title: NTP Technical Report on the Toxicity Studies of 3,3',4,4'-Tetrachloroazoxybenzene (CAS No. 21232-47-3) Administered by Gavage to F344/N Rats and B6C3F1 Mice.
abstract: 3,3',4,4'-Tetrachloroazoxybenzene is not commercially manufactured but is present as a contaminant of 3,4-dichloroaniline and its herbicidal derivative Diuron(R). In addition, environmental contamination occurs when 3,3',4,4'-tetrachloroazoxybenzene is formed by the photolysis and biolysis of 3,4-dichloroaniline. 3,3',4,4'-Tetrachloroazoxybenzene was nominated by the United States Environmental Protection Agency for toxicity testing based on concerns over the potential for human exposure, the structural resemblance to 2,3,7,8-tetrachlorodibenzo-p-dioxin, and the reported dioxin-like effects of 3,3',4,4'-tetrachloroazoxybenzene. The toxicity of 3,3',4,4'-tetrachloroazoxybenzene was evaluated in 16-day and 13-week gavage studies in male and female F344/N rats and B6C3F1 mice. In addition to histopathology, evaluations included hematology (rats only), clinical chemistry, thyroid hormone analyses (rats only), hepatic cell proliferation (rats only), cytochrome P(450)1A immunohistological staining in the liver (rats only), and assessments of male reproductive endpoints and estrous cycle length. Additional genetic toxicology studies included mutagenicity tests in Salmonella typhimurium and the determination of micronuclei in mouse bone marrow and peripheral blood erythrocytes. In the 16-day studies, groups of five male and five female rats received 3,3',4,4'-tetrachloroazoxybenzene in corn oil by gavage at doses of 0, 12.5, 32, 80, 200, or 500 mg per kg body weight, 5 days a week. Groups of five male and five female mice received 0, 1, 3.2, 10, 32, or 100 mg/kg in corn oil by gavage, 5 days a week. Major effects in rats included increases in liver and lung weights, and decreases in mean body weights and body weight gains, heart weights, and thymus weights. Effects in mice included increases in liver weights and decreases in thymus weights. No effects on survival were observed. Treatment-related lesions included cytoplasmic alteration of hepatocytes, splenic hematopoietic cell proliferation, thymic atrophy, and nephropathy in rats and thymic atrophy, splenic hematopoietic cell proliferation, and hepatic foci of inflammation and necrosis in mice. In the 13-week studies, groups of 10 male and 10 female rats and mice received 3,3',4,4'- tetrachloroazoxybenzene in corn oil by gavage at doses of 0, 0.1, 1, 3, 10, or 30 mg/kg, 5 days a week. In the 13-week rat study, all males and seven females in the 30 mg/kg groups died. Decreases in final mean body weights and body weight gains were observed in 3 and 10 mg/kg males and 10 and 30 mg/kg females. Decreased thymus weights, accompanied by thymic atrophy observed microscopically, were observed at doses of 1 mg/kg or greater in males and females. Increased liver weights were observed in males and females administered 1 mg/kg or greater, and hepatic cytochrome P(450)1A staining was increased in 1 and 3 mg/kg males and 3, 10, and 30 mg/kg females. In addition, a responsive anemia and decreases in platelet counts were observed in dosed male and female rats. A marked decrease in circulating thyroxine concentrations was observed in dosed males and females. In spite of this sharp decrease, thyroid-stimulating hormone concentrations were marginally increased. A decrease in epididymal spermatozoal motility was observed in all dosed groups tested. In 10 mg/kg females, the estrous cycle length was increased. Major effects included increased incidences of hyperplasia of the forestomach in 3, 10, and 30 mg/kg males and 10 and 30 mg/kg females. Increased incidences of centrilobular degeneration and hematopoietic cell proliferation were observed in the liver of dosed males and females. Furthermore, chronic active inflammation of the lung vasculature and hematopoietic cell proliferation in the spleen were observed in dosed males and females. The increased severities of cardiomyopathy and nephropathy in males and the incidences of cardiomyopathy and nephropathy and severity of cardiomyopathy in females were 3,3',4,4'-tetrachloroazoxybenzene related. In the 13-week mouse study, the major effects included increases in liver weights in males administered 3 mg/kg or greater and females administered 1 mg/kg or greater. Hyperplasia of the forestomach and dilatation of hair follicles were observed in 10 and 30 mg/kg males and 30 mg/kg females. Furthermore, thymus weights were decreased in males administered 3 mg/kg or greater and in 10 and 30 mg/kg females. Increased incidences of centrilobular hypertrophy of hepatocytes were observed in 10 and 30 mg/kg males and females. Increased incidences of hematopoietic cell proliferation in the spleen were observed in 30 mg/kg males and in 10 and 30 mg/kg females. Increases in the incidences of thymocyte necrosis were observed in 10 mg/kg males and in 10 and 30 mg/kg females. The incidences of splenic pigmentation were increased in all dosed groups of males, and the severity of pigmentation increased with increasing dose in males and females. 3,3',4,4'-Tetrachloroazoxybenzene was not mutagenic in S. typhimurium strain TA97, TA98, TA100, or TA1535 with or without induced S9 metabolic activation enzymes. It did not induce significant increases in micronucleated erythrocytes in a three-exposure male mouse bone marrow micronucleus test up to dose levels of 200 mg/kg, but results of a 13-week peripheral blood micronucleus test conducted in male and female mice were positive. In summary, 3,3',4,4'-tetrachloroazoxybenzene caused typical dioxin-like effects, including thymic atrophy, increased liver weights, induction of hepatic cytochrome P(450)1A, and decreased mean body weight gains. Furthermore, a marked decrease in circulating thyroxine concentrations was observed in male and female rats, even at the lowest dose (0.1 mg/kg) in female rats. A decrease in epididymal sperm motility was observed at all doses in rats. Effects on the hematopoietic system occurred at doses including and lower than those that caused histopathologic alterations in the liver. A no-observable-adverse-effect-level (NOAEL) was not reached in rats. In male and female mice, the NOAEL was 1 and 0.1 mg/kg, respectively. Furthermore, treatment-related effects included increased incidences of hyperplasia of the forestomach epithelium in rats and mice, chronic active inflammation of the vasculature of the lung in rats, increased incidences and/or severities of cardiomyopathy and nephropathy in rats, and dilatation of the hair follicles in mice. Comparison of various dioxin-like effects in these studies with those reported in the literature indicate that 3,3',4,4'- tetrachloroazoxybenzene is six to two orders of magnitude less potent than 2,3,7,8-tetrachlorodibenzo-p-dioxin.