In Vitro Metabolic Activation of Thiabendazole via 5-Hydroxythiabendazole: Identification of a Glutathione Conjugate of 5-Hydroxythiabendazole

Abstract

Thiabendazole (TBZ) (Fig. 1) is a broad-spectrum antihelmintic used for treatment of parasitic infections in animals and humans and an agricultural fungicide for postharvest treatment of fruits and vegetables (Walton et al., 1999;Groten et al., 2000). It is recognized as a potent nephrotoxin that causes tubular necrosis, leading to severe kidney damage and a teratogen that results in impairment of mouse limb development and selective toxicity to the embryo (Ogata et al., 1984;Mizutani et al., 1990;Tada et al., 1992;Fujitani et al., 1999). Irreversible binding of TBZ-related radioactivity to macromolecules and tissue protein in the mouse embryo has also been observed after oral dosing of [ 14 C]TBZ to pregnant mice (Yoneyama and Ichikawa, 1986). In humans, several isolated cases of hepatotoxicity, such as intrahepatic cholestasis or micronodular cirrhosis, have also been reported after TBZ administration (Manivel et al., 1987;Bion et al., 1995). It is currently believed that TBZ-induced nephrotoxicity is a result of P450-dependent oxidative cleavage of the thiazole moiety in TBZ to a proximate toxicant, thioformamide ( Fig. 1) (Mizutani et al., 1994). However, the mechanisms responsible for TBZ-induced hepatotoxicity in humans and teratogenicity in preclinical species are not clear.TBZ is extensively metabolized in animals and humans (Tocco et al., 1966). The primary route of metabolism is the CYP1A2-catalyzed formation of 5-hydroxythiabendazole (5-OHTBZ), which is further converted to a glucuronide and a sulfate conjugate and is eliminated in the urine and bile (Wilson et al., 1973;Rey-Grobellet et al., 1996; Coulet et al., 1998a,b). Other metabolites such as 4-hydroxythiabendazole, 2-acetylbenzimidazole, N-methylthiabendazole, and benzimidazole have also been detected (Fujitani et al., 1991). Recent studies by Coulet et al. (2000) have shown good correlation in the covalent binding of the radiolabel to proteins and the formation of 5-OHTBZ in incubations of [ 14 C]TBZ with cultured human and rabbit hepatocytes. These studies have also shown that treatment of CYP1A2-expressing liver cells with isolated [14 C]5-OHTBZ leads to significant covalent binding of the radiolabel (Coulet et al., 2000). Furthermore, 5-OHTBZ has been described to possess the same teratogenic potential as the parent drug TBZ, as demonstrated in a limb-bud culture system (Tsuchiya et al., 1987

Mechanism-Based Inactivation of Cytochrome P450 3A4 by Mibefradil through Heme Destruction

“…The benzimidazole moiety of mibefradil also has the potential to be involved in the observed inactivation of CYP3A4. Hydroxylation at the 5-position of the benzene ring can produce inactivating species through quinone imine formation, which can subsequently form glutathione and/or P450 apoprotein adducts (Dalvie et al, 2006). In addition, hydroxylation of the benzimidazole could result in the formation of a highly reactive arene oxide intermediate, which could similarly form an apoprotein adduct and inactivate the enzyme (Delatour et al, 1984;Uetrecht, 2003).…”

ChemInform Abstract: The Biochemistry of Drug Metabolism — An Introduction. Part 4. Reactions of Conjugation and Their Enzymes

“…4.104) percent of the conjugate to be formed nonenzymatically, depending on enzymatic efficiency. Microsomal incubations of thiabendazole or its 5-OH metabolite in the presence of GSH afforded the glutathione conjugate 4.459 whose formation was consistent with the intermediate quinoneimine 4.458 [315]. 4.104).…”

Mechanism of teratogenesis: Electron transfer, reactive oxygen species, and antioxidants

“…Thiabendazole (TBZ), an antihelminthic agent, is teratogenic, nephrotoxic, and hepatotoxic (Dalvie et al, 2006). The 5‐hydroxy derivative (5‐OHTBZ) is further oxidized to quinone imine.…”