| Summary: | Three new 6-methyl-2-oxo-1,2-dihydroquinoline-3-carbaldehyde-thiosemicarbazones-N-4-substituted pro-ligands and their Cu(II) complexes (<b>1</b>, -NH<sub>2</sub>; <b>2</b>, -NHMe; <b>3</b>, -NHEt) have been prepared and characterized. In both the X-ray structures of <b>1</b> and <b>3</b>, two crystallographically independent complex molecules were found that differ either in the nature of weakly metal-binding species (water in <b>1a</b> and nitrate in <b>1b</b>) or in the co-ligand (water in <b>3a</b> and methanol in <b>3b</b>). Electron Paramagnetic Resonance (EPR) measurements carried out on complexes <b>1</b> and <b>3</b> confirmed the presence of such different species in the solution. The electrochemical behavior of the pro-ligands and of the complexes was investigated, as well as their biological activity. Complexes <b>2</b> and <b>3</b> exhibited a high cytotoxicity against human tumor cells and 3D spheroids derived from solid tumors, related to the high cellular uptake. Complexes <b>2</b> and <b>3</b> also showed a high selectivity towards cancerous cell lines with respect to non-cancerous cell lines and were able to circumvent cisplatin resistance. Via the Transmission Electron Microscopy (TEM) imaging technique, preliminary insights into the biological activity of copper complexes were obtained.
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