F.A. Smith

Physiologically based pharmacokinetics and the risk assessment process for methylene chloride

Methylene chloride (dichloromethane, DCM) is metabolized by two pathways: one dependent on oxidation by mixed function oxidases (MFO) and the other dependent on glutathione S-transferases (GST). A physiologically based pharmacokinetic (PB-PK) model based on knowledge of these pathways was used to describe the metabolism of DCM in four mammalian species (mouse, rat, hamster, and humans). Kinetic constants for the model were derived from in vivo experiments or the literature. The model was constructed to distinguish contributions from the two pathways of metabolism in lung and liver tissue, and to permit extrapolation from rodents to humans. Model validation was conducted by comparing predicted blood concentration time-course data in rats, mice, and humans with experimental data from these species. The tumor incidence in two chronic studies of DCM toxicity in mice was correlated with various measures of target tissue dose calculated with the PB-PK model. Tumor incidence correlated well with tissue AUC (area under the concentration/time curve) and amount of DCM metabolized by the GST pathway. However, tumor incidence did not correlate with the amount of DCM metabolized by the MFO pathway. Because of its low chemical reactivity, DCM is unlikely to be directly involved in carcinogenesis. Consequently, metabolism of DCM by GST appears to be important in carcinogenesis. The PB-PK model was used to estimate target doses of presumed toxic chemical species in humans exposed to DCM by inhalation or by drinking water. Target tissue doses in humans exposed to low concentrations of DCM are 140- to 170-fold lower (inhalation) or 50- to 210-fold lower (drinking water) than would be expected from the linear extrapolation and body surface area factors which have been used in conventional risk assessment methods (D. V. Singh, H. L. Spitzer, and P. D. White (1985). Addendum to the Health Assessment Document for Dichloromethane (Methylene Chloride). EPA/600/8-82/004F). The PB-BK analysis thus suggests that conventional risk analyses greatly overestimate the risk in humans exposed to low concentrations of DCM. PB-PK considerations provide a scientific basis for risk assessment, improve experimental design in chronic studies, and structure collection of quantitative metabolic constants required for risk assessment.

Three-generation reproduction study of rats given 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the diet.

Abstract A three-generation reproduction study was conducted to evaluate the effects of chronic, low-level ingestion of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Sprague-Dawley rats were maintained continuously on diets providing dose levels of 0, 0.001, 0.01, or 0.1 μg TCDD/kg/day. No significant toxicity was noted in the f0 rats of either sex during the 90 days of TCDD ingestion prior to mating. Significant decreases in fertility and neonatal survival were observed in the f0 generation rats receiving 0.1 μg TCDD/kg/day; these effects precluded continuation of this high dose level in subsequent generations. At 0.01 μg TCDD/kg/day, fertility was significantly decreased in the f1 and f2 but not f0 generations. Other indications of toxicity seen at 0.01 μg TCDD/kg/day included decreases in litter size at birth, gestation survival (proportion of pups born alive), and neonatal survival and growth. Among the rats receiving 0.001 μg TCDD/kg/day, no effect on fertility, litter size at birth, or postnatal body weight was observed in any generation. No consistent effect on neonatal survival was observed at 0.001 μg TCDD/kg/day. In summary, the reproductive capacity of rats ingesting TCDD was clearly affected at dose levels of 0.01 and 0.1 μg TCDD/kg/day, but not at 0.001 μg TCDD/kg/day, through three successive generations.

Teratogenicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in CF-1 mice☆

Abstract The effect of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) on the developing embryo and fetus of CF-1 mice has been evaluated. Pregnant CF-1 mice were given TCDD by oral gavage on Days 6 through 15 of gestation at dosages of 0, 0.001, 0.01, 0.1, 1, and 3 μg/kg/day. Little or no maternal toxicity was observed at any dosage. Cleft palate and dilated renal pelvis were found at 3.0 μg/kg/day. Cleft palate was found at 1.0 μg/kg/day. No malformations were found at the intermediate dosages of 0.1 or 0.01 μg/kg/day. Teratogenic effects observed, i.e., cleft palate and dilated renal pelvis, were comparable to those seen in studies utilizing other strains of mice. The incidence of malformations was not statistically significant at the 0.1-μg/kg/day dosage and below. The experimental nonteratogenic dosage for TCDD in the developing embryo and fetus of CF-1 mice was estimated to be 0.1 μg/kg/day.

Acute, pharmacokinetic, and subchronic toxicological studies of 2,4-dichlorophenoxyacetic acid.

The single-dose oral LD50 values in Fischer 344 rats for technical-grade, 2,4-dichlorophenoxyacetic acid (2,4-D), esters, and salts ranged from 553 mg/kg (isobutyl ester in females) to 1090 mg/kg (dimethylamine salt in males). The LD50 values for the acid, esters, or salts, when expressed as acid equivalents, were consistent which suggests that the acute toxicity was due to 2,4-D per se. Acute dermal LD50 values in rabbits for the acid, esters, and salts were greater than 2000 mg/kg. Overall, these results indicate that the acute oral and dermal toxicity of 2,4-D are low. Pharmacokinetics were evaluated in male Fischer 344 rats given single oral doses of 10, 25, 50, 100, or 150 mg 2,4-[14C]D/kg. The amount of 2,4-D in the plasma, kidney, and urine 6 hr postdosing indicated that the urinary elimination of 2,4-D was saturated in male rats given oral doses in excess of 50 mg/kg. Subchronic dietary studies in male and female Fischer 344 rats used dose levels of 0, 15, 60, 100, or 150 mg/kg/day of purified or technical-grade 2,4-D acid for 13 weeks. Body weight gains were decreased for both sexes at the higher dose levels of purified and technical-grade 2,4-D acid. Kidney weights were increased in all treated male rats and in females given the higher three dose levels of purified 2,4-D. Treatment-related cytoplasmic alterations were present in the renal proximal tubules of most rats given 60 mg/kg/day and higher of purified or technical-grade 2,4-D; a few females given 15 mg/kg/day also had slight alterations in the cytoplasm of the proximal tubules. A dose-related degenerative change was identified in the descending proximal renal tubules of all male rats given the highest three dose levels of either test material and some given 15 mg/kg/day. Dose levels of 100 or 150 mg/kg/day of either compound for both sexes produced minimal swelling and increased staining homogeneity in the liver cells and were associated with a slight elevation of liver weight and serum glutamic pyruvic transaminase activity. Higher dose levels of technical-grade and purified 2,4-D decreased total serum tetraiodothyronine levels in female rats, however, the morphology of the thyroid gland was normal. The no-observed-effect level (NOEL) was less than 15 mg/kg/day for both purified and technical-grade 2,4-D acid.

The effects of maternally inhaled vinyl chloride on embryonal and fetal development in mice, rats, and rabbits.

Abstract These studies evaluated the effects of inhaled vinyl chloride on mouse, rat, and rabbit embryonal and fetal development. Groups of pregnant CF-1 mice, Sprague-Dawley rats and New Zealand white rabbits were exposed to 500 ppm of vinyl chloride 7 hr daily during the period of major organogenesis. Subsequently, other groups of mice were similarly exposed to 50 ppm of vinyl chloride and rats and rabbits were exposed to 2500 ppm of vinyl chloride. While maternal toxicity was observed, vinyl chloride alone did not cause significant embryonal or fetal toxicity and was not teratogenic in any of the species at the concentrations tested. Maternal toxicity was more prominent among mice than among rats and rabbits. Simultaneous exposure of some of the pregnant animals to vinyl chloride by inhalation plus 15% ethanol in the drinking water resulted in toxic effects greater than those associated with exposure to vinyl chloride alone in the three species. The maternal toxicity was enhanced to an extent greater than the embryotoxicity.

Results of a reproduction study in rats fed diets containing hexachlorobutadiene

Abstract Hexachlorobutadiene (HCBD) is a by-product of certain processes associated with the chlorination of hydrocarbons. Very small amounts of HCBD may also be produced in chlorine cells. This study evaluated the effects of HCBD on reproduction in rats. Groups of male and female adult rats were fed diets containing HCBD at dose levels of 0, 0.2, 2.0, or 20 mg/kg/day for 90 days prior to mating, 15 days during mating, and subsequently throughout gestation and lactation. Signs of toxicity among the adult rats were observed at the two higher dose levels and included decreased weight gain and food consumption as well as alterations in kidney structure. There was no effect on pregnancy or neonatal survival and development. The body weight of neonates at the time of weaning, 21 days of age, was slightly but significantly less than that of control litters. This effect was found only at the high dose level. No toxic effects were observed among the adults at a dose level of 0.2 mg/kg/day or among the neonates at dose levels of 0.2 or 2.0 mg/kg/day.

Three-generation reproduction study of rats ingesting 2,4,5-trichlorophenoxyacetic acid in the diet

Male and female 4–6-wk-old Sprague-Dawley rats (F0) were fed lab chow containing 2,4,5-T to provide dosage levels of 0, 3, 10 or 30 mg/kg/day for 90 days and were then bred. At day 21 of lactation, pups were randomly selected for the following generation (F1) and the rest were autopsied. Subsequent matings were conducted to produce F2, F3a and F3b litters, successive generations being fed from weaning on the appropriate test or control diet. Fertility was decreased in the matings for the F3b litters in the group on the 10-mg/kg/day dose level. Postnatal survival was significantly decreased in the F2 litters of the 10-mg/kg group and the F1′ F2 and F3a litters of the 30-mg/kg/day group. The relative liver weight of weanlings was significantly increased in the F2′ F3a and Fb litters of the 30-mg/kg/day group. A significant decrease in relative thymus weight was seen only in the Fb generation of this dosage group. Thus, dose levels of 2,4,5-T that were sufficiently high to cause signs of toxicity in neonates had no effect on the reproductive capacity of rats, except for a tendency toward a reduction in postnatal survival at a dose level of 30 mg/kg/day. Reproduction was not impaired at the lowest dose level (3 mg/kg/day).

Teratologic evaluation of 3,6-dichloropicolinic acid in rats and rabbits

3,6-Dichloropicolinic acid (clopyralid), a new herbicide, was evaluated for teratogenic potential in Fischer 344 rats and New Zealand White rabbits. Rats were given 0, 15, 75, or 250 mg clopyralid/kg/day by gavage on Days 6-15 of gestation while rabbits were given 0, 110, or 250 mg clopyralid/kg/day on Days 6-18 of gestation. Maternal toxicity, as evidenced by decreased body weight gain, was observed among pregnant rats in the 250-mg/kg/day group. No evidence of maternal toxicity was observed among treated rabbits. A teratogenic effect was not detected in either species.