Herbicidal phenylalkylureas as possible mutagens: II. Chemical basis of mutagenic activity

The herbicide monuron (N-(4-chlorophenyl)-N′, N′-dimethylurea)—a recognized carcinogen—and some of its congeners have been found also to induce back mutations in Salmonella typhimurium (Ames test), as well as micronuclei in mouse bone marrow cells and an inhibition of testicular DNA synthesis (DSI test) in mice. As both the Ames test and the DSI test correlate well with chemical carcinogenicity, urea herbicides in general might be suspected of carcinogenic activity. In order to investigate the potential carcinogenic mode of action of these substances we looked for possible activation mechanisms. The reaction sequence N-oxidation-esterification-carbonium ion formation seemed the only one able to fit the observed structure-activity relationships. Chemical synthesis of the proposed N-acetoxy compounds provided the means for testing this hypothesis. Only the N-acetoxy-, but neither the N-hydroxy- nor the unchanged parent ureas, exhibited a direct alkylating action as measured by the NBP-assay. The DSI reactivity pattern of several positional isomers could be shown to coincide with the theoretical predictions, as did the stability of the chloroisomers in aqueous solution at pH 7. Furthermore the N-acetoxy derivatives could be demonstrated to behave in the Ames test as directly acting mutagens, while the activity of the parent compounds is dependent on the metabolic activation by an S-9-liver fraction. The enhancing action of Acetyl-CoA on the S-9 mediated mutagenicity of these ureas can then be regarded as the final proof for the proposed reaction sequence.