Subtleties of structure-agonist versus antagonist relationships of opioid peptides and peptidomimetics

The development of novel delta opioid antagonists and delta opioid agonists structurally derived from the prototype delta antagonist TIPP (H-Tyr-Tic-Phe-Phe-OH), is reviewed. Both delta antagonists and delta agonists with extraordinary potency and unprecedented delta receptor selectivity were discovered. Some of them are already widely used as pharmacological tools and are also of interest as potential therapeutic agents for use in analgesia. The results of the performed structure-activity studies revealed that the delta antagonist versus delta agonist behavior of this class of compounds depended on very subtle structural differences in diverse locations of the molecule. These observations can be best explained with a receptor model involving a number of different inactive and active receptor conformations.

Side chain methyl substitution in the delta-opioid receptor antagonist TIPP has an important effect on the activity profile

The delta-opioid antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP-OH) or its C-terminal amide analogue was systematically modified topologically by substitution of each amino acid residue by all stereoisomers of the corresponding beta-methyl amino acid. The potency and selectivity (delta- vs mu- and kappa-opioid receptor) were evaluated by radioreceptor binding assays. Agonist or antagonist potency were assayed in the mouse vas deferens and in the guinea pig ileum. In the TIPP analogues containing L-beta-methyl amino acids the influence on delta-receptor affinity and on delta-antagonist potency is limited, the [(2S,3R)-beta-MePhe3]TIPP-OH analogue being among the most potent delta-antagonists reported. In the D-beta-methyl amino acid series, the [D-beta-MeTic2] analogues are delta-selective antagonists whereas [D-Tic2]TIPP-NH2 is a delta-agonist. NMR studies did not indicate any influence of the beta-methyl substituent on the conformation of the Tic residue. The [(2R,3S)-beta-MePhe3]TIPP-NH2 is a potent delta-agonist, its C-terminal carboxylic acid analogue being more delta-selective but displaying partial agonism in both the delta- and mu-bioassay. These results constitute further examples of a profound influence of beta-methyl substitution on the potency, selectivity, and signal transduction properties of a peptide.

Enkephalin analogs containing 4,4-difluoro-2-aminobutyric acid: synthesis and fluorine effect on the biological activity

Analogs of Met-enkephalin and [D-Pen2, D-Pen5]enkephalin (DPDPE) containing the partially fluorinated amino acid 4,4-difluoro-2-aminobutyric acid (DFAB) in the 2- or 3-position of the peptide sequence were synthesized and their opioid activities and receptor selectivities were determined in vitro. The linear fluorinated [D-DFAB2, Met5-NH2]enkephalin showed mu and delta agonist potencies comparable to those of natural [Leu5]enkephalin. The partially fluorinated DPDPE analogs behaved differently as compared with their non-fluorinated correlates. While L-amino acid substitution in position 3 of DPDPE usually resulted in higher delta agonist potency than D-amino acid substitution. [D-DFAB3]DPDPE turned out to be a more potent delta agonist than [L-DFAB3]DPDPE. Furthermore, [D-DFAB3]DPDPE showed over 100-fold higher delta agonist potency than [D-Abu3]DPDPE (Abu = 2-aminobutyric acid), indicating that the fluorine substituents interact favorably with a delta opioid receptor subsite.

Synthesis of cyclic dipeptide templates, their incorporation into peptides and studies on their conformational and biological properties

This study investigated the diastereoselective synthesis of three dipeptide templates 1, 2 and 3, which may be regarded as conformationally restricted analogs of H-Gly-Xaa-OH, in which Xaa constitutes an aromatic amino acid. Bond formation between alpha-C of Gly and the aromatic moiety was achieved by proton-catalyzed intramolecular electrophilic aromatic substitution. The absolute configuration of the dipeptide templates was determined by single-crystal X-ray crystallography or by nuclear Overhauser enhancement measurements. A protective group strategy was elaborated to allow their incorporation into peptide sequences by liquid phase as well as by solid-phase peptide synthesis. The templates were used to generate an enkephalin analog 15, a modified peptidic neurokinin antagonist 20 and two dermorphin derivatives (24 and 33). Molecular dynamic simulations with 15 and 20 revealed the preference for a turn-like motif for 15. The biological activity, as investigated by respective receptor binding and functional assays, was strongly diminished with all four derivatives, indicating that their receptor-relevant molecular geometries lie outside the examined conformational space.

The receptor-bound conformation of H-Tyr-Tic-(Phe-Phe)-OH-related delta-opioid antagonists contains all trans peptide bonds

Two different models for the receptor-bound conformation of delta-opioid peptide antagonists containing the N-terminal dipeptide segment H-Tyr-Tic (Tic = 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) have been proposed. Both models are based on spatial overlap of the Tyr1 and Tic2 aromatic rings and N-terminal amino group with the corresponding aromatic rings and nitrogen atom of the nonpeptide delta-antagonist naltrindole. However, in one model the peptide bond between the Tyr1 and Tic2 residues assumes the trans conformation, whereas in the other it is in the cis conformation. To distinguish between these two models, we prepared the two peptides H-Tyr(psi)[CH2NH]Tic-Phe-Phe-OH and H-Tyr(psi)[CH2NH]MeTic-Phe-Phe-OH (MeTic = 3-methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) in which a cis peptide bond between the Tyr and Tic (or MeTic) residues is sterically forbidden. Both compounds turned out to be moderately potent delta-opioid antagonists in the mouse vas deferens assay. A molecular mechanics study performed with both peptides resulted in low-energy conformations in which the torsional angle ("omega1") of the reduced peptide bond between Tyr and Tic (or MeTic) had a value of 180 degrees (trans conformation) and which were in good agreement with the proposed model with all trans peptide bonds. Furthermore, this study confirmed that neither of these two peptides could assume low-energy conformations in which "omega1" had a value of 0 degrees (cis conformation). Conformers with that same bond in the gauche conformation ("omega1" = -60 degrees) were also identified, but were higher in energy and showed no spatial overlap with naltrindole. On the basis of these results it is concluded that the receptor-bound conformation of delta-peptide antagonists containing an N-terminal H-Tyr-Tic-dipeptide segment must have all trans peptide bonds.

Synthesis of a novel side-chain to side-chain cyclized enkephalin analogue containing a carbonyl bridge

A novel type of cyclic opioid peptide analogue, cyclo[N epsilon,N epsilon'-carbonyl-D-Lys2,Lys5]enkephalinamide, was prepared from a linear precursor peptide. The peptide was synthesized on the Merrified resin and also by a combination of the solid-phase technique and the classical method in solution. In both cases the cyclization was performed by reaction of bis(4-nitrophenyl)carbonate with the free side-chain amino groups of the two lysine residues. The described method permits the convenient preparation of novel peptide analogues cyclized via a ureido group incorporating the side-chain amino groups of two alpha, omega-diamino acid residues. The cyclic enkephalin analogue containing a 21-membered ring structure showed preference for mu over delta opioid receptors in opioid bioassays in vitro.

Structural modifications of the N-terminal tetrapeptide segment of [D-Ala2]deltorphin I: effects on opioid receptor affinities and activities in vitro and on antinociceptive potency

A series of deltorphin I analogs containing D- or L-N-methylalanine (MeAla), D- or L-proline (Pro), alpha-aminoisobutyric acid (Aib), sarcosine (Sar) or D-tert-leucine (Tle) in place of D-Ala2, or phenylalanine in place of Tyr1, was synthesized. The opioid activity profiles of these peptides were determined in mu and delta opioid receptor-representative binding assays and bioassays in vitro as well as in the rat tail flick test in vivo. In comparison with the deltorphin I parent, both the L- and the D-MeAla2-analog were slightly more potent delta agonists in the mouse vas deferens (MDV) assay, and the D-MeAla2-analog showed two-fold higher antinociceptive potency in the analgesic test. In view of the fact that deltorphin analogs with an unsubstituted L-amino acid residue in the 2-position generally lack opioid activity, the observed high delta opioid potency of [L-MeAla2]deltorphin I is postulated to be due to the demonstrated presence of a conformer with a cis Tyr1-MeAla2 peptide bond, since the cis conformer allows for a spatial arrangement of the pharmacophoric moieties in the N-terminal tripeptide segment similar to that in active deltorphin analogs containing a D-amino acid residue in the 2-position. Substitution of Aib in the 2-position led to a compound, H-Tyr-Aib-Phe-Asp-Val-Val-Gly-NH2, which displayed lower delta receptor affinity than the parent peptide but higher delta selectivity and, surprisingly, three times higher antinociceptive potency. The D- and L-Pro2-, Sar2- and D-Tle2-analogs showed much reduced delta receptor affinities and were inactive in the tail flick test. Replacement of Tyr1 in deltorphin I with Phe produced a 32-fold decrease in delta receptor affinity but only a 7-fold drop in antinociceptive potency.

Cyclic morphiceptin analogs: cyclization studies and opioid activities in vitro

Attempts were undertaken to develop cyclic beta-casomorphin-5 analogs with improved opioid activity profiles by deletion of the glycine residue in position 5, leading to analogs structurally related to the opioid peptide morphiceptin (H-Tyr-Pro-Phe-Pro-NH2). The tetrapeptide sequence Boc-Tyr(tBu)-D-Xaa-Phe-Yaa-OH (Xaa = Lys, Orn, A2bu; Yaa = Pro in L- or D-configuration) was used to study the influence of ring size and chirality on the yield of cyclization between the omega-amino group of Xaa and the C-terminal carboxyl group. In all cases the cyclization reaction was performed under identical experimental conditions to allow a direct comparison with regard to yield and homogeneity. The reaction products were purified by crystallization and liquid chromatography, and were characterized by HPLC, TLC, electrospray mass spectrometry and 1H-NMR spectroscopy. In none of the reactions performed with the cyclization precursors containing proline in the L-configuration could a cyclic monomer be detected, and the cyclodimer (7-9) was the exclusive product in each case. Cyclodimerization was also the favored reaction in the attempted formation of the 11-membered ring of the cyclic [D-A2bu2, D-Pro4]-morphiceptin analog 12, since only traces of the monomer were found. In the case of both the [D-Lys2, D-Pro4]-analog 10 and the [D-Orn2, D-Pro4]-analog 11, the cyclomonomer/cyclodimer ratio was about 80:20. The cyclic monomers 10 and 11 showed high opioid activity in the mu-receptor-representative guinea pig ileum assay (IC50 = 2-5 nM) and in the delta-receptor representative mouse vas deferens assay (IC50 = 50-60 nM), whereas the potency of the cyclodimers was 2-3 orders of magnitude lower in both in vitro bioassays.

Effect of aromatic amino acid substitutions in the 3-position of cyclic beta-casomorphin analogues on mu-opioid agonist/delta-opioid antagonist properties

The beta-casomorphin-5 analog H-Tyr-c[-D-Orn-2-Nal-D-Pro-Gly-] (2-Nal = 2-naphthylalanine) was the first reported cyclic opioid peptide with mixed mu agonist/delta antagonist properties [R. Schmidt et al. (1994) J. Med. Chem. 37, 1136-1144]. The 2-Nal3 residue in this peptide was replaced with benzothienylalanine (Bta) (3), His(Bzl) (4), Tyr(Bzl) (5), 4'-benzoylphenylalanine (Bpa) (6), 4'-benzylphenylalanine (Bzp) (7), thyronine (Thy) (8), thyroxine (Thx) (9), 4'-biphenylalanine (Bip) (10), 4'-biphenylglycine (Bpg) (12) and 3,3-diphenylalanine (Dip) (14), and the in vitro opioid activity profiles of the resulting compounds were determined in mu and delta receptor-representative binding assays and bioassays. Analogues 3, 12 and 14 were full agonists in the mu receptor-representative guinea-pig ileum (GPI) assay and also were agonists in the delta receptor-representative mouse vas deferens (MVD) assay. The agonist effects of the latter compounds in the MVD assay were antagonized by the highly selective delta antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP), indicating that they were triggered by delta receptor activation. The Bzp3- and Bip3- containing peptides 7 and 10 turned out to be mu antagonists against the mu selective agonist H-Tyr-D-Ala-Phe-Phe-NH2 in the GPI assay. The other analogues were weak partial mu agonists which displayed remarkably decreased mu receptor affinity as compared to parent peptide 1. Compounds 4-10 were found to be delta antagonists in the MVD assay. Analogues 4 and 9 exhibited delta antagonist potency similar to that of parent peptide 1, while compounds 5-8 and 10 showed 3-12-fold higher delta antagonist potency against DPDPE and deltorphin I and, in most cases, increased delta receptor affinity. These results indicate that the delta receptor tolerates bulky aromatic side chains in the 3-position of cyclic beta-casomorphin analogs with either delta agonist or delta antagonist properties. However, these compounds displayed drastically reduced mu receptor affinity in nearly all cases.

Synthesis and bioactivity studies of 1-adamantanamine derivatives of peptides

Small enkephalin-related peptides containing a 1-adamantanamine moiety coupled through an amide linkage at the C-terminus were synthesized. Several of the compounds showed high mu opioid activity and mu receptor selectivity. The new adamantanamine derivatives were also examined for antiviral activity against HIV-1 in a cell culture system. Some of them inhibited syncytia formation even when the antigen assay gave evidence for viral replication.

Tyrosine-iodination converts the delta-opioid peptide antagonist TIPP to an agonist

The binding properties and pharmacological activities of H-Tyr(3'-I)-Tic-Phe-Phe-OH ([Tyr(3'-I)1]TIPP) were studied. Similar to the delta-opioid receptor antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP), [Tyr(3'-I)1]TIPP is a selective and potent ligand at delta-opioid receptors. The displacement curve of [3H]diprenorphine binding by [Tyr(3'-I)1]TIPP was shifted to the right in the presence of Na+ and 5'-guanylylimidodiphosphate, suggesting that it acted as a delta-opioid receptor agonist. [Tyr(3'-I)1]TIPP also behaved as a full agonist in the mouse vas deferens assay and its effect was both naloxone- and TIPP-reversible. These data show that monoiodination at the 3'-position of the N-terminal tyrosine aromatic ring of TIPP converted it from a potent and selective antagonist to a full agonist at delta-opioid receptors.

Structure-activity relationships of dermorphin analogues containing N-substituted amino acids in the 2-position of the peptide sequence

A series of dermorphin analogues containing an N-alkylated amino-acid residue Xaa in the 2-position of the peptide sequence was synthesized (Xaa = N-methylalanine, proline, pipecolic acid, N-methylphenylalanine, 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid [Tic]). These peptides have the potential of assuming a cis Tyr1-Xaa2 peptide bond. Their in vitro opioid activity profiles were determined in mu- and delta-receptor-representative binding assays and bioassays. Aside from [D-Pro2]dermorphin, all analogues showed high affinity for mu- and/or delta-opioid receptors. Whereas most compounds were found to be full mu-agonists in the guinea pig ileum (GPI) assay, [Tic2]dermorphin (compound 7) was a partial mu-agonist. Replacement of Gly4 in 7 with Phe resulted in an analogue (8) with weak mu-antagonist activity. Furthermore, analogues 7 and 8 both were potent delta-antagonists (Ke = 3-40 nM) against the delta-agonists Leu-enkephalin, DPDPE and deltorphin I in the mouse vas deferens (MVD) assay. Compound 3, containing L-Pro in the 2-position, turned out to be one of the most mu-receptor-selective linear dermorphin analogues reported to date. Low-temperature HPLC experiments using micropellicular octadecyl silica as stationary phase revealed conformational heterogeneity of the dermorphin analogues which was ascribed to cis-trans isomerization around the Tyr1-Xaa2- and Tyr5-Pro6 peptide bonds. In the case of analogue 7 four separate peaks corresponding to the four possible isomers were apparent at -5 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Highly potent side chain-main chain cyclized dermorphin-deltorphin analogues: an integrated approach including synthesis, bioassays, NMR spectroscopy and molecular modelling

Our continuing efforts to study structure-activity relationships of peptide opioids have resulted in the synthesis of a series of cyclic opioids related to dermorphins and deltorphins. The biological activities of the compounds have been determined and the conformational analyses carried out using 1H-NMR spectroscopy and molecular modelling. The three compounds in the series Tyr-c[D-Orn-Phe-Ala], Tyr-c[D-Lys-Phe-Ala], and Tyr-c[A2bu-Phe-Ala-Leu] are cyclized via a lactam bridge from the side-chain of the residue at the second position with the carboxyl terminus of each compound. The molecules incorporate 12-, 13- and 14-membered rings, respectively. They include a phenylalanine at the third position which is a distinguishing characteristic of dermorphins and deltorphins. The guinea pig ileum and mouse vas deferens assays show that the compounds are highly active at both mu- and delta-opioid receptors. The compounds are all highly effective antinociceptive agents as measured by the intrathecal rat hot plate test. Conformational analyses of the molecules indicate that they can adopt topochemical arrays required for bioactivity at both mu- and delta-receptors which explains their high activity in both guinea pig ileum and mouse vas deferens in vitro assays. The results support our models for mu- and delta-receptor activity for constrained peptide opioids.

Six highly active mu-selective opioid peptides identified from two synthetic combinatorial libraries

Two synthetic combinatorial libraries (SCLs) were prepared, each composed of 52,128,400 L-amino acid hexapeptides, one with and the other without an N-terminal acetyl moiety. The two libraries were used in conjunction with an iterative selection process to identify individual peptides capable of inhibiting the binding of the mu-selective opioid peptide [3H]-[D-Ala2,MePhe4,Gly-ol5]enkephalin to rat brain homogenates. As reported previously, when using the nonacetylated SCL the first five residues identified corresponded exactly to methionine- and leucine-enkephalin, both of which are endogenous opioid peptides. The iterative identification process has now been completed for two additional mixtures found to have activity in the initial screening of this SCL. Two new series unrelated to the enkephalins have been identified: YPFGFO-NH2 and WWPKHO-NH2 (where O = one of the 20 L-amino acids). Individual peptides from each of these were found to be agonists at the mu receptor and have high affinity (IC50 values of the most active peptides were 10-15 nM) and selectivity for the mu receptor. In addition to the acetalins (described previously), two new series have now been identified from the acetylated library: Ac-FRWWYO-NH2 and Ac-RWIG-WO-NH2 (IC50 values of the most active peptides were 5-30 nM). Ac-FRWWYM-NH2 was determined to be an agonist at the mu receptor, whereas Ac-RWIGWR-NH2 was found to be an antagonist at this receptor.

An all D-amino acid opioid peptide with central analgesic activity from a combinatorial library

A synthetic combinatorial library containing 52,128,400 D-amino acid hexapeptides was used to identify a ligand for the mu opioid receptor. The peptide, Ac-rfwink-NH2, bears no resemblance to any known opioid peptide. Simulations using molecular dynamics, however, showed that three amino acid moieties have the same spatial orientation as the corresponding pharmacophoric groups of the opioid peptide PLO17. Ac-rfwink-NH2 was shown to be a potent agonist at the mu receptor and induced long-lasting analgesia in mice. Analgesia produced by intraperitoneally administered Ac-rfwink-NH2 was blocked by intracerebroventricular administration of naloxone, demonstrating that this peptide may cross the blood-brain barrier.

Cyclic beta-casomorphin analogues with mixed mu agonist/delta antagonist properties: synthesis, pharmacological characterization, and conformational aspects

Analogues of the potent and moderately mu-opioid-receptor-selective cyclic beta-casomorphin-5 derivative H-Tyr-c[-D-Orn-Phe-D-Pro-Gly-] (2) were prepared by conventional solution synthesis. Replacement of the Phe3 residue by 2-naphthylalanine (2-Nal) led to a peptide (4) with high affinity for both mu and delta opioid receptors. This compound turned out to be an agonist in the mu-receptor-representative guinea pig ileum (GPI) assay but a moderately potent antagonist against various delta agonists in the delta-receptor-representative mouse vas deferens (MVD) assay. It thus represents the first known cyclic opioid peptide analogue with mixed mu agonist/delta antagonist properties. Interestingly, replacement of 2-Nal3 in compound 4 with 1-naphthylalanine (1-Nal) resulted in an analogue (5) showing high affinity for mu receptors and a full agonist effect in the MVD assay that was mediated by both mu and delta receptors. Substitution of Trp for Phe3 in 2 (compound 8) was well tolerated at both receptors and led to an analogue with agonist activity in both the GPI and MVD assays. Variation of the peptide ring size in 4 was achieved by substitution of D-Orn2 with D-Lys (compound 6) or D-2,4-diaminobutyric acid (compound 7). Analogue 6 was also a mixed mu agonist/delta antagonist with somewhat lower potency than 4, whereas compound 7 displayed mu agonist and partial delta agonist properties. Further reduction of the peptide ring size, as achieved by deletion of the Gly5 residue, produced a compound (9) which was a full agonist in both bioassays. Conformational analysis of analogues 2, 4, and 5 by 1H NMR spectroscopy and molecular mechanics studies suggested that the overall conformation of parent compound 2 and the 2-Nal-containing peptide 4 was similar, while the side-chain orientation of 1-Nal in peptide 5 was different. These results suggest that the delta antagonist properties of analogue 4 may not be due to a difference in its overall conformation as compared to the agonist 2 but may be a direct effect of the 2-naphthyl moiety per se preventing proper alignment of the peptide for receptor activation.

Acetylated glucopyranosyl esters of enkephalins

Acetylated D-glucopyranosyl esters of enkephalins were prepared by two different fragment condensation procedures involving direct participation of imidazole in the ester linkage formation. By both methods anomeric mixtures of D-glucosyl esters were obtained and resolved by column chromatography. Depending on coupling conditions, racemization of either the C-terminal or the penultimate amino acid residue of the enkephalin molecule occurred. The glucoconjugates with inverted stereochemistry were quantitated and separated from the main product by reversed-phase high-performance liquid chromatography. The opioid agonist potencies of the synthesized glucopyranosyl esters of enkephalins on electrically stimulated guinea pig ileum and mouse vas deferens preparations were determined in comparison with [Leu5]enkephalin.

Acetalins: opioid receptor antagonists determined through the use of synthetic peptide combinatorial libraries

A synthetic peptide combinatorial library made up of 52,128,400 hexapeptides, each having an acetyl group at the N terminus and an amide group on the C terminus, was screened to find compounds able to displace tritiated [D-Ala2,MePhe4,Gly-ol5]enkephalin from mu opioid receptor binding sites in crude rat brain homogenates. Individual peptides with mu receptor affinity were found using an iterative process for successively determining the most active peptide mixtures. Upon completion of this iterative process, the three peptides with the highest affinity were Ac-RFMWMT-NH2, Ac-RFMWMR-NH2, and Ac-RFMWMK-NH2. These peptides showed high affinity for mu and kappa 3 opioid receptors, somewhat lower affinity for delta receptors, weak affinity for kappa 1 receptors, and no affinity for kappa 2 receptors. They were found to be potent mu receptor antagonists in the guinea pig ileum assay and relatively weak antagonists in the mouse vas deferens assay. These peptides represent a class of opioid receptor ligands that we have termed acetalins (acetyl plus enkephalin).

TIPP[psi]: a highly potent and stable pseudopeptide delta opioid receptor antagonist with extraordinary delta selectivity

Pseudopeptide analogues of the delta opioid antagonists H-Tyr-Tic-Phe-Phe-OH (TIPP) and H-Tyr-Tic-Phe-OH (TIP) containing a reduced peptide bond between the Tic2 and Phe3 residues were synthesized. The two compounds, H-Tyr-Tic psi [CH2NH]Phe-Phe-OH (TIPP [psi]) and H-Tyr-Tic psi-[CH2NH]Phe-OH (TIP [psi]), were tested in mu-, delta-, and kappa-receptor-selective binding assays and in the guinea pig ileum (GPI) and mouse vas deferens (MVD) bioassays. In comparison with their respective parent peptides, both pseudopeptide analogues showed increased delta antagonist potency in the MVD assay, higher delta receptor affinity and further improved delta receptor selectivity. The more potent compound, TIPP [psi], displayed subnanomolar delta receptor affinity and in direct comparisons with other selective delta ligands was shown to have unprecedented delta specificity (Ki mu/Ki delta = 10,500). Furthermore, this compound turned out to be highly stable against enzymatic degradation and, unlike other delta antagonists, showed no mu or kappa antagonist properties. TIPP [psi] is likely to find wide use as a pharmacological tool in opioid research.

Methionine-enkephalin related glycoconjugates. Synthesis and biological activity

A series of glycoconjugates, in which [Met5]enkephalin or [D-Ala2,Met5]enkephalin have been linked through an ester bond to the HO-6 of various D-glycopyranose moieties, were synthesized by classical solution methods. The biological activities of these compounds were determined on selective pharmacological preparations: guinea pig ileum and mouse vas deferens for opioid activity, and two mouse cell lines, fibroblasts L929 and melanoma B16BL6, to study the influence on growth processes. The results reported in this study demonstrate the differential effect of the carbohydrate part in enkephalin-related glycoconjugates on receptor recognition. In addition, synthesized neo-glycopeptides stimulate growth of the examined mouse cell lines, whereas parent peptide demonstrated some growth inhibitory properties. Full growth curves showed a dose-dependent effect at concentrations of 10(-7) to 10(-10) M.

A topochemical approach to explain morphiceptin bioactivity

A topochemical model to explain the bioactivity of morphiceptin (Tyr1-Pro2-Phe3-Pro4-NH2) was developed by taking account of accessible conformations around rotatable bonds which define relative spatial arrangements of opioid pharmacophores, the amine and phenolic groups of tyrosine and the aromatic ring of phenylalanine, necessary for receptor recognition. For this purpose, 1H-NMR measurements and computer simulations were extensively carried out on 10 stereoisomeric analogs related to morphiceptin: Tyr-Pro-(L and D)-Phe- (L and D)-Pro-NH2; Tyr-Pro-(L and D)-(NMe)Phe-(L and D)-Pro-NH2; Tyr-(NMe)Ala-Phe-D-Pro-NH2; and Tyr-Ala-Phe-D-Pro-NH2. These analogs are structurally close to one another but display various opiate potencies from highly active to inactive. The conformation of each rotatable bond has been specifically identified by measuring accessible space for the analogs, in which the difference in composition is observed in the specific site affecting only the conformation around the target bond. The most interesting characteristic of the model is a requirement of a cis amide bond linking residues 1 and 2. The model also requires the side chains in a trans conformation (chi 1 = 180 degrees) for the Tyr and Phe residues. The distances between the three pharmacophores, d1 (Tyr N to Tyr OH), d2 (Tyr N to the center of the aromatic ring of the third residue), and d3 (Tyr OH to the center of the aromatic ring of the third residue), were found to be approximately 8, approximately 7, and approximately 11-13 A, respectively. This model should aid in pharmaceutical design of peptide and nonpeptide ligands with opioid potencies.

Differential stereochemical requirements of mu vs. delta opioid receptors for ligand binding and signal transduction: development of a class of potent and highly delta-selective peptide antagonists

Opioid peptide analogs consisting entirely of aromatic amino acid residues and containing conformationally restricted phenylalanine derivatives in position 2 of the peptide sequence were synthesized and pharmacologically characterized in vitro. Both diastereoisomers of H-Tyr-(D or L)-NMePhe-Phe-Phe-NH2 (NMePhe is N alpha-methylphenylalanine) were mu-receptor-selective, were full agonists in the mu-receptor-representative guinea pig ileum assay, and were partial agonists in the mouse vas deferens assay, with the L-NMePhe2 analog displaying somewhat higher intrinsic activity than the D-NMePhe2 analog. Further conformational restriction at position 2 in the sequence, as achieved through substitution of D- or L-tetrahydro-3-isoquinoline carboxylic acid (Tic), produced a configuration-dependent differential effect on receptor selectivity and intrinsic activity, leading to a potent mu-selective mu agonist (the D-Tic2 analog) with increased intrinsic activity in the mouse vas deferens assay and to a potent delta-selective delta antagonist (the L-Tic2 analog). These results demonstrate that imposition of conformational constraints in a peptide not only may alter receptor selectivity but also may decrease, totally abolish, or even enhance intrinsic activity. The tetrapeptide H-Tyr-Tic-Phe-Phe-NH2 was a moderately potent full agonist in the guinea pig ileum assay and, thus, represents a compound with mixed mu-agonist/delta-antagonist properties. The corresponding peptide with a free C-terminal carboxyl group H-Tyr-Tic-Phe-Phe-OH showed high delta-receptor affinity (Ki delta = 1.2 nM), unprecedented delta selectivity (Ki mu/Ki delta = 1410), high potency as delta antagonist (Ke = 3-8 nM against various delta agonists in the mouse vas deferens assay) and, unlike other delta antagonists, had no mu-antagonist properties. The tripeptides H-Tyr-Tic-Phe-OH and H-Tyr-Tic-Phe-NH2 were also delta antagonists.

Conformationally restricted deltorphin analogues

Conformationally restricted deltorphin analogues were synthesized either through incorporation of cyclic phenylalanine analogues in position 2 or 3 of the peptide sequence or through various side chain-to-side chain cyclizations. Compounds were tested in mu-, delta-, and kappa-receptor selective binding assays and in the guinea pig ileum (GPI) and mouse vas deferens (MVD) bioassays. Replacement of Phe3 in [D-Ala2]deltorphin I with 2-aminoindan-2-carboxylic acid (Aic) or L- or D-2-aminotetralin-2-carboxylic acid (Atc) resulted in agonist compounds which retained the high delta receptor selectivity of the parent peptide. Substitution of a tetrahydroisoquinoline-3-carboxylic acid (Tic) residue in the 2-position of [D-Ala2]deltorphin I and of [Phe4,Nle6]deltorphin produced a partial delta agonist, H-Tyr-Tic-Phe-Asp-Val-Val-Gly-NH2, and a pure delta antagonist, H-Tyr-Tic-Phe-Phe-Leu-Nle-Asp-NH2, respectively. The latter antagonist displayed high delta selectivity (Ki mu/Ki delta = 502) and was a potent antagonist against selective delta agonists in the MVD assay (Ke congruent to 10 nM). Various [D-Ala2]-deltorphin I analogues cyclized between the side chains of Orn (or Lys) and Asp (or Glu) residues substituted in positions 2 and 4, 4 and 7, and 2 and 7 were essentially nonselective. Comparison with corresponding N-terminal tetrapeptide analogues revealed that the C-terminal tripeptide segment in the deltorphin heptapeptides made a crucial contribution to delta affinity and delta selectivity in the case of the agonist peptides but not in the case of the antagonist.