Bioassay of Styrene for Possible Carcinogenicity (CAS No.100-42-5)

Styrene, a widely used intermediate in the manufacture of plastics, elastomers, and resins, was selected for bioassay by the National Cancer Institute because of the widespread use of this compound and a lack of adequate carcinogenicity data. A bioassay for the possible carcinogenicity of styrene was conducted using Fischer 344 rats and B6C3F1 mice. Styrene was administered by gavage to groups of 50 male and 50 female animals of each species. Forty rats of each sex and twenty mice of each sex were placed on test as vehicle controls. The high, medium, and low dosages of styrene administered to rats were, respectively, 2,000, 1,000, and 500 mg/kg. The high and low dosages administered to mice were 300 and 150 mg/kg, respectively. The compound was administered for 78 weeks to high and medium dose rats, for 103 weeks to low dose rats, and for 78 weeks to mice. The period of compound administration was followed by an observation period of 27 weeks for high and medium dose rats, 1 week for low dose rats, and 13 weeks for mice. Mortality among male and female high dose rats was significantly higher than that among their respective vehicle controls. In response to this elevated and early mortality, an additional dosed group of each sex was included in the chronic bioassay. No significant positive association was apparent between dosage and mortality among any other dosed rat groups. For mice, there was a significant positive association between mortality and the dosages of styrene administered to males, but not to females. Adequate numbers of animals in all groups, except for the high dose male and female rats, survived sufficiently long to be at risk from late-developing tumors. Slight dose-related mean body weight depression was apparent when male rats and female mice were compared to their respective vehicle controls, indicating that the dosages administered to these animals during the chronic bioassay may have approximated the maximum tolerated dosages. There was no distinct depression in mean body weight when dosed female rats anddosed male mice were compared to their respective vehicle controls. However, since there was significant accelerated mortality among high dose female rats, it is possible that the dosage administered to the medium dose female rats may have exceeded the maximum tolerated dosage. In male mice, there was a significant positive association between styrene dosage and the incidences of a combination of adenomas and carcinomas of the lung. This finding was supported by the high dose to control Fisher exact comparison. However, the variation of the incidence of these neoplasms in historical control male mice at this laboratory does not permit a firm conclusion of carcinogenicity. There was no significant difference between tumor incidence at any other site in male mice, or at any site in rats or female mice, when dosed groups were compared to vehicle controls. The findings of an increased incidence of a combination of adenomas and carcinomas of the lung provided suggestive evidence for the carcinogenicity of styrene in male B6C3F1 mice. However, it is concluded that, under the conditions of this bioassay, no convincing evidence for the carcinogenicity of the compound was obtained in Fischer 344 rats or B6C3F1 mice of either sex. Levels of Evidence of Carcinogenicity: Male Rats: Negative Female Rats: Negative Male Mice: Equivocal Female Mice: Negative Synonyms: ethenylbenzene; vinylbenzene; vinylbenzol; styrolene; styrol; styrole; styropol; styropor; styron; cinnamene; cinnamol; phenethylene; phenylethylene; phenylethene