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Carcinogenicity

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Description of key information

Inhalation (equivalent to OECD 453), rat: LOAEC (carcinogenicity) = 54 mg/m³ (liver tumour development)

Inhalation (equivalent to OECD 453), rat: LOAEC (systemic) = 54 mg/m³ (spleen effects)

Inhalation (equivalent to OECD 453), rat: LOAEC (local) = 54 mg/m³ (degenerative changes of olfactory epithelium in nasal turbinates)

Inhalation (equivalent to OECD 453), mouse: LOAEC (carcinogenicity) = 54 mg/m³ (liver tumour development)

Inhalation (equivalent to OECD 453), mouse: LOAEC (systemic) = 54 mg/m³ (liver effects)

Inhalation (equivalent to OECD 453), mouse: LOAEC (local) = 54 mg/m³ (degenerative changes of olfactory epithelium in nasal turbinates)

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

Referenceopen allclose all

Endpoint:
carcinogenicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
EPA OTS 798.3300 (Carcinogenicity)
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 453 (Combined Chronic Toxicity / Carcinogenicity Studies)
GLP compliance:
yes
Species:
rat
Strain:
Fischer 344
Sex:
male/female
Details on test animals or test system and environmental conditions:
TEST ANIMALS
- Source: Charles River, Kingston, NY

Route of administration:
inhalation: vapour
Type of inhalation exposure (if applicable):
whole body
Vehicle:
unchanged (no vehicle)
Mass median aerodynamic diameter (MMAD):
>= 2.3 - <= 2.6 µm
Geometric standard deviation (GSD):
2.8
Analytical verification of doses or concentrations:
yes
Details on analytical verification of doses or concentrations:
Gas chromatographic confirmation of exposure levels was performed prestudy and every 6 months during the study.
Duration of treatment / exposure:
6 h/day, 5 days/week
Frequency of treatment:
3, 12, 18 or 26 months
Dose / conc.:
15 ppm (nominal)
Remarks:
54 mg/m³ analytical concentration
Dose / conc.:
75 ppm (nominal)
Remarks:
270 mg/m³ analytical concentration
Dose / conc.:
374 ppm (nominal)
Remarks:
1350 mg/m³ analytical concentration
No. of animals per sex per dose:
80/sex/dose
Control animals:
yes, concurrent no treatment
Details on study design:
Following approximately 3, 12 and 18 months of exposure, up to 10 animals/sex/group were sacrificed, selected organs were weighed and organ/body and organ/brain weight ratios calculated. Following approximately 26 months of exposure, all survivors were sacrificed, selected organs were weighed and organ/body and organ/brain weight ratios calculated. Histopathological evaluation of selected tissues was performed for all control (0 ppm) and high-dose (374 ppm) animals and sentinel animals and all animals in the low-dose (15 ppm) and mid-dose (75 ppm) groups which were found dead or sacrificed in a moribund condition prior to study termination. In addition, the eyes, liver, lungs, nasopharyngeal tissues, ovaries, spleen and testes were examined for all animals in groups dosed at 15 and 75 ppm.
Observations and examinations performed and frequency:
CAGE SIDE OBSERVATIONS: Yes
- Time schedule: daily

DETAILED CLINICAL OBSERVATIONS: Yes
- Time schedule: Pretest and weekly thereafter

BODY WEIGHT: Yes
- Time schedule for examinations: Once pretest, weekly through week 13, monthly thereafter, and just prior to terminal sacrifice.

OPHTHALMOSCOPIC EXAMINATION: Yes
- Time schedule for examinations: Pretest and prior to scheduled necropsy

HAEMATOLOGY and CLINICAL CHEMISTRY: Yes
- Hematology and clinical chemistry parameters were evaluated for up to 10 animals/sex/group sacrificed at month 3, 12 and 18 and at study termination. Differential white blood cell counts were analyzed for all survivors at month 12 and 18 and at termination of the study.

URINALYSIS: No data

NEUROBEHAVIOURAL EXAMINATION: No data


Sacrifice and pathology:
SACRIFICE:
At the interim sacrifices (3, 12 and 18 months), at least 9 animals/sex/group were sacrificed by exsanguination following carbon dioxide inhalation. After 26 months, all survivors were sacrificed.

GROSS PATHOLOGY: Yes
- At necropsy, selected organs were weighed (brain, adrenals, kidneys, liver, lungs, ovaries, spleen, and testes with epididymides).
- Organ/body and organ/brain weight ratios were calculated.
- Complete macroscopic examination of selected tissues were conducted.

HISTOPATHOLOGY: Yes
- Following routine processing in neutral buffered 10% formalin and hematoxylin-eosin staining, sections of the following tissues were examined histologically: adrenal glands, aorta, bone and marrow (sternum), brain (medulla/pons, cerebrum and cerebellum), clitoral gland, epididymides, esophagus, exorbital lacrimal gland, eyes, gallbladder (if present), heart, kidneys, larynx, cecum, colon, rectum, liver, lungs, mesenteric and mediastinal lymph nodes, mammary gland, nasoturbinates, sciatic nerve, ovaries, pancreas, preputial gland, pituitary, prostate, mandibular salivary glands, duodenum, jejunum, ileum, spleen, stomach, testes, thymic region, thyroid/parathyroid, trachea, urinary bladder, uterus, vagina, tissue masses and macroscopic lesions.
- When appropriate, microsopic findings were graded with respect to severity for a relative comparison among the exposure groups. Severity scores were based on the subjective assessments of degree of morphological change and the approximate percentage of the tissue section affected. In each section examined, the severity scores defining the approximate extent of tissue involvement were: Minimal (up to 2%); Slight ( 2-10%); Moderate (10-30%); Moderately Severe ( 30 to 70%); and Extreme (over 70%).
Statistics:
- Hematology and clinical chemistry paramters, in-life and terminal body wts, and abosolute and relative organ wt data were evaluated by the appropriate one-way analysis of variance (ANOVA) technique, followed by a multiple comparison procedure, if needed. Bartlett's test was performed to determine if groups had equal variance. If the variances were equal, parmetric procedures were used; if not, nonparametric procedures were used. The standard parametric procedure was the standard one-way ANOVA using F-distribution to assess significance. If significant differences among the means were indicated, Dunnett's test was used to determine which means were significantly different from the control. If a nonparametric procedure for testing equality of means was needed, the Kruskal-Wallis test was used, and if differences were indicated, Dunn's summed rank test was ued to determine which treatments differed from control. A statistical test for trend in the dose levels was also performed. In the parmetric case, standard regression techniques with a test for trend and lack of fit were used. In the nonparametric case, Jonckheere's test for montonic trend was used. Bartlett's test for equal variance was conducted at the 1% 2-sided risk level. All other statistical tests were conducted at the 5% and 1% 2-sided risk levels.
- Statistical analysis of tumor incidence data was performed using contingency tables. First, a standard chi-square analysis was performed to determine if the tumor incidence differed among the groups tested. Second, each group was compared to the control group using a 2 x 2 Fisher exact test and the significance level was corrected via the Bonferroni inequality test to assure an overall test of the stated significance level. Third, Armitage's test for linear trend in the exposure groups was performed.
Clinical signs:
no effects observed
Description (incidence and severity):
- There were no physical observations which were considered MEKO related.
- At termination of the study, in the control group survivorship was 34% in the males and 60% in the females. There was no significant difference in survivorship among any of the exposure groups including control. In most cases, survivorship for the MEKO exposed groups was slightly greater than for the control group.
Mortality:
no mortality observed
Body weight and weight changes:
effects observed, treatment-related
Description (incidence and severity):
- Mean body weights and body weight gains from study initiation were significantly elevated by exposure to MEKO in both the males and the females. After 13 weeks of exposure, the 374 ppm males were 13% heavier than the control males and the females were 4% heavier.
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:
effects observed, treatment-related
Description (incidence and severity):
- Ophthalmoscopic examinations of the animals found a treatment-exaggerated incidence of corneal dystrophy and opacities at 18 and 26 months, but not at 3 or 12 months. The dystrophic changes seen in the 374 ppm group were far more severe than in other groups. This increase was probably a result of MEKO exaggerating a strain-related condition already present.
Haematological findings:
effects observed, treatment-related
Description (incidence and severity):
- At the 3 month sacrifice in the 374 ppm group, methemoglobin was elevated in the males from 0.4 to 1.2%; hemoglobin was decreased 4%; erythrocytes were decreased 7%; mean corpuscular volume was increased 2%; mean corpuscular hemoglobin concentration was decreased 4%; platelets were increased 25% and leukocyte counts were increased 6%. Similar effects were seen in the females. The differences were still statistically significantly different at 12 months in the 374 ppm group but tolerance or adaptation seemed to occur for the effects. Most were no longer significantly different by 18 months in the males or 24 months in both sexes.
Clinical biochemistry findings:
effects observed, treatment-related
Description (incidence and severity):
- Alanine aminotransferase was the only parameter that was consistently affected by MEKO exposure in males. It was significantly decreased at 3, 12 and 18 months by as much as 64 % (12 months). There was also a trend for decreased aspartate aminotransferase in the males, but the difference from controls was only statistically significant at 12 months (54%).
Urinalysis findings:
not examined
Behaviour (functional findings):
not examined
Organ weight findings including organ / body weight ratios:
effects observed, treatment-related
Description (incidence and severity):
- MEKO-related increases in absolute and relative organ weights were seen in the liver, spleen and testes. At three months in the 374 ppm group, liver weights were elevated about 18% and spleen weights were elevated by about 33%. Tolerance or adaptation occurred and the liver and spleen differences decreased over time. However the increase in testes weight did not. At study termination the 374 ppm group's testes weighed 82% more than the control group's.
Gross pathological findings:
effects observed, treatment-related
Description (incidence and severity):
- Macroscopic findings at 3 and 12 months occurred either sporadically or with similar incidence in treated and control animals and were not considered to be related to treatment.
- At 18 months, tan/red discoloration of the liver was observed in 2 of 9 male and 2 of 10 female rats in the 75 ppm group and 6 of 9 males rats in the 374 ppm group and appeared to be treatment-related.
- An increase in the incidence of enlarged testes was observed in male rats from the 75 and 374 ppm groups.
- At 26 months, opacity of the eye and red/tan discoloration of the liver occurred with greater incidence in males and females in the 374 ppm group.
- In the males at 374 ppm, an increased incidence of nodules/masses of the liver and enlarged testes were observed.
- The incidence of enlarged spleens was decreased in the 374 ppm group relative to all of the other groups.
Histopathological findings: non-neoplastic:
effects observed, treatment-related
Description (incidence and severity):
- Treatment-related macroscopic findings were not observed at 3 or 12 months. At 18 months an increased incidence of red/tan discoloration of the liver and enlarged testes in treated animals appeared to be treatment related. In the chronic study (24 months and all unscheduled deaths), an increased incidence of red/tan discoloration and nodules/masses of the liver, enlarged testes, and opacity enlarged spleens in animals of Group 374 ppm appeared to be treatment related.
- There were a number of treatment related microscopic findings. Congestion of the spleen with pigment in reticuloendothelial cells and extramedullary hematopoiesis appeared to be treatment related in the 374 ppm animals at 3 months, 12 months and 18 months sacrifices. However, at the terminal sacrifice these findings were masked by the high incidence of spontaneous occuring mononuclear cell leukemia in animals other than the 374 ppm animals and could not be evaluated.
- Findings which appeared treatment related at 12 and 18 months and in the chronic study were seen in the liver and nasal turbinates. The liver changes were increased incidence of basophilic foci and hepatocellular vacuoles and decreased incidence of hyperplasia/proliferation of the biliary duct and peribiliary fibrosis. The turbinate changes were degenerative changes of olfactory epithelium eosinophilic/basophilic material/erythrocytes in the lumen of nasal turbinate section 2, 3 and 4; and a decrease in the incidence of eosinophilic droplets in olfactory epithelium in treated animals.
Histopathological findings: neoplastic:
effects observed, treatment-related
Description (incidence and severity):
- Findings which appeared treatment related only in the chronic study animals were seen in the liver. The liver changes were increased incidence of hepatocellular carcinoma and adenoma and spongiosis hepatis. The incidence of liver carcinoma only observed in male rat was control (0/50); 15 ppm (0/50), 75 ppm (1/50); 374 ppm (12/50). No liver tumors were evident up to 18 months of exposure and only liver tumors were considered to be a result of MEKO exposure, ie MEKO caused no other types of tumors.

Tumor incidence in Rats (50 rats/group) at 26 Months

Tumor Type Control 15 ppm 75 ppm 374 ppm

MALES
Liver adenomas 0 2 5* 18*
Liver carcinomas 0 0 1 12*

FEMALES
Liver adenomas 0 0 2 4
Liver carcinomas 0 0 0 0

* Statistically significant incidence

Fibroadenomas in mammary gland in males was observed in 2/50, 2/50, 4/50, 9/50 in 0, 15, 75, and 374 ppm, respectively. The incidence of fibroadenomas was statistically significant in the high dose group.
Fibroadenomas in mammary gland in females was observed in 10/50, 7/50, 9/50, 17/50 in 0, 15, 75, and 374 ppm, respectively. The incidence of fibroadenomas was not statistically significant.
Dose descriptor:
LOAEC
Remarks:
carcinogenicity
Effect level:
54 mg/m³ air (analytical)
Based on:
test mat.
Sex:
male/female
Basis for effect level:
histopathology: neoplastic
Dose descriptor:
LOAEC
Remarks:
systemic toxicity
Effect level:
54 mg/m³ air
Based on:
test mat.
Sex:
male/female
Basis for effect level:
histopathology: non-neoplastic
Dose descriptor:
LOAEC
Remarks:
local
Effect level:
54 mg/m³ air
Based on:
test mat.
Sex:
male/female
Basis for effect level:
histopathology: non-neoplastic
Critical effects observed:
yes
Lowest effective dose / conc.:
54 other: mg/m³ air (analytical)
System:
hepatobiliary
Organ:
liver
Treatment related:
yes
Dose response relationship:
yes
Conclusions:
Under the inhalation exposure conditions of this study, methyl ethyl ketoxime (MEKO) was a liver oncogen in male F-344 rats at a vapor concentration of 15 ppm.
Endpoint:
carcinogenicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
EPA OTS 798.3300 (Carcinogenicity)
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 453 (Combined Chronic Toxicity / Carcinogenicity Studies)
GLP compliance:
yes
Species:
mouse
Strain:
CD-1
Sex:
male/female
Details on test animals or test system and environmental conditions:
TEST ANIMALS
- Source: Charles River, Kingston, NY
Route of administration:
inhalation: vapour
Type of inhalation exposure (if applicable):
whole body
Vehicle:
unchanged (no vehicle)
Mass median aerodynamic diameter (MMAD):
>= 2.1 - <= 2.7 µm
Geometric standard deviation (GSD):
3.4
Analytical verification of doses or concentrations:
yes
Details on analytical verification of doses or concentrations:
Gas chromatographic confirmation of exposure levels was performed prestudy and every 6 months during the study.
Duration of treatment / exposure:
3, 12 and 18 months
Frequency of treatment:
6 hours/day, 5 days/week
Dose / conc.:
15 ppm (nominal)
Remarks:
54 mg/m³ analytical concentration
Dose / conc.:
75 ppm (nominal)
Remarks:
270 mg/m³ analytical concentration
Dose / conc.:
374 ppm (nominal)
Remarks:
1350 mg/m³ analytical concentration
No. of animals per sex per dose:
60/sex/dose
Control animals:
yes, concurrent no treatment
Details on study design:
Following approximately 3 and 12 months of exposure, up to 10 animals/sex/group were sacrificed, selected organs were weighed and organ/body and organ/brain weight ratios calculated. Following approximately 18 months of exposure, all survivors were sacrificed, selected organs were weighed and organ/body and organ/brain weight ratios calculated. Histopathological evaluation of selected tissues was performed for all control (0 ppm) and high-dose (374 ppm) animals and sentinel animals and all animals in the low-dose (15 ppm) and mid-dose (75 ppm) groups which were found dead or sacrificed in a moribund condition prior to study termination. In addition, the eyes, liver, lungs, nasopharyngeal tissues, ovaries, spleen and testes were examined for all animals in groups dosed at 15 and 75 ppm.
Observations and examinations performed and frequency:
CAGE SIDE OBSERVATIONS: Yes
- Time schedule: daily

DETAILED CLINICAL OBSERVATIONS: Yes
- Time schedule: Pretest and weekly thereafter

BODY WEIGHT: Yes
- Time schedule for examinations: Once pretest, weekly through week 13, monthly thereafter, and just prior to terminal sacrifice.

OPHTHALMOSCOPIC EXAMINATION: Yes
- Time schedule for examinations: Pretest and prior to scheduled necropsy

HAEMATOLOGY and CLINICAL CHEMISTRY: Yes
- Hematology and clinical chemistry parameters were evaluated for up to 10 animals/sex/group sacrificed at month 3, 12 and 18 and at study termination. Differential white blood cell counts were analyzed for all survivors at month 12 and 18 and at termination of the study.

URINALYSIS: No data

NEUROBEHAVIOURAL EXAMINATION: No data
Sacrifice and pathology:
SACRIFICE:
At the interim sacrifices (12 months), at least 9 animals/sex/group were sacrificed by exsanguination following carbon dioxide inhalation. After 18 months, all survivors were sacrificed.

GROSS PATHOLOGY: Yes
- At necropsy, selected organs were weighed (brain, adrenals, kidneys, liver, lungs, ovaries, spleen, and testes with epididymides).
- Organ/body and organ/brain weight ratios were calculated.
- Complete macroscopic examination of selected tissues were conducted.

HISTOPATHOLOGY: Yes
- Following routine processing in neutral buffered 10% formalin and hematoxylin-eosin staining, sections of the following tissues were examined histologically: adrenal glands, aorta, bone and marrow (sternum), brain (medulla/pons, cerebrum and cerebellum), clitoral gland, epididymides, esophagus, exorbital lacrimal gland, eyes, gallbladder (if present), heart, kidneys, larynx, cecum, colon, rectum, liver, lungs, mesenteric and mediastinal lymph nodes, mammary gland, nasoturbinates, sciatic nerve, ovaries, pancreas, preputial gland, pituitary, prostate, mandibular salivary glands, duodenum, jejunum, ileum, spleen, stomach, testes, thymic region, thyroid/parathyroid, trachea, urinary bladder, uterus, vagina, tissue masses and macroscopic lesions.
- When appropriate, microsopic findings were graded with respect to severity for a relative comparison among the exposure groups. Severity scores were based on the subjective assessments of degree of morphological change and the approximate percentage of the tissue section affected. In each section examined, the severity scores defining the approximate extent of tissue involvement were: Minimal (up to 2%); Slight ( 2-10%); Moderate (10-30%); Moderately Severe ( 30 to 70%); and Extreme (over 70%).
Statistics:
- Hematology and clinical chemistry paramters, in-life and terminal body wts, and abosolute and relative organ wt data were evaluated by the appropriate one-way analysis of variance (ANOVA) technique, followed by a multiple comparison procedure, if needed. Bartlett's test was performed to determine if groups had equal variance. If the variances were equal, parmetric procedures were used; if not, nonparametric procedures were used. The standard parametric procedure was the standard one-way ANOVA using F-distribution to assess significance. If significant differences among the means were indicated, Dunnett's test was used to determine which means were significantly different from the control. If a nonparametric procedure for testing equality of means was needed, the Kruskal-Wallis test was used, and if differences were indicated, Dunn's summed rank test was ued to determine which treatments differed from control. A statistical test for trend in the dose levels was also performed. In the parmetric case, standard regression techniques with a test for trend and lack of fit were used. In the nonparametric case, Jonckheere's test for montonic trend was used. Bartlett's test for equal variance was conducted at the 1% 2-sided risk level. All other statistical tests were conducted at the 5% and 1% 2-sided risk levels.
- Statistical analysis of tumor incidence data was performed using contingency tables. First, a standard chi-square analysis was performed to determine if the tumor incidence differed among the groups tested. Second, each group was compared to the control group using a 2 x 2 Fisher exact test and the significance level was corrected via the Bonferroni inequality test to assure an overall test of the stated significance level. Third, Armitage's test for linear trend in the exposure groups was performed.
Clinical signs:
no effects observed
Description (incidence and severity):
- There were no physical observations which were considered MEKO related.
- At termination of the study, in the control group survivorship was 43% in the males and 61% in the females. There was no significant difference in survivorship among any of the exposure groups including control. In most cases, survivorship for the MEKO exposed groups was slightly greater than for the control group.
Mortality:
no mortality observed
Body weight and weight changes:
no effects observed
Description (incidence and severity):
- No significant mean body weight or body weight gain changes in any test group compared to the control group.
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:
no effects observed
Description (incidence and severity):
- Ophthalmoscopic examinations of the animals found no MEKO treatment-related effects.
Haematological findings:
effects observed, treatment-related
Description (incidence and severity):
- At the 12 month interim sacrifice, methemoglobin was elevated from 0.2% in the controls to 0.5% in the 374 ppm group males. In the females methemoglobin did not appear to be effected but there was an increase in platelets in the 374 ppm group (35%) and a significant decrease in mean corpuscular hemoglobin concentration at 76 ppm (2.7%) and 374 ppm (3.3%).
Clinical biochemistry findings:
effects observed, treatment-related
Description (incidence and severity):
- Statistically significant changes were seen in some clinical chemistry parameters evaluated at the 12 month interim sacrifice. In the 374 ppm group males, there was an decrease in chloride (4%), and increase in creatinine (50%), and increase in total protein (18%) and an increase in albumin (32%). None of these parameters were changed in the females or showed a dose-related increase. However, because they occurred in the high-exposure group they may be MEKO related.
Urinalysis findings:
not examined
Behaviour (functional findings):
not examined
Organ weight findings including organ / body weight ratios:
effects observed, treatment-related
Description (incidence and severity):
- At the 12 month interim sacrifice, liver organ weights were significantly increased (17%) in the females at 374 ppm. At termination of the study there were no related effects on organ weights.
Gross pathological findings:
no effects observed
Description (incidence and severity):
- There were no MEKO related macroscopic findings.
Histopathological findings: non-neoplastic:
effects observed, treatment-related
Description (incidence and severity):
- Microscopically, findings which appeared to be related to treatment included changes in the nasal turbinates and in the liver. In the turbinates, degenerative and reparative changes were observed. These included desquamation of olfactory epithelium, dilation of submucosal glands debris and inflammatory cells in the gland and in the nasal lumen and with proliferation of squamous or respiratory epithelium. In some areas the hypertrophic cells from the glands appeared to be extending to the luminal surface and replacing the lost epithelium. A NOEL for this finding could not be obtained. The liver changes, indicating hepatoxicity, included pigment in reticuloendothelial cells, necrosis, centrilobular, hepatocellular hypertrophy and granulomatous inflammation.
Histopathological findings: neoplastic:
effects observed, treatment-related
Description (incidence and severity):
- There was also an increase in liver carcinomas in the 374 ppm male group relative to control and the other exposure groups.

Tumor Incidence in Mice (60 mice/group) at 18 Months

Tumor Type Control 15 ppm 75 ppm 374 ppm

MALES
Liver adenomas 4(1) 11(1) 10(2) 11(1) (within historical control range)
Liver carcinomas 2(0) 2(0) 1(0) 10*(2)

FEMALES
Liver adenomas 0 0 1 3
Liver carcinomas 0 0 0 1

* Statistically significant incidence
Numbers in brackets indicates incidence at the 12 month interim sacrifice.
Dose descriptor:
LOAEC
Remarks:
carcinogenicity
Effect level:
54 mg/m³ air (analytical)
Based on:
test mat.
Sex:
male
Basis for effect level:
histopathology: neoplastic
histopathology: non-neoplastic
Dose descriptor:
LOAEC
Remarks:
systemic toxicity
Effect level:
54 mg/m³ air
Based on:
test mat.
Sex:
male/female
Basis for effect level:
histopathology: non-neoplastic
Dose descriptor:
LOAEC
Remarks:
local
Effect level:
54 mg/m³ air
Based on:
test mat.
Sex:
male/female
Basis for effect level:
histopathology: non-neoplastic
Critical effects observed:
yes
Lowest effective dose / conc.:
54 other: mg/m³
System:
hepatobiliary
Organ:
liver
Treatment related:
yes
Dose response relationship:
yes
Conclusions:
Under the inhalation exposure conditions of this study, MEKO produced changes in the olfactory epithelium in both sexes of mice in exposed groups and was a liver oncogen in male CD-1 mice at a vapor concentration of 15 ppm.
Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
NOAEC
54 mg/m³
Study duration:
chronic
Species:
rat
Quality of whole database:
Both studies were performed under GLP conditions and have a Klimisch score of 1.
System:
hepatobiliary
Organ:
liver

Carcinogenicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Justification for classification or non-classification

The available data on carcinogenicity of the test substance meet the criteria for classification as Carc. Cat. 1B (H350) according to Regulation (EC) 1272/2008.

Additional information

A carcinogenicity inhalation study with rats and mice is available (Newton et al., 2001). CD-1 mice (50/sex/group) and F-344 rats (50/sex/group) were exposed 6 h/day, 5 days/week for 18 (mice) or 26 months (rats) via whole-body inhalation exposures to target vapor concentrations of 0, 15, 75, and 375 ppm (corresponding to 0, 54, 270 and 1350 mg/m³). Satellite groups of rats and mice (10/sex/group/interval) were exposed for 12 months (mice) and 3, 12, or 18 months (rats) to evaluate chronic toxicity. Methyl ethyl ketone (MEK), a possible hydrolysis product of the test substance, was present at less than 1%.

In rats, effects on blood parameters indicative of haemolytic anaemia and compensatory haematopoieseis were seen at 1350 mg/m³ at 3 months and 12 months but not at 18 months in males and not at 26 months in either sex. Effects to the spleen (increased organ weight, extramedullary haematopoiesis and haemosiderosis) were seen at the same dose level. Spleen weights were increased in both sexes at 3 and 12 months, at 18 months in females only, but were not increased in either sex at 26 months. Histopathological effects persisted at the same time points as the spleen weights, but could not be accurately determined at termination due to leukemia. Liver weights were also increased at the highest dose. At 18 and 24 months as well as for all unscheduled deaths, an increased incidence of red/tan discoloration of the liver was observed. Nodules and masses could also be seen macroscopically in the liver at 24 months. Effects observed in the liver of the rats included decreases in the incidence of both peribiliary fibrosis and hyperplasia/proliferation of the biliary duct, an increase of spongiosis hepatitis in males, and an increase in the incidence of intracytoplasmic vacuoles and hepatocellular basophilic foci. The effects on the liver were generally most profound in the high-exposure groups and, with the exception of the spongiosis hepatitis, occurred in both sexes. An increase in hepatocellular adenomas occurred in the male rats at 54, 270 and 1350 mg/m³ (statistically significant starting at 270 mg/m³), and hepatocellular carcinomas in the male rats at 270 and1350 mg/m³ (statistically significant at 1350 mg/m³).

A high incidence of cataracts and corneal dystrophy occurred in both control and exposed rats, with an earlier appearance and slightly higher incidence for these ocular lesions in exposed animals compared to controls. Degenerative and reparative changes of the olfactory epithelium in the nasal turbinates, primarily limited to the dorsal meatus, occurred in rats at 270 and 1350 mg/m³).

The effects on the blood were not as clear for mice as for rats. There was an increase in methaemoglobin at 12 months only in males. Other erythrocyte parameters were generally unaffected in both sexes and spleen weights and spleen pathology were not affected (there was no 3 month sacrifice and the study was terminated at 18 months). In addition, in the mice, liver changes included increased incidences of pigment in reticuloendothelial cells, centrilobular hypertrophy, granulomatous inflammation, and a slightly increased incidence of necrosis (270 and 1350 mg/m³). A statistically significant increase in hepatocellular carcinomas occurred in male mice at 1350 mg/m³. An increase in liver adenomas in male mice was observed starting at 54 mg/m³, however those were not statistically significant and within historical control range. Degenerative and reparative changes of the olfactory epithelium in the nasal turbinates, primarily limited to the dorsal meatus, occurred in mice at 54, 270 and 1350 mg/m³.

Based on the results of the study, a LOAEC for carcinogenicity was set to 54 mg/m³ for male rats and mice.

 

In both species, the liver tumors appeared relatively late in the life of the animals, with no significant increase in tumors at 12 months of exposure in mice and at 18 months of exposure in rats. Lifespan shortening was not observed, as exposed animals survived generally as well as, or slightly better than, the controls. A NOAEC for systemic toxicity was set at 54 mg/m³ for rats, based on increased incidence of hepatocellular adenomas (males only) and degenerative and reparative changes of the olfactory epithelium in nasal turbinates, while for mice the same concentration level was a LOAEC based on the effects on the olfactory epithelium.

 

The strongest evidence of a carcinogenic effect following exposure to the test substance is provided by the inhalation studies showing increased liver tumours in exposed male rats and male mice.
No such tumours were seen in control animals. These observations, common to both species, provide relatively strong evidence of a causal relationship between the test substance exposure and a carcinogenic response. Whereas an increased tumour rate was only seen at the highest the test substance exposure level employed in the mouse study, it was seen in the mid and high exposure groups in rats. Female rats and female mice appeared less sensitive to this hepatocarcinogenic effect of the test substance. The evidence is considerably weaker in females, with only a small increase in (benign) adenoma at the top dose (in female mice and female rats), observed in the absence of statistical significance.

Overall, long-term inhalation to vapours of the test substance led to a carcinogenic effect in both rats and mice. There were statistically significant increases in benign and malignant tumours in the livers of male rats and in malignant liver tumours in male mice exposed to the test substance.
No such tumours were seen in control rats and the tumour rates in the control mice were low. There were also increases in hepatocellular adenoma in female rats and mice exposed to high levels of the test substance, relative to the concurrent controls, but these findings were not statistically significant. There were no increased levels of malignant liver tumours seen in female rats or mice.

Regarding the available results with respect to the relevance of the findings to humans, tumours of the liver observed in animals are relevant for human hazard assessment.

As the data on carcinogenicity were from two different studies, in two different species, and there are no unresolved questions regarding the adequacy of the design, conduct or interpretation of these studies, there is no reason to consider the strength of evidence to be limited. Generally, this is sufficient to justify classification of a substance in category 1B for carcinogenicity.