TACN-based chelators with pendant azaheterocyclic arms for stable radiocopper chelation

Objectives We have developed a chelating system for radiocopper based on 1,4,7-triazacyclononane (TACN) with three pendant five-membered azaheterocycles for metal chelation. Initial screening was performed with Cu-64 for evaluation of the chelating properties.

Methods Radiolabeling efficiencies of 1,4,7-tris((1H-imidazol-2-yl)methyl)-1,4,7-triazonane (NOTI), 1,4,7-tris((1-methyl-1H-imidazol-2-yl)methyl)-1,4,7-triazonane (NOTI-Me), and 1,4,7-tris(thiazol-2-ylmethyl)-1,4,7-triazonane (NOTThia) were determined in ammonium acetate buffer pH 8.0 in dependence of the chelator concentration by analytical RP-HPLC. The stability/inertness of Cu-64 labeled chelates was assessed in human serum at 37 °C and by nat. Cu exchange experiments at r.t. for up to 36 h. The lipophilicity of Cu-64 labeled chelates was determined by partition between water/octanol. Furthermore, X-ray diffraction studies were performed on single crystals of the nat. Cu complexes.

Results All chelators radiolabeled in quantitative yield in < 1 min at r.t. at a concentration of 0.1 µM producing specific activities of 60-120 MBq/nmol. Cu-64 complexes showed marginal decomposition in human serum for up to 19 h (< 1%) and minimal exchange in the presence of five-fold excess of nat. Cu compared to chelator concentration for up to 36 h (< 1%). Cu-64 labeled chelators are highly hydrophilic with log P values for NOTI, NOTI-Me, and NOTThia of -3.35 ± 0.12, -3.18 ± 0.29, and -3.36 ± 0.25, respectively. In the solid state, the copper center exhibits a strongly distorted-pseudo-prismatic CuN₃N’₃ polyhedron similar to the Cu-NOTA structure .

Conclusions This class of chelators exhibits extremely fast labeling kinetics under mild conditions and proves to be of high stability/inertness in vitro. The results strongly support the development of bifunctional counterparts for conjugation of targeting vectors and further testing in vivo.

Research Support CheMatech, Dijon (France) for providing samples of TACN as well as Dr. Harald Scherer and Dr. Manfred Keller of the Chemistry Department at the University Freiburg (Germany) for NMR measurements.