The contribution of metronidazole and two metabolites to the mutagenic activity detected in urine of treated humans and mice.

The urine of two patients receiving therapeutic doses of the trichomonacide, metronidazole, was analyzed for mutagenic activity using the histidine auxotroph TA1535 of Salmonella typhimurium. The activity detected in the urine was significantly higher than could be accounted for by the presence of the administered drug. Chromatographic analysis of the urine indicated the presence of the metabolite 1-(2-hydroxyethyl)-2-hydroxymethyl-5-nitroimidazole, which when tested in vitro with TA1535 was found to be ten times more active than metronidazole. An additional urinary metabolite, 1-acetic acid-2-methyl-5-nitroimadazole, was found to be inactive when similarly tested. The in vitro mutagenic activity of metronidazole and the two metabolites was unchanged by the addition of phenobarbital- or Aroclor-induced rat liver homogenate to the test system. In addition, metronidazole and the hydroxymethyl metabolite reverted S. typhimurium TA100 but not TA1537, TA1538, or TA98, and the acetic acid metabolite failed to revert any of the tester strains. In studies with mice, metronidazole was required in excess of the human dose in order for significant amounts of the hydroxymethyl metabolite to be detected in the urine. Urine from mice pretreated with the hepatotoxin, carbon tetrachloride, prior to the administration of metronidazole demonstrated approximately a 50% reduction in mutagenic activity, and the formation of the urinary metabolites was inhibited. These findings indicate the production of metabolites from the parent compound by the liver of the intact animal which could not be determined by use of the standard in vitro liver homogenate system.