1,3-butadiene: cancer, mutations, and adducts. Part III: In vivo mutation of the endogenous hprt genes of mice and rats by 1,3-butadiene and its metabolites.

1,3-Butadiene (BD), an important chemical used mainly in the production of synthetic rubber, is a potent carcinogen in mice, a weak carcinogen in rats, and a suspected carcinogen in humans. To provide a better understanding of the mutagenic mechanisms involved in interspecies differences in BD-induced carcinogenesis, studies were conducted in rodents to test two hypotheses: (a) the mutagenic potency of BD at the hypoxanthine-guanine phosphoribosyltransferase (hprt) locus of T lymphocytes (T cells) can be used to quantify interspecies differences in BD-induced carcinogenicity in exposed rodents and (b) comparison of the mutagenic potency and specificity of BD and racemic mixtures of two epoxy metabolites, 1,2-epoxy-3-butene (BDO) and 1,2,3,4-diepoxybutane (BDO2), at the hprt locus of T cells can be used to define the relative contribution of each intermediate to observed BD mutagenicity in each species. The first hypothesis was investigated by determining the effects of exposure duration and elapsed time after exposures on hprt mutant frequencies (MFs) in T cells from thymus and spleen of female B6C3F1 mice and F344 rats (4 to 5 weeks old). In this study, rodents were exposed by inhalation to 0 or 1,250 parts per million (ppm) BD for up to 2 weeks, or to 0 or 625 ppm BD for up to 4 weeks (with all exposures 6 hours/day, 5 days/week). The second hypothesis was examined by defining the effects of exposure concentration and elapsed time after exposures on the hprt MFs in splenic T cells from mice and rats exposed by inhalation to BD (0, 20, 62.5, or 625 ppm), BDO (0, 2.5, or 25 ppm), or BDO2 (0, 2, or 4 ppm) for 4 weeks (all exposures 6 hours/day, 5 days/week). The hprt MFs were measured weekly or biweekly using the T cell cloning assay for up to 10 weeks after the last exposure. The mutagenic potency of BD (represented by the difference in the areas under the mutant T cell "manifestation" curves [or the "change in MFs over time"] of exposed versus control animals) was significantly greater in mice (4.4-fold) than in rats following 2 weeks of exposure to 1,250 ppm BD. Mutagenic potency in mice was 8.5-fold greater than that in rats following 4 weeks of exposure to 625 ppm BD. These hprt MF data provide the first evidence that BD is mutagenic in the rat, albeit the mutagenic response was significantly less than that observed in similarly exposed mice. In addition, the MF data from the two exposure-duration studies indicate that both exposure concentration and exposure duration are important in determining the magnitude of the mutagenic response to BD. The relative contribution of BDO versus BDO2 to overall BD mutagenicity was evaluated by exposing mice and rats to carefully chosen concentrations of BD and racemic mixtures of BDO and BDO2 (that is, 62.5, 2.5, and 4.0 ppm, respectively) and comparing the mutagenic potency of each compound when comparable blood levels of metabolites were achieved. The resulting MF data indicate that (+/-)-BDO2 is a major contributor to the mutagenicity of BD in mice at lower BD exposure levels (< or = 62.5 ppm), whereas other metabolites and stereochemical configurations are responsible for mutations in BD-exposed rats and for the incremental mutagenic effects at higher exposure levels in mice. Molecular analysis of hprt cDNA from expanded T cell clones from control and BD-exposed mice demonstrated an increased frequency of large deletions in exposed animals (p = 0.016), presumably associated with in situ formation of (+/-)-BDO2, meso-BDO2, or both. Results of these mutagenicity experiments, along with data from collaborative studies of DNA adducts from the same animals, should provide a better understanding of the interspecies variation in carcinogenic response to BD and improve the extrapolation of rodent data to the estimation of cancer risk in exposed persons.