Spectroelectrochemical and Computational Studies on the Mechanism of Hypoxia Selectivity of Copper Radiopharmaceuticals

Abstract

Detailed chemical, spectroelectrochemical and computational studies have been used to investigate the mechanism of hypoxia selectivity of a range of copper radiopharmaceuticals. A revised mechanism involving a delicate balance between cellular uptake, intracellular reduction, reoxidation, protonation and ligand dissociation is proposed. This mechanism accounts for observed differences in the reported cellular uptake and washout of related copper bis(thiosemicarbazonato) complexes. Three copper and zinc complexes have been characterised by X-ray crystallography and the redox chemistry of a series of copper complexes has been investigated by using electronic absorption and EPR spectroelectrochemistry. Time-dependent density functional theory (TD-DFT) calculations have also been used to probe the electronic structures of intermediate species and assign the electronic absorption spectra. DFT calculations also show that one-electron oxidation is ligand-based, leading to the formation of cationic triplet species. In the absence of protons, metal-centred one-electron reduction gives the reduced anionic copper(I) species, [CuIATSM](-), and for the first time it is shown that molecular oxygen can reoxidise this anion to give the neutral, lipophilic parent complexes, which can wash out of cells. The electrochemistry is pH dependent and in the presence of stronger acids both chemical and electrochemical reduction leads to quantitative and rapid dissociation of copper(I) ions from the mono- or diprotonated complexes, [CuIATSMH] and [Cu(I)ATSMH2]+. In addition, a range of protonated intermediate species have been identified at lower acid concentrations. The one-electron reduction potential, rate of reoxidation of the copper(I) anionic species and ease of protonation are dependent on the structure of the ligand, which also governs their observed behaviour in vivo.

Pathophysiologic Correlation Between 62Cu-ATSM and 18F-FDG in Lung Cancer

“…The reduced copper(I) dissociates from the ATSM complex and eventually is trapped irreversibly in tissue in an overreduced environment. Recent studies based on advanced spectroelectrochemical techniques and computational work indicate that the hypoxia selectivity of copper-ATSM arises because of a delicate equilibrium in which the rates of reduction (most likely enzyme-mediated), reoxidation, and protonation are fast relative to the rate of pH-mediated ligand dissociation (27,28). Thus, the accumulation of copper-ATSM may not directly depict oxygen tension but rather a high NADH-NADPH concentration with dynamic changes in intracellular pH that result from the overreduction of hypoxic tissues still having a viable mitochondrial or microsomal enzyme system.…”

Reactivity of Thiosemicarbazides with Redox Active Metal Ions: Controlled Formation of Coordination Complexes versus Heterocyclic Compounds

“…We have investigated the coordination chemistry of bis(thiosemicarbazone) ligands at length in the context of the hypoxic selectivity of the copper complexes. [26][27][28][29] We have found that the synthesis of both the free ligands and the complexes is complicated by the intraligand formation of cyclic products arising from coupling of the two thiosemicarbazone limbs. [30] For monothiosemicarbazones, interligand cyclisation occurs on reaction with methanolic ferric chloride or cupric perchlorate under reflux to yield heterocyclic compounds.…”

A Comparison of the Behavior of 64Cu-Acetate and 64Cu-ATSM In Vitro and In Vivo

“…Experimental and in silico studies indicated that variables such as pH also influence the stability of the reduced Cu(I) species and affect the reduction, reoxidation, and ligand dissociation events (6)(7)(8)(9). Although hypoxia selectivity for 64 Cu-ATSM is evident in vitro and in vivo, some more recent studies found that the relationship between oxygenation status and uptake and retention of 64 Cu-ATSM varied with cell and tumor line (10,11).…”