Regular ArticleDevelopment of a Physiologically-Based Pharmacokinetic Model for Perchloroethylene Using Tissue Concentration-Time Data
Abstract
The tissue disposition of perchloroethylene (PCE) was characterized experimentally in rats in order to (1) obtain input parameters from in vivo data for the development of a physiologically based pharmacokinetic (PBPK) model, and (2) use the PBPK model to predict the deposition of PCE in a variety of tissues following inhalation exposure. For the derivation of model input parameters, male Sprague-Dawley rats received a single bolus of 10 mg PCE/kg body wt in polyethylene glycol 400 by ia injection through an indwelling carotid arterial cannula. Other male Sprague-Dawley rats inhaled 500 ppm PCE for 2 hr in dynamic exposure inhalation chambers. Serial samples of brain, liver, kidney, lung, heart, skeletal muscle, perirenal fat, and blood were taken for up to 72 hr following ia injection, during the 2-hr inhalation exposure, and for up to 72 hr postexposure. Blood and tissue PCE concentrations were analyzed using a gas chromatography headspace technique. Following ia administration, the tissues exhibited similar terminal elimination half-lives (t). As comparable tissue ts are consistent with a blood-flow-limited model, tissue:blood partition coefficients were calculated for noneliminating compartments by division of the area under the tissue concentration-time curve (AUC) by the blood AUC. Liver PCE concentration versus time data were employed in the calculation of in vivo metabolic rate constants. A PBPK model was developed using these parameters derived from the ia data set and used to predict tissue PCE concentrations during and following PCE inhalation. Predicted tissue levels were in close agreement with the levels measured over time in the seven tissues and in blood. Tissue concentration-time data can thus provide valuable input for parameter estimation and for validation of PBPK model simulations, as long as independent in vivo data sets are used for each step.
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Development and evaluation of a harmonized physiologically based pharmacokinetic (PBPK) model for perchloroethylene toxicokinetics in mice, rats, and humans
2011, Toxicology and Applied PharmacologyCitation Excerpt :As expected, the optimized parameters had increased oxidation and decreased conjugation, resulting in a clear improvement in model fit across all the data (see Fig. 6B, and Table 8 calibration data residuals). For rat calibration data, the poorest fits are to the fraction of retained perc metabolized and rate of perc exhalation (both from Reitz et al., 1996) and perc in various tissues (from Dallas et al., 1994a, 1994c; and Warren et al., 1996), with residual errors of 2- to 3-fold. As with mice, it is unclear how to completely reconcile the extremely well-fit closed chamber data with the more poorly fit C-14 data from Reitz et al. (1996), but inter-study variation cannot be ruled out.
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.
As part of a project designed to develop a framework for extrapolating acute central nervous system (CNS) effects of hydrocarbon solvents in animals to humans, experimental studies were conducted in rats and human volunteers in which acute CNS effects were measured and toxicokinetic data were collected. A complex hydrocarbon solvent, white spirit (WS) was used as a model solvent and two marker compounds for WS, 1,2,4-trimethyl benzene (TMB) and n-decane (NDEC), were analyzed to characterize internal exposure after WS inhalation. Toxicokinetic data on blood and brain concentrations of the two marker compounds in the rat, together with in vitro partition coefficients were used to develop physiologically based pharmacokinetic (PBPK) models for TMB and NDEC. The rat models were then allometrically scaled to obtain models for inhalatory exposure for man. The human models were validated with blood and alveolar air kinetics of TMB and NDEC, measured in human volunteers. Using these models, it was predicted that external exposures to WS in the range of 344–771 mg/m3 would produce brain concentrations similar to those in rats exposed to 600 mg/m3 WS, the no effect level (NOEL) for acute CNS effects. Assuming similar brain concentration–effect relations for humans and rats, the NOEL for acute CNS effects in humans should be in this range. The prediction was consistent with data from a human volunteer study in which the only statistically significant finding was a small change in the simple reaction time test following 4 h exposure to approximately 570 mg/m3 WS. Thus, the data indicated that the results of animal studies could be used to predict a no effect level for acute CNS depression in humans, consistent with the framework described above.
Volatile organic compounds inhibit human and rat neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes
2005, Toxicology and Applied PharmacologyThe relative sensitivity of rats and humans to volatile organic compounds (VOCs) such as toluene (TOL) and perchloroethylene (PERC) is unknown and adds to uncertainty in assessing risks for human exposures to VOCs. Recent studies have suggested that ion channels, including nicotinic acetylcholine receptors (nAChRs), are targets of TOL effects. However, studies comparing TOL effects on human and rat ligand-gated ion channels have not been conducted. To examine potential toxicodynamic differences between these species, the sensitivity of human and rat nAChRs to TOL was assessed. Since PERC has similar effects, in vivo, to TOL, effects of PERC on nAChR function were also examined. Two-electrode voltage-clamp techniques were utilized to measure acetylcholine-induced currents in neuronal nAChRs (α4β2, α3β2, and α7) expressed in Xenopus oocytes. PERC (0.065 mM) inhibited α7 nAChR currents by 60.1 ± 4.0% (human, n = 7) and 40 ± 3.5% (rat, n = 5), and inhibited α4β2 nAChR currents by 42.0 ± 5.2% (human, n = 6) and 52.2 ± 5.5% (rat, n = 8). Likewise, α3β2 nAChRs were significantly inhibited by 62.2 ± 3.8% (human, n = 7) and 62.4 ± 4.3% (rat, n = 8) in the presence of 0.065 mM PERC. TOL also inhibited both rat and human α7, α4β2, and α3β2 nAChRs. Statistical analysis indicated that although there was not a species (human vs. rat) difference with PERC (0.0015–0.065 mM) or TOL (0.03–0.9 mM) inhibition of α7, α4β2, or α3β2 nAChRs, all receptor types were more sensitive to PERC than TOL. These results demonstrate that human and rat nACh receptors represent a sensitive target for VOCs. This toxicodynamic information will help decrease the uncertainty associated with animal to human extrapolations in the risk assessment of VOCs.
Glutathione conjugation of perchloroethylene in rats and mice in vitro: Sex-, species-, and tissue-dependent differences
1998, Toxicology and Applied PharmacologyPerchloroethylene (Per)-induced nephrotoxicity and nephrocarcinogenicity have been associated with metabolism by the glutathione (GSH) conjugation pathway to formS-(1,2,2-trichlorovinyl)glutathione (TCVG). Formation of TCVG was determined in incubations of Per and GSH with isolated renal cortical cells and hepatocytes from male and female Fischer 344 rats and with renal and hepatic cytosol and microsomes from male and female Fischer 344 rats and B6C3F1 mice. The goal was to assess the role of metabolism in the sex and species dependence of susceptibility to Per-induced toxicity. A key finding was that GSH conjugation of Per occurs in kidney as well as in liver. Although amounts of TCVG formation in isolated kidney cells and hepatocytes from male and female rats were generally similar, TCVG formation in subcellular fractions showed marked sex, species, and tissue dependence. This may be due to the presence of multiple pathways for metabolism in intact cells, whereas only the GSH conjugation pathway is active in the subcellular fractions under the present assay conditions. TCVG formation in kidney and liver subcellular fractions from both male rats and mice were invariably higher than corresponding values in female rats and mice. Amounts of TCVG formation in rat liver subcellular fractions were approximately 10-fold higher than in corresponding fractions from rat kidney. Although rats are more susceptible to Per-induced renal tumors than mice, amounts of TCVG formation were 7- to 10-fold higher in mouse kidney subcellular fractions and 2- to 5-fold higher in mouse liver subcellular fractions of both sexes compared to corresponding fractions from the rat. Hence, although the higher amounts of TCVG formation in liver and kidney from male rats correspond to their higher susceptibility to Per-induced renal tumors compared with female rats, the markedly higher amounts of TCVG formation in mice compared with rats suggest that other enzymatic or transport steps in the handling of Per in mice contribute to their relatively low susceptibility to Per-induced renal tumors.
Uptake, distribution, and elimination of carbon tetrachloride in rat tissues following inhalation and ingestion exposures
1997, Toxicology and Applied PharmacologyCarbon tetrachloride (CCl4) has been studied extensively for its hepatotoxic effects. There is a paucity of information, however, about its tissue deposition following administration by different routes and patterns of exposure. The specific objective of this study was to delineate the uptake, distribution, and elimination of CCl4in tissues of rats subjected to equivalent oral and inhalation exposures. Male Sprague–Dawley rats (325–375 g) were exposed to 1000 ppm CCl4for 2 hr. The total absorbed dose (179 mg CCl4/kg bw) was administered to other groups of rats as a single oral bolus or by constant gastric infusion over a period of 2 hr. Animals were terminated at selected time intervals during and postexposure and tissues (liver, kidney, lung, brain, fat, skeletal muscle, spleen, heart, and GI tract) removed for measurement of their CCl4content by headspace gas chromatography. CCl4levels in all tissues were much lower in the gastric infusion group than in the oral bolus and inhalation groups. Inhalation resulted in relatively high tissue CCl4concentrations, because inhaled chemicals enter the arterial circulation and are transported directly to organs throughout the body. It seems logical that the liver should accumulate more CCl4following ingestion than following inhalation. This did not prove to be the case when comparing liver AUC values for the gastric infusion and inhalation groups. Substantially lower CCl4concentrations in the liver of animals in the gastric infusion group appeared to be due to very rapid metabolic clearance of the relatively small amounts of CCl4entering the liver over the 2-hr infusion period. It was hypothesized that the capacity of first-pass hepatic and pulmonary elimination could be exceeded, if CCl4were given as a single, large oral bolus. Indeed, deposition of CCl4in all tissues was greater in the oral bolus group than in the gastric infusion group. The time courses of uptake and elimination of CCl4appeared to be governed largely by a tissue's rate of blood perfusion and lipid content. CCl4was rapidly taken up, for example, by the brain and liver. These organs’ CCl4content then diminished, as CCl4was metabolized and redistributed to adipose tissue. CCl4accumulated slowly, but to very high concentrations, in fat and remained elevated for a prolonged period. Thus, concentrations of CCl4in some tissues may not be reflective of blood levels. The most appropriate measure of internal dose for CCl4acute hepatotoxicity appears to be the area under tissue concentration versus time curve from 0 to 30 min. Tissue time–course data sets are essential for the refinement and validation of physiological models for CCl4and other volatile organic chemicals.
Use of a physiologically based model to predict systemic uptake and respiratory elimination of perchloroethylene
1994, Toxicology and Applied PharmacologyThe pharmacokinetics of inhaled perchloroethylene (PCE) were studied in male Sprague-Dawley rats to characterize the pulmonary absorption and elimination of the volatile organic chemical (VOC). The direct measurements of the time course of PCE in the blood and breath were used to evaluate the ability of a physiologically based pharmacokinetic (PBPK) model to predict systemic uptake and elimination of PCE. Fifty or 500 ppm PCE was inhaled for 2 hr through a miniaturized one-way breathing valve by unanesthetized male Sprague-Dawley rats of 325-375 g. Serial samples of the inhaled and exhaled breath streams, as well as arterial blood, were collected during and following PCE inhalation and analyzed by headspace gas chromatography. PCE-exhaled breath concentrations increased rapidly to near steady state (i.e., within 20 min) and were directly proportional to the inhaled concentration. Uptake of PCE into the blood was also rapid, but blood levels continued to increase progressively over the course of the 2-hr exposure at both exposure levels. Cumulative uptake, or total absorbed dose, was not proportional to the exposure level. A PBPK model was developed from in vivo parameters determined from tissue concentration-time data in a companion ia study (Dallas et al., 1994, Toxicol. Appl. Pharmacol.128, 50-59). PCE concentrations in the blood and exhaled breath during and following PCE inhalation were well predicted by the PBPK; model. Despite species differences in blood:air and lung:air partition coefficients, the model was used to account for similar levels of PCE and other VOCs in the expired air of rats and humans. The model also accurately simulated percentage uptake and cumulative uptake of PCE over time. The model′s ability to predict systemically absorbed doses of PCE under a variety of exposure scenarios should be useful in assessment of risks in occupational and environmental settings.