Characterization of the mechanism of reduction of trans-diamminetetrachloroplatinum(IV) by l-cysteine and dl-homocysteine

Abstract

The interactions between Pt(IV) anticancer prodrugs incorporating two ammines/amines in trans positions in their equatorial planes and some important thiols have not been exploited to date. In this work, the reduction of one such Pt(IV) prodrug, namely trans-[Pt(NH3)2Cl4], by two thiol-containing amino acids l-cysteine (Cys) and dl-homocysteine (Hcy) which are prevalent in human plasma has been characterized by stopped-flow spectroscopic and ESI high-resolution mass spectral methods. The reduction process obeys overall second-order kinetics. The dependencies of the observed second-order rate constants k′ on pH have been established between pH 4.03 and 11.24. Mass spectral analysis indicates that cystine and homocystine are the dominant products for the Cys and Hcy oxidations, respectively. The suggested reaction mechanism involves all possible protolytic species of Cys/Hcy, which attack one of the two apically coordinated chlorides in parallel (all as rate-determining steps), leading to a Cl+ transfer to the attacking sulfur atom. The rate expression has been derived, and the rate constants for the rate-determining steps have been calculated. Features of the reduction process are discussed based on the obtained rate constants. The overall kinetic and mechanistic picture enables an in-depth understanding of the reduction process of this type of Pt(IV) anticancer prodrug.