Search

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.

1,3-Butadiene (BD), an important commodity chemical used in the production of synthetic rubber, is carcinogenic in B6C3F1 mice and Sprague-Dawley rats, raising concern for potential carcinogenicity in humans. Mice are more sensitive than rats to the carcinogenic effects of BD. Metabolic activation of BD to form the putative DNA-reactive metabolites, butadiene monoxide (BMO) and butadiene diepoxide (BDE), is mediated by cytochrome P450. Detoxication of the epoxides occurs by glutathione S-transferase-catalyzed conjugation with glutathione and hydrolysis by epoxide hydrolase. Species differences in metabolic activation and detoxication most likely contribute to the difference in carcinogenic potency of BD by modulating the circulating blood levels of the epoxides. This study measured the in vivo concentrations of BD, BMO and BDE in the blood of male Sprague-Dawley rats and B6C3F1 mice during and following 6 h nose-only exposure to inhaled BD at 62.5, 625 or 1250 p.p.m. BD. Blood samples for BD and BMO (> or = 3 samples/time point) were collected at 2, 3, 4 and 6 h of exposure. Blood samples for BDE were collected at 3 and 6 h of exposure. After exposure, blood samples for BD, BMO and BDE were collected at 2-10 min intervals up to 30 min post-exposure. BD was quantified by gas chromatography using a vial headspace equilibration technique. BD epoxides were extracted into methylene chloride and quantified by gas chromatography-mass spectrometry. The concentration of BD in blood was not directly proportional to the inhaled concentration of BD, suggesting that the uptake of BD was saturable at the highest inhaled concentration. In both rats and mice, BD and BMO blood levels were at steady-state at 2, 3, 4 and 6 h of exposure, and declined rapidly after removal from exposure to BD. Steady-state blood concentrations of BD were 2.4, 37 and 58 microM in mice and 1.3, 18 and 37 microM in rats exposed to 62.5, 625 and 1250 p.p.m. BD respectively. Both species formed BMO from BD. In mice the respective steady-state BMO concentrations in blood were 0.6, 3.7 and 8.6 microM, compared to BMO blood concentrations in rats of 0.07, 0.94 and 1.3 microM. Mice, but not rats, had quantifiable levels of BDE in the blood. The peak concentrations of BDE in the blood of mice at 6 h were 0.65, 1.9 and 2.5 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhaled 1,3-butadiene, a monomer used in the production of synthetic rubber and other resins, is metabolized to mutagenic and carcinogenic epoxide intermediates. A physiologically based pharmacokinetic model of the uptake, tissue distribution, and metabolism of butadiene was constructed to determine if the biochemical kinetic constants obtained from in vitro studies are consistent with the observed in vivo uptake and metabolism. The model includes compartments for lung, blood, fat, liver, other rapidly perfused tissues ('viscera') and slowly perfused tissues. Metabolism of butadiene was assumed to occur in viscera in addition to lung and liver. Enzymatic reaction rate equations for the formation of 1,2-epoxy-3-butene, for hydrolysis of this epoxide, and for its conjugation with glutathione were also included. Physiological and biochemical parameters for the mouse, rat and human were obtained from the literature; they were not adjusted to produce a fit to experimental data. The model was used to test the hypothesis that differences in uptake and clearance of butadiene by the three species are due to differences in the activities of the metabolizing enzymes. The model reproduces whole-body observations for the mouse and rat. It predicts that inhalation uptake of butadiene and formation and retention of epoxybutene are controlled to a much greater extent by physiological parameters than by biochemical parameters and that storage in the fat represents a significant fraction of the retained butadiene. Accumulation of epoxybutene in the blood is predicted to be higher in mice than in rats or humans, but accumulation of the epoxide intermediate in the liver is predicted to be highest in humans. The epoxide tissue concentrations predicted by the model do not, by themselves, correlate with tumor incidence in mice and rats, indicating that other factors are crucial for carcinogenesis induced by butadiene.

Acrylonitrile (ACN) has been shown to cause tumors of the brain, stomach and Zymbal's gland in rats in several bioassays, but it has not been tested in other species. The carcinogenic risk of humans exposed to ACN is unclear. ACN is metabolized in the liver to 2-cyanoethylene oxide (CEO), which is believed to be the proximate or ultimate carcinogenic species. Therefore, the kinetics of CEO formation were studied with liver and lung microsomes from mice and humans using a GC-MS assay for CEO, and the data were compared with previously obtained kinetic parameters for rat microsomal enzymes. The rate of CEO formation by human liver microsomes was comparable to that of rat liver microsomes, but less than that of mouse liver microsomes. Liver microsomes produced more CEO than lung microsomes with all three species. CEO formation by microsomes from mice was approximately 4 times greater than that by microsomes from rats or humans, suggesting that mice would have higher CEO concentrations in blood than rats after ACN exposure. However, after oral administration of ACN, the concentration of CEO in mouse blood was one-third that in rat blood at all doses and time points examined. These results show that CEO circulates via the blood, providing exposure to distant sites. The blood concentrations of CEO do not appear to correlate with rates of microsomal CEO formation. This suggests that species differences in the detoxication of CEO may play an important role in determining circulating CEO concentrations and distant organ exposure.

A new method has been developed for determination of styrene oxide-globin adducts. The technique takes advantage of the reaction between alkylated globin and Raney nickel, which cleaves the carbon-sulfur bond in the styrene oxide-cysteine adduct to form 1-phenylethanol (1-PE) and 2-phenylethanol (2-PE). These alcohols are then reacted with pentafluorobenzoyl chloride and analyzed by GC-ECD. The method appears useful for biological monitoring of individuals exposed to styrene and, potentially, to other chemicals or their electrophilic metabolites which can react with cysteine residues in available proteins. The detection limit of the method, which is 0.04 nmol adducts/sample, indicates that it should be possible to detect adducts in the blood of people who are occupationally exposed to at least 18 mg/m3 of styrene. Analysis of globin from human whole blood which had been modified with [14C]styrene oxide indicated that 6% of the total globin adducts were detected. The method was applied to human and rat blood which had been treated with styrene oxide in vitro and to blood from rats given a single i.p. dose of styrene in vivo. Results from these experiments indicate that 77 times more adducts were detected at a given dose from rat globin than from human globin and that only 0.015% of the styrene dose was bioavailable as styrene oxide in the blood of rats. The reaction with Raney nickel is conducted at 5 degrees C to minimize unfavorable side reactions, such as degradation of 1- and 2-PE and conversion of styrene glycol to 1- and 2-PE. The optimal amount of Raney nickel was found to be 5-6 g/g globin. Since the recovery of 1-PE was not reproducible, only 2-PE is used for quantitation.

Both styrene and its presumed active metabolite styrene oxide show dose response as potent inducers of sister chromatid exchanges (SCEs) in human lymphocyte cultures. The SCE inducing and clastogenic capacity of styrene in lymphocytes in vitro can be explained by gas chromatographically measurable increase of styrene oxide in styrene treated cultures.