Carcinogenicity of mutagens: predictive capability of the Salmonella mutagenesis assay for rodent carcinogenicity

A total of 224 chemicals that have been tested in long-term studies for carcinogenicity in rats and mice by the National Cancer Institute and the National Toxicology Program were tested for mutagenicity in Salmonella typhimurium. Correlations between mutagenicity and carcinogenicity were examined. The influences of chemical structure, rodent species and organ responses, and bacterial strain responses on the carcinogenesis/mutagenesis correlations were also examined. Not all carcinogens induced tumors in both rats and mice. A clear mutagenic or equivocal mutagenic response in Salmonella was predictive for 77% of the carcinogens or equivocal carcinogens, although only 54% of the 149 carcinogens or equivocal carcinogens were mutagens, and 58% of the nonmutagens were carcinogens or equivocal carcinogens. The proportion of mutagens and equivocal mutagens that were not carcinogenic or equivocal was 23%. There was no apparent way to distinguish the mutagenic carcinogens from the mutagenic noncarcinogens by the responses of the specific Salmonella strains. The proportions of different chemical classes in the data base strongly affected the correlations; 40% of the chlorinated carcinogens were mutagens, whereas 75% of the amines and 100% of the nitro-containing carcinogens were mutagens. Because 29% of the chemicals (30% of the carcinogens) were chlorinated, the poor correlation of this class was reflected in the overall correlation. It is concluded that the use of the Salmonella mutagenicity assay is warranted for the identification of carcinogens, but not for noncarcinogens. The proportion of carcinogens detected as mutagens is dependent on the specific classes of chemicals tested and on the rodent species used to define the carcinogens.     

Carcinogenicity studies of fluoxetine hydrochloride in rats and mice.

The antidepressant drug fluoxetine HCl was tested for carcinogenicity in three well designed and controlled studies in Fischer rats and C57BL/6 x C3H F1 mice. The compound was administered to the animals for 24 months at dietary doses of approximately 0, 0.5, 2.0, or 10.0 mg/kg body weight in rats and 1.0, 5.0, or 10.0 mg/kg in mice. The highest dose tested was a maximum tolerated dose for both species as evidenced by clinical signs (rats and mice) and some mortality (mice) referable to central nervous system pharmacological effects, decreased weight gain (rats), and histopathological changes of phospholipidosis (rats) and hepatic fatty change (mice). There was no evidence of an increased incidence of any type of unusual or commonly occurring spontaneous neoplasm in either rats or mice. There were statistically significant decreases in a few commonly occurring neoplasms. The data reported herein provide convincing evidence that fluoxetine is neither a complete carcinogen nor a tumor promoter.     

Carcinogenicity of sulfuric acid in rats and mice

An International Agency for Research on Cancer (IARC) committee recognized aerosol of sulphuric acid as a human carcinogen on the basis of epide-miological studies. No experimental studies on the carcinogenicity, either of sulfuric acid aerosol or of sulfuric acid itself was available. Our aim was to determine whether sulfuric acid is a causal or modifying factor in carcinogenesis, especially in the respiratory tract. We used two species of laboratory animals (both sexes) - 315 Wistar rats and 219 CBAxC57Bl mice in a long term experimental study. The rats were treated with sulfuric acid (maximal tolerated doses, by chronic intratracheal instillations or by gastric intubations) and/or benzo(a)pyrene (by intratracheal instillations). The mice were treated with sulfuric acid (by chronic gastric intubations) and/or urethane (by intraperitoneal injections). We observed the animals throughout their lives and performed gross and microscopic examination of all organs. The results of the first year of study did not provide clear evidence either for sulfuric acid carcinogenicity or for co-carcinogenicity. However, in the second year tumors appeared in those organs where sulfuric acid acted directly. A modifying (stimulating) effect of sulfuric acid on carcinogenesis induced with benzo(a)pyren was observed in rats. Sulfuric acid did not influence lung carcinogenesis induced with urethane in mice.     

Alkaline DNA fragmentation in vivo: borderline or negative results obtained respectively with 7,12-dimethylbenz[a]anthracene and benzo[a]pyrene

Using the in vivo DNA damage alkaline elution assay, a satisfactory correlation with carcinogenicity in the same target organ has been previously shown for a variety of chemical agents. This work was intended to enlarge the exploration of the predictivity of this test. Benzo[a]pyrene (BP) was found negative for damage to liver DNA of mice and rats, and 7,12-dimethylbenz[a]anthracene (DMBA) negative for damage to liver and bone marrow DNA of mice and slightly positive for damage to mammary gland DNA of young female rats. The results were found to be correlated with the extension of DNA arlyation in target organs in similar experimental conditions. From carcinogenicity data reported in the Survey of Compounds Which Have Been Tested for Carcinogenic Activity (vols. 1961-1973) BP and DMBA were both found to be essentially negative as liver carcinogens; however, DMBA was a potent carcinogen in inducing mammary tumors.     

Inter-species comparisons of carcinogenicity.

The carcinogenicity of 250 chemicals in 2 species, usually the rat and the mouse, was obtained from the published literature through 3 independent sources. Of the 250 compounds listed, 38% were non-carcinogenic in both rats and mice, and 44% were carcinogenic in both species. A total of 43 compounds had different results in the two species, 21 (8%) being carcinogenic in mice only, 17 (7%) in rats only and 5 (2%) having differing results from other species. A comparison of the major target organs affected by chemicals carcinogenic in both species revealed that 64% of the chemicals studied produced cancer at the same site. This comparison of carcinogenic activity in 2 species suggests that extrapolation from results in a single-animal study to man may be subject to substantial errors.     

Analysis of DNA adducts in rats exposed to pentachlorophenol

Pentachlorophenol (PCP) is a widely used biocide that has been reported to be hepatocarcinogenic in mice. Its effects in rats are equivocal, but the liver clearly is not a target organ for carcinogenesis. The carcinogenic effects of PCP in mice may relate to reactive oxygen species generated during metabolism. PCP is known to increase the hydroxyl radical-derived DNA lesion, 8-oxodeoxyguanosine (ohdG), in the liver of exposed mice. To investigate whether the generation of oxidative DNA damage and direct DNA adducts may explain the species difference in carcinogenicity, we have analyzed ohdG in hepatic DNA from PCP-exposed rats. Rats were exposed acutely to PCP for 1 or 5 days. Tissues also were obtained from a 27 week interim sacrifice of the 2 year National Toxicology Program carcinogenesis bioassay. We used HPLC with electrochemical array detection for ohdG analysis. Single or 5 day exposure to PCP (up to 120 or 60 mg/kg/day, respectively) did not increase ohdG. Dietary exposure to 1000 p.p.m. PCP (equivalent to 60 mg/kg/day) for 27 weeks induced a 2-fold increase in ohdG (1.8 versus 0.91x10(-6) in controls). In parallel, formation of direct DNA adducts was analyzed by 32P-post-labeling following nuclease P1 adduct enrichment. We detected two major DNA adducts with relative adduct labeling of 0.78x10(7) adducts per total nucleotides. One of these adducts was found to co-migrate with the adduct induced by the metabolite, tetrachloro-1,4-benzoquinone. We observed differences in DNA adduct formation between acute and chronic studies, with acute studies not inducing any detectable amount of DNA adducts. These results indicated that chronic, but not acute exposure to PCP increased ohdG and direct adducts in hepatic DNA. As the same exposure conditions that enhanced ohdG did not produce liver cancer in rats, the generation of reactive oxygen species, oxidative DNA damage and direct DNA adducts is not sufficient for the induction of hepatocarcinogenesis by PCP in the rat.     

Long-term Exposure to the Anti-inflammatory Agent Phenylbutazone Induces Kidney Tumors in Rats and Liver Tumors in Mice

Long-term toxicity and carcinogenicity of phenylbutazone, a nonsteroidal anti-inflammatory drug, were evaluated in F344/N rats and B6C3F1 mice. In 2-year studies, phenylbutazone was given in corn oil by gavage 5 days per week to groups of 50 rats of each sex at doses of 0, 50, or 100 mg/kg body weight, and to groups of 50 mice at doses of 0, 150, or 300 mg/kg body weight. Body weights and survival were similar among groups. Major target organs are kidneys in rats, and liver in mice. Kidney: inflammation, papillary necrosis, and mineralization in both sexes of rats, and hyperplasia and dilatation of the pelvis epithelium, and cysts in female rats. Uncommon tubular cell tumors of the kidney were found in 13 exposed rats: 5 in the 50 mg group and 4 in the 100 mg group of males; 4 in dosed female rats; none in controls. In female rats, dose-related increases in hyperplasia of the pelvis transitional epithelium, and 2 carcinomas were discovered. Urinary bladder: papillomas of the transitional epithelium were seen in 2 low-dose male and in 1 low-dose female rats. Forestomach: ulcers in rats, with acanthosis, hyperkeratosis, and basal cell hyperplasia in female rats; however, no neoplasms were associated with these lesions. Liver: primarily in male mice exposed to. phenylbutazone, hemorrhage, centrilobular cytomegaly and karyomegaly, fatty metamorphosis, cellular degeneration, and coagulative necrosis were seen; clear cell foci were observed in male mice. In summary, under the conditions of these 2-year oral intubation studies, phenylbutazone is associated with renal carcinogenicity in rats, as evidenced by increases in tubular cell neoplasms in both sexes. Evidence of carcinogenicity for male mice was shown by increased incidences and multiplicity of liver tumors. No carcinogenic activity was found for female mice.     

Nasal cavity neoplasia in F344/N rats and (C57BL/6 x C3H)F1 mice inhaling propylene oxide for up to two years

Propylene oxide (CAS: 75-56-9) was studied for potential carcinogenicity and chronic toxicity by inhalation in F344/N rats and (C57BL/6 x C3H)F1 mice. Groups of 50 animals of each sex were exposed to 0, 200, or 400 ppm propylene oxide for 6 hours/day, 5 days/week, for up to 103 weeks. Survival decreased in mice exposed to propylene oxide; the decrease was significant (P less than .005) in mice exposed to 400 ppm. Survival of exposed rats was comparable to that of controls. Mean body weight of rats and mice exposed to 400 ppm propylene oxide decreased, when compared to that of controls, during the 2d year of exposure. Exposure to propylene oxide for up to 2 years induced inflammatory and proliferative responses in nasal cavity of both species. There was clear evidence of carcinogenicity in mice exposed to 400 ppm propylene oxide; 10 of 50 males and 5 of 50 females had hemangiomas or hemangiosarcomas of the nasal submucosa. Papillary adenomas involving the nasal respiratory epithelium and underlying submucosal glands were observed in 3 female rats and 2 male rats exposed to 400 ppm propylene oxide.     

Carcinogenic potency of alkylating agents in rodents and humans.

Alkylating agents are known to produce second tumors in cancer patients treated for their primary cancer. Since therapeutic doses are high and the pharmacokinetics of the drugs are thoroughly studied, these agents provide a unique opportunity to compare intrinsic carcinogenic potency between experimental animals and humans. We have examined the carcinogenicity of melphalan, chlorambucil, and cyclophosphamide in causing leukemia in patients treated for cancer or polycythemia vera and lymphosarcoma in rats and mice. A good correlation among species is observed when the carcinogenic potency is based on the total lifetime exposure to active species derived from these drugs.     

Epidemiological and mechanistic data suggest that 1, 3-butadiene will not be carcinogenic to humans at exposures likely to be encountered in the environment or workplace

1, 3-Butadiene (BD) is a carcinogen in both rats and mice withmice being substantially more sensitive than rats. It is notknown if BD poses a carcinogenic risk for humans. Findings fromexposure assessment studies indicate that potential industrialexposure to BD in monomer, polymer, and end-user industriesis typically <2 p.p.m. Epidemiologic studies of persons occupationallyexposed to BD are inconclusive. In vitro metabolism of BD inrats, mice and human tissues indicate that there are significantquantitative species differences in the metabolic activationof BD to butadiene monoepoxide (BMO) and butadiene diepoxide(BDE) and the detoxication of BMO. Activation/detoxica-tionratios calculated using in vitro kinetic constants reveal thatratios in mice were 12-fold greater than rats and humans. Inrats and mice exposed to BD, concentrations of BMO in bloodand tissues of mice were up to 14-fold higher than in rats andBDE was only detected in mice thereby providing a strong argumentfor why mice are highly sensitive to BD carcinogenicity. Thefact that human tissues do not appear to metabolize BMO to BDEto any significant extent suggest that humans may not be sensitiveto BD carcinogenicity. In mice, BDE is a more potent carcinogenthan BMO. BDE is mutagenic in vitro at the hprt locus in humanTK6 lymphoblasts at concentrations that were 100-fold less thanthe concentration of BMO required to yield a similar mutationfrequency. Importantly, the concentrations of BDE that weregenotoxic in vitro are nearly identical to the concentrationsof BDE measured in blood and tissues of mice exposed to BD byinhalation. BD is genotoxic in mice, but not rats, followinginhalation exposure and this is paralleled by species differencesin observed tumor susceptibility. BD is not genotoxic in occupationally-exposedworkers. The genetic basis for BD carcinogenicity appears tobe primarily through induction of point mutations and deletionevents mediated via the potent genotoxic metabolite, BDE. Thegenotoxic endpoints induced by BDE (e.g., deletion and pointmutations) rather than BMO (e.g., point mutations) likely representthe underlying mechanism responsible for the striking speciesdifferences observed in the genotoxicity and carcinogenicityof BD in mice versus rats. In summary, the preponderance ofevidence which includes both epidemiological and mechanisticdata in mice, rats, and humans strongly suggests that BD willnot be carcinogenic to humans at occupational or environmentalexposures. Any cancer risk assessment for BD should use in vitrohuman tissue metabolic data and in vitro and in vivo rat datafor estimation of human cancer risks.     

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 hd 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.     

Carcinogenicity studies of masheri, a pyrolysed product of tobacco

The carcinogenicity of two commonly used brown and black varietiesof masheri, a pyrolysed tobacco product, was studied by feedingthe masheri through the diet at a 10% level to three differentanimal species of both sexes. In Sprague—Dawley rats,only brown masheri was used, while in Swiss mice and Syriangolden hamsters both varieties were used. In all the three species,forestomach papillomas were induced as a result of masheri treatment.In rats, 37% of animals showed forestomach papillomas whilein mice and hamsters the incidence was 42–47% and 25–43%,respectively. No malignant changes were observed in any of thegroups except 2/23 male hamsters showed forestomach carcinomain the black masheri diet group.     

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

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

Disposition of butadiene monoepoxide and butadiene diepoxide in various tissues of rats and mice following a low-level inhalation exposure to 1,3-butadiene

1,3-Butadiene (BD), a chemical used extensively in the productionof styrene-butadiene rubber, is carcinogenic in Sprague-Dawleyrats and B6C3F₁ mice. Chronic inhalation studies revealed profoundspecies differences in the potency and organ-site specificityof BD carcinogenesis between rats and mice. BD is a potent carcinogenin mice and a weak carcinogen in rats. Previous studies fromour laboratory and others have shown marked differences betweenrats and mice in the metabolism of BD, which may account forspecies differences in carcinogenicity. The purpose of the presentstudy was to examine the production and disposition of two mutagenicBD metabolites, butadiene monoepoxide (BDO) and butadiene diepoxide(BDO₂), in blood and other tissues of rats and mice during andfollowing inhalation exposures to a target concentration of62.5 p.p.m. BD. BDO was increased above background in blood,bone marrow, heart, lung, fat, spleen and thymus tissues ofmice after 2 h and 4 h exposures to BD. In rats, levels of BDOwere increased in blood, fat, spleen and thymus tissues. Noincreases in BDO were observed in rat lungs. BDO₂, the moremutagenic of the two epoxides, was increased in the blood ofrats and mice at 2 and 4 h after initiation of exposure to BD.In mice, BDO₂ was detected in all tissues examined immediatelyfollowing the 4 h exposure. This metabolite was detected inheart, lung, fat, spleen and thymus of rats, but at levels 40-to 160-fold lower than those seen in mice. Immediately afterthe 4 h exposure, blood levels of BDO₂ were 204±15 pmol/gfor mice but were 41-fold lower for rats. In the sensitive mousetarget organs, heart and lungs, levels of BDO₂ exceeded BDOlevels immediately after the exposure. This study shows thatthe levels of BD epoxides are markedly greater in the mouseBD target organs. The high concentrations of BDO₂ in these organssuggest that this compound may be particularly important inBD-induced carcinogenesis. Thus, although BD is oxidativelymetabolized by similar metabolic pathways in rats and mice,the substantial quantitative differences in tissue levels ofmutagenic epoxides between species may be responsible for theincreased sensitivity of mice to BD-induced carcinogenicity.     

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.     

Carcinogenicity of chrysazin in large intestine and liver of mice

The carcinogenicity of chrysazin (1,8-dihydroxy-9,10-anthracenedione) was examined by dietary administration to C3H/HeN mice. All of the effective mice (17) which were given 0.2% chrysazin diet and which survived more than 500 days developed adenomatous hyperplasia with cystic glands of the cecum. Similar lesions were also seen in the colon of mice in this group. These intestinal lesions were not obtained in any effective mouse (19) of the control group. The incidence of hepatocellular carcinoma of mice given chrysazin (4/17) was significantly higher than that of the controls (0/19). These results indicate that chrysazin is carcinogenic in mice as well as in rats. Some mechanistic aspects of the causation of these intestinal lesions and liver neoplasms are also discussed.     

Neoplasms observed in untreated and corn oil gavage control groups of F344/N rats and (C57BL/6N X C3H/HeN)F1 (B6C3F1) mice

Control data on F344/N rats and (C57BL/6N X C3H/HeN)F1 (B6C3F1) mammary tumor virus-free mice from the National Toxicology Program (NTP) were examined to determine if animals receiving corn oil by gavage showed tumor incidences that differed from those of untreated control animals. Analyses of these data were adjusted for interlaboratory variability, time-related trends, and supplier effects. Two biologically significant effects were found: Male F344/N control rats receiving corn oil by gavage showed a higher (P less than .05) incidence of pancreatic acinar cell adenoma and a lower (P less than .001) incidence of leukemia (primarily mononuclear cell leukemia) than did the corresponding untreated controls. The increased incidences of pancreatic acinar cell adenoma seen in male rats administered corn oil by gavage were associated with elevated body weights observed in these animals relative to untreated controls. Female F344 rats and male and female B6C3F1 mice showed little or no evidence of a difference in tumor incidence between corn oil gavage-treated and untreated controls. A review of nearly 300 carcinogenesis studies done by the National Cancer Institute (NCI) and the NTP revealed that there were no corn oil gavage studies in which increased incidences of pancreatic acinar cell tumors or leukemia in male F344/N rats were the sole evidence of the carcinogenicity of a test chemical. Thus use of corn oil appears to have little impact on the interpretation of NCI-NTP carcinogenicity studies.     

Thyroid tumours in rats and hepatomas in mice after griseofulvin treatment.

Griseofulvin, an antibiotic used to treat dermatophystosis, was tested for carcinogenicity in mice, rats and hamsters. Three groups of mice and rats were given the drug in powdered diet in alternating 5-week periods for life, at dose levels of 3.0%, 1.5% and 0.3% (mice) and 2.0%, 1.0% and 0.2% (rats). A group of mice and 3 groups of hamsters received continuous daily treatment for life with griseofulvin at 3.0%, 1.5%, 0.3% and 0.1% dose levels respectively. A significant incidence of hepatic tumours was observed at the 2 higher treatment levels in mice. Also, statistically significant rates (P less than or equal to 0.001 and/or P less than or equal to 0.020) of thyroid tumours, indicating a dose-response, were recorded in male rats at the 2.0%, 1.0%, and 0.2% dose levels, and in females at the 2.0% and 1.0% dose levels. Hamsters did not develop neoplasms in response to treatment at any level.     

Induction of Renal Cell Tumors in Rats and Mice, and Enhancement of Hepatocellular Tumor Development in Mice after Long-term Hydroquinone Treatment

Hydroquinone (HQ) was administered to F344 rats and B6C3F₁ mice of both sexes at a level of 0.8% in the diet for two years. This treatment induced renal tubular hyperplasia as well as adenomas, predominantly in males of both species, and was associated with chronic nephropathy in rats. In addition, the occurrence of epithelial hyperplasia of the renal papilla was increased in male rats. Foci of cellular alteration of the liver were significantly reduced in number by HQ in rats, but in contrast, were increased in mice, where development of hepatocellular adenoma was also enhanced in males. The incidence of squamous cell hyperplasia of the forestomach epithelium was significantly higher in mice of both sexes given HQ than in the controls, but no corresponding increase in tumor development was observed. The present study strongly indicates potential renal carcinogenicity of HQ in male rats and hepatocarcinogenicity in male mice. Thus, it is possible that HQ, which is present in the human environment, may play a role in cancer development in man.     

Species differences in sodium o-phenylphenate induction of urinary bladder lesions

The effects of sodium o-phenylphenate (Na-OPP) treatment on urinary bladder epithelium were examined in male F344 rats, B6C3F1 mice, Syrian golden hamsters and Hartley guinea pigs. Na-OPP was incorporated into diet at a dose of 2% and administered for 4, 8, 12, 24, 36 or 48 weeks. Simple and papillary or nodular (PN) hyperplasias were evident on light microscopy and pleomorphic microvilli demonstrated by scanning electron microscopy were only observed in rats, the lesions becoming more advanced with continued chemical feeding. In mice, hamsters and guinea pigs, proliferative lesions relating to Na-OPP administration were not observed. No significant differences in urinary pH, osmolality or crystal formation were apparent between the various animal species. Since carcinogenicity has been demonstrated for Na-OPP in rats but not in mice, the present findings suggest that Na-OPP might not exert urinary bladder carcinogenic potential in hamsters and guinea pigs.