The Pharmacological Heterogeneity of Nepenthone Analogs in Conferring Highly Selective and Potent κ-Opioid Agonistic Activities

Discovery of an Ortho-Substituted N-Cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaine Derivative as a Selective and Potent Kappa Opioid Receptor Agonist with Subsided Sedative Effect

Research into kappa opioid receptor (KOR) agonists with attenuated central-nervous-system side effects is a critical focus for developing productive and safe analgesics. Herein, a series of ortho-substituted N-cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaines were designed, synthesized, and subjected to bioassays. Compound 7a exhibited high subtype selectivity and potent agonistic activity toward KOR (KOR, Ki = 3.9 nM, MOR/KOR = 270, DOR/KOR = 1075; [35S]GTPγS binding, EC50 = 3.4 nM). Additionally, this compound exhibited robust and persistent antinociceptive effects in rodent models with different animal strains (hot plate test, ED50 = 0.20-0.30 mg/kg, i.p.; abdominal constriction test, ED50 = 0.20-0.60 mg/kg, i.p.), with its KOR-mediated mechanism for antinociception firmly established. Notably, compound 7a, unlike conventional KOR agonists, displayed minimal sedation and aversion at the antinociceptive ED50 dose. This feature addresses a crucial limitation in existing KOR agonists, positioning compound 7a as a promising novel therapeutic agent.

Morphinan Evolution: The Impact of Advances in Biochemistry and Molecular Biology

Morphinan-based opioids, derived from natural alkaloids like morphine, codeine and thebaine, have long been pivotal in managing severe pain. However, their clinical utility is marred by significant side effects and high addiction potential. This review traces the evolution of the morphinan scaffold in light of advancements in biochemistry and molecular biology, which have expanded our understanding of opioid receptor pharmacology. We explore the development of semi-synthetic and synthetic morphinans, their receptor selectivity and the emergence of biased agonism as a strategy to dissociate analgesic properties from undesirable effects. By examining the molecular intricacies of opioid receptors and their signaling pathways, we highlight how receptor-type selectivity and signaling bias have informed the design of novel analgesics. This synthesis of historical and contemporary perspectives provides an overview of the morphinan landscape, underscoring the ongoing efforts to mitigate the problems facing opioids through smarter drug design. We also highlight that most morphinan derivatives show a preference for the G protein pathway, although detailed experimental comparisons are still necessary. This fact underscores the utility of the morphinan skeleton in future opioid drug discovery.

Historical perspectives and recent advances in small molecule ligands of selective/biased/multi-targeted μ/δ/κ opioid receptor (2019-2022)

The opioids have been used for more than a thousand years and are not only the most widely prescribed drugs for moderate to severe pain and acute pain, but also the preferred drugs. However, their non-analgesic effects, especially respiratory depression and potential addiction, are important factors that plague the safety of clinical use and are an urgent problem for pharmacological researchers to address. Current research on analgesic drugs has evolved into different directions: de-opioidization; application of pharmacogenomics to individualize the use of opioids; development of new opioids with less adverse effects. The development of new opioid drugs remains a hot research topic, and with the in-depth study of opioid receptors and intracellular signal transduction mechanisms, new research ideas have been provided for the development of new opioid analgesics with less side effects and stronger analgesic effects. The development of novel opioid drugs in turn includes selective opioid receptor ligands, biased opioid receptor ligands, and multi-target opioid receptor ligands and positive allosteric modulators (PAMs) or antagonists and the single compound as multi-targeted agnoists/antagonists for different receptors. PAMs strategies are also getting newer and are the current research hotspots, including the BMS series of compounds and others, which are extensive and beyond the scope of this review. This review mainly focuses on the selective/biased/multi-targeted MOR/DOR/KOR (mu opioid receptor/delta opioid receptor/kappa opioid receptor) small molecule ligands and involves some cryo-electron microscopy (cryoEM) and structure-based approaches as well as the single compound as multi-targeted agnoists/antagonists for different receptors from 2019 to 2022, including discovery history, activities in vitro and vivo, and clinical studies, in an attempt to provide ideas for the development of novel opioid analgesics with fewer side effects.

Reversal of subtype-selectivity and function by the introduction of a para-benzamidyl substituent to N-cyclopropylmethyl nornepenthone

The discovery and development of novel μ-opioid receptor (MOR) antagonists is a significant area to combat Opioid Use Disorder (OUD). In this work, a series of para-substituted N-cyclopropylmethyl-nornepenthone derivatives were designed and synthesized and pharmacologically assayed. Compound 6a was identified as a selective MOR antagonist both in vitro and in vivo. Its molecular basis was elucidated using molecular docking and MD simulations. A subpocket on the extracellular side of the TM2 domain of MOR, in particular the residue Y^2.64, was proposed to be responsible for the reversal of subtype selectivity and functional reversal of this compound.

Is a Highly Selective and Potent κ Opioid Receptor (KOR) Agonist with an Unexpected Nonreduction in Locomotor Activity

Undue central nervous system (CNS) side effects including dysphoria and sedation remain to be a challenge for the development of κ opioid receptor (KOR) agonists as effective and safe analgesics. On the basis of our previous work on morphinan-based KOR agonists, a series of 7α-methyl-7β-substituted northebaine derivatives were designed, synthesized, and biologically assayed. Among others, compound 4a (SLL-627) has been identified as a highly selective and potent KOR agonist both in vitro and in vivo, and its molecular basis was also examined and discussed. Besides low liability to conditioned place aversion (CPA) test, treatment of SLL-627 was associated with a nonreduction in locomotor activity, compared to most of the other arylacetamide- or morphinan-based KOR agonists which generally exhibited apparently sedative effects. This unexpected finding provides new insights to dissociate analgesia from sedation for future discovery of innovative KOR agonists.

Novel selective κ agonists SLL-039 and SLL-1206 produce potent antinociception with fewer sedation and aversion

SLL-039 (N-cyclopropylmethyl-7α-4'-(N'-benzoyl) amino-phenyl-6,14-endoethano-tetrahydronorthebaine) and SLL-1206 (N-cyclopropylmethyl-7α-3'-(p-methoxybenzyl) amino-phenyl-6,14-endoethano-tetrahydronorthebaine) are two 4,5-epoxymorphinan-based high selective κ receptor agonists that we recently discovered. In the present study we characterized their pharmacological properties in comparison with arylacetamide-based typical κ agonist U50,488H. We showed that both SLL-039 and SLL-1206 produced potent and long-lasting antinociceptive actions in three different rodent models of pain via activation of κ opioid receptor. In hot-plate assay, the antinociceptive potency of SLL-039 and SLL-1206 increased about 11-and 17.3-fold compared to U50,488H and morphine, respectively, with ED₅₀ values of 0.4 mg/kg. Following repeated administration, SLL-1206, SLL-039, and U50,488H all developed analgesic tolerance tested in hot-plate assay. U50,488H and SLL-039 produced antipruritic effects in a dose-dependent manner, whereas SLL-1206 displayed some antipruritic effects only at very low doses. In addition, SLL-1206 was capable of decreasing morphine-induced physical dependence. More importantly, SLL-039 and SLL-1206 at effective analgesic doses did not cause sedation and conditioned place aversion (CPA), whereas U50,488H did. In comparison with SLL-039, SLL-1206 caused similar antinociceptive responses, but fewer sedation and CPA. In conclusion, our results suggest that SLL-039 and SLL-1206 have potential to be developed as novel analgesic agents, and 4,5-expoxymorphinan scaffold is an attractive structure for the development of selective κ agonists with fewer side effects.

Diels-Alder Adducts of Morphinan-6,8-Dienes and Their Transformations

6,14-ethenomorphinans are semisynthetic opiate derivatives containing an ethylene bridge between positions 6 and 14 in ring-C of the morphine skeleton that imparts a rigid molecular structure. These compounds represent an important family of opioid receptor ligands in which the 6,14-etheno bridged structural motif originates from a [4 + 2] cycloaddition of morphinan-6,8-dienes with dienophiles. Certain 6,14-ethenomorphinans having extremely high affinity for opioid receptors are often non-selective for opioid receptor subtypes, but this view is now undergoing some revision. The agonist 20R-etorphine and 20R-dihydroetorphine are several thousand times more potent analgesics than morphine, whereas diprenorphine is a high-affinity non-selective antagonist. The partial agonist buprenorphine is used as an analgesic in the management of post-operative pain or in substitution therapy for opiate addiction, sometimes in combination with the non-selective antagonist naloxone. In the context of the current opioid crisis, we communicated a summary of several decades of work toward generating opioid analgesics with lesser side effects or abuse potential. Our summary placed a focus on Diels-Alder reactions of morphinan-6,8-dienes and subsequent transformations of the cycloadducts. We also summarized the pharmacological aspects of radiolabeled 6,14-ethenomorphinans used in molecular imaging of opioid receptors.

Discovery of an M-Substituted N-Cyclopropylmethyl-7α-phenyl-6,14-endoethanotetrahydronorthebaine as a Selective, Potent, and Orally Active κ-Opioid Receptor Agonist with an Improved Central Nervous System Safety Profile

The search for selective kappa opioid receptor (κOR) agonists with an improved safety profile is an area of interest in opioid research. In this work, a series of m-substituted analogs were designed, synthesized, and assayed, resulting in the identification of compound 6c (SLL-1206) as a κOR agonist with single-digit nanomolar activities. The subtype selectivity of compound 6c appeared to be a consequence of an enormous decrease in the affinity for μOR and δOR, rather than a significant increase in the affinity for κOR, which was not the case for SLL-039, another selective and potent κOR agonist identified in our previous work. Besides reduced central nervous system effects, SLL-1206 exhibited substantially improved physicochemical and pharmacokinetic properties compared with SLL-039, with increases of over 20-fold in aqueous solubility and approximately 40-fold in oral bioavailability in rats.

Structural Characterization of KOR Inactive and Active States for 3D Pharmacology and Drug Discovery

The structure of the human kappa opioid receptor (KOR) in complex with the long-acting antagonist JDTic was solved crystallographically in 2012 and, along with structures of other opioid receptors, revolutionized our understanding of opioid system function and pharmacology. More recently, active state KOR structure was also determined, giving important insights into activation mechanisms of the receptor. In this review, we will discuss how the understanding of atomistic structures of KOR established a key platform for deciphering details of subtype and functional selectivity of KOR-targeting ligands and for discovery of new chemical probes with potentially beneficial pharmacological profiles.

Exploration of the SAR Connection between Morphinan- and Arylacetamide-Based κ Opioid Receptor (κOR) Agonists Using the Strategy of Bridging

κ opioid receptor (κOR) is a subtype of opioid receptors, and there are two major κOR agonists currently available, morphinans and arylacetamides, which are structurally distinct from each other. Numerous efforts had been made to correlate these series of compounds in order to establish a consensus binding pattern for κOR agonists. Unfortunately, no morphinan-based agent with an arylacetamidyl substituent has been identified as a κOR agonist with a pharmacological profile similar to arylacetamides. Since the recently described morphinan-based compound SLL-039 was identified as a selective and potent κOR agonist that contains a unique benzamidyl substituent in structure similar to arylacetamides, numerous arylacetamidyl substituents were introduced to this scaffold to examine whether the structure-activity relationships (SARs) of arylacetamides in conferring κOR agonistic activities could be reproduced by these analogues. Thus, a series of N-cyclopropylmethyl-7α-arylacetamidylphenyl-6,14-endoethanotetrahydronorthebaine analogues were designed, synthesized, and assayed for biological activities. Among these compounds, compound 4j with a 3',4'-dimethylphenylacetamidyl substituent showed a single digit low nanomolar affinity to the κOR and relatively high subtype selectivity in binding assays, but this profile was not reproduced in functional assays. In contrast, compound 4i displayed moderately selective κOR agonistic activities in functional assays, which was inconsistent with its nonselective nature in binding assays. Overall, introduction of an arylacetamidyl substituent to the morphinan-based scaffold was associated with pharmacological diversity in both binding and functional activities on opioid receptors in vitro. The resultant SARs were inconsistent with that of classical arylacetamides as κOR agonists, despite bearing a similar arylacetamidyl substituent in the structure. Therefore, the arylacetamidyl substituent of the morphinan-based scaffold was found to be disconnected from that of arylacetamides in conferring κOR activities.

The Use of Computational Approaches in the Discovery and Mechanism Study of Opioid Analgesics

Opioid receptors that belong to class A G protein-coupled receptors (GPCRs) are vital in pain control. In the past few years, published high-resolution crystal structures of opioid receptor laid a solid basis for both experimental and computational studies. Computer-aided drug design (CADD) has been established as a powerful tool for discovering novel lead compounds and for understanding activation mechanism of target receptors. Herein, we reviewed the computational-guided studies on opioid receptors for the discovery of new analgesics, the structural basis of receptor subtype selectivity, agonist interaction mechanism, and biased signaling mechanism.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Discovery of a Highly Selective and Potent κ Opioid Receptor Agonist from N-Cyclopropylmethyl-7α-phenyl-6,14-endoethanotetrahydronorthebaines with Reduced Central Nervous System (CNS) Side Effects Navigated by the Message-Address Concept

Effective and safe analgesics represent an unmet medical need for the treatment of acute and chronic pain. A series of N-cyclopropylmethyl-7α-phenyl-6,14-endoethanotetrahydronorthebaines were designed, synthesized, and assayed, leading to the discovery of a benzylamine derivative (compound 4, SLL-039) as a highly selective and potent κ opioid agonist (κ, K_i = 0.47 nM, κ/μ = 682, κ/δ = 283), which was confirmed by functional assays in vitro and antinociceptive assays in vivo. The in vivo effect could be blocked by pretreatment with the selective κ antagonist nor-BNI. Moreover, this compound did not induce sedation, a common dose limiting effect of κ opioid receptor agonists, at its analgesic dose compared to U50,488H. The dissociation of sedation/antinociception found in SLL-039 was assumed to be correlated with the occupation of its benzamide motif in a unique subsite involving V118^2.63, W124^EL1, and E209^EL2.