Mutagenicity of the racemic mixtures of butadiene monoepoxide and butadiene diepoxide at the Hprt locus of T-lymphocytes following inhalation exposures of female mice and rats

Abstract

The purpose of this study was to determine if Hprt mutant frequency (Mf) data from rodents exposed directly to individual epoxy metabolites of 1,3-butadiene (BD) can be used to identify the relative significance of each intermediate in the mutagenicity of BD in mice vs. rats. To this end, the relative contributions of the racemic mixtures of BD monoepoxide (BDO) and BD diepoxide (BDO(2)) to BD-induced mutagenicity was investigated by exposing mice and rats to selected concentrations of BDO and BDO(2) (i.e., 2.5 and 4.0 ppm, respectively) and comparing the mutagenic potency of each intermediate to that of BD (at 62.5 ppm) when comparable blood levels of metabolites are achieved (in the mouse). Female B6C3F1 mice and F344 rats (4-5 weeks old) were exposed to rac-BDO (0, 2.5, or 25 ppm) or (+/-)-BDO(2) (0, 2, 4 ppm) by inhalation for 4 weeks (6 h/day, 5 days/week), and then groups of control and exposed animals (n=3-12/group) were necropsied at multiple time points post-exposure for measuring Hprt Mfs in splenic lymphocytes (via the T-cell cloning assay) and estimating mutagenic potencies (represented by the difference in the areas under the mutant T-cell 'manifestation' curves of treated vs. control animals). The resulting Mf data, along with the extant metabolism data, suggest that at lower BD exposures (</=62.5 ppm) (+/-)-BDO(2) is a major contributor to the mutagenicity of BD in mice, whereas other metabolites and stereochemical configurations are responsible for mutations in BD-exposed rats and for the incremental mutagenic effects at higher BD exposures in mice. These studies indicate that additional work is needed to determine more definitively the relative contributions of these and other metabolites and stereochemical forms to BD-induced mutagenicity. Also, the novel approach of measuring mutagenic potencies as the change in Hprt Mfs over time in T-cells of exposed vs. control animals, as used in this study, can be valuable for predicting the potential role of these intermediates in each species.