Carcinogenicity of inhaled vanadium pentoxide in F344/N rats and B6C3F1 mice.

Vanadium pentoxide (V2O5) is a slightly soluble compound found in airborne particle emissions from metallurgical works and oil and coal burning. Because the carcinogenic potential of V2O5 was not known, F344/N rats and B6C3F1 mice (N=50/sex/species) were exposed to V2O5 at concentrations of 0, 0.5 (rats only), 1, 2, or 4 (mice only) mg/m3, by whole-body inhalation for 2 years. The survival and body weights of rats were minimally affected by exposure to V2O5. The survival and body weights of male mice exposed to 4 mg/m3 and body weights of all exposed groups of female mice were lower than the controls. Alveolar/bronchiolar (A/B) neoplasms occurred in male rats exposed to 0.5 and 2 mg/m3 at incidences exceeding the National Toxicology Program (NTP) historical control ranges. A marginal increase in A/B neoplasms was also observed in female rats exposed to 0.5 mg/m3. Increases in chronic inflammation, interstitial fibrosis, and alveolar and bronchiolar hyperplasia/metaplasia and squamous metaplasia were observed in exposed male and female rats. A/B neoplasms were significantly increased in all groups of exposed mice. As with rats, increases in chronic inflammation, interstitial fibrosis, and alveolar and bronchiolar epithelial hyperplasia were observed in mice exposed to V2O5. Thus, V2O5 exposure was a pulmonary carcinogen in male rats and male and female mice. The marginal tumor response in the lungs of female rats could not be attributed conclusively to exposure to V2O5. These responses were noted at and slightly above the OSHA permissible occupational exposure limit of 0.5 mg/m3 (dust) (National Institute for Occupational Safety and Health, NIOSH Pocket Guide to Chemical Hazards, U.S. Department of Health and Human Services, Washington, DC, 1997, p. 328).

Toxicology and carcinogenesis studies of microencapsulated citral in rats and mice.

Citral, a widely used natural ingredient, is added to foods and cosmetics as a flavoring and fragrance agent. Male and female F344/N rats and B6C3F1 mice were exposed to microencapsulated citral in the feed for 14 weeks or two years. All studies included untreated and vehicle control groups. In the 14-week studies, rats and mice were given diets containing 3,900, 7,800, 15,600, or 31,300 ppm citral. In rats, food consumption was reduced in the two highest dose groups. In mice an apparent increase in food consumption was observed, but was due to mice scattering the feed. Body weights of all treated animals were less than controls. All rats and four male mice were killed moribund in the high dose groups. In rats, forestomach and kidney lesions were observed. At the higher doses, lesions observed in the bone marrow, testes, and thymus in rats and in the ovary in mice were considered related to inanition and resultant moribundity. In the two-year studies, rats were exposed to 1,000, 2,000, or 4,000 ppm citral. Body weights were reduced in the 4,000 ppm rats. Mice were exposed to 500, 1,000, or 2,000 ppm citral. Body weights in the 1,000 and 2,000 ppm groups were reduced. No neoplasms were attributed to citral in rats or mice. Malignant lymphoma occurred with a positive trend and was significantly greater than controls in female mice in the 2,000 ppm group. However, the incidences were within the NTP historical control range and could not be clearly related to citral administration.

The role of oxidative stress in indium phosphide-induced lung carcinogenesis in rats.

Indium phosphide (IP), widely used in the microelectronics industry, was tested for potential carcinogenicity. Sixty male and 60 female Fischer 344 rats were exposed by aerosol for 6 h/day, 5 days/week, for 21 weeks (0.1 or 0.3 mg/m(3); stop exposure groups) or 105 weeks (0 or 0.03 mg/m(3) groups) with interim groups (10 animals/group/sex) evaluated at 3 months. After 3-month exposure, severe pulmonary inflammation with numerous infiltrating macrophages and alveolar proteinosis appeared. After 2 years, dose-dependent high incidences of alveolar/bronchiolar adenomas and carcinomas occurred in both sexes; four cases of squamous cell carcinomas appeared in males (0.3 mg/m(3)), and a variety of non-neoplastic lung lesions, including simple and atypical hyperplasia, chronic active inflammation, and squamous cyst, occurred in both sexes. To investigate whether inflammation-related oxidative stress functioned in the pathogenesis of IP-related pulmonary lesions, we stained lungs of control and high-dose animals immunohistochemically for four markers indicative of oxidative stress: inducible nitric oxide synthase (i-NOS), cyclooxygenase-2 (COX-2), glutathione-S-transferase Pi (GST-Pi), and 8-hydroxydeoxyguanosine (8-OHdG). Paraffin-embedded samples from the 3-month and 2-year control and treated females were used. i-NOS and COX-2 were highly expressed in inflammatory foci after 3 months; at 2 years, all four markers were expressed in non-neoplastic and neoplastic lesions. Most i-NOS staining, mainly in macrophages, occurred in chronic inflammatory and atypical hyperplastic lesions. GST-Pi and 8-OHdG expression occurred in cells of carcinoma epithelium, atypical hyperplasia, and squamous cysts. These findings suggest that IP inhalation causes pulmonary inflammation associated with oxidative stress, resulting in progression to atypical hyperplasia and neoplasia.

A retrospective analysis of background lesions and tissue accountability for male accessory sex organs in Fischer-344 rats.

Because the paired lobes (ventral, dorsal, lateral, and anterior) of the rat prostate have not been consistently sampled in many carcinogenicity and toxicity studies, comparison among different investigations has been compromised. The lack of specific site identification for prostatic lesions further lessens the value of incidences reported. We present here the lobe-specific incidences and degree of severity of background prostatic, seminal vesicular, and ampullary glandular lesions in 1768 control Fischer-344 rats from 35 recent National Toxicology Program 2-year carcinogenicity and toxicity studies conducted in 4 laboratories. The dorsal and lateral lobes were combined and considered the dorsolateral lobe where inflammation, epithelial degeneration, mucinous cysts, and edema were observed. Inflammation in the dorsolateral lobes was significantly associated with pituitary gland adenoma whose prolactin was suggested to play an important role in pathogenesis of prostatic inflammation. Epithelial degeneration, epithelial hyperplasia, inflammation, edema, and adenoma were conspicuous in the ventral lobes. Inflammation and edema occurred in the anterior lobes (coagulating glands). Inflammation, dilatation, epithelial hyperplasia, edema, and adenoma were observed in the seminal vesicles. Inflammation was also present in the ampullary glands. We suggest an optimal embedment and trimming method in rat prostate and seminal vesicle to ensure adequate, consistent sampling.

Effects of glutaraldehyde in a 2-year inhalation study in rats and mice.

Whole-body inhalation toxicology and carcinogenicity studies were performed with the widely used fixative and cold-sterilant glutaraldehyde. Groups of 50 male and female F344/N rats and B6C3F(1) mice were exposed to glutaraldehyde (rats: 0, 250, 500, or 750 ppb; mice: 0, 62.5, 125, or 250 ppb) 6 h/day, 5 days/week, for 104 weeks. Survival of 500- and 750-ppb female rats was less than that of controls. Mean body weights of all exposed groups of male rats, 500- and 750-ppb female rats, and 250-ppb female mice were generally less than those of controls. No exposure-related neoplastic lesions were observed in either rats or mice. Non-neoplastic lesions were limited primarily to the most anterior region of the nasal cavity. In rats, hyperplasia and inflammation of the squamous epithelium; hyperplasia, goblet cell hyperplasia, inflammation, and squamous metaplasia of the respiratory epithelium; and hyaline degeneration of the olfactory epithelium were observed. In mice, the nasal lesions were qualitatively similar to those in rats. Squamous metaplasia of the respiratory epithelium was observed in both sexes of mice while female mice also had inflammation and hyaline degeneration of the respiratory epithelium. In contrast to the nasal carcinogen formaldehyde, no neoplastic lesions were observed after inhalation exposure to glutaraldehyde. However, exposure to glutaraldehyde resulted in considerable non-neoplastic lesions in the noses of rats and mice.

Examination of low-incidence brain tumor responses in F344 rats following chemical exposures in National Toxicology Program carcinogenicity studies.

Neoplasms in the brain are uncommon in control Fischer 344 (F344) rats; they occur at a rate of less than 1% in 2-yr toxicity/carcinogenicity studies. Furthermore, only 10 of nearly 500 studies conducted by the National Toxicology Program (NTP) showed any evidence of chemically related neoplastic effects in the brain. Generally, the brain tumor responses were considered equivocal, because the characteristics of potential neurocarcinogenic agents (such as statistically significant increased incidences, decreased latency and/or survival, and demonstration of dose-response relationships) were not observed. A thorough examination, including comparisons with a well-established historical database, is often critical in evaluating rare brain tumors. Chemicals that gave equivocal evidence of brain tumor responses were generally associated with carcinogenicity at other sites, and many chemicals were mutagenic when incubated with metabolic activating enzymes. Other factors that were supportive of the theory that marginal increases in brain tumor incidence were related to chemical exposure were that (a) some of the tumors were malignant, (b) no brain neoplasms were observed in concurrent controls from some studies, and/or (c) brain tumors were also seen following exposure to structurally related chemicals. In 2-yr studies in F344 rats (studies conducted by the NTP), equivocal evidence of carcinogenicity was observed for the following 9 chemicals: isoprene, bromoethane, chloroethane, 3,3'-dimethylbenzidine dihydrochloride, 3,3'-dimethoxybenzidine dihydrochloride, furosemide, C.I. direct blue 15, diphenhydramine hydrochloride, and 1-H-benzotriazole. Glycidol was the only chemical evaluated by the NTP with which there was clear evidence of brain tumor induction in F344 rats. Clarification of the potential neurocarcinogenic risks of chemicals that produce equivocal evidence of a brain tumor response in conventional 2-yr rodent studies may be aided by the use of transgenic mouse models that exhibit genetic alterations that reflect those present in human brain tumors as well as by the use of in utero exposures.

The association between severe nephropathy and pheochromocytoma in the male F344 rat -- the National Toxicology Program experience.

The possible correlation between the severity of chronic progressive glomerulonephropathy (CPN) and the incidence of adrenal pheochromocytoma was examined in selected studies of male Fischer 344 (F344) rats at the National Toxicology Program (NTP). The NTP historical control database was first examined in order to determine whether there was association between the severity of CPN and the occurrence of adrenal pheochromocytoma in unexposed animals. Following this analysis, the 125 most recent NTP studies conducted in F344 rats were examined in order to determine how frequently chemicals that cause increased severity of CPN showed an increased incidence of pheochromocytoma. Finally, we examined the association between the incidence of pheochromocytoma and the severity of CPN in those NTP studies with chemically related increased rates of pheochromocytoma. In control male F344 rats surviving beyond 21 mo, the incidence of adrenal pheochromocytoma was consistently higher in animals with more severe CPN. This association was significant (p < 0.05) both for 900 NTP inhalation study controls and 900 NTP feeding study controls. An association was not consistently observed when dosed groups were considered. Although 22% (28/125) of NTP studies reported a chemically related increased severity of CPN, only 3 of these reported a corresponding significant increase in the incidence of pheochromocytoma. Of 6 NTP studies that reported increased incidence of pheochromocytoma, animals with pheochromocytoma from 5 of those studies had some degree of increased severity of CPN. However, the estimated strength of the correlation with the severity of CPN varied from study to study and was often quite different from that indicated by an analysis of the more extensive NTP control databases. The possible correlation between the severity of CPN and the incidence of pheochromocytoma may influence interpretation of carcinogenic effects observed at this site.

Effect of 13 week magnetic field exposures on DMBA-initiated mammary gland carcinomas in female Sprague-Dawley rats.

Several studies suggest that exposure to 50 Hz magnetic fields may promote chemically induced breast cancer in rats. Groups of 100 female Sprague-Dawley rats were initiated with four weekly 5 mg gavage doses of 7,12-dimethylbenz[a]anthracene (DMBA) starting at 50 days of age. After the first weekly DMBA administration, exposure to ambient fields (sham exposed), 50 Hz magnetic fields at either 1 or 5 G field intensity or 60 Hz fields at 1 G for 18.5 h/day, 7 days/week was initiated. Exposure continued for 13 weeks. A vehicle control group without DMBA was included. In a second study, using lower doses of DMBA, groups of 100 female Sprague-Dawley rats were initiated with four weekly doses of 2 mg of DMBA starting at 50 days of age followed, after the first weekly DMBA administration, by exposure to ambient fields (sham exposed) or 50 Hz magnetic fields at either 1 or 5 G field intensity for 18.5 h/day, 7 days/week for 13 weeks. Rats were weighed and palpated weekly for the presence of tumors. There was no effect of magnetic field exposure on body weight gains or on the time of appearance of mammary tumors in either study. At the end of 13 weeks, the animals were killed and the mammary tumors counted and measured. Mammary gland masses found grossly were examined histologically. In the first 13 week study, the mammary gland carcinoma incidences were 92, 86, 96 and 96% for the DMBA controls, 1 G, 50 Hz, 5 G, 50 Hz and 1 G, 60 Hz groups, respectively. The total numbers of carcinomas were 691, 528 (P < 0. 05, decrease), 561 and 692 for the DMBA controls, 1 G, 50 Hz, 5 G, 50 Hz and 1 G, 60 Hz groups, respectively. In study 2, the mammary gland carcinoma incidences were 43, 48 and 38% for the DMBA controls, 1 G, 50 Hz and 5 G, 50 Hz groups, respectively. The total numbers of carcinomas were 102, 90 and 79 for the DMBA controls, 1 G, 50 Hz and 5 G, 50 Hz groups, respectively. There was no effect of magnetic field exposure on tumor size either by in-life palpation or by measurement at necropsy in either study. There was no evidence that 50 or 60 Hz magnetic fields promoted breast cancer in these studies in female rats. These studies do not support the hypothesis that magnetic field exposure promotes breast cancer in this DMBA rat model.

Chronic toxicity/oncogenicity evaluation of 60 Hz (power frequency) magnetic fields in F344/N rats.

A 2-yr whole-body exposure study was conducted to evaluate the chronic toxicity and possible oncogenicity of 60 Hz (power frequency) magnetic fields in rats. Groups of 100 male and 100 female F344/N rats were exposed continuously to pure, linearly polarized, transient-free 60 Hz magnetic fields at flux densities of 0 Gauss (G) (sham control), 20 milligauss (mG), 2 G, and 10 G; an additional group of 100 male and 100 female F344/N rats received intermittent (1 hr on/1 hr off) exposure to 10 G fields. Mortality patterns, body weight gains throughout the study, and the total incidence and number of malignant and benign tumors in all groups exposed to magnetic fields were similar to those found in sex-matched sham controls. Statistically significant increases in the combined incidence of C-cell adenomas and carcinomas of the thyroid were seen in male rats chronically exposed to 20 mG and 2 G magnetic fields. These increases were not seen in male rats exposed continuously or intermittently to 10 G fields or in female rats at any magnetic field exposure level. No increases in the incidence of neoplasms, which have been identified in epidemiology studies as possible targets of magnetic field action (leukemia, breast cancer, and brain cancer), were found in any group exposed to magnetic fields. There was a decrease in leukemia in male rats exposed to 10 G intermittent fields. The occurrence of C-cell tumors at the 2 lower field intensities in male rats is interpreted as equivocal evidence of carcinogenicity; data from female rats provides no evidence of carcinogenicity in that sex. These data, when considered as a whole, are interpreted as indicating that chronic exposure to pure linearly polarized 60 Hz magnetic fields has little or no effect on cancer development in the F344/N rat.

Chronic toxicity/oncogenicity evaluation of 60 Hz (power frequency) magnetic fields in B6C3F1 mice.

A 2-yr whole-body exposure study was conducted to evaluate the chronic toxicity and possible oncogenicity of 60 Hz (power frequency) magnetic fields in mice. Groups of 100 male and 100 female B6C3F1 mice were exposed to pure, linearly polarized, transient-free 60 Hz magnetic fields at flux densities of 0 Gauss (G) (sham control), 20 milligauss (mG), 2 G, and 10 G; an additional group of 100 male and 100 female B6C3F1 mice received intermittent (1 hr on/1 hr off) exposure to 10 G fields. A small but statistically significant increase in mortality was observed in male mice exposed continuously to 10 G fields; mortality patterns in all other groups of mice exposed to magnetic fields were comparable to those found in sex-matched sham controls. Body weight gains and the total incidence and number of malignant and benign tumors were similar in all groups. Magnetic field exposure did not increase the incidence of neoplasia in any organ, including those sites (leukemia, breast cancer, and brain cancer) that have been identified in epidemiology studies as possible targets of magnetic field action. A statistically significant decrease in the incidence of malignant lymphoma was observed in female mice exposed continuously to 10 G fields, and statistically significant decreases in the incidence of lung tumors were seen in both sexes exposed continuously to 2 G fields. These data do not support the hypothesis that chronic exposure to pure, linearly polarized 60 Hz magnetic fields is a significant risk factor for neoplastic development in mice.

Disseminated thrombosis and bone infarction in female rats following inhalation exposure to 2-butoxyethanol.

Groups of 10 male and 10 female F344/N rats were exposed to 0, 31, 62.5, 125, 250, and 500 ppm of 2-butoxyethanol (BE) by inhalation, 6 hr/day, 5 days/wk, for 13 wk. Four moribund female rats from the 500 ppm group were sacrificed during the first 4 days of exposure, and 1 moribund female from the same group was sacrificed during week 5. Dark irregular mottling and/or loss of the distal tail were noted in sacrificed moribund rats. Similar gross lesions were noted in the terminally sacrificed females exposed to 500 ppm BE. Histologic changes noted in the day 4 sacrificed moribund rats included disseminated thrombosis involving the coccygeal vertebrae, cardiac atrium, lungs, liver, pulp of the incisor teeth, and the submucosa of the anterior section of the nasal cavity. Alterations noted in coccygeal vertebrae from the 500 ppm sacrificed moribund rats included ischemic necrosis and/or degeneration of bone marrow cells, bone-lining cells, osteocytes (within cortical and trabecular bone), and chondrocytes (both articular and growth plate), changes that are consistent with an infarction process. The moribund female rat that was sacrificed during week 5 and those female rats treated with 500 ppm and sacrificed following 13 wk of treatment lacked thrombi, but they had coccygeal vertebral changes consistent with prior infarction and transient or complete bone growth arrest. No bone lesions or thrombi were noted in the male rats treated with the same doses of BE. In conclusion, exposure to 500 ppm BE vapors caused acute disseminated thrombosis and bone infarction in female rats. Possible pathogenic mechanisms are discussed.

Inhalation toxicity and carcinogenicity studies of cobalt sulfate.

Cobalt sulfate is a water-soluble cobalt salt with a variety of industrial and agricultural uses. Several cobalt compounds have induced sarcomas at injection sites in animals, and reports have suggested that exposure to cobalt-containing materials may cause lung cancer in humans. The present studies were done because no adequate rodent carcinogenicity studies had been performed with a soluble cobalt salt using a route relevant to occupational exposures. Groups of 50 male and 50 female F344/N rats and B6C3F1 mice were exposed to aerosols containing 0, 0.3, 1.0, or 3.0 mg/m3 cobalt sulfate hexahydrate, 6 h/day, 5 days/week, for 104 weeks. Survival and body weights of exposed rats and mice were generally unaffected by the exposures. In rats, proteinosis, alveolar epithelial metaplasia, granulomatous alveolar inflammation, and interstitial fibrosis were observed in the lung in all exposed groups. Nonneoplastic lesions of the nose and larynx were also attributed to exposure to all concentrations of cobalt sulfate. In 3.0 mg/m3 male rats and in female rats exposed to 1.0 or 3.0 mg/m3, the incidences of alveolar/bronchiolar neoplasms were increased over those in the control groups. Lung tumors occurred with significant positive trends in both sexes. The incidences of adrenal pheochromocytoma in 1.0 mg/m3 male rats and in 3.0 mg/m3 female rats were increased. Nonneoplastic lesions of the respiratory tract were less severe in mice than in rats. In mice, alveolar/bronchiolar neoplasms in 3.0 mg/m3 males and females were greater than those in the controls, and lung tumors occurred with significantly positive trends. Male mice had liver lesions consistent with a Helicobacter hepaticus infection. Incidences of liver hemangiosarcomas were increased in exposed groups of male mice; however, because of the infection, no conclusion could be reached concerning an association between liver hemangiosarcomas and cobalt sulfate. In summary, exposure to cobalt sulfate by inhalation resulted in increased incidence of alveolar/bronchiolar neoplasms and a spectrum of inflammatory, fibrotic, and proliferative lesions in the respiratory tracts of male and female rats and mice. Adrenal pheochromocytomas were increased in female rats, and possibly in male rats.

Effect of 26 week magnetic field exposures in a DMBA initiation-promotion mammary gland model in Sprague-Dawley rats.

Several studies have suggested that exposure to 50 Hz magnetic fields promote chemically induced breast cancer in rats. Groups of 100 female Sprague-Dawley rats were initiated with a single 10 mg gavage dose of 7,12-dimethylbenz[a]anthracene (DMBA) at 50 days of age followed by exposure to ambient fields (sham exposed), 50 Hz magnetic fields at either 1 or 5 Gauss (G) field intensity or 60 Hz fields at 1 G for 18.5 h/day, 7 days/week for 26 weeks. A vehicle control group without DMBA was included. Rats were palpated weekly for the presence of tumors. There was no effect of magnetic field exposure on body weight gains or the time of appearance of mammary tumors. At the end of 26 weeks, the animals were killed and the mammary tumors counted and measured. Mammary gland masses found grossly were examined histologically. The mammary gland carcinoma incidence was 96, 90, 95 and 85% (P < 0.05, decrease) for the DMBA controls, 1 G 50 Hz, 5 G 50 Hz and 1 G 60 Hz groups, respectively. The total numbers of carcinomas were 649, 494 (P < 0.05, decrease), 547 and 433 (P < 0.05, decrease) for the DMBA controls, 1 G 50 Hz, 5 G 50 Hz and 1 G 60 Hz groups, respectively. The number of fibroadenomas varied from 276 to 319, with the lowest number in the 1 G 60 Hz exposure group. Measurement of the tumors revealed no difference in tumor size between groups. In this breast cancer initiation-promotion study in female Sprague-Dawley rats, there was no evidence that 50 or 60 Hz magnetic fields promoted breast cancer under the conditions of this assay. This study does not support the hypothesis that magnetic field exposure can promote breast cancer in this rat model.

Impact of Helicobacter hepaticus infection in B6C3F1 mice from twelve National Toxicology Program two-year carcinogenesis studies.

Male and female B6C3F1 mice from 12 National Toxicology Program (NTP) 2-yr carcinogenesis studies were found to be infected with Helicobacter hepaticus. Many of the male mice from 9 of these studies had an associated hepatitis (affected studies). Helicobacter hepaticus has been reported to be associated with an increased incidence of hepatitis and hepatocellular neoplasms in the A/JCr male mouse. We attempted to determine if the data from the Helicobacter-affected NTP B6C3F1 mouse studies were compromised and unsuitable for cancer hazard identification. The incidences of neoplasms of the liver (both hepatocellular and hemangiosarcoma) but not of other organs in control male B6C3F1 mice were increased in affected studies as compared with control males from unaffected studies. The increased incidence of hepatocellular neoplasms was observed in those males exhibiting H. hepaticus-associated hepatitis. Other observations further differentiated control male mice from affected and unaffected studies. H-ras codon 61 CAA to AAA mutations were less common in liver neoplasms from males from affected studies as compared with historical and study controls. In addition, increases in cell proliferation rates and apoptosis were observed in the livers of male mice with H. hepaticus-associated hepatitis. These data support the hypothesis that the increased incidence of liver neoplasms is associated with H. hepaticus and that hepatitis may be important in the pathogenesis. Therefore, interpretation of carcinogenic effects in the liver of B6C3F1 mice may be confounded if there is H. hepaticus-associated hepatitis.

Effect of individual versus group caging on the incidence of pituitary and Leydig cell tumors in F344 rats: proposed mechanism.

Recently, an increase in pituitary tumor (pars distalis adenoma) incidence, and decrease in testicular interstitial cell tumor incidence, has been noted in F344 rats, in 2 year National Toxicology Program dermal and inhalation studies. One of the factors that may have contributed to this correlation is the difference in housing protocols. Rats in inhalation and dermal toxicity studies are singly caged, in contrast to other types of studies in which rats are group-caged, such as dosed-feed, dosed-water, or gavage studies. We propose that stress, related to individual caging, particularly among males, directly impairs testosterone synthesis and produces Leydig cell atrophy which leads to a feedback increase in the synthesis of luteinizing hormone by the anterior pituitary. This is followed by anterior pituitary cell functional hypertrophy, hyperplasia, and eventually neoplasia. It is known that individual caging of male rats produces a stress response associated with increased serum corticosteroids. The testicular interstitial cells (Leydig cells) have specific receptors for the glucocorticoid hormones. The Leydig cell enzyme 11-beta-hydroxysteroid dehydrogenase (11-beta-HSD) inactivates gluococorticoids; however, prolonged stress depletes this enzyme, enabling the gluococorticoids to impair steroidogenesis and eventually to lead to compensatory pituitary proliferations, including neoplasms.

Spontaneous neoplasm incidences in Fischer 344 rats and B6C3F1 mice in two-year carcinogenicity studies: a National Toxicology Program update.

Spontaneous neoplasm rates were determined for control Fischer 344 (F344) rats and B6C3F1 mice from 2-yr rodent carcinogenicity studies carried out by the National Toxicology Program (NTP). The most frequently occurring neoplasms in untreated male F344 rats were testicular adenoma (89.1%), mononuclear cell leukemia (50.5%), adrenal gland pheochromocytoma (31.9%), and pituitary gland neoplasms (30.4%). For untreated female F344 rats, the most frequently occurring neoplasms were pituitary gland neoplasms (54.2%), mammary gland fibroadenoma (41.2%), and mononuclear cell leukemia (28.1%). The most frequently occurring neoplasms in untreated male B6C3F1 mice were liver adenoma/carcinoma (42.2%), lung adenoma/carcinoma (20.5%), and malignant lymphoma (8.3%). For untreated female B6C3F1 mice, the most frequently occurring neoplasms were liver adenoma/carcinoma (23.6%), malignant lymphoma (20.9%), and pituitary gland adenoma/carcinoma (14.8%). The tumor rates observed in feeding study (untreated) and inhalation study (chamber) control rats were generally similar. The major exceptions were pituitary gland tumors and testicular adenoma in male F344 rats. The overall incidence of testicular adenoma was much lower in chamber controls (69.4%) than in feeding study controls (89.1%), whereas pituitary gland neoplasm showed the opposite trend (60.7% vs 30.4%). The most likely explanation for this difference is related to the individual housing of chamber controls and the group housing of feeding study controls. Differences in diagnostic criteria may influence reported tumor rates. To ensure consistency and comparability of tumor diagnosis from study to study, the NTP uses rigorous histopathology quality assurance and peer review procedures. Biological factors such as body weight may also affect tumor incidence. For example, increased body weights are associated with increased incidences of certain site-specific neoplasms, especially pituitary gland and mammary gland neoplasms in rats and liver tumors in mice. The presence of Helicobacter hepaticus has been associated with an increased incidence of liver neoplasms in male B6C3F1 mice. Other factors that may produce differences in control tumor rates from study to study include diet, environmental factors, genetic drift, study duration, and survival differences. The NTP database provides historical control data that may be useful in the evaluation of possible chemically related changes in tumor incidence. However, it is essential that the study being evaluated be comparable to those in the NTP database with respect to those factors that are known to influence tumor occurrence.

Theophylline-induced mesenteric periarteritis in F344/N rats.

The toxicity and carcinogenic potential of theophylline (an alkaloid bronchodilator drug) was investigated in male and female F344/N rats in 16-day, 14-week, and 2-year gavage and feeding studies. In 16-day studies, rats were fed diets containing 0, 500, 1000, 2000, 4000, and 8000 ppm of theophylline or given 0, 12.5 (twice daily), 25 (once daily), 50 (once daily), 50 (twice daily), 100 (once daily), 200 (once daily), 200 (twice daily), and 400 (once daily) mg theophylline/kg body weight in corn oil by gavage. In 14-week studies, rats were fed diets containing 0, 1000, 2000, and 4000 ppm theophylline or given 0, 37.5, 75, and 150 mg/kg body weight theophylline in corn oil by gavage. In 2-year gavage studies, rats were given 0, 7.5, 25, and 75 mg/kg body weight in corn oil. In 16-day gavage studies, treatment-related periarteritis occurred in arteries of the pancreas and adjacent to the mesenteric lymph nodes of early death male and female rats given 400 mg/kg once daily. In the 14-week studies, treatment-related periarteritis occurred at similar sites and in male rats exposed to 75 and 150 mg/kg, and in all exposed female rats (gavage studies), in females exposed to 1000 ppm, and in both sexes exposed to 2000 and 4000 ppm (feeding studies). In the 2-year study, chronic periarteritis was significantly increased only in the males receiving 75 mg/kg of theophylline. The adventitia, media and intima of medium- and large-sized mesenteric arteries were involved. Similar to other vasodilator chemicals, the pathogenesis of theophylline-induced vascular lesions may be a consequence of hemodynamic changes induced in the vascular wall.

An update of the National Toxicology Program database on nasal carcinogens.

Nearly 500 long-term rodent carcinogenicity studies carried out by the National Cancer Institute and the National Toxicology Program were examined, and 12 chemicals were identified that produced nasal tumors: allyl glycidol ether, p-cresidine, 1,2-dibromo-3-chloropropane, 1,2-dibromoethane, 2,3-dibromo-1-propanol, dimethylvinyl chloride, 1,4-dioxane, 1,2-epoxybutane, iodinated glycerol, procarbazine, propylene oxide, and 2,6-xylidine. All 12 of these chemicals produced nasal tumors in rats, and 5 also produced nasal tumors in mice. Most of the nasal carcinogens (1) produced tumor increases in both sexes, (2) produced tumors at other sites as well, (3) had significantly reduced survival at doses that were carcinogenic, and (4) were genotoxic. Only 5 of the 12 nasal carcinogens were administered by inhalation. A variety of different types of nasal cavity tumors were produced, and specific tumor rates are given for those chemicals causing multiple tumor types. Increased incidences of nasal neoplasms were often accompanied by suppurative/acute inflammation, epithelial/focal hyperplasia and squamous metaplasia. However, high incidences of these nonneoplastic nasal lesions were also frequently seen in inhalation studies showing no evidence of nasal carcinogenicity, suggesting that in general nasal carcinogenesis is not associated with the magnitude of chronic toxicity observed at this site.

Body weight-tumor incidence correlations in long-term rodent carcinogenicity studies.

Associations between body weight and tumor incidence and among the incidences of selected site-specific tumors were examined for more than 4,000 male and female Fischer-344 rats and B6C3F1 mice in the National Toxicology Program historical control database. Incidences of certain site-specific tumors, most notably mammary gland and pituitary gland tumors in rats and liver tumors in mice, were shown to have a strong positive correlation with 52-wk body weight. Using individual animal data, logistic regression models were derived for predicting site-specific tumor incidence as a function of 52-wk body weight, age, and other factors. This association between body weight and tumor incidence can explain many of the decreased tumor incidences observed in National Toxicology Program carcinogenicity studies. Body weight differences between dosed and control groups can also mask carcinogenic effects for those sites sensitive to body weight changes. Thus, when designing long-term rodent carcinogenicity studies, measures should be taken to minimize potential body weight differences between dosed and control groups. There were a number of small but significant negative correlations among tumor incidences, reflecting primarily the lethality of the tumors in question. None of these correlations (nor the 2 small positive correlations found) are likely to have any impact on the interpretation of experimental results. However, the high negative correlation between pituitary gland and testis tumors in male Fischer-344 rats cannot be dismissed so easily, does not reflect tumor lethality, and is currently being studied further.

Alteration in cell kinetics in control B6C3F1 mice infected with Helicobacter hepaticus.

The discovery of Helicobacter hepaticus infection, H. hepaticus hepatitis, and increased incidence of liver tumors in control males from several recent National Toxicology Program B6C3F1 mouse carcinogenicity bioassays raised questions regarding the suitability of these bioassays for hazard identification. The purpose of this study was to determine if changes in cell proliferation and death at terminal sacrifice might be linked to the increased liver tumor incidences among control males. In control males, enhanced rates of hepatocyte proliferation, as assessed by immunostaining for proliferating cell nuclear antigen (PCNA), and apoptosis, as assessed from hematoxylin and eosin- and TUNEL-stained preparations, were seen in 3 bioassays with H. hepaticus hepatitis. One bioassay with H. hepaticus infection without attendant hepatitis and one bioassay without H. hepaticus or hepatitis did not have elevated rates of hepatocyte proliferation or apoptosis. There was no significant effect on PCNA cell proliferation indices or apoptosis in females. The present findings are indicative of a clear association between the presence of H. hepaticus infection with attendant hepatitis, increased cell proliferation and apoptosis, and increased incidences of hepatocellular neoplasia in males but not in females. Thus, the interpretation of liver tumor responses in H. hepaticus-infected studies is considered to be confounded in male mice. The lack of enhanced cell proliferation or hepatocellular neoplasia in control females suggests that bioassay results from females are valid for hazard identification. Furthermore, the absence of enhanced cell proliferation in lungs and kidneys of male and females suggests that neoplastic effects at these sites are not exacerbated by H. hepaticus infection.