* Department of Environmental Health Science, University of Georgia, Athens, Georgia 30602; Battelle, Pacific Northwest Division, Sequim, Washington 98382; and
ManTech Environmental Technology, Inc., Dayton, Ohio 45437
Received June 10, 2004; accepted September 7, 2004
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ABSTRACT |
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Key Words: (36) DCA; PBPK; GSTzeta; suicide inhibition; rat; mouse.
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INTRODUCTION |
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DCA has been proposed as one of the key metabolites responsible for TCE-induced liver carcinogenicity in mice (Bull, 2000). DCA causes a variety of adverse effects in humans and animals. Effects of DCA treatment in humans have been limited to transient central neuropathy, peripheral neuropathy (tingling in fingers and toes and nerve conduction changes), and metabolic changes (U.S. EPA, 2003
). Noncancer effects in animals include metabolic effects (Davis, 1990
; Evans and Stacpoole, 1982
), hepatic toxicity (Bhat et al., 1991
; Sanchez and Bull, 1990
), reproductive/developmental toxicity (Linder et al., 1997
; Smith et al., 1992
; Toth et al., 1992
), and neurotoxicity (Moser et al., 1999
). DCA has been shown to increase both incidence and multiplicity of hepatic adenomas and adenocarcinomas in male (Bull et al., 1990
; Daniel et al., 1992; DeAngelo et al., 1991
, 1999
; Ferreira-Gonzalez et al., 1995
; Herren-Freund et al., 1987
) and female B6C3F1 mice (Pereira, 1996
; Pereira and Phelps, 1996
; U.S. EPA, 1991
) and in male F344 rats (DeAngelo et al., 1996
; Richmond et al., 1995
).
Pharmacokinetic studies of DCA have been performed in mice (e.g., Gonzalez-Leon, et al., 1999; Schultz et al., 2002
), rats (e.g., Gonzalez-Leon et al., 1997
; Saghir and Schultz, 2002
) and humans (e.g., Curry et al., 1985
, 1991
). Metabolic clearance of DCA from the blood is very rapid. Pharmacokinetic differences exist between routes of DCA intake, with concentration-time profiles in rats exhibiting secondary peaks of DCA after oral gavage dosing (Saghir and Schultz, 2002
). In animal studies, only a small fraction of DCA is found in feces and urine, although the amount of DCA in urine was reported to increase with DCA dose (Lin et al., 1993
).
The primary metabolic pathway for DCA biotransformation is glutathione-dependent oxidative dechlorination of DCA to glyoxylate which is catalyzed by glutathione transferase zeta (GSTZ1-1; Tong et al., 1998b). The existence of a second metabolic pathway for DCA involving reductive dechlorination to monochloroacetic acid (MCA) has been proposed previously (Larson and Bull, 1992
; Stacpoole et al., 1990
). However, Ammini and colleagues (2003)
reported finding no appreciable MCA in the urine of GSTzeta knockout mice exposed to 50 mg/kg DCA by oral gavage. Alternatively, kinetic studies in male rats using the chiral dihaloacetates, bromochloro and fluorochloroacetate (BCA, FCA), provide indirect evidence that di-haloacetates are metabolized by an additional glutathione-dependent pathway that also forms glyoxylate (Schultz and Sylvester, 2001
). Specifically, the initial rate of metabolism for the (+) BCA stereoisomer is not altered after loss of GSTzeta activity due to prior DCA exposure, which is in contrast to () BCA and (,+) FCA (Schultz and Sylvester, 2001
). Based on the (,+) BCA data, this additional pathway was estimated to account for less than 20% of the overall metabolic clearance for di-haloacetates. Further information regarding the specific mechanism(s) for this pathway remain to be elucidated.
Interestingly, DCA has been shown to inhibit its own metabolism in rodents (Gonzalez-Leon et al., 1997, 1999
; Schultz et al., 2002
) and humans (Curry et al., 1991
). The inhibition has been proposed to occur from the formation of a covalent bond between GSH and DCA which results in a S-(
-chlorocarboxymethyl)glutathione intermediate (Anderson et al., 1999
; Wempe et al., 1999
). This intermediate then gives rise to a carbonium
sulfonium ion that either undergoes hydrolysis to form glyoxylate, regenerating GSH, or alternatively, reacts with GSTzeta to covalently modify and inactivate the enzyme (Tzeng et al., 2000
). Although prolonged exposure to DCA greatly decreases its metabolism, biotransformation is still the primary pathway for DCA elimination (Gonzalez-Leon et al., 1997
, Saghir and Schultz, 2002
), which is also consistent with the existence of a secondary metabolic pathway that is not altered by loss of GSTzeta activity.
Barton and colleagues (1999) developed a simple physiologically based pharmacokinetic (PBPK) model to describe blood concentrations of DCA in mice. This model consists of three compartments: body, liver, and gastrointestinal tract lumen and describes administration of DCA via iv injection, oral gavage, or by drinking water and includes both urinary and metabolic clearance of DCA. Suicide inhibition of DCA metabolism was described empirically as a function of ingestion rates. The transition from uninhibited to inhibited metabolism was assumed to occur linearly over the drinking water concentrations of 0.021.0 g/l given for 2 weeks. In this model, metabolism was not completely inhibited at concentrations >1 g/l.
Abbas and Fisher (1997) included a submodel of DCA in their PBPK model describing TCE absorption, metabolism, and clearance in mice. The DCA submodel tracks metabolically derived DCA in lung, body, kidney, and liver compartments. The sole source of clearance of DCA was metabolism, which was described as a saturable process.
The Abbas and Fisher DCA PBPK model did not include a description of suicide inhibition of DCA. The empirical nature of the description of suicide inhibition in the Barton PBPK model limits its predictive ability under conditions of partial inhibition, varying drinking water scenarios (e.g., longer or shorter exposure period, longer or shorter recovery time), or other routes of exposure (e.g., suicide inhibition due to prior bolus exposure).
Our objective was to develop a PBPK model for DCA in rodents, which includes a description for inactivation of GSTzeta and corresponding inhibition in the metabolism of DCA. Our approach was to utilize the wide array of both published and unpublished pharmacokinetic datasets for DCA focusing exclusively on intravenous and drinking water routes of exposure. Sole use of intravenous pharmacokinetic studies obviates the need for more complicated descriptions of DCA oral absorption kinetics and is more applicable to describing metabolically derived DCA from TCE. Drinking water is the major route of exposure for the general population to DCA (U.S. EPA, 2003). Eventually the DCA PBPK model is expected to help address the risks posed from ingestion of DCA-contaminated drinking water directly or from ingestion of chlorinated solvents that are expected to be metabolized to DCA, such as TCE.
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MATERIALS AND METHODS |
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Chemicals. All chemicals were purchased from Sigma Chemical Company.
Drinking water experiment. Treated mice (n = 5/time point) were provided with drinking water containing 2.0 g/l of DCA neutralized to pH 7 with NaOH. Treated rats (n = 5/time point) were provided with drinking water containing 0.5 or 2.0 g/l DCA neutralized to pH 7 with NaOH. An aliquot of drinking water solution was sampled daily for the duration of the study and analyzed by gas chromatography to assure the DCA solution was stable and the concentration was accurate. Control rats and mice were placed on drinking water containing sodium chloride at the same concentration as the dose group. Drinking water was contained in 60-ml glass tubes capped with rubber stoppers. The drinking tubes were designed to prevent dripping. Water bottles were weighed daily to determine volume of consumption and dose rate. After 14 days rats and mice were taken off treated drinking water and given pure water. Mice were sacrificed by CO2 asphyxiation at various time points up to 8 h posttreatment. Blood samples were collected via the inferior vena cava, and whole livers were removed. Rats were bled at various time points up to 8 h via the lateral tail vein using a 25 gauge needle fitted with a heparinized capillary tube. At the last time point the rats were sacrificed by CO2 inhalation, and blood and liver were collected as with the mice. For all animals a 200-mg portion of the liver was removed for analysis.
Sample preparation and analysis. 200 ml of blood were mixed with 20% lead acetate to quench any metabolism (Ketcha et al., 1996) and stored at 80°C. 200-mg liver samples were quenched with 400 ml of 20% lead acetate, homogenized, and stored at 80°C pending analysis. DCA was esterified and quantified in the blood and liver samples by gas chromatography (Abbas and Fisher, 1997
).
IV dosing experiment. DCA blood concentrations following an iv dose of 5 mg/kg DCA to pretreated (2 g/l DCA in drinking water for 14 days) mice were collected and used for validation of the mouse model. Methodological details are the same as described by Schultz and colleagues (2002).
Published data. Original data files (Saghir and Schultz, 2002) containing DCA plasma time courses in naïve and DCA pretreated (0.2 g/l in drinking water for 7 days) rats following iv exposures ranging from 0.05 to 20 mg/kg were used for rat model development and validation. Plasma and whole blood were used interchangeably based on the blood/plasma ratio of 0.91 for DCA measured by Schultz and colleagues for rats (1999)
. A time course for inhibition of immunoreactive GSTzeta protein levels in rats following ip exposure to 38.7 mg/kg DCA was digitized from the work of Anderson and colleagues (1999)
using Grab It! (Datatrend Software, Raleigh, NC) and used in model development.
Original data files for DCA whole blood time courses in naïve and pretreated (2 g/l DCA in drinking water for 14 days) mice following iv exposures to 20 mg/kg and recovery times ranging from 6 to 48 h collected by Schultz and colleagues (2002) were utilized for mouse model development. DCA blood and liver time courses in mice during exposure to DCA at 2 g/l in drinking water measured by Schultz and colleagues (2002)
were also used. Please consult the respective original publications for methodological details.
Statistics. Daily water consumption amounts were analyzed for statistical differences using Analysis of Variance (ANOVA), = 0.05. Multiple comparisons were performed using least significant differences,
= 0.05.
PBPK model structure. Figure 1 shows the basic structure of the DCA PBPK model. DCA was tracked in kidney, liver, slowly and rapidly perfused tissues. Rapid venous equilibration was assumed for the blood. IV dosing was described as an infusion directly into the blood. Ingested DCA in drinking water was delivered directly into the liver. The rate of DCA dosing (mg/h) was calculated by multiplying the drinking water concentration (mg/ml) by the rate of drinking water consumption (ml/h). The time-dependent rate of drinking water consumption was calculated by multiplying the average daily water consumption (ml) by the time-dependent percentage of total daily water intake (1/h). Water intake percentages in 0.5-h intervals were obtained for female B6C3F1 mice from the publication of Yuan (1993). Scaled average daily water consumption rates in Table 1 were calculated by dividing measured average daily water consumption levels presented in Table 2 by the
power of mean body weights. Since average daily water consumption was significantly reduced in rats exposed to 2 g/l and 0.5 g/l DCA in drinking water, dose-specific water consumption values were used to calculate a scaled average daily water consumption rate for rats exposed to 0.5 or 2 g/l DCA. DCA clearance was attributed solely to metabolism. Urinary clearance of DCA was not included, as only a small fraction (0.6%) of total DCA doses has been measured unchanged in the urine following a 28.2 mg/kg oral gavage dose of DCA (Lin et al., 1993
). However, for modeling higher doses of DCA, urinary excretion should be considered.
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Fixed model parameters. Species-specific physiological parameters, displayed in Table 1, were taken from the work of Brown and colleagues (1997) with the exception of cardiac output rates, which were taken from the work of Arms and Travis (1988)
. DCA tissue:blood partition coefficients as determined by Abbas and Fisher (1997)
in B6C3F1 mice and shown in Table 3 were used for both mice and rats. The Michaelis-Menten affinity constant, Km, was set to the value estimated by Tong and colleagues (1998a)
using an in vitro liver cytosol enzyme-activity assay. The degradation rate was set at a value estimated previously from an immunoreactive GSTzeta protein time course following iv exposure to 38.7 mg/kg DCA (Anderson et al., 1999
).
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IV plasma time courses in naïve rats (Saghir and Schultz, 2002) following administration of 1, 5, or 20 mg/kg DCA were used to estimate an intermediate Vmax0C value. The first-order inhibition rate, kd, was set to an initial estimate of 2.1 h1, and the linear metabolism rate, kfc, was set to 0 h1 for alternative model 1. The first-order inhibition rate, kd, was then fit to an immunoreactive GSTzeta protein time course (Anderson et al., 1999
) with Vmax0C at its intermediate estimated value and kfc set again to 0 h1. Next, DCA blood time courses following 14 days exposure to 0.5 and 2 g/l DCA in drinking water were simulated with both Vmax0C and kd set to their intermediate estimated values. Finally, a global parameter estimation, using intermediate estimates and alternate values as starting values, was run fitting both Vmax0C and kd to all datasets (three naïve rat time courses, one protein time course, two drinking water time courses). Parameters were estimated by maximum likelihood techniques as implemented in ACSL Math V 2.5.4 (Aegis Technologies, Huntsville, AL). The Nelder-Mead algorithm was used for likelihood estimation. The error model was set to relative. Three different starting values that resulted in successful conclusion of the Nelder-Mead algorithm were used for each adjustable parameter.
For alternative model 2, the same parameter estimation methodology was performed, with the exception that the linear metabolism rate, kfc, was set to an initial estimate of 1.05 h1 during stepwise estimation of Vmax0C and kfc. The linear metabolism rate, kfc, was fit to the DCA blood time course following 14 days exposure to 0.5 and 2 g/l DCA in drinking water with both Vmax0C and kfc set to their intermediate estimated values. A global parameter estimation, using intermediate estimates and alternate values as starting values, was run fitting all three estimated model parameters (Vmax0C, kd, kfC) to all datasets (three naïve rat time courses, one protein time course, two drinking water time courses).
A similar fitting methodology was attempted to parameterize the mouse PBPK model. Instabilities in the log-likelihood function used for parameter estimation in ACSL Math resulted in parameter estimates that were irreproducible from varying starting values and judged inferior to initial and final visual estimates. Thus, the decision was made to fit parameters visually rather than by maximum likelihood. For models 1 and 2, a value for Vmax0C was fit to an iv time course in naïve mice (20 mg/kg DCA) (Schultz et al., 2002). The degradation rate for the mouse was kept at the value estimated from the rat GSTzeta protein time course. The first-order inhibition rate was then fit to time courses in mice following administration of 20 mg/kg with recovery times ranging from 6 to 48 h (Schultz et al., 2002
). Subsequently, for model 2, the linear metabolism rate, kfc, was fit to the DCA blood time courses from mice previously treated for 14 days with 2 g/l DCA in drinking water that either continued (Schultz, et al., 2002
) or were removed from treatment (current study). This parameter estimation sequence was iterated until no further adjustment of model parameters was deemed necessary for visual agreement (two iterations total). Final parameter estimates for metabolic and suicide inhibition parameters are given in Table 4.
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Validation. The calibrated rat model, was used to simulate blood concentrations of DCA in pretreated (0.2 g/l in drinking water for 7 days) rats following iv doses ranging from 0.05 to 20 mg/kg DCA and compared to time courses collected by Saghir and Schultz (2002) for validation purposes. The mouse model, using visual parameter estimates, was used to simulate blood concentrations of DCA in pretreated (2 g/l in drinking water for 14 days) mice following an iv dose of 5 mg/kg DCA and compared to measured values from the current study for validation.
The PBPK model was also used to simulate liver concentrations in rats after 14 days exposure to 0.5 or 2 g/l DCA and in mice after 14 days exposure to 2 g/l DCA. The simulation results were compared to measured time points from the current study to assess the model's ability to predict liver concentrations in addition to blood.
Simulations of the rates of metabolism for both metabolic pathways. The PBPK model was used to simulate 14 days pretreatment with drinking water at 0, 0.2, or 2 g/l in rats or mice with a 16-h recovery period followed by a 20-mg/kg iv dose. Control water consumption rates were assumed for both rats and mice at the DCA 0.2 g/l concentration, and for mice at the 2 g/l DCA concentration. The calculated 2 g/l DCA water consumption rate based on the current study was used for rats at the 2 g/l concentration to reflect the decreased drinking behavior of rats at this concentration. The total rate of metabolism and the individual rates of metabolism for the GSTzeta-mediated (pathway 1) and secondary, nonspecific (pathway 2) pathways were recorded at 3 min post iv exposure. Simulations predicted that 3 min post exposure was when the rate of total metabolism was at a maximum.
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RESULTS |
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Model Discrimination
Alternative model 2 which included a secondary metabolic pathway gave a significantly better description of DCA plasma levels and percent reduction in GSTzeta protein levels in the rat (p < 0.005). Parameter estimates for Vmax0C (79.1 mg/h/kg3/4, model 1; 77.5 mg/h/kg3/4, model 2) and kd (2.45 1/h, model 1; 2.48 1/h, model 2) were quite similar Alternative model 2 clearly improved descriptions of DCA plasma levels in rats following DCA treatment in drinking water. Accumulation of DCA in the plasma was predicted by alternative model 1, which was not seen in the data. Accumulation of DCA in the plasma was also predicted by alternative model 1 in the mouse, but this was also not seen in the data. All subsequent results discussed and presented below are from alternative model 2, with secondary DCA metabolism unless otherwise noted.
Rat IV Simulations
Initial calibration of the initial maximal rate for GSTzeta-mediated metabolism, Vmax0C, to naïve rat plasma data shown in Figure 2 yielded an estimate of 71.7 mg/h/kg0.75. For this concentration range (1 to 20 mg/kg), simulations were insensitive to subsequent changes in the inhibition constant, kd. The final parameter estimate, 77.5 mg/h/kg0.75, obtained from simultaneous estimation of Vmax0C, kd, and kfC was somewhat larger than the initial estimate given above. Plasma DCA concentrations were well predicted following iv administration of 20 mg/kg as shown in Figure 2. However, clearance of DCA from the plasma at the 1- and 5-mg/kg doses was underpredicted.
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DISCUSSION |
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A second proposed metabolic pathway representing a nonspecific pathway of DCA metabolism was necessary to successfully predict DCA blood levels following 14 days of treatment with DCA in drinking water in both rats and mice. Without this second metabolic pathway, the model predicted accumulation of DCA in the blood, which was not observed. A linear description of the proposed nonspecific pathway was successful, using a first-order rate constant. The first-order rate constant was similar between species (1.15 for rat vs. 1.0 for mouse). This metabolic pathway may represent reductive chlorination of DCA to MCA or metabolic clearance by an additional GST isozyme. The results of this work show that the secondary metabolic pathway for DCA is important quantitatively in subchronic dosing scenarios such as the ones shown in Figures 4a and 8a. Under suicide inhibition conditions, the relative percentage of metabolism attributed to the secondary pathway increases, and this pathway becomes more critical for DCA elimination. The presence and further identification of this secondary pathway should be validated by further kinetic studies in the recently developed GSTzeta knockout rodents (Fernandez-Canon et al., 2002).
A degradation rate (and by calculation, a corresponding zero-order resynthesis rate) for GSTzeta estimated from GSTzeta rat protein levels was used in this PBPK model assuming the rate of maximal GSTzeta-mediated metabolism (Vmax) was proportional to the amount of GSTzeta enzyme present. The same rate was assumed in mice and worked adequately enough that major species differences in GSTzeta degradation would not be expected from the results of this modeling exercise. The fact that independently derived degradation rate and Km values were able to adequately predict DCA blood levels in rats and mice lends confidence to the mechanistic nature of this PBPK model. An extensive database (five iv time courses with concentrations ranging from 0.05 to 20 mg/kg in pretreated rats) was available to validate the rat PBPK model blood concentration predictions with adequate success. There were less data available (one 5-mg/kg iv time course in pretreated mice) to validate the mouse model and, thus, less ability to judge the quality of the mouse PBPK model predictions.
More research is needed currently to understand the metabolic pathway and the stoichiometric yield of DCA from the metabolism of TCE. Substantial uncertainty exists in DCA blood levels following TCE dosing because of artifactual production of DCA (Ketcha et al., 1996). Once such research is completed, the PBPK model developed in this work could be incorporated into an existing TCE model (e.g., Clewell et al., 2000
; Fisher, 2000
) to more accurately predict DCA blood and liver dosimetry as a function of TCE dose. Reducing uncertainty in DCA dosimetry predictions would aid in reducing uncertainty in TCE dose-response assessment calculations (e.g., liver cancer slope factor), which are based, in part, on PBPK-model predicted dose metrics. A refined DCA submodel would also help elucidate the contribution of DCA to TCE-induced liver carcinogenicity in mice. Such reductions in uncertainty would have implications for TCE human health risk assessment.
Likewise, the ability of this DCA PBPK model to accommodate different drinking water exposure scenarios (length of exposure, concentration in drinking water, time of recovery) will increase its accuracy and ability to predict DCA blood and liver dose metrics that could be utilized in DCA risk assessment. Chronic treatment of B6C3F1 mice with high concentrations of DCA in drinking water (2 g/l and higher) have resulted in hepatocellular carcinoma in approximately 95% of the animals (Bull et al., 1990; DeAngelo et al., 1991
). Chronic treatment of male F344 rats with a mean daily concentration of 1.6 g/l DCA resulted in a 21% hepatocellular carcinoma incidence (DeAngelo et al., 1996
). Pharmacokinetic studies following 14 days treatment with concentrations of DCA of 2 g/l in drinking water in mice (Schultz et al., 2002
) and rats (Gonzalez-Leon et al., 1997
) show that the rate of DCA GSTzeta metabolism is significantly inhibited in both species. Thus, suicide inhibition of DCA GSTzeta-mediated metabolism is occurring at dose levels that clearly increase cancer incidence in rodent cancer bioassays. If the suicide inhibition occurs for the entire 2 years of the bioassay is an open question, since a recent study has shown age may act as a modifying factor, allowing older animals to adapt and be less sensitive to DCA suicide inhibition (Schultz et al., 2002
). If DCA is the proximate carcinogen, suicide inhibition would increase DCA potency. Alternatively, if a metabolite of DCA is the bad actor, suicide inhibition of DCA would be protective. More mechanistic information is necessary to fully understand the implications of suicide inhibition for DCA cancer risk assessment.
The current cancer dose response assessment for DCA included in the toxicological review of DCA (U.S. EPA, 2003) utilizes human equivalent doses derived from the doses used in animal bioassays. Once developed and validated, a human PBPK model for DCA could be used along with the rodent DCA model in this current work to predict internal dose metrics and refine the animal-to-human extrapolation in the DCA dose-response assessment in a similar manner to the draft 2001 TCE risk assessment (U.S. EPA, 2001
). In addition, a PBPK model for DCA including suicide inhibition will allow for the inclusion of polymorphism-specific enzyme inhibition constants. Observed differences in polymorphic variants of recombinant human GSTZ-1 may indicate that subpopulations may differ in their susceptibility to toxicity from DCA (Tzeng et al., 2000
). The development of our PBPK model utilized data collected from three laboratories (Wright Patterson Air Force Base, Batelle Northwest, and University of Rochester Medical Center) which lends confidence to its future application in both cancer and noncancer DCA dose-response assessment.
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ACKNOWLEDGMENTS |
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NOTES |
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1 To whom correspondence should be addressed at Environmental Health Science, University of Georgia, Athens, GA 30601. E-mail: dkeys{at}uga.edu.
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