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.

Opioid research in the time of the opioid crisis

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.

Delta Opioid Receptor-Mediated Antidepressant-Like Effects of Diprenorphine in Mice

Major depressive disorder is a highly common disorder, with a lifetime prevalence in the United States of approximately 21%. Traditional antidepressant treatments are limited by a delayed onset of action and minimal efficacy in some patients. Ketamine is effective and fast-acting, but there are concerns over its abuse liability. Thus, there is a need for safe, fast-acting antidepressant drugs. The opioid buprenorphine shows promise but also has abuse liability due to its mu-agonist component. Preclinical evidence indicates that the delta-opioid system contributes to mood disorders, and delta-opioid agonists are effective in preclinical models of depression- and anxiety-like states. In this study, we test the hypothesis that the mu-opioid antagonist diprenorphine by virtue of its partial delta opioid agonist activity may offer a beneficial profile for an antidepressant medication without abuse liability. Diprenorphine was confirmed to bind with high affinity to all three opioid receptors, and functional experiments for G protein activation verified diprenorphine to be a partial agonist at delta- and kappa-opioid receptors and a mu-antagonist. Studies in C57BL/6 mice demonstrated that an acute dose of diprenorphine produced antidepressant-like effects in the tail suspension test and the novelty-induced hypophagia test that were inhibited in the presence of the delta-selective antagonist, naltrindole. Diprenorphine did not produce convulsions, a side effect of many delta agonists but rather inhibited convulsions caused by the full delta agonist SNC80; however, diprenorphine did potentiate pentylenetetrazole-induced convulsions. Diprenorphine, and compounds with a similar pharmacological profile, may provide efficient and safe rapidly acting antidepressants. SIGNIFICANCE STATEMENT: The management of major depressive disorder, particularly treatment-resistant depression, is a significant unmet medical need. Here we show that the opioid diprenorphine, a compound with mu-opioid receptor antagonist activity and delta- and kappa-opioid receptor partial agonist activities, has rapid onset antidepressant-like activity in animal models. Diprenorphine and compounds with a similar pharmacological profile to diprenorphine should be explored as novel antidepressant drugs.

PACAP and acetylcholine cause distinct Ca2+ signals and secretory responses in chromaffin cells

The adrenomedullary chromaffin cell transduces chemical messages into outputs that regulate end organ function throughout the periphery. At least two important neurotransmitters are released by innervating preganglionic neurons to stimulate exocytosis in the chromaffin cell-acetylcholine (ACh) and pituitary adenylate cyclase activating polypeptide (PACAP). Although PACAP is widely acknowledged as an important secretagogue in this system, the pathway coupling PACAP stimulation to chromaffin cell secretion is poorly understood. The goal of this study is to address this knowledge gap. Here, it is shown that PACAP activates a Gαs-coupled pathway that must signal through phospholipase C ε (PLCε) to drive Ca2+ entry and exocytosis. PACAP stimulation causes a complex pattern of Ca2+ signals in chromaffin cells, leading to a sustained secretory response that is kinetically distinct from the form stimulated by ACh. Exocytosis caused by PACAP is associated with slower release of peptide cargo than exocytosis stimulated by ACh. Importantly, only the secretory response to PACAP, not ACh, is eliminated in cells lacking PLCε expression. The data show that ACh and PACAP, acting through distinct signaling pathways, enable nuanced and variable secretory outputs from chromaffin cells.

Opioid Antagonists from the Orvinol Series as Potential Reversal Agents for Opioid Overdose

The opioid crisis continues to claim many lives, with a particular issue being the ready availability and use (whether intentional or accidental) of fentanyl and fentanyl analogues. Fentanyl is both potent and longer-acting than naloxone, the standard of care for overdose reversal, making it especially deadly. Consequently, there is interest in opioid reversal agents that are better able to counter its effects. The orvinol series of ligands are known for their high-affinity binding to opioid receptors and often extended duration of action; generally, compounds on this scaffold show agonist activity at the kappa and the mu-opioid receptor. Diprenorphine is an unusual member of this series being an antagonist at mu and only a partial agonist at kappa-opioid receptors. In this study, an orvinol antagonist, 14, was designed and synthesized that shows no agonist activity in vitro and is at least as good as naloxone at reversing the effects of mu-opioid receptor agonists in vivo.

Finding the Perfect Fit: Conformational Biosensors to Determine the Efficacy of GPCR Ligands

G protein-coupled receptors (GPCRs) are highly druggable targets that adopt numerous conformations. A ligand's ability to stabilize specific conformation(s) of its cognate receptor determines its efficacy or ability to produce a biological response. Identifying ligands that produce different receptor conformations and potentially discrete pharmacological effects (e.g., biased agonists, partial agonists, antagonists, allosteric modulators) is a major goal in drug discovery and necessary to develop drugs with better effectiveness and fewer side effects. Fortunately, direct measurements of ligand efficacy, via receptor conformational changes are possible with the recent development of conformational biosensors. In this review, we discuss classical efficacy models, including the two-state model, the ternary-complex model, and multistate models. We describe how nanobody-, transducer-, and receptor-based conformational biosensors detect and/or stabilize specific GPCR conformations to identify ligands with different levels of efficacy. In particular, conformational biosensors provide the potential to identify and/or characterize therapeutically desirable but often difficult to measure conformations of receptors faster and better than current methods. For drug discovery/development, several recent proof-of-principle studies have optimized conformational biosensors for high-throughput screening (HTS) platforms. However, their widespread use is limited by the fact that few sensors are reliably capable of detecting low-frequency conformations and technically demanding assay conditions. Nonetheless, conformational biosensors do help identify desirable ligands such as allosteric modulators, biased ligands, or partial agonists in a single assay, representing a distinct advantage over classical methods.

OREX-1038: a potential new treatment for pain with low abuse liability and limited adverse effects

Drugs targeting mu opioid receptors are the mainstay of clinical practice for treating moderate-to-severe pain. While they can offer excellent analgesia, their use can be limited by adverse effects, including constipation, respiratory depression, tolerance, and abuse liability. Multifunctional ligands acting at mu opioid and nociceptin/orphanin FQ peptide receptors might provide antinociception with substantially improved adverse-effect profiles. This study explored one of these ligands, OREX-1038 (BU10038), in several assays in rodents and nonhuman primates. Binding and functional studies confirmed OREX-1038 to be a low-efficacy agonist at mu opioid and nociceptin/orphanin FQ peptide receptors and an antagonist at delta and kappa opioid receptors with selectivity for opioid receptors over other proteins. OREX-1038 had long-acting antinociceptive effects in postsurgical and complete Freund's adjuvant (CFA)-induced thermal hyperalgesia assays in rats and a warm water tail-withdrawal assay in monkeys. OREX-1038 was active for at least 24 h in each antinociception assay, and its effects in monkeys did not diminish over 22 days of daily administration. This activity was coupled with limited effects on physiological signs (arterial pressure, heart rate, and body temperature) and no evidence of withdrawal after administration of naltrexone or discontinuation of treatment in monkeys receiving OREX-1038 daily. Over a range of doses, OREX-1038 was only transiently self-administered, which diminished rapidly to nonsignificant levels; overall, both OREX-1038 and buprenorphine maintained less responding than remifentanil. These results support the concept of dual mu and nociceptin/orphanin FQ peptide receptor partial agonists having improved pharmacological profiles compared with opioids currently used to treat pain.

In silico analysis of SARS-CoV-2 proteins as targets for clinically available drugs

The ongoing pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) requires treatments with rapid clinical translatability. Here we develop a multi-target and multi-ligand virtual screening method to identify FDA-approved drugs with potential activity against SARS-CoV-2 at traditional and understudied viral targets. 1,268 FDA-approved small molecule drugs were docked to 47 putative binding sites across 23 SARS-CoV-2 proteins. We compared drugs between binding sites and filtered out compounds that had no reported activity in an in vitro screen against SARS-CoV-2 infection of human liver (Huh-7) cells. This identified 17 "high-confidence", and 97 "medium-confidence" drug-site pairs. The "high-confidence" group was subjected to molecular dynamics simulations to yield six compounds with stable binding poses at their optimal target proteins. Three drugs-amprenavir, levomefolic acid, and calcipotriol-were predicted to bind to 3 different sites on the spike protein, domperidone to the Mac1 domain of the non-structural protein (Nsp) 3, avanafil to Nsp15, and nintedanib to the nucleocapsid protein involved in packaging the viral RNA. Our "two-way" virtual docking screen also provides a framework to prioritize drugs for testing in future emergencies requiring rapidly available clinical drugs and/or treating diseases where a moderate number of targets are known.

Mixed-solvent molecular dynamics simulation-based discovery of a putative allosteric site on regulator of G protein signaling 4

Regulator of G protein signaling 4 (RGS4) is an intracellular protein that binds to the G_α subunit ofheterotrimeric G proteins and aids in terminating G protein coupled receptor signaling. RGS4 has been implicated in pain, schizophrenia, and the control of cardiac contractility. Inhibitors of RGS4 have been developed but bind covalently to cysteine residues on the protein. Therefore, we sought to identify alternative druggable sites on RGS4 using mixed-solvent molecular dynamics simulations, which employ low concentrations of organic probes to identify druggable hotspots on the protein. Pseudo-ligands were placed in consensus hotspots, and perturbation with normal mode analysis led to the identification and characterization of a putative allosteric site, which would be invaluable for structure-based drug design of non-covalent, small molecule inhibitors. Future studies on the mechanism of this allostery will aid in the development of novel therapeutics targeting RGS4.

Allosteric Modulator Leads Hiding in Plain Site: Developing Peptide and Peptidomimetics as GPCR Allosteric Modulators

Allosteric modulators (AMs) of G-protein coupled receptors (GPCRs) are desirable drug targets because they can produce fewer on-target side effects, improved selectivity, and better biological specificity (e.g., biased signaling or probe dependence) than orthosteric drugs. An underappreciated source for identifying AM leads are peptides and proteins-many of which were evolutionarily selected as AMs-derived from endogenous protein-protein interactions (e.g., transducer/accessory proteins), intramolecular receptor contacts (e.g., pepducins or extracellular domains), endogenous peptides, and exogenous libraries (e.g., nanobodies or conotoxins). Peptides offer distinct advantages over small molecules, including high affinity, good tolerability, and good bioactivity, and specific disadvantages, including relatively poor metabolic stability and bioavailability. Peptidomimetics are molecules that combine the advantages of both peptides and small molecules by mimicking the peptide's chemical features responsible for bioactivity while improving its druggability. This review 1) discusses sources and strategies to identify peptide/peptidomimetic AMs, 2) overviews strategies to convert a peptide lead into more drug-like "peptidomimetic," and 3) critically analyzes the advantages, disadvantages, and future directions of peptidomimetic AMs. While small molecules will and should play a vital role in AM drug discovery, peptidomimetics can complement and even exceed the advantages of small molecules, depending on the target, site, lead, and associated factors.

The Buprenorphine Analogue BU10119 Attenuates Drug-Primed and Stress-Induced Cocaine Reinstatement in Mice

There are no Food and Drug Administration-approved medications for cocaine use disorder, including relapse. The μ-opioid receptor (MOPr) partial agonist buprenorphine alone or in combination with naltrexone has been shown to reduce cocaine-positive urine tests and cocaine seeking in rodents. However, there are concerns over the abuse liability of buprenorphine. Buprenorphine's partial agonist and antagonist activity at the nociception receptor (NOPr) and κ-opioid receptor (KOPr), respectively, may contribute to its ability to inhibit cocaine seeking. Thus, we hypothesized that a buprenorphine derivative that exhibits antagonist activity at MOPr and KOPr with enhanced agonist activity at the NOPr could provide a more effective treatment. Here we compare the pharmacology of buprenorphine and two analogs, BU10119 and BU12004, in assays for antinociception and for cocaine- and stress-primed reinstatement in the conditioned place preference paradigm. In vitro and in vivo assays showed that BU10119 acts as an antagonist at MOPr, KOPr, and δ-opioid receptor (DOPr) and a partial agonist at NOPr, whereas BU12004 showed MOPr partial agonist activity and DOPr, KOPr, and NOPr antagonism. BU10119 and buprenorphine but not BU12004 lessened cocaine-primed reinstatement. In contrast, BU10119, BU12004, and buprenorphine blocked stress-primed reinstatement. The selective NOPr agonist SCH221510 but not naloxone decreased cocaine-primed reinstatement. Together, these findings are consistent with the concept that NOPr agonism contributes to the ability of BU10119 and buprenorphine to attenuate reinstatement of cocaine-conditioned place preference in mice. The findings support the development of buprenorphine analogs lacking MOPr agonism with increased NOPr agonism for relapse prevention to cocaine addiction. SIGNIFICANCE STATEMENT: There are no Food and Drug Administration-approved medications for cocaine use disorder. Buprenorphine has shown promise as a treatment for cocaine relapse prevention; however, there are concerns over the abuse liability of buprenorphine. Here we show a buprenorphine analogue, BU10119, which lacks μ-opioid receptor agonism and inhibits cocaine-primed and stress-primed reinstatement in a conditioned place-preference paradigm. The results suggest the development of BU10119 for the management of relapse to cocaine seeking.

Mice Expressing Regulators of G protein Signaling-insensitive Gαo Define Roles of μ Opioid Receptor Gαo and Gαi Subunit Coupling in Inhibition of Presynaptic GABA Release

Regulators of G protein signaling (RGS) proteins modulate signaling by G protein-coupled receptors. Using a knock-in transgenic mouse model with a mutation in Gαo that does not bind RGS proteins (RGS-insensitive), we determined the effect of RGS proteins on presynaptic μ opioid receptor (MOR)-mediated inhibition of GABA release in the ventrolateral periaqueductal gray (vlPAG). The MOR agonists [d-Ala², N-MePhe⁴, Gly-ol]-enkephalin (DAMGO) and met-enkephalin (ME) inhibited evoked inhibitory postsynaptic currents (eIPSCs) in the RGS-insensitive mice compared with wild-type (WT) littermates, respectively. Fentanyl inhibited eIPSCs similarly in both WT and RGS-insensitive mice. There were no differences in opioid agonist inhibition of spontaneous GABA release between the genotypes. To further probe the mechanism underlying these differences between opioid inhibition of evoked and spontaneous GABA release, specific myristoylated Gα peptide inhibitors for Gαo₁ and Gαi_1-3 that block receptor-G protein interactions were used to test the preference of agonists for MOR-Gα complexes. The Gαo₁ inhibitor reduced DAMGO inhibition of eIPSCs, but Gαi_1-3 inhibitors had no effect. Both Gαo₁ and Gαi_1-3 inhibitors separately reduced fentanyl inhibition of eIPSCs but had no effects on ME inhibition. Gαi_1-3 inhibitors blocked the inhibitory effects of ME and fentanyl on miniature postsynaptic current (mIPSC) frequency, but both Gαo₁ and Gαi_1-3 inhibitors were needed to block the effects of DAMGO. Finally, baclofen-mediated inhibition of GABA release is unaffected in the RGS-insensitive mice and in the presence of Gαo₁ and Gαi_1-3 inhibitor peptides, suggesting that GABA_B receptor coupling to G proteins in vlPAG presynaptic terminals is different than MOR coupling. SIGNIFICANCE STATEMENT: Presynaptic μ opioid receptors (MORs) in the ventrolateral periaqueductal gray are critical for opioid analgesia and are negatively regulated by RGS proteins. These data in RGS-insensitive mice provide evidence that MOR agonists differ in preference for Gαo versus Gαi and regulation by RGS proteins in presynaptic terminals, providing a mechanism for functional selectivity between agonists. The results further define important differences in MOR and GABA_B receptor coupling to G proteins that could be exploited for new pain therapies.

SAR Matrices Enable Discovery of Mixed Efficacy μ-Opioid Receptor Agonist Peptidomimetics with Simplified Structures through an Aromatic-Amine Pharmacophore

We previously described the development of potent μ-opioid receptor (MOR)-agonist/δ-opioid receptor (DOR)-antagonist peptidomimetic ligands as an approach toward effective analgesics with reduced side effects. In this series, a tetrahydroquinoline (THQ) or substituted phenyl is employed to link two key pharmacophore elements, a dimethyltyrosine amino acid and typically an aromatic pendant. Using new and previously reported analogues, we constructed a structure-activity relationship (SAR) matrix that probes the utility of previously reported amine pendants. This matrix reveals that the MOR-agonist/DOR-antagonist properties of these ligands do not change when a tetrahydroisoquinoline (THIQ) pendant is used, despite removal of substituents on the core phenyl ring. Based on this observation, we retained the THIQ pendant and replaced the phenyl core with simpler aliphatic chain structures. These simpler analogues proved to be potent MOR-agonists with high variability in their effects at the DOR and the κ-opioid receptor (KOR). These data show that the amine of the THIQ pendant may be a novel pharmacophore element that favors high MOR-efficacy, whereas the aromatic ring of the THIQ pendant may produce high MOR-potency. Combined, the two pharmacophores within the THIQ pendant may be a structurally efficient means of converting opioid peptides and peptidomimetics into potent and efficacious MOR-agonists.

Pharmacological Properties of δ-Opioid Receptor-Mediated Behaviors: Agonist Efficacy and Receptor Reserve

δ-Opioid receptor (δ-receptor) agonists produce antihyperalgesia, antidepressant-like effects, and convulsions in animals. However, the role of agonist efficacy in generating different δ-receptor-mediated behaviors has not been thoroughly investigated. To this end, efficacy requirements for δ-receptor-mediated antihyperalgesia, antidepressant-like effects, and convulsions were evaluated by comparing the effects of the partial agonist BU48 and the full agonist SNC80 and changes in the potency of SNC80 after δ-receptor elimination. Antihyperalgesia was measured in a nitroglycerin-induced thermal hyperalgesia assay. An antidepressant-like effect was evaluated in the forced swim test. Mice were observed for convulsions after treatment with SNC80 or the δ-opioid receptor partial agonist BU48. Ligand-induced G protein activation was measured by [³⁵S]guanosine 5'-O-[γ-thio]triphosphate binding in mouse forebrain tissue, and δ-receptor number was measured by [³H]D-Pen^2,5-enkephalin saturation binding. BU48 produced antidepressant-like effects and convulsions but antagonized SNC80-induced antihyperalgesia and G protein activation. The potency of SNC80 was shifted to the right in δ-receptor heterozygous knockout mice and naltrindole-5'-isothiocyanate-treated mice, and the magnitude of potency shift differed across assays, with the largest shift occurring in the thermal hyperalgesia assay, followed by the forced swim test and then convulsion observation. Naltrindole antagonized these SNC80-induced behaviors with similar potencies, suggesting that these effects are mediated by the same type of δ-receptor. These data suggest that δ-receptor-mediated behaviors display a rank order of efficacy requirement, with antihyperalgesia having the highest requirement, followed by antidepressant-like effects and then convulsions. These findings further our understanding of the pharmacological mechanisms mediating the in vivo effects of δ-opioid receptor agonists. SIGNIFICANCE STATEMENT: δ-Opioid receptor (δ-receptor) agonists produce antihyperalgesia, antidepressant-like effects, and convulsions in animal models. This study evaluates pharmacological properties, specifically the role of agonist efficacy and receptor reserve, underlying these δ-receptor-mediated behaviors. These data suggest that δ-receptor-mediated behaviors display a rank order of efficacy requirement, with antihyperalgesia having the highest requirement, followed by antidepressant-like effects and then convulsions.

In vitro pharmacology of fentanyl analogs at the human mu opioid receptor and their spectroscopic analysis

Opioids are widely misused and account for almost half of overdose deaths in the United States. The cost in terms of lives, health care, and lost productivity is significant and has been declared a national crisis. Fentanyl is a highly potent mu opioid receptor (MOR) agonist and plays a significant role in the current opioid epidemic; fentanyl and its analogs (fentalogs) are increasingly becoming one of the biggest dangers in the opioid crisis. The availability of fentalogs in the illicit market is thought to play a significant role in the recent increase in opioid-related deaths. Although there is both rodent homolog in vivo and in vitro data for some fentalogs, prior to this publication very little was known about the pharmacology of many of these illicit compounds at the human MOR (hMOR). Using gas chromatography-mass spectrometry, nuclear magnetic resonance spectroscopy, and in vitro assays, this study describes the spectral and pharmacological properties of 34 fentalogs. The reported spectra and chemical data will allow for easy identification of novel fentalogs in unknown or mixed samples. Taken together these data are useful for law enforcement and clinical workers as they will aid in the identification of fentalogs in unknown samples and can potentially be used to predict physiological effects after exposure.

Aromatic-Amine Pendants Produce Highly Potent and Efficacious Mixed Efficacy μ-Opioid Receptor (MOR)/δ-Opioid Receptor (DOR) Peptidomimetics with Enhanced Metabolic Stability

We previously reported a novel SAR campaign that converted a metabolically unstable series of μ-opioid receptor (MOR) agonist/δ-opioid receptor (DOR) antagonist bicyclic core peptidomimetics with promising analgesic activity and reduced abuse liabilities into a more stable series of benzylic core analogues. Herein, we expanded the SAR of that campaign and determined that the incorporation of amines into the benzylic pendant produces enhanced MOR-efficacy in this series, whereas the reincorporation of an aromatic ring into the pendant enhanced MOR-potency. Two compounds, which contain a piperidine (14) or an isoindoline (17) pendant, retained the desired opioid profile in vitro, possessed metabolic half-lives of greater than 1 h in mouse liver microsomes (MLMs), and were active antinociceptive agents in the acetic acid stretch assay (AASA) at subcutaneous doses of 1 mg/kg.

Regulator of G-Protein Signaling (RGS) Protein Modulation of Opioid Receptor Signaling as a Potential Target for Pain Management

Opioid drugs are the gold standard for the management of pain, but their use is severely limited by dangerous and unpleasant side effects. All clinically available opioid analgesics bind to and activate the mu-opioid receptor (MOR), a heterotrimeric G-protein-coupled receptor, to produce analgesia. The activity of these receptors is modulated by a family of intracellular RGS proteins or regulators of G-protein signaling proteins, characterized by the presence of a conserved RGS Homology (RH) domain. These proteins act as negative regulators of G-protein signaling by serving as GTPase accelerating proteins or GAPS to switch off signaling by both the Gα and βγ subunits of heterotrimeric G-proteins. Consequently, knockdown or knockout of RGS protein activity enhances signaling downstream of MOR. In this review we discuss current knowledge of how this activity, across the different families of RGS proteins, modulates MOR activity, as well as activity of other members of the opioid receptor family, and so pain and analgesia in animal models, with particular emphasis on RGS4 and RGS9 families. We discuss inhibition of RGS proteins with small molecule inhibitors that bind to sensitive cysteine moieties in the RH domain and the potential for targeting this family of intracellular proteins as adjuncts to provide an opioid sparing effect or as standalone analgesics by promoting the activity of endogenous opioid peptides. Overall, we conclude that RGS proteins may be a novel drug target to provide analgesia with reduced opioid-like side effects, but that much basic work is needed to define the roles for specific RGS proteins, particularly in chronic pain, as well as a need to develop newer inhibitors.

Novel Dimethyltyrosine-Tetrahydroisoquinoline Peptidomimetics with Aromatic Tetrahydroisoquinoline Substitutions Show in Vitro Kappa and Mu Opioid Receptor Agonism

The dimethyltyrosine-tetrahydroisoquinoline (Dmt-Tiq) scaffold was originally developed in the production of selective delta opioid receptor (DOR) antagonists. Installation of a 7-benzyl pendant on the tetrahydroisoquinoline core of this classic opioid scaffold introduced kappa opioid receptor (KOR) agonism. Further modification of this pendant resulted in retention of KOR agonism and the addition of mu opioid receptor (MOR) partial agonism, a bifunctional profile with potential to be used in the treatment of cocaine addiction.

The δ-opioid receptor positive allosteric modulator BMS 986187 is a G-protein-biased allosteric agonist

BACKGROUND AND PURPOSE

The δ-opioid receptor is an emerging target for the management of chronic pain and depression. Biased signalling, the preferential activation of one signalling pathway over another downstream of δ-receptors, may generate better therapeutic profiles. BMS 986187 is a positive allosteric modulator of δ-receptors. Here, we ask if BMS 986187 can directly activate the receptor from an allosteric site, without an orthosteric ligand, and if a signalling bias is generated.

EXPERIMENTAL APPROACH

We used several clonal cell lines expressing δ-receptors, to assess effects of BMS 986187 on events downstream of δ-receptors by measuring G-protein activation, β-arrestin 2 recruitment, receptor phosphorylation, loss of surface receptor expression, ERK1/ERK2 phosphorylation, and receptor desensitization.

KEY RESULTS

BMS 986187 is a G protein biased allosteric agonist, relative to β-arrestin 2 recruitment. Despite showing direct and potent G protein activation, BMS 986187 has a low potency to recruit β-arrestin 2. This appears to reflect the inability of BMS 986187 to elicit any significant receptor phosphorylation, consistent with low receptor internalization and a slower onset of desensitization, compared with the full agonist SNC80.

CONCLUSIONS AND IMPLICATIONS

This is the first evidence of biased agonism mediated through direct binding to an allosteric site on an opioid receptor, without a ligand at the orthosteric site. Our data suggest that agonists targeting δ-receptors, or indeed any GPCR, through allosteric sites may be a novel way to promote signalling bias and thereby potentially produce a more specific pharmacology than can be observed by activation via the orthosteric site.

Structural Simplification of a Tetrahydroquinoline-Core Peptidomimetic μ-Opioid Receptor (MOR) Agonist/δ-Opioid Receptor (DOR) Antagonist Produces Improved Metabolic Stability

We have previously reported a series of μ-opioid receptor (MOR) agonist/δ-opioid receptor (DOR) antagonist ligands to serve as potential nonaddictive opioid analgesics. These ligands have been shown to be active in vivo, do not manifest withdrawal syndromes or reward behavior in conditioned-place preference assays in mice, and do not produce dependence. Although these attributes are promising, these analogues exhibit poor metabolic stability in mouse liver microsomes, likely due to the central tetrahydroquinoline scaffold in this series. As such, a structure-activity relationship (SAR) campaign was pursued to improve their metabolic stability. This resulted in a shift from our original bicyclic tetrahydroquinoline core to a monocyclic benzylic-core system. By eliminating one of the rings in this scaffold and exploring the SAR of this new core, two promising analogues were discovered. These analogues (5l and 5m) had potency and efficacy values at MOR better or comparable to morphine, retained their DOR-antagonist properties, and showed a 10-fold improvement in metabolic stability.

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.

The Role of G-proteins and G-protein Regulating Proteins in Depressive Disorders

Progress toward new antidepressant therapies has been relatively slow over the past few decades, with the result that individuals suffering from depression often struggle to find an effective treatment – a process often requiring months. Furthermore, the neural factors that contribute to depression remain poorly understood, and there are many open questions regarding the mechanism of action of existing antidepressants. A better understanding of the molecular processes that underlie depression and contribute to antidepressant efficacy is therefore badly needed. In this review we highlight research investigating the role of G-proteins and the regulators of G-protein signaling (RGS) proteins, two protein families that are intimately involved in both the genesis of depressive states and the action of antidepressant drugs. Many antidepressants are known to indirectly affect the function of these proteins. Conversely, dysfunction of the G-protein and RGS systems can affect antidepressant efficacy. However, a great deal remains unknown about how these proteins interact with antidepressants. Findings pertinent to each individual G-protein and RGS protein are summarized from in vitro, in vivo, and clinical studies.

Synthesis and Pharmacological Evaluation of Novel C-8 Substituted Tetrahydroquinolines as Balanced-Affinity Mu/Delta Opioid Ligands for the Treatment of Pain

The use of opioids for the treatment of pain, while largely effective, is limited by detrimental side effects including analgesic tolerance, physical dependence, and euphoria, which may lead to opioid abuse. Studies have shown that compounds with a μ-opioid receptor (MOR) agonist/δ-opioid receptor (DOR) antagonist profile reduce or eliminate some of these side effects including the development of tolerance and dependence. Herein we report the synthesis and pharmacological evaluation of a series of tetrahydroquinoline-based peptidomimetics with substitutions at the C-8 position. Relative to our lead peptidomimetic with no C-8 substitution, this series affords an increase in DOR affinity and provides greater balance in MOR and DOR binding affinities. Moreover, compounds with carbonyl moieties at C-8 display the desired MOR agonist/DOR antagonist profile whereas alkyl substitutions elicit modest DOR agonism. Several compounds in this series produce a robust antinociceptive effect in vivo and show antinociceptive activity for greater than 2 h after intraperitoneal administration in mice.

Allostery at opioid receptors: modulation with small molecule ligands

Opioid receptors are 7-transmembrane domain receptors that couple to heterotrimeric G proteins. The endogenous ligands for opioid receptors are peptides which bind to the orthosteric site on the receptors. The μ-opioid receptor is the target for opioid analgesics, while the δ-opioid receptor has been suggested as a target for pain management, migraine and depression. Similarly, κ-opioid receptors are involved in pain and depression and nociceptin receptors in pain and mood behaviours. However, exogenous orthosteric ligands for opioid receptors cause a myriad of on-target side effects. Recently, selective allosteric ligands for μ- and δ-opioid receptors have been described. These compounds bind to a site on the receptor distinct from the orthosteric site. Occupation of this allosteric site leads to modulation of orthosteric ligand binding affinity and/or efficacy. Allosteric modulators may be positive, negative or silent (neutral) (PAMs, NAMs or SAMs respectively). PAMs may have in vivo activity by enhancing the activity of exogenous drugs or endogenous opioid peptides. Enhancing endogenous opioid peptide activity maintains the temporal and spatial distribution of these molecules but improves, and potentially qualitatively changes, activity at their cognate receptors which could limit side effects compared with traditional opioid drugs. In this review, we describe the rationale and promise for the development of allosteric modulators for opioid receptors, the discovery of selective allosteric modulators, the identification of potential allosteric sites on opioid receptors and the mode of action of the modulators.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Measuring ligand efficacy at the mu-opioid receptor using a conformational biosensor

The intrinsic efficacy of orthosteric ligands acting at G-protein-coupled receptors (GPCRs) reflects their ability to stabilize active receptor states (R*) and is a major determinant of their physiological effects. Here, we present a direct way to quantify the efficacy of ligands by measuring the binding of a R*-specific biosensor to purified receptor employing interferometry. As an example, we use the mu-opioid receptor (µ-OR), a prototypic class A GPCR, and its active state sensor, nanobody-39 (Nb39). We demonstrate that ligands vary in their ability to recruit Nb39 to µ-OR and describe methadone, loperamide, and PZM21 as ligands that support unique R* conformation(s) of µ-OR. We further show that positive allosteric modulators of µ-OR promote formation of R* in addition to enhancing promotion by orthosteric agonists. Finally, we demonstrate that the technique can be utilized with heterotrimeric G protein. The method is cell-free, signal transduction-independent and is generally applicable to GPCRs.

In vivo effects of μ-opioid receptor agonist/δ-opioid receptor antagonist peptidomimetics following acute and repeated administration

BACKGROUND AND PURPOSE

Agonists at μ-opioid receptors (μ-receptors) are used for pain management but produce adverse effects including tolerance, dependence and euphoria. The co-administration of a μ-receptor agonist with a δ-opioid receptor (δ-receptor) antagonist has been shown to produce antinociception with reduced development of some side effects. We characterized the effects of three μ-receptor agonist/δ-receptor antagonist peptidomimetics in vivo after acute and repeated administration to determine if this profile provides a viable alternative to traditional opioid analgesics.

EXPERIMENTAL APPROACH

Three μ-receptor agonist / δ-receptor antagonist peptidomimetics, AAH8, AMB46 and AMB47, and morphine were evaluated for the development of tolerance and dependence after 5 days of twice daily treatment with escalating doses of drug (10-50 mg·kg^-1 ). Antinociceptive effects were measured in the warm water tail withdrawal assay before and after repeated drug treatment. Physical dependence was evaluated by naltrexone-precipitated withdrawal jumping. The rewarding effects of AAH8 were evaluated using a conditioned place preference (CPP) assay with twice daily conditioning sessions performed for 5 days.

KEY RESULTS

Morphine, AAH8, AMB47 and AMB46 all demonstrated acute antinociceptive effects, but repeated administration only produced tolerance in animals treated with morphine and AMB46. Injection of naltrexone precipitated fewer jumps in mice treated repeatedly with AAH8 as compared with morphine, AMB47 or AMB46. Conditioning with morphine, but not AAH8, produced significant CPP.

CONCLUSIONS AND IMPLICATIONS

AAH8 may be a better alternative than traditional opioid analgesics, producing antinociception with less development of tolerance and dependence and may be less rewarding than morphine.

Role of signalling molecules in behaviours mediated by the δ opioid receptor agonist SNC80

BACKGROUND AND PURPOSE

GPCRs exist in multiple conformations that can engage distinct signalling mechanisms which in turn may lead to diverse behavioural outputs. In rodent models, activation of the δ opioid receptor (δ-receptor) has been shown to elicit antihyperalgesia, antidepressant-like effects and convulsions. We recently showed that these δ-receptor-mediated behaviours are differentially regulated by the GTPase-activating protein regulator of G protein signalling 4 (RGS4), which facilitates termination of G protein signalling. To further evaluate the signalling mechanisms underlying δ-receptor-mediated antihyperalgesia, antidepressant-like effects and convulsions, we observed how changes in Gα_o or arrestin proteins in vivo affected behaviours elicited by the δ-receptor agonist SNC80 in mice.

EXPERIMENTAL APPROACH

Transgenic mice with altered expression of various signalling molecules were used in the current studies. Antihyperalgesia was measured in a nitroglycerin-induced thermal hyperalgesia assay. Antidepressant-like effects were evaluated in the forced swim test. Mice were also observed for convulsive activity following SNC80 treatment.

KEY RESULTS

In Gα_o RGS-insensitive heterozygous knock-in mice, the potency of SNC80 to produce antihyperalgesia and antidepressant-like effects was enhanced with no change in SNC80-induced convulsions. Conversely, in Gα_o heterozygous knockout mice, SNC80-induced antihyperalgesia was abolished while antidepressant-like effects and convulsions were unaltered. No changes in SNC80-induced behaviours were observed in arrestin 3 knockout mice. SNC80-induced convulsions were potentiated in arrestin 2 knockout mice.

CONCLUSIONS AND IMPLICATIONS

Taken together, these findings suggest that different signalling molecules may underlie the convulsive effects of the δ-receptor relative to its antihyperalgesic and antidepressant-like effects.

Role of the guanine nucleotide binding protein, Gαo, in the development of morphine tolerance and dependence

RATIONALE

The use of morphine and other opioids for chronic pain is limited by the development of analgesic tolerance and physical dependence. Morphine produces its effects by activating the μ opioid receptor, which couples to Gαi/o-containing heterotrimeric G proteins. Evidence suggests that the antinociceptive effects of morphine are mediated by Gαo. However, the role of Gαo in the development of morphine tolerance and dependence is unknown.

OBJECTIVE

The objective of the study is to evaluate the contribution of Gαo to the development of morphine tolerance and dependence in mice.

METHODS

129S6 mice lacking one copy of the Gαo gene (Gαo +/-) were administered morphine acutely or chronically. Mice were examined for tolerance to the antinociceptive action of morphine using the 52 °C hot plate as the nociceptive stimulus and for dependence by evaluating the severity of naltrexone-precipitated withdrawal. Wild-type littermates of the Gαo +/- mice were used as controls. Changes in μ receptor number and function were determined in midbrain and hindbrain homogenates using radioligand binding and μ agonist-stimulated [35S]GTPγS binding, respectively.

RESULTS

Following either acute or chronic morphine treatment, all mice developed antinociceptive tolerance and physical dependence, regardless of genotype. With chronic morphine treatment, Gαo +/- mice developed tolerance faster and displayed more severe naltrexone-precipitated withdrawal in some behaviors than did wild-type littermates. Morphine tolerance was not associated with changes in μ receptor number or function in brain homogenates from either wild-type or Gαo +/- mice.

CONCLUSIONS

These data suggest that the guanine nucleotide binding protein Gαo offers some protection against the development of morphine tolerance and dependence.

Pharmacologic Evidence for a Putative Conserved Allosteric Site on Opioid Receptors

Allosteric modulators of G protein-coupled receptors, including opioid receptors, have been proposed as possible therapeutic agents with enhanced selectivity. BMS-986122 is a positive allosteric modulator (PAM) of the μ-opioid receptor (µ-OR). BMS-986187 is a structurally distinct PAM for the δ-opioid receptor (δ-OR) that has been reported to exhibit 100-fold selectivity in promoting δ-OR over μ-OR agonism. We used ligand binding and second-messenger assays to show that BMS-986187 is an effective PAM at the μ-OR and at the κ-opioid receptor (κ-OR), but it is ineffective at the nociceptin receptor. The affinity of BMS-986187 for δ-ORs and κ-ORs is approximately 20- to 30-fold higher than for μ-ORs, determined using an allosteric ternary complex model. Moreover, we provide evidence, using a silent allosteric modulator as an allosteric antagonist, that BMS-986187 and BMS-986122 bind to a similar region on all three traditional opioid receptor types (µ-OR, δ-OR, and κ-OR). In contrast to the dogma surrounding allosteric modulators, the results indicate a possible conserved allosteric binding site across the opioid receptor family that can accommodate structurally diverse molecules. These findings have implications for the development of selective allosteric modulators.

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.

Modulation of opioid receptor affinity and efficacy via N-substitution of 9β-hydroxy-5-(3-hydroxyphenyl)morphan: Synthesis and computer simulation study

The enantiomers of a variety of N-alkyl-, N-aralkyl-, and N-cyclopropylalkyl-9β-hydroxy-5-(3-hydroxyphenyl)morphans were synthesized employing cyanogen bromide and K2CO3 to improve the original N-demethylation procedure. Their binding affinity to the μ-, δ-, and κ-opioid receptors (ORs) was determined and functional (GTPγ35S) assays were carried out on those with reasonable affinity. The 1R,5R,9S-enantiomers (1R,5R,9S)-(-)-5-(3-hydroxyphenyl)-2-(4-nitrophenethyl)-2-azabicyclo[3.3.1]nonan-9-ol (1R,5R,9S-16), (1R,5R,9S)-(-) 2-cinnamyl-5-(3-hydroxyphenyl)-2-azabicyclo[3.3.1]nonan-9-ol (1R,5R,9S-20), and (1R,5R,9S)-(-)-5-(3-hydroxyphenyl)-2-(4-(trifluoromethyl)phenethyl)-2-azabicyclo[3.3.1]nonan-9-ol (1R,5R,9S-15), had high affinity for the μ-opioid receptor (e.g., 1R,5R,9S-16: Ki=0.073, 0.74, and 1.99nM, respectively). The 1R,5R,9S-16 and 1R,5R,9S-15 were full, high efficacy μ-agonists (EC50=0.74 and 18.5nM, respectively) and the former was found to be a partial agonist at δ-OR and an antagonist at κ-OR, while the latter was a partial agonist at δ-OR and κ-OR in the GTPγ35S assay. The enantiomer of 1R,5R,9S-16, (+)-1S,5S,9R-16 was unusual, it had good affinity for the μ-OR (Ki=26.5nM) and was an efficacious μ-antagonist (Ke=29.1nM). Molecular dynamics simulations of the μ-OR were carried out with the 1R,5R,9S-16 μ-agonist and the previously synthesized (1R,5R,9S)-(-)-5-(9-hydroxy-5-(3-hydroxyphenyl-2-phenylethyl)-2-azabicyclo[3.3.1]nonane (1R,5R,9S-(-)-NIH 11289) to provide a structural basis for the observed high affinities and efficacies. The critical roles of both the 9β-OH and the p-nitro group are elucidated, with the latter forming direct, persistent hydrogen bonds with residues deep in the binding cavity, and the former interacting with specific residues via highly structured water bridges.

The role of regulator of G protein signaling 4 in delta-opioid receptor-mediated behaviors

RATIONALE

Regulator of G protein signaling (RGS) proteins act as negative modulators of G protein signaling. RGS4 has been shown to negatively modulate G protein signaling mediated by the delta opioid receptor (DOPr) in vitro. However, the role of RGS4 in modulating DOPr-mediated behaviors in vivo has not been elucidated.

OBJECTIVE

The aim of this study was to compare the ability of the DOPr agonist SNC80 to induce DOPr-mediated antinociception, antihyperalgesia, antidepressant-like effects, and convulsions in wild-type and RGS4 knockout mice.

METHODS

Antinociception was assessed in the acetic acid stretch assay. Antihyperalgesia was measured in a nitroglycerin-induced thermal hyperalgesia assay. Antidepressant-like effects were evaluated in the forced swim and tail suspension tests. Mice were also observed for convulsive activity post-SNC80 treatment. SNC80-induced phosphorylation of MAP kinase in striatal tissue from RGS4 wild-type and knockout mice was quantified by Western blot. DOPr number from forebrain tissue was measured using [³H]DPDPE saturation binding.

RESULTS

Elimination of RGS4 potentiated SNC80-induced antinociception and antihyperalgesia. SNC80-induced antidepressant-like effects were potentiated in RGS4 knockout mice in the forced swim test but not in the tail suspension test. Additionally, RGS4 knockout did not alter SNC80-induced convulsions. SNC80-induced phosphorylation of MAP kinase was potentiated in striatum from RGS4 knockout mice. Loss of RGS4 did not affect total DOPr number.

CONCLUSIONS

Overall, these findings demonstrate that reduction of RGS4 functionally may increase the therapeutic index of SNC80. These results provide the first evidence of differential regulation of DOPr-mediated behaviors by RGS proteins and G protein signaling pathways.

Benzylideneoxymorphone: A new lead for development of bifunctional mu/delta opioid receptor ligands

Opioid analgesic tolerance remains a considerable drawback to chronic pain management. The finding that concomitant administration of delta opioid receptor (DOR) antagonists attenuates the development of tolerance to mu opioid receptor (MOR) agonists has led to interest in producing bifunctional MOR agonist/DOR antagonist ligands. Herein, we present 7-benzylideneoxymorphone (6, UMB 246) displaying MOR partial agonist/DOR antagonist activity, representing a new lead for designing bifunctional MOR/DOR ligands.

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.

Dynamic Coupling and Allosteric Networks in the α Subunit of Heterotrimeric G Proteins

G protein α subunits cycle between active and inactive conformations to regulate a multitude of intracellular signaling cascades. Important structural transitions occurring during this cycle have been characterized from extensive crystallographic studies. However, the link between observed conformations and the allosteric regulation of binding events at distal sites critical for signaling through G proteins remain unclear. Here we describe molecular dynamics simulations, bioinformatics analysis, and experimental mutagenesis that identifies residues involved in mediating the allosteric coupling of receptor, nucleotide, and helical domain interfaces of Gαi. Most notably, we predict and characterize novel allosteric decoupling mutants, which display enhanced helical domain opening, increased rates of nucleotide exchange, and constitutive activity in the absence of receptor activation. Collectively, our results provide a framework for explaining how binding events and mutations can alter internal dynamic couplings critical for G protein function.

Rapid Synthesis of Boc-2',6'-dimethyl-l-tyrosine and Derivatives and Incorporation into Opioid Peptidomimetics

The unnatural amino acid 2',6'-dimethyl-l-tyrosine has found widespread use in the development of synthetic opioid ligands. Opioids featuring this residue at the N-terminus often display superior potency at one or more of the opioid receptor types, but the availability of the compound is hampered by its cost and difficult synthesis. We report here a short, three-step synthesis of Boc-2',6'-dimethyl-l-tyrosine (3a) utilizing a microwave-assisted Negishi coupling for the key carbon-carbon bond forming step, and employ this chemistry for the expedient synthesis of other unnatural tyrosine derivatives. Three of these derivatives (3c, 3d, 3f) have not previously been examined as Tyr(1) replacements in opioid ligands. We describe the incorporation of these tyrosine derivatives in a series of opioid peptidomimetics employing our previously reported tetrahydroquinoline (THQ) scaffold, and the binding and relative efficacy of each of the analogues at the three opioid receptor subtypes: mu (MOR), delta (DOR), and kappa (KOR).

Further Optimization and Evaluation of Bioavailable, Mixed-Efficacy μ-Opioid Receptor (MOR) Agonists/δ-Opioid Receptor (DOR) Antagonists: Balancing MOR and DOR Affinities

In a previously described peptidomimetic series, we reported the development of bifunctional μ-opioid receptor (MOR) agonist and δ-opioid receptor (DOR) antagonist ligands with a lead compound that produced antinociception for 1 h after intraperitoneal administration in mice. In this paper, we expand on our original series by presenting two modifications, both of which were designed with the following objectives: (1) probing bioavailability and improving metabolic stability, (2) balancing affinities between MOR and DOR while reducing affinity and efficacy at the κ-opioid receptor (KOR), and (3) improving in vivo efficacy. Here, we establish that, through N-acetylation of our original peptidomimetic series, we are able to improve DOR affinity and increase selectivity relative to KOR while maintaining the desired MOR agonist/DOR antagonist profile. From initial in vivo studies, one compound (14a) was found to produce dose-dependent antinociception after peripheral administration with an improved duration of action of longer than 3 h.

Ligand-Based Discovery of a New Scaffold for Allosteric Modulation of the μ-Opioid Receptor

With the hope of discovering effective analgesics with fewer side effects, attention has recently shifted to allosteric modulators of the opioid receptors. In the past two years, the first chemotypes of positive or silent allosteric modulators (PAMs or SAMs, respectively) of μ- and δ-opioid receptor types have been reported in the literature. During a structure-guided lead optimization campaign with μ-PAMs BMS-986121 and BMS-986122 as starting compounds, we discovered a new chemotype that was confirmed to display μ-PAM or μ-SAM activity depending on the specific substitutions as assessed by endomorphin-1-stimulated β-arrestin2 recruitment assays in Chinese Hamster Ovary (CHO)-μ PathHunter cells. The most active μ-PAM of this series was analyzed further in competition binding and G-protein activation assays to understand its effects on ligand binding and to investigate the nature of its probe dependence.

Asymmetric synthesis and in vitro and in vivo activity of tetrahydroquinolines featuring a diverse set of polar substitutions at the 6 position as mixed-efficacy μ opioid receptor/δ opioid receptor ligands

We previously reported a small series of mixed-efficacy μ opioid receptor (MOR) agonist/δ opioid receptor (DOR) antagonist peptidomimetics featuring a tetrahydroquinoline scaffold and showed the promise of this series as effective analgesics after intraperitoneal administration in mice. We report here an expanded structure-activity relationship study of the pendant region of these compounds and focus in particular on the incorporation of heteroatoms into this side chain. These analogues provide new insight into the binding requirements for this scaffold at MOR, DOR, and the κ opioid receptor (KOR), and several of them (10j, 10k, 10m, and 10n) significantly improve upon the overall MOR agonist/DOR antagonist profile of our previous compounds. In vivo data for 10j, 10k, 10m, and 10n are also reported and show the antinociceptive potency and duration of action of compounds 10j and 10m to be comparable to those of morphine.

Structural insights into µ-opioid receptor activation

Activation of the μ-opioid receptor (μOR) is responsible for the efficacy of the most effective analgesics. To understand the structural basis for μOR activation, we obtained a 2.1 Å X-ray crystal structure of the μOR bound to the morphinan agonist BU72 and stabilized by a G protein-mimetic camelid-antibody fragment. The BU72-stabilized changes in the μOR binding pocket are subtle and differ from those observed for agonist-bound structures of the β₂ adrenergic receptor (β₂AR) and the M2 muscarinic receptor (M2R). Comparison with active β₂AR reveals a common rearrangement in the packing of three conserved amino acids in the core of the μOR, and molecular dynamics simulations illustrate how the ligand-binding pocket is conformationally linked to this conserved triad. Additionally, an extensive polar network between the ligand-binding pocket and the cytoplasmic domains appears to play a similar role in signal propagation for all three GPCRs.

Selectivity and anti-Parkinson's potential of thiadiazolidinone RGS4 inhibitors

Many current therapies target G protein coupled receptors (GPCR), transporters, or ion channels. In addition to directly targeting these proteins, disrupting the protein-protein interactions that localize or regulate their function could enhance selectivity and provide unique pharmacologic actions. Regulators of G protein signaling (RGS) proteins, especially RGS4, play significant roles in epilepsy and Parkinson's disease. Thiadiazolidinone (TDZD) inhibitors of RGS4 are nanomolar potency blockers of the biochemical actions of RGS4 in vitro. Here, we demonstrate the substantial selectivity (8- to >5000-fold) of CCG-203769 for RGS4 over other RGS proteins. It is also 300-fold selective for RGS4 over GSK-3β, another target of this class of chemical scaffolds. It does not inhibit the cysteine protease papain at 100 μM. CCG-203769 enhances Gαq-dependent cellular Ca(2+) signaling in an RGS4-dependent manner. TDZD inhibitors also enhance Gαi-dependent δ-OR inhibition of cAMP production in SH-SY-5Y cells, which express endogenous receptors and RGS4. Importantly, CCG-203769 potentiates the known RGS4 mechanism of Gαi-dependent muscarinic bradycardia in vivo. Furthermore, it reverses raclopride-induced akinesia and bradykinesia in mice, a model of some aspects of the movement disorder in Parkinson's disease. A broad assessment of compound effects revealed minimal off-target effects at concentrations necessary for cellular RGS4 inhibition. These results expand our understanding of the mechanism and specificity of TDZD RGS inhibitors and support the potential for therapeutic targeting of RGS proteins in Parkinson's disease and other neural disorders.

Discovery, synthesis, and molecular pharmacology of selective positive allosteric modulators of the δ-opioid receptor

Allosteric modulators of G protein-coupled receptors (GPCRs) have a number of potential advantages compared to agonists or antagonists that bind to the orthosteric site of the receptor. These include the potential for receptor selectivity, maintenance of the temporal and spatial fidelity of signaling in vivo, the ceiling effect of the allosteric cooperativity which may prevent overdose issues, and engendering bias by differentially modulating distinct signaling pathways. Here we describe the discovery, synthesis, and molecular pharmacology of δ-opioid receptor-selective positive allosteric modulators (δ PAMs). These δ PAMs increase the affinity and/or efficacy of the orthosteric agonists leu-enkephalin, SNC80 and TAN67, as measured by receptor binding, G protein activation, β-arrestin recruitment, adenylyl cyclase inhibition, and extracellular signal-regulated kinases (ERK) activation. As such, these compounds are useful pharmacological tools to probe the molecular pharmacology of the δ receptor and to explore the therapeutic potential of δ PAMs in diseases such as chronic pain and depression.

C7β-methyl analogues of the orvinols: the discovery of kappa opioid antagonists with nociceptin/orphanin FQ peptide (NOP) receptor partial agonism and low, or zero, efficacy at mu opioid receptors

Buprenorphine is a successful analgesic and treatment for opioid abuse, with both activities relying on its partial agonist activity at mu opioid receptors. However, there is substantial interest in its activities at the kappa opioid and nociceptin/orphanin FQ peptide receptors. This has led to an interest in developing compounds with a buprenorphine-like pharmacological profile but with lower efficacy at mu opioid receptors. The present article describes aryl ring analogues of buprenorphine in which the standard C20-methyl group has been moved to the C7β position, resulting in ligands with the desired profile. In particular, moving the methyl group has resulted in far more robust kappa opioid antagonist activity than seen in the standard orvinol series. Of the compounds synthesized, a number, including 15a, have a profile of interest for the development of drug abuse relapse prevention therapies or antidepressants and others (e.g., 8c), as analgesics with a reduced side-effect profile.