Registration Dossier

Administrative data

Description of key information

In a two-year combined chronic toxicity and oncogenicity whole body vapor inhalation study in Fischer 344 rats, conducted to GLP (Research and Consulting company Ltd, 2005), the key effect relevant to carcinogenicity was an increase in Leydig cell tumours at all concentrations, but they are considered to be a spontaneous finding as they are common to Fischer 344 rats and were also observed in control animals. It was thought that HMDS might accelerate progression to this common tumour. Therefore the NOAEC for carcinogenicity, relevant to humans is ≥5000 ppm (33200 mg/m³).

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
Reference
Endpoint:
carcinogenicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Study period:
20.08.1997 to 30.08.2005
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Reason / purpose:
reference to other study
Qualifier:
according to
Guideline:
OECD Guideline 453 (Combined Chronic Toxicity / Carcinogenicity Studies)
Qualifier:
according to
Guideline:
EPA OTS 798.3320 (Combined Chronic Toxicity / Carcinogenicity)
GLP compliance:
yes
Species:
rat
Strain:
Fischer 344
Sex:
male/female
Details on test animals and environmental conditions:
TEST ANIMALS
- Source: Charles River Laboratories, Raleigh, N. Carolina, USA
- Age at study initiation: Approximately 6 weeks
- Weight at study initiation: Males: 84.9 to 129.2 g: Females: 73.4 to 114.8 g.
- Fasting period before study: None
- Housing: Groups of five of the same sex in Makrolon type IV cages (except during exposure)
- Diet (e.g. ad libitum): Ad libitum (except during exposure)
- Water (e.g. ad libitum): Ad libitum (except during exposure)
- Acclimation period: ten days


ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22± 3
- Humidity (%): 40-70
- Air changes (per hr): 10-15
- Photoperiod (hrs dark / hrs light): 12/12


IN-LIFE DATES: From: 01.09.1997 To: 14.09.1999
Route of administration:
inhalation
Type of inhalation exposure (if applicable):
whole body
Vehicle:
unchanged (no vehicle)
Details on exposure:
GENERATION OF TEST ATMOSPHERE / CHAMBER DESCRIPTION
- Exposure apparatus: Housed in groups of two of the same sex in stainless steel wire cages in sealed chambers used for group isolation.
- Method of holding animals in test chamber: cage
- Source and rate of air: No data
- Method of conditioning air: No data
- Temperature, humidity, pressure in air chamber: 22± 3oC, 40-60%, negative pressure of approximately 2-3 mm
- Air flow rate: No data
- Air change rate: 12-15 changes/hour
- Treatment of exhaust air: No data


TEST ATMOSPHERE
- Brief description of analytical method used: Gas chromatography
- Samples taken from breathing zone: No data
Analytical verification of doses or concentrations:
yes
Details on analytical verification of doses or concentrations:
The concentration in each chamber of the dose groups was determined daily at least six times during the exposure period, starting 20 - 30 minutes after the beginning of generation and continued until pump off (at about 5.5 hours). Analytical concentrations were determined by gas chromatography analysis.
Duration of treatment / exposure:
12 or 24 months
Frequency of treatment:
6 hours/day, 5 days/week
Post exposure period:
12 months in Subgroup B
Remarks:
Doses / Concentrations:
0.68-0.75, 2.64-2.89, 10.0-11.4 and 32.5-34.8 mg/l
Basis:
nominal conc.
Remarks:
Doses / Concentrations:
97.1-107.1, 391.1-428.1, 1509.4-1720.8 and 4894.6-5241.0 ppm
Basis:
nominal conc.
Remarks:
Doses / Concentrations:
0.7, 2.7, 10.6 and 33.2 mg/l
Basis:
other: target conc.
Remarks:
Doses / Concentrations:
100, 400, 1600, and 5000 ppm
Basis:
other: target conc.
No. of animals per sex per dose:
Subgroup A.  20 rats/sex/group
Subgroup B. 20 rats/sex/group
Subgroup C. 65 rats/sex/group
Additionally, 10 males and 10 females were used for disease surveillance.
Control animals:
yes, concurrent no treatment
Details on study design:
- Dose selection rationale: Based on a 13-week whole body inhalation study in Fischer-344 rats. The choice of the highest concentration was based upon safety comsiderations and was set at half the Lower Explosive Limit for HMDS.
- Rationale for animal assignment (if not random): random, stratified by body weight
- Rationale for selecting satellite groups: None given
- Post-exposure recovery period in satellite groups: 12 months in subgroup B, none in subgroups A and B
Positive control:
None
Observations and examinations performed and frequency:
CAGE SIDE OBSERVATIONS: Yes
- Time schedule: Daily


DETAILED CLINICAL OBSERVATIONS: Yes
- Time schedule: Twice per week. Observations included but were not limited to changes in behavior, somatomotor activity, body position, respiration, alteration of skin (including skin lesions and hair loss), nose and eye. Additionally, palpation for early detection of non-visible masses was performed on a weekly basis from 6 months after start of exposure until termination. Special attention was given to mass development for each visible and palpable mass. The time of onset, location, size, appearance and progression of mass were recorded.

BODY WEIGHT: Yes
- Time schedule for examinations: Day 1 and 7 of acclimitisation, immediately before the first treatment, once weekly for the first 14 weeks, then every two weeks.

FOOD CONSUMPTION:
- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: Yes (g/animal/day recorded).

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
- Timescale and animals examined: During the acclimitisation period animals were conducted on 20 animals/group/sex from subgroup C. Towards the end of the first year and towards study termination examinations were performed on the last 20 animals/group/sex from subgroup C (some 400ppm females not examined).

HAEMATOLOGY: Yes
- Time schedule for collection of blood: after 3, 6 and 12 months of exposure in subgroup C.
- Anaesthetic used for blood collection: Yes (ether)
- Animals fasted: Yes, for 17 hours before sampling.
- How many animals: 20
- Parameters checked in No.1 were examined.


CLINICAL CHEMISTRY: Yes
- Time schedule for collection of blood: after 3, 6 and 12 months of exposure in subgroup C.
- Animals fasted: Yes, for 17 hours before sampling.
- How many animals: 20
- Parameters checked in table No.1 were examined.


URINALYSIS: Yes
- Time schedule for collection of urine: after 3, 6 and 12 months of exposure in subgroup C.
- Metabolism cages used for collection of urine: Yes
- Animals fasted: Yes, urine collected during pre-blood sampling fasting period.
- Parameters checked in table No.1 were examined.


NEUROBEHAVIOURAL EXAMINATION: No
Sacrifice and pathology:
GROSS PATHOLOGY: Yes (see table 2)
HISTOPATHOLOGY: Yes (see table 2)
Other examinations:
None reported.
Statistics:
Analysis was two-tailed for significance levels of 5% and 1%. Analysis of body weight, food consumption, as well as organ weights and clinical pathology (not differential white cell counts) were analysed by a one way analysis of variance followed by comparison of the control group to each treated group by Dunnett's test. The Steel-test was applied instead of the Dunnett's test for the other parameters in the clinical laboratory investigations. Histopathology data were analysed independently using SAS/BASE and SAS/STAT software.
Clinical signs:
no effects observed
Mortality:
no mortality observed
Body weight and weight changes:
effects observed, treatment-related
Description (incidence and severity):
effects in females; no effects in males
Food consumption and compound intake (if feeding study):
effects observed, treatment-related
Description (incidence and severity):
effects in females; no effects in males
Food efficiency:
not examined
Water consumption and compound intake (if drinking water study):
not examined
Ophthalmological findings:
effects observed, treatment-related
Haematological findings:
no effects observed
Clinical biochemistry findings:
no effects observed
Urinalysis findings:
no effects observed
Behaviour (functional findings):
not examined
Organ weight findings including organ / body weight ratios:
effects observed, treatment-related
Gross pathological findings:
effects observed, treatment-related
Description (incidence and severity):
enlarged liver in females at 5000 ppm
Histopathological findings: non-neoplastic:
effects observed, treatment-related
Histopathological findings: neoplastic:
effects observed, treatment-related
Description (incidence and severity):
benign Leydig cell tumours were statistically significant
Details on results:
CLINICAL SIGNS AND MORTALITY: There were no treatment-related clinical signs. There were no treatment-related deaths observed during the study. During the first year, 12 animals out of 1050 (two at 100 ppm, three at each 400 and 1600 ppm and four at 5000 ppm) died. Although, there were no deaths in the control group and the number of deaths was marginally higher in the higher exposure group, the distribution was considered to be incidental. Of the 12 animals that died prematurely during the first year 1 animal was from subgroup A, 2 animals were from subgroup B and 9 animals were from subgroup C. During the second year, the mortality rate was similar in control and treated animals and there were no differences between subgroup B and subgroup C animals.

BODY WEIGHT AND WEIGHT GAIN:  In males, the difference in mean body weight noted between the exposed animals and the respective controls were considered to be unrelated to the test article treatment. In  groups 4 (1600 ppm) and 5 (5000 ppm) females, the mean body weight were significantly reduced from the respective control values on 27 occasions for subgroup A (12 months of exposure) and 29 occasions for subgroup C (two years exposure). However, the differences were marginal (-2 to -8% from control) and there was up to -4% difference in body weight from control before the start of exposures at week 1. In addition, there was no exposure-response relationship in body weight between the groups and there was no confirmation of a similar response in the subgroup B (12 months exposure and 12 months recovery).  This finding is probably related with a slight reduction in food consumption noted in these animals which was considered to be test article related. 

FOOD CONSUMPTION: Slight reduction in food consumption noted in females of two highest concentration groups, which was considered to be test article related. No such effect observed at lower concentrations.

OPHTHALMOSCOPIC EXAMINATION: After one year and towards the end of the two year exposure, all examined animals (100%) showed corneal opacity.   

HAEMATOLOGY: The changes noted in the hematological parameters between 400 and 5000 ppm consisted of occasional slightly reduced hemoglobin concentration, hematocrit and mean cell hemoglobin concentration and increased mean cell volume.  These findings were not toxicologically significant.

CLINICAL CHEMISTRY: The increase in urea and creatinine and imbalances of electrolytes in the urine may be related to the morphological tubular changes noted in the kidneys. Other findings in the clinical biochemistry included an increase in lipids and proteins and changes in enzyme activities suggest metabolic adaptive changes, primarily related to the liver. These findings were not toxicologically significant.

URINALYSIS: No treatment-related findings.

ORGAN WEIGHTS: The increased relative testes weights in group 5 (5000 ppm) subgroup A males and of the absolute and relative testes weights in subgroup C males of the mid-high (1600 ppm) and high exposure levels (5000 ppm) were considered to be test article-related. These findings also correlated with the results of the histopathological examination. In males from subgroup A, an increase in absolute and relative liver weight at 5000 ppm and the relative liver weight at 1600 ppm were noted. However, there were no correlated findings in the histopathological examination. Therefore, the increase liver weight in males but not in females after the one year of exposure could be the consequence of a transient metabolic adaptation without any toxicological relevance. This finding was not detected in subgroup C animals of both sexes. Statistically significant increase in relative kidney weight was observed in males of high exposure group (5000 ppm) after one year of exposure (subgroup A). In other subgroups (B and C) the increase in kidney weights was not consistent between the different exposure levels. 

GROSS PATHOLOGY:  In subgroup A (one year of exposure), an enlarged liver was noted in group 5 (5000 ppm).  In subgroup C (two years exposure), nodules in the kidneys were present in 3 males from group 4 (1600 ppm) and 7 males from group 5 (5000 ppm).  In group C, the incidence of enlarged testes was higher in all treated male groups than in the control group. The observation in groups 4 and 5 correlated with recorded increase in testicular weight for these groups and the results of the histopathological examination.

HISTOPATHOLOGY: NON-NEOPLASTIC: At interim sacrifice after one year exposure as well as at terminal sacrifice after two years, a variety of non-neoplastic and neoplastic findings were noted in this study which were considered to be incidental findings commonly observed in rats of this age and strain and which did not distinguish exposed animals from controls. There were no microscopic findings associated with the noted increase in liver weight.  The increase in kidney weight correlated with an increase in the incidence/severity of chronic nephropathy only in the 24 month treatment group males and females. Calcified cylinders in the papillary tubules were noted in 19 male animals each of group 4 and 5 following one year of exposure. The incidence of this finding was noted in 24 males of group 3, 63 males in group 4 and all 65 males of group 5 following two year sacrifice. The severity of this alteration was dose-dependent. Tubular granular casts and tubular hyaline casts were noted in male animals of all groups, including controls. The incidence of these casts was statistically significantly increased in males of high exposure group. Cortical mineralization was noted in the male and female animals in groups 3, 4 and 5. The incidence of transitional cell hyperplasia of the renal papilla was statistically increased in males of groups 4 and 5 in subgroup C.  

Animals sacrificed after one year of exposure (subgroup A): Kidneys:  Increased incidence of calcified cylinders in the tubules of the papilla in males at 1600 and 5000 ppm; increased incidence of tubular hyaline casts in males of group 5 (5000 ppm) and females of group 4 (1600 ppm). Testes:  Multifocal Leydig cell hyperplasia present in all males. An increased severity of Leydig cell hyperplasia was noted in all groups exposed to HMDS. Nasal Cavity:  Increased incidence of eosinophilic inclusions in the male olfactory and respiratory epithelium. Animals sacrificed one year of exposure and one year of recovery (subgroup B): Kidneys: Increased incidence of calcified cylinders in the tubules of the papilla in males of groups 3, 4 and 5; increased incidence of transitional cell hyperplasia in males of groups 4 and 5; increased incidence of tubular hyaline casts in males of group 5. Nasal cavity: Increased incidence of eosinophilic inclusions in olfactory/respiratory epithelium in males of groups 4 and 5 and in females of groups 2 to 5. Sternum: Increased incidence of enchondral hyperostosis in males of group 5 whereas the incidence decreased in females of groups 2 to 5.         

Animals sacrificed after two years of exposure (subgroup C): Kidneys: Calcified cylinders in males of groups 3 (400 ppm), 4 (1600 ppm) and 5 (5000 ppm); transitional cell hyperplasia observed in males of groups 4 and 5; increased incidence of tubular pigment deposits in males of groups 4 and 5 whereas the incidence decreased in females of group 2 (100 ppm) and 5 (5000 ppm). Lungs: Increased incidence of alveolar histiocytosis in females of groups 4 and 5; increased focal alveolitis in males and females of group 5; increased foreign body granuloma in males of group 5. Nasal Cavity: Increased incidence and severity of eosinophilic inclusions in the olfactory/respiratory epithelium in males of groups 4 and 5. Sternum/Femur: Increased incidence and severity of enchondral hyperostosis in the male sternum of group 5 whereas the incidence decreased in both sexes and femur of females in groups 2 to 5 and in the femur of males in groups 2 to 4. Bone marrow: Increased incidence of atrophy in females of group 5.

HISTOPATHOLOGY: NEOPLASTIC: In the testes, there was statistically significantly increased incidence of Leydig cell tumors at all exposure levels when compared to controls after one year of exposure (subgroup A). The majority of animals in subgroups B (one year of exposure and one year of recovery) and C (two years of exposure) developed Leydig cell tumors regardless of exposure. From the neoplastic findings diagnosed in this study, the occurrence of renal tubular adenomas and carcinomas in males of groups 4 and 5 after two years of exposure are not excluded as possibly related to the exposure to the test article, since these findings did not occur in control group 1(0 ppm), groups 2 (100 ppm) and 3 (400 ppm).


Relevance of carcinogenic effects / potential:
Carcinogenic effects are considered species-specific and not relevant to humans.
Dose descriptor:
other: NOAEC (human)
Effect level:
>= 5 000 ppm
Sex:
male/female
Basis for effect level:
other: No effects relevant to humans.
Remarks on result:
other: Effect type: toxicity (migrated information)
Dose descriptor:
other: NOAEC (human)
Effect level:
>= 5 000 ppm
Sex:
male/female
Remarks on result:
other: Effect type: carcinogenicity (migrated information)
Dose descriptor:
other: NOAEC (rat)
Effect level:
100 ppm
Sex:
male/female
Basis for effect level:
other: chronic Nephropathy (species-specific)
Remarks on result:
other: Effect type: toxicity (migrated information)
Dose descriptor:
NOAEC
Sex:
male
Basis for effect level:
other: Leydig Cell Tumors
Remarks on result:
not determinable
Remarks:
no NOAEC identified. Effect type:carcinogenicity (migrated information)

Vapor inhalation exposure of Fischer 344 rats to HMDS for up to two years produced calcified cylinders in tubules of the

kidney papilla in the male rats. However, these effects in the kidneys, do not adversely affect the survival of the

animals, and may be considered as a starting change of renal toxicity. The occurrence of renal tubular adenomas and

carcinomas in some males may be related to test article exposure. A follow-up study showed that an alpha 2

u-globulin mediated mechanism was responsible for the observed nephropathy and kidney neoplasia in the male rats.

The inconsistency with the increase or decrease hyperostosis between sexes and between different bones (sternum and

femur), as well as the lack of dose-response in females, suggests there is little toxicological significance to these

findings. The increased incidence of eosinophillic inclusions in the olfactory epithelium may be related to

nasal irritation caused by mildly irritant chemicals and are also commonly seen in ageing rats. Test article-related

increase of the testicular weight was correlated with the macroscopic finding of increased incidence of enlarged

testes in all treated groups. Increased severity in Leydig cell hyperplasia and increased incidence in Leydig cell

tumors were observed at all exposure levels after one year. Leydig cell tumors occur spontaneously and are very common

in Fischer 344 rats. In fact, following the 2 year sacrifice, almost all males, both control and treated, showed this finding. The one-year data indicate that HMDS exposure accelerated progression of this common tumor. No microscopic findings were considered to be related to HMDS exposures.

Conclusions:
In a two-year combined chronic toxicity and oncogenicity whole body vapor inhalation study in Fischer 344 rats, conducted to GLP (reliability score 1), the target organs of hexamethyldisiloxane were kidney, nasal cavity and testes. Effects in the kidneys were considered species specific (alpha-2
u-globulin mediated mechanism). Effects in the nasal cavity were considered to be irritative. Effects on the testes (Leydig cell tumours) were increased following exposure to all concentrations, but they are considered to be a spontaneous finding as they are common to Fischer 344 rats and were also observed in control animals. It was thought that HMDS might accelerate progression to this common tumour. Therefore the NOAEL for systemic effects relevant to humans is ≥5000 ppm (33.2 mg/l).

Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
NOAEC
33 200 mg/m³
Study duration:
chronic
Species:
rat

Carcinogenicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

In a two-year combined chronic toxicity and oncogenicity whole body vapour inhalation study (Research and Consulting Company Ltd, 2005) Fischer 344 rats were exposed to HMDS 6 hours/day, 5 days/week at a target concentration of 700, 2700, 10600 or 33200 mg/m3. The animals were divided into three subgroups: Subgroup A  (20 rats/sex/group), exposed for 12 months with a treatment-free observation period of 12 months; Subgroup B (20 rats/sex/group), exposed for 12 months; Subgroup C (65 rats/sex/group), exposed for 24 months. The animals were examined in detail in accordance with OECD test guideline 453.

HMDS produced calcified cylinders in tubules of the kidney papilla in the male rats. However, these effects in the kidneys, do not adversely affect the survival of the animals, and may be considered as a starting change of renal toxicity. The occurrence of renal tubular adenomas and carcinomas in some males may be related to test substance exposure. A follow-up study showed that an alpha 2 u-globulin mediated mechanism was responsible for the observed nephropathy and kidney neoplasia in the male rats. The increased incidence of eosinophillic inclusions in the olfactory epithelium might be related to nasal irritation caused by mildly irritant chemicals and are also commonly seen in ageing rats. Test article-related increase of the testicular weight was correlated with the macroscopic finding of increased incidence of enlarged testes in all treated groups. Increased severity in Leydig cell hyperplasia and increased incidence in Leydig cell tumors were observed at all exposure levels after one year. Leydig cell tumors occur spontaneously and are very common in Fischer 344 rats. In fact, following the 2 year sacrifice, almost all males, both control and treated, showed this finding. The one-year data indicate that HMDS exposure accelerated progression of this common tumor. No microscopic findings were considered to be related to HMDS exposures. Therefore the NOAEC for carcinogenicity relevant to humans is 33200 mg/m3.


Justification for classification or non-classification

The available data suggest that hexamethyldisiloxane does not need to be classified for carcinogenic effects

according toRegulation (EC) No 1272/2008.