Thirteen-week study of toxicity of fiber-like multi-walled carbon nanotubes with whole-body inhalation exposure in rats.

Cancer development due to fiber-like straight type of multi-walled carbon nanotubes (MWCNTs) has raised concerns for human safety because of its shape similar to asbestos. To set concentrations of MWCNT for a rat carcinogenicity study, we conducted a 13-week whole body inhalation study. F344 male and female rats, 6-week-old at the commencement of the study, were exposed by whole-body inhalation to MWCNT at concentrations of 0, 0.2, 1 and 5 mg/m(3) with a generation and exposure system utilizing the cyclone sieve method. Measured concentrations in the exposure chambers were 0.20 ± 0.02, 1.01 ± 0.11 and 5.02 ± 0.25 mg/m(3) for 13 weeks. The MMAD (GSD) of MWCNT were 1.4-1.6 µm (2.3-3.0), and mean width and length were 94.1-98.0 nm and 5.53-6.19 µm, respectively, for each target concentration. Lung weights were increased 1.2-fold with 1 mg/m(3) and 1.3-fold with 5 mg/m(3) in both sexes compared to the controls. In the bronchoalveolar lavage fluid (BALF) analyses, inflammatory parameters were increased concentration-dependently in both sexes from 0.2 mg/m(3). Granulomatous changes in the lung were induced at 1 and 5 mg/m(3) in females and even at 0.2 mg/m(3) in males. Focal fibrosis of the alveolar wall was observed in both sexes at 1 mg/m(3) or higher. Inflammatory infiltration in the visceral pleural and subpleural areas was induced only at 5 mg/m(3). In conclusion, we determined 0.2 mg/m(3) as the low-observed-adverse-effect level (LOAEL) for respiratory tract toxicity in the present inhalation exposure study of rats.

Development of a new multi-walled carbon nanotube (MWCNT) aerosol generation and exposure system and confirmation of suitability for conducting a single-exposure inhalation study of MWCNT in rats.

Because the primary route of human exposure to multi-walled carbon nanotube (MWCNT) is via inhalation, a new dry MWCNT aerosol generation and exposure system for whole-body inhalation exposure using a cyclone and sieve has been developed. The system was tested for operational performance at 0.2, 1 and 5 mg/m(3). Additionally, it was examined whether this system can be employed in animal whole-body inhalation studies by exposing rats to MWCNT aerosol for 6 h at 5 mg/m(3). The system could consistently provide aerosols with a similar particle size distribution and configuration at all the target exposure concentrations. Almost all MWCNTs were fibrous, and the presence of many well-dispersed, nano-sized particles was confirmed. Additionally, the animal study revealed that large amounts of MWCNTs were inhaled into the lung, resulting in an inflammatory response, with increased LDH and albumin levels, and granulomatous change. Therefore, the aerosol generation and exposure system appears useful for MWCNT inhalation studies using rats.

Lack of micronucleus induction activity of ethyl tertiary-butyl ether in the bone marrow of F344 rats by sub-chronic drinking-water treatment, inhalation exposure, or acute intraperitoneal injection.

Ethyl tertiary-butyl ether (ETBE) is an oxygenated gasoline additive synthesized from ethanol and isobutene that is used to reduce CO2 emissions. To support the Kyoto Protocol, the production of ETBE has undergone a marked increase. Previous reports have indicated that exposure to ETBE or methyl tertiary-butyl ether resulted in liver and kidney tumors in rats and/or mice. These reports raise concern about the effects of human exposure being brought about by the increased use of ETBE. The present study was conducted to evaluate the genotoxicity of ETBE using micronucleus induction of polychromatic erythrocytes in the bone marrow of male and female rats treated with ETBE in the drinking-water at concentrations of 0, 1,600, 4,000 or 10,000 ppm or exposed to ETBE vapor at 0, 500, 1,500 or 5,000 ppm for 13 weeks. There were no significant increases in micronucleus induction in either the drinking water-administered or inhalation-administered groups at any concentration of ETBE; although, in both groups red blood cells and hemoglobin concentration were slightly reduced in the peripheral blood in rats administered the highest concentration of ETBE. In addition, two consecutive daily intraperitoneal injections of ETBE at doses of 0, 250, 500 or 1,000 mg/kg did not increase the frequency of micronucleated bone marrow cells in either sex; all rats receiving intraperitoneal injections of ETBE at a dose of 2,000 mg/kg died after treatment day 1. These data suggest that ETBE is not genotoxic in vivo.

Novel method using hybrid markers: development of an approach for pulmonary measurement of multi-walled carbon nanotubes.

BACKGROUND: Multi-walled carbon nanotubes (MWCNT)s are suspected to induce pulmonary and pleural cancers due to their asbestos-like configurations. Therefore, accurate measurement of inhaled nanotubes in target organs is crucial for assessing cancer risk. Conventionally, nanotubes are measured after combustion at high temperature for conversion into CO2; however, the sensitivity is poor and the method lacks versatility. We have therefore developed a novel approach using hybrid markers for nanotube analysis, featuring high sensitivity and the capacity to conduct repeated analyses. The method involves adsorption of markers to nanotubes, followed by their desorption and assessment by means of high performance liquid chromatography (HPLC). METHODS: Recovery of MWCNT from rat lungs was conducted, and pulmonary MWCNT amounts were determined using rats intratracheally-exposed to MWCNT aerosol at 5 mg/m3 for 6 hours/day. RESULTS: The correlation coefficient for the calibration curve of MWCNT weight and the HPLC area was 0.9991. Consequently, the lower quantitation limit yielded was 0.2 µg. The recovery was 92-98% at approximately 0.4-2.0 µg demonstrating that MWCNTs in the lung could be measured accurately and precisely. CONCLUSIONS: We have developed a novel method using a hybrid marker approach for nanotube analysis, featuring very high sensitivity and the capacity to conduct repeated analyses. We further confirmed correlations between the amounts of nanotubes and markers and pulmonary nanotube measurement demonstrated that trace amounts could be detected with values closely relating to the administered dose, verifying that the method is sensitive and precise.

Two-week Toxicity of Multi-walled Carbon Nanotubes by Whole-body Inhalation Exposure in Rats.

To evaluate pulmonary toxicity of multi-walled carbon nanotubes (MWCNTs), F344 rats of both sexes were exposed by inhalation to 0.2, 1 or 5 mg/m(3) MWCNT aerosol for 6 h/day, 5 days/week for 2 weeks using a whole-body exposure system. At the end of the 2-week exposure period, one-half of the rats were necropsied, and at the end of an additional 4-week postexposure period, the remaining rats were necropsied. MWCNTs were deposited in the lungs of all MWCNT-exposed groups and mostly remained in the lungs throughout the 4-week postexposure period. Granulomatous changes in the lung were found in the rats exposed to 5 mg/m(3) MWCNTs, and these changes were slightly aggravated at the end of the 4-week postexposure period. In the bronchoalveolar lavage fluid (BALF), the numbers of neutrophils, percentages of bi- and multinucleated alveolar macrophages, levels of ALP activity and concentrations of total protein and albumin were elevated in the rats exposed to 1 and 5 mg/m(3) MWCNTs. At the end of the 4-week postexposure period, the values of the BALF parameters tended to remain elevated. In addition, goblet cell hyperplasias in the nasal cavity and nasopharynx were observed in the rats exposed to 1 and 5 mg/m(3) MWCNTs, but these lesions had largely regressed by the end of the postexposure period. Based on the histopathological and inflammatory changes, the no-observed-adverse-effect level (NOAEL) for inhalation of MWCNTs for 2 weeks was 0.2 mg/m(3).

Subchronic toxicity and carcinogenicity studies of 1,2-dichloropropane inhalation to mice.

The subchronic toxicity and carcinogenicity of 1,2-dichloropropane (DCP) in male and female B6D2F1 mice exposed to DCP by inhalation for 13 weeks or for 2 years was investigated. The DCP concentrations used were 50, 100, 200, 300 or 400 ppm (v/v) in the 13-week study, and 32, 80 or 200 ppm (v/v) in the 2-year study. Thirteen weeks inhalation exposure of mice to DCP caused death in the mice exposed to 300 ppm and above, and was found to induce hemolytic anemia and lesions of the liver, forestomach and heart. Two years exposure to DCP significantly increased the combined incidence of bronchiolo-alveolar adenomas and carcinomas in females and marginally increased the incidence of Harderian gland adenomas in males. As non-neoplastic lesion, atrophy and respiratory metaplasia in the olfactory epithelium, and respiratory metaplasia in the submucosal gland of the nasal cavity were increased. Thus, two years inhalation exposure to DCP is carcinogenic in female mice and there is a marginal evidence of carcinogenicity in males.

Inhalation carcinogenicity of 1,1,1-trichloroethane in rats and mice.

Carcinogenicity of 1,1,1-trichloroethane (TCE) was examined by an inhalation exposure of F344 rats and BDF1 mice of both sexes to TCE at 0, 200, 800 or 3200 ppm for 6 h/d, 5 d/week for 104 weeks. In male rats, the incidences of bronchiolo-alveolar adenomas and peritoneal mesotheliomas were significantly increased in the 800 and 3200 ppm-exposed groups, respectively. The incidence of bronchiolo-alveolar adenomas in the 3200 ppm-exposed groups exceeded the range of historical control data in the Japan Bioassay Research Center. In female rats, the tumor incidences were not increased in any organs of the TCE-exposed groups. In male mice, a significant positive trend with dose was shown for incidences of bronchiolo-alveolar carcinomas, combined incidences of bronchiolo-alveolar adenomas/carcinomas and hepatocellular adenomas. The incidence of Harderian gland adenomas was significantly increased in the 3200 ppm-exposed group, and malignant lymphomas of spleen at this highest dose exceeded the range of historical control data. In female mice, the combined incidence of bronchiolo-alveolar adenomas/carcinomas was significantly increased in the 3200 ppm-exposed group, and the incidences of hepatocellular adenomas and combined incidences of hepatocellular adenomas/carcinomas were significantly increased in the 200, 800 and 3200 ppm-exposed groups with dose dependence except the combined incidence of hepatocellular adenomas/carcinomas in the 200 ppm-exposed group. The incidences of bronchiolo-alveolar adenomas in the 3200 ppm-exposed group and combined incidences of hepatocellular adenomas/carcinomas in the 200 ppm-exposed groups exceeded the ranges of historical control data. Thus, this study provided clear evidence of inhalation carcinogenicity for TCE in both rats and mice.

Hepatotumorigenicity of ethyl tertiary-butyl ether with 2-year inhalation exposure in F344 rats.

Carcinogenicity of ethyl tertiary-butyl ether (ETBE) was examined with inhalation exposure using F344/DuCrlCrlj rats. Groups of 50 male and 50 female rats, 6 week old at commencement, were exposed to ETBE at 0, 500, 1,500 or 5,000 ppm (v/v) in whole-body inhalation chambers for 6 h/day, 5 days/week for 104 weeks. A significant increase in the incidence of hepatocellular adenomas was indicated in males exposed at 5,000 ppm, but not in females at any concentration. In addition, significantly increased incidences of eosinophilic and basophilic cell foci were observed in male rats at 5,000 ppm. Regarding non-neoplastic lesions, rat-specific changes were observed in kidney, with an increase in the severity of chronic progressive nephropathy in both sexes at 5,000 ppm. Increased incidences of urothelial hyperplasia of the pelvis were observed at 1,500 ppm and above, and mineral deposition was apparent in the renal papilla at 5,000 ppm in males. There were no treatment-related histopathological changes observed in any other organs or tissues in either sex. The present 2-year inhalation study demonstrated hepatotumorigenicity of ETBE in male, but not in female rats.

Inhalation carcinogenicity and chronic toxicity of indium-tin oxide in rats and mice.

OBJECTIVES: Carcinogenicity and chronic toxicity of indium-tin oxide (ITO) were examined by inhalation exposure of rats and mice to ITO aerosol. METHODS: Fifty mice of both sexes were exposed to ITO at 0 (control), 0.01, 0.03 or 0.1 mg/m(3) for 6 h/day, 5 day/wk for 104 wk, and 50 rats of both sexes were exposed to 0, 0.01 or 0.03 mg/m(3) ITO for the same time period. The repeated exposure of 50 rats of both sexes to 0.1 mg/m(3) ITO was discontinued at the 26th wk, followed by clean air exposure for the remaining 78 wk. RESULTS: In rats, incidences of bronchiolo-alveolar adenomas and carcinomas, bronchiolo-alveolar hyperplasia, alveolar wall fibrosis and thickened pleural wall, alveolar proteinosis and infiltrations of alveolar macrophages and inflammatory cells were significantly increased. Combined incidences of malignant lung tumors in male rats and total lung tumors in male and female rats were significantly increased at exposure to 0.01 mg/m(3) ITO. In mice, no carcinogenic response occurred, but thickened pleural wall, alveolar proteinosis and alveolar macrophage infiltration were induced. Mice were less susceptible to ITO than rats. The lung content of indium was the greatest, followed by the spleen, kidney and liver. Blood indium levels increased dose-dependently. CONCLUSIONS: There was clear evidence of carcinogenicity of inhaled ITO in male and female rats but not clear evidence in mice, together with occurrence of the chronic pulmonary lesions in both rats and mice.

Pulmonary toxicity in mice by 2- and 13-week inhalation exposures to indium-tin oxide and indium oxide aerosols.

OBJECTIVES: Inhalation toxicities of indium-tin oxide (ITO) and indium oxide (IO) in mice were characterized in comparison with those previously reported in rats. METHODS: B6C3F(1) mice of both sexes were exposed by inhalation to ITO or IO aerosol for 6 h/day, 5 day/wk for 2 wk at 0, 0.1, 1, 10 or 100 mg/m(3) or 13 wk at 0, 0.1or 1 mg/m(3). RESULTS: ITO and IO particles were deposited in the lung, mediastinal lymph node (MLN) and nasal-associated lymphoid tissue. Alveolar proteinosis, infiltrations of alveolar macrophages and inflammatory cells and increased lung weight were induced by 2- and 13-week exposures to ITO and IO, while alveolar epithelial hyperplasia occurred only in the 2-week exposures. Thickened pleural wall, hyperplastic MLN, extramedullary hematopoiesis in the spleen and increased levels of erythrocyte parameters were induced by 13-week exposure to ITO. The ITO- and IO-induced pulmonary lesions were milder in mice than those previously reported in rats, and the fibrotic lesions were different between these two species. Indium levels in the lung and pooled blood were analyzed in the mice exposed to ITO and IO for 13 wk. In the 13-week inhalation exposure of mice to ITO, alveolar proteinosis and significantly increased lung weight were induced at the same exposure concentration as the current threshold limit value for indium and its compounds.

Two- and 13-week inhalation toxicities of indium-tin oxide and indium oxide in rats.

OBJECTIVES: Two- and 13-week inhalation toxicities of indium-tin oxide (ITO) and indium oxide (IO) were characterized for risk assessments of workers exposed to ITO. METHODS: F344 rats of both sexes were exposed by inhalation to ITO or IO aerosol for 6 h/day, 5 day/wk for 2 wk at 0, 0.1, 1, 10 or 100 mg/m(3) or 13 wk at 0, 0.1 or 1 mg/m(3). An aerosol generator and inhalation exposure system was constructed. RESULTS: Blood and lung contents of indium were elevated in a dose-related manner in the ITO- and IO-exposed rats. ITO and IO particles were deposited in the lung, mediastinal lymph node and nasal-associated lymphoid tissue. Exposures to ITO and IO induced alveolar proteinosis, infiltrations of alveolar macrophages and inflammatory cells and alveolar epithelial hyperplasia in addition to increased lung weight. ITO affected the lung more severely than IO did. Fibrosis of alveolar wall developed and some of these lesions worsened at the end of the 26-week post-exposure period. CONCLUSIONS: Persistent pulmonary lesions including alveolar proteinosis and macrophage infiltration occurred after 2- and 13-week inhalation exposures of rats to ITO and IO. Fibrosis of alveolar wall developed later. These lesions occurred after ITO exposure at the same concentration as the current occupational exposure limit in the USA and at blood indium levels below the biological exposure index in Japan for indium.

Inhalation carcinogenicity and toxicity of 1,2-dichloropropane in rats.

The toxicity and carcinogenicity of 1,2-dichloropropane (DCP) were examined by inhalation exposure of male and female F344 rats to DCP for either 13 wk or 2 years. In the 13-wk study, the DCP concentrations used were 125, 250, 500, 1000, or 2000 ppm (v/v), and in the 2-year study the DCP concentrations were 80, 200, or 500 ppm (v/v). Thirteen-week exposure to DCP induced hyperplasia in the respiratory epithelium and atrophy of the olfactory epithelium at 125 ppm and above. At the higher levels of exposure, hemolytic anemia and lesions of liver and adrenal gland were observed. Two-year exposure to DCP significantly increased incidences of papilloma in the nasal cavity of male and female rats exposed to 500 ppm DCP. In addition, three cases of esthesioneuroepithelioma were observed in the DCP-exposed male rats. Total nasal tumors increased in a concentration-dependent manner. Hyperplasia of the transitional epithelium and squamous cell hyperplasia, both of which were morphologically different from the hyperplasia of the respiratory epithelium observed in the 13-wk exposure study, occurred in a concentration-dependent manner; these lesions are considered to be preneoplastic lesions. Atrophy of the olfactory epithelium, inflammation of the respiratory epithelium, and squamous cell metaplasia were also seen in the 2-year study. These results demonstrate that DCP is a nasal carcinogen in rats. Lifetime cancer risks for humans exposed to DCP in the ambient air and work environment were quantitatively estimated, using both nonthreshold and threshold approaches, with the data obtained from the 2-year study.

Two-year inhalation study of carcinogenicity and chronic toxicity of 1,4-dioxane in male rats.

Carcinogenicity and chronic toxicity of 1,4-dioxane were examined by inhalation exposure of 50 male F344 rats to 1,4-dioxane vapor at 0 (clean air), 50, 250, or 1250 ppm (v/v) for 6 h/day, 5 days/wk, and 104 wk. Survival rates of 250 and 1250 ppm-exposed groups were decreased near the end of the 2-yr exposure period, due probably to the occurrence of malignant tumors. A statistically significant but marginal decrement of terminal body weight (<10%) was found in the 1250 ppm-exposed group, suggesting slight systemic toxicity. Significant changes in plasma levels of AST, ALT, ALP, and gamma-GTP and relative weight of the liver occurred in the 1250 ppm-exposed group. Dose-dependent and statistically significant increases in incidences of nasal squamous cell carcinomas, hepatocellular adenomas, and peritoneal mesotheliomas were found primarily in the 1250 ppm-exposed group. The incidences of renal cell carcinomas, fibroadenomas in the mammary gland, and adenomas in the Zymbal gland were also increased dose-dependently. Preneoplastic lesions occurred in the nasal cavity and liver of the 1,4-dioxane-exposed groups. As nonneoplastic lesions, the significantly increased incidences of nuclear enlargement, atrophy, and respiratory metaplasia in the nasal cavity were noted at 50 ppm and above. A LOAEL (lowest observed adverse effect level) was determined at 50 ppm for the nasal endpoint of general chronic toxicity. This study provides clear evidence of carcinogenicity for 1,4-dioxane in male rats. A cytotoxic-proliferative and in vivo genotoxic mode of action is suggested to operate in 1,4-dioxane-induced carcinogenesis.

Thirteen-week inhalation toxicity of 1,4-dioxane in rats.

Thirteen-week inhalation toxicity of 1,4-dioxane was examined by repeated inhalation exposure of male and female F344 rats to 0 (control), 100, 200, 400, 800, 1600, 3200, or 6400 ppm (v/v) 1,4-dioxane vapor for 6 h/day and 5 days/wk. All the 6400-ppm-exposed males and females died during the first week. Terminal body weight decreased, and relative weights of liver, kidney, and lung increased. AST increased in the 200 ppm-and 3200-ppm-exposed females, and ALT increased in the 3200-ppm-exposed males and females. Nuclear enlargement of nasal respiratory epithelial cells occurring in the 100-ppm-exposed males and females was the most sensitive, followed by the enlarged nuclei in the olfactory, tracheal, and bronchial epithelia. 1,4-Dioxane-induced liver lesions occurred at higher exposure concentrations than the nasal lesions did, and were characterized by single-cell necrosis and centrilobular swelling of hepatocytes in males and females. Glutathione S-transferase placental form (GST-P) positive liver foci were observed in the 1600-ppm-exposed females and 3200-ppm-exposed males and females, which are known as a preneoplastic lesion in rat hepatocarcinogenesis. Plasma levels of 1,4-dioxane increased linearly with an increase in the concentrations of exposure to 400 ppm and above. The enlarged nuclei in the nasal epithelia and the GST-P-positive liver foci were discussed in light of the possible development of nasal and hepatic tumors by long-term inhalation exposure to 1,4-dioxane. A lowest-observed-adverse-effect level (LOAEL) was determined at 100 ppm for the nasal endpoint in both male and female rats.

Inhalation carcinogenicity and chronic toxicity of carbon tetrachloride in rats and mice.

Carcinogenicity and chronic toxicity of carbon tetrachloride were examined by inhalation exposure of 50 F344 rats and 50 BDF1 mice of both sexes to carbon tetrachloride at 0 (clean air), 5, 25, or 125 ppm (v/v) for 6 h/day, 5 days/wk, for 104 wk. Incidences of hepatocellular adenomas and carcinomas in rats and mice of both sexes and of adrenal pheochromocytomas in mice of both sexes were significantly increased dose-dependently. Hepatocellular carcinomas and cirrhosis significantly occurred in the 125-ppm-exposed rats of both sexes, and 3 cases of hepatocellular carcinomas and increased incidences of hepatic altered cell foci were noted in the 25-ppm-exposed female rats. Hepatocellular carcinomas were induced in mice of both sexes at 25 and 125 ppm, and hepatocellular adenomas occurred in females at 5 ppm without any degenerative or necrotic change in hepatocytes. Hepatocellular carcinomas metastasized to the lung. The chronic hepatotoxicity was characterized by cirrhosis, fibrosis, and fatty change in rats, and ceroid deposition, bile-duct proliferation, and hydropic change in mice. Survival rates were decreased in the 125-ppm-exposed rats and mice of both sexes and in the 25-ppm-exposed female mice, in association with decreased body weights. The decreased survival rates were considered to be causally related to both various tumors including hepatocellular carcinomas and severe chronic progressive nephropathy in rats and to hepatocellular carcinomas in mice. This study provided clear evidence of carcinogenicity for carbon tetrachloride in rats and mice. A cytotoxic-proliferative and genotoxic mode of action for carbon tetrachloride-induced hepatocarcinogenesis was suggested.

Thirteen-week inhalation toxicity of carbon tetrachloride in rats and mice.

Subchronic toxicity of carbon tetrachloride (CCl4) was examined by inhalation exposure of F344 rats and BDF1 mice of both sexes to 0, 10, 30, 90, 270 or 810 ppm (v/v) CCl4 vapor for 13 wk (6 h/d and 5 d/wk). In the high exposure levels at 270 and 810 ppm, altered cell foci in the livers of both rats and mice, and fibrosis and cirrhosis in the rat liver were observed. Hematoxylin and eosin-stained altered cell foci of rats were recognized as glutathione-S-transferase placental form (GST-P) positive foci, which are preneoplastic lesions of hepatocarcinogenesis. The most sensitive endpoint of CCl4-induced toxicity was fatty change with large droplets in rats of both sexes and male mice, and cytoplasmic globules in male mice, as well as increased relative liver weight in male rats. Those endpoints were manifested at 10 ppm and the LOAEL was determined as 10 ppm for the hepatic endpoints in rats and mice. Enhanced cytolytic release of liver transaminases into plasma in rats and mice and its close association with hepatic collapse in mice were observed at medium and high levels of inhalation exposure. Both CCl4-induced hematotoxicity and nephrotoxicity were observed in both rats and mice, but those toxicities were manifested at higher exposure concentrations than hepatotoxicity. The LOAEL for the hepatic endpoint and the GST-P-stained altered cell foci provide relevant animal data for reconsidering the occupational exposure limit val1ue of 5 ppm for CCl4 and strengthen the evidence of CCl4-induced hepatocarcinogenicity which is used in its carcinogenicity classification.

Thirteen-week inhalation toxicity of p-dichlorobenzene in mice and rats.

Subchronic inhalation toxicity of p-dichlorobenzene (p-DCB) was examined by exposing BDF1 mice and F344 rats of both sexes (6 h/d and 5 d/wk) to inhalation of 25, 55, 120, 270 or 600 ppm (v/v) p-DCB vapor for 13 wk. The exposure to p-DCB vapor retarded the growth rate in the male mice, and induced hepatotoxicity in the mice and rats of both sexes and renal and hematological toxicity in the male rats. Hepatotoxicity was characterized by increased liver weight, hepatocellular hypertrophy, and increased serum levels of total cholesterol. Liver necrosis and increased serum levels of AST and ALT were observed in the exposed mice, whereas these changes, which indicate hepatocellular death, did not occur in any of the exposed rats. p-DCB-induced renal lesions occurred only in the male rats. Hyaline droplets were observed in the proximal tubular epithelial cells, and were stained positively with anti-alpha2u-globulin, suggesting excessive accumulation of alpha2u-globulin in the epithelial cells. Granular casts were formed in the tubular lumen, resulting from the necrotic desquamation of the renal tubular epithelium. Papillary mineralization in the renal pelvis and increased serum levels of BUN and creatinine were noted. These renal changes indicated alpha2u-globulin nephropathy. Decreases in red blood cell counts, hemoglobin concentration, hematocrit and mean corpuscular volume and increased spleen weight occurred in the exposed male rats. The NOAEL was 120 ppm for the hepatic endpoint in mice and for the renal endpoint in rats. The maximum tolerated dose for a 2-yr bioassay inhalation study of rodent carcinogenicity was estimated to be 300 ppm, based on the present results.

Carcinogenicity and chronic toxicity after inhalation exposure of rats and mice to N,N-dimethylformamide.

Carcinogenicity and chronic toxicity of N,N-Dimethylformamide (DMF) were examined by inhalation exposure of groups of 50 rats and 50 mice of both sexes to DMF vapor at a concentration of 0, 200, 400 or 800 ppm (v/v) for 6 h/d, 5 d/wk, for 104 wk. In rats, incidences of hepatocellular adenomas and carcinomas significantly increased in the 400 and 800 ppm-exposed groups and in the 800 ppm-exposed group, respectively. The hepatocellular adenoma did not increase significantly in the 400 ppm-exposed female rats, but its incidence exceeded a range of historical control data in the Japan Bioassay Research Center (JBRC). In mice, incidences of hepatocellular adenomas and carcinomas significantly increased in all the DMF-exposed groups. Incidence of hepatoblastomas significantly increased in the 200 and 400 ppm-exposed male mice, and 4 cases of hepatoblastomas in the 400 ppm-exposed female mice and the 800 ppm-exposed male mice exceeded the range of historical control data of the JBRC. Incidences of altered cell foci increased in the liver of exposed rats and mice in an exposure concentration-related manner, and those foci were causally related to the hepatocellular tumors. Liver weights increased in both rats and mice exposed to DMF at 200 ppm and above. Increased levels of gamma-GTP, ALT, AST and total bilirubin in exposed rats of both sexes and AST and ALT in exposed mice of both sexes were noted. It was concluded that 2-yr inhalation exposure to DMF increased incidences of hepatocellular adenomas and carcinomas in rats and incidences of hepatocellular adenomas, carcinomas and hepatoblastomas in mice, and that hepatocarcinogenicity of DMF was more potent in mice than in rats.

Toxicity due to 2- and 13-wk inhalation exposures of rats and mice to N, N-dimethylformamide.

In order to better characterize the toxicity of N,N-dimethylformamide (DMF) and to provide its basic toxicity data for risk assessment of workers exposed to DMF, F344 rats and BDF1 mice of both sexes were exposed by inhalation (6 h/d x 5 d/wk) to 100, 200, 400, 800 or 1,600 ppm DMF for 2 wk, and 50, 100, 200, 400 or 800 ppm DMF for 13 wk. Three male and 7 female rats died during the 2-wk exposure to 1,600 ppm DMF, but no death of the exposed rats or mice occurred under any other exposure conditions. Massive, focal and single cell necroses were observed in the liver of DMF-exposed rats and mice. The massive necrosis associated with the centrilobular fibrosis occurred at the highest exposure concentration. The single cell necrosis was associated with fragmentation of the nucleoli as well as an increased mitotic figure. The 13-wk exposures of rats and mice to DMF were characterized by increases in the relative liver weight and the incidence of the centrilobular hepatocellular hypertrophy as well as increased serum levels of AST, ALT, LDH, total cholesterol and phospholipid. Lower confidence limits of the benchmark dose yielding the response with a 10% extra risk (BMDL10) were determined for the relative liver weight and the incidence of hepatocellular hypertrophy of the 13-wk exposed animals. The BMDL10 resulted in 1 ppm for the increased relative liver weight of male rats and mice and 17 ppm for the hepatocellular hypertrophy of male mice.