Homology modeling and molecular dynamics simulations of the active state of the nociceptin receptor reveal new insights into agonist binding and activation

Analgesic Peptides: From Natural Diversity to Rational Design

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

Discovery and structure-activity relationships (SAR) of a novel class of 2-substituted N-piperidinyl indole-based nociceptin opioid receptor ligands

The development of SAR around substituted N-piperidinyl indole-based nociceptin opioid receptor (NOP) ligands led to the discovery of a novel series of 2-substituted N-piperidinyl indoles that provide both selective NOP full agonists and bifunctional NOP full agonists-μ opioid (MOP) receptor partial agonists. 2-substituted N-piperidinyl indoles have improved potency at the NOP receptor and are NOP full agonists, compared to our previously reported 3-substituted N-piperidinyl indoles that are selective NOP partial agonists. SAR in this series of 2-substituted N-piperidinyl indoles shows that 2-substitution versus 3-substitution on the indole moiety affects their intrinsic activity and opioid receptor selectivity. Molecular docking of these 2-substituted N-piperidinyl indoles in an active-state NOP homology model and MOP receptor structures provides a rationale for the differences observed in the binding, functional profiles and selectivity of 2-substituted versus 3-substituted N-piperidinyl indoles.

Structures of the entire human opioid receptor family

Opioids are effective analgesics, but their use is beset by serious side effects, including addiction and respiratory depression, which contribute to the ongoing opioid crisis. The human opioid system contains four opioid receptors (μOR, δOR, κOR, and NOPR) and a set of related endogenous opioid peptides (EOPs), which show distinct selectivity toward their respective opioid receptors (ORs). Despite being key to the development of safer analgesics, the mechanisms of molecular recognition and selectivity of EOPs to ORs remain unclear. Here, we systematically characterize the binding of EOPs to ORs and present five structures of EOP-OR-Gi complexes, including β-endorphin- and endomorphin-bound μOR, deltorphin-bound δOR, dynorphin-bound κOR, and nociceptin-bound NOPR. These structures, supported by biochemical results, uncover the specific recognition and selectivity of opioid peptides and the conserved mechanism of opioid receptor activation. These results provide a structural framework to facilitate rational design of safer opioid drugs for pain relief.

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

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

Spotlight on Nociceptin/Orphanin FQ Receptor in the Treatment of Pain

In our society today, pain has become a main source of strain on most individuals. It is crucial to develop novel treatments against pain while focusing on decreasing their adverse effects. Throughout the extent of development for new pain therapies, the nociceptin/orphanin FQ receptor (NOP receptor) has appeared to be an encouraging focal point. Concentrating on NOP receptor to treat chronic pain with limited range of unwanted effects serves as a suitable alternative to prototypical opioid morphine that could potentially lead to life-threatening effects caused by respiratory depression in overdose, as well as generate abuse and addiction. In addition to these harmful effects, the uprising opioid epidemic is responsible for becoming one of the most disastrous public health issues in the US. In this article, the contributing molecular and cellular structure in controlling the cellular trafficking of NOP receptor and studies that support the role of NOP receptor and its ligands in pain management are reviewed.

In silico analysis of energy interactions between nociceptin/orfanin FQ receptor and two antagonists with potential antidepressive action

This study addresses the binding energies of NOPR-ligand complexes and presents the main amino acid residues involved in the interaction between these complexes.

Differential barcoding of opioid receptors trafficking

Over the past several years, studies have highlighted the δ-opioid receptor (DOPr) as a promising therapeutic target for chronic pain management. While exhibiting milder undesired effects than most currently prescribed opioids, its specific agonists elicit effective analgesic responses in numerous animal models of chronic pain, including inflammatory, neuropathic, diabetic, and cancer-related pain. However, as compared with the extensively studied μ-opioid receptor, the molecular mechanisms governing its trafficking remain elusive. Recent advances have denoted several significant particularities in the regulation of DOPr intracellular routing, setting it apart from the other members of the opioid receptor family. Although they share high homology, each opioid receptor subtype displays specific amino acid patterns potentially involved in the regulation of its trafficking. These precise motifs or "barcodes" are selectively recognized by regulatory proteins and therefore dictate several aspects of the itinerary of a receptor, including its anterograde transport, internalization, recycling, and degradation. With a specific focus on the regulation of DOPr trafficking, this review will discuss previously reported, as well as potential novel trafficking barcodes within the opioid and nociceptin/orphanin FQ opioid peptide receptors, and their impact in determining distinct interactomes and physiological responses.

Efficiency of Homology Modeling Assisted Molecular Docking in G-protein Coupled Receptors

BACKGROUND

Molecular docking is in regular practice to assess ligand affinity on a target protein crystal structure. In the absence of protein crystal structure, the homology modeling or comparative modeling is the best alternative to elucidate the relationship details between a ligand and protein at the molecular level. The development of accurate homology modeling (HM) and its integration with molecular docking (MD) is essential for successful, rational drug discovery.

OBJECTIVE

The G-protein coupled receptors (GPCRs) are attractive therapeutic targets due to their immense role in human pharmacology. The GPCRs are membrane-bound proteins with the complex constitution, and the understanding of their activation and inactivation mechanisms is quite challenging. Over the past decade, there has been a rapid expansion in the number of solved G-protein-coupled receptor (GPCR) crystal structures; however, the majority of the GPCR structures remain unsolved. In this context, HM guided MD has been widely used for structure-based drug design (SBDD) of GPCRs.

METHODS

The focus of this review is on the recent (i) developments on HM supported GPCR drug discovery in the absence of GPCR crystal structures and (ii) application of HM in understanding the ligand interactions at the binding site, virtual screening, determining receptor subtype selectivity and receptor behaviour in comparison with GPCR crystal structures.

RESULTS

The HM in GPCRs has been extremely challenging due to the scarcity in template structures. In such a scenario, it is difficult to get accurate HM that can facilitate understanding of the ligand-receptor interactions. This problem has been alleviated to some extent by developing refined HM based on incorporating active /inactive ligand information and inducing protein flexibility. In some cases, HM proteins were found to outscore crystal structures.

CONCLUSION

The developments in HM have been highly operative to gain insights about the ligand interaction at the binding site and receptor functioning at the molecular level. Thus, HM guided molecular docking may be useful for rational drug discovery for the GPCRs mediated diseases.

Nociceptin/Orphanin FQ Peptide Receptor-Related Ligands as Novel Analgesics

Despite similar distribution patterns and intracellular events observed in the nociceptin/ orphanin FQ peptide (NOP) receptor and other opioid receptors, NOP receptor activation displays unique pharmacological profiles. Several researchers have identified a variety of peptide and nonpeptide ligands to determine the functional roles of NOP receptor activation and observed that NOP receptor- related ligands exhibit pain modality-dependent pain processing. Importantly, NOP receptor activation results in anti-nociception and anti-hypersensitivity at the spinal and supraspinal levels regardless of the experimental settings in non-human primates (NHPs). Given that the NOP receptor agonists synergistically enhance mu-opioid peptide (MOP) receptor agonist-induced anti-nociception, it has been hypothesized that dual NOP and MOP receptor agonists may display promising functional properties as analgesics. Accumulating evidence indicates that the mixed NOP/opioid receptor agonists demonstrate favorable functional profiles. In NHP studies, bifunctional NOP/MOP partial agonists (e.g., AT-121, BU08028, and BU10038) exerted potent anti-nociception via NOP and MOP receptor activation; however, dose-limiting adverse effects associated with the MOP receptor activation, including respiratory depression, itch sensation, physical dependence, and abuse liability, were not observed. Moreover, a mixed NOP/opioid receptor agonist, cebranopadol, presented promising outcomes in clinical trials as a novel analgesic. Collectively, the dual agonistic actions on NOP and MOP receptors, with appropriate binding affinities and efficacies, may be a viable strategy to develop innovative and safe analgesics.

Structure-Based SAR in the Design of Selective or Bifunctional Nociceptin (NOP) Receptor Agonists

The nociceptin opioid receptor (NOP), the fourth member of the opioid receptor family, and its endogenous peptide ligand, nociceptin or orphanin FQ (N/OFQ), play a vital role in several central nervous system pathways regulating pain, reward, feeding, anxiety, motor control and learning/memory. Both selective NOP agonists as well as bifunctional agonists at the NOP and mu opioid receptor (MOP) have potential therapeutic applications in CNS disorders related to these processes. Using Surflex-Dock protocols, we conducted a computational structure-activity study of four scaffold classes of NOP ligands with varying NOP-MOP selectivity. By docking these compounds into the orthosteric binding sites within an active-state NOP homology model, and an active-state MOP crystal structure, the goal of this study was to use a structure-based drug design approach to modulate NOP affinity and NOP vs. MOP selectivity. We first docked four parent compounds (no side chain) to determine their binding interactions within the NOP and MOP binding pockets. Various polar sidechains were added to the heterocyclic A-pharmacophore to modulate NOP ligand affinity. The substitutions mainly contained a 1-2 carbon chain with a polar substituent such as an amine, alcohol, sulfamide, or guanidine. The SAR analysis is focused on the impact of structural changes in the sidechain, such as chain length, hydrogen bonding capability, and basic vs neutral functional groups on binding affinity and selectivity at both NOP and MOP receptors. This study highlights structural modifications that can be leveraged to rationally design both selective NOP and bifunctional NOP-MOP agonists with different ratios of functional efficacy.

Potential of Nociceptin/Orphanin FQ Peptide Analogs for Drug Development

Nociceptin receptor (NOP) belongs to the family of opioid receptors but was discovered and characterized much later than the so called classical opioid receptors, μ, δ and κ (or MOP, DOP and KOP, resp.). Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand of this receptor and it controls numerous important functions in the central nervous system and in the periphery, so its analogs may be developed as innovative drugs for the treatment of a variety of conditions and pathological states. Availability of potent and selective ligands with high affinity to NOP receptor is essential to fully understand the role of NOP-N/OFQ system in the body, which in turn may lead to designing novel therapeutics. Here, we have focused on reviewing the structure of potent peptide-based agonists, antagonists, biased analogs and bivalent ligands that target NOP receptor.

Biased Agonism at Nociceptin/Orphanin FQ Receptors: A Structure Activity Study on N/OFQ(1-13)-NH2

Nociceptin/orphanin FQ (N/OFQ) controls different biological functions via selective stimulation of the N/OFQ peptide (NOP) receptor. The pleiotropic actions of N/OFQ may limit the development of NOP ligands as innovative drugs in different therapeutic areas. The pharmacological concept of functional selectivity (aka biased agonism) might be useful for amplifying beneficial actions and/or counteracting side effects. Thus, molecules with large bias factors toward G protein or β arrestin are required for investigating the translational value of NOP biased modulation. Herein, the biased behavior of a heterogeneous library of NOP-targeting peptide derivatives was evaluated in vitro with the aim to provide possible insights into the structural determinants that govern the selective activation of G protein versus β-arrestin. Our results demonstrate that lipidation of N/OFQ(1-13)-NH2 is a useful strategy for obtaining G protein biased agonists for the NOP receptor.

Discovery and Structure-Activity Relationships of Nociceptin Receptor Partial Agonists That Afford Symptom Ablation in Parkinson's Disease Models

A novel series of C(3)-substituted piperdinylindoles were developed as nociceptin opioid receptor (NOP) partial agonists to explore a pharmacological hypothesis that NOP partial agonists would afford a dual pharmacological action of attenuating Parkinson's disease (PD) motor symptoms and development of levodopa-induced dyskinesias. SAR around the C-3 substituents investigated effects on NOP binding, intrinsic activity, and selectivity and showed that while the C(3)-substituted indoles are selective, high affinity NOP ligands, the steric, polar, and cationic nature of the C-3 substituents affected intrinsic activity to afford partial agonists with a range of efficacies. Compounds 4, 5, and 9 with agonist efficacies between 25% and 35% significantly attenuated motor deficits in the 6-OHDA-hemilesioned rat model of PD. Further, unlike NOP antagonists, which appear to worsen dyskinesia expression, these NOP partial agonists did not attenuate or worsen dyskinesia expression. The NOP partial agonists and their SAR reported here may be useful to develop nondopaminergic treatments for PD.

Biochemical and pharmacological investigation of novel nociceptin/OFQ analogues and N/OFQ-RYYRIK hybrid peptides

The endogenous ligand nociceptin (N/OFQ) and a positively charged synthetic peptide RYYRIK are both selective for the nociceptin opioid receptor (NOPr). Despite their structural dissimilarity, N/OFQ and RYYRIK compete for the same binding site of NOP receptor possessing full and partial agonistic character, respectively. In the view of the message-address concept, hybrid peptide constructs were probed for the NOP receptor combining different regions of N/OFQ and RYYRIK related peptide sequences. Nine novel nociceptin- or Ac-RYYRIK-NH2 peptide variants or hybrid peptides were synthesized and characterized. Peptides P2 and P8 contain fragments of native N/OFQ. The other seven analogues (P1, P3-7, P9) are composed of Ac-RYYRIK-NH2 fragments and parts of the original nociceptin sequence. The analogues were characterized in receptor binding assays and G-protein activation experiments on rat brain membranes, as well as by electrically stimulated mouse vas deferens bioassay. In receptor binding assays ligands P2, P4, P6 (Ki 0.37 nM) and P7 showed higher affinity (Ki 0.65 nM, 0.6 nM, 0.37 nM and 0.44 nM, respectively) for NOP receptor than their parent compounds N/OFQ (Ki 2.8 nM) or Ac-RYYRIK-NH2 (Ki 4.2 nM). In [35S]GTPγS binding experiments P2 and P3 behaved as full agonists. The other variants exhibited partial agonist properties characterized by submaximal stimulatory effects. In mouse vas deferens bioassay only P2 showed agonist activity. P4, P5, P6 inhibited the biological activity of N/OFQ more effectively than the NOP receptor selective antagonist JTC-801. In summary, hybrid peptides P4, P5 and P6 proved to be NOP receptor partial agonists even antagonists, while P2 peptide retained the full agonist property.

Microswitches for the Activation of the Nociceptin Receptor Induced by Cebranopadol: Hints from Microsecond Molecular Dynamics

Cebranopadol (CBP) is a novel analgesic acting as agonist at the nociceptin (NOP) and μ-opioid (MOP) receptors, exhibiting high potency and efficacy as an antinociceptive and antihypersensitive drug. The binding conformation and the dynamical interactions of CBP with the NOP receptor have been investigated by molecular docking, molecular dynamics (MD) in the microsecond time scale, and hybrid quantum mechanics/molecular mechanics (QM/MM). CBP binds to the NOP receptor as a bidentate ligand of the aspartate D130^3,32 by means of both its fluoroindole and dimethyl nitrogens. Starting from the known crystal structure of the inactive state of the receptor, in complex with the antagonist compound-24 (NOP-C24) the comparative analysis of 1 μs MD trajectories of the NOP-C24 complex itself and the NOP_free and NOP-CBP complexes provides new insights on the already known microswitches related to receptor activation, in the frame of the extended ternary complex model. The agonist acts by destabilizing the inactive conformation of the NOP receptor, by inducing a conformational change of M134^3,36, which allows W276^6,48 to flip around its χ₂ dihedral, going in close proximity to the receptor hydrophobic core (T138^3,40, P227^5,50, F272^6,44), which is known to be fundamental for the activation of the opioid receptors. A complete rational picture is also provided for the role of N133^3,35 and W276^6,48 undergoing critical conformational changes related to an anticooperativity effect, i.e. the well-known role of sodium as negative modulator of agonist binding. Finally, the movement of residue Y319^7,53 belonging to the NPxxY motif is also induced by the binding of the agonist in the inactive state, opening a gate for a water channel just as upon receptor activation.

NOP-Targeted Nonpeptide Ligands

The development of nonpeptide systemically active small-molecule NOP-targeted ligands has contributed tremendously to validating the NOP receptor as a promising target for therapeutics. Although a NOP-targeted compound is not yet approved for clinical use, a few NOP ligands are in clinical trials for various indications. Both successful and failed human clinical trials with NOP ligands provide opportunities for rational development of new and improved NOP-targeted compounds. A few years after the discovery of the NOP receptor in 1994, and its de-orphanization upon discovery of the endogenous peptide nociceptin/orphanin FQ (N/OFQ) in 1995, there was a significant effort in the pharmaceutical industry to discover nonpeptide NOP ligands from hits obtained from high-throughput screening campaigns of compound libraries. Depending on the therapeutic indication to be pursued, NOP agonists and antagonists were discovered, and some were optimized as clinical candidates. Advances such as G protein-coupled receptor (GPCR) structure elucidation, functional selectivity in ligand-driven GPCR activation, and multi-targeted ligands provide new scope for the rational design of novel NOP ligands fine-tuned for successful clinical translation. This article reviews the field of nonpeptide NOP ligand drug design in the context of these exciting developments and highlights new optimized nonpeptide NOP ligands possessing interesting functional profiles, which are particularly attractive for several unmet clinical applications involving NOP receptor pharmacomodulation.

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.

NOP-Targeted Peptide Ligands

The nociceptin/orphanin FQ (N/OFQ)-N/OFQ peptide (NOP) receptor system is widely distributed at both the peripheral and central level where it modulates important biological functions with increasing therapeutic implications. This chapter wants to provide a comprehensive and updated overview focused on the available structure-activity relationship studies on NOP receptor peptide ligands developed through different rational approaches. Punctual modifications and cyclizations of the N/OFQ sequence have been properly combined furnishing potent NOP selective ligands with different pharmacological activities (full and partial agonists, pure antagonists) and enhanced metabolic stability in vivo. The screening of peptide libraries provided a second family of NOP ligands that have been successfully optimized. Moreover, recent findings suggest the possibility to apply different multimerization strategies for the realization of multi-target NOP/opioid receptor ligands or tetrabranched N/OFQ derivatives with extraordinarily prolonged duration of action in vivo. The diverse approaches led to the identification of important pharmacological tools along with drug candidates currently in clinical development such as Rec 0438 (aka UFP-112) for the treatment of overactive bladder and SER 100 (aka ZP120) for the clinical management of systolic hypertension.

Probing ligand recognition of the opioid pan antagonist AT-076 at nociceptin, kappa, mu, and delta opioid receptors through structure-activity relationships

Few opioid ligands binding to the three classic opioid receptor subtypes, mu, kappa and delta, have high affinity at the fourth opioid receptor, the nociceptin/orphanin FQ receptor (NOP). We recently reported the discovery of AT-076 (1), (R)-7-hydroxy-N-((S)-1-(4-(3-hydroxyphenyl)piperidin-1-yl)-3-methylbutan-2-yl)-1,2,3,4-tetrahydroisoquinoline-3-carboxamide, a pan antagonist with nanomolar affinity for all four subtypes. Since AT-076 binds with high affinity at all four subtypes, we conducted a structure-activity relationship (SAR) study to probe ligand recognition features important for pan opioid receptor activity, using chemical modifications of key pharmacophoric groups. SAR analysis of the resulting analogs suggests that for the NOP receptor, the entire AT-076 scaffold is crucial for high binding affinity, but the binding mode is likely different from that of NOP antagonists C-24 and SB-612111 bound in the NOP crystal structure. On the other hand, modifications of the 3-hydroxyphenyl pharmacophore, but not the 7-hydroxy Tic pharmacophore, are better tolerated at kappa and mu receptors and yield very high affinity multifunctional (e.g. 12) or highly selective (e.g. 16) kappa ligands. With the availability of the opioid receptor crystal structures, our SAR analysis of the common chemotype of AT-076 suggests rational approaches to modulate binding selectivity, enabling the design of multifunctional or selective opioid ligands from such scaffolds.

Structure- and conformation-activity studies of nociceptin/orphanin FQ receptor dimeric ligands

The peptide nociceptin/orphanin FQ (N/OFQ) and the N/OFQ receptor (NOP) constitute a neuropeptidergic system that modulates various biological functions and is currently targeted for the generation of innovative drugs. In the present study dimeric NOP receptor ligands with spacers of different lengths were generated using both peptide and non-peptide pharmacophores. The novel compounds (12 peptide and 7 nonpeptide ligands) were pharmacologically investigated in a calcium mobilization assay and in the mouse vas deferens bioassay. Both structure- and conformation-activity studies were performed. Results demonstrated that dimerization did not modify the pharmacological activity of both peptide and non-peptide pharmacophores. Moreover, when dimeric compounds were obtained with low potency peptide pharmacophores, dimerization recovered ligand potency. This effect depends on the doubling of the C-terminal address sequence rather than the presence of an additional N-terminal message sequence or modifications of peptide conformation.

Pharmacological studies on the NOP and opioid receptor agonist PWT2-[Dmt1]N/OFQ(1-13)

An innovative chemical strategy named peptide welding technology (PWT) has been developed for the facile synthesis of tetrabranched peptides. [Dmt¹]N/OFQ(1-13)-NH₂ acts as a universal agonist for nociceptin/orphanin FQ (N/OFQ) and classical opioid receptors. The present study investigated the pharmacological profile of the PWT derivative of [Dmt¹]N/OFQ(1-13)NH₂ (PWT2-[Dmt¹]) in several assays in vitro and in vivo after spinal administration in monkeys subjected to the tail withdrawal assay. PWT2-[Dmt¹] mimicked the effects of [Dmt¹]N/OFQ(1-13)-NH₂ displaying full agonist activity, similar affinity/potency and selectivity at human recombinant N/OFQ (NOP) and opioid receptors in receptor binding, stimulation of [³⁵S]GTPγS binding, calcium mobilization in cells expressing chimeric G proteins, and BRET studies for measuring receptor/G-protein and receptor/β-arrestin 2 interaction. In vivo in monkeys PWT2-[Dmt¹] elicited dose-dependent and robust antinociceptive effects being more potent and longer lasting than [Dmt¹]N/OFQ(1-13)-NH₂. The analgesic action of PWT2-[Dmt¹] was sensitive to the NOP receptor antagonist J-113397, but not naltrexone. Thus, the present study demonstrated that the tetrabranched derivative of [Dmt¹]N/OFQ(1-13)-NH₂ obtained with the PWT technology maintains the in vitro pharmacological profile of the parent peptide but displays higher potency and longer lasting action in vivo.

Difference and Influence of Inactive and Active States of Cannabinoid Receptor Subtype CB2: From Conformation to Drug Discovery

Cannabinoid receptor 2 (CB2), a G protein-coupled receptor (GPCR), is a promising target for the treatment of neuropathic pain, osteoporosis, immune system, cancer, and drug abuse. The lack of an experimental three-dimensional CB2 structure has hindered not only the development of studies of conformational differences between the inactive and active CB2 but also the rational discovery of novel functional compounds targeting CB2. In this work, we constructed models of both inactive and active CB2 by homology modeling. Then we conducted two comparative 100 ns molecular dynamics (MD) simulations on the two systems-the active CB2 bound with both the agonist and G protein and the inactive CB2 bound with inverse agonist-to analyze the conformational difference of CB2 proteins and the key residues involved in molecular recognition. Our results showed that the inactive CB2 and the inverse agonist remained stable during the MD simulation. However, during the MD simulations, we observed dynamical details about the breakdown of the "ionic lock" between R131(3.50) and D240(6.30) as well as the outward/inward movements of transmembrane domains of the active CB2 that bind with G proteins and agonist (TM5, TM6, and TM7). All of these results are congruent with the experimental data and recent reports. Moreover, our results indicate that W258(6.48) in TM6 and residues in TM4 (V164(4.56)-L169(4.61)) contribute greatly to the binding of the agonist on the basis of the binding energy decomposition, while residues S180-F183 in extracellular loop 2 (ECL2) may be of importance in recognition of the inverse agonist. Furthermore, pharmacophore modeling and virtual screening were carried out for the inactive and active CB2 models in parallel. Among all 10 hits, two compounds exhibited novel scaffolds and can be used as novel chemical probes for future studies of CB2. Importantly, our studies show that the hits obtained from the inactive CB2 model mainly act as inverse agonist(s) or neutral antagonist(s) at low concentration. Moreover, the hit from the active CB2 model also behaves as a neutral antagonist at low concentration. Our studies provide new insight leading to a better understanding of the structural and conformational differences between two states of CB2 and illuminate the effects of structure on virtual screening and drug design.

Nociceptin/Orphanin FQ Receptor Structure, Signaling, Ligands, Functions, and Interactions with Opioid Systems

The NOP receptor (nociceptin/orphanin FQ opioid peptide receptor) is the most recently discovered member of the opioid receptor family and, together with its endogenous ligand, N/OFQ, make up the fourth members of the opioid receptor and opioid peptide family. Because of its more recent discovery, an understanding of the cellular and behavioral actions induced by NOP receptor activation are less well developed than for the other members of the opioid receptor family. All of these factors are important because NOP receptor activation has a clear modulatory role on mu opioid receptor-mediated actions and thereby affects opioid analgesia, tolerance development, and reward. In addition to opioid modulatory actions, NOP receptor activation has important effects on motor function and other physiologic processes. This review discusses how NOP pharmacology intersects, contrasts, and interacts with the mu opioid receptor in terms of tertiary structure and mechanism of receptor activation; location of receptors in the central nervous system; mechanisms of desensitization and downregulation; cellular actions; intracellular signal transduction pathways; and behavioral actions with respect to analgesia, tolerance, dependence, and reward. This is followed by a discussion of the agonists and antagonists that have most contributed to our current knowledge. Because NOP receptors are highly expressed in brain and spinal cord and NOP receptor activation sometimes synergizes with mu receptor-mediated actions and sometimes opposes them, an understanding of NOP receptor pharmacology in the context of these interactions with the opioid receptors will be crucial to the development of novel therapeutics that engage the NOP receptor.

The nociceptin receptor (NOPR) and its interaction with clinically important agonist molecules: a membrane molecular dynamics simulation study

The nociceptin receptor (NOPR) is an orphan G protein-coupled receptor that contains seven transmembrane helices. NOPR has a distinct mechanism of activation, though it shares a significant homology with other opioid receptors. Previously there have been reports on homology modeling of NOPR and also molecular dynamics simulation studies for a short period. Recently the crystal structure of NOPR was reported. In this study, we analyzed the time dependent behavior of NOPR docked with clinically important agonist molecules such as NOP (natural agonist) peptide and compound 10 (SCH-221510 derivative) using molecular dynamics simulations (MDS) for 100 ns. Molecular dynamics simulations of NOPR-agonist complexes allowed us to refine the system and to also identify stable structures with better binding modes. Structure activity relationships (SAR) for SCH221510 derivatives were investigated and reasons for the activities of these derivatives were determined. Our molecular dynamics trajectory analysis of NOPR-peptide and NOPR-compound 10 complexes found residues to be crucial for binding. Mutagenesis studies on the residues identified from our analysis could prove useful. Our results could also provide useful information in the structure-based drug design of novel and potent agonists targeting NOPR.

Structure-based virtual screening of the nociceptin receptor: hybrid docking and shape-based approaches for improved hit identification

The antagonist-bound crystal structure of the nociceptin receptor (NOP), from the opioid receptor family, was recently reported along with those of the other opioid receptors bound to opioid antagonists. We recently reported the first homology model of the ‘active-state’ of the NOP receptor, which when docked with ‘agonist’ ligands showed differences in the TM helices and residues, consistent with GPCR activation after agonist binding. In this study, we explored the use of the active-state NOP homology model for structure-based virtual screening to discover NOP ligands containing new chemical scaffolds. Several NOP agonist and antagonist ligands previously reported are based on a common piperidine scaffold. Given the structure–activity relationships for known NOP ligands, we developed a hybrid method that combines a structure-based and ligand-based approach, utilizing the active-state NOP receptor as well as the pharmacophoric features of known NOP ligands, to identify novel NOP binding scaffolds by virtual screening. Multiple conformations of the NOP active site including the flexible second extracellular loop (EL2) loop were generated by simulated annealing and ranked using enrichment factor (EF) analysis and a ligand–decoy dataset containing known NOP agonist ligands. The enrichment factors were further improved by combining shape-based screening of this ligand–decoy dataset and calculation of consensus scores. This combined structure-based and ligand-based EF analysis yielded higher enrichment factors than the individual methods, suggesting the effectiveness of the hybrid approach. Virtual screening of the CNS Permeable subset of the ZINC database was carried out using the above-mentioned hybrid approach in a tiered fashion utilizing a ligand pharmacophore-based filtering step, followed by structure-based virtual screening using the refined NOP active-state models from the enrichment analysis. Determination of the NOP receptor binding affinity of a selected set of top-scoring hits resulted in identification of several compounds with measurable binding affinity at the NOP receptor, one of which had a new chemotype for NOP receptor binding. The hybrid ligand-based and structure-based methodology demonstrates an effective approach for virtual screening that leverages existing SAR and receptor structure information for identifying novel hits for NOP receptor binding. The refined active-state NOP homology models obtained from the enrichment studies can be further used for structure-based optimization of these new chemotypes to obtain potent and selective NOP receptor ligands for therapeutic development.

Modelling of ORL1 receptor-ligand interactions

An opioid receptor like (ORL1) receptor is one of a family of G-protein-coupled receptors (GPCR); it represents a new pharmaceutical target with extensive therapeutic potential for the regulation of important biological functions such as nociception, mood disorders, drug abuse, learning or cardiovascular control. Although the crystal structure of the inactive form of the ORL1 receptor has been determined, little is known about its activation. By using X-ray structures of the β2-adrenegic receptor in its inactive (2RH1) and active (3P0G) states as templates, inactive and active homology models of the ORL1 receptor were constructed. Structurally diverse sets of strongly binding antagonists and agonists were docked with both ORL1 receptor forms. The major receptor-ligand interactions responsible for antagonist and agonist binding were identified. Although both sets of ligands, agonists and antagonists, bind to the same region of the receptor, they occupy partially different binding pockets. Agonists bind to the inactive receptor in a slightly different manner than antagonists. This difference is more pronounced in binding to the active ORL1 receptor model and points to the amino acids at the extracellular end of TM6, suggesting that this region is important for receptor-activation.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The therapeutic potential of nociceptin/orphanin FQ receptor agonists as analgesics without abuse liability

Although mu opioid (MOP) receptor agonists are the most commonly used analgesics for the treatment of moderate to severe pain in the clinic, the side effects of MOP agonists such as abuse liability limit their value as a medication. Research to identify novel analgesics without adverse effects is pivotal to advance the health care of humans. The nociceptin/orphanin FQ peptide (NOP) receptor, the fourth opioid receptor subtype, mediates distinctive actions in nonhuman primates which suggests the possibility that activity at this receptor may result in strong analgesia in the absence of virtually all of the side effects associated with MOP agonists. The present review highlights the recent progress of pharmacological studies of NOP-related ligands in primates. Selective NOP agonists, either peptidic or nonpeptidic, produce full analgesia in various assays in primates, when delivered systemically or intrathecally. Yet small molecule NOP agonists do not serve as reinforcers, indicating a lack of abuse liability. Given that NOP agonists have low abuse liability and that coactivation of NOP and MOP receptors produces synergistic antinociception, it is worth developing bifunctional NOP/MOP ligands. The outcomes of these studies and recent developments provide new perspectives to establish a translational bridge for understanding the biobehavioral functions of NOP receptors in primates and for facilitating the development of NOP-related ligands as a new generation of analgesics without abuse liability in humans.