Butan-2-one O,O',O''-(vinylsilylidyne)trioxime

Carcinogenicity

Administrative data

Endpoint:

carcinogenicity: inhalation

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Data source

Reference

Reference Type:

publication

Title:

A chronic inhalation/oncogenicity study of methyethylketoxime in rats and mice

Author:

Newton PE

Year:

2001

Bibliographic source:

Inhalation Toxicology 13(12): 1093-1116, 2001

Materials and methods

Test guideline

Test guideline 1

Qualifier:

according to guideline

Guideline:

EPA OTS 798.3300 (Carcinogenicity)

Test guideline 2

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 453 (Combined Chronic Toxicity / Carcinogenicity Studies)

GLP compliance:

yes

Test material

Test material information

Constituent 1

Reference substance name:

Butanone oxime

EC Number:

202-496-6

EC Name:

Butanone oxime

Cas Number:

96-29-7

Molecular formula:

C4H9NO

IUPAC Name:

butan-2-one oxime

Test material form:

liquid

Test animals

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

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22ºC (chamber)
- Humidity (%): 46% (chamber)

Administration / exposure

Route of administration:

inhalation: vapour

Type of inhalation exposure (if applicable):

whole body

Vehicle:

unchanged (no vehicle)

Details on exposure:

GENERATION OF TEST ATMOSPHERE / CHAMBER DESCRIPTION
- Exposure apparatus: stainless-steel and glass 10 m3 chambers (Hartford, Aberdeen, MD).
- Method of holding animals in test chamber: The animals were individually housed during the exposures
in open mesh cages. Each animal’s location in the chamber was rotated on a weekly basis throughout the study.
- Source and rate of air: Chamber airflow entered tangentially through a turret into the top of the chambers to facilitate air mixing within the chamber. The chambers were operated dynamically at flow rates that provided 1 complete air change every 4.6 min with a 99% equilibrium time of 23 min.
- Method of conditioning air: not reported.
- Temperature, humidity, pressure in air chamber: 22°C temperature and 46% relative humidity. Slight negative pressure (–0.5 cm H2O) relative to the surrounding area.
- Method of particle size determination: Factory-calibrated TSI aerodynamic particle sizer
- Treatment of exhaust air: incineration

TEST ATMOSPHERE
- Brief description of analytical method used: The exposure atmospheres were measured at least four times per exposure using an infrared spectrophotometer (Wilks, MIRAN, Foxboro, MA). Gas chromatographic confirmation of the exposure levels was performed prestudy and every 6 months during the study.
- Samples taken from breathing zone: yes, samples were collected from the left and right sides, front and back, top and bottom, and the normal sampling portal.

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

Vapor samples, collected in 2 tandem, 15-ml, methanol-filled impingers, were injected (1 μl) and analyzed on a gas chromatograph (Hewlett Packard 5980 Series II) equipped with a flame-ionization detector and a DB-1701 column (30 m × 0.32 mm ID) with a 1-μm film thickness. The injector and detector temperatures were 250°C and 300°C, respectively. The column followed a ramped temperature profile. The initial temperature was 40°C for 1 min followed by a 8°C/min ramp to 150°C for 5 min. The lower limit of quantification was 2.6 ppm.

Gas chromatographic analysis for methyl ethyl ketone (MEK), a possible MEKO hydrolysis product, was conducted using samples collected on charcoal tubes. The samples were extracted in methanol and analyzed using the same gas chromatographic conditions.
Chamber samples were collected before any animal exposures were conducted and every 6 months during the study to demonstrate MEKO homogeneity throughout the exposure chamber.

In addition, to measure for the presence of MEKO as an aerosol, particle size distribution and mass concentration measurements were performed daily for the first 2 wk and monthly thereafter in all chambers, including control, using a factory-calibrated TSI aerodynamic particle sizer.

Duration of treatment / exposure:

3, 12, 18 or 26 months

Frequency of treatment:

6 h/day, 5 days/week

Doses / concentrations

Doses / concentrations 1

Dose / conc.:

0 ppm (nominal)

Doses / concentrations 2

Dose / conc.:

15 ppm (nominal)

Remarks:

15 ± 1 ppm = 54 mg/m3 (analytical concentration)

Doses / concentrations 3

Dose / conc.:

75 ppm (nominal)

Remarks:

75± 2 ppm = 270 mg/m3 (analytical concentration)

Doses / concentrations 4

Dose / conc.:

375 ppm (nominal)

Remarks:

374± 10 ppm = 1350 mg/m3 (analytical concentration)

No. of animals per sex per dose:

50

Control animals:

yes, concurrent no treatment

Details on study design:

- Dose selection rationale:
In a range-finding study, F-344 rats and CD-1 mice (Charles River, Kingston, NY), 10/species/sex/group, were exposed via whole-body inhalation to MEKO for 6 h/day, 5 days/wk for 4 wk at target exposure levels of 0, 25, 100, and 400 ppm. MEKO-related findings in the rats at 400 ppm in this study included a 0.5% increase in methemoglobin, a threefold increase in reticulocytes, increased numbers of nucleated erythrocytes and polychromia, 10% decreases in hemoglobin, hematocrit, erythrocytes, and mean corpuscular hemoglobin concentration (MCHC), 10% increase in mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH), 30% increase in platelets, and 13% increase in total leukocytes. In addition, there were approximately 30% increases in liver and spleen weights. The mice were not as sensitive, with little change seen in the hematological parameters except for methemoglobin concentration, which increased 1 to 2%. Liver and spleen weights were increased by about 30%.

Subsequently, a supplemental study was conducted with 10 animals/sex/ species/group exposed via whole-body exposure, 6 h/day, 5 days/wk for 8 wk, at target exposure levels of 0 and 1000 ppm. Similar but larger changes were seen in this study. Methemoglobin concentration increased by 3.5% in the rats and 6% in the mice. Also in the rats, hemoglobin and hematocrit levels decreased about 20% and platelets, MCV, MCH, and MCHC levels increased about 20%. There was a fourfold increase in reticulocyte count. Leukocytes increased about 25%. Similar but less severe changes in these hematological parameters were seen in the mice. Spleen weights tripled in both species, and liver weights increased by up to 50%. There were also 5 to 10% increases in the heart, kidney, and lung weights. Based upon the results of both probe studies, 375 ppm was considered to be a maximum tolerated dose for the present chronic study for rats. Also, these results were the base for another study with mice. The mid- and low-exposure levels were set proportionately lower at 75 and 15 ppm.

Examinations

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.

Results and discussion

Results of examinations

Clinical signs:

no effects observed

Description (incidence and severity):

There were no physical observations which were considered MEKO related.

Dermal irritation (if dermal study):

not examined

Mortality:

no mortality observed

Description (incidence):

At termination of the study, in the control group survivorship was 35% in the males and 64% 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.

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%).

Endocrine findings:

not examined

Urinalysis findings:

not examined

Behaviour (functional findings):

not examined

Immunological 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 testes weights in the 374 ppm group males were 82% greater than that of the controls.

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.

Neuropathological findings:

not examined

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, i.e. MEKO caused no other types of tumors.

Relevance of carcinogenic effects / potential:

Tumors associated with MEKO exposure were limited to the liver of males. The tumors appeared relatively late in the life of the animals and were not life-threatening. It is of interest that in this study, the mean body weight of the high-exposure male rats was about 13% heavier than the controls through most of the study. The incidence of liver carcinomas was significantly increased in this group. The female rats did not have this weight difference and showed no increased incidence of liver carcinomas. Studies involving dietary restriction have found a correlation between increased body weight and spontaneous tumors. Seilkop (1994) estimated that a 10 to 15% decrease in control rat body weights relative to treated weights would produce a 19.6 to 38.4% false positive rate for liver tumor production in rats.

Effect levels

Key result

Dose descriptor:

LOAEC

Effect level:

75 ppm (analytical)

Based on:

test mat.

Sex:

male/female

Basis for effect level:

histopathology: neoplastic

Target system / organ toxicity

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

75 ppm

System:

hepatobiliary

Organ:

liver

Treatment related:

yes

Dose response relationship:

yes

Relevant for humans:

not specified

Any other information on results incl. tables

Table 1. Percent survival after 18 months in mice and 26 months in rats

MEKO exposure (ppm)	Mice		Rats
	Males	Females	Males	Female
0	43	61	34	60
15	57	51	37	58
75	52	62	27	60
374	48	65	43	76

 

Table 2. After 26 months of inhalation exposure to MEKO, incidence of tumors in Fischer 344 rats for which over 5% of the animals in any group had a specific tumor type

	F-344 Rat tumor incidence
Tumor type	0 ppm	15 ppm	75 ppm	374 ppm
Males
Adrenal pheochromocytoma	9	8	10	14
Liver adenoma	0	2	5a	18b
Liver carcinoma	0	0	1	12b
Combined liver adenomas and carcinomas	0	2	6b	27b
Lymphoreticular mononuclear-cell leukemia	16	19	19	4b
Mammary gland fibroadenoma	2	2	4	9a
Mesentery mesotelioma	1	1	0	4
Pituitary adenoma	36	18a	30	24a
Pituitary carcinoma	1	4	1	0
Skin fibroma	1	3	7a	2
Testicular adenoma	50	54	57	58
Thyroid adenoma	12	5	2a	10
Thyroid carcinoma	0	0	1	4
Females
Liver adenoma	0	0	2	4
Lymphoreticular mononuclear-cell leukemia	22	13	10a	1b
Mammary-gland fibroadenoma	10	7	9	17
Pituitary adenoma	36	22a	27	33
Thyroid adenoma	7	6	2	8
Uterine endometrial polyp	15	10	11	22

Note. Data are for n = 50. The combined groupings reflect animals with at least one tumor of the type mentioned. Animals with more than one tumor type cause the combined incidence to be less than the sum of the individual incidences.

a) Statistically significant vs. control, p < .05.

b) Statistically significant vs. control, p < .01.

 

Table 3. Incidence of corneal dystrophy and cataracts in F-344 rats after 26 months of inhalation exposure to MEKO

	Corneal dystrophy			Cataracts of all types
Group	Males	Females	Total	Males	Females	Total
0 ppm	76%	84%	81%	53%	45%	48%
15 ppm	95%	100%	98%	15%	45%	33%
75 ppm	88%	100%	83%	75%	59%	65%
374 ppm	91%	100%	97%	64%	83%	76%

 

Table 4. Hematological effects of MEKO exposure after 3, 12, and 18 months of inhalation exposure (male rats)

Month	MEKO (ppm)	Met Hgb	Hgb	Hct	RBC	Retic	Plat	MCV	MCH	MCHC	WBC
3	0	0.4 ± 0.2	15.9 ± 0.4	43.3 ± 1.4	9.22 ± 0.21	1.1 ± 0.4	7.43 ± 0.39	47.0 ± 1.0	17.3 ± 0.2	36.8 ± 0.6	5.58 ± 1.33
	15	0.5 ± 0.2	16.0 ± 0.5	43.5 ± 1.3	9.35 ± 0.23	1.1 ± 0.3	7.70 ± 0.38	46.5 ± 0.9	17.1 ± 0.1	36.9 ± 0.7	5.74 ± 0.91
	75	0.5 ± 0.1	16.0 ± 0.5	43.9 ± 1.3	9.46 ± 0.23^a	1.2 ± 0.3	8.02 ± 0.37^b	46.4 ± 1.0	16.9 ± 0.2^b	36.5 ± 0.8	6.96 ± 1.12^a
	374	1.2 ± 0.2^b	15.2 ± 0.4^b	42.8 ± 1.2	8.61 ± 0.19^b	1.1 ± 0.4	9.97 ± 0.47^b	49.8 ± 1.2^b	17.6 ± 0.3^b	35.5 ± 0.5^b	6.64 ± 0.84
12	0	0.2 ± 0.3	15.7 ± 0.5	41.7 ± 1.3	9.21 ± 0.45	2.5 ± 0.3	7.87 ± 1.92	45.4 ± 1.9	17.0 ± 0.7	37.6 ± 0.4	6.25 ± 1.34
	15	0.4 ± 0.4	15.5 ± 0.5	41.4 ± 1.6	9.14 ± 0.35	2.8 ± 0.8	7.28 ± 0.77	45.2 ± 1.0	16.9 ± 0.4	37.5 ± 0.5	6.01 ± 0.67
	75	0.5 ± 0.4	15.3 ± 0.3	40.9 ± 1.3	9.15 ± 0.24	2.6 ± 0.4	8.68 ± 1.56	44.7 ± 0.5	16.7 ± 0.3	37.4 ± 0.3	6.96 ± 1.60
	374	0.5 ± 0.4	14.7 ± 0.3^b	39.6 ± 1.2^b	8.59 ± 0.14^b	2.9 ± 0.4	9.28 ± 0.39^b	46.1 ± 1.3	17.1 ± 0.3	37.1 ± 0.6	6.87 ± 0.77
18	0	0.0 ± 0.1	15.5 ± 0.7	43.3 ± 2.0	8.54 ± 0.71	1.7 ± 0.7	6.40 ± 0.83	50.9 ± 2.5	18.2 ± 0.8	35.8 ± 0.3	7.05 ± 3.67
	15	0.0 ± 0.1	15.5 ± 0.6	43.6 ± 1.9	8.70 ± 0.28	1.6 ± 0.6	6.62 ± 0.49	50.0 ± 1.3	17.8 ± 0.3	35.6 ± 0.5	6.68 ± 2.29
	75	0.2 ± 0.5	14.3 ± 3.0	40.2 ± 7.7	7.86 ± 1.81	3.1 ± 4.3	8.30 ± 3.06	52.0 ± 4.3	18.3 ± 0.7	35.3 ± 1.5	6.85 ± 1.91
	374	0.4 ± 0.4	14.9 ± 0.7	42.1 ± 1.7	8.58 ± 0.39	1.9 ± 0.6	7.67 ± 0.63^b	49.0 ± 0.6^a	17.3 ± 0.2^b	35.3 ± 0.3	6.86 ± 1.69

Note. MEKO, methylethylketoxime (ppm); Met Hgb, methemoglobin (%); Hgb, hemoglobin (g/dl); Hct, hematocrit (%); RBC, erythrocytes (10⁶/µl); Retic, reticulocytes (%); Plat, platelets (100 T/µl); MCV, mean corpuscular volume (fl); MCH, mean corpuscular hemoglobin (pg); MCHC, mean corpuscular hemoglobin concentration (g/dl); WBC, white blood cells (thousands/µl). Data are (mean ± SD), n = 10.

a) Statistically significant vs. control, p < .05.

b) Statistically significant vs. control, p < .01.

 

Table 5. Hematological effects of MEKO exposure after 3, 12, and 18 months of inhalation exposure (female rats)

Month	MEKO (ppm)	Met Hgb	Hgb	Hct	RBC	Retic	Plat	MCV	MCH	MCHC	WBC
3	0	0.4 ± 0.2	16.1 ± 0.4	43.6 ± 1.1	8.60 ± 0.21	1.0 ± 0.3	7.96 ± 0.53	50.6 ± 0.5	18.7 ± 0.1	36.9 ± 0.4	3.64 ± 0.72
	15	0.3 ± 0.2	16.0 ± 0.4	43.9 ± 0.9	8.63 ± 0.19	1.0 ± 0.2	7.79 ± 0.36	50.8 ± 0.5	18.5 ± 0.1	36.5 ± 0.4	4.17 ± 0.80
	75	0.5 ± 0.2	15.9 ± 0.3	44.0 ± 1.5	8.58 ± 0.18	1.0 ± 0.2	8.51 ± 0.42	51.3 ± 0.9	18.6 ± 0.1	36.2 ± 0.6^a	3.69 ± 0.64
	374	1.1 ± 0.2^b	14.9 ± 1.2^b	42.0 ± 4.2	7.65 ± 0.65^b	1.0 ± 0.3	8.70 ± 0.98^b	54.8 ± 1.8^b	19.4 ± 0.2^b	35.6 ± 1.3^b	4.86 ± 0.61^b
12	0	0.2 ± 0.3	15.8 ± 0.3	42.4 ± 1.1	8.40 ± 0.13	2.2 ± 0.2	7.19 ± 0.55	50.5 ± 0.7	18.8 ± 0.1	37.2 ± 0.5	2.94 ± 0.83
	15	0.2 ± 0.2	15.9 ± 0.3	42.4 ± 0.9	8.47 ± 0.17	2.1 ± 0.5	7.33 ± 0.33	50.1 ± 0.8	18.7 ± 0.2	37.4 ± 0.3	3.16 ± 0.53
	75	0.3 ± 0.3	15.6 ± 0.3	41.8 ± 1.1	8.36 ± 0.16	2.1 ± 0.5	7.27 ± 0.36	49.9 ± 1.0	18.7 ± 0.2	37.4 ± 0.6	3.32 ± 0.65
	374	0.7 ± 0.3^b	14.2 ± 0.4^b	38.7 ± 1.1^b	7.39 ± 0.17^b	2.7 ± 0.5	8.27 ± 0.65^b	52.3 ± 1.2^b	19.3 ± 0.2^b	36.8 ± 0.8	3.61 ± 0.56
18	0	0.0 ± 0.1	15.7 ± 0.5	43.8 ± 1.2	8.32 ± 0.19	1.3 ± 0.4	5.97 ± 0.78	52.6 ± 0.8	18.9 ± 0.4	35.8 ± 0.3	4.12 ± 1.12
	15	0.0 ± 0.0	15.8 ± 0.3	44.4 ± 0.9	8.39 ± 0.16	1.4 ± 0.5	6.02 ± 0.45	53.0 ± 0.3	18.9 ± 0.2	35.6 ± 0.4	3.89 ± 0.64
	75	0.1 ± 0.2	15.7 ± 0.2	43.9 ± 0.8	8.34 ± 0.12	1.6 ± 0.3	5.95 ± 0.27	52.7 ± 0.8	18.8 ± 0.2	35.7 ± 0.2	3.80 ± 0.75
	374	0.2 ± 0.4	14.9 ± 0.3^b	42.3 ± 1.1^b	7.78 ± 0.20^b	1.7 ± 0.3	6.88 ± 0.45^b	54.4 ± 0.7^b	19.2 ± 0.2^a	35.2 ± 0.3^b	4.88 ± 0.94

Note. MEKO, methylethylketoxime (ppm); Met Hgb, methemoglobin (%); Hgb, hemoglobin (g/dl); Hct, hematocrit (%); RBC, erythrocytes (10⁶/µl); Retic, reticulocytes (%); Plat, platelets (100 T/µl); MCV, mean corpuscular volume (fl); MCH, mean corpuscular hemoglobin (pg); MCHC, mean corpuscular hemoglobin concentration (g/dl); WBC, white blood cells (thousands/µl). Data are mean ± SD, n = 10.

a) Statistically significant vs. control, p < .05.

b) Statistically significant vs. control, p < .01.

 

Table 6. Organ weights in male rats after exposure to MEKO for 3, 12, 18, or 26 months

Month	MEKO (ppm)	Liver			Spleen
		Wt	Wt/TBW	Wt/Brn	Wt	Wt/TBW	Wt/Brn
3	0	6.828 ± 0.475	2.51 ± 0.08	3.78 ± 0.27	0.496 ± 0.048	1.82 ± 0.12	2.74 ± 0.22
	15	7.259 ± 0.464	2.60 ± 0.06	3.86 ± 0.23	0.535 ± 0.56	1.91 ± 0.13	2.84 ± 0.28
	75	7.824 ± 0.502^b	2.78 ± 0.11^b	4.23 ± 0.27^b	0.537 ± 0.066	1.91 ± 0.18	2.91 ± 0.40
	374	8.374 ± 0.466^b	3.00 ± 0.01^b	4.71 ± 0.35^b	0.666 ± 0.049^b	2.39 ± 0.15^b	3.74 ± 0.22^b
12	0	10.126 ± 0.763	2.73 ± 0.23	5.26 ± 0.45	0.645 ± 0.069	1.73 ± 0.16	3.35 ± 0.41
	15	10.342 ± 0.584	2.66 ± 0.11	5.36 ± 0.38	0.649 ± 0.048	1.67 ± 0.09	3.36 ± 0.23
	75	10.376 ± 1.128	2.77 ± 0.12	5.34 ± 0.71	0.681 ± 0.076	1.82 ± 0.12	3.50 ± 0.44
	374	11.900 ± 0.602^b	2.99 ± 0.06^b	6.16 ± 0.40^b	0.877 ± 0.072^b	2.21 ± 0.18^b	4.54 ± 0.36^b
18	0	11.801 ± 1.406	3.04 ± 0.34	6.14 ± 0.79	1.189 ± 0.976	3.05 ± 2.47	6.28 ± 5.35
	15	11.453 ± 1.301	2.85 ± 0.22	5.86 ± 0.81	0.961 ± 0.199	2.39 ± 0.42	4.92 ± 1.11
	75	12.968 ± 1.216	3.17 ± 0.16	6.79 ± 0.72	1.167 ± 0.321	2.86 ± 0.79	6.13 ± 1.74
	374	12.963 ± 1.652	3.11 ± 0.20	6.46 ± 0.85	1.094 ± 0.199	2.63 ± 0.38	5.47 ± 1.05
26	0	14.394 ± 3.191	4.25 ± 0.81	7.25 ± 1.50	1.380 ± 0.705	4.12 ± 2.07	0.69 ± 0.34
	15	15.526 ± 3.653	4.75 ± 1.05	8.00 ± 1.97	3.113 ± 3.560	10.03 ± 12.51	1.63 ± 1.93
	75	17.000 ± 5.154	5.23 ± 1.56	8.50 ± 2.57	4.331 ± 6.617	13.00 ± 19.56	2.14 ± 3.25
	374	17.040 ± 5.822^a	5.20 ± 1.28	8.81 ± 2.99	1.258 ± 0.359	3.70 ± 0.83	0.64 ± 0.18

Note. MEKO, methylethylketoxime (ppm); Wt, organ weight (g); TBW, total body weight; Brn, brain weight. Data are mean ± SD, n = 10 except at month 26 when n ranged from 14 to 22.

a) These weights were from all animals without any macroscopic evidence of tumors.

b) Significantly different from control, p < .01.

 

Table 7. Organ weights in female rats after exposure to MEKO for 3, 12, 18, or 26 months

Month	MEKO (ppm)	Liver			Spleen
		Weight (g)	Wt/TBW	Wt/Brn	Weight (g)	Wt/TBW	Wt/Brn
3	0	4.211 ± 0.292	2.63 ± 0.07	2.49 ± 0.17	0.350 ± 0.040	2.19 ± 0.21	2.07 ± 0.26
	15	4.282 ± 0.267	2.70 ± 0.09	2.54 ± 0.18	0.343 ± 0.41	2.16 ± 0.22	2.03 ± 0.25
	75	4.329 ± 0.294	2.68 ± 0.07	2.57 ± 0.19	0.338 ± 0.45	2.11 ± 0.31	2.01 ± 0.27
	374	4.743 ± 0.375^b	2.79 ± 0.11^b	2.75 ± 0.19^b	0.459 ± 0.057^b	2.70 ± 0.24^b	2.66 ± 0.33^b
12	0	5.366 ± 0.393	2.62 ± 0.13	3.01 ± 0.17	0.392 ± 0.047	1.91 ± 0.16	2.20 ± 0.24
	15	5.214 ± 0.515	2.58 ± 0.11	2.89 ± 0.28	0.398 ± 0.039	1.97 ± 0.14	2.21 ± 0.23
	75	5.696 ± 0.474	2.73 ± 0.13	3.19 ± 0.22	0.448 ± 0.043^a	2.15 ± 0.20^a	2.51 ± 0.21^a
	374	5.450 ± 0.436	2.78 ± 0.09^a	3.10 ± 0.22	0.531 ± 0.062^b	2.71 ± 0.28^b	3.03 ± 0.37^b
18	0	7.051 ± 0.696	2.90 ± 0.14	3.95 ± 0.50	0.451 ± 0.082	1.86 ± 0.30	2.52 ± 0.44
	15	6.802 ± 0.810	2.76 ± 0.20	3.84 ± 0.46	0.504 ± 0.080	2.06 ± 0.39	2.84 ± 0.44
	75	7.248 ± 0.639	2.89 ± 0.18	4.20 ± 0.33	0.472 ± 0.074	1.90 ± 0.39	2.73 ± 0.40
	374	7.658 ± 1.076	2.99 ± 0.29	4.31 ± 0.69	0.587 ± 0.092^b	2.28 ± 0.20^a	3.30 ± 0.57^b
26	0	10.556 ± 1.418	4.09 ± 0.63	5.82 ± 0.85	1.080 ± 0.927	4.30 ± 4.05	5.90 ± 4.86
	15	10.534 ± 2.006	3.78 ± 0.57	5.85 ± 1.22	1.205 ± 1.501	4.35 ± 5.42	6.64 ± 8.21
	75	10.343 ± 2.183	3.71 ± 0.63	5.55 ± 1.17	1.072 ± 1.456	3.82 ± 4.92	5.70 ± 7.68
	374	11.197 ± 1.691	3.98 ± 0.48	6.08 ± 0.92	0.816 ± 0.155	2.91 ± 0.50	4.44 ± 0.87

Note. MEKO, methylethylketoxime (ppm); Wt, organ weight (g); TBW, total body weight; Brn, brain weight. Data are mean ± SD, n = 10 except at month 26 when n ranged from 29 to 39.

a) Statistically significant vs. control, p < .05.

b) Statistically significant vs. control, p < .01.

 

Table 8. Testes weight changes in male rats after exposure to MEKO for 3, 12, 18 or 26 months

Month	MEKO (ppm)	Testes
		Wt	Wt/TBW	Wt/Brn
3	0	2.750 ± 0.124	1.01 ± 0.04	1.52 ± 0.08
	15	2.852 ± 0.109	1.02 ± 0.06	1.52 ± 0.03
	75	2.828 ± 0.120	1.01 ± 0.05	1.53 ± 0.07
	374	2.895 ± 0.102^b	1.04 ± 0.03	1.63 ± 0.09^b
12	0	3.030 ± 0.247	0.81 ± 0.07	1.57 ± 0.09
	15	3.116 ± 0.408	0.80 ± 0.10	1.61 ± 0.22
	75	3.335 ± 1.006	0.91 ± 0.10	1.71 ± 0.48
	374	3.255 ± 0.148	0.82 ± 0.04	1.69 ± 0.11
18	0	3.442 ± 0.671	8.87 ± 1.74	1.80 ± 0.42
	15	3.586 ± 0.785	8.94 ± 1.77	1.83 ± 0.41
	75	4.127 ± 0.931	10.08 ± 2.10	2.16 ± 0.49
	374	4.918 ± 1.467^b	11.82 ± 3.38^b	2.46 ± 0.75^a
26	0	5.556 ± 2.325	1.68 ± 0.77	2.82 ± 1.19
	15	6.501 ± 3.790	1.95 ± 1.10	3.32 ± 1.92
	75	7.559 ± 1.842	2.36 ± 0.75	3.80 ± 1.05
	374	10.136 ± 3.766^b	2.96 ± 0.98^b	5.13 ± 1.80^b

Note. MEKO, methylethylketoxime (ppm); Wt, organ weight (g); TBW, total body weight; Brn, Brain weight. Data are mean ± SD, n = 10 except at 26 months where n ranged from 14 to 22.

a) Statistically significant vs. control, p < .05.

b) Statistically significant vs. control, p < .01.

 

 

 

 

Applicant's summary and conclusion

Conclusions:

Under the inhalation exposure conditions of this study, methyl ethyl ketoxime (MEKO) was demonstrated a liver oncogen in male F-344 rats at a vapor concentration of 75 ppm.

Executive summary:

A chronic inhalation toxicity/oncogenicity study of methylethylketoxime (MEKO) was performed in accordance with Guideline EPA OTS 798.3300 and under GLP principles. F-344 rats (50/ sex/ group) were exposed 6 h/day, 5 days/wk for 26 months via whole-body inhalation exposures to target vapor concentrations of 0, 15, 75, and 375 ppm (actual concentrations of 0, 15 ± 1, 75 ± 2, or 374 ± 10 ppm) . Satellite groups ( 10/ sex/ group/ interval) were exposed for 3, 12, or 18 months to evaluate chronic toxicity. Treatment related effects included increased body weight (male rats only), methemoglobin formation, hematology and clinical chemistry changes, increased liver weight, and increased spleen and testes weights. A high incidence of cataracts and corneal dystrophy occurred in both control and MEKO-exposed rats, with an earlier appearance and slightly higher incidence for these ocular lesions in MEKO-exposed animals compared to controls. Degenerative and reparative changes of the olfactory epithelium in the nasal turbinates, primarily limited to the dorsal meatus, occurred at levels of 75 and 374 ppm. MEKO-related findings in the rat included congestion of the spleen with pigment in reticuloendothelial cells and extramedullary hematopoiesis and a decreased incidence of lymphoreticular mononuclear cell leukemia. 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 hepatis 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 hepatis, occurred in both sexes. An increase in hepatocellular adenomas occurred in the male rats at 75 and 374 ppm, and hepatocellular carcinomas in the male rats at 374 ppm. The liver tumors appeared relatively late in the life of the animals, with no significant increase in tumors at 18 months of exposure in rats. Lifespan shortening was not observed, as MEKO-exposed animals survived generally as well as, or slightly better than, the controls.