Synthesis, copper(II) complexation, 64Cu-labeling, and bioconjugation of a new bis(2-pyridylmethyl) derivative of 1,4,7-triazacyclononane

A new ligand derivative of 1,4,7-triazacyclononane (TACN), 2-[4,7-bis(2-pyridylmethyl)-1,4,7-triazacyclononan-, -yl]acetic acid (6), has been synthesized and its complexation behavior toward Cu2+ ions investigated. The ligand 6 has been characterized by spectroscopic methods, and a molecular structure of a corresponding Cu(II) complex has been elucidated by single-crystal X-ray analysis. The suitability of 6 for conjugation to peptide substrates has been shown by amide coupling of 6 to the stabilized derivative of bombesin (BN), beta Ala- beta Ala-[Cha , Nle(14) ]BN(7-14), to give the conjugate 8. The free ligand 6 and the bioconjugate 8 were labeled with Cu-64(2+), and the resulting complexes, Cu-64 subset of(6) and Cu-64 subset of(8), were found to be stable in the presence of a large (:xcess of a competing ligand (cyclam) or copper-seeking superoxide dismutase (SOD), as well as in rat plasma. Biodistribution studies of Cu-64 subset of(8) in Wistar rats showed a high activity uptake into the pancreas (5.76 +/- 0.25 SUV, 5 min p.i.; 3.93 0.25 SUV, I It p.i.), which is the organ with high levels of gastrin-releasing peptide receptor (GRPR). This receptor is overexpressed in a large number of breast and prostate carcinomas. The novel Cu-64 subset of(6) complex had a dominating influence on the nonspecific activity biodistribution of its BN conjugate, since the distribution data of Cu-64 subset of(6) are similar to those of Cu-64 subset of(8). The Cu-64 complexes exhibited a low activity itceumulation in the liver tissue and an extensive renal clearance, which was distinctively different to the biodistribution Of (CuCl2)-Cu-64, suggesting that Cu-64 subset of(6) does not undergo significant demetalation, but rather exhibits high in vivo stability.