Polyoxovanadates are a subclass of polyoxometalates (POMs) known to interact with proteins and to present anticancer, antimicrobial, and antiviral activities. Herein, we aimed to pursue the study of the breast anticancer activity of a mixed-valence polyoxovanadate, [Cl@V^V₇V^IV₈O₃₆]⁶⁻ (V₁₅) against MCF-7 and MDA-MB-231 cancer cell lines and to analyze its Ca²⁺-ATPase inhibition potential. ⁵¹V NMR and UV-Vis/NIR studies of V₁₅ indicated its stability in HEPES and RPMI media. For the Ca²⁺-ATPase activity, V₁₅ showed an IC₅₀ value of 14.2 μM and a mixed type of inhibition. The electrostatic potential map of V₁₅ and other POMs were correlated with the enzyme activity inhibition. V₁₅ also exhibited cytotoxicity against MDA-MB-231 (IC₅₀ = 17.2 μM) and MCF-7 (IC₅₀ = 15.1 μM) breast cancer cell lines. Using V₁₅ concentrations equivalent to half and 1/4 of the IC_50, it was observed that MDA-MB-231 cell migration was reduced by 90 and 70%, after 24 h, respectively. Moreover, V₁₅ caused morphological changes from fusiform to an epithelial-like (amoeboid) shape. Finally, V₁₅ induced the increase in RIPK1, MLKL, and RIPK3 gene expression, up to 3, 10, and 15-fold, respectively, pointing out that the mechanisms of cell death in the triple-negative breast cancer cell line may occur by necroptosis.

The mixed-valence polyoxovanadate (MV-POV) (NBu4)5[I@VIV10VV8O42] (V18), where NBu4+ = tetra-n-butylammonium, was synthesized, forming dark green films with good adherence to glass and poly(vinyl chloride), PVC, directly from the supernatant of the reaction. V18 solid was characterized by elemental analysis, spectroscopic techniques, thermal analysis and scanning electron microscopy (SEM). V18 showed an irreversible phase transition from microcrystalline to amorphous at 66 °C. Flexible, translucid, green films, named PVC+V18, were prepared from a solution of PVC and V18 (1 wt.%, cyclohexanone) through solvent-casting and spin coating methods. Spectroscopic analysis indicated that V18 retained its polynuclear structure within PVC. The glass transition temperature (Tg) of PVC decreased from 82.6 to 30.3 °C after V18 incorporation, revealing a plasticizing effect. Energy dispersive spectroscopy (EDS) mapping and atomic force microscopy (AFM) analysis showed homogeneous films with low roughness (mean roughness (Ra) = 0.288 nm). The direct and indirect optical energy gaps of PVC reduce from 5.20 to 4.73 eV and from 4.38 to 3.45 eV, respectively. Molecular docking simulations revealed a strong interaction, of 112.6 kcal mol–1, between the polymeric chain and V18, primarily mediated by C–H···O hydrogen bonds. Thermal studies of PVC+V18 films showed an irreversible color change from green to black (30 h at 100 °C; 20 min at 170 °C).