Nitromethane

Carcinogenicity

Description of key information

There was clear evidence of carcinogenic activity of nitromethane in female F344/N rats based on increased incidences of mammary gland fibroadenomas and carcinomas following exposure to 188 and 375 ppm. There was clear evidence of carcinogenic activity in female B6C3F1 mice exposed to 188 ppm (469 mg/m3), based on increased incidences of liver neoplasms (primarily adenomas) and harderian gland adenomas and carcinomas.  Increased incidences of alveolar/bronchiolar adenomas and carcinomas in male and female mice exposed to 188 or 375 ppm nitromethane were also considered to be related to chemical administration.

Key value for chemical safety assessment

Carcinogenicity: via oral route

Endpoint conclusion

Endpoint conclusion:

no study available

Carcinogenicity: via inhalation route

Link to relevant study records

Reference

Endpoint:

carcinogenicity: inhalation

Type of information:

experimental study

Adequacy of study:

key study

Study period:

circa 1997

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: The study was conducted according to test guidelines and in accordance with GLP

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 451 (Carcinogenicity Studies)

GLP compliance:

yes

Species:

rat

Strain:

Fischer 344

Sex:

male/female

Details on test animals or test system and environmental conditions:

The rats were 4 weeks old on receipt from the supplier. They were quarantined for 13-14 days before use. Five animals per sex were randomly selected for parasite evaluation and gross examination for evidence of disease. At the end of the study, serologic analyses were performed on 5 sentinel rats/sex. Water and food were available ad libitum (except during exposure, when food was withheld).

Rats and mice were housed individually. Water was available ad libitum; feed was available ad libitum except during exposure periods. Cages were rotated within the inhalation chambers weekly.

Route of administration:

inhalation: vapour

Type of inhalation exposure (if applicable):

whole body

Vehicle:

air

Details on exposure:

Vapor generation: The test material was held in a stainless-steel reservoir under a nitrogen blanket. The material was pumped through a liquid distribution manifold of stainless steel tubing to heated-wick vaporizers. One set of dual vaporizers supplied vapor to all chambers. The vapor-laden air was transferred through the distribution line and diluted with HEPA- and charcoal-filtered air. Theree-way valves in the chamber inlet ducts allowed nitromethane vapors to be diverted to the exhaust until a stable concentration of test material was built up in the distribution line. At each chamber, vapor moving through the inlet duct was further diluted with filtered air to the appropriate concentration of test material with a metered three-way valve. A small particle detector was placed in the chambers to measure concentrations of aerosol. No particle counts above the minimum resolvable level (200 particles/cm3) were detected.

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

Chamber concentrations were monitored with an one-line gas chromatograph (GC). The monitor was coupled with the inhalation chanbers by a computer-controlled 12-port stream select valve. The GC was calibrated comparing chamber concentration data to data from grab samples analyzed by an off-line GC. The grab samples were collected in bubblers containing dimethylformamide. The off-line GC was calibrated with gravimetrically prepared nitromethane standards. Chamber concentration uniformity was maintained throughout the study.

Buildup and decay rates for chamber concentrations were determined with and without animals in the chambers. The time to achieve 90% of the target concentration was 5-17 minutes and the time for decay to 10% of the target concentration was 13-19 minutes. Studies of nitromethane degradation and monitoring for impurities were conducted throughout the studies by comparing bubbler samples to a reference sample. No significant degradation was observed during the studies.

Duration of treatment / exposure:

6 hours/day

Frequency of treatment:

5 days/week for 103 weeks

Post exposure period:

None

Remarks:

Doses / Concentrations:
0, 94, 188, or 375 ppm
Basis:
nominal conc.

No. of animals per sex per dose:

50 male and 50 female

Control animals:

yes, concurrent vehicle

Details on study design:

Study conduct: Groups of 50 animals/sex were exposed to 0, 94, 188 or 375 ppm test material by inhalation, 6 hours and 12 minutes per day, 5 days per week for 103 weeks. All animals were observed twice daily. Clinical findings were redorded monthly through week 91, and then every 2 weeks until the end of the studies. Animals were weighed at the beginning of the study, weekly through week 12, monthly from week 15 through week 91, every 2 weeks thereafter and at the end of the study.

Positive control:

Not applicable.

Observations and examinations performed and frequency:

All animals were observed twice daily. Clinical findings were redorded monthly through week 91, and then every 2 weeks until the end of the studies. Animals were weighed at the beginning of the study, weekly through week 12, monthly from week 15 through week 91, every 2 weeks thereafter and at the end of the study.

Sacrifice and pathology:

Complete necropsies were performed on all animals. At necropsy, all organs and tissues were examined for gross lesions and the gross lesions, tissue masses, adrenal gland, bone and bone marrow, brain, clitoral gland, epididymis, esophagus, heart, kidney, large intestine, larynx, liver, lung, lymph nodes, mammary gland, nose, ovary, pancreas, parathyroid, pituitary, preputial gland, prostate, salivary gland, seminal vesicle, skin, small intestine, spinal cord and sciatic nerve (in controls and high dose animals), spleen, stomach, testis, thymus, thyroid, trachea, urinary bladder and uterus were fixed in 10% neutral buffered formalin, sectioned, stained and examined microscopically. Samples of each paired organ were examined. Complete histopathological examinations were performed on all animals. The sciatic nerves and spianl cords from approximately 15 animals/sex in the control and 375 ppm groups were examined.

Microscopic evaluations were completed by the study laboratory pathologist. A quality assessment pathologist reviewed the lung, nasal cavity, all neoplasms, the kidneys of males, the mammary glands of females, and the sciatic nerve samples. The NTP Pathology Working group chairperson reviewed selected tissues and addressed any inconsistencies between the two pathologists.

Other examinations:

No additional information available.

Statistics:

Statistical analyses: The probability of survival was estimated by the product-limit procedure of Kaplan and Meier (J Am Stat Assoc 53:457-481, 1958). Animals found dead of other than natural causes were removed from the analyses. Survival data were tested for equality using Cox's test (J R Stat Soc B34:187-220, 1972) and dose-related trends were identified using Tarone's test (Biometrika 62:679-682).

Logistic regression was used to analyze neoplasm indices. This analysis assumed that the diagnosed neoplasms were discovered as the result of death from an unrelated cause and did not affect the risk of death. Both linear and quadratic terms in time were incorporated initially, and the quadratic term was eliminated if the fit of the model was not significantly enhanced. The neoplasm indices of exposed and control animals were compared on the basis of the likelihood score test for the regression coefficient of dose. Data were also analyzed using the life table test (Cox, 1972, Tarone, 1975, see above), the Fisher exact test and the Cochran-Armitage trend test (Statisitcal Methods in Medical Research, John Wiley and Sons, 1971, p. 362-365).

Tests of significance included for neoplasms included pairwise comparisons of each exposed group with controls and a test for an overall dose-related trend. Continuity-corrected tests were used in the analysis of neoplasm incidence and reported P values are one-sided. The Seilkop model (Fund Appl Toxicol 24:247-259) was used to evaluate the possible impact of survival and body weight differences on the incidences of mammary gland tumors in females.

A logistic regression analysis was used to analyze incidences of nonneoplastic lesions. Body weight data were analyzed with the parametric multiple comparison procedures of Dunnett (J Am Stat Assoc 50:1096-1121, 1955) and Williams (Biometrics 27:103-117, 1971 and Biometrics 28:519-531, 1972).

Clinical signs:

no effects observed

Mortality:

no mortality observed

Body weight and weight changes:

no effects observed

Food consumption and compound intake (if feeding study):

not specified

Food efficiency:

not specified

Water consumption and compound intake (if drinking water study):

not specified

Ophthalmological findings:

not specified

Haematological findings:

not specified

Clinical biochemistry findings:

not specified

Urinalysis findings:

not specified

Behaviour (functional findings):

not specified

Organ weight findings including organ / body weight ratios:

not specified

Gross pathological findings:

effects observed, treatment-related

Description (incidence and severity):

masses on shoulder and torso

Histopathological findings: non-neoplastic:

no effects observed

Histopathological findings: neoplastic:

effects observed, treatment-related

Details on results:

Test material concentrations: The average (+/- SD) concentrations of test material analyzed in chambers containing nominal concentrations of 94, 188 and 375 ppm were 94 +/- 4, 188 +/- 7 and 375 +/- 10 ppm. The total number of samples taken from each chamber ranged from 5575-5791. Since analytical concentrations were very close to nominal concentrations, results are reported according to nominal concentrations.

Effects at 375 ppm: There was no effect of treatment on survival or male body weights at this or lower concentrations. There were no indications of hindlimb paralysis at this or lower concentrations. From week 23 to the end of the study, the mean body weight of females was 6% greater than that of controls. Clinical findings (masses on shoulder and torso) were consistent with mammary gland neoplasms. The incidences of fibroadenoma (72%), fibroadenoma or adenoma (combined)(72%), carcinoma (22%), adenoma or carcinoma (26%), and fibroadenoma, adenoma, or carcinoma (combined)(82%) in the mammary gland were increased with respect to those of controls (38%, 40%, 4%, 8% and 42%, respectively). The incidences of these tumors also were greater than those of historical controls. There was no increase in mammary gland neoplasms in males. The incidence of mononuclear cell leukemia in females (18%) was lower than that of controls (44%).


Effects at 188 ppm: From week 23 to the end of the study, the mean body weight of females was 3-4% greater than that of controls. Clinical findings (masses on shoulder and torso) were consistent with mammary gland neoplasms. The incidences of fibroadenoma (66%), fibroadenoma or adenoma (combined)(66%), and fibroadenoma, adenoma, or carcinoma (combined)(68%) in the mammary gland were increased with respect to those of controls (38%, 40% and 42%, respectively). The incidences of these tumors also were greater than those of historical controls. There was no increase in mammary gland neoplasms in males.

Effects at 94 ppm: No treatment-related effects were observed.

Relevance of carcinogenic effects / potential:

Nitromethane is an animal carcinogen with unknown relevance to humans.

Dose descriptor:

NOEC

Effect level:

375 ppm

Based on:

test mat.

Sex:

male

Basis for effect level:

other: Highest concentration examined.

Remarks on result:

other:

Remarks:

Effect type: other: toxicity/carcinogenicity (migrated information)

Dose descriptor:

NOEC

Effect level:

94 ppm

Based on:

test mat.

Sex:

female

Remarks on result:

other:

Remarks:

Effect type: other: toxicity/carcinogenicity (migrated information)

Dose descriptor:

LOEC

Effect level:

188 ppm

Based on:

test mat.

Sex:

female

Basis for effect level:

other: The incidences of fibroadenoma was 66% in the 188 ppm exposed females vs 38% in the control group.

Remarks on result:

other: Effect type: carcinogenicity (migrated information)

It was stated in the methods section that a Selikop model was used to evaluate the effect of body weight on tumor incidences. However, the results of this analysis with regard to incidences of mammary gland tumors in females were not listed. The authors stated that the elevated body weights of females in the 188 and 375 ppm groups could not account for the markedly increased incidences of mammary gland neoplasms. While the incidence of mammary gland tumors was near the upper bound of the historical control range (16%-46%), it was similar to the 46% rate predicted by the Selikop logistic regression model

Conclusions:

Under the conditions of these 2-year inhalation studies, there was no evidence of carcinogenic activity of nitromethane in male F344/N rats exposed to 94, 188, or 375 ppm. There was clear evidence of carcinogenic activity of nitromethane in female F344/N rats based on increased incidences of mammary gland fibroadenomas and carcinomas.

Executive summary:

The carcinogenic potential of nitromethane was examined in Fischer 344 rats. Groups of 50 male and 50 female rats were exposed to 0, 94, 188, or 375 ppm nitromethane by inhalation, 6 hours per day, 5 days per week, for 103 weeks. There were no significant differences in survival rates between exposed and control male or female rats. The mean body weights of females in the 375 ppm group was slightly greater than that of the control group; the mean body weights of exposed males were generally similar to the mean body weight of the controls throughout the study. Clinical findings were consistent with incidences of mammary gland neoplasms in females exposed to 188 or 375 ppm; no hindlimb paralysis, as occurred in rats in the 13-week study, was observed in male or female rats in the 2-year study. The incidences of mammary gland fibroadenoma and fibroadenoma, adenoma, or carcinoma (combined) in female rats in the 188 and 375 ppm groups were significantly greater than in the controls (incidence of mammary gland fibroadenomas was 38, 42, 66 and 72% in the 0, 94, 188 and 375 ppm groups, respectively). Under the conditions of these 2-year inhalation studies, there was no evidence of carcinogenic activity of nitromethane in male F344/N rats exposed to 94, 188, or 375 ppm. There was clear evidence of carcinogenic activity of nitromethane in female F344/N rats based on increased incidences of mammary gland fibroadenomas and carcinomas.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

LOAEC

469 mg/m³

Study duration:

chronic

Species:

rat

Quality of whole database:

Good

Carcinogenicity: via dermal route

Endpoint conclusion

Endpoint conclusion:

no study available

Justification for classification or non-classification

Based on an NTP study, nitromethane can cause cancer in laboratory animals. Therefore nitromethane is classified as a catagory 2 carcinogen according to GHS.

Additional information

Rat inhalation study

The carcinogenic potential of nitromethane was examined in Fischer 344 rats. Groups of 50 male and 50 female rats were exposed to 0, 94, 188, or 375 ppm nitromethane by inhalation, 6 hours per day, 5 days per week, for 103 weeks. There were no significant differences in survival rates between exposed and control male or female rats. The mean body weights of females in the 375 ppm group was slightly greater than that of the control group; the mean body weights of exposed males were generally similar to the mean body weight of the controls throughout the study. Clinical findings were consistent with incidences of mammary gland neoplasms in females exposed to 188 or 375 ppm; no hindlimb paralysis, as occurred in rats in the 13-week study, was observed in male or female rats in the 2-year study. The incidences of mammary gland fibroadenoma and fibroadenoma, adenoma, or carcinoma (combined) in female rats in the 188 and 375 ppm groups were significantly greater than in the controls (incidence of mammary gland fibroadenomas was 38, 42, 66 and 72% in the 0, 94, 188 and 375 ppm groups, respectively). Under the conditions of these 2-year inhalation studies, there was no evidence of carcinogenic activity of nitromethane in male F344/N rats exposed to 94, 188, or 375 ppm. There was clear evidence of carcinogenic activity of nitromethane in female F344/N rats based on increased incidences of mammary gland fibroadenomas and carcinomas. 

Male and female Long-Evans rats were exposed to vapors of nitromethane (NM) at either 100 ppm or 200 ppm. The animals were exposed seven hours per day, five days per week for two years. Control groups of rats were also housed in a similar inhalation chamber, but NM was not introduced into the chamber. The animals were observed daily for signs of pharmacologic or toxicologic effect and body weights were recorded periodically. At the two-year termination of the exposure period, clinical laboratory examination (serum chemistry and hematology) were obtained on selected animals and all surviving animals were sacrificed. All animals were subjected to a thorough histopathologic examination. During the study there were no pharmacologic effects from exposure to NM at either 100 ppm or 200 ppm. There was no effect on mortality on either sex at either exposure level. Body weights of male rats exposed to NM were not significantly different from control rats, but the body weights of female rats of both exposure groups were slightly less than their controls. There was no effect of exposure of rats of either sex to either level of NM on hematology. There were no clinically significant effects on serum chemistry although small increases in serum creatinine were observed in both sexes. There were no effects of exposure to NM on organ weights. There were no significant differences in the non-neoplastic or neoplastic pathology related to exposure to NM.

Mouse inhalation study

Groups of 50 male and 50 female mice were exposed to 0, 188, 375 or 750 ppm nitromethane by inhalation, 6 hours/day, 5 days/week for 103 weeks. The survival rate of females in the 750 ppm group was marginally greater than that of the controls. The mean body weights of exposed females were generally slightly greater than the mean body weights of the controls during the study but were generally similar to the mean body weight of the controls at the end of the study. The mean body weights of exposed males were similar to those of the controls throughout the study. Clinical findings included swelling around the eyes and exophthalmos in exposed males and females; these findings were coincident with harderian gland neoplasms. The incidences of harderian gland adenoma and adenoma or carcinoma (combined) in exposed mice increased with increasing exposure concentration and were significantly greater in males and females in the 375 and 750 ppm groups than those in the controls. The incidences of harderian gland carcinoma in males and females in the 375 and 750 ppm groups were also slightly greater than those in the controls. Female mice in the 188 and 750 ppm groups had significantly greater incidences of hepatocellular adenoma and hepatocellular adenoma or carcinoma (combined) than the controls. The incidences of liver eosinophilic focus increased with increasing exposure concentration, and incidences in the 375 and 750 ppm groups were significantly greater than the control incidence. The incidences of alveolar/bronchiolar carcinoma in male mice in the 750 ppm group and female mice in the 375 ppm group were significantly greater than those in the controls. Females in the 750 ppm group also had a significantly greater incidence of alveolar/bronchiolar adenoma or carcinoma (combined) and a slightly greater incidence of alveolar/bronchiolar adenoma than the controls. Females in the 375 ppm group had a significantly greater incidence of cellular infiltration of histiocytes in the lung than the controls. The incidences of degeneration and metaplasia of the olfactory epithelium and hyaline degeneration of the respiratory epithelium were significantly greater in exposed male and female mice than those in the controls. Additionally, males in the 375 and 750 ppm groups had significantly greater incidences of inflammation of the nasolacrimal duct than did the controls. In summary, there was clear evidence of carcinogenic activity of nitromethane in male B6C3F1 mice based on increased incidences of harderian gland adenomas and carcinomas. There was clear evidence of carcinogenic activity in female B6C3F1 mice, based on increased incidences of liver neoplasms (primarily adenomas) and harderian gland adenomas and carcinomas. Increased incidences of alveolar/bronchiolar adenomas and carcinomas in male and female mice exposed to nitromethane were also considered to be related to chemical administration.

Justification for selection of carcinogenicity via inhalation route endpoint:

The study was conducted according to test guidelines and in accordance with GLP

Carcinogenicity: via inhalation route (target organ): glandular: mammary gland