Peptide-based radiopharmaceuticals: Future tools for diagnostic imaging of cancers and other diseases

Small synthetic receptor-binding peptides are the agents of choice for diagnostic imaging and radiotherapy of cancers due to their favorable pharmacokinetics. Molecular modification techniques permit the synthesis of a variety of bioactive peptides with chelating groups, without compromising biological properties. Various techniques have been developed that allow efficient and site-specific labeling of peptides with clinically useful radionuclides such as (99m)Tc, (123)I, (111)In, and (18)F. Among them, (99m)Tc is the radionuclide of choice because of its excellent chemical and imaging characteristics. Recently, many (99m)Tc-labeled peptides have proven to be useful imaging agents. Beside (99m)Tc-labeled peptides, several peptides radiolabeled with (111)In and (123)I have been prepared and characterized. In addition, (18)F-labeled peptides hold clinical potential due to their ability to quantitatively detect and characterize a variety of human diseases using positron-emission tomography. The availability of this wide range of peptides labeled with different radionuclides offers multiple diagnostic and therapeutic applications. Various receptors are over-expressed in particular tumor types and peptides binding to these receptors can be used to visualize tumor lesions scintigraphically. Thus, radiolabeled peptides have potential use as carriers for the delivery of radionuclides to tumors, infarcts, and infected tissues for diagnostic imaging and radiotherapy. Many radiolabeled peptides are currently under investigation to determine their potential as imaging agents. These peptides are designed mainly for thrombus, tumor, and infection/inflammation imaging. This article presents recent developments in small synthetic peptides for imaging of thrombosis, tumors, and infection/inflammation.