Physiologically based modeling of the toxicokinetic interaction between toluene and m-xylene in the rat

Modeling the impact of heat stress on the toxicokinetics of toluene and acetone

Many workers can be exposed simultaneously to heat and volatile chemicals. In a controlled human exposure study, it was observed that an increase in ambient temperature was associated with increased blood concentrations for acetone and toluene. Based on the expected changes in physiological parameters that occur with an increase in ambient temperature, we aimed to develop a PBPK model for acetone and toluene that could account for the impact of temperature on the kinetics of these solvents. Changes in temperature-dependent physiological parameters (i.e. blood flows, cardiac output, alveolar ventilation) based on recent measurements in volunteers were introduced in the PBPK models to simulate observed blood concentrations for different temperature exposure conditions. Because initial simulations did not adequately predict solvent kinetics at any temperature, the most sensitive parameter (alveolar ventilation; Qp) was, therefore, optimized on experimental acetone blood concentrations to obtain a relationship with temperature. The new temperature-dependent Qp relationship gave Qp values consistent with the literature and estimated a mean increase of 19% at 30 °C (wet bulb globe temperature) compared to 21 °C. The integration of a new temperature-dependent Qp relationship in the PBPK toluene model yielded adequate simulations of the experimental data for toluene in blood, exhaled air and urine. With further validation with other solvents, the temperature-dependant PBPK model could be a useful tool to better assess the risks of simultaneous exposure to volatile chemicals and heat stress and interpret biomonitoring data in workers as well as in the general population. TRN: NCT02659410, Registration date: January 15, 2016.

Application of physiologically-based pharmacokinetic modeled toluene blood concentration in the assessment of short term exposure limits

Toluene is a volatile hydrocarbon with solvent applications in several industries. Acute neurological effects in workers exposed to toluene have been reported in various publications. To inform the basis for a toluene Short Term Exposure Limit (STEL), studies of toluene-exposed workers were modeled using customized exposure scenarios within an existing physiologically-based pharmacokinetic (PBPK) model to simulate blood concentrations during individual studies. Maximum simulated blood concentration ranged from 0.3 to 1.7 (mean = 0.74 mg/L, median = 0.73, upper 95th percentile = 1.07) at the studies identified No Observed Adverse Effect Concentration (NOAEC). Maximum simulated blood concentration ranged from 0.7 to 4.1 mg/L (mean = 1.81, median = 1.63, lower 95th percentile = 0.92) at the studies identified Lowest Observed Adverse Effect Concentration (LOAEC). The maximum blood concentration for a 100 ppm STEL-like simulation was 0.4 mg/L, at the lower end of the NOAEC range and below the 95th percentile of the LOAEC. Therefore, it appears that a STEL <100 ppm would be unnecessary to protect workers due to peak occupational exposures to toluene.

Case study on the impact of the source of metabolism parameters in next generation physiologically based pharmacokinetic models: Implications for occupational exposures to trimethylbenzenes

Physiologically based pharmacokinetic (PBPK) models are a means of making important linkages between exposure assessment and in vitro toxicity. A key constraint on rapid application of PBPK models in risk assessment is traditional reliance on substance-specific in vivo toxicokinetic data to evaluate model quality. Bounding conditions, in silico, in vitro, and chemical read-across approaches have been proposed as alternative sources for metabolic clearance estimates. A case study to test consistency of predictive ability across these approaches was conducted using trimethylbenzenes (TMB) as prototype chemicals. Substantial concordance was found among TMB isomers with respect to accuracy (or inaccuracy) of approaches to estimating metabolism; for example, the bounding conditions never reproduced the human in vivo toxicokinetic data within two-fold. Using only approaches that gave acceptable prediction of in vivo toxicokinetics for the source compound (1,2,4-TMB) substantially narrowed the range of plausible internal doses for a given external dose for occupational, emergency response, and environmental/community health risk assessment scenarios for TMB isomers. Thus, risk assessments developed using the target compound models with a constrained subset of metabolism estimates (determined for source chemical models) can be used with greater confidence that internal dosimetry will be estimated with accuracy sufficient for the purpose at hand.

Role of Physiologically Based Kinetic modelling in addressing environmental chemical mixtures - A review

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The availability and applicability of Physiologically Based Kinetic (PBK) models for mixtures is reviewed.

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PBK models can support risk assessment of mixtures by incorporating the toxicokinetic processes.

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Quantitative structure-activity relationship (QSAR) models can be used to fill data gaps in PBK modelling.

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PBK models for mixtures can be improved by including various types of interactions.

The role of Physiologically Based Kinetic (PBK) modelling in assessing mixture toxicology has been growing for the last three decades. It has been widely used to investigate and address interactions in mixtures. This review describes the current state-of-the-art of PBK models for chemical mixtures and to evaluate the applications of PBK modelling for mixtures with emphasis on their role in chemical risk assessment. A total of 35 mixture PBK models were included after searching web resources (Scopus, PubMed, Web of Science, and Google Scholar), screening for duplicates, and excluding articles based on eligibility criteria. Binary mixtures and volatile organic compounds accounted for two-thirds of the chemical mixtures identified. The most common exposure route and modelled system were found to be inhalation and rats respectively. Twenty two (22) models were for binary mixtures, 5 for ternary mixtures, 3 for quaternary mixtures, and 5 for complex mixtures. Both bottom-up and top-down PBK modelling approaches are described. Whereas bottom-up approaches are based on a series of binary interactions, top-down approaches are based on the lumping of mixture components. Competitive inhibition is the most common type of interaction among the various types of mixtures, and usually becomes a concern at concentrations higher than environmental exposure levels. It leads to reduced biotransformation that either means a decrease in the amount of toxic metabolite formation or an increase in toxic parent chemical accumulation. The consequence is either lower or higher toxicity compared to that estimated for the mixture based on the additivity principle. Therefore, PBK modelling can play a central role in predicting interactions in chemical mixture risk assessment.

Predictions of bisphenol A hepatic clearance in the isolated perfused rat liver (IPRL): impact of albumin binding and of co-administration with naproxen

1. This study aimed (i) to characterise hepatic clearance (CL) of bisphenol A (BPA) and naproxen (NAP) administered alone or in binary mixtures to highlight the influence of a binding to albumin (ALB) using an isolated perfused rat liver (IPRL) system; and (ii) to compare results of prediction algorithms with measured clearance rates. 2. The IPRL system and liver microsomes were used to determine the metabolic constants of BPA and NAP either in the presence or absence of ALB. In this study, the IPRL was used as proxy for the in vivo situation. Accordingly, diverse in vitro-to-in vivo and in vivo-to-in vivo extrapolations (IVIVEs) were made to predict CL of BPA determined in situ/in vivo with ALB from metabolic data determined without ALB by using different binding correction methods (i.e., direct and conventional scaling as well as a novel scaling considering an ALB-facilitated uptake mechanism). 3. The addition of ALB significantly influenced the liver kinetics of BPA and NAP either administered alone or in binary mixtures, which was reflected in the Michaelis-Menten constants. Analysis of concomitant exposures of BPA and NAP gave a fully competitive inhibition. Furthermore, the IVIVE method based on the ALB-facilitated uptake mechanism provided the most accurate predictions of CL_{in vivo} as compared with the other IVIVE methods when the impact of ALB is considered. 4. Our findings support the notion that high binding to ALB reduces the biotransformation of BPA and NAP when administered alone or in mixtures in the IPRL system. However, the free drug concentration in liver in vivo is probably higher than expected since the IVIVE method based on a potential ALB-facilitated uptake mechanism is the most robust prediction method. Overall, this study should improve the physiologically-based pharmacokinetic (PBPK) modelling of chemical-drug interactions.

Advancing In Vitro-In Vivo Extrapolations of Mechanism-Specific Toxicity Data Through Toxicokinetic Modeling

International legislation, such as the European REACH regulation (registration, evaluation, authorization, and restriction of chemicals), mandates the assessment of potential risks of an ever-growing number of chemicals to the environment and human health. Although this legislation is considered one of the most important investments in consumer safety ever, the downside is that the current testing strategies within REACH rely on extensive animal testing. To address the ethical conflicts arising from these increased testing requirements, decision-makers, such as the European Chemicals Agency (ECHA), are committed to Russel and Burch's 3R principle (i.e., reduction, replacement, refinement) by demanding that animal experiments should be substituted with appropriate alternatives whenever possible. A potential solution of this dilemma might be the application of in vitro bioassays to estimate toxic effects using cells or cellular components instead of whole organisms. Although such assays are particularly useful to assess potential mechanisms of toxic action, scientists require appropriate methods to extrapolate results from the in vitro level to the situation in vivo. Toxicokinetic models are a straightforward means of bridging this gap. The present chapter describes different available options for in vitro-in vivo extrapolation (IVIVE) of mechanism-specific effects focused on fish species and also reviews the implications of confounding factors during the conduction of in vitro bioassays and their influence on the optimal choice of different dose metrics.

The application of global sensitivity analysis in the development of a physiologically based pharmacokinetic model for m-xylene and ethanol co-exposure in humans

Global sensitivity analysis (SA) was used during the development phase of a binary chemical physiologically based pharmacokinetic (PBPK) model used for the analysis of m-xylene and ethanol co-exposure in humans. SA was used to identify those parameters which had the most significant impact on variability of venous blood and exhaled m-xylene and urinary excretion of the major metabolite of m-xylene metabolism, 3-methyl hippuric acid. This analysis informed the selection of parameters for estimation/calibration by fitting to measured biological monitoring (BM) data in a Bayesian framework using Markov chain Monte Carlo (MCMC) simulation. Data generated in controlled human studies were shown to be useful for investigating the structure and quantitative outputs of PBPK models as well as the biological plausibility and variability of parameters for which measured values were not available. This approach ensured that a priori knowledge in the form of prior distributions was ascribed only to those parameters that were identified as having the greatest impact on variability. This is an efficient approach which helps reduce computational cost.

Assessing human variability in kinetics for exposures to multiple environmental chemicals: a physiologically based pharmacokinetic modeling case study with dichloromethane, benzene, toluene, ethylbenzene, and m-xylene

The objective of this study was to compare the magnitude of interindividual variability in internal dose for inhalation exposure to single versus multiple chemicals. Physiologically based pharmacokinetic models for adults (AD), neonates (NEO), toddlers (TODD), and pregnant women (PW) were used to simulate inhalation exposure to "low" (RfC-like) or "high" (AEGL-like) air concentrations of benzene (Bz) or dichloromethane (DCM), along with various levels of toluene alone or toluene with ethylbenzene and xylene. Monte Carlo simulations were performed and distributions of relevant internal dose metrics of either Bz or DCM were computed. Area under the blood concentration of parent compound versus time curve (AUC)-based variability in AD, TODD, and PW rose for Bz when concomitant "low" exposure to mixtures of increasing complexities occurred (coefficient of variation (CV) = 16-24%, vs. 12-15% for Bz alone), but remained unchanged considering DCM. Conversely, AUC-based CV in NEO fell (15 to 5% for Bz; 12 to 6% for DCM). Comparable trends were observed considering production of metabolites (AMET), except for NEO's CYP2E1-mediated metabolites of Bz, where an increased CV was observed (20 to 71%). For "high" exposure scenarios, Cmax-based variability of Bz and DCM remained unchanged in AD and PW, but decreased in NEO (CV= 11-16% to 2-6%) and TODD (CV= 12-13% to 7-9%). Conversely, AMET-based variability for both substrates rose in every subpopulation. This study analyzed for the first time the impact of multiple exposures on interindividual variability in toxicokinetics. Evidence indicates that this impact depends upon chemical concentrations and biochemical properties, as well as the subpopulation and internal dose metrics considered.

An integrated QSAR-PBPK modelling approach for predicting the inhalation toxicokinetics of mixtures of volatile organic chemicals in the rat

The objective of this study was to predict the inhalation toxicokinetics of chemicals in mixtures using an integrated QSAR-PBPK modelling approach. The approach involved: (1) the determination of partition coefficients as well as V(max) and K(m) based solely on chemical structure for 53 volatile organic compounds, according to the group contribution approach; and (2) using the QSAR-driven coefficients as input in interaction-based PBPK models in the rat to predict the pharmacokinetics of chemicals in mixtures of up to 10 components (benzene, toluene, m-xylene, o-xylene, p-xylene, ethylbenzene, dichloromethane, trichloroethylene, tetrachloroethylene, and styrene). QSAR-estimated values of V(max) varied compared with experimental results by a factor of three for 43 out of 53 studied volatile organic compounds (VOCs). K(m) values were within a factor of three compared with experimental values for 43 out of 53 VOCs. Cross-validation performed as a ratio of predicted residual sum of squares and sum of squares of the response value indicates a value of 0.108 for V(max) and 0.208 for K(m). The integration of QSARs for partition coefficients, V(max) and K(m), as well as setting the K(m) equal to K(i) (metabolic inhibition constant) within the mixture PBPK model allowed to generate simulations of the inhalation pharmacokinetics of benzene, toluene, m-xylene, o-xylene, p-xylene, ethylbenzene, dichloromethane, trichloroethylene, tetrachloroethylene and styrene in various mixtures. Overall, the present study indicates the potential usefulness of the QSAR-PBPK modelling approach to provide first-cut evaluations of the kinetics of chemicals in mixtures of increasing complexity, on the basis of chemical structure.

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.

Physiologically based pharmacokinetic modeling of cyclohexane as a tool for integrating animal and human test data

This report describes a physiologically based pharmacokinetic model for cyclohexane and its use in comparing internal doses in rats and volunteers following inhalation exposures. Parameters describing saturable metabolism of cyclohexane are measured in rats and used along with experimentally determined partition coefficients. The model is evaluated by comparing predicted blood and brain concentrations to data from studies in rats and then allometrically scaling the results to humans. Levels of cyclohexane in blood and exhaled air are measured in human volunteers and compared with model values. The model predicts that exposure of volunteers to cyclohexane at levels of 4100 mg/m(3) ( approximately 1200 ppm) will result in brain levels similar to those in rats exposed to 8000 mg/m(3) (the no-effect level for acute central nervous system effects). There are no acute central nervous system effects in humans exposed to 860 mg/m(3), consistent with model predictions that current occupational exposure levels for cyclohexane protect against acute central nervous system effects.

Physiologically based modelling and prediction of drug interactions

A major challenge for drug development and environmental or occupational health is the prediction of pharmacokinetic and pharmacodynamic interactions between drugs, natural chemicals or environmental contaminants. This article reviews briefly past developments in the area of physiologically based pharmacokinetic (PBPK) modelling of interactions. It also demonstrates a systems biology approach to the question, and the capabilities of new software tools to facilitate that development. Individual Systems Biology Markup Language models of metabolic pathways can now be automatically merged and coupled to a template PBPK pharmacokinetic model, using for example the GNU MCSim software. The global model generated is very efficient and able to simulate the interactions between a theoretically unlimited number of substances. Development time and the number of model parameter increase only linearly with the number of substances considered, even though the number of possible interactions increases exponentially.

A PBPK Modeling Assessment of the Competitive Metabolic Interactions of JP-8 Vapor with Two Constituents,m-Xylene and Ethylbenzene

Jet Propellant 8 (JP-8) is a kerosene-based jet fuel used in the military and is composed of hundreds of hydrocarbons. A PBPK model was developed to assess the metabolic interactions of JP-8 vapor on two prominent constituents of JP-8 vapor, m-xylene (XYL) and ethylbenzene (EBZ). A limited number of rats were exposed to JP-8 vapor in a Leach chamber for 4 h to 380, 1100, or 2700 mg/m3 (total hydrocarbon). Several individual hydrocarbons were monitored in the chamber atmosphere, including XYL, EBZ, and the total hydrocarbon concentration. Blood and liver were harvested and analyzed by a novel headspace SPME/GC-MS method that allowed for identification of individual hydrocarbons and low limits of detection. The PBPK model was able to describe the metabolic interactions between XYL, EBZ, and a lumped aromatic fraction of JP-8 vapor estimated to be 18 to 25% of the fuel vapor. Competitive inhibition of XYL and EBZ metabolism was observed for JP-8 vapor inhalation exposures of 1100 and 2700 mg/m3. Future inhalation studies with jet fuel include aerosol exposures and expansion of the PBPK models to include other hydrocarbons such as n-alkanes and upper respiratory tract dosimetry of aerosol droplets.

Modeling the toxicokinetics of inhaled toluene in rats: influence of physical activity and feeding status

Toluene is found in petroleum-based fuels and used as a solvent in consumer products and industrial applications. The critical effects following inhalation exposure involve the brain and nervous system in both humans and experimental animals, whether exposure duration is acute or chronic. The goals of this physiologically based pharmacokinetic (PBPK) model development effort were twofold: (1) to evaluate and explain the influence of feeding status and activity level on toluene pharmacokinetics utilizing our own data from toluene-exposed Long Evans (LE) rats, and (2) to evaluate the ability of the model to simulate data from the published literature and explain differing toluene kinetics. Compartments in the model were lung, slowly and rapidly perfused tissue groups, fat, liver, gut, and brain; tissue transport was blood-flow limited and metabolism occurred in the liver. Chemical-specific parameters and initial organ volumes and blood flow rates were obtained from the literature. Sensitivity analysis revealed that the single most influential parameter for our experimental conditions was alveolar ventilation; other moderately influential parameters (depending upon concentration) included cardiac output, rate of metabolism, and blood flow to fat. Based on both literature review and sensitivity analysis, other parameters (e.g., partition coefficients and metabolic rate parameters) were either well defined (multiple consistent experimental results with low variability) or relatively noninfluential (e.g. organ volumes). Rats that were weight-maintained compared to free-fed rats in our studies could be modeled with a single set of parameters because feeding status did not have a significant impact on toluene pharmacokinetics. Heart rate (HR) measurements in rats performing a lever-pressing task indicated that the HR increased in proportion to task intensity. For rats acclimated to eating in the lab during the day, both sedentary rats and rats performing the lever-pressing task required different alveolar ventilation rates to successfully predict the data. Model evaluation using data from diverse sources together with statistical evaluation of the resulting fits revealed that the model appropriately predicted blood and brain toluene concentrations with some minor exceptions. These results (1) emphasize the importance of experimental conditions and physiological status in explaining differing kinetic data, and (2) demonstrate the need to consider simulation conditions when estimating internal dose metrics for toxicity studies in which kinetic data were not collected.

Modeling VOC-odor exposure risk in livestock buildings

This paper describes a novel idea of linking models of exposure, internal dosimetry, and health effects. Risk assessment approach that integrates predicted odor caused by volatile organic compounds (VOC-odor) of toluene/xylene concentrations in human tissues leads to predict exposure risks in livestock buildings. First, VOC transport model was developed to calculate airborne toluene/xylene concentrations. Based on a physiologically based pharmacokinetic (PBPK) model, concentrations within five compartments representing lung, liver, fat, slowly perfused tissues, and rapidly perfused tissues could be quantified. By using a pharmacodynamic (PD) Hill model, we can optimally fit data from rat and human experiments to reconstruct dose-response relationships for accounting human health effects from nose poke and eye irritation. Results demonstrated that peak tissue concentration occurring at 5-10h in that fat contains the highest concentration than other tissues at around 4ppm of toluene and 1.8ppm of xylene. The EC(10) values are 114 and 232ppm, whereas expected risks are estimated to be 0.71% and 0.26% of human exposure to toluene and xylene, respectively. Risk analyses indicate that inhalation exposure in livestock buildings poses no significant threat to human health under the present environmental conditions. This method provides a rigorous and effective approach to relate target tissue concentration to human nose poke or eye irritation. We suggest that our probabilistic framework and methods be taken seriously because they produce general conclusions that are more robust and could offer a risk-management framework for discussion of future establishment of limits for respiratory exposure to VOC-odor.

Occupational toluene exposure induces cytochrome P450 2E1 mRNA expression in peripheral lymphocytes

Print workers are exposed to organic solvents, of which the systemic toxicant toluene is a main component. Toluene induces expression of cytochrome P450 2E1 (CYP2E1), an enzyme involved in its own metabolism and that of other protoxicants, including some procarcinogens. Therefore, we investigated the association between toluene exposure and the CYP2E1 response, as assessed by mRNA content in peripheral lymphocytes or the 6-hydroxychlorzoxazone (6OH-CHZ)/chlorzoxazone (CHZ) quotient (known as CHZ metabolic ratio) in plasma, and the role of genotype (5 -flanking region RsaI/PstI polymorphic sites) in 97 male print workers. The geometric mean (GM) of toluene concentration in the air was 52.80 ppm (10-760 ppm); 54% of the study participants were exposed to toluene concentrations that exceeded the maximum permissible exposure level (MPEL). The GM of urinary hippuric acid at the end of a work shift (0.041 g/g creatinine) was elevated relative to that before the shift (0.027 g/g creatinine; p < 0.05). The GM of the CHZ metabolic ratio was 0.33 (0-9.3), with 40% of the subjects having ratios below the GM. However, the average CYP2E1 mRNA level in peripheral lymphocytes was 1.07 (0.30-3.08), and CYP2E1 mRNA levels within subjects correlated with the toluene exposure ratio (environmental toluene concentration:urinary hippuric acid concentration) (p = 0.014). Genotype did not alter the association between the toluene exposure ratio and mRNA content. In summary, with further validation, CYP2E1 mRNA content in peripheral lymphocytes could be a sensitive and noninvasive biomarker for the continuous monitoring of toluene effects in exposed persons.

Neurobehavioural evaluation and kinetics of inhalation of constant or fluctuating toluene concentrations in human volunteers

The health risks of inhalation exposure to volatile organic solvents may not only depend on the total external dose, but also on the pattern of exposure. It has been suggested that exposure to regularly occurring peak concentrations may have a stronger impact on the brain than constant exposure at the same average level. Recent animal experimental studies conducted in our laboratory using relatively high concentrations of toluene have shown different effects on discrimination performance and motor activity during and after exposure, depending on the exposure scenario. Relevance of these findings for man was evaluated in a volunteer study in which 11 healthy men (age 20-49 years) were exposed by inhalation for 4h to either a constant concentration of 40ppm toluene or to three 30-min exposure peaks at 110ppm during this 4h period. Selected tests from the Neurobehavioural Evaluation System (NES) were performed repeatedly during and after exposure. Blood concentrations of toluene as well as urinary o-cresol excretion were measured at relevant time points. The results show that toluene concentration in blood increased during constant exposure and fluctuated during occupationally relevant peak exposures. Presumably, brain concentrations showed similar qualitative patterns. No clear changes were observed on neurobehavioural measures of motor performance, attention, perceptual coding and memory, or on measures of mood and affect. The exposure conditions do not seem to induce significant acute changes in central nervous system function similar to those observed at much higher concentrations in animals, although a statistical correlation was found between one motor performance test (Finger Tapping Test with alternating hands) and blood toluene concentrations. Urinary o-cresol excretion appeared to be significantly higher during the first 2h after exposure.

Styrene and ethylene glycol have a synergetic effect on lipid peroxidation that is better protected than repaired by CoQ10

Previous study of a group of 22 workers occupationally exposed to styrene, ethylene glycol and their mixture at a paint and lacquer industry indicated significantly elevated concentration of malondialdehyde with 4-hydroxynonenal (MDA+4-HNE) in the blood plasma, successfully decreased with coenzyme Q10 (CoQ10) supplementation. The aim of present study is to evaluate whether the exposure to styrene or/and ethylene glycol could be responsible for the increase in MDA level. The mechanism of a single solvent action and the mixture was examined, specially whether it is connected with hydroxyl radical (*OH) generation. It was also investigated whether coenzyme Q10 could be considered as a protective (given before the solvents) or repairing (given after the solvents) agent in oxidative stress caused by the solvents. The results indicate that ethylene glycol nor styrene increase MDA and *OH, but as a mixture give synergetic interaction, elevating MDA and *OH concentration to a statistically significant extent. Coenzyme Q10 at a dose of 3.0 microg/ml only protects, but does not repair increased lipid peroxidation caused by ethylene glycol with styrene. In order to obtain both a protective and repairing effect, a concentration of 12.0 microg/ml CoQ is needed.

Incorporation of therapeutic interventions in physiologically based pharmacokinetic modeling of human clinical case reports of accidental or intentional overdosing with ethylene glycol

Although occupational uses of the high production volume (HPV) chemical ethylene glycol (EG) have not been associated with adverse effects, there are case reports where humans have either intentionally or accidentally ingested large quantities of EG, primarily from antifreeze. The acute toxicity of EG can proceed through three stages, each associated with a different metabolite: central nervous system depression (ethylene glycol), cardiopulmonary effects associated with metabolic acidosis (glycolic acid), and ultimately renal toxicity (oxalic acid), depending on the total amounts consumed and the effectiveness of therapeutic interventions. A physiologically based pharmacokinetic (PBPK) model developed in a companion paper (Corley et al., 2005). Development of a physiologically based pharmacokinetic model for ethylene glycol and its metabolite, glycolic acid, in rats and humans. Toxicol. Sci., in press 2005) was refined in this study to include clinically relevant treatment regimens for EG poisoning such as hemodialysis or metabolic inhibition with either ethanol or fomepizole. Such modifications enabled the model to describe data from several human case reports, confirming the ability of the previous model to describe the pharmacokinetics of EG and its metabolite, glycolic acid, in humans across a broad range of doses and multiple exposure routes. By integrating the case report data sets with controlled studies in this PBPK model, it was demonstrated that fomepizole, if administered early enough in a clinical situation, can be more effective than ethanol or hemodialysis in preventing the metabolism of EG to more toxic metabolites. Hemodialysis remains an important option, however, if treatment is instituted after a significant amount of EG is metabolized or if renal toxicity has occurred.

A spreadsheet program for modeling quantitative structure-pharmacokinetic relationships for inhaled volatile organics in humans

The extent and profile of target tissue exposure to toxicants depend upon the pharmacokinetic processes, namely, absorption, distribution, metabolism and excretion. The present study developed a spreadsheet program to simulate the pharmacokinetics of inhaled volatile organic chemicals (VOCs) in humans based on information from molecular structure. The approach involved the construction of a human physiologically-based pharmacokinetic (PBPK) model, and the estimation of its parameters based on quantitative structure-property relationships (QSPRs) in an Excel spreadsheet. The compartments of the PBPK model consisted of liver, adipose tissue, poorly perfused tissues and richly perfused tissues connected by circulating blood. The parameters required were: human physiological parameters such as cardiac output, breathing rate, tissue volumes and tissue blood flow rates (obtained from the biomedical literature), tissue/air partition coefficients (obtained using QSPRs developed with rat data), blood/air partition coefficients (Pb) and hepatic clearance (CL). Using literature data on human Pb and CL for several VOCs (alkanes, alkenes, haloalkanes and aromatic hydrocarbons), multi-linear additive QSPR models were developed. The numerical contributions to human Pb and CL were obtained for eleven structural fragments (CH3, CH2, CH, C, C [double bond] C, H, Cl, Br, F, benzene ring, and H in the benzene ring structure). Using these data as input, the PBPK model written in an Excel spreadsheet simulated the inhalation pharmacokinetics of ethylbenzene (33 ppm, 7 h) and dichloromethane (100 ppm, 6 h) in humans exposed to these chemicals. The QSPRs developed in this study should be useful for predicting the inhalation pharmacokinetics of VOCs in humans, prior to testing and experimentation.

Magnitude and mechanistic determinants of the interspecies toxicokinetic uncertainty factor for organic chemicals

The interspecies uncertainty factor, UF(AH), of 10 was recently subdivided into two components to account separately for interspecies differences in toxicokinetics and toxicodynamics (UF(AH-TK)=3.16, UF(AH-TD)=3.16). Even though the UF(AH) in its composite or dissociated form is used during the health risk assessment of systemic toxicants, there is no convincing scientific basis to justify the use of the same UF for all chemicals. In this study, we use equations that describe the toxicokinetics of chemicals at steady-state to characterize the magnitude and mechanistic determinants of UF(AH-TK) for several volatile organic chemicals (VOCs). Further, algorithms have been developed to determine the magnitude of the components of UF(AH-TK), namely the UF(AH-TK-ABS) (accounting for interspecies differences in dose absorbed during identical inhalation exposure conditions), UF(AH-TK-MET) (referring to the factor by which the blood concentration of unchanged parent chemical differs from one species to another, due to metabolic clearance, when both species receive identical doses) and UF(AH-TK-DIS) (reflecting the magnitude of difference in chemical concentrations distributed in target tissues of two species when the arterial blood concentration in both species is identical). The results show that the body weight, the rate of alveolar ventilation, the fraction of cardiac output flowing to the liver, partition coefficients (blood:air and tissue:blood), and the hepatic extraction ratio are the only parameters that play a critical role in the extrapolation of tissue and blood concentrations across species. Further, the magnitude of the UF(AH-TK-ABS) (means+/-SD, 0.19+/-0.04), UF(AH-TK-MET) (means+/-SD, 0.24+/-0.05) and UF(AH-TK-DIS) (mean range: 1.76-0.93) obtained in this study for several VOCs compares well with that obtained previously using physiologically based toxicokinetic models.

Seeking solutions to chemical mixtures challenges in public health

The Agency for Toxic Substances and Disease Registry (ATSDR) identifies people near hazardous waste sites who are at potential health risk because of their exposure to environmental chemicals. Nearly, 2000 chemicals have been associated with such sites. Residents of U.S. communities are potentially exposed to hazardous substances through air, soil, drinking water, and food. The agency has determined that more than 73 million people live within a 4-mile radius of waste sites. More than 14 million Americans live within 1 mile of a National Priorities List site, of which 11% are 7 years of age or younger, 12% are 64 years of age or older, 24% are women of childbearing age, and 25% are minorities. The lack of adequate environmental sampling and information on human exposures often restricts ATSDR's evaluation and assessment activities. Assessing human exposure with its attendant health risks and outcomes is complex because many populations have a wide range of reported illnesses, and generally exposures are to mixtures of chemicals. This prompted ATSDR to consider mixtures issues more in depth and to establish a formal mixtures assessment and research program in 1994. In this paper, we present an overview of the agency activities, the genesis, legislative mandates, and pertinence of the mixtures program including applied research and the development of methods for evaluating the impact of multiple-chemical exposure. On the basis of 20-year experience of evaluating and researching environmental chemical mixtures at waste sites, ATSDR convened the International Conference on Chemical Mixtures (ICCM) in 2002. The conference was supported by several federal agencies and scientific organizations and attended by international and national experts. The conference addressed broad topics such as prevalence of exposures to chemical mixtures, importance of interactions at environmentally relevant levels, validity of assuming additivity (dose or response) as default for mixtures assessment, and promising avenues in the three broad areas, viz., research, assessment, and computational tools.

Chemical mixture toxicology: from descriptive to mechanistic, and going on to in silico toxicology

Because of the pioneering vision of certain leaders in the biomedical field, the last two decades witnessed rapid advances in the area of chemical mixture toxicology. Earlier studies utilized conventional toxicology protocol and methods, and they were mainly descriptive in nature. Two good examples might be the parallel series of studies conducted by the U.S. National Toxicology Program and TNO in The Netherlands, respectively. As a natural course of progression, more and more sophistication was incorporated into the toxicology studies of chemical mixtures. Thus, at least the following seven areas of scientific achievements in chemical mixture toxicology are evident in the literature: (a) the application of better and more robust statistical methods; (b) the exploration and incorporation of mechanistic bases for toxicological interactions; (c) the application of physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) modeling; (d) the studies on more complex chemical mixtures; (e) the use of science-based risk assessment approaches; (f) the utilization of functional genomics; and (g) the application of technology. Examples are given for the discussion of each of these areas. Two important concepts emerged from these studies and they are: (1) dose-dependent toxicologic interactions; and (2) "interaction thresholds". Looking into the future, one of the most challenging areas in chemical mixture research is finding the answer to the question "when one tries to characterize the health effects of chemical mixtures, how does one deal with the infinite number of combination of chemicals, and other possible stressors?" Undoubtedly, there will be many answers from different groups of researchers. Our answer, however, is first to focus on the finite (biological processes) rather than the infinite (combinations of chemical mixtures and multiple stressors). The idea is that once we know a normal biological process(es), all stimuli and insults from external stressors are merely perturbations of the normal biological process(es). The next step is to "capture" the biological process(es) by integrating the recent advances in computational technology and modern biology. Here, the computer-assisted Reaction Network Modeling, linked with PBPK modeling, offers a ray of hope to dealing with the complex biological systems.

Implications of chemical mixtures in public health practice

The Agency for Toxic Substances and Disease Registry (ATSDR) is a federal public health agency that investigates and strives to prevent human health problems produced by exposure to toxic chemicals and their mixtures in the environment. Most human exposures involving toxic chemicals or mixtures are thought to originate from environmental and occupational sources; however, concurrent exposures are also likely from other sources, such as prescription and nonprescription drugs, indoor air pollutants, alcohol, and tobacco smoke. Thus, in evaluating the potential hazard following exposure to environmental mixtures, ATSDR not only considers the inherent joint toxicity of the mixture but also the influence of environmental, demographic, occupational, and lifestyle factors. To foster these goals, ATSDR has pursued a Mixtures Research and Assessment Program that consists of three component efforts: trend analysis, joint toxicity assessment, and experimental testing. Through trend analysis, ATSDR sets priorities for environmental mixtures of concern for which joint toxicity assessments are conducted as needed. If data are not available to conduct appropriate assessments, a research agenda is pursued through established extramural mechanisms. Ultimately, the data generated are used to support ATSDR's work at sites involving exposure to chemical mixtures. This pragmatic approach allows testable hypotheses or research needs to be identified and resolved and enhances our understanding of the mechanisms of joint toxicity. Several collaborative and cooperative efforts with national and international organizations such as the Toxicology and Nutrition Office, the Netherlands, and the Department of Energy are being pursued as part of these activities. ATSDR also develops guidance manuals to consistently and accurately apply current methodologies for the joint toxicity assessment of chemicals. Further, expert panels often are assembled to resolve outstanding scientific issues or obtain expert advice on pertinent issues. Recently, the need for studies on chemical mixtures has been proposed as one of the six priority areas the agency identified in its agenda for public health environmental research. This has been reinforced through the agency's close work with communities whose leaders have spoken passionately about their concern for information on exposures to chemical mixtures. The five other priority research areas the agency identified are exposure, susceptible populations, communities and tribal involvement, evaluation/surveillance of health effects, and health promotion/prevention.

Six interaction profiles for simple mixtures

The Agency for Toxic Substances and Disease Registry (ATSDR) has a program for chemical mixtures that encompasses research on chemical mixtures toxicity, health risk assessment, and development of innovative computational methods. ATSDR prepared a guidance document that instructs users on how to conduct health risk assessment on chemical mixtures (Guidance Manual for the Assessment of Joint Toxic Action of Chemical Mixtures). ATSDR also developed six interaction profiles for chemical mixtures. Two profiles were developed for persistent environmental chemicals that are often found in contaminated fish and also can be detected in human breast milk. The mixture included chlorinated dibenzo-p-dioxins, hexachlorobenzene, dichlorodiphenyl dichloroethane, methyl mercury, and polychlorinated biphenyls. Two profiles each were developed for mixtures of metals and mixtures of volatile organic chemicals (VOCs) that are frequently found at hazardous waste sites. The two metal profiles dealt with (a) lead, manganese, zinc, and copper; and (b) arsenic, cadmium, chromium, and lead; the two VOCs mixtures dealt with (a) 1,1,1-trichloroethane, 1,1-dichloroethane, trichloroethylene, and tetrachloroethylene; and (b) benzene, ethylbenzene, toluene, and xylenes (BTEX). Weight-of-evidence methodology was used to assess the joint toxic action for most of the mixtures. Physiologically based pharmacokinetic modeling was used for BTEX. In most cases, a target-organ toxicity dose modification of the hazard index approach is recommended for conducting exposure-based assessments of noncancer health hazards.

Evaluation of the dermal bioavailability of aqueous xylene in F344 rats and human volunteers

Xylene is a clear, colorless liquid used as a solvent in the printing, rubber, and leather industries and is commonly found in paint thinners, paints, varnishes, and adhesives. Although humans are most likely to be exposed to xylene via inhalation, xylene is also found in well and surface water. Therefore, an assessment of the dermal contribution to total xylene uptake is useful for understanding human exposures. To evaluate the significance of these exposures, the dermal absorption of o-xylene was assessed in F344 male rats and human volunteers using a combination of real-time exhaled breath analysis and physiologically based pharmacokinetic (PBPK) modeling. Animals were exposed to o-xylene dermally. Immediately following the initiation of exposure, individual animals were placed in a glass off-gassing chamber and exhaled breath was monitored. Human volunteers participating in the study placed both legs into a stainless steel hydrotherapy tub containing an initial concentration of approximately 500 microg/L o-xylene. Exhaled breath was continually analyzed from each volunteer before, during, and after exposure to track absorption and subsequent elimination of the compound in real time. In both animal and human studies, a PBPK model was used to estimate the dermal permeability coefficient (K(p)) to describe each set of exhaled breath data. Rat skin was found to be approximately 12 times more permeable to aqueous o-xylene than human skin. The estimated human and rat aqueous o-xylene K(p) values were 0.005 +/- 0.001 cm/h and 0.058 +/- 0.009 cm/h, respectively.

Repeated ingestion of 2-butoxyethanol: case report and literature review

BACKGROUND

Ethylene glycol monobutyl ether (2-butoxyethanol) is not commonly associated with significant human poisoning. Exposures are usually through occupational contact and typically involve inhalation injury. Animal studies report severe hemolysis occurring in rats and mice. Rare published human cases give varied descriptions of the clinical course associated with 2-butoxyethanol poisoning including reports of metabolic acidosis, ethylene glycol production, oxaluria, renal failure, and anemia. We report a case of two separate ingestions (80 to 100 grams) of a glass cleaner concentrate containing 22% 2-butoxyethanol, and its primary metabolite butoxyacetic acid.

CASE REPORT

An 18-year-old male ingested 360-480 mL of 22% 2-butoxyethanol on two separate occasions. Approximately 10hours after the first ingestion, the patient developed severe CNS depression, metabolic acidosis, hematuria, and mild elevation of hepatic enzymes. He was treated initially with ethanol therapy but continued to deteriorate and was started on hemodialysis. Approximately 10 days after discharge, the patient ingested 480 mL of the same product and received ethanol and hemodialysis within four hours of ingestion. During his second admission the patient did not develop the delayed severe CNS depression or profound metabolic acidosis. Clinically significant hemolytic anemia, oxaluria, ethylene glycol production, and renal failure were not noted in either episode. The patient recovered on both occasions without sequelae.

CONCLUSION

Hemodialysis may be an effective treatment intervention for managing severe acute 2-butoxyethanol intoxication, however, further investigation is warranted.

Evaluation of the dermal absorption of aqueous toluene in F344 rats using real-time breath analysis and physiologically based pharmacokinetic modeling

Toluene is a ubiquitous chemical that is commonly used for its solvent properties in industry and manufacturing, and is a component of many paint products. Because of its widespread use, there is potential for both occupational and nonoccupational dermal exposure to toluene. To understand the significance of these exposures, the dermal bioavailability of toluene was assessed in F344 male rats using a combination of real-time exhaled breath analysis and physiologically based pharmacokinetic (PBPK) modeling. Animals were exposed to toluene at 0.5 or 0.2 mg/ml aqueous concentration (0.05% or 0.02%) using a 2.5-cm-diameter occluded glass patch system attached to a clipper-shaved area on the back of the rat. Immediately following exposure, individual animals were placed in a glass off-gassing chamber and exhaled breath was monitored as chamber concentration in real time using an ion-trap mass spectrometer (MS/MS). The real-time exhaled breath profile clearly demonstrated the rapid absorption of toluene, with peak chamber concentrations observed within 1 h from the start of exposure. The PBPK model describing the exposure and off-gassing chamber was used to estimate a dermal permeability coefficient (K(p)) to describe each set of exhaled breath data. Regardless of exposure level, a single K(p) value of 0.074 +/- 0.005 cm/h provided a good fit to all data sets. These rat studies using aqueous toluene will form the basis for comparing the dermal bioavailability of toluene in various paint products and may ultimately aid in understanding human health risk under a variety of exposure scenarios.

Physiological-model-based derivation of the adult and child pharmacokinetic intraspecies uncertainty factors for volatile organic compounds

The intraspecies uncertainty factor (UF(HH)=10x) is used in the determination of the reference dose or reference concentration and accounts for the pharmacokinetic and pharmacodynamic heterogeneity within the human population. The Food Quality Protection Act of 1996 mandated the use of an additional uncertainty factor (UF(HC)=10x) to take into account potential pre- and postnatal toxicity and lack of completeness of the data with respect to exposure and toxicity to children. There is no conclusive experimental or theoretical justification to support or refute the magnitude of the UF(HH) and UF(HC) nor any conclusive evidence to suggest that a factor of 100 is needed to account for intrahuman variability. This study presents a new chemical-specific method for estimating the pharmacokinetic (PK) component of the interspecies uncertainty factor (UF(HH-PK) and UF(HC-PK)) for volatile organic compounds (VOCs). The approach utilizes validated physiological-based pharmacokinetic (PBPK) models and simplified physiological-model-based algebraic equations to translate ambient exposure concentration to tissue dose in adults and children the ratio of which is the UF(HH-PK) and UF(HC-PK). The results suggest that: (i) the UF(HH-PK) and UF(HC-PK) are chemical specific; (ii) for the chemicals used in this study there is no significant difference between UF(HH-PK) and UF(HC-PK); (iii) the magnitude of UF(HH-PK) and UF(HC-PK) varies between 0.033 and 2.85 with respect to tissue and blood concentrations; (iv) the body weight, the rate of ventilation, the fraction of cardiac output flowing to the liver, the blood : air partition coefficient, and the hepatic extraction ratio are the only parameters that play a critical role in the variability of tissue and blood doses within species; and (v) the magnitude of the UF(HH-PK) and UF(HC-PK) obtained with the simplified steady-state equations is essentially the same with that obtained with PBPK models. Overall, this study suggests that no adult-children differences in the parent chemical concentrations of the VOCs are likely to be observed during inhalation exposures. The physiological-model-based approaches used in the present study to estimate the UF(HH-PK) and UF(HC-PK) provide a scientific basis for their magnitude. They can replace the currently used empirical default approaches to provide chemical-specific UF(HH-PK) in future risk assessments.

Validation of a physiological modeling framework for simulating the toxicokinetics of chemicals in mixtures

The objective of this study was to investigate the usefulness of a physiologically based toxicokinetic (PBTK) modeling framework for simulating the kinetics of chemicals in mixtures of varying complexities and composition. The approach involved the simulation of the kinetics of components in two situations: (i) when one of the mixture components was substituted with another (i.e., benzene in the benzene (B)-toluene (T)-ethyl benzene (E)-m-xylene (X) mixture was substituted with dichloromethane (D)), and (ii) when another chemical was added to the existing four-chemical mixture model (i.e., when D was added to the existing BTEX mixture model). In both cases, differing compositions of mixtures were used to obtain simulations and to generate experimental data on kinetics for validation purposes. Since the quantitative and qualitative mechanisms of interaction among B, T, E, and X have already been established, the mechanisms of binary interactions between D and the BTEX components (e.g., competitive, noncompetitive, or uncompetitive metabolic inhibition) were investigated in the present study. The analysis of rat blood kinetic data (4-h inhalation exposures, 50-200 ppm each) to all binary combinations (D-B, D-T, D-E, and D-X) investigated in the present study was suggestive of competitive metabolic inhibition as the plausible interaction mechanism. By incorporating the newly estimated values of metabolic inhibition constant (K(i)) for each of these binary combinations within the five-chemical PBTK model (i.e., for the DBTEX mixture), the model adequately predicted the venous blood kinetics of chemicals in rats following a 4-h inhalation exposure to various mixtures (mixture 1:100 ppm of D and 50 ppm each of T, E, and X; mixture 2: 100 ppm each of D, T, E, and X; mixture 3: 100 ppm of D and 50 ppm each of B, T, E, and X; mixture 4: 100 ppm each of D, B, T, E, and X). The results of the present study suggest that the PBTK model framework is useful for conducting extrapolations of the kinetics of chemicals from one mixture to another differing in complexity and composition, based on mechanistic considerations of interactions elucidated at the binary level.

Physiological modeling of the toxicokinetic interactions in a quaternary mixture of aromatic hydrocarbons

The available data on binary interactions are yet to be considered within the context of mixture risk assessments because of our inability to predict the effect of a third or fourth chemical in the mixture on the interacting binary pairs. Physiologically based toxicokinetic (PBTK) models represent a framework that can be potentially used for predicting the impact of multiple interactions on component kinetics at any level of complexity. The objective of this study was to develop and validate an interaction-based PBTK model for simulating the toxicokinetics of the components of a quaternary mixture of aromatic hydrocarbons [benzene (B), toluene (T), ethylbenzene (E), m-xylene (X)] in the rat. The methodology consisted of: (1) obtaining and refining the validated individual chemical PBTK models from the literature, (2) interconnecting all individual chemical PBTK models at the level of liver on the basis of the mechanism of binary chemical interactions (e.g., competitive, noncompetitive, or uncompetitive metabolic inhibition), and (3) comparing the a priori predictions of the interaction-based model to corresponding experimental data on venous blood concentrations of B, T, E, and X during mixture exposures. The analysis of blood kinetics data from inhalation exposures (4 h, 50-200 ppm each) of rats to all binary combinations of B, T, E, and X was suggestive of competitive metabolic inhibition as the plausible interaction mechanism. The metabolic inhibition constant (K(i)) for each binary combination was quantified and incorporated within the mixture PBTK model. The binary interaction-based PBTK model predicted adequately the inhalation toxicokinetics of all four components in rats following exposure to mixtures of BTEX (50 ppm each of B, T, E, and X, 4 h; 100 ppm each of B, T, E and X, 4 h; 100 ppm B + 50 ppm each of T, E, and X, 4 h). The results of the present study suggest that data on interactions at the binary level alone are required and sufficient for predicting the kinetics of components in complex mixtures.

Toluene and styrene intoxication route in the rat cochlea

It is well established that organic solvents such as toluene and styrene are ototoxic in the rat; however, the intoxication route used to reach the organ of Corti is still questionable. The distribution of toluene and styrene in various tissues of Long-Evans rats (n = 2 x 8) was studied after inhalation of either 1750 ppm toluene or 1750 ppm styrene for 10 h (6 consecutive h + 4 h the following day). At the end of the solvent exposures, blood, brain, auditory nerves, the organ of Corti, cerebrospinal (CSF), and inner ear fluids (IEF) were sampled or removed to measure the rates of solvent uptake in each tissue by gas chromatography. Results indicate that CSF and IEF were free from detectable solvents, whereas the organ of Corti, the nerves, and the brain were contaminated. Therefore, both toluene- and styrene-induced hearing losses are caused by tissue intoxication rather than by fluid contamination. It is proposed that the outer sulcus is used as an intoxication route to reach the organ of Corti.

Physiologically based modeling of 1,2,4-trimethylbenzene inhalation toxicokinetics

A physiologically based toxicokinetic model was developed for inhalation exposure of 1,2,4-trimethylbenzene (TMB) in man. The model consists of six compartments for TMB and one compartment for the metabolite 3,4-dimethylhippuric acid (DMHA). Based on previous experimental findings from human exposures to TMB, liver metabolism was divided in two pathways, one of the first order and one of the Michaelis-Menten type. Muscle tissue was split in two compartments to account for working and resting muscle tissues during bicycle exercise. The model was used to investigate how various factors influence potential biomarkers of exposure, i.e., TMB in blood and exhaled air and DMHA in urine. Increasing the work load from rest to moderate exercise (100 W) more than doubled all biomarker levels end of shift. The effect on next morning levels was even more pronounced, illustrated by a fivefold increase in the DMHA excretion rate. Simulations of five daily 8-h exposures suggest that biomarker levels end of shift remain fairly constant whereas the levels prior to shift increase gradually during the week. This suggests that end of shift levels reflect the exposure of the same day whereas levels Friday morning reflect exposure during the entire working week. Simulations with randomly generated exposures show that the variability due to fluctuating exposure is lower next morning than end of shift. End of shift exhalation rate of TMB is more sensitive to fluctuation than TMB in venous blood and DMHA in urine. Biomarker levels for 25 ppm exposure at different sampling times are given.

A comparison of 1H8- and 2H8-toluene toxicokinetics in men

1. To examine the bioequivalence of an isotope-labelled tracer to study toxicant disposition, we conducted 33 controlled human exposures to a mixture of 50 ppm 1H8-toluene and 50 ppm 2H8-toluene for 2 h, and measured concentrations in blood and breath, and metabolite levels in urine for 100 h post-exposure. 2. A physiologically based kinetic (PBK) model found that compared with 1H8-toluene, 2H8-toluene had a 6.4+/-13% (mean+/-SD) lower AUC, a 6.5+/-13% higher systemic clearance (1.46+/-0.27 versus 1.38+/-0.25 l/h-kg), a 17+/-22% larger terminal volume of distribution (66.4+/-14 versus 57.2+/-10 l/kg) and a 9.7+/-26% longer terminal half-life (38+/-12 versus 34+/-10 h) (p < 0.05 for all comparisons). 3. The higher 2H8-toluene clearance may have been due to an increased rate of ring oxidation, consistent with the 17% higher observed fraction of 2H5- versus 1H5-cresol metabolites in urine. 4. The larger terminal volume and half-lives for 2H8-toluene suggested a higher adipose tissue/blood partition coefficient. 5. Observed isotope differences were small compared with interindividual differences in 1H8-toluene kinetics from previous studies. 6. The PBK model allowed us to ascribe observed isotope differences in solvent toxicokinetics to underlying physiologic mechanisms.

Approaches to developing alternative and predictive toxicology based on PBPK/PD and QSAR modeling

Systematic toxicity testing, using conventional toxicology methodologies, of single chemicals and chemical mixtures is highly impractical because of the immense numbers of chemicals and chemical mixtures involved and the limited scientific resources. Therefore, the development of unconventional, efficient, and predictive toxicology methods is imperative. Using carcinogenicity as an end point, we present approaches for developing predictive tools for toxicologic evaluation of chemicals and chemical mixtures relevant to environmental contamination. Central to the approaches presented is the integration of physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) and quantitative structure--activity relationship (QSAR) modeling with focused mechanistically based experimental toxicology. In this development, molecular and cellular biomarkers critical to the carcinogenesis process are evaluated quantitatively between different chemicals and/or chemical mixtures. Examples presented include the integration of PBPK/PD and QSAR modeling with a time-course medium-term liver foci assay, molecular biology and cell proliferation studies. Fourier transform infrared spectroscopic analyses of DNA changes, and cancer modeling to assess and attempt to predict the carcinogenicity of the series of 12 chlorobenzene isomers. Also presented is an ongoing effort to develop and apply a similar approach to chemical mixtures using in vitro cell culture (Syrian hamster embryo cell transformation assay and human keratinocytes) methodologies and in vivo studies. The promise and pitfalls of these developments are elaborated. When successfully applied, these approaches may greatly reduce animal usage, personnel, resources, and time required to evaluate the carcinogenicity of chemicals and chemical mixtures.

ATSDR evaluation of health effects of chemicals. V. Xylenes: health effects, toxicokinetics, human exposure, and environmental fate

Xylenes, or dimethylbenzenes, are among the highest-volume chemicals in production. Common uses are for gasoline blending, as a solvent or component in a wide variety of products from paints to printing ink, and in the production of phthalates and polyester. They are often encountered as a mixture of the three dimethyl isomers, together with ethylbenzene. As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals found at Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) sites that are of greatest concern for public health purposes. These profiles comprehensively summarize toxicological and environmental information. This article constitutes the release of the bulk of this profile (ATSDR, 1995) into the mainstream scientific literature. An extensive listing of known human and animal health effects, organized by route, duration, and end point, is presented. Toxicological information on toxicokinetics, biomarkers, interactions, sensitive subpopulations, reducing toxicity after exposure, and relevance to public health is also included. Environmental information encompasses physical properties, production and use, environmental fate, levels seen in the environment, analytical methods, and a listing of regulations. ATSDR, as mandated by CERCLA (or Superfund), prepares these profiles to inform and assist the public.

Physiologically based pharmacokinetic modeling of a ternary mixture of alkyl benzenes in rats and humans

The objective of the present study was to develop a physiologically based pharmacokinetic (PBPK) model for a ternary mixture of alkyl benzenes [toluene (TOL), m-xylene (XYL), and ethylbenzene (EBZ)] in rats and humans. The approach involved the development of the mixture PBPK model in the rat and extrapolation to humans by substituting rat physiological parameters and blood:air partition coefficients in the model with those of humans, scaling maximal velocity for metabolism on the basis of body weight0.75 and keeping all other model parameters species-invariant. The development of the PBPK model for the ternary mixture in the rat was accomplished by initially validating or refining the existing PBPK models for TOL, XYL, and EBZ and linking the individual chemical models via the hepatic metabolism term. Accordingly, the Michaelis-Menten equation for each solvent was modified to test four possible mechanisms of metabolic interaction (i.e., no interaction, competitive inhibition, noncompetitive inhibition, and uncompetitive inhibition). The metabolic inhibition constant (Ki) for each binary pair of alkyl benzenes was estimated by fitting the binary chemical PBPK model simulations to previously published data on blood concentrations of TOL, XYL, and EBZ in rats exposed for 4 hr to a binary combination of 100 or 200 ppm of each of these solvents. Competitive metabolic inhibition appeared to be the most plausible mechanism of interaction at relevant exposure concentrations for all binary mixtures of alkyl benzenes in the rat (Ki,TOL-XYL = 0.17; Ki,TOL-EBZ = 0.79; Ki,XYL-TOL = 0.77; Ki,XYL-EBZ = 1.50; Ki,EBZ-TOL = 0.33; Ki,EBZ-XYL = 0.23 mg/L). Incorporating the Ki values obtained with the binary chemical mixtures, the PBPK model for the ternary mixture simulated adequately the time course of the venous blood concentrations of TOL, XYL, and EBZ in rats exposed to a mixture containing 100 ppm each of these solvents. Following the validation of the ternary mixture model in the rat, it was scaled to predict the kinetics of TOL, XYL, and EBZ in blood and alveolar air of human volunteers exposed for 7 hr to a combination of 17, 33, and 33 ppm, respectively, of these solvents. Model simulations and experimental data obtained in humans indicated that exposure to atmospheric concentrations of TOL, XYL, and EBZ that remain within the permissible concentrations for a mixture would not result in biologically significant modifications of their pharmacokinetics. Overall, this study demonstrates the utility of PBPK models in the prediction of the kinetics of components of chemical mixtures, by accounting for mechanisms of binary chemical interactions.

Integrated approaches for the analysis of toxicologic interactions of chemical mixtures

Although an overwhelmingly large portion of the resources in toxicologic research is devoted to single chemical studies, the toxicology of chemical mixtures, not single chemicals, is the real issue regarding health effects of environmental and/or occupational exposure to chemicals. The relative lack of activities in the area of toxicology of chemical mixtures does not suggest ignorance of the importance of the issue by the toxicology community. Instead, it is a reflection of the difficulty, complexity, and controversy surrounding this area of research. Until recently, much of the literature on the toxicology of chemical mixtures has been either very focused on certain specific interaction studies or slanted toward broad-based, relatively vague theoretical deliberation. The typical interaction study involved binary mixtures at relatively high dose levels with acute toxicities as endpoints. Although the theoretical papers have been valuable contributions, little is available on actual, practical experimental approaches toward a systematic solution of this immensely complex area of research. We present here a broad discussion on the important issues of the toxicology of chemical mixtures. First, we provide some background information with respect to the problem and significance of toxicology of chemical mixtures in relation to some of the real life issues. Second, we review and compare the existing experimental approaches relevant to toxicologic interactions of chemical mixtures. Third, we propose three integrated approaches that involve the combination of physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) modeling with: (1) Monte Carlo simulation, (2) median effect principle (MEP), and (3) response surface methodology (RSM). These modeling approaches, coupled with very focused mechanistically based toxicology studies, could be the basis for solving the problems of toxicology and risk assessment of chemical mixtures.

Application of physiologically based pharmacokinetic modelling to combination toxicology

Non-additive toxicity has been demonstrated in laboratory animals for a large number of temporally separated or concurrent multiple chemical exposures. These exposures are typically at concentrations higher than those found in the environment, leading to the question of the applicability of the results to the human situation. Physiologically based pharmacokinetic (PBPK) modelling has been applied successfully to single chemicals; its utility for extrapolation across species and dose has been demonstrated. Use of PBPK modelling in the study of chemical mixtures is increasing although still limited. The use of PBPK modelling by various investigators in the field of combination toxicology is reviewed. PBPK modelling has been used to examine: the role of increased metabolism in non-additive toxicity resulting from temporally separated exposures; the influence of the time interval separating two chemical exposures; and the role of inhibition of metabolism in concurrent exposure to two chemicals. In summary, development of a PBPK or PBPK/pharmacodynamic model for a combined exposure provides a basis for extrapolation across species, route and dose, and a useful tool for risk assessment.