Hexamethylcyclotrisiloxane

Endpoint summary

Description of key information

In the key 90-day inhalation repeated dose toxicity study, conducted according to OECD Test Guideline 413 and in compliance with GLP, the NOAEC for systemic toxicity, concluded by the study authors, was 150 ppm (1400 mg/m3) hexamethylcyclotrisiloxane vapour

(D3; CAS 541-05-9; EC No. 208-765-4) for male and female rats based on clinical signs, body weight changes, effects on blood and clinical biochemistry parameters, organ weights and histopathology at 600 ppm and higher. The test substance-related effects at concentrations up to 150 ppm were considered transitory, adaptive/metabolic, not clearly adverse or giving no evidence of organ toxicity. The reviewer of the study, however, concluded that the true NOAEC for systemic toxicity in male and female rats is at least 600 ppm (equivalent to 5459 mg/m3) based on not fully reversible statistically significant changes in body weights, haematology and blood chemistry parameters seen in 2500 ppm recovery male rats as well as not fully reversible statistically significantly increased liver / body weight ratio seen in recovery males and females from 2500 ppm group.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - systemic effects

Link to relevant study records

Reference

Endpoint:

sub-chronic toxicity: inhalation

Type of information:

experimental study

Adequacy of study:

key study

Study period:

from 6 May 1999 to 3 Sept 1999

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 413 (Subchronic Inhalation Toxicity: 90-Day Study)

Deviations:

no

GLP compliance:

yes

Limit test:

no

Species:

rat

Strain:

Fischer 344

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: not stated
- Age at study initiation: 9 wk
- Weight at study initiation: 118-141 g (m); 93-121 g (f)
- Housing: suspended wire mesh cages, individually housed
- Diet: standard diet ad libitum
- Water: drinking water ad libitum (except during exposure)

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 17-25
- Humidity (%): 29-72
- Photoperiod (hrs dark / hrs light): 12 h/ 12 h

IN-LIFE DATES: From: 1999-06-06 To: 1999-09-03

Route of administration:

inhalation: vapour

Type of inhalation exposure:

nose only

Vehicle:

other: unchanged (no vehicle)

Remarks on MMAD:

MMAD / GSD: not applicable

Details on inhalation exposure:

GENERATION OF TEST ATMOSPHERE / CHAMBER DESCRIPTION
- Method of holding animals in test chamber: exposure tube
- Source and rate of air: conditioned, filtered compressed air
- Air flow rate: 19-31 lpm

TEST ATMOSPHERE
- Brief description of analytical method used: gas chromatography
- Samples taken from breathing zone: taken from 5 locations in the exposure chamber 1/hr

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

target concentrations: 0, 15, 150, 600, 2500 ppm
measured concentrations: 0, 14.9, 150, 601, 2519 ppm

Duration of treatment / exposure:

90 days, 6 hr/day, 5 days/wk

Frequency of treatment:

daily (excluding weekends)

Dose / conc.:

0 ppm (analytical)

Dose / conc.:

14.9 ppm (analytical)

Dose / conc.:

150 ppm (analytical)

Dose / conc.:

601 ppm (analytical)

Dose / conc.:

2 519 ppm (analytical)

No. of animals per sex per dose:

10 for the 15, 150 and 600 ppm groups
20 for the 2500 ppm groups
25 for controls

Details on study design:

Post-exposure recovery period in satellite groups: 4-wk (0, 2500 ppm)

Positive control:

No

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes
- Time schedule: 2x/day, 1x/day at weekends

BODY WEIGHT: Yes
- Time schedule for examinations: weekly

FOOD CONSUMPTION:
- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: consumption determined weekly

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

WATER CONSUMPTION: No

OPHTHALMOSCOPIC EXAMINATION: Yes
- Time schedule for examinations: start and completion of study
- Dose groups that were examined: all

HAEMATOLOGY: Yes
- Time schedule for collection of blood: at sacrifice
- Anaesthetic used for blood collection: Yes (sodium pentobarbital)
- Animals fasted: Yes
- How many animals: all
- Parameters examined: See Table 1

CLINICAL CHEMISTRY: Yes
- Time schedule for collection of blood: at sacrifice
- Animals fasted: Yes
- How many animals: all
- Parameters examined: See Table 1

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No

Sacrifice and pathology:

GROSS PATHOLOGY: Yes (see Table 2)
HISTOPATHOLOGY: Yes (see Table 2)
ORGAN WEIGHTS: Yes (see Table 2)

Other examinations:

None.

Statistics:

Mean and standard deviations were calculated for all measured parameters. Group means were compared by ANOVA. Where significant effects were identified a further comparison was made with the control group using Dunnett’s test. The level of statistical significance was p<=0.05 in all cases.

Clinical signs:

effects observed, treatment-related

Description (incidence and severity):

Test substance-related clinical signs were limited to the 600 and 2500 ppm groups for both sexes. There was an exposure-dependent increased incidence of wet ventral abdominal and dorsal rump fur in both of these groups. In addition, the incidence of yellow discoloured inguinal fur, red material around the mouth and rough hair coat were increased in the 2500 ppm group. These signs were primary observed immediately following daily exposure completion. No other test substance-related clinical signs were observed.

Mortality:

mortality observed, treatment-related

Description (incidence):

No deaths occurred.

Body weight and weight changes:

effects observed, treatment-related

Description (incidence and severity):

Body weight gain was statistically significantly reduced in the 2500 ppm males during the first three weeks of exposure. No other test substance-related effects on body weight were observed. The reduced body weight gain in the first three weeks of the study in males at 2500 ppm, resulted in statistically significantly lower group mean body weight gain in 2500 ppm male group relative to the control group throughout the treatment period. There were no statistically significant or remarkable differences in female body weight for any of the dose levels.
During the recovery period, mean body weight in the 2500 ppm males remained statistically significantly lower than the control group; however, body weight gain upon cessation of treatment was increased significantly. Mean body weight and body weight gain for the 2500 ppm females during recovery were similar to the control group.

Food consumption and compound intake (if feeding study):

no effects observed

Description (incidence and severity):

Food consumption was unaffected by the test substance exposure.

Food efficiency:

not examined

Water consumption and compound intake (if drinking water study):

not examined

Ophthalmological findings:

no effects observed

Description (incidence and severity):

No effects observed.

Haematological findings:

effects observed, treatment-related

Description (incidence and severity):

Haemoglobin, haematocrit, mean corpuscular volume and mean corpuscular haemoglobin were statistically significantly decreased and platelets were statistically significantly increased for the 2500 ppm males at the end of the 13-week exposure period. Erythrocytes were also decreased for the 2500 ppm males but the change was not statistically significant. Also, exposure dependent and statistically significant decreases in erythrocytes, haemoglobin and haematocrit were present for the 600 ppm males. In addition, the incidence of several atypical cells (anisocytes, poikilocytes, microcytes and hypochromic) was increased for the 2500 ppm males and the incidence of microcytes was somewhat increased for the 2500 ppm females at the end of exposure. Also, for the 2500 ppm females at the end of the treatment period, mean corpuscular volume, mean corpuscular haemoglobin and mean corpuscular haemoglobin concentration were statistically decreased, but the differences from control were slight and there were no effects on erythrocytes, haemoglobin or haematocrit. The only other statistically significant value at the end of the exposure period was an increased percentage of mature neutrophils for the 2500 ppm females. The increase was not large and the total white blood cell count was not increased. Therefore, these differences from the control group were not attributed to the treatment.
At recovery evaluation, haemoglobin, haematocrit, mean corpuscular volume and mean corpuscular haemoglobin remained slightly decreased and platelets remained slightly increased in the 2500 ppm males. The differences from the control group were statistically significant, but the magnitude of the differences was less than at the end of the exposure period. The incidence of several atypical cells (anisocytes, microcytes and hypochromic) remained elevated for the 2500 ppm males; no differences from control in red cell morphology were present for the 2500 ppm females. Also, in the 2500 ppm males, the erythrocyte count and the percentage of nucleated red blood cells were increased statistically significantly at the recovery evaluation. Mean corpuscular volume and mean corpuscular haemoglobin were statistically significantly decreased at the recovery evaluation in the 2500 ppm females, but, as at the end of exposure there were no effects on erythrocytes, haemoglobin or haematocrit, and the differences were not large (<3%). Statistically significantly elevated platelets were seen for the 2500 ppm females, but the differences were not large and no effect on platelets count was seen at the end of the exposure period. The only other statistically significantly different values at the recovery evaluation were decreased leukocytes, lymphocytes and mean corpuscular haemoglobin concentration for the 2500 ppm males. However, the differences were not large and the values were within the normal limits for these parameters. None of these changes were considered to be associated with exposure to the test substance.

Clinical biochemistry findings:

effects observed, treatment-related

Description (incidence and severity):

In the 13-week clinical chemistry evaluation, albumin, globulin and total protein (TP) and gamma glutamyl transpeptidase (GGT) were statistically significantly increased and chloride was statistically significantly decreased for both sexes in the 2500 ppm group. Potassium and calcium were statistically significantly increased for the 2500 ppm males and triglycerides were statistically significantly increased for the 2500 ppm females. Glucose was statistically significantly decreased for the 600 and 2500 ppm males. Exposure-related, but not statistically significant decreases in alkaline phosphatase (AP) were present in all male groups and in the 150, 600 and 2500 ppm females.
Other statistically significantly differences at the end of the 13-week exposure period included decreased sodium in the 2500 ppm males, decreased alanine aminotransferase (ALT) in the 150 and 600 ppm females, decreased aspartate aminotransferase (AST) in the 600 ppm males and the 150 and 2500 ppm females and increased cholesterol in the 15 and 150 ppm females. These differences were of slight magnitude and were not exposure-dependent and were therefore not considered to be associated with exposure to the test substance.
At the recovery evaluation, AP remained statistically significantly decreased for the 2500 ppm males, but less so than at 13 weeks, and the 2500 ppm female AP level was comparable to the control. All other clinical chemistry changes observed at 13 weeks had returned to levels comparable to the control at the recovery evaluation; therefore, the effects seen at 13 weeks were reversible upon cessation of exposure. There were no other test substance-related changes in clinical chemistry.

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):

Liver weight was increased in all treated groups on an absolute, relative to body weight and relative to brain weight basis at the end of the exposure period. Although the differences were not statistically significant, an exposure-dependent response was observed. The increases for the 15 ppm females were slight. In the 2500 ppm group, thymus weight was decreased in both sexes and adrenal gland weight was increased in 2500 ppm females only, when evaluated on an absolute, relative to body weight and relative to brain weight basis; not all of these differences were statistically significant.
At the recovery necropsy, liver weight was comparable to control for the 2500 ppm males; although significantly higher when evaluated relative to body weight, the percent difference was 6.4%, which directly parallels the final body weight that was 6.3% lower than control group. Recovery liver weight for the 2500 ppm females was still elevated approximately 10% above the control value; this increase was statistically significant only when evaluated relative to body weight, and the difference was much lower than that observed at the 13-week necropsy (approximately 70%). Adrenal weight for the 2500 ppm females was also still somewhat increased, but the difference was not statistically significant. And, as for the liver, the magnitude of the increase was much less than that seen at the 13-week necropsy, i.e. approximately 14% at recovery versus approximately 35% at the end of exposure. No effects were apparent on the thymus weight. All effects seen on organ weights at the end of exposure exhibited substantial partial to full recovery and were therefore reversible upon cessation of exposure. There were no other test substance-related changes or differences in organ weights.

Gross pathological findings:

no effects observed

Description (incidence and severity):

No test-substance related gross necropsy findings.

Neuropathological findings:

not examined

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Description (incidence and severity):

At the end of the exposure period, diffuse hepatocytic hypertrophy was observed in all 600 and 2500 ppm animals and in 7/10 and 5/10 150 ppm males and females, respectively. The severity of this finding was exposure dependent. Hepatocytic hypertrophy in the centrilobular region was observed in a single 600 ppm male, 9/10 150 ppm males and 6/10 15 ppm males. The severity of this finding was exposure-dependent at the 15 and 150 ppm exposure levels, and minimal for all six 15 ppm rats in which it was seen. Centrilobular hypertrophy was not specifically noted for any females. Diffuse hypertrophy was characterised by uniformly increased size of hepatocytes. Centrilobular hypertrophy was diagnosed when only hepatocytes in the centrilobular to midzonal regions were affected, or when the severity in the centrilobular region was markedly different from the periportal region. Thus, the regional distribution of hepatocytic hypertrophy appeared to be exposure dependent. No hepatocytic hypertrophy was evident in the 2500 ppm males or females at the recovery necropsy. Thus, this change was fully reversible within 28 days following cessation of exposure.
Adrenal cortical cell vacuolation was seen for essentially all rats on the study, including the controls, at both the end of exposure and recovery. The severity of this finding was higher in the 600 and 2500 ppm groups (both sexes) at the end of exposure, but similar between the control and 2500 ppm groups at recovery. This change was fully reversible at the highest exposure level given (2500 ppm) within 28 days after cessation of exposure.

Histopathological findings: neoplastic:

no effects observed

Details on results:

The test substance induced effects at all doses but up to 150 ppm these were considered transitory, adaptive/metabolic, not clearly adverse or giving no evidence of organ toxicity.

Key result

Dose descriptor:

NOAEC

Effect level:

600 ppm

Based on:

test mat.

Sex:

male/female

Basis for effect level:

body weight and weight gain

clinical biochemistry

haematology

organ weights and organ / body weight ratios

Remarks on result:

other: equivalent to 5459 mg/m3

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

1 500 ppm (analytical)

System:

other: effects on body weight gain, heamatology and clinical chemistry parameters, and liver / body weight ratio

Organ:

liver

Treatment related:

yes

Dose response relationship:

yes

Relevant for humans:

not specified

Conclusions:

In the 90-day inhalation repeated dose toxicity study, conducted according to OECD Test Guideline 413 and in compliance with GLP, the NOAEC for systemic toxicity, concluded by the study authors, was 150 ppm (1400 mg/m3) for male and female rats based on clinical signs, body weight changes, effects on blood and clinical biochemistry parameters, organ weights and histopathology at 600 ppm and higher. The test substance-related effects at concentrations up to 150 ppm were considered transitory, adaptive/metabolic, not clearly adverse or giving no evidence of organ toxicity. The reviewer of the study, however, concluded that the true NOAEC for systemic toxicity in male and female rats is at least 600 ppm (equivalent to 5459 mg/m3) based on not fully reversible statistically significant changes in body weights, haematology and blood chemistry parameters seen in 2500 ppm recovery male rats as well as not fully reversible statistically significantly increased liver / body weight ratio seen in recovery males and females from 2500 ppm group.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEC

5 459 mg/m³

Study duration:

subchronic

Species:

rat

Repeated dose toxicity: inhalation - local effects

Link to relevant study records

Reference

Endpoint:

sub-chronic toxicity: inhalation

Type of information:

experimental study

Adequacy of study:

key study

Study period:

from 6 May 1999 to 3 Sept 1999

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 413 (Subchronic Inhalation Toxicity: 90-Day Study)

Deviations:

no

GLP compliance:

yes

Limit test:

no

Species:

rat

Strain:

Fischer 344

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: not stated
- Age at study initiation: 9 wk
- Weight at study initiation: 118-141 g (m); 93-121 g (f)
- Housing: suspended wire mesh cages, individually housed
- Diet: standard diet ad libitum
- Water: drinking water ad libitum (except during exposure)

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 17-25
- Humidity (%): 29-72
- Photoperiod (hrs dark / hrs light): 12 h/ 12 h

IN-LIFE DATES: From: 1999-06-06 To: 1999-09-03

Route of administration:

inhalation: vapour

Type of inhalation exposure:

nose only

Vehicle:

other: unchanged (no vehicle)

Remarks on MMAD:

MMAD / GSD: not applicable

Details on inhalation exposure:

GENERATION OF TEST ATMOSPHERE / CHAMBER DESCRIPTION
- Method of holding animals in test chamber: exposure tube
- Source and rate of air: conditioned, filtered compressed air
- Air flow rate: 19-31 lpm

TEST ATMOSPHERE
- Brief description of analytical method used: gas chromatography
- Samples taken from breathing zone: taken from 5 locations in the exposure chamber 1/hr

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

target concentrations: 0, 15, 150, 600, 2500 ppm
measured concentrations: 0, 14.9, 150, 601, 2519 ppm

Duration of treatment / exposure:

90 days, 6 hr/day, 5 days/wk

Frequency of treatment:

daily (excluding weekends)

Dose / conc.:

0 ppm (analytical)

Dose / conc.:

14.9 ppm (analytical)

Dose / conc.:

150 ppm (analytical)

Dose / conc.:

601 ppm (analytical)

Dose / conc.:

2 519 ppm (analytical)

No. of animals per sex per dose:

10 for the 15, 150 and 600 ppm groups
20 for the 2500 ppm groups
25 for controls

Details on study design:

Post-exposure recovery period in satellite groups: 4-wk (0, 2500 ppm)

Positive control:

No

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes
- Time schedule: 2x/day, 1x/day at weekends

BODY WEIGHT: Yes
- Time schedule for examinations: weekly

FOOD CONSUMPTION:
- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: consumption determined weekly

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

WATER CONSUMPTION: No

OPHTHALMOSCOPIC EXAMINATION: Yes
- Time schedule for examinations: start and completion of study
- Dose groups that were examined: all

HAEMATOLOGY: Yes
- Time schedule for collection of blood: at sacrifice
- Anaesthetic used for blood collection: Yes (sodium pentobarbital)
- Animals fasted: Yes
- How many animals: all
- Parameters examined: See Table 1

CLINICAL CHEMISTRY: Yes
- Time schedule for collection of blood: at sacrifice
- Animals fasted: Yes
- How many animals: all
- Parameters examined: See Table 1

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No

Sacrifice and pathology:

GROSS PATHOLOGY: Yes (see Table 2)
HISTOPATHOLOGY: Yes (see Table 2)
ORGAN WEIGHTS: Yes (see Table 2)

Other examinations:

None.

Statistics:

Mean and standard deviations were calculated for all measured parameters. Group means were compared by ANOVA. Where significant effects were identified a further comparison was made with the control group using Dunnett’s test. The level of statistical significance was p<=0.05 in all cases.

Clinical signs:

effects observed, treatment-related

Description (incidence and severity):

Test substance-related clinical signs were limited to the 600 and 2500 ppm groups for both sexes. There was an exposure-dependent increased incidence of wet ventral abdominal and dorsal rump fur in both of these groups. In addition, the incidence of yellow discoloured inguinal fur, red material around the mouth and rough hair coat were increased in the 2500 ppm group. These signs were primary observed immediately following daily exposure completion. No other test substance-related clinical signs were observed.

Mortality:

mortality observed, treatment-related

Description (incidence):

No deaths occurred.

Body weight and weight changes:

effects observed, treatment-related

Description (incidence and severity):

Body weight gain was statistically significantly reduced in the 2500 ppm males during the first three weeks of exposure. No other test substance-related effects on body weight were observed. The reduced body weight gain in the first three weeks of the study in males at 2500 ppm, resulted in statistically significantly lower group mean body weight gain in 2500 ppm male group relative to the control group throughout the treatment period. There were no statistically significant or remarkable differences in female body weight for any of the dose levels.
During the recovery period, mean body weight in the 2500 ppm males remained statistically significantly lower than the control group; however, body weight gain upon cessation of treatment was increased significantly. Mean body weight and body weight gain for the 2500 ppm females during recovery were similar to the control group.

Food consumption and compound intake (if feeding study):

no effects observed

Description (incidence and severity):

Food consumption was unaffected by the test substance exposure.

Food efficiency:

not examined

Water consumption and compound intake (if drinking water study):

not examined

Ophthalmological findings:

no effects observed

Description (incidence and severity):

No effects observed.

Haematological findings:

effects observed, treatment-related

Description (incidence and severity):

Haemoglobin, haematocrit, mean corpuscular volume and mean corpuscular haemoglobin were statistically significantly decreased and platelets were statistically significantly increased for the 2500 ppm males at the end of the 13-week exposure period. Erythrocytes were also decreased for the 2500 ppm males but the change was not statistically significant. Also, exposure dependent and statistically significant decreases in erythrocytes, haemoglobin and haematocrit were present for the 600 ppm males. In addition, the incidence of several atypical cells (anisocytes, poikilocytes, microcytes and hypochromic) was increased for the 2500 ppm males and the incidence of microcytes was somewhat increased for the 2500 ppm females at the end of exposure. Also, for the 2500 ppm females at the end of the treatment period, mean corpuscular volume, mean corpuscular haemoglobin and mean corpuscular haemoglobin concentration were statistically decreased, but the differences from control were slight and there were no effects on erythrocytes, haemoglobin or haematocrit. The only other statistically significant value at the end of the exposure period was an increased percentage of mature neutrophils for the 2500 ppm females. The increase was not large and the total white blood cell count was not increased. Therefore, these differences from the control group were not attributed to the treatment.
At recovery evaluation, haemoglobin, haematocrit, mean corpuscular volume and mean corpuscular haemoglobin remained slightly decreased and platelets remained slightly increased in the 2500 ppm males. The differences from the control group were statistically significant, but the magnitude of the differences was less than at the end of the exposure period. The incidence of several atypical cells (anisocytes, microcytes and hypochromic) remained elevated for the 2500 ppm males; no differences from control in red cell morphology were present for the 2500 ppm females. Also, in the 2500 ppm males, the erythrocyte count and the percentage of nucleated red blood cells were increased statistically significantly at the recovery evaluation. Mean corpuscular volume and mean corpuscular haemoglobin were statistically significantly decreased at the recovery evaluation in the 2500 ppm females, but, as at the end of exposure there were no effects on erythrocytes, haemoglobin or haematocrit, and the differences were not large (<3%). Statistically significantly elevated platelets were seen for the 2500 ppm females, but the differences were not large and no effect on platelets count was seen at the end of the exposure period. The only other statistically significantly different values at the recovery evaluation were decreased leukocytes, lymphocytes and mean corpuscular haemoglobin concentration for the 2500 ppm males. However, the differences were not large and the values were within the normal limits for these parameters. None of these changes were considered to be associated with exposure to the test substance.

Clinical biochemistry findings:

effects observed, treatment-related

Description (incidence and severity):

In the 13-week clinical chemistry evaluation, albumin, globulin and total protein (TP) and gamma glutamyl transpeptidase (GGT) were statistically significantly increased and chloride was statistically significantly decreased for both sexes in the 2500 ppm group. Potassium and calcium were statistically significantly increased for the 2500 ppm males and triglycerides were statistically significantly increased for the 2500 ppm females. Glucose was statistically significantly decreased for the 600 and 2500 ppm males. Exposure-related, but not statistically significant decreases in alkaline phosphatase (AP) were present in all male groups and in the 150, 600 and 2500 ppm females.
Other statistically significantly differences at the end of the 13-week exposure period included decreased sodium in the 2500 ppm males, decreased alanine aminotransferase (ALT) in the 150 and 600 ppm females, decreased aspartate aminotransferase (AST) in the 600 ppm males and the 150 and 2500 ppm females and increased cholesterol in the 15 and 150 ppm females. These differences were of slight magnitude and were not exposure-dependent and were therefore not considered to be associated with exposure to the test substance.
At the recovery evaluation, AP remained statistically significantly decreased for the 2500 ppm males, but less so than at 13 weeks, and the 2500 ppm female AP level was comparable to the control. All other clinical chemistry changes observed at 13 weeks had returned to levels comparable to the control at the recovery evaluation; therefore, the effects seen at 13 weeks were reversible upon cessation of exposure. There were no other test substance-related changes in clinical chemistry.

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):

Liver weight was increased in all treated groups on an absolute, relative to body weight and relative to brain weight basis at the end of the exposure period. Although the differences were not statistically significant, an exposure-dependent response was observed. The increases for the 15 ppm females were slight. In the 2500 ppm group, thymus weight was decreased in both sexes and adrenal gland weight was increased in 2500 ppm females only, when evaluated on an absolute, relative to body weight and relative to brain weight basis; not all of these differences were statistically significant.
At the recovery necropsy, liver weight was comparable to control for the 2500 ppm males; although significantly higher when evaluated relative to body weight, the percent difference was 6.4%, which directly parallels the final body weight that was 6.3% lower than control group. Recovery liver weight for the 2500 ppm females was still elevated approximately 10% above the control value; this increase was statistically significant only when evaluated relative to body weight, and the difference was much lower than that observed at the 13-week necropsy (approximately 70%). Adrenal weight for the 2500 ppm females was also still somewhat increased, but the difference was not statistically significant. And, as for the liver, the magnitude of the increase was much less than that seen at the 13-week necropsy, i.e. approximately 14% at recovery versus approximately 35% at the end of exposure. No effects were apparent on the thymus weight. All effects seen on organ weights at the end of exposure exhibited substantial partial to full recovery and were therefore reversible upon cessation of exposure. There were no other test substance-related changes or differences in organ weights.

Gross pathological findings:

no effects observed

Description (incidence and severity):

No test-substance related gross necropsy findings.

Neuropathological findings:

not examined

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Description (incidence and severity):

At the end of the exposure period, diffuse hepatocytic hypertrophy was observed in all 600 and 2500 ppm animals and in 7/10 and 5/10 150 ppm males and females, respectively. The severity of this finding was exposure dependent. Hepatocytic hypertrophy in the centrilobular region was observed in a single 600 ppm male, 9/10 150 ppm males and 6/10 15 ppm males. The severity of this finding was exposure-dependent at the 15 and 150 ppm exposure levels, and minimal for all six 15 ppm rats in which it was seen. Centrilobular hypertrophy was not specifically noted for any females. Diffuse hypertrophy was characterised by uniformly increased size of hepatocytes. Centrilobular hypertrophy was diagnosed when only hepatocytes in the centrilobular to midzonal regions were affected, or when the severity in the centrilobular region was markedly different from the periportal region. Thus, the regional distribution of hepatocytic hypertrophy appeared to be exposure dependent. No hepatocytic hypertrophy was evident in the 2500 ppm males or females at the recovery necropsy. Thus, this change was fully reversible within 28 days following cessation of exposure.
Adrenal cortical cell vacuolation was seen for essentially all rats on the study, including the controls, at both the end of exposure and recovery. The severity of this finding was higher in the 600 and 2500 ppm groups (both sexes) at the end of exposure, but similar between the control and 2500 ppm groups at recovery. This change was fully reversible at the highest exposure level given (2500 ppm) within 28 days after cessation of exposure.

Histopathological findings: neoplastic:

no effects observed

Details on results:

The test substance induced effects at all doses but up to 150 ppm these were considered transitory, adaptive/metabolic, not clearly adverse or giving no evidence of organ toxicity.

Key result

Dose descriptor:

NOAEC

Effect level:

600 ppm

Based on:

test mat.

Sex:

male/female

Basis for effect level:

body weight and weight gain

clinical biochemistry

haematology

organ weights and organ / body weight ratios

Remarks on result:

other: equivalent to 5459 mg/m3

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

1 500 ppm (analytical)

System:

other: effects on body weight gain, heamatology and clinical chemistry parameters, and liver / body weight ratio

Organ:

liver

Treatment related:

yes

Dose response relationship:

yes

Relevant for humans:

not specified

Conclusions:

In the 90-day inhalation repeated dose toxicity study, conducted according to OECD Test Guideline 413 and in compliance with GLP, the NOAEC for systemic toxicity, concluded by the study authors, was 150 ppm (1400 mg/m3) for male and female rats based on clinical signs, body weight changes, effects on blood and clinical biochemistry parameters, organ weights and histopathology at 600 ppm and higher. The test substance-related effects at concentrations up to 150 ppm were considered transitory, adaptive/metabolic, not clearly adverse or giving no evidence of organ toxicity. The reviewer of the study, however, concluded that the true NOAEC for systemic toxicity in male and female rats is at least 600 ppm (equivalent to 5459 mg/m3) based on not fully reversible statistically significant changes in body weights, haematology and blood chemistry parameters seen in 2500 ppm recovery male rats as well as not fully reversible statistically significantly increased liver / body weight ratio seen in recovery males and females from 2500 ppm group.

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

In the key 90-day inhalation repeated dose toxicity study, conducted according to OECD Test Guideline 413 and in compliance with GLP (Dow Corning Corporation, 2001), three groups, each comprised of 10 male and 10 female rats, were exposed to the target concentrations of 0, 15, 150, 600 and 2500 ppm hexamethylcyclotrisiloxane vapours. Additional 5 male and 5 female rats were included in the 0 and 2500 exposure group and were used as recovery animals. All rats were exposed to the test concentrations for 6 hours per day, 5 days per week for 13 weeks. After the 13-week exposure period, 10 rats per sex per group were euthanised and examined at necropsy. The remaining 10 rats in the control and high concentration group were maintained without treatment for additional 4 weeks, non-exposure period, after which they were euthanised and examined at necropsy. Endpoints evaluated included survival, clinical condition, body weight, food consumption, ophthalmic examination, haematology, clinical chemistry, selected organ weights, gross necropsy and microscopic examination of selected tissues.

 

No deaths occurred during the study period.

 

Test substance-related clinical signs were limited to the 600 and 2500 ppm groups for both sexes. There was an exposure-dependent increased incidence of wet ventral abdominal and dorsal rump fur in both of these groups. In addition, the incidence of yellow discoloured inguinal fur, red material around the mouth and rough hair coat were increased in the 2500 ppm group. These signs were primary observed immediately following daily exposure completion. No other test substance-related clinical signs were observed. Body weight gain was statistically significantly reduced in the 2500 ppm males during the first three weeks of exposure. No other test substance-related effects on body weight were observed. The reduced body weight gain in the first three weeks of the study in males at 2500 ppm, resulted in statistically significantly lower group mean body weight gain in 2500 ppm male group relative to the control group throughout the treatment period. There were no statistically significant or remarkable differences in female body weight for any of the dose levels.

 

During the recovery period, mean body weight in the 2500 ppm males remained statistically significantly lower than the control group; however, body weight gain upon cessation of treatment was increased significantly. Mean body weight and body weight gain for the 2500 ppm females during recovery were similar to the control group. Food consumption was unaffected by the test substance exposure. Haemoglobin, haematocrit, mean corpuscular volume and mean corpuscular haemoglobin were statistically significantly decreased and platelets were statistically significantly increased for the 2500 ppm males at the end of the 13-week exposure period. Erythrocytes were also decreased for the 2500 ppm males but the change was not statistically significant. Also, exposure dependent and statistically significant decreases in erythrocytes, haemoglobin and haematocrit were present for the 600 ppm males. In addition, the incidence of several atypical cells (anisocytes, poikilocytes, microcytes and hypochromic) was increased for the 2500 ppm males and the incidence of microcytes was somewhat increased for the 2500 ppm females at the end of exposure. Also, for the 2500 ppm females at the end of the treatment period, mean corpuscular volume, mean corpuscular haemoglobin and mean corpuscular haemoglobin concentration were statistically decreased, but the differences from control were slight and there were no effects on erythrocytes, haemoglobin or haematocrit. The only other statistically significant value at the end of the exposure period was an increased percentage of mature neutrophils for the 2500 ppm females. The increase was not large and the total white blood cell count was not increased. Therefore, these differences from the control group were not attributed to the treatment.

 

At recovery evaluation, haemoglobin, haematocrit, mean corpuscular volume and mean corpuscular haemoglobin remained slightly decreased and platelets remained slightly increased in the 2500 ppm males. The differences from the control group were statistically significant, but the magnitude of the differences was less than at the end of the exposure period. The incidence of several atypical cells (anisocytes, microcytes and hypochromic) remained elevated for the 2500 ppm males; no differences from control in red cell morphology were present for the 2500 ppm females. Also, in the 2500 ppm males, the erythrocyte count and the percentage of nucleated red blood cells were increased statistically significantly at the recovery evaluation. Mean corpuscular volume and mean corpuscular haemoglobin were statistically significantly decreased at the recovery evaluation in the 2500 ppm females, but, as at the end of exposure there were no effects on erythrocytes, haemoglobin or haematocrit, and the differences were not large (<3%). Statistically significantly elevated platelets were seen for the 2500 ppm females, but the differences were not large and no effect on platelets count was seen at the end of the exposure period. The only other statistically significantly different values at the recovery evaluation were decreased leukocytes, lymphocytes and  mean corpuscular haemoglobin concentration for the 2500 ppm males. However, the differences were not large and the values were within the normal limits for these parameters. None of these changes were considered to be associated with exposure to the test substance.

 

In the 13-week clinical chemistry evaluation, albumin, globulin and total protein (TP) and gamma glutamyl transpeptidase (GGT) were statistically significantly increased and chloride was statistically significantly decreased for both sexes in the 2500 ppm group. Potassium and calcium were statistically significantly increased for the 2500 ppm males and triglycerides were statistically significantly increased for the 2500 ppm females. Glucose was statistically significantly decreased for the 600 and 2500 ppm males. Exposure-related, but not statistically significant decreases in alkaline phosphatase (AP) were present in all male groups and in the 150, 600 and 2500 ppm females.

 

Other statistically significantly differences at the end of the 13-week exposure period included decreased sodium in the 2500 ppm males, decreased alanine aminotransferase (ALT) in the 150 and 600 ppm females, decreased aspartate aminotransferase (AST) in the 600 ppm males and the 150 and 2500 ppm females and increased cholesterol in the 15 and 150 ppm females. These differences were of slight magnitude and were not exposure-dependent and were therefore not considered to be associated with exposure to the test substance.

 

At the recovery evaluation, AP remained statistically significantly decreased for the 2500 ppm males, but less so than at 13 weeks, and the 2500 ppm female AP level was comparable to the control. All other clinical chemistry changes observed at 13 weeks had returned to levels comparable to the control at the recovery evaluation; therefore, the effects seen at 13 weeks were reversible upon cessation of exposure. There were no other test substance-related changes in clinical chemistry.

 

Liver weight was increased in all treated groups on an absolute, relative to body weight and relative to brain weight basis at the end of the exposure period. Although the differences were not statistically significant, an exposure-dependent response was observed. The increases for the 15 ppm females were slight. In the 2500 ppm group, thymus weight was decreased in both sexes and adrenal gland weight was increased in 2500 ppm females only, when evaluated on an absolute, relative to body weight and relative to brain weight basis; not all of these differences were statistically significant.

 

At the recovery necropsy, liver weight was comparable to control for the 2500 ppm males; although significantly higher when evaluated relative to body weight, the percent difference was 6.4%, which directly parallels the final body weight that was 6.3% lower than control group. Recovery liver weight for the 2500 ppm females was still elevated approximately 10% above the control value; this increase was statistically significant only when evaluated relative to body weight, and the difference was much lower than that observed at the 13-week necropsy (approximately 70%). Adrenal weight for the 2500 ppm females was also still somewhat increased, but the difference was not statistically significant. And, as for the liver, the magnitude of the increase was much less than that seen at the 13-week necropsy, i.e. approximately 14% at recovery versus approximately 35% at the end of exposure. No effects were apparent on the thymus weight. All effects seen on organ weights at the end of exposure exhibited substantial partial to full recovery and were therefore reversible upon cessation of exposure. There were no other test substance-related changes or differences in organ weights.

 

There were no test-substance related gross necropsy findings.

 

At the end of the exposure period, diffuse hepatocytic hypertrophy was observed in all 600 and 2500 ppm animals and in 7/10 and 5/10 150 ppm males and females, respectively. The severity of this finding was exposure dependent. Hepatocytic hypertrophy in the centrilobular region was observed in a single 600 ppm male, 9/10 150 ppm males and 6/10 15 ppm males. The severity of this finding was exposure-dependent at the 15 and 150 ppm exposure levels, and minimal for all six 15 ppm rats in which it was seen. Centrilobular hypertrophy was not specifically noted for any females. Diffuse hypertrophy was characterised by uniformly increased size of hepatocytes. Centrilobular hypertrophy was diagnosed when only hepatocytes in the centrilobular to midzonal regions were affected, or when the severity in the centrilobular region was markedly different from the periportal region. Thus, the regional distribution of hepatocytic hypertrophy appeared to be exposure dependent. No hepatocytic hypertrophy was evident in the 2500 ppm males or females at the recovery necropsy. Thus, this change was fully reversible within 28 days following cessation of exposure.

 

Adrenal cortical cell vacuolation was seen for essentially all rats on the study, including the controls, at both the end of exposure and recovery. The severity of this finding was higher in the 600 and 2500 ppm groups (both sexes) at the end of exposure, but similar between the control and 2500 ppm groups at recovery. This change was fully reversible at the highest exposure level given (2500 ppm) within 28 days after cessation of exposure.

 

The NOAEC for systemic toxicity, concluded by the study authors, was 150 ppm (1400 mg/m3) for male and female rats based on clinical signs, body weight changes, effects on blood and clinical biochemistry parameters, organ weights and histopathology at 600 ppm and higher. The test substance-related effects at concentrations up to 150 ppm were considered transitory, adaptive/metabolic, not clearly adverse or giving no evidence of organ toxicity. The reviewer of the study, however, concluded that the true NOAEC for systemic toxicity is at least 600 ppm (equivalent to 5459 mg/m3) based on not fully reversible statistically significant changes in body weights, haematology and blood chemistry parameters seen in 2500 ppm recovery male rats as well as not fully reversible statistically significantly increased liver / body weight ratio seen in recovery males and females from 2500 ppm group.

  

In a two-week dose range-finding study by the inhalation route, not conducted according to OECD Test Guidelines or in compliance with GLP, hexamethylcyclotrisiloxane vapours were administered to male and female rats via 6-hour daily nose only inhalation exposure at concentrations of 0, 15, 300, 1500 and 2500 ppm. The clinical signs observed included of wet inguinal fur in male and female rats at concentrations of 1500 ppm and higher. Reduced body weight gain was observed for males at 2500 ppm. Blood effects consistent of decreased hemoglobin and hematocrit and increased platelets in males at 2500 ppm. Reduced alkaline phosphatase was recorded in all treated groups, however the biological significance of this finding was unclear. Increased absolute and relative liver weights were seen in males and females at 300 ppm group and higher. Increased adrenal weight was noted in females at 1500 ppm and higher and males at 2500 ppm. Increased heart weight was noted in females at 2500 ppm. There were no abnormal findings at necropsy (Dow Corning Corporation, 2000).

  

In a 28-day repeated dose toxicity study by the inhalation route, conducted according to OECD Test Guideline 412 and in compliance with GLP, 5 male and 5 female rats were exposed via nose-only inhalation to hexamethylcyclotrisiloxane aerosol with a considerable amount of test substance present as vapour at concentrations of 0, 0.084, 0.945, 9.041 mg/l (Laboratory of Pharmacology and Toxicology, 1992). The animals were exposed daily for 6 hours per day, 7 days a week for 4 weeks. Following exposure, 5 male and female additional control and high dose animals served as recovery group and were maintained for 4 weeks without treatment. Three males and one female from the high dose group were found dead between study days 13 and 16. Symptoms noted on days prior to death included dyspnoea, ataxia, reduced reflexes and/or piloerection. At autopsy of dead animals showed diffuse hyperaemia and petechial haemorrhages in the lungs. Microscopically congestion of the kidneys, lungs and liver, as well as pulmonary (perivascular) edema were observed in the dead animals.  

 

No clinical signs of toxicity were noted in the control, low- or mid-dose animals. In the high-dose animals, slight temporary sedation after the 6-hour exposure was noted and slight transient corneal opacity was observed during the latter weeks of exposure in these animals. There were no treatment-related effects on behaviour, body weight, food consumption, ophthalmology, haematology or clinical chemistry, organ weights and macroscopic/histopathological findings. All previously high-dosed rats remained unremarkable compared to controls during and after the 4-week recovery period. 

 

To evaluate local tolerance to the treatment, the lungs, bronchi and nasal cavity of all rats were examined. There were no signs of toxicity at 0.084 mg/L. At the intermediate concentration of 0.945 mg/L, a slight haemorrhagic encrustation of the nose was observed in all rats in the first week, and the number exhibiting this symptom varied from day 8 to the end of the study. During the last three weeks the maximum number of animals were affected on day 19. This encrustation was still visible immediately before the necropsy (3/5 males; 3/5 females). At the highest concentration of 9.041 mg/L the haemorrhagic encrustation was noted in several to all animals, with the maximum incidence observed between test days 15 and 28. Macroscopic inspection during dissection revealed no local changes except for light-red (seromucous) secretion in two high-dosed male animals. Microscopic examination of the nasal cavity revealed slight inflammatory changes in 3/5 high exposure males. No inflammatory reactions were noted in the control or remaining test article-treated animals. No test article-related changes were noted microscopically in the lungs of the control, low- or mid-concentration treated animals. At the high dose, an increased incidence of pulmonary perivascular round cell infiltration was detected (6 of 10 animals). In the lungs of a few or several high-exposure rats vascular congestion, (perivascular) edema, hyperplasia of pulmonary arteries and/or emphysema were observed. Following the recovery period, none of the previously high-dosed rats showed macroscopically visible local changes after discontinuation of dosing. Microscopic examination revealed aggregation of macrophages in the lungs of 4 males and slight pulmonary perivascular round cell infiltrates in 1 female and 2 male rats. In addition, a slight focal mucosal inflammation of the nasal cavity was detected in one male. These changes are consistent with respiratory tract irritation. The 4-week recovery period may have been too short to allow for complete reversibility of the local effects.

 

The NOAEC for systemic effects was 0.945 mg/l based on mortality, clinical signs and effects in necropsy at the high concentration; the LOAEC for local effects was 0.084 mg/l based on nasal and upper respiratory tract irritation.

  

In a two-week inhalation repeated dose toxicity study in rats, intended to examine the potential for hepatic microsomal enzyme induction in rats exposed to hexamethylcyclotrisiloxane, there was considerable evidence of microsomal enzyme induction at concentrations of 300 ppm (equivalent to 2.73 mg/l) and higher as well as an induced increased liver weight at 1500 ppm in male rats and 300 ppm and higher in female rats. The similarity of the cytochrome P450 expression profile to that of the phenobarbital-treated positive controls lead the investigators to describe hexamethylcyclotrisiloxane as ‘phenobarbital-like’ (Dow Corning Corporation, 2002).

  

In a combined repeated dose toxicity study with the reproduction / developmental toxicity screening test, conducted according to OECD Test Guideline 422 and in compliance with GLP (Dow Corning Corporation, 2002), Sprague-Dawley male and female rats were exposed to hexamethylcyclotrisiloxane vapor via whole body inhalation at concentrations of 0, 100, 500 or 2500 ppm (approximately 0.61, 4.5 or 22.8 mg/L). The animals were exposed 6 hours per day, 7 days per week for up to 39 days. Females in each exposure level were divided into a toxicity group (10 animals/group) and a reproductive group (10 animals/group). A single group of males (10 animals/group) was used for both the toxicity and reproductive phases of the study. Males and toxicity group females were treated for 28 and 29 days, respectively. Reproductive group females were treated for 14 days prior to the mating period, during the mating period and then up to and including gestation day 19.

 

In general, whole-body inhalation exposure of male and female Sprague-Dawley rats to hexamethylcyclotrisiloxane vapour at concentrations of up to 2500 ppm for 28-29 consecutive days was well-tolerated, as suggested by minimal effects on body weight and food consumption, the lack of significant clinical signs of toxicity, and minimal effects on clinical pathology parameters. Increased liver weight and centrilobular hepatocellular hypertrophy were demonstrated in both sexes exposed to 2500 ppm. These effects are considered to be an adaptive response characteristic of this class of cyclic siloxanes. No liver effects were identified at lower exposure levels.

 

Protein droplet nephropathy was observed by histopathology in the kidneys of males exposed to 500 and 2500 ppm. One male rat exposed to 100 ppm was identified with a very slight lesion of the same nature. The finding was corroborated for males in the 2500 ppm exposure group by organ weight and clinical chemistry changes. Females were unaffected. This nephropathy, affecting only male rats, is consistent in incidence and histomorphology with that caused by excessive accumulation of α2µ-globulin in renal cortical tubules. This mechanism is specific to male rats and not associated with human risk.

 

In males exposed to 2500 ppm, there were statistically significant decreases in the absolute weight of epididymides (10 %) and in the absolute (30 %) and relative (27 %) weight of the seminal vesicles. Slight atrophy of the seminal vesicles was identified microscopically in four of the 10 males in the 2500 ppm exposure group. The epididymides, testes, and prostate from exposed rats were histologically normal.

 

At the 2500 ppm exposure level there was reduced motor activity in both sexes and a reduced “reaction to handling” in females. There was no histomorphologic evidence of neurotoxicity at any exposure level.

 

The NOAEC for systemic effects relevant to humans was 500 ppm (equivalent to 4.55 mg/l) in male and female rats based on decreased body weight, decreased seminal vesicles weight and histopathological findings in seminal vesicles at 2500 ppm. Increased liver weight and centrilobular hepatocellular hypertrophy were observed in male and female rats in the 2500 ppm group, which were considered to be an adaptive response to the test substance treatment and non-adverse. Kidney effects such as organ weight changes at 2500 ppm and protein droplet nephropathy at 500 and 2500 ppm, which also correlated with clinical chemistry changes, were seen in male rats. These changes were considered to be caused by a2u-globulin retention in renal cortical tubules, a mechanism specific to male rats and not associated with human risk.

  

In a 28-day oral repeated dose toxicity study in rats, conducted according to a protocol similar to OECD Test Guideline 407 and in compliance with GLP (Dow Corning Corporation, 1990b), undiluted hexamethylcyclotrisiloxane was administered 5 days per week by oral gavage to 6 male and 6 female rats at dose of 1500 mg/kg bw/day for 28 days. There were no deaths or treatment-related clinical signs of toxicity. There were no effects on food consumption and body weight gain. The only treatment-related effect was a statistically significant increase in liver weight in males and females, but this effect was concluded by the study reviewer to be an adaptive change rather than a toxic effect. There were no gross pathological changes in any organs or tissue. At microscopic examination, all treated males had markedly increased incidence of hyaline droplets in proximal tubule epithelium of the kidneys. Three of the treated males had evidence of intracanalicular and/or intracellular bile stasis, with granulomatous cholangitis in one and multifocal hepatocellular necrosis in another. No treatment-related histopathologic changes were noted in females. An additional histopathology review (Dow Corning Corporation, 2010) of the liver tissue slides from the study was conducted to determine if treatment-related effects, such as liver pigment accumulation and periportal vacuolation, described in more recent studies with silicon-based materials were present in this older study. No clear increase in periportal lipid vacuolation or perilobular fatty change was observed in any of the slides. In the study report dark brown pigment, called bile stasis with an attendant granulomatous cholangitis, was identified in the bile ducts from about half (3/6) of the treated male rats by the Dow Corning Corporation (1990b) pathologist. During the additional histopathology review of the liver tissue slides, under polarised light some of the pigment accumulations showed birefringence, but this finding was not consistent in size or between animals. This pigment accumulation was accompanied by bile duct proliferation and chronic inflammation and therefore considered to be an adverse finding. It was also noted in the additional histopathology review that bile stasis is a process, not a material; what would be seen in such a case is bile pigment (bile) that has accumulated due to intra- or extra-hepatic obstruction. A special stain (Hall’s) that can positively identify bile was not used to characterise the material during the study histopathology evaluation. In addition, the study authors hypothesized that the presumptive bile stasis was secondary to dosing accidents involving the lung. Overall, the additional histopathology review concluded that the study authors misidentified the brown pigment seen in the tissue slides. The review also confirmed presence of treatment-related adverse brown pigment accompanied by bile duct proliferation and chronic inflammation in the livers of male rats treated with 1500 mg hexamethylcyclotrisiloxane/kg bw/day for 28 days. Therefore, the LOAEL for systemic toxicity was concluded by the study reviewer to be 1500 mg/kg bw/day (the highest dose tested). A NOAEL for this effect could not be established.

  

In a 14-day oral repeated dose toxicity study in rats, not conducted according to OECD Test Guideline or in compliance with GLP (Dow Corning Corporation, 1990a), hexamethylcyclotrisiloxane in 0.5% methocel A4M in distilled water was administered by oral gavage to 8 male and 8 female rats at dose levels of 0, 25, 100, 400 and 1600 mg/kg bw/day; exposure occurred 5 days per week for 2 weeks. The purpose of the study was to investigate whether hexamethylcyclotrisiloxane increases liver weights in a similar way to that seen for other oligomeric cyclosiloxanes.

 

There were no deaths or treatment-related clinical signs of toxicity. There were no effects on body weight gain. Clear test article-related increases in liver weights were seen in male rats at 100 mg/kg bw/day and higher and in female rats at 1600 mg/kg bw/day. There was slight evidence of liver weight increases in male rats at the lowest tested dose of 25 mg/kg bw/day. The biological significance of the liver weight increase was said to be unclear. There were no gross pathological changes in any organs or tissue. The NOAEL for liver weight was 25 mg/kg bw/day for male rats and 400 mg/kg bw/day for female rats.

Data for the final hydrolysis product, dimethylsilanediol

Data for the final hydrolysis product, dimethylsilanediol (DMSD), have been included in the dossier to allow risk characterisation for exposure of humans via the environment.

In the combined repeated dose toxicity study with the reproduction / developmental toxicity screening test with dimethylsilanediol, conducted according to OECD Test Guideline 422 and in compliance with GLP, dimethylsilanediol was administered by oral gavage in corn oil for 28 (toxicity group females) or 29 (males) days to 10 rats/sex/group (exception, female 50 mg/kg group where N=9) at 0, 50, 250 or 500 mg/kg bw/day (Dow Corning Corporation, 2009). A single group of males was used for both the toxicity and reproductive phases of the study. Reproductive group females were treated (10 rats/dose group) for 14 days prior to the mating period, during the mating period and through post-partum day 3. Clinical observations were performed daily immediately following exposure. Body weight measurements were performed weekly. All animals received a detailed physical examination once before the first dose (to allow for within-subject comparisons), and weekly thereafter. Additional body weights on reproductive females were obtained on gestational days (GD) 0, 7, 14, and 20, within 24 hours of parturition, and on post-partum day four. Individual food consumption was recorded at least weekly, except during the cohabitation period. Functional observational battery (FOB) and motor activity evaluations were performed on males and toxicity group females once prior to initiation of exposures and during the 4th week of exposure. Blood samples for haematology and serum chemistry evaluations were collected at the scheduled necropsy from males and toxicity group females. Complete necropsies were performed on the males and the toxicity group females and selected organs were weighed. Microscopic examination was performed on protocol specified tissues on all toxicity group animals from the control and 500 mg/kg bw/day dose groups. Target tissues examined from the low- and mid-dose levels included liver, lung, prostate gland and thyroid gland from male rats and liver and lung from female rats.

Mating was initiated after the first two weeks of exposure by pairing reproductive group females with males of the same treatment group until positive evidence of mating was obtained. Reproductive and developmental parameters evaluated included evidence of mating, pregnancy, duration of gestation, mean litter size, mean live litter size, mean litter weight, and mean ratio of live births/litter size. Dams and pups were euthanized on post-partum day 4 and examined for external gross lesions. The number of corpora lutea, and the number of uterine implantation sites were determined for all reproductive group females.

Oral gavage administration of dimethylsilanediol to male and female Sprague-Dawley rats at concentrations of up to 500 mg/kg bw/day for 28 (toxicity females) or 29 (males) consecutive days was generally well tolerated. For the toxicity group males at 250 and 500 mg/kg bw/day significant soiling was observed (abdominal region and urogenital soiling). Soiling of the muzzle was a significant abnormal observation in the toxicity group females at 500 mg/kg bw/day. Both abdominal soiling and urogenital soiling were significant abnormal observations in the reproductive group females at 500 mg/kg bw/day.

There were no statistically significant differences across exposure groups in the mean body weights on any day for toxicity group females and reproductive toxicity group females. In the male group there was a significant difference across treatment groups in week 2, however, there was not a significant difference between control and any of the treatment groups for that week. With respect to body weight gain, male group 4 animals had a significant decrease in body weight gain during week 4 and in total gain from day 1 to 29. For toxicology females there was significant decrease in body weight gain during week 3. There were no statistically significant differences in body weight gain for the reproductive females in any of the treatment groups during any of the measured intervals. There were no differences in the average daily food consumption between control and treatment groups for the reproductive females group or the toxicity male and female groups for any of the measured time periods. There were no significant differences of toxicological significance between the control and treatment groups in either sex for the FOB ranked tests. There were no significant differences between either male or female treatment groups and their respective controls for the FOB continuous test and motor activity. There were no treatment-related changes associated administration on rat neurobiological function as evaluated with FOB and motor activity parameters. The significant changes that were noted in haematological parameters and prothrombin times for toxicity group males and females were within or slightly below historical control values. The significant changes that were noted in clinical chemistry parameters for toxicity group males and females were within or slightly below historical control values. The increased liver weights in toxicity males and females at 250 and 500 mg/kg bw/day correlated with the histopathologic finding of centrilobular hypertrophy. There were no other treatment-related differences in organ weights, absolute and relative for toxicity group males and females.

There were three primary effects of the test article observed in the liver, including centrilobular hypertrophy in both sexes, periportal hepatocellular vacuolation (microvascular lipidosis, females only), and brown pigment accumulation (males only) which was accompanied by chronic inflammation and bile duct hyperplasia. Centrilobular hypertrophy is considered an adaptive change. Hepatic lipidosis, unless severe, is generally considered non-adverse. Hepatic brown pigment accompanied by chronic inflammation and bile duct hyperplasia is considered an adverse effect. Follicular cell hypertrophy was observed in the thyroid gland of mid- and high-dose male rats. This may reflect an adaptive secondary effect and adverse in the rat, but the mechanism is generally not applicable to species with significant levels of thyroid binding globulin (Capen,et al., 2002). Lung (males and females) and prostate gland were considered possible target tissues; however, further examination and inclusion of animals from the mid- and low-dose groups did not support this interpretation. There were no treatment-related effects apparent for any of the reproductive endpoints: gestation length, litter size, litter weight, ratio live births/litter size, litter sex ratio, number of implantation sites, number of corpora lutea, mating and fertility indices.

Based on the results of this study, the systemic toxicity NOAEL (No-Observed-Adverse-Effect-Level) for dimethylsilanediol in rats via oral administration in corn oil is considered to be 250 mg/kg bw/day based on hepatic brown pigment accumulation in and around the bile ducts, with associated bile duct hyperplasia and chronic inflammation at 500 mg/kg bw/day. In the absence of adverse effects on reproductive or developmental parameters in this study, a NOAEL of ≥500 mg/kg bw/day is assigned for reproductive and developmental toxicity.

Justification for classification or non-classification

On the basis of a 90-day inhalation study LOAEL of 5.5 mg/l, D3 is not classified for specific target organ toxicity according to Regulation (EC) No 1272/2008.