Emerging Topics in Metal Complexes: Pharmacological Activity.

This Special Issue (SI), "Emerging Topics in Metal Complexes: Pharmacological Activity", includes reports updating our knowledge on metals with multidirectional biological properties and metal-containing compounds/complexes for their potential therapeutic applications, with a focus on strategies improving their pharmacological features [...].

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.

Biological Consequences of Vanadium Effects on Formation of Reactive Oxygen Species and Lipid Peroxidation.

Lipid peroxidation (LPO), a process that affects human health, can be induced by exposure to vanadium salts and compounds. LPO is often exacerbated by oxidation stress, with some forms of vanadium providing protective effects. The LPO reaction involves the oxidation of the alkene bonds, primarily in polyunsaturated fatty acids, in a chain reaction to form radical and reactive oxygen species (ROS). LPO reactions typically affect cellular membranes through direct effects on membrane structure and function as well as impacting other cellular functions due to increases in ROS. Although LPO effects on mitochondrial function have been studied in detail, other cellular components and organelles are affected. Because vanadium salts and complexes can induce ROS formation both directly and indirectly, the study of LPO arising from increased ROS should include investigations of both processes. This is made more challenging by the range of vanadium species that exist under physiological conditions and the diverse effects of these species. Thus, complex vanadium chemistry requires speciation studies of vanadium to evaluate the direct and indirect effects of the various species that are present during vanadium exposure. Undoubtedly, speciation is important in assessing how vanadium exerts effects in biological systems and is likely the underlying cause for some of the beneficial effects reported in cancerous, diabetic, neurodegenerative conditions and other diseased tissues impacted by LPO processes. Speciation of vanadium, together with investigations of ROS and LPO, should be considered in future biological studies evaluating vanadium effects on the formation of ROS and on LPO in cells, tissues, and organisms as discussed in this review.

Rationalizing the Decavanadate(V) and Oxidovanadium(IV) Binding to G-Actin and the Competition with Decaniobate(V) and ATP.

The experimental data collected over the past 15 years on the interaction of decavanadate(V) (V10O286-; V10), a polyoxometalate (POM) with promising anticancer and antibacterial action, with G-actin, were rationalized by using several computational approaches (docking, density functional theory (DFT), and molecular dynamics (MD)). Moreover, a comparison with the isostructural and more stable decaniobate(V) (Nb10O286-; Nb10) was carried out. Four binding sites were identified, named α, ß, γ, and δ, the site α being the catalytic nucleotide site located in the cleft of the enzyme at the interface of the subdomains II and IV. It was observed that the site α is preferred by V10, whereas Nb10 is more stable at the site ß; this indicates that, differently from other proteins, G-actin could contemporaneously bind the two POMs, whose action would be synergistic. Both decavanadate and decaniobate induce conformational rearrangements in G-actin, larger for V10 than Nb10. Moreover, the binding mode of oxidovanadium(IV) ion, VIVO2+, formed upon the reduction of decavanadate(V) by the -SH groups of accessible cysteine residues, is also found in the catalytic site α with (His161, Asp154) coordination; this adduct overlaps significantly with the region where ATP is bound, accounting for the competition between V10 and its reduction product VIVO2+ with ATP, as previously observed by EPR spectroscopy. Finally, the competition with ATP was rationalized: since decavanadate prefers the nucleotide site α, Ca2+-ATP displaces V10 from this site, while the competition is less important for Nb10 because this POM shows a higher affinity for ß than for site α. A relevant consequence of this paper is that other metallodrug-protein systems, in the absence or presence of eventual inhibitors and/or competition with molecules of the organism, could be studied with the same approach, suggesting important elements for an explanation of the biological data and a rational drug design.

The Aquaporin-3-Inhibiting Potential of Polyoxotungstates.

Polyoxometalates (POMs) are of increasing interest due to their proven anticancer activities. Aquaporins (AQPs) were found to be overexpressed in tumors bringing particular attention to their inhibitors as anticancer drugs. Herein, we report for the first time the ability of polyoxotungstates (POTs), such as of Wells-Dawson P2W18, P2W12, and P2W15, and Preyssler P5W30 structures, to affect aquaporin-3 (AQP3) activity and impair melanoma cell migration. The tested POTs were revealed to inhibit AQP3 function with different effects, with P2W18, P2W12, and P5W30 being the most potent (50% inhibitory concentration (IC50) = 0.8, 2.8, and 3.2 µM), and P2W15 being the weakest (IC50 > 100 µM). The selectivity of P2W18 toward AQP3 was confirmed in yeast cells transformed with human aquaglyceroporins. The effect of P2W12 and P2W18 on melanoma cells that highly express AQP3 revealed an impairment of cell migration between 55% and 65% after 24 h, indicating that the anticancer properties of these compounds may in part be due to the blockage of AQP3-mediated permeability. Altogether, our data revealed that P2W18 strongly affects AQP3 activity and cancer cell growth, unveiling its potential as an anticancer drug against tumors where AQP3 is highly expressed.

Polyoxometalates as Potential Next-Generation Metallodrugs in the Combat Against Cancer.

Polyoxometalates (POMs) are an emerging class of inorganic metal oxides, which over the last decades demonstrated promising biological activities by the virtue of their great diversity in structures and properties. They possess high potential for the inhibition of various tumor types; however, their unspecific interactions with biomolecules and toxicity impede their clinical usage. The current focus of the field of biologically active POMs lies on organically functionalized and POM-based nanocomposite structures as these hybrids show enhanced anticancer activity and significantly reduced toxicity towards normal cells in comparison to unmodified POMs. Although the antitumor activity of POMs is well documented, their mechanisms of action are still not well understood. In this Review, an overview is given of the cytotoxic effects of POMs with a special focus on POM-based hybrid and nanocomposite structures. Furthermore, we aim to provide proposed mode of actions and to identify molecular targets. POMs are expected to develop into the next generation of anticancer drugs that selectively target cancer cells while sparing healthy cells.

An EXAFS Approach to the Study of Polyoxometalate-Protein Interactions: The Case of Decavanadate-Actin.

EXAFS and XANES experiments were used to assess decavanadate interplay with actin, in both the globular and polymerized forms, under different conditions of pH, temperature, ionic strength, and presence of ATP. This approach allowed us to simultaneously probe, for the first time, all vanadium species present in the system. It was established that decavanadate interacts with G-actin, triggering a protein conformational reorientation that induces oxidation of the cysteine core residues and oxidovanadium (VIV) formation. The local environment of vanadium's absorbing center in the [decavanadate-protein] adducts was determined, a V-SCys coordination having been verified experimentally. The variations induced in decavanadate's EXAFS profile by the presence of actin were found to be almost totally reversed by the addition of ATP, which constitutes a solid proof of decavanadate interaction with the protein at its ATP binding site. Additionally, a weak decavanadate interplay with F-actin was suggested to take place, through a mechanism different from that inferred for globular actin. These findings have important consequences for the understanding, at a molecular level, of the significant biological activities of decavanadate and similar polyoxometalates, aiming at potential pharmacological applications.

Unveiling the agonistic properties of Preyssler-type Polyoxotungstates on purinergic P2 receptors.

The Preyssler-type polyoxotungstate ({P5W30}) belongs to the family of polyanionic metal-oxides formed by group V and VI metal ions, such as V, Mo and W, commonly known as polyoxometalates (POMs). POMs have demonstrated inhibitory effect on a significant number of ATP-binding proteins in vitro. Purinergic P2 receptors, widely expressed in eukaryotic cells, contain extracellularly oriented ATP-binding sites and play many biological roles with health implications. In this work, we use the immortalized mouse hippocampal neuronal HT-22 cells in culture to study the effects of {P5W30} on the cytosolic Ca2+ concentration. Changes in cytosolic Ca2+ concentration were monitored using fluorescence microscopy of HT-22 cells loaded with the fluorescent Ca2+ indicator Fluo3. 31P-Nuclear magnetic resonance measurements of {P5W30} indicate its stability in the medium used for cytosolic Ca2+ measurements for over 30 min. The findings reveal that addition of {P5W30} to the extracellular medium induces a sustained increase of the cytosolic Ca2+ concentration within minutes. This Ca2+ increase is triggered by extracellular Ca2+ entry into the cells and is dose-dependent, with a half-of-effect concentration of 0.25 ± 0.05 µM {P5W30}. In addition, after the {P5W30}-induced cytosolic Ca2+ increase, the transient Ca2+ peak induced by extracellular ATP is reduced up to 100% with an apparent half-of-effect concentration of 0.15 ± 0.05 µM {P5W30}. Activation of metabotropic purinergic P2 receptors affords about 80% contribution to the increase of Fluo3 fluorescence elicited by {P5W30} in HT-22 cells, whereas ionotropic receptors contribute, at most, with 20%. These results suggest that {P5W30} could serve as a novel agonist of purinergic P2 receptors.

Metals and Metal Complexes in Diseases with a Focus on COVID-19: Facts and Opinions.

In the present Special Issue on "Metals and Metal Complexes in Diseases with a Focus on COVID-19: Facts and Opinions", an attempt has been made to include reports updating our knowledge of elements considered to be potential candidates for therapeutic applications and certain metal-containing species, which are extensively being examined towards their potential biomedical use due to their specific physicochemical properties [...].

Repurposing Therapeutic Drugs Complexed to Vanadium in Cancer.

Repurposing drugs by uncovering new indications for approved drugs accelerates the process of establishing new treatments and reduces the high costs of drug discovery and development. Metal complexes with clinically approved drugs allow further opportunities in cancer therapy-many vanadium compounds have previously shown antitumor effects, which makes vanadium a suitable metal to complex with therapeutic drugs, potentially improving their efficacy in cancer treatment. In this review, covering the last 25 years of research in the field, we identified non-oncology-approved drugs suitable as ligands to obtain different vanadium complexes. Metformin-decavanadate, vanadium-bisphosphonates, vanadyl(IV) complexes with non-steroidal anti-inflammatory drugs, and cetirizine and imidazole-based oxidovanadium(IV) complexes, each has a parent drug known to have different medicinal properties and therapeutic indications, and all showed potential as novel anticancer treatments. Nevertheless, the precise mechanisms of action for these vanadium compounds against cancer are still not fully understood.

Evaluation of Gold Complexes to Address Bacterial Resistance, Quorum Sensing, Biofilm Formation, and Their Antiviral Properties against Bacteriophages.

The worldwide increase in antibiotic resistance poses a significant challenge, and researchers are diligently seeking new drugs to combat infections and prevent bacterial pathogens from developing resistance. Gold (I and III) complexes are suitable for this purpose. In this study, we tested four gold (I and III) complexes, (1) chlorotrimethylphosphine gold(I); (2) chlorotriphenylphosphine gold(I); (3) dichloro(2-pyridinecarboxylate) gold (III); and (4) 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene gold(I) chloride, for their antibacterial, antibiofilm, antiviral, and anti-quorum sensing activities. Results reveal that 1 significantly inhibits Escherichia coli DSM 1077 and Staphylococcus aureus ATCC 6538, while 2, 3, and 4 only inhibit S. aureus ATCC 6538. The minimum inhibitory concentration (MIC) of 1 for S. aureus ATCC 6538 is 0.59 µg/mL (1.91 µM), and for methicillin-resistant S. aureus strains MRSA 12 and MRSA 15, it is 1.16 µg/mL (3.75 µM). For E. coli DSM 1077 (Gram-negative), the MIC is 4.63 µg/mL (15 µM), and for multi-resistant E. coli I731940778-1, it is 9.25 µg/mL (30 µM). Complex 1 also disrupts biofilm formation in E. coli and S. aureus after 6 h or 24 h exposure. Moreover, 1 and 2 inhibit the replication of two enterobacteria phages. Anti-quorum sensing potential still requires further clarification. These findings highlight the potential of gold complexes as effective agents to combat bacterial and viral infections.

The Preyssler-Type Polyoxotungstate Exhibits Anti-Quorum Sensing, Antibiofilm, and Antiviral Activities.

The increase in bacterial resistance to antibiotics has led researchers to find new compounds or find combinations between different compounds with potential antibacterial action and with the ability to prevent the development of antibiotic resistance. Polyoxotungstates (POTs) are inorganic clusters that may fulfill that need, either individually or in combination with antibiotics. Herein, we report the ability of the polyoxotungstates (POTs) with Wells-Dawson P2W18, P2W17, P2W15, and Preyssler P5W30 type structures to differently affect Gram-negative and Gram-positive microorganisms, either susceptible or resistant to antibiotics. The compound P5W30 showed the highest activity against the majority of the tested bacterial strains in comparison with the other tested POTs (P2W15, P2W17 and P2W18) that did not show inhibition zones for the Gram-negative bacteria, A. baumanii I73775, E. coli DSM 1077, E. coli I73194, K. pneumoniae I7092374, and P. aeruginosa C46281). Generally, the results evidenced that Gram-positive bacteria are more susceptible to the POTs tested. The compound P5W30 was the one most active against S. aureus ATCC 6538 and MRSA16, reaching <0.83 mg·mL−1 (100 µM) and 4.96 mg·mL−1 (600 µM), respectively. Moreover, it was verified by NMR spectroscopy that the most promising POT, P5W30, remains intact under all the experimental conditions, after 24 h at 37 °C. This prompted us to further evaluate the anti-quorum sensing activity of P5W30 using the biosensor Chromobacterium violaceum CV026, as well as its antibiofilm activity both individually and in combination with the antibiotic cefoxitin against the methicillin-resistant Staphylococcus aureus 16 (MRSA16). P5W30 showed a synergistic antibacterial effect with the antibiotic cefoxitin and chloramphenicol against MRSA16. Moreover, the antibiofilm activity of P5W30 was more pronounced when used individually, in comparison with the combination with the antibiotic cefoxitin. Finally, the antiviral activity of P5W30 was tested using the coliphage Qß, showing a dose-dependent response. The maximum inactivation was observed at 750 µM (6.23 mg·mL−1). In sum, P5W30 shows anti-quorum sensing and antibiofilm activities besides being a potent antibacterial agent against S. aureus and to exhibit antiviral activities against enteric viruses.

Decavanadate and metformin-decavanadate effects in human melanoma cells.

Decavanadate is a polyoxometalate (POMs) that has shown extensive biological activities, including antidiabetic and anticancer activity. Importantly, vanadium-based compounds as well as antidiabetic biguanide drugs, such as metformin, have shown to exert therapeutic effects in melanoma. A combination of these agents, the metformin-decavanadate complex, was also recognized for its antidiabetic effects and recently described as a better treatment than the monotherapy with metformin enabling lower dosage in rodent models of diabetes. Herein, we compare the effects of decavanadate and metformin-decavanadate on Ca2+-ATPase activity in sarcoplasmic reticulum vesicles from rabbit skeletal muscles and on cell signaling events and viability in human melanoma cells. We show that unlike the decavanadate-mediated non-competitive mechanism, metformin-decavanadate inhibits Ca2+-ATPase by a mixed-type competitive-non-competitive inhibition with an IC50 value about 6 times higher (87 µM) than the previously described for decavanadate (15 µM). We also found that both decavanadate and metformin-decavanadate exert antiproliferative effects on melanoma cells at 10 times lower concentrations than monomeric vanadate. Western blot analysis revealed that both, decavanadate and metformin-decavanadate increased phosphorylation of extracellular signal-regulated kinase (ERK) and serine/threonine protein kinase AKT signaling proteins upon 24 h drug exposure, suggesting that the anti-proliferative activities of these compounds act independent of growth-factor signaling pathways.

Inhibition of SERCA and PMCA Ca2+-ATPase activities by polyoxotungstates.

Plasma membrane calcium ATPases (PMCA) and sarco(endo) reticulum calcium ATPases (SERCA) are key proteins in the maintenance of calcium homeostasis. Herein, we compare for the first time the inhibition of SERCA and PMCA calcium pumps by several polyoxotungstates (POTs), namely by Wells-Dawson phosphotungstate anions [P2W18O62]6- (intact, {P2W18}), [P2W17O61]10- (monolacunary, {P2W17}), [P2W15O56]12- (trilacunary, {P2W15}), [H2P2W12O48]12- (hexalacunary, {P2W12}), [H3P2W15V3O62]6- (trivanadium-substituted, {P2W15V3}) and by Preyssler-type anion [NaP5W30O110]14- ({P5W30}). The speciation in the solutions of tested POTs was investigated by 31P and 51V NMR spectroscopy. The tested POTs inhibited SERCA Ca2+-ATPase activity, whereby the Preyssler POT showed the strongest effect, with an IC50 value of 0.37 µM. For {P2W17} and {P2W15V3} higher IC50 values were determined: 0.72 and 0.95 µM, respectively. The studied POTs showed to be more potent inhibitors of PMCA Ca2+-ATPase activity, with lower IC50 values for {P2W17}, {P5W30} and {P2W15V3}.

Inhibition of Na+/K+- and Ca2+-ATPase activities by phosphotetradecavanadate.

Polyoxometalates (POMs) are promising inorganic inhibitors for P-type ATPases. The experimental models used to study the effects of POMs on these ATPases are usually in vitro models using vesicles from several membrane sources. Very recently, some polyoxotungstates, such as the Dawson anion [P2W18O62]6-, were shown to be potent P-type ATPase inhibitors; being active in vitro as well as in ex-vivo. In the present study we broaden the spectrum of highly active inhibitors of Na+/K+-ATPase from basal membrane of epithelial skin to the bi-capped Keggin-type anion phosphotetradecavanadate Cs5.6H3.4PV14O42 (PV14) and we confront the data with activity of other commonly encountered polyoxovanadates, decavanadate (V10) and monovanadate (V1). The X-ray crystal structure of PV14 was solved and contains two trans-bicapped α-Keggin anions HxPV14O42(9-x)-. The anion is built up from the classical Keggin structure [(PO4)@(V12O36)] capped by two [VO] units. PV14 (10 µM) exhibited higher ex-vivo inhibitory effect on Na+/K+-ATPase (78%) than was observed at the same concentrations of V10 (66%) or V1 (33%). Moreover, PV14 is also a potent in vitro inhibitor of the Ca2+-ATPase activity (IC50 5 µM) exhibiting stronger inhibition than the previously reported activities for V10 (15 µM) and V1 (80 µM). Putting it all together, when compared both P-typye ATPases it is suggested that PV14 exibited a high potential to act as an in vivo inhibitor of the Na+/K+-ATPase associated with chloride secretion.

Binding modes of decavanadate to myosin and inhibition of the actomyosin ATPase activity.

Decavanadate, a vanadate oligomer, is known to interact with myosin and to inhibit the ATPase activity, but the putative binding sites and the mechanism of inhibition are still to be clarified. We have previously proposed that the decavanadate (V(10)O(28)(6-)) inhibition of the actin-stimulated myosin ATPase activity is non-competitive towards both actin and ATP. A likely explanation for these results is that V(10) binds to the so-called back-door at the end of the Pi-tube opposite to the nucleotide-binding site. In order to further investigate this possibility, we have carried out molecular docking simulations of the V(10) oligomer on three different structures of the myosin motor domain of Dictyostelium discoideum, representing distinct states of the ATPase cycle. The results indicate a clear preference of V(10) to bind at the back-door, but only on the "open" structures where there is access to the phosphate binding-loop. It is suggested that V(10) acts as a "back-door stop" blocking the closure of the 50-kDa cleft necessary to carry out ATP-gamma-phosphate hydrolysis. This provides a simple explanation to the non-competitive behavior of V(10) and spurs the use of the oligomer as a tool to elucidate myosin back-door conformational changes in the process of muscle contraction.

Vanadate proliferative and anti-mineralogenic effects are mediated by MAPK and PI-3K/Ras/Erk pathways in a fish chondrocyte cell line.

We recently reported proliferative and anti-mineralogenic effects of vanadate on fish chondrocytes and here we investigate the signalling pathways associated with these effects. Our data show that vanadate stimulates chondrocyte proliferation through the MAPK pathway, using signalling mechanisms similar to those used by IGF-1, while it inhibits chondrocyte differentiation/mineralization through a putative PI-3K/Ras/Erk signalling, a pathway shared with insulin. Our data also suggest that vanadate impairs ECM mineralization not only by interfering with regulatory pathways but also by inhibiting enzymatic activity of ALP. Finally, this work provides additional evidence for the conservation, throughout evolution, of mechanisms regulating chondrocyte proliferation and differentiation.

Sarcoplasmic reticulum calcium ATPase is inhibited by organic vanadium coordination compounds: pyridine-2,6-dicarboxylatodioxovanadium(V), BMOV, and an amavadine analogue.

The general affinity of the sarcoplasmic reticulum (SR) Ca (2+)-ATPase was examined for three different classes of vanadium coordination complexes including a vanadium(V) compound, pyridine-2,6-dicarboxylatodioxovanadium(V) (PDC-V(V)), and two vanadium(IV) compounds, bis(maltolato)oxovanadium(IV) (BMOV), and an analogue of amavadine, bis( N-hydroxylamidoiminodiacetato)vanadium(IV) (HAIDA-V(IV)). The ability of vanadate to act either as a phosphate analogue or as a transition-state analogue with enzymes' catalysis phosphoryl group transfer suggests that vanadium coordination compounds may reveal mechanistic preferences in these classes of enzymes. Two of these compounds investigated, PDC-V(V) and BMOV, were hydrolytically and oxidatively reactive at neutral pH, and one, HAIDA-V(IV), does not hydrolyze, oxidize, or otherwise decompose to a measurable extent during the enzyme assay. The SR Ca (2+)-ATPase was inhibited by all three of these complexes. The relative order of inhibition was PDC-V(V) > BMOV > vanadate > HAIDA-V(IV), and the IC 50 values were 25, 40, 80, and 325 microM, respectively. Because the observed inhibition is more potent for PDC-V(V) and BMOV than that of oxovanadates, the inhibition cannot be explained by oxovanadate formation during enzyme assays. Furthermore, the hydrolytically and redox stable amavadine analogue HAIDA-V(IV) inhibited the Ca (2+)-ATPase less than oxovanadates. To gauge the importance of the lipid environment, studies of oxidized BMOV in microemulsions were performed and showed that this system remained in the aqueous pool even though PDC-V(V) is able to penetrate lipid interfaces. These findings suggest that the hydrolytic properties of these complexes may be important in the inhibition of the calcium pump. Our results show that two simple coordination complexes with known insulin enhancing effects can invoke a response in calcium homeostasis and the regulation of muscle contraction through the SR Ca (2+)-ATPase.

The antibacterial activity of polyoxometalates: structures, antibiotic effects and future perspectives.

Polyoxometalates (POMs) are, mostly anionic, metal oxide compounds that span a wide range of tunable physical and chemical features rendering them very interesting for biological purposes, an continuously emerging but little explored field. Due to their biological and biochemical effects, including antitumor, -viral and -bacterial properties, POMs and POM-based systems are considered as promising future metallodrugs. In this article, we focus on the antibacterial activity of POMs and their therapeutic potential in the battle against bacteria and their increasing resistance against pharmaceuticals. Recent advances in the synthesis of POMs are highlighted, with emphasis on the development and properties of biologically active POM-based hybrid and nanocomposite structures. By analysing the antibacterial activity and structure of POMs, putative mode of actions are provided, including potential targets for POM-protein interactions, and a structure-activity-relationship was established for a series of POMs against two bacteria, namely Helicobacter pylori and Streptococcus pneumoniae.

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.