The metabolic activation of ronidazole [(1-methyl-5-nitroimidazole-2-yl)-methyl carbamate] to reactive metabolites by mammalian, cecal bacterial and T. foetus enzymes

Activation of tinidazole, an antiprotozoal drug to a mutagen by mammalian liver S9

Tinidazole was found to display much higher mutagenic activity compared to metronidazole in Salmonella strain TA100 and YG1029. Under anaerobiosis, the specific activity of this nitroimidazole was enhanced, at least, by about 1.75-fold in TA100 and several fold in TA100NR relative to aerobic conditions. The mutagenicity in the latter strain with S9 mix became further increased by 2.5-fold under anaerobiosis, indicating the role of oxygen sensitive bacterial and mammalian S9 nitroreductases in the activation of the drug. The mutagenicity of the drug was slightly lowered in TA100/1,8-DNP6 (O-acetyltransferase deficient), YG1029 (O-acetyltransferase overexpressing) and TA100 in the presence of pentachlorophenol (PCP), an O-acetyltransferase inhibitor. These results rule out the possible involvement of N-acetoxyarylamine pathway in the metabolic activation of these nitroimidazoles.

Drug residue formation from ronidazole, a 5-nitroimidazole. VIII. Identification of the 2-methylene position as a site of protein alkylation

Ronidazole protein-bound adducts were generated by the in vitro anaerobic incubation of [2-methylene-14C]ronidazole with microsomes from the livers of male rats. Acid hydrolysis of the protein adducts yielded an imidazole ring fragment bearing the radiolabel and an amino acid residue derived from the proteins. This fragment has been identified as carboxymethylcysteine by co-chromatography of the amino acid and its dansyl derivative with known standards under a variety of conditions. The carboxymethylcysteine was estimated to represent at least 15% of the radioactivity derived from the protein-bound adducts and provides unequivocal evidence that nucleophilic attack by protein cysteine thiols occurred at the 2-methylene position of ronidazole.

Mechanism of reductive activation of a 5-nitroimidazole by flavoproteins: model studies with dithionite

The flavoprotein nitroreductases NADPH:cytochrome P-450 reductase and xanthine oxidase catalyzed the cofactor-dependent anaerobic nitro group reduction and covalent binding to protein sulfhydryl groups of the 5-nitroimidazole substrate ronidazole [1-methyl-5-nitroimidazole-2-yl)-methyl carbamate). Studies with variously radiolabeled ronidazole molecules demonstrated that the imidazole ring was intact while greater than 80% of the C-4 3H and 2-carbamoyl group were lost from the covalently bound product. The stoichiometry of cofactor consumption during the enzyme-catalyzed reduction of the substrate could not be determined, so a model nitroreductase system which utilized dithionite as the reductant and agarose-immobilized cysteine as the target for alkylation was developed. Two moles of dithionite was consumed per mole of substrate for maximal reduction of uv absorbance due to the nitro group, for maximal release of C-4 3H, and for maximal covalent binding to agarose-immobilized cysteine. These results indicate that four electrons are required for the reductive activation of the substrate, consistent with formation of a hydroxylamine reactive intermediate. Covalent binding of variously radiolabeled substrate molecules after dithionite reduction exhibited the same labeling pattern as flavoprotein-catalyzed covalent binding, suggesting that covalent binding is mediated by the same species in both chemical and biological systems. The data are consistent with a mechanism where the substrate undergoes four-electron reduction to form a hydroxylamine, which is susceptible to nucleophilic attack at C-4. When water attacks C-4, the 2-carbamoyl group can eliminate to form a Michael-like acceptor which adds thiols at the 2-methylene position.

Chemical and biological properties of acetyl derivatives of the hydroxylamino reduction products of metronidazole and dimetridazole

Metronidazole and related 5-nitroimidazoles undergo reduction of their nitro group apparently to produce such reactive species as 5-hydroxylaminoimidazoles. To define the role of these species we have sought ways to prepare them by the catalytic reduction of metronidazole, dimetridazole and flunidazole. Although their respective 5-hydroxylaminoimidazoles were too unstable to be isolated directly, their O,N-diacetyl derivatives were isolable. Of these, the diacetyl derivative of the hydroxylamine derived from dimetridazole, O,N-diacetyl-1,2-dimethyl-5-hydroxylaminoimidazole (DiacDMH), was used for further study. DiacDMH was converted to its monoacetyl derivative, N-acetyl-1,2-dimethyl-5-hydroxylaminoimidazole (AcDMH), by enzymatic deacylation. Both DiacDMH and AcDMH were examined for bactericidal activity against such strains as Bacteroides fragilis, Clostridium perfringens, and Escherichia coli strain SR58, which are known to be sensitive to dimetridazole, as well as a variety of other bacteria. No bactericidal activity was detected, even in the presence of deacetylating enzymes. As the 5-hydroxylaminoimidazole itself could not be shown to form in these bacterial incubations, it remains uncertain whether or not the hydroxylamino functionality of a 5-nitroimidazole has bactericidal activity.