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

Novel Pt(IV) complexes to overcome multidrug resistance in gastric cancer by targeting P-glycoprotein

Systematic toxicity and drug resistance significantly limited FDA-approved platinum drugs for further clinical applications. In order to reverse the resistance (MDR) and enhance their anticancer efficiency, four Pt(IV) complexes (12-15) conjugating with P-glycoprotein (P-gp) inhibitors were designed and synthesized. Among them, complex 14 (IC₅₀ = 3.37 μM) efficiently reversed cisplatin resistance in SGC-7901/CDDP cell line and increased selectivity index (6.9) against normal HL-7702 cell line. Detailed mechanisms in SGC-7901/CDDP cells assays revealed that complex 14 efficiently induced apoptosis via down-regulating expression of P-gp for enhanced intracellular uptake of platinum, arrested cells at G2/M phase, induced DNA damage and initiated mitochondrial apoptosis pathway. Further in vivo studies demonstrated that the enhanced accumulation of complex 14 contributed to tumor inhibition of 75.6% in SGC-7901/CDDP xenografts, which was much higher than cisplatin (25.9%) and oxaliplatin (43%). Moreover, the low systematic toxicity made 14 a potential novel P-gp-mediated MDR modulator.

Development of Natural Product-Conjugated Metal Complexes as Cancer Therapies

Platinum-based drugs have revolutionized cancer care, but are unfortunately associated with various adverse effects. Meanwhile, natural product scaffolds exhibit multifarious bioactivities and serve as an attractive resource for cancer therapy development. Thus, the conjugation of natural product scaffolds to metal complexes becomes an attractive strategy to reduce the severe side effects arising from the use of metal bearing drugs. This review aims to highlight the recent examples of natural product-conjugated metal complexes as cancer therapies with enhanced selectivity and efficacy. We discuss the mechanisms and features of different conjugate complexes and present an outlook and perspective for the future of this field.

Exploring the Hydrolytic Behavior of the Platinum(IV) Complexes with Axial Acetato Ligands

Platinum(IV) complexes are generally thought to be kinetically inert, and are expected to be stable enough to resist premature aquation before entering the cancer cells. Nevertheless, in this work, complex 2 with axial acetato ligands can hydrolyze relatively quickly under biologically relevant conditions with a half-life of 91.7 min, resulting in the loss of the equatorial chlorido ligand. Further study indicated that the fast hydrolysis of complex 2 may be attributed to the strong σ-donor ability of N-isopropyl-1R,2R-diaminocyclohexane, and an increasing σ-donor ability of the amine group can promote the hydrolysis rate of the corresponding platinum(IV) complex. The experiment results were proven by the corresponding DFT calculation. Our study can help to re-evaluate the aqueous properties of the platinum(IV) complexes with axial acetate, which may be less inert to hydrolysis than expected under biologically relevant conditions.

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.

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.

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.

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.

Insights into the acid-base properties of Pt(IV)-diazidodiam(m)inedihyroxido complexes from multinuclear NMR spectroscopy

Platinum(IV) am(m)ine complexes are of interest as potential anticancer pro-drugs, but there are few reports of their acid-base properties. We have studied the acid-base properties of three photoactivatable anticancer platinum(IV)-diazidodiam(m)ine complexes (cis,trans,cis-[Pt(IV)(N(3))(2)(OH)(2)(NH(3))(2)], trans,trans,trans-[Pt(IV)(N(3))(2)(OH)(2)(NH(3))(2)], and cis,trans-[Pt(IV)(N(3))(2)(OH)(2)(en)]) using multinuclear NMR methods and potentiometry. In particular, the combination of both direct and indirect techniques for the detection of (15)N signals has allowed changes of the chemical shifts to be followed over the pH range 1-11; complementary (14)N NMR studies have been also carried out. A distinct pK(a) value of approximately 3.4 was determined for all the investigated complexes, involving protonation/deprotonation reactions of one of the axial hydroxido groups, whereas a second pH-dependent change for the three complexes at approximately pH 7.5 appears not to be associated with a loss of an am(m)ine or hydroxido proton from the complex. Our findings are discussed in comparison with the limited data available in the literature on related complexes.

Specific DNA structural attributes modulate platinum anticancer drug site selection and cross-link generation

Heavy metal compounds have toxic and medicinal potential through capacity to form strong specific bonds with macromolecules, and the interaction of platinum drugs at the major groove nitrogen atom of guanine bases primarily underlies their therapeutic activity. By crystallographic analysis of transition metal-and in particular platinum compound-DNA site selectivity in the nucleosome core, we establish that steric accessibility, which is controlled by specific structural parameters of the double helix, modulates initial guanine-metal bond formation. Moreover, DNA conformational features can be linked to both similarities and distinctions in platinum drug adduct formation between the naked and nucleosomal DNA states. Notably, structures that facilitate initial platinum-guanine bond formation can oppose cross-link generation, rationalizing the occurrence of long-lived therapeutically ineffective monofunctional adducts. These findings illuminate DNA structure-dependent reactivity and provide a novel framework for understanding metal-double helix interactions, which should facilitate the development of improved chromatin-targeting medicinal agents.

Comparative activities of nickel(II) and zinc(II) complexes of asymmetric [NN'O] ligands as 26S proteasome inhibitors

In this study, we compare the proteasome inhibition capabilities of two anticancer candidates, [Ni(L(IA))(2)] (1) and [Zn(L(IA))(2)] (2), where L(IA-) is the deprotonated form of the ligand 2,4-diiodo-6-(((2-pyridinylmethyl)amino)methyl)phenol. Species 1 contains nickel(II), a considerably inert ion that favors covalency, whereas 2 contains zinc(II), a labile transition metal ion that favors predominantly ionic bonds. We report on the synthesis and characterization of 1 and 2 using various spectroscopic, spectrometric, and structural methods. Furthermore, the pharmacological effects of 1 and 2, along with those of the salts NiCl(2) and ZnCl(2), were evaluated in vitro and in cultured human cancer cells in terms of their proteasome-inhibitory and apoptotic cell-death-inducing capabilities. It is shown that neither NiCl(2) nor 1 have the ability to inhibit the proteasome activity at any sustained levels. However, ZnCl(2) and 2 showed superior inhibitory activity versus the chymotrypsin-like activity of both the 26S proteasome (IC(50) = 5.7 and 4.4 micromol/L, respectively) and the purified 20S proteasome (IC(50) = 16.6 and 11.7 micromol/L, respectively) under cell-free conditions. Additionally, inhibition of proteasomal activity in cultured prostate cancer cells by 2 was associated with higher levels of ubiquitinated proteins and apoptosis. Treatment with either the metal complex or the salt was relatively nontoxic toward human normal cells. These results strengthen the current working hypothesis that fast ligand dissociation is required to generate an [ML(IA)](+) pharmacophore, capable of interaction with the proteasome. This interaction, possibly via N-terminal threonine amino acids present in the active sites, renders the proteasome inactive. Our results present a compelling rationale for 2 along with its gallium(III) and copper(II) congeners to be further investigated as potential anticancer drugs that act as proteasome inhibitiors.