Current status and perspectives in peptide receptor radiation therapy

Regulatory peptide receptors are overexpressed in numerous human cancers. The specific receptor binding property of peptides can be exploited by their labelling with a radionuclide and their use as carriers to guide the radioactivity to the tissues expressing their specific receptors. During the past decade, radiolabelled receptor-binding peptides have emerged as an important class of radiopharmaceuticals for tumour diagnosis and therapy. The first and most successful imaging agents to date are somatostatin analogues which are routinely used for somatostatin receptor scintigraphy. Peptide receptor radionuclide therapy (PRRT) with (90)Y-DOTA-octreotide and (177)Lu-DOTA-octreotate in neuroendocrine tumours (NETs) results in symptomatic improvement, prolonged survival, and enhanced quality of life. The results in terms of tumour regression are very encouraging with few and mostly mild side effects when patients are carefully selected and kidney protective agents are given. Nevertheless much profit can be gained from improving the available receptor-targeting strategies and developing new strategies. In this review, the current state of clinical use of radiolabelled peptides for diagnosis and therapy of NETs is presented. In addition, potential directions for optimization and future developments are discussed. These include the optimization of peptide analogues or derivatives, increasing the access and binding on specific receptors on the tumour sites, increasing radiotoxicity profile and multimodality strategies. Other peptides such as minigastrin, glucagon-like peptide-1 (GLP-1), cholecystokinin (CCK), bombesin (BN)/gastrin-releasing peptide (GRP), substance P, neurotensin (NT), neuropeptide Y (NPY) and RGD peptides are promising for PRRT and currently under preclinical and clinical development.