6-Nitrobenzimidazole, a heterocyclic aromatic compound used in photographic developers, was selected for bioassay by the National Cancer Institute because of the suspect status of aromatic nitro-compounds.
A bioassay for possible carcinogenicity of 6-nitrobenzimidazole was conducted using Fischer 344 rats and B6C3F1 mice. 6-Nitrobenzimidazole was administered in the feed, at either of two concentrations, to groups of 50 male and 50 female animals of each species. The dietary concentrations used in the chronic bioassay were 0.5 and 0.12 percent for the high and low dose rats, respectively, and 0.24 and 0.12 percent for the high and low dose mice, respectively. After a 78-week period of compound administration, observation of the rats continued for up to an additional 29 weeks and observation of the mice continued for an additional 18 weeks. For each species and each dosed group, 49 or 50 animals of each sex were placed on test as controls.
There were no significant positive associations between the administered dietary concentrations of 6-nitrobenzimidazole and mortality in either sex of rats or mice. In all groups adequate numbers of animals survived sufficiently long to be at risk from late-developing tumors.
Among both male and female mice, the incidences of hepatocellular carcinomas in high dose groups were statistically significant relative to controls.
Among rats of both sexes, nonneoplastic lesions of the eyes and of the Harderian glands appeared to be associated with administration of 6-nitrobenzimidazole. No neoplasms, however, were attributed to compound administration.
Under the conditions of this bioassay, dietary administration of 6-nitrobenzimidazole was not carcinogenic to Fischer 344 rats; however, the compound was carcinogenic to B6C3F1 mice, causing hepatocellular carcinomas in both sexes.