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Description of key information

The NOAEC for methyl isobutyl ketone (MIBK) from a GLP 2-year whole-body inhalation study in F344N rats (equivalent to OECD Test Guideline 451) was determined to be 450 ppm.   A similar NOAEC of 450 ppm was determined for B6C3F1 mice in a concurrent 2-year study.  Ninety-day inhalation studies in Fischer 344 rats and B6C3F1 mice also were conducted and a NOAEC of 1002 ppm was established for both species.  In an oral (gavage) subchronic toxicity study in Sprague-Dawley rats, the no-observed adverse effect level (NOAEL) was established at 250 mg/kg body weight/day. 
No dermal repeat-dose toxicity studies have been conducted.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records
Reference
Endpoint:
sub-chronic toxicity: oral
Type of information:
experimental study
Adequacy of study:
key study
Study period:
1986
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Well documented. According to acceptable guidelines.
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 408 (Repeated Dose 90-Day Oral Toxicity Study in Rodents)
Deviations:
yes
Remarks:
- humidity was >70% in 2 instances
GLP compliance:
yes
Limit test:
no
Species:
rat
Strain:
Sprague-Dawley
Sex:
male/female
Route of administration:
oral: gavage
Vehicle:
corn oil
Analytical verification of doses or concentrations:
yes
Duration of treatment / exposure:
13 weeks
Frequency of treatment:
7 days/week
Remarks:
Doses / Concentrations:
0, 50, 250, or 1000 mg/kg bw/day
Basis:
actual ingested
No. of animals per sex per dose:
30/sex/dose
Control animals:
yes, concurrent vehicle
Observations and examinations performed and frequency:
Non-fasted body weights recorded on Test Day 1 and weekly thereafter.

Individual food consumption data were collected on a weekly basis.

Moribundity/mortality checks were performed twice daily (morning and afternoon).

Test animals were examined once/week for clinical signs of toxicity.

Ophthalmology examinations were performed during the quarantine period and in Test Week 13.

Hematology, clinical chemistry, and urinalysis were performed on 10 rats/sex/dose during Test Week 7 (interim clinical pathology) and on Test Week 14.
Sacrifice and pathology:
Necropsy evaluations were performed on 10 rats/sex/dose during Test Week 7 (interim necropsy) and on all remaining animals on Test Week 14.

Histological tissue examinations were conducted on the control and high-dose animals.
Statistics:
Statistics: one-way ANOVA with Dunnett's t-test.
Clinical signs:
effects observed, treatment-related
Mortality:
mortality observed, treatment-related
Body weight and weight changes:
effects observed, treatment-related
Food consumption and compound intake (if feeding study):
effects observed, treatment-related
Food efficiency:
not examined
Water consumption and compound intake (if drinking water study):
not examined
Ophthalmological findings:
no effects observed
Haematological findings:
effects observed, treatment-related
Clinical biochemistry findings:
effects observed, treatment-related
Urinalysis findings:
effects observed, treatment-related
Behaviour (functional findings):
not examined
Organ weight findings including organ / body weight ratios:
effects observed, treatment-related
Gross pathological findings:
effects observed, treatment-related
Histopathological findings: non-neoplastic:
effects observed, treatment-related
Histopathological findings: neoplastic:
not examined
Details on results:
CLINICAL SIGNS AND MORTALITY
Reversible lethargy was observed in rats of both sexes receiving 1000 mg/kg-day (but not at lower dose levels) for a few hours following dosing and reportedly decreased in incidence and severity during the study.
Two female rats administered 1000 mg/kg bw/d died during the study (one during Test Week 6 and the other during Test Week 13. These animals demonstrated mottled lungs at gross necropsy. In addition, the liver and kidneys were dark red for one of the animals). A control female also was found dead on Test Week 5.

BODY WEIGHT AND WEIGHT GAIN
Males in the high-dose group showed a slight (9%) but significantly decreased mean body weight gain as compared to controls during the last 2 weeks of exposure, whereas female body weight gain was significantly increased during 5 of the last 6 weeks of exposure.

FOOD CONSUMPTION AND COMPOUND INTAKE (if feeding study)
Both male and female food consumption was significantly increased during the second half of the exposure period.

OPHTHALMOSCOPIC EXAMINATION
No ophthalmic lesions were noted.

HAEMATOLOGY
The only potentially exposure-related hematological effects observed were slight but statistically significant increases in hemoglobin (+6%) and hematocrit (+8%) at terminal sacrifice in females administered 1000 mg/kg-day and a 15% decrease in lymphocyte count in high-dose males at terminal sacrifice.

CLINICAL CHEMISTRY
250 mg/kg-day: increased (+39%) serum glutamic-pyruvic transaminase (SGPT) in female rats at the terminal sacrifice.
1000 mg/kg-day: increased SGPT (+73%, interim; +34%, terminal) in females as compared to controls, increased serum alkaline phosphatase (+84%, interim) in females, increased serum cholesterol in males (+30%, interim) and females (+59%, interim; +65%, final), decreased albumin/globulin ratio in males (-16%, interim), and minimally increased serum total protein in females (+9%, interim; +10%, terminal), increased blood-urea-nitrogen (BUN) in males (+37%, interim), increased serum potassium in males (+34%, terminal), decreased serum glucose in males (-27%, terminal), and a reported increase in urinary protein and ketones in males and females at terminal sacrifice (summary data were not provided).

URINALYSIS
Increased quantities of urinary protein and ketones and elevated specific gravity were generally seen for high-dose and, to a lesser extent, mid-dose animals of both sexes at both the interim and teminal kill periods. An elevated number of epithelial cells also occurred for high- and mid-dose males at the interim kills.

NEUROBEHAVIOUR
No data

ORGAN WEIGHTS
- 250 mg/kg bw/d
Liver: increased terminal absolute (+34%, males; +39%, females) and relative (+42%, males; +38%, females) weights
Kidneys: increased terminal absolute or relative kidney weights in males and females, ranging from 6 to 12% over controls

- 1000 mg/kg bw/d
Liver: increased terminal absolute (+34%, males; +39%, females) and relative (+42%, males; +38%, females) liver weights,
Kidneys: increased terminal absolute and relative kidney weights (from 25 to 34% in males and from 20 to 22% in females) as compared to controls
Adrenals: increased relative weights in male (+29%) and female (+11%) rats
Testis: slightly increased relative weights (+9%) in males

GROSS PATHOLOGY
Gross necropsy failed to suggest treatment-related lesions.

HISTOPATHOLOGY: NON-NEOPLASTIC
1000mg/kg bw/d: increased incidence of male rats with mild nephropathy (multifocally distributed swollen or hyperchromatic and flattened renal cortical tubular epithelial cells) at 1000 mg/kg-day (16/20) as compared to controls (4/20) but no increase in such lesions in females.
Dose descriptor:
NOEL
Effect level:
50 mg/kg bw/day (actual dose received)
Sex:
male/female
Basis for effect level:
other: No treatment-related effects of any kind
Dose descriptor:
NOAEL
Effect level:
250 mg/kg bw/day (actual dose received)
Sex:
male/female
Basis for effect level:
other: Increases in relative kidney weights for male and females rats administered MIBK at doses of 250 mg/kg bw/d but without histological lesions
Dose descriptor:
LOAEL
Effect level:
1 000 mg/kg bw/day (actual dose received)
Sex:
male/female
Basis for effect level:
other: Based on kidney changes, hepatomegaly and alterations in clinical chemistry and urinalysis parameters
Critical effects observed:
not specified
Conclusions:
The NOAEL was estimated to be 250 mg/kg bw/d, based on increases in relative kidney weights for male and female rats administered MIBK at doses of 250 mg/kg bw/d but without histological lesions. Effects at higher doses included kidney changes, hepatomegaly and alterations in clinical chemistry and urinalysis parameters. No treatment-related effects of any kind were observed at 50 mg/kg-day.
Executive summary:

Groups of 30 male and 30 female Sprague-Dawley rats were administered MIBK by gavage in corn oil at daily dose levels of 0 (vehicle control), 50, 250, or 1000 mg/kg-day for 13 consecutive weeks and evaluated for exposure-related changes in body weight, food consumption, mortality, clinical signs, ophthalmological parameters, and terminal organ weights (heart, liver, spleen, brain, kidney, gonads, adrenals, thyroid, and parathyroid). The following evaluations were conducted in rats from each exposure level at interim (week 7) and final sacrifices: hematology, clinical chemistry, urinalysis, and comprehensive gross pathology. All tissue samples collected during gross necropsy in high-dose and control rats were evaluated for histopathologies, and kidney samples were also histologically evaluated in mid-dose rats.

Reversible lethargy was observed in rats of both sexes receiving 1000 mg/kg-day (but not at lower dose levels) for a few hours following dosing and reportedly decreased in incidence and severity during the study. Males in the high-dose group showed a slight (9%) but significantly decreased mean body weight gain as compared to controls during the last 2 weeks of exposure, whereas female body weight gain was significantly increased during 5 of the last 6 weeks of exposure. Both male and female food consumption was significantly increased during the second half of the exposure period. The only potentially exposure-related hematological effects observed were slight but statistically significant increases in hemoglobin (+6%) and hematocrit (+8%) at terminal sacrifice in females administered 1000 mg/kg-day and a 15% decrease in lymphocyte count in high-dose males at terminal sacrifice.

The lowest hepatic effect level that was observed in the oral exposure studies was 250 mg/kg-day for increased (+39%) serum glutamic-pyruvic transaminase (SGPT) in female rats at the terminal sacrifice. The following changes suggestive of adverse liver effects were observed at 1000 mg/kg-day at either interim and/or final sacrifice: increased SGPT (+73%, interim; +34%, terminal) in females as compared to controls, increased serum alkaline phosphatase (+84%, interim) in females, increased serum cholesterol in males (+30%, interim) and females (+59%, interim; +65%, final), increased terminal absolute (+34%, males; +39%, females) and relative (+42%, males; +38%, females) liver weights, decreased albumin/globulin ratio in males (-16%, interim), and minimally increased serum total protein in females (+9%, interim; +10%, terminal).

The only renal effect occurring at 250 mg/kg-day was increased terminal absolute or relative kidney weights in males and females, ranging from 6 to 12% over controls. The following changes suggestive of adverse kidney effects were observed at 1000 mg/kg-day: increased terminal absolute and relative kidney weights (from 25 to 34% in males and from 20 to 22% in females) as compared to controls, increased blood-urea-nitrogen (BUN) in males (+37%, interim), increased serum potassium in males (+34%, terminal), decreased serum glucose in males (-27%, terminal), and a reported increase in urinary protein and ketones in males and females at terminal sacrifice (summary data were not provided). Histological examination of kidney tissues revealed an increased incidence of male rats with mild nephropathy (multifocally distributed swollen or hyperchromatic and flattened renal cortical tubular epithelial cells) at 1000 mg/kg-day (16/20) as compared to controls (4/20) but no increase in such lesions in females.

Significantly increased relative adrenal weights in male (+29%) and female (+11%) rats and slightly increased relative testis weights (+9%) in males were also observed at 1000 mg/kgday. No exposure-related histopathologic lesions were evident in the liver or adrenal glands nor in any other tissue that was examined, aside from the kidney. The NOAEL was estimated to be 250 mg/kg bw/d, based on increases in relative kidney weights for male and female rats administered MIBK at doses of 250 mg/kg bw/d but without histological lesions. Effects at higher doses included kidney changes, hepatomegaly and alterations in clinical chemistry and urinalysis parameters. No treatment-related effects of any kind were observed at 50 mg/kg-day.

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
NOAEL
250 mg/kg bw/day
Study duration:
subchronic
Species:
rat

Repeated dose toxicity: inhalation - systemic effects

Link to relevant study records
Reference
Endpoint:
chronic toxicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2000-2002
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Guideline study without detailed documentation
Reason / purpose for cross-reference:
reference to same study
Qualifier:
equivalent or similar to guideline
Guideline:
other: OECD Guideline 451 (Carcinogenicity Studies)
Deviations:
no
GLP compliance:
yes
Limit test:
no
Species:
rat
Strain:
other: F344/N
Sex:
male/female
Details on test animals or test system and environmental conditions:
TEST ANIMALS
- 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


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


Route of administration:
inhalation: vapour
Type of inhalation exposure:
whole body
Vehicle:
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

Chambers:
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:
yes
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
Remarks:
Doses / Concentrations:
450, 900, or 1800 ppm
Basis:
analytical conc.
Remarks:
Doses / Concentrations:
1843, 3686 and 7373 mg/m3
Basis:
analytical conc.
No. of animals per sex per dose:
50/sex/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:
CAGE SIDE OBSERVATIONS: Yes
- Time schedule: Twice daily

DETAILED CLINICAL OBSERVATIONS: Yes
- 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.

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

FOOD EFFICIENCY:
- 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

WATER CONSUMPTION: No

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: No

CLINICAL CHEMISTRY: No

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No

Sacrifice and pathology:
GROSS PATHOLOGY: Yes
HISTOPATHOLOGY: Yes.
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.
Statistics:
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.

Clinical signs:
effects observed, treatment-related
Mortality:
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.
Dose descriptor:
NOAEC
Effect level:
450 ppm (analytical)
Sex:
male/female
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
Critical effects observed:
not specified
Conclusions:
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. 

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
NOAEC
1 843 mg/m³
Study duration:
chronic
Species:
rat

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion
Endpoint conclusion:
no adverse effect observed

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion
Endpoint conclusion:
no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

Inhalation exposure

Rat studies

In a key whole body 2-year inhalation study in Fischer 344 rats, animals (50/sex/group) were exposed to MIBK at concentrations of 0 (control), 450 ppm, 900 ppm, or 1800 ppm (0, 1843, 3686 and 7373 mg/m3) for 6 hours per day, 5 days per week for 2 years (NTP, 2007; Stout et al., 2008). 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. 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 ofa2µ-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.

In a supporting study(Dodd and Eisler, 1983; Phillips et al, 1987), groups of 14 male and 14 female Fischer 344 rats were exposed to measured mean concentrations of 0, 50, 252, and 1002 ppm (0, 205, 1033, and 4106 mg/m3) MIBK for 6 hrs/day, 5 days/week, for 14 weeks and sacrificed the animals following their final exposure day. The following endpoints were evaluated: clinical signs, body weights, organ weights (kidneys, heart, liver, lungs, and testes), urinalysis, hematology, serum chemistry (including glucose and hepatic enzyme levels), complete gross pathology, targeted histopathology (nasal cavity, trachea, liver, kidneys, and lungs) in all animals and complete histopathology in control and high-exposure groups. No effects of any kind were observed in rats of either sex at 205 mg/m3. Terminal body weights were significantly increased in female rats at >=1033 mg/m3. Platelet numbers in male rats were significantly increased at 4106 mg/m3by 13% over controls, and eosinophil number in female rats was significantly decreased at 4106 mg/m3by 57% as compared to controls. Serum cholesterol in male rats was significantly increased at the 1033 and 4106 mg/m3exposure levels by 23 and 35%, respectively, as compared to controls. Male rats showed a significant increase in absolute (+13) and relative (+9%) liver weight at 4106 mg/m3. No histological lesions were observed in the liver and no changes were seen in serum liver enzymes and bilirubin in any exposure group; thus, the observed liver enlargement may have been an adaptive response to increased hepatic metabolic activity rather than a toxic effect. Urine glucose was significantly increased in male rats at 1033 mg/m3 (+37%) and 4106 mg/m3(+55%) and in female rats at 4106 mg/m3(+26%). Significantly increased urine protein (+28%) was also observed in male rats at 4106 mg/m3. The only renal histological lesion observed was hyaline droplet formation in all male rats; the severity of the lesion generally increased with exposure level. The U. S. EPA has concluded that renal alpha2µ-globulin hyaline droplet formation is unique to male rats and is probably not relevant to humans for the purposes of risk assessment (U. S. EPA. (1991) Alpha2µ-globulin: Association with chemically induced renal toxicity and neoplasia in the male rat. Risk Assessment Forum. EPA/625/3-91/019F).

In a supporting study (McEwen et al., 1970), the Wright-Patterson Air Force Base Aerospace Medical Research Laboratory conducted a subchronic inhalation toxicity study in male Wistar albino rats that were exposed to 410 mg/m3MIBK vapour [100 ppm] for 90 days in an altitude chamber. The untreated control group was maintained in a separate altitude chamber. Statistically significant increases in liver and kidney weights and organ-to-body weight ratios for these tissues were noted in exposed rats. Microscopic examination of the kidneys revealed hyaline droplet degeneration of the proximal tubules (with occasional foci of tubular necrosis) in all of the exposed rats, including those that were removed from the inhalation chamber after 15, 22, 28, 71 and 85 days. The authors noted a trend towards a linear progression of hyaline droplet degeneration during exposure, but this pattern was not seen in all treatment groups. Moreover, the hyaline droplets appeared to increase in size with time. This observation was thought to have resulted from the coalescence of smaller droplets. Microscopic examination of rat kidneys removed after 15 days of exposure indicated a gradual reversion of tubular damage with time. Kidney damage was completely reversed in rats observed up to 60 days after exposure. Recovery MIBK-induced kidney lesions was also noted in rats that were serially killed to study reversibility after 90 days of exposure. However, recovery was not as rapid as that noted in animals exposed for shorter periods.

In a supporting study(Dodd et alk., 1982; Phillips et al., 1987), groups of six male and six female rats were exposed for 6 hrs/day, 5 days/week, for 9 days to measured concentrations of 0, 101, 501, or 1996 ppm (0, 410, 2053, or 8178 mg/m3) MIBK. Groups were evaluated for changes in clinical signs, body weight, organ weights (liver, lungs, kidneys, and testes), ophthalmology, gross pathology, and histopathology. In the highest dose group (1996 ppm), an increase in liver weight (as % of body weight) and a significant increase in both absolute and relative kidney weights were noted in male and female rats. Epithelial regeneration of the proximal convoluted tubules was also noted at 1996 ppm. No ophthalmological lesions or alterations in body weight resulted from exposure to 1996 ppm MIBK. In the 501 ppm exposure group, a non significant increase in kidney weight and a statistically signiticant increase in liver weight were observed in male rats, but not in female rats. In both 501 and 1996 ppm exposure groups, hyaline droplet formation was observed in the kidneys of male rats. No microscopie abnormalities were noted in rats exposed to 101 ppm MIBK.

Mouse studies

In a supporting 2-year inhalation study in B6C3F1 mice(NTP, 2007; Stout et al., 2008), animals (50/sex/group) were administered MIBK at concentrations of 0 (control), 450 ppm, 900 ppm, or 1800 ppm (0, 1843, 3686 and 7373 mg/m3) for 6 hours per day, 5 days per week for 2 years. This GLP study was equivalent to OECD Test Guideline 451. Survival of male and female mice was similar to controls. There were no clinical findings observed related to MIBK exposure. Mean body weights of male mice were generally similar to the controls throughout the study. Mean body weights of females exposed to 1800 ppm MIBK were 9-16% less than the controls. Eosinophilic foci in the liver were increased in all exposed groups of female mice, and the increases over the controls were significant in the 450 and 1800 ppm groups; this lesion was not significantly increased in exposed male mice. Hepatocellular adenomas and carcinomas were reported in males and females at various doses. 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 reported neoplastic effects in the liver of female mice (multiple adenomas) at higher dose levels. Subsequent investigations (Geteret al., 2009) reported that MIBK-related hepatocellular findings in mice may be due to induction of cytochrome P450 enzymes following activation of the mouse constitutive androstane receptor (CAR) in a manner that is similar to Phenobarbital-like compounds. The authors of the study noted that a carcinogenic effect in mice that can be attributed to Phenobarbital-like activation of CAR is not relevant to humans.

In a supporting study(Dodd and Eisler, 1983; Phillips et al, 1987), groups of 14 male and 14 female B6C3F1 mice to measured mean concentrations of 0, 50, 252, and 1002 ppm (0, 205, 1033, and 4106 mg/m3) MIBK for 6 hrs/day, 5 days/week, for 14 weeks and sacrificed the animals following their final exposure day. The following endpoints were evaluated: clinical signs, body weights, organ weights (kidneys, heart, liver, lungs, and testes), hematology, complete gross pathology, targeted histopathology (nasal cavity, trachea, liver, kidneys, and lungs) in all animals and complete histopathology in control and high-exposure groups. No effects of any kind were observed in either sex at 205 mg/m3. Mouse hematology was unaffected at all exposure levels. Male mice showed a significant increase in absolute (+7%) and relative (+11%) liver weight at 4106 mg/m3; absolute, but not relative, liver weight was also slightly increased in male mice (+8%) at 1033 mg/m3. No histological lesions were observed in the liver. Based on the data, it was concluded that biologically important toxic effects were not apparent in male and female B6C3F1 mice exposed to MIBK vapor at concentrations of 0, 50, 252, or 1002 ppm during a 14-week exposure.

In a supporting study(Dood et al., 1982; Phillips et al., 1987), groups of six male and six female B6C3F1 mice were exposed for 6 hrs/day, 5 days/week, for 9 days to measured concentrations of 0, 101, 501, or 1996 ppm (0, 410, 2053, or 8178 mg/m3) MIBK. At the highest exposure concentration (1996 ppm MIBK), an increase in liver weight (as a % of body weight) was observed in female mice, but not in male mice. A signiticant increase in both absolute and relative kidney weights (females) and a decrease in relative kidney weight (males) were also noted in the 1996 ppm exposure group. No ophthalmological lesions or alterations in body weight resulted from exposure to 1996 ppm MIBK. No statistically significant effects on liver weight, kidney weight, or other organ weights were observed in mice exposed to 501 ppm MIBK. At a concentration of 101 ppm, a statistically signiticant decrease in liver weight (as a % of body weight) was observed in male mice but not in female mice. No signiticant changes in kidney weight or other organ weights were noted in male or female mice exposed to 101 ppm MIBK. Compared to controls, no statistically significant histologic lesions were observed at any of the concentrations tested.

Oral route

In a key study (Mulligan, 1986), groups of 30 male and 30 female Sprague-Dawley rats were administered MIBK by gavage in corn oil at daily dose levels of 0 (vehicle control), 50, 250, or 1000 mg/kg-day for 13 consecutive weeks and evaluated for exposure-related changes in body weight, food consumption, mortality, clinical signs, ophthalmological parameters, and terminal organ weights (heart, liver, spleen, brain, kidney, gonads, adrenals, thyroid, and parathyroid). The following evaluations were conducted in rats from each exposure level at interim (week 7) and final sacrifices: hematology, clinical chemistry, urinalysis, and comprehensive gross pathology. All tissue samples collected during gross necropsy in high-dose and control rats were evaluated for histopathologies, and kidney samples were also histologically evaluated in mid-dose rats. Reversible lethargy was observed in rats of both sexes receiving 1000 mg/kg-day (but not at lower dose levels) for a few hours following dosing and reportedly decreased in incidence and severity during the study. Males in the high-dose group showed a slight (9%) but significantly decreased mean body weight gain as compared to controls during the last 2 weeks of exposure, whereas female body weight gain was significantly increased during 5 of the last 6 weeks of exposure. Both male and female food consumption was significantly increased during the second half of the exposure period. The only potentially exposure-related hematological effects observed were slight but statistically significant increases in hemoglobin (+6%) and hematocrit (+8%) at terminal sacrifice in females administered 1000 mg/kg-day and a 15% decrease in lymphocyte count in high-dose males at terminal sacrifice. The lowest hepatic effect level that was observed in the oral exposure studies was 250 mg/kg-day for increased (+39%) serum glutamic-pyruvic transaminase (SGPT) in female rats at the terminal sacrifice. The following changes suggestive of adverse liver effects were observed at 1000 mg/kg-day at either interim and/or final sacrifice: increased SGPT (+73%, interim; +34%, terminal) in females as compared to controls, increased serum alkaline phosphatase (+84%, interim) in females, increased serum cholesterol in males (+30%, interim) and females (+59%, interim; +65%, final), increased terminal absolute (+34%, males; +39%, females) and relative (+42%, males; +38%, females) liver weights, decreased albumin/globulin ratio in males (-16%, interim), and minimally increased serum total protein in females (+9%, interim; +10%, terminal). Only renal effect occurring at 250 mg/kg-day was increased terminal absolute or relative kidney weights in males and females, ranging from 6 to 12% over controls. The following changes suggestive of adverse kidney effects were observed at 1000 mg/kg-day: increased terminal absolute and relative kidney weights (from 25 to 34% in males and from 20 to 22% in females) as compared to controls, increased blood-urea-nitrogen (BUN) in males (+37%, interim), increased serum potassium in males (+34%, terminal), decreased serum glucose in males (-27%, terminal), and a reported increase in urinary protein and ketones in males and females at terminal sacrifice (summary data were not provided). Histological examination of kidney tissues revealed an increased incidence of male rats with mild nephropathy (multifocally distributed swollen or hyperchromatic and flattened renal cortical tubular epithelial cells) at 1000 mg/kg-day (16/20) as compared to controls (4/20) but no increase in such lesions in females. Significantly increased relative adrenal weights in male (+29%) and female (+11%) rats and slightly increased relative testis weights (+9%) in males were also observed at 1000 mg/kgday. No exposure-related histopathologic lesions were evident in the liver or adrenal glands nor in any other tissue that was examined, aside from the kidney. The NOAEL was estimated to be 250 mg/kg bw/d, based oniIncreases in relative kidney weights for male and female rats administered MIBK at doses of 250 mg/kg bw/d but without histological lesions. Effects at higher doses included kidney changes, hepatomegaly and alterations in clinical chemistry and urinalysis parameters. No treatment-related effects of any kind were observed at 50 mg/kg-day.


Justification for selection of repeated dose toxicity via oral route - systemic effects endpoint:
Key study

Justification for selection of repeated dose toxicity inhalation - systemic effects endpoint:
Key study

Repeated dose toxicity: via oral route - systemic effects (target organ) urogenital: kidneys

Repeated dose toxicity: inhalation - systemic effects (target organ) urogenital: kidneys

Justification for classification or non-classification

The substance does not meet the criteria for classification and labelling for this endpoint, as set out in Regulation (EC) NO. 1272/2008.

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