Design and Synthesis of Xanthone Analogs Conjugated with Aza-aromatic Substituents as Promising G-Quadruplex Stabilizing Ligands and their Selective Cancer Cell Cytotoxic Action.

We have examined the stabilization of higher order non-canonical G-quadruplex (G4) DNA structures formed by the G-rich sequences in the promoter region of oncogenes by newly synthesized, novel nitrogen-containing aromatics conjugated to xanthone moiety. Compounds with N-heterocyclic such as pyridine (XNiso), benzimidazole (XBIm), quinoxaline (XQX) and fluorophore dansyl (XDan) substituents showed greater effectiveness on the stabilization of the G4 DNA as well as selective cancer cell (mainly A549 cells) cytotoxicity over normal cells both in terms of UV-Vis spectral titrations and cytotoxicity assay respectively. Both fluorescence spectral titrimetric measurements and circular dichroism (CD) melting experiments further substantiated the G4 stabilization phenomenon by these small molecular ligands. In polymerase stop assay, the formation of stable G4 structures in the promoter of oncogenes and halting of DNA synthesis in the presence of the above-mentioned compounds was demonstrated by using oncogene promoter as the DNA synthesis template. Apoptosis-mediated cell death of the cancer cells was proved by Annexin V-PI dual staining assay and cell cycle arrest occurred in the S phase of the cell cycles. The plausible mode of binding involves the stacking of the xanthone core on the G4 DNA plane with the possibility of interaction with the 5'-overhang as indicated by Molecular Dynamics (MD) simulation studies.