Comparison of the disposition of butadiene epoxides in Sprague–Dawley rats and B6C3F₁ mice following a single and repeated exposures to 1,3-butadiene via inhalation¹

Abstract

1,3-Butadiene (BD), a compound used extensively in the rubber industry, is a potent carcinogen in mice and a weak carcinogen in rats in chronic carcinogenicity bioassays. While many chemicals are known to alter their own metabolism after repeated exposures, the effect of exposure prior to BD on its in vivo metabolism has not been reported. The purpose of the present research was to examine the effect of repeated exposure to BD on tissue concentrations of two mutagenic BD metabolites, butadiene monoepoxide (BDO) and butadiene diepoxide (BDO₂). Concentrations of BD epoxides were compared in several tissues of rats and mice following a single exposure or ten repeated exposures to a target concentration of 62.5 ppm BD. Female Sprague–Dawley rats and female B6C3F₁ mice were exposed to BD for 6 h or 6 h×10 days. BDO and BDO₂ were quantified in blood and several other tissues following preparation by cryogenic vacuum distillation and analysis by multidimensional gas chromatography–mass spectrometry. Blood and lung BDO concentrations did not differ significantly (P≤0.05) between the two exposure regimens in either species. Following multiple exposures to BD, BDO levels were 5- and 1.6-fold higher (P≤0.05) in mammary tissue and 2- and 1.4-fold higher in fat tissue of rats and mice, respectively, as compared with single exposures. BDO₂ levels also increased in rat fat tissue following multiple exposures to BD. However, in mice, levels of this metabolite decreased by 15% in fat, by 28% in mammary tissue and by 34% in lung tissue following repeated exposures to BD. The finding that the mutagenic epoxide BDO, which is the precursor to the highly mutagenic BDO₂, accumulates in rodent fat may be important in assessing the potential risk to humans from inhalation of BD.

Introduction

1,3-Butadiene (BD), an industrial chemical used in the production of polybutadiene and styrene-butadiene rubber, is among the top 50 chemicals produced in the US (Chemical and Engineering News, 1994, Chemical and Engineering News, 1995). While the likelihood for occupational exposures of workers in the rubber industry to BD is great, exposures of <2 ppm (8 h time-weighted average) are typical in occupational settings in the US (Fajen et al., 1990). Furthermore, the presence of BD in cigarette smoke, automobile exhaust and gasoline formulations results in low-level exposures of most of the population to this compound (Miller, 1978). The US Environmental Protection Agency (EPA) has listed BD as one of the hazardous air pollutants in the 1990 Clean Air Act Amendment (Environmental Protection Agency, 1991).

Chronic studies indicated that BD is carcinogenic in both rats and mice, but that profound differences in tumor types and susceptibility to BD-induced tumors exist. Only slight increases in pancreatic exocrine neoplasms, Leydig cell tumors and mammary gland and thyroid neoplasms were observed in Sprague–Dawley rats exposed to 1000 and 8000 ppm BD for 6 h/day, 5 days/week for 2 years (Owen et al., 1987). Conversely, tumors were induced in B6C3F₁ mice following chronic exposures ranging from 6.25 to 625 ppm. Predominant tumors included alveolar–bronchiolar carcinomas, hemangiosarcomas of the heart, lymphomas and neoplasms of the forestomach, Harderian gland, preputial gland, liver, mammary gland and ovary (Melnick et al., 1990). Although the mechanism of these species differences in BD-induced carcinogenicity is not thoroughly understood, data from our laboratory (Bechtold et al., 1995, Thornton-Manning et al., 1995a) and others (Himmelstein et al., 1994, Himmelstein et al., 1995) have suggested that these differences may be due to species-related differences in the production and disposition of reactive BD epoxide metabolites.

The initial step of BD metabolism is the production of butadiene monoepoxide (BDO) by cytochrome P450 enzymes (Bolt et al., 1983, Schmidt and Loeser, 1985, Duescher and Elfarra, 1994). Human cytochrome P450 isoforms CYP2E1 and CYP2A6 have been shown to catalyze the formation of BDO (Csanády et al., 1992, Duescher and Elfarra, 1994). BDO is further oxidized by cytochrome P450 enzyme(s) to form butadiene diepoxide (BDO₂) (Malvoisin and Roberfroid, 1982). Recombinant human isoforms CYP2E1 and CYP3A4 have been shown to metabolize BDO to BDO₂ (Seaton et al., 1995). Both epoxides are also likely substrates for glutathione S-transferases and epoxide hydrolases (Csanády et al., 1992, Boogaard et al., 1996).

In vivo inhalation studies with BD have shown BDO in the blood of laboratory animals after short-term exposures (Bond et al., 1986, Dahl et al., 1991, Himmelstein et al., 1994, Himmelstein et al., 1995, Bechtold et al., 1995, Thornton-Manning et al., 1995a, Thornton-Manning et al., 1995b). Recent studies have shown that BDO₂ is present in high concentrations in the blood of mice following inhalation exposures ranging from 62.5 to 1250 ppm BD (Himmelstein et al., 1994, Bechtold et al., 1995, Thornton-Manning et al., 1995a). BDO₂ is also present in the blood of rats following low-level exposures, but at concentrations that are about 40-fold lower than those found in mice (Thornton-Manning et al., 1995a).

Because metabolism may be important in the ultimate carcinogenic outcome of BD, it is important to evaluate potential alterations in metabolic pathways that may contribute to the production of carcinogenic metabolites. One such alteration may result from repeated exposures to BD, such as that which would be encountered in occupational settings. It is well-established that many chemicals can interact with xenobiotic-metabolizing enzymes to alter their own metabolism, as well as that of other exogenous or endogenous substrates. Most studies examining BD metabolism have used single acute exposures. The purpose of the present study was to evaluate the disposition of BD metabolites in several tissues of rats and mice following a single inhalation exposure to a target concentration of 62.5 ppm BD and ten repeated exposures (5 days/week for 2 weeks) to the same concentration.