Hepatobiliary transporters and drug-induced cholestasis


  • Potential conflict of interest: Nothing to report.


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. (HEPATOLOGY 2006;44:778–787.)

Drug-induced liver injury is an important clinical problem accounting for approximately 2%-5% of hospitalizations for jaundice, 10% of cases of hepatitis in all adults, and more than 40% of hepatitis cases in adults over 50 years of age.1 A variety of clinical presentations may be seen in patients who develop drug-induced hepatotoxicity, ranging from asymptomatic mild biochemical abnormalities to an acute illness with jaundice that resembles viral hepatitis.2, 3 According to the consensus conference of the Council for International Organization of Medical Sciences, drug-induced liver injury can be classified into hepatocellular, cholestatic, and mixed types of liver damage depending on serum biochemistry markers.4 Whereas hepatocellular injury is mainly characterized by the elevation of serum aminotransferases, cholestatic liver damage is reflected by increased levels of alkaline phosphatase, γ-glutamyltranspeptidase, and conjugated bilirubin in serum.

For most hepatocellular forms of drug-induced liver injury, the underlying pathophysiological mechanism is poorly understood. Only a minority of cases can be related to intrinsic hepatotoxins. Most cases are still attributed to idiosyncratic reactions, where immunoallergic mechanisms resulting from hypersensitivity and aberrant metabolism of the suspected drug are thought to represent the predominant pathophysiological pathways. In contrast, it has become evident that many cases of drug-induced cholestatic injury result from a drug- or metabolite-mediated inhibition of hepatobiliary transporter systems expressed at the two polar surface domains of liver cells. Whereas basolateral uptake transporters are important in controlling hepatic drug and toxin exposure, apical or canalicular transporters are responsible for hepatic drug clearance as well as for the secretion of bile salts and other bile constituents across the canalicular membrane of hepatocytes into bile. Any drug-mediated functional disturbance of these processes can lead to an intracellular accumulation of potentially harmful bile constituents resulting in the development of cholestatic liver cell damage. In addition to the direct inhibitory effect of certain drugs on canalicular efflux transporters, hepatocellular transporter expression and function might be altered by environmental and genetic factors contributing to the development of drug-induced cholestasis in susceptible individuals. Furthermore, bile ductular reabsorption of bile salts and drugs and cholehepatic shunting might contribute to changes in bile composition and contribute to hepatic accumulation of drugs and toxins. This review summarizes current knowledge about the function of hepatocellular uptake and efflux systems involved in bile formation and discusses environmental and genetic risk factors that might affect the function of these systems.

Physiology of Hepatobiliary Transport and Bile Formation

Hepatic uptake and efflux processes involved in bile formation are maintained by distinct transport systems expressed at the two polar surface domains of liver cells. After canalicular secretion, bile composition undergoes further modification in the bile canaliculi, involving reabsorption and secretion processes maintained by apical and basolateral transport system in cholangiocytes. Figure 1 shows a scheme of hepatocellular and bile ductular transport proteins involved in uptake and efflux of endogenous and exogenous (xenobiotic) cholephilic compounds.

Figure 1.

Bile salt transporters in human liver and cholangiocytes. Efflux transporters (blue symbols): BSEP, bile salt export pump; MDR, multidrug resistance protein; MRP, multidrug resistance–associated protein; ABCG5/8; BCRP, breast cancer resistance protein; Ostα/Ostβ. Uptake transporters (red symbols): ASBT, apical sodium dependent bile salt transporter; NTCP, sodium taurocholate cotransporting polypeptide; OATP, organic anion-transporting polypeptide; OCT, organic cation transporter; OAT, organic anion transporter.

Hepatic Transport Systems

Properties and Function of Basolateral (Sinusoidal) Transporters.

Sodium-dependent and sodium-independent transport pathways have been identified to play a key role in hepatic uptake of endogenous and exogenous substances from sinusoidal blood plasma (Fig. 1). The sodium-dependent pathway is represented by the sodium taurocholate cotransporting polypeptide NTCP (SLC10A1) (reviewed by Hagenbuch and Dawson5), the substrate specificity of which is essentially limited to conjugated bile salts and certain sulfated steroids. NTCP accounts for more than 80% of conjugated (i.e., taurocholate and glycocholate) but less than 50% of unconjugated (i.e., cholate) bile salt uptake.5 In contrast, the sodium-independent pathway is represented by different members of the superfamily of organic anion-transporting polypeptides (OATP/SLCO) (reviewed by Hagenbuch and Meier6). In human liver, the highest expressions are found for OATP1B1 (SLCO1B1) and its 80% sequence homologue OATP1B3 (SLCO1B3), both of which are predominantly if not exclusively expressed in the liver. With the exception of OATP2B1 (SLCO2B1), the substrate specificity of which seems to be limited to bromosulphophtalein and steroid sulfates, OATP1A2 (SLCO1A2), OATP1B1, and OATP1B3 exhibit overlapping transport activities for conjugated and unconjugated bile salts, bromosulphophtalein, neutral steroids, steroid sulfates and glucuronides, and selected organic cations.6 Furthermore, numerous drugs are substrates of OATPs, including the antihistamine fexofenadine, opioid peptides, digoxin, the HMG CoA-reductase inhibitor pravastatin, the angiotensin-converting enzyme inhibitor enalapril, and the antimetabolite methotrexate.6 In addition, OATP1B1 and OATP1B3 mediate the uptake of the hepatotoxins phalloidin and microcystin into human liver,7–9 while hepatic uptake of amanitin, the most dangerous natural toxins causing hepatic failure seems to be exclusively mediated by OATP1B3.10

In addition, the sodium-independent uptake systems involve the organic anion and organic cation transporter family of solute carriers (SLC22), belonging to a gene family separate from the OATPs. OAT2 (SLC22A7) is the only transporter of the OAT/OCT family expressed in human liver and is believed to be liver-specific (reviewed by Koepsell and Endou11). OCT1 (SCL22A1) is expressed in human liver as well as in kidneys, small intestine and colon.11 In contrast to hepatic OATPs the exact role of OAT2 and OCT1 for hepatic uptake of drugs and bile constituents remains to be established.

In addition to these uptake systems, the basolateral hepatocyte membrane also localizes several adenosine triphosphate (ATP)-dependent efflux pumps. These transporters belong to the family of multidrug resistance–associated proteins (MRPs) (ABCC), which are multispecific transporters for different organic anions (reviewed by Homolya et al.12). Among the MRP family of ATP-binding cassette (ABC) transporters, MRP1 (ABCC1), MRP3 (ABCC3), MRP4 (ABCC4), MRP5 (ABCC5), and MRP6 (ABCC6) have been implicated in the cellular efflux of drug-glutathione, -glucuronide, and -sulfate conjugates (MRP1); the efflux of bile salts (MRP3); and the transport of nucleoside analog drugs such as zidovudine, lamivudine, and stavudine (MRP4) and cyclic adenosine and guanosine monophosphate, as well as methotrexate and the purine analogs 6-mercaptopurine and 6-thioguanine (MRP4 and MRP5).13 The physiological substrates of MRP6 are yet unknown.

Regulation of Basolateral Transporters.

Hepatocellular transport systems are subject to extensive transcriptional and posttranscriptional regulation, allowing adaptational changes in response to the intracellular accumulation of bile salts (reviewed by Eloranta and Kullak-Ublick14 and Trauner and Boyer15). During cholestasis, the sodium taurocholate–cotransporting polypeptide NTCP is suppressed through farnesoid X receptor (FXR)-mediated induction of small heterodimeric partner 1, thereby preventing the hepatocyte from further accumulating toxic bile salts.16, 17 Similarly, the expression of OATP1B1 is downregulated during cholestasis through bile acid–mediated activation of small heterodimeric partner 1, which leads to a repression of hepatocyte nuclear factor 1α, the major transcriptional activator of OATP1B1.18, 19 In contrast, cholestasis leads to an FXR-mediated activation of hepatic OATP1B3,20 which might constitute an escape mechanism promoting the hepatocellular clearance of xenobiotics during cholestasis. On the posttranscriptional level, sodium-dependent and -independent hepatocellular uptake systems are mainly regulated by cyclic adenosine monophosphate–mediated dephosphorylation processes, which is controlled by phophoinositide-3-kinase/protein kinase B.21–23 Furthermore, PDZK1 was demonstrated to be a critical determinant for the proper subcellular localization and function of rat Oatp1a1.24

The transcriptional regulation of basolaterally expressed MRPs is not fully elucidated. Studies in mice support the notion that Mrp3 and Mrp4 are induced through a pregnane X receptor–mediated pathway.25

Properties and Function of Apical (Canalicular) Transporters.

The secretion of bile salts and xenobiotics across the canalicular membrane of hepatocytes is mediated by various ABC transporters (Fig. 1). With the exception of FIC1 (ATP8B1), which is thought to play a role in the regulation of the enterohepatic bile acid pool and in the elimination of hydrophobic substances from the enterohepatic circulation,26 canalicular transporters involved in bile formation and hepatic drug clearance belong to different members of the superfamily of ABC transporters. These include members of the family of multidrug resistance (MDR) P-glycoproteins such as MDR1 (ABCB1), MDR3 (ABCB4), and the bile salt export pump (BSEP) (ABCB11). In addition, the canalicular membrane localizes the multidrug resistance–associated protein 2 (MRP2) (ABCC2) and the ABC half transporters breast cancer resistance protein (BCRP) (ABCG2) and the cholesterol flippase ABCG5 and ABCG8 (ABCG5 and ABCG8).

Within the family of multidrug resistance proteins, BSEP and MDR3 are two highly conserved members, which are involved in the secretion of cholephilic compounds from the liver cell into the bile canaliculus (reviewed by Kullak-Ublick27 and Meier and Stieger28). BSEP constitutes the predominant bile salt efflux system of hepatocytes and mediates the cellular excretion of numerous conjugated bile salts such as taurine- or glycine-conjugated cholate, chenodeoxycholate, and deoxycholate.29, 30 MDR3 was shown to function as an ATP-dependent phospholipid flippase, translocating phosphatidylcholine from the inner to the outer leaflet of the canalicular membrane (reviewed by van Erpecum31). Canalicular phospholipids are then solubilized by canalicular bile salts to form mixed micelles, thereby protecting cholangiocytes from the detergent properties of bile salts. In addition to these processes, MRP2, the only canalicular member of the multidrug resistance–associated protein family, mediates the canalicular transport of glucuronidated and sulfated bile salts. MRP2 is the main driving force for bile salt–independent bile flow through canalicular excretion of reduced glutathione. Furthermore, MRP2 transports a wide spectrum of organic anions, including bilirubin diglucuronide, glutathione conjugates, leukotriene C4, and divalent bile salt conjugates, as well as drug substrates such as cancer chemotherapeutic agents, uricosurics, and antibiotics (reviewed by Borst et al.32).

The exact contribution of MDR1 to hepatic bile formation remains to be established, but it is thought to contribute to the canalicular excretion of drugs and other xenobiotics into bile. Its broad substrate specificity and its physiological expression in various tissues with excretory and protective functions make MDR1 one of the major determinants of drug disposition and toxicity. Substrates are neutral and positively charged organic compounds and include various chemotherapeutic and immunosuppressant agents, antiarrhythmic drugs, HIV protease inhibitors, and antifungals.33, 34

The ABC half transporter breast cancer resistance protein BCRP (ABCG2) shows the highest expression levels in mammary epithelium and placenta, where it plays an important role in conferring a multidrug resistance phenotype against a variety of xenobiotics. Recently, BCRP has been shown to transport sulfated bile salt conjugates such as taurolithocholate sulfate in vitro35 and to be involved in the biliary excretion of drugs such as pitavastatin.36 It might therefore be speculated that BCRP contributes to the hepatocellular excretion of bile salts and xenobiotics. Furthermore, the heterodimeric transporter ABCG5/ABCG8 (ABCG5 and ABCG8) has been identified as the apical transport system involved in the hepatobiliary excretion of plant sterols and cholesterol (reviewed by Klett and Patel37 and Kosters et al.38). Overexpression of ABCG5/ABCG8 in transgenic mice led to an increase in biliary cholesterol secretion and a reduced intestinal absorption of dietary cholesterol, providing strong evidence for ABCG5/ABCG8 being involved in hepatocellular secretion and intestinal efflux of cholesterol.37 However, the possible role of these ABC half transporters for hepatic drug clearance and the development of drug-induced cholestasis remain to be determined.

Regulation of Canalicular Transporters.

Transcriptional regulation of BSEP and MDR3 is mediated by FXR.39–41 FXR-mediated activation of BSEP and MDR3 leads to increased bile salt efflux and the formation of mixed micelles in the biliary tree during cholestatic episodes, thereby preventing toxic effects of bile salts on hepatocytes and cholangiocytes. In addition, FXR has been shown to induce MRP2 expression in human hepatocytes, which might constitute another compensatory mechanism during cholestasis.42 In contrast, MDR1 is upregulated via the pregnane X receptor, which in addition to endogenous ligands is activated by different xenobiotics such as rifampicin or St. John's Wort.43–45 This pathway is part of a general cellular detoxification mechanism, because MDR1 is the key transporter protein involved in the cellular efflux of numerous drugs and xenobiotics. Transcriptional regulation of BCRP most likely involves the aryl hydrocarbon receptor and the epidermal growth factor, whereas ABCG5 and ABCG8 are direct targets of the oxysterol-dependent liver X receptor α and β.46

Posttranscriptional targeting of Bsep, Mdr2 (the rat homologue of human MDR3), and Mrp2 to the canalicular membrane47–49 is mediated by phophoinositide-3-kinase and protein kinase C isoforms.

Bile Ductular Transporters

Properties and Function of Basolateral and Apical Cholangiocyte Transporters.

Uptake of bile salts from canalicular bile into cholangiocytes is mediated by the apical sodium-dependent bile salt transporter (SCL10A2) (Fig. 1). The apical sodium-dependent bile salt transporter belongs to the superfamily of solute carriers and is identical with the gene product expressed in the terminal ileum of the small intestine.5 Furthermore, the uptake of bile salts involves the organic anion-transporting polypeptide 1A2 (OATP1A2), which belongs to the OATP superfamily of sodium-independent solute transporters (SCLO; former nomenclature SLC21).6

After their uptake into cholangiocytes, bile salts are effluxed at the basolateral cholangiocyte membrane into the peribiliary plexus via an anion exchange mechanism.50 From here, bile salts reach the portal circulation and undergo the cholehepatic shunt pathway. MRP3, a basolaterally expressed member of the family of multidrug resistance–associated proteins contributes to the efflux of bile salts from cholangiocytes.51 Moreover, MRP2 was recently localized in gallbladder-derived biliary epithelial cells, where it might contribute to taurocholate homeostasis.51 In addition, a splicing variant of rat apical sodium-dependent bile salt transporter could be localized to the basolateral membrane of cholangiocytes, where it is proposed to function as a bile salt efflux protein. However, the contribution of this truncated protein to bile salt efflux in human cholangiocytes has not been established.52 Furthermore, the heterodimeric organic solute transporter OSTα/OSTβ was recently found to be expressed in the basolateral membrane of cholangiocytes, where it is thought to contribute to bile acid and sterol transport into the peribiliary plexus.53


ATP, adenosine triphosphate; MRP, multidrug resistance–associated protein; ABC, ATP-binding cassette; FXR, farnesoid X receptor; MDR, multidrug resistance; BSEP, bile salt export pump; BCRP, breast cancer resistance protein.

Role of Hepatocellular Transporter Systems in Drug-Induced Cholestasis

Animal models and in vitro studies of drug-induced liver injury reveal different carrier-related mechanisms that might be relevant to the development of hepatic damage (Fig. 2). Basolateral transport processes play an important role in controlling hepatic exposure to drugs and hepatotoxin and hence determine the amount of drug and drug metabolites reaching the canalicular membrane. Accordingly, increased hepatic uptake of xenobiotics might be associated with hepatic damage. On the apical hepatocyte membrane, decreased canalicular transporter function could lead to intracellular accumulation of bile constituents and xenobiotics with consecutive toxic liver cell damage.54 Furthermore, there is speculation as to whether bile ductular reabsorption of bile salts and drugs and cholehepatic shunting could result in changes in bile composition and promote hepatic drug accumulation.

Figure 2.

Transporter-related mechanisms in the development of liver injury. Red and green fonts designate potentially harmful or protective transporter related mechanisms, respectively.

However, in many cases of drug-induced cholestasis, altered transporter function coexists with other mechanism of liver injury such as immunoallergic reaction. Circulating antigen–antibody complexes in immunoallergic hepatic insult are known to injure several hepatic structures. For instance, in ductular forms of drug-induced liver injury such as vanishing bile duct syndrome, a drug or drug metabolite is thought to trigger immune response against biliary epithelium.55 It might also be speculated that immunoallergic reactions might indirectly alter bile formation through impairment of hepatocellular transporter function. In the latter case, the functional state of hepatocellular transporter proteins might be essential to determine the susceptibility to such superimposed damage.

Environmental Risk Factors

Drug–Transporter Interactions.

The possible impact of a drug-mediated inhibition of basolateral transport processes for the development of cholestasis has not yet been studied in detail. A study investigating the effect of rifampin on basolateral OATP function could demonstrate a significant inhibition of transport activity only for OATP1B3, whereas OATP1A2, OATP1B1, and OATP1B2 function is unaffected.56 No data are available on a possible drug-mediated functional alteration of other basolaterally expressed transport systems. However, there is speculation that inhibition of OATP-mediated uptake of hepatotoxins such as amanitin, microcystin, and phalloidin could prevent toxic liver injury. For instance, OATP1B3-mediated uptake of amanitin was inhibited in transected MDCKII cells by OATP1B3 substrates and inhibitors such as cyclosporine and rifampicin as well as by some antidotes used in the past for the treatment of human amatoxin poisoning.10 Inhibition of hepatocellular uptake transport might therefore constitute a promising approach for the treatment of human poisoning with certain hepatotoxins.

On the canalicular side, various drugs are thought to cause cholestasis through inhibition of BSEP function (Fig. 3). In vitro studies with rat Bsep revealed that cyclosporine, rifampin, bosentan, troglitazone, and glibenclamide all inhibit ATP-dependent taurocholate transport.57–59 Whereas most of these drugs directly cis-inhibit BSEP function in a competitive manner, estradiol 17β-glucuronide and progesterone metabolites indirectly trans-inhibit Bsep after secretion into the bile canaliculus by Mrp2.57 On the other hand, verapamil, cyclosporine, and vinblastine showed in vitro transport by MDR3, which could potentially lead to concentration-dependent inhibition of phospholipid flippase activity.60 An additional mechanism of cholestatic cell damage was recently proposed by Fouassier et al.,61 who investigated the effect of bosentan on Mrp2-mediated canalicular bile formation. It could be shown that bosentan stimulates and significantly increases Mrp2-dependent bilirubin excretion and bile salt–independent bile flow, while biliary lipid secretion was profoundly inhibited and uncoupled from bile salt secretion. Inhibition of biliary lipid secretion was not seen in Mrp2-deficient TR rats, which suggests that translocation of organic anions across the canalicular membrane is a prerequisite for the occurrence of the uncoupling effect. Thus, Mrp2-induced choleresis might dilute bile salts in the bile canaliculi below the concentration required for solubilization of phosphatidylcholine and cholesterol. Consequently, decreased biliary phospholipid secretion does not necessarily mean a defect in the canalicular phospholipid flippase MDR3 but could also be explained by a physicochemical disequilibrium in bile composition.

Figure 3.

Mechanisms of BSEP inhibition. Abbreviations: BSEP, bile salt export pump; MRP2, multidrug resistance–associated protein 2.

In contrast, data supporting a contribution of MDR1 to hepatic drug clearance are not yet available. However, such a contribution could be suspected based on the important role of MDR1 as a source of many clinically relevant drug interactions in other tissues with excretory function, such as small intestinal enterocytes or the proximal tubulus cells of the kidney.62 Because MDR1 transports numerous drugs with hepatotoxic potential such as amiodarone, HIV protease inhibitors, or chemotherapeutics,34 it can be speculated that altered hepatic MDR1 function at the canalicular membrane might contribute to decreased biliary elimination of xenobiotics, thereby promoting hepatotoxicity.

In addition to these hepatic sources of drug toxicity, it is conceivable that drug reabsorption by cholangiocytes and cholehepatic shunting contributes to the development of hepatic damage. For instance, the nonsteroidal anti-inflammatory drug sulindac was shown to induce hypercholeresis in rats, a phenomenon that is explained by cholehepatic shunting.63 The resulting hepatic accumulation of sulindac and the inhibition of canalicular bile salt transport might contribute to the potential of this drug to produce cholestasis.64, 65

Age and Sex.

It is well established that age over 50 years and female sex are associated with an increased risk to develop drug-induced hepatic damage, and it was therefore speculated as to whether these factors affect the expression of hepatocellular transporters. A recent study investigating the extent of interindividual variability in the canalicular expression of BSEP, MDR3, MRP2, and MDR1 in over 100 samples of human liver tissue could demonstrate significant interindividual variability of canalicular transporter expression, with 15%-20% of individuals being classified as low or very low expressers of at least one of the investigated proteins.66 However, this variability could not be related to demographic data such as age or sex. Differences in the susceptibility to develop drug-induced cholestasis could therefore not be related to age and sex differences in baseline expression levels of canalicular transporter proteins.

Preexisting Liver Disease.

It is well known that preexisting liver disease is associated with a worse outcome of drug-induced hepatotoxicity. However, it is still controversial whether preexisting liver disease is also a susceptibility factor for the development of hepatotoxic drug side effects. There is indication that underlying liver pathology might be associated with altered expression and function of canalicular transporter proteins. For instance, cholestatic alcoholic hepatitis leads to reduced hepatic messenger RNA and protein expression levels of NTCP and BSEP, whereas messenger RNA levels of MRP2, MRP3, MDR1, and MDR3 remain unchanged. In contrast, early stages of primary biliary cirrhosis (stages I and II) and mild cholestasis were not associated with changes in hepatocellular transporter expression.66–68 The impact of such disease-associated adaptive changes in hepatocellular transporter expression on cholestatic drug side effects has not yet been investigated; however, it can be speculated that downregulation of BSEP in certain pathological conditions constitutes an additional risk factor for the development of drug-induced cholestatic liver injury.

Genetic Risk Factors

In addition to these environmental risk factors, genetics are a major determinant of hepatocellular transporter function. Only limited information is so far available on the functional consequences of genetic variation in basolateral transporter systems. Tirona et al.69 identified a total of 14 nonsynonymous SLC1B1 SNPs encoding OATP1B1 in a population of African Americans and European Americans, 6 of which exhibited reduced in vitro uptake of the OATP1B1 substrates estrone-3-sulfate and estradiol-17β-glucuronide. OATP1B1 genetic variants have also been associated with interindividual differences in hepatic disposition of pravastatin and irinotecan, respectively.70–73 Although the impact of these observations for the development of cholestasis remains to be studied, these data indicate that polymorphic OATP1B1 function and expression are a determinant of hepatic exposure to OATP1B1 substrates. High OATP expression phenotypes might therefore constitute a risk factor for the development of hepatotoxicity due to high hepatocellular uptake of drugs and hepatotoxins, whereas low expression phenotypes should be protective.

On the canalicular side, mutations in the ABCB11 and ABCB4 genes encoding BSEP and MDR3 are a well-established cause of inherited cholestatic syndromes such as progressive and benign forms of familial cholestasis.74–76 Furthermore, mutations and polymorphisms in these two genes have been associated with intrahepatic cholestasis of pregnancy and might contribute to the individual risk to develop primary biliary cirrhosis and primary sclerosing cholangitis.77–80 Patients with intrahepatic cholestasis of pregnancy or benign forms of familial cholestasis were occasionally reported to exhibit increased susceptibility to certain drugs. For instance, increased susceptibility to oral contraceptives or postmenopausal hormone replacement therapy is a frequent phenomenon in patients with intrahepatic cholestasis of pregnancy,78, 81 while different anti-inflammatory drugs were suspected to induce cholestatic episodes in a patient with benign recurrent intrahepatic cholestasis.74 These observations favor the concept that a genetically determined canalicular transporter deficiency is the common pathophysiological denominator for the development of cholestasis under different extrinsic and intrinsic challenges in affected patients. In addition to such disease-causing mutations, a frequent polymorphism in ABCB11 has recently been associated with a threefold increased risk to develop cholestatic side effects under treatment with different drugs such as β-lactam antibiotics, oral contraceptives, psychotropic drugs, and proton pump inhibitors.82 Interestingly, this polymorphism, which leads to a valine-to-alanine exchange at the highly conserved position 444 of the BSEP protein, has also been associated with decreased hepatic BSEP content in human liver tissue samples, offering a mechanistic explanation for this observation.66 It can therefore be speculated that ABCB11 and ABCB4 genetic variants found to be associated with different cholestatic conditions also predispose to the occurrence of cholestasis under treatment with certain drugs.

Mutations in ABCC2 encoding MRP2 results in the Dubin-Johnson syndrome, a disease characterized by conjugated hyperbilirubinemia. Overt jaundice was observed in patients with subclinical Dubin-Johnson syndrome under treatment with oral contraceptives,83 probably due to a metabolite-induced competitive inhibition of bilirubin diglucuronide excretion. As for BSEP and MDR3, a genetically determined MRP2 deficiency syndrome therefore also predisposes to hepatic drug side effects. No association was found between the presence of two frequent ABCC2 polymorphisms and the susceptibility for cholestatic drug side effects (unpublished data). Such an association was suspected based on the observation that hepatic MRP2 expression was significantly influenced by these ABCC2 polymorphisms in healthy liver tissue.66

Functionally relevant genetic polymorphisms affecting drug disposition have also been described for ABCB1 encoding MDR1. For instance, decreased MDR1 function has been found with a synonymous polymorphism in exon 26 (C3435T). Homozygous carriers of this polymorphism, which is linked to a nonsynonymous polymorphism in exon 21 (G2677T), were initially shown to exhibit increased plasma levels of the MDR1 substrate digoxin.84, 85 Furthermore, the C3435T polymorphism could be associated with differences in treatment outcome in HIV infection and acute lymphatic leukemia.86–88 However, study results remain controversial, because some investigators found increased MDR1 expression and function in carriers of the variant alleles in positions 3435 and 2677,89 whereas others failed to show functional differences between the two groups.90, 91 Therefore, the impact of these polymorphisms on the canalicular excretion of drugs and drug metabolites remains to be delineated.

Recent studies have also identified frequently occurring nonsynonymous polymorphisms in the ABC half transporter genes ABCG5/ABCG8 and ABCG2. Although some of these variants result in significantly altered transport capacity and substrate handling of the encoded proteins, a possible role of these polymorphisms for hepatic drug handling and the development of liver injury has not been studied.92, 93


From the examples delineated in this article, it is apparent that drug-induced dysfunction of bile salt transporters is one important cause of drug-induced cholestatic side effects. In addition to the direct or indirect inhibition of canalicular transporter function by different drugs, it has become increasingly evident that genetics are a major determinant of hepatocellular transporter function and expression, thereby determining an individual's susceptibility to develop cholestasis. The challenge in the future will consist in integrating these environmental and genetic determinants of drug toxicity in a comprehensive system, allowing the cautious use of problematic drugs in susceptible patients.