Design, synthesis, and biological evaluation of a new class of small molecule peptide mimetics targeting the melanocortin receptors

Strategies for converting turn-motif and cyclic peptides to small molecules for targeting protein-protein interactions

The development of small molecules that interact with protein-protein interactions is an ongoing challenge. Peptides offer a starting point in the drug discovery process for targeting protein-interactions due to their larger, more flexible structure and the structurally diverse properties that allow for a greater interaction with the protein. The techniques for rapidly identifying potent cyclic peptides and turn-motif peptides are highly effective, but this potential has not yet transferred to approved drug candidates. By applying the properties of the peptide-protein interaction the development of small molecules for drug discovery has the potential to be more efficient. In this review, we discuss the methods that allow for the unique binding properties of peptides to proteins, and the methods deployed to transfer these qualities to potent small molecules.

Demonstration of a Common DPhe7 to DNal(2')7 Peptide Ligand Antagonist Switch for Melanocortin-3 and Melanocortin-4 Receptors Identifies the Systematic Mischaracterization of the Pharmacological Properties of Melanocortin Peptides

Melanocortin peptides containing a 3-(2-naphthyl)-d-alanine residue in position 7 (DNal(2')⁷), reported as melanocortin-3 receptor (MC3R) subtype-specific agonists in two separate publications, were found to lack significant MC3R agonist activity. The cell lines used at the University of Arizona for pharmacological characterization of these peptides, consisting of HEK293 cells stably transfected with human melanocortin receptor subtypes MC1R, MC3R, MC4R, or MC5R, were then obtained and characterized by quantitative polymerase chain reaction (PCR). While the MC1R cell line correctly expressed only hMCR1, the three other cell lines were mischaracterized with regard to receptor subtype expression. The demonstration that a 3-(2-naphthyl)-d-alanine residue in position 7, irrespective of the melanocortin peptide template, results primarily in the antagonism of MC3R and MC4R then allowed us to search the published literature for additional errors. The erroneously characterized DNal(2')⁷-containing peptides date back to 2003; thus, our analysis suggests that systematic mischaracterization of the pharmacological properties of melanocortin peptides occurred.

ACTH(6-9)PGP Peptide Protects SH-SY5Y Cells from H2O2, tert-Butyl Hydroperoxide, and Cyanide Cytotoxicity via Stimulation of Proliferation and Induction of Prosurvival-Related Genes

Stabilized melanocortin analog peptide ACTH(6-9)PGP (HFRWPGP) possesses a wide range of neuroprotective activities. However, its mechanism of action remains poorly understood. In this paper, we present a study of the proproliferative and cytoprotective activity of the adrenocorticotropic hormone fragment 6-9 (HFRW) linked with the peptide prolyine-glycyl-proline on the SH-SY5Y cells in the model of oxidative stress-related toxicity. The peptide dose-dependently protected cells from H2O2, tert-butyl hydroperoxide, and KCN and demonstrated proproliferative activity. The mechanism of its action was the modulation of proliferation-related NF-κB genes and stimulation of prosurvival NRF2-gene-related pathway, as well as a decrease in apoptosis.

Incorporation of Agouti-Related Protein (AgRP) Human Single Nucleotide Polymorphisms (SNPs) in the AgRP-Derived Macrocyclic Scaffold c[Pro-Arg-Phe-Phe-Asn-Ala-Phe-dPro] Decreases Melanocortin-4 Receptor Antagonist Potency and Results in the Discovery of Melanocortin-5 Receptor Antagonists

While the melanocortin receptors (MCRs) are known to be involved in numerous biological pathways, the potential roles of the MC5R have not been clearly elucidated in humans. Agouti-related protein (AgRP), an MC3R/MC4R antagonist and MC4R inverse agonist, contains an exposed β-hairpin loop composed of six residues (Arg-Phe-Phe-Asn-Ala-Phe) that is imperative for binding and function. Within this active loop of AgRP, four human missense polymorphisms were deposited into the NIH Variation Viewer database. These polymorphisms, Arg111Cys, Arg111His, Phe112Tyr, and Ala115Val (AgRP full-length numbering), were incorporated into the peptide macrocycles c[Pro¹-Arg²-Phe³-Phe⁴-Xaa⁵-Ala⁶-Phe⁷-dPro⁸], where Xaa was Dap⁵ or Asn⁵, to explore the functional effects of these naturally occurring substitutions in a simplified AgRP scaffold. All peptides lowered potency at least 10-fold in a cAMP accumulation assay compared to the parent sequences at the MC4Rs. Compounds MDE 6-82-3c, ZMK 2-82, MDE 6-82-1c, ZMK 2-85, and ZMK 2-112 are also the first AgRP-based chemotypes that antagonize the MC5R.

Antibacterial sorbicillin and diketopiperazines from the endogenous fungus Penicillium sp. GD6 associated Chinese mangrove Bruguiera gymnorrhiza

One new sorbicillin derivative, 2-deoxy-sohirnone C (1), one new diketopiperazine alkaloid, 5S-hydroxynorvaline-S-Ile (2), and two naturally occurring diketopiperazines, 3S-hydroxylcyclo(S-Pro-S-Phe) (3) and cyclo(S-Phe-S-Gln) (4), together with three known compounds were isolated from the Chinese mangrove endophytic fungus Penicillium sp. GD6. Their structures were determined on the basis of extensive spectroscopic analyses and by comparison with literature data. The absolute configuration of 3-hydroxyl moiety in 3 was determined by Mosher's method, while the absolute stereochemistry of 2 and 4 was established by comparison with the CD spectra of natural and synthesized diketopiperazines. Compound 1 showed moderate antibacterial activity against Methicillin-resistant Staphylococcus aureus with a MIC value of 80 μg·mL^-1.

Concise diastereoselective synthesis of calcaripeptide C via asymmetric transfer hydrogenation/Pd-induced chiral allenylzinc as a key reaction

The synthesis of the cyclodepsipeptide calcaripeptide C was accomplished with an overall yield of 10.7% by a catalytic asymmetric transfer hydrogenation (ATH) together with Marshall's allenylation as pivotal reactions.

Design of peptide and peptidomimetic ligands with novel pharmacological activity profiles

Peptide hormones and neurotransmitters are of central importance in most aspects of intercellular communication and are involved in virtually all degenerative diseases. In this review, we discuss physicochemical approaches to the design of novel peptide and peptidomimetic agonists, antagonists, inverse agonists, and related compounds that have unique biological activity profiles, reduced toxic side effects, and, if desired, the ability to cross the blood-brain barrier. Designing ligands for specific biological and medical needs is emphasized, as is the close collaboration of chemists and biologists to maximize the chances for success. Special emphasis is placed on the use of conformational (ϕ-ψ space) and topographical (χ space) considerations in design.

Adventures in peptides and science with students! The joys of research

Working with students at the undergraduate, graduate and postdoctoral levels has brought me great joy and satisfaction. Each student is a unique human being, so each provides an important opportunity for learning, creativity and accomplishment in research and scholarship. In this reflection, several examples of this scientific process are discussed in the context of solving scientific problems which led to novel and critical scientific insights.

Design of novel melanocortin receptor ligands: multiple receptors, complex pharmacology, the challenge

The major pharmacophore for the melanocortin 1, 3, 4 and 5 receptors is the sequence -His-Phe-Arg-Trp-. There is a need for potent, biologically stable, receptor selective ligands, both agonists and antagonists, for these receptors. In this report we briefly examine the structural and biophysical approaches we have taken to develop selective agonist and antagonist ligands that can cross (or not) the blood brain barrier. Remaining questions and unmet needs are also discussed.

Approaches to the rational design of selective melanocortin receptor antagonists

INTRODUCTION

When establishing the physiological roles of specific receptors in normal and disease states, it is critical to have selective antagonist ligands for each receptor in a receptor system with several subtypes. The melanocortin receptors have five subtypes referred to as the melanocortin 1 receptor, melanocortin 2 receptor, melanocortin 3 receptor, melanocortin 4 receptor and melanocortin 5 receptor, and they are of critical importance for many aspects of human health and disease.

AREAS COVERED

This article reviews the current efforts to design selective antagonistic ligands for the five human melanocortin receptors summarizing the currently published orthosteric and allosteric antagonists for each of these receptors.

EXPERT OPINION

Though there has been progress, there are still few drugs available that address the many significant biological activities and diseases that are associated with these receptors, which is possibly due to the lack of receptor selectivity that these designed ligands are currently showing. The authors believe that further studies into the antagonists' 3D conformational and topographical properties in addition to future mutagenesis studies will provide greater insight into these ligands which could play a role in the treatment of various diseases in the future.

Solid-phase peptide head-to-side chain cyclodimerization: discovery of C(2)-symmetric cyclic lactam hybrid α-melanocyte-stimulating hormone (MSH)/agouti-signaling protein (ASIP) analogues with potent activities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the pharmacophores of the agouti-signaling protein (ASIP), an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. The designed hybrid ASIP/MSH pharmacophore was explored in monomeric cyclic, and cyclodimeric templates. The monomeric cyclic disulfide series yielded peptides with hMC3R-selective non-competitive binding affinities. The direct on-resin peptide lactam cyclodimerization yielded nanomolar range (25-120 nM) hMC1R-selective full and partial agonists in the cyclodimeric lactam series which demonstrates an improvement over the previous attempts at hybridization of MSH and agouti protein sequences. The secondary structure-oriented pharmacophore hybridization strategy will prove useful in development of unique allosteric and orthosteric melanocortin receptor modulators. This report also illustrates the utility of peptide cyclodimerization for the development of novel GPCR peptide ligands.

Substitution of arginine with proline and proline derivatives in melanocyte-stimulating hormones leads to selectivity for human melanocortin 4 receptor

A new series of melanotropin analogues with His or Arg residues in the core pharmacophores of MTII, SHU9119, and Ac-NDP-gamma-MSH-NH(2) replaced by Pro or trans-/cis-4-guanidinyl-Pro derivatives were designed and synthesized to introduce selectivity toward the human melanocortin 4 receptor (hMC4R). Analogues 1, 2, 3, 6, 7, 8 were found to be hMC4R selective. Second messenger studies have demonstrated that analogues 1 and 2 are insurmountable inhibitors of MTII agonist activity at the hMC4R. Molecular modeling studies suggest that the hMC4R selectivity is due to a beta-turn shift induced by the Pro ring that makes the global minimum structures of these analogues resemble the NMR solution structure of the hASIP melanocortin receptor binding motif. Substitution of His in MTII also provided functional selectivity for the hMC3R or the hMC4R. These findings are important for a better understanding of the selectivity mechanism at the hMC3R/hMC4R and the development of therapeutic ligands selectively targeting the hMC4R.

Beta-turn secondary structure and melanocortin ligands

The melanocortin pathway has emerged during this past decade as an important target area for the discovery and development of therapeutic agents related to obesity and type 2 diabetes. This peptide-G-protein coupled receptor (GPCR) pathway has evolved from peptide-based ligands to small molecules possessing a variety of different molecular scaffolds. Herein, we summarize the originating hypothesis of the importance of the reverse beta-turn secondary structure for agonist ligand potency at the melanocortin receptors and how that information was utilized for the discovery of small molecules based upon this type of turn structure.

Melanotropins as drugs for the treatment of obesity and other feeding disorders: potential and problems

Current biological and pharmacological evidence suggests that the melanocortin 4 and melanocortin 3 receptors which are seven transmembrane G-protein coupled receptors (GPCRs) are involved in various aspects of energy balance and feeding behaviors in animals including humans. The natural endogenous ligands for these receptors are products of the gene pro-opiomelanocortin (POMC), and include alpha-melanocyte stimulating hormone, gamma-melanocyte stimulating hormone and perhaps other modified products of POMC. Thus well designed agonists and antagonists of these ligands might serve as drugs for the treatment of feeding disorders. However, these melanotropin peptides also can have other biological activities that involve the MC3R and MC4R, and these other biological properties will need to be modulated in ligands that are likely to be useful drugs for feeding disorders. Current progress in these areas with special emphasis on the MC3R will be discussed along with possible new directions that might be fruitful in these important aspects of contemporary biology and medicine.

The first preparative solution phase synthesis of melanotan II

Melanotan II is a synthetic cyclic heptapeptide used to prevent a sunlight-induced skin cancer by stimulating the skin tanning process. In this paper we report the first solution phase synthesis of the title compound. The hexapeptide sequence has been assembled by [(2+2)+1+1] scheme. After removing the orthogonal protection, a carbodiimide mediated lactamization, involving the ε-amino group of lysine and γ-carboxy group of aspartic acid, led to a cyclic intermediate. Appending N-acetylnorleucine concluded the assembly of melanotan II molecule. Protection of the lateral groups in arginine and tryptophan was omitted for atom and step economy reasons. The total synthesis of melanotan II was accomplished in 12 steps with 2.6% overall yield, affording >90% pure peptide without using preparative chromatography.

Targeting melanocortin receptors: an approach to treat weight disorders and sexual dysfunction

The melanocortin system has multifaceted roles in the control of body weight homeostasis, sexual behaviour and autonomic functions, and so targeting this pathway has immense promise for drug discovery across multiple therapeutic areas. In this Review, we first outline the physiological roles of the melanocortin system, then discuss the potential of targeting melanocortin receptors by using MC3 and MC4 agonists for treating weight disorders and sexual dysfunction, and MC4 antagonists to treat anorectic and cachectic conditions. Given the complexity of the melanocortin system, we also highlight the challenges and opportunities for future drug discovery in this area.

Structure-antifungal activity relationship of His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2 and analogues

The synthesis, in vitro evaluation and conformational study of His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH(2) and analogues acting as antifungal agents are reported. Among them, His-Phe-Lys-Trp-Gly-Arg-Phe-Val-NH(2) exhibited a moderate but significant antifungal activity against Cryptococcus neoformans, Candida albicans and Candida tropicalis. A theoretical study allows us to propose a biologically relevant conformation for these octapeptides acting as antifungal agents. In addition, these theoretical calculations allow us to determine the minimal structural requirements to produce the antifungal response and can provide a guide for the design of compounds with this biological activity.

Structure-activity relationships of cyclic lactam analogues of alpha-melanocyte-stimulating hormone (alpha-MSH) targeting the human melanocortin-3 receptor

A variety of dicarboxylic acid linkers introduced between the alpha-amino group of Pro(6) and the -amino group of Lys(10) of the cyclic lactam alpha-melanocyte-stimulating hormone (alpha-MSH)-derived Pro(6)-D-Phe(7)/D-Nal(2')(7)-Arg(8)-Trp(9)-Lys(10)-NH2 pentapeptide template lead to nanomolar range and selective hMC3R agonists and antagonists. Replacement of the Pro(6) residue and the dicarboxylic acid linker with 2,3-pyrazine-dicarboxylic acid furnished a highly selective nanomolar range hMC3R partial agonist (analogue 12, c[CO-2,3-pyrazine-CO-D-Phe-Arg-Trp-Lys]-NH2, EC50 = 27 nM, 70% max cAMP) and an hMC3R antagonist (analogue 13, c[CO-2,3-pyrazine-CO-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 23 nM). Modeling experiments suggest that 2,3-pyrazinedicarboxylic acid stabilizes a beta-turn-like structure with the D-Phe/D-Nal(2') residues, which explains the high potency of the corresponding peptides. Placement of a Nle residue in position 6 produced a hMC3R/hMC5R antagonist (analogue 15, c[CO-(CH 2)2-CO-Nle-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 12 and 17 nM, respectively), similarly to the previously described cyclic gamma-melanocyte-stimulating hormone (gamma-MSH)-derived hMC3R/hMC5R antagonists. These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-3 receptor.

Design, synthesis and biological evaluation of ligands selective for the melanocortin-3 receptor

The processed products of the proopiomelanocortin gene (ACTH, alpha-MSH, beta-MSH, gamma-MSH, etc.) interact with five melanocortin receptors, the MC1R, MC2R, MC3R, MC4R, and MC5R to modulate and control many important biological functions crucial for good health both peripherally (as hormones) and centrally (as neurotransmitters). Pivotal biological functions include pigmentation, adrenal function, response to stress, fear/flight, energy homeostasis, feeding behavior, sexual function and motivation, pain, immune response, and many others, and are believed to be involved in many disease states including pigmentary disorders, adrenal disorders, obesity, anorexia, prolonged and neuropathic pain, inflammatory response, etc. The melanocortin-3 receptor (MC3R) is found primarily in the brain and spinal cord and also in the periphery, and its biological functions are still not well understood. Here we review some of the biological functions attributed to the MC3R, and then examine in more detail efforts to design and synthesize ligands that are potent and selective for the MC3R, which might help resolve the many questions still remaining about its function. Though some progress has been made, there is still much to be done in this critical area.