Kinetic Investigation Of Reductive Activation Processes Of Pt (IV) Anticancer Prodrugs

To date,50%of chemotherapeutic drugs are platinum-based anticancer drugs.Therefore,platinum anticancer drugs play an important role in the treatment of cancer.However,platinum（II）anticancer drugs such as cisplatin,carboplatin,oxaliplatin have serious toxic side effects and drug resistance in clinic.Pt（IV）anticancer drugs with octahedral configuration can reduce this side effects and drug resistance.Therefore,designing and synthesizing low-toxic,high-activity Pt（IV）anticancer drugs has became a hot research field.Reductive activation of the Pt（IV）anticancer drug was occoured firstly when it play an anticaner activity.Therefore,the study of reduction process of Pt（IV）anticancer drug is important for designing a highly effective Pt（IV）anticancer drugs.Cysteine,homocysteine and glutathione are the three most important thiols in the human body and play a leading role in the reductive activation of Pt（IV）anticancer drugs.In this work,two Pt（IV）complexes,cis,cis,trans-[Pt（NH₃）₂Cl₂Br₂]and cis,cis,trans-[Pt（dach）Cl₂（OOCCH₃）Cl]were synthesized.We here reported a stopped-flow kinetic study of the reduction of two Pt（IV）complexes by cysteine,homocysteine and glutathione.The reaction products were identified by ESI-MS.Reaction stoichiometry was determined by spectrophotometric titration.Finally,the reaction mechanism of the Pt（IV）reduction was proposed.The Values of rate constant of the rate determining steps were calculated by a curve fitting method.Kinetic studies demonstrated that the reactions between three thiols and cis,cis,trans-[Pt（NH₃）₂Cl₂Br₂]and cis,cis,trans-[Pt（dach）Cl₂（OOCCH₃）Cl]are overall second-order with the first-order in[Pt（IV）]and[thiol]_tot.The second-order rate constants k′were increased dramatically with the increase in pH value of reaction media.Thiols were oxidized to their intermolecular disulfides.Mechanisms were proposed for the reductions by analysis of the reaction rate and products.When cis,cis,trans-[Pt（NH₃）₂Cl₂Br₂]reduced by thiols,a“bromine-bridge”was formed by-SH or-S^-interaction with axial bromide ligand of Pt（IV）complex,and then electron transfer was occurred,followed by generation of transient bromothiol and sulfenylbromide species,which were reacted with the excess of thiols,leading to formation of thiol-disulfides.The Pt（IV）complex was reduced to cisplatin.When cis,cis,trans-[Pt（dach）Cl₂（OOCCH₃）Cl]reduced by thiols,-SH or-S^-was interacted with axial chloride ligand of Pt（IV）complex affording a“chloride-bridge”,by which electron transfer was occurred,giving the transient bromothiol and sulfenylbromide species,which were reacted with the excess of thiols,leading to formation of thiol-disulfides.At the same time,the Pt（IV）complex losed its axial chloride ligand and acetate ligand.Overall second-order rate laws were deduced from the reaction mechanisms.Values of rate constant of the rate determining steps were obtaind by fitting k′-a_H.The reactivity of cysteinie,homocysteine and glutathione for reduction of cis,cis,trans-[Pt（NH₃）₂Cl₂Br₂]and cis,cis,trans-[Pt（dach）Cl₂（OOCCH₃）Cl]were compared by the second-order rate constants.A reactivity trend of k′_Cys>k′_GSH>k′_Hcy is found,illustrating that the reactivity is Cys>GSH>Hcy.By comparing the values of rate constant of the rate determining steps of reduction of several Pt（IV）complexes by three thiols,reduction rate was found to be related to the axial ligand and non-leaving ligand of the Pt（IV）complex.When the two axial positions of a Pt（IV）compelx were occupied by Br^-,Cl^-and CH₃COO^-,the reduction rate was increased with the trend of Br^->Cl^->CH₃COO^-.Non-leaving ligands influence the reduction rate with the trend of thiazole>cyclohexylamine≈NH₃1,2-diaminocyclohexane.