In vivo effects of naproxen, salicylic acid, and valproic acid on the pharmacokinetics of trichloroethylene and metabolites in rats.

It was recently demonstrated that some drugs modulate in vitro metabolism of trichloroethylene (TCE) in humans and rats. The objective was to assess in vivo interactions between TCE and three drugs: naproxen (NA), valproic acid (VA), and salicylic acid (SA). Animals were exposed to TCE by inhalation (50 ppm for 6 h) and administered a bolus dose of drug by gavage, equivalent to 10-fold greater than the recommended daily dose. Samples of blood, urine, and collected tissues were analyzed by headspace gas chromatography coupled to an electron capture detector for TCE and metabolites (trichloroethanol [TCOH] and trichloroacetate [TCA]) levels. Coexposure to NA and TCE significantly increased (up to 50%) total and free TCOH (TCOHtotal and TCOHfree, respectively) in blood. This modulation may be explained by an inhibition of glucuronidation. VA significantly elevated TCE levels in blood (up to 50%) with a marked effect on TCOHtotal excretion in urine but not in blood. In contrast, SA produced an increase in TCOHtotal levels in blood at 30, 60, and 90 min and urine after coexposure. Data confirm in vitro observations that NA, VA, and SA affect in vivo TCE kinetics. Future efforts need to be directed to evaluate whether populations chronically medicated with the considered drugs display greater health risks related to TCE exposure.

Bayesian population analysis of a harmonized physiologically based pharmacokinetic model of trichloroethylene and its metabolites.

Bayesian population analysis of a harmonized physiologically based pharmacokinetic (PBPK) model for trichloroethylene (TCE) and its metabolites was performed. In the Bayesian framework, prior information about the PBPK model parameters is updated using experimental kinetic data to obtain posterior parameter estimates. Experimental kinetic data measured in mice, rats, and humans were available for this analysis, and the resulting posterior model predictions were in better agreement with the kinetic data than prior model predictions. Uncertainty in the prediction of the kinetics of TCE, trichloroacetic acid (TCA), and trichloroethanol (TCOH) was reduced, while the kinetics of other key metabolites dichloroacetic acid (DCA), chloral hydrate (CHL), and dichlorovinyl mercaptan (DCVSH) remain relatively uncertain due to sparse kinetic data for use in this analysis. To help focus future research to further reduce uncertainty in model predictions, a sensitivity analysis was conducted to help identify the parameters that have the greatest impact on various internal dose metric predictions. For application to a risk assessment for TCE, the model provides accurate estimates of TCE, TCA, and TCOH kinetics. This analysis provides an important step toward estimating uncertainty of dose-response relationships in noncancer and cancer risk assessment, improving the extrapolation of toxic TCE doses from experimental animals to humans.

A human physiologically based pharmacokinetic model for trichloroethylene and its metabolites, trichloroacetic acid and free trichloroethanol.

Nine male and eight female healthy volunteers were exposed to 50 or 100 ppm trichloroethylene vapors for 4 h. Blood, urine, and exhaled breath samples were collected for development of a physiologically based pharmacokinetic (PBPK) model for trichloroethylene and its two major P450-mediated metabolites, trichloroacetic acid and free trichloroethanol. Blood and urine were analyzed for trichloroethylene, chloral hydrate, free trichloroethanol and trichloroethanol glucuronide, and trichloroacetic acid. Plasma was analyzed for dichloroacetic acid. Trichloroethylene was also measured in exhaled breath samples. Trichloroethylene, free trichloroethanol, and trichloroacetic acid were found in blood samples of all volunteers and only trace amounts of dichloroacetic acid (4-12 ppb) were found in plasma samples from a few volunteers. Trichloroethanol glucuronide and trichloroacetic acid were found in urine of all volunteers. No chloral hydrate was detected in the volunteers. Gender-specific PBPK models were developed with fitted urinary rate constant values for each individual trichloroethylene exposure to describe urinary excretion of trichloroethanol glucuronide and trichloroacetic acid. Individual urinary excretion rate constants were necessary to account for the variability in the measured cumulative amount of metabolites excreted in the urine. However, the average amount of trichloroacetic acid and trichloroethanol glucuronide excreted in urine for each gender was predicted using mean urinary excretion rate constant values for each sex. A four-compartment physiological flow model was used for the metabolites (lung, liver, kidney, and body) and a six-compartment physiological flow model was used for trichloroethylene (lung, liver, kidney, fat, and slowly and rapidly perfused tissues). Metabolic capacity (Vmaxc) for oxidation of trichloroethylene was estimated to be 4 mg/kg/h in males and 5 mg/kg/h in females. Metabolized trichloroethylene was assumed to be converted to either free trichloroethanol (90%) or trichloroacetic acid (10%). Free trichloroethanol was glucuronidated forming trichloroethanol glucuronide or converted to trichloroacetic acid via back conversion of trichloroethanol to chloral (trichloroacetaldehyde). Trichloroethanol glucuronide and trichloroacetic acid were then excreted in urine. Gender-related pharmacokinetic differences in the uptake and metabolism of trichloroethylene were minor, but apparent. In general, the PBPK models for the male and female volunteers provided adequate predictions of the uptake of trichloroethylene and distribution of trichloroethylene and its metabolites, trichloroacetic acid and free trichloroethanol. The PBPK models for males and females consistently overpredicted exhaled breath concentrations of trichloroethylene immediately following the TCE exposure for a 2- to 4-h period. Further research is needed to better understand the biological determinants responsible for the observed variability in urinary excretion of trichloroethanol glucuronide and trichloroacetic acid and the metabolic pathway resulting in formation of dichloroacetic acid.

The extent of dichloroacetate formation from trichloroethylene, chloral hydrate, trichloroacetate, and trichloroethanol in B6C3F1 mice.

Conflicting data have been published related to the formation of dichloroacetate (DCA) from trichloroethylene (TRI), chloral hydrate (CH), or trichloroacetic acid (TCA) in B6C3F1 mice. TCA is usually indicated as the primary metabolic precursor to DCA. Model simulations based on the known pharmacokinetics of TCA and DCA predicted blood concentrations of DCA that were 10- to 100-fold lower than previously published reports. Because DCA has also been shown to form as an artifact during sample processing, we reevaluated the source of the reported DCA, i.e., whether it was metabolically derived or formed as an artifact. Male B6C3F1 mice were dosed with TRI, CH, trichloroethanol (TCE), or TCA and metabolic profiles of each were determined. DCA was not detected in any of these samples above the assay LOQ of 1.9 microM of whole blood. In order to slow the clearance of DCA, mice were pretreated for 2 weeks with 2 g/liter of DCA in their drinking water. Even under this pretreatment condition, no DCA was detected from a 100 mg/kg i.v. dose of TCA. Although there is significant uncertainty in the amount of DCA that could be generated from TRI or its metabolites, our experimental data and pharmacokinetic model simulations suggest that DCA is likely formed as a short-lived intermediate metabolite. However, its rapid elimination relative to its formation from TCA prevents the accumulation of measurable amounts of DCA in the blood.

A physiologically based pharmacokinetic model for trichloroethylene and its metabolites, chloral hydrate, trichloroacetate, dichloroacetate, trichloroethanol, and trichloroethanol glucuronide in B6C3F1 mice.

A six-compartment physiologically based pharmacokinetic (PBPK) model for the B6C3F1 mouse was developed for trichloroethylene (TCE) and was linked with five metabolite submodels consisting of four compartments each. The PBPK model for TCE and the metabolite submodels described oral uptake and metabolism of TCE to chloral hydrate (CH). CH was further metabolized to trichloroethanol (TCOH) and trichloroacetic acid (TCA). TCA was excreted in urine and, to a lesser degree, metabolized to dichloroacetic acid (DCA). DCA was further metabolized. The majority of TCOH was glucuronidated (TCOG) and excreted in the urine and feces. TCOH was also excreted in urine or converted back to CH. Partition coefficient (PC) values for TCE were determined by vial equilibrium, and PC values for nonvolatile metabolites were determined by centrifugation. The largest PC values for TCE were the fat/blood (36.4) and the blood/air (15.9) values. Tissue/blood PC values for the water-soluble metabolites were low, with all PC values under 2.0. Mice were given bolus oral doses of 300, 600, 1200, and 2000 mg/kg TCE dissolved in corn oil. At various time points, mice were sacrificed, and blood, liver, lung, fat, and urine were collected and assayed for TCE and metabolites. The 1200 mg/kg dose group was used to calibrate the PBPK model for TCE and its metabolites. Urinary excretion rate constant values were 0. 06/hr/kg for CH, 1.14/hr/kg for TCOH, 32.8/hr/kg for TCOG, and 1. 55/hr/kg for TCA. A fecal excretion rate constant value for TCOG was 4.61/hr/kg. For oral bolus dosing of TCE with 300, 600, and 2000 mg/kg, model predictions of TCE and several metabolites were in general agreement with observations. This PBPK model for TCE and metabolites is the most comprehensive PBPK model constructed for P450-mediated metabolism of TCE in the B6C3F1 mouse.

Enterohepatic recirculation of trichloroethanol glucuronide as a significant source of trichloroacetic acid. Metabolites of trichloroethylene.

Trichloroacetic acid (TCA) is a metabolite of trichloroethylene (TRI) thought to contribute to its hepatocarcinogenic effects in mice. Recent studies have shown that peak blood concentrations of TCA do not occur until approximately 12 hr after an oral dose of TRI; however, blood concentrations of TRI reach a maximum within 1 hr and is nondetectable after 2 hr. The objective of this study was to examine quantitatively enterohepatic recirculation of trichloroethanol (TCEOH) and TCA as a possible mechanism responsible for the delayed production of TCA. Jugular vein, duodenum, and bile duct-cannulated Fischer 344 rats were used, with the collection of blood, bile, urine, and feces samples after intraduodenal and intravenous dosing of animals with TRI, TCEOH, and TCA. Samples were analyzed by GC for TCA, total TCEOH, and free TCEOH. The results show that, after an intravenous dose of TCEOH (100 mg/kg), 36% of the TCEOH in blood is attributable to enterohepatic recirculation. With the same treatment, 76% of the TCA in blood is attributable to enterohepatic recirculation of metabolites. Peak concentrations of total TCEOH in bile, after an intraduodenal dose of TRI, are over 5 times higher than peak concentrations of total TCEOH in systemic blood. Peak concentrations of TCEOH glucuronide in bile are approximately 200 times higher than peak concentrations of TCEOH glucuronide in systemic blood.

Combining physiologically based pharmacokinetic modeling with Monte Carlo simulation to derive an acute inhalation guidance value for trichloroethylene.

Using the Monte Carlo method and physiologically based pharmacokinetic modeling, an occupational inhalation exposure to trichloroethylene consisting of 7 h of exposure per day for 5 days was simulated in populations of men and women of 5000 individuals each. The endpoint of concern for occupational exposure was drowsiness. The toxicologic condition leading to drowsiness was assumed to be high levels of both trichloroethanol and trichloroethylene. Therefore, the output of the simulation or dose metric was the maximum value of the sum of the concentration of trichloroethylene in blood and the concentration of trichloroethanol within its volume of distribution occurring within 1 week of exposure. The distributions of the dose metric in the simulated populations were lognormal. To protect 99% of a worker population, a concentration of 30 ppm over a 7-h period of the work day should not be exceeded. Subjecting a susceptible individual (the 99th percentile of the dose metric) to 200 ppm (the ACGIH short-term exposure limit or STEL) for 15 min twice a day over a work week necessitates a 2.5-h rest in fresh air following the STEL exposure to allow the blood concentrations of trichloroethylene and trichloroethanol to drop to levels that would not cause drowsiness. Both the OSHA PEL and the ACGIH TLV are greater than the value of 30 ppm derived here. As well as suggesting a new occupational guidance value, this study provides an example of this method of guidance value derivation.

N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine and 2,2,2-trichloroethanol: two novel metabolites of tetrachloroethene in humans after occupational exposure.

The excretion of tetrachloroethene metabolites in urine was studied in occupationally exposed workers to identify and quantify metabolites formed by glutathione conjugation and by cytochrome P450 oxidation of tetrachloroethene in humans. The glutathione conjugation pathway has been implicated in the chronic toxicity and possible tumorigenicity of tetrachloroethene to the kidney in rats. The biosynthesis of S-(1,2,2-trichlorovinyl)glutathione and N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine in humans had not been demonstrated. In this study, we investigated the biotransformation of tetrachloroethene in humans occupationally exposed during dry cleaning. Tetrachloroethene concentrations in the air of the dry cleaning shop were 50 +/- 4 ppm; two individuals were exposed for 8 hr daily and two individuals were exposed for 4 hr daily. In urine samples collected from the individuals at the beginning and at the end of the work week, N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine and 2,2,2-trichloroethanol as tetrachloroethene metabolites in humans were identified by GC/MS. The concentrations of N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine in the urine of the individuals were not significantly different at the start and at the end of the work week; however, concentrations of both N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine and 2,2,2-trichloro compounds (trichloroacetic acid and 2,2,2-trichloroethanol) as a marker for cytochrome P450-mediated metabolism were proportional to the length of daily tetrachloroethene exposure. A remarkable difference in the excretion pattern of 2,2,2-trichloro compounds, the major tetrachloroethene metabolites, was observed. Trichloroacetic acid and 2,2,2-trichloroethanol were present in the urine of two of the exposed individuals. Only 2,2,2-trichloroethanol was identified as a major urinary tetrachloroethene metabolite in two other individuals who did not excrete detectable amounts of trichloroacetic acid. The obtained results indicate that humans also have the ability to biosynthesize nephrotoxic glutathione S-conjugates from tetrachloroethene; however, when compared with rats, the human capacity for the biosynthesis of N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine seems to be lower.

Phase I pharmacokinetics study of high-dose fotemustine and its metabolite 2-chloroethanol in patients with high-grade gliomas.

The pharmacokinetics of high-dose fotemustine followed by autologous bone-marrow transplantation during a phase I-II clinical trial in 24 patients with glioblastoma or astrocytoma (grade III-IV) was investigated. Plasma levels of fotemustine were determined by high-performance liquid chromatography (HPLC) and UV detection. The metabolite, 2-chloroethanol, was simultaneously followed in six patients by gag liquid chromatography and electron capture detection (GLC-ECD) assay. The drug was given as a 1-h infusion on 2 consecutive days. In all, 40 pharmacokinetic determinations of fotemustine were made at dose levels ranging from 2 x 300 to 2 x 500 mg/m2. Plasma drug elimination was best described by a bi-exponential model, with short distribution and elimination half-lives of 4.15 +/- 2.57 and 28.8 +/- 12.1 min being observed, respectively. No significant difference in half-lives or clearance was seen between the first and the second administration. During dose escalation, the mean area under the concentrationtime curve (AUC) increased from 5.96 +/- 2.89 to 12.22 +/- 3.95 mg l-1 h. Drug clearance was independent of the dose given and equal to 109 +/- 65 l/h, indicating no possible saturation of metabolism and elimination mechanisms at these high-dose levels. The metabolite 2-chloroethanol appeared very early in plasma samples. Its elimination was rapid and rate-limited by the kinetics of the parent compound, giving the same apparent terminal half-life. A close relationship between AUC and C45 values was evidenced (r = 0.890). Associated with the stability of fotemustine kinetic parameters, this could be used in future studies for individual dose adjustment, particularly for high-dose fractionated regimens.

Sedative/hypnotic effects of chloral hydrate in the neonate: trichloroethanol or parent drug?

Although the metabolism and pharmacokinetics of chloral hydrate (CH) have been reported, there have been no attempts to correlate CH or its metabolite, trichloroethanol (TCE) with the sedative or hypnotic effects. In order to determine whether plasma concentrations of CH or TCE reflect the sedative/hypnotic effects, a sedation/agitation scale was developed. Based on the results of the present study, the sedative/hypnotic effects of TCE cannot be ruled out completely. However, in the neonate, the parent drug CH seems to have a more important role than has been previously suggested from human research.

Determination of chloral hydrate metabolism in adult and neonate biological fluids after single-dose administration.

A simple, rapid and sensitive electron-capture gas chromatographic method has been developed for the simultaneous determination of chloral hydrate, trichloroethanol and trichloroacetic acid in biological fluids. The described method is applicable to single-dose pharmacokinetic studies of chloral hydrate in the adult. The method also meets the important requirement of using very small sample volumes and is sufficiently sensitive and reliable for disposition studies in the neonate.

Intestinal absorption of chloral hydrate, free trichloroethanol and trichloroacetic acid in dogs.

In order to examine the intestinal absorption of chloral hydrate (CH), free trichloroethanol (F-TCE) and trichloroacetic acid (TCA), an intestinal circulation system in dogs was developed using jejunal, ileal and colonic loops, and solutions of CH, F-TCE and TCA were circulated within them. The concentrations of these substances and their metabolites in the serum, urine, bile and circulates were then measured. In all groups, the fraction of water absorbed from the intestine was about 10% of the administered volume two hours after administration. The absorbed fraction of CH was about 50% in the jejunum and ileum, and about 40% in the colon. The absorbed fraction of F-TCE was about 60% in the jejunum, 50-60% in the ileum and about 40% in the colon, while the figures for TCA were about 40-50% in the jejunum and about 30-40% in the ileum and colon. The combined biliary and urinary excretion ratios of the administered substances and their respective metabolites to the total amounts absorbed from the intestine were about 25-30% for F-TCE, 10-15% for CH and 0.1-0.2% for TCA in all parts of the intestine two hours after administration.

The absorption of trichloroethylene and its metabolites from the urinary bladder of anesthetized dogs.

In order to examine the absorption of trichloroethylene (TRI) and its metabolites from the urinary bladder of dogs, we injected TRI and its metabolites, i.e., chloral hydrate (CH), free trichloroethanol (F-TCE), trichloroacetic acid (TCA) and conjugated trichloroethanol (Conj-TCE), into the urinary bladder of anesthetized dogs, and measured the agents and their respective metabolites in the blood or serum, urine and bile. The percentage of water absorbed from the urinary bladder was 10-20% 2 h after the administration of all substances. The percentage of agents absorbed was 60-70% for the TRI and TCA groups, and 50-60% for the CH, F-TCE and Conj-TCE groups 2 h after administration. The combined urinary and biliary excretion rates of the absorbed materials from the urinary bladder 2 h after administration were 46% for F-TCE, 30% for CH, 6% for Conj-TCE and 0.5-1.0% for TRI and TCA. Urinary re-excretion rates of the total excreted amounts were 65-70% in TRI, CH and F-TCE groups, about 50% in TCA and 99% in Conj-TCE group. It is possible that all of the substances administered, particularly F-TCE, are metabolized to Conj-TCE in the urinary bladder.

Pharmacokinetics of chloral hydrate poisoning treated with hemodialysis and hemoperfusion.

In a severe case of chloral hydrate intoxication treated with combined hemodialysis and hemoperfusion the pharmacokinetics of the metabolites trichloroethanol (TCE), trichloroethanol glucuronide (TCE-Glu) and trichloroacetic acid (TCA) were studied. Indications of delayed absorption and some slowing of metabolism were found. At a blood flow rate of 200 ml/min clearances by hemodialysis and hemoperfusion, respectively, in ml/min were estimated to be 188 and 156 for TCE, 184 and 181 for TCE-Glu, 142 and 91 for TCA. Clearance by hemoperfusion declined with time. The half-lives of TCE and TCA were 3.2 and 4.3 hours during combined hemodialysis and hemoperfusion. After termination of treatment the half-life of TCE was 12.8 hours, whereas TCA was metabolized so slowly, that no reliable calculation could be performed. We conclude that hemodialysis and hemoperfusion are equally and highly efficient in the treatment of chloral hydrate poisoning, but hemoperfusion may increase the risk of gastric bleeding more than hemodialysis. Hemodialysis may therefore be preferable and should be tried in spite of low blood pressure.