Silicate(2-), hexafluoro-, disodium, reaction products with lithium magnesium sodium silicate

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics

Type of information:

calculation (if not (Q)SAR)

Remarks:

Migrated phrase: estimated by calculation

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Assessment based on existing toxicological data

Reason / purpose for cross-reference:

other: see Remarks

Remarks:

general substance background and toxicokinetics summary

Conclusions:

Interpretation of results (migrated information): no bioaccumulation potential based on study results

Reference 2

Endpoint:

dermal absorption in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Justification for type of information:

See Document "Laponite Analog justification-BL_6_30_2020.pdf" in Section 13.2 - Toxicokinetic assessment for Synthetic fluorohectorites for justification of read-across.

Qualifier:

according to guideline

Guideline:

OECD Guideline 428 (Skin Absorption: In Vitro Method)

Version / remarks:

OECD Guideline 428, Skin Absorption: in vitro Method, adopted 13 April 2004

Deviations:

no

Principles of method if other than guideline:

The test item Laponite XLG - Silicic acid, lithium, magnesium, sodium salt was investigated in vitro on its absorption and penetration properties on human skin. For all 12 replicates 7 different donors were used. The analyzed compound was Lithium.
For the determination of the dermal absorption/percutanous penetration properties of the test substance skin pieces were mounted onto Franz chambers and after checking the skin integrity, a finite dose of the test preparation (10 µL) was applied onto the skin and was left on the skin for an exposure period of 24 hours under non-occluded conditions in a practice relevant manner. Afterwards the test preparation was removed by washing the skin with extraction solution.
Benzoic acid and 2-Ethylhexyl trans-4-methoxycinnamate are used as positive and negative control substances known to permeate or not permeate the skin to demonstrate the performance of the system.

GLP compliance:

yes (incl. QA statement)

Specific details on test material used for the study:

The test item for this study was Laponite Type 2 rather than this substance, which is Laponite Type 1. The structure of Type 1 and Type 2 are essentially the same but Type 1 has structural fluorine whereas Type 2 does not. The nano particle size of these two substances in aqueous dispersion is very similar and it is expected that they would both behave in a similar way when applied to the skin. The use of Laponite Type 2 has been assessed as being a good surrogate substance for read across in this case.
Identity: Laponite XLG - Silicic acid, lithium, magnesium, sodium salt
Description: 3 % (w/w) Laponite XLG in deionised water
Batch No.: Bx 11-249
Content of test substance: 809 ng lithium in 10 mg test item
Purity: not applicable, product is a dispersion
Stability in water: > 1 year
Storage: At room temperature
Expiry date: December 2012

Species:

other: human skin taken from the abdoment

Details on test animals or test system and environmental conditions:

Human skin was provided dermatomized by Biopredic, Rennes (France).
Whole skin with a thickness of 425 - 557um and free of any adipose tissue was used. The surface of the skin in contact to the test item was 1.0 cm².
7 donors were used in this study.
A limited amount of the test preparation corresponding to realistic in use conditions was applied to the surface of the skin. According to the guideline cited the application of the test item to the skin should mimic realistic conditions. 10 µL of the test item were applied to the skin. 

Details on in vitro test system (if applicable):

For the determination of the dermal absorption/percutanous penetration properties of the test substance skin pieces were mounted onto Franz chambers and after checking the skin integrity, a finite dose of the test preparation (10 µL) was applied onto the skin and was left on the skin for an exposure period of 24 hours under non-occluded conditions in a practice relevant manner. Afterwards the test preparation was removed by washing the skin with extraction solution.

Signs and symptoms of toxicity:

not examined

Dermal irritation:

not examined

Key result

Time point:

24 h

Dose:

10 µL of the test preparation (3 % (w/w) Laponite XLG in deionised water ) were applied on the skin in the respective donor chamber. The application of the test item to the skin should mimic realistic conditions.

Parameter:

percentage

Absorption:

1.8 %

It is assumed that substances penetrating the stratum corneum of the skin during the exposure time diffuse into the dermis and/or the receptor solution over the 24 h monitoring time since penetration of the outer layer of the skin is considered as the rate limiting step.

The amount of penetrated test substance was determined by its concentration in the collected samples. For a worst case consideration the amounts of test item found in the receptor vials and in the extracts of the remaining skin after tape stripping were considered to have penetrated the skin respectively to be systemically available.

The amounts of test substance measured in the combined washing solutions, in the extracts of the skin areas not in contact with the receptor fluid are considered not to have passed the skin. An overall mass balance of Lithium was calculated.

The various samples solutions from the skin dermal absorption assay were analyzed by AAS for the presence of Lithium. The LLOQ for Lithium was 1.00 ng/mL in receptor solution and extraction solution. Four chambers examined for Lithium did not meet the acceptance criteria (chambers 1, 4 and 5 of experiment 1, and chamber 6 of experiment 2) due to recovery not in the range of 85-115 %. So in total 8 chambers met the requirements of the study. So in total 8 chambers were used for calculation of dermal delivery of Lithium.

Lithium was detected in only two fractions relevant for dermal absorption, which are the extract of the dermis and epidermis, but not in the receptor fluid samples. In the samples where no Lithium was detected for worst case considerations a Lithium concentration of 1.00 ng/mL (LLOQ) was assumed. So a major part of the reported dermal delivery comes only from this calculation.

In conclusion, it can be stated that under the reported conditions, 15.6 ng/cm² (1.80 % of applied dose) had penetrated the skin and are considered as bioavailable portion 

Conclusions:

The test item Laponite XLG - Silicic acid, lithium, magnesium, sodium salt was investigated in vitro on its absorption and penetration properties on human skin. For all 12 replicates 7 different donors were used. The analyzed compound was Lithium.
Two experiments were performed with human skin samples which were stored frozen until use and under static non-occluded conditions. Thus the test substance was analysed in 6 replicates per experiment (12 replicates in total).
The thickness of the skin used was 425 - 557 um. The blank samples (KBL, at 0 hours) were collected immediately after filling the donor chambers at the maximal flow rate of the pump prior to application of the test item. The conductivity across the skin samples of each chamber was determined before treatment and after the last sampling as a measure of skin integrity. The integrity of the skin was demonstrated prior to application and only skin samples within the acceptable range of ≤900 µS/cm were used. In addition, no major impairment on the skin layer was detectable after incubation with the test item.
10 µL (corresponding to approx. 809 ng/cm2 Lithium, theoretical value) of the test preparation were applied on each skin sample, left on the skin for 24 hours and then washed off using 5% HNO3 (extraction solution).
The stratum corneum was removed from the skin by stripping 10 times, and extracted with extraction solution. The epidermis was separated from the dermis using heat separation. Both skin compartments were extracted with extraction solution. Analysis for the presence of Lithium was carried out by means of AAS (atom absorption spectroscopy). The LLOQ was determined as 1.00 ng/mL for Lithium in PBS (receptor solution) and extraction solution.
8 out of 12 chambers met the acceptance criteria and were used for the assessment of the absorption and penetration properties. Four chambers examined for Lithium did not meet the acceptance criteria (chambers 1, 4 and 5 of experiment 1, and chamber 6 of experiment 2) due to recovery not in the range of 85-115 %. So in total 8 chambers met the requirements of the study.
Lithium was detected in only two fractions relevant for dermal absorption, which are the extract of the dermis and epidermis, but not in the receptor fluid samples. In the samples where no Lithium was detected for worst case considerations a Lithium concentration of 1.00 ng/mL (LLOQ) was assumed. So a major part of the reported dermal delivery comes only from this calculation.

Executive summary:

In conclusion, it can be stated that under the reported conditions, 15.6 ng/cm² (1.80 % of applied dose) had penetrated the skin and are considered as bioavailable portion. It is assessed from this anlysis that a negligible amount of Laponite XLG in dispersed (nano) form has penetrated the skin barrier and as such should not be considered as being bioavailable via dermal exposure.

Description of key information

Since the test material has a low potential for absorption by any route it means that the test material will not be readily bioavailable. The majority of any test material that is ingested orally is likely to pass through the gastrointestinal (GI) tract unchanged and be excreted in the faeces. Any small amount of constituents from the test material that are absorbed by the gut will enter the essential elemental pool along with those that are absorbed from the daily nutritional requirement of elements and therefore are not considered to be of any toxicological significance. Na, Mg and Si are found naturally in the human body. Lithium and fluoride are the only ions that are less common but fluoride is found due to fluoridation of water and dental products. The level of fluoride that potentially could be absorbed is not likely to be greater than levels humans are already exposed to

A review of publicly available information has indicated that lithium is not expected to bioaccumulate and its human toxicity is low. Large doses of lithium (up to 10 mg/L in serum) are given to patients with bipolar disorder. A provisional recommended daily intake of 14.3 μg/kg body weight lithium for an adult has been suggested. Lithium does not accumulate in the body since daily doses are required to maintain effect.

In conclusion, the test material has a low potential for any absorption by inhalation, oral ingestion and dermal absorption. It is not assessed as being necessary to carry out further animal testing to confirm these conclusions. No toxicological classification is necessary. Bioaccumulation of fluoride is not expected at exposure levels to the substance during manufacturing and certainly not in downstream formulations.

For dermal absorption or penetration of this substance in nano form once totally dispersed in aqueous medium, it can be stated that under the reported conditions for Laponite XLG, type 2, which is a good surrogate substance for read-across, 15.6 ng/cm² (1.80 % of applied dose) had penetrated the skin and are considered as bioavailable portion. It is assessed from this anlysis that a negligible amount of Laponite XLG in dispersed (nano) form has penetrated the skin barrier and as such should not be considered as being bioavailable via dermal exposure.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - dermal (%):

1.8

Additional information

The test material, Silicate(2-), hexafluoro-, disodium, reaction products with lithium magnesium sodium silicate, (EC no 285-349-9) is a monoconstituent substance and is an inorganic layered silicate structure with a unit cell of the following composition: Na_0.7[Mg_5.3Li_0.7Si₈O₂₀(OH)_0.0F_4.0].

It is an inorganic synthetic version of hectorite clay, which is smectite clay that commonly occurs in nature.

It is available as a white odourless free flowing powder with a particle size in the 1 - 250 micron range.

In a water solubility study (carried out by means of dialysis because of the unusual nature of this material) the nominal loading rate of 0.1 g/l confirmed that this substance was essentially insoluble in water at its natural pH of 8.4 and whilst not dissolving in water it disperses to form a clear sol(McCarthy & Doyle 2010-a)

In addition a hydrolysis study, again using a dialysis method to determine the release of component ions from the structure, confirmed that no hydrolysisof lithium and magnesiumoccurred at pH values from 4 – 8.4. Results indicated dissociation of lithium and magnesium at pH below 4 and significant dissociation at pH 1.(McCarthy & Doyle 2010-b).

Fluoride present in the structure is bound and not readily available at the natural pH of this material in water (8.4 @ 0.5% w/w). The fluoride content of the substance is around 10% (variable, dependant on processing condition and level of the insoluble impurity neighborite, NaMgF₃). Of that 10% ~ 1 - 2% appears as soluble fluoride ions, measured as “free fluoride” by means of a fluoride ion specific electrode method at a pH of 6.0.

The free fluoride is thought to be more accessible due to the fact that it is situated at the edges of the layers within the clay structure or present in small quantities as F^-ions ex processing.

Fluoride availability from this substance during the process of ingestion and inhalation has been determined in the laboratory by the determination of solubility in simulated gastric fluid (pH 1.4 @ 37°C for 2 hours) and alveolar lung fluid (pH 7.2 @ 37°C for 7 days)(O’Connor & Wooley 2011-c).

At pH of 1.4, the structure disintegrates and therefore all the fluoride is made accessible.

At pH 7.2 under these conditions, ~2% of the total fluoride in the material has been made accessible, about 1/10^thof the value expected at this pH for free fluoride measurement.

Ionic fluoride is known to have toxic properties for ingestion and inhalation, and is also known to bioaccumulate in the body at higher than normal exposure to this substance. Fluoride in small concentrations is readily accessible through fluoridated drinking water, toothpastes, mouthwashes etc and is beneficial at low concentrations. Adult humans can be exposed to around 25mgF/day (usually orally) from such sources. 

Inhalation

Particle size analysis of a typical sample showed that 80% of particles were smaller than 100µm and ~ 16% of particles were smaller than 10 µm in the respirable fraction(Doyle 2010-d).  

In the acute inhalation study of Laponite B (silicate(2-), hexafluoro-, disodium, reaction products with lithium magnesium sodium silicate) the LC₅₀was reported to be >1660 mg/m³(C J Hardy & G C Clarke 1983-e & Z Berczy, L Cobb, C Cherry 1975-f).

Note:The level of dust in the air as supplied by the methodology of the test could not be produced at a higher concentration than 1.66mg/l, this was the highest attainable level. At this level, there was no mortality or signs of toxicity exhibited by the exposed rats. As it was not possible to sufficiently high concentrations in the atmosphere to cause adverse effects, classification is not considered necessary.  The maximum achieved concentration of 1.66 mg/l is used as part of the derivation of no effect levels

Although silicate(2-), hexafluoro-, disodium, reaction products with lithium magnesium sodium silicate has the potential to be inhaled due to its particle size distribution it does not exhibit inhalation toxicity at a high dose in the rat. Since the test material has virtually no solubility in alveolar fluid at a pH of around 6, apart from the low level of fluoride release, it is considered the inhaled test material will not be absorbed as such and the ionic fluoride absorption would be well within the restraints of the DNEL for this type of material as explained below.

For sodium fluoride (NaF: CAS 7681-49-4), the DNEL for inhalation for industrial workers has been set in the REACH registration dossier at 2.5 mg/m3 and the OEL is also 2.5 mg/m3 (as F).

The substance, silicate(2-), hexafluoro-, disodium, reaction products with lithium magnesium sodium silicate, has been classed as a “nuisance dust” and at the worst case scenario for nuisance dust, the maximum level allowed over an 8 hour period is 10mg/m³. This would mean that workers inhalation of total available fluoride from this material would be ~ 1.0 mg/m3, well below the allowable DNEL for ionic fluoride, and as has been proven, the level of accessible fluoride in the lung would be ~0.02mg/m³which is of negligible concern.

In addition, the process dust on the manufacturing facility is kept to a minimum by use of LEV’s (local exhaust ventilation) and housekeeping. The levels normally seen are of the order of <1mg/m³, which would reduce the level of accessible fluoride by a further factor of 10. Contribution of potentially absorbed fluoride into the bloodstream from inhalation of Laponite type 1 to the “normal” daily dose of F ingested by adults would be insignificant.

Ingestion

The acute oral median lethal dose (LD50) of the test material was estimated to be > 2000 mg/kg bw(A Pooles, 2011-g & Huntigdon 1982-h). As such, the test material has been demonstrated to have a very low potential for toxicity by oral absorption.

It may have been expected that this material could exhibit some oral toxicity based on the fluoride content of the substance and the fact that at the pH of stomach fluid, virtually all of the bound fluoride in the structure becomes available, but this was not demonstrated by the acute oral test in rats.

Laponite type 1 products are generally used in surface coatings such as paints and abrasive based paper, and to a lesser extent in cosmetics. It is not used in any product intended for ingestion or that would be used orally. The level of Laponite used in such preparations for viscosity modification and surface properties is low, usually of the order of around 1-2%. At this level any release of fluoride that is ingested by mistake is not expected to be harmful. A chronic oral study has not been carried out on this substance to verify this assumption and it is not assessed necessary to prescribe such testing at this time.

Workers who are exposed to this material during manufacture are more likely to inhale the material through dust levels in the process area where inhaled material could potentially become “ingested” in very small amounts through dust being adsorbed in the nose and throat. There has been no evidence of fluoride or lithium toxicology in any workers during the history of manufacture over the past 40 years

Dermal

The LD50 in a dermal toxicity study of Laponite type 2 (a similar read-across substance) was > 2000 mg/kg(Nitka 1988-i).For an inorganic powder, this is not an expected route of exposure. Workers use hand and body protection as a routine to prevent daily contact with the material.

Levels of Laponite type 1 used in cosmetics are low, (~1% in a formulation) and testing has shown the material to be either skin irritant or a skin sensitiser.

 

Conclusions

Since the test material has a low potential for absorption by any route it means that the test material will not be readily bioavailable. The majority of any test material that is ingested orally is likely to pass through the gastrointestinal (GI) tract unchanged and be excreted in the faeces. Any small amount of constituents from the test material that are absorbed by the gut will enter the essential elemental pool along with those that are absorbed from the daily nutritional requirement of elements and therefore are not considered to be of any toxicological significance. Na, Mg and Si are found naturally in the human body. Lithium and fluoride are the only ions that are less common.

A review of publicly available information has indicated that lithium is not expected to bioaccumulate and its human toxicity is low. Large doses of lithium (up to 10 mg/L in serum) are given to patients with bipolar disorder. A provisional recommended daily intake of 14.3 μg/kg body weight lithium for an adult has been suggested. Lithium does not accumulate in the body since daily doses are required to maintain effect. (Journal of the American College of Nutrition, Vol. 21, No. 1, 14-21 (2002))

Fluoride is present in drinking water and also in most toothpaste. The level of fluoride that potentially could be absorbed is not likely to be greater than levels humans are already exposed to

In conclusion, the test material has a low potential for any absorption by inhalation, oral ingestion and dermal absorption. It is not assessed as being necessary to carry out further animal testing to confirm these conclusions. No toxicological classification is necessary.