Dimethylbis(octadecyloxy)silane

Endpoint summary

Key value for chemical safety assessment

Genetic toxicity in vitro

Description of key information

Gene mutation (Bacterial reverse mutation assay / Ames test): negative with and without metabolic activation in Salmonella typhimurium strains TA 98, TA 100, TA 1535 or TA 1537 (OECD 471, 1983) (CCR, 1996).
Gene mutation (Bacterial reverse mutation assay / Ames test): read across from trimethyl(octadecyloxy)silane in stearyl alcohol: negative with and without metabolic activation in Salmonella typhimurium strains TA1535, TA1537, TA100, TA98 and E. coli WP2uvrA (Charles River, 2016).

These studies were conducted according to an appropriate OECD guidelines, and in compliance with GLP.

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed (negative)

Additional information

Dimethylbis(octadecyloxy)silane has been tested for mutagenicity to bacteria, in a study conducted according to the OECD TG 471 and compliant with GLP (CCR, 1996). No evidence of a test-substance related increase in the number of revertants was observed with or without metabolic activation in Salmonella typhimurium strains TA 98, TA 100, TA 1535 or TA 1537 (without a fifth strain) in the initial or the repeat experiments up to cytotoxic/limit concentrations. Appropriate positive, solvent and negative controls were included and gave the expected results. It is concluded that the test substance is negative for mutagenicity to bacteria under the conditions of the test.

No data are available for an appropriate fifth strain of bacteria to detect cross-linking or oxidising mutagens, therefore data are read across from the related substance trimethyl(octadecyloxy)silane. Trimethyl(octadecyloxy)silane in stearyl alcohol was tested in a valid bacterial reverse mutation assay, conducted according to OECD TG 471 and in compliance with GLP using  Salmonella typhimurium strains (TA1535, TA1537, TA98 and TA100) and Escherichia coli (WP2uvrA). No increase in the number of revertant was observed in any test strains , with or without metabolic activation, when tested up to precipitating concentrations. Appropriate positive and solvent controls were included and gave the expected results. It is concluded that the test substance is negative for mutagenicity to bacteria under the conditions of the test.

Read-across justification

There are bacterial mutagenicity data available for dimethylbis(octadecyloxy)silane (CAS 29043-70-7). However, the available data do not include fifth strain to detect mutagenicity via cross-linking. Therefore, read-across data are used to fulfil Annex requirements. This document describes the analogue approach for fulfilling this endpoint by read-across from the source substance, Reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol (EC 943-282-9), according to the Read-across Assessment Framework (RAAF).

Read-across is proposed in accordance with RAAF Scenario 2: “This scenario covers the analogue approach for which the read-across hypothesis is based on different compounds which have the same type of effect(s). For the REACH information requirement under consideration, the effects obtained in a study conducted with one source substance are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect(s) or absence of effect is predicted. The predicted strength of the effects may be similar or based on a worst case.”

The read-across justification is presented (Table 1) according to RAAF scenario 2 assessment elements (AE) as outlined in Table B1 of the RAAF1:

Table 1: RAAF scenario 2 assessment elements (AE) as given in Appendix B (Table B1) of the RAAF1

AE A.1	Characterisation of source substance
AE A.2	Link of structural similarity and differences with the proposed Prediction
AE A.3	Reliability and adequacy of the source study
AE 2.1	Compounds the test organism is exposed to
AE 2.2	Common underlying mechanism, qualitative aspects
AE 2.3	Common underlying mechanism, quantitative aspects
AE 2.4	Exposure to other compounds than to those linked to the prediction
AE 2.5	Occurrence of other effects than covered by the hypothesis and Justification
AE A.4	Bias that influences the prediction

1.       AE A.1 Identity and characterisation of the source substance

The source substance reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol is composed of 35-48% stearyl alcohol (octadecanol) and 50-65% trimethyl(octadecyloxy)silane. Trimethyl(octadecyloxy)silane has a single silicon atom connected to one octadecyloxy group and three methyl groups. The hydrolysis half-life for the substance has been predicted using a validated QSAR estimation method as >15 hours at pH 7 and 20-25°C. The hydrolysis products are trimethylsilanol and octadecanol. The source substance has log Kow of 9 at 20°C (QSAR), water solubility of 2.2E-06 mg/L at 20°C (QSAR) and vapour pressure of 2.7E-03 Pa at 25°C (QSAR).

2.       AE A.2 Link of structural similarities and differences with the proposed prediction

The target and source substance have similar physico-chemical properties as well as hydrolysis rates (Table 2). The source substance reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol is composed of 35-48% stearyl alcohol (octadecanol) and 50-65% trimethyl(octadecyloxy)silane. Trimethyl(octadecyloxy)silane has a single silicon atom connected to one octadecyloxy group and three methyl groups, while the target substance dimethylbis(octadecyloxy)silane (CAS 29043-70-7) has two octadecyloxy groups and two methyl groups bonded to a silicon atom. The hydrolysis half-lives for both substances have been predicted using a validated QSAR estimation method. Hydrolysis half-lives are estimated to be >15 hours at pH 7 and 20-25°C. Hydrolysis in vivo is expected to be slow due to the high adsorption potential and low water solubility of the substances. Both substances hydrolyse to form similar silicon-containing products trimethylsilanol or dimethylsilanediol, for source and target substance, respectively, and the same non-silicon hydrolysis product, octadecanol.

Table 2: Physico-chemical properties

Property	Source substance	Target substance
Substance name	Reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol	Dimethylbis(octadecyloxy)silane
CAS number	Not applicable	29043-70-7
Hydrolysis half-life	> 15 hours at pH 7	>15 hours at pH 7
Silanol hydrolysis product	trimethylsilanol	dimethylsilanediol
Non-Si hydrolysis product	octadecanol	octadecanol
LogKow value (parent)	9 at 20°C (QSAR)	9 at 20°C (QSAR)
LogKow value (Si-product)	-0.4 at 20^oC	-0.4 at 20^oC
Vapour pressure (parent)	2.7E-03 Pa at 25°C (QSAR)	4E-04 Pa at 25°C
Vapour pressure (Si-product)	7.0Pa at 25^oC	7.0Pa at 25^oC
Water solubility (parent)	2.2E-06 mg/L at 20°C (QSAR)	2.5E-14 mg/L at 20°C (QSAR)
Water solubility (Si-product)	1E+06mg/l at 20^oC	1E+06mg/l at 20^oC

3.       AE A.3 Reliability and adequacy of the source study

Reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol was tested in a valid bacterial reverse mutation assay, conducted according to OECD TG 471 and in compliance with GLP using Salmonella typhimurium strains (TA1535, TA1537, TA98 and TA100) and Escherichia coli (WP2uvrA). No increase in the number of revertant was observed in any test strains, with or without metabolic activation, when tested up to precipitating concentrations. Appropriate positive and solvent controls were included and gave the expected results. It is concluded that the test substance is negative for mutagenicity to bacteria under the conditions of the test (Charles River, 2016).

4.       AE A.4 Bias that influences the prediction

Data on Reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol are read-across to the registered substance dimethylbis(octadecyloxy)silane (CAS 29043-70-7). The source substance and the target substance have structural similarity and similar physico-chemical properties. They also have similar silicon-containing hydrolysis products and the same non-Si hydrolysis product. Therefore, their toxicological properties are considered to be similar, with identical genetic toxicity effects. No other data for relevant substances are available.

5.       AE A.2.1 Compounds the test organism is exposed to

The source substance as well as the target substance have >15-hour half-lives at pH 7. It is considered that the test organism is exposed largely to the parent substances but some exposure to the hydrolysis products trimethylsilanol, dimethylsilanediol and octadecanol is possible. None of these substances are known to be of toxicological concern.

 

Trimethylsilanol has been tested for mutagenicity to bacteria, in a study which was conducted according to OECD TG 471, compliant with GLP (Dow Corning Corporation, 1995)). No evidence of a test-substance related increase in the number of revertants was observed with or without metabolic activation in Salmonella typhimurium strains TA 98, TA 100, TA 1535 or TA 1537, or E. coli WP2 uvrA, in the initial or the repeat experiments using the plate incorporation method up to limit concentrations. Appropriate positive and solvent controls were included and gave expected results. It is concluded that the test substance is negative for mutagenicity to bacteria under the conditions of the test.

No genetic toxicity data are available for dimethylsilanediol. However, it is structurally similar to trimethylsilanol and its genotoxic potential is expected to be similar. Trimethylsilanol has one hydroxyl and three methyl groups attached to a silicon atom, while dimethylsilanediol has two hydroxyl and two methyl groups attached to a silicon atom.

Publicly available data on octadecanol concludes that the substance is negative for mutagenicity in bacteria in a reliable study conducted according to OECD guideline 471 and under GLP.

6.       AE A.2.2 and A.2.3 Common underlying mechanism, qualitative and quantitative aspects

Mutagenicity data in bacteria are available for the target substance dimethylbis(octadecyloxy)silane (CAS 29043-70-7), however only four bacterial strains were used (S. typhimurium TA 1535, TA 1537, TA 98 and TA 100). Therefore, data are read-across from the structurally analogous substance Reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol. Both substances have similar hydrolysis products. Moreover, they have similar physico-chemical properties. Thus, both substances are expected to have similar genetic toxicity profiles. No mutagenic effect in bacteria was reported for the target and source substance.

7.       AE 2.4 Exposure to other compounds than to those linked to the prediction

Neither the target substance, dimethylbis(octadecyloxy)silane (CAS 29043-70-7), nor the source substance, Reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol have impurities of toxicological concern.

The purity in the bacterial mutagenicity study with the target substance was not reported.  

The purity in the bacterial mutagenicity study with the source substance, Reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol, was reported to be > 98.5 % combined stearyl alcohol and trimethyl(octadecyloxy)silane.

AE 2.5 Occurrence of Other Effects than Covered by the Hypothesis and Justification

Not relevant

Justification for classification or non-classification

Based on reliable bacterial mutagenicity data, dimethylbis(octadecyloxy)silane is not classified for mutagenicity according to Regulation (EC) No 1272/2008.