TACN-derived ligands as highly stable biofunctionalizable chelators for copper(II)

The tridentate macrocycle 1,4,7-triazacyclononane (TACN) forms stable complexes especially with Cu(II) whereby the metal ion lying out of the plane defined by the three nitrogen atoms. The donor atoms are oriented in such a way as to maximize orbital overlap and thereby produce complexes with very high stabilities. The introduction of further donor groups on the ligand skeleton, such as pyridine units, significantly influences the thermodynamic stability as well as the kinetic inertness of the metal complexes formed. We have developed a ligand scaffold based on bis(2-pyridylmethyl)triazacyclononane (DMPTACN) [1]. This structure allows for the introduction of linker groups, such as carboxylic acids, maleimide or isothiocyanate, thereby facilitating coupling of targeting molecules (see Figure).
TACN ligands containing one or two pendant 2-picolyl arms prefer the formation of square-pyramidal coordination geometry with copper(II). A hexadentate ligand with two picoline coordination groups as well as a carboxylic functionality (DMPTACN-COOH) enforces a six-coordinate copper(II) complex having a distorted octahedral structure. It has been found that relevant peptide conjugates of this hexadentate bis(2-pyridylmethyl)-TACN-acetic acid derivative can readily form radiocopper complexes under physiologically relevant conditions and show high in vivo stability. Such radiolabeled peptides are thus attractive candidates for radiopharmaceutical applications.
We want to present the synthesis of peptide conjugates with DMPTACN derivatives capable of gastrin releasing peptide receptor (GRPR) and epidermal growth factor receptor (EGFR) targeting. Both types of receptors are overexpressed on different cancer cells. In vitro binding characteristics of [⁶⁴Cu]Cu^II-labeled DMPTACN-peptide conjugates in GRPR and EGFR overexpressing cancer cells (PC-3, FaDu, A431) will be presented. Small animal PET studies confirmed a high extent of tumor accumulation in NMRI nu/nu mice bearing the human prostate tumor PC-3. Derived from that, it can be concluded that ^64CuII complexes of DMPTACN-peptide conjugates have considerable potential for tumor imaging, since these peptide derivatives can effectively display receptor-rich tissue in vivo.
[1] G. Gasser, L. Tjioe, B. Graham, M. J. Belousoff, S. Juran, M. Walther, J.-U. Künstler, R. Bergmann, H. Stephan, L. Spiccia, Bioconjugate Chem. 2008, 9, 719-730.