Bis(amido)bis(oxinate)diamine Ligands for theranostic radiometals

With the interest in radiometal-containing diagnostic and therapeutic pharmaceuticals increasing rapidly, appropriate ligands to coordinate completely and stably said radiometals is essential. Reported here are two novel, bis(amido)bis(oxinate)diamine ligands, H₂amidohox (2,2'-(ethane-1,2-diylbis(((8-hydroxyquinolin-2-yl)methyl)azanediyl))diacetamide) and H₂amidoC3hox (2,2'-(propane-1,3-diylbis(((8-hydroxyquinolin-2-yl)methyl)azanediyl))diacetamide), that combine two 8-hydroxyquinoline and amide donor groups and differ by one carbon in their 1,2-ethylenediamine vs. 1,3-diaminopropane backbones, respectively. Both ligands have been thoroughly studied via metal complexation, solution thermodynamics and radiolabeling with three radiometal ions: [^nat/64Cu]Cu²⁺, [^nat/111In]In³⁺, and [^nat/203Pb]Pb²⁺. X-ray crystallography determined the structures of the hexacoordinated Cu²⁺-ligand complexes, indicating a better fit of Cu²⁺ to the H₂amidohox binding pocket. Concentration dependent radiolabeling with [⁶⁴Cu]Cu²⁺ was successfully quantitative as low as 1 μM with H₂amidohox and 10 μM with H₂amidoC3hox within 5 min at room temperature. However, [⁶⁴Cu][Cu(amidohox)] maintained higher kinetic inertness against a superoxide dismutase enzyme-challenge assay and ligand challenges compared to the [⁶⁴Cu][Cu(amidoC3hox)] counterpart. Similarly, H₂amidohox had significantly higher radiochemical conversion with both [¹¹¹In]In³⁺ (97% at 1 μM) and [²⁰³Pb]Pb²⁺ (97% at 100 μM) under mild conditions compared to H₂amidoC3hox (76% with [¹¹¹In]In³⁺ at 1 μM and 0% with [²⁰³Pb]Pb²⁺). By studying non-radioactive and radioactive complexation with both ligands, a comprehensive understanding of the coordination differences between two- and three‑carbon diamine backbones is discussed. Overall, the ethylenediamine backbone of H₂amidohox proves to be superior in rapid, mild radiolabeling and kinetic inertness towards competing ligands and proteins.