Human bombesin receptors are a group of the GPCR family; three subtypes of this receptors are BB1 receptor that binds neuromedin (NMB) with high affinity, BB2 receptor that binds gastrin releasing peptide (GRP), and an orphan receptor designated as bombesin receptor subtype-3 (BRS-3). BRS-3 is classified in the bombesin receptor family, because of its high homology with the other two bombesin receptors. It shares 51% homology with BB2 (GRP) receptor and 47% homology with the BB1 (NMB) receptor. On investigating expressions of bombesin receptor subtypes in human tumors, it has been observed that BB2 receptor is predominately expressed in gastrinomas, breast and prostate cancers; BB1 is expressed in bronchial, intestinal, and thymic cancers; whereas all three receptors are expressed in NSCLC, SCLC, bronchial carcinoids, and large-cell neuroendocrine cancers.
