2,6-Xylidine is a chemical intermediate used principally in the production of dyes. It is also a component of tobacco smoke, a degradation product of aniline-based pesticides, and a metabolite of certain drugs, particularly the xylide group of local anesthetics. The National Toxicology Program (NTP) sponsored single-administration, 2-week, and 13-week studies of 2,6-xylidine by gavage in F344/N rats. The U.S. Environmental Protection Agency (EPA) sponsored short-term gavage studies and 10-week range-finding studies in Charles River CD rats (a Sprague Dawley-derived strain). A carcinogenesis study of 2,6-xylidine was initiated by the EPA , which designed and monitored the study during the 2-year exposure period. The NTP then assumed responsibility for the study, conducting terminal kill, necropsy, histopathologic evaluation, data analysis, and report preparation.
Oral LD50 values of 1.2-1.3 g/kg were calculated for F344/N and Charles River CD rats administered single doses of 2,6-xylidine. Marginally toxic effects occurred in the hepatic, renal, and hematopoietic systems of dosed rats in the single-administration, 2-week, 10-week, and 13-week studies.
The 56 male and 56 female Charles River CD rats used in the 104-week carcinogenesis studies were the offspring of animals fed diets containing 0, 300, 1,000, or 3,000 ppm 2,6-xylidine before breeding, during pregnancy, and through the lactation period. The concentrations of 2,6-xylidine offered to animals in the 104-week studies were the same as those given to their parents.
During most of the 2-year studies, high dose male and female rats showed a reduction (greater than 10%) in body weight gain. Survival in the high dose male rats was significantly reduced (P<0.001) relative to that in controls. Survival also was reduced in the 1,000-ppm group. There was no significant relationship between concentration and mortality in female rats, but mortality was high for all groups of female rats during the second year of the study.
The epithelium of the nasal cavity was the primary site of compound-related neoplastic and nonneoplastic lesions. The incidences of both papillomas and carcinomas of the nasal cavity were significantly increased in high dose male and female rats. Carcinomas or adenocarcinomas (combined) occurred in 28/56 high dose males, 24/56 high dose females, and 1/56 mid dose females. Papillary adenomas occurred in 10/56 high dose males, 2/56 mid dose males, and 6/56 high dose females. None occurred in the other groups. The carcinomas were highly invasive and frequently destroyed the nasal turbinates and nasal septum. Metastasis to the brain was present in 5/56 male and 7/56 females high dose rats.
Malignant mesenchymal tumors were observed in the nasal cavity. Rhabdomyosarcomas occurred in two high dose male rats and two high dose female rats. These rare malignant tumors have not been previously reported at this site in Sprague Dawley rats. Malignant mixed tumors having features of adenocarcinomas and rhabdomyosarcomas were reported in one high dose male and one high dose female rat. One undifferentiated sarcoma was seen in a high dose female rat. The nonneoplastic lesions observed in the nasal cavity included acute inflammation, epithelial hyperplasia, and squamous metaplasia.
The incidences of subcutaneous tissue fibromas were increased in high dose male and female rats (male: control, 0/56; low dose, 1/56; mid dose, 2/56; high dose, 4/56; female: 0/56; 2/56; 1/56; 4/56) and were dose related. Subcutaneous fibrosarcomas were observed in three high dose females, one high dose male, one mid dose female, one low dose male, and one control female.
A significant dose-related increase occurred in the incidence of female rats with neoplastic nodules of the liver (0/56; 1/56; 2/56; 4/55). This increase was significant in the high dose group by the incidental tumor test.
Conclusions
Under the conditions of these 2-year feed studies, 2,6-xylidine was clearly carcinogenic for male and female Charles River CD rats, causing significant increases in the incidences of adenomas and carcinomas of the nasal cavity. A rhabdomyosarcoma, a rare tumor of the nasal cavity, was observed in dosed rats of each sex. In addition, the increased incidences of subcutaneous fibromas and fibrosarcomas in male and female rats and the increased incidence of neoplastic nodules of the liver in female rats may have been related to the administration of 2,6-xylidine.