Isaindigotone derivatives: A new class of highly selective ligands for telomeric G-quadruplex DNA

Four isaindigotone derivatives (5a,b and 6a,b) designed as telomeric G-quadruplex ligands have been synthesized and characterized. The unfused aromatic rings in these compounds allow a flexible and adaptive conformation in G-quadruplex recognition. The interaction of human telomeric G-quadruplex DNA with these designed ligands was explored by means of FRET melting, fluorescence titration, CD spectroscopy, continuous variation, and molecular modeling studies. Our results showed that the adaptive scaffold might not only allow the ligands to well occupy the G-quartet but also perfectly bind to the grooves of the G-quadruplex. The synergetic effect of the multiple binding modes might be responsible for the improved binding ability and high selectivity of these ligands toward G-quadruplex over duplex DNA. Long-term exposure of HL60 and CA46 cancer cells to compound 5a showed a remarkable decrease in population growth, cellular senescence phenotype, and shortening of the telomere length, which is consistent with the behavior of an effective telomeric G-quadruplex ligand and telomerase inhibitor.