Proton NMR conformational analysis of cyclic beta-casomorphin analogues of the type Tyr-cyclo[N omega-D-Orn-Xaa-Yaa-Gly-]

A conformational study of the cyclic beta-casomorphin-5 analogues H-Tyr-cyclo[-D-Orn-2-Nal-Pro-Gly-] (1) (mu-selective agonist; 2-Nal = 2 naphthylalanine), H-Tyr-cyclo[-D-Orn-2-Nal-D-Pro-Gly-] (2) (mixed mu agonist/delta antagonist) and H-Tyr-cyclo[-D-Orn-Phe-D-Pro-Gly-] (3) (highly potent mu and delta agonist) has been carried out using 1H NMR spectroscopy. A complete assignment of the proton resonances of the three pentapeptides has been achieved. Compound 1 was shown to exist in two conformations, a major one (90%) characterized by a cis amide bond between 2-Nal3 and Pro4, and a minor one (10%) showing cis amide bonds both between D-Orn2 and 2-Nal3 and between 2-Nal3 and Pro4. Peptides 2 and 3 each showed only one conformer with all-trans peptide bonds in both cases. Temperature dependence studies of the amide proton chemical shifts indicated the existence of several intramolecular hydrogen bonds in the case of compounds 2 and 3 but not in the case of peptide 1. The backbone conformations of 2 and 3 were found to be similar, both being characterized by two consecutive gamma turns around the D-Pro4 and D-Orn2 residues, respectively, and by a D-Orn2-CO<--HN delta-D-Orn2 hydrogen bond. Altogether, the overall backbone conformation and the preferred side chain conformation were found to be roughly similar for the three title peptides. For all three compounds a close proximity between the aromatic moiety of the 3-position residue (2-Nal or Phe) and the D(or L)-Pro4 residue was established on the basis of ROESY experiments. The examination of low energy conformations obtained in molecular modelling studies by taking into account the various experimentally found NMR parameters (NOEs, vicinal H,H coupling constants, torsion angles, H-bonds) led to proposals of the solution conformation for each peptide. These conformations are in close agreement with a pharmacophore model for mu opioid receptor binding compounds.

Influence of sample pH on the conformational backbone dynamics of a pseudotripeptide (H-Tyr-Tic psi [CH2-NH]Phe-OH) incorporating a reduced peptide bond: an NMR investigation

In the present paper we investigate the influence of sample pH on the conformational and dynamical properties of the pseudotripeptide H-Tyr-Tic psi [CH2-NH]Phe-OH (TIP[psi]; Tic: 1,2,3,4,-tetrahydroisoquinoline-3-carboxylic acid) using various one- and two-dimensional nmr techniques in conjunction with molecular modeling. Studies were conducted at three different pH levels corresponding to the zwitterionic peptide containing a formal positive charge (pH 3.1), the deprotonated molecule (pH 9.1), and a situation at neutral pH (pH 7.2) involving both protonated and deprotonated states of the reduced peptide bond. Analysis of the one-dimensional1H-nmr spectra reveals that in solution TIP[psi] is in slow dynamic exchange between conformations containing cis and trans configurations of the Tyr-Tic bond. An nmr pH dependence study of the cis:trans ratio indicated that the exchange process was governed by the protonation state of the reduced bone amine. From the nmr data, reduced peptide bond pK alpha values of 6.5 and 7.5 were determined for the cis and trans conformers, respectively. It was concluded that conformations containing a trans Tyr-Tic bond are stabilized at low pH by an intramolecular hydrogen bond between the Tyr carbonyl and the reduced peptide bond protonated amine. This observation was corroborated by molecular mechanics investigations that revealed low energy trans structures compatible with nmr structural data, and furthermore, were consistently characterized by the existence of a strong N+ H ... O = C interaction closing a seven-membered cycle. The dynamics of cis-trans isomerization about the Tyr-Tic peptide bond were probed by nmr exchange experiments. The selective presaturation of exchanging resonances carried out at several temperatures between 50 and 70 degrees C allowed the determination of isomerization rate constants as well as thermodynamic activation parameters. delta G not equal to values were in close agreement with the cis-->trans energy barrier found in X-Pro peptide fragments (approximately 83 kJ/mol). A large entropic barrier determined for the trans-->cis conversion of TIP[psi] (5.7 JK-1 mol-1 at pH 3.1;6.5 J K-1 mol-1 at pH 9.1) is discussed in terms of decreased solvent molecular ordering around the conformers possessing a trans Tyr-Tic bond. Evidence that the neutral form of the reduced peptide bond gains rigidity upon protonation was obtained from relaxation measurements in the rotating frame. T1 rho measurements of several protons in the vicinity of the reduced peptide bond were made as a function of spin-lock field.(ABSTRACT TRUNCATED AT 400 WORDS)

Attenuation of morphine tolerance and dependence with the highly selective delta-opioid receptor antagonist TIPP[psi]

We examined the effects of i.c.v. treatment with naltrindole, and the two highly selective peptide delta-opioid receptor antagonists H-Tyr-Tic-Phe-Phe-OH (TIPP) and H-Tyr-Tic psi [CH2-NH]-Phe-Phe- OH (TIPP[psi]), on the development of morphine tolerance and dependence. Each treatment significantly decreased naloxone-precipitated withdrawal, with TIPP[psi] reducing the most symptoms. TIPP[psi], but neither naltrindole nor TIPP, attenuated the development of analgesic tolerance in the tail-flick test. These results suggest that delta-opioid receptors are critically involved in the development of morphine tolerance and dependence.

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.

Synthesis and binding characteristics of tritiated TIPP[psi], a highly specific and stable delta opioid antagonist

The pseudopeptide H-Tyr-Tic psi [CH2-NH]Phe-Phe-OH (TIPP[psi]) is a delta opioid antagonist with high delta receptor affinity and unprecedented delta selectivity. TIPP[psi] was radiolabelled by catalytic tritiation of its precursor [Tyr(3',5'-I2)1]TIPP[psi]. The resulting radioligand, [3H]TIPP[psi], had a specific activity of 1.77 TBq/mmol (47.9 Ci/mmol) and showed high stability against enzymatic degradation. [3H]TIPP[psi] binding to rat brain membranes was saturable and Scatchard analysis indicated a single binding site with a Kd of 0.98 nM and a Bmax of 105.4 fmol/mg. A study of [3H]TIPP[psi] binding displacement by various receptor-selective opioids showed the expected rank order of potency (delta >> mu > kappa). [3H]TIPP[psi] represents an excellent new radioligand for delta receptor labelling studies in vitro and in vivo.

Comparative analysis of various proposed models of the receptor-bound conformation of H-Tyr-Tic-Phe-OH related delta-opioid antagonists

A molecular mechanics study (grid search and energy minimization) was performed with six delta opioid peptide antagonists containing a tetrahydroisoquinoline-3-carboxylic acid (Tic) residue in the 2-position of the peptide sequence. Compounds examined were the highly potent and selective TIP(P) peptides H-Tyr-Tic-Phe-OH (TIP), H-Tyr-Tic psi[CH2-NH]Phe-OH (TIP[psi]), H-Tyr-Tic-Phe-Phe-OH (TIPP), and H-Tyr-Tic psi[CH2-NH]Phe-Phe-OH (TIPP[psi]), and the weakly active analogues H-Tyr-Tic-NH2 and H-Tyr-Tic-Ala-OH. Low energy conformers of the peptides were examined for their compatibility with three proposed models of the delta receptor-bound conformation. Model 1, based on spatial overlap of the Tyr1 and Phe3 aromatic rings and N-terminal amino group of the peptides with the corresponding aromatic rings and nitrogen atom of the nonpeptide delta-antagonist naltrindole, was ruled out because of the demonstrated importance of the Tic2 aromatic ring for delta antagonism and because of the somewhat elevated energies of the conformers consistent with this model. Models of the receptor-bound conformation based on superimposition of the Tyr1 and Tic2 aromatic rings and N-terminal amino group of the peptides with the corresponding moieties in naltrindole included an all-trans peptide bond conformer [model 2, proposed by B.C. Wilkes and P.W. Schiller (1994) Biopolymers, Vol. 34, pp. 1213-1219] and a conformer with a cis peptide bond between the Tyr1 and Tic2 residues (model 3, originally proposed by P.A. Temussi et al. [(1994) Biochemical and Biophysical Research Communications, Vol. 198, pp. 933-939]. For all six peptides low energy conformers consistent with both model 2 and model 3 were identified; however, peptide conformers corresponding to model 2 showed better coplanarity of the Tyr1 aromatic ring and the phenol ring in naltrindole than peptide conformers corresponding to model 3. Both models remain plausible candidate structures for the receptor-bound conformation of delta antagonists of the TIP(P) class. TIP(P) analogues containing additional conformational constraints need to be developed in order to arrive at a unique model.

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.

TIPP[psi], a highly selective delta ligand

TIPP[psi] is a new delta-selective opioid peptide antagonist. In the current study, we have explored its selectivity against the mu and kappa receptor subtypes. Against [3H]DPDPE binding, TIPP[psi] is quite potent, with a Ki value of < 1 nM, confirming its potent activity at delta receptors. In contrast, its Ki values against mu 1, mu 2, kappa 1, kappa 2 and kappa 3 binding sites are all > 5 microM. DPDPE also is delta-selective. It labels delta sites > 25-fold more potently than mu 1 receptors and is even more selective against the other subtypes. However, this selectivity does not compare to the delta/mu 1 selectivity of TIPP[psi] which exceeds 15,000. This far higher selectivity, coupled with its antagonist properties, gives TIPP[psi] a number of advantages over previously reported delta-selective compounds. We have utilized these advantages to develop an improved mu 1 binding assay using TIPP[psi].

Theoretical conformational analysis of the opioid delta antagonist H-Tyr-Tic-Phe-OH and the mu agonist H-Tyr-D-Tic-Phe-NH2

A molecular mechanics study (grid search and energy minimization) of the highly delta receptor-selective delta opioid antagonist H-Tyr-Tic-Phe-OH (TIP; Tic: tetrahydroisoquinoline-3-carboxylic acid) resulted in four low energy conformers with energies within 2 kcal/mol of that of the lowest energy structure. These four conformers contain trans peptide bonds only and represent compact structures showing various patterns of aromatic ring stacking. The centrally located Tic residue imposes several conformational constraints on the N-terminal dipeptide segment; however, the results of molecular dynamics simulations indicated that this tripeptide still shows some structural flexibility, particularly at the Phe3 residue. Analogous studies performed with the structurally related mu receptor-selective mu agonist H-Tyr-D-Tic-Phe-NH2 resulted in low energy structures that were also compact but showed patterns of ring stacking different from those obtained with TIP. Superimposition of low energy conformers of TIP and H-Tyr-D-Tic-Phe-NH2 revealed that the Phe3 residues of the L-Tic- and the D-Tic peptide were always located on opposite sides of the plane defined by the Tic residue, thus providing an explanation for the distinct activity profiles of the two compounds in structural terms. Attempts to demonstrate spatial overlap between the pharmacophoric moieties of low energy conformers of TIP and the nonpeptide delta antagonist naltrindole were made by superimposing either the Tyr1 and Tic2 aromatic rings and the N-terminal amino group or the Tyr1 and Phe3 aromatic rings and the N-terminal amino group of the peptide with the corresponding aromatic rings and nitrogen atom in the alkaloid structure.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Synthesis and binding characteristics of the highly specific, tritiated delta opioid antagonist [3H]TIPP

The recently discovered highly specific delta opioid antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP) was radiolabelled by catalytic tritiation of its precursor [Tyr (3', 5'-I2)1]TIPP. The tritiated ligand labelled rat brain membrane binding sites with a Kd value of 0.64 nM and a Bmax of 82 fmol/mg protein. Binding competition experiments with various unlabelled mu-, delta- and kappa-opioid receptor ligands resulted in mu/delta and kappa/delta selectivity ratios of 600 and 9100, respectively, and thus confirmed the previously established high delta specificity of TIPP. Because of its high delta receptor affinity, extraordinary delta selectivity and low non-specific binding, [3H]TIPP represents an excellent new radioligand for the study of delta opioid receptor interactions.

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.