Conjugation of DOTA-like chelating agents to peptides and radiolabeling with trivalent metallic isotopes.

Peptides can be labeled with various trivalent radiometals for imaging or targeted radionuclide-therapy applications. The peptide is first conjugated to a chelating agent that is able to form stable complexes with the radionuclide of interest. This conjugation step can be carried out as part of the solid-phase peptide synthesis, or it can be undertaken in the solution phase after synthesis and purification of the peptide. The latter route, described here, involves reacting a molar excess of the activated tri-tert-butyl ester-derivatized chelator with a designated free amino group of a peptide analog, in which all other reactive amines are protected, in the presence of a coupling agent. The conjugate molecule is then purified prior to deprotection and further purification by HPLC. The product can be radiolabeled by addition of a suitable metal salt, followed, if necessary, by removal of the unchelated metal. The entire process of conjugation, purification and radiolabeling should take approximately 12.5 h.