Steps To Percutaneous Absorption

A cosmetic that comes in contact with human skin will be absorbed into and through the skin. The components of the cosmetic will respond to the chemical and physical laws of nature, which direct the absorption process. Examples of this are solubility, partition coefficients, and molecular weight. The skin presents a barrier, both physical structure and chemical composition. A cosmetic component will transverse from a lipophilic stratum corneum to a more progressively hydrophilic epidermis, dermis, and blood microcirculation.

Percutaneous absorption has been defined as a series of steps [1]. Table 1 lists our current knowledge of these steps. Step 1 is the vehicle containing the chemical(s) of interest. There is a partitioning of the chemical from the vehicle to the skin. This initiates a series of absorption and excretion kinetics that are influenced by a variety of factors, such as regional and individual variation. These factors moderate the absorption and excretion kinetics [2].

Once a chemical has been absorbed through the skin, it enters the systemic circulation of the body. Here, the pharmacokinetics of the chemical define body interactions. This is illustrated for [14C]hydroquinone in vivo in man, where plasma radioactivity was measured ipsilaterally (next to the dose site) and contralaterally (in the opposite arm) after a topical dose. Thirty minutes after the dose, the hydroquinone has been absorbed through the skin and has reached a near-peak plasma concentration (Fig. 1) [3]. Figure 2 shows

Table 1 Steps to Percutaneous Absorption

Vehicle

Absorption kinetics Skin site of application Individual variation Skin condition Occlusion

Drug concentration and surface area Multiple-dose application Time Excretion kinetics

Effective cellular and tissue distribution Substantivity (nonpenetrating surface adsorption) Wash and rub resistance/decontamination Volatility Binding

Anatomical pathways

Cutaneous metabolism

Quantitative structure activity relationships

Decontamination

Dose accountability

Models

Figure 1 Plasma radioactivity is detected in human volunteers 30 minutes after [14C]hydroqui-none is applied to skin. Ipsilateral is blood taken near the site of dosing, and contralateral is from the other arm. Hydroquinone is rapidly absorbed into and through human skin.

a 10 20 30

a 10 20 30

TIME (HOURS)

Figure 2 Hydroquinone is applied to human skin. Wash recovery with time decreases because hydroquinone is being absorbed into and through human skin. At the same time, tape strips of the skin surface show a rise in stratum-corneum content of hydroquinone. It is a dynamic process;hydroquinone disappears from the skin surface, appears and increases in the stratum corneum, and then appears in the blood.

hydroquinone disappearance from the surface of the skin (decreased wash recovery) and concurrent appearance in the stratum corneum (obtained from skin tape strips) [3]. As the cosmetic component transverses the skin, the chemical can be exposed to skin enzymes, which are capable of altering the chemical structure through metabolism [3].

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