Salphen metal complexes as potential anticancer agents: interaction profile and selectivity studies toward the three G-quadruplex units in the KIT promoter

DNA G-rich sequences can organize in four-stranded structures called G-quadruplexes (G4s). These motifs are enriched in significant sites within the human genomes, including telomeres and promoters of cancer related genes. For instance, KIT proto-oncogene promoter, associated with diverse cancers, contains three adjacent G4 units, namely Kit2, SP, and Kitt. Aiming at finding new and selective G-quadruplex binders, we have synthesized and characterized five non-charged metal complexes of Pt(II), Pd(II), Ni(II), Cu(II) and Zn(II) of a chlorine substituted Salphen ligand. The crystal structure of the Pt(II) and Pd(II) complexes was determined by XRPD. FRET measurements indicated that Pt(II) and Pd(II) compounds stabilize Kit1 and Kit2 G4s but not SP, telomeric and double stranded DNA. Spectroscopic investigations (UV-Vis, circular dichroism and fluorescence) suggested the Cu(II) complex as the most G4-selective compound. Interestingly, docking simulations indicate that the synthesized compounds fit groove binding pockets of both Kit1 and Kit2 G4s. Moreover, they exhibited dose-dependent cytotoxic activity in MCF-7, HepG2 and HeLa cancer cells.