Agonist properties of putative small-molecule somatostatin sst2 receptor-selective antagonists

The availability of antagonist ligands for somatostatin receptors is very limited, with those that are available often displaying agonist properties or limited receptor subtype selectivity. Hay et al. [Bioorg. Med. Chem. Lett. 11 (2001) 2731] recently described the development of small-molecule somatostatin receptor subtype 2 (sst(2)) selective compounds. This study investigates the binding affinity and functional characteristics of two of those antagonists (2 and 3) and the agonist compound, from which they were derived (1). In radioligand binding studies using the agonist radioligands [125I][Tyr(11)]SRIF-14 (Ala-Gly-c[Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-(125I-Tyr)-Thr-Ser-Cys]-OH), [125I]LTT-SRIF-28 ([Leu(8),DTrp(22),125I-Tyr(25)]SRIF-28; Ser-Ala-Asn-Ser-Asn-Pro-Ala-Leu-Ala-Pro-Arg-Glu-Arg-Lys-Ala-Gly-c[Cys-Lys-Asn-Phe-Phe-DTrp-Lys-Thr-(125I-Tyr)-Thr-Ser-Cys]-OH), [125I]CGP 23996 (c[Lys-Asu-Phe-Phe-Trp-Lys-Thr-(125I-Tyr)-Thr-Ser]), [125I][Tyr(3)]octreotide (DPhe-c[Cys-(125I-Tyr)-DTrp-Lys-Thr-Cys]-Thr-OH) and [125I][Tyr(10)]cortistatin-14 (Pro-c[Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-(125I-Tyr)-Ser-Ser-Cys]-Lys) at human recombinant somatostatin receptors expressed in Chinese hamster lung fibroblast (CCL39) cells and native rat cortex, the compounds bound with high affinity (pK(d) 6.8-9.7) and selectivity to human sst(2) receptors. Some affinity was also observed for sst(5) labelled by [125I][Tyr(3)]octreotide and [125I]CGP 23996. In functional studies at human sst(2) receptors expressed in Chinese hamster ovary (CHO) cells, both the agonist 1 and the two putative antagonists 2 and 3 concentration dependently inhibited forskolin-stimulated adenylate cyclase and stimulated luciferase reporter gene expression, with similar efficacy to the natural ligand somatotropin release inhibiting factor (SRIF)-14. Compound 1 had similar potency to SRIF-14, which was in the nanomolar range, whereas 2 and 3 were 10-100-fold less potent. The intrinsic activity of 2 and 3 was too high to allow antagonist studies to be carried out. In conclusion, in contrast to previous findings, all three compounds are potent agonists at recombinant human sst(2) receptors.