Highly efficient method for ¹²⁵I-radiolabeling of biomolecules using inverse-electron-demand Diels-Alder reaction

In this report, we present a rapid and highly efficient method for radioactive iodine labeling of trans-cyclooctene group conjugated biomolecules using inverse-electron-demand Diels-Alder reaction. Radioiodination reaction of the tetrazine structure was carried out using the stannylated precursor 2 to give ¹²⁵I-labeled product ([¹²⁵I]1) with high radiochemical yield (65 ± 8%) and radiochemical purity (>99%). For radiolabeling application of [¹²⁵I]1, trans-cyclooctene derived cRGD peptide and human serum albumin were prepared. These substrates were reacted with [¹²⁵I]1 under mild condition to provide the radiolabeled products [¹²⁵I]6 and [¹²⁵I]8, respectively, with excellent radiochemical yields. The biodistribution study of [¹²⁵I]8 in normal ICR mice showed significantly lower thyroid uptake values than that of ¹²⁵I-labeled human serum albumin prepared by a traditional radiolabeling method. Therefore [¹²⁵I]8 will be a useful radiolabeled tracer in various molecular imaging and biological studies. Those results clearly demonstrate that [¹²⁵I]1 will be used as a valuable prosthetic group for radiolabeling of biomolecules.