Despite its efficient antitumor activity against a wide spectrum of human cancers, the clinical use of the antibiotic streptonigrin is severely limited by its strong cytotoxic side effects. Structural characterization and mechanistic elucidation of the drug is a prerequisite for the development of a rational approach to improve the chemotherapeutic properties of the antibiotic. Although the mechanism of antitumor activity of streptonigrin is unknown at present, it is generally agreed that the semiquinone radical intermediate, other radical intermediates involving perhaps oxygen and metals including the paramagnetic ions Co++, Cu++, and Mn++ play a key role. The proposed project will employ electron paramagnetic resonance to detail the structure of the semiquinone radical as a function of solvent system, search for and, if possible, identify other free radical intermediates, and study the role that the metals play. Substituted 5,8-quinolinequinones will be synthesized to be studied as streptonigrin models and isotopic substitution will be employed to aid in the structural determinations. Hyperfine coupling constants will be assigned from which spin densities will be calculated. Electron spin echospectroscopy and an extension of the method of Leigh, to measure metal-radical distances, will be employed to add further structural detail. Finally, the structure of the complexes of the diamagnetic ions Cd++ and Zn++ of the semiquinone will be characterized by electron paramagnetic resonance. Isotopically enriched Cd113 will be employed to measure the superhyperfine coupling to this nucleus and to assess the degree of complexation. Structural elucidation of the free radicals and their metal complexes will provide an important tool to study the redox chemistry of the drug. Development of the electron spin echo and Leigh methodologies may prove to be useful in the study of other similar antibiotics.
