A kinetic analysis of oxidation of the antioxidant <Emphasis Type="Italic">N</Emphasis>-acetyl-<Emphasis Type="SmallCaps">l</Emphasis>-cysteine (NAC) by Pt(IV) complexes

N-acetyl-l-cysteine (NAC) is an antioxidant and a supplement and has been demonstrated to have protective effects for a variety of toxic effects of heavy metals. Although previous works have shown that NAC can ameliorate the severe toxic effects of cisplatin, there is a lack of understanding of the interactions between NAC and Pt(IV)-based prodrugs. In this work, the oxidation of NAC by a cisplatin prodrug (cis-Pt(NH₃)₂Cl₄]), by a prototype of Pt(IV) anticancer drug ormaplatin (Pt(dach)Cl₄]) and by a model compound (trans-PtCl₂(CN)₄]^2–) was characterized in detail. NAC was oxidized to NAC-disulfide as identified by mass spectrometric analysis. Time-resolved spectral and stopped-flow kinetic measurements were carried out over a wide pH range, demonstrating that the oxidation followed overall second-order kinetics. The observed second-order rate constants k′ versus pH profiles were established. A reaction mechanism was deduced, involving three parallel rate-determining steps; conceivable transition states were also proposed for these steps. Rate constants of the rate-determining steps, obtained from the simulations of rate equation to the k′–pH profiles, were largely correlated with the electron density on the sulfur atom in NAC. The Pt(IV) prodrugs can execute oxidative stress in the biological systems of the human body by direct oxidation of relevant molecules, similar to HOCl/OCl^? and chloroamines. Instead, the oxidative stress involved in the severe toxic effects of cisplatin is produced via a different mode. NAC could be a chemoprotecting agent also for the Pt(IV) anticancer drugs if recent drug delivery technologies are used.