NTP Toxicology and Carcinogenesis Studies of Xylenes (Mixed) (60% m-Xylene, 14% p-Xylene, 9% o-Xylene, and 17% Ethylbenzene) (CAS No. 1330-20-7) in F344/N Rats and B6C3F1 Mice (Gavage Studies).
The technical grade of xylenes (mixed) (hereafter termed xylenes) contains the three isomeric forms and ethylbenzene (percentage composition shown above). The annual production for 1985 was approximately 7.4 x 108 gallons. Xylenes is used as a solvent and a cleaning agent and as a degreaser and is a constituent of aviation and automobile fuels. Xylenes is also used in the production of benzoic acid, phthalate anhydride, and isophthalic and terephthalic acids as well as their dimethyl esters. Toxicology and carcinogenesis studies of xylenes were conducted in laboratory animals because a large number of workers are exposed and because the long- term effects of exposure to xylenes were not known. Exposure for the present studies was by gavage in corn oil. In single-administration studies, groups of five F344/N rats and B6C3F1 mice of each sex received 500, 1,000, 2,000, 4,000, or 6,000 mg/kg. Administration of xylenes caused deaths at 6,000 mg/kg in rats and mice of each sex and at 4,000 mg/kg in male rats. In rats, clinical signs observed within 24 hours of dosing at 4,000 mg/kg included prostration, muscular incoordination, and loss of hind limb movement; these effects continued through the second week of observation. Tremors, prone position, and slowed breathing were recorded for mice on day 3, but all mice appeared normal by the end of the 2- week observation period. In 14- day studies, groups of five rats of each sex were administered 0, 125, 250, 500, 1,000, or 2,000 mg/kg, and groups of five mice of each sex received 0, 250, 500, 1,000, 2,000, or 4,000 mg/kg. Chemical- related mortality occurred only at 2,000 mg/kg in rats and at 4,000 mg/kg in mice. Rats and mice exhibited shallow breathing and prostration within 48 hours following dosing at 2,000 mg/kg. These signs persisted until day 12 for rats, but no clinical signs were noted during the second week for mice. In 13- week studies, groups of 10 rats of each sex received 0, 62.5, 125, 250, 500, or 1,000 mg/kg, and groups of 10 mice of each sex received 0, 125, 250, 500, 1,000, or 2,000 mg/kg. No deaths or clinical signs of toxicity were recorded in rats. However, high dose male rats gained 15% less weight and females 8% less weight than did the vehicle controls. Two female mice died at the 2,000 mg/kg dose. Lethargy, short and shallow breathing, unsteadiness, tremors, and paresis were observed for both sexes in the 2,000 mg/kg group within 5- 10 minutes after dosing and lasted for 15- 60 minutes. Two- year toxicology and carcinogenesis studies were conducted by administering 0, 250, or 500 mg/kg xylenes in corn oil by gavage to groups of 50 F344/N rats of each sex, 5 days per week for 103 weeks. Groups of 50 B6C3F1 mice of each sex were administered 0, 500, or 1,000 mg/kg xylenes on the same schedule. Although the mortality was dose related in male rats (final survival: vehicle control, 36/50; low dose, 26/50; high dose, 20/50), many of the early deaths in the dosed males were gavage related. Body weights of the high dose male rats were 5%- 8% lower than those of the vehicle controls after week 59. The mean body weights of low dose and vehicle control male rats and those of dosed and vehicle control female rats were comparable. Survival rates of female rats and both sexes of dosed mice were not significantly different from those of the vehicle controls. The mean weights of dosed male and female mice were comparable to those of the vehicle controls. Hyperactivity lasting 5- 30 minutes was observed in high dose mice after dosing, beginning after week 4 and continuing through week 103. At no site was the incidence of nonneoplastic or neoplastic lesions in dosed rats or mice of either sex considered to be related to the administration of xylenes. Neither xylenes nor any of its components (o- xylene, m-xylene, p- xylene, or ethylbenzene) were mutagenic when tested with or without metabolic activation in Salmonella typhimurium strains TA100, TA1535, TA97, or TA98 with the preincubation protocol. In addition, ethylbenzene was tested in cytogenetic assays using cultured Cetic assays using cultured Chinese hamster ovary cells both with and without metabolic activation; neither sister- chromatid exchanges nor chromosomal aberrations were induced by ethylbenzene. An audit of the experimental data was conducted for the 2-year studies of xylenes. No data discrepancies were found that influenced the final interpretations. Under the conditions of these 2-year gavage studies, there was no evidence of carcinogenicity of xylenes (mixed) for male or female F344/N rats given 250 or 500 mg/kg or for male or female B6C3F1 mice given 500 or 1,000 mg/kg.