In vitro toxicity and genotoxicity assessment of disinfection by-products, organic N-chloramines.

Disinfection by-products (DBPs) are of concern to both water industries and health authorities. Although several classes of DBPs have been studied, and there are regulated safe levels in disinfected water for some, a large portion of DBPs are not characterized, and need further investigation. Organic N-chloramines are a group of DBPs, which can be formed during common disinfection processes such as chlorination and chloramination, but little is known in terms of their toxicological significance if consumed in drinking water. Only a few in vitro studies using bacterial assays have reported some genotoxic potential of organic N-chloramines, largely in the context of inflammatory processes in the body rather than exposure through drinking water. In this study, we investigated 16 organic N-chloramines produced by chlorination of model amino acids and amines. It was found that within the drinking water-relevant micromolar concentration range, four compounds were both cytotoxic and genotoxic to mammalian cells. A small reduction of cellular GSH was also observed in the treatment with these four compounds, but not of a magnitude to account for the cytotoxicity and genotoxicity. The results presented in this study demonstrate that some organic N-chloramines, at low concentrations that might be present in disinfected water, can be harmful to mammalian cells.

Kinetics of chloral hydrate and its metabolites in male human volunteers.

Chloral hydrate (CH) is a short-lived intermediate in the metabolism of trichloroethylene (TRI). TRI, CH, and two common metabolites, trichloroacetic acid (TCA) and dichloroacetic acid (DCA) have been shown to be hepatocarcinogenic in mice. To better understand the pharmacokinetics of these metabolites of TRI in humans, eight male volunteers, aged 24-39, were administered single doses of 500 or 1,500 mg or a series of three doses of 500 mg given at 48 h intervals, in three separate experiments. Blood and urine were collected over a 7-day period and CH, DCA, TCA, free trichloroethanol (f-TCE), and total trichloroethanol (T-TCE=trichloroethanol and trichloroethanol-glucuronide [TCE-G]) were measured. DCA was detected in blood and urine only in trace quantities (<2 microM). TCA, on the other hand, had the highest plasma concentration and the largest AUC of any metabolite. The TCA elimination curve displayed an unusual concentration-time profile that contained three distinct compartments within the 7-day follow-up period. Previous work in rats has shown that the complex elimination curve for TCA results largely from the enterohepatic circulation of TCE-G and its subsequent conversion to TCA. As a result TCA had a very long residence time and this, in turn, led to a substantial enhancement of peak concentrations following the third dose in the multiple dose experiment. Approximately 59% of the AUC of plasma TCA following CH administration is produced via the enterohepatic circulation of TCE-G. The AUC for f-TCE was found to be positively correlated with serum bilirubin concentrations. This effect was greatest in one subject that was found to have serum bilirubin concentrations at the upper limit of the normal range in all three experiments. The AUC of f-TCE in the plasma of this individual was consistently about twice that of the other seven subjects. The kinetics of the other metabolites of CH was not significantly modified in this individual. These data indicate that individuals with a more impaired capacity for glucuronidation may be very sensitive to the central nervous system depressant effects of high doses of CH, which are commonly attributed to plasma levels of f-TCE.

Dichloroacetate toxicokinetics and disruption of tyrosine catabolism in B6C3F1 mice: dose-response relationships and age as a modifying factor.

Dichloroacetate (DCA) is a rodent carcinogen commonly found in municipal drinking water supplies. Toxicokinetic studies have established that elimination of DCA is controlled by liver metabolism, which occurs by the cytosolic enzyme glutathione-S-transferase-zeta (GST-zeta). DCA is also a mechanism based inhibitor of GST-zeta, and a loss in GST-zeta enzyme activity occurs following repeated doses or prolonged drinking water exposures. GST-zeta is identical to an enzyme that is part of the tyrosine catabolism pathway known as maleylacetoacetate isomerase (MAAI). In this pathway, GST-zeta plays a critical role in catalyzing the isomerization of maleylacetoacetate to fumarylacetoacetate. Disruption of tyrosine catabolism has been linked to increased cancer risk in humans. We studied the elimination of i.v. doses of DCA to young (10 week) and aged (60 week) mice previously treated with DCA in their drinking water for 2 and 56 weeks, respectively. The diurnal change in blood concentrations of DCA was also monitored in mice exposed to three different drinking water concentrations of DCA (2.0, 0.5 and 0.05 g/l). Additional experiments measured the in vitro metabolism of DCA in liver homogenates prepared from treated mice given various recovery times following treatment. The MAAI activity was also measured in liver cytosol obtained from treated mice. Results indicated young mice were the most sensitive to changes in DCA elimination after drinking water treatment. The in vitro metabolism of DCA was decreased at all treatment rates. Partial restoration ( approximately 65% of controls) of DCA elimination capacity and hepatic GST-zeta activity occurred after 48 h recovery from 14 d 2.0 g/l DCA drinking water treatments. Recovery from treatments could be blocked by interruption of protein synthesis with actinomycin D. MAAI activity was reduced over 80% in liver cytosol from 10-week-old mice. However, MAAI was unaffected in 60-week-old mice. These results indicate that in young mice, inactivation and re-synthesis of GST-zeta is a highly dynamic process and that exogenous factors that deplete or reduce GST-zeta levels will decrease DCA elimination and may increase the carcinogenic potency of DCA. As mice age, the elimination capacity for DCA is less affected by reduced liver metabolism and mice appear to develop some toxicokinetic adaptation(s) to allow elimination of DCA at rates comparable to naive animals. Reduced MAAI activity alone is unlikely to be the carcinogenic mode of action for DCA and may in fact, only be important during the early stages of DCA exposure.

Differential effects of dihalogenated and trihalogenated acetates in the liver of B6C3F1 mice.

Haloacetates are produced in the chlorination of drinking water in the range 10--100 microg l(-1). As bromide concentrations increase, brominated haloacetates such as bromodichloroacetate (BDCA), bromochloroacetate (BCA) and dibromoacetate (DBA) appear at higher concentrations than the chlorinated haloacetates: dichloroacetate (DCA) or trichloroacetate (TCA). Both DCA and TCA differ in their hepatic effects; TCA produces peroxisome proliferation as measured by increases in cyanide-insensitive acyl CoA oxidase activity, whereas DCA increases glycogen concentrations. In order to determine whether the brominated haloacetates DBA, BCA and BDCA resemble DCA or TCA more closely, mice were administered DBA, BCA and BDCA in the drinking water at concentrations of 0.2--3 g l(-1). Both BCA and DBA caused liver glycogen accumulation to a similar degree as DCA (12 weeks). The accumulation of glycogen occurred in cells scattered throughout the acinus in a pattern very similar to that observed in control mice. In contrast, TCA and low concentrations of BDCA (0.3 g l(-1)) reduced liver glycogen content, especially in the central lobular region. The high concentration of BDCA (3 g l(-1)) produced a pattern of glycogen distribution similar to that in DCA-treated and control mice. This effect with a high concentration of BDCA may be attributable to the metabolism of BDCA to DCA. All dihaloacetates reduced serum insulin levels. Conversely, trihaloacetates had no significant effects on serum insulin levels. Dibromoacetate was the only brominated haloacetate that consistently increased acyl-CoA oxidase activity and rates of cell replication in the liver. These results further distinguish the effects of the dihaloacetates from those of peroxisome proliferators like TCA.

Toxicokinetics of bromodichloroacetate in B6C3F1 mice.

The oral and i.v. elimination kinetics were investigated for bromodichloroacetate (BDCA), a haloacetate found in drinking water. The BDCA was administered at a dose of 5, 20 and 100 mg kg-1 to B6C3F1 mice and appears to distribute to the total body water with a mean volume of distribution of 427 +/- 79 ml kg-1. It is subject to first-pass hepatic metabolism with a range of bioavailabilities of 0.28-0.73. A mean terminal half-life of 1.37 +/- 0.21 h. was calculated from the two lower doses of both i.v. and oral administration. Non-linear behavior was exhibited at doses greater than 20 mg kg-1, with a much higher than expected area under the curve (AUC), a decrease in total body clearance (CL(b)) and an increase in the terminal half-life to 2.3 h at the highest dose. The average CL(b) was 220 ml h(-1) kg-1 for the lower two doses but decreased to 156 ml h(-1) kg-1 at the high dose. The BDCA is primarily eliminated by metabolism, with only 2.4% of the parent dose being recovered in the urine at the high dose. The unbound renal clearance, as calculated from the high dose, was 15.0 ml h(-1) kg-1. The BDCA is moderately bound to plasma proteins (f(u) = 0.28) and preferentially distributes to the plasma with a blood/plasma ratio of 0.88.

Effects of dichloroacetate (DCA) on serum insulin levels and insulin-controlled signaling proteins in livers of male B6C3F1 mice.

DCA is hepatocarcinogenic in rodents. At carcinogenic doses, DCA causes a large accumulation of liver glycogen. Thus, we studied the effects of DCA treatment on insulin levels and expression of insulin-controlled signaling proteins in the liver. DCA treatment (0.2-2.0 g/l in drinking water for 2 weeks) reduced serum insulin levels. The decrease persisted for at least 8 weeks. In livers of mice treated with DCA for 2-, 10-, and 52-week periods, insulin receptor (IR) protein levels were significantly depressed. Additionally, protein kinase B (PKBalpha) expression decreased significantly with DCA treatment. In normal liver, glycogen levels were increased as early as at 1 week, and this effect preceded changes in insulin and IR and PKBalpha. In contrast to normal liver, IR protein was elevated in DCA-induced liver tumors relative to that in liver tissue of untreated animals and to an even greater extent when compared to adjacent normal liver in the treated animal. Mitogen-activated protein kinase (MAP kinase) phosphorylation was also increased in tumor tissue relative to normal liver tissue and tissue from untreated controls. These data suggest that normal hepatocytes down-regulate insulin-signaling proteins in response to the accumulation of liver glycogen caused by DCA. Furthermore, these results suggest that the initiated cell population, which does not accumulate glycogen and is promoted by DCA treatment, responds differently from normal hepatocytes to the insulin-like effects of this chemical. The differential sensitivity of the 2 cell populations may contribute to the tumorigenic effects of DCA in the liver.

Trapping and identification of the dichloroacetate radical from the reductive dehalogenation of trichloroacetate by mouse and rat liver microsomes.

A key question in the risk assessment of trichloroethylene (TRI) is the extent to which its carcinogenic effects might depend on the formation of dichloroacetate (DCA) as a metabolite. One of the metabolic pathways proposed for the formation of DCA from TRI is by the reductive dehalogenation of trichloroacetate (TCA), via a free radical intermediate. Although proof of this radical has been elusive, the detection of fully dechlorinated metabolites in the urine and the formation of lipid peroxidation by-products in microsomal incubations with TCA argue for its existence. We report here the trapping of the dichloroacetate radical with the spin-trapping agent PBN, and its identification by GC/MS. The PBN/dichloroacetate radical adduct was found to undergo an intramolecular rearrangement during its extraction into organic solvent. An internal condensation reaction between the acetate and the nitroxide radical moieties is hypothesized to form a cyclic adduct with the elimination of an OH radical. The PBN/dichloroacetate radical adduct has been identified by GC/MS in both a chemical Fenton system and in rodent microsomal incubations with TCA as substrate.

Mode of action of liver tumor induction by trichloroethylene and its metabolites, trichloroacetate and dichloroacetate.

Trichloroethylene (TCE) induces liver cancer in mice but not in rats. Three metabolites of TCE may contribute--chloral hydrate (CH), dichloroacetate (DCA), and trichloroacetate (TCA). CH and TCA appear capable of only inducing liver tumors in mice, but DCA is active in rats as well. The concentrations of TCA in blood required to induce liver cancer approach the mM range. Concentrations of DCA in blood associated with carcinogenesis are in the sub-microM range. The carcinogenic activity of CH is largely dependent on its conversion to TCA and/or DCA. TCA is a peroxisome proliferator in the same dose range that induces liver cancer. Mice with targeted disruptions of the peroxisome proliferator-activated receptor alpha (PPAR-alpha) are insensitive to the liver cancer-inducing properties of other peroxisome proliferators. Human cells do not display the responses associated with PPAR-alpha that are observed in rodents. This may be attributed to lower levels of expressed PPAR-alpha in human liver. DCA treatment produces liver tumors with a different phenotype than TCA. Its tumorigenic effects are closely associated with differential effects on cell replication rates in tumors, normal hepatocytes, and suppression of apoptosis. Growth of DCA-induced tumors has been shown to arrest after cessation of treatment. The DCA and TCA adequately account for the hepatocarcinogenic responses to TCE. Low-level exposure to TCE is not likely to induce liver cancer in humans. Higher exposures to TCE could affect sensitive populations. Sensitivity could be based on different metabolic capacities for TCE or its metabolites or result from certain chronic diseases that have a genetic basis.

In vivo MRI measurements of tumor growth induced by dichloroacetate: implications for mode of action.

Dichloroacetate (DCA) is an important by-product of the chlorination of drinking water that produces liver cancer in rodents. Assessment of the risk that results from concentrations that occur in drinking water will be dependent upon the mode of action held responsible for these tumors. A study by Stauber and Bull [Stauber, A.J. and Bull, R. J (1997) Differences in phenotype and cell replicative behavior of hepatic tumors inducted by dichloroacetate (DCA) and trichloroacetate (TCA). Toxicol. Appl. Pharmacol. 144, 235-246] in mice treated with DCA demonstrated a lesion distribution that was skewed towards many small, altered foci of cells that are assumed to be precursor lesions [EPA, (1996). U.S. Environmental Protection Agency: Proposed Guidelines for carcinogen risk assessment; notice. Fed. Reg. 61, pp. 17960-10811]. The present study was designed to determine the extent to which the tumorigenic effects of DCA could be explained by its effect on tumor growth rates (i.e. tumor promoting activity). In vivo magnetic resonance imaging (MRI) allowed accurate determination of growth rates of individual lesions in mice that had been treated with DCA in drinking water at 2 g/l. Out of thirty treated mice, ten were found to have hepatic tumors detectable by MRI at 48 weeks of treatment. These tumor-bearing animals were assigned to two groups matched on the size of lesions observed by in vivo MR1. Treatment with DCA continued in one group of five mice and was stopped in the other. For both groups, tumor growth rates were determined by measuring changes in size of all lesions greater than 1 mm(3) in volume during a 14-day period. Removal of DCA treatment resulted in growth rates that could not be distinguished from zero across all lesion sizes represented in the sample. These data are in agreement with previous observations of DCAs effects on replication rates within tumors (Stauber and Bull, (1997)). Tumor growth rates observed in animals maintained on treatment decreased with lesion volume in a manner that is consistent with a stochastic Gompertz birth-death process proposed by Tan [Tan, W.Y. (1986) A stochastic Gompertz birth-death process. Stat. Prob. Lett. 4, 25-28]. Parameters of this model obtained by fitting measured growth rates were used to predict the lesion-size distribution expected after one year of DCA treatment. The shape of the predicted lesion-size distribution was similar to that observed by Stauber and Bull (Stauber and Bull, (1997)) in mice sacrificed after 40 weeks of DCA treatment. We conclude that the effects of DCA on the division and/or death rates of spontaneously initiated cells can account for the predominance of small lesions in DCA-treated animals.

Effect of enterohepatic circulation on the pharmacokinetics of chloral hydrate and its metabolites in F344 rats.

Chloral hydrate (CH) is a commonly found disinfection by-product in water purification, a metabolite of trichloroethylene, and a sedative/hypnotic drug. CH and two of its reported metabolites, trichloroacetic acid (TCA) and dichloroacetic acid (DCA), are hepatocarcinogenic in mice. Another metabolite of CH, trichloroethanol (TCE), is also metabolized into TCA, and the enterohepatic circulation (EHC) of TCE maintains a pool of metabolite for the eventual production of TCA. To gain insight on the effects of EHC on the kinetics of CH and on the formation of TCA and DCA, dual cannulated F344 rats were infused with 12, 48, or 192 mg/kg of CH and the blood, bile, urine, and feces were collected over a 48-h period. CH was cleared rapidly (>3000 ml/h/kg) and displayed biphasic elimination kinetics, with the first phase being elimination of the dose and the second phase exhibiting formation rate-limited kinetics relative to its TCE metabolite. The effects of EHC on metabolite kinetics were only significant at the highest dose, resulting in a 44% and 17% decrease in the area under the curve (AUC) of TCA and TCE, respectively. The renal clearance of CH, free TCE (f-TCE), and TCA of 2, 2.7, and 38 ml/h/kg, respectively, indicates an efficient reabsorption mechanism for all of these small chlorinated compounds. DCA was detected at only trace levels (<2 microM) as a metabolite of CH, TCA, or TCE.

Comparative toxicokinetics of chlorinated and brominated haloacetates in F344 rats.

Chloro, bromo, and mixed bromochloro haloacetates (HAs) are by-products of drinking water disinfection and are hepatocarcinogenic in rodents. We compared the toxicokinetics of a series of di-HAs, dichloro (DCA), bromochloro (BCA), dibromo (DBA) and tri-HAs: trichloro (TCA), bromodichloro (BDCA), chlorodibromo (CDBA), and tribromo (TBA) after iv and oral dosing (500 micrometer/kg) in male F344 rats. The blood concentrations of the HAs after iv injection declined in a bi-exponential manner with a short but pronounced distributive phase. The structural features that had the greatest influence on the disposition of HAs were substitution of a halogen for a hydrogen and the degree of bromine substitution. All di-HAs had blood elimination half-lives of less than 4 h (DCA > DBA, BCA) compared to the tri-HAs, which had half-lives that varied from 0.6 to 8.0 h (TCA > BDCA > CDBA > TBA). The urinary excretion of all di-HAs was low and accounted for less than 3% of the dose in contrast to the tri-HAs, where urinary excretion accounted for at least 30% of the dose. Toxicokinetic analysis indicated the steady-state apparent volume of distribution varied between 301 and 881 ml/kg among the HAs, but the variation was not statistically significant (P > 0.17). The blood concentration-time profiles for all di-HAs after oral dosing was complex and exhibited multiple peaks. This did not appear to be due to enterohepatic recirculation, as bile duct cannulated animals also displayed similar profiles. In contrast, the profiles for the tri-HAs did not exhibit multiple peaking after oral dosing and could be described using a one-compartment pharmacokinetic model. The oral bioavailability of the HAs varied between 30% (DBA) and 116% (TCA), depending on the number of halogen substituents and the degree of bromine substitution. In general, three patterns of elimination for the HAs can be broadly described: low metabolism with moderate renal clearance (TCA), high metabolism and renal clearance (BDCA, CDBA, TBA), and high metabolism, low renal clearance (DCA, BCA, DBA).

Effect of pre-treatment with dichloroacetic or trichloroacetic acid in drinking water on the pharmacokinetics of a subsequent challenge dose in B6C3F1 mice.

Dichloroacetate (DCA) and trichloroacetate (TCA) are prominent by-products of chlorination of drinking water. Both chemicals have been shown to be hepatic carcinogens in mice. Prior work has demonstrated that DCA inhibits its own metabolism in rats and humans. This study focuses on the effect of prior administration of DCA or TCA in drinking water on the pharmacokinetics of a subsequent challenge dose of DCA or TCA in male B6C3F1 mice. Mice were provided with DCA or TCA in their drinking water at 2 g/l for 14 days and then challenged with a 100 mg/kg i.v. (non-labeled) or gavage (14C-labeled) dose of DCA or TCA. The challenge dose was administered after 16 h fasting and removal of the haloacetate pre-treatment. The haloacetate blood concentration-time profile and the disposition of 14C were characterized and compared with controls. The effect of pre-treatment on the in vitro metabolism of DCA in hepatic S9 was also evaluated. Pre-treatment with DCA caused a significant increase in the blood concentration-time profiles of the challenge dose of DCA. No effect on the blood concentration-time profile of DCA was observed after pre-treatment with TCA. Pre-treatment with TCA had no effect on subsequent doses of DCA. Pre-treatment with DCA did not have a significant effect on the formation of 14CO2 from radiolabeled DCA. In vitro experiments with liver S9 from DCA-pre-treated mice demonstrated that DCA inhibits it own metabolism. These results indicate that DCA metabolism in mice is also susceptible to inhibition by prior treatment with DCA, however the impact on clearance is less marked in mice than in F344 rats. In contrast, the metabolism and pharmacokinetics of TCA is not affected by pre-treatment with either DCA or TCA.

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.

Effects of dichloroacetate on glycogen metabolism in B6C3F1 mice.

Dichloroacetate (DCA) is a by-product of drinking water chlorination. Administration of DCA in drinking water results in accumulation of glycogen in the liver of B6C3F1 mice. To investigate the processes affecting liver glycogen accumulation, male B6C3F1 mice were administered DCA in drinking water at levels varying from 0.1 to 3 g/l for up to 8 weeks. Liver glycogen synthase (GS) and glycogen phosphorylase (GP) activities, liver glycogen content, serum glucose and insulin levels were analyzed. To determine whether effects were primary or attributable to increased glycogen synthesis, some mice were fasted and administered a glucose challenge (20 min before sacrifice). DCA treatments in drinking water caused glycogen accumulation in a dose-dependent manner. The DCA treatment in drinking water suppressed the activity ratio of GS measured in mice sacrificed at 9:00 AM, but not at 3:00 AM. However, net glycogen synthesis after glucose challenge was increased with DCA treatments for 1-2 weeks duration, but the effect was no longer observed at 8 weeks. Degradation of glycogen by fasting decreased progressively as the treatment period was increased, and no longer occurred at 8 weeks. A shift of the liver glycogen-iodine spectrum from DCA-treated mice was observed relative to that of control mice, suggesting a change in the physical form of glycogen. These data suggest that DCA-induced glycogen accumulation at high doses is related to decreases in the degradation rate. When DCA was administered by single intraperitoneal (i.p.) injection to naïve mice at doses of 2-200 mg/kg at the time of glucose challenge, a biphasic response was observed. Doses of 10-25 mg/kg increased both plasma glucose and insulin concentrations. In contrast, very high i.p. doses of DCA (> 75 mg/kg) produced progressive decreases in serum glucose and glycogen deposition in the liver. Since the blood levels of DCA produced by these higher i.p. doses were significantly higher than observed with drinking water treatment, we conclude that apparent differences with data of previous investigations is related to substantial differences in systemic dose and/or dose-time relations.

Workshop report. Health risks of drinking water chlorination by-products: report of an expert working group.

Studies of water chlorination by-products have suggested a possible increased risk of bladder and colon cancers, as well as adverse reproductive and developmental effects such as increased spontaneous abortion rates and fetal anomalies. A workshop for an expert working group was convened to advise Health Canada on the need for further action. Participants were given background papers and a set of key questions to review prior to the meeting. At the workshop, experts presented an overview of what was known to date on water chlorination by-products from toxicologic studies, epidemiologic studies of cancer and adverse reproductive/developmental effects, and risk assessment. This paper summarizes the information provided in the background papers and presentations, describes the consensus arrived at regarding assessment of evidence for level of risk and presents a number of suggestions for future research.

Comparison of the effects of iodine and iodide on thyroid function in humans.

Concerns have been raised over the use of iodine for disinfecting drinking water on extended space flights. Most fears revolve around effects of iodide on thyroid function. iodine (I2) is the form used in drinking-water disinfection. Risk assessments have treated the various forms of iodine as if they were toxicologically equivalent. Recent experiments conducted in rats found that administration of iodine as I- (iodide) versus I2 had opposite effects on plasma thyroid hormone levels. I2-treated animals displayed elevated thyroxine (T4) and thyroxine/triiodothyronine (T/T3) ratios, whereas those treated with I- displayed no change or reduced plasma concentrations of T4 at concentrations in drinking water of 30 or 100 mg/L. The study herein was designed to assess whether similar effects would be seen in humans as were observed in rats. A 14-d repeated-dose study utilizing total doses of iodine in the two forms at either 0.3 or 1 mg/kg body weight was conducted with 33 male volunteers. Thyroid hormones evaluated included T4, T3, and thyroid-stimulating hormone (TSH). TSH was significantly increased by the high dose of both I2 and I-, as compared to the control. Decreases in T4 were observed with dose schedules with I- and I2, but none were statistically significant compared to each other, or compared to the control. This human experiment failed to confirm the differential effect of I2 on maintenance of serum T4 concentrations relative to the effect of I- that was observed in prior experiments in rats. However, based on the elevations in TSH, there should be some concern over the potential impacts of chronic consumption of iodine in drinking water.

Dichloroacetate and trichloroacetate promote clonal expansion of anchorage-independent hepatocytes in vivo and in vitro.

Dichloroacetate (DCA) and trichloroacetate (TCA) are hepatocarcinogenic by-products of water chlorination and metabolites of several industrial solvents. To determine whether DCA and TCA promote the clonal expansion of anchorage-independent liver cells in vitro, a modification of the soft agar assay (over agar assay) was utilized to quantitate growth and analyze phenotype of anchorage-independent hepatocellular colonies. Hepatocytes from naïve male B6C3F1 mice were isolated and cultured with 0-2.0 mM DCA or TCA over agar for 10 days, at which time colonies of eight cells or more were scored. Both DCA and TCA promoted the formation of anchorage-independent colonies in a dose-dependent manner. Immunocytochemical analysis using a c-Jun antibody demonstrated that colonies promoted by DCA were primarily c-Jun+, whereas TCA-promoted colonies were primarily c-Jun-. This corresponds to the differences in c-Jun immunoreactivity reported in tumors induced by DCA and TCA. Neither DCA nor TCA induced c-Jun expression in hepatocyte monolayers, indicating that these haloacetates selectively affect subpopulations of anchorage-independent hepatocyts. The latency of colony formation was decreased by the concentration of DCA, although the same number of colonies appeared after 25 days in culture at all DCA concentrations used. The plating density of hepatocytes also affected colony formation. At lower cell densities, promotion of colony formation by DCA was significantly reduced. Pretreatment of male B6C3F1 mice with 0.5 g/liter DCA in drinking water resulted in a fourfold increase in in vitro colony formation above hepatocytes isolated from naïve mice, suggesting that DCA is promoting the clonal expansion of anchorage-independent hepatocytes in vivo. Results from this study indicate that DCA and TCA promote the survival and growth of initiated cells. Furthermore, results from over agar assays reflect observations made in vivo, indicating this assay provides a valid means to investigate the mechanism by which chemicals promote clonal expansion of initiated hepatocytes.

Physiologically-based pharmacokinetic model for trichloroethylene considering enterohepatic recirculation of major metabolites.

Trichloroacetic acid (TCA) is a major metabolite of trichloroethylene (TRI) thought to contribute to its hepatocarcinogenic effects in mice. Recent studies have shown that peak blood concentrations of TCA in rats do not occur until approximately 12 hours following an oral dose of TRI. However, blood concentrations of TRI reach a maximum within an hour and are nondetectable after 2 hours. The results of a study which examined the enterohepatic recirculation (EHC) of the principle TRI metabolites was used to develop a physiologically-based pharmacokinetic model for TRI, which includes enterohepatic recirculation of its metabolites. The model quantitatively predicts the uptake, distribution and elimination of TRI, trichloroethanol, trichloroethanol-glucuronide, and TCA and includes production of metabolites through the enterohepatic recirculation pathway. Physiologic parameters used in the model were obtained from the literature. Parameters for TRI metabolism were taken from Fisher et al. Other kinetic parameters were found in the literature or estimated from experimental data. The model was calibrated to data from experiments of an earlier study where TRI was orally administered. Verification of the model was conducted using data on the enterohepatic recirculation of TCEOH and TCA, chloral hydrate data (infusion doses) from Merdink, and TRI data from Templin and Larson and Bull.

Effect of pretreatment with dichloroacetate or trichloroacetate on the metabolism of bromodichloroacetate.

Haloacetates are a common class of water chlorination by-products. Depending on the amount of bromide in the source water, varying amounts of chlorinated, brominated, and mixed bromochloro haloacetates are produced. When administered to rodents, haloacetates have been shown to increase formation of thiobarbituric acid-reactive substances and 8-hydroxydeoxyguanosine levels in the liver. These responses appear to be modified by prior treatment. To examine potential mechanisms that account for these modifications in oxidative stress, the ability of trichloroacetate (TCA) or dichloroacetate (DCA) pretreatment to alter the metabolism of bromodichloroacetate (BDCA) and the disposition of its metabolites was examined in male B6C3F1 mice. Two-week pretreatment with 1 g/L DCA and TCA in the drinking water of mice alters the initial hepatic metabolism of BDCA and the further metabolism of its metabolite DCA. DCA pretreatment inhibits cytosolic metabolism of both 1 mM DCA or BDCA up to 70%. In contrast, DCA pretreatment stimulates hepatic microsomal BDCA metabolism 1.3-fold but has little effect on microsomal metabolism of DCA. Increased microsomal metabolism of BDCA appears to be attributable to the induction of a metabolic pathway that produces CO2 and bromodichloromethane (BDCM) as metabolites. TCA pretreatment inhibits BDCA metabolism up to 70% in the cytosol and 30% in microsomes but has little effect on DCA metabolism. These results indicate that the hepatic metabolism of the haloacetate becomes quite complex at the high doses that have been employed in cancer bioassays. BDCA serves as a good example, because it is metabolized to at least two carcinogenic metabolites that have different modes of action, BDCM and DCA. As doses approach those that induce cancer in mice, the proportion of and amounts of these metabolites as a fraction of the dose administered will change substantially. This article demonstrates that those interactions will occur from mixed treatment with haloacetates as well.

Pharmacokinetics and metabolism of dichloroacetate in the F344 rat after prior administration in drinking water.

The effect of prior administration of dichloroacetate (DCA) in drinking water on the pharmacokinetics of DCA in male F344 rats was studied. Rats were provided with DCA in their drinking water at 0.2 and 2.0 g/liter for 14 days and then challenged with iv bolus iv or gavage doses of [14C1,2]DCA, 16 hr after pretreatment withdrawal. The blood concentration-time profiles of DCA and the disposition of 14C was characterized and compared with controls. The effect of pretreatment on the in vitro metabolism of DCA in hepatic cytosol was also evaluated. Pretreatment caused a significant increase in the blood concentration and AUC0-->infinity of DCA (433.3 versus 2406 microg ml-1 hr). Pharmacokinetic analysis indicated that pretreatment significantly decreased total body clearance (267.4 versus 42.7 ml hr-1 kg-1), which was largely due to decreased metabolism since only modest differences in the urinary clearance of DCA were observed. Pretreatment significantly decreased the formation of 14CO2 after both iv and oral doses of [14C]DCA. The decrease in CO2 formation was also observed after pretreatment with DCA at 0.2 g/liter. Pretreatment also increased the urinary elimination of DCA and several metabolites, particularly glycolate. The in vitro experiments demonstrated that DCA pretreatment inhibited the conversion of DCA to glyoxylate, oxalate, and glycolate in hepatic cytosol. These results indicate that DCA has an auto-inhibitory effect on its metabolism and that pharmacokinetic studies using single doses in naïve rats will underestimate the concentration of DCA at the target tissue during chronic or repeated exposures.