INTRODUCTION
Topical medications for acne and other dermatological conditions are designed to penetrate skin layers and reach their target sites in a biologically active form at sufficient concentrations to elicit clinical effects. Improving drug delivery to an intended site is critical for increasing clinical effectiveness and decreasing side effects.1 A drug’s vehicle significantly impacts skin penetration and subsequent delivery of active ingredients2; thus, several methods have been developed to assess permeation properties of topical drugs to both evaluate and optimize novel drug formulations.3
Current standards for assessing topical drug penetration utilize in vitro and ex vivo techniques, including diffusion cell assays, but these techniques have limited clinical translatability. In a traditional diffusion cell assay, such as the Franz cell, drug is applied to explanted skin stretched over a collection chamber, and drug that passes through the skin into the collection chamber is sampled for analysis.3 Although this technique is useful, it has several inherent limitations. First, diffusion cell assays poorly reproduce complex biological systems, as excised skin is nonviable and does not possess active blood flow or functional enzymes.4 Second, only the drug that passes through the skin is collected; such transdermal delivery precludes the ability to measure drug within distinct compartments of the skin itself.1 In living skin, drug that passes completely through epidermis and dermis is quickly removed from the skin by the abundant dermal vasculature, and has systemic rather than dermal effects.5 Because the target for topical dermatologics is in the skin itself, drug passing into systemic circulation becomes clinically useless. Thus, given that skin is enzymatically active, and deposition can occur at different positions within the epidermis and dermis, new methods that allow for in vivo assessment of drug deposition within the skin are necessary to effectively evaluate topical drug formulations.
Tape stripping is a relatively non-invasive and well-tolerated method for the serial collection of tissue samples from progressively deeper skin layers.6 Since its early use as a tool for investigating stratum corneum anatomy and barrier function,6 improvements in analytical techniques have
Current standards for assessing topical drug penetration utilize in vitro and ex vivo techniques, including diffusion cell assays, but these techniques have limited clinical translatability. In a traditional diffusion cell assay, such as the Franz cell, drug is applied to explanted skin stretched over a collection chamber, and drug that passes through the skin into the collection chamber is sampled for analysis.3 Although this technique is useful, it has several inherent limitations. First, diffusion cell assays poorly reproduce complex biological systems, as excised skin is nonviable and does not possess active blood flow or functional enzymes.4 Second, only the drug that passes through the skin is collected; such transdermal delivery precludes the ability to measure drug within distinct compartments of the skin itself.1 In living skin, drug that passes completely through epidermis and dermis is quickly removed from the skin by the abundant dermal vasculature, and has systemic rather than dermal effects.5 Because the target for topical dermatologics is in the skin itself, drug passing into systemic circulation becomes clinically useless. Thus, given that skin is enzymatically active, and deposition can occur at different positions within the epidermis and dermis, new methods that allow for in vivo assessment of drug deposition within the skin are necessary to effectively evaluate topical drug formulations.
Tape stripping is a relatively non-invasive and well-tolerated method for the serial collection of tissue samples from progressively deeper skin layers.6 Since its early use as a tool for investigating stratum corneum anatomy and barrier function,6 improvements in analytical techniques have
