G-quadruplex DNA: A potential target for anticancer drug design

Human telomeres contain many repeats of the guanine rich hexamer (TTAGGG), the oligonucleotides (TTAGGG)4 form G-quadruplex DNA structures in the presence of physiological concentration of K+ ions.; A new class of compounds has recently been identified that bind to and stabilize the quadruplex DNA structures found at the end of human chromosomes. These have been referred to as G-quadruplex interactive agents, or QIAs. Recent interest has focused on their potential application in cancer chemotherapy. Increasing the stability of the quadruplex region is expected to inhibit the progression of telomerase, the enzyme that adds repetitive DNA sequence to the end of human chromosomes. About 90% of cancer cells are known to exhibit telomerase activity.; In the current study, the binding and selectivity of a series of porphyrin (polycyclic compounds) derivatives has been explored as potential G-quadruplex interactive agents using spectroscopic (absorption and/or fluorescence) and biological (gel electrophoresis) approaches. A universal fluorescence-binding assay has been developed for measuring the binding and specificity of potential QIAs for the G-quadruplex structures. This approach is independent of the spectroscopic properties of the ligand, and involves use of a fluorescent DNA quadruplex sequence. (TTAGGG)₄ oligonucleotides have been synthesized using solid-phase synthesis where the 5th and 11th guanine residues of the human telomeric sequence have been replaced by a fluorescent pteridine derivative, 6-methyl-8-(2-deoxy-D-ribofuranosyl) isoxanthopterin (6MI), which serve as guanine analog.; From spectroscopic analysis, combined with dialysis studies and melting temperature measurements, we found that the N-substituted anionic porphyrins, N-methyl meso porphyrin IX (NMM) and Cobalt(III) mesoporphyrin IX (Co(III)MPIX), are able to stabilize quadruplex structures and have a relatively high binding affinity and high selectivity for quadruplex DNA over single strand DNA, double strand DNA, and triple strand DNA. Attempts to increase selectivity of cationic porphyrin for quadruplex DNA by substitution of the TMP ring side chains showed little effect. Addition of saccharide moieties, for improved cellular uptake was{09} also examined.