The Impact of Emerging Bioconjugation Chemistries on Radiopharmaceuticals

The use of radiolabeled antibodies, immunoglobulin fragments, and other proteins are an increasingly important sector of research for diagnostic imaging and targeted radiotherapy in nuclear medicine. As with all radiopharmaceuticals, efficient radiochemistry is a prerequisite to clinical translation. For proteins, variations in the primary amino acid sequence, the secondary structures, and tertiary folds, as well as differences in the size, charge, polarity, lipophilicity, and the presence of posttranslational modifications, add complexity to the system. The choice of radionuclide or chelate, and its impact on the thermodynamic, kinetic, and metabolic stability of a radiotracer, has attracted much attention but the chemistry by which the radionuclide is conjugated to the protein scaffold is of equal importance. Recently, a wealth of creative advances in protein ligation methods based on chemical, photochemical, and enzyme-mediated processes has emerged. As radiochemists explore alternative bioconjugation strategies, this article considers their potential impact on radiotracer design.