Drug residue formation from ronidazole, a 5-nitro-imidazole. IV. The role of the microflora.

When radioactive 1-methyl-5-nitroimidazole-2-methanol carbamate, ronidazole, labeled at the 4,5-ring positions was administered orally to germ-free and conventional rats, a much larger fraction of the radioactivity was excreted in the feces of the conventional animals. Determination of the total radioactive residues present in the carcass, blood, plasma, liver, fat and kidney 5 days after dosing indicated that the carcass of the germ-free animals contained a greater quantity of residue than that of conventional rats. On the other hand, the blood of the conventional animals contained a much higher level of radioactivity than that of the germ-free animals. These results show that while the microflora influence the distribution of the drug their presence is not obligating for the formation of persistent tissue residues in rats dosed with ronidazole.

Drug residue formation from ronidazole, a 5-nitroimidazole. I. Characterization of in vitro protein alkylation.

The metabolic activation of [14C]ronidazole by rat liver enzymes to metabolite(s) bound to macromolecules was investigated. The alkylation of protein by [14C]ronidazole metabolite(s) was catalyzed most efficiently by rat liver microsomes, in the absence of oxygen utilizing NADPH as a source of reducing equivalents. Based on a comparison of total ronidazole metabolized versus the amount bound to microsomal protein, approximately one molecule alkylates microsomal protein for every 20 molecules of ronidazole metabolized. Protein alkylation was strongly inhibited by sulfhydryl-containing compounds such as cysteine and glutathione whereas methionine had no effect. Based on HPLC analysis of ronidazole, cysteine was found not to inhibit microsomal metabolism of ronidazole ruling out a decrease in the rate of production of the reactive metabolite(s) as the mechanism of cysteine inhibition.

Problems associated with the use of cycloheximide to distinguish between animal drug residues bound to protein and those incorporated into protein.

The user of cycloheximide to distinguish between covalently-bound drug residues in animals and residues due to the incorporation of drug fragments into endogenous molecules was explored. The results indicated that cycloheximide prevented the absorption of both glycine and ronidazole from the gastro-intestinal tract, an effect that complicates its use in the characterization of drug residues in animals.

Tumor-initiating ability of the twelve monomethylbenz[a]anthracenes.

The 12 possible monomethylbenz[a]anthracenes (MBAs) were examined for their tumor-initiating activity in the classical two-stage initiation-promotion experiment. Based on the average number of tumors/mouse, 7-MBA was the most tumorigenic compound of the series causing 4.9 papillomas/mouse at an initiating dose of 400 nmol/mouse. At this same dose level 8-MBA and 12-MBA were equally potent causing 1.0 papillomas/mouse. 6- an 9-MBAs caused -0.6 papillomas/mouse at this dose level. These data are in general agreement with previous experiments using other model systems. Of interest is the low tumorigenicity of the 1-, 2-, 3-and 4-MBAs which has been cited in support of the bay region theory of polycyclic aromatic hydrocarbon carcinogenesis. The tumor-initiating ability of anthracene and 9,10-dimethylanthracene was also examined. Neither of these compounds possessed tumor-initiating activity.

Tumor-initiating activity of the dihydrodiols of 8-methylbenz[a]anthracene and 8-hydroxymethylbenz[a]anthracene.

The tumor-initiating activity of the 3,4-dihydrodiols (diols) as well as other metabolites of 8-methylbenz[a]anthracene (8-MBA) and 8-hydroxymethylbenz[a]anthracene (8-OHMBA) were examined in the classical 2-stage initiation-promotion model on mouse skin. The 3,4-diol of 8-MBA caused 2.2 times as many papillomas/mouse as did 8-MBA. The 3,4-diol of 8-OHMBA was not more tumorigenic than either 8-MBA or 8-OHMBA. None of the other diols tested, including the 5,6- and 8,9-diol of 8-MBA and the 5,6 and 10,11-diol of 8-OHMBA were remarkably tumorigenic. These data indicate that the 3,4-diol of 8-MBA is a good candidate as a proximate carcinogen of 8-MBA and further suggest that the bay region 3,4-diol-1,2-epoxide is a likely ultimate carcinogen of this compound on mouse skin.