Design and pharmacology of peptoids and peptide–peptoid hybrids based on the melanocortin agonists core tetrapeptide sequence

Dimeric peptoids as antibacterial agents

Building upon our previous study on peptoid-based antibacterials which showed good activity against Gram-positive bacteria only, herein we report the synthesis of 34 dimeric peptoid compounds and the investigation of their activity against Gram-positive and Gram-negative pathogens. The newly designed peptoids feature a di-hydrophobic moiety incorporating phenyl, bromo-phenyl, and naphthyl groups, combined with variable lengths of cationic units such as amino and guanidine groups. The study also underscores the pivotal interplay between hydrophobicity and cationicity in optimizing efficacy against specific bacteria. The bromophenyl dimeric guanidinium peptoid compound 10j showed excellent activity against S. aureus 38 and E. coli K12 with MIC of 0.8 μg mL-1 and 6.2 μg mL-1, respectively. Further investigation into the mechanism of action revealed that the antibacterial effect might be attributed to the disruption of bacterial cell membranes, as suggested by tethered bilayer lipid membranes (tBLMs) and cytoplasmic membrane permeability studies. Notably, these promising antibacterial agents exhibited negligible toxicity against mammalian red blood cells. Additionally, the study explored the potential of 12 active compounds to disrupt established biofilms of S. aureus 38. The most effective biofilm disruptors were ethyl and octyl-naphthyl guanidinium peptoids (10c and 10 k). These compounds 10c and 10 k disrupted the established biofilms of S. aureus 38 with 51 % at 4x MIC (MIC = 17.6 μg mL-1 and 11.2 μg mL-1) and 56 %-58 % at 8x MIC (MIC = 35.2 μg mL-1 and 22.4 μg mL-1) respectively. Overall, this research contributes insights into the design principles of cationic dimeric peptoids and their antibacterial activity, with implications for the development of new antibacterial compounds.

Short Tryptamine-Based Peptoids as Potential Therapeutics for Microbial Keratitis: Structure-Function Correlation Studies

Peptoids are peptidomimetics that have attracted considerable interest as a promising class of antimicrobials against multi-drug-resistant bacteria due to their resistance to proteolysis, bioavailability, and thermal stability compared to their corresponding peptides. Staphylococcus aureus is a significant contributor to infections worldwide and is a major pathogen in ocular infections (keratitis). S. aureus infections can be challenging to control and treat due to the development of multiple antibiotic resistance. This work describes short cationic peptoids with activity against S. aureus strains from keratitis. The peptoids were synthesized via acid amine-coupling between naphthyl-indole amine or naphthyl-phenyl amine with different amino acids to produce primary amines (series I), mono-guanidines (series II), tertiary amine salts (series III), quaternary ammonium salts (series IV), and di-guanidine (series V) peptoids. The antimicrobial activity of the peptoids was compared with ciprofloxacin, an antibiotic that is commonly used to treat keratitis. All new compounds were active against Staphylococcus aureus S.aureus 38. The most active compounds against S.aur38 were 20a and 22 with MIC = 3.9 μg mL−1 and 5.5 μg mL−1, respectively. The potency of these two active molecules was investigated against 12 S. aureus strains that were isolated from microbial keratitis. Compounds 20a and 22 were active against 12 strains with MIC = 3.2 μg mL−1 and 2.1 μg mL−1, respectively. There were two strains that were resistant to ciprofloxacin (Sa.111 and Sa.112) with MIC = 128 μg mL−1 and 256 μg mL−1, respectively. Compounds 12c and 13c were the most active against E. coli, with MIC > 12 μg mL−1. Cytoplasmic membrane permeability studies suggested that depolarization and disruption of the bacterial cell membrane could be a possible mechanism for antibacterial activity and the hemolysis studies toward horse red blood cells showed that the potent compounds are non-toxic at up to 50 μg mL−1.

Peptoid NPhe4 in AGRP-Based c[Pro1-Arg2-Phe3-Phe4-Xxx5-Ala6-Phe7-DPro8] Scaffolds Maintain Mouse MC4R Antagonist Potency

The melanocortin receptors are involved in numerous physiological functions and are regulated by agonists derived from the proopiomelanocortin gene transcript and two endogenous antagonists, agouti and agouti-related protein (AGRP). The key binding and functional determinant of AGRP, an MC3R and MC4R antagonist, is an Arg-Phe-Phe tripeptide sequence located on an exposed hexapeptide (Arg-Phe-Phe-Asn-Ala-Phe) loop. It has previously been observed that cyclizing this sequence through a DPro-Pro motif (c[Pro¹-Arg²-Phe³-Phe⁴-Asn⁵-Ala⁶-Phe⁷-DPro⁸]) resulted in a macrocyclic scaffold with MC4R antagonist activity, with increased MC4R potency when a diaminopropionic acid (Dap) residue is substituted at position 5. In this report, a series of 11 single-peptoid substitutions were performed in the AGRP-derived macrocycles. While most peptoid substitutions decreased MC4R antagonist potency, it was observed that NPhe⁴ (compounds 4 and 11) or NDab⁵ (diaminobutyric acid, compound 7) maintained MC4R antagonist potency. The NPhe⁴ substitutions also resulted in MC5R antagonist and inverse agonist activity equipotent to the parent scaffolds. These data may be used in the design of future MC4R and MC5R antagonist leads and probes that possess increased metabolic stability due to the presence of peptoid residues.

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.

Progress with peptide scanning to study structure-activity relationships: the implications for drug discovery

INTRODUCTION

Peptides have gained renewed interest as candidate therapeutics. However, to bring them to a broader clinical use, challenges such as the rational optimization of their pharmacological properties remain. Peptide scanning techniques offer a systematic framework to gain information on the functional role of individual amino acids of a peptide. Due to progress in mastering new chemical synthesis routes targeting amino acid backbone, they are currently diversified. Structure-activity relationship (SAR) analyses such as alanine- or enantioneric- scanning can now be supplemented by N-substitution, lactam cyclisation- or aza-amino scanning procedures addressing not only SAR considerations but also the peptide pharmacological properties.

AREAS COVERED

This review highlights the different scanning techniques currently available and illustrates how they can impact drug discovery.

EXPERT OPINION

Progress in peptide scanning techniques opens new perspectives for peptide drug development. It comes with the promise of a paradigm change in peptide drug design in which peptide drugs will be closer to the parent peptides. However, scanning still remains assimilable to a trial and error strategy that could benefit from being combined with specific in silico approaches that start reaching maturity.

Peptides and Peptide Analogs to Inhibit Protein-Protein Interactions

Protein-protein interactions are governed by relatively few amino acid residues at the binding interface. Peptides derived from these protein regions may serve as mimics of one of the interaction partners in structural studies or as inhibitors to disrupt the respective interaction and investigate its biological consequences. Inhibitory peptides may also be lead structures for drug development if the respective protein-protein interaction is essential for a pathogen or disease mechanism. Binding peptides may be systematically derived from one of the binding partners or found in the screen of combinatorial peptide libraries. Molecular modelling based on structural data helps to refine existing peptides or even design novel binding peptides. This chapter gives an outline of the binding peptide discovery process and subsequent chemical modifications to further enhance affinity and specificity and to increase stability against degradation in vivo. Examples from the past three decades illustrate the great diversity of applications for protein binding peptides and peptide analogs.

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.

Peptoids and peptide-peptoid hybrid biopolymers as peptidomimetics

Peptoids (oligomers of N-substituted glycine residues) and peptide-peptoid hybrid polymers (peptomers) are interesting classes of compounds mimicking structure and function of biologically active peptides. The oligomeric peptidomimetics such as peptoids are particularly important compounds since they provide access to an enormous molecular diversity, by variation of the building blocks. The modular structure of peptoids, ease of synthesis, and high compatibility with existing peptide chemistry synthetic protocols, make peptoids and peptoid-containing peptidomimetics ideal tools for structure-activity and drug discovery related studies.

Association of melanocortin 1 receptor gene (MC1R) polymorphisms with skin reflectance and freckles in Japanese

Most studies on the genetic basis of human skin pigmentation have focused on people of European ancestry and only a few studies have focused on Asian populations. We investigated the association of skin reflectance and freckling with genetic variants of melanocortin 1 receptor (MC1R) gene in Japanese. DNA samples were obtained from a total of 653 Japanese individuals (ages 19-40 years) residing in Okinawa; skin reflectance was measured using a spectrophotometer and freckling status was determined for each individual. Lightness index (L*) and freckling status were not correlated with age, body mass index or ancestry (Ryukyuan or Main Islanders of Japan). Among the 10 nonsynonymous variants that were identified by direct sequencing of the coding region of MC1R, two variants--R163Q and V92M--with the derived allele frequencies of 78.6 and 5.5%, respectively, were most common. Multiple regression analysis showed that the 163Q allele and the presence of nonsynonymous rare variants (allele frequencies <5%) were significantly associated with an increase in sex-standardized skin lightness (L* of CIELAB (CIE 1976 (L*a*b*) color space)) of the inner upper arm. Relative to the 92V allele, the 92M allele was significantly associated with increased odds of freckling. This is the first study to show an association between the 163Q allele and skin reflectance values; this association indicated that light-toned skin may have been subjected to positive selection in East Asian people.

Peptoid-Peptide hybrid ligands targeting the polo box domain of polo-like kinase 1

We replaced the amino terminal Pro residue of the Plk1 polo-box-domain-binding pentapeptide (PLHSpT) with a library of N-alkyl-Gly "peptoids", and identified long-chain tethered phenyl moieties giving greater than two-orders-of-magnitude affinity enhancement. Further simplification by replacing the peptoid residue with appropriate amides gave low-nanomolar affinity N-acylated tetrapeptides. Binding of the N-terminal long-chain phenyl extension was demonstrated by X-ray co-crystal data.

Peptides and proteins as a continuing exciting source of inspiration for peptidomimetics

Despite their enormous diversity in biological function and structure, peptides and proteins are endowed with properties that have induced and stimulated the development of peptidomimetics. Clearly, peptides can be considered as the "stem" of a phylogenetic molecular development tree from which branches of oligomeric peptidomimetics such as peptoids, peptidosulfonamides, urea peptidomimetics, as well as β-peptides have sprouted. It is still a challenge to efficiently synthesize these oligomeric species, and study their structural and biological properties. Combining peptides and peptidomimetics led to the emergence of peptide-peptidomimetic hybrids in which one or more (proteinogenic) amino acid residues have been replaced with these mimetic residues. In scan-like approaches, the influence of these replacements on biological activity can then be studied, to evaluate to what extent a peptide can be transformed into a peptidomimetic structure while maintaining, or even improving, its biological properties. A central issue, especially with the smaller peptides, is the lack of secondary structure. Important approaches to control secondary structure include the introduction of α,α-disubstituted amino acids, or (di)peptidomimetic structures such as the Freidinger lactam. Apart from intra-amino acid constraints, inter-amino acid constraints for formation of a diversity of cyclic peptides have shaped a thick branch. Apart from the classical disulfide bridges, the repertoire has been extended to include sulfide and triazole bridges as well as the single-, double- and even triple-bond replacements, accessible by the extremely versatile ring-closing alkene/alkyne metathesis approaches. The latter approach is now the method of choice for the secondary structure that presents the greatest challenge for structural stabilization: the α-helix.

Incorporation of a bioactive reverse-turn heterocycle into a peptide template using solid-phase synthesis to probe melanocortin receptor selectivity and ligand conformations by 2D 1H NMR

By use of a solid-phase synthetic approach, a bioactive reverse turn heterocycle was incorporated into a cyclic peptide template to probe melanocortin receptor potency and ligand structural conformations. The five melanocortin receptor isoforms (MC1R-MC5R) are G-protein-coupled receptors (GPCRs) that are regulated by endogenous agonists and antagonists. This pathway is involved in pigmentation, weight, and energy homeostasis. Herein, we report novel analogues of the chimeric AGRP-melanocortin peptide template integrated with a small molecule moiety to probe the structural and functional consequences of the core His-Phe-Arg-Trp peptide domain using a reverse-turn heterocycle. A series of six compounds are reported that result in inactive to full agonists with nanomolar potency. Biophysical structural analysis [2D (1)H NMR and computer-assisted molecular modeling (CAMM)] were performed on selected analogues, resulting in the identification that these peptide-small molecule hybrids possessed increased flexibility and fewer discrete conformational families compared to the reference peptide and result in a novel template for further structure-function studies.

Modulation of human estrogen receptor α activity by multivalent estradiol-peptidomimetic conjugates

Estradiol-peptidomimetic conjugates (EPCs) are linear, sequence-specific peptoid oligomers that site-specifically display multiple copies of 17β-estradiol (E2), a ligand for the human estrogen receptor α (hERα). We evaluate the ability of multivalent EPCs to activate hERα-mediated transcription. EPCs activated the hERα in both a length- and valence-dependent manner, with the highest levels of activation generated by divalent peptoid 6-mers, divalent 18-mers, and trivalent 9-mers. Hexavalent EPCs did not activate hERα, but instead blocked E2-mediated hERα activation. The physicochemical features of EPCs can be precisely tuned, which may allow the generation of a library of chemical tools for modulating specific effects of estrogens.

Peptide-peptoid hybrids based on (1-11)-parathyroid hormone analogs

A series of peptide-peptoid hybrids, containing N-substituted glycines, were synthesized based on the H-Aib-Val-Aib-Glu-Ile-Gln-Leu-Nle-His-Gln-Har-NH(2) (Har = Homoarginine) as the parent parathyroid hormone (1-11) analog. The compounds were pharmacologically characterized in their agonistic activity at the parathyroid hormone 1 receptor.

Solid-phase synthesis of N-substituted glycine oligomers (alpha-peptoids) and derivatives

Peptoids (N-substituted polyglycines and extended peptoids with variant backbone amino-acid monomer units) are oligomeric synthetic polymers that are becoming a valuable molecular tool in the biosciences. Of particular interest are their applications to the exploration of peptoid secondary structures and drug design. Major advantages of peptoids as research and pharmaceutical tools include the ease and economy of synthesis, highly variable backbone and side-chain chemistry possibilities. At the same time, peptoids have been demonstrated as highly active in biological systems while resistant to proteolytic decay. This review with 227 references considers the solid-phase synthetic aspects of peptoid preparation and utilization up to 2010 from the instigation, by R. N. Zuckermann et al., of peptoid chemistry in 1992.

Peptoid-Peptide hybrid backbone architectures

Peptidomimetic oligomers and foldamers have received considerable attention for over a decade, with beta-peptides and the so-called peptoids (N-alkylglycine oligomers) representing prominent examples of such architectures. Lately, hybrid or mixed backbones consisting of both alpha- and beta-amino acids (alpha/beta-peptides) have been investigated in some detail as well. The present Minireview is a survey of the literature concerning hybrid structures of alpha-amino acids and peptoids, including beta-peptoids (N-alkyl-beta-alanine oligomers), and is intended to give an overview of this area of research within the field of peptidomimetic science.

Biological activities of guanidine compounds

BACKGROUND

The guanidine group defines chemical and physicochemical properties of many compounds of medical interest and guanidine-containing derivatives constitute a very important class of therapeutic agents suitable for the treatment of a wide spectrum of diseases.

OBJECTIVE

To review the most important pharmacological properties, mechanisms of action and therapeutic uses of simple guanidine derivatives, cyclic analogues of guanidines as well as peptides, peptidomimetics and peptoids incorporating arginine.

METHODS

The review presents both the recent patent literature and original papers dealing with guanidine derivatives that show interesting biological activity and emphasizes the newest developing drugs.

CONCLUSION

Recent achievements in the synthesis of guanidine-containing molecules with diverse chemical, biochemical and pharmacological properties make them of great importance to the design and development of novel drugs acting at CNS, anti-inflammatory agents, inhibitors of Na(+)/H(+) exchanger, inhibitors of NO synthase, antithrombotic, antidiabetic and chemotherapeutic agents as well as guanidinium-based transporters and vectors.

Passage of Trojan peptoids into plant cells

Efficient drug delivery is essential for many therapeutic applications. In this context, Trojan peptoids have attracted attention as powerful tools to deliver bioactive molecules into living cells. Certain cell-penetrating peptides, peptide mimetics, and peptoids have been shown to be endowed with a transport function and the structural features of this function have been characterized. However, most of the research has been done by using mammalian cell cultures as model organisms and the actual cellular mechanism of membrane passage has not been elucidated. Plant cells, which are encased in a cellulosic cell wall and differ in membrane composition, represent an alternative experimental system to address this issue, but so far, have attracted only little attention for both peptide- and peptoid-based carrier systems. Moreover, efficient delivery of nonproteinaceous bioactive macromolecules into living plant cells could complement genetic engineering in biotechnological applications, such as metabolic engineering and molecular farming. In the present study, we investigated carrier peptoids with or without guanidinium side chains with regard to their uptake into plant cells, the cellular mechanism of uptake, and intracellular localization. We can show that in contrast to polyamine peptoids (polylysine-like) fluorescently labeled polyguanidine peptoids (polyarginine-like) enter rapidly into tobacco BY-2 cells without affecting the viability of these cells. A quantitative comparison of this uptake with endocytosis of fluorescently labeled dextranes indicates that the main uptake of the guanidinium peptoids occurs between 30-60 min after the start of incubation and clearly precedes endocytosis. Dual visualization with the endosomal marker FM4-64 shows that the intracellular guanidinium peptoid is distinct from endocytotic vesicles. Once the polyguanidine peptoids have entered the cell, they associate with actin filaments and microtubules. By pharmacological manipulation of the cytoskeleton we tested whether the association with the cytoskeleton is necessary for uptake, and observed that the actin inhibitor latrunculin B as well as the microtubule inhibitor oryzalin impaired uptake and intracellular spread of the guanidinium carrier to a certain extent. These findings are discussed with respect to the potential mechanisms of uptake and with respect to the potential of Trojan peptoids as tools for metabolic engineering in plant biotechnology.

Hydrazone- and hydrazide-containing N-substituted glycines as peptoid surrogates for expedited library synthesis: application to the preparation of Tsg101-directed HIV-1 budding antagonists

Replacing the Pro6 in the p6(Gag)-derived 9-mer "P-E-P-T-A-P-P-E-E" with N-substituted glycine (NSG) residues is problematic. However, incorporation of hydrazone amides ("peptoid hydrazones") can be readily achieved in library fashion. Furthermore, reduction of these hydrazones to N-substituted "peptoid hydrazides" affords a facile route to library diversification. This approach is demonstrated by application to Tsg101-binding compounds designed as potential HIV budding antagonists. [reaction: see text]

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.

A novel and selective beta-melanocyte-stimulating hormone-derived peptide agonist for melanocortin 4 receptor potently decreased food intake and body weight gain in diet-induced obese rats

alphaMSH has generally been accepted as the endogenous ligand for melanocortin 4 receptor (MC4R), which plays a major role in energy homeostasis. Targeting MC4R to develop antiobesity agents, many investigators have performed a structure-activity relationship (SAR) studies based on alphaMSH structure. In this report, we performed a SAR study using human betaMSH (5 - 22) (DEGPYRMEHFRWGSPPKD, peptide 1) as a lead sequence to develop potent and selective agonists for MC4R and MC3R. The SAR study was begun with a truncation of N terminus of betaMSH (5 - 22) together with acetylation of the N terminus and amidation of the C terminus of the peptide. Introduction of a cyclic disulfide constrain and replacement of L-Phe with D-Phe afforded a super potent agonist (peptide 5). Furthermore truncation at the C terminus generated a small and potent MC4R and MC3R agonist (Ac-YRcyclo[CEHdFRWC]amide, peptide 6), which exhibited no MC5R and greatly reduced MC1R activity. Molecular modeling of Ac-YRcyclo[CEHdFRWC]amide (peptide 6) revealed that Arg2 in the peptide formed a salt bridge with Glu4. Subcutaneous or intracerebroventricular administration of peptide 6 in rats showed potent in vivo efficacy as evidenced by its effects in reducing energy balance, increasing fat use, and decreasing weight gain in both acute and chronic rat metabolic studies. Furthermore, the antiobesity effect by peptide 6 was manifested only in wild-type but not MC4R-deficient mice, indicating that antiobesity effects of the peptide were attributed largely through MC4R but not MC3R agonist activity of the peptide.

N-alkylated dipeptide amides and related structures as imitations of the melanocortins' active core

Thirty-three low molecular mass structures combining both peptide and peptoid features were prepared and tested on human melanocortin receptors MC1,3-5R. Most of them displayed low micromolar activity with preference for diamines, guanidino and 2-naphthyl derivatives compared to monoacetylated, amino and 3-indolyl counterparts. Some contained L- or D-histidine residues, but the change did not influence affinity. QSAR modelling yielded excellent models for the MC3-5 receptors explaining R2Y=0.89-0.91 and predicting Q2=0.77-0.80 of the affinity variations. One compound displayed MC1R selectivity (13-fold and more). An NMR study of showed that it exists as a mixture of four rotamers at its tertiary amide bonds. Comparisons with earlier data for melanocortin core tetrapeptide analogues indicate that the novel peptide-peptoids interact with the melanocortin receptors in a different way.

Peptoid-peptide hybrids as potent novel melanocortin receptor ligands

All possible peptoid-peptide hybrids of an MC4 receptor agonist were synthesized and investigated on cells expressing different melanocortin (MC) receptor subtypes and for rat grooming behavior. In general, receptor selectivity remained while affinity and potency were decreased. The length of the functional group of Trp was more important for MC3 and MC5 than for MC4 receptor binding. In general, the potency of the peptoid-peptide hybrids to increase rat excessive grooming behavior correlated well with MC4 receptor pharmacology.