Bombesin (BBN or BN)-like peptide is an analog of human gastrin-releasing peptide (GRP) that binds to GRP receptors (GRP-R) (1). Both GRP and BN share an amidated C-terminus sequence homology of seven amino acids (Trp-Ala-Val-Gly-His-Leu-Met-NH2). BN peptides induce such biological responses as secretion of adrenal, pituitary, and gastrointestinal hormones; gastric acid secretion; modulation of neuronal firing rate; and regulation of smooth muscle contraction (2, 3). BN-Related peptide receptors can be divided into four subtypes: GRP-R (BB2, BRS-2), neuromedin B receptor (NMB-R, BB1, BRS-1), the orphan receptor BB3-R (BRS-3), and the amphibian receptor BB4-R (1). Several human cancers, such as prostate, breast, lung, colon, and pancreatic cancers, overexpress receptors for BN-like peptides. The BN-like peptides have been radiolabeled with different radionuclides for in vivo imaging of various cancers (4-7).
Litorin (pGlu-Gln-Trp-Ala-Val-Gly-His-Phe-Met-NH2), an amphibian BN peptide derivative, is found to stimulate the contraction of smooth muscle, to stimulate gastrin, gastric acid, and pancreatic secretion, and to suppress the nutriment in in vivo experiments (1). 99mTc-Litorin was developed for non-invasive imaging of tumors with overexpressed GRP-R (8).