In vitro and ex vivo photostability studies on Flumethasone and Fluocinolone acetonide

Flumethasone and Fluocinolone acetonide are synthetic corticosteroids used topically for the short-term treatment of various inflammatory skin diseases such as eczema, eye, nose, and ear infections, stomatitis and psoriasis. As other 11-hydroxy corticosteroids, they have shown to be sensitive to light. Previous studies have demonstrated their reactivity under UV light (UVB>UVA) both in organic and aqueous solvents and their main photoproducts have been isolated and characterized. For this high instability to UV irradiation, these drugs may lose their therapeutic activity. Indeed, their packaging instructions always indicate to protect them from light during storage. Moreover, the same problem may arise when patients are exposed to solar radiation. In order to verify the instability of these molecules when applied on the skin under UV light, the photodegradation of the two drugs is studied both in vitro (aqueous solution and commercial formulation) and in the pig skin; this ex vivo model, very close to the human skin, was used to study the fate of drugs in this organ when exposed to light. Both the glucocorticosteroids applied on the pig skin showed high photoinstability under UVB irradiation (up to 40 % for Flumethasone and to 30 % for Fluocinolone acetonide, under 10J/cm2 of UVB). The photoproducts formed in the skin were the same found in vitro, except for the hydroperoxide of Fluocinolone. The very high reactivity of this photoproduct towards biological substrates (i.e. proteins and lipids) could be the reason of the lack of its detection.