Peptides serve as effective drugs and the contrast agents in the clinic today. However the inherent drawbacks of peptide structures can limit their efficacy as drugs. To overcome this we have been developing new methods to create ‘tailor-made’ peptides and peptide mimetics with improved pharmacological and physical properties. In this work we introduce novel peptide and small molecule conjugated molecules for the earlier diagnosis and treatment of melanoma.
Melanotropin; Human Melanocortin Receptors; Melanoma; Selective Ligands for MC receptors; MTII; THIQ
The major pharmacophore for the melanocortin 1,3,4 and 5 receptors is the sequence -His-Phe-Arg-Trp-. There is a need for potent, biologically stable, receptor selective ligands, both agonists and antagonists, for these receptors. In this report we briefly examine the structural and biophysical approaches we have taken to develop selective agonist and antagonist ligands that can cross (or not) the blood brain barrier. Remaining questions and unmet needs are also discussed.
Melanocortin Receptor Ligands SAR; Selective Ligands for MCRs; Melanocortin Receptor Selective Ligands; Orthosteric and Allosteric Ligands; Agonists and Antagonists for MCRs
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.
α-MSH; Agouti-related protein; human melanocortin receptors; hybrid pharmacophore; macrocyclic peptide
Melanocortin 4 receptor (MC4R) plays an important role in the regulation of food intake and glucose homeostasis. Synthetic nonpeptide compound N- (3R)-1 4-tetrahydroisoquinolinium-3-ylcarbonyl -(1R)-1-(4-chlorobenzyl)-2- 4-cyclohexyl-4-(1H-1,2,4-triazol-1-ylmethyl)piperidin-1-yl -2-oxoethylamine (THIQ) is a potent agonist at MC4R but not at hMC2R. In this study, we utilized two approaches (chimeric receptor and site-directed mutagenesis) to narrow down the key amino acid residues of MC4R responsible for THIQ binding and signaling. Cassette substitutions of the second, third, fourth, fifth, and sixth transmembrane regions (TMs) of the human MC4R (hMC4R) with the homologous regions of hMC2R were constructed. Our results indicate that the cassette substitutions of these TMs of the hMC4R with homologous regions of the hMC2R did not significantly alter THIQ binding affinity and potency except the substitution of the hMC4R TM3, suggesting that the conserved amino acid residues in these TMs of the hMC4R are main potential candidates for THIQ binding and signaling while non conserved residues in TM3 of MC4R may also be involved. Nineteen MC4R mutants were then created, including 13 conserved amino acid residues and 6 non-conserved amino acid residues. Our results indicate that seven conserved residue [E100 (TM2), D122 (TM3), D126 (TM3), F254 (TM6), W258 (TM6), F261 (TM6), H264 (TM6)] are important for THIQ binding and three non-conserved residues [N123 (TM3), I129 (TM3) and S131 (TM3)] are involved in THIQ selectivity. In conclusion, our results suggest that THIQ utilize both conserved and non-conserved amino acid residues for binding and signaling at hMC4R and non conserved residues may be responsible for MC4R selectivity.
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.
Agouti-signalling protein; melanocortin receptors; cyclodimerization; C2-symmetry; macrocyclic peptide
Multiple N-methylation is a novel technology to improve bioavailability of peptides and increase receptor subtype selectivity. This technique has been applied here to the superpotent but non-selective cyclic peptide MT-II. A library of all possible 31 backbone N-methylated derivatives has been synthesized and tested for binding and activation at melanocortin receptor subtypes 1, 3, 4 and 5. It turned out that selectivity is improved with every introduced N-methyl group, resulting in several N-methylated selective and potent agonists for the hMC1R. The most potent of these derivatives is N-methylated on four out of five amide bonds in the cyclic structure. Its solution structure indicates a strongly preferred backbone conformation which resembles other a-MSH analogs but possesses much less flexibility and in addition distinct differences in the spatial arrangement of individual amino acid side chains.
The melanocortin receptor (MCR) subtype family is a member of the GPCR superfamily and each of them has a different pharmacological profile regarding the relative potency of the endogenous and synthetic melanocortin peptides. α-MSH and ACTH are endogenous nonselective agonists for MC1R, MC3R, MC4R and MC5R. In this study, we examined the role of Phe7 in ACTH on Human (h)MC1R, MC3R and MC4R binding and signaling. Our results indicate that substitution of the Phe7 with DNal (2’)7 in ACTH1-24 has different pharmacological profile from that of substitution of the Phe7 with DNal (2’)7 in MSH at hMC1R, hMC3R and hMC4R. NDNal (2’)7-ACTH1-24 is an agonist at hMC3R and hMC4R which did not switch peptide from agonist to antagonist at hMC3R and hMC4R. Further experiments indicate that NDNal (2’)7-ACTH1-17 is the minimal peptide required for hMC3R and hMC4R activation. Single amino acid substitution studies of DNal (2’)7 ACTH1-17 indicate that the amino acid residues 15, 16 and 17 in NDNal (2’)7-ACTH1-17 are crucial for hMC3R and hMC4R activation. Substitutions of these amino acid residues reduced or abolished agonist activity at hMC3R and hMC4R. Conformational studies revealed a new β-turn (-Arg8-Trp9-Gly10-Lys11-) in NDNal (2’)7-ACTH1-17, compared to the β-turn like structure at NDP-α-MSH (–His6-DPhe7-Arg8-Trp9-). Our results suggest that NDP-α-MSH and NDNal (2’)7-ACTH1-17 does not share the same binding site; highly basic C terminal fragment Lys15-Lys16-Arg17 of NDNal (2’)7-ACTH1-17 induced a new β-turn and this shift contributed the selective agonist activity at hMC3R and hMC4R.
POMC; MCR; agonist; GPCR; receptor activation
A new series of melanotropin analogues with His or Arg residues in the core pharmacophores of MTII, SHU9119 and Ac-NDP-γ-MSH-NH2 replaced by Pro or trans-/cis- 4-guanidinyl-Pro derivatives were designed and synthesized to introduce selectivity toward the human melanocortin 4 receptor (hMC4R). Analogues 1, 2, 3, 6, 7, 8 were found to be hMC4R selective. Second messenger studies have demonstrated that analogues 1 and 2 are insurmountable inhibitors of MTII agonist activity at the hMC4R. Molecular modeling studies suggest that the hMC4R selectivity is due to a β-turn shift induced by the Pro ring that makes the global minimum structures of these analogues resemble the NMR solution structure of the hASIP melanocortin receptor binding motif. Substitution of His in MTII also provided functional selectivity for the hMC3R or the hMC4R. These findings are important for a better understanding of the selectivity mechanism at the hMC3R/hMC4R, and the development of therapeutic ligands selectively targeting the hMC4R.
A procedure has been developed for directly depositing membrane fragments derived from bacterial (chromatophores from Rhodopseudomonas sphaeroides) and mammalian cells (μ-opioid receptor- and MC4 receptor-transfected HEK cells, and rat trigeminal ganglion cells) onto the silica surface of a plasmon-waveguide resonance (PWR) spectrometer. Binding of ligands (cytochrome c2 for the chromatophores, the peptide agonists DAMGO and Melanotan-II that are specific for the μ-opioid and MC4 receptors, and two non-peptide agonists that are specific for the CB1 receptor) to these membrane fragments has been observed and characterized with high sensitivity using PWR spectral shifts. The KD values obtained are in excellent agreement with conventional pharmacological assays and with prior PWR studies using purified receptors inserted into deposited lipid bilayer membranes. These studies provide a new tool for obtaining useful biological information about receptor-mediated processes in real biological membranes.
G-protein coupled receptors; bacterial chromatophores; transfected HEK cells; rat trigeminal ganglion; μ-opioid receptor; cannabinoid CB1 receptor; melanocortin-4 receptor
Differentiation of the physiological role of the melanocortin receptor 5 MC5R from that of other melanocortin receptors will require development of high affinity and selective antagonists. To date, a few synthetic antagonist ligands active at hMC5 receptor are available, but most do not have appreciable selectivity. With the aim to gain more potent and selective antagonists for the MC5R ligands, we have designed, synthesized, and pharmacologically characterized a series of alkylthioaryl-bridged macrocyclic peptide analogues derived from MT-II and SHU9119. These 20-membered macrocycles were synthesized by a tandem combination using solid phase peptide synthesis and microwave-assisted reactions. Biological assays for binding affinities and adenylate cyclase activities for the hMC1R, hMC3R, hMC4R, and hMC5R showed that three analogues, compounds, 9, 4, and 7, are selective antagonists at the hMC5 receptor. In particular, compound 9 (PG-20N) is a selective and competitive hMC5R antagonist, with IC50 of 130 ± 11 nM, and a pA2 value of 8.3, and represents an important tool for further biological investigations of the hMC5R. Compounds 4 and 7 (PG14N, PG17N) show potent and selective allosteric inhibition at hMC5R with IC50 values of 38 ± 3 nM and 58 ± 6 nM, respectively. Compound 9 will be used to further investigate and more clearly understand the physiological roles played by the MC5 receptor in humans and other animals.
Melanocortin 4 receptor (MC4R) plays an important role in the regulation of food intake and body weight. To determine the molecular basis of human MC4R (hMC4R) responsible for α-melanocortin-stimulating hormone (α-MSH) binding, in this study, we utilized both receptor domain exchange and site-directed mutagenesis studies to investigate the molecular determinants of hMC4R responsible for α-MSH binding and signaling. α-MSH is a potent agonist at hMC4R but not at hMC2R. Cassette substitutions of the second, third, fourth, fifth, and sixth transmembrane regions (TM) of the hMC4R with the homologous regions of hMC2R were performed and α-MSH binding and signaling were examined. Our results indicate that each chimeric receptor was expressed at the cell surface and the expression levels remain similar to that of the wild-type receptor. The cassette substitutions of the second, fourth, fifth, and sixth TMs of the hMC4R with homologous regions of the hMC2R did not significantly alter α-MSH binding affinity and potency except substitution of the TM3 of the hMC4R, suggesting that the conserved residues in TMs of the hMC4R are crucial for α-MSH binding and signaling. Further mutagenesis studies indicate that conserved residues Glu100 in TM2, Asp122, Asp126 in TM3 and Trp258, Phe261, His264 in TM6 are involved in α-MSH binding and signaling. In conclusion, our results suggest that the conserved residues in the TM2, TM3, and TM6 of the hMC4R are responsible for α-MSH binding and signaling.
The melanocortin receptor (MCR) subtype family is a member of the GPCR superfamily and each of them has a different pharmacological profile regarding the relative potency of the endogenous and synthetic melanocortin peptides. Substitution of Trp with DNaI (2′) in -γ-MSH resulted in the loss of binding affinity and potency at hMC4R. However, the molecular mechanism of this ligand selectivity is unclear. In this study, we utilized chimeric receptors and site-directed mutagenesis approaches to investigate the molecular basis of MC4R responsible for peptide [Pro5, DNaI (2′)8]-γ-MSH selectivity. Cassette substitutions of the second, third, fourth, fifth, and sixth TM of the human MC4R (hMC4R) with the homologous regions of hMC1R were constructed and the binding affinity of peptide [Pro5, DNaI (2′)8]-γ-MSH at these chimeric receptors was evaluated. Our results indicate that the cassette substitutions of TM2, TM3, TM4 and TM5 of hMC4R with homologous regions of the hMC1R did not significantly increase peptide [Pro5, DNaI (2′)8]-γ-MSH binding affinity and potency but substitution of the TM6 of the hMC4R with the same region of the hMC1R significantly enhances [Pro5, DNaI (2′)8]-γ-MSH binding affinity and potency. Further site-directed mutagenesis study indicates that four amino acid residues, Phe267, Tyr268, Ile269 and Ser270, in TM6 of the hMC4R may play an important role in [Pro5, DNaI (2′)-γ-MSH selective activity at MC4R.
MC4R; γ-MSH; MC1R; Obesity; GPCR; Agonist
We have identified compound 1 as a novel ligand for opioid and melanocortin (MC) receptors, which is derived from the overlapping of a well known structure for the δ opioid receptor, 2,6-dimethyltyrosine (Dmt)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic), and a small molecule for the MC receptor, Tic-DPhe(p-Cl)-piperidin-4-yl-N-phenyl-propionamide. Ligand 1 showed that there is an overlapping pharmacophore between opioid and MC receptors through the Tic residue. The ligand displayed high biological activities at the δ opioid receptor (Ki = 0.38 nM in binding assay, EC50 = 0.48 nM in GTP-γ-S binding assay, IC50 = 74 nM in MVD) as an agonist instead of an antagonist and showed selective binding affinity (IC50 = 2.3 μM) at the MC-3 receptor rather than at the MC-5 receptor. A study of the structure-activity relationships demonstrated that the residues in positions 2, 3, and the C-terminus act as a pharmacophore for the MC receptors, and the residues in positions 1 and 2 act as a pharmacophore for the opioid receptors. Thus, this structural construct can be used to prepare chimeric structures with adjacent or overlapping pharmacophores for opioid and MC receptors.
opioid receptor; melanocortin receptor; anti-opioid effect; multi-target drug; overlapping pharmacophores; antinociception; side effect; Dmt-Tic; fentanyl
Hypothalamic POMC neurons regulate energy balance via interactions with brain melanocortin receptors (MC-Rs). POMC neurons express the MC3-R which can function as an inhibitory autoreceptor in vitro. We now demonstrate that central activation of MC3-R with icv infusion of the specific MC3-R agonist, [D-Trp8]-γ-MSH, transiently suppresses hypothalamic Pomc expression and stimulates food intake in rats. Conversely, we also show that icv infusion of a low dose of a selective MC3-R antagonist causes a transient decrease in feeding and weight gain. These data support a functional inhibitory role for the MC3-R on POMC neurons that leads to changes in food intake.
POMC; MSH; melanocortin-3-receptor; feeding
A variety of dicarboxylic acid linkers introduced between the α-amino group of Pro6 and the ε-amino group of Lys10 of the cyclic lactam α-melanocyte-stimulating hormone (α-MSH)-derived Pro6-D-Phe7/D-Nal(2′)7-Arg8-Trp9-Lys10-NH2 pentapeptide template lead to nanomolar range and selective hMC3R agonists and antagonists. Replacement of the Pro6 residue and the dicarboxylic acid linker with 2,3-pyrazine-dicarboxylic acid furnished a highly selective nanomolar range hMC3R partial agonist (analogue 12, c[CO-2,3-pyrazine-CO-D-Phe-Arg-Trp-Lys]-NH2, EC50 = 27 nM, 70% max cAMP) and an hMC3R antagonist (analogue 13, c[CO-2,3-pyrazine-CO-D-Nal(2′)-Arg-Trp-Lys]-NH2, IC50 = 23 nM). Modeling experiments suggest that 2,3-pyrazinedicarboxylic acid stabilizes a β-turn-like structure with the D-Phe/D-Nal(2′) residues, which explains the high potency of the corresponding peptides. Placement of a Nle residue in position 6 produced a hMC3R/hMC5R antagonist (analogue 15, c[CO-(CH2)2-CO-Nle-D-Nal(2′)-Arg-Trp-Lys]-NH2, IC50 = 12 and 17 nM, respectively), similarly to the previously described cyclic γ-melanocyte-stimulating hormone (γ-MSH)-derived hMC3R/hMC5R antagonists. These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-3 receptor.
Melanocortin hormones and neurotransmitters regulate a vast array of physiologic processes by interacting with five G-protein-coupled melanocortin receptor types. In the present study, we have systematically studied the regulation of individual human melanocortin receptor wild subtypes using a synthetic rhodamine-labeled human melanotropin agonist and antagonist, arrestins fused to green fluorescent protein in conjunction with two-photon fluorescence laser scanning microscopy and confocal microscopy. Stimulation of the melanocortin receptors by its cognate agonist triggered rapid arrestin recruitment and receptor internalization for all four human melanocortin receptors examined. Antagonists-bound melanocortin receptors, on the other hand, did not recruit β-arrestins, and remained in the cell membrane even after long-term (30 min) treatment. Agonist-mediated internalization of all melanocortin receptor subtypes was sensitive to inhibitors of clathrin-dependent endocytosis, but not to caveolae inhibitors. In summary, agonist-mediated internalization of all subtypes of melanocortin receptors are dependent upon β-arrestin-mediated clathrin-coated pits, whereas, β-arrestin-2 conjugated green fluorescence protein (β-arrestin-2-GFP) recruitment is not dependent on protein kinase A activation. Real time two-photon fluorescence laser scanning microscopy is a most powerful tool to study the dynamic processes in living cells and tissues, without inflicting significant and often lethal damage to the specimen.
β-arrestin; green fluorescence protein; melanocortin receptors; MTII; rhodamine; SHU-9119; two-photon fluorescence laser scanning microscopy
In search of new selective antagonists and/or agonists for the human melanocortin receptor subtypes hMC1R to hMC5R to elucidate the specific biological roles of each GPCR, we modified the structures of the superagonist MT-II (Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH2) and the hMC3R/hMC4R antagonist SHU9119 (Ac-Nle-c[Asp-His-D-Nal(2′)-Arg-Trp-Lys]-NH2) by replacing the His-D-Phe and His-D-Nal(2′) fragments in MT-II and SHU9119, respectively, with Aba-Xxx (4-amino-1,2,4,5-tetrahydro-2-benzazepin-3-one-Xxx) dipeptidomimetics (Xxx = D-Phe/pCl-D-Phe/D-Nal(2′)). Employment of the Aba mimetic yielded novel selective high affinity hMC3R and hMC3R/hMC5R antagonists.
Human melanocortin receptors; 4-Amino-1,2,4,5-tetrahydro-2-benzazepin-3-ones; Cyclic lactam analogues; Conformational restrictions; hMC3R/hMC5R antagonists
Recently we have demonstrated that replacing His6 by constrained amino acids in the well known antagonist SHU9119 resulted in potent and selective antagonist ligands especially at the hMC3R and hMC4 receptors. With the aim to further explore position 6 in the sequence of SHU9119 and MT-II, we have designed, synthesized, and pharmacologically characterized a series of peptide analogues of MT-II and SHU9119 at the human melanocortin receptors subtypes MC3R, MC4R and MC5R. All these peptides were modified at position 6 with constrained amino acids which are commercially available. In this study we have identified new selective ligands for the hMC4R, and an antagonist for the hMC3/hMC4 receptors. Additionally, we have discovered an interesting new selective antagonist at the hMC3R, Ac-Nle-c[Asp-βAla-DNal(2′)-Arg-Trp-Lys]-NH2 (2, PG-106) which represents an important tool in further biological investigations of the hMC3R. PG-106 will be useful in further efforts to differentiate the substructural features responsible for selectivity at the hMC3R, hMC4R, and hMC5R.
Melanocortin receptors; Melanotropins; Structure-activity relationships; Cyclic melanotropins; Receptor selective melanotropin antagonists
The processed products of the proopiomelanocortin gene (ACTH, α-MSH, β-MSH, γ-MSH, etc.) interact with five melanocortin receptors, the MC1R, MC2R, MC3R, MC4R, and MC5R to modulate and control many important biological functions crucial for good health both peripherally (as hormones) and centrally (as neurotransmitters). Pivotal biological functions include pigmentation, adrenal function, response to stress, fear/flight, energy homeostasis, feeding behavior, sexual function and motivation, pain, immune response, and many others, and are believed to be involved in many disease states including pigmentary disorders, adrenal disorders, obesity, anorexia, prolonged and neuropathic pain, inflammatory response, etc. The roeianocortin-3 receptor (MC3R) is found primarily in the brain and spinal cord and also in the periphery, and its biological functions are still not well understood. Here we review some of the biological functions attributed to the MC3R, and then examine in more detail efforts to design and synthesize ligands that are potent and selective for the MC3R, which might help resolve the many questions still remaining about its function. Though some progress has been made, there is still much to be done in this critical area.
The effects of the linker arm rigidity and size on melanocortin receptor selectivity were explored in a series of compounds using cyclic lactam α-melanocyte-stimulating hormone template. A variety of dicarboxylic acid linkers introduced between the α-amino group of His6 and the ɛ-amino group of Lys10 lead to high-affinity, selective human melanocortin receptor-1 and -5 (hMC1R and hMC5R) antagonists. The incorporation of hydrophilic functions into the linker arm was found to be unfavorable for both binding potency and receptor selectivity. Analogs 8 and 9 containing highly conformationally constrained hydrophobic linkers (m- and p-phthalic acids) were found to be selective nanomolar range hMC1R antagonists (IC50 = 7 and 4 nM, respectively), whereas the employment of a small conformationally constrained linker (maleic acid) resulted in a high-affinity (IC50 = 19 nM) and selective hMC5R antagonist (analog 12). These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-1 and -5 receptors.
α-melanocyte-stimulating hormone; antagonist; human melanocortin-1 receptor; human melanocortin-5 receptor; macrocyclic; melanocortin; peptide; All, allyl; Alloc, allyloxycarbonyl; Boc, tert-butyloxycarbonyl; Fmoc, fluorenylmethoxycarbonyl; CH3CN, acetonitrile; DCM, dichloromethane; DIPEA, diisopropylethylamine; DMF, N,N-dimethylformamide; DIC, diisopropyl carbodiimide; HBTU, 2-(1H-benzotriazole-1-yl)-1, 1, 3, 3-tetramethyluronium hexafluorophosphate; HOBt, N-hydroxybenzotriazole; hMCR, human melanocortin receptor; MSH, melanocyte-stimulating hormone; Nal(2′), 2′-naphthylalanine; Pbf, 2, 2, 4, 6, 7-pentamethyldihydrobenzofuran-5-sulfonyl; PyBOP, benzotriazol-1-yloxy-tris-pyrrolidinophosphonium hexafluorophosphate; TFA, trifluoroacetic acid; Trt, trityl; SPPS, solid-phase peptide synthesis; RP-HPLC, reverse-phase high-performance liquid chromatography; hMC1R, human melanocortin-1 receptor; α-MSH, Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2; NDP-α-MSH, Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2
A new bicyclic template has been developed for the synthesis of peptide
mimetics. Straightforward synthetic steps, starting from amino acids, allow the
facile construction of a wide range of analogs. This system was designed to
target the melanocortin receptors (MCRs), with functional group selection based
on a known pharmacophore and guidance from molecular modeling to rationally
identify positional and stereochemical isomers likely to be active. The
functions of hMCRs are critical to myriad biological activities, including
pigmentation, steroidogenesis, energy homeostasis, erectile activity, and
inflammation. These G-protein-coupled receptors (GPCRs) are targets for drug
discovery in a number of areas, including cancer, pain, and obesity
therapeutics. All compounds from this series tested to date are antagonists
which bind with high affinity. Importantly, many are highly selective for a
particular MCR subtype, including some of the first completely hMC5R-selective
Melanocortins; Peptide mimetics; GPCRs
Intensive efforts have been made to develop potent and selective ligands
for certain human melanocortin receptors as possible treatments for obesity and
sexual dysfunction due to the role of these receptors in feeding behavior,
energy homeostasis, sexual function, etc. A number of novel α-MSH
analogues were designed and synthesized primarily on the basis of our previous
MTII NMR structure. In these peptide analogues, a disulfide or lactam bridge
between residues at positions 5 and 8 was used as a conformational constraint to
enhance the β-turn spanning His6 and
d-Phe7, while the pharmacophore group in Arg8 was
mimicked via Nα-alkylation of residues 8
or 9 with the guanidinylbutyl group. Biological assays for binding affinities
and adenylate cyclase activities for the hMC1R,
hMC3R, hMC4R, and hMC5R
showed that three analogues have good binding affinity for the
hMC4R (0.7–4.1 nM), but have no binding affinity up
to 10 μM at the other three melanocortin receptors. Interestingly,
the three hMC4R selective analogues display only 50% binding
efficiency, suggesting there is allosteric modulation of the melanocortin-4
receptor. These analogues were found to act as antagonists of the
hMC4R. This result represents a discovery of very selective
peptide-based antagonists for the hMC4R. The high selectivity
may be due to the strong conformational constraint via ring contraction as
compared to MTII, and the rigid conformation preferred by these new ligands
allows them to recognize only the hMC4R, but not to activate the second
messenger. The MTII NMR structure-based design thus not only examined the
structural model of melanocortin ligands, but also yielded new biologically
unique α-MSH analogues.
New modalities providing safe and effective treatment of pain, especially prolonged pathological pain, have not appeared despite much effort. In this mini-review/overview we suggest that new paradigms of drug design are required to counter the underlying changes that occur in the nervous system that may elicit chronic pain states. We illustrate this approach with the example of designing, in a single ligand, molecules that have agonist activity at μ and δ opioid receptors and antagonist activities at cholecystokinin (CCK) receptors. Our findings thus far provide evidence in support of this new approach to drug design. We also report on a new biophysical method, plasmon waveguide resonance (PWR) spectroscopy, which can provide new insights into information transduction in G-protein coupled receptors (GPCRs) as illustrated by the δ opioid receptor.
drug design; neuropathic pain; bifunctional ligands; plasmon waveguide resonance spectroscopy; GPCRs; opioid receptors; cholecystokinin receptors
A series of cyclic lactam analogues of γ-MSH (H-Tyr1-Val2-Met3-Gly4-His5-Phe6-Arg7-Trp8-Asp9-Arg10-Phe11-Gly12-OH) with a bulky hydrophobic residue in the direct proximity to the pharmacophore (Xaa-d-Phe/d-Nal(2′)-Arg-Trp) were designed and synthesized by solid-phase methods. A variety of amino acids with a broad range of hydrophobic/hydrophilic properties was introduced in position 5 to further explore their complementary role in receptor selectivity. Biological evaluation of these peptides revealed several analogues with potent hMC3R agonist and hMC3R/hMC5R antagonist activities, and good receptor selectivity. Analogue 4, c[Nle-Arg-d-Phe-Arg-Trp-Glu]-NH2, was found to be a very potent and selective hMC3R agonist (EC50 = 1.2 nM, 112% act). In addition, analogue 13, c[Nle-Val-d-Nal(2′)-Arg-Trp-Glu]-NH2, was identified as an hMC3R/hMC5R antagonist with the best selectivity against the hMC4R in this series (pA2(hMC3R) = 8.4; pA2(hMC5R) = 8.7). These results indicate the significance of steric factors in melanocortin receptor selectivity and suggest that introduction of bulky residues in the direct proximity to the melanocortin pharmacophore is an effective approach to design of novel hMC3R and hMC5R selective ligands.
Cyclic melanotropin peptides, designed with an aromatic amino acid substitution at the N-terminal position of the MT-II-type scaffold, were prepared by solid-phase peptide synthesis and evaluated for their ability to bind to and activate human melanocortin-1, -3, -4, and -5 receptors. The structure–activity studies of these MT-II analogues have identified a selective antagonist at the hMC4R (H-Phe-c[Asp-Pro-d-Nal(2′)-Arg-Trp-Gly-Lys]-NH2, pA2 = 8.7), a selective partial agonist at the hMC4R (H-d-Nal(2′)-c[Asp-Pro-d-Phe-Arg-Trp-Gly-Lys]-NH2, IC50 = 11 nM, EC50 = 56 nM), and a selective partial agonist at the hMC3R (H-D-Phe-c[Asp-Pro-d-Phe-Arg-Trp-Lys]-NH2, IC50 = 3.7 nM, EC50 = 4.9 nM). Aromatic amino acid substitution at the N-terminus in conjuction with the expansion of the 23-membered cyclic lactam MT-II scaffold to a 26-membered scaffold by addition of a Gly residue in position 10 leads to melanotropin peptides with enhanced receptor selectivity.
Melanotropins; N-Terminal aromatic amino acid substitution; Cyclic peptide lactams; Structure-activity; Receptor selectivity; Melanotropin receptors