The role of different cytochrome P450 isoforms in in vitro chloroform metabolism

Evaluating the impact of anatomical and physiological variability on human equivalent doses using PBPK models

Addressing human anatomical and physiological variability is a crucial component of human health risk assessment of chemicals. Experts have recommended probabilistic chemical risk assessment paradigms in which distributional adjustment factors are used to account for various sources of uncertainty and variability, including variability in the pharmacokinetic behavior of a given substance in different humans. In practice, convenient assumptions about the distribution forms of adjustment factors and human equivalent doses (HEDs) are often used. Parameters such as tissue volumes and blood flows are likewise often assumed to be lognormally or normally distributed without evaluating empirical data for consistency with these forms. In this work, we performed dosimetric extrapolations using physiologically based pharmacokinetic (PBPK) models for dichloromethane (DCM) and chloroform that incorporate uncertainty and variability to determine if the HEDs associated with such extrapolations are approximately lognormal and how they depend on the underlying distribution shapes chosen to represent model parameters. We accounted for uncertainty and variability in PBPK model parameters by randomly drawing their values from a variety of distribution types. We then performed reverse dosimetry to calculate HEDs based on animal points of departure for each set of sampled parameters. Corresponding samples of HEDs were tested to determine the impact of input parameter distributions on their central tendencies, extreme percentiles, and degree of conformance to lognormality. This work demonstrates that the measurable attributes of human variability should be considered more carefully and that generalized assumptions about parameter distribution shapes may lead to inaccurate estimates of extreme percentiles of HEDs.

Global optimization of the Michaelis-Menten parameters using physiologically-based pharmacokinetic (PBPK) modeling and chloroform vapor uptake data in F344 rats

Objective: To quantify metabolism, a physiologically based pharmacokinetic (PBPK) model for a volatile compound can be calibrated with the closed chamber (i.e. vapor uptake) inhalation data. Here, we introduce global optimization as a novel component of the predictive process and use it to illustrate a procedure for metabolic parameter estimation.Materials and methods: Male F344 rats were exposed in vapor uptake chambers to initial concentrations of 100, 500, 1000, and 3000 ppm chloroform. Chamber time-course data from these experiments, in combination with optimization using a chemical-specific PBPK model, were used to estimate Michaelis-Menten metabolic constants. Matlab^® simulation software was used to integrate the mass balance equations and to perform the global optimizations using MEIGO (MEtaheuristics for systems biology and bIoinformatics Global Optimization - Version 64 bit, R2016A), a toolbox written for Matlab®. The cost function used the chamber time-course data and least squares to minimize the difference between data and simulation values.Results and discussion: The final values estimated for V_max (maximum metabolic rate) and K_m (affinity constant) were 1.2 mg/h and a range between 0.0005 and 0.6 mg/L, respectively. Also, cost function plots were used to analyze the dose-dependent capacity to estimate V_max and K_m within the experimental range used. Sensitivity analysis was used to assess identifiability for both parameters and show these kinetic data may not be sufficient to identify K_m.Conclusion: In summary, this work should help toxicologists interested in optimization techniques understand the overall process employed when calibrating metabolic parameters in a PBPK model with inhalation data.

Overview of Disinfection By-products and Associated Health Effects

The presence of chemical compounds formed as disinfection by-products (DBPs) is widespread in developed countries, and virtually whole populations are exposed to these chemicals through ingestion, inhalation, or dermal absorption from drinking water and swimming pools. Epidemiological evidence has shown a consistent association between long-term exposure to trihalomethanes and the risk of bladder cancer, although the causal nature of the association is not conclusive. Evidence concerning other cancer sites is insufficient or mixed. Numerous studies have evaluated reproductive implications, including sperm quality, time to pregnancy, menstrual cycle, and pregnancy outcomes such as fetal loss, fetal growth, preterm delivery, and congenital malformation. The body of evidence suggests only minor effects from high exposure during pregnancy on fetal growth indices such as small for gestational age (SGA) at birth. Populations highly exposed to swimming pools such as pool workers and professional swimmers show a higher prevalence of respiratory symptoms and asthma, respectively, although the direction of the association, and thus causality, is not clear among professional swimmers. The risk of asthma, wheezing, eczema, and other respiratory outcomes among children attending swimming pools has been the object of extensive research. Early studies suggested a positive association, while subsequent larger studies found no correlations or showed a protective association. Future research should develop methods to evaluate the effects of the DBP mixture and the interaction with personal characteristics (e.g., genetics, lifestyle), clarify the association between swimming pools and respiratory health, evaluate the occurrence of DBPs in low- and middle-income countries, and evaluate outcomes suggested by animal studies that have not been considered in epidemiological investigations.

Predictors of blood trihalomethane concentrations in NHANES 1999-2006

BACKGROUND

Trihalomethanes (THMs) are water disinfection by-products that have been associated with bladder cancer and adverse birth outcomes. Four THMs (bromoform, chloroform, bromodichloromethane, dibromochloromethane) were measured in blood and tap water of U.S. adults in the National Health and Nutrition Examination Survey (NHANES) 1999-2006. THMs are metabolized to potentially toxic/mutagenic intermediates by cytochrome p450 (CYP) 2D6 and CYP2E1 enzymes.

OBJECTIVES

We conducted exploratory analyses of blood THMs, including factors affecting CYP2D6 and CYP2E1 activity.

METHODS

We used weighted multivariable regressions to evaluate associations between blood THMs and water concentrations, survey year, and other factors potentially affecting THM exposure or metabolism (e.g., prescription medications, cruciferous vegetables, diabetes, fasting, pregnancy, swimming).

RESULTS

From 1999 to 2006, geometric mean blood and water THM levels dropped in parallel, with decreases of 32%-76% in blood and 38%-52% in water, likely resulting, in part, from the lowering of the total THM drinking water standard in 2002-2004. The strongest predictors of blood THM levels were survey year and water concentration (n = 4,232 total THM; n = 4,080 bromoform; n = 4,582 chloroform; n = 4,374 bromodichloromethane; n = 4,464 dibromochloromethane). We detected statistically significant inverse associations with diabetes and eating cruciferous vegetables in all but the bromoform model. Medications did not consistently predict blood levels. Afternoon/evening blood samples had lower THM concentrations than morning samples. In a subsample (n = 230), air chloroform better predicted blood chloroform than water chloroform, suggesting showering/bathing was a more important source than drinking.

CONCLUSIONS

We identified several factors associated with blood THMs that may affect their metabolism. The potential health implications require further study.

Micronuclei in bone marrow and liver in relation to hepatic metabolism and antioxidant response due to coexposure to chloroform, dichloromethane, and toluene in the rat model

Genotoxicity in cells may occur in different ways, direct interaction, production of electrophilic metabolites, and secondary genotoxicity via oxidative stress. Chloroform, dichloromethane, and toluene are primarily metabolized in liver by CYP2E1, producing reactive electrophilic metabolites, and may also produce oxidative stress via the uncoupled CYP2E1 catalytic cycle. Additionally, GSTT1 also participates in dichloromethane activation. Despite the oxidative metabolism of these compounds and the production of oxidative adducts, their genotoxicity in the bone marrow micronucleus test is unclear. The objective of this work was to analyze whether the oxidative metabolism induced by the coexposure to these compounds would account for increased micronucleus frequency. We used an approach including the analysis of phase I, phase II, and antioxidant enzymes, oxidative stress biomarkers, and micronuclei in bone marrow (MNPCE) and hepatocytes (MNHEP). Rats were administered different doses of an artificial mixture of CLF/DCM/TOL, under two regimes. After one administration MNPCE frequency increased in correlation with induced GSTT1 activity and no oxidative stress occurred. Conversely, after three-day treatments oxidative stress was observed, without genotoxicity. The effects observed indicate that MNPCE by the coexposure to these VOCs could be increased via inducing the activity of metabolism enzymes.

Application of an updated physiologically based pharmacokinetic model for chloroform to evaluate CYP2E1-mediated renal toxicity in rats and mice

Physiologically based pharmacokinetic (PBPK) models are tools for interpreting toxicological data and extrapolating observations across species and route of exposure. Chloroform (CHCl(3)) is a chemical for which there are PBPK models available in different species and multiple sites of toxicity. Because chloroform induces toxic effects in the liver and kidneys via production of reactive metabolites, proper characterization of metabolism in these tissues is essential for risk assessment. Although hepatic metabolism of chloroform is adequately described by these models, there is higher uncertainty for renal metabolism due to a lack of species-specific data and direct measurements of renal metabolism. Furthermore, models typically fail to account for regional differences in metabolic capacity within the kidney. Mischaracterization of renal metabolism may have a negligible effect on systemic chloroform levels, but it is anticipated to have a significant impact on the estimated site-specific production of reactive metabolites. In this article, rate parameters for chloroform metabolism in the kidney are revised for rats, mice, and humans. New in vitro data were collected in mice and humans for this purpose and are presented here. The revised PBPK model is used to interpret data of chloroform-induced kidney toxicity in rats and mice exposed via inhalation and drinking water. Benchmark dose (BMD) modeling is used to characterize the dose-response relationship of kidney toxicity markers as a function of PBPK-derived internal kidney dose. Applying the PBPK model, it was also possible to characterize the dose response for a recent data set of rats exposed via multiple routes simultaneously. Consistent BMD modeling results were observed regardless of species or route of exposure.

Evidence of bioactivation of halomethanes and its relation to oxidative stress response in Chirostoma riojai, an endangered fish from a polluted lake in Mexico

Halomethanes (HMs) are produced autochthonously in water bodies through the action of ultraviolet light in the presence of HM precursors, such as dissolved organic carbon and halogens. In mammals, toxic effects induced by HMs are diverse and include oxidative stress, which is also induced by divalent and polyvalent metals; however, in fish little information is available on HM metabolism and its possible consequences at the population level. In the present study, high CYP 2E1 and GST theta-like activities were found in viscera of the Toluca silverside Chirostoma riojai from Lake Zumpango (LZ; central Mexico). Formaldehyde, one of the HM metabolites, was correlated with CYP 2E1 activity and also induced lipid peroxidation in viscera. Hepatic CYP 2E1 activity was correlated with GST theta-like activity, suggesting the coupling of both pathways of HM bioactivation and its consequent oxidative damage. Sediment metals, among others, were also responsible for oxidative stress, particularly iron, lead, arsenic and manganese. However, under normal environmental conditions, the antioxidant enzymes of this species sustain catalysis adapted to oxidative stress. Findings suggest that this fish species apparently has mechanisms of adaptation and recovery that enable it to confront toxic agents of natural origin, such as metals and other substances formed through natural processes, e.g., HMs. This has allowed C. riojai to colonize LZ despite the high sensitivity of this species to xenobiotics of anthropogenic origin.

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.

Incorporating pharmacokinetic differences between children and adults in assessing children's risks to environmental toxicants

Children's risks from environmental toxicant exposure can be affected by pharmacokinetic factors that affect the internal dose of parent chemical or active metabolite. There are numerous physiologic differences between neonates and adults that affect pharmacokinetics including size of lipid, and tissue compartments, organ blood flows, protein binding capacity, and immature function of renal and hepatic systems. These factors combine to decrease the clearance of many therapeutic drugs, which can also be expected to occur with environmental toxicants in neonates. The net effect may be greater or lesser internal dose of active toxicant depending upon how the agent is distributed, metabolized, and eliminated. Child/adult pharmacokinetic differences decrease with increasing postnatal age, but these factors should still be considered in any children's age group, birth through adolescence, for which there is toxicant exposure. Physiologically based pharmacokinetic (PBPK) models can simulate the absorption, distribution, metabolism, and excretion of xenobiotics in both children and adults, allowing for a direct comparison of internal dose and risk across age groups. This review provides special focus on the development of hepatic cytochrome P-450 enzymes (CYPs) in early life and how this information, along with many factors unique to children, can be applied to PBPK models for this receptor population. This review describes a case study involving the development of neonatal PBPK models for the CYP1A2 substrates caffeine and theophylline. These models were calibrated with pharmacokinetic data in neonates and used to help understand key metabolic differences between neonates and adults across these two drugs.

The metabolic rate constants and specific activity of human and rat hepatic cytochrome P-450 2E1 toward toluene and chloroform

Chloroform (CHCl3) is a near-ubiquitous environmental contaminant, a by-product of the disinfection of drinking water sources and a commercially important compound. Standards for safe exposure have been established based on information defining its toxicity, which is mediated by metabolites. The metabolism of CHCl3 is via cytochrome P-450 2E1 (CYP2E1)-mediated oxidation to phosgene, which is known to obey a saturable mechanism. CYP2E1 is a highly conserved form, expressed in all mammalian systems studied, and is responsible for the metabolism of a great many low-molecular-weight (halogenated) compounds. However, the Michaelis-Menten rate constants for CHCl3 oxidation have not been derived in vitro, and the specific activity of CYP2E1 toward CHCl3 has not been reported. In this investigation with microsomal protein (MSP), apparent Vmax values of 27.6 and 28.3 nmol/h/mg MSP and apparent K(m) values of 1 and 0.15 microM in rats and human organ donors, respectively, were demonstrated. The specific activity of CYP2E1 toward CHCl3 in rats and humans was 5.29 and 5.24 pmol/min/pmol CYP2E1, respectively. Toluene metabolism to benzyl alcohol (BA), another CYP2E1-dependent reaction, was also highly dependent on CYP2E1 content in humans, and was more efficient than was CHCl3 metabolism. The specific activity of human CYP2E1 toward toluene metabolism in human MSP was 23 pmol/min/pmol CYP2E1. These results demonstrate that differences in CYP2E1 content of MSP among individuals and between species are largely responsible for observed differences in toluene and CHCl3 metabolism in vitro.

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.

Kinetic parameters of OPT pesticide desulfuration by c-DNA expressed human CYPs

The role of different cytochrome P450 isoforms (CYPs) in the desulfuration of four organophosphorothionate pesticides (OPTs), namely diazinon (DIA), azinphos-methyl (AZ), chlorpyrifos (CPF) and parathion (PARA), at OPT levels representative of actual human exposure has been investigated. For this purpose c-DNA expressed human CYPs and a method, based on acetylcholinesterase (AChE) inhibition, able to detect nM levels of oxon have been used. Our results indicate that the four tested OPTs at low concentration were mainly desulfurated by CYP2B6, 2C19 and 1A2, showing K(m) values in the range 0.8-5 μM and the highest efficiency (intrinsic clearance (ICL)) values. CYP3A4 was generally endowed with high K(m) and resulted linear up to 25-100 μM OPT, concentrations saturating the most efficient CYPs. The tentative extrapolation of the relative contribution of single CYPs, taking into account the average content of different isoforms in the human liver, indicate that CYP1A2 is the major responsible for oxon formation. Indeed this CYP catalyses the 50-90% of desulfuration reaction, depending on the OPT. As CYP3A4 activity is not completely saturated up to 100 μM OPT, and due to the high hepatic content, its contribution to oxon formation may result relevant in poisoning episodes, when individuals are exposed at high doses of OPTs.

Hepatoprotection by dimethyl sulfoxide. III. Role of inhibition of the bioactivation and covalent bonding of chloroform

Dimethyl sulfoxide (DMSO) has previously been shown to have the ability to attenuate chloroform (CHCl(3))-induced liver injury in the naive rat even when administered 24 h after the toxicant. These studies were undertaken to determine if the protective action by late administration of DMSO is due to an inhibition of the bioactivation of CHCl(3). This was done by comparing the cytochrome P450 inhibitors, diallyl sulfide (DAS), and aminobenzotriazole (ABT) to DMSO for their protective efficacy when administered 24 h after CHCl(3) exposure. In addition, (14)CHCl(3) was utilized to measure the effect of DMSO and ABT on the covalent binding of CHCl(3) in the liver following their late administration. Male Sprague-Dawley rats (300-350 g) received 0.75 ml/kg CHCl(3) po. Twenty-four hours later, they received ip injection of 2 ml/kg DMSO, 100 mg/kg DAS, or 30 mg/kg ABT. Plasma ALT activities and quantitation of liver injury by light microscopy at 48 h after CHCl(3) dosing indicated that all three treatments were equally effective at protecting the liver. A detailed study of the time course of injury development indicated that the protective action of DMSO was occurring within 10 h of its administration. Therefore, in the radiolabel studies, rats were killed 24-34 h after receiving 0.75 ml/kg CHCl(3) (30 microCi/kg (14)CHCl(3)) po. Treatment with ABT at 24 h after (14)CHCl(3) dosing decreased the covalent binding of (14)C to hepatic protein by 35% and reduced the amount of (14)C in the blood by 50% by 10 h after its administration. DMSO treatment did not significantly affect any of these parameters. The lack of effect by late administration of DMSO on the covalent binding of CHCl(3) would indicate that DMSO may offer protection by mechanisms other than inhibition of the bioactivation of CHCl(3). These studies also indicate that specific cytochrome P450 inhibitors may be of benefit in clinical situations to help treat the delayed onset hepatitis that can result following poisoning with an organohalogen, even if the antidotes are administered a number of hours after the initial exposure.

Cytochrome P450 2E1 is the primary enzyme responsible for low-dose carbon tetrachloride metabolism in human liver microsomes

We examined which human CYP450 forms contribute to carbon tetrachloride (CCl(4)) bioactivation using hepatic microsomes, heterologously expressed enzymes, inhibitory antibodies and selective chemical inhibitors. CCl(4) metabolism was determined by measuring chloroform formation under anaerobic conditions. Pooled human microsomes metabolized CCl(4) with a K(m) of 57 microM and a V(max) of 2.3 nmol CHCl(3)/min/mg protein. Expressed CYP2E1 metabolized CCl(4) with a K(m) of 1.9 microM and a V(max) of 8.9 nmol CHCl(3)/min/nmol CYP2E1. At 17 microM CCl(4), a monoclonal CYP2E1 antibody inhibited 64, 74 and 83% of the total CCl(4) metabolism in three separate human microsomal samples, indicating that at low CCl(4) concentrations, CYP2E1 was the primary enzyme responsible for CCl(4) metabolism. At 530 microM CCl(4), anti-CYP2E1 inhibited 36, 51 and 75% of the total CCl(4) metabolism, suggesting that other CYP450s may have a significant role in CCl(4) metabolism at this concentration. Tests with expressed CYP2B6 and inhibitory CYP2B6 antibodies suggested that this form did not contribute significantly to CCl(4) metabolism. Effects of the CYP450 inhibitors alpha-naphthoflavone (CYP1A), sulfaphenazole (CYP2C9) and clotrimazole (CYP3A) were examined in the liver microsome sample that was inhibited only 36% by anti-CYP2E1 at 530 microM CCl(4). Clotrimazole inhibited CCl(4) metabolism by 23% but the other chemical inhibitors were without significant effect. Overall, these data suggest that CYP2E1 is the major human enzyme responsible for CCl(4) bioactivation at lower, environmentally relevant levels. At higher CCl(4) levels, CYP3A and possibly other CYP450 forms may contribute to CCl(4) metabolism.

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.

Dexamethasone ameliorates oxidative DNA damage induced by benzene and LPS in mouse bone marrow

Mice were grouped to receive vehicle, dexamethasone (DEX), lipopolysaccharide (LPS), benzene (BZ, 200 mg/kg) and combinations: LPS + DEX, BZ + DEX, LPS + BZ, LPS + DEX + BZ. The DNA damage in bone marrow cells from BZ group was enhanced 2.8-fold measured by nuclear 8-hydroxy-2 '-deoxyguanosine (8-oxodG) and 1.4-fold measured by Comet score (index of DNA breaks) (p < 0.05). In the BZ + DEX group, 8-oxodG level and the Comet score were lowered to 65% and 76% respectively of that in the BZ group (p < 0.05). The BZ + LPS caused a 3.9-fold increase in 8-oxodG and a 1.6-fold increase in the Comet score (p < 0.05). The LPS + DEX + BZ lowered 8-oxodG level and the Comet score to 50% and 78% of the values in the LPS + BZ group, respectively (p < 0.05). Nitrate/nitrite levels in serum were higher after BZ + LPS treatment than after all other treatments. Both 8-oxodG level and the Comet scores were correlated to the serum nitrate/nitrite level across all the treatments (r = 0.55, p < 0.01 and r = 0.69, p < 0.01, respectively). In bone marrow cells the 8-oxodG correlated with the Comet scores (r = 0.80, p < 0.01). We conclude that DEX administration can reduce the DNA damage from BZ treatment and from the combination of BZ and LPS. The correlation of DNA damage with nitrate/nitrite indicates the possible involvement of reactive nitrogen species (RNS) in the interaction between BZ and the inflammatory reaction stimulated by LPS. The 8-oxodG determination is more sensitive than strand break analysis by the Comet assay in bone marrow in vivo in mice for measuring the BZ-induced DNA damage.