In the 16-day studies, rats and mice received 0, 125, 250, 500, 1,000, or 2,000 mg/kg dimethoxane in corn oil per day. Deaths occurred in rats and in male mice that received 2,000 mg/kg. Body weights of rats and mice were similar to those of vehicle controls. Compound-related clinical signs were not seen in surviving rats. Hemorrhage and necrosis of the stomach were observed in rats in the 2,000 mg/kg group which died before the end of the studies. Lesions of the forestomach, including inflammation, hyperplasia, hyperkeratosis, and ulceration, occurred in rats that received 250-2,000 mg/kg. Mice that received 500-2,000 mg/kg dimethoxane had lesions of the forestomach including erosion, ulceration, hyperplasia, and hyperkeratosis. Forestomach lesions were not seen at 125 or 250 mg/kg.
No compound-related deaths occurred in rats. Doses used were 0, 31, 62, 125, 250, or 500 mg/kg dimethoxane in corn oil by gavage. The final mean body weights of rats that received 500 mg/kg were 17% lower than that of vehicle controls for males and 5% lower for females. Ulceration, inflammation, and acanthosis with hyperkeratosis of the stratified squamous epithelium of the forestomach were seen in rats that received 500 mg/kg. Forestomach lesions were not seen in males that received 31 mg/kg or in females that received 31, 62, or 125 mg/kg.
All mice lived to the end of the studies (doses used were 0, 31, 62, 125, 250, or 500 mg/kg dimethoxane in corn oil by gavage). Final mean body weights of dosed and vehicle control mice were similar. Minimal-to- mild acanthosis and hyperkeratosis of the squamous epithelium of the forestomach were seen in 4/10 high dose male and 1/10 high dose female mice.
Because of the forestomach lesions observed in rats and mice and reduced body weight observed for male rats, doses selected for the 2-year studies were 0, 62.5, or 125 mg/kg dimethoxane in corn oil, given by gavage 5 days per week to groups of 60 male rats; 0, 125, or 250 mg/kg to groups of 60 female rats; and 0, 250, or 500 mg/kg to groups of 58 or 60 mice of each sex. Ten animals per sex and species from each dose group were killed 15 months after initiation of the studies to determine toxicity, pre-neoplastic lesions, and early induced neoplasia.
Minimal diffuse acanthosis and hyperplasia of the forestomach were seen in 7/10 female rats at 250 mg/kg, 7/10 males at 125 mg/kg, and 1/9 male and 1/9 female vehicle controls. Acanthosis of the forestomach was seen in 7/10 male and 6/10 female mice at 500 mg/kg. Harderian gland adenomas were seen in one high dose male and one high dose female mouse. A harderian gland adenocarcinoma was seen in a second high dose female mouse. No compound-related effects were observed for clinical chemical or hematologic values or for organ weights for rats or mice.
Body Weight and Survival in the Two-Year Studies:
Mean body weights of dosed and vehicle control rats and mice of each sex were generally similar. No significant differences in survival were observed between any groups of rats (male: vehicle control, 23/50; low dose, 28/50; high dose, 21/50; female: 30/50; 31/50; 24/50) or mice (male: 33/50; 27/48; 29/50; female: 36/50; 35/50; 34/50).
Nonneoplastic and Neoplastic Effects in the Two-Year Studies:
At no site was a significantly increased incidence of neoplastic lesions observed in dosed male or female rats or in dosed female mice. Acanthosis and hyperkeratosis were increased in the forestomach of high dose rats; acanthosis, hyperkeratosis, focal hyperplasia, and chronic active inflammation were increased in the forestomach of dosed mice. The incidence of squamous cell papillomas of the forestomach was increased in high dose male mice (vehicle control, 2/47; low dose, 3/47; high dose, 7/50). A squamous cell carcinoma of the forestomach was present in another high dose male mouse. Although the incidence of squamous cell papillomas in the high dose group was not significantly different from that in the vehicle controls, the incidence exceeded the highest observed in historical corn oil gavage vehicle controls (3/49). Other than a single squamous cell papilloma in the esophagus of a low dose male mouse, no hyperplastic or neoplastic lesions were seen outside the stomach of dosed mice which could be related to the administration of dimethoxane. Despite the observation of three harderian gland neoplasms in mice killed at 15 months, no increase in the incidences of harderian gland neoplasms was seen in dosed mice in the 2-year studies (male: 2/48; 2/48; 2/48; female: 2/48; 0/49; 2/50).
Dimethoxane was mutagenic in strain TA100 of S. typhimurium in the presence but not the absence of exogenous metabolic activation; it was not mutagenic in strains TA98, TA1535, or TA1537 with or without activation. Dimethoxane induced SCEs and chromosomal aberrations in CHO cells both with or without exogenous metabolic activation. Dimethoxane induced sex-linked recessive lethal mutations in Drosophila when administered by abdominal injection to adult males; no induction of reciprocal translocations was observed in adult males after injection of dimethoxane.
Under the conditions of these 2-year corn oil gavage studies, there was no evidence of carcinogenic activity of dimethoxane for male F344/N rats receiving 62.5 or 125 mg/kg or for female F344/N rats receiving 125 or 250 mg/kg per day. There was equivocal evidence of carcinogenic activity of dimethoxane for male B6C3F1 mice, as indicated by an increased incidence of forestomach neoplasms. There was no evidence of carcinogenic activity for female B6C3F1 mice receiving 250 or 500 mg/kg per day. Acanthosis and hyperkeratosis occurred at increased incidences in the forestomach of high dose rats. Inflammation, acanthosis with hyperkeratosis, and focal hyperplasia occurred at increased incidences in the forestomach of dosed mice.