Effects of N-Substitutions on the Tetrahydroquinoline (THQ) Core of Mixed-Efficacy μ-Opioid Receptor (MOR)/δ-Opioid Receptor (DOR) Ligands

SAR Matrices Enable Discovery of Mixed Efficacy μ-Opioid Receptor Agonist Peptidomimetics with Simplified Structures through an Aromatic-Amine Pharmacophore

We previously described the development of potent μ-opioid receptor (MOR)-agonist/δ-opioid receptor (DOR)-antagonist peptidomimetic ligands as an approach toward effective analgesics with reduced side effects. In this series, a tetrahydroquinoline (THQ) or substituted phenyl is employed to link two key pharmacophore elements, a dimethyltyrosine amino acid and typically an aromatic pendant. Using new and previously reported analogues, we constructed a structure-activity relationship (SAR) matrix that probes the utility of previously reported amine pendants. This matrix reveals that the MOR-agonist/DOR-antagonist properties of these ligands do not change when a tetrahydroisoquinoline (THIQ) pendant is used, despite removal of substituents on the core phenyl ring. Based on this observation, we retained the THIQ pendant and replaced the phenyl core with simpler aliphatic chain structures. These simpler analogues proved to be potent MOR-agonists with high variability in their effects at the DOR and the κ-opioid receptor (KOR). These data show that the amine of the THIQ pendant may be a novel pharmacophore element that favors high MOR-efficacy, whereas the aromatic ring of the THIQ pendant may produce high MOR-potency. Combined, the two pharmacophores within the THIQ pendant may be a structurally efficient means of converting opioid peptides and peptidomimetics into potent and efficacious MOR-agonists.

Behavioral effects of benzylideneoxymorphone (BOM), a low efficacy µ opioid receptor agonist and a δ opioid receptor antagonist

RATIONALE

Opioids remain the drugs of choice for treating moderate to severe pain, although adverse effects often limit use. Drugs acting concomitantly as agonists at μ opioid receptors and antagonists at δ opioid receptors produce antinociceptive effects with a reduced profile of adverse effects; one such drug, benzylideneoxymorphone (BOM), might further limit adverse effects because it appears to have lower pharmacological efficacy than other μ opioid receptor agonists.

OBJECTIVES

The current study compared the acute behavioral effects of BOM with the effects of other μ opioid receptor agonists.

METHODS

Discriminative stimulus and rate-decreasing effects were studied in 1 group of 7 rats discriminating 3.2 mg/kg morphine while responding under a fixed-ratio 10 schedule of food presentation. Antinociceptive effects were determined in a second group of 8 rats using a warm water tail withdrawal procedure. Reinforcing effects were evaluated in a third group of 12 rats with a history of remifentanil self-administration.

RESULTS

BOM produced morphine-lever responding and both discriminative stimulus and rate-decreasing effects were antagonized by naltrexone. BOM did not markedly increase tail-withdrawal latencies from water maintained at 50 °C and did not substantially attenuate the antinociceptive effects of morphine. BOM was not self-administered and did not change remifentanil self-administration.

CONCLUSIONS

Some effects of BOM (e.g., discriminative stimulus effects) appear to be mediated by μ opioid receptors; however, BOM is not self-administered by rats, suggesting that it might have limited abuse liability and a reduced profile of adverse effects compared with currently prescribed opioids.

The search for opioid analgesics with limited tolerance liability

Reducing the well-known side effects of opioids prescribed to treat chronic pain remains unresolved, despite extensive research in this field. Among several options to tackle this problem the synthesis of multifunctional compounds containing hybridized structures gained a lot of interest. Recently, extensively investigated are combinations of opioid agonist and antagonist pharmacophores embodied in a single molecule. To this end, agonism at the μ opioid receptor (MOR) with simultaneous antagonism at the δ opioid receptor (DOR) emerged as a promising avenue to obtaining novel analogs devoid of serious adverse effects associated with morphine-based analgesics. In this review we covered up-to-date research on the synthesis of peptide-based ligands with MOR agonist/DOR antagonist profile.

Aromatic-Amine Pendants Produce Highly Potent and Efficacious Mixed Efficacy μ-Opioid Receptor (MOR)/δ-Opioid Receptor (DOR) Peptidomimetics with Enhanced Metabolic Stability

We previously reported a novel SAR campaign that converted a metabolically unstable series of μ-opioid receptor (MOR) agonist/δ-opioid receptor (DOR) antagonist bicyclic core peptidomimetics with promising analgesic activity and reduced abuse liabilities into a more stable series of benzylic core analogues. Herein, we expanded the SAR of that campaign and determined that the incorporation of amines into the benzylic pendant produces enhanced MOR-efficacy in this series, whereas the reincorporation of an aromatic ring into the pendant enhanced MOR-potency. Two compounds, which contain a piperidine (14) or an isoindoline (17) pendant, retained the desired opioid profile in vitro, possessed metabolic half-lives of greater than 1 h in mouse liver microsomes (MLMs), and were active antinociceptive agents in the acetic acid stretch assay (AASA) at subcutaneous doses of 1 mg/kg.

Structure-Activity Relationships of 7-Substituted Dimethyltyrosine-Tetrahydroisoquinoline Opioid Peptidomimetics

The opioid receptors modulate a variety of biological functions, including pain, mood, and reward. As a result, opioid ligands are being explored as potential therapeutics for a variety of indications. Multifunctional opioid ligands, which act simultaneously at more than one type of opioid receptor, show promise for use in the treatment of addiction, pain, and other conditions. Previously, we reported the creation of bifunctional kappa opioid receptor (KOR) agonist/mu opioid receptor (MOR) partial agonist ligands from the classically delta opioid receptor (DOR) antagonist selective dimethyltyrosine-tetrahydroisoquinoline (Dmt-Tiq) scaffold through the addition of a 7-benzyl pendant on the tetrahydroisoquinoline ring. This study further explores the structure–activity relationships surrounding 7-position pendants on the Dmt-Tiq scaffold. Some analogues maintain a KOR agonist/MOR partial agonist profile, which is being explored in the development of a treatment for cocaine addiction. Others display a MOR agonist/DOR antagonist profile, which has potential to be used in the creation of a less addictive pain medication. Ultimately, we report the synthesis and in vitro evaluation of novel opioid ligands with a variety of multifunctional profiles.

Toward a Universal μ-Agonist Template for Template-Based Alignment Modeling of Opioid Ligands

Opioid ligands are a large group of G-protein-coupled receptor ligands possessing high structural diversity, along with complicated structure-activity relationships (SARs). To better understand their structural correlations as well as the related SARs, we developed the innovative template-based alignment modeling in our recent studies on a variety of opioid ligands. As previously reported, this approach showed promise but also with limitations, which was mainly attributed to the small size of morphine as a template. With this study, we set out to construct an artificial μ-agonist template to overcome this limitation. The newly constructed template contained a largely extended scaffold, along with a few special μ-features relevant to the μ-selectivity of opioid ligands. As demonstrated in this paper, the new template showed significantly improved efficacy in facilitating the alignment modeling of a wide variety of opioid ligands. This report comprises of two main parts. Part 1 discusses the general construction process and the structural features as well as a few typical examples of the template applications and Part 2 focuses on the template refinement and validation.

Novel Dimethyltyrosine-Tetrahydroisoquinoline Peptidomimetics with Aromatic Tetrahydroisoquinoline Substitutions Show in Vitro Kappa and Mu Opioid Receptor Agonism

The dimethyltyrosine-tetrahydroisoquinoline (Dmt-Tiq) scaffold was originally developed in the production of selective delta opioid receptor (DOR) antagonists. Installation of a 7-benzyl pendant on the tetrahydroisoquinoline core of this classic opioid scaffold introduced kappa opioid receptor (KOR) agonism. Further modification of this pendant resulted in retention of KOR agonism and the addition of mu opioid receptor (MOR) partial agonism, a bifunctional profile with potential to be used in the treatment of cocaine addiction.

A Novel Mu-Delta Opioid Agonist Demonstrates Enhanced Efficacy With Reduced Tolerance and Dependence in Mouse Neuropathic Pain Models

Numerous studies have demonstrated a physiological interaction between the mu opioid receptor (MOR) and delta opioid receptor (DOR) systems. A few studies have shown that dual MOR-DOR agonists could be beneficial, with reduced tolerance and addiction liability, but are nearly untested in chronic pain models, particularly neuropathic pain. In this study, we tested the MOR-DOR agonist SRI-22141 in mice in the clinically relevant models of HIV Neuropathy and Chemotherapy-Induced Peripheral Neuropathy (CIPN). SRI-22141 was more potent than morphine in the tail flick pain test and had equal or enhanced efficacy versus morphine in both neuropathic pain models, with significantly reduced tolerance. SRI-22141 also produced no jumping behavior during naloxone-precipitated withdrawal in CIPN or naïve mice, suggesting that SRI-22141 produces little to no dependence. SRI-22141 also reduced tumor necrosis factor-α and cyclooxygenase-2 in CIPN in the spinal cord, suggesting an anti-inflammatory mechanism of action. The DOR-selective antagonist naltrindole strongly reduced CIPN efficacy and anti-inflammatory activity in the spinal cord, without affecting tail flick antinociception, suggesting the importance of DOR activity in these models. Overall, these results provide compelling evidence that MOR-DOR agonists could have strong efficacy with reduced side effects and an anti-inflammatory mechanism in the treatment of neuropathic pain. PERSPECTIVE: This study demonstrates that a MOR-DOR dual agonist given chronically in chronic neuropathic pain models has enhanced efficacy with strongly reduced tolerance and dependence, with a further anti-inflammatory effect in the spinal cord. This suggests that MOR-DOR dual agonists could be effective treatments for neuropathic pain with reduced side effects.

Multivalent peptide and peptidomimetic ligands for the treatment of pain without toxicities and addiction

The current opioid crisis has created a tragic problem in medicine and society. Pain is the most ubiquitous and costly disease in society and yet all of our "treatments" have toxicities, especially for prolonged use. However, there are several alternatives that have been discovered in the past fifteen years that have been demonstrated in animals to have none of the toxicities of current drugs. Many of the compounds are multivalent and have novel biological activity profiles. Unfortunately, none of these have been in clinical trials in humans, perhaps because they were discovered in academic laboratories. A review of these novel chemicals are given in this paper.

Structural Simplification of a Tetrahydroquinoline-Core Peptidomimetic μ-Opioid Receptor (MOR) Agonist/δ-Opioid Receptor (DOR) Antagonist Produces Improved Metabolic Stability

We have previously reported a series of μ-opioid receptor (MOR) agonist/δ-opioid receptor (DOR) antagonist ligands to serve as potential nonaddictive opioid analgesics. These ligands have been shown to be active in vivo, do not manifest withdrawal syndromes or reward behavior in conditioned-place preference assays in mice, and do not produce dependence. Although these attributes are promising, these analogues exhibit poor metabolic stability in mouse liver microsomes, likely due to the central tetrahydroquinoline scaffold in this series. As such, a structure-activity relationship (SAR) campaign was pursued to improve their metabolic stability. This resulted in a shift from our original bicyclic tetrahydroquinoline core to a monocyclic benzylic-core system. By eliminating one of the rings in this scaffold and exploring the SAR of this new core, two promising analogues were discovered. These analogues (5l and 5m) had potency and efficacy values at MOR better or comparable to morphine, retained their DOR-antagonist properties, and showed a 10-fold improvement in metabolic stability.

Dual Pharmacophores Explored via Structure-Activity Relationship (SAR) Matrix: Insights into Potent, Bifunctional Opioid Ligand Design

Short-acting μ-opioid receptor (MOR) agonists have long been used for the treatment of severe, breakthrough pain. However, selective MOR agonists including fentanyl and morphine derivatives are limited clinically due to high risks of dependence, tolerance, and respiratory depression. We recently reported the development of a long-acting, bifunctional MOR agonist/δ-opioid receptor (DOR) antagonist analgesic devoid of tolerance or dependence in mice (AAH8, henceforth referred to as 2B). To address the need for short-acting treatments for breakthrough pain, we present a series of novel, short-acting, high-potency MOR agonist/DOR antagonist ligands with antinociceptive activity in vivo. In this study, we utilized a two-dimensional structure-activity relationship matrix to identify pharmacological trends attributable to combinations of two key pharmacophore elements within the chemotype. This work enhances our ability to modulate efficacy at MOR and DOR, accessing a variety of bifunctional profiles while maintaining high affinity and potency at both receptors.

Bifunctional opioid receptor ligands as novel analgesics

Prolonged treatment of chronic severe pain with opioid analgesics is frought with problematic adverse effects including tolerance, dependence, and life-threatening respiratory depression. Though these effects are mediated predominately through preferential activation of μ opioid peptide (μOP) receptors, there is an emerging appreciation that actions at κOP and δOP receptors contribute to the observed pharmacologic and behavioral profile of μOP receptor agonists and may be targeted simultaneously to afford improved analgesic effects. Recent developments have also identified the related nociceptin opioid peptide (NOP) receptor as a key modulator of the effects of μOP receptor signaling. We review here the available literature describing OP neurotransmitter systems and highlight recent drug and probe design strategies.

The in vivo antinociceptive and μ-opioid receptor activating effects of the combination of N-phenyl-2',4'-dimethyl-4,5'-bi-1,3-thiazol-2-amines and naloxone

Morphine is widely used for the treatment of severe pain. This analgesic effect is mediated principally by the activation of μ-opioid receptors (MOR). However, prolonged activation of MOR also results in tolerance, dependence, addiction, constipation, nausea, sedation, and respiratory depression. To address this problem, we sought alternative ways to activate MOR - either by use of novel ligands, or via a novel activation mechanism. To this end, a series of compounds were screened using a sensitive CHO-K1/MOR/Gα15 cell-based FLIPR^® calcium high-throughput screening (HTS) assay, and the bithiazole compound 5a was identified as being able activate MOR in combination with naloxone. Structural modifications of 5a resulted in the discovery of lead compound 5j, which could effectively activate MOR in combination with the MOR antagonist naloxone or naltrexone. In vivo, naloxone in combination with 100 mg/kg of compound 5j elicited antinociception in a mouse tail-flick model with an ED₅₀ of 17.5 ± 4 mg/kg. These results strongly suggest that the mechanism by which the 5j/naloxone combination activates MOR is worthy of further study, as its discovery has the potential to yield an entirely novel class of analgesics.

Synthesis and Biological Evaluation of Fentanyl Analogues Modified at Phenyl Groups with Alkyls

A series of fentanyl analogues modified at the phenyl group of the phenethyl with alkyl and/or hydroxyl and alkoxy, and the phenyl group in the anilido moiety replaced with benzyl or substituted benzyl, were synthesized. The in vitro opioid receptor functional activity of these compounds was evaluated by assessment of their ability to modulate forskolin-stimulated cAMP accumulation and by their ability to induce β-arrestin2 recruitment. Compound 12 is a potent μ-opioid (MOP) receptor agonist, a potent κ-opioid (KOP) receptor antagonist with weak β-arrestin2 recruitment activity. Compounds 10 and 11 are potent MOP receptor agonists with weak δ-opioid (DOP) receptor antagonist activity and moderate KOP receptor antagonist activity as well as weak β-arrestin2 recruitment activity at the MOP receptor. These compounds are promising leads for discovery of potent opioid analgesics with reduced side effects relative to clinically available strong opioid analgesics.

Asymmetric aerobic decarboxylative Povarov reactions of N-aryl α-amino acids with methylenephthalimidines via cooperative photoredox and chiral Brønsted acid catalysis

This work realizes a new synthetic utility of methylenephthalimidines as a 1,2-synthon and the synthesis of valuable chiral isoindolin-1-ones featuring a 3,3-spiro-tetrahydroquinoline-based stereocenter.

DARK Classics in Chemical Neuroscience: Morphine

As the major psychoactive agent in opium and direct precursor for heroin, morphine is a historically critical molecule in chemical neuroscience. A structurally complex phenanthrene alkaloid produced by Papaver somniferum, morphine has fascinated chemists seeking to disentangle pharmacologically beneficial analgesic effects from addiction, tolerance, and dependence liabilities. In this review, we will detail the history of morphine, from the first extraction and isolation by Sertürner in 1804 to the illicit use of morphine and proliferation of opioid use and abuse disorders currently ravaging the United States. Morphine is a molecule of great cultural relevance, as the agent that single-handedly transformed our understanding of pharmacognosy, receptor dynamics, and substance abuse and dependence disorders.

Intramolecular Umpolung Allylation of Imines

An intramolecular umpolung allylation of imines is reported. This reaction occurs via the intermediacy of 2-azaallyl anions. It could proceed either under transition-metal-catalyzed conditions or under transition-metal-free conditions. Importantly, this approach afforded trans-3-vinyl-4-aminochromanes with high diastereoselectivity, while conventional, nonumpolung methods often display high cis-selectivity.

Synthesis and Pharmacological Evaluation of Novel C-8 Substituted Tetrahydroquinolines as Balanced-Affinity Mu/Delta Opioid Ligands for the Treatment of Pain

The use of opioids for the treatment of pain, while largely effective, is limited by detrimental side effects including analgesic tolerance, physical dependence, and euphoria, which may lead to opioid abuse. Studies have shown that compounds with a μ-opioid receptor (MOR) agonist/δ-opioid receptor (DOR) antagonist profile reduce or eliminate some of these side effects including the development of tolerance and dependence. Herein we report the synthesis and pharmacological evaluation of a series of tetrahydroquinoline-based peptidomimetics with substitutions at the C-8 position. Relative to our lead peptidomimetic with no C-8 substitution, this series affords an increase in DOR affinity and provides greater balance in MOR and DOR binding affinities. Moreover, compounds with carbonyl moieties at C-8 display the desired MOR agonist/DOR antagonist profile whereas alkyl substitutions elicit modest DOR agonism. Several compounds in this series produce a robust antinociceptive effect in vivo and show antinociceptive activity for greater than 2 h after intraperitoneal administration in mice.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Placement of Hydroxy Moiety on Pendant of Peptidomimetic Scaffold Modulates Mu and Kappa Opioid Receptor Efficacy

In an effort to expand the structure–activity relationship (SAR) studies of a series of mixed-efficacy opioid ligands, peptidomimetics that incorporate methoxy and hydroxy groups around a benzyl or 2-methylindanyl pendant on a tetrahydroquinoline (THQ) core of the peptidomimetics were evaluated. Compounds containing a methoxy or hydroxy moiety in the o- or m-positions increased binding affinity to the kappa opioid receptor (KOR), whereas compounds containing methoxy or hydroxy groups in the p-position decreased KOR affinity and reduced or eliminated efficacy at the mu opioid receptor (MOR). The results from a substituted 2-methylindanyl series aligned with the findings from the substituted benzyl series. Our studies culminated in the development of 8c, a mixed-efficacy MOR agonist/KOR agonist with subnanomolar binding affinity for both MOR and KOR.

Novel Molecular Strategies and Targets for Opioid Drug Discovery for the Treatment of Chronic Pain

Opioid drugs like morphine and fentanyl are the gold standard for treating moderate to severe acute and chronic pain. However, opioid drug use can be limited by serious side effects, including constipation, tolerance, respiratory suppression, and addiction. For more than 100 years, we have tried to develop opioids that decrease or eliminate these liabilities, with little success. Recent advances in understanding opioid receptor signal transduction have suggested new possibilities to activate the opioid receptors to cause analgesia, while reducing or eliminating unwanted side effects. These new approaches include designing functionally selective ligands, which activate desired signaling cascades while avoiding signaling cascades that are thought to provoke side effects. It may also be possible to directly modulate downstream signaling through the use of selective activators and inhibitors. Separate from downstream signal transduction, it has also been found that when the opioid system is stimulated, various negative feedback systems are upregulated to compensate, which can drive side effects. This has led to the development of multi-functional molecules that simultaneously activate the opioid receptor while blocking various negative feedback receptor systems including cholecystokinin and neurokinin-1. Other novel approaches include targeting heterodimers of the opioid and other receptor systems which may drive side effects, and making endogenous opioid peptides druggable, which may also reduce opioid mediated side effects. Taken together, these advances in our molecular understanding provide a path forward to break the barrier in producing an opioid with reduced or eliminated side effects, especially addiction, which may provide relief for millions of patients.

A one-pot multicomponent approach to a new series of morphine derivatives and their biological evaluation

Morphine derivatives displaying a mixed MOR/DOR biological characteristic were synthesized through a one-pot multicomponent approach.

Benzylideneoxymorphone: A new lead for development of bifunctional mu/delta opioid receptor ligands

Opioid analgesic tolerance remains a considerable drawback to chronic pain management. The finding that concomitant administration of delta opioid receptor (DOR) antagonists attenuates the development of tolerance to mu opioid receptor (MOR) agonists has led to interest in producing bifunctional MOR agonist/DOR antagonist ligands. Herein, we present 7-benzylideneoxymorphone (6, UMB 246) displaying MOR partial agonist/DOR antagonist activity, representing a new lead for designing bifunctional MOR/DOR ligands.

Discovery, structure-activity relationship studies, and anti-nociceptive effects of N-(1,2,3,4-tetrahydro-1-isoquinolinylmethyl)benzamides as novel opioid receptor agonists

μ-Opioid receptor (MOR) agonists are analgesics used clinically for the treatment of moderate to severe pain, but their use is associated with severe adverse effects such as respiratory depression, constipation, tolerance, dependence, and rewarding effects. In this study, we identified N-({2-[(4-bromo-2-trifluoromethoxyphenyl)sulfonyl]-1,2,3,4-tetrahydro-1-isoquinolinyl}methyl)cyclohexanecarboxamide (1) as a novel opioid receptor agonist by high-throughput screening. Structural modifications made to 1 to improve potency and blood-brain-barrier (BBB) penetration resulted in compounds 45 and 46. Compound 45 was a potent MOR/KOR (κ-opioid receptor) agonist, and compound 46 was a potent MOR and medium KOR agonist. Both 45 and 46 demonstrated a significant anti-nociceptive effect in a tail-flick test performed in wild type (WT) B6 mice. The ED₅₀ value of 46 was 1.059 mg/kg, and the brain concentrations of 45 and 46 were 7424 and 11696 ng/g, respectively. Accordingly, compounds 45 and 46 are proposed for lead optimization and in vivo disease-related pain studies.