Enzyme-Induction Dependent Bioactivation of Troglitazone and Troglitazone Quinone In Vivo

Abstract

Troglitazone (TGZ), a 2,4-thiazolidinedione antidiabetic, causes hepatotoxicity in 1.9% of patients. TGZ is an inducer of, and substrate for, hepatic P450 3A. Microsomal metabolism yields a benzoquinone (TGZQ) and reactive intermediates. Kassahun et al. [Kassahun et al. (2001) Chem. Res. Toxicol. 14, 62-70] have trapped the intermediates as thioester, thioether, and disulfide conjugates of glutathione and found five conjugates in rat bile. The thioether was substituted in the chromane moiety. We have investigated the effect of the P450 3A inducer, dexamethasone (DEX), on metabolism of TGZ and TGZQ in rats and assessed the compounds' cytotoxicity. TGZ-glucuronide and sulfonate were confirmed as principal biliary metabolites of TGZ (50 mg/kg, iv). Bile from noninduced animals also contained a TGZ-glutathione thioether adduct (ML3) but it was substituted in the thiazolidinedione moiety. Pretreatment with DEX (50 mg/kg/day for 3 days) resulted in a 2-5-fold increase in the biliary concentration of ML3 and a 2-fold increase in the concentration of TGZQ, which was commensurate with the induction of hepatic P450 3A. Three of the known glutathione-conjugated metabolites were also found. TGZQ (50 mg/kg, iv) was metabolized to an analogue of one of the TGZ-glutathione thioesters and a glutathione adduct of TGZQ hydroquinone after DEX pretreatment. TGZ quinol glucuronide was a biliary metabolite of TGZ and TGZQ. Its formation would represent deactivation of TGZQ. TGZ was toxic to rat hepatocytes and Hep-G2 cells at concentrations exceeding 50 and 25 microM, respectively, after 24 h. In contrast, TGZQ was nontoxic to rat hepatocytes and toxic to Hep G2 cells only at concentrations exceeding 100 microM. Our results show that TGZQ as well as TGZ yields reactive metabolites in vivo, and that bioactivation is enhanced by induction of P450 3A. However, hepatotoxicity is unlikely to be due to either TGZQ or its metabolites.

Metabolic Activation of Troglitazone: Identification of a Reactive Metabolite and Mechanisms Involved

“…5. This adduct may be the same as ML3 or MET1/2, which was reported to be the major adduct formed in rats or in human hepatocytes, as judged based on the similarity of the MS fragmentation pattern (Tettey et al, 2001;Prabhu et al, 2002). Collectively, the major TGZ-GSH adduct is determined to be 5-glutathionyl-5-[4-(6-hydroxy-2,5,7,8-tetramethylchroman-2-ylmethoxy)benzyl]-thiazolidine-2,4-dione.…”

“…Tettey et al (2001) and Prabhu et al (2002) reported a major GSH adduct of TGZ formed in rats (ML3) and in human hepatocytes (MET1/2) with a mass spectral pattern similar to that of M5, but the assignment of the position attached to GSH was different. The structure of M5 proposed by Kassahun et al (2001) resulted from the addition of GSH to an o-quinone methide derivative of TGZ.…”

“…The structure of M5 proposed by Kassahun et al (2001) resulted from the addition of GSH to an o-quinone methide derivative of TGZ. However, the structures of ML3 or MET1/2 proposed by Tettey et al (2001) and Prabhu et al (2002) resulted from the addition of the GSH to either the thiazolidinedione ring or the adjacent methylene bridge. The exact structure of the major GSH conjugate of TGZ has yet to be determined.…”

Troglitazone and liver injury: In search of answers

“…43 Likewise, the quinone M3 was not toxic to cultured rat hepatocytes even at concentrations of 100 mol/L (approximately 50-fold the liver concentration). 67 …”

“…42,43 However, in susceptible individuals, M3 conceivably could react with proteins, RNA, or DNA to induce liver injury. Also, M3-derived reactive intermediates are covalently bound to microsomal protein and glutathione, 67,161 but the significance of these adducts in the pathogenesis of TGZ-associated hepatoxicity is unknown.…”