Shuying Huo

Characterization of the reaction products, kinetics and mechanism of oxidation of the drug captopril by platinum( iv ) complexes

Captopril, the first pharmaceutical drug designed, synthesized, and used primarily for the treatment of hypertension and congestive heart failure, is an angiotensin-converting-enzyme inhibitor and is also an antioxidant. On the other hand, the kinetic and mechanistic aspects for the oxidation of captopril are not well understood. The oxidation of captopril by cisplatin prodrug and a model compound, cis-[Pt(NH3)2Cl4] and trans-[PtCl2(CN)4]2−, was thus investigated in this work. A stopped-flow spectrometer was employed to follow the oxidation kinetics over a wide pH-range under the pseudo first-order conditions of [captopril] ≫ [Pt(IV)]. The oxidation by the Pt(IV) complexes displayed a second-order character, first-order each in [Pt(IV)] and in [captopril], whereas the metal-ion-catalyzed autooxidation made a very minor contribution to the overall kinetics in acidic media and was negligible in neutral media. Captopril was oxidized to form the captopril-disulfide as identified by ESI mass spectrometry under the conditions of the kinetic measurements. In the proposed reaction mechanism, the Pt(IV) complexes are reduced by the three protolytic captopril species in parallel as rate-determining steps, generating reactive species of chlorothiol and/or sulfenylchloride. The reactive species will be rapidly trapped, either directly or indirectly, by another molecule of captopril to form captopril-disulfide. The rate constants for the rate-determining steps have been derived, demonstrating that the fully deprotonated captopril is about 105 to 106 times more reactive than its corresponding thiol form toward the Pt(IV) complexes.

Kinetic characterization of the interactions of trans-dichloro-platinum(IV) anticancer prodrugs and a model compound with thiosulfate

Sodium thiosulfate has been utilized as a rescuing agent for relief of the toxic effects of cisplatin and carboplatin. In this work, we characterized the kinetics of reactions of the trans-dichloro-platinum(IV) complexes cis-[Pt(NH3)2Cl4], ormaplatin [Pt(dach)Cl4] and trans-[PtCl2(CN)4]2− (anticancer prodrugs and a model compound) with thiosulfate at biologically important pH. An overall second-order rate law was established for the reduction of trans-[PtCl2(CN)4]2− by thiosulfate, and varying the pH from 4.45 to 7.90 had virtually no influence on the reaction rate. In the reactions of thiosulfate with cis-[Pt(NH3)2Cl4] and with [Pt(dach)Cl4], the kinetic traces displayed a fast reduction step followed by a slow substitution involving the intermediate Pt(II) complexes. The reduction step also followed second-order kinetics. Reductions of cis-[Pt(NH3)2Cl4] and [Pt(dach)Cl4] by thiosulfate proceeded with similar rates, presumably due to their similar configurations, whereas the reduction of trans-[PtCl2(CN)4]2− was about 1,000 times faster. A common reduction mechanism is suggested, and the transition state for the rate-determining step has been delineated. The activation parameters are consistent with transfer of Cl+ from the platinum(IV) center to the attacking thiosulfate in the rate-determining step.

Reduction of ormaplatin by a dithiol model compound for the active site of thioredoxin: stopped-flow kinetic analysis

Ormaplatin ([Pt(dach)Cl4]) is a prototypical of Pt(IV) anticancer prodrug; its reduction by 3,6-dioxa-1,8-octanedithiol (DODT, a model compound for the active site of thioredoxin), has been characterized at 25.0xa0°C and in a wide pH range by rapid scan spectral and kinetic analysis. The reduction follows overall second-order kinetics, being first-order each in [Pt(IV)] and in [DODT]; the observed second-order constant k′ versus pH profile has been established. A 1:1 reaction stoichiometry is obtained, indicating that DODT is oxidized to its intramolecular disulfide; these characteristics are consistent with the redox properties of thioredoxin. A reaction mechanism has been proposed which involves the three protolytic species of DODT attacking [Pt(dach)Cl4] in parallel as rate-determining steps. The rate equation is derived from the reaction mechanism and rate constants of the rate-determining steps have been evaluated. The thiol–thiolate and dithiolate forms of DODT are 105–106 times more reactive than the dithiol form. Comparisons of the rate constants highlight that the electron density of the sulfur atom plays a key role in determining the reactivity of DODT toward ormaplatin and similar Pt(IV) compounds. It is concluded that the reduced thioredoxin is a strong competitor to other thiol-containing compounds in the reduction processes of Pt(IV) anticancer prodrugs.

Reactivity of the glutathione species towards the reduction of ormaplatin (or tetraplatin)

The reduction of ormaplatin (tetraplatin), a prototype for Pt(IV) anticancer prodrugs, by glutathione (GSH) was kinetically characterized over a wide pH range at 25.0°C and 1.0M ionic strength. The reduction follows overall second-order kinetics, giving rise to the oxidized glutathione as the oxidation product, which was identified by high-resolution mass spectrometry. The reaction mechanism put forward involves parallel attacks by all the GSH species on the Pt(IV) prodrug as rate-determining steps. All rate constants for the rate-determining steps have been derived for the first time, enabling the construction of the reactivity of GSH species versus their pH distribution diagram. The diagram clearly displays that only one out of the five GSH species is the mainly responsible for the reduction of ormaplatin at the physiological pH of 7.4.

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

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.

A platinum(IV)-based metallointercalator: synthesis, cytotoxicity, and redox reactions with thiol-containing compounds

A Pt(IV)-based metallointercalator, trans-dichloro(1,10-phenanthroline)(ethylenediamine)platinum(IV) trans-[PtCl2(phen)(en)]Cl2, was synthesized and characterized by 1H NMR, ESI–MS, and elemental analysis. Three human carcinoma cell lines Hela, MCF-7, and A549 were employed for in vitro cytotoxicity evaluations using the standard MTT assay. Based on the measured IC50 values, the cytotoxicity of this metallointercalator is very similar to that of cisplatin. Reduction of trans-[PtCl2(phen)(en)]2+ by l-glutathione and 3,6-dioxa-1,8-octanedithiol was studied by 1H NMR and ESI–MS; the Pt(IV) complex was reduced to its Pt(II) counterpart. The oxidation products of l-glutathione and 3,6-dioxa-1,8-octanedithiol were identified as the glutathione disulfide and the cyclic intramolecular disulfide, respectively. Mechanisms for these reductions are proposed.

Kinetics and mechanism of oxidation of the drug intermediate 1-(2-Hydroxyethyl)piperidine by Bis(hydrogenperiodato)argentate(III)

1-(2-Hydroxyethyl)piperidine (HEP) is involved in many drugs and drug leads, but its oxidation mechanisms are poorly understood. The oxidation of HEP by bis(hydrogenperiodato)-argentate(III) ([Ag(HIO6)2]5-;) in aqueous alkaline medium was shown, by electrospray ionization mass spectrometry (ESI-MS), to generate piperidine and formaldehyde as the major products. The reaction was monitored spectrophotometrically in the 25.0 to 40.0 oC range revealing that the oxidation followed a first-order kinetics in [Ag(III)] and a fractional-order in [HEP]. A rate law and a reaction mechanism were proposed based on the study of the dependency of the pseudo-first-order rate constants, kobsd, on [OHˉ] and on [IO4ˉ]tot (total concentration of periodate). The mechanism involves the formation of a periodato-Ag(III)-HEP ternary complex, whose decomposition generates Ag(I) by means of two pathways: one independent and another facilitated by OHˉ. The reaction rate constants and associated equilibrium constants as well as the activation parameters of the rate-determining steps were calculated.

Synthesis, characterization, and cytotoxicity of Pt(IV) complexes containing 1,10-phenanthroline and 2,2′-bipyridine and diaminocyclohexane ligands

Four platinum(IV) complexes containing intercalating ligands [1,10-phenanthroline (phen) and 2,2′-bipyridine (bpy)] and ancillary ligands [(1S,2S)-diaminocyclohexane (SS-DACH) and (1R,2R)-diaminocyclohexane (RR-DACH)] were synthesized and characterized by 1H nuclear magnetic resonance, electrospray ionization mass spectrometry, X-ray crystal structure analysis, elemental analysis, ultraviolet absorption spectroscopy, circular dichroism spectroscopy, and electrochemical analysis. The reactions between [Pt(phen)(SS-DACH)Cl2]2+ and glutathione and Ac-CPFC-NH2 were investigated by high-performance liquid chromatography. [Pt(phen)(SS-DACH)Cl2]2+ was reduced to its corresponding Pt(II) complex [Pt(phen)(SS-DACH)]2+, while glutathione and Ac-CPFC-NH2 were oxidized to glutathione-disulfide and a peptide containing an intramolecular disulfide bond, respectively. The cytotoxicities of the Pt(IV) complexes against a human non-small cell lung cancer cell line (A549) and the corresponding cisplatin-resistant cell line (A549cisR) were evaluated. These Pt(IV) complexes showed a higher activity toward A549 and A549cisR than did cisplatin. Also, the cytotoxicities of the Pt(IV) complexes were higher for A549cisR than for A549 cells. Moreover, the cytotoxicities of the (SS-DACH)-liganded platinum complexes were higher than those of the (RR-DACH)-liganded platinum complexes in either A549 or A549cisR cells. Phen-liganded platinum complexes were more cytotoxic than the bpy-liganded platinum complexes. The cytotoxicities of these Pt(IV) complexes had no correlation with reduction potentials.

A kinetic analysis of oxidation of the antioxidant N-acetyl-l-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.

Kinetics and mechanism of oxidation of 3,6-dioxa-1,8-octanedithiol and dl-dithiothreitol by a platinum(IV) complex

A Pt(IV) complex [PtCl2(en)(heda)]Cl2 (hedaxa0=xa0N-(2-hydroxyethyl)-ethylenediamine) has been synthesized and characterized by ESI–MS, 1H NMR, elemental analysis, and X-ray crystal structure analysis. Oxidations of dithiol compounds 3,6-dioxa-1,8-octanedithiol (DODT) and dl-dithiothreitol (DTT) by this complex were studied. The oxidation products were characterized as ten- and six-membered ring compounds for DODT and DTT, respectively, both containing an intramolecular disulfide bond. The kinetics of the oxidation reactions was followed by stopped-flow spectrophotometry over a wide pH range. The oxidations display an overall second-order character, being first-order each in [Pt(IV)] and in [dithiol]. A mechanism involving three parallel rate-determining steps is proposed, and the rate constants of the rate-determining steps have been evaluated.