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 Cu-ll(btsc) and Zn(II)ATSM (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 Cu-ll(btsc) by H2O2 catalyzed by either myeloperoxidase or horseradish peroxidase, by HOCI 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 are demonstrated. Unlike reduction, the oxidative reactions proceed by irreversible ligand oxidation, culminating in release of Cu(II). 2-Pyridylazoresorcinol complexation was used to demonstrate that Cu(II) release by reaction with peroxynitrite species involved rate-limiting homolysis of the peroxy O-O bond to generate secondary oxidizing radicals (NO2 center dot, (OH)-O-center dot, and CO3 center dot-). Because the potentials for Cu-ll(btsc) oxidation and reduction are ligand-dependent, varying by as much as 200 mV, it is clearly advantageous in designing therapeutic methodolOgies for specific treatments to identify the operative Cu-release pathway.