Cyclic hexapeptide analogs of somatostatin containing bridge modifications. Syntheses and conformational analyses

The cyclic hexapeptide c[Pro6-Phe7-D-Trp8-Lys9-Thr10-Phe11] displays higher bioactivity than native somatostatin in inhibiting the release of growth hormone. The superscript numbers refer to the location of the residues in native somatostatin. To investigate the structural role played by the Phe11-Pro6 bridging region, we have synthesized a series of cyclic hexapeptide analogs of somatostatin incorporating peptidomimetics and retro-inverso modifications at the bridging region. Among them, two analogs contain the retro-inverso modification mAla6-gPhe11 at the bridging region, and five analogs contain 2-aminocyclopentane carboxylic acid (2-Ac5c) and 1-aminocyclopentane carboxylic acid (1-Ac5c) as proline mimetics. The conformational preferences of these analogs have been studied using 1H-NMR and computer simulations. All of these analogs maintain conformations similar to those of the parent cyclic hexapeptide around the Phe7-D-Trp8-Lys9-Thr10 tetrapeptide region consisting of a beta II' turn. However, they display different conformational features around the bridging region. The R-mAla analog and the five Ac5c analogs show only a trans amide bond for Phe11-Pro6 in the bridging region, while the S-mAla analog displays a cis/trans isomerization for the same amide linkage in the bridging region. The R-mAla and the five Ac5c analogs do not bind to the somatostatin receptor, while the S-mAla analog displays a high binding activity. Applying our recently proposed model for bioactivity of somatostatin analogs, we examined the structure-bioactivity relationships for these somatostatin analogs. This investigation provides valuable insight into the structural role played by the bridging region.