Polyoxometalates Impact as Anticancer Agents

Polyoxometalates (POMs) are oxoanions of transition metal ions, such as V, Mo, W, Nb, and Pd, forming a variety of structures with a wide range of applications. Herein, we analyzed recent studies on the effects of polyoxometalates as anticancer agents, particularly their effects on the cell cycle. To this end, a literature search was carried out between March and June 2022, using the keywords "polyoxometalates" and "cell cycle". The effects of POMs on selected cell lines can be diverse, such as their effects in the cell cycle, protein expression, mitochondrial effects, reactive oxygen species (ROS) production, cell death and cell viability. The present study focused on cell viability and cell cycle arrest. Cell viability was analyzed by dividing the POMs into sections according to the constituent compound, namely polyoxovanadates (POVs), polyoxomolybdates (POMos), polyoxopaladates (POPds) and polyoxotungstates (POTs). When comparing and sorting the IC₅₀ values in ascending order, we obtained first POVs, then POTs, POPds and, finally, POMos. When comparing clinically approved drugs and POMs, better results of POMs in relation to drugs were observed in many cases, since the dose required to have an inhibitory concentration of 50% is 2 to 200 times less, depending on the POMs, highlighting that these compounds could become in the future an alternative to existing drugs in cancer therapy.

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

{BiW8 O30 } Exerts Antitumor Effect by Triggering Pyroptosis and Upregulating Reactive Oxygen Species

We successfully synthesized {BiW8 }, a 10-nuclear heteroatom cluster modified {BiW8 O30 }. At 24 h post-incubation, the IC50 values of {BiW8 } against HUVEC, MG63, RD, Hep3B, HepG2, and MCF7 cells were 895.8, 127.3, 344.3, 455.0, 781.3, and 206.3 μM, respectively. The IC50 value of {BiW8 } on the MG63 cells was more than 2-fold lower than that of the other raw materials. Through morphological and functional features, we demonstrated pyroptosis as a newly identified mechanism of cell death induced by {BiW8 }. {BiW8 } increased 2-fold reactive oxygen species (ROS) levels in MG63 cells at 24 h post-incubation. Compared with 0 h, the glutathione (GSH) content decreased by 59, 65, 75, 94, and 97 % at 6, 12, 24, 36 and 48 h post-incubation, respectively. Furthermore, multiple antitumor mechanisms of {BiW8 } were identified via transcriptome analysis and chemical simulation, including activation of pyroptosis, suppression of GSH generation, depletion of GSH, and inhibition of DNA repair.

Metal-Coordinated Supramolecular Self-Assemblies for Cancer Theranostics

Metal-coordinated supramolecular nanoassemblies have recently attracted extensive attention as materials for cancer theranostics. Owing to their unique physicochemical properties, metal-coordinated supramolecular self-assemblies can bridge the boundary between traditional inorganic and organic materials. By tailoring the structural components of the metal ions and binding ligands, numerous multifunctional theranostic nanomedicines can be constructed. Metal-coordinated supramolecular nanoassemblies can modulate the tumor microenvironment (TME), thus facilitating the development of TME-responsive nanomedicines. More importantly, TME-responsive organic-inorganic hybrid nanomaterials can be constructed in vivo by exploiting the metal-coordinated self-assembly of a variety of functional ligands, which is a promising strategy for enhancing the tumor accumulation of theranostic molecules. In this review, recent advancements in the design and fabrication of metal-coordinated supramolecular nanomedicines for cancer theranostics are highlighted. These supramolecular compounds are classified according to the order in which the coordinated metal ions appear in the periodic table. Furthermore, the prospects and challenges of metal-coordinated supramolecular self-assemblies for both technical advances and clinical translation are discussed. In particular, the superiority of TME-responsive nanomedicines for in vivo coordinated self-assembly is elaborated, with an emphasis on strategies that enhance the accumulation of functional components in tumors for an ideal theranostic outcome.

Organo-functionalized polyoxovanadates: crystal architecture and property aspects

Polyoxovanadates (POVs), as one of the most prominent members of polyoxometalates (POMs), have been subject to extensive studies by virtue of their aesthetically intriguing structures and potential applications in catalysis, magnetism, and optics, among others. In recent years, organo-functionalized POVs have received considerable attention due to the combination of the advantages of POVs with the importance of organic species. In this review, the key developments of polyoxovanadates and, particularly, the achievements that are related to polyoxovanadates modified with organic ligands and transition metal-organic ligand are summarized. Herein, we systematically introduce the structural features of organo-functionalized POVs and their main applications involved in the magnetism and catalysis aspects. Finally, the current challenges and future prospects in the design, synthesis, and property investigation of polyoxovanadates are also discussed.

How to not build a cage: endohedral functionalization of polyoxometalate-based metal-organic polyhedra

Introducing functionalities into the interior of metal-organic cage complexes can confer properties and utilities (e.g. catalysis, separation, drug delivery, and guest recognition) that are distinct from those of unfunctionalized cages. Endohedral functionalization of such cage molecules, for decades, has largely relied on modifying their organic linkers to covalently append targeted functional groups to the interior surface. We herein introduce an effective coordination method to bring in functionalities at the metal sites instead, for a set of polyhedral cages where the nodes are in situ formed polyoxovanadate clusters, [VIV 6O6(OCH3)9(μ6-SO4)(COO)3]2-. Replacing the central sulfates of these hexavanadate clusters with more strongly coordinating phosphonate groups allows the installation of functionalities within the cage cavities. Organophosphonates with phenyl, biphenyl, and terphenyl tails were examined for internalization. Depending on the size/shape of the cavities, small phosphonates can fit into the molecular containers whereas larger ones inhibit or transform the framework architecture, whereby the first non-cage complex was isolated from a reaction that otherwise would lead to entropically favored regular polyhedra cages. The results highlight the complex and dynamic nature of the self-assembly process involving polyoxometalates and the scope of molecular variety accessible by the introduction of endo functional groups.

Inhibition of Mitochondrial ATP Synthesis and Regulation of Oxidative Stress Based on {SbW8 O30 } Determined by Single-Cell Proteomics Analysis

The 10-nuclear heteroatom cluster modified {SbW₈ O₃₀ } was successfully synthesized and exhibited inhibitory activity (IC₅₀ =0.29 μM). Based on proteomics analysis, Na₄ Ni₂ Sb₂ W₂ -SbW₈ inhibited ATP production by affecting the expression of 16 related proteins, hindering metabolic functions in vivo and cell proliferation due to reactive oxygen species (ROS) stress. In particular, the low expression of FAD/FMN-binding redox enzymes (relative expression ratio of the experimental group to the control=0.43843) could be attributed to the redox mechanism of Na₄ Ni₂ Sb₂ W₂ -SbW₈ , which was consistent with the effect of polyoxometalates (POMs) and FMN-binding proteins on ATP formation. An electrochemical study showed that Na₄ Ni₂ Sb₂ W₂ -SbW₈ combined with FMN to form Na₄ Ni₂ Sb₂ W₂ -SbW₈ -2FMN complex through a one-electron process of the W atoms. Na₄ Ni₂ Sb₂ W₂ -SbW₈ acted as catalase and glutathione peroxidase to protect the cell from ROS stress, and the inhibition rates were 63.3 % at 1.77 μM of NADPH and 86.06 % at 10.62 μM of 2-hydroxyterephthalic acid. Overall, our results showed that POMs can be specific oxidative/antioxidant regulatory agents.

PtIV- or MoVI-substituted decavanadates inhibit the growth of Mycobacterium smegmatis

Inhibitory effects of two monosubstituted decavanadates by Pt^IV in monoplatino(IV)nonavanadate(V) ([H₂Pt^IVV₉O₂₈]^5-, V₉Pt), and by Mo^IV in monomolybdo(VI)nonavanadate(V) ([Mo^VIV₉O₂₈]^5-,V₉Mo) were investigated against the growth of Mycobacterium smegmatis with the EC₅₀ values of 0.0048 mM and 0.015 mM, respectively. These compare to the reported inhibitory value for decavanadate ([V₁₀O₂₈]^6-/[HV₁₀O₂₈]^5-, V₁₀) on Mycobacterium smegmatis (EC₅₀ = 0.0037 mM). Time-dependent ⁵¹V NMR spectroscopic studies were carried out for all three polyanions in aqueous solution, biological medium (7H9), heated and non-heated supernatant to evaluate their stability in their respective media, monitor their hydrolysis to form various oxovanadates over time and calculate the EC₅₀ values. These studies allow us to calculate adjusted and maximum EC₅₀ for the polyoxovanadate (POV) present in solution at the beginning of the study when there is most intact anion in the media and thus the EC₅₀ values represent the initial effects of the POVs. The results have shown that V₁₀ is 1.3 times more potent than V₉Pt and 4 times more potent than V₉Mo, indicating that the inhibitory effects of monosubstituted polyanions are related to the V₁₀ structure. We attributed the minor differences in the growth inhibitory effects to the differences in charges (5- vs 6-) of V₉Pt and V₉Mo compared to V₁₀ and/or the differences in the chemical composition. We concluded that the potency of the growth inhibition by V₁₀ is mainly due to the chemical properties of the vanadium and the decametalate's unique structure even though the presence of the Mycobacterium smegmatis facilitate hydrolysis of the anions. SYNOPSIS: Two decavanadate derivatives, monoplatino(IV)nonavanadate(V) ([H₂Pt^IVV₉O₂₈]^5-), monomolybdo(VI)nonavanadate(V) ([Mo^VIV₉O₂₈]^5-) and decavanadate are more potent growth inhibitors of Mycobacterium smegmatis than monomeric vanadate. The spectroscopic characterization carried out in the growth medium led to the conclusion that both the decavanadate structure and its properties are important for its growth effects.

Polyoxovanadate-iodobodipy supramolecular assemblies: new agents for high efficiency cancer photochemotherapy

Two novel polyoxovanadate-iodoBodipy supramolecular assemblies, named as (2I-BDP-C6)2V6 and (2I-BDP-C6)3V10, were first synthesized by the self-assembly of anionic hexavanadate and decavanadate with cationic iodoBodipy for photochemotherapy, respectively. The mechanisms for synergistic photochemotherapy of the anion-cation pairs were determined. In particular, (2I-BDP-C6)3V10 can effectively kill lung cancer cells (HepG2) by synergetic chemotherapy as well as photodynamic therapy.