Cobalt(III) Chaperone Complexes of Curcumin: Photoreduction, Cellular Accumulation and Light-Selective Toxicity towards Tumour Cells

Light-activated prodrugs offer the potential for highly selective tumour targeting. However, the application of many photoactivated chemotherapeutics is limited by a requirement for oxygen, or for short activation wavelengths that can damage surrounding tissue. Herein, we present a series of cobalt(III)-curcumin prodrugs that can be activated by visible light under both oxygenated and hypoxic conditions. Furthermore, the photoproduct can be controlled by the activation wavelength: green light yields free curcumin, whereas blue light induces photolysis of curcumin to a phototoxic product. Confocal fluorescence microscopy and phototocytotoxicity studies in DLD-1 and MCF-7 tumour cells demonstrated that the cobalt(III) prodrugs are nontoxic in the dark but accumulate in significant concentrations in the cell membrane. When cells were treated with light for 15 min, the cytotoxicity of the cobalt complexes increased by up to 20-fold, whereas free curcumin exhibited only a two-fold increase in cytotoxicity. The nature of the ancillary ligand and cobalt reduction potential were found to strongly influence the stability and biological activity of the series.