Synthesis and biological evaluation of 14-alkoxymorphinans. 20. 14-phenylpropoxymetopon: an extremely powerful analgesic

Recent Chemical and Pharmacological Developments on 14-Oxygenated-N-methylmorphinan-6-ones

Adequate pain management, particularly chronic pain, remains a major challenge associated with modern-day medicine. Current pharmacotherapy offers unsatisfactory long-term solutions due to serious side effects related to the chronic administration of analgesic drugs. Morphine and structurally related derivatives (e.g., oxycodone, oxymorphone, buprenorphine) are highly effective opioid analgesics, mediating their effects via the activation of opioid receptors, with the mu-opioid receptor subtype as the primary molecular target. However, they also cause addiction and overdose deaths, which has led to a global opioid crisis in the last decades. Therefore, research efforts are needed to overcome the limitations of present pain therapies with the aim to improve treatment efficacy and to reduce complications. This review presents recent chemical and pharmacological advances on 14-oxygenated-N-methylmorphinan-6-ones, in the search of safer pain therapeutics. We focus on drug design strategies and structure-activity relationships on specific modifications in positions 5, 6, 14 and 17 on the morphinan skeleton, with the goal of aiding the discovery of opioid analgesics with more favorable pharmacological properties, potent analgesia and fewer undesirable effects. Targeted molecular modifications on the morphinan scaffold can afford novel opioids as bi- or multifunctional ligands targeting multiple opioid receptors, as attractive alternatives to mu-opioid receptor selective analgesics.

Peripherally acting opioid analgesics and peripherally-induced analgesia

The management of pain, particularly chronic pain, is still an area of medical need. In this context, opioids remain a gold standard for the treatment of pain. However, significant side effects, mainly of central origin, limit their clinical use. Here, we review recent progress to improve the therapeutic and safety profiles of opioids for pain management. Characterization of peripheral opioid-mediated pain mechanisms have been a key component of this process. Several studies identified peripheral µ, δ, and κ opioid receptors (MOR, DOR, and KOR, respectively) and nociceptin/orphanin FQ (NOP) receptors as significant players of opioid-mediated antinociception, able to achieve clinically significant effects independently of any central action. Following this, particularly from a medicinal chemistry point of view, main efforts have been directed towards the peripheralization of opioid receptor agonists with the objective of optimizing receptor activity and minimizing central exposure and the associated undesired effects. These activities have allowed the characterization of a great variety of compounds and investigational drugs that show low central nervous system (CNS) penetration (and therefore a reduced side effect profile) yet maintaining the desired opioid-related peripheral antinociceptive activity. These include highly hydrophilic/amphiphilic and massive molecules unable to easily cross lipid membranes, substrates of glycoprotein P (a extrusion pump that avoids CNS penetration), nanocarriers that release the analgesic agent at the site of inflammation and pain, and pH-sensitive opioid agonists that selectively activate at those sites (and represent a new pharmacodynamic paradigm). Hopefully, patients with pain will benefit soon from the incorporation of these new entities.

Molecular Docking, Molecular Dynamics, and Structure-Activity Relationship Explorations of 14-Oxygenated N-Methylmorphinan-6-ones as Potent μ-Opioid Receptor Agonists

Among opioids, morphinans are of major importance as the most effective analgesic drugs acting primarily via μ-opioid receptor (μ-OR) activation. Our long-standing efforts in the field of opioid analgesics from the class of morphinans led to N-methylmorphinan-6-ones differently substituted at positions 5 and 14 as μ-OR agonists inducing potent analgesia and fewer undesirable effects. Herein we present the first thorough molecular modeling study and structure–activity relationship (SAR) explorations aided by docking and molecular dynamics (MD) simulations of 14-oxygenated N-methylmorphinan-6-ones to gain insights into their mode of binding to the μ-OR and interaction mechanisms. The structure of activated μ-OR provides an essential model for how ligand/μ-OR binding is encoded within small chemical differences in otherwise structurally similar morphinans. We reveal important molecular interactions that these μ-agonists share and distinguish them. The molecular docking outcomes indicate the crucial role of the relative orientation of the ligand in the μ-OR binding site, influencing the propensity of critical non-covalent interactions that are required to facilitate ligand/μ-OR interactions and receptor activation. The MD simulations point out minor differences in the tendency to form hydrogen bonds by the 4,5α-epoxy group, along with the tendency to affect the 3–7 lock switch. The emerged SARs reveal the subtle interplay between the substituents at positions 5 and 14 in the morphinan scaffold by enabling the identification of key structural elements that determine the distinct pharmacological profiles. This study provides a significant structural basis for understanding ligand binding and μ-OR activation by the 14-oxygenated N-methylmorphinan-6-ones, which should be useful for guiding drug design.

Exploring pharmacological activities and signaling of morphinans substituted in position 6 as potent agonists interacting with the μ opioid receptor

BACKGROUND

Opioid analgesics are the most effective drugs for the treatment of moderate to severe pain. However, they also produce several adverse effects that can complicate pain management. The μ opioid (MOP) receptor, a G protein-coupled receptor, is recognized as the opioid receptor type which primarily mediates the pharmacological actions of clinically used opioid agonists. The morphinan class of analgesics including morphine and oxycodone are of main importance as therapeutically valuable drugs. Though the natural alkaloid morphine contains a C-6-hydroxyl group and the semisynthetic derivative oxycodone has a 6-carbonyl function, chemical approaches have uncovered that functionalizing position 6 gives rise to a range of diverse activities. Hence, position 6 of N-methylmorphinans is one of the most manipulated sites, and is established to play a key role in ligand binding at the MOP receptor, efficacy, signaling, and analgesic potency. We have earlier reported on a chemically innovative modification in oxycodone resulting in novel morphinans with 6-acrylonitrile incorporated substructures.

RESULTS

This study describes in vitro and in vivo pharmacological activities and signaling of new morphinans substituted in position 6 with acrylonitrile and amido functions as potent agonists and antinociceptive agents interacting with MOP receptors. We show that the presence of a 6-cyano group in N-methylmorphinans has a strong influence on the binding to the opioid receptors and post-receptor signaling. One 6-cyano-N-methylmorphinan of the series was identified as the highest affinity and most selective MOP agonist, and very potent in stimulating G protein coupling and intracellular calcium release through the MOP receptor. In vivo, this MOP agonist showed to be greatly effective against thermal and chemical nociception in mice with marked increased antinociceptive potency than the lead molecule oxycodone.

CONCLUSION

Development of such novel chemotypes by targeting position 6 provides valuable insights on ligand-receptor interaction and molecular mode of action, and may aid in identification of opioid therapeutics with enhanced analgesic properties and fewer undesirable effects.

Development of 5-Substituted N-Methylmorphinan-6-ones as Potent Opioid Analgesics with Improved Side-Effect Profile

One of the most important functions of the opioid system is the control of pain. Among the three main opioid receptor classes (μ, δ, κ), the μ (MOR) is the main type targeted for pharmacotherapy of pain. Opioid analgesics such as morphine, oxycodone and fentanyl are agonists at the MOR and are the mainstay for the treatment of moderate-to-severe pain. However, adverse effects related to opioid use are severe and often lead to early discontinuation and inadequate analgesia. The development of more effective and safer medications for the management of pain still remains a major direction in pharmaceutical research. Chemical approaches towards the identification of novel MOR analgesics with reduced side effects include structural modifications of 14-alkoxy-N-methylmorphinan-6-ones in key positions that are important for binding, selectivity, potency, and efficacy at opioid receptors. This paper describes a representative strategy to improve the therapeutic usefulness of opioid analgesics from the morphinan class of drugs by targeting position 5. The focus is on chemical and biological studies and structure-activity relationships of this series of ligands. We report on 14-alkoxymorphinan-6-ones having a methyl and benzyl group at position 5 as strong opioid antinociceptive agents with reduced propensity to cause undesired effects compared to morphine although interacting selectively with MORs.

Deconstructing 14-phenylpropyloxymetopon: minimal requirements for binding to mu opioid receptors

A series of phenylpropyloxyethylamines and cinnamyloxyethylamines were synthesized as deconstructed analogs of 14-phenylpropyloxymetopon and analyzed for opioid receptor binding affinity. Using the Conformationally Sampled Pharmacophore modeling approach, we discovered a series of compounds lacking a tyrosine mimetic, historically considered essential for μ opioid binding. Based on the binding studies, we have identified the optimal analogs to be N-methyl-N-phenylpropyl-2-(3-phenylpropoxy)ethanamine, with 1520 nM, and 2-(cinnamyloxy)-N-methyl-N-phenethylethanamine with 1680 nM affinity for the μ opioid receptor. These partial opioid structure analogs will serve as the novel lead compounds for future optimization studies.

Most recent developments and modifications of 14-alkylamino and 14-alkoxy-4,5-epoxymorphinan derivatives

The 14-position of natural opiates (e.g. morphine) are unsubstituted, however synthetic approaches have uncovered that functionalizing position 14 gives rise to a wide range of diverse activities. This review focuses on SAR of the position, with the aim of aiding in the search for opioid analgesics with improved clinical profiles.

Consensus 3D model of μ-opioid receptor ligand efficacy based on a quantitative Conformationally Sampled Pharmacophore

Despite being studied for over 30 years, a consensus structure-activity relationship (SAR) that encompasses the full range peptidic and nonpeptidic μ-opioid receptor ligands is still not available. To achieve a consensus SAR the Conformationally Sampled Pharmacophore (CSP) method was applied to develop a predictive model of the efficacy of μ-opioid receptor ligands. Emphasis was placed on predicting the efficacy of a wide range of agonists, partial agonists, and antagonists as well as understanding their mode of interaction with the receptor. Inclusion of all accessible conformations of each ligand, a central feature of the CSP method, enabled structural features between diverse μ-opioid receptor ligands that dictate efficacy to be identified. The models were validated against a diverse collection of peptidic and nonpeptidic ligands, including benzomorphans, fentanyl (4-anilinopiperidine), methadone (3,3-diphenylpropylamines), etonitazene (benzimidazole derivatives), funaltrexamine (C6-substituted 4,5-epoxymorphinan), and herkinorin. The model predicts (1) that interactions of ligands with the B site, as with the 19-alkyl substituents of oripavines, modulate the extent of agonism; (2) that agonists with long N-substituents, as with fentanyl and N-phenethylnormorphine, can bind in an orientation such that the N substitutent interacts with the B site that also allows the basic N-receptor Asp interaction essential for agonism; and (3) that the μ agonist herkinorin, that lacks a basic nitrogen, binds to the receptor in a manner similar to the traditional opioids via interactions mediated by water or a ion. Importantly, the proposed CSP model can be reconciled with previously published SAR models for the μ receptor.

Synthesis and pharmacological activities of 6-glycine substituted 14-phenylpropoxymorphinans, a novel class of opioids with high opioid receptor affinities and antinociceptive potencies

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 vitro and in vivo pharmacological profile of the 5-benzyl analogue of 14-methoxymetopon, a novel mu opioid analgesic with reduced propensity to alter motor function

Opioids are the most effective analgesics for pain management, and efficient pain control is a therapeutic priority. Herein, we describe the synthesis and pharmacological activities of the 5-benzyl analogue of the μ opioid analgesic 14-methoxymetopon (14-MM). The result of the replacement of the 5-methyl in 14-MM with a benzyl group on in vitro opioid receptor binding and functional profiles, and in vivo behavioural properties, i.e. nociception and motor activity, was investigated. In rodent brain membranes, the 5-benzyl derivative showed high affinity at the μ opioid receptor and decreased interaction with δ and κ receptors, hence displaying a similar binding profile as 14-MM. It displayed potent agonist activity in vitro and in vivo. In in vitro guanosine-5′-O-(3-[³⁵S]thio)-triphosphate ([³⁵S]GTPγS) binding assay, it activated G-proteins in rat brain membranes through a μ opioid receptor-mediated mechanism having significantly enhanced potency compared to DAMGO (d-Ala²,Me-Phe⁴,Gly-ol⁵]enkephalin), and to the μ opioid agonist morphinans 14-MM, 14-O-methyloxymorphone (14-OMO) and morphine. In vivo, the 5-benzyl analogue of 14-MM elicited dose-dependent and naloxone-sensitive antinociceptive effects in hot-plate and tail-flick tests in mice after subcutaneous (s.c.) administration. Its analgesic potency was comparable to 14-MM, and was 50-fold higher than that of morphine. Contrary to morphine, 14-MM and 14-OMO, no motor dysfunction was produced by the new opioid in the mouse rotarod test at any of the tested doses. In summary, the 5-benzyl analogue of 14-MM emerged as a novel potent μ opioid antinociceptive agent with reduced propensity to cause unwanted motor impairment.

Morphinans and isoquinolines: acetylcholinesterase inhibition, pharmacophore modeling, and interaction with opioid receptors

Following indications from pharmacophore-based virtual screening of natural product databases, morphinan and isoquinoline compounds were tested in vitro for acetylcholinesterase (AChE) inhibition. After the first screen, active and inactive compounds were used to build a ligand-based pharmacophore model in order to prioritize compounds for biological testing. Among the virtual hits tested, the enrichment of actives was significantly higher than in a random selection of test compounds. The most active compounds were biochemically tested for their activity on mu, delta, and kappa opioid receptors.

Synthesis of 14-alkoxymorphinan derivatives and their pharmacological actions

Among opioids, morphinans play an important role as therapeutically valuable drugs. They include pain relieving agents such as naturally occurring alkaloids (e.g. morphine, codeine), semisynthetic derivatives (e.g. oxycodone, oxymorphone, buprenorphine), and synthetic analogs (e.g. levorphanol). Currently used opioid analgesics also share a number of severe side effects, limiting their clinical usefulness. The antagonist morphinans, naloxone and naltrexone are used to treat opioid overdose, opioid dependence, and alcoholism. All these opioid drugs produce their biological actions through three receptor types, mu, delta, and kappa, belonging to the G-protein-coupled receptor family. Considerable effort has been put forward to understand the appropriate use of opioid analgesics, while medicinal chemistry and opioid pharmacology have been continuously engaged in the search for safer, more efficacious and nonaddicting opioid compounds, with the final goal to reduce complications and to improve patient compliance. Toward this goal, recent advances in chemistry, ligand-based structure activity relationships and pharmacology of 14-alkoxymorphinans are reviewed in this chapter. Current developments of different structural patterns of 14-alkoxymorphinans as research tools and their potential therapeutic opportunities are also summarized.

14-Methoxymetopon, a highly potent mu opioid agonist, biphasically affects ethanol intake in Sardinian alcohol-preferring rats

RATIONALE

Increased opioidergic activity is thought to increase the propensity to consume ethanol. However, the dose monotonicity and receptor subtype for this effect remain uncertain. 14-methoxymetopon is a centrally acting, selective micro opioid receptor agonist with greater systemic antinociceptive potency than morphine and a putatively improved therapeutic index.

OBJECTIVE

To determine whether 14-methoxymetopon influenced voluntary ethanol intake in Sardinian alcohol-preferring (sP) rats.

METHODS

Male sP rats with continuous 2-bottle choice access to ethanol (10% v/v) or water were subjects. The effects of systemic 14-methoxymetopon administration (2, 5, 12.25, 30 micro/kg, s.c.) on 4-h ethanol intake were determined. The ability of naltrexone (50 micro/kg, s.c.), an opioid antagonist, to block actions of 14-methoxymetopon (12.25, 30 micro/kg, s.c.) was examined as were the effects of 14-methoxymetopon (12.25 micro/kg, s.c.) on self-administered blood alcohol levels (BALs) and clearance of a passive ethanol bolus (1 g/kg). Finally, the effects of central 14-methoxymetopon administration (0.0003-100 ng, i.c.v.) on 4-h ethanol intake were evaluated.

RESULTS

Systemic 14-methoxymetopon very potently and dose-dependently suppressed ethanol and food intake for 30 min, followed by a greater, longer-lasting, and behaviorally specific increase in ethanol intake. The increased ethanol intake led to threefold higher BALs, was naltrexone-reversible, and not due to altered ethanol clearance. Intracerebroventricular 14-methoxymetopon administration rapidly altered ethanol intake per an inverted U-shaped dose-response function, increasing it at a 10 pg dose, while suppressing it at a 10,000-fold higher dose.

CONCLUSIONS

The novel mu analgesic increases ethanol intake, a potential therapeutic liability, and results suggest a non-monotonic influence of brain mu opioid receptor stimulation on ethanol intake.

Local peripheral antinociceptive effects of 14-O-methyloxymorphone derivatives in inflammatory and neuropathic pain in the rat

Antinociception achieved after peripheral administration of opioids has opened a new approach to the treatment of severe and chronic pain. Additionally, opioid analgesics with restricted access to the central nervous system could improve safety of opioid drugs used in clinical practice. In the present study, peripheral components of antinociceptive actions of 6-amino acid-substituted derivatives of 14-O-methyloxymorphone were investigated after local intraplantar (i.pl.) administration in rat models of inflammatory and neuropathic pain. Their antinociceptive activities were compared with those of morphine, the classical mu-opioid receptor agonist. Intraplantar administration of morphine and the 6-amino acid derivatives produced dose-dependent reduction of formalin-induced flinching of the inflamed paw, without significant effect on the paw edema. Local i.pl. administration of the new derivatives in rats with neuropathic pain induced by sciatic nerve ligation produced antiallodynic and antihyperalgesic effects; however, the antinociceptive activity was lower than that observed in inflammatory pain. In both models, the 6-amino acid derivatives and morphine at doses that produced analgesia after i.pl. administration were systemically (s.c.) much less active indicating that the antinociceptive action is due to a local effect. Moreover, the local opioid antinociceptive effects were significantly attenuated by naloxone methiodide, a peripherally acting opioid receptor antagonist, demonstrating that the effect was mediated by peripheral opioid receptors. The present data indicate that the peripherally restricted 6-amino acid conjugates of 14-O-methyloxymorphone elicit antinociception after local administration, being more potent in inflammatory than in neuropathic pain. Opioid drugs with peripheral site of action can be an important target for the treatment of long lasting pain.

Effect of a 6-cyano substituent in 14-oxygenated N-methylmorphinans on opioid receptor binding and antinociceptive potency

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.

Mechanistic diversity of the van Leusen reaction applied to 6-ketomorphinans and synthetic potential of the resulting acrylonitrile substructures

Tosylmethyl isocyanide was used to convert 7,8-didehydro-6-ketomorphinans to 6,7-didehydromorphinan-6-carbonitriles with retainment of the 4,5-epoxy ring. However, ring opening occurred in the presence of NaH giving 5,6,7,8-tetradehydromorphinan-6-carbonitriles. Addition of nucleophiles such as Li diisopropylamide or Grignard reagents to the acrylonitrile substructure yielded ring-opened 5,6-didehydro products. Seven products were characterized by X-ray crystal structure analysis and revealed insight into the mechanistic diversity of the van Leusen reaction.

Synthesis and biological evaluation of 14-alkoxymorphinans. 22.(1) Influence of the 14-alkoxy group and the substitution in position 5 in 14-alkoxymorphinan-6-ones on in vitro and in vivo activities

Novel 14-alkoxy-substituted (e.g. allyloxy, benzyloxy, naphthylmethoxy) morphinan-6-one derivatives were synthesized and biologically evaluated. Compounds 6-9 and 11 displayed affinities in the subnanomolar range to mu opioid receptors which were comparable to 14-O-methyloxymorphone (1) and 14-methoxymetopon (3), and higher than oxymorphone (2). Opioid binding affinity was sensitive to the character and length of the substituent in position 14. In smooth muscle preparations they behaved as potent agonists. Antinociceptive potencies of compounds 6-11 in the hot-plate test after sc administration in mice were considerably greater than the potency of morphine. In the colonic propulsion test, the most potent analgesic compound 7 showed negligible constipating activity at the analgesic dose. These findings provide further evidence that the nature of the substituent at position 14 has a major impact on the abilities of morphinans to interact with opioid receptors. Introduction of a 5-methyl group has no significant effect on in vitro biological activities, but resulted in decreased antinociceptive potency.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation, condensation with formaldehyde and rearrangement, some of which do not contain as isoquinoline system, together with napthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids with the structures of new bases, together with their reactions and syntheses, are reported. The literature from July 2003 to June 2004 is reviewed, with 145 references cited.

Synthesis and Biological Evaluation of 14-Alkoxymorphinans. 21. Novel 4-Alkoxy and 14-Phenylpropoxy Derivatives of the μ Opioid Receptor Antagonist Cyprodime

The synthesis, biological, and pharmacological evaluation of novel derivatives of cyprodime are described. Their binding affinities at mu, delta, and kappa opioid receptors were evaluated using receptor binding assay. It was observed that the affinity of these compounds was sensitive to the character and length of the substituent in position 4. Further prolongation of the 4-alkoxy group of cyprodime (1) and its 4-butoxy analogue 2 is detrimental for the mu opioid receptor affinity. Introduction of an arylalkoxy group at C-4 does not increase mu affinity in the case of benzyloxy, while a phenylpropoxy group reduces mu affinity. The delta and kappa affinities were also reduced compared to the reference compounds. A significant increase in the affinity at the mu opioid receptors was achieved by introducing a 14-phenylpropoxy group. Increases in the affinity at delta and kappa receptors were also observed. These findings provide further evidence that the nature of the substituent at position 14 has a major impact on the abilities of morphinans to interact with opioid receptors. In the [(35)S]GTPgammaS binding assay, all tested compounds were partial agonists at mu and delta receptors. Compounds 8 and 17 showed antagonism at kappa receptors, while compound 7 exhibited some partial agonist activity at this receptor. The novel derivatives of cyprodime containing a 14-phenylpropoxy group acted as potent antinociceptives. When tested in vivo, compounds 7, 8, and 17 were considerably more potent than morphine, with phenol 7 showing the highest antinociceptive potency (21-fold in the hot plate test, 38-fold in the tail flick test, and 300-fold in the paraphenylquinone writhing test) in mice. Introduction of a 14-phenylpropoxy substituent leads to a profound alteration in the pharmacological profile of this class of compounds.