Copper(II) Complexes with Isomeric Morpholine-Substituted 2-Formylpyridine Thiosemicarbazone Hybrids as Potential Anticancer Drugs Inhibiting Both Ribonucleotide Reductase and Tubulin Polymerization: The Morpholine Position Matters

Abstract

The development of copper(II) thiosemicarbazone complexes as potential anticancer agents, possessing dual functionality as inhibitors of R2 ribonucleotide reductase (RNR) and tubulin polymerization by binding at the colchicine site, presents a promising avenue for enhancing therapeutic effectiveness. Herein, we describe the syntheses and physicochemical characterization of four isomeric proligands H 2 L 3 –H 2 L 6 , with the methylmorpholine substituent at pertinent positions of the pyridine ring, along with their corresponding Cu(II) complexes 3–6. Evidently, the position of the morpholine moiety and the copper(II) complex formation have marked effects on the in vitro antiproliferative activity in human uterine sarcoma MES-SA cells and the multidrug-resistant derivative MES-SA/Dx5 cells. Activity correlated strongly with quenching of the tyrosyl radical (Y•) of mouse R2 RNR protein, inhibition of RNR activity in the cancer cells, and inhibition of tubulin polymerization. Insights into the mechanism of antiproliferative activity, supported by experimental results and molecular modeling calculations, are presented.

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

“…As shown in Table 6 , among the 1 : 1 complexes only the square-planar Pd( ii ) complex 8 showed appreciable inhibitory activity. The square-pyramidal Zn( ii ) complex 7, structurally closely related to the recently reported Cu( ii ) counterpart endowed with appreciable tubulin polymerization inhibitory activity (IC 50 = 7.0 μM), 21 did not show significant inhibitory activity even at 20 μM. Within the bis-ligand complexes, compound 4 showed the highest inhibitory activity so far, indicating that the metal complex stoichiometry and metal identity play an important role.…”

“…Intrigued by the ability of recently reported Cu( ii ) complex with HL 1 to inhibit the polymerization of purified tubulin, 21 we performed similar assays with complexes 1 , 2 , 4 and 6–8 . As a reference for comparison, combretastatin A-4 (CA-4) and Cu(HL 1 )Cl 2 were used.…”

“…aEach experiment was performed 2–3 times, and SD's are presented.bCombretastatin A-4.cTaken from ref. 21.…”

“…Second, the drug can be altered in cells to become more active or less active. As for the Cu( ii ) complex with HL 1 21 complex formation with Fe( iii ) and Pd( ii ) resulted in significant enhancement of the ability of the proligands to inhibit tubulin assembly. The active metal complexes 4 and 8 were further investigated for their abilities to inhibit the binding of [ 3 H]colchicine to tubulin at two different concentrations (5 and 25 μM), with tubulin and colchicine at 0.5 and 5 μM concentrations, respectively ( Table 6 ).…”

“…19,20 In addition, recently we described copper( ii ) complexes with isomeric morpholine-thiosemicarbazone hybrids as the first transition metal complexes of thiosemicarbazones (TSCs) with a 1 : 1 metal-to-ligand ratio, and these complexes bind to tubulin in the colchicine site. 21 Cu( ii ) complexes featuring hybrid TSCs, with the morpholine moiety at each of the four available positions of the pyridine ring and a potentially redox active 2,6-dimethylphenol unit at the end nitrogen atom of the thiosemicarbazide fragment, exhibited significant anticancer activity against human uterine sarcoma MES-SA cells and the multidrug resistant derivative MES-SA/Dx5 cells, with IC 50 values ranging from 1.4 μM to 13.1 μM. Notably, the compound bearing the morpholine moiety at position 6 of the pyridine ring exhibited the greatest antiproliferative activity (the lowest IC 50 values) in the cancer cell lines and inhibited tubulin polymerization by binding to the colchicine site.…”