The reductions of Pt(iv) anticancer prodrugs [Pt(dach)Cl4] (ormaplatin/tetraplatin), cis-[Pt(NH3)2Cl4], and cis,cis,trans-[Pt(NH3)2Cl2Br2] by the several dominant reductants in human plasma have been characterized kinetically in this work, including l-ascorbic acid (Asc), l-glutathione (GSH), l-cysteine (Cys), dl-homocysteine (Hcy), and a dipeptide Gly-Cys. All the reductions follow an overall second-order kinetics, being first-order each in [Pt(iv)] and in the [reductant]. A general reactivity trend of Asc < Hcy < Cys-Gly < GSH < Cys is clearly revealed for the reductions of [Pt(dach)Cl4] and [Pt(NH3)2Cl4] at 37.0 °C and pH 7.40. Analysis of the observed second-order rate constants k' implies that these Pt(iv) prodrugs have a very short lifetime (less than a minute) in human plasma and can hardly enter into cells before reduction and that Asc might not play a dominant role in the reduction process among the reductants. The reductions of [Pt(dach)Cl4] and [Pt(NH3)2Cl4] by Asc have been studied in a wide pH range, and a reaction mechanism has been proposed involving parallel reductions of the Pt(iv) complexes by the Asc protolytic species. Moreover, a halide-bridged (inner-sphere) electron transfer mode for the rate-determining steps is discussed in detail; several lines of evidence strongly bolster this type of electron transfer. Furthermore, the observed activation parameters corresponding to k' have been measured around pH 7.40. Analysis of the established k'-pH profiles indicates that k' is a composite of at least three parameters in the pH range of 5.74-7.40 and the measured activation parameters in this range do not correspond to a single rate-determining step. Consequently, the isokinetic relationship reported previously using the measured ΔH(‡) and ΔS(‡) in the above pH range might be an artifact since the relationship is not justified anymore when our new data are added.

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A complex reaction mechanism of oxidation of the anti-tubercular prodrug isoniazid (isonicotinic hydrazide, INH) by [IrCl] as a model for redox processes of such drugs in biological systems has been studied in aqueous solution as a function of pH between 0 and 8.5. Similar experiments have been performed with its isomer nicotinic hydrazide (NH). All reactions are overall second-order, first-order in [IrCl] and hydrazide, and the observed second-order rate constants k' have been determined as a function of pH. Spectrophotometric titrations indicate a stoichiometry of [Ir(iv)] : [hydrazide] = 4 : 1. HPLC analysis shows that the oxidation product of INH is isonicotinic acid. The derived reaction mechanism, based on rate law, time-resolved spectra and stoichiometry, involves parallel attacks by [IrCl] on all four protolytic species of INH and NH as rate-determining steps, depending on pH. These steps are proposed to generate two types of hydrazyl free radicals. These radicals react further in three rapid consecutive processes, leading to the final oxidation products. Rate constants for the rate-determining steps have been determined for all protolytic species I-IV of INH and NH. They are used to calculate reactivity-pH diagrams. These diagrams demonstrate that for both systems, species IV is ca. 10 times more reactive in the redox process than the predominant species III at the physiological pH of 7.4. Thus, species IV will be the main reactant, in spite of the fact that its concentration at this pH is extremely low, a fact that has not been considered in previous work. The results indicate that pH changes might be an important factor in the activation process of INH in biological systems also, and that in such systems this process most likely is more complicated than previously assumed.

L-Selenomethionine (SeMet), the predominant form of selenium acquired from the diet by humans, has been used as a supplement, and exhibit some important functions like cancer prevention and antioxidative defense. Its interactions with Pt(II) anticancer drugs have been characterized, but its redox reactions with platinum(IV) anticancer prodrugs have not been exploited. In this work, the oxidation of SeMet by Pt(IV) anticancer model compounds trans-[PtX2(CN)4](2-) (X = Cl, Br) was characterized. A stopped-flow spectrometer was used to record the rapid scan spectra and to follow the reaction kinetics over a wide pH range. An overall second-order rate law was derived: -d[Pt(IV)]/dt = k'[Pt(IV)][SeMet], where k' pertains to the observed second-order rate constants. The k'-pH profiles showed that k' increased only about 6 times even though the solution pH was varied from 0.25 to 10.5. The redox stoichiometry was determined as Δ[Pt(IV)]/Δ[SeMet] = 1 : (1.07 ± 0.07), suggesting that SeMet was oxidized to selenomethionine selenoxide. The selenoxide together with its hydrated form was identified explicitly by high resolution mass spectral analysis. A reaction mechanism was proposed which encompassed three parallel rate-determining steps relying on the protolytic species of SeMet. Rate constants of the rate-determining steps were obtained from the simulations of the k'-pH profiles. Activation parameters were determined for the reactions of the zwitterionic form of SeMet with the Pt(IV) complexes. A bridged electron transfer process is delineated in the rate-determining steps and several lines of evidence support the bridged electron transfer mode. Strikingly, reduction of [PtX2(CN)4](2-) by SeMet is 3.7 × 10(3)-5.7 × 10(4) times faster than that by L-methionine. Some potential biological consequences resulting from the strikingly fast reduction are discussed.

Pt(IV) anticancer active complexes are commonly regarded as prodrugs, and the reduction of the prodrugs to their Pt(II) analogs is the activation process. The reduction of a cisplatin prodrug cis-[Pt(NH 3 ) 2 Cl 4 ] and a carboplatin prodrug cis,trans-[Pt(cbdca)(NH 3 ) 2 Cl 2 ] by DL-homocysteine (Hcy) has been investigated kinetically in a wide pH range in this work. The reduction process follows overall second-order kinetics: −d[Pt(IV)]/dt = k [Hcy] tot [Pt(IV)], where [Hcy] tot stands for the total concentration of Hcy and k pertains to the observed second-order rate constants. The k versus pH profiles have been established for both prodrugs. Spectrohotometric titrations reveal a stoichiometry of [Pt(IV)]: [Hcy] tot = 1:2; homocystine is identified as the major oxidation product of Hcy by high-resolution mass spectrometry. A reaction mechanism has been proposed, which involves all the four protolysis species of Hcy attacking the Pt(IV) prodrugs in parallel. Moreover, these parallel attacks are the rate-determining steps, resulting in a Cl + transfer from the Pt(IV) prodrugs to the attacking sulfur atom. Rate constants An Accumet Basic AB15 Plus pH meter, equipped with an Accumet AccutupH R combination pH electrode (Fisher Scientific, Pittsburgh, PA), was used to measure the pH values of buffer solutions. Each time, the electrode was calibrated using the standard buffers

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