Development of nociceptin receptor (NOP) agonists and antagonists

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.

Mechanistic Characterization of the Pharmacological Profile of HS-731, a Peripherally Acting Opioid Analgesic, at the µ-, δ-, κ-Opioid and Nociceptin Receptors

Accumulated preclinical and clinical data show that peripheral restricted opioids provide pain relief with reduced side effects. The peripherally acting opioid analgesic HS-731 is a potent dual μ-/δ-opioid receptor (MOR/DOR) full agonist, and a weak, partial agonist at the κ-opioid receptor (KOR). However, its binding mode at the opioid receptors remains elusive. Here, we present a comprehensive in silico evaluation of HS-731 binding at all opioid receptors. We provide insights into dynamic interaction patterns explaining the different binding and activity of HS-731 on the opioid receptors. For this purpose, we conducted docking, performed molecular dynamics (MD) simulations and generated dynamic pharmacophores (dynophores). Our results highlight two residues important for HS-731 recognition at the classical opioid receptors (MOR, DOR and KOR), particular the conserved residue 5.39 (K) and the non-conserved residue 6.58 (MOR: K, DOR: W and KOR: E). Furthermore, we assume a salt bridge between the transmembrane helices (TM) 5 and 6 via K2275.39 and E2976.58 to be responsible for the partial agonism of HS-731 at the KOR. Additionally, we experimentally demonstrated the absence of affinity of HS-731 to the nociceptin/orphanin FQ peptide (NOP) receptor. We consider the morphinan phenol Y1303.33 responsible for this affinity lack. Y1303.33 points deep into the NOP receptor binding pocket preventing HS-731 binding to the orthosteric binding pocket. These findings provide significant structural insights into HS-731 interaction pattern with the opioid receptors that are important for understanding the pharmacology of this peripheral opioid analgesic.

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.

Comprehensive overview of biased pharmacology at the opioid receptors: biased ligands and bias factors

One of the main challenges in contemporary medicinal chemistry is the development of safer analgesics, used in the treatment of pain. Currently, moderate to severe pain is still treated with the "gold standard" opioids whose long-term often leads to severe side effects. With the discovery of biased agonism, the importance of this area of pharmacology has grown exponentially over the past decade. Of these side effects, tolerance, opioid misuse, physical dependence and substance use disorder (SUD) stand out, since these have led to many deaths over the past decades in both USA and Europe. New therapeutic molecules that induce a biased response at the opioid receptors (MOR, DOR, KOR and NOP receptor) are able to circumvent these side effects and, consequently, serve as more advantageous therapies with great promise. The concept of biased signaling extends far beyond the already sizeable field of GPCR pharmacology and covering everything would be vastly outside the scope of this review which consequently covers the biased ligands acting at the opioid family of receptors. The limitation of quantifying bias, however, makes this a controversial subject, where it is dependent on the reference ligand, the equation or the assay used for the quantification. Hence, the major issue in the field of biased ligands remains the translation of the in vitro profiles of biased signaling, with corresponding bias factors to in vivo profiles showing the presence or the lack of specific side effects. This review comprises a comprehensive overview of biased ligands in addition to their bias factors at individual members of the opioid family of receptors, as well as bifunctional ligands.

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.

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.

NOP receptor pharmacological profile - A dynamic mass redistribution study

The Nociceptin/Orphanin FQ (N/OFQ) peptide NOP receptor is coupled to pertussis toxin (PTX)-sensitive G proteins (G_i/o) whose activation leads to the inhibition of both cAMP production and calcium channel activity, and to the stimulation of potassium currents. The label free dynamic mass redistribution (DMR) approach has been demonstrated useful for investigating the pharmacological profile of G protein-coupled receptors. Herein, we employ DMR technology to systematically characterize the pharmacology of a large panel of NOP receptor ligands. These are of peptide and non-peptide nature and display varying degrees of receptor efficacy, ranging from full agonism to pure antagonism. Using Chinese hamster ovary (CHO) cells expressing the human NOP receptor we provide rank orders of potency for full and partial agonists as well as apparent affinities for selective antagonists. We find the pharmacological profile of NOP receptor ligands to be similar but not identical to values reported in the literature using canonical assays for G_i/o-coupled receptors. Our data demonstrate that holistic label-free DMR detection can be successfully used to investigate the pharmacology of the NOP receptor and to characterize the cellular effects of novel NOP receptor ligands.

Patent Highlights February-March 2018

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research development.

NOP Ligands for the Treatment of Anxiety and Mood Disorders

Many studies point toward the nociceptin/orphanin FQ (N/OFQ) and the N/OFQ peptide receptor (NOP) as targets for the development of innovative drugs for treating anxiety- and mood-related disorders. Evidence supports the view that the activation of NOP receptors with agonists elicits anxiolytic-like effects, while its blockade with NOP antagonists promotes antidepressant-like actions in rodents. Genetic studies showed that NOP receptor knockout mice display an antidepressant-like phenotype, and NOP antagonists are inactive in these animals. In contrast, the genetic blockade of NOP receptor signaling generally displays an increase of anxiety states in the elevated plus-maze test. In this chapter we summarized the most relevant findings of NOP receptor ligands in the modulation of anxiety and mood disorders, and the putative mechanisms of action are discussed.

Multifunctional Opioid Ligands

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

In vitro pharmacological characterization of a novel unbiased NOP receptor-selective nonpeptide agonist AT-403

Nociceptin/orphanin FQ (N/OFQ) regulates several biological functions via selective activation of the N/OFQ receptor (NOP), a member of the opioid receptor family. We recently identified a new high affinity and highly selective NOP agonist AT-403. In this study, we characterized the functional profile of AT-403 and compared it to other known nonpeptide NOP agonists Ro 65-6570, Ro 2q, SCH-221510, MCOPPB, AT-202 and SCH-486757, using the following assays: GTPγ[³⁵ S] stimulated binding, calcium mobilization assay in cells-expressing human NOP or classical opioid receptors and chimeric G proteins, bioluminescence resonance energy transfer (BRET) based assay for studying NOP receptor interaction with G protein and arrestin, and the electrically stimulated mouse vas deferens bioassay. All compounds behaved as NOP full agonists consistently showing the following rank order of potency MCOPPB > AT-403 > Ro 65-6570 = Ro 2q > SCH-221510 >  AT-202 > SCH-486757. AT-403 and MCOPPB displayed the highest NOP selectivity both at human and murine receptors. Interestingly, while all the other nonpeptide NOP agonists displayed bias toward G protein-mediated signaling in the BRET assay, AT-403, similar to the natural ligand N/OFQ, behaved as an unbiased agonist, activating G-protein-mediated function as well as arrestin recruitment. AT-403 may be a useful nonpeptide tool compound to study the pharmacology of NOP activation in disease states.

Bifunctional opioid/nociceptin hybrid KGNOP1 effectively attenuates pain-related behaviour in a rat model of neuropathy

A bifunctional peptide containing an opioid and nociceptin receptor-binding pharmacophore, H-Dmt-D-Arg-Aba-β-Ala-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2 (KGNOP1), was tested for its analgesic properties when administered intrathecally in naïve and chronic constriction injury (CCI)-exposed rats with neuropathy-like symptoms. KGNOP1 significantly increased the acute pain threshold, as measured by the tail-flick test, and also increased the threshold of a painful reaction to mechanical and thermal stimuli in CCI-exposed rats. Both of the effects could be blocked by pre-administration of [Nphe1]-Nociceptin (1-13)-NH2 (NPhe) or naloxone, antagonists for nociceptin and opioid receptors, respectively. This led us to conclude that KGNOP1 acts as a dual opioid and nociceptin receptor agonist in vivo. The analgesic effect of KGNOP1 proved to be more powerful than clinical drugs such as morphine and buprenorphine. Repeated daily intrathecal injections of KGNOP1 led to the development of analgesic tolerance, with the antiallodynic action being completely abolished on day 6. Nevertheless, the development of tolerance to the antihyperalgesic effect was delayed in comparison to morphine, which lost its efficacy as measured by the cold plate test after 3days of daily intrathecal administration, whereas KGNOP1 was efficient up to day 6. A single intrathecal injection of morphine to KGNOP1-tolerant rats did not raise the pain threshold in any of the behavioural tests; in contrast, a single intrathecal dose of KGNOP1 significantly suppressed allodynia and hyperalgesia in morphine-tolerant rats.

Bifunctional peptide-based opioid agonist/nociceptin antagonist ligand for dual treatment of nociceptive and neuropathic pain

Drugs able to treat both nociceptive and neuropathic pain effectively without major side effects are lacking. We developed a bifunctional peptide-based hybrid (KGNOP1) that structurally combines a mu-opioid receptor agonist (KGOP1) with antinociceptive activity and a weak nociceptin receptor antagonist (KGNOP3) with anti-neuropathic pain activity. We investigated KGNOP1-related behavioral effects after intravenous administration in rats by assessing thermal nociception, cold hyperalgesia in a model of neuropathic pain induced by chronic constriction injury of the sciatic nerve, and plethysmography parameters including inspiratory time (TI) and minute ventilation (VM) in comparison to the well-known opioid analgesics, tramadol and morphine. Time-course and dose-dependent effects were investigated for all behavioral parameters to determine the effective doses 50% (ED50). Pain-related effects on cold hyperalgesia were markedly increased by KGNOP1 as compared to KGNOP3 and tramadol (ED50: 0.0004, 0.32, and 12.1 μmol/kg, respectively), whereas effects on thermal nociception were significantly higher with KGNOP1 as compared to morphine (ED50: 0.41 and 14.7 μmol/kg, respectively). KGNOP1 and KGOP1 produced a larger increase in TI and deleterious decrease in VM in comparison to morphine and tramadol (ED50(TI): 0.63, 0.52, 12.2, and 50.9 μmol/kg; ED50(VM): 0.57, 0.66, 10.6, and 50.0 μmol/kg, respectively). Interestingly, the calculated ratios of anti-neuropathic pain/antinociceptive to respiratory effects revealed that KGNOP1 was safer than tramadol (ED50 ratio: 5.44 × 10 vs 0.24) and morphine (ED50 ratio: 0.72 vs 1.39). We conclude that KGNOP1 is able to treat both experimental neuropathic and nociceptive pain, more efficiently and safely than tramadol and morphine, respectively, and thus should be a candidate for future clinical developments.

In vitro functional characterization of novel nociceptin/orphanin FQ receptor agonists in recombinant and native preparations

Nociceptin/Orphanin FQ (N/OFQ) regulates several biological functions via selective activation of the N/OFQ receptor (NOP). In this study novel nonpeptide NOP ligands were characterized in vitro in receptor binding and [³⁵S]GTPγS stimulated binding in membranes of cells expressing human NOP and classical opioid receptors, calcium mobilization assay in cells coexpressing the receptors and chimeric G proteins, bioluminescence resonance energy transfer (BRET) based assay for studying NOP receptor interaction with G protein and arrestin, the electrically stimulated mouse vas deferens and the mouse colon bioassays. The action of the AT compounds were compared with standard NOP agonists (N/OFQ and Ro 65-6570) and the NOP selective antagonist SB-612111. AT compounds displayed high NOP affinity and behaved as NOP agonists in all the functional assays consistently showing the following rank order of potency AT-127≥AT-090≥AT-035>AT-004= AT-001. AT compounds behaved as NOP full agonists in the calcium mobilization and mouse colon assays and as partial agonists in the [³⁵S]GTPγS and BRET assays. Interestingly AT-090 and AT-127, contrary to standard nonpeptide agonists that display G protein biased agonism, behaved as an unbiased agonists. AT-090 and AT-127 displayed higher NOP selectivity than Ro 65-6570 at native mouse receptors. AT-090 and AT-127 might be useful pharmacological tools for investigating the therapeutic potential of NOP partial agonists.

Bifunctional Peptide-Based Opioid Agonist-Nociceptin Antagonist Ligands for Dual Treatment of Acute and Neuropathic Pain

Herein, the opioid pharmacophore H-Dmt-d-Arg-Aba-β-Ala-NH2 (7) was linked to peptide ligands for the nociceptin receptor. Combination of 7 and NOP ligands (e.g., H-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2) led to binding affinities in the low nanomolar domain. In vitro, the hybrids behaved as agonists at the opioid receptors and antagonists at the nociceptin receptor. Intravenous administration of hybrid 13a (H-Dmt-d-Arg-Aba-β-Ala-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2) to mice resulted in potent and long lasting antinociception in the tail-flick test, indicating that 13a was able to permeate the BBB. This was further supported by a cell-based BBB model. All hybrids alleviated allodynia and hyperalgesia in neuropathic pain models. Especially with respect to hyperalgesia, they showed to be more effective than the parent compounds. Hybrid 13a did not result in significant respiratory depression, in contrast to an equipotent analgesic dose of morphine. These hybrids hence represent a promising avenue toward analgesics for the dual treatment of acute and neuropathic pain.

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.

A Novel Nociceptin Receptor Antagonist LY2940094 Inhibits Excessive Feeding Behavior in Rodents: A Possible Mechanism for the Treatment of Binge Eating Disorder

Nociceptin/orphanin FQ (N/OFQ), a 17 amino acid peptide, is the endogenous ligand of the ORL1/nociceptin-opioid-peptide (NOP) receptor. N/OFQ appears to regulate a variety of physiologic functions including stimulating feeding behavior. Recently, a new class of thienospiro-piperidine-based NOP antagonists was described. One of these molecules, LY2940094 has been identified as a potent and selective NOP antagonist that exhibited activity in the central nervous system. Herein, we examined the effects of LY2940094 on feeding in a variety of behavioral models. Fasting-induced feeding was inhibited by LY2940094 in mice, an effect that was absent in NOP receptor knockout mice. Moreover, NOP receptor knockout mice exhibited a baseline phenotype of reduced fasting-induced feeding, relative to wild-type littermate controls. In lean rats, LY2940094 inhibited the overconsumption of a palatable high-energy diet, reducing caloric intake to control chow levels. In dietary-induced obese rats, LY2940094 inhibited feeding and body weight regain induced by a 30% daily caloric restriction. Last, in dietary-induced obese mice, LY2940094 decreased 24-hour intake of a high-energy diet made freely available. These are the first data demonstrating that a systemically administered NOP receptor antagonist can reduce feeding behavior and body weight in rodents. Moreover, the hypophagic effect of LY2940094 is NOP receptor dependent and not due to off-target or aversive effects. Thus, LY2940094 may be useful in treating disorders of appetitive behavior such as binge eating disorder, food choice, and overeating, which lead to obesity and its associated medical complications and morbidity.

The Importance of Ligand-Receptor Conformational Pairs in Stabilization: Spotlight on the N/OFQ G Protein-Coupled Receptor

Understanding the mechanism by which ligands affect receptor conformational equilibria is key in accelerating membrane protein structural biology. In the case of G protein-coupled receptors (GPCRs), we currently pursue a brute-force approach for identifying ligands that stabilize receptors and facilitate crystallogenesis. The nociceptin/orphanin FQ peptide receptor (NOP) is a member of the opioid receptor subfamily of GPCRs for which many structurally diverse ligands are available for screening. We observed that antagonist potency is correlated with a ligand's ability to induce receptor stability (Tm) and crystallogenesis. Using this screening strategy, we solved two structures of NOP in complex with top candidate ligands SB-612111 and C-35. Docking studies indicate that while potent, stabilizing antagonists strongly favor a single binding orientation, less potent ligands can adopt multiple binding modes, contributing to their low Tm values. These results suggest a mechanism for ligand-aided crystallogenesis whereby potent antagonists stabilize a single ligand-receptor conformational pair.

Pharmacological Profile of Nociceptin/Orphanin FQ Receptors Interacting with G-Proteins and β-Arrestins 2

Nociceptin/orphanin FQ (N/OFQ) controls several biological functions by selectively activating an opioid like receptor named N/OFQ peptide receptor (NOP). Biased agonism is emerging as an important and therapeutically relevant pharmacological concept in the field of G protein coupled receptors including opioids. To evaluate the relevance of this phenomenon in the NOP receptor, we used a bioluminescence resonance energy transfer technology to measure the interactions of the NOP receptor with either G proteins or β-arrestin 2 in the absence and in presence of increasing concentration of ligands. A large panel of receptor ligands was investigated by comparing their ability to promote or block NOP/G protein and NOP/arrestin interactions. In this study we report a systematic analysis of the functional selectivity of NOP receptor ligands. NOP/G protein interactions (investigated in cell membranes) allowed a precise estimation of both ligand potency and efficacy yielding data highly consistent with the known pharmacological profile of this receptor. The same panel of ligands displayed marked differences in the ability to promote NOP/β-arrestin 2 interactions (evaluated in whole cells). In particular, full agonists displayed a general lower potency and for some ligands an inverted rank order of potency was noted. Most partial agonists behaved as pure competitive antagonists of receptor/arrestin interaction. Antagonists displayed similar values of potency for NOP/Gβ₁ or NOP/β-arrestin 2 interaction. Using N/OFQ as reference ligand we computed the bias factors of NOP ligands and a number of agonists with greater efficacy at G protein coupling were identified.

Discovery of the first small-molecule opioid pan antagonist with nanomolar affinity at mu, delta, kappa, and nociceptin opioid receptors

The trans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine scaffold is a known pharmacophore for mu opioid (MOP), kappa opioid (KOP), and delta opioid (DOP) receptor antagonists; however, it has not been explored in nociceptin opioid (NOP/ORL-1) receptor ligands. We recently found that the selective KOP antagonist JDTic, (3R)-7-hydroxy-N-((1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl)-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide, containing this opioid antagonist pharmacophore, has significant binding affinity at the NOP receptor (Ki 16.67 ± 0.76 nM), with no intrinsic activity in the [(35)S]GTPγS functional assay. Since this is the first ligand containing the trans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine opioid antagonist pharmacophore to have affinity for the NOP receptor, we explored the structural determinants of its NOP binding affinity. When rational chemical modifications of JDTic were carried out, based on our previously established NOP pharmacophoric structure-activity relationship (SAR) model, most modifications led to a significant decrease in NOP and opioid binding affinity compared to JDTic. Interestingly, however, removal of the 3,4-dimethyl groups of the trans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine antagonist scaffold of JDTic increased the binding affinity at NOP by 10-fold (Ki 1.75 ± 0.74 nM) while maintaining comparable affinity for KOP, MOP, and DOP receptors (Ki 1.14 ± 0.63, 1.67 ± 0.6, and 19.6 ± 1.3 nM, respectively). In vitro functional efficacy studies using the [(35)S]GTPγS assay showed that this compound AT-076 functions as an antagonist at all four opioid receptors. Detailed characterization of the antagonist activity of AT-076 shows that it has a noncompetitive antagonist profile at the NOP and KOP receptors (insurmountable antagonism), but is a potent competitive antagonist at the MOP and DOP receptors, with Ke values 3-6-fold more potent than those of JDTic. AT-076 is the first opioid pan antagonist with high affinity at all four opioid receptor subtypes. Our SAR studies show that the 3,4-dimethyl groups of the well-known trans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine opioid antagonist scaffold may be removed without significant loss in binding affinity or antagonist potency to obtain an opioid pan antagonist such as AT-076.

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.

Discovery of a novel series of orally active nociceptin/orphanin FQ (NOP) receptor antagonists based on a dihydrospiro(piperidine-4,7'-thieno[2,3-c]pyran) scaffold

Nociceptin/OFQ (N/OFQ) is a 17 amino acid peptide that is the endogenous ligand for the ORL1/NOP receptor. Nociceptin appears to regulate a host of physiological functions such as biological reactions to stress, anxiety, mood, and drug abuse, in addition to feeding behaviors. To develop tools to study the function of nociceptin and NOP receptor, our research effort sought to identify orally available NOP antagonists. Our effort led to the discovery of a novel chemical series based on the dihydrospiro(piperidine-4,7'-thieno[2,3-c]pyran) scaffold. Herein we show that dihydrospiro(piperidine-4,7'-thieno[2,3-c]pyran)-derived compounds are potent NOP antagonists with high selectivity versus classical opioid receptors (μ, δ, and κ). Moreover, these compounds exhibit sufficient bioavailability to produce a high level of NOP receptor occupancy in the brain following oral administration in rats.

Nociceptin/orphanin FQ receptor antagonists as innovative antidepressant drugs

Nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP) were identified in the mid 90s as a novel peptidergic system structurally related to opioids. A growing body of preclinical evidence suggests that blockade of NOP receptors evokes antidepressant-like actions. These have been explored using a range of compounds (peptide and non peptide antagonists), across different species (rat and mouse) and assays (behavioral despair and chronic mild stress) suggesting a robust and consistent antidepressant-like effect. Moreover, rats and mice knockout for the NOP receptor gene display an antidepressant-like phenotype in behavioral despair assays. Electrophysiological, immunohistochemical and neurochemical studies point to an important role played by monoaminergic systems, particularly 5-HTergic, in mediating the antidepressant-like properties of NOP antagonists. However other putative mechanisms of action, including modulation of the CRF system, circadian rhythm and a possible neuroendocrine-immune control might be involved. A close relationship between the N/OFQ-NOP receptor system and stress responses is well described in the literature. Stressful situations also alter endocrine, behavioral and neurochemical parameters in rats and chronic administration of a NOP antagonist restored these alterations. Interestingly, clinical findings showed that plasma N/OFQ levels were significantly altered in major and post-partum depression, and bipolar disease patients. Collectively, data in the literature support the notion that blockade of NOP receptor signaling could be a novel and interesting strategy for the development of innovative antidepressants.

Probiotics VSL#3 protect against development of visceral pain in murine model of irritable bowel syndrome

BACKGROUND AND AIMS

Irritable bowel syndrome (IBS) is linked to post-inflammatory and stress-correlated factors that cause changes in the perception of visceral events. Probiotic bacteria may be effective in treating IBS symptoms. Here, we have investigated whether early life administration of VSL#3, a mixture of 8 probiotic bacteria strains, protects against development of visceral hypersensitivity driven by neonatal maternal separation (NMS), a rat model of IBS.

METHODS

Male NMS pups were treated orally with placebo or VSL#3 from days 3 to 60, while normal, not separated rats were used as controls. After 60 days from birth, perception of painful sensation induced by colorectal distension (CRD) was measured by assessing the abdominal withdrawal reflex (score 0-4). The colonic gene expression was assessed by using the Agilent Whole Rat Genome Oligo Microarrays platform and confirmed by real time PCR.

RESULTS

NMS rats exhibited both hyperalgesia and allodynia when compared to control rats. VSL#3 had a potent analgesic effect on CRD-induced pain without changing the colorectal compliance. The microarray analysis demonstrated that NMS induces a robust change in the expression of subsets of genes (CCL2, NOS3, THP1, NTRK1, CCR2, BDRKRB1, IL-10, TNFRSF1B, TRPV4, CNR1 and OPRL1) involved in pain transmission and inflammation. TPH1, tryptophan hydroxylase 1, a validated target gene in IBS treatment, was markedly upregulated by NMS and this effect was reversed by VSL#3 intervention.

CONCLUSIONS

Early life administration of VSL#3 reduces visceral pain perception in a model of IBS and resets colonic expression of subsets of genes mediating pain and inflammation.

TRANSCRIPT PROFILING

Accession number of repository for expression microarray data is GSE38942 (http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE38942).

Deciphering intracellular localization and physiological role of nociceptin and nocistatin

Nociceptin and nocistatin are endogenous ligands of G protein coupled receptor family. Numerous techniques have been used to study the diverse parameters including, localization, distribution and ultrastructure of these peptides. The majority of the study parameters are based on their physiological roles in different organ systems. The present study presents an overview of the different methods used for the study of nociceptin, nocistatin and their receptors. Nociceptin has been implicated in many physiological functions including, nociception, locomotion, stressed-induced analgesia, learning and memory, neurotransmitter and hormone release, renal function, neuronal differentiation, sexual and reproductive behavior, uterine contraction, feeding, anxiety, gastrointestinal motility, cardiovascular function, micturition, cough, hypoxic-ischemic brain injury, diuresis and sodium balance, temperature regulation, vestibular function, and mucosal transport. It has been noted that the use of light and electron microscopy was less frequent, though it may be one of the most promising tools to study the intracellular localization of these neuropeptides. In addition, more studies on the level of circulating nociceptin and nocistatin are also necessary for investigating their clinical roles in health and disease. A variety of modern tools including physiological, light and electron microscopy (EM) are needed to decipher the extent of intracellular localization, tissue distribution and function of these peptides. The intracellular localization of nociceptin and nocistatin will require a high resolution transmission EM capable of identifying these peptides and other supporting molecules that co-localize with them. A tracing technique could also elucidate a possible migratory ability of nociceptin and nocistatin from one cellular compartment to the other.

[Dmt1]N/OFQ(1-13)-NH2: a potent nociceptin/orphanin FQ and opioid receptor universal agonist

BACKGROUND AND PURPOSE

Intrathecally (i.t.) administered nociceptin/orphanin FQ (N/OFQ) evokes antinociceptive effects in rodents. Recent studies in monkeys demonstrated that i.t. co-application of N/OFQ and morphine elicits synergistic antinociceptive actions suggesting mixed N/OFQ peptide (NOP) and μ opioid receptor agonists as innovative spinal analgesics. Thus, novel N/OFQ related peptides were synthesized in order to identify and pharmacologically characterize a mixed NOP/ μ opioid receptor agonist.

EXPERIMENTAL APPROACH

The following in vitro assays were used: calcium mobilization in cells expressing the human NOP or classical opioid receptors and chimeric G proteins, receptor and [(35)S]-GTPγS binding, [(35)S]-GTPγS binding in rat spinal cord membranes, guinea pig ileum bioassay. In vivo experiments were performed in monkeys using the tail withdrawal assay.

KEY RESULTS

From calcium mobilization studies [Dmt(1)]N/OFQ(1-13)-NH(2) was selected as the most potent and least selective compound. The mixed NOP/opioid full agonist activity and high affinity of [Dmt(1)]N/OFQ(1-13)-NH(2) was confirmed at human recombinant receptors in receptor binding, calcium mobilization and/or [(35)S]-GTPγS binding studies, at rat spinal cord receptors in [(35)S]-GTPγS binding experiments, and at guinea pig receptors inhibiting neurogenic contractions in the ileum. In vivo in the tail withdrawal assay in monkeys i.t. [Dmt(1) ]N/OFQ(1-13)-NH(2) was able to elicit robust and long-lasting antinociceptive effects.

CONCLUSIONS AND IMPLICATIONS

Collectively, these results demonstrate that [Dmt(1)]N/OFQ(1-13)-NH(2) behaves as NOP/opioid receptor universal agonist and substantiate the suggestion that such mixed ligands are worthy of development as innovative spinal analgesics.

Structural requirements of N-substituted spiropiperidine analogues as agonists of nociceptin/orphanin FQ receptor

The nociceptin/orphanin FQ (NOP) receptor is involved in a wide range of biological functions, including pain, anxiety, depression and drug abuse. Especially, its agonists have great potential to be developed into anxiolytics. In this work, both the ligand- and receptor-based three-dimensional quantitative structure–activity relationship (3D-QSAR) studies were carried out using comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) techniques on 103 N-substituted spiropiperidine analogues as NOP agonists. The resultant optimal ligand-based CoMSIA model exhibited Q² of 0.501, R²_ncv of 0.912 and its predictive ability was validated by using an independent test set of 26 compounds which gave R²_pred value of 0.818. In addition, docking analysis and molecular dynamics simulation (MD) were also applied to elucidate the probable binding modes of these agonists. Interpretation of the 3D contour maps, in the context of the topology of the active site of NOP, provided insight into the NOP-agonist interactions. The information obtained from this work can be used to accurately predict the binding affinity of related agonists and also facilitate the future rational design of novel agonists with improved activity.

Novel helix-constrained nociceptin derivatives are potent agonists and antagonists of ERK phosphorylation and thermal analgesia in mice

The nociceptin opioid peptide receptor (NOP, NOR, ORL-1) is a GPCR that recognizes nociceptin, a 17-residue peptide hormone. Nociceptin regulates pain transmission, learning, memory, anxiety, locomotion, cardiovascular and respiratory stress, food intake, and immunity. Nociceptin was constrained using an optimized helix-inducing cyclization strategy to produce the most potent NOP agonist (EC50 = 40 pM) and antagonist (IC50 = 7.5 nM) known. Alpha helical structures were measured in water by CD and 2D (1)H NMR spectroscopy. Agonist and antagonist potencies, evaluated by ERK phosphorylation in mouse neuroblastoma cells natively expressing NOR, increased 20-fold and 5-fold, respectively, over nociceptin. Helix-constrained peptides with key amino acid substitutions had much higher in vitro activity, serum stability, and thermal analgesic activity in mice, without cytotoxicity. The most potent agonist increased hot plate contact time from seconds up to 60 min; the antagonist prevented this effect. Such helix-constrained peptides may be valuable physiological probes and therapeutics for treating some forms of pain.