Oxidative Release of Copper from Pharmacologic Copper Bis(thiosemicarbazonato) Compounds

Intracellular delivery of therapeutic or analytic copper from copper bis-thiosemicabazonato complexes is generally described in terms of mechanisms involving one-electron reduction to the Cu(I) analogue by endogenous reductants, thereby rendering the metal ion labile and less strongly coordinating to the bis-thiosemicarbazone (btsc) ligand. However, electrochemical and spectroscopic studies described herein indicate that one-electron oxidation of CuII(btsc) and ZnIIATSM (btsc = diacetyl-bis(4-methylthiosemicarbazonato)) complexes occurs within the range of physiological oxidants, leading to the likelihood that unrecognized oxidative pathways for copper release also exist. Oxidations of CuII(btsc) by H2O2 catalyzed by either myeloperoxidase or horseradish peroxidase, by HOCl and taurine chloramine (which are chlorinating agents generated primarily in activated neutrophils from MPO-catalyzed reactions), and by peroxynitrite species (ONOOH, ONOOCO2–) that can form under certain conditions of oxidative stress...