Proliferative lesions of the exocrine pancreas: relationship to corn oil gavage in the National Toxicology Program.

A microscopic review of pancreata from corn oil vehicle control and untreated control F344/N male rats in thirty-seven 2-year carcinogenesis studies was conducted to determine the extent and strength of the association of proliferative exocrine pancreatic lesions with corn oil gavage. The incidence of focal basophilic cellular change was similar in both untreated and vehicle control groups and was unrelated to corn oil gavage. The overall incidences of focal acinar hyperplasia and acinar adenoma were about five times greater in male rats that received the corn oil than in untreated rats (12.6 and 4.9% vs. 2.6 and 0.9%). This association was not consistent for each study group of vehicle controls. Over one-third (7/20) of the vehicle control groups had incidences of hyperplasia and adenoma no greater than the average rate for untreated male rats. There was no relationship between incidences of proliferative acinar lesions and the animal laboratory, the animal source, and the brand, lot, or peroxide level of the corn oil. The incidences of focal acinar hyperplasia and acinar adenoma were related to maximum mean body weights attained by the groups during the course of the study.

Bromodichloromethane, a trihalomethane that produces neoplasms in rodents.

Bromodichloromethane, a trihalomethane found in water supplies after chlorination, was administered by gavage in corn oil to male and female F344/N rats and B6C3F1 mice for up to 2 years at dose levels of 0, 50, or 100 mg/kg to rats, 0, 25, or 50 mg/kg to male mice, and 0, 75, or 150 mg/kg to female mice. Survival at 2 years in rats and in male mice was comparable among groups and was greater than 50% at the termination of the experiment. Survival in female mice was greater than 50% in all groups until week 84 but was reduced toward the end of the study because of ovarian abscesses in some female mice. There was clear evidence of carcinogenicity in males and females of both species as shown by increased incidences of tubular cell adenomas and adenocarcinomas in the kidney and adenocarcinomas and adenomatous polyps in the large intestine in male and female rats, increased incidences of tubular cell adenomas and adenocarcinomas in the kidney of male mice, and increased incidences of hepatocellular adenomas and carcinomas in female mice. Of the three trihalomethanes studied to date in the National Cancer Institute/National Toxicology Program (chloroform, chlorodibromomethane, or bromodichloromethane) bromodichloromethane caused the widest spectrum of neoplasms in rodents.

Proliferative lesions of the exocrine pancreas in male F344/N rats.

While the rat pancreas is susceptible to experimental cancer induction, the spontaneous incidence of pancreatic cancer in this species is reported to be very low. However, we observed unusually high incidences of focal acinar hyperplasia and acinar adenoma in vehicle control male F344/N rats of some NCI/NTP 2-year toxicological studies. The vehicle in these studies was corn oil given by gavage. Focal acinar hyperplasia, acinar adenoma, and acinar carcinoma (found rarely) represent a continuous spectrum of proliferative lesions of the exocrine pancreas. While the carcinomas have clear morphological indications of malignancy, the biological behavior of focal acinar hyperplasia and acinar adenoma is not known. Although induction of acinar carcinomas is considered clear evidence of carcinogenicity of a test chemical, significantly increased incidences in treated rats of acinar adenomas but not carcinomas provides some evidence of carcinogenicity. The association of acinar hyperplasia and adenoma with vegetable oil gavage complicates the interpretation when marginally elevated incidences of these lesions are observed in rats administered the test chemical in vegetable oil vehicle. Studies of the biological behavior of exocrine pancreatic lesions in male rats would be helpful in assessing the significance of their presence when found after test compound administration.

Decreases in spontaneous tumors in rats and mice after treatment with amphetamine.

Toxicology and carcinogenesis studies of dl-amphetamine sulfate, a drug used in the treatment of weight control, narcolepsy, and behavioral syndromes in children, were performed in F344/N rats and B6C3F1 mice. In these studies, amphetamine was administered for 2 years at doses of 0, 20, or 100 ppm in the feed to groups of 50 animals/dose/sex/species. The average amount of amphetamine consumed per day was estimated to be 1 or 5 mg/kg for low or high dose rats, 4 or 30 mg/kg for low or high dose male mice, and 3 or 19 mg/kg for low or high dose female mice. Survival was similar in dosed and control groups. The most notable effect of long-term treatment with this drug was the reduction of body weight in comparison to controls, and reduction in spontaneous tumors including pheochromocytomas of the adrenal gland in male rats, fibroadenomas of the mammary gland in female rats, adenomas of the anterior pituitary gland in male and female rats and female mice, endometrial stromal polyps of the uterus of female rats, adenomas or carcinomas of the liver in male and female mice, adenomas of the Harderian gland in male and female mice, and adenomas or carcinomas of the lung in male and female mice. Decreases in spontaneous tumors have previously been seen in 2-year rodent studies in groups of animals that have a reduced body weight in comparison to controls, but the spectrum of reduction in spontaneous neoplasms after treatment with amphetamine is broader than has previously been observed.

Two-year inhalation carcinogenesis studies of methyl methacrylate in rats and mice: inflammation and degeneration of nasal epithelium.

Methyl methacrylate (MMA), a liquid monomer, is used as a chemical intermediate in the manufacture of plexiglass and other acrylic products and as "bone cement" in orthopedic and dental surgery. Toxicology and carcinogenesis inhalation studies of MMA were conducted because of: (1) widespread human exposure; (2) evidence of mutagenicity; and (3) inadequacy of previously conducted long-term oral, dermal, and inhalation studies. Groups of 50 male F344/N rats were exposed to MMA by inhalation at 0, 500, or 1000 ppm, female F344/N rats at 0, 250, or 500 ppm, and male and female B6C3F1 mice at 0, 500, or 1000 ppm, 6 h a day, 5 days a week for 102 weeks. Survival rates of male and female rats and mice exposed to MMA were similar to those of their respective controls. Body weights were reduced in the low and high dose male (3-6% and 5-10%, respectively) and female (5-7% and 8-10%) rats exposed to MMA for more than 80 weeks and in male (7-19% and 6-17%) and female (0-13% and 0-17%) mice for more than 20 weeks. Inhalation exposure of MMA for 102 weeks did not induce any increased incidences of neoplasms in male or female rats or mice. Non-neoplastic lesions in the nasal cavity of MMA-exposed rats and mice were significantly increased and these included inflammation and degeneration of the olfactory epithelium of MMA-exposed male and female rats and inflammation, hyperplasia, cytoplasmic inclusions in the respiratory epithelium, and degeneration of the olfactory epithelium in male and female mice.

Respiratory tract lesions in F344/N rats and B6C3F1 mice after inhalation exposure to 1,2-epoxybutane.

1,2-Epoxybutane, a short-chain epoxide used as a stabilizer in chlorinated hydrocarbon solvents, was administered by inhalation exposure as a vapor 6 h/day, 5 day/week, for 24 months at exposure concentrations of 0, 200 or 400 ppm to F344/N rats and 0, 50, or 100 ppm to B6C3F1 mice. Survival of all groups of rats was 50% or greater until week 98 but was reduced in exposed groups by the end of the study. Survival in male mice was comparable among groups. Survival in female mice was greater than 50% until week 86, but was then reduced in the high-exposure group of mice. Exposure-related inflammatory, degenerative, and proliferative lesions occurred in the nasal cavity of both rats and mice. Seven papillary adenomas occurred in the nasal passages of high-exposure male rats and 2 in the nasal passages of high-exposure female rats. Alveolar/bronchiolar adenoma or carcinoma (combined) occurred with increased incidence in exposed male rats relative to controls. No exposure-related neoplastic lesions were seen in mice. After inhalation exposure, 1,2-epoxybutane was carcinogenic in rodents as were other epoxides or related compounds including propylene oxide, 1,3-butadiene, and ethylene oxide. The site of carcinogenic activity was considered to be related to length of the carbon chain.

Toxicity of monochloroacetic acid administered by gavage to F344 rats and B6C3F1 mice for up to 13 weeks.

Groups of 20 rats and 20 mice of each sex were administered monochloroacetic acid (MCAA) once daily, 5 days per week, in water by gavage for up to 13 weeks. Doses used were 0, 30, 60, 90, 120, or 150 mg/kg for rats and 0, 25, 50, 100, 150, or 200 mg/kg for mice. Compound-related deaths occurred at the four highest dose levels in rats and at the highest dose level in mice. Mean body weights of treated groups of rats and mice surviving until the end of the study were similar to those of the controls. A dose-related increase in blood urea nitrogen, alanine aminotransferase, aspartate aminotransferase, as well as a dose-related increase in the relative liver and kidney weights was observed in rats but not in mice. A dose-related increase in the incidence and severity of cardiomyopathy occurred in rats. This lesion may be related to the inhibition of heart mitochondrial aconitase activity. No compound-related lesions were observed in mice. The results of this study indicate that F344 rats are more sensitive than B6C3F1 mice; sexes within the species were equally sensitive. The no-observable-effect level was estimated as 30 mg MCAA/kg body weight for rats and 100 mg MCAA/kg body weight for mice.

Benzyl acetate carcinogenicity, metabolism, and disposition in Fischer 344 rats and B6C3F1 mice.

Carcinogenesis studies of benzyl acetate (a fragrance and flavoring agent) were conducted in F344 rats and B6C3F1 mice. The chemical was given in corn oil by gavage once daily, 5 days/week for 103 weeks, to groups of 50 animals of each sex and species. For rats the doses were 0, 250, and 500 mg/kg body weight and for mice the doses were 0, 500, and 1000 mg/kg. Mean body weights of control and dosed rats and mice were not affected adversely by benzyl acetate. The survival of control and low dose female mice was lower than that of the high dose group. A genital tract infection may have contributed to the reduced survival. No other significant difference in survival was observed for dosed rats or mice. Benzyl acetate was absorbed from the gastrointestinal tract of rats and mice, with approximately 90% of the administered dose recovered as various metabolites in the urine within 24 h. The primary metabolite was hippuric acid, with minor amounts of a mercapturic acid, and one or more unidentified metabolites. This capacity for absorption, metabolism, and disposition was unaffected by the amount or number of doses administered. Under the conditions of these studies, benzyl acetate administration was associated with an increased incidence of acinar cell adenoma of the exocrine pancreas in male F344/N rats. No evidence of carcinogenicity was found for female F344/N rats. For male and female B6C3F1 mice there was evidence of carcinogenicity, in that benzyl acetate caused an increased incidence of hepatocellular neoplasms (particularly adenomas) and squamous cell neoplasms of the forestomach.

The sequential development of cancer: a morphological perspective.

Cancer development proceeds through sequential or contemporaneous morphological changes from normal, preneoplastic, and premalignant lesions to highly malignant neoplasms. The morphological continuum that comprises cancer development is usually divided into diagnostic categories of hyperplasia (or dysplasia), benign neoplasia, and malignant neoplasia based on perceived biological behavior. Although a morphological continuum may be evident from the histological evaluation of preneoplastic and neoplastic lesions, it is not axiomatic that all preneoplastic or benign lesions progress. The probability of regression or progression from one category to another, or the rates at which these might occur, are seldom known for spontaneous or induced neoplasms. Host factors as well as exogenous stimuli may influence these events. The concept of neoplastic progression and the limitations of our knowledge of the biological behavior of preneoplastic lesions and benign neoplasms are important considerations in the interpretation of pathology data from carcinogenicity studies.

Toxicity and carcinogenicity of nitrofurazone in F344/N rats and B6C3F1 mice.

Toxicology and carcinogenesis studies were conducted by feeding diets containing nitrofurazone (99% pure) to groups of F344/N rats and B6C3F1 mice for 14 days, 13 wk or 2 yr. In the 14-day studies, in which doses ranged from 630 to 10,000 ppm, nitrofurazone was more toxic to mice than to rats. Accordingly, in the 13-wk studies, doses for rats ranged from 150 to 2500 ppm and for mice from 70 to 1250 ppm. At the higher doses, convulsive seizures and gonadal hypoplasia were observed in both species. Evidence of toxicity in rats also included degenerative arthropathy. For the 2-yr studies, rats were exposed to 0, 310 or 620 ppm nitrofurazone and the survival of male rats given 620 ppm was lower than that of controls (33/50, 30/50 and 20/50 in the control, 310- and 620-ppm groups, respectively). Nitrofurazone administration increased the incidences of mammary gland fibroadenomas in female rats (8/49, 36/50 and 36/50 in the control, 310- and 620-ppm groups, respectively). In male rats it was associated with a marginal increase in sebaceous gland adenomas and trichoepitheliomas of the skin, mesotheliomas of the tunica vaginalis, and tumours of the perputial gland. Nitrofurazone caused testicular degeneration (atrophy of germinal epithelium and aspermatogenesis) in rats, and degeneration of vertebral and knee articular cartilage in rats of both sexes. In mice, dietary concentrations of nitrofurazone for the 2-yr studies were 0, 150 or 310 ppm. In mice of each sex, nitrofurazone administration induced stimulus-sensitive convulsive seizures, primarily during the first year of study. In male mice, there was no evidence of any chemically-related carcinogenic effects, but there was a treatment-related decrease in survival (39/50, 31/50 and 27/50 in the control, 150- and 310-ppm groups, respectively). In female mice nitrofurazone induced ovarian lesions with increased incidences of benign mixed tumours (0/47, 17/50 and 20/50 in control, low- and high-dose groups, respectively) and granulosa cell tumours (1/47, 4/50 and 9/50 in control, low- and high-dose groups, respectively).

Toxicity and carcinogenicity of hydroquinone in F344/N rats and B6C3F1 mice.

Toxicology and carcinogenesis studies were conducted by administering hydroquinone (more than 99% pure) by gavage to groups of F344/N rats and B6C3F1 mice of each sex for 14 days, 13 wk or 2 yr. 14-day studies were conducted by administering hydroquinone in corn oil to rats at doses ranging from 63 to 1000 mg/kg body weight and to mice at doses ranging from 31 to 500 mg/kg, 5 days/wk. In the 13-wk studies, doses for rats and mice ranged from 25 to 400 mg/kg. At those doses showing some indication of toxicity in the 14-day and 13-wk studies, the central nervous system, forestomach and liver were identified as target organs in both species and renal toxicity was observed in rats. Based on these results, 2-yr studies were conducted by administering 0, 25 or 50 mg hydroquinone/kg in deionized water by gavage to groups of 65 rats of each sex, 5 days/wk. Groups of 65 mice of each sex were given 0, 50 or 100 mg/kg on the same schedule. 10 rats and 10 mice from each group were killed and evaluated after 15 months. Mean body weights of high-dose male rats and high-dose mice were approx. 5-14% lower than those of controls during the second half of the study. No differences in survival were observed between dosed and control groups of rats or mice. Nearly all male rats and most female rats in all vehicle control and exposed groups had nephropathy, which was judged to be more severe in high-dose male rats. Hyperplasia of the renal pelvic transitional epithelium and renal cortical cysts were increased in male rats. Tubular cell hyperplasia of the kidney was seen in two high-dose male rats, and renal tubular adenomas were seen in 4/55 low-dose and 8/55 high-dose male rats; none was seen in vehicle controls or in female rats. Mononuclear cell leukaemia in female rats occurred with increased incidences in the dosed groups (vehicle control, 9/55; low dose, 15/55; high dose, 22/55). Compound-related lesions observed in the liver of high-dose male mice included anisokaryosis, syncytial alteration and basophilic foci. The incidences of hepatocellular neoplasms, primarily adenomas, were increased in dosed female mice (3/55; 16/55; 13/55). Follicular cell hyperplasia of the thyroid gland was increased in dosed mice.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental infection of conventional dogs with canine parvovirus.

Four 6-week-old conventional pups were inoculated with a parvovirus (PV) isolated from the feces of a dog with naturally occurring enteritis. Blood for hematologic studies, virus isolation (VI), and antibody titration and feces for VI and negative-contrast electron microscopy were collected on day 0 and daily until necropsy. Beginning at postinoculation day 2, necropsies were done and specimens were collected for immunofluorescence, VI, and light microscopic examination. The PV infection was confirmed by VI, immunofluorescence, electron microscopy, and seroconversion. Clinical illness was not observed in inoculated pups, although mild intestinal lesions similar to those of naturally occurring PV enteritis were found. The failure to elicit severe disease in conventional pups indicates that one or more factors, such as intercurrent enteric or systemic infections, immune status, age, nutrition, virulence of virus, dose of infectious virus, and route of inoculation influence the clinical and pathologic manifestations of PV infection.

Suppurative abomasitis associated with Clostridium septicum infection.

A 1-week-old calf and two 1-month-old lambs from different farms in eastern South Dakota had severe abomasitis characterized by diffuse suppurative inflammation, edema, and emphysema. Clostridium septicum in the abomasal wall of each was identified by immunofluorescence and isolation. The lesions and microbiologic findings established the disease as braxy, a condition infrequently reported in the United States and thought to affect sheep only.

Gender differences in animal bioassays for carcinogenicity.

Animal bioassays for carcinogenicity are essential components of occupational health studies. Animal data that have been collected under controlled experimental conditions provide definitive information about the carcinogenic activities of individual substances or defined mixtures and their relative potencies in the test species. Such information serves as a frame of reference for clinical and epidemiologic studies, pointing to potential adverse health effects and to the types of substances that might produce them. This article alerts the occupational and environmental health communities to 20 substances that produced breast tumors, 13 substances that produced uterine tumors, and 8 substances that produced ovarian tumors in long-term National Toxicology Program animal studies. Each of the substances also produced neoplasms at other body sites. Follow-up studies of molecular measures of exposure and response in people and in animals will reduce the uncertainties of transspecies extrapolations.

Lesions associated with coccidiosis in nursing piglets.

Of 45 piglets with diarrhea, 28 had coccidiosis, with no evidence of concurrent viral infection. Villous atrophy and necrotic enteritis were the characteristic lesions, and were more severe in piglets with combined viral and coccidial infections than with coccidiosis alone. Necrotic enteritis presumably was caused by bacterial invasion of the villous lamina propria at foci denuded of epithelium by coccidia, viruses or both. Consistent lesions associated with coccidia in piglets not infected by other primary enteric pathogens suggest that coccidia are the cause of significant clinical disease in nursing piglets 6 to 15 days old.

Peer review in carcinogenicity bioassays: uses/abuses.

"Truth is the essence of science," and "peer review" is a key element in assuring the "truth". Peer review is particularly important with regard to diagnostic pathology in bioassays used for regulatory purposes and approval of drugs and medical devices for at least 2 reasons: (a) If a study is designed correctly and conducted properly, the bottom-line results rely almost entirely on pathological interpretations, and (b) diagnostic pathology is a subjective science that relies on the training and experience of the pathologist and therefore is subject to individual bias. "Bias" can be introduced during the collection and preparation of pathological materials, use of inconsistent terminology during the pathology evaluation and interpretation of the results. Peer review can help in reducing this bias. However, peer review is also subject to bias by knowledge of treatment groups, selection of inappropriate slides for review, in the type of question asked of the reviewers, and in the selection of the reviewers. When done correctly, pathology peer review can be an effective and important part of a process to assure the results of a study, but when done improperly it can cloud the issue and have a negative impact on the credibility of the study.

Toxicity and carcinogenicity of chronic exposure to tris(2-chloroethyl)phosphate.

Previous short-term studies of tris(2-chloroethyl)phosphate (TRCP), a flame retardant used in industrial and consumer products, demonstrated that repeated administration of 350 mg TRCP/kg body wt by oral gavage resulted in necrosis of pyramidal neurons in the CA1 region of the hippocampus of F344 rats, but not in B6C3F1 mice. The 2-year studies reported here were designed to characterize the chronic toxicity and potential carcinogenicity of TRCP in each sex of F344 rats and B6C3F1 mice. Groups of 60 rats per sex received 0, 44, or 88 mg/kg by oral gavage, once per day, 5 days per week, for up to 103 weeks. Groups of 60 mice per sex received 0, 175, or 350 mg/kg by oral gavage on the same dosing schedule. Each of these groups contained 10 animals which were euthanized at 66 weeks. The principal toxic effects of chronic exposure of rats to TRCP occurred in the brain and kidney. In contrast to the findings in the 16-week studies, a hippocampal lesion was not observed in the brain, although degenerative lesions were widely distributed in the gray and white matter of the brain stem and cerebral cortex of high-dose female and, to a lesser extent, male rats. These findings suggest that the hippocampal necrosis may be dependent upon the size of the individual doses or may have a pathogenesis different from that of the lesions in the brain stem and cerebral cortex. The other primary effect of chronic exposure was a dose-dependent increased incidence of renal tubule hyperplasia and adenoma.(ABSTRACT TRUNCATED AT 250 WORDS)

Occurrence and relevance of chemically induced benign neoplasms in long-term carcinogenicity studies.

Recent carcinogenicity studies conducted and evaluated by the National Toxicology Program/National Institute of Environmental Health Sciences were examined to determine the frequency of chemically increased incidences of neoplasia. Many of the chemicals originally selected for study were chosen because of an a priori suggestion that they might be carcinogens. Of the 143 chemical studies evaluated, usually involving male and female rats and mice, 42 (29%) did not induce any neoplasms, 20 (14%) gave marginal or equivocal neoplastic responses, and 81 (57%) showed positive neoplastic responses in one or more of the 524 species-gender experiments. Of these 81 positive studies, 60 (74%) were considered positive based on malignant neoplasia, 16 (20%) were positive due primarily to benign neoplasia, but had supporting evidence of malignant neoplasia in the same organ/tissue, and 5 (6%) were positive based only on benign neoplasia. These five chemicals are a) allyl isothiocyanate (transitional cell papillomas of the urinary bladder in male rats), b) 2-amino-4-nitrophenol (tubular cell adenomas of the kidney in male rats), c) asbestos intermediate range chrysotile (adenomatous polyps of the large intestine in male rats), d) decabromodiphenyl oxide (neoplastic nodules of the liver in male and female rats), and e) nitrofurazone (fibroadenomas of the mammary gland in female rats and benign mixed tumors and granulosa cell tumors of the ovary in female mice). For all but one of these lesions (mammary gland), the occurrence in historic controls is low. Thus, only 5 of the 143 chemicals studied (3.5%) induced benign neoplasia alone, and those observed benign neoplasms are known to progress to malignancy. Accordingly, we consider chemically induced benign neoplasia to be an important indicator of a chemical's carcinogenic potential in rodents, and believe it should continue to be made an integral part of the overall weight-of-the-evidence evaluation process for identifying potential human health hazards.