Endomorphin Analogs Research Articles

Cyclic Glycopeptide Analogs of Endomorphin-1 Provide Highly Effective Antinociception in Male and Female Mice.

Opioids acting at the mu opioid receptor (MOR) remain the most effective treatment for moderate to severe pain, but their use is limited by serious side effects. We have shown that a cyclized analog of endomorphin-1 provided pain relief comparable to that of morphine with reduction or absence of several side effects, including abuse liability. Glycosylation can promote penetration of cellular barriers. Here we developed cyclic glycopeptide endomorphin (glycoEM) analogs as drug candidates for potent and long-lasting analgesia. The analogs were assessed in receptor binding and functional assays and for blood-brain barrier penetration by microdialysis and MS. Two of the analogs showed MOR selectivity and more potent and longer lasting antinociception than morphine in male and female mice. Comparable antinociception occurred at A2 doses 5-fold lower (20-fold on a molar basis) than morphine doses. The results support further study of the glycoEMs for clinical application.

Comparison of Morphine and Endomorphin Analog ZH853 for Tolerance and Immunomodulation in a Rat Model of Neuropathic Pain

µ-Opioid receptor agonists, the gold standard for analgesia, come with significant side effects when used chronically. Tolerance, defined as the decrease in analgesic activity after repeated use, remains a vital therapeutic obstacle as it increases the likelihood of dose escalation and potentially lethal side effects like respiratory depression. Previous experiments have shown that the endomorphin-1 analog, ZH853, is a specific µ-opioid receptor agonist with reduced side effects like tolerance and glial activation following chronic central administration in pain-naive animals. Here, we investigated the effects of chronic, peripheral administration of µ-opioid receptor agonists following neuropathic injury. Though µ-opioids are effective at reducing neuropathic pain, they are not recommended for first-line treatment due to negative side effects. Compared with chronic morphine, chronic ZH853 treatment led to decreased tolerance and reduced glial activation. Following twice-daily intravenous injections, morphine was less potent and had a shorter duration of antinociception compared with ZH853. Chronic morphine, but not chronic ZH853, elevated markers of activation/inflammation of astrocytes (glial fibrillary acidic protein), microglia (ionized calcium-binding adapter molecule 1), the proinflammatory cytokine tumor necrosis factor-α, and phosphorylated mitogen-activated protein (MAP) kinase p38 (pp38). By contrast, chronic ZH853 reduced ionized calcium-binding adapter molecule 1 and tumor necrosis factor-α relative to both morphine and vehicle, suggesting anti-inflammatory properties with respect to these markers. Glial fibrillary acidic protein and pp38 were not significantly different from vehicle but were significantly lower than morphine. This study demonstrates the effectiveness of chronic ZH853 for providing analgesia in a neuropathic pain state with reduced tolerance compared with morphine, potentially due to reductions in spinal glial activation. PerspectiveNeuropathic pain is generally undertreated and resistant to medication, and side-effects limit opioid treatment. Here, we show that, compared with an equiantinociceptive dose of morphine, chronic intravenous administration of endomorphin analog ZH853 led to prolonged antiallodynia, reduced tolerance, and inhibition of spinal cord neuroinflammation in male spared nerve-injured rats.

Novel endomorphin analogues CEMR-1 and CEMR-2 produce potent and long-lasting antinociception with a favourable side effect profile at the spinal level.

Endomorphins have shown great promise as pharmaceutics for the treatment of pain. We have previously confirmed that novel endomorphin analogues CEMR-1 and CEMR-2 behaved as potent μ agonists and displayed potent antinociceptive activities at the supraspinal and peripheral levels. The present study was undertaken to evaluate the antinociceptive properties of CEMR-1 and CEMR-2 following intrathecal (i.t.) administration. Furthermore, their antinociceptive tolerance and opioid-like side effects were also determined. The spinal antinociceptive effects of CEMR-1 and CEMR-2 were determined in a series of pain models, including acute radiant heat paw withdrawal test, spared nerve injury-induced neuropathic pain, complete Freund's adjuvant-induced inflammatory pain, visceral pain and formalin pain. Antinociceptive tolerance was evaluated in radiant heat paw withdrawal test. Spinal administration of CEMR-1 and CEMR-2 produced potent and prolonged antinociceptive effects in acute pain. CEMR-1 and CEMR-2 may produce their antinociception through distinct μ receptor subtypes. These two analogues also exhibited significant analgesic activities in neuropathic, inflammatory, visceral and formalin pain at the spinal level. It is noteworthy that CEMR-1 showed non-tolerance-forming analgesic properties, while CEMR-2 exhibited substantially reduced antinociceptive tolerance. Furthermore, both analogues displayed no or reduced side effects on conditioned place preference response, physical dependence, locomotor activity and gastrointestinal transit. The present investigation demonstrated that CEMR-1 and CEMR-2 displayed potent and long-lasting antinociception with a favourable side effect profile at the spinal level. Therefore, CEMR-1 and CEMR-2 might serve as promising analgesic compounds with minimal opioid-like side effects.

Endomorphin-2 analogs with C-terminal esterification display potent antinociceptive effects in the formalin pain test in mice

Previously, we have investigated three C-terminal esterified endomorphin-2 (EM-2) analogs EM-2-Me, EM-2-Et and EM-2-Bu with methyl, ethyl and tert-butyl ester modifications, respectively. These analogs produced significant antinociception in acute pain at the spinal and supraspinal levels, with reduced tolerance and gastrointestinal side effects. The present study was undertaken to determine the analgesic effects and opioid mechanisms of these three analogs in the formalin pain test. Our results demonstrated that intracerebroventricular (i.c.v.) administration of 0.67–20 nmol EM-2 analogs EM-2-Me, EM-2-Et and EM-2-Bu produced dose-dependent antinociceptive effects in both phase Ⅰ and phase Ⅱ of formalin pain. EM-2-Me and EM-2-Bu displayed more potent antinociception than morphine. Especially, EM-2-Bu exhibited the highest antinociception in phase Ⅱ of formalin pain, with the ED50 value being 2.1 nmol. Naloxone (80 nmol, i.c.v.) completely antagonized the antinociceptive effects of EM-2-Me, EM-2-Et and EM-2-Bu (20 nmol, i.c.v.) in both phase I and phase Ⅱ of formalin pain, suggesting a central opioid mechanism. Nevertheless, the antinociception induced by EM-2-Me might be involved in the release of dynorphin A, which subsequently acted on κ- opioid receptor. EM-2-Bu produced the antinociception probably by the direct activation of both μ- and δ-opioid receptors. EM-2-Me, EM-2-Et and EM-2-Bu also produced significant analgesic effects after peripheral administration, and the central opioid receptors were involved. Furthermore, EM-2-Bu had no influence on the locomotor activity after i.c.v. injection. The present investigation demonstrated that C-terminal esterified modifications of EM-2 will be beneficial for developing novel therapeutics in formalin pain.

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.

Endomorphin analog ZH853 shows low reward, tolerance, and affective-motivational signs of withdrawal, while inhibiting opioid withdrawal and seeking

Currently available μ-opioid receptor agonist pharmacotherapies for opioid use disorder possess adverse effects limiting their use and, despite treatment, rates of relapse remain high. We previously showed that endomorphin analog ZH853 had no effect in rodent models that predict abuse liability in humans. Here we extended these findings by examining dependence liability and reinforcing properties in female rats and male rats with previous opioid exposure. The potential use of ZH853 in managing opioid use disorder was evaluated by examining its effect on opioid-seeking behavior and withdrawal. We found that ZH853 did not induce locomotor activation in male and female mice and was not self-administered by female rats. Relative to morphine, ZH853 led to similar somatic signs of withdrawal, but low affective-motivational signs of withdrawal, and absent changes in ventral tegmental area K(+)-Cl(−) co-transporter expression associated with reward dysregulation. The low abuse liability of ZH853 was further supported in oxycodone self-administering male rats, where ZH853 substitution extinguished opioid-seeking behavior. ZH853 priming also did not reinstate morphine conditioned place preference. Lastly, ZH853 inhibited oxycodone-seeking behavior during relapse after forced abstinence and decreased the expression of morphine withdrawal. These findings suggest the potential use of ZH853 as a safer opioid medication for long-term treatment of pain and opioid use disorder.

Novel Cyclic Endomorphin Analogues with Multiple Modifications and Oligoarginine Vector Exhibit Potent Antinociception with Reduced Opioid-like Side Effects.

Endomorphins (EMs) are potent pharmaceuticals for the treatment of pain. Herein, we investigated several novel EM analogues with multiple modifications and oligoarginine conjugation. Our results showed that analogues 1-6 behaved as potent μ-opioid agonists and enhanced stability and lipophilicity. Analogues 5 and 6 administered centrally and peripherally induced significant and prolonged antinociceptive effects in acute pain. Both analogues also produced long-acting antiallodynic effects against neuropathic and inflammatory pain. Furthermore, they showed a reduced acute antinociceptive tolerance. Analogue 6 decreased the extent of chronic antinociceptive tolerance, and analogue 5 exhibited no tolerance at the supraspinal level. Particularly, they displayed nontolerance-forming antinociception at the peripheral level. In addition, analogues 5 and 6 exhibited reduced or no opioid-like side effects on gastrointestinal transit, conditioned place preference (CPP), and motor impairment. The present investigation established that multiple modifications and oligoarginine-vector conjugation of EMs would be helpful in developing novel analgesics with fewer side effects.

Selective MOR activity of DAPEA and Endomorphin-2 analogues containing a (R)-γ-Freidinger lactam in position two

The use of α-amino-γ lactam of Freidinger (Agl) may serve as an impressive method to increase the biological stability of peptides and an appropriate tool to elucidate their structure-activity relationships. The endomorphin-2 (EM-2) and [D-Ala2, des-Leu5] enkephalin amide (DAPEA) are two linear opioid tetrapeptides agonists of MOR and MOR/DOR respectively. Herein, we investigated the influence of the incorporation of (R/S)-Agl in position 2 and 3 on the biological profile of the aforementioned products in vitro and in vivo. Receptor radiolabeled displacement and functional assays were used to measure in vitro the binding affinity and receptors activation of the novel analogues. The mouse tail flick and formalin tests allowed to observe their antinociceptive effect in vivo. Data revealed that peptide A2D was able to selectively bind and activate MOR with a potent antinociceptive effect after intracerebroventricular (i.c.v.) administration, performing better than the parent compounds EM-2 and DAPEA. Molecular docking calculations helped us to understand the key role exerted by the Freidinger Agl moiety in A2D for the interaction with the MOR binding pocket.

Contribution of the μ opioid receptor and enkephalin to the antinociceptive actions of endomorphin-1 analogs with unnatural amino acid modifications in the spinal cord

Endomorphin analogs containing unnatural amino acids have demonstrated potent analgesic effects in our previous studies. In the present study, the differences in antinociception and the mechanisms thereof for analogs 1-3 administered intracerebroventricularly and intrathecally were explored. All analogs at different routes of administration produced potent analgesia compared to the parent peptide endomorphin-1. Multiple antagonists and antibodies were used to explore the mechanisms of action of these analogs, and it was inferred that analogs 1-3 stimulated the μ opioid receptor to induce antinociception. Moreover, the antibody data suggested that analog 2 may induce the release of immunoreactive [Leu5]-enkephaline and [Met5]-enkephaline to produce a secondary component of antinociception at the spinal level and analog 3 may stimulate the the release of immunoreactive [Met5]-enkephaline at the spinal level. Finally, analogs 2 and 3 produced no acute tolerance in the spinal cord. We hypothesize that the unique characteristics of the endomorphin analogs result from their capacities to stimulate the release of endogenous antinociceptive substances.

The spinal anti-allodynic effects of endomorphin analogs with C-terminal hydrazide modification in neuropathic pain model

The present study was undertaken to further investigate the spinal anti-allodynic effects of endomorphins (EMs) and their C-terminal hydrazide modified analogs EM-1-NHNH2 and EM-2-NHNH2 in the spared nerve injury (SNI) model of neuropathic pain in mice. Our results demonstrated that intrathecal (i.t.) administration of endomorphin-1 (EM-1), endomorphin-2 (EM-2), EM-1-NHNH2 and EM-2-NHNH2 produced potent anti-allodynic effects ipsilaterally in neuropathic pain model. Judging from the area under the curve (AUC) values, these two analogs exhibited higher antinociception than their parent peptides. Moreover, they also displayed significant antinociceptive effects in the contralateral paw administered intrathecally. Interestingly, EM-1 and its analog EM-1-NHNH2 displayed their antinociception probably by μ2-opioid receptor subtype since the μ1-opioid receptor antagonist naloxonazine didn’t significantly block the anti-allodynia of EM-1 and EM-1-NHNH2, which implied a same opioid mechanism. However, the anti-allodynia induced by EM-2, but not EM-2-NHNH2 was significantly reduced by both μ1-opioid antagonist, naloxonazine and κ-antagonist, nor-binaltorphamine (nor-BNI), indicating multiple opioid receptors were involved in the anti-allodynic effects of EM-2. Most importantly, EM-1-NHNH2 decreased the antinociceptive tolerance, and EM-2-NHNH2 displayed non-tolerance-forming antinociception. Therefore, C-terminal amide to hydrazide conversion changed the spinal antinociceptive profiles of EMs in neuropathic pain. The present investigation is of great value in the development of novel opioid therapeutics against neuropathic pain.

Endomorphin analog exhibited superiority in alleviating neuropathic hyperalgesia via weak activation of NMDA receptors

Morphine is a key drug for the treatment of pain but its side effects limit its clinical application. MEL-0614, an endomorphin-1 analog, has fewer side effects than morphine in addition to its powerful analgesic effect. In this study, we measured the effect of morphine and MEL-0614 on hyperalgesia (7days) and neuropathic allodynia (14days) after thermal, mechanical, and cold stimulation. We found that after four and eight consecutive days of intrathecal administration (1, 3, and 10nmol), morphine induced severe hyperalgesia and neuropathic allodynia, respectively. MEL-0614 did not induce hyperalgesia at low doses (1 and 3nmol) and had a mitigating effect on morphine-induced neuropathic exacerbations in spared nerve injury mice. Hyperalgesia was blocked by Dynorphin A (1-17) antibody but not by an opioid receptor antagonist. To explore the reasons for the different results of morphine and MEL-0614, we used quantitative PCR and immunofluorescence to explore the effects of both on N-methyl-D-aspartate (NMDA) receptor subtype 2B (NR2B), microglia marker iba-1, and inflammatory mediators. After 8days of consecutive administration, morphine (10nmol) promoted an increase in the number of NR2B, iba-1, and inflammatory mediators in the spinal cord of mice. MEL-0614 (10nmol) had no significant effect on these factors, and after co-administration with morphine, the expression of NR2B, iba-1, and inflammatory mediators was lower than that with morphine injection alone. Our research showed the advantage of MEL-0614 in terms of hyperalgesia and neuropathic allodynia, which may provide clinical relief of hyperalgesia and neuropathic allodynia caused by morphine.

Endomorphin-2 Analog Inhibits the Growth of DLD-1 and RKO Human Colon Cancer Cells by Inducing Cell Apoptosis.

BackgroundIn developed countries, colon cancer is a leading cause of cancer-associated mortality. Dietary changes have resulted in an increased incidence of colon cancer in Asia. This study aimed to investigate the effects of the structural analog of endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH2) on human colon cancer cells in vitro.Material/MethodsHuman DLD-1 and RKO colon cancer cells and CCD-18Co normal human colonic fibroblasts were treated with increasing doses of the structural analog of endomorphin-2. Cells underwent the MTT assay, fluorescence confocal flow cytometry, and Hoechst 33258 staining to investigate cell proliferation, the cell cycle, and apoptosis. Western blot was used to measure the expression levels of poly(ADP-ribose) polymerase-1 (PARP-1), cytochrome c, caspase-3, and caspase-9. The 2′,7′-dichlorofluorescein diacetate (DCFH-DA) fluorescence method measured reactive oxygen species (ROS).ResultsCell proliferation of DLD-1 and RKO cells was inhibited by the endomorphin-2 analog in a dose-dependent manner, and a 100 μM dose reduced DLD-1 and RKO cell proliferation by 28% and 23%, respectively, at 72 h. Endomorphin-2 analog induced cell apoptosis and the generation of ROS, activated caspase-3 and caspase-9, and increased the levels of p53 and cytochrome c release, and down-regulated of Akt activation in DLD-1 and RKO cells in a dose-dependent manner. Treatment of the DLD-1 and RKO cells with the endomorphin-2 analog increased the expression of Bax and reduced the expression of Bcl-2.ConclusionsEndomorphin-2 analog inhibited colon cancer cell proliferation, activated apoptosis, and down-regulated Akt phosphorylation of human DLD-1 and RKO colon cancer cells in vitro in a dose-dependent manner.

Design, synthesis, and biological activity of new endomorphin analogs with multi-site modifications

Endomorphin (EM)-1 and EM-2 are the most effective endogenous analgesics with efficient separation of analgesia from the risk of adverse effects. Poor metabolic stability and ineffective analgesia after peripheral administration were detrimental for the use of EMs as novel clinical analgesics. Therefore, here, we aimed to establish new EM analogs via introducing different bifunctional d-amino acids at position 2 of [(2-furyl)Map4]EMs. The combination of [(2-furyl)Map4]EMs with D-Arg2 or D-Cit2 yielded analogs with enhanced binding affinity to the μ-opioid receptor (MOR) and increased stability against enzymatic degradation (t1/2 > 300 min). However, the agonistic activities of these analogs toward MOR were slightly reduced. Similar to morphine, peripheral administration of the analog [D-Cit2, (2-furyl)Map4]EM-1 (10) significantly inhibited the pain behavior of mice in multiple pain models. In addition, this EM-1 analog was associated with reduced tolerance, less effect on gastrointestinal mobility, and no significant motor impairment. Compared to natural EMs, the EM analogs synthesized herein had enhanced metabolic stability, bioavailability, and analgesic properties.

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.

Discriminative Stimulus and Low Abuse Liability Effects of Novel Endomorphin Analogs Suggest a Potential Treatment Indication for Opioid Use Disorder.

Opioid dependence can be difficult to manage using existing pharmacotherapies. A long-acting opioid with low abuse liability that substitutes for a shorter-acting opioid may improve treatment of opioid use disorders (OUDs). We recently characterized an endomorphin (EM) analog (ZH853) that produced a longer duration of antinociception compared with morphine, but did not produce self-administration or several other adverse effects preclinically. Here, we further characterized ZH853 in tests of antinociception, abuse liability, and drug discrimination. A conditioned place preference (CPP) procedure, that included a locomotor activity assessment, was used to test abuse liability in rats. Subsequently, dopamine (DA) cell-somas located in the ventral tegmental area (VTA) from these rats were assessed by size using immunohistochemistry and Stereo Investigator software. A hot-plate antinociception test in male and female mice confirmed central penetration. Morphine-substitution effects of several EM analogs (ZH850, ZH831, and ZH853) were tested in a drug discrimination (DD) procedure in rats. Morphine produced dose-dependent CPP and locomotor sensitization and reduced the size of DA cell somas in VTA, whereas ZH853 did not produce any of these effects relative to control. The antinociceptive effects of ZH853 were μ-receptor selective since β-funaltrexamine antagonized these effects. Rats responded on a morphine-trained lever when injected with ZH831 and ZH853 during DD experiments. The favorable morphine-substitution effects of these EM analogs relative to their low abuse liability indicate promising novel compounds that may improve treatment of OUD. SIGNIFICANCE STATEMENT: In this experiment, we investigated the preclinical effects of novel endomorphin analogs for use as substitution therapies for opioid use disorder, a problem that has contributed to an opioid overdose epidemic. Several endomorphin analogs substituted for morphine without producing adverse effects, including reward behaviors associated with abuse liability. These compounds have the potential to become important additional tools to treat opioid use disorders.

Morphine immunomodulation prolongs inflammatory and postoperative pain while the novel analgesic ZH853 accelerates recovery and protects against latent sensitization

BackgroundNumerous studies have identified the proinflammatory, pronociceptive effects of morphine which ultimately exacerbate pain. Our novel endomorphin analog ZH853 does not produce proinflammatory effects on its own and gives potent, long-lasting analgesia. This study investigates whether ZH853’s lack of interaction with the neuroimmune system reduces the risk of prolonged pain.MethodsAdult male Sprague-Dawley rats were subjected to one of two treatment paradigms. Either (1) chronic pain followed by chronic treatment with morphine, ZH853 or vehicle, or (2) chronic drug administered prior to pain induction. Complete Freund’s adjuvant (CFA) was injected or paw incision surgery was performed on the left hind plantar foot pad. Drugs were administered through Alzet osmotic minipumps at a rate of 1 μl/h for 5 days at appropriate doses based on prior experiments. Animals were tested for mechanical allodynia and thermal hyperalgesia using von Frey filaments and the Hargreaves apparatus, respectively. Additionally, several gait parameters were measured using the CatWalk XT. When all animals had recovered from pain, 1 mg/kg of naltrexone was administered to test for development of latent sensitization (LS). A second set of animals was used to investigate dorsal horn inflammation following CFA and drug treatment. ANOVAs were used to assess differences between drug treatment groups.ResultsAs expected, morphine increased and prolonged pain in all experiments compared to vehicle treatment. However, ZH853 treatment reduced the overall time spent in pain and the severity of pain scores compared to morphine. ZH853 not only reduced inflammation versus morphine treatment but also, in some instances, acted as an anti-inflammatory drug compared to vehicle treatment. Finally, ZH853 prevented the development of LS while vehicle- and morphine-treated animals showed robust relapse to pain.ConclusionsZH853 has a favorable side effect profile versus morphine and provides superior analgesia in a number of pain states. We now know that chronic use of this compound reduces time spent in a chronic pain state, the opposite of common opioids like morphine, and reduces the risk of LS, making ZH853 an excellent candidate for clinical development in humans for inflammatory and postoperative pain.

Innovative Opioid Peptides and Biased Agonism: Novel Avenues for More Effective and Safer Analgesics to Treat Chronic Pain

Chronic pain states are clinically relevant and yet unsolved conditions impacting on quality of life and representing an important social and economic burden; these diseases are poorly treated with the currently available drugs, being urgent the need of innovative analgesics. In this frame, novel analogues of endomorphin-1 and dermorphin emerge as promising starting points to develop innovative, more effective analgesics to treat neuropathic pain. An extensive and structured search of bibliographic databases for peer-reviewed research literature was undertaken using focused review questions; all the retrieved papers were published on prestigious international journals by the experts of the field and were carefully analyzed to collect all the information and data necessary to the conceptual framework of this review. One hundred papers were included in this review; forty-one defined the up-to-date findings on neuropathic pain etiopathogenesis and its currently available treatment options. Thirty-five papers described all the advantages and drawbacks of using endomorphin-1 (23) or dermorphin (12) in the frame of neuropathic pain and outlined the most relevant advances in developing endomorphin-1 and dermorphin analogs useful as potential, innovative analgesics. Twenty-four papers provided the latest insights into exploiting biased agonism at opioid receptor as an innovative strategy to develop more effective and safer analgesics. This review reports that innovative opioid peptides will be of great help in better understanding the multifaceted scenario of neuropathic pain treatment, providing very interesting opportunities for the identification of novel and more effective opioid analgesics to be employed as medications.

Endomorphin-1 analogs with oligoarginine-conjugation at C-terminus produce potent antinociception with reduced opioid tolerance in paw withdrawal test

For clinical use, it is essential to develop potent endomorphin (EM) analogs with reduced antinociceptive tolerance. In the present study, the antinociceptive activities and tolerance development of four potent EM-1 analogs with C-terminal oligoarginine-conjugation was evaluated and compared in the radiant heat paw withdrawal test. Following intracerebroventricular (i.c.v.) administration, all analogs 1–4 produced potent and prolonged antinociceptive effects. Notably, analogs 2 and 4 with the introduction of D-Ala in position 2 exhibited relatively higher analgesic potencies than those of analogs 1 and 3 with β-Pro substitution, consistent with their μ-opioid binding characteristic. In addition, at a dose of 50 μmol/kg, endomorphin-1 (EM-1) failed to produce any significant antinociceptive activity after peripheral administration, whereas analogs 1–4 induced potent antinociceptive effects with an increased duration of action. Herein, our results indicated the development of antinociceptive tolerance to EM-1 and morphine at the supraspinal level on day 7. By contrast, analogs 1–4 decreased the antinociceptive tolerance. Furthermore, subcutaneous (s.c.) administration of morphine at 50 μmol/kg also developed the antinociceptive tolerance, whereas the extent of tolerance developed to analogs 1–4 was largely reduced. Especially, analog 4 exhibited non-tolerance-forming antinociception after peripheral administration. The present investigation gave the evidence that C-terminal conjugation of EM-1 with oligoarginine vector will facilitate the development of novel opioid analgesics with reduced opioid tolerance.

Constraining Endomorphin-1 by β,α-Hybrid Dipeptide/Heterocycle Scaffolds: Identification of a Novel κ-Opioid Receptor Selective Partial Agonist.

Herein we present the expedient synthesis of endomorphin-1 analogues containing stereoisomeric β2-homo-Freidinger lactam-like scaffolds ([Amo2]EM), and we discuss opioid receptor (OR) affinity, enzymatic stability, functional activity, in vivo antinociceptive effects, and conformational and molecular docking analysis. Hence, H-Tyr-Amo-Trp-PheNH2 resulted to be a new chemotype of highly stable, selective, partial KOR agonist inducing analgesia, therefore displaying great potential interest as a painkiller possibly with reduced harmful side effects.

MEL-N16: A Series of Novel Endomorphin Analogs with Good Analgesic Activity and a Favorable Side Effect Profile.

Opioid peptides are neuromodulators that bind to opioid receptors and reduce pain sensitivity. Endomorphins are among the most active endogenous opioid peptides, and they have good affinity and selectivity toward the μ opioid receptor. However, their clinical usage is hindered by their inability to cross the blood-brain barrier and their poor in vivo activity after peripheral injection. In order to overcome these defects, we have designed and synthesized a series of novel endomorphin analogs with multiple site modifications. Radioligand binding, cAMP accumulation, and β-arrestin-2 recruitment assays were employed to determine the activity of synthesized endomorphin analogs toward opioid receptors. The blood-brain barrier permeability and antinociceptive effect of these analogs were determined in several rodent models of acute and persistent pain. In addition, the side effects of the analogs were examined. The radioligand binding assay and functional activity examination indicated that the MEL-N16 series of compounds were more active agonists against μ opioid receptor than were the parent peptides. Notably, the analogs displayed biased downstream signaling toward G-protein pathways over β-arrestin-2 recruitment. The analogs showed highly potent antinociceptive effects in the tested nociceptive models. In comparison with endomorphins, the synthesized analogs were better able to penetrate the blood-brain barrier and exerted their pain regulatory activity in the central nervous system after peripheral injection. These analogs also have lower tendency to cause side effects than morphine does at similar or equal antinociceptive doses. The MEL-N16 compounds have highly potent and efficacious analgesic effects in various pain models with a favorable side effect profile.