Three new 6-methyl-2-oxo-1,2-dihydroquinoline-3-carbaldehyde-thiosemicarbazones-N-4-substituted pro-ligands and their Cu(II) complexes ( , -NH ; , -NHMe; , -NHEt) have been prepared and characterized. In both the X-ray structures of and , two crystallographically independent complex molecules were found that differ either in the nature of weakly metal-binding species (water in and nitrate in ) or in the co-ligand (water in and methanol in ). Electron Paramagnetic Resonance (EPR) measurements carried out on complexes and 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 and exhibited a high cytotoxicity against human tumor cells and 3D spheroids derived from solid tumors, related to the high cellular uptake. Complexes and 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.