The TIPP opioid peptide family: development of delta antagonists, delta agonists, and mixed mu agonist/delta antagonists

The discovery of the prototype delta opioid antagonists TIPP (H-Tyr-Tic-Phe-Phe-OH) and TIP (H-Tyr-Tic-Phe-OH) in 1992 was followed by extensive structure-activity relationship studies, leading to the development of analogues that are of interest as pharmacological tools or as potential therapeutic agents. Stable TIPP-derived delta opioid antagonists with subnanomolar delta receptor binding affinity and extraordinary delta receptor selectivity include TIPP[Psi] (H-Tyr-TicPsi[CH(2)NH]Phe-Phe-OH] and TICP[Psi] (H-Tyr-TicPsi[CH(2)NH]Cha-Phe-OH); Cha: cyclohexylalanine), which are widely used in opioid research. Theoretical conformational analyses in conjunction with the pharmacological characterization of conformationally constrained TIPP analogues led to a definitive model of the receptor-bound conformation of H-Tyr-Tic-(Phe-Phe)-OH-related delta opioid antagonists, which is characterized by all-trans peptide bonds. Further structure-activity studies revealed that the delta antagonist vs delta agonist behavior of TIP(P)-derived compounds depended on very subtle structural differences in diverse locations of the molecule and suggested a delta receptor model involving a number of different inactive receptor conformations. A further outcome of these studies was the identification of a new class of potent and very selective dipeptide delta agonists of the general formula H-Tyr-Tic-NH-X (X = arylalkyl), which are of interest for drug development because of their low molecular weight and lipophilic character. Most interestingly, TIPP analogues containing a C-terminal carboxamide group displayed a mixed mu agonist/delta antagonist profile, and thus were expected to be analgesics with a low propensity to produce tolerance and physical dependence. This turned out to be the case with the TIPP-derived mu agonist/delta antagonist DIPP-NH(2)[Psi] (H-Dmt-TicPsi[CH(2)NH]Phe-Phe-NH(2)); Dmt: 2',6'- dimethyltyrosine).

Matrix-assisted laser desorption/ionization mass spectrometric quantification of the mu opioid receptor agonist DAMGO in ovine plasma

The synthetic opioid peptide analog Tyr-D-Ala-Gly-N-methyl-Phe-Gly-ol (DAMGO), which is a mu opioid receptor-selective agonist, was quantified in ovine plasma samples with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), using delayed extraction and a reflectron. The internal standard was pentadeuterated DAMGO. Timed-ion selection was used to select the precursor ion. The analysis of the post-source decay fragments improved the detection sensitivity, and the use of the precursor-product ion relationship optimized the specificity. For plasma samples, the inter-assay variability of this method was 6.4% (n = 79) and the intra-assay variability was 6.0% (n = 10). The variability for controls was 3.4% (n = 43). The profile of DAMGO amount versus time was determined in sheep plasma, and the corresponding pharmacokinetic data were calculated.

Optically active aromatic amino acids. Part VI. Synthesis and properties of (Leu5)-enkephalin analogues containing O-methyl-L-tyrosine1 with ring substitution at position 3'

Twelve new [Tyr(Me)1, Leu5]-enkephalin analogues with substituents at position 3' of the Tyr ring have been synthesized using traditional solution methods. The substituents were -CO2H, -CONH2, -CO2Me, -(E)-CH=NOH, -(E)-CH=NOMe and CH2OH. The analogues were C-terminated with methyl esters, amides or as free acids. In the in vitro biological assays a remarkable agonist activity to the opiate receptor mu in guinea pig ileum (GPI) relative to Leu-ENK was shown by the following: Leu-ENK, 100; [Tyr(Me)(3'-CO2Me)1, Leu-OMe5]-ENK (I), 8.1; [Tyr(Me)(3'-(E)-CH=NOH)1, Leu-OMe5]-ENK (VI), 26.2; [Tyr(Me)(3'-(E)-CH=NOH)1, Leu-OH5]-ENK (VII), 2.9; [Tyr(Me)(3'-(E)-CH=NOH)1, Leu-NH2(5)]-ENK (VIII), 4.7; and [Tyr(Me)(3'-CH2OH)1, Leu-OMe5]-ENK (X), 5.6. The agonist effect was naltrexone- or naloxone-reversible. The masking of the hydroxyl group in (E)-hydroxyiminomethyl group of analogue (VI) by O-methylation has totally abolished its GPI agonist activity. It seems that the (E)-CH=NOH group shows affinity and plays an analogous role to the phenol group Tyr1 in leucine-enkephalin and in the tyramine group of the opiate alkaloids. The analogues: [Tyr(Me)(3'-CO2Me)1, Leu-OMe5]-ENK (I), [Tyr(Me)(3'-CO2H)1, Leu-OMe5]-ENK (II), [Tyr(Me)(3'-CO2Me)1, Leu-NH2(5)]-ENK (III), [Tyr(Me)(3'-CO2H)1, Leu-NH2(5)]-ENK (IV), [Tyr(Me)(3'-CONH2)1, Leu-NH2(5)]-ENK (V), [Tyr(Me)(3'-(E)-CH=NOH)1, Leu-OMe5]-ENK (VI), [Tyr(Me)(3'-(E)-CH=NOH)1, Leu-OH5]-ENK (VII), [Tyr(Me)(3'-(E)-CH=NOH)1, Leu-NH2(5)]-ENK (VIII), [Tyr(Me)(3'-(E)-CH=NOMe)1, Leu-OMe5]-ENK (IX), [Tyr(Me)(3'-CH2OH)1, Leu-OMe5]-ENK (X), [Tyr(Me)(3'-CH2OH)1, Leu-OH5]-ENK (XI) and [Tyr(Me)(3'-CH2OH)1, Leu-NH2(5)]-ENK (XII) under testing had no significant agonist activity to the enkephalinergic receptor in mouse vas deferens (MVD). All methyl esters of synthesized analogues of [Leu5]-ENK showed higher activity to mu receptors than structurally identical C-terminal amides. It is a surprising result since usually C-terminate amides are stronger agonists than C-terminate esters.

An enzymatically stable kyotorphin analog induces pain in subattomol doses

Intraplantar injection of the enzymatically stable, N-methylated kyotorphin analog Tyr(NMe)-Arg-OH produced marked and sharp nociceptive flexor responses in a dose-dependent manner. A significant response was observed with this compound at a dose of 0. 01 amol (6000 molecules). Tyr(NMe)-Arg-OH-nociception was completely blocked by the kyotorphin antagonist leucyl-arginine and its enzymatically stable, N-methylated analog, as well as by CP-99994, a specific neurokinin 1 antagonist. These findings suggest that the nociceptive effect produced by Tyr(NMe)-Arg-OH in subattomol doses occurs via specific interaction with the kyotorphin receptor and that the extraordinary potency observed may result from amplification through local substance P release.

Novel ligands lacking a positive charge for the delta- and mu-opioid receptors

Recently we reported using minilibraries to replace Lys(9) [somatostatin (SRIF) numbering] of the potent somatostatin agonist L-363,301 (c[-Pro-Phe-D-Trp-Lys-Thr-Phe-]) to generate the potent neurokinin receptor (NK-1) antagonist c[-Pro-Phe-D-Trp-p-F-Phe-Thr-Phe-]. This novel cyclic hexapeptide did not bind the SRIF receptor. Thus, a single mutation converted L-363,301, a SRIF agonist with potency ca. 2-8 times the potency of SRIF in laboratory animals,(24) into a selective NK-1 receptor antagonist with an IC(50) of 2 nM in vitro. During the screening of the same libraries for ligands of the delta-opioid receptor, we identified four compounds (1-4) which represent a new class of delta-opioid antagonists, some of which were also NK-1 receptor antagonists. The most potent delta-opioid antagonist, c[-Pro-1-Nal-D-Trp-Tyr-Thr-Phe-] (2), showed a K(e) value of 128 nM in the mouse vas deferens assay and a delta-receptor binding affinity constant of 152 nM in the rat brain membrane binding assay. These results are of interest because they represent a novel class of delta-opioid antagonists and, like two previously reported delta-opioid antagonists, they lack a positive charge. To examine further the requirement for a positive charge in the delta-opioid ligands, we prepared two analogues of the beta-casomorphin-derived mixed mu-agonist/delta-antagonist, H-Dmt-c[-D-Orn-2-Nal-D-Pro-Gly-] (7), in which we eliminated the positive charge either through formylation of the primary amino group (5) or by the deletion of this N-terminal amino group (6). These latter compounds proved to be delta-opioid antagonists with K(e) values in the 16-120 nM range, as well as fairly potent mu-opioid antagonists (K(e) approximately 200 nM). These six compounds provide the most convincing evidence to date that there is no requirement for a positive charge in mu- and delta-opioid receptor antagonists. In addition, cyclic hexapeptide 4 lacks a phenolic hydroxyl group. Taken together, these data suggest that the prevailing assumptions about delta- and mu-opioid receptor binding need revision and that the receptors for these opioid ligands have much in common with the NK-1 and somatostatin receptors.

The opioid mu agonist/delta antagonist DIPP-NH(2)[Psi] produces a potent analgesic effect, no physical dependence, and less tolerance than morphine in rats

Opioid compounds with mixed mu agonist/delta antagonist properties are expected to be analgesics with low propensity to produce tolerance and dependence. In an effort to strengthen the mu agonist component of the mixed mu agonist/delta antagonist H-Tyr-Tic-Phe-Phe-NH(2) (TIPP-NH(2)), analogues containing structurally modified tyrosine residues in place of Tyr(1) were synthesized. Among the prepared compounds, H-Dmt-Tic-Phe-Phe-NH(2) (DIPP-NH(2); Dmt = 2',6'-dimethyltyrosine) and H-Dmt-TicPsi[CH(2)NH]Phe-Phe-NH(2) (DIPP-NH(2)[Psi]) retained a mixed mu agonist/delta antagonist profile, as determined in the guinea pig ileum and mouse vas deferens assays, whereas H-Tmt-Tic-Phe-Phe-NH(2) (Tmt = N,2',6'-trimethyltyrosine) was a partial mu agonist/delta antagonist and H-Tmt-TicPsi[CH(2)NH]Phe-Phe-NH(2) was a mu antagonist/delta antagonist. DIPP-NH(2)[Psi] showed binding affinities in the subnanomolar range for both mu and delta receptors in the rat brain membrane binding assays, thus representing the first example of a balanced mu agonist/delta antagonist with high potency. In the rat tail flick test, DIPP-NH(2)[Psi] given icv produced a potent analgesic effect (ED(50) = 0.04 microg), being about 3 times more potent than morphine (ED(50) = 0.11 microg). It produced less acute tolerance than morphine but still a certain level of chronic tolerance. Unlike morphine, DIPP-NH(2)[Psi] produced no physical dependence whatsoever upon chronic administration at high doses (up to 4.5 microg/h) over a 7-day period. In conclusion, DIPP-NH(2)[Psi] fulfills to a large extent the expectations based on the mixed mu agonist/delta antagonist concept with regard to analgesic activity and the development of tolerance and dependence.

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.

Synthesis and binding characteristics of [3H] H-Tyr-Ticpsi[CH2-NH] Cha-Phe-OH, a highly specific and stable delta-opioid antagonist

Substitution of the Phe3 aromatic ring in H-Tyr-Ticpsi[CH2-NH]Phe-Phe-OH with cyclohexylalanine (Cha) has been reported to result in a compound, H-Tyr-Ticpsi[CH2-NH]Cha-Phe-OH (TICP[psi]), showing substantially increased delta-opioid antagonist potency and high delta selectivity. TICP[psi] was radiolabeled by catalytic tritiation of its precursor Tyr(3',5'-I2)1TICP[psi]. Binding characteristics of the new tritiated pseudopeptide were determined using the radioligand binding assay in rat brain membranes. On the basis of the results of saturation binding studies performed at 25 degrees C, an equilibrium dissociation constant (Kd) of 0.35 nM and a receptor density (Bmax) of 112 fmol/mg protein were calculated. This new tritiated ligand exhibits high affinity for delta-opioid receptors, whereas its binding to mu and kappa receptors is weak. A study of [H3]TICP[psi] binding displacement by various receptor-selective opioids showed the following rank order of potency: delta > kappa = mu. These receptor binding characteristics of the ligand, together with its high specific radioactivity (41.3 Ci/mmol) and stability, makes it a useful tool for labeling delta-opioid receptors, both in vitro and in vivo.

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.

Aggregation behaviour and Zn2+ binding properties of secretin

Dynamic light scattering measurements were carried out on secretin in aqueous solution (2 mM; pH 5.0). The results indicated that the molecule exists as a fairly compact hexamer under these solution conditions. Secondary structural properties of the secretin hexameric complex were evaluated using CD and NMR spectroscopy. Specifically, the spectral properties of secretin in water were examined as a function of peptide concentration. Results from the analyses indicated a 2-fold increase (17-32%) in alpha-helical content within the region Ser11-Arg21 as the peptide concentration was increased from 0.1 to 2 mM. Displacement of the alphaH proton chemical shifts relative to random coil values did not alter significantly with increasing peptide concentration. This observation confirmed that the length of the helical segment is independent of peptide concentration between 0.1 and 2 mM. The nature of the helix was furthermore determined as amphipathic, and thus the potential for a cooperative intermolecular association through the apolar helical face of individual monomers was indicated. These findings suggest that secretin aggregates into symmetric hexamers at millimolar concentrations and, furthermore, that the helical domain is stabilized through this intermolecular association. The potential for secretin to bind divalent cations, including Ca2+ and Zn2+, was also examined by CD1 and NMR spectroscopy. The results revealed that Zn2+ specifically coordinates to the His1 and Asp3 residues of each secretin monomer without disrupting the peptide's helical structure, whereas Ca2+ did not exhibit any interaction with the peptide hormone. It was concluded from these studies that secretin may be stored in a hexameric form within its secretory tissues and that zinc may play a role in the storage of secretin through a specific interaction with the N-terminal histidine and aspartic acid residues.

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.

The octapeptide angiotensin II adopts a well-defined structure in a phospholipid environment

The conformational properties adopted by angiotensin II in a phospholipid micelle solution were studied by NMR spectroscopy and molecular modelling. The octapeptide was found to assume a well-defined hairpin structure with its C- and N-termini approaching to within 0.76 nm of each other. Three of the residues had fixed side chain configurations; Tyr4 (g+), His6 (g-) and Val3 (g-). Consequently, the His6 and Tyr4 aromatic rings were consistently close together. Conformers containing a cis His6-Pro7 peptide bond were observed for the peptide in a purely aqueous sample but completely disappeared when lipid vesicles were added to the sample. This result is explained by the existence of a very stable hydrogen bond between the Phe8 NH and the His6 carbonyl group of the lipid-solvated trans isomer, resulting in the formation of an inverse gamma turn centered on Pro7. 1H-NMR selective line broadening was apparent for several of the angiotensin II protons upon titration of an aqueous sample with less than stoichiometric amounts of 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer vesicles. The data obtained were consistent with the structure derived for micelle-bound angiotensin II, indicating that conformations adopted by the peptide hormone in the presence of micelles and lipid-bilayer vesicles are similar.

Interaction of agonist peptide [3H]Tyr-D-Ala-Phe-Phe-NH2 with mu-opioid receptor in rat brain and CHO-mu/1 cell line

Opioid receptor binding properties of [3H]Tyr-D-Ala-Phe-Phe-NH2 (TAPP) were characterized in rat brain and Chinese hamster ovary (CHO) cells expressing the rat mu-receptor. In rat brain, [3H]TAPP labeled a single class of opioid sites with a dissociation constant (Kd) of 0.31 nM and maximal number of binding sites (Bmax) of 119 fmol/mg protein. In CHO-mu/1 cell membranes, the Kd and Bmax values were 0.78 nM and 1806 fmol/mg protein, respectively. Binding to rat brain was demonstrated to be pharmacologically identical to that obtained with CHO-mu/1 cell membranes and modulated by Na+ ions and guanine nucleotides. The high affinity and selectivity of [3H]TAPP together with its low non-specific binding make this radioligand a useful tool for labeling the native and cloned mu-opioid receptor.

In vivo disposition of dermorphin analog (DALDA) in nonpregnant and pregnant sheep

Although synthetic opioid peptide analogs have been used extensively to study the functional roles of opioid receptors, little is known about their in vivo disposition. Our goal was to develop novel opioid drugs with limited transfer across the placenta. DALDA (Tyr-D-Arg-Phe-Lys-NH2) is a potent and highly selective mu agonist that is quite polar because of its 3+ charge at physiological pH. It can therefore be expected that the distribution of DALDA across the placenta would be highly restricted. In this study, we determined the pharmacokinetics and placental transfer of DALDA after systemic administration in sheep. DALDA was infused intravenously to four nonpregnant and four pregnant sheep at a dose of 0.6 mg/kg/hr for 4 hr. Steady state plasma levels of DALDA were 5436 +/- 464 ng/ml in nonpregnant sheep and 5214 +/- 661 ng/ml in pregnant sheep. A one-compartment open model provided an excellent fit for nonpregnant and pregnant plasma data. The apparent volume of distribution was estimated to be 45.6 +/- 4.4 and 59.2 +/- 7.9 ml/kg in nonpregnant and pregnant animals, respectively. There was no difference in the elimination half-life of DALDA in nonpregnant (1.4 +/- 0.1 hr) and pregnant (1.7 +/- 0.2 hr) animals, and clearance was also similar in nonpregnant (23.1 +/- 1.7 ml/kg/hr) and pregnant (23.7 +/- 1.3 ml/kg/hr) animals. These data suggest that the distribution of DALDA is restricted to plasma volume and that its disposition is not altered in pregnancy. DALDA was not detected in any of the fetal plasma samples (< 50 ng/ml), indicating that fetal plasma concentration is < 1% of maternal concentration. The highly restricted placental distribution of DALDA suggests that it may be a promising opioid drug for obstetrical use.

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.

Role of hydrophobic substituents in the interaction of opioid Tyr-Tic dipeptide analogs with dodecylphosphocholine micelles. Molecular partitioning in model membrane systems

The conformational properties of three Tyr-Tic-NH-R dipeptide analogs [where R = (CH2)2-Ph, (CH2)3-Ph or (CH2)2-cHx; Ph = phenyl; cHx = cyclohexyl and Tic = tetrahydroisoquinoline-3-carboxylic acid] have been investigated in purely aqueous solution and in the presence of fully deuterated dodecylphosphocholine micelles. H-Tyr-Tic-NH-(CH2)2-Ph is an opioid delta-agonist, whereas H-Tyr-Tic-NH-(CH2)3-Ph is a fairly potent delta-antagonist. H-Tyr-Tic-NH-(CH2)2-cHx is a less potent delta-antagonist. 1H-NMR spectra revealed that conformers containing cis and trans configurations of the Tyr-Tic peptide bond were present in all compounds in H2O and the H2O/lipid solvent. Analyses of the NMR data for the compounds in H2O indicate that in all three dipeptides the C-terminal substituent is flexible and the Tyr-side-chain adopts a trans orientation in most of the conformations. This promotes a compact Tyr-Tic structure. NOE patterns observed for the compounds in the micelle solution indicate that Tyr has an even greater tendency to assume a trans side chain configuration in the biphasic-solvent system. This feature was more pronounced in the trans conformers than in the cis conformers. Specific lipid-peptide interactions were indicated by NOESY spectra acquired for micelle samples incorporating 20% (by mass) protonated lipid. According to the obtained NOE data, Tyr and Tic form an aromatic cluster which preferentially inserts into the lipid interior of the micelle for the trans conformers of all three dipeptides and for the cis conformer of H-Tyr-Tic-NH-(CH2)2-Ph. For the cis isomers, partitioning of the C-terminal substituents into the lipid phase exhibited more diverse behaviour. The cis conformers of H-Tyr-Tic-NH-(CH2)3-Ph and H-Tyr-Tic-NH-(CH2)2-cHx preferentially anchor to the micelle via their C-terminal substituent, while the corresponding region in H-Tyr-Tic-NH-(CH2)2-Ph remains flexible and immersed in the aqueous phase. The inconsistent mode of peptide-micelle interaction observed for cis conformers of the three compounds studied is explained in terms of differences in their dipeptide-substituent hydrophobicities. The more apolar the substituent, the greater its tendency to preferentially insert into the lipid core of the micelle. Amide-proton temperature coefficients measured for the three peptides revealed differences amongst the cis and trans isomers. The amide proton in the trans conformer of each compound is highly exposed to the aqueous phase in both solvent systems studied, whereas the cis NH proton of each peptide is only partially exposed. These results demonstrate that a subtle structural modification of an active peptide analog can result in dramatic changes of its biological activity and its mode of partitioning at a membrane surface.

Renal effects of TAPP, a highly selective mu-opioid agonist

1. The effect of i.v. administration of TAPP, a highly selective and exclusively peripherally-acting mu-opioid receptor agonist, on urine output, urinary sodium, potassium and cyclic GMP, and on plasma immunoreactive atrial natriuretic factor (IR-ANF) levels was studied in conscious normally hydrated female rats (200-250 g). 2. TAPP treatment produced a significant dose-dependent increase of urine output and urinary sodium, potassium and cyclic GMP excretion during the first hour. The highest TAPP dose used (2.5 mg kg-1. body weight) elicited a 10 fold elevation of urine output from 0.23 +/- 0.06 ml h-1 to 2.5 +/- 0.3 ml h-1 (n = 18) accompanied by augmented sodium [from 17.0 +/- 4.7 mu Eq h-1 to 79 +/- 12.7 mu Eq h-1, n = 18 (P < 0.001)], potassium [from 9.5 +/- 2.5 mu Eq h-1 to 39.4 +/- 6.6 mu Eq h-1, n = 18 (P < 0.005)], and cyclic GMP excretion [from 191 +/- 21 pmol h-1 to 1340 +/- 322 pmol h-1, n = 18 (P < 0.001)]. Plasma IR-ANF rose from 22 +/- 4 pg ml-1 to 508 +/- 22 pg ml-1 (n = 18) (P < 0.001) 5 min after administration of TAPP (2500 micrograms kg-1). 3. TAPP lowered systemic blood pressure, also in a dose-related manner, 1-5 min after injection. This decrease in blood pressure was transient and did not last more than 10 min. 4. Pretreatment with the opioid antagonist naloxone (0.8 mg per rat) abolished the diuretic, natriuretic and kaliuretic effect of TAPP (250 micrograms kg-1); urine output dropped from 1.16 +/- 0.15 ml h-1, n = 12, to the control value of 0.15 +/- 0.06 ml h-1, n = 12 (P < 0.001), sodium excretion fell from 57.5 +/- 11 mu Eq h-1, to 21.3 +/- 8.5 mu Eq h-1, n = 12 (P < 0.001), and potassium excretion decreased from 45.4 +/- 9.7 mu Eq h-1, n = 12, to 16.1 +/- 7.0 mu Eq h-1, (P < 0.001). 5. Pretreatment with anti-ANF serum (0.4 ml) abolished the diuretic effect of TAPP: urine output diminished significantly from 1.93 +/- 0.28 to 0.88 +/- 0.29 ml h-1 (P < 0.01) (n = 6). The TAPP-induced diuretic action, increased sodium/potassium excretion and elevated urinary cyclic GMP levels were also reversed by anti-ANF antibodies. 6. Since TAPP is totally unable to cross the blood-brain barrier, the ensemble of these observations led to the conclusion that the diuretic, natriuretic, kaliuretic and hypotensive effects produced by this mu-opioid agonist through interaction with peripheral mu-opioid receptors occur via ANF release.

Distinct conformational preferences of three cyclic beta-casomorphin-5 analogs determined using NMR spectroscopy and theoretical analysis

The conformational properties of three cyclic beta-casomorphin analogs based on the general formula H-Tyr-c[-D-Orn-2-Nal-D-Pro-Xaa-] (2-Nal = 2-naphthylalanine; Xaa = D-Ala, Sar or NMe-Ala) in DMSO solution were investigated using NMR spectroscopy in conjunction with molecular modeling techniques. The D-Ala5- and Sar5-analogs (compounds 1 and 2 respectively) are potent mixed mu-agonist/delta-antagonists with high mu- and delta-opioid receptor affinities, whereas the NMe-Ala5-analog (compound 3) is a potent mu-agonist and a weak partial delta-agonist. Distinct conformational differences emerged for the three compounds studied. Flexibility in the bare ring structures was found to increase in the order 3 < 2 < 1. The increased structural rigidity of 3 may be responsible for its low delta-receptor affinity as compared to the two other analogs. A low fractional population of conformers containing two cis peptide bonds was found for compound 2 but not for analog 1 or 3. Initial evidence for this observation was obtained from NMR differential line-broadening experiments and later confirmed by molecular mechanics simulations. Comparison of the temperature dependence of amide proton chemical shifts acquired for the three cyclic analogs indicate a large degree of intramolecular hydrogen bonding for 1 but not for the other two peptides.

Isolation and identification of peptide conformers by reversed-phase high-performance liquid chromatography and NMR at low temperature

Peptide conformers with one or more rotationally hindered peptide bonds due to the presence of proline and/or another N-substituted amino acid residue in the molecule were separated by reversed-phase chromatography at low temperatures, isolated and identified by NMR. The scope of this investigation included the cis-trans isomers of the dipeptides Leu-Pro, Phe-Pro and Tyr-Pro as well as conformers of opioid peptides containing proline and/or the proline-like Tic (1,2,3,4-tetrahydro-isoquinoline-3-carboxylic acid) residues: Tyr-Pro-Phe (beta-casomorphin 1-3 fragment), Tyr-Tic-Phe-Phe, Try-Pro-Phe-Pro-Gly (beta-casomorphin-5), Tyr-Tic-Phe-Phe-Val-Val-Gly-NH2 and Tyr-Tic-Phe-Gly-Tyr-Pro-Ser-NH2. Chromatography with micropellicular and totally porous octadecylated silica stationary phases and aqueous methanol under isocratic elution conditions resulted in well separated peaks of the rotational isomers at sufficiently low temperatures. Preparative RP-HPLC was carried out with eluents containing water and methanol, both deuterated, and the effluent fractions containing each isomer were collected for further investigation. The conformational states of the peptide isomers upon separation were conserved by storing the effluent fractions in liquid nitrogen. The Leu-Pro, Phe-Pro, Tyr-Pro and Tyr-Pro-Phe conformers were identified by one- and two-dimensional NMR spectroscopy at -15 degrees C. Upon comparing the NMR spectra of the isomers, for these peptides the retention order of the conformers was unambiguously established: in each case the trans, conformer is eluted before the cis conformer. On the basis of NMR data obtained the beta-casomorphin-5, which contains two proline residues, the elution order of its four conformers was established by NMR spectroscopy of the fractions obtained by RP-HPLC at low temperature as trans-trans (least retained), trans-cis, cis-cis and cis-trans (most retained).