Inhalation pharmacokinetics of 1,2-epoxybutene-3 reveal species differences between rats and mice sensitive to butadiene-induced carcinogenesis

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Abstract
Comparative investigations of inhalation pharmacokinetics of 1,2-epoxybutene-3 (vinyl oxirane, the primary reactive intermediate of butadiene) revealed major differences in metabolism of this compound between rats and mice. Whereas in rats no indication of saturation kinetics of epoxybutene metabolism could be observed up to exposure concentrations of 5000 ppm, in mice saturation of epoxybutene metabolism becomes apparent at atmospheric concentrations of about 500 ppm. The estimated maximal metabolic rate (Vmax) in mice for epoxybutene was only 350 μmol×h−1×kg−1 (rats: >2600 μmol× h−1×kg−1). In the lower concentration range where first order metabolism applies (up to about 500 ppm) epoxybutene is metabolized by mice at higher rates compared to rats (metabolic clearance per kg body weight, mice: 24900 ml×h−1, rats: 13400 ml×h−1). Under these conditions the steady state concentration of epoxybutene in the mouse is about 10 times that in the rat. When mice are exposed to high concentrations of butadiene (>2000 ppm; conditions of saturation of butadiene metabolism; closed exposure system) epoxybutene is exhaled by the animals, and its concentration in the gas phase increases with exposure time. At about 10 ppm epoxybutene signs of acute toxicity are observed. When rats are exposed to butadiene under similar conditions, the epoxybutene concentration reaches a plateau at about 4 ppm. Under these conditions hepatic non-protein sulfhydryl compounds are virtually depleted in mice but not in rats. We conclude that in addition to the higher rate of metabolism of butadiene in mice, limited detoxification and consequently accumulation of its primary reactive intermediate epoxybutene may be a major determinant for the higher susceptibility of mice to butadiene-induced carcinogenesis.