Insights into the toxicokinetics and toxicodynamics of 1,3-butadiene

1,3 Butadiene (BD) is a colorless gas used in the production of synthetic rubber and plastics. BD is carcinogenic in rats and mice, however, there are striking species differences in cancer potency and spectrum of tumors, with mice being more susceptible to tumor induction than rats. Epidemiology studies suggest an excess incidence of leukemia in workers in the styrene-butadiene rubber industry. Consideration of mechanisms of BD carcinogenicity can provide insights into differences in cancer potency between rodents and serve to elucidate the extent to which BD exposure may cause cancer in humans. Mechanistic research in the areas of biochemical toxicology, molecular biology, molecular dosimetry, and susceptibility factors can impact BD cancer risk assessment for humans. This research has focused on quantitating species differences in the metabolism of BD and BD epoxides, defining molecular lesions produced by BD epoxides, identifying biomarkers for BD exposure to explore metabolic pathways in humans, and determining potential risk factors for sensitive subpopulations. BD is activated by P450 isozymes, including CYP2E1, to at least two genotoxic metabolites, epoxybutene (EB) and diepoxybutane (DEB). Dosimetry data from several laboratories on EB and DEB following inhalation exposure to BD indicate that blood concentrations of EB were four-eight-fold higher in mice compared with rats and that blood concentrations of DEB were 25-100-fold higher in mice than in rats. The higher levels of these two DNA-reactive metabolites in mice compared with rats probably contribute to the species differences in carcinogenic effects of BD between mice and rats. In vitro metabolism studies of BD in rats, mice, and human tissues indicate that there are significant quantitative species differences in the metabolic activation of BD to EB and DEB and the detoxication of EB and DEB. Activation/detoxication ratios calculated using in vitro kinetic constants reveal that ratios in mice were greater than in both rats and humans. In vitro data are consistent with in vivo dosimetry data and cancer potency for rodents, and suggest that humans may be at a decreased risk. Data on mutagenicity and mutational spectra of BD epoxides show mechanistic differences between EB- and DEB-induced mutational events suggesting involvement of DEB in the development of cancer. Concentrations of DEB that are genotoxic in vitro are within the range of concentrations measured in mice in vivo, whereas concentrations of EB that are genotoxic in vitro are ten-100-fold greater than concentrations observed in vivo. Characterization of molecular events indicate that EB-induced genotoxicity is due to point mutations and small deletions, while DEB induces point mutations, small deletions, and large-scale deletions involving many base pairs. The extent to which epoxybutanediol is involved in BD carcinogenesis is not known. Molecular dosimetry studies in rodents and humans have focused on urinary metabolites and DNA and hemoglobin adducts. Data from these studies are consistent with in vivo and in vitro metabolism data providing further support for the differences in metabolic activation and deactivation of BD and BD epoxides across species and the role of DEB in tumor development. Research on potential susceptibility factors points to other P450 isozymes, in addition to CYP2E1, that are involved in both the metabolic activation and mutagenicity of BD. Taken together, mechanistic data on BD toxicokinetics and toxicodynamics provide an integrated insight into critical steps in initiation of cancer, metabolites responsible for cancer, sensitive biomarkers for exposure, and potential risk factors for individual susceptibility. Available evidence suggests that BD is unlikely to be a human carcinogen at the low exposure concentrations currently encountered in the environment or workplace.

Sites and mechanisms for uptake of gases and vapors in the respiratory tract

Inhalation is a common route by which individuals are exposed to toxicants. The air contains a multitude of gases and vapors that are brought into the respiratory tract with each breath. Depending upon the physical and chemical characteristics of the toxicant, the respiratory tract can be considered as a target organ in addition to a portal of entry. Sufficient information is not always available on the fate or effects of an inhaled gas or vapor. Two physiochemical principles, water solubility and reactivity, can be used to predict the site of uptake of gases and vapors in the respiratory tract and potential mechanisms for reaction with respiratory tract tissue and absorption into the blood. Four model compounds, formaldehyde, ozone, dibasic esters, and butadiene are discussed as examples of how knowledge of aqueous solubility and chemical reactivity can help toxicologists predict sites and mechanisms by which inhaled gases and vapors interact with respiratory tract tissues.

Inhibition of cytochrome P450 2E1 decreases, but does not eliminate, genotoxicity mediated by 1,3-butadiene

1,3-Butadiene (BD), a rodent carcinogen, is metabolized to mutagenic and DNA-reactive epoxides. In vitro data suggest that this oxidation is mediated by cytochrome P450 2E1 (CYP2E1). In this study, we tested the hypothesis that oxidation of BD by CYP2E1 is required for genotoxicity to occur. Inhalation exposures were conducted with B6C3F1 mice using a closed-chamber technique, and the maximum rate of butadiene oxidation was estimated. The total amount of butadiene metabolized was then correlated with the frequency of micronuclei (MN). Three treatment groups were used: (1) mice with no pretreatment; (2) mice pretreated with 1,2-trans-dichloroethylene (DCE), a specific CYP2E1 inhibitor; and (3) mice pretreated with 1-aminobenzotriazole (ABT), an irreversible inhibitor of cytochromes P450. Mice in all 3 groups were exposed to an initial BD concentration of 1100 ppm, and the decline in concentration of BD in the inhalation chamber with time, due to uptake and metabolism of BD, was monitored using gas chromatography. A physiologically based pharmacokinetic model was used to analyze the gas uptake data, estimate V(max) for BD oxidation, and compute the total amount of BD metabolized. Model simulations of the gas uptake data predicted the maximum rate of BD oxidation would be reduced by 60% and 100% for the DCE- and ABT-pretreated groups, respectively. Bone marrow was harvested 24 h after the onset of the inhalation exposure and analyzed for frequency of micronuclei in polychromatic erythrocytes (MN-PCE). The frequency of MN-PCE per 1000 PCE in mice exposed to BD was 28.2 +/- 3.1, 19.8 +/- 2.5, and 12.3 +/- 1.9, for the mice with no pretreatment, DCE-pretreated mice and ABT-pretreated mice, respectively. Although inhibition of CYP2E1 decreased BD-mediated genotoxicity, it did not completely eliminate genotoxic effects. These data suggest that other P450 isoforms may contribute significantly to the metabolic activation of BD and resultant genotoxicity.

Analysis of respiratory exchange of methanol in the lung of the monkey using a physiological model

A physiologically based pharmacokinetic (PBPK) model was developed for the monkey, to account for fractional systemic uptake of inhaled methanol vapors in the lung. Fractional uptake of inhaled [14C]-methanol was estimated using unreported exhaled breath time course measurements of [14C]-methanol from the D.C. Dorman et al. (1994, Toxicol Appl Pharmacol. 128, 229-238) lung-only exposure study. The cumulative amount of [14C]-methanol exhaled was linear with respect to exposure duration (0.5 to 2 h) and concentration (10 to 900 ppm). The model estimated that forty to eighty-one percent of the of inhaled [14C]-methanol delivered to the lung was taken into systemic circulation in female Cynomolgus monkeys exposed for two h to 10-900 ppm of [14C]-methanol. There was no apparent trend between the percent of inhaled [14C]-methanol absorbed systemically and the [14C]-methanol exposure concentration. Model simulations were conducted using a single saturable Michaelis-Menten equation with Vmaxc, the metabolic capacity set to 15.54 mg/kg/h and Km, the affinity constant, to 0.66 mg/l. The [14C]-methanol blood concentrations were variable across [14C]-methanol exposure groups and the PBPK model tended to over-predict systemic clearance of [14C]-methanol. Accounting for fractional uptake of inhaled polar solvents is an important consideration for risk assessment of inhaled polar solvents.

Effects of a thirteen-week inhalation exposure to ethyl tertiary butyl ether on fischer-344 rats and CD-1 mice

The 1990 Clean Air Act Amendments require that oxygenates be added to automotive fuels to reduce emissions of carbon monoxide and hydrocarbons. One potential oxygenate is the aliphatic ether ethyl tertiary butyl ether (ETBE). Our objective was to provide data on the potential toxic effects of ETBE. Male and female Fisher 344 rats and CD-1 mice were exposed to 0 (control), 500, 1750, or 5000 ppm of ETBE for 6 h/day and 5 days/wk over a 13-week period. ETBE exposure had no effect on mortality and body weight with the exception of an increase in body weights of the female rats in the 5000-ppm group. No major changes in clinical pathology parameters were noted for either rats or mice exposed to ETBE for 6 (rats only) or 13 weeks. Liver weights increased with increasing ETBE-exposure concentration for both sexes of rats and mice. Increases in kidney, adrenal, and heart (females only) weights were noted in rats. Degenerative changes in testicular seminiferous tubules were observed in male rats exposed to 1750 and 5000 ppm but were not seen in mice. This testicular lesion has not been reported previously for aliphatic ethers. Increases in the incidence of regenerative foci, rates of renal cell proliferation, and alpha2u-globulin containing protein droplets were noted in the kidneys of all treated male rats. These lesions are associated with the male rat-specific syndrome of alpha2u-globulin nephropathy. Increases in the incidence of centrilobular hepatocyte hypertrophy and rates of hepatocyte cell proliferation were seen in the livers of male and female mice in the 5000-ppm group, consistent with a mitogenic response to ETBE. These two target organs for ETBE toxicity, mouse liver and male rat kidney, have also been reported for methyl tertiary butyl ether and unleaded gasoline.

Quantitative and qualitative differences in the metabolism of 14C-1,3-butadiene in rats and mice: relevance to cancer susceptibility

1,3-Butadiene (butadiene) is a potent carcinogen in mice, but not in rats. Metabolic studies may provide an explanation of these species differences and their relevance to humans. Male Sprague-Dawley rats and B6C3F1 mice were exposed for 6 h to 200 ppm [2,3-14C]-butadiene (specific radioactivity [sa] 20 mCi/mmol) in a Cannon nose-only system. Radioactivity in urine, feces, exhaled volatiles and 14C-CO2 were measured during and up to 42 h after exposure. The total uptake of butadiene by rats and mice under these experimental conditions was 0.19 and 0.38 mmol (equivalent to 3.8 and 7.5 mCi) per kg body weight, respectively. In the rat, 40% of the recovered radioactivity was exhaled as 14C-CO2, 70% of which was trapped during the 6-h exposure period. In contrast, only 6% was exhaled as 14C-CO2 by mice, 3% during the 6-h exposure and 97% in the 42 h following cessation of exposure. The formation of 14C-CO2 from [2,3-14C]-labeled butadiene indicated a ready biodegradability of butadiene. Radioactivity excreted in urine accounted for 42% of the recovered radioactivity from rats and 71% from mice. Small amounts of radioactivity were recovered in feces, exhaled volatiles and carcasses. Although there was a large measure of commonality, the exposure to butadiene also led to the formation of different metabolites in rats and mice. These metabolites were not found after administration of [4-14C]-1,2-epoxy-3-butene to animals by i.p. injection. The results show that the species differences in the metabolism of butadiene are not simply confined to the quantitative formation of epoxides, but also reflect a species-dependent selection of metabolic pathways. No metabolites other than those formed via an epoxide intermediate were identified in the urine of rats or mice after exposure to 14C-butadiene. These findings may have relevance for the prediction of butadiene toxicity and provide a basis for a revision of the existing physiologically based pharmacokinetic models.

A physiological model for tert-amyl methyl ether and tert-amyl alcohol: hypothesis testing of model structures

The oxygenate tert-amyl methyl ether (TAME) is a gasoline fuel additive used to reduce carbon monoxide in automobile emissions. To evaluate the relative health risk of TAME as a gasoline additive, information is needed on its pharmacokinetics and toxicity. The objective of this study was to use a physiologically-based pharmacokinetic (PBPK) model to describe the disposition of TAME and its major metabolite, tert-amyl alcohol (TAA), in male Fischer-344 rats. The model compartments for TAME and TAA were flow-limited. The TAME physiological model had 6 compartments: lung, liver, rapidly perfused tissues, slowly perfused tissues, fat, and kidney. The TAA model had 3 compartments: lung, liver, and total-body water. The 2 models were linked through metabolism of TAME to TAA in the liver. Model simulations were compared with data on blood concentrations of TAME and TAA taken from male Fischer-344 rats during and after a 6-hour inhalation exposure to 2500, 500, or 100 ppm TAME. The PBPK model predicted TAME pharmacokinetics when 2 saturable pathways for TAME oxidation were included. The TAA model, which included pathways for oxidation and glucuronide conjugation of TAA, underpredicted the experimental data collected at later times postexposure. To account for biological processes occurring during this time, three hypotheses were developed: nonspecific binding of TAA, diffusion-limited transport of TAA, and enterohepatic circulation of TAA glucuronide. These hypotheses were tested using three different model structures. Visual inspection and statistical evaluation involving maximum likelihood techniques indicated that the model incorporating nonspecific binding of TAA provided the best fit to the data. A correct model structure, based upon experimental data, statistical analyses, and biological interpretation, will allow a more accurate extrapolation to humans and, consequently, a greater understanding of human risk from exposure to TAME.

Influence of gender and acetone pretreatment on benzene metabolism in mice exposed by nose-only inhalation

Benzene (BZ) requires oxidative metabolism catalyzed by cytochrome P-450 2E1 (CYP 2E1) to exert its hematotoxic and genotoxic effects. We previously reported that male mice have a two-fold higher maximum rate of BZ oxidation compared with female mice; this correlates with the greater sensitivity of males to the genotoxic effects of BZ as measured by micronuclei induction and sister chromatid exchanges. The aim of this study was to quantitate levels of BZ metabolites in urine and tissues, and to determine whether the higher maximum rate of BZ oxidation in male mice would be reflected in higher levels of hydroxylated BZ metabolites in tissues and water-soluble metabolites in urine. Male and female B6C3F, mice were exposed to 100 or 600 ppm 14C-BZ by nose-only inhalation for 6 h. An additional group of male mice was pretreated with 1% acetone in drinking water for 8 d prior to exposure to 600 ppm BZ; this group was used to evaluate the effect of induction of CYP 2E1 on urine and tissue levels of BZ and its hydroxylated metabolites. BZ, phenol (PHE), and hydroquinone (HQ) were quantified in blood, liver, and bone marrow during exposure and postexposure, and water-soluble metabolites were analyzed in urine in the 48 h after exposure. Male mice exhibited a higher flux of BZ metabolism through the HQ pathway compared with females after exposure to either 100 ppm BZ (32.0 2.03 vs. 19.8 2.7%) or 600 ppm BZ (14.7 1.42 vs. 7.94 + 0.76%). Acetone pretreatment to induce CYP 2E1 resulted in a significant increase in both the percent and mass of urinary HQ glucuronide and muconic acid in male mice exposed to 600 ppm BZ. This increase was paralleled by three- to fourfold higher steady-state concentrations of PHE and HQ in blood and bone marrow of acetone-pretreated mice compared with untreated mice. These results indicate that the higher maximum rate of BZ metabolism in male mice is paralleled by a greater proportion of the total flux of BZ through the pathway for HQ formation, suggesting that the metabolites formed along this pathway may be responsible for the genotoxicity observed following BZ exposure.

A comparison of physiologically based pharmacokinetic model predictions and experimental data for inhaled ethanol in male and female B6C3F1 mice, F344 rats, and humans

Ethanol is added to unleaded gasoline as an oxygenate to decrease carbon monoxide automobile emissions. This introduces inhalation as a new possible route of environmental exposure to humans. Knowledge of the pharmacokinetics of inhaled ethanol is critical for adequately assessing the dosimetry of this chemical in humans. The purpose of this study was to characterize the pharmacokinetics of inhaled ethanol in male and female B6C3F1 mice and F344 rats and to develop a physiologically based pharmacokinetic (PBPK) model for inhaled ethanol in mice, rats, and humans. During exposure to 600 ppm for 6 hr, steady-state blood ethanol concentrations (BEC) were reached within 30 min in rats and within 5 min in mice. Maximum BEC ranged from 71 microM in rats to 105 microM in mice. Exposure to 200 ppm ethanol for 30 min resulted in peak BEC of approximately 25 microM in mice and approximately 15 microM in rats. Peak BEC of about 10 microM were measured following exposure to 50 ppm in female rats and male and female mice, while blood ethanol was undetectable in male rats. No sex-dependent differences in peak BEC at any exposure level were observed. Species-dependent differences were found following exposure to 200 and 600 ppm. A blood flow limited PBPK model for ethanol inhalation was developed in mice, rats, and humans which accounted for a fractional absorption of ethanol. Compartments for the model included the pulmonary blood and air, brain, liver, fat, and rapidly perfused and slowly perfused tissues. The PBPK model accurately simulated BEC in rats and mice at all exposure levels, as well as BEC reported in human males in previously published studies. Simulated peak BEC in human males following exposure to 50 and 600 ppm ranged from 7 to 23 microM and 86 and 293 microM, respectively. These results illustrate that inhalation of ethanol at or above the concentrations expected to occur upon refueling results in minimal BEC and are unlikely to result in toxicity.

Pharmacokinetics of methanol and formate in female cynomolgus monkeys exposed to methanol vapors

The 1990 Clean Air Act Amendments contain mandates for reduced automotive emissions and add new requirements for the use of alternative fuels such as methanol to reduce certain automotive pollutants. Methanol is acutely toxic in humans at relatively low doses, and the potential for exposure to methanol will be increased if it is used in automotive fuel. Formate is the metabolite responsible for neurotoxic effects of acute methanol exposure. Since formate metabolism is dependent on folate, potentially sensitive folate-deficient subpopulations, such as pregnant women, may accumulate formate and be at higher risk from low-level methanol exposure. Our objective was to determine the pharmacokinetics of 14C-methanol and 14C-formate in normal and folate-deficient monkeys after exposure to 14C-methanol vapors at environmentally relevant concentrations: below the threshold limit value (TLV), at the TLV of 200 parts per million (ppm), and above the TLV. Four normal adult female cynomolgus monkeys were individually anesthetized with isoflurane, and each was exposed by endotracheal intubation to 10, 45, 200, or 900 ppm 14C-methanol for 2 hours. Concentrations of the inhaled and exhaled 14C-methanol, blood concentrations of 14C-methanol and 14C-formate, exhaled 14C-carbon dioxide (14CO2), and respiratory parameters were measured during exposure. After exposure, 14C-methanol and 14CO2 exhaled, 14C-methanol and 14C-formate excreted in urine, and 14C-methanol and 14C-formate in blood were quantified. The amounts of exhaled 14C-methanol and 14CO2, blood concentrations of 14C-methanol and 14C-formate, and 14C-methanol and 14C-formate excreted in urine were linearly related to methanol exposure concentration. For all exposures, blood concentrations of 14C-methanol-derived formate were 10 to 1000 times lower than endogenous blood formate concentrations (100 to 200 mM) reported for monkeys and were several orders of magnitude lower than levels of formate known to be toxic. Since the metabolism of formate in primates depends on the availability of tetrahydrofolate, the same four monkeys were next placed on a folate-deficient diet until folate concentrations in red blood cells consistent with moderate folate deficiency (29 to 107 ng/mL) were achieved. Monkeys were then reexposed to the highest exposure concentration, 900 ppm 14C-methanol, for a similar 2-hour period, and again the pharmacokinetic data described above were obtained. Even with a reduced folate status, monkeys exposed to 900 ppm methanol for 2 hours had peak concentrations of methanol-derived formate that were well below the endogenous levels of formate. Although these results represent only a single exposure and therefore preclude broad generalizations, they do suggest the body contains sufficient folate stores to effectively detoxify small doses of methanol-derived formate from exogenous sources, such as those that might occur during normal use of automotive fuel.

Disposition of butadiene epoxides in Sprague-Dawley rats

1,2-Epoxybutene (BMO) and diepoxybutane (BDE) are metabolic products of 1,3-butadiene in rodents. Both BMO and BDE are suspect in the development of tumors in rats and mice. To understand the distribution and elimination of these compounds in the absence of the rate-limiting production from butadiene, the pharmacokinetics of BMO and BDE in blood were determined in adult male Sprague-Dawley rats following intravenous administration. All animals were dually cannulated in these studies. For the BMO studies, rats were dosed with 71, 143, or 286 mumol/kg BMO (n = 3 for each dose group). For the BDE studies, rats were dosed with 523 mumol/kg BDE (n = 3). All animals tolerated the BMO and BDE doses without grossly observable adverse effects. Blood was drawn at predetermined time points and extracted in methylene chloride. BDE and BMO concentrations were quantitated by gas chromatography or gas chromatography/mass spectrometry. The BMO distribution half-lives were short and ranged from 1.4 min at the lowest dose to 1.8 min at the highest dose. Volume of distribution at steady state ranged from 0.53 +/- 0.17 to 0.59 +/- 0.31 l/kg. Systemic clearances ranged from 67 +/- 17 to 114 +/- 20 ml/min per kg. The terminal elimination half-lives were also short and ranged from 5.7 to 8.5 min among the doses. The pharmacokinetic parameters after an i.v. dose of 523 mumol/kg BDE were a distribution half-life of 2.7 min, terminal elimination T1/2 of 14 min, volume of distribution at steady state of 0.73 +/- 0.06 l/kg, and systemic clearance of 76 +/- 8 ml/min per kg. These pharmacokinetic parameters demonstrate the similarity between disposition of the two epoxides in rats, that include a rapid distribution after i.v. administration into a small extravascular body compartment as well as a rapid elimination from blood. These pharmacokinetic data provide useful blood clearance information for assessing the critical physiological and biochemical determinants underlying the disposition of butadiene epoxides.

Physiologically based pharmacokinetic modeling of 1,3-butadiene, 1,2-epoxy-3-butene, and 1,2:3,4-diepoxybutane toxicokinetics in mice and rats

1,3-Butadiene (BD) is a more potent tumor inducer in mice than in rats. BD also shows striking differences in metabolic activation, with substantially higher blood concentrations of 1,2:3,4-diepoxybutane (butadiene diepoxide; BDE) in BD-exposed mice than in similarly exposed rats. The objective of this study was to develop a single mechanistic model structure capable of describing BD disposition in both species. To achieve this objective, known pathways of 1,2-epoxy-3-butene (butadiene monoepoxide; BMO) and BDE metabolism were incorporated into a physiologically based pharmacokinetic model by scaling rates determined in vitro. With this model structure, epoxide clearance was underestimated for both rats and mice. Improved simulation of blood epoxide concentrations was achieved by addition of first-order metabolism in the slowly perfused tissues, verified by simulation of data on the time course for BMO elimination after i.v. injection of BMO. Blood concentrations of BD were accurately predicted for mice and rats exposed by inhalation to constant concentrations of BD. However, if all BD was assumed to be metabolized to BMO, blood concentrations of BMO were overpredicted. By assuming that only a fraction of BD metabolism produces BMO, blood concentrations of BMO could be predicted over a range of BD exposure concentrations for both species. In vitro and in vivo studies suggest an alternative cytochrome P-450-mediated pathway for BD metabolism that does not yield BMO. Including an alternative pathway for BD metabolism in the model also gave accurate predictions of blood BDE concentrations after inhalation of BD. Blood concentrations of BMO and BDE observed in both mice and rats are best explained by the existence of an alternative pathway for BD metabolism which does not produce BMO.

Mechanistic considerations in benzene physiological model development

Benzene, an important industrial solvent, is also present in unleaded gasoline and cigarette smoke. The hematotoxic effects of benzene in humans are well documented and include aplastic anemia, pancytopenia, and acute myelogenous leukemia. However, the risks of leukemia at low exposure concentrations have not been established. A combination of metabolites (hydroquinone and phenol, for example) may be necessary to duplicate the hematotoxic effect of benzene, perhaps due in part to the synergistic effect of phenol on myeloperoxidase-mediated oxidation of hydroquinone to the reactive metabolite benzoquinone. Because benzene and its hydroxylated metabolites (phenol, hydroquinone, and catechol) are substrates for the same cytochrome P450 enzymes, competitive interactions among the metabolites are possible. In vivo data on metabolite formation by mice exposed to various benzene concentrations are consistent with competitive inhibition of phenol oxidation by benzene. In vitro studies of the metabolic oxidation of benzene, phenol, and hydroquinone are consistent with the mechanism of competitive interaction among the metabolites. The dosimetry of benzene and its metabolites in the target tissue, bone marrow, depends on the balance of activation processes such as enzymatic oxidation and deactivation processes such as conjugation and excretion. Phenol, the primary benzene metabolite, can undergo both oxidation and conjugation. Thus the potential exists for competition among various enzymes for phenol. Zonal localization of phase I and phase II enzymes in various regions of the liver acinus also impacts this competition. Biologically based dosimetry models that incorporate the important determinants of benzene flux, including interactions with other chemicals, will enable prediction of target tissue doses of benzene and metabolites at low exposure concentrations relevant for humans.

Reduction of benzene metabolism and toxicity in mice that lack CYP2E1 expression

Transgenic CYP2E1 knockout mice (cyp2e1-/-) were used to investigate the involvement of CYP2E1 in the in vivo metabolism of benzene and in the development of benzene-induced toxicity. After benzene exposure, absence of CYP2E1 protein was confirmed by Western blot analysis of mouse liver samples. For the metabolism studies, male cyp2e1-/- and wild-type control mice were exposed to 200 ppm benzene, along with a radiolabeled tracer dose of [14C]benzene (1.0 Ci/mol) by nose-only inhalation for 6 hr. Total urinary radioactivity and all radiolabeled individual metabolites were reduced in urine of cyp2e1-/- mice compared to wild-type controls during the 48-hr period after benzene exposure. In addition, a significantly greater percentage of total urinary radioactivity could be accounted for as phenylsulfate conjugates in cyp2e1-/- mice compared to wild-type mice, indicating the importance of CYP2E1 in oxidation of phenol following benzene exposure in normal mice. For the toxicity studies, male cyp2e1-/-, wild-type, and B6C3F1 mice were exposed by whole-body inhalation to 0 ppm (control) or 200 ppm benzene, 6 hr/day for 5 days. On Day 5, blood, bone marrow, thymus, and spleen were removed for evaluation of micronuclei frequencies and tissue cellularities. No benzene-induced cytotoxicity or genotoxicity was observed in cyp2e1-/- mice. In contrast, benzene exposure resulted in severe genotoxicity and cytotoxicity in both wild-type and B6C3F1 mice. These studies conclusively demonstrate that CYP2E1 is the major determinant of in vivo benzene metabolism and benzene-induced myelotoxicity in mice.

Physiologically based pharmacokinetic modeling of blood and tissue epoxide measurements for butadiene

In vitro and in vivo butadiene (BD) metabolism data from laboratory animals were integrated into a rodent physiologically based pharmacokinetic (PBPK) model with flow- and diffusion-limited compartments. The resulting model describes experimental data from multiple sources under scenarios such as closed chamber inhalation and nose-only flow-through inhalation exposures. Incorporation of diurnal glutathione (GSH) variation allows accurate simulation of GSH changes observed in air control nose-only exposures and BD exposures. An isolated tissue model based on rate parameters determined in vitro predicts the decrease in epoxide concentrations in intact animals during the time lag between exsanguination and tissue removal for tissues capable of epoxide biotransformation, providing a better indication of in vivo dosimetry. Further refinements of the model are required relative to model predictions of an important BD metabolite, diepoxybutane.

Metabolism of butadiene by mice, rats, and humans: a comparison of physiologically based toxicokinetic model predictions and experimental data

1,3-Butadiene is a carcinogen in rats and mice, with mice being substantially more sensitive than rats. Our recent research is directed toward obtaining a better understanding of the cancer risk of butadiene in humans by evaluating species-dependent differences in the formation of the toxic metabolites epoxybutene and diepoxybutane. The recent data include in vitro studies on butadiene metabolism using tissues from humans, rats, and mice as well as experimental data and physiological model predictions for butadiene in blood and butadiene epoxides in blood, lung, and liver after exposure of rats and mice to inhaled butadiene. The findings suggest that humans would be more like rats and less like mice regarding the formation of butadiene epoxides. These research findings permit a reassessment of some default options that are used in carcinogen risk assessments. The research approach employed can be a useful strategy for developing mechanistic and toxicokinetic data to supplant default assumptions used in carcinogen risk assessments.

The use of toxicologic data in mechanistic risk assessment: 1,3-butadiene as a case study

The National Research Council (NRC) recently published a report. Science and Judgment in Risk Assessment, that critiqued the current approaches to characterizing human cancer risks from exposure to chemicals. One issue raised in the report relates to the use of default options for quantitation of cancer risks. Default options are general guidelines that can be used for risk assessment when specific information about a chemical is absent. Research on 1,3-butadiene represents an interesting case study in which existing knowledge on this chemical indicates that two default options may no longer be tenable: (1) humans are as sensitive as the most sensitive animal species, and (2) the rate of metabolism is a function of body surface area rather than inherent species differences in metabolic capacity. Butadiene, a major commodity chemical used in the production of synthetic rubber, is listed as one of 189 hazardous air pollutants under the 1990 Clean Air Act Amendments. Butadiene is a carcinogen in rats and mice, with mice being substantially more sensitive than rats. The extent to which butadiene poses a cancer risk to humans exposed to this chemical is uncertain. Butadiene requires metabolic activation to DNA-reactive epoxides to exert its mutagenic and carcinogenic effects. Research is directed toward obtaining a better understanding of the cancer risks of butadiene in humans by evaluating species-dependent differences in the formation of the toxic butadiene epoxide metabolites, epoxybutene and diepoxybutane. The data include in-vitro studies on butadiene metabolism using tissues from humans, rats, and mice as well as experimental data and physiological model predictions for butadiene in blood and butadiene epoxides in blood, lung, and liver after exposure of rats and mice to inhaled butadiene. The findings suggest that humans are more like rats and less like mice regarding the formation of butadiene epoxides. The research approach employed can be a useful strategy for developing mechanistic and toxicokinetic data to supplant default options used in carcinogen risk assessments for butadiene.

Development of physiologically based pharmacokinetic model for methyl tertiary-butyl ether and tertiary-butanol in male Fisher-344 rats

Methyl tertiary-butyl ether (MTBE) and its metabolite tertiary-butanol (TBA) both cause renal tumors in chronically exposed male rats. Knowledge of the kinetic behavior of MTBE and TBA in rats and its comparison to the kinetics of these chemicals in humans will aid in assessing human risk. The objective of this study was to develop a physiologically based pharmacokinetic (PBPK) model for MTBE and TBA in rats that will form the basis for a human model. Physiological parameters such as blood flows, tissue volumes, and alveolar ventilation were obtained from the literature. Chemical-specific parameters such as the solubility of MTBE and TBA in blood and selected tissues and metabolic rate constants to describe whole-body metabolism of MTBE in rats were measured using vial equilibration and gas uptake techniques, respectively. MTBE metabolism was described in the model as occurring through two saturable pathways. The model was able to predict gas uptake data (100 to 2000 ppm starting concentrations) and levels of MTBE in blood of rats exposed to MTBE by inhalation (400 to 8000 ppm, 6 hr), i.v. (40 mg/kg), and oral (40 or 400 mg/kg) administration. Two different models to describe the dosimetry of TBA in a rat were tested for their ability to predict TBA blood levels after MTBE exposure. TBA blood levels were predicted best at low MTBE exposure concentrations using a two-compartment model. The pharmacokinetics of TBA appear to be far more complex than those of MTBE, and additional experimental data on TBA distribution and elimination will be necessary to refine the submodel. With a quantitative description of the important determinants of MTBE and TBA dosimetry understood, a better assessment of the potential toxic and cancer risk for humans exposed to MTBE can be made.

Differences in rates of benzene metabolism correlate with observed genotoxicity

Benzene (BZ) requires oxidative metabolism via cytochrome P450 2E1 (CYP 2E1) to exert its hematotoxic and genotoxic effects. Male mice are two- to threefold more sensitive to the genotoxic effects of BZ as measured by micronuclei induction and sister chromatid exchanges. The purpose of our study was to investigate sex-related differences in the metabolism of BZ, phenol (PHE) and hydroquinone (HQ) in order to understand the metabolic basis for sex-dependent differences in BZ genotoxic susceptibility in mice. Rates of BZ oxidation were quantitated using closed chamber gas uptake studies with male and female B6C3F1 mice exposed to initial low (400-500 ppm), intermediate (1200-1300 ppm), and high (2600-2800 ppm) BZ concentrations. Acetone-pretreated and diethyldithiocarbamate-pretreated male mice were also studied to determine the extent to which induction and inhibition of CYP 2E1, respectively, would alter in vivo BZ oxidation rates. Elimination of PHE and HQ from blood was also compared in male and female mice to complement previously reported data on sex-related differences in urinary excretion of conjugated metabolites following iv administration of PHE. Based on PBPK model analysis, the optimized rate of metabolism (Vmax) of BZ was almost twofold higher in male mice (14.0 mumol/hr-kg) than in female mice (7.9 mumol/hr-kg); both male and female mice gas-uptake data were well fit with a KM of 3.0 microM. Pretreatment of male mice with 1% acetone in drinking water for 8 days to specifically induce CYP 2E1 enhanced the rate of BZ oxidation by approximately fivefold (Vmax = 75 mumol/hr-kg), while diethyldithiocarbamate pretreatment (320 mg/kg ip 30 min prior to uptake study) completely inhibited BZ oxidation (Vmax = 0 mumol/hr-kg). Thus, both pretreatment regimens are potentially useful investigative tools to study the metabolic basis for benzene toxicity. Elimination of PHE from blood was significantly faster in male mice, while elimination of HQ did not differ between male and female mice. Previous data indicated male mice produce more of the oxidized and conjugated metabolite, HQ glucuronide, after PHE administration, suggesting that HQ production from PHE is greater in male mice. Taken together, these data support the hypothesis that the greater sensitivity of male mice to the genotoxic effects of BZ compared to females is a function of greater oxidative metabolism toward both BZ and PHE in male mice. These data also suggest that differences in hepatic human CYP 2E1 activity may be an important factor to consider when evaluating human risk for benzene-induced hematotoxic and genotoxic effects.

Benzene: a case study in parent chemical and metabolite interactions

Benzene, an important industrial solvent, is also present in unleaded gasoline and cigarette smoke. The hematotoxic effects of benzene in humans are well documented and include aplastic anemia and pancytopenia, and acute myelogenous leukemia. A combination of metabolites (hydroquinone and phenol for example) is apparently necessary to duplicate the hematotoxic effect of benzene, perhaps due in part to the synergistic effect of phenol on myeloperoxidase-mediated oxidation of hydroquinone to the reactive metabolite benzoquinone. Since benzene and its hydroxylated metabolites (phenol, hydroquinone and catechol) are substrates for the same cytochrome P450 enzymes, competitive interactions among the metabolites are possible. In vivo data on metabolite formation by mice exposed to various benzene concentrations are consistent with competitive inhibition of phenol oxidation by benzene. In vitro studies of the metabolic oxidation of benzene, phenol and hydroquinone are consistent with the mechanism of competitive interaction among the metabolites. The dosimetry of benzene and its metabolites in the target tissue, bone marrow, depends on the balance of activation processes such as enzymatic oxidation and deactivation processes such as conjugation and excretion. Phenol, the primary benzene metabolite, can undergo both oxidation and conjugation. Thus, the potential exists for competition among various enzymes for phenol. However, zonal localization of Phase I and Phase II enzymes in various regions of the liver acinus regulates this competition. Biologically-based dosimetry models that incorporate the important determinants of benzene flux, including interactions with other chemicals, will enable prediction of target tissue doses of benzene and metabolites at low exposure concentrations relevant for humans.

Health risk assessment of chemical mixtures from a research perspective

Human exposures to chemicals in the environment and workplace typically involve chemical mixtures. One of the key risk assessment issues for mixtures is that of extrapolation from high to low dose. Observation of an interaction among chemicals in a mixture at high concentrations in animals does not necessarily mean that the same effect, in type or magnitude, will be significant in humans exposed to lower concentrations of the mixture. Physiologically based toxicokinetic (PBTK) models can be used to assist in the extrapolation from high to low dose. Mechanisms observed in animals such as competitive inhibition of xenobiotic metabolism (e.g., butadiene and styrene or benzene and toluene) can be incorporated into PBTK models. The models can then be used to predict the magnitude of the interactive effects at high and low exposure concentrations. The most relevant predictions can then be tested using selected experiments. A research strategy involving hypothesis generation through quantitative modeling and testing through laboratory-based experiments may be the most effective strategy for addressing the complex issue of human health risks from exposures to chemical mixtures.

Recent developments in methanol toxicity

The disposition of methanol and its putative toxic metabolite formate has been studied in humans, non-human primates, and rodents after exposure to high, neurotoxic doses. The rate at which rodents detoxify formate is more rapid than that of primates. Formate, an endogenous biological substrate, is detoxified by metabolism to CO2 via a tetrahydrofolate-(THF) dependent pathway. Species with high hepatic THF levels, such as rodents, are less sensitive to the neurotoxic effects of large methanol doses compared with species with low THF levels, such as primates. Data on the capacity of primates to detoxify formate derived from inhalation of low levels of methanol are critical for assessing human risk from methanol fuels. Female cynomolgus monkeys exposed to low concentrations of [14C]methanol (10-200 ppm) for 2 h have blood levels of methanol-derived formate that are 100- to 1000-fold lower than endogenous levels of formate. Healthy human volunteers exposed at rest or during exercise to 200 ppm methanol for 6 h or exposed to 20 mg/kg orally have elevated blood levels of methanol, but blood formate concentrations are not significantly increased above endogenous concentrations. Deficiencies in THF may prolong blood levels of formate and increase the likelihood of toxic effects. Limited studies in non-human primates with low THF levels exposed to 900 ppm methanol for 2 h have shown that concentrations of methanol-derived formate in blood remain below endogenous levels. Thus human populations may not be at added risk of neurotoxic effects resulting from exposure to low levels of methanol.

The application of physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) modeling to understanding the mechanism of action of hazardous substances

Much of toxicology research is focused on elucidating the nature of the mechanisms through which various xenobiotics exert their toxic effects. The central issue in extrapolating laboratory experiments to the human situation is whether mechanisms which are operative in laboratory animals are similar to mechanisms operating in humans. The underlying assumption is that understanding mechanisms permits rational extrapolation between species, across routes of exposure, or from high to low doses. There are two general classes of mechanisms of action. First, there are the mechanisms that result in the translation of an exposure concentration to the effective dose at the target site. The mechanisms that are operative at a pharmacokinetic level include those that are physiologically driven and those that are metabolically based. Second are mechanisms through which the dose at the target site elicits the ultimate adverse response. These are pharmacodynamic in nature and refer to the action of the effective dose at the target site. Altered gene regulation, cytotoxicity, and cell proliferation are examples of processes involving potential adverse effects at the target site. A quantitative understanding of the mechanisms involved in going from exposure to dose and dose to response can aid in answering the question of whether or not these mechanisms in animals and humans are similar or different.

In vitro conjugation of benzene metabolites by human liver: potential influence of interindividual variability on benzene toxicity

In addition to industrial sources, benzene is present in the environment as a component of cigarette smoke and automobile emissions. Toxicity of benzene most likely results from oxidative metabolism of benzene to reactive products. However, susceptibility to these toxic effects may be related to a balance between activation (phase I) and detoxication (phase II) reactions. In the present study, we have estimated kinetic parameters of the two major detoxication reactions for benzene metabolites--phenol sulfation and hydroquinone glucuronidation--in liver subcellular fractions from 10 humans, and single samples from mice and rats. The extent of oxidative metabolism of benzene by these liver samples has been reported previously. Here, initial rates of phenol sulfation varied 3-fold (range 0.309-0.919 nmol/mg protein/min) among human samples. Measured rates were faster in rats (1.195 nmol/mg protein/min) than in mice (0.458 nmol/mg protein/min). Initial rates of hydroquinone glucuronidation by human samples also varied 3-fold (range 0.101-0.281 nmol/mg protein/min). Hydroquinone glucuronidation was more rapid by mouse microsomes (0.218 nmol/mg protein/min) than by rat microsomes (0.077 nmol/mg protein/min). To integrate interindividual differences in various enzyme activities, a physiological compartmental model was developed that incorporates rates of both conjugation reactions and oxidation reactions. Model equations were solved for steady-state concentrations of phenol and hydroquinone attained in human, mouse and rat blood during continuous exposure to benzene (0.01 microM in blood). Among the 10 human subjects, steady-state concentrations of phenol varied 6-fold (range 0.38-2.17 nM) and steady-state concentrations of hydroquinone varied 5-fold (range 6.66-31.44 nM). Predicted steady-state concentrations of phenol were higher in mice compared with rats (2.28 and 0.83 nM respectively). Likewise, higher steady-state concentrations of hydroquinone were predicted in mice than in rats (42.44 and 17.99 nM respectively). Predicted steady-state concentrations of phenol and hydroquinone in mice were higher than predictions for the 10 human subjects, whereas predicted concentrations for rats fell among the human values. As such, our results underscore the importance of considering the balance between activation and detoxication reactions in the elimination of toxicants. Model simulations suggest that both phase I and phase II pathways influence the relative risk from exposure to benzene.

Dose-, route-, and sex-dependent urinary excretion of phenol metabolites in B6C3F1 mice

Phenol is the major oxidized metabolite of benzene, a known human leukemogen and ubiquitous environmental pollutant. Unlike benzene, phenol does not induce tumors in mice following oral exposure; benzene also exhibits sex-related differences in genotoxicity to bone marrow cells that are not observed following phenol administration. We studied the urinary excretion of phenol metabolites in mice as a means to further investigate the metabolic basis for differences in benzene- and phenol-induced toxicity. Male and female B6C3F1 mice (n = 3/group) were exposed to 15, 40, 100, or 225 mumol [14C]phenol/kg by i.v. tail vein injection (6 microCi/mouse). First-pass intestinal metabolism of phenol was evaluated by comparison of urinary excretion of phenol metabolites following i.v. administration with additional groups of male mice that received the same dose levels by oral gavage. Mice were placed in glass metabolism cages, and urine was collected over dry ice for 48 h. Urinary metabolites were separated by high-pressure liquid chromatography (HPLC) and quantified by liquid scintillation spectrometry. Urinary excretion of conjugated metabolites of phenol was dose-dependent in both male and female mice administered phenol by i.v. injection or gavage. The major urinary metabolites of phenol were phenol sulfate (PS), phenol glucuronide (PG), and hydroquinone glucuronide (HQG). Sulfation was the dominant pathway at all dose levels, but decreased as a percent of the excreted dose with a concomitant increase in glucuronidation as the dose level increased. Male mice consistently excreted a higher proportion of phenol as the oxidized conjugated metabolite, HQG, compared to female mice, suggesting that male mice oxidize phenol to hydroquinone more rapidly than female mice. Increased oxidation of phenol to hydroquinone by male mice compared to female mice is consistent with both the greater sensitivity of male mice to the genotoxic effects of benzene and the greater potency of hydroquinone compared to phenol as a genotoxicant. Intestinal conjugation of phenol prior to absorption was significant only at low doses and thus alone does not provide an explanation for the lack of carcinogenicity of phenol in bioassays conducted at much higher dose levels.

Pharmacokinetic modeling approaches for describing the uptake, systemic distribution, and disposition of inhaled chemicals

A fundamental relationship in toxicology is that an external chemical exposure leading to an internal tissue dose can result in an adverse biological response. An understanding of these relationships in experimental animals is often used to extrapolate and predict the potential risk to humans following exposure to toxic chemicals. The exposure-dose-response relationships for volatile compounds inhaled by the lungs are complicated by the fact that many toxic effects caused by these chemicals have been identified in tissues and organ systems other than the lungs. Pharmacokinetic modeling approaches have been devised to quantitate the relationships between inhaled concentrations of volatile compounds and the resulting critical tissue doses in experimental animals. These animal models have also been extrapolated to predict chemical disposition in humans for estimation of human health risks. This communication reviews three pharmacokinetic descriptions, each representing different levels of complexity, that have been used to assess chemical disposition of inhaled, volatile chemicals. The mathematical structures, assumptions, data needs, and risk assessment capabilities of each modeling approach are described.

Age-related changes in benzene disposition in male C57BL/6N mice described by a physiologically based pharmacokinetic model

A physiologically based pharmacokinetic (PBPK) model was developed to describe the disposition of benzene in 3- and 18-month C57BL/6N mice and to examine the relevant physiologic and/or biochemical parameters governing previously observed age-related changes in the disposition of benzene. The model developed was based on that of Medinsky et al. (Toxicol. Appl. Pharmacol. 99 (1989) 193-206), with the inclusion of an additional rate constant for urinary elimination of benzene metabolites. Experimentally determined tissue partition coefficients for benzene in 3- and 18-month mice, as well as actual body weights and fat compartment volumes, were included as part of the model. Model simulations were conducted for oral exposure of 3-month mice to 10 and 200 mg benzene/kg and for oral exposure of 18-month mice to 10 and 150 mg benzene/kg. Total amount of benzene metabolized, as well as metabolism of benzene to specific metabolites and their elimination, was simulated. Modeling results for total amount of benzene metabolites eliminated in urine over a 24-h period at 10 mg/kg showed that a greater total amount of benzene metabolites would be excreted by 18-month versus 3-month old mice. At saturating doses of 150 and 200 mg/kg, total amount of benzene metabolites excreted 24 h post-dose was predicted to be equivalent in 18-month mice and 3-month old mice, but the rate of elimination over time was shown to be decreased in 18-month vs. 3-month mice. Decreased urinary elimination of total benzene metabolites was simulated by a smaller renal elimination rate constant in 18-month vs. 3-month mice, which is consistent with decreased renal blood flow noted in aging rodents. These model predictions were consistent with observed in vitro and in vivo experimental data. Model simulations for production of specific metabolites from benzene and elimination in urine agreed well with experimental data in showing no significant age-related changes in formation of benzene metabolites, with the exception of hydroquinone conjugates. Model simulations and experimental data showed decreased total urinary elimination of hydroquinone conjugates in 18-month vs. 3-month mice. The change in hydroquinone conjugate elimination with age was simulated in modeling experiments as an age-related increase in Km for production of hydroquinone conjugates from benzene. The results of this study indicate that age-related changes in physiology are primarily responsible for altered disposition of benzene in aged mice and suggest that concentrations for toxicity of benzene and/or metabolites may differ in target tissues of aged mice.

1,3-Butadiene: linking metabolism, dosimetry, and mutation induction

There is increasing concern for the potential adverse health effects of human exposures to chemical mixtures. To better understand the complex interactions of chemicals within a mixture, it is essential to develop a research strategy which provides the basis for extrapolating data from single chemicals to their behavior within the chemical mixture. 1,3-Butadiene (BD) represents an interesting case study in which new data are emerging that are critical for understanding interspecies differences in carcinogenic/genotoxic response to BD. Knowledge regarding mechanisms of BD-induced carcinogenicity provides the basis for assessing the potential effects of mixtures containing BD. BD is a multisite carcinogen in B6C3F1 mice and Sprague-Dawley rats. Mice exhibit high sensitivity relative to the rat to BD-induced tumorigenesis. Since it is likely that BD requires metabolic activation to mutagenic reactive epoxides that ultimately play a role in carcinogenicity of the chemical, a quantitative understanding of the balance of activation and inactivation is essential for improving our understanding and assessment of human risk following exposure to BD and chemical mixtures containing BD. Transgenic mice exposed to 625 ppm BD for 6 hr/day for 5 days exhibited significant mutagenicity in the lung, a target organ for the carcinogenic effect of BD in mice. In vitro studies designed to assess interspecies differences in the activation of BD and inactivation of BD epoxides reveal that significant differences exist among mice, rats, and humans. In general, the overall activation/detoxication ratio for BD metabolism was approximately 10-fold higher in mice compared to rats or humans.(ABSTRACT TRUNCATED AT 250 WORDS)

Critical issues in benzene toxicity and metabolism: the effect of interactions with other organic chemicals on risk assessment

Benzene, an important industrial solvent, is also present in unleaded gasoline and cigarette smoke. The hematotoxic effects of benzene are well documented and include aplastic anemia and pancytopenia. Some individuals exposed repeatedly to cytotoxic concentrations of benzene develop acute myeloblastic anemia. It has been hypothesized that metabolism of benzene is required for its toxicity, although administration of no single benzene metabolite duplicates the toxicity of benzene. Several investigators have demonstrated that a combination of metabolites (hydroquinone and phenol, for example) is necessary to duplicate the hematotoxic effect of benzene. Enzymes implicated in the metabolic activation of benzene and its metabolites include the cytochrome P450 monooxygenases and myeloperoxidase. Since benzene and its hydroxylated metabolites (phenol, hydroquinone, and catechol) are substrates for the same cytochrome P450 enzymes, competitive interactions among the metabolites are possible. In vivo data on metabolite formation by mice exposed to various benzene concentrations are consistent with competitive inhibition of phenol oxidation by benzene. Other organic molecules that are substrates for cytochrome P450 can inhibit the metabolism of benzene. For example, toluene has been shown to inhibit the oxidation of benzene in a noncompetitive manner. Enzyme inducers, such as ethanol, can alter the target tissue dosimetry of benzene metabolites by inducing enzymes responsible for oxidation reactions involved in benzene metabolism. The dosimetry of benzene and its metabolites in the target tissue, bone marrow, depends on the balance of activation processes, such as enzymatic oxidation, and deactivation processes, like conjugation and excretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of inhaled [14C]methanol and methanol-derived [14C]formate in normal and folate-deficient cynomolgus monkeys

Large-scale use of methanol (MeOH) as an automotive fuel may increase exposure of the public to MeOH vapor, necessitating the need for additional data for an adequate human health risk assessment. Formate is accepted as the toxic metabolite of MeOH, its metabolism is folate-dependent, and potentially sensitive folate-deficient subpopulations (e.g., pregnant women) exist that may be at higher risk to low-level methanol exposure. This study determined the pharmacokinetics of [14C]MeOH and [14C]formate in normal and folate-deficient (FD) monkeys following inhalation of environmentally relevant concentrations of [14C]MeOH. Four normal adult female cynomolgus monkeys were anesthetized (isoflurane) and exposed by lung-only inhalation to 10, 45, 200, and 900 ppm [14C]MeOH for 2 hr. Monkeys were then placed on a FD diet until folate concentrations consistent with moderate deficiency (29-107 ng/ml) developed in red blood cells and then reexposed to 900 ppm (900-FD) for 2 hr. Average (+/- SD) end-of-exposure blood [14C]MeOH concentrations were 0.65 +/- 0.3, 3.0 +/- 0.8, 21 +/- 16, 106 +/- 84, and 211 +/- 71 microM, while average (+/- SD) peak blood [14C]formate concentrations were 0.07 +/- 0.02, 0.25 +/- 0.09, 2.3 +/- 2.9, 2.8 +/- 1.7, and 9.5 +/- 4.7 microM following MeOH inhalation at 10, 45, 200, 900, and 900-FD ppm, respectively. The blood concentration of [14C]MeOH-derived formate from all exposures was 10 to 1000 times lower than the endogenous blood formate concentration (0.1 to 0.2 mM) reported for monkeys. These results suggest that low-level exposure to MeOH would not result in elevated blood formate concentrations in humans under short-term exposure conditions.

Benzene metabolism by human liver microsomes in relation to cytochrome P450 2E1 activity

Low levels of benzene from sources including cigarette smoke and automobile emissions are ubiquitous in the environment. Since the toxicity of benzene probably results from oxidative metabolites, an understanding of the profile of biotransformation of low levels of benzene is critical in making a valid risk assessment. To that end, we have investigated metabolism of a low concentration of [14C]benzene (3.4 microM) by microsomes from human, mouse and rat liver. The extent of phase I benzene metabolism by microsomal preparations from 10 human liver samples and single microsomal preparations from both mice and rats was then related to measured activities of cytochrome P450 (CYP) 2E1. Measured CYP 2E1 activities, as determined by hydroxylation of p-nitrophenol, varied 13-fold (0.253-3.266 nmol/min/mg) for human samples. The fraction of benzene metabolized in 16 min ranged from 10% to 59%. Also at 16 min, significant amounts of oxidative metabolites were formed. Phenol was the main metabolite formed by all but two human microsomal preparations. In those samples, both of which had high CYP 2E1 activity, hydroquinone was the major metabolite formed. Both hydroquinone and catechol formation showed a direct correlation with CYP 2E1 activity over the range of activities present. A simulation model was developed based on a mechanism of competitive inhibition between benzene and its oxidized metabolites, and was fit to time-course data for three human liver preparations. Model calculations for initial rates of benzene metabolism ranging from 0.344 to 4.442 nmol/mg/min are directly proportional to measured CYP 2E1 activities. The model predicted the dependence of benzene metabolism on the measured CYP 2E1 activity in human liver samples, as well as in mouse and rat liver samples. These results suggest that differences in measured hepatic CYP 2E1 activity may be a major factor contributing to both interindividual and interspecies variations in hepatic metabolism of benzene. Validation of this system in vivo should lead to more accurate assessment of the risk of benzene's toxicity following low-level exposure.

In vivo metabolism of butadiene by mice and rats: a comparison of physiological model predictions and experimental data

1,3-Butadiene (BD), a rodent carcinogen, is metabolized to mutagenic and potentially DNA-reactive epoxides, including butadiene monoepoxide (BMO) and butadiene diepoxide. A physiological model containing five tissue groups (liver, lung, fat, slowly perfused tissues and rapidly perfused tissues) and blood was developed to describe uptake and metabolism of inhaled BD and BMO. Maximal rates for hepatic and pulmonary metabolism of BD and hepatic metabolism of BMO incorporated into the model were extrapolated from in vitro data (Csanády et al., Carcinogenesis, 13, 1143-1153, 1992). Apparent enzyme affinities used in the model were identified to the values measured in vitro. Model stimulations for BD and BMO uptake were compared to results from experiments in which groups of male Sprague-Dawley rats and B6C3F1 mice were exposed to initial concentrations of 50-5000 p.p.m. BD in closed chamber experiments and published data on BMO uptake by rats and mice. Metabolic rate constants extrapolated from in vitro data stimulated both BMO and BD uptake from closed chambers. The Vmax for hepatic metabolism of BD extrapolated from in vitro studies was 62 mumol/kg/h for rats and 340 mumol/kg/h for mice, while the Vmax for pulmonary metabolism of BD was 1.0 and 22 for rats and mice, respectively. These results demonstrate the usefulness of data derived in vitro for predicting in vivo behavior. Model simulations were also conducted in which only hepatic metabolism of BD was incorporated. These simulations underestimated BD uptake for mice, but not rats. Inclusion of in vitro-derived rates of pulmonary metabolism of BD into the model improved the fit to the data for mice. Since mice, but not rats, develop lung tumors after exposure to BD, these results point to the need for further characterize the metabolic capacity and target cells in the lung for BD and its metabolites. Once characterized, these models can be extended to predict in vivo behavior of BD in humans.

Benzene and phenol metabolism by mouse and rat liver microsomes

Benzene, an important industrial solvent and constituent of unleaded gasoline, causes leukemia and aplastic anemia in humans. Mice are more sensitive than rats to benzene toxicity, though neither species has been shown to respond consistently with benzene-induced leukemia. Benzene biotransformation in liver to phenol, hydroquinone, catechol and/or muconaldehyde is thought to be necessary for its hematotoxicity and/or genotoxicity. Our goal is to develop a mathematical simulation model capable of describing the pathways and kinetics of benzene metabolism by rat and mouse liver microsomes and to assess the role of species metabolic differences in species sensitivity. Microsomes were incubated with 4 microM [U-14C]-benzene or 4 microM [U-14C]phenol. Metabolite production was quantified by extraction into ethyl acetate, HPLC separation and liquid scintillation spectroscopy. After 45 min, mouse liver microsomes converted 20% of the benzene to phenol, 31% to hydroquinone and 2% to catechol. Rat liver microsomes converted 23% of benzene to phenol, 8% to hydroquinone and 0.5% to catechol. Production of hydroquinone and catechol continued for 90 min for mouse liver microsomes, while production by rat liver microsomes had virtually ceased by 90 min. Muconic acid production by mouse liver microsomes was < 0.2% and < 0.04% from benzene and phenol respectively after 90 min. A quantitative simulation model was constructed to describe the in vitro metabolism of benzene, incorporating the reaction sequences: benzene-->phenol-->catechol-->trihydroxybenzene and phenol-->hydroquinone-->trihydroxybenzene. In the model, all of the reaction steps are assumed to be catalyzed by the same enzyme(s), cytochrome(s) P450, and benzene, phenol, hydroquinone and catechol in solution are all assumed to compete, through reversible binding, for the same reaction site(s) on cytochrome(s) P450. The simulation model accurately described both the benzene and phenol kinetic data, supporting this proposed mechanism. In particular, this model suggests that the observed inhibition of benzene on phenol metabolism, and of phenol on benzene metabolism, occurs through competition for a common reaction site, which can also bind catechol and hydroquinone.

Physiologically based modeling of 2-butoxyethanol disposition in rats following different routes of exposure

2-Butoxyethanol (BE) is widely used as a solvent in coatings and other consumer products and has shown hematotoxicity in laboratory animals. To provide a physiological basis for extrapolating toxicokinetic data observed in rats to humans, a blood flow rate-limited, physiologically based pharmacokinetic model was developed to describe the distribution and metabolism of BE in rats following drinking water, dermal, and inhalation exposures. The major urinary metabolite, butoxyacetic acid, represented 45 to 60% of the absorbed dose in all three routes of exposure. Other identified urinary metabolites in our studies included ethylene glycol and BE-glucuronide. A model formulation of the possible metabolic pathways based on the experimental data was proposed. The amounts of individual urinary metabolites were used to develop the model. Metabolic constants were estimated by fitting the data within the constraints of in vitro measurements. The model explained the change of profiles of urinary metabolites in different exposure routes by taking into account the differences in absorption rate and by incorporating a minor pathway for metabolism by skin. Sensitivity analysis showed that metabolic constants and blood flow rate to liver had a relatively larger influence on the production of urinary metabolites than the organ volume or the partition coefficient for BE.

Advances in biologically based models for respiratory tract uptake of inhaled volatiles

Physiologically based pharmacokinetic models for volatile organic chemicals typically describe the respiratory tract as a single compartment in which chemicals in the alveolar air space and the arterial blood are in instantaneous equilibrium. These models also assume that the distribution of chemical in the air-stream is uniform throughout the respiratory tract and that uptake is significant only in the alveolar region. A functional role for the upper respiratory tract in the uptake of volatile chemicals has been largely ignored. While these models have worked well for chemicals with low aqueous solubility in biological fluids, systemic uptake of highly soluble volatiles is overestimated. Thus, there is a significant effort to describe the critical determinants for uptake of soluble chemicals and to formulate models with more biologically relevant descriptions of respiratory tract structure and function. Investigators have addressed this problem from several viewpoints. Airflow patterns in the respiratory tract, regional metabolism, diffusion-dependent uptake, and the cyclic nature of respiration are now being incorporated into current models. Use of dosimetric models that incorporate relevant biology for inhaled chemicals will ultimately result in more meaningful human risk assessments.

Acute methanol toxicity in minipigs

The pig has been proposed as a potential animal model for methanol-induced neuro-ocular toxicosis in humans because of its low liver tetrahydrofolate levels and slower rate of formate metabolism compared to those of humans. To examine the validity of this animal model, 12 4-month-old female minipigs (minipig YU) were given a single oral dose of water or methanol at 1.0, 2.5, or 5.0 g/kg body wt by gavage (n = 3 pigs/dose). Dose-dependent signs of acute methanol intoxication, which included mild CNS depression, tremors, ataxia, and recumbency, developed within 0.5 to 2.0 hr, and resolved by 52 hr. Average maximum methanol concentrations in plasma, of 3100 +/- 700 (SD), 6200 +/- 2300, and 15,200 +/- 900 micrograms/ml were reached within 0.5 to 4 hr following methanol administration in animals given 1.0, 2.5, or 5.0 g methanol/kg, respectively. The mean initial elimination half-lives of methanol were 9.0 +/- 1.6, 22.4 +/- 6.1, and 18.9 +/- 4.3 hr, for 1, 2.5, and 5.0 g/kg doses, respectively. In 3 minipigs, a transient increase in plasma formate concentration (1.74-3.40 mEq/liter vs control = 0.5 +/- 0.3 mEq/liter) occurred 4 to 30 hr following methanol administration. Methanol- and formate-dosed pigs did not develop optic nerve lesions, toxicologically significant formate accumulation, or metabolic acidosis. Based on results following a single dose, female minipigs do not appear to be overtly sensitive to methanol and thus may not be a suitable animal model for acute methanol-induced neuro-ocular toxicosis.

Effect of dose on the disposition of methoxyethanol, ethoxyethanol, and butoxyethanol administered dermally to male F344/N rats

The glycol ethers methoxyethanol (ME), ethoxyethanol (EE), and butoxyethanol (BE) are widely used in industrial and household products. Rodent studies indicate the ME and EE are potentially toxic compounds causing teratogenic, fetotoxic, hematotoxic, and testicular effects. Exposure of rodents to high concentrations of BE resulted in anemia due to hemolysis of blood cells, leukopenia, hemoglobinuria, and liver and kidney damage. The purpose of this study was to determine the uptake, metabolism, and excretion of dermally administered glycol ethers as a function of the externally applied dose. Three different amounts of the 14C-labeled glycol ethers (450-4000 mumole/kg) were applied to same-sized areas on the clipped backs of F344/N rats, and nonoccluded percutaneous absorption was measured. The rates of excretion of the 14C-labeled parent compound and metabolites by different routes were measured, as well as the amount of 14C remaining in the carcass. Within the dose range studied, the absorption and metabolism of these three glycol ethers by F344/N rats was linearly related to the dermally applied dose. The absorption of all three glycol ethers was approximately 20-25%, regardless of the chain length of the alkyl group or the dose administered. The majority of the absorbed dose was excreted in the urine. Feces and exhaled CO2 represented minor routes of excretion. The alkoxyacetic acid was a major metabolite for all three glycol ethers. The formation of small amounts of ethylene glycol indicated cleavage of the ether bond.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of exposure concentration on the disposition of inhaled butoxyethanol by F344 rats

The glycol ethers are a class of solvents widely used due to their range of vapor pressures and miscibility in aqueous and organic media. Butoxyethanol (BE) causes anemia and lowered hematocrits in rats due to direct hemolysis of red blood cells. Exposure to BE is most likely to occur by dermal contact or by inhalation. In this paper, we report the uptake, metabolism, and excretion of BE following 6-hr exposure at different inhaled concentrations. The uptake and metabolism of BE were essentially linear up to 438 ppm. The majority of the inhaled butoxy-[14C]ethanol was eliminated in the urine with butoxyacetic acid (BAA) being the major urinary metabolite, accompanied by lesser amounts of ethylene glycol and BE glucuronide. A small proportion (5-8%) of the retained BE was exhaled as 14CO2. Most (greater than 80%) of the [14C]BE-derived material in blood was in the plasma. BAA was the major metabolite of BE in plasma. Ratios of ethylene glycol to BAA in plasma were higher than those in urine. The BE-derived 14C in plasma rapidly became associated with the acid-precipitable (protein) fraction, probably due to binding of metabolites to proteins or incorporation of the BE metabolites into the carbon pool. These results indicate that, in rats, overall metabolism of BE to BAA, the hemolytic metabolite, was linearly related to the exposure concentration up to a concentration that caused severe toxicity (438 ppm). Assuming that the toxicity of inhaled BE is directly proportional to the formation of BAA, the toxicity of inhaled BE can be expected to be linearly related to the exposure concentration up to exposure concentrations that cause mortality.

Particle-associated polycyclic aromatic hydrocarbons--a reappraisal of their possible role in pulmonary carcinogenesis

According to the model presented here, large aggregates of inert dust forming in the lung can drastically increase retention of polycyclic aromatic hydrocarbons (PAHs) that are either adsorbed or precipitated onto the dust particles. Experiments designed to demonstrate the carcinogenicity of PAHs in animals require that large amounts of PAHs be administered to induce lung cancer. If the PAHs are attached to an inert carrier dust before instillation, aggregation of particles is a very significant factor in the retention of PAHs in the lung. A slow release of particle-associated PAHs would result in a prolonged exposure to surrounding tissues from a limited number of administrations. Because an increased incidence of tumors has been observed following these types of exposures, it has been concluded that the increased retention of PAHs due to their association with inert particles is an important factor in PAH-induced pulmonary carcinogenesis. However, large aggregates of inert dust containing crystalline PAHs are unlikely to form with the much lower doses typical of human exposures. Inhaled particles are more likely to deposit and react with the surrounding lung medium without interference from other particles. Our model demonstrates that, under more typical, low-dose exposure conditions, particle-associated PAHs will be released rapidly from the particles. Sustained exposure of target tissues to PAHs will result from repeated exposures, not from increased retention due to association of PAHs with their carrier particles. This distinction is very important, because in high-dose, instillation experiments in animals, critical doses to cells, and thus tumors, should occur at the sites at which particles are retained; whereas, the much lower, but more frequent exposures common for humans should instead lead to tumors at the sites at which particles are initially deposited, but not necessarily retained.

Disposition of inhaled isoprene in B6C3F1 mice

Isoprene (2-methyl-1,3-butadiene) is the monomeric unit of widely occurring natural products called terpenes. Isoprene is widely used in industry with nearly 1.1 million pounds produced in the United States in 1987. The purpose of this investigation was to determine the toxicokinetics of inhaled isoprene in B6C3F1 mice and to compare the data to previously published toxicokinetic data in F344 rats (A. R. Dahl, L. S. Birnbaum, J. A. Bond, P. G. Gervasi, and R. F. Henderson, 1987. Toxicol. Appl. Pharmacol. 89, 237-248). The comparative toxicokinetics in the two species will be useful for extrapolation of rodent toxicity data to humans. Male B6C3F1 mice were exposed to nominal concentrations of 20, 200, and 2000 ppm isoprene or [14C]isoprene for up to 6 hr. For all exposures, steady-state levels of isoprene were reached rapidly (i.e., within 15 to 30 min) after the onset of exposure. The mean (+/- SE) steady-state blood levels of isoprene (identified by headspace analysis) for the 20, 200, and 2000 ppm exposures were 24.8 +/- 3.3, 830 +/- 51, and 6800 +/- 400 ng isoprene/ml blood, respectively. At the two higher exposure concentrations, the increases in blood levels of isoprene were proportional to the increases in air concentrations of isoprene. There was approximately a 2.3-fold decrease in the retained 14C/inhaled 14C ratio with increasing exposure concentration. Depending on the exposure concentration, from 52% (20 ppm isoprene) to 73% (2000 ppm isoprene) of the metabolite-associated (nonisoprene) radioactivity was excreted in the urine over a 64-hr postexposure period. 14CO2 exhalation after the end of the 6-hr exposure was minimal (2%) at the 20 ppm exposure and increased up to 18% at the higher isoprene exposure concentrations. These data suggest that metabolism of isoprene in mice is nonlinear within the range of exposure concentrations used in this study. Hemoglobin adduct formation reached near-maximum between 200 and 2000 ppm isoprene exposure concentration, consistent with our conclusion that pathways for metabolism of isoprene were saturated. Isoprene metabolites were present in blood after inhalation of isoprene at all concentrations studied. There were substantial differences in the toxicokinetics of inhaled isoprene in mice compared to rats. In mice, fractional retention of inhaled isoprene, which reflects, in part, metabolism of isoprene, was linearly related to exposure concentrations up to 200 ppm but decreased at 2000 ppm; in rats, fractional retention of inhaled isoprene decreased with increasing exposure concentration over a range of exposures from 8 to 1500 ppm.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo deposition of ultrafine aerosols in the nasal airway of the rat

We studied the deposition of ultrafine aerosols, ranging in geometric diameter from 0.005 to 0.1 microns, in the nasal airway of Fischer-344/N rats, at inspiratory flow rates of 200, 300, 400, and 600 ml/min. Simultaneously, we measured the pressure drop across the rat nasal airway. The purpose was to determine whether the in vivo deposition of ultrafine aerosols in the rat nasal airway is the same as the deposition observed in rat nasal casts. At a flow rate of 400 ml/min, corresponding to the normal mean inspiratory flow rate of the rat, deposition efficiency increased from 6 to 58%, when the particle diameter decreased from 0.1 to 0.005 microns. For 0.005-microns-diameter particles, the deposition efficiency decreased from 68 to 52% when the flow rate was increased from 200 to 600 ml/min. These results agree well with those from previous experiments with nasal casts, which indicated that diffusion is the dominant mechanism for deposition of ultrafine aerosols. The pressure drop in the nasal airway of the rat increased almost linearly with flow rate, from 73 Pa at 200 ml/min to 247 Pa at 600 ml/min. These values are within the range of those obtained in previous experiments with nasal casts, although the pressure drop in casts increased as a power greater than 1 with flow rate. The results of our study support the use of nasal airway casts to estimate the in vivo deposition of ultrafine aerosols.

The retention of polycyclic aromatic hydrocarbons in the bronchial airways and in the alveolar region--a theoretical comparison

Several experiments indicate that physical transport phenomena such as molecular diffusion and partitioning between aqueous and lipid phases have a profound influence on the pulmonary retention of polycyclic aromatic hydrocarbons (PAHs). Because the average distance of diffusion between the air interface and the capillary blood is only about 0.5 microm in the alveoli, whereas in the bronchi it probably exceeds 50 microm, there should be a fundamental difference between the bronchial airways and the alveolar region in the retention of PAHs. A theoretical model was developed to simulate the retention of lipophilic substances in the two regions of the lung. Results show that a substance like benzo[a]pyrene, a PAH, may be retained for hours in the bronchi, compared to less than 1 min in the alveoli. This predicted dramatic difference in retention could explain the characteristic, biphasic pattern in the pulmonary clearance of PAHs observed in many animal experiments, but more importantly, it could also explain the fact that human lung cancers occur predominantly in the bronchi, although only a small fraction of inhaled particles carrying PAHs are deposited there.

Measurement of steady-state blood concentrations in B6C3F1 mice exposed by inhalation to vinylidene fluoride

The purpose of this study was to obtain data on blood concentrations of vinylidene fluoride (VDF), an important plastics monomer in B6C3F1 mice during inhalation exposure. A new method for sampling blood from mice during the exposures was developed. The technique used a pernasal exposure tube with an outer, sliding cylinder that allowed access to the heart through the thorax. Blood was removed from an anesthetized mouse via heart-puncture while the animal was being exposed to VDF. Concentrations of VDF were measured in blood of mice during 6-h exposures to nominal concentrations of 250, 3750, or 15,000 ppm VDF. A physiological model developed to simulate blood levels of VDF in rats was adapted for mice by incorporating physiologically realistic parameters for mice where appropriate (alveolar ventilation, cardiac output, blood flow to organs, and organ volumes) and by assuming that chemical-specific parameters such as tissue/blood partition coefficients determined for rats could also be applied to mice. Measured steady-state levels of VDF in blood of mice increased with increasing exposure concentration. For both the 15,000 and 3750 ppm VDF exposures, the experimentally determined data fell within the 95% confidence interval predicted by the physiological model. For the 250 ppm VDF exposure, the experimentally determined values for VDF in blood were lower than what was predicted by the model. Model predictions indicated that for mice, as observed for rats, levels of VDF would rise very rapidly, reaching steady-state within minutes of exposure, and that at the end of exposure, blood levels will decline rapidly. At the two lowest exposure concentrations, we were unable to detect VDF in blood taken 15 min or longer after cessation of exposure, suggesting that the post-exposure levels were at or below our limit of detection which was 4 ng VDF/ml blood. For the 15,000 ppm exposure VDF could be detected in blood up to 15 min post exposure.

Benzene hemoglobin adducts in mice and rats: characterization of formation and physiological modeling

Benzene is a myelotoxin and a human leukemogen. Humans are exposed to this compound, both occupationally and environmentally. This study was conducted to determine whether formation of benzene-derived adducts with blood hemoglobin (Hb) can be used as a biomarker of exposure to benzene. B6C3F1 mice and F344/N rats were given 0.1 to 10,000 mumol [14C]benzene/kg body wt, orally. Twenty-four hours later, animals were euthanized, and globin was isolated from blood samples. The globin was analyzed by liquid scintillation spectrometry for the presence of [14C]benzene-derived adducts. Hb adduct formation was linear with respect to dose for amounts of up to 500 mumol [14C]benzene/kg body wt, for both rodent species. Within this linear dose-response range, mice formed adducts from [14C]benzene approximately 3.5 times less efficiently [0.022 +/- 0.010 (pmol adducts/mg globin)/(mumol/kg body wt dose)] than did rats [0.076 +/- 0.014 (pmol adducts)/(mumol/kg body wt dose)]. Benzene-derived Hb adducts also accumulated linearly when mice and rats were given up to three daily doses of 500 mumol [14C]benzene/kg body wt. These data were used to develop a physiological model for benzene-derived Hb adduct formation. Both first-order and saturable pathways for adduct formation were incorporated. The results showed that the model simulated the levels of Hb adducts in both mice and rats after oral exposures to benzene and predicted the levels of Hb adducts present after inhalation exposure. These studies suggest that Hb adducts might be useful biomarkers for human exposures to benzene.

Effect of inhaled azodicarbonamide on F344/N rats and B6C3F1 mice with 2-week and 13-week inhalation exposures

Azodicarbonamide (ADA), a compound used in the baking and plastics industries, has been reported to cause pulmonary sensitization and dermatitis in people. Two-week repeated and 13-week subchronic inhalation exposures of F344/N rats and B6C3F1 mice to ADA were conducted to determine the toxicity of inhaled ADA. The mean air concentrations of ADA in the 2-week studies were 207, 102, 52, 9.4, or 2.0 mg/m3. No exposure-related mortality nor abnormal clinical signs were observed in rats or mice during or after exposure. The terminal body weights were slightly depressed in the highest exposure group. Liver weights were lower in male rats exposed to 200 mg ADA/m3. No significant lesions were noted on either gross or histologic evaluation of rats or mice. In the 13-week subchronic study, the mean air concentrations of ADA were 204, 100, or 50 mg/m3. No mortality or clinical signs related to exposure were observed. The terminal body weights of exposed rats were not significantly different from those of control rats but were significantly depressed in mice exposed to 100 or 200 mg ADA/m3. No histopathological lesions were noted in mice. Lung weights were increased and enlarged mediastinal and/or tracheobronchial lymph nodes were noted in rats exposed to 50 mg ADA/m3. No exposure-related lesions were observed microscopically in rats exposed to 100 or 200 mg ADA/m3. All rats in the 50 mg ADA/m3 exposure group only had lung lesions that consisted of perivascular cuffing with lymphocytes and a multifocal type II cell hyperplasia, suggesting a possible immune reaction to an antigen in the lung. Viral titers for rats exposed to 50 mg ADA/m3 were negative for Sendai virus and pneumonia virus of mice, which produce similar lesions. The possibility of an unknown viral antigen causing this lesion cannot be eliminated. Lung tissue from male rats was analyzed for ADA and biurea, the major metabolite of ADA. No ADA was detected. The amount of biurea in the lungs increased nonlinearly with increasing exposure concentration, suggesting that clearance was somewhat impaired with repeated exposures. However, even at the highest exposure concentration, this amount of biurea was less than 1% of the estimated total ADA deposited over the exposure period. In summary, ADA is rapidly cleared from the lungs, even when inhaled at concentrations up to 200 mg/m3. Exposure to ADA for up to 13 weeks did not appear to be toxic to rodents.

Disposition of three glycol ethers administered in drinking water to male F344/N rats

The glycol ethers 2-methoxyethanol (ME), 2-ethoxyethanol (EE), and 2-butoxyethanol (BE) are widely used solvents in industrial and consumer applications. The reproductive, teratogenic, and hematotoxic effects of the glycol ethers are due to the alkoxyacetic acid metabolites of these compounds. The effect of alkyl group length on disposition of these three glycol ethers was studied in male F344/N rats allowed access for 24 hr to 2-butoxy[U-14C]ethanol, 2-ethoxy[U-14C]ethanol, or 2-methoxy[U-14C]ethanol in drinking water at three doses (180 to 2590 ppm), resulting in absorbed doses ranging from 100 to 1450 mumols/kg body wt. Elimination of radioactivity was monitored for 72 hr. The majority of the 14C was excreted in urine or exhaled as CO2. Less than 5% of the dose was exhaled as unmetabolized glycol ether. Distinct differences in the metabolism of the glycol ethers as a function of alkyl chain length were noted. For BE 50-60% of the dose was eliminated in the urine as butoxyacetic acid and 8-10% as CO2; for EE 25-40% was eliminated as ethoxyacetic acid and 20% as CO2; for ME 34% was eliminated as methoxyacetic acid and 10-30% as CO2. Ethylene glycol, a previously unreported metabolite of these glycol ethers, was excreted in urine, representing approximately 10, 18, and 21% of the dose for BE, EE, and ME, respectively. Thus, for longer alkyl chain lengths, a smaller fraction of the administered glycol ether was metabolized to ethylene glycol and CO2. Formation of ethylene glycol suggests that dealkylation of the glycol ethers occurs prior to oxidation to alkoxyacetic acid and, as such, represents an alternate pathway in the metabolism of these compounds that does not involve formation of the toxic acid metabolite.

Azodicarbonamide: methods for the analysis in tissues of rats and inhalation disposition

1. A method has been developed for measuring azodicarbonamide (ADA) and its metabolite biurea in tissues of rat. The method is based on the reaction of ADA with triphenylphosphine; the derivative so formed was isolated and quantified using reversed-phase h.p.l.c. Quantification was by u.v. detection with 14C-ADA as internal standard. Biurea was measured by oxidation to ADA, followed by treatment as described above. 2. When biurea was added to tissues at 100-400 micrograms, recoveries of 92-125% were observed. In contrast, recoveries of ADA added to tissues were generally much less than 100% and could not be reliably determined. The inability to quantify ADA added to tissues was ascribed to its rapid and facile reduction by tissue sulphydryl groups. 3. When rats were exposed to ADA aerosol concentrations of 200, 100, 50 and 0 mg/m3 for 13 weeks by inhalation, a non-linear dose-dependent accumulation of biurea was observed in lungs. No ADA was detected in lungs. Neither biurea nor ADA could be detected in kidneys.