Comparison of average estimated metabolic rates for styrene in previously exposed and unexposed groups with pharmacokinetic modelling

Estimation of absorption of aromatic hydrocarbons diffusing from interior materials in automobile cabins by inhalation toxicokinetic analysis in rats

Aromatic hydrocarbons, as well as aliphatic hydrocarbons, diffusing from interior materials in automotive cabins are the most common compounds contributing to interior air pollution. In this study, the amounts of seven selected aromatic hydrocarbons absorbed by a car driver were estimated by evaluating their inhalation toxicokinetics in rats. Measured amounts of these substances were injected into a closed chamber system containing a rat, and the concentration changes in the chamber were examined. The toxicokinetics of the substances were evaluated on the basis of the concentration-time course using a nonlinear compartment model. The amounts absorbed in humans at actual concentrations in automobile cabins without ventilation were extrapolated from the results obtained from rats. The absorbed amounts estimated for a driver during a 2 h drive were as follows (per 60 kg of human body weight): 30 microg for toluene (interior median concentration, 40 microg m(-3) in our previous study), 10 microg for ethylbenzene (12 microg m(-3)), 6 microg for o-xylene (10 microg m(-3)), 8 microg for m-xylene (11 microg m(-3)), 9 microg for p-xylene (11 microg m(-3)), 11 microg for styrene (11 microg m(-3)) and 27 microg for 1,2,4-trimethylbenzene (24 microg m(-3)). Similarly, in a cabin where air pollution was marked, the absorbed amount of styrene (654 microg for 2 h in a cabin with an interior maximum concentration of 675 microg m(-3)) was estimated to be much higher than those of other substances. This amount (654 microg) was approximately 1.5 times the tolerable daily intake of styrene (7.7 microg kg(-1) per day) recommended by the World Health Organization.

Exterior exposure estimation using a one-compartment toxicokinetic model with blood sample measurements

Exposure assessment of individuals exposed to certain chemicals plays an important role in the analysis of occupational-as well as environmental-health problems. Biological monitoring, as an alternative to direct environmental measurements, may be applied to relate the exterior exposure with the amount of individual intake. In this paper, we estimate individuals' (inhalation) exposure retrospectively from their blood concentrations via a simplified one-compartment toxicokinetic model. Considering stochastic variations to the toxicokinetic model, the solution to the resultant stochastic differential equation (SDE), together with measurement error, is transformed into a dynamic linear state-space model. The unknown model parameters and the mean inhalation concentration are then estimated via Markov Chain Monte Carlo (MCMC) simulations. The proposed method is used in the analysis of the styrene data (Wang et al. in Occup Environ Med 53:601-605, 1996) to backward estimate the inhalation concentration, assuming it is unknown. The data analysis showed that the internal stochastic variations, often ignored in toxicokinetic model analysis, outweighed in standard deviation almost twice that of the measurement error. Also, the simulation results showed that the method performed relatively well to the approach considering measurement error only.

Estimation of interindividual variation in oxidative metabolism of dichloromethane in human volunteers

A modified version of the original physiologically based pharmacokinetic (PBPK) model by Andersen et al. (1987) has been developed and used in conjunction with previously published human kinetic data for dichloromethane (DCM) metabolism and to assess interindividual variability in the rate of oxidative metabolism. Time-course data for 13 volunteers (10 males, 3 females) exposed to one or more concentrations of DCM (50 ppm, 100 ppm, 150 ppm, or 200 ppm) for 7.5h were used to optimize the maximal rate of hepatic metabolism (V(maxC)) through the cytochrome P450 pathway for each individual. DCM breath and blood concentrations were used, along with carboxyhemoglobin concentrations in blood and carbon monoxide (CO) concentrations in exhaled breath, to estimate the model parameters. Significant improvements in model fit were achieved when extrahepatic oxidative metabolism of DCM was added to the model structure. The 13 individual V(maxC) values ranged from 7.1 to 23.6 mg/h/kg0.7 and appeared to be bimodally distributed. The distribution was not sex related and may be related to differential CYP2E1 induction. A comparison of the observed variation in V(maxC) values to other estimates of variability in the rate of oxidative metabolism and human CYP2E1 activity suggest a relatively narrow range in human hepatic activity toward DCM.

Two-sample scale tests for comparison of metabolic rates for styrene in previously exposed and unexposed groups

Statistical analysis procedure in two-sample treatment difference, as well as alternative hypothesis, may play a central role in statistical inference, especially in small sample size case. In this paper, simple non-parametric two-sample permutation tests for scale difference based on the Hodges-Lehmann estimator are reformulated, and are applied to a study examining whether previous styrene exposure increases (thus a one-sided alternative hypothesis rather than a two-sided one) the human liver's metabolic ability to convert styrene into styrene oxide, in which a three-compartment physiologically based pharmacokinetic (PBPK) model was used to compare the estimated metabolic constant kappaamong the previously exposed and unexposed groups. Contrary to the previous conclusion, the proposed tests for scale difference identified from a mixed-effects model showed a significant result.

Toxicokinetics of organic solvents: a review of modifying factors

This article reviews, with an emphasis on human experimental data, factors known or suspected to cause changes in the toxicokinetics of organic solvents. Such changes in the toxicokinetic pattern alters the relation between external exposure and target dose and thus may explain some of the observed individual variability in susceptibility to toxic effects. Factors shown to modify the uptake, distribution, biotransformation, or excretion of solvent include physical activity (work load), body composition, age, sex, genetic polymorphism of the biotransformation, ethnicity, diet, smoking, drug treatment, and coexposure to ethanol and other solvents. A better understanding of modifying factors is needed for several reasons. First, it may help in identifying important potential confounders and eliminating negligible ones. Second, the risk assessment process may be improved if different sources of variability between external exposures and target doses can be quantitatively assessed. Third, biological exposure monitoring may be also improved for the same reason.