apaziquone and hydroquinone

The factors influencing the kinetics of the oxygen-sensitive reduction of indolequinones, including those bearing leaving groups in the (indol-3-yl)methyl position, have been studied. The hydroquinones derived from some representative indolequinones were found to autoxidize slowly in oxygenated solution at rates (effective rate constant with O2 approximately 40-300 M-1 s-1) that cannot compete with the reductive elimination of leaving groups. The rates of reaction between hydroquinone and O2 were even slower in the presence of approximately 4 microM superoxide dismutase (effective rate constant approximately 2-7 M-1 s-1), indicating the role of superoxide radicals in hydroquinone autoxidation. Since the release of the leaving groups from the hydroquinones is not significantly oxygen-sensitive, tumour selectivity requires specific reduction by enzymes that are overexpressed in some tumours. Conversely, the release of leaving groups from semiquinone radicals is inhibited by oxygen too efficiently unless the semiquinone reacts with targets on a timescale of milliseconds. Modification of redox properties has been explored with the aim of changing this oxygen sensitivity. The new 2-phenylindolequinones are approximately 60-100 mV higher in reduction potential than 2-alkyl derivatives but this is insufficient to decrease the rate of electron transfer from semiquinone to oxygen to a degree which might confer hypoxia-selective cytotoxicity. These results are discussed in the context of toxicity of EO9 and related compounds towards hypoxic rather than anoxic cells.

Topics: Antineoplastic Agents; Aziridines; Cell Hypoxia; Chromatography, High Pressure Liquid; Humans; Hydroquinones; Indolequinones; Indoles; Kinetics; Molecular Structure; Oxidation-Reduction; Oxygen; Pulse Radiolysis; Quinones; Structure-Activity Relationship; Superoxide Dismutase; Superoxides

The properties of the semiquinone radical from [3-hydroxy-5-aziridinyl-1-methyl-2-(1H-indole-4,7-indi one)-prop-beta-en-alpha-ol], EO9, have been studied using pulse-radiolysis techniques. The reduction potential of the semiquinone of EO9 at pH7.4, E(EO9/EO9-), is -253 +/- 6 mV and hence this quinone can be readily reduced by one-electron reducing enzymes such as cytochrome P450 reductase and xanthine oxidase. However, the radical is unstable in the presence of oxygen (k = 1.3 +/- 0.15 x 10(8) M-1 s-1). The semiquinone radicals and the hydroquinone are in equilibrium although the formation of the hydroquinone is favoured t physiologically relevant pH. The hydroquinone of EO9 is also unstable in the presence of oxygen and it is predicted that in fully aerated solutions, its half life is 1.5 +/- 0.3 seconds. These results are discussed in view of the selective cytotoxicity of EO9 and its ability to undergo bioreductive activation by one-electron reducing enzymes and DT-diaphorase.

Topics: Antineoplastic Agents; Aziridines; Drug Stability; Free Radicals; Half-Life; Hydrogen-Ion Concentration; Hydroquinones; Indolequinones; Indoles; Oxidation-Reduction; Oxygen; Pulse Radiolysis; Spectrophotometry