Isobutene is produced during the fractionation of refinery gases or through the catalytic cracking of methyl-t-butyl ether. Isobutene is primarily used to produce diisobutylene, trimers, butyl rubber, and other polymers. In addition, it is used in the production of isooctane, high-octane aviation gasoline, methyl-t-butyl ether, and copolymer resins with butadiene and acrylonitrile. Isobutene was selected for evaluation because of the potential for human exposure due to its large production volume and the lack of adequate data on its carcinogenic potential. The toxicity and carcinogenicity of isobutene were determined in male and female F344/N rats and B6C3F1 mice exposed to isobutene (greater than 98% pure) by inhalation for 14 weeks or 2 years. The mutagenicity of isobutene was assessed in Salmonella typhimurium, and the frequency of micronuclei was determined in the peripheral blood of mice exposed by inhalation for 14 weeks.
Fourteen-week studies
Groups of 10 male and 10 female F344/N rats and B6C3F1 mice were exposed to isobutene at concentrations of 0, 500, 1,000, 2,000, 4,000, or 8,000 ppm 6 hours per day, 5 days per week, for 14 weeks. Concentrations greater than 8,000 ppm isobutene were not used because of the danger of explosion. All rats and mice survived to the end of the study. The final mean body weights and body weight gains of all exposed groups were similar to those of the chamber controls. No exposure-related gross lesions were observed in male or female rats or mice at necropsy. Microscopically, minimal hypertrophy of goblet cells lining the nasopharyngeal duct in the most caudal nose section was observed in some rats in each exposed group of males and females.
Two-year studies
Based on the lack of significant exposure-related toxicologic effects in the 14-week rat and mouse studies, 8,000 ppm was selected as the highest exposure concentration in the 2-year studies. Concentrations of 0, 500, 2,000, and 8,000 ppm were selected for rats and mice with the 500 and 2,000 ppm selection based on published metabolic elimination rates for Sprague-Dawley rats and B6C3F1 mice.
Rats
Groups of 50 male and 50 female F344/N rats were exposed to isobutene at concentrations of 0, 500, 2,000, or 8,000 ppm 6 hours per day, 5 days per week, for 105 weeks. Survival of exposed males and females was similar to that of the chamber controls. Mean body weights of exposed groups were generally similar to those of the chamber controls throughout the study.
Two-hydroxyisobutyric acid - biomarker of exposure
2-Hydroxyisobutyric acid (HIBA), the major urinary metabolite of isobutene, was measured in the urine of male and female rats as an indicator of isobutene exposure at 6, 12, and 18 months. The amount of HIBA excreted increased with increasing exposure concentration. However, when HIBA concentration was normalized to isobutene exposure concentration, the relative amount of HIBA excreted decreased with increasing exposure concentration, implying nonlinear kinetics.
Pathology findings
The incidence of thyroid gland follicular cell carcinoma in male rats exposed to 8,000 ppm was increased compared to the chamber control group and exceeded the historical control range. The incidences of hyaline degeneration of the olfactory epithelium were marginally increased in exposed rats; however, the severities of hyaline degeneration increased with increasing exposure concentration in males and females.
Mice
Groups of 50 male and 50 female B6C3F1 mice were exposed to isobutene at concentrations of 0, 500, 2,000, or 8,000 ppm 6 hours per day, 5 days per week, for 105 weeks. Survival of exposed males and females was similar to that of the chamber controls. Mean body weights of exposed mice were generally similar to those of the chamber controls throughout the study except for female mice exposed to 2,000 or 8,000 ppm, which weighed slightly less than chamber controls from about week 52 until week 92.
Two-hydroxyisobutyric acid - biomarker of exposure
HIBA was measured in the urine of male and female mice as an indicator of isobutene exposure at 6, 12,and 18 months. The amount of HIBA excreted increased with increasing exposure concentration. However, when HIBA concentration was normalized to isobutene exposure concentration, the relative amount of HIBA excreted decreased with increasing exposure concentration, implying nonlinear kinetics.
Pathology findings
The incidences of hyaline degeneration of the respiratory epithelium in all groups of exposed males and females were significantly greater than those in the chamber control groups. The incidences of hyaline degeneration of the olfactory epithelium in 2,000 and 8,000 ppm mice were greater than those in the chamber controls.
Genetic toxicology
Isobutene was not mutagenic in any of four strains of S. typhimurium, with or without S9 metabolic activation, and no increase in the frequency of micronucleated erythrocytes was seen in peripheral blood of male or female mice treated with isobutene by inhalation for 14 weeks.
Conclusions
Under the conditions of these 2-year inhalation studies, there was some evidence of carcinogenic activity of isobutene in male F344/N rats based on an increased incidence of follicular cell carcinoma of the thyroid gland. There was no evidence of carcinogenic activity of isobutene in female F344/N rats or male or female B6C3F1 mice exposed to 500, 2,000, or 8,000 ppm.
Exposure to isobutene by inhalation for 2 years resulted in increased incidences and/or severities of nasal lesions including hyaline degeneration of the olfactory epithelium in male and female rats and mice and hyaline degeneration of the respiratory epithelium in male and female mice.