Hexamethylcyclotrisiloxane

Genetic toxicity: in vivo

Administrative data

Endpoint:

in vivo mammalian somatic cell study: cytogenicity / bone marrow chromosome aberration

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1980-11-25 to 1981-03-23

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

1982

Report date:

1982

Materials and methods

Test guideline

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 475 (Mammalian Bone Marrow Chromosome Aberration Test)

Deviations:

yes

Remarks:

number of cells scored for MI; presentation of results

Principles of method if other than guideline:

Method: other

GLP compliance:

yes

Type of assay:

chromosome aberration assay

Test material

Test material information

Constituent 1

Reference substance name:

Hexamethylcyclotrisiloxane

EC Number:

208-765-4

EC Name:

Hexamethylcyclotrisiloxane

Cas Number:

541-05-9

Molecular formula:

C6H18O3Si3

IUPAC Name:

hexamethyl-1,3,5,2,4,6-trioxatrisilinane

Test animals

Species:

rat

Strain:

Sprague-Dawley

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River
- Age at study initiation: 50-60 days
- Weight at study initiation: 200-440 g
- Assigned to test groups randomly: [no/yes, under following basis: ]
- Fasting period before study:
- Housing: 6/cage
- Diet: ad libitum
- Water: ad libitum
- Acclimation period:


ENVIRONMENTAL CONDITIONS
- Temperature (°C): 20
- Humidity (%): 50
- Air changes (per hr): no information
- Photoperiod (hrs dark / hrs light): 12/12

Administration / exposure

Route of administration:

intraperitoneal

Vehicle:

Vehicle: DMSO

Duration of treatment / exposure:

6, 24 or 48 hours

Frequency of treatment:

Single ip injection of test and positive and solvent controls

Doses / concentrations

Doses / concentrations 1

Dose / conc.:

125 mg/kg bw (total dose)

Doses / concentrations 2

Dose / conc.:

225 mg/kg bw (total dose)

Doses / concentrations 3

Dose / conc.:

300 mg/kg bw (total dose)

Doses / concentrations 4

Dose / conc.:

400 mg/kg bw (total dose)

Doses / concentrations 5

Dose / conc.:

515 mg/kg bw (total dose)

Doses / concentrations 6

Dose / conc.:

1 080 mg/kg bw (total dose)

No. of animals per sex per dose:

5 male animals per dose

Control animals:

yes, concurrent vehicle

Positive control(s):

-cyclophosphamide
- Justification for choice of positive control(s): none given in report
- Route of administration: ip injection
- Doses / concentrations: 21 mg/kg bw

Examinations

Tissues and cell types examined:

Bone marrow

Details of tissue and slide preparation:

CRITERIA FOR DOSE SELECTION: initial cytotoxicity assay.
The doses used in the cytogenetic studies were based on the range-finding experiments. Animals were injected IP with hexamethylcyclotrisiloxane prepared in glass tubes at doses of 1081, 721, 360, 180, and 72 mg/kg. No deaths were observed. A second range-finding experiment was not performed. All subsequent cytogenetic studies were performed using polypropylene tubes to prepare samples.

TREATMENT AND SAMPLING TIMES ( in addition to information in specific fields): Animals were sacrificed at 6, 24, or 48 hours after injection. Colchicine was injected 2 to 2.5 hours before sacrifice at a final dose of approximately 1.5 mg/kg.

DETAILS OF SLIDE PREPARATION: Approximately four slides were prepared from each animal. Slides were fixed, stained with Giesma and permanently mounted.

METHOD OF ANALYSIS: Suitable cells (with properly condensed and well spread chromosomes) were photographed. Projected photographs were examined for chromosomal breaks or gaps, chromatid breaks or gaps, complex rearrangements, polyploidy and large translocation or deletions. Small translocation or deletions could not be detected.

Statistics:

If the compound did not cause chromosomal damage, then the distribution of breaks per animal for all the negative control and test animals is expected to follow the Poisson distribution. A comparison of the expected and observed distribution values was performed using the Chi2 test as a measure of "goodness to fit". The Wilcoxon test was used as a non-parametric test to compare the distribution of breaks per animal between the negative control animals and the highest test doses. Details of both statistical evaluations are given in the text, Statistical Methods, Snedecor, G.W. and Cochran, W.G., Iowa State University Press, Ames, Iowa, 1980.

Results and discussion

Test results

Sex:

male

Genotoxicity:

negative

Toxicity:

no effects

Remarks:

on mitotic index. Deaths occurred at highest dose.

Vehicle controls validity:

valid

Negative controls validity:

not applicable

Positive controls validity:

valid

Additional information on results:

For the cytogenetic experiments, hexamthylcyclotrisiloxane prepared in polypropylene tubes produced mortalities at high dose levels. The dosage groups analyzed in the cytogenetic studies were based on the maximum tolerated dose for each sacrifice group. In the 48-hour sacrifice group, doses of 1080 mg/kg killed 5 of 6 animals and 515 mg/kg killed 1 of 6 animals. In the 24-hour group, the highest dose level (400 mg/kg) killed 3 of 6 animals, but all 6 animals injected with 300 mg/kg lived. In the 6-hour assay, all of the animals injected with the highest dose, 300 mg/kg, lived.

There was no evidence that hexamethylcyclotrisiloxane induced chromosomal damage in rats following IP injection. A summary of the total number of cells analyzed for each dose at each time of sacrifice is summarized as follows: For animals injected with 125 mg/kg the frequencies of breaks were 1.40%, 2.02%, and 1.99% for the three times of sacrifice (6, 24, and 48 hours, respectively); frequencies of 0.71%, 2.17%, and 2.35% were seen for animals injected with 225 mg/kg at the three times of sacrifice. For animals dosed with 300 mg/kg, the frequencies of breaks were 2.07% and 1.01% at 6 and 24 hours; 0.90% breaks were seen with animals dosed with 400 mg/kg at 24 hours; and 1.94% breaks were seen in animals injected with 515 mg/kg at 48 hours. No complex rearrangements such as quadriradials, triradials, or ring chromosomes were detected in animals injected with the test substance. For the negative control animals, the frequencies of breaks were 0.85%, 0.58%, and 2.23% for sacrifice times of 6, 24, and 48 hours, respectively. The animals injected with cyclophosphamide had both a large number of breaks and complex rearrangements. A comparison of the frequencies of breaks for doses of hexamethylcyclotrisiloxane with the negative control at the three time points shows no significant differences. These frequencies are similar to those recorded for control animals in previous testing. The frequency of breaks in previous negative control animals ranged from 0 to 2.58%, a range compatible with all the results obtained in this project. The conclusion based on comparison to historical data is confirmed by a statistical analysis using the Wilcoxon test that demonstrated that there was no difference between the number of breaks per animal for the negative control groups and the test groups. The use of the Chi2 test as a measure of "goodness to fit" indicated that the distribution of breaks per animal did not closely follow the Poisson distribution. However, the Wilcoxon test demonstrated that there was no difference between the number of breaks per animal for the negative control groups and the test groups.

Any other information on results incl. tables

Data for the following dose groups is summarized below:
a. 48-hour sacrifice time: 515, 225, and 125 mg/kg

b. 24-hour sacrifice time: 400, 300, 225, and 125 mg/kg

c. 6-hour sacrifice time: 300, 225, and 125 mg/kg


Summary of Chromosome Aberration Data at three respective
intervals
                                        
Test   Dose   No.                
Chem. (mg/kg) cells   G*  B* O*  MI*(%)
DMSO:
6 hrs  --   142     1     1       0      4.6
              101     0     2       0      5.4
              116     5     1       0      5.8
              120     1     1       0      7.3
              107     0     0       0      4.7

24 hrs* --    120     0     0       0      7.3
              101     2     0       0      3.7
              103     1     2       0      3.0
              95      0     1       0      2.7
              94      0     0       0      6.8

48 hrs  --  117     0     0       0      4.8
              108     1     3       0      5.7
              92      0     5       0      3.5
              113     2     4       0      7.4
              107     0     0       0      5.2

Test material:
6 hrs  300  112     0     3       0      3.7
              120     1     1       0      4.8
              113     0     3       0      4.3
              115     0     0       0      3.4
              120     0     5       0      3.5
       225  114     3     0       0      6.9
              108     0     1       0      4.9
              120     0     0       0      3.3
              109     1     3       0      2.9
              110     2     3       0      4.6
       125  98      1     1       0      6.6
              107     2     1       0      7.9
              106     2     1       0      4.9
              118     2     3       0      3.6
24 hrs 400    117     0     1       0      4.9
              106     3     1       0      5.9
       300  121     0     2       0      3.0
              115     0     0       0      9.9
              136     3     0       0      5.1
              108     3     1       0      7.9
              114     4     3       0      5.0
       225  104     0     1       0      10.0
              119     0     0       0      8.3 
              115     0     4       0      5.8
              138     0     3       0      7.8
              122     1     5       0      8.9
       125  113     0     0       0      7.7
              121     0     2       0      7.8
              110     0     0       0      4.4
              131     0     6       0      5.1
              119     0     4       0      6.0
48 hrs 515  106     3     4       0      9.9   
              109     0     0       0      6.7
              146     2     3       0      12.4
       225  102     1     3       0      6.4
              115     0     3       0      6.1
              109     0     3       0      9.3
              109     0     0       0      6.8
              103     0     1       0      6.4
              100     0     5       0      5.4
       125  112     0     3       0      9.5
              119     1     4       0      6.9
              43      0     0       0      5.0
              118     0     2       0      5.1
              111     0     1       0      3.1
            
Cyclophosphamide
24 hrs   21 109     ND    >64     7Quad  4.2
                                    3Tri
              120     ND    >67     2Quad  5.4
                                    1Tri
              107     ND    >48     2Quad  2.5
                                    2Tri
---
*: G=Gaps; B=Breaks; O=Other; MI=Mitotic Index
ND = not determined
Quad = quadriradial
Tri = Triradial