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Formation of a mutagenic drug metabolite by intestinal microorganisms
Abstract:
A new broad-spectrum antiparasitic agent, 4-isothiocyano-4'-nitrodiphenylamine, is devoid of mutagenic activity in vitro, either alone or in the presence of activating enzymes of rat liver. However, six species of mammals receiving this drug excrete as as yet unidentified mutagenic metabolite. Several observations suggested that one or several constituents of the enteric bacterial flora, rather than the metabolic activities of the host, are involved in the formation of this mutagen. Unequivocal demonstration for such a mechanism was provided by germ-free rats that do not form this metabolite, in contrast to their conventional littermates. Only a relatively moderate and apparently quite selective reduction in the total number of microorganisms of the intestinal flora is needed to elminate this mutagenic transformation. For example, following administration of a single dose of erythromycin or erythromycylamine, conversion of the isothiocyanate to a mutagen can be prevented completely, while antiparastitic activity is maintained. There is no obligatory association between chemotherapeutic activity and the formation of the mutagenic metabolite, and these two activities can be dissociated completely. This suggests a new approach for increasing the safety of pharmacological agents.
Insights
A new antiparasitic drug is safe in vitro, but forms a mutagen in mammals. Gut bacteria, not the host, create this mutagen, which can be prevented by antibiotics like erythromycin.
Area of Science:
- Pharmacology
- Microbiology
- Toxicology
Background:
- A novel broad-spectrum antiparasitic agent, 4-isothiocyano-4'-nitrodiphenylamine, exhibits no in vitro mutagenicity.
- Mammalian metabolism of this drug results in the excretion of an unidentified mutagenic metabolite.
- Evidence suggests enteric bacterial flora, not host metabolism, is responsible for mutagen formation.
Purpose of the Study:
- To investigate the mechanism of mutagenic metabolite formation from 4-isothiocyano-4'-nitrodiphenylamine in mammals.
- To determine the role of gut microbiota in the in vivo mutagenicity of the antiparasitic agent.
- To explore strategies for dissociating antiparasitic activity from mutagen formation.
Main Methods:
- In vitro mutagenicity assays with and without rat liver activating enzymes.
- Administration of the drug to various mammalian species and analysis of excreted metabolites.
- Comparison of metabolite formation in germ-free rats versus conventional littermates.
- Administration of antibiotics (erythromycin, erythromycylamine) to assess their impact on mutagen formation.
Main Results:
- Germ-free rats did not produce the mutagenic metabolite, unlike conventional rats, confirming the role of gut bacteria.
- Selective reduction of intestinal flora, e.g., with erythromycin, prevented mutagen formation while maintaining antiparasitic efficacy.
- The formation of the mutagenic metabolite is not obligatory for the drug's antiparasitic activity.
Conclusions:
- The mutagenic metabolite of 4-isothiocyano-4'-nitrodiphenylamine is formed by enteric bacteria, not host metabolism.
- Modulating gut microbiota offers a novel strategy to enhance the safety of this antiparasitic agent.
- Pharmacological agents can be designed to separate therapeutic effects from toxic metabolite production.