OATP1B1 POLYMORPHISM IS A MAJOR DETERMINANT OF SERUM BILIRUBIN LEVEL BUT NOT ASSOCIATED WITH RIFAMPICIN-MEDIATED BILIRUBIN ELEVATION

Combination of UGT1A1 polymorphism and baseline plasma bilirubin levels in predicting the risk of antipsychotic-induced dyslipidemia in schizophrenia patients

The prolonged usage of atypical antipsychotic drugs (AAPD) among individuals with schizophrenia often leads to metabolic side effects such as dyslipidemia. These effects not only limit one's selection of AAPD but also significantly reduce compliance and quality of life of patients. Recent studies suggest that bilirubin plays a crucial role in maintaining lipid homeostasis and may be a potential pre-treatment biomarker for individuals with dyslipidemia. The present study included 644 schizophrenia patients from two centers. Demographic and clinical characteristics were collected at baseline and 4 weeks after admission to investigate the correlation between metabolites, episodes, usage of AAPDs, and occurrence of dyslipidemia. Besides, we explored the combined predictive value of genotypes and baseline bilirubin for dyslipidemia by employing multiple PCR targeted capture techniques to sequence two pathways: bilirubin metabolism-related genes and lipid metabolism-related genes. Our results indicated that there existed a negative correlation between the changes in bilirubin levels and triglyceride (TG) levels in patients with schizophrenia. Among three types of bilirubin, direct bilirubin in the baseline (DBIL-bl) proved to be the most effective in predicting dyslipidemia in the ROC analysis (AUC = 0.627, p < 0.001). Furthermore, the odds ratio from multinomial logistic regression analysis showed that UGT1A1*6 was a protective factor for dyslipidemia (ß = -12.868, p < 0.001). The combination of baseline DBIL and UGT1A1*6 significantly improved the performance in predicting dyslipidemia (AUC = 0.939, p < 0.001). Schizophrenia patients with UGT1A1*6 mutation and a certain level of baseline bilirubin may be more resistant to dyslipidemia and have more selections for AAPD than other patients.

Synthetic augmentation of bilirubin metabolism in human pluripotent stem cell-derived liver organoids

UGT1A1 (UDP glucuronosyltransferase family 1 member A1) is the primary enzyme required for bilirubin conjugation, which is essential for preventing hyperbilirubinemia. Animal models lack key human organic anion transporting polypeptides with distinct epigenetic control over bilirubin metabolism, necessitating a human model to interrogate the regulatory mechanism behind UGT1A1 function. Here, we use induced pluripotent stem cells to develop human liver organoids that can emulate conjugation failure phenotype. Bilirubin conjugation assays, chromatin immunoprecipitation, and transcriptome analysis elucidated the role of glucocorticoid antagonism in UGT1A1 activation. This antagonism prevents the binding of transcriptional repressor MECP2 at the expense of NRF2 with associated off-target effects. Therefore, we introduced functional GULO (L-gulonolactone oxidase) in human organoids to augment intracellular ascorbate for NRF2 reactivation. This engineered organoid conjugated more bilirubin and protected against hyperbilirubinemia when transplanted in immunosuppressed Crigler-Najjar syndrome rat model. Collectively, we demonstrate that our organoid system serves as a manipulatable model for interrogating hyperbilirubinemia and potential therapeutic development.

Effect of Hepatic Impairment on the Pharmacokinetics and Pharmacodynamics of Cilofexor, a Selective Nonsteroidal Farnesoid X Receptor Agonist

Cilofexor is a nonsteroidal farnesoid X receptor agonist in clinical development for treatment of nonalcoholic steatohepatitis. This work characterized the pharmacokinetics, pharmacodynamics, safety, and tolerability of cilofexor in participants with normal hepatic function or hepatic impairment (HI). Participants with stable mild, moderate, or severe HI (Child-Pugh [CP] A, B, or C, respectively, [n = 10/group]) and healthy matched controls with normal hepatic function received a single oral dose of cilofexor (30 mg for CP-A or B; 10 mg for CP-C) with a standardized meal. Overall, 56 participants received cilofexor and completed the study. Cilofexor area under the plasma concentration-time curve was 76%, 2.5-fold, and 6.3-fold higher in participants with mild, moderate, or severe HI, respectively, relative to the area under the plasma concentration-time curve in matched participants with normal hepatic function. Cilofexor unbound fraction was 38%, 2-fold, and 3.16-fold higher in participants with mild, moderate, and severe HI, respectively, relative to participants with normal hepatic function. Moderate correlations were identified between cilofexor exposure and CP score or laboratory tests components of CP score. Serum 7α-hydroxy-4-cholesten-3-one and plasma fibroblast growth factor 19 were similar in participants with mild, moderate, or severe HI and participants with normal hepatic function. Cilofexor was generally well tolerated; all cilofexor-related adverse events were mild in severity. Cilofexor can be administered to patients with mild HI without dose adjustment. Caution and dose modification are warranted when administering cilofexor to patients with moderate or severe HI.

Endogenous Biomarkers for SLC Transporter-Mediated Drug-Drug Interaction Evaluation

Membrane transporters play an important role in the absorption, distribution, metabolism, and excretion of xenobiotic substrates, as well as endogenous compounds. The evaluation of transporter-mediated drug-drug interactions (DDIs) is an important consideration during the drug development process and can guide the safe use of polypharmacy regimens in clinical practice. In recent years, several endogenous substrates of drug transporters have been identified as potential biomarkers for predicting changes in drug transport function and the potential for DDIs associated with drug candidates in early phases of drug development. These biomarker-driven investigations have been applied in both preclinical and clinical studies and proposed as a predictive strategy that can be supplanted in order to conduct prospective DDIs trials. Here we provide an overview of this rapidly emerging field, with particular emphasis on endogenous biomarkers recently proposed for clinically relevant uptake transporters.

Prediction of Drug-Induced Hyperbilirubinemia by In Vitro Testing

Bilirubin, the end product of heme catabolism, is produced continuously in the body and may reach toxic levels if accumulates in the serum and tissues; therefore, a highly efficient mechanism evolved for its disposition. Normally, unconjugated bilirubin enters hepatocytes through the uptake transporters organic anion transporting polypeptide (OATP) 1B1 and 1B3, undergoes glucuronidation by the Phase II enzyme UDP glucuronosyltransferase 1A1 (UGT1A1), and conjugated forms are excreted into the bile by the canalicular export pump multidrug resistance protein 2 (MRP2). Any remaining conjugated bilirubin is transported back to the blood by MRP3 and passed on for uptake and excretion by downstream hepatocytes or the kidney. The bile salt export pump BSEP as the main motor of bile flow is indirectly involved in bilirubin disposition. Genetic mutations and xenobiotics that interfere with this machinery may impede bilirubin disposition and cause hyperbilirubinemia. Several pharmaceutical compounds are known to cause hyperbilirubinemia via inhibition of OATP1Bs, UGT1A1, or BSEP. Herein we briefly review the in vitro prediction methods that serve to identify drugs with a potential to induce hyperbilirubinemia. In vitro assays can be deployed early in drug development and may help to minimize late-stage attrition. Based on current evidence, drugs that behave as mono- or multispecific inhibitors of OATP1B1, UGT1A1, and BSEP in vitro are at risk of causing clinically significant hyperbilirubinemia. By integrating inhibition data from in vitro assays, drug serum concentrations, and clinical reports of hyperbilirubinemia, predictor cut-off values have been established and are provisionally suggested in this review. Further validation of in vitro readouts to clinical outcomes is expected to enhance the predictive power of these assays.

The involvement of human organic anion transporting polypeptides (OATPs) in drug-herb/food interactions

Organic anion transporting polypeptides (OATPs) are important transporter proteins that are expressed at the plasma membrane of cells, where they mediate the influx of endogenous and exogenous substances including hormones, natural compounds and many clinically important drugs. OATP1A2, OATP2B1, OATP1B1 and OATP1B3 are the most important OATP isoforms and influence the pharmacokinetic performance of drugs. These OATPs are highly expressed in the kidney, intestine and liver, where they determine the distribution of drugs to these tissues. Herbal medicines are increasingly popular for their potential health benefits. Humans are also exposed to many natural compounds in fruits, vegetables and other food sources. In consequence, the consumption of herbal medicines or food sources together with a range of important drugs can result in drug-herb/food interactions via competing specific OATPs. Such interactions may lead to adverse clinical outcomes and unexpected toxicities of drug therapies. This review summarises the drug-herb/food interactions of drugs and chemicals that are present in herbal medicines and/or food in relation to human OATPs. This information can contribute to improving clinical outcomes and avoiding unexpected toxicities of drug therapies in patients.

Influence of Uridine Diphosphate Glucuronosyltransferase Family 1 Member A1 and Solute Carrier Organic Anion Transporter Family 1 Member B1 Polymorphisms and Efavirenz on Bilirubin Disposition in Healthy Volunteers

Chronic administration of efavirenz is associated with decreased serum bilirubin levels, probably through induction of UGT1A1 We assessed the impact of efavirenz monotherapy and UGT1A1 phenotypes on total, conjugated, and unconjugated serum bilirubin levels in healthy volunteers. Healthy volunteers were enrolled into a clinical study designed to address efavirenz pharmacokinetics, drug interactions, and pharmacogenetics. Volunteers received multiple oral doses (600 mg/day for 17 days) of efavirenz. Serum bilirubin levels were obtained at study entry and 1 week after completion of the study. DNA genotyping was performed for UGT1A1 [*80 (C>T), *6 (G>A), *28 (TA₇), *36 (TA₅), and *37 (TA₈)] and for SLCO1B1 [*5 (521T>C) and *1b (388A>G] variants. Diplotype predicted phenotypes were classified as normal, intermediate, and slow metabolizers. Compared with bilirubin levels at screening, treatment with efavirenz significantly reduced total, conjugated, and unconjugated bilirubin. After stratification by UGT1A1 phenotypes, there was a significant decrease in total bilirubin among all phenotypes, conjugated bilirubin among intermediate metabolizers, and unconjugated bilirubin among normal and intermediate metabolizers. The data also show that UGT1A1 genotype predicts serum bilirubin levels at baseline, but this relationship is lost after efavirenz treatment. SLCO1B1 genotypes did not predict bilirubin levels at baseline or after efavirenz treatment. Our data suggest that efavirenz may alter bilirubin disposition mainly through induction of UGT1A1 metabolism and efflux through multidrug resistance-associated protein 2. SIGNIFICANCE STATEMENT: Efavirenz likely alters the pharmacokinetics of coadministered drugs, potentially causing lack of efficacy or increased adverse effects, as well as the disposition of endogenous compounds relevant in homeostasis through upregulation of UGT1A1 and multidrug resistance-associated protein 2. Measurement of unconjugated and conjugated bilirubin during new drug development may provide mechanistic understanding regarding enzyme and transporters modulated by the new drug.

Organic Anion Transporter Polypeptide 1B1 Polymorphism Modulates the Extent of Drug-Drug Interaction and Associated Biomarker Levels in Healthy Volunteers

Understanding transporter‐mediated drug–drug interactions is an integral part of risk assessment in drug development. Recent studies support the use of hexadecanedioate (HDA), tetradecanedioate (TDA), coproporphyrin (CP)‐I, and CP‐III as clinical biomarkers for evaluating organic anion‐transporting polypeptide (OATP)1B1 (SLCO1B1) inhibition. The current study investigated the effect of OATP1B1 genotype c.521T>C (OATP1B1‐Val174Ala) on the extent of interaction between cyclosporin A (CsA) and pravastatin, and associated endogenous biomarkers of the transporter (HDA, TDA, CP‐I, and CP‐III), in 20 healthy volunteers. The results show that the levels of each clinical biomarker and pravastatin were significantly increased in plasma samples of the volunteers following administration of pravastatin plus CsA compared with pravastatin plus placebo. The overall fold change in the area under the concentration–time curve (AUC) and maximum plasma concentration (C_max) was similar among the four biomarkers (1.8–2.5‐fold, paired t‐test P value < 0.05) in individuals who were homozygotes or heterozygotes of the major allele, c.521T. However, the fold change in AUC and C_max for HDA and TDA was significantly abolished in the subjects who were c.521‐CC, whereas the respective fold change in AUC and C_max for pravastatin and CP‐I and CP‐III were slightly weaker in individuals who were c.521‐CC compared with c.521‐TT/TC genotypes. In addition, this study provides the first evidence that SLCO1B1 c.521T>C genotype is significantly associated with CP‐I but not CP‐III levels. Overall, these results suggest that OATP1B1 genotype can modulate the effects of CsA on biomarker levels; the extent of modulation differs among the biomarkers.

Organic Anion Transporting Polypeptides: Emerging Roles in Cancer Pharmacology

The organic anion transporting polypeptides (OATPs) are a superfamily of drug transporters involved in the uptake and disposition of a wide array of structurally divergent endogenous and exogenous substrates, including steroid hormones, bile acids, and commonly used drugs, such as anti-infectives, antihypertensives, and cholesterol lowering agents. In the past decade, OATPs, primarily OATP1A2, OATP1B1, and OATP1B3, have emerged as potential mediators of chemotherapy disposition, including drugs such as methotrexate, doxorubicin, paclitaxel, docetaxel, irinotecan and its important metabolite 7-ethyl-10-hydroxycamptothecin, and certain tyrosine kinase inhibitors. Furthermore, OATP family members are polymorphic and numerous studies have shown OATP variants to have differential uptake, disposition, and/or pharmacokinetics of numerous drug substrates with important implications for interindividual differences in efficacy and toxicity. Additionally, certain OATPs have been found to be overexpressed in a variety of human solid tumors, including breast, liver, colon, pancreatic, and ovarian cancers, suggesting potential roles for OATPs in tumor development and progression and as novel targets for cancer therapy. This review focuses on the emerging roles for selected OATPs in cancer pharmacology, including preclinical and clinical studies suggesting roles in chemotherapy disposition, the pharmacogenetics of OATPs in cancer therapy, and OATP overexpression in various tumor tissues with implications for OATPs as therapeutic targets.

CYP2C9 and OATP1B1 genetic polymorphisms affect the metabolism and transport of glimepiride and gliclazide

The therapeutic use of glimepiride and gliclazide shows substantial inter-individual variation in pharmacokinetics and pharmacodynamics in human populations, which might be caused by genetic differences among individuals. The aim of this study was to assess the effect of CYP2C9 and OATP1B1 genetic polymorphisms on the metabolism and transport of glimepiride and gliclazide. The uptake of glimepiride and gliclazide was measured in OATP1B1*1a, *5 and *15-HEK293T cells, and their metabolism was measured using CYP2C9*1, *2 and *3 recombinase by LC-MS. Glimepiride in OATP1B1*1a, *5 and *15-HEK293T cells had V_max values of 155 ± 18.7, 80 ± 9.6, and 84.5 ± 8.2 pmol/min/mg, while gliclazide had V_max values of 15.7 ± 4.6, 7.2 ± 2.5, and 8.7 ± 2.4 pmol/min/mg, respectively. The clearance of glimepiride and gliclazide in OATP1B1*5 and *15 was significantly reduced compared to the wild-type. Glimepiride in the presence of CYP2C9*1, *2 and *3 recombinase had V_max values of 21.58 ± 7.78, 15.69 ± 5.59, and 9.17 ± 3.03 nmol/min/mg protein, while gliclazide had V_max values of 15.73 ± 3.11, 10.53 ± 4.06, and 6.21 ± 2.94 nmol/min/mg protein, respectively. The clearance of glimepiride and gliclazide in CYP2C9*2 and *3 was significantly reduced compared to the wild-type. These findings collectively indicate that OATP1B1*5 and *15 and CYP2C9*2 and *3 have a significant effect on the transport and metabolism of glimepiride and gliclazide.

Endogenous Metabolites-Mediated Communication Between OAT1/OAT3 and OATP1B1 May Explain the Association Between SLCO1B1 SNPs and Methotrexate Toxicity

Although OATP1B1 is not expressed in the kidney, polymorphisms in SLCO1B1 have been associated with methotrexate clearance or toxicity. This unexpected pharmacogenetic association may reflect remote communication between liver and kidney transporters. This study confirms the pharmacogenetic association with methotrexate toxicity in adult patients with hematological malignancies. Using a targeted urinary metabolomics approach, we identified 38 and 34 metabolites which were differentially excreted between wildtype and carriers of the c.388A>G or c.521T>C variant alleles, respectively, half of them being associated with methotrexate toxicity. These metabolites mainly consisted of fatty acid derivatives and microbiota catabolites, including glycine conjugates and other uremic toxins, all known OATs substrates. These results suggest that dysfunction of a transporter affects the excretion profile of endogenous or exogenous substrates, possibly through metabolite-mediated interactions involving other transport systems, even in distant organs. This opens the way for better comprehension of complex pharmacokinetics and transporter-mediated drug-drug or nutrient-drug interactions.

Pharmacokinetics and pharmacogenetics of anti-tubercular drugs: a tool for treatment optimization?

INTRODUCTION

WHO global strategy is to end tuberculosis epidemic by 2035. Pharmacokinetic and pharmacogenetic studies are increasingly performed and might confirm their potential role in optimizing treatment outcome in specific settings and populations. Insufficient drug exposure seems to be a relevant factor in tuberculosis outcome and for the risk of phenotypic resistance. Areas covered: This review discusses available pharmacokinetic and pharmacogenetic data of first and second-line antitubercular agents in relation to efficacy and toxicity. Pharmacodynamic implications of optimized drugs and new options regimens are reviewed. Moreover a specific session describes innovative investigations on drug penetration. Expert opinion: The optimal use of available antitubercular drugs is paramount for tuberculosis control and eradication. Whilst trials are still on-going, higher rifampicin doses should be reserved to treatment for tubercular meningitis. Therapeutic Drug Monitoring with limiting sampling strategies is advised in patients at risk of failure or with slow treatment response. Further studies are needed in order to provide definitive recommendations of pharmacogenetic-based individualization: however lower isoniazid doses in NAT2 slow acetylators and higher rifampicin doses in individuals with SLCO1B1 loss of function genes are promising strategies. Finally in order to inform tailored strategies we need more data on tissue drug penetration and pharmacological modelling.

Membrane Transporters for Bilirubin and Its Conjugates: A Systematic Review

Background: Bilirubin is a highly-hydrophobic tetrapyrrole which binds to plasma albumin. It is conjugated in the liver to glucuronic acid, and the water-soluble glucuronides are excreted in urine and bile. The membrane transporters of bilirubin diglucuronide are well-known. Still undefined are however the transporters performing the uptake of bilirubin from the blood into the liver, a process known to be fast and not rate-limited. The biological importance of this process may be appraised by considering that in normal adults 200–300 mg of bilirubin are produced daily, as a result of the physiologic turnover of hemoglobin and cellular cytochromes. Nevertheless, research in this field has yielded controversial and contradicting results. We have undertaken a systematic review of the literature, believing in its utility to improve the existing knowledge and promote further advancements.

Methods: We have sourced the PubMed database until 30 June 2017 by applying 5 sequential searches. Screening and eligibility criteria were applied to retain research articles reporting results obtained by using bilirubin molecules in membrane transport assays in vitro or by assessing serum bilirubin levels in in vivo experiments.

Results: We have identified 311 articles, retaining 44, reporting data on experimental models having 6 incremental increases of complexity (isolated proteins, membrane vesicles, cells, organ fragments, in vivo rodents, and human studies), demonstrating the function of 19 membrane transporters, encoded by either SLCO or ABC genes. Three other bilirubin transporters have no gene, though one, i.e., bilitranslocase, is annotated in the Transporter Classification Database.

Conclusions: This is the first review that has systematically examined the membrane transporters for bilirubin and its conjugates. Paradoxically, the remarkable advancements in the field of membrane transport of bilirubin have pointed to the elusive mechanism(s) enabling bilirubin to diffuse into the liver as if no cellular boundary existed.

Recent advance in the pharmacogenomics of human Solute Carrier Transporters (SLCs) in drug disposition

Drug pharmacokinetics is influenced by the function of metabolising enzymes and influx/efflux transporters. Genetic variability of these genes is known to impact on clinical therapies. Solute Carrier Transporters (SLCs) are the primary influx transporters responsible for the cellular uptake of drug molecules, which consequently, impact on drug efficacy and toxicity. The Organic Anion Transporting Polypeptides (OATPs), Organic Anion Transporters (OATs) and Organic Cation Transporters (OCTs/OCTNs) are the most important SLCs involved in drug disposition. The information regarding the influence of SLC polymorphisms on drug pharmacokinetics is limited and remains a hot topic of pharmaceutical research. This review summarises the recent advance in the pharmacogenomics of SLCs with an emphasis on human OATPs, OATs and OCTs/OCTNs. Our current appreciation of the degree of variability in these transporters may contribute to better understanding the inter-patient variation of therapies and thus, guide the optimisation of clinical treatments.

Metabolomic and Genome-wide Association Studies Reveal Potential Endogenous Biomarkers for OATP1B1

Transporter-mediated drug-drug interactions (DDIs) are a major cause of drug toxicities. Using published genome-wide association studies (GWAS) of the human metabolome, we identified 20 metabolites associated with genetic variants in organic anion transporter, OATP1B1 (P < 5 × 10-8 ). Of these, 12 metabolites were significantly higher in plasma samples from volunteers dosed with the OATP1B1 inhibitor, cyclosporine (CSA) vs. placebo (q-value < 0.2). Conjugated bile acids and fatty acid dicarboxylates were among the metabolites discovered using both GWAS and CSA administration. In vitro studies confirmed tetradecanedioate (TDA) and hexadecanedioate (HDA) were novel substrates of OATP1B1 as well as OAT1 and OAT3. This study highlights the use of multiple datasets for the discovery of endogenous metabolites that represent potential in vivo biomarkers for transporter-mediated DDIs. Future studies are needed to determine whether these metabolites can serve as qualified biomarkers for organic anion transporters. Quantitative relationships between metabolite levels and modulation of transporters should be established.

Inherited disorders of bilirubin clearance

Inherited disorders of hyperbilirubinemia may be caused by increased bilirubin production or decreased bilirubin clearance. Reduced hepatic bilirubin clearance can be due to defective (i) unconjugated bilirubin uptake and intrahepatic storage, (ii) conjugation of glucuronic acid to bilirubin (e.g., Gilbert syndrome, Crigler-Najjar syndrome, Lucey-Driscoll syndrome, breast milk jaundice), (iii) bilirubin excretion into bile (Dubin-Johnson syndrome), or (iv) conjugated bilirubin re-uptake (Rotor syndrome). In this review, the molecular mechanisms and clinical manifestations of these conditions are described, as well as current approaches to diagnosis and therapy.

Role of extrahepatic UDP-glucuronosyltransferase 1A1: Advances in understanding breast milk-induced neonatal hyperbilirubinemia

Newborns commonly develop physiological hyperbilirubinemia (also known as jaundice). With increased bilirubin levels being observed in breast-fed infants, breast-feeding has been recognized as a contributing factor for the development of neonatal hyperbilirubinemia. Bilirubin undergoes selective metabolism by UDP-glucuronosyltransferase (UGT) 1A1 and becomes a water soluble glucuronide. Although several factors such as gestational age, dehydration and weight loss, and increased enterohepatic circulation have been associated with breast milk-induced jaundice (BMJ), deficiency in UGT1A1 expression is a known cause of BMJ. It is currently believed that unconjugated bilirubin is metabolized mainly in the liver. However, recent findings support the concept that extrahepatic tissues, such as small intestine and skin, contribute to bilirubin glucuronidation during the neonatal period. We will review the recent advances made towards understanding biological and molecular events impacting BMJ, especially regarding the role of extrahepatic UGT1A1 expression.

Evaluation of cynomolgus monkeys for the identification of endogenous biomarkers for hepatic transporter inhibition and as a translatable model to predict pharmacokinetic interactions with statins in humans

Inhibition of hepatic transporters such as organic anion transporting polypeptides (OATPs) 1B can cause drug-drug interactions (DDIs). Determining the impact of perpetrator drugs on the plasma exposure of endogenous substrates for OATP1B could be valuable to assess the risk for DDIs early in drug development. As OATP1B orthologs are well conserved between human and monkey, we assessed in cynomolgus monkeys the endogenous OATP1B substrates that are potentially suitable to assess DDI risk in humans. The effect of rifampin (RIF), a potent inhibitor for OATP1B, on plasma exposure of endogenous substrates of hepatic transporters was measured. From the 18 biomarkers tested, RIF (18 mg/kg, oral) caused significant elevation of plasma unconjugated and conjugated bilirubin, which may be attributed to inhibition of cOATP1B1 and cOATP1B3 based on in vitro to in vivo extrapolation analysis. To further evaluate whether cynomolgus monkeys are a suitable translational model to study OATP1B-mediated DDIs, we determined the inhibitory effect of RIF on in vitro transport and pharmacokinetics of rosuvastatin (RSV) and atorvastatin (ATV). RIF strongly inhibited the uptake of RSV and ATV by cOATP1B1 and cOATP1B3 in vitro. In agreement with clinical observations, RIF (18 mg/kg, oral) significantly decreased plasma clearance and increased the area under the plasma concentration curve (AUC) of intravenously administered RSV by 2.8- and 2.7-fold, and increased the AUC and maximum plasma concentration of orally administered RSV by 6- and 10.3-fold, respectively. In contrast to clinical findings, RIF did not significantly increase plasma exposure of either intravenous or orally administered ATV, indicating species differences in the rate-limiting elimination pathways.

Gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid-enhanced magnetic resonance imaging: A potential utility for the evaluation of regional liver function impairment following transcatheter arterial chemoembolization

The present study aimed to evaluate regional liver function impairment following transcatheter arterial chemoembolization (TACE), assessed by magnetic resonance imaging (MRI) enhanced by gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid (Gd-EOB-DTPA). Additionally, this study evaluated the associations between signal intensity and various clinical factors. A prospective study was conducted between March 2012 and May 2013 with a total of 35 patients. Gd-EOB-DTPA-enhanced MRI was performed 3–5 days after TACE therapy. The signal to noise ratio (SNR) was subsequently calculated for healthy liver tissue regions and peritumoral regions, prior to and 20 min after Gd-EOB-DTPA administration. The correlation between clinical factors and relative SNR was assessed using Pearson’s correlation coefficient or Spearman’s rank correlation coefficient. Prior to Gd-EOB-DTPA administration, the SNR values showed no significant difference (t=1.341, P=0.191) in healthy liver tissue regions (50.53±15.99; range, 11.25–83.46) compared with peritumoral regions (49.81±15.85; range, 12.34–81.53). On measuring at 20 min following Gd-EOB-DTPA administration, the SNR in healthy liver tissue regions (82.55±33.33; range, 31.45–153.02) was significantly higher (t=3.732, P<0.001) compared with that in peritumoral regions (75.77±27.41; range, 31.42–144.49). The relative SNR in peritumoral regions correlated only with the quantity of iodized oil used during TACE therapy (r=0.528, P=0.003); the age, gender, diameter and blood supply of the tumor, or Child-Pugh class of the patient did not correlate with relative SNR. Gd-EOB-DTPA-enhanced MRI may be an effective way to evaluate regional liver function impairment following TACE therapy.

What do drug transporters really do?

Potential drug-drug interactions mediated by the ATP-binding cassette (ABC) transporter and solute carrier (SLC) transporter families are of clinical and regulatory concern. However, the endogenous functions of these drug transporters are not well understood. Discussed here is evidence for the roles of ABC and SLC transporters in the handling of diverse substrates, including metabolites, antioxidants, signalling molecules, hormones, nutrients and neurotransmitters. It is suggested that these transporters may be part of a larger system of remote communication ('remote sensing and signalling') between cells, organs, body fluid compartments and perhaps even separate organisms. This broader view may help to clarify disease mechanisms, drug-metabolite interactions and drug effects relevant to diabetes, chronic kidney disease, metabolic syndrome, hypertension, gout, liver disease, neuropsychiatric disorders, inflammatory syndromes and organ injury, as well as prenatal and postnatal development.

Effect of drug transporter pharmacogenetics on cholestasis

INTRODUCTION

The liver is the central place for the metabolism of drugs and other xenobiotics. In the liver cell, oxidation and conjugation of compounds take place, and at the same time, bile formation helps in extrusion of these compounds via the biliary route. A large number of transporters are responsible for drug uptake into the liver cell and excretion into bile or efflux to the sinusoidal blood.

AREAS COVERED

Genetic variants of these transporters and their transactivators contribute to changes in drug handling and are also responsible for cholestatic syndromes of different severity. This review summarizes the current knowledge regarding the influence of these genetic changes. The review covers progressive hereditary cholestatic syndromes as well as recurrent or transient cholestatic syndromes such as drug-induced liver injury, intrahepatic cholestasis of pregnancy, and benign recurrent intrahepatic cholestasis.

EXPERT OPINION

Polymorphisms in transporter genes are frequent. For clinically relevant cholestatic syndromes, it often requires a combination of genetic variants or acquired triggers such as pregnancy or drug treatment. In combination with other pathogenetic aspects, genetic variants in drug transporters may contribute to our understanding of not only cholestatic diseases such as primary sclerosing cholangitis or primary biliary cirrhosis, but also the natural course of chronic liver disease in general.

Inherited disorders of bilirubin transport and conjugation: new insights into molecular mechanisms and consequences

Inherited disorders of bilirubin metabolism might reduce bilirubin uptake by hepatocytes, bilirubin conjugation, or secretion of bilirubin into bile. Reductions in uptake could increase levels of unconjugated or conjugated bilirubin (Rotor syndrome). Defects in bilirubin conjugation could increase levels of unconjugated bilirubin; the effects can be benign and frequent (Gilbert syndrome) or rare but severe, increasing the risk of bilirubin encephalopathy (Crigler-Najjar syndrome). Impairment of bilirubin secretion leads to accumulation of conjugated bilirubin (Dubin-Johnson syndrome). We review the genetic causes and pathophysiology of disorders of bilirubin transport and conjugation as well as clinical and therapeutic aspects. We also discuss the possible mechanisms by which hyperbilirubinemia protects against cardiovascular disease and the metabolic syndrome and the effects of specific genetic variants on drug metabolism and cancer development.

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.

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.

The impact of SLCO1B1 genetic polymorphisms on neonatal hyperbilirubinemia: a systematic review with meta-analysis

OBJECTIVE

To determine whether three variants (388 G>A, 521 T>C, and 463 C>A) of the solute carrier organic anion transporter family member 1B1 (SLCO1B1) are associated with neonatal hyperbilirubinemia.

DATA SOURCE

The China National Knowledge Infrastructure and MEDLINE databases were searched. The systematic review with meta-analysis included genetic studies which assessed the association between neonatal hyperbilirubinemia and 388 G>A, 521 T>C, and 463 C>A variants of SLCO1B1 between January of 1980 and December of 2012. Data selection and extraction were performed independently by two reviewers.

SUMMARY OF THE FINDINGS

Ten articles were included in the study. The results revealed that SLCO1B1 388 G>A is associated with an increased risk of neonatal hyperbilirubinemia (OR, 1.39; 95% CI, 1.07-1.82) in Chinese neonates, but not in white, Thai, Latin American, or Malaysian neonates. The SLCO1B1 521 T>C mutation showed a low risk of neonatal hyperbilirubinemia in Chinese neonates, while no significant associations were found in Brazilian, white, Asian, Thai, and Malaysian neonates. There were no significant differences in SLCO1B1 463 C>A between the hyperbilirubinemia and the control group.

CONCLUSION

This study demonstrated that the 388 G>A mutation of the SLCO1B1 gene is a risk factor for developing neonatal hyperbilirubinemia in Chinese neonates, but not in white, Thai, Brazilian, or Malaysian populations; the SLCO1B1 521 T>C mutation provides protection for neonatal hyperbilirubinemia in Chinese neonates, but not in white, Thai, Brazilian, or Malaysian populations.

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.

UGT1A1 genetic analysis as a diagnostic aid for individuals with unconjugated hyperbilirubinemia

OBJECTIVE

To assess the clinical utility of UGT1A1 genetic testing and describe the spectrum and prevalence of UGT1A1 variations identified in pediatric unconjugated hyperbilirubinemia (UCH), and to characterize specific genotype-phenotype relationships in suspected Gilbert and Crigler-Najjar syndromes.

STUDY DESIGN

A retrospective study was conducted to review clinical information and UGT1A1 genotyping data from 181 pediatric patients referred for UCH. In silico analyses were performed to aid in the assessment of novel UGT1A1 variants.

RESULTS

Overall, 146/181 pediatric patients had at least one heterozygous UGT1A1 functional variant. Identified UGT1A1 variants included 17 novel variants, 7 rare star alleles, and 1 rare variant. There were 129 individuals who possessed the TA7 (*28) promoter repeat and 15 individuals who possessed the *6 (c.211G > A) variation. Out of the 104 individuals with accompanying bilirubin levels, 41 individuals did not have identifiable UGT1A1 variants that explained their UCH, although glucose-6-phosphate dehydrogenase deficiency and other causes of UCH could not be ruled out.

CONCLUSION

Much of the observed UCH could be attributed to variation at the UGT1A1 locus, and UGT1A1 testing helped to substantiate a genetic diagnosis, thereby aiding in individual and family disease management. Although UGT1A1 variation plays a large role in UCH, genetic assessment of UGT1A1 alone may not be comprehensive. Assessment of additional genes may also be useful to evaluate genetic causes for UCH.

Physiological and biochemical basis of clinical liver function tests: a review

OBJECTIVE

To review the literature on the most clinically relevant and novel liver function tests used for the assessment of hepatic function before liver surgery.

BACKGROUND

Postoperative liver failure is the major cause of mortality and morbidity after partial liver resection and develops as a result of insufficient remnant liver function. Therefore, accurate preoperative assessment of the future remnant liver function is mandatory in the selection of candidates for safe partial liver resection.

METHODS

A MEDLINE search was performed using the key words "liver function tests," "functional studies in the liver," "compromised liver," "physiological basis," and "mechanistic background," with and without Boolean operators.

RESULTS

Passive liver function tests, including biochemical parameters and clinical grading systems, are not accurate enough in predicting outcome after liver surgery. Dynamic quantitative liver function tests, such as the indocyanine green test and galactose elimination capacity, are more accurate as they measure the elimination process of a substance that is cleared and/or metabolized almost exclusively by the liver. However, these tests only measure global liver function. Nuclear imaging techniques ((99m)Tc-galactosyl serum albumin scintigraphy and (99m)Tc-mebrofenin hepatobiliary scintigraphy) can measure both total and future remnant liver function and potentially identify patients at risk for postresectional liver failure.

CONCLUSIONS

Because of the complexity of liver function, one single test does not represent overall liver function. In addition to computed tomography volumetry, quantitative liver function tests should be used to determine whether a safe resection can be performed. Presently, (99m)Tc-mebrofenin hepatobiliary scintigraphy seems to be the most valuable quantitative liver function test, as it can measure multiple aspects of liver function in, specifically, the future remnant liver.

Potential clinical factors affecting hepatobiliary enhancement at Gd-EOB-DTPA-enhanced MR imaging

OBJECTIVE

The objective was to clarify the clinical factors that might affect the degree of hepatic parenchymal enhancement at gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid (Gd-EOB-DTPA)-enhanced magnetic resonance (MR) imaging.

MATERIALS AND METHODS

A total of 84 patients with (n=63) and without chronic liver disease (n=21) underwent Gd-EOB-DTPA-enhanced MR imaging. Contrast-enhanced MR images of hepatobiliary phase (HP) were obtained at 20 min after Gd-EOB-DTPA administration. The relative enhancement (RE) of liver parenchyma at 20 min HP was calculated from region of interest measurements at each patient. Then, these results were correlated with various clinical parameters using Pearson correlation coefficient or Spearman rank correlation coefficient. Furthermore, the predictor of the degree of hepatic parenchymal enhancement was determined using multiple regression analysis.

RESULTS

The presence or absence of chronic liver disease (P=.002), ascites (P=.005) and splenomegaly (P=.027), and the values of prothrombin activity (P=.008), total bilirubin (T-Bil) (P=.001), albumin (P=.001), aspartate aminotransferase (AST) (P=.002) and cholinesterase (P=.007) were significantly correlated with the RE of liver parenchyma at 20 min HP. Among these parameters, increases of T-Bil (P=.011 to .028) and AST (P=.018 to .049) were predictors of decreased hepatic parenchymal enhancement.

CONCLUSIONS

Hepatic parenchymal enhancement of Gd-EOB-DTPA was affected by various clinical parameters. Impaired hepatobiliary enhancement may be predicted by routine biochemical tests, such as T-Bil and AST.

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.

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.

Functions of OATP1A and 1B transporters in vivo: insights from mouse models

Organic anion-transporting polypeptides (OATPs) are a superfamily of uptake transporters that mediate the cellular uptake of a broad range of endogenous and exogenous compounds. Of these OATP transporters, members of the 1A and 1B subfamilies have broad substrate specificities. Because they are mainly expressed in liver, kidney and small intestine, OATP1A and 1B transporters can have a major impact on the pharmacokinetics of many drugs. To study their role in physiology and drug disposition, several mouse models lacking functional expression of one or more OATPs have been generated. This review discusses recent findings for these models that have led to new insights into the impact of OATP1A and 1B transporters on pharmacokinetics and toxicokinetics, and on bilirubin detoxification and bile acid handling in normal liver physiology.

Mayo Genome Consortia: a genotype-phenotype resource for genome-wide association studies with an application to the analysis of circulating bilirubin levels

OBJECTIVE

To create a cohort for cost-effective genetic research, the Mayo Genome Consortia (MayoGC) has been assembled with participants from research studies across Mayo Clinic with high-throughput genetic data and electronic medical record (EMR) data for phenotype extraction.

PARTICIPANTS AND METHODS

Eligible participants include those who gave general research consent in the contributing studies to share high-throughput genotyping data with other investigators. Herein, we describe the design of the MayoGC, including the current participating cohorts, expansion efforts, data processing, and study management and organization. A genome-wide association study to identify genetic variants associated with total bilirubin levels was conducted to test the genetic research capability of the MayoGC.

RESULTS

Genome-wide significant results were observed on 2q37 (top single nucleotide polymorphism, rs4148325; P=5.0 × 10(-62)) and 12p12 (top single nucleotide polymorphism, rs4363657; P=5.1 × 10(-8)) corresponding to a gene cluster of uridine 5'-diphospho-glucuronosyltransferases (the UGT1A cluster) and solute carrier organic anion transporter family, member 1B1 (SLCO1B1), respectively.

CONCLUSION

Genome-wide association studies have identified genetic variants associated with numerous phenotypes but have been historically limited by inadequate sample size due to costly genotyping and phenotyping. Large consortia with harmonized genotype data have been assembled to attain sufficient statistical power, but phenotyping remains a rate-limiting factor in gene discovery research efforts. The EMR consists of an abundance of phenotype data that can be extracted in a relatively quick and systematic manner. The MayoGC provides a model of a unique collaborative effort in the environment of a common EMR for the investigation of genetic determinants of diseases.

Frequency of the SLCO1B1 388A>G and the 521T>C polymorphism in Tanzania genotyped by a new LightCycler®-based method

PURPOSE

The 388A>G and the 521T>C polymorphism of the SLCO1B1 gene affect the activity of the uptake transporter OATP1B1, thus influencing kinetics, safety, and efficacy of substrate drugs. To evaluate the impact of these polymorphisms in populations of different ethnic origins, it is important to know their frequencies and to develop fast and reliable methods for genotyping. We therefore established a new, high-throughput method and determined the genotype and allelic frequencies of these polymorphisms in Tanzanians, for which the frequencies were unknown thus far.

METHODS

New LightCycler® 480-based methods with hybridization probes were established and used to genotype 366 Tanzanian and 236 European individuals for 388A>G (rs2306283) and 521T>C (rs4149056) in the SLCO1B1 gene. The methods were validated by direct sequencing of the polymerase chain reaction (PCR) products of heterozygous individuals and checked for intrarun precision repeatability, interrun precision reproducibility, robustness, and deviations from the Hardy-Weinberg equilibrium.

RESULTS

The new methods allow unambiguous identification of the corresponding genotypes. There was a clear difference in allelic distribution between Tanzanians and Europeans, with the 388A>G (rs2306283) being much more prevalent in Tanzanians (87% vs 41%) and the 521T>C (rs4149056) being very rare in this African population (6% vs 17%).

CONCLUSIONS

We developed robust and high-throughput methods to genotype common SLCO1B1 allelic variants with the LightCycler® 480. In Tanzanians, we identified the highest frequency of the 388A>G and 521T>C polymorphisms ever reported from black populations. The clinical relevance of SLCO1B1 genetic variation in the African population remains to be investigated.

Organic anion transporter 1B1 (SLCO1B1) polymorphism and gallstone formation: High incidence of Exon4 CA genotype in female patients in North India

AIM

  Gallstone disease is an important cause of abdominal morbidity Organic anion transport protein 1B1 (OATP1B1) (encoded by SLCO1B1) is a major transporter protein for bile salt uptake in enterohepatic circulation of bile salts. Disturbance in this pathway can decrease relative concentration of bile salts in gallbladder and may lead to formation of gallstones. We investigated role of SLCO1B1 polymorphisms [(Exon4 C > A (Pro155Thr; rs11045819) and Ex6 + 40T > C (Val174Ala; rs4149056)] in conferring interindividual susceptibility to gallstone disease.

METHODS

  A total of 173 healthy controls and 226 gallstone patients (USG positive) were recruited. Genotyping was done by using standard polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) method.

RESULTS

  The observed control frequencies of both polymorphisms of SLCO1B1 gene [(Exon4 C > A (Pro155Thr; rs11045819) and Ex6 + 40T > C (Val174Ala; rs4149056)] were in agreement with Hardy-Weinberg equilibrium. The frequency CA genotype and A allele of Exon4 C > A polymorphism was higher in gallstones patients (12.4% and 6.2%) as compared to controls (5.2% and 2.6%) which was statistically significant [(P = 0.029; OR = 2.31; 95% CI = 1.1-5.0); (P = 0.034; OR = 2.22; 95% CI = 1.1-4.8)], respectively). However, distribution of genotypes and alleles of Ex6 + 40T > C polymorphism was almost similar between gallstone patients and controls. Haplotype analysis showed frequency of A,T haplotype consisting of was significantly higher in gallstone patients as compared to controls and was imposing risk for the disease (P = 0.036; OR = 2.34; 95% CI = 1.0-5.1).

CONCLUSION

  These results suggest that SLCO1B1 Exon4 C > A polymorphism confers increased risk for gallstone disease in North Indian population.

Loci from a genome-wide analysis of bilirubin levels are associated with gallstone risk and composition

BACKGROUND & AIMS

Genome-wide association studies have mapped loci that are associated with serum levels of bilirubin. Bilirubin is a major component of gallstones so we investigated whether these variants predict gallstone bilirubin content and overall risk for gallstones.

METHODS

Loci that were identified in a meta-analysis to attain a genome-wide significance level of a P value less than 1.0×10(-7) (UGT1A1, SLCO1B1, LST-3TM12, SLCO1A2) were analyzed in 1018 individuals with known gallstone composition. Gallstone risk was analyzed in 2606 German choleystecomized individuals and 1121 controls and was replicated in 210 cases and 496 controls from South America.

RESULTS

By using the presence of bilirubin as a phenotype, variants rs6742078 (UGT1A1; P = .003), rs4149056 (SLCO1B1; P = .003), and rs4149000 (SLCO1A2; P = .015) were associated with gallstone composition. In regression analyses, only UGT1A1 and SLCO1B1 were independently retained in the model. UGT1A1 (rs6742078; P = .018) was associated with overall gallstone risk. In a sex-stratified analysis, only male carriers of rs6742078 had an increased risk for gallstone disease (P = 2.1×10(-7); odds ratio(recessive), 2.34; P(women) = .47). The sex-specific association of rs6742078 was confirmed in samples from South America (P(men) = .046; odds ratio(recessive), 2.19; P(women) = .96).

CONCLUSIONS

The UGT1A1 Gilbert syndrome variant rs6742078 is associated with gallstone disease in men; further studies are required regarding the sex-specific physiology of bilirubin and bile acid metabolism. Variants of ABCG8 and UGT1A1 are the 2 major risk factors for overall gallstone disease, they contribute a population attributable risk of 21.2% among men.

Several conserved positively charged amino acids in OATP1B1 are involved in binding or translocation of different substrates

OATP1B1 and 1B3 are related transporters mediating uptake of numerous compounds into hepatocytes. A putative model of OATP1B3 with a "positive binding pocket" containing conserved positively charged amino acids was predicted (Meier-Abt et al. J Membr Biol 208:213-227, 2005). Based on this model, we tested the hypothesis that these positive amino acids are important for OATP1B1 function. We made mutants and measured surface expression and uptake of estradiol-17β-glucuronide, estrone-3-sulfate and bromosulfophthalein in HEK293 cells. Two of the mutants had low surface expression levels: R181K at 10% and R580A at 30% of wild-type OATP1B1. A lysine at position 580 (R580K) rescued the expression of R580A. Mutations of several amino acids resulted in substrate-dependent effects. The largest changes were seen for estradiol-17β-glucuronide, while estrone-3-sulfate and bromosulfophthalein transport were less affected. The wild-type OATP1B1 K (m) value for estradiol-17β-glucuronide of 5.35 ± 0.54 μM was increased by R57A to 30.5 ± 3.64 μM and decreased by R580K to 0.52 ± 0.18 μM. For estrone-3-sulfate the wild-type high-affinity K (m) value of 0.55 ± 0.12 μM was increased by K361R to 1.8 ± 0.47 μM and decreased by R580K to 0.1 ± 0.04 μM. In addition, R580K reduced the V (max) values for all three substrates to <25% of wild-type OATP1B1. Mutations at intracellular K90, H92 and R93 mainly affected V (max) values for estradiol-17β-glucuronide uptake. In conclusion, the conserved amino acids R57, K361 and R580 seem to be part of the substrate binding sites and/or translocation pathways in OATP1B1.

Serum bilirubin and genes controlling bilirubin concentrations as biomarkers for cardiovascular disease

BACKGROUND

Serum bilirubin has been consistently shown to be inversely related to cardiovascular disease (CVD). Recent studies showed serum bilirubin to be associated with CVD-related factors such as diabetes, metabolic syndrome, and body mass index. Although the association of serum bilirubin with CVD has been found in both retrospective and prospective studies, less information is available on the role of genes that control bilirubin concentrations and their association with CVD.

CONTENT

In this review, we provide detailed information on the identity of the major genes that control bilirubin concentrations and their association with serum bilirubin concentrations and CVD risk. We also update the results of the major studies that have been performed on the association between serum bilirubin, CVD, and CVD-related diseases such as diabetes or metabolic syndrome. Studies consistently indicate that bilirubin concentrations are inversely associated with different types of CVD and CVD-related diseases. A conditional linkage study indicates that UGT1A1 is the major gene controlling serum bilirubin concentrations, and this finding has been confirmed in recent genomewide association studies. Studies also indicate that individuals homozygous for UGT1A1*28 have a significantly lower risk of developing CVD than carriers of the wild-type alleles.

SUMMARY

Serum bilirubin has a protective effect on CVD and CVD-related diseases, and UGT1A1 is the major gene controlling serum bilirubin concentrations. Pharmacologic, nonpharmacologic, or genetic interventions that increase serum bilirubin concentrations could provide more direct evidence on the role of bilirubin in CVD prevention.

Organic anion transporting polypeptide 1a/1b-knockout mice provide insights into hepatic handling of bilirubin, bile acids, and drugs

Organic anion transporting polypeptides (OATPs) are uptake transporters for a broad range of endogenous compounds and xenobiotics. To investigate the physiologic and pharmacologic roles of OATPs of the 1A and 1B subfamilies, we generated mice lacking all established and predicted mouse Oatp1a/1b transporters (referred to as Slco1a/1b-/- mice, as SLCO genes encode OATPs). Slco1a/1b-/- mice were viable and fertile but exhibited markedly increased plasma levels of bilirubin conjugated to glucuronide and increased plasma levels of unconjugated bile acids. The unexpected conjugated hyperbilirubinemia indicates that Oatp1a/1b transporters normally mediate extensive hepatic reuptake of glucuronidated bilirubin. We therefore hypothesized that substantial sinusoidal secretion and subsequent Oatp1a/1b-mediated reuptake of glucuronidated compounds can occur in hepatocytes under physiologic conditions. This alters our perspective on normal liver functioning. Slco1a/1b-/- mice also showed drastically decreased hepatic uptake and consequently increased systemic exposure following i.v. or oral administration of the OATP substrate drugs methotrexate and fexofenadine. Importantly, intestinal absorption of oral methotrexate or fexofenadine was not affected in Slco1a/1b-/- mice. Further analysis showed that rifampicin was an effective and specific Oatp1a/1b inhibitor in controlling methotrexate pharmacokinetics. These data indicate that Oatp1a/1b transporters play an essential role in hepatic reuptake of conjugated bilirubin and uptake of unconjugated bile acids and drugs. Slco1a/1b-/- mice will provide excellent tools to study further the role of Oatp1a/1b transporters in physiology and drug disposition.

Genome-wide association of serum bilirubin levels in Korean population

A large-scale, genome-wide association study was performed to identify genetic variations influencing serum bilirubin levels using 8841 Korean individuals. Significant associations were observed at UGT1A1 (rs11891311, P = 4.78 x 10(-148)) and SLCO1B3 (rs2417940, P = 1.03 x 10(-17)), which are two previously identified loci. The two single-nucleotide polymorphisms (SNPs) were replicated (rs11891311, P = 3.18 x 10(-15)) or marginally significant (rs2417940, P = 8.56 x 10(-4)) in an independent cohort of 1096 individuals. In a conditional analysis adjusted for the top UGT1A1 variant (rs11891311), another variant in UGT1A1 (rs4148323, P = 1.22 x 10(-121)) remained significant; this suggests that in UGT1A1 at least two independent genetic variations influence the bilirubin levels in the Korean population. The protein coding variant rs4148323, which is monomorphic in European-derived populations, may be specifically associated with serum bilirubin levels in Asians (P = 2.56 x 10(-70)). The SLCO1B3 variant (rs2417940, P = 1.67 x 10(-18)) remained significant in a conditional analysis for the top UGT1A1 variant. Interestingly, there were significant differences in the associated variations of SLCO1B3 between Koreans and European-derived populations. While the variant rs2417940 at intron 7 of SLCO1B3 was more significantly associated in Koreans, variants rs17680137 (P = 0.584) and rs2117032 (P = 2.76 x 10(-5)), two of the top-ranked SNPs in European-derived populations, did not reach the genome-wide significance level. Also, variants in SLCO1B1 did not reach genome-wide significance in Koreans. Our result supports the idea that there are considerable ethnic differences in genetic association of bilirubin levels between Koreans and European-derived populations.