Gallium(III) complexes of α-N-heterocyclic piperidylthiosemicarbazones: Synthesis, structure-activity relationship, cellular uptake and activation of caspases-3/7/9.
Two types of α-N-heterocyclic piperidylthiosemicarbazone ligands and related Ga(III) complexes were synthesized. The structure of Ga4 and Ga5 were characterized by X-ray single crystal diffraction. We generated the related α-N-heterocycliperidinylthiosemicarbazone analogs to examine the effect of aldehydes or ketones in the Schiff base. The antitumor activity of both type ligands increased after coordination with gallium. Interestingly, the antitumor activity of gallium complexes containing pyridyl groups (first type ligands) is higher than that of pyrazine group-containing (second type ligands) complexes. Gallium complexes significantly depleted cellular iron, resulting in upregulation of transferrin receptor-1 and downregulation of ferritin. They also effectively activate the caspase family proteins (caspase-3/7/9), promote the release of cytochromes from the mitochondria, and ultimately lead to apoptosis.