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Administrative data

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

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
2012
Report date:
2012

Materials and methods

Test guideline
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)

Test material

Constituent 1
Chemical structure
Reference substance name:
Silicic acid, lithium magnesium sodium salt
EC Number:
258-476-2
EC Name:
Silicic acid, lithium magnesium sodium salt
Cas Number:
53320-86-8
Molecular formula:
Na0.7+ [(Si8Mg5.5Li0.3) O20(OH)4]0.7-
IUPAC Name:
Silicic acid, lithium magnesium sodium salt
Test material form:
solid: particulate/powder
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

Test animals

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. 

Administration / exposure

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.

Results and discussion

Signs and symptoms of toxicity:
not examined
Dermal irritation:
not examined
Percutaneous absorption
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 %

Any other information on results incl. tables

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 

Applicant's summary and conclusion

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.

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