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 body tissues, mainly adipose, resulting in chronic lifetime human exposure. Since human exposure to DLCs always involves 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.
2,3,7,8-Tetrachlorodibenzo-p-dioxin (
TCDD),
2,3,4,7,8-pentachlorodibenzofuran (PeCDF), and
3,3',4,4',5-pentachlorobiphenyl (
PCB 126) are not manufactured commercially other than for scientific research purposes. The main sources of
TCDD and PeCDF releases into the environment are from
metal smelting, refining, and processing; combustion and incineration sources; chemical manufacturing and processing; biological and photochemical processes; and existing reservior sources that reflect past releases. PCB mixtures were commercially produced and used in the electric power industry as dielectric insulating fluids in transformers and capacitors and used in hydraulic fluids, plastics, and paints.
TCDD, PeCDF, and
PCB 126 were selected for study by the National Toxicology Program as part of the
dioxin TEF evaluation to assess the
cancer risk posed by
complex mixtures of
polychlorinated dibenzodioxins (PCDDs),
polychlorinated dibenzofurans (PCDFs), and
PCBs. The
dioxin TEF evaluation includes conducting multiple 2-year rat bioassays to evaluate the relative chronic toxicity and carcinogenicity of DLC's, structurally related
PCBs, and mixtures of these compounds. Female Harlan Sprague-Dawley rats were administered a mixture of
TCDD, PeCDF, and
PCB 126 (henceforth referred to as the TEF mixture) in
corn oil:
acetone (99:1) by gavage for 14, 31, or 53 weeks or 2 years. While one of the aims of the
dioxin TEF evaluation was a comparative analysis across studies, in this Technical Report only the results of the present study of the mixture of
TCDD, PeCDF, and
PCB 126 are presented and discussed. 2-YEAR STUDY: Groups of 81 female rats were administered 10, 22, 46, or 100 ng toxic equivalents (TEQ)/kg
body weight in
corn oil:
acetone (99:1) by gavage, 5 days per week, for up to 105 weeks; a group of 81 vehicle control female rats received the
corn oil/
acetone vehicle alone. Actual doses used for each compound in the mixture were: for 10 ng TEQ/kg: 3.3 ng/kg
TCDD, 6.6 ng/kg PeCDF, and 33.3 ng/kg
PCB 126; for 22 ng TEQ/kg: 7.3 ng/kg
TCDD, 14.5 ng/kg PeCDF, and 73.3 ng/kg
PCB 126; for 46 ng TEQ/kg: 15.2 ng/kg
TCDD, 30.4 ng/kg PeCDF, and 153 ng/kg
PCB 126; and for 100 ng TEQ/kg: 33 ng/kg
TCDD, 66 ng/kg PeCDF, and 333 ng/kg
PCB 126. Up to 10 rats per group were evaluated at 14, 31, or 53 weeks. Survival of all dosed groups of rats was similar to that of the vehicle control group. Mean
body weights of the 22 and 46 ng TEQ/kg groups were less than those of the vehicle control groups after week 69 of the study. Mean
body weights of the 100 ng TEQ/kg group were less than those of the vehicle control group after week 37 of the study.
Thyroid Hormone Concentrations: Alterations in serum
thyroid hormone concentrations were evaluated at the 14-, 31-, and 53-week interim evaluations. At 14, 31, and 53 weeks, there were dose-dependent reductions in total serum and free
thyroxine concentrations. There were dose-dependent increases in serum
triiodothyronine concentrations at 14 and 31 weeks. No changes in serum
thyroid stimulating hormone concentrations were observed at any time point. Hepatic Cell Proliferation Data: To evaluate hepatocyte replication, analysis of labeling of replicating hepatocytes with
5-bromo-2'-deoxyuridine was conducted at the interim evaluations. At 14 weeks, no effects on the hepatocellular labeling index were observed in the dosed groups compared to the vehicle controls. At 31 and 53 weeks, the hepatocellular labeling index was significantly higher in the 46 and 100 ng TEQ/kg groups compared to the vehicle controls.
Cytochrome P450 Enzyme Activities: To evaluate the expression of known
dioxin-responsive genes, CYP1A1-associated 7-ethoxyresorufin-O-deethylase (
EROD) activity and CYP1A2-associated acetanilide-4-hydroxylase (A4H) activity were evaluated at the interim time points. Liver and lung
EROD (
CYP1A1) activities and hepatic A4H (
CYP1A2) activities were significantly greater in all dosed groups than in the vehicle controls at all interim evaluations (14, 31, and 53 weeks). Determinations of
TCDD, PeCDF, and
PCB 126 Concentrations in Tissues: Tissue concentrations of
TCDD, PeCDF, and
PCB 126 were analyzed in the fat, liver, lung, and blood at each interim evaluation and at the end of the 2-year study (105 weeks). The highest concentrations of
TCDD, PeCDF, and
PCB 126 were observed in the liver followed by fat. Liver and fat concentrations of
TCDD, PeCDF, and
PCB 126 at each interim evaluation and at 105 weeks were higher in groups with increasing doses of the mixture and generally increased with duration of dosing. In the lung, PeCDF was present at detectable concentrations in the 46 and 100 ng TEQ/kg groups at 14 and 31 weeks. Measurable concentrations of
TCDD and
PCB 126 were observed at 14 and 31 weeks in the lung of rats in all dosed groups with the highest concentrations observed in the 100 ng TEQ/kg group. At 53 weeks, concentrations of
TCDD, PeCDF, and
PCB 126 in the lung generally increased with increasing dose. At 105 weeks, detectable concentrations of
TCDD, PeCDF, and
PCB 126 in the lung were observed in all dosed groups. In blood,
TCDD and
PCB 126 concentrations at 14 and 31 weeks generally increased with increasing dose. Blood concentrations of PeCDF were detectable in the 46 and 100 ng TEQ/kg groups at 14 weeks and at 22 ng TEQ/kg or greater at 31 weeks. At 53 and 105 weeks, concentrations of
TCDD, PeCDF, and
PCB 126 in blood generally increased with increasing dose and duration of dosing. Pathology and Statistical Analyses: Relative liver weights were significantly increased in all dosed groups at 14, 31, and 53 weeks and correlated with increased incidences of hepatocellular
hypertrophy. Increasing duration of exposure led to an increase in the spectrum, incidence, and severity of nonneoplastic effects. The only significant effect at 14 weeks was increased incidences of hepatocellular
hypertrophy. At 53 weeks, there was a significant effect on the incidences of hepatocellular
hypertrophy, multinucleated hepatocytes, pigmentation, focal fatty change, bile duct
hyperplasia, and toxic hepatopathy. At 2 years, there were significant increases in the incidences of
hepatocellular adenoma and
cholangiocarcinoma of the liver. There was an increase in hepatic toxicity characterized by increases in the incidences of numerous nonneoplastic lesions including hepatocyte
hypertrophy, multinucleated hepatocytes, pigmentation,
inflammation, diffuse fatty change, bile duct
hyperplasia, oval cell
hyperplasia, nodular
hyperplasia, eosinophilic focus, cholangiofibrosis,
bile duct cysts,
necrosis, portal
fibrosis, mixed cell focus, and toxic hepatopathy. In the lung, there were dose-dependent increases in the incidences of bronchiolar
metaplasia of the alveolar epithelium at 53 weeks and at 2 years and squamous
metaplasia at 2 years. At 2 years, there was a dose-dependent increase in the incidences of cystic keratinizing
epithelioma. In the pancreas, there were increases in the incidences of numerous nonneoplastic lesions including arterial chronic active
inflammation, acinar cytoplasmic vacuolization, acinar
atrophy, chronic active
inflammation, and duct dilatation. At 2 years, incidences of acinar
adenoma or
acinar carcinoma that exceeded the historical control ranges were seen in all dosed groups except the 100 ng TEQ/kg group. Treatment-related increases in the incidences of nonneoplastic lesions were seen in other organs including
hyperplasia, cystic degeneration,
atrophy, and cytoplasmic vacuolization of the adrenal cortex; gingival squamous
hyperplasia of the oral mucosa; squamous
metaplasia of the uterus;
atrophy of the thymus (incidence and severity); chronic active
inflammation of the ovary; nephropathy of the kidney (incidence and severity);
cardiomyopathy; bone marrow
hyperplasia; transitional epithelium of the urinary bladder; chronic active
inflammation of the mesenteric artery; and follicular cell
hypertrophy of the thyroid gland. (ABSTRACT TRUNCATED).