title: NTP Toxicology and Carcinogenesis Studies of CS2 (94% o-Chlorobenzalmalononitrile, CAS No. 2698-41-1) in F344/N Rats and B6C3F1 Mice (Inhalation Studies).
abstract: CS2 (94% o-chlorobenzalmalononitrile [CS]; 5% Cab-O-Sil(R). colloidal silica; 1% hexamethyldisilizane), an eye and respiratory irritant, is used as an aerosol to control riots. NTP Toxicology and Carcinogenesis studies were conducted by exposing groups of F344/N rats and B6C3F1 mice of each sex for 6 hours per day, 5 days per week for 2 weeks, 13 weeks, or 2 years, to a CS2 aerosol. Genetic toxicology studies with CS2 were conducted in Salmonella typhimurium, mouse lymphoma cells, and Chinese hamster ovary (CHO) cells. Fourteen-Day Studies: At exposure concentrations of 0, 1, 3, 10, 30, or 100 mg/m3 CS2, all rats exposed to 30 or 100 mg/m3 and all mice exposed to 10, 30, or 100 mg/m3 died before the end of the studies. Compound-related clinical signs observed included erythema, blepharospasm, listlessness, nasal discharge, and mouse breathing. Thirteen-Week Studies: At exposure concentrations of 0, 0.4, 0.75, 1.5, 3, or 6 mg/m3, 1/10 male rats exposed to 6 mg/m3 died before the end of the studies. Final mean body weights of rats exposed to 1.5 mg/m3 or more were 17%-44% lower than that of controls for males and 10%-24% lower for females. The absolute and relative thymus weights were reduced for exposed male and female rats, particularly at 6 mg/m3. Compound-related lesions of the nasal passage in rats included focal erosion with regenerative hyperplasia and squamous metaplasia of the respiratory epithelium and suppurative inflammation. Acute inflammation and hyperplasia of the respiratory epithelium were seen in the larynx and trachea of some exposed rats. All mice exposed to 6 mg/m3 and 1/10 males and 1/10 females exposed to 3 mg/m3 died before the end of the studies. Final mean body weights of mice exposed to 3 mg/m3 were 13% lower than that of controls for males and 9% lower for females. Compound-related lesions of the nasal passage in mice included squamous metaplasia of the nasal respiratory epithelium and inflammation. Based on these results, CS2 exposure concentrations for the 2-year studies were 0, 0.075, 0.25, or 0.75 mg/m3 for 6 hours per day, 5 days per week for 105 weeks for groups of 50 rats of each sex. Groups of 50 mice of each sex were exposed to 0, 0.75, or 1.5 mg/m3 on the same schedule. Body Weights and Survival in the Two-Year Studies: Final mean body weights of rats exposed to 0.75 mg/m3 were 7%-11% lower than those of controls. Final mean body weights of mice exposed to CS2 were lower than those of controls (male: 5% and 9%; female: 10% and 17%). No compound-related clinical signs were observed. No significant differences in survival were seen for any group of rats or mice of either sex. Nonneoplastic and Neoplastic Effects in the Two-Year Studies: Compound-related nonneoplastic lesions occurred in the nasal passage of exposed rats and mice. In exposed rats, hyperplasia and squamous metaplasia of the respiratory epithelium and degeneration of the olfactory epithelium with ciliated columnar and/or squamous metaplasia were observed. Focal chronic inflammation and proliferation of the periosteum of the turbinate bones were increased slightly in rats at the top exposure concentration. Suppurative inflammation with hyperplasia and squamous metaplasia of the respiratory epithelium occurred in exposed mice. There were no compound-related increased incidences of neoplasms in rats or mice exposed to CS2. In exposed female mice, there were pronounced decreases in the incidences of adenomas of the pituitary pars distalis (control, 13/47; 0.75 mg/m3, 5/46; 1.5 mg/m3, 1/46) and decreased incidences of malignant lymphomas (21/50; 12/50; 8/50). Genetic Toxicology: The responses in Salmonella gene mutation tests with CS2 were equivocal in one laboratory for strain TA100 in the absence of exogenous metabolic activation (S9) and equivocal in another laboratory for TA97 with S9; in all other strains tested, CS2 was clearly negative with or without S9. CS2 induced trifluorothymidine resistance in mouse L5178Y/TK lymphoma cells in the absence of S9; it was not tested with S9. CS2 induced both sister chromatid exchangeslymphoma cells in the absence of S9; it was not tested with S9. CS2 induced both sister chromatid exchanges and chromosomal aberrations in CHO cells with and without S9. Conclusions: Under the conditions of these inhalation studies, there was no evidence of carcinogenic activity of CS2 for male or female F344/N rats exposed to 0.075, 0.25, or 0.75 mg/m3 in air for up to 2 years. There was no evidence of carcinogenic activity for male or female B6C3F1 mice exposed to 0.75 or 1.5 mg/m3 in air for up to 2 years. Concentration-related decreases in the incidences of pituitary gland adenomas and lymphomas were observed in female mice. Exposure to CS2 caused degeneration and squamous metaplasia of the olfactory epithelium, hyperplasia and metaplasia of the respiratory epithelium, and proliferation of the periosteum of the nasal passage of rats. In mice, exposure to this compound caused suppurative inflammation and hyperplasia and squamous metaplasia of the respiratory epithelium of the nasal passage.