Topical Drug Bioavailability, Bioequivalence, and Penetration

pp 155-162

The Human Skin Blanching Assay for Topical Corticosteroid Bioavailability Assessment

  • Eric W. SmithAffiliated withSchool of Pharmaceutical Sciences, Rhodes University
  • , Eric MeyerAffiliated withPharmacy Department, The University of Sydney
  • , John M. HaighAffiliated withSchool of Pharmaceutical Sciences, Rhodes University

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The human skin blanching assay (often called the vasoconstrictor assay) has been used for nearly 30 years as a means of qualitatively assessing the topical availability and potency of corticosteroids. This bioassay uses the skin-whitening side effect of this class of drugs to estimate the rate and extent of corticosteroid diffusion to the dermal vasculature, the intensity of this whiteness correlating directly with the topical availability of the drug. This versatile bioassay may be applied in a number of test modes: to evaluate the topical potency of newly synthesized corticosteroid molecules, to assess the effects of the delivery vehicle formulation on the delivery of the drug to the skin, to assess the efficacy of penetration enhancers, or to rank the potency of commercial formulations for clinical and registration purposes. The latter has probably been the most widespread use of the assay, as it is now well established that incorporating identical concentrations of the same drug into two different topical vehicles (chemical equivalency) does not necessarily produce topically bioequivalent dosage forms. Various forms of the blanching assay have been used to compare drug release from ointments,1,2 creams, and gels,2,3 and for the comparison of generic formulations to trade-name “equivalents.”4–6 The major problem that arises with this data is that the diversity in experimental methodology practiced by the various researchers7–9 makes interstudy comparison of results difficult.