A review of synthetic strategies for macrocyclic bifunctional chelators in molecular imaging and radiopharmaceutical applications

Abstract

Metal complexes play indispensable roles in modern medicine, underpinning diagnostic techniques such as magnetic resonance imaging and nuclear imaging, as well as targeted radionuclide therapy. Their clinical success relies on chelators that form thermodynamically stable and kinetically inert metal complexes, thereby minimizing the release of toxic free metal ions in vivo. Although acyclic chelators were among the first ligands used clinically, their limited kinetic inertness prompted the development of macrocyclic polyaminopolycarboxylate chelators, whose preorganized cavities confer exceptional complex stability. To enable site-specific conjugation to peptides, antibodies, and other biological targeting vectors, these macrocycles must be functionalized into bifunctional chelating agents. Despite their widespread application, a comprehensive overview of the synthetic methodologies for preparing these bifunctional chelators has been lacking. This review comprehensively surveys the synthetic strategies for macrocyclic polyaminopolycarboxylate chelators used in molecular imaging and targeted radionuclide therapy. Particular emphasis is placed on cyclen-, 1,4,7-triazacyclononane-, cyclam-, and cage-based platforms, including the DOTA, NOTA, TETA, PCTA, DiamSar, and AmBaSar families and their structurally related derivatives. The review covers the synthesis of the parent macrocycles and their selective functionalization into bifunctional chelators for conjugation to biological targeting vectors. It further examines the synthesis of derivatives bearing amino, thiol, maleimide, N -hydroxysuccinimide ester, alkyne, and benzyl functionalities, with particular emphasis on protecting-group strategies, regioselective functionalization, purification methods, and reaction conditions that govern regioselectivity and yield. By consolidating these methodologies, the review provides a practical framework for the rational design and synthesis of bifunctional chelators for diagnostic and therapeutic radiopharmaceutical applications. The selective functionalization of macrocyclic polyaminopolycarboxylate chelators relies on carefully designed protection and deprotection strategies, with orthogonal protecting groups such as tert-butyloxycarbonyl, carboxybenzyl, and tert -butyl esters playing key roles in obtaining regioisomerically pure intermediates. The introduction of diverse functional groups, including amines, thiols, maleimides, N -hydroxysuccinimide esters, and alkynes, enables efficient conjugation to biological targeting vectors through amide coupling, Michael addition, and click chemistry. Collectively, the synthetic methodologies summarized in this review provide a robust foundation for the development of next-generation bifunctional chelators and theranostic radiopharmaceuticals with expanded applications in precision molecular imaging and targeted radionuclide therapy.