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| Issue 23 |  |
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| Newsletter | Tape stripping in combination with spectroscopic measurements: qualitative analysis of the penetration behavior of sunscreens and drugs into the skin | Jürgen Lademann |
Tape stripping is a frequently used method to investigate the penetration of topically applied substances into the skin. Substances are applied on the skin, adhesive films are put on the treated skin areas and are taken off again after a certain application time. The tapes contain corneocytes as well as proportions of the applied substance.
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The representation of the functional connection between the amount of the substance applied on the individual tapes and the strip number allows us to make statements about the penetration behavior. A disadvantage of the tape stripping method is that the amount of the corneocytes taken changes per stripping in depence on the test persons. Furthermore, the type of the substance applied and the penetration time have an influence on the amount of corneocytes on the individual tapes. Thus, a qualitative and quantitative determination of the penetration kinetics in relation to the profile of the horny layer is not possible.
Penetration of sunscreen products into the horny layer
In the "Experimental and Applied Physiology of the Skin" Department of the Charité Dermatological University Hospital a method was developed which allows to determine quantatively the amount of corneocytes on the individual tapes spectroscopically with the aid of a specially developed UV/VIS Spectrometer (Perkin Elmer) [1]. It thus becomes possible to assign the amount of the detected substances to the relative thickness of the horny layer, as demonstrated in Figure 1 for a topically applied filter substance which covers the short-wave as well as the long-wave UV-A region (320 to 400 nm) with a maximum at 360 nm (2).
Each horizontal line in this figure represents a tape. The distance between the lines corresponds to the amount of corneocytes on the individual tapes. In the present case the stratum corneum was almost completely removed. Thus a "cross section" through the horny layer is obtained, which demonstrates the penetration of the filter substance. As Figure 1 shows, the UV filter substance is located in the upper layers of the horny layer. Therefore, it does not make sense in all cases to remove the stratum corneum completely. For comparative studies with various sunscreen products it is in most cases sufficient to remove the first 10 to 15 tapes only.
Investigations of the lateral spreading of topically applied substances
The combined investigation method is not only applied to analyze the penetration of substances into the horny layer but also to investigate the local spreading of substances on the skin. Here, tapes are removed from within and from outside an area on which the substances to be investigated have been topically applied. A comparison of the penetration profiles from within and from outside of the treated skin areas allows us to come to conclusions about the lateral ways of penetration [2]. Figure 2 shows the proportion of the UV filter substance Parsolâ in percentages, which was detected with the aid of the above described test method after six hours from within and from outside of the applied skin area. The intensity of the lateral spreading is clearly determined by the type of emulsion and the filter substance as well as by the skin properties of the test persons.
Penetration of topically applied drugs
Although the application of the tape stripping method combined with the spectroscopic determination of the amount of corneocytes is only limited to the horny layer, it may also be applied for an analysis of the penetration kinetics of topically applied drugs. These substances should penetrate through the skin and should develop their effects in the area of the living cells. In relation with studies of the penetration kinetics of steroids (Clobetasol) into the skin it could be shown that the horny layer has a definite reservoir function which determines the penetration kinetics [3].
Figure 3 a and b show two penetration profiles of Clobetasol in various formulations (4). 20 tapes were removed from the underarms of the same test person. It become obvious that it is particularly necessary to relate the detected substance amount per tape to the relative thickness of the horny layer and not to the strip number, because the amount of the corneocytes per tape may differ up to one hundred percent in dependence of the type of formulation. A clear correlation could be shown between the detected penetration kinetics and the clinical appearance in the form of a blanching effect (vasoconstriction).
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Figure 3: Penetration profile of Clobetasol propionate in various emulsions
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The tape stripping method combined with the spectroscopic determination of the amount of corneocytes was taken up by the FDA's "Guidance for Industry -Topical Dermatological Drug Product NDAs and ANDAs - In Vivo Bioavailability, Bioequivalence, In Vitro Release, and Associated Studies" as recommended test method.
References
1. Weigmann, H.-J., Lademann, J., Meffert, H., Schaefer, H., Sterry, W.: Determination of the horny layer profile by tape stripping in combination with optical spectroscopy in the visible range as a prerequisite to quantify percutaneous absorption. Skin Pharm., 12, 1998, 34-45
2. INCI Name: Butyl Methoxydibenzoyl Methane
3. Baumann, M., Weigmann, H.-J., Lademann, J.: Determination of UV filter concentration in relation to the tape number - determination of the recovery rate; in Report EC Project SMT4-CT-2152, Measurements to assess sunscreen efficacy in industrial research, Berlin September 1999, p. 74 -79
4. Clobetasol Propionate Gel, 0.05 percent
5. Weigmann, H.-J., Lademann, J., v.Pelchrzim, R., Sterry, W., Hagemeister, T., Molzahn, R., Schäfer, M., Linscheid, M., Schaefer, H.: Bioavailability of clobetasol propionate - quantification of drug concentrations in the stratum corneum by dermatopharmacokinetics using tape stripping. Skin Pharm., 12, 1998, 46-53
Author
Dr.-Ing. Jürgen Lademann
Priv.-Doz. Dr. Dr.-Ing. Jürgen Lademann is head of the Department "Experimental and Applied Physiology of the Skin" at the Charité Dermatological University Hospital in Berlin.
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