Unravelling the antitumoral potential of novel bis(thiosemicarbazonato) Zn(II) complexes: structural and cellular studies

The development of pharmacologically active compounds based on bis(thiosemicarbazones) (BTSC) and on their coordination to metal centers constitutes a promising field of research. We have recently explored this class of ligands and their Cu(II) complexes for the design of cancer theranostics agents with enhanced uptake by tumoral cells. In the present work, we expand our focus to aliphatic and aromatic BTSC Zn(II) complexes bearing piperidine/morpholine pendant arms. The new complexes ZnL1-ZnL4 were characterized by a variety of analytical techniques, which included single-crystal X-ray crystallography for ZnL2 and ZnL3. Taking advantage of the fluorescent properties of the aromatic complexes, we investigated their cellular uptake kinetics and subcellular localization. Furthermore, we tried to elucidate the mechanism of action of the cytotoxic effect observed in human cancer cell line models. The results show that the aliphatic complexes (ZnL1 and ZnL2) have a symmetrical structure, while the aromatic counterparts (ZnL3 and ZnL4) have an asymmetrical nature. The cytotoxic activity was higher for the aromatic BTSC complexes, as well as the cellular uptake, evaluated by measurement of intracellular Zn accumulation. Among the most active complexes, ZnL3 presented the fastest uptake kinetics and lysosomal localization assessed by live-cell microscopy. Detailed studies of its impact on cellular production of reactive oxygen species and impairment of lysosomal membrane integrity reinforced the influence of the pendant piperidine in the biological performance of aromatic BTSC Zn(II) complexes.