A Refined Model for the Somatostatin Pharmacophore:  Conformational Analysis of Lanthionine−Sandostatin Analogs

Abstract

We report the conformational analysis of a series of analogs of sandostatin (octreotide, D-Phe1-c[Cys2-Phe3-D-Trp4-Lys5-Thr6-Cys 7]-Thr8-ol) using 1H NMR spectroscopy and molecular modeling. Two active compounds in which the disulfide group is replaced by a monosulfide (lanthionine) bridge (D-Phe1-c[AlaL2-Phe3-D-Trp4-Lys5-Thr6-A laL7]-Thr8-ol and D-Phe1-c[AlaL2-Phe3-D-Trp4-Lys5-Thr6-Al aL7]-Thr8-NH2, where AlaL denotes each of the lanthionine amino acid ends linked by the monosulfide bridge) show different mSSTR2b/rSSTR5 receptor selectivities as compared to sandostatin. These new results have enabled us to reveal features of the somatostatin pharmacophore common to the model previously proposed in our laboratory on the basis of main chain and side chain chiral methylation studies. In addition, our studies provide new insight into the role of the disulfide bridge and of Thr8 in binding potency. We also show that the lanthionine group is a good mimetic of beta-VI turns and can be incorporated in sandostatin analogs maintaining the essential secondary structural features of sandostatin. These results facilitate the design of new sandostatin peptidomimetics.

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

“…Interestingly, Goodman and coworkers illustrated that in SRIF analogues, which bind selectively to sst 2 2/3/5 , the side chains of DTrp 8 , Lys 9 , Phe 7 and Phe 2 constitute the most essential elements necessary for binding. 15,16 In their pharmacophore model, DTrp 8 and Lys 9 were at a close proximity of ∼5 Å, which is similar to the sst 2 pharmacophore. The aromatic ring at position 7, Phe 7 is farther away from DTrp 8 (7 to 9 Å) and Lys 9 (9 to 11 Å), and is not present in the sst 2 pharmacophore.…”

“…On the basis of these 3D structures of the free peptides, a pharmacophore model was proposed for SRIF analogues binding to sst 2/5 (and sst 3 ). [15][16][17][18] In this model, the side chains and the relative spatial arrangement of DPhe 2 , Phe 7 , DTrp 8 and Lys 9 constitute the most essential elements necessary for binding ( Figure 3C). The side chain of DTrp 8 is in close proximity to the side chain of Lys 9 (∼5 Å), whereas the side chain of Phe 7 is about 7 − 9 Å away from the side chain of DTrp 8 and 9 − 11 Å from the side chain of Lys 9 .…”

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 differs from 1 in that the two cysteines are substituted by homocysteines (Hcy) at position 3 and 14 resulting in a disulfide bridge with 22 atoms in the cycle instead of 20 atoms. While 1 binds to the sst 2/5 receptors with high binding affinity (IC 50 = 1.9 nM and 5.1 nM, respectively) and to sst 3 with moderate binding affinity (IC 50 = 39 nM), 3 binds more selectively to sst 2 with comparable high binding affinity as 1 (IC 50 = 4.9 nM) but with much less binding affinity to sst 3 and sst 5 (IC 50 = 452 nM and 109 nM, respectively). On the other hand, 3 also binds to sst 4 to some extent (IC 50 = 115 nM) ( Table 1).…”

The cyclization of peptides and depsipeptides

“…The only reported total synthesis in the solution phase is by Shiba et al [212], with the cyclized sulfur bridges being formed by extrusion of sulfur from cystine, followed by fragment condensation. In progressing to solid phase techniques, lanthionine bridges have been made compatible with peptide cyclization on an oxime resin [213,214], and using a biomimetic approach of assembling linear peptides containing cysteine and dehydroalanine residues, and forming the sulfur bridges by intramolecular 1,4-addition of the cysteine SH to the dehydroalanine [215,216]. In the more recent solid phase approach [217], an analogue of the ring C of nisin has been introduced via a pre-formed sulfur bridge as part of a triply orthogonal protecting group strategy summarized in Scheme 23.…”