Metabolic and non-metabolic factors determining troglitazone hepatotoxicity: a review.

Troglitazone (TGZ), a thiazolidinedione class of antidiabetic agent, causes serious idiosyncratic hepatotoxicity. TGZ is metabolized into reactive metabolites that covalently bind to cellular macromolecules, one of which is oxidation at the chromane ring, a unique structure of TGZ, and another involves oxidative cleavage of the thiazolidinedione ring, a structure common to less hepatotoxic antidiabetics, rosiglitazone and pioglitazone. TGZ is cytotoxic to HepG2 cells and rat and human hepatocytes. However, the role of the reactive metabolite on the TGZ toxicity is controversial, because there was no correlation of the generation of the reactive metabolites with susceptibility to the TGZ cytotoxicity, and chemical inhibitors of drug metabolizing enzymes could not protect the cells against the toxicity. Mitochondrial dysfunction, especially mitochondrial permeability transition, may be a pathophysiological event, which is mediated by TGZ itself and is a major non-metabolic factor. Other events such as apoptosis and PPARgamma-dependent steatosis could be also mediated by TGZ, while inhibition of bile salt export pump, a cause of TGZ-induced cholestasis, may be caused by the TGZ sulfate. In conclusion, although the TGZ is biotransformed into chemically reactive metabolites, there is currently no potential evidence for involvement of the reactive metabolite in the TGZ-induced liver injury.