Radio-labeled receptor-binding peptides: a new class of radiopharmaceuticals

Radio-labeled receptor-binding peptides have emerged as an important class of radiopharmaceuticals. In vertebrates, these peptides transmit their biological function by binding to their specific receptor on the target cell surface. This specific receptor-binding property is exploited when the radio-labeled peptide is used as a radiopharmaceutical. The high-binding affinity for its receptor facilitates retention of the peptide in receptor-expressing tissues, whereas its relatively small size facilitates rapid clearance from the blood and other nontarget tissues. Receptor-binding peptides labeled with gamma-emitters can be used to visualize receptor-positive cells in vivo. In addition, when labeled with beta- or alpha-emitters, these peptides can be used for peptide-receptor radionuclide therapy. Various receptors are overexpressed on particular tumor types, and peptides binding to these receptors can be used to visualize tumor lesions scintigraphically. Furthermore, peptides binding receptors on granulocytes can be used to image infectious and inflammatory foci, whereas peptides binding receptors on activated thrombocytes can be used for thrombus imaging. Here, the peptide analogs that are under development for these applications are reviewed.