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

Glycosylated neuropeptides: a new vista for neuropsychopharmacology?

The application of endogenous neuropeptides (e.g., enkephalins) as analgesics has been retarded by their poor stability in vivo and by their inability to effectively penetrate the blood-brain barrier (BBB). Effective BBB transport of glycosylated enkephalins has been demonstrated in several labs now. Analgesia (antinociception) levels greater than morphine, and with reduced side effects have been observed for several glycopeptides related to enkephalin. Somewhat paradoxically, enhanced BBB transport across this lipophilic barrier is achieved by attaching water-soluble carbohydrate groups to the peptide moieties to produce biousian glycopeptides that can be either water-soluble or membrane bound. Transport is believed to rely on an endocytotic mechanism (transcytosis), and allows for systemic delivery and transport of the water-soluble glycopeptides. Much larger endorphin/dynorphin glycopeptide analogs bearing amphipathic helix address regions also have been shown to penetrate the BBB in mice. This holds forth the possibility of transporting much larger neuropeptides across the BBB, which may encompass a wide variety of receptors beyond the opioid receptors.

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.

New 2',6'-dimethyl-L-tyrosine (Dmt) opioid peptidomimetics based on the Aba-Gly scaffold. Development of unique mu-opioid receptor ligands

The Aba-Gly scaffold, incorporated into Dmt-Tic ligands (H-Dmt-Tic-Gly-NH-CH2-Ph, H-Dmt-Tic-Gly-NH-Ph, H-Dmt-Tic-NH-CH2-Bid), exhibited mixed micro/delta or delta opioid receptor activities with micro agonism. Substitution of Tic by Aba-Gly coupled to -NH-CH2-Ph (1), -NH-Ph (2), or -Bid (Bid=1H-benzimidazole-2-yl) (3) shifted affinity (Ki(micro)=0.46, 1.48, and 19.9 nM, respectively), selectivity, and bioactivity to micro-opioid receptors. These compounds represent templates for a new class of lead opioid agonists that are easily synthesized and suitable for therapeutic pain relief.

[d-1-Nal4]endomorphin-2 is a potent μ-opioid receptor antagonist in the aequorin luminescence-based calcium assay

A functional assay, based on aequorin-derived luminescence triggered by receptor-mediated changes in Ca(2+) levels, was used to examine relative potency and efficacy of the micro-opioid receptor antagonists. A series of position 3- and 4-substituted endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2)) analogues containing D-3-(1-naphthyl)-alanine (D-1-Nal) or D-3-(2-naphthyl)-alanine (D-2-Nal), which were previously shown to reverse antinociception induced by endomorphin-2 in the in vivo hot-plate test in mice, was tested in the aequorin luminescence-based calcium assay to examine their micro-opioid antagonist potency in vitro. A recombinant mammalian cell line expressing the micro-opioid receptor together with a luminescent reporter protein, apoaequorin, was used in the study. The results obtained in this functional assay indicated that analogues with D-1-Nal or D-2-Nal substitutions in position 4 of endomorphin-2 are strong micro-opioid receptor antagonists, while those substituted in position 3 are partial agonists. Exceptional antagonist potency in the calcium assay was observed for [D-1-Nal(4)]endomorphin-2. The pA(2) value for this analogue was 7.95, compared to the value of 8.68 obtained for the universal, non-selective opioid antagonist of the alkaloid structure, naloxone. The obtained results were compared with the data from the hot-plate test in mice. In that in vivo assay [D-1-Nal(4)]endomorphin-2 was also the most potent analogue of the series.

Conversion of the potent delta-opioid agonist H-Dmt-Tic-NH-CH(2)-bid into delta-opioid antagonists by N(1)-benzimidazole alkylation(1)

N(1)-Alkylation of 1H-benzimidizole of the delta agonist H-Dmt-Tic-NH-CH(2)-Bid with hydrophobic, aromatic, olefinic, acid, ethyl ester, or amide (1-6) became delta antagonists (pA(2)=8.52-10.14). delta- and micro-Opioid receptor affinities were high (K(i)delta=0.12-0.36 nM and K(i)micro=0.44-1.42 nM). Only delta antagonism (pA(2)=8.52-10.14) was observed; micro agonism (IC(50)=30-450 nM) was not correlated with changes in alkylating agent or delta antagonism, and some compounds yielded mixed delta antagonism/micro agonism.

The role of crystallography in drug design

Structure and function are intimately related. Nowhere is this more important than the area of bioactive molecules. It has been shown that the enantioselectivity of an enzyme is directly related to its chirality. X-ray crystallography is the only method for determining the "absolute" configuration of a molecule and is the most comprehensive technique available to determine the structure of any molecule at atomic resolution. Results from crystallographic studies provide unambiguous, accurate, and reliable 3-dimensional structural parameters, which are prerequisites for rational drug design and structure-based functional studies.

From the potent and selective mu opioid receptor agonist H-Dmt-d-Arg-Phe-Lys-NH(2) to the potent delta antagonist H-Dmt-Tic-Phe-Lys(Z)-OH

H-Dmt-d-Arg-Phe-Lys-NH(2) ([Dmt(1)]DALDA) binds with high affinity and selectivity to the mu opioid receptor and is a potent and long-acting analgesic. Substitution of d-Arg in position 2 with Tic and masking of the lysine amine side chain by Z protection and of the C-terminal carboxylic function instead of the amide function transform a potent and selective mu agonist into a potent and selective delta antagonist H-Dmt-Tic-Phe-Lys(Z)-OH. Such a delta antagonist could be used as a pharmacological tool.

Opioid peptide-derived analgesics

Two recent developments of opioid peptide-based analgesics are reviewed. The first part of the review discusses the dermorphin-derived, cationic-aromatic tetrapeptide H-Dmt-D-Arg-Phe-Lys-NH(2) ([Dmt(1)]DALDA, where Dmt indicates 2',6'-dimethyltyrosine), which showed subnanomolar mu receptor binding affinity, extraordinary mu receptor selectivity, and high mu agonist potency in vitro. In vivo, [Dmt(1)]DALDA looked promising as a spinal analgesic because of its extraordinary antinociceptive effect (3000 times more potent than morphine) in the mouse tail-flick assay, long duration of action (4 times longer than morphine), and lack of effect on respiration. Unexpectedly, [Dmt(1)]DALDA also turned out to be a potent and long-acting analgesic in the tail-flick test when given subcutaneously (s.c.), indicating that it is capable of crossing the blood-brain barrier. Furthermore, little or no cross-tolerance was observed with s.c. [Dmt(1)]DALDA in morphine-tolerant mice. The second part of the review concerns the development of mixed mu agonist/delta antagonists that, on the basis of much evidence, are expected to be analgesics with a low propensity to produce tolerance and physical dependence. The prototype pseudopeptide H-Dmt-TicPsi[CH(2)NH]Phe-Phe-NH(2) (DIPP-NH(2)[Psi], where Tic indicates 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) showed subnanomolar mu and delta receptor binding affinities and the desired mu agonist/delta antagonist profile in vitro. DIPP-NH(2)[Psi] produced a potent analgesic effect after intracerebroventricular administration in the rat tail-flick assay, no physical dependence, and less tolerance than morphine. The results obtained with DIPP-NH(2)[Psi] indicate that mixed mu agonist/delta antagonists look promising as analgesic drug candidates, but compounds with this profile that are systemically active still need to be developed.

Structural function of C-terminal amidation of endomorphin

To investigate the structural function of the C-terminal amide group of endomorphin-2 (EM2, H-Tyr-Pro-Phe-Phe-NH(2)), an endogenous micro-opioid receptor ligand, the solution conformations of EM2 and its C-terminal free acid (EM2OH, H-Tyr-Pro-Phe-Phe-OH) in TFE (trifluoroethanol), water (pH 2.7 and 5.2), and aqueous DPC (dodecylphosphocholine) micelles (pH 3.5 and 5.2) were investigated by the combination of 2D (1)H-NMR measurement and molecular modelling calculation. Both peptides were in equilibrium between the cis and trans rotamers around the Tyr--Pro w bond with population ratios of 1 : 1 to 1 : 2 in dimethyl sulfoxide, TFE and water, whereas they predominantly took the trans rotamer in DPC micelle, except in EM2OH at pH 5.2, which had a trans/cis rotamer ratio of 2 : 1. Fifty possible 3D conformers were generated for each peptide, taking different electronic states depending on the type of solvent and pH (neutral and monocationic forms for EM2, and zwitterionic and monocation forms for EM2OH) by the dynamical simulated annealing method, under the proton-proton distance constraints derived from the ROE cross-peak intensities. These conformers were then roughly classified into four groups of two open [reverse S (rS)- and numerical 7 (n7)-type] and two folded (F1- and F2-type) conformers according to the conformational pattern of the backbone structure. Most EM2 conformers in neutral (in TFE) and monocationic (in water and DPC micelles) forms adopted the open structure (mixture of major rS-type and minor n7-type conformers) despite the trans/cis rotamer form. On the other hand, the zwitterionic EM2OH in TFE, water and DPC micelles showed an increased population of F1- and F2-type folded conformers, the population of which varied depending on their electronic state and pH. Most of these folded conformers took an F1-type structure similar to that stabilized by an intramolecular hydrogen bond of (Tyr1)NH(3) (+)...COO(-)(Phe4), observed in its crystal structure. These results show that the substitution of a carboxyl group for the C-terminal amide group makes the peptide structure more flexible and leads to the ensemble of folded and open conformers. The conformational requirement of EM2 for binding to the micro-opioid receptor and the structural function of the C-terminal amide group are discussed on the basis of the present conformational features of EM2 and EM2OH and a possible model for binding to the micro-opioid receptor, constructed from the template structure of rhodopsin.

Novel endomorphin-2 analogs with mu-opioid receptor antagonist activity

A series of position 4-substituted endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) analogs containing 3-(1-naphthyl)-alanine (1-Nal) or 3-(2-naphthyl)-alanine (2-Nal) in L- or D-configuration, was synthesized. The opioid activity profiles of these peptides were determined in the mu-opioid receptor representative binding assay and in the Guinea-Pig Ileum assay/Mouse Vas Deferens assay (GPI/MVD) bioassays in vitro, as well as in the mouse hot-plate test of analgesia in vivo. In the binding assay the affinity of all new analogs for the mu-opioid receptor was reduced compared with endomorphin-2. The two most potent analogs were [D-1-Nal(4)]- and [D-2-Nal4]endomorphin-2, with IC50 values 14 +/- 1.25 and 19 +/- 2.1 nM, respectively, compared with 1.9 +/- 0.21 nM for endomorphin-2. In the GPI assay these analogs were found to be weak antagonists and they were inactive in the MVD assay. The in vitro GPI assay results were in agreement with those obtained in the in vivo hot-plate test. Antinociception induced by endomorphin-2 was reversed by concomitant intracerebroventricula (i.c.v.) administration of [D-1-Nal4]- and [D-2-Nal4]-endomorphin-2, indicating that these analogs were mu-opioid antagonists. Their antagonist activity was compared with that of naloxone. At a dose 5 microg per animal naloxone almost completely inhibited antinociceptive action of endomorphin-2, while [D-1-Nal4]endomorphin-2 in about 46%.

[Aladan3]TIPP: a fluorescent delta-opioid antagonist with high delta-receptor binding affinity and delta selectivity

Fluorescent analogues of the potent and highly selective delta-opioid antagonist TIPP (H-Tyr-Tic-Phe-Phe-OH) and TIP (H-Tyr-Tic-Phe-OH) containing the exceptionally environmentally sensitive fluorescent amino acid beta-(6'-dimethylamino-2'-naphthoyl)alanine (Aladan [Ald]) in place of Phe3 were synthesized. The Ald3- and D-Ald3 analogues of TIPP and TIP all retained delta-opioid antagonist properties. The most potent analogue, [Ald3]TIPP, showed a K(e) value of 2.03 nM in the mouse vas deferens assay and five times higher delta vs. mu selectivity (K(i)mu/K(i)delta = 7930) than the TIPP parent peptide in the opioid receptor binding assays. Theoretical conformational analyses of [Ald3]TIPP and [Ald3]TIP using molecular mechanics calculations resulted in a number of low-energy conformers, including some showing various patterns of aromatic ring stacking and others with the Ald side chain and a carbonyl group (fluorescence quencher) in close proximity. These ensembles of low-energy conformers are in agreement with the results of steady-state fluorescence experiments (fluorescence emission maxima and quantum yields) and fluorescence decay measurements (fluorescence lifetime components), which indicated that the fluorophore was either engaged in intramolecular hydrophobic interactions or in proximity of a fluorescence quencher (e.g., a carbonyl group). These fluorescent TIP(P) delta-opioid antagonists represent valuable pharmacological tools for various applications, including studies on membrane interactions, binding to receptors, cellular uptake and intracellular distribution, and tissue distribution.

A new structural motif for mu-opioid antagonists

On the basis of the structural features of the Dmt-Tic pharmacophore, a new motif leading to a fairly potent mu-opioid antagonist is described. This motif contains the 4-amino-1,2,4,5-tetrahydro-2-benzazepine-3-one skeleton as a substitute for the Tic residue, which provides the conformational constraint compatible with the mu-opioid receptor. The stereoselective synthesis of four stereoisomers is performed starting from homochiral 2',6'-dimethyltyrosine (Dmt) and o-aminomethylphenylalanine.

Highly selective fluorescent analogue of the potent delta-opioid receptor antagonist Dmt-Tic

A fluorescent tripeptide probe derived by coupling fluorescein to H-Dmt-Tic-Glu-NH2 was developed to interact with delta-opioid receptors with high affinity (Ki = 0.035 nM) and selectivity (Ki(mu)/Ki(delta) = 4371). It acts as an irreversible delta-opioid receptor antagonist, and binding to NG108-15 cells is blocked by the standard nonpeptidic delta-opioid receptor antagonist naltrindole. This probe should prove useful in the study of the distribution of delta-opioid receptors in tissues and the internalization of opioid peptides during signal transduction.

Inverse agonism and neutral antagonism at wild-type and constitutively active mutant delta opioid receptors

The delta opioid receptor modulates nociceptive and emotional behaviors. This receptor has been shown to exhibit measurable spontaneous activity. Progress in understanding the biological relevance of this activity has been slow, partly due to limited characterization of compounds with intrinsic negative activity. Here, we have used constitutively active mutant (CAM) delta receptors in two different functional assays, guanosine 5'-O-(3-thio)triphosphate binding and a reporter gene assay, to test potential inverse agonism of 15 delta opioid compounds, originally described as antagonists. These include the classical antagonists naloxone, naltrindole, 7-benzylidene-naltrexone, and naltriben, a new set of naltrindole derivatives, H-Tyr-Tic-Phe-Phe-OH (TIPP) and H-Tyr-TicPsi[CH2N]Cha-Phe-OH [TICP(Psi)], as well as three 2',6'-dimethyltyrosine-1,2,3,4-tetrahydroquinoline-3-carboxylate (Dmt-Tic) peptides. A reference agonist, SNC 80 [(+)-4-[(alphaR)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide], and inverse agonist, ICI 174864 (N,N-diallyl-Tyr-Aib-Aib-Phe-Leu), were also included. In a screen using wild-type and CAM M262T delta receptors, naltrindole (NTI) and close derivatives were mostly inactive, and TIPP behaved as an agonist, whereas Dmt-Tic-OH and N,N(CH3)2-Dmt-Tic-NH2 showed inverse agonism. The two latter compounds showed negative activity across 27 CAM receptors, suggesting that this activity was independent from the activation mechanism. These two compounds also exhibited nanomolar potencies in dose-response experiments performed on wild-type, M262T, Y308H, and C328R CAM receptors. TICP(Psi) exhibited strong inverse agonism at the Y308H receptor. We conclude that the stable N,N(CH3)2-Dmt-Tic-NH2 compound represents a useful tool to explore the spontaneous activity of delta receptors, and NTI and novel derivatives behave as neutral antagonists.

G protein activation and cyclic AMP modulation by naloxone benzoylhydrazone in distinct layers of rat olfactory bulb

1 Naloxone benzoylhydrazone (NalBzoH) has initially been developed as an agonist of the pharmacologically defined kappa3-opioid receptor and has recently been employed as an antagonist at the opioid receptor-like (ORL1) receptor. In the present study, we investigated the ability of NalBzoH to elicit agonist-like effects on receptor signalling in distinct layers of rat olfactory bulb, a brain region where we have demonstrated the presence of opioid and ORL1 receptors coupled to both stimulation and inhibition of cyclic AMP formation. 2 In membranes of the olfactory nerve-glomerular layer (ON-GL), external plexiform layer (EPL) and granule cell layer (GRL), NalBzoH elicited a concentration-dependent stimulation of guanosine-5'-O-(3-[35S]-thio)triphosphate ([35S]GTPgammaS) binding with pEC50 values ranging from 7.36 to 7.86, whereas the kappa1-opioid receptor agonists (-)-U-50,488 and U-69,593 were inactive. 3 In membranes of GRL, but not ON-GL and EPL, NalBzoH stimulated basal adenylyl cyclase activity by 40% with a pEC50 of 8.14, and significantly potentiated the net enzyme stimulation elicited by corticotropin-releasing hormone and pituitary adenylate cyclase-activating peptide 38. Pertussis toxin prevented the NalBzoH stimulations of [35S]GTPgammaS binding and adenylyl cyclase activity. 4 In membranes of EPL and GRL, but not ON-GL, NalBzoH elicited a concentration-dependent inhibition of forskolin-stimulated adenylyl cyclase activity with pEC50 values of 8.07 and 8.08, respectively. 5 At concentrations that completely blocked the actions of nociceptin/orphanin FQ (N/OFQ), the ORL1 receptor antagonists CompB and [Nphe1]N/OFQ(1-13)NH2 failed to antagonize either the stimulatory or the inhibitory effect of NalBzoH on cyclic AMP formation. Similarly, the kappa1-opioid receptor antagonist nor-binaltorphimine counteracted the NalBzoH effects with relatively low potencies (pKi values=7.67-8.09). 6 Conversely, the selective delta-opioid receptor antagonist TIPP (pKi=9.10) and the selective mu-opioid receptor antagonist CTAP (pKi=8.27) reduced the inhibitory effect of NalBzoH by 70 and 30%, respectively. Moreover, TIPP and CTAP potently inhibited the NalBzoH stimulation of cyclic AMP, each antagonist maximally causing 50% blockade of the agonist response. 7These data demonstrate that in the olfactory bulb NalBzoH activates receptor signalling by acting through delta- and mu-opioid receptors and independently of ORL1 and kappa1-opioid receptors.

Rationale, Design, and Synthesis of Novel Phenyl Imidazoles as Opioid Receptor Agonists for Gastrointestinal Disorders

A small series of novel, imidazoles 4 have been prepared that exhibit very good binding affinities for the delta and mu opioid receptors (ORs), as well as demonstrate potent agonist functional activity at the delta OR. Representative imidazole 4a (K(i) delta = 0.9 nM; K(i) mu = 55 nM; K(i) kappa = 124 nM; EC(50) delta =13-25 nM) was further profiled for OR related in vivo effects. Compound 4a reduced gastrointestinal (GI) propulsive motility in a dose-dependent and naloxone-reversible manner, based on the results of the mouse glass bead expulsion test (3, 5, and 10 mg/kg, ip) and the mouse fecal pellet output test (1 and 3 mg/kg, ip). Compound 4a showed no analgesic activity as measured by the mouse abdominal irritant test (MAIT) when dosed at 100 mg/kg, sc, but did show significant MAIT activity at doses of both 10 microg (40% inhibition) and 100 microg (100% inhibition) when dosed intracerebroventricularly (icv). Taken together, these in vivo results suggest that 4a acts peripherally when dosed systemically, and that these prototypical compounds may prove promising as medicinal leads for GI indications.

Direct influence of C-terminally substituted amino acids in the Dmt-Tic pharmacophore on delta-opioid receptor selectivity and antagonism

A series of 17 analogues were developed on the basis of the general formula H-Dmt-Tic-NH-CH(R)-R' (denotes chirality; R = charged, neutral, or aromatic functional group; R' = -OH or -NH(2)). These compounds were designed to test the following hypothesis: the physicochemical properties of third-residue substitutions C-terminal to Tic in the Dmt-Tic pharmacophore modify delta-opioid receptor selectivity and delta-opioid receptor antagonism through enhanced interactions with the mu-opioid receptor. The data substantiate the following conclusions: (i) all compounds had high receptor affinity [K(i)(delta) = 0.034-1.1 nM], while that for the mu-opioid receptor fluctuated by orders of magnitude [K(i)(mu) = 15.1-3966 nM]; (ii) delta-opioid receptor selectivity [K(i)(mu)/K(i)(delta)] declined 1000-fold from 22,600 to 21; (iii) a C-terminal carboxyl group enhanced selectivity but only as a consequence of the specific residue; (iv) amidated, positive charged residues [Lys-NH(2) (6), Arg-NH(2) (7)], and a negatively charged aromatic residue [Trp-OH (11)] enhanced mu-opioid affinity [K(i)(mu) = 17.0, 15.1, and 15.7 nM, respectively], while Gly-NH(2) (8), Ser-NH(2) (10), and His-OH (12) were nearly one-tenth as active; and (v) D-isomers exhibited mixed effects on mu-opioid receptor affinity (2' << 3' < 4' < 1' < 5') and decreased delta-selectivity in D-Asp-NH(2) (1') and D-Lys(Ac)-OH (5'). The analogues exhibited delta-opioid receptor antagonism (pA(2) = 6.9-10.07) and weak mu-opioid receptor agonism (IC(50) > 1 microM) except H-Dmt-Tic-Glu-NH(2) (3), which was a partial delta-opioid receptor agonist (IC(50) = 2.5 nM). Thus, these C-terminally extended analogues indicated that an amino acid residue containing a single charge, amino or guanidino functionality, or aromatic group substantially altered the delta-opioid receptor activity profile (selectivity and antagonism) of the Dmt-Tic pharmacophore, which suggests that the C-terminal constituent plays a major role in determining opioid receptor activity as an "address domain".

Modeling and simulation of the human delta opioid receptor

A model for the human delta opioid receptor has been generated via sequence alignment, structure building using the crystal structure of bovine rhodopsin as a template, and refinement by molecular dynamics simulation. The model building suggested that, in addition to the previously postulated interaction between D128 and Y308, an internal salt bridge also exists between residues D128 and R192, both of which are conserved in all the opioid receptors. The model and salt bridge were then shown to be stable during a 20-nsec simulation in a lipid bilayer. It is therefore proposed that both of these interactions play a role in stabilizing the inactive state of the receptor. The model is also used in an effort to rationalize many of the mutational studies performed on delta opioid receptors, and to suggest a plausible explanation for the differences between known delta opioid agonists and antagonists.

Dmt and opioid peptides: a potent alliance

The introduction of the Dmt (2',6'-dimethyl-L-tyrosine)-Tic pharmacophore into the design of opioid ligands produced an extraordinary family of potent delta-opioid receptor antagonists and heralded a new phase in opioid research. First reviewed extensively in 1998, the incorporation of Dmt into a diverse group of opioid molecules stimulated the opioid field leading to the development of unique analogues with remarkable properties. This overview will document the crucial role played by this residue in the proliferation of opioid peptides with high receptor affinity (K(i) equal to or less than 1 nM) and potent bioactivity. The discussion will include the metamorphosis between delta-opioid receptor antagonists to delta-agonists based solely on subtle structural changes at the C-terminal region of the Dmt-Tic pharmacophore as well as their behavior in vivo. Dmt may be considered promiscuous due to the acquisition of potent mu-agonism by dermorphin and endomorphin derivatives as well as by a unique class of opioidmimetics containing two Dmt residues separated by alkyl or pyrazinone linkers. Structural studies on the Dmt-Tic compounds were enhanced tremendously by x-ray diffraction data for three potent and biologically diverse Dmt-Tic opioidmimetics that led to the development of pharmacophores for both delta-opioid receptor agonists and antagonists. Molecular modeling studies of other unique Dmt opioid analogues illuminated structural differences between delta- and mu-receptor ligand interactions. The future of these compounds as therapeutic applications for various medical syndromes including the control of cancer-associated pain is only a matter of time and perseverance.

A chimeric opioid peptide with mixed μ agonist/δ antagonist properties

There is evidence to indicate that opioid compounds with mixed mu agonist/delta antagonist properties are analgesics with low propensity to produce tolerance and physical dependence. A chimeric peptide containing the potent and selective mu agonist H-Dmt-D-Arg-Phe-Lys-NH2 ([Dmt1]DALDA) (Dmt=2',6'-dimethyltyrosine) and the potent and selective delta antagonist H-Tyr-TicPsi[CH2-NH]Cha-Phe-OH (TICP[Psi]) (Cha=cyclohexylalanine), connected 'tail-to-tail' via a short linker, was synthesized using a combination of solid-phase and solution techniques. The resulting peptide, H-Dmt-->D-Arg-->Phe-->Lys-NH-CH2-CH2-NH-Phe<--Cha[NH-CH2]PsiTic<--Tyr-H, showed the expected mu agonist/delta antagonist profile in the guinea-pig ileum and mouse vas deferens assays. Its mu and delta receptor binding affinities were in the low nanomolar range, as determined in rat brain membrane binding assays.

Functional properties of Pfr(Tic)amide and BIBP3226 at human neuropeptide FF2 receptors

The functional characteristics of two putative neuropeptide FF (NPFF) antagonists, BIBP3226 and PFR(Tic)amide, on the human neuropeptide FF receptor subtype 2 (hNPFF2) were investigated. Surprisingly, PFR(Tic)amide was shown to exhibit agonist properties in the [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding assay. The efficacy of PFR(Tic)amide was significantly greater than that of (1DMe)Y8Fa, a stable analog of NPFF, and PFR(Tic)amide can therefore be classified as a 'super-agonist'. BIBP3226 did act as a reversible competitive antagonist on the hNPFF2 receptor. However, high concentrations of BIBP3226 also non-specifically increased [35S]GTP-gammaS binding. The usefulness of BIBP3226 as an antagonist tool on the NPFF receptor is thus limited.

Synthesis and evaluation of derivatives of leucine enkephalin as potential affinity labels for δ opioid receptors

As part of an effort to develop peptide-based affinity labels for opioid receptors, [Leu(5)]enkephalin (LeuEnk) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), potent agonists for delta receptors, were selected as the parent peptides for further modification. The affinity label derivatives were prepared using standard Fmoc solid-phase peptide synthesis in conjunction with Fmoc-Phe(p-NHAlloc) (Fmoc: 9-flourenylmethoxycarbonyl;) and selective modification of the p-amino group on this residue. The electrophilic isothiocyanate and bromoacetamide groups were introduced into the para position of Phe(4); the corresponding free amine-containing peptides were also prepared for comparison. The pure peptides were evaluated in radioligand binding assays using Chinese hamster ovary (CHO) cells expressing delta and micro opioid receptors. Modification of Phe(4) in LeuEnk and DTLET significantly decreased delta-receptor binding affinity (40 to >2,000-fold). Among the synthesized analogues, [Phe(p-NH(2))(4)]DTLET showed the highest delta-receptor binding affinity (IC(50) = 39 nM) and enhanced selectivity for delta receptors compared to DTLET while other derivatives exhibited much lower delta receptor affinity. The differences in affinities between the two series of analogues and between the derivatives of LeuEnk and N,N-dibenzyl[Leu(5)]Enk reported previously suggest subtle differences in interactions of Phe(4) with delta receptors depending on other modifications in the sequences.

DPI-3290 [(+)-3-((α-R)-α-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N-(3-fluorophenyl)-N-methylbenzamide]. II. A Mixed Opioid Agonist with Potent Antinociceptive Activity and Limited Effects on Respiratory Function

Allyl-2,5-dimethyl-1-piperazines have been of interest as analgesic agents for the management of moderate-to-severe pain. In this study, we compared the antinociceptive properties and respiratory depressant activity of one such agent, (+)-3-((alpha-R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N-(3-fluorophenyl)-N-methylbenzamide (DPI-3290), with those of established narcotic analgesics, morphine and fentanyl. Intravenous administration of DPI-3290 in conscious laboratory rats increased antinociception in a dose-dependent manner with a corresponding ED(50) value of 0.05 +/- 0.0072 mg/kg. Simultaneous measurement of arterial blood gas in animals treated with DPI-3290 demonstrated dose-dependent increases in pCO2 with an ED(50) value of 0.91 +/- 0.22 mg/kg. In comparison, morphine and fentanyl increased antinociception in rats with ED(50) values of 2.01 +/- 0.0005 and 0.0034 +/- 0.00024 mg/kg, respectively, and the ED(50) value for morphine-induced changes in pCO2 was 4.23 +/- 0.72 mg/kg, whereas the ED(50) value for fentanyl-induced changes in pCO2 was 0.0127 +/- 0.0035 mg/kg. A separate series of experiments were designed to examine the effects of DPI-3290 on mu-opioid receptor induced antinociception and hypercapnia. Intravenous bolus doses of DPI-3290 that ranged from 0.2 to 1.0 mg/kg had no effect on antinociception mediated by alfentanil (2 microg/kg/min i.v.) but reduced hypercapnia by approximately 50%. Results from these studies demonstrate the equivalent antinociceptive efficacy of DPI-3290, morphine, and fentanyl but dramatic differences in the hypercapnia that antinociceptive doses of these drugs produce. When measured simultaneously, DPI-3290 had an 18.2-fold difference in the ratio comparing the ED(50) value for antinociception with the ED(50) value for changes in pCO2; this ratio was 2.1 for morphine and 3.7 for fentanyl. Furthermore, DPI-3290 reduced the alfentanil-mediated hypercapnia without any effect on antinociception. Together, the balanced opioid agonist activity of DPI-3290 may provide a means of powerful analgesia while mitigating the mu-opioid receptor-mediated hypercapnia.

Synthesis and biological evaluation of 14-alkoxymorphinans. 20. 14-phenylpropoxymetopon: an extremely powerful analgesic

The synthesis and the biological and pharmacological evaluation of several 14-phenylpropoxy analogues of 14-methoxymetopon are described. Most of the new compounds were nonselective and exhibited binding affinities in the subnanomolar or low nanomolar range at opioid receptors mu, kappa, delta), with 14-phenylpropoxymetopon (PPOM; 7) displaying the highest affinity for all three opioid receptor types. The most striking finding of this study is that the derivatives from the novel series of N-methyl-14-phenylpropoxymorphinans acted as extremely powerful antinociceptives with potencies higher than that of 14-methoxymetopon (1) and even etorphine. 14-Phenylpropoxymetopon (PPOM; 7) showed considerably increased potency in the in vivo assays in mice (25-fold in the tail-flick assay, 10-fold in the hot-plate assay, and 2.5-fold in the paraphenylquinone writhing test) when compared to etorphine, while it was equipotent to dihydroetorphine in the hot-plate assay and the paraphenylquinone writhing test and ca. twice as potent in the tail-flick assay than this reference compound. The 3-O-alkyl ethers of PPOM, compounds 6 and 8, showed less potency in in vivo assays, but partly surpassed the potency of the 3-OH analogue 14-methoxymetopon (1).

Unique high-affinity synthetic mu-opioid receptor agonists with central- and systemic-mediated analgesia

Unique opioid mimetic substances containing identical N-terminal aromatic residues separated by an unbranched alkyl chain containing two to eight methylene groups were developed. Regardless of the length of interposing alkyl chain, the bis-Tyr and bis-Phe compounds were inactive; however, replacement by a single Dmt (2',6'-dimethyl-L-tyrosine) residue enhanced activity by orders of magnitude. Moreover, the bis-Dmt compounds were another 10-fold more potent with an optimum intra-aromatic ring distance of about four to six methylene units. 1,4-Bis(Dmt-NH)butane (7) had high mu-opioid receptor affinity (K(i) = 0.041 nM) and functional mu-opioid agonist bioactivity (IC(50) = 5.3 nM) with in vivo central (intracerebroventricular) and systemic (subcutaneous) analgesia in mice (1.5- to 2.5-fold greater than and 10-12% relative to morphine, respectively); these activities were reversed by naloxone to the same degree. It appears that the bis-Dmt compounds indiscriminately act as both message and address domains.

Conversion of delta-, kappa- and mu-receptor selective opioid peptide agonists into delta-, kappa- and mu-selective antagonists

2',6'-Dimethyl substitution of the Tyr(1) residue of opioid agonist peptides and deletion of the positively charged N-terminal amino group or its replacement with a methyl group has recently been shown to represent a general structural modification to convert opioid peptide agonists into antagonists. This conversion requires the syntheses of opioid peptide analogues containing either 3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid (Dhp) or (2S)-2-methyl-3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid [(2S)-Mdp] in place of Tyr(1). Using this approach, delta-, kappa- and mu-selective opioid peptide agonist peptides were successfully converted into corresponding delta-, kappa- and mu-selective antagonists, whereby receptor selectivity was often maintained or even improved. Thus, two (2S)-Mdp(1)-analogues of the delta-selective cyclic enkephalin analogue H-Tyr-c[D-Pen-Gly-Phe(pF)-Pen]-Phe-OH turned out to be potent and selective delta antagonists. Most successful was the development of kappa antagonists derived from dynorphin A (Dyn A), including the highly potent and selective kappa-antagonist [(2S)-Mdp(1)]Dyn A(1-11)-NH(2) (dynantin) and the enzymatically stable octapeptide analogue [(2S)-Mdp(1),MeArg(7),D-Leu(8)]Dyn A(1-8)-NH(2). The (2S)-Mdp(1)-analogues of dynorphin B and alpha-neoendorphin also were kappa antagonists and may be useful as pharmacological tools in studies of kappa receptor subtypes. Finally, the Dhp(1)-analogues of the mu-selective cyclic enkephalin analogue H-Tyr-c[N(epsilon ),N(beta)-carbonyl-D-Lys(2),Dap(5)]enkephalinamide and of endomorphin-2 were moderately potent mu opioid antagonists.

Hetaryleneaminopolyols and hetarylenecarbopeptoids: a new type of glyco- and peptidomimetics. Syntheses and studies on solution conformation and dynamics

Ready access to a new class of oligomers has been demonstrated by the synthesis of hetaryleneaminopolyols and hetarylenecarbopeptoids using 3-hydroxymethyl-5-(4-amino-4-deoxy-d-arabinotetritol-1-yl)-2-methylfuran and 5-(4-amino-4-deoxy-d-arabinotetritol-1-yl)-2-methyl-3-furoic acid as novel scaffolds. The conformational behavior of peptidomimetics 22, 23, 25, 26, and 36 have been analyzed by NMR spectroscopy and extensive molecular dynamics simulations. MD simulations using the GB/SA continuum solvent model for water and the MM3 force field provide a population distribution of conformers which satisfactorily agrees with the experimental NMR data for the torsional degrees of freedom of the molecule.

X-ray studies on ligands

Advances in x-ray crystallographic data collection, structure solution, and refinement/validation have reduced the time required and expanded the range of samples amenable to x-ray crystallographic studies. Consequently, we can now collect complete atomic resolution data sets on physically smaller crystals and solve larger problems by direct methods beyond what could have been accomplished even five years ago. Applying these improved methods to the study of opioid ligands has enhanced our knowledge of the opioid pharmacophore. Despite considerable progress, it is still difficult to define the pharmacophoric parameters required for highly selective and potent opioid peptides. In part this is due to the conformational flexibility remaining even in conformationally constrained peptides.

A solid-phase synthetic strategy for labeled peptides: synthesis of a biotinylated derivative of the delta opioid receptor antagonist TIPP (Tyr-Tic-Phe-Phe-OH)

A general solid-phase synthetic strategy for labeled peptides was developed and used to prepare a biotinylated derivative of the delta opioid receptor antagonist TIPP (Tyr-Tic-Phe-Phe-OH). A monoprotected hydrophilic diamine linker was attached to an aldehyde-containing solid-phase resin by reductive amination, followed by introduction of biotin and peptide synthesis to yield Tyr-Tic-Phe-Phe-Asp-NH(CH(2)CH(2)O)(2)CH(2)CH(2)NH-biotin (2). The high delta receptor affinity and selectivity of 2 demonstrate the applicability of this design approach for labeled peptide derivatives.

Synthesis and evaluation of potential affinity labels derived from endomorphin-2

In an attempt to identify potential peptide-based affinity labels for opioid receptors, endomorphin-2 (Tyr-Pro-Phe-PheNH2), a potent and selective endogenous ligand for mu-opioid receptors, was chosen as the parent peptide for modification. The tetrapeptide analogs were prepared using standard Fmoc-solid phase peptide synthesis in conjunction with incorporation of Fmoc-Phe(p-NHAlloc) and modification of the p-amino group. The electrophilic groups isothiocyanate and bromoacetamide were introduced into the para position on either Phe3 or Phe4; the corresponding free amine-containing peptides were also prepared for comparison. The peptides bearing an affinity label group and their free amine analogs were evaluated in a radioligand-binding assay using Chinese hamster ovary (CHO) cells expressing mu- and delta-opioid receptors. Modification on Phe4 was better tolerated than on Phe3 for mu-receptor binding. Among the analogs tested, [Phe(p-NH2)4]endomorphin-2 showed the highest affinity (IC50 = 37 nm) for mu-receptors. The Phe(p-NHCOCH2Br)4 analog displayed the highest mu-receptor affinity (IC50 = 158 nm) among the peptides containing an affinity label group. Most of the compounds exhibited negligible binding affinity for delta-receptors, similar to the parent peptide.

(2S,3R) beta-methyl-2',6'-dimethyltyrosine-L-tetrahydroisoquinoline-3-carboxylic acid [(2S,3R)TMT-L-Tic-OH] is a potent, selective delta-opioid receptor antagonist in mouse brain

The constrained opioid peptide (2S,3R)beta-methyl-2',6'-dimethyltyrosine-L-tetrahydroisoquinoline-3-carboxylic acid [(2S,3R)TMT-L-Tic-OH] exhibits high affinity and selectivity for the delta-opioid receptors (). In the present study, we examined the pharmacological properties of (2S,3R)TMT-L-Tic-OH in mouse brain. A 5'-O-(3-[(35)S]thiotriphosphate) ([(35)S]GTP gamma S) binding assay was used to determine the effect of (2S,3R)TMT-L-Tic-OH on G protein activity in vitro, in mouse brain membranes. delta- (SNC80; (+)-4-[(alpha R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxy-benzyl]-N,N-diethyl-benzamide) or mu- (DAMGO; [D-Ala(2), Me-Phe(4),Gly(ol)(5)]enkephalin) selective opioid full agonists stimulated [(35)S]GTP gamma S binding in mouse brain membranes 150 +/- 4.5% and 152 +/- 5.7% over the basal level, respectively. (2S,3R)TMT-L-Tic-OH did not influence basal [(35)S]GTP gamma S binding in mouse brain membranes but dose dependently shifted the dose-response curve of SNC80 to the right, with a K(e) value of 3.6 +/- 0.7 nM. In contrast, (2S,3R)TMT-L-Tic-OH had no effect on the dose-response curve of the mu-selective opioid agonist, DAMGO. Warm water (55 degrees C) tail-flick and radiant heat paw-withdrawal tests were used to determine the in vivo nociceptive properties of (2S,3R)TMT-L-Tic-OH in the mouse. Intracerebroventricular injection of (2S,3R)TMT-L-Tic-OH had no significant effect on withdrawal latencies in either nociceptive tests. (2S,3R)TMT-L-Tic-OH (30 nmol/mouse) attenuated deltorphin II- but not DAMGO-mediated antinociception (40 +/- 13 and 100% of maximal possible effect, respectively) when administered intracerebroventricularly 10 min before the agonist. Taken together these results suggest that (2S,3R)TMT-L-Tic-OH is a potent highly selective neutral delta-opioid antagonist in mouse brain.

The structure of an endomorphin analogue incorporating 1-aminocyclohexane-1-carboxlylic acid for proline is similar to the beta-turn of Leu-enkephalin

Endomorphin (EM2, Tyr-Pro-Phe-Phe-NH(2)) can assume various conformations related to cis/trans-rotamers of the amide linkage of Tyr-Pro. To control isomerization, restricted or flexible components have been introduced at the Pro position. We focused on [Chx(2)]EM2, an EM2 analogue substituting 1-aminocyclohexane-1-carboxlylic acid (Chx) for Pro. X-ray diffraction analysis revealed that [Chx(2)]EM2 is folded into the trans-form of Tyr-Chx. The manner of folding resembled that seen in D-TIPP, an EM analogue incorporating tetrahydroisoquinoline carboxylic acid, as well as the beta-turn of Leu-enkephalin. Selectivity for the opioid mu-receptor was fairly well conserved by [Chx(2)]EM, suggesting that the folded form is important for mu-selectivity.

Solid phase synthesis and evaluation of Tyr-Tic-Phe-Phe(p-NHCOCH(2)Br) ([Phe(p-bromoacetamide)(4)]TIPP), a potent affinity label for delta opioid receptors

Derivatives of the delta opioid receptor selective peptide Tyr-Tic-Phe-Phe-OH (TIPP) containing a p-bromoacetamide moiety on the phenyl ring of Phe(3) or Phe(4) were prepared by solid phase synthesis. [Phe(p-NHCOCH(2)Br)(4)]TIPP exhibited high affinity for cloned delta receptors (IC(50) = 5.4 nM), and incubation with only 2.5 nM resulted in 85% wash resistant inhibition of radioligand binding to delta receptors. Therefore, this peptide is a potent affinity label for further study of delta opioid receptors.

CCK(2) receptor antagonists containing the conformationally constrained phenylalanine derivatives, including the new amino acid Xic

The conformationally constrained analogues of phenylalanine, tetrahydroisoquinoline-3-carboxylic acid (Tic), Sic, Hic and Nic, and the new amino acid Xic have been incorporated into a potent and highly selective cholecystokinin-2 (CCK(2)) receptor antagonist (2) in place of the phenylalanine residue, producing compounds 15a-e. High selectivities for CCK(2) over CCK(1) were observed for compounds 15a-e. The in vitro profile of the analogue containing the Nic residue (15d) was identical to that of compound 2, whereas the alternative conformational constraints resulted in a significant loss of affinity. The apparent advantage of Nic in the context of these CCK(2) ligands was subsequently demonstrated to be statistically significant.

Agonist, antagonist, and inverse agonist characteristics of TIPP (H-Tyr-Tic-Phe-Phe-OH), a selective delta-opioid receptor ligand

Recent evidence indicates that the well established delta-opioid antagonist TIPP (H-Tyr-Tic-Phe-Phe-OH) also displays agonist activity in several cellular models. Therefore, it is possible that TIPP, and structurally related compounds, might represent a novel class of opioid agonists exhibiting unique characteristics. The purpose of this study was to examine the properties of TIPP at selected points of the signal transduction pathway (i.e., receptor binding, G-protein activation, and effector regulation) in GH(3)DORT cells (GH(3) cells expressing delta-opioid receptors) and compare them with that of an established delta-opioid agonist, [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE). DPDPE exhibited properties of an agonist in all assays. In contrast, TIPP demonstrated characteristics of an agonist, antagonist, or inverse agonist, depending on the step in the signal transduction cascade examined and the assay conditions employed. In receptor binding assays, the addition of guanine nucleotides and sodium ions increased the affinity of TIPP for delta-opioid receptors in both membrane preparations and digitonin-permeabilized cells, which is characteristic of an inverse agonist. In assays measuring G-protein activation, TIPP failed to stimulate guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding in membrane preparations, which is consistent with an antagonist profile. However, when using cells semi-permeabilized with digitonin, TIPP exhibited properties of an agonist, producing concentration-dependent, antagonist-reversible stimulation of [(35)S]GTPgammaS binding. Finally, in assays examining regulation of the intracellular effector adenylyl cyclase, TIPP exhibited characteristics of an agonist, producing inhibition of enzyme activity in both membrane preparations and whole cells. Therefore, although DPDPE and TIPP act similarly as agonists to regulate the intracellular effector adenylyl cyclase, they demonstrate significant differences in the signal transduction cascade preceding this final point of convergence.

Design, synthesis, and evaluation of opioid analogues with non-peptidic beta-turn scaffold: enkephalin and endomorphin mimetics

We have identified a mu-selective opioid receptor agonist without a cationic amino group in the molecule from libraries of bicyclic beta-turn peptidomimetics. The biologically active conformation of the lead is proposed to mimic an endomorphin type III 4 --> 1 beta-turn conformation.

Crystal structure of biphalin sulfate: a multireceptor opioid peptide

Biphalin is a dimeric opioid peptide, composed of two tetrapeptides connected 'tail-to-tail', that exhibits a high affinity for all three opioid receptor types (i.e. mu, delta and kappa). This study presents the X-ray crystal structure of biphalin sulfate and compares it to other opioids that interact with the same biological targets. Both halves of the molecule have a folded backbone conformation but differ significantly from one another. Residues 1-4 in biphalin, which compare well with the delta selective opioid peptide DADLE, fold into a random coil. Residues 5-8, which can be fit to the mu selective peptide D-TIPP-NH2, exhibit a fairly normal type III' beta bend. Biphalin also exhibits structural similarities with two naltrexone analogs, naltrexonazine and norbinaltorphamine, that are specific to mu and kappa receptor sites.

Transport and metabolism of opioid peptides across BeWo cells, an in vitro model of the placental barrier

In keeping with the advance of biotechnology, cell culture becomes an important tool for investigating the transport and the metabolism phenomena. A cell line of human origin, the BeWo choriocarcinoma cell line, was used for the study of the transport and metabolism of opioid peptides across the in vitro model of the placental barrier. Opioid peptides, both naturally occurring and their synthetic analogs, are of interest to be developed as potent analgesics and were included in this study. The apparent permeability coefficients (Pe)s of the peptides containing 4-11 amino acid or analog residues were in the range of 0.23-14.6 x 10(-5) cm/s. The (Pe)s of these peptides were comparable to those of sucrose or dextrans, hydrophilic markers. The (Pe)s of low molecular weight (MW) peptides was not dependent on their MW or molecular size, whereas an inversely linear correlation between (Pe)s and molecular size was observed with the larger peptides. Molecular sieving of the BeWo monolayer restricted the transport of the peptides with MW> or =1033 Da or molecular size > or =6.6 A. Membrane partitioning ability and charge of the peptides were also investigated and found to be the minor factors regulating the extent of peptide permeation. Contrasting to the transport of Tyr-[D-pen-Gly-Phe-D-Pen] (DPDPE) peptide analog across the blood-brain barrier, the transport of DPDPE across the BeWo monolayers were not indicated to be via carrier-mediated transport. The major transport pathway of the opioid peptides across the BeWo monolayers was found to be via paracellular route. In metabolism studies, aminopeptidase was found to be a major enzyme type responsible for the degradation of naturally occurring peptides but not for the synthetic analogs. The finding obtained from the present study reveals the applicability of the BeWo cell line as an in vitro model for investigating placental transport and metabolism of opioid peptides.

Evaluation of the Dmt-Tic pharmacophore: conversion of a potent delta-opioid receptor antagonist into a potent delta agonist and ligands with mixed properties

Analogues of the 2',6'-dimethyl-L-tyrosine (Dmt)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic) pharmacophore were prepared to test the hypothesis that a "spacer" and a third aromatic center in opioid peptides are required to convert a delta-antagonist into ligands with delta-agonist or with mixed delta-antagonist/mu-agonist properties. Potent delta-agonists and bifunctional compounds with high delta- and mu-opioid receptor affinities were obtained by varying the spacer length [none, NH-CH(2), NH-CH(2)-CH(2), Gly-NH-CH(2)] and C-terminal aromatic nucleus [1H-benzimidazole-2-yl, phenyl (Ph) and benzyl groups]. C-terminal modification primarily affected mu-opioid receptor affinities, which increased maximally 1700-fold relative to the prototype delta-antagonist H-Dmt-Tic-NH(2) and differentially modified bioactivity. In the absence of a spacer (1), the analogue exhibited dual delta-agonism (pEC(50), 7.28) and delta-antagonism (pA(2), 7.90). H-Dmt-Tic-NH-CH(2)-1H-benzimidazole-2-yl (Bid) (2) became a highly potent delta-agonist (pEC(50), 9.90), slightly greater than deltorphin C (pEC(50), 9.56), with mu-agonism (pE(50), 7.57), while H-Dmt-Tic-Gly-NH-CH(2)-Bid (4) retained potent delta-antagonism (pA(2), 9.0) but with an order of magnitude less mu-agonism. Similarly, H-Dmt-Tic-Gly-NH-Ph (5) had nearly equivalent high delta-agonism (pEC(50), 8.52) and mu-agonism (pEC(50), 8.59), while H-Dmt-Tic-Gly-NH-CH(2)-Ph (6) whose spacer was longer by a single methylene group exhibited potent delta-antagonism (pA(2), 9.25) and very high mu-agonism (pEC(50), 8.57). These data confirm that the distance between the Dmt-Tic pharmacophore and a third aromatic nucleus is an important criterion in converting Dmt-Tic from a highly potent delta-antagonist into a potent delta-agonist or into ligands with mixed delta- and mu-opioid properties.

Synthesis and biological activities of YkFA analogues: effects of position 4 substitutions and altered ring size on in vitro opioid activity

Substitution in position 4 of the potent opioid peptide YkFA with aliphatic hydrophobic residues resulted in compounds that retained low nanomolar activities at both mu and delta opioid receptors, while ring contraction by incorporation of diaminobutyric acid in position 2 resulted in a more pronounced decrease in potency at both receptors for the psi[CH(2)NH] pseudopeptide as compared to the all amide parent.

Conformational comparison of mu-selective endomorphin-2 with its C-terminal free acid in DMSO solution, by 1H NMR spectroscopy and molecular modeling calculation

In order to make clear the structural role of the C-terminal amide group of endomorphin-2 (EM2, H-Tyr-Pro-Phe-Phe-NH2), an endogenous mu-receptor ligand, in the biological function, the solution conformations of endomorphin-2 and its C-terminal free acid (EM2OH, H-Tyr-Pro-Phe-Phe-OH), studied using two-dimensional 1H NMR measurements and molecular modeling calculations, were compared. Both peptides were in equilibrium between the cis and trans isomers around the Tyr-Pro omega bond in a population ratio of approximately/= 1:2. The lack of significant temperature and concentration dependence of NH protons suggested that the NMR spectra reflected the conformational features of the respective molecules themselves. Fifty possible 3D structures for the each isomer were generated by the dynamical simulated annealing method under the proton-proton distance constraints derived from the ROE cross-peaks. These energy-minimized conformers, which were all in the phi torsion angles estimated from J(NHCalphaH) coupling constants within +/- 30 degrees, were then classified in groups one or two according to the folding backbone structures. All trans and cis EM2 conformers adopt an open conformation in which their extended backbone structures are twisted at the Pro2-Phe3 moiety. In contrast, the trans and cis conformers of EM2OH show conformational variation between the 'bow'-shaped extended and folded backbone structures, although the cis conformers of its zwitterionic form are refined into the folded structure of the close disposition of C- and N-terminal groups. These results indicate clearly that the substitution of carboxyl group for C-terminal amide group makes the peptide flexible. The conformational requirement for mu-receptor activation has been discussed based on the active form proposed for endomorphin-1 and by comparing conformational features of EM2 and EM2OH.

Novel C-terminus modifications of the Dmt-Tic motif: a new class of dipeptide analogues showing altered pharmacological profiles toward the opioid receptors

The design, synthesis and pharmacological evaluation of a novel class of Dmt-Tic dipeptide analogues are described. These resulting analogues bearing different C-terminal functionalities were found to bind to the human delta receptor with high affinity. One specific class of dipeptides bearing urea/thiourea functionalities showed partial to full activation of the delta receptor. Several dipeptides also showed good binding affinities with full activation of the human kappa receptor, a novel property for those ligands.

Probing the shape of a hydrophobic pocket in the active site of delta-opioid antagonists

The change of selectivity and the induction of antagonism by the insertion of 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic) in the second position of several opioid peptides have led to the interpretation of Tyr-Tic as a specific message domain for delta-opioid antagonists and to the discovery of dipeptides with substantial opioid activity. Selectivity and activity increase enormously when Tyr is substituted by 2',6'-dimethyl tyrosine (Dmt), hinting that the side chain of Dmt fits a hydrophobic cavity of the receptor very tightly and precisely. We have investigated the specificity of this fit by systematic changes of the substituents on the aromatic ring of ryr. Mono- and disubstitutions different from 2',6'- invariably lead to catastrophic decreases of activity. The only substitution compatible with retention of substantial antagonism is 2-methyl. An analysis of the conformational properties of all analogues reveals that substitutions do not affect the global shape of the molecule significantly. Accordingly, it is possible to use the shape of the different side chains to map the hydrophobic cavity of the receptor. The resulting complementary image is funnel shaped.

Synthesis and evaluation of isothiocyanate-containing derivatives of the delta-opioid receptor antagonist Tyr-Tic-Phe-Phe (TIPP) as potential affinity labels for delta-opioid receptors

Derivatives of the delta-opioid receptor-selective peptide antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP) containing an isothiocyanate moiety at the para position of either Phe(3) or Phe(4) were prepared as potential affinity labels for delta-opioid receptors. The synthesis was accomplished using a general solution-phase synthetic procedure which allows for introduction of affinity labeling groups late in the synthesis of a variety of small peptide substrates. The target peptides and their corresponding amines were then evaluated in radioligand binding experiments using Chinese hamster ovary (CHO) cells expressing delta- and mu-opioid receptors. The peptides [Phe(p-NCS)(3)]TIPP (2) and [Phe(p-NCS)(4)]TIPP (4) showed affinity for delta-receptors comparable to the parent compound TIPP (IC(50) = 12 and 5 nM, respectively, vs 6 nM for TIPP). Both peptides 2 and 4 were able to inhibit radioligand binding to delta-receptors in a wash-resistant manner at a concentration of 10 nM. Therefore, the peptides [Phe(p-NCS)(3)]TIPP (2) and [Phe(p-NCS)(4)]TIPP (4) represent two affinity labels that may prove useful in the study of delta-opioid receptors.

Extended TIP(P) analogues as precursors for labeled delta-opioid receptor ligands

Tyr-Tic-Phe-Phe-OH (TIPP) and the shorter Tyr-Tic-Phe-OH (TIP) peptides are potent and highly selective antagonists at the delta-opioid receptor and, therefore, are ideal candidates for the attachment of labels to assist in the study of delta-opioid receptors. Peptides extended at the C-terminus with residues which can be used as handles for further modification and/or labeling (i.e. Asx, Glx, and Lys) were synthesized. The TIPP-D/L-Asx/Glx derivatives exhibited similar delta-receptor affinity to TIPP (K(i) = 5-10 nM vs K(i) = 6 nM), and neither the location of the carboxylic acid moiety nor the stereochemistry of the C-terminal residue significantly affected the delta-receptor affinity of these derivatives. Extension of TIPP with an additional residue did not increase mu-receptor affinity, even though the position of the acidic group, which imparts delta-receptor selectivity to TIPP, was shifted relative to the carboxylic acid moiety of TIPP. The delta-receptor affinities of the TIP-D/L-Asx/Glx derivatives were found to be influenced mainly by the position of the carboxylic acid function rather than the stereochemistry of the C-terminal residue. TIP(P)-D/L-Lys(Ac)-OH derivatives exhibited moderate delta-receptor affinity (K(i)(delta) = 16-28 nM). The most potent compounds found in the extended TIP(P) series were TIPP-D-Gln-OH and TIP-D-Gln-OH (K(i)(delta) = 5 nM) which had similar affinities to TIPP.

Assessment of substitution in the second pharmacophore of Dmt-Tic analogues

The Dmt-Tic pharmacophore exhibits potent delta-opioid receptor antagonism. Analogues with substitutions in the second pharmacophore with (1, 1') or without a COOH function (2-9) were synthesized: several had high delta affinity (1', 2, 7, and 9), but exhibited low to non-selectivity toward mu receptors similar to H-Dmt-Tic-amide and H-Dmt-Tic-ol. Functional bioactivity indicated high delta antagonism (pA2 7.4-7.9) (1', 2, and 9) and modest mu agonism, pEC50 (6.1-6.3) (1', 2, 8, and 9), but with Emax values analogous to dermorphin. These Dmt-Tic analogues with mixed delta antagonist/mu agonist properties would appear to be better candidates as analgesics than pure mu agonists.