Platinum(II)-Salphen Complexes as DNA Binders and Photosensitisers

Current anticancer therapies suffer from issues such as off-target side effects and the emergence of drug resistance; therefore, the discovery of alternative therapeutic approaches is vital. These can include the development of drugs with different modes of action, and the exploration of new biomolecular targets. For the former, there has been increasing interest in drugs that are activated by an external stimulus (e. g. light irradiation) to generate cytotoxic chemicals such as reactive oxygen species (ROS). For the latter, significant efforts are being directed to explore non-canonical DNA and RNA structures (e. g. guanine-quadruplexes), as alternative biomolecular targets. Herein we report the synthesis of a library of 21 new platinum(II)-Salphen complexes (square planar platinum(II) complexes coordinated to tetradentate O,N,N,O-Schiff base ligands), and the investigation, for all complexes, of their photophysical and photochemical properties, their interactions with duplex and quadruplex DNA, and their cytotoxicity against HeLa cancer cells both in the dark and upon light irradiation. Thanks to the intrinsic phosphorescence of the platinum(II) complexes, confocal microscopy was used for six of the complexes to determine their cellular permeability and localisation in two cancer cell lines (HeLa and U2OS). Altogether, these studies have allowed us to identify two lead platinum(II) complexes with high guanine-quadruplex DNA affinity and selectivity, good cell permeability and nuclear localisation, and high cytotoxicity against HeLa cancer cells upon irradiation with no detected cytotoxicity in the dark.