Bioassay of a Mixture of 1,2,3,6,7,8-Hexachlorodibenzo-p-dioxin and 1,2,3,7,8,9-Hexachlorodibenzo-p-dioxin (Gavage) for Possible Carcinogenicity (CAS No. 57653-85-7,CAS No. 19408-74-3).
Hexachlorodibenzo-p-dioxins (HCDD) are formed during the manufacture of certain chlorophenols. They have been found in trichlorophenol, tetrachlorophenol, and pentachlorophenol and in the chlorophenol-derived herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). From 1967 to 1970, the concentration of HCDD in commercial pentachlorophenol ranged from 0.03 to 38 ppm. Since then, HCDD levels in pentachlorophenol have been less than 1 ppm. A bioassay of a mixture of 1,2,3,6,7,8- and 1,2,3,7,8,9-hexachlorodibenzo-p-dioxin (HCDD) for possible carcinogenicity was conducted by administering the test material by gavage to Osborne-Mendel rats and B6C3F1 mice for 104 weeks. Fifty rats and 50 mice of each sex were administered HCDD suspended in a vehicle of 9:1 corn oil-acetate 2 days per week for 104 weeks at doses of 1.25, 2.5, or 5 &mgr;g/kg/wk for rats and male mice and 2.5, 5, or 10 &mgr;g/kg/wk for female mice. Seventy-five rats and 75 mice of each sex served as vehicle controls. In addition, one untreated control group containing 25 rats and 25 mice of each sex was present in the HCDD treatment room, and one untreated control group containing 25 rats and 25 mice of each sex was present in the vehicle control room. All surviving animals were killed at 105 to 108 weeks. In rats, a dose-related depression in mean body weight gain became evident in the males after week 68 of the bioassay and in the females after week 33. In mice, the mean body weight gain in the dosed groups was comparable with that of the vehicle control groups. No other toxic clinical signs were reported in either the rats or the mice. Administration of HCDD had no adverse effect on the survival of either species. In male rats, hepatocellular carcinomas or neoplastic nodules occurred at low incidences that were dose related (P=0.003). In a direct comparison, the incidence of these tumors in the high-dose group was higher (P=0.022) than that in the corresponding vehicle-control groups, but the Bonferroni requirement of P=0.017 for the multiple comparison of three dosed groups with a control group was not met. In female rats, hepatocellular carcinomas, adenomas, or neoplastic nodules occurred at incidences that were dose related (P<0.001), and in direct comparisons the incidences of these tumors in the mid-and high-dosed groups were significantly higher (P=0.006 and P<0.001, respectively) than those in the corresponding vehicle-control group. In male mice, hepatocellular carcinomas or adenomas occurred at incidences that were dose related (P=0.001), and in a direct comparison the incidence of these tumors in the high-dose group was significantly higher (P=0.001) than that in the corresponding vehicle-control group. In female mice, hepatocellular carcinomas or adenomas occurred at incidences that were dose-related (P=0.002), and the incidence of these tumors in the high-dose group was significantly higher (P=0.004) than that in the corresponding vehicle-control group. Complex nonneoplastic toxic liver lesions were seen in all dosed groups of rats and mice. Compound-associated hyperplastic lesions of the lung were also found in both male and female rats. Under the conditions of this bioassay, HCDD administered by gavage was carcinogenic, causing increased incidences of hepatocellular carcinomas or neoplastic nodules in female Osborne-Mendel rats and inducing hepatocellular carcinomas and adenomas in male and female B6C3F1 mice. HCDD was not demonstrated to be carcinogenic for male rats. Levels of Evidence of Carcinogenicity: Male Rats: Equivocal Female Rats: Positive Male Mice: Positive Female Mice: Positive Synonym: HCDD