1,3-Butadiene (BD) is a known rodent and human carcinogen that is metabolized mainly by P450 2E1 to three epoxides, 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane (DEB) and 1,2-epoxy-3,4-butanediol (EB-diol). The individual epoxides vary up to 200-fold in their mutagenic potency, with DEB being the most mutagenic metabolite. It is important to understand the internal formation of the individual epoxides to assign the relative risk for each metabolite and to understand the molecular mechanisms responsible for major species differences in carcinogenicity. We have conducted extensive exposure-biomarker studies on mice, rats and humans. Using low exposures that range from current occupational levels to human exposures from tobacco smoke has provided evidence that mice are very different from humans, with mice forming ~200 times more DEB than humans at exposures of 0.1–1.5 ppm BD. While no gender differences have been noted in mice and rats for globin adducts or N-7 guanine adducts, female rats and mice had 2–3-fold higher Hprt mutations and DNA-DNA cross-links, suggesting a gender difference in DNA repair. Numerous molecular epidemiology studies have evaluated globin adducts and Hprt mutations, SCEs and chromosomal abnormalities. None of the blinded studies have shown evidence of human genotoxicity at current occupational exposures and studies of globin adducts have shown similar or lower formation of adducts in females than males. If one calculates the EB dose-equivalents for the three species, mice clearly differ from rats and humans, being ~44 and 174 times greater than rats and humans, respectively. These data provide a scientific basis for improved risk assessment of BD.

Abstract1,3-Butadiene is an important industrial chemical used in the production of synthetic rubber and is also found in gasoline and combustion products. It is a multispecies, multisite carcinogen in rodents, with mice being the most sensitive species. 1,3-Butadiene is metabolized to several epoxides that form DNA and protein adducts. Previous analysis of 1,2,3-trihydroxybutyl-valine globin adducts suggested that most adducts resulted from 3-butene-1,2-diol metabolism to 3,4-epoxy-1,2-butanediol, rather than from 1,2;3,4-diepoxybutane. To specifically examine metabolism of 1,3-butadiene to 1,2;3,4-diepoxybutane, the formation of the 1,2;3,4-diepoxybutane-specific adduct N,N-(2,3-dihydroxy-1,4-butadiyl)-valine was evaluated in mice treated with 3, 62.5, or 1250 ppm 1,3-butadiene for 10 days and rats exposed to 3 or 62.5 ppm 1,3-butadiene for 10 days, or to 1000 ppm 1,3-butadiene for 90 days, using a newly developed immunoaffinity liquid chromatography tandem mass spectrometry assay. In addition, 2-hydroxy-3-butenyl-valine and 1,2,3-trihydroxybutylvaline adducts were determined. The analyses of several adducts derived from 1,3-butadiene metabolites provided new insight into species and exposure differences in 1,3-butadiene metabolism. Mice formed much higher amounts of N,N-(2,3-dihydroxy-1,4-butadiyl)-valine than rats. The formation of 2-hydroxy-3-butenyl-valine and N,N-(2,3-dihydroxy-1,4-butadiyl)-valine was similar in mice exposed to 3 or 62.5 ppm 1,3-butadiene, whereas 2-hydroxy-3-butenyl-valine was 3-fold higher at 1250 ppm. In both species, 1,2,3-trihydroxybutyl-valine adducts were much higher than 2-hydroxy-3-butenyl-valine and N,N-(2,3-dihydroxy-1,4-butadiyl)-valine. Together, these data show that 1,3-butadiene is primarily metabolized via the 3-butene-1,2-diol pathway, but that mice are much more efficient at forming 1,2;3,4-diepoxybutane than rats, particularly at low exposures. This assay should also be readily adaptable to molecular epidemiology studies on 1,3-butadiene-exposed workers

1,3-Butadiene (BD) is an important industrial chemical that is classified as a human carcinogen. BD carcinogenicity has been attributed to its metabolism to several reactive epoxide metabolites and formation of the highly mutagenic 1,2:3,4-diepoxybutane (DEB) has been hypothesized to drive mutagenesis and carcinogenesis at exposures experienced in humans. We report herein the formation of DEB-specific N,N-(2,3-dihydroxy-1,4-butadiyl)valine (pyr-Val) in BD-exposed workers as a biomarker of DEB formation. pyr-Val was determined in BD monomer and polymer plant workers that had been previously analyzed for several other biomarkers of exposure and effect. pyr-Val was detected in 68 of 81 (84%) samples ranging from 0.08 to 0.86 pmol/g globin. Surprisingly, pyr-Val was observed in 19 of 23 administrative control subjects not known to be exposed to BD, suggesting exposure from environmental sources of BD. The mean ± SD amounts of pyr-Val were 0.11 ± 0.07, 0.16 ± 0.12, and 0.29 ± 0.20 pmol/g globin in the controls, monomer, and polymer workers, respectively, clearly demonstrating formation of DEB in humans. The amounts of pyr-Val found in this study suggest that humans are much less efficient in the formation of DEB than mice or rats at similar exposures. Formation of pyr-Val was more than 50-fold lower than has been associated with increased mutagenesis in rodents. The results further suggest that formation of DEB relative to other epoxides is significantly different in the highest exposed polymer workers compared with controls and BD monomer workers. Whether this is due to saturation of metabolic formation or increased GST-mediated detoxification could not be determined.

1,3-Butadiene (BD) is a known rodent and human carcinogen that is metabolized mainly by P450 2E1 to three epoxides, 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane (DEB), and 1,2-epoxy-3,4-butanediol. The individual epoxides vary up to 200-fold in their mutagenic potency, with DEB being the most mutagenic metabolite. It is important to understand the internal formation of the individual epoxides to assign the relative risk for each metabolite and to understand the molecular mechanisms responsible for extensive species differences in carcinogenicity. This study presents a comprehensive exposure-response for the formation of the DEB-specific N,N-(2,3-dihydroxy-1,4-butadiyl)valine (pyr-Val) in mice and rats. Using nano-ultra high pressure liquid chromatography-tandem-mass spectrometry allowed analysis of pyr-Val in mice and rats exposed to BD as low as 0.1 and 0.5 ppm BD, respectively, and demonstrated significant differences in the amounts and exposure-response of pyr-Val formation. Mice formed 10- to 60-fold more pyr-Val compared to rats at similar exposures. The formation of pyr-Val increased with exposures, and the formation was most efficient with regard to formation per parts per million BD at low exposures. While formation at higher exposures appeared linear in mice, in rats formation saturated at exposures > or = 200 ppm for 10 days. In rats, amounts of pyr-Val were lower after 20 days than after 10 days of exposure, suggesting that the lifespan of rat erythrocytes may be shortened following exposure to BD. This research supports the hypothesis that the lower susceptibility of rats to BD-induced carcinogenesis results from greatly reduced formation of DEB following exposure to BD.

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