Two-year inhalation toxicity study of propylene in F344/N rats and B6C3F1 mice

Chronic toxicity studies of propylene were conducted by exposing groups of 50 F344/N rats and 50 B6C3F1 mice of each sex in chambers to air containing the chemical in concentrations of 5000 and 10,000 ppm, 6 hr per day, 5 days per week, for 103 weeks. Groups of 50 rats and 50 mice of each sex in similar chambers received clean air only on the same schedule and served as controls. Survival and mean body weights of exposed and control male and female rats and mice were similar. In exposed rats, increased incidences of nonneoplastic lesions were observed in the nasal cavity. These consisted of epithelial hyperplasia in female rats exposed to the high concentration, and squamous metaplasia in female rats exposed to both concentrations and in male rats exposed to the low concentration. In addition, inflammatory changes characterized by an influx of lymphocytes, macrophages, and granulocytes into the submucosa and by granulocytes into the lumen occurred in male rats of both exposure groups. There was no evidence of nasal cavity lesions in exposed mice, suggesting a species difference in sensory irritation to propylene. There were no treatment-related increases or decreases in tumor incidence in the exposed groups relative to controls for either rats or mice. These data suggest that inhaled propylene induces signs of nasal cavity toxicity in rats but not in mice, and that the chemical is not carcinogenic to either species at the concentrations tested.     

Effect of subchronic formaldehyde inhalation on minute volume and nasal deposition in Sprague-Dawley rats

Since respiratory depression during formaldehyde (HCHO) inhalation is an important mechanism in reducing the dose received and potentially the toxicity in the nasal passages of exposed animals, this study was conducted to determine if changes in the pattern of minute volume response and nasal deposition occurred in nosepiece challenges to rats after long-term repeated exposures to HCHO. Male Sprague-Dawley rats were exposed to 0, 0.5, 3, or 15 ppm HCHO for 6 h/d, 5 d/wk, for 8 or 16 wk. The preexposed animals and age-specific controls were then submitted to a HCHO nosepiece challenge at the same concentration that was received in the subchronic exposure. Very high nasal deposition was demonstrated in all measurements. There was a diminished maximum minute volume depression in the 16-wk group relative to the 8-wk group. The difference in response was not statistically associated with the subchronic preexposure concentration. The substantial recovery of all initially depressed responses that occurred during the challenges probably diminished the impact of the decreased maximum responses on the resulting nasal deposition over the course of the long-term exposures.     

Isobutyraldehyde Administered by Inhalation (Whole Body Exposure) for up to Thirteen Weeks or Two Years Was a Respiratory Tract Toxicant but Was Not Carcinogenic in F344/N Rats and B6C3F1 Mice

Isobutyraldehyde (a chemical structurally related to formaldehydeand used as a flavoring agent) was studied for toxicity andcarcinogenicity by exposing male and female F344/N rats andB6C3F₁ mice. Animals were exposed to isobutyraldehyde vapors6 h per day, 5 days per week for up to 13 weeks or 2 years.In the 13-week studies, groups of 10 male and 10 female F344/Nrats and B6C3F₁ mice were exposed to concentrations of 0, 500,1000, 2000, 4000, or 8000 ppm. Chemical-related body weightdepression and deaths occurred in rats and mice exposed to 4000and 8000 ppm. Necrosis of the epithelium accompanied with acuteinflammatory reaction was observed in the nasal turbinate, larynx,and trachea of rats exposed to 8000 ppm. Exposure of rats to4000 ppm resulted in metaplasia of the nasal respiratory epithelium,inflammation, degeneration of the olfactory epithelium, andosteodystrophy of the nasal turbinate bone. In the 13-week mousestudy, exposure to 8000 ppm or 4000 ppm resulted in necrosisof the epithelium lining of the nasal turbinates. Osteodystrophyof the nasal turbinate bone and squamous metaplasia of the nasalrespiratory epithelium were noted in mice exposed 4000 ppm.Degeneration of the olfactory epithelium was noted in malesexposed 2000 ppm and in females exposed to 4000 ppm. In the2-year studies, groups of 50 male and 50 male F344/N rats andB6C3F1 were exposed to concentrations isobutyraldehyde vaporsof 0, 500, 1000, or 2000 ppm 6 h per day, 5 days per week. Therewere no differences in survival rates or mean body weights betweenexposed groups and control rats. Survival of male mice exposedto 2000 ppm and mean body weights of female mice exposed to1000 or 2000 ppm were lower than those of the of the controls.No increase in neoplasm incidence was observed in rats and micein the 2-year studies that could be attributed to isobutyraldehydeexposure. Chemical-related nonneoplastic lesions were limitedto the nose of rats and mice. They included squamous metaplasiaof the respiratory epithelium (rats), suppurative inflammation(rats), and olfactory epithelial degeneration (rats and mice)at 1000 and 2000 ppm.     

Chronic Toxicity and Oncogenicity Inhalation Study with Vinyl Acetate in the Rat and Mouse

Vinyl acetate was evaluated for chronic toxicity and oncogenicityin male and female rats and mice in a 104-week study. Targetconcentrations were 0, 50, 200, and 600 ppm. The study includedinterim terminations at approximately 53 and 83 weeks and agroup whose exposure was terminated at 70 weeks and alloweda 15-week recovery period. Over the course of the exposures,body weight gain was consistently depressed in all 600 ppm groupsand in the 200 ppm mice. Except for female rats of the 600 ppmexposure group, recovery animals showed significant improvementsin weight gain relative to controls. There were no changes inhematological parameters of either species that could be unequivocallyrelated to treatment. The only effect noted on clinical chemicalparameters during the study were decreases in blood glucosein the 600 ppm females. There were no adverse effects on survivalin either species. Increases in lung weight were noted in ratsand mice primarily in the 600 ppm groups. These changes wereassociated with bronchial exfoliation, macrophage accumulation,and fibrous plaques and buds extending into the airway lumen,and bronchial/bronchiolar epithelial disorganization. The mostsignificant histopathological changes were noted in the nasalcavity. In the olfactory epithelium of both rats and mice, themain nonneoplastic changes included epithelial atrophy, regenerativeeffects (squamous metaplasia and respiratory metaplasia of olfactoryepithelium), basal cell hyperplasia, and epithelial nest-likeinfolds. No nonneoplastic changes were observed in the respiratoryepithelium of rats, while squamous metaplasia at the naso/maxilloturbinateregion was prevalent in mice. Nonneoplastic changes were similarin the recovery groups. Oncogenic responses to vinyl acetateexposure were mainly confined to the nasal cavity in rats andincluded endo- and exophytic papillomas, squamous cell carcinoma,carcinoma in situ in olfactory regions, and endophytic papillomain respiratory regions. Squamous cell carcinomas were also foundeither in areas normally covered by cuboidal epithelium or areasof unknown origin. One squamous cell carcinoma was found inthe larynx of a rat of the 600 ppm groups. One squamous cellcarcinoma was found in the lung of a mouse of the 600 ppm group.The no-observable- adverse-effect level for all effects was50 ppm in both species. The tumorigenic response appears tobe nonlinear. The nonlinear dose response and the unique natureof the rodent nasal cavity suggest that specific risk extrapolationmodels should be developed for vinyl acetate.     

Two-Year Inhalation Exposure of Female and Male B6C3F1 Mice and F344 Rats to Chlorine Gas Induces Lesions Confined to the Nose

Chlorine gas is a respiratory irritant in both animals and humansthat produces concentration-dependent responses ranging fromminor irritation to death. Female and male B6C3F1 mice and F344rats were exposed to chlorine gas for up to 2 years to determinechronic toxicity and carcinogenicity. Groups of approximately70 each of female and male mice and rats were exposed to 0,0.4, 1.0, or 2.5 ppm chlorine gas for 6 hr/day, 5 days/week(mice and male rats), or 3 alternate days/week (female rats)for 2 years, with an interim necropsy of rats at 12 months (10rats/sex/concentration group). A complete necropsy was performedon all animals. Histological examination was performed on allorgans from high-concentration and control animals and selectedtarget organs from mid-and low-concentration groups. Exposure-dependentlesions were confined to the nasal passages in all sex and speciesgroups. Chlorine-induced lesions, which were most severe inthe anterior nasal cavity, included respiratory and olfactoryepithelial degeneration, septal fenestration, mucosal inflammation,respiratory epithelial hyperplasia, squamous metaplasia andgoblet cell hypertrophy and hyperplasia, and secretory metaplasiaof the transitional epithelium of the lateral meatus. Intracellularaccumulation of eosinophilic proteinaceous material was alsoa prominent response involving the respiratory, transitional,and olfactory epithelia, and in some cases the squamous epitheliumof the nasal vestibule. Many of these nasal lesions exhibitedan increase in incidence and/or severity that was related tochlorine exposure concentration and were statistically significantlyincreased at all chlorine concentrations studied. Male miceand female rats appeared more sensitive to chlorine than femalemice and male rats, respectively. The reasons for the sex differenceswithin a species were not determined. Interspecies differencesin regional dosimetry and site-specific tissue susceptibilityto chlorine exposure should be taken into account when usingthese data for accurate assessment of potential human healthrisks. The incidence of neoplasia was not increased by exposure,indicating that inhaled chlorine in rats and mice is an upperrespiratory tract toxicant but not a carcinogen.     

DNA--Protein Cross-links and Cell Replication at Specific Sites in the Nose of F344 Rats Exposed Subchronically to Formaldehyde

Chronic exposures to high concentrations (>6 ppm) of formaldehyde(HCHO) induce cell proliferation, squamous metaplasia, and squamouscell carcinomas in F344 rats. To assess the cancer risk associatedwith HCHO exposure, DNA-protein cross-links (DPX) formed ina single exposure of naive (previously unexposed) rats and monkeyshave been used as a surrogate for the internal dose. Since thequantity of DPX may differ in subchronically exposed animals,the effects of preexposure to HCHO on the acute DPX yield (concentrationof DPX following a single exposure) and the cumulative DPX yield(concentration of DPX following repeated exposures) were determined.Male F344 rats were preexposed (PE) to 0.7, 2, 6, or 15 ppmof HCHO (6 hr/day, 5 days/week, 11 weeks + 4 days). Naive (N)rats were exposed to room air. On the 5th day of the 12th week,PE and N rats were simultaneously exposed (3 hr) to H¹⁴CHO atthe same concentrations used for preexposure. Acute DPX yieldsand cell replication (incorporation of ¹⁴C into DNA) were determinedin the mucosal lining of the nasal lateral meatus (LM) (hightumor site in HCHO bioassay) and the medial and posterior meatuses(M:PM) (low tumor site in bioassay). DPX yields in the LM wereapproximately sixfold higher than in the M:PM. At 0.7 and 2ppm, no differences between PE and N rats were detected in eithertissue. At 6 and 15 ppm, acute DPX yields in the LM of PE ratswere approximately half those of N rats, but no differenceswere detected in the M:PM. Cell proliferation was induced inPE rats at 6 ppm (LM only) and especially at 15 ppm (LM andM:PM). Cumulative DPX yields were measured indirectly by determiningthe decrease in extractability of DNA from proteins. PE ratswere preexposed to 6 or 10 ppm as above, while N rats were exposedto room air. Both groups (PE and N) were then exposed (3 hr)to the same concentration of unlabeled HCHO. DPX yields increasedin a concentration-dependent manner in both groups, but theyields were smaller in PE than N rats, suggesting that no accumulationof DPX occurred in PE rats. The results demonstrate that atconcentrations <2 ppm, N and PE rats are equivalent withrespect to the formation of DPX. At concentrations >6 ppm,N and PE rats are not equivalent, but the impact of this high-doseeffect on low-dose cancer risk estimates derived with the linearizedmultistage model is small.     

Dimethylethanolamine: Acute, 2-Week, and 13-Week Inhalation Toxicity Studies in Rats

Dimethylethanolamine (DMEA) is a volatile, water-soluble aminethat has applications in the chemical and pharmaceutical industries.These studies evaluated the acute and subchronic inhalationtoxicity of DMEA. Acute (4-hr) exposures of Wistar rats to DMEAvapor resulted in an LC5O value (95% confidence limits) of 1641(862–3125)ppm. Clinical signs of nasal and ocular irritation, respiratorydistress, and body weight loss were observed in rats exposedto 1668 ppm DMEA and higher. In the 2-week study, F-344 ratsexposed to 98, 288, or 586 ppm DMEA for 9 days (6 hr/day) duringan 11-day period also exhibited signs of respiratory and ocularirritation (except the 98 ppm group). All animals of the 586ppm group and 4 of 15 male rats of the 288 ppm group died. Bodyweight values for the 288 ppm group were reduced to about 75%of preexposure values, while the 98 ppm group gained 35% lessweight than controls. Statistically significant differencesin clinical pathology parameters (288 ppm group) and in organweight values (288 and 98 ppm groups) probably resulted fromthe decreased food consumption and not from specific targetorgan toxicity. In the groups evaluated histologically (the98 and 288 ppm groups) the eye and nasal mucosa were the primarytarget organs. In the 13-week subchronic study, F-344 rats wereexposed to 0, 8, 24, or 76 ppm DMEA for 6 hr/day, 5 days/weekfor 13 weeks. The principal exposure-related changes were transientcorneal opacity in the 24 and 76 ppm groups; decreased bodyweight gain for the 76 ppm group; and histopathologic lesionsof the respiratory and olfactory epithelium of the anteriornasal cavity of the 76 ppm group and of the eye of several 76ppm group females. Rats maintained for a 5-week recovery periodonly exhibited histological lesions of the nasal tissue, withthe lesions being decreased in incidence and severity. DMEAacts primarily as an ocular and upper respiratory tract irritantand toxicant at vapor concentrations of 76 ppm, while 24 ppmor less produced no biologically significant toxicity in rats.Thus, 24 ppm was considered to be the no-observable-effect level.     

Ultrastructural Evaluation of Acute Nasal Toxicity in the Rat Respiratory Epithelium in Response to Formaldehyde Gas

Ultrastructural Evaluation of Acute Nasal Toxicity in the RatRespiratory Epithelium in Response to Formaldehyde Gas. MONTEIRO-RIVIERE,N. A., AND POPP, J. A. (1986). Fundam. Appl. Toxicol. 6, 251–262.Ultrastructural lesions were induced by formaldehyde (HCHO)gas in the rat nasal respiratory epithelium. Male F-344 rats,7–9 weeks old, were exposed to 0.5 or 2 ppm (6 hr/day)for 1 or 4 days and to 6 ppm (6 hr/day) of HCHO for 1 day andsacrificed immediately or 18 hr after 1, 2, or 4 days of exposure.Other groups were exposed to 15 ppm (6 hr/day) of HCHO for 1and 2 days. Ultrastructural changes to 0.5 or 2 ppm were limitedto altered cilia with wing-like projections occasionally seenon the tips of the ciliary shafts. Autophagic vacuoles werepresent in some of the basal cells while neutrophils were seenin the basal and suprabasal layers after 1 day exposure to 6ppm of formaldehyde. Hypertrophy of goblet and ciliated cellswere noted in animals exposed to 6 ppm of formaldehyde and sacrificed18 hours after 1,2, or 4 days of exposure. Some nonciliatedcells formed apical blebs containing an abundance of SER. Ciliated-mucouscells were observed after 2 and 4 days of exposure to 6 ppmof formaldehyde. Nonkeratinized squamous cells containing microfilamentswere seen as early as 4 days after exposure to 6 ppm and at1 and 2 days after exposure to 15 ppm. Loss of microvilli inciliated cells occurred in all exposure levels. At 15 ppm for1 and 2 days, nucleolar segregation was observed in basal andcuboidal cells and internalized cilia were noted. These resultsdemonstrate that short-term exposure to 6 or 15 ppm of HCHOcaused respiratory epithelial injury which was not cell specific,but was dose related in severity.     

Inhalation toxicity studies of the alpha,beta-unsaturated ketones: ethyl vinyl ketone

The National Toxicology Program is conducting a chemical class study to investigate the structure-activity relationships for the toxicity of alpha,beta-unsaturated ketones. Ethyl vinyl ketone (EVK) was selected for study because it is a representative straight-chain aliphatic alpha,beta-unsaturated ketone with extensive use and widespread exposure. Short-term inhalation studies of EVK were conducted to provide toxicity data for comparison with the related alpha,beta-unsaturated ketones 2-cyclohexene-1-one (CHX) and methyl vinyl ketone (MVK). These data will be used in designing chronic toxicity and carcinogenicity studies of these ketones. Male and female F344 rats and B6C3F1 mice were exposed to 0, 2, 4, or 8 ppm EVK 6 h/day, 5 days/wk for 13 wk. The nasal cavity was the major target organ of EVK in both rats and mice. Pathologic findings in both the olfactory and respiratory epithelium were observed. Lesions consisted primarily of olfactory epithelial necrosis, atrophy and regeneration, and/or hyperplasia and squamous metaplasia of the respiratory epithelium. Squamous metaplasia of the respiratory epithelium was present in all rats and mice exposed to 4 and 8 ppm EVK, and these lesions were more severe in rats than in mice. Few systemic effects were observed in rats and mice exposed to EVK. A transient decrease in total leukocytes due to decrements in lymphocyte and monocyte populations was present in male rats after exposure to 8 ppm for 3 and 21 days; however, this effect was not present after exposure for 13 wk. There were no chemical-related effects on micronucleus formation in mice, or on sperm motility and vaginal cytology in either species. EVK, like other alpha,beta-unsaturated ketones, is a reactive, direct-acting gaseous irritant with toxicity limited primarily to the upper respiratory tract.     

The Toxicity of Dimethylamine in F-344 Rats and B6C3F1 Mice following a 1-Year Inhalation Exposure

The Toxicity of Dimethylamine in F-344 Rats and B6C3F1 Micefollowing a 1-Year Inhalation Exposure (1985). BUCKLEY, L. A.,MORGAN, K. T., SWENBERG, J. A., JAMES, R. A., HAMM, T. E., JR.,and BARROW, C. S. Fundam. Appl. Toxicol. 5, 341–352. Dimethylamineis a widely used commodity chemical, for which there are fewchronic toxicity data. Male and female F-344 rats and B6C3F1mice were exposed by inhalation to 0, 10, 50, or 175 ppm dimethylamine(DMA) for 6 hr/day, 5 days/week for 12 months. Groups of 9–10male and female rats and mice were necropsied after 6 and 12months of exposure. No male mice were sacrificed at 12 monthsdue to a high incidence of early deaths in that group. The meanbody weight gain of rats and mice exposed to 175 ppm DMA wasdepressed to approximately 90% of control after 3 weeks of exposure.The only other treatment-related changes were concentration-relatedlesions in the nasal passages. Two distinct locations in thenose were affected: the respiratory epithelium in the anteriornasal passages, and the olfactory epithelium, especially thatlining the anterior dorsal meatus. There was focal destructionof the anterior nasoturbinate and nasal septum, local inflammation,and focal squamous metaplasia of the respiratory epitheliumin rats and mice. Mild goblet cell hyperplasia was observedonly in rats. The olfactory epithelium exhibited extensive lossof sensory cells with less damage to sustentacular cells. Therewas also loss of olfactory nerves, hypertrophy of Bowman's glands,and distension of the ducts of these glands by serocellulardebris in regions underlying degenerating olfactory epithelium.At the 175-ppm exposure level, rats had more extensive olfactorylesions than mice, with hyperplasia of small basophilic cellsadjacent to the basement membrane being present in rats butnot mice. After 12 months of exposure to 10 ppm DMA, minimalloss of olfactory sensory cells and their axons in olfactorynerve bundles was observed in the nasal passages of a few ratsand mice. These results indicate that the olfactory sensorycell is highly sensitive to the toxic effects of DMA, with minorlesions being produced in rodents even at the current thresholdlimit value of 10 ppm.     

Dichloromethane (methylene chloride): metabolism to formaldehyde and formation of DNA-protein cross-links in B6C3F1 mice and Syrian golden hamsters.

Dichloromethane (DCM) is metabolized via a glutathione transferase (GST)-dependent pathway to formaldehyde (HCHO), a mutagenic compound that could play an important role in the carcinogenic effects of DCM observed in the liver and lungs of B6C3F1 mice at 2000 and 4000 ppm. Syrian hamsters metabolize DCM more slowly than mice via this pathway, and hamsters exposed to 3500 ppm showed no apparent carcinogenic response. The possible formation of DNA-protein cross-links (DPX) from DCM in both species was examined. Male mice and hamsters were pre-exposed for 2 days (6 hr/day) to 4000 ppm of DCM and on the third day were exposed (6 hr) to a decaying concentration (4500 to 2500 ppm) of [14C]DCM. DPX were detected in mouse liver, but not in mouse lung, hamster liver, or hamster lung. The failure to detect DPX in mouse lung does not exclude their possible formation in a subpopulation of lung cells. Metabolic incorporation of 14C derived from [14C]DCM into DNA suggested a higher rate of turnover of some mouse lung cells than of hamster lung cells, but no large difference in the turnover rates of liver cells in the two species under these conditions. These results demonstrate that HCHO derived from DCM can form DNA-protein cross-links in the liver of the B6C3F1 mouse. The formation of DPX is dependent on the activity of the GST pathway, and species such as hamsters and humans having much lower rates of DCM metabolism via this pathway may not generate toxicologically significant concentrations of HCHO and DPX.     

Respiratory pathology in rats and mice after inhalation of 1,2-dibromo-3-chloropropane or 1,2 dibromoethane for 13 weeks

Seventy F344 rats and 144 B6C3F1 mice were subdivided into seven groups. Three groups were each exposed via inhalation to 1, 5, or 25 ppm of 1,2-dibromo-3-chloropropane (DBCP) for 6 h per day, 5 days per week for 13 weeks. Three additional groups were each similarly exposed to 3, 15, or 75 ppm of 1,2-Dibromoethane (EDB). The remaining group was exposed to room air under the same conditions. At 13 weeks, rats and mice showed severe necrosis and atrophy of the olfactory epithelium in the nasal cavity after inhalation of 5 or 25 ppm DBCP and 75 ppm EDB. Lower concentrations induced squamous cell metaplasia, hyperplasia and cytomegaly of the epithelium of the respiratory nasal turbinals. Squamous metaplasia, hyperplasia and cytomegaly of the epithelium was also seen in larynx, trachea, bronchi and bronchioles. Other compound related toxic lesions in rats were seen in the liver, kidney and testes.     

Subchronic and developmental toxicity studies of n-butyl propionate vapor in rats

Two inhalation studies were conducted to evaluate the possible subchronic and developmental toxic effects of n-butyl propionate. In the subchronic study, Sprague-Dawley rats (15/sex/group) were exposed to 0, 250, 750, or 1500 ppm vapor for 6 h/d, 5 d/wk for 13 wk. Five of the rats per sex per group were held after the final exposure for an 8-wk recovery period. Standard parameters of subchronic toxicity were measured throughout the study, and at the end of exposure and recovery periods, necropsies were performed, organs weighed, and tissues processed for microscopic examination. Exposure did not produce marked treatment-related deaths or adversely affect clinical signs, hematology, clinical chemistries, organ weights, or the histology of major visceral organs. The only systemic toxic effects were significant decreases in body weight, body weight gain, and feed consumption that occurred in 1500 ppm group rats. Morphologic changes were limited to the nasal cavity as evidenced by a concentration-related increased incidence and severity of olfactory epithelium degeneration in rats of the 750 and 1500 ppm groups. These degenerative microscopic alterations were primarily confined to the olfactory epithelium within the dorsal portion of the medial meatus, with lesser involvement of the olfactory mucosae overlying the tips of some of the adjacent ethmoturbinates. Both the systemic and nasal cavity effects appeared reversible after exposure ceased. In the developmental toxicity study, pregnant Sprague-Dawley rats (24/group) were exposed to 0, 500, 1000, or 2000 ppm vapor for 6 h/d on gestation d 6-15 and sacrificed on gestation d 20. All treatment-group dams exhibited significant reductions in body weight, body weight gain, and feed consumption. Gestational parameters were equivalent across all groups and there were no treatment-related developmental or teratogenic effects. The no-observed-adverse effects levels (NOAELs) determined for nbutyl propionate were 250 ppm for subchronic toxicity (based on the olfactory epithelium degeneration) and 22000 ppm for developmental toxicity (no developmental effects at top dose tested). Under the conditions of this study, a NOAEL was not determined for maternal toxicity.     

Upper respiratory tract toxicity of inhaled methylvinyl ketone in F344 rats and B6C3F1 mice.

The National Toxicology Program is conducting a chemical class study to investigate the structure-activity relationships for the toxicity of alpha,beta-unsaturated ketones. Methylvinyl ketone (MVK) was selected for study because it is a representative straight-chain aliphatic alpha,beta-unsaturated ketone and because of its extensive use and widespread exposure. Short-term inhalation studies of MVK were conducted to provide toxicity data for comparison with other alpha,beta-unsaturated ketones and for use in designing chronic toxicity and carcinogenicity studies. In 2-week studies, rats and mice were exposed to 0, 0.25, 0.5, 1, 2, 4, or 8 ppm MVK 6 h/day, 5 days/week for 12 exposures. Morbidity and early deaths occurred in all male and female rats after 1 exposure and in 2 male mice after 10 exposures to 8 ppm. Rats exhibited nasal cavity toxicity and lung necrosis at 4 ppm. No toxicity was observed in animals exposed to less than 2 ppm. Based on these results a 13-week study was conducted at 0, 0.5, 1, and 2 ppm MVK. As observed in the 2-week study, the nasal cavity was the main target organ and rats were more sensitive than mice. Respiratory and olfactory epithelial necrosis were prominent by day 21 in the rat. At study termination these lesions were still evident but not as severe as noted earlier. Additionally, changes such as olfactory epithelial regeneration and metaplasia (respiratory) as well as respiratory epithelial hyperplasia and metaplasia (squamous) were clearly evident. Nasal lesions in mice were limited to a subtle squamous metaplasia of transitional and/or respiratory epithelium covering predominantly the tips of naso- and maxilloturbinates in Levels I and II. A transient, leukopenia was observed in rats exposed to 2 ppm, however, this effect was not present after 13 weeks of exposure. In mice, leukocyte counts were significantly decreased at all exposure concentrations after 13 weeks of exposure. Absolute testicular and epididymal weights and sperm counts were decreased at the high dose only. MVK can be characterized as a reactive, direct-acting gaseous irritant. MVK exposure causes the same nasal cavity lesions as the cyclic alpha,beta-unsaturated ketone, 2-cyclohexen-1-one, although at lower exposure concentrations.     

INHALATION TOXICITY STUDIES OF THE α,β-UNSATURATED KETONES: Ethyl Vinyl Ketone

The National Toxicology Program is conducting a chemical class study to investigate the structure-activity relationships for the toxicity ofα,β -unsaturated ketones. Ethyl vinyl ketone (EVK) was selected for study because it is a representative straight-chain aliphaticα,β -unsaturated ketone with extensive use and widespread exposure. Short-term inhalation studies of EVK were conducted to provide toxicity data for comparison with the relatedα,β -unsaturated ketones 2-cyclohexene-1-one (CHX) and methyl vinyl ketone (MVK). These data will be used in designing chronic toxicity and carcinogenicity studies of these ketones. Male and female F344 rats and B6C3F1 mice were exposed to 0, 2, 4, or 8 ppm EVK 6 h/day, 5 days/wk for 13 wk. The nasal cavity was the major target organ of EVK in both rats and mice. Pathologic findings in both the olfactory and respiratory epithelium were observed. Lesions consisted primarily of olfactory epithelial necrosis, atrophy and regeneration, and/or hyperplasia and squamous metaplasia of the respiratory epithelium. Squamous metaplasia of the respiratory epithelium was present in all rats and mice exposed to 4 and 8 ppm EVK, and these lesions were more severe in rats than in mice. Few systemic effects were observed in rats and mice exposed to EVK. A transient decrease in total leukocytes due to decrements in lymphocyte and monocyte populations was present in male rats after exposure to 8 ppm for 3 and 21 days; however, this effect was not present after exposure for 13 wk. There were no chemicalrelated effects on micronucleus formation in mice, or on sperm motility and vaginal cytology in either species. EVK, like otherα,β -unsaturated ketones, is a reactive, directacting gaseous irritant with toxicity limited primarily to the upper respiratory tract.     

Toxicity of Divinylbenzene-55 for B6C3F1 Mice in a Two-Week Inhalation Study

Divinylbenzene (DVB) is a crosslinking monomer used primarilyfor copolymerization with styrene to produce ion-exchange resins.The toxicity of inhaled DVB was investigated because of thepotential for worker exposure and the structural similarityof DVB to styrene, a potential carcinogen. Male and female B6C3F1mice were exposed to 0, 25, 50, or 75 ppm DVB for 6 hr/day,5 days/week for up to 2 weeks. Six mice/sex/dose group werekilled after 3, 5, and 10 exposures and six mice/sex in the75 ppm group were killed 7 days after 10 exposures. The mostsevere effects occurred in the nasal cavity and liver, withless severe effects occurring in the kidneys. In the nasal cavityolfactory epithelium acute necrosis and inflammation were presentat early time points followed by regeneration, architecturalreorganization, and focal respiratory metaplasia by 7 days afterthe last exposure. Olfactory epithelial changes were concentration-dependentwith extensive involvement at 75 ppm and peripheral sparingat 25 ppm. There was also necrosis and regeneration of olfactory-associatedBowman's glands as well as the lateral nasal (Steno's) glands.Hepatocellular centrilobular (CL) necrosis was observed onlyin the 75 ppm dose group and was similar to that caused by styrene.A time-dependent progression was observed, characterized byCL degeneration after 1 exposure, necrosis after 3 and 5 exposures,and chronic inflammation with CL karyomegaly after 10 exposuresand 7 days after the 10th exposure. Hepatic GSH levels weredecreased in a dose-dependent manner. In the kidneys, transienttubular damage was observed in some male mice exposed to 75ppm, and appeared to be a response to DVB-induced tubular epithelialinjury.     

Chronic toxicity and oncogenicity bioassay of inhaled ethyl acrylate in Fischer 344 rats and B6C3F1 mice

Male and female Fischer 344 rats and B6C3F1 mice were exposed to 0, 25 or 75 ppm (0, 0.10 or 0.31 mg/l) ethyl acrylate vapors, 6 hours per day, 5 days per week, for a total of 27 months. Additional rats and mice were exposed to 225 ppm (0.92 mg/l) for 6 months and then held for 21 additional months post-exposure. Histopathologic changes in olfactory portions of the nasal mucosa were present in animals in all of these three exposure groups. These microscopic exposure-related changes were concentration-dependent, primarily in terms of distribution of the lesions within the nasal cavity. Generally those areas of the nasal mucosa normally lined by olfactory epithelium were altered, while the regions lined by respiratory epithelium were relatively unaffected. There was no indication of an oncogenic response in any organ or tissue in either rats or mice. A follow-up study in which Fischer 344 rats and B6C3F1 mice were exposed to 5 ppm (0.02 mg/l) for 24-months revealed no treatment-related changes in the nasal mucosa.     

Propylene glycol monomethyl ether acetate (PGMEA) metabolism,disposition, and short-term vapor inhalation toxicity studies

Male Fischer 344 rats were given a single po dose of approximately 8.7 mmol/kg of [1-¹⁴C]propylene glycol monomethyl ether acetate (PGMEA) or exposed to 3000 ppm [1-¹⁴C]PGMEA for 6 hr. After dosing, expired air, excreta, and tissues were analyzed for ¹⁴C activity, and metabolites in urine were isolated and identified. Approximately 64% of the administered ¹⁴C activity was eliminated as ¹⁴CO₂ and about 24% was excreted in urine within 48 hr after a single po dose of radiolabeled PGMEA. Similarly, 53% was eliminated as ¹⁴CO₂ and 26% was excreted in urine within 48 hr after the inhalation exposure. Propylene glycol, propylene glycol monomethyl ether (PGME), and the sulfate and glucuronide conjugates of PGME were identified as urinary metabolites after po dosing, as well as after inhalation exposure to PGMEA. The urinary metabolite profile and disposition of [¹⁴C]PGMEA were nearly identical to results previously obtained with propylene glycol monomethyl ether (PGME), indicating that PGMEA is rapidly and extensively hydrolyzed to PGME in vivo. A short-term vapor inhalation toxicity study in which male and female Fischer 344 rats and B6C3F1 mice were exposed to 0, 300, 1000, or 3000 ppm PGMEA confirmed that there were no substantial differences in the systemic effects of PGMEA as compared to PGME. However, histopathologic examination did reveal changes in the olfactory portions of the nasal mucosa of rats and mice exposed to PGMEA, which may be related to acetic acid resulting from hydrolysis of PGMEA in the nasal epithelium.     

Toxicology and carcinogenesis studies of microencapsulated trans-cinnamaldehyde in rats and mice.

trans-Cinnamaldehyde is a widely used natural ingredient that is added to foods and cosmetics as a flavoring and fragrance agent. Male and female F344/N rats and B6C3F(1) mice were exposed to microencapsulated trans-cinnamaldehyde in the feed for three months or two years. All studies included untreated and vehicle control groups. In the three-month studies, rats and mice were given diets containing 4100, 8200, 16,500, or 33,000 ppm trans-cinnamaldehyde. In rats, feed consumption was reduced in all exposed groups. In mice, feed consumption was reduced in the highest dose groups. Body weights of all treated males were less than controls. Body weights were reduced in female rats exposed to 16,500 or 33,000 ppm and female mice exposed to 8200 ppm or greater. All rats survived to the end of the study but some male mice in the highest dose groups died due to inanition from unpalatability of the dosed feed. The incidence of squamous epithelial hyperplasia of the forestomach was significantly increased in rats exposed to 8200 ppm or greater and female mice exposed to 33,000 ppm. In mice, the incidence of olfactory epithelial degeneration of the nasal cavity was significantly increased in males and females exposed to 16,500 ppm and females exposed to 33,000 ppm. In the two-year studies, rats and mice were exposed to 1000, 2100, or 4100 ppm trans-cinnamaldehyde. Body weights were reduced in mice exposed to 2100 ppm and in rats and mice exposed to 4100 ppm. In rats, hippuric acid excretion was dose proportional indicating that absorption, metabolism, and excretion were not saturated. No neoplasms were attributed to trans-cinnamaldehyde in rats or mice. Squamous cell papillomas and carcinomas of the forestomach were observed in male and female mice but the incidences were within the NTP historical control range and were not considered to be related to trans-cinnamaldehyde exposure.     

SUBCHRONIC AND DEVELOPMENTAL TOXICITY STUDIES OF n-BUTYL PROPIONATE VAPOR IN RATS

Two inhalation studies were conducted to evaluate the possible subchronic and developmental toxic effects of n -butyl propionate. In the subchronic study, Sprague-Dawley rats (15/sex/group) were exposed to 0, 250, 750, or 1500 ppm vapor for 6 h/d, 5 d/wk for 13 wk. Five of the rats per sex per group were held after the final exposure for an 8- wk recovery period. Standard parameters of subchronic toxicity were measured throughout the study, and at the end of exposure and recovery periods, necropsies were performed, organs weighed, and tissues processed for microscopic examination. Exposure did not produce marked treatment-related deaths or adversely affect clinical signs, hematology, clinical chemistries, organ weights, or the histology of major visceral organs. The only systemic toxic effects were significant decreases in body weight, body weight gain, and feed consumption that occurred in 1500 ppm group rats. Morphologic changes were limited to the nasal cavity as evidenced by a concentration-related increased incidence and severity of olfactory epithelium degeneration in rats of the 750 and 1500 ppm groups. These degenerative microscopic alterations were primarily confined to the olfactory epithelium within the dorsal portion of the medial meatus, with lesser involvement of the olfactory mucosae overlying the tips of some of the adjacent ethmoturbinates. Both the systemic and nasal cavity effects appeared reversible after exposure ceased. In the developmental toxicity study, pregnant Sprague-Dawley rats (24/group) were exposed to 0, 500, 1000, or 2000 ppm vapor for 6 h/d on gestation d 6?15 and sacrificed on gestation d 20. All treatment-group dams exhibited significant reductions in body weight, body weight gain, and feed consumption. Gestational parameters were equivalent across all groups and there were no treatment-related developmental or teratogenic effects. The no-observed-adverse effects levels (NOAELs) determined for n butyl propionate were 250 ppm for subchronic toxicity (based on the olfactory epithelium degeneration) and 2000 ppm for developmental toxicity (no developmental effects at top dose tested). Under the conditions of this study, a NOAEL was not determined for maternal toxicity.