Structural determinants of opioid and NOP receptor activity in derivatives of buprenorphine

The unique pharmacological profile of buprenorphine has led to its considerable success as an analgesic and as a treatment agent for drug abuse. Activation of nociceptin/orphanin FQ peptide (NOP) receptors has been postulated to account for certain aspects of buprenorphine’s behavioural profile. In order to investigate the role of NOP activation further, a series of buprenorphine analogues has been synthesised with the aim of increasing affinity for the NOP receptor. Binding and functional assay data on these new compounds indicate that the area around C20 in the orvinols is key to NOP receptor activity, with several compounds displaying higher affinity than buprenorphine. One compound, 1b, was found to be a mu opioid receptor partial agonist of comparable efficacy to buprenorphine, but with higher efficacy at NOP receptors

BU08073 a buprenorphine analog with partial agonist activity at μ-receptors <em> in vitro </em>but long-lasting opioid antagonist activity <i>in vivo</i> in mice

BACKGROUND AND PURPOSE: Buprenorphine is a potent analgesic with high affinity at μ, δ and κ and moderate affinity at nociceptin opioid (NOP) receptors. Nevertheless, NOP receptor activation modulates the in vivo activity of buprenorphine. Structure activity studies were conducted to design buprenorphine analogues with high affinity at each of these receptors and to characterize them in in vitro and in vivo assays. EXPERIMENTAL APPROACH: Compounds were tested for binding affinity and functional activity using [(35)S]GTPγS binding at each receptor and a whole-cell fluorescent assay at μ receptors. BU08073 was evaluated for antinociceptive agonist and antagonist activity and for its effects on anxiety in mice. KEY RESULTS: BU08073 bound with high affinity to all opioid receptors. It had virtually no efficacy at δ, κ and NOP receptors, whereas at μ receptors, BU08073 has similar efficacy as buprenorphine in both functional assays. Alone, BU08073 has anxiogenic activity and produces very little antinociception. However, BU08073 blocks morphine and U50,488-mediated antinociception. This blockade was not evident at 1 h post-treatment, but is present at 6 h and remains for up to 3–6 days. CONCLUSIONS AND IMPLICATIONS: These studies provide structural requirements for synthesis of ‘universal’ opioid ligands. BU08073 had high affinity for all the opioid receptors, with moderate efficacy at μ receptors and reduced efficacy at NOP receptors, a profile suggesting potential analgesic activity. However, in vivo, BU08073 had long-lasting antagonist activity, indicating that its pharmacokinetics determined both the time course of its effects and what receptor-mediated effects were observed. LINKED ARTICLES: This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-

Comparison of the Antinociceptive and Antirewarding Profiles of Novel Bifunctional Nociceptin Receptor/μ-Opioid Receptor Ligands: Implications for Therapeutic Applications

The nociceptin receptor (NOPr), a member of the opioid receptor family, is a target for the treatment of pain and drug abuse. Nociceptin/orphanin FQ (N/OFQ), the endogenous peptide for NOPr, not only modulates opioid antinociception, but also blocks the rewarding effects of several abused drugs, such as morphine, cocaine, and amphetamine. We hypothesized that NOPr agonists, with bifunctional activity at the μ-opioid receptor (MOPr), may function as nonaddicting analgesics or as drug abuse medications. Bifunctional small-molecule NOPr agonists possessing different selectivities and efficacies at MOPr were evaluated in an acute thermal antinociception assay, and for their ability to induce conditioned place preference (CPP) and their effect on morphine-induced CPP. 1-(1-Cyclooctylpiperidin-4-yl)-indolin-2-one) (SR14150), a high-affinity NOPr partial agonist, with low MOPr affinity and efficacy, produced analgesia that was naloxone-reversible. SR14150 did not induce CPP alone, nor did it attenuate morphine-induced CPP. 3-Ethyl-1-(1-(4-isopropylcyclohexyl)piperidin-4-yl)-indolin-2-one (SR16507), which has high affinity for both NOPr and MOPr, full agonist activity at NOPr, and partial agonist activity at MOPr, was also a potent analgesic and produced CPP alone, but also modestly attenuated morphine CPP. 1-(1-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)piperidinl-4-yl)-indolin-2-one (SR16835), a NOPr full agonist and low-affinity MOPr partial agonist, was not antinociceptive, did not produce CPP alone, but attenuated morphine CPP. Our results suggest that NOPr full-agonist activity is required to modulate opioid-induced reward, whereas a bifunctional NOPr/MOPr partial agonist profile may be suitable as a nonaddicting analgesic. The opioid-modulating effects of the NOPr ligands may be used effectively to produce better medications for treatment of drug abuse and pain

Nociceptin/Orphanin FQ Receptor Activation Attenuates Antinociception Induced by Mixed Nociceptin/Orphanin FQ/μ-Opioid Receptor Agonists

Activation of brain nociceptin/orphanin FQ (NOP) receptors leads to attenuation of μ-opioid receptor (MOP receptor)-mediated antinociception. Buprenorphine, a high-affinity partial MOP receptor agonist also binds to NOP receptors with 80 nM affinity. The buprenorphine-induced inverted U-shaped dose-response curve for antinociception may be due to NOP receptor activation, given that, in the presence of the NOP receptor antagonist, 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (J113397), or in NOP receptor knockout mice, buprenorphine has a steeper dose-response curve and acts as a full agonist. To further explore the involvement of the direct activation of NOP receptors by buprenorphine and other compounds that activate both NOP and MOP receptors, the antinociceptive effects of 1-(1-(2,3,3α,4,5,6-hexahydro-1H-phenalen-1-yl)piperidin-4-yl)-indolin-2-one. (SR16435), 3-ethyl-1-(1-(4-isopropylcyclohexyl)piperidin-4-yl)-indolin-2-one (SR16507), buprenorphine, pentazocine, and morphine, compounds with varying levels of MOP and NOP receptor affinity and efficacy, were assessed in mice using the tail-flick assay. The ability of the selective NOP receptor antagonist (−)-cis-1-methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111) to potentiate antinociception induced by the above compounds was examined to investigate whether activation of NOP receptors leads to attenuation of MOP receptor-mediated antinociception. SB-612111 potentiated antinociception induced by buprenorphine and the other mixed NOP/MOP receptor agonists SR16435 and SR16507. However, SB-612111 had no effect on pentazocine or morphine antinociception, two compounds with no NOP receptor-binding affinity. These results further support the hypothesis that activation of NOP receptors can lead to attenuation of MOP receptor-mediated antinociception elicited by mixed NOP/MOP receptor compounds such as buprenorphine, SR16435, and SR16507 and that, although buprenorphine has low efficacy in vitro, it has significant NOP receptor agonist activity in vivo

The First Universal Opioid Ligand, (2S)-2-[(5R,6R,7R,14S)-N-cyclopropylmethyl-4,5-epoxy-6,14-ethano-3-hydroxy-6-methoxymorphinan-7-yl]-3,3-dimethylpentan-2-ol (BU08028): Characterization of the In Vitro Profile and In Vivo Behavioral Effects in Mouse Models of Acute Pain and Cocaine-Induced Reward

Certain behavioral features of buprenorphine, including a bell-shaped curve for antinociception and attenuation of alcohol consumption, are thought to be mediated by activation of nociceptin/orphanin FQ peptide (NOP) receptors, despite moderate affinity and low efficacy at NOP receptors. We hypothesized that ligands with buprenorphine's physical properties, but possessing increased NOP receptor affinity and efficacy, would improve the profile as a drug abuse medication and reduce addiction liability. Using this strategy, we designed several compounds with universally high affinity, i.e., less than 10 nM at μ, δ, κ, and NOP receptors. Among these, (2S)-2-[(5R,6R,7R,14S)-N-cyclopropylmethyl-4,5-epoxy-6,14-ethano-3-hydroxy-6-methoxymorphinan-7-yl]-3,3-dimethylpentan-2-ol (BU08028) has high affinity at all opioid receptors and increased NOP receptor efficacy in vitro in the [35S]GTPγS binding assay, however, while still being a partial agonist. In vivo, BU08028 was evaluated in an acute thermal antinociception assay, for its ability to induce conditioned place preference (CPP), and for its effect on cocaine-induced CPP. BU08028 is a very potent long-lasting analgesic. It produces an increase in locomotor activity and a significant CPP. As a pretreatment to cocaine, BU08028 does not alter cocaine CPP but causes a further increase in cocaine-induced locomotor activity. The analgesic, rewarding, and stimulant effects are probably caused by μ receptor stimulation. It is likely that with BU08028, a partial agonist at both NOP and μ receptors, μ-mediated activity overpowers NOP-mediated effects. Thus, it is possible that a different buprenorphine analog that is a universal high-affinity opioid ligand but with “full agonist” activity at NOP may counteract traditional opioid-mediated effects such as antinociception and reward

AT-1001: A high-affinity α3β4 nAChR ligand with novel nicotine-suppressive pharmacology

BACKGROUND AND PURPOSE: The α3β4 subtype of nicotinic acetylcholine receptors (nAChRs) has been implicated in mediating nicotine reinforcement processes. AT-1001 has been recently described as a high-affinity and selective α3β4 nAChR antagonist that blocks nicotine self-administration in rats. The aim of this study was to investigate the mechanism of action underlying the nicotine-suppressive effects of AT-1001. EXPERIMENTAL APPROACH: Effects of AT-1001 were determined using in vitro assays and rat models of nicotine addiction, and compared with varenicline. KEY RESULTS: AT-1001 and its analogue AT-1012 were functionally selective as antagonists for α3β4 over α4β2 nAChRs, but not to the same extent as the binding selectivity, and had partial agonist activity at α3β4 nAChRs. In contrast, varenicline was a partial agonist at α4β2, a weak agonist at α3β4 and inhibited α4β2 at a much lower concentration than it inhibited α3β4 nAChRs. AT-1001 and varenicline also had very different in vivo properties. Firstly, AT-1001 did not exhibit reinforcing properties per se while varenicline was self-administered. Secondly, systemic treatment with AT-1001 did not induce reinstatement of nicotine seeking but rather attenuated reinstatement induced by varenicline, as well as nicotine. Finally, unlike varenicline, AT-1001 selectively blocked nicotine self-administration without altering alcohol lever pressing as assessed in an operant co-administration paradigm. CONCLUSIONS AND IMPLICATIONS: These findings describe a more complex AT-1001 in vitro profile than previously appreciated and provide further support for the potential of AT-1001 and congeners as clinically useful compounds for smoking cessation, with a mechanism of action distinct from currently available medications