Triapine Derivatives Act as Copper Delivery Vehicles to Induce Deadly Metal Overload in Cancer Cells

Abstract

Thiosemicarbazones continue to attract the interest of researchers as potential anticancer drugs. For example, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone, or triapine, is the most well-known representative of this class of compounds that has entered multiple phase I and II clinical trials. Two new triapine derivatives <b>HL¹</b> and <b>HL²</b> were prepared by condensation reactions of 2-pyridinamidrazone and S-methylisothiosemicarbazidium chloride with 3-<i>N</i>-(<i>tert</i>-butyloxycarbonyl) amino-pyridine-2-carboxaldehyde, followed by a Boc-deprotection procedure. Subsequent reaction of <b>HL¹</b> and <b>HL²</b> with CuCl₂·2H₂O in 1:1 molar ratio in methanol produced the complexes <b>[Cu^II(HL¹)Cl₂]·H₂O</b> (<b>1·H₂O</b>) and <b>[Cu^II(HL²)Cl₂]</b> (<b>2</b>). The reaction of <b>HL²</b> with Fe(NO₃)₃∙9H₂O in 2:1 molar ratio in the presence of triethylamine afforded the complex <b>[Fe^III(L²)₂]NO₃∙0.75H₂O</b> (<b>3∙0.75H₂O</b>), in which the isothiosemicarbazone acts as a tridentate monoanionic ligand. The crystal structures of <b>HL¹</b>, <b>HL²</b> and metal complexes <b>1</b> and <b>2</b> were determined by single crystal X-ray diffraction. The UV-Vis and EPR spectroelectrochemical measurements revealed that complexes <b>1</b> and <b>2</b> underwent irreversible reduction of Cu(II) with subsequent ligand release, while <b>3</b> showed an almost reversible electrochemical reduction in dimethyl sulfoxide (DMSO). Aqueous solution behaviour of <b>HL¹</b> and <b>1,</b> as well as of <b>HL²</b> and its complex <b>2</b>, was monitored as well. Complexes <b>1</b>-<b>3</b> were tested against ovarian carcinoma cells, as well as noncancerous embryonic kidney cells, in comparison to respective free ligands, triapine and cisplatin. While the free ligands <b>HL¹</b> and <b>HL²</b> were devoid of antiproliferative activity, their respective metal complexes showed remarkable antiproliferative activity in a micromolar concentration range. The activity was not related to the inhibition of ribonucleotide reductase (RNR) R2 protein, but rather to cancer cell homeostasis disturbance-leading to the disruption of cancer cell signalling.