Efficient Synthesis of a Family of Bifunctional Chelators Based on the PCTA[12] Macrocycle Suitable for Bioconjugation

PCTA[12] is a 12-membered tetraaza-macrocyclic ligand that incorporates a pyridine unit within the macrocyclic ring and three acetate pendant arms. Unlike DOTA and NOTA chelators, PCTA is a recent entry to the field of macrocyclic polyaminocarboxylate ligands available to complex a variety of M2+/M3+ ions for biomedical applications such as diagnostic and radiotherapeutic. Despite the promising properties of its chelates, only a few of bifunctional chelating agents (BFCAs) derived from PCTA have been described so far. Based on our very recent methodology for the preparation of PCTA[12] itself, we report here the efficient synthesis of several BFCAs derived from PCTA bearing a free reactive function group, mainly devoted to conjugation purposes: ester, carboxylic acid, alcohol, aliphatic amine, aromatic amine, maleimide, bromo or azide functions. These functions were introduced either on the 4-position of the pyridine ring or on the methylene carbon atom of the central acetate chelating arm, while keeping the three carboxylate groups available for metal chelation. Moreover, two of these BFCAs-PCTA were used for conjugation with a tetrapeptide (cholecystokinin analogue), a bioactive molecule (biotin), or a solid support (silica gel).