Nitroxide delivery system for Nrf2 activation and skin protection

Abstract Cyclic nitroxides are a large group of compounds composed of diverse stable radicals also known as synthetic antioxidants. Although nitroxides are valuable for use in several skin conditions, in in vivo conditions they have several drawbacks, such as nonspecific dispersion in normal tissue, preferential renal clearance and rapid reduction of the nitroxide to the corresponding hydroxylamine. However, these drawbacks can be easily addressed by encapsulating the nitroxides within microemulsions. This approach would allow nitroxide activity and therefore their valuable effects (e.g. activation of the Keap1-Nrf2-EpRE pathway) to continue. In this work, nitroxides were encapsulated in a microemulsion composed of biocompatible ingredients. The nanometric size and shape of the vehicle microemulsion and nitroxide microemulsion displayed high similarity, indicating that the stability of the microemulsions was preserved. Our studies demonstrated that nitroxide microemulsions were more potent inducers of the Keap1-Nrf2-EpRE pathway than the free nitroxides, causing the activation of phase II enzymes. Moreover, microemulsions containing nitroxides significantly reduced UVB-induced cytotoxicity in the skin. Understanding the mechanism of this improved activity may expand the usage of many other Nrf2 modulating molecules in encapsulated form, as a skin protection strategy against oxidative stress-related conditions.