Structural studies of [2',6'-dimethyl-L-tyrosine1]endomorphin-2 analogues: enhanced activity and cis orientation of the Dmt-Pro amide bond

Functional selectivity of EM-2 analogs at the mu-opioid receptor

The mu opioid receptor agonists are the most efficacious pain controlling agents but their use is accompanied by severe side effects. More recent developments indicate that some ligands can differentially activate receptor downstream pathways, possibly allowing for dissociation of analgesia mediated through the G protein from the opioid-related side effects mediated by β-arrestin pathway. In an effort to identify such biased ligands, here we present a series of thirteen endomorphin-2 (EM-2) analogs with modifications in positions 1, 2, and/or 3. All obtained analogs behaved as mu receptor selective agonists in calcium mobilization assay carried out on cells expressing opioid receptors and chimeric G proteins. A Bioluminescence Resonance Energy Transfer (BRET) approach was employed to determine the ability of analogs to promote the interaction of the mu opioid receptor with G protein or β-arrestin 2. Nearly half of the developed analogs showed strong bias towards G protein, in addition four compounds were nearly inactive towards β-arrestin 2 recruitment while blocking the propensity of EM-2 to evoke mu-β-arrestin 2 interaction. The data presented here contribute to our understanding of EM-2 interaction with the mu opioid receptor and of the transductional propagation of the signal. In addition, the generation of potent and selective mu receptor agonists strongly biased towards G protein provides the scientific community with novel tools to investigate the in vivo consequences of biased agonism at this receptor.

Vagal apnea and hypotension evoked by systemic injection of an antinociceptive analogue of endomorphin-2

PK20M (Dmt-D-Lys-Phe-Phe-OH) is a novel modified endomorphin-2 (EM-2) peptide producing strong dose- and time-dependent antinociceptive activity. Yet its prototype, endogenous EM-2, has been reported to trigger respiratory and vascular effects such as apnea and hypotension. The purpose of this study was to investigate the potency of the PK20M to evoke respiratory and cardiovascular responses in comparison to endogenous endomorphins. The engagement of the vagal pathway and μ opioid receptors in mediation of these responses was investigated. The effects of intravenous injections of PK20M, EM-1, and EM-2 were studied in anaesthetized, spontaneously breathing rats. The main dose-dependent effect of all endomorphins in the intact rats was immediate apnea, blood pressure and heart rate decrease. PK20M produced apnea in at least half of the intact animals in a much smaller dose than EM-1 and EM-2. The effects of all compounds were abrogated by pre-treatment with MNLX, a peripherally acting μ receptor antagonist. Cervical vagotomy eliminated arrest of breathing in the case of each tested compound. Hypotension was reduced by vagi section only after EM-1 and EM-2 administration. Our results demonstrated that apnea and bradycardia caused by systemic injection of all endomorphins were mediated via activation of μ vagal opioid receptors. The hypotension depended on intact vagi nerves only in the case of EM-1 and EM-2, whereas PK20M decreased blood pressure via other mechanisms outside vagal innervation. Modified opioid agonist is more potent in evoking extended hypotension; at the same time, it produces an arrest of breathing less frequently than its prototype EM-2.

Endomorphin-2 analogs containing modified tyrosines: Biological and theoretical investigation of the influence on conformation and pharmacological profile

New analogs of the endogenous opioid agonist endomorphin-2 (EM-2, H-Tyr-Pro-Phe-Phe-NH₂) have been obtained by introducing modified tyrosines at the position 1 of the sequence. For all analogs, the cis/trans conformation ratio about the tyramine-Pro amide bond, lipophilicity, receptor affinities, and functional activities, have been determined. Among the novel derivatives, [Dmt(3'-Cl)]¹EM-2 (4) stood out for its subnanomolar μ-opioid receptor affinity and potent agonist activity, superior to that of the parent peptide EM-2. Hybrid quantum mechanics/molecular mechanics docking computations supported the cis tyramine-Pro bioactive conformation, and allowed us to analyze the contribution of the substituents of the "message" tyramine to binding, highlighting the role of halogen-bonding in the higher receptor affinity of peptide 4.

Targeting Neuropathic Pain: Pathobiology, Current Treatment and Peptidomimetics as a New Therapeutic Opportunity

There is a huge need for pharmaceutical agents for the treatment of chronic Neuropathic Pain (NP), a complex condition where patients can suffer from either hyperalgesia or allodynia originating from central or peripheral nerve injuries. To date, the therapeutic guidelines include the use of tricyclic antidepressants, serotonin-noradrenaline reuptake inhibitors and anticonvulsants, beside the use of natural compounds and non-pharmacological options. Unfortunately, these drugs suffer from limited efficacy and serious dose-dependent adverse effects. In the last decades, the heptapeptide SP1-7, the major bioactive metabolite produced by Substance P (SP) cleavage, has been extensively investigated as a potential target for the development of novel peptidomimetic molecules to treat NP. Although the physiological effects of this SP fragment have been studied in detail, the mechanism behind its action is not fully clarified and the target for SP1-7 has not been identified yet. Nevertheless, specific binding sites for the heptapeptide have been found in brain and spinal cord of both mouse and rats. Several Structure-Affinity Relationship (SAR) studies on SP1-7 and some of its synthetic analogues have been carried out aiming to developing more metabolically stable and effective small molecule SP1-7-related amides that could be used as research tools for a better understanding of the SP1-7 system and, in a longer perspective, as potential therapeutic agents for future treatment of NP.

Structural basis for bifunctional peptide recognition at human δ-opioid receptor

Bi-functional μ- and δ- opioid receptor (OR) ligands are potential therapeutic alternatives to alkaloid opiate analgesics with diminished side effects. We solved the structure of human δ-OR bound to the bi-functional δ-OR antagonist and μ-OR agonist tetrapeptide H-Dmt(1)-Tic(2)-Phe(3)-Phe(4)-NH₂ (DIPP-NH₂) by serial femtosecond crystallography, revealing a cis-peptide bond between H-Dmt(1) and Tic(2). The observed receptor-peptide interactions are critical to understand the pharmacological profiles of opioid peptides, and to develop improved analgesics.

Endomorphin analogues with mixed μ-opioid (MOP) receptor agonism/δ-opioid (DOP) receptor antagonism and lacking β-arrestin2 recruitment activity

Analogues of endomorphin (Dmt-Pro-Xaa-Xaa-NH2) modified at position 4 or at positions 4 and 3, and tripeptides (Dmt-Pro-Xaa-NH2) modified at position 3, with various phenylalanine analogues (Xaa=Trp, 1-Nal, 2-Nal, Tmp, Dmp, Dmt) were synthesized and their effects on in vitro opioid activity were investigated. Most of the peptides exhibited high μ-opioid (MOP) receptor binding affinity (KiMOP=0.13-0.81nM), modest MOP-selectivity (Kiδ-opioid (DOP)/KiMOP=3.5-316), and potent functional MOP agonism (GPI, IC50=0.274-249nM) without DOP and κ-opioid (KOP) receptor agonism. Among them, compounds 7 (Dmt-Pro-Tmp-Tmp-NH2) and 9 (Dmt-Pro-1-Nal-NH2) were opioids with potent mixed MOP receptor agonism/DOP receptor antagonism and devoid of β-arrestin2 recruitment activity. They may offer a unique template for the discovery of potent analgesics that produce less respiratory depression, less gastrointestinal dysfunction and that have a lower propensity to induce tolerance and dependence compared with morphine.

Characterization of opioid activities of endomorphin analogs with C-terminal amide to hydrazide conversion

Previously, we have synthesized an endomorphin-2 (EM-2) analog with C-terminal amide to hydrazide conversion, exhibiting slightly lower μ-affinity than EM-2. In the present study, the influence of C-terminal amide group to hydrazide conversion on the in vitro and in vivo opioid activities of EMs was evaluated. Our results demonstrated that C-terminal amide to hydrazide conversion of EMs did not markedly change their μ-opioid receptor binding affinities. Nevertheless, EM-2-NHNH2 decreased guinea pig ileum (GPI) and mouse vas deferens (MVD) potencies by about 10- and 5-fold compared to the parent compound, respectively. It is noteworthy that EM-1-NHNH2 exhibited the highest antinociception after intracerebroventricular (i.c.v.) injection, about 1.5-fold more potent than EM-1, but with moderate colonic contractile and expulsive effects, comparable with EM-1. Additionally, though EM-2-NHNH2 showed a slightly lower antinociceptive effect than EM-2, at higher doses (i.c.v., 1.5 and 5 nmol/mouse) the inhibitory effects of colonic propulsion were significantly attenuated, which would be helpful in the development of suitable μ-opioid therapeutics, but without some undesirable side effects. Therefore, the present results gave the evidence that C-terminal amide to hydrazide conversion of EMs may play an important role in the regulation of opioid activities.

Developmental potential for endomorphin opioidmimetic drugs

Morphine, which is agonist for μ-opioid receptors, has been used as an anti-pain drug for millennia. The opiate antagonists, naloxone and naltrexone, derived from morphine, were employed for drug addiction and alcohol abuse. However, these exogenous agonists and antagonists exhibit numerous and unacceptable side effects. Of the endogenous opioid peptides, endomorphin(EM)-1 and endomorphin(EM)-2 with their high μ-receptor affinity and exceptionally high selectivity relative to δ- and κ-receptors in vitro and in vivo provided a sufficiently sequence-flexible entity in order to prepare opioid-based drugs. We took advantage of this unique feature of the endomorphins by exchanging the N-terminal residue Tyr¹ with 2′,6′-dimethyl-l-tyrosine (Dmt) to increase their stability and the spectrum of bioactivity. We systematically altered specific residues of [Dmt¹]EM-1 and [Dmt¹]EM-2 to produce various analogues. Of these analogues, [N-allyl-Dmt¹]EM-1 (47) and [N-allyl-Dmt¹]EM-2 (48) exhibited potent and selective antagonism to μ-receptors: they completely inhibited naloxone- and naltrexone-induced withdrawal from following acute morphine dependency in mice and reversed the alcohol-induced changes observed in sIPSC in hippocampal slices. Overall, we developed novel and efficacious opioid drugs without deleterious side effects that were able to resist enzymatic degradation and were readily transported intact through epithelial membranes in the gastrointestinal tract and the blood-brain-barrier.

2',6'-dimethylphenylalanine: a useful aromatic amino Acid surrogate for tyr or phe residue in opioid peptides

Two aromatic amino acids, Tyr(1) and Phe(3) or Phe(4), are important structural elements in opioid peptides because they interact with opioid receptors. The usefulness of an artificial amino acid residue 2',6'-dimethylphenylalanine (Dmp) was investigated as an aromatic amino acid surrogate for several opioid peptides, including enkephalin, dermorphin, deltorphin, endomorphin, dynorphin A, and nociceptin peptides. In most peptides, substitutions of Phe(3) by a Dmp residue produced analogs with improved receptor-binding affinity and selectivity, while the same substitution of Phe(4) induced markedly reduced receptor affinity and selectivity. Interestingly, replacement of Tyr(1) by Dmp produced analogs with unexpectedly high affinity or produced only a slight drop in receptor affinity and bioactivity for most peptides. Thus, Dmp is also a useful surrogate for the N-terminal Tyr residue in opioid peptides despite the lack of a phenolic hydroxyl group, which is considered necessary for opioid activity. The Dmp(1)-substituted analogs are superior to 2',6'-dimethyltyrosine (Dmt)(1)-substituted analogs for high receptor selectivity since the latter generally have poor receptor selectivity. Thus, Dmp is very useful as an aromatic amino acid surrogate in opioid peptides and may be useful for developing other novel peptide mimetics with high receptor specificity.

Engineering endomorphin drugs: state of the art

INTRODUCTION

Although endomorphins-1 (EM-1; H-Tyr-Pro-Phe-Trp-NH(2)) and -2 (EM-2; H-Tyr-Pro-Phe-Phe-NH(2)) are primarily considered agonists for the μ-opioid receptor (MOR), systematic alterations to specific residues provided antagonists and ligands with mixed μ/δ-opioid properties, suitable for application to health-related topics. While the application of endomorphins as antinociceptive agents and numerous biological endpoints were experimentally delineated in laboratory animals and in vitro, clinical use is currently absent. However, structural alterations provide enhanced stability; formation of MOR antagonists or mixed and dual μ/δ-acting ligands could find considerable therapeutic potential.

AREAS COVERED

This review attempts to succinctly provide insight on the development and bioactivity of endomorphin analogues during the past decade. Rational design approaches will focus on the engineering of endomorphin agonists, antagonists and mixed ligands for their application as a multi-target ligand.

EXPERT OPINION

Aside from alleviating pain, EM analogues open new horizons in the treatment of medical syndromes involving neural reward mechanisms and extraneural regulation effects on homeostasis. Highly selective MOR antagonists may be promising to reduce inflammation, attenuate addiction to drugs and excess consumption of high-caloric food, ameliorate alcoholism, affect the immune system and combat opioid bowel dysfunction.

Endomorphins in rheumatoid arthritis, osteoarthritis, and experimental arthritis

The opioid tetrapeptides endomorphins (EM)-1 and EM-2 are widely expressed in central nervous system and immune tissues of rats and humans. Their analgesic properties are well characterized but they also have anti-inflammatory properties. EM-1 significantly attenuated the onset of hindpaw inflammation in adjuvant-induced arthritis in rats. Immunohistochemical staining demonstrated the presence of EMs in T cells, macrophages, and fibroblasts in synovial tissues from patients with osteo- or rheumatoid arthritis (RA). In an ex vivo superfusion system, EM-1 potently inhibited the release of proinflammatory cytokines interleukin (IL)-6 and IL-8 from synovial tissues from patients with osteo- or RA. These results demonstrate that EMs are endogenously synthesized within human immune cells and have the potential to act as potent therapeutic agents in the treatment of chronic inflammatory disease. We discuss the clinical potential for EM analogues chemically modified to resist proteolytic degradation and identify modified protease-resistant analogues with enhanced bioactivity.

Discovery of dipeptides with high affinity to the specific binding site for substance P1-7

Substance P 1-7 (SP(1-7), H-Arg-Pro-Lys-Pro-Gln-Gln-Phe-OH) is the major bioactive metabolite of substance P. The interest in this heptapeptide originates from the observation that it modulates, and in certain cases opposes the effects of the parent peptide, e.g., the nociceptive effect. The mu-opioid receptor agonist endomorphin-2 (EM-2, H-Tyr-Pro-Phe-Phe-NH(2)) has been found to also interact with the specific binding site of SP(1-7) with only a 10-fold lower affinity compared to the native peptide. Considering the smaller size of EM-2 compared to the target heptapeptide, it was selected as a lead compound in the development of low-molecular-weight ligands to the SP(1-7) binding site. An alanine scan and truncation study led to the unexpected discovery of the dipeptide H-Phe-Phe-NH(2) (K(i) = 1.5 nM), having equal affinity as the endogenous heptapeptide SP(1-7.) Moreover, the studies show that the C-terminal phenylalanine amide is crucial for the affinity of the dipeptide.

In vitro and in vivo characterization of opioid activities of C-terminal esterified endomorphin-2 analogs

Previously, we have synthesized a series of endomorphin-2 (EM-2) analogs by the substitution of C-terminal amide group. In the present study, to further our knowledge of the influence of C-terminal esterified modification on the pharmacological activities, we investigated the in vitro and in vivo opioid activities of C-terminal esterified EM-2 analogs 1-3. Our results showed that the ED(50) values on contractions of the longitudinal muscle of distal colon induced by analogs 1-3 were about 1.5-fold higher, 2- and 8-fold lower than EM-2, respectively. In addition, intravenous (i.v.) injections of analogs 1 and 2 dose-dependently decreased the system arterial pressure (SAP) and heart rate (HR) in anesthetized rats, but the degree of the hypotension and bradycardia was significantly smaller relative to the parent. Moreover, analog 3 was almost ineffective. Nevertheless, all these analogs produced potent antinociception in the tail-flick test after intracerebroventricular (i.c.v.) injection, and this antinociception was inhibited by naloxone, indicating an opioid mechanism. In summary, these results gave the evidence that the conversion of C-terminal amide to esterified modification may play an important role in the regulation of opioid affinities and pharmacological activities.

Synthesis and in vitro evaluation of a library of modified endomorphin 1 peptides

Endomorphin 1 (Endo-1=Tyr-Pro-Trp-Phe-NH(2)), an endogenous opioid with high affinity and selectivity for mu-opioid receptors, mediates acute and neuropathic pain in rodents. To overcome metabolic instability and poor membrane permeability, the N- and C-termini of Endo-1 were modified by lipoamino acids (Laa) and/or sugars, and 2',6'-dimethyltyrosine (Dmt) replacement of Tyr. Analogues were assessed for mu-opioid receptor affinity, inhibition of cAMP accumulation, enzymatic stability, and permeability across Caco-2 cell monolayers. C-terminus modification decreased receptor affinity, while N-terminus C8-Laa improved stability and permeability with slight change in receptor affinity. Dmt provided a promising lead compound: [C8Laa-Dmt[1]]-Endo-1 is nine times more stable (t(1/2)=43.5min), >8-fold more permeable in Caco-2 cell monolayers, and exhibits 140-fold greater mu-opioid receptor affinity (K(imu)=0.08nM).

[Dmt1, d-1-Nal3]morphiceptin, a novel opioid peptide analog with high analgesic activity

The morphiceptin-derived peptide [Dmt1, d-1-Nal3]morphiceptin, labeled mu-opioid receptor (MOP) with very high affinity and selectivity in the receptor binding assays. In the mouse hot plate test, [Dmt1, d-1-Nal3]morphiceptin given intracerebroventricularly (i.c.v.) produced profound supraspinal analgesia, being approximately 100-fold more potent than the endogenous MOP receptor ligand, endomorphin-2. The antinociceptive effect of this new analog lasted up to 120min. Thus, [Dmt1, d-1-Nal3]morphiceptin is an interesting and extraordinarily potent analgesic, raising the possibility of novel approaches in the design of clinically useful drugs for pain treatment.

Internalisation of the mu-opioid receptor by endomorphin-1 and leu-enkephalin is dependant on aromatic amino acid residues

The opioid receptor system in the central nervous system controls a number of physiological processes, most notably pain. However, most opioids currently available have a variety of side-effects as well as exhibiting tolerance. Tolerance is most likely to be a complex phenomenon, however, the role of receptor internalisation is thought to play a crucial role. In this study, we examined the role of aromaticity in ligand-mediated receptor internalisation of the mu-opioid receptor (MOPR). These studies show that the amount of receptor internalisation may be dependant on the amphiphilicity of the ligand. Specifically, deletion of the C-terminus aromatic residues of endomorphin 1, particularly tryptophan reduces receptor-mediated internalisation whilst the addition of tryptophan within the enkephalin sequence increases receptor internalisation and decreases tolerance.

Design and synthesis of opioidmimetics containing 2',6'-dimethyl-L-tyrosine and a pyrazinone-ring platform

Twelve 2',6'-dimethyl-L-tyrosine (Dmt) analogues linked to a pyrazinone platform were synthesized as 3- or 6-[H-Dmt-NH(CH(2))(n)],3- or 6-R-2(1H)-pyrazinone (n=1-4). 3-[H-Dmt-NH-(CH(2))(4)]-6-beta-phenethyl-5-methyl-2(1H)-pyrazinone 11 bound to mu-opioid receptors with high affinity (K(i)mu=0.13 nM; K(i)delta/K(i)mu=447) with mu-agonism (GPI IC(50)=15.9 nM) and weak delta-antagonism (MVD pA(2)=6.35). Key factors affecting opioid affinity and functional bioactivity are the length of the aminoalkyl chain linked to Dmt and the nature of the R residue. These data present a simplified method for the formation of pyrazinone opioidmimetics and new lead compounds.

Bifunctional [2',6'-dimethyl-L-tyrosine1]endomorphin-2 analogues substituted at position 3 with alkylated phenylalanine derivatives yield potent mixed mu-agonist/delta-antagonist and dual mu-agonist/delta-agonist opioid ligands

Endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH2) and [Dmt1]EM-2 (Dmt = 2',6'-dimethyl-l-tyrosine) analogues, containing alkylated Phe3 derivatives, 2'-monomethyl (2, 2'), 3',5'- and 2',6'-dimethyl (3, 3', and 4', respectively), 2',4',6'-trimethyl (6, 6'), 2'-ethyl-6'-methyl (7, 7'), and 2'-isopropyl-6'-methyl (8, 8') groups or Dmt (5, 5'), had the following characteristics: (i) [Xaa3]EM-2 analogues exhibited improved mu- and delta-opioid receptor affinities. The latter, however, were inconsequential (Kidelta = 491-3451 nM). (ii) [Dmt1,Xaa3]EM-2 analogues enhanced mu- and delta-opioid receptor affinities (Kimu = 0.069-0.32 nM; Kidelta = 1.83-99.8 nM) without kappa-opioid receptor interaction. (iii) There were elevated mu-bioactivity (IC50 = 0.12-14.4 nM) and abolished delta-agonism (IC50 > 10 muM in 2', 3', 4', 5', 6'), although 4' and 6' demonstrated a potent mixed mu-agonism/delta-antagonism (for 4', IC50mu = 0.12 and pA2 = 8.15; for 6', IC50mu = 0.21 nM and pA2 = 9.05) and 7' was a dual mu-agonist/delta-agonist (IC50mu = 0.17 nM; IC50delta = 0.51 nM).

Synthesis of 3,6-bis[H-Tyr/H-Dmt-NH(CH2)m,n]-2(1H)pyrazinone derivatives: function of alkyl chain length on opioid activity

Dimeric opioid analogues linked to a pyrazinone platform, 3-[Tyr/Dmt-NH(CH2)m]-6-[Tyr/Dmt-NH(CH2)n]-2(1H)-pyrazinone (m, n=3 or 4), were synthesized. The Tyr-containing compound (m=4, n=3) exhibited mu-receptor affinity (K(i)mu; 7.58 nM) comparable to that of morphine, while the Dmt derivatives exhibited considerably higher affinity (K(i)mu; 0.021-0.051 nM) with corresponding agonism (IC50=1.79-4.93 nM). Interestingly one compound (m=4, n=3) revealed modest delta-opioid agonism; the converse analogue (m=3, n=4), however, was inactive in MVD assay.

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.

Opioid receptor binding and antinociceptive activity of the analogues of endomorphin-2 and morphiceptin with phenylalanine mimics in the position 3 or 4

Endomorphin-2 (EM-2) and morphiceptin are the same class of putative mu-opioid receptor ligands. To investigate the effectiveness of phenylglycine (Phg, L or D) and homophenylalanine (Hfe) as the surrogates of phenylalanine in the position 3 and/or 4 of them, a series of their analogues were synthesized. Opioid receptor binding affinities were determined. Two analogues, [Hfe3]EM-2 and [Phg4] (EM-2/morphiceptin), showed different but potent antinociceptive activity in mouse hot-plate test, the results combined with their half-lives of degradation by mouse brain homogenate could also present some evidence to the in vivo degradative mechanism of EM-2.

Potent in vivo antinociception and opioid receptor preference of the novel analogue [Dmt1]endomorphin-1

[Dmt1]Endomorphin-1 is a novel analogue of the potent mu-opioid agonist endomorphin-1. Given the physiological role of endomorphin-1 in vivo, this compound was investigated to determine if the antinociception occurred through systemic, supraspinal or in a combination of both neuronal pathways. This compound exhibited a potent dose-dependent effect intracerebroventricularly in both spinal and supraspinal regions, and was blocked by opioid antagonist naloxone, which verified the involvement of opioid receptors. Specific opioid antagonists characterized the apparent receptor type: beta-funaltrexamine (mu1/mu2-irreversible antagonist) equally inhibited spinal- and central-mediated antinociception; on the other hand, naloxonazine (mu1-subtype) was ineffective in both neural pathways and naltrindole (delta-selective antagonist) partially (26%), though not significantly, blocked only the spinal-mediated antinociception. Therefore, spinal antinociception was primarily triggered by mu2-subtypes without involvement of mu1-opioid receptors; however, although a slight enhancement of antinociception by delta-receptors cannot be completely ruled out since functional bioactivity indicated mixed mu-agonism/delta-antagonism. In terms of the CNS action, [Dmt1]endomorphin-1 appears to act through mu2-opioid receptor subtypes.

Opioid ligands with mixed mu/delta opioid receptor interactions: an emerging approach to novel analgesics

Opioids are widely used in the treatment of severe pain. The clinical use of the opioids is limited by serious side effects such as respiratory depression, constipation, development of tolerance, and physical dependence and addiction liabilities. Most of the currently available opioid analgesics exert their analgesic and adverse effects primarily through the opioid mu receptors. A large number of biochemical and pharmacological studies and studies using genetically modified animals have provided convincing evidence regarding the existence of modulatory interactions between opioid mu and delta receptors. Several studies indicate that delta receptor agonists as well as delta receptor antagonists can provide beneficial modulation to the pharmacological effects of mu agonists. For example, delta agonists can enhance the analgesic potency and efficacy of mu agonists, and delta antagonists can prevent or diminish the development of tolerance and physical dependence by mu agonists. On the basis of these observations, the development of new opioid ligands possessing mixed mu agonist/delta agonist profile and mixed mu agonist/delta antagonist profile has emerged as a promising new approach to analgesic drug development. A brief overview of mu-delta interactions and recent developments in identification of ligands possessing mixed mu agonist/delta agonist and mu agonist/delta antagonist activities is provided in this report.

Enantioselective Synthesis of a Phenylalanine Library Containing Alkyl Groups on the Aromatic Moiety: Confirmation of Stereostructure by X-Ray Analysis

Six phenylalanine analogues containing 2'-methyl-, 2',6'-dimethyl-, 2'-ethyl-6'-methyl-, 2'-isopropyl-6'-methyl-, 2',4',6'-trimethyl-, and 3',5'-dimethyl-L-phenylalanine were synthesized enantioselectively through asymmetric hydrogenation of acetamidoacrylate derivatives. Enzymatic digestion and X-ray analysis supported the L-configuration of the phenylalanine derivatives obtained.

New series of potent delta-opioid antagonists containing the H-Dmt-Tic-NH-hexyl-NH-R motif

Heterodimeric compounds H-Dmt-Tic-NH-hexyl-NH-R (R=Dmt, Tic, and Phe) exhibited high affinity to delta- (K(i)delta=0.13-0.89nM) and mu-opioid receptors (K(i)mu=0.38-2.81nM) with extraordinary potent delta antagonism (pA(2)=10.2-10.4). These compounds represent the prototype for a new class of structural homologues lacking mu-opioid receptor-associated agonism (IC(50)=1.6-5.8muM) based on the framework of bis-[H-Dmt-NH]-alkyl (Okada, Y.; Tsuda, Y.; Fujita, Y.; Yokoi, T.; Sasaki, Y.; Ambo, A.; Konishi, R.; Nagata, M.; Salvadori, S.; Jinsmaa, Y.; Bryant, S. D.; Lazarus, L. H. J. Med. Chem.2003, 46, 3201), which exhibited both high mu affinity and bioactivity.

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.

Structure–activity relationships of novel endomorphin-2 analogues with N–O turns induced by α-aminoxy acids

Endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH2, EM-2) is a putative endogenous mu-opioid receptor ligand. To study the structure-activity relationship against its receptor, we introduced N-O turns into EM-2 and got the analogues with potent affinities for mu-opioid receptor. Our results indicated that N-O turn structures at the Pro2-aminoxy-Phe3 position of EM-2 analogues played important roles for their affinities. These novel analogues with N-O turns provided a new approach to develop potent analgesics related to EM-2.

Morphiceptin Analogues Containing a Dipeptide Mimetic Structure: An Investigation on the Bioactive Topology at the μ-Receptor

We describe the design, the conformational behavior, and the biological activity at the mu-opioid receptor of new morphiceptin analogues. In these analogues a recently described dipeptide mimetic structure replaces both the N- and the C-terminal Xaa-Pro dipeptide of morphiceptin. Conformational investigation on the most active analogue, compared to the parent peptide, indicates a high degree of structural tolerance within the mu-opioid receptor binding site. In fact, our results indicate that only the location and the relative orientation of the side chains of the aromatic pharmacophoric residues represent the indispensable structural features for mu-receptor binding. To reach such topological arrangement, opioid peptides can adopt different conformations and configurations. In particular, opioid peptides bearing a proline residue as spacer between the two aromatic residues can adopt, in the active state, both cis and trans configurations at the Tyr(1)-Pro(2) amide bond, each of them with the appropriate backbone and side chains orientations.

Structure–activity relationship of the novel bivalent and C-terminal modified analogues of endomorphin-2

Endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2)) is a putative endogenous mu-opioid receptor ligand. To develop potent analgesics with less side effects related to it, we used the methods of dimerization and C-terminal modification. Through dimerization we got the 'balanced agonists' with potent analgesic activity and we have developed the structure-activity relationship between the selectivity and the distance of the two tyrosine pharmacophores. Modification at the C-terminal increased the selectivity of endomorphin-2 to mu-opioid receptor with binding affinity conserved.

Studies on the structure–activity relationship of 2′,6′-dimethyl-l-tyrosine (Dmt) derivatives: bioactivity profile of H–Dmt–NH–CH3

The 2',6'-dimethyl-l-tyrosine (Dmt) enhances receptor affinity, functional bioactivity and in vivo analgesia of opioid peptides. To further investigate its direct influence on these opioid parameters, we developed a series of compounds (H-Dmt-NH-X). Among them, H-Dmt-NH-CH(3) showed the highest affinity (K(i)mu=7.45 nM) equal to that of morphine, partial mu-opioid agonism (E(max)=66.6%) in vitro and a moderate antinociception in mice.

Differentiation of opioid receptor preference by [Dmt1]endomorphin-2-mediated antinociception in the mouse

The potent opioid [Dmt1]endomorphin-2 (Dmt-Pro-Phe-Phe-NH2) differentiated between the opioid receptor subtypes responsible for the antinociception elicited by endomorphin-2 in mice. Antinociception, induced by the intracerebroventricular administration of [Dmt1]endomorphin-2 and inhibited by various opioid receptor antagonists [naloxone, naltrindole, beta-funaltrexamine, naloxonazine], was determined by the tail-flick (spinal effect) and hot-plate (supraspinal effect) tests. The opioid receptor subtypes involved in [Dmt1]endomorphin-2-induced antinociception differed between these in vivo model paradigms: naloxone (non-specific opioid receptor antagonist) and beta-funaltrexamine (irreversible mu1/mu2-opioid receptor antagonist) blocked antinociception in both tests, although stronger inhibition occurred in the hot-plate than the tail-flick test suggesting involvement of other opioid receptors. Consequently, we applied naloxonazine (mu1-opioid receptor antagonist) that significantly blocked the effect in the hot-plate test and naltrindole (delta-opioid receptor antagonist), which was only effective in the tail-flick test. The data indicated that [Dmt1]endomorphin-2-induced spinal antinociception was primarily mediated by both mu2- and delta-opioid receptors, while a supraspinal mechanism involved only mu1/mu2-subtypes.

Development of Potent μ-Opioid Receptor Ligands Using Unique Tyrosine Analogues of Endomorphin-2

Six analogues of tyrosine, which contained alkyl groups at positions 2', 3', and 6', either singly or in combination on the tyramine ring, were investigated for their effect on the opioid activity of [Xaa(1)]endomorphin-2 (EM-2). The opioid analogues displayed the following characteristics: (i) high mu-opioid receptor affinity [K(i)(mu) = 0.063-2.29 nM] with selectivity [K(i)(delta)/K(i)(mu)] ranging from 46 to 5347; (ii) potent functional mu-opioid agonism [GPI assay (IC(50) = 0.623-0.924 nM)] and with a correlation between delta-opioid receptor affinities and functional bioactivity using MVD; (iii) intracerebroventricular administration of [Dmt(1)]- (14) and [Det(1)]EM-2 (10) produced a dose-response antinociception in mice, with the former analogue more active than the latter; and (iv) a marked shift occurred from the trans-orientation at the Tyr(1)-Pro(2) bond to a cis-conformer compared to that observed previously with [Dmt(1)]EM-2 (14) (Okada et al. Bioorg. Med. Chem. 2003, 11, 1983-1984) except [Mmt(1)]EM-2 (7). The active profile of the [Xaa(1)]EM-2 analogues indicated that significant modifications on the tyramine ring are possible while high biological activity is maintained.

Development of potent bifunctional endomorphin-2 analogues with mixed mu-/delta-opioid agonist and delta-opioid antagonist properties

The C terminus of endomorphin-2 (EM-2) analogues (Tyr-Pro-Phe-NH-X) was modified with aromatic, heteroaromatic, or aliphatic groups (X = phenethyl,benzyl, phenyl, naphthyl, pyridyl, quinolyl, isoquinolyl, tert-butyl, cyclohexyl, or adamantyl; 3-18) to study their effect on opioid activity. Only 9 (1-naphthyl), 11 (5-quinolyl), 16 (cyclohexyl), and 18 (2-adamantyl) exhibited mu-opioid receptor affinity in the nanomolar range (K(i) = 2.41-6.59 nM), which, however, was 3- to 10-fold less than the parent peptide. Replacement of Tyr(1) by Dmt (2',6'-dimethyl-l-tyrosine) (19-32) exerted profound effects: (i) acquisition of high mu-opioid receptor affinity (K(i) = 0.11-0.52 nM) except 23 (Ph); (ii) presence of potent functional mu-opioid receptor agonism (IC(50) < 1 nM) for 19 ([Dmt(1)]EM-2), 27 (1-naphthyl), 29 (5-quinolyl), and 32 (5-isolquinolyl); (iii) association of weak delta-opioid antagonist activity (pA(2) = 5.41-7.18) except 19 ([Dmt(1)]EM-2), 20 (H), 27 (1-naphthyl), and in particular 29 (5-quinolyl) with its potent delta-agonism (IC(50) = 0.62 nM, pA(2) = 5.88); (iv) production of antinociception after ic administration of 32 (5-isoquinolyl) in mice, a bioactivity absent in the corresponding Tyr(1) analogue (14); and (v) preferential cis orientation (cis/trans = 3:2 to 7:3) at the Dmt-Pro amide bond, in contrast to the Tyr-Pro amide trans orientation (cis/trans = 1:2 to 1:3). Thus, [Dmt(1)]EM-2 analogues with hydrophobic C-terminal extensions provide model compounds with potent mu-opioid receptor bioactivity and dual functional agonism.

Conformational analysis and μ-opioid receptor affinity of short peptides, endomorphin models in a low polarity solvent

Peptide carbamates containing the sequence H-Pro-Trp-PheNH2 showed in CDCl3 restricted conformations stabilized by the presence of a gamma-turn. To test the reliability of the peptides as endomorphin conformational models, we measured the affinities for mu-receptors labelled with [3H]-DAMGO. In particular, Cbz-Pro-Trp-PheNH2 displayed a nanomolar affinity.