Some effects of trichloroethylene on mouse lungs and livers

Physiologically based pharmacokinetic modeling of inhaled trichloroethylene and its oxidative metabolites in B6C3F1 mice

A physiologically based pharmacokinetic (PBPK) model for inhaled trichloroethylene (TCE) was developed for B6C3F1 mice. Submodels described four P450-mediated metabolites of TCE, which included chloral hydrate (CH), free and glucuronide-bound trichloroethanol (TCOH-f and TCOH-b), trichloroacetic acid (TCA), and dichloroacetic acid (DCA). Inhalation time course studies were carried out for calibration of the model by exposing mice to TCE vapor concentrations of either 100 or 600 ppm for 4 h. At several time points, mice were euthanized and blood, liver, kidney, lung, and fat were collected and analyzed for TCE and its oxidative metabolites. Peak blood TCE concentrations were 0.86 and 7.32 microgram/mL, respectively, in mice exposed to 100 and 600 ppm TCE. The model overpredicted the mixed venous blood and tissue concentrations of TCE for mice of both exposure groups. Fractional absorption of inhaled TCE was proposed to explain the discrepancy between the model predictions and the TCE blood time course data. When fractional absorption (53%) of inhaled TCE was incorporated into the model, a comprehensive description of the uptake, distribution, and clearance of TCE in the blood was obtained. Fractional uptake of inhaled TCE was further verified by collecting TCE in exhaled breath following a 4-h constant concentration exposure to TCE and validation was provided by testing the model against TCE blood concentrations from an independent data set. The submodels adequately simulated the distribution and clearance kinetics of CH and TCOH-f in blood and the lungs, TCOH-b in the blood, and TCA and DCA, which were respectively detected for up to 43 and 14 h postexposure in blood and livers of mice exposed to 600 ppm TCE. This is the first extensive tissue time course study of the major metabolites of TCE following an inhalation exposure to TCE and the PBPK model predictions were in good general agreement with the observed kinetics of the oxidative metabolites formed in mice exposed to TCE concentrations of 100 and 600 ppm.

Aldehyde dismutation catalyzed by pulmonary carbonyl reductase: kinetic studies of chloral hydrate metabolism to trichloroacetic acid and trichloroethanol

The kinetics of the NAD(P)(+)-linked aldehyde dismutation by pulmonary carbonyl reductase of guinea pig were studied using a highly hydrated substrate, chloral hydrate (CH). The enzyme irreversibly converted the substrate into trichloroacetic acid (TCA) and trichloroethanol (TCE) in the presence of the reduced or oxidized cofactors, of which NAD(P)+ gave a higher reaction rate than did NAD(P)H, and the concentration ratios of the two products (TCA plus TCE) to CH utilized were 1:1. In the NAD(P)(+)-linked reaction TCA was the predominant product and its amount was compatible with that of TCE plus NAD(P)H produced, whereas in the NAD(P)H-linked reaction equal amounts of TCA and TCE were formed and the cofactor was little oxidized. These results suggest that the enzyme oxidized the hydrated aldehydes to TCA with NAD(P)+ as the cofactor and reduced the unhydrated aldehyde to TCE with NAD(P)H. The steady-state kinetic measurements in the NADP(+)-linked CH oxidation were consistent with an ordered Bi Bi mechanism which is the same as that for the secondary alcohol oxidation by the enzyme. The dehydrogenase activity was inhibited competitively with respect to CH by a secondary alcohol substrate, propan-2-ol. The CH and propan-2-ol dehydrogenase activities were similarly inactivated by 2,4,6-trinitrobenzene-sulfonate, and NADP(H), several cofactor analogs and a cofactor-competitive inhibitor, Cibacron blue dye, protected against the inactivation, which suggest that lysine residues are essential for catalysis.

Damage and repair of mouse bronchial epithelium following acute inhalation of trichloroethylene

The time-course of damage and repair in the bronchial epithelium of the mouse was studied morphometrically after a single inhalation of trichloroethylene (TCE). Mice were exposed for 30 min to 500, 1000, 2000, 3500, or 7000 ppm of TCE under controlled conditions and sacrificed after 2 and 24 h, and 2, 5, and 7 days. At all TCE concentrations, the damage consisted of dilation of endoplasmic reticulum and was selective for nonciliated cells throughout the bronchial tree. Damage was quantitatively evaluated by counting the percentage of vacuolated cells. Injury was maximal 24 h after exposure and was significantly (p less than 0.05) higher in bronchi with a diameter smaller than 0.4 mm (67.0% +/- 8.5%) in comparison with larger bronchi (34.0% +/- 13.2%). Cell turnover was studied by autoradiography after [3H]thymidine injection. Following TCE inhalation, the percentage of [3H]thymidine labeled cells, which in the controls was virtually zero, increased to a maximum of 13.5% +/- 6.8% (average at all doses) 48 h after treatment and then rapidly decreased to baseline values at 7 days (0.2% +/- 0.1%). Repair of some cells occurred after 24 h, and at 48 h the percentage of vacuolated cells decreased and became significantly related (p less than 0.01) to the administered TCE dose (2.4% +/- 1.4% and 21.2% +/- 6.0% average in the 500 and 7000 ppm groups, respectively). Repair of bronchial epithelium took place by both recovery of less damaged cells and proliferation of residual nonciliated cells, leading to restoration of the normal ratio between ciliated and nonciliated cells.

Metabolism, toxicity, and carcinogenicity of trichloroethylene

Lifetime cancer or unit risk estimates for TRI have been calculated by the EPA on the basis of metabolized dose-tumor incidence relationships. Previously, it was common practice to directly extrapolate exposure dose-tumor incidence data from laboratory animal studies to predict cancer risks in humans. Such direct species-to-species extrapolations, however, do not take into account potentially important species differences in systemic uptake, tissue distribution, metabolism, deposition at the site(s) of action, and elimination. The consideration and use of pharmacokinetic and metabolic data can significantly reduce, though not eliminate, uncertainties inherent in species-to-species, route-to-route, and high- to low-dose extrapolations. The total amount of TRI metabolized was considered in the most recent EPA Health Assessment Document for Trichloroethylene to be the effective dose (EFD) producing tumors. Exposure dose-metabolism relationships were determined from direct measurement data in inhalation and oral dosing studies in mice and rats. The magnitude of TRI metabolism in these two species closely approximated body surface area. Thus, it was assumed that the amount of TRI metabolized per square meter of surface area was equivalent among species when calculating human equivalent doses from the animal data. Direct measurement data from an inhalation study in humans were used to calculate the amount of TRI metabolized and the unit risk estimate when a person inhales 1 microgram TRI per cubic meter continuously for 24 h. The EPA Cancer Assessment Group (CAG) elected to use this risk estimate for TRI in air, since it was calculated on the basis of a human metabolized dose rather than unit risk estimates based on animal studies. The current survey of literature and ongoing research uncovered no new animal or human studies in which TRI metabolites were directly measured, which would be any more suitable for use in estimating the total metabolized dose of TRI. On the basis of information now available, it is appropriate to continue to use the total amount of TRI metabolized as the EFD producing tumors in the liver. Use of the total amount metabolized represents an important "step in the right direction" in reducing uncertainties in interspecies extrapolations of data on a chemical such as TRI. TRI is believed to be metabolically activated to a reactive intermediate(s), although the identity of the intermediate(s) is unclear. There is evidence that formation of reactive intermediate(s) and TRI hepatotoxicity are directly proportional to the overall extent of TRI metabolism.(ABSTRACT TRUNCATED AT 400 WORDS)

Lung injury induced by trichloroethylene

Trichloroethylene (TCE) produced bronchiolar damage when administered to mice. Administration of 2000 mg/kg caused injury in Clara cells of the bronchiolar epithelium, which was observed at 24 h following TCE treatment; increase of the dosage to 2500 mg/kg induced additionally, alterations in alveolar Type II cells of the parenchyma. Specifically, lamellar bodies were reduced in number and microvilli displayed distorted protrusions. The increase in severity of cellular injury with higher dosages of TCE coincided with increased accumulation of pulmonary calcium and lengthened anesthesia recovery times following TCE-induced anesthesia. Time-course studies conducted with 2000 mg/kg demonstrated rapid and marked reduction in pulmonary microsomal cytochrome P-450 content and aryl hydrocarbon hydroxylase activity. Significant decreases were observed as early as 1 h, and the levels were still depressed at 24 h following TCE treatment. Hepatic necrosis was relatively mild at the dosages of TCE examined. These results demonstrate that TCE is pneumotoxic and affects Clara and alveolar Type II cells.