Thiazolidinedione Bioactivation:  A Comparison of the Bioactivation Potentials of Troglitazone, Rosiglitazone, and Pioglitazone Using Stable Isotope-Labeled Analogues and Liquid Chromatography Tandem Mass Spectrometry

Troglitazone, a thiazolidinedione (TZD) type insulin sensitizer for the treatment of diabetes, was withdrawn from the U.S. market after several fatal cases of hepatotoxicity. Although the mechanism(s) of these idiosyncratic adverse reactions are not completely understood, circumstantial evidence suggests at least a partial contribution of reactive metabolite formation. Despite isolated case reports of hepatotoxicity, the other TZD derivatives pioglitazone and rosiglitazone are comparatively safe. Herein, we report on the bioactivation potential of these drugs and their TZD ring isotope-labeled 2-15N-3,4,5-13C3 analogues in rat and human liver microsomes supplemented with glutathione (GSH). Screening for GSH adducts as surrogate markers for reactive intermediate formation was performed by liquid chromatography tandem mass spectrometry. Chemical characterization of the GSH conjugates was conducted by acquisition of their respective product ion spectra and the comparison between unlabeled and stable isotope-labeled TZD derivatives. The data suggest that all drugs undergo bioactivation processes via a common metabolic activation on the TZD ring, yielding disulfide type GSH conjugates as evidenced by the loss of labeled positions in the TZD moiety. Additional bioactivation processes leading to GSH adducts not involving TZD ring scission were evident for troglitazone. In human liver microsomes at low substrate concentrations, only troglitazone yielded a predominant GSH adduct not involving TZD ring scission. This property may contribute, together with other factors such as the relatively high dose administered as well as its potential to induce hepatic cholestasis and oxidative stress, to the hepatotoxicity of this drug.