Search

Position 6 of the morphinan skeleton plays a key role in the μ-opioid receptor (MOR) activity in vitro and in vivo. We describe the consequence of the 6-carbonyl group deletion in N-methylmorphinan-6-ones 1-4 on ligand-MOR interaction, signaling, and antinociception. While 6-desoxo compounds 1a, 2a, and 4a show similar profiles to their 6-keto counterparts, the 6-desoxo-14-benzyloxy substituted 3a displays significantly increased MOR binding and agonist potency and a distinct binding mode compared with its analogue 3.

We previously reported on a series of small molecules targeting the κ-opioid (KOP) receptor featuring a diphenethylamine scaffold and showed the promise of these ligands as effective analgesics with reduced liability for adverse effects. This study expands the structure-activity relationships on our original series by presenting several modifications in the lead compounds 1 (HS665) and 2 (HS666). A library of new diphenethylamines was designed, synthesized, and pharmacologically evaluated. In comparison with 1 and 2, the KOP receptor affinity, selectivity, and agonist activity were modulated by introducing bulkier N-substituents, a 2-fluoro substitution, and additional hydroxyl groups at positions 3' and 4'. Several analogues showed subnanomolar affinity and excellent KOP receptor selectivity acting as full or partial agonists, and one as an antagonist. The new diphenethylamines displayed antinociceptive efficacies with increased potencies than U50,488, 1 and 2 in the writhing assay and without inducing motor dysfunction after sc administration in mice.

Background and Purpose: The κ receptor has a central role in modulating neurotransmission in central and peripheral neuronal circuits that subserve pain and other behavioural responses. Although κ receptor agonists do not produce euphoria or lead to respiratory suppression, they induce dysphoria and sedation. We hypothesized that brain-penetrant κ receptor ligands possessing biased agonism towards G protein signalling over β-arrestin2 recruitment would produce robust antinociception with fewer associated liabilities., Experimental Approach: Two new diphenethylamines with high κ receptor selectivity, HS665 and HS666, were assessed following i.c.v. administration in mouse assays of antinociception with the 55°C warm-water tail withdrawal test, locomotor activity in the rotorod and conditioned place preference. The [ 35 S]-GTPγS binding and β-arrestin2 recruitment in vitro assays were used to characterize biased agonism., Key Results: HS665 (κ receptor agonist) and HS666 (κ receptor partial agonist) demonstrated dose-dependent antinociception after i.c.v. administration mediated by the κ receptor. These highly selective κ receptor ligands displayed varying biased signalling towards G protein coupling in vitro, consistent with a reduced liability profile, reflected by reduced sedation and absence of conditioned place aversion for HS666., Conclusions and Implications: HS665 and HS666 activate central κ receptors to produce potent antinociception, with HS666 displaying pharmacological characteristics of a κ receptor analgesic with reduced liability for aversive effects correlating with its low efficacy in the β-arrestin2 signalling pathway. Our data provide further understanding of the contribution of central κ receptors in pain suppression, and the prospect of dissociating the antinociceptive effects of HS665 and HS666 from κ receptor-mediated adverse effects., (© 2017 The Authors. British Journal of Pharmacology published by John Wiley & Sons Ltd on behalf of British Pharmacological Society.)

Morphine and structurally related derivatives are highly effective analgesics, and the mainstay in the medical management of moderate to severe pain. Pharmacological actions of opioid analgesics are primarily mediated through agonism at the µ opioid peptide (MOP) receptor, a G protein-coupled receptor. Position 17 in morphine has been one of the most manipulated sites on the scaffold and intensive research has focused on replacements of the 17-methyl group with other substituents. Structural variations at the N-17 of the morphinan skeleton led to a diversity of molecules appraised as valuable and potential therapeutics and important research probes. Discovery of therapeutically useful morphine-like drugs has also targeted the C-6 hydroxyl group, with oxymorphone as one of the clinically relevant opioid analgesics, where a carbonyl instead of a hydroxyl group is present at position 6. Herein, we describe the effect of N-substituent variation in morphine and oxymorphone on in vitro and in vivo biological properties and the emerging structure-activity relationships. We show that the presence of a N-phenethyl group in position 17 is highly favorable in terms of improved affinity and selectivity at the MOP receptor, potent agonism and antinociceptive efficacy. The N-phenethyl derivatives of morphine and oxymorphone were very potent in stimulating G protein coupling and intracellular calcium release through the MOP receptor. In vivo, they were highly effective against acute thermal nociception in mice with marked increased antinociceptive potency compared to the lead molecules. It was also demonstrated that a carbonyl group at position 6 is preferable to a hydroxyl function in these N-phenethyl derivatives, enhancing MOP receptor affinity and agonist potency in vitro and in vivo. These results expand the understanding of the impact of different moieties at the morphinan nitrogen on ligand-receptor interaction, molecular mode of action and signaling, and may be instrumental to the development of new opioid therapeutics.

Here we report on the design, synthesis, and biological characterization of novel κ opioid (KOP) receptor ligands of diphenethylamines. In opioid receptor binding and functional assays, the N-cyclobutylmethyl substituted derivative 4 (HS665) showed the highest affinity and selectivity for the KOP receptor and KOP agonist potency. Compound 4 inhibited acetic acid induced writhing after subcutaneous administration in mice via KOP receptor-mediated mechanisms, being equipotent as an analgesic to the KOP agonist U50,488.

The synthesis and the effect of a combination of 6-glycine and 14-phenylpropoxy substitutions in N-methyl- and N-cycloproplymethylmorphinans on biological activities are described. Binding studies revealed that all new 14-phenylpropoxymorphinans (11-18) displayed high affinity to opioid receptors. Replacement of the 14-methoxy group with a phenylpropoxy group led to an enhancement in affinity to all three opioid receptor types, with most pronounced increases in δ and κ activities, hence resulting in a loss of μ receptor selectivity. All compounds (11-18) showed potent and long-lasting antinociceptive effects in the tail-flick test in rats after subcutaneous administration. For the N-methyl derivatives 13 and 14, analgesic potencies were in the range of their 14-methoxy analogues 9 and 10, respectively. Even derivatives 15-18 with an N-cyclopropylmethyl substituent acted as potent antinociceptive agents, being several fold more potent than morphine. Subcutaneous administration of compounds 13 and 14 produced significant and prolonged antinociceptive effects mediated through peripheral opioid mechanisms in carrageenan-induced inflammatory hyperalgesia in rats.

In a continued effort to find new substitution patterns in morphinans that would produce strong antinociception while inducing lesser side effects, 4,5-oxygen bridge-opened 6-cyano-substituted N-methylmorphinans (1-3) were synthesized. All compounds showed high affinities in the low nanomolar range to the mu opioid receptor and decreased interaction with delta and kappa receptors, thus being mu selective. When tested in vivo, the 6-cyanomorphinanas acted as potent antinociceptive agents which were either more active or equipotent to their 6-keto analogues 4-6.

Opioid analgesics with restricted access to the central nervous system represent a new approach to the treatment of severe pain with an improved safety profile. The objective of this study was to investigate the peripheral and central components of the antinociceptive actions of the 6-amino acid conjugates (glycine, alanine, and phenylalanine) of 14-O-methyloxymorphone. Their antinociceptive activities were compared with those of the centrally penetrating mu-opioid agonists morphine, fentanyl, and 14-O-methyloxymorphone. In the tail-flick test in rats, the 6-amino acid conjugates were 45- to 1170-fold more potent than morphine after i.c.v. administration and 19- to 209-fold after s.c. administration. They showed potencies similar to fentanyl after s.c. administration and were more potent after i.c.v. application. The time course of action was different between s.c. and i.c.v. administration, with significant long-lasting effects after i.c.v. administration. Systemic administration of the peripherally selective opioid antagonist naloxone methiodide antagonized the effects after s.c. but not after i.c.v. administration in the tail-flick test. Subcutaneous 6-amino acid derivatives also elicited antihyper-analgesic effects in the formalin test in rats, which were reversed by systemically administered naloxone methiodide. Although morphine exerts its analgesic effects by central and peripheral mechanisms, the investigated new opioids interact primarily with peripheral opioid receptors after s.c. administration. The present data indicate that the 6-amino acid conjugates of 14-O-methyloxymorphone have limited access to the central nervous system and can mediate antinociception at peripheral sites. Also, they might find clinical application when the central actions of opioids are unwanted.

14-Methoxymetopon is a potent opioid analgesic. When given systemically, it is approximately 500-fold more active than morphine. However, this enhanced potency is markedly increased with either spinal or supraspinal administration, where its analgesic activity is more than a million-fold greater than morphine. It was mu-opioid receptor selective in binding assays and its analgesia was blocked only by mu-opioid receptor-selective antagonists. Yet, it had a different selectivity profile than either morphine or morphine-6beta-glucuronide. Unlike morphine, 14-methoxymetopon was antagonized by 3-O-methylnaltrexone, it was sensitive to antisense probes targeting exons 1, 2 and 8 of the opioid receptor gene and was inactive both spinally and supraspinally in CXBK mice. Although it retarded gastrointestinal transit, it displayed a ceiling effect with no dose lowering transit by more than 65%, in contrast to the complete inhibition of transit by morphine. These finding demonstrate that 14-methoxymetopon is a highly potent mu-opioid with a pharmacological profile distinct from that of the traditional mu-opioid morphine.

Evidence suggests that the antinociceptive effects of selective delta-opioid receptor agonists may involve an activation of the mu-receptor in some experimental conditions. The aim of this study was to clarify the receptors involved in the antinociceptive responses of the selective and systemically active delta-opioid receptor agonist Tyr-D-Ser-(O-tert-butyl)-Gly-Phe-Leu-Thr-(O-tert-butyl) (BUBU). The antinociception induced by systemic (i.v.) or central (i.c.v.) administration of BUBU was measured in the hot plate (jumping and paw lick latencies) and tail immersion tests in mice. In both tests, the responses were more intense when BUBU was administered by central route. The pre-treatment with the mu-opioid receptor antagonist cyprodime blocked the effects induced by central BUBU in the hot plate and tail immersion tests. The delta-opioid receptor antagonist naltrindole had no effect on BUBU-induced antinociception in the hot plate but decreased BUBU responses in the tail immersion test. Further evidence for this dual receptor action of BUBU was demonstrated by using antisense oligodeoxynucleotides. Thus, a reduction in central BUBU-induced antinociception was observed in the tail immersion test after the administration of antisense probes that selectively blocked the expression of mu- or delta-opioid receptors. These findings clearly indicate using a dual pharmacological and molecular approach that BUBU mediates its antinociceptive effects via activation of both mu- and delta-opioid receptors.

The newly synthesized 14-alkoxymetopon derivatives, 14-methoxymetopon, 14-ethoxymetopon, 14-methoxy-5-methyl-morphinone, exhibit high affinity for the naloxone binding sites in rat brain. A substantial decrease in affinity was observed, in the presence of NaCl indicating a high degree of agonist activity. All three 14-alkoxymetopon derivatives displayed high affinity for [3H][D-Ala2,(Me)Phe4,Gly-ol5]enkephalin ([3H]DAMGO) binding sites, much less potency toward delta sites and were the least effective at kappa sites. Isolated tissue studies using the guinea pig ileum preparation confirmed their high agonist potency. Following administration the new compounds produced naloxone reversible antinociceptive effects and were 130-300 times more potent than morphine in the acetic acid induced abdominal constriction model in the mouse, and the hot plate and tail flick tests in the rat. The compounds also produced dose-dependent muscle rigidity, and potentiated barbiturate-induced narcosis. The in vivo apparent pA2 values for naloxone against 14-ethoxymetopon and morphine were similar in analgesia, suggesting an interaction with the same (mu) receptor site. The dependence liability of 14-alkoxymetopon derivatives in the withdrawal jumping test was less pronounced than that of morphine in either rats or mice, similar to tolerance to the their analgesic action. It is concluded that the 14-alkoxymetopon derivatives studied are selective and potent agonists at mu opioid receptors, with reduced dependence liability.