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Dermal absorption

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Administrative data

Endpoint:
dermal absorption in vitro / ex vivo
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: non-guideline study, scientifically acceptable, well documented, please refer to IUCLID section 13 for read across justification.

Data source

Reference
Reference Type:
publication
Title:
Metabolism of vitamin E during skin permeation
Author:
AE-RI Cho Lee and Kakuji Tojo
Year:
1996
Bibliographic source:
J. Soc. Cosmet. Chem.,47, 85-95

Materials and methods

Test guideline
Qualifier:
no guideline required
GLP compliance:
not specified

Test material

Reference
Name:
Unnamed
Type:
Constituent
Type:
Constituent
Details on test material:
(3H)2-dl-alpha-Tocopherol, 2mCi/mM
Radiolabelling:
yes

Administration / exposure

Details on in vitro test system (if applicable):
HPLC analysis:
When the mobile phase of methanol:chloroform (90:10) was used for vitamin E analysis the vitamin E peak was very sharp at a retention time of about two minutes. To separate vitamin E and its metabolites the water fraction of the mobile phase was increased to prolong the retention time, resulting in better separation of vitamin E from its metabolites. Methanol:water (95:5) was selected as a mobile phase

Radioactivity counting:
A liquid scintillation counter (LSC) was used to quantify the concentration of radiolabeled vitamin E. For the assay, the samples were withdrawn and mixed well with scintillation cocktail. Time was calculated from the corrected values of dpm and the specific activity (dpm/mg) of radiolabeled vitamin E in the donor solution at time zero and the concentration of vitamin E in the receptor solution at each sampling.

Permeation study:
A freshly excised full thickness of abdominal skin of a female hairless mouse (5-7 weeks old, Jackson Lab. HRS/J Strain) was mounted between the half cells of the in vitro skin permeation system. Vitamin E (13 mg/ml) or [3H]2-dl-alpha-tocopherol with vitamin E in silicon fluid was employed as a donor solution. An aqueous solution of Tween-80 (5 mM) was used as a receptor solution. The solubility of vitamin E in silicone fluid was 39 mg/ml. To increase the solubility of vitamin E in receptor solution, maintaining a sink condition during the permeation experiment, Tween-80 was used. Tween-80 can increase the solubility of the lipophilic compound by incorporating vitamin E into a micelle. For the receptor solution, 5 mM Tween-80 was chosen after considering the critical micelle concentration of Tween-80 in water (0.4 mM) and the possible skin damage by the surfactant at high concentration. The donor and receptor solutions were then charged in each cell compartment. At predetermined time intervals, 30 ul of receptor solution was withdrawn and assayed for the drug concentration with HPLC or liquid scintillation counting.

Metabolism of vitamin E in the skin during the permeation study:
In vitro skin permeation experiments of radiolabeled vitamin E were performed as mentioned above. A 70 ul sample was withdrawn from the receptor cell at predetermined time intervals and analyzed with HPLC in line with the liquid scintillation counter.

Results and discussion

Any other information on results incl. tables

Permeation profile:

Vitamin E permeated after a very long lag time ( ca. 48 hr), and the permeation profiles were not consistent. Although the average profile increased with time, the individual permeation profile were quite variable. Initially, it was suspected that this small permeation rate might be due to the low solubility (Cs: 18 mg/ml) of vitamin E in the receptor solution (40 % PEG). When 5 mM Tween-80 solution was employed as a receptor solution( Cs: 2.26 mg/ml), which raised the solubility of vitamin E by a

factor of 100, the same skin permeation profile was obtained. This suggested that the low skin permeation rate of vitamin E is not due to its low solubility in receptor solution. To characterize the long lag times and small permeation profiles a permeation experiment of vitamin E was conducted using radiolabeled as well as non-labeled vitamin E.

 

Comparison of skin permeation profiles of vitamin E between HPLC and radiotracer analysis:

The LSC determined the total amount of radioactivity to permeate the skin while HPLC analysis can separate vitamin E from other metabolites. The radiolabeled vitamin E promptly penetratetd the skin, while non-labeled vitamin E appeared after a remarkably long time lag (ca.48 hr). If the long time lag observed in HPLC analysis is caused by skin binding, since the

Binding also increased lag time, the permeation profile of radiolabeled vitamin E should be similar to that from HPLC analysis. This was not the case. It is therefore suggested that the long time lag may be due to significant metabolism of vitamin E during the initial period of time, when the enzyme responsible for vitamin E metabolism was still very active. During the initial period of time, most vitamin E in the skin may be metabolized. When the time elapsed the enzymes are deactivated

gradually and intact vitamin E appeared.

 

Identification of Vitamin E and its metabolites during skin permeation:

Most of the radioactivity of radiolabeled vitamin E was recovered with the same retention time as vitamin E, and so this tritium-labeled vitamin E was used for this study. With this technique, it was possible to exclude the interference caused by endogenous skin compounds that may leach out from the skin. This technique also allowed for the metabolic pathway of radiolabeled alpha tocopherol during the skin permeation study to be traced. The authors conclude that vitamin E was metabolized to a more hydrophilic metabolite, which may be an alpha tocopherol quinone. The disappearance of peaks from time to time in the HPLC chromatograms, implied that there was some interaction between the dermis of the skin and vitamin E in receptor solution. This may be caused by back diffusion or further metabolism taking place by the enzyme leached from the dermis. The authors stated that the skin is capable of vitamin E metabolism and that vitamin E is metabolized toits more hydrophilic metabolites which may be more easily excreted from the body.

Applicant's summary and conclusion

Conclusions:
Percutaneous absorption of vitamin E has been investigated. The radiolabeled vitamin E promptly penetrated the skin while nonlabeled vitamin E appeared after a long time lag. HPLC analysis in line with UV and a liquid scintillation counter showed that vitamin E was extensively metabolized to alpha-tocopherol quinone and more hydrophilic metabolites in the hairless mouse skin. Since vitamin E undergoe significant metabolism during skin permeation, the data with radiolabeled vitamin E does not necessarily represent the characteristics of intact vitamin E itself. Total radioactivity has to be identified from the possible metabolites.
Executive summary:

Metabolism of vitamin E (alpha-tocopherol) in hairless mouse skin was investigated. An in vitro skin permeation study was performed with [3H]-1,3 -alpha-tocopherol. hairless mous skin was mounted betwee the half cells of the Valia-Chien diffusion cell, and the total radioactivity in the receptor solution as a function of time was identified with HPLC in line with a UV spectrometer and a liquid scintillation counter. Significant amounts of radioactivity were recovered as alpha-tocopherol quinone and more hydrophilic metabolites. To confirm the metabolism of vitamin E in the skin, a stability study of vitamin E in the receptor solution without skin was performed, and most of the radioactivity was recovered as alpha-tocopherol, suggesting that major metabolism takes place in the skin, not in the receptor solution. The present study suggests that vitamin E undergoes very extensive metabolism during the skin permeation process and that the skin possesses the enzymes responsible for vitamin E metabolism like other tissues.