Study of the Reactions of Platinum Macrocyclic Complex Ions with Free Radicals Formed by Pulse Radiolysis of Aqueous Media

Radiosensitization of DNA by Cisplatin Adducts Results from an Increase in the Rate Constant for the Reaction with Hydrated Electrons and Formation of Pt(I)

Pulse radiolysis measurements of the decay of hydrated electrons in solutions containing different concentrations of the oligonucleotide GTG with and without a cisplatin adduct show that the presence of a cisplatin moiety accelerates the reaction between hydrated electrons and the oligonucleotide. The rate constant of the reaction is found to be 2.23 × 10(10) mol(-1) L s(-1), which indicates that it is diffusion controlled. In addition, we show for the first time the formation of a Pt(I) intermediate as a result of the reaction of hydrated electrons with GTG-cisplatin. A putative reaction mechanism is proposed, which may form the basis of the radiosensitization of cancer cells in concomitant chemoradiation therapy with cisplatin.

Free Radical-Induced Redox Chemistry of Platinum-Containing Anti-cancer Drugs

Arrhenius parameters for the reactions of oxidizing hydroxyl radicals and reducing hydrated electrons with cisplatin, transplatin and carboplatin in aqueous solution have been determined using pulsed electron radiolysis and absorption spectroscopy techniques. Under physiological pH and chloride concentration conditions, hydroxyl radical reaction rate constants of (9.99 +/- 0.20) x 10(9), (8.38 +/- 0.55) x 10(9), and (6.03 +/- 0.08) x 10(9) M(-1) s(-1) at 24.0, 20.7 and 24.0 degrees C, respectively, with corresponding activation energies of 12.79 +/- 0.57, 13.88 +/- 1.14, and 14.35 +/- 0.56 kJ mol(-1) for these three reactions, were determined. These oxidations of cisplatin and transplatin to form a Pt(III) transient are significantly faster than reported previously at room temperature. The lower rate constant for carboplatin is consistent with hydroxyl radical reaction partitioning between reaction at the platinum center and the cyclobutanedicarboxylate ligand. The equivalent reductive hydrated electron reaction rate constants measured were (1.99 +/- 0.04) x 10(10) (24.0 degrees C), (1.77 +/- 0.08) x 10(10) (22.0 degrees C), and (8.92 +/- 0.06) x 10(9) M(-1) s(-1) (24.0 degrees C), with corresponding activation energies of 15.75 +/- 1.00, 19.74 +/- 1.82, and 19.99 +/- 0.34 kJ mol(-1). Again, the values determined for cisplatin and transplatin are faster than reported; however, all three values are consistent with direct reduction of the platinum center to form a Pt(I) moiety.