Bleomycins (BLM) are a family of glycopeptide antitumor drugs that inflict double strand breaks in cellular DNA in presence of metal ions (like Fe2+) and O2. Coordination of the metal ions to the drug produces metallobleomycins (M-BLMs) which are believed to cleave the sugar framework of DNA via a free radical mechanism. The molecular basis of the drug action remains rather obscure due to the absence of precise structural information on the coordination spheres of the metals in M-BLMs. This proposal will undertake a systematic synthetic analogue approach to M-BLMs in order to establish (a) the coordination structures of M-BLMs and (b) the characteristics of the metal-centered chemical reactions which are presumably associated with the drug action. In our analogue approach, selected transition metal complexes of several designed organic fragments (ligands) resembling the metal-chelating locus of BLM will be synthesized and structurally characterized by X-ray crystallography. So far, no structural data on M-BLM analogues are available. The solution structures of these complexes in presence of buffer, dithiothreitol, ascorbate etc. will be verified by UV-vis, 1H and 13C NMR (both one- and two-dimensional) spectroscopy. A systematic approach of this kind will result in precise correlation of the spectral parameters of M-BLMs with their coordination structures. Our studies will also reveal the mode of binding of the various donor groups of BLM to different metals in different oxidation states. Conditions for free radical generation by these analogues when reacted with O2, H2O2, or light will be set and the nature of the free radical(s) will be studied by spin- trapping techniques. The structures of the radical-forming metallospecies will also be explored by spectroscopy. An intercalator (tailed ethidium) will be attached to specific analogues to direct the radical(s) to DNA substrates. The pattern and course of DNA damage and site specificity (if any) will be monitored by gel electrophoresis using restriction fragments of plasmid DNA. Results from these experiments will (i) ascertain the mode of drug action of M-BLMs (and possibly other metal-ion-induced free radical-forming drugs) and (ii) will generate a set of DNA-specific inorganic drugs that induces DNA strand breaks. The results will also be useful in the future design of new metallodrugs for chemotherapy.
