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Toxicological information

Repeated dose toxicity: inhalation

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Administrative data

chronic toxicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Guideline study without detailed documentation
Reason / purpose for cross-reference:
reference to same study

Data source

Referenceopen allclose all

Reference Type:
study report
Reference Type:
Toxicity and carcinogenicity of methyl isobutyl ketone in F344N rats and B6C3F1 mice following 2-year inhalation exposure
Stout MD, Herbert RA, Kissling GE, Suarez F, Roycroft JH, Chhabra RS & Bucher JR
Bibliographic source:
Toxicology, 244, 209-219

Materials and methods

Test guideline
equivalent or similar to guideline
other: OECD Guideline 451 (Carcinogenicity Studies)
GLP compliance:
Limit test:

Test material

Constituent 1
Chemical structure
Reference substance name:
EC Number:
EC Name:
Cas Number:
Molecular formula:
Details on test material:
- Name of test material (as cited in study report): Methyl isobutyl ketone (MIBK)
- Analytical purity: >99%
- Storage condition of test material: Room temperature

Test animals

other: F344/N
Details on test animals or test system and environmental conditions:
- Source: Taconic Laboratory Animals and Services (Germantown, NY)
- Age at study initiation: approximately 6 weeks old
- Housing: Individually housed
- Diet (e.g. ad libitum): NTP-2000 non-purified diet (irradiated wafers; obtained from Zeigler Brothers, Inc.) was available ad libitum except during exposure periods and changed weekly.
- Water (e.g. ad libitum): tap water was available ad libitum via an automatic watering system.
- Acclimation period: approximately 2 weeks

- Temperature (°C): 23 ± 3°C
- Humidity (%): 55 ±15%
- Air changes (per hr): 15±2
- Photoperiod (hrs dark / hrs light): 12/12

Administration / exposure

Route of administration:
inhalation: vapour
Type of inhalation exposure:
whole body
clean air
Details on inhalation exposure:
Chamber Air Supply Filters:
Single HEPA, changed annually; charcoal (RSE, Inc., New Baltimore, MI); and Purafil (Environmental Systems, Lynnwood, WA) not changed

Stainless steel (Lab Products, Inc., Harford Systems Division, Aberdeen, MD), changed weekly

Chamber Environment:
Temperature: 75° ± 3° F Relative humidity: 55% ± 15% Room fluorescent light: 12 hours/day Chamber air changes: 15 ± 2/hour
Analytical verification of doses or concentrations:
Details on analytical verification of doses or concentrations:
The MIBK concentrations in the exposure chambers were monitored by an on-line gas chromatograph approximately every 28 min. Buildup and decay rates for chamber vapor concentrations were determined with animals present in the chambers and the time to achieve 90% of the target concentration after the beginning of vapor generation (T90) was 12 min. Evaluations of chamber uniformity and persistence and monitoring for MIBK degradation impurities were conducted periodically throughout the studies by gas chromatography. Chamber uniformity was maintained and no degradation was detected.

Duration of treatment / exposure:
2 years
Frequency of treatment:
6h/day, 5 days per week
Doses / concentrationsopen allclose all
Doses / Concentrations:
450, 900, or 1800 ppm
analytical conc.
Doses / Concentrations:
1843, 3686 and 7373 mg/m3
analytical conc.
No. of animals per sex per dose:
Control animals:
yes, sham-exposed
Details on study design:
- Rationale for animal assignment (if not random): random
Positive control:
Not applicable


Observations and examinations performed and frequency:
- Time schedule: Twice daily

- Time schedule: Weekly for the first 13 weeks, monthly until the last four months of the studies, every 2 weeks thereafter, and at the end of the studies.

- Time schedule for examinations: Body weights were recorded initially and then weekly for the first 13 weeks, monthly until the last four months of the studies, every 2 weeks thereafter, and at the end of the studies.

- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: No

- Body weight gain in kg/food consumption in kg per unit time X 100 calculated as time-weighted averages from the consumption and body weight gain data: No







Sacrifice and pathology:
Complete histopathology was performed on all rats. In addition to gross lesions and tissue masses, the following tissues were examined: adrenal gland, bone with marrow, brain, clitoral gland, esophagus, eye, harderian gland, heart and aorta, large intestine (cecum, colon, rectum), small intestine (duodenum, jejunum, ileum), kidney, larynx, liver, lung and bronchi, lymph nodes (mandibular, mesenteric, bronchial, mediastinal), mammary gland, nose, ovary, pancreas, parathyroid gland, pituitary gland, preputial gland, prostate gland, salivary glands, skin, spleen, stomach (forestomach and glandular), testis with epididymis and seminal vesicle, thymus, thyroid gland, trachea, urinary bladder, and uterus.
The probability of survival was estimated by the product limit procedure of Kaplan and Meier (1958). Statistical analyses for possible dose-related effects on survival used Cox (1972) method for testing two groups for equality and Tarone (1975) life table test to identify dose-related trends. All reported P values for the survival analyses are two sided. Average severity values were analyzed for significance with the Mann–Whitney U test. The poly-k test was used to assess neoplasm and non-neoplastic lesion prevalence. This test is a survival-adjusted quantal-response procedure that modifies the Cochran–Armitage linear trend test to take survival differences into account. Unless otherwise specified, a value of k = 3 was used in the analysis of site-specific lesions. Tests of significance included pairwise comparisons of each exposed group with controls and a test for an overall exposure related trend. Continuity-corrected poly-3 tests were used in the analysis of lesion incidence, and reported P values are one sided.

Results and discussion

Results of examinations

Clinical signs:
effects observed, treatment-related
mortality observed, treatment-related
Body weight and weight changes:
effects observed, treatment-related
Food consumption and compound intake (if feeding study):
not examined
Food efficiency:
not examined
Water consumption and compound intake (if drinking water study):
not examined
Ophthalmological findings:
not examined
Haematological findings:
not examined
Clinical biochemistry findings:
not examined
Urinalysis findings:
not examined
Behaviour (functional findings):
not examined
Organ weight findings including organ / body weight ratios:
not specified
Gross pathological findings:
not specified
Histopathological findings: non-neoplastic:
effects observed, treatment-related
Histopathological findings: neoplastic:
effects observed, treatment-related
Details on results:
Survival of the 1800 ppm males was significantly less than that of the controls (0 ppm, 32/50; 450 ppm, 28/49; 900 ppm, 25/50; 1800 ppm, 19/50). Mean body weights of 900 ppm male rats were 6–8% less after week 97 and 1800 ppm males were 5–8% less after week 89, than those of the chamber control group. Mean body weights and survival (0 ppm, 35/50; 450 ppm, 34/50; 900 ppm, 26/50; 1800 ppm, 32/50) of all exposed groups of females were similar to those of the controls.

Chronic progressive nephropathy (CPN) similar to that which occurs in aged rats was observed in males and females in all groups, including controls (Tables 1 and 2). In males, significant increases in incidence were observed at 1800 ppm and in severity at all exposure concentrations. The incidence of mineralization was also significantly increased at all exposure concentrations in males; severity was generally increased in exposed groups (Table 1). In female rats, increased incidences of CPN were significant in all exposed groups. The average severity of CPN ranged from minimal to mild and was increased in exposed females at 1800 ppm. In both sexes, changes consisted of a spectrum of lesions that included varying degrees of renal tubule dilatation with and without hyaline (proteinaceous) casts, multifocal degeneration, regeneration, and hypertrophy of the tubular epithelium; thickening of the tubular and glomerular basement membranes; glomerulosclerosis; interstitial fibrosis; and variable infiltrates of mononuclear inflammatory cells within the interstitium. Minimal CPN affected less than 10% of the renal parenchyma, and consisted of focal to multifocal regenerative renal tubules surrounded by a thickened basement membrane. These regenerative tubules were small and lined by cuboidal basophilic epithelial cells. Mild CPN affected approximately 10–39% of the renal parenchyma and consisted of multifocal clusters of regenerative renal tubules, tubules that contained protein, glomeruli with thickened basement membranes, and scattered infiltrates of predominantly lymphocytes and macrophages. Moderate CPN had similar but more severe and widespread changes including glomerular atrophy and variable interstitial fibrosis. Marked CPN was diffuse and of greater severity. Mineralization was generally of minimal to mild severity and consisted of linear deposits of lamellated mineral within the lumen or epithelial cells of the collecting tubules of the renal papilla. There were exposure concentration-related increases in minimal to mild transitional epithelial hyperplasia in the renal pelvis of male rats, which were significant at 900 and 1800 ppm (Table 1). Transitional epithelial hyperplasia consisted of focal proliferation of the transitional epithelium lining the renal pelvis; the affected epithelium appeared thickened and often formed papillary projections into the urinary space. In the single section analysis (standard evaluation) of the kidney in males (Table 1), increases in renal tubule hyperplasia were significantat 450 and 1800 ppm, and the severities in these groups were elevated. There were also slight increases in renal tubule adenoma, carcinoma, and adenoma or carcinoma (combined). There were significant positive trends for adenomas and adenomas or carcinomas (combined). Although not statistically significant, the incidences of renal tubule adenoma and renal tubule adenoma or carcinoma (combined) in the 900 and 1800 ppm groups and renal tubule carcinoma in the 1800 ppm group exceeded the historical ranges for chamber controls from inhalation studies fed NTP-2000 diet. In the extended evaluation of the kidneys (Table 1), additional renal tubule hyperplasias were observed in all exposed groups such that in the combined single and step section analysis, the incidences of hyperplasia in exposed groups were significantly greater than in the controls. Additional renal tubule adenomas were observed in all groups including the controls. No additional renal tubule carcinomas were observed. In the combined single and step section analysis of renal neoplasms, there were significant positive trends for renal adenoma and adenoma or carcinoma (combined) and the incidences of these lesions were significantly increased at 1800 ppm. Hyperplasia occurred as single or multiple expanded cortical tubules composed of increased numbers of tubular epithelial cells arranged in multiple layers that partially or completely filled the tubule. Renal tubule adenomas were discrete, highly cellular, proliferative lesions that were larger than focal hyperplasias (generally greater than the combined diameter of five normal-sized renal tubules). Adenomas tended to have a more complex structure than hyperplasias and were characterized by closely packed tubules and solid nests composed of cells with large vesicular nuclei and abundant pale eosinophilic cytoplasm which sometime contained clear vacuoles. Renal tubule carcinomas were highly cellular, expansive and invasive masses composed of large basophilic to amphophilic cells that formed large multilayered tubular structures, solid nests, and sheets. Renal mesenchymal tumors occurred in two female rats in the 1800 ppm group (Table 2). Both neoplasms observed in this study were single, small to medium sized masses with poorly defined margins and were composed of sheets of mature mesenchymal (spindle) cells that infiltrated the inner cortex, medulla, and renal pelvis encircling and sequestering glomeruli, tubules, and collecting ducts. Lesions at sites other than the kidney were also observed in males. Mononuclear cell leukemia (0 ppm, 25/50; 450 ppm, 26/50; 900 ppm, 32/50; 1800 ppm, 35/50) increasedwith a significant positive trend and at 1800 ppm, the increase was significant and exceeded the historical ranges for chamber controls from inhalation studies fed NTP-2000 diet (188/399, 47±10%; range 32–66%). Adrenal medulla hyperplasia was also significantly increased at 1800 ppm (0 ppm, 13/50; 450 ppm, 18/48; 900 ppm, 18/50; 1800 ppm, 24/50). There were also exposure-related increases in benign or malignant pheochromocytoma (combined) of the adrenal gland in male rats (0 ppm, 8/50; 450 ppm, 9/48; 900 ppm, 11/50; 1800 ppm, 14/50). However, these increases were not significant and were within the historical ranges for chamber controls from inhalation studies fed NTP-2000 diet (69/398, 17±7%; range 10–28%), although the incidence in the 1800 ppm group was the upper limit of the historical range.

Effect levels

Dose descriptor:
Effect level:
450 ppm (analytical)
Basis for effect level:
other: = 1840 mg/m3, non-neoplastic lesions observed in the kidneys at higher dose levels and the irrelevance to humans of the tumour types observed in the kidneys of male rats

Target system / organ toxicity

Critical effects observed:
not specified

Applicant's summary and conclusion

A NOAEC was not reported by the study authors. Review of the study data suggests that a NOAEC of 450 ppm can be derived for neoplastic and non-neoplastic lesions, based on the non-neoplastic lesions observed in the kidneys at higher dose levels and the irrelevance to humans of the tumour types observed in the kidneys of male rats.
Executive summary:

In a whole body 2-year inhalation study in Fischer 344 rats, animals (50/sex/group) were administered the read-across substance, MIBK, at concentrations of 0 (control), 450, 900 or 1800 ppm for 6 hours per day, 5 days per week for 2 years. This GLP study was equivalent to OECD Test Guideline 451. Mortality was observed in all groups administered test article. However, survival was significantly decreased in males administered MIBK at 1800 ppm as compared to controls. Mean body weights also were decreased in males administered 900 ppm and 1800 ppm as compared to controls. The mean body weights and survival in treated females were similar to controls. The primary target of MIBK toxicity was the kidney in rats. Briefly, chronic progressive nephropathy (CPN) similar to that which occurs in aged rats also was observed in all rats (including controls). There were treatment related significant increases in both the incidence (1800 ppm) and severity in all exposed groups. Kidney lesions that typically accompany CPN also were reported in males exposed to 900 ppm and 1800 ppm MIBK. The kidney lesions observed were suggestive of a2µ-globulin nephropathy (specific to male rat), a mechanism of xenobiotic-induced renal carcinogenesis for which there is no human counterpart. A NOAEC was not identified by the authors. Review of the study data suggests that a NOAEC of 450 ppm (1843 mg/m3) can be derived for neoplastic and non-neoplastic lesions, based on the non-neoplastic lesions observed in the kidneys at higher dose levels and the irrelevance to humans of the tumour types observed in the kidneys of male rats.