Ring substituted and other conformationally constrained tyrosine analogues of [D-Pen2,D-Pen5]enkephalin with delta opioid receptor selectivity

A Multi-Angle Approach to Predict Peptide-GPCR Complexes: The N/OFQ-NOP System as a Successful AlphaFold Application Case Study

With nearly 700 structures solved and a growing number of customized structure prediction algorithms being developed at a fast pace, G protein-coupled receptors (GPCRs) are an optimal test case for validating new approaches for the prediction of receptor active state and ligand bioactive conformation complexes. In this study, we leveraged the availability of hundreds of peptide GPCRs in the active state and both classical homology and artificial intelligence (AI) based protein modeling combined with docking and AI-based peptide structure prediction approaches to predict the nociceptin/orphanin FQ-NOP receptor active state complex (N/OFQ-NOPa). The In Silico generated hypotheses were validated via the design, synthesis, and pharmacological characterization of novel linear N/OFQ(1-13)-NH2 analogues, leading to the discovery of a novel antagonist (3B; pKB = 6.63) bearing a single ring-constrained residue in place of the Gly2-Gly3 motif of the N/OFQ message sequence (FGGF). While the experimental validation was ongoing, the availability of the Cryo-EM structure of the predicted complex enabled us to unambiguously validate the generated hypotheses. To the best of our knowledge, this is the first example of a peptide-GPCR complex predicted with atomistic accuracy (full complex Cα RMSD < 1.0 Å) and of the N/OFQ message moiety being successfully modified with a rigid scaffold.

Conformationally constrained histidines in the design of peptidomimetics: strategies for the χ-space control

A successful design of peptidomimetics must come to terms with χ-space control. The incorporation of χ-space constrained amino acids into bioactive peptides renders the χ¹ and χ² torsional angles of pharmacophore amino acids critical for activity and selectivity as with other relevant structural features of the template. This review describes histidine analogues characterized by replacement of native α and/or β-hydrogen atoms with alkyl substituents as well as analogues with α, β-didehydro unsaturation or C^α-C^β cyclopropane insertion (ACC derivatives). Attention is also dedicated to the relevant field of β-aminoacid chemistry by describing the synthesis of β²- and β³-models (β-hHis). Structural modifications leading to cyclic imino derivatives such as spinacine, aza-histidine and analogues with shortening or elongation of the native side chain (nor-histidine and homo-histidine, respectively) are also described. Examples of the use of the described analogues to replace native histidine in bioactive peptides are also given.

Biological and conformational evaluation of bifunctional compounds for opioid receptor agonists and neurokinin 1 receptor antagonists possessing two penicillamines

Neuropathic pain states and tolerance to opioids can result from system changes in the CNS, such as up-regulation of the NK1 receptor and substance P, lead to antiopioid effects in ascending or descending pain-signaling pathways. Bifunctional compounds, possessing both the NK1 antagonist pharmacophore and the opioid agonist pharmacophore with delta-selectivity, could counteract these system changes to have significant analgesic efficacy without undesirable side effects. As a result of the introduction of cyclic and topological constraints with penicillamines, 2 (Tyr-cyclo[d-Pen-Gly-Phe-Pen]-Pro-Leu-Trp-NH-[3',5'-(CF(3))(2)-Bzl]) was found as the best bifunctional compound with effective NK1 antagonist and potent opioid agonist activities, and 1400-fold delta-selectivity over the mu-receptor. The NMR structural analysis of 2 revealed that the relative positioning of the two connected pharmacophores as well as its cyclic and topological constraints might be responsible for its excellent bifunctional activities as well as its significant delta-opioid selectivity. Together with the observed high metabolic stability, 2 could be considered as a valuable research tool and possibly a promising candidate for a novel analgesic drug.

Improving metabolic stability by glycosylation: bifunctional peptide derivatives that are opioid receptor agonists and neurokinin 1 receptor antagonists

In order to obtain a metabolically more stable analgesic peptide derivative, O-beta-glycosylated serine (Ser(Glc)) was introduced into TY027 (Tyr-d-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-3',5'-Bzl(CF(3))(2)) which was a previously reported bifunctional compound with delta/micro opioid agonist and neurokinin-1 receptor antagonist activities and with a half-life of 4.8 h in rat plasma. Incorporation of Ser(Glc) into various positions of TY027 gave analogues with variable bioactivities. Analogue 6 (Tyr-d-Ala-Gly-Phe-Nle-Pro-Leu-Ser(Glc)-Trp-NH-3',5'-Bzl(CF(3))(2)) was found to have effective bifunctional activities with a well-defined conformation with two beta-turns based on the NMR conformational analysis in the presence of DPC micelles. In addition, 6 showed significant improvement in its metabolic stability (70 + or - 9% of 6 was intact after 24 h incubation in rat plasma). This improved metabolic stability, along with its effective and delta selective bifunctional activities, suggests that 6 could be an interesting research tool and possibly a promising candidate as a novel analgesic drug.

Chemoselective nitration of phenols with tert-butyl nitrite in solution and on solid support

tert-Butyl nitrite was identified as a safe and chemoselective nitrating agent that provides preferentially mononitro derivatives of phenolic substrates in the presence of potentially competitive functional groups. On the basis of our control experiments, we propose that the reaction proceeds through the formation of O-nitrosyl intermediates prior to C-nitration via homolysis and oxidation. The reported nitration method is compatible with tyrosine-containing peptides on solid support in the synthesis of fluorogenic substrates for characterization of proteases.

The biological activity and metabolic stability of peptidic bifunctional compounds that are opioid receptor agonists and neurokinin-1 receptor antagonists with a cystine moiety

In order to improve metabolic stability, a ring structure with a cystine moiety was introduced into TY027 (Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-[3',5'-(CF(3))(2)Bzl]), which is a lead compound of our developing bifunctional peptide possessing opioid agonist and NK1 antagonist activities. TY038 (Tyr-cyclo[D-Cys-Gly-Phe-Met-Pro-D-Cys]-Trp-NH-[3',5'-(CF(3))(2)Bzl]) was found as a highly selective delta opioid agonist over mu receptor in conventional tissue-based assays, together with an effective NK1 antagonist activity and good metabolic stability with more than 24h half life in rat plasma.

Synthesis of stable and potent delta/mu opioid peptides: analogues of H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-OH by ring-closing metathesis

Ring-closing metathesis has emerged as a powerful tool in organic synthesis for generating cyclic structures via C-C double bond formation. Recently, it has been successfully used in peptide chemistry for obtaining cyclic molecules bridged through an olefin unit in place of the usual disulfide bond. Here, we describe this approach for obtaining cyclic olefin bridged analogues of H-Tyr-c[D-Cys-Gly-Phe-Cys]-OH. The synthesis of the new ligands was performed using the second generation Grubbs' catalyst. The resulting cis-8 (cDADAE) and trans-9 (tDADAE) were fully characterized and tested at delta, mu, and kappa opioid receptors. Also the linear precursor 13 (lDADAE) and the hydrogenated derivative 11 (rDADAE) also were tested. All the cyclic products containing a olefinic bond are slightly selective but highly active and potent for the delta and mu opioid receptors. Activity toward the kappa opioid receptors was absent or very low.

Development of Potent μ-Opioid Receptor Ligands Using Unique Tyrosine Analogues of Endomorphin-2

Six analogues of tyrosine, which contained alkyl groups at positions 2', 3', and 6', either singly or in combination on the tyramine ring, were investigated for their effect on the opioid activity of [Xaa(1)]endomorphin-2 (EM-2). The opioid analogues displayed the following characteristics: (i) high mu-opioid receptor affinity [K(i)(mu) = 0.063-2.29 nM] with selectivity [K(i)(delta)/K(i)(mu)] ranging from 46 to 5347; (ii) potent functional mu-opioid agonism [GPI assay (IC(50) = 0.623-0.924 nM)] and with a correlation between delta-opioid receptor affinities and functional bioactivity using MVD; (iii) intracerebroventricular administration of [Dmt(1)]- (14) and [Det(1)]EM-2 (10) produced a dose-response antinociception in mice, with the former analogue more active than the latter; and (iv) a marked shift occurred from the trans-orientation at the Tyr(1)-Pro(2) bond to a cis-conformer compared to that observed previously with [Dmt(1)]EM-2 (14) (Okada et al. Bioorg. Med. Chem. 2003, 11, 1983-1984) except [Mmt(1)]EM-2 (7). The active profile of the [Xaa(1)]EM-2 analogues indicated that significant modifications on the tyramine ring are possible while high biological activity is maintained.

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

The challenge of peptide and peptidomimetic research is the development of methods and techniques to improve the biological properties of native peptides and to convert peptide ligands into non-peptide compounds. Improved biological properties of peptides includes enhancement of stability, potency, and receptor selectivity, for both in vivo and in vitro applications. The design of a ligand with specific activity and desired biological properties is a complex task, and, to accomplish this objective, knowledge about putative interactions between a ligand and the corresponding receptor will be valuable. This includes interactions for both the binding and signal transduction processes. Structure-activity relationship (SAR) studies involve systematic modification of a lead peptide and are designed to provide insight into potential interactions involved in the formation of the ligand-receptor complex. It is desirable to have knowledge about both favorable and unfavorable processes that may occur in putative ligand-receptor interactions that result in either receptor stimulation or inhibition. Herein, we discuss various SAR studies that have involved melanocortin peptides over three decades and the information these studies have provided to the melanocortin field.

A versatile chemo-enzymatic route to enantiomerically pure beta-branched alpha-amino acids

A series of diastereoisomers of beta-methyl-beta-phenylalanine analogues 1a-f have been prepared in enantiomerically pure form using a combination of chemo- and biocatalysis. Starting from l-threonine methyl ester 2, a range of beta,beta-disubstituted didehydroamino acids were obtained as their (Z)-isomers 6a-f. Asymmetric hydrogenation of these alkenes, using either the [Rh(R,R)-Et-DuPhos(COD)]BF4 or [Rh(S,S)-Et-DuPhos(COD)]BF4 catalyst, followed by hydrolysis yielded two of the four possible sets of diastereoisomers of the beta-branched amino acid. Subsequent stereoinversion, using a stereoselective amino acid oxidase in combination with a nonselective reducing agent, furnished the remaining two sets of diastereomers.

Effects of the substitution of Phe4 in the opioid peptide [D-Ala8]dynorphin A-(1-11)NH2

Phenylalanine at position 4 of the peptide dynorphin A (Dyn A) is an important residue for opioid receptor affinity and activity, but there is very little information available on the structure-activity relationships or conformational preference of this residue for interaction with kappa-opioid receptors. Based on the hypothesis that the spatial orientation of the aromatic ring at position 4 of Dyn A is important for opioid receptor affinity and selectivity, a series of Dyn A analogues with various Phe derivatives substituted at position 4 were synthesized and evaluated for their opioid receptor affinity and activity. The L- and D-Homophe4 (homophenylalanine) analogues of [D-Ala8]Dyn A-(1-11)NH2 were compared to the (R)- and (S)-Atc4 (2-aminotetralin-2-carboxylic acid) derivatives (Aldrich et al. Chirality 2001, 13, 125-129). [l-Homophe4,D-Ala8]Dyn A-(1-11)NH2 exhibited higher kappa-opioid receptor affinity than the D-Homophe4 isomer, while [(R)-Atc4,D-Ala8]Dyn A-(1-11)NH2 exhibited higher kappa-opioid receptor affinity than the (S)-Atc4 isomer. Comparing the structure of Atc to those of Phe and Homophe, these results suggest that the Atc isomers are functioning more as constrained Homophe rather than Phe analogues in these Dyn A derivatives. The higher kappa-opioid receptor affinity of the (R)-Atc4 analogue suggests that Phe4 of Dyn A most likely adopts a gauche (-) or trans conformation in the kappa-opioid receptor binding site. Comparison of [D-Ala8]Dyn A-(1-11)NH2 derivatives containing Aic4 (2-aminoindan-2-carboxylic acid) and Tic4 (1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) with the peptides containing their acyclic counterparts alpha-MePhe4 and N-MePhe4, respectively, suggest that the loss in opioid receptor affinity seen for the Aic4 and Tic4 analogues is probably due to an improper orientation of the aromatic ring in these residues. Most of the analogues in this series showed much lower affinity for delta-opioid receptors than the parent peptide, suggesting that kappa- and delta-opioid receptors have distinct binding pockets for the residue at position 4 of Dyn A. All of the analogues with high affinity for kappa-opioid receptors exhibited full agonist activity in the adenylyl cyclase assay using cloned kappa-opioid receptors, indicating that changes in the position or orientation of the phenyl ring in this residue did not alter the ability of the peptides to activate the receptor.

pKa and volume of residue one influence delta/mu opioid binding: QSAR analysis of tyrosine replacement in a nonselective deltorphin analogue

[Gly(4)]deltorphin (Tyr-D-Ala-Phe-Gly-Val-Val-Gly-NH(2)) is a nonselective analogue of the opioid heptapeptides isolated from Phyllomedusa amphibian skin. Its nonselective nature allows for simultaneous characterization of the effects of sequence modification on both delta (delta) and mu (mu) receptor binding. The N-terminal regions of opioid peptides are considered to be responsible for receptor recognition, and the tyrosine at position one is relatively intolerant to alteration. In order to further investigate the role of the phenolic hydroxyl group in receptor interaction, a series of peptides was synthesized in which the position-one tyrosine residue was replaced with analogues of varying electronic, steric, and acid/base character, including ring-substituted tyrosines, para-substituted phenylalanines, and other nonaromatic and heterocyclic amino acids. The effects of these replacements on delta and mu receptor affinities were measured and then analyzed through quantitative structure-activity relationship (QSAR) calculations. Results support a dual hydrogen bond donor/acceptor role for the Tyr(1) hydroxyl moiety, with less acidic hydroxyl groups exhibiting stronger binding to opioid receptors. In addition, steric bulk in the Tyr(1) position independently strengthens mu and possibly delta binding, presumably by either a ligand conformational effect or enhanced van der Waals interactions with a 'loose' receptor site. The pK(a) effect is stronger on delta than on mu binding, generating an increase in delta selectivity with increasing residue-one pK(a).

Stereoinversion of β- and γ-substituted α-amino acids using a chemo-enzymatic oxidation–reduction procedure

Both D- and L-beta- and gamma-substituted alpha-amino acids can be interconverted to their respective L- and D- diastereoisomers by treatment with an enantioselective amino acid oxidase and a chemical reducing agent.

Dynorphin A analogs containing a conformationally constrained phenylalanine derivative in position 4: reversal of preferred stereochemistry for opioid receptor affinity and discrimination of kappa vs. delta receptors

Analogs of the opioid peptide [D-Ala8]dynorphin A-(1-11)NH2 containing optically pure (R)- and (S)-2-aminotetralin-2-carboxylic acid (Atc) in position 4 were synthesized and evaluated for opioid receptor affinity. These peptides are the first reported dynorphin A analogs containing a conformationally constrained amino acid in place of the important aromatic residue Phe4. By incorporating resolved Atc isomers, the opioid receptor affinity and the stereochemistry of the constrained residue could be unambiguously correlated. Both Dyn A analogs containing Atc in position 4 retained nanomolar affinity for kappa and mu opioid receptors. Unexpectedly the peptide containing (R)-Atc, corresponding to a conformationally constrained D-Phe analog, displaying higher affinity for both kappa and mu receptors than the peptide containing (S)-Atc. In contrast [D-Phe4,D-Ala8]Dyn A-(1-11)NH2 exhibited significantly lower affinity for kappa and mu receptors than the parent peptide, as expected. Conformational restriction of the Phe4 sidechain or incorporation of D-Phe in position 4 had the largest effect on delta receptor affinity, yielding compounds with negligible affinity for these receptors. Thus, there appear to be distinctly different structural requirements for this residue for kappa vs. delta receptors, and it is possible to completely distinguish between these two receptors by changing a single residue in Dyn A.

Conformational restriction of the Tyr53 side-chain in the decapeptide HE

A series of conformationally restricted analogs of the hen egg lysozyme (HEL) decapeptide 52-61 in which the conformationally flexible Tyr53 residue was replaced by several more constrained tyrosine and phenylalanine analogs was prepared. Among these tyrosine and phenylalanine analogs were 1,2,3,4-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid (Htc), 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic), 4-amino- 1,2,4,5-tetrahydro-8-hydroxy-2-benzazepine-3-one (Hba), 4-amino-1,2,4,5-tetrahydro-2-benzazepine-3-one (Aba), 2-amino-6-hydroxytetralin-2-carboxylic acid (Hat) and 2-amino-5-hydroxyindan-2-carboxylic acid (Hai) in which the rotations around Calpha-Cbeta and Cbeta-Cgamma were restricted because of cyclization of the side-chain to the backbone. Synthesis of Pht-Hba-Gly-OH using a modification of the Flynn and de Laszlo procedure is described. Analogs of beta-methyltyrosine (beta-MeTyr) in which the side-chains were biased to particular side-chain torsional angles because of substitution at the beta-hydrogens were also prepared. These analogs of HEL[52-61] peptide were tested for their ability to bind to the major histocompatibility complex class II I-Ak molecule and to be recognized in this context by two T-cell hybridomas, specific for the parent peptide HEL[52-61]. The data showed that the conformation and also the configuration of the Tyr53 residue influenced both the binding of the peptide to I-Ak and the recognition of the peptide/I-Ak complex by a T-cell receptor.

Synthesis and conformational studies of peptidomimetics containing furanoid sugar amino acids and a sugar diacid

Furanoid sugar amino acids (1) were synthesized and used as dipeptide isosteres to induce interesting turn structures in small linear peptides. They belong to a new variety of designed hybrid structures that carry both amino and carboxyl groups on rigid furanose sugar rings. Four such molecules, 6-amino-2,5-anhydro-6-deoxy-D-gluconic acid (3, Gaa) and its mannonic (4, Maa), idonic (5, Iaa), and a 3,4-dideoxyidonic (6, ddIaa) congeners were synthesized. The synthesis followed a novel reaction path in which an intramolecular 5-exo S(N)2 opening of the hexose-derived terminal aziridine ring in 2 by the gamma-benzyloxy oxygen with concomitant debenzylation occurred during pyridinium dichromate oxidation of the primary delta-hydroxyl group to carboxyl function, leading to the formation of furanoid sugar amino acid frameworks in a single step. Incorporation of these furanoid sugar amino acids into Leu-enkephalin replacing its Gly-Gly portion gave analogues 8-11. Detailed structural analysis of these molecules by circular dichroism (CD) and various NMR techniques in combination with constrained molecular dynamics (MD) simulations revealed that two of these analogues, 8a and 10a, have folded conformations composed of an unusual nine-membered pseudo beta-turn-like structure with a strong intramolecular H-bond between LeuNH --> sugarC3-OH. This, in turn, brings the two aromatic rings of Tyr and Phe in close proximity, a prerequisite for biological activities of opioid peptides. The analgesic activities of 8a,b determined by mouse hot-plate and tail-clip methods were similar to that of Leu-enkephalin methyl ester. The syn disposition of the beta-hydroxycarboxyl motif on the sugar rings appears to be the driving force to nucleate the observed turn structures in some of these molecules (8 and 10). Repetition of the motif on both sides of a furanose ring resulted in a novel molecular design of sugar diacid, 2,5-anhydro-D-idaric acid (7, Idac). Bidirectional elongation of the diacid moieties of 7 with identical peptide strands led to the formation of a C2-symmetric reverse-turn mimetic 12 which displayed a very ordered structure consisting of identical intramolecular H-bonds at two ends between LeuNH --> sugar-OH, the same as in 8 and 10.

Assessment of stereoselectivity of trimethylphenylalanine analogues of delta-opioid [D-Pen(2),D-Pen(5)]-enkephalin

[D-Pen(2),D-Pen(5)]-Enkephalin (DPDPE) is an enzymatically stable delta-opioid receptor-selective peptide, which was modified by the trimethylation of the Phe(4) residue to give beta-methyl-2', 6'-dimethylphenylalanine (TMP), resulting in four conformations : (2R,3S)-beta-Phe-DPDPE, (2R,3R)-beta-Phe-DPDPE, (2R, 3S)-beta-Phe-DPDPE, and (2S,3R)-beta-Phe-DPDPE. Synthesis was by solid-phase techniques using enantiomerically pure amino acids to give the four optically pure diastereoisomer peptides. The potency and selectivity (delta- versus mu-opioid receptor) were evaluated by radioreceptor binding in rat brain, with a mu/delta ratio decrease for all TMP conformations, compared with the parent compound (DPDPE). Octanol/buffer distribution analysis showed enhanced lipophilicity of all TMP forms, with a sixfold enhancement associated with (2S,3S)-TMP. In situ vascular perfusion in anesthetized rats showed a 1.6-fold (p < 0.01) increase in the ratio of brain uptake for (2S,3S)-TMP and a 1.5-fold (p < 0.01) decrease in uptake for (2R,3R)-TMP. Saturability of (2S,3S)-TMP was shown (p < 0.01) against 100 microM unlabeled DPDPE, showing a shared nondiffusionary transport system. P-glycoprotein affinity was shown in situ for the parent and (2S,3S)-TMP (p < 0.01). Protein binding capacity of the TMP compounds in rat plasma and in situ mammalian bovine serum albumin-Ringer showed (2R,3S)-TMP and (2S,3R)-TMP with the lowest degree of protein binding (p < 0.01), and (2S,3S)-TMP and (2R,3R)-TMP with comparable affinities to DPDPE. Analgesia, via intravenous administration, showed significantly reduced (p < 0.01) end effect and time course for (2R,3R)-TMP, (2R,3S)-TMP, and (2S, 3R)-TMP as compared with DPDPE. These results demonstrate that topographical modification in a conformationally restricted peptide can significantly modulate potency and receptor selectivity, binding capacity, enzymatic stability, lipophilicity, P-glycoprotein affinity, and blood-brain barrier permeability, resulting in a change of bioavailability, and thereby provides insight for future peptide drug design.

Design of peptides, proteins, and peptidomimetics in chi space

Peptide and protein biological activities depend on their three dimensionals structures in the free state and when interacting with their receptors/acceptors. The backbone conformations such as alpha-helix, beta-sheet, beta-turn, and so forth provide critical templates for the three-dimensional structure, but the overall shape and intrinsic stereoelectronic properties of the peptide or protein important for molecular recognition, signal transduction, enzymatic specificity, immunomodulation, and other biological effects depend on arrangement of the side chain groups in three-dimensional chi space (their chi 1, chi 2, etc. torsional angles). In this paper we explore approaches to the de novo design of polypeptides and peptidomimetics with biased or specific conformational/topographical properties in chi space. We consider computational and experimental methods that can be used to examine the effects of specific structural modifications in constraining side chain groups of amino acid residues and their similarities in chi space to the natural amino acids to evaluate what sort of mimetics are likely to mimic normal amino acids. We then examine some of the asymmetric synthetic methods that are being developed to obtain the amino acid mimetics. Finally, we consider selected examples in the literature where these specialized amino acids have been incorporated in biologically active peptides and the specific insights they have provided regarding the topographical requirements for bioactive peptide potency, selectivity, and other biochemical and pharmacological properties. Constraints in chi space show great promise as useful tools in peptide, protein, and peptidomimetic de novo design of structures and pharmacophores with specific stereostructural, biochemical and biological properties.

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.

A three-dimensional model of the delta-opioid pharmacophore: comparative molecular modeling of peptide and nonpeptide ligands

A comparative molecular modeling study of delta-opioid ligands was performed under the assumption that potent peptide and nonpeptide agonists may have common three-dimensional (3D) arrangement of pharmacophore groups upon binding to the delta-receptor. Low-energy conformations of the agonists 7-spiroindanyloxymorphone (SIOM) and 2-methyl-4a-alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12, 12a-alpha-octahydro-quinolino[2,3,3-g]isoquinoline (TAN-67), and a partial agonist oxomorphindole (OMI) were determined by high-temperature molecular dynamics (MD). A good spatial overlap was found for the pharmacophore groups of SIOM, TAN-67, and OMI, including the basic nitrogen, phenol hydroxyl, and two aromatic ring. Based on this overlap we proposed a 3D pharmacophore model for nonpeptide delta-opioid agonists with a distance of 7.0 +/- 1.3 A between the two aromatic rings and of 8.2 +/- 1.0 A between the nitrogen and phenyl ring. The potent and highly delta-opioid receptor selective agonist [(2S,3R)-TMT(1)]DPDPE, which shares global backbone constraints of the 14-membered disulfide cycle and a strong preference for the trans rotamer of the TMT(1) side chain, was chosen as a peptide template of the delta-opioid pharmacophore. Extensive MD simulations at 300 K with the AMBER force field were performed for [(2S,3R)-TMT(1)]DPDPE and the less potent [(2S, 3S)-TMT(1)]DPDPE analogue. Multiple MD trajectories were collected for each peptide starting from the x-ray structures of DPDPE and [L-Ala(3)]DPDPE and from models proposed in the literature. Low-energy MD conformations were filtered by the nonpeptide pharmacophore query and then directly superimposed with SIOM, OMI, and TAN-67. Two conformers of [(2S,3R)-TMT(1)]DPDPE that showed the best overlap with the nonpeptide pharmacophore (rms deviation </= 1. 0 A for N,O atoms and centroids of two aromatic rings) were selected as possible delta-receptor binding conformations. These conformations have similar backbone structures, and trans rotamers of the TMT(1) side-chain group. They are reasonably close to the crystal structure of [L-Ala(3)]DPDPE, and differ significantly from the crystal structure of DPDPE. The conformer with a gauche(-) rotamer of Phe(4) is most consistent with structure-activity relationships of delta-opioid peptides. The proposed 3D models were used for rational design of new nonpeptide delta-receptor ligands.

Deltorphin II analogues with 6-hydroxy-2-aminotetralin-2-carboxylic acid in position 1

Two approaches to the design of very active and highly selective delta opioid peptides were used to obtain new deltorphin analogues with altered hydrophobic and stereoelectronic properties. Deltorphin II analogues were synthesized with the substitution of Ile instead of Val at positions 5 and 6 in the address domain and 6-hydroxy-2-aminotetralin-2-carboxylic acid (Hat) instead of Tyr(1) in the message domain. In the radioreceptor-binding studies, in which type-specific tritiated opioid ligands were used, (R)- and (S)-Hat-deltorphins exhibited similar K(i) values, revealing high delta selectivity. The peptides displayed agonist properties in the in vitro bioassay, with IC(50) values in the subnanomolar range in the mouse vas deferens assay and in the micromolar or higher range in the guinea pig ileum assay, again demonstrating a high selectivity toward delta receptors. The agonist property of the new ligands was confirmed by means of [(35)S]GTPgammaS-binding experiments in membranes of the rat frontal cortex.

Synthesis of (2S,3R)-beta-methyltyrosine catalyzed by tyrosine phenol-lyase

A one-step enzymatic synthesis of the conformationally restrained tyrosine analog (2S,3R)-beta-methyltyrosine is reported. This synthesis extends the preparative chemistry associated with tyrosine phenol-lyase. This beta-methyltyrosine derivative was shown to be an efficient protein tyrosine kinase substrate, suggesting that conformational restraint may ultimately be used to enhance tyrosine kinase recognition of substrates.

Binding to delta and mu opioid receptors by deltorphin I/II analogues modified at the Phe3 and Asp4/Glu4 side chains: a report of 32 new analogues and a QSAR study

The synthesis and binding affinities of 32 X3Gly4 dual-substitution analogues of the natural opioid heptapeptides deltorphin I and II are reported. A multiple regression QSAR analysis was performed using those results along with literature data for the X3Asp4 and Phe3X4 side chain analogues. Fitting to a three-term potential well model with hydrophobic and van der Waals attraction terms and a steric repulsion term indicates that the delta and mu receptor sites for binding the residue three side chain are similar, and that the binding interaction is primarily van der Waals and secondarily hydrophobic. Further analysis indicates that both sites are more constrained with respect to side chain length than width or thickness, and the mu site appears to be somewhat larger. A binding model consistent with these findings pictures the native third residues Phe ring laying on a step notched out of the receptor surface, pointing toward the back (riser) of the step, and sandwiched between the receptor and ligand. However, the binding sites for the residue four side chains are quite different on delta and mu receptors. Binding to the delta site appears to involve both electrostatic attraction (probably to a partial positive charge) and van der Waals attraction, but not necessarily hydrogen bonding, and more constraint with respect to side chain length than width or thickness. In contrast, there is no evidence for any kind of binding attraction between the side chain of residue four and the mu site, which acts more as steric repulsion site, as though the space that is a pocket on the delta receptor is filled in on the mu receptor. A regression model based only on steric repulsion by van der Waals bulk and/or the effective bulk of a hydration layer accounts for over 80% of the residue four related variation in mu affinity.

Evolution of the Dmt-Tic pharmacophore: N-terminal methylated derivatives with extraordinary delta opioid antagonist activity

The delta opioid antagonist H-Dmt-Tic-OH (2',6'-dimethyl-L-tyrosyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) exhibits extraordinary delta receptor binding characteristics [Ki delta = 0.022 nM; Ki mu/Ki delta = 150,000] and delta antagonism (pA2 = 8.2; Ke = 5.7 nM). A change in chirality of Dmt at C alpha (1, 2, 6, 8, 10, 13) curtailed delta receptor parameters, while replacement of its alpha-amino function by a methyl group (3) led to inactivity; Tyr-Tic analogues 4 and 11 weakly interacted with delta receptors. N-Alkylation of H-Dmt-Tic-OH and H-Dmt-Tic-Ala-OH with methyl groups produced potent delta-opioid ligands with high delta receptor binding capabilities and enhanced delta antagonism: (i) N-Me-Dmt-Tic-OH 5 had high delta opioid binding (Ki delta = 0.2 nM), elevated delta antagonism on mouse vas deferens (MVD) (pA2 = 8.5; Ke = 2.8 nM), and nondetectable mu activity with guinea pig ileum (GPI). (ii) N,N-Me2-Dmt-Tic-OH (12) was equally efficacious in delta receptor binding (Ki delta = 0.12 nM; Ki mu/Ki delta = 20000), but delta antagonism rose considerably (pA2 = 9.4; Ke = 0.28 nM) with weak mu antagonism (pA2 = 5.8; Ke = 1.58 microM; GPI/MVD = 1:5640). N-Me-(9) and N,N-Me2-Dmt-Tic-Ala-OH (15) also augmented delta opioid receptor binding, such that 15 demonstrated high affinity (Ki delta = 0.0755 nM) and selectivity (Ki mu/Ki delta = 20132) with exceptional antagonist activity on MVD (pA2 = 9.6; Ke = 0.22 nM) and weak antagonism on GPI (pA2 = 5.8; Ke = 1.58 microM; GPI/MVD = 1:7180). Although the amidated dimethylated dipeptide analogue 14 had high Ki delta (0.31 nM) and excellent antagonist activity (pA2 = 9.9; Ke = 0.12 nM), the increased activity toward mu receptors in the absence of a free acid function at the C-terminus revealed modest delta selectivity (Ki mu/Ki delta = 1655) and somewhat comparable bioactivity (GPI/MVD = 4500). Thus, the data demonstrate that N,N-(Me)2-Dmt-Tic-OH (12) and N,N-Me2-Dmt-Tic-Ala-OH (15) retained high delta receptor affinities and delta selectivities and acquired enhanced potency in pharmacological bioassays on MVD greater than that of other peptide or non-peptide delta antagonists.

beta-Methylation of the Phe7 and Trp9 melanotropin side chain pharmacophores affects ligand-receptor interactions and prolonged biological activity

Topographically modified melanotropin side chain pharmacophore residues Phe7 and Trp9 in a cyclic peptide template (Ac-Nle4-c[Asp-His-Xaa7-Arg-Yaa9-Lys]-NH2) and Phe7 in a linear peptide template (Ac-Ser-Tyr-Ser-Nle4-Glu-His-Xaa7-Arg-Trp-Gly-Lys-Pro-Val-NH2) result in differences in potency and prolonged biological activity in the frog and lizard skin bioassays. These topographic modifications included the four isomers of beta-methylphenylalanine (beta-MePhe)7 and beta-methyltryptophan (beta-MeTrp)9 and the two isomers of 1,2,3,4-tetrahydro-beta-carboline (Tca)9 Modifications in the cyclic template resulted in up to a 1000-fold difference in potency for the beta-MePhe7 stereoisomeric peptides; up to a 476-fold difference in potency resulted for the beta-MeTrp9 peptides, and about a 50-fold difference between the Tca9-containing peptides. Up to a 40-fold difference in potency resulted for the beta-MePhe7 stereoisomeric peptides using the linear template in these assays. The relative potency ranking for modifications in the cyclic template of beta-MePhe7 were 2R,3S > 2S,3S = 2S,3R > 2R,3R in the frog assay and 2S,3R > 2R,3S > 2S,3S > 2R,3R in the lizard assay. The relative potencies for modifications in the cyclic template of beta-MeTrp9 were 2R,3S > 2R,3R > 2S,3S > > 2S,3R in the frog assay and 2S,3S = 2R,3R > 2R,3S > 2S,3R in the lizard assay. The relative potencies for modifications in the cyclic template of Tca9 were DTca > LTca in both assays. Significant differences in prolonged (residual) activities were also observed for these modified peptides and were dependent upon stereochemistry of the beta-methyl amino acid, peptide template, and bioassay system. Furthermore, comparisons of beta-MeTrp9 stereoisomeric peptides on the frog, lizard, and human MC1 receptors suggest that structure-activity relationships on both the classical frog and lizard skin bioassays do not necessarily predict corresponding SAR profiles for the human melanocortin receptors, indicating a remarkable species specificity of the MC1 receptor requirements.

Synthesis and biological properties of beta-MePhe3 analogues of deltorphin I and dermenkephalin: influence of biased chi 1 Phe3 residues on peptide recognition for delta-opioid receptors

Using the method of conformational constraint, we have designed and synthesized analogues of deltorphin I and dermenkephalin containing each of the four stereoisomers (2S,3S; 2S,3R; 2R,3S; 2R,3R) of the unusual amino acid beta-methylphenylalanine in position three. The potency and selectivity of these analogues were evaluated by radioreceptor binding assays in the rat brain using [3H]CTOP (mu-ligand) and[3H]p-C1Phe4]DPDPE (delta-ligand), and by bioassay using the mouse vas deferens (delta-receptor assay) and guinea pig ileum (mu-receptor assay) assays. The substitution of a beta-MePhe for Phe3 in deltorphin I and dermenkephalin has a large and variable effect on the bioactivities of the synthesized analogues. The synthesized analogues are somewhat less potent than the native peptides. Both [(2S,3R)-beta-MePhe3]deltorphin and [(2S,3R)-beta-MePhe3] dermenkephalin are more selective, however, and interact essentially specifically with the receptor in the binding assays and bioassays. The bioassay data in vitro of the synthesized analogues of deltorphin I and dermenkephalin follow the same general trends as the receptor binding data. These results demonstrate that topographical modifications of the side-chain conformation of critical structural moieties in a ligand can significantly modulate both the potency and receptor selectivity for ligands that have multiple sites of biological activity, and they illustrate that this approach has general application to peptide and peptidomimetic ligand design.

Opioid peptides: simultaneous delta agonism and mu antagonism in somatostatin analogues

Four isomers of the Somatostatin analogue H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) were made with beta-MePhe in position 1 and assayed for opioid binding in rat brain, biological activity in MVD and GPI bioassays, and antinociception in mouse warm-water tail flick assays. The analogues displayed varying potencies and biological activities including: simultaneous delta receptor agonism/mu receptor antagonism, mu receptor antagonism, and delta receptor agonism. These analogues demonstrated that the N-terminal residue is important for receptor potency/selectivity and signal transduction. These analogues my represent leads to therapeutic agents that yield analgesia via delta agonist effects, yet lack side effects associated with mu activity.

Tachykinin NK-1 receptor probed with constrained analogues of substance P

The action of rotameric probes introduced either in position 7 or 8 in the sequence of substance P (SP) was investigated, i.e. L-tetrahydroisoquinoleic acid (Tic), L-fluorenylglycine (Flg), L-diphenylalanine (Dip), the diastereoisomers of L-1-Indanylglycine (Ing) and L-benz[f]indanylglycine (Bfi), the Z- and E-isomers of dehydrophenylalanine and dehydronaphthylalanine (delta ZPhe, delta EPhe, delta ZNal, ENal) and L-O,O'-dimethylphenylalanine (Dmp). The aim of this study was the topographical characterization of the binding subsites of human NK-1 receptor expressed in CHO cells, especially the S7 and S8 subsites, corresponding to residues Phe7 and Phe8 of substance P. According to the binding potencies of these substituted-SP analogues, the S7 binding subsite is smaller than the S8 subsite: the S7 subsite accepts only one aromatic nucleus, while the S8 can accommodate three coplanar nuclei altogether. These findings are compatible with the idea that the S8 binding subsite may reside in the extracellular loops of the hNK-1 receptor. NK-1 agonists bind to human NK-1 receptor and activate the production of both inositol phosphates and cyclic AMP. As already quoted for septide, [pGlu6, Pro9]SP(6-11), discrepancies are observed between affinity (K1) and activity (EC50) values for IPs production. While a weak correlation between K1 and EC50 values for IPs production could be found (r = 0.70), an excellent correlation could be demonstrated between their affinities (K1) and their potencies (EC50) for cAMP production (r = 0.97). The high potency (EC50) observed for "septide-like' molecules on PI hydrolysis, compared to their affinity is not an artefact related to the high level of NK-1 receptors expressed on CHO cells since a good correlation was found between EC50 values obtained for PI hydrolysis and those measured for spasmogenic activity in guinea pig ileum bioassay (r = 0.94).

Topographical amino acid substitution in position 10 of glucagon leads to antagonists/partial agonists with greater binding differences

The role of position 10 in the beta-turn region of glucagon was investigated by substituting chiral constrained amino acids and other modifications in the N-terminal region. A series of glucagon analogues have been designed and synthesized by incorporating beta-methylphenylalanine isomers (2S,3S, 2S,3R, 2R,3R, and 2R,3S) at position 10 in order to explore the structural and topographical requirements of the glucagon receptor, and, in addition, utilizing previous studies which indicated that antagonism could be enhanced by modifications (des-His1, Glu9) and a bulky group at position 5. The structures of the new analogues are as follows: [des-His1,-Tyr5,Glu9]glucagon-NH2 (II), [des-His1,Tyr5,Glu9,Phe10]glucagon-NH2 (III), [des-His1,Tyr5,Glu9,-Ala10]glucagon-NH2 (IV), [des-His1,Tyr5,Glu9,(2S,3R)-beta-MePhe10]glucagon-NH2 (V), [des-His1,-Tyr5,Glu9,(2S,3S)-beta-MePhe10]glucagon-NH2 (VI), [des-His1,Tyr5,Glu9,D-Tyr10]glucagon-NH2 (VII), [des-His1,Tyr5,Glu9,D-Phe10]glucagon-NH2 (VIII), [des-His1,Tyr5,Glu9,D-Ala10]glucagon-NH2 (IX), [des-His1,Tyr5,Glu9,(2R,3R)-beta-MePhe10]glucagon-NH2 (X), and [des-His1,Tyr5,Glu9,(2R,3S)-beta-MePhe10]glucagon-NH2 (XI). These analogues led to dramatically different changes in in vitro binding affinities for glucagon receptors. Their receptor binding potencies IC50 values (nM) are 2.3 (II), 4.1 (III), 395.0 (IV), 10.0 (V), 170.0 (VI), 74.0 (VII), 34.5 (VIII), 510.0 (IX), 120.0 (X), and 180.0 (XI). Analogues II, III, V, VI, and XI were found to be weak partial agonists/partial antagonists with maximum stimulation between 5%-9%, while the other compounds (IV and VII-X) were antagonists unable to activate the adenylate cyclase system even at concentrations as high as 10(-5) M. In competition experiments, all of the analogues caused a right shift of the glucagon-stimulated adenylate cyclase dose-response curve. The pA2 values were 6.60 (II), 6.85 (III), 6.20 (IV), 6.20 (V), 6.10 (VI), 6.50 (VII), 6.20 (VIII), 5.85 (IX), 6.20 (X), and 6.00 (XI). Putative topographical requirements of the glucagon receptor for the aromatic side chain conformation in position 10 of glucagon antagonists are discussed.

Development of a model for the delta-opioid receptor pharmacophore: 3. Comparison of the cyclic tetrapeptide, Tyr-c[D-Cys-Phe-D-Pen]OH with other conformationally constrained delta-receptor selective ligands

We have previously proposed a model of the delta-opioid receptor bound conformation for the cyclic tetrapeptide, Tyr-c[D-Cys-Phe-D-Pen]OH (JOM-13) based on its conformational analysis and from conformation-affinity relationships observed for its analogues with modified first and third residues. To further verify the model, it is compared here with results of conformational and structure-activity studies for other known conformationally constrained delta-selective ligands: the cyclic pentapeptide agonist, Tyr-c[D-Pen-Gly-Phe-D-Phe]OH (DPDPE): the peptide antagonist, Tyr-Tic-Phe-PheOH (TIPP); the alkaloid agonist, 7-spiroindanyloxymorphone (SIOM); and the related alkaloid antagonist, oxymorphindole (OMI). A candidate delta-bound conformer is identified for DPDPE that provides spatial overlap of the functionally important N-terminal NH3+ and C-terminal COO- groups and the aromatic rings of the Tyr and Phe residues in both cyclic peptides. It is shown that all delta-selective ligands considered have similar arrangements of their pharmacophoric elements, i.e., the tyramine moiety and a second aromatic ring (i.e., the rings of Phe3, Phe4, and Tic2 residues in JOM-13, DPDPE, and TIPP, respectively; the indole ring system in OMI, and the indanyl ring system in SIOM). The second aromatic rings, while occupying similar regions of space throughout the analogues considered, have different orientations in agonists and antagonists, but identical orientations in peptide and alkaloid ligands with the same agonistic or antagonistic properties. These results agree with the previously proposed binding model for JOM-13, are consistent with the view that delta-opioid agonists and antagonists share the same binding site, and support the hypothesis of a similar mode of binding for opioid peptides and alkaloids.

Conformational analysis of beta-methyl-para-nitrophenylalanine stereoisomers of cyclo[D-Pen2, D-Pen5]enkephalin by NMR spectroscopy and conformational energy calculations

Solution conformations of beta-methyl-para-nitrophenylalanine4 analogues of the potent delta-opioid peptide cyclo[D-Pen2, D-Pen5]enkephalin (DPDPE) were studied by combined use of nmr and conformational energy calculations. Nuclear Overhauser effect connectivities and 3JHNC alpha H coupling constants measured for the (2S, 3S)-, (2S, 3R)-, and (2R, 3R)-stereoisomers of [beta-Me-p-NO2Phe4]DPDPE in DMSO were compared with low energy conformers obtained by energy minimization in the Empirical Conformational Energy Program for Peptides (ECEPP/2) force field. The conformers that satisfied all available nmr data were selected as probable solution conformations of these peptides. Side-chain rotamer populations, established using homonuclear (3JH alpha H beta) and heteronuclear (3JH alpha C gamma) coupling constants and 13C chemical shifts, show that the beta-methyl substituent eliminates one of the three staggered rotamers of the torsion angle chi 1 for each stereoisomer of the beta-Me-p-NO2Phe4. Similar solution conformations were suggested for the L-Phe4-containing (2S, 3S)- and (2S, 3R)-stereoisomers. Despite some local differences, solution conformations of L- and D-Phe4-containing analogues have a common shape of the peptide backbone and allow similar orientations of the main delta-opioid pharmacophores. This type of structure differs from several models of the solution conformations of DPDPE, and from the model of biologically active conformations of DPDPE suggested earlier. The latter model is allowed for the potent (2S, 3S)- and (2S, 3R)-stereoisomers of [beta-Me-p-NO2Phe4]DPDPE, but it is forbidden for the less active (2R, 3R)- and (2R, 3S)-stereoisomers. It was concluded that the biologically active stereoisomers of [beta-Me-p-NO2Phe4]DPDPE in the delta-receptor-bound state may assume a conformation different from their favorable conformations in DMSO.

Conformational restriction of Tyr and Phe side chains in opioid peptides: information about preferred and bioactive side-chain topology

The side chain of Tyr and Phe was fixed into the gauche(-) or gauche(+) conformation by using the Tic Htc structures, and into the trans conformation by using an aminobenzazepine-type (Aba) structure. When incorporated into dermorphin or deltorphin II, the Tic and Htc analogues all showed a large decrease in both mu and delta affinities and activities. Fixation of Phe(3) in the trans rotamer resulted in a large increase in delta affinity in the dermorphin analogue, whereas in the [Aba(3)-Gly(4)] deltorphin II analogue, good delta affinity is maintained despite the removal of the Glu side chain. Whereas several authors propose a gauche(-) preferred conformation for the Phe(3) side chain, these results suggest a trans conformation at the delta receptor. The use of these conformationally constrained residues for evaluating the preferred solution conformation in the flexible N-terminal tripeptide Tyr-D-Ala-Phe is illustrated. The (1)H-nmr parameters--chemical shift, temperature dependence, and nuclear Overhauser effects to the D-Ala(2) methyl protons in the different analogues--provide direct evidence to confirm the proposed sandwich conformation in the native peptides.