Photocytotoxic Copper(II) Complexes with Schiff-Base Scaffolds for Photodynamic Therapy

Photodynamic therapy (PDT) is a promising and minimally invasive method for the treatment of superficial diseases, and photosensitizers with high phototoxicity indices (defined as (IC₅₀^dark )/(IC₅₀^irradiation )) are essential for the development of ideal photosensitizing properties for this technology. Herein, we report a series of photocytotoxic copper(II) complexes [Cu(^R QYMP)(dppn)] (^R QYMP=N,N,O-tridentate Schiff-base derivatives, dppn=benzo[i]dipyrido[3,2-a;2',3'-c]phenazine), the structures of which have been confirmed by mass spectrometry and FTIR spectroscopy. X-ray crystallography revealed that the CuN₄ O core of the [Cu(^cumyl QYMP)(dppn)](ClO₄ ) complex (3) has a distorted square-pyramidal geometry. Phototoxicity indices of 329 against human squamous cell carcinoma (SCC15) and 296 against basal cell carcinoma (BCC) cell lines have been determined with [Cu(^3-OMe QYMP)(dppn)](ClO₄ ) (4). This can be attributed to the formation of reactive oxygen species, cell apoptosis, and caspase-3 activation, indicating high potential of complex 4 as a photosensitizer candidate in PDT. Thus, copper complexes bearing suitable Schiff-base ligands with a dppn co-ligand may be considered for the design of efficient metal-based anticancer agents for PDT.