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Background/purposeThe superficial layer on the skin surface, known as the acid mantle, comprises a mixture of sebum, sweat, corneocyte debris and constituents of natural moisturizing factor. Thus, the phrase ‘residual skin surface components’ (RSSC) is an appropriate term for the mixture of substances recovered from the skin surface. There is no general agreement about the effects of ethnicity, gender and age on RSSC. The aim of this human volunteer study was to evaluate RSSC in relation to ethnicity, gender and age. A suitable acquisition medium for RSSC collection was identified and samples of RSSC were subsequently analysed using gas chromatography-mass spectrometry (GC-MS) and gravimetry.MethodsA total of 315 volunteers participated in the study from a range of self-declared ethnic backgrounds. Six acquisition media were compared to determine the most suitable media for RSSC collection. The effect of age, gender and ethnicity on RSSC collection was evaluated by gravimetric analysis while GC-MS was used to determine the composition of RSSC.ResultsOf the six candidate materials assessed, cigarette paper provided the most practical and reproducible sample acquisition medium. There was no significant difference in the amount of RSSC collected when based on gender and ethnicity and no significant correlation between RSSC recovery and age. Up to 49 compounds were detected from human RSSC when analysed by GC-MS.ConclusionsThe results of the present study suggest that RSSC can be effectively collected using cigarette paper and analysed by GC-MS. Ethnicity, gender and age had no significant impact on the quantity of RSSC recovered from the skin surface.

This study investigated the in vitro nail permeability of penetrants of varying lipophilicity-caffeine (CF, logP -0.07), methylparaben (MP, logP 1.96) and terbinafine (TBF, logP 3.3) and the effect of 2 novel penetration enhancers (PEs), thioglycolic acid (TA) and urea hydrogen peroxide (urea H(2)O(2)) on their permeation. Studies were conducted using full thickness human nail clippings and ChubTur((R)) diffusion cells and penetrants were applied as saturated solutions. The rank order of steady-state penetrant flux through nails without PE application (MP>CF>TBF) suggested a greater sensitivity to penetrant molecular weight rather than logP. TA increased the flux of CF and MP approximately 4- and approximately 2-fold, respectively, whilst urea H(2)O(2) proved ineffective at enhancing permeability. The sequential application of TA followed by urea H(2)O(2) increased TBF and CF flux ( approximately 19- and approximately 4-fold, respectively) but reversing the application order of the PEs was only mildly effective at increasing just MP flux ( approximately 2-fold). Both nail PEs are likely to function via disruption of keratin disulphide bonds and the associated formation of pores that provide more 'open' drug transport channels. Effects of the PEs were penetrant specific, but the use of a reducing agent (TA) followed by an oxidising agent (urea H(2)O(2)) dramatically improved human nail penetration.

The skin is the main barrier between the internal body environment and the external one. The characteristics of this barrier and its properties are able to modify and affect drug delivery and chemical toxicity parameters. Therefore, it is not surprising that permeability of many different compounds has been measured through several in vitro and in vivo techniques. Moreover, many different in silico approaches have been used to identify the correlation between the structure of the permeants and their permeability, to reproduce the skin behavior, and to predict the ability of specific chemicals to permeate this barrier. A significant number of issues, like interlaboratory variability, experimental conditions, dataset building rationales, and skin site of origin and hydration, still prevent us from obtaining a definitive predictive skin permeability model. This review wants to show the main advances and the principal approaches in computational methods used to predict this property, to enlighten the main issues arised and to address the challenges to develop in future research.