Synthesis of a substance P antagonist with a somatostatin scaffold: factors affecting agonism/antagonism at GPCRs and the role of pseudosymmetry

The beta-D-glucose scaffold as a beta-turn mimetic

Activity and selectivity are typically the first considerations when designing a drug. However, absorption, distribution, metabolism, excretion, and toxicity (ADMET) are equally important considerations. Peptides can provide a combination of potent binding and exquisite selectivity, as evidenced by their pervasive use as enzymes, hormones, and signaling agents within living systems. In particular, peptidic turn motifs are key elements of molecular recognition. They may be found at the exposed surfaces of globular proteins, where they are available for binding interactions with other peptides and small molecules. However, despite these advantages, peptides often make poor drugs. The amide backbone is subject to rapid enzymatic proteolysis, resulting in short half-lives. Furthermore, the ability of the amide backbone to hydrogen bond with water restricts its ability to cross membranes and, consequentially, results in poor oral bioavailability. Accordingly, the development of nonpeptidic scaffolds that mimic peptidic turn motifs represents a promising means of converting peptidic agents into more drugable molecules. In this Account, we describe the design and synthesis of beta-turn mimetics that use a beta-D-glucose scaffold, the first use of a sugar scaffold for this purpose. Somatostatin (SRIF) is a small protein (14 amino acid residues) human hormone; a shorter (6 amino acid residues) synthetic peptide, L-363,301, is a fully peptidal agonist. These two cyclic peptides share the beta-turn motif comprising Phe(7)-Trp(8)-Lys(9)-Thr(10) (d-Trp(8) in the case of L-363,301), of which the tryptophan and lysine residues in the i + 1 and i + 2 positions, respectively, are critical for binding. In 1988, we initiated a program that tested and validated the then-novel proposition that the beta-D-glucose scaffold can mimic the beta-turn in L-363,301. The beta-D-glucose scaffold proved to be an attractive mimic of a beta-turn in part because it permits the convenient attachment of amino acid side chains via facile etherification reactions, rather than carbon-carbon bond formations; it is also an inexpensive starting material with well-defined stereochemistry. From the beginning, biological assays were used alongside physical measurements to assess the relevance of the design. Our first two synthetic targets, compounds 6 and 7, bound the SRIF receptors on benchmark (AtT-20) cells, albeit weakly, consistent with the objective of the design. Subsequently, a better ligand (8) and two congeners were found to be agonists at the SRIF receptors, providing convincing evidence that the peptide backbone is not required for receptor binding or signal transduction. The unexpectedly high level of receptor affinity of selected analogs, as well as the fortuitous discovery that our peptidomimetics were active against several chemically distinct receptors, led us to hypothesize that these monosaccharides could access multiple potential binding modes. Our later studies of this sugar scaffold confirmed this property, which we termed pseudosymmetry, whereby multiple similar but nonidentical motifs are displayed within a single analog. We propose the presence of pseudosymmetry to be an element of privilege and an advantage for lead discovery.

Strategies for targeting protein-protein interactions with synthetic agents

The development of small-molecule modulators of protein-protein interactions is a formidable goal, albeit one that possesses significant potential for the discovery of novel therapeutics. Despite the daunting challenges, a variety of examples exists for the inhibition of two large protein partners with low-molecular-weight ligands. This review discusses the strategies for targeting protein-protein interactions and the state of the art in the rational design of molecules that mimic the structures and functions of their natural targets.

Designing scaffolds of peptides for phage display libraries

Phage display is a powerful method for the discovery of peptide ligands that are used for analytical tools, drug discovery, and target validations. Phage display technology can produce a huge number of peptides and generate novel peptide ligands. Recently, phage display technology has successfully managed to create peptide ligands that bind to pharmaceutically difficult targets such as the erythropoietin receptor. As a result of the structural analysis of their ligands, we found that the conformational design of peptides in library is important for selecting high-affinity ligands that bind to every target from a phage peptide library. Key issues concern constraints on the conformation of peptides on the phage and the development of chemically synthesized peptides derived from peptides on phage. This review discusses studies related to the conformation of peptides selected from phage display peptide libraries in addition to the conversion from peptides to non-peptides.

Exploring privileged structures: the combinatorial synthesis of cyclic peptides

Head-to-tail cyclic peptides have been reported to bind to multiple, unrelated classes of receptor with high affinity. They may therefore be considered to be privileged structures. This review outlines the strategies by which both macrocyclic cyclic peptides and cyclic dipeptides or diketopiperazines have been synthesised in combinatorial libraries. It also briefly outlines some of the biological applications of these molecules, thereby justifying their inclusion as privileged structures.

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.

Successful bridging from a peptide to a non peptide antagonist at the human tachykinin NK-2 receptor

Non peptide products have been found to show nanomolar binding and functional affinities at the human tachykinin NK-2 receptor. The new antagonists do not possess stereogenic centers and their thermal behaviour in solution is featured by a peculiar set of conformational stereoisomers. A macroscopic viewpoint is preferentially adopted to rationalize the obtained results.

Analogs of Substance P modified at the C-terminus which are both agonist and antagonist of the NK-1 receptor depending on the second messenger pathway

The initial goal of this study was to analyze, using photolabeling, the interactions between Substance P and its tachykinin NK-1 receptor. Therefore, the photoreactive amino acid para-benzoyl-phenylalanine (pBzl)Phe was incorporated into the Substance P sequence from position 4 to 11 leading to Bapa0[(pBzl)Phex]SP analogs. Biotinyl sulfone-5-aminopentanoic acid (Bapa) was introduced in order to purify the covalent complex. These photoreactive SP analogs were first assayed for their affinity for the two binding sites associated with the NK-1 receptor, as well as for their potency in activating the phospholipase C and adenylate cyclase pathways. All analogs photoreactive from position 4 to 11 have moderate to high affinity for the two NK-1 receptor-binding sites, except for the analog modified at position 7. This affinity could be correlated to their potency to activate the phospholipase C and adenylate cyclase pathways, except for the analog photoreactive at position 11. Bapa0[(pBzl)Phe11]SP was found to be an agonist in the phospholipase C pathway and an antagonist in the adenylate cyclase pathway, other analogs modified at position 11 were therefore analyzed. Among these, Bapa0[Pro9, (pBzl)Hcy(O2)11]SP is a partial agonist, whereas Bapa0[Hcy(ethylaminodansyl)11]SP is a full agonist in the phospholipase C pathway, the two analogs being antagonist in the adenylate cyclase pathway. These results show that analogs of SP can be simultaneously agonist at one binding site and antagonist at the other binding site associated with the NK-1 receptor.

Exploring privileged structures: the combinatorial synthesis of cyclic peptides

Head-to-tail cyclic peptides have been reported to bind to multiple, unrelated classes of receptor with high affinity. They may therefore be considered to be privileged structures. This review outlines the strategies by which both macrocyclic cyclic peptides and cyclic dipeptides or diketopiperazines have been synthesised in combinatorial libraries. It also briefly outlines some of the biological applications of these molecules, thereby justifying their inclusion as privileged structures.

Small molecule somatostatin receptor subtype-2 antagonists

The first potent small molecule sst2 antagonists are reported. Altering known sst2 agonist molecules yielded compounds with high sst2 binding affinity and full antagonist activity. Compound 7a, for example, displaced somatostatin binding to the sst2 receptor with an IC(50)=2.9 nM and antagonized somatostatin action with an IC(50)=29nM.

Design and synthesis of novel scaffolds for drug discovery: hybrids of beta-D-glucose with 1,2,3,4-tetrahydrobenzo[e][1,4]diazepin-5-one, the corresponding 1-oxazepine, and 2- and 4-pyridyldiazepines

[structure: see text]. We describe the syntheses of novel tricyclic scaffolds that incorporate a fusion of a substituted pyranose ring with the seven-membered rings of 1,2,3,4-tetrahydrobenzo[e][1,4]diazepin-5-one and the corresponding oxazepine and pyridyldiazepine to generate the benzodiazepines, and the related heterocycles. In each instance, the pyranose rings contain three protected hydroxyls, suitable for selective derivatization.