Disposition of inhaled isobutene in F344/N rats

Isobutene (2-methylpropene) (CAS No. 115-11-7) is a gas widely used in the chemical manufacturing industry. As an aid to planning long-term toxicity studies, research was conducted to determine the effect of exposure concentrations on the absorption and metabolism of isobutene in F344/N rats. Male F344/N rats (11-15 weeks of age) were exposed for 2 hr to 0, 40, 400, or 4000 ppm isobutene, and a time-course evaluation of blood levels of isobutene was performed using headspace analysis methods. Blood levels of isobutene were linearly related to exposure concentrations between 40 and 400 ppm but increased in a supralinear fashion at the highest concentration, suggesting that the capacity of the rats to metabolize isobutene had been exceeded. Total uptake, excretion patterns, and metabolic conversions were studied in rats exposed for up to 6 hr to 0, 2, 40, 400, or 4000 ppm [14C]isobutene. Absorption of the inhaled isobutene was approximately 8% up to 40 ppm isobutene, but decreased at the higher concentrations. The amount of isobutene metabolized per ppm.hr of exposure was also linear up to 40 ppm but decreased at higher concentrations. Over 90% of the absorbed isobutene was metabolized at exposure concentrations up to 400 ppm, but the exposure to approximately 4000 ppm isobutene resulted in approximately 20% of the absorbed dose exhaled as the unmetabolized isobutene. Two urinary metabolites were identified as isobutenediol and 2-hydroxyisobutyric acid. Two other urinary metabolites were tentatively identified as sulfate conjugates of isobutenediol. Based on these studies, linear dose-response relationships would be expected in chronic toxicity studies for exposures up to 40 ppm isobutene. Additional studies would be required to determine if repeated exposures would induce higher metabolic capacities in the exposed rats.