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Carcinogenicity

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Description of key information

clear evidence of carcinogenicity for female mice (uterus carcinomas)
equivocal evidence of carcinogenicity for female mice (hepatocellular adenomas and carcinomas)
equivocal evidence of carcinogenicity for male (alveolar/bronchial adenomas and carcinomas); study was considered inadequate for carcinogenicity because of reduced survival in the exposed group
equivocal evidence of carcinogenicity for male rats (increased incidences of skin tumours)
equivocal evidence of carcinogenicity for male and female rats (increased incidences of brain astrocytoma)

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
Study period:
March 1982 - Feb 1984
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Reason / purpose for cross-reference:
reference to same study
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 451 (Carcinogenicity Studies)
Deviations:
yes
Remarks:
only one dose level; haematology, clinical biochemistry, urinalysis not performed
GLP compliance:
not specified
Species:
mouse
Strain:
B6C3F1
Sex:
male/female
Details on test animals or test system and environmental conditions:
TEST ANIMALS
- Source: Frederick Cancer Research Facility (Frederick, MD, USA)
- Age at study initiation: 9 weeks
- Weight at study initiation: males: 23.5 - 23.9 g; females: 18.7 - 19.1 g
- Housing: individually in stainless steel wire cages (Hazleton Systems, Inc., Aberdeen, MD, USA); no bedding
- Diet (e.g. ad libitum): NIH 07 Rat and Mouse Ration (Zeigler Bros., Inc., Gardners, PA, USA); available ad libitum during nonexposure periods
- Water (e.g. ad libitum): automatic watering system (Edstrom Industries, Waterford, WI, USA); available ad libitum
- Acclimation period: 21 days


ENVIRONMENTAL CONDITIONS
- Temperature (°C): 15.6 - 28.3 (mean 24.4)
- Humidity (%): 38 - 88 (mean 60%)
- Air changes (per hr): 10
- Photoperiod (hrs dark / hrs light): 12 / 12


IN-LIFE DATES: From: February 1982 To: February 1984
Route of administration:
inhalation: gas
Type of inhalation exposure (if applicable):
whole body
Vehicle:
air
Details on exposure:
GENERATION OF TEST ATMOSPHERE / CHAMBER DESCRIPTION
- Exposure apparatus: vapour generation system; the liquid to be vaporized was forced under pressure, at a metered rate, directly from the shipping container into a stainless steel boiler that was maintained at about 32 °C by a controlled-temperature water bath. The vapour was routed through a gas metering valve and a purge/expose valve into a pipe at the chamber inlet, where the vapour was mixed with dilution air entering the chamber. Uniformity in the exposure chamber was noted as the mean values of the concentrations were within approximately 10% of the target concentration at all 12 positions samples within the chamber. There was no evidence of decomposition of chloroethane in the exposure atmospheres.



TEST ATMOSPHERE
- Brief description of analytical method used: chamber concentrations were determined by GC-FID. The calibration by the monitor was confirmed and corrected two times per month by checking the calibration against volumetrically prepared gas standards.


Analytical verification of doses or concentrations:
yes
Details on analytical verification of doses or concentrations:
mean of chamber concentration: 15048 +/- 641 ppm
Duration of treatment / exposure:
100 weeks
Frequency of treatment:
6 hours/day; 5 days per week
Post exposure period:
no
Dose / conc.:
15 000 ppm (nominal)
Remarks:
corresponding to 39577 mg/m³
No. of animals per sex per dose:
50
Control animals:
yes, sham-exposed
Details on study design:
- Dose selection rationale: Although no chemically related toxic effects were observed in the short-term studies, concerns about potential flammability and the explosion hazard led to the selection of 0 and 15000 ppm as the exposure concentrations for male and female rats for the 2-year studies.
Observations and examinations performed and frequency:
CAGE SIDE OBSERVATIONS: Yes
- Time schedule: two times per day

DETAILED CLINICAL OBSERVATIONS: No

BODY WEIGHT: Yes
- Time schedule for examinations: once per week for the first 12 weeks of the study and once per month thereafter

FOOD CONSUMPTION AND COMPOUND INTAKE:
- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: No
- Compound intake calculated as time-weighted averages from the consumption and body weight gain data: 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

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: No

CLINICAL CHEMISTRY: No

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No


Sacrifice and pathology:
GROSS PATHOLOGY: Yes
HISTOPATHOLOGY: Yes
Tissues examined included: adrenal glands, brain, bronchial lymph nodes, caecum, clitoral or preputial gland, colon, duodenum, oesophagus, gallbladder, gross lesions, heart, illeum, jejunum, kidneys, larynx, liver, lungs and mainstem bronchi, mammary gland, mandibular lymph nodes, nose, pancreas, parathyroid glands, pituitary gland, prostate/testes/epididymis or ovaries/uterus, rectum, salivary glands, skin, spleen, sternebrae including marrow, stomach, thymus, thyroid gland, tissue masses with regional lymph nodes, trachea, and urinary bladder
Statistics:
Survival analyses: estimated by the product-limit procedure of Kaplan and Meier (1985) and presented in the form of graphs. Statistical analyses for a possible compound-related effect on survival used the method of Cox (1972). All reported P values for the survival analysis are two-sided.

Calculation of incidence: the incidence of neoplastic or non-neoplastic lesions is given as the ratio of the number of animals bearing such lesions at a specific anatomic site to the number of animals in which that site was examined.

Analysis of tumor incidence: The majority of tumors in this study were considered to be incidental to the cause of death or not rapidly lethal. Thus, the primary statistical method used was a logistic regression analysis, which assumed that the diagnosed tumors were discovered as a result of death from an unrelated cause and thus did not affect the risk of death. In this approach, tumor prevalence was modelled as a logistic function of chemical exposure and time. Both linear and quadratic terms in time were incorporated initially, and the quadratic term was eliminated if it did not significantly enhance the fit of the model. The exposed and control groups were compared on the basis of the likelihood score test for the regression coefficient of dose. This method of adjusting for intercurrent mortality is the prevalence analysis of Dinse and Lagakos (1983), further described and illustrated by Dinse and Haseman (1986). When tumors are incidental, this comparison of the time-specific tumor prevalences also provides a comparison of the time-specific tumor incidences (McKnight and Crowley, 1984).
In addition to logistic regression, alternative methods of statistical analysis were used (life table test (Cox, 1972) and Fisher exact test (Gart et al., 1979)).
Tests of significance include pairwise comparisons of the exposed group with controls. Continuity-corrected tests were used in the analysis of tumor incidence and reported P values are one-sided.
Details on results:
CLINICAL SIGNS AND MORTALITY
Survival of exposed male mice was significantly lower than that of controls after week 48; after week 72, survival was reduced to 50%. Because of the reduced number of exposed male mice surviving to the end of the study and the absence of obvious carcinogenic effects, this study was considered inadequate for determination of carcinogenicity.
Survival of exposed female mice after week 82 was significantly lower than that of controls; the majority of exposed females died as a result of chloroethane-induced carcinomas of the uterus.
Exposed females were hyperactive during the daily exposure period. The hyperactivity was most intense at the start of each exposure day and was characterized by the animal's running and climbing about the cages. Activity returned to normal soon after exposure ended.


BODY WEIGHT AND WEIGHT GAIN
Mean body weights of exposed female mice were generally similar to those of controls throughout the study in spite of the increased activity. Mean body weights of exposed male mice were up to 13% higher than those of controls throughout the study.



HISTOPATHOLOGY: NON-NEOPLASTIC
During the study greater than normal incidences of non-neoplastic urogenital lesions were seen in individually housed control and exposed male mice and may have contributed to the reduced survival. Exposed mice were more severely affected than controls, as indicated during formal clinical observations and histopathological review. The condition was generally described as a preputial infection with ascending urinary tract infection. The etiology of this condition could not be determined and the apparent contribution of chloroethane exposure to the incidence or the severity of the lesions (inflammation, abscesses, ulceration, in some cases necrosis and bladder distension) is not understood.


HISTOPATHOLOGY: NEOPLASTIC (if applicable)
A highly significant incidence (86%) of uterine carcinomas of endometrial origin, clearly associated with chloroethane exposure, was observed in exposed female mice. The tumors were highly malignant and invaded the myometrium of the uterus; 34 metastasized to a wide variety of organs, primarily lung (23), ovary (22), lymph nodes (18), kidney (8), adrenal gland (8), pancreas (7), urinary bladder (7), mesentery (7), spleen (5), heart (4), and toa lesser extent, colon (2), stomach (1), gallbladder (1), small intestine (1), ureter (1) and liver (1). Although one control female mouse did have a carcinoma of the uterus, this was not considered to be of endometrial origin and was morphologically different from those occuring in exposed mice.

The incidence of alveolar/bronchiolar tumors of the lung in exposed male mice was significantly greater than that in control mice (adenomas: control 3/50, exposed 8/48; adenomas or carcinomas combined: control 5/50, exposed 10/48). Although these neoplasms are relatively common in male mice (historical incidence in chamber controls: 75/348, 22%), the potential expression of alveolar/bronchiolar neoplasms in exposed male mice was probably reduced by the poor survival of exposed animals This is supported by the fact that adenomas in exposed mice were detected as early as day 409, whereas adenomas or carcinomas in control mice were not observed until day 700. However, the association of exposure to chloroethane is not clear, especially since there were no supporting non-neoplastic lesions in the lungs of exposed mice and no neoplasms were seen in exposed female mice. In addition, the remainder of the respiratory tract, including the nasal cavity was unaffected by chloroethane exposure.

The incidence of hepatocellular carcinomas in female mice exposed to chloroethane was significantly greater than that in controls (control 3/49; exposed 8/48). Another exposed female had a hepatocellular adenoma. The incidence of hepatocellular carcinomas and adenoma combined in exposed females (17%) is greater than the historical incidence in chamber controls from the study laboratory (29/347, 8%) or in untreated control females from NTP studies (184/2032, 9%). The one adenoma reported in an exposed female mouse was observed on day 590. It is possible, that if survival of exposed females had not been reduced as a result of chloroethane-induced uterine carcinomas, the incidence of hepatocellular neoplasm might have been greater.

There were no compound related reproductive effects found during histopathological examination of animals exposed to 15000ppm.

Tumour findings in mice

MICE

control

15000 ppm

male

female

male

female

Survival rate

28/50

11/50

32/50

2/50

 

Uterine carcinomas of endometrical gland origin

 

0/49a

 

43/50 (86%)

 

Hepatocellular adenoma or carcinoma

15/50 (30%)

3/49 (6%)

11/50 (22%)

8/48 (17%)

 

Alveolar/Bronchiolar adenoma or carcinoma

5/50 (10%)

0/50

10/48 (21%)

0/50

a: one chamber control mouse had a uterine carcinoma not of endometrial origin

Endpoint:
carcinogenicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Study period:
March 1982 - March 1984
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Reason / purpose for cross-reference:
reference to same study
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 451 (Carcinogenicity Studies)
Deviations:
yes
Remarks:
only one dose level; haematology, clinical biochemistry, urinalysis not performed
GLP compliance:
not specified
Species:
rat
Strain:
other: F344/N
Sex:
male/female
Details on test animals or test system and environmental conditions:
TEST ANIMALS
- Source: Frederick Cancer Research Facility (Frederick, MD, USA)
- Age at study initiation: 8 weeks
- Weight at study initiation: males: 167 - 169 g; females: 129 g
- Housing: individually in stainless steel wire cages (Hazleton Systems, Inc., Aberdeen, MD, USA); no bedding
- Diet (e.g. ad libitum): NIH 07 Rat and Mouse Ration (Zeigler Bros., Inc., Gardners, PA, USA); available ad libitum during nonexposure periods
- Water (e.g. ad libitum): automatic watering system (Edstrom Industries, Waterford, WI, USA); available ad libitum
- Acclimation period: 21 days


ENVIRONMENTAL CONDITIONS
- Temperature (°C): 15.6 - 28.3 (mean 24.4)
- Humidity (%): 38 - 88 (mean 60%)
- Air changes (per hr): 10
- Photoperiod (hrs dark / hrs light): 12 / 12


IN-LIFE DATES: From: February 1982 To: March 1984
Route of administration:
inhalation: gas
Type of inhalation exposure (if applicable):
whole body
Vehicle:
air
Details on exposure:
GENERATION OF TEST ATMOSPHERE / CHAMBER DESCRIPTION
- Exposure apparatus: vapour generation system; the liquid to be vaporized was forced under pressure, at a metered rate, directly from the shipping container into a stainless steel boiler that was maintained at about 32 °C by a controlled-temperature water bath. The vapour was routed through a gas metering valve and a purge/expose valve into a pipe at the chamber inlet, where the vapour was mixed with dilution air entering the chamber.
Uniformity in the exposure chamber was noted as the mean values of the concentrations were within approximately 10% of the target concentration at all 12 positions samples within the chamber. There was no evidence of decomposition of chloroethane in the exposure atmospheres.


TEST ATMOSPHERE
- Brief description of analytical method used: chamber concentrations were determined by GC-FID. The calibration by the monitor was confirmed and corrected two times per month by checking the calibration against volumetrically prepared gas standards.


Analytical verification of doses or concentrations:
yes
Details on analytical verification of doses or concentrations:
mean of chamber concentration: 15051 +/- 636 ppm
Duration of treatment / exposure:
102 weeks (2 years)
Frequency of treatment:
6 hours/day; 5 days per week
Post exposure period:
no
Dose / conc.:
15 000 ppm (nominal)
Remarks:
corresponding to 39577 mg/m³
No. of animals per sex per dose:
50
Control animals:
yes, sham-exposed
Details on study design:
- Dose selection rationale: Although no chemically related toxic effects were observed in the short-term studies, concerns about potential flammability and the explosion hazard led to the selection of 0 and 15000 ppm as the exposure concentrations for male and female rats for the 2-year studies.
Observations and examinations performed and frequency:
CAGE SIDE OBSERVATIONS: Yes
- Time schedule: two times per day

DETAILED CLINICAL OBSERVATIONS: No

BODY WEIGHT: Yes
- Time schedule for examinations: once per week for the first 12 weeks of the study and once per month thereafter

FOOD CONSUMPTION AND COMPOUND INTAKE:
- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: No
- Compound intake calculated as time-weighted averages from the consumption and body weight gain data: 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

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: No

CLINICAL CHEMISTRY: No

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No


Sacrifice and pathology:
GROSS PATHOLOGY: Yes
HISTOPATHOLOGY: Yes
Tissues examined included: adrenal glands, brain, bronchial lymph nodes, caecum, clitoral or preputial gland, colon, duodenum, oesophagus, gross lesions, heart, illeum, jejunum, kidneys, larynx, liver, lungs and mainstem bronchi, mammary gland, mandibular lymph nodes, nose, pancreas, parathyroid glands, pituitary gland, prostate/testes/edpididymis or ovaries/uterus, rectum, salivary glands, skin, spleen, sternebrae including marrow, stomach, thymus, thyroid gland, tissue masses with regional lymph nodes, trachea, and urinary bladder
Statistics:
Survival analyses: estimated by the product-limit procedure of Kaplan and Meier (1985) and presented in the form of graphs. Statistical analyses for a possible compound-related effect on survival used the method of Cox (1972). All reported P values for the survival analysis are two-sided.

Calculation of incidence: the incidence of neoplastic or non-neoplastic lesions is given as the ratio of the number of animals bearing such lesions at a specific anatomic site to the number of animals in which that site was examined.

Analysis of tumor incidence: The majority of tumors in this study were considered to be incidental to the cause of death or not rapidly lethal. Thus, the primary statistical method used was a logistic regression analysis, which assumed that the diagnosed tumors were discovered as a result of death from an unrelated cause and thus did not affect the risk of death. In this approach, tumor prevalence was modelled as a logistic function of chemical exposure and time. Both linear and quadratic terms in time were incorporated initially, and the quadratic term was eliminated if it did not significantly enhance the fit of the model. The exposed and control groups were compared on the basis of the likelihood score test for the regression coefficient of dose. This method of adjusting for intercurrent mortality is the prevalence analysis of Dinse and Lagakos (1983), further described and illustrated by Dinse and Haseman (1986). When tumors are incidental, this comparison of the time-specific tumor prevalences also provides a comparison of the time-specific tumor incidences (McKnight and Crowley, 1984).
In addition to logistic regression, alternative methods of statistical analysis were used (life table test (Cox, 1972) and Fisher exact test (Gart et al., 1979)).
Tests of significance include pairwise comparisons of the exposed group with controls. Continuity-corrected tests were used in the analysis of tumor incidence and reported P values are one-sided.
Details on results:
CLINICAL SIGNS AND MORTALITY
No compound-related clinical signs were observed.
Although survival of exposed and control male rats was unusually low at the end of the study, there were no statistically significant differences in survival between exposed and control groups of either sex (male: control 16/50, exposed 8/50; female: control 31/50, exposed 22/50). At week 90, survival for rats was not unusually low. Survival for male rats was 37/50 (control) and 31/50 (exposed) and for females, 43/50 (control) and 33/50 (exposed). At week 95, survival in all groups was at or above 48%; therefore, these studies are considered adequate for evaluation of carcinogenicity.

BODY WEIGHT AND WEIGHT GAIN
Mean body weights of exposed female rats were generally 5% - 10% lower than those of controls from week 11 to week 42 and 6% - 13% lower from week 47 to the end of the study.
Mean body weights of exposed male rats were 4% - 8% lower than those of controls after week 33.


HISTOPATHOLOGY: NON-NEOPLASTIC + NEOPLASTIC
Astrocytoma (uncommon malignant glial cell tumors) of the brain were observed in three exposed female rats and gliosis (a non-neoplastic proliferation of glial cells) was seen in a fourth. The three female rats with malignant astrocytoma died before the end of the study; the tumors may have been the primary cause of death. Although the incidence of malignant astrocytoma is not statistically increased versus the concurrent controls, it is significant (P<0.05) relative to the incidence observed in chamber controls from previous studies at this laboratory (1/297) and also relative to the historical control incidence of glial cell tumors in untreated control female F344/N rats (23/1969). However, the highest incidence observed in a single untreated control group is 3/50.
Three male rats had primary tumors of glial cell origin; a malignant oligodendroglioma in one unexposed control rat and a benign oligodendroglioma and a malignant astrocytoma in two exposed rats.

Low incidences of several types of skin neoplasms (trichoepitheliomas 1/50, sebaceous gland adenomas 1/50, and basal cell carcinomas 3/50) occurred only in exposed males. All are epithelial tumors that arise from the epidermis or adnexal structures. The incidence of each of these morphologic types of skin tumors in exposed rats is not significantly greater than that in controls, but the combined incidence (5/50) is greater than the mean historical incidence of epithelial skin tumors for chamber controls from the study laboratory (2/300, 0.7%) and the historical incidence in untreated controls in previous NTP non-inhalation studies (30/1936, 2%). Keratoacanthomas were observed in 4/50 control and 2/50 exposed male rats; squamous cell carcinomas were observed in 0/50 control and 2/50 exposed males. The combined incidence of these two tumors was not significantly greater than that in controls. Although the skin is directly exposed to chloroethane vapour, the epithelial tumors can not be related with certainty to chloroethane exposure because the marginally increased incidence in the exposed group is not statistically significant and the neoplasms were of various morphologic types. Skin tumors were not seen in female rats.
There were no compound related reproductive effects found during histopathological examination of animals exposed to 15000 ppm.


OTHER FINDINGS
The incidences of mononuclear cell leukaemia in exposed male and female rats was marginally greater than those in controls (male: control, 33/50; exposed, 36/50; female: control, 20/50; exposed; 25/50). Because mononuclear cell leukaemia is a common tumor with variable incidences, the marginal increases in the incidences of leukaemia were not considered biologically significant.

Tumour findings in rats

RATS

control

15000 ppm

male

female

male

female

Survival rate

16/50

31/50

8/50

22/50

 

Brain: Astrocytoma malignant

0/50

0/50

1/50 (2%)

3/50 (6%)

 

Skin: trichoepitheliomas

0/50

0/50

1/50 (2%)

0/50

Skin: sebaceous gland adenomas

0/50

0/50

1/50 (2%)

0/50

Skin: basal cell carcinomas

0/50

0/50

3/50 (6%)

1/50 (2%)

Skin: Keratoacanthoma

4/50 (8%)

0/50

2/50 (4%)

0/50

Skin: squamous cell carcinoma

0/50

0/50

2/50 (4%)

0/50

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
NOAEC
39 577 mg/m³
Study duration:
chronic
Species:
mouse
Quality of whole database:
The available information comprises an adequate and reliable study, and is thus sufficient to fulfil the standard information requirements set out in Annex VII, 8.5, of Regulation (EC) No 1907/2006.
System:
female reproductive system
Organ:
uterus

Carcinogenicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Justification for classification or non-classification

CLP: carcinogenicity category 2

Additional information

Under normal conditions chloromethane exists as a gas. Therefore the only relevant route of exposure is via inhalation.

In a carcinogenicity study equivalent to OECD Guideline 451 F344/N rats (50 animals per sex) were exposed to 0 or 15000 ppm chloroethane 6 hours/day, 5 days/week for 102 weeks (NTP, 1989).

Although survival of exposed and control male rats was unusually low at the end of the study, there were no statistically significant differences in survival between exposed and control groups of either sex (male: control 16/50, exposed 8/50; female: control 31/50, exposed 22/50). Survival at week 90 for male rats was 37/50 (control) and 31/50 (exposed) and for females, 43/50 (control) and 33/50 (exposed). At week 95, survival in all groups was at or above 48%; therefore, these studies are considered adequate for evaluation of carcinogenicity. The high incidences of mononuclear cell leukaemia may have contributed to the high mortality.

No compound-related clinical signs were observed.

Mean body weights of exposed female rats were generally 5% - 10% lower than those of controls from week 11 to week 42 and 6% - 13% lower from week 47 to the end of the study.

Mean body weights of exposed male rats were 4% - 8% lower than those of controls after week 33.

Astrocytoma (uncommon malignant glial cell tumours) of the brain were observed in three exposed female rats and gliosis (a non-neoplastic proliferation of glial cells) was seen in a fourth. Although the incidence of malignant astrocytoma is not statistically increased versus the concurrent controls, it is significant (P<0.05) relative to the incidence observed in chamber controls from previous studies at this laboratory (1/297) and also relative to the historical control incidence of glial cell tumors in untreated control female F344/N rats (23/1969). However, the highest incidence observed in a single untreated control group is 3/50.

Three male rats had primary tumours of glial cell origin; a malignant oligodendroglioma in one unexposed control rat and a benign oligodendroglioma and a malignant astrocytoma in two exposed rats. 

Low incidences of several types of skin neoplasms (trichoepitheliomas 1/50, sebaceous gland adenomas 1/50, and basal cell carcinomas 3/50) occurred only in exposed males. All are epithelial tumors that arise from the epidermis or adnexal structures. The incidence of each of these morphologic types of skin tumors in exposed rats is not significantly greater than that in controls, but the combined incidence (5/50) is greater than the mean historical incidence of epithelial skin tumors for chamber controls from the study laboratory (2/300, 0.7%) and the historical incidence in untreated controls in previous NTP non-inhalation studies (30/1936, 2%). Keratoacanthoma were observed in 4/50 control and 2/50 exposed male rats; squamous cell carcinomas were observed in 0/50 control and 2/50 exposed males. The combined incidence of these two tumors was not significantly greater than that in controls. Although the skin is directly exposed to chloroethane vapour, the epithelial tumors can not be related with certainty to chloroethane exposure because the marginally increased incidence in the exposed group is not statistically significant and the neoplasms were of various morphologic types. Skin tumors were not seen in female rats.

In summary, under the conditions of this 2-year inhalation study, there was equivocal evidence of carcinogenic activity of chloroethane for male F344/N rats, as indicated by benign and malignant epithelial neoplasm of the skin. For female F344/N rats, there was equivocal evidence of carcinogenic activity, as indicated by three uncommon malignant astrocytoma of the brain in the exposed group.

 

B6C3F1 mice were exposed to chloroethane under the same conditions as the F344/N rats in a 100-week inhalation study (NTP, 1989).

Survival of exposed male mice was significantly lower than that of controls after week 48; after week 72, survival was reduced to 50%. Non-neoplastic urogenital lesions were seen in individually housed control and exposed male mice and may have contributed to the reduced survival.

Survival of exposed female mice after week 82 was significantly lower than that of controls; the majority of exposed females died as a result of chloroethane-induced carcinomas of the uterus.

Exposed females were hyperactive during the daily exposure period.

Mean body weights of exposed female mice were generally similar to those of controls throughout the study in spite of the increased activity. Mean body weights of exposed male mice were up to 13% higher than those of controls throughout the study.

A highly significant incidence (86%, control 0/49, exposed 43/50) of uterine carcinomas of endometrial origin, clearly associated with chloroethane exposure, was observed in exposed female mice. The tumours were highly malignant and invaded the myometrium of the uterus; 34 metastasized to a wide variety of organs. Although one control female mouse did have a carcinoma of the uterus, this was not considered to be of endometrial origin and was morphologically different from those occurring in exposed mice.

Two marginally increased incidences of other neoplasms (aveolar/bronchiolar and hepatocellular) were observed in male and female mice.

The incidence of alveolar/bronchiolar tumors of the lung in exposed male mice was significantly greater than that in control mice (adenomas: control 3/50, exposed 8/48; adenomas or carcinomas combined: control 3/49, exposed 7/48). The association of exposure to chloroethane is not clear, especially since there were no supporting non-neoplastic lesions in the lungs of exposed mice and no neoplasms were seen in exposed female mice. In addition, the remainder of the respiratory tract, including the nasal cavity was unaffected by chloroethane exposure.

The incidence of hepatocellular carcinomas in female mice exposed to chloroethane was significantly greater than that in controls (control 3/49; exposed 8/48). Another exposed female had a hepatocellular adenoma. The incidence of hepatocellular carcinomas and adenoma combined in exposed females (17%) is greater than the historical incidence in chamber controls from the study laboratory (29/347, 8%) or in untreated control females from NTP studies (184/2032, 9%). The one adenoma reported in an exposed female mouse was observed on day 590. It is possible, that if survival of exposed females had not been reduced as a result of chloroethane-induced uterine carcinomas, the incidence of hepatocellular neoplasm might have been greater.

In summary, under the conditions of this 2-year inhalation study, there was clear evidence of carcinogenic activity for female B6C3F1 mice as indicated by the carcinomas of the uterus. A marginally increased incidence of hepatocellular neoplasms was observed in the exposed group of females. The study in male mice was considered to be an inadequate study of carcinogenicity because of the reduced survival in the exposed group. However, there was an increased incidence of alveolar/bronchiolar neoplasms in the lung.

The species and sex-specific tumour formation (uterine carcinoma) in female mice cannot be explained via a genotoxic mechanism, since no genotoxic potential was considered for chloroethane in vivo.

Endometrial uterine tumours are known to be caused by hormonal imbalances in experimental animals as well as in humans (Gargas et al., 2008).

Bucher et al. (1995) investigated the possible relationship of changes in blood concentrations of sex hormones to uterine carcinoma formation by examining the oestrous cycle of mice prior to and during a 21-day exposure to 15000 ppm chloroethane. There were no consistent changes either in the oestrous cycle nor in the measurement of serum oestradiol and progesteron attributable to chloroethane exposure. Thus Bucher et al. concluded that early changes in circulating sex hormones are not important contributing factors in the uterine neoplasia caused by chloroethane. However, large variations in measured values, with standard errors of about 20 -70% of the mean values, made statistical comparison difficult, thus the results by Bucher et al. cannot be said to refute the hypothesis, that estrogen unopposed by sufficient progesterone can lead to inappropriate stimulation of the endometrium and eventual tumorigenesis (Gargas et al., 2008).

Studies on the role of oxidation by cytochrome P-450 dependent monooxygenases and of glutathione provide a standpoint for a possible explanation of the carcinogenicity of chloroethane in female mice. The species differences regarding the oxidative metabolism are not significant enough to make the metabolite acetaldehyde accountable for the carcinogenic effect of chloroethane (BUA, 1997). Clear species differences were observed in the GSH-dependent metabolism. This was manifested for examples by the observation, that the mouse metabolizes chloroethane via the GSH-S-transferase pathway by about a half of an order of magnitude faster than the rat (Fedtke et al., 1994) and that high chloroethane concentrations are mainly metabolized via the GSH-conjugation in the mouse, whereas in rats at high chloroethane concentrations exhalation of the unchanged substance predominates (Pottenger et al., 1991 and 1992). In addition the urine metabolites differ between rats and mice after inhalation of chloroethane (Fedtke et al., 1995). However, a connection between the tumorigenicity and the GSH-dependent metabolism of chloroethane can be postulated, a causal relationship currently is not provable (BUA, 1997).

Holder (2008) take into account, that stress reaction of hyperkinesis was observed only during 15000 ppm gas exposure, but not when exposure ceased. Test rodents other than female mice did not exhibit a pattern of visible stress nor did they have a carcinogenic response to chloroethane. Unremitting stress has been documented to contribute a feedback to the hypothalamus which stimulates the hypothalamic-pituitary-axis (HPA), which in turn, induces the adrenal glands. High adrenal production of corticosteroids could adversely promote endometrial cells to cancer in mice.

References:

BUA (Beratergremium für umweltrelevante Altstoffe) (1997) Chloroethane. Report 210, Supplementary reports IV, S. Hirzel Verlag Stuttgart

Gargas, M.L. et al. (2008) Physiologically based pharmacokinetic modeling of chloroethane disposition in mice, rats and women. Toxicological Sciences 104(1):54 -66

Holder, J.W. (2008) Analysis of chloroethane toxicity and carcinogenicity including a comparison with bromoethane. Toxicol. Ind. Health 24(10):655 -675 (as cited in Pubmed, 2010)