Quantification of drugs within the skin is essential for topical and transdermal delivery research. Over the last two decades, horizontal sectioning, consisting of tape stripping throughout the stratum corneum, has become one of the traditional investigative techniques. Tape stripping of human stratum corneum is widely used as a method for studying the kinetics and penetration depth of drugs.
The Food and Drug Administration released a draft guidance proposing a Dermatopharmacokinetic method for evaluating bioavailability and/or bioequivalence of topical dermatological drug products. As specified in this document, the method measures topically applied drug levels in the outermost layer of the skin, the stratum corneum, as a function of time post-application and postremoval of the formulation, so as to generate a stratum corneum concentration versus time profile. The stratum corneum is collected by successive application and removal of adhesive tape providing a minimally invasive technique by which the drug's concentration in the skin can be determined. The Dermatopharmacokinetic method assumes that: (i) in normal circumstances, the stratum corneum is the rate-determining barrier to percutaneous absorption, (ii) the stratum corneum concentration of drug is directly related to that which diffuses into the underlying viable epidermis, and (iii) Stratum corneum drug levels are more useful and relevant for assessing local, dermatological efficacy than plasma concentrations.
This paper shows the applications of the tape stripping technique to evaluate drug penetration through the skin as well as stratum corneum composition and physiology, underlining its versatile application in the area of topical and transdermal drugs.