Inhibition of human organic anion transporting polypeptide OATP 1B1 as a mechanism of drug-induced hyperbilirubinemia

Development of a Pharmacokinetic Model That Accounts for the Plasma Concentrations of Conjugated and Unconjugated Bilirubin Observed in a Variety of Disease States

Introduction

For a large variety of liver pathologies, the plasma unconjugated (UB) and conjugated (CB) bilirubin concentrations appear to be coupled. For example, in alcoholic cirrhosis, UB and CB are roughly the same over a large range of total bilirubin, requiring an initial massive increase (about 40-fold) in plasma CB to reach the level of UB and then similar increases in UB and CB as the disease progresses. This coupling has been either unrecognized or ignored and this paper is the first attempt to try to explain it quantitatively in terms of known hepatic cell metabolic and membrane transport properties.

Methods

A simplified pharmacokinetic model is developed and applied to a variety of hyperbilirubinemic pathologies. A central feature of the model is based on the recent observation that double knockout of the rat OATP1A and OATP1B hepatic transporters produces a roughly 400-fold increase in plasma CB, indicating that there is a normal rapid recycling of CB from the cell to the plasma with reuptake via OATP. We use the experimental rat Km of OATP CB transport to show that OATP uptake becomes saturated at relatively low plasma CB concentrations, decreasing uptake, and producing massive (up to 1000-fold) increases in CB in some pathologies. It is assumed that UB and CB are competing for the OATP transporter, producing the increased plasma UB that is observed in "pure" CB pathologies.

Results

The model accurately describes the clinically observed UB and CB for pure UB (Gilbert's, hemolytic anemia) and CB (Dubin-Johnson, Rotor syndrome, biliary atresia) pathologies as well as in cirrhosis.

Conclusion

This model is a preliminary, first attempt to quantitatively describe UB and CB pharmacokinetics. It is hoped that it will stimulate more detailed measurements and analysis.

Generation of a Cell Line Selectively Producing Functionally Active OATP1B1 Transporter

The solute carrier organic anion transporter family member, OATP1B1, is one of the most important transporter proteins, which mediate penetration of many endogenous substances and xenobiotics into hepatocytes. A model system providing expression of the functional protein is needed to assess interaction of OATP1B1 with various substances. Based on the HEK293 cells, we obtained the HEK293-OATP1B1 cell line, constitutively expressing the SLCO1B1 gene encoding the OATP1B1 transporter. Expression of the SLCO1B1 gene was confirmed by real-time PCR analysis and Western blotting. Functionality of the transporter was assessed by the transport of atorvastatin, which is a substrate of OATP1B1. Cells of the resulting cell line, which selectively express the functionally active recombinant OATP1B1 transporter, can be used to study functions of the protein and to test drugs for being substrates, inducers, and inhibitors of OATP1B1, and to assess the risks of drug interactions.

A possible case of bictegravir-associated severe unconjugated hyperbilirubinemia

BACKGROUND

Bictegravir (BIC) co-formulated with emtricitabine (FTC) and tenofovir alafenamide (TAF) is approved by Federal Food and Drug Administration in 2018 for both treatment-naïve and experienced persons living with HIV (PLWH).

CASE PRESENTATION

A young man with recently diagnosed human immunodeficiency virus (HIV) infection presented with jaundice. Blood work was significant for mild anemia and grade 4 unconjugated hyperbilirubinemia. A comprehensive evaluation for hemolytic anemia failed to reveal any etiology. Other causes of hyperbilirubinemia were negative. Four months prior, patient was started on antiretroviral therapy with a single tablet regimen containing bictegravir/emtricitabine/tenofovir alafenamide (BIC/FTC/TAF), brand name Biktarvy®, and the medication was suspected to be the cause. The medication was held, and the hyperbilirubinemia improved.

CONCLUSION

Severe hyperbilirubinemia can be found in the patient using BIC/FTC/TAF. The data for this adverse reaction is scarce, and more studies are needed on this possible side effect. The mechanism of unconjugated hyperbilirubinemia by INSTI remains undefined.

Gadoxetic acid-enhanced magnetic resonance imaging predicts hyperbilirubinemia induced by glecaprevir during hepatitis C virus treatment

Glecaprevir is a substrate for organic anion-transporting polypeptide (OATP) 1B1/1B3, which transports bilirubin. Hyperbilirubinemia is an adverse event during anti-hepatitis C virus treatment with glecaprevir and pibrentasvir. Gadoxetic acid is also transported by OATP1B1/1B3, and we aimed to evaluate whether gadoxetic acid-enhanced magnetic resonance (MR) imaging was associated with glecaprevir trough concentrations (C_trough). We further determined whether this was predictive of hyperbilirubinemia development in a cohort of 33 patients. The contrast enhancement index (CEI), a measure of hepatic enhancement effect on the hepatobiliary image, was assessed. Glecaprevir C_trough was determined 7 days after administration. Five of the 33 patients (15%) developed Common Terminology Criteria for Adverse Events grade ≥ 2 hyperbilirubinemia. We found a negative relationship between CEI and C_trough (r = − 0.726, p < 0.001). The partial correlation coefficient between CEI and C_trough was − 0.654 (p < 0.001), while excluding the effects of albumin, FIB-4 index, and indirect bilirubin at baseline. The C_trough was significantly higher in patients with hyperbilirubinemia than in those without (p = 0.008). In multivariate analysis, CEI ≤ 1.71 was an independent factor influencing the development of hyperbilirubinemia (p = 0.046). Our findings indicate that gadoxetic acid MR imaging can help predict glecaprevir concentration and development of hyperbilirubinemia.

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.

Is It Time to Use Modeling of Cellular Transporter Homeostasis to Inform Drug-Drug Interaction Studies: Theoretical Considerations

Mathematical modeling has been an important tool in pharmaceutical research for 50 + years and there is increased emphasis over the last decade on using modeling to improve the efficiency and effectiveness of drug development. In an earlier commentary, we applied a multiscale model linking 6 scales (whole body, tumor, vasculature, cell, spatial location, time), together with literature data on nanoparticle and tumor properties, to demonstrate the effects of nanoparticle particles on systemic disposition. The current commentary used a 4-scale model (cell membrane, intracellular organelles, spatial location, time) together with literature data on the intracellular processing of membrane receptors and transporters to demonstrate disruption of transporter homeostasis can lead to drug-drug interaction (DDI) between victim drug (VD) and perpetrator drug (PD), including changes in the area-under-concentration-time-curve of VD in cells that are considered significant by the US Food and Drug Administration (FDA). The model comprised 3 computational components: (a) intracellular transporter homeostasis, (b) pharmacokinetics of extracellular and intracellular VD/PD concentrations, and (c) pharmacodynamics of PD-induced stimulation or inhibition of an intracellular kinetic process. Model-based simulations showed that (a) among the five major endocytic processes, perturbation of transporter internalization or recycling led to the highest incidence and most extensive DDI, with minor DDI for perturbing transporter synthesis and early-to-late endosome and no DDI for perturbing transporter degradation and (b) three experimental conditions (spatial transporter distribution in cells, VD/PD co-incubation time, extracellular PD concentrations) were determinants of DDI detection. We propose modeling is a useful tool for hypothesis generation and for designing experiments to identify potential DDI; its application further aligns with the model-informed drug development paradigm advocated by FDA.

Ursodeoxycholic acid markedly promotes the absorption of microemulsion-formulated cyclosporine A: A case report

WHAT IS KNOWN AND OBJECTIVE

Cyclosporine A (CyA) causes intrahepatic biliary stasis via inhibition of bile acid excretion through the bile salt export pump. We report a case of a patient in whom ursodeoxycholic acid (UDCA) markedly promoted the absorption of microemulsion-formulated CyA.

CASE SUMMARY

The patient was a 22-year-old Japanese man diagnosed with stage 3 aplastic anaemia. He was treated with CyA, and 2 h post-dose (C2) was increased by UDCA.

WHAT IS NEW AND CONCLUSION

A remarkable interaction was observed between CyA and UDCA. This is a valuable finding for improving the treatment strategies for haematological disorders.

In Vitro Liver Toxicity Testing of Chemicals: A Pragmatic Approach

The liver is among the most frequently targeted organs by noxious chemicals of diverse nature. Liver toxicity testing using laboratory animals not only raises serious ethical questions, but is also rather poorly predictive of human safety towards chemicals. Increasing attention is, therefore, being paid to the development of non-animal and human-based testing schemes, which rely to a great extent on in vitro methodology. The present paper proposes a rationalized tiered in vitro testing strategy to detect liver toxicity triggered by chemicals, in which the first tier is focused on assessing general cytotoxicity, while the second tier is aimed at identifying liver-specific toxicity as such. A state-of-the-art overview is provided of the most commonly used in vitro assays that can be used in both tiers. Advantages and disadvantages of each assay as well as overall practical considerations are discussed.

Associations among plasma concentrations of regorafenib and its metabolites, adverse events, and ABCG2 polymorphisms in patients with metastatic colorectal cancers

PURPOSE

The association between the pharmacokinetics and pharmacodynamics of regorafenib, a multiple tyrosine kinase inhibitor, remains unclear. This study assessed the trough plasma concentrations (C_trough) of regorafenib and its N-oxide (M2) and N-oxide/desmethyl (M5) metabolites, and evaluated the associations among these levels, adverse events, and pharmacokinetic-related genetic polymorphisms in patients with metastatic colorectal cancer.

METHODS

The C_trough levels of regorafenib and its metabolites were assessed in a single-center, prospective, observational study, 7 days after the initial treatment. The correlation between those values and adverse events was then examined. In addition, the genetic polymorphisms of ABCG2, SLCO1B1, and UGT1A9 were determined and evaluated for associations with the levels of regorafenib, M2, and M5.

RESULTS

We analyzed 43 patients who received regorafenib 40-120 mg/day; among them, 35 patients started at 120 mg/day. With regard to bilirubin increase, the C_trough values of regorafenib were significantly higher in the group with grade ≥ 2 than in groups with grades 0 and 1 (p = 0.010). The M5 C_trough levels were significantly associated with the severity of hypertension or rash (p < 0.05). In a multivariate analysis, the M5 C_trough values and age were significant predictors of severe rash. Lastly, significant differences were noted in the M5 concentration-to-dose ratio values between the patients with ABCG2 421A/A and ABCG2 421C/A or C/C polymorphisms (p = 0.035).

CONCLUSION

This study showed that the C_trough of regorafenib was associated with bilirubin increase, and also clarified for the first time that the C_trough of M5 was significantly correlated with hypertension and severe rash.

Potent and Specific Inhibition of NTCP-Mediated HBV/HDV Infection and Substrate Transporting by a Novel, Oral-Available Cyclosporine A Analogue

Analogues of the natural product cyclosporine A (CsA) were developed and assessed as antivirals against infection of hepatitis B virus (HBV) and its satellite hepatitis D virus (HDV). An analogue termed 27A exhibits potent inhibition of HBV/HDV infection by specifically blocking viral engagement to its cellular receptor NTCP, while it lacks immunosuppressive activity found in natural CsA. Intraperitoneal injection or oral intake of 27A protects HDV-susceptible mouse model from HDV infection. 27A serves as a promising lead for the development of novel anti-HDV/HBV agents.

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.

Isolated Elevated Bilirubin

http://aasldpubs.onlinelibrary.wiley.com/hub/journal/10.1002/(ISSN)2046-2484/video/15-4-reading-shroff-maddur a video presentation of this article http://aasldpubs.onlinelibrary.wiley.com/hub/journal/10.1002/(ISSN)2046-2484/video/15-4-interview-shroff-maddur an interview with the author https://www.wileyhealthlearning.com/Activity/7088613/disclaimerspopup.aspx questions and earn CME.

Comparison of the predictive outcomes for anti-tuberculosis drug-induced hepatotoxicity by different machine learning techniques

BACKGROUND

The study compared the predictive outcomes of artificial neural network, support vector machine and random forest on the occurrence of anti-tuberculosis drug-induced hepatotoxicity.

METHODS

The clinical and genomic data of patients treated with anti-tuberculosis drugs at Taipei Medical University-Wanfang Hospital were used as training sets, and those at Taipei Medical University-Shuang Ho Hospital served as test sets. Features were selected through a univariate risk factor analysis and literature evaluation. The accuracy, sensitivity, specificity, and the area under the receiver operating characteristic curve were calculated to compare the traditional, genomic, and combined models of the three techniques.

RESULTS

Nine models were created with 7 clinical factors and 4 genotypes. Artificial neural network with clinical and genomic factors exhibited the best performance, with an accuracy of 88.67%, a sensitivity of 80%, and a specificity of 90.4% for the test set. The area under the receiver operating characteristic curve of this best model reached 0.894 for training set and 0.898 for test set, which was significantly better than 0.801 for training set and 0.728 for test set by support vector machine and 0.724 for training set and 0.718 for test set by random forest.

CONCLUSIONS

Artificial neural network with clinical and genomic data can become a clinical useful tool in predicting anti-tuberculosis drug-induced hepatotoxicity. The machine learning technique can be an innovation to predict and prevent adverse drug reaction.

Overview of organic anion transporters and organic anion transporter polypeptides and their roles in the liver

Organic anion transporters (OATs) and organic anion transporter polypeptides (OATPs) are classified within two SLC superfamilies, namely, the SLC22A superfamily and the SLCO superfamily (formerly the SLC21A family), respectively. They are expressed in many tissues, such as the liver and kidney, and mediate the absorption and excretion of many endogenous and exogenous substances, including various drugs. Most are composed of 12 transmembrane polypeptide chains with the C-terminus and the N-terminus located in the cell cytoplasm. OATs and OATPs are abundantly expressed in the liver, where they mainly promote the uptake of various endogenous substrates such as bile acids and various exogenous drugs such as antifibrotic and anticancer drugs. However, differences in the locations of glycosylation sites, phosphorylation sites, and amino acids in the OAT and OATP structures lead to different substrates being transported to the liver, which ultimately results in their different roles in the liver. To date, few articles have addressed these aspects of OAT and OATP structures, and we study further the similarities and differences in their structures, tissue distribution, substrates, and roles in liver diseases.

Associations among regorafenib concentrations, severe adverse reactions, and ABCG2 and OATP1B1 polymorphisms

PURPOSE

The ability of predicting severe adverse reactions caused by regorafenib is important. We evaluated regorafenib concentrations for adverse reaction risks and assessed the relevance of laboratory values and gene polymorphisms.

METHODS

A total of 28 Japanese cancer patients who were treated with regorafenib were evaluated for the steady state of serum regorafenib concentrations and adverse reactions for 28 days. In addition, we determined the association of regorafenib concentrations with ABCG2 and OATP1B1 polymorphisms, which are regorafenib transporters.

RESULTS

Regorafenib concentrations were significantly higher in the group with Grade 2 or higher total bilirubin elevation and thrombocytopenia compared with the group with grades 0 or 1 [3.45 (2.18-7.31) vs. 1.76 (0.26-2.77) µg/mL, P = 0.01 and 3.45 (2.12-7.31) vs. 1.76 (0.26-2.77) µg/mL, P = 0.02, respectively]. A strong association was noted between serum regorafenib concentrations and total bilirubin levels, but the physical and genetic factors predicting regorafenib pharmacokinetics could not be clarified.

CONCLUSIONS

Regorafenib concentrations were associated with total bilirubin elevation and thrombocytopenia. Total serum bilirubin could be a useful marker when estimating regorafenib pharmacokinetics.

In vitro UGT1A1 inhibition by tyrosine kinase inhibitors and association with drug-induced hyperbilirubinemia

PURPOSE

Hyperbilirubinemia has been observed in patients treated with tyrosine kinase inhibitor (TKI) drugs. Therefore, it would be beneficial to understand whether there is a relationship between inhibition of uridine-5'-diphosphate glucuronosyltransferase (UGT) 1A1 activity and observed bilirubin elevations in TKI drug-treated patients. UGT1A1 is responsible for the glucuronidation of bilirubin which leads to its elimination in the bile.

METHODS

To examine this question, an in vitro glucuronidation assay was developed to determine the inhibitory effect of TKI drugs employing human liver microsomes (HLM) with varying UGT1A1 activity. Utilizing β-estradiol as the UGT1A1 probe substrate, 20 TKI drugs were evaluated at concentrations that represent clinical plasma levels. Adverse event reports were searched to generate an empirical Bayes geometric mean (EGBM) score for clinical hyperbilirubinemia with the TKI drugs.

RESULTS

Erlotinib, nilotinib, regorafenib, pazopanib, sorafenib and vemurafenib had IC₅₀ values that were lower than their clinical steady-state C_max concentrations. These TKI drugs had high incidences of hyperbilirubinemia and higher EBGM scores. The IC₅₀ values and C_max/IC₅₀ ratios correlated well with EBGM scores for hyperbilirubinemia (P < 0.005). For the TKI drugs with higher incidence of hyperbilirubinemia in Gilbert syndrome patients, who have reduced UGT1A1 activity, six of eight had smaller ratios in the low UGT1A1 activity microsomes than the wild-type microsomes for drugs, indicating greater sensitivity to the drugs in this phenotype.

CONCLUSIONS

These results suggest that in vitro UGT1A1 inhibition assays have the potential to predict clinical hyperbilirubinemia.

Identification of novel cell-impermeant fluorescent substrates for testing the function and drug interaction of Organic Anion-Transporting Polypeptides, OATP1B1/1B3 and 2B1

Organic Anion-Transporting Polypeptides are multispecific membrane proteins that regulate the passage of crucial endobiotics and drugs across pharmacological barriers. OATP1B1 and OATP1B3 have been described to play a major role in the hepatic uptake of statins, antivirals and various chemotherapeutics; whereas the pharmacological role of the ubiquitously expressed OATP2B1 is less well characterized. According to current industry standards, in vitro testing for susceptibility to OATP1B1 and 1B3 mediated transport is recommended for drug candidates that are eliminated in part via the liver. Here we show that human OATP1B1, 1B3 and 2B1 transport a series of commercially available viability dyes that are generally believed to be impermeable to intact cells. We demonstrate that the intracellular accumulation of Zombie Violet, Live/Dead Green, Cascade Blue and Alexa Fluor 405 is specifically increased by OATPs. Inhibition of Cascade Blue or Alexa Fluor 405 uptake by known OATP substrates/inhibitors yielded IC₅₀ values in agreement with gold-standard radioligand assays. The fluorescence-based assays described in this study provide a new tool for testing OATP1B/2B1 drug interactions.

Systems pharmacology modeling of drug-induced hyperbilirubinemia: Differentiating hepatotoxicity and inhibition of enzymes/transporters

Elevations in serum bilirubin during drug treatment may indicate global liver dysfunction and a high risk of liver failure. However, drugs also can increase serum bilirubin in the absence of hepatic injury by inhibiting specific enzymes/transporters. We constructed a mechanistic model of bilirubin disposition based on known functional polymorphisms in bilirubin metabolism/transport. Using physiologically based pharmacokinetic (PBPK) model-predicted drug exposure and enzyme/transporter inhibition constants determined in vitro, our model correctly predicted indinavir-mediated hyperbilirubinemia in humans and rats. Nelfinavir was predicted not to cause hyperbilirubinemia, consistent with clinical observations. We next examined a new drug candidate that caused both elevations in serum bilirubin and biochemical evidence of liver injury in rats. Simulations suggest that bilirubin elevation primarily resulted from inhibition of transporters rather than global liver dysfunction. We conclude that mechanistic modeling of bilirubin can help elucidate underlying mechanisms of drug-induced hyperbilirubinemia, and thereby distinguish benign from clinically important elevations in serum bilirubin.

Linking organic anion transporting polypeptide 1B1 and 1B3 (OATP1B1 and OATP1B3) interaction profiles to hepatotoxicity - The hyperbilirubinemia use case

Hyperbilirubinemia is a pathological condition of excessive accumulation of conjugated or unconjugated bilirubin in blood. It has been associated with neurotoxicity and non-neural organ dysfunctions, while it can also be a warning of liver side effects. Hyperbilirubinemia can either be a result of overproduction of bilirubin due to hemolysis or dyserythropoiesis, or the outcome of impaired bilirubin elimination due to liver transporter malfunction or inhibition. There are several reports in literature that inhibition of organic anion transporting polypeptides 1B1 and 1B3 (OATP1B1 and OATP1B3) might lead to hyperbilirubinemia. In this study we created a set of classification models for hyperbilirubinemia, which, besides physicochemical descriptors, also include the output of classification models of human OATP1B1 and 1B3 inhibition. Models were based on either human data derived from public toxicity reports or animal data extracted from the eTOX database VITIC. The generated models showed satisfactory accuracy (68%) and area under the curve (AUC) for human data and 71% accuracy and 70% AUC for animal data. However, our results did not indicate strong association between OATP inhibition and hyperbilirubinemia, neither for humans nor for animals.

Is the unbound concentration of atazanavir of interest in therapeutic drug monitoring?

To date, therapeutic drug monitoring (TDM) is carried out with antiretrovirals and is usually based on total concentrations (C_t ). However, for some patients, TDM does not reflect efficacy or the avoidance of toxicity as is the case for atazanavir (ATV), a HIV protease inhibitor. As the unbound concentration (C_u ) is the pharmacological active form, the aim of the study was to evaluate the value of C_u and the unbound fraction (f_u , f_u = C_u /C_t ) for the TDM of ATV. The variability of C_u and the corresponding f_u of ATV was explored in 43 patients treated with ATV for an average of 13.5 months. C_u was determined by coupling ultrafiltration and liquid chromatography. As ATV is highly bound to alpha-1 acid glycoprotein (AAG), the correlation between f_u and AAG was also evaluated. The viral load was monitored to evaluate the patients' virologic response, while total plasma bilirubin and unconjugated plasma bilirubin were used as biomarkers of ATV toxicity. Median trough C_u and C_t were 37.9 μg/L (Interquartile range (IQR) 20.6-94.9 μg/L) and 628.6 μg/L (IQR 362.7-1078.1 μg/L), respectively. f_u , C_u and C_t showed high variability, but the f_u variability was not correlated with the AAG level. The unbound concentration and fraction were unrelated to the virologic response (P = 0.21 and P = 0.65 for C_u and f_u , respectively) nor to the unconjugated bilirubin (Pearson correlation coefficient (ρ), ρ = 0.22; P = 0.17 for C_u ). Neither total nor unbound concentrations of ATV fully explained hyperbilirubinaemia or virologic failure. From this study, we conclude that unbound ATV did not appear to be more relevant than C_t .

Role and modulation of drug transporters in HIV-1 therapy

Current treatment of human immunodeficiency virus type-1 (HIV-1) infection involves a combination of antiretroviral drugs (ARVs) that target different stages of the HIV-1 life cycle. This strategy is commonly referred to as highly active antiretroviral therapy (HAART) or combined antiretroviral therapy (cART). Membrane-associated drug transporters expressed ubiquitously in mammalian systems play a crucial role in modulating ARV disposition during HIV-1 infection. Members of the ATP-binding cassette (ABC) and solute carrier (SLC) transporter superfamilies have been shown to interact with ARVs, including those that are used as part of first-line treatment regimens. As a result, the functional expression of drug transporters can influence the distribution of ARVs at specific sites of infection. In addition, pathological factors related to HIV-1 infection and/or ARV therapy itself can alter transporter expression and activity, thus further contributing to changes in ARV disposition and the effectiveness of HAART. This review summarizes current knowledge on the role of drug transporters in regulating ARV transport in the context of HIV-1 infection.

Drug transporters in tissues and cells relevant to sexual transmission of HIV: Implications for drug delivery

Efflux and uptake transporters of drugs are key regulators of the pharmacokinetics of many antiretroviral drugs. A growing body of literature has revealed the expression and functionality of multiple transporters in female genital tract (FGT), colorectal tissue, and immune cells. Drug transporters could play a significant role in the efficacy of preventative strategies for HIV-1 acquisition. Pre-exposure prophylaxis (PrEP) is a promising strategy, which utilizes topically (vaginally or rectally), orally or other systemically administered antiretroviral drugs to prevent the sexual transmission of HIV to receptive partners. The drug concentration in the receptive mucosal tissues and target immune cells for HIV is critical for PrEP effectiveness. Hence, there is an emerging interest in utilizing transporter information to explain tissue disposition patterns of PrEP drugs, to interpret inter-individual variability in PrEP drug pharmacokinetics and effectiveness, and to improve tissue drug exposure through modulation of the cervicovaginal, colorectal, or immune cell transporters. In this review, the existing literature on transporter expression, functionality and regulation in the transmission-related tissues and cells is summarized. In addition, the relevance of transporter function for drug delivery and strategies that could exploit transporters for increased drug concentration at target locales is discussed. The overall goal is to facilitate an understanding of drug transporters for PrEP optimization.

Identification of Novel Inhibitors of Organic Anion Transporting Polypeptides 1B1 and 1B3 (OATP1B1 and OATP1B3) Using a Consensus Vote of Six Classification Models

Organic anion transporting polypeptides 1B1 and 1B3 are transporters selectively expressed on the basolateral membrane of the hepatocyte. Several studies reveal that they are involved in drug–drug interactions, cancer, and hyperbilirubinemia. In this study, we developed a set of classification models for OATP1B1 and 1B3 inhibition based on more than 1700 carefully curated compounds from literature, which were validated via cross-validation and by use of an external test set. After combining several sets of descriptors and classifiers, the 6 best models were selected according to their statistical performance and were used for virtual screening of DrugBank. Consensus scoring of the screened compounds resulted in the selection and purchase of nine compounds as potential dual inhibitors and of one compound as potential selective OATP1B3 inhibitor. Biological testing of the compounds confirmed the validity of the models, yielding an accuracy of 90% for OATP1B1 and 80% for OATP1B3, respectively. Moreover, at least half of the new identified inhibitors are associated with hyperbilirubinemia or hepatotoxicity, implying a relationship between OATP inhibition and these severe side effects.

Fasiglifam (TAK-875) Inhibits Hepatobiliary Transporters: A Possible Factor Contributing to Fasiglifam-Induced Liver Injury

Fasiglifam (TAK-875), a selective G-protein-coupled receptor 40 agonist, was developed for the treatment of type 2 diabetes mellitus; however, its development was terminated in phase III clinical trials because of liver safety concerns. Our preliminary study indicated that intravenous administration of 100 mg/kg of TAK-875 increased the serum total bile acid concentration by 3 to 4 times and total bilirubin levels by 1.5 to 2.6 times in rats. In the present study, we examined the inhibitory effects of TAK-875 on hepatobiliary transporters to explore the mechanisms underlying its hepatotoxicity. TAK-875 decreased the biliary excretion index and the in vitro biliary clearance of d₈-taurocholic acid in sandwich-cultured rat hepatocytes, suggesting that TAK-875 impaired biliary excretion of bile acids, possibly by inhibiting bile salt export pump (Bsep). TAK-875 inhibited the efflux transporter multidrug resistance-associated protein 2 (Mrp2) in rat hepatocytes using 5 (and 6)-carboxy-2',7'-dichlorofluorescein as a substrate. Inhibition of MRP2 was further confirmed by reduced transport of vinblastine in Madin-Darby canine kidney cells overexpressing MRP2 with IC₅₀ values of 2.41 μM. TAK-875 also inhibited the major bile acid uptake transporter Na(+)/taurocholate cotransporting polypeptide (Ntcp), which transports d₈-taurocholic acid into rat hepatocytes, with an IC₅₀ value of 10.9 μM. TAK-875 significantly inhibited atorvastatin uptake in organic anion transporter protein (OATP) 1B1 and OATP1B3 cells with IC₅₀ values of 2.28 and 3.98 μM, respectively. These results indicate that TAK-875 inhibited the efflux transporter MRP2/Mrp2 and uptake transporters Ntcp and OATP/Oatp, which may affect bile acid and bilirubin homeostasis, resulting in hyperbilirubinemia and cholestatic hepatotoxicity.

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.

Utility of bilirubins and bile acids as endogenous biomarkers for the inhibition of hepatic transporters

It is useful to identify endogenous substrates for the evaluation of drug-drug interactions via transporters. In this study, we investigated the utility of bilirubins, substrates of OATPs and MRP2, and bile acids and substrates of NTCP and BSEP, as biomarkers for the inhibition of transporters. In rats administered 20 and 80 mg/kg rifampicin, the plasma levels of bilirubin glucuronides were elevated, gradually decreased, and almost returned to the baseline level at 24 hours after administration without an elevation of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). This result indicates the transient inhibition of rOatps and/or rMrp2. Although the correlation between free plasma concentrations and IC50 values of rOatps depended on the substrates used in the in vitro studies, the inhibition of rOatps by rifampicin was confirmed in the in vivo study using valsartan as a substrate of rOatps. In rats administered 10 and 30 mg/kg cyclosporin A, the plasma levels of bile acids were elevated and persisted for up to 24 hours after administration without an elevation of ALT and AST. This result indicates the continuous inhibition of rNtcp and/or rBsep, although there were differences between the free plasma or liver concentrations and IC50 values of rNtcp or rBsep, respectively. This study suggests that the monitoring of bilirubins and bile acids in plasma is useful in evaluating the inhibitory potential of their corresponding transporters.

Association of serum bilirubin and non-alcoholic fatty liver disease: A feasible therapeutic avenue?

AIM: To the look at the current strength of evidence and the potential application of anti-oxidants in this setting.

METHODS: Two electronic databases (PubMed and Web of Knowledge) were searched to January 2013 to find studies addressing serum bilirubin levels in non-alcoholic fatty liver disease (NAFLD). The search used key word combinations in relation to NAFLD and serum bilirubin specific to human adults only. After screening selected studies were reviewed in depth by two independent reviewers. Data synthesis with further meta-analysis was planned but not possible due to the heterogeneity of the outcome measures in these studies.

RESULTS: Out of 416 studies screened only seven studies were considered suitable for inclusion. All seven studies consistently reported an inverse association of bilirubin with NAFLD despite the heterogeneous sample of studies. Only two studies were prospective. No negative studies were found.

CONCLUSION: Most studies suggest a correlation between high bilirubin levels of any type are inversely correlated with NAFLD. But to date most of these studies have been poorly designed to allow meaningful conclusions, except one cohort study. There is a need for a large prospective cohort study in multiple populations to test this hypothesis fully before mechanistic associations can be established and therapeutic options of the apparent anti-oxidant effect of bilirubin be explored in NAFLD. Furthermore these studies should include analysis of UGT1A1 gene to expound upon underlying cause of unconjugated hyperbilirubinaemia.

Mechanisms underlying benign and reversible unconjugated hyperbilirubinemia observed with faldaprevir administration in hepatitis C virus patients

Faldaprevir, an investigational agent for hepatitis C virus treatment, is well tolerated but associated with rapidly reversible, dose-dependent, clinically benign, unconjugated hyperbilirubinemia. Multidisciplinary preclinical and clinical studies were used to characterize mechanisms underlying this hyperbilirubinemia. In vitro, faldaprevir inhibited key processes involved in bilirubin clearance: UDP glucuronosyltransferase (UGT) 1A1 (UGT1A1) (IC50 0.45 µM), which conjugates bilirubin, and hepatic uptake and efflux transporters, organic anion-transporting polypeptide (OATP) 1B1 (IC50 0.57 µM), OATP1B3 (IC50 0.18 µM), and multidrug resistance-associated protein (MRP) 2 (IC50 6.2 µM), which transport bilirubin and its conjugates. In rat and human hepatocytes, uptake and biliary excretion of [(3)H]bilirubin and/or its glucuronides decreased on coincubation with faldaprevir. In monkeys, faldaprevir (≥20 mg/kg per day) caused reversible unconjugated hyperbilirubinemia, without hemolysis or hepatotoxicity. In clinical studies, faldaprevir-mediated hyperbilirubinemia was predominantly unconjugated, and levels of unconjugated bilirubin correlated with the UGT1A1*28 genotype. The reversible and dose-dependent nature of the clinical hyperbilirubinemia was consistent with competitive inhibition of bilirubin clearance by faldaprevir, and was not associated with liver toxicity or other adverse events. Overall, the reversible, unconjugated hyperbilirubinemia associated with faldaprevir may predominantly result from inhibition of bilirubin conjugation by UGT1A1, with inhibition of hepatic uptake of bilirubin also potentially playing a role. Since OATP1B1/1B3 are known to be involved in hepatic uptake of circulating bilirubin glucuronides, inhibition of OATP1B1/1B3 and MRP2 may underlie isolated increases in conjugated bilirubin. As such, faldaprevir-mediated hyperbilirubinemia is not associated with any liver injury or toxicity, and is considered to result from decreased bilirubin elimination due to a drug-bilirubin interaction.

Inhibitory effect of medicinal plant-derived carboxylic acids on the human transporters hOAT1, hOAT3, hOATP1B1, and hOATP2B1

A significant number of organic carboxylic acids have been shown to influence the absorption and distribution of drugs mediated by organic anion transporters (OATs). In this study, uptake experiments were performed to assess the inhibitory effects of cinnamic acid, ferulic acid, oleanolic acid, deoxycholic acid, and cynarin on hOAT1, hOAT3, hOATP1B1, and hOATP2B1. After a drug-drug interaction (DDI) investigation, cinnamic acid, ferulic acid, deoxycholic acid, and cynarin were found and validated to inhibit hOAT1 in a competitive manner, and deoxycholic acid was found to be an inhibitor of all four transporters. The apparent 50% inhibitory concentrations of cinnamic acid, ferulic acid, deoxycholic acid, and cynarin were estimated to be 133.87, 3.69, 90.03 and 6.03 μmol·L(-1) for hOAT1, respectively. The apparent 50% inhibitory concentrations of deoxycholic acid were estimated to be 9.57 μmol·L(-1) for hOAT3, 70.54 μmol·L(-1) for hOATP1B1, and 168.27 μmol·L(-1) for hOATP2B1. Because cinnamic acid, ferulic acid, and cynarin are ingredients of food or food additives, the present study suggests there are new food-drug interactions to be disclosed. In addition, deoxycholic acid may be used as a probe for studying the correlation of OATs and OATPs.

Hepatobiliary transporters in drug-induced cholestasis: a perspective on the current identifying tools

INTRODUCTION

Impaired bile formation leads to the accumulation of cytotoxic bile salts in hepatocytes and, consequently, cholestasis and severe liver disease. Knowledge of the role of hepatobiliary transporters, especially the bile salt export pump (BSEP), in the pathogenesis of cholestasis is continuously increasing.

AREAS COVERED

This review provides an introduction into the role of these transport proteins in bile formation. It addresses the clinical relevance and pathophysiologic consequences of altered functions of these transporters by genetic mutations and drugs. In particular, the current practical aspects of identification and mitigation of drug candidates with liver liabilities employed during drug development, with an emphasis on preclinical screening for BSEP interaction, are discussed.

EXPERT OPINION

Within the potential pathogenetic mechanisms of acquired cholestasis, the inhibition of BSEP by drugs is well established. Interference of a new compound with BSEP transport activity should raise a warning sign to conduct follow-up experiments and to monitor liver function during clinical development. A combination of in vitro screening for transport interaction, in silico predicting models, and consideration of physicochemical and metabolic properties should lead to a more efficient screening of potential liver liability.

Genome-wide association study to characterize serum bilirubin elevations in patients with HCV treated with GS-9256, an HCV NS3 serine protease inhibitor

BACKGROUND

Protease inhibitors for the treatment of HCV can cause mild and reversible elevations of unconjugated bilirubin. We sought to characterize genetic determinants of bilirubin elevations using a genome-wide approach among patients with genotype 1 HCV who received combination therapy that included GS-9256, a novel potent inhibitor of HCV NS3 serine protease, as part of a Phase IIb trial.

METHODS

Of the 200 patients sampled, 176 had confirmed European ancestry and were included in the analysis. Infinium HumanOmni5BeadChip (Illumina, Inc., San Diego, CA, USA) was used for genotyping. A categorical analysis of low (grade 0-1) versus high (grade 2-4) bilirubin toxicity grade and a quantitative trait locus mapping of peak bilirubin concentrations was performed.

RESULTS

A total of 4,466,809 genetic markers were analysed. No single variant showed a statistically significant association with observed bilirubin elevations in this patient population. In a targeted analysis of single nucleotide polymorphisms in genes known to be involved in bilirubin transport, no significant differences in allele frequency between high and low bilirubin toxicity grade were observed.

CONCLUSIONS

These results indicate that risk for bilirubin elevation in patients receiving GS-9256 is unlikely to be strongly influenced by common genetic variants with large effects. The current study cannot rule out a role for common variants of weak effect, or a more complex model, including multiple contributing factors, such as rare variants and as yet unidentified environmental influences.

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.

Extended mathematical model for "in vivo" quantification of the interaction betweeen atazanavir and bilirubin

The objective of the present work was to conduct an "in vivo" analysis of the atazanavir-bilirubin interaction. We developed a new mathematical approach to PK/PDPK models for competitive interaction based on the Michaelis-Menten equation, which was applied to patients with polymorphisms in the gene for UDP-glucuronosyltransferase 1A1 (UGT1A1). Atazanavir is known to induce concentration-dependent increases in bilirubin plasma levels. Thus, we employed our mathematical model to analyse rises in steady state atazanavir and bilirubin concentrations, ultimately plotting a nomogram for detection of suboptimal atazanavir exposure. Application of our model revealed that an absolute value or a steady state increase in bilirubin falling below 3.8Φ µmol/L (where Φ is a correction factor, =1 for UGT1A1 wild type and ≠1 for UGT1A1 variants) could be used to predict suboptimal atazanavir exposure and treatment failure. Thus, we have successfully established a new mathematical approach for pharmacodynamic-pharmacokinetic modelling of the interaction between atazanavir and bilirubin, as it relates to genetic variants of UGT1A1. Taken together, our findings indicate that bilirubin plasma levels represent a valuable marker of atazanavir exposure.

Atazanavir-bilirubin interaction: a pharmacokinetic-pharmacodynamic model

Purpose

The aim of this work was to analyze the atazanavir–bilirubin relationship, using a new mathematical approach to pharmacokinetic–pharmacodynamic models, for competitive drug interactions based on Michaelis–Menten equations.

Patients and methods

Because atazanavir induces an increase of plasma bilirubin levels, in a concentration-dependent manner, we developed a mathematical model, based on increments of atazanavir and bilirubin concentrations at steady state, in HIV infected (HIV⁺) patients, and plotted the corresponding nomogram for detecting suboptimal atazanavir exposure.

Results

By applying the obtained model, the results indicate that an absolute value or an increment of bilirubin at steady state below 3.8 μmol/L, are predictive of suboptimal atazanavir exposure and therapeutic failure.

Conclusion

We have successfully implemented a new mathematical approach to pharmacokinetic–pharmacodynamic model for atazanavir–bilirubin interaction. As a result, we found that bilirubin plasma levels constitute a good marker of exposure to atazanavir and of viral suppression.

Assessment of gadoxetate DCE-MRI as a biomarker of hepatobiliary transporter inhibition

Drug-induced liver injury (DILI) is a clinically important adverse drug reaction, which prevents the development of many otherwise safe and effective new drugs. Currently, there is a lack of sensitive and specific biomarkers that can be used to predict, assess and manage this toxicity. The aim of this work was to evaluate gadoxetate-enhanced MRI as a potential novel biomarker of hepatobiliary transporter inhibition in the rat. Initially, the volume fraction of extracellular space in the liver was determined using gadopentetate to enable an estimation of the gadoxetate concentration in hepatocytes. Using this information, a compartmental model was developed to characterise the pharmacokinetics of hepatic uptake and biliary excretion of gadoxetate. Subsequently, we explored the impact of an investigational hepatobiliary transporter inhibitor on the parameters of the model in vivo in rats. The investigational hepatobiliary transporter inhibitor reduced both the rate of uptake of gadoxetate into the hepatocyte, k1 , and the Michaelis-Menten constant, Vmax , characterising its excretion into bile, whereas KM values for biliary efflux were increased. These effects were dose dependent and correlated with effects on plasma chemistry markers of liver dysfunction, in particular bilirubin and bile acids. These results indicate that gadoxetate-enhanced MRI provides a novel functional biomarker of inhibition of transporter-mediated hepatic uptake and clearance in the rat. Since gadoxetate is used clinically, the technology has the potential to provide a translatable biomarker of drug-induced perturbation of hepatic transporters that may also be useful in humans to explore deleterious functional alterations caused by transporter inhibition.

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.

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.

In vitro OATP1B1 and OATP1B3 inhibition is associated with observations of benign clinical unconjugated hyperbilirubinemia

1.  Transient benign unconjugated hyperbilirubinemia has been observed clinically with several drugs including indinavir, cyclosporine, and rifamycin SV. Genome-wide association studies have shown significant association of OATP1B1 and UGT1A1 with elevations of unconjugated bilirubin, and OATP1B1 inhibition data correlated with clinical unconjugated hyperbilirubinemia for several compounds. 2.  In this study, inhibition of OATP1B3 and UGT1A1, in addition to OATP1B1, was explored to determine whether one measure offers value over the other as a potential prospective tool to predict unconjugated hyperbilirubinemia. OATP1B1 and OATP1B3-mediated transport of bilirubin was confirmed and inhibition was determined for atazanavir, rifampicin, indinavir, amprenavir, cyclosporine, rifamycin SV and saquinavir. To investigate the intrinsic inhibition by the drugs, both in vivo Fi (fraction of intrinsic inhibition) and R-value (estimated maximum in vivo inhibition) for OATP1B1, OATP1B3 and UGT1A1 were calculated. 3.  The results indicated that in vivo Fi values >0.2 or R-values >1.5 for OATP1B1 or OATP1B3, but not UGT1A1, are associated with previously reported clinical cases of drug-induced unconjugated hyperbilirubinemia. 4.  In conclusion, inhibition of OATP1B1 and/or OATP1B3 along with predicted human pharmacokinetic data could be used pre-clinically to predict potential drug-induced benign unconjugated hyperbilirubinemia in the clinic.

Hepatocellular organic anion-transporting polypeptides (OATPs) and multidrug resistance-associated protein 2 (MRP2) are inhibited by silibinin

Silibinin has been reported to be a promising compound for hepatitis C treatment of nonresponders to standard treatment. Although administered silibinin is well tolerated, increased serum bilirubin levels have been observed during high-dose i.v. silibinin therapy. The mechanism of silibinin-induced hyperbilirubinemia in humans, however, has not been identified so far. The aim of this study was to investigate the effect of silibinin on hepatocellular uptake and efflux transport systems for organic anions to elucidate the cause of silibinin-induced hyperbilirubinemia. Therefore, the effect of silibinin on transport activity of the hepatocellular uptake transporters organic anion-transporting polypeptides (OATPs) OATP1B1, OATP1B3, and OATP2B1, as well as Na(+)-taurocholate cotransporting polypeptide (NTCP) and of the efflux transporters multidrug resistance-associated protein 2 (MRP2) and bile-salt export pump (BSEP) was studied. The effect of silibinin on OATPs and NTCP function was studied in stable transfected Chinese hamster ovary cells using the radiolabeled model substrates estrone-3-sulfate and dehydroepiandrosteronesulfate for OATPs and taurocholate for NTCP. Interaction of silibinin with MRP2 and BSEP was measured in vesicles isolated from Sf21 or Sf9 insect cells expressing these transporters using either estradiol-17β-glucuronide or taurocholate as substrates. OATP1B1, OATP1B3, and OATP2B1 were inhibited by silibinin, with OATP1B1 being inhibited by (a) complex mechanism(s). An inhibitory effect was also seen for MRP2. In contrast, the bile acid transporters NTCP and BSEP were not affected by silibinin. We concluded that silibinin-induced hyperbilirubinemia may be caused by an inhibition of the bilirubin-transporting OATPs and the efflux-transporter MRP2.

Structure-activity relationship for FDA approved drugs as inhibitors of the human sodium taurocholate cotransporting polypeptide (NTCP)

The hepatic bile acid uptake transporter sodium taurocholate cotransporting polypeptide (NTCP) is less well characterized than its ileal paralog, the apical sodium dependent bile acid transporter (ASBT), in terms of drug inhibition requirements. The objectives of this study were (a) to identify FDA approved drugs that inhibit human NTCP, (b) to develop pharmacophore and Bayesian computational models for NTCP inhibition, and (c) to compare NTCP and ASBT transport inhibition requirements. A series of NTCP inhibition studies were performed using FDA approved drugs, in concert with iterative computational model development. Screening studies identified 27 drugs as novel NTCP inhibitors, including irbesartan (Ki = 11.9 μM) and ezetimibe (Ki = 25.0 μM). The common feature pharmacophore indicated that two hydrophobes and one hydrogen bond acceptor were important for inhibition of NTCP. From 72 drugs screened in vitro, a total of 31 drugs inhibited NTCP, while 51 drugs (i.e., more than half) inhibited ASBT. Hence, while there was inhibitor overlap, ASBT unexpectedly was more permissive to drug inhibition than was NTCP, and this may be related to NTCP possessing fewer pharmacophore features. Findings reflected that a combination of computational and in vitro approaches enriched the understanding of these poorly characterized transporters and yielded additional chemical probes for possible drug-transporter interaction determinations.