Dose-dependent kinetics of inhaled methylethylketone in man

Sex differences in urinary levels of several biological indicators of exposure: a simulation study using a compartmental-based toxicokinetic model

Toxicokinetic modeling is a useful tool to describe or predict the behavior of a chemical agent in the human or animal organism. A general model based on four compartments was developed in a previous study to quantify the effect of human variability on a wide range of biological exposure indicators. The aim of this study was to adapt this existing general toxicokinetic model to three organic solvents--methyl ethyl ketone, 1-methoxy-2-propanol, and 1,1,1,-trichloroethane--and to take into account sex differences. In a previous human volunteer study we assessed the impact of sex on different biomarkers of exposure corresponding to the three organic solvents mentioned above. Results from that study suggested that not only physiological differences between men and women but also differences due to sex hormones levels could influence the toxicokinetics of the solvents. In fact the use of hormonal contraceptive had an effect on the urinary levels of several biomarkers, suggesting that exogenous sex hormones could influence CYP2E1 enzyme activity. These experimental data were used to calibrate the toxicokinetic models developed in this study. Our results showed that it was possible to use an existing general toxicokinetic model for other compounds. In fact, most of the simulation results showed good agreement with the experimental data obtained for the studied solvents, with a percentage of model predictions that lies within the 95% confidence interval varying from 44.4 to 90%. Results pointed out that for same exposure conditions, men and women can show important differences in urinary levels of biological indicators of exposure. Moreover, when running the models by simulating industrial working conditions, these differences could be even more pronounced. A general and simple toxicokinetic model, adapted for three well-known organic solvents, allowed us to show that metabolic parameters can have an important impact on the urinary levels of the corresponding biomarkers. These observations give evidence of an interindividual variability, an aspect that should have its place in the approaches for setting limits of occupational exposure.

Quantitative property-property relationships for computing occupational exposure limits and vapour hazard ratios of organic solvents

Vapour Hazard Ratio (VHR) is used in solvent substitution to select the best replacement option regarding overexposure potential of solvents. However, VHR calculations are limited by the availability of Occupational Exposure Limits (OELs). The overall objective of this study was to develop quantitative property-property relationship (QPPR) approaches for computing OELs, in view of supporting the derivation of VHRs for solvents without OELs. QPPRs were developed for estimating OELs using a database of 88 solvents which have health-based Time-Weighted Average (TWA) OELs published by the American Conference of Governmental Industrial Hygienists (ACGIH). Three surrogates of biotic lipid : air partition coefficients [n-octanol : air (K(oa)), olive oil : air (K(oila)) and fat : air (K(fa))] were selected for evaluating the descriptive/predictive relationship with OELs for solvents with local modes of action. For solvents with systemic modes of action, the prediction of OEL needs to consider quantitative differences in toxicokinetics (i.e. kinetic variability factor, KVF) and toxicological potency (i.e. effective internal concentration, EIC). The n-octanol : water (K(ow)), the oil : water (K(oilw)) and the fat : water (K(fw)) partition coefficients were selected for evaluating the relationship with EICs. For local modes of action, K(oa) is the most accurate predictor of OELs [OEL (ppm) = 10(((-0.45 x log K(oa)) + 3.65)); n = 21, r²= 0.71, PRESS/SSY = 0.36, F = 45.5 with p < 0.001] and the mean (±SD) (range) of the recommended to predicted OELs was 1.04 ± 0.61 (0.2-2.5). For systemic modes of action, KVFs and EICs vary in a range from 0.73 to 41.4 µmol L⁻¹ and 1.20-848 µmol L⁻¹, respectively. K(ow) is an accurate predictor of calculated EICs [EIC (µmol L⁻¹) = 10 (((-1.16 x log K(ow)) + 3.65)); n = 27, r²= 0.88, PRESS/SSY = 0.12, F = 181 with p < 0.001] and 50% of the predicted OEL values were within a factor of two of the recommended TWA OELs. Overall, 61% and 87% of the predicted VHRs were within a factor of two and five, respectively, of the calculated VHRs. The QPPR models developed in this study represent potentially useful tools for estimating provisional OELs for solvents lacking such guideline values. These provisional OELs are developed only to support initial estimations of VHR for dealing with the challenge of solvent substitution where relative values rather than absolute values of OEL and vapour pressure guide the hygienist in making pragmatic decisions for managing occupational health hazards.

Sub-chronic toxicity of low concentrations of industrial volatile organic pollutants in vitro

Organic solvents form an important class of pollutants in the ambient air and have been associated with neurotoxicity and immunotoxicity in humans. Here we investigated the biological effects of sub-chronic exposure to industrially important volatile organic solvents in vitro. Jurkat T cells were exposed to toluene, n-hexane and methyl ethyl ketone (MEK) individually for 5 days and solvent exposure levels were confirmed by headspace gas chromatography. A neuroblastoma cell line (SH-SY5Y) was exposed to toluene for the same period. Following exposure, cells were harvested and toxicity measured in terms of the following endpoints: membrane damage (LDH leakage), perturbations in intracellular free Ca(2+), changes in glutathione redox status and dual-phosphorylation of MAP kinases ERK1/2, JNK and p38. The results show that sub-chronic exposure to the volatile organic solvents causes membrane damage, increased intracellular free calcium and altered glutathione redox status in both cell lines. However, acute and sub-chronic solvent exposure did not result in MAP kinase phosphorylation. Toxicity of the solvents tested increased with hydrophobicity. The lowest-observed-adverse-effect-levels (LOAELs) measured in vitro were close to blood solvent concentrations reported for individuals exposed to the agents at levels at or below their individual threshold limit values (TLVs).

Gases and organic solvents in urine as biomarkers of occupational exposure: a review

A brief review of urine analysis in studies of occupational exposure to volatile organic compounds and gases is provided. Analysis of exhaled breath for volatile compounds does not have a long history in occupational medicine. A number of studies has been undertaken since the 1980s, and the methods are well enough accepted to be put forward as biological equivalents of threshold limit values (TLVs) for some volatile organic compounds (VOCs) such as acetone; methanol; methyl ethyl ketone (MEK); methyl isobutyl ketone (MIBK); tetrahydrofurane; dichloromethane. In the last 20 years many scientific articles have shown that the urinary concentrations of unchanged solvents are correlated with environmental exposure and could be used for biological monitoring. The use of urine analysis of unchanged solvents in occupational applications is not yet widespread. Nonetheless, in the short time since its application, a number of important discoveries has been made, and the future appears bright for this branch of analysis. In this paper, the basic concepts and methodology of urine analysis are briefly presented with a critical revision of the literature on this matter. The excretion mechanisms of organic solvents in urine are discussed, with regard to biological variability, and the future directions of research are described.

Development of a physiologically based pharmacokinetic model for methyl ethyl ketone in F344 rats

A physiologically based pharmacokinetic (PBPK) model to describe the absorption, distribution, metabolism, and elimination of methyl ethyl ketone (MEK) in rats was developed. Partition coefficients were experimentally determined in rat tissues and blood samples using an in vitro vial equilibration technique. These solubility ratios were in agreement with previous human-based estimates that MEK is uniformly soluble within all tissues. The in vivo metabolism of MEK was evaluated using groups of three F344 male rats exposed to 100-2000 ppm MEK in a closed, recirculating gas uptake system. An optimal fit of a family of uptake curves was obtained by adjusting Michaelis-Menten metabolic constants, Km (affinity), and Vmax (capacity) using the PBPK model. At the highest chamber concentration, the uptake curve could not be modeled without the addition of a first-order (Kfo) metabolic pathway. Pretreatment with pyrazole, an inhibitor of oxidative microsomal metabolism, decreased the slope of the gas uptake curve but did not abolish metabolism. Optimal model fit to the gas uptake curve from pyrazole-pretreated animals required the apparent Km to be increased roughly 50 times the value determined in naive rats. The completed PBPK model was evaluated against real-time exhaled breath data collected from rats receiving an intravenous (iv) injection of MEK via a jugular vein cannula. Model simulation of the iv-treated animals required alveolar ventilation to be reduced 30% in order to match the data. Exhaled breath profiles from animals treated with MEK by oral gavage or intraperitoneal (ip) injection were evaluated and absorption rates was determined. Development of a comprehensive PBPK model for MEK in rats is the first step toward future extrapolations to apply to humans.

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.

Physiologically based pharmacokinetic and pharmacodynamic modeling in health risk assessment and characterization of hazardous substances

Recent advances in physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) modeling have introduced novel approaches for evaluating toxicological problems. Because PBPK models are amenable to extrapolation of tissue dosimetry, they are increasingly being applied to chemical risk assessment. A comprehensive listing of PBPK/PD models for environmental chemicals developed to date is referenced. Salient applications of PBPK/PD modeling to health risk assessments and characterization of hazardous substances are illustrated with examples.

Osmolar gap with minimal acidosis in combined methanol and methyl ethyl ketone ingestion

Methyl ethyl ketone is a common solvent but data on overdose in humans are scarce. We report a case of co-ingestion of methyl ethyl ketone together with methanol associated with a hyperosmolar coma without anion gap metabolic acidosis. Blood levels of methyl ethyl ketone and its metabolite, 2-butanol, indicated that this solvent did contribute approximately 20 mosm/L to an observed osmolar gap of 99 mosm/L. At the levels detected, methyl ethyl ketone may have inhibited methanol metabolism, contributing to the low serum formate (1.3 mmol/L) and normal anion gap despite a blood methanol of 67 mmol/L.

Effect of various exposure scenarios on the biological monitoring of organic solvents in alveolar air. I. Toluene and m-xylene

The present study was undertaken to investigate the influence of different exposure scenarios on the elimination of toluene and m-xylene in alveolar air and other biological fluids in human volunteers. The study was also aimed at establishing the effectiveness of physiologically based toxicokinetic models in predicting the value of biological monitoring data after exposure to toluene and m-xylene. Two adult male and two adult female white volunteers were exposed by inhalation, in a dynamic, controlled-environment exposure chamber, to various concentrations of toluene (21-66 ppm) or m-xylene (25-50 ppm) in order to establish the influence of exposure concentration, duration of exposure, variation of concentration within day, and work load on respective biological exposure indices. The concentrations of unchanged solvents in end-exhaled air and in blood as well as the urinary excretion of hippuric acid and m-methyl-hippuric acid were determined. The results show that doubling the exposure concentration for both solvents led to a proportional increase in the concentrations of unchanged solvents in alveolar air and blood at the end of a 7-h exposure period. Cumulative urinary excretion of the respective metabolites exhibited a nearly proportional increase. Adjustment of exposure concentration to account for a prolongation of the duration of exposure resulted in essentially identical cumulative urinary excretion of the metabolites. Induced within-day variations in the exposure concentration led to corresponding but not proportional changes in alveolar concentration for both solvents, depending on whether or not sampling preceded or followed peak exposure to solvent. At the end of repeated 10-min periods of physical exercise at 50 W, alveolar air concentrations of both solvents were increased by 40%. Experimental data collected during the present study were adequately simulated by physiologically based toxicokinetic modeling. These results suggest that alveolar air solvent concentration is a reliable index of exposure to both toluene and m-xylene under various experimental exposure scenarios. For clinical situations likely to be encountered in the workplace, physiologically based toxicokinetic modeling appears to be a useful tool both for developing strategies of biological monitoring of exposure to volatile organic solvents and for predicting alveolar air concentrations under a given set of exposure conditions.

The closed chamber technique--uptake, endogenous production, excretion, steady-state kinetics and rates of metabolism of gases and vapors

The "closed chamber technique" (CCT) is presented. It allows investigation of pharmacokinetics of volatile substances in vivo in animals and in man and in vitro using tissue fractions. During the exposure period only the atmospheric concentrations of the substance are measured. The concentration-time data obtained are pharmacokinetically analyzed by a two compartment model describing uptake, endogenous production and excretion of the unchanged substance and its metabolic elimination. Using this model, pharmacokinetics of ethylene have been determined in rats and man. For both species, the results compared well with an estimation based on an allometric species scaling. Furthermore, the applicability of CCT is demonstrated in vivo on several other gases and vapors of solvents, e.g. trichloroethylene and 1,1,1-trichloroethane, and in vitro on 1,2-epoxybutene-3.

Development and utilization of physiologically based pharmacokinetic models for toxicological applications

Recent advances in physiologically based pharmacokinetic (PB-PK) modeling have introduced novel approaches for evaluating toxicological problems. Because PB-PK models are amenable to extrapolation of tissue dosimetry, they are increasingly being applied to chemical risk assessment. This paper reviews the development of PB-PK modeling for toxicological applications. It briefly compares and contrasts the fundamental differences between conventional compartmental analysis and PB-PK modeling. The theory and principles, data requirements and the methodologies to obtain them, and the steps to construct PB-PK models are described. A comprehensive listing of PB-PK models for environmental chemicals developed to date is referenced. Salient applications of PB-PK modeling to toxicological problems are illustrated with examples. Finally, the uncertainties and limitations in PB-PK modeling are also discussed.