Melanocortin ligands: 30 years of structure-activity relationship (SAR) studies

Diagnostic Procedures for Inflammatory Bowel Disease: Laboratory, Endoscopy, Pathology, Imaging, and Beyond

Diagnosing inflammatory bowel disease (IBD) can often be challenging, and differentiating between Crohn's disease and ulcerative colitis can be particularly difficult. Diagnostic procedures for IBD include laboratory tests, endoscopy, pathological tests, and imaging tests. Serological and stool tests can be easily performed in an outpatient setting and provide critical diagnostic clues. Although endoscopy is an invasive procedure, it offers essential diagnostic information and allows for tissue biopsy and therapeutic procedures. Video capsule endoscopy and device-assisted enteroscopy are endoscopic procedures used to evaluate the small bowel. In addition to endoscopy, magnetic resonance imaging, computed tomography, and ultrasound (US) are valuable tools for small bowel assessment. Among these, US is noninvasive and easily utilized, making its use highly practical in daily clinical practice. Endoscopic biopsy aids in the diagnosis of IBD and is crucial for assessing the histological activity of the disease, facilitating a thorough evaluation of disease remission, and aiding in the development of treatment strategies. Recent advances in artificial intelligence hold promise for enhancing various aspects of IBD management, including diagnosis, monitoring, and precision medicine. This review compiles current procedures and promising future tools for the diagnosis of IBD, providing comprehensive insights.

5-Hydroxypyrroloindoline Affords Tryptathionine and 2,2'-bis-Indole Peptide Staples: Application to Melanotan-II

With peptides increasingly favored as drugs, natural product motifs, namely the tryptathionine staple, found in amatoxins and phallotoxins, and the 2,2'-bis-indole found in staurosporine represent unexplored staples for unnatural peptide macrocycles. We disclose the efficient condensation of a 5-hydroxypyrroloindoline with either a cysteine-thiol or a tryptophan-indole to form a tryptathionine or 2-2'-bis-indole staple. Judicious use of protecting groups provides for chemoselective stapling using α-MSH, which provides a basis for investigating both chemoselectivity and affinity. Both classes of stapled peptides show nanomolar Ki's, with one showing a sub-nanomolar Ki value.

Melanocortin 3,5 receptors immunohistochemical expression in colonic mucosa of inflammatory bowel disease patients: A matter of disease activity?

BACKGROUND

Melanocortin 3 and 5 receptors (i.e., MC3R and MC5R) belong to the melanocortin family. However, data regarding their role in inflammatory bowel diseases (IBD) are currently unavailable.

AIM

This study aims to ascertain their expression profiles in the colonic mucosa of Crohn's disease (CD) and ulcerative colitis (UC), aligning them with IBD disease endoscopic and histologic activity.

METHODS

Colonic mucosal biopsies from CD/UC patients were sampled, and immunohistochemical analyses were conducted to evaluate the expression of MC3R and MC5R. Colonic sampling was performed on both traits with endoscopic scores (Mayo endoscopic score and CD endoscopic index of severity) consistent with inflamed mucosa and not consistent with disease activity (i.e., normal appearing mucosa).

RESULTS

In both CD and UC inflamed mucosa, MC3R (CD: + 7.7 fold vs normal mucosa, P < 0.01; UC: + 12 fold vs normal mucosa, P < 0.01) and MC5R (CD: + 5.5 fold vs normal mucosa, P < 0.01; UC: + 8.1 fold vs normal mucosa, P < 0.01) were significantly more expressed compared to normal mucosa.

CONCLUSION

MC3R and MC5R are expressed in the colon of IBD patients. Furthermore, expression may differ according to disease endoscopic activity, with a higher degree of expression in the traits affected by disease activity in both CD and UC, suggesting a potential use of these receptors in IBD pharmacology.

The Melanocortin System in Inflammatory Bowel Diseases: Insights into Its Mechanisms and Therapeutic Potentials

The melanocortin system is a complex set of molecular mediators and receptors involved in many physiological and homeostatic processes. These include the regulation of melanogenesis, steroidogenesis, neuromodulation and the modulation of inflammatory processes. In the latter context, the system has assumed importance in conditions of chronic digestive inflammation, such as inflammatory bowel diseases (IBD), in which numerous experiences have been accumulated in mouse models of colitis. Indeed, information on how such a system can counteract colitis inflammation and intervene in the complex cytokine imbalance in the intestinal microenvironment affected by chronic inflammatory damage has emerged. This review summarises the evidence acquired so far and highlights that molecules interfering with the melanocortin system could represent new drugs for treating IBD.

Setmelanotide optimization through fragment-growing, molecular docking in-silico method targeting MC4 receptor

Obesity has emerged as a global issue, but with the complex structures of multiple related important targets and their agonists or antagonists determined, the mechanism of ligand-protein interaction may offer new chances for developing new generation agonists anti-obesity. Based on the molecule surface of the cryo-EM protein structure 7AUE, we tried to replace D-Ala3 with D-Met in setmelanotide as the linker site for fragment-growing with De novo evolution. The simulation results indicate that the derivatives could improve the binding abilities with the melanocortin 4 receptor and the selectivity over the melanocortin 1 receptor. The improved selectivity of the newly designed derivatives is mainly due to the shape difference of the molecular surface at the orthosteric peptide-binding pocket between melanocortin 4 receptor and melanocortin 1 receptor. The new extended fragments could not only enhance the binding affinities but also function as a gripper to seize the pore, making it easier to balance and stabilize the other component of the new derivatives. Although it is challenging to synthesize the compounds designed in silico, this study may perhaps serve as a trigger for additional anti-obesity research.Communicated by Ramaswamy H. Sarma.

Structural Understanding of Peptide-Bound G Protein-Coupled Receptors: Peptide-Target Interactions

The activation of G protein-coupled receptors (GPCRs) is triggered by ligand binding to their orthosteric sites, which induces ligand-specific conformational changes. Agonists and antagonists bound to GPCR orthosteric sites provide detailed information on ligand-binding modes. Among these, peptide ligands play an instrumental role in GPCR pharmacology and have attracted increased attention as therapeutic drugs. The recent breakthrough in GPCR structural biology has resulted in the remarkable availability of peptide-bound GPCR complexes. Despite the several structural similarities shared by these receptors, they exhibit distinct features in terms of peptide recognition and receptor activation. From this perspective, we have summarized the current status of peptide-bound GPCR structural complexes, largely focusing on the interactions between the receptor and its peptide ligand at the orthosteric site. In-depth structural investigations have yielded valuable insights into the molecular mechanisms underlying peptide recognition. This study would contribute to the discovery of GPCR peptide drugs with improved therapeutic effects.

MC4R biased signalling and the conformational basis of biological function selections

The MC4R, a GPCR, has long been a major target for obesity treatment. As the most well‐studied melanocortin receptor subtype, the evolutionary knowledge pushes the drug development and structure–activity relationship (SAR) moving forward. The past decades have witnessed the evolution of scientists' view on GPCRs gradually from the control of a single canonical signalling pathway via a bilateral ‘active‐inactive’ model to a multi‐state alternative model where the ligands' binding affects the selection of the downstream signalling. This evolution brings the concept of biased signalling and the beginning of the next generation of peptide drug development, with the aim of turning from receptor subtype specificity to signalling pathway selectivity. The determination of the value structures of the MC4R revealed insights into the working mechanism of MC4R activation upon binding of agonists. However, new challenge has risen as we seek to unravel the mystery of MC4R signalling selection. Thus, more biased agonists and ligands with representative biological functions are needed to solve the rest of the puzzle.

Structural insights into ligand recognition and activation of the melanocortin-4 receptor

Melanocortin-4 receptor (MC4R) plays a central role in the regulation of energy homeostasis. Its high sequence similarity to other MC receptor family members, low agonist selectivity and the lack of structural information concerning MC4R-specific activation have hampered the development of MC4R-seletive therapeutics to treat obesity. Here, we report four high-resolution structures of full-length MC4R in complex with the heterotrimeric G_s protein stimulated by the endogenous peptide ligand α-MSH, FDA-approved drugs afamelanotide (Scenesse™) and bremelanotide (Vyleesi™), and a selective small-molecule ligand THIQ, respectively. Together with pharmacological studies, our results reveal the conserved binding mode of peptidic agonists, the distinctive molecular details of small-molecule agonist recognition underlying receptor subtype selectivity, and a distinct activation mechanism for MC4R, thereby offering new insights into G protein coupling. Our work may facilitate the discovery of selective therapeutic agents targeting MC4R.

Melanocortin 1 Receptor Agonists Based on a Bivalent, Bicyclic Peptide Framework

We have designed a new class of highly potent bivalent melanocortin receptor ligands based on the nature-derived bicyclic peptide sunflower trypsin inhibitor 1 (SFTI-1). Incorporation of melanotropin pharmacophores in each of the two turn regions of SFTI-1 resulted in substantial gains in agonist activity particularly at human melanocortin receptors 1 and 3 (hMC1R/hMC3R) compared to monovalent analogues. In in vitro binding and functional assays, the most potent molecule, compound 6, displayed low picomolar agonist activity at hMC1R (pEC₅₀ > 10.3; EC₅₀ < 50 pM; pK_i: 10.16 ± 0.04; K_i: 69 ± 5 pM) and is at least 30-fold more selective for this receptor than for hMC3R, hMC4R, or hMC5R. The results are discussed in the context of structural homology models of hMCRs in complex with the developed bivalent ligands.

Structure-Based Design of Melanocortin 4 Receptor Ligands Based on the SHU-9119-hMC4R Cocrystal Structure†

The melanocortin receptors (MC1R-MC5R) belong to class A G-protein-coupled receptors (GPCRs) and are known to have receptor-specific roles in normal and diseased states. Selectivity for MC4R is of particular interest due to its involvement in various metabolic disorders, including obesity, feeding regulation, and sexual dysfunctions. To further improve the potency and selectivity of MC4R (ant)agonist peptide ligands, we designed and synthesized a series of cyclic peptides based on the recent crystal structure of MC4R in complex with the well-characterized antagonist SHU-9119 (Ac-Nle⁴-c[Asp⁵-His⁶-DNal(2')⁷-Arg⁸-Trp⁹-Lys¹⁰]-NH₂). These analogues were pharmacologically characterized in vitro, giving key insights into exploiting binding site subpockets to deliver more selective ligands. More specifically, the side chains of the Nle⁴, DNal(2')⁷, and Trp⁹ residues in SHU-9119, as well as the amide linkage between the Asp⁵ and Lys¹⁰ side chains, were found to represent structural features engaging a hMC4R/hMC3R selectivity switch.

Synthesis, characterization and optimization of in vitro properties of NIR-fluorescent cyclic α-MSH peptides for melanoma imaging

Melanoma are malignant tumors derived from melanocytes being responsible for the majority of skin cancer deaths with an increasing rate of incidence. The Melanocortin-1 receptor (MC1R) has been recognized as a molecular target for melanoma detection. Here, we report on the development and optimization of molecular probes which are based on novel conjugates of near-infrared (NIR) fluorescent indocyanine dyes and an MC1R-targeting peptide intended for optical fluorescence imaging enabling an early, specific, accurate and sensitive diagnosis of malignant melanomas. The introduction of anionic groups into the aromatic ring of the indolenine substructure of the conjugated dyes has shown to result in a strong fluorescence in aqueous solution and a concomitant increase of binding affinities of the peptide conjugates to the target receptor. The length and flexibility of the PEG chain introduced as a linker, as well as the nature of its attachment to the dye also affect the binding affinities, albeit to a lower extent. The conjugates have been successfully applied in the MC1R-specific staining of B16F10 melanoma cells, both in cell cultures and in microtome sections of solid tumors.

Biodistribution and Multicompartment Pharmacokinetic Analysis of a Targeted α Particle Therapy

Targeted α particle therapy (TAT) is ideal for treating disease while minimizing damage to surrounding nontargeted tissues due to short path length and high linear energy transfer (LET). We developed a TAT for metastatic uveal melanoma, targeting the melanocortin-1 receptor (MC1R), which is expressed in 94% of uveal melanomas. Two versions of the therapy are being investigated: ²²⁵Ac-DOTA-Ahx-MC1RL (²²⁵Ac-Ahx) and ²²⁵Ac-DOTA-di-d-Glu-MC1RL (²²⁵Ac-di-d-Glu). The biodistribution (BD) from each was studied and a multicompartment pharmacokinetic (PK) model was developed to describe drug distribution rates. Two groups of 16 severe combined immunodeficient (SCID) mice bearing high MC1R expressing tumors were intravenously injected with ²²⁵Ac-Ahx or ²²⁵Ac-di-d-Glu. After injection, four groups (n = 4) were euthanized at 24, 96, 144, and 288 h time points for each cohort. Tumors and 13 other organs were harvested at each time point. Isomeric γ spectra were measured in tissue samples using a scintillation γ detector and converted to α activity using factors for γ ray abundance per α decay. Time activity curves were calculated for each organ. A five-compartment PK model was built with the following compartments: blood, tumor, normal tissue, kidney, and liver. This model is characterized by a system of five ordinary differential equations using mass action kinetics, which describe uptake, intercompartmental transitions, and clearance rates. The ordinary differential equations were simultaneously solved and fit to experimental data using a genetic algorithm for optimization. The BD data show that both compounds have minimal distribution to organs at risk other than the kidney and liver. The PK parameter estimates had less than 5% error. From these data, ²²⁵Ac-Ahx showed larger and faster uptake in the liver. Both compounds had comparable uptake and clearance rates for other compartments. The BD and PK behavior for two targeted radiopharmaceuticals were investigated. The PK model fit the experimental data and provided insight into the kinetics of the compounds systematically.

MODELLING OF 3D-STRUCTURES OF THE RARE MELANOCORTIN-1-RECEPTOR MUTATIONS ASSOCIATED TO MELANISM IN THE BANANAQUIT

Melanism in plumage color is often associated to the single nucleotide polymorphism of the melanocortin-1-receptor (MC1R). Despite the striking association between the substitution of a Glutamic-acid by for a Lysine at position 92 on the MC1R protein and a completely black plumage, an in-depth understanding of the effect of missense mutations on the conformational change and behavior of the MC1R in the lipid bilayer caused by the absence of a crystal structure is lacking. We examine the structural basis for receptor activation using DNA sequences from the GenBank to perform in silicoprotein homology-based modeling. Our tridimensional model shows that the Alanine for a 179-Threoninesubstitution is a structural complement of the charge-reversing effect associated to the substitution of a Glutamic-acid by for a Lysine at position 92 on the MC1R. We proposed the possibility of gradual evolution in stability and electrostatic properties of the MC1R by the sequential accumulation of these two rare substitutions. These two rare substitutions further perturb physical-chemical properties that may be necessary folding requirements of the constitutively active MC1R forms without altering of ligand binding affinity. The computational coarse-grained molecular dynamics of the MC1R binding affinities to the melanocyte-stimulating hormone predicted the disparity in ligand binding amongalleles. We speculate that the disparity in structural constraints and ligand binding among the alleles within heterozygous individuals may contribute as a mechanism to the plumage color variation in the Coereba flaveola.

Fluorescent Isoindole Crosslink (FlICk) Chemistry: A Rapid, User-friendly Stapling Reaction

The stabilization of peptide secondary structure via stapling is a ubiquitous goal for creating new probes, imaging agents, and drugs. Inspired by indole-derived crosslinks found in natural peptide toxins, we employed ortho-phthalaldehydes to create isoindole staples, thus transforming inactive linear and monocyclic precursors into bioactive monocyclic and bicyclic products. Mild, metal-free conditions give an array of macrocyclic α-melanocyte-stimulating hormone (α-MSH) derivatives, of which several isoindole-stapled α-MSH analogues (K_i ≈1 nm) are found to be as potent as α-MSH. Analogously, late-stage intra-annular isoindole stapling furnished a bicyclic peptide mimic of α-amanitin that is cytotoxic to CHO cells (IC₅₀ =70 μm). Given its user-friendliness, we have termed this approach FlICk (fluorescent isoindole crosslink) chemistry.

Radiochemical and radiopharmacological characterization of a 64 Cu-labeled α-MSH analog conjugated with different chelators

Radiolabeled α-melanocyte-stimulating hormone (α-MSH) derivatives have a high potential for diagnosis and treatment of melanoma, because of high specificity and binding affinity to the melanocortin-1 receptor (MC1R). Hence, the α-MSH-derived peptide NAP-NS1 with a β-Ala linker (ε-Ahx-β-Ala-Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH₂ ) was conjugated to different chelators: either to NOTA (p-SCN-Bn-1,4,7-triazacyclononane-1,4,7-triacetic acid), to a hexadentate bispidine carbonate derivative (dimethyl-9-(((4-nitrophenoxy)carbonyl)oxy)-2,4-di(pyridin-2-yl)-3,7-bis(pyridin-2-ylmethyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylate), or to DMPTACN (p-SCN-Ph-bis(2-pyridyl-methyl)-1,4,7-triaza-cyclononane), labeled with ⁶⁴ Cu, and investigated in terms of radiochemical and radiopharmacological properties. For the three ⁶⁴ Cu-labeled conjugates negligible transchelation, suitable buffer and serum stability, as well as appropriate water solubility, was determined. The three conjugates exhibited high binding affinity (low nanomolar range) in murine B16F10, human MeWo, and human TXM13 cells. The B_max values of [⁶⁴ Cu]Cu-bispidine-NAP-NS1 ([⁶⁴ Cu]Cu-2) and [⁶⁴ Cu]Cu-DMPTACN-NAP-NS1 ([⁶⁴ Cu]Cu-3) were higher than those of [⁶⁴ Cu]Cu-NOTA-NAP-NS1 ([⁶⁴ Cu]Cu-1), implying that different charged chelate units might have an impact on binding capacity. Preliminary in vivo biodistribution studies suggested the main excretion pathway of [⁶⁴ Cu]Cu-1 and [⁶⁴ Cu]Cu-3 to be renal, while that of [⁶⁴ Cu]Cu-2 seemed to be both renal and hepatobiliary. An initial moderate uptake in the kidney decreased clearly after 60 minutes. All three ⁶⁴ Cu-labeled conjugates should be considered for further in vivo investigations using a suitable xenograft mouse model.

Biomedical applications of radioiodinated peptides

The overexpression of peptide receptors in certain tumors as compared to endogeneous expression levels represents the molecular basis for the design of peptide-based tools for targeted nuclear imaging and therapy. Receptor targeting with radiolabelled peptides became a very important imaging and/or therapeutic approach in nuclear medicine and oncology. A great variety of peptides has been radiolabelled with clinical relevant radionuclides, such as radiometals and radiohalogens. However, to the best of our knowledge concise and updated reviews providing information about the biomedical application of radioiodinated peptides are still missing. This review outlines the synthetic efforts in the preparation of radioiodinated peptides highlighting the importance of radioiodine in nuclear medicine, giving an overview of the most relevant radioiodination strategies that have been employed and describes relevant examples of their use in the biomedical field.

Bacterial Protein Mimetic of Peptide Hormone as a New Class of Protein- based Drugs

Specific peptide molecules classified as hormones, neuropeptides and cytokines are involved in intercellular signaling regulating various physiological processes in all organs and tissues. This justifies the peptidergic signaling as an attractive pharmacological target. Recently, a protein mimetic of a peptide hormone has been identified in Escherichia coli suggesting the potential use of specific bacterial proteins as a new type of peptide-like drugs. We review the scientific rational and technological approaches leading to the identification of the E. coli caseinolytic protease B (ClpB) homologue protein as a conformational mimetic of α-melanocyte-stimulating hormone (α-MSH), a melanocortin peptide critically involved in the regulation of energy homeostasis in humans and animals. Theoretical and experimental backgrounds for the validation of bacterial ClpB as a potential drug are discussed based on the known E. coli ClpB amino acid sequence homology with α-MSH. Using in silico analysis, we show that other protein sources containing similar to E. coli ClpB α-MSH-like epitopes with potential biological activity may exist in Enterobacteriaceae and in some Brassicaceae. Thus, the original approach leading to the identification of E. coli ClpB as an α-MSH mimetic protein can be applied for the identification of mimetic proteins of other peptide hormones and development of a new type of peptide-like protein-based drugs.

Molecular Imaging and Radionuclide Therapy of Melanoma Targeting the Melanocortin 1 Receptor

Melanoma is a deadly disease at late metastatic stage, and early diagnosis and accurate staging remain the key aspects for managing melanoma. The melanocortin 1 receptor (MC1 R) is overexpressed in primary and metastatic melanomas, and its endogenous ligand, the α-melanocyte-stimulating hormone (αMSH), has been extensively studied for the development of MC1 R-targeted molecular imaging and therapy of melanoma. Natural αMSH is not well suited for this purpose due to low stability in vivo. Unnatural amino acid substitutions substantially stabilized the peptide, while cyclization via lactam bridge and metal coordination further improved binding affinity and stability. In this study, we summarized the development and the in vitro and in vivo characteristics of the radiolabeled αMSH analogues, including ^99mTc-, ¹¹¹In-, ⁶⁷ Ga-, or ¹²⁵I-labeled αMSH analogues for imaging with single-photon emission computed tomography; ⁶⁸Ga-, ⁶⁴Cu-, or ¹⁸F-labeled αMSH analogues for imaging with positron emission tomography; and ¹⁸⁸Re-, ¹⁷⁷Lu-, ⁹⁰Y-, or ²¹²Pb-labeled αMSH analogues for radionuclide therapy. These radiolabeled αMSH analogues showed promising results with high tumor uptake and rapid normal tissue activity clearance in the preclinical model of B16F1 and B16F10 mouse melanomas. These results highlight the potential of using radiolabeled αMSH analogues in clinical applications for molecular imaging and radionuclide therapy of melanoma.

Autoantibodies reactive to adrenocorticotropic hormone can alter cortisol secretion in both aggressive and nonaggressive humans

Violent aggression in humans may involve a modified response to stress, but the underlying mechanisms are not well understood. Here we show that naturally present autoantibodies reactive to adrenocorticotropic hormone (ACTH) exhibit distinct epitope-binding profiles to ACTH peptide in subjects with a history of violent aggression compared with controls. Namely, while nonaggressive male controls displayed a preferential IgG binding to the ACTH central part (amino acids 11-24), subjects who had committed violent acts of aggression had IgG with increased affinity to ACTH, preferentially binding to its N terminus (amino acids 1-13). Purified IgGs from approximately half of the examined sera were able to block ACTH-induced cortisol secretion of human adrenal cells in vitro, irrespective of the source of sample (from a control subject or a violent aggressor). Nevertheless, in the resident-intruder test in mice, i.p. injection of residents with ACTH and IgG from aggressive subjects, but not from control subjects, shortened latency for the first attack against intruders. Immunohistochemical screening of violent aggressors' sera on rat brain and pituitary sections did not show IgG binding to ACTH-producing cells, but 4 of 16 sera revealed selective binding to a nonidentified antigen in vasopressinergic neurons of the hypothalamic paraventricular and supraoptic nuclei. Thus, the data show that ACTH-reactive plasmatic IgGs exhibit differential epitope preference in control and violently aggressive subjects. These IgGs can modulate ACTH-induced cortisol secretion and, hence, are involved in the regulation of the stress response. However, the possible role of ACTH-reactive autoantibodies in aggressive behavior needs further investigation.

Key amino acid residue in Melanocortin-1 receptor (melanocyte α-MSH receptor) for ligand selectivity

The melanocortin-1 receptor (MC1R) is a subtype of the melanocortin receptor family and NDP-α-MSH is a non-selective agonist for MC1R. The core sequence of NDP-α-MSH, His-Phe-Arg-Trp, is important for ligand binding and biological activities at the melanocortin receptor subtypes (MCRs). A recent study indicates that Trp⁹ in NDP-α-MSH plays an important role in ligand selectivity. Deletion of Trp⁹ in NDP-α-MSH (des-Trp⁹-NDP-α-MSH) resulted in loss of agonist activity at MC4R, although remains agonist activity at MC1R. The molecular basis for this receptor ligand selectivity is unknown. In this study we examined what region of the MC1R is responsible for des-NDP-α-MSH selectivity. Our results indicate that (1) substitution of TM3 of MC4R with the corresponding region of MC1R switches des-Trp⁹-NDP-α-MSH from no activity to agonist; (2) des-Trp⁹-NDP-α-MSH exhibits agonistic activity at the L133M mutation of the MC4R; and (3) substitution of non-conserved amino acid residue M128 in TM3 of MC1R significantly reduced des-Trp⁹-NDP-α-MSH agonist activity. Our results demonstrate that amino acid residue 128 in TM3 of MC1R, or amino acid residue L133 in TM3 of the MC4R, play crucial roles in ligand des-Trp⁹-NDP-α-MSH selectivity at MC1R or MC4R.

NMR Insights into the Structure-Function Relationships in the Binding of Melanocortin Analogues to the MC1R Receptor

Linear and cyclic analogues of the α-melanocyte stimulating hormone (α-MSH) targeting the human melanocortin receptor 1 (MC1R) are of pharmacological interest for detecting and treating melanoma. The central sequence of α-MSH (His-Phe-Arg-Trp) has been identified as being essential for receptor binding. To deepen current knowledge on the molecular basis for α-MSH bioactivity, we aimed to understand the effect of cycle size on receptor binding. To that end, we synthesised two macrocyclic isomeric α-MSH analogues, c[NH-NO₂-C₆H₃-CO-His-DPhe-Arg-Trp-Lys]-Lys-NH₂ (CycN-K6) and c[NH-NO₂-C₆H₃-CO-His-DPhe-Arg-Trp-Lys-Lys]-NH₂ (CycN-K7). Their affinities to MC1R receptor were determined by competitive binding assays, and their structures were analysed by ¹H and ¹³C NMR. These results were compared to those of the previously reported analogue c[S-NO₂-C₆H₃-CO-His-DPhe-Arg-Trp-Cys]-Lys-NH₂ (CycS-C6). The MC1R binding affinity of the 22-membered macrocyclic peptide CycN-K6 (IC₅₀ = 155 ± 16 nM) is higher than that found for the 25-membered macrocyclic analogue CycN-K7 (IC₅₀ = 495 ± 101 nM), which, in turn, is higher than that observed for the 19-membered cyclic analogue CycS-C6 (IC₅₀ = 1770 ± 480 nM). NMR structural study indicated that macrocycle size leads to changes in the relative dispositions of the side chains, particularly in the packing of the Arg side chain relative to the aromatic rings. In contrast to the other analogues, the 22-membered cycle’s side chains are favorably positioned for receptor interaction.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.

Melanoma targeting with [99mTc(N)(PNP3)]-labeled α-melanocyte stimulating hormone peptide analogs: Effects of cyclization on the radiopharmaceutical properties

The purpose of this study was to evaluate the effect of cyclization on the biological profile of a [^99mTc(N)(PNP3)]-labeled α-melanocyte stimulating hormone peptide analog. A lactam bridge-cyclized H-Cys-Ahx-βAla³-c[Lys⁴-Glu-His-D-Phe-Arg-Trp-Glu¹⁰]-Arg¹¹-Pro-Val-NH₂ (NAP-NS2) and the corresponding linear H-Cys-Ahx-βAla-Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH₂ (NAP-NS1) peptide were synthetized, characterized by ESI-MS spectroscopy and their melanocortin-1 receptor (MC1R) binding affinity was determined in B16/F10 melanoma cells. The consistent [^99mTc(N)(PNP3)]-labeled compounds were readily obtained in high specific activity and their stability and biological properties were assessed. As an example, the chemical identity of [^99mTc(N)(NAP-NS1)(PNP3)]⁺ was confirmed by carrier added experiments supported by radio/UV HPLC analysis combined with ESI(+)-MS. Compared with the linear peptide, cyclization negatively affected the biological properties of NAP-NS2 peptide by reducing its binding affinity for MC1R and by decreasing the overall excretion rate of the corresponding [^99mTc(N)(PNP3)]-labeled peptide from the body as well as its in vivo stability. [^99mTc(N)(NAP-NS1)(PNP3)]⁺ was evaluated for its potential as melanoma imaging probe in murine melanoma model. Data from in vitro and in vivo studies on B16/F10 melanoma model of [^99mTc(N)(NAP-NS1)(PNP3)]⁺ clearly evidenced that the radiolabeled linear peptide keeps its biological properties up on the conjugation to the [^99mTc(N)(PNP3)]-building block. The progressive increase of the tumor-to-nontarget ratios over the time indicates a quite stable interaction between the radio-complex and the MC1R.

Synthesis and Pharmacology of α/β(3)-Peptides Based on the Melanocortin Agonist Ac-His-dPhe-Arg-Trp-NH2 Sequence

The melanocortin-3 and -4 receptors are expressed in the brain and play key roles in regulating feeding behavior, metabolism, and energy homeostasis. In the present study, incorporation of β(3)-amino acids into a melanocortin tetrapeptide template was investigated. Four linear α/β(3)-hybrid tetrapeptides were designed with the modifications at the Phe, Arg, and Trp residues in the agonist sequence Ac-His-dPhe-Arg-Trp-NH2. The most potent mouse melanocortin-4 receptor (mMC4R) agonist, Ac-His-dPhe-Arg-β(3)hTrp-NH2 (8) showed 35-fold selectivity versus the mMC3R. The study presented here has identified a new template with heterogeneous backbone for designing potent and selective melanocortin receptor ligands.

1,4-disubstituted-[1,2,3]triazolyl-containing analogues of MT-II: design, synthesis, conformational analysis, and biological activity

Side chain-to-side chain cyclizations represent a strategy to select a family of bioactive conformations by reducing the entropy and enhancing the stabilization of functional ligand-induced receptor conformations. This structural manipulation contributes to increased target specificity, enhanced biological potency, improved pharmacokinetic properties, increased functional potency, and lowered metabolic susceptibility. The Cu^I-catalyzed azide–alkyne 1,3-dipolar Huisgen’s cycloaddition, the prototypic click reaction, presents a promising opportunity to develop a new paradigm for an orthogonal bioorganic and intramolecular side chain-to-side chain cyclization. In fact, the proteolytic stable 1,4- or 4,1-disubstituted [1,2,3]triazolyl moiety is isosteric with the peptide bond and can function as a surrogate of the classical side chain-to-side chain lactam forming bridge. Herein we report the design, synthesis, conformational analysis, and functional biological activity of a series of i-to-i+5 1,4- and 4,1-disubstituted [1,2,3]triazole-bridged cyclopeptides derived from MT-II, the homodetic Asp⁵ to Lys¹⁰ side chain-to-side chain bridged heptapeptide, an extensively studied agonist of melanocortin receptors.

Molecular mechanisms involved in interleukin 1-beta (IL-1β)-induced memory impairment. Modulation by alpha-melanocyte-stimulating hormone (α-MSH)

Pro-inflammatory cytokines can affect cognitive processes such as learning and memory. Particularly, interleukin-1β (IL-1β) influences the consolidation of hippocampus-dependent memories. We previously reported that administration of IL-1β in dorsal hippocampus impaired contextual fear memory consolidation. Different mechanisms have been implicated in the action of IL-1β on long-term potentiation (LTP), but the processes by which this inhibition occurs in vivo remain to be elucidated. We herein report that intrahippocampal injection of IL-1β induced a significant increase in p38 phosphorylation after contextual fear conditioning. Also, treatment with SB203580, an inhibitor of p38, reversed impairment induced by IL-1β on conditioned fear behavior, indicating that this MAPK would be involved in the effect of the cytokine. We also showed that IL-1β administration produced a decrease in glutamate release from dorsal hippocampus synaptosomes and that treatment with SB203580 partially reversed this effect. Our results indicated that IL-1β-induced impairment in memory consolidation could be mediated by a decrease in glutamate release. This hypothesis is sustained by the fact that treatment with d-cycloserine (DCS), a partial agonist of the NMDA receptor, reversed the effect of IL-1β on contextual fear memory. Furthermore, we demonstrated that IL-1β produced a temporal delay in ERK phosphorylation and that DCS administration reversed this effect. We also observed that intrahippocampal injection of IL-1β decreased BDNF expression after contextual fear conditioning. We previously demonstrated that α-MSH reversed the detrimental effect of IL-1β on memory consolidation. The present results demonstrate that α-MSH administration did not modify the decrease in glutamate release induced by IL-1β. However, intrahippocampal injection of α-MSH prevented the effect on ERK phosphorylation and BDNF expression induced by IL-1β after contextual fear conditioning. Therefore, in the present study we determine possible molecular mechanisms involved in the impairment induced by IL-1β on fear memory consolidation. We also established how this effect could be modulated by α-MSH.

Immunofluorescence evidence of melanotrophs in the pituitary of four odontocete species. An immunohistochemical study and a critical review of the literature

Cetaceans share peculiar features of their pituitary glands, with a complete separation of pars distalis and pars nervosa by a dural septum and the absence of an intermediate lobe and cleft. In most mammals the pars intermedia is the main source of circulating α-melanocyte stimulating hormone (α-MSH), derived from a large precursor called proopiomelanocortin (POMC), which also generates adrenocorticotropic hormone (ACTH) in the adenohypophysis. The lack of an intermediate lobe in cetaceans led us to investigate whether their glands are able to produce α-MSH, and if this hormone is secreted by a distinct population of melanotrophs or by corticotrophs in the pars distalis. Immunofluorescence evidences seem to support the first assumption, with ACTH-immunoreactive (-ir) elements rarely overlapping with α-MSH-ir ones. The discovery of a population of true melanotrophs in the hypophysis of some odontocetes underscores the need for further research on the melanocortin system of cetaceans.

Peptide scanning for studying structure-activity relationships in drug discovery

Peptide-based therapeutics have grown in importance over the last few decades. Furthermore, peptides have been extensively used as lead compounds in the drug discovery process to investigate the nature of chemical space required for molecular recognition and activity at a variety of targets. This critical commentary reviews scanning techniques, which employ natural and non-proteinogenic amino acids to facilitate understanding of structural requirements for peptide biological activity. The value of sequence analysis by such methods is highlighted by examples, in which the elements for peptide affinity and activity have been elucidated and employed to prepare peptidomimetic leads for drug development.

Structure-activity relationships of peptides incorporating a bioactive reverse-turn heterocycle at the melanocortin receptors: identification of a 5800-fold mouse melanocortin-3 receptor (mMC3R) selective antagonist/partial agonist versus the mouse melanocortin-4 receptor (mMC4R)

The melanocortin-3 (MC3) and melanocortin-4 (MC4) receptors regulate energy homeostasis, food intake, and associated physiological conditions. The melanocortin-4 receptor (MC4R) has been studied extensively. Less is known about specific physiological roles of the melanocortin-3 receptor (MC3R). A major obstacle to this lack of knowledge is attributed to a limited number of identified MC3R selective ligands. We previously reported a spatial scanning approach of a 10-membered thioether-heterocycle ring incorporated into a chimeric peptide template that identified a lead nM MC4R ligand. Upon the basis of those results, 17 compounds were designed and synthesized that focused upon modification in the pharmacophore domain. Notable results include the identification of a 0.13 nM potent 5800-fold mMC3R selective antagonist/slight partial agonist versus a 760 nM mMC4R full agonist (ligand 11). Biophysical experiments (two-dimensional (1)H NMR and computer-assisted molecular modeling) of this ligand resulted in the identification of an inverse γ-turn secondary structure in the ligand pharmacophore domain.

Evaluation of novel 99mTc(I)-labeled homobivalent α-melanocyte-stimulating hormone analogs for melanocortin-1 receptor targeting

Aiming to apply the multivalency concept to melanoma imaging, we have assessed the in vivo melanocortin type 1 receptor (MC1R)-targeting properties of (99m)Tc(I)-labeled homobivalent peptide conjugates which contain copies of the α-melanocyte-stimulating hormone (α-MSH) analog [Ac-Nle(4), Asp(5), D-Phe(7), Lys(11)]α-MSH4-11 separated by linkers of different length (L(2) nine atoms and L(3) 14 atoms). The MC1R-binding affinity of L(2) and L(3) is significantly higher than that of the monovalent conjugate L(1). Metallation of these conjugates yielded the complexes fac-[M(CO)(3)(k(3)-L)](+) (M is (99m)Tc/Re; 1/1a, L is L(1); 2/2a, L is L(2); 3/3a, L is L(3)), with IC(50) values in the subnanomolar and nanomolar range. The MC1R-mediated internalization of 2 and 3 is higher than that of 1 in B16F1 melanoma cells. Biodistribution studies in melanoma-bearing mice have shown low nonspecific accumulation with a tumor uptake that correlates with IC(50) values. However, no correlation between tumor uptake and valency was found. Nevertheless, 2 displayed the highest tumor retention, and the best tumor to nontarget organ ratios.

A sucrose-derived scaffold for multimerization of bioactive peptides

A spherical molecular scaffold bearing eight terminal alkyne groups was synthesized in one step from sucrose. One or more copies of a tetrapeptide azide, either N(3)(CH(2))(5)(CO)-His-DPhe-Arg-Trp-NH(2) (MSH4) or N(3)(CH(2))(5)(CO)-Trp-Met-Asp-Phe-NH(2) (CCK4), were attached to the scaffold via the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Competitive binding assays using Eu-labeled probes based on the superpotent ligands Ser-Tyr-Ser-Nle-Glu-His-DPhe-Arg-Trp-Gly-Lys-Pro-Val-NH(2) (NDP-α-MSH) and Asp-Tyr-Met-Gly-Trp-Met-Asp-Phe-NH(2) (CCK8) were used to study the interactions of monovalent and multivalent MSH4 and CCK4 constructs with Hek293 cells engineered to overexpress MC4R and CCK2R. All of the monovalent and multivalent MSH4 constructs exhibited binding comparable to that of the parental ligand, suggesting that either the ligand spacing was inappropriate for multivalent binding, or MSH4 is too weak a binder for a second 'anchoring' binding event to occur before the monovalently-bound construct is released from the cell surface. In contrast with this behavior, monovalent CCK4 constructs were significantly less potent than the parental ligand, while multivalent CCK4 constructs were as or more potent than the parental ligand. These results are suggestive of multivalent binding, which may be due to increased residence times for monovalently bound CCK4 constructs on the cell surface relative to MSH4 constructs, the greater residence time being necessary for the establishment of multivalent binding.

Adrenocorticotropic tumor cells transplanted into mouse embryos affect pancreatic histogenesis

A wide range of individual differences exist in the total number of functional and structural units in each organ, such as β cells in pancreatic islands, and these units are the basis of the organ's overall function, including its functional reserve. The endocrine environment may influence organ histogenesis, during which functional and structural units are formed and increase in number. We analyzed the effects of a continuous high level of adrenocorticotropic hormone (ACTH) and/or secondarily induced glucocorticoid on histogenesis of the pancreas in mouse embryos. Pituitary tumor-derived AtT20 cells, which secrete ACTH continuously, were injected subcutaneously into mouse embryos at embryonic day (E) 12.5, and the embryos were allowed to develop exo utero until E18.5 (AtT20 group). E18.5 AtT20 group embryos with high ACTH levels (23.74 ± 6.19 ng/mL vs control group, 0.48 ± 0.40 ng/mL, P < 0.05) were examined for the effects on histogenesis of the pancreas. Using serial sections of the E18.5 pancreas, we stereologically measured the volumes, and counted total cell numbers and numbers of mitotic or pyknotic cells of the whole pancreas, endocrine and exocrine cells, and glucagon-immunopositive α cells and insulin-immunopositive β cells in the endocrine part. Although the volumes of the whole pancreas and exocrine part did not change significantly, in the AtT20 group the endocrine part was significantly larger, with fewer pyknotic cells and lower ratios of α and β cells than in the control group. These results suggest that the high level of ACTH and/or glucocorticoid affects histogenesis of the pancreas.

Melanocortin-1 receptor-targeting with radiolabeled cyclic α-melanocyte-stimulating hormone analogs for melanoma imaging

Melanoma is a type of skin cancer known for its high aggressiveness, early dissemination of metastases, and poor prognosis once metastasized. Thus, early diagnosis of melanoma is a key issue for increasing patient survival. The overexpression of melanocortin-1 receptors (MC1R) in isolated melanoma cells and melanoma tissues led to the radiolabeling of several linear and cyclic MC analogs for melanoma imaging or therapy. Cyclization of α-melanocyte stimulating hormone (α-MSH) peptides has been successfully used to improve binding affinity and in vivo stability of peptides. Herein, we describe the different peptide cyclization strategies recently reported for radiolabeled α-MSH analogs and discuss how such strategies affect MC1R binding affinity, pharmacokinetic profile, and MC1R-melanoma imaging. This review also highlights how the nature of the radiometal and labeling approach influence those properties. Among the cyclized α-MSH peptides reported, (99m)Tc/(111)In-labeled metal-cyclized and lactam bridge-cyclized peptides displayed the highest melanoma and lowest renal uptake values in B16/F1 melanoma-bearing mice and became the most promising tools to be further explored as potential melanoma imaging probes.

Cyclic lactam hybrid α-MSH/Agouti-related protein (AGRP) analogues with nanomolar range binding affinities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the topographical similarities between the pharmacophores of Agouti related protein (AGRP) an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. When employed in two different 23-membered macrocyclic lactam peptide templates, the designed hybrid AGRP/MSH pharmacophore yielded non-competitive ligands with nanomolar range binding affinities. The topography-based pharmacophore hybridization strategy will prove useful in development of unique non-competitive melanocortin receptor modulators.

Structure, function and regulation of the melanocortin receptors

Melanocortin receptors belong to the seven-transmembrane (TM) domain proteins that are coupled to G-proteins and signaled through intracellular cyclic adenosine monophosphate. Many structural features conserved in other G-protein coupled receptors (GPCRs) are found in the melanocortin receptors. There are five melanocortin receptor subtypes and each of the melanocortin receptor subtypes has a different pattern of tissue expression and has its own profile regarding the relative potency of different melanocortin peptides. α-, β-, and γ-MSH and ACTH are known endogenous agonist ligands for the melanocortin receptors. Agouti and AgRP are the only known naturally occurring antagonists of the melanocortin receptors. We have examined the molecular basis of all five human melanocortin receptors for different ligand binding affinities and potencies using chimeric and mutated receptors. Our studies indicate that human melanocortin MC(1) receptor, human melanocortin MC(3) receptor, human melanocortin MC(4) receptor and human melanocortin MC(5) receptor utilize orthosteric sites for non selective agonists, α-MSH and NDP-α-MSH, high affinity binding and utilize allosteric sites for selective agonist or antagonist binding. Furthermore, our results indicate that molecular determinants of human melanocortin MC(2) receptor for ACTH binding and signaling are different from that of other melanocortin receptors. Many studies also indicate that agonists can induce different conformation changes of melanocortin receptors, which then lead to the activation of different signaling pathways, even when the expression level of receptor and the strength of stimulus-response coupling are the same. This finding may provide new information for the design of drugs for targeting melanocortin receptors.

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.

Allosteric ligands for G protein-coupled receptors: a novel strategy with attractive therapeutic opportunities

Allosteric receptor ligands bind to a recognition site that is distinct from the binding site of the endogenous messenger molecule. As a consequence, allosteric agents may attach to receptors that are already transmitter-bound. Ternary complex formation opens an avenue to qualitatively new drug actions at G protein-coupled receptors (GPCRs), in particular receptor subtype selective potentiation of endogenous transmitter action. Consequently, suitable exploitation of allosteric recognition sites as alternative molecular targets could pave the way to a drug discovery paradigm different from those aimed at mimicking or blocking the effects of endogenous (orthosteric) receptor activators. The number of allosteric ligands reported to modulate GPCR function is steadily increasing and some have already reached routine clinical use. This review aims at introducing into this fascinating field of drug discovery and at providing an overview about the achievements that have already been made. Various case examples will be discussed in the framework of GPCR classification (family A, B, and C receptors). In addition, the behavior at muscarinic receptors of hybrid derivatives incorporating both an allosteric and an orthosteric fragment in a common molecular skeleton will be illustrated.

Alpha-melanocyte stimulating hormone: production and degradation

Proopiomelanocortin (POMC) is a polypeptide hormone precursor that is expressed in the brain and in peripheral tissues such as in the pituitary gland, immune system, and skin. In the brain, POMC is processed to form several peptides including alpha-melanocyte stimulating hormone (α-MSH). alpha-MSH is expressed in the hypothalamic arcuate nucleus and in the nucleus tractus solitarius of the brainstem where it has a crucial role in the regulation of metabolic functions. Specifically, α-MSH is an anorexigenic peptide. Its production and maturation processes have been shown to be regulated according to the metabolic condition of the organism. This review summarizes our current knowledge on α-MSH processing including its maturation and degradation processes and pharmacological aspects of its manipulation.

Interactions of the melanocortin-4 receptor with the peptide agonist NDP-MSH

Melanocortin-4 receptor (MC4R) has an important regulatory role in energy homeostasis and food intake. Peptide agonists of the MC4R are characterized by the conserved sequence His₆-Phe₇-Arg₈-Trp₉, which is crucial for their interaction with the receptor. This investigation utilized the covalent attachment approach to identify receptor residues in close proximity to the bound ligand [Nle₄,d-Phe₇]melanocyte-stimulating hormone (NDP-MSH), thereby differentiating between residues directly involved in ligand binding and those mutations that compromise ligand binding by inducing conformational changes in the receptor. Also, recent X-ray structures of G-protein-coupled receptors were utilized to refine a model of human MC4R in the active state (R^⁎), which was used to generate a better understanding of the binding mode of the ligand NDP-MSH at the atomic level.

The mutation of residues in the human MC4R—such as Leu106 of extracellular loop 1, and Asp122, Ile125, and Asp126 of transmembrane (TM) helix 3, His264 (TM6), and Met292 (TM7)—to Cys residues produced definitive indications of proximity to the side chains of residues in the core region of the peptide ligand. Of particular interest was the contact between d-Phe₇ on the ligand and Ile125 of TM3 on the MC4R. Additionally, Met292 (TM7) equivalent to Lys(7.45) (Ballesteros numbering scheme) involved in covalently attaching retinal in rhodopsin is shown to be in close proximity to Trp₉.

For the first time, the interactions between the terminal regions of NDP-MSH and the receptor are described. The amino-terminus appears to be adjacent to a series of hydrophilic residues with novel interactions at Cys196 (TM5) and Asp189 (extracellular loop 2). These interactions are reminiscent of sequential ligand binding exhibited by the β₂-adrenergic receptor, with the former interaction being equivalent to the known interaction involving Ser204 of the β₂-adrenergic receptor.