Registration Dossier

Administrative data

Description of key information

In the key 28-day inhalation study (Harlan, 2012) conducted to OECD 422 and to GLP, inhalation with read-across substance and structural analogue, 2,4,6,8-tetramethylcyclotetrasiloxane, the reported NOAEC for 2,4,6,8-tetramethylcyclotetrasiloxane for systemic toxicity following inhaled administration in male and female rats was 100 ppm.

A 28 day repeat dose study is available for the read-across substance trimethoxy(methyl)silane. This substance hydrolyses rapidly to methylsilanetriol, which is also the proposed final hydrolysis product of 2,4,6,8,10-pentamethylcyclopentasiloxane. This study is used as the starting point for the calculation of an oral DNEL for consumers. The hydrolysis product is relevant as exposure is via the environment, where hydrolysis would have occurred.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
NOAEL
50 mg/kg bw/day
Study duration:
subacute
Species:
rat
System:
other: cardiovascular; haematopoietic; digestive; endocrine systems
Organ:
duodenum
jejunum
liver
thymus
adrenal glands
thyroid gland

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
NOAEC
984 mg/m³
Study duration:
subchronic
Species:
rat
System:
urinary
Organ:
kidney
bladder

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion
Endpoint conclusion:
no study available

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion
Endpoint conclusion:
no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

There are no studies on repeated dose toxicity for the submission substance, however, data are available for the source substances 2,4,6,8 -tetramethylcyclotetrasiloxane (CAS 2370-88-9) (key) and trimethoxy(methyl)silane (CAS 1185-55-3) (supporting).

Inhalation exposure

In a key 28-day inhalation study (Harlan, 2012) conducted to OECD 422 and to GLP, inhalation with read-across substance and structural analogue, 2, 4, 6, 8-tetramethylcyclotetrasiloxane (CAS 2370-88-9), it was not tolerated at the high dose concentration of 3000 ppm and resulted in the early death of three females and one male. After reduction of the high-dose level to 2000 ppm, two further females and a male were found dead.

 

Clinical laboratory investigations, necropsy and histopathology results all indicate that the urinary tract was a target organ for 2,4,6,8-tetramethylcyclotetrasiloxane. During macroscopic and microscopic examinations, stones/granular deposits in the urinary tract, injury, inflammation and dilation of urethra, urinary bladder, ureter or kidney were found in males and females which died before scheduled termination but also in most of the survivors at the dose level of 3000/2000 ppm and some males and females at the dose level of 1000 ppm. Changes in the urinary tract were still observed in both sexes at the high-dose level after the recovery period. In addition, in males and some females at the dose level of 3000/2000 ppm, higher kidney/body weight ratio was noted and increased blood concentration of urea was recorded in some males at the high-dose level. Uremia was considered to be a direct cause of the pre-term deaths or, alternatively, to cause heart failure and consequent deaths of males and females at the high-dose level. Impairment of the urinary tract observed in males and females at the dose levels of 3000/2000 and 1000 ppm was considered to be adverse.

 

In addition, the thyroid gland was also considered to be a target organ for the test item-related toxicity. Amorphous material in the colloid of the thyroid gland were observed in males of all treatment groups with dose dependency in its severity and incidence and was considered to have been caused by a metabolic change in the thyroid gland although this was considered not to be adverse.

 

Treatment with 2,4,6,8-tetramethylcyclotetrasiloxane also resulted in reductions in food consumption in males at all dose levels and in females at the dose level of 3000/2000 ppm and body weight gains and body weights at 3000/2000 ppm in both sexes.

 

Based on the results of this study the NOAEC for 2,4,6,8-tetramethylcyclotetrasiloxane for systemic toxicity following inhaled administration in male and female rats is 100 ppm.

A 90-day whole-body repeated dose toxicity study via inhalation route was also available for trimethoxy(methyl)silane (CAS 1185-55-3) which hydrolyses to the same silanol hydrolysis product as the registered and the source substance, methylsilanetriol. The study was conducted according to OECD TG 413 and in compliance with GLP. Trimethoxy(methyl)silane was administered to rats six hours per day, five days per week. The NOAEC of 100 ppm (0.56 mg/l) was based on an increased incidence of grossly observed urinary bladder calculi along with kidney dilation at the 400 ppm exposure concentration. The results of this study support the findings of the key study on 2, 4, 6, 8-tetramethylcyclotetrasiloxane, which adds weight to the evidence that the effects observed are due to methysilanetriol rather than the parent substances.

Oral exposure

There are no studies on repeated dose toxicity after oral administration for the submission substance. However, a supporting 28-day repeat dose study conducted to OECD 422 and GLP is available via the oral route for the read-across substance trimethoxy(methyl)silane (CAS 1185-55-3). This substance hydrolyses rapidly to methylsilanetriol (predicted half-life approximately 5 seconds at pH 2 and 37.5°C), which is also the final hydrolysis product of 2,4,6,8,10-pentamethylcyclopentasiloxane. Exposure to trimethoxy(methyl)silane was associated with organ weight and/or histomorphological changes in males (liver, thymus, thyroid, duodenum, jejunum, and red blood cell) and females (liver, thyroid, duodenum, jejunum, and adrenal gland) at dose levels at or above 250 mg/kg bw/day.  A marked increase in prothrombin time was observed for males at 250 and 1000 mg/kg bw/day whereas females were unaffected. Exposure was also associated with increased blood platelet concentration for males and females at 1000 mg/kg bw/day. These data support a NOAEL for the toxicity phase of the study of 50 mg/kg bw/day. This study is used as the starting point for the calculation of an oral DNEL for consumers. The hydrolysis product is relevant as exposure is via the environment, where hydrolysis would have occurred.

A 90-day repeat dose study via the oral route with the read-across substance 2,4,6,8-tetramethylcyclotetrasiloxane is planned by the Registrants.

READ-ACROSS JUSTIFICATION

To reduce animal testing REACH recommends to make use of a read-across approach where appropriate based on similarity in properties relevant for the specific endpoint. In the case of repeated dose toxicity and reproductive toxicity relevant properties are structural similarity as well as physicochemical and basic toxicological parameters with similar values. In the following paragraphs the read-across approach for 2,4,6,8,10-pentamethylcyclopentasiloxane (HD5) from 2,4,6,8-tetramethylcyclotetrasiloxane (HD4) is evaluated point by point.

Read-across hypothesis

The read-across hypothesis is that the systemic toxicity of the registered and read-across substances results from exposure to the ultimate products of hydrolysis, which are the same for both substances, RAAF Scenario 1 applies. The hypothesis is based on the following:

• The hydrolytic instability of the parent substances, HD4 and HD5.

• The relevance of the silanol hydrolysis products to the systemic toxicity of the substances. Read-across from trimethoxy(methyl)silane (CAS 1185-55-3) has also been included to support this hypothesis and to demonstrate that the observed effects are route specific. Trimethoxy(methyl)silane (CAS 1185-55-3) hydrolyses to the same silanol hydrolysis product, methylsilanetriol, as the source and target substances.

• The similarity in structure of the two parent substances.

• The similar physicochemical properties of the two parent substances which feed into toxicokinetic models.

The hydrolysis half-life of the substance has been measured using an accepted method. HD5 and HD4 hydrolyse very rapidly (measured half-life 4.2 min at pH 7 and 25 °C; calculated half-life of ≤5 s at pH 2 and 37.5°C for HD5; measured half-life 2.2 min at pH 7 and 25°C; calculated half-life of ≤5 s at pH 2 and 37.5°C for HD4; see the attached document), forming methylsilanediol. Subsequently the Si-H bonds breakdown to give hydrogen and methylsilanetriol, with a half-life of 6-8 hours at pH 7 and 25°C. Investigations are in progress to confirm the hydrolysis pathway; the results of these investigations are taken into account in the proposed testing strategy. The read-across substance has been selected as the most appropriate based on chemical structure for which data are or will be available.

The read-across substance is predictive for the toxicological profile of the registered substance as structure and key physical chemical parameters are very similar and predict similar toxicokinetic behaviour. Both substances react rapidly both in vivo and ex vivo to common hydrolysis products. Rapid excretion of the hydrolysis product via urine can be expected for both substances.

The hypothesis is supported by physicochemical and toxicological data on the substances, and the rapid hydrolysis of both substances to the same products.

The supporting source substance, trimethoxy(methyl)silane (CAS 1185-55-3)hydrolyses to the same silanol hydrolysis product, methylsilanetriol, as the registered substance HD5 and the source substance HD4 with measured hydrolysis half-life of 2.2 hours at pH 7and 25°C; calculated half-life of ≤5 s at pH 2 and 37.5°C.

Analogue approach justification

(a) Structural similarity

The registered substance and the read-across substance are members of an analogue group of Siloxanes with no functional groups attached (see Section 1.4 of the CSR).

HD5 is a cyclic siloxane with five silicon atoms linked by five oxygen atoms; each silicon atom is substituted by a methyl group and a hydrogen atom. HD4 is the equivalent methyl/hydrogen substituted siloxane with four silicons and four oxygens in the ring. Both are part of the analogue group of siloxanes with no functional groups attached (log Kow>4). In establishing this proposed analogue group, the relevant properties and chemistry were given due consideration.

Both HD4 and HD5 hydrolyse rapidly (both have calculated half-lives of ≤5 s at pH 2 and 37.5°C, the conditions relevant to oral exposure). The intermediate hydrolysis products in both cases are a series of linear silanediols, H4L4-diol, H3L3-diol and H2L2-diol, followed by methylsilanediol; HD5 produces in addition H5L5-diol as the first hydrolysis product. The final hydrolysis product is methylsilanetriol and hydrogen gas is produced as a by-product of the reaction. Hydrolysis is discussed in detail in the attached document of the CSR

(b) Similar physicochemical properties

A data matrix is attached in Section 13 of the IUCLID dossier, and the key physicochemical parameters are summarised in the table below:

Table: Key physicochemical properties and repeated-dose toxicological data

CAS Number

6166-86-5

2370-88-9

1185-55-3

Abbreviation

HD5

HD4

MTMS

Chemical Name

2,4,6,8,10-pentamethylcyclopentasiloxane

2,4,6,8-tetramethylcyclotetrasiloxane

Trimethoxy(methyl)silane

Si hydrolysis product

Methylsilanetriol

Methylsilanetriol

Methylsilanetriol

Molecular weight

300.6

240.5

136.22

log Kow(parent)

6.33

5.54

0.7

log Kow(silanol hydrolysis product - methylsilanetriol)

-2.4

-2.4

-2.4

Water solubility (parent)

3.3 mg/l

0.6 mg/l

9.1E+04 mg/l

Water solubility (silanol hydrolysis product - methylsilanetriol)

1E+06 mg/l

1E+06 mg/l

1E+06 mg/l

Vapour pressure (parent)

91 Pa

479 Pa

3000 Pa

Vapour pressure (silanol hydrolysis product - methylsilanetriol)

0.05 Pa

0.05 Pa

0.05 Pa

Hydrolysis t1/2at pH 7 and 25°C

4.2 minutes

2.2 minutes

2.2 hours

Hydrolysis t1/2at pH 2 and 37.5°C

≤5 s

≤5 s

≤5 s

Acute oral toxicity LD50 (mg/kg bw)

>2000 (OECD 420)

(Reconsile, 2010b)

>2000 (OECD 420)

(Reconsile, 2010a)

> 11685 (no guideline followed, standard acute method)

(Mellon, 1963)

Repeated dose toxicity: inhalation

No study available

NOAEC = 100ppm (male/female) Bladder stones and histopathological findings in urinary tract at 3000/2000 and 1000 ppm, plus reversible reduction of food consumption and reversible pathology change in thyroid gland in males at 100 ppm

(OECD 422)

(Harlan Laboratories, 2012)

NOAEC = 100ppm (male/female)

Decreased activity, soiling around muzzle, abdomen and urogenital regions with gross pathological findings including dilation of kidneys and urinary bladder with calculus in bladder. Absolute adrenal gland weights were statistically increased in females and kidney weights were increased in males(OECD 413)

(Dow Corning Corporation, 2008)

Repeated dose toxicity: oral

No study available

OECD TG 408 study planned

NOAEL=50mg/kg bw/day

Organ weight and/or histomorphological changes in liver, thymus, thyroid, duodenum, jejunum, adrenal gland and red blood cell, increase in prothrombin time, increased blood platelet concentration

(OECD 422)

(Dow Corning Corporation, 2005)

(c) Toxicokinetics

Using data based on the physicochemical properties of the registered and read-across substances and their hydrolysis products in algorithms that are the basis of many computer-based physiologically based pharmacokinetic or toxicokinetic (PBTK) prediction models, reasonable predictions or statements have been made about their potential absorption, distribution, metabolism and excretion (ADME) properties (see section 5.1). Because the physicochemical properties of the registered and read-across substances are very similar (see Table 5.6.1), they are predicted to have very similar ADME properties.

Following inhalation exposure, based on a QPSR which estimates a blood:air partition coefficient of <1, there may be some absorption into the systemic circulation, although significant uptake would be expected for the final hydrolysis product, methylsilanetriol. Similarly, following dermal exposure, absorption and therefore systemic exposure to the registered and read-across substances is predicted to be minimal, and this is also predicted to be the case for the final hydrolysis product.

Although systemic exposure to the registered or read-across substances via inhaled or dermal routes is unlikely to occur as described above, should it happen then it is predictedthey will distribute primarily into fat and to a much lesser extent into brain, liver, kidney and muscle. Distribution into the main body compartments in predicted to be minimal for the final hydrolysis product.

Following oral exposure, both substances hydrolyse very quickly to silanediols, with a half-life of ≤5 seconds for each step (see the attached document). Compared to the typical gastric emptying half-life for liquids in the order of 11 - 30 minutes for humans and 77 minutes for rats (RIVM,http: //www. interspeciesinfo. com/Interspecies/Stomach), several hydrolysis half-lives would therefore have occurred and absorption in the intestine would almost exclusively relate to common hydrolysis products in both cases.

However, in an in vitro study conducted to assess the rate and extent of hydrolysis and condensation reactions of HD4 and HD5 in simulated rat gastric conditions (DCC, 2014) no reaction products were observed after 4 hours, with >99% of the original concentration of HD4 or HD5 remaining. Although this result would appear to indicate that after oral administration in the rat the substances do not hydrolyse at similar rates to the same hydrolysis product, further analysis of the methodology used shows that as the concentrations used far exceed the solubility of the substances (particularly at the high concentration employed), the result is not unexpected. This is because it was noted that the test substance and buffer solutions had formed two distinct layers so only a small proportion of the test substance was in contact with the aqueous medium and therefore subject to hydrolysis. However, regardless of the very low soluble fraction of either substance at these concentrations, some hydrolysis would have occurred resulting in the common hydrolysis product. Furthermore, as a result of the lipophilic nature of both substances, intestinal absorption via micellular uptake would occur and once into the more aqueous systemic environment at lower concentrations, hydrolysis of both substances to the common hydrolysis product would have occurred. The amount of uptake by such a mechanism is likely to be similar for HD4 and HD5. Therefore, at exposure concentrations more relevant to potential human exposure than the simulation, the soluble fractions of HD4 and HD5 would be much higher and they would hydrolyse at similar rates to the common final hydrolysis product, methylsilanetriol

Athough long duration systemic exposure to the registered and read-across substances is unlikely, should it occur, they will rapidly hydrolyse and the hydrolysis products will beeffectively eliminated via the kidneys in urine so accumulation will not occur.

(d) Acute toxicity

Neither the registered or read-across substance showed any evidence of acute toxicity. This is discussed further in section 5.2, and the data for HD4 and HD5 is shown in Table 5.6.3 above.

 (e) Multiconstituent nature of the commercial product

The registered substance is one constituent (approximately 40-45%) of a multiconstituent reaction mass; the pure substance is not commercially available. The other constituent (approximately 50-55%) of the commercially available substance is the read-across substance, 2,4,6,8-tetramethylcyclotetrasiloxane (HD4).The only impurity present above 1% but less than 5% is another siloxane with Si-H and Si-Me bonds that is predicted to hydrolyse very rapidly to the same products as HD4 and HD5. Further details are not provided here for reasons of confidentiality.

(f) Bias that influences the prediction

Neither the registered nor the source substances showed any evidence of acute toxicity.

The oral and inhalation repeated dose studies for the supporting substance trimethoxy(methyl)silane (CAS 1185-55-3) show that there is a difference between effects observed after inhalation exposure and those observed after oral exposure to the same substance, indicating different target organs. It can be concluded that the effects seen after repeated exposure are route specific, and the difference in effects between the oral study with trimethoxy(methyl)silane and the inhalation study with HD4 do not contradict the hypothesis that the same breakdown products derived from different parent compounds would cause the same type of effects.

The inhalation studies on the supporting source substance trimethoxy(methyl)silane and the source substance HD4 show a similarity of effects, supporting the hypothesis that the same breakdown products derived from different parent compounds would cause the same type of effects by the same route of exposure. The source substance HD4 is to be tested by the oral route, which has been identified by ECHA as the appropriate route of exposure for both the source substance HD4 and the target substance HD5. Therefore it is considered that similar effects will be seen for the registered substance, HD5, which produces the same ultimate hydrolysis products and similar intermediate hydrolysis products as HD-4.

Conclusion

Based on the similar chemical structure and similar toxicokinetic behaviour read across of long-term (oral) mammalian toxicity data for 2,4,6,8-tetramethylcyclotetrasiloxaneto 2,4,6,8,10-pentamethylcyclopentasiloxane is considered to be valid, based on all currently available information.


Justification for classification or non-classification

Based on the available inhalation data with read-across substance 2,4,6,8-tetramethylcyclotetrasiloxane, 2,4,6,8,10-pentamethylcyclopentasiloxane is not classified for adverse effects following repeated inhaled exposures according to Regulation (EC) No 1272/2008.