Photorelease and Cellular Delivery of Mitocurcumin from Its Cytotoxic Cobalt(III) Complex in Visible Light

Ternary cobalt­(III) complexes of curcumin (Hcur) and mitocurcumin [Hmitocur, a dicationic bis­(triphenylphosphonium) derivative of curcumin] having a tetradentate phenolate-based ligand (H₂L), namely, [Co­(cur)­(L)] (1) and [Co­(mitocur)­(L)]­Cl₂ (2), were prepared and structurally characterized, and their photoinduced cytotoxicity was studied. The diamagnetic cobalt­(III) complexes show an irreversible Co­(III)–Co­(II) redox response and a quasireversible curcuminoid-based reduction near −1.45 and −1.74 V SCE, respectively, in DMF/0.1 M [ⁿBu₄N]­(ClO₄). The complexes exhibit a curcumin/mitocurcumin-based absorption band near 420 nm. Complex 1 was structurally characterized by X-ray crystallography. The structure contains the metal in a CoN₂O₄ distorted octahedral coordination arrangement with curcumin binding to the metal in its enolic form. Binding to cobalt­(III) increases the hydrolytic stability of curcumin. Complex 2, having a dicationic curcuminoid, shows significant cellular uptake and photoinduced cytotoxicity compared to its curcumin analogue 1. The dicationic cobalt­(III) complex 2 has significantly better cellular uptake and bioactivity than the neutral species 1. Complex 2 with mitochondrial localization releases the mitocurcumin dye upon exposure to visible light (400–700 nm) in human breast cancer MCF-7 cells through photoreduction of cobalt­(III) to cobalt­(II). Complex 2 displays a remarkable photodynamic therapy (PDT) effect, giving an IC₅₀ value of ∼3.9 μM in visible light (400–700 nm) in MCF-7 cells while being much less toxic in the dark (>50 μM). The released mitocurcumin acts as a phototoxin, generating intracellular reactive oxygen species (ROSs). The overall process leads to light-controlled delivery of a curcuminoid (mitocur) into the tumor cells while the dye alone suffers from hydrolytic instability and poor bioavailability.