The use of biologically active substances with anti-inflammatory properties such as corticosteroids has increased considerably in the last few decades. Particularly, the compound we are interested in, prednisolone (Predn), is a glucocorticoid with high biological activity. This compound absorbs UV radiation and may participate in photochemical processes, which can result in its own decomposition. These processes could result in the formation of free radicals or reactive oxygen species (ROS). On these grounds, the kinetic and mechanistic aspects of the direct photodegradation of Predn have been studied in aqueous medium under different atmospheric conditions by stationary and time-resolved techniques. The mechanism involved in the photodegradation has been elucidated. Predn is capable of generating the excited triplet state ³Predn* as a result of UVB light absorption. In the presence of oxygen, ³Predn* allows the formation of ROS, of which O₂(¹Δ_g) (Φ_Δ = 0.014), H₂O₂ and the radical OH˙ stand out. The latter is generated from the spontaneous dismutation of O₂˙⁻ and subsequent homolytic cleavage, photochemically promoted, of H₂O₂. Predn undergoes unimolecular photodegradation reactions under an inert argon atmosphere. In this study we found that in the presence of oxygen, the Predn photo-consumption is improved. This implies that the attack by ROS involves a very important additional contribution to the photodegradation of Predn under aerobic conditions.

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.

The photodegradation of ceftriaxone (CFT) in aqueous solution induced by UVB irradiation was studied in the presence and in the absence of oxygen. These selected conditions simulate some natural situations of contaminated water, where dissolved oxygen and the fraction of UV radiation that penetrates the terrestrial atmosphere could participate in the photodegradative process. CFT undergoes unimolecular photodegradation when irradiated with UVB light, and in an oxygen atmosphere the photodegradation process increases due to the action of reactive oxygen species (ROS), which are generated by photosensitization processes that involve the CFT triplet excited state (³CFT*). We report here for the first time the transient absorption spectrum, the lifetime and the quantum yield of ³CFT*. In the photodegradative mechanisms, hydroxyl radicals, superoxide radical anions and hydrogen peroxide are generated and participate in the CFT photooxidation. Bioassays on the Vero cell line demonstrated that the CFT photoproducts generated in the presence and in the absence of oxygen are not cytotoxic. A decrease in the bactericidal capacity of CFT after UVB exposure against S. aureus strains was also observed. The photoproducts obtained under photooxidation mechanisms mediated by ROS suffer a greater loss of antimicrobial activity. These results, from an ecological point of view, are very interesting since the action of UVB light represents an adequate method for natural CFT degradation in water because it allows a decrease in the generation of multi-resistance to antibiotics by bacteria. Also, the presence of photolysis products does not show toxic effects on living organisms.