Oxidation of guanosine derivatives by a platinum(IV) complex: internal electron transfer through cyclization

Platinum(IV) Complexes of the 1,3,5-Triamino Analogue of the Biomolecule Cis-Inositol Designed as Innovative Antineoplastic Drug Candidates

Metal complexes occupy a special place in the field of treatment and diagnostics. Their main advantages stem from the possibility of fine-tuning their thermodynamic properties and kinetic behavior in the biological milieu by applying different approaches such as properly constructed inner coordination sphere, appropriate choice of ligands, metal oxidation state, redox potential, etc., which are specific to these compounds. Here we discuss the design and synthesis of two octahedral cationic Pt(IV) complexes of the tridentate ligand all-cis-2,4,6-triaminocyclohexane-1,3,5-triol (taci) with composition, fac-[Pt(taci)I3]+, 1 and bis-[Pt(taci)2]4+, 2 as well as the potential for their application as antineoplastic agents. The complexes have been isolated in a solid state as: fac-[Pt(taci)I3]I·3H2O (1A), fac-[Pt(taci)I3]I (1B), fac-[Pt(taci)I3]I·2DMF (1C), bis-[Pt(taci)2](CO3)2·6H2O (2A) by changing the acidity of the reaction systems, the molar ratios of the reagents and the counterions, and by re-crystallization. The ligand taci is coordinated through the NH2-groups, each molecule occupying three coordination places in the inner coordination sphere of Pt(IV). Monitoring of the hydrolysis processes of 1A and 2A at different acidity showed that while 2A remained stable over the study period, the I--ions in 1A were successively substituted, with the main product under physiologically mimetic conditions being fac,cis-[Pt(taci)I(OH)2]+ (h2). The antiproliferative tests involved eight cancer cell models, among which chemosensitive (derived from leukemias and solid tumors) and chemoresistant human Acute myeloid leukemia lines (HL-60/Dox, HL-60/CDDP), as well as the non-malignant kidney' cells HEK-293T showed that the complexes 1A and 2A are characterized by a fundamentally different profile of chemosensitivity and spectrum of cytotoxic activity compared to cisplatin. The new Pt(IV) complexes were shown to be more effective in selectively inhibiting the proliferation of human malignant cells compared to cisplatin. Remarkable activity was recorded for 1A, which showed an effect (IC50 = 8.9 ± 2.4) at more than 16-fold lower concentration than cisplatin (IC50 = 144.4 ± 9.8) against the resistant cell line HL-60/CDDP. In parallel, 1A exhibited virtually the same cytotoxic effect against the parental HL-60 cells (IC50 = 9.0 ± 1.2), where cisplatin displays comparable chemosensitivity (IC50 = 8.3 ± 0.8). The determined resistance indices (RI~1) show unequivocally that the resistant lines are sensitive to both compounds tested; therefore, they are capable of overcoming the mechanisms of cisplatin resistance. The structural features of these compounds and their promising pharmacological properties justify their inclusion in the group of "non-classical metal-based antitumor compounds" and are a prerequisite for the admission of alternative mechanisms of action.

Glycation of Lys-16 and Arg-5 in amyloid-β and the presence of Cu2+ play a major role in the oxidative stress mechanism of Alzheimer's disease

Extensive research has linked the amyloid-beta (Aβ) peptide to neurological dysfunction in Alzheimer's disease (AD). Insoluble Aβ plaques in the AD patient brain contain high concentrations of advanced glycation end-products (AGEs) as well as transition metal ions. This research elucidated the roles of Aβ, sugars, and Cu²⁺ in the oxidative stress mechanism of AD at the molecular level. Mass spectral (MS) analysis of the reactions of Aβ with two representative sugars, ribose-5-phosphate (R5P) and methylglyoxal (MG), revealed Lys-16 and Arg-5 as the primary glycation sites. Quantitative analysis of superoxide [Formula: see text] production by a cyt c assay showed that Lys-16 generated four times as much [Formula: see text] as Arg-5. Lys-16 and Arg-5 in Aβ_1-40 are both adjacent to histidine residues, which are suggested to catalyze glycation. Additionally, Lys-16 is close to the central hydrophobic core (Leu-17-Ala-21) and to His-13, both of which are known to lower the pKa of the residue, leading to increased deprotonation of the amine and an enhanced glycation reactivity compared to Arg-5. Gel electrophoresis results indicated that all three components of AD plaques-Aβ_1-40, sugars, and Cu²⁺-are necessary for DNA damage. It is concluded that the glycation of Aβ_1-40 with sugars generates significant amounts of [Formula: see text], owing to the rapid glycation of Lys-16 and Arg-5. In the presence of Cu²⁺, [Formula: see text] converts to hydroxyl radical (HO·), the source of oxidative stress in AD.

Side Reactions with an Equilibrium Constraint: Detailed Mechanism of the Substitution Reaction of Tetraplatin with dGMP as a Starting Step of the Platinum(IV) Reduction Process

Two possible pathways of the substitution reaction within the reduction process of the Pt^IV(DACH)Cl₄ by dGMP are compared: associative reaction course and autocatalytic Basolo-Pearson mechanisms. Since two forms: single-protonated and fully deprotonated phosphate group of dGMP are present in equilibrium at neutral and mildly acidic solutions, consideration of a side reactions scheme with acido-basic equilibrium-constraint is a very important model for obtaining reliable results. The examined complexes are optimized at the B3LYP-GD3BJ/6-31G(d) level with the COSMO implicit solvation model and Klamt's radii used for cavity construction. Energy characteristics and thermodynamics for all reaction branches are determined using the B3LYP-GD3BJ/6-311++G(2df,2pd)/IEF-PCM/scaled-UAKS level with Wertz's entropy corrections. Rate constants are estimated for each individual branch according to Eyring's transition state theory (TST), averaged according to equilibrium constraint and compared with available experimental data. The determined reaction barriers of the autocatalytic pathway fairly correspond with experimental values. Furthermore, autocatalytic reaction of tetraplatin and its two analogues complexes [Pt^IV(en)Cl₄ and Pt^IV(NH₃)₂Cl₄] are explored and compared with measured data in order to examined general reaction descriptors.

The Next Generation of Platinum Drugs: Targeted Pt(II) Agents, Nanoparticle Delivery, and Pt(IV) Prodrugs

The platinum drugs, cisplatin, carboplatin, and oxaliplatin, prevail in the treatment of cancer, but new platinum agents have been very slow to enter the clinic. Recently, however, there has been a surge of activity, based on a great deal of mechanistic information, aimed at developing nonclassical platinum complexes that operate via mechanisms of action distinct from those of the approved drugs. The use of nanodelivery devices has also grown, and many different strategies have been explored to incorporate platinum warheads into nanomedicine constructs. In this Review, we discuss these efforts to create the next generation of platinum anticancer drugs. The introduction provides the reader with a brief overview of the use, development, and mechanism of action of the approved platinum drugs to provide the context in which more recent research has flourished. We then describe approaches that explore nonclassical platinum(II) complexes with trans geometry or with a monofunctional coordination mode, polynuclear platinum(II) compounds, platinum(IV) prodrugs, dual-threat agents, and photoactivatable platinum(IV) complexes. Nanoparticles designed to deliver platinum(IV) complexes will also be discussed, including carbon nanotubes, carbon nanoparticles, gold nanoparticles, quantum dots, upconversion nanoparticles, and polymeric micelles. Additional nanoformulations, including supramolecular self-assembled structures, proteins, peptides, metal-organic frameworks, and coordination polymers, will then be described. Finally, the significant clinical progress made by nanoparticle formulations of platinum(II) agents will be reviewed. We anticipate that such a synthesis of disparate research efforts will not only help to generate new drug development ideas and strategies, but also will reflect our optimism that the next generation of approved platinum cancer drugs is about to arrive.

Reduction Process of Tetraplatin in the Presence of Deoxyguanosine Monophosphate (dGMP): A Computational DFT Study

The reduction mechanism of [Pt(IV) (dach)Cl4 ] (dach=diaminocyclohexyl) in the presence of dGMP was studied. The first step is substitution of a chloro ligand by dGMP, followed by nucleophilic attack of a phosphate or sugar oxygen atom to the C8-position of guanine. Subsequent reduction forms the [Pt(II) (dach)Cl2 ] complex. The whole process is completed by a hydrolysis. Two different pathways for the substitution reaction were examined: a direct associative and a Basolo-Pearson autocatalytic mechanism. All the explored structures were optimized at the B3LYP-D3/6-31G(d) level and by using the COSMO solvation model with Klamt's radii. Single-point energetics was determined at the B3LYP-GD3BJ/6-311++G(2df,2pd)/PCM/scaled-UAKS level. Activation barriers were used for an estimation of the rate constants and these were compared with experimental values. It was found that the rate-determining step is the nucleophilic attack with a slightly faster performance in the 3'-dGMP branch than in the case of 5'-dGMP with activation barriers of 21.1 and 20.4 kcal mol(-1) (experimental: 23.8 and 23.2 kcal mol(-1) ). The reduction reaction is connected with an electron flow from guanine. The product of the reduction reaction is a chelate structure, which dissociates within the last reaction step, that is, a hydrolysis reaction. The whole redox process (substitution, reduction, and hydrolysis) is exergonic by 34 and 28 kcal mol(-1) for 5'-dGMP and 3'-dGMP, respectively.

Oxidation of 5'-dGMP, 5'-dGDP, and 5'-dGTP by a platinum(IV) complex

We previously reported that a Pt(IV) complex, [Pt(IV)(dach)Cl4] [trans-d,l-1,2-diaminocyclohexanetetrachloroplatinum(IV)] binds to the N7 of 5'-dGMP (deoxyguanosine-5'-monophosphate) at a relatively fast rate and oxidizes it to 8-oxo-5'-dGMP. Here, we further studied the kinetics of the oxidation of 5'-dGMP by the Pt(IV) complex. The electron transfer rate constants between 5'-dGMP and Pt(IV) in [H8-5'-dGMP-Pt(IV)] and [D8-5'-dGMP-Pt(IV)] were similar, giving a small value of the kinetic isotope effect (KIE: 1.2 ± 0.2). This small KIE indicates that the deprotonation of H8 in [H8-5'-dGMP-Pt(IV)] is not involved in the rate-determining step in the electron transfer between guanine (G) and Pt(IV). We also studied the reaction of 5'-dGDP (deoxyguanosine-5'-diphosphate) and 5'-dGTP (deoxyguanosine-5'-triphosphate) with the Pt(IV) complex. Our results showed that [Pt(IV)(dach)Cl4] oxidized 5'-dGDP and 5'-dGTP to 8-oxo-5'-dGDP and 8-oxo-5'-dGTP, respectively, by the same mechanism and kinetics as for 5'-dGMP. The Pt(IV) complex binds to N7 followed by a two-electron inner sphere electron transfer from G to Pt(IV). The reaction was catalyzed by Pt(II) and occurred faster at higher pH. The electron transfer was initiated by either an intramolecular nucleophilic attack by any of the phosphate groups or an intermolecular nucleophilic attack by free OH(-) in the solution. The rates of reactions for the three nucleotides followed the order: 5'-dGMP > 5'-dGDP > 5'-dGTP, indicating that the bulkier the phosphate groups are, the slower the reaction is, due to the larger steric hindrance and rotational barrier of the phosphate groups.

Folate conjugation to polymeric micelles via boronic acid ester to deliver platinum drugs to ovarian cancer cell lines

In this study, a novel technique was used for the reversible attachment of folic acid on the surface of polymeric micelles for a tumor-specific drug delivery system. The reversible conjugation is based on the interaction between phenylboronic acid (PBA) and dopamine to form a borate ester. The conjugation is fast and efficient and in vitro experiments via confocal fluorescent microscopy show that the linker is stable in for several hours. Reversible addition-fragmentation chain transfer (RAFT) polymerization was used to synthesize two various sized water-soluble block copolymer of oligoethylene glycol methylether methacylate and methyl acrylic acid (POEGMEMA(35)-b-PMAA(200) and POEGMEMA(26)-b-PMAA(90)). The platinum drug, oxoplatin, was then subsequently attached to the polymer via ester formation leading to platinum loading of 12 wt % as determined by TGA. The platinum-induced amphiphilic block copolymers that consequently led to the formation of micelles of sizes 150 and 20 nm in an aqueous environment with the longer PMAA block forming larger micelles. The small micelles were in addition cross-linked using 1,8-diaminooctane to further stabilize their structure. The targeting ability of folate conjugated polymeric micelles was investigated against two types of tumor cell lines: A549 (-FR) and OVCAR-3 (+FR). The cell line growth inhibitory efficacy of material synthesized was evaluated by using SRB method. The results revealed that folate conjugated micelles showed higher activity in FR + OVCAR-3 cells but not in FR - A549 cells. Similar results were obtained for both small and large micelles without the conjugation of folate. Comparing large and small micelles it can be observed that larger micelles are more efficient, which has been attributed to the lower stability of the smaller micelles. Micelle stabilization via cross-linking could indeed increase the toxicity of the drug carrier.

Theoretical investigation into the mechanism of 3'-dGMP oxidation by [Pt(IV)Cl4(dach)]

The mechanism for the oxidation of 3'-dGMP by [PtCl(4)(dach)] (dach = diaminocyclohexane) in the presence of [PtCl(2)(dach)] has been investigated using density functional theory. We find that the initial complexation, i.e., the formation of [PtCl(3)(dach)(3'-dGMP)], is greatly assisted by the reaction of the encounter pair [PtCl(2)(dach)···3'-dGMP] with [PtCl(4)(dach)], leading to migration of an axial chlorine ligand from platinum(IV) to platinum(II). A dinuclear platinum(II)/platinum(IV) intermediate could not be found, but the reaction is predicted to pass through a platinum(III)/platinum(III) transition structure. A cyclization process, i.e., C8-O bond formation, from [PtCl(3)(dach)(3'-dGMP)] occurs through an intriguing phosphate-water-assisted deprotonation reaction, analogous to the opposite of a proton shuttle mechanism. Followed by this, the guanine moiety is oxidized via dissociation of the Pt(IV)-Cl(ax) bond, and the cyclic ether product is finally formed after deprotonation. We have provided rationalizations, including molecular orbital explanations, for the key steps in the process. Our results help to explain the effect of [PtCl(4)(dach)] on the complexation step and the effect of a strong hydroxide base on the cyclization reaction. The overall reaction cycle is intricate and involves autocatalysis by a platinum(II) species.

Density functional theory studies on the oxidation of 5'-dGMP and 5'-dAMP by a platinum(IV) complex

Density functional theory has been used to investigate the oxidation of a guanine nucleotide by platinum(IV), a process that can be important in the degradation of DNA. For the first time, we have provided a comprehensive mechanism for all of the steps in this process. A number of intermediates are predicted to occur but with short lifetimes that would make them difficult to observe experimentally. A key step in the mechanism is electron transfer from guanine to platinum(IV), and we show that this is driven by the loss of a chloride ligand from the platinum complex after nucleophilic attack of 5'-phosphate to C8 of guanine. We have investigated several different initial platinum(IV) guanine adducts and shown that the adduct formed from replacement of an axial chlorine ligand in the platinum(IV) complex undergoes oxidation more easily. We have studied adenine versus guanine adducts, and our results show that oxidation of the former is more difficult because of disruption of the aromatic π system that occurs during the process. Finally, our results show that the acidic hydrolysis step to form the final oxidized product occurs readily via an initial protonation of N7 of the guanine.

Different rotamer states of cytosine nucleobases in heteronuclear PtPd-, PtPd2, and Pt2Pd2Ag complexes derived from [Pt(2,2'-bpy)(1-MeC-N3)2]2+ (1-MeC = 1-methylcytosine): first examples of species with head-head oriented 1-MeC(-) ligands

[Pt(2,2'-bpy)(1-MeC-N3)(2)](NO(3))(2) (1) (2,2'-bpy = 2,2'-bipyridine; 1-MeC = 1-methylcytosine) exists in water in an equilibrium of head-tail and head-head rotamers, with the former exceeding the latter by a factor of ca. 20 at room temperature. Nevertheless, 1 reacts with (en)Pd(II) (en = ethylenediamine) to give preferentially the dinuclear complex [Pt(2,2'-bpy)(1-MeC(-)-N3,N4)(2)Pd(en)](NO(3))(2)·5H(2)O (2) with head-head arranged 1-methylctosinato (1-MeC(-)) ligands and Pd being coordinated to two exocyclic N4H(-) positions. Addition of AgNO(3) to a solution of 2 leads to formation of a pentanuclear chain compound [{Pt(2,2'-bpy)(1-MeC(-))(2)Pd(en)}(2)Ag](NO(3))(5)·14H(2)O (5) in which Ag(+) cross-links two cations of 2 via the four available O2 sites of the 1-MeC(-) ligands. 2 and 5 appear to be the first X-ray structurally characterized examples of di- and multinuclear complexes derived from a Pt(II) species with two cis-positioned cytosinato ligands adopting a head-head arrangement. (tmeda)Pd(II) (tmeda = N,N,N',N'-tetramethylethylenediamine) and (2,2'-bpy)Pd(II) behave differently toward 1 in that in their derivatives the head-tail orientation of the 1-MeC(-) nucleobases is retained. In [Pt(2,2'-bpy)(1-MeC(-))(2){Pd(2,2'-bpy)}(2)](NO(3))(4)·10H(2)O (4), both (2,2'-bpy)Pd(II) entities are pairwise bonded to N4H(-) and O2 sites of the two 1-MeC(-) rings, whereas in [Pt(2,2'-bpy)(1-MeC(-))(2){Pd(tmeda)}(2)(NO(3))](NO(3))(3)·5H(2)O (3) only one of the two (tmeda)Pd(II) units is chelated to N4H(-) and O2. The second (tmeda)Pd(II) is monofunctionally attached to a single N4H(-) site. On the basis of these established binding patterns, ways to the formation of mixed Pt/Pd complexes and possible intermediates are proposed. The methylene protons of the en ligand in 2 are special in that they display two multiplets separated by 0.64 ppm in the (1)H NMR spectrum.

Synthesis, characterization and antiproliferative activity on mesothelioma cell lines of bis(carboxylato)platinum(IV) complexes based on picoplatin

The synthesis and characterization of a series of picoplatin-based (picoplatin = [PtCl(2)(mpy)(NH(3))], mpy = 2-methylpyridine), Pt(iv) complexes with axial carboxylato ligands of increasing length are reported. The synthesis is based on the oxidation with hydrogen peroxide of picoplatin to give the cis,cis,trans-[PtCl(2)(mpy)(NH(3))(OH)(2)] intermediate and then its transformation into the dicarboxylato complexes cis,cis,trans-[PtCl(2)(mpy)(NH(3))(RCOO)(2)] (R = CH(3)(CH(2))(n), n = 0-4) with the corresponding anhydride. Pt(iv) complexes with n = 0-2 were selected to be tested on four malignant pleural mesothelioma (MPM) cell lines, on human mesothelial cells (HMC), and on the cisplatin-sensitive ovarian A2780 cell line along with cisplatin as a metallo-drug reference. In general, the longer the axial chain, the more cytotoxic and selective the Pt(IV) complex is. Pt(IV) analogs show good activity on the MPM cell lines, approaching or in some case bypassing that of cisplatin and represent quite promising drug candidates for the treatment of tumors whose chemoresistance is mainly based on glutathione overexpression, such as MPM.

Oxidative DNA damage mediated by transition metal ions and their complexes

DNA damage by redox-active metal complexes depends on the interaction of the metal complex with DNA together with the mechanism of oxygen activation. Weak interaction, tight binding, and direct involvement of DNA in the coordination sphere of the metal are described. Metal complexes acting through the production of diffusing radicals and metal complexes oxidizing DNA by metal-centered active species are compared. Metal complexes able to form high-valent metal-oxo species in close contact with DNA and perform DNA oxidation in a way reminiscent of enzymatic chemistry are the most elegant systems.

Two-electron oxidation of deoxyguanosine by a Ru(III) complex without involving oxygen molecules through disproportionation

Among the many mechanisms for the oxidation of guanine derivatives (G) assisted by transition metals, Ru(III) and Pt(IV) metal ions share basically the same principle. Both Ru(III)- and Pt(IV)-bound G have highly positively polarized C8-H's that are susceptible to deprotonation by OH(-), and both undergo two-electron redox reactions. The main difference is that, unlike Pt(IV), Ru(III) is thought to require O(2) to undergo such a reaction. In this study, however, we report that [Ru(III)(NH(3))(5)(dGuo)] (dGuo = deoxyguanosine) yields cyclic-5'-O-C8-dGuo (a two-electron G oxidized product, cyclic-dGuo) without O(2). In the presence of O(2), 8-oxo-dGuo and cyclic-dGuo were observed. Both [Ru(II)(NH(3))(5)(dGuo)] and cyclic-dGuo were produced from [Ru(III)(NH(3))(5)(dGuo)] accelerated by [OH(-)]. We propose that [Ru(III)(NH(3))(5)(dGuo)] disproportionates to [Ru(II)(NH(3))(5)(dGuo)] and [Ru(IV)(NH(3))(4)(NH(2)(-))(dGuo)], followed by a 5'-OH attack on C8 in [Ru(IV)(NH(3))(4)(NH(2)(-))(dGuo)] to initiate an intramolecular two-electron transfer from dGuo to Ru(IV), generating cyclic-dGuo and Ru(II) without involving O(2).

Oxidation of a guanine derivative coordinated to a Pt(IV) complex initiated by intermolecular nucleophilic attacks

In this study we report that fac-[Pt(IV)(dach)(9-EtG)Cl(3)](+) (dach = d,l-1,2-diaminocyclohexane, 9-EtG = 9-ethylguanine) in high pH (pH 12) or phosphate solution (pH 7.4) produces 8-oxo-9-EtG and Pt(II) species. The reaction in H(2)(18)O revealed that the oxygen atom in hydroxide or phosphate ends up at the C8 position of 8-oxo-G. The kinetics of the redox reaction was first order with respect to both Pt(IV)-G and free nucleophiles (OH(-) and phosphate). The oxidation of G initiated by hydroxide was approximately 30∼50 times faster than by phosphate in 100 mM NaCl solutions. The large entropy of activation of OH(-1) (ΔS(‡) = 26.6 ± 4.3 J mol(-1) K(-1)) due to the smaller size of OH(-) is interpreted to be responsible for the faster kinetics compared to phosphate (ΔS(‡) = -195.5 ± 11.1 J mol(-1) K(-1)). The enthalpy of activation for phosphate reaction is more favorable relative to the OH(-) reaction (ΔH(‡) = 35.4 ± 3.5 kJ mol(-1) for phosphate vs. 96.6 ± 11.4 kJ mol(-1) for OH(-1)). The kinetic isotope effect of H8 was determined to be 7.2 ± 0.2. The rate law, kinetic isotope effect, and isotopic labeling are consistent with a mechanism involving proton ionization at the C8 position as the rate determining step followed by two-electron transfer from G to Pt(IV).

Unusual DNA binding modes for metal anticancer complexes

DNA is believed to be the primary target for many metal-based drugs. For example, platinum-based anticancer drugs can form specific lesions on DNA that induce apoptosis. New platinum drugs can be designed that have novel modes of interaction with DNA, such as the trinuclear platinum complex BBR3464. Also it is possible to design inert platinum(IV) pro-drugs which are non-toxic in the dark, but lethal when irradiated with certain wavelengths of light. This gives rise to novel DNA lesions which are not as readily repaired as those induced by cisplatin, and provides the basis for a new type of photoactivated chemotherapy. Finally, newly emerging ruthenium(II) organometallic complexes not only bind to DNA coordinatively, but also by H-bonding and hydrophobic interactions triggered by the introduction of extended arene rings into their versatile structures. Intriguingly osmium (the heavier congener of ruthenium) reacts differently with DNA but can also give rise to highly cytotoxic organometallic complexes.

Comparison of the electronic properties, and thermodynamic and kinetic parameters of the aquation of selected platinum(II) derivatives with their anticancer IC50 indexes

Three potential anticancer agents {trans-[PtCl(2)(NH(3))(thiazole)], cis-[PtCl(2)(NH(3))(piperidine)], and PtCl(2)(NH(3))(cyclohexylamine) (JM118)} were explored and compared with cisplatin and the inactive [PtCl(dien)](+) complex. Basic electronic properties, bonding and stabilization energies were determined, and thermodynamic and kinetic parameters for the aquation reaction were estimated at the B3LYP/6-311++G(2df,2pd) level of theory. Since the aquation process represents activation of these agents, the obtained rate constants were compared with the experimental IC(50) values for several tumor cells. Despite the fact that the processes in which these drugs are involved and the way in which they affect cells are very complex, some correlations can be deduced.

Importance of platinum(II)-assisted platinum(IV) substitution for the oxidation of guanosine derivatives by platinum(IV) complexes

Guanosine derivatives with a nucleophilic group at the 5' position (G-5') are oxidized by the Pt (IV) complex Pt( d, l)(1,2-(NH 2) 2C 6H 10)Cl 4 ([Pt (IV)(dach)Cl 4]). The overall redox reaction is autocatalytic, consisting of the Pt (II)-catalyzed Pt (IV) substitution and two-electron transfer between Pt (IV) and the bound G-5'. In this paper, we extend the study to improve understanding of the redox reaction, particularly the substitution step. The [Pt (II)(NH 3) 2(CBDCA-O,O')] (CBDCA = cyclobutane-1,1-dicarboxylate) complex effectively accelerates the reactions of [Pt (IV)(dach)Cl 4] with 5'-dGMP and with cGMP, indicating that the Pt (II) complex does not need to be a Pt (IV) analogue to accelerate the substitution. Liquid chromatography/mass spectroscopy (LC/MS) analysis showed that the [Pt (IV)(dach)Cl 4]/[Pt (II)(NH 3) 2(CBDCA-O,O')]/cGMP reaction mixture contained two Pt (IV)cGMP adducts, [Pt (IV)(NH 3) 2(cGMP)(Cl)(CBDCA-O,O')] and [Pt (IV)(dach)(cGMP)Cl 3]. The LC/MS studies also indicated that the trans, cis-[Pt (IV)(dach)( (37)Cl) 2( (35)Cl) 2]/[Pt (II)(en)( (35)Cl) 2]/9-EtG mixture contained two Pt (IV)-9-EtG adducts, [Pt (IV)(en)(9-EtG)( (37)Cl)( (35)Cl) 2] and [Pt (IV)(dach)(9-EtG)( (37)Cl)( (35)Cl) 2]. These Pt (IV)G products are predicted by the Basolo-Pearson (BP) Pt (II)-catalyzed Pt (IV)-substitution scheme. The substitution can be envisioned as an oxidative addition reaction of the planar Pt (II) complex where the entering ligand G and the chloro ligand from the axial position of the Pt (IV) complex are added to Pt (II) in the axial positions. From the point of view of reactant Pt (IV), an axial chloro ligand is thought to be substituted by the entering ligand G. The Pt (IV) complexes without halo axial ligands such as trans, cis-[Pt(en)(OH) 2Cl 2], trans, cis-[Pt(en)(OCOCF 3) 2Cl 2], and cis, trans, cis-[Pt(NH 3)(C 6H 11NH 2)(OCOCH 3) 2Cl 2] ([Pt (IV)(a,cha)(OCOCH 3) 2Cl 2], satraplatin) did not react with 5'-dGMP. The bromo complex, [Pt (IV)(en)Br 4], showed a significantly faster substitution rate than the chloro complexes, [Pt (IV)(en)Cl 4] and [Pt (IV)(dach)Cl 4]. The results indicate that the axial halo ligands are essential for substitution and the Pt (IV) complexes with larger axial halo ligands have faster rates. When the Pt (IV) complexes with different carrier ligands were compared, the substitution rates increased in the order [Pt (IV)(dach)Cl 4] < [Pt (IV)(en)Cl 4] < [Pt (IV)(NH 3) 2Cl 4], which is in reverse order to the carrier ligand size. These axial and carrier ligand effects on the substitution rates are consistent with the BP mechanism. Larger axial halo ligands can form a better bridging ligand, which facilitates the electron-transfer process from the Pt (II) to Pt (IV) center. Smaller carrier ligands exert less steric hindrance for the bridge formation.

Spontaneous Reduction of Mixed 2,2′-Bipyridine/Methylamine/Chloro Complexes of PtIV in Water in the Presence of Light Is Accompanied by Complex Isomerization, Loss of Methylamine, and Formation of a Strong Oxidant, Presumably HOCl

Three 2,2'-bipyridine (2,2'-bpy) complexes of Pt(IV) have been synthesized, characterized by X-ray crystallography, and their solution behavior in D(2)O studied by (1)H NMR spectroscopic analysis: mer-[PtCl(3)(2,2'-bpy)(MeNH(2))]ClH(2)O (4), trans-[PtCl(2)(2,2'-bpy)(MeNH(2))(2)]Cl(2) (5), and trans-[Pt (2,2'-bpy)(MeNH(2))(2)(OH)(2)]Cl(2) (6; MeNH(2)=methylamine). Complexes 4 and 5 undergo hydrolysis of the Cl(-) ions, both in the dark and daylight, as evident from a drop in the pH value. Two solvolysis products were detected in the case of 4, which is indicative of species with equatorial and axial OH(-) groups. The hydrolysis reaction of 5 implies that an axial Cl(-) group is replaced by an OH(-) moiety; in contrast, 6 remains virtually unaffected. Ordinary daylight, in particular irradiation with a 50-W halogen lamp, initially causes ligand-isomerization processes, which are followed by the reduction of 4 and 5 to Pt(II) species. This reduction of 4 and 5 is accompanied by the formation of hypochlorous acid, as demonstrated qualitatively in the decoloration test of indigo, and loss of MeNH(2), which is particularly pronounced in the case of 5. The formation of Pt(II) compounds is established on the basis of the J coupling constants of (195)Pt with selected (1)H NMR resonances. The results obtained herein are possibly also relevant to the chemistry of Cl-containing Pt(IV) antitumor agents and their reactions with DNA.

Kinetics and mechanism of the oxidation of guanosine derivatives by Pt(IV) complexes

The kinetics of redox reactions of the PtIV complexes trans-Pt(d,l)(1,2-(NH2)2C6H10)Cl4 ([PtIVCl4(dach)]) and Pt(NH2CH2CH2NH2)Cl4 ([PtIVCl4(en)]) with 5'- and 3'-dGMP (G) have been studied. These redox reactions involve substitution followed by an inner-sphere electron transfer. The substitution is catalyzed by PtII and follows the classic Basolo-Pearson PtII-catalyzed PtIV-substitution mechanism. We found that the substitutution rates depend on the steric hindrance of PtII, G, and PtIV with the least sterically hindered PtII complex catalyzing at the highest rate. 3'-dGMP undergoes substitution faster than 5'-dGMP, and [PtIVCl4(en)] substitutes faster than [PtIVCl4(dach)]. The enthalpies of activation of the substitution, DeltaH double dagger s, of 3'-dGMP is only 70% greater than that of 5'-dGMP (50.4 vs 30.7 kJ mol(-1)), but the entropy of activation of the substitution, DeltaS double dagger s, of 3'-dGMP is much greater than that of 5'-dGMP (-59.4 vs -129.5 J K(-1) mol(-1)), indicating that steric hindrance plays a major role in the substitution. The enthalpy of activation of electron transfer, DeltaH double dagger e, of 3'-dGMP is smaller than that of 5'-dGMP (88.8 vs 137.8 kJ mol(-1)). The entropy of activation of electron transfer, DeltaS double dagger e, of 3'-dGMP is negative, but that of 5'-dGMP is positive (-27.8 vs +128.8 J K-1 mol-1). The results indicate that 5'-hydroxo has less rotational barrier than 5'-phosphate, but it is geometrically unfavorable for internal electron transfer. The electron-transfer rate also depends on the reduction potential of PtIV. Because of its higher reduction potential, [PtIVCl4(dach)] has a faster electron transfer than [PtIVCl4(en)].

Synthesis, characterization and biological activity of trans-platinum(II) and trans-platinum(IV) complexes with 4-hydroxymethylpyridine

The synthesis and chemical characterization of two new trans platinum complexes, trans-[PtCl(2)NH(3)(4-hydroxymethylpyridine)] (1) and trans-[PtCl(4)NH(3)(4-hydroxymethylpyridine)] (2) are described. Their ability to interact with 5'-GMP by themselves and in the presence of reducing agents in the case of trans-[PtCl(4)NH(3)(4-hydroxymethylpyridine)] were tested. Circular dichroism, electrophoretic mobility in agarose gel, and atomic force microscopy studies showed that the interaction of complex 1 with DNA is stronger than that of complex 2. Cytotoxicity tests against HL-60 tumor cells also showed higher activity for trans-[PtCl(2)NH(3)(4-hydroxymethylpyridine)] than for trans-[PtCl(4)NH(3)(4-hydroxymethylpyridine)]. Complex 1 presents similar behavior to cisplatin, but with a lower IC(50) at 24 h. Complex 1 also showed high apoptosis induction.