Development of an updated PBPK model for trichloroethylene and metabolites in mice, and its application to discern the role of oxidative metabolism in TCE-induced hepatomegaly

Trichloroethylene (TCE) is a lipophilic solvent rapidly absorbed and metabolized via oxidation and conjugation to a variety of metabolites that cause toxicity to several internal targets. Increases in liver weight (hepatomegaly) have been reported to occur quickly in rodents after TCE exposure, with liver tumor induction reported in mice after long-term exposure. An integrated dataset for gavage and inhalation TCE exposure and oral data for exposure to two of its oxidative metabolites (TCA and DCA) was used, in combination with an updated and more accurate physiologically-based pharmacokinetic (PBPK) model, to examine the question as to whether the presence of TCA in the liver is responsible for TCE-induced hepatomegaly in mice. The updated PBPK model was used to help discern the quantitative contribution of metabolites to this effect. The update of the model was based on a detailed evaluation of predictions from previously published models and additional preliminary analyses based on gas uptake inhalation data in mice. The parameters of the updated model were calibrated using Bayesian methods with an expanded pharmacokinetic database consisting of oral, inhalation, and iv studies of TCE administration as well as studies of TCE metabolites in mice. The dose-response relationships for hepatomegaly derived from the multi-study database showed that the proportionality of dose to response for TCE- and DCA-induced hepatomegaly is not observed for administered doses of TCA in the studied range. The updated PBPK model was used to make a quantitative comparison of internal dose of metabolized and administered TCA. While the internal dose of TCA predicted by modeling of TCE exposure (i.e., mg TCA/kg-d) showed a linear relationship with hepatomegaly, the slope of the relationship was much greater than that for directly administered TCA. Thus, the degree of hepatomegaly induced per unit of TCA produced through TCE oxidation is greater than that expected per unit of TCA administered directly, which is inconsistent with the hypothesis that TCA alone accounts for TCE-induced hepatomegaly. In addition, TCE-induced hepatomegaly showed a much more consistent relationship with PBPK model predictions of total oxidative metabolism than with predictions of TCE area-under-the-curve in blood, consistent with toxicity being induced by oxidative metabolites rather than the parent compound. Therefore, these results strongly suggest that oxidative metabolites in addition to TCA are necessary contributors to TCE-induced liver weight changes in mice.     

Characterizing uncertainty and population variability in the toxicokinetics of trichloroethylene and metabolites in mice, rats, and humans using an updated database, physiologically based pharmacokinetic (PBPK) model, and Bayesian approach

We have developed a comprehensive, Bayesian, PBPK model-based analysis of the population toxicokinetics of trichloroethylene (TCE) and its metabolites in mice, rats, and humans, considering a wider range of physiological, chemical, in vitro, and in vivo data than any previously published analysis of TCE. The toxicokinetics of the “population average,” its population variability, and their uncertainties are characterized in an approach that strives to be maximally transparent and objective. Estimates of experimental variability and uncertainty were also included in this analysis. The experimental database was expanded to include virtually all available in vivo toxicokinetic data, which permitted, in rats and humans, the specification of separate datasets for model calibration and evaluation. The total combination of these approaches and PBPK analysis provides substantial support for the model predictions. In addition, we feel confident that the approach employed also yields an accurate characterization of the uncertainty in metabolic pathways for which available data were sparse or relatively indirect, such as GSH conjugation and respiratory tract metabolism. Key conclusions from the model predictions include the following: (1) as expected, TCE is substantially metabolized, primarily by oxidation at doses below saturation; (2) GSH conjugation and subsequent bioactivation in humans appear to be 10- to 100-fold greater than previously estimated; and (3) mice had the greatest rate of respiratory tract oxidative metabolism as compared to rats and humans. In a situation such as TCE in which there is large database of studies coupled with complex toxicokinetics, the Bayesian approach provides a systematic method of simultaneously estimating model parameters and characterizing their uncertainty and variability. However, care needs to be taken in its implementation to ensure biological consistency, transparency, and objectivity.     

Trichloroacetic acid: updated estimates of its bioavailability and its contribution to trichloroethylene-induced mouse hepatomegaly

Trichloroacetic acid (TCA) is a common drinking water disinfection byproduct that produces a spectrum of liver effects, including hepatomegaly and liver tumors, in mice. It is also an oxidative metabolite of trichloroethylene (TCE), a solvent used in degreasing with widespread environmental exposure, which also produces hepatomegaly and liver tumors in mice. Physiologically based pharmacokinetic (PBPK) modeling of TCE and TCA can be used to quantitatively compare the dose-responses for hepatomegaly for these two chemicals on the basis of internal TCA dose, and thereby test the hypothesis that TCA could fully explain TCE-induced hepatomegaly. Previously, using a PBPK model calibrated using kinetic data from i.v. and gavage dosing of TCA and from TCA produced from TCE, it was concluded that TCA accounted for only about one-fifth of the degree of hepatomegaly produced by TCE. However, recently available data suggest a non-linear change in internal TCA dose attributed to a dose-dependent fractional absorption of TCA administered in drinking water, the primary route of exposure of TCA both environmentally and in experimental toxicity studies. Therefore, in the present reanalysis, the PBPK modeling of TCA was updated using these data and the comparison between TCA- and TCE-induced hepatomegaly was revisited using updated internal dose predictions. With respect to updated PBPK modeling results, incorporating less than complete absorption of TCA administered in drinking water substantially improves the PBPK model fit to the newly available data, based on goodness-of-fit comparison. However, inter-experimental variability is high, with nearly complete absorption estimated for some studies. With respect to the comparison of TCA and TCA-induced hepatomegaly, this reanalysis predicts that TCA can account for roughly one-third to one-half of the effect observed with TCE - greater than previously reported, but still inconsistent with TCA being the sole active moiety for this effect. However, given uncertainty as to the precise degree of contribution of TCA and due to high inter-experimental variability in estimated fractional absorption, a more precise quantitative estimate of the relative contribution of TCA may obtained through an appropriate experiment in mice simultaneously measuring TCA kinetics and TCE- and TCA-induced hepatomegaly.     

Species differences in the metabolism of trichloroethylene to the carcinogenic metabolites trichloroacetate and dichloroacetate.

Differing rates and extent of trichloroethylene (TCE) metabolism have been implicated as being responsible for varying sensitivities of mice and rats to the hepatocarcinogenic effects of TCE. Recent data indicate that the induction of hepatic tumors in mice may be attributed to the metabolites trichloroacetate (TCA) and/or dichloroacetate (DCA). The present study was directed at determining whether mice and rats varied in (1) the peak blood concentrations, (2) the area under the blood concentration over time curves (AUC) for TCE and metabolites in blood, and (3) the net excretion of TCE to these metabolites in urine in the dose range used in the cancer bioassays of TCE, and to contrast the kinetic parameters observed for TCE-derived TCA and DCA with those obtained following direct administration of TCA and DCA. Blood and urine samples were collected over 72 hr from rats and mice after a single oral dose of TCE of 1.5 to 23 mmol/kg. The AUC values from the blood concentration with time profiles of TCE, TCA, and trichloroethanol (TCOH) were similar for Sprague-Dawley rats and B6C3F1 mice. Likewise, the percentages of initial TCE dose recovered as the urinary metabolites TCA and TCOH were comparable. Nevertheless, the peak blood concentrations of TCE, TCA, and TCOH observed in mice were much greater than those in rats, while the residence time of TCE and metabolites was prolonged in rats relative to that of mice. DCA was detected in the blood of mice but not in rats. The blood concentrations of DCA observed in mice given a carcinogenic dose of TCE (15 mmol/kg) were of the same magnitude as those observed with carcinogenic doses of DCA. In conclusion, the net metabolism of TCE to TCA and TCOH was similar in rats and mice. The initial rates of metabolism of TCE to TCA, however, were much higher in mice, especially as the TCE dose was increased, leading to greater concentrations of TCA and DCA in mice approximated those produced by carcinogenic doses of the chlorinated acetates makes it highly likely that both compounds play a role in the induction of hepatic tumors in mice by TCE.     

Identification of trichloroethylene and its metabolites in human seminal fluid of workers exposed to trichloroethylene.

We have investigated the potential of the male reproductive tract to accumulate trichloroethylene (TCE) and its metabolites, including chloral, trichloroethanol (TCOH), trichloroacetic acid (TCA), and dichloroacetic acid (DCA). Human seminal fluid and urine samples from eight mechanics diagnosed with clinical infertility and exposed to TCE occupationally were analyzed. In in vivo experimental studies, TCE and its metabolites were determined in epididymis and testis of mice exposed to TCE (1000 ppm) by inhalation for 1 to 4 weeks. In other studies, incubations of monkey epididymal microsomes were performed in the presence of TCE and NADPH. Our results showed that seminal fluid from all eight subjects contained TCE, chloral, and TCOH. DCA was present in samples from two subjects, and only one contained TCA. TCA and/or TCOH were also identified in urine samples from only two subjects. TCE, chloral, and TCOH were detected in murine epididymis after inhalation exposure with TCE for 1 to 4 weeks. Levels of TCE and chloral were similar throughout the entire exposure period. TCOH levels were similar at 1 and 2 weeks but increased significantly after 4 weeks of TCE exposure. Chloral was identified in microsomal incubations with TCE in monkey epididymis. CYP2E1, a P450 that metabolizes TCE, was localized in human and monkey epididymal epithelium and testicular Leydig cells. These results indicated that TCE is metabolized in the reproductive tract of the mouse and monkey. Furthermore, TCE and its metabolites accumulated in seminal fluid, and suggested associations between production of TCE metabolites, reproductive toxicity, and impaired fertility.     

Development and evaluation of a harmonized physiologically based pharmacokinetic (PBPK) model for perchloroethylene toxicokinetics in mice, rats, and humans

This article reports on the development of a “harmonized” PBPK model for the toxicokinetics of perchloroethylene (tetrachloroethylene or perc) in mice, rats, and humans that includes both oxidation and glutathione (GSH) conjugation of perc, the internal kinetics of the oxidative metabolite trichloroacetic acid (TCA), and the urinary excretion kinetics of the GSH conjugation metabolites N-Acetylated trichlorovinyl cysteine and dichloroacetic acid. The model utilizes a wider range of in vitro and in vivo data than any previous analysis alone, with in vitro data used for initial, or “baseline,” parameter estimates, and in vivo datasets separated into those used for “calibration” and those used for “evaluation.” Parameter calibration utilizes a limited Bayesian analysis involving flat priors and making inferences only using posterior modes obtained via Markov chain Monte Carlo (MCMC). As expected, the major route of elimination of absorbed perc is predicted to be exhalation as parent compound, with metabolism accounting for less than 20% of intake except in the case of mice exposed orally, in which metabolism is predicted to be slightly over 50% at lower exposures. In all three species, the concentration of perc in blood, the extent of perc oxidation, and the amount of TCA production is well-estimated, with residual uncertainties of ~ 2-fold. However, the resulting range of estimates for the amount of GSH conjugation is quite wide in humans (~ 3000-fold) and mice (~ 60-fold). While even high-end estimates of GSH conjugation in mice are lower than estimates of oxidation, in humans the estimated rates range from much lower to much higher than rates for perc oxidation. It is unclear to what extent this range reflects uncertainty, variability, or a combination. Importantly, by separating total perc metabolism into separate oxidative and conjugative pathways, an approach also recommended in a recent National Research Council review, this analysis reconciles the disparity between those previously published PBPK models that concluded low perc metabolism in humans and those that predicted high perc metabolism in humans. In essence, both conclusions are consistent with the data if augmented with some additional qualifications: in humans, oxidative metabolism is low, while GSH conjugation metabolism may be high or low, with uncertainty and/or interindividual variability spanning three orders of magnitude. More direct data on the internal kinetics of perc GSH conjugation, such as trichlorovinyl glutathione or tricholorvinyl cysteine in blood and/or tissues, would be needed to better characterize the uncertainty and variability in GSH conjugation in humans.     

Bayesian calibration of a physiologically based pharmacokinetic/pharmacodynamic model of carbaryl cholinesterase inhibition

Carbaryl, an N-methyl carbamate (NMC), is a common insecticide that reversibly inhibits neuronal cholinesterase activity. The objective of this work was to use a hierarchical Bayesian approach to estimate the parameters in a physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model from experimental measurements of carbaryl in rats. A PBPK/PD model was developed to describe the tissue dosimetry of carbaryl and its metabolites (1-naphthol and "other hydroxylated metabolites") and subsequently to predict the carbaryl-induced inhibition of cholinesterase activity, in particular in the brain and blood. In support of the model parameterization, kinetic tracer studies were undertaken to determine total radioactive tissue levels of carbaryl and metabolites in rats exposed by oral or intravenous routes at doses ranging from 0.8 to 9.2 mg/kg body weight. Inhibition of cholinesterase activity in blood and brain was also measured from the exposed rats. Markov Chain Monte Carlo (MCMC) calibration of the rat model parameters was implemented using prior information from literature for physiological parameter distributions together with kinetic and inhibition data on carbaryl. The posterior estimates of the parameters displayed at most a twofold deviation from the mean. Monte Carlo simulations of the PBPK/PD model with the posterior distribution estimates predicted a 95% credible interval of tissue doses for carbaryl and 1-naphthol within the range of observed data. Similar prediction results were achieved for cholinesterase inhibition by carbaryl. This initial model will be used to determine the experimental studies that may provide the highest added value for model refinement. The Bayesian PBPK/PD modeling approach developed here will serve as a prototype for developing mechanism-based risk models for the other NMCs.     

Pharmacokinetics for regulatory risk analysis: The case of trichloroethylene

Physiologically based pharmacokinetic (PBPK) models describing the uptake, metabolism, and excretion of volatile organic compounds (VOCs) are now proposed for use in regulatory health-risk assessment. A steady-state analysis of one such model is shown to provide simple, convenient predicted relationships between an applied dose and the corresponding toxicologically effective, metabolized dose for certain VOCs like trichloroethylene (TCE). A version of this PBPK model was fit to data on human metabolism of TCE to urinary metabolites in chronically exposed workers, yielding a direct estimate of PBPK parameters governing human capacity to metabolize TCE. It is shown that this estimate is consistent with others based on experimental studies of TCE metabolism in humans exposed to TCE by inhalation for short periods. These results are applied to human cancer-risk assessment using rodent bioassay data on TCE-induced tumorigenesis.     

Metabolism and toxicity of trichloroethylene in epididymis and testis

The widespread occupational exposure to trichloroethylene (TCE) led us to test the hypothesis that TCE causes toxicity in the male reproductive system. We also investigated mechanisms mediating the potential cytotoxic response. Mice were exposed to TCE (1000 ppm) by inhalation for 6 h/day for 5 days/week for a total of 19 days. Exposure after the first week was interspersed by a "weekend." To estimate internal exposure, we measured the TCE metabolites, trichloroacetic acid (TCA) and trichloroethanol (TCOH), in urine at Days 4, 9, 14, and 19. Urinary excretion of TCOH was significantly higher than TCA; levels of TCOH and TCA significantly increased by the second and third week, respectively. Cytochrome P450 2E1 (CYP2E1), an enzyme involved in TCE metabolism, was localized in the epididymal epithelium and testicular Leydig cells, and was found at higher levels in the former than the latter. Immunoblotting confirmed that CYP2E1 protein was present in greater amounts in epididymis than in testis. p-Nitrophenol hydroxylation, a CYP2E1 catalytic activity, was also higher in the epididymis than in the testis. Chloral, a major TCE metabolite, was generated in microsomal incubations at significantly higher levels in epididymis than in testis. Antibody inhibition of CYP2E1 reduced chloral formation, which was more pronounced in epididymis than in testis. After 4 weeks of TCE exposure, damage to the epididymis was manifested as sloughing of epithelial cells. These results indicated that TCE is metabolized in the male reproductive tract, leading to adverse effects that are more severe in the epididymis than in the testis.     

Evaluation of uncertainty in input parameters to pharmacokinetic models and the resulting uncertainty in output

Physiologically-based pharmacokinetic (PBPK) models may be used to predict the concentrations of parent chemical or metabolites in tissues, resulting from specified chemical exposures. An important application of PBPK modeling is in assessment of carcinogenic risks to humans, based on animal data. The parameters of a PBPK model may include metabolic parameters, blood/air and tissue/blood partition coefficients, and physiological parameters, such as organ weights and blood flow rates. Uncertainty in estimates of these parameters results in uncertainty regarding tissue concentrations and resulting risks. Data are reviewed relevant to the quantification of these uncertainties, for a PBPK model-based risk assessment for tetrachloroethylene. Probability distributions are developed to express uncertainty in model parameters, and uncertainties are propagated by a sequence of operations that simulates processes recognized as contributing to estimates of human risk. Distributions of PBPK model output and human risk estimates are used to characterize uncertainty resulting from uncertainty in model parameters.     

Modulation of trichloroethylene in vitro metabolism by different drugs in rats

Trichloroethylene (TCE) is a widely used chemical to which humans are frequently exposed. Toxicological interactions with drugs are among factors having the potential to modulate the toxicity of TCE. The aim of this study was to identify metabolic interactions between TCE and 14 widely used drugs in rat suspended hepatocytes and characterize the strongest using microsomal assays (oxidation and/or glucuronidation). The concentrations of TCE and its metabolites, trichloroethanol (TCOH) and trichloroacetate (TCA), were measured by gas chromatography with injection headspace coupled to mass spectrometry (GC–MS). Results in hepatocyte incubations show that selected drugs can be segregated into four groups: group 1: drugs causing no significant interactions (five drugs: amoxicillin, carbamazepine, ibuprofen, mefenamic acid and ranitidine); group 2: increasing both TCE metabolites (two drugs: naproxen and salicylic acid); group 3: decreasing both TCE metabolites (five drugs: acetaminophen, gliclazide, valproic acid, cimetidine and diclofenac) and group 4: affecting only one (two drugs: erythromycin and sulphasalazine). Naproxen and salicylic acid (group 2) and acetaminophen, gliclazide and valproic acid (from group 3) presented the strongest interactions (i.e. drugs changing metabolite levels by 50% or more). For group 2 drugs, characterization in rat microsomes confirmed interaction with naproxen only, which was found to partially competitively inhibit TCOH glucuronidation (K_i = 211.6 μM). For group 3 selected drugs, confirmation was positive only for gliclazide (K_i = 58 μM for TCOH formation) and valproic acid (K_i = 1215.8 μM for TCA formation and K_i = 932.8 μM for TCOH formation). The inhibition was found to be partial non competitive for both drugs. Our results confirm the existence of interactions between TCE and a variety of widely used drugs. Further efforts are undertaken to determine if these interactions are plausible in humans and if they can impact the risk of toxicity of TCE in medicated population.     

Revised assessment of cancer risk to dichloromethane: part I Bayesian PBPK and dose-response modeling in mice

The current USEPA cancer risk assessment for dichloromethane (DCM) is based on deterministic physiologically based pharmacokinetic (PBPK) modeling involving comparative metabolism of DCM by the GST pathway in the lung and liver of humans and mice. Recent advances in PBPK modeling include probabilistic methods and, in particular, Bayesian inference to quantitatively address variability and uncertainty separately. Although Bayesian analysis of human PBPK models has been published, no such efforts have been reported specifically addressing the mouse, apart from results included in the OSHA final rule on DCM. Certain aspects of the OSHA model, however, are not consistent with current approaches or with the USEPA's current DCM cancer risk assessment. Therefore, Bayesian analysis of the mouse PBPK model and dose-response modeling was undertaken to support development of an improved cancer risk assessment for DCM. A hierarchical population model was developed and prior parameter distributions were selected to reflect parameter values that were considered the most appropriate and best available. Bayesian modeling was conducted using MCSim, a publicly available software program for Markov Chain Monte Carlo analysis. Mean posterior values from the calibrated model were used to develop internal dose metrics, i.e., mg DCM metabolized by the GST pathway/L tissue/day in the lung and liver using exposure concentrations and results from the NTP mouse bioassay, consistent with the approach used by the USEPA for its current DCM cancer risk assessment. Internal dose metrics were 3- to 4-fold higher than those that support the current USEPA IRIS assessment. A decrease of similar magnitude was also noted in dose-response modeling results. These results show that the Bayesian PBPK model in the mouse provides an improved basis for a cancer risk assessment of DCM.     

Effects of oral exposure to trichloroethylene on female reproductive function

The present study was conducted to determine if subchronic oral exposure to trichloroethylene (TCE) influenced female reproductive performance, and if TCE or major metabolites trichloroacetic acid (TCA) and trichloroethanol (TCOH) preferentially accumulated in female reproductive organs or neonatal tissues. Female Long-Evans hooded rats were exposed to vehicle (corn-oil), 10, 100 or 1000 mg/kg/day by gavage for 2 weeks before mating and throughout mating to day 21 of pregnancy. Gas chromatography analysis of tissues from females at the end of premating exposure indicated that TCE levels were uniformly high in fat, adrenals and ovaries across treatment groups, while uterine tissue had relatively high levels of TCA. Female fertility, however, was not influenced in any treatment group. In the 1000 mg/kg/day group, 5 out of 23 females died and weight gain was significantly depressed throughout the treatment period. Neonatal survival was significantly depressed in this group alone, with the majority of deaths occurring among female offspring at the time of birth. TCA levels in blood, liver, and milk contents of the stomach in female but not male neonates increased across treatment groups. These results indicate that oral exposure to TCE at levels below those causing limiting maternal toxicity had no influence on pregnancy outcome, and that the accumulation of TCE and TCA in ovaries, adrenals and uteri had no influence on mating success.     

Trichloroethanol up-regulates matrix metalloproteinase-9 and tissue inhibitor of metalloproteinase-1 in HaCaT cells

Occupational trichloroethylene (TCE) exposure could induce generalized skin hypersensitivity reactions complicated with severe liver dysfunctions. Active extracellular matrix degradation and remodeling are involved in the skin hypersensitivity reaction induced by chemical exposure. In the present study, we have compared the effects of in vitro exposure to trichloroethanol (TCOH) and trichloroacetic acid (TCA) of a keratinocyte cell line (HaCaT). The modulation of matrix metalloproteinases (MMPs) was selected as marker of sensitization. HaCaT cells were treated with different concentrations of TCOH or TCA up to 6 days. The gelatinolyic activities of MMP-2 and MMP-9 were detected by gelatin-zymography. MMP-2, tissue inhibitor of metalloproteinase (TIMP)-2, MMP-9 and TIMP-1 mRNAs were analyzed by real-time PCR and MMP-9 and TIMP-1 proteins were tested by Western blotting. A dose–effect relationship between TCOH treatment and MMP-9 activity, mRNA and protein expression levels was found in HaCaT cells. TCOH also induced up-regulation of TIMP-1 mRNA and protein. We found no such effects in HaCaT cells treated with TCA. Moreover, previously published literatures on patch tests suggested that TCOH could induce moderately positive reactions at low concentrations in hypersensitivity patients caused by occupational TCE exposure. In summary, these observations indicated that TCOH might play an important role in TCE-induced skin hypersensitivity.     

Pharmacokinetic modeling: prediction and evaluation of route dependent dosimetry of bisphenol A in monkeys with extrapolation to humans

A physiologically based pharmacokinetic (PBPK) model was developed for bisphenol A (BPA) in adult rhesus monkeys using intravenous (iv) and oral bolus doses of 100 μg d6-BPA/kg (Doerge et al., 2010). This calibrated PBPK adult monkey model for BPA was then evaluated against published monkey kinetic studies with BPA. Using two versions of the adult monkey model based on monkey BPA kinetic data from Doerge et al. (2010) and Taylor et al. (2011), the aglycone BPA pharmacokinetics were simulated for human oral ingestion of 5 mg d16-BPA per person (Völkel et al., 2002). Völkel et al. were unable to detect the aglycone BPA in plasma, but were able to detect BPA metabolites. These human model predictions of the aglycone BPA in plasma were then compared to previously published PBPK model predictions obtained by simulating the Völkel et al. kinetic study. Our BPA human model, using two parameter sets reflecting two adult monkey studies, both predicted lower aglycone levels in human serum than the previous human BPA PBPK model predictions. BPA was metabolized at all ages of monkey (PND 5 to adult) by the gut wall and liver. However, the hepatic metabolism of BPA and systemic clearance of its phase II metabolites appear to be slower in younger monkeys than adults. The use of the current non-human primate BPA model parameters provides more confidence in predicting the aglycone BPA in serum levels in humans after oral ingestion of BPA.     

Pharmacokinetic analysis of trichloroethylene metabolism in male B6C3F1 mice: Formation and disposition of trichloroacetic acid, dichloroacetic acid, S-(1,2-dichlorovinyl)glutathione and S-(1,2-dichlorovinyl)-l-cysteine

Trichloroethylene (TCE) is a well-known carcinogen in rodents and concerns exist regarding its potential carcinogenicity in humans. Oxidative metabolites of TCE, such as dichloroacetic acid (DCA) and trichloroacetic acid (TCA), are thought to be hepatotoxic and carcinogenic in mice. The reactive products of glutathione conjugation, such as S-(1,2-dichlorovinyl)-l-cysteine (DCVC), and S-(1,2-dichlorovinyl) glutathione (DCVG), are associated with renal toxicity in rats. Recently, we developed a new analytical method for simultaneous assessment of these TCE metabolites in small-volume biological samples. Since important gaps remain in our understanding of the pharmacokinetics of TCE and its metabolites, we studied a time-course of DCA, TCA, DCVG and DCVG formation and elimination after a single oral dose of 2100 mg/kg TCE in male B6C3F1 mice. Based on systemic concentration-time data, we constructed multi-compartment models to explore the kinetic properties of the formation and disposition of TCE metabolites, as well as the source of DCA formation. We conclude that TCE-oxide is the most likely source of DCA. According to the best-fit model, bioavailability of oral TCE was ∼ 74%, and the half-life and clearance of each metabolite in the mouse were as follows: DCA: 0.6 h, 0.081 ml/h; TCA: 12 h, 3.80 ml/h; DCVG: 1.4 h, 16.8 ml/h; DCVC: 1.2 h, 176 ml/h. In B6C3F1 mice, oxidative metabolites are formed in much greater quantities (∼ 3600 fold difference) than glutathione-conjugative metabolites. In addition, DCA is produced to a very limited extent relative to TCA, while most of DCVG is converted into DCVC. These pharmacokinetic studies provide insight into the kinetic properties of four key biomarkers of TCE toxicity in the mouse, representing novel information that can be used in risk assessment.     

Toxicokinetics of inhaled trichloroethylene and tetrachloroethylene in humans at 1 ppm: empirical results and comparisons with previous studies.

Trichloroethylene (TRI) and tetrachloroethylene (TETRA) are solvents that have been widely used in a variety of industries, and both are widespread environmental contaminants. In order to provide a better basis for understanding their toxicokinetics at environmental exposures, seven human volunteers were exposed by inhalation to 1 ppm of TRI or TETRA for 6 h, with biological samples collected for analysis during exposure and up to 6-days postexposure. Concentrations of TRI, TETRA, free trichloroethanol (TCOH), total TCOH (free TCOH plus glucuronidated TCOH), and trichloroacetic acid (TCA) were determined in blood and urine; TRI and TETRA concentrations were measured in alveolar breath. Toxicokinetic time courses and empirical analyses of classical toxicokinetic parameters were compared with those reported in previous human volunteer studies, most of which involved exposures that were at least 10-fold higher. Qualitatively, TRI and TETRA toxicokinetics were consistent with previous human studies. Quantitatively, alveolar retention and clearance by exhalation were similar to those found previously but blood and urine data suggest a number of possible toxicokinetic differences. For TRI, data from the current study support lower apparent blood-air partition coefficients, greater apparent metabolic clearance, less TCA production, and greater glucuronidation of TCOH as compared to previous studies. For TETRA, the current data suggest TCA formation that is similar or slightly lower than that of previous studies. Variability and uncertainty in empirical estimates of total TETRA metabolism are substantial, with confidence intervals among different studies substantially overlapping. Relative contributions to observed differences from concentration-dependent toxicokinetics and interindividual and interoccasion variability remain to be determined.