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

The Signature Sequence Region of the Human Drug Transporter Organic Anion Transporting Polypeptide 1B1 Is Important for Protein Surface Expression

The organic anion transporting polypeptides (OATPs) encompass a family of membrane transport proteins responsible for the uptake of xenobiotic compounds. Human organic anion transporting polypeptide 1B1 (OATP1B1) mediates the uptake of clinically relevant compounds such as statins and chemotherapeutic agents into hepatocytes, playing an important role in drug delivery and detoxification. The OATPs have a putative 12-transmembrane domain topology and a highly conserved signature sequence (human OATP1B1: DSRWVGAWWLNFL), spanning the extracellular loop 3/TM6 boundary. The presence of three conserved tryptophan residues at the TM interface suggests a structural role for the sequence. This was investigated by site-directed mutagenesis of selected amino acids within the sequence D251E, W254F, W258/259F, and N261A. Transport was measured using the substrate estrone-3-sulfate and surface expression detected by luminometry and confocal microscopy, facilitated by an extracellular FLAG epitope. Uptake of estrone-3-sulfate and the surface expression of D251E, W254F, and W258/259F were both significantly reduced from the wild type OATP1B1-FLAG in transfected HEK293T cells. Confocal microscopy revealed that protein was produced but was retained intracellularly. The uptake and expression of N261A were not significantly different. The reduction in surface expression and intracellular protein retention indicates a structural and/or membrane localization role for these signature sequence residues in the human drug transporter OATP1B1.

The intestinal absorption of folates

The properties of intestinal folate absorption were documented decades ago. However, it was only recently that the proton-coupled folate transporter (PCFT) was identified and its critical role in folate transport across the apical brush-border membrane of the proximal small intestine established by the loss-of-function mutations identified in the PCFT gene in subjects with hereditary folate malabsorption and, more recently, by the Pcft-null mouse. This article reviews the current understanding of the properties of PCFT-mediated transport and how they differ from those of the reduced folate carrier. Other processes that contribute to the transport of folates across the enterocyte, along with the contribution of the enterohepatic circulation, are considered. Important unresolved issues are addressed, including the mechanism of intestinal folate absorption in the absence of PCFT and regulation of PCFT gene expression. The impact of a variety of ions, organic molecules, and drugs on PCFT-mediated folate transport is described.

Vitamin E-drug interactions: molecular basis and clinical relevance

Vitamin E (α-, β-, γ- and δ-tocopherol and -tocotrienol) is an essential factor in the human diet and regularly taken as a dietary supplement by many people, who act under the assumption that it may be good for their health and can do no harm. With the publication of meta-analyses reporting increased mortality in persons taking vitamin E supplements, the safety of the micronutrient was questioned and interactions with prescription drugs were suggested as one potentially underlying mechanism. Here, we review the evidence in the scientific literature for adverse vitamin E-drug interactions and discuss the potential of each of the eight vitamin E congeners to alter the activity of drugs. In summary, there is no evidence from animal models or randomised controlled human trials to suggest that the intake of tocopherols and tocotrienols at nutritionally relevant doses may cause adverse nutrient-drug interactions. Consumption of high-dose vitamin E supplements ( ≥  300 mg/d), however, may lead to interactions with the drugs aspirin, warfarin, tamoxifen and cyclosporine A that may alter their activities. For the majority of drugs, however, interactions with vitamin E, even at high doses, have not been observed and are thus unlikely.

Phase I and II metabolism and MRP2-mediated export of bosentan in a MDCKII-OATP1B1-CYP3A4-UGT1A1-MRP2 quadruple-transfected cell line

BACKGROUND AND PURPOSE

Hepatic uptake (e.g. by OATP1B1), phase I and II metabolism (e.g. by CYP3A4, UGT1A1) and subsequent biliary excretion (e.g. by MRP2) are key determinants for the pharmacokinetics of numerous drugs. However, stably transfected cell models for the simultaneous investigation of transport and phase I and II metabolism of drugs are lacking.

EXPERIMENTAL APPROACH

A newly established quadruple-transfected MDCKII-OATP1B1-CYP3A4-UGT1A1-MRP2 cell line was used to investigate metabolism and transcellular transport of the endothelin receptor antagonist bosentan.

KEY RESULTS

Intracellular accumulation of bosentan equivalents (i.e. parent compound and metabolites) was significantly lower in all cell lines expressing MRP2 compared to cell lines lacking this transporter (P < 0.001). Accordingly, considerably higher amounts of bosentan equivalents were detectable in the apical compartments of cell lines with MRP2 expression (P < 0.001). HPLC and LC-MS measurements revealed that mainly unchanged bosentan accumulated in intracellular and apical compartments. Furthermore, the phase I metabolites Ro 48-5033 and Ro 47-8634 were detected intracellularly in cell lines expressing CYP3A4. Additionally, a direct glucuronide of bosentan could be identified intracellularly in cell lines expressing UGT1A1 and in the apical compartments of cell lines expressing UGT1A1 and MRP2.

CONCLUSIONS AND IMPLICATIONS

These in vitro data indicate that bosentan is a substrate of UGT1A1. Moreover, the efflux transporter MRP2 mediates export of bosentan and most likely also of bosentan glucuronide in the cell system. Taken together, cell lines simultaneously expressing transport proteins and metabolizing enzymes represent additional useful tools for the investigation of the interplay of transport and metabolism of drugs.

Prediction of organic anion-transporting polypeptide 1B1- and 1B3-mediated hepatic uptake of statins based on transporter protein expression and activity data

Organic anion-transporting polypeptides (OATP) 1B1 and OATP1B3 are drug transporters mediating the active hepatic uptake of their substrates. Because they exhibit overlapping substrate specificities, the contribution of each isoform to the net hepatic uptake needs to be considered when predicting drug-drug interactions. The relative contribution of OATP1B1- and OATP1B3-mediated uptake of statins into hepatocytes was estimated based on either relative transporter protein expression data or relative activity data. Therefore, kinetics of eight statins and OATP1B1- and OATP1B3-specific reference substrates was determined in OATP1B1- and OATP1B3-expressing human embryonic kidney 293 cells and in human cryopreserved hepatocytes. Absolute OATP1B1 and OATP1B3 protein abundance was determined by liquid chromatography-tandem mass spectrometry in all expression systems. Transporter activity data generated in recombinant cell lines were extrapolated to hepatocyte values using relative transporter expression factors (REF) or relative activity factors (RAF). Our results showed a pronounced OATP1B1 and comparatively low OATP1B3 protein expression in the investigated hepatocyte lot. Based on REF scaling, we demonstrated that the active hepatic uptake clearances of reference substrates, atorvastatin, pravastatin, rosuvastatin, and simvastatin were well predicted within twofold error, demonstrating that OATP1B1 and OATP1B3 were major contributors. For other statins, the net hepatic uptake clearance was underpredicted, suggesting the involvement of other hepatic uptake transporters. Summarized, we showed that REF- and RAF-based predictions were highly similar, indicating a direct transporter expression-activity relationship. Moreover, we demonstrated that the REF-scaling method provided a powerful tool to quantitatively assess the transporter-specific contributions to the net uptake clearance of statins in hepatocytes.

Organic anion transporting polypeptides and organic cation transporter 1 contribute to the cellular uptake of the flavonoid quercetin

Flavonoids such as quercetin and kaempferol mediate several health protective effects, e.g., anticancer effects. They are inhibitors of organic anion transporting polypeptides (OATP) and organic cation transporters (e.g., OCT2). However, little is known whether such transporters contribute to the cellular uptake of flavonoids. Therefore, we investigated the cellular uptake of kaempferol and quercetin using HEK293 cell lines stably expressing different human OATPs or OCT1. Kaempferol was not a substrate of any of the investigated transporters (OATP1A2, OATP1B1, OATP1B3, OATP2A1, OATP2B1, OATP3A1, OATP4A1, OATP5A1, and OCT1). Quercetin showed a significantly higher uptake into the HEK293-OATP1A2, HEK293-OATP2A1, HEK293-OATP2B1, and HEK293-OCT1 cells compared to control cells. The OATP1A2-, OATP2B1-, and OCT1-mediated quercetin uptake was inhibited by known inhibitors such as naringin, cyclosporin A, and quinidine, respectively. The cellular accumulation of quercetin into HEK293-OATP2A1 cells was not inhibited by prostaglandin E2 and diclofenac. The ionophore carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) reduced the net uptake of quercetin by increasing the uptake in the HEK293-control cells and causing no significant change in the HEK293-OATP2B1 cells indicating that quercetin follows the FCCP-driven proton flux through the plasma membrane. In addition to passive diffusion, the SLC transporters OATP1A2, OATP2B1, and OCT1 contribute to cellular accumulation of quercetin.

Clinical application of transcriptional activators of bile salt transporters

Hepatobiliary bile salt (BS) transporters are critical determinants of BS homeostasis controlling intracellular concentrations of BSs and their enterohepatic circulation. Genetic or acquired dysfunction of specific transport systems causes intrahepatic and systemic retention of potentially cytotoxic BSs, which, in high concentrations, may disturb integrity of cell membranes and subcellular organelles resulting in cell death, inflammation and fibrosis. Transcriptional regulation of canalicular BS efflux through bile salt export pump (BSEP), basolateral elimination through organic solute transporters alpha and beta (OSTα/OSTβ) as well as inhibition of hepatocellular BS uptake through basolateral Na(+)-taurocholate cotransporting polypeptide (NTCP) represent critical steps in protection from hepatocellular BS overload and can be targeted therapeutically. In this article, we review the potential clinical implications of the major BS transporters BSEP, OSTα/OSTβ and NTCP in the pathogenesis of hereditary and acquired cholestatic syndromes, provide an overview on transcriptional control of these transporters by the key regulatory nuclear receptors and discuss the potential therapeutic role of novel transcriptional activators of BS transporters in cholestasis.

Role of solute carriers in response to anticancer drugs

Membrane transporters play critical roles in moving a variety of anticancer drugs across cancer cell membrane, thereby determining chemotherapy efficacy and/or toxicity. The retention of anticancer drugs in cancer cells is the result of net function of efflux and influx transporters. The ATP-binding cassette (ABC) transporters are mainly the efflux transporters expressing at cancer cells, conferring the chemo-resistance in various malignant tumors, which has been well documented over the past decades. However, the function of influx transporters, in particular the solute carriers (SLC) in cancer cells, has only been recently well recognized to have significant impact on cancer therapy. The SLC transporters not only directly bring anticancer agents into cancer cells but also serve as the uptake mediators of essential nutrients for tumor growth and survival. In this review, we concentrate on the interaction of SLC transporters with anticancer drugs and nutrients, and their impact on chemo-sensitivity or -resistance of cancer cells. The differential expression patterns of SLC transporters between normal and tumor tissues may be well utilized to achieve specific delivery of chemotherapeutic agents.

Introduction of aromatic ring-containing substituents in cyclic nucleotides is associated with inhibition of toxin uptake by the hepatocyte transporters OATP 1B1 and 1B3

Analogs of the cyclic nucleotides cAMP and cGMP have been extensively used to mimic or modulate cellular events mediated by protein kinase A (PKA), Exchange protein directly activated by cAMP (Epac), or protein kinase G (PKG). We report here that some of the most commonly used cyclic nucleotide analogs inhibit transmembrane transport mediated by the liver specific organic anion transporter peptides OATP1B1 and OATP1B3, unrelated to actions on Epac, PKA or PKG. Several cAMP analogs, particularly with 8-pCPT-substitution, inhibited nodularin (Nod) induced primary rat hepatocyte apoptosis. Inhibition was not mediated by PKA or Epac, since increased endogenous cAMP, and some strong PKA- or Epac-activating analogs failed to protect cells against Nod induced apoptosis. The cAMP analogs inhibiting Nod induced hepatocyte apoptosis also reduced accumulation of radiolabeled Nod or cholic acid in primary rat hepatocytes. They also inhibited Nod induced apoptosis in HEK293 cells with enforced expression of OATP1B1 or 1B3, responsible for Nod transport into cells. Similar results were found with adenosine analogs, disconnecting the inhibitory effect of certain cAMP analogs from PKA or Epac. The most potent inhibitors were 8-pCPT-6-Phe-cAMP and 8-pCPT-2′-O-Me-cAMP, whereas analogs like 6-MB-cAMP or 8-Br-cAMP did not inhibit Nod uptake. This suggests that the addition of aromatic ring-containing substituents like the chloro-phenyl-thio group to the purines of cyclic nucleotides increases their ability to inhibit the OATP-mediated transport. Taken together, our data show that aromatic ring substituents can add unwanted effects to cyclic nucleotides, and that such nucleotide analogs must be used with care, particularly when working with cells expressing OATP1B1/1B3, like hepatocytes, or intact animals where hepatic metabolism can be an issue, as well as certain cancer cells. On the other hand, cAMP analogs with substituents like bromo, monobutyryl were non-inhibitory, and could be considered an alternative when working with cells expressing OATP1 family members.

Transport and killing mechanism of a novel camptothecin-deoxycholic acid derivate on hepatocellular carcinoma cells

Abstract Camptothecin-20(s)-O-glycine ester-[N-(3'α, 12'α-dihydroxy-24'-carbonyl-5'β-cholan)] (A2), 10-(3'α,12'α-dihydroxy-5'β-cholan-24'-carboxyl)-(20 s)-camptothecin (C2), and 10-O-(3-O-(3'α, 12'α-dihydroxy-24'-carbonyl-5'β-cholan)-propyl)-(20S)-camptothecin (D2) are novel camptothecin-deoxycholic acid analogues. MTT assays were performed to assess the anticancer activity of these compounds against hepatocellular carcinoma SMMC-7721, breast carcinoma MCF-7, and colorectal carcinoma HCT-116 cells. A2 had a high killing ability on SMMC-7721 cells selectively, but C2 and D2 did not exhibit selectivity with regard to SMMC-7721 killing. Uptake assays were performed in an effort to elucidate the transport mechanisms of A2 into SMMC-7721 cells. A2 increased the mRNA expression of OATP1B3 (an organic anion-transporting polypeptide) and uptake of A2 was inhibited by rifampin (inhibitor of OATP1B3), which indicated that the transporter-mediated transport of A2 was mediated by OATP1B3. In addition, according to the western blot and apoptosis assays, we found that A2 killed SMMC-7721 cells by inducing cell apoptosis mainly via an AIF (apoptosis-inducing factor) pathway and a caspase-dependent mitochondria apoptosis pathway.

Cysteine scanning mutagenesis of transmembrane domain 10 in organic anion transporting polypeptide 1B1

Organic anion transporting polypeptide (OATP) 1B1 is an important drug transporter expressed in human hepatocytes. Previous studies have indicated that transmembrane (TM) domain 2, 6, 8, 9, and in particular 10 might be part of the substrate binding site/translocation pathway. To explore which amino acids in TM10 are important for substrate transport, we mutated 34 amino acids individually to cysteines, expressed them in HEK293 cells, and determined their surface expression. Transport activity of the two model substrates estrone-3-sulfate and estradiol-17β-glucuronide as well as of the drug substrate valsartan for selected mutants was measured. Except for F534C and F537C, all mutants were expressed at the plasma membrane of HEK293 cells. Mutants Q541C and A549C did not transport estradiol-17β-glucuronide and showed negligible estrone-3-sulfate transport. However, A549C showed normal valsartan transport. Pretreatment with the anionic and cell impermeable sodium (2-sulfonatoethyl)methanethiosulfonate (MTSES) affected the transport of each substrate differently. Pretreatment of L545C abolished estrone-3-sulfate uptake almost completely, while it stimulated estradiol-17β-glucuronide uptake. Further analyses revealed that mutant L545C in the absence of MTSES showed biphasic kinetics for estrone-3-sulfate that was converted to monophasic kinetics with a decreased apparent affinity, explaining the previously seen inhibition. In contrast, the apparent affinity for estradiol-17β-glucuronide was not changed by MTSES treatment, but the V_max value was increased about 4-fold, explaining the previously seen stimulation. Maleimide labeling of L545C was affected by preincubation with estrone-3-sulfate but not with estradiol-17β-glucuronide. These results strongly suggest that L545C is part of the estrone-3-sulfate binding site/translocation pathway but is not directly involved in binding/translocation of estradiol-17β-glucuronide.

The roles of MRP2, MRP3, OATP1B1, and OATP1B3 in conjugated hyperbilirubinemia

Increased concentrations of bilirubin glucuronides in blood plasma indicate hepatocellular dysfunction. Elucidation of the transport processes of bilirubin conjugates across the basolateral (sinusoidal) and the canalicular plasma membrane domains of hepatocytes has decisively contributed to our current understanding of the molecular basis of conjugated hyperbilirubinemia in human liver diseases. Under normal conditions, unconjugated bilirubin is taken up into hepatocytes by transporters of the organic anion-transporting polypeptide (OATP) family, followed by conjugation with glucuronic acid, and ATP-dependent transport into bile. This efflux across the canalicular membrane is mediated by multidrug resistance protein 2 (MRP2 or ABCC2), which is a 190-kDa glycoprotein transporting with high affinity and efficiency monoglucuronosyl bilirubin and bisglucuronosyl bilirubin into bile. MRP2 is hereditarily deficient in human Dubin-Johnson syndrome. Under pathophysiological conditions such as cholestatic liver injury and MRP2 inhibition, the basolateral efflux pump multidrug resistance protein 3 (MRP3 or ABCC3) is responsible for the occurrence of conjugated hyperbilirubinemia. MRP3 is a glycoprotein with a similar molecular mass as MRP2, with 48% amino acid identity, and with overlapping substrate specificity. Human MRP3 is the only basolateral efflux pump shown to transport bilirubin glucuronides. In human and rat hepatocytes, MRP3/Mrp3 is strongly upregulated under conditions of cholestasis and MRP2 deficiency. This is in line with the concept that basolateral efflux pumps of the hepatocyte compensate for impaired canalicular efflux of compounds into bile and contribute to balance the rate of uptake or synthesis of compounds in hepatocytes with the capacity for efflux into bile.

Effects of polymorphisms in ABCG2, SLCO1B1, SLC10A1 and CYP2C9/19 on plasma concentrations of rosuvastatin and lipid response in Chinese patients

AIM

This study examined whether the ABCG2 421C>A polymorphism and variants in other genes potentially related to the pharmacokinetics of rosuvastatin influenced the plasma concentration of rosuvastatin in Chinese patients with hypercholesterolemia.

PATIENTS & METHODS

Overnight fasting blood samples were collected from 291 patients who had received a rosuvastatin 10 mg night-time dose for at least 4 weeks. Plasma concentrations of rosuvastatin and N-desmethyl rosuvastatin were quantified using liquid chromatography tandem mass spectrometry.

RESULTS

In subjects with the ABCG2 421AA genotype (n = 39), the mean plasma concentrations of rosuvastatin and its metabolite were 63 and 41% greater than the values in those with the 421CA genotype (n = 108) and 120 and 99% greater than in those with the 421CC genotype (n = 129). The plasma concentrations of rosuvastatin were associated (r = -0.194; p = 0.001) with the percentage reduction in low-density lipoprotein cholesterol with rosuvastatin, but the association was not significant after adjusting for the ABCG2 421C>A polymorphism. The SLCO1B1 521T>C polymorphism was associated with increased plasma concentrations of rosuvastatin and impaired N-demethylation of rosuvastatin, but had no impact on its lipid-lowering effect. Polymorphisms in CYP2C9, CYP2C19 and SLC10A1 had minimal effects.

CONCLUSION

These findings suggest that the increased plasma concentrations of rosuvastatin in Chinese patients are associated with increased lipid-lowering effects and lower doses of rosuvastatin should be effective in subjects with the ABCG2 421C>A variant.

In vivo disposition of doxorubicin is affected by mouse Oatp1a/1b and human OATP1A/1B transporters

Organic anion-transporting polypeptides (OATPs) are important drug uptake transporters, mediating distribution of substrates to several pharmacokinetically relevant organs. Doxorubicin is a widely used anti-cancer drug extensively studied for its interactions with various drug transporters, but not OATPs. Here, we investigated the role of OATP1A/1B proteins in the distribution of doxorubicin. In vitro, we observed ∼ 2-fold increased doxorubicin uptake in HEK293 cells overexpressing human OATP1A2, but not OATP1B1 or OATP1B3. In mice, absence of Oatp1a/1b transporters led to up to 2-fold higher doxorubicin plasma concentrations and 1.3-fold higher plasma AUC. Conversely, liver AUC and liver-to-plasma ratios of Oatp1a/1b(-/-) mice were 1.4-fold and up to 4.1-fold lower than in wild-type mice, respectively. Decreased doxorubicin levels in the small intestinal content reflected those in the liver, indicating a reduced biliary excretion of doxorubicin in Oatp1a/1b(-/-) mice. These results demonstrate important control of doxorubicin plasma clearance and hepatic uptake by mouse Oatp1a/1b transporters. This is unexpected, as the fairly hydrophobic weak base doxorubicin is an atypical OATP1A/1B substrate. Interestingly, transgenic liver-specific expression of human OATP1A2, OATP1B1 or OATP1B3 could partially rescue the increased doxorubicin plasma levels of Oatp1a/1b(-/-) mice. Hepatic uptake and bile-derived intestinal excretion of doxorubicin were completely reverted to wild-type levels by OATP1A2, and partially by OATP1B1 and OATP1B3. Thus, doxorubicin is transported by hepatocyte-expressed OATP1A2, -1B1 and -1B3 in vivo, illustrating an unexpectedly wide substrate specificity. These findings have possible implications for the uptake, disposition, therapy response and toxicity of doxorubicin, also in human tumors and tissues expressing these transporters.

OATP1B3 is expressed in pancreatic β-islet cells and enhances the insulinotropic effect of the sulfonylurea derivative glibenclamide

Organic anion transporting polypeptide OATP1B3 is a membrane-bound drug transporter that facilitates cellular entry of a variety of substrates. Most of the previous studies focused on its hepatic expression and function in hepatic drug elimination. In this study, we report expression of OATP1B3 in human pancreatic tissue, with the abundance of the transporter localized in the islets of Langerhans. Transport studies using OATP1B3-overexpressing MDCKII cells revealed significant inhibition of the cellular uptake of the known substrate cholecystokinin-8 in the presence of the insulinotropic antidiabetes compounds tolbutamide, glibenclamide, glimepiride, and nateglinide and identified glibenclamide as a novel substrate of OATP1B3. Sulfonylurea derivatives exert their insulinotropic effect by binding to the SUR1 subunit of the KATP channels inducing insulin secretion in β-cells. Here, we show that transient overexpression of human OATP1B3 in a murine β-cell line (MIN6)-which exhibits glucose and glibenclamide-sensitive insulin secretion-significantly enhances the insulinotropic effect of glibenclamide without affecting glucose-stimulated insulin secretion. Taken together, our data provide evidence that the drug transporter OATP1B3 functions as a determinant of the insulinotropic effect of glibenclamide on the tissue level. Changes in transport activity based on drug-drug interactions or genetic variability may therefore influence glibenclamide efficacy.

Pharmacogenetics of methotrexate in acute lymphoblastic leukaemia: why still at the bench level?

PURPOSE

The antifolate drug methotrexate (MTX) was introduced into clinical practice about 60 years ago and remains an important component of different acute lymphoblastic leukemia (ALL) treatment protocols. It acts by inhibiting several enzymes in the folate pathway, thereby resulting in the disruption of folate homeostasis. To date, treatment regimens have not been personalized despite there being experimental evidence that gene polymorphisms of folate metabolizing enzymes affect MTX response. The aim of this review was to evaluate the influence of genetic polymorphisms of the enzymes involved in the MTX pathway on ALL treatment outcomes and identify factors underlining the failure to personalize MTX therapy.

METHODS

We conducted a literature search in PUBMED and Google Scholar using the following key words: methotrexate, polymorphism, acute lymphoblastic leukemia, pharmacogenetics, pharmacogenomics and personalized medicine.

RESULTS

The reasons for the failure to personalize MTX therapy may be due to (1) most studies involving single-center, small-sized cohorts, (2) differences in MTX dose across different protocols, (3) failure to consider minimal residual disease as a risk factor for post-induction treatment, (4) differences in outcome criteria between studies and (5) failure to consider the folate levels of a patient before initiation of MTX therapy. Although high-throughput techniques allow the mapping of thousands of genetic polymorphisms in a single run, it remains a major challenge to dissect out folate-metabolizing enzymes which have a high impact on the efficacy and toxicity of MTX and which, therefore, could be the targets for intervention.

CONCLUSIONS

Prospective pharmacogenetic studies which consider all of the above-mentioned factors should be undertaken to facilitate the design of personalized MTX treatment for ALL patients.

Chronic cholestatic liver diseases: clues from histopathology for pathogenesis

Chronic cholestatic liver diseases include fibrosing cholangiopathies such as primary biliary cirrhosis or primary sclerosing cholangitis. These and related cholangiopathies clearly display pathologies associated with (auto)immunologic processes. As the cholangiocyte's apical membrane is exposed to the toxic actions of the bile fluid, the interaction of bile with cholangiocytes and the biliary tree in general must be considered to completely understand the pathogenesis of cholangiopathies. While the molecular processes involved in the hepatocellular formation of bile are well understood in both normal and pathophysiologic conditions, those in the bile ducts of normal liver and in livers with cholangiopathies lag behind. This survey highlights key mechanisms known to date that are important for the formation of bile by hepatocytes and its modification by the biliary tree. It also delineates the clinical pathophysiologic findings for cholangiopathies and puts them in perspective with current experimental models to reveal the pathogenesis of cholangiopathies and develop novel therapeutic approaches.

The SLCO (former SLC21) superfamily of transporters

The members of the organic anion transporting polypeptide superfamily (OATPs) are classified within the SLCO solute carrier family. All functionally well characterized members are predicted to have 12 transmembrane domains and are sodium-independent transport systems that mediate the transport of a broad range of endo- as well as xenobiotics. Substrates are mainly amphipathic organic anions with a molecular weight of more than 300Da, but some of the known transported substrates are also neutral or even positively charged. Among the well characterized substrates are numerous drugs including statins, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, antibiotics, antihistaminics, antihypertensives and anticancer drugs. Based on their amino acid sequence identities, the different OATPs cluster into families (in general with more than 40% amino acid sequence identity) and subfamilies (more than 60% amino acid identity). With the sequencing of genomes from different species and the computerized prediction of encoded proteins more than 300 OATPs can be found in the databases, however only a fraction of them have been identified in humans, rodents, and some additional species important for pharmaceutical research like the rhesus monkey (Macaca mulatta), the dog (Canis lupus familiaris) and the pig (Sus scrofa). These OATPs form 6 families (OATP1-OATP6) and 13 subfamilies. In this review we try to summarize what is currently known about OATPs with respect to endogenous substrates, tissue distribution, transport mechanisms, regulation of expression, structure-function relationship and mutations and polymorphisms.

Organic anion-transporting polypeptides: a novel approach for cancer therapy

Organic anion-transporting polypeptides (OATPs) encoded by the SLCO genes constitute an important transporter superfamily that mediates transmembrane transport of various clinical drugs and endogenous nutrients. Eleven human OATPs with different transport functions are expressed in various tissues. Bile acids, steroid hormone conjugates, prostaglandins, testosterone and thyroid hormones that promote cell proliferation are typical substrates of OATPs. Many important clinical drugs have been identified as substrates of OATP1B1, OATP1B3, OATP2B1 and OATP1A2. Liver-specific OATP1B1 and OATP1B3 as well as testis-specific OATP6A1 are expressed in malignancies and can act as biomarkers for many tumours. Various studies have shown the associations of genetic polymorphisms in OATP genes with the uptake pharmacokinetics of their substrates. Because of their abundant expression in tumours and their high transport activity for many cancer drugs, OATPs should be considered as important therapeutic targets in anti-cancer drug design.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

In vitro characterization of axitinib interactions with human efflux and hepatic uptake transporters: implications for disposition and drug interactions

Axitinib is an inhibitor of tyrosine kinase vascular endothelin growth factor receptors 1, 2, and 3. The ATP-binding cassette (ABC) and solute carrier (SLC) transport properties of axitinib were determined in selected cellular systems. Axitinib exhibited high passive permeability in all cell lines evaluated (Papp ≥ 6 × 10(-6) cm/s). Active efflux was observed in Caco-2 cells, and further evaluation in multidrug resistance gene 1 (MDR1) or breast cancer resistance protein (BCRP) transfected Madin-Darby canine kidney cells type 2 (MDCK) cells indicated that axitinib is at most only a weak substrate for P-glycoprotein (P-gp) but not BCRP. Axitinib showed incomplete inhibition of P-gp-mediated transport of digoxin in Caco-2 cells and BCRP transport of topotecan in BCRP-transfected MDCK cells with IC50 values of 3 μM and 4.4 μM, respectively. Axitinib (10 mg) did not pose a risk for systemic drug interactions with P-gp or BCRP per regulatory guidance. A potential risk for drug interactions through inhibition of P-gp and BCRP in the gastrointestinal tract was identified because an axitinib dose of 10 mg divided by 250 mL was greater than 10-fold the IC50 for each transporter. However, a GastroPlus simulation that considered the low solubility of axitinib resulted in lower intestinal concentrations and suggested a low potential for gastrointestinal interactions with P-gp and BCRP substrates. Organic anion transporting polypeptide 1B1 (OATP1B1) and OATP1B3 transfected human embryonic kidney 293 (HEK293) cells transported axitinib to a minor extent but uptake into suspended hepatocytes was not inhibited by rifamycin SV suggesting that high passive permeability predominates. Mouse whole-body autoradiography revealed that [(14)C]axitinib-equivalents showed rapid absorption and distribution to all tissues except the brain. This suggests that efflux transport of axitinib may occur at the mouse blood-brain barrier.

Clinical significance of organic anion transporting polypeptides (OATPs) in drug disposition: their roles in hepatic clearance and intestinal absorption

Organic anion transporting polypeptide (OATP) family transporters accept a number of drugs and are increasingly being recognized as important factors in governing drug and metabolite pharmacokinetics. OATP1B1 and OATP1B3 play an important role in hepatic drug uptake while OATP2B1 and OATP1A2 might be key players in intestinal absorption and transport across blood-brain barrier of drugs, respectively. To understand the importance of OATPs in the hepatic clearance of drugs, the rate-determining process for elimination should be considered; for some drugs, hepatic uptake clearance rather than metabolic intrinsic clearance is the more important determinant of hepatic clearances. The importance of the unbound concentration ratio (liver/blood), K(p,uu) , of drugs, which is partly governed by OATPs, is exemplified in interpreting the difference in the IC(50) of statins between the hepatocyte and microsome systems for the inhibition of HMG-CoA reductase activity. The intrinsic activity and/or expression level of OATPs are affected by genetic polymorphisms and drug-drug interactions. Their effects on the elimination rate or intestinal absorption rate of drugs may sometimes depend on the substrate drug. This is partly because of the different contribution of OATP isoforms to clearance or intestinal absorption. When the contribution of the OATP-mediated pathway is substantial, the pharmacokinetics of substrate drugs should be greatly affected. This review describes the estimation of the contribution of OATP1B1 to the total hepatic uptake of drugs from the data of fold-increases in the plasma concentration of substrate drugs by the genetic polymorphism of this transporter. To understand the importance of the OATP family transporters, modeling and simulation with a physiologically based pharmacokinetic model are helpful.

The molecular and biophysical characterization of the human blood-nerve barrier: current concepts

The internal microenvironment in peripheral nerves is highly regulated in order to maintain normal axonal impulse transmission to or from the central nervous system. In humans, this regulation is facilitated by specialized tight junction (TJ)-forming endoneurial microvascular endothelial cells and perineurial myofibroblasts that form multiple concentric layers around nerve fascicles. The endoneurial endothelial cells come in direct contact with circulating blood and, thus, can be considered the blood-nerve barrier (BNB). Studies on the molecular and biophysical properties of the human BNB in vivo or in situ are limited. Owing to the recent isolation of primary human endoneurial endothelial cells and the development of simian virus 40 large T-antigen immortalized cell lines, data are emerging on the structural and functional characteristics of these cells. These data aim to increase our understanding of how solutes, macromolecules, nutrients and hematogenous leukocytes gain access into or are restricted from the endoneurium of peripheral nerves. These concepts have clinical relevance in understanding normal peripheral nerve homeostasis, the response of peripheral nerves to external insult and stresses such as drugs and toxins and the pathogenesis of peripheral neuropathies. This review discusses current knowledge in this nascent and exciting field of microvascular biology.

The effect of breast cancer resistance protein, multidrug resistant protein 1, and organic anion-transporting polypeptide 1B3 on the antitumor efficacy of the lipophilic camptothecin 7-t-butyldimethylsilyl-10-hydroxycamptothecin (AR-67) in vitro

AR-67 (7-t-butyldimethylsilyl-10-hydroxycamptothecin) is a lipophilic camptothecin analog, currently under early stage clinical trials. Transporters are known to have an impact on the disposition of camptothecins and on the response to chemotherapeutics in general due to their expression in tumor tissues. Therefore, we investigated the interplay between the breast cancer resistance protein (BCRP), multidrug resistant protein 1 (MDR1), and organic anion-transporting polypeptide (OATP) 1B1/1B3 transporters and AR-67 and their impact on the toxicity profile of AR-67. Using cell lines expressing the aforementioned transporters, we showed that the lipophilic AR-67 lactone form is a substrate for efflux transporters BCRP and MDR1. Additionally, OATP1B1 and OATP1B3 facilitated the uptake of AR-67 carboxylate in SLCO1B1- and SLCO1B3-transfected cell systems compared with the mock-transfected ones. Notably, both BCRP and MDR1 conferred resistance to AR-67 lactone. Prompted by recent studies showing increased OATP1B3 expression in certain cancer types, we investigated the effect of OATP1B3 expression on cell viability after exposure to AR-67 carboxylate. OATP1B3-expressing cells had increased carboxylate uptake as compared with mock-transfected cells but were not sensitized because the intracellular amount of lactone was 50-fold higher than that of carboxylate and comparable between OATP1B3-expressing and OATP1B3-nonexpressing cells. In conclusion, BCRP- and MDR1-mediated efflux of AR-67 lactone confers resistance to AR-67, but OATP1B3-mediated uptake of the AR-67 carboxylate does not sensitize OATP1B3-expressing tumor cells.

Transporters and drug-drug interactions: important determinants of drug disposition and effects

Uptake and efflux transporters determine plasma and tissue concentrations of a broad variety of drugs. They are localized in organs such as small intestine, liver, and kidney, which are critical for drug absorption and elimination. Moreover, they can be found in important blood-tissue barriers such as the blood-brain barrier. Inhibition or induction of drug transporters by coadministered drugs can alter pharmacokinetics and pharmacodynamics of the victim drugs. This review will summarize in particular clinically observed drug-drug interactions attributable to inhibition or induction of intestinal export transporters [P-glycoprotein (P-gp), breast cancer resistance protein (BCRP)], to inhibition of hepatic uptake transporters [organic anion transporting polypeptides (OATPs)], or to inhibition of transporter-mediated [organic anion transporters (OATs), organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATEs), P-gp] renal secretion of xenobiotics. Available data on the impact of nutrition on transport processes as well as genotype-dependent, transporter-mediated drug-drug interactions will be discussed. We will also present and discuss data on the variable extent to which information on the impact of transporters on drug disposition is included in summaries of product characteristics of selected countries (SPCs). Further work is required regarding a better understanding of the role of the drug metabolism-drug transport interplay for drug-drug interactions and on the extrapolation of in vitro findings to the in vivo (human) situation.

Sorafenib hepatobiliary disposition: mechanisms of hepatic uptake and disposition of generated metabolites

Sorafenib is an orally active tyrosine kinase inhibitor used in the treatment of renal and hepatocellular carcinoma. This study was designed to establish whether transport proteins are involved in the hepatic uptake of sorafenib and to determine the extent of biliary excretion of sorafenib and its metabolites in human hepatocytes. Initial uptake was assessed in freshly isolated, suspended human hepatocytes in the presence of inhibitors and modulators. [(14)C]Sorafenib (1 µM) uptake at 4°C was reduced by about 61-63% of the uptake at 37°C, suggesting a high degree of passive diffusion. Hepatocyte uptake of [(14)C]sorafenib was not Na(+) dependent or influenced by the organic anion transporter 2 inhibitor ketoprofen. However, initial [(14)C]sorafenib hepatocyte uptake was reduced by 46 and 30% compared with control values in the presence of the organic anion transporting polypeptide inhibitor rifamycin SV and the organic cation transporter (OCT) inhibitor decynium 22, respectively. [(14)C]Sorafenib (0.5-5 µM) uptake was significantly higher in hOCT1-transfected Chinese hamster ovary cells compared with mock cells, and inhibited by the general OCT inhibitor, 1-methyl-4-phenylpryidinium. OCT1-mediated uptake was saturable with a Michaelis-Menten constant of 3.80 ± 2.53 µM and a V(max) of 116 ± 42 pmol/mg/min. The biliary excretion index and in vitro biliary clearance of sorafenib (1 µM) in sandwich-cultured human hepatocytes were low (∼11% and 11 ml/min/kg, respectively). Results suggest that sorafenib uptake in human hepatocytes occurs via passive diffusion, by OCT1, and by organic anion transporting polypeptide(s). Sorafenib undergoes modest biliary excretion, predominantly as a glucuronide conjugate(s).

Expanding role of pharmacogenomics in the management of cardiovascular disorders

Cardiovascular disease is a leading cause of death worldwide. Many pharmacologic therapies are available that aim to reduce the risk of cardiovascular disease but there is significant inter-individual variation in drug response, including both efficacy and toxicity. Pharmacogenetics aims to personalize medication choice and dosage to ensure that maximum clinical benefit is achieved whilst side effects are minimized. Over the past decade, our knowledge of pharmacogenetics in cardiovascular therapies has increased significantly. The anticoagulant warfarin represents the most advanced application of pharmacogenetics in cardiovascular medicine. Prospective randomized clinical trials are currently underway utilizing dosing algorithms that incorporate genetic polymorphisms in cytochrome P450 (CYP)2C9 and vitamin k epoxide reductase (VKORC1) to determine warfarin dosages. Polymorphisms in CYP2C9 and VKORC1 account for approximately 40 % of the variance in warfarin dose. There is currently significant controversy with regards to pharmacogenetic testing in anti-platelet therapy. Inhibition of platelet aggregation by aspirin in vitro has been associated with polymorphisms in the cyclo-oxygenase (COX)-1 gene. However, COX-1 polymorphisms did not affect clinical outcomes in patients prescribed aspirin therapy. Similarly, CYP2C19 polymorphisms have been associated with clopidogrel resistance in vitro, and have shown an association with stent thrombosis, but not with other cardiovascular outcomes in a consistent manner. Response to statins has been associated with polymorphisms in the cholesterol ester transfer protein (CETP), apolipoprotein E (APOE), 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, calmin (CLMN) and apolipoprotein-CI (APOC1) genes. Although these genes contribute to the variation in lipid levels during statin therapy, their effects on cardiovascular outcomes requires further investigation. Polymorphisms in the solute carrier organic anion transporter 1B1 (SLCO1B1) gene is associated with increased statin exposure and simvastatin-induced myopathy. Angiotensin-converting enzyme (ACE) inhibitors and β-adrenoceptor antagonists (β-blockers) are medications that are important in the management of hypertension and heart failure. Insertion and deletion polymorphisms in the ACE gene are associated with elevated and reduced serum levels of ACE, respectively. No significant association was reported between the polymorphism and blood pressure reduction in patients treated with perindopril. However, a pharmacogenetic score incorporating single nucleotide polymorphisms (SNPs) in the bradykinin type 1 receptor gene and angiotensin-II type I receptor gene predicted those most likely to benefit and suffer harm from perindopril therapy. Pharmacogenetic studies into β-blocker therapy have focused on variations in the β1-adrenoceptor gene and CYP2D6, but results have been inconsistent. Pharmacogenetic testing for ACE inhibitor and β-blocker therapy is not currently used in clinical practice. Despite extensive research, no pharmacogenetic tests are currently in clinical practice for cardiovascular medicines. Much of the research remains in the discovery phase, with researchers struggling to demonstrate clinical utility and validity. This is a problem seen in many areas of therapeutics and is because of many factors, including poor study design, inadequate sample sizes, lack of replication, and heterogeneity amongst patient populations and phenotypes. In order to progress pharmacogenetics in cardiovascular therapies, researchers need to utilize next-generation sequencing technologies, develop clear phenotype definitions and engage in multi-center collaborations, not only to obtain larger sample sizes but to replicate associations and confirm results across different ethnic groups.

Germline genetic variations in methotrexate candidate genes are associated with pharmacokinetics, toxicity, and outcome in childhood acute lymphoblastic leukemia

The pharmacogenetics of methotrexate (MTX) was investigated in a large cohort of pediatric patients with acute lymphoblastic leukemia (ALL). Four hundred ninety-nine children with ALL from the ALL-BFM (Berlin-Frankfurt-Münster) 2000 trial who received 1996 courses of MTX at 5 g/m(2) were genotyped for 8 single nucleotide polymorphisms in 5 candidate genes of the MTX/folate pathway. Patients' MTX pharmacokinetics, MTX toxicities, and outcomes were correlated with the genotypes. The interindividual variability in MTX kinetics had a substantial genetic component between 68% and 75%. The SLCO1B1 rs4149056 variant was significantly associated with MTX kinetics. In a multiple regression model, MTX area under the concentration time curve (AUC)0-48h increased by 26% (P < .001) per SLCO1B1 rs4149056 C allele. MTX AUC0-48h was a significant predictor of overall toxic adverse events during MTX courses (R(2) = 0.043; P < .001), whereas the thymidylate synthase rs34743033 tandem repeat polymorphism was predictive of stomatitis (R(2) = 0.018; P = .009), a frequent side effect of high-dose MTX. Multiple Cox regression analyses revealed an association of minimal residual disease (hazard ratio 7.3; P < .001) and methylenetetrahydrofolate reductase rs1801131 (hazard ratio 3.1; P = .015) with event-free survival in the ALL-BFM 2000 study population. Genetic variations substantially influence the kinetics and response to high-dose MTX therapy in childhood ALL.

Interaction of silymarin flavonolignans with organic anion-transporting polypeptides

Organic anion-transporting polypeptides (OATPs) are multispecific transporters mediating the uptake of endogenous compounds and xenobiotics in tissues that are important for drug absorption and elimination, including the intestine and liver. Silymarin is a popular herbal supplement often used by patients with chronic liver disease; higher oral doses than those customarily used (140 mg three times/day) are being evaluated clinically. The present study examined the effect of silymarin flavonolignans on OATP1B1-, OATP1B3-, and OATP2B1-mediated transport in cell lines stably expressing these transporters and in human hepatocytes. In overexpressing cell lines, OATP1B1- and OATP1B3-mediated estradiol-17β-glucuronide uptake and OATP2B1-mediated estrone-3-sulfate uptake were inhibited by most of the silymarin flavonolignans investigated. OATP1B1-, OATP1B3-, and OATP2B1-mediated substrate transport was inhibited efficiently by silymarin (IC₅₀ values of 1.3, 2.2 and 0.3 µM, respectively), silybin A (IC₅₀ values of 9.7, 2.7 and 4.5 µM, respectively), silybin B (IC₅₀ values of 8.5, 5.0 and 0.8 µM, respectively), and silychristin (IC₅₀ values of 9.0, 36.4, and 3.6 µM, respectively). Furthermore, silymarin, silybin A, and silybin B (100 µM) significantly inhibited OATP-mediated estradiol-17β-glucuronide and rosuvastatin uptake into human hepatocytes. Calculation of the maximal unbound portal vein concentrations/IC₅₀ values indicated a low risk for silymarin-drug interactions in hepatic uptake with a customary silymarin dose. The extent of silymarin-drug interactions depends on OATP isoform specificity and concentrations of flavonolignans at the site of drug transport. Higher than customary doses of silymarin, or formulations with improved bioavailability, may increase the risk of flavonolignan interactions with OATP substrates in patients.

Drug-induced liver injury: the role of drug metabolism and transport

Many studies have pinpointed the significant contribution of liver-mediated drug metabolism and transport to the complexity of drug-induced liver injury (DILI). Phase I cytochrome P450 (CYP450) enzymes can lead to altered drug metabolism and formation of toxic metabolites, whilst Phase II enzymes are also associated with DILI. The emerging role of hepatic transporters in regulating the movement of endogenous and exogenous chemicals (e.g., bile acids and drugs) across cellular and tissue membranes is critical in determining the pathophysiology of liver disease as well as drug toxicity and efficacy. Genetic and environmental factors can have a significant impact on drug metabolism and transporter proteins, consequently increasing the risk of DILI in susceptible individuals. The assessment of these factors therefore represents an important approach for predicting and preventing DILI, by better understanding the pharmacological profile of a specific drug. This review focuses on the mechanisms of DILI associated with drug metabolism and hepatic transport, and how they can be influenced by underlying factors.

Potential of the novel antiretroviral drug rilpivirine to modulate the expression and function of drug transporters and drug-metabolising enzymes in vitro

The objective of this study was to assess the drug-drug interaction potential of the new non-nucleoside reverse transcriptase inhibitor (NNRTI) rilpivirine in vitro. The following were evaluated: P-glycoprotein (P-gp/ABCB1) inhibition by calcein assay; breast cancer resistance protein (BCRP/ABCG2) inhibition by pheophorbide A efflux; and inhibition of organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 by 8-fluorescein-cAMP uptake. Inhibition of cytochrome P450 enzymes was assessed using commercially available kits. Substrate characteristics were evaluated by growth inhibition assays in MDCKII cells overexpressing particular ABC transporters. Induction of drug-metabolising enzymes and transporters was quantified by real-time RT-PCR in LS180 cells, and activation of pregnane X receptor (PXR) by a reporter gene assay. Rilpivirine significantly inhibited P-gp (IC(50) = 13.1 ± 6.8 μmol/L), BCRP (IC(50) = 1.5 ± 0.3 μmol/L), OATP1B1 (IC(50) = 4.1 ± 1.8 μmol/L), OATP1B3 (IC(50) = 6.1 ± 0.9 μmol/L), CYP3A4 (IC(50) = 1.3 ± 0.6 μmol/L), CYP2C19 (IC(50) = 2.7 ± 0.3 μmol/L) and CYP2B6 (IC(50) = 4.2 ± 1.6 μmol/L). Growth inhibition assays indicate that rilpivirine is not a substrate of P-gp, BCRP, or multidrug resistance-associated proteins 1 and 2. In LS180 cells, rilpivirine induced mRNA expression of ABCB1, CYP3A4 and UGT1A3, whereas ABCC1, ABCC2, ABCG2, OATP1B1 and UGT1A9 were not induced. Moreover, rilpivirine was a PXR activator. In conclusion, rilpivirine inhibits and induces several relevant drug-metabolising enzymes and drug transporters, but owing to its low plasma concentrations it is most likely less prone to drug-drug interactions than older NNRTIs.

Interaction profile of macitentan, a new non-selective endothelin-1 receptor antagonist, in vitro

Macitentan is a new non-selective endothelin-1 receptor antagonist under development for the treatment of pulmonary arterial hypertension. Information on the potential for macitentan to influence the pharmacokinetics of concomitantly administered drugs by inhibition or induction of drug metabolising enzymes or drug transporters is sparse. We therefore studied the potential of macitentan to inhibit and induce critical targets of drug metabolism and drug distribution (transporters) in vitro. Induction was quantified at the mRNA level by real-time RT-PCR in LS180 cells and revealed that macitentan significantly induced mRNA expression of cytochrome P450 3A4 (CYP3A4), P-glycoprotein (P-gp, ABCB1), solute carrier of organic anions 1B1 (SLCO1B1), and uridinediphosphate-glucuronosyltransferase 1A3 (UGT1A9). By means of a reporter gene assay our study establishes macitentan as a potent activator of pregnane X receptor (PXR). Inhibition of drug transporters was evaluated by using transporter over-expressing cell lines and fluorescent specific substrates of the respective transporters and revealed that macitentan is an inhibitor of P-gp, breast cancer resistance protein (BCRP), SLCO1B1, and SLCO1B3. Using commercial kits macitentan was demonstrated to be a moderate inhibitor of CYP3A4 and CYP2C19. In conclusion our data provide a comprehensive analysis of the interaction profile of macitentan with drug metabolising and transporting enzymes in vitro. Although macitentan has a similar or higher potency for induction and inhibition of drug metabolising enzymes and transporters than bosentan, its low plasma concentrations and minimal accumulation in the liver suggest that it will be markedly less prone to drug-drug interactions than bosentan.

Genetics is a major determinant of expression of the human hepatic uptake transporter OATP1B1, but not of OATP1B3 and OATP2B1

BACKGROUND

Organic anion transporting polypeptide (OATP) 1B1, OATP1B3, and OATP2B1 (encoded by SLCO1B1, SLCO1B3, SLCO2B1) mediate the hepatic uptake of endogenous compounds like bile acids and of drugs, for example, the lipid-lowering atorvastatin, thereby influencing hepatobiliary elimination. Here we systematically elucidated the contribution of SLCO variants on expression of the three hepatic OATPs under consideration of additional important covariates.

METHODS

Expression was quantified by RT-PCR and immunoblotting in 143 Caucasian liver samples. A total of 109 rare and common variants in the SLCO1B3-SLCO1B1 genomic region and the SLCO2B1 gene were genotyped by MALDI-TOF mass spectrometry and genome-wide SNP microarray technology. SLCO1B1 haplotypes affecting hepatic OATP1B1 expression were associated with pharmacokinetic data of the OATP1B1 substrate atorvastatin (n = 82).

RESULTS

Expression of OATP1B1, OATP1B3, and OATP2B1 at the mRNA and protein levels showed marked interindividual variability. All three OATPs were expressed in a coordinated fashion. By a multivariate regression analysis adjusted for non-genetic and transcription covariates, increased OATP1B1 expression was associated with the coding SLCO1B1 variant c.388A > G (rs2306283) even after correction for multiple testing (P = 0.00034). This held true for haplotypes harboring c.388A > G but not the functional variant c.521T > C (rs4149056) associated with statin-related myopathy. c.388A > G also significantly affected atorvastatin pharmacokinetics. SLCO variants and non-genetic and regulatory covariates together accounted for 59% of variability of OATP1B1 expression.

CONCLUSIONS

Our results show that expression of OATP1B1, but not of OATP1B3 and OATP2B1, is significantly affected by genetic variants. The SLCO1B1 variant c.388A > G is the major determinant with additional consequences on atorvastatin plasma levels.

Targeted drug delivery to treat pain and cerebral hypoxia

Limited drug penetration is an obstacle that is often encountered in treatment of central nervous system (CNS) diseases including pain and cerebral hypoxia. Over the past several years, biochemical characteristics of the brain (i.e., tight junction protein complexes at brain barrier sites, expression of influx and efflux transporters) have been shown to be directly involved in determining CNS permeation of therapeutic agents; however, the vast majority of these studies have focused on understanding those mechanisms that prevent drugs from entering the CNS. Recently, this paradigm has shifted toward identifying and characterizing brain targets that facilitate CNS drug delivery. Such targets include the organic anion-transporting polypeptides (OATPs in humans; Oatps in rodents), a family of sodium-independent transporters that are endogenously expressed in the brain and are involved in drug uptake. OATP/Oatp substrates include drugs that are efficacious in treatment of pain and/or cerebral hypoxia (i.e., opioid analgesic peptides, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors). This clearly suggests that OATP/Oatp isoforms are viable transporter targets that can be exploited for optimization of drug delivery to the brain and, therefore, improved treatment of CNS diseases. This review summarizes recent knowledge in this area and emphasizes the potential that therapeutic targeting of OATP/Oatp isoforms may have in facilitating CNS drug delivery and distribution. Additionally, information presented in this review will point to novel strategies that can be used for treatment of pain and cerebral hypoxia.

Lack of association between SLCO1B1 polymorphism and the lipid-lowering effects of atorvastatin and simvastatin in Chinese individuals

PURPOSE

There is significant inter-individual variability in the lipid-lowering effects of atorvastatin and simvastatin. Our goal was to investigate the impact of SLCO1B1 genetic polymorphism on the lipid-lowering effects of atorvastatin and simvastatin.

METHODS

We recruited 363 unrelated hyperlipidemic patients with the CYP3A4 1/1, CYP3A5 1/1, and CYP3AP1 1/1 genotypes: 189 of these were treated with atorvastatin and 174 were treated with simvastatin as a single-agent therapy (20 mg day(-1) orally) for 4 weeks. The genotyping of SLCO1B1 c.521T > C (p.V174A, OATP-C5) was performed with allele-specific polymerase chain reaction (AS-PCR), and PCR restriction fragment length polymorphism (RFLP) was performed to detect the carriers of SLCO1B1 c.388A > G (p.N130D, OATP-C1b). Serum triglyceride (TGs), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C) levels were determined before and after treatment.

RESULTS

The frequencies of the SLCO1B1 521T > C and 388A > G variant alleles in Chinese hyperlipidemic patients were found to be 16.2% and 72.1% respectively. After treatment with 20 mg simvastatin or atorvastatin daily for 4 weeks, TC, TG, and LDL-C concentrations were lower than at baseline, on average, by 18.1 ± 3.7%, 25.8 ± 9.7%, 27.7 ± 5.4% in the simvastatin-treated group, and 17.5 ± 3.7%, 22.6 ± 8.6%, 27.5 ± 5.5% in the atorvastatin-treated group respectively, and the mean relative reduction in serum HDL cholesterol did not reach statistical significance (-1.0 ± 10.9%, 0.5 ± 9.3%). However, no significant differences were observed in the lipid-lowering effects of atorvastatin and simvastatin between subjects with different SLCO1B1 genotypes.

CONCLUSION

The SLCO1B1 521T > C and 388A > G variants were found to be relatively common in Chinese patients with essential hyperlipidemia. These frequencies were found to be similar to those observed in healthy Chinese and Japanese individuals, but significantly different from Caucasians and blacks. SLCO1B1 521T > C and 388A > G polymorphisms may not be associated with the lipid-lowering effects of atorvastatin and simvastatin.

N-Glycosylation dictates proper processing of organic anion transporting polypeptide 1B1

Organic anion transporting polypeptides (OATPs) have been extensively recognized as key determinants of absorption, distribution, metabolism and excretion (ADME) of various drugs, xenobiotics and toxins. Putative N-glycosylation sites located in the extracellular loops 2 and 5 is considered a common feature of all OATPs and some members have been demonstrated to be glycosylated proteins. However, experimental evidence is still lacking on how such a post-translational modification affect the transport activity of OATPs and which of the putative glycosylation sites are utilized in these transporter proteins. In the present study, we substituted asparagine residues that are possibly involved in N-glycosylation with glutamine residues and identified three glycosylation sites (Asn134, Asn503 and Asn516) within the structure of OATP1B1, an OATP member that is mainly expressed in the human liver. Our results showed that Asn134 and Asn516 are used for glycosylation under normal conditions; however, when Asn134 was mutagenized, an additional asparagine at position 503 is involved in the glycosylation process. Simultaneously replacement of all three asparagines with glutamines led to significantly reduced protein level as well as loss of transport activity. Further studies revealed that glycosylation affected stability of the transporter protein and the unglycosylated mutant was retained within endoplasmic reticulum.

Inhibition of hepatitis C virus RNA translation by antisense bile acid conjugated phosphorothioate modified oligodeoxynucleotides (ODN)

BACKGROUND

The 5'-noncoding region (5'NCR) of the HCV-genome comprises an internal ribosome entry site essential for HCV-translation/replication. Phosphorothioate oligodeoxynucleotides (tS-ODN) complementary to this region can inhibit HCV-translation in vitro. In this study, bile acid conjugated tS-ODN were generated to increase cell-selective inhibition of 5'NCR-dependent HCV-translation.

METHODS

Different bile acid conjugated tS-ODN complementary to the HCV5'NCR were selected for their inhibitory potential in an in vitro transcription/translation assay. To analyze OATP (organic anion transporting polypeptides)-selective uptake of bile acid conjugated ODN, different hepatoma cells were stably transfected with the OATP1B1-transporter and primary human hepatocytes were used. An adenovirus encoding the HCV5'NCR fused to the luciferase gene (Ad-GFP-NCRluc) was generated to quantify 5'NCR-dependent HCV gene expression in OATP-overexpressing hepatoma cells and in vivo.

RESULTS

A 17mer phosphorothioate modified ODN (tS-ODN4_13) complementary to HCV5'NCR was able to inhibit 5'NCR-dependent HCV-translation in an in vitro transcription/translation test system by more than 90% and it was also effective in Huh7-cells containing the HCV subgenomic replicon. Conjugation to taurocholate (tS-ODN4_13T) significantly increased selective ODN uptake by primary human hepatocytes and by OATP1B1-expressing HepG2-cells compared to parental HepG2-cells. Correspondingly, tS-ODN4_13T significantly inhibited HCV gene expression in liver-derived OATP1B1-expressing HepG2- or CCL13-cells up to 70% compared to unconjugated tS-ODN and compared to mismatch taurocholate coupled tS-ODN. In vivo, tS-ODN4_13T showed also a trend to block 5'NCR-dependent HCV gene expression.

CONCLUSIONS

The tested taurocholate conjugated 17mer antisense ODN complementary to HCV5'NCV showed an increased and selective uptake by hepatocytes and liver-derived cells through OATP-mediated transport resulting in enhanced specific inhibition of HCV gene expression in vitro and in vivo. Thus, this novel approach may represent a promising strategy to improve antisense approaches with ODN in the control of hepatitis C infection.

An immortalized human blood-nerve barrier endothelial cell line for in vitro permeability studies

Solute and macromolecular transport studies may elucidate nutritional requirements and drug effects in healthy and diseased peripheral nerves. Endoneurial endothelial cells are specialized microvascular cells that form the restrictive blood-nerve barrier (BNB). Primary human endoneurial endothelial cells (pHEndECs) are difficult to isolate, limiting their widespread availability for biomedical research. We developed a simian virus-40 large T-antigen (SV40-LTA) immortalized human BNB cell line via stable transfection of low passage pHEndECs and observed continuous growth in culture for >45 population doublings. As observed with pHEndECs, the immortalized BNB endothelial cells were Ulex Europaeus agglutinin-1-positive and endocytosed low density lipoprotein, but lost von Willebrand factor expression. Glucose transporter-1, P-glycoprotein (P-gp), γ-glutamyl transpeptidase (γ-GT), large neutral amino acid transporter-1 (LAT-1), creatine transporter (CRT), and monocarboxylate transporter-1 (MCT-1) mRNA expression were retained at all passages with loss of alkaline phosphatase (AP) expression after passages 16-20. Compared with an SV40-LTA immortalized human blood-brain barrier endothelial cell line, there was increased γ-GT protein expression, equivalent expression of organic anion transporting polypeptide-C (OATP-C), organic anion transporter 3 (OAT-3), MCT-1, and LAT-1, and reduced expression of AP, CRT, and P-gp by the BNB cell line at passage 20. Further studies demonstrated lower transendothelial electrical resistance (~181 vs. 191 Ω cm(2)), equivalent permeability to fluoresceinated sodium (4.84 vs. 4.39 %), and lower permeability to fluoresceinated high molecular weight (70 kDa) dextran (0.39 vs. 0.52 %) by the BNB cell line. This cell line retained essential molecular and biophysical properties suitable for in vitro peripheral nerve permeability studies.

Evaluation of organic anion-transporting polypeptide 1B1 and CYP3A4 activities in primary human hepatocytes and HepaRG cells cultured in a dynamic three-dimensional bioreactor system

The long-term stability of liver cell functions is a major challenge when studying hepatic drug transport, metabolism, and toxicity in vitro. The aim of the present study was to investigate organic anion-transporting polypeptide (OATP) 1B1 and CYP3A4 activities in fresh primary human hepatocytes and differentiated cryopreserved HepaRG cells when cultured in a three-dimensional (3D) bioreactor system. OATP1B1 activity was determined by loss from media experiments of [(3)H]estradiol-17β-D-glucuronide and atorvastatin acid (ATA) for up to 7 days in culture. ATA metabolite formation was determined at days 3 to 4 to evaluate CYP3A4 activity. Overall, the results showed that freshly isolated human hepatocytes inoculated in the bioreactor retained OATP1B1 activity for at least 7 days, whereas in HepaRG cells no OATP1B1 activity was observed beyond day 2. The activity data were in agreement with immunohistochemical stainings, which showed that OATP1B1 protein expression was preserved for at least 9 days in fresh human hepatocytes, whereas OATP1B1 was expressed markedly lower in HepaRG cells after 9 days in culture. Fresh human hepatocytes and HepaRG cells exhibited similar CYP3A4 activity in bioreactor culture, and immunohistochemical stainings supported these findings. Activity and mRNA expression of OATP1B1 and CYP3A4 in primary human hepatocytes compared with HepaRG cells in fresh suspensions were in agreement with data obtained in bioreactor culture. In conclusion, freshly isolated human hepatocytes cultured in a 3D bioreactor system preserve both OATP1B1 and CYP3A4 activities, allowing long-term in vitro studies on drug disposition and toxicity.