Exploring Guanidinium Group Involvement in Hordatine Interactions with the G-Quadruplex Motif Within the c-MYC Promoter Region

G-quadruplexes (G4s) are four-stranded DNA or RNA structures formed by guanine-rich sequences. They occur in functional regions of the genomic material, including the promoter part of genes, regulatory region, and telomeric threads. G4s play a key role in various biological processes, including transcription, replication, and telomere maintenance. Guanidine-containing derivatives can bind to G-quadruplexes, either by intercalating into the structure or by interacting with the grooves or loops. The binding can stabilize the G-quadruplex, potentially affecting its biological function. In this paper, the ability of guanidinium-containing hordatines to interact with G4 was evaluated. Analogues lacking the guanidinium group or showing the benzofuran system instead of the dihydrobenzofuran core were prepared and tested as well. NMR titration and docking calculations were used to probe the binding of the compounds to G4 of c-MYC oncogene. Spectroscopic analyses were consistent with a significant interaction of benzofurans 3 and 4 at the 5′-end and 3′-end tetrads and with the formation of ligand/G-quadruplex complexes with a 2:1 stoichiometry. The resulting data were supported by docking simulations. Cytotoxic activity was evaluated on a model of U2OS osteosarcoma (ATCC HTB-96) and breast cancer (MDA-MB-231) cell lines, further highlighting the key role of the guanidinium fragment and the benzofuran core in the G-quadruplex stabilization.