Partition coefficients between human blood or adipose tissue and air for aromatic solvents

Prediction of the Partition Coefficient between Adipose Tissue and Blood for Environmental Chemicals: From Single QSAR Models to an Integrated Approach

The adipose tissue:blood partition coefficient is a key-endpoint to predict the pharmacokinetics of chemicals in humans and animals, since other organ:blood affinities can be estimated as a function of this parameter. We performed a search in the literature to select all the available rat in vivo data. This approach resulted into two improvements to existing models: a homogeneous definition of the endpoint and an expanded data collection. The resulting dataset was used to develop QSAR models as a function of linear and non-linear algorithms. Several applicability domain definitions were assessed and the definition corresponding to a good balance between performance and coverage was retained. We assessed the pertinence of combining single models into integrated approaches to increase the accuracy in predictions. The best integrated model outperformed the single models and it was characterized by an external mean absolute error (MAE) equal to 0.26, while preserving an adequate coverage (84 %). This performance is comparable to experimental variability and it highlights the pertinence of the integrated model.

Blood/air distribution of volatile organic compounds (VOCs) in a nationally representative sample

Volatile organic compounds (VOCs) in human blood are an effective biomarker of environmental exposure and are closely linked to health outcomes. Unlike VOC concentrations in air, which are routinely collected, blood VOC data are not as readily available. This study aims to develop the quantitative relationship between air and blood VOCs by deriving population-based blood/air distribution coefficients (popKs) of ten common VOCs in the general U.S. population. Air and human blood samples were collected from 364 adults aged 20-59 years in 1999-2000 National Health and Nutrition Examination Survey (NHANES). Determinants of popKs were identified using weighted multivariate regression models. In the non-smoking population, median popKs ranged from 3.1 to 77.3, comparable to values obtained in the laboratory. PopKs decreased with increasing airborne VOC concentrations. Smoking elevated popKs by 1.5-3.5 times for aromatic compounds, but did not affect the popKs for methyl tert-butyl ether (MTBE) or chlorinated compounds. Drinking water concentration was a modifier of MTBE's popK. Age, gender, body composition, nor ethnicity affected popKs. PopKs were predictable using linear models with air concentration as the independent variable for both adults and children. This is the first study to estimate blood/air distribution coefficients using simultaneous environmental and biological monitoring on a national population sample. This study was also the first to determine the blood/air distribution coefficient of p-dichlorobenzene, a compound frequently found in indoor environments. These results have applications in exposure assessment, pharmacokinetic analysis, physiologically-based pharmacokinetic (PBPK) modeling, and uncertainty analysis.

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.

Determination of exposure to benzene, toluene and xylenes in Turkish primary school children by analysis of breath and by environmental passive sampling

Benzene, toluene, m/p-xylene and o-xylene (BTX) are toxic volatile organic compounds and ubiquitous air pollutants. Smoking and consumer products are indoor sources of BTX, whereas traffic and industrial activities are primary sources contributing to outdoor levels of BTX. The aim of this study was to characterize exposure of children to BTX by personal air sampling using diffusive samplers and by analysis of end-exhaled air. For this study, 101 children of 10-11 years of age were recruited from four primary schools in Southern Turkey during the warm season (May 2008). Two schools were situated in a residential area near primary and secondary iron and steel works (Payas) and two schools were located in a non-industrialized city (Iskenderun). The children and their parents were visited at home for an interview and to identify possible sources of BTX in the residence. Median concentrations of benzene determined by diffusive samplers were higher in Payas (4.1 microg/m(3)) than in Iskenderun (2.7 microg/m(3), p<0.001). For toluene, no differences were observed, whereas for xylene isomers air concentrations tended to be lower for children living in Payas. The median end-exhaled air concentrations were 8.2, 29, 3.8, and 5.7 pmol/L for benzene, toluene, m/p-xylene and o-xylene, respectively (Payas), and 6.9, 25, 4.9, and 6.0 pmol/L, respectively (Iskenderun). Concentrations of toluene in end-exhaled air were 50% higher in children living with household members who smoked indoors (p<0.05) and benzene in end-exhaled air was more than 3-fold higher for those children who were exposed to tobacco smoke inside a vehicle (p<0.001). End-exhaled concentrations of benzene were also higher in children living in a residence with an attached garage (p<0.05). These exposure modifying factors were not identified when using the results obtained with diffusive samplers.

Endogenous volatile organic compounds in breath and blood of healthy volunteers: examining breath analysis as a surrogate for blood measurements

To investigate the premise that levels of endogenous volatile organic compounds (VOC) in breath reflect those in blood, the concentration of acetone and isoprene were measured in radial arterial blood, peripheral venous blood and breath samples from ten healthy volunteers. Coefficients of repeatability as a percentage of mean are less than 30% in breath but greater than 70% in blood. The volunteer-mean ratios of arterial to venous blood concentration are 1.4 (0.9-2.1) for acetone and 0.55 (0.3-1.0) for isoprene. Concentration in breath showed a significant inter-subject correlation with concentration in arterial blood (CAB) for acetone but not for isoprene. Arterial blood/breath ratios are 580 (280-1060) for acetone and 0.47 (0.22-0.77) for isoprene. The sample-mean blood/breath ratio was used to calculate an estimate of CAB and the standard deviation of this estimate was lower than that of arterial blood measured directly. For most subjects, estimated CAB was within uncertainty limits of the actual CAB. Owing to the poor repeatability of VOC concentrations from consecutive blood samples, and the capacitive effects of the lung, this study suggests that breath VOC measurements may provide a more consistent measure than blood measurements for investigating underlying physiological function or pathology within individuals.

Analysis of algorithms predicting blood:air and tissue:blood partition coefficients from solvent partition coefficients for prevalent components of JP-8 jet fuel

Algorithms predicting tissue and blood partition coefficients (PCs) from solvent properties were compared to assess their usefulness in a petroleum mixture physiologically based pharmacokinetic/pharmacodynamic model. Measured blood:air and tissue:blood PCs for rat and human tissues were sought from literature resources for 14 prevalent jet fuel (JP-8) components. Average experimental PCs were compared with predicted PCs calculated using algorithms from 9 published sources. Algorithms chosen used solvent PCs (octanol:water, saline or water:air, oil:air coefficients) due to the relative accessibility of these parameters. Tissue:blood PCs were calculated from ratios of predicted tissue:air and experimental blood:air values (PCEB). Of the 231 calculated values, 27% performed within +/- 20% of the experimental PC values. Physiologically based equations (based on water and lipid components of a tissue type) did not perform as well as empirical equations (derived from linear regression of experimental PC data) and hybrid equations (physiological parameters and empirical factors combined) for the jet fuel components. The major limitation encountered in this analysis was the lack of experimental data for the selected JP-8 constituents. PCEB values were compared with tissue:blood PCs calculated from ratios of predicted tissue:air and predicted blood:air values (PCPB). Overall, 68% of PCEB values had smaller absolute % errors than PCPB values. If calculated PC values must be used in models, a comparison of experimental and predicted PCs for chemically similar compounds would estimate the expected error level in calculated values.

Modeling interchild differences in pharmacokinetics on the basis of subject-specific data on physiology and hepatic CYP2E1 levels: A case study with toluene

The objective of the present study was to evaluate the magnitude of interindividual variability in the internal dose of toluene in children of various age groups, on the basis of subject-specific hepatic CYP2E1 content and physiology. The methodology involved the use of a previously validated physiologically based pharmacokinetic (PBPK) model, in which the intrinsic clearance for hepatic metabolism (CL(int)) was expressed in terms of the CYP2E1 content. The adult toluene PBPK model, with enzyme content-normalized CL(int), facilitated the calculation of child-specific CL(int) based on knowledge of hepatic CYP2E1 protein levels. The child-specific physiological parameters, except liver volume, were computed with knowledge of age and body weight, whereas physicochemical parameters for toluene were kept age-invariant based on available data. The actual individual-specific liver volume (autopsy data) was also included in the model. The resulting model was used to simulate the blood concentration profiles in children exposed by inhalation, to 1 ppm toluene for 24 h. For this exposure scenario, the area under the venous blood concentration vs. time curve (AUC) ranged from 0.30 to 1.01 microg/ml x h in neonates with low CYP2E1 concentration (<3.69 pmol/mg protein). The simulations indicated that neonates with higher levels of CYP2E1 (4.33 to 55.93 pmol/mg protein) as well as older children would have lower AUC (0.16 to 0.43 microg/ml x h). The latter values were closer to those simulated for adults. Similar results were also obtained for 7 h exposure to 17 ppm toluene, a scenario previously evaluated in human volunteers. The interindividual variability factor for each subgroup of children and adults, calculated as the ratio of the 95th and 50th percentile values of AUC, was within a factor of 2. The 95th percentile value of the low metabolizing neonate group, however, was greater than the mean adult AUC by a factor of 3.9. This study demonstrates the feasibility of incorporating subject-specific data on hepatic CYP2E1 content and physiology within PBPK models for evaluating the age, interchild and population variability of internal dose for use in risk assessment of inhaled volatile organics.

Ethanol-Induced Increase in the Metabolic Clearance of 1,1,1-Trichloroethane in Human Volunteers

This study evaluated the effect of moderate doses of ethanol over a short period of time on the toxicokinetics of an organic solvent, 1,1,1-trichloroethane. A group of 10 moderate drinkers were recruited and exposed via inhalation for 2 h to a low concentration of 1,1,1-trichloroethane (175 ppm) on two separate occasions. Subjects were administered ethanol (0.35 g/kg body weight) on each of the 7 days preceding one of the exposures. Blood and urine samples were collected during and following each exposure, with blood analyzed for 1,1,1-trichloroethane and urine analyzed for the metabolites of 1,1,1-trichloroethane: trichloroethanol and trichloroacetic acid. Prior ethanol consumption resulted in a significant increase in apparent metabolic clearance of 1,1,1-trichloroethane (mean increase = 25.4%). The results of this study demonstrate that ethanol consumption over time can affect the rate at which an organic solvent is cleared through metabolism in humans. For chemicals with toxic metabolic products, this inductive effect of ethanol consumption on the rate of biotransformation could be potentially harmful to exposed individuals. Metabolic clearance of compounds with high hepatic extraction may not be affected by enzyme induction as it is likely that these compounds are essentially completely metabolized while passing through the liver.

A physiologically based toxicokinetic model of inhalation exposure to xylenes in Caucasian men

Widespread exposure to the volatile aromatic hydrocarbons, ortho-, meta-, and para-xylene occurs in many industries including the manufacture of plastics, pharmaceuticals, and synthetic fibers. This paper describes the development of a physiologically based toxicokinetic model using biomonitoring data to quantify the kinetics of ortho-, meta-, and para-xylenes. Serial blood concentrations of deuterium-labeled xylene isomers were obtained over 4 days after 37 controlled, 2h inhalation exposures to different concentrations of the isomers. Peak toxicant concentrations in blood occurred in all subjects at the termination of exposure. Systemic clearance averaged 116 L/h+/-34 L/h, 117 L/h+/-23 L/h, and 129 L/h+/-33 L/h for ortho-, para-, and meta-xylene, respectively. The half-life of each toxicant in the terminal phase (>90 h post-exposure) was fit by the model, yielding values of 30.3+/-10.2 h for para-xylene, 33.0+/-11.7 h for meta-xylene and 38.5+/-18.2 h for ortho-xylene. Significant isomeric differences were found (p<0.05) for toxicant half-life, clearance and extrahepatic metabolism. Inter-individual variability seen in this study suggests that airborne concentration guidelines may not protect all workers. A Biological Exposure Index is preferred for this purpose since it is integrative and reflective of inter-individual kinetic variability.

The use of a physiologically based pharmacokinetic model to evaluate deconvolution measurements of systemic absorption

BACKGROUND

An unknown input function can be determined by deconvolution using the systemic bolus input function (r) determined using an experimental input of duration ranging from a few seconds to many minutes. The quantitative relation between the duration of the input and the accuracy of r is unknown. Although a large number of deconvolution procedures have been described, these routines are not available in a convenient software package.

METHODS

Four deconvolution methods are implemented in a new, user-friendly software program (PKQuest, http://www.pkquest.com). Three of these methods are characterized by input parameters that are adjusted by the user to provide the "best" fit. A new approach is used to determine these parameters, based on the assumption that the input can be approximated by a gamma distribution. Deconvolution methodologies are evaluated using data generated from a physiologically based pharmacokinetic model (PBPK).

RESULTS AND CONCLUSIONS

The 11-compartment PBPK model is accurately described by either a 2 or 3-exponential function, depending on whether or not there is significant tissue binding. For an accurate estimate of r the first venous sample should be at or before the end of the constant infusion and a long (10 minute) constant infusion is preferable to a bolus injection. For noisy data, a gamma distribution deconvolution provides the best result if the input has the form of a gamma distribution. For other input functions, good results are obtained using deconvolution methods based on modeling the input with either a B-spline or uniform dense set of time points.

PKQuest: volatile solutes - application to enflurane, nitrous oxide, halothane, methoxyflurane and toluene pharmacokinetics

BACKGROUND: The application of physiologically based pharmacokinetic models (PBPK) to human studies has been limited by the lack of the detailed organ information that is required for this analysis. PKQuest is a new generic PBPK that is designed to avoid this problem by using a set of "standard human" default parameters that are applicable to most solutes. RESULTS: PKQuest is used to model the human pharmacokinetics of the volatile solutes. A "standard human" value for the lipid content of the blood and each organ (klip) was chosen. This set of klip and the oil/water partition coefficient then specifies the organ/blood partition for each organ. Using this approach, the pharmacokinetics of inert volatile solute is completely specified by just 2 parameters: the water/air and oil/water partition coefficients. The model predictions of PKQuest were in good agreement with the experimental data for the inert solutes enflurane and nitrous oxide and the metabolized solutes halothane and toluene. METHODS: The experimental data that was modeled was taken from previous publications. CONCLUSIONS: This approach greatly increases the predictive power of the PBPK. For inert volatile solutes the pharmacokinetics are determined just from the water/air and oil/water partition coefficient. Methoxyflurane cannot be modeled by this PBPK because the arterial and end tidal partial pressures are not equal (as assumed in the PBPK). This inequality results from the "washin-washout" artifact in the large airways that is established for solutes with large water/air partition coefficients.PKQuest and the worked examples are available on the web www.pkquest.com.

Study of brain uptake and biodistribution of [11C]toluene in non-human primates and mice

Inhalant abuse is a rapidly growing health problem particularly among adolescents. Yet we know little about the neural mechanisms underlying the abuse liability of inhalants, particularly when compared to other addictive drugs. Specifically, our understanding of the relationship between the regional brain phamacokinetics and features classically associated with drug reinforcement is lacking. Under the hypothesis that the abuse liability of toluene can be related to its pharmacokinetic properties and the pattern of regional brain uptake, we developed the methodology for radiolabeling and purifying [11C]toluene for use in PET studies. Here we report the regional brain distribution and kinetics of the widely abused solvent toluene in non-human primates and the whole body biodistribution in mice. To our knowledge, this is the first reported study of the in vivo brain pharmacokinetics of labeled toluene in non-human primates. Rapid uptake of radioactivity into striatal and frontal regions was followed by rapid clearance from the brain. Concurrent findings in rodents indicate similar radio-tracer kinetics, with excretion through kidneys and liver. Taken together, our data provides insight into pharmacokinetic features possibly associated with the abuse liability of toluene.

Bayesian estimation of variability in adipose tissue blood flow in man by physiologically based pharmacokinetic modeling of inhalation exposure to toluene

Due to the lipophilicity of many xenobiotics, the perfusion of fat tissue is of special interest in physiologically based pharmacokinetic (PBPK) modeling. In order to estimate inter- and intra-individual variability in fat tissue blood flow with exercise, a population PBPK model for toluene was fitted to experimental data from subjects exposed to toluene vapors (Carlsson, A., 1982. Exposure to toluene: uptake, distribution and elimination in man. Scand. J. Work Environ. Health 8, 43-55). Six male volunteers were exposed to 80 ppm toluene for two hours during rest and moderate to heavy exercise (50-150 W). Extensive data collection was made, including sampling of arterial blood, exhaled breath and subcutaneous fat tissue. The model was simultaneously fitted to the time courses of toluene in arterial blood, exhaled breath, and subcutaneous fat in the six individuals by Markov chain Monte Carlo (MCMC) simulation. In order to describe the experimental observations in subcutaneous fat accurately, the fat compartment was split in two. According to the analysis, the increased perfusion of perirenal fat associated with physical workload was best described if it was set to the same, elevated, level during all exercise levels, rather than scaled directly to the increase in oxygen uptake. No increase in subcutaneous fat perfusion could be detected at these exposure conditions.

Empirical relations predicting human and rat tissue:air partition coefficients of volatile organic compounds

Based on the hypothesis that tissue partitioning of volatile organic compounds (VOCs) is due to lipophilic and hydrophilic interactions with tissue components, empirical relations are established between olive oil (P(oil:air)), saline (P(saline:air)), and tissue partition coefficients (P(tissue:air)) for human and rat tissues. Reported values of partition coefficients of a wide range of VOCs with distinct chemical structures (n = 137) have been compiled from the literature. Bilinear regression analysis shows that partition coefficients of VOCs in human blood, brain, fat, liver, kidney, and muscle tissues are well described by a linear combination of P(oil:air) and P(saline:air) with tissue-specific regression coefficients. The regression coefficient associated with the hydrophilic component of VOC partitioning in rat tissues is systematically higher than that of human tissues. For the human model, tissue concentrations calculated from predicted partition coefficients are generally within a factor 4 of tissue concentrations calculated from experimentally observed partition coefficients. These results demonstrate that, without prior knowledge of tissue composition, it is possible to obtain estimates of human tissue partition coefficients of VOCs with an accuracy that is in the same range as that commonly used in risk assessment.

Modeling the acute neurotoxicity of styrene

Styrene is a widely used industrial solvent associated with acute neurotoxicity. To investigate the relationships between exposure, blood concentrations, and the appearance of neurotoxic effects, four healthy males were exposed to styrene concentrations of 5-200 ppm in four different exposure-time profiles. A digit recognition test and P300 event-related evoked potential were used to measure neurologic function. A physiologically based kinetic (PBK) model generated close predictions of measured styrene blood concentrations, in the range of 0.01-12 mg/L, from this and 21 previous studies. Simulated peak brain concentration, durationXaverage exposure, and peak exposure level were predictive of toxicity. Central nervous system effects were expected at a blood concentration near 2.4 mg/L. A standard of 20 ppm was expected to protect styrene-exposed workers from acute central nervous system toxicity under light work conditions.