FLUORESCENCE SPECTROSCOPY - A RAPID, NONINVASIVE METHOD FOR MEASUREMENT OF SKIN SURFACE THICKNESS OF TOPICAL AGENTS

We report the quantification of skin surface thickness of topical agen ts by in vivo fluorescence spectroscopy, and demonstrate its potential uses for assessment of application technique and substantivity. A ser ies of studies were performed on forearm skin of eight normal subjects using three creams which have intrinsic fluorescence: a sunscreen (Ne utrogena SPF15 waterproof cream). an antiseptic (Hewlett's cream) and a steroid (Trimovate (clobetasone butyrate) cream), initially, the dos e-response relationship was established for each agent by applying a s eries of five doses (0.5-8 mu l/cm(2)) and measuring cream fluorescenc e using appropriate excitation and emission wavelengths. Next, the inf luence of application technique was examined by comparing light applic ation of cream with firm rubbing, Substantivity of the three creams wa s assessed on dry skin by taking fluorescence measurements over 8 h. F inally, water resistance of 2 mu l/cm(2) of sunscreen and antiseptic c ream were compared by measuring fluorescence after each of four water immersions, The fluorescence intensity was strongly correlated with th e logarithm of surface density, r = 1.0, 0.92 and 0.98 for sunscreen, antiseptic and steroid creams, respectively, allowing derivation of a simple expression for equivalent thickness. Surface thickness of each cream was lower following firm rubbing compared with light application (P < 0.01). The rate constants for reduction of surface density of th e three creams with time on dry skin were not significantly different. However, on washed skin, the rate constant was higher for Hewlett's t han Neutrogena cream (0.503 and 0.243 h, respectively, P = 0.02), with a higher rate for each cream on wet compared with dry skin (P < 0.001 ). Hence, fluorescence spectroscopy is a simple, rapid method for meas urement of cream thickness in vivo. The many potential applications in dermatology include quantitative assessment of application technique and substantivity of topical agents.