*4.1.3.2 NT receptor-targeting peptides*

Neurotensin (NT) with 13 amino acids in its structure is expressed in the central nervous system, peripheral tissues, and gastrointestinal system [72]. Despite overexpression of neurotensin receptors in 75% of ductal pancreatic carcinomas, NT receptor expression of endocrine pancreatic tumors, pancreatitis, and normal pancreatic tissue is negative (somatostatin positive) [85]. In a clinical study using 111In-labeled DTPA and DOTA-conjugated NT analogues, a number of specific changes were made to unnatural amino acids, indicating that the C-terminus plays a key role in the binding and biological properties of the peptide, and there is not a critical change in receptor binding activity of the peptide [72]. Because of high renal involvement of 111In-NT analogues, PRRT is not considered appropriate with these analogues. With the discovery of three times more stable 99Tcm-labeled NT (NT-XIX) analogue, a better tumor-to-kidney ratio, higher tumor involvement, and higher kidney excretion were achieved. 99Tcm-Demotensin, which was a high-stability NT analogue, has a higher tumor-to-intestinal and tumor-to-liver ratios, but the most appropriate ratios for imaging pancreatic tumors were obtained with the 111In-labeled analogues [86].
