Oxazole-containing macrocycles, which include the
natural product telomestatin, represent a promising class of
anticancer agents that target G-quadruplex DNA. Two synthetic hexaoxazole-containing
macrocyclic compounds (HXDV and HXLV-AC) have been characterized with regard to their cytotoxic activities versus human
cancer cells, as well as the mode, thermodynamics, and specificity with which they bind to the intramolecular (3+1) G-quadruplex structural motif formed in the presence of K+
ions by human telomeric
DNA. Both compounds exhibit cytotoxic activities versus human lymphoblast (RPMI 8402) and oral
carcinoma (KB3-1) cells, with associated IC50 values ranging from 0.4 to 0.9microM. The compounds bind solely to the quadruplex
nucleic acid form, but not to the duplex or triplex form. Binding to the quadruplex is associated with a stoichiometry of two
ligand molecules per
DNA molecule, with one
ligand molecule binding to each end of the host quadruplex via a nonintercalative "terminal capping" mode of interaction. For both compounds, quadruplex binding is primarily entropy driven, while also being associated with a negative change in heat capacity. These thermodynamic properties reflect contributions from favorable
ligand-induced alterations in the loop configurational entropies of the quadruplex, but not from changes in net hydration. The stoichiometry and mode of binding revealed by our studies have profound implications with regard to the number of
ligand molecules that can potentially bind the 3-overhang region of human telomeric
DNA.