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Today, crossing the stratum corneum (SC) may be a challenge for cosmetic actives as well as non-actives, such as carriers or excipients used in cosmetics or pharmaceutical products, which also can act in the skin. For this, the chemical must penetrate the skin, making its pharmacological aspects essential. Knowing how an ingredient penetrates and its speed of penetration, formulators can control what penetrates and modulates the pharmacodynamic response by adapting the nature and quantity of the active administered. However, the current understanding of percutaneous penetration and parameters that can influence it remains a sub judice area. Ethnicity or pigmentation, for instance, can be implied in percutaneous absorption for which several studies, described here, have been conducted to clarify their roles.
Percutaneous penetration in general has been evaluated using both invasive and noninvasive techniques. Thus, the assessment of the penetration of several compounds through the skin can be evaluated indirectly via several extrapolations, including from SC thickness or its hydration. Tape-stripping is a technique that evaluates skin penetration by measuring the compound quantity present in tapes after their removal. On the other hand, invasive techniques, described later, allow for a direct skin penetration evaluation. More, in vitro studies using Franz cells are also performed to directly assess skin penetration. However, it was not until the late 1980s that interethnic differences in penetration specifically were investigated. Penetration can be studied via pharmacokinetics, i.e., assessing the flux of actives in living organisms over time, or via pharmacodynamics, i.e., assessing a biologic effect induced by the compound depending on the compound amount applied on skin. These two perspectives and results associated are summarized here.
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