Versatile new bis(thiosemicarbazone) bifunctional chelators: Synthesis, conjugation to bombesin(7-14)-NH₂, and copper-64 radiolabeling

New bifunctional derivatives of diacetyl-bis(4-methylthiosemicarbazone) (H2atsm) have been prepared by a selective transamination reaction of a new dissymmetric bis(thiosemicarbazone) precursor H₂L¹. The new derivatives contain an aliphatic carboxylic acid (H₂L² and H₂L³), t-butyl carbamate (H₂L ⁴), or ammonium ion (H₂L⁵) functional group. The new ligands and copper(II) complexes have been characterized by NMR spectroscopy, mass spectrometry, and microanalysis. The complex CuII(L ⁴) was structurally characterized by X-ray crystallography and shows the metal center to be in an N₂S₂ distorted square planar coordination geometry. Electrochemical measurements show that the copper(II) complexes undergo a reversible reduction attributable to a Cu(II)/Cu(I) process. The ligands and the copper(II) complexes featuring a carboxylic acid functional group have been conjugated to the tumor targeting peptide bombesin(7-14)- NH₂. The bifunctional peptide conjugates were radiolabeled with copper-64 in the interest of developing new positron emission tomography (PET) imaging agents. The conjugates were radiolabeled with copper- 64 rapidly in high radiochemical purity (> 95%) at room temperature under mild conditions and were stable in a cysteine and histidine challenge study.