New sst4/5-selective somatostatin peptidomimetics based on a constrained tryptophan scaffold

Synthesis of Azepinoindoles via Pd-Catalyzed C(sp2)-H Imidoylative Cyclization Reactions

An efficient and convenient method for the construction of diverse free (N-H)-benzazepinoindoles by Pd-catalyzed C(sp²)-H imidoylative cyclization of 3-(2-isocyanobenzyl)-1H-indoles was developed. The reaction shows a wide substrate scope and can be scaled up, providing a practical route to valuable bioactive azepinoindoles.

A computational study of somatostatin subtype-4 receptor agonist binding

The somatostatin subtype-4 receptor (sst4) is highly expressed in neocortical and hippocampal areas, which are affected by amyloid beta accumulation. Sst4 agonists enhance downstream activity of amyloid beta peptide catabolism through neprilysin and may slow the progression of Alzheimer’s disease (AD). Sst4 is a G protein coupled receptor (GPCR), the structure of which has yet to be resolved. A newly constructed sst4 homology model, along with a previously reported model-built sst4 receptor structure, were used in the present study to gain insights into binding requirements of sst4 agonists employing a set of compounds patented by Boehringer Ingelheim. Besides aiming at delineating binding at the macromolecular level of these recently disclosed compounds, our objectives included the generation of a quantitative structure-activity relationship (QSAR) global model to explore the relationship between chemical structure and affinity. Through the implementation of model building, docking, and QSAR, plausible correlations between structural properties and the binding affinity are established. This study sheds light on understanding binding requirements at the sst4 receptor.

Graphical abstract

Novel Drug-Like Somatostatin Receptor 4 Agonists are Potential Analgesics for Neuropathic Pain

Somatostatin released from the capsaicin-sensitive sensory nerves mediates analgesic and anti-inflammatory effects via the somatostatin sst4 receptor without endocrine actions. Therefore, sst4 is considered to be a novel target for drug development in pain including chronic neuropathy, which is an emerging unmet medical need. Here, we examined the in silico binding, the sst4-linked G-protein activation on stable receptor expressing cells (1 nM to 10 μM), and the effects of our novel pyrrolo-pyrimidine molecules in mouse inflammatory and neuropathic pain models. All four of the tested compounds (C1-C4) bind to the same binding site of the sst4 receptor with similar interaction energy to high-affinity reference sst4 agonists, and they all induce G-protein activation. C1 is the more efficacious (γ-GTP-binding: 218.2% ± 36.5%) and most potent (EC50: 37 nM) ligand. In vivo testing of the actions of orally administered C1 and C2 (500 µg/kg) showed that only C1 decreased the resiniferatoxin-induced acute neurogenic inflammatory thermal allodynia and mechanical hyperalgesia significantly. Meanwhile, both of them remarkably reduced partial sciatic nerve ligation-induced chronic neuropathic mechanical hyperalgesia after a single oral administration of the 500 µg/kg dose. These orally active novel sst4 agonists exert potent anti-hyperalgesic effect in a chronic neuropathy model, and therefore, they can open promising drug developmental perspectives.

Side Chain Cyclized Aromatic Amino Acids: Great Tools as Local Constraints in Peptide and Peptidomimetic Design

Constraining the conformation of flexible peptides is a proven strategy to increase potency, selectivity, and metabolic stability. The focus has mostly been on constraining the backbone dihedral angles; however, the correct orientation of the amino acid side chains (χ-space) that constitute the peptide pharmacophore is equally important. Control of χ-space utilizes conformationally constrained amino acids that favor, disfavor, or exclude the gauche (-), the gauche (+), or the trans conformation. In this review we focus on cyclic aromatic amino acids in which the side chain is connected to the peptide backbone to provide control of χ¹- and χ²-space. The manifold applications for cyclized analogues of the aromatic amino acids Phe, Tyr, Trp, and His within peptide medicinal chemistry are showcased herein with examples of enzyme inhibitors and ligands for G protein-coupled receptors.

Conformational and thermodynamic properties of non-canonical α,α-dialkyl glycines in the peptaibol Alamethicin: molecular dynamics studies

In this work, we investigate the structure, dynamic and thermodynamic properties of noncanonical disubstituted amino acids (α,α-dialkyl glycines), also known as non-natural amino acids, in the peptaibol Alamethicin. The amino acids under study are Aib (α-amino isobutyric acid or α-methyl alanine), Deg (α,α-diethyl glycine), Dpg (α,α-dipropyl glycine), Dibg (α,α-di-isobutyl glycine), Dhg (α,α-dihexyl glycine), DΦg (α,α-diphenyl glycine), Dbzg (α,α-dibenzyl glycine), Ac6c (α,α-cyclohexyl glycine), and Dmg (α,α-dihydroxymethyl glycine). It is hypothesized that these amino acids are able to induce well-defined secondary structure in peptidomimetics. To test this hypothesis, new peptidomimetics of Alamethicin were constructed by replacing the native Aib positions of Alamethicin by one or more new α,α-dialkyl glycines. Dhg and Ac6c demonstrated the capacity to induce well-defined α-helical structures. Dhg and Ac6c also promote the thermodynamic stabilization of these peptides in a POPC model membrane and are better alternatives to the Aib in Alamethicin. These noncanonical amino acids also improved secondary structure properties, revealing preorganization in water and maintenance of α helical structure in POPC. We show that it is possible to optimize the helicity and thermodynamic properties of native Alamethicin, and we suggest that these amino acids could be incorporated in other peptides with similar structural effect.

Tetrahydro-β-carboline-based spirocyclic lactam as type II' β-turn: application to the synthesis and biological evaluation of somatostatine mimetics

The synthesis of novel spirocyclic lactams, embodying D-tryptophan (Trp) amino acid as the central core and acting as peptidomimetics, is presented. It relies on the strategic combination of Seebach's self-reproduction of chirality chemistry and Pictet-Spengler condensation as key steps. Investigation of the conformational behavior by molecular modeling, X-ray crystallography, and NMR and IR spectroscopies suggests very stable and highly predictable type II' β-turn conformations for all compounds. Relying on this feature, we also pursued their application to two potential mimetics of the hormone somatostatin, a pharmaceutically relevant natural peptide, which contains a Trp-based type II' β-turn pharmacophore.

A structure-based approach to understanding somatostatin receptor-4 agonism (sst4)

It has been reported that somatostatin receptor subtypes 4 and 5 would be high-impact templates for homology modeling if their 3D structures became available. We have generated a homology model of the somatostatin receptor subtype 4 (sst4), using the newest active state β(2) adrenoreceptor crystal structure, and subsequently docked a variety of agonists into the model-built receptor to elucidate the binding modes of reported agonists. Using experimental restraints, we were able to explain observed activity profiles. We propose two binding modes that can consistently explain findings for high-affinity agonists and reason why certain structures display low affinities for the receptor.

Design, synthesis, and biological evaluation of benzodiazepine-based SUMO-specific protease 1 inhibitors

As the best-characterized ubiquitin-like protein (UBL), small ubiquitin-related modifier (SUMO) was found to conjugate with a number of proteins to regulate cellular functions including transcription, signal transduction, and cell cycle. While E1, E2 and E3 ligases are responsible for the forward SUMOylation reaction, SUMO-specific proteases (SENPs) reversibly remove SUMO from the SUMOylated proteins. Recently, SENP1 was found to be a potential therapeutic target for the treatment of prostate cancers, but the design and synthesis of its inhibitors have not been reported. We designed and synthesized a series of benzodiazepine-based SENP1 inhibitors, and they showed inhibitory activity as good as IC(50)=9.2μM (compound 38). The structure-activity relationship was also discussed.

Design of novel neurokinin 1 receptor antagonists based on conformationally constrained aromatic amino acids and discovery of a potent chimeric opioid agonist-neurokinin 1 receptor antagonist

A screening of conformationally constrained aromatic amino acids as base cores for the preparation of new NK1 receptor antagonists resulted in the discovery of three new NK1 receptor antagonists, 19 [Ac-Aba-Gly-NH-3',5'-(CF(3))(2)-Bn], 20 [Ac-Aba-Gly-NMe-3',5'-(CF(3))(2)-Bn], and 23 [Ac-Tic-NMe-3',5'-(CF(3))(2)-Bn], which were able to counteract the agonist effect of substance P, the endogenous ligand of NK1R. The most active NK1 antagonist of the series, 20 [Ac-Aba-Gly-NMe-3',5'-(CF(3))(2)-Bn], was then used in the design of a novel, potent chimeric opioid agonist-NK1 receptor antagonist, 35 [Dmt-D-Arg-Aba-Gly-NMe-3',5'-(CF(3))(2)-Bn], which combines the N terminus of the established Dmt(1)-DALDA agonist opioid pharmacophore (H-Dmt-D-Arg-Phe-Lys-NH(2)) and 20, the NK1R ligand. The opioid component of the chimeric compound 35, that is, Dmt-D-Arg-Aba-Gly-NH(2) (36), also proved to be an extremely potent and balanced μ and δ opioid receptor agonist with subnanomolar binding and in vitro functional activity.

Conformationally constrained opioid ligands: the Dmt-Aba and Dmt-Aia versus Dmt-Tic scaffold

Replacement of the constrained phenylalanine analogue 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic) in the opioid Dmt-Tic-Gly-NH-Bn scaffold by the 4-amino-1,2,4,5-tetrahydro-indolo[2,3-c]azepin-3-one (Aia) and 4-amino-1,2,4,5-tetrahydro-2-benzazepin-3-one (Aba) scaffolds has led to the discovery of novel potent mu-selective agonists (Structures 5 and 12) as well as potent and selective delta-opioid receptor antagonists (Structures 9 and 15). Both stereochemistry and N-terminal N,N-dimethylation proved to be crucial factors for opioid receptor selectivity and functional bioactivity in the investigated small peptidomimetic templates. In addition to the in vitro pharmacological evaluation, automated docking models of Dmt-Tic and Dmt-Aba analogues were constructed in order to rationalize the observed structure-activity data.