TR-544 - Abstract - National Toxicology Program

TR-544 - Abstract

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TR-544
Toxicology and Carcinogenesis Studies of Dibromoacotonitrile (CAS No. 3252-43-5) in F344/N Rats and B6C3F1 Mice (Drinking Water Studies)

PLEASE NOTE: The following abstract has been extracted from the DRAFT technical report to be reviewed by the National Toxicology Program Board of Scientific Counselors Technical Reports Review Subcommittee on February 27-28, 2008 (see the meeting page). When this report becomes final the entire report will be available in pdf format on the NTP website.

Draft Abstract

Chemical Formula: C HBr N -- Molecular Weight: 198.84

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 (98.5% 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 in males and 2, 4, 7, 12, or 19 mg/kg in 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 was 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 in males and 2, 3, 10, 14, or 22 mg/kg in 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 in males and 1, 2, 4, 7, or 13 mg/kg in 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 in 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 in males and 2, 4, or 8 mg/kg in 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) 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 to males and 3, 6, or 11 mg/kg to 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 Salmonella typhimurium or Escherichia 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 Drosophila 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 and female 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.

Exposure to dibromoacetonitrile for 2 years caused increased incidences of epithelial hyperkeratosis in the esophagus of male and female rats, 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

Summary of the 2-Year Carcinogenesis and Genetic Toxicology Studies of Dibromoacetonitrile

	Male F344/N Rats	Female F344/N Rats	Male B6C3F1 Mice	Female B6C3F1 Mice
Concentrations in drinking water	0, 50, 100, or 200 mg/L	0, 50, 100, or 200 mg/L	0, 50, 100, or 200 mg/L	0, 50, 100, or 200 mg/L
Body weights	200 mg/L group 7% less than control group after 1 year	Exposed groups similar to control group	200 mg/L group 7% less than control group after 8 weeks	200 mg/L group 7% less than control group after 8 weeks and 14% less after 1 year
Survival Rates	31/50, 33/50, 25/50, 35/50	29/50, 35/50, 29/50, 31/50	40/50, 40/50, 35/50, 42/50	36/50, 36/50, 43/50, 47/50
Nonneoplastic effects	Esophagus: epithelial hyperkeratosis (6/48, 8/50, 34/50, 46/50)	Esophagus: epithelial hyperkeratosis (10/49, 8/50, 28/50, 48/50) Glandular Stomach: glandular ectasia (10/50, 4/50, 10/50, 26/50); Tongue: squamous epithelial hyperplasia (1/50, 1/50, 2/50, 6/50)	Forestomach: squamous epithelial hyperplasia (1/50, 4/50, 1/50, 6/50)	None
Neoplastic effects	Oral Cavity (oral mucosa or tongue): squamous cell papilloma or squamous cell carcinoma (0/50, 0/50, 2/50, 5/50) Glandular Stomach: adenoma (0/50, 0/50, 0/50, 2/50)	Oral Cavity (oral mucosa or tongue): squamous cell papilloma (1/50, 0/50, 4/50, 0/50)	Forestomach: squamous cell papilloma or squamous cell carcinoma (0/50, 1/50, 0/50, 5/50)	Forestomach: squamous cell papilloma (1/50, 0/50, 5/50, 14/50)
Equivocal findings	None	Skin: squamous cell papilloma, keratoacanthoma, basal cell adenoma, or basal cell carcinoma (0/50, 0/50, 3/50, 4/50)	Liver: hepatoblastoma (1/50, 7/50, 3/50, 2/50); hepatocellular adenoma, hepatocellular carcinoma, or hepatoblastoma (37/50, 46/50, 43/50, 42/50)	None
Level of evidence of carcinogenic activity	Clear evidence	Some evidence	Clear evidence	Clear evidence
Genetic toxicology
Salmonella typhimurium and Escherichia coli gene mutations:		Positive in E. coli WP uvrA/pKM101 with S9; weakly positive in strains TA97, TA100, and TA1535, with S9; negative in TA98 and TA1537 with and without S9
Sex-linked recessive lethal mutations Drosophila melanogaster:		No induction of sex-linked recessive lethal mutations
Micronucleated erythrocytes Mouse peripheral blood in vivo:		Negative