5-<i>N</i>-Methylated Quindoline Derivatives as Telomeric G-Quadruplex Stabilizing Ligands: Effects of 5-<i>N</i> Positive Charge on Quadruplex Binding Affinity and Cell Proliferation

A series of 5-N-methyl quindoline (cryptolepine) derivatives (2a−x) as telomeric quadruplex ligands was synthesized and evaluated. The designed ligands possess a positive charge at the 5-N position of the aromatic quindoline scaffold. The quadruplex binding of these compounds was evaluated by circular dichroism (CD) spectroscopy, fluorescence resonance energy transfer (FRET) melting assay, polymerase chain reaction (PCR) stop assay, nuclear magnetic resonance (NMR), and molecular modeling studies. Introduction of a positive charge not only significantly improved the binding ability but also induced the selectivity toward antiparallel quadruplex, whereas the nonmethylated derivatives tended to stabilize hybrid-type quadruplexes. NMR and molecular modeling studies revealed that the ligands stacked on the external G-quartets and the positively charged 5-N atom could contribute to the stabilizing ability. Long-term exposure of human cancer cells to 2r showed a remarkable cessation in population growth and cellular senescence phenotype and accompanied by a shortening of the telomere length.