Synthesis, G-Quadruplex Stabilisation, Docking Studies, and Effect on Cancer Cells of Indolo[3,2-b]quinolines with One, Two, or Three Basic Side Chains

G-quadruplex (G4) DNA structures in telomeres and oncogenic promoter regions are potential targets for cancer therapy, and G4 ligands have been shown to modulate telomerase activity and oncogene transcription. Herein we report the synthesis and G4 thermal stabilisation effects, determined by FRET melting assays, of 20 indolo[3,2-b]quinolines mono-, di-, and trisubstituted with basic side chains. Molecular modelling studies were also performed in an attempt to rationalise the ligands binding poses with G4. Overall, the results suggest that ligand binding and G4 DNA thermal stabilisation increase with an N5-methyl or a 7-carboxylate group and propylamine side chains, whereas selectivity between G4 and duplex DNA appears to be modulated by the number and relative position of basic side chains. From all the indoloquinoline derivatives studied, the novel trisubstituted compounds 3d and 4d, bearing a 7-(aminoalkyl)carboxylate side chain, stand out as the most promising compounds; they show high G4 thermal stabilisation (T-m values between 17 and 8 degrees C) with an inter-G4 T-m trend of Hsp90A>KRas21R approximate to F21T>c-Kit2, 10-fold selectivity for G4 over duplex DNA, and 100-fold selectivity for the HCT116 cancer cell line (IC50 and IC90: <10M) over primary rat hepatocytes. Compounds 3d and 4d also decreased protein expression levels of Hsp90 and KRas in HCT116 cancer cells.