Novel method for studying photolability of topical formulations: a case study of titanium dioxide stabilization of ketoprofen

Sunlight may decompose active substances and excipients in pharmaceuticals. This may cause formulation problems as well as induce adverse skin reactions. The photodecomposition of topical preparations may occur on the skin surface, but also deeper in the skin after penetration of light into the viable tissues. The aim of the present study was to investigate whether microparticles of titanium dioxide could protect against photodecomposition using ketoprofen as a photolabile model substance. The results showed quality differences between titanium dioxide, where surface-coated particles were superior to pharmaceutical grades in reducing the degradation in vitro. The protective effect was also studied in humans. The skin was treated for 3 h with the gels and then exposed to ultraviolet (UV) light (11.7 J/cm2 UVA and 5.4 mJ/cm2 UVB). Layers of the stratum corneum were then removed by consecutive tape strippings and assayed for content of ketoprofen. The remaining amount was higher in the different stratum corneum compartments after treatment with a gel containing 4% coated titanium dioxide compared with a transparent gel. Thus, surface-coated microparticles of titanium dioxide may well be of clinical benefit in protecting photolabile drug substances against sunlight.