Colin N. Park

Results of a two-year chronic toxicity and oncogenicity study of 2,3,7,8-tetrachlorodibenzo-p-dioxin in rats

Rats were maintained for 2 years on diets supplying 0.1, 0.01, and 0.001 μg of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)/kg/day. Analysis of these diets indicated 2200, 210, and 22 parts per trillion (ppt) of TCDD. Ingestion of 0.1 μg/kg/day caused an increased incidence of hepatocellular carcinomas and squamous cell carcinomas of the lung, hard palate/nasal turbinates, or tongue, whereas a reduced incidence of tumors of the pituitary, uterus, mammary glands, pancreas, and adrenal gland was noted. Other indications of toxicity at this dose level included increased mortality, decreased weight gain, slight depression of erythroid parameters, increased urinary excretion of porphyrins and δ-aminolevulinic acid, along with increased serum activities of alkaline phosphatase, γ-glutamyl transferase and glutamic-pyruvic transaminase. Gross and histopathologic changes were noted in the hepatic, lymphoid, respiratory, and vascular tissues. The primary hepatic ultrastructural change at this high dose level was proliferation of the rough endoplasmic reticulum. Terminal liver and fat samples from rats at this high dose level contained 24,000 and 8100 ppt of TCDD, respectively. Rats given 0.01 μg/kg/day for 2 years had a lesser degree of toxicity than that seen at the highest dose level. This included increased urinary excretion of porphyrins in females, liver lesions (including hepatocellular nodules), and lung lesions (including focal alveolar hyperplasia). Terminal liver and fat samples from rats of this dose level contained 5100 and 1700 ppt of TCDD, respectively. Ingestion of 0.001 μg of TCDD/kg/day (∼22 ppt in the diet) caused no effects considered to be of any toxicologic significance. At this lower dose level, terminal liver and fat samples each contained 540 ppt of TCDD. These data indicate that continuous doses of TCDD sufficient to induce severe toxicity increased the incidence of some types of tumors, while reducing other types. During the 2-year study in rats, no increase in tumors occurred in those rats receiving sufficient TCDD to induce slight or no manifestations of toxicity.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD): results of a 13-week oral toxicity study in rats.

Abstract Rats were given 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at dosages of 0, 0.001, 0.01, 0.1, or 1.0 μg/kg, 5 days/wk for 13 wk. Doses of 1.0 μg TCDD/kg/day caused some mortality, inactivity, decreased body weights and food consumption, icterus, increased serum bilirubin and alkaline phosphatase, pathomorphologic changes in the liver, lymphoid depletion of the thymus and other lymphoid organs, increased urinary excretion of porphyrins and delta-aminolevulinic acid, and minimal alterations of some hematopoietic components. Morphological evidence of a functional suppression of the reproductive organs was consistent with either a direct toxic effect of this dose of TCDD, or an indirect toxic effect associated with the poor physical condition of these rats. Doses of 0.1 μg TCDD/kg/day caused decreased body weights and food consumption, and slight degrees of liver degeneration and lymphoid depletion. Other effects seen only in females given this dose level included increases in urinary excretion of coproporphyrin and delta-aminolevulinic acid and increases in serum alkaline phosphatase and bilirubin. Effects seen only in males given this dose level included a depression of some hematologic parameters (packed cell volume, red blood cells, and hemoglobin). In rats given 0.01 or 0.001 μg TCDD/kg/day, all parameters were essentially unaffected, except for a slight increase in the mean liver-to-body-weight ratio in rats given 0.01 μg TCDD/kg/day. This slight increase in relative liver weight was not considered of any toxicological significance. These data indicate that no discernible ill effects occurred in rats given 0.01 or 0.001 μg TCDD/kg 5 days/wk for 13 wk.

Resolution of dose-response toxicity data for chemicals requiring metabolic activation: Example—Vinyl chloride☆

Abstract The toxicity of many chemicals results from biotransformation products formed from the chemical rather than from the chemical per se. In such cases, the incremental response may become diminishingly smaller with increasing dose or exposure because activation of the chemical to the toxic form follows apparent Michaelis—Menten rather than apparent first-order kinetics. To illustrate this concept, rats were exposed to concentrations ranging from 1.4 to 4600 ppm of vinyl chloride for 6 hr, and the total amount metabolized was determined. The amount metabolized followed apparent Michaelis—Menten kinetics. For rats, the logarithmic probability incidence of angiosarcoma versus the amount of vinyl chloride metabolized rather than the exposure concentration of vinyl chloride is linear. Assuming no threshold in spite of evidence to the contrary, extrapolation of the data below the range of doses causing experimentally observable responses predicted an incidence of 0.01% hepatic angiosarcoma in rats exposed to 4.6 ppm of vinyl chloride. Theoretical extension of the extrapolation to humans after adjusting for metabolic and body mass differences was undertaken. The theoretical extrapolation for man exposed daily for 8 hr to 1 ppm suggests an incidence of 1.5 per 100,000,000. This theoretical incidence, although a likely overestimate because of a less than predicted incidence in men exposed to 200 ppm and greater, as well as evidence for a threshold in rats, is less than that expected to occur spontaneously. The concepts evolved from this analysis reveals why pharmacokinetics must be considered in designing toxicology experiments as well as in interpretation of the resulting data.

Incorporation of in vitro enzyme data into the physiologically-based pharmacokinetic (PB-PK) model for methylene chloride: implications for risk assessment.

Physiologically-based pharmacokinetic (PB-PK) models provide a mechanism for reducing the uncertainty inherent in extrapolating the results of animal toxicity tests to man. This paper discusses a technique for incorporating data from in vitro studies of xenobiotic metabolism into in vivo PB-PK models. Methylene chloride is used as an example, and carcinogenic risk estimates incorporating PB-PK principles are presented.

1,4-Dioxane. I. Results of a 2-year ingestion study in rats

Abstract Four groups of rats, 60/sex/level, were maintained on drinking water containing 0, 1.0, 0.1, or 0.01% 1,4-dioxane for up to 716 days. Male and female rats receiving 1% dioxane (equivalent to approximately 1015 and 1599 mg/kg/day, respectively) showed decreases in body weight gains, survival rates, and water consumption. Hepatocellular and renal tubular degenerative changes, accompanied by regenerative activity, were similar to those reported in previous studies following exposure to toxic levels of dioxane. Hepatocellular and nasal carcinomas, occurring at this dose level, were considered related to the lifetime exposure to these massive toxic dosages of dioxane. Male and female rats receiving 0.1% dioxane (equivalent to approximately 94 and 148 mg/kg/day, respectively) in the drinking water had variable degrees of renal and hepatic degenerative changes, but there was no indication of treatment-related tumor occurrence. Male and female rats receiving 0.01% dioxane in the drinking water (equivalent to approximately 9.6 and 19.0 mg/kg/day, respectively) showed no evidence of tumor formation or other toxic effects considered to be related to treatment. These data indicate a dose response for the toxicity of dioxane.

Risk of angiosarcoma in workers exposed to vinyl chloride as predicted from studies in rats

Dose-response data for the induction of angiosarcoma in rats exposed to various levels of vinyl chloride (VC) together with attendant biotransformation data were used to estimate the risk of developing angiosarcoma in persons exposed to VC. Since a biotransformation product of VC, not VC per se, is responsible for the induction of angiosarcoma, the body surface area of people relative to rats was used to estimate the dose of the carcinogen biotransformed from VC by the former. Four models were used to extrapolate the data. Using a probit model, 10 hepatic angiosarcomas were predicted to occur in a recently reported epidemiological cohort of 9677 workers whereas five have occurred. Linear models and that based on the equation, Risk = 1 − e−βx, where x = dose, do not appear as reliable. For an 8-hr day, 5 days/week, 35-year time-weighted-average exposure of 1 ppm, the predicted incidence of hepatic angiosarcoma using the probit model is 1.5 × 10−8.

A Cross-Validation Approach to Sample Size Determination for Regression Models

Abstract A cross-validation approach to the a priori determination of sample size requirements and the a posteriori estimates of the validity of a derived regression equation is developed for regression models, where sampling from multivariate normal populations is discussed in particular. Tables of sample size estimates are presented for the random model and their applications illustrated. An algorithm is given to obtain tables for the fixed model directly from those for the random case.

Carcinogenic risk assessment: Ethylene dibromide

Abstract Ethylene dibromide has been shown to result in an increased incidence of gastric tumors in rats following intubation at 40 mg/kg/day. The U.S. Environmental Protection Agency (EPA) has used a one-hit carcinogenic model with parameters derived from this bioassay in rats to estimate the risk of cancer in humans arising from inhalation exposure to ethylene dibromide. The EPA estimated an almost 100% lifetime incidence of cancer to be expected in workers exposed to 0.4 ppm of ethylene dibromide at citrus fumigation centers. This communication reports a test of the validity of that prediction based on a comparison of the incidence of cancer predicted by the method used by the EPA with that observed in a group of 156 workers employed in the production of ethylene dibromide. The one-hit model, as used by the EPA, predicted a total of 85 tumors above the normal background incidence in this group of employees. A total of eight tumors has been observed. Therefore, use of this model to predict carcinogenic response in humans appears to result in highly exaggerated risk estimates.

Quantitative Risk Assessment: State-of-the-Art for Carcinogenesis

Abstract A critical evaluation of the use of statistical models in carcinogenic risk assessment is made with emphasis on the strengths and weaknesses of current practice. The objective is to bring together information from the fields of toxicology and statistics to develop a sound scientific basis for making risk decisions. It is emphasized that risk assessment is a complex, multifaceted process that is not easily quantified and at present must be based on qualitative as well as on quantitative information. It is evident that dose-response modeling of tumor incidence data from animal studies is of limited value in estimating human risk associated with low-dose chemical exposures. The models take into account only one part of the complex process and have no accepted basis in biology. The key decision points, qualitative factors, and quantitative considerations are identified and discussed, and a risk assessment framework that incorporates these inputs is presented. A model that incorporates the tumorigenic...

Quantitative risk assessment: State-of-the-art for carcinogenesis

A critical evaluation of the use of statistical models in carcinogenic risk assessment is made with emphasis on the strengths and weaknesses of current practice. The objective is to bring together information from the fields of toxicology and statistics to develop a sound scientific basis for making risk decisions. It is emphasized that risk assessment is a complex, multifaceted process that is not easily quantified and, at present, must be based on qualitative as well as quantitative information. It is evident that dose-response modeling of tumor incidence data from animal studies is of limited value in estimating human risk associated with low-dose chemical exposures. The models take into account only one part of the complex process and have no accepted basis in biology. The key decision points, qualitative factors, and quantitative considerations are identified and discussed and a risk assessment framework that incorporates these inputs is presented. A model that incorporates the tumorigenic dose-response information as well as the qualitative and quantitative biological factors that affect the estimate of risk is proposed. It is concluded that much work needs to be done before a completely quantitative approach to risk assessment is to be useful; in particular pharmacokinetic modeling should be pursued more aggressively.