Toxicology and carcinogenesis studies of 3,3',4,4'-tetrachloroazobenzene (TCAB) (CAS No. 14047-09-7) in Harlan Sprague-Dawley rats and B6C3F1 mice (gavage studies).

Abstract

3,3',4,4'-Tetrachloroazobenzene (TCAB) is not commercially manufactured but is formed as an unwanted by-product in the manufacture of 3,4-dichloroaniline and its herbicidal derivatives Propanil, Linuron, and Diuron. It occurs from the degradation of chloroanilide herbicides (acylanilides, phenylcarbamates, and phenylureas) in soil by peroxide-producing microorganisms; and is formed by the photolysis and biolysis of 3,4-dichloroaniline. Humans may be exposed to TCAB during the manufacture as well as the application of herbicides containing TCAB as a contaminant. TCAB was nominated by the United States Environmental Protection Agency for toxicity and carcinogenicity testing based on its structural and biological similarity to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the potential for human exposure from the consumption of crops contaminated with 3,4-dichloroaniline-derived herbicides. Male and female Harlan Sprague-Dawley rats and B6C3F1 mice were administered TCAB (at least 97.8% pure) in corn oil:acetone (99:1) by gavage for 3 months (rats only) or 2 years. 3-MONTH STUDY IN RATS: Groups of 10 male and 10 female Harlan Sprague-Dawley rats were administered 0.1, 0.3, 1, 3, 10, 30, or 100 mg TCAB/kg body weight in corn oil:acetone (99:1) by gavage, 5 days a week, for 14 weeks; groups of 10 male and 10 female rats received the corn oil:acetone vehicle alone. Special study groups of 30 (dosed groups) or 6 (vehicle control group) female Harlan Sprague-Dawley rats were administered 0.1, 3, or 100 mg TCAB/kg body weight in corn oil:acetone (99:1) by gavage, 5 days a week, for 13 weeks; vehicle controls received the corn oil:acetone vehicle alone. All male and female rats survived to the end of the study. Terminal mean body weights of males were not significantly different from vehicle controls in any group. Terminal mean body weights of females administered 10 mg/kg or greater were significantly less than those of the vehicle controls. Mean body weight gains of all dosed groups of females were significantly less than those of the vehicle controls. The hematology results indicate that TCAB induced a microcytic normochromic responsive anemia in male Sprague-Dawley rats. Serum concentrations of total thyroxine (T4) and free T4 were significantly decreased in a dose-related manner in all dosed groups in both sexes compared to their respective vehicle controls; total triiodothyronine (T3) and thyroid stimulating hormone (TSH) concentrations were generally unaffected. There were no statistically significant differences in the BrdU labeling indices in the liver of males or females exposed to TCAB compared to their respective vehicle controls. Significant induction of hepatic 7-ethoxyresorufin-O-deethylase (EROD) and 7-pentoxyresorufin-O-deethylase activities was observed in all dosed groups of males and females. Significant induction of hepatic acetanilide-4-hydroxylase activity was observed in males exposed to 3 mg/kg or greater and all treated groups of females. EROD activities in the lung generally increased with increasing dose and were significantly greater in all treated groups of males and females compared to their respective vehicle controls. The highest concentrations of TCAB were observed in fat tissue with lower concentrations in the liver and lung. TCAB concentrations were significantly increased in a dose-dependent manner in all tissues from dosed groups relative to vehicle controls. At the end of the 3-month study, absolute and relative liver weights were significantly greater than those of the vehicle controls in all dosed groups of males and in females administered 10 mg/kg or greater. Absolute and relative lung weights were significantly greater in 100 mg/kg males and 3 mg/kg or greater females. Absolute and relative right kidney and spleen weights were generally significantly greater for all dosed groups of males. Absolute thymus weights of 10 mg/kg or greater males and absolute and relative thymus weights of 1 mg/kg or greater females were significantly less than those of the vehicle controls. In the liver, the incidences of midzonal to diffuse hepatocytic hypertrophy in males administered 1 mg/kg or greater and in females administered 10 mg/kg or greater were significantly greater than the vehicle control incidences. Hematopoietic cell proliferation occurred in most males administered 3 mg/kg or greater and most females administered 10 mg/kg or greater. The incidences of midzonal hepatocytic cytoplasmic fatty vacuolization were significantly increased in males administered 3 mg/kg or greater. In the lung, significantly increased incidences of bronchiolar metaplasia of the alveolar epithelium and interstitial mononuclear cell infiltration occurred in 10, 30, and 100 mg/kg males. The incidence of interstitial mononuclear cell infiltration was also significantly increased in 100 mg/kg females. Significantly increased incidences of hematopoietic cell proliferation of the spleen occurred in males administered 10 mg/kg or greater. The incidences of hemosiderin pigment of the spleen were significantly increased in 10 mg/kg or greater females. Atrophy in the thymus was significantly increased in all dosed groups of females, except the 0.1 mg/kg group, and in males administered 10 mg/kg or greater. 2-YEAR STUDY IN RATS: Groups of 50 male and 50 female Harlan Sprague-Dawley rats were administered 10, 30, or 100 mg TCAB/kg body weight in corn oil:acetone (99:1) by gavage, 5 days a week, for 2 years; groups of 50 male and 50 female rats received the corn oil:acetone vehicle alone. The survival of all dosed groups of males was significantly less than that of the vehicle controls. Mean body weights of 100 mg/kg males were less than those of the vehicle control group throughout the study. Mean body weights of 30 mg/kg males were 6% less than those of the vehicle control group after week 24, and those of 10 mg/kg males were 7% less than the vehicle control group after week 80. Mean body weights of 100 mg/kg females were less than those of the vehicle control group throughout the study, and those of 30 mg/kg females were 6% less than the vehicle control group after week 36. In the lung, the incidences of multiple cystic keratinizing epithelioma and single or multiple cystic keratinizing epithelioma (combined) in males and females were significantly increased in all dosed groups (except multiple epithelioma in 10 mg/kg females). Significantly increased incidences of pigmentation, alveolar epithelium squamous metaplasia (except 10 mg/kg females), and alveolar epithelium bronchiolar metaplasia occurred in all dosed groups of males and females. The incidences of histiocytic cellular infiltration in all dosed groups of males were significantly increased. In the liver, the incidences of cholangiocarcinoma (single or multiple) occurred in a positive trend in males and were significantly greater than that in the vehicle control group; the incidence in 100 mg/kg females was also increased. A significant dose-related increase in hepatic toxicity was observed in dosed rats and was characterized by increased incidences of numerous lesions including hepatocyte hypertrophy, centrilobular degeneration, hepatocellular necrosis, pigmentation, fatty change, bile duct hyperplasia, oval cell hyperplasia, nodular hyperplasia, hematopoietic cell proliferation, eosinophilic focus, mixed cell focus, multinucleated hepatocytes, bile duct cyst, toxic hepatopathy, and cholangiofibrosis. Significantly increased incidences of gingival squamous cell carcinoma within the oral mucosa occurred in 10 mg/kg males and 100 mg/kg males and females. The incidences of gingival squamous hyperplasia and cystic keratinizing hyperplasia in dosed groups of males and females were generally significantly increased. The incidences of follicular cell adenoma (single or multiple) of the thyroid gland in 30 and 100 mg/kg males were significantly greater than that in the vehicle control group. The incidences of follicular cell hypertrophy, follicular cell hyperplasia, and inflammation were significantly increased in 30 and 100 mg/kg males. Three incidences of single or multiple squamous cell papilloma of the forestomach occurred in 100 mg/kg females, and single incidences of squamous cell carcinoma of the forestomach occurred in 10 and 100 mg/kg females. Significantly increased incidences of epithelial hyperplasia occurred in all dosed groups of males and females. There were three incidences of malignant schwanomma in the thoracic cavity in 100 mg/kg males and a single incidence in 30 mg/kg males. In the adrenal cortex of 30 and 100 mg/kg females, there were slightly increased incidences of adenoma. In all dosed groups of males, the incidences of degeneration, cytoplasmic vacuolization, and hyperplasia of the zona fasciculata were significantly increased. Increased incidences and severities of necrosis occurred in 30 and 100 mg/kg males. Incidences of cytoplasmic vacuolation in 10 and 100 mg/kg females and hyperplasia of the zona fasciculata in 30 mg/kg females were significantly greater than those in the vehicle controls. Numerous nonneoplastic effects were seen in other organs including atrophy, acinar cytoplasmic vacuolization, and inflammation of the pancreas; blood vessel inflammation; lymphoid follicle atrophy and pigmentation of the spleen; pigmentation and atrophy of the mesenteric lymph node; germinal epithelial degeneration of the testes; and inflammation of the nose. 2-YEAR STUDY IN MICE: Groups of 50 male and 50 female mice were administered 3, 10, or 30 mg TCAB/kg body weight in corn oil:acetone (99:1) by gavage, 5 days a week, for 2 years; groups of 50 male and 50 female rats received the corn oil:acetone vehicle alone. Survival of 10 and 30 mg/kg males and 30 mg/kg females was significantly less than that of vehicle controls. All 30 mg/kg males died before the end of the study. (ABSTRACT TRUNCATED)