1,3-Butadiene is produced in large volumes for use in the manufacture of synthetic rubber and of thermoplastic resins. In previous inhalation studies conducted by the NTP (NTP, 1984) there was clear evidence of multiple organ carcinogenicity in male and female mice exposed to 625 or 1,250 ppm 1,3-butadiene for 60 or 61 weeks. To better characterize exposure-response relationships for neoplasms and nonneoplastic lesions, toxicology and carcinogenesis studies were conducted by exposing groups of male and female B6C3F1 mice to air containing 1,3-butadiene (greater than 99% pure) for up to 2 years. An additional study in male B6C3F1 mice, in which exposure to 1,3-butadiene was stopped after limited exposure periods (13, 26, 40, or 52 weeks), was performed to assess the effects of varying concentration and duration of exposure on the incidences of 1,3-butadiene-induced neoplasms. In vitro genetic toxicology studies were conducted in Salmonella typhimurium and mouse lymphoma cells. In vivo genetic effects were assayed in germ cells of male Drosophila melanogaster and in bone marrow and peripheral blood cells of B6C3F1 mice.
Groups of 70 male and 70 female mice were exposed to air containing 0, 6.25, 20, 62.5, or 200 ppm 1,3-butadiene for 6 hours per day, 5 days per week for up to 2 years; groups of 90 male and 90 female mice were exposed to 625 ppm 1,3-butadiene on the same schedule. Up to 10 animals from each group were examined after 9 and 15 months of exposure.
Survival and Body Weight in the 2-Year Studies
Two-year survival was decreased for males and females exposed to concentrations of 20 ppm or above, primarily due to the development of chemical-related malignant neoplasms. No female mice exposed to 200 or 625 ppm or males exposed to 625 ppm survived to the end of the studies (males: 35/50, 39/50, 24/50, 22/50, 4/50, 0/70; females: 37/50, 33/50, 24/50, 11/50, 0/50, 0/70). Mean body weights of exposed male and female mice were similar to those of the controls.
Hematologic Effects in the 2-Year Studies
Hematologic parameters were evaluated after 9 and 15 months of exposure. At 9 months, decreases in erythrocyte counts, hemoglobin concentration, and packed red cell volume were observed in male mice exposed to 62.5 ppm or above and in female mice exposed to 200 or 625 ppm. Mean erythrocyte volume was increased in male mice exposed to 625 ppm and in females exposed to 200 or 625 ppm. At 15 months, decreases in erythrocyte counts, hemoglobin concentration, and packed red cell volume and increases in mean erythrocyte volume were observed in male and female mice exposed to 625 ppm.
Neoplasms and Nonneoplastic Lesions in the 2-Year Studies
Exposure of mice to 1,3-butadiene induced benign and malignant neoplasms at multiple sites. Statistically significant increases in the incidences of neoplasms at one or more sites were seen at concentrations of 20 ppm and higher in males and 6.25 ppm and higher in females. There was no exposure level in this study at which a significant carcinogenic response was not observed. Statistically significant increases occurred in the incidences of malignant lymphoma; histiocytic sarcoma; cardiac hemangiosarcoma; harderian gland adenoma; hepatocellular adenoma and carcinoma; alveolar/bronchiolar adenoma and carcinoma; mammary gland carcinoma, adenoacanthoma, and malignant mixed tumor (females only); benign and malignant ovarian granulosa cell tumor; and forestomach squamous cell papilloma and carcinoma.
Low incidences of uncommon neoplasms also occurred in exposed male and female mice, including intestinal carcinomas in males, renal tubule adenomas in males and females, skin sarcomas (all types combined) in females, and Zymbal's gland adenomas and carcinomas in females.
Lymphocytic lymphomas appeared as early as week 23 and were the principal cause of death of male and female mice exposed to 625 ppm 1,3-butadiene. The early and extensive development of lethal lymphocytic lymphomas in mice exposed to 625 ppm resulted in a reduced number of mice at risk for neoplasms developing later at other sites. Exposure-response relationships for 1,3-butadiene-induced neoplasms were more clearly characterized at concentrations below 625 ppm and after adjustment for intercurrent mortality.
Increased incidences of nonneoplastic lesions in exposed mice included bone marrow atrophy; testicular atrophy; ovarian atrophy, angiectasis, germinal epithelial hyperplasia, and granulosa cell hyperplasia; uterine atrophy; cardiac endothelial hyperplasia and mineralization; alveolar epithelial hyperplasia; forestomach epithelial hyperplasia; and harderian gland hyperplasia.
The stop-exposure study consisted of groups of 50 male mice exposed to 1,3-butadiene at concentrations of 200 ppm for 40 weeks, 625 ppm for 13 weeks, 312 ppm for 52 weeks, or 625 ppm for 26 weeks. After the exposures were completed, these groups were placed in control chambers for the remainder of the 2-year study. The total exposure of 1,3-butadiene (concentration times duration of exposure) of the 13- and 40-week stop-exposure groups was approximately 8,000 ppm-weeks, while that of the 26- and 52-week stop-exposure groups was approximately 16,000 ppm-weeks.
The survival of all stop-exposure groups was markedly lower than that of the controls. The incidences of lymphocytic lymphoma, histiocytic sarcoma, cardiac hemangiosarcoma, alveolar/bronchiolar adenoma and carcinoma, forestomach squamous cell papilloma and carcinoma, hepatocellular adenoma, harderian gland adenoma and adenocarcinoma, and preputial gland carcinoma were significantly increased. Neoplasms were induced at most of these sites after only 13 weeks of exposure to 1,3-butadiene. Additionally, low numbers of malignant gliomas and neuroblastomas of the brain and Zymbal's gland carcinomas occurred in one or more stop-exposure groups.
At similar total exposures, the incidence of lymphocytic lymphoma was greater with exposure to a higher concentration of 1,3-butadiene for a short time compared with exposure to a lower concentration for an extended period (34% at 625 ppm for 13 weeks versus 12% at 200 ppm for 40 weeks; 60% at 625 ppm for 26 weeks versus 8% at 312 ppm for 52 weeks).
1,3-Butadiene has been tested both in vitro and in vivo for mutagenic activity. In vitro, positive results were obtained in the Salmonella typhimurium gene mutation assay with strain TA1535; mutagenic activity was not observed in other S. typhimurium strains (TA100, TA97, and TA98). 1,3-Butadiene was negative in the mouse lymphoma assay for induction of trifluorothymidine resistance in L5178Y cells with and without S9.
In vivo, 1,3-butadiene did not induce sex-linked recessive lethal mutations in germ cells of male Drosophila melanogaster; however, it did induce significant increases in chromosomal aberrations and sister chromatid exchanges in bone marrow cells of mice exposed for 2 weeks by inhalation. In addition, significant increases in micronucleated erythrocytes were observed in peripheral blood samples obtained from male and female mice exposed to 1,3-butadiene for 2 or 13 weeks or 15 months by inhalation.
The previous inhalation studies of 1,3-butadiene (TR-288) in male and female B6C3F1 mice provided clear evidence of carcinogenicity at exposure concentrations of 625 or 1,250 ppm. The present inhalation studies - 2-year exposures of 6.25, 20, 62.5, 200, or 625 ppm or shorter duration exposures of 200, 312, or 625 ppm - provide a better characterization of the concentration-dependent responses for 1,3-butadiene-induced neoplasms and nonneoplastic lesions. The present studies confirmed the clear evidence of carcinogenicity of 1,3-butadiene in male B6C3F1 mice based on increased incidences of neoplasms in the hematopoietic system, heart, lung, forestomach, liver, harderian gland, preputial gland, brain, and kidney. There was clear evidence of carcinogenicity of 1,3-butadiene in female B6C3F1 mice based on increased incidences of neoplasms in the hematopoietic system, heart, lung, forestomach, liver, harderian gland, ovary, and mammary gland.
Low incidences of intestinal carcinomas in male mice, Zymbal's gland carcinomas in male and female mice, and renal tubule adenomas and skin sarcomas in female mice may also have been related to administration of 1,3-butadiene.