Novel and existing data for a future physiological toxicokinetic model of ethylene and its metabolite ethylene oxide in mouse, rat, and human.

The olefin ethylene is a ubiquitously found gas. It originates predominantly from plants, combustion processes and industrial sources. In mammals, inhaled ethylene is metabolized by cytochrome P450-dependent monooxygenases, particularly by cytochrome P450 2E1, to ethylene oxide, an epoxide that directly alkylates proteins and DNA. Ethylene oxide was mutagenic in vitro and in vivo in insects and mammals and carcinogenic in rats and mice. A physiological toxicokinetic model is a most useful tool for estimating the ethylene oxide burden in ethylene-exposed rodents and humans. The only published physiological toxicokinetic model for ethylene and metabolically produced ethylene oxide is discussed. Additionally, existing data required for the development of a future model and for testing its predictive accuracy are reviewed and extended by new gas uptake studies with ethylene and ethylene oxide in B6C3F1 mice and with ethylene in F344 rats.

Ethylene oxide in blood of ethylene-exposed B6C3F1 mice, Fischer 344 rats, and humans.

The gaseous olefin ethylene (ET) is metabolized in mammals to the carcinogenic epoxide ethylene oxide (EO). Although ET is the largest volume organic chemical worldwide, the EO burden in ET-exposed humans is still uncertain, and only limited data are available on the EO burden in ET-exposed rodents. Therefore, EO was quantified in blood of mice, rats, or 4 volunteers that were exposed once to constant atmospheric ET concentrations of between 1 and 10 000 ppm (rodents) or 5 and 50 ppm (humans). Both the compounds were determined by gas chromatography. At ET concentrations of between 1 and 10 000 ppm, areas under the concentration-time curves of EO in blood (µmol × h/l) ranged from 0.039 to 3.62 in mice and from 0.086 to 11.6 in rats. At ET concentrations ≤ 30 ppm, EO concentrations in blood were 8.7-fold higher in rats and 3.9-fold higher in mice than that in the volunteer with the highest EO burdens. Based on measured EO concentrations, levels of EO adducts to hemoglobin and lymphocyte DNA were calculated for diverse ET concentrations and compared with published adduct levels. For given ET exposure concentrations, there were good agreements between calculated and measured levels of adducts to hemoglobin in rats and humans and to DNA in rats and mice. Reported hemoglobin adduct levels in mice were higher than calculated ones. Furthermore, information is given on species-specific background adduct levels. In summary, the study provides most relevant data for an improved assessment of the human health risk from exposure to ET.

1,2:3,4-Diepoxybutane in blood of male B6C3F1 mice and male Sprague-Dawley rats exposed to 1,3-butadiene.

The important industrial chemical 1,3-butadiene (BD; CAS Registry Number: 106-99-0) is a potent carcinogen in B6C3F1 mice and a weak one in Sprague-Dawley rats. This difference is mainly attributed to the species-specific burden by the metabolically formed 1,2:3,4-diepoxybutane (DEB). However, only limited data exist on the DEB blood burden of rodents at BD concentrations below 100 ppm. Considering this, DEB concentrations were determined in the blood of mice and rats immediately after 6h exposures to various constant concentrations of BD of between about 1 and 1200 ppm. Immediately after its collection, blood was injected into a vial that contained perdeuterated DEB (DEB-D(6)) as internal standard. Plasma samples were prepared and treated with sodium diethyldithiocarbamate that derivatized metabolically produced DEB and DEB-D(6) to their bis(dithiocarbamoyl) esters, which were then analyzed by high performance liquid chromatography coupled with an electrospray ionization tandem mass spectrometer. DEB concentrations in blood versus BD exposure concentrations in air could be described by one-phase exponential association functions. Herewith calculated (±)-DEB concentrations in blood increased in mice from 5.4 nmol/l at 1 ppm BD to 1860 nmol/l at 1250 ppm BD and in rats from 1.2 nmol/l at 1 ppm BD to 92 nmol/l at 200 ppm BD, at which exposure concentration 91% of the calculated DEB plateau concentration in rat blood was reached. This information on the species-specific blood burden by the highly mutagenic DEB helps to explain why the carcinogenic potency of BD in rats is low compared to that in mice.

Quantitative investigation on the metabolism of 1,3-butadiene and of its oxidized metabolites in once-through perfused livers of mice and rats.

The industrial chemical 1,3-butadiene (BD) is a potent carcinogen in mice and a weak one in rats. This difference is generally related to species-specific burdens by the metabolites 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane (DEB), and 3,4-epoxy-1,2-butanediol (EBD), which are all formed in the liver. Only limited data exist on BD metabolism in the rodent liver. Therefore, metabolism of BD, its epoxides, and the intermediate 3-butene-1,2-diol (B-diol) was studied in once-through perfused livers of male B6C3F1 mice and Sprague-Dawley rats. In BD perfusions, predominantly EB and B-diol were found (both species). DEB and EBD were additionally detected in mouse livers. Metabolism of BD showed saturation kinetics (both species). In EB perfusions, B-diol, EBD, and DEB were formed with B-diol being the major metabolite. Net formation of DEB was larger in mouse than in rat livers. In both species, hepatic clearance (Cl(H)) of EB was slightly smaller than the perfusion flow. In DEB perfusions, EBD was formed as a major metabolite. Cl(H) of DEB was 61% (mouse) and 73% (rat) of the perfusion flow. In the B-diol-perfused rat liver, EBD was formed as a minor metabolite. Cl(H) of B-diol was 53% (mouse) and 34% (rat) of the perfusion flow. In EBD-perfused rat livers, Cl(H) of EBD represented only 22% of the perfusion flow. There is evidence for qualitative species differences with regard to the enzymes involved in BD metabolism. The first quantitative findings in whole livers showing intrahepatic first-pass metabolism of BD and EB metabolites will improve the risk estimation of BD.

Styrene-7,8-oxide burden in ventilated, perfused lungs of mice and rats exposed to vaporous styrene.

Styrene (ST) is an important industrial chemical. In long-term inhalation studies, ST-induced lung tumors in mice but not in rats. To test the hypothesis that the lung burden by the reactive metabolite styrene-7,8-oxide (SO) would be most relevant for the species-specific tumorigenicity, we investigated the SO burden in isolated lungs of male Sprague-Dawley rats and in-situ prepared lungs of male B6C3F1 mice ventilated with air containing vaporous ST and perfused with a modified Krebs-Henseleit buffer (37 degrees C). Styrene vapor concentrations were determined in air samples collected in the immediate vicinity of the trachea. They were almost constant during each experiment. Styrene exposures ranged from 50 to 980 ppm (rats) and from 40 to 410 ppm (mice). SO was quantified from the effluent perfusate. Lungs of both species metabolized ST to SO. After a mathematical translation of the ex-vivo data to ventilation and perfusion conditions as they are occurring in vivo, a species comparison was carried out. At ST concentrations of up to 410 ppm, mean SO levels in mouse lungs ranged up to 0.45 nmol/g lung, about 2 times higher than in rat lungs at equal conditions of ST exposure. We conclude that the species difference in the SO lung burden is too small to consider the genotoxicity of SO as sufficient for explaining the fact that only mice developed lung tumors when exposed to ST. Another cause is considered as driving force for lung tumor development in the mouse.