Biophysical characterization and antineoplastic activity of new bis(thiosemicarbazonato) Cu(II) complexes

Aiming to explore alternative mechanisms of cellular uptake and cytotoxicity, we have studied a new family of copper(II) complexes (CuL¹-CuL⁴) with bis(thiosemicarbazone) (BTSC) ligands containing pendant protonable cyclic amines (morpholine and piperidine). Herein, we report on the synthesis and characterization of these new complexes, as well as on their biological performance (cytotoxic activity, cellular uptake, protein and DNA binding), in comparison with the parental Cu^IIATSM (ATSM=diacetyl-bis(N4-methylthiosemicarbazonate) complex without pendant cyclic amines. The new compounds have been characterized by a range of analytical techniques including ESI-MS, IR spectroscopy, cyclic voltammetry, reverse-phase HPLC and X-ray spectroscopy. In vitro cytotoxicity studies revealed that the copper complexes are cytotoxic, unlike the corresponding ligands, with a similar potency to that of CuATSM. Unlike CuATSM, the new complexes were able to circumvent cisplatin cross-resistance. The presence of the protonable cyclic amines did not lead to an enhancement of the interaction of the complexes with human serum albumin or calf thymus DNA. However, CuL¹-CuL⁴ showed a remarkably augmented cellular uptake compared with CuATSM, as proved by uptake, internalization and externalization studies that were performed using the radioactive congeners ⁶⁴CuL¹-⁶⁴CuL⁴. The enhanced cellular uptake of CuL¹-CuL⁴ indicates that this new family of Cu^IIBTSC complexes deserves to be further evaluated in the design of metallodrugs for cancer theranostics.