Phosgene: a metabolite of chloroform

Efficient Cu Decorated Inorganic B12P12 Nanoclusters for Sensing Toxic COCl2 Gas: A Detailed DFT Study

Gas sensing materials have been widely explored recently owing to their versatile environmental and agriculture monitoring applications. Phosgene (COCl2) is a toxic and harmful gas, therefore, a reliable and sensitive technique is required for monitoring its quantity in the atmosphere. In this study, pure as well as copper decorated B[Formula: see text]P[Formula: see text](Cu-BP) nanoclusters were analyzed using DFT method to investigate their specific potential for phosgene gas adsorption. Cu interaction resulted in three optimized geometries S1, S2 and S3 with interaction energies of [Formula: see text]234.52[Formula: see text]kJ/mol, [Formula: see text]214.59[Formula: see text]kJ/mol and [Formula: see text]266.45[Formula: see text]kJ/mol, respectively. In all these three cases, the COCl2 prefers to interact at the top of the cage. The phosgene molecule (COCl2) interacts with bare nanocage at a distance of 3.22[Formula: see text]Å with interaction energy of [Formula: see text]6.22[Formula: see text]kJ/mol, while the observed interaction energies of phosgene at Cu decorated B[Formula: see text]P[Formula: see text] are [Formula: see text]76.90[Formula: see text]kJ/mol, [Formula: see text]119.03[Formula: see text]kJ/mol and [Formula: see text]29.60[Formula: see text]kJ/mol, respectively. To observe the variations in electronic structure, fermi level, molecular electrostatic potential (MEP), frontier molecular orbitals (FMOs), natural bonding orbital ([Formula: see text]), softness, hardness, chemical potential and electrophilicity are calculated before and after phosgene adsorption. Energy gap reduce significantly after phosgene adsorption from 2.31[Formula: see text]eV, 2.05[Formula: see text]eV and 2.46[Formula: see text]eV to 1.54[Formula: see text]eV, 1.57[Formula: see text]eV and 2.45[Formula: see text]eV, respectively. Results of all analysis suggested that decoration of Cu significantly enhanced the adsorption power of B[Formula: see text]P[Formula: see text] nan-cluster for COCl2 molecule. Therefore, the Cu-decorated B[Formula: see text]P[Formula: see text] nanocages are considered as potential candidates for application in COCl2 sensors.

Epidemiology and Toxicology of Volatile Organic Chemical Contaminants in Water Absorbed Through the Skin

This paper provides a general introduction to the occurrence, epidemiology, and toxicity of some of the most common contaminants of water supplies, the volatile organic chemicals (VOCs). The VOCs are formed from the reaction of chlorine during disinfection with naturally occurring carbon in the form of humic acids. The VOCs may also enter water supplies as a result of manufacturing, processing, distribution, and urban and agricultural run off. Their occurrence is summarized in this paper.

No epidemiologic studies examine the health effects where skin is the sole route of exposure. However, in several studies skin is one of the routes of exposure for VOCs. These are summarized in this paper.

Finally, the toxicity of some of the more important VOCs is summarized. Where possible, similarities in toxicity between individual members of this class of chemical contaminants are noted. There are striking similarities of toxicity of various VOCs in the liver, kidney, and hematopoietic system. These similarities should be considered as skin exposure models are being developed.

Health effects from swimming training in chlorinated pools and the corresponding metabolic stress pathways

Chlorination is the most popular method for disinfecting swimming pool water; however, although pathogens are being killed, many toxic compounds, called disinfection by-products (DBPs), are formed. Numerous epidemiological publications have associated the chlorination of pools with dysfunctions of the respiratory system and with some other diseases. However, the findings concerning these associations are not always consistent and have not been confirmed by toxicological studies. Therefore, the health effects from swimming in chlorinated pools and the corresponding stress reactions in organisms are unclear. In this study, we show that although the growth and behaviors of experimental rats were not affected, their health, training effects and metabolic profiles were significantly affected by a 12-week swimming training program in chlorinated water identical to that of public pools. Interestingly, the eyes and skin are the organs that are more directly affected than the lungs by the irritants in chlorinated water; instead of chlorination, training intensity, training frequency and choking on water may be the primary factors for lung damage induced by swimming. Among the five major organs (the heart, liver, spleen, lungs and kidneys), the liver is the most likely target of DBPs. Through metabolomics analysis, the corresponding metabolic stress pathways and a defensive system focusing on taurine were presented, based on which the corresponding countermeasures can be developed for swimming athletes and for others who spend a lot of time in chlorinated swimming pools.

Microbial degradation of chloroform

Chloroform (CF) is largely produced by both anthropogenic and natural sources. It is detected in ground and surface water sources and it represents the most abundant halocarbon in the atmosphere. Microbial CF degradation occurs under both aerobic and anaerobic conditions. Apart from a few reports describing the utilization of CF as a terminal electron acceptor during growth, CF degradation was mainly reported as a cometabolic process. CF aerobic cometabolism is supported by growth on short-chain alkanes (i.e., methane, propane, butane, and hexane), aromatic hydrocarbons (i.e., toluene and phenol), and ammonia via the activity of monooxygenases (MOs) operatively divided into different families. The main factors affecting CF cometabolism are (1) the inhibition of CF degradation exerted by the growth substrate, (2) the need for reductant supply to maintain MO activity, and (3) the toxicity of CF degradation products. Under anaerobic conditions, CF degradation was mainly associated to the activity of methanogens, although some examples of CF-degrading sulfate-reducing, fermenting, and acetogenic bacteria are reported in the literature. Higher CF toxicity levels and lower degradation rates were shown by anaerobic systems in comparison to the aerobic ones. Applied physiological and genetic aspects of microbial cometabolism of CF will be presented along with bioremediation perspectives.

Sex-related differences in renal toxicodynamics in rodents

INTRODUCTION

An issue yet to be addressed, in the investigation of the xenobiotic toxicity, is a detailed characterization of the sex differences in toxicological responses. The 'sex issue' is particularly significant in nephrotoxicology as the kidney is a relevant target organ for xenobiotics and few studies have approached this subject in the past. There is a strong need to improve our understanding regarding the influence of sex in toxicology, given their increased requirement to establish the limits of exposure to chemicals in the environment and at work.

AREAS COVERED

In this review, the authors provide the reader with the current knowledge of sex differences in kidney toxicity for rats and mice. To make the review easier to consult, these studies have been organized according to the class of xenobiotic.

EXPERT OPINION

From the analysis of the present knowledge emerges a dramatic need for information on sex differences in xenobiotics toxicity. Although animals are reasonably good predictors of adverse renal effects in patients, there is need to identify alternative methods (e.g. in vitro/ex vivo) to better study sex differences in organ toxicity.

Oxidation of chloroform in an aerobic soil exposed to natural gas

Acclimation of a sandy soil to an air-natural gas mixture stimulated the biological oxidation of chloroform to carbon dioxide. Acetylene and methane inhibited chloroform oxidation. Chloroform oxidation continued up to 31 days in the absence of methane. Chloroform oxidation rates increased at chloroform concentrations up to 5 mug g of soil.

Chloroform Cometabolism by Butane-Grown CF8, Pseudomonas butanovora, and Mycobacterium vaccae JOB5 and Methane-Grown Methylosinus trichosporium OB3b

Chloroform (CF) degradation by a butane-grown enrichment culture, CF8, was compared to that by butane-grown Pseudomonas butanovora and Mycobacterium vaccae JOB5 and to that by a known CF degrader, Methylosinus trichosporium OB3b. All three butane-grown bacteria were able to degrade CF at rates comparable to that of M. trichosporium. CF degradation by all four bacteria required O(inf2). Butane inhibited CF degradation by the butane-grown bacteria, suggesting that butane monooxygenase is responsible for CF degradation. P. butanovora required exogenous reductant to degrade CF, while CF8 and M. vaccae utilized endogenous reductants. Prolonged incubation with CF resulted in decreased CF degradation. CF8 and P. butanovora were more sensitive to CF than either M. trichosporium or M. vaccae. CF degradation by all three butane-grown bacteria was inactivated by acetylene, which is a mechanism-based inhibitor for several monooxygenases. Butane protected all three butane-grown bacteria from inactivation by acetylene, which indicates that the same monooxygenase is responsible for both CF and butane oxidation. CF8 and P. butanovora were able to degrade other chlorinated hydrocarbons, including trichloroethylene, 1,2-cis-dichloroethylene, and vinyl chloride. In addition, CF8 degraded 1,1,2-trichloroethane. The results indicate the potential of butane-grown bacteria for chlorinated hydrocarbon transformation.

Cometabolism of chlorinated solvents by nitrifying bacteria: kinetics, substrate interactions, toxicity effects, and bacterial response

Pure cultures of ammonia-oxidizing bacteria, Nitrosomonas europaea, were exposed to trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), chloroform (CF), 1,2-dichloroethane (1,2-DCA), or carbon tetrachloride (CT), in the presence of ammonia, in a quasi-steady-state bioreactor. Estimates of enzyme kinetics constants, solvent inactivation constants, and culture recovery constants were obtained by simultaneously fitting three model curves to experimental data using nonlinear optimization techniques and an enzyme kinetics model, referred to as the inhibition, inactivation, and recovery (IIR) model, that accounts for inhibition of ammonia oxidation by the solvent, enzyme inactivation by solvent product toxicity, and respondent synthesis of new enzyme (recovery). Results showed relative enzyme affinities for ammonia monooxygenase (AMO) of 1,1-DCE approximately TCE > CT > NH(3) > CF > 1,2-DCA. Relative maximum specific substrate transformation rates were NH(3) > 1,2-DCA > CF > TCE approximately 1,1-DCE > CT (=0). The TCE, CF, and 1,1-DCE inactivated the cells, with 1,1-DCE being about three times more potent than TCE or CF. Under the conditions of these experiments, inactivating injuries caused by TCE and 1,1-DCE appeared limited primarily to the AMO enzyme, but injuries caused by CF appeared to be more generalized. The CT was not oxidized by N. europaea while 1,2-DCA was oxidized quite readily and showed no inactivation effects. Recovery capabilities were demonstrated with all solvents except CF. A method for estimating protein yield, the relationship between the transformation capacity model and the IIR model, and a condition necessary for sustainable cometabolic treatment of inactivating substrates are presented. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 520-534, 1997.

A Green Method for Processing Polymers using Dense Gas Technology

Dense CO2 can be used as an environmentally-benign polymer processing medium because of its liquid-like densities and gas-like mass transfer properties.In this work, polymer bio-blends of polycarbonate (PC), a biocompatible polymer, and polycaprolactone (PCL), a biodegradable polymer were prepared. Dense CO2 was used as a reaction medium for the melt-phase PC polymerization in the presence of dense CO2-swollen PCL particles and this method was used to prepare porous PC/PCL blends. To extend the applicability of dense CO2 to the biomedical industry and polymer blend processing, the impregnation of ibuprofen into the blend was conducted and subsequent dissolution characteristics were observed.

Application of key events analysis to chemical carcinogens and noncarcinogens

The existence of thresholds for toxicants is a matter of debate in chemical risk assessment and regulation. Current risk assessment methods are based on the assumption that, in the absence of sufficient data, carcinogenesis does not have a threshold, while noncarcinogenic endpoints are assumed to be thresholded. Advances in our fundamental understanding of the events that underlie toxicity are providing opportunities to address these assumptions about thresholds. A key events dose-response analytic framework was used to evaluate three aspects of toxicity. The first section illustrates how a fundamental understanding of the mode of action for the hepatic toxicity and the hepatocarcinogenicity of chloroform in rodents can replace the assumption of low-dose linearity. The second section describes how advances in our understanding of the molecular aspects of carcinogenesis allow us to consider the critical steps in genotoxic carcinogenesis in a key events framework. The third section deals with the case of endocrine disrupters, where the most significant question regarding thresholds is the possible additivity to an endogenous background of hormonal activity. Each of the examples suggests that current assumptions about thresholds can be refined. Understanding inter-individual variability in the events involved in toxicological effects may enable a true population threshold(s) to be identified.

Diabetes and hypertriglyceridemia modify the mode of acetaminophen-induced hepatotoxicity and nephrotoxicity in rats and mice

Certain disease conditions can modify drug-induced toxicities, which, in turn, may cause a medication-related health crisis. Therefore, preclinical investigations into the alterations in drug-induced toxicities using appropriate disease animal models are very important. This paper reviews the reported data related to the effects of diabetes and hypertriglyceridemia, common lifestyle-related diseases in a modern society, on acetaminophen (APAP)-induced hepatotoxicity and nephrotoxicity in rats and mice. It has generally been reported that diabetes protects rats and mice from APAP-induced hepatotoxicity and there are several reports that help to speculate on the effects of diabetes on APAP-induced nephrotoxicity. In fructose-induced hypertriglyceridemic rats, hepatotoxicity of APAP becomes apparently less severe, whereas nephrotoxicity of APAP becomes significantly more severe. The mechanisms of alteration of APAP-induced hepatorenal toxicity under diabetic and hypertriglyceridemic conditions are also discussed in this paper.

Effects of concentrated drinking water injection on glutathione and glutathione-dependent enzymes in liver of Cyprinus carpio L

Two drinking water production plants located in North Italy, collecting water from the River Po (Plants 1 and 2) were chosen for this study. Water samples were collected before and after the disinfection process and at two points along the piping system. Water samples were concentrated by the solid-phase extraction system and injected intraperitoneally into specimens of Cyprinus carpio. The concentration of water samples was 3 l/equiv. In order to assess the effects of the water samples on carp liver, total glutathione and glutathione-dependent enzymes, such as glutathione S-transferase, glutathione peroxidase, glutathione reductase and glyoxalase I, were measured following this treatment for 6 days at two experimental times (3 and 6 days). Both water plant-treated carp showed a general increase of the enzymatic activities of glutathione S-transferase, and glutathione reductase which might be employed as potential biomarkers of oxidative stress induced by disinfected river water. Plant 1-treated carp showed higher glyoxalase I and glutathione levels and lower glutathione peroxidase activity. A depleted level of total glutathione and of glyoxalase I for specimens of water plant 2 (for both experimental times), without correlation with the distances in the pipeline, suggests that river plant water can also lead to potentially adverse effects on selected biochemical parameters in C. carpio.

Mechanisms of chloroform-induced hepatotoxicity: oxidative stress and mitochondrial permeability transition in freshly isolated mouse hepatocytes

The role of mitochondrial permeability transition (MPT) and oxidative stress in chloroform toxicity was determined in freshly isolated female B6C3F1 mouse hepatocytes. Incubation of chloroform (12 mM) with hepatocytes resulted in cell death (alanine aminotransferase release and propidium iodide fluorescence). Chloroform had volatilized from the incubation and glutathione was depleted by 1 h; however, toxicity was not significantly different between control and chloroform-incubated cells. Hepatocytes were washed and reincubated in fresh media at 1 h. Subsequent reincubation of chloroform-treated hepatocytes resulted in significant toxicity at 3-5 h. Inclusion of the MPT inhibitor cyclosporine A or the antioxidant N-acetylcysteine (NAC) in the reincubation media at 1 h prevented toxicity. Confocal microscopy studies with the dye calcein AM indicated MPT that was blocked by cyclosporine A or NAC. Fluorescence microscopy studies utilizing JC-1 indicated loss of mitochondrial membrane potential, which was also blocked by cyclosporine A or NAC. Dichlorofluorescein fluorescence increased during the reincubation phase, indicating increased oxidative stress, and the increase was blocked by cyclosporine A. Since oxidative stress may occur by peroxynitrite, its role in toxicity was examined. Either of the nitric oxide synthase inhibitors N(G)-methyl-L-arginine (L-NMMA) and 7-nitroindazole (7-NI) at 1 h blocked toxicity. Western blot analysis of hepatocytes for 3-nitrotyrosine in proteins, a biomarker of peroxynitrite, indicated one major nitrated protein at 81 kD. Nitration of this protein was inhibited by cyclosporine A, L-NMMA, 7-NI, or NAC. The data indicate that chloroform-induced cell death occurs in two phases: a metabolic phase characterized by glutathione depletion, and an oxidative phase characterized by MPT and protein nitration.

Mechanism of chloroform-induced renal toxicity: non-involvement of hepatic cytochrome P450-dependent metabolism

Chloroform causes hepatic and renal toxicity in a number of species. In vitro studies have indicated that chloroform can be metabolized by P450 enzymes in the kidney to nephrotoxic intermediate, although direct in vivo evidence for the role of renal P450 in the nephrotoxicity has not been reported. This study was to determine whether chloroform renal toxicity persists in a mouse model with a liver-specific deletion of the P450 reductase (Cpr) gene (liver-Cpr-null). Chloroform-induced renal toxicity and chloroform tissue levels were compared between the liver-Cpr-null and wild-type mice at 24 h following differing doses of chloroform. At a chloroform dose of 150 mg/kg, the levels of blood urea nitrogen (BUN) were five times higher in the exposed group than in the vehicle-treated one for the liver-Cpr-null mice, but they were only slightly higher in the exposed group than in the vehicle-treated group for the wild-type mice. Severe lesions were found in the kidney of the liver-Cpr-null mice, while only mild lesions were found in the wild-type mice. At a chloroform dose of 300 mg/kg, severe kidney lesions were observed in both strains, yet the BUN levels were still higher in the liver-Cpr-null than in the wild-type mice. Higher chloroform levels were found in the tissues of the liver-Cpr-null mice. These findings indicated that loss of hepatic P450-dependent chloroform metabolism does not protect against chloroform-induced renal toxicity, suggesting that renal P450 enzymes play an essential role in chloroform renal toxicity.

Use of a physiologically based pharmacokinetic model to identify exposures consistent with human biomonitoring data for chloroform

Biomonitoring data provide evidence of human exposure to environmental chemicals by quantifying the chemical or its metabolite in a biological matrix. To better understand the correlation between biomonitoring data and environmental exposure, physiologically based pharmacokinetic (PBPK) modeling can be of use. The objective of this study was to use a combined PBPK model with an exposure model for showering to estimate the intake concentrations of chloroform based on measured blood and exhaled breath concentrations of chloroform. First, the predictive ability of the combined model was evaluated with three published studies describing exhaled breath and blood concentrations in people exposed to chloroform under controlled showering events. Following that, a plausible exposure regimen was defined combining inhalation, ingestion, and dermal exposures associated with residential use of water containing typical concentrations of chloroform to simulate blood and exhaled breath concentrations of chloroform. Simulation results showed that inhalation and dermal exposure could contribute substantially to total chloroform exposure. Next, sensitivity analysis and Monte Carlo analysis were performed to investigate the sources of variability in model output. The variability in exposure conditions (e.g., shower duration) was shown to contribute more than the variability in pharmacokinetics (e.g., body weight) to the predicted variability in blood and exhaled breath concentrations of chloroform. Lastly, the model was used in a reverse dosimetry approach to estimate distributions of exposure consistent with concentrations of chloroform measured in human blood and exhaled breath.

Hepatic antioxidant enzymes and total glutathione of Cyprinus carpio exposed to three disinfectants, chlorine dioxide, sodium hypochlorite and peracetic acid, for superficial water potabilization

This study was carried out in order to assess the effects of disinfectant-treatment on antioxidant response of Cyprinus carpio L. Therefore, enzymatic activities of glutathione S-transferases, glyoxalase I, glyoxalase II, glutathione peroxidases, glutathione reductase, catalase and total glutathione content of carp liver, exposed to surface water treated with three disinfectants for potabilization, sodium hypochlorite, chlorine dioxide and peracetic acid were investigated. Specimens of carp were exposed in four experimental tanks supplied with a continuous water flow from Lake Trasimeno (Italy), three of them treated with constant concentration of sodium hypochlorite, chlorine dioxide and peracetic acid, for 10 and 20 days, while the control tank was supplied with untreated lake water. Differences in biochemical parameters were observed in specimens following exposure to these disinfectants and mainly, chlorine compounds induced marked biochemical variations of carp liver, compared to those induced by peracetic acid treatment. Our results showed that antioxidant parameters of Cyprinus carpio could be used as biomarkers of oxidative stress when this species is exposed to disinfectants for water potabilization.

Characterization of the metabolic interaction between trihalomethanes and chloroacetic acids using rat liver microsomes

The aim of this study was to investigate the in vitro metabolism of trihalomethanes (THMs) in the presence of trichloroacetic acid (TCA), dichloracetic acid (DCA), monochloroacetic acid (MCA), and 4-methylpyrazole (4-MP) using liver microsomes from male Sprague-Dawley rats. Using the vial equilibration technique, initial experiments were carried out with starting concentrations of approximately 40 ppm THMs and 12-22 mM chloroacetic acids. The results indicated a mutual metabolic inhibition between THMs present as binary or quaternary mixtures. Although DCA and MCA had no influence on THMs, TCA produced a marked inhibition of the metabolism of all THMs: chloroform (CHCl3) (55%), bromodichloromethane (BDCM) (34%), dibromochloromethane (DBCM) (30%), and bromoform (TBM) (23%). The presence of 4-MP also reduced THM metabolism, the importance of which decreased in the following order: CHCl3 > BDCM > DBCM = TBM. In further vial equilibration experiments, using 9-140 ppm as starting concentrations of THMs, enzyme kinetic parameters (i.e., Michaelis constant, K(m), and maximum velocity, V(max)) were determined both in the absence and in the presence of TCA (12.2 mM). Results are consistent with a competitive inhibition between TCA and CHCl3, whereas the metabolic inhibition of BDCM and TMB by TCA was non-competitive. As for DBCM, results suggest a more complex pattern of inhibition. These results suggest that CYP2E1 is involved in the metabolism of THMs as well as in the metabolic interaction between THMs and TCA.

Hepatotoxicity and mechanism of action of haloalkanes: carbon tetrachloride as a toxicological model

The use of many halogenated alkanes such as carbon tetrachloride (CCl4), chloroform (CHCl3) or iodoform (CHI3), has been banned or severely restricted because of their distinct toxicity. Yet CCl4 continues to provide an important service today as a model substance to elucidate the mechanisms of action of hepatotoxic effects such as fatty degeneration, fibrosis, hepatocellular death, and carcinogenicity. In a matter of dose,exposure time, presence of potentiating agents, or age of the affected organism, regeneration can take place and lead to full recovery from liver damage. CCl4 is activated by cytochrome (CYP)2E1, CYP2B1 or CYP2B2, and possibly CYP3A, to form the trichloromethyl radical, CCl3*. This radical can bind to cellular molecules (nucleic acid, protein, lipid), impairing crucial cellular processes such as lipid metabolism, with the potential outcome of fatty degeneration (steatosis). Adduct formation between CCl3* and DNA is thought to function as initiator of hepatic cancer. This radical can also react with oxygen to form the trichloromethylperoxy radical CCl3OO*, a highly reactive species. CCl3OO* initiates the chain reaction of lipid peroxidation, which attacks and destroys polyunsaturated fatty acids, in particular those associated with phospholipids. This affects the permeabilities of mitochondrial, endoplasmic reticulum, and plasma membranes, resulting in the loss of cellular calcium sequestration and homeostasis, which can contribute heavily to subsequent cell damage. Among the degradation products of fatty acids are reactive aldehydes, especially 4-hydroxynonenal, which bind easily to functional groups of proteins and inhibit important enzyme activities. CCl4 intoxication also leads to hypomethylation of cellular components; in the case of RNA the outcome is thought to be inhibition of protein synthesis, in the case of phospholipids it plays a role in the inhibition of lipoprotein secretion. None of these processes per se is considered the ultimate cause of CCl4-induced cell death; it is by cooperation that they achieve a fatal outcome, provided the toxicant acts in a high single dose, or over longer periods of time at low doses. At the molecular level CCl4 activates tumor necrosis factor (TNF)alpha, nitric oxide (NO), and transforming growth factors (TGF)-alpha and -beta in the cell, processes that appear to direct the cell primarily toward (self-)destruction or fibrosis. TNFalpha pushes toward apoptosis, whereas the TGFs appear to direct toward fibrosis. Interleukin (IL)-6, although induced by TNFalpha, has a clearly antiapoptotic effect, and IL-10 also counteracts TNFalpha action. Thus, both interleukins have the potential to initiate recovery of the CCl4-damaged hepatocyte. Several of the above-mentioned toxication processes can be specifically interrupted with the use of antioxidants and mitogens, respectively, by restoring cellular methylation, or by preserving calcium sequestration. Chemicals that induce cytochromes that metabolize CCl4, or delay tissue regeneration when co-administered with CCl4 will potentiate its toxicity thoroughly, while appropriate CYP450 inhibitors will alleviate much of the toxicity. Oxygen partial pressure can also direct the course of CCl4 hepatotoxicity. Pressures between 5 and 35 mmHg favor lipid peroxidation, whereas absence of oxygen, as well as a partial pressure above 100 mmHg, both prevent lipid peroxidation entirely. Consequently, the location of CCl4-induced damage mirrors the oxygen gradient across the liver lobule. Mixed halogenated methanes and ethanes, found as so-called disinfection byproducts at low concentration in drinking water, elicit symptoms of toxicity very similar to carbon tetrachloride, including carcinogenicity.

Evaluation of the influence of chloroacetic acids on the pharmacokinetics of trihalomethanes in the rat

Chloroacetic acids (monochloroacetic acid [MCA], dichloroacetic acid [DCA], and trichloroacetic acid [TCA]) and trihalomethanes (THMs: chloroform [CHCl(3)], bromodichloromethane [BDCM], dibromochloromethane [DBCM], and bromoform [TBM]) are common by-products of the chlorination of drinking water. The purpose of this study was to evaluate the influence of chloroacetic acids on the pharmacokinetics of trihalomethanes in the male Sprague-Dawley rat. In the first series of studies, groups of 5 animals were given, by intravenous injections, a single dose of 0.125 mmol/kg of one of the four THMs. Additional groups received a binary mixture containing 0.125 mmol/kg of a THM plus 0.125 mmol/kg of a chloroacetic acid. The venous blood concentrations of unchanged THMs were measured by headspace gas chromatography from 5 min to 6 h postadministration. The areas under the blood concentration versus time curves (AUCs) of CHCl(3), BDCM, and DBCM were increased by a factor of 3.5, 1.6, and 2, respectively, by coadministration of TCA. DCA coadministration resulted in an increase in the AUC of DBCM (x2.5) and TBM (x1.3), whereas MCA modified the Cmax (x1.5) and AUC (x1.8) of BDCM and the AUC of DBCM (x2.5). In the second series of experiments, animals received either a single dose of 0.03125 mmol/kg of one of the four THMs, a mixture containing 0.03125 mmol/kg of each of the four THMs (total dose = 0.125 mmol/kg), or a mixture containing 0.03125 mmol/kg of each of the four THMs plus 0.125 mmol/kg of either TCA or DCA. Results indicated that the AUCs of CHCl(3), BDCM, DBCM, and TBM were increased during coadministration compared to single administrations (+2.5-fold). Combined administration of the four THMs with TCA, and not DCA, resulted in an increase of the AUCs of THMs (CHCl(3): x11.7; BDCM, DBCM, and TBM: x3.9) and an increase in the Cmax of CHCl(3) (x1.9). Overall, these results indicate that, at the dose levels tested in this study, TCA alters the blood concentration profiles of THMs.

Metabolism of chloroform in the human liver and identification of the competent P450s

The oxidative and reductive cytochrome P450 (P450)-mediated chloroform bioactivation has been investigated in human liver microsomes (HLM), and the role of human P450s have been defined by integrating results from several experimental approaches: cDNA-expressed P450s, selective chemical inhibitors and specific antibodies, correlation studies in a panel of phenotyped HLM. HLM bioactivated CHCl(3) both oxidatively and reductively. Oxidative reaction was characterized by two components, suggesting multiple P450 involvement. The high affinity process was catalyzed by CYP2E1, as clearly indicated by kinetic studies, correlation with chlorzoxazone 6-hydroxylation (r = 0.837; p < 0.001), and inhibition by monoclonal antihuman CYP2E1 and 4-methylpyrazole. The low affinity phase of oxidative metabolism was essentially catalyzed by CYP2A6. This conclusion was supported by the correlation with coumarin 7-hydroxylase (r = 0.777; p < 0.01), inhibition by coumarin and by the specific antibody, in addition to results with heterologously expressed P450s. Chloroform oxidation was poorly dependent on pO(2), whereas the reductive metabolism was highly inhibited by O(2). The production of dichloromethyl radical was significant only at CHCl(3) concentration > or =1 mM, increasing linearly with substrate concentration. CYP2E1 was the primary enzyme involved in the reductive reaction, as univocally indicated by all the different approaches. The reductive pathway seems to be scarcely relevant in the human liver, since it is active only at high substrate concentrations, and in strictly anaerobic conditions. The role of human CYP2E1 in CHCl(3) metabolism at low levels, typical of actual human exposure, provides insight into the molecular basis for eventual difference in susceptibility to chloroform-induced effects due to either genetic, pathophysiological, or environmental factors.

Chloroform: exposure estimation, hazard characterization, and exposure-response analysis

Chloroform has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to chloroform principally through inhalation of indoor air, particularly during showering, and through ingestion of tap water. Data on concentrations of chloroform in various media were sufficient to serve as the basis for development of deterministic and probabilistic estimates of exposure for the general population in Canada. On the basis of data acquired principally in studies in experimental animals, chloroform causes hepatic and renal tumors in mice and renal tumors in rats. The weight of evidence indicates that chloroform is likely carcinogenic only at concentrations that induce the obligatory precursor lesions of cytotoxicity and proliferative regenerative response. Since this cytotoxicity is primarily related to rates of formation of reactive, oxidative metabolites, dose response has been characterized in the context of rates of formation of reactive metabolites in the target tissue. Results presented here are from a "hybrid" physiologically based pharmacokinetic (PBPK) animal model that was revised to permit its extension to humans. The relevant measure of exposure response, namely, the mean rate of metabolism in humans associated with a 5% increase in tumor risk (TC05), was estimated on the basis of this PBPK model and compared with tissue dose measures resulting from 24-h multimedia exposure scenarios for Canadians based on midpoint and 95th percentiles for concentrations in outdoor air, indoor air, air in the shower compartment, air in the bathroom after showering, tap water, and food. Nonneoplastic effects observed most consistently at lowest concentrations or doses following repeated exposures of rats and mice to chloroform are cytotoxicity and regenerative proliferation. As for cancer, target organs are the liver and kidney. In addition, chloroform has induced nasal lesions in rats and mice exposed by both inhalation and ingestion at lowest concentrations or doses. The mean rate of metabolism associated with a 5% increase in fatty cysts estimated on the basis of the PBPK model was compared with tissue dose measures resulting from the scenarios already described, and lowest concentrations reported to induce cellular proliferation in the nasal cavities of rats and mice were compared directly with midpoint and 95th percentile estimates of concentrations of chloroform in indoor air in Canada. The degree of confidence in the underlying database and uncertainties in estimates of exposure and in characterization of hazard and dose response are delineated.

Correlation of a specific mitochondrial phospholipid-phosgene adduct with chloroform acute toxicity

The dose and time dependence of formation of a specific adduct between mitochondrial phospholipid and phosgene have been determined in the liver of Sprague-Dawley (SD) rats as well as in the liver and kidney of B6C3F1 mice after dosing with chloroform. Rats were induced with phenobarbital or non-induced. Determination of tissue glutathione (GSH) and of serum markers of hepatotoxicity and nephrotoxicity was also carried out. With dose-dependence experiments, a strong correlation between the formation of the specific phospholipid adduct, GSH depletion and organ toxicity could be evidenced in all the organs studied. With non-induced SD rats, no such effects could be induced up to a dose of 740 mg/kg. Time-course studies with B6C3F1 mice indicated that the specific adduct formation took place at very early times after chloroform dosing and was concurrent with GSH depletion. The adduct formed during even transient GSH depletion (residual level: 30% of control) and persisted after restoration of GSH levels. Following a chloroform dose at the hepatotoxicity threshold (150 mg/kg), the elimination of the adduct in the liver occurred within 24 h and correlated with the recovery of ALT, which was slightly increased (12 times) after treatment. Following a moderately nephrotoxic dose (60 mg/kg), the renal adduct persisted longer than 48 h, when a 100% increase in blood urea nitrogen and a 40% increase in serum creatinine indicated the onset of organ damage. The formation of the adduct in the liver mitochondria of B6C3F1 mice was associated with the decrease of phosphatidyl-ethanolamine (PE), in line with previous results in rat liver indicating that the adduct results from the reaction of phosgene with PE. The adduct levels implicated the reaction of phosgene with about 50% PE molecules in the liver mitochondrial membrane of phenobarbital-induced SD rats and of about 10% PE molecules of the inner mitochondrial membrane of the liver of B6C3F1 mice. The association of this adduct with the toxic effects of chloroform makes it a very good candidate as the primary critical alteration in the sequence of events leading to cell death caused by chloroform.

The utility of PBPK in the safety assessment of chloroform and carbon tetrachloride

Occupational exposure limits (OELs) for individual substances are established on the basis of the available toxicological information at the time of their promulgation, expert interpretation of these data in light of industrial use, and the framework in which they sit. In the United Kingdom, the establishment of specific OELs includes the application of uncertainty factors to a defined starting point, usually the NOAEL from a suitable animal study. The magnitude of the uncertainty factors is generally determined through expert judgment including a knowledge of workplace conditions and management of exposure. PBPK modeling may help in this process by informing on issues relating to extrapolation between and within species. This study was therefore designed to consider how PBPK modeling could contribute to the establishment of OELs. PBPK models were developed for chloroform (mouse and human) and carbon tetrachloride (rat and human). These substances were chosen for examination because of the extent of their toxicological databases and availability of existing PBPK models. The models were exercised to predict the rate (chloroform) or extent (carbon tetrachloride) of metabolism of these substances, in both rodents and humans. Monte Carlo analysis was used to investigate the influence of variability within the human and animal model populations. The ratio of the rates/extent of metabolism predicted for humans compared to animals was compared to the uncertainty factors involved in setting the OES. Predictions obtained from the PBPK models indicated that average rat and mouse metabolism of carbon tetrachloride and chloroform, respectively, are much greater than that of the average human. Application of Monte Carlo analysis indicated that even those people who have the fastest rates or most extensive amounts of metabolism in the population are unlikely to generate the levels of metabolite of these substances necessary to produce overt toxicity in rodents. This study highlights the value that the use of PBPK modeling may add to help inform and improve toxicological aspects of a regulatory process.

Chlorinated methanes and liver injury: highlights of the past 50 years

The chlorinated methanes, particularly carbon tetrachloride and chloroform, are classic models of liver injury and have developed into important experimental hepatoxicants over the past 50 years. Hepatocellular steatosis and necrosis are features of the acute lesion. Mitochondria and the endoplasmic reticulum as target sites are discussed. The sympathetic nervous system, hepatic hemodynamic alterations, and role of free radicals and biotransformation are considered. With carbon tetrachloride, lipid peroxidation and covalent binding to hepatic constituents have been dominant themes over the years. Potentiation of chlorinated methane-induced liver injury by alcohols, aliphatic ketones, ketogenic compounds, and the pesticide chlordecone is discussed. A search for explanations for the potentiation phenomenon has led to the discovery of the role of tissue repair in the overall outcome of liver injury. Some final thoughts about future research are also presented.

Laboratory of Chemical Pharmacology, National Heart, Lung, and Blood Institute, NIH: a short history

The Laboratory of Chemical Pharmacology (LCP) began in 1950 as the Section of Pharmacology within the National Heart Institute, the National Institutes of Health. Its first chief was Bernard B. Brodie, considered by many to be one of the fathers of modern pharmacology. Since its inception, LCP has made many significant contributions to the fields of pharmacology and toxicology. LCP was among the first to study (a) the effects of drugs on the turnover of serotonin and norepinephrine in brain and other tissues, (b) the absorption of drugs from the gastrointestinal tract and their passage across the blood-brain barrier, (c) the oxidation and reduction of drugs and other foreign compounds by liver microsomal enzymes (later known as the cytochrome P450 enzymes) and inhibitors and inducers of these enzymes, (d) the formation of toxic chemically reactive metabolites of drugs and other foreign compounds, and (e) mechanisms of immunological responses. Approximately 300 scientists worked in LCP during its existence, and they and their collaborators published more than 1,300 papers. This is a short history of the people who worked in it and of their contributions to biomedical sciences.

Chloropicrin dechlorination in relation to toxic action

Chloropicrin (CCl3NO2) is a widely used soil fumigant with an unknown mechanism of acute toxicity. We investigated the possible involvement of dechlorination in CCl3NO2 toxicity by considering its metabolism, inhibition of pyruvate and succinate dehydrogenases, cytotoxicity in cultured cells, and interaction with hemoproteins. In a newly discovered pathway, CCl3NO2 is metabolized to thiophosgene, which is characterized as the cyclic cysteine adduct (raphanusamic acid) in the urine of mice. CCl3NO2 inhibits porcine heart pyruvate dehydrogenase complex (IC-50 4 microM) and mouse liver succinate dehydrogenase complex (IC-50 13 microM), whereas its dehalogenated metabolites (CHCl2NO2 and CH2ClNO2) are more than 10 times less effective. The inhibitory potency of CCl3NO2 for these dehydrogenase complexes is similar to that of captan, folpet, and dichlone fungicides (IC-50 2-6 microM). CCl3NO2 cytotoxicity with Hepa 1c1c7+ mouse hepatoma cells (IC-50 9 microM) is not correlated with glutathione depletion. Mice treated intraperitoneally with CCl3NO2 at 50 mg/kg but not with an equivalent dose of CHCl2NO2 show increased concentrations of oxyhemoglobin in liver. The acute toxicity of CCl3NO2 in mice is due to the parent compound or metabolites other than CHCl2NO2 or CH2ClNO2 and may be associated with inhibition of the pyruvate dehydrogenase complex and elevated oxyhemoglobin.

Comparative characterization of CHCl(3) metabolism and toxicokinetics in rodent strains differently susceptible to chloroform-induced carcinogenicity

A comparative kinetic study in B6C3F1 mice, Osborne-Mendel (OM) and Sprague-Dawley (SD) rats has been undertaken with the major aim to determine the extent of covalent binding of chloroform reactive metabolites produced in vivo through oxidative and/or reductive metabolism in the target organs of chloroform carcinogenicity. Some additional kinetic observations of chloroform biotransformation were also collected comparatively. Expiration of [14C]-CO(2) showed that chloroform metabolism went to saturation in all tested rodent strains. In the B6C3F1 mouse maximal rates of approximately 135 µmol [14C]-CO(2)/kg b.w./h were reached at a dose of approximately 150 mg/kg, while in the two rat strains saturation occurred at a dose of approximately 60 mg/kg, with a maximal rate of approximately 40 µmol [14C]-CO(2)/kg b.w./h. At doses of 150-180 mg/kg b.w., limited differences were found in the distribution and elimination of [14C]-chloroform in the liver and kidney. Species differences have been found in the kinetics of alkali-extractable radioactivity in the blood. The levels of adducts of electrophilic intermediates with the polar heads (PH) of phospholipids (PL) showed a limited variability accross the rodents tested and did not correlate with the species and organ susceptibility to chloroform carcinogenicity. The levels of adducts of radical intermediates with the fatty acyl chains (FC) of PL were much lower than the PH adducts in all the samples analyzed; at the carcinogenicity bioassay doses, statistically significant levels of hepatic FC adducts were present only in the B6C3F1 mouse, where chloroform is hepatocarcinogenic. The observations in the rat kidney were suggestive of the formation of electrophilic reactive metabolites, presumably different from phosgene and associated with an initial chloroform reduction.

Metabolism of chloroform by cytochrome P450 2E1 is required for induction of toxicity in the liver, kidney, and nose of male mice

Chloroform is a nongenotoxic-cytotoxic liver and kidney carcinogen and nasal toxicant in some strains and sexes of rodents. Substantial evidence indicates that tumor induction is secondary to events associated with cytolethality and regenerative cell proliferation. Therefore, pathways leading to toxicity, such as metabolic activation, become critical information in mechanism-based risk assessments. The purpose of this study was to determine the degree to which chloroform-induced cytotoxicity is dependent on the cytochromes P450 in general and P450 2E1 in particular. Male B6C3F(1), Sv/129 wild-type (Cyp2e1+/+), and Sv/129 CYP2E1 knockout (Cyp2e1-/- or Cyp2e1-null) mice were exposed 6 h/day for 4 consecutive days to 90 ppm chloroform by inhalation. Parallel control and treated groups, excluding Cyp2e1-null mice, also received an i.p. injection (150 mg/kg) of the irreversible cytochrome P450 inhibitor 1-aminobenzotriazole (ABT) twice on the day before exposures began and 1 h before every exposure. Cells in S-phase were labeled by infusion of BrdU via an implanted osmotic pump for 3.5 days prior to necropsy, and the labeling index was quantified immunohistochemically. B6C3F(1) and Sv/129 wild-type mice exposed to chloroform alone had extensive hepatic and renal necrosis with significant regenerative cell proliferation. These animals had minimal toxicity in the nasal turbinates with focal periosteal cell proliferation. Administration of ABT completely protected against the hepatic, renal, and nasal toxic effects of chloroform. Induced pathological changes and regenerative cell proliferation were absent in these target sites in Cyp2e1-/- mice exposed to 90 ppm chloroform. These findings indicate that metabolism is obligatory for the development of chloroform-induced hepatic, renal, and nasal toxicity and that cytochrome P450 2E1 appears to be the only enzyme responsible for this cytotoxic-related metabolic conversion under these exposure conditions.

Influence of oral administration of a quaternary mixture of trihalomethanes on their blood kinetics in the rat

Trihalomethanes (THMs; chloroform, bromoform, bromodichloromethane, dibromochloromethane), formed as by-products of chlorine disinfection, are found to occur in combination in drinking water supplies. THMs are metabolized by cytochromes P-450 and are likely substrates of CYP2E1. Therefore, it is possible that mixed exposure results in toxicokinetic interactions among THMs. The toxicokinetics of THMs during mixed exposures has not been investigated previously. The purpose of this study was to characterize the blood kinetics of the four THMs administered singly or in combination in the rat. A single dose of 0.25 mmol/kg or 0.5 mmol/kg b.w., of each THM alone, or of a quaternary mixture containing 0.25 mmol/kg of each THM (total dose of 1.0 mmol/kg) was administered by gavage. The venous blood concentrations of the THMs were measured by headspace gas chromatography (GC) at 20, 40, 60, 120, 180, 270 and 360 min post-administration. Results showed a nonlinear relationship between the area under the blood concentration versus time curves (AUCs) and administered doses of THMs, suggesting that metabolism is saturated in this dose range. The venous blood concentrations of THMs following administration of the quaternary mixture were significantly higher compared to single exposures. The altered kinetics of THMs during combined exposures is consistent with the occurrence of mutual inhibition of their hepatic metabolism. Simulation exercises conducted with physiologically based toxicokinetic models support metabolic inhibition as the possible mechanism of the interaction among THMs. The data reported in this study provide the starting point for evaluating the significance of interactions among THMs in the risk assessment process.

Effects of singly administered betaine on hepatotoxicity of chloroform in mice

Effects of a single dose of betaine on the chloroform-induced hepatotoxicity were examined in adult male ICR mice. Administration of betaine (1000 mg/kg, ip) 1 to 7 hr prior to a chloroform challenge (0.25 ml/kg, ip) resulted in remarkable enhancement of hepatotoxicity as indicated by increases in serum sorbitol dehydrogenase (SDH), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities. The potentiation of hepatotoxicity was most significant when mice were treated with betaine 4 hr earlier than chloroform. However, a 24 hr prior administration of betaine protected the animals from induction of the chloroform hepatotoxicity. Thus, its effect appeared to be highly dependent on the time lapse from the betaine pretreatment to the challenge of mice with chloroform. Betaine treated either 4 or 24 hr prior to sacrifice did not alter the hepatic contents of cytochrome P-450, cytochrome b5, or NADPH cytochrome P-450 reductase activity. Accordingly the hepatic microsomal p-nitroanisole O-demethylase, aminopyrine N-demethylase, or p-nitrophenol hydroxylase activities were not influenced by the betaine pretreatment. Betaine was shown not to affect any of the enzyme activities associated with glutathione (GSH) conjugation reaction, such as glutathione S-transferases (GSTs), glutathione disulfide (GSSG) reductase and GSH peroxidase irrespective of the time of its administration. When betaine was administered to mice 2-6 hr prior to sacrifice, hepatic GSH level, but not plasma GSH, was decreased significantly. Enhancement of the chloroform hepatotoxicity by betaine correlated well with the reduction in hepatic GSH levels. Both hepatic and plasma GSH levels were elevated in mice 24 hr following the betaine treatment. The results suggest that betaine affects induction of the chloroform hepatotoxicity by modulating the availability of hepatic GSH, which appears to be associated with its role in the transsulfuration pathway in the liver.

Chloroform-induced cytolethality in freshly isolated male B6C3F1 mouse and F-344 rat hepatocytes

Chloroform is carcinogenic in rodents but is not mutagenic or DNA reactive. Chloroform-induced hepatocarcinogenesis in rodents is believed to be secondary to events associated with cytotoxicity and cell proliferation. Understanding the mechanisms of chloroform toxicity may provide insights into the mechanisms of carcinogenicity. The goal of these studies was to characterize the cytotoxicity of chloroform in male B6C3F1 mouse and F-344 rat hepatocytes in vitro. We used an in vitro suspension-culture system that reproduced the exposure of the liver to chloroform and the expression of toxicity in vivo. Simulations of a physiologically based dosimetry model for chloroform indicated that the livers of mice and rats were exposed to chloroform concentrations up to 5 mM for 3 h after hepatotoxic doses of chloroform. Freshly isolated male mouse and rat hepatocytes were exposed to chloroform in sealed flasks and then cultured for 24 h as monolayers. Following a 2- or 3-h exposure in suspension, chloroform induced concentration-dependent cytotoxicity (lactate dehydrogenase release) in culture at concentrations higher than 1 mM. Cytolethality was not increased under reduced oxygen tension, indicating that reductive metabolism does not contribute to chloroform-induced toxicity. A threshold of 1 mM chloroform was also found for glutathione (GSH) depletion, with a 50% depletion at 3.8 mM after 2 h. Addition of dithiothreitol, a reducing agent, did not prevent chloroform-induced toxicity, indicating that oxidation of sulfhydryl groups is not critical for toxicity. The lack of protein sulfhydryl group depletion is consistent with this conclusion. Cotreatment with the cytochrome P450 inhibitor 1-phenylimidazole prevented both cytolethality and GSH depletion, indicating that metabolism is necessary for chloroform-induced toxicity. Both species exhibited similar sensitivity toward chloroform toxicity, indicating that toxicity is not sufficient to explain different susceptibility in heptocarcinogenicity. As chloroform metabolism is saturated in the micromolar range, our results indicate that both metabolism and exposure of the liver cells to high concentrations of chloroform are required for toxicity.

Enhanced nephrotoxicity of acetaminophen in fructose-induced hypertriglyceridemic rats: contribution of oxidation and deacetylation of acetaminophen to an enhancement of nephrotoxicity

Fructose-induced hypertriglyceridemic Sprague-Dawley (SD) rats become resistant to hepatotoxicity and susceptible to nephrotoxicity of acetaminophen (APAP) as compared with normal SD rats. Fischer-344 rats, which are susceptible to APAP nephrotoxicity, have two toxic metabolic pathways involving cytochrome P450-dependent oxidation of APAP to N-acetyl-p-benzoquinone imine (NAPQI) and P450-independent deacetylation of APAP to p-aminophenol (PAP). SD rats, however, have only the former pathway. This study was undertaken to investigate whether alterations in the metabolic pathways of APAP and in the intrinsic susceptibility to toxic metabolites are responsible for an enhancement of APAP nephrotoxicity in the fructose-pretreated SD-rats. In the non-pretreated rats, the inhibition of APAP oxidation by the MFO inhibitor, piperonyl butoxide, and deacetylation by carboxyesterase inhibitor, bis(p-nitrophenyl)phosphate, did not alter APAP-induced renal lesions. In contrast, these inhibitors protected the fructose-pretreated rats from APAP-induced renal lesions. Since there were no differences in the severity of gentamicin-, chloroform, and 45 min-ischemia/reperfusion-induced renal lesions between the non-pretreated and the fructose-pretreated rats, it is unlikely that the increased intrinsic susceptibility to chemicals and their metabolites in the fructose-pretreated rats is a major factor in the enhancement of APAP nephrotoxicity. These results indicate that the enhancement of APAP nephrotoxicity in the fructose-pretreated rats is due, at least in part, to an alteration in metabolic pathways of APAP.

Chloroform mode of action: implications for cancer risk assessment

Risk assessment methodology, particularly pertaining to potential human carcinogenic risks from exposures to environmental chemicals, is undergoing intense scrutiny from scientists, regulators, and legislators. The current practice of estimating human cancer risk is based almost exclusively on extrapolating the results of chronic, high-dose studies in rodents to estimate potential risk in humans. However, many scientists are questioning whether the logic used in this current risk assessment methodology is the best way to safeguard public health. A major tool of human cancer risk assessment is the linearized multistage (LMS) model. The LMS model has been identified as an aspect of risk assessment that could be improved. One way to facilitate this improvement is by developing a way to incorporate a carefully derived, more biologically relevant mechanism of action data on carcinogenesis. Recent data on chloroform indicate that the dose-response relationship for chloroform-induced tumors in rats and mice is nonlinear, based upon events secondary to cell necrosis and subsequent regeneration as the likely mode of action for the carcinogenic effects of chloroform. In light of these data, there is a sound scientific basis for removing some of the uncertainty that accompanies current cancer risk assessments of chloroform. The following points summarize the critical data: (1) a substantial body of data demonstrates a lack of direct in vivo or in vitro genotoxicity of chloroform; (2) chloroform induces liver and kidney tumors in long-term rodent cancer bioassays only at doses that induce frank toxicity at these target sites; (3) the chloroform doses required to produce tumors in susceptible species exceed the MTD, often by a considerable margin; (4) cytotoxicity and compensatory cell proliferation are associated with the chloroform doses required to induce liver or kidney tumors in susceptible rodent species; (5) there are no instances of chloroform-induced tumors that are not preceded by this pattern of dose-dependent toxic responses; (6) it is biologically plausible that cytolethality leads to chronically stimulated cell proliferation and related events such as inflammation and growth stimulation which act to initiate and promote the carcinogenic process; and (7) the consistently linked cellular events of cytolethality and subsequent cell proliferation, for which doses of no adverse effect have been clearly shown to exist, are one of the biological prerequisites for chloroform-induced tumors in animals. Based on these data, it is inappropriate to extrapolate cancer risk from high doses that produce necrosis and regenerative cell proliferation to low doses that do not with a model that presumes genotoxicity and a linear dose-response relationship. The weight of the scientific evidence concerning chloroform-induced tumors in rodents is consistent with and supports a cancer risk assessment methodology based on mode of action as the basis for establishing regulatory standards for this compound.

Glutathione S-transferase-mediated mutagenicity of trihalomethanes in Salmonella typhimurium: contrasting results with bromodichloromethane off chloroform

Trihalomethanes (THMs) are the most prevalent disinfection by-products identified in chlorinated drinking water. Among the THMs, chloroform (CHCl3) generally occurs at the highest concentration in finished water, but the concentrations of each of the brominated THMs (CHBrCl2, CHBr2Cl, and CHBr3) can exceed that of CHCl3. Each of these four THMs was carcinogenic in rodents in chronic oral dosing studies. This study assessed THM mutagenicity in a strain of Salmonella typhimurium TA1535 that was transfected with rat theta-class glutathione S-transferase T1-1 (+GST). The +GST strain and its nontransfected parent strain (-GST) were employed in a plate-incorporation assay and exposed for 24 hr to the vapor of individual THMs at concentrations up to 25,600 ppm in sealed Tedlar bags. Base-substitution revertants were produced in the +GST strain in a dose-dependent fashion by CHBrCl2 but not by CHCl3. At 4800 ppm CHBrCl2, which produced a calculated agar concentration of 0.67 mM, there were 419 +/- 75 revertants per plate compared to a spontaneous level of 23 +/- 5. CHCl3 produced a doubling of revertants only at the two highest concentrations tested (19,200 and 25,600 ppm). These results indicate that bromination of THMs confers the capability for theta-class GST-mediated transformation to mutagenic intermediates at low substrate concentrations, suggesting the possibility of a similar activation route in humans. Further, the very low affinity of the GSH-dependent pathway for CHCl3 demonstrates that different THMs can induce adverse effects via different mechanisms, indicating that risk evaluations of THMs should not treat members of this class as if they shared a common mode of action.

In vitro quantitative determination of phospholipid adducts of chloroform intermediates in hepatic and renal microsomes from different rodent strains

We have comparatively studied in vitro the oxidative and reductive pathways of chloroform metabolism in hepatic and renal microsomes of rodent strains used for carcinogenicity testing (B6C3F1 mice, Osborne Mendel and Sprague Dawley rats). To this aim we exploited the regioselective binding of phosgene to phospholipid (PL) polar heads and of dichloromethyl radical to PL fatty acyl chains, using a method based on the chemical transmethylation of PL adducts, followed by phase partitioning of the resulting products (De Biasi et al., 1992). The analysis of results let us to conclude at first that a (14)C label partitioning by 89.2 (±6.5)% or 13.7 (±5.0)% in the aqueous phase is typical of the PL adduct with phosgene (PL-PHOS) or with dichloromethyl radical (PL-RAD), respectively. Metabolism of 0.1 mM CHCl(3) was mainly oxidative in all the samples, being hepatic microsomes more active than renal ones by about one order of magnitude and levels of CHCl(3)-derived PL adducts in B6C3F1 mouse liver microsomes higher than in rat samples. At 5 mM CHCl(3), total levels of PL adducts in renal microsomes reached levels almost similar to those found in liver microsomes. However, while B6C3F1 mouse kidney microsomes produced both reactive metabolites, similarly as the hepatic samples, Osborne Mendel rat kidney microsomes bioactivated CHCl(3) only reductiveiy, producing the radical. The relevance of this finding depends on the fact that phosgene is known to be the major cause of CHCl(3) toxicity, based on data with the rat liver and mouse liver and kidney, while nephrotoxicity in rats occurs with minimal production of COCl(2). Chloroform reductive bioactivation may therefore provide a reasonable explanation for the toxicity of chloroform to the rat kidney. The same finding may be of interest in elucidating the metabolic reasons of the chloroform-induced kidney tumors in Osborne Mendel rats.

Ketone potentiation of haloalkane-induced hepato- and nephrotoxicity. II. Implication of monooxygenases

Previous results in Sprague-Dawley rats indicate that acetone (A), methyl ethyl ketone (MEK), and methyl isobutyl ketone (MiBK) pretreatment (3 d, po) at dosages of 6.8 and 13.6 mmol/kg potentiate CCl4 hepatotoxicity and CHCl3 nephrotoxicity, respectively. The potentiation potency profile observed was MiBK > A > MEK for liver and A > MEK > or = MiBK for kidney toxicity (Raymond & Plaa, 1995). In the present study, hepatic and renal microsomes from A-, MEK-, and MiBK-pretreated rats (6.8 or 13.6 mmol/kg) were examined for cytochrome P-450 content, substrate-specific monooxygenase activity (aminopyrine and benzphetamine N-demethylase, aniline hydroxylase) and in vitro covalent binding of 14CHCl3 and 14CCl4. Of the three ketones, only MiBK significantly increased P-450 content of liver and renal cortical microsomes. Similarly, 14CCl4 covalent binding under aerobic and anaerobic conditions was significantly increased by MiBK pretreatment only. 14CHCl3 covalent binding by renal cortical microsomes was significantly increased only under aerobic conditions by MiBK pretreatment. MiBK (13.6 mmol/kg) increased (threefold) aminopyrine N-demethylation in both liver and kidney, but only benzphetamine N-demethylation (two-fold, at 6.8 and 13.6 mmol/kg) in liver; A and MEK had no effect on either monooxygenase. All ketones at dosages of 6.8 and 13.6 mmol/kg increased aniline hydroxylation in liver (two-fold) and kidney (fivefold). Comparable profiles for P-450 induction, haloalkane covalent binding, and aminopyrine or benzphetamine N-demethylase activity were observed in liver and kidney microsomes. This profile was consistent with the ketone potentiation potency ranking profile observed in vivo for liver but not kidney injury. These findings affirm the importance of ketone-enhanced bioactivation for potentiation of CCl4 hepatotoxicity but suggest an alternative mechanism for CHCl3 nephrotoxicity.

Multiple activation of chloroform in kidney microsomes from male and female DBA/2J mice

Microsomes from the renal cortex of DBA/2J mice can metabolize chloroform through oxidative and reductive pathways, similar to hepatic microsomes. The oxidative or reductive nature of CHCl3 activation is strictly dependent on the oxygenation of the incubation mixture, as indicated by the formation of qualitatively different adducts to phospholipids (PLs). The protein and lipid binding levels measured in kidney microsomes from control females differed significantly from the binding levels observed with kidney microsomes from male and testosterone-treated female DBA/2J mice in aerobic conditions only. Therefore, the sex-dependent CHCl3-induced acute nephrotoxicity seems related only with the oxidative CHCl3 activation. The levels of adducts to PL polar heads and to protein showed a strict correlation with each other. Therefore, the assay of adducts to PL polar heads may be used as a substitute for the assay of adducts to protein. This might be especially convenient when studying the effects of both phosgene and the trichloromethyl radicals.

The contribution of electrophilic and radicalic intermediates to phospholipid adducts formed by halomethanes in vivo

The different production of phosgene and free-radicals from CHCl3 and CCl4 was determined in vitro and in vivo, by measuring the regioselective binding to the two intermediates to phospholipid (PL) molecules. Results clearly indicated that this assay can be successfully used to selectively detect electrophilic and radicalic metabolites produced in vivo and selectively quantitate their adducts. The in vivo biotransformation of CCl4, similarly to the in vitro situation, resulted in the formation of radicals only, the contribution of phosgene to the structural damage of PL being negligible. These findings allowed us to rule out the hypothesis of substantial formation of radicalic intermediates from CHCl3 in phenobarbital (PB)-pretreated Sprague-Dawley (SD) rats, derived from in vitro data. While the role of reduced glutathione (GSH) in preventing COCl2-derived damages seems to be less important in vivo than in vitro, it is not possible to rule out the action of radical scavenging systems in decreasing the level of adducts with fatty acyl chains (FC) of PL measured in vivo.

Dose and route dependency of metabolism and toxicity of chloroform in ethanol-treated rats

The effects of a single dose of ethanol on the metabolism and toxicity of chloroform administered to rats per os (p.o.), intraperitoneally (i.p.), or by inhalation (inh) at different doses were investigated. Rats that had been given either ethanol (2 g/kg) or vehicle (water) alone at 4 p.m. on the previous day were challenged with chloroform at 10 a.m. p.o. (0.01, 0.2, or 0.4 g/kg), i.p. (0, 0.1, 0.2, or 0.4 g/kg), or inh (for 6 h each at 0, 50, 100, or 500 ppm). The ethanol treatment, which had no influence on the intake of food and water, increased chloroform metabolism in vitro about 1.5-fold with no significant influence on liver glutathione content. The treatment had a dose-dependent effect on the metabolism and toxicity of chloroform, and the effect differed depending on the route of administration. Compared at the same dose level, the area under the curve (AUC) of blood chloroform concentration was invariably smaller following p.o. than i.p. administration. In accordance with this, chloroform administered p.o. caused more deleterious hepatic damage than the same amount of chloroform administered i.p. Although ethanol treatment had no significant influence on the AUC at any dose by any route of administration, the toxicity of p.o.-administered chloroform was significantly higher in ethanol-treated rats than in control rats at a dose as low as 0.1 g/kg, whereas no significant difference was observed in toxicity between both groups of rats at such a low dose administered i.p.(ABSTRACT TRUNCATED AT 250 WORDS)

Toxicity of bromodichloromethane in female rats and mice after repeated oral dosing

The carcinogenic water disinfection byproduct, bromodichloromethane (BDCM), produces renal and hepatic toxicity in rodents in acute and subchronic studies. In the present investigation, female rats and mice (n = 6) were dosed daily for 5 consecutive days with BDCM (dissolved in an aqueous, 10% Emulphor solution) by gavage. Rats received 75, 150 and 300 mg BDCM/kg body weight/day and mice received 75 and 150 mg BDCM/kg body weight/day. Two rats in the 300 mg/kg/day treatment group died on day 5. On day 6, the animals were sacrificed and serum samples were taken for analysis of indicators of hepatic and renal toxicity. Livers and kidneys were excised and samples taken for histopathological evaluation. Portions of the livers were also utilized to produce microsomes for analysis of cytochrome P450 enzyme activities and total P450 content. Total hepatic cytochrome P450 was decreased in rats dosed with 150 and 300 mg BDCM/kg body weight/day, but was not significantly affected in BDCM-treated mice. Serum lactate (LDH) and sorbitol (SDH) dehydrogenase, aspartate aminotransferase (AST), creatinine and blood urea nitrogen were increased above those of controls in rats dosed with 300 mg BDCM/kg/day. These data suggested that hepatic and renal damage had occurred in this treatment group. This was confirmed by histopathological analyses which revealed that lesions occurred in both hepatic and renal tissues from rats dosed with 150 and 300 mg BDCM/kg/day. The hepatic lesions were centrilobular and primarily consisted of vacuolar degeneration. The hepatotoxicity indicators alanine aminotransferase (ALT) and SDH were increased in mice dosed with 150 mg BDCM/kg/day. However, no histopathological lesions were observed in these animals. This study shows that BDCM is both hepatotoxic and nephrotoxic to female rats after repeated dosing, but is only weakly hepatotoxic to female mice at the administered doses. Also, reduced activities of hepatic cytochrome P450 were observed in rats, but not mice. These species differences in toxicity and xenobiotic metabolizing enzyme inhibition caused by BDCM suggest that an understanding of the mechanism of toxicity of this compound will be critical when extrapolating rodent toxicity data to humans for this environmental pollutant.

The mechanism of chloroform toxicity in isolated rat hepatocytes

Chloroform (CHCl3) is widely used in the manufacture of drugs, cosmetics, plastics and cleaning agents. It is also found in chlorinated drinking water. This study was designed to investigate the toxic effect of CHCl3 on isolated male rat hepatocytes using several toxicity parameters. The hepatocytes were isolated by a collagenase perfusion technique and the cell viability was determined by Trypan blue exclusion. The leakage of cytosolic enzymes such as aspartate transaminase (AST) and alanine transaminase (ALT) after treatment with CHCl3 was measured. Reduced glutathione content (GSH) and its related enzymes, glutathione reductase (GSH-Rx) and glutathione peroxidase (GSH-Px), were also evaluated to study the effect of CHCl3 on hepatocytes. Exposure to 100 and 1000 ppm CHCl3 results in a significant decrease in cell after 30 min incubation. However, the effect of 1 and 10 ppm concentrations was observed at 60 min incubation. AST leakage was significantly increased in all treatment groups, while ALT was significantly increased at 100 and 1000 ppm CHCl3 after 60 and 30 min, respectively. As early as 15 min, GSH was decreased significantly at 1000 ppm, but at 100 and 10 ppm CHCl3 the decrease in GSH began after 30 and 120 min, respectively. GSH-Px activity did not changed. However, the activity of GSH-Rx was significantly decreased at 1000 ppm CHCl3 and at the same time GSH content was decreased. The data indicate that the toxic effect of CHCl3 was dose- and time-dependent. The degree of GSH depletion correlated with increased cytotoxicity and decreased GSH-Rx activity due to CHCl3.

Physiological pharmacokinetics and cancer risk assessment

There has been considerable progress in recent years in developing physiological models for the pharmacokinetics of toxic chemicals and in the application of these models in cancer risk assessment. Physiological pharmacokinetic models consist of a number of individual compartments, based on the anatomy and physiology of the mammalian organism of interest, and include specific parameters for metabolism, tissue binding, and tissue reactivity. Because of the correspondence between these compartments and specific tissues or groups of tissues, these models are particularly useful for predicting the doses of biologically active forms of toxic chemicals at target tissues under a wide variety of exposure conditions and in different animal species, including humans. Due to their explicit characterization of the biological processes governing pharmacokinetic behaviour, these models permit more accurate predictions of the dose of active metabolites reaching target tissues in exposed humans and hence of potential cancer risk. In addition, physiological models also permit a more direct evaluation of the impact of parameter uncertainty and inter-individual variability in cancer risk assessment. In this article, we review recent developments in physiologic pharmacokinetic modeling for selected chemicals and the application of these models in carcinogenic risk assessment. We examine the use of these models in integrating diverse information on pharmacokinetics and pharmacodynamics and discuss challenges in extending these pharmacokinetic models to reflect more accurately the biological events involved in the induction of cancer by different chemicals.