Work

Redox and structural determinants of mitochondrial complex III inhibition by triphenylphosphonium-conjugated atovaquone analogs

Year: 2026

Type: article

Abstract: Growing evidence demonstrates that cancer cell mitochondria are functional and represent promising therapeutic targets. Mitochondrial drug delivery strategies frequently use triphenylphosphonium (TPP⁺) conjugated compounds, which accumulate in cancer cell mitochondria due to their elevated negative membrane potential. TPP+-conjugated agents inhibit mitochondrial electron transport chain complexes I/II, thereby suppressing mitochondrial respiration and proliferation in cancer cells. Recent reports indicate that introducing electron-withdrawing substituents, such as trifluoromethyl, into the TPP⁺ phenyl rings of mitochondria-targeted agents modulates electron density around the phosphorus atom, thereby enhancing mitochondrial uncoupling activity and cancer cell proliferation. To fully understand the effect of electronic substituents on TPP+-conjugated compounds with respect to redox modulation, binding to mitochondrial complex III, we used the mitochondria-targeted atovaquone (ATO).ATO, a hydroxy-1,4-naphthoquinone and the only FDA-approved inhibitor of mitochondrial complex III, is undergoing clinical trials for cancer treatment. Here, we synthesized a new series of ATO-based mitochondria-targeted derivatives (MitoR-ATO) incorporating electron-donating and electron-withdrawing substituents on the TPP⁺ moiety. We evaluated their effects on superoxide generation by EPR spin-trapping, redox mechanisms by pulse radiolysis, mitochondrial oxygen consumption by the Seahorse XF-96 analyzer, Mito-ATO binding at the complex III Qi site by computational analysis, and cancer cell growth using the IncuCyte Live-Cell Imaging system. Surprisingly, several MitoR-ATO derivatives are potent inhibitors of mitochondrial complex III, through binding at the complex III Qi site.Results from these studies establish the structure–activity relationships governing the electronic substituent effects on TPP+-containing Mito-ATO with respect to redox properties, complex III binding, and cancer cell proliferation. The therapeutic implications of this study for cancer treatment are also discussed.

Source: Redox Biology

DOI: 10.1016/j.redox.2026.104370