Synthesis and biological evaluation of cyclic endomorphin-2 analogs

Peptide-derived ligands for the discovery of safer opioid analgesics

Drugs targeting the μ-opioid receptor (MOR) remain the most efficacious analgesics for the treatment of pain, but activation of MOR with current opioid analgesics also produces harmful side effects, notably physical dependence, addiction, and respiratory depression. Opioid peptides have been accepted as promising candidates for the development of safer and more efficacious analgesics. To develop peptide-based opioid analgesics, strategies such as modification of endogenous opioid peptides, development of multifunctional opioid peptides, G protein-biased opioid peptides, and peripherally restricted opioid peptides have been reported. This review seeks to provide an overview of the opioid peptides that produce potent antinociception with much reduced side effects in animal models and highlight the potential advantages of peptides as safer opioid analgesics.

Analgesic Peptides: From Natural Diversity to Rational Design

Pain affects one-third of the global population and is a significant public health issue. The use of opioid drugs, which are the strongest painkillers, is associated with several side effects, such as tolerance, addiction, overdose, and even death. An increasing demand for novel, safer analgesic agents is a driving force for exploring natural sources of bioactive peptides with antinociceptive activity. Since the G protein-coupled receptors (GPCRs) play a crucial role in pain modulation, the discovery of new peptide ligands for GPCRs is a significant challenge for novel drug development. The aim of this review is to present peptides of human and animal origin with antinociceptive potential and to show the possibilities of their modification, as well as the design of novel structures. The study presents the current knowledge on structure-activity relationship in the design of peptide-based biomimetic compounds, the modification strategies directed at increasing the antinociceptive activity, and improvement of metabolic stability and pharmacodynamic profile. The procedures employed in prolonged drug delivery of emerging compounds are also discussed. The work summarizes the conditions leading to the development of potential morphine replacements.

Synthesis, Biological Activity and Molecular Docking of Chimeric Peptides Targeting Opioid and NOP Receptors

Recently, mixed opioid/NOP agonists came to the spotlight for their favorable functional profiles and promising outcomes in clinical trials as novel analgesics. This study reports on two novel chimeric peptides incorporating the fragment Tyr-c[D-Lys-Phe-Phe]Asp-NH₂ (RP-170), a cyclic peptide with high affinity for µ and κ opioid receptors (or MOP and KOP, respectively), conjugated with the peptide Ac-RYYRIK-NH₂, a known ligand of the nociceptin/orphanin FQ receptor (NOP), yielding RP-170-RYYRIK-NH₂ (KW-495) and RP-170-Gly₃-RYYRIK-NH₂ (KW-496). In vitro, the chimeric KW-496 gained affinity for KOP, hence becoming a dual KOP/MOP agonist, while KW-495 behaved as a mixed MOP/NOP agonist with low nM affinity. Hence, KW-495 was selected for further in vivo experiments. Intrathecal administration of this peptide in mice elicited antinociceptive effects in the hot-plate test; this action was sensitive to both the universal opioid receptor antagonist naloxone and the selective NOP antagonist SB-612111. The rotarod test revealed that KW-495 administration did not alter the mice motor coordination performance. Computational studies have been conducted on the two chimeras to investigate the structural determinants at the basis of the experimental activities, including any role of the Gly₃ spacer.

Synthesis, Pharmacological Evaluation, and Computational Studies of Cyclic Opioid Peptidomimetics Containing β3-Lysine

Our formerly described pentapeptide opioid analog Tyr-c[D-Lys-Phe-Phe-Asp]NH₂ (designated RP-170), showing high affinity for the mu (MOR) and kappa (KOR) opioid receptors, was much more stable than endomorphine-2 (EM-2) in the rat brain homogenate and displayed remarkable antinociceptive activity after central (intracerebroventricular) and peripheral (intravenous ) administration. In this report, we describe the further modification of this analog, which includes the incorporation of a β³-amino acid, (R)- and (S)-β³-Lys, instead of D-Lys in position 2. The influence of such replacement on the biological properties of the obtained analogs, Tyr-c[(R)-β³-Lys-Phe-Phe-Asp]NH₂ (RP-171) and Tyr-c[(S)-β³-Lys-Phe-Phe-Asp]NH₂, (RP-172), was investigated in vitro. Receptor radiolabeled displacement and functional calcium mobilization assays were performed to measure binding affinity and receptor activation of the new analogs. The obtained data revealed that only one of the diastereoisomeric peptides, RP-171, was able to selectively bind and activate MOR. Molecular modeling (docking and molecular dynamics (MD) simulations) suggests that both compounds should be accommodated in the MOR binding site. However, in the case of the inactive isomer RP-172, fewer hydrogen bonds, as well as instability of the canonical ionic interaction to Asp¹⁴⁷, could explain its very low MOR affinity.

Design, Synthesis and Functional Analysis of Cyclic Opioid Peptides with Dmt-Tic Pharmacophore

The opioid receptors are members of the G-protein-coupled receptor (GPCR) family and are known to modulate a variety of biological functions, including pain perception. Despite considerable advances, the mechanisms by which opioid agonists and antagonists interact with their receptors and exert their effect are still not completely understood. In this report, six new hybrids of the Dmt-Tic pharmacophore and cyclic peptides, which were shown before to have a high affinity for the µ-opioid receptor (MOR) were synthesized and characterized pharmacologically in calcium mobilization functional assays. All obtained ligands turned out to be selective antagonists of the δ-opioid receptor (DOR) and did not activate or block the MOR. The three-dimensional structural determinants responsible for the DOR antagonist properties of these analogs were further investigated by docking studies. The results indicate that these compounds attach to the DOR in a slightly different orientation with respect to the Dmt-Tic pharmacophore than Dmt-TicΨ[CH₂-NH]Phe-Phe-NH₂ (DIPP-NH₂[Ψ]), a prototypical DOR antagonist peptide. Key pharmacophoric contacts between the DOR and the ligands were maintained through an analogous spatial arrangement of pharmacophores, which could provide an explanation for the predicted high-affinity binding and the experimentally observed functional properties of the novel synthetic ligands.

Pharmacological Profile and Molecular Modeling of Cyclic Opioid Analogues Incorporating Various Phenylalanine Derivatives

Peptide-based agonists of the μ opioid receptor (μOR) are promising therapeutic candidates for pain relief with reduced side effects compared to morphine. A deep understanding of μOR-ligand interactions is necessary for future design of peptide-based opioid analgesics. To explore the requirements of the μOR binding pocket, eight new analogues of our cyclic peptide Tyr-c[d-Lys-Phe-Phe-Asp]NH₂ displaying high μOR affinity were synthesized, in which Phe in either the third or fourth position was replaced by various derivatives of this amino acid (β³ -Phe, homoPhe, β³ -homoPhe and PhGly). The aim of this research was to examine the structural effects of such modifications on the bioactivity, and both experimental and theoretical methods were used. The binding of the cyclic analogues to all three OR types (μ, δ, κ) was assessed by radioligand competitive binding assay, and their functional activity was determined in a calcium mobilization assay. In order to provide structural hypotheses explaining the obtained experimental affinities, the complexes of the cyclic peptides with μOR were subjected to molecular modeling.

Intracerebroventricular administration of CYX-6, a potent μ-opioid receptor agonist, a δ- and κ-opioid receptor antagonist and a biased ligand at μ, δ & κ-opioid receptors, evokes antinociception with minimal constipation and respiratory depression in rats in contrast to morphine

Mu opioid receptor (MOPr) agonists are thought to produce analgesia via modulation of G-protein-coupled intracellular signalling pathways whereas the β-arrestin2 pathway is proposed to mediate opioid-related adverse effects. Here, we report the antinociception, constipation and respiratory depressant profile of CYX-6, a potent MOPr agonist that is also a delta and a kappa opioid receptor (DOPr/KOPr) antagonist and that lacks β-arrestin2 recruitment at each of the MOPr, DOPr and the KOPr. In anaesthetised male Sprague Dawley rats, an intracerebroventricular (i.c.v.) guide cannula was stereotaxically implanted. After 5-7 days post-surgical recovery, rats received a single i.c.v. bolus dose of CYX-6 (3-30 nmol), morphine (100 nmol) or vehicle. Antinociception was assessed using the warm water tail flick test (52.5 ± 0.5 °C). Constipation was assessed using the charcoal meal gut motility test and the castor oil-induced diarrhoea test. Respiratory depression was measured by whole-body plethysmography in awake, freely moving animals, upon exposure to a hypercapnic gas mixture (8% CO2, 21% O2 and 71% N2). The intrinsic pharmacology of CYX-6 given by the i.c.v. route in rats showed that it produced dose-dependent antinociception. It also produced respiratory stimulation rather than depression and it had a minimal effect on intestinal motility in contrast to the positive control, morphine. CYX-6 is an endomorphin-2 analogue that dissociates antinociception from constipation and respiratory depression in rats. Our findings provide useful insight to inform the discovery and development of novel opioid analgesics with a superior tolerability profile compared with morphine.

Synthesis and Pharmacological Evaluation of Hybrids Targeting Opioid and Neurokinin Receptors

Morphine, which acts through opioid receptors, is one of the most efficient analgesics for the alleviation of severe pain. However, its usefulness is limited by serious side effects, including analgesic tolerance, constipation, and dependence liability. The growing awareness that multifunctional ligands which simultaneously activate two or more targets may produce a more desirable drug profile than selectively targeted compounds has created an opportunity for a new approach to developing more effective medications. Here, in order to better understand the role of the neurokinin system in opioid-induced antinociception, we report the synthesis, structure–activity relationship, and pharmacological characterization of a series of hybrids combining opioid pharmacophores with either substance P (SP) fragments or neurokinin receptor (NK1) antagonist fragments. On the bases of the in vitro biological activities of the hybrids, two analogs, opioid agonist/NK1 antagonist Tyr-[d-Lys-Phe-Phe-Asp]-Asn-d-Trp-Phe-d-Trp-Leu-Nle-NH₂ (2) and opioid agonist/NK1 agonist Tyr-[d-Lys-Phe-Phe-Asp]-Gln-Phe-Phe-Gly-Leu-Met-NH₂ (4), were selected for in vivo tests. In the writhing test, both hybrids showed significant an antinociceptive effect in mice, while neither of them triggered the development of tolerance, nor did they produce constipation. No statistically significant differences in in vivo activity profiles were observed between opioid/NK1 agonist and opioid/NK1 antagonist hybrids.

Antinociceptive potency of a fluorinated cyclopeptide Dmt-c[D-Lys-Phe-p-CF3-Phe-Asp]NH2

Opioid peptides and opiate drugs such as morphine, mediate their analgesic effects, but also undesired side effects, mostly through activation of the mu opioid receptor. However, delta- and kappa-opioid receptors can also contribute to the analgesic effects of opioids. Recent findings showed that simultaneous activation of multiple opioid receptors may result in additional analgesia with fewer side effects. Here, we evaluated the pharmacological profile of our formerly developed mixed mu/kappa-opioid receptor ligands, Dmt-c[D-Lys-Phe-Phe-Asp]NH₂ (C-36) and Dmt-c[D-Lys-Phe-p-CF₃-Phe-Asp]NH₂ (F-81). The ability of these peptides to cross the blood–brain barrier was tested in the parallel artificial membrane permeability (PAMPA) assay. On the basis of the hot-plate test in mice after central and peripheral administration, analog F-81 was selected for the anti-nociceptive and anti-inflammatory activity assessment after peripheral administration.

Tryptophan-Containing Non-Cationizable Opioid Peptides - a new chemotype with unusual structure and in vivo activity

Recently, a new family of opioid peptides containing tryptophan came to the spotlight for the absence of the fundamental protonable tyramine 'message' pharmacophore. Structure-activity relationship investigations led to diverse compounds, characterized by different selectivity profiles and agonist or antagonist effects. Substitution at the indole of Trp clearly impacted peripheral/central antinociceptivity. These peculiarities prompted to gather all the compounds in a new class, and to coin the definition 'Tryptophan-Containing Non-Cationizable Opioid Peptides', in short 'TryCoNCOPs'. Molecular docking analysis suggested that the TryCoNCOPs can still interact with the receptors in an agonist-like fashion. However, most TryCoNCOPs showed significant differences between the in vitro and in vivo activities, suggesting that opioid activity may be elicited also via alternative mechanisms.

Ring size in cyclic endomorphin-2 analogs modulates receptor binding affinity and selectivity

The study reports the solid-phase synthesis and biological evaluation of a series of new side chain-to-side chain cyclized opioid peptide analogs of the general structure Tyr-[D-Xaa-Phe-Phe-Asp]NH2, where Xaa = Lys (1), Orn (2), Dab (3), or Dap (4) (Dab = 2,4-diaminobutyric acid, Dap = 2,3-diaminopropionic acid), containing 17- to 14-membered rings. The influence of the ring size on binding to the MOP, DOP and KOP opioid receptors was studied. In general, the reduction of the size of the macrocyclic ring increased the selectivity for the MOP receptor. The cyclopeptide incorporating Xaa = Lys displayed subnanomolar MOP affinity but modest selectivity over the KOP receptor, while the analog with the Orn residue showed increased affinity and selectivity for MOP. The analog with Dab was a weak MOP agonist and did not bind to the other two opioid receptors. Finally, the peptide with Xaa = Dap was completely MOP receptor-selective with subnanomolar affinity. Interestingly, the deletion of one Phe residue from 1 led to the 14-membered Tyr-c[D-Lys-Phe-Asp]NH2 (5), a potent and selective MOP receptor ligand. The in vitro potencies of the new analogs were determined in a calcium mobilization assay performed in Chinese Hamster Ovary (CHO) cells expressing human recombinant opioid receptors and chimeric G proteins. A good correlation between binding and the functional test results was observed. The influence of the ring size, solid support and the N-terminal protecting group on the formation of cyclodimers was studied.

Cyclic endomorphin analogs in targeting opioid receptors to achieve pain relief

Endomorphins, the endogenous ligands of the µ-opioid receptor, are attractive candidates for opioid-based pain-relieving agents. These tetrapeptides, with their remarkable affinity for the µ-opioid receptor, display favorable antinociceptive activity when injected directly into the brain of experimental animals. However, the application of endomorphins as clinical analgesics has been impeded by their instability in body fluids and inability to reach the brain after systemic administration. Among numerous modifications of the endomorphin structure aimed at improving their pharmacological properties, cyclization can be viewed as an interesting option. Here, we have summarized recent advances in obtaining endomorphin-based cyclic peptide analogs.

Synthesis of mixed opioid affinity cyclic endomorphin-2 analogues with fluorinated phenylalanines

As part of our continuing studies on the structure-activity relationships of cyclic pentapeptides based on the structure of endomorphin-2 (EM-2), we report here the synthesis and biological activities of a new series of analogues of a general sequence Tyr/Dmt-c[d-Lys-Phe-Phe-Asp]NH2 (where Dmt = 2',6'-dimethyltyrosine), incorporating fluorinated amino acids: 4-fluorophenylalanine (4-F-Phe), 2,4-difluorophenylalanine (2,4-F-Phe), or 4-trifluoromethylphenylalanine (4-CF3-Phe) instead of the Phe residue in position 3 or 4. Depending on the fluorinated amino acid residue and its position in the sequence, analogues were mixed, high affinity MOP/KOP receptor agonists, MOP/DOP/KOP agonists, or selective KOP agonists. The in vitro potencies and efficacies of all novel analogues were assessed in calcium mobilization assay. The most potent analogues, Dmt-c[d-Lys-Phe-4-F-Phe-Asp]NH2 and Dmt-c[d-Lys-Phe-2,4-F-Phe-Asp]NH2, were tested in vivo in the mouse hot-plate test. They produced strong antinociceptive effect not only after intracerebroventricular but also after intraperitoneal injection, indicating that they were able to cross the blood-brain barrier.

Pharmacological characterization of endomorphin-2-based cyclic pentapeptides with methylated phenylalanine residues

As part of our continuing studies on the structure-activity relationships of cyclic pentapeptides based on the structure of endomorphin-2, we report here the synthesis and biological activities of a new series of analogs incorporating 2', 3' or 4'-methylphenylalanine (MePhe) residues into positions 3 or 4 of the parent cyclopeptide, Dmt-c[d-Lys-Phe-Phe-Asp]NH2 (Dmt=2',6'-dimethyltyrosine). Analogs with MePhe in position 4 showed a row of magnitude increased μ-opioid receptor (MOP receptor) affinity as compared with a parent compound. The in vitro potencies of the new analogs were determined in calcium mobilization assay performed in Chinese Hamster Ovary (CHO) cells expressing human recombinant opioid receptors and chimeric G proteins. All analogs were strong μ/κ (MOP/KOP) receptor agonists and weak δ (DOP) receptor agonists. In the in vivo hot-plate test in mice, the MePhe(4)-modified peptides showed remarkable antinociceptive activity after intracerebroventricular (i.c.v.) administration which was most likely due to the concomitant activation of more than one opioid receptor type.

Antinociceptive effect of D-Lys(2), Dab(4)N-(ureidoethyl)amide, a new cyclic 1-4 dermorphin/deltorphin analog

BACKGROUND

A preliminary evaluation of antinociceptive activity of a new cyclic dermorphin/deltorphin tetrapeptide analog restricted via a urea bridge and containing C-terminal ureidoethylamid {[H-Tyr-d-Lys(&(1))-Phe-Dab(&(2))-CH2CH2NHCONH2][&(1)CO&(2)]} (cUP-1) revealed a significant and long-lasting increase of pain threshold to thermal stimulation after systemic application. The current studies were aimed at further evaluation of cUP-1 activity in animal models of somatic and visceral pain. The influence of cUP-1 on motor functions was also investigated.

METHODS

The influence of cUP-1 (0.5-2mgkg(-1), iv) on nociceptive threshold to mechanical pressure and analgesic efficacy in formalin and acetic acid-induced writhing tests were estimated. The antinociceptive effect of cUP-1 was compared to that of morphine (MF). The influence of cUP-1 (1, 4 and 8mgkg(-1), iv) on locomotor activity, motor coordination and muscle strength was estimated using open field and rota-rod tests and a grip strength measurement.

RESULTS

Administration of cUP-1 in doses of 1 and 2mgkg(-1) elicited a significant increase of nociceptive threshold to mechanical pressure. MF applied in the same doses induced an antinociceptive effect only at the higher dose (2mgkg(-1)). There were no marked differences between the effect of cUP-1 and MF at each dose, at relative time points. In the writhing test and both phases of the formalin test, cUP-1 showed a significant, dose-dependent antinociceptive effect which did not markedly differ from that of MF. cUP-1 did not significantly affect motor functions of mice.

CONCLUSIONS

Systemic application of cUP-1 elicited a dose-dependent antinociceptive effect. The analgesic efficacy of cUP-1 on mechanical nociception, visceral and formalin-induced pain was comparable to that of MF. cUP-1 did not impair motor functions of mice.

Orexigenic effects of endomorphin-2 (EM-2) related to decreased CRH gene expression and increased dopamine and norepinephrine activity in the hypothalamus

Endomorphin-1 (EM-1) and endomorphin-2 (EM-2) are opioid peptides which are selective partial agonists of μ-opioid receptor. We studied the effects of EM-2 injected into the arcuate nucleus (ARC) of the hypothalamus on feeding behavior and gene expression of orexigenic [agouti-related peptide (AgRP), neuropeptide Y (NPY) and orexin-A] and anorexigenic [cocaine and amphetamine-regulated transcript (CART), corticotrophin releasing hormone (CRH) and proopiomelanocortin (POMC)] peptides in male Wistar rats fed a standard laboratory diet. Furthermore, we evaluated the effects of EM-2 on dopamine (DA), norepinephrine (NE) and serotonin (5-hydroxytryptamine, 5-HT) steady state concentrations, in the hypothalamus. 64 rats (16 for each group of treatment) were injected into the ARC, at 9.00 am, with either vehicle or EM-2 (0.50-0.75 μmol/kg) or EM-2 (0.50 μmol/kg) plus β-funaltrexamine (0.20 μmol/kg). Food intake was recorded through 24h following injection, and hypothalamic DA, NE, 5-HT levels and neuropeptide gene expression were evaluated 24h after EM-2 administration. Compared to vehicle, EM-2 significantly increased food intake, throughout 24h post-injection. Furthermore, EM-2 treatment led to a significant increase of DA and NE concentrations and a decrease of CRH mRNA levels. On the other hand, β-funaltrexamine administration reverted both feeding stimulatory and neuromodulatory effects induced by EM-2. We can conclude that the orexigenic effect of μ-opioid receptor activation by EM-2 could be related to both inhibition of CRH and stimulation of dopamine and norepinephrine levels, in the hypothalamus.

Lipo-endomorphin-1 derivatives with systemic activity against neuropathic pain without producing constipation

To enhance the drug-like properties of the endogenous opioid peptide endomorphin-1 (1 = Tyr-Pro-Trp-Phe-NH(2)), the N-terminus of the peptide was modified with 2-aminodecanoic acid, resulting in compound 3. Tyr in compound 1 was replaced with 2,6-dimethyltyrosine yielding compound 2. Derivative 2 was also substituted with 2-aminodecanoic acid producing compound, 4. Lipoamino acid-modified derivatives showed improved metabolic stability and membrane permeability while maintaining high μ-opioid (MOP) receptor binding affinity and acting as a potent agonist. In vivo studies showed dose-dependent antinociceptive activity following intravenous (i.v.) administration of compounds 3 and 4 in a chronic constriction injury (CCI)-rat model of neuropathic pain with ED(50) values of 1.22 (± 0.93) and 0.99 (± 0.89) µmol/kg, respectively. Pre-treatment of animals with naloxone hydrochloride significantly attenuated the anti-neuropathic effects of compound 3, confirming the key role of opioid receptors in mediating antinociception. In contrast to morphine, no significant constipation was produced following i.v. administration of compound 3 at 16 µmol/kg. Furthermore, following chronic administration of equi-potent doses of compound 3 and morphine to rats, there was less antinociceptive tolerance for compound 3 compared with morphine.

The cis-4-amino-L-proline residue as a scaffold for the synthesis of cyclic and linear endomorphin-2 analogues

Endomorphin-2 (EM-2: Tyr-Pro-Phe-Phe-NH(2)) is an endogenous tetrapeptide that combines potency and efficacy with high affinity and selectivity toward the μ opioid receptor, the most responsible for analgesic effects in the central nervous system. The presence of the Pro(2) represents a crucial factor for the ligand structural and conformational properties. Proline is in fact an efficient stereochemical spacer, capable of inducing favorable spatial orientation of aromatic rings, a key factor for ligand recognition and interaction with receptors. Here the Pro(2) has been replaced by 4(S)-NH(2)-2(S)-proline (cAmp), a proline/GABA cis-chimera residue. This bivalent amino acid maintains the capacity to influenc the tetrapeptide conformation and offers the opportunity to generate new linear models and unusually constrained cyclic analogues characterized by an N-terminal Tyr bearing a free α-amino group. The results indicate that the new analogues do not show affinity for both δ and κ opioid receptors and bind only poorly to the μ receptors (for cyclopeptide 9: K(i)(μ) = 660 nM; GPI (IC(50)) = 1.4% at 1 μM; for linear tetrapeptide acid 13: K(i)(μ) = 2000 nM; GPI (IC(50)) = 0% at 1 μM; for linear tetrapeptide amide 15: K(i)(μ) = 310 nM; GPI (IC(50)) = 894 nM).

Short cyclic peptides derived from the C-terminal sequence of α1-antitrypsin exhibit significant anti-HIV-1 activity

Serpin A1 (α1-AT), the largest subgroup of serpins, presents in human plasma at high concentration and plays important regulatory roles in physiological and pathological processes. Accumulated evidence suggests that α1-AT may play a role in controlling HIV-1 infection. In this study, we designed and synthesized a set of short linear peptides derived from the C-terminal sequence of α1-AT. Since none of them showed significant anti-HIV-1 activity, we proceeded to synthesize four short cyclic peptides having 7 amino acids, and we found that three of them exhibited significant anti-HIV-1 activity. One of these cyclic peptides, designated CPM, inhibited HIV-1 entry and infection at low μM level, indicating that these short cyclic peptides could serve as leads for the development of novel anti-HIV-1 therapeutics.

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.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Spinal or systemic TY005, a peptidic opioid agonist/neurokinin 1 antagonist, attenuates pain with reduced tolerance

BACKGROUND AND PURPOSE

The use of opioids in treating pain is limited due to significant side effects including somnolence, constipation, analgesic tolerance, addiction and respiratory depression. Pre-clinical studies have shown that neurokinin 1 (NK(1) ) receptor antagonists block opioid-induced antinociceptive tolerance and may inhibit opioid-induced rewarding behaviours. Here, we have characterized a bifunctional peptide with both opioid agonist and NK(1) antagonist pharmacophores in a rodent model of neuropathic pain.

EXPERIMENTAL APPROACH

Rats were evaluated for behavioural responses to both tactile and thermal stimuli in either an uninjured, sham- or nerve-injured state. TY005 (Tyr-DAla-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bn(CF(3) )(2) ) was delivered spinally or systemically to assess the antinociceptive effects after acute exposure. Motor skills were evaluated using the rotarod test to determine potential sedative effects. Spinal TY005 was given chronically to sham- or nerve-injured animals to determine the development of tolerance.

KEY RESULTS

Bolus injections of TY005 produced dose-dependent antinociception in non-injured animals and alleviated nerve injury-induced thermal and tactile hypersensitivities (i.e. antihyperalgesia) more effectively than morphine. Sedative effects were not evident from the rotarod test at doses that were antihyperalgesic, nor at doses threefold higher. Repeated administration of TY005 did not lead to the development of antihyperalgesic tolerance or alter sensory thresholds.

CONCLUSIONS AND IMPLICATIONS

Collectively, the data suggest that opioid agonist/NK(1) antagonist bifunctional peptides represent a promising novel approach to the management of chronic pain without the development of tolerance, reducing the need for escalation of doses and unwanted side effects associated with opiates alone.

Synthesis and antinociceptive effects of endomorphin-1 analogs with C-terminal linked by oligoarginine

Endomorphins (EMs) cannot be delivered into the central nervous system (CNS) in sufficient quantity to elicit antinociception when given systemically because they are severely restricted by the blood-brain barrier (BBB). In the present study, we investigated herein a series of EM-1 analogs with C-terminal linked by oligoarginine in order to improve the brain delivery and antinociception after systemic administration. Indeed, all these analogs decreased the opioid receptor affinity and in vitro pharmacological activity. Moreover, analogs 4, 7-9 produced a less potent antinociceptive activity after intracerebroventricular (i.c.v.) administration, with the ED(50) values about 11- to 13-fold lower potencies than that of EM-1. Nevertheless, our results revealed that EM-1 failed to induce any significant antinociception at a dose of 50μmol/kg after subcutaneous (s.c.) administration, whereas equimolar dose of these four analogs produced a little low but significant antinociceptive effects. Naloxone (10nmol/kg, i.c.v.) significantly blocked the antinociceptive effects, indicating an opioid and central mechanism. These results demonstrated that C-terminal of EM-1 linked to oligoarginine improved the brain delivery, eliciting potent antinociception following peripheral administration.