3-Amino-4-ethoxyacetanilide, an azo dye intermediate closely related to the para-aminophenol analgesics, was selected for bioassay by the National Cancer Institute because of the increased incidence of bladder cancer reported among workers in the dye manufacturing industry.
A bioassay of 3-amino-4-ethoxyacetanilide for possible carcinogenicity was conducted using Fischer 344 rats and B6C3F1 mice. 3-Amino-4-ethoxyacetanilide was administered in the feed, at either of two concentrations, to groups of approximately 50 male and 50 female animals of each species. The dietary concentrations used in the chronic bioassay for low and high dose rats were 0.4 and 1.5 percent, respectively. The dietary concentrations used for low and high dose mice were 0.4 and 0.8 percent, respectively. After a 78-week period of chemical administration, observation of rats continued for up to 18 weeks. For each species, 50 animals of each sex were placed on test as controls for low dose groups and approximately 50 animals of each sex were placed on test as controls for high dose groups.
In both species, adequate numbers of animals in all groups survived sufficiently long enough to be at risk from late-developing tumors.
Among rats the only clearly compound-related lesion was hemosiderosis of the thyroid gland. No neoplastic lesions were statistically significant in dosed rats.
Among mice the incidence of follicular-cell carcinomas of the thyroid gland was significant for high dose males. An elevated incidence of thyroid hyperplasia was observed in each dosed group. Hemosiderosis of the thyroid cells were found in nearly all dosed mice, but not in any control mice.
Under the conditions of this bioassay, 3-amino-4-ethoxyacetanilide was carcinogenic in male B6C3F1 mice, causing follicular-cell carcinomas of the thyroid gland. Evidence provided by this bioassay was insufficient to establish the carcinogenicity of 3-amino-4-ethoxyacetanilide in female mice or in Fischer 344 rats of either sex.