014 In‐vivo determination of UVA sunscreen efficacy using diffuse reflectance spectroscopy

A variety of in vitro and in vivo methods have been proposed to evaluate the UVA efficacy of sunscreens. Diffuse reflectance spectroscopy (DRS) is a recent approach that allows measuring sunscreen protectiveness in the UVA portion of the spectrum. FDA representatives have shown a marked interest for the development of this alternative method. The absorption spectrum of the product is obtained by measuring the change in skin reflection due to the product. From this absorption spectrum the UVA efficacy of a sunscreen product can then be calculated for an appropriate UV source and a given skin response action spectrum. One major advantage of this test method is that the UVA protection is determined directly on human skin, thus accounting for sunscreen/skin surface and emulsion interactions encountered in vivo, but not in vitro. Both DRS and in vivo persistent pigment darkening (PPD) method were used to compare the UVA efficacy of various UVA sunscreens (such as oxybenzone, avobenzone, ecamsule and zinc oxide at different concentrations) and of marketed products. A significant correlation was found between DRS and PPD results only for photostable products. To get a correlation with PPD results with photo-unstable products, a pre-exposure of the products applied on skin was found necessary before performing the DRS measurements. The DRS method allows evaluating both the UVA absorption spectrum of the products and their broadness of absorption and can thus be considered as a powerful tool for the in-vivo evaluation of sunscreen UVA protective efficacy.