G-quadruplex DNA targeted metal complexes acting as potential anticancer drugs

Although cisplatin and its analogues have been widely utilized as anticancer metallodrugs in clinics, their serious side effects and damage to normal tissues cannot be avoided because cisplatin kills cancer cells by attacking genomic DNA. Thus the design of metallodrugs possessing different actions of anticancer mechanism is promising. G-quadruplex nucleic acid, which is formed by self-assembly of guanine-rich nucleic acid sequences, has recently been considered as an attractive target for anticancer drug design. The basic unit of a G-quadruplex is a G-quartet, a planar motif generated from four guanine residues pairing together through Hoogsteen like hydrogen bonds. DNA G-quadruplex (G4) structures exist in the chromosomal telomeric sequences and the promoter regions of numerous genes, including oncogenetic promoters. Formation of G4 structures within the 3'-overhang of telomeric DNA can inhibit the telomerase activity, which is silent in normal cells but up-regulated in most cancer cells, thus significantly shortening telomeres and preventing cancer cell proliferation and immortalization. Intramolecular G4 structures formed within the oncogene promoter regions can effectively inhibit oncogenen transcription and expression. Thus rational design of small molecular ligands to selectively interact, stabilize or cleave G4 structures is a promising strategy for developing potent anti-cancer drugs with selective toxicity towards cancer cells over normal ones. This review will highlight the recent development of G4-interacting metal complexes, termed G4-ligands, discussing their binding modes with G-quadruplex DNA and their potential to serve as anticancer drugs in the medical field.