Toxicology and carcinogenesis studies of sodium nitrite (CAS NO. 7632-00-0) in F344/N rats and B6C3F1 mice (drinking water studies).
UNLABELLED: Sodium nitrite is used as a color fixative and preservative in meats and fish. It is also used in manufacturing diazo dyes, nitroso compounds, and other organic compounds; in dyeing and printing textile fabrics and bleaching fibers; in photography; as a laboratory reagent and a corrosion inhibitor; in metal coatings for phosphatizing and detinning; and in the manufacture of rubber chemicals. Sodium nitrite also has been used in human and veterinary medicine as a vasodilator, a bronchial dilator, an intestinal relaxant, and an antidote for cyanide poisoning. Sodium nitrite was nominated by the FDA for toxicity and carcinogenesis studies based on its widespread use in foods. Male and female F344/N rats and B6C3F1 mice were exposed to sodium nitrite (99% pure) in drinking water for 14 weeks or 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium, rat and mouse bone marrow, and mouse peripheral blood. 14-WEEK STUDY IN RATS: Groups of 10 male and 10 female rats were exposed to 0, 375, 750, 1500, 3,000, or 5000 ppm sodium nitrite (equivalent to average daily doses of approximately 30, 55, 115, 200, or 310 mg sodium nitrite/kg body weight to males and 40, 80, 130, 225, or 345 mg/kg to females) in drinking water for 14 weeks. Clinical pathology study groups of 15 male and 15 female rats were exposed to the same concentrations for 70 or 71 days. One female exposed to 3000 ppm died before the end of the study. Body weights of males exposed to 3000 or 5000 ppm and females exposed to 5000 ppm were significantly less than those of the controls. Water consumption by 5000 ppm males and 3000 and 5000 ppm females was less than that by the controls at weeks 2 and 14. Clinical findings related to sodium nitrite exposure included brown discoloration in the eyes and cyanosis of the mouth, tongue, ears, and feet of males exposed to 3000 or 5000 ppm and of females exposed to 1500 ppm or greater. Reticulocyte counts were increased in males and females exposed to 3000 or 5000 ppm. The erythron was decreased on day 19 but increased by week 14 in males and females exposed to 5000 ppm. Methemoglobin concentrations were elevated in almost all exposed groups throughout the 14 week study; a no-observed-adverse-effect level was not achieved. The relative kidney and spleen weights of males and females exposed to 3000 or 5000 ppm were significantly greater than those of the controls. Sperm motility in 1500 and 5000 ppm males was significantly decreased. Increased erythropoietic activity in the bone marrow of exposed males and females was observed. The incidences of squamous cell hyperplasia of the forestomach in 5000 ppm males and females were significantly increased. 14-WEEK STUDY IN MICE: Groups of 10 male and 10 female B6C3F1 mice were exposed to 0, 375, 750, 1500, 3000, or 5000 ppm sodium nitrite (equivalent to average daily doses of approximately 90, 190, 345, 750, or 990 mg/kg to males and 120, 240, 445, 840, or 1230 mg/kg to females) in drinking water for 14 weeks. Body weights of males exposed to 5000 ppm were significantly less than those of the controls. Water consumption by males exposed to 1500 ppm or greater was slightly less than that by the controls at week 13. Relative spleen weights of 3000 and 5000 ppm males and absolute and relative heart, kidney, liver, and spleen weights of females exposed to 3000 or 5000 ppm were greater than those of the control groups. Sperm motility was decreased in 5000 ppm males, and the estrous cycles of 1500 and 5000 ppm females were significantly longer than in the controls. There were increased incidences of squamous cell hyperplasia of the forestomach in 5000 ppm males and females, extramedullary hematopoiesis of the spleen in 3000 and 5000 ppm males and 1500 ppm or greater females, and degeneration of the testis in 3000 and 5000 ppm males. 2-YEAR STUDY IN RATS: Groups of 50 male and 50 female rats were exposed to 0, 750, 1500, or 3000 ppm sodium nitrite (equivalent to average daily doses of approximately 35, 70, or 130 mg/kg to males and 40, 8d 40, 80, or 150 mg/kg to females) in drinking water for 2 years. For toxicokinetic studies of plasma nitrite and blood methemoglobin, 10 male and 10 female special study rats were exposed to the same concentrations for 12 months. Survival of exposed groups was similar to that of the controls. Mean body weights of males and females exposed to 3000 ppm were less than those of the controls throughout the study. Water consumption by males and females exposed to 3000 ppm was less than that by the controls throughout the study, and that by the other exposed groups was generally less after week 14. The incidences of hyperplasia of the forestomach epithelium in males and females exposed to 3000 ppm were significantly greater than those in the control groups. The incidence of fibroadenoma of the mam mary gland was significantly increased in females exposed to 1500 ppm, and the incidences of multiple fibroadenoma were increased in 750 ppm and 1500 ppm females; however, these neoplasms occur with a high background incidence, and no increase was seen in the 3000 ppm group. The incidences of mononuclear cell leukemia were significantly decreased in males and females exposed to 1500 or 3000 ppm. 2-YEAR STUDY IN MICE: Groups of 50 male and 50 female B6C3F1 mice were exposed to 0, 750, 1500, or 3000 ppm sodium nitrite (equivalent to average daily doses of approximately 60, 120, or 220 mg/kg to males and 45, 90, or 165 mg/kg to females) in drinking water for 2 years. Survival of exposed groups was similar to that of the controls; mean body weights of 3000 ppm females were less than those of the controls throughout the study. Exposed groups generally consumed less water than the control groups. The incidences of squamous cell papilloma or carcinoma (combined) in the forestomach of female mice occurred with a positive trend. The incidence of hyperplasia of the glandular stomach epithelium was significantly greater in 3000 ppm males than in the controls. GENETIC TOXICOLOGY: Sodium nitrite was mutagenic in Salmonella typhimurium strain TA100, with and without Aroclor 1254-induced hamster and rat liver S9 enzymes; no mutagenicity was observed in strain TA98. Results of acute bone marrow micronucleus tests with sodium nitrite in male rats and mice by intraperitoneal injection were negative. In addition, a peripheral blood micronucleus assay conducted with mice from the 14-week study gave negative results. CONCLUSIONS: Under the conditions of this 2-year drinking water study, there was no evidence of carcinogenic activity of sodium nitrite in male or female F344/N rats exposed to 750, 1500, or 3000 ppm. There was no evidence of carcinogenic activity of sodium nitrite in male B6C3F1 mice exposed to 750, 1500, or 3000 ppm. There was equivocal evidence of carcinogenic activity of sodium nitrite in female B6C3F1 mice based on the positive trend in the incidences of squamous cell papilloma or carcinoma (combined) of the forestomach. Exposure to sodium nitrite in drinking water resulted in increased incidences of epithelial hyperplasia in the forestomach of male and female rats and in the glandular stomach of male mice. Decreased incidences of mononuclear cell leukemia occurred in male and female rats.