Are the metal identity and stoichiometry of metal complexes important for colchicine site binding and inhibition of tubulin polymerization?

Quite recently we discovered that copper(II) complexes with isomeric morpholine-thiosemicarbazone hybrid ligands show good cytotoxicity in cancer cells and that the molecular target responsible for this activity might be tubulin. In order to obtain better lead drug candidates, we opted to exploit the power of coordination chemistry to (i) assemble structures with globular shape to better fit the colchicine pocket and (ii) vary the metal ion. We report the synthesis and full characterization of bis-ligand cobalt(III) and iron(III) complexes with 6-morpholinomethyl-2-formylpyridine 4 N -(4-hydroxy-3,5-dimethylphenyl)-3-thiosemicarbazone (HL1), 6-morpholinomethyl-2-acetylpyridine 4 N -(4-hydroxy-3,5-dimethylphenyl)-3-thiosemicarbazone (HL2), and 6-morpholinomethyl-2-formylpyridine 4 N -phenyl-3-thiosemicarbazone (HL3), and mono -ligand nickel(II), zinc(II) and palladium(II) complexes with HL1, namely [Co III (HL 1 )(L 1 )](NO 3 ) 2 (1), [Co III (HL 2 )(L 2 )](NO 3 ) 2 (2), [Co III (HL 3 )(L 3 )](NO 3 ) 2 (3), [Fe III (L 2 ) 2 ]NO 3 (4), [Fe III (HL 3 )(L 3 )](NO 3 ) 2 (5), [Ni II (L 1 )]Cl (6), [Zn(L 1 )Cl] (7) and [Pd II (HL 1 )Cl]Cl (8). We discuss the effect of the metal identity and metal complex stoichiometry on in vitro cytotoxicity and antitubulin activity. The high antiproliferative activity of complex 4 correlated well with inhibition of tubulin polymerization. Insights into the mechanism of antiproliferative activity were supported by experimental results and molecular docking calculations.