Mu-Opioid receptor biased ligands: A safer and painless discovery of analgesics?

Progress in agonist therapy for substance use disorders: Lessons learned from methadone and buprenorphine

Substance use disorders (SUD) are serious public health problems worldwide. Although significant progress has been made in understanding the neurobiology of drug reward and the transition to addiction, effective pharmacotherapies for SUD remain limited and a majority of drug users relapse even after a period of treatment. The United States Food and Drug Administration (FDA) has approved several medications for opioid, nicotine, and alcohol use disorders, whereas none are approved for the treatment of cocaine or other psychostimulant use disorders. The medications approved by the FDA for the treatment of SUD can be divided into two major classes - agonist replacement therapies, such as methadone and buprenorphine for opioid use disorders (OUD), nicotine replacement therapy (NRT) and varenicline for nicotine use disorders (NUD), and antagonist therapies, such as naloxone for opioid overdose and naltrexone for promoting abstinence. In the present review, we primarily focus on the pharmacological rationale of agonist replacement strategies in treatment of opioid dependence, and the potential translation of this rationale to new therapies for cocaine use disorders. We begin by describing the neural mechanisms underlying opioid reward, followed by preclinical and clinical findings supporting the utility of agonist therapies in the treatment of OUD. We then discuss recent progress of agonist therapies for cocaine use disorders based on lessons learned from methadone and buprenorphine. We contend that future studies should identify agonist pharmacotherapies that can facilitate abstinence in patients who are motivated to quit their illicit drug use. Focusing on those that are able to achieve abstinence from cocaine will provide a platform to broaden the effectiveness of medication and psychosocial treatment strategies for this underserved population. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Do Toxic Synthetic Cannabinoid Receptor Agonists Have Signature in Vitro Activity Profiles? A Case Study of AMB-FUBINACA

Recreational consumption of synthetic cannabinoid receptor agonists (SCRAs) is a growing crisis in public health in many parts of the world. AMB-FUBINACA is a member of this class of drugs and is responsible for a large proportion of SCRA-related toxicity both in New Zealand and internationally. Strikingly, little is currently known about the mechanisms by which SCRAs exert toxic effects or whether their activity through the CB1 cannabinoid receptor (the mediator of cannabinoid-related psychoactivity) is sufficient to explain clinical observations. The current study therefore set out to perform a basic molecular pharmacology characterization of AMB-FUBINACA (in comparison to traditional research cannabinoids CP55,940, WIN55,212-2, and Δ9-THC) in fundamental pathways of receptor activity, including cAMP inhibition, pERK activation, ability to drive CB1 internalization, and ability to induce translocation of β-arrestins-1 and -2. Activity pathways were then compared by operational analysis to indicate whether AMB-FUBINACA may be a biased ligand. Results revealed that AMB-FUBINACA is highly efficacious and potent in all pathways assayed. However, surprisingly, bias analysis suggested that Δ9-THC, not AMB-FUBINACA, may be a biased ligand, with it being less active in both arrestin pathways than predicted by the activity of the other ligands tested. These data may help predict molecular characteristics of SCRAs. However, more research is required to determine whether these molecular effects manifest in toxicity at tissue/system level.

Acupuncture/Electroacupuncture as an Alternative in Current Opioid Crisis

Opioid drugs are the first line of defense in severe pain but the adverse effects associated with opioids are considered as a serious issue worldwide. Acupuncture/electroacupuncture is a type of Chinese medicine therapy which is an effective analgesic therapy, well documented in animals and human studies. Electroacupuncture stimulation could release endogenous opioid peptides causing analgesia in a variety of pain models. It can be used as an alternative therapy to control the opioid crisis.

The Mechanisms Involved in Morphine Addiction: An Overview

Opioid use disorder is classified as a chronic recurrent disease of the central nervous system (CNS) which leads to personality disorders, co-morbidities and premature death. It develops as a result of long-term administration of various abused substances, along with morphine. The pharmacological action of morphine is associated with its stimulation of opioid receptors. Opioid receptors are a group of G protein-coupled receptors and activation of these receptors by ligands induces significant molecular changes inside the cell, such as an inhibition of adenylate cyclase activity, activation of potassium channels and reductions of calcium conductance. Recent data indicate that other signalling pathways also may be involved in morphine activity. Among these are phospholipase C, mitogen-activated kinases (MAP kinases) or β-arrestin. The present review focuses on major mechanisms which currently are considered as essential in morphine activity and dependence and may be important for further studies.

Opioid receptors: drivers to addiction?

Drug addiction is a worldwide societal problem and public health burden, and results from recreational drug use that develops into a complex brain disorder. The opioid system, one of the first discovered neuropeptide systems in the history of neuroscience, is central to addiction. Recently, opioid receptors have been propelled back on stage by the rising opioid epidemics, revolutions in G protein-coupled receptor research and fascinating developments in basic neuroscience. This Review discusses rapidly advancing research into the role of opioid receptors in addiction, and addresses the key questions of whether we can kill pain without addiction using mu-opioid-receptor-targeting opiates, how mu- and kappa-opioid receptors operate within the neurocircuitry of addiction and whether we can bridge human and animal opioid research in the field of drug abuse.

Balancing opioid analgesia with the risk of nonmedical opioid use in patients with cancer

The current opioid crisis has brought renewed attention and scrutiny to opioid prescriptions. When patients receiving opioid therapy for pain engage in nonmedical opioid use (NMOU) or diversion, untoward consequences can occur. New evidence suggests that patients with cancer might be at a higher risk of NMOU than was previously thought, but clinical evidence still supports the use of opioid analgesics as the gold standard to treat cancer-related pain, creating a dilemma in patient management. Clinicians are encouraged to adopt a universal precautions approach to patients with cancer receiving opioids, which includes screening all patients; discussing the risks, benefits, adverse effects and alternatives of opioid therapy; and providing education on safe use, storage and disposal. Use of urine drug tests, prescription drug monitoring programmes and close observation of behaviours related to opioid use help to ensure treatment adherence, detect NMOU and support therapeutic decision-making. These measures can optimize the risk-benefit ratio while supporting safe opioid use. In this Review, we examine the role of opioids in cancer pain, the risk of substance use disorder and methods to achieve the right balance between the two in order to ensure safe opioid use.

Possible biased analgesic of hydromorphone through the G protein-over β-arrestin-mediated pathway: cAMP, CellKey™, and receptor internalization analyses

Morphine, fentanyl, and oxycodone are widely used as analgesics, and recently hydromorphone has been approved in Japan. Although all of these are selective for μ-opioid receptors (MORs) and have similar structures, their analgesic potencies and adverse effects (AEs) are diverse. Recent molecular analyses of MOR signaling revealed that the G protein-mediated signaling pathway causes analgesic effects and the β-arrestin-mediated signaling pathway is responsible for AEs. We used several cell-based analyses that selectively measure cellular responses activated by either G protein- or β-arrestin-mediated pathways. GloSensor™ cAMP, CellKey™, and receptor internalization assays were performed with four different types of cells stably expressing differentially labelled MOR. EC₅₀ values measured by cAMP and CellKey™ assays had potencies in the order fentanyl ≤ hydromorphone < morphine ≤ oxycodone, all also exhibiting full agonist responses. However, in the internalization assay, only fentanyl elicited a full agonist response. Hydromorphone had the strongest potency next to fentanyl; however, contribution of the β-arrestin-mediated pathway was small, suggesting that its effect could be biased toward the G protein-mediated pathway. Based on these properties, hydromorphone could be chosen as an effective analgesic.

Prevention of Opioid Abuse and Treatment of Opioid Addiction: Current Status and Future Possibilities

Prescription opioid medications have seen a dramatic rise in misuse and abuse, leading regulators and scientists to develop policies and abuse-deterrent technologies to combat the current opioid epidemic. These abuse-deterrent formulations (ADFs) are intended to deter physical and chemical tampering of opioid-based products, while still providing safe and effective delivery for therapeutic purposes. Even though formulations with varying abuse-deterrent technologies have been approved, questions remain about their effectiveness. While these formulations provide a single means to combat the epidemic, a greater emphasis should be placed on formulations for treatment of addiction and overdose to help those struggling with opioid dependence. This article analyzes various ADFs currently in clinical use and explores potential novel systems for treatment of addiction and prevention of overdose.

Perioperative opioid analgesia-when is enough too much? A review of opioid-induced tolerance and hyperalgesia

Opioids are a mainstay of acute pain management but can have many adverse effects, contributing to problematic long-term use. Opioid tolerance (increased dose needed for analgesia) and opioid-induced hyperalgesia (paradoxical increase in pain with opioid administration) can contribute to both poorly controlled pain and dose escalation. Hyperalgesia is particularly problematic as further opioid prescribing is largely futile. The mechanisms of opioid tolerance and hyperalgesia are complex, involving μ opioid receptor signalling pathways that offer opportunities for novel analgesic alternatives. The intracellular scaffold protein β-arrestin-2 is implicated in tolerance, hyperalgesia, and other opioid side-effects. Development of agonists biased against recruitment of β-arrestin-2 could provide analgesic efficacy with fewer side-effects. Alternative approaches include inhibition of peripheral μ opioid receptors and blockade of downstream signalling mechanisms, such as the non-receptor tyrosine kinase Src or N-methyl-D-aspartate receptors. Furthermore, it is prudent to use multimodal analgesic regimens to reduce reliance on opioids during the perioperative period. In the third paper in this Series we focus on clinical and mechanism-based understanding of tolerance and opioid-induced hyperalgesia, and discuss current and future strategies for pain management.

Current strategies toward safer mu opioid receptor drugs for pain management

INTRODUCTION

Pain relief remains a major public health challenge. The most efficient available painkillers are opioids targeting the mu opioid receptor (MOR). MORs are expressed in the areas of the brain [including pain and respiratory centers] that are important for processing reward and aversion. Thus, MOR activation efficiently alleviates severe pain, but the concomitant reward and respiratory depressant effects pose a threat; patients taking opioids potentially develop opioid addiction and high risk for overdose. Areas covered: Ongoing efforts to generate safer opioid analgesics are reviewed here. The design of biased compounds that trigger MOR induced G protein over β-arrestin signaling, peripheral opioids, drugs targeting MORs in heteromers and drugs enhancing endogenous opioid activity are discussed. Expert opinion: There is evidence that throttling MOR signaling may lead to an era of opioids that are truly efficient painkillers with lower side effects and risk of overdose. However, few of the drugs derived from the advanced approaches outlined here, are getting approval by regulatory committees for use in clinical settings. Thus, there is an urgent need to (i) better clarify mechanisms underlying the hazardous physiological effects of MOR activation, and (ii) fully validate the safety of these new MOR-based therapies.

Strategies for the discovery of biased GPCR ligands

G-protein-coupled receptors (GPCRs) represent important drug targets with complex pharmacological characteristics. Biased signaling represents one important dimension, describing ligand-dependent shifts of naturally imprinted signaling profiles. Because biased GPCR modulators provide potential therapeutic benefits including higher efficiencies and reduced adverse effects, the identification of such ligands as drug candidates is highly desirable. This review aims to provide an overview of the challenges and strategies in the discovery of biased ligands. We show different approaches for biased ligand discovery in the example of G-protein-biased opioid analgesics and discuss possibilities to design biased ligands by targeting extracellular receptor regions.

A bifunctional-biased mu-opioid agonist-neuropeptide FF receptor antagonist as analgesic with improved acute and chronic side effects

Opioid analgesics, such as morphine, oxycodone, and fentanyl, are the cornerstones for treating moderate to severe pain. However, on chronic administration, their efficiency is limited by prominent side effects such as analgesic tolerance and dependence liability. Neuropeptide FF (NPFF) and its receptors (NPFF1R and NPFF2R) are recognized as an important pronociceptive system involved in opioid-induced hyperalgesia and analgesic tolerance. In this article, we report the design of multitarget peptidomimetic compounds that show high-affinity binding to the mu-opioid receptor (MOPr) and NPFFRs. In vitro characterization of these compounds led to identification of KGFF03 and KGFF09 as G-protein-biased MOPr agonists with full agonist or antagonist activity at NPFFRs, respectively. In agreement with their biased MOPr agonism, KGFF03/09 showed reduced respiratory depression in mice, as compared to the unbiased parent opioid agonist KGOP01. Chronic subcutaneous administration of KGOP01 and KGFF03 in mice rapidly induced hyperalgesia and analgesic tolerance, effects that were not observed on chronic treatment with KGFF09. This favorable profile was further confirmed in a model of persistent inflammatory pain. In addition, we showed that KGFF09 induced less physical dependence compared with KGOP01 and KGFF03. Altogether, our data establish that combining, within a single molecule, the G-protein-biased MOPr agonism and NPFFR antagonism have beneficial effects on both acute and chronic side effects of conventional opioid analgesics. This strategy can lead to the development of novel and potent antinociceptive drugs with limited side effects on acute and chronic administration.

The in vivo antinociceptive and μ-opioid receptor activating effects of the combination of N-phenyl-2',4'-dimethyl-4,5'-bi-1,3-thiazol-2-amines and naloxone

Morphine is widely used for the treatment of severe pain. This analgesic effect is mediated principally by the activation of μ-opioid receptors (MOR). However, prolonged activation of MOR also results in tolerance, dependence, addiction, constipation, nausea, sedation, and respiratory depression. To address this problem, we sought alternative ways to activate MOR - either by use of novel ligands, or via a novel activation mechanism. To this end, a series of compounds were screened using a sensitive CHO-K1/MOR/Gα15 cell-based FLIPR^® calcium high-throughput screening (HTS) assay, and the bithiazole compound 5a was identified as being able activate MOR in combination with naloxone. Structural modifications of 5a resulted in the discovery of lead compound 5j, which could effectively activate MOR in combination with the MOR antagonist naloxone or naltrexone. In vivo, naloxone in combination with 100 mg/kg of compound 5j elicited antinociception in a mouse tail-flick model with an ED₅₀ of 17.5 ± 4 mg/kg. These results strongly suggest that the mechanism by which the 5j/naloxone combination activates MOR is worthy of further study, as its discovery has the potential to yield an entirely novel class of analgesics.

Synthesis and Biological Evaluation of Fentanyl Analogues Modified at Phenyl Groups with Alkyls

A series of fentanyl analogues modified at the phenyl group of the phenethyl with alkyl and/or hydroxyl and alkoxy, and the phenyl group in the anilido moiety replaced with benzyl or substituted benzyl, were synthesized. The in vitro opioid receptor functional activity of these compounds was evaluated by assessment of their ability to modulate forskolin-stimulated cAMP accumulation and by their ability to induce β-arrestin2 recruitment. Compound 12 is a potent μ-opioid (MOP) receptor agonist, a potent κ-opioid (KOP) receptor antagonist with weak β-arrestin2 recruitment activity. Compounds 10 and 11 are potent MOP receptor agonists with weak δ-opioid (DOP) receptor antagonist activity and moderate KOP receptor antagonist activity as well as weak β-arrestin2 recruitment activity at the MOP receptor. These compounds are promising leads for discovery of potent opioid analgesics with reduced side effects relative to clinically available strong opioid analgesics.

Emerging Insights into Mu Opioid Pharmacology

Opioid analgesics, most of which act through mu opioid receptors, have long represented valuable therapeutic agents to treat severe pain. Concerted drug development efforts for over a 100 years have resulted in a large variety of opioid analgesics used in the clinic, but all of them continue to exhibit the side effects, especially respiratory depression, that have long plagued the use of morphine. The recent explosion in fatalities resulting from overdose of prescription and synthetic opioids has dramatically increased the need for safer analgesics, but recent developments in mu receptor research have provided new strategies to develop such drugs. This chapter reviews recent advances in developing novel opioid analgesics from an understanding of mu receptor structure and function. This includes a summary of the mechanism of agonist binding deduced from the crystal structure of mu receptors. It will also highlight the development of novel agonist mechanisms, including biased agonists, bivalent ligands, and allosteric modulators of mu receptor function, and describe how receptor phosphorylation modulates these pathways. Finally, it will summarize research on the alternative pre-mRNA splicing mechanisms that produces a multiplicity of mu receptor isoforms. Many of these isoforms exhibit different pharmacological specificities and brain circuitry localization, thus providing an opportunity to develop novel drugs with increased therapeutic windows.

Endogenous opioid system dysregulation in depression: implications for new therapeutic approaches

The United States is in the midst of an opioid addiction and overdose crisis precipitated and exacerbated by use of prescription opioid medicines. The majority of opioid prescriptions are dispensed to patients with comorbid mood disorders including major depressive disorder (MDD). A growing body of research indicates that the endogenous opioid system is directly involved in the regulation of mood and is dysregulated in MDD. This involvement of the endogenous opioid system may underlie the disproportionate use of opioids among patients with mood disorders. Emerging approaches to address endogenous opioid dysregulation in MDD may yield novel therapeutics that have a low or absent risk of abuse and addiction relative to µ-opioid agonists. Moreover, agents targeting the endogenous opioid system would be expected to yield clinical benefits qualitatively different from conventional monaminergic antidepressants. The development of safe and effective agents to treat MDD-associated endogenous opioid dysregulation may represent a distinct and currently underappreciated means of addressing treatment resistant depression with the potential to attenuate the on-going opioid crisis.

In Vitro Pharmacological Characterization of Buprenorphine, Samidorphan, and Combinations Being Developed as an Adjunctive Treatment of Major Depressive Disorder

A combination of buprenorphine (BUP) and samidorphan (SAM) at a 1:1 (mg/mg) fixed-ratio dose is being investigated as an adjunctive treatment of major depressive disorder (BUP/SAM, ALKS 5461). Both [3H]BUP and [3H]SAM bound to the μ-, κ-, and δ-opioid receptors (MOR, KOR, and DOR, respectively) with Kd values of 3 nM or less. [3H]BUP dissociated from the MOR more slowly than [3H]SAM did. In the [35S]GTPγS assay, BUP was a partial agonist at the MOR, KOR, and DOR. SAM was an antagonist at the MOR and a partial agonist at the KOR and DOR. The pharmacology of the combination of SAM and BUP was characterized at ratios like the molar ratios of both compounds at steady state in humans. In all assessments, SAM reduced the efficacy of BUP at the MOR without altering its potency. At the KOR, SAM had no significant effect on the activity of BUP. In bioluminescent resonance energy transfer assays, SAM, naltrexone, and naloxone were partial agonists when the MOR was coupled to the Gα oB and Gα z, and were antagonists when coupled to Gα i At the KOR, SAM was a partial agonist activating Gα oA and Gα oB and a full agonist in stimulating Gα z SAM inhibited BUP's recruitment of β-arrestin to the MOR, suggesting an attenuation of BUP's efficacy in activating G proteins correlated with an inhibition of β-arrestin recruitment. The collective data suggest that SAM attenuates the efficacy of BUP under all conditions tested at the MOR and DOR but had little effect on BUP activity at the KOR.

Polymorphisms associated with fentanyl pharmacokinetics, pharmacodynamics and adverse effects

Fentanyl is an agonist of the μ-opioid receptor commonly used in the treatment of moderate-severe pain. In order to study whether pharmacogenetics explains some of the variability in the response to fentanyl, several genes related to fentanyl receptors, transporters and metabolic enzymes have been analysed. Thirty-five healthy volunteers (19 men and 16 women) receiving a single 300 μg oral dose of fentanyl were genotyped for 9 polymorphisms in cytochrome P450 (CYP) enzymes (CYP3A4 and CYP3A5), ATP-binding cassette subfamily B member 1 (ABCB1), opioid receptor mu 1 (OPRM1), catechol-O-methyltransferase (COMT) and adrenoceptor beta 2 (ADRB2) by real-time PCR. Fentanyl concentrations were measured by ultra-performance liquid chromatography combined with tandem mass spectrometry (UPLC-MS/MS). Fentanyl pharmacokinetics is affected by sex. Carriers of the CYP3A4*22 allele, which is known to reduce the mRNA expression, showed higher area under the concentration-time curve (AUC) and lower clearance (Cl) values. Although this finding might be of importance, its validity needs to be confirmed in other similar settings. Furthermore, carriers of the ABCB1 C1236T T/T genotype presented a lower AUC and higher Cl, as well as lower half-life (T_1/2 ). As volunteers were blocked with naltrexone, the effect of fentanyl on pharmacodynamics might be biased; however, we could observe that fentanyl had a hypotensive effect. Moreover, ADRB2 C523A A allele carriers showed a tendency towards reducing systolic blood pressure. Likewise, OPRM1 and COMT minor allele variants were risk factors for the development of somnolence. CYP3A5*3, ABCB1 C3435T and ABCB1 G2677T/A were not associated with fentanyl's pharmacokinetics, pharmacodynamics and safety profile.

DARK Classics in Chemical Neuroscience: Fentanyl

Fentanyl rose to prominence as an alternative analgesic to morphine nearly 50 years ago; today, fentanyl has re-emerged as a dangerous recreational substance. The increased potency and analgesic effect of fentanyl are advantageous in the treatment of pain but are also responsible for the rise in unintentional opioid overdose deaths. In response to this crisis, fentanyl, its analogues, and even precursors are under heightened regulatory scrutiny. Despite this controversial history, derivatization of fentanyl has resulted in numerous synthetic analogues that provide valuable insights into opioid receptor binding and signaling events. In this review, the impact of fentanyl on chemical neuroscience is shown through its synthesis and properties, manufacturing, metabolism, pharmacology, approved and off-label indications, adverse effects, and the responsibility it has in the opioid epidemic.

In vitro and in vivo Pharmacological Activities of 14-O-Phenylpropyloxymorphone, a Potent Mixed Mu/Delta/Kappa-Opioid Receptor Agonist With Reduced Constipation in Mice

Pain, particularly chronic pain, is still an unsolved medical condition. Central goals in pain control are to provide analgesia of adequate efficacy and to reduce complications associated with the currently available drugs. Opioids are the mainstay for the treatment of moderate to severe pain. However, opioid pain medications also cause detrimental side effects, thus highlighting the need of innovative and safer analgesics. Opioids mediate their actions via the activation of opioid receptors, with the mu-opioid receptor as the primary target for analgesia, but also for side effects. One long-standing focus of drug discovery is the pursuit for new opioids exhibiting a favorable dissociation between analgesia and adverse effects. In this study, we describe the in vitro and in vivo pharmacological profiles of the 14-O-phenylpropyl substituted analog of the mu-opioid agonist 14-O-methyloxymorphone (14-OMO). The consequence of the substitution of the 14-O-methyl in 14-OMO with a 14-O-phenylpropyl group on in vitro binding and functional activity, and in vivo behavioral properties (nociception and gastrointestinal motility) was investigated. In binding studies, 14-O-phenylpropyloxymorphone (POMO) displayed very high affinity at mu-, delta-, and kappa-opioid receptors (Ki values in nM, mu:delta:kappa = 0.073:0.13:0.30) in rodent brain membranes, with complete loss of mu-receptor selectivity compared to 14-OMO. In guinea-pig ileum and mouse vas deferens bioassays, POMO was a highly efficacious and full agonist, being more potent than 14-OMO. In the [35S]GTPγS binding assays with membranes from CHO cells expressing human opioid receptors, POMO was a potent mu/delta-receptor full agonist and a kappa-receptor partial agonist. In vivo, POMO was highly effective in acute thermal nociception (hot-plate test, AD50 = 0.7 nmol/kg) in mice after subcutaneous administration, with over 70- and 9000-fold increased potency than 14-OMO and morphine, respectively. POMO-induced antinociception is mediated through the activation of the mu-opioid receptor, and it does not involve delta- and kappa-opioid receptors. In the charcoal test, POMO produced fourfold less inhibition of the gastrointestinal transit than 14-OMO and morphine. In summary, POMO emerges as a new potent mixed mu/delta/kappa-opioid receptor agonist with reduced liability to cause constipation at antinociceptive doses.

A bifunctional nociceptin and mu opioid receptor agonist is analgesic without opioid side effects in nonhuman primates

Misuse of prescription opioids, opioid addiction, and overdose underscore the urgent need for developing addiction-free effective medications for treating severe pain. Mu opioid peptide (MOP) receptor agonists provide very effective pain relief. However, severe side effects limit their use in the clinical setting. Agonists of the nociceptin/orphanin FQ peptide (NOP) receptor have been shown to modulate the antinociceptive and reinforcing effects of MOP agonists. We report the discovery and development of a bifunctional NOP/MOP receptor agonist, AT-121, which has partial agonist activity at both NOP and MOP receptors. AT-121 suppressed oxycodone's reinforcing effects and exerted morphine-like analgesic effects in nonhuman primates. AT-121 treatment did not induce side effects commonly associated with opioids, such as respiratory depression, abuse potential, opioid-induced hyperalgesia, and physical dependence. Our results in nonhuman primates suggest that bifunctional NOP/MOP agonists with the appropriate balance of NOP and MOP agonist activity may provide a dual therapeutic action for safe and effective pain relief and treating prescription opioid abuse.

Current Concepts and Treatments of Schizophrenia

Schizophrenia is a debilitating mental illness which involves three groups of symptoms, i.e., positive, negative and cognitive, and has major public health implications. According to various sources, it affects up to 1% of the population. The pathomechanism of schizophrenia is not fully understood and current antipsychotics are characterized by severe limitations. Firstly, these treatments are efficient for about half of patients only. Secondly, they ameliorate mainly positive symptoms (e.g., hallucinations and thought disorders which are the core of the disease) but negative (e.g., flat affect and social withdrawal) and cognitive (e.g., learning and attention disorders) symptoms remain untreated. Thirdly, they involve severe neurological and metabolic side effects and may lead to sexual dysfunction or agranulocytosis (clozapine). It is generally agreed that the interactions of antipsychotics with various neurotransmitter receptors are responsible for their effects to treat schizophrenia symptoms. In particular, several G protein-coupled receptors (GPCRs), mainly dopamine, serotonin and adrenaline receptors, are traditional molecular targets for antipsychotics. Comprehensive research on GPCRs resulted in the exploration of novel important signaling mechanisms of GPCRs which are crucial for drug discovery: intentionally non-selective multi-target compounds, allosteric modulators, functionally selective compounds and receptor oligomerization. In this review, we cover current hypotheses of schizophrenia, involving different neurotransmitter systems, discuss available treatments and present novel concepts in schizophrenia and its treatment, involving mainly novel mechanisms of GPCRs signaling.

Emerging strategies targeting CB2 cannabinoid receptor: Biased agonism and allosterism

During these last years, the CB₂ cannabinoid receptor has emerged as a potential anti-inflammatory target in diseases such as multiple sclerosis, amyotrophic lateral sclerosis, Huntington's disease, ischemic stroke, autoimmune diseases, osteoporosis, and cancer. However, the development of clinically useful CB₂ agonists reveals to be very challenging. Allosterism and biased-signaling mechanisms at CB₂ receptor may offer new avenues for the development of improved CB₂ receptor-targeted therapies. Although there has been some exploration of CB₁ receptor activation by new CB₁ allosteric or biased-signaling ligands, the CB₂ receptor is still at initial stages in this domain. In an effort to understand the molecular basis behind these pharmacological approaches, we have analyzed and summarized the structural data reported so far at CB₂ receptor.

In vitro and in vivo activity of cyclopeptide Dmt-c[d-Lys-Phe-Asp]NH2, a mu opioid receptor agonist biased toward β-arrestin

Morphine and related drugs, which are the most effective analgesics for the relief of severe pain, act through activating opioid receptors. The endogenous ligands of these receptors are opioid peptides which cannot be used as antinociceptive agents due to their low bioactivity and stability in biological fluids. The major goal of opioid research is to understand the mechanism of action of opioid receptor agonists in order to improve therapeutic utility of opioids. Analgesic effects of morphine are mediated mostly through activation of the mu opioid receptor. However, in the search for safer and more effective drug candidates, analogs with mixed opioid receptor profile gained a lot of interest. Recently, the concept of biased agonists able to differentially activate GPCR downstream pathways, became a new approach in the design of novel drug candidates. It is hypothesized that compounds promoting G-protein signaling may produce analgesia while β-arrestin recruitment may be responsible for opioid side effects. In this report we showed that replacement of the tyrosine residue in the mu-selective ligand Tyr-c[d-Lys-Phe-Asp]NH₂ with 2',6'-dimethyltyrosine (Dmt) produced a cyclopeptide Dmt-c[d-Lys-Phe-Asp]NH₂ with mu/delta opioid receptor agonist profile. This analog showed improved antinociception in the hot-plate test, probably due to the simultaneous activation of mu and delta receptors but also significantly inhibited the gastrointestinal transit. Using the bioluminescence resonance energy transfer (BRET) assay it was shown that this analog was a mu receptor agonist biased toward β-arrestin. β-Arrestin-dependent signaling is most likely responsible for the observed inhibition of gastrointestinal motility exerted by the novel cyclopeptide.

The novel μ-opioid receptor agonist PZM21 depresses respiration and induces tolerance to antinociception

BACKGROUND AND PURPOSE

PZM21 is a novel μ-opioid receptor ligand that has been reported to induce minimal arrestin recruitment and be devoid of the respiratory depressant effects characteristic of classical μ receptor ligands such as morphine. We have re-examined the signalling profile of PZM21 and its ability to depress respiration.

EXPERIMENTAL APPROACH

G protein (Gi ) activation and arrestin-3 translocation were measured in vitro, using BRET assays, in HEK 293 cells expressing μ receptors. Respiration (rate and tidal volume) was measured in awake, freely moving mice by whole-body plethysmography, and antinociception was measured by the hot plate test.

KEY RESULTS

PZM21 (10-9 - 3 × 10-5  M) produced concentration-dependent Gi activation and arrestin-3 translocation. Comparison with responses evoked by morphine and DAMGO revealed that PZM21 was a low efficacy agonist in both signalling assays. PZM21 (10-80 mg·kg-1 ) depressed respiration in a dose-dependent manner. The respiratory depression was due to a decrease in the rate of breathing not a decrease in tidal volume. On repeated daily administration of PZM21 (twice daily doses of 40 mg·kg-1 ), complete tolerance developed to the antinociceptive effect of PZM21 over 3 days but no tolerance developed to its respiratory depressant effect.

CONCLUSION AND IMPLICATIONS

These data demonstrate that PZM21 is a low efficacy μ receptor agonist for both G protein and arrestin signalling. Contrary to a previous report, PZM21 depresses respiration in a manner similar to morphine, the classical opioid receptor agonist.

A real time screening assay for cannabinoid CB1 receptor-mediated signaling

The cannabinoid CB1 receptor is expressed throughout the central nervous system where it functions to regulate neurotransmitter release and synaptic plasticity. While the CB1 receptor has been identified as a target for both natural and synthetic cannabinoids, the specific downstream signaling pathways activated by these various ligands have not been fully described. In this study, we developed a real-time membrane potential fluorescent assay for cannabinoids using pituitary AtT20 cells that endogenously express G protein-gated inward rectifier K⁺ (GIRK) channels and were stably transfected with the CB1 receptor using a recombinant lentivirus. In whole-cell patch clamp experiments application of the cannabinoid agonist WIN 55,212-2 to AtT20 cells expressing the CB1 receptor (AtT20/CB1) activated GIRK currents that were blocked by BaCl₂. WIN 55,212-2 activation of the GIRK channels was associated with a time- and concentration-dependent (EC₅₀ = 309 nM) hyperpolarization of the membrane potential in the AtT20/CB1 cells when monitored using a fluorescent membrane potential-sensitive dye. The WIN 55,212-2-induced fluorescent signal was inhibited by pretreatment of the cells with either the GIRK channel blocker tertiapin-Q or the CB1 receptor antagonist SR141716. The cannabinoids displayed a response of WIN 55,212-2 ≈ anandamide (AEA) > CP 55,940 > Δ⁹-tetrahydrocannabinol (THC) when maximal concentrations of the four ligands were tested in the assay. Thus, the AtT20/CB1 cell fluorescent assay will provide a straightforward and efficient methodology for examining cannabinoid-stimulated G_i signaling.

Can oliceridine (TRV130), an ideal novel µ receptor G protein pathway selective (µ-GPS) modulator, provide analgesia without opioid-related adverse reactions?

All drugs have both favorable therapeutic and untoward adverse effects. Conventional opioid analgesics possess both analgesia and adverse reactions, such as nausea, vomiting, and respiratory depression. The opioid ligand binds to µ opioid receptor and non-selectively activates two intracellular signaling pathways: the G protein pathway induce analgesia, while the β-arrestin pathway is responsible for the opioid-related adverse reactions. An ideal opioid should activate the G protein pathway while deactivating the β-arrestin pathway. Oliceridine (TRV130) has a novel characteristic mechanism on the action of the µ receptor G protein pathway selective (µ-GPS) modulation. Even though adverse reactions (ADRs) are significantly attenuated, while the analgesic effect is augmented, the some residual ADRs persist. Consequently, a G protein biased µ opioid ligand, oliceridine, improves the therapeutic index owing to increased analgesia with decreased adverse events. This review article provides a brief history, mechanism of action, pharmacokinetics, pharmacodynamics, and ADRs of oliceridine.

Microglia Express Mu Opioid Receptor: Insights From Transcriptomics and Fluorescent Reporter Mice

Background: Microglia activation contributes to chronic pain and to the adverse effects of opiate use such as analgesic tolerance and opioid-induced hyperalgesia. Both mu opioid receptor (MOR) encoded by Oprm1/OPRM1 gene and toll like receptor 4 (TLR4) have been reported to mediate these morphine effects and a current question is whether microglia express the Oprm1 transcript and MOR protein. The aim of this study was to characterize Oprm1-MOR expression in naive murine and human microglia, combining transcriptomics datasets previously published by other groups with our own imaging study using the Cx3cr1-eGFP-MOR-mCherry reporter mouse line. Methods: We analyzed microglial Oprm1/OPRM1 expression obtained from transcriptomics datasets, focusing on ex vivo studies from adult wild-type animals and adult post-mortem human cerebral cortex. Oprm1, as well as co-regulated gene sets were examined. The expression of MOR in microglia was also investigated using our novel fluorescent Cx3cr1-eGFP-MOR-mcherry reporter mouse line. We determined whether CX3cR1-eGFP positive microglial cells expressed MOR-mCherry protein by imaging various brain areas including the Frontal Cortex, Nucleus Accumbens, Ventral Tegmental Area, Central Amygdala, and Periaqueductal Gray matter, as well as spinal cord. Results: Oprm1 expression was found in all 12 microglia datasets from mouse whole brain, in 7 out of 8 from cerebral cortex, 3 out of 4 from hippocampus, 1 out of 1 from striatum, and 4 out of 5 from mouse or rat spinal cord. OPRM1 was expressed in 16 out of 17 microglia transcriptomes from human cerebral cortex. In Cx3cr1-eGFP-MOR-mCherry mice, the percentage of MOR-positive microglial cells ranged between 35.4 and 51.6% in the different brain areas, and between 36.8 and 42.4% in the spinal cord. Conclusion: The comparative analysis of the microglia transcriptomes indicates that Oprm1/OPRM1 transcripts are expressed in microglia. The investigation of Cx3cr1-eGFP-MOR-mCherry mice also shows microglial expression of MOR proteinin the brain and spine. These results corroborate functional studies showing the actions of MOR agonists on microglia and suppression of these effects by MOR-selective antagonists or MOR knockdown.

Management of Opioid Addiction With Opioid Substitution Treatments: Beyond Methadone and Buprenorphine

With the opioid crisis in North America, opioid addiction has come in the spotlight and reveals the weakness of the current treatments. Two main opioid substitution therapies (OST) exist: buprenorphine and methadone. These two molecules are mu opioid receptor agonists but with different pharmacodynamic and pharmacokinetic properties. In this review, we will go through these properties and see how they could explain why these medications are recognized for their efficacy in treating opioid addiction but also if they could account for the side effects especially for a long-term use. From this critical analysis, we will try to delineate some guidelines for the design of future OST.