Multiple-site carcinogenicity of benzene in Fischer 344 rats and B6C3F1 mice.

Toxicology and carcinogenesis studies of benzene (CAS No. 71-43-2; greater than 99.7% pure) were conducted in groups of 60 F344/N rats and 60 B6C3F1 mice of each sex for each of three exposure doses and vehicle controls. These composite studies on benzene were designed and conducted because of large production volume and widespread human exposure, because of the epidemiologic association with leukemia, and because previous experiments were considered inadequate or inconclusive for determining carcinogenicity in laboratory animals. Using the results from 17-week studies, doses for the 2-year studies were selected based on clinical observations (tremors in higher dosed mice), on clinical pathologic findings (lymphoid depletion in rats and leukopenia in mice), and on body weight effects. Doses of 0, 50, 100, or 200 mg/kg body weight benzene in corn oil were administered by gavage to male rats, 5 days per week, for 103 weeks. Doses of 0, 25, 50, or 100 mg/kg benzene in corn oil were administered by gavage to female rats and to male and female mice for 103 weeks. Ten animals in each of the 16 groups were killed at 12 months, and necropsies were performed. Hematologic profiles were performed at 3-month intervals. For the 2-year studies, mean body weights of the top dose groups of male rats and of both sexes of mice were lower than those of the controls. Survivals of the top dose group of rats and mice of each sex were reduced; however, at week 92 for rats and week 91 for mice, survival was greater than 60% in all groups; most of the dosed animals that died before week 103 had neoplasia. Compound-related nonneoplastic or neoplastic effects on the hematopoietic system, Zymbal gland, forestomach, and adrenal gland were found both for rats and mice. Further, the oral cavity was affected in rats, and the lung, liver, Harderian gland, preputial gland, ovary, and mammary gland were affected in mice. Under the conditions of these 2-year gavage studies, there was clear evidence of carcinogenicity of benzene in male F344/N rats, female F344/N rats, male B6C3F1 mice, and female B6C3F1 mice. In male rats, benzene caused increased incidences of Zymbal gland carcinomas, squamous cell papillomas and squamous cell carcinomas of the oral cavity, and squamous cell papillomas and squamous cell carcinomas of the skin. In female rats, benzene caused increased incidences of Zymbal gland carcinomas and squamous cell papillomas and squamous cell carcinomas of the oral cavity.(ABSTRACT TRUNCATED AT 400 WORDS)

Thirteen-week inhalation toxicity of N,N-dimethylformamide in F344/N rats and B6C3F1 mice.

Male and female F-344 rats and B6C3F1 mice (10/sex/group) were exposed to N,N-dimethylformamide (DMF) by whole body inhalation exposure at 0, 50, 100, 200, 400, or 800 ppm, 6 h/day, 5 days/week, for 13 weeks. A concentration-dependent depression in body weight occurred in rats of both sexes at 400 (6-11%) and 800 ppm (20-22%). In contrast, all weight changes in both sexes of mice were within 10% of controls. No rats died, while 5 mice died from nonexposure-related causes. Relative liver weights were significantly increased at all DMF concentrations in both sexes and both species. Activities of serum sorbitol dehydrogenase (SDH) were statistically increased in male and female rats (200 to 800 ppm) on study days 4, 24, and 91 (13 weeks). Activities of alanine aminotransferase (ALT) and isocitrate dehydrogenase (ICD) were statistically increased in both sexes of rats exposed to 800 ppm DMF at all time points. Cholesterol (CHOL) levels were statistically increased in male and female rats (50-800 ppm) at all sampling time points. Levels of total bile acids (TBA) were statistically increased in both sexes of rats (400-800 ppm) on days 24 and 91. Centrilobular hepatocellular necrosis (minimal to moderate) was seen in rats of both sexes exposed at 400 and 800 ppm, with the lesions more severe in females. Centrilobular hepatocellular hypertrophy (minimal to mild) was found in all groups of DMF-exposed male mice, and in female mice exposed at 100-800 ppm. For male and female rats the no-observed-adverse-effect concentration (NOAEC) for microscopic liver injury was 200 ppm. The NOAEC was 50 ppm for female mice, but an NOAEC based upon the absence of microscopic liver injury was not determined in male mice.

Comparative gavage subchronic toxicity studies of o-chloroaniline and m-chloroaniline in F344 rats and B6C3F1 mice.

ortho-Chloroaniline (o-CA) andmeta-chloroaniline (m-CA) are chemical intermediates for pigment production in the textile industry. Comparative subchronic gavage studies were conducted to determine the effect of structure on toxicity.o-CA orm-CA was administered to 10 animals/sex/species in deionized water at dosages of 0, 10, 20, 40, 80, and 160 mg/kg for 13 weeks. Blood samples for clinical pathology were collected after 3 and 23 days in rats and at study termination (Day 93) in rats and mice. No mortalities occurred that could be directly attributed to treatment. Transient clinical signs of toxicity observed after dosing included cyanosis in rats and ataxia and tremors in mice. Methemoglobin formation was directly related to dosage (rats and mice) and duration of treatment (rats). At study termination, Heinz body formation in erythrocytes in association with decreased hemoglobin, hematocrit, and red blood cell count was a prominent treatment-related effect. Enlarged spleens (gross necropsy observation) and increased spleen weight were treatment effects of each chemical in both species. Microscopic lesions typical of increased red blood cell production were found in hematopoietic tissues (bone marrow, spleen, and liver), while lesions due to increased red cell destruction were found in these tissues and also the kidneys (rats). Microscopic changes were more frequently seen and severe, and involved more body organs, in rats than mice, and in m-CA-treated animals thano-CA-treated animals. Sex differences in lesion incidence/severity were not evident.

Hematological effects in F344 rats and B6C3F1 mice during the 13-week gavage toxicity study of methylene blue trihydrate.

Methylene blue trihydrate is used widely as a dye and therapeutic agent. Methylene blue was administered by gavage to 30 animals/sex/dose group in a 0.5% aqueous methylcellulose suspension at doses of 0, 25, 50, 100, and 200 mg/kg. Blood samples from 10 animals/sex/dose group were collected at the end of study weeks 1, 6, and 13. Methylene blue treatment resulted in methemoglobin formation and oxidative damage to red blood cells, leading to a regenerative anemia and a variety of tissue and biochemical changes secondary to erythrocyte injury. An early change was a dose-related increase in methemoglobin, where the response of rats and mice was similar in magnitude. Mice appeared to be more sensitive than rats to the formation of Heinz bodies and the development of anemia that was characterized by a decrease in hemoglobin, hematocrit, and erythrocyte count. Splenomegaly was apparent in all treated mice and in the 100 mg/kg (males only) and 200 mg/kg rats at necropsy. There was a dose-related increase in absolute and relative spleen weight for both species. Microscopic examination revealed increased splenic hematopoiesis in all mice treatment groups and in rats at the 50 mg/kg dose level and above. Splenic congestion and bone marrow hyperplasia were also observed in these rat-dose groups. Mice at the higher doses showed hematopoiesis in the liver and accumulation of hemosiderin in Kupffer cells. These gross and microscopic findings are consistent with the development of hemolytic anemia. A dose-related increase in the reticulocyte count during study weeks 6 and 13 suggested a compensatory response to anemia.

Chronic inhalation oncogenicity study of isoprene in B6C3F1 mice.

The oncogenic potential of isoprene as affected by concentration, length of daily exposure, and weeks of exposure over the life-span of the animal, as independent variables, was evaluated. Ten groups were exposed for 8 h/day, 5 days/week as follows (ppm-weeks): 0-80, 10-80, 70-40, 70-80, 140-40, 280-20, 280-80, 700-80, 2200-40, 2200-80. Two groups were exposed for 4 h/day: 2200-20, 2200-80. Groups were held until 96 or 105 weeks on study. The concentration x time (duration of exposure) values provided a series of theoretically equivalent exposure hazards. There was an exposure-related increased incidence of liver, lung, Harderian gland and forestomach tumors, and hemangiosarcomas and histiocytic sarcomas. The LOEL appeared to be 70 ppm. These results are similar to the profile of tumors seen in 1,3-butadiene (BD)-exposed mice without the early onset of T-cell lymphoma as seen with BD. Isoprene appears to be about one order of magnitude less potent than BD in mice. Statistical analyses indicated that the product of isoprene concentration, and length/duration of exposure was not a sufficient basis for predicting tumor risk at any site. Extrapolation of tumor probability between the high and low doses based on cumulative exposure was not appropriate and could not be justified by statistical models. A threshold effect level and strong nonlinearities with respect to concentration appeared to exist for tumor development in this study.

Carcinogenicity studies of oxazepam in mice.

Oxazepam is a benzodiazepine widely used as a sedative-hypnotic and antianxiety drug. In chronic studies, groups of 60 male and 60 female Swiss-Webster (SW) or B6C3F1 mice received oxazepam in feed at concentrations of 0,2500, or 5000 ppm. Additional groups of 60 male and female B6C3F1 mice received 125 ppm in feed to allow for study of mice with serum concentrations of oxazepam similar to those achieved in humans taking a therapeutic dose. At 57 weeks, treatment-related mortality of exposed SW mice caused the study to be terminated. Enhanced systemic amyloidosis contributing to heart failure was considered the principal cause of death. Hepatocellular adenomas and carcinomas were increased in exposed SW mice. Survival of B6C3F1 mice receiving 2500 and 5000 ppm oxazepam was also lower than that of controls. Early deaths were due to increased incidences of hepatoblastoma and hepatocellular carcinoma, and nearly all mice receiving 2500 or 5000 ppm developed hepatocellular neoplasia. An increase in follicular cell hyperplasia of the thyroid gland occurred in all exposed groups of B6C3F1 mice, and thyroid gland follicular cell adenoma was increased in exposed females. Further studies of the capacity of oxazepam to induce liver cell mitogenesis and an evaluation of the frequency of activated H- and K-ras oncogenes in the liver tumors of B6C3F1 mice has shown that many of the neoplastic and nonneoplastic responses of mice to oxazepam resemble those observed with phenobarbital.

Results and conclusions of the National Toxicology Program's rodent carcinogenicity studies with sodium fluoride.

The US National Toxicology Program (NTP) has conducted toxicity and carcinogenicity studies with sodium fluoride administered in the drinking water to F344/N rats and B6C3F1 mice. The drinking water concentrations used in the 2-year studies were 0, 25, 100, or 175 ppm sodium fluoride (equivalent to 0, 11, 45 or 79 ppm fluoride). Survival and weight gains of rats and mice were not affected by fluoride treatment. Animals receiving sodium fluoride developed effects typical of dental fluorosis, and female rats given 175 ppm had increased osteosclerosis. There were no increases in neoplasms in female rats or in male or female mice that were attributed to sodium fluoride administration. There was equivocal evidence of carcinogenic activity of sodium fluoride in male rats based on the occurrence of a small number of osteosarcomas in treated animals.

Toxicity of diethanolamine. 2. Drinking water and topical application exposures in B6C3F1 mice.

Toxicology studies of diethanolamine were conducted in male and female B6C3F1 mice to characterize and compare effects of exposure in the drinking water with those caused by topical application and to compare responses in mice to those observed in rats. Each study consisted of five dose groups plus controls and the size of each group was 10 animals per sex. Doses of diethanolamine ranged from 630 to 10,000 ppm in the drinking water study (approximately equivalent to daily doses of 100-1700 mg kg-1 in males and 140-1100 mg kg-1 in females) and from 80 to 1250 mg kg-1 in the topical application study. Exposure to diethanolamine caused dose-dependent toxic effects in the liver (hepatocellular cytological alterations and necrosis), kidney (nephropathy and tubular epithelial necrosis in males), heart (cardiac myocyte degeneration) and skin (site of application: ulceration, inflammation, hyperkeratosis, and acanthosis). Cytological alterations in the liver consisted of multiple hepatocyte changes, including enlarged cells that were frequently multinucleated, increased nuclear pleomorphism, increased eosinophilia and disruption of hepatic cords. A no-observed-adverse-effect level (NOAEL) was not achieved for hepatocellular cytological alterations or for acanthosis in the skin.