Book of Abstracts: Albany 2011
June 14-18 2011
©Adenine Press (2010)
Platinum(II) Phenanthroimidazoles for G-quadruplex Targeting: The Effect of Structure on Binding Affinity, Selectivity and Telomerase Inhibition
G-quadruplexes have gained recognition as viable targets for chemotherapeutic drug design, based on their ability to interfere with cancer cell proliferation (1,2). These higher order DNA structures, held together by Hoogsteen hydrogen bonds, result from the folding of a guanine (G) rich DNA sequence in the presence of potassium or sodium cations (3). G-quadruplex forming sequences have been identified throughout the human genome in telomeres, promoter regions of oncogenes, nuclease hypersensitivity regions and untranslated regions of RNA (4,5). From this list, one highly investigated G-quadruplex target for drug design is the telomere. Small molecules that promote the folding of the human telomeric sequence, (TTAGGG)n, into a G-quadruplex structure can result in biologically relevant phenomena, such as the loss of telomere integrity through disruption of the shelterin complex of proteins (6,7). or the prevention of telomere elongation by the reverse transcriptase enzyme telomerase (8). Both of these events have profound effects on cancer cell proliferation thereby accomplishing one of the main goals of chemotherapy, to halt tumor growth.
We have previously shown that platinum phenanthroimidazole-based binders are good stabilizers of the intermolecular T4G4T4 G-quadruplex motif (9). These complexes possess optimal geometries for targeting this structure and are substitutionally inert. Their synthesis is facile and highly modular, lending itself to the ready generation of compound libraries to maximize affinity and selectivity. However, upon initial studies involving the binding of the phenyl  and naphthyl  derivatives to the intramolecular motif based on the human telomere, we discovered that a significant twist within the ligand itself may prevent favorable π-stacking interactions of the naphthyl  ligand with the G-quadruplex. We hypothesized that incorporating an internal hydrogen bond within our phenanthroimidazole ligands may reduce the twist, resulting in a planar ligand surface for optimal overlap with the G-quadruplex.
Through the use of molecular modeling, circular dichroism, and fluorescence displacement assays, we have shown that phenanthroimidazole platinum(II) complexes template and stabilize G-quadruplex forming sequences based on the human telomeric repeat, (TTAGGG)n with the greatest stabilization from the indoyl  derivative (G4DC50 = 0.53μM). However, while the incorporation of the internal hydrogen bond does increase binding strength to the quadruplex motif, it also tends to reduce selectivity between quadruplex and duplex DNA. We found that the introduction of a chloride modification to the phenanthroimidazole core, as well as a sidearm with protonable sites restores selectivity while also increasing binding strength to the quadruplex motif (G4DC50 = 0.28μM for ). We also show complexes [1-4] are able to inhibit telomerase through the TRAP-LIG assay, thus verifying that phenanthroimidazole-based platinum(II) complexes can elicit antiproliferative effects and act as telomere disruption agents.
Department of Chemistry