Effect of Methimazole, an FMO Substrate and Competitive Inhibitor, on the Neurotoxicity of 3,3′-Iminodipropionitrile in Male Rats

Abstract

This study was designed to examine the role of flavin-containing monooxygenase (FMO) on the auditory and vestibular neurotoxicity of 3,3'-iminodipropionitrile (IDPN) using the FMO substrate and competitive inhibitor methimazole (MMI). Specifically, the purpose was to block the FMO-mediated conversion of IDPN to the putative neurotoxic metabolite N-hydroxy3,3'-iminodipropionitrile (HOIDPN). In three separate experiments, adult male Long-Evans hooded rats were administered (ip) saline (vehicle), MMI, IDPN, or HOIDPN individually, or a combination of IDPN and MMI or HOIDPN and MMI. Animals were observed daily for signs of the ECC syndrome (excitation with choreiform and circling movements) for 10 days. One to 2 weeks after exposure, a battery of behavioral tests was used to examine vestibular and auditory function. MMI completely blocked the neurotoxicity associated with a 600 mg/kg dose of IDPN and partially blocked the effects of a 1000 mg/kg dose of IDPN. In contrast, MMI failed to block, and instead increased, the neurotoxicity associated with HOIDPN. These data suggest that FMOmediated metabolism of IDPN is necessary for the generation of a metabolite responsible for the vestibular and auditory neurotox- icitieS. C 1997 Society of ToThe synthetic organonitrile 3,3'-iminodipropionitrile (IDPN) is known to produce the ECC syndrome excitation with cho-' This article has been reviewed by the National Health Effects and Environmental Research Laboratory, U.S. Environmental Protection Agency, and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. This work was supported in part by the North Carolina Agricultural Research Service (M.B.G.), NIEHS Grant ES-00044 (M.B.G.), and NIH Grant NS22688 (L.M.S.).2 To whom correspondence should be addressed at Neurotoxicology Division [MD-74B] USEPA, R.T.P., NC 27711. E-mail: crofton.kevin® EPAMAIL.EPA.GOV. reiform and circling movements (Delay, 1952; Thullier and Burger, 1954;Selye, 1957) through degeneration of the vestibular system (Slagel and Hartman, 1965;Llorens et al., 1993;Llorens and Dememes, 1994). IDPN has also been associated with auditory and olfactory damage, and proximal axonopathy (Chou and Hartman, 1964;Griffin et al., 1978;Crofton and Knight, 1991;Genter et al, 1992Genter et al, , 1994aCrofton et al., 1994). Currently, the role of metabolism in IDPNinduced neurotoxicity is unclear.Theories on the activation of LDPN have involved three possible metabolic pathways: (1) cytochrome P450 metabolism, (2) monoamine oxidase (MAO) metabolism, and (3) flavin-containing monooxygenase (FMO) metabolism (e.g., Jacobson et al., 1987). It has also been suggested that IDPN is the toxic compound and needs no hepatic bioactivation (Llorens and Crofton, 1991). Tanii et al. (1984) first demonstrated that the neurotoxicity of some related neurotoxic nitriles (e.g., 2-pentenenitrile) was increased by inactivation of hepatic P450s with carbon tetrachloride (CC1 4 ) pretreatment. After observing enhanced v...

Flavin-containing monooxygenase 1-catalysedN,N-dimethylamphetamineN-oxidation

“…The chemical inhibitors used were as follows: methimazole for FMOs (Nace et al 1997;Kim & Ziegler 2000), imipramine for FMO1 (Dixit & Roche 1984;Cashman, 2000), and trimethylamine for FMO3 (Lang & Rettie 2000). DMA was incubated with rat liver microsomal protein (1 mg ml −1 ) in the presence of each inhibitor.…”

Cytochrome P450 and Non-Cytochrome P450 Oxidative Metabolism: Contributions to the Pharmacokinetics, Safety, and Efficacy of Xenobiotics

“…The vestibular and auditory neurotoxicities that result after administration of IDPN are also attributed to FMO-catalyzed conversion of the compound to the putative N-hydroxy-3,39-iminodipropionitrile (HOIDPN; Fig. 2B) (Jacobson et al, 1987;Morandi et al, 1987;Nace et al, 1997). This proposal is based on the fact that coadministration of methimazole, an FMO1/FMO3 inhibitor, with IDPN prevents these IDPN-mediated adverse events.…”

Vestibular toxicity of cis-2-pentenenitrile in the rat

“…Evidence that the vestibular toxicity depends on CYP bioactivation is also available for allylnitrile (Boadas-Vaello et al, 2009), but the ototoxic metabolite has not been identified. In the case of IDPN, the data available support the conclusion that flavin monooxygenase-mediated generation of N-hydroxy-IDPN is a bioactivation step (Morandi et al, 1987;Nace et al, 1997), but conclusive identification of the ototoxic metabolite is lacking.…”