Peptide receptor radionuclide therapy.

On their plasma membranes, cells express receptor proteins with high affinity for regulatory peptides, such as somatostatin. Changes in the density of these receptors during disease, for example, overexpression in many tumors, provide the basis for new imaging methods. The first peptide analogues successfully applied for visualization of receptor-positive tumors were radiolabeled somatostatin analogues. The next step was to label these analogues with therapeutic radionuclides for peptide receptor radionuclide therapy (PRRT). Results from preclinical and clinical multicenter studies already have shown an effective therapeutic response when using radiolabeled somatostatin analogues to treat receptor-positive tumors. Infusion of positively charged amino acids reduces kidney uptake, enlarging the therapeutic window. For PRRT of CCK-B receptor-positive tumors, such as medullary thyroid carcinoma, radiolabeled minigastrin analogues currently are being successfully applied. The combination of different therapy modalities holds interest as a means of improving the clinical therapeutic effects of radiolabeled peptides. The combination of different radionuclides, such as (177)Lu- and (90)Y-labeled somatostatin analogues, to reach a wider tumor region of high curability, has been described. A variety of other peptide-based radioligands, such as bombesin and NPY(Y(1)) analogues, receptors for which are expressed on common cancers such as prostate and breast cancer, are currently under development and in different phases of (pre)clinical investigation. Multireceptor tumor targeting using the combination of bombesin and NPY(Y(1)) analogues is promising for scintigraphy and PRRT of breast carcinomas and their lymph node metastases.