The First Anticancer Tris(pyrazolyl)borate Molybdenum(IV) Complexes: Tested in Vitro and in Vivo-A Comparison of O,O-, S,O-, and N,N-Chelate Effects

Abstract

The synthesis, characterization and biological activity of molybdenum(IV) complexes containing Trofimenko's scorpionato ligand, hydrotris(3-isopropylpyrazolyl)borate (Tp^iPr ), in addition to varying biologically active as well as other conventional ligands is described. Ligands employed include (O,O-) (S,O-) (N,N-) donors that have been successfully coordinated to the molybdenum center by means of oxygen-atom transfer (OAT) reactions from the known Mo^VI starting material, Tp^iPr MoO₂ Cl. The synthesized complexes were characterized by standard analytical methods and where possible by X-ray diffraction analysis. The aqueous stability of the compounds was studied by means of UV/Vis spectroscopy and the impact of the attached ligand scaffolds on the oxidation potentials (Mo^IV to Mo^V ) was studied by cyclic voltammetry. Utilizing polyvinylpyrrolidone (PVP) as a solubilizing agent, adequate aqueous solubility for biological tests was obtained. Anticancer activity tests and preliminary mode of action studies have been performed in vitro and in vivo.

Keywords: anticancer; biological tests; in vivo experiments; molybdenum; tris(pyrazolyl)borate.

Figure 1

General structure for hydrotris(pyrazol‐1H‐yl)borate ligands.

Scheme 1

Synthesis of complexes 1–6.

Figure 2

Crystal structures of compounds 1 (left), 5 (center), and 6 (right), drawn with 50 % displacement ellipsoids. Hydrogen atoms are omitted for clarity.

Figure 3

Cyclic voltammogram of the O,O‐chelates, compounds 1 and 4 in DMF referenced to the NHE.

Figure 4

Representative scatter plots differentiating apoptotic and necrotic from viable cells based on annexin V‐FITC and PI staining after 24 h incubation with complexes 2 (10 μm) and 3 (10 μm) and flow cytometric analysis. (Q1) necrotic cells; (Q2) late apoptotic cells; (Q3) early apoptotic cells; (Q4) viable cells.

Figure 5

Plasmid DNA interaction studies with molybdenum(IV) complexes (2 and 3) were performed by agarose (1 %) gel electrophoresis. The pUC19 plasmid was incubated with compounds 2 and 3 (50 μm) for 15 min, 30 min, 1 h, 2 h, 4 h and 6 h at 37 °C in TE buffer. C _6h corresponds to an untreated control incubated for 6 h, while C _0h corresponds to an untreated control at the beginning of the experiment.

Figure 6

In vivo anticancer activity. CT‐26 cells were injected subcutaneously in the right flank of BALB/c mice. Mice were treated on the days indicated by ▾ intraperitoneal with 5 mg kg⁻¹ 2 or 3. Tumor volumes were calculated as described in the material and method section. Each experimental group contained four animals. Data are means ± SEM Statistical analysis was performed by two‐way ANOVA with Bonferroni post‐test (*, p<0.05; **, p<0.01; compared to control mice).