Toxicology and carcinogenesis studies of 4-vinylcyclohexene (greater than 98% pure), a dimer of 1,3-butadiene present in the off-gasses from tire curing, were conducted by administering the chemical in corn oil by gavage 5 days per week at doses of 0, 200, or 400 mg/kg body weight to groups of 50 F344/N rats and B6C3F1 mice of each sex for 103 weeks. Doses selected for the 2-year studies were based on survival, body weight gains, and histopathologic effects observed during the 14-day and 13 week studies.
All rats and most mice in the 14-day studies died when administered doses greater than or equal to 1,250 mg/kg, although no compound-related gross or histopathologic effects were observed. Final body weights were reduced in the 13-week studies in male rats receiving doses greater than or equal to 400 mg/kg of 4-vinylcyclohexene, in female rats receiving 800 mg/kg, and in female mice receiving 600 mg/kg. Extensive mortality was observed only in mice dosed at 1,200 mg/kg. Compound-related histopathologic effects in the 13-week studies included hyaline droplet degeneration of the proximal convoluted tubules of the kidney in dosed male rats, the severity of which was dose related, and a reduction in the number of primary follicles and mature graafian follicles in the ovaries of female mice receiving 1,200 mg/kg of 4-vinylcyclohexene. No compound-related gross or histopathologic effects were evident in dosed female rats or male mice in the 13-week studies.
Many dosed rats died early in the 2-year studies (male: vehicle control, 17/50; low dose, 37/50; high dose, 45/50; female: vehicle control, 10/50; low dose, 22/50; high dose, 36/50; P<0.001 for all groups except low dose female rats, for which P=0.022). The poor survival of dosed male and female rats reduced the sensitivity of the studies for detecting the possible carcinogenic effects of 4-vinylcyclohexene. Mean body weights of dosed rats were comparable to those of their respective vehicle controls, except for high dose males late in the study. Survival of high dose mice of each sex was lower (P<0.001) than that of the vehicle controls, whereas survival of low dose mice of each sex was comparable to that of the vehicle controls. Mean body weights of high dose mice of each sex were generally lower than those of the vehicle controls throughout most of the 2-year studies.
Administration of 4-vinylcyclohexene to F344/N rats by gavage for 2 years was associated with a slightly increased incidence of epithelial hyperplasia of the forestomach (1/50; 3/50; 5/47) and squamous cell papillomas or carcinomas (combined) of the skin in high dose males (0/50; 1/50; 4/50). Low dose female rats, whose survival was more similar to that of the vehicle controls, had a marginally increased incidence of adenomas or squamous cell carcinomas (combined) of the clitoral gland (1/50; 5/50; 0/49).
In B6C3F1 mice, administration of 4-vinylcyclohexene for two years by gavage was associated with mild, acute inflammatory lesions and epithelial hyperplasia of theforestomach, especially in males (0/47; 7/50; 7/46), and with an increased incidence of a number of other nonneoplastic lesions, including lung congestion in high dose males and females, splenic red pulp atrophy in high dose males, congestion of the adrenal gland in high dose females, and cytologic alteration of the adrenal cortex in low dose and high dose females.
The incidences of uncommon ovarian neoplasms were markedly increased (P<0.01) in both groups of dosed female mice (mixed tumor, benign: 0/49; 25/48, 52%; 11/47, 23%; granulosa cell tumor or carcinoma [combined]: 1/49, 2%; 10/48, 21%; 13/47, 28%). In addition, a slight increase in the incidence of adrenal gland adenomas in high dose females was observed (0/50; 3/49, 6%; 4/48, 8%). The extensive mortality seen in the high dose male mice confounded interpretation of the increased incidences of malignant lymphomas and alveolar/bronchiolar adenomas or carcinomas (combined) of the lung seen in these animals surviving to the end of the study (malignant lymphomas: 3/37, 8%; 5/39, 13%; 4/7, 57%; alveolar/bronchiolar adenomas or carcinomas [combined]: 3/37, 8%; 9/39, 23%; 3/7, 43%).
4-Vinylcyclohexene was not mutagenic in Salmonella typhimurium strains TA100, TA1535, TA1537, or TA98 in the presence or absence of Aroclor 1254-induced male Sprague-Dawley rat or male Syrian hamster liver S9 when tested according to the preincubational protocol. However, several of its metabolites, including 4-vinylcyclohexene diepoxide, have been shown to be mutagenic in Salmonella and/or induce chromosomal damage in vitro.
An audit of the experimental data was conducted for these 2-year carcinogenesis studies on 4-vinylcyclohexene. No data discrepancies were found that influenced the final interpretations.
4-Vinylcyclohexene was administered by gavage in corn oil to F344/N rats and B6C3F1 mice of each sex at doses of 200 or 400 mg/kg for 103 weeks. Under these conditions, the 2-year gavage studies of 4-vinylcyclohexene in male and female rats and male mice were considered inadequate studies of carcinogenicity because of extensive and early mortality at the high dose or at both doses and the lack of conclusive evidence of a carcinogenic effect. There was clear evidence of carcinogenicity of 4-vinylcyclohexene for female mice, as shown by markedly increased incidences of uncommon ovarian neoplasms at both doses. In addition, the increased incidence of adrenal gland adenomas in high dose female mice may have been related to the administration of 4-vinylcyclohexene.