Photochemical study of the highly used corticosteroids dexamethasone and prednisone. Effects of micellar confinement and cytotoxicity analysis of photoproducts
In order to investigate the effect of solar UV light on pharmacological compounds considered as emerging environmental pollutants, we studied the kinetic, mechanistic and toxicological aspects of the direct photolysis of two highly used anti-inflammatory corticosteroids: dexamethasone and prednisone. The photochemical experiments were done in aqueous media under different atmospheric conditions, with the purpose of discerning the participation of oxygen in the photodegradative processes. Moreover, the influence of direct micelles of different surfactants on the behavior of the corticosteroids under UV light was evaluated, and the cytotoxicity of the photoproducts toward the Vero cell line was tested as well. Through static and dynamic spectroscopic techniques, it was possible to detect a dependence between the rate of the photodegradation and the availability of oxygen, since both corticosteroids are capable of photosensitizing the generation of reactive oxygen species, mainly singlet oxygen and the hydroxyl radical, by generating their respective triplet excited state (characterized for the first time in this work). Dexamethasone and prednisone are inert to the action of singlet oxygen but sensitive to the effect of the hydroxyl radical. Also, unimolecular photochemical reactions, namely Norrish type I and II photo-rearrangements, occur due to the presence of carbonyl functional groups in the molecular structures. In the presence of direct micelles of anionic, cationic and non-ionic surfactants, a photoprotective effect was observed for all the studied corticosteroids. The protection phenomenon was attributed to the interaction of the molecules with the micellar interfaces and their overall distribution inside the non-polar micellar core. This behavior seems to be in a close relationship with the dipolar moment of each molecule. On the other hand, the in vitro cytotoxicity bioassays, MTT metabolism and neutral red uptake, reveal that the corticosteroids and their photoproducts obtained under different atmospheric conditions induce cell metabolic alterations at the mitochondrial level, and this effect could be more significant than lysosomal damage. In most cases, photoproducts are found to have greater toxicity than the original drugs. Studies on micellar influence and cytotoxicity also included the corticosteroid prednisolone, whose photodegradation has been previously studied in homogeneous medium.