In vivo characterization of MMP-2200, a mixed δ/μ opioid agonist, in mice

Design and synthesis of oxytocin glycosides for the treatment of pain and substance use disorder

Peptide drugs are a promising alternative to classical small molecule therapeutics with diverse applications, ranging from antibiotic resistant infection to prostate cancer. Oxytocin (OT) is a highly evolutionarily conserved peptide neurohormone and has been of interest for pharmaceutical use since 1909. Despite their increased safety profile relative to most small molecule drugs, peptides are poor candidates based on the pharmacokinetic (PK) properties from their peptide nature. Broad application of OT as a drug has been limited by these same PK issues. Several strategies have been proposed to overcome these limitations, among them glycosylation, which was used in combination with other sequence modifications to produce robust antinociception in mouse models, increased selectivity and potency at the OT receptor, and improved stability in rats.

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.

Identification and Pharmacological Characterization of a Low-Liability Antinociceptive Bifunctional MOR/DOR Cyclic Peptide

Peptide-based opioid ligands are important candidates for the development of novel, safer, and more effective analgesics to treat pain. To develop peptide-based safer analgesics, we synthesized a mixture-based cyclic pentapeptide library containing a total of 24,624 pentapeptides and screened the mixture-based library samples using a 55 °C warm water tail-withdrawal assay. Using this phenotypic screening approach, we deconvoluted the mixture-based samples to identify a novel cyclic peptide Tyr-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt), which produced dose- and time-dependent antinociception with an ED50 (and 95% confidence interval) of 0.70 (0.52-0.97) mg/kg i.p. mediated by the mu-opioid receptor (MOR). Additionally, higher doses (≥3 mg/kg, i.p.) of CycloAnt antagonized delta-opioid receptors (DOR) for at least 3 h. Pharmacological characterization of CycloAnt showed the cyclic peptide did not reduce breathing rate in mice at doses up to 15 times the analgesic ED50 value, and produced dramatically less hyperlocomotion than the MOR agonist, morphine. While chronic administration of CycloAnt resulted in antinociceptive tolerance, it was without opioid-induced hyperalgesia and with significantly reduced signs of naloxone-precipitated withdrawal, which suggested reduced physical dependence compared to morphine. Collectively, the results suggest this dual MOR/DOR multifunctional ligand is an excellent lead for the development of peptide-based safer analgesics.

Modification and Delivery of Enkephalins for Pain Modulation

Chronic pain negatively affects patient's quality of life and poses a significant economic burden. First line pharmaceutical treatment of chronic pain, including NSAIDs or antidepressants, is often inefficient to reduce pain, or produces intolerable adverse effects. In such cases, opioids are frequently prescribed for their potent analgesia, but chronic opioid use is also frequently associated with debilitating side effects that may offset analgesic benefits. Nonetheless, opioids continue to be widely utilized due to the lack of effective alternative analgesics. Since their discovery in 1975, a class of endogenous opioids called enkephalins (ENKs) have been investigated for their ability to relieve pain with significantly reduced adverse effects compared to conventional opioids. Their low metabolic stability and inability to cross biological membranes, however, make ENKs ineffective analgesics. Over past decades, much effort has been invested to overcome these limitations and develop ENK-based pain therapies. This review summarizes and describes chemical modifications and ENK delivery technologies utilizing ENK conjugates, nanoparticles and ENK gene delivery approaches and discusses valid lessons, challenges, and future directions of this evolving field.

Nitro-benzylideneoxymorphone, a bifunctional mu and delta opioid receptor ligand with high mu opioid receptor efficacy

Introduction: There is a major societal need for analgesics with less tolerance, dependence, and abuse liability. Preclinical rodent studies suggest that bifunctional ligands with both mu (MOPr) and delta (DOPr) opioid peptide receptor activity may produce analgesia with reduced tolerance and other side effects. This study explores the structure-activity relationships (SAR) of our previously reported MOPr/DOPr lead, benzylideneoxymorphone (BOM) with C7-methylene-substituted analogs. Methods: Analogs were synthesized and tested in vitro for opioid receptor binding and efficacy. One compound, nitro-BOM (NBOM, 12) was evaluated for antinociceptive effects in the warm water tail withdrawal assay in C57BL/6 mice. Acute and chronic antinociception was determined, as was toxicologic effects on chronic administration. Molecular modeling experiments were performed using the Site Identification by Ligand Competitive Saturation (SILCS) method. Results: NBOM was found to be a potent MOPr agonist/DOPr partial agonist that produces high-efficacy antinociception. Antinociceptive tolerance was observed, as was weight loss; this toxicity was only observed with NBOM and not with BOM. Modeling supports the hypothesis that the increased MOPr efficacy of NBOM is due to the substituted benzylidene ring occupying a nonpolar region within the MOPr agonist state. Discussion: Though antinociceptive tolerance and non-specific toxicity was observed on repeated administration, NBOM provides an important new tool for understanding MOPr/DOPr pharmacology.

Strategies towards safer opioid analgesics-A review of old and upcoming targets

Opioids continue to be of use for the treatment of pain. Most clinically used analgesics target the μ opioid receptor whose activation results in adverse effects like respiratory depression, addiction and abuse liability. Various approaches have been used by the field to separate receptor-mediated analgesic actions from adverse effects. These include biased agonism, opioids targeting multiple receptors, allosteric modulators, heteromers and splice variants of the μ receptor. This review will focus on the current status of the field and some upcoming targets of interest that may lead to a safer next generation of analgesics. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Brain Penetrant, but not Peripherally Restricted, Synthetic Cannabinoid 1 Receptor Agonists Promote Morphine-Mediated Respiratory Depression

Introduction: Cannabis acceptance and use continues to rise despite the gaps in knowledge regarding the mechanisms of cannabinoids and the endocannabinoid system in many physiological functions, including respiratory influence. Methods: With recent evidence of cannabinoid receptor 1 (CB1R) presence in the collection of respiratory neurons in the brainstem, as well as in the peripheral lung tissue, it is vital that the mechanisms involved in central and peripheral CB1R modulation of respiratory function be delineated. In this study we sought to define the roles of central versus peripheral CB1R activation on respiratory depression alone and in combination with morphine using whole body plethysmography. Results: We show that the peripherally restricted CB1 agonist (4-{2-[-(1E)-1[(4-propylnaphthalen-1-yl)methylidene]-1H-inden-3yl]ethyl}morpholine [PrNMI] 0.3, 0.6, and 1 mg/kg) does not induce respiratory depression, while our previous studies showed that a central penetrating synthetic cannabinoid does induce respiratory depression. Significantly, the combination of morphine with the peripheral CB1 agonist, PrNMI, attenuated morphine-induced respiratory depression. Conclusions: These studies support that a peripherally restricted CB1R agonist may be a unique strategy to attenuate the respiratory depression associated with opioid therapy.

Peptides as Pharmacological Carriers to the Brain: Promises, Shortcomings and Challenges

Central nervous system (CNS) diseases are among the most difficult to treat, mainly because the vast majority of the drugs fail to cross the blood-brain barrier (BBB) or to reach the brain at concentrations adequate to exert a pharmacological activity. The obstacle posed by the BBB has led to the in-depth study of strategies allowing the brain delivery of CNS-active drugs. Among the most promising strategies is the use of peptides addressed to the BBB. Peptides are versatile molecules that can be used to decorate nanoparticles or can be conjugated to drugs, with either a stable link or as pro-drugs. They have been used to deliver to the brain both small molecules and proteins, with applications in diverse therapeutic areas such as brain cancers, neurodegenerative diseases and imaging. Peptides can be generally classified as receptor-targeted, recognizing membrane proteins expressed by the BBB microvessels (e.g., Angiopep2, CDX, and iRGD), "cell-penetrating peptides" (CPPs; e.g. TAT_47-57, SynB1/3, and Penetratin), undergoing transcytosis through unspecific mechanisms, or those exploiting a mixed approach. The advantages of peptides have been extensively pointed out, but so far few studies have focused on the potential negative aspects. Indeed, despite having a generally good safety profile, some peptide conjugates may display toxicological characteristics distinct from those of the peptide itself, causing for instance antigenicity, cardiovascular alterations or hemolysis. Other shortcomings are the often brief lifetime in vivo, caused by the presence of peptidases, the vulnerability to endosomal/lysosomal degradation, and the frequently still insufficient attainable increase of brain drug levels, which remain below the therapeutically useful concentrations. The aim of this review is to analyze not only the successful and promising aspects of the use of peptides in brain targeting but also the problems posed by this strategy for drug delivery.

Peptidomimetics and Their Applications for Opioid Peptide Drug Discovery

Despite various advantages, opioid peptides have been limited in their therapeutic uses due to the main drawbacks in metabolic stability, blood-brain barrier permeability, and bioavailability. Therefore, extensive studies have focused on overcoming the problems and optimizing the therapeutic potential. Currently, numerous peptide-based drugs are being marketed thanks to new synthetic strategies for optimizing metabolism and alternative routes of administration. This tutorial review briefly introduces the history and role of natural opioid peptides and highlights the key findings on their structure-activity relationships for the opioid receptors. It discusses details on opioid peptidomimetics applied to develop therapeutic candidates for the treatment of pain from the pharmacological and structural points of view. The main focus is the current status of various mimetic tools and the successful applications summarized in tables and figures.

Differential Effects of a Novel Opioid Ligand UTA1003 on Antinociceptive Tolerance and Motor Behaviour

Analgesic tolerance is a major problem in the clinic for the maintenance of opioid-induced long-term pain relief. Opioids with mixed activity on multiple opioid receptors promise reduced antinociceptive tolerance in preclinical studies, but these compounds typically show poor bioavailability upon oral, subcutaneous, intraperitoneal, or intravenous administration. We designed UTA1003 as a novel opioid that acts as a mu (MOP) and kappa (KOP) opioid receptor agonist and a partial agonist for delta (DOP) opioid receptor. In the present study, its antinociceptive effects, as well as its effects on antinociceptive tolerance and motor behaviour, were investigated in male rats. Acute antinociception was measured before (basal) and at different time points after subcutaneous injection of UTA1003 or morphine using the tail flick and hot plate assays. Various motor behavioural activities, including horizontal locomotion, rearing, and turning, were automatically measured in an open-field arena. The antinociceptive and behavioural effects of repeated administration of UTA1003 and morphine were determined over eight days. UTA1003 induced mild antinociceptive effects after acute administration but induced no tolerance after repeated treatment. Importantly, UTA1003 co-treatment with morphine prevented antinociceptive tolerance compared to morphine alone. UTA1003 showed less motor suppression than morphine in both acute and sub-chronic treatment regimens, while it did not affect morphine-induced motor suppression or hyper-excitation. Based on these activities, we speculate that UTA1003 crosses the blood-brain barrier after subcutaneous administration and, therefore, could be developed as a lead molecule to avoid opioid-induced antinociceptive tolerance and motor suppression. Further structural modifications to improve its antinociceptive effects, toxicity profile, and ADME parameters are nevertheless required.

Analgesic Opioid Ligand Discovery Based on Nonmorphinan Scaffolds Derived from Natural Sources

Strong opioid analgesics, including morphine, are the mainstays for treating moderate to severe acute pain and alleviating chronic cancer pain. However, opioid-related adverse effects, including nausea or vomiting, sedation, respiratory depression, constipation, pruritus (itch), analgesic tolerance, and addiction and abuse liability, are problematic. In addition, the use of opioids to relieve chronic noncancer pain is controversial due to the "opioid crisis" characterized by opioid misuse or abuse and escalating unintentional death rates due to respiratory depression. Hence, considerable research internationally has been aimed at the "Holy Grail" of the opioid analgesic field, namely the discovery of novel and safer opioid analgesics with improved opioid-related adverse effects. In this Perspective, medicinal chemistry strategies are addressed, where structurally diverse nonmorphinan-based opioid ligands derived from natural sources were deployed as lead molecules. The current state of play, clinical or experimental status, and novel opioid ligand discovery approaches are elaborated in the context of retaining analgesia with improved safety and reduced adverse effects, especially addiction liability.

Design and Synthesis of Brain Penetrant Glycopeptide Analogues of PACAP With Neuroprotective Potential for Traumatic Brain Injury and Parkinsonism

There is an unmet clinical need for curative therapies to treat neurodegenerative disorders. Most mainstay treatments currently on the market only alleviate specific symptoms and do not reverse disease progression. The Pituitary adenylate cyclase-activating polypeptide (PACAP), an endogenous neuropeptide hormone, has been extensively studied as a potential regenerative therapeutic. PACAP is widely distributed in the central nervous system (CNS) and exerts its neuroprotective and neurotrophic effects via the related Class B GPCRs PAC1, VPAC1, and VPAC2, at which the hormone shows roughly equal activity. Vasoactive intestinal peptide (VIP) also activates these receptors, and this close analogue of PACAP has also shown to promote neuronal survival in various animal models of acute and progressive neurodegenerative diseases. However, PACAP's poor pharmacokinetic profile (non-linear PK/PD), and more importantly its limited blood-brain barrier (BBB) permeability has hampered development of this peptide as a therapeutic. We have demonstrated that glycosylation of PACAP and related peptides promotes penetration of the BBB and improves PK properties while retaining efficacy and potency in the low nanomolar range at its target receptors. Furthermore, judicious structure-activity relationship (SAR) studies revealed key motifs that can be modulated to afford compounds with diverse selectivity profiles. Most importantly, we have demonstrated that select PACAP glycopeptide analogues (2LS80Mel and 2LS98Lac) exert potent neuroprotective effects and anti-inflammatory activity in animal models of traumatic brain injury and in a mild-toxin lesion model of Parkinson's disease, highlighting glycosylation as a viable strategy for converting endogenous peptides into robust and efficacious drug candidates.

Modulating β-arrestin 2 recruitment at the δ- and μ-opioid receptors using peptidomimetic ligands

μ-Opioid receptor agonists provide potent and effective acute analgesia; however, their therapeutic window narrows considerably upon repeated administration, such as required for treating chronic pain. In contrast, bifunctional μ/δ opioid agonists, such as the endogenous enkephalins, have potential for treating both acute and chronic pain. However, enkephalins recruit β-arrestins, which correlate with certain adverse effects at μ- and δ-opioid receptors. Herein, we identify the C-terminus of Tyr-ψ[(Z)CF[double bond, length as m-dash]CH]-Gly-Leu-enkephalin, a stable enkephalin derivative, as a key site to regulate bias of both δ- and μ-opioid receptors. Using in vitro assays, substitution of the Leu⁵ carboxylate with amides (NHEt, NMe₂, N^CyPr) reduced β-arrestin recruitment efficacy through both the δ-opioid and μ-opioid, while retaining affinity and cAMP potency. For this series, computational studies suggest key ligand-receptor interactions that might influence bias. These findings should enable the discovery of a range of tool compounds with previously unexplored biased μ/δ opioid agonist pharmacological profiles.

Aiming at Ideal Therapeutics-MOPr/DOPr or MOPr-DOPr Heteromertargeting Ligand

BACKGROUND AND OBJECTIVE

The recent alarming reports related to "opioid crisis" necessitate the development of safer and effective analgesics without unwanted side effects. Thus, there needs to be an alternative target or strategy for the development of drugs for the treatment of opioid use/abuse. As one of the novel targets, in these two decades, ligands targeting opioid receptor "heteromerization" including mu-opioid receptor (MOPr)-delta opioid receptor (DOPr) heteromer have been proposed and the pharmacological advancement of reduced side effects has been broadly accepted and well recognized. In this review, some of the ligands targeting both MOPr and DOPr or MOPr-DOPr heteromers are introduced especially focusing on their pharmacological effects in vivo.

CONCLUSION

It has been found that most of those ligands possess potent antinociceptive activity (as much as or higher than that of morphine) with reduced side effects such as tolerance. In addition, some of them are also able to reduce or prevent physiological withdrawal symptoms observed under chronic opioid use. Importantly, there are an increasing number of evidence that show changes in heteromer expression in various pathological animal models and these strongly argue for targeting heteromers for the development of the next generation of pain medication in the near future.

An Effective and Safe Enkephalin Analog for Antinociception

Opioids account for 69,000 overdose deaths per annum worldwide and cause serious side effects. Safer analgesics are urgently needed. The endogenous opioid peptide Leu-Enkephalin (Leu-ENK) is ineffective when introduced peripherally due to poor stability and limited membrane permeability. We developed a focused library of Leu-ENK analogs containing small hydrophobic modifications. N-pivaloyl analog KK-103 showed the highest binding affinity to the delta opioid receptor (68% relative to Leu-ENK) and an extended plasma half-life of 37 h. In the murine hot-plate model, subcutaneous KK-103 showed 10-fold improved anticonception (142%MPE·h) compared to Leu-ENK (14%MPE·h). In the formalin model, KK-103 reduced the licking and biting time to ~50% relative to the vehicle group. KK-103 was shown to act through the opioid receptors in the central nervous system. In contrast to morphine, KK-103 was longer-lasting and did not induce breathing depression, physical dependence, and tolerance, showing potential as a safe and effective analgesic.

Organic Solution Advanced Spray-Dried Microparticulate/Nanoparticulate Dry Powders of Lactomorphin for Respiratory Delivery: Physicochemical Characterization, In Vitro Aerosol Dispersion, and Cellular Studies

The purpose of this study was to formulate Lactomorphin (MMP2200) in its pure state as spray-dried(SD) powders, and with the excipient Trehalose as co-spray-dried(co-SD) powders; for intranasal and deep lung administration with Dry Powder Inhalers (DPI). Lactomorphin is a glycopeptide which was developed for the control of moderate to severe pain. Particles were rationally designed and produced by advanced spray drying particle engineering in a closed mode from a dilute organic solution. Comprehensive physicochemical characterization using different analytical techniques was carried out to analyze the particle size, particle morphology, particle surface morphology, solid-state transitions, crystallinity/non-crystallinity, and residual water content. The particle chemical composition was confirmed using attenuated total reflectance-Fourier-transform infrared (ATR-FTIR), and Confocal Raman Microscopy (CRM) confirmed the particles' chemical homogeneity. The solubility and Partition coefficient (LogP) of Lactomorphin were determined by the analytical and computational methodology and revealed the hydrophilicity of Lactomorphin. A thermal degradation study was performed by exposing samples of solid-state Lactomorphin to a high temperature (62 °C) combined with zero relative humidity (RH) and to a high temperature (62 °C) combined with a high RH (75%) to evaluate the stability of Lactomorphin under these two different conditions. The solid-state processed particles exhibited excellent aerosol dispersion performance with an FDA-approved human DPI device to reach lower airways. The cell viability resazurin assay showed that Lactomorphin is safe up to 1000 μg/mL on nasal epithelium cells, lung cells, endothelial, and astrocyte brain cells.

The Delta-Specific Opioid Glycopeptide BBI-11008: CNS Penetration and Behavioral Analysis in a Preclinical Model of Levodopa-Induced Dyskinesia

In previous work we evaluated an opioid glycopeptide with mixed μ/δ-opioid receptor agonism that was a congener of leu-enkephalin, MMP-2200. The glycopeptide analogue showed penetration of the blood-brain barrier (BBB) after systemic administration to rats, as well as profound central effects in models of Parkinson's disease (PD) and levodopa (L-DOPA)-induced dyskinesia (LID). In the present study, we tested the glycopeptide BBI-11008 with selective δ-opioid receptor agonism, an analogue of deltorphin, a peptide secreted from the skin of frogs (genus Phyllomedusa). We tested BBI-11008 for BBB-penetration after intraperitoneal (i.p.) injection and evaluated effects in LID rats. BBI-11008 (10 mg/kg) demonstrated good CNS-penetrance as shown by microdialysis and mass spectrometric analysis, with peak concentration levels of 150 pM in the striatum. While BBI-11008 at both 10 and 20 mg/kg produced no effect on levodopa-induced limb, axial and oral (LAO) abnormal involuntary movements (AIMs), it reduced the levodopa-induced locomotor AIMs by 50% after systemic injection. The N-methyl-D-aspartate receptor antagonist MK-801 reduced levodopa-induced LAO AIMs, but worsened PD symptoms in this model. Co-administration of MMP-2200 had been shown prior to block the MK-801-induced pro-Parkinsonian activity. Interestingly, BBI-11008 was not able to block the pro-Parkinsonian effect of MK-801 in the LID model, further indicating that a balance of mu- and delta-opioid agonism is required for this modulation. In summary, this study illustrates another example of meaningful BBB-penetration of a glycopeptide analogue of a peptide to achieve a central behavioral effect, providing additional evidence for the glycosylation technique as a method to harness therapeutic potential of peptides.

Cannabinoid-2 Agonism with AM2301 Mitigates Morphine-Induced Respiratory Depression

Introduction: An escalating number of fatalities resulting from accidental opioid overdoses typically attributed to respiratory depression continue to define the opioid epidemic. Opioid respiratory depression results from a decrease in reflexive inspiration within the preBötzinger complex in the brainstem. Objective: Cannabinoid receptor agonism is reported to enhance opioid analgesia, yet whether cannabinoids enhance or inhibit opioid-induced respiratory depression is unknown. Methods: Studies herein sought to define the roles of cannabinoid-1 receptor (CB1R) and cannabinoid-2 receptor (CB2R) on respiratory depression using selective agonists alone and in combination with morphine in male mice. Results: Using whole body plethysmography, the nonselective CB1R and CB2R agonist (Δ⁹-tetrahydrocannabinol) and the CB1R synthetic cannabinoid, AM356, induced respiratory depression, whereas the well-published selective CB2 agonist, JWH 133, and the novel CB2 agonist (AM2301) did not. Moreover, a selective CB2R agonist (AM2301) significantly attenuated morphine sulfate-induced respiratory depression. Conclusion: Notably, findings suggest that attenuation of opioid-induced respiratory depression relies on CB2R activation, supporting selective CB2R agonism as an opioid adjunct therapy.

The Intriguing Effects of Substituents in the N-Phenethyl Moiety of Norhydromorphone: A Bifunctional Opioid from a Set of "Tail Wags Dog" Experiments

(−)-N-Phenethyl analogs of optically pure N-norhydromorphone were synthesized and pharmacologically evaluated in several in vitro assays (opioid receptor binding, stimulation of [³⁵S]GTPγS binding, forskolin-induced cAMP accumulation assay, and MOR-mediated β-arrestin recruitment assays). “Body” and “tail” interactions with opioid receptors (a subset of Portoghese’s message-address theory) were used for molecular modeling and simulations, where the “address” can be considered the “body” of the hydromorphone molecule and the “message” delivered by the substituent (tail) on the aromatic ring of the N-phenethyl moiety. One compound, N-p-chloro-phenethynorhydromorphone ((7aR,12bS)-3-(4-chlorophenethyl)-9-hydroxy-2,3,4,4a,5,6-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7(7aH)-one, 2i), was found to have nanomolar binding affinity at MOR and DOR. It was a potent partial agonist at MOR and a full potent agonist at DOR with a δ/μ potency ratio of 1.2 in the ([³⁵S]GTPγS) assay. Bifunctional opioids that interact with MOR and DOR, the latter as agonists or antagonists, have been reported to have fewer side-effects than MOR agonists. The p-chlorophenethyl compound 2i was evaluated for its effect on respiration in both mice and squirrel monkeys. Compound 2i did not depress respiration (using normal air) in mice or squirrel monkeys. However, under conditions of hypercapnia (using air mixed with 5% CO₂), respiration was depressed in squirrel monkeys.

The Therapeutic Effectiveness of Full Spectrum Hemp Oil Using a Chronic Neuropathic Pain Model

BACKGROUND

Few models exist that can control for placebo and expectancy effects commonly observed in clinical trials measuring 'Cannabis' pharmacodynamics. We used the Foramen Rotundum Inflammatory Constriction Trigeminal Infraorbital Nerve injury (FRICT-ION) model to measure the effect of "full-spectrum" whole plant extracted hemp oil on chronic neuropathic pain sensitivity in mice.

METHODS

Male BALBc mice were submitted to the FRICT-ION chronic neuropathic pain model with oral insertion through an incision in the buccal/cheek crease of 3 mm of chromic gut suture (4-0). The suture, wedged along the V2 trigeminal nerve branch, creates a continuous irritation that develops into secondary mechanical hypersensitivity on the snout. Von Frey filament stimuli on the mouse whisker pad was used to assess the mechanical pain threshold from 0-6 h following dosing among animals (n = 6) exposed to 5 μL of whole plant extracted hemp oil combined with a peanut butter vehicle (0.138 mg/kg), the vehicle alone (n = 3) 7 weeks post-surgery, or a naïve control condition (n = 3).

RESULTS

Mechanical allodynia was alleviated within 1 h (d = 2.50, p < 0.001) with a peak reversal effect at 4 h (d = 7.21, p < 0.001) and remained significant throughout the 6 h observation window. There was no threshold change on contralateral whisker pad after hemp oil administration, demonstrating the localization of anesthetic response to affected areas.

CONCLUSION

Future research should focus on how whole plant extracted hemp oil affects multi-sensory and cognitive-attentional systems that process pain.

Behavioral pharmacology of the mixed-action delta-selective opioid receptor agonist BBI-11008: studies on acute, inflammatory and neuropathic pain, respiration, and drug self-administration

RATIONALE AND OBJECTIVES

The present study characterized the behavioral pharmacology of a novel, mixed-action delta-selective (78:1) opioid receptor agonist, BBI-11008. This glycopeptide drug candidate was tested in assays assessing antinociception (acute, inflammatory, and neuropathic pain-like conditions) and side-effect endpoints (respiratory depression and drug self-administration).

RESULTS

BBI-11008 had a 78-fold greater affinity for the delta opioid receptor than the mu receptor, and there was no binding to the kappa opioid receptor. BBI-11008 (3.2-100; 10-32 mg kg-1, i.v.) and morphine (1-10; 1-3.2 mg kg-1, i.v.) produced antinociceptive and anti-allodynic effects in assays of acute thermal nociception and complete Freund's adjuvant (CFA)-induced inflammatory pain, with BBI-11008 being less potent than morphine in both assays. BBI-11008 (1-18 mg kg-1, i.v.) had similar efficacy to gabapentin (10-56 mg kg-1, i.v.) in a spinal nerve ligation (SNL) model of neuropathic pain. In the respiration assay, with increasing %CO2 exposure, BBI-11008 produced an initial increase (32 mg kg-1, s.c.) and then decrease (56 mg kg-1, s.c.) in minute volume (MV) whereas morphine (3.2-32 mg kg-1, s.c.) produced dose-dependent decreases in MV. In the drug self-administration procedure, BBI-11008 did not maintain self-administration at any dose tested.

CONCLUSIONS

These results suggest that the glycopeptide drug candidate possesses broad-spectrum antinociceptive and anti-allodynic activity across a range of pain-like conditions. Relative to morphine or fentanyl, the profile for BBI-11008 in the respiration and drug self-administration assays suggests that BBI-11008 may have less pronounced deleterious side effects. Continued assessment of this compound is warranted.

A Novel Mu-Delta Opioid Agonist Demonstrates Enhanced Efficacy With Reduced Tolerance and Dependence in Mouse Neuropathic Pain Models

Numerous studies have demonstrated a physiological interaction between the mu opioid receptor (MOR) and delta opioid receptor (DOR) systems. A few studies have shown that dual MOR-DOR agonists could be beneficial, with reduced tolerance and addiction liability, but are nearly untested in chronic pain models, particularly neuropathic pain. In this study, we tested the MOR-DOR agonist SRI-22141 in mice in the clinically relevant models of HIV Neuropathy and Chemotherapy-Induced Peripheral Neuropathy (CIPN). SRI-22141 was more potent than morphine in the tail flick pain test and had equal or enhanced efficacy versus morphine in both neuropathic pain models, with significantly reduced tolerance. SRI-22141 also produced no jumping behavior during naloxone-precipitated withdrawal in CIPN or naïve mice, suggesting that SRI-22141 produces little to no dependence. SRI-22141 also reduced tumor necrosis factor-α and cyclooxygenase-2 in CIPN in the spinal cord, suggesting an anti-inflammatory mechanism of action. The DOR-selective antagonist naltrindole strongly reduced CIPN efficacy and anti-inflammatory activity in the spinal cord, without affecting tail flick antinociception, suggesting the importance of DOR activity in these models. Overall, these results provide compelling evidence that MOR-DOR agonists could have strong efficacy with reduced side effects and an anti-inflammatory mechanism in the treatment of neuropathic pain. PERSPECTIVE: This study demonstrates that a MOR-DOR dual agonist given chronically in chronic neuropathic pain models has enhanced efficacy with strongly reduced tolerance and dependence, with a further anti-inflammatory effect in the spinal cord. This suggests that MOR-DOR dual agonists could be effective treatments for neuropathic pain with reduced side effects.

Simultaneous Activation of Mu and Delta Opioid Receptors Reduces Allodynia and Astrocytic Connexin 43 in an Animal Model of Neuropathic Pain

Neuropathic pain is a chronic condition triggered by lesions to the somatosensory nervous system in which pain stimuli occur spontaneously or as pathologically amplified responses. In this scenario, the exchange of signaling molecules throughout cell-to-cell and cell-to-extracellular environment communications plays a key role in the transition from acute to chronic pain. As such, connexin 43 (Cx43), the core glial gap junction and hemichannel-forming protein, is considered a triggering factor for disease chronicization in the central nervous system (CNS). Drugs targeting μ opioid receptors (MOR) are currently used for moderate to severe pain conditions, but their use in chronic pain is limited by the tolerability profile. δ opioid receptors (DOR) have become attractive targets for the treatment of persistent pain and have been associated with the inhibition of pain-sustaining factors. Moreover, it has been shown that simultaneous targeting of MOR and DOR leads to an improved pharmacological fingerprint. Herein, we aimed to study the effects of the benzomorphan ligand LP2, a multitarget MOR/DOR agonist, in an experimental model of neuropathic pain induced by the unilateral sciatic nerve chronic constriction injury (CCI) on male Sprague-Dawley rats. Results showed that LP2 significantly ameliorated mechanical allodynia from the early phase of treatment up to 21 days post-ligatures. We additionally showed that LP2 prevented CCI-induced Cx43 alterations and pro-apoptotic signaling in the CNS. These findings increase the knowledge of neuropathic pain development and the role of spinal astrocytic Cx43, suggesting new approaches for the treatment of neuropathic pain.

Receptor Ligands as Helping Hands to L-DOPA in the Treatment of Parkinson's Disease

Levodopa (LD) is the most effective drug in the treatment of Parkinson’s disease (PD). However, although it represents the “gold standard” of PD therapy, LD can cause side effects, including gastrointestinal and cardiovascular symptoms as well as transient elevated liver enzyme levels. Moreover, LD therapy leads to LD-induced dyskinesia (LID), a disabling motor complication that represents a major challenge for the clinical neurologist. Due to the many limitations associated with LD therapeutic use, other dopaminergic and non-dopaminergic drugs are being developed to optimize the treatment response. This review focuses on recent investigations about non-dopaminergic central nervous system (CNS) receptor ligands that have been identified to have therapeutic potential for the treatment of motor and non-motor symptoms of PD. In a different way, such agents may contribute to extending LD response and/or ameliorate LD-induced side effects.

Dual Pharmacophores Explored via Structure-Activity Relationship (SAR) Matrix: Insights into Potent, Bifunctional Opioid Ligand Design

Short-acting μ-opioid receptor (MOR) agonists have long been used for the treatment of severe, breakthrough pain. However, selective MOR agonists including fentanyl and morphine derivatives are limited clinically due to high risks of dependence, tolerance, and respiratory depression. We recently reported the development of a long-acting, bifunctional MOR agonist/δ-opioid receptor (DOR) antagonist analgesic devoid of tolerance or dependence in mice (AAH8, henceforth referred to as 2B). To address the need for short-acting treatments for breakthrough pain, we present a series of novel, short-acting, high-potency MOR agonist/DOR antagonist ligands with antinociceptive activity in vivo. In this study, we utilized a two-dimensional structure-activity relationship matrix to identify pharmacological trends attributable to combinations of two key pharmacophore elements within the chemotype. This work enhances our ability to modulate efficacy at MOR and DOR, accessing a variety of bifunctional profiles while maintaining high affinity and potency at both receptors.

Simultaneous targeting of MOR/DOR: A useful strategy for inflammatory pain modulation

Development of new analgesics endowed with mu/delta opioid receptor (MOR/DOR) activity represents a promising alternative to MOR selective compounds because of their better therapeutic and tolerability profile. Lately, we have synthetized the MOR/DOR agonist LP2 that showed a long lasting antinociceptive activity in the tail flick test, an acute pain model. Here, we investigate whether LP2 is also effective in the mouse formalin test, a model of inflammatory pain sustained by mechanisms of central sensitization. Moreover, we evaluated a possible peripheral component of LP2 analgesic activity. Different doses of LP2 were tested after either intraperitoneal (i.p.) or intraplantar (i.pl.) administration. LP2 (0.75-1.00 mg/kg, i.p.), dose-dependently, counteracted both phases of the formalin test after i.p. administration. The analgesic activity of LP2 (0.75-1.00 mg/kg) was completely blocked by a pretreatment with the opioid antagonist naloxone (3 mg/kg, i.p.). Differently, the pretreatment with naloxone methiodide (5 mg/kg, i.p.), a peripherally restricted opioid antagonist, completely blocked the lower analgesic dose of LP2 (0.75 mg/kg) but only partially relieved the antinociceptive effects of LP2 at the dose of 1.00 mg/kg, thus revealing a peripheral analgesic component of LP2. I.pl. injections of LP2 (10-20 μg/10 μl) were also performed to investigate a possible effect of LP2 on peripheral nerve terminals. Nociceptive sensitization, which occur both at peripheral and central level, is a fundamental step for pain chronicization, thus LP2 is a promising drug for pain conditions characterized by nociceptive sensitization.

Synthesis, Biological, and Structural Explorations of New Zwitterionic Derivatives of 14- O-Methyloxymorphone, as Potent μ/δ Opioid Agonists and Peripherally Selective Antinociceptives

Herein, the synthesis and pharmacological characterization of an extended library of differently substituted N-methyl-14-O-methylmorphinans with natural and unnatural amino acids and three dipeptides at position 6 that emerged as potent μ/δ opioid receptor (MOR/DOR) agonists with peripheral antinociceptive efficacy is reported. The current study adds significant value to our initial structure–activity relationships on a series of zwitterionic analogues of 1 (14-O-methyloxymorphone) by targeting additional amino acid residues. The new derivatives showed high binding and potent agonism at MOR and DOR in vitro. In vivo, the new 6-amino acid- and 6-dipeptide-substituted derivatives of 1 were highly effective in inducing antinociception in the writhing test in mice after subcutaneous administration, which was antagonized by naloxone methiodide demonstrating activation of peripheral opioid receptors. Such peripheral opioid analgesics may represent alternatives to presently available drugs for a safer pain therapy.

The combination of the opioid glycopeptide MMP-2200 and a NMDA receptor antagonist reduced l-DOPA-induced dyskinesia and MMP-2200 by itself reduced dopamine receptor 2-like agonist-induced dyskinesia

Dopamine (DA)-replacement therapy utilizing l-DOPA is the gold standard symptomatic treatment for Parkinson's disease (PD). A critical complication of this therapy is the development of l-DOPA-induced dyskinesia (LID). The endogenous opioid peptides, including enkephalins and dynorphin, are co-transmitters of dopaminergic, GABAergic, and glutamatergic transmission in the direct and indirect striatal output pathways disrupted in PD, and alterations in expression levels of these peptides and their precursors have been implicated in LID genesis and expression. We have previously shown that the opioid glycopeptide drug MMP-2200 (a.k.a. Lactomorphin), a glycosylated derivative of Leu-enkephalin mediates potent behavioral effects in two rodent models of striatal DA depletion. In this study, the mixed mu-delta agonist MMP-2200 was investigated in standard preclinical rodent models of PD and of LID to evaluate its effects on abnormal involuntary movements (AIMs). MMP-2200 showed antiparkinsonian activity, while increasing l-DOPA-induced limb, axial, and oral (LAO) AIMs by ∼10%, and had no effect on dopamine receptor 1 (D1R)-induced LAO AIMs. In contrast, it markedly reduced dopamine receptor 2 (D2R)-like-induced LAO AIMs. The locomotor AIMs were reduced by MMP-2200 in all three conditions. The N-methyl-d-aspartate receptor (NMDAR) antagonist MK-801 has previously been shown to be anti-dyskinetic, but only at doses that induce parkinsonism. When MMP-2200 was co-administered with MK-801, MK-801-induced pro-parkinsonian activity was suppressed, while a robust anti-dyskinetic effect remained. In summary, the opioid glycopeptide MMP-2200 reduced AIMs induced by a D2R-like agonist, and MMP-2200 modified the effect of MK-801 to result in a potent reduction of l-DOPA-induced AIMs without induction of parkinsonism.

Evaluation of morphine-like effects of the mixed mu/delta agonist morphine-6-O-sulfate in rats: Drug discrimination and physical dependence

Morphine-6-O-sulfate (M6S) is as a mixed-action mu/delta (μ/δ) opioid receptor agonist with high potency and analgesic efficacy. These studies used assays of drug discrimination and schedule-controlled responding to assess abuse-liability, tolerance, and physical dependence as compared to morphine in rats. Attempts to train 0.3 mg/kg (IP) M6S from saline failed, but all rats rapidly acquired the discrimination when the training dose was changed to 3.0 mg/kg morphine, and substitution tests showed that morphine and fentanyl both fully substituted for the training dose, M6S and M3A6S (3-O-acetyl ester of M6S) only partially substituted, and salvinorin A did not elicit morphine-like effects. Tolerance to response rate-decreasing effects was studied in rats administered either 1.0 or 3.0 mg/kg morphine or M6S before food-reinforced operant sessions. At both unit doses, tolerance to M6S-elicited rate suppression developed more slowly than tolerance to morphine-induced reductions in response rates. To assess dependence, rats were maintained on 1.0 mg/kg morphine or 1.0 mg/kg M6S until food-reinforced response rates were stable for at least 5 days. Rats were then administered saline or increasing doses of the opioid antagonist naltrexone (NTX) (0.3, 1.0, 3.0, or 10.0 mg/kg) in order to determine antagonist-precipitated withdrawal. NTX precipitated withdrawal was similar in both morphine-maintained and M6S-maintained rats. In conclusion, the mixed μ/δ agonist activity of M6S failed to completely protect against the development of physical dependence, but delayed tolerance development to behavioral effects and resulted in decreased morphine-like subjective effects, perhaps implying a decreased abuse liability over μ agonists.

Delta/mu opioid receptor interactions in operant conditioning assays of pain-depressed responding and drug-induced rate suppression: assessment of therapeutic index in male Sprague Dawley rats

RATIONALE AND OBJECTIVES

Although delta/mu receptor interactions vary as a function of behavioral endpoint, there have been no assessments of these interactions using assays of pain-depressed responding. This is the first report of delta/mu interactions using an assay of pain-depressed behavior.

METHODS

A mult-cycle FR10 operant schedule was utilized in the presence of (nociception) and in the absence of (rate suppression) a lactic acid inflammatory pain-like manipulation. SNC80 and methadone were used as selective/high efficacy delta and mu agonists, respectively. Both SNC80 and methadone alone produced a dose-dependent restoration of pain-depressed responding and dose-dependent response rate suppression. Three fixed ratio mixtures, based on the relative potencies of the drugs in the nociception assay, also produced dose-dependent antinociception and sedation. Isobolographic analysis indicated that all three mixtures produced supra-additive antinociceptive effects and simply additive sedation effects.

CONCLUSIONS

The therapeutic index (TI) inversely varied as a function of amount of SNC80 in the mixture, such that lower amounts of SNC80 produced a higher TI, and larger amounts produced a lower TI. Compared to literature using standard pain-elicited assays, the orderly relationship between SNC80 and TI reported here may be a unique function of assessing pain-depressed behavior.

Multifunctional Opioid Ligands

The opioid receptor system plays a major role in the regulation of mood, reward, and pain. The opioid receptors therefore make attractive targets for the treatment of many different conditions, including pain, depression, and addiction. However, stimulation or blockade of any one opioid receptor type often leads to on-target adverse effects that limit the clinical utility of a selective opioid agonist or antagonist. Literature precedent suggests that the opioid receptors do not act in isolation and that interactions among the opioid receptors and between the opioid receptors and other proteins may produce clinically useful targets. Multifunctional ligands have the potential to elicit desired outcomes with reduced adverse effects by allowing for the activation of specific receptor conformations and/or signaling pathways promoted as a result of receptor oligomerization or crosstalk. In this chapter, we describe several classes of multifunctional ligands that interact with at least one opioid receptor. These ligands have been designed for biochemical exploration and the treatment of a wide variety of conditions, including multiple kinds of pain, depression, anxiety, addiction, and gastrointestinal disorders. The structures, pharmacological utility, and therapeutic drawbacks of these classes of ligands are discussed.

Placement of Hydroxy Moiety on Pendant of Peptidomimetic Scaffold Modulates Mu and Kappa Opioid Receptor Efficacy

In an effort to expand the structure–activity relationship (SAR) studies of a series of mixed-efficacy opioid ligands, peptidomimetics that incorporate methoxy and hydroxy groups around a benzyl or 2-methylindanyl pendant on a tetrahydroquinoline (THQ) core of the peptidomimetics were evaluated. Compounds containing a methoxy or hydroxy moiety in the o- or m-positions increased binding affinity to the kappa opioid receptor (KOR), whereas compounds containing methoxy or hydroxy groups in the p-position decreased KOR affinity and reduced or eliminated efficacy at the mu opioid receptor (MOR). The results from a substituted 2-methylindanyl series aligned with the findings from the substituted benzyl series. Our studies culminated in the development of 8c, a mixed-efficacy MOR agonist/KOR agonist with subnanomolar binding affinity for both MOR and KOR.

Evaluation of Analgesia, Tolerance, and the Mechanism of Action of Morphine-6-O-Sulfate Across Multiple Pain Modalities in Sprague-Dawley Rats

BACKGROUND

Morphine-6-O-sulfate (M6S) is a mixed μ/δ-opioid receptor (OR) agonist and potential alternative to morphine for treatment of chronic multimodal pain.

METHODS

To provide more support for this hypothesis, the antinociceptive effects of M6S and morphine were compared in tests that access a range of pain modalities, including hot plate threshold (HPT), pinprick sensitivity threshold (PST) and paw pressure threshold tests.

RESULTS

Acutely, M6S was 2- to 3-fold more potent than morphine in HPT and PST tests, specifically, derived from best-fit analysis of dose-response relationships of morphine/M6S half-effective dose (ED50) ratios (lower, upper 95% confidence interval [CI]) were 2.8 (2.0-5.8) in HPT and 2.2 (2.1, 2.4) in PST tests. No differences in analgesic drug potencies were detected in the PPT test (morphine/M6S ED50 ratio 1.2 (95% CI, 0.8-1.4). After 7 to 9 days of chronic treatment, tolerance developed to the antinociceptive effects of morphine, but not to M6S, in all 3 pain tests. Morphine-tolerant rats were not crosstolerant to M6S. The antinociceptive effects of M6S were not sensitive to κ-OR antagonists. However, the δ-OR antagonist, naltrindole, blocked M6S-induced antinociception by 55% ± 4% (95% CI, 39-75) in the HPT test, 94% ± 4% (95% CI, 84-105) in the PST test, and 5% ± 17% (95% CI, -47 to 59) or 51% ± 14% (95% CI, 14-84; 6 rats per each group) in the paw pressure threshold test when examined acutely or after 7 days of chronic treatment, respectively.

CONCLUSIONS

Activity via δ-ORs thus appears to be an important determinant of M6S action. M6S also exhibited favorable antinociceptive and tolerance profiles compared with morphine in 3 different antinociceptive assays, indicating that M6S may serve as a useful alternative for rotation in morphine-tolerant subjects.

Heat-shock protein 90 (Hsp90) promotes opioid-induced anti-nociception by an ERK mitogen-activated protein kinase (MAPK) mechanism in mouse brain

Recent advances in developing opioid treatments for pain with reduced side effects have focused on the signaling cascades of the μ-opioid receptor (MOR). However, few such signaling targets have been identified for exploitation. To address this need, we explored the role of heat-shock protein 90 (Hsp90) in opioid-induced MOR signaling and pain, which has only been studied in four previous articles. First, in four cell models of MOR signaling, we found that Hsp90 inhibition for 24 h with the inhibitor 17-N-allylamino-17-demethoxygeldanamycin (17-AAG) had different effects on protein expression and opioid signaling in each line, suggesting that cell models may not be reliable for predicting pharmacology with this protein. We thus developed an in vivo model using CD-1 mice with an intracerebroventricular injection of 17-AAG for 24 h. We found that Hsp90 inhibition strongly blocked morphine-induced anti-nociception in models of post-surgical and HIV neuropathic pain but only slightly blocked anti-nociception in a naive tail-flick model, while enhancing morphine-induced precipitated withdrawal. Seeking a mechanism for these changes, we found that Hsp90 inhibition blocks ERK MAPK activation in the periaqueductal gray and caudal brain stem. We tested these signaling changes by inhibiting ERK in the above-mentioned pain models and found that ERK inhibition could account for all of the changes in anti-nociception induced by Hsp90 inhibition. Taken together, these findings suggest that Hsp90 promotes opioid-induced anti-nociception by an ERK mechanism in mouse brain and that Hsp90 could be a future target for improving the therapeutic index of opioid drugs.

Novel Molecular Strategies and Targets for Opioid Drug Discovery for the Treatment of Chronic Pain

Opioid drugs like morphine and fentanyl are the gold standard for treating moderate to severe acute and chronic pain. However, opioid drug use can be limited by serious side effects, including constipation, tolerance, respiratory suppression, and addiction. For more than 100 years, we have tried to develop opioids that decrease or eliminate these liabilities, with little success. Recent advances in understanding opioid receptor signal transduction have suggested new possibilities to activate the opioid receptors to cause analgesia, while reducing or eliminating unwanted side effects. These new approaches include designing functionally selective ligands, which activate desired signaling cascades while avoiding signaling cascades that are thought to provoke side effects. It may also be possible to directly modulate downstream signaling through the use of selective activators and inhibitors. Separate from downstream signal transduction, it has also been found that when the opioid system is stimulated, various negative feedback systems are upregulated to compensate, which can drive side effects. This has led to the development of multi-functional molecules that simultaneously activate the opioid receptor while blocking various negative feedback receptor systems including cholecystokinin and neurokinin-1. Other novel approaches include targeting heterodimers of the opioid and other receptor systems which may drive side effects, and making endogenous opioid peptides druggable, which may also reduce opioid mediated side effects. Taken together, these advances in our molecular understanding provide a path forward to break the barrier in producing an opioid with reduced or eliminated side effects, especially addiction, which may provide relief for millions of patients.

Role of the sugar moiety on the opioid receptor binding and conformation of a series of enkephalin neoglycopeptides

Glycosylation by simple sugars is a drug discovery alternative that has been explored with varying success for enhancing the potency and bioavailability of opioid peptides. Long ago we described two O-glycosides having either β-Glucose and β-Galactose of (d-Met², Pro⁵)-enkephalinamide showing one of the highest antinociceptive activities known. Here, we report the resynthesis of these two analogs and the preparation of three novel neoglycopeptide derivatives (α-Mannose, β-Lactose and β-Cellobiose). Binding studies to cloned zebrafish opioid receptors showed very small differences of affinity between the parent compound and the five glycopeptides thus suggesting that the nature of the carbohydrate moiety plays a minor role in determining the binding mode. Indeed, NMR conformational studies, combined with molecular mechanics calculations, indicated that all glycopeptides present the same major conformation either in solution or membrane-like environment. The evidences provided here highlight the relevance for in vivo activity of the conjugating bond between the peptide and sugar moieties in opioid glycopeptides.

Antinociceptive effects of the 6-O-sulfate ester of morphine in normal and diabetic rats: Comparative role of mu- and delta-opioid receptors

This study determined the antinociceptive effects of morphine and morphine-6-O-sulfate (M6S) in both normal and diabetic rats, and evaluated the comparative role of mu-opioid receptors (mu-ORs) and delta-opioid receptors (delta-ORs) in the antinociceptive action of these opioids. In vitro characterization of mu-OR and delta-OR-mediated signaling by M6S and morphine in stably transfected Chinese hamster ovary (CHO-K1) cells showed that M6S exhibited a 6-fold higher affinity for delta-ORs and modulated G-protein and adenylyl cyclase activity via delta-ORs more potently than morphine. Interestingly, while morphine acted as a full agonist at delta-ORs in both functional assays examined, M6S exhibited either partial or full agonist activity for modulation of G-protein or adenylyl cyclase activity, respectively. Molecular docking studies indicated that M6S but not morphine binds equally well at the ligand binding site of both mu- and delta-ORs. In vivo analgesic effects of M6S and morphine in both normal and streptozotocin-induced diabetic Sprague-Dawley rats utilizing the hot water tail flick latency test showed that M6S produced more potent antinociception than morphine in both normal rats and diabetic rats. This difference in potency was abrogated following antagonism of delta- but not mu- or kappa (kappa-ORs) opioid receptors. During 9days of chronic treatment, tolerance developed to morphine-treated but not to M6S-treated rats. Rats that developed tolerance to morphine still remained responsive to M6S. Collectively, this study demonstrates that M6S is a potent and efficacious mu/delta opioid analgesic with a delayed tolerance profile when compared to morphine in both normal and diabetic rats.

PERSPECTIVE

This study demonstrates that M6S acts at both mu- and delta-ORs, and adds to the growing evidence that the use of mixed mu/delta opioid agonists in pain treatment may have clinical benefit.

Effects of N-Substitutions on the Tetrahydroquinoline (THQ) Core of Mixed-Efficacy μ-Opioid Receptor (MOR)/δ-Opioid Receptor (DOR) Ligands

N-Acetylation of the tetrahydroquinoline (THQ) core of a series of μ-opioid receptor (MOR) agonist/δ-opioid receptor (DOR) antagonist ligands increases DOR affinity, resulting in ligands with balanced MOR and DOR affinities. We report a series of N-substituted THQ analogues that incorporate various carbonyl-containing moieties to maintain DOR affinity and define the steric and electronic requirements of the binding pocket across the opioid receptors. 4h produced in vivo antinociception (ip) for 1 h at 10 mg/kg.

Isocyanide-Based Multicomponent Reactions for the Synthesis of Heterocycles

Multicomponent reactions (MCRs) are extremely popular owing to their facile execution, high atom-efficiency and the high diversity of products. MCRs can be used to access various heterocycles and highly functionalized scaffolds, and thus have been invaluable tools in total synthesis, drug discovery and bioconjugation. Traditional isocyanide-based MCRs utilize an external nucleophile attacking the reactive nitrilium ion, the key intermediate formed in the reaction of the imine and the isocyanide. However, when reactants with multiple nucleophilic groups (bisfunctional reactants) are used in the MCR, the nitrilium intermediate can be trapped by an intramolecular nucleophilic attack to form various heterocycles. The implications of nitrilium trapping along with widely applied conventional isocyanide-based MCRs in drug design are discussed in this review.

Synthesis of Carfentanil Amide Opioids Using the Ugi Multicomponent Reaction

We report a novel approach to synthesize carfentanil amide analogues utilizing the isocyanide-based four-component Ugi multicomponent reaction. A small library of bis-amide analogues of carfentanil was created using N-alkylpiperidones, aniline, propionic acid, and various aliphatic isocyanides. Our lead compound showed high affinity for mu (MOR) and delta opioid receptors (DOR) with no appreciable affinity for kappa (KOR) receptors in radioligand binding assays. The compound was found to be a mixed MOR agonist/partial DOR agonist in [(35)S]GTPγS functional assays, and it showed moderate analgesic potency in vivo. The compound showed no visible signs of physical dependence or constipation in mice. In addition, it produced less respiratory depression than morphine. Most mixed MOR/DOR opioids reported in the literature are peptides and thereby systemically inactive. Our approach utilizing a multicomponent reaction has the promise to deliver potent and efficacious small-molecule analgesics with potential clinical utility.

Structural Requirements for CNS Active Opioid Glycopeptides

Glycopeptides related to β-endorphin penetrate the blood-brain barrier (BBB) of mice to produce antinociception. Two series of glycopeptides were assessed for opioid receptor binding affinity. Attempts to alter the mu-selectivity of [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO)-related glycopeptides by altering the charged residues of the amphipathic helical address were unsuccessful. A series of pan-agonists was evaluated for antinociceptive activity (55 °C tail flick) in mice. A flexible linker was required to maintain antinociceptive activity. Circular dichroism (CD) in H2O, trifluoroethanol (TFE), and SDS micelles confirmed the importance of the amphipathic helices (11s → 11sG → 11) for antinociception. The glycosylated analogues showed only nascent helices and random coil conformations in H2O. Chemical shift indices (CSI) and nuclear Overhauser effects (NOE) with 600 MHz NMR and CD confirmed helical structures in micelles, which were rationalized by molecular dynamics calculations. Antinociceptive studies with mice confirm that these glycosylated endorphin analogues are potential drug candidates that penetrate the BBB to produce potent central effects.

CNS active O-linked glycopeptides

Naturally occurring glycopeptides and glycoproteins play important roles in biological processes. Glycosylation is one of the most common post-translational modifications in vivo. Glycopeptides are involved in cell signaling and sorting, providing cell surface markers for recognition. From the drug design and synthesis perspective, modification of a peptide through glycosylation results in increased bioavailability and bioactivity of glycopeptides in living systems with negligible toxicity of degradation products. Glycopeptide synthesis can be accomplished through incorporation of a glycosylated amino acid in solid phase peptide synthesis (SPPS) to form the desired peptide, or via incorporation of sugar-amino acid moieties. Additionally, research indicates that glycosylation increases penetration of the blood-brain barrier (BBB) by peptides, which may lead to novel therapeutics for neurological disorders. Recent applications of glycopeptides have focused on the in vivo central nervous system (CNS) effects after peripheral administration of centrally active peptides modified with various carbohydrates.

The mixed-action delta/mu opioid agonist MMP-2200 does not produce conditioned place preference but does maintain drug self-administration in rats, and induces in vitro markers of tolerance and dependence

Previous work in our laboratories provides preclinical evidence that mixed-action delta/mu receptor glycopeptides have equivalent efficacy for treating pain with reduced side effect profiles compared to widely used mu agonist analgesics such as morphine. This study evaluated the rewarding and reinforcing effects of a lead candidate, mixed-action delta/mu agonist MMP-2200, using a conditioned place preference assay as well as a drug self-administration procedure in rats. In place conditioning studies, rats underwent a 2-week conditioning protocol and were then tested for chamber preference. Rats receiving MMP-2200, at previously determined analgesic doses, could not distinguish between the drug and saline-paired chamber, whereas rats receiving the opioid agonist morphine showed a strong preference for the morphine-paired chamber. In self-administration studies, rats were trained to respond for the high efficacy mu opioid receptor agonist fentanyl on an FR5 schedule of reinforcement. Following complete dose-response determinations for fentanyl, a range of doses of MMP-2200 as well as morphine were tested. Relative to the mu agonist morphine, MMP-2200 maintained a significantly lower number of drug infusions. To begin investigating potential molecular mechanisms for the reduced side effect profile of MMP-2200, we also examined βarrestin2 (βarr2) recruitment and chronic MMP-2200 induced cAMP tolerance and super-activation at the human delta and mu receptors in vitro. MMP-2200 efficaciously recruited βarr2 to both receptors, and induced cAMP tolerance and super-activation equivalent to or greater than morphine at both receptors. The in vivo findings suggest that MMP-2200 may be less reinforcing than morphine but may have some abuse potential. The reduced side effect profile cannot be explained by reduced βarr2 recruitment or reduced cAMP tolerance and superactivation at the monomeric receptors in vitro.

Behavioral and cellular pharmacology characterization of 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(isoquinoline-3'-carboxamido)morphinan (NAQ) as a mu opioid receptor selective ligand

Mu opioid receptor (MOR) selective antagonists and partial agonists have been used for the treatment of opioid abuse and addiction. Our recent efforts on the identification of MOR antagonists have provided several novel leads displaying interesting pharmacological profiles. Among them, 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-[(3'-isoquinolyl)acetamido]morphinan (NAQ) showed sub-nanomolar binding affinity to the MOR with significant selectivity over the delta opioid receptor (DOR) and the kappa opioid receptor (KOR). Its central nervous system penetration capacity together with marginal agonism in the MOR-GTPγS binding assay made it a very interesting molecule for developing novel opioid abuse and addiction therapeutic agents. Therefore, further pharmacological characterization was conducted to fully understand its biological profile. At the molecular and cellular level, NAQ not only induced no translocation of β-arrestin2 to the MOR, but also efficaciously antagonized the effect of DAMGO in MOR-βarr2eGFP-U2OS cells in the β-arrestin2 recruitment assay. At the in vivo level, NAQ displayed a potent inhibition of the analgesic effect of morphine in the tail-flick assay (ID50=1.19 mg/kg). NAQ (10 mg/kg) also significantly decreased the hyper-locomotion induced by acute morphine without inducing any vertical jumps. Meanwhile NAQ precipitated lesser withdrawal symptoms in morphine dependent mice than naloxone. In conclusion, NAQ may represent a new chemical entity for opioid abuse and addiction treatment.

Development of a bioavailable μ opioid receptor (MOPr) agonist, δ opioid receptor (DOPr) antagonist peptide that evokes antinociception without development of acute tolerance

We have previously described a cyclic tetrapeptide, 1, that displays μ opioid receptor (MOPr) agonist and δ opioid receptor (DOPr) antagonist activity, a profile associated with a reduced incidence of opioid tolerance and dependence. Like many peptides, 1 has poor bioavailability. We describe here an analogue of 1 with an added C-terminal β-glucosylserine residue, Ser(β-Glc)NH2, a modification that has previously been shown to improve bioavailability of opioid peptides. The resulting peptide, 4, exhibits full antinociceptive efficacy in the mouse warm water tail withdrawal assay after intraperitoneal administration with potency similar to that of morphine. Further, 4 does not give rise to acute tolerance and thus represents a promising lead for the development of opioid analgesics with reduced side effects.

Can amphipathic helices influence the CNS antinociceptive activity of glycopeptides related to β-endorphin?

Glycosylated β-endorphin analogues of various amphipathicity were studied in vitro and in vivo in mice. Opioid binding affinities of the O-linked glycopeptides (mono- or disaccharides) and unglycosylated peptide controls were measured in human receptors expressed in CHO cells. All were pan-agonists, binding to μ-, δ-, or κ-opioid receptors in the low nanomolar range (2.2-35 nM K(i)'s). The glycoside moiety was required for intravenous (i.v.) but not for intracerebroventricular (i.c.v.) activity. Circular dichroism and NMR indicated the degree of helicity in H2O, aqueous trifluoroethanol, or micelles. Glycosylation was essential for activity after i.v. administration. It was possible to manipulate the degree of helicity by the alteration of only two amino acid residues in the helical address region of the β-endorphin analogues without destroying μ-, δ-, or κ-agonism, but the antinociceptive activity after i.v. administration could not be directly correlated to the degree of helicity in micelles.