Styrene-7,8-oxide in blood of workers exposed to styrene

An integrated approach to biomonitoring exposure to styrene and styrene-(7,8)-oxide using a repeated measurements sampling design

The aim of this work was to investigate urinary analytes and haemoglobin and albumin adducts as biomarkers of exposure to airborne styrene (Sty) and styrene-(7,8)-oxide (StyOX) and to evaluate the influence of smoking habit and genetic polymorphism of metabolic enzymes GSTM1 and GSTT1 on these biomarkers. We obtained three or four air and urine samples from each exposed worker (eight reinforced plastics workers and 13 varnish workers), one air and urine samples from 22 control workers (automobile mechanics) and one blood sample from all subjects. Median levels of exposure to Sty and StyOX, respectively, were 18.2 mg m(-3) and 133 microg m(-3) for reinforced plastics workers, 3.4 mg m(-3) and 12 microg m(-3) for varnish workers, and <0.3 mg m(-3) and <5 microg m(-3) for controls. Urinary levels of styrene, mandelic acid, phenylglyoxylic acid, phenylglycine (PHG), 4-vinylphenol (VP) and mercapturic acids (M1+M2), as well as cysteinyl adducts of serum albumin (but not those of haemoglobin) were significantly associated with exposure status (controls<exposed workers). Also, levels of VP and M1+M2 were significantly affected by smoking, and levels of M1+M2 were significantly affected by GSTM1 polymorphisms. Multiple linear regression analyses of the subject-specific (logged) metabolite levels across exposed workers showed that Sty was a significant predictor for all urinary analytes while StyOX was a significant predictor of PHG only. Interestingly, the log scale regression coefficients for Sty in these models were significantly less than one for all metabolites except M1+M2. This suggests that the natural scale relationships between levels of all Sty metabolites, except M1+M2, displayed downward concavity with increasing Sty exposure, suggestive of saturable metabolism. Levels of the protein adducts were not associated with exposure to either Sty or StyOX among exposed subjects.

Styrene Metabolism, Genotoxicity, and Potential Carcinogenicity

This report reviews styrene biotransformation, including minor metabolic routes, and relates metabolism to the genotoxic effects and possible styrene-related carcinogenicity. Styrene is shown to require metabolic activation in order to become notably genotoxic and styrene 7,8-oxide is shown to contribute quantitatively by far the most (in humans more than 95%) to the genotoxicity of styrene, while minor ring oxidation products are also shown to contribute to local toxicities, especially in the respiratory system. Individual susceptibility depending on metabolism polymorphisms and individual DNA repair capacity as well as the dependence of the nonlinearity of the dose-response relationships in the species in question and the consequences for risk evaluation are analyzd.

Critical appraisal of the expression of cytochrome P450 enzymes in human lung and evaluation of the possibility that such expression provides evidence of potential styrene tumorigenicity in humans

Styrene is widely used with significant human exposure, particularly in the reinforced plastics industry. In mice it is both hepatotoxic and pneumotoxic, and this toxicity is generally thought to be associated with its metabolism to styrene oxide. Styrene causes lung tumors in mice but not in rats. The question is how the tumorigenic effect in mouse lung may relate to the human. This review examines the comparison of the metabolic activation rates (1) between the liver and lung and (2) for the lung, between the rodent and human. Emphasis is placed on the specific cytochromes P450 present in the lungs of humans and what role they might play in the bioactivation of styrene and other compounds. In general, pulmonary metabolism is very slow compared to hepatic metabolism. Furthermore, metabolic rates in humans are slow compared to those in rats and mice. There is a wide difference in what specific cytochromes P450 investigators have reported as being present in human lung which makes comparisons, both inter-species and inter-organ, difficult. The general low activity for cytochrome P450 activity in the lung, especially for CYP2F1, the human homolog for CYP2F2 which has been identified in mice as being primarily responsible for styrene metabolism, argues against the hypothesis that human lung would produce enough styrene oxide to damage pulmonary epithelial cells leading to cell death, increased cell replication and ultimately tumorigenicity, the presumed mode of action for styrene in the production of the mouse lung tumors.

Comparison of genotoxic potency of styrene 7,8-oxide with gamma radiation and human cancer risk estimation of styrene using the rad-equivalence approach

Styrene is suspected to cause lympho-hematopoietic malignancies through the formation of styrene 7,8-oxide. However, we are still unable to calculate the cancer risk for workers exposed to styrene using epidemiological data. The aims of this study were to determine the blood dose after styrene exposure and to compare the genotoxic potency of styrene 7,8-oxide and gamma radiation in order to calculate the cancer risk by means of the rad-equivalence approach. Leucocytes of 20 individuals were exposed to 0, 0.1, 0.2 or 0.3 mM styrene 7,8-oxide (1 h) or 1, 2 or 3 gray (=100, 200, 300 rad) gamma radiation. Genotoxicity was evaluated with the cytokinesis-block micronucleus assay. Comparison of the two slopes of the regression lines between micronuclei and dose revealed a genotoxic potency for styrene 7,8-oxide of 37 rad/mMh, corresponding with a median value derived from mutagenicity studies (1, 37, 208 rad/mMh). At exposure levels of 1 ppm styrene, a blood styrene 7,8-oxide concentration between 0.03 x 10(-)(6) and 0.42 x 10(-)(6) mM is to be expected using data of toxicokinetic models and human exposure studies. With the cancer risk per unit dose of gamma radiation as benchmark, we calculated a lifetime risk of acquiring a fatal lympho-hematopoietic cancer of 0.17 in 10(3) workers (between 0.037 x 10(-)(3) and 5.0 x 10(-)(3)) exposed to 20 ppm styrene during 40 years.

Predictors of occupational exposure to styrene and styrene-7,8-oxide in the reinforced plastics industry

OBJECTIVE

To identify demographic and work related factors that predict blood levels of styrene and styrene-7,8-oxide (SO) in the fibreglass reinforced plastics (FRP) industry.

METHODS

Personal breathing-zone air samples and whole blood samples were collected repeatedly from 328 reinforced plastics workers in the Unuted States between 1996 and 1999. Styrene and its major metabolite SO were measured in these samples. Multivariable linear regression analyses were applied to the subject-specific levels to explain the variation in exposure and biomarker levels.

RESULTS

Exposure levels of styrene were approximately 500-fold higher than those of SO. Exposure levels of styrene and SO varied greatly among the types of products manufactured, with an 11-fold range of median air levels among categories for styrene and a 23-fold range for SO. Even after stratification by job title, median exposures of styrene and SO among laminators varied 14- and 31-fold across product categories. Furthermore, the relative proportions of exposures to styrene and SO varied among product categories. Multivariable regression analyses explained 70% and 63% of the variation in air levels of styrene and SO, respectively, and 72% and 34% of the variation in blood levels of styrene and SO, respectively. Overall, air levels of styrene and SO appear to have decreased substantially in this industry over the last 10-20 years in the US and were greatest among workers with the least seniority.

CONCLUSIONS

As levels of styrene and SO in air and blood varied among product categories in the FRP industry, use of job title as a surrogate for exposure can introduce unpredictable measurement errors and can confound the relation between exposure and health outcomes in epidemiology studies. Also, inverse relations between the intensity of exposure to styrene and SO and years on the job suggest that younger workers with little seniority are typically exposed to higher levels of styrene and SO than their coworkers.

Mortality patterns among workers exposed to styrene in the reinforced plastic boatbuilding industry: an update

BACKGROUND

Mortality was updated through 1998 for 5,204 workers exposed to styrene between 1959 and 1978 at two reinforced plastic boatbuilding plants. The a priori hypothesis: leukemia and lymphoma excesses would be found.

METHODS

Standardized mortality ratios (SMR) and 95% confidence intervals (CI) used Washington State and U.S. rates.

RESULTS

Overall, 860 deaths occurred (SMR 1.09, CI 1.02-1.17), with excess mortality for esophageal cancer (n = 12, SMR 2.30, CI 1.19-4.02), prostate cancer (n = 24, SMR 1.71, CI 1.09-2.54), and accidents (n = 99, SMR 1.26, CI 1.02-1.53). Among 2,062 highly exposed workers, urinary tract cancer (n = 6, SMR 3.44, CI 1.26-7.50) and respiratory disease (n = 12, SMR 2.54, CI 1.31-4.44) rates were elevated. Urinary tract cancer SMR increased with duration of employment.

CONCLUSIONS

We found no excess leukemia or lymphoma mortality. Unanticipated excess urinary tract cancer and respiratory disease mortality, possibly associated with styrene exposure, are difficult to interpret and could be chance findings.

Effect of epoxide hydrolase and glutathione S-tranferase genotypes on the induction of micronuclei and DNA damage by styrene-7,8-oxide in vitro

Styrene is one of the most important organic chemicals used worldwide. Its main metabolite, styrene-7,8-oxide (SO), is considered responsible for the genotoxic effects associated with exposure to styrene. SO is detoxified by hydrolysis catalyzed by epoxide hydrolase (EH), or, to a minor extent, by conjugation mediated by glutathione S-transferases (GSTs). The purpose of the present study was to investigate whether EH (exons 3 and 4), GSTP1 (exons 5 and 6), GSTM1 and GSTT1 polymorphisms have any influence on the genotoxicity of SO in human leukocytes. Peripheral leukocytes from 30 healthy donors were exposed to SO (50 and 200 micro M) and genotoxicity was evaluated by means of the micronucleus (MN) test and alkaline comet assay, using 1% DMSO as solvent control. When EH genotypes were classified in low, medium, and high with respect to the expected EH activity, an increase in induced comet tail length was observed with decreasing EH activity in SO-exposed cells. An increase was seen in induced MN frequency in EH low-activity donors. These findings are consistent with the detoxifying activity of this enzyme. In addition, increases in MN frequencies for GSTP1 *A/*B and *A/*C genotypes with regard to the wild-type homozygous *A/*A genotype were detected. This may be due to a low detoxifying activity as a consequence of altered SO affinity of the variant protein, but must be confirmed using homozygote variant individuals, not included in this study. No clear results were obtained for GSTM1 or GSTT1 genotypes, even when performing the analysis after grouping individuals with the same expected EH activity, probably due to the minor role that glutathione conjugation plays in styrene metabolism. The present in vitro findings using human leukocytes suggest that polymorphisms in EH, and, to a lesser extent, in GSTP1, may influence induction of cytogenetic and DNA damage by SO.

A toxicokinetic model for styrene and its metabolite styrene-7,8-oxide in mouse, rat and human with special emphasis on the lung

Styrene (ST) occurs ubiquitously in the environment and it is an important industrial chemical. After its uptake by the exposed mammalian organism, ST is oxidized to styrene-7,8-oxide (SO) by cytochrome P450 dependent monooxygenases. This reactive intermediate is further metabolized by epoxide hydrolase (EH) and glutathione S-transferase (GST). In long-term animal studies, ST induced lung tumors in mice but not in rats. Considering the lung to be the relevant target organ for ST induced carcinogenicity in mice, we extended a previously developed physiological toxicokinetic model in order to simulate the lung burden with ST and SO in the ST exposed mouse, rat and human. The new model describes oral and pulmonary uptake of ST, its distribution into various tissues, its exhalation and its metabolism to SO in lung and liver. It also simulates the distribution of the produced SO into the tissues and its EH and GST mediated metabolism in liver and in lung. In both organs the ST induced GSH consumption is described together with the formation of adducts to hemoglobin and to DNA of lymphocytes in ST exposed mice, rats and humans. The model includes compartments for arterial, venous and pulmonary blood, liver, muscle, fat, richly perfused tissues and lung. The latter organ is represented by two compartments, namely by the conducting and the alveolar zone. The physiological description of the pulmonary compartments relies on measured alveolar retentions, literature values of surface area of capillary endothelium, of the thickness of the tissue 'air-to-plasma', of the partition coefficient lung:blood and of metabolic parameters of ST and SO measured in pulmonary cell fractions of rodents and humans. Simulations of average pulmonary GSH levels in ST exposed rodents agree with measured data. The model predicts a significant GSH depletion (40%) in the conducting zone of mice exposed for 6 h to a ST concentration of only 20 ppm. In the conducting zone of rats, exposure to 200 ppm ST results in a loss of GSH of about 15% only. In humans, a pulmonary GSH reduction does not occur. The highest average pulmonary SO concentrations are predicted for mice, somewhat lower values for rats and by far the lowest ones for humans. Following steady state exposure to 20 ppm ST, the average SO concentration in mouse lungs is expected to be only three times higher than in rats. This difference diminishes to a factor of less than two at 70 ppm. In humans exposed to 20 ppm ST for 8 h, the average pulmonary SO burden of 0.016 micromol/kg is predicted to be about 17 and 50 times smaller than the corresponding values for rat and mouse. In agreement with reported values, pulmonary DNA adduct levels in rodents exposed to 160 ppm ST were simulated to be similar in rats and mice. In summary, there was no dramatic difference in the calculated average pulmonary SO burden between both animal species. However, pulmonary GSH loss was by far more expressed in ST exposed mice than rats. Since the model was validated on all available ST/SO data in mice, rats and humans, we consider it to be useful for estimating the risk resulting from exposure to ST.

Spectrum of styrene-induced DNA adducts: the relationship to other biomarkers and prospects in human biomonitoring

Styrene is an important industrial chemical that has shown genotoxicity in many toxicology assays. This is believed to be related to the DNA-binding properties of styrene-7,8-oxide (SO), a major metabolite of styrene. In this review, we have summarized knowledge on various aspects of styrene genotoxicity, especially in order to understand the formation and removal of primary DNA lesions, and the usefulness of biomarkers for risk assessment. Biological significances of specific DNA adducts and their role in the cascade of genotoxic events are discussed. Links between markers of external and internal exposure are evaluated, as well as metabolic aspects leading to the formation of DNA adducts and influencing biomarkers of biological effect. Finally, we suggest a design of a population study, which may contribute to our understanding genotoxic events in the exposure either to single xenobiotic or complex mixture.

Estimation of a possible tumorigenic risk of styrene from daily intake via food and ambient air

Concerns of a tumorigenic risk of styrene (ST) originate from the findings that styrene (ST) is metabolized to the genotoxic intermediate styrene-7,8-oxide (SO). Therefore, it was hypothesized that results of animal long-term studies with ST and SO together with the SO tissue burden are sufficient for conducting a 'worst case' estimate of the tumorigenic risk of ST. On this basis we predicted the excess human lifetime risk for lung tumors (p(EXL)) and the highest possible risk for other systemic tumors (p(HPS)) resulting from daily intake of ST via food and ambient air. As measures for p(EXL) the mean lifetime concentration of SO in the transitional zone of the lung and for p(HPS) the mean lifetime concentration of SO in blood were calculated using a physiological toxicokinetic model. For a daily oral intake of 12 microST, p(EXL) was obtained to be between 5x10(-9) and 2x10(-8) and p(HPS) to be between 7x10(-9) and 2x10(-8). Lifetime risks calculated for continuous exposure to 3 microg/m(3) ST in ambient air were between 8x10(-7) and 3x10(-6) (p(EXL)) and between 2x10(-8) and 4x10(-8) (p(HPS)). Although these values indicate very low risks, the actual risks are expected to be even by far smaller. This is discussed in detail for lung tumorigenesis.

Determination of styrene and styrene-7,8-oxide in human blood by gas chromatography-mass spectrometry

Methods of isotope-dilution gas chromatography-mass spectrometry (GC-MS) are described for the determination of styrene and styrene-7,8-oxide (SO) in blood. Styrene and SO were directly measured in pentane extracts of blood from 35 reinforced plastics workers exposed to 4.7-97 ppm styrene. Using positive ion chemical ionization, styrene could be detected at levels greater than 2.5 microg/l blood and SO at levels greater than 0.05 microg/l blood. An alternative method for measurement of SO employed reaction with valine followed by derivatization with pentafluorophenyl isothiocyanate and analysis via negative ion chemical ionization GC-MS-MS (SO detection limit=0.025 microg/l blood). The detection limits for SO by these two methods were 10-20-fold lower than gas chromatographic assays reported earlier, based upon either electron impact MS or flame ionization detection. Excellent agreement between the two SO methods was observed for standard calibration curves while moderate to good agreement was observed among selected reinforced plastics workers (n = 10). Levels of styrene in blood were found to be proportional to the corresponding air exposures to styrene, in line with other published relationships. Although levels of SO in blood, measured by the direct method, were significantly correlated with air levels of either styrene or SO among the reinforced plastics workers, blood concentrations were much lower than previously reported at a given exposure to styrene. The two assays for SO in blood appear to be unbiased and to have sufficient sensitivity and specificity for applications involving workers exposed to styrene and SO during the manufacture of reinforced plastics.

Styrene oxide in blood, hemoglobin adducts, and urinary metabolites in human volunteers exposed to (13)C(8)-styrene vapors

Styrene is used in the manufacture of plastics and polymers and in the boat-building industry. The major metabolic route for styrene in rats, mice, and humans involves conversion to styrene-7,8-oxide (SO). The purpose of this study was to evaluate blood SO, SO-hemoglobin (SO-Hb) adducts, and urinary metabolites in styrene-exposed human volunteers and to compare these results with data previously obtained for rodents. Four healthy male volunteers were exposed for 2 h during light physical exercise to 50 ppm (13)C(8)-styrene vapor via a face mask. Levels and time profiles of styrene in exhaled air, blood, and urine (analyzed by GC) and urinary excretion patterns of mandelic acid and phenylglyoxylic acid in urine (analyzed by HPLC) were comparable to previously published volunteer studies. Maximum levels of SO in blood (measured by GC-MS) of 2.5-12.2 (average 6.7) nM were seen after 2 h, i.e., in the first sample collected after exposure had ended. The styrene blood level in humans was about 1.5 to 2 times higher than in rats and 4 times higher than in mice for equivalent styrene exposures. In contrast the SO levels in human blood was approximately fourfold lower than in mice. The level of hydroxyphenethylvaline (determined by GC-MS-MS) in pooled blood collected after exposure was estimated as 0.3 pmol/g globin corresponding to a SO-Hb adduct increment of about 0.003 pmol/g and ppmh. NMR analyses of urine showed that a major portion (> 95%) of the excreted (13)C-derived metabolites was derived from hydrolysis of SO, while only a small percentage of the excreted metabolites (< 5%) was derived from metabolism via phenylacetaldehyde. Signals consistent with metabolites derived from other pathways of styrene metabolism in rodents (such as glutathione conjugation with SO or ring epoxidation) were not detected.

New aspects in genotoxic risk assessment of styrene exposure--a working hypothesis

Styrene is one of the most important plastic monomers worldwide. Styrene-7,8-oxide (SO), the major in-vivo metabolite of styrene, is classified as probably carcinogenic to humans and carcinogenic in rodents. Biological monitoring of exposure to styrene is usually carried out by determination of mandelic acid and phenylglyoxylic acid, the two main styrene metabolites in urine. SO binds covalently to human plasma protein and haemoglobin. The ability of SO to induce DNA adducts and DNA strand-breaks has been well documented. Recently in-vitro results showed that SO may disrupt the pre-existing oxidative status in white blood cells. This disruption would alter the balance between oxidants and antioxidants in cells. Styrene exposure can also result in oxidative DNA damage. A significant increase of 8-hydroxy-2;-deoxyguanosine (8-OHdG) has been found in white blood cells of styrene-exposed workers. According to these findings we propose a new hypothesis for the genotoxic risk assessment of styrene. Depletion of glutathione and increase in lipid peroxidation, similarity in the decrease of high molecular weight (HMW) DNA fragments after SO exposure compared to hydrogen peroxide (H(2)O(2)) exposure, oxidative DNA damage (increased amounts of 8-OHdG and an increased level of DNA strand-breaks) following styrene or SO exposure are due to oxidative stress which can be a result of the imbalance between oxidants and antioxidants. Formation of protein-, RNA- and DNA-adducts, changes in DNA repair capacity and styrene metabolism following styrene exposure could cause this imbalance between oxidants and antioxidants. Oxidative stress seems to be the basis for genotoxic risk assessment of styrene.

Proposed changes in the classification of carcinogenic chemicals in the work area

Carcinogenic chemicals in the work area are currently classified into three categories in Section III of the German List of MAK and BAT Values. This classification is based on qualitative criteria and reflects essentially the weight of evidence available for judging the carcinogenic potential of the chemicals. It is proposed that these Categories--IIIA1, IIIA2, and IIIB--be retained as Categories 1, 2, and 3, to conform with EU regulations. On the basis of our advancing knowledge of reaction mechanisms and the potency of carcinogens, it is now proposed that these three categories be supplemented with two additional categories. The essential feature of substances classified in the new categories is that exposure to these chemicals does not convey a significant risk of cancer to man, provided that an appropriate exposure limit (MAK value) is observed. It is proposed that chemicals known to act typically by nongenotoxic mechanisms and for which information is available that allows evaluation of the effects of low-dose exposures be classified in Category 4. Genotoxic chemicals for which low carcinogenic potency can be expected on the basis of dose-response relationships and toxicokinetics and for which risk at low doses can be assessed will be classified in Category 5. The basis for a better differentiation of carcinogens is discussed, the new categories are defined, and possible criteria for classification are described. Examples for Category 4 (1,4-dioxane) and Category 5 (styrene) are presented. The proposed changes in classifying carcinogenic chemicals in the work area are presented for further discussion.

Changes in high molecular weight DNA fragmentation following human blood exposure to styrene-7,8-oxide

Styrene-7,8-oxide (S7,8O) is the major in vivo metabolite of styrene and is a genotoxic agent potentially carcinogenic to humans. It is known to cause DNA strand breaks and adducts. We studied high molecular weight DNA fragmentation in white blood cells following incubation of blood with S7,8O from individuals with no previous exposure to this compound. To better understand the effects of S7,8O, we also examined blood exposed to hydrogen peroxide (H2O2) a typical oxidant that is linked to oxidative stress. All individuals in this study showed a variable reduction in high molecular weight DNA fragments determined by laser densitometry compared to untreated controls for both S7,8O and H2O2 treated samples. This decrease was independent of the concentration and length of exposure of blood to S7,8O and H2O2. An increase in low molecular weight DNA fragments from samples treated with S7,8O and H2O2, compared to untreated samples was also noted. Similarities in the reduction of HMW-DNA fragments after S7,8O and H2O2 exposure suggested a similar mechanism of HMW-DNA damage. It was surmised that S7,8O exposure in blood may induce high molecular weight DNA fragmentation due to oxidative stress.

Toxicokinetic models for volatile industrial chemicals and reactive metabolites

Two approaches of compartmental toxicokinetic modeling of gaseous compounds are presented which are suitable for kinetic analysis of concentration-time data measured in the air of closed exposure systems. The first approach is based on a two-compartment model with physiological gas uptake, the second on a physiologically-based toxicokinetic model. Both models can be used for the description of inhalation, accumulation, exhalation and metabolism of gaseous compounds together with the toxicokinetics of metabolites. Interspecies extrapolation is based on physicochemical, physiological and biochemical parameters. The advantage of the two-compartment model is its limited number of variables and its experimentally easy applicability. Its disadvantage is the impossibility to predict tissue specific concentrations. The advantage of the physiologically-based model is its usability for predictions and for the description of tissue specific concentrations. However, it entails great effort, since a series of parameters has to be determined before meaningful model calculations can be carried out.

A physiologic pharmacokinetic model for styrene and styrene-7,8-oxide in mouse, rat and man

Concern about the carcinogenic potential of styrene (ST) is due to its reactive metabolite, styrene-7,8-oxide (SO). To estimate the body burden of SO resulting from various scenarios, a physiologically based pharmacokinetic (PBPK) model for ST and its metabolite SO was developed. This PBPK model describes the distribution and metabolism of ST and SO in the rat, mouse and man following inhalation, intravenous (i.v.), oral (p.o.) and intraperitoneal (i.p.) administration of ST or i.v., p.o. and i.p. administration of SO. Its structure includes the oxidation of ST to SO, the intracellular first-pass hydrolysis of SO catalyzed by epoxide hydrolase and the conjugation of SO with glutathione. This conjugation is described by an ordered sequential ping-pong mechanism between glutathione, SO and glutathione S-transferase. The model was based on a PBPK model constructed previously to describe the pharmacokinetics of butadiene with its metabolite butadiene monoxide. The equations of the original model were revised to refer to the actual tissue concentration of chemicals instead of their air equivalents used originally. Blood:air and tissue:blood partition coefficients for ST and SO were determined experimentally and have been published previously. Metabolic parameters were taken from in vitro or in vivo measurements. The model was validated using various data sets of different laboratories describing pharmacokinetics of ST and SO in rodents and man. In addition, the influences of the biochemical parameters, alveolar ventilation and blood:air ventilation and blood:air partition coefficient for ST on the pharmacokinetics of ST and SO were investigated by sensitivity analysis.(ABSTRACT TRUNCATED AT 250 WORDS)