Synthesis and receptor binding of opioid peptide analogues containing β3‐homo‐amino acids

β‐Amino acids containing hybrid peptides and β‐peptides show great potential as peptidomimetics. In this paper we describe the synthesis and affinity toward the µ‐ and δ‐opioid receptors of β‐peptides, analogues of Leu‐enkephalin, deltorphin I, dermorphin and α,β‐hybrides, analogues of deltorphin I. Substitution of α‐amino acid residues with β³‐homo‐amino acid residues, in general resulted in decrease of affinity to opioid receptors. However, the incorporation β³h‐D ‐Ala in position 2 or β³hPhe in position 3 of deltorphin I resulted in potent and selective ligand for δ‐opioid receptor. The NMR studies of β‐deltorphin I analogue suggest that conformational motions in the central part of the peptide backbone are partially restricted and some conformational preferences can be expected. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Use of hydrophilic diamines for bridging of two opioid peptide pharmacophores. Synthesis and receptor binding of two new analogues.

The bivalent ligand approach, which assumes that two pharmacophores are connected by a spacer, was used to design receptor type-selective ligands for opioid receptors. The first two opioid peptide bivalent ligands with different spacer lengths containing different numbers of hydroxyl groups, (Tyr-D-Ala-Gly-Phe-NH-CH2-CHOH-)2 (Tyr-D-Ala-Gly-Phe-NH-CH2-CHOH-CHOH-)2, were synthesized and their binding to mu, delta, and kappa opioid receptors was characterized. Both analogues were found to possess high opioid in vitro activities. The length of the hydrophilic spacer does not affect the affinity for delta receptors, whereas shorter spacer length increases affinity for mu and even more so for kappa receptors. Thus receptor type-selective peptides for opioid receptors can be designed using the bivalent approach.     

6-Hydroxy-1,2,3,4-tetrahydro-isoquinoline-3-carboxylic acid mimics active conformation of tyrosine in opioid peptides

6-Hydroxy-1,2,3,4-tetrahydro-isoquinoline-3-carboxylic acid (6Htc) has been proposed as a rigid mimic of tyrosine conformation in opioid ligand-receptor complex. The significant receptor binding to mu and delta opioid receptors of respective analogues of deltorphin, dermorphin, and endomorphin with D,L-6Htc prove initial prediction.     

Synthesis, conformation and biological activity of dermorphin and deltorphin I analogues containing N-alkylglycine in place of residues in position 1, 3, 5 and 6.

Syntheses are described of new dermorphin and [D-Ala2]deltorphin I analogues in which the phenylalanine, the tyrosine or the valine residues have been substituted by the corresponding N-alkylglycine residues. Structural investigations by CD measurements in different solvents and preliminary pharmacological experiments were carried out on the resulting peptide-peptoid hybrids. The contribution from aromatic side chain residues is prominent in the CD spectra of dermorphin analogues and the assignment of a prevailing secondary structure could be questionable. In the CD spectra of deltorphin analogues the aromatic contribution is lower and the dichroic curves indicate the predominance of random conformer populations. The disappearance of the aromatic contribution in the [Ntyr1,D-Ala2]-deltorphin spectrum could be explained in terms of high conformational freedom of the N-terminal residue. The kinetics of degradation of the synthetic peptoids digestion by rat and human plasma enzymes were compared with that of [Leu5]-enkephalin. The binding to opioid receptors was tested on crude membrane preparations from CHO cells stably transfected with the mu- and delta-opioid receptors. The biological potency of peptoids was compared with that of dermorphin in GPI preparations and with that of deltorphin I in MVD preparations. All the substitutions produced a dramatic decrease in the affinity of the peptide-peptoid hybrids for both the mu- and delta-opioid receptors. Nval5 and/or Nval6 containing hybrids behaved as mu-opioid receptor agonists and elicit a dose-dependent analgesia (tail-flick test) when injected i.c.v. in rats.     

Topographical requirements for delta opioid ligands: presence of a carboxyl group in position 4 is not critical for deltorphin high delta receptor affinity and analgesic activity.

To investigate the role of the carboxyl group in deltorphin molecules, we have synthesized three new analogues in which the acidic amino acid residues in position 4 of the deltorphins were replaced by non-acidic but hydrophilic amino acids residues. The three analogues, [Ser4]-, [Gln4]-, and [Cys4]-deltorphin, all are as potent or more potent than either deltorphin I or II at delta opioid receptors and possess good delta selectivities. The excellent correlation between their in vitro delta receptor potencies and their intrathecal antinociception activity forms a strong argument for involvement of those receptors in spinal nociceptive modulation in the rats.     

Ligand Photo-Isomerization Triggers Conformational Changes in iGluR2 Ligand Binding Domain

Neurological glutamate receptors bind a variety of artificial ligands, both agonistic and antagonistic, in addition to glutamate. Studying their small molecule binding properties increases our understanding of the central nervous system and a variety of associated pathologies. The large, oligomeric multidomain membrane protein contains a large and flexible ligand binding domains which undergoes large conformational changes upon binding different ligands. A recent application of glutamate receptors is their activation or inhibition via photo-switchable ligands, making them key systems in the emerging field of optochemical genetics. In this work, we present a theoretical study on the binding mode and complex stability of a novel photo-switchable ligand, ATA-3, which reversibly binds to glutamate receptors ligand binding domains (LBDs). We propose two possible binding modes for this ligand based on flexible ligand docking calculations and show one of them to be analogues to the binding mode of a similar ligand, 2-BnTetAMPA. In long MD simulations, it was observed that transitions between both binding poses involve breaking and reforming the T686-E402 protein hydrogen bond. Simulating the ligand photo-isomerization process shows that the two possible configurations of the ligand azo-group have markedly different complex stabilities and equilibrium binding modes. A strong but slow protein response is observed after ligand configuration changes. This provides a microscopic foundation for the observed difference in ligand activity upon light-switching.     

N‐terminal guanidinylation of the cyclic 1,4‐ureido‐deltorphin analogues: the synthesis,receptor binding studies,and resistance to proteolytic digestion

The synthesis of a series of N‐guanidinylated cyclic ureidopeptides, analogues of 1,4‐ureido‐deltorphin/dermorphine tetrapeptide is described. The δ‐ and μ‐opioid receptor affinity of new guanidinylated analogues and their non‐guanidinylated precursors was determined by the displacement radioligand binding experiments. Our results indicate that the guanidinylation of cyclic 1,4‐ureidodeltorphin peptide analogues does not exhibit a uniform influence on the opioid receptor binding properties, similarly as reported earlier for some linear peptides. All analogues were also tested for their in vitro resistance to proteolysis during incubation with large excess of chymotrypsin, pepsin, and papain by means of mass spectroscopy. Guanidinylated ureidopeptides 1G–4G showed mixed μ agonist/δ agonist properties and high enzymatic stability indicating their potential as therapeutic agents for treatment of pain. Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.     

Structure activity relationship studies of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(isoquinoline-3′-carboxamido)morphinan (NAQ) analogues as potent opioid receptor ligands: Preliminary results on the role of electronic characteristics for affinity and function

17-Cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(isoquinoline-3′-carboxamido)morphinan (NAQ) was previously designed following the ‘message-address’ concept and was identified as a potent and highly selective mu opioid receptor (MOR) ligand based on its pharmacological profile. We here report the preliminary structure activity relationship (SAR) studies of this novel lead compound. For the new ligands synthesized as NAQ analogues, their binding assay results showed that a longer spacer and a saturated ring system of the side chain were unfavorable for their MOR selectivity over the kappa and delta opioid receptors. In contrast, substitutions with different electronic properties at either 1′- or 4′-position of the isoquinoline ring of the side chain were generally acceptable for reasonable MOR selectivity. The majority of NAQ analogues retained low efficacy at the MOR compared to NAQ in the ³⁵S-GTP[γS] binding assays while electron-withdrawing groups at 1′-position of the isoquinoline ring induced higher MOR stimulation than electron-donating groups did. In summary, the electronic characteristics of substituents at 1′- or 4′-position of the isoquinoline ring in NAQ seem to be critical and need to be further tuned up to achieve higher MOR selectivity and lower MOR stimulation.     

Synthesis and binding characteristics of the highly delta-specific new tritiated opioid peptide, [3H]deltorphin II.

A radiolabelled form of deltorphin II was synthesized by catalytic tritiation using [p-IPhe3]-deltorphin II as a precursor. The ligand labels rat brain membranes with a Kd value of 1.9 nM, and the Bmax was found to be 92 fmol/mg protein. This new tritiated ligand exhibits high affinity for the delta opioid binding site, whereas its binding to the mu type is weak and extremely low for the kappa type. Mu/delta and kappa/delta selectivity ratios were about 900 and 10,000, respectively. The highly delta selective binding properties of this new radioligand suggest that it could serve as an excellent tool for investigating the delta opioid receptors in various species.     

Affinity labeling of delta opioid receptors by an enkephalin-derivative alkylating agent, DSLET-Mal

Opioid binding properties of Tyr-D-Ser-Gly-Phe-Leu-Thr-NH-NH-Gly-Mal (DSLET-Mal), a novel enkephalin-framed affinity label, was determined in rat brain membranes. In competition studies the ligand showed high affinity for the delta opioid sites, labelled by [(3)H][Ile(5,6)]deltorphin II (K(i) = 8 nM), whereas its binding to the mu ([(3)H]DAMGO) and kappa ([(3)H]EKC) sites was weaker. Preincubation of the rat brain membranes with DSLET-Mal at micromolar concentrations resulted in a wash-resistant and dose-dependent inhibition of the [(3)H][Ile(5,6)]deltorphin II binding sites (96% blocking at 10 microM concentration). Intracerebroventricular (ICV) administration of DSLET-Mal reduced the density of delta opioid receptors and had no effect on mu and kappa receptors, as determined by saturation binding studies. [Ile(5, 6)]deltorphin II-stimulated [(35)S]GTPgammaS binding was determined in membrane preparations of different brain areas of the ICV-treated animals. In both frontal cortex and hippocampus DSLET-Mal significantly decreased G protein activation by the delta agonist, having no effect on DAMGO stimulated [(35)S]GTPgammaS binding. DSLET-Mal had qualitatively similar effects on both receptor binding and G protein activation. These characteristics of the compound studied suggest that DSLET-Mal can serve as an affinity label for further studies of the delta-opioid receptors.     

Conformation-activity relationships of opioid peptides with selective activities at opioid receptors

The discovery of endogenous opioid peptides 25 years ago opened up a new chapter in efforts to understand the origins and control of pain, its relationships to other biological functions, including inflammatory and other immune responses, and the relationships of opioid peptides and their receptors to a variety of undesirable or toxic side effects often associated with the nonpeptide opiates such as morphine including addiction, constipation, a variety of neural toxicities, tolerance, and respiratory depression. For these investigations the need for potent and highly receptor selective agonists and antagonists has been crucial since they in principle allow one to distinguish unequivocally the roles of the different opioid receptors (mu, delta, and kappa) in the various biological and pathological roles of the opioid peptides and their receptors. Conformational and topographical constraint of the linear natural endogenous opioid peptides has played a major role in developing peptide ligands with high selectivity for mu, delta, and kappa receptors, and in understanding the conformational, topographical, and stereoelectronic structural requirements of the opioid peptides for their interactions with opioid receptors. In turn, this had led to insights into the three-dimensional pharmacophore for opioid receptors. In this article we review and discuss some of the developments that have led to potent, selective, and stable peptide and peptidomimetic ligands that are highly potent and selective, and that have delta agonist, mu antagonist, and kappa agonist biological activities (other authors in this issue will discuss the development of other types of activities and selectivities). These have led to ligands that provide unique insight into opioid pharmacophores and the critical roles opioid ligands and receptor scan play in pain, addiction, and other human maladies.     

Design and synthesis of trivalent ligands targeting opioid,cholecystokinin, and melanocortin receptors for the treatment of pain

It has been known that co-administration of morphine with either cholecystokinin (CCK) receptor or melanocortin (MC) receptor antagonists enhance morphine’s analgesic efficacy by reducing serious side effects such as tolerance and addiction.1, 2, 3, 4 Considering these synergistic effects, we have designed trivalent ligands in which all three different pharmacophores for opioid, CCK, and MC receptors are combined in such a way as to conserve their own topographical pharmacophore structures. These ligands, excluding the cyclic compound, were synthesized by solid phase synthesis using Rink-amide resin under microwave assistance in very high yields. These trivalent ligands bind to their respective receptors well demonstrating that the topographical pharmacophore structures for the three receptors were retained for receptor binding. Ligand 10 was a lead compound to show the best biological activities at all three receptors.     

Complementarity of delta opioid ligand pharmacophore and receptor models

The elaboration of a pharmacophore model for the delta opioid receptor selective ligand JOM-13 (Tyr-c[D-Cys-Phe-D-Pen]OH) and the parallel, independent development of a structural model of the delta receptor are summarized. Although the backbone conformation of JOM-13's tripeptide cycle is well defined, considerable conformational lability is evident in the Tyr(1) residue and in the Phe(3) side chain, key pharmacophore elements of the ligand. Replacement of these flexible features of the ligand by more conformationally restricted analogues and subsequent correlation of receptor binding and conformational properties allowed the number of possible binding conformations of JOM-13 to be reduced to two. Of these, one was chosen as more likely, based on its better superposition with other conformationally constrained delta receptor ligands. Our model of the delta opioid receptor, constructed using a general approach that we have developed for all rhodopsin-like G protein-coupled receptors, contains a large cavity within the transmembrane domain that displays excellent complementarity in both shape and polarity to JOM-13 and other delta ligands. This binding pocket, however, cannot accommodate the conformer of JOM-13 preferred from analysis of ligands, alone. Rather, only the "alternate" allowed conformer, identified from analysis of the ligands but "disfavored" because it does not permit simultaneous superposition of all pharmacophore elements of JOM-13 with other delta ligands, fits the binding site. These results argue against a simple view of a single, common fit to a receptor binding site and suggest, instead, that at least some binding site interactions of different ligands may differ.     

Design and bioactivities of melanotropic peptide agonists and antagonists: Design based on a conformationally constrained somatostatin template

Summary α-Melanotropin and ACTH, POMC peptides, initiate biological activity by interaction with the classical pigment cell (α-MSH receptor, MC1R) and adrenal gland (ACTH receptor, MC2R) melanocortin receptors, respectively. The recently discovered MC3R, MC4R and MC5R receptors provide new targets and new biological functions for POMC peptides. We have developed conformationally constrained α-melanotropin peptides that interact with all of these receptors as agonists and antagonists and are examining new approaches to obtain highly selective ligands for each of these melanocortin receptors. Previously, we had converted somatostatin-derived peptides into potent and highly selective analogues that act as antagonists at the μ opioid receptors. Using the reverse turn template that came out of these studies, we have designed, de novo, agonist and antagonist peptide analogues that interact with melanocortin receptors.     

Synthesis and binding properties of deltorphin I analogues containing (R) and (S)-alpha-hydroxymethylnaphtylalanine

New analogues of deltorphin I (DT I), in which the phenylalanine residue in position 3 is substituted with amphiphilic alpha,alpha-disubstituted amino acid enantiomers, (R) and (S)-alpha-hydroxymethylnaphtylalanine, were synthesized and tested for mu and delta opioid receptor affinity and selectivity. Although both analogues have lower affinity to delta receptors than DT I, they both expressed specificity to delta receptors.     

Binding mode characterization of 6α- and 6β-N-heterocyclic substituted naltrexamine derivatives via docking in opioid receptor crystal structures and site-directed mutagenesis studies: Application of the ‘message–address’ concept in development of mu opioid receptor selective antagonists

Highly selective opioid receptor antagonists are essential pharmacological probes in opioid receptor structural characterization and opioid agonist functional studies. Currently, there is no highly selective, nonpeptidyl and reversible mu opioid receptor antagonist available. Among a series of naltrexamine derivatives that have been designed and synthesized, two compounds, NAP and NAQ, were previously identified as novel leads for this purpose based on their in vitro and in vivo pharmacological profiles. Both compounds displayed high binding affinity and selectivity to the mu opioid receptor. To further study the interaction of these two ligands with the three opioid receptors, the recently released opioid receptor crystal structures were employed in docking studies to further test our original hypothesis that the ligands recognize a unique ‘address’ domain in the mu opioid receptor involving Trp318 that facilitates their selectivity. These modeling results were supported by site-directed mutagenesis studies on the mu opioid receptor, where the mutants Y210A and W318A confirmed the role of the latter in binding. Such work not only enriched the ‘message–address’ concept, also facilitated our next generation ligand design and development.     

Mutational analysis of the structure and function of opioid receptors

The cloning of the opioid receptors allows the investigation of receptor domains involved in the peptidic and nonpeptidic ligand interaction and activation of the opioid receptors. Receptor chimera studies and mutational analysis of the primary sequences of the opioid receptors have provided insights into the structural domains required for the ligand recognition and receptor activation. In the current review, we examine the current reports on the possible involvement of extracellular domains and transmembrane domains in the high-affinity binding of peptidic and nonpeptidic ligands to the opioid receptor. The structural requirement for the receptors' selectivity toward different ligands is discussed. The receptor domains involved in the activation and subsequent cellular regulation of the receptors' activities as determined by mutational analysis will also be discussed. Finally, the validity of the conclusions based on single amino acid mutations is examined.     

The Relationship Between Structure and Activity Among Opioid Peptides

Summary Since the discovery and isolation of the endogenous opioid peptides Leu- and Met-enkephalin, structural studies have been focused on deducing the bioactive conformation of the peptide ligands. Theoretically, linear peptides can have many different backbone conformations, yet early, X-ray studies on enkephalin and its analogues showed only two different backbone conformations: extended and single β-bend. More recent reports include a third conformation for Leu-enkephalin and constrained opioid peptides from two ‘new’ classes (i.e. cyclic and ‘allaromatic’ peptides). In this report the relationship between solid-state X-ray structure and opioid peptide activity is examined. The N-terminal amine nitrogen and the two aromatic rings have previously been identified as structural features important to the biological activity of opioid peptides. From X-ray studies we find that the distances between the centroids of the aromatic rings, and between the N-terminal amine nitrogen and the centroid of the phenylalanine ring, vary over a large range. There is a discernible relationship, however, between the separation of the two rings and their orientation that correlates with activity.     

Interleukin 1 reduces opioid binding in guinea pig brain

Interleukin 1 (IL1) is a macrophage-derived polypeptide which signals neurons in the preoptic-anterior hypothalamus to initiate fever and the acute-phase glycoprotein response. Recently, increases in cerebrospinal fluid and hypothalamic levels of β-endorphin have been reported during endotoxin (LPS)- and IL1-induced fevers, suggesting that this opioid may participate in the modulation of IL1 effects in the CNS. In this study, we investigated whether purified (human) IL1 influences the specific binding of three prototypic opioid agonists (2-D-alanine-5-L-methionineamide, DAME; (−)-ethylketocyclazocine, EKC; dihydromorphine, DHM) and one antagonist (naloxone) to opioid receptor-enriched membrane preparations in cerebral cortex, hypothalamus, midbrain, pons, medulla, and cerebellum of guinea pig brain. IL1 reduced the binding of these ligands to their receptors during a 30-min incubation. The extent of IL1 inhibition of a given ligand for its binding sites varied according to the brain region; within some regions, the extent of this inhibition also varied with the four ligands tested. But in cortex the effect of IL1 on the specific binding of DHM is dose-dependent. Similar results were obtained with crude homologous IL1. S. enteritidis endotoxin, suspended in pyrogen-free saline at concentrations from 4 to 36 μg/ml, did not inhibit the binding of these opioid ligands to their receptors in any brain region. These results indicate that IL1 interacts with the opiate receptors in guinea pig brain. This interaction, moreover, is not limited to the hypothalamus alone, the primary site of the pyrogenic action of IL1, but also occurs in other brain regions.     

Ligands for kappa-opioid and ORL1 receptors identified from a conformationally constrained peptide combinatorial library.

We have screened a synthetic peptide combinatorial library composed of 2 x 10(7) beta-turn-constrained peptides in binding assays on four structurally related receptors, the human opioid receptors mu, delta, and kappa and the opioid receptor-like ORL1. Sixty-six individual peptides were synthesized from the primary screening and tested in the four receptor binding assays. Three peptides composed essentially of unnatural amino acids were found to show high affinity for human kappa-opioid receptor. Investigation of their activity in agonist-promoted stimulation of [(35)S]guanosine 5'-3-O-(thio)triphosphate binding assay revealed that we have identified the first inverse agonist as well as peptidic antagonists for kappa-receptors. To fine-tune the potency and selectivity of these kappa-peptides we replaced their turn-forming template by other turn mimetic molecules. This "turn-scan" process allowed the discovery of compounds with modified selectivity and activity profiles. One peptide displayed comparable affinity and partial agonist activity toward all four receptors. Interestingly, another peptide showed selectivity for the ORL1 receptor and displayed antagonist activity at ORL1 and agonist activity at opioid receptors. In conclusion, we have identified peptides that represent an entirely new class of ligands for opioid and ORL1 receptors and exhibit novel pharmacological activity. This study demonstrates that conformationally constrained peptide combinatorial libraries are a rich source of ligands that are more suitable for the design of nonpeptidal drugs.