Application of quantitative time-lapse imaging (QTLI) for evaluation of Mrp2-based drug–drug interaction induced by liver metabolites

Induction of open-form bile canaliculus formation by hepatocytes for evaluation of biliary drug excretion

Biliary excretion is a major drug elimination pathway that affects their efficacy and safety. The currently available in vitro sandwich-cultured hepatocyte method is cumbersome because drugs accumulate in the closed bile canalicular lumen formed between hepatocytes and their amounts cannot be mealsured directly. This study proposes a hepatocyte culture model for the rapid evaluation of drug biliary excretion using permeation assays. When hepatocytes are cultured on a permeable support coated with the cell adhesion protein claudins, an open-form bile canalicular lumen is formed at the surface of the permeable support. Upon application to the basolateral (blood) side, drugs appear on the bile canalicular side. The biliary excretion clearance of several drugs, as estimated from the obtained permeabilities, correlates well with the reported in vivo biliary excretion clearance in humans. Thus, the established model is useful for applications in the efficient evaluation of biliary excretion during drug discovery and development.

Current Research Method in Transporter Study

Transporters play an important role in the absorption, distribution, metabolism, and excretion (ADME) of drugs. In recent years, various in vitro, in situ/ex vivo, and in vivo methods have been established for studying transporter function and drug-transporter interaction. In this chapter, the major types of in vitro models for drug transport studies comprise membrane-based assays, cell-based assays (such as primary cell cultures, immortalized cell lines), and transporter-transfected cell lines with single transporters or multiple transporters. In situ/ex vivo models comprise isolated and perfused organs or tissues. In vivo models comprise transporter gene knockout models, natural mutant animal models, and humanized animal models. This chapter would be focused on the methods for the study of drug transporters in vitro, in situ/ex vivo, and in vivo. The applications, advantages, or limitations of each model and emerging technologies are also mentioned in this chapter.

Sandwich-Cultured Hepatocytes for Mechanistic Understanding of Hepatic Disposition of Parent Drugs and Metabolites by Transporter-Enzyme Interplay

Functional interplay between transporters and drug-metabolizing enzymes is currently one of the hottest topics in the field of drug metabolism and pharmacokinetics. Uptake transporter-enzyme interplay is important to determine intrinsic hepatic clearance based on the extended clearance concept. Enzyme and efflux transporter interplay, which includes both sinusoidal (basolateral) and canalicular efflux transporters, determines the fate of metabolites formed in the liver. As sandwich-cultured hepatocytes (SCHs) maintain metabolic activities and form a canalicular network, the whole interplay between uptake and efflux transporters and drug-metabolizing enzymes can be investigated simultaneously. In this article, we review the utility and applicability of SCHs for mechanistic understanding of hepatic disposition of both parent drugs and metabolites. In addition, the utility of SCHs for mimicking species-specific disposition of parent drugs and metabolites in vivo is described. We also review application of SCHs for clinically relevant prediction of drug-drug interactions caused by drugs and metabolites. The usefulness of mathematical modeling of hepatic disposition of parent drugs and metabolites in SCHs is described to allow a quantitative understanding of an event in vitro and to develop a more advanced model to predict in vivo disposition.

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.

Sandwich-Cultured Hepatocytes as a Tool to Study Drug Disposition and Drug-Induced Liver Injury

Sandwich-cultured hepatocytes (SCH) are metabolically competent and have proper localization of basolateral and canalicular transporters with functional bile networks. Therefore, this cellular model is a unique tool that can be used to estimate biliary excretion of compounds. SCH have been used widely to assess hepatobiliary disposition of endogenous and exogenous compounds and metabolites. Mechanistic modeling based on SCH data enables estimation of metabolic and transporter-mediated clearances, which can be used to construct physiologically based pharmacokinetic models for prediction of drug disposition and drug-drug interactions in humans. In addition to pharmacokinetic studies, SCH also have been used to study cytotoxicity and perturbation of biological processes by drugs and hepatically generated metabolites. Human SCH can provide mechanistic insights underlying clinical drug-induced liver injury (DILI). In addition, data generated in SCH can be integrated into systems pharmacology models to predict potential DILI in humans. In this review, applications of SCH in studying hepatobiliary drug disposition and bile acid-mediated DILI are discussed. An example is presented to show how data generated in the SCH model were used to establish a quantitative relationship between intracellular bile acids and cytotoxicity, and how this information was incorporated into a systems pharmacology model for DILI prediction.

Nature and uses of fluorescent dyes for drug transporter studies

INTRODUCTION

Drug transporters are now recognized as major players involved in pharmacokinetics and toxicology. Methods for assessing their activity are important to consider, particularly owing to regulatory requirements with respect to inhibition of drug transporter activity and prediction of drug-drug interactions. In this context, the use of fluorescent-dye-based transport assays is likely to deserve attention.

AREAS COVERED

This review provides an overview of the nature of fluorescent dye substrates for ATP-binding cassette and solute carrier drug transporters. Their use for investigating drug transporter activity in cultured cells and clinical hematological samples, drug transporter inhibition, drug transporter imaging and drug transport at the organ level are summarized.

EXPERT OPINION

A wide range of fluorescent dyes is now available for use in various aspects of drug transporter studies. The use of these dyes for transporter analyses may, however, be hampered by classic pitfalls of fluorescence technology, such as quenching. Transporter-independent processes such as passive diffusion of dyes through plasma membrane or dye sequestration into subcellular compartments must also be considered, as well as the redundant handling by various distinct transporters of some fluorescent probes. Finally, standardization of dye-based transport assays remains an important on-going issue.

Mathematical modeling of the in vitro hepatic disposition of mycophenolic acid and its glucuronide in sandwich-cultured human hepatocytes

In recent years, it has become increasingly important to test the safety of circulating metabolites of novel drugs as part of drug discovery and development programs. Accordingly, it is essential to develop suitable methods for identifying the major metabolites and their disposition in animal species and in humans. Mycophenolic acid (MPA), a selective inosine-5'-monophosphate dehydrogenase (IMPDH) inhibitor, is metabolized by glucuronidation and enterohepatic circulation of MPA-glucuronides is an important factor in the continuous systemic exposure of MPA. In humans, about 90% of the administered MPA dose is finally excreted as MPA phenyl-glucuronide (MPAG) in urine. Notably, the plasma concentration of MPAG is much higher than that of MPA. These factors suggest that, after its formation in hepatocytes, MPAG is excreted into bile and is also transported across the basolateral membrane to enter the circulation. In the present study, we performed metabolic/hepatobiliary transport studies of MPA and MPAG using sandwich-cultured human hepatocytes (SCHH) and constructed mathematical models of their hepatic disposition. We also performed vesicular transport studies to identify which human multidrug resistance-associated proteins (MRPs) are involved in the transport of MPAG from hepatocytes. MPAG was a preferred substrate for the biliary excretion transporter MRP2 and the hepatic basolateral transporters MRP3 and MRP4 in conventional and metabolic/hepatobiliary transport studies using SCHH and vesicular transport studies using human MRP-expressing membrane vesicles. The resulting mathematical model suggested that the basolateral transport plays an important role in the hepatic disposition of MPAG formed in hepatocytes. Our findings suggest that mathematical modeling of metabolic/hepatobiliary transport studies using SCH will provide useful information for determining the fate of metabolites formed in hepatocytes.

Sandwich-cultured hepatocytes: utility for in vitro exploration of hepatobiliary drug disposition and drug-induced hepatotoxicity

INTRODUCTION

The sandwich-cultured hepatocyte (SCH) model has become an invaluable in vitro tool for studying hepatic drug transport, metabolism, biliary excretion and toxicity. The relevant expression of many hepatocyte-specific functions together with the in vivo-like morphology favor SCHs over other preclinical models for evaluating hepatobiliary drug disposition and drug-induced hepatotoxicity.

AREAS COVERED

In this review, the authors highlight recommended procedures required for reproducibly culturing hepatocytes in sandwich configuration. It also provides an overview of the SCH model characteristics as a function of culture time. Lastly, the article presents a summary of the most prominent applications of the SCH model, including hepatic drug clearance prediction, drug-drug interaction potential and drug-induced hepatotoxicity.

EXPERT OPINION

When human (cryopreserved) hepatocytes are used to establish sandwich cultures, the model appears particularly valuable to quantitatively investigate clinically relevant mechanisms related to in vivo hepatobiliary drug disposition and hepatotoxicity. Nonetheless, the SCH model would largely benefit from better insight into the fundamental cell signaling mechanisms that are critical for long-term in vitro maintenance of the hepatocytic phenotype. Studies systematically exploring improved cell culture conditions (e.g., co-cultures or extracellular matrix modifications), as well as in vitro work identifying key transcription factors involved in hepatocyte differentiation are currently emerging.