Targeted tumor diagnosis and therapy with peptide hormones as radiopharmaceuticals

Regulatory, receptor-binding peptides could be considered as future agents of choice for diagnostic imaging and therapy of cancers because their receptors are overexpressed in various human cancer cells. Peptides exhibit several advantages over classical macromolecules or drugs, e.g., from the chemical point of view: they are easy to synthesize and can withstand harsh chemical conditions which are required for chelation and radiolabeling. From the biological point of view, peptides exhibit fast blood clearance and high target-to-background ratios through receptor-mediated internalization. Furthermore, they are effective carriers for the delivery of cytotoxic drugs to target the affected tissues, thus avoiding normal cells from non-specific toxicity of anticancer agents. Owing to these features, radiolabeled receptor-binding peptides have emerged as a new class of radiopharmaceuticals for tumor scintigraphy and, more recently, to treat cancers by using peptide receptor radiation therapy (PRRT). The challenge in this scenario is to modify bioactive peptide hormones and to synthesize new sequences with improved metabolic stability without affecting the receptor binding properties after labeling with a chelator for incorporation of a radiometal. At the present time, however, the radiolabeled cholecystokinin-2 (CCK2)- and octreotide somatostatin-receptor selective analogs are the only examples that are being used in clinical practice. Other peptides such as neurotensin-, substance P-, gastrin-releasing peptide-, glucagons-like peptide 1 and neuropeptide Y (NPY) are under investigation to target breast, prostate, ovary, pancreas and brain tumors, in which overexpression of these peptide receptors has been reported. Among these peptides, neuropeptide Y (NPY) seems to be a very promising candidate because the change in its subtype receptor expression correlates with neoplastic changes. Here, we summarize the variety of experiences gained in the development of various peptide analogs, chelator/radiolabeling techniques for applications in tumor imaging and therapy.