Impact of Metal Coordination on Cytotoxicity of 3-Aminopyridine-2-carboxaldehyde Thiosemicarbazone (Triapine) and Novel Insights into Terminal Dimethylation

The first metal complexes of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine) were synthesized. Triapine, was prepared by a novel three-step procedure in 64% overall yield. In addition. a series of related ligands, namely, 2-formylpyridine thiosemicarbazone, 2-acetylpyridine thiosemicarbazone, 2-pyridineformamide thiosemicarbazone, and their N-4-dimethylated derivatives (including the N-4-dimethylated analogue of Triapine) were prepared, along with their corresponding gallium(III) and iron(III) complexes with the general formula [M(L)(2)](-) where HL is the respective thiosemicarbazone. The compounds were characterized by elemental analysis, H-1 and C-13 NMR, IR land UV-vis spectroscopies, mass spectrometry, and cyclic voltammetry. In addition. Triapine and its iron(III) and gallium(III) complexes were studied by X-ray crystallography. All ligands and complexes were tested for their in vitro antiproliferative activity in two human cancer cell lines (41M and SK-BR-3), and structure-activity relationships were established. In general, the coordination to gallium(III) increased the cytotoxicity while the iron(III) complexes show reduced cytotoxic activity compared to the metal-free thiosemicarbazones. Selected compounds were investigated for the capacity of inhibiting ribonucleotide reductase by incorporation of H-3-cytidine into DNA.