Sulindac is excreted into bile by a canalicular bile salt pump and undergoes a cholehepatic circulation in rats

Investigating bile acid-mediated cholestatic drug-induced liver injury using a mechanistic model of multidrug resistance protein 3 (MDR3) inhibition

Inhibition of the canalicular phospholipid floppase multidrug resistance protein 3 (MDR3) has been implicated in cholestatic drug-induced liver injury (DILI), which is clinically characterized by disrupted bile flow and damage to the biliary epithelium. Reduction in phospholipid excretion, as a consequence of MDR3 inhibition, decreases the formation of mixed micelles consisting of bile acids and phospholipids in the bile duct, resulting in a surplus of free bile acids that can damage the bile duct epithelial cells, i.e., cholangiocytes. Cholangiocytes may compensate for biliary increases in bile acid monomers via the cholehepatic shunt pathway or bicarbonate secretion, thereby influencing viability or progression to toxicity. To address the unmet need to predict drug-induced bile duct injury in humans, DILIsym, a quantitative systems toxicology model of DILI, was extended by representing key features of the bile duct, cholangiocyte functionality, bile acid and phospholipid disposition, and cholestatic hepatotoxicity. A virtual, healthy representative subject and population (n = 285) were calibrated and validated utilizing a variety of clinical data. Sensitivity analyses were performed for 1) the cholehepatic shunt pathway, 2) biliary bicarbonate concentrations and 3) modes of MDR3 inhibition. Simulations showed that an increase in shunting may decrease the biliary bile acid burden, but raise the hepatocellular concentrations of bile acids. Elevating the biliary concentration of bicarbonate may decrease bile acid shunting, but increase bile flow rate. In contrast to competitive inhibition, simulations demonstrated that non-competitive and mixed inhibition of MDR3 had a profound impact on phospholipid efflux, elevations in the biliary bile acid-to-phospholipid ratio, cholangiocyte toxicity, and adaptation pathways. The model with its extended bile acid homeostasis representation was furthermore able to predict DILI liability for compounds with previously studied interactions with bile acid transport. The cholestatic liver injury submodel in DILIsym accounts for several processes pertinent to bile duct viability and toxicity and hence, is useful for predictions of MDR3 inhibition-mediated cholestatic DILI in humans.

Current Models for Predicting Drug-induced Cholestasis: The Role of Hepatobiliary Transport System

Drug-induced cholestasis is the main type of liver disorder accompanied by high morbidity and mortality. Evidence for the role of hepatobiliary pumps in the cholestasis patho-mechanism is constantly increasing. Recognition of the interactions of chemical agents with these transporters at the initial phases of drug discovery can help develop new drug candidates with low cholestasis potential. This review delivers an outline of the role of these transport proteins in bile creation. It addresses the pathophysiological mechanism for drug-induced cholestasis. In-vitro models, including cell-based and membrane-based approaches and In-vivo models such as genetic knockout animals, are considered. The benefits and restrictions of each model are discussed in this review. Current understandings into the cellular and molecular process that control the activity of hepatobiliary pumps have directed to a better understanding of the pathophysiology of drug-induced cholestasis. A combination of in-vitro monitoring for transport interaction, in-silico predicting systems, and consideration of and metabolic and physicochemical properties must cause more effective monitoring of possible liver problems.

Non-steroidal anti-inflammatory drugs (NSAIDs) and organ damage: A current perspective

Owing to the efficacy in reducing pain and inflammation, non-steroidal anti-inflammatory drugs (NSAIDs) are amongst the most popularly used medicines confirming their position in the WHO's Model List of Essential Medicines. With escalating musculoskeletal complications, as evident from 2016 Global Burden of Disease data, NSAID usage is evidently unavoidable. Apart from analgesic, anti-inflammatory and antipyretic efficacies, NSAIDs are further documented to offer protection against diverse critical disorders including cancer and heart attacks. However, data from multiple placebo-controlled trials and meta-analyses studies alarmingly signify the adverse effects of NSAIDs in gastrointestinal, cardiovascular, hepatic, renal, cerebral and pulmonary complications. Although extensive research has elucidated the mechanisms underlying the clinical hazards of NSAIDs, no review has extensively collated the outcomes on various multiorgan toxicities of these drugs together. In this regard, the present review provides a comprehensive insight of the existing knowledge and recent developments on NSAID-induced organ damage. It precisely encompasses the current understanding of structure, classification and mode of action of NSAIDs while reiterating on the emerging instances of NSAID drug repurposing along with pharmacophore modification aimed at safer usage of NSAIDs where toxic effects are tamed without compromising the clinical benefits. The review does not intend to vilify these 'wonder drugs'; rather provides a careful understanding of their side-effects which would be beneficial in evaluating the risk-benefit threshold while rationally using NSAIDs at safer dose and duration.

Current insights in the complexities underlying drug-induced cholestasis

Drug-induced cholestasis (DIC) poses a major challenge to the pharmaceutical industry and regulatory agencies. It causes both drug attrition and post-approval withdrawal of drugs. DIC represents itself as an impaired secretion and flow of bile, leading to the pathological hepatic and/or systemic accumulation of bile acids (BAs) and their conjugate bile salts. Due to the high number of mechanisms underlying DIC, predicting a compound's cholestatic potential during early stages of drug development remains elusive. A profound understanding of the different molecular mechanisms of DIC is, therefore, of utmost importance. Although many knowledge gaps and caveats still exist, it is generally accepted that alterations of certain hepatobiliary membrane transporters and changes in hepatocellular morphology may cause DIC. Consequently, liver models, which represent most of these mechanisms, are valuable tools to predict human DIC. Some of these models, such as membrane-based in vitro models, are exceptionally well-suited to investigate specific mechanisms (i.e. transporter inhibition) of DIC, while others, such as liver slices, encompass all relevant biological processes and, therefore, offer a better representation of the in vivo situation. In the current review, we highlight the principal molecular mechanisms associated with DIC and offer an overview and critical appraisal of the different liver models that are currently being used to predict the cholestatic potential of drugs.

Evaluation of Drug Biliary Excretion Using Sandwich-Cultured Human Hepatocytes

Evaluation of hepatobiliary transport of drugs is an important challenge, notably during the development of new molecular identities. In this context, sandwich-cultured human hepatocytes (SCHH) have been proposed as an interesting and integrated tool for predicting in vitro biliary excretion of drugs. The present review was therefore designed to summarize key findings about SCHH, including their establishment, their main functional features and their use for the determination of canalicular transport and the prediction of in vivo biliary clearance and hepatobiliary excretion-related drug-drug interactions. Reviewed data highlight the fact that SCHH represent an original and probably unique holistic in vitro approach to predict biliary clearance in humans, through taking into account sinusoidal drug uptake, passive drug diffusion, drug metabolism and sinusoidal and canalicular drug efflux. Limits and proposed refinements for SCHH-based analysis of drug biliary excretion, as well as putative human alternative in vitro models to SCHH are also discussed.

Histopathological challenges in suspected drug-induced liver injury

When a patient with suspected drug-induced liver injury (DILI) undergoes liver biopsy, the pathologist is confronted with two major challenges. The first and most important is to establish the pattern(s) of injury which are present. Patterns of injury represent stereotypical responses of an organ to injury and relate to specific aetiologies of liver damage. The pattern of injury and the histological details of that injury can then be analysed with respect to the patient's intercurrent diseases and medication history. The specific expertise of the pathologist can be used to weigh the prospect of DILI against the likelihood of other explanations of injury. The second challenge is to characterize specific types of injury and the severity of injury, both of which may have importance for clinical decision-making and prognosis. The pathologist's report should convey both an accurate description of the pathology as well its interpretation.

Simvastatin intolerance genetic determinants: some features in ethnic Uzbek patients with coronary artery disease

Introduction

The objective is to study the influence of CYP3A5 (6986A>G), CYP2C9 (430C>T), CYP2C9 (1075A>C), SLCO1B1 (521T>C) and BCRP (ABCG2, 421C>A) gene polymorphisms on the development of simvastatin intolerance in ethnic Uzbek patients with coronary artery disease (CAD).

Material and methods

The case group contained 50 patients with clinical simvastatin-induced intolerance symptoms; the control group contained 50 patients without side-effects. Genotyping was performed by means of the PCR-RFLP method.

Results

Among 37 patients with simvastatin-induced liver symptoms the *3/*3 genotype of the CYP3A5 gene (p = 0.0001) and variant genotype of the CA BCRP gene were observed more frequently than in the control group (p = 0.0001). However, when the 13 patients who had statin-associated muscle symptoms (SAMS) were compared with the control group (n = 50), it was found that in the case group the 3*/3* genotype of the CYP3A5 gene (OR = 8.6; 95% CI: 2.1-34.1; p = 0.003) and C allele carriers of the gene polymorphism SLCO1B1 (OR = 3.54; 95% CI: 1.35-9.27; Χ² = 5.7; p = 0.017) were predominant.

Conclusions

The *3/*3 genotype of the CYP3A5 (6986A>G) gene and CA genotype of the BCRP (ABCG2, 421C>A) gene were associated with simvastatin-induced liver symptoms in ethnic Uzbek CAD patients, whereas in patients with simvastatin-associated muscle symptoms (SAMS), the combination of *3/*3 genotype of CYP3A5 (6986A> G) and carriage of the C allele of the SLCO1B1 gene polymorphism was predominant.

New therapeutic concepts in bile acid transport and signaling for management of cholestasis

The identification of the key regulators of bile acid (BA) synthesis and transport within the enterohepatic circulation has revealed potential targets for pharmacological therapies of cholestatic liver diseases. Novel drug targets include the bile BA receptors, farnesoid X receptor and TGR5, the BA-induced gut hormones, fibroblast growth factor 19 and glucagon-like peptide 1, and the BA transport systems, apical sodium-dependent bile acid transporter and Na⁺ -taurocholate cotransporting polypeptide, within the enterohepatic circulation. Moreover, BA derivatives undergoing cholehepatic shunting may allow improved targeting to the bile ducts. This review focuses on the pathophysiological basis, mechanisms of action, and clinical development of novel pharmacological strategies targeting BA transport and signaling in cholestatic liver diseases. (Hepatology 2017;65:1393-1404).

Potential of nor-Ursodeoxycholic Acid in Cholestatic and Metabolic Disorders

24-nor-ursodeoxycholic acid (norUDCA) is a side-chain shortened derivate of ursodeoxycholic acid (UDCA). Since norUDCA is only ineffectively conjugated with glycine or taurine, it has specific physicochemical and therapeutic properties distinct from UDCA. Nonamidated norUDCA undergoes cholehepatic shunting enabling 'ductular targeting' and inducing a bicarbonate-rich hypercholeresis, with cholangioprotective effects. At the same time it has direct anti-inflammatory, antilipotoxic, anti fibrotic, and antiproliferative properties targeting various liver cell populations. norUDCA appears to be one of the most promising novel treatment approaches targeting the liver and the bile duct system at multifactorial and multicellular levels. This review article is a summary of a lecture given at the XXIII International Bile Acid Meeting (Falk Symposium 194) on 'Bile Acids as Signal Integrators and Metabolic Modulators' held in Freiburg, October 8-9, 2014, and summarizes the recent progress with norUDCA as a novel therapeutic approach in cholestatic and metabolic (liver) disorders.

Biosynthesis and trafficking of the bile salt export pump, BSEP: therapeutic implications of BSEP mutations

The bile salt export pump (BSEP, ABCB11) is the primary transporter of bile acids from the hepatocyte to the biliary system. This rate-limiting step in bile formation is essential to the formation of bile salt dependent bile flow, the enterohepatic circulation of bile acids, and the digestion of dietary fats. Mutations in BSEP are associated with cholestatic diseases such as progressive familial intrahepatic cholestasis type 2 (PFIC2), benign recurrent intrahepatic cholestasis type 2 (BRIC2), drug-induced cholestasis, and intrahepatic cholestasis of pregnancy. Development of clinical therapies for these conditions necessitates a clear understanding of the cell biology of biosynthesis, trafficking, and transcriptional and translational regulation of BSEP. This chapter will focus on the molecular and cell biological aspects of this critical hepatic membrane transporter.

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Drug-induced cholestasis

Cholestasis caused by drugs is an important differential diagnosis in patients presenting with a biochemical cholestatic pattern. The extent of serologic tests and radiological imaging depends on the clinical context. The underlying condition of the patient and detailed information on drug use, results of rechallenge, and the documented hepatotoxicity of the drug are important to establish a diagnosis of drug-induced liver injury (DILI). Most cases of cholestatic DILI are mild, but in rare cases, ductopenia and cholestatic cirrhosis can develop. Approximately 10% of patients with cholestatic jaundice caused by drugs develop liver failure.

Drug-induced cholestasis

Recent progress in understanding the molecular mechanisms of bile formation and cholestasis have led to new insights into the pathogenesis of drug-induced cholestasis. This review summarizes their variable clinical presentations, examines the role of transport proteins in hepatic drug clearance and toxicity, and addresses the increasing importance of genetic determinants, as well as practical aspects of diagnosis and management.

Hepatotoxicity related to antirheumatic drugs

Antirheumatic agents are among commonly used drugs associated with adverse hepatic reactions. Sulfasalazine and azathioprine are among the most important causes of acute hepatotoxicity. Because such a large number of people take NSAIDs, even the rare occurrence of hepatotoxicity from these agents might contribute substantially to the total burden of drug-induced liver disease. A wide spectrum of hepatotoxic effects is described with antirheumatic drugs. Studies investigating genetic susceptibility to diclofenac hepatotoxicity have expanded our understanding of the potential drug-specific, class-specific and general factors involved in its pathogenesis, and methotrexate-associated liver disease demonstrates the interaction between drug, host and environmental factors that determines the likelihood and magnitude of liver disease. Infliximab therapy is associated with typical drug-induced autoimmune hepatitis. Although validated causality assessment methods have been used to objectively assess the strength of the association between a drug and a clinical event, in practice the diagnosis of drug-induced liver injury (DILI) involves a clinical index of suspicion, pattern recognition, the establishment of a temporal relationship between drug exposure and the adverse event, and the exclusion of alternative explanations for the clinical presentation. Detailed understanding of genetic and environmental factors underlying an individual's susceptibility would enable risk reduction and potentially primary prevention of hepatotoxicity.

Risk factors for idiosyncratic drug-induced liver injury

Idiosyncratic drug-induced liver injury (DILI) is a rare disorder that is not related directly to dosage and little is known about individuals who are at increased risk. There are no suitable preclinical models for the study of idiosyncratic DILI and its pathogenesis is poorly understood. It is likely to arise from complex interactions among genetic, nongenetic host susceptibility, and environmental factors. Nongenetic risk factors include age, sex, and other diseases (eg, chronic liver disease or human immunodeficiency virus infection). Compound-specific risk factors include daily dose, metabolism characteristics, and propensity for drug interactions. Alcohol consumption has been proposed as a risk factor for DILI from medications, but there is insufficient evidence to support this. Many studies have explored genetic defects that might be involved in pathogenesis and focused on genes involved in drug metabolism and the immune response. Multicenter databases of patients with DILI (the United States Drug Induced Liver Injury Network, DILIGEN, and the Spanish DILI registry) are important tools for clinical and genetic research. A genome-wide association study of flucloxacillin hepatotoxicity has yielded groundbreaking results and many similar studies are underway. Nonetheless, DILI is challenging to investigate because of its rarity, the lack of experimental models, the number of medications that might cause it, and challenges to diagnosis.

Drug-induced liver injury: insights from genetic studies

Drug-induced liver injury (DILI) is an increasing health problem and a challenge for physicians, regulatory bodies and the pharmaceutical industry, not only because of its potential severity and elusive pathogenesis but also because it is often inaccurately diagnosed, commonly missed entirely and more often not reported. The general view is that idiosyncratic DILI, which is not predictable whether based on the pharmacology of the drug or on the dose administered, is determined by the presence in the recipient of variants in, or expression of, genes coding for key metabolic pathways and/or the immune response, and the interaction of these genetic variants with environmental variables. Furthermore, idiosyncratic DILI is an example of a complex-trait disease with two or more susceptibility loci, as reflected by the frequency of genetic variants in the population often being higher than the occurrence of significant liver injury. Polymorphisms of bioactivation/toxification pathways via the CYP450 enzymes (Phase I), detoxification reactions (Phase II) and excretion/transport (Phase III), together with immunological factors that might determine DILI are reviewed. Challenges such as gene-trait association studies and whole-genome studies, and future approaches to the study of DILI are explored. Better knowledge of the candidate genes involved could provide further insight for the prospective identification of susceptible patients at risk of developing drug-induced hepatotoxicity, development of new diagnostic tools and new treatment strategies with safer drugs.

Biochemical mechanisms in drug-induced liver injury: Certainties and doubts

Drug-induced liver injury is a significant and still unresolved clinical problem. Limitations to knowledge about the mechanisms of toxicity render incomplete the detection of hepatotoxic potential during preclinical development. Several xenobiotics are lipophilic substances and their transformation into hydrophilic compounds by the cytochrome P-450 system results in production of toxic metabolites. Aging, preexisting liver disease, enzyme induction or inhibition, genetic variances, local O₂ supply and, above all, the intrinsic molecular properties of the drug may affect this process. Necrotic death follows antioxidant consumption and oxidation of intracellular proteins, which determine increased permeability of mitochondrial membranes, loss of potential, decreased ATP synthesis, inhibition of Ca²⁺-dependent ATPase, reduced capability to sequester Ca²⁺ within mitochondria, and membrane bleb formation. Conversely, activation of nucleases and energetic participation of mitochondria are the main intracellular mechanisms that lead to apoptosis. Non-parenchymal hepatic cells are inducers of hepatocellular injury and targets for damage. Activation of the immune system promotes idiosyncratic reactions that result in hepatic necrosis or cholestasis, in which different HLA genotypes might play a major role. This review focuses on current knowledge of the mechanisms of drug-induced liver injury and recent advances on newly discovered mechanisms of liver damage. Future perspectives including new frontiers for research are discussed.

Bile acids: trying to understand their chemistry and biology with the hope of helping patients

An informal review of the author's five decades of research on the chemistry and biology of bile acids in health and disease is presented. The review begins with a discussion of bile acid structure and its remarkable diversity in vertebrates. Methods for tagging bile acids with tritium for metabolic or transport studies are summarized. Bile acids solubilize polar lipids in mixed micelles; progress in elucidating the structure of the mixed micelle is discussed. Extensive studies on bile acid metabolism in humans have permitted the development of physiological pharmacokinetic models that can be used to simulate bile acid metabolism. Consequences of defective bile acid biosynthesis and transport have been clarified, and therapy has been developed. Methods for measuring bile acids have been improved. The rise and fall of medical and contact dissolution of cholesterol gallstones is chronicled. Finally, principles of therapy with bile acid agonists and antagonists are given. Advances in understanding bile acid biology and chemistry have helped to improve the lives of patients with hepatobiliary or digestive disease.

Lessons from the toxic bile concept for the pathogenesis and treatment of cholestatic liver diseases

Alterations in bile secretion at the hepatocellular and cholangiocellular levels may cause cholestasis. Formation of 'toxic bile' may be the consequence of abnormal bile composition and can result in hepatocellular and/or bile duct injury. The canalicular phospholipid flippase (Mdr2/MDR3) normally mediates biliary excretion of phospholipids, which normally form mixed micelles with bile acids and cholesterol to protect the bile duct epithelium from the detergent properties of bile acids. Mdr2 knockout mice are not capable of excreting phospholipids into bile and spontaneously develop bile duct injury with macroscopic and microscopic features closely resembling human sclerosing cholangitis. MDR3 mutations have been linked to a broad spectrum of hepatobiliary disorders in humans ranging from progressive familial intrahepatic cholestasis in neonates to intrahepatic cholestasis of pregnancy, drug-induced cholestasis, intrahepatic cholelithiasis, sclerosing cholangitis and biliary cirrhosis in adults. Other examples for bile injury due to the formation of toxic bile include the cholangiopathy seen in cystic fibrosis, after lithocholate feeding (in mice) and vanishing bile duct syndromes induced by drugs and xenobiotics. Therapeutic strategies for cholangiopathies may target bile composition/toxicity and the affected bile duct epithelium itself, and ideally should also have anti-cholestatic, anti-fibrotic and anti-neoplastic properties. Ursodeoxycholic acid (UDCA) shows some of these properties, but is of limited efficacy in the treatment of human cholangiopathies. By contrast to UDCA, its side chain-shortened homologue norUDCA undergoes cholehepatic shunting leading to a bicarbonate-rich hypercholeresis. Moreover, norUDCA has anti-inflammatory, anti-fibrotic and anti-proliferative effects, and stimulates bile acid detoxification. Upcoming clinical trials will have to demonstrate whether norUDCA or other side chain-modified bile acids are also clinically effective in humans. Finally, drugs for the treatment of cholangiopathies may target bile toxicity via nuclear receptors (FXR, PPARalpha) regulating biliary phospholipid and bile acid excretion.

Drug-induced liver injury

Drug-induced liver injury is a frequent cause of hepatic dysfunction. Reliably establishing whether the liver disease was caused by a drug requires the exclusion of other plausible causes and the search for a clinical drug signature. The drug signature consists of the pattern of liver test abnormality, the duration of latency to symptomatic presentation, the presence or absence of immune-mediated hypersensitivity and the response to drug withdrawal. Determination of causality also includes an evaluation of individual susceptibility to drug-induced liver injury. This susceptibility is governed by both genetic and environmental factors. Components of the drug signature in conjunction with certain risk factors have been incorporated into formal scoring systems that are predictive of the likelihood of drug-induced liver injury. The most validated scoring system is the Roussel-Uclaf causality assessment method, which nonetheless retains certain imperfections. Mitigating the potential for drug-induced liver injury is achieved by the identification of toxicity signals during clinical trials and the monitoring of liver tests in clinical practice. There are three signals of liver toxicity in clinical trials: (i) a statistically significant doubling (or more) in the incidence of serum alanine aminotransferase (ALT) elevation >3 x the upper limit of normal (ULN); (ii) any incidence of serum ALT elevation >8-10 x ULN; and (iii) any incidence of serum ALT elevation >3 x ULN accompanied by a serum bilirubin elevation >2 x ULN. Monitoring of liver tests in clinical practice has shown unconvincing efficacy, but where a benefit-risk analysis would favour continued therapy, monthly monitoring may have some benefit compared with no monitoring at all. With rare exception, treatment of drug-induced liver injury is principally supportive. Drug toxicity is the most common cause of acute liver failure, defined as a prolonged prothrombin time (international normalised ratio > or =1.5) and any degree of mental alteration occurring <26 weeks after the onset of illness in a patient without pre-existing cirrhosis. A patient who meets these criteria must be evaluated for liver transplantation. The pathogenesis of drug-induced liver injury can be examined on the basis of the two principal patterns of injury. The hepatocellular pattern is characterised by a predominant rise in the level of transaminases and results from the demise of hepatocytes by means of either apoptosis or necrosis. The cholestatic pattern is characterised by a predominant rise of the serum alkaline phosphatase level and usually results from injury to the bile ductular cells either directly by the drug or its metabolite, or indirectly by an adaptive immune response.

Nonsteroidal anti-inflammatory drug-induced hepatotoxicity

Nonsteroidal anti-inflammatory drugs are among the most common drugs associated with drug-induced liver injury, with an estimated incidence of between 3 and 23 per 100,000 patient years. Nimesulide, sulindac, and diclofenac seem to be associated with the highest risk and the only risk factor consistently identified is the concomitant use of other hepatotoxic drugs. Diclofenac-induced liver injury is a paradigm for drug-related hepatotoxicity. Recent studies suggest that genetic factors favoring the formation and accumulation of the reactive acylglucuronide metabolite of diclofenac and an enhanced immune response to the metabolite-protein adducts are associated with increased susceptibility to hepatotoxicity.

Intestinal tract injury by drugs: Importance of metabolite delivery by yellow bile road

Drug secretion into bile is typically considered a safe route of clearance. However, biliary delivery of some drugs or their reactive metabolites to the intestinal tract evokes adverse consequences due to direct toxic actions or indirect disruption of intestinal homeostasis. Biliary concentration of the chemotherapy agent 5-fluorodeoxyuridine (FUDR) and other compounds is associated with bile duct damage while enterohepatic cycling of antibiotics contributes to the disruptions of gut flora that produce diarrhea. The goal of this review is to describe key evidence that biliary delivery is an important factor in the intestinal injury caused by representative drugs. Emphasis will be given to 3 widely used drugs whose reactive metabolites are plausible causes of small intestinal injury, namely the nonsteroidal anti-inflammatory drug (NSAID) diclofenac, the immunosuppressant mycophenolic acid (MPA), and the chemotherapy agent irinotecan. Capsule endoscopy and other sensitive diagnostic techniques have documented a previously unappreciated, high prevalence of small intestinal injury among NSAID users. Clinical use of MPA and irinotecan is frequently associated such severe intestinal injury that dosage must be reduced. Observations from clinical and experimental studies have defined key events in the pathogenesis of these drugs, including roles for multidrug resistance-associated protein 2 (MRP2) and other transporters in biliary secretion and adduction of enterocyte proteins by reactive acyl glucuronide metabolites as a likely mechanism for intestinal injury. New strategies for minimizing the adverse intestinal consequences of irinotecan chemotherapy illustrate how basic information about key events in the biliary secretion of drugs and the nature of their proximate toxicants can lead to safer protocols for drugs.

Rofecoxib-induced hepatotoxicity: a forgotten complication of the coxibs

Rofecoxib is a member of the coxib family of nonsteroidal anti-inflammatory drugs that selectively inhibit cyclooxygenase-2. Although the coxibs are generally well-tolerated, rofecoxib was recently withdrawn from the market due to concerns regarding cardiovascular safety. Rare cases of hepatic injury attributable to the coxibs have been reported. In the present study, two additional cases of severe hepatotoxicity are described in patients with cholestatic symptoms and abnormal liver biochemistry, shortly following the initiation of rofecoxib for arthritic complaints. In both cases, liver histology was compatible with drug-induced hepatotoxicity, and rapid clinical and biochemical improvements were observed following rofecoxib discontinuation. With new coxibs and expanding indications on the horizon, physicians in all areas of practice must be aware of this disorder and consider it in any patient who develops hepatic dysfunction after taking a coxib.

Hepatobiliary transporters and drug-induced cholestasis

Drug-induced liver injury is an important clinical problem with significant morbidity and mortality. Whereas for most hepatocellular forms of drug-induced hepatic injury the underlying pathophysiological mechanism is poorly understood, there is increasing evidence that cholestatic forms of drug-induced liver damage result from a drug- or metabolite-mediated inhibition of hepatobiliary transporter systems. In addition to their key role in determining hepatic drug exposure and clearance, the coordinated action of these transport systems is essential for bile formation and the biliary secretion of cholephilic compounds and xenobiotics. Any drug-mediated functional disturbance of these processes can lead to an intracellular accumulation of potentially harmful bile constituents and result in the development of cholestatic liver cell damage. In addition to direct drug-mediated inhibition of hepatocellular transport, function of these transporters can be altered by pre-existing hepatic disease and genetic factors, which contribute to the development of drug-induced cholestasis in susceptible individuals. This review summarizes current knowledge about the function of hepatobiliary uptake and efflux systems and discusses factors that might predispose to drug-induced cholestasis.

Inhibition of bile acid transport across Na+/taurocholate cotransporting polypeptide (SLC10A1) and bile salt export pump (ABCB 11)-coexpressing LLC-PK1 cells by cholestasis-inducing drugs

Vectorial transport of bile acids across hepatocytes is a major driving force for bile flow, and bile acid retention in the liver causes hepatotoxicity. The basolateral and apical transporters for bile acids are thought to be targets of drugs that induce cholestasis. Previously, we constructed polarized LLC-PK1 cells that express both a major bile acid uptake transporter human Na+/taurocholate cotransporting polypeptide (SLC10A1) (NTCP) and the bile acid efflux transporter human bile salt export pump (ABCB 11) (BSEP) and showed that monolayers of such cells can be used to characterize vectorial transcellular transport of bile acids. In the present study, we investigated whether cholestasis-inducing drugs could inhibit bile acid transport in such cells. Because fluorescent substrates allow the development of a high-throughput screening method, we examined the transport by NTCP and BSEP of fluorescent bile acids as well as taurocholate. The aminofluorescein-tagged bile acids, chenodeoxycholylglycylamidofluorescein and cholylglycylamidofluorescein, were substrates of both NTCP and BSEP, and their basal-to-apical transport rates across coexpressing cell monolayers were 4.3 to 4.5 times those of the vector control, although smaller than for taurocholate. The well known cholestatic drugs, rifampicin, rifamycin SV, glibenclamide, and cyclosporin A, reduced the basal-to-apical transport and the apical efflux clearance of taurocholate across NTCP- and BSEP-coexpressing cell monolayers. Further analysis indicated that the drugs inhibited both NTCP and BSEP. Our study suggests that such coexpressing cells can provide a useful system for the identification of inhibitors of these two transport systems, including potential drug candidates.

Transporters and drug discovery: why, when, and how

Drug transporters are now increasingly recognized as important determinants of variable drug disposition and response. In addition, transporter associated problems appear to be occurring with greater frequency during the drug discovery and development process. What has not been clear is whether drug transporter related issues are a truly new problem, or whether such issues had existed all along, but were previously unrecognized or ignored. In this review, a brief overview of key drug transporters will be outlined. In addition, a commentary on specific issues of relevance to pharmaceutical sciences in terms of the role and relevance of drug transporters to the drug discovery and development process is provided.

Hepatobiliary Disposition in Primary Cultures of Dog and Monkey Hepatocytes

Hepatobiliary transporters are a major route for elimination of xenobiotics and endogenous products. In vitro hepatobiliary models have been reported for human and rat, but not for the other preclinical species used in safety evaluation. We have established methodologies for culturing dog and monkey hepatocytes with optimal bile canalicular formation and function, using a sandwich culture comprising rigid collagen substratum and gelled collagen overlay. Hepatic uptake utilizing sinusoidal transporters and biliary excretion through canalicular transporters were assessed using the bile salt taurocholate, salicylate (negative control), and the Bsep inhibitors cyclosporin A (CsA) and glyburide. There was significant taurocholate and salicylate canalicular efflux in dog and monkey hepatocytes, although the amount of salicylate transported was one thousandth that of taurocholate. Species differences were observed, as glyburide significantly inhibited taurocholate uptake in monkey (64% at 10 microM) but not dog hepatocytes, and inhibited taurocholate efflux in dog (100% at 10 microM) but not monkey hepatocytes. CsA did not inhibit bile salt uptake and significantly inhibited canalicular efflux in dog (at 0.1 microM) and monkey (at 1 and 10 microM) hepatocyte cultures. These results suggest that glyburide is a bile salt uptake inhibitor in monkey but not in dog hepatocytes and that CsA inhibits bile salt canalicular efflux but not basolateral uptake in these species. We have established dog and monkey hepatocytes in sandwich culture with intact bile canalicular formation and function. The differences observed in taurocholate transport between dog and monkey hepatocytes may be indicative of in vivo species differences.

24-norUrsodeoxycholic acid is superior to ursodeoxycholic acid in the treatment of sclerosing cholangitis in Mdr2 (Abcb4) knockout mice

BACKGROUND & AIMS

Current therapy for primary sclerosing cholangitis is of limited efficacy. Multidrug resistance gene 2 knockout mice (Mdr2(-/-)) represent a well-characterized model for sclerosing cholangitis. Experiments were performed to test in such mice the therapeutic effects of 24-norUrsodeoxycholic acid, a C(23) homologue of ursodeoxycholic acid with 1 fewer methylene group in its side chain.

METHODS

Mdr2(-/-) mice were fed a diet containing 24-norUrsodeoxycholic acid (0.5% wt/wt) or ursodeoxycholic acid (0.5% wt/wt) as a clinical comparator for 4 weeks; controls received standard chow. Effects on serum liver tests, liver histology, markers of inflammation and fibrosis, and bile acid transport and metabolism were compared. 24-norUrsodeoxycholic acid metabolism was studied in serum, liver, bile, and urine.

RESULTS

24-norUrsodeoxycholic acid markedly improved liver tests and liver histology and significantly reduced hydroxyproline content and the number of infiltrating neutrophils and proliferating hepatocytes and cholangiocytes. 24-norUrsodeoxycholic acid underwent extensive phase I/II metabolism (hydroxylation, sulfation, and glucuronidation), thereby increasing the hydrophilicity of biliary bile acid secretion. There was a coordinated induction of bile acid detoxifying enzymes (Cyp2b10, Cyp3a11, and Sult2a1) and efflux pumps (Mrp3 and Mrp4). Ursodeoxycholic acid, in contrast, increased alanine transaminase and alkaline phosphatase levels, had no significant effects on hydroxyproline content, and induced biliary transporters and detoxification enzymes to a much smaller extent than 24-norUrsodeoxycholic acid.

CONCLUSIONS

24-norUrsodeoxycholic acid ameliorates sclerosing cholangitis in Mdr2(-/-) mice. Its therapeutic mechanisms involve (1) increasing the hydrophilicity of biliary bile acids, (2) stimulating bile flow with flushing of injured bile ducts, and (3) inducing detoxification and elimination routes for bile acids.

Novel biotransformation and physiological properties of norursodeoxycholic acid in humans

Experiments were performed in 2 volunteers to define the biotransformation and physiological properties of norursodeoxycholic acid (norUDCA), the C(23) (C(24)-nor) homolog of UDCA. To complement the in vivo studies, the biotransformation of norUDCA ex vivo using precision-cut human liver slices was also characterized. In the human studies, both a tracer dose given intravenously and a physiological dose (7.9 mmol, 3.0 g) given orally were excreted equally in bile and urine. By chromatography and mass spectrometry, the dominant biotransformation product of norUDCA in bile and urine was the C-23 ester glucuronide. Little N-acyl amidation (with glycine or taurine) occurred. The oral dose induced a sustained bicarbonate-rich hypercholeresis, with total bile flow averaging 20 microL/kg/min, a rate extrapolating to 2 L/d. The increased bile flow was attributed to cholehepatic shunting of norUDCA as well to the lack of micelles in bile. Phospholipid and cholesterol secretion relative to bile acid secretion decreased during secretion of norUDCA and its metabolites, presumably also because of the absence of micelles in canalicular bile. When incubated with human liver slices, norUDCA was glucuronidated, whereas UDCA was conjugated with glycine or taurine. In conclusion, in humans, norUDCA is glucuronidated rather than amidated. In humans, but not animals, there is considerable renal elimination of the C-23 ester glucuronide, the dominant metabolite. NorUDCA ingestion induces a bicarbonate-rich hypercholeresis and evokes less phospholipid and cholesterol secretion into bile than UDCA. Molecules that undergo cholehepatic shunting should be powerful choleretics in humans.

Transport by vesicles of glycine- and taurine-conjugated bile salts and taurolithocholate 3-sulfate: a comparison of human BSEP with rat Bsep

The bile salt export pump (BSEP) of hepatocyte secretes conjugated bile salts across the canalicular membrane in an ATP-dependent manner. The biliary bile salts of human differ from those of rat in containing a greater proportion of glycine conjugates and taurolithocholate 3-sulfate (TLC-S). In the present study, the transport properties of hBSEP and rBsep were investigated using membrane vesicles from HEK293 cells infected with recombinant adenoviruses containing hBSEP or rBsep cDNA. ATP-dependent uptake of radiolabeled glycine-, taurine-conjugated bile salts, and [(3)H]cholate was observed when hBSEP or rBsep was expressed. Comparison of initial uptake rates indicated that for both transporters, taurine-conjugated bile salts were transported more rapidly than glycine-conjugated bile salts, however, hBSEP transported glycine conjugates to an extent that was approximately 2-fold greater than rBsep. In addition, [(3)H]TLC-S was significantly transported by hBSEP, and hardly transported by rBsep. The mean K(m) value for the uptake of [(3)H]TLC-S by hBSEP was 9.5+/-1.5 microM, a value similar to that for hMRP2 (8.2+/-1.3 microM). In conclusion, both hBSEP and rBsep transport taurine-conjugated bile salts better than glycine-conjugated bile salts, but hBSEP transports glycine conjugates to a greater extent as compared to rBsep. TLC-S, which is present in human bile but not rodent bile, is more avidly transported by hBSEP compared with rBsep.

Detoxification of lithocholic acid, a toxic bile acid: relevance to drug hepatotoxicity

Lithocholic acid, a monohydroxy, secondary bile acid, is formed by bacterial 7-dehydroxylation of the primary bile acid chenodeoxycholic acid (CDCA) and of the secondary bile acid ursodeoxycholic acid (UDCA). Lithocholic acid and its precursor CDCA are toxic when fed to the rabbit, rhesus monkey, and baboon, but not when CDCA, as well as UDCA, is used for therapeutic purposes in man. Older studies showed that the species specific toxicity of lithocholic acid could be explained by efficient sulfation of lithocholic acid in man and in chimpanzee, but not in the rabbit, rhesus monkey, or baboon. Rodents detoxify lithocholic acid by hydroxylation, but this does not occur in species in which it is toxic. Recent studies suggest that lithocholic acid induces its own detoxification by activating nuclear receptors to promote transcription of genes encoding sulfotransferase. In addition, work with CaCo2 cells suggest that lithocholic acid may undergo sulfation in the enterocyte and be effluxed back into the intestinal lumen. The evolution of trihydroxy bile acids in vertebrates may have occurred to decrease the formation of lithocholic acid. Lithocholic acid is a rare example of a toxic endobiotic; a variety of mechanisms have evolved to solve the problem of efficient detoxification.

Conditioning of liver grafts with prostaglandins improves bile acid transport

INTRODUCTION

Conditioning of liver grafts by bolus pretreatment with prostaglandins has been previously demonstrated to improve hepatic bile flow. However, the underlying mechanisms have not been investigated. To elucidate whether improved bile flow after prolonged ischemia is due to maintained bile acid secretion or due to increased paracellular permeability, we performed a study using increasing doses of the marker acid taurocholate in the isolated perfused rat liver system.

METHODS

Livers were harvested from adult Lewis rats and stored for 24 hours in UW solution. Pretreatment of livers was performed 1 minute before preservation. One group received prostaglandin I2, the second group received prostaglandin E1, and the control group was treated with saline. After 24 hours of cold storage the grafts were investigated in the isolated perfused rat liver system by perfusion with an oxygenated Krebs-Ringer-Henseleit buffer. Increasing doses of the radiolabeled marker bile acid taurocholate were infused to investigate bile acid transport.

RESULTS

Bile flow and bile acid output were increased by pretreatment of the livers with prostaglandin I2 and prostaglandin E1, as compared to the control group. More specifically, the maximum transport rate was tripled by prostaglandin I2 and by prostaglandin E1 preconditioning of liver grafts, in comparison to the control group (P < .01 vs prostaglanin I2 and E1).

CONCLUSION

The results clearly demonstrate that increased bile flow after conditioning of liver grafts with prostaglandins is not due to increased paracellular permeability but is based on markedly improved bile acid output.

Enterohepatic bile salt transporters in normal physiology and liver disease

The vectorial transport of bile salts from blood into bile is essential for the generation of bile flow, solubilization of cholesterol in bile, and emulsification of lipids in the intestine. Major transport proteins involved in the enterohepatic circulation of bile salts include the hepatocellular bile salt export pump (BSEP, ABCB11), the apical sodium-dependent bile salt transporter (ASBT, SLC10A2) in cholangiocytes and enterocytes, the sodium-dependent hepatocyte bile salt uptake system NTCP (SLC10A1), the organic anion transporting polypeptides OATP-C (SLC21A6), OATP8 (SLC21A8) and OATP-A (SLC21A3), and the multidrug resistance protein MRP3 (ABCC3). Synthesis and transport of bile salts are intricately linked processes that undergo extensive feedback and feed-forward regulation by transcriptional and posttranscriptional mechanisms. A key regulator of hepatocellular bile salt homeostasis is the bile acid receptor/farnesoid X receptor FXR, which activates transcription of the BSEP and OATP8 genes and of the small heterodimer partner 1 (SHP). SHP is a transcriptional repressor that mediates bile acid-induced repression of the bile salt uptake systems rat Ntcp and human OATP-C. A nuclear receptor that activates rodent Oatp2 (Slc21a5) and human MRP2 (ABCC2) is the pregnane X receptor/steroid X receptor PXR/SXR. Intracellular trafficking and membrane insertion of bile salt transporters is regulated by lipid, protein, and extracellular signal-related kinases in response to physiologic stimuli such as cyclic adenosine monophosphate or taurocholate. Finally, dysfunction of individual bile salt transporters such as BSEP, on account of genetic mutations, steric inhibition, suppression of gene expression, or disturbed signaling, is an important cause of cholestatic liver disease.

Hepatotoxicity associated with non-steroidal anti-inflammatory drugs

NSAIDs are one of most frequently prescribed agents in clinical practice. Whereas hepatotoxicity is a rare complication of most NSAIDs (typically 1 to 10 per 100,000 persons exposed), the high level of usage means that these drugs cause liver disease. Because of their divergent chemical structures, the mechanisms and clinicopathological manifestations of hepatotoxicity vary widely. The reactive metabolite syndrome, in which serious rash, eosinophilia, and other forms of tissue injury are common, may be incited by several NSAIDs, including newer agents. Women, people aged more than 50 years, and for some drugs, the type of arthritis, may be risk factors for drug-induced liver injury. The spectrum of NSAID-drug related hepatotoxicity continues to expand, with reports of interactive toxicity in adults with hepatitis C and recognition of rare cases of liver disease associated with non-selective, selective, and preferential COX-2 inhibitors. Better outcomes require people taking NSAIDs to be aware of possible drug reactions involving the liver, and prescribers should be vigilant for early symptoms of hepatotoxicity so that incriminated agents are discontinued promptly.

Drug-induced cholestasis

Drug-induced cholestasis is a common entity, seen with numerous classes of pharmacological agents. A high index of suspicion is required for the correct diagnosis. Different clinical syndromes may be recognized, with variable degrees of hepatitis in association with cholestasis. The most important aspect of treatment is prompt discontinuation of the offending drug. Several agents have been used for symptomatic relieve of the pruritus associated with cholestasis, including cholestyramine, ursodeoxycholic acid, and opiate antagonists, with limited results. Prognosis is usually good, with few cases of prolonged cholestasis leading to vanishing bile duct syndrome. Liver failure may rarely occur if diagnosis goes unrecognized and the inciting drug is not withdrawn.

Fluorescent substrates of sister-P-glycoprotein (BSEP) evaluated as markers of active transport and inhibition: evidence for contingent unequal binding sites

PURPOSE

Although sister-P-glycoprotein (SPGP, BSEP) is closely related to P-glycoprotein, it is much more selective in distribution and substrate recognition. Moreover, because inhibition or lack of BSEP function has severe consequences including cholestasis, hepatotoxicity, exposure to toxic xenobiotics, and drug interactions, in vitro methods are necessary for quantifying and characterizing specific inhibition of BSEP. Therefore, the objective is to discern a method and quantitatively characterize several example BSEP inhibitors.

METHODS

With fluorescent markers having been used successfully to evaluate and quantify inhibition of P-gp-mediated transport, this study evaluates several compounds for specific cell retention caused by BSEP inhibitors. In addition to the several compounds asserted to be BSEP inhibitors, the compounds suggested to be BSEP substrates might also inhibit BSEP competitively. Retained fluorescence of possible BSEP substrates was measured by a flow cell cytometer using transfected cells presenting the BSEP transporter specifically and abundantly.

RESULTS

Several compounds were shown to inhibit BSEP active transport of the fluorescent substrates dihydrofluorescein and bodipy. The inhibition potency was quantified (i.e., cyclosporin A IC50 approximately 7 microM), revealing incongruent relative sensitivities among the substrate markers, with H2FDA generally the most sensitive of the series of substrate markers evaluated.

CONCLUSIONS

The inconsistent sensitivities of the transport markers (H2FDA and bodipy) were reminiscent of the apparent multiple binding site behaviors observed for P-gp and could indicate opposing and unequal yet interacting binding sites akin to those of P-gp. Nonetheless, notable differences between P-gp and BSEP in marker substrate recognition/transport were apparent despite the observed overlap in xenobiotic recognition and transport. Thus far the most potent inhibitors seem to be cyclosporin, tamoxifen, and valinomycin. There are likely to be much more potent inhibitors, and other substrates also may be more sensitive to inhibition of transport.

The influence of sulindac on patients with primary biliary cirrhosis that responds incompletely to ursodeoxycholic acid: a pilot study

OBJECTIVES

In 30% of patients with primary biliary cirrhosis (PBC) ursodeoxycholic acid (UDCA) causes full biochemical normalization, while 70% are incomplete responders. The only differences between the two groups are the significantly higher cholestasis indices in the incomplete responders. In these patients we investigated whether the strongly choleretic sulindac together with UDCA is superior to UDCA monotherapy.

DESIGN AND METHODS

Twenty-three patients with PBC incompletely responding to UDCA monotherapy were entered in the open label study for 12 months. Eleven patients (stage II, seven; III, two; and IV, two) received UDCA (10-15 mg/kg/day) plus sulindac (100-300 mg/day) (Group I). Twelve patients (stage I, six; II, four; III, one; and IV, one) were treated with UDCA alone (Group II). Liver biochemistry, analysis of antimitochondrial, antinuclear, smooth muscle, and liver-kidney-microsomal antibodies, ultrasonography and gastroscopy were done in regular intervals.

RESULTS

In Group I all liver indices, IgG, IgM and IgA significantly improved although pretreatment data and stages of the disease tended to be higher than in Group II. In five patients of Group I liver histology improved slightly. Sulindac was well tolerated. The biochemical indices did not further improve on UDCA monotherapy.

CONCLUSIONS

Sulindac in combination with UDCA further improves liver biochemistries in patients with PBC who responded incompletely to UDCA alone.

Mechanisms of NSAID-induced hepatotoxicity: focus on nimesulide

Nonsteroidal anti-inflammatory drugs (NSAIDs) have been associated with idiosyncratic hepatotoxicity in susceptible patients. The molecular mechanisms underlying this toxicity have not yet been fully elucidated. However, experimental evidence suggests that they include increased concentration of the drugs in the hepatobiliary compartment, formation of reactive metabolites that covalently modify proteins and produce oxidative stress, and mitochondrial injury. Genetic and/or acquired patient factors can either augment the pathways leading to hepatic toxicity or impede the protective and detoxifying pathways. An example is nimesulide, a selective cyclo-oxygenase-2 inhibitor widely used for the treatment of inflammatory and pain conditions, which has been recently associated with rare but serious and unpredictable adverse reactions in the liver (increases in serum aminotransferase activities, hepatocellular necrosis, and/or intrahepatic cholestasis). Similar to other drugs causing idiosyncratic hepatotoxicity, both the molecule and the patient contribute to the hazard. Here, the weakly acidic sulfonanilide drug undergoes bioreductive metabolism of the nitroarene group to reactive intermediates that have been implicated in oxidative stress, covalent binding, and mitochondrial injury. It is only in a small number of susceptible patients, however, that genetic or nongenetic factors will cause this potential toxicity to become clinically manifest. In view of the very large recipient population, the incidence of nimesulide-induced liver injury has been low (approximately 0.1 per 100,000 patients treated). Although this estimation is based on spontaneous reporting data versus sales units and needs correction due to the classical bias of this system, the type and incidence of these rare but severe hepatic adverse reactions are comparable to that of other NSAIDs.

The pathophysiology of cholestasis with special reference to primary biliary cirrhosis

Cholestasis in primary biliary cirrhosis results from impairment of bile flow either by reduced transport at the level of the canaliculi or by disturbed bile flow through damaged intrahepatic bile ductules. Whatever its cause, the expression of hepatic transport proteins will be affected. In cholestatic rats: the expression of the multispecific organic anion transporter mrp2 is decreased; the bile salt export pump bsep and the phospholipid transporter mdr2 are less affected; the carrier protein for hepatic uptake of bile salts ntcp is sharply down-regulated; Mrp3, a basolateral ATP-dependent transporter for glucuronides and bile salts, is upregulated. Thus, bile salts that cannot exit the hepatocyte because of the cholestasis are effectively removed across the basolateral membrane. These may be adaptive responses in defence against overloading of hepatocytes with cytotoxic bile salts. These responses show that the expression of hepatic transporter proteins is highly regulated. This occurs by transcriptional and post-transcriptional mechanisms. Primary biliary cirrhosis starts as a disease of the small intrahepatic bile ducts and therefore the experimental evidence for 'cross-talk' between hepatocytes and cholangiocytes is of great interest for this disease and needs to be further investigated. New insights in bile physiology may enable the development of new therapies for cholestatic liver diseases as primary biliary cirrhosis.