Metal complexes quietly power much of modern medicine. Magnetic resonance imaging, nuclear imaging scans, and targeted radionuclide therapy all depend on metal ions held firmly in place by chelators, molecular cages that grip a metal atom at multiple points. The clinical value of these complexes rests on two properties: thermodynamic stability, which favors complex formation, and kinetic inertness, which prevents the metal from escaping once bound. Without both, toxic free metal ions can be released in the body, undermining diagnosis and endangering patients.
A new open-access review published in EJNMMI Radiopharmacy and Chemistry on 28 August 2026 addresses a long-standing gap in this field. Although acyclic chelators were among the first ligands used clinically, their limited kinetic inertness pushed chemists toward macrocyclic polyaminopolycarboxylate chelators, whose preorganized ring cavities confer exceptional complex stability. To attach these cages to peptides, antibodies, and other biological targeting vectors, the macrocycles must be converted into bifunctional chelating agents, molecules that bind a metal on one side and carry a chemical handle for conjugation on the other. According to the review, a comprehensive overview of the synthetic methodologies for preparing these bifunctional chelators had been lacking until now.
The review surveys the synthetic strategies for macrocyclic polyaminopolycarboxylate chelators used in molecular imaging and targeted radionuclide therapy, with particular emphasis on four platform families: cyclen-based, 1,4,7-triazacyclononane-based, cyclam-based, and cage-based structures. These platforms underpin the DOTA, NOTA, TETA, PCTA, DiamSar, and AmBaSar chelator families and their structurally related derivatives.
The synthesis of the parent macrocycles is only the starting point. The real challenge lies in selective functionalization, the process of converting a symmetric macrocycle into a bifunctional chelator without scrambling the positions of its reactive groups. The review examines derivatives bearing amino, thiol, maleimide, N-hydroxysuccinimide ester, alkyne, and benzyl functionalities, each of which enables a different conjugation route to biological targeting vectors. Amino groups support amide coupling, maleimides and thiols enable Michael addition, and alkynes open the door to click chemistry, a family of fast, selective reactions widely used to join molecular fragments under gentle conditions.
Central to achieving this selectivity are protecting-group strategies. The review concludes that the selective functionalization of these macrocyclic chelators relies on carefully designed protection and deprotection sequences, with orthogonal protecting groups such as tert-butyloxycarbonyl, carboxybenzyl, and tert-butyl esters playing key roles in obtaining regioisomerically pure intermediates. Orthogonal groups can be removed independently of one another, allowing a chemist to unmask one reactive site while leaving others masked.
The review also details the practical factors that govern regioselectivity and yield, including reaction conditions and purification methods. By assembling these procedures in one place, the authors provide a practical framework for the rational design and synthesis of bifunctional chelators for diagnostic and therapeutic radiopharmaceutical applications.
The significance of this synthetic toolkit extends across the radiopharmaceutical pipeline. In diagnostic imaging, the same chemistry attaches positron-emitting or gamma-emitting isotopes to targeting vectors for nuclear imaging.
The review’s conclusion frames these methodologies as a foundation for future work. Collectively, the synthetic approaches summarized provide a robust basis for developing next-generation bifunctional chelators and theranostic radiopharmaceuticals, agents that combine diagnostic and therapeutic functions in a single targeting system. The functional group diversity documented in the review, spanning amines, thiols, maleimides, N-hydroxysuccinimide esters, and alkynes, supplies the chemical vocabulary needed to build such matched pairs.
As a review article, the work consolidates existing literature rather than reporting new experimental results, and its value lies in organization and synthesis rather than novel findings. For chemists designing the next generation of molecular imaging agents and targeted radionuclide therapies, the review offers a consolidated map of which synthetic routes exist, which protecting group combinations succeed, and which reaction conditions deliver the regioselectivity and yield that clinical translation requires.
Subject of Research: Synthetic strategies for macrocyclic bifunctional chelators used in molecular imaging and targeted radionuclide therapy
Article Title: A review of synthetic strategies for macrocyclic bifunctional chelators in molecular imaging and radiopharmaceutical applications
Article References: Bayat, F. (2026). A review of synthetic strategies for macrocyclic bifunctional chelators in molecular imaging and radiopharmaceutical applications. EJNMMI Radiopharmacy and Chemistry. https://doi.org/10.1186/s41181-026-00491-7
Image Credits: AI Generated
DOI: 10.1186/s41181-026-00491-7
Keywords: bifunctional chelators, macrocyclic chelators, DOTA, NOTA, radiopharmaceuticals, molecular imaging, targeted radionuclide therapy, protecting groups, regioselectivity, theranostics, click chemistry, EJNMMI Radiopharmacy and Chemistry
News Source: Bethany Barker. (November 6, 2026). How Bifunctional Chelators Bind Medical Radioisotopes: A Synthetic Roadmap. Scienmag.
