Synthesis of 6-Amino Acid Substituted Derivatives of the Highly Potent Analgesic 14-O-Methyloxymorphone

Peripheralization Strategies Applied to Morphinans and Implications for Improved Treatment of Pain

Opioids are considered the most effective analgesics for the treatment of moderate to severe acute and chronic pain. However, the inadequate benefit/risk ratio of currently available opioids, together with the current 'opioid crisis', warrant consideration on new opioid analgesic discovery strategies. Targeting peripheral opioid receptors as effective means of treating pain and avoiding the centrally mediated side effects represents a research area of substantial and continuous attention. Among clinically used analgesics, opioids from the class of morphinans (i.e., morphine and structurally related analogues) are of utmost clinical importance as analgesic drugs activating the mu-opioid receptor. In this review, we focus on peripheralization strategies applied to N-methylmorphinans to limit their ability to cross the blood-brain barrier, thus minimizing central exposure and the associated undesired side effects. Chemical modifications to the morphinan scaffold to increase hydrophilicity of known and new opioids, and nanocarrier-based approaches to selectively deliver opioids, such as morphine, to the peripheral tissue are discussed. The preclinical and clinical research activities have allowed for the characterization of a variety of compounds that show low central nervous system penetration, and therefore an improved side effect profile, yet maintaining the desired opioid-related antinociceptive activity. Such peripheral opioid analgesics may represent alternatives to presently available drugs for an efficient and safer pain therapy.

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.

Discovery, Structure-Activity Relationship, and Mechanistic Studies of 1-((3R,4S)-3-((Dimethylamino)methyl)-4-hydroxy-4-(3-methoxyphenyl)piperidin-1-yl)-2-(2,4,5-trifluorophenyl)ethan-1-one as a Novel Potent Analgesic

Management of moderate to severe pain relies heavily on opioid analgesics such as morphine, oxycodone, and fentanyl in clinics. However, their prolonged use was associated with undesirable side effects. Many new strategies to reduce side effects have been proposed, but not without disadvantages. Using a hot plate model as a phenotypic screening method, our studies identified (3R,4S)-9d with a new scaffold as a potent analgesic with ED₅₀ values of 0.54 mg/kg and 0.021 mg/kg in hot plate and antiwrithing models, respectively. Mechanistic studies showed that it elicited its analgesic effect via the active metabolite (3R,4S)-10a. The mechanism of (3R,4S)-10a-induced activation of the μ opioid receptor (MOR) was proposed by means of molecular dynamics (MD) simulation.

Synthesis, Biological, and Structural Explorations of New Zwitterionic Derivatives of 14- O-Methyloxymorphone, as Potent μ/δ Opioid Agonists and Peripherally Selective Antinociceptives

Herein, the synthesis and pharmacological characterization of an extended library of differently substituted N-methyl-14-O-methylmorphinans with natural and unnatural amino acids and three dipeptides at position 6 that emerged as potent μ/δ opioid receptor (MOR/DOR) agonists with peripheral antinociceptive efficacy is reported. The current study adds significant value to our initial structure–activity relationships on a series of zwitterionic analogues of 1 (14-O-methyloxymorphone) by targeting additional amino acid residues. The new derivatives showed high binding and potent agonism at MOR and DOR in vitro. In vivo, the new 6-amino acid- and 6-dipeptide-substituted derivatives of 1 were highly effective in inducing antinociception in the writhing test in mice after subcutaneous administration, which was antagonized by naloxone methiodide demonstrating activation of peripheral opioid receptors. Such peripheral opioid analgesics may represent alternatives to presently available drugs for a safer pain therapy.

Synthesis, Pharmacology, and Molecular Docking Studies on 6-Desoxo-N-methylmorphinans as Potent μ-Opioid Receptor Agonists

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.

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.

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.

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.

DAMGO and 6beta-glycine substituted 14-O-methyloxymorphone but not morphine show peripheral, preemptive antinociception after systemic administration in a mouse visceral pain model and high intrinsic efficacy in the isolated rat vas deferens

Peripheral micro-opioid receptors (MOR) have emerged as important components of inhibitory nociceptive pathways. Here, the antinociceptive effects of MOR agonists, the 6beta-glycine derivative of 14-O-methyloxymorphone (HS-731), DAMGO and morphine were evaluated in a mouse model of visceral pain. The abdominal acetic acid-induced writhing test was used to examine the peripheral, preemptive antinociceptive opioid action on visceral nociception. HS-731 administered subcutaneously (s.c.) or intracerebroventricularly (i.c.v.) dose-dependently and completely inhibited writhing, being 24-598-fold more potent, depending on the administration route, than two selective MOR agonists, the enkephalin analogue [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]enkephalin (DAMGO) and morphine. A longer duration of action (2-3 h) was induced by HS-731 given before acetic acid, while shorter effect was produced by morphine (30-60 min) and DAMGO (30-45 min). The antinociceptive effects of systemic opioids were reversed by the s.c. opioid antagonist, naloxone. Blocking of central MOR by the selective MOR antagonist D-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP, i.c.v.) resulted in a significant reduction of antinociception of s.c. morphine, whereas it completely failed to antagonize the effects of systemic HS-731 or DAMGO. In in vitro studies, HS-731 and DAMGO, but not morphine showed high intrinsic efficacy, naltrexone-sensitive agonist effect at MOR of the rat vas deferens. These data demonstrate that selective activation of peripheral MOR by systemic s.c. HS-731 or DAMGO produces potent peripheral, preemptive visceral antinociception, while morphine's effects are mediated primarily through central mechanisms. Our findings support the role of peripheral MOR in the pathology of pain states involving sensitization of peripheral nociceptors.

Peripherally mediated antinociception of the mu-opioid receptor agonist 2-[(4,5alpha-epoxy-3-hydroxy-14beta-methoxy-17-methylmorphinan-6beta-yl)amino]acetic acid (HS-731) after subcutaneous and oral administration in rats with carrageenan-induced hindpaw inflammation

Opioids induce analgesia by activating opioid receptors not only within the central nervous system but also on peripheral sensory neurons. This study investigated peripherally mediated antinociception produced by the mu-opioid receptor agonist 2-[(4,5alpha-epoxy-3-hydroxy-14beta-methoxy-17-methylmorphinan-6beta-yl)amino]acetic acid (HS-731) after s.c. and oral administration in rats with carrageenan-induced hindpaw inflammation. Antinociceptive effects after s.c. administration were assessed 3 h after intraplantar carrageenan injection and compared with those of centrally acting mu-opioid agonists 14-methoxymetopon and morphine. Opioid agonists caused dose-dependent increases in inflamed paw withdrawal latencies to mechanical and thermal stimulation. The time course of action was different, in that HS-731 (20 microg/kg s.c.) produced significant long-lasting effects up to 4 h after administration, whereas 14-methoxymetopon (20 microg/kg) and morphine (2 mg/kg) reached their peak of action at 10 to 30 min, and their effect declined rapidly thereafter. Subcutaneous administration of the peripherally selective opioid antagonist naloxone methiodide inhibited antinociception elicited by HS-731 (20 microg/kg s.c.), whereas it was ineffective against 14-methoxymetopon (20 microg/kg s.c.). Moreover, the antinociception produced by 100 microg/kg s.c. HS-731 was dose-dependently reversed by s.c. naloxone methiodide. This indicates that HS-731 preferentially activates peripheral opioid receptors, whereas 14-methoxymetopon mediates analgesia via central mechanisms. Orally administered HS-731 significantly reduced hyperalgesia in the inflamed paw induced by carrageenan, which was reversible by s.c. administered naloxone methiodide. These results show that systemic (s.c. and oral) treatment with the mu-opioid agonist HS-731 produces potent and long-lasting antinociception through peripheral mechanisms in rats with carrageenan-induced hindpaw inflammation.

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.

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.

In vitro opioid activity profiles of 6-amino acid substituted derivatives of 14-O-methyloxymorphone

A series of 6-amino acid conjugates (glycine, alanine and phenylalanine) of the highly potent opioid analgesic 14-O-methyloxymorphone was developed in an effort to obtain agonists that would have potentially limited ability to cross the blood-brain barrier. Binding studies revealed that all derivatives displayed high affinities (0.77-2.58 nM) at the mu-opioid receptor in rat brain membranes. They were potent agonists in mouse vas deferens preparation (IC(50)=5.52-26.8 nM). While the alpha-amino acid epimers are favoured by mu-opioid receptors, the beta-epimers proved to have increased interaction with delta-sites. Only the beta-phenylalanine conjugate showed some preference for delta- over mu-opioid receptors and delta-opioid receptor agonist activity. The relatively high delta-opioid receptor affinity of this analogue was also predicted by molecular modelling studies. The newly developed ionizable derivatives could find clinical applications as potent analgesics without the adverse actions of centrally acting opioids.

Peripheral versus central antinociceptive actions of 6-amino acid-substituted derivatives of 14-O-methyloxymorphone in acute and inflammatory pain in the rat

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.