Cytotoxic effects of novel polyoxotungstates and a platinum compound on human cancer cell lines

Promising application of polyoxometalates in the treatment of cancer, infectious diseases and Alzheimer's disease

As shown in studies conducted in recent decades, polyoxometalates (POMs), as inorganic metal oxides, have promising biological activities, including antitumor, anti-infectious and anti-Alzheimer’s activities, due to their special structures and properties. However, some side effects impede their clinical applications to a certain extent. Compared with unmodified POMs, POM-based inorganic–organic hybrids and POM-based nanocomposite structures show significantly enhanced bioactivity and reduced side effects. In this review, we introduce the biological activities of POMs and their derivatives and highlight the side effects of POMs on normal cells and organisms and their possible mechanisms of action. We then propose a development direction for overcoming their side effects. POMs are expected to constitute a new generation of inorganic metal drugs for the treatment of cancer, infectious diseases, and Alzheimer's disease.

Graphical abstract

Synthesis and crystal structure of a new hybrid organic–inorganic material containing neutral molecules, cations and heptamolybdate anions

In the crystal structure of hexa­kis­(2-methyl-1H-imidazol-3-ium) hepta­molybdate 2-methyl-1H-imidazole disolvate dihydrate,(C₄H₇N₂)₆[Mo₇O₂₄]·2C₄H₆N₂·2H₂O, [Mo₇O₂₄]⁶⁻ hepta­molybdate anions, 2-methyl­imidazolium cations, neutral 2-methyl­imidazole mol­ecules and water mol­ecules are linked through hydrogen bonds into a three-dimensional network.

The title compound, hexa­kis­(2-methyl-1H-imidazol-3-ium) hepta­molybdate 2-methyl-1H-imidazole disolvate dihydrate, (C₄H₇N₂)₆[Mo₇O₂₄]·2C₄H₆N₂·2H₂O, was prepared from 2-methyl­imidazole and ammonium hepta­molybdate tetra­hydrate in acid solution. The [Mo₇O₂₄]⁶⁻ hepta­molybdate cluster anion is accompanied by six protonated (C₄H₇N₂)⁺ 2-methyl­imidazolium cations, two neutral C₄H₆N₂ 2-methyl­imidazole mol­ecules and two water mol­ecules of crystallization. The cluster consists of seven distorted MoO₆ octa­hedra sharing edges or vertices. In the crystal, the components are linked by N—H⋯N, N—H⋯O, O—H⋯O, N—H⋯(O,O) and O—H⋯(O,O) hydrogen bonds, generating a three-dimensional network. Weak C—H⋯O inter­actions consolidate the packing.

The P-type ATPase inhibiting potential of polyoxotungstates

Polyoxometalates (POMs) are transition metal complexes that exhibit a broad diversity of structures and properties rendering them promising for biological purposes. POMs are able to inhibit a series of biologically important enzymes, including phosphatases, and thus are able to affect many biochemical processes. In the present study, we analyzed and compared the inhibitory effects of nine different polyoxotungstates (POTs) on two P-type ATPases, Ca2+-ATPase from skeletal muscle and Na+/K+-ATPase from basal membrane of skin epithelia. For Ca2+-ATPase inhibition, an in vitro study was performed and the strongest inhibitors were determined to be the large heteropolytungstate K9(C2H8N)5[H10Se2W29O103] (Se2W29) and the Dawson-type POT K6[α-P2W18O62] (P2W18) exhibiting IC50 values of 0.3 and 0.6 μM, respectively. Promising results were also shown for the Keggin-based POTs K6H2[CoW11TiO40] (CoW11Ti, IC50 = 4 μM) and Na10[α-SiW9O34] (SiW9, IC50 = 16 μM), K14[As2W19O67(H2O)] (As2W19, IC50 = 28 μM) and the lacunary Dawson K12[α-H2P2W12O48] (P2W12, IC50 = 11 μM), whereas low inhibitory potencies were observed for the isopolytungstate Na12[H4W22O74] (W22, IC50 = 68 μM) and the Anderson-type Na6[TeW6O24] (TeW6, IC50 = 200 μM). Regarding the inhibition of Na+/K+-ATPase activity, for the first time an ex vivo study was conducted using the opercular epithelium of killifish in order to investigate the effects of POTs on the epithelial chloride secretion. Interestingly, 1 μM of the most potent Ca2+-ATPase inhibitor, Se2W29, showed only a minor inhibitory effect (14% inhibition) on Na+/K+-ATPase activity, whereas almost total inhibition (99% inhibition) was achieved using P2W18. The remaining POTs exhibited similar inhibition rates on both ATPases. These results reveal the high potential of some POTs to act as P-type ATPase inhibitors, with Se2W29 showing high selectivity towards Ca2+-ATPase.

Synthesis, cytotoxicity and antitumour mechanism investigations of polyoxometalate doped silica nanospheres on breast cancer MCF-7 cells

Polyoxometalates (POMs) have shown the potential anti-bacterial, anti-viral and anti-tumor activities. In order to improve their physiological stability and antitumour activity for medical application, K2Na[AsIIIMo6O21(O2CCH2NH3)3]·6H2O doped silica nanospheres (POM@SiO2) with diameters of ~40 nm have been synthesized by the water-in-oil microemulsion method in this study. The obtained spheres were morphologically uniform nanosized and nearly monodispersed in solution. The nanoparticles had high entrapment efficiency, which was upto 46.2% by the inductively coupled plasma mass spectrometry (ICP-MS) analysis and POMs slowly released from the nanospheres both in the PH 7.4 and 5.5 phosphate buffer saline (PBS) solutions in 60 h. The in vitro MTT assays of particles on MCF-7 cell line (a human breast adenocarcinoma cell line) exhibited enhanced antitumor activity compared to that of plain polyoxometalate. The IC50 value of the POM@SiO2 nanoparticles was 40.0 μg/mL at 24 h calculated by the encapsulated POM concentration, which was much lower comparing to that of 2.0 × 104 μg/mL according to the pure POM. And the SiO2 shells showed low inhibitory effect at the corresponding concentration. Confocal images further indicated the cell morphology changes and necrosis. Flow cytometric analysis showed nanoparticles induced the apoptosis by arresting the cells in S phase and western blot analysis indicated they promoted apoptosis by inhibiting the Bcl-2 protein. Moreover, the study of interactions between human serum albumin (HSA) and the nanoparticles indicated the fluorescence quenching was static, and the nanoparticles were likely to bind to HSA and changed its conformation.

Bi-amino surface functionalized polyoxometalate nanocomposite as an environmentally friendly catalyst: synthesis and dye degradation

In this paper, cobalt ferrite magnetic nanoparticle was modified by N-(2-aminoethyl)-3-(trimethoxysilyl) propyl amine. Polyoxometalate was immobilized on the modified bi-amino surface functionalized nanoparticle to prepare the environmentally friendly catalyst nanoparticle (EFCN). The synthesized EFCN was characterized using Fourier transform infrared, scanning electron microscopy and X-ray powder diffraction. The EFCN was used for photocatalytic dye degradation. Acid Red 18 and Direct Red 81 were used as model dyes. The effect of catalyst dosage, dye concentration and salt on dye degradation was studied. Dye degradation increases by catalyst dosage. The results showed that decolorization followed a zero-order kinetic model.

Target Delivery of a Novel Antitumor Organoplatinum(IV)-Substituted Polyoxometalate Complex for Safer and More Effective Colorectal Cancer Therapy In Vivo

An inactive organoplatinum(IV)-substituted polyoxometalate is developed as an efficient and nontoxic prodrug with significant potential for treating human colorectal cancers. Further encapsulation of Pt(IV) -PW11 with DSPE-PEG2000 nanoparticles (NPs) enables targeted delivery and controlled release of inactive prodrug. Such Pt(IV) -PW11 -DSPE-PEG2000 NPs are highly efficient in inhibiting cellular growth of HT29 cells and treating human colorectal cancer in mice, superior to classic cisplatin.

The Synthesis and Characterization of Aromatic Hybrid Anderson-Evans POMs and their Serum Albumin Interactions: The Shift from Polar to Hydrophobic Interactions

Four aromatic hybrid Anderson polyoxomolybdates with Fe³⁺ or Mn³⁺ as the central heteroatom have been synthesized by using a pre-functionalization protocol and characterized by using single-crystal X-ray diffraction, FTIR, ESI-MS, ¹H NMR spectroscopy, and elemental analysis. Structural analysis revealed the formation of (TBA)₃[FeMo₆O₁₈{(OCH₂)₃CNHCOC₆H₅}₂]⋅3.5 ACN (TBA-FeMo₆-bzn; TBA=tetrabutylammonium, ACN=acetonitrile, bzn=TRIS-benzoic acid alkanolamide, TRIS–R=(HOCH₂)₃C–R)), (TBA)₃[FeMo₆O₁₈{(OCH₂)₃CNHCOC₈H₇}₂]⋅2.5 ACN (TBA-FeMo₆-cin; cin=TRIS-cinnamic acid alkanolamide), (TBA)₃[MnMo₆O₁₈{(OCH₂)₃CNHCOC₆H₅}₂]⋅3.5 ACN (TBA-MnMo₆-bzn), and (TBA)₃[MnMo₆O₁₈{(OCH₂)₃CNHCOC₈H₇}₂]⋅2.5 ACN (TBA-MnMo₆-cin). To make these four compounds applicable in biological systems, an ion exchange was performed that gave the water-soluble (up to 80 mm) sodium salts Na₃[FeMo₆O₁₈{(OCH₂)₃CNHCOC₆H₅}₂] (Na-FeMo₆-bzn), Na₃[FeMo₆O₁₈{(OCH₂)₃CNHCOC₈H₇}₂] (Na-FeMo₆-cin), Na₃[MnMo₆O₁₈{(OCH₂)₃CNHCOC₆H₅}₂] (Na-MnMo₆-bzn), and Na₃[MnMo₆O₁₈{(OCH₂)₃CNHCOC₈H₇}₂] (Na-MnMo₆-cin). The hydrolytic stability of the sodium salts was examined by applying ESI-MS in the pH range of 4 to 9. Sodium dodecylsulfate–polyacrylamide gel electrophoresis (SDS-PAGE) showed that human and bovine serum albumin (HSA and BSA) remain intact in solutions that contain up to 100 equivalents of the sodium salts over more than 4 d at 20 °C. Tryptophan (Trp) fluorescence quenching was applied to study the interactions between the sodium salts and HSA and BSA at pH 5.5 and 7.4. The quenching constants were extracted by using Stern–Volmer analysis, which suggested the formation of a 1:1 POM–protein complex in all samples. It is suggested that the aromatic hybrid POM approaches subdomain IIA of HSA and exhibits hydrophobic interactions with its hydrophobic tails, whereas the Anderson core is stabilized through electrostatic interactions with polar amino acid side chains from, for example, subdomain IB.

The use of polyoxometalates in protein crystallography - An attempt to widen a well-known bottleneck

Polyoxometalates (POMs) are discrete polynuclear metal-oxo anions with a fascinating variety of structures and unique chemical and physical properties. Their application in various fields is well covered in the literature, however little information about their usage in protein crystallization is available. This review summarizes the impact of the vast class of POMs on the formation of protein crystals, a well-known (frustrating) bottleneck in macromolecular crystallography, with the associated structure elucidation and a particular emphasis focused on POM's potential as a powerful crystallization additive for future research. The Protein Data Bank (PDB) was scanned for protein structures with incorporated POMs which were assigned a PDB ligand ID resulting in 30 PDB entries. These structures have been analyzed with regard to (i) the structure of POM itself in the immediate protein environment, (ii) the kind of interaction and position of the POM within the protein structure and (iii) the beneficial effects of POM on protein crystallography apparent so far.

Polyoxometalate-Based Organic-Inorganic Hybrids as Antitumor Drugs

Polyoxometalates (POMs) have shown encouraging antitumor activity. However, their cytotoxicity in normal cells and unspecific interactions with biomolecules are two major obstacles that impede the practical applications of POMs in clinical cancer treatment. Derivatization of POMs with more biocompatible organic ligands is expected to cause a synergetic effect and achieve improved bioactivity and biospecificity. Herein, the synthesis of an amphiphilic organic-inorganic hybrid is reported by grafting a long-chain organoalkoxysilane lipid onto a POM. The amphiphilic POM hybrid could spontaneously assemble into the vesicles and exhibits enhanced antitumor activity for human colorectal cancer cell lines (HT29) compared to that of parent POMs. This detailed study reveals that the amphiphilic nature of POM hybrids enables the as-formed vesicles to easily bind to the cell membranes and then be uptaken by the cells, thus leading to a substantial increase in antitumor activity. Such prominent antitumor action is mostly accomplished via cell apoptosis, which ultimately results in cell death. Our finding demonstrates that novel POM hybrids-based drugs with increased bioactivity could be obtained by decorating POMs with selective organic ligands.

Synthesis, structure, self-assembly and genotoxicity evaluation of a series of Mn-Anderson cluster based polyoxometalate–organic hybrids

A new series of POM–organic hybrids have been developed which show less genotoxicity compared to the parent polyoxometalate cluster.

Synthesis of chitosan-coated polyoxometalate nanoparticles against cancer and its metastasis

HeLa cells, before and after treatment with nanoparticles.

Synthesis and Characterization of H3PW12O40 and H3PMo12O40 Nanoparticles by a Simple Method

In this study H₃PW₁₂O₄₀·9H₂O and H₃PMo₁₂O₄₀·6H₂O (HPA) particles were changed into nano forms by heat-treatment in an autoclave as a simple, repaid, inexpensive and one step method. The particle size of these nanoparticles was around 25 nm. The as-synthesized nanostructures were characterized by dynamic light scattering, X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and inductively coupled plasma analyzer. Thermal stability of nanoparticles was surveyed by thermal gravimeter analyse. Acidity of prepared nanoparticles was investigated by pyridine adsorption method. Results showed rising acidity by declining particle size of HPA.

Synthesis of Fe3O4@silica/poly(N-isopropylacrylamide) as a novel thermo-responsive system for controlled release of H3PMo12O40 nano drug in AC magnetic field

In this work, a new method is introduced for synthesis of nano drug for the first time. H₃PMo₁₂O₄₀ and Fe₃O₄@SiO₂/poly(N-isopropylacrylamide), were prepared as nano drug and magneto thermally responsive nano-carrier respectively. Then the released behavior of H₃PMo₁₂O₄₀ nano-drug from this thermo-responsive carrier was investigated in an AC magnetic field. When a drug particle is broken up to nanometer range, the total surface area is increased; therefore the rate of dissolution and the rate of release are increased. The as-synthesized nanostructures were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and Fourier transform infrared spectroscopy (FT-IR). Furthermore, experimental condition which lead to the released profile of H₃PMo₁₂O₄₀ nano-drug from Fe₃O₄@SiO₂/poly(N-isopropylacrylamide), such as strength of magnetic field (H), temperature (T), particle size of drug and content of loaded drug were tested. Increasing the strength of magnetic field, temperature and content of loaded drug, the rate of drug release was also increased.

Targeted delivery of polyoxometalate nanocomposites

Polyoxometalate/carboxymethyl chitosan nanocomposites with an average diameter of 130 nm are synthesized and labeled with fluorescein isothiocyanate (FITC) for a combined drug-carrier and cellular-monitoring approach. [Eu(β(2) -SiW(11) O(39) )(2) ](13-) /CMC nanospheres as a representative example do not display cytotoxicity for POM concentrations up to 2 mg mL(-1) . Cellular uptake of fluoresecently labelled {EuSiW(11) O(39) }/FITC-CMC nanoparticles is monitored with confocal laser scanning microscopy. Nanoparticle uptake occurs after incubation times of around 1 h and no cyctotoxic effects are observed upon prolonged treatment. The preferential location of the POM/CMC nanocomposites in the perinuclear region is furthermore verified with transmission electron microscopy investigations on unlabeled nanoparticles. Therefore, this approach is a promising dual strategy for the safe cellular transfer and monitoring of bioactive POMs.

Antiviral Activity of Metal-Containing Polymers-Organotin and Cisplatin-Like Polymers

Polymers containing platinum and to a lesser extent tin, have repeatedly demonstrated antitumor activity in vitro and in vivo against a variety of cell and tumor types. The mechanisms responsible for the antitumor activity include inducing a delay in cell proliferation and sister chromatid exchanges blocking tumor growth. As most DNA and some RNA viruses require, and even induce, infected cells to initiate DNA replication and subsequent cell division, compounds with antitumor activity will very likely also possess antiviral activity. This article examines the use of metal-containing polymers as a novel class of antivirals.

HIV-1 protease inhibition potential of functionalized polyoxometalates

Polyoxometalates (POMs) are interesting biomedical agents due to their versatile anticancer and antiviral properties, such as remarkable anti-HIV activity. Although POMs are tunable and easily accessible inorganic drug prototypes in principle, their full potential can only be tapped by enhancing their biocompatibility, for example, through organic functionalization. We have therefore investigated the HIV-1 protease inhibition potential of functionalized Keggin- and Dawson-type POMs with organic side chains. Their inhibitory performance was furthermore compared to other POM types, and the buffer dependence of the results is discussed. In addition, chemical shift mapping NMR experiments were performed to exclude POM–substrate interactions. Whereas the introduction of organic side chains into POMs is a promising approach in principle, the influence of secondary effects on the reaction system also merits detailed investigation.

Bis[4,4′-(propane-1,3-diyl)dipiperidinium] β-octamolybdate(VI)

The title compound, bis­[4,4′-(propane-1,3-di­yl)­dipiperidin­ium] β-octa­molybdate(VI), (C₁₃H₂₈N₂)₂[Mo₈O₂₆], was produced by hydro­thermal reaction of an acidified aqueous solution of Na₂MoO₄·2H₂O and 4,4′-trimethyl­ene­dipiperidine (L). The structure of the title compound consists of β-octa­molybdate(VI) anion clusters and protonated [H₂L]²⁺ cations. The octa­molybdate anion is located around an inversion center. N—H⋯O hydrogen bonds between the cations and anions ensure the cohesion of the structure and result in a three-dimensional network.

Structural characterization and reactivity of gamma-octamolybdate functionalized by proline

The reaction of molybdate and dl-proline at pH 3.4 results in the formation of a Na(4)[Mo(8)O(26)(proO)(2)] x 22H(2)O complex (pro=proline) in which two proline ligands are attached to molybdenum(VI) ions via monodentate coordination of the carboxylate groups. The structure of the complex was determined by single crystal X-ray diffraction and by combination of (1)H, (13)C and (95)Mo NMR spectroscopy techniques in solution. The structure of the complex is strongly dependent on the pH. At native pH 3.4 the octamolybdate-type structure seems to be present in solution, but the increase of pH to 5.8 resulted in a rearrangement of the structure to a heptamolybdate-type structure. At physiological pH, the polyoxometalate framework was completely dissociated into the monomeric MoO(4)(2-) unit. The reactivity of the Na(4)[Mo(8)O(26)(proO)(2)] x 22H(2)O towards the hydrolysis of ATP was tested at different pH values. While in solution at pH 3.4 the hydrolysis proceeded to yield AMP (adenosine monophosphate) and ADP (adenosine diphosphate) in nearly equal amounts, reaction mixture at pH 5.8 gave ADP as the only product of hydrolysis after 24h of reaction. At neutral pH, the hydrolysis of ATP was slower, but it proceeded to yield 75% of ADP after 48 h of reaction.