A Mitochondria-Targeted Triphenylphosphonium-Conjugated Nitroxide Functions as a Radioprotector/Mitigator

Jiang, J., Stoyanovsky, D. A., Belikova, N. A., Tyurina, Y. Y., Zhao, Q., Tungekar, M. A., Kapralova, V., Huang, Z., Mintz, A. H., Greenberger, J. S. and Kagan, V. E. A Mitochondria-Targeted Triphenylphosphonium-Conjugated Nitroxide Functions as a Radioprotector/Mitigator. Radiat. Res. 172, 706-717 (2009). Removal of excessive mitochondrial reactive oxygen species by electron scavengers and antioxidants is a promising therapeutic strategy to reduce the detrimental effects of radiation exposure. Here we exploited triphenylphosphonium (TPP) cation as a means to target nitroxide radicals to mitochondria. We synthesized a library of TPP-conjugated nitroxides and tested their radioprotective effects in gamma-irradiated mouse embryo cells and human epithelial BEAS-2B cells. Cells were incubated with conjugates either before or after irradiation. We found that [2-(1-oxyl-2,2,6,6-tetramethyl-piperidin-4-ylimino)-ethyl]-triphenyl-phosphonium (TPEY-Tempo) significantly blocked radiation-induced apoptosis as revealed by externalization of phosphatidylserine on the cell surface and inhibition of cytochrome c release from mitochondria. Using electron paramagnetic resonance, we showed that TPEY-Tempo was integrated into cells and mitochondria, where it underwent one-electron reduction to hydroxylamine. TPEY-Tempo acted as an electron scavenger that prevented superoxide generation and cardiolipin oxidation in mitochondria. Finally, TPEY-Tempo increased the clonogenic survival rate of irradiated cells. The cellular integration efficiencies of nonradioprotective TPP conjugates, including Mito-Tempo (Alexis, San Diego, CA), were markedly lower, although these homologues were integrated into isolated succinate-energized mitochondria to a similar extent as TPEY-Tempo. We conclude that mitochondrial targeting of TPP-conjugated nitroxides represents a promising approach for the development of novel radioprotectors. (C) 2009 by Radiation Research Society