2-pyridylthioureas:: Novel nonpeptide somatostatin agonists with SST4 selectivity

Somatostatin [somatotropin release-inhibiting factor (SRIF)] is a cyclic tetradecapeptide that is a potent inhibitor of growth hormone (GH) secretion from the anterior pituitary. In addition to the inhibitory effects on GH-release, SRIF-14 and SRIF-28, a 28-amino acid form of SRIF extended from the N-terminal end, inhibit the release of a variety of other peptides including glucagon, insulin, and gastrin, and both peptides act as neurotransmitters and neuromodulators in the central nervous system and the periphery. SRIF exerts its potent inhibitory effects following binding to high affinity SRIF receptors (ssts) that have been identified on target tissues. The recent cloning of five ssts has confirmed that the effects of SRIF are mediated by a family of G protein-coupled receptors (sst(1-5)). Based on structural and pharmacological properties sst(2), sst(3), and sst(5) belong to the SRIF1 receptor subclass, and the sst(1) and sst(4) subtypes comprise the SRIF2 subclass. The major difference between these two subclasses is that SRIF1 receptors bind octapeptide and hexapeptide SRIF-14 analogs with high affinity, while SRIF2 receptors bind these analogs with drastically reduced affinity. A screening program was initiated to identify a lead nonpeptide with affinity for sst(1-5) receptors. The search focused on a scaffold with the following attachments: (1) a heteroaromatic nucleus to mimic the Trp(8) residue, (2) a nonheteroaromatic nucleus to mimic Phe(7), and (3) a primary amine or other basic group to mimic the Lys(9) residue of SRIF-14. Using these criteria, a novel thiourea (NNC 26-9100, 17) was discovered as a structural lead. The key fragments in this compound are a heteroaromatic moiety (pyridine), an aromatic group, and a basic imidazole group connected through a thiourea scaffold. Compound 17 exhibited a K-i = 6 nM at sst(4) receptors with a 100-fold sst(4)/sst(2) selectivity and was shown to be a full agonist at this receptor subtype. This article will review the literature on the design and development of nonpeptide somatostatin receptor ligands and the therapeutic potential of these agents. Furthermore, our work on the development of 2-pyridyrthioureas as sst(4) receptor agonists will be described.