Genetic Determinants of Drug-induced Cholestasis and Intrahepatic Cholestasis of Pregnancy

Christiane Pauli-Magnus, M.D.; Peter J. Meier, M.D.; Bruno Stieger, Ph.D.

Disclosures

Semin Liver Dis. 2010;30(2):147-159. 

In This Article

Pathophysiology of Drug-induced Cholestasis

Prior to their adverse action on hepatocytes, drugs need to be taken up into the cells. A large variety of drugs is entering hepatocytes via the OATPs expressed in the basolateral hepatocyte membrane.[10,26,27] It was recently found that the transport activity of OATPs may be directly modulated by physiologic substrates such as prostaglandins[128] or estrone-3-sulfate,[129] as well as the drug clotrimazole[130] or the drug metabolite estradiol-17β-glucuronide.[131] Such interactions may potentially lead to different intracellular drug concentrations at comparable serum levels.

Unfortunately, the underlying pathogenetic mechanisms of drug-induced liver injury often remain enigmatic. After the cloning of rat Bsep, it could be directly demonstrated that drugs known to be leading to cholestatic liver injury, such as cyclosporine,[132,133] are competitive inhibitors of Bsep. Hence, this mechanism is the likely cause of cholestasis from drugs such as cyclosporine, rifamycin SV, rifampicin, and glibenclamide.[37,44] The Ki values of Bsep inhibition in the Sf9 cell expression system compare favorably with the Km values obtained in isolated rat liver canalicular plasma membrane vesicles.[37] Such inhibition of Bsep leads to intracellular retention of bile salts in hepatocytes, which at elevated concentrations are cytotoxic to hepatocytes.[134] Bosentan is a dual endothelin receptor antagonist, which is pharmacologically active together with one of its main metabolites. Bosentan elimination is via the biliary route predominantly. Bosentan and its metabolite enter hepatocytes by OATP1B1- and OATP1B3-mediated transport.[135] In clinical trials, it was found that bosentan caused asymptomatic, reversible transaminase elevations in some patients.[136] The incidence of bosentan-induced liver injury was dose-dependent and increases in plasma bile salt levels of affected individuals correlated with the administered dose of bosentan. Furthermore, individuals, who were taking glyburide together with bosentan, showed a higher incidence of liver injury than patients with a bosentan monotherapy. Experiments with rat and human BSEP expressed in Sf9 cell vesicles identified bosentan as a competitive inhibitor of BSEP.[45,136] Rats treated with bosentan displayed an elevation of plasma bile salt levels, which further increases upon coadministration of glibenclamide.[136] Hence, as serum bile salt levels in patients positively correlated with the bosentan dose and as the serum liver parameters after stopping of bosentan spontaneous normalized, it can be concluded that bosentan acts as a competitive BSEP inhibitor. This inhibition of BSEP seems to be rather specific, as no elevation of serum bilirubin was observed.[136] Most interestingly, in a follow-up investigation of the cholestatic mechanism of bosentan in rats it was found that contrary to the expectations bosentan leads to a stimulation of bile flow.[137] The increased bile flow was not caused by an increased bile salt output, but was associated with an increased glutathione and bicarbonate secretion. This stimulation of bile flow was not observed in TR rats, which lack functional Mrp2. Hence, bosentan not only directly affects the function of Bsep as a competitive inhibitor, but also exerts indirect effects, which depend on Mrp2. In vitro characterization of rat and human Mrp2/MRP2 as well as Bsep/BSEP expressed in Sf9 cells confirmed the inhibition by bosentan of both isoforms of Bsep/BSEP.[138] Furthermore, this study demonstrated a direct stimulation of Mrp2/MRP2 transport activity by bosentan. This finding most likely presents the molecular explanation for the observed increase of bile salt-independent bile flow in rats. Stimulation of MRP2 activity is not unique for bosentan.

Other examples include sulfinpyrazone, penicillin G, or indomethacin.[139] The consequence of drug-induced activation of MRP2 may be a lowering of the bile salt concentration in the canaliculus below a (yet unknown) threshold value followed by an alteration in canalicular phospholipid and cholesterol secretion.[140] Taken together, BSEP can either be inhibited directly by drug from the cytoplasm or indirectly, most probably from the canalicular side. This latter process seems to need the presence of functional MRP2.

In addition, indirect mechanisms of BSEP inhibition requiring MRP2 have been described, such as for estradiol-17β-glucuronide, bosentan, and for the HER1/HER2 inhibitor PKI166.[37,138,141] In cases where the acquired liver disease is caused by bland cholestasis, this process is rapidly reversible upon discontinuation of the drug, as illustrated for bosentan.[136]

Taken together, many drugs as well as endogenous steroid metabolites have the potential to interfere with transport activity of BSEP. As this includes direct and indirect inhibition of BSEP as well as regulation of its carrier density in the canalicular membrane, the actual mechanism of drug-induced reduction of BSEP activity may be complex for a given substance.

Troglitazone is a drug, which was withdrawn from the market due to its hepatotoxicity. The exact molecular mechanisms of its toxicity remains somewhat enigmatic, but a consensus has emerged that troglitazone is mainly toxic to mitochondria.[142,143] In addition to its direct adverse action on mitochondria, troglitazone administration leads to an acute reduction to bile flow in rats. Hence, troglitazone is also a cholestatic drug.[144] Troglitazone is mainly metabolized into troglitazone sulfate in rats, which is subsequently excreted into bile.[145] Both the parent compound and its sulfated metabolite are competitive inhibitors of Bsep in rat canalicular plasma membrane vesicles. Recently, troglitazone was also demonstrated to be an inhibitor of dog and human BSEP.[146,147] Taken together, troglitazone can negatively impact mitochondria via direct toxicity as well as by inhibiting BSEP, which in turn leads to an accumulation of bile salts in hepatocytes. By themselves, they are at elevated intracellular concentrations toxic to mitochondria.[134] Often, drug-induced liver injury results not only in cholestatic but in mixed (hepatocellular and cholestatic) liver injury.[117] A study investigating a potential class effect of thiazolidinediones on Bsep found that both rosiglitazone and ciglitazone are inhibitors of ATP-dependent taurocholate transport into rat canalicular plasma membrane vesicles.[147] This is a strong indication that the toxicity of troglitazone requires multiple mechanisms for exerting its cholestatic potential.

In summary, drug-induced cholestatic liver injury is a complex pathophysiologic entity including both direct and indirect effects of BSEP inhibition.

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