Radiolabelled peptides and low molecular weight proteins in metabolic diseases.

During the last decade there has been a tremendous effort to develop labelled peptides for diagnosis and therapy. The main goal has been to develop tumor imaging/therapeutic agents, as well as peptides directed to the study of thrombosis and infection. Relatively few efforts have been made to develop peptides directed to the study of metabolic diseases. Ideally, a peptide suitable for the study of metabolism should be constructed keeping in mind the following characteristics: a) preserved affinity constant, b) preserved or improved specificity for its binding site, c) increased biological half-life in comparison with the parent peptide, d) labelling with a g or positron emitter whose physical half-life fits with the biological half-life, e) strong binding of the nuclide to the molecule so that it cannot be released after internalization. In this paper some of the peptides or low molecular weight proteins along with some analogues which have been employed in experimental studies and in humans are reviewed, with major emphasis on amyloid seekers, insulin and leptin. Many of these radiopharmaceuticals have been labelled with iodine isotopes, however their in vivo application suffer of severe limitations due to rapid release of iodine after internalization. On the other hand, new perspectives are opened by new radiofluorination techniques, which offer the unique advantage to quantify organ uptake and kinetics, parameters which are of paramount importance in metabolic studies.