An in vivo method to assess the photostability of UV filters in a sunscreen.

An in vivo method was developed in order to study the photostability of UV filters in a sunscreen. This method assesses sunscreen photostability in the emulsion after direct interaction with the skin. For this purpose UV filters were recovered from volunteers' forearms by using the stripping technique, then extracted from tapes and quantified by high-performance liquid chromatography (HPLC). The photostability of the filters tested was evaluated by comparing the amount of filters recovered from the strippings of UV-irradiated skin (40 minimal erythema dose, or MED) versus non-irradiated skin. Sequential analysis of several successive tapes reflected the distribution profile of the filters in the stratum corneum. Photochemical change was observed for one filter: it was shown to undergo a photochemical modification with the appearance of an additional HPLC peak. Moreover, UV filters tested displayed a high affinity for the stratum corneum but presented different distribution profiles. This in vivo method takes into account the interaction of the sunscreen agents with the stratum corneum. Furthermore, unlike spectrometric methods usually used for photostability assessment, it gives quantitative data for each individual filter of a finished product by using an HPLC technique.