Bioassay of azobenzene for possible carcinogenicity.
A bioassay of azobenzene for possible carcinogenicity was conducted by administering the test chemical in feed to F344 rats and B6C3F1 mice. Groups of 50 rats of each sex were administered azobenzene at one of two doses, either 200 or 400 ppm, for 105 or 106 weeks. Matched controls consisted of 20 untreated rats of each sex. All surviving rats were killed at the end of administration of the test chemical. Groups of 50 male mice were administered azobenzene at one of two doses, either 200 or 400 ppm, for 105 weeks. Groups of 50 female mice were administered the test chemical at one of two doses, initially 400 or 800 ppm, for 38 weeks. Because of excessively lowered body weights in the dosed groups of the females, doses for the females were then reduced to 100 and 400 ppm, respectively, and administration at the lowered doses was continued for 67 or 68 weeks. The time-weighted average doses for the female mice were either 208 or 545 ppm. Matched controls consisted of 20 untreated mice of each sex. All surviving mice were killed at the end of administration of the test chemical. Mean body weights of dosed rats and mice of each sex were lower than those of corresponding controls, and were generally dose related throughout the bioassay. Mortality was dose related in the male rats and the female mice, but was not significantly affected in either the female rats or the male mice. Survival was 70% or greater at week 90 on study in all dosed and control groups of each species and sex; thus, sufficient numbers of animals were at risk in all groups for the development of late-appearing tumors. In rats, a large number of sarcomas, including fibrosarcomas, hemangiosarcomas, and osteosarcomas in both males and females and malignant hemangiopericytomas in females, occurred in the spleen and other abdominal organs at incidences that were dose related in each sex (P<0.001) and that in direct comparisons were significantly higher (P<0.001) in the high-dose groups of each sex than in the corresponding control groups (males: controls, 0/20, low-dose 6/49, high-dose 31/49; females: controls 0/20, low-dose 5/50, high-dose 21/50). In mice, no tumors occurred in either males or females at incidences that were significantly higher in the dosed groups than in the corresponding control groups. It is concluded that under the conditions of this bioassay, azobenzene was carcinogenic (sarcomagenic) for F344 rats, inducing various types of sarcomas in the spleen and other abdominal organs of both males and females. The test chemical was not carcinogenic for B6C3F1 mice of either sex.