Dioxin toxic equivalency factor evaluation overview
Polyhalogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have the ability to bind to and activate the ligand-activated transcription factor, the aryl hydrocarbon receptor (AhR). Structurally related compounds that bind to the AhR and exhibit biological actions similar to TCDD are commonly referred to as "dioxin-like compounds" (DLCs). Ambient human exposure to DLCs occurs through the ingestion of foods containing residues of DLCs that bioconcentrate through the food chain. Due to their lipophilicity and persistence, once internalized they accumulate in adipose tissue resulting in chronic lifetime human exposure.
Since human exposure to DLCs always occurs as a complex mixture, the toxic equivalency factor (TEF) methodology has been developed as a mathematical tool to assess the health risk posed by complex mixtures of these compounds. The TEF methodology is a relative potency scheme that ranks the dioxin-like activity of a compound relative to TCDD, which is the most potent congener. This allows for the estimation of the potential dioxin-like activity of a mixture of chemicals, based on a common mechanism of action involving an initial binding of DLCs to the AhR.
The toxic equivalency of DLCs was nominated for evaluation because of the widespread human exposure to DLCs and the lack of data on the adequacy of the TEF methodology for predicting relative potency for cancer risk. To address this, the National Toxicology Program conducted a series of 2-year bioassays in female Harlan Sprague-;Dawley rats to evaluate the chronic toxicity and carcinogenicity of DLCs and structurally related polychlorinated biphenyls (PCBs) and mixtures of these compounds.
Polychlorinated biphenyls (PCBs) and their mixtures including 2,3',4,4',5-pentachlorobiphenyl (PCB 118) were produced commercially before 1977 for the electric industry as dielectric insulating fluids for transformers and capacitors. Manufacture and use of these chemicals were stopped because of increased PCB residues in the environment, but they continue to be released into the environment through the use and disposal of products containing PCBs, as by-;products during the manufacture of certain organic chemicals, during combustion of some waste materials, and during atmospheric recycling. This PCB 118 study was conducted as part of the dioxin TEF evaluation that included multiple 2-year rat bioassays to evaluate the relative chronic toxicity and carcinogenicity of DLCs, structurally related PCBs, and mixtures of these compounds. Female Harlan Sprague-Dawley rats were administered PCB 118 (at least 99% pure) in corn oil:acetone (99:1) by gavage for 14, 31, or 53 weeks or 2 years.
Groups of 80 female rats were administered 100, 220, 460, 1,000, or 4,600 ;µg PCB 118/kg body weight in corn oil:acetone (99:1) by gavage, 5 days per week, for up to 105 weeks; a group of 80 vehicle control female rats received the corn oil/acetone vehicle alone. Groups of 30 female rats received 10 or 30 µg/kg for up to 53 weeks only. Up to 10 rats per group were evaluated at 14, 31, or 53 weeks. A stop-exposure group of 50 female rats was administered 4,600 µg/kg PCB 118 in corn oil:acetone (99:1) by gavage for 30 weeks then the vehicle for the remainder of the study.
Survival of all dosed groups of rats was similar to that of the vehicle control group. Mean body weights of 1,000 ;µg/kg rats were 7% less than those of the vehicle controls after week 36, and those of the 4,600 µg/kg core study and stop-exposure groups were 7% less than those of the vehicle controls after week 7. Following cessation of treatment, the body weight gain in the stop-exposure group was similar to that of the vehicle control group.
In general, exposure to PCB 118 lead to dose-dependent decreases in the concentrations of serum total thyroxine (T4) and free T4 in all dosed groups. There were no effects on triiodothyronine or thyroid stimulating hormone levels in any dosed groups evaluated at the 14-, 31-, and 53-week interim evaluations. There were increases in hepatic cell proliferation in the 4,600 µg/kg group at 14, 31, and 53 weeks. Administration of PCB 118 led to dose-dependent increases in CYP1A1-associated 7-ethoxyresorufin-O-deethylase, CYP1A2-associated acetanilide;4-hydroxylase, and CYP2B-associated pentoxyresorufin-O-deethylase activities at the 14-, 31-, and 53-week interim evaluations. Analysis of PCB 118 concentrations in dosed groups showed dose- and duration of dosing-dependent increases in fat, liver, lung, and blood. The highest concentrations were seen in fat at 2 years with lower concentrations observed in the liver, lung, and blood.
At the 53-week interim evaluation, three 4,600 µg/kg rats had liver cholangiocarcinoma and one had hepatocellular adenoma. At 2 years, there were significant treatment;-related increases in the incidences of cholangiocarcinoma and hepatocellular adenoma. Four incidences of hepatocholangioma occurred in the 4,600 µg/kg core study group.
At 2 years, a significant dose-related increase in hepatic toxicity was observed and was characterized by increased incidences of numerous lesions including hepatocyte hypertrophy, inflammation, oval cell hyperplasia, pigmentation, multinucleated hepatocyte, eosinophilic and mixed cell foci, diffuse fatty change, toxic hepatopathy, nodular hyperplasia, necrosis, bile duct hyperplasia and cyst, and cholangiofibrosis. The incidences of these lesions were often decreased in the 4,600 µg/kg stop-exposure group compared to the 4,600 µg/kg core study group.
In the lung at 2 years, a significantly increased incidence of cystic keratinizing epithelioma occurred in the 4,600 µg/kg core study group compared to the vehicle control group incidence. Incidences of bronchiolar metaplasia of the alveolar epithelium were significantly increased in the groups administered 460 µg/kg or greater, and the incidence of squamous metaplasia was significantly increased in the 4,600 µg/kg core study group.
The incidence of carcinoma of the uterus in the 4,600 µg/kg stop-exposure group was significantly greater than those in the vehicle control and 4,600 µg/kg core study groups at 2 years. A marginal increase in squamous cell carcinoma occurred in the 220 µg/kg group.
At 2 years, there were marginally increased incidences of exocrine pancreatic adenoma or carcinoma in the 460, 1,000, and 4,600 µg/kg core study groups.
Numerous nonneoplastic effects were seen in other organs including: adrenal cortical atrophy and cytoplasmic vacuolization, pancreatic acinar cell cytoplasmic vacuolization and arterial chronic active inflammation, follicular cell hypertrophy of the thyroid gland, inflammation and respiratory epithelial hyperplasia of the nose, and kidney pigmentation.
Under the conditions of this 2-year gavage study, there was clear evidence of carcinogenic activity of PCB 118 in female Harlan Sprague-Dawley rats based on increased incidences of neoplasms of the liver (cholangiocarcinoma, hepatocholangioma, and hepatocellular adenoma) and cystic keratinizing epithelioma of the lung. Occurrences of carcinoma in the uterus were considered to be related to the administration of PCB 118. Occurrences of squamous cell carcinoma of the uterus and acinar neoplasms of the pancreas may have been related to administration of PCB 118.