Design and biological evaluation of novel antitumor agents with mechanisms of action against topoisomerase II and/or G-quadruplexes

DNA is the molecular target for many of the alkylating agents that are used in the clinic and which have significantly increased the survival rate of cancer patients. Unfortunately, these drugs are not generally selective against tumor cells and adversely affect certain critical cellular functions of normal and tumor cells alike. In contrast to these non-specific DNA alkylating agents, agents that interact with protein-DNA complexes and secondary DNA structures have a greater selectivity for cancer cells over normal cells. The aims of this study are to provide the rationale for designing novel anticancer agents that target proteinDNA complexes and to provide insight into the molecular mechanism of action of telomestatin, a natural product that targets secondary DNA structures. A-62176 and psorospermin were used as a basis for the design of two series of novel potent anticancer agents that target protein-DNA complexes. A- 62176 is a fluoroquinolone analogue that shows good activity against a number of cancer cell lines. In the current study four new fluoroquinoanthroxazines were designed and evaluated. From this study, two fluoroquinoanthroxazines with contrasting profiles of either potent G-quadruplex interaction with weak topoisomerase II poisoning effect or vice versa were identified. Also, a second series of psorospermin/A-62176 hybrid compounds that make irreversible DNA adducts with the topoisomerase II-DNA complexes was designed on the basis of the similarity of the psorospermin and A-62176 with respect to their structure and interaction with DNA. Psorospermin, a natural product isolated from the African plant Psorospermum febrifugum. These novel hybrid compounds showed enhanced DNA alkylating activity and exhibited significant activity in vitro against various tumor cell lines. Telomestatin is a natural product isolated from Streptomyces anulatus 3533-SV4. Although telomerase is considered to be the target for telomestatin, the precise mechanism of action of this compound has not yet been elucidated. The present study demonstrates that telomestatin interacts quite specifically with the human telomeric intramolecular G-quadruplexes, and thereby inhibits telomerase activity. The importance of the selectivity of telomestatin and TMPyP4 for intramolecular versus intermolecular G-quadruplex structures in mediating the corresponding biological effects was also investigated.