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Abstract Title:

Toxicology and carcinogenesis studies of dibromoacetonitrile (CAS No. 3252-43-5) in F344/N rats and B6C3F1 mice (drinking water studies).

Abstract Source:

Natl Toxicol Program Tech Rep Ser. 2010 Jun(544):1-194. PMID: 20725153

Abstract Author(s):

[No authors listed]

Abstract:

Dibromoacetonitrile is formed as a result of the reaction of chlorine oxidizing compounds (e.g., chlorine gas, hypochlorous acid, and hypochlorite) with natural organic matter, particularly nitrogen-containing organic compounds, in water containing bromine; it is also a by-product of ozone disinfection. Dibromoacetonitrile is not produced on a large industrial scale. Thus, chlorinated drinking water is the primary source of human exposure to dibromoacetonitrile. The United States Environmental Protection Agency nominated dibromoacetonitrile for carcinogenicity studies as part of an interagency initiative to characterize the potential chronic toxicity and carcinogenicity of exposure to various water disinfection by-products in drinking water. Dibromoacetonitrile was selected as a representative of the family of haloacetonitriles. The drinking water route mimics the major pathway of human exposure to dibromoacetonitrile. Male and female F344/N rats and B6C3F1 mice were exposed to dibromoacetonitrile (98.6% pure) in drinking water for 2 weeks, 3 months, or 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium, Escherichia coli, Drosophila melanogaster, and mouse peripheral blood erythrocytes. 2-WEEK STUDY IN RATS Groups of five male and five female rats were exposed to drinking water containing 0, 12.5, 25, 50, 100, or 200 mg/L dibromoacetonitrile for 15 days (equivalent to average daily doses of approximately 2, 3, 7, 12, or 18 mg dibromoacetonitrile/kg body weight for males and 2, 4, 7, 12, or 19 mg/kg for females). All rats survived to the end of the study. The mean body weights of 200 mg/L males were significantly less than those of the control group. Water consumption decreased in an exposure-related manner. Atrophy of the testicular germinal epithelium occurred in two males exposed to 200 mg/L. 2-WEEK STUDY IN MICE Groups of five male and five female mice were exposed to drinking water containing 0, 12.5, 25, 50, 100, or 200 mg/L dibromoacetonitrile for 15 days (equivalent to average daily doses of approximately 2, 4, 8, 15, or 21 mg/kg for males and 2, 3, 10, 14, or 22 mg/kg for females). All mice survived to the end of the study. Mean body weights of all exposed groups were similar to those of the controls. Water consumption by mice exposed to 200 mg/L was less than that by the controls. The liver weights of females exposed to 50, 100, or 200 mg/L were significantly decreased. No lesions were attributed to exposure to dibromoacetonitrile. 3-MONTH STUDY IN RATS Groups of 10 male and 10 female rats were exposed to drinking water containing 0, 12.5, 25, 50, 100, or 200 mg/L dibromoacetonitrile for 3 months (equivalent to average daily doses of approximately 1, 2, 3, 6, or 11 mg/kg for males and 1, 2, 4, 7, or 13 mg/kg for females). All rats survived to the end of the study. The mean body weights of 200 mg/L females were significantly less than those of the control group. Water consumption by 200 mg/L rats was less than that by the control groups. No histopathologic lesions were attributed to exposure to dibromoacetonitrile. 3-MONTH STUDY IN MICE Groups of 10 male and 10 female mice were exposed to drinking water containing 0, 12.5, 25, 50, 100, or 200 mg/L dibromoacetonitrile for 3 months (equivalent to average daily doses of approximately 2, 3, 6, 11, or 18 mg/kg for males and females). All mice survived to the end of the study. The mean body weights of exposed groups were similar to those of the controls. Water consumption by 200 mg/L females was less than that by the controls. No histopathologic lesions were attributed to exposure to dibromoacetonitrile. 2-YEAR STUDY IN RATS Groups of 50 male and 50 female rats were exposed to drinking water containing 0, 50, 100, or 200 mg/L dibromoacetonitrile for 105 to 106 weeks (equivalent to average daily doses of approximately 2, 4, or 7 mg/kg for males and 2, 4, or 8 mg/kg for females). Survival of all exposed groups of rats was similar to that of the controls. Mean body weights of 200 mg/L males were approximately 7% less than those of the controls during the second year of the study. Water consumption by the 100 and 200 mg/L groups was generally less than that by the controls throughout the study. The combined incidence of squamous cell papilloma or carcinoma of the oral mucosa or tongue was significantly increased in 200 mg/L males. The incidence of squamous cell papilloma in the oral mucosa or tongue was increased in 100 mg/L females, but not significantly. Oral cavity neoplasms are uncommon in untreated F344/N rats, occurring at a mean incidence of 1% or less in males and females. The incidence of squamous epithelial hyperplasia of the tongue was increased in 200 mg/L females. The latter lesion is considered to be part of the continuum of proliferative changes in oral cavity neoplasia. Glandular stomach adenomas were observed in two 200 mg/L males. This is a rare neoplasm that has not been seen in nearly 2,000 historical control male F344/N rats. The incidence of glandular ectasia of the glandular stomach in 200 mg/L females was significantly increased. The incidences of epithelial hyperkeratosis of the esophagus were significantly increased in 100 and 200 mg/L males and females. The incidences of squamous cell papilloma or keratoacanthoma (combined) and squamous cell papilloma, keratoacanthoma, basal cell adenoma, or basal cell carcinoma (combined) of the skin occurred with a positive trend in female rats. 2-YEAR STUDY IN MICE Groups of 50 male and 50 female mice were exposed to drinking water containing 0, 50, 100, or 200 mg/L dibromoacetonitrile/L for 105 to 106 weeks (equivalent to average daily doses of approximately 4, 7, or 13 mg/kg for males and 3, 6, or 11 mg/kg for females). Survival of female mice exposed to 100 or 200 mg/L was significantly greater than that of the controls. Mean body weights of 200 mg/L males and females were less than those of the controls throughout most of the study. Water consumption by exposed groups was also less than that of controls throughout most of the study. The incidence of squamous cell papilloma or carcinoma (combined) of the forestomach was significantly increased in 200 mg/L males. The incidence of squamous cell papilloma of the forestomach was significantly increased in 200 mg/L females. Squamous cell neoplasms of the forestomach are uncommon in control male and female B6C3F1 mice, occurring at a mean incidence of about 1% to 2%. The incidences of epithelial hyperplasia of the forestomach were slightly increased in 50 and 200 mg/L males. The latter lesion is considered to be part of the continuum of proliferative changes in forestomach neoplasia. The incidence of hepatoblastoma was significantly increased in 50 mg/L males; the incidences of hepatocellular adenoma, hepatocellular carcinoma, or hepatoblastoma (combined) were significantly increased in 50 and 100 mg/L males. GENETIC TOXICOLOGY Dibromoacetonitrile was tested in five independent bacterial mutagenicity assays, each using multiple tester strains of S. typhimurium or E. coli, and mutagenic responses were seen in all five assays, primarily in S. typhimurium strain TA100 in the presence of exogenous metabolic activation (S9), but sometimes also in TA97, TA1535, and E. coli Wp2uvrA/pKM101 with S9. Induced hamster liver S9 enzymes were most effective in generating the mutagenic metabolite, although some very weak activity was also seen with induced rat liver S9 at some of the laboratories. In vivo, dibromoacetonitrile did not induce sex-linked recessive lethal mutations in germ cells of male D. melanogaster exposed by feeding or by injection, and no increases in the frequencies of micronucleated erythrocytes were observed in peripheral blood of male or female mice administered dibromoacetonitrile for 3 months in drinking water. CONCLUSIONS Under the conditions of these 2-year drinking water studies there was clear evidence of carcinogenic activity of dibromoacetonitrile in male rats based on increased incidences of squamous cell papillomas or carcinomas of the oral cavity; adenomas in the glandular stomach of male rats were also considered to be exposure-related. There was some evidence of carcinogenic activity of dibromoacetonitrile in female rats based on an increased incidence of squamous cell papillomas of the oral cavity; increased incidences of basal cell or squamous cell neoplasms of the skin in female rats may have been related to dibromoacetonitrile exposure. There was clear evidence of carcinogenic activity of dibromoacetonitrile in male mice based on increased incidences of squamous cell papillomas or carcinomas of the forestomach. Increased incidences of neoplasms in the liver of male mice may have been related to dibromoacetonitrile exposure. There was clear evidence of carcinogenic activity of dibromoacetonitrile in female mice based on increased incidences of squamous cell papillomas of the forestomach. Exposure to dibromoacetonitrile for 2 years caused increased incidences of epithelial hyperkeratosis in the esophagus of male and female rats, glandular ectasia of the glandular stomach and squamous epithelial hyperplasia of the tongue in female rats, and squamous epithelial hyperplasia of the forestomach in male mice. Synonym: 2,2-dibromoacetonitrile.

Study Type : Review

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