ToxSci Advance Access originally published online on June 26, 2006
Toxicological Sciences 2006 93(1):22-33; doi:10.1093/toxsci/kfl048
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A Dermatotoxicokinetic Model of Human Exposures to Jet Fuel
* Department of Environmental Sciences and Engineering, School of Public Health, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7431; and CIIT Centers for Health Research, 6 Davis Drive PO Box 12137, Research Triangle Park, North Carolina 27709-2137
1 To whom correspondence should be addressed at Department of Environmental Sciences and Engineering, School of Public Health, The University of North Carolina at Chapel Hill, CB#7431, Rosenau Hall, Chapel Hill, NC 27599-7431. Fax: (919) 966-4711. E-mail: leena_french{at}unc.edu.
Received February 27, 2006; accepted June 16, 2006
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ABSTRACT |
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Workers, both in the military and the commercial airline industry, are exposed to jet fuel by inhalation and dermal contact. We present a dermatotoxicokinetic (DTK) model that quantifies the absorption, distribution, and elimination of aromatic and aliphatic components of jet fuel following dermal exposures in humans. Kinetic data were obtained from 10 healthy volunteers following a single dose of JP-8 to the forearm over a surface area of 20 cm2. Blood samples were taken before exposure (t = 0 h), after exposure (t = 0.5 h), and every 0.5 h for up to 3.5 h postexposure. The DTK model that best fit the data included five compartments: (1) surface, (2) stratum corneum (SC), (3) viable epidermis, (4) blood, and (5) storage. The DTK model was used to predict blood concentrations of the components of JP-8 based on dermal-exposure measurements made in occupational-exposure settings in order to better understand the toxicokinetic behavior of these compounds. Monte Carlo simulations of dermal exposure and cumulative internal dose demonstrated no overlap among the low-, medium-, and high-exposure groups. The DTK model provides a quantitative understanding of the relationship between the mass of JP-8 components in the SC and the concentrations of each component in the systemic circulation. The model may be used for the development of a toxicokinetic modeling strategy for multiroute exposure to jet fuel.
Key Words: toxicokinetics; skin; exposure assessment; jet fuel.
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INTRODUCTION |
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Occupational and environmental exposure assessments aim to quantify the uptake of toxicants through inhalation, ingestion, and dermal contact with environmental media (e.g., air, soil and water). Historically, inhalation exposures have been the focus of occupational and environmental exposure assessments. For the most part, the dermal route has been neglected, making it difficult to assess its importance relative to other routes of exposure. However, recent interest in dermal-exposure assessment has been spawned by the decreasing trend in inhalation exposures to chemicals in the workplace (Sartorelli, 2002
; Schneider et al., 2000; Semple, 2004, 2005).
Current dermal-exposure assessment strategies make use of three types of sampling techniques based on surrogate skin, removal, or a fluorescent tracer (Fenske, 1993; Nylander-French, 2003). One of the more promising methods is tape stripping (a removal technique), which is a method of direct sampling of the amount of chemical in the stratum corneum (SC) (Chao and Nylander-French, 2004; Chao et al., 2005, 2006; Fent et al., in press; Jacobi et al., 2005; Loffler et al., 2004; Nylander-French, 2000; Surber et al., 1999). In tape stripping, part of the SC is removed, and the rate and extent of dermal absorption are quantified. Tape stripping has been used in bioequivalence studies of therapeutic agents (Loden et al., 2004; Pershing et al., 2002, 2003; Shah, 2001) and for measurement of dermal exposures to jet fuel in occupational-exposure settings (Chao et al., 2005, 2006).
The tape-strip sampling technique has not been fully utilized partly due to inconsistent standardization. For example, normalization of the mass measured on tape strips is done by dividing the mass of chemicals on tape strips by the mass of SC removed (Reddy et al., 2002) or by the keratin content (Chao and Nylander-French, 2004). A standard approach for reporting tape-strip data has not yet been determined. In addition, there is insufficient evidence that tape-strip measurements from different exposure groups (i.e., high-, medium-, or low-exposure) are representative of exposure based on internal dose metrics (e.g., blood and urine biological markers of exposure). A paucity of studies has demonstrated an association between dermal exposures measured using the tape-strip technique and urinary metabolite levels in humans under occupational-exposure conditions (Chao et al., 2006). Clearly, demonstrating that tape-strip measurements are predictive of internal dose can be of benefit to exposure assessment and epidemiological studies.
Quantification of the absorption, distribution, metabolism, and elimination of chemicals that may come into contact with the skin is necessary for assessing exposures and human health risks associated with occupational and environmental exposures to chemicals. Membrane models have been used to quantify the toxicokinetic behavior of chemicals in the skin (Pirot et al., 1997; Reddy et al., 2002). In membrane models, skin is defined as a homogeneous membrane and the diffusion across the membrane is described by Fick's first law of diffusion:
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C is the concentration gradient across the membrane (mg/cm3), and x is the thickness of the membrane (cm). The main limitation to applying membrane models to tape-strip sampled data is that the mathematical treatment is difficult and cumbersome. Alternatively, compartmental models have been developed to quantitatively describe the dermal absorption and penetration of chemicals (Guy et al., 1985; Qiao et al., 2000; Shatkin and Brown, 1991; Williams and Riviere, 1995). However, a compartmental model of the skin that relates tape-strip measurements to internal dose has not been reported. Though compartmental models are not strictly representative of a complex tissue like the skin, they are easier to implement and can be combined with systemic compartmental models for quantitation of the toxicokinetic behavior of chemicals following dermal exposures.
In this study, we evaluated four different compartmental models of dermal exposure to jet-propulsion fuel 8 (JP-8). JP-8 is a fuel used extensively in military vehicles by member states of North Atlantic Treaty Organization (Subcommittee on Jet-Propulsion Fuel 8, 2003). Dermal exposure to JP-8, which occurs occupationally, may contribute to systemic levels of various aromatic and aliphatic hydrocarbon components (Chao et al., 2006; Serdar et al., 2004). We report in this paper data-based compartmental models (Andersen, 1991). Data-based compartmental models of the skin have been referred to as dermatotoxicokinetic (DTK) models (Qiao et al., 2000); for consistency, we will also use this terminology. The main objective of our study was to construct a DTK model of the skin that quantitatively characterized and predicted the toxicokinetic behavior of JP-8 following controlled dermal exposures. A second objective was to examine the effect of dermal-exposure variability on blood concentrations of JP-8 components. The following chemical components of JP-8 were examined: naphthalene, 1-methyl naphthalene, 2-methyl naphthalene, decane, undecane, and dodecane. Data derived from tape-strip and blood samples that were collected from dermal only–exposed human volunteers provided the basis for model development. The results of our study are intended to facilitate the development of a modeling strategy for multiroute exposures to jet fuel.
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METHODS |
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Study population.
Ten volunteers (five females and five males) were recruited for this study. The average age of the volunteers was 27 years and ranged from 21 to 37 years. Seven of the study volunteers were Caucasian. One volunteer was Asian, another African American, and one was mixed Asian-Caucasian. The mean body mass index (BMI) was 21 kg/m2 and ranged from 19 to 25 kg/m2. Approval for this study was obtained from the Institutional Review Board on research involving human subjects (School of Public Health, The University of North Carolina at Chapel Hill, Chapel Hill, NC).
Experimental design.
Exposures were conducted inside a chamber with dimensions of 20.3 cm width x 20.3 cm length x 18.8 cm height and a total volume of 7706 cm3 (Chao and Nylander-French, 2004). The volunteer's forearm was placed palm up inside the exposure chamber, and two aluminum application wells (dimensions 2.5 x 4.0 cm = 10 cm2 per well) were pressed against the skin to prevent JP-8 from spreading during the experiment. A total of 1 ml of neat JP-8 was applied to the skin. The exposure chamber was sealed for the duration of the 0.5-h experiment. Immediately at the end of the 0.5-h exposure period, the exposure sites were wiped with gauze pads and tape stripped 10 times. Tape strips were placed in 10 ml of acetone containing 1 µg/ml of internal standards (naphthalene-d8 and dodecane-d26). All tape-strip samples were stored in 20 ml vials (I-CHEM, Rockwood, TN) and refrigerated at 4°C. Blood samples were drawn from the unexposed arm prior to exposure and at the following time intervals: 0.5, 1, 1.5, 2, 2.5, 3, and 3.5 h. All blood samples were collected in 6-ml test tubes containing sodium heparin (Vacutainer, Franklin Lakes, NJ). The blood samples were transferred to 15-ml centrifuge tubes (Fisherbrand, Pittsburgh, PA) and stored at – 80°C.
Chemical analysis.
Tape-strip samples were analyzed by gas chromatography-mass spectrometry (GC-MS). A Thermoquest Trace GC (Thermo Electron Corporation, Austin, TX), coupled with a Combi Pal autosampler (CTC Analytics, Zwingen, Switzerland) and a Finnigan Polaris Q quadrupole ion trap MS (Thermo Electron Corporation) in electron ionization mode, was used for the chemical analysis. Separation of the sample was done with a fused-silica capillary column, 30 m x 0.25 mm internal diameter, coated with a mixture of 5% diphenyl:95% dimethyl polysiloxane (0.25 µm film thickness, RTX-5MS, Restek Corporation, Bellefonte, PA). The oven temperature for analysis was 40°C (3 min) and then 10°C/min to 290°C (10 min). One microliter of sample was directly injected into the inlet. The ions used for quantitation were m/z 128 (naphthalene), 142 (1-methyl and 2-methyl naphthalene), 136 (naphthalene-d8), 71 (n-decane, n-undecane, n-dodecane), and 98 (dodecane-d26).
Blood samples were analyzed using headspace solid-phase microextraction (HS-SPME) and the GC-MS system described above. Modifications were made from published methods (Cardinali et al., 2000; Waidyanatha et al., 2003). Whole blood (1 ml) was added to a 10-ml headspace vial with crimp top (Microliter, Suwanee, GA) containing 3 ml of deionized water, 2 g of NaCl, and 2 µl of 5 ng/ml internal standard. The vial was put into an aluminum block (CTC Analytics) for heating and agitating at 45°C and 250 rpm. After 1 min, a 100-µm polydimethyl-siloxane fiber (Supelco, Bellefonte, PA) was inserted into the headspace of the vial for 20 min. The needle containing the SPME fiber was withdrawn and introduced into the inlet of the GC for 20 min. The inlet temperature was set at 230°C; an IceBlue septum (Restek Corporation) and a liner with an internal diameter of 0.8 mm (SGE, Austin, TX) were used. All other GC-MS conditions were the same as for the analysis of tape-strip samples.
Basic DTK model.
The initial DTK model (model A) developed to study the disposition of naphthalene, 1-methyl naphthalene, 2-methyl naphthalene, decane, undecane, and dodecane is linear with four first-order rate constants and three compartments representing the surface, skin, and blood (Fig. 1). AsclXtreme version 2.0 (Aegis Technologies Group, Inc, Austin, TX) simulation software was used to implement the DTK models (model code is available upon request). Diffusion of chemicals across the SC is described by kds. Thus, the rate of input to skin is kds times the amount (defined as DERMDOSE, i.e., the first tape strip) of the chemical applied to the skin:
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Refined DTK model.
The epidermis, which performs much of the barrier function of skin, is made up of two very different layers: the SC and the viable epidermis (VE). The SC consists of dead cells in a hydrophobic environment. The VE, on the other hand, is comprised of viable cells in a hydrophilic environment. Based on these biological differences between the two main protective layers of the skin, two-compartment DTK models are more realistic and may predict the toxicokinetic behavior of chemicals better than one-compartment models. This has been demonstrated successfully for dermal exposures to chemicals in an aqueous vehicle (Shatkin and Brown, 1991
). In our study, two different two-compartment DTK models were evaluated (models C and D). In both models, the diffusion of chemicals across the SC is described by kds and diffusion across the VE is described by kdv.
Furthermore, because the aromatic and aliphatic hydrocarbons naphthalene, 1-methyl naphthalene, 2-methyl naphthalene, decane, undecane, and dodecane are lipophilic compounds, they are stored in fat tissues throughout the body. The fat:blood partition coefficient for naphthalene is 160 (Willems et al., 2001) and 25 for decane (Perleberg et al., 2004). To account for the storage of JP-8 components in fat, the basic DTK model was altered to include a storage compartment, and distribution rate constant k2 and redistribution rate constant k–2.
Toxicokinetic analysis.
Prior to the calibration process, the structural identifiability of the DTK models was investigated. For a model to be structurally identifiable from the data, the associated input-output parameters must differ (condition I), and the data should contain sufficient information to uniquely determine the parameters (condition II) (Slob et al., 1997). It is important to distinguish between two types of identifiability problems: "structural" and "statistical." Statistical identifiability problems are related to the variance of observations or parameter estimates, and the availability of sufficient information to distinguish between alternative model structures (Rothenberg, 1971; Slob et al., 1997). In our study, both the conditions for structural identifiability were fulfilled. We discuss the statistical analysis of the DTK models later (see the "Results" section).
The DTK models were formulated by simultaneously fitting the tape-strip and blood data. Parameters were optimized using the method of simulated annealing (Xcellon, 2004). Simulated annealing is a global optimization method that optimizes parameter values without being influenced by the initial values. This optimization protocol is more robust than others (Goffe et al., 1994; Levitt, 2002). However, a major limitation of simulated annealing is the longer computational time required. Default settings for simulated annealing were adjusted to include fewer maximum iterations (1000) and maximum function evaluations (5000) in order to decrease the optimization time. Initial values of rate constants (Table 1) were obtained by averaging literature values (Guy et al., 1985; Qiao et al., 2000; Williams and Riviere, 1995). The first tape strip from each study participant was treated as the dose to the skin (Table 2). The baseline blood concentration was not subtracted from blood concentrations at t > 0 min but included as the initial concentration in the blood compartment because the volunteers may have had recent environmental exposures to similar compounds. Consequently, baseline concentration may not be an accurate (or true) baseline since the baseline concentrations may decrease after breathing cleaner indoor air. The volume of blood (Vb) was estimated using allometric relationships (Davies and Morris, 1993). The equation is Vb = 72.447 x (body weight in kg)1.007 and was used to calculate the mass of chemicals in the blood. The baseline amount in the skin (AS0) compartment was calculated using the following algebraic expression:
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Simulations of occupational-exposure scenarios were performed using the final model. The exposure scenarios were based on a field study that measured dermal exposures to naphthalene in the U.S. Air Force personnel (Chao et al., 2005
). Naphthalene was used as a surrogate for JP-8 exposure because it (1) is abundant in JP-8, (2) is readily absorbed into the blood, and (3) is only a minor component in confounding sources of exposure, such as cigarette smoke and gasoline exhaust (Rustemeier et al., 2002; Serdar et al., 2003). The first set of simulations involved predicting the concentration-time profile for a specific field–exposure scenario. An average whole-body (surface area = 2 m2) dermal-exposure level of 2018 ng/m2 was used as the input in the simulations. The duration of exposure was 240 min. The dose to the skin was set to that of naphthalene, and exposure concentrations for other compounds were estimated using the ratios reported in Table 2 because occupational-exposure measurements were not available for these compounds. It was assumed that the ratios remained constant over time. After the exposure was complete, the dose was set to zero and the simulation was continued for up to 8 h.
In the second set of simulations, a Monte Carlo method was used to examine the impact of exposure variability on cumulative internal dose. The geometric mean and SD of the whole-body dermal exposure to naphthalene were 344 ± 4 ng/m2 for the low-exposure group, 483 ± 4 ng/m2 for the medium-exposure group, and 4188 ± 10 ng/m2 for the high-exposure group (as published by Chao et al., 2005, 2006). These values were used to specify the input distribution functions for the simulations. The distribution of the input parameters was lognormal and the number of simulations was set at 100. The model output was cumulative internal dose (DOSEc), i.e., the area under the blood concentration-time curve. DOSEc was calculated by integrating the blood concentration for the duration of the simulation:
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RESULTS |
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Model Selection: Visual Inspection
Simulations of the blood concentrations of naphthalene, 1-methyl naphthalene, 2-methyl naphthalene, decane, undecane, and dodecane were compared to experimentally measured blood concentrations in individual study volunteers. One of the volunteers was a 23-year-old Caucasian male with a BMI of 25 kg/m2 (Fig. 2). For this individual, the maximum concentrations in blood (Cmax) occurred shortly after the end of exposure (tmax
30 min) for all chemicals but dodecane, which occurred at tmax 60 min. The peak concentrations were 0.8, 0.5, 1, 3.5, 1.8, and 3.3 ng/ml for naphthalene, 1-methyl naphthalene, 2-methyl naphthalene, decane, undecane, and dodecane, respectively. Cmax was sustained for up to t = 100 min for decane and dodecane. The blood concentrations of all chemicals at t > 0 min did not return to baseline levels. The models B and D seemed to fit naphthalene and 1-methyl naphthalene data the best; all models fit the 2-methyl naphthalene time course equally well. For the aliphatic components, no model stood out as being superior to others, although model A seemed to fit the dodecane data the best. None of the models was able to predict the sustained blood concentration observed in the dodecane time course.
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), model A (- - - -), model B (- - - -), model C (----), and model D (----).The simulations and the observed data for the individual discussed above were compared to results from a different individual (Fig. 3). This individual was a 24-year-old Caucasian female with a BMI of 22 kg/m2. The toxicokinetic behavior of aromatic and aliphatic hydrocarbons was very different for this volunteer. The peak concentrations occurred at tmax
60 min for naphthalene (Cmax = 0.3 ng/ml), at tmax 100 min for 1-methyl naphthalene (Cmax = 0.3 ng/ml), and at tmax 60 min for 2-methyl naphthalene (Cmax = 0.3 ng/ml); peak concentrations for all aliphatic components occurred at tmax 30 min. The Cmax were 2.5, 1.2, and 3.5 ng/ml for decane, undecane, and dodecane, respectively. Concentrations in blood at t > 0 min did not reach baseline levels for aromatic components; however, concentrations at t > 0 min did reach baseline concentrations for the aliphatic components. Model C seemed to fit the naphthalene data the best. Models A, C, and D fit 2-methyl naphthalene data equally well. Model fits to 1-methyl naphthalene data were poor. For the aliphatic components, model D fit most datasets; however, the peak concentration for decane was not predicted by any of the models.
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Model Selection: Statistical Evaluation
In addition to the visual inspection of model predictions and time-course data, a likelihood ratio (LR) test was used to provide a statistically rigorous comparison of the models (Andersen et al., 2001
; Pinheiro and Bates, 2000). One model is said to be nested within another model if it represents a special case of that model. If L2 is the maximum likelihood of the model with additional parameters and L1 is the maximum likelihood of the model with fewer parameters, then L2 > L1 and, correspondingly, log L2 > log L1. The LR test statistic is
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distribution with f degrees of freedom. The p value is compared to 
, which is the significance level for accepting or rejecting the null hypothesis. All models were compared to model A at a significance level of = 0.1. The degrees of freedom for the LR test is equal to the number of additional parameters in the more complex model. Thus, with the addition of a storage compartment (model B), two additional degrees of freedom were introduced (k2 and k–2). Splitting the skin compartment into the SC and VE (model C) introduced one more degree of freedom (kdv). Model D introduced three more degrees of freedom as a result of splitting the skin and addition of a storage compartment (kdv, k2, and k–2). Overall, model B resulted in significantly improved fits to the data for 15% of the study volunteers. When models C and D were compared to model A, 25 and 40% of the volunteers had improved fits, respectively. Based on both visual inspection and statistical evaluation of the models, model D was selected as the model that best fit the kinetic data. The optimized rate constants for aromatic and aliphatic components are reported in Table 3. Some interesting patterns were observed. First, kds is four orders of magnitude smaller than kdv. This was expected since the SC is the rate-limiting barrier to hydrophobic compounds, which are not as limited by the VE. Second, the distribution rate constant k2 is larger than the redistribution rate constant k–2, which is consistent with the expected properties of the storage compartment. Finally, the elimination rate constant ke is consistently larger for aliphatic components.
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Simulations of Dermal Exposures to JP-8
Simulations of whole-body dermal exposure to JP-8 were conducted to examine the toxicokinetic behavior of aromatic and aliphatic components of JP-8 under an occupational-exposure setting (Fig. 4). The parameters for the simulations were an exposure duration of 240 min (4 h) and dermal exposure to 2018 ng/m2 of naphthalene for a 70-kg person. After the end of 240-min exposure, the simulations were extended for an additional 4 h. Since dermal exposures to other components were not measured by Chao et al. (2005)
, exposures to 1-methyl naphthalene, 2-methyl naphthalene, decane, undecane, and dodecane were estimated using the ratios reported in Table 2. The model-predicted Cmax were 3.6 x 10–3, 8.6 x 10–4, 3.9 x 10–3, 4.1 x 10–3, 1.4 x 10–2, and 1.1 x 10–2 ng/ml for naphthalene, 1-methyl naphthalene, 2-methyl naphthalene, decane, undecane, and dodecane, respectively. All peak concentrations occurred at tmax = 240 min. The model predictions of blood concentration of all components drop sharply at the end of the exposure. The concentrations at the end of the simulation were 54, 58, 37, 10, 44, and 44% of Cmax for naphthalene, 1-methyl naphthalene, 2-methyl naphthalene, decane, undecane, and dodecane, respectively.
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The mass of aromatic and aliphatic components of JP-8 in the SC, VE, and storage compartments were predicted to characterize the toxicokinetic behavior of these compounds (Fig. 5). In these simulations, naphthalene and decane were treated as chemical tracers for the aromatic and aliphatic hydrocarbons, respectively. Predictions were made for the same simulation conditions as described above. The SC achieved a steady state while the other compartments did not. In the storage compartment, the ratio of mass at t = 8 h to the mass at tmax is 56 and 10% for aromatic and aliphatic components, respectively. Conversely, the ratios are 53 and 9% for aromatic and aliphatic components in the VE, respectively, and 0% for both aromatic and aliphatic components in the SC.
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Dermal-Exposure Variability
Monte Carlo methods were used to examine the distribution of the cumulative internal dose (DOSEc) for naphthalene in blood resulting from variability of dermal-exposure levels in the low-, medium-, and high-exposure groups. The input parameters were the geometric mean and SD of the whole-body dermal exposure to naphthalene. These were 344 ± 4 ng/m2 for the low-exposure group, 483 ± 4 ng/m2 for the medium-exposure group, and 4188 ± 10 ng/m2 for the high-exposure group. The distribution of the input parameters was specified as lognormal and 100 iterations were performed (Table 4). The DOSEc for naphthalene at all exposure levels was normally distributed and did not intersect one another. Further simulations of daily 4-h exposure to low, medium, and high levels of naphthalene were conducted to examine the time-course profiles of naphthalene in blood from repeated exposures (Fig. 6). The DOSEc for low, medium, and high dermal exposures are 1.6, 2.3, and 19.6 ng·h/ml, respectively.
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DISCUSSION |
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Chemicals deposited on the skin can evaporate from the surface, be absorbed into deeper layers of the skin, be metabolized, or penetrate the skin for distribution to other tissues. Understanding the fate of chemicals deposited on the skin is an important step in assessing the associated risks. Therefore, the current study presents a DTK model that can be used to quantify the toxicokinetic behavior of chemicals following dermal exposure to a complex chemical mixture such as JP-8. This study represents the first attempt at deriving toxicokinetic parameters for dermal exposure to JP-8 in humans.
In the current study, a two-compartment model of the skin was developed and parameters were estimated for dermal exposure to JP-8. We found that, although one-compartment models fit some observed data, a two-compartment model of the skin fits the data better from all 10 study volunteers. In our model, the two compartments represented the SC and VE, thus representing a more biologically plausible description of the skin. Physiologically, the SC and VE have very different physical characteristics, primarily in that the SC is "hydrophobic" and the VE is "hydrophilic." Our estimates of kds and kdv supported the two-compartment model structure; kds was four orders of magnitude less than kdv, suggesting slower diffusion across the SC than the VE. This was not unexpected since the SC is rate limiting for the absorption and penetration of chemicals (Madison, 2003; Marks, 2004; Pirot et al., 1997). It is well known that chemicals move across the SC by passive diffusion (Frasch and Barbero, 2003; Mitragotri, 2003). The VE, however, consists of an aqueous medium with proteins that may be involved in metabolism and active transport of compounds into the systemic circulation (Boderke et al., 2000; Poet and McDougal, 2002). The rate of transport across the VE is dependent on blood flow, interstitial fluid movement, and interaction with VE constituents. Conversely, the rate of transport of a chemical across the SC is dependent on the thickness of the SC, the concentration gradient, level of hydration, and the diffusion coefficient of the compound.
The penetration of chemicals across the skin is quantified experimentally using diffusion cells and dermatomed skin from rats and pigs (McDougal and Robinson, 2002; McDougal et al., 2000; Muhammad et al., 2004; Sartorelli et al., 1998). In these experiments, the chemical of interest is applied to a donor cell and the receptor compartment is sampled at different time points. The cumulative quantity of chemical collected in the receptor compartment is plotted as a function of time. The flux value is obtained from the slope of the cumulative amount of chemical permeated versus time plot. Apparent permeability coefficient (Kp) values are computed using the following expression (McDougal and Boeniger, 2002):
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; McDougal and Boeniger, 2002); however, steady-state conditions are difficult to maintain experimentally in vivo. Previously, the flux value was estimated for aromatic and aliphatic components of JP-8 in humans from the slope of the linear portion of the cumulative mass of chemical in blood per cm2 versus time curve (Kim et al., 2006). Kp was calculated using Equation 10. This calculation did not take into account the mass in the storage compartment and the mass cleared from the blood, and therefore, underestimated Kp. Here, we have revised our previous estimates of Kp using the final DTK model. The rate of input in this DTK model is equivalent to the product of the permeability coefficient (Kp), the exposed surface area (Aexp), and the concentration of the chemical component in JP-8 (CJP-8) (McCarley and Bunge, 2001; McDougal and Boeniger, 2002):
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) but within an order of magnitude reported by McDougal et al. (2000). These results suggest that reasonable estimates of the permeability coefficient for aromatic and aliphatic components of JP-8 may be derived from in vitro studies.
Although the SC is the main rate-limiting barrier for the penetration of chemicals, the VE can also affect the amount of parent compound available for systemic circulation by binding chemicals and/or metabolizing them to a more water-soluble form. Although we did not include skin metabolism in our DTK model, we quantified the disposition of chemicals in the VE. The net rate of change of the amount of chemical in the VE depends on the rate of input from the SC, rate of efflux to the systemic circulation, and rate of input from the systemic circulation. Therefore, the MBDE for the VE is
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). The rate constant k1 has been expressed as Qve/(Vve x Pve:b) where Qve is the volumetric flow rate to the exposed area of skin, Vve is the volume of the exposed VE, and Pve:b is the solubility of the chemical in the blood relative to the VE, i.e., VE:blood partition coefficient. The rate constant k–1 is represented as Qve/Vb. Rearranging, Pve:b, therefore, equals (Vb/Vve) x (k–1/k1). Using this expression, we estimated an average Pve:b of 5 for aromatic compounds and 3 for aliphatic compounds. The Pve:b for 1-methyl naphthalene was larger than other components of JP-8; when this value was taken out of the calculation, we estimated a Pve:b of 3 also for the aromatic components. Our estimates were similar to Pve:b reported in the literature. McCarley and Bunge (2001) calculated Pve:b using the following equation:
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Another major change we made to the basic DTK model was the addition of a storage compartment. JP-8 is a mixture of lipophilic hydrocarbons, which suggests that sequestration of JP-8 components in fat stores throughout the body is very likely. Chemicals are stored in fat by dissolution in neutral fats, which constitute from about 20 to 50% of the body weight (Klaassen, 1996). Thus, considerable amounts of chemicals with high fat:blood partition coefficients will be stored in body fat. Also, blood proteins such as albumin can bind with chemicals and serve as a depot protein. Further, the liver and kidneys both have a high capacity for metabolism and/or binding chemicals before elimination by active transport (Klaassen, 1996). Few DTK models have included a storage compartment for toxicants. Qiao et al. (2000) included a storage compartment to quantify the toxicokinetic behavior of p-nitrophenol in swine using a 12-compartment model. They found that the peripheral compartment was able to store a constant amount of p-nitrophenol, and the excess was redistributed to the systemic circulation for elimination. In our study, simulations of dermal exposure to JP-8 showed that the storage capacity of the SC compared to the VE is small, while as expected, the storage compartment is a larger reservoir for exogenous chemicals.
For the six aromatic and aliphatic components of JP-8 that we examined in our study, there are three significant clearance pathways: metabolism, exhalation, and urination. Urinary clearance is not considered to be a major pathway for chemicals with high Kow since they will be reabsorbed efficiently through the tubular cells of the nephron back into the systemic circulation. The metabolic clearance of all the components of JP-8, mostly from the liver, is not well understood. The metabolism of naphthalene, however, has been studied extensively (ATSDR, 1995; Buckpitt and Bahnson, 1986; U.S. EPA, 1998). Phase I metabolism of naphthalene involves oxygenation by the cytochrome P450 monooxygenases (e.g., CYP 1A1) to naphthalene-1,2-oxide. This epoxide rearranges to 1- or 2-naphthol or undergoes phase II reactions to form more water-soluble compounds for eventual urinary clearance. The metabolism of aliphatic components, such as dodecane, is not well understood, but they are thought to undergo oxidation reaction to a ketone through an intermediate hydroxylation step (Subcommittee on Jet-Propulsion Fuel 8, 2003). Clearance by exhalation is the other major loss mechanism for volatile organics. Pulmonary clearance can be approximated by cardiac output times the ratio 1/(1 + Pb:a) where Pb:a is the blood:air partition coefficient (Andersen et al., 2001). For naphthalene (Pb:a 571), pulmonary clearance is 0.2% of cardiac output, whereas for decane (Pb:a 5) it is 16%. The difference in pulmonary clearance between the aromatic and aliphatic components may explain the difference in estimates of ke between aliphatic (ke = 0.11 min–1) and aromatic (ke = 0.04 min–1) components of JP-8.
We constructed and evaluated a DTK model based on empirical data that may be used to quantify the absorption, distribution, and elimination of aromatic and aliphatic compounds following dermal exposure to JP-8. The final optimized model was used to simulate the time course of naphthalene and other components of JP-8 using information collected in an occupational-exposure assessment study (Chao et al., 2005, 2006). We observed that the predicted cumulative internal dose was distinctly different among low-, medium-, and high-exposure groups. This supports the hypothesis that dermal-exposure measurements made by tape-strip sampling may be used in exposure assessment studies. We note that biological variability will introduce more variation into the distribution of DOSEc. In our study, we could not examine inter- and/or intra-individual variation in the disposition of JP-8 components due to the limited sample size. Further study is required to better estimate inter- and intra-individual variability in exposure measurements for different work processes. We have described here a DTK model that will be useful for the development of a toxicokinetic modeling strategy for multiroute exposures to jet fuel in humans.
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NOTES |
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Disclaimer: The authors certify that all research involving human subjects was done under full compliance with all government policies and the Helsinki Declaration.
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ACKNOWLEDGMENTS |
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The authors express special thanks to Dr Stephen Rappaport for constructive comments and to Dr Louise Ball for allowing the use of her laboratory for the dermal-exposure study. We would also like to acknowledge Drs Suramya Waidyanatha and Joachim Pleil for their assistance with the chemical analysis. The study was funded by National Institute of Environmental Health Sciences P42-ES05948.
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REFERENCES |
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