1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane

Endpoint summary

Description of key information

The key skin sensitisation study is read-across from the structurally analogous substance 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982-82-9). The study, conducted according to OECD TG 406, and in compliance with GLP, reports the test substance, 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982-82-9), to be not sensitising (Dow Corning Corporation, 2000).

Key value for chemical safety assessment

Skin sensitisation

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed (not sensitising)

Additional information:

The key skin sensitisation study is read-across from the structurally analogous substance 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982-82-9). The study, conducted according to OECD test guideline 406, and in compliance with GLP, reports the test substance, 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982-82-9), to be not sensitising to guinea pig skin (Dow Corning Corporation, 2000).

At induction, 3 intradermal injections of Freund's Complete Adjuvant diluted in water, 1 % (v/v) of test material in Alembicol D, or 1 % (v/v) test material in a 50: 50 mixture of Freund's Complete Adjuvant and Alembicol D, were made intracutaneously to the scapular region of 20 test guinea pigs (day 1). One week later, 0.4 mL of undiluted test substance was applied epicutaneously to the same area for 48 hours, kept in contact with the skin under impermeable plastic adhesive tape.

At challenge (two weeks after topical induction application), 0.2 mL of 25 % and 50 % test material in Alembicol D were applied topically to the left flank of each test animal, kept in contact with the skin for 24 hours under impermeable dressing. Dermal reactions were evaluated at 24 and 48 hours after removal of the test material. Changes in body weight were recorded on day 1 and the last day of the observation period. At the end of the study period, all the animals were sacrificed and necropsy was performed.

No mortality occurred during the observation period. Expected body weight gain was observed in all the animals. No dermal reactions (erythema or oedema) were noted in any of the test animals at 24 and 48 hours after challenge exposure.

Appropriate negative and positive controls were used and provided expected results.

READ-ACROSS JUSTIFICATION

To reduce animal testing REACH recommends to make use of a read-across approach where appropriate based on the high accordance in properties relevant for the specific endpoint. In the case of skin sensitisation, the relevant properties are structural similarity and the physical-chemical parameters in the same range. In the following paragraphs the proposed read-across from 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982-82-9) to the registered substance, tris(trimethylsiloxy)phenylsilane (CAS 2116-84-9), is evaluated point by point.

Read-across hypothesis

The hypothesis is that the target (registration) substance and the source (read-across) substances have similar toxicological properties because they are structurally similar and hydrolyse at comparable rates to similar silanol hydrolysis products:

• Phenylsilanetriol and trimethylsilanol (registered substance);

• Diphenyl(methyl)silanol and dimethylsilanediol 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane).

Read-across justification

The predicted hydrolysis half-lives of the registered substance, tris(trimethylsiloxy)phenylsilane (CAS 2116-84-9), are >1.9 h at pH 4, >200 h (>8.3 d) at pH 7 and >2.0 at pH 9 and 20-25°C (QSAR). At pH 4 (closest to the known pH of the skin, 5.5), the hydrolysis rate is >1.9 h. The products of hydrolysis are phenylsilanetriol and trimethylsilanol.

The read-across substance 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982-82-9) has predicted hydrolysis half-lives of >200 h at pH 7, approximately 1.9 h at pH 4, 1.4 h at pH 5, and 2.0 h at pH 9 and 25°C. The products of hydrolysis are dimethylsilanediol and diphenyl(methyl)silanol.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at pH 7 and increase as the pH is raised or lowered.

Analogue approach justification

(a) Structural similarity

The registration and read-across substance are structurally similar and are members of an analogue group of alkoxysilanes and silanols. Both substances are linear or branched siloxanes. All silicon atoms present are fully substituted with methyl and phenyl groups. The registration substance, tris(trimethylsiloxy)phenylsilane (CAS 2116-84-9) is a branched siloxane chain with four silicon atoms, connected by three oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with three methyl or one phenyl group.

The read-across substance is a linear siloxane, with three silicon atoms connected by two oxygen atoms. The terminal silicon atoms are substituted with two phenyl and one methyl group. The substance 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982-82-9) has two methyl groups on the central silicon atom.

Both substances produce phenyl and/or methyl substituted silicon hydrolysis products. See table below for detail of hydrolysis products.

Si hydrolysis products of submission substance and relevant read-across substances

Substance
	Phenylsilanetriol	Diphenyl(methyl)silanol	Methyl(phenyl)silanediol	Trimethylsilanol	Dimethylsilanediol
Tris(trimethylsiloxy)phenylsilane (CAS 2116-84-9)	Yes	-	-	Yes	-
1,3,3,5-Tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982-82-9)	-	Yes	-	-	Yes

(b) Similar physicochemical characteristics

Structural similarity and physicochemical properties are relevant to the skin irritation potential of a substance. The parent substances hydrolyse at a similar rate and to similar silanol hydrolysis products. The key physicochemical parameters are summarised below.

Table 5.2.3: Key physicochemical parameters

	Target (registration substance)	Source (read-across substance)
CAS number	2116-84-9	3982-82-9
EC number	218-320-6	223-620-5
Chemical Name	tris(trimethylsiloxy)phenylsilane	1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane
Molecular weight  (gmol^-1)	156.21	484.82
log Kow(parent)	9.0 at 20°C (QSAR)	9.0 at 20°C (QSAR)
Water solubility (parent)	0.0066 mg/l at 23°C (measured)	5.9E-09 mg/l at 23°C (QSAR)
Hydrolysis at pH 4, 20-25°C	>1.9 h	>1.9 h
Vapour pressure (parent)	0.09 Pa at 25°C (QSAR)	5.2E-07 Pa

(c) Discussion of the hydrolysis rates of the substances

The predicted half-lives for the registration substance tris(trimethylsiloxy)phenylsilane (CAS 2116-84-9) (1.9 - 3.6 h) and the read-across substance 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982-82-9) at pH 5.5 are comparable. Both substances will be hydrolysed to similar silanol hydrolysis products.

Based on the structural similarity and similar physicochemical properties of the silicon hydrolysis products, their sensitisation potential is expected to also be similar. Therefore, the read-across is considered valid.

Respiratory sensitisation

Endpoint conclusion

Endpoint conclusion:

no study available

Justification for classification or non-classification

Based on the available data for 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS 3982 -82 -9), no classification for skin sensitisation is required for the registered substance according to Regulation (EC) No. 1272/2008.