Novel sst₂-Selective Somatostatin Agonists. Three-Dimensional Consensus Structure by NMR

Abstract

The three-dimensional NMR structures of six octapeptide agonist analogues of somatostatin (SRIF) in the free form are described. These analogues, with the basic sequence H-DPhe/Phe 2 -c[Cys 3 -Xxx 7 -DTrp 8 -Lys 9 -Thr 10 -Cys 14 ]-Thr-NH 2 (the numbering refers to the position in native SRIF), with Xxx 7 being Ala/Aph, exhibit potent and highly selective binding to human SRIF type 2 (sst 2 ) receptors. The backbone of these sst 2 -selective analogues have the usual type-II' β-turn reported in the literature for sst 2/3/5 -subtype-selective analogues. Correlating biological results and NMR studies led to the identification of the side chains of DPhe 2 , DTrp 8 and Lys 9 as the necessary components of the sst 2 pharmacophore. This is the first study to show that the aromatic ring at position 7 (Phe 7 ) is not critical for sst 2 binding and that it plays an important role in sst 3 and sst 5 binding. This pharmacophore is therefore different from that proposed by others for sst 2/3/5 analogues.

Ring Size in Octreotide Amide Modulates Differently Agonist versus Antagonist Binding Affinity and Selectivity

“…With the available conformational flexibility in the backbone of the analogue with DHcy at position 3 and Hcy at position 14, one would expect the aromatic side chain of Cpa 2 to fit both sst 2 and sst 5 pharmacophores. The binding data of 5 compared to those of 4 show a 50-fold loss of binding affinity to sst 2 , equal affinity to sst 5 , a 3-fold loss to sst 3 and some gain of binding affinity to sst 1 and sst 4 (Table 1). Hence, the binding data could only be explained based on the 3D structure, which shows that the bulkier Nal group at the C-terminus extends further away in the plane of the peptide backbone, which probably prevents the analogue from binding to both sst 2 and sst 5 .…”

“…Analogue 3 has the type-II' β-turn similar to the sst 2 -selective analogues and the side chains of DPhe 2 , DTrp 8 and Lys 9 are in the plane of the backbone of the analogue, just like in 1 (Figure 3). The flexibility in the side chain of DPhe 2 along with the backbone flexibility due to the longer Hcy at positions 3 and 14, explains the low binding affinity of this analogue to sst 4 and sst 5 receptors. Analogue 5, with DHcy at position 3 has an inverse γ-turn around residue DTrp 8 .…”

Abdominal surgery inhibits circulating acyl ghrelin and ghrelin-O-acyltransferase levels in rats: role of the somatostatin receptor subtype 2

“…26 and the sst2 antagonist S-406-028, H2N-pNO2Phe-DCys-Tyr-DAph(Cbm)-Lys-Thr-Cys-2Nal-NH2, molecular weight 1,208.5, compound 4 in Ref. 11 (Clayton Foundation Laboratories, Salk Institute, La Jolla, CA) were synthesized and purity assessed as previously described (11,26).…”

Brain somatostatin receptor 2 mediates the dipsogenic effect of central somatostatin and cortistatin in rats: role in drinking behavior

“…Therefore, we first determined whether somatostatin agonists injected intracerebroventricularly induce orexigenic or dipsogenic effect in the light phase without access to water or food, respectively, to dissociate these two components. Then, we determined the sst subtype(s) involved in the dipsogenic effect using sst subtype-specific peptide analogs, namely the sst 1 agonist S-406-062, sst 2 agonist S-346-011, sst 4 agonist S-315-297, and the sst 2 antagonist S-406-028 recently developed (9,17,19,27). The brain angiotensinergic mechanism is well known to regulate thirst and fluid balance (22), and one report indicates that this pathway plays a role in the dipsogenic effect of intracerebroventricular octreotide (30).…”