Evidence for site-specific bioactivation of alachlor in the olfactory mucosa of the Long-Evans rat

Alachlor (2-chloro-2',6'-diethyl-N-[methoxymethyl]-acetanilide) is a restri cted-use chloracetanilide herbicide which has been shown previously to prod uce a dose-dependent incidence of olfactory mucosal tumors in rats followin g chronic dietary exposure. However, the mechanism of alachlor carcinogenic ity is poorly understood. Alachlor was administered ip to male Long-Evans r ats for up to 28 days at doses that are carcinogenic in chronic studies in order to study olfactory lesion development and alterations in cell prolife ration. Neither treatment-related olfactory mucosal lesions nor regenerativ e cell proliferation, as assessed with BrdU labeling, was detected. In vitr o genotoxicity studies using Salmonella typhimurium strain TA100 showed tha t alachlor was non-mutagenic in the absence of metabolic activation. When p re-incubated with an olfactory mucosal S9 activation system, alachlor induc ed a weak, dose-dependent mutagenic response at 500-1250 mu g/plate, with t oxicity at higher doses. In contrast, an S9 activation system derived from nasal respiratory mucose, the tissue physically juxtaposed with the olfacto ry mucosa but reportedly not susceptible to alachlor-induced tumors, did no t produce a mutagenic response for alachlor or the positive control. Thus, this result suggested site-specificity of alachlor activation consistent wi th the target site of carcinogenicity. The mutagenicity of alachlor to Salm onella, in the presence of an olfactory mucosal-activating system, was conf irmed by a limited positive response in the mouse lymphoma assay. Here ther e were increases in small colony mutants (indicative of chromosomal effects ) as well as large colony mutants (which reflect gene mutations). This stud y suggests that target tissue bioactivation of alachlor results in the form ation of one or more mutagenic metabolite(s), which may be critical in alac hlor-induced nasal tumorigenesis.