Click-Chemistry-Mediated Synthesis of Selective Melanocortin Receptor 4 Agonists

Ψ and χ Angle Constrains at the C-Terminus Trp Position of the Melanotropin Tetrapeptide Ac-His-d-Phe-Arg-Trp-NH2 Lead to Potent and Selective Agonists at hMC1R and hMC4R

Melanocortin receptors (MCRs) are a family of G protein-coupled receptors that regulate important physiological functions. Yet, drug development targeting MCRs is hindered by potential side effects due to a lack of receptor subtype-selective ligands with bioavailability. Here, we report novel synthetic pathways to introduce Ψ and χ angle constraints at the C-terminus Trp position of the nonselective prototype tetrapeptide agonist Ac-His-d-Phe-Arg-Trp-NH₂. With these conformational constraints, peptide 1 (Ac-His-d-Phe-Arg-Aia) shows improved selectivity at hMC1R, with an EC₅₀ of 11.2 nM for hMC1R and at least 15-fold selectivity compared to other MCR subtypes. Peptide 3 (Ac-His-pCF₃-d-Phe-Arg-Aia) is a potent and selective hMC4R agonist with an EC₅₀ of 4.1 nM at hMC4R and at least ninefold selectivity. Molecular docking studies reveal that the Ψ and χ angle constraints force the C-terminal Aia residue to flip and interact with TM6 and TM7, a feature that we hypothesize leads to the receptor subtype selectivity.

Targeting Melanocortin Receptors Using SNAr-Type Macrocyclization: A Doubly Orthogonal Route to Cyclic Peptide Conjugates

While a range of strategies exist to accomplish peptide macrocyclization, they are frequently limited by the need for orthogonal protection or provide little opportunity for structural diversification. We have evaluated an efficient macrocyclization method that employs nucleophilic aromatic substitution (S_NAr) to create thioether macrocycles. This versatile macrocyclization, orthogonal to conventional peptide synthesis, can be performed in solution on unprotected peptidomimetics or on resin-bound peptides with side-chain protection in place. We show that the electron-withdrawing groups present in the products can be further utilized in subsequent orthogonal reactions to alter the peptide properties or to add prosthetic groups. The macrocyclization strategy was applied to the design of melanocortin ligands, generating a library of potent melanocortin agonists that exhibit distinct subtype selectivity.

Tuning peptide structure and function through fluorobenzene stapling

Cyclic peptides are promising next-generation therapeutics with improved biological stability and activity. A catalyst-free stapling method for cysteine-containing peptides was developed. This enables fine-tuning of the macrocycle by using the appropriate regioisomers of fluorobenzene linkers. Stapling was performed on the unprotected linear peptide or, more conveniently, directly on-resin after peptide synthesis. NMR spectroscopy and circular dichroism studies demonstrate that the type of stapling can tune the secondary structures of the peptides. The method was applied to a set of potential agonists for melanocortin receptors, generating a library of macrocyclic potent ligands with ortho , meta or para relationships between the thioethers. Their small but significant difference in potency and efficacy demonstrates how the method allows facile fine-tuning of macrocyclic peptides towards biological targets from the same linear precursor.

Antistress Action of Melanocortin Derivatives Associated with Correction of Gene Expression Patterns in the Hippocampus of Male Rats Following Acute Stress

Natural melanocortins (MCs) have been used in the successful development of drugs with neuroprotective properties. Here, we studied the behavioral effects and molecular genetic mechanisms of two synthetic MC derivatives-ACTH(4-7)PGP (Semax) and ACTH(6-9)PGP under normal and acute restraint stress (ARS) conditions. Administration of Semax or ACTH(6-9)PGP (100 μg/kg) to rats 30 min before ARS attenuated ARS-induced behavioral alterations. Using high-throughput RNA sequencing (RNA-Seq), we identified 1359 differentially expressed genes (DEGs) in the hippocampus of vehicle-treated rats subjected to ARS, using a cutoff of >1.5 fold change and adjusted p-value (Padj) < 0.05, in samples collected 4.5 h after the ARS. Semax administration produced > 1500 DEGs, whereas ACTH(6-9)PGP administration led to <400 DEGs at 4.5 h after ARS. Nevertheless, ~250 overlapping DEGs were identified, and expression of these DEGs was changed unidirectionally by both peptides under ARS conditions. Modulation of the expression of genes associated with biogenesis, translation of RNA, DNA replication, and immune and nervous system function was produced by both peptides. Furthermore, both peptides upregulated the expression levels of many genes that displayed decreased expression after ARS, and vice versa, the MC peptides downregulated the expression levels of genes that were upregulated by ARS. Consequently, the antistress action of MC peptides may be associated with a correction of gene expression patterns that are disrupted during ARS.

ACTH 6-9-PGP improves memory consolidation processes in rats

Introduction: The His-Phe-Arg-Trp sequence corresponding to the 6-9th amino acid residue of the adrenocorticotropic hormone molecule (ACTH6-9) is the critical pharmacophore of all endogenous melanocortin receptor agonists. In order to effects prolongation it may be stabilized by the addition of the amino acid sequence Pro-Gly-Pro (PGP) to the C-terminus. The aim of this work was to study the effect of ACTH6-9-PGP (HFRWPGP) on the processes of memory consolidation in the model of passive avoidance conditioning in comparison with ACTH4-7-PGP effects.

Materials and methods: The study was carried out on the model of passive avoidance conditioning. The effects of ACTH6-9-PGP were studied after its intraperitoneal injection to male Wistar rats at doses of 0.5, 5, 50, 150, and 450 μg/kg 15 minutes before the experiment, whereas the effects of ACTH4-7-PGP– under the similar conditions at doses of 50, 150, and 450 μg/kg.

Results and discussion: It was found that ACTH6-9-PGP had a pronounced stimulating effect on the memory consolidation process in the dose range from 0.5 μg/kg to 150 μg/kg, significantly increasing the latent period of an animal entering the dark chamber. Administration of ACTH4-7-PGP led to an improvement in the consolidation processes of the acquired conditioned reflex at the doses of 50 μg/kg and 450 μg/kg.

Conclusion: The range of effective doses of ACTH6-9-PGP is lower than that of ACTH4-7-PGP, which indicates the greater activity of HFRWPGP sequence in relation to memory consolidation processes and allows considering this peptide as a promising molecule for creating nootropic pharmacological drugs.

The melanocortin pathway and energy homeostasis: From discovery to obesity therapy

Background

Over the past 20 years, insights from human and mouse genetics have illuminated the central role of the brain leptin-melanocortin pathway in controlling mammalian food intake, with genetic disruption resulting in extreme obesity, and more subtle polymorphic variations influencing the population distribution of body weight. At the end of 2020, the U.S. Food and Drug Administration (FDA) approved setmelanotide, a melanocortin 4 receptor agonist, for use in individuals with severe obesity due to either pro-opiomelanocortin (POMC), proprotein convertase subtilisin/kexin type 1 (PCSK1), or leptin receptor (LEPR) deficiency.

Scope of review

Herein, we chart the melanocortin pathway's history, explore its pharmacology, genetics, and physiology, and describe how a neuropeptidergic circuit became an important druggable obesity target.

Major conclusions

Unravelling the genetics of the subset of severe obesity has revealed the importance of the melanocortin pathway in appetitive control; coupling this with studying the molecular pharmacology of compounds that bind melanocortin receptors has brought a new obesity drug to the market. This process provides a drug discovery template for complex disorders, which for setmelanotide took 25 years to transform from a single gene into an approved drug.

C-Terminal lactamization of peptides

Solid-phase synthesis of peptides (SPPS) with release through formation of C-terminal γ-, δ-, or ε-lactams is presented. The natural products ciliatamide A and C were synthesized in up to 90% yield. Peptides carrying C-terminal lactams were shown to possess increased bio-stability and comparable biological activity as compared to the parent non-lactamized peptide amides.

Heteroaryl Rings in Peptide Macrocycles

This Review is devoted to the chemistry of macrocyclic peptides having heterocyclic fragments in their structure. These motifs are present in many natural products and synthetic macrocycles designed against a particular biochemical target. Thiazole and oxazole are particularly common constituents of naturally occurring macrocyclic peptide molecules. This frequency of occurrence is because the thiazole and oxazole rings originate from cysteine, serine, and threonine residues. Whereas other heteroaryl groups are found less frequently, they offer many insightful lessons that range from conformational control to receptor/ligand interactions. Many options to develop new and improved technologies to prepare natural products have appeared in recent years, and the synthetic community has been pursuing synthetic macrocycles that have no precedent in nature. This Review attempts to summarize progress in this area.

Recent applications of click chemistry in drug discovery

Introduction: Click chemistry has been exploited widely in the past to expedite lead discovery and optimization. Indeed, Copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry is a bioorthogonal reaction of widespread utility throughout medicinal chemistry and chemical biology. Areas covered: The authors review recent applications of CuAAC click chemistry to drug discovery based on the literature published since 2013. Furthermore, the authors provide the reader with their expert perspectives on the area including their outlook on future developments. Expert opinion: Click chemistry reactions are an important part of the medicinal chemistry toolbox and offer substantial advantages to medicinal chemists in terms of overcoming the limitations of useful chemical synthesis, increasing throughput, and improving the quality of compound libraries. To explore new chemical spaces for drug-like molecules containing a high degree of structural diversity, it may be useful to merge the diversity-oriented synthesis and 'privileged' substructure-based strategy with bioorthogonal reactions using sophisticated automation and flow systems to improve productivity. Large compound libraries obtained in this way should be of great value for the discovery of bioactive compounds and therapeutic agents.

On resin click-chemistry-mediated synthesis of novel enkephalin analogues with potent anti-nociceptive activity

Here, we report the chemical synthesis of two DPDPE analogues 7a (NOVA1) and 7b (NOVA2). This entailed the solid-phase synthesis of two enkephalin precursor chains followed by a Cu^I-catalyzed azide-alkyne cycloaddition, with the aim of improving in vivo analgesic efficacy versus DPDPE. NOVA2 showed good affinity and selectivity for the μ-opioid receptor (K_I of 59.2 nM, EC₅₀ of 12.9 nM, E_Max of 87.3%), and long lasting anti-nociceptive effects in mice when compared to DPDPE.

Discovery of Polypharmacological Melanocortin-3 and -4 Receptor Probes and Identification of a 100-Fold Selective nM MC3R Agonist versus a μM MC4R Partial Agonist

The centrally expressed melanocortin-3 and melanocortin-4 receptors (MC3R and MC4R, respectively) are established targets to treat diseases of positive- and negative-energy homeostasis. We previously reported [ Doering , S. R. ; J. Med. Chem. 2017 , 60 , 4342 - 4357 ] mixture-based positional scanning approaches to identify dual MC3R agonist and MC4R antagonist tetrapeptides. Herein, 46 tetrapeptides were chosen for MC3R agonist screening selectivity profiles, synthesized, and pharmacologically characterized at the mouse melanocortin-1, -3, -4, and -5 receptors. Substitutions to the tetrapeptide template were selected solely based on MC3R agonist potency from the mixture-based screen. This study resulted in the discovery of compound 42 (Ac-Val-Gln-(pI)DPhe-DTic-NH₂), a full MC3R agonist that is 100-fold selective for the MC3R over the μM MC4R partial agonist pharmacology. This compound represents a first-in-class MC3R selective agonist. This ligand will serve as a useful in vivo molecular probe for the investigation of the roles of the MC3R and MC4R in diseases of dysregulated energy homeostasis.

MC4R Agonists: Structural Overview on Antiobesity Therapeutics

The melanocortin-4 receptor (MC4R) regulates adipose tissue formation and energy homeostasis, and is believed to be a monogenic target for novel antiobesity therapeutics. Several research efforts targeting this receptor have identified potent and selective agonists. While viable agonists have been characterized in vitro, undesirable side effects frequently appeared during clinical trials. The most promising candidates have diverse structures, including linear peptides, cyclic peptides, and small molecules. Herein, we present a compilation of potent MC4R agonists and discuss the pivotal structural differences within those molecules that resulted in good selectivity for MC4R over other melanocortins. We provide insight on recent progress in the field and reflect on directions for development of new agonists.