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Chloroprene (2-chloro-1,3-butadiene) is a high production chemical used almost exclusively in the production of polychloroprene (neoprene) elastomer, Because of its structural similarity to 1,3-butadiene, a trans-species carcinogen, inhalation studies were performed with chloroprene to evaluate its carcinogenic potential in rats and mice. Groups of 50 male and female F344/N rats and 50 male and female B6C3F1 mice were exposed to 0, 12.8, 32 or 80 p.p.m. chloroprene (6 h/day, 5 days/week) for 2 years. Under these conditions, chloroprene was carcinogenic to the oral cavity, thyroid gland, lung, kidney and mammary gland of rats, and to the lung, circulatory system (hemiangiomas, and hemangiosarcomas), Harderian gland, kidney, forestomach, liver, mammary gland, skin, mesentery and Zymbal's gland of mice. Survival adjusted tumor rates in mice were fit to a Weibull model for estimation of the shape of the dose-response curves, estimation of ED10 values (the estimated exposure concentration associated with an increased cancer risk of 10%) and comparison of these parameters with those for 1,3-butadiene. Butadiene has been identified as a potent carcinogen in mice and has been associated with increased risk of lymphatic and hematopoietic cancer in exposed workers. Shape parameter values for most of the neoplastic effects of chloroprene and 1,3-butadiene were consistent with linear or supralinear responses in the area near the lowest tested exposures. The most potent carcinogenic effect of 1,3-butadiene was the induction of lung neoplasms in female mice, which had an ED10 value of 0.3 p.p.m. Since the ED10 value for that same response in chloroprene exposed mice was also 0.3 p.p.m., we conclude that the carcinogenic potency of chloroprene in mice is similar to that of 1,3-butadiene. Cancer potency of chloroprene is greater in the mouse lung than in the rat lung, but greater in the rat kidney than in the mouse kidney and nearly equivalent in the mammary gland of each species. [ABSTRACT FROM PUBLISHER]

C60 fullerene (C60), or buckminsterfullerene, is a spherical arrangement of 60 carbon atoms, having a diameter of approximately 1 nm, and is produced naturally as a by-product of combustion. Due to its small size, C60 has attracted much attention for use in a variety of applications; however, insufficient information is available regarding its toxicological effects. The effects on respiratory toxicity and immunotoxicity of C60 aggregates (50 nm [nano-C60] and 1 μm [micro-C60] diameter) were examined in B6C3F1/N mice and Wistar Han rats after nose-only inhalation for 13 weeks. Exposure concentrations were selected to allow for data evaluations using both mass-based and particle surface area-based exposure metrics. Nano-C60 exposure levels selected were 0.5 and 2 mg/m 3 (0.033 and 0.112 m 2 /m 3 ), while micro-C60 exposures were 2, 15 and 30 mg/m 3 (0.011, 0.084 and 0.167 m 2 /m 3 ). There were no systemic effects on innate, cell-mediated, or humoral immune function. Pulmonary inflammatory responses (histiocytic infiltration, macrophage pigmentation, chronic inflammation) were concentration-dependent and corresponded to increases in monocyte chemoattractant protein (MCP)-1 (rats) and macrophage inflammatory protein (MIP)-1α (mice) in bronchoalveolar lavage (BAL) fluid. Lung overload may have contributed to the pulmonary inflammatory responses observed following nano-C60 exposure at 2 mg/m 3 and micro-C60 exposure at 30 mg/m 3 . Phenotype shifts in cells recovered from the BAL were also observed in all C60-exposed rats, regardless of the level of exposure. Overall, more severe pulmonary effects were observed for nano-C60 than for micro-C60 for mass-based exposure comparisons. However, for surface-area-based exposures, more severe pulmonary effects were observed for micro-C60 than for nano-C60, highlighting the importance of dosimetry when evaluating toxicity between nano- and microparticles., Competing Interests: Declaration of Interest This work was supported [in part] by the Intramural Research Program of the National Institutes of Health (NIH), National Institute of Environmental Health Sciences (NIEHS) and by NTP Contracts N01-ES-55538 and N01-ES-5553.

C60 fullerenes (C60) are spherical structures consisting of 60 carbon atoms that are generated via combustion from both natural and anthropogenic sources. C60 are also synthesized intentionally for industrial applications. Individual C60 structures have an approximate diameter of 1nm; however, C60 readily forms aggregates and typically exist as larger particles that range from nanometers to micrometers in diameter. In this report, lung and extrapulmonary tissue deposition and lung clearance of C60 nanoparticles (nano-C60, 50nm) and microparticles (micro-C60, 1μm) were examined in Wistar Han rats and B6C3F1/N mice after nose-only inhalation for 90 days. Exposure concentrations were 0.5 and 2mg/m(3) (nano-C60) and 2, 15, and 30mg/m(3) (micro-C60). For both C60 particle sizes, the C60 lung burden increased proportionally to exposure concentration. The C60 lung burden was greater in both species at all time points following exposure to nano-C60 particle exposure compared to micro-C60 exposure at the common exposure concentration 2mg/m(3). The calculated C60 particle lung retention half-times were similar for both nano-C60 and micro-C60 exposure at 2mg/m(3) in male mice (15-16 days). In contrast, in male rats, the half-time of C60 particles following nano-C60 exposure (61 days) was roughly twice as long as the half-time following micro-C60 exposure (27 days) at the same exposure concentration (2mg/m(3)) and was similar to the clearance following micro-C60 exposure at higher exposure concentrations (15 and 30mg/m(3)). C60 was detected in bronchial lymph nodes but the burden was not quantified due to the high variability in the data. C60 concentrations were below the experimental limit of quantitation (ELOQ) in liver, spleen, blood, brain and kidney tissues. These tissue burden data provide information for comparison between nanometer and micrometer sized C60 particle exposure and will aid in the interpretation of toxicity data., (Published by Elsevier Ireland Ltd.)

Occupational exposure to cobalt is of widespread concern due to its use in a variety of industrial processes and the occurrence of occupational disease. Due to the lack of toxicity and carcinogenicity data following exposure to cobalt, and questions regarding bioavailability following exposure to different forms of cobalt, the NTP conducted two chronic inhalation exposure studies in rats and mice, one on soluble cobalt sulfate heptahydrate, and a more recent study on insoluble cobalt metal. Herein, we compare and contrast the toxicity profiles following whole-body inhalation exposures to these two forms of cobalt. In general, both forms were genotoxic in the Salmonella T98 strain in the absence of effects on micronuclei. The major sites of toxicity and carcinogenicity in both chronic inhalation studies were the respiratory tract in rats and mice, and the adrenal gland in rats. In addition, there were distinct sites of toxicity and carcinogenicity noted following exposure to cobalt metal. In rats, carcinogenicity was observed in the blood, and pancreas, and toxicity was observed in the testes of rats and mice. Taken together, these findings suggest that both forms of cobalt, soluble and insoluble, appear to be multi-site rodent carcinogens following inhalation exposure., (Published by Elsevier Ireland Ltd.)

Two-year 1-bromopropane (1-BP) inhalation studies were conducted because of the potential for widespread exposure, the lack of chronic toxicity and carcinogenicity data, and the known carcinogenicity of structurally related compounds. Male and female F344/N rats and B6C3F1/N mice were exposed by inhalation to 0, 62.5 (mice only), 125, 250, or 500 (rats only) ppm 1-BP for 6 hr/day, 5 days/week for 105 weeks. Exposure of male and female rats to 1-BP resulted in significantly increased incidences of adenomas of the large intestine and skin neoplasms. In male rats, the incidence of malignant mesothelioma of the epididymis was statistically significantly increased at 500 ppm, but the biological significance of this common lesion is unclear. Incidences of pancreatic islet adenoma in male rats were significantly increased at all concentrations relative to concurrent controls but were within the historical control range for inhalation studies. There was no evidence of carcinogenic activity of 1-BP in male B6C3F1 mice; however, significantly increased incidences of alveolar/bronchiolar neoplasms of the lung were present in female mice. Exposure to 1-BP also resulted in increased incidences of nonneoplastic lesions in the nose of rats and mice, the larynx of rats and male mice, the trachea of female rats and male and female mice, and the lungs of mice. Inflammatory lesions with Splendore Hoeppli (S-H) material were present primarily in the nose and skin of exposed male and female rats, indicating that 1-BP caused immunosuppression.

Methyl isobutyl ketone (MIBK) is primarily used as a denaturant for rubbing alcohol, as a solvent and in the manufacture of methyl amyl alcohol. Inhalation of vapors is the most likely route of exposure in the work place. In order to evaluate the potential of MIBK to induce toxic and carcinogenic effects following chronic exposure, groups of 50 male and 50 female F344/N rats and B6C3F1 mice were exposed to MIBK at concentrations of 0, 450, 900, or 1800ppm by inhalation, 6h/day, 5 days per week for 2 years. Survival was decreased in male rats at 1800ppm. Body weight gains were decreased in male rats at 900 and 1800ppm and in female mice at 1800ppm. The primary targets of MIBK toxicity and carcinogenicity were the kidney in rats and the liver in mice. In male rats, there was increased mineralization of the renal papilla at all exposure concentrations. The incidence of chronic progressive nephropathy (CPN) was increased at 1800ppm and the severity was increased in all exposed groups. There were also increases in renal tubule hyperplasia at all exposure concentrations, and in adenoma and adenoma or carcinoma (combined) at 1800ppm; these lesions are thought to represent a continuum in the progression of proliferative lesions in renal tubule epithelium. These increases may have resulted from the increased severity of CPN, either through alpha2micro-globulin-dependent or -independent mechanisms. An increase in mononuclear cell leukemia at 1800ppm was an uncertain finding. Adrenal medulla hyperplasia was increased at 1800ppm, and there was a positive trend for increases in benign or malignant pheochromocytomas (combined). In female rats, there were increases in the incidence of CPN in all exposure concentrations and in the severity at 1800ppm, indicating that CPN was increased by mechanisms in addition to those related to alpha2micro-globulin. There were renal mesenchymal tumors, which have not been observed in historical control animals, in two female rats at 1800ppm. The relationship of these tumors to exposure to MIBK was uncertain. Hepatocellular adenomas, and adenoma or carcinoma (combined) were increased in male and female mice exposed to 1800ppm. There were also treatment-related increases in multiple adenomas in both sexes.

Dibromoacetic acid (DBA) is a water disinfection byproduct formed by the reaction of chlorine oxidizing compounds with natural organic matter in water containing bromide. Male and female F344/N rats and B6C3F(1) mice were exposed to DBA in drinking water for 2 weeks (N=5), 3 months (N=10), or 2 years (N=50). Concentrations of DBA in drinking water were 0, 125, 250, 500, 1000, and 2000mg/L in the 2-week and 3-month studies, and 0, 50, 500, and 1000mg/L in the 2-year studies. Toxic effects of DBA in the prechronic studies were detected in the liver (hepatocellular cytoplasmic vacuolization in rats and mice) and testes (delayed spermiation and atypical residual bodies in male rats and mice, and atrophy of the germinal epithelium in rats). In the 2-year studies, neoplasms were induced at multiple sites in rats and mice exposed to DBA; these included mononuclear cell leukemia and abdominal cavity mesothliomas in rats, and neoplasms of the liver (hepatocellular adenoma or carcinoma and hepatoblastoma) and lung (alveolar adenoma or carcinoma) in mice. The increase in incidence of hepatocellular neoplasms in male mice was significant even at the lowest exposure concentration of 50mg/L, which is equivalent to an average daily dose of approximately 4mg/kg. These studies provide critical information for future re-evaluations of health-based drinking water standards for haloacetic acids.

Metabolism studies are crucial for data interpretation from rodent toxicity and carcinogenicity studies. Metabolism studies are usually conducted in 6 to 8 week old rodents. Long-term studies often continue beyond 100 weeks of age. The potential for age-related changes in transcript levels of genes encoding for enzymes associated with metabolism was evaluated in the liver of male F344/N rats at 32, 58, and 84 weeks of age. Differential expression was found between the young and old rats for genes whose products are involved in both phase I and phase II metabolic pathways. Thirteen cytochrome P450 genes from CYP families 1-3 showed alterations in expression in the older rats. A marked age-related decrease in expression was found for 4 members of the Cyp3a family that are critical for drug metabolism in the rat. Immunohistochemical results confirmed a significant decrease in Cyp3a2 and Cyp2c11 protein levels with age. This indicates that the metabolic capacity of male rats changes throughout a long-term study. Conducting multiple hepatic microarray analyses during the conduct of a long-term study can provide a global view of potential metabolic changes that might occur. Alterations that are considered crucial to the interpretation of long-term study results could then be confirmed by subsequent metabolic studies.

Chromium picolinate monohydrate (CPM) is a synthetic compound heavily marketed to consumers in the United States for use as a dietary supplement for muscle building and weight loss. The National Toxicology Program (NTP) tested the toxicity of this compound based on the potential for widespread consumer exposure and lack of information about its toxicity. Groups of 10 male and 10 female F344/N rats and B6C3F(1) mice were exposed to 0, 80, 240, 2000, 10,000, or 50,000 ppm CPM in feed for 13 weeks. CPM administration produced no effect on body weight gain or survival of rats or mice. Organ weights and organ/body weight ratios in exposed animals were generally unaffected by CPM. No compound-related changes in hematology and clinical chemistry parameters were observed. There were no histopathological lesions attributed to CPM in rats or mice.

Propylene glycol mono-t-butyl ether (PGMBE) is used as a solvent in a variety of commercial applications. Male and female F344/N rats and B6C3F(1) mice were exposed to PGMBE by whole-body inhalation for 2 or 14 weeks (0, 75, 150, 300, 600, or 1200 ppm) or 2 years (0, 75, 300, or 1200 ppm); male NBR rats were exposed for 2 weeks. The kidney and the liver were targets of PGMBE toxicity in rats. Renal lesions suggestive of alpha(2u)-globulin nephropathy were observed in male F344/N, in the 2 and 14-week studies, no kidney lesions were seen in NBR rats. In the 2-year study, male rats displayed exposure-related nonneoplastic lesions in the kidney, and may have shown marginal increases in tubular neoplasms. In the liver, the incidences of hepatocellular adenomas occurred with a positive trend in male rats, and may have been related to PGMBE exposure. In mice of both sexes, the major target of PGMBE toxicity was the liver. In the 2-week study, liver weights and in the 14-week study, liver weights and the incidences of centrilobular hypertrophy were increased. In the 2-year study, the incidences of exposure-related hepatocellular adenoma, adenoma or carcinoma combined, and hepatoblastoma occurred with a positive trend, and were significantly increased in 1200 ppm groups. In summary, exposure to PGMBE resulted in nonneoplastic lesions of the kidney characteristic of alpha(2u)-globulin nephropathy, and may have increased renal tubular neoplasms in male rats. Exposure to PGMBE also produced increases in hepatic tumors in male and female mice.

Female F344 rats and B6C3F1 mice were exposed to vanadium pentoxide (V2O5) at concentrations of 0, 0.5, 1, or 2 mg/m3 (rats) and 0, 1, 2, or 4 mg/m3 (mice) for 6 h/day, 5 days/week (for up to 18 months), by whole-body inhalation. Lung weights and lung burdens of vanadium were determined for exposed animals after 1, 5, and 12 days and after 1, 2, 6, 12, and 18 months of V2O5 exposure. Blood vanadium concentrations were determined at 1, 2, 6, 12, and 18 months for all animals including controls. A model that assumed a first-order deposition rate and a first-order elimination rate for vanadium was employed to fit the lung burden data. Comparisons between exposed groups indicated a progressive increase in lung weight with exposure concentration and time on exposure for both species. The vanadium lung burdens appeared to reach steady state in the lowest exposure groups (0.5 and 1 mg/m3 for rats and mice, respectively) but showed a decline in the higher exposure groups. This deposition pattern was similar between rats and mice but the maximum lung burdens were observed at different times (1 or 2 months in mice vs. 6 months in rats). The vanadium deposition rate decreased faster in mice, while the elimination half-lives of vanadium lung burdens were about six- to nine-fold shorter in mice than in rats at 1 and 2 mg/m3. Thus, the retention of vanadium in the lungs at 18 months was lower in mice (approximately 2% retained) compared with rats (13-15% retained) at the common exposure concentrations of 1 and 2 mg/m3. The lung burden data were approximately proportional to the exposure concentration in both species, likely due to concomitant decreases in deposition and elimination to a similar extent with increasing exposure. The area under the lung burden versus time curves and the area under the blood concentration (control-normalized) versus time curves were also proportional to exposure concentration. The progression of pathological changes in the lung with exposure and time is thought to affect the pattern and/or extent of vanadium deposition in the lungs following repeated exposures to V2O5.

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).

Decalin (decahydronaphthalene) is an industrial solvent known to cause alpha2u-globulin nephropathy in male rats. Studies were conducted using decalin (mixture of cis and trans isomers) to (1) characterize systemic elimination of decalin in rats and mice and (2) evaluate disposition of decalin, its metabolites, and kidney alpha2u-globulin in young and old rats of both sexes following a single 6-h whole-body inhalation exposure at up to 400 ppm decalin. Additionally, a separate group of young male F344/N rats were administered either cis- or trans-decalin iv at doses up to 20 mg/kg to assess disposition of each isomer, its metabolites, and kidney alpha2u-globulin. Decalin was eliminated from blood in a dose-dependent manner, regardless of sex, age, or species. C0 and AUC infinity increased supra-proportionally with exposure concentration. Mice were more efficient in eliminating decalin than rats at lower exposure concentrations, but nonlinear elimination kinetics were more noticeable at 400 ppm. Sex differences in blood decalin elimination were observed in rats; females had a consistently higher AUC infinity at all exposure concentrations. There was a dose-dependent increase in kidney decalin, decalone, and alpha2u-globulin in male rats exposed to decalin. Kidney alpha2u-globulin and decalone concentrations in old male rats were substantially lower than those in young males, but were similar to those observed in all (young and old) females. Compared to old males and all females, young male rats had significantly lower urinary decalol concentrations, but higher kidney decalin, decalone, and alpha2u-globulin concentrations. Administration of decalin to male rats as either the cis or trans isomer revealed that more cis -decalone is produced per unit dose as compared to trans-decalone, and that more trans-decalin accumulated in the kidney (as alpha2u-globulin-ligand complexes) compared to cis-decalin. These patterns of isomer-specific metabolism were also reflected in the cis/trans ratios of decalin in blood, as well as urinary decalol metabolites. The ratio of alpha2u-globulin to the total amount of decalin plus decalone measured in the male rat kidney was approximately 1.0. Therefore, alpha2u-globulin was a key factor in the accumulation of decalin and decalone in kidneys of young male rats, decalin and decalone were practically absent in all females and in old males.

Decalin (decahydronaphthalene) is a widely used industrial solvent known to cause male rat-specific alpha2u-globulin nephropathy. In this project, 13-week and two-year inhalation studies of decalin were conducted consecutively in both sexes of F344/N rats. The key objectives were to (1) characterize the 13-week toxicity of decalin in rats, with an emphasis on nephropathy in males; (2) compare the kidney concentrations of decalin, 2-decalone, and alpha2u-globulin in males over 2 to 13 weeks of decalin exposure; and (3) correlate male rat nephropathy observed in the 13-week study with renal carcinogenicity in the two-year study. F344 rats (M/F) were exposed via whole-body inhalation to 0, 25, 50, 100, 200, or 400 ppm decalin for 13 weeks. Urine was collected at weeks 2 and 6 for creatinine and decalol analyses and at week 12 for clinical urinalysis. Right kidneys were collected from male rats at weeks 2 and 6 and from both sexes at week 13, homogenates were prepared using the whole kidney, and these homogenates were analyzed for alpha2u-globulin, decalin, and 2-decalone. Left kidneys were evaluated for histopathology and cell proliferation utilizing a proliferating cell nuclear antigen technique and counting proximal renal tubular epithelial cells to determine cell labeling indices. Necropsies and histopathologic evaluations were performed at week 13. Decalin exposure caused increases in kidney weight, urinalysis parameters (protein, AST, LDH), kidney alpha2u-globulin concentration, and proximal convoluted renal tubular cell proliferation in males. These changes were accompanied by microscopic lesions (accumulation of hyaline droplets in cortical tubules, regeneration of proximal tubular epithelium, and granular casts in medullary tubules) clearly linked to alpha2u-globulin nephropathy. Both decalin and 2-decalone were related to increased alpha2u-globulin in male kidneys. Kidney concentrations of decalin, 2-decalone, and alpha2u-globulin in exposed females were negligible, while females excreted greater amounts of decalol metabolites in urine than males at weeks 2 and 6. There were no exposure-related microscopic lesions in females. For chronic exposure, F344 rats were exposed via whole-body inhalation to 0, 25, 50 (males only), 100, or 400 ppm decalin for two years. Chronic exposure induced a spectrum of nonneoplastic and neoplastic lesions in the renal cortex of males, ranging from regenerative lesions of chronic nephropathy to tubular carcinomas. Incidences of renal tubular adenoma, tubular carcinoma, combined tubular adenomas and carcinomas, cortical tubular hyperplasia, hyaline droplet accumulation, hyperplasia of pelvic epithelium, and mineralization in renal papilla were increased in exposed males compared to controls. There was a clear increase in the mean severity of chronic nephropathy in decalin-exposed males. It was concluded that the carcinogenic effect on the renal cortical epithelium of male rats exposed to decalin was related to increased turnover of this epithelium, resulting from the cytotoxic effects of alpha2u-globulin accumulation in the renal cortical tubular cell cytoplasm.

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.

Female Fischer 344 (F344)/N rats (10 per exposure group) were exposed to 2-butoxyethanol (BE) vapors (0, 31, 62.5, 125, 250, or 500 ppm 6 h/d, 5 d/wk, for 13 weeks) to characterize its prechronic toxicity. Dental lesions consisting of bilateral multifocal dental pulp thrombosis, pulp infarction, and odontoblast infarction were noted in the maxillary incisors of 3 of 4 rats from the 500-ppm group that were sacrificed when moribund during the first week of exposure. In addition, 1 rat from the 500-ppm group that was sacrificed on day 32 had similar unilateral incisor lesions but with additional findings consistent with a unilateral maxillary incisor fracture. In contrast, rats sacrificed after 13 weeks of exposure lacked dental lesions. In conclusion, BE has the potential to cause pulp thrombosis and odontoblast infarction in female rats. The apparent variability in response to BE noted in moribund sacrificed vs terminally sacrificed rats was attributed to development of tolerance to BE-induced hemolysis and subsequent incisor regeneration.

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.

2-Butoxyethanol (2BE) is used extensively in the production of cleaning agents and as a general solvent. It is primarily metabolized in the liver to 2-butoxyacetic acid (2BAA), which is excreted in urine. The objective of this study was to develop a physiologically based pharmacokinetic (PBPK) model describing the toxicokinetic behavior of 2BE and 2BAA in different species following repeated, long-term exposures. The PBPK model was first developed for short-term 2BE exposure to male rats. Allometric scaling was employed to estimate physiological and biochemical model parameters based on body weight. To accommodate differences in 2BE toxicokinetics in female rats, a higher Vmax for 2BE metabolism to 2BAA, higher plasma protein binding sites for 2BAA, and lower Vmax for 2BAA excretion through the kidney were incorporated into the model. For mice, a higher Vmax for 2BE metabolism to 2BAA for both sexes and higher plasma protein binding sites for 2BAA for female mice were also incorporated into the model. Subsequently, the model was expanded to simulate 2BE and 2BAA toxicokinetics for long-term, repeated exposures by incorporating time-dependent changes in model parameters. To reflect physiological/biochemical changes in animals during a chronic exposure, parameters for cardiac output, body composition, metabolic capacity, protein binding, or capacity of renal excretion were adjusted over time depending on species and sex. Sensitivity analysis was performed to better understand how sensitive model responses were to uncertainties in input parameters. The resulting PBPK model was used to simulate toxicokinetic data acquired during a 2-year inhalation toxicity and carcinogenicity study in male and female F344/N rats and B6C3F1 mice., (Copyright 1998 Academic Press.)

2-Butoxyethanol (2BE) is used extensively in the production of cleaning agents and solvents. It is primarily metabolized in the liver to 2-butoxyacetic acid (2BAA), which is believed to be responsible for 2BE toxicities associated with hemolysis of red blood cells. The objective of the study was to characterize the systemic disposition of 2BE and 2BAA in rats and mice during 2-year 2BE inhalation toxicity studies. Male and female F344 rats and B6C3F1 mice (6-7 weeks old) were exposed to target 2BE concentrations of 0, 31.2, 62.5, or 125 ppm (rats), or 0, 62.5, 125, or 250 ppm (mice), by whole-body inhalation for 6 h/day, 5 days/week for up to 18 months. Postexposure blood samples were collected after 1 day, 2 weeks, and 3, 6, 12, and 18 months of exposure. Postexposure 16-h urine samples were collected after 2 weeks and 3, 6, 12, and 18 months of exposure. A separate set of mice was kept in the control chamber and exposed to 2BE for 3 weeks when they were approximately 19 months old. Postexposure blood samples were collected after 1 day and 3 weeks of exposure and 16-h urine samples were collected after 2 weeks of exposure from these aged mice. Blood samples were analyzed for both 2BE and 2BAA and urine samples were analyzed for 2BAA using GC/MS, and their kinetic parameters were estimated through the curve-fitting method using SAS. Systemically absorbed 2BE was rapidly cleared from blood (t1/2-RAT < 10 min; t1/2-MOUSE < 5 min after the 1-day exposure) independent of exposure concentration. Proportional increases in AUC2BE relative to increases in exposure concentration indicated linear 2BE kinetics. In contrast, the rate of 2BAA elimination from blood decreased as the exposure concentration increased. Nonproportional increases in AUC2BAA also indicated that 2BAA is eliminated following dose-dependent, nonlinear kinetics. Overall, mice eliminated both 2BE and 2BAA from blood faster than rats. Sex-related differences in 2BAA elimination were most significant with rats, in that females were less efficient in clearing 2BAA from the blood. Differences in renal excretion of 2BAA are possibly responsible for the sex-related difference in the 2BAA blood profiles in rats. As exposure continued, the rates of elimination for both 2BE and 2BAA decreased in both species, resulting in longer residence times in the blood. When 19-month-old naive mice were exposed to 125 ppm, 2BE was rapidly cleared from the systemic circulation, exhibiting clearance profiles similar to young mice. However, old mice eliminated 2BAA from blood > 10 times slower than young mice after 1-day of exposure. This delayed elimination of 2BAA in old mice was less obvious after 3 weeks of exposure, suggesting that there might be other factors in addition to the age of animals that could influence the apparent difference in 2BAA kinetics between old and young mice. It was concluded that the elimination kinetics of 2BE and 2BAA following repeated 2BE exposure appear to be dependent on species, sex, age, time of exposure, as well as the exposure concentration., (Copyright 1998 Academic Press.)

Inhalation studies of molybdenum trioxide (MoO3) were conducted because of its wide use in industry, human exposure, and lack of data on carcinogenicity. Groups of 50 male and 50 female F344/N rats and B6C3F1 mice were exposed to MoO3 by inhalation at 0, 10, 30, or 100 mg/m3, 6 h/day, 5 days/week, for 2 years. In both rats and mice, survival and mean body weights of exposed groups of males and females were similar to those of their respective controls. There were significant exposure-dependent increases in blood molybdenum concentration in exposed rats and mice. There were no toxicological differences in bone density or curvature between exposed animals and their respective controls. In rats, dose-dependent increases in incidence of hyaline degeneration in the nasal olfactory epithelium and squamous metaplasia of the epithelium lining the base of the epiglottis were observed. The incidence of alveolar/bronchiolar adenoma or carcinoma (combined) was marginally increased in males but not in females compared with controls. In mice, the incidences of squamous metaplasia of the epithelium lining the base of the epiglottis, hyperplasia of the laryngeal epithelium, and metaplasia of the alveolar epithelium were significantly increased in all exposed males and females compared with controls. The incidence of alveolar/bronchiolar adenoma or carcinoma (combined) in exposed groups of males and females was significantly greater than that in the control groups.

Tetrafluoroethylene (TFE) was evaluated for carcinogenicity in inhalation studies because of its high use in the production of Teflon. There was clear evidence of hepatocarcinogenic activity in B6C3F1 mice after 2 yr of TFE exposure. The present study was designed to characterize the mutation profiles of H- and K-ras oncogenes in liver neoplasms in mice after exposure to 0, 312, 625, or 1,250 ppm TFE. ras mutations were identified by restriction fragment length polymorphism, single-stranded conformation polymorphism analysis, and direct sequencing of polymerase chain reaction amplified-DNA isolated from frozen or paraffin-embedded liver neoplasms. A low frequency (15%, 9/59) of H-ras codon 61 mutations was detected in hepatocellular neoplasms when compared with the higher frequency (59% of this study and 56% of historical data) in spontaneously occurring liver neoplasms. There was no difference in the mutation frequency or spectrum among exposure groups or between benign and malignant hepatocellular neoplasms. K-ras mutations at codons 12, 13, and 61 and H-ras mutations at codon 117 were not detected in hepatocellular neoplasms. These data suggest that TFE-induced hepatocellular neoplasms are developed by pathways that are mostly independent of ras mutations. The ras mutation frequency and spectrum were similar to those of the structurally related chemical tetrachloroethylene.

Tetrahydrofuran (THF) is a widely used industrial solvent and was selected for carcinogenesis studies by the National Toxicology Program (NTP) because of its potential for widespread occupational exposure in humans and a lack of information on animal toxicity and carcinogenicity. Groups of 50 male and 50 female F344/N rats and B6C3F1 mice were exposed to 0, 200, 600, or 1800 ppm THF by inhalation, 6 h per day, 5 days per week, for 105 weeks. Survival and mean body weights of male and female rats exposed to THF were comparable to that of the controls. No clinical findings or nonneoplastic lesions related to THF exposure were observed in male or female rats. The incidences of renal tubule epithelial adenoma or carcinoma (combined) in exposed male rats occurred with a positive trend, and in males exposed to 600 and 1800 ppm exceeded the historical range for controls in 2-year NTP inhalation studies. There were no other neoplastic lesions related to THF exposure observed in male or female rats. After week 36, the survival of male mice exposed to 1800 ppm was significantly lower than that of the controls. Mean body weights of male and female mice exposed to THF were similar to those of the controls throughout the study. Male mice exposed to 1800 ppm were observed in a state of narcosis during and up to 1 h after the exposure periods. Nonneoplastic lesions related to THF exposure were not observed in male or female mice. The neoplastic lesions related to THF exposure were seen in female mice only. In female mice exposed to 1800 ppm, the incidences of hepatocellular neoplasms were significantly greater than those in the controls. In conclusion, there was some evidence of carcinogenic activity of THF in male F344/N rats due to increased incidences of adenoma or carcinoma (combined) of the kidney at the 600 and 1800 ppm exposure levels. There was clear evidence of carcinogenic activity in female B6C3F1 mice based on increased incidences of hepatocellular neoplasms at the 1800 ppm exposure level. THF was not carcinogenic in female rats or male mice exposed at 200, 600, or 1800 ppm., (Copyright 1998 Academic Press.)

As with 1,3-butadiene (BD), inhalation exposure of B6C3F1 mice to isoprene (2-methyl-1,3-butadiene) caused a macrocytic anemia; induced increases in sister chromatid exchanges in bone marrow cells and in levels of micronucleated erythrocytes in peripheral blood; and produced degeneration of the olfactory epithelium, forestomach epithelial hyperplasia, and testicular atrophy. Most notable was the finding that like BD, isoprene induced neoplasms in the liver, lung, Harderian gland, and forestomach of mice. The carcinogenic effects of isoprene were observed after a 26-week exposure (6 h/day, 5 days/week) of male mice to 700 ppm or higher concentrations of isoprene followed by a 26-week recovery period. Unlike BD, isoprene did not induce lymphomas or hemangiosarcomas of the heart in mice under these conditions nor did it induce chromosomal aberrations in mouse bone marrow cells. No toxicological effects were evident in rats exposed for 13 weeks to either isoprene or BD at concentrations up to 7000 ppm or 8000 ppm, respectively. Interstitial cell hyperplasia of the testis was observed in male F344 rats exposed to 7000 ppm isoprene for 26 weeks, and following a 26-week recovery period, there was a marginal increase in benign testicular interstitial cell tumors.

To evaluate the toxicity and carcinogenic potential of long-term exposure to ozone, B6C3F1 mice were exposed by whole-body inhalation to 0, 0.12, 0.5, or 1.0 ppm and 0, 0.5, or 1.0 ppm ozone for 24 or 30 mo (lifetime), respectively. The incidence of alveolar/ bronchiolar adenomas and carcinomas (combined) increased (p < 0.05) in female mice exposed to 1.0 ppm for 24 or 30 mo and marginally increased (p > 0.05) in male mice exposed to concentrations of 0.5 or 1.0 ppm. An increased incidence of nonneoplastic lesions were observed in the nasal cavities and in the centriacinar region of the lung of mice exposed to 0.5 or 1.0 ppm for 24 and 30 mo. Nasal cavity lesions were mild and included hyaline degeneration, hyperplasia, squamous metaplasia, fibrosis and suppurative inflammation of the transitional and respiratory epithelium of the lateral wall, and atrophy of the olfactory epithelium. Lung lesions included replacement of the epithelium of the alveolar ducts and adjacent alveolar septa with epithelium similar to that normally found in terminal bronchioles (metaplasia) and associated alveolar histiocytosis. Based on the results of these studies, we conclude that inhalation exposure of B6C3F1 mice to ozone for 24 or 30 mo (a) is carcinogenic in female B6C3F1 mice exposed to 1.0 ppm of ozone based on an increased incidence of alveolar/bronchiolar adenoma or carcinoma and (b) results in mild, site-specific, nonneoplastic lesions in the nasal cavity and centriacinar lung of male and female mice exposed to 0.5 or 1.0 ppm of ozone for 2 yrs, which persist with continued exposure to 30 mo. It is uncertain whether or not the marginal increase (p > 0.05) of alveolar/bronchiolar neoplasms in male B6C3F1 mice resulted from exposure to ozone.

Chloroprene (2-chloro-1,3-butadiene) is a high production chemical used almost exclusively in the production of polychloroprene (neoprene) elastomer. Because of its structural similarity to isoprene (2-methyl-1,3-butadiene) and to 1,3-butadiene, a potent trans-species carcinogen, inhalation studies were performed on chloroprene to characterize its toxicological potential and to provide a basis for selecting exposure concentrations for chronic toxicity and carcinogenicity studies. Thirteen-week inhalation toxicology studies were conducted in male and female F344 rats and B6C3F(1) mice at exposure concentrations of 0, 5, 12, 32 or 80 ppm (6 h/day; 5 days/week). A 200 ppm exposure group was also included for rats only, because a previous study showed that this concentration of chloroprene is lethal to mice. In mice, exposure to 80 ppm chloroprene caused a marginal decrease in body weight gain in males and epithelial hyperplasia of the forestomach in males and females. This lesion has been observed in mice exposed to isoprene or 1,3-butadiene. In rats, exposure to 80 ppm chloroprene or higher concentrations caused degeneration and metaplasia of the olfactory epithelium and exposure to 200 ppm caused anemia, hepatocellular necrosis and reduced sperm motility. These lesions have not been observed in rats exposed to isoprene or 1,3-butadiene. The profile of toxic effects of chloroprene is considerably different from that of isoprene or 1,3-butadiene; this may be due to differences in exposure concentrations that were used in toxicology studies of these compounds and /or to the influence of the chlorine substitution on the toxicokinetics of these compounds, on their biotransformation, or on the reactivity of metabolic intermediates with tissue macromolecules.

Isoprene, the 2-methyl analogue of 1,3-butadiene, is a high production chemical used largely in the manufacture of synthetic rubber and is the major endogenous hydrocarbon exhaled in human breath. Thirteen-week inhalation toxicology studies of isoprene were conducted in male and female F344 rats and B6C3F1 mice at exposure concentrations of 0, 70, 220, 700, 2200, and 7000 ppm (6 h/day; 5 days/week). In addition, 26-week inhalation studies at the same exposure levels, followed by a 26-week recovery period, were conducted in male rats and mice. The 13-week exposures produced no discernible exposure-related toxic effects in rats. Interstitial cell hyperplasia of the testis was observed in all male rats in the 7000 ppm group after 26 weeks of exposure; following the 26-week recovery period the only effect in rats was a marginal increase in benign testicular interstitial cell tumors. In mice, isoprene induced toxic and carcinogenic effects at multiple organ sites. Following the 26-week exposure and 26-week recovery periods, incidences of neoplastic lesions in the liver, lung, forestomach, and harderian gland were significantly increased. Neoplastic effects were observed at 700 ppm and higher exposures. Non-neoplastic lesions in mice exposed to isoprene included spinal cord degeneration, testicular atrophy, degeneration of the olfactory epithelium, and epithelial hyperplasia of the forestomach. A partial hindlimb paralysis and a nonresponsive macrocytic anemia were also seen in mice. Most of the toxic and carcinogenic effects caused by isoprene, as well as the species' difference in response, had been observed after inhalation exposures to 1,3-butadiene.

The purpose of this study was to evaluate the toxicity and potential carcinogenicity or cocarcinogenicity of ozone exposure in rats. Fischer-344/N (F-344/N) rats were exposed 6 hr/day, 5 days/wk, to 0, 0.12, 0.5, or 1.0 ppm ozone by inhalation for 2-yr and lifetime exposures. The cocarcinogenicity study included subcutaneous administration of 0, 0.1, or 1.0 mg/kg body weight of 4-(N-nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK) and inhalation of 0 or 0.5 ppm ozone to male rats. NNK was administered by subcutaneous injections 3 times per week for the first 20 wk with ozone inhalation exposure. The ozone inhalation exposure was for 2 yr (104 wk), including the first 20 wk of NNK treatment and continuing for 84 wk after the last NNK injection. Ozone exposure caused a concentration-related increase in inflammation of the centriacinar region of the lung. There was also increased fibrosis and an extension of the bronchiolar epithelium in these centriacinar regions to involve the proximal alveoli. There was no increased incidence of neoplasms at any site, including the lung, that was associated with ozone exposure. Rats administered 1.0 mg/kg body weight NNK alone had an increased incidence of bronchiolar/alveolar neoplasms, but this effect was not enhanced by ozone exposure. Ozone exposure for 2 yr and lifetime was associated with site-specific toxic alterations in the nasal passage and lung similar to those previously described for short-term exposures. While there was significant attenuation of the pulmonary lesions as compared to short-term exposures, lesions persisted in the lifetime study and there was evidence of a mild progressive fibrosis. We conclude that under the conditions of these studies: (a) ozone exposure is not carcinogenic to either male or female F-344/N rats, (b) ozone does not enhance the incidence of pulmonary neoplasms in F-344/N rats exposed to a known pulmonary carcinogen (NNK), and (c) mild site-specific toxic lesions characteristic of ozone exposure persist in the nasal passage and lung throughout the lifetime of the rat with continued ozone exposure.

Isoprene (2-methyl-1,3-butadiene) was selected for toxicologic evaluations because of its structural similarity to 1,3-butadiene, a potent rodent carcinogen. Two-week inhalation toxicology studies of isoprene were conducted in F344 rats and B6C3F1 mice at exposure concentrations of 0, 438, 875, 1750, 3500, or 7000 ppm. For rats, there were no chemically related changes in survival, body weight gain, clinical signs, hematologic or clinical chemistry parameters, or gross or microscopic lesions. Exposure of mice to isoprene did not produce mortalities and only caused a decrease in body weight gain for male mice in the 7000 ppm exposure group; however, hematologic changes and microscopic lesions including testicular atrophy, olfactory epithelial degeneration, and forestomach epithelial hyperplasia were observed in isoprene-exposed mice. Similar toxicologic effects have been previously observed in B6C3F1 mice exposed to 1,3-butadiene. A species difference in susceptibility between rats and mice exposed to isoprene was evident in these short-term exposure studies.

1,3-Butadiene, a large-production volume chemical used mainly in the manufacture of synthetic rubber, was found to induce multiple-organ carcinogenicity in male and female B6C3F1 mice at exposure concentrations (625 and 1250 ppm) equivalent to and below the OSHA standard of 1000 ppm. Since this study was terminated after 60 weeks of exposure because of reduced survival due to fatal tumors, and because dose-response relationships for 1,3-butadiene-induced neoplastic and nonneoplastic lesions were not clearly established, a second long-term inhalation study of 1,3-butadiene in B6C3F1 mice was conducted at lower exposure concentrations, ranging from 6.25 to 625 ppm. Both the histopathological findings from animals dying through week 65 and the results of evaluations of animals exposed for 40 and 65 weeks are presented in this report. Exposure to 1,3-butadiene caused a regenerative anemia at concentrations of 62.5 ppm and higher. Testicular atrophy was induced at 625 ppm, and ovarian atrophy was observed at 20 ppm and higher. During the first 50 weeks of the study, lymphocytic lymphoma was the major cause of death of mice exposed to 625 ppm 1,3-butadiene. Neoplasms of the heart, forestomach, lung, Harderian gland, mammary gland, ovary, and liver were frequently observed in 1,3-butadiene-exposed mice that died between week 40 and week 65 of the study. Studies in which exposure to 1,3-butadiene was stopped after limited periods were also included to assess the relationship between exposure levels and duration of exposures on the outcome of 1,3-butadiene-induced carcinogenicity. In these studies, lymphocytic lymphomas were induced in male mice exposed to 625 ppm 1,3-butadiene for only 13 weeks. The incidence of lymphocytic lymphoma in male mice exposed to 625 ppm 1,3-butadiene for 26 weeks was two times that in mice exposed to 625 ppm for 13 weeks. However, when the exposure concentration was reduced by half to 312 ppm and the exposure duration extended to 52 weeks, the incidence of lymphocytic lymphoma was reduced by 90%. Thus, the multiple of the exposure concentration times the exposure duration did not predict the incidence of lymphocytic lymphoma in mice. The early mortalities resulting from lymphocytic lymphomas in male mice exposed to 625 ppm 1,3-butadiene limited the expression of tumors at other sites. A clearer dose-response for 1,3-butadiene-induced neoplasia should be apparent from experiments in mice exposed to lower concentrations of this chemical for 2 years.

Daily fluctuations in plasma calcium, phosphate and their radionuclides (injected 5 or more days previously) were determined in rats maintained on a closely controlled feeding and light schedule. Male rats (150-300 g) were trained to a 7a.m.-7p.m. "light" schedule with food available 9 p.m. to 9 a.m. All prarmeters dropped rapidly at the start of each feeding period and then rose during the day. The daily changes in radionuclide concentrations were several orders of magnitude greater than for the stable ions. Continuous access to food produced an earlier fall (5 p.m.) in all plasma values. In thyroidectomized rats (with parathyroid transplants) plasma calcium and phosphate remained relatively constant during the 24 h period. If the time of availability of food was moved 6 h earlier (no change in light cycle), the drop in these plasma values also occurred 6 h earlier. Closer examination of these daily changes indicated that all values fell at least 1 h prior to feeding. In fasted rats, plasma calcium and phosphate concentrations fell as usual; however, 45Ca and 32P rose instead of falling. It was concluded that, in normal rats, changes in endogenous calcitonin secretion may actually increase rather than decrease daily pertubations of plasma calcium and phosphate. Daily changes in these values are influenced by a condition reflex developed due to the feeding habits of the rats. Finally, it is suggested that intestinal hormones rather than calcium may be the primary control of calcitonin secretion, and that the gastrointestinal tract plays a major role, in addition to absorptive activity, in both calcium and phosphate homeostasis.

A centrifugal analyzer and a spectrophotometer were compared for routine analysis of xenobiotic metabolizing enzymes glutathione (GSH) peroxidase, GSH-S transferase, and GSH reductase. Lung, liver, and kidney from 60-day-old male rats were used as the source of enzymes. Linear regression analysis was used to assess the accuracy and precision of the centrifugal analyzer method in measuring enzyme activities. Biologically and statistically, the centrifugal analyzer proved to be acceptable for routine measurement of these GSH-dependent enzymes.

Criteria for the selection of aerosol concentrations to be used in inhalation studies assessing the toxicity and carcinogenicity of chemical substances were discussed by the authors in a meeting sponsored by the National Toxicology Program. Concepts in the design of aerosol inhalation studies emerged from that meeting and are being communicated through this publication. Inhalation studies assessing the toxicity and carcinogenicity of aerosols have often used maximum exposure levels on the basis of technological feasibility. Evidence has now accumulated that the amount of pulmonary burden of deposited particles impacts on particle clearance above some as yet not well-defined exposure concentration. The sequelae are such that lung clearance decreases with increased particulate burden to the point of approaching complete cessation. This paper focuses on the major determinants in establishing maximal aerosol concentrations for use in inhalation toxicity studies with special emphasis on experimental design features to assess lung retention. The subject matter of this paper is a rapidly developing area in terms of knowledge. Accordingly, the contents of this article are intended as guidelines and not as absolute rules for the conduct and interpretation of inhalation exposure studies.

Arachidonic acid initiates the irreveresible aggregation of human platelets on conversion to the bicyclic prostaglandin endoperoxides, PGG2 and PGH2. An enzyme in arterial walls catalyzes the conversion of PGG2 and PGH2 to PGX, which inhibits human platelet aggregation. Preincubation with monocyclic peroxides (3-(alpha-hydroxyethyl)-1,2-dioxane, 3-(alpha-hydroxypropyl)-1,2-dioxolane or 3-methyl-3-(hydroxymethyl)-1,2-dioxolane) completely inhibited arachidonic acid initiated aggregation. Similarly, two analogs of PGH2, (15S)-hydroxy-9 alpha, 11 alpha-(epoxymethano)prosta-5Z, 13E-dienoic and (15S)-hydroxy-11 alpha, 9 alpha-(epoxymethano)prosta-5Z, 13E-dienoic acids, initiated irreversible aggregation of platelets. but were completely blocked by the monocyclic peroxides. Aggregation initiated by ADP or epinephrine was also completely inhibited by the cyclic peroxides. Aggregation of human platelets appears initiated through an endoperoxide receptor which can combine with either the natural bicyclic prostaglandin peroxides or the synthetic monocyclic peroxides. Natural inhibitors, such as PGX, may well be monocyclic endoperoxides similar to the compounds studied here.