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

Synthesis and pharmacological characterization of a novel, highly potent, peptidomimetic delta-opioid radioantagonist, [3H]Tyr-Tic-(2S,3R)-beta-MePhe-Phe-OH

[(3)H]Tyr-Tic-(2S,3R)-beta-MePhe-Phe-OH (where Tic: 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) with a specific radioactivity of 53.7 Ci/mmol was synthesized and characterized in receptor binding assays at 25 degrees C in rat brain membranes. The specific binding was saturable and displayed high affinity, with a K(D) of 0.16+/-0.005 nM and B(max) of 85.9+/-6.3 fmol/mg protein. NaCl increased its affinity by about 4-fold in membranes of rat brain and Chinese Hamster Ovary Cells stably transfected with the human delta-opioid receptors (hDOR-CHO) showing that the new ligand is an antagonist. The prototypic delta-opioid ligands were much more potent than mu- or kappa-specific ligands in competition assays. The autoradiographic distribution of the binding sites of the new ligand agreed with the known locations of the delta-opioid receptors in rat brain. The unlabeled new ligand was about 7-fold more potent than the parent peptide in competing for the binding sites of [(3)H]Tyr-Tic-(2S,3R)-beta-MePhe-Phe-OH in rat brain membranes. Likewise, the threo-beta-methyl analog was 3.8-fold more potent than the parent compound in antagonizing the effect of DPDPE in the [(35)S]GTPgammaS functional assay in hDOR-CHO membranes. The new, highly potent, conformationally constrained antagonist may be a valuable pharmacological tool in understanding the structural and topographical requirements of peptide ligand binding to the delta-opioid receptors.

Novel diastereomeric opioid tetrapeptides exhibit differing pharmacological activity profiles

A novel opioid peptide antagonist analogue, [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe, derived from the potent, delta-receptor selective TIPP tetrapeptide (Tyr-Tic-Phe-Phe) series was synthesized and radiolabeled by catalytic tritiation of its iodinated precursor peptide. The purified radioprobe exhibited a specific activity of 2.15 TBq/mmol (58 Ci/mmol). The novelty of this compound is that it contains structurally modified tyrosine residue (2',6'-dimethyltyrosine, Dmt1) replacing tyrosine (Tyr1) at the N-terminus, and beta-methyl substituted phenylalanine (betaMePhe3) at the third position. As the configuration of betaMePhe3 side-chain might be different due to diastereomerism, and accordingly can alter the biological activity, both unlabeled threo (2S,3R and 2R,3S) diastereomeric analogues were also prepared and included in this study. The affinity and selectivity (delta-opioid versus mu-opioid receptor) were evaluated by radioreceptor binding assays. Agonist or antagonist potencies were determined in [35S]GTPgammaS binding experiments using Chinese Hamster Ovary (CHO) cells selectively expressing delta- or mu-opioid receptors. The equilibrium binding of the radiolabeled peptide derivative [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe to rat brain membranes was saturable and the Scatchard analysis indicated a single binding site with a Kd of 0.3 nM and a Bmax of 127 fmol/mg protein. A study of [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe binding displacement by various receptor-type specific opioid ligands showed the rank order of competitor's potency delta > mu > kappa, suggesting selective labeling of opioid delta-sites. In the functional tests, the (2S,3R) and (2R,3S) peptides exhibited partial agonist behaviour by weakly stimulating regulatory G-proteins in CHO cell membranes transfected with different receptors. Both isomers were quite weak partial agonists at the delta-receptor and reasonable partial agonists at the mu-receptor, with a prevalence of (2S,3R) over (2R,3S) for the mu-receptor. Consistent with these observations both stereomers competitively inhibited the stimulation of [35S]GTPgammaS binding induced by the prototype delta-agonist peptide (pClPhe4)-DPDPE in delta(m) CHO cell membranes, and still the (2S,3R) compound exerted more potent delta-antagonist effect. [3H]Dmt-Tic-(2S,3R)betaMePhe-Phe represents a high affinity new radioligand and also constitute further example of the influence of beta-methyl substitution on the potency and selectivity of TIPP analogues, thus becoming a valuable biochemical and pharmacological tool in opioid research.

Opioid receptor binding characteristics and structure-activity studies of novel tetrapeptides in the TIPP (Tyr-Tic-Phe-Phe) series

The development of the prototype synthetic delta-opioid receptor antagonist peptides TIPP [(H-Tyr-Tic-Phe- Phe-OH); Tic: tetrahydroisoquinoline-3-carboxylic acid] and TIPPpsi (H-Tyr-psiTic-Phe-Phe-OH) by Schiller and coworkers was followed by extensive structure-activity relationship studies, leading to the emergence of numerous analogs that are of pharmacological interest. Eight novel diastereomeric compounds in this peptide family were designed, prepared, and tested biologically to gain structure-activity relationship information. The new multisubstituted tetrapeptide analogs contain both a 2',6'-dimethyltyrosine residue at the N-terminus and beta-methyl-cyclohexylalanine at the third position as replacements for the original first tyrosine and the third phenylalanine, respectively. These derivatives wear either free acidic (-COOH) or amidated (-CONH2) C-terminal. The potency and delta- versus mu-opioid receptor selectivity were evaluated by in vitro radioreceptor-binding assays, while the intrinsic G-protein-activating efficacy of these analogs was tested in [35S]GTPgammaS-binding assays using rat brain membranes or Chinese hamster ovary cells stably expressing mu- or delta-opioid receptors. The analogs showed delta-antagonist selectivity with differences regarding their isomeric forms, and these analogs containing a C-terminal carboxamide group displayed a mixed mu-agonist/delta-antagonist profile, thus they are expected to be safer analgesics with a low propensity to produce tolerance and physical dependence. These results constitute further examples of the influence of beta-methyl substitution and C-terminal amidation on potency, selectivity, and signal transduction properties of TIPP-related peptides as well as they represent valuable pharmacological tools for opioid research.

Pharmacological and functional biochemical properties of d-Ala2-d-Nle5-enkephalin-Arg-Phe

Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN) a synthetic analogue of the endogenous Met-enkephalin-Arg-Phe (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF), was investigated in radioligand binding assays, [(35)S]GTPgammaS stimulation experiments as well as in in vivo algesiometric tests. Binding properties of [(3)H]DADN were measured in crude membrane fractions of rat spinal cord tissues and in homogenates of Chinese hamster ovary (CHO) cells selectively expressing delta-, kappa-or micro-opioid receptors. The highest affinity for [(3)H]DADN binding was observed in membranes from CHO cells transfected with micro-opioid receptors confirming the micro-selectivity of the peptide. Unlabeled DADN was also investigated in functional biochemical experiments by measuring opioid receptor-mediated G-protein activation in rat brain membrane fractions. The peptide stimulated the activity of the regulatory G-proteins in a concentration dependent manner, and the stimulation was efficiently inhibited in the presence of micro-receptor specific antagonist ligands further supporting the selectivity profile of DADN. Intrathecally administered DADN produced a dose-related, naloxone-reversible antinociception in rat hot water tail-flick tests. Among the selective opioid antagonists tested, the delta-selective naltrindole (NTI) and the kappa-specific norbinaltorphimine (norBNI) showed only slight blocking effects compared with naloxone. The results obtained in the in vitro agonist-stimulated [(35)S]GTPgammaS binding assays are in good agreement with the opioid agonist effect seen in the in vivo pain test.

Characterization of Specific Opioid Binding Sites in Neural Membranes from the Myenteric Plexus of Porcine Small Intestine

Delta- and kappa-opioid receptors (OPRs), but not micro-OPRs, are expressed in the myenteric plexus of the porcine distal small intestine. In a subpopulation of myenteric neurons, delta- and kappa-OPRs seem to be colocalized and may functionally interact. In this study, radioligand binding was used to characterize myenteric OPR populations in detail. The nonselective OPR antagonist [3H]diprenorphine bound to a single, high-affinity site in myenteric neural membrane homogenates. Naloxone displaced 65 and 59% of [3H]diprenorphine binding from this site in Na(+)-free Tris and Krebs-HEPES buffers, respectively. Naltrexone-derived delta- and kappa-OPR antagonists, including naltriben, 7-benzylidenenaltrexone, nor-binaltorphimine, and 5'-guanidinonaltrindole, displaced [3H]diprenorphine from two distinct binding sites to levels similar to that of naloxone. The selective delta-OPR ligands Tyr-1,2,3,4-tetrahydroisoquinoline-Phe-Phe-OH (TIPP), [D-Pen2,D-Pen5]enkephalin (DPDPE), [D-Ala2, Glu4]deltorphin II, and (+)-4-[(alphaR)-alpha((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl-3-methoxybenzyl)-N,N-diethylbenzamide (SNC-80) and the kappa-OPR agonist (D-(5alpha,7alpha,8beta)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxoaspiro-(4,5)dec-8-yl) benzeneacetamide (U-69,593) displaced [3H]diprenorphine from three independent binding sites; these included high-affinity delta- and kappa-OPR sites, and a residual binding site. Residual [3H]diprenorphine binding was displaced by the selective kappa-OPR antagonist nor-binaltorphimine after saturation of delta and kappa sites, respectively, with DPDPE and U-69,593. The residual binding site displayed low affinity for delta- and kappa-OPR agonists and TIPP, as well as moderate affinity for naltrexone-derived ligands, properties reminiscent of delta-/kappa-OPR heterodimers.

Characterization of [125I]AR-M100613, a high-affinity radioligand for delta opioid receptors

AR-M100613 ([I]-Dmt-c[-D-Orn-2-Nal-D-Pro-D-Ala-]) is the iodinated analog of a cyclic casomorphin previously shown to be a potent antagonist at the delta opioid receptor. Specific [125I]AR-M100613 binding to rat whole brain membranes was saturable, reversible, and best fit to a one-site model (Kd = 0.080 +/- 0.008 nM, Bmax = 45.2 +/- 4.4 fmol/mg protein). [125I]AR-M100613 binding was displaced with high affinity by the delta opioid receptor ligands SNC-80, Deltorphin II and DPDPE but not the mu or kappa-selective receptor ligands DAMGO and U69593. Residual non-selective binding of [125I]AR-M 100613 to mu opioid receptors is blocked by the addition of CTOP to the assay buffer. [35S]GTPgammaS binding assays indicate that AR-M100613 is a potent, selective, and reversible antagonist for delta opioid receptors in rat brain membranes. The high-affinity, high specific activity, low nonspecific binding and antagonist profile of [125I]AR-M100613 favor its use as a radiochemical probe for delta opioid receptors.

Mu-opioid receptor specific antagonist cyprodime: characterization by in vitro radioligand and [35S]GTPgammaS binding assays

The use of compounds with high selectivity for each opioid receptor (mu, delta and kappa) is crucial for understanding the mechanisms of opioid actions. Until recently non-peptide mu-opioid receptor selective antagonists were not available. However, N-cyclopropylmethyl-4,14-dimethoxy-morphinan-6-one (cyprodime) has shown a very high selectivity for mu-opioid receptor in in vivo bioassays. This compound also exhibited a higher affinity for mu-opioid receptor than for delta- and kappa-opioid receptors in binding assays in brain membranes, although the degree of selectivity was lower than in in vitro bioassays. Cyprodime has recently been radiolabelled with tritium resulting in high specific radioactivity (36.1 Ci/mmol). We found in in vitro binding experiments that this radioligand bound with high affinity (K(d) 3. 8+/-0.18 nM) to membranes of rat brain affording a B(max) of 87. 1+/-4.83 fmol/mg. Competition studies using mu, delta and kappa tritiated specific ligands confirmed the selective labelling of cyprodime to a mu-opioid receptor population. The mu-opioid receptor selective agonist [D-Ala(2),N-MePhe(4),Gly(5)-ol]enkephalin (DAMGO) was readily displaced by cyprodime (K(i) values in the low nanomolar range) while the competition for delta- ([D-Pen(2), D-Pen(5)]enkephalin (DPDPE)) and kappa- (5alpha,7alpha, 8beta-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro(4, 5)dec-8-yl]-benzene-acetamide (U69,593)) opioid receptor selective compounds was several orders of magnitude less. We also found that cyprodime inhibits morphine-stimulated [35S]GTPgammaS binding. The EC(50) value of morphine increased about 500-fold in the presence of 10 microM cyprodime. These findings clearly indicate that cyprodime is a useful selective antagonist for mu-opioid receptor characterization.

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.

Slide-binding characterization and autoradiographic localization of delta opioid receptors in rat and mouse brains with the tetrapeptide antagonist [3H]TIPP

Slide-binding and autoradiographic studies were performed on cryostat sections from brains of adult Sprague-Dawley rats and BALB C mice to describe the binding characteristics of the tetrapeptide [3H]TIPP, an antagonist with high specificity and affinity for the delta opioid receptors. Steady-state binding of [3H]TIPP to cryostat sections of brain paste was reached in 120-180 min of incubation. Specific [3H]TIPP binding resulted in maximal numbers of binding sites (Bmax) of 15.59 and 23.91 fmol/mg protein, and dissociation constants (Kd) of 0.46 and 0.85 nM for rat and mouse brain paste sections, respectively. TIPP displayed the highest affinity for delta opioid receptors in inhibiting specific [3H]TIPP binding, with IC50 values of 0.82 nM and 0.14 nM in rat and mouse brain sections, respectively. While DPDPE was also effective in displacing the specific binding of [3H]TIPP (IC50 = 3.18 +/- 0.53 nM and 0.63 +/- 0.42 nM in rat and mouse brain paste sections, respectively), other subclass-selective or nonopioid ligands were much less effective, or ineffective. Autoradiographic localization of [3H]TIPP binding revealed the characteristic distribution of delta opioid receptors in both species. In consequence of its antagonistic nature, and of its unnatural amino acid residue, which makes this ligand more resistant to biodegradation, [3H]TIPP is a superior ligand for evaluation of the binding characteristics and autoradiogaphic distribution of the delta opioid receptors.

Synthesis and binding characteristics of the highly selective radiolabelled deltorphin analogues containing 2-aminotetralin-2-carboxylic acid in position 3

Following the description of [3H]Ile5,6deltorphin II, when it was reported that changes in hydrophobicity at positions 5 and 6 give rise to analogues with increased delta-receptor affinity and selectivity, new conformationally restricted deltorphin analogues were designed. A synthetic amino acid, 2-aminotetralin-2-carboxylic acid (Atc), was introduced at position 3 instead of Phe in Ile5,6deltorphin I and II, and the resultant compounds were prepared in tritiated form. Opioid binding sites specific for [3H]S-Atc3,Ile5,6deltorphin I and [3H]R-Atc3,Ile5,6deltorphin II were characterized in rat brain membranes. Their binding was saturable, stereoselective and inhibited by delta-selective ligands with high potency. They labelled single class of opioid sites at 35 degrees C with high affinity (Kd approximately 0.3 nM), Bmax values of 130 fmol/mg protein, and very low non-specific binding was observed. Both tritiated deltorphin analogues showed delta-receptor specificity in rat brain, therefore they could represent excellent new radioligands for investigating the complexity of the opioid receptor systems.

Effects of mu- and delta-opioid-receptor antagonists on the stimulus properties of cholecystokinin

Melton and Riley recently reported that the relatively selective mu-opioid-antagonist naloxone potentiated the stimulus properties of the gut peptide cholecystokinin (CCK). To assess whether such opioid potentiation is limited to activity at the mu-receptor subtype, in the present experiment the effects of the highly selective delta-antagonist naltrindole on CCK's stimulus properties were examined. Because in the initial report of naloxone's potentiation of CCK a relatively high, nonphysiologic dose of CCK (i.e., 13 micrograms/kg) was used as the training drug, in the current analysis subjects were trained to discriminate 5.6 micrograms/kg CCK from its vehicle and the assessments and comparisons of the effects of naloxone and naltrindole were based on this dose. Specifically, rats were administered 5.6 micrograms/kg CCK before saccharin-LiCl pairings and the CCK vehicle before saccharin alone. With such training, they rapidly acquired the drug discrimination, avoiding saccharin consumption when it was preceded by CCK and consuming the same saccharin solution when it was preceded by its vehicle. In subsequent generalization tests, doses of CCK that were ineffective in suppressing saccharin consumption (i.e., did not substitute for the training dose of CCK) did result in the suppression of saccharin consumption when combined with doses of the mu antagonist naloxone that alone had no effect on saccharin intake. On the other hand, the highly selective delta-opioid-receptor antagonist naltrindole was ineffective in potentiating the effects of CCK. Specifically, when naltrindole was combined with ineffective doses of CCK, subjects drank at control levels. The ability of naloxone to potentiate CCK's stimulus effects is consistent with a range of other demonstrations of the role of the mu-opioid-receptor subtype in CCK-opioid interactions, although the specific basis for the interaction remains unknown. Given recent findings on the effects of delta agonists and antagonists on CCK-induced activity, the failure of naltrindole to potentiate CCK's stimulus effects may be due to the absence of delta activity within this preparation, rather than the absence of delta mediation of CCK-opioid interactions in general.

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.

Binding characteristics of the novel highly selective delta agonist, [3H]IIe5,6deltorphin II

Following the description of the [3H]deltorphin II, it has been reported that the modification of deltorphin II with the substitution of Val5,6 residues by the more hydrophobic IIe5,6 residues leads to an increased affinity and selectivity. The IIe5,6 deltorphin II (Tyr-D-Ala-Phe-Gly-IIe-IIe-HH2) was tritiated by catalytic dehalogenation and labelled rat brain membrane sites with a Kd value of 0.40 nM and a Bmax of 121 fmol/mg protein. Competition binding experiments with various unlabelled subtype specific opioid receptor ligands resulted in mu/delta and kappa/delta selectivity ratios of 2400 and 18,000 respectively. Due to its high delta receptor affinity, delta selectivity and very low non-specific binding (< 20%), [3H]IIe5,6 deltorphin II, is a very useful tool for the identification and characterisation of delta opioid receptors.