Physiological Modeling of the Relative Contributions of Styrene-7,8-oxide Derived from Direct Inhalation and from Styrene Metabolism to the Systemic Dose in Humans

Rogelio Tornero-Velez and Stephen M. Rappaport,1

Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599–7400

Received May 16, 2001; accepted August 6, 2001


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX
 REFERENCES
 
Workers in the reinforced plastics industry are exposed to large quantities of styrene and to small amounts of the carcinogen, styrene-7,8-oxide (SO), in air. Since SO is also the primary metabolite of styrene, we modified a published physiologically based pharmacokinetic (PBPK) model to investigate the relative contributions of inhaled SO and metabolically derived SO to the systemic levels of SO in humans. The model was tested against air and blood measurements of styrene and SO from 252 reinforced plastics workers. Results suggest that the highly efficient first-pass hydrolysis of SO via epoxide hydrolase in the liver greatly reduces the systemic availability of SO formed in situ from styrene. In contrast, airborne SO, absorbed via inhalation, is distributed to the systemic circulation, thereby avoiding such privileged-access metabolism. The best fit to the model was obtained when the relative systemic availability (the ratio of metabolic SO to absorbed SO per unit exposure) equaled 2.75 x 10-4, indicating that absorbed SO contributed 3640 times more SO to the blood than an equivalent amount of inhaled styrene. Since the ratio of airborne styrene to SO rarely exceeds 1500 in the reinforced plastics industry, this indicates that inhalation of SO presents a greater hazard of cytogenetic damage than inhalation of styrene. We conclude that future studies should assess exposures to airborne SO as well as styrene.

Key Words: bioavailability; inhalation; physiologically based modeling; reinforced plastics; styrene; styrene-7,8-oxide.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX
 REFERENCES
 
While much is known regarding the kinetics of styrene in humans (Bond, 1989Go; Droz 1983; Filser, 1992Go; Löf and Johanson, 1993Go; Löf et al., 1986Go; Marhuenda et al., 1997Go; Perbellini, 1988; Ramsey and Andersen 1984Go; Ramsey and Young 1978Go; Sumner and Fennel, 1994; Wigaeus, 1983), the kinetics of styrene's genotoxic metabolite, styrene-7,8-oxide (SO), are not as well understood. Since SO is a known carcinogen (McConnell and Swenberg, 1994Go), the disposition of SO following metabolism of styrene (primarily in the liver) is a key determinant of the potential carcinogenicity of styrene. Indeed, the International Agency for Research on Cancer (IARC) has classified styrene as a possible carcinogen (2B), based largely on its metabolism to SO (IARC, 1994Go). However, SO is efficiently detoxified prior to release from the human hepatocyte (Csanády et al., 1994Go; Korn et al., 1994Go; Rappaport et al., 1996Go) thereby lessening the risk associated with exposure to styrene.

Assessment of the bioavailability of SO from styrene exposure is further complicated by coexposure to airborne SO, which arises from oxidation of styrene during the manufacture of reinforced plastics. While concentrations of SO in air are typically 1/500–1/1500 of those of styrene (Pfäffli and Säämänen, 1993Go, Tornero-Velez et al., 2000Go), the high systemic availability of inhaled SO could offset its low level of exposure compared to styrene (Rappaport et al., 1996Go). Pharmacokinetic studies involving ethylene and butadiene show that only minor fractions of the metabolized olefins become systemically available as the corresponding epoxides (Csanády et al., 1996Go, 2000Go; Filser and Bolt, 1984Go; Johanson and Filser, 1993Go,1996Go; Kohn and Melnick, 2000Go).

The notion that kinetic factors can influence bioavailability is well documented in the literature. Intestinal and hepatic first-pass effects are particularly important in this regard. For example, blood ethanol concentrations are much higher following iv administration rather than po dosing (Julkunen et al., 1985Go). Similarly, the immediate detoxication of SO following hepatic metabolism of styrene (more than 85% of the absorbed dose of styrene is metabolized to SO in humans [Bond, 1989Go]) constitutes a hepatic-first pass effect.

The magnitude of the hepatic first-pass effect can be gauged by comparing the area under the SO blood concentration-time curve (AUCSO) following exposure to styrene [AUCSO(styrene)] with that following equivalent exposure to SO [AUCSO(SO)]. Since levels of hemoglobin and albumin adducts in blood reflect the AUC of electrophilic species under reasonable assumptions (Ehrenberg et al., 1974Go), these markers may be used to determine bioavailability. Measurements of SO adducts of blood proteins by Rappaport et al. (1993, 1996) indicated that the ratio AUCSO(styrene)/AUCSO(SO) was about 2% in Sprague-Dawley rats (following ip administration) and about 0.05% in reinforced plastics workers. The (human) estimate of AUCSO(styrene)/AUCSO(SO) {cong} 0.05% was complicated by the simultaneous exposures of the workers to both SO and styrene. Since the contributions to blood of "metabolic SO" (derived from styrene) and "absorbed SO" (from inhalation of SO) are indistinguishable, statistical methods were needed to ascribe the corresponding fractions of the AUC (for details see Rappaport et al., 1996Go). Here we describe an alternative approach involving physiologically based pharmacokinetic (PBPK) modeling to estimate AUCSO(styrene)/ AUCSO(SO).

PBPK modeling refers to the development of mathematical descriptions of the uptake and disposition of chemicals based on biological determinants of these processes (Andersen, 1991Go). These determinants include physiochemical properties (partition coefficients), rates of biochemical reactions, and a large body of physiological data (Krishnan and Andersen, 1994Go). PBPK models offer biologically plausible approaches to quantify bioavailability (Pastino and Conolly, 2000Go) that can be used to predict the relative influences of metabolic SO and absorbed SO to the systemic dose of SO in vivo.

Csanády et al. (1994) described a PBPK model to characterize the disposition of styrene and SO. This model included metabolism of styrene to SO and elimination of SO via hydrolysis or conjugation with glutathione (GSH) but did not allow for direct absorption of SO into the blood from airborne exposure. Although the model of Csanády et al. (1994) was validated with measurements of styrene in air and SO in blood of reinforced plastics workers, simultaneous coexposure to SO was not evaluated (Korn et al., 1994Go). The current investigation describes modifications of the Csanády et al. (1994) model to investigate the relative contributions of absorbed and metabolic SO to AUCSO among reinforced plastics workers. The model is tested against air and blood measurements of both styrene and SO from 252 workers.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX
 REFERENCES
 
Subjects.
Biological and environmental samples reflecting personal exposure to styrene and SO were obtained from 328 reinforced plastics workers, employed in 17 factories in the Pacific Northwest of the U.S. These facilities produced a variety of products including boats, trucks, pipes, and storage tanks. Surveys were conducted between December of 1996 and October of 1999 among workers who were predominately male, nonsmokers, Caucasians, and reasonably young (median age of 34.4 years with an interquartile range of 28.6–43.4 years).

Environmental samples.
Passive monitors were used to measure personal exposures to styrene and SO repeatedly among the 328 workers (described in Tornero-Velez et al. [2000]). Large ranges of daily exposures were observed for both styrene (range: < 1 to 167 ppm [n = 1394]) and SO (range: < 1 to 187 ppb [n = 1386]). Both sets of measurements were consistent with levels reported from similar reinforced plastics facilities (summarized in Nylander French et al., 1999Go; Rappaport et al., 1996Go; Yeowell-O'Connell et al., 1996Go).

Biological samples.
A method for measuring styrene and SO in blood, based upon gas chromatography-mass spectrometry with positive-ion-chemical ionization, was developed and applied to the samples (described in Tornero-Velez et al., 2001Go). This method permitted measurement of SO in blood at levels as low as 0.05 µg/l, a limit that is 10-fold lower than those previously reported. Large ranges of blood levels were observed for both compounds (styrene: < 0.001 to 2.05 mg/l [n = 488]; SO: < 0.05–0.49 µg/l [n = 302]). While blood-styrene levels were generally consistent with those reported in other studies, the corresponding levels of SO in blood were considerably lower than the few previously reported measurements (summarized in Tornero-Velez et al., 2001Go).

PBPK model.
Csanády et al. (1994) described a PBPK model for styrene and SO in rats, mice, and humans that assumed that all SO was derived from styrene metabolism (hereafter "basic model"). We modified that model to include uptake of SO by inhalation (hereafter "extended model"). Although Csanády et al. (1994) did not consider SO gas-exchange, they reported a blood:air partition coefficient for SO, {lambda}SO, of 2370 (SE = 499, n = 3), allowing for consideration of SO uptake via inhalation in the extended model. For both styrene and SO, the basic and extended models assume instantaneous equilibrium between circulating blood and alveolar air, as determined by the blood:air partition coefficients.

A schematic of the extended model is shown in Figure 1Go. The systemic circulation distributes styrene and SO to the liver (the only metabolizing tissue), fat, muscle, and the richly perfused tissues, which are all assumed to be homogenous well-mixed compartments with human physiological constants shown in Table 1Go. The chemical-dependent physiochemical and biochemical constants are shown in Table 2Go. Mass-balance equations were written to describe the rates of change in styrene and SO concentrations for each compartment, based upon Johanson and Filser (1993) and Csanády et al. (1994); see Appendix. With the exception of KmIH for epoxide hydrolase, the fitting procedure for which is discussed below, the parameters employed in the extended model are the same as those in the basic model. The model was coded in Matlab (The Mathworks Inc., Natick, MA) and solutions were obtained with the Matlab stiff ordinary differential equation solver ode15s.



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FIG. 1. Schematic of the extended PBPK model to study the disposition of inhaled styrene and styrene-7,8-oxide.

 

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TABLE 1 Human-Specific Physiological Parameters Used in Extended PBPK Model
 

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TABLE 2 Human-Specific Physiochemical and Metabolic Parameters Used in the Extended PBPK Model
 
Metabolic reactions for both the basic and extended models were all modeled in terms of saturable processes. Activation of styrene to SO via cytochrome P-450 has affinity, Kmmo (0.010 mmol/l), and capacity, Vmaxmo (3.6 mmol/h per 70 kg human). The value for Kmmo was 30 times lower than that initially determined in vitro from suspensions of liver microsomes (Mendrala et al., 1993Go), even after accounting for the blood:buffer partition coefficient of 3.4 in humans (Csanády et al., 1994Go). The value of Vmaxmo was determined by fitting concentration-time courses of styrene in humans, but was found to be within the range of values extrapolated allometrically from rats and mice (1.9–3.7 mmol/h per 70 kg body weight). The fitted value was also comparable to the value of 1.77 mmol/h (per 83 kg) used by Ramsey and Andersen (1984). Conjugation of SO by glutathione S-transferases (GST) has capacity VmaxGST (0.028 mmol/h per g liver), which was determined in vitro (Mendrala et al., 1993Go) after accounting for the concentration of cytosolic protein in human liver, and affinity, KmSO (2.5 mmol/l), which was derived from rodent-fitted values (Csanády et al., 1994Go). The affinity of GST towards GSH, KmGSH (0.1 mmol/l), was determined by fitting of butadiene kinetics in rats and mice (Johanson and Filser, 1993Go). The turnover of cytosolic GSH was modeled with zero-order production and first-order elimination, with rate constant, KdGSH (0.2 l/h) determined from butadiene kinetics in rats (Johanson and Filser, 1993Go).

Elimination of SO by epoxide hydrolase (EH) was modeled with a postulated "intrahepatic" (IH) compartment, a complex containing both cytochrome P-450 (styrene -> SO) and EH (SO -> styrene glycol) in the subcellular endoplasmic recticulum. This conceptualization stems from the work of Oesch and Daly (1972) and has been employed in models describing butadiene kinetics in rats and mice (Csanády et al., 1996Go; Johanson and Filser, 1993Go,1996Go; Kohn and Melnick, 2000Go). The postulated mechanism dictates that SO, formed in situ within the IH compartment, reaches EH faster than SO, diffusing from the cytosol. This diffusive transport of SO was modeled with an effective intrahepatic flow, QIH, between the cytosol and the IH compartment. Because QIH was not experimentally accessible, it was derived from the flow-dependent relation between apparent and intrinsic clearance (for details, see Johanson and Filser, 1993Go). Two Michaelis-Menten constants are specified, an apparent constant of affinity Kmapp (0.01 mmol/l), obtained from suspensions of liver microsomes in buffer (Mendrala et al., 1993Go), and an intrinsic constant of affinity KmIH (0.001 mmol/l) set to 10% of Kmapp. In this formulation, the measured enzyme constant (Kmapp) pertains to detoxication of cytosolic SO, while the derived constant (KmIH) governs the elimination of in situ SO; both are required parameters.

Because KmIH was not accessible through in vitro experimentation, Csanády et al. (1994) estimated this parameter by fitting the basic model to in vivo human data from Korn et al. (1994). That study provided end-of-shift concentrations of SO in blood and the corresponding shift-averaged concentrations of styrene in air for 14 workers in the reinforced plastics industry (Korn et al., 1994Go). By setting KmIH/Kmapp = 10%, Csanády et al. (1994) effectively calibrated their model to the blood levels of SO reported by Korn et al. (1994). In contrast, both Kmapp and VmaxEH were determined from in vitro experimentation (Mendrala et al., 1993Go).

Calibration of the intrinsic KmIH.
Under the extended model, SO enters the blood from hepatic metabolism of styrene at a lower rate than from direct absorption following inhalation. As shown in Figure 2Go, we postulate that a small fraction of metabolic SO eludes the endoplasmic recticulum membrane complex and enters the cytosol where it is either conjugated with GSH or enters the systemic circulation. On the other hand, we postulate that absorbed SO is immediately available to the systemic circulation. Thus, in the extended model, the ratio KmIH/Kmapp reflects the difference in the rates of detoxication of metabolic SO versus absorbed SO. Under the basic model, SO was more rapidly eliminated from the IH compartment than the cytosol, as reflected by the ratio KmIH/Kmapp, which was set to 10% by Csanády et al. (1994). However, preliminary applications of the extended model indicated that a value of KmIH/Kmapp = 10% was much too great to adequately explain the data from our study. Values of KmIH/Kmapp = 1 and 0.5% were assigned, based upon results from SO-albumin adducts reported by Rappaport et al. (1996).



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FIG. 2. Schematic illustrating reduced bioavailability of metabolic SO compared to absorbed (inhaled) SO. Abbreviations: EH, epoxide hydrolase; GST, glutathione S-transferase; S, styrene; SG, styrene glycol; SO, styrene-7,8-oxide; SOSG, glutathione conjugate of SO.

 
Approximate steady-state blood-SO.
The systemic availability of metabolic and absorbed SO was modeled under conditions of approximate steady-state by using as input daily 8 h exposures of 50 ppm styrene and 50 ppb SO under a typical occupational regimen (8 h/day, 5 days/week) for 12 days. Predicted end-of-shift levels of SO in blood were found to increase 4.25% from the first Monday (3.06 µg/l) to the first Friday of exposure (3.19 µg/l). The buildup over the course of a week rapidly approached a limit of 2.28% after several weeks of exposure, such that the daily end-of-shift blood level of SO reached at least 99.6% of its maximum value by the second Friday (day 12). Repeating this exercise with the fitted model resulted in a more rapid approach to steady state. The AUCSO was computed by integrating the concentration of SO in venous blood over the course of the exposure regimen followed by 60 days of zero exposure to styrene and SO, which was needed to clear the fat compartment.

Relative systemic availability.
The relative contributions of cumulative exposures to styrene and SO to the systemic dose of SO were computed using the following formula:


((1))
(Since SO-albumin adducts should reflect the AUCSO over several weeks, Equation 1Go is analogous to the relationship presented by Rappaport et al. [1996].) Since the input concentrations of styrene and SO were of the same duration, the exposure level (ppm) rather than cumulative exposure (ppm-h) was used in calculations of RSAstyrene:SO. Input styrene concentrations of 1, 10, 25, 50, and 75 ppm were used, representing the 8th, 53rd, 78th, 92nd, and 98th percentiles of the distribution of the workers' geometric means in the current study. Corresponding input SO exposures were selected at a factor of 1/500 of the styrene exposures (2, 20, 50, 100, and 150 ppb), representing the 10th, 58th, 92nd, 99th, and > 99th percentiles from the current study. For all exposure scenarios, the extended PBPK model was invoked with the mean parameter values listed in Tables 1Go and 2Go.

Sensitivity analysis.
The sensitivity of SO kinetics to styrene and SO parameters in the extended model was evaluated through normalized sensitivity coefficients (NSC), representing the fractional change in output associated with a fractional change in the input parameter (Clewell et al., 2000Go). A change of 1% in the model parameter was applied to ensure discovery of the true local sensitivity. A value of NSC greater than 1.0 would indicate that there is amplification of error from the input to the output. To evaluate the separate influences of absorbed and metabolic SO, sensitivity analysis was conducted for inhaled SO, inhaled styrene, and for the combined exposure. The output parameter was the concentration of SO in venous blood, following 8 h of exposure to 9.14 ppm styrene or 17.1 ppb SO, or both (each based on median values from the current study).

Monte Carlo evaluation of styrene kinetics.
The styrene portion of the model was evaluated by Monte Carlo analysis. This procedure randomly selected a set of parameter values from the population distributions and ran the PBPK model (8 h exposure to 2, 10, or 50 ppm styrene) to obtain the end-of-shift blood-styrene. This process was repeated with 200 iterations per exposure scenario. The population distributions for the model parameters were assumed to be either lognormal or normal (Portier and Kaplan, 1989Go), as shown in Tables 1Go and 2Go. With the exception of Vmaxmo, the distributions of model parameters were truncated at ± 3 standard deviations to eliminate values that were biologically implausible (Bois et al., 1996Go). The maximum rate of styrene oxidation to SO, Vmaxmo, was bounded at ± 2 standard deviations due to its high coefficient of variation (CV) of 50%. For model stability, alveolar ventilation was set equal to cardiac output (Thomas et al., 1996Go). In order to maintain mass balance, fractional blood flows (required to sum to 1), were calculated by setting the flow of the rapidly perfused tissues equal to the difference between cardiac output and the remaining compartment tissues (Thomas et al., 1996Go). Similarly, the sum of the individual tissue volumes was constrained to be equal to 0.87 x body volume, equivalent to the volume of tissue participating in mass balance when the model was parameterized with mean values. This was achieved by setting the volume of muscle tissue equal to the difference between body mass and the remaining tissue volumes. Partition coefficients associated with the equilibration of styrene were not included in the model because a preliminary sensitivity analysis indicated that predicted levels of styrene in blood were not sensitive to these parameters.

Analysis of data.
Data were analyzed using SAS statistical software V8.03 (SAS Institute, Cary, NC). Due to skew in exposure distributions the geometric mean (GM) was employed unless otherwise indicated, and was estimated as GM = X, where X represents the mean of the logarithms (base e) of the measurements. The overall ratio of airborne styrene to SO was estimated by taking the ratio of GMs. Since the GM exposure to styrene among these workers was 9.14 ppm while that to SO was 17.1 ppb, a typical worker was exposed to 534 times more styrene than SO, consistent with previous observations (Nylander-French et al., 1999Go; Yeowell-O'Connell et al., 1996Go). The relationship between SO in blood and styrene in air was based on GMs of the subject-specific GMs, averaged by facility. To reduce the effects of measurement error, only facilities with a minimum of 10 subjects were used; this restricted the useable number of facilities from 17 to 8 and the number of subjects from 328 to 252. The relationship between styrene in blood and styrene in air was based on daily end-of-shift measurements in blood and daily average exposures for this subset of 252 subjects. Of the repeated exposure measurements per subject, only measurements corresponding to the day of blood collection were used; this restricted the useable number of air-blood pairs to 348 (221 subjects). For comparison of Monte Carlo simulations with observed data, only measurements above the limit of detection were employed (190 subjects, 297 measurements).


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX
 REFERENCES
 
Modifying the PBPK model of Csanády et al. (1994) for uptake of SO by inhalation allowed for comparison of metabolic and absorbed SO, under scenarios where styrene concentrations were 500 times those of SO. Simulations were initially performed with KmIH/Kmapp = 10% as in the basic model. Table 3Go shows that AUCso was proportional to exposure to either styrene (1–75 ppm) or SO (2–150 ppb), indicating linear kinetics over these ranges. For SO exposures between 2 and 150 ppb, the slopes SO = AUCso/SO [ppm]) were independent of exposure SO varied less than 1%). For exposure to styrene between 1 and 75 ppm the corresponding slopes increased slightly with exposure (about 7%), suggesting slightly reduced intrinsic clearance in detoxication of SO. As shown in Table 3Go, the ratio of slopes (ßstyreneSO) provided an estimate of RSAstyrene:SO ranging from 5.27 x 10-3 (at 1 ppm styrene and 2 ppb SO) to 5.64 x 10-3 (75 ppm styrene and 150 ppb SO). At the median exposure of 9.14 ppm styrene, RSAstyrene:SO = 5.28 x 10-3, indicating that direct absorption of SO contributed 189 times more SO to the blood than metabolism of an equivalent amount of styrene when KmIH/Kmapp = 10%. Interestingly, this result was insensitive to the ratio of styrene:SO in air (RSAstyrene:SO was 5.28 x 10-3 when airborne SO was 1/1000th the styrene concentration).


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TABLE 3 Relative Systemic Availability (RSA) of Inhaled Styrene and SO
 
The greater systemic availability of absorbed SO was not attributable to the 50 fold greater blood:air partition coefficient of SO relative to that for styrene ({lambda}BSO = 2370 vs. {lambda}BST = 48) because uptake of SO was limited by pulmonary flow. For example, reducing {lambda}BSO from 2370 to 48 resulted in only a 4.2% lowering of the SO blood concentration. Rather, the greater availability of absorbed SO was related to the intrahepatic first-pass effect, reflected in the ratio KmIH/Kmapp. Figure 3Go shows a linear relationship between RSAstyrene:SO and KmIH/Kmapp (least-squares slope = 5.50 x 10-2). Hence, for KmIH/Kmapp equal to 10, 1, or 0.5%, airborne SO was 182, 1820, or 3640 times more efficient than styrene, respectively, in contributing to systemic SO. For each scenario, 10, 1, or 0.5%, the slopes, ßSO and ßstyrene (numerator and denominator of RSAstyrene:SO), varied less than 1 and 7%, respectively.



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FIG. 3. Relative systemic availability (RSAstyrene:SO) as a function of the ratio KmIH/Kmapp; the line represents the least-squares regression where RSAstyrene:SO = 5.5 x 10-2(KmIH/Kmapp), r2 = 0.99.

 
Figure 4Go shows the relationship between ratios of end-of-shift SO in blood (µg/l) and styrene in air (ppm) superimposed on the same modeled relationships with KmIH/Kmapp = 10% (A), 1% (B), or 0.5% (C). In each plot, the top line simulates coexposure to SO at 1/500th the air concentration of styrene while the bottom line represents SO at 1/1500th the styrene concentration. For the 8 factories in our study, the GM air concentrations of SO in air ranged from 1/337th to 1/1070th of the styrene concentrations. When the air concentration of SO was 1/1000th that of styrene (midway between the simulated values), styrene contributed 82% of the dose of SO in blood when KmIH/Kmapp = 10% (Case A) but only 18% of the dose when KmIH/Kmapp = 0.5% (Case C).



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FIG. 4. The relationship between end-of-shift SO in blood (µg/l) and styrene in air (ppm) when KmIH/Kmapp = 10% (A), 1% (B), or 0.5% (C). In each plot, the top line represents coexposure to SO at 1/500th the styrene concentration and the bottom line simulates coexposure to SO at 1/1500th the styrene concentration. Superimposed on each plot are 8 facility-specific means including data from a total of 252 subjects.

 
Sensitivity analysis (Fig. 5Go) revealed that the level of SO in venous blood was more greatly influenced by hydrolysis of SO via EH (inhaled styrene: NSC [VmaxEH] = –0.96, NSC [KmIH] = 1.0) than by production of SO via CYP450 (all exposures: NSC [VmaxMO] < 0.07, NSC [Kmmo] < 0.05). The model output was minimally affected by changes in glutathione conjugation (all exposure scenarios: NSC [all parameters] < 0.04). Interestingly, the model was also sensitive to the partition coefficient of SO between liver and blood only with respect to metabolic SO but not absorbed SO (styrene exposure: NSC [{lambda}LSO] = -0.98; SO exposure: NSC [{lambda}LSO] = -0.046), indicating the importance of the liver-blood equilibrium to the residence time of SO in the liver.



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FIG. 5. Normalized sensitivity coefficients for PBPK model predictions of end-of-shift SO in blood.

 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX
 REFERENCES
 
To study the disposition of inhaled SO in humans, we altered the basic model of Csanády et al. (1994) to include pulmonary uptake of SO in our extended model. An important feature of both models is the ability to explain low levels of circulating SO through a detoxifying shunt for the newly formed epoxide. This mechanism, which derives from the model of butadiene and butadiene monoepoxide by Johanson and Filser (1993), is postulated to reside in an IH compartment associated with a complex of P-450 and EH enzymes (Fig. 2Go). The predicted clearance of SO in rats, mice, and humans approached the higher observed values when KmIH < Kmapp, thus, for fixed Kmapp, lower values of KmIH signify increased channeling of the newly formed SO from P450 to EH.

By relying upon an IH compartment, both the basic and extended models achieve the same objective as the recent "privileged access" model of Kohn and Melnick (2000). Although Kohn and Melnick (2000) refuted the notion of an IH compartment, they recognized evidence of a transient complex between P450 and EH residing in the endoplasmic recticulum (Oesch and Daly, 1972Go; Etter et al., 1991Go; Oesch, 1973Go, cited by Johanson and Filser, 1993Go). Furthermore, Kohn and Melnick (2000) proposed that the bound substrate had greater affinity for EH than cytosolic substrate as reflected by the ratio R = Kmbound/Kmcytosolic. Thus, even though the privileged access model and the IH compartment model are based on different mechanistic hypotheses, both attempt to model sequestration of the newly formed epoxide and, therefore, include an analogous parameter, i.e., KmIH/Kmapp (in the IH compartment model) or R (in the privileged access model), which limits export of SO formed in situ.

The intrinsic affinity of SO for EH, KmIH, was identified as the sole parameter for calibration of our extended model for several reasons. First, sensitivity analysis indicated that KmIH had a major influence on human SO kinetics whereas that of glutathione conjugation was minimal. Second, KmIH could not be estimated through experimental in vitro techniques whereas VmaxEH, had been estimated experimentally (Mendrala et al., 1993Go). And finally, in developing the basic model, Csanády et al. (1994) did not include concurrent SO exposure when fitting KmIH to the human data of Korn et al. (1994); although direct exposure to SO undoubtedly occurred among these reinforced plastics workers, the importance of this route was not recognized until 2 years later (Rappaport et al., 1996Go; Yeowell-O'Connell, 1996).

By varying the ratio KmIH/Kmapp it was possible to fit the model to aggregated data from 8 reinforced plastics facilities as shown in Figure 4Go. The best fit occurred when KmIH/Kmapp = 0.5% (Figure 4Go, Case C), which corresponds to a value of RSAstyrene:SO = 2.75 x 10-4. Hence, inhalation of SO was predicted to produce 3640 times more SO in blood than an equivalent exposure to styrene. Based on statistical analysis of the relative contributions of SO-albumin adducts (reflecting the dose of SO to blood) in a single reinforced plastics facility, Rappaport et al. (1996) estimated the RSAstyrene:SO to be 4.65x10-4 {approx} 0.05% (95 CI: 0%, 0.10%), which is plausible based upon our fitted model. Since the ratio of airborne styrene to SO rarely exceeds 1500:1, this suggests that direct inhalation of SO poses a greater hazard to reinforced plastics workers than inhalation of styrene.

In their determination of parameters to describe SO kinetics in rodents, Csanády et al. (1994) employed data from po, ip, and iv administration of SO. They were able to adequately model SO kinetics in rats with KmIH/Kmapp = 10%. However, in mice a reasonable fit could only be achieved by reducing Kmapp from 0.74 mmol/l (determined in vitro by Mendrala et al., 1993Go) to 0.09 mmol/l, while simultaneously setting KmIH equal to 0.009 mmol/l. This effectively reduced KmIH to 1.2% of the value of Kmapp determined in vitro. For humans, this suggests that it is plausible for the true value of KmIH to be 1% of Kmapp. Indeed, the current study and the prior study of Rappaport et al. (1996) point to KmIH/Kmapp being near 0.5–1%, suggesting that direct absorption of airborne SO contributes 1800–3600 times more SO to the blood than metabolism of airborne styrene. Had Csanády et al. (1994) considered uptake of SO, convergence of KmIH/Kmapp with our fit (0.5–1%) would have been attained if the airborne ratio of styrene to SO had been about 250:1 in the factory that provided the data for their calibration (Korn et al., 1994Go). For the 8 factories in our study, individual ratios of styrene:SO ranged from 337 to 1070. Given such variability among reinforced plastics factories, a ratio of airborne styrene:SO of 250: 1 is certainly plausible.

Some uncertainty persists in the modeling of absorbed SO. In particular, inhalation of SO might be subject to presystemic removal in the airways via reactions in the lung. For a number of gases the measured rate of elimination was less than that predicted by a simple inhalation model, which assumed instantaneous equilibration of the chemical between air and blood (Csanády et al., 1994Go,1996Go,2000Go; Filser et al., 1996Go; Gargas et al., 1986Go; Johanson and Filser, 1992Go; Medinsky et al., 1994Go; Reitz et al., 1990Go). This effect has been ascribed to adsorption of the chemical on the walls of the upper airways (Johanson and Filser, 1992Go). To explain the discrepancy between observed concentrations of ethylene oxide in exhaled breath and model predictions, Csanády et al. (2000) modified their PBPK model to account for adsorption of ethylene oxide in the upper respiratory tract.

Absorbed and metabolic SO have markedly different susceptibilities to lung and liver metabolism. Glutathione S-transferase resides outside of the IH compartment (Fig. 2Go), offering a potentially important mechanism for removal of absorbed SO. On the contrary, sensitivity analysis showed that levels of SO in venous blood were insensitive to elimination by glutathione conjugation. This may reflect uncertainties in the model, stemming from a lack of experimental data. Only recently have the mercapturic acid conjugates arising from glutathione conjugation of SO been measured among reinforced plastics workers (Ghittori et al., 1997Go). As these data were not available to Csanády et al., 1994, many of the parameters describing glutathione conjugation in humans were based on rodent kinetics (KmGSH, KmSO, KdGSH). The maximum metabolic rate VmaxGST was determined in vitro by Mendrala et al. (1993) from subcellular fractions of human liver cells but with a great deal of imprecision (due to low sensitivity, assays were conducted with high substrate concentration, and even then GST activity was barely detectable in 2 of 5 samples). Although the urinary levels of mercapturic acid conjugates are low, they may aid in delineating subtleties in the dispositions of absorbed versus metabolic SO. However, in order to evaluate this conjecture VmaxGST and the corresponding values of KmGSH and KmSO need to be determined with better precision.

Sensitivity analysis also indicated that the liver:blood partition coefficient for SO, i.e., {lambda}LSO, was an important determinant of SO in blood. This parameter represents the availability of SO to the liver where it can be acted upon by hepatic GST or epoxide hydrolase. Any bias in the measurement of {lambda}LSO would lead to error in the model prediction (a modest bias of –10% in {lambda}LSO would lead to a corresponding bias of 16.4% in the predicted level of SO in blood). Although the reactivity of SO (the half-life in human blood in vitro is 42 min [Yeowell-O'Connell et al., 1997Go]) would seem to present experimental difficulties in the determination of {lambda}LSO, this potential source of error was not addressed by Csanády et al. (1994).

Since styrene is the source of metabolic SO, we investigated the extended model's description of styrene kinetics through Monte Carlo analysis and compared the predictions with extensive empirical data (Fig. 6Go). The obvious agreement between observed and predicted values point to prior validation of that portion of the PBPK model by several investigators (Filser et al., 1993Go; Löf and Johanson, 1993Go; Löf et al., 1986Go; Ramsey and Andersen, 1984Go). However, the importance of styrene kinetics to the corresponding SO kinetics is dubious since our sensitivity analysis revealed a minimal effect of styrene metabolism on levels of SO in venous blood, given coexposure to styrene and SO in air.



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FIG. 6. The relationship between end-of-shift styrene in blood (mg/l) and styrene in air (ppm) (n = 297) superimposed on Monte Carlo analysis of the styrene portion of model. The bars represent the 5th, 50th, and 95th percentiles of the distribution of simulated styrene in blood.

 
In summary, simulations conducted with the extended PBPK model illustrated the marked susceptibility of metabolic SO to first-pass clearance. In contrast, low-level airborne SO was not subject to first-pass losses and, thus, contributed significantly to the systemic dose of SO. The results suggest that future studies should assess the risk of exposure to airborne SO as well as styrene to cytogenetic effects among workers.


    APPENDIX
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX
 REFERENCES
 
Mass Balance Differential Equations
The extended model is based on the PBPK model described by Johanson and Filser (1993) and Csanády et al. (1994), modified to include uptake of SO by inhalation. The equations (for abbreviations refer to Tables 1 Goand 2Go) described below specify the model implemented in this study. The concentrations of styrene (S) and SO in the arterial blood were calculated assuming a steady state between alveolar air and blood (Ramsey and Andersen, 1984Go).


((A1))
where x refers to S or SO, and {lambda}X is the corresponding blood:air partition coefficient. From Equation A1, the following equations are derived for the concentration of S or SO in arterial blood:


((A2))


((A3))
By setting the venous blood flow equal to the cardiac output, the concentration of S and SO in the mixed venous blood is given by:


((A4))


((A5))
where the subscript i refers to the tissue group (richly perfused tissue, fat, muscle, or liver). The concentration of S or SO in nonmetabolizing compartments is described by:


((A6))
The metabolism of S is assumed to take place exclusively in the liver according to Michaelis-Menten kinetics. The concentration change of S in the liver is given by:


((A7))
where,


((A8))

The capacity of cytochrome P450 for oxidation of S, Vmaxmo** (mmol/h) = Vmaxmo (mmol/h per g liver) x VL (kg) x 103 g/kg. The Michaelis-Menten constant is specified for liver tissue by Kmmo** = Kmmo x {lambda}LSO, where Kmmo was determined for venous blood from the liver.

The metabolism of SO occurs in the cytosol (liver compartment) and in the endoplasmic recticulum (IH compartment). The SO concentration in the IH compartment arises from metabolism of S, diffusive flow of SO to/from the liver cytosol, QIH, and detoxication of SO via metabolic clearance of SO, CLSO. These processes result in the mass balance equation:


((A9))
where VIH is the volume of the IH compartment. Steady state is assumed between the IH and liver compartments. Setting Equation A9 equal to zero C (dCihSO/dt = 0) and solving for the concentration of SO in the IH compartment results in:


((A10))
where QIH is based on the following flow-dependent relationship between apparent and intrinsic clearance:


((A11))
The metabolic clearance of SO in the IH compartment follows Michaelis-Menten kinetics giving:


((A12))
Based on these equations, the concentration of SO in the IH compartment is


((A13))
where,


((A14))
2

The change of SO concentration in the liver is controlled by transport of SO via the systemic circulation, diffusion of SO between the cytosol and the IH compartment, and conjugation of SO with glutathione according to:


((A15))
where,


((A16))

The capacity of GST, VmaxGST* (mmol/h) = VmaxGST (mmol/h per g liver) x VL(kg) x 103 g/kg. The turnover of GSH in the cytosol is described by zero-order production and first order elimination. Conjugation of GSH with SO was modeled according to:


((A17))


    ACKNOWLEDGMENTS
 
We thank Dr. Diana Echeverria for providing expert support with the collection of air and blood samples from the exposed subjects in the reinforced plastics facilities. We also appreciate the thoughtful review of a draft of this manuscript by Dr. Melvin E. Andersen. We also acknowledge the assistance of Dr. Lisa Sweeney for discussion pertaining to this work. This work was supported by grant R01CA69463 from the National Cancer Institute and through training grant T32ES07018 of the National Institute of Environmental Health Sciences.


    NOTES
 
1 To whom correspondence should be addressed. Fax: (919) 966-4711. E-mail: stephen_rappaport{at}unc.edu. Back

2 In personal communication, Dr. Lisa Sweeney noted a minor error in this equation as documented by Csanády et al. (1994). The investigators reported the equation as "a," where a = c – KmSO + KmlH. Back


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX
 REFERENCES
 
Andersen, M. E. (1991). Physiological modelling of organic compounds. Ann. Occup. Hyg. 35, 309–321.[ISI][Medline]

Arms, A. D., and Travis, C. C. (1988). Reference Physiological Parameters in Pharmacokinetic Modeling. NTIS PB-196019. Office of Health and Environmental Assessment, U. S. Environmental Protection Agency, Washington, DC.

Bois, F. Y., Jackson, E. T., Pekari, K., and Smith M. T. (1996). Population toxicokinetics of benzene. Environ. Health Perspect. 104(Suppl.), 1405–1411.[ISI][Medline]

Bond, J. A. (1989). Review of the toxicology of styrene. Crit. Rev. Toxicol. 19, 227–249.[Medline]

Clewell, H. J., Gentry, P. R., Covington, T. R., and Gearhart, J. M. (2000). Development of a physiologically based pharamacokinetic model of trichloroethylene and its metabolites for use in risk assessment. Environ. Health Perspect. 108(Suppl.), 283–305.

Csanády, G. A., Denk, B., Pütz, C., Kreuzer, P. E., Kessler, W., Baur, C., Gargas, M. L., and Filser, J. G. (2000). A physiological toxicokinetic model for exogenous and endogenous ethylene and ethylene oxide in rat, mouse, and human: Formation of 2-hydroxyethyl adducts with hemoglobin and DNA. Toxicol. Appl. Pharmacol.165, 1–26.[ISI][Medline]

Csanády, G. A., Kreuzer, P. E., Baur, C., and Filser, J. G. (1996). A physiological toxicokinetic model for 1,3-butadiene in rodents and man: Blood concentrations of 1,3-butadiene, its metabolically formed epoxides, and of haemoglobin adducts—relevance of glutathione depletion. Toxicology 113, 300–305.[ISI][Medline]

Csanády, G. A., Mendrala, A. L., Nolan, R. J., and Filser, J. G. (1994). A physiologic pharmacokinetic model for styrene and styrene-7,8-oxide in mouse, rat and man. Arch. Toxicol. 68, 143–157.[ISI][Medline]

Droz, P. O., and Guillemin, M. P. (1983). Human styrene exposure. V. Development of a model for biological monitoring. Int. Arch. Occup. Environ. Health 53, 19–36.[ISI][Medline]

Ehrenberg, L., Hiesche, K. D., Osterman-Golkar, S., and Wenneberg, I. (1974). Evaluation of genetic risks of alkylating agents: Tissue doses in the mouse from air contaminated with ethylene oxide. Mutat. Res. 24, 83–103.[ISI][Medline]

Etter, H.-U., Richter, C., Ohta, Y., Winterhalter, K. H., Sasabe, H., and Kawato, S. (1991). Rotation and interaction with epoxide hydrase of cytochrome P-450 in proteoliposomes. J. Biol. Chem. 266, 18600–18605.[Abstract/Free Full Text]

Filser, J. G. (1992). The closed chamber technique—uptake, endogenous production, excretion, steady-state kinetics and rates of metabolism of gases and vapors. Arch. Toxicol. 55, 219–223.

Filser, J. G., and Bolt, H. M. (1984). Inhalation pharmacokinetics based on gas uptake studies. VI. Comparative evaluation of ethylene oxide and butadiene monoxide as exhaled reactive metabolites of ethylene and 1,3-butadiene in rats. Arch. Toxicol. 55, 219–223.[ISI][Medline]

Filser, J. G., Csanády, G. A., Denk, B., Hartmann, M., Kauffmann, A., Kessler, W., Kreuzer, P. E., Putz, C., Shen, J. H., and Stei, P. (1996). Toxicokinetics of isoprene in rodents and humans. Toxicology 113, 278–287.[ISI][Medline]

Filser, J. G., Johanson, G., Kessler, W., Kreuzer, P. E., Stei, P., Baur, C., and Csanády, G. A. (1993). A pharmacokinetic model to describe toxicokinetic interactions between 1,3-butadiene and styrene in rats: Predictions for human exposure. In Butadiene and Styrene Assessment of Health Hazards, IARC Scientific Publication No. 127 (M. Sorsa, K. Peltonen, H. Vainio, and K. Hemminki, Eds.) pp. 65–78. International Agency for Research on Cancer, Lyon.

Gargas, M. L., Andersen, M. E., and Clewell, H. J., 3rd. (1986). A physiologically based simulation approach for determining metabolic constants from gas uptake data. Toxicol. Appl. Pharmacol. 86, 341–352.[ISI][Medline]

Ghittori, S., Maestri, L., Imbriani, M., Capodaglio, E., and Cavalleri, A. (1997). Urinary excretion of specific mercapturic acids in workers exposed to styrene. Amer. J. Ind. Med. 31, 636–644.[ISI][Medline]

IARC (1994). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Vol. 60: Some industrial chemicals, pp. 321–346. International Agency for Research on Cancer, Lyon, France.

Johanson, G., and Filser, J. G. (1992). Experimental data from closed chamber gas uptake studies in rodents suggest lower uptake rate of chemical than calculated from literature values on alveolar ventilation. Arch. Toxicol. 66, 291–295.[ISI][Medline]

Johanson, G., and Filser J. G. (1996). PBPK model for butadiene metabolism to epoxides: Quantitative species differences in metabolism. Toxicology 113, 40–47.[ISI][Medline]

Johanson, G., and Filser, J. G. (1993). A physiologically based pharmacokinetic model for butadiene and its metabolite butadiene monoxide in rat and mouse and its significance for risk extrapolation. Arch. Toxicol. 67, 151–163.[ISI][Medline]

Julkunen, R. J., Di Padova, C., and Lieber, C. S. (1985). First pass metabolism of ethanol—a gastrointestinal barrier against the systemic toxicity of ethanol. Life Sci. 37, 567–573.[ISI][Medline]

Kohn, M. C., and Melnick, R. L. (2000). The privileged access model of 1,3-butadiene disposition. Environ. Health Perspect.108(Suppl. 5), 911–917.

Korn, M., Gfrorer, W., Filser, J. G., and Kessler, W. (1994). Styrene-7,8-oxide in blood of workers exposed to styrene. Arch. Toxicol. 68,524–527. [published erratum appears in Arch. Toxicol. 1994, 69(1), 72][ISI][Medline]

Krishnan, K., and Andersen, M. E. (1994). Physiologically based pharmacokinetic modeling in toxicology. In Principles and Methods of Toxicology, 3rd ed. (A.W. Hayes, Ed.), pp. 149–188. Ravens Press, New York.

Löf, A., and Johanson, G. (1993). Dose-dependent kinetics of inhaled styrene in man. In Butadiene and Styrene Assessment of Health Hazards, IARC Scientific Publication No. 127 (M. Sorsa, K. Peltonen, H. Vainio, and K. Hemminki, Eds.), pp. 15–26. International Agency for Research on Cancer, Lyon.

Löf, A., Lundgren, E., and Nordqvist, M. B. (1986). Kinetics of styrene in workers from a plastics industry after controlled exposure: A comparison with subjects not previously exposed. Br. J. Ind. Med. 43, 537–543.[ISI][Medline]

Marhuenda, D., Prieto, M. J., Periago, J. F., Marti, J., Perbellini, L., and Cardona, A. (1997). Biological monitoring of styrene exposure and possible interference of acetone co-exposure. Int. Arch. Occup. Environ. Health 69, 455–460.[ISI][Medline]

McConnell, E. E., and Swenberg, J. A. (1994). Review of styrene and styrene oxide long-term animal studies. Crit. Rev. Toxicol. 24(Suppl.), S49–55.[ISI][Medline]

Medinsky, M. A., Leavens, T. L., Csanády, G. A., Gargas, M. L., Bond J. A. (1994). In vivo metabolism of butadiene by mice and rats: A comparison of physiological model predictions and experimental data. Carcinogenesis 15, 1329–1340.[Abstract]

Mendrala, A. L., Langvardt, P. W., Nitschke, K. D., Quast, J. F., and Nolan, R. J. (1993). In vitro kinetics of styrene and styrene oxide metabolism in rat, mouse, and human. Arch. Toxicol. 67, 18–27.[ISI][Medline]

Nylander-French, L. A., Kupper, L. L., and Rappaport, S. M. (1999). An investigation of factors contributing to styrene and styrene-7,8-oxide exposures in the reinforced plastics industry. Ann. Occup. Hyg. 43, 99–105.[Abstract/Free Full Text]

Oesch, F. (1973). Mammalian epoxide hydrases: Inducible enzymes catalyzing the inactivation of carcinogenic and cytotoxic metabolites derived from aromatic and olefinic compounds. Xenobiotica. 3, 305–340.[ISI][Medline]

Oesch, F., and Daly, J. (1972). Conversion of naphthalene to trans-naphthalene dihydrodiol: Evidence for the presence of a coupled aryl monooxygenase-epoxide hydrase system in hepatic microsomes. Biochem. Biophys. Res. Commun. 46, 1713–1720.[ISI][Medline]

Pastino, G. M., and Conolly, R. B. (2000). Application of a physiologically based pharmacokinetic model to estimate the bioavailibility of ethanol in male rats: Distinction between gastric and hepatic pathways of metabolic clearance. Toxicol. Sci. 55, 256–265[Abstract/Free Full Text]

Perbellini, L., Mozzo, P., Turri, P. V., Zedde, A., and Brugnone, F. (1988). Biological exposure index of styrene suggested by a physiologico-mathematical model. Int. Arch. Occup. Environ. Health 60, 187–193.[ISI][Medline]

Pfäffli, P., and Säämänen, A. (1993). The occupational scene of styrene. In Butadiene and Styrene Assessment of Health Hazards, IARC Scientific Publication No. 127 (M. Sorsa, K. Peltonen, H. Vainio, and K. Hemminki, Eds.), pp. 15–26. International Agency for Research on Cancer, Lyon.

Portier, C. J., and Kaplan, N. L. (1989). Variability of safe dose estimates when using complicated models of the carcinogenic process. A case study: Methylene chloride. Fund. Appl. Toxicol. 13, 533–544.[ISI][Medline]

Ramsey, J. C., and Andersen, M. E. (1984). A physiologically based description of the inhalation pharmacokinetics of styrene in rats and humans. Toxicol. Appl. Pharmacol. 73, 159–175.[ISI][Medline]

Ramsey, J. C., and Young, J. D. (1978). Pharmacokinetics of inhaled styrene in rats and humans. Scand. J. Work Environ. Health 4, 84–91.[ISI][Medline]

Rappaport, S. M., Ting, D., Jin, Z., Yeowell-O'Connell, K., Waidyanatha, S., and McDonald, T. (1993). Application of Raney nickel to measure adducts of styrene oxide with hemoglobin and albumin. Chem. Res. Toxicol. 6, 238–244.[ISI][Medline]

Rappaport, S. M., Yeowell-O'Connell, K., Bodell, W., Yager, J. W., and Symanski, E. (1996). An investigation of multiple biomarkers among workers exposed to styrene and styrene-7,8-oxide. Cancer Res. 56, 5410–5416.[Abstract]

Reitz, R. H., Mendrala, A. L., Corley, R. A., Quast, J. F., Gargas, M. L., Andersen, M. E., Staats, D. A., and Connolly, R. B. (1990). Estimating the risk of liver cancer associated with human exposures to chloroform using physiologically based pharmacokinetic modeling. Toxicol. Appl. Pharmacol. 105, 443–459.[ISI][Medline]

Schmidt, R. F., and Thews, G. (1977). Physiolgie des Menschen [Human Physiology]. Springer, Berlin Heidelberg, New York.

Schwegler, U. (1991). Pharmacokinetik von Styrol bei Ratte und Maus [Pharmacokinetics of Styrene in Rat and Mice]. Institut fur Toxikologie, GSF. GSF-Bericht 15/91.

Sumner, S. J., and Fennell, T. R. (1994). Review of the metabolic fate of styrene. Crit. Rev. Toxicol. 24(Suppl.), S11–S33.[ISI][Medline]

Thomas, R. S., Bigelow, P. L., Keefe, T. J., and Yang, R. S. (1996). Variability in biological exposure indices using physiologically based pharmacokinetic modeling and Monte Carlo simulation. Am. Ind. Hyg. Assoc. J. 57, 23–32.[ISI][Medline]

Tornero-Velez, R., Waidyanatha, S., Echeverria, D., and Rappaport, S. M. (2000). Measurement of styrene-7,8-oxide and other oxidation products of styrene in air. J. Environ. Monit. 2, 111–117.[ISI][Medline]

Tornero-Velez, R., Waidyanatha, S., Pérez, H. L., Osterman-Golkar, S., Echeverria, D., and Rappaport, S. M. (2001). Determination of styrene and styrene-7,8-oxide in human blood by gas chromatography-mass spectrometry. J. Chromatogr. B. Biomed. Sci. Appl. 757, 59–68.[Medline]

Vujtovic-Ockenga, N., Baur, C., Kessler, W., and Filser, J. G. (1993). Direct determination of partition coefficients tissue/blood of non-volatile substances – example: styrene-7,8-oxide. Naunyn-Schmiedeberg's Arch. Toxicol. 347(Suppl.), R6.

Wigaeus, E., Löf, A., Bjurstrom, R., and Nordqvist, M. B. (1983). Exposure to styrene. Uptake, distribution, metabolism and elimination in man. Scand. J. Work Environ. Health 9, 479–488.[ISI][Medline]

Yeowell-O'Connell, K., Jin, Z., and Rappaport, S. M. (1996). Determination of albumin and hemoglobin adducts in workers exposed to styrene and styrene oxide. Cancer Epidemiol. Biomarkers Prev. 5, 205–215.[Abstract]

Yeowell-O'Connell, K., Pauwels, W., Severi, M., Jin, Z., Walker, M. R., Rappaport, S. M., and Veulemans, H. (1997). Comparison of styrene-7,8-oxide adducts formed via reaction with cysteine, N-terminal valine and carboxylic acid residues in human, mouse and rat hemoglobin. Chem. Biol. Interact. 106, 67–85.[ISI][Medline]





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