Studies on the Mechanism of Hypoxic Selectivity in Copper Bis(Thiosemicarbazone) Radiopharmaceuticals

Abstract

Copper diacetyl-bis(N4-methylthiosemicarbazone), Cu(II)ATSM, is a promising agent for imaging hypoxic tissue. Here we present results that provide insight into the chemical and electronic properties underlying previously observed structure-activity relationships. Density functional theory (DFT) calculations on the electronic structures and molecular orbitals of a series of 13 Cu(II)bis(thiosemicarbazone) analogues with different alkylation patterns and with fixed geometries based on the known structure of Cu(II)PTSM showed that the LUMO and the next lowest orbital were very close in energy, and their energy order was strikingly dependent on the ligand alkylation pattern in a way that correlated with hypoxia-selectivity and redox potentials. The LUMOs of Cu(II)ATSM and other hypoxia-selective analogues were predominantly metal-based (leading to a singlet reduced species) while the LUMOs of Cu(II)PTSM and other nonselective analogues were predominantly ligand-based (leading to a triplet reduced species). Upon relaxation of the geometric constraint and full optimization in both Cu(II)ATSM and Cu(II)GTS, the metal-based orbital became the LUMO, and the singlet was the thermodynamically preferred form of the reduced species. Chemical and electrochemical investigation showed that all Cu(II) complexes were reducible, but Cu(I)PTSM and other nonselective analogues dissociated immediately upon reduction with release of ligand (detected by UV-vis) while Cu(I)ATSM and other hypoxia-selective analogues did not. Instead they were rapidly re-oxidized to the Cu(II) complex by molecular oxygen. The reversible electrochemical reduction of nonselective complexes Cu(II)PTSM and Cu(II)GTS became irreversible in the presence of weak acid, whereas that of Cu(II)ATSM was unaffected. In light of these results we present a model to explain the structure-activity relationships on the basis of electronic structure and molecular vibrations.

Increasing Cu bioavailability inhibits Aβ oligomers and tau phosphorylation

“…Importantly, once Cu II (gtsm) is exposed to the intracellular reducing environment, Cu II is reduced to Cu I , causing it to dissociate from the ligand and to increase Cu bioavailability. The negative control Cu II (atsm) does not reduce and dissociate under normal intracellular conditions, and therefore does not increase Cu bioavailability (18,(23)(24)(25)(26). We found both metal complexes increased cellular Cu levels several hundredfold ( Fig.…”

PET imaging of cardiac hypoxia: Opportunities and challenges

“…If the lifetime of the stable complex inside the cell is thought of as a cycling between reduction and oxidation, re-oxidation of the unstable complex from Cu(I) to Cu(II) by molecular oxygen is likely to be a key regulator of whether these tracers dissociate and deposit their radiocopper, or diffuse back out of the cell unchanged. This has been shown to be possible chemically in a non-biological system, but as yet, this has not been demonstrated in live cells [115]. The relative importance of these two mechanisms is currently unknown.…”

“…One further potential modifier of BTSC complex dissociation is protonation, which may be more prevalent during acidosis [96,115,116]. While it has been shown in vitro that Cu-BTSC complexes are stable under acidic but oxygenated conditions [113,115], and at this time there is no experimental data to directly support cellular retention of radiocopper by protonation alone, density functional theory calculations by Holland et al suggest that a decrease in cellular pH may protonate the unstable Cu(I) complex, and increase its rate of dissociation [116,117].…”

“…While it has been shown in vitro that Cu-BTSC complexes are stable under acidic but oxygenated conditions [113,115], and at this time there is no experimental data to directly support cellular retention of radiocopper by protonation alone, density functional theory calculations by Holland et al suggest that a decrease in cellular pH may protonate the unstable Cu(I) complex, and increase its rate of dissociation [116,117]. If it occurred, double protonation would also decrease the complexes' lipophilicity, tending to decrease their washout rates, increase their retention times within the cell, and hence increase their chances of interacting with intracellular reducing species [96].…”

Synthesis and characterizations of some new tetra thiosemicarbazones and their cyclization reactions; tetra 4-methyl-5-etoxycarbonyl-2,3-dihydro-1,3-thiazole and tetra 2-acetylamino-4-acetyl-4,5-dihydro-1,3,4-thiodiazole derivatives

“…14 In another study, the bis(thiosemicarbazone) complexes of copper have shown special promise as radiopharmaceuticals, as illustrated by a per fusion imaging agent. [15][16][17][18][19] In the recent studies, appear to be a structural class with anti-pox virus activity. 20 Isatin derivatives such as methisazone (marboran), the β-thiosemicarbazone of N-methyl isatin, have been described as smallpox chemoprophylactic agents.…”