Radiometallated peptides targeting guanylate cyclase C and the urokinase-type plasminogen activator receptor

Cholecystokinin and gastrin receptors targeting in gastrointestinal cancer

Cholecystokinin and Gastrin are amongst the first gastrointestinal hormone discovered. In addition to classical actions (contraction of gallbladder, growth and secretion in the stomach and pancreas), these also act as growth stimulants for gastrointestinal malignancies and cell lines. Growth of these tumours is inhibited by antagonists of the cholecystokinin and gastrin receptors. These receptors provides most promising approach in clinical oncology and several specific radiolabelled ligands have been synthesized for specific tumour targeting and therapy of tumours overexpressing these receptors. Therefore, definition of the molecular structure of the receptor involved in the autocrine/paracrine loop may contribute to novel therapies for gastrointestinal cancer. Hence, this review tries to focus on the role and distribution of these hormones and their receptors in gastrointestinal cancer with a brief talk about the clinical trial using available agonist and antagonist in gastrointestinal cancers.

Radiopeptide imaging and therapy in the United States

Radiolabeled peptides targeted against receptors on the cell surface have been shown to be remarkably specific and effective in the diagnosis and therapy of malignant disease. Much of the early work in this field took place outside the United States, but in recent years the research effort within the United States has accelerated. Most of the initial studies in the United States focused on somatostatin receptor ligands. (111)In-pentetreotide was approved in 1994 and has been used extensively in the diagnosis and management of a wide variety of neuroendocrine tumors, particularly carcinoid. This work was extended to (99m)Tc-labeled analogs, and the most successful, (99m)Tc-depreotide, was approved in 1999. This agent was found to be accurate in the diagnosis of lung cancer, but it was not particularly successful because it was supplanted by (18)F-FDG imaging with positron tomography. More recently, studies with (68)Ga-labeled somatostatin analogs were initiated in the United States. This effort was delayed relative to that in other parts of the world because of difficulty in obtaining the necessary generators and regulatory uncertainty, both of which are less of a problem currently. Several ligands are being developed to image melanoma through targeting of the melanocyte-stimulating hormone receptor. Other ligands are being developed to use the arginine-glycine-aspartate oligopeptide to target angiogenesis and to use bombesin analogs to target the gastrin-releasing peptide receptor for the diagnosis and potential therapy of prostate cancer. Peptide dimers that target 2 receptors simultaneously are also being constructed, potentially increasing the selectivity of the approach significantly. Radiopeptide therapy has been explored with these ligands, initially with high-dose (111)In-pentetreotide. This step has been followed by U.S. participation in several trials with (90)Y-, (177)Lu-, and (188)Re-labeled analogs. Some of these agents are now available clinically outside the United States, and it is important to design and conduct the appropriate trials so that this therapy can be offered within the United States.