Methods For Percutaneous Absorption

Ideally, information on the dermal absorption of a particular compound in humans is best obtained through studies performed on humans. However, because many compounds are potentially toxic, or it is not convenient to test them in humans, studies can be performed using other techniques. Percutaneous absorption has been measured by two major methods: (1) in vitro diffusion cell techniques, and (2) in vivo determinations, both of which generally use radiolabeled compounds. To ensure their applicability to the clinical situation, the relevance of studies using these techniques must constantly be challenged [4].

In vitro techniques involve placing a piece of human skin in a diffusion chamber containing a physiological receptor fluid. The compound under investigation is applied to one side of the skin. The compound is then assayed at regular intervals on the other side of the skin. The skin may be intact, dermatomed, or separated into epidermis and dermis; however, separating skin with heat will destroy skin viability. The advantages of

the in vitro techniques are that they are easy to use and results are obtained quickly. Their major disadvantage is the limited relevance of the conditions present in the in vitro system to those found in humans.

Percutaneous absorption in vivo is usually determined by the indirect method of measuring radioactivity in excreta after the topical application of a labeled compound. In human studies, the plasma level of a topically applied compound is usually extremely low—often below assay detection. For this reason, tracer methodology is used. After the topical application of the radiolabeled compound, the total amount of radioactivity excreted in urine or in urine plus feces is determined. The amount of radioactivity retained in the body or excreted by a route not assayed (CO2) is corrected for by determining the amount of radioactivity excreted after parenteral administration. Absorption represents the amount of radioactivity excreted, expressed as percentage of the applied dose. Percutaneous absorption can also be assessed by the ratio of the areas under the concentration-versus-time curves after the topical and intravenous administration of a radiolabeled component. The metabolism of a compound by the skin as it is absorbed will not be detected by this method. A biological response, such as vasoconstriction after the topical application of steroids, has also been used to assess dermal absorption in vivo [4].

An emerging method is that of skin tape stripping. After washing, consecutive stratum corneum tape strips exhibit a profile, such as that for estradiol (Fig. 3) in human stratum corneum. The first few strips have higher estradiol content because they contain residual surface estradiol. Tape stripping can show a profile of a cosmetic within skin

Tape Striping Human Volunteers Lotion
Figure 3 Estradiol is applied to human skin, then washed 24 hours after dosing. Tape strips (consecutive 1 -10 in some areas) show a concentration pattern of estradiol through the stratum corneum.

over a time course. In addition, the chemical content of the tape Strippings can be used to compare bioavailability of competing products. Proof can be obtained by using this technique to observe which products penetrate skin faster and deeper.

11 Habits To Make or Break For Soft Flawless Skin

11 Habits To Make or Break For Soft Flawless Skin

Habits to Break and Habits to Maintain for Dazzling Skin. As you all know, our skin is the obvious appearance of who or what we are, or perhaps would like to be. However, it is more than just a simple mask.

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