Registration Dossier

Administrative data

Key value for chemical safety assessment

Additional information

Information is available from reliable studies for all the required in vitro endpoints. Additional information is available for the hydrolysis product, trimethylsilanol, from an in vitro cytogenicity study, and in vivo assays for clastogenicity to somatic and to germ cells. Hexamethyldisilazane hydrolyses very rapidly to trimethylsilanol (hydrolysis half-life at pH 7 and 1.5°C: ≤ 0.5 min (measured)); the other hydrolysis products, ammonium ions, are not expected to contribute to genetic toxicity, based on information in the public domain (OECD, 2003, 2004, 2005). It is considered, therefore, that read-across to the registered substance is scientifically justified.

No test substance related increase in the number of revertants relative to the solvent control was observed when hexamethyldisilazane was tested in S. typhimurium strains TA 98, TA 100, TA 1535, TA 1537 and TA 102 in accordance with OECD 471 and in compliance with GLP (Covance, 1997a). The results obtained with and without metabolic activation in the initial plate incorporation test and in the repeat pre-incubation assay were in agreement. Appropriate solvent and positive controls were included and gave expected results. The test substance was therefore considered to be negative for mutagenicity to bacteria under the conditions of this test. A number of supporting bacterial mutagenicity studies are available and all produced a negative result.

Hexamethyldisilazane has been tested in human lymphocyte cells according to OECD 473 under GLP (Covance, 1997b). The proportions of cells with aberrations (cells with structural aberrations including gaps, cells with structural aberrations excluding gaps and polyploid, endoreduplicated or hyperdiploid cells) were examined in relation to historical negative control (normal ranges). Frequencies of cells with numerical aberrations fell within historical negative control ranges under most treatment conditions. The number of aberrant cells fell outside the normal range in a single culture at the highest concentration following treatment for 20 hours without metabolic activation, but the increase was small and was not seen in the replicate culture, and was associated with lower cell viability, so it is not considered to be of biological significance. The number of aberrant cells fell outside the normal range in a single culture at each of the two concentrations analysed following treatment for 44 hours without metabolic activation, but because the increase was small and not seen in replicate cultures, and was also associated with lower cell viability, the effect was considered to be of no biological significance. In addition, sampling at this time (approximately 3.3 times the normal cell cycle length) is outside the requirements of method B.10. Mutagenicity - in vitro mammalian chromosome aberration test in Regulation (EC) 440/2008, which is an additional reason for considering the slight increase of no biological significance. It is therefore concluded that the test substance is negative for cytogenicity in human lymphocytes under the conditions of the test.

The conclusion from the key cytogenicity study, (Covance, 1997b), is supported by findings in a second study, where a weak positive response was observed in Chinese hamster ovary cells in the first trial without metabolic activation using a non-standard protocol (NTP 1995). This response was not replicated in the second trial, and the study result is reported to be negative.

Results are available from an in vitro study in which hexamethyldisilazane was tested for mutagenicity to L5178Y TK+/- cells in the presence and absence of metabolic activation according to OECD 476 and under GLP (Microbiological Associates, 1991). No test substance related increase in mutant frequency relative to solvent control was observed in any of the three trials conducted in this study. In addition, there was no evidence of an increase in small colonies when treated cultures were compared to solvent cultures. It is concluded that the test substance is neither mutagenic nor clastogenic under the conditions of the test.

Additional information on the potential for cytogenicity of the registered substance is available from an in vitro cytogenicity study on the silanol hydrolysis product, trimethylsilanol, CAS 1066-40-6, which was tested according to a protocol that is similar to OECD 473 for cytogenicity to mammalian cells using mouse lymphoma L5178Y cells and testing up to cytotoxic concentrations (Litton Bionetics, 1978). No increase in the number of cells with two or more aberrations was observed in the presence of metabolic activation. A dose-related increase in the frequency of aberrations per cell was reported in the absence of metabolic activation, and it was concluded by the authors of the report that this represented evidence for clastogenicity, though the authors observed that these effects could be the result of cytotoxic stress on the cells. The number of cells with two or more aberrations per cell was not increased. The number of cells with aberrations was not reported. It is concluded by the reviewer that trimethylsilanol does not induce biologically significant aberrations in mouse lymphoma L5178Y cells with or without metabolic activation under the conditions of the test.

The negative conclusion reached after careful consideration of the results obtained in the key in vitro cytogenicity study on hexamethyldisilazane is supported by the lack of evidence of increase in small colonies in the key in vitro mammalian mutagenicity study on the registered substance. Induction of small colonies in such studies is associated with chemicals that induce gross chromosome aberrations (Regulation EC 440/2008 citing Yandell et al, 1990, Mutation Res., 229, p. 89-102). Additional supporting data come from testing on the hydrolysis product trimethylsilanol, CAS 1066-40-6, which include, in addition to a negative interpretation of an in vitro study, in vivo cytogenicity studies on somatic and germ cells.

Trimethylsilanol has been tested in an in vivo chromosome aberration study conducted according to a protocol that is similar to OECD 475 (Bioassay Systems Corporation, 1982). The test substance did not cause a statistically significant, dose related increase in chromosome breaks or aberrations when administered by intraperitoneal injection to Sprague Dawley rats up to the maximum tolerated dose. It is concluded that the test substance is not clastogenic in rat bone marrow cells under the conditions of the test. Trimethylsilanol has been tested in an in vivo rodent dominant lethal assay according to OECD 421 (1981) and equivalent to OECD 478 (Dow Corning Corporation, 1983). The test substance when administered to male rats at three dose levels, five days per week over eight weeks, gave no evidence of inducing chromosomal damage in germinal tissue; there was no substance related effect on fertility index, pre-implantation loss or resorption rate. A clear dominant lethal syndrome was evident in the positive control group. Although the test material was administered at a high enough level to induce transient sedation without toxicity, no reduction in male fertility or increase in dominant lethality was obtained. It is concluded that the test substance is negative for genetic toxicity to germ cells under the conditions of the test.

OECD (2003): SIDS Initial Assessment Report for SIAM 17, Arona, Italy, 11 -14 November 2003, Ammonium Chloride, CAS 12125 -02 -9.

OECD (2004): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Ammonium sulfate, CAS 7783 -20 -2.

OECD (2005): SIDS Initial Assessment Report for SIAM 20, Paris, France, 19 -21 April 2005, Persulfates, CAS 7727 -54 -0.


Justification for selection of genetic toxicity endpoint
The most recent of the most reliable studies were selected as key. They were conducted according to OECD guidelines and in compliance with GLP.

Short description of key information:
In vitro
Gene mutation (Bacterial reverse mutation assay / Ames test): negative with and without activation in in S. typhimurium strains TA 98, TA 100, TA 1535, TA 1537 and TA 102 (OECD TG 471) Covance, 1997a).
Cytogenicity in mammalian cells: negative in cultured human lymphocytes (OECD TG 473) (Covance 1997b).
Cytogenicity in mammalian cells: the hydrolysis product trimethylsilanol, CAS 1066-40-6: negative in L5178Y mouse lymphoma cells (similar to OECD TG 473) (Litton Bionetics 1978).
Mutagenicity in mammalian cells: negative in L5178Y mouse lymphoma cells (similar to OECD TG 476) (Microbiological Associates, 1991).
In vivo:
Mammalian Bone Marrow Chromosome Aberration Test in rat (ip study): the hydrolysis product trimethylsilanol, CAS 1066-40-6: negative (similar to OECD TG 475) (Bioassay Systems Corporation (1982)).
Rat dominant lethal Assay (oral study): the hydrolysis product trimethylsilanol, CAS 1066-40-6: negative (similar to OECD TG 478) (Dow Corning Corporation, 1983).

Endpoint Conclusion: No adverse effect observed (negative)

Justification for classification or non-classification

Based on the available in vitro and in vivo genotoxicity data, hexamethyldisilazane is not classified for mutagenicity according to Regulation (EC) 1272/2008.

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