N-terminal fatty acylated His-dPhe-Arg-Trp-NH(2) tetrapeptides: influence of fatty acid chain length on potency and selectivity at the mouse melanocortin receptors and human melanocytes

Lipopeptides as tools in catalysis, supramolecular, materials and medicinal chemistry

Lipopeptides are amphiphilic peptides in which an aliphatic chain is attached to either the C or N terminus of peptides. Their self-assembly - into micelles, vesicles, nanotubes, fibres or nanobelts - leads to applications in nanotechnology, catalysis or medicinal chemistry. Self-organization of lipopeptides is dependent on both the length of the lipid tail and the amino acid sequence, in which the chirality of the peptide sequence can be transmitted into the supramolecular species. This Review describes the use of lipopeptides to design synthetic advanced dynamic supramolecular systems, nanostructured materials or self-responsive delivery systems in the area of medical biotechnology. We examine the influence of external stimuli, the ability of lipopeptide-derived structures to adapt over time and their application as medicinal agents with antibacterial, antifungal, antiviral or anticancer activities. Finally, we discuss the catalytic efficiency of lipopeptides, with the aim of building minimal synthetic enzymes, and recent efforts to incorporate metals into lipopeptide assemblies.

Discovery of a Long-Acting Parathyroid Hormone 1-34 Analogue to Treat Hypoparathyroidism

Hypoparathyroidism (HP) is a rare disease with clinical manifestations of hypocalcemia and hyperphosphatemia, resulting from deficient or absent parathyroid hormone (PTH) secretion. Conventional treatment for patients with HP involves extensive calcium and vitamin D supplementation. In 2015, PTH1-84 was approved by the United States Food and Drug Administration as an adjunct for HP patients who cannot be well-controlled on conventional treatment. However, PTH1-84 therapy requires a daily injection, leading to poor patient compliance. The purpose of this study was to develop a long-acting PTH1-34 analogue by increasing its affinity to albumin. Three PTH1-34 variants were generated by substituting two of the three lysine (Lys) residues with arginine, reserving a single Lys as the modification site in each sequence. A series of side chains, containing fatty acid, deoxycholic acid, or biotin groups, were synthesized to modify these PTH1-34 variants by using a solid-liquid phase synthesis approach. In vitro bioactivity and albumin affinity tests were used to screen these new PTH1-34 analogues. Finally, Lys27-AAPC was selected from 69 synthesized analogues as a candidate therapeutic compound because it retained potency and exhibited a high albumin-binding capacity. In pharmacodynamic experiments, Lys27-AAPC demonstrated enhanced and prolonged efficacy in serum calcium elevating relative to PTH1-84. Moreover, a lyophilized powder for injection containing Lys27-AAPC was developed for further testing and represented a potential long-acting HP treatment.

Molecular determinants of ACTH receptor for ligand selectivity

The adrenocorticotropic hormone (ACTH) receptor, known as the melanocortin-2 receptor (MC2R), plays a key role in regulating adrenocortical function. ACTH receptor is a subtype of the melanocortin receptor family which is a member of the G-protein coupled receptor (GPCR) superfamily. ACTH receptor has unique characteristics among MCRs. α-MSH, β-MSH, γ-MSH and ACTH are agonists for MCRs but only ACTH is the agonist for ACTH receptor. In addition, the melanocortin receptor accessory protein (MRAP) is required for ACTH receptor expression at cell surface and function. In this review, we summarized the information available on the relationship between ACTH and ACTH receptor and provide the latest understanding of the molecular basis of the ACTH receptor responsible for ligand selectivity and function.

A review of lipidation in the development of advanced protein and peptide therapeutics

The use of biologics (peptide and protein based drugs) has increased significantly over the past few decades. However, their development has been limited by their short half-life, immunogenicity and low membrane permeability, restricting most therapies to extracellular targets and administration by injection. Lipidation is a clinically-proven post-translational modification that has shown great promise to address these issues: improving half-life, reducing immunogenicity and enabling intracellular uptake and delivery across epithelia. Despite its great potential, lipidation remains an underutilized strategy in the clinical translation of lead biologics. We review how lipidation can overcome common challenges in biologics development as well as highlight gaps in our understanding of the effect of lipidation on therapeutic efficacy, where increased research and development efforts may lead to next-generation drugs.

Discovery of Melanocortin Ligands via a Double Simultaneous Substitution Strategy Based on the Ac-His-dPhe-Arg-Trp-NH2 Template

The melanocortin system regulates an array of diverse physiological functions including pigmentation, feeding behavior, energy homeostasis, cardiovascular regulation, sexual function, and steroidogenesis. Endogenous melanocortin agonist ligands all possess the minimal messaging tetrapeptide sequence His-Phe-Arg-Trp. Based on this endogenous sequence, the Ac-His¹-dPhe²-Arg³-Trp⁴-NH₂ tetrapeptide has previously been shown to be a useful scaffold when utilizing traditional positional scanning approaches to modify activity at the various melanocortin receptors (MC1-5R). The study reported herein was undertaken to evaluate a double simultaneous substitution strategy as an approach to further diversify the Ac-His¹-dPhe²-Arg³-Trp⁴-NH₂ tetrapeptide with concurrent introduction of natural and unnatural amino acids at positions 1, 2, or 4, as well as an octanoyl residue at the N-terminus. The designed library includes the following combinations: (A) double simultaneous substitution at capping group position (Ac) together with position 1, 2, or 4, (B) double simultaneous substitution at positions 1 and 2, (C) double simultaneous substitution at positions 1 and 4, and (D) double simultaneous substitution at positions 2 and 4. Several lead ligands with unique pharmacologies were discovered in the current study including antagonists targeting the neuronal mMC3R with minimal agonist activity and ligands with selective profiles for the various melanocortin subtypes. The results suggest that the double simultaneous substitution strategy is a suitable approach in altering melanocortin receptor potency or selectivity or converting agonists into antagonists and vice versa.

Discovery of Mixed Pharmacology Melanocortin-3 Agonists and Melanocortin-4 Receptor Tetrapeptide Antagonist Compounds (TACOs) Based on the Sequence Ac-Xaa1-Arg-(pI)DPhe-Xaa4-NH2

The centrally expressed melanocortin-3 and -4 receptors (MC3R/MC4R) have been studied as possible targets for weight management therapies, with a preponderance of studies focusing on the MC4R. Herein, a novel tetrapeptide scaffold [Ac-Xaa¹-Arg-(pI)DPhe-Xaa⁴-NH₂] is reported. The scaffold was derived from results obtained from a MC3R mixture-based positional scanning campaign. From these results, a set of 48 tetrapeptides were designed and pharmacologically characterized at the mouse melanocortin-1, -3, -4, and -5 receptors. This resulted in the serendipitous discovery of nine compounds that were MC3R agonists (EC₅₀ < 1000 nM) and MC4R antagonists (5.7 < pA₂ < 7.8). The three most potent MC3R agonists, 18 [Ac-Arg-Arg-(pI)DPhe-Tic-NH₂], 1 [Ac-His-Arg-(pI)DPhe-Tic-NH₂], and 41 [Ac-Arg-Arg-(pI)DPhe-DNal(2')-NH₂] were more potent (EC₅₀ < 73 nM) than the melanocortin tetrapeptide Ac-His-DPhe-Arg-Trp-NH₂. This template contains a sequentially reversed "Arg-(pI)DPhe" motif with respect to the classical "Phe-Arg" melanocortin signaling motif, which results in pharmacology that is first-in-class for the central melanocortin receptors.

Melanocortin antagonist tetrapeptides with minimal agonist activity at the mouse melanocortin-3 receptor

The melanocortin system regulates many important functions in the body. There are five melanocortin G protein-coupled receptor subtypes known to date. Herein, we report a structure-activity relationship (SAR) study of a tetrapeptide lead discovered through a double substitution strategy at the melanocortin core His-Phe-Arg-Trp sequence. Several compounds were identified with micromolar agonist activity at the mouse melanocortin-1 (mMC1R) and mouse melanocortin-5 receptor (mMC5R) subtypes, weak antagonist activity at the mouse melanocortin-3 receptor (mMC3R), and potent antagonist activity at the mouse melanocortin-4 receptor (mMC4R). Two compounds (2 and 3) were nanomolar mMC4R antagonists with no mMC3R antagonist activity observed. Additionally, we identified three tetrapeptide MC3R antagonists (1, 6, and 7) that possess minimal mMC3R agonist activity only at 100 μM, not commonly observed for mMC3R/mMC4R antagonists. These novel molecular templates have the potential as molecular probes to better differentiate the roles of the centrally expressed MC3 and MC4 receptors.

Third transmembrane domain of the adrenocorticotropic receptor is critical for ligand selectivity and potency

The ACTH receptor, known as the melanocortin-2 receptor (MC2R), plays an important role in regulating and maintaining adrenocortical function. MC2R is a subtype of the melanocortin receptor (MCR) family and has unique characteristics among MCRs. Endogenous ACTH is the only endogenous agonist for MC2R, whereas the melanocortin peptides α-, β-, and γ-melanocyte-stimulating hormone and ACTH are full agonists for all other MCRs. In this study, we examined the molecular basis of MC2R responsible for ligand selectivity using ACTH analogs and MC2R mutagenesis. Our results indicate that substitution of Phe(7) with D-Phe or D-naphthylalanine (D-Nal(2')) in ACTH(1-24) caused a significant decrease in ligand binding affinity and potency. Substitution of Phe(7) with D-Nal(2') in ACTH(1-24) did not switch the ligand from agonist to antagonist at MC2R, which was observed in MC3R and MC4R. Substitution of Phe(7) with D-Phe(7) in ACTH(1-17) resulted in the loss of ligand binding and activity. Molecular analysis of MC2R indicated that only mutation of the third transmembrane domain of MC2R resulted in a decrease in D-Phe ACTH binding affinity and potency. Our results suggest that Phe(7) in ACTH plays an important role in ligand selectivity and that the third transmembrane domain of MC2R is crucial for ACTH selectivity and potency.

Position and length of fatty acids strongly affect receptor selectivity pattern of human pancreatic polypeptide analogues

Pancreatic polypeptide (PP) is a satiety-inducing gut hormone targeting predominantly the Y4 receptor within the neuropeptide Y multiligand/multireceptor family. Palmitoylated PP-based ligands have already been reported to exert prolonged satiety-inducing effects in animal models. Here, we suggest that other lipidation sites and different fatty acid chain lengths may affect receptor selectivity and metabolic stability. Activity tests revealed significantly enhanced potency of long fatty acid conjugates on all four Y receptors with a preference of position 22 over 30 at Y1 , Y2 and Y5 receptors. Improved Y receptor selectivity was observed for two short fatty acid analogues. Moreover, [K(30)(E-Prop)]hPP2-36 (15) displayed enhanced stability in blood plasma and liver homogenates. Thus, short chain lipidation of hPP at key residue 30 is a promising approach for anti-obesity therapy because of maintained selectivity and a sixfold increased plasma half-life.

Peptide modifications differentially alter G protein-coupled receptor internalization and signaling bias

Although G protein-coupled receptors (GPCRs) are targeted by more clinically used drugs than any other type of protein, their ligand development is particularly challenging. Humans have four neuropeptide Y receptors: hY1R and hY5R are orexigenic, while hY2R and hY4R are anorexigenic, and represent important anti-obesity drug targets. We show for the first time that PEGylation and lipidation, chemical modifications that prolong the plasma half-lives of peptides, confer additional benefits. Both modifications enhance pancreatic polypeptide preference for hY2R/hY4R over hY1R/hY5R. Lipidation biases the ligand towards arrestin recruitment and internalization, whereas PEGylation confers the opposite bias. These effects were independent of the cell system and modified residue. We thus provide novel insights into the mode of action of peptide modifications and open innovative venues for generating peptide agonists with extended therapeutic potential.

Identification and in vivo and in vitro characterization of long acting and melanocortin 4 receptor (MC4-R) selective α-melanocyte-stimulating hormone (α-MSH) analogues

We report in vitro and in vivo data of new α-melanocyte-stimulating hormone (α-MSH) analogues which are N-terminal modified with a long chain fatty acid derivative. While keeping the pharmacophoric motif (d-Phe-Arg-Trp) fixed, we tried to improve selectivity and physicochemical parameters like solubility and stability of these analogues by replacing amino acids further away from the motif. Receptor specific changes in binding affinity to the melanocortin receptors were observed between the acetyl derivatives and the fatty acid analogues. Furthermore, amino acids at the N-terminal of α-MSH (Ser-Tyr-Ser) not considered to be part of the pharmacophore were found to have an influence on the MC4/MC1 receptor selectivity. While the acetyl analogues have an in vivo effect for around 7 h, the long chain fatty acid analogues have an effect up to 48 h in an acute feeding study in male Sprague-Dawley rats after a single subcutaneous administration.

Modified low molecular weight cyclic peptides as mimetics of BDNF with improved potency, proteolytic stability and transmembrane passage in vitro

We recently reported the development of the BDNF mimetic peptide cyclo-[dPAKKR] 1 which promotes the survival of cultured sensory neurons via a trkB independent mechanism [Fletcher, J. M.; Morton, C. M.; Zwar, R. A.; Murray, S. S.; O'Leary, P. D.; Hughes, R. A. J. Biol. Chem.2008, 283, 33375]. In the present study we prepared a series of hydrophobically-modified analogues of 1 with an eye to improving its pharmacokinetic properties. Select members of this second generation of compounds showed improved biological activity, stability in plasma, and an ability to cross model biological membranes.

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.

Parallel Synthesis of a Lipopeptide Library by Hydrazone-Based Chemical Ligation

alpha-Melanocyte-stimulating hormone (alpha-MSH) is an endogeneous linear tridecapeptide with potential application for the modulation of skin tanning. To evaluate the interest of introducing a lipid moiety onto this peptide, we developed an efficient chemoselective parallel method to prepare a large series of analogues of alpha-melanocortin with high purity, varying the nature or the relative position of the lipid moiety. Two sets of building blocks containing lipidic alpha-oxo-aldehydes or alpha-hydrazinoacetyl peptides were combined to obtain a 102-membered library of amphiphilic alpha-MSH analogues. This library was pharmacologically tested at 1 x 10(-7) M for the ability to induce AMPc production in M4Be melanoma cell line after stimulation of the human melanocortin MC1 receptor. Among theses lipopeptides, 84 compounds exhibited an AMPc induction higher than Melitane, a patented alpha-MSH agonist. These results provide strong evidence of the interest of introduction of a lipid tail for the pharmacomodulation of bioactive peptides.

Identification of arginine analogues as antagonists and agonists for the melanocortin-4 receptor

In the present study, conducted to explore potent and small molecular melanocortin-4 (MC4) receptor ligands, we found that tripeptide 3a, containing a D-Phe-Arg-2-Nal (Nal; naphthylalanine) sequence, exhibited a moderate affinity for the MC4 receptor. Structural optimization led to the identification of a compound with a high affinity for the MC4 receptor, namely, tripeptide 3e, which showed a 70-fold higher affinity for the MC4 receptor than the lead compound 3a. Moreover, in an effort to further reduce the peptidic characters of tripeptide 3e, we found that dipeptide 3g exhibited a relatively high affinity for the MC4 receptor. Furthermore, in these analogues, the substituted position (1' vs. 2') of the naphthyl ring of Nal residue at position 7 was found to be important for the differentiation of agonist and antagonist activity. The synthesis and structure-activity relationships of the arginine analogues as MC4 receptor ligands were described in this paper.

Molecular identification of the human melanocortin-2 receptor responsible for ligand binding and signaling

The melanocortin-2 receptor (MC2R), also known as the adrenocorticotropic hormone (ACTH) receptor, plays an important role in regulating and maintaining adrenocortical function, specifically steroidogenesis. Mutations of the human MC2R (hMC2R) gene have also been identified in humans with familial glucocorticoid deficiency; however, the molecular basis responsible for hMC2R ligand binding and signaling remains unclear. In this study, both truncated ACTH peptides and site-directed mutagenesis studies were used to determine molecular mechanisms of hMC2R binding ACTH and signaling. Our results indicate that ACTH1-16 is the minimal peptide required for hMC2R binding and signaling. Mutations of common melanocortin receptor family amino acid residues E80 in transmembrane domain 2 (TM2), D107 in TM3, F178 in TM4, F235 and H238 in TM6, and F258 in TM7 significantly reduced ACTH-binding affinity and signaling. Furthermore, mutations of unique amino acids D104 and F108 in TM3 and F168 and F178 in TM4 significantly decreased ACTH binding and signaling. In conclusion, our results suggest that the residues in TM2, TM3, and TM6 of hMC2R share similar binding sites with other MCRs but the residues identified in TM4 and TM7 of hMC2R are unique and required for ACTH selectivity. Our study suggests that hMC2R may have a broad binding pocket in which both conserved and unique amino acid residues are required, which may be the reason why alpha-MSH was not able to bind hMC2R.

Structure-activity relationships of melanocortin agonists containing the benzimidazole scaffold

The melanocortin system has been implicated in regulating various physiological processes including pigmentation, energy homeostasis, obesity, steroidogenesis cardiovascular, and exocrine gland function. The five melanocortin receptors that belong to the super family of G protein-coupled receptors are stimulated by naturally occurring agonists. The aim of this research was focused on the design, synthesis, and pharmacological characterization of melanocortin ligands that contain the 1,2,5-trisubstituted benzimidazole scaffold. A series of benzimidazole analogues, with three points of diversity at positions 1, 2, and 5, were designed, synthesized, pharmacologically assayed at the mouse melanocortin receptors MC1R, MC3R, MC4R, and MC5R and resulted in ligands possessing a range of agonist activity from nm to no stimulation at up to 100 microM concentrations. This study demonstrates that the benzimidazole structure template can be appended with key melanocortin agonist amino acids for the design melanocortin receptor agonist ligands.

Structure–activity relationships of novel piperazines as antagonists for the melanocortin-4 receptor

During the investigation of antagonists for the MC4 receptor, we found that 10ab having a naphthyl group showed almost the same binding affinity for the MC4 receptor as that of the lead compound 1 with a benzoyl group. We also developed a new type of compounds, namely, bis-piperazines, and found that the bis-piperazines 10 exhibited a high affinity for the MC4 receptor. In particular, (-)-10bg exhibited the highest affinity for the MC4 receptor with an IC50 value of 8.13nM. In this paper, we present the design, synthesis, and structure-activity relationships of the novel bis-piperazines as MC4 receptor antagonists.

On-Line Synthesis of Pseudopeptide Library Incorporating a Benzodiazepinone Turn Mimic: Biological Evaluation on MC1 Receptors

Alpha melanocyte stimulating hormone (alpha-MSH) is a widely distributed hormone. This tridecapeptide exhibits various biological activities mediated through different receptors. alpha-MSH binds to the melanocortin-1 receptor (MC1-R), mainly expressed in keratinocytes and melanocytes, inducing melanogenesis and anti-inflammatory processes. The central His-Phe-Arg-Trp tetrapeptide sequence of alpha-MSH is known to form a turn in the bioactive conformation. To find new potent analogs of alpha-MSH, we decided to introduce non-peptide building blocks in the alpha-MSH sequence. Molecular modeling studies showed that two amino acids of the central core sequence could be replaced by the benzodiazepinone building block without loosing the beta-turn conformation. Benzodiazepines are well-known pharmacophores exhibiting a wide scope of biological activities and are described as constrained dipeptide mimics templates. Although numerous synthetic pathways leading to benzodiazepinones have been described in literature, no methodology has 1,4-benzodiazepine-2,5-diones building blocks bearing a free carboxylic acid function and a protected amino function suitable for incorporation into peptide sequences. In this study, we report the synthesis of peptides with a benzodiazepinone moiety obtained directly during the course of solid-phase peptide synthesis (SPPS). This "on-line" strategy leads to the generation of a 54-member pseudo-peptide library of alpha-MSH analogs. After LC/MS purification, binding assays were performed on the MC1 receptor leading to the discovery of several micromolar ligands.

Melanoma prevention strategy based on using tetrapeptide α‐MSH analogs that protect human melanocytes from UV‐induced DNA damage and cytotoxicity

Melanoma is the deadliest form of skin cancer, with no cure for advanced disease. We propose a strategy for melanoma prevention based on using analogs of alpha-melanocyte stimulating hormone (alpha-MSH) that function as melanocortin 1 receptor (MC1R) agonists. Treatment of human melanocytes with alpha-MSH results in stimulation of eumelanin synthesis, reduction of apoptosis that is attributable to reduced hydrogen peroxide generation and enhanced repair of DNA photoproducts. These effects should contribute to genomic stability of human melanocytes, thus preventing their malignant transformation to melanoma. Based on these findings, we synthesized and tested the effects of 3 tetrapeptide alpha-MSH analogs, Ac-His-D-Phe-Arg-Trp-NH2, n-Pentadecanoyl- and 4-Phenylbutyryl-His-D-Phe-Arg-Trp-NH2, on cultured human melanocytes. The latter two analogs were more potent than the former, or alpha-MSH, in stimulating the activity of tyrosinase, thus melanogenesis, reducing apoptosis and release of hydrogen peroxide and enhancing repair of DNA photoproducts in melanocytes exposed to UV radiation (UVR). The above analogs are MC1R agonists, as their effects were abrogated by an analog of agouti signaling protein, the physiological MC1R antagonist, and were absent in melanocytes expressing loss-of-function MC1R. Analogs, such as 4-Phenylbutyryl-His-D-Phe-Arg-Trp-NH2 with prolonged and reversible effects, can potentially be developed into topical agents to prevent skin photocarcinogenesis, particularly melanoma.