Radiolabeled peptides in diagnosis and tumor imaging: clinical overview

The authors briefly review radiopeptides currently approved for use in the United States. They present a short review of the peptide somatostatin's actions and also note the five somatostatin receptors (SSTRs) to which the peptide and its synthetic analogs octreotide, lanreotide, and vapreotide bind. The many conditions besides neuroendocrine tumors having SSTRs are listed. Labeled octreotide and the other two analogues have a strong affinity for SSTR2 and SSTR5, which thereby produce positive imaging. The various neuroendocrine tumors best imaged by somatostatin receptor scintigraphy (SRS) are discussed, and the exceptions (insulinoma and medullary thyroid carcinoma) are noted to be seen better with labeled VIP and (99m)Tc-dimethylsuccinic acid (DMSA), respectively. SRS and VIP receptor scintigraphy are also noted to image many nonneuroendocrine tumors, which often have appropriate receptors. Several of the currently emerging and very effective new imaging techniques are described. These include (99m)Tc-DMSA for medullary thyroid carcinoma, (18)F dihydroxyphenylalanine positron emission tomography, and C(11) 5-hydroxytryptophan positron emission tomography scanning for all neuroendocrine tumor, but especially carcinoid tumor, metastases. The special role of SRS in identifying gastric carcinoid tumors in hypergastrinemic patients is reviewed. Various pitfalls in interpreting SRS are presented and receptor-enhancing techniques described. Besides use of SRS (mainly Octreoscan, Mallinckrodt Medical, St. Louis, MO) only for detecting and localizing primary tumors and metastases for staging, there are many additional special uses for clinical management of SRS-positive tumors. These include the intraoperative use of the handheld gamma-detecting probe. A brief enumeration is given of the most promising of other non-SST G-protein-coupled receptors and ligands currently under development. Finally, we have posed a number of questions for which answers are needed in the immediate future to facilitate better imaging. Extrapolations of current knowledge and experience with radiolabeled peptide pharmaceutical imaging are converted to reasonable speculations of anticipated future developments in this field.