This proposal describes a mechanistic study of the process of reductive alkylation by which many synthetic and naturally occurring quinones are proposed to exert cytotoxicity. Also described is the design of new reductive alkylating quinones having the active sites of purine-utilizing enzymes as their target. Reductive alkylation is carried out by certain quinone systems containing an appropriately positioned leaving group when in their hydroquinone forms. Alkylation could occur either by formation of a hydroquinone stablized carbonium ion or a reactive quinone methide. Both mechanisms have been proposed for the antitumor antibiotic Mitomycin C which when activated by an intracellular quinone reductase preferably alkylates the DNA of hypoxic tumor cells. Cytotoxicity of Mitomycin C and other quinone antibiotics may also pertain to the generation of reactive oxygen species by cycling between the hydroquinone and quinone forms via a semiquinone intermediate. In this proposal the possible formation of a quinone methiode from the semiquinone form by elimination of the leaving group as a radical is considered. This mechanism is consistent with observations cited in the literature and could unify the two postulated mechanisms for cytotoxicity mentioned above. Kinetic methodologies for establishing the presence of a quinone methide or carbonium ion in various hydroquinone systems under non-biological conditions are presented. From this study a knowledge of the structural requirements for reductive alkylation will be gained with which new antitumor agents may designed. The imidazo [4,5-g] quinazoline-4,9-dione system may be described as an extended purine able to be functionalized to act as a reductive alkylator. Based on precedents, entry of this system into the active sites of xanthine oxidase or guanase is proposed. The potential utility of these compounds in cancer chemotherapy is that alkylation of these enzymes will occur only under reducing conditions. Since certain purine antitumor agents are degraded by these enzymes, such agents will be potentiated in hypoxic tumors only. Similarly, reductive alkylators of key enzymes in purine de novo synthesis may be designed. This aspect of the project will involve synthetic, kinetic, and enzyme binding studies.
| Islam, I; Skibo, E B; Dorr, R T et al. (1991) Structure-activity studies of antitumor agents based on pyrrolo[1,2-a]benzimidazoles: new reductive alkylating DNA cleaving agents. J Med Chem 34:2954-61 |
| Dempcy, R O; Skibo, E B (1991) Rational design of quinazoline-based irreversible inhibitors of human erythrocyte purine nucleoside phosphorylase. Biochemistry 30:8480-7 |
| Lee, C H; Skibo, E B (1987) Active-site-directed reductive alkylation of xanthine oxidase by imidazo[4,5-g]quinazoline-4,9-diones functionalized with a leaving group. Biochemistry 26:7355-62 |
| Skibo, E B; Gilchrist, J H; Lee, C H (1987) Electronic probes of the mechanism of substrate oxidation by buttermilk xanthine oxidase: role of the active-site nucleophile in oxidation. Biochemistry 26:3032-7 |
| Skibo, E B (1986) Noncompetitive and irreversible inhibition of xanthine oxidase by benzimidazole analogues acting at the functional flavin adenine dinucleotide cofactor. Biochemistry 25:4189-94 |
