title: Bioassay of 5-nitro-o-toluidine for possible carcinogenicity (CAS No. 99-55-8).
abstract: A bioassay of 5-nitro-o-toluidine for possible carcinogenicity was conducted using Fischer 344 rats and B6C3F1 mice. 5-Nitro-o-toluidine was administered in the feed, at either of two concentrations, to groups of 50 male and 50 female animals of each species. The time-weighted average high and low dietary concentrations of 5-nitro-o-toluidine were, respectively, 0.01 and 0.005 percent for rats, and 0.23 and 0.12 percent for mice. After a 78-week period of compound administration, observation of the rats continued for an additional 30 to 31 weeks and observation of the mice continued for up to an additional 20 weeks. For each species, 50 animals of each sex were placed on test as controls and fed only the basal diet. There were no significant positive associations between the concentration of 5-nitro-o-toluidine administered and mortality in rats or mice of either sex, and adequate numbers of animals in all groups survived sufficiently long to be at risk from late-developing tumors. No unusual tumors were observed in rats of either sex and no convincing statistical evidence was provided for a significant positive association between compound administration and the incidence of any neoplasm in rats. Among mice there was a statistically significant positive association between administration of the chemical and the incidences of hepatocellular carcinomas in both males and females. The combined incidence of hemangiomas and hemangiosarcomas in male mice and the incidence of hemangiosarcomas in female mice were not statistically significant in relation to their respective control groups. However, they were considered to be associated with compound administration since they rarely occur in untreated B6C3F1 mice. Under the conditions of this bioassay 5-nitro-o-toluidine was carcinogenic in B6C3F1 mice, causing hepatocellular carcinomas in both sexes, an increase in the combined incidence of hemangiomas and hemangiosarcomas in male mice, and an increased incidence of hemangiosarcomas in female mice. The compound was not carcinogenic in Fischer 344 rats.