Registration Dossier
Registration Dossier
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EC number: 242-171-6 | CAS number: 18293-82-8
Endpoint summary
Administrative data
Key value for chemical safety assessment
Additional information
There are no genetic toxicity data for the registered substance so data have been read-across from the structurally similar read-across substance isobutyl(trimethoxy)silane.
A reliable bacterial mutagenicity study was conducted according to draft protocols 419 and 420 which are similar to OECD TG 471, in which no evidence of mutagenicity was observed with or without metabolic activation when isobutyl(trimethoxy)silane was tested in Salmonella typhimurium strains TA 98, TA 100, TA 1535, TA1537 and E. coli WP2 (Dow Corning 1987).
Read-across approach
Non-testing methods including read-across from surrogate substances are able to provide information on mutagenic toxicity (REACH Guidance part 07a, R.7.7.3). In the case of genetic toxicity the presence or absence of functional groups that are known to be related to genetic toxicity is considered important, as the presence or absence of reactive groups and molecular substructures is associated with mutagenic and carcinogenic properties of chemicals (Benigni et al, 2008). Consideration is therefore given to the structural similarity, particularly presence or absence of structural alerts for genetic toxicity, when selecting surrogate substances for genetic toxicity endpoints. Additional information on read across approach is given in a supporting report (PFA 2013aa) attached in Section 13 of the IUCLID 5 dossier. In the following paragraphs the proposed read-across for the above endpoints for isobutyl(trimethoxy)silane (CAS 18395-30-7) to isobutyl(dimethoxy)methylsilane (CAS 18293-82-8) is evaluated point by point.
Read-across hypothesis
The read-across hypothesis is that the source and target substances have similar genetic toxicity properties because they are structurally very similar.
They are structurally similar and are members of a structural class of alkoxysilane substances. The registered substance, isobutyl(dimethoxy)methylsilane, has two methoxy groups bound to silicon, the read-across substance, isobutyl(trimethoxy)silane, has three methoxy groups. The registration and read-across substance both have an identical hydrocarbon side-chain (isobutyl) bound to the silicon, and the registration substance also has a methyl group attached. Both substances hydrolyse rapidly to produce the similar silicon-containing hydrolysis products, isobutyl(methyl)silanediol or (2-methylpropyl)silanetriol, and methanol. Neither substance nor the hydrolysis products have structural alerts for genetic toxicity, therefore read across from the analogous substance isobutyl(trimethoxy)silane (CAS 18395-30-7) with a similar hydrolysis product is considered representative of the toxicity of the target substance isobutyl(dimethoxy)methylsilane (CAS 18293-82-8).They share the common hydrolysis product methanol, which does not contribute to genotoxicity based on publicly available information. This is discussed further below.
Read-across justification
The calculated half-lives of the registered substance, isobutyl(dimethoxy)methylsilane (CAS 18293-82-8), are 0.2 hours at pH 4, 1.7 hours at pH 7 and 0.04 hours at pH 9 and 25°C. At 37.5ºC and pH 7 (relevant for in vitro assays) the calculated hydrolysis half-life is approximately 0.6 hours. The products of hydrolysis are isobutyl(methyl)silanediol and methanol.
The read-across substance, isobutyl(trimethoxy)silane (CAS 18395-30-7), has a calculated hydrolysis half-life of 0.2 hours at pH 4, 4.1 hours at pH 7 and 0.1 hours at pH 9 and 25°C. At 37.5ºC and pH 7 (relevant for in vitro assays) the calculated hydrolysis half-life is approximately 1.5 hours. The products of hydrolysis are (2-methylpropyl)silanetriol and methanol.
Analogue approach justification
(a) Structural similarity
The registration and read-across substance are structurally similar and are members of a structural class of alkoxysilane substances. The registered substance, isobutyl(dimethoxy)methylsilane, has two methoxy groups bound to silicon, the read-across substance, isobutyl(trimethoxysilane), has three methoxy groups. The registration and read-across substance both have an identical hydrocarbon side-chain (isobutyl) bound to the silicon, and the registration substance also has a methyl group attached. Both substances hydrolyse rapidly to produce the similar silicon-containing hydrolysis products, isobutyl(methyl)silanediol or (2-methylpropyl)silanetriol, and methanol.
(b) Structural alerts for genotoxicity
Neither isobutyl(dimethoxy)methylsilane (CAS 18293-82-8) nor isobutyl(trimethoxy)silane (CAS 18395-30-7) has structural alerts for genotoxicity (Benigni et al, 2008).
(c) Discussion of toxicity of the non-silanol hydrolysis product
Methanol has been tested in vitro in bacterial and mammalian mutagenicity assays and in vivo in micronucleus and chromosome aberration assays. The majority of the results were negative (OECD, 2004). In the ECHA disseminated dossier for methanol, the conclusions of all the key in vitro studies and the weight of evidence of the in vivo assays are negative.
Benigni et al. (2008).The Benigni/Bossa rule base for mutagenicity and carcinogenicity JR Scientific report EUR 23241 EN
OECD (2004): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1.
PFA (2013aa). Peter Fisk Associates, Genotox Analogue Report, PFA.300.004.001
Justification for selection of genetic toxicity endpoint
Study was conducted to a design similar to an appropriate OECD test guideline and in compliance with GLP.
Short description of key information:
Gene mutation (Bacterial reverse mutation assay / Ames test): negative with and without activation in all strains tested (similar to OECD TG 471) (Dow Corning 1987).
Endpoint Conclusion: No adverse effect observed (negative)
Justification for classification or non-classification
Based on the available in vitro read-across data from read-across substance isobutyl(trimethoxy)silane, the registered substance isobutyl(dimethoxy)methylsilane does not require classification for mutagenicity according to Regulation (EC) No. 1272/2008.
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