Human physiologic factors in respiratory uptake of 1,3-butadiene

Exposure to 1,3-Butadiene in the U.S. Population: National Health and Nutrition Examination Survey 2011-2016

1,3-Butadiene is a volatile organic compound with a gasoline-like odour that is primarily used as a monomer in the production of synthetic rubber. The International Agency for Research on Cancer has classified 1,3-butadiene as a human carcinogen. We assessed 1,3-butadiene exposure in the U.S. population by measuring its urinary metabolites N-acetyl-S-(3,4-dihydroxybutyl)-L-cysteine (34HBMA), N-acetyl-S-(1-hydroxymethyl-2-propenyl)-L-cysteine (1HMPeMA), N-acetyl-S-(2-hydroxy-3-butenyl)-L-cysteine (2HBeMA), and N-acetyl-S-(4-hydroxy-2-buten-1-yl)-L-cysteine (4HBeMA). Urine samples from the 2011 to 2016 National Health and Nutrition Examination Survey were analysed for 1,3-butadiene metabolites using ultrahigh-performance liquid chromatography/tandem mass spectrometry. 34HBMA and 4HBeMA were detected in >96% of the samples; 1HMPeMA and 2HBeMA were detected in 0.66% and 9.84% of the samples, respectively. We used sample-weighted linear regression models to examine the influence of smoking status (using a combination of self-reporting and serum-cotinine data), demographic variables, and diet on biomarker levels. The median 4HBeMA among exclusive smokers (31.5 µg/g creatinine) was higher than in non-users (4.11 µg/g creatinine). Similarly, the median 34HBMA among exclusive smokers (391 µg/g creatinine) was higher than in non-users (296 µg/g creatinine). Furthermore, smoking 1-10, 11-20, and >20 cigarettes per day (CPD) was associated with 475%, 849%, and 1143% higher 4HBeMA (p < 0.0001), respectively. Additionally, smoking 1-10, 11-20, and >20 CPD was associated with 33%, 44%, and 102% higher 34HBMA (p < 0.0001). These results provide significant baseline data for 1,3-butadiene exposure in the U.S. population, and demonstrate that tobacco smoke is a major exposure source.

Environmental exposure to dioxin-like compounds and the mortality risk in the U.S. population

BACKGROUND

Little is known about the mortality risk associated with chronic dioxin exposure in the general U.S. populations.

OBJECTIVE

To explore the association between dioxin-like chemicals and mortality risk in a large population-based cohort study.

METHODS

The analysis included 2361 subjects aged 40 years or older from the 1999-2004 National Health and Nutrition Examination Survey (NHANES). Exposure to a mixture of dioxin-like chemicals, including dibenzo-p-dioxins, dibenzofurans, and polychlorinated biphenyls was estimated using toxic equivalency values (TEQs) calculated with 2005 World Health Organization toxic equivalency factors. All-cause and cause-specific mortalities were obtained from the NHANES-linked follow-up data through December 31, 2006. Cox proportional-hazards models were applied to assess the associations of interest.

RESULTS

A total of 242 deaths occurred during the follow-up period, including 75 from cardiovascular disease and 72 from cancer. There was an increased mortality risk associated with logarithmically expressed dioxin TEQs for all-cause deaths (hazard ratio=1.19, 95% confidence interval=1.02-1.39, p=0.02). Similar graded dose-response trends were found for cardiovascular and cancer mortality which did not reach statistical significance.

CONCLUSIONS

In general, higher dioxin exposure is associated with an increased mortality risk among subjects aged 40 and above. The cause-specific analyses and responsible mechanisms will require further investigation.

A stochastic whole-body physiologically based pharmacokinetic model to assess the impact of inter-individual variability on tissue dosimetry over the human lifespan

Physiologically based pharmacokinetic (PBPK) models have proven to be successful in integrating and evaluating the influence of age- or gender-dependent changes with respect to the pharmacokinetics of xenobiotics throughout entire lifetimes. Nevertheless, for an effective application of toxicokinetic modelling to chemical risk assessment, a PBPK model has to be detailed enough to include all the multiple tissues that could be targeted by the various xenobiotics present in the environment. For this reason, we developed a PBPK model based on a detailed compartmentalization of the human body and parameterized with new relationships describing the time evolution of physiological and anatomical parameters. To take into account the impact of human variability on the predicted toxicokinetics, we defined probability distributions for key parameters related to the xenobiotics absorption, distribution, metabolism and excretion. The model predictability was evaluated by a direct comparison between computational predictions and experimental data for the internal concentrations of two chemicals (1,3-butadiene and 2,3,7,8-tetrachlorodibenzo-p-dioxin). A good agreement between predictions and observed data was achieved for different scenarios of exposure (e.g., acute or chronic exposure and different populations). Our results support that the general stochastic PBPK model can be a valuable computational support in the area of chemical risk analysis.

Liquid chromatography electrospray ionization tandem mass spectrometry analysis method for simultaneous detection of trichloroacetic acid, dichloroacetic acid, S-(1,2-dichlorovinyl)glutathione and S-(1,2-dichlorovinyl)-L-cysteine

Trichloroethylene (TCE, CAS 79-01-6) is a widely used industrial chemical, and a common environmental pollutant. TCE is a well-known carcinogen in rodents and is classified as "probably carcinogenic to humans". Several analytical methods have been proposed for detection of TCE metabolites in biological media utilizing derivatization-free techniques; however, none of them is suitable for simultaneous detection of both oxidative metabolites and glutathione conjugates of TCE in small volume biological samples. Here, we report a new combination of methods for assessment of major TCE metabolites: dichloroacetic acid (DCA), trichloroacetic acid (TCA), S-(1,2-dichlorovinyl)-L-cysteine (DCVC), and S-(1,2-dichlorovinyl) glutathione (DCVG). First, DCA and TCA were extracted with ether. Second, the remaining aqueous fraction underwent solid phase extraction for DCVC and DCVG. Then, DCA and TCA were measured by hydrophilic interaction liquid chromatography ion exchange negative electrospray ionization tandem mass spectrometry, while DCVC and DCVG were measured by reverse phase positive electrospray ionization tandem mass spectrometry. This method was applied successfully to measure all 4 TCE metabolites in as little as 50 microl of serum from mice orally exposed to TCE (2100 mg/kg, 2h). Serum concentrations (mean+/-standard deviation) of the TCE metabolites obtained with this method are comparable or equivalent to those previously reported in the literature: DCA, 0.122+/-0.014 nmol/ml (limit of detection: 0.01 nmol/ml); TCA, 256+/-30 nmol/ml (0.4 nmol/ml); DCVG, 0.037+/-0.015 nmol/ml (0.001 nmol/ml); DCVC, 0.0024+/-0.0009 nmol/ml (0.001 nmol/ml). This method opens new opportunities to increase throughput and decrease number of animals required for mechanistic studies on TCE in rodents.

Quantifying heterogeneity in exposure-risk relationships using exhaled breath biomarkers for 1,3-butadiene exposures

The health effects of human exposure to 1,3-butadiene (BD) have been extensively studied using both epidemiological and animal toxicology approaches. However, various data and knowledge gaps remain, one of which is an understanding of the human heterogeneity in BD dosimetry. The objective of our study was to better understand the role of individual variability in delivered tissue dose. We designed a study of laboratory exposures of a relatively large group of healthy human subjects. Subjects were then exposed to 2.0 ppm BD through a face mask for 20 min, followed by 40 min of breathing clean air. Exhaled breath concentrations of BD were measured at ten time points during and after exposure, and a three-compartment physiologically based pharmacokinetic (PBPK) model was used to quantify the kinetic behavior of BD. We implemented a Markov chain Monte Carlo procedure to fit the model to the experimental data, and used global sensitivity analysis techniques to examine the sensitivity of exhaled breath concentrations to PBPK model parameters. Uptake during exposure was strongly influenced by rebreathing of exhaled BD during exposure; inclusion of rebreathing in the model simulations resulted in a 21% increase in the amount of BD retained in the body. We found that uptake ranged from 38% to 77% across individuals. We measured considerable intra-individual variability from 11 subjects who underwent the testing twice. Most of this variation stemmed from phase I metabolism of BD, which varied by as much as a factor of 2.6 within individuals. Overall, we have sought to quantify the sources of inter- and intra-individual variabilities in the pharmacokinetic behavior of BD. The results of our research may impact the current framework for biomarker and pharmacokinetic studies by improving our understanding of the sources of heterogeneity in response to chemical exposures.

Relationships between levels of volatile organic compounds in air and blood from the general population

The relationships between levels of volatile organic compounds (VOCs) in blood and air have not been well characterized in the general population where exposure concentrations are generally at parts per billion levels. This study investigates relationships between the levels of nine VOCs, namely, benzene, chloroform, 1,4-dichlorobenzene, ethylbenzene, methyl tert-butyl ether (MTBE), tetrachloroethene, toluene, and m-/p- and o-xylene, in blood and air from a stratified random sample of the general US population. We used data collected from 354 participants, including 89 smokers and 265 nonsmokers, aged 20-59 years, who provided samples of blood and air in the National Health and Nutrition Examination Survey (NHANES) 1999-2000. Demographic and physiological characteristics were obtained from self-reported information; smoking status was determined from levels of serum cotinine. Multiple linear regression models were used to investigate the relationships between VOC levels in air and blood, while adjusting for effects of smoking and demographic factors. Although levels of VOCs in blood were positively correlated with the corresponding air levels, the strength of association (R(2)) varied from 0.02 (ethylbenzene) to 0.68 (1,4-DCB). Also the blood-air relationships of benzene, toluene, ethylbenzene, and the xylenes (BTEX) were influenced by smoking, exposure-smoking interactions, and by gender, age, and BMI, whereas those of the other VOCs were not. Interestingly, the particular exposure-smoking interaction for benzene was different from those for toluene, ethylbenzene, and the xylenes. Whereas smokers retained more benzene in their blood at increasing exposure levels, they retained less toluene, ethylbenzene, and xylenes at increasing exposure levels. Investigators should consider interaction effects of exposure levels and smoking when exploring the blood-air relationships of the BTEX compounds in the general population.

QSAR modeling of blood:air and tissue:air partition coefficients using theoretical descriptors

Human blood:air, human and rat tissue (fat, brain, liver, muscle, and kidney):air partition coefficients of a diverse set of organic compounds were correlated and predicted using structural descriptors by employing CODESSA-PRO and ISIDA programs. Four and five descriptor regression models developed using CODESSA-PRO were validated on three different test sets. Overall, these models have reasonable values of correlation coefficients (R(2)) and leave-one-out correlation coefficients (R(cv)(2)): R(2) = 0.881-0.983; R(cv)(2) = 0.826-0.962. Calculations with ISIDA resulted in models based on atom/bond sequences involving two to three atoms with statistical parameters that were similar to those of models obtained with CODESSA-PRO (R(2) = 0.911-0.974; R(cv)(2) = 0.831-0.936). A mixed pool of molecular and fragment descriptors did not lead to significant improvement of the models.

Incorporating pharmacokinetic differences between children and adults in assessing children's risks to environmental toxicants

Children's risks from environmental toxicant exposure can be affected by pharmacokinetic factors that affect the internal dose of parent chemical or active metabolite. There are numerous physiologic differences between neonates and adults that affect pharmacokinetics including size of lipid, and tissue compartments, organ blood flows, protein binding capacity, and immature function of renal and hepatic systems. These factors combine to decrease the clearance of many therapeutic drugs, which can also be expected to occur with environmental toxicants in neonates. The net effect may be greater or lesser internal dose of active toxicant depending upon how the agent is distributed, metabolized, and eliminated. Child/adult pharmacokinetic differences decrease with increasing postnatal age, but these factors should still be considered in any children's age group, birth through adolescence, for which there is toxicant exposure. Physiologically based pharmacokinetic (PBPK) models can simulate the absorption, distribution, metabolism, and excretion of xenobiotics in both children and adults, allowing for a direct comparison of internal dose and risk across age groups. This review provides special focus on the development of hepatic cytochrome P-450 enzymes (CYPs) in early life and how this information, along with many factors unique to children, can be applied to PBPK models for this receptor population. This review describes a case study involving the development of neonatal PBPK models for the CYP1A2 substrates caffeine and theophylline. These models were calibrated with pharmacokinetic data in neonates and used to help understand key metabolic differences between neonates and adults across these two drugs.

Diepoxybutane induces caspase and p53-mediated apoptosis in human lymphoblasts

Diepoxybutane (DEB) is the most potent metabolite of the environmental chemical 1,3-butadiene (BD), which is prevalent in petrochemical industrial areas. BD is a known mutagen and human carcinogen, and possesses multiorgan systems toxicity that includes bone marrow depletion, spleen, and thymus atrophy. Toxic effects of BD are mediated through its epoxy metabolites. In working towards elucidating the cellular and molecular mechanisms of BD toxicity, we investigated the ability of DEB to induce apoptosis in human lymphoblasts. DEB induced a concentration and exposure time-dependent apoptosis, which accounted for the DEB-induced loss of cell viability observed in TK6 lymphoblasts. The DEB-induced apoptosis was inhibited by inhibitors of caspases 3 and 9. The role of p53 in mediating the DEB-induced apoptosis was also investigated. DEB induced elevated p53 levels in direct correlation to the extent of DEB-induced apoptosis, as the concentration of DEB increased up to 5 microM. The extent of DEB-induced apoptosis was dramatically higher in TK6 lymphoblasts as compared to the genetically paired p53-deficient NH32 lymphoblasts under the same experimental conditions. Our results confirm and extend observations on the occurrence of apoptosis in DEB exposed cells, and demonstrate for the first time the elevation of p53 levels in human lymphoblasts in response to DEB exposure. In addition, our results demonstrate for the first time that DEB-induced apoptosis is mediated by caspases 3 and 9, as well as the p53 protein. It is possible that DEB-induced apoptosis may explain BD-induced bone marrow depletion, spleen and thymus atrophy in BD-exposed animals.

Association of the blood/air partition coefficient of 1,3-butadiene with blood lipids and albumin

Pulmonary gas uptake is a function of the blood solubility of a vapor, indicated by the blood/air partition coefficient. We hypothesized that blood lipid compositions are associated with the blood/air partition coefficients of lipophilic toxic vapors such as 1,3-butadiene. Our goal was to investigate cross-sectional and longitudinal relationships of blood triglycerides, total cholesterol, and albumin to the blood/air partition coefficient of butadiene. We collected blood samples from 24 subjects at three time points: a fasting baseline and 2 and 4 hr after drinking a standardized high-fat milk shake (107 g fat, 80 g sugar, and 27 g protein). The blood/air partition coefficient was determined using the closed vial-equilibrium technique. Triglycerides and total cholesterol were analyzed by an enzymatic method, and albumin was analyzed with an immunoassay technique. We used multiple linear regression and general linear models to examine the cross-sectional and longitudinal relationship, respectively. The results showed that the blood/air partition coefficient of butadiene was cross-sectionally associated only with triglycerides at baseline, and longitudinally related to baseline triglycerides, total cholesterol, and the change in triglycerides over time. The blood/air partition coefficient of butadiene increased, on average, by approximately 20% and up to 40% for subjects with borderline higher triglyceride levels after ingestion of a standardized milk shake. In addition, a time factor beyond lipids was also significant in predicting the blood/air partition coefficient of butadiene. This may represent the effects of other unmeasured parameters related to time or time of day on the blood/air partition coefficient of butadiene. Because the blood/air partition coefficient is a major determinant of gas uptake, ingestion of a high fat meal before this type of exposure may significantly increase an individual's absorbed dose, possibly increasing the risk of adverse effects.