In vivo doses of butadiene epoxides as estimated from in vitro enzyme kinetics by using cob(I)alamin and measured hemoglobin adducts: an inter-species extrapolation approach.

1,3-Butadiene (BD) is a rodent and human carcinogen. In the cancer tests, mice have been much more susceptible than rats with regard to BD-induced carcinogenicity. The species-differences are dependent on metabolic formation/disappearance of the genotoxic BD epoxy-metabolites that lead to variations in the respective in vivo doses, i.e. "area under the concentration-time curve" (AUC). Differences in AUC of the most gentoxic BD epoxy-metabolite, diepoxybutane (DEB), are considered important with regard to cancer susceptibility. The present work describes: the application of cob(I)alamin for accurate measurements of in vitro enzyme kinetic parameters associated with BD epoxy-metabolites in human, mouse and rat; the use of published data on hemoglobin (Hb) adduct levels of BD epoxides from BD exposure studies on the three species to calculate the corresponding AUCs in blood; and a parallelogram approach for extrapolation of AUC of DEB based on the in vitro metabolism studies and adduct data from in vivo measurements. The predicted value of AUC of DEB for humans from the parallelogram approach was 0.078 nM · h for 1 ppm · h of BD exposure compared to 0.023 nM · h/ppm · h as calculated from Hb adduct levels observed in occupational exposure. The corresponding values in nM · h/ppm · h were for mice 41 vs. 38 and for rats 1.26 vs. 1.37 from the parallelogram approach vs. experimental exposures, respectively, showing a good agreement. This quantitative inter-species extrapolation approach will be further explored for the clarification of metabolic rates/pharmacokinetics and the AUC of other genotoxic electrophilic compounds/metabolites, and has a potential to reduce and refine animal experiments.