* Battelle Pacific Northwest Division, Richland, Washington 99352; and Louisiana State University Health Sciences Center, Shreveport, Louisiana
1 To whom correspondence should be addressed at Biological Monitoring & Modeling, P.O. Box 999, MSIN P7-59, Richland, WA 99352. Fax: 5093769064. E-mail: rick.corley{at}pnl.gov.
Received October 26, 2004; accepted February 12, 2005
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ABSTRACT |
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Key Words: ethylene glycol; glycolic acid; PBPK modeling; fomepizole; ethanol; hemodialysis.
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INTRODUCTION |
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For developmental toxicity, human relevance is less clear. Although all laboratory mammals and humans appear to metabolize EG similarly, and all have the potential to form the proximate toxicant, glycolic acid, developmental toxicity has been observed only in rodent species (rats and mice). Ethylene glycol does not cause developmental toxicity in non-rodent species (i.e., rabbit), and there have not been any reported cases of human developmental effects induced by EG. In one recent review, the likelihood of developmental toxicity occurring in humans through occupational or consumer exposures was considered negligible, primarily because of the high dose rates needed to produce this effect in rodents (CERHR, 2004). The same review recommended the development of a PBPK model for rodents and humans to facilitate inter-species extrapolations and the dose-rate effects on pharmacokinetics that are critical to developmental toxicity.
In a companion study to this report (Corley et al., 2005), a physiologically based pharmacokinetic (PBPK) model for EG and its major metabolite responsible for developmental toxicity, glycolic acid, was developed for adult rats and humans. However, only limited controlled exposure studies, below levels leading to the saturation of metabolic and clearance processes, were available to confirm the ability of the model to simulate human exposures (i.e., validate the human PBPK model). Therefore, the purpose of the present study was to extend the human PBPK model developed in the companion study to include the major treatment regimens used in accidental or intentional overdosing case reports. Although human case reports are often confounded by inadequate descriptions of the total amounts of EG consumed, the presence of other chemicals or drugs, or the effect of therapeutic regimens or toxicity, many of the case reports summarized in Table 1 included relatively extensive analyses of the kinetics of EG and/or glycolic acid to confirm the diagnosis and monitor the effectiveness of the various treatments. Such data are potentially useful for model validations once the effects of various treatment regimens have been adequately accounted for.
Thus, the initial human PBPK model developed in the companion study (Corley et al., 2005) was modified to include the basic treatment regimens that have been shown to alter the kinetics of EG and glycolic acid: metabolic inhibition by ethanol or fomepizole (4-methyl pyrazole, 4-MP), and hemodialysis. The other major treatment for EG intoxication that affects kinetics, gastric lavage, was handled simplistically by reducing the total dose of EG available for absorption. Once the model was modified to include therapeutic interventions, the additional data sets summarized in Table 1 provided a unique opportunity for additional validation of the human PBPK model for high oral doses, where metabolism or clearance processes can be saturated. The resulting model can also serve as a useful tool in the evaluation of the effectiveness of various treatment regimens on the kinetics of EG and glycolic acid.
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MATERIALS AND METHODS |
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All of the human case reports listed in Table 1 were the result of intentional or accidental oral ingestion of large amounts of EG. The absorption of EG into gastrointestinal tissues of humans was described as a first-order process as was used in the rat PBPK model of Corley et al. (2005):
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As observed in the rat pharmacokinetic studies (especially fasted vs. non-fasted animals) described in the companion study (Corley et al., 2005), the oral absorption rate constant, KAS, can be affected by a number of factors including the amount of food in the stomach or other drugs or liquids consumed. Generally, a default value of KAS of 1 h1 was used in all human simulations, with occasional increases necessary to fit the peak blood concentration of EG (if the dose and time of dosing were well-documented) as noted in the Results. The values for KAS were all within the range of values (15 h1) used for fasted versus non-fasted rats (Corley et al., 2005
). For those case reports with poor documentation of the total amounts of EG consumed, the total dose was estimated by fitting the model to the initial peak blood concentration of EG. When vomiting occurred or gastric lavage was included in the treatment regimen, the total dose (if estimated) was also reduced to fit the initial blood concentration of EG.
Inhibition of the metabolism of EG by ethanol or fomepizole was described with the competitive metabolism equation, as described by Tardif et al. (1993). Using ethanol inhibition as an example, the rate of metabolism of EG (RateEG, mg/h) was described by:
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The inhibitory constant, KIEtOH, is the same as the Km for ethanol metabolism. Although Pastino et al. (2000) reported a range of Kms for ethanol metabolism (0.0534 mM) for different isoforms, the average value of 2.7 mM (124.2 mg/l) used in the ethanol PBPK model of Pastino et al. (1997)
was used as the inhibition constant for ethanol metabolism because the phenotypes of the individuals described in the case reports were unknown.
The study-specific time course for the inhibition of EG metabolism by ethanol was modeled using Discrete Schedules in SimuSolv (Registered trademark of The Dow Chemical Company, Midland, MI) where the inhibition constant, KIEtOH, was switched from a very high value (e.g., 109) that effectively results in no inhibition, to 124.2 mg/l for ethanol for the duration of each therapy. The average concentration of ethanol (CVEtOH) reported in each case report was used to complete the inhibition calculation.
The same equations were used to describe the inhibition of EG metabolism by fomepizole (4-MP), which is nearly complete at therapeutic concentrations (e.g., 150200 µM or
12.316.4 mg/l; Brent et al., 1999
) because of its very low KI. KIs for fomepizole inhibition of alcohol dehydrogenases have been reported to be on the order of 0.082.75 µM in rats, cats, dogs, and humans (Connally, et al., 2000
; Li and Theorell, 1969
; Reynier, 1969
); a value of 0.2 µM, determined by Li and Theorell (1969)
for purified human liver alcohol dehydrogenase, was used in all human simulations, although any KI in the reported range for fomepizole would have worked equally well because of the very high Km for EG metabolism (23.8 mM).
Hemodialysis was also scheduled according to the treatment regimens used in each case report using the Discrete function of SimuSolv. Hemodialysis was modeled simply as a partitioning of venous blood into dialysate according to:
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Simulations of human case reports.
As summarized in Table 1, there are several accidental or intentional human poisoning case reports in the literature where either EG or its metabolites (glycolic acid or oxalic acid) were analyzed in human blood or urine to confirm diagnosis and follow the progression of treatments. To validate the human PBPK model, the actual reported conditions from each case report were used to initiate simulations. If not reported, body weights were assumed to be 70 kg for an adult male and 58 kg for an adult female (ICRP, 1975). If vomiting occurred or gastric lavage was used in treatment regimens, the total amount of EG consumed (if estimated in the case report) was arbitrarily reduced to fit the initial (usually peak) blood concentration of EG with no further adjustment. As discussed above, the default, first-order absorption rate constant of 1 h1 was used in all but two simulations, where it was raised to 2.5 h1 (Harry et al., 1994
) or 5 h1 (Hewlett and McMartin, 1986
) to decrease the time needed to simulate the peak concentration of EG in blood. Actual average blood levels of ethanol and fomepizole were used in the model, if reported, assuming the average levels were reached instantaneously and maintained as a constant for the duration of each therapy. If actual blood concentrations were not determined, typical therapeutic blood concentrations (e.g., 1350 mg/l for ethanol and 15 mg/l for fomepizole) were used in the simulations. A similar approach was used to simulate hemodialysis. If the blood flow rate used in hemodialysis was not reported, a typical value of 15 l/h was used for the duration of each therapy. No other adjustments were made to physiological or biochemical parameters to improve the fit to the data. Subject demographics (e.g., age, weight, sex, race or ethnicity) and treatment regimens, when reported, were included in each case description.
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RESULTS |
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Eder et al. (1998) described the pharmacokinetics of EG in a 58-year-old man who consumed
20 oz of antifreeze. Treatments consisted of activated charcoal, ethanol, and hemodialysis. Serum ethanol concentrations were maintained between 77 and 194 mg/dl Simulations were conducted assuming a 70 kg body weight, average ethanol concentrations (135.5 mg/dl), and an assumed hemodialysis rate of 15 l/h. Initial simulations using dose levels estimated by the authors to be 20 oz (which is
591 ml, not 56 ml as reported) of 50% EG did not match the blood levels as shown in Figure 2. Because one empty and one half-full container of reportedly half-strength antifreeze was found with the patient, the actual amount consumed was unknown. Therefore, the dose was increased from 4700 to 7000 mg/kg to fit the initial peak blood concentration. This change in the total dose resulted in an overall improvement in simulating the complex kinetics of EG in this patient after both metabolic inhibition and hemodialysis.
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In the first of the individual cases described in detail by Brent et al., a 73-year-old man (68 kg) ingested an unknown amount of antifreeze. In the second case, a 35-year-old woman also consumed an unknown amount of antifreeze and ethanol. Simulations were conducted using individual hemodialysis schedules and average fomepizole blood concentrations (15 mg/l) for each patient and are shown in Figure 7. Dose levels in each case (3250 and 5000 mg/kg for case 1 and 2, respectively) were adjusted to fit the initial blood concentration of EG. Using the individual treatment scheduled resulted in an excellent fit of the model to the complex kinetics of EG in both cases. However, the model overpredicted the concentration of glycolic acid in the blood of the 73-year-old man (Cmax observed was 772 mg/l versus 1476 mg/l predicted) although the predicted rate of clearance of glycolic acid was consistent with the data.
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DISCUSSION |
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In each of these cases, best estimates were made on the amounts of EG consumed (or the amounts reported), with the dosage adjusted downward if the case reports described vomiting or gastric lavage to fit the initial blood concentrations of EG without further modifications to the model. This downward adjustment to the dose due to vomiting or gastric lavage can be verified in those instances where either the concentrations of glycolic acid in blood or the amounts of EG eliminated in urine were also determined. In these cases, adjusting the total dose available to be absorbed and scheduling the appropriate treatment regimen results in a simulation that fits the pattern of EG clearance from blood, the overall pattern of the formation and clearance of glycolic acid, and the elimination of EG and glycolic acid in urine. Although the model may not perfectly simulate each human case report, given all the uncertainties associated with poisoning cases that are difficult to control, the overall ability to simulate these often drastic situations was remarkably good. Thus, the simulations served to confirm that the general PBPK model structures and parameters derived from in vitro and controlled in vivo studies, as described in the companion paper by Corley et al. (2005), are capable of providing reasonable simulations of human exposures to EG over a broad range of doses that are critical for assessing human health risks.
The current PBPK model can also be used to compare various treatment regimens commonly used in treating EG poisoning to evaluate their effects on pharmacokinetics of EG and glycolic acid. For example, as shown in Figure 8, fomepizole is a more effective inhibitor of the metabolism of EG to glycolic acid than ethanol because of its significantly lower KI when each simulation is conducted at normal therapeutic levels for each treatment. The simulation indicates that fomepizole treatment should result in a more rapid lowering of blood glycolic acid concentrations, along with a lower urinary glycolate excretion. These results would imply a more complete inhibition of metabolism by fomepizole compared to ethanol, as indicated by the slower elimination of EG from the blood (Fig. 8a) and higher excretion in the urine (Fig. 8c).
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Hemodialysis is also an effective treatment for decreasing the body burdens of EG and glycolic acid as shown in Figure 9. The simulations indicate a rapid lowering of serum EG and glycolate concentrations, which corresponds to scenarios reported in human clinical cases in which dialysis has been used (Barceloux et al., 1999; Brent et al., 1999
). The lower serum glycolate level is important in reversing the metabolic acidosis and hence toxicity (Jacobsen et al., 1984
), while the decreased body burden of EG lowers the potential for toxicity in these patients and allows for a shorter hospital stay. As such, hemodialysis has been an often-used component of EG therapy, even though it is an invasive procedure with possible adverse effects.
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Lastly, the modifications to the human PBPK described in this article to include various treatment regimens used clinically to treat accidental or intentional ingestions of EG enabled Corley et al. (2005) to compare internal dose surrogates of the intermediate metabolite, glycolic acid, over a broad dose range in rats and humans with a greater degree of confidence. Work is in progress to develop a more explicit gestational model to describe the disposition of glycolic acid in rat embryos, and additional data are being collected in an effort to improve the description of oxalic acid and calcium oxalate dosimetry in the kidneys of rats and humans that could be used to further refine internal dose surrogates for human health risk assessments.
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SUPPLEMENTARY MATERIAL |
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ACKNOWLEDGMENTS |
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