCFT) and the demonstration that this transporter is mutated in hereditary folate malabsorption, an autosomal recessive disorder, the molecular basis for this low-pH transport activity is now understood. This review focuses on the properties of PCFT and briefly addresses the two other folate-specific transporters along with other facilitative and ATP-binding cassette (ABC) transporters with folate transport activities. The role of these transporters in the vectorial transport of folates across epithelia is considered.

Contribution of organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 to hepatic uptake of nateglinide, and the prediction of drug–drug interactions via these transporters

Objectives

We have investigated the contributions of organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 to the hepatic uptake of nateglinide, and the possibility of drug–drug interactions via these transporters.

Methods

Uptake studies using transporter-expressing HEK293 cells and cryopreserved human hepatocytes were performed to examine the contributions of each transporter. Inhibition studies using cryopreserved human hepatocytes were performed to examine the possibility of drug–drug interactions.

Key findings

The rate of saturable hepatic uptake of nateglinide using human hepatocytes was 47.6%. A certain increase in uptake was observed in the examination using transporter-expressing HEK293 cells, indicating contributions of OATP1B1 and OATP1B3 to hepatic nateglinide uptake. The 50% inhibitory concentration (IC50) values of nateglinide using cryopreserved human hepatocytes for uptake of estrone 3-sulfate (substrate of OATP1B1), and cholecystokinin octapeptide (substrate of OATP1B3) were 168 and 17.4 µmol/l, respectively. Moreover, ciclosporin inhibited saturable hepatic uptake of nateglinide with an IC50 value of 6.05 µmol/l. The calculated 1 + Iin,max,u/IC50 values for inhibition of OATP1B1 and OATP1B3 by nateglinide, and the inhibition of saturable uptake of nateglinide by ciclosporin, were all close to 1, indicating a low clinical risk of drug–drug interaction with nateglinide taken up via OATP1B1 and OATP1B3.

Conclusions

OATP1B1 and OATP1B3 may have contributed to the hepatic uptake of nateglinide, but the possibility of drug–drug interactions appeared to be low.

Transporter-mediated drug-drug interactions with oral antidiabetic drugs

Uptake transporters (e.g., members of the SLC superfamily of solute carriers) and export proteins (e.g., members of the ABC transporter superfamily) are important determinants for the pharmacokinetics of drugs. Alterations of drug transport due to concomitantly administered drugs that interfere with drug transport may alter the kinetics of drug substrates. In vitro and in vivo studies indicate that many drugs used for the treatment of metabolic disorders and cardiovascular diseases (e.g., oral antidiabetic drugs, statins) are substrates for uptake transporters and export proteins expressed in the intestine, the liver and the kidney. Since most patients with type 2 diabetes receive more than one drug, transporter-mediated drug-drug interactions are important molecular mechanisms leading to alterations in oral antidiabetic drug pharmacokinetics with the risk of adverse drug reactions. This review focuses on uptake transporters of the SLCO/SLC21 (OATP) and SLC22 (OCT/OAT) family of solute carriers and export pumps of the ABC (ATP-binding cassette) transporter superfamily (especially P-glycoprotein) as well as the export proteins of the SLC47 (MATE) family and their role for transporter-mediated drug-drug interactions with oral antidiabetic drugs.

Effect of organic anion-transporting polypeptide 1B1 (OATP1B1) polymorphism on the single- and multiple-dose pharmacokinetics of enalapril in healthy Chinese adult men

BACKGROUND

Enalapril is an angiotensin-converting enzyme (ACE) inhibitor approved for the treatment of mild to severe hypertension and congestive heart failure. There is evidence that enalapril may be an organic anion-transporting polypeptide 1B1 (OATP1B1) substrate, suggesting that genetic polymorphisms of the OATP1B1 gene may play a role in causing the interindividual pharmacokinetic differences of this drug.

OBJECTIVE

The purpose of this study was to investigate the functional significance of the OATP1B1 genetic polymorphism on the pharmacokinetics of enalapril and its active metabolite enalaprilat in healthy Chinese adult male participants.

METHODS

This was a single-center, open-label, single- and multiple-dose study conducted in healthy Chinese male participants. Each participant received a single oral dose of 10 mg enalapril under fasting conditions, followed by enalapril 10 mg/d for 7 days. In the single-dose phase, sequential blood samples were collected from 0 to 24 hours after drug administration. In the multiple-dose phase, samples were obtained before drug administration on days 4, 5, 6, and 7; on day 7, samples were collected from 0 to 72 hours after drug administration. An HPLC-MS/MS method was used to determine plasma concentrations of enalapril and enalaprilat. A polymerase chain reaction technique was used for genotyping of 2 single nucleotide polymorphisms (SNPs) of the OATP1B1 gene: T521C and A388G. The pharmacokinetic parameters of enalapril and enalaprilat were then compared according to genotype groups, using 1-way ANOVA, except for T(max) in which the Mann-Whitney test or Kruskal-Wallis test was used.

RESULTS

The study included 32 healthy Han Chinese male participants (age range, 18-28 years; weight range, 50.0-80.0 kg; height range,159-182.0 cm). Twenty-six were OATP1B1*15 noncarriers (homozygous for 521TT), the others were *15 carriers with at least one 521 T>C mutant allele. After single and multiple oral doses of 10 mg enalapril, plasma concentrations of enalapril in *15 noncarriers were lower than that in *15 carriers, with significant difference in area under the curve at steady state (AUC(ss)) between *15 noncarriers and *15 carriers (P = 0.048) in the multiple-dose phase. There were no significant differences in enalapril's AUC(0-24), C(max), or the ratio of the AUC(0-24h) in the single-dose study to the AUC(ss) (R(ac)) between the *15 carriers and noncarriers. In contrast to enalapril, the mean AUC(0-24h) and C(max) of enalaprilat in *15 noncarriers was significantly higher than those in *15 carriers (P = 0.040 and P = 0.027, respectively) in the single-dose phase. There were no significant differences in enalaprilat's AUC(ss) or C(maxss) between the 2 groups in the multiple-dose phase. For the 3 groups classified according to the effect of A388G variant in all subjects homozygous for 521T (TT), *1a/*1a, *1a/*1b, and *1b/*1b, no significant difference was found in AUC(0-24h), C(max), and T(max) of enalapril and enalaprilat.

CONCLUSIONS

In this small population of healthy Chinese men, the OATP1B1*15 allele and T521C variant appeared to be an important determinant of the pharmacokinetics of enalapril. There were significant differences between the *15 carriers and noncarriers in enalapril's AUC(ss) and enalaprilat's AUC(0-24h), C(max), and R(ac). However, there were no significant differences in enalapril's AUC(0-24), C(max), or enalaprilat's AUC(ss), C(maxss) between the *15 carriers and noncarriers.

Loss of organic anion transporting polypeptide 1a1 increases deoxycholic acid absorption in mice by increasing intestinal permeability

Deoxycholic acid (DCA) is a known hepatotoxicant, a tissue tumor promoter, and has been implicated in colorectal cancer. Male mice are more susceptible to DCA toxicity than female mice. Organic anion transporting polypeptide 1a1 (Oatp1a1), which is known to transport bile acids (BAs) in vitro, is predominantly expressed in livers of male mice. In addition, the concentrations of DCA and its taurine conjugate (TDCA) are increased in serum of Oatp1a1-null mice. To investigate whether Oatp1a1 contributes to the gender difference in DCA toxicity in mice, wild-type (WT) and Oatp1a1-null mice were fed a 0.3% DCA diet for 7 days. After feeding DCA, Oatp1a1-null mice had 30-fold higher concentrations of DCA in both serum and livers than WT mice. Feeding DCA caused more hepatotoxcity in Oatp1a1-null mice than WT mice. After feeding DCA, Oatp1a1-null mice expressed higher BA efflux-transporters (bile salt-export pump, organic solute transporter (Ost)α/β, and multidrug resistance-associated protein [Mrp]2) and lower BA-synthetic enzymes (cytochrome P450 [Cyp]7a1, 8b1, 27a1, and 7b1) in livers than WT mice. Intravenous administration of DCA and TDCA showed that lack of Oatp1a1 does not decrease the plasma elimination of DCA or TDCA. After feeding DCA, the concentrations of DCA in ileum and colon tissues are higher in Oatp1a1-null than in WT mice. In addition, Oatp1a1-null mice have enhanced intestinal permeability. Taken together, the current data suggest that Oatp1a1 does not mediate the hepatic uptake of DCA or TDCA, but lack of Oatp1a1 increases intestinal permeability and thus enhances the absorption of DCA in mice.

The expression and function of organic anion transporting polypeptides in normal tissues and in cancer

Organic anion transporting polypeptides (OATPs) are members of the SLCO gene superfamily of proteins. The 11 human OATPs are classified into 6 families and subfamilies on the basis of their amino acid sequence similarities. OATPs are expressed in several epithelial tissues throughout the body and transport mainly amphipathic molecules with molecular weights of more than 300 kDa. Members of the OATP1 and OATP2 families are functionally the best-characterized OATPs. Among these are the multispecific OATP1A2, OATP1B1, OATP1B3, and OATP2B1. They transport various endo- and xenobiotics, including hormones and their conjugates as well as numerous drugs such as several anticancer agents. Recent reports demonstrate that some OATPs are up- or downregulated in several cancers and that OATP expression might affect cancer development. On the basis of the findings summarized in this review, we propose that OATPs could be valuable targets for anticancer therapy.

The expression and function of organic anion transporting polypeptides in normal tissues and in cancer

Organic anion transporting polypeptides (OATPs) are members of the SLCO gene superfamily of proteins. The 11 human OATPs are classified into 6 families and subfamilies on the basis of their amino acid sequence similarities. OATPs are expressed in several epithelial tissues throughout the body and transport mainly amphipathic molecules with molecular weights of more than 300 kDa. Members of the OATP1 and OATP2 families are functionally the best-characterized OATPs. Among these are the multispecific OATP1A2, OATP1B1, OATP1B3, and OATP2B1. They transport various endo- and xenobiotics, including hormones and their conjugates as well as numerous drugs such as several anticancer agents. Recent reports demonstrate that some OATPs are up- or downregulated in several cancers and that OATP expression might affect cancer development. On the basis of the findings summarized in this review, we propose that OATPs could be valuable targets for anticancer therapy.

Expression of SLCO1B3 is associated with intratumoral cholestasis and CTNNB1 mutations in hepatocellular carcinoma

Recent studies have shown that intratumoral cholestasis is a hallmark of CTNNB1 mutations in hepatocellular carcinomas (HCC). Here, we analyzed the expressions of genes involved in bile acid and bilirubin metabolism and their correlation with the mutational status of CTNNB1 in a series of HCC. The expressions of CYP7A1 and CYP27A1, which encode rate-limiting enzymes in bile acid synthesis, were unaltered or only marginally increased in CTNNB1-mutated HCC compared with those in HCC with wild-type CTNNB1. Among the genes involved in bile acid and bilirubin transport, the expression of SLCO1B3 was significantly elevated in HCC with CTNNB1 mutations, whereas the expression of ABCC4 was elevated in HCC with wild-type CTNNB1. Immunohistochemistry confirmed the frequent expression of SLCO1B3 in CTNNB1-mutated HCC at the protein level, but not in most HCC with wild-type CTNNB1. Immunohistochemistry for MRP4 (encoded by ABCC4) partly agreed with ABCC4 expression, but most cases did not express detectable levels of MRP4. Notably, all HCC with bile accumulation, including those without CTNNB1 mutations, expressed SLCO1B3, suggesting that SLCO1B3 expression, rather than CTNNB1 mutation, is the critical determinant of intratumoral cholestasis. As SLCO1B3 is involved in the uptake of a number of chemotherapeutic and diagnostic agents, SLCO1B3 expression and the status of CTNNB1 mutation might need to be considered in the drug delivery to HCC.

Influence of non-steroidal anti-inflammatory drugs on organic anion transporting polypeptide (OATP) 1B1- and OATP1B3-mediated drug transport

The transporter-mediated uptake of drugs from blood into hepatocytes is a prerequisite for intrahepatic drug action or intracellular drug metabolism before excretion. Therefore, uptake transporters, e.g., members of the organic anion transporting polypeptide (OATP) family are important determinants of drug pharmacokinetics. Highly and almost exclusively expressed in hepatocytes are the OATP family members OATP1B1 (SLCO1B1) and OATP1B3 (SLCO1B3). Drug substrates of OATP1B1 and OATP1B3 include antibiotics and HMG-CoA reductase inhibitors (statins). It has been demonstrated that administration of two or more drugs that are substrates for these hepatic uptake transporters may lead to transporter-mediated drug-drug interactions, resulting in altered transport kinetics for drug substrates. In this study we investigated whether non-steroidal anti-inflammatory drugs (NSAIDs) and paracetamol interact with OATP1B1 and OATP1B3 using the standard substrate BSP and the drug substrate pravastatin. Using human embryonic kidney cells stably expressing OATP1B1 or OATP1B3, we demonstrated that bromosulfophthalein uptake was inhibited by diclofenac, ibuprofen. and lumiracoxib. Of interest, pravastatin uptake was stimulated by these NSAIDs, and for ibuprofen we determined activation constants (EC₅₀ values) of 64.0 and 93.1 μM for OATP1B1- and OATP1B3-mediated uptake, respectively. Furthermore, we investigated whether NSAIDs were also substrates for OATP1B1 and OATP1B3 and demonstrated that only diclofenac was significantly transported by OATP1B3, whereas all other NSAIDs investigated were not substrates for these uptake transporters. These results demonstrated that drugs may interact with transport proteins by allosteric mechanisms without being substrates and, therefore, not only uptake inhibition but also allosteric-induced modulation of transport function may be an important mechanism in transporter-mediated drug-drug interactions.

Fully automated microinjection system for Xenopus laevis oocytes with integrated sorting and collection

Microinjection is the most flexible transfection method in terms of choice of reagents to inject into cells. But this method lacks the high throughput to compete with less flexible methods like chemical- or viral-based approaches. Various approaches have been pursued to increase the throughput by automating the microinjection process. However, these approaches focused solely on the microinjection itself and disregarded the tasks before and after the injection, which also belong to the critical time path of the whole process, that is, sorting out viable cells from a cell suspension, placing the cell for injection, and collecting the cell after the injection. In the approach with our XenoFactor, we demonstrate a system capable of running the whole process automatically. By optimizing the XenoFactor for Xenopus laevis oocytes, we could demonstrate the successful automated injection. Starting from a suspension with a mixture of defolliculated oocytes at different stages and quality levels, the manual approach requires 1 day in total for the preparation of 400 microinjected oocytes. The XenoFactor takes only 4h for the same amount and delivers injected oocytes of reproducible quality and without the fatigue symptoms experienced during the manual approach.

Interactions of green tea catechins with organic anion-transporting polypeptides

Organic anion-transporting polypeptides (OATPs) are multispecific transporters that mediate the uptake of numerous drugs and xenobiotics into cells. Here, we examined the effect of green tea (Camellia sinensis) catechins on the function of the four OATPs expressed in human enterocytes and hepatocytes. Uptake of the model substrate estrone-3-sulfate by cells expressing OATP1A2, OATP1B1, OATP1B3, or OATP2B1 was measured in the absence and presence of the four most abundant flavonols found in green tea. Uptake by OATP1A2, OATP1B1, and OATP2B1 was inhibited by epicatechin gallate (ECG) and epigallocatechin gallate (EGCG) in a concentration-dependent way. In contrast, OATP1B3-mediated uptake of estrone-3-sulfate was strongly stimulated by EGCG at low substrate concentrations. The effect of EGCG on OATP1B3 was also studied with additional substrates: uptake of estradiol-17β-glucuronide was unchanged, whereas uptake of Fluo-3 was noncompetitively inhibited. Both ECG and EGCG were found to be substrates of OATP1A2 (K(m) values of 10.4 and 18.8 μM, respectively) and OATP1B3 (34.1 and 13.2 μM, respectively) but not of OATP1B1 or OATP2B1. These results indicate that two of the major flavonols found in green tea have a substantial effect on the function of OATPs expressed in enterocytes and hepatocytes and can potentially alter the pharmacokinetics of drugs and other OATP substrates. In addition, the diverse effects of EGCG on the transport of other OATP1B3 substrates suggest that different transport/binding sites are involved.

SLCO1B1 polymorphism and oral antidiabetic drugs

Organic anion-transporting polypeptide 1B1 (OATP1B1; gene: SLCO1B1) is an influx transporter expressed on the sinusoidal membrane of human hepatocytes, where it mediates the uptake of its substrates from blood into liver. In vitro, the SLCO1B1 c.521T>C (p.Val174Ala) single-nucleotide polymorphism (SNP) has been associated with reduced and the c.388A>G (p.Asn130Asp) SNP with both enhanced and reduced transport activity of OATP1B1. In vivo in humans, the c.521C allele (present in SLCO1B1*5 and *15 haplotypes) is associated with decreased hepatic uptake and increased plasma concentrations of several OATP1B1 substrates. The SLCO1B1*1B (c.388G-c.521T) haplotype is associated with enhanced hepatic uptake and decreased plasma concentrations of some OATP1B1 substrates. The SLCO1B1 c.521CC genotype has been associated with an about 60-190% increased, and the SLCO1B1*1B/*1B genotype with an about 30% decreased area under the plasma concentration-time curve of repaglinide. Moreover, SLCO1B1 polymorphism can affect the extent of interaction between OATP1B1 inhibitors and repaglinide. Accordingly, SLCO1B1 genotyping may help in choosing the optimal starting dose of repaglinide. In Chinese individuals, the SLCO1B1 c.521C allele has been associated with increased plasma concentrations of nateglinide, but the association could not be replicated in Caucasians. SLCO1B1 genotype has had no effect on the pharmacokinetics of rosiglitazone, pioglitazone or their metabolites. The hepatic uptake of metformin is mediated by organic cation transporters 1 and 3, and the liver is not important for the elimination or action of the dipeptidylpeptidase 4 inhibitors sitagliptin, vildagliptin and saxagliptin. Therefore, SLCO1B1 polymorphism unlikely affects the response to these antidiabetics. Possible effects of SLCO1B1 polymorphism on sulfonylureas remain to be investigated.

Organic anion transporting polypeptide 1B1: a genetically polymorphic transporter of major importance for hepatic drug uptake

The importance of membrane transporters for drug pharmacokinetics has been increasingly recognized during the last decade. Organic anion transporting polypeptide 1B1 (OATP1B1) is a genetically polymorphic influx transporter expressed on the sinusoidal membrane of human hepatocytes, and it mediates the hepatic uptake of many endogenous compounds and xenobiotics. Recent studies have demonstrated that OATP1B1 plays a major, clinically important role in the hepatic uptake of many drugs. A common single-nucleotide variation (coding DNA c.521T>C, protein p.V174A, rs4149056) in the SLCO1B1 gene encoding OATP1B1 decreases the transporting activity of OATP1B1, resulting in markedly increased plasma concentrations of, for example, many statins, particularly of active simvastatin acid. The variant thereby enhances the risk of statin-induced myopathy and decreases the therapeutic indexes of statins. However, the effect of the SLCO1B1 c.521T>C variant is different on different statins. The same variant also markedly affects the pharmacokinetics of several other drugs. Furthermore, certain SLCO1B1 variants associated with an enhanced clearance of methotrexate increase the risk of gastrointestinal toxicity by methotrexate in the treatment of children with acute lymphoblastic leukemia. Certain drugs (e.g., cyclosporine) potently inhibit OATP1B1, causing clinically significant drug interactions. Thus, OATP1B1 plays a major role in the hepatic uptake of drugs, and genetic variants and drug interactions affecting OATP1B1 activity are important determinants of individual drug responses. In this article, we review the current knowledge about the expression, function, substrate characteristics, and pharmacogenetics of OATP1B1 as well as its role in drug interactions, in parts comparing with those of other hepatocyte-expressed organic anion transporting polypeptides, OATP1B3 and OATP2B1.

Effects of SLCO1B1 polymorphisms on the pharmacokinetics and pharmacodynamics of repaglinide in healthy Chinese volunteers

PURPOSE

Repaglinide is commonly used in the treatment of patients with type 2 diabetes mellitus to reduce postprandial hyperglycemia. The objective of this research was to study the effects of SLCO1B1 polymorphisms on the pharmacokinetics and pharmacodynamics of repaglinide in healthy Chinese volunteers.

METHODS

A total of 22 healthy young male participants were recruited from a pool of pharmacogenetically characterized participants genotyped for SLCO1B1, CYP3A4, and CYP2C8 SNPs by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Volunteers with CYP2C8*3 and CYP3A4*4 alleles were excluded from the clinical study. Then selected volunteers took part in the clinical pharmacokinetic study, receiving 2 mg repaglinide.

RESULTS

Healthy participants with SLCO1B1*1A/*1B or *1A/*1A genotype and SLCO1B1 *15/*1A or *5/*1A genotype had significantly higher AUC(0-∞) than participants with SLCO1B1*1B/*1B genotype, with the former showing an increase over the latter of 39.81 and 42.09%, respectively (P = 0.028, 0.032). The clearance in the former two genotype groups was significantly attenuated (by 27.39 and 28.55%, respectively) compared with individuals with SLCO1B1*1B/*1B genotype (P = 0.015, 0.019). No significant differences in blood glucose-lowering effect were observed among three genotype groups.

CONCLUSIONS

SLCO1B1*1B/*1B genotype is associated with reduced pharmacokinetic exposure after a single dose oral administration of 2 mg repaglinide, including decreased AUC(0-∞) and increased clearance of repaglinide. Moreover, this polymorphism of SLCO1B1 has significant influence on the pharmacokinetics of repaglinide, but no effects on its pharmacodynamics.

Frequencies of single-nucleotide polymorphisms of SLCO1A2, SLCO1B3 and SLCO2B1 genes in a Finnish population

Organic anion transporting polypeptides 1A2, 1B3 and 2B1 (OATP1A2, OATP1B3 and OATP2B1) are expressed in tissues important for pharmacokinetics, and mediate the cellular influx of various endogenous and exogenous compounds, including drugs. The aim of the study was to investigate the frequencies of single-nucleotide polymorphisms (SNP) of SLCO1A2, SLCO1B3 and SLCO2B1 in a Finnish population. The distribution of nine non-synonymous SLCO1A2, SLCO1B3 and SLCO2B1 SNPs was determined in 552 healthy Finnish Caucasian participants by using allelic discrimination with TaqMan 5'nuclease assays. The SLCO1A2 c.38T>C (p.Ile13Thr) and c.516C>T (p.Glu172Asp) SNPs were found with variant allele frequencies of 12.9% (95% confidence interval: 11.0-15.0) and 7.2% (5.8-8.8). The variant allele frequencies of SLCO1B3 c.334T>G (p.Ser112Ala), c.699G>A (p.Met233Ile) and c.767G>C (p.Gly256Ala) were 77.0% (74.4-79.4), 76.9% (74.3-79.3) and 12.8% (10.9-14.9), respectively. None of the participants carried the SLCO1B3 c.1309G>A (p.Gly437Ser) SNP. The SLCO2B1 c.601G>A (p.Val201Met), c.935G>A (p.Arg312Gln) and c.1457C>T (p.Ser486Phe) variant allele frequencies were 2.1% (1.4-3.1), 13.6% (11.7-15.7) and 2.8% (2.0-4.0), respectively. The SLCO1B3 c.334T>G and c.699G>A SNPs were in a nearly complete linkage disequilibrium (r² = 0.99, D' = 1.00), all other SNP pairs showed only a weak correlation. In conclusion, non-synonymous sequence variations of SLCO1A2, SLCO1B3 and SLCO2B1 occur at high frequencies in the Finnish population.

Characterization of digoxin uptake in sandwich-cultured human hepatocytes

Digoxin is a drug that is commonly used to treat congestive heart failure. Because of digoxin's narrow therapeutic index, patients are susceptible to drug-drug interaction-mediated cardiotoxicity. Digoxin is primarily cleared renally; however, a significant component of clearance is due to multidrug resistance 1-mediated transport into bile. Digoxin is reported to be actively transported into human hepatocytes by the organic anion-transporting polypeptide 1B3 (OATP1B3); however, further characterization has not been fully described. The purpose of this study was to investigate the hepatic uptake mechanisms of [(3)H]digoxin using sandwich-cultured human hepatocytes (SCHH) and transporter-expressing cells. Digoxin uptake in SCHH involves both a saturable (carrier-mediated) process and a passive (nonsaturable) process. At low concentrations, the saturable component exhibited an apparent K(m) of 2.39 μM and a V(max) of 4.49 pmol/(min · mg protein). The calculated passive diffusion clearance was 1.25 μl/(min · mg protein). Uptake of [(3)H]digoxin in SCHH was not inhibited by a variety of substrates or inhibitors for OATP1B1, OATP1B3, OATP2B1, organic anion transporter 2, organic cation transporter 1, and monocarboxylate transporter 8. Cytochalasin B, which inhibits glucose transporters, did not significantly inhibit digoxin uptake, whereas the flavonoids quercetin and rutin inhibited uptake by ∼50%. Nonlabeled digoxin inhibited [(3)H]digoxin uptake by ∼50%. Studies with OATP-transfected human embryonic kidney cells or oocytes showed that digoxin is not a substrate of OATP1B1, OATP2B1, or OATP1B3. In conclusion, the data suggest that digoxin uptake in SCHH involves both saturable and passive processes. The saturable process is mediated by an as yet undetermined digoxin transporter(s).

Uptake transporters of the human OATP family: molecular characteristics, substrates, their role in drug-drug interactions, and functional consequences of polymorphisms

Organic anion transporting polypeptides (OATPs, gene family: SLC21/SLCO) mediate the uptake of a broad range of substrates including several widely prescribed drugs into cells. Drug substrates for members of the human OATP family include HMG-CoA-reductase inhibitors (statins), antibiotics, anticancer agents, and cardiac glycosides. OATPs are expressed in a variety of different tissues including brain, intestine, liver, and kidney, suggesting that these uptake transporters are important for drug absorption, distribution, and excretion. Because of their wide tissue distribution and broad substrate spectrum, altered transport kinetics, for example, due to drug-drug interactions or due to the functional consequences of genetic variations (polymorphisms), can contribute to the interindividual variability of drug effects. Therefore, the molecular characteristics of human OATP family members, the role of human OATPs in drug-drug interactions, and the in vitro analysis of the functional consequences of genetic variations in SLCO genes encoding OATP proteins are the focus of this chapter.

Membrane drug transporters and chemoresistance in human pancreatic carcinoma

Pancreatic cancer ranks among the tumors most resistant to chemotherapy. Such chemoresistance of tumors can be mediated by various cellular mechanisms including dysregulated apoptosis or ineffective drug concentration at the intracellular target sites. In this review, we highlight recent advances in experimental chemotherapy underlining the role of cellular transporters in drug resistance. Such contribution to the chemoresistant phenotype of tumor cells or tissues can be conferred both by uptake and export transporters, as demonstrated by in vivo and in vitro data. Our studies used human pancreatic carcinoma cells, cells stably transfected with human transporter cDNAs, or cells in which a specific transporter was knocked down by RNA interference. We have previously shown that 5-fluorouracil treatment affects the expression profile of relevant cellular transporters including multidrug resistance proteins (MRPs), and that MRP5 (ABCC5) influences chemoresistance of these tumor cells. Similarly, cell treatment with the nucleoside drug gemcitabine or a combination of chemotherapeutic drugs can variably influence the expression pattern and relative amount of uptake and export transporters in pancreatic carcinoma cells or select for pre-existing subpopulations. In addition, cytotoxicity studies with MRP5-overexpressing or MRP5-silenced cells demonstrate a contribution of MRP5 also to gemcitabine resistance. These data may lead to improved strategies of future chemotherapy regimens using gemcitabine and/or 5-fluorouracil.