Cardioprotective effects of idebenone do not involve ROS scavenging: Evidence for mitochondrial complex I bypass in ischemia/reperfusion injury.

Novel therapeutic strategies to treat mitochondrial deficiencies in acute coronary syndromes are needed. Complex I of the mitochondrial electron transport system is damaged following ischemia/reperfusion (I/R) injury. This disruption contributes to aberrant electron transport, diminished bioenergetics, an altered redox environment, and mitochondrial damage involved in tissue injury. In this study, we determined the cardiac and mitochondrial effects of idebenone, a benzoquinone currently in several clinical trials with purported 'antioxidant' effects. We employed complimentary models of ischemia/reperfusion injury in perfused hearts, permeabilized cardiac fibers, isolated mitochondria, and in cells to elucidate idebenone's cardioprotective mechanism(s). In ex vivo whole hearts, infarct size was markedly reduced with post-ischemic idebenone treatment (25 ± 5% area at risk, AAR) compared to controls (56 ± 6% AAR, P < .05). Several parameters of hemodynamic function were also significantly improved after idebenone treatment. Parallel studies of anoxia/reoxygenation were conducted using isolated mitochondria and permeabilized ventricular fibers. In isolated mitochondria, we simultaneously monitored respiration and ROS emission. Idebenone treatment modestly elevated succinate-derived HO production when compared to vehicle control (1.34 ± 0.05 vs 1.21 ± 0.05%, HO/O respectively, P < .05). Isolated mitochondria subjected to anoxia/reoxygenation demonstrated higher rates of respiration with idebenone treatment (2360 ± 69 pmol/s*mg) versus vehicle control (1995 ± 101 pmol/s*mg). Both mitochondria and permeabilized cardiac fibers produced high rates of HO after anoxia/reoxygenation, with idebenone showing no discernable attenuation on HO production. These insights were further investigated with studies in mitochondria isolated from reperfused ventricle. The profound decrease in complex-I dependent respiration after ischemia/reperfusion (701 ± 59 pmolO/s*mg compared to 1816 ± 105 pmol O/s*mg in normoxic mitochondria) was attenuated with idebenone treatment (994 ± 76 vs pmol O/s*mg, P < .05). Finally, the effects of idebenone were determined using permeabilized cell models with chemical inhibition of complex I. ADP-dependent oxidative phosphorylation capacity was significantly higher in complex-I inhibited cells treated acutely with idebenone (89.0 ± 4.2 pmol/s*million cells versus 70.1 ± 8.2 pmol/s*million cells in untreated cells). Taken together, these data indicate that the cardioprotective effects of idebenone treatment do not involve ROS-scavenging but appear to involve augmentation of the quinone pool, thus providing reducing equivalents downstream of complex I. As this compound is already in clinical trials for other indications, it may provide a safe and useful approach to mitigate ischemia/reperfusion injury in patients.