A Novel Mitragynine Analog with Low-Efficacy Mu Opioid Receptor Agonism Displays Antinociception with Attenuated Adverse Effects

Metabolic engineering of yeast for de novo production of kratom monoterpene indole alkaloids

Monoterpene indole alkaloids (MIAs) from Mitragyna speciosa ("kratom"), such as mitragynine and speciogynine, are promising novel scaffolds for opioid receptor ligands for treatment of pain, addiction, and depression. While kratom leaves have been used for centuries in South-East Asia as stimulant and pain management substance, the biosynthetic pathway of these psychoactives have only recently been partially elucidated. Here, we demonstrate the de novo production of mitragynine and speciogynine in Saccharomyces cerevisiae through the reconstruction of a five-step synthetic pathway from common MIA precursor strictosidine comprising fungal tryptamine 4-monooxygenase to bypass an unknown kratom hydroxylase. Upon optimizing cultivation conditions, a titer of ∼290 μg/L kratom MIAs from glucose was achieved. Untargeted metabolomics analysis of lead production strains led to the identification of numerous shunt products derived from the activity of strictosidine synthase (STR) and dihydrocorynantheine synthase (DCS), highlighting them as candidates for enzyme engineering to further improve kratom MIAs production in yeast. Finally, by feeding fluorinated tryptamine and expressing a human tailoring enzyme, we further demonstrate production of fluorinated and hydroxylated mitragynine derivatives with potential applications in drug discovery campaigns. Altogether, this study introduces a yeast cell factory platform for the biomanufacturing of complex natural and new-to-nature kratom MIAs derivatives with therapeutic potential.

Oxa-Iboga alkaloids lack cardiac risk and disrupt opioid use in animal models

Ibogaine and its main metabolite noribogaine provide important molecular prototypes for markedly different treatment of substance use disorders and co-morbid mental health illnesses. However, these compounds present a cardiac safety risk and a highly complex molecular mechanism. We introduce a class of iboga alkaloids - termed oxa-iboga - defined as benzofuran-containing iboga analogs and created via structural editing of the iboga skeleton. The oxa-iboga compounds lack the proarrhythmic adverse effects of ibogaine and noribogaine in primary human cardiomyocytes and show superior efficacy in animal models of opioid use disorder in male rats. They act as potent kappa opioid receptor agonists in vitro and in vivo, but exhibit atypical behavioral features compared to standard kappa opioid agonists. Oxa-noribogaine induces long-lasting suppression of morphine, heroin, and fentanyl intake after a single dose or a short treatment regimen, reversal of persistent opioid-induced hyperalgesia, and suppression of opioid drug seeking in rodent relapse models. As such, oxa-iboga compounds represent mechanistically distinct iboga analogs with therapeutic potential.

Signaling Modulation Mediated by Ligand Water Interactions with the Sodium Site at μOR

The mu opioid receptor (μOR) is a target for clinically used analgesics. However, adverse effects, such as respiratory depression and physical dependence, necessitate the development of alternative treatments. Recently we reported a novel strategy to design functionally selective opioids by targeting the sodium binding allosteric site in μOR with a supraspinally active analgesic named C6guano. Presently, to improve systemic activity of this ligand, we used structure-based design, identifying a new ligand named RO76 where the flexible alkyl linker and polar guanidine guano group is swapped with a benzyl alcohol, and the sodium site is targeted indirectly through waters. A cryoEM structure of RO76 bound to the μOR-Gi complex confirmed that RO76 interacts with the sodium site residues through a water molecule, unlike C6guano which engages the sodium site directly. Signaling assays coupled with APEX based proximity labeling show binding in the sodium pocket modulates receptor efficacy and trafficking. In mice, RO76 was systemically active in tail withdrawal assays and showed reduced liabilities compared to those of morphine. In summary, we show that targeting water molecules in the sodium binding pocket may be an avenue to modulate signaling properties of opioids, and which may potentially be extended to other G-protein coupled receptors where this site is conserved.

Structure-Guided Design of Partial Agonists at an Opioid Receptor

The persistence of chronic pain and continuing overdose deaths from pain-relieving opioids targeting μ opioid receptor (μOR) have fueled the need for reliable long-term analgesics which use different targets and mechanisms. The δ opioid receptor (δOR) is a potential alternative target for non-addictive analgesics to alleviate chronic pain, made more attractive by its lack of respiratory depression associated with μOR agonists. However, early δOR full agonists were found to induce seizures, precluding clinical use. Partial δOR agonists may offer more controlled activation of the receptor compared to full agonists, but the development of such ligands has been hindered by uncertainty over the molecular mechanism mediating partial agonism. Using a structure-based approach, we explored the engagement of the sodium binding pocket in δOR and developed a bitopic ligand, C6-Quino, predicted to be a selective δOR partial agonist. Functional studies of C6-Quino revealed that it displayed δOR partial agonist activity at both G-protein and arrestin pathways. Its interaction with the sodium pocket was confirmed and analyzed using a single particle cryo-EM. Additionally, C6-Quino demonstrated favorable chemical and physiological properties like oral activity, and analgesic activity in multiple chronic pain models. Notably, μOR-related hyperlocomotion and respiratory depression, and δOR-related convulsions, were not observed at analgesic doses of C6-Quino. This fundamentally new approach to designing δOR ligands provides a blueprint for the development of partial agonists as safe analgesics and acts as a generic method to optimize signaling profiles of other Class A GPCRs.

A µ-opioid receptor modulator that works cooperatively with naloxone

The µ-opioid receptor (µOR) is a well-established target for analgesia1, yet conventional opioid receptor agonists cause serious adverse effects, notably addiction and respiratory depression. These factors have contributed to the current opioid overdose epidemic driven by fentanyl2, a highly potent synthetic opioid. µOR negative allosteric modulators (NAMs) may serve as useful tools in preventing opioid overdose deaths, but promising chemical scaffolds remain elusive. Here we screened a large DNA-encoded chemical library against inactive µOR, counter-screening with active, G-protein and agonist-bound receptor to 'steer' hits towards conformationally selective modulators. We discovered a NAM compound with high and selective enrichment to inactive µOR that enhances the affinity of the key opioid overdose reversal molecule, naloxone. The NAM works cooperatively with naloxone to potently block opioid agonist signalling. Using cryogenic electron microscopy, we demonstrate that the NAM accomplishes this effect by binding a site on the extracellular vestibule in direct contact with naloxone while stabilizing a distinct inactive conformation of the extracellular portions of the second and seventh transmembrane helices. The NAM alters orthosteric ligand kinetics in therapeutically desirable ways and works cooperatively with low doses of naloxone to effectively inhibit various morphine-induced and fentanyl-induced behavioural effects in vivo while minimizing withdrawal behaviours. Our results provide detailed structural insights into the mechanism of negative allosteric modulation of the µOR and demonstrate how this can be exploited in vivo.

Biased signalling in analgesic research and development

Ligand bias offers a novel means to improve the therapeutic profile of drugs. With regard to G protein-coupled receptors involved in analgesia, it could be advantageous to develop such drugs if the analgesic effect is mediated by a different cellular signalling pathway than the adverse effects associated with the drug. Whilst this has been explored over a number of years for the μ receptor, it remains unclear whether this approach offers significant benefit for the treatment of pain. Nevertheless, the development of biased ligands at other G protein-coupled receptors in the CNS does offer some promise for the development of novel analgesic drugs in the future. Here we summarise and discuss the recent evidence to support this.

Peniditerpenoids A and B: Oxidized Indole Diterpenoids with Osteoclast Differentiation Inhibitory Activity from a Mangrove-Sediment-Derived Penicillium sp

An unprecedented di-seco-indole diterpenoid, peniditerpenoid A (1), and a rare N-oxide-containing indole diterpenoid derivative, peniditerpenoid B (2), together with three known ones (3-5), were obtained from the mangrove-sediment-derived fungus Penicillium sp. SCSIO 41411. Their structures were determined by the analysis of spectroscopic data, quantum chemical calculations, and X-ray diffraction analyses. Peniditerpenoid A (1) inhibited lipopolysaccharide-induced NF-κB with an IC50 value of 11 μM and further effectively prevented RANKL-induced osteoclast differentiation in bone marrow macrophages. In vitro studies demonstrated that 1 exerted significant inhibition of NF-κB activation in the classical pathway by preventing TAK1 activation, IκBα phosphorylation, and p65 translocation. Furthermore, 1 effectively reduced the level of NFATc1 activation, resulting in the attenuation of osteoclast differentiation. Our findings suggest that 1 holds promise as an inhibitor with significant potential for the treatment of diseases related to osteoporosis.

The μ-opioid receptor-mediated Gi/o protein and β-arrestin2 signaling pathways both contribute to morphine-induced side effects

The μ-opioid receptor-biased agonist theory holds that Gio protein signaling mediates the analgesic effect of opioids and the related side effects via the β-arrestin2 signaling pathway. A series of μ-opioid-biased agonists have been developed in accordance with this theory, and the FDA has approved TRV130 (as a representative of biased agonists) for marketing. However, several reports have raised the issue of opioid side effects associated with the use of agonists. In this study, five permeable peptides were designed to emulate 11 S/T phosphorylation sites at the μ-opioid receptor (MOR) carboxyl-terminal. In vitro experiments were performed to detect the activation level of G proteins from the cAMP inhibition assay and the β-arrestin2 recruitment by the BRET assay. Designed peptides might effectively interfere with the activation of the Gio and β-arrestin2 pathways when combined with morphine. The resulting morphine-induced tolerance, respiratory inhibition, and constipation in mice showed that the β-arrestin2 pathway was responsible for morphine tolerance while the Gio signaling pathway was involved with respiratory depression and constipation and that these side effects were significantly related to phosphorylation sites S363 and T370. This study may provide new directions for the development of safer and more effective opioid analgesics, and the designed peptides may be an effective tool for exploring the mechanism by which μ-opioid receptors function, with the potential of reducing the side effects that are associated with clinical opioid treatment.

A Platform for the Synthesis of Corynantheine-Type Corynanthe Alkaloids

Corynantheine-type alkaloids are major components of the Mitragyna speciosa, also known as kratom, that exhibit unique pharmacological activity. However, no universal method to prepare these alkaloids has been reported. Disclosed herein is a catalytic, asymmetric platform that enables rapid access to corynantheine alkaloids. The first enantioselective total synthesis of (-)-corynantheidine pseudoindoxyl is described. The first asymmetric syntheses of (+)-corynoxine and (-)-corynoxine B were also achieved, along with enantioselective syntheses of (-)-corynantheidol and (-)-corynantheidine. Through this work, all series of corynantheine alkaloids including indole, spirooxindole, and pseudoindoxyl can now be accessed in the laboratory, enabling comprehensive biological investigation of kratom alkaloids to be undertaken.

Discovery of a novel series of potent carbonic anhydrase inhibitors with selective affinity for μ Opioid receptor for Safer and long-lasting analgesia

In this study, we investigated the development of dual-targeted ligands that bind to both μ-opioid receptor (MOR) and carbonic anhydrase (CA) enzymes, using fentanyl structure as a template. We synthesized and evaluated 21 novel compounds with dual-targeted affinity identifying the lead candidate compound 8, showing selective affinity for MOR and potent inhibition of several cytosolic CA isoforms. By means of repeated treatment of 3 daily administrations for 17 days, fentanyl (0.1 mg/kg, subcutaneously) led to tolerance development, pain threshold alterations and withdrawal symptoms in CD-1 mice, as well as astrocyte and microglia activation in the dorsal horn of the lumbar spinal cord. In contrast, compound 8 (0.32 mg/kg s.c.) maintained stable during days its analgesic effect at the higher dose tested with fewer withdrawal symptoms, allodynia development and glial cells activation. Our results suggest that targeting both MOR and CA enzymes can lead to the development of new class of potent analgesic agents with fewer side effects and reduced tolerance development. Further studies are needed to explore the potential mechanisms underlying these effects and to further optimize the therapeutic potential of these compounds.

IUPHAR themed review: Opioid efficacy, bias, and selectivity

Drugs acting at the opioid receptor family are clinically used to treat chronic and acute pain, though they represent the second line of treatment behind GABA analogs, antidepressants and SSRI's. Within the opioid family mu and kappa opioid receptor are commonly targeted. However, activation of the mu opioid receptor has side effects of constipation, tolerance, dependence, euphoria, and respiratory depression; activation of the kappa opioid receptor leads to dysphoria and sedation. The side effects of mu opioid receptor activation have led to mu receptor drugs being widely abused with great overdose risk. For these reasons, newer safer opioid analgesics are in high demand. For many years a focus within the opioid field was finding drugs that activated the G protein pathway at mu opioid receptor, without activating the β-arrestin pathway, known as biased agonism. Recent advances have shown that this may not be the way forward to develop safer analgesics at mu opioid receptor, though there is still some promise at the kappa opioid receptor. Here we discuss recent novel approaches to develop safer opioid drugs including efficacy vs bias and fine-tuning receptor activation by targeting sub-pockets in the orthosteric site, we explore recent works on the structural basis of bias, and we put forward the suggestion that Gα subtype selectivity may be an exciting new area of interest.

Molecular insights into GPCR mechanisms for drugs of abuse

Substance abuse is on the rise, and while many people may use illicit drugs mainly due to their rewarding effects, their societal impact can range from severe, as is the case for opioids, to promising, as is the case for psychedelics. Common with all these drugs' mechanisms of action are G protein-coupled receptors (GPCRs), which lie at the center of how these drugs mediate inebriation, lethality, and therapeutic effects. Opioids like fentanyl, cannabinoids like tetrahydrocannabinol, and psychedelics like lysergic acid diethylamide all directly bind to GPCRs to initiate signaling which elicits their physiological actions. We herein review recent structural studies and provide insights into the molecular mechanisms of opioids, cannabinoids, and psychedelics at their respective GPCR subtypes. We further discuss how such mechanistic insights facilitate drug discovery, either toward the development of novel therapies to combat drug abuse or toward harnessing therapeutic potential.

Total Synthesis and Structural Plasticity of Kratom Pseudoindoxyl Metabolites

Mitragynine pseudoindoxyl, a kratom metabolite, has attracted increasing attention due to its favorable side effect profile as compared to conventional opioids. Herein, we describe the first enantioselective and scalable total synthesis of this natural product and its epimeric congener, speciogynine pseudoindoxyl. The characteristic spiro-5-5-6-tricyclic system of these alkaloids was formed through a protecting-group-free cascade relay process in which oxidized tryptamine and secologanin analogues were used. Furthermore, we discovered that mitragynine pseudoindoxyl acts not as a single molecular entity but as a dynamic ensemble of stereoisomers in protic environments; thus, it exhibits structural plasticity in biological systems. Accordingly, these synthetic, structural, and biological studies provide a basis for the planned design of mitragynine pseudoindoxyl analogues, which can guide the development of next-generation analgesics.

Integration of discovery and engineering in plant alkaloid research: Recent developments in elucidation, reconstruction, and repurposing biosynthetic pathways

Plants synthesize tens of thousands of bioactive nitrogen-containing compounds called alkaloids, including some clinically important drugs in modern medicine. The discovery of new alkaloid structures and their metabolism in plants have provided ways to access these rich sources of bioactivities including new-to-nature compounds relevant to therapeutic and industrial applications. This review discusses recent advances in alkaloid biosynthesis discovery, including complete pathway elucidations. Additionally, the latest developments in the production of new and established plant alkaloids based on either biosynthesis or semisynthesis are discussed.

One-pot synthesis of 4-(imidazol-1-yl)indole derivatives through a sequential dearomatization and Ag-catalyzed cyclization/Cs2CO3-mediated addition/aromatization reaction

A convenient one-pot assembly of 4-(imidazol-1-yl)indole derivatives from easily accessible o-alkynylanilines and imidazoles has been developed. The sequential dearomatization and Ag(I)-catalyzed cyclization/Cs₂CO₃-mediated conjugate addition/aromatization cascade reactions exhibit high efficiency and excellent selectivity. The combined use of a silver(I) salt and cesium carbonate is significant for facilitating this domino transformation. The 4-(imidazol-1-yl)indole products could be easily converted to the corresponding derivatives and might be valuable in biological chemistry and medicinal science.

Climbing behavior by mice as an endpoint for preclinical assessment of drug effects in the absence and presence of pain

This study evaluated climbing in mice as a tool to assess the expression and treatment of pain-related behavioral depression in male and female ICR mice. Mice were videotaped during 10-min sessions in a vertical plexiglass cylinder with wire mesh walls, and "Time Climbing" was scored by observers blind to treatments. Initial validation studies demonstrated that baseline climbing was stable across repeated days of testing and depressed by intraperitoneal injection of dilute lactic acid (IP acid) as an acute pain stimulus. Additionally, IP acid-induced depression of climbing was blocked by the positive-control non-steroidal anti-inflammatory drug (NSAID) ketoprofen but not by the negative control kappa opioid receptor agonist U69593. Subsequent studies examined effects of single-molecule opioids (fentanyl, buprenorphine, naltrexone) and of fixed-proportion fentanyl/naltrexone mixtures (10:1, 3.2:1, and 1:1) that vary in their efficacy at the mu opioid receptor (MOR). Opioids administered alone produced a dose- and efficacy-dependent decrease in climbing, and fentanyl/naltrexone-mixture data indicated that climbing in mice is especially sensitive to disruption by even low-efficacy MOR activation. Opioids administered as a pretreatment to IP acid failed to block IP acid-induced depression of climbing. Taken together, these findings support the utility of climbing in mice as an endpoint to evaluate candidate-analgesic effectiveness both to (a) produce undesirable behavioral disruption when the test drug is administered alone, and (b) produce a therapeutic blockade of pain-related behavioral depression. The failure of MOR agonists to block IP acid-induced depression of climbing likely reflects the high sensitivity of climbing to disruption by MOR agonists.

Insights into distinct signaling profiles of the µOR activated by diverse agonists

Drugs targeting the μ-opioid receptor (μOR) are the most effective analgesics available but are also associated with fatal respiratory depression through a pathway that remains unclear. Here we investigated the mechanistic basis of action of lofentanil (LFT) and mitragynine pseudoindoxyl (MP), two μOR agonists with different safety profiles. LFT, one of the most lethal opioids, and MP, a kratom plant derivative with reduced respiratory depression in animal studies, exhibited markedly different efficacy profiles for G protein subtype activation and β-arrestin recruitment. Cryo-EM structures of μOR-Gi1 complex with MP (2.5 Å) and LFT (3.2 Å) revealed that the two ligands engage distinct subpockets, and molecular dynamics simulations showed additional differences in the binding site that promote distinct active-state conformations on the intracellular side of the receptor where G proteins and β-arrestins bind. These observations highlight how drugs engaging different parts of the μOR orthosteric pocket can lead to distinct signaling outcomes.

Endogenous opiates and behavior: 2021

This paper is the forty-fourth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2021 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonizts 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).

Modified Akuamma Alkaloids with Increased Potency at the Mu-opioid Receptor

Akuammine (1) and pseudoakuammigine (2) are indole alkaloids found in the seeds of the akuamma tree (Picralima nitida). Both alkaloids are weak agonists of the mu opioid receptor (μOR); however, they produce minimal effects in animal models of antinociception. To probe the interactions of 1 and 2 at the opioid receptors, we have prepared a collection of 22 semisynthetic derivatives. Evaluation of this collection at the μOR and kappa opioid receptor (κOR) revealed structural-activity relationship trends and derivatives with improved potency at the μOR. Most notably, the introduction of a phenethyl moiety to the N1 of 2 produces a 70-fold increase in potency and a 7-fold increase in selectivity for the μOR. The in vitro potency of this compound resulted in increased efficacy in the tail-flick and hot-plate assays of antinociception. The improved potency of these derivatives highlights the promise of exploring natural product scaffolds to probe the opioid receptors.

Structures of the entire human opioid receptor family

Opioids are effective analgesics, but their use is beset by serious side effects, including addiction and respiratory depression, which contribute to the ongoing opioid crisis. The human opioid system contains four opioid receptors (μOR, δOR, κOR, and NOPR) and a set of related endogenous opioid peptides (EOPs), which show distinct selectivity toward their respective opioid receptors (ORs). Despite being key to the development of safer analgesics, the mechanisms of molecular recognition and selectivity of EOPs to ORs remain unclear. Here, we systematically characterize the binding of EOPs to ORs and present five structures of EOP-OR-Gi complexes, including β-endorphin- and endomorphin-bound μOR, deltorphin-bound δOR, dynorphin-bound κOR, and nociceptin-bound NOPR. These structures, supported by biochemical results, uncover the specific recognition and selectivity of opioid peptides and the conserved mechanism of opioid receptor activation. These results provide a structural framework to facilitate rational design of safer opioid drugs for pain relief.

Structure-based design of bitopic ligands for the µ-opioid receptor

Mu-opioid receptor (µOR) agonists such as fentanyl have long been used for pain management, but are considered a major public health concern owing to their adverse side effects, including lethal overdose1. Here, in an effort to design safer therapeutic agents, we report an approach targeting a conserved sodium ion-binding site2 found in µOR3 and many other class A G-protein-coupled receptors with bitopic fentanyl derivatives that are functionalized via a linker with a positively charged guanidino group. Cryo-electron microscopy structures of the most potent bitopic ligands in complex with µOR highlight the key interactions between the guanidine of the ligands and the key Asp2.50 residue in the Na+ site. Two bitopics (C5 and C6 guano) maintain nanomolar potency and high efficacy at Gi subtypes and show strongly reduced arrestin recruitment-one (C6 guano) also shows the lowest Gz efficacy among the panel of µOR agonists, including partial and biased morphinan and fentanyl analogues. In mice, C6 guano displayed µOR-dependent antinociception with attenuated adverse effects, supporting the µOR sodium ion-binding site as a potential target for the design of safer analgesics. In general, our study suggests that bitopic ligands that engage the sodium ion-binding pocket in class A G-protein-coupled receptors can be designed to control their efficacy and functional selectivity profiles for Gi, Go and Gz subtypes and arrestins, thus modulating their in vivo pharmacology.

Ligand-Free Signaling of G-Protein-Coupled Receptors: Relevance to μ Opioid Receptors in Analgesia and Addiction

Numerous G-protein-coupled receptors (GPCRs) display ligand-free basal signaling with potential physiological functions, a target in drug development. As an example, the μ opioid receptor (MOR) signals in ligand-free form (MOR-μ*), influencing opioid responses. In addition, agonists bind to MOR but can dissociate upon MOR activation, with ligand-free MOR-μ* carrying out signaling. Opioid pain therapy is effective but incurs adverse effects (ADRs) and risk of opioid use disorder (OUD). Sustained opioid agonist exposure increases persistent basal MOR-μ* activity, which could be a driving force for OUD and ADRs. Antagonists competitively prevent resting MOR (MOR-μ) activation to MOR-μ*, while common antagonists, such as naloxone and naltrexone, also bind to and block ligand-free MOR-μ*, acting as potent inverse agonists. A neutral antagonist, 6β-naltrexol (6BN), binds to but does not block MOR-μ*, preventing MOR-μ activation only competitively with reduced potency. We hypothesize that 6BN gradually accelerates MOR-μ* reversal to resting-state MOR-μ. Thus, 6BN potently prevents opioid dependence in rodents, at doses well below those blocking antinociception or causing withdrawal. Acting as a ‘retrograde addiction modulator’, 6BN could represent a novel class of therapeutics for OUD. Further studies need to address regulation of MOR-μ* and, more broadly, the physiological and pharmacological significance of ligand-free signaling in GPCRs.

Opioid signaling and design of analgesics

Clinical treatment of acute to severe pain relies on the use of opioids. While their potency is significant, there are considerable side effects that can negatively affect patients. Their rise in usage has correlated with the current opioid epidemic in the United States, which has led to more than 70,000 deaths per year (Volkow and Blanco, 2021). Opioid-related drug development aims to make target compounds that show strong potency but with diminished side effects. Research into pharmaceuticals that could act as potential alternatives to current pains medications has relied on mechanistic insights of opioid receptors, a class of G-protein coupled receptors (GPCRs), and biased agonism, a common phenomenon among pharmaceutical compounds where downstream effects can be altered at the same receptor via different agonists. Opioids function typically by binding to an active site on the extracellular portion of opioid receptors. Once activated, the opioid receptor initiates a G-protein signaling pathway and/or the β-arrestin2 pathway. The proposed concept for the development of safe analgesics around mu and kappa opioid receptor subtypes has focused on not recruiting β-arrestin2 (biased agonism) and/or having low efficacy at the receptor (partial agonism). By altering chemical motifs on a common scaffold, chemists can take advantage of biased agonism as well as create compounds with low intrinsic efficacy for the desired treatments. This review will focus on ligands with bias profile, signaling aspects of the receptor and probe into the structural basis of receptor that leads to bias and/or partial agonism.

Kratom: History, pharmacology, current user trends, adverse health effects and potential benefits

Kratom (Mitragyna speciosa Korth.) is a tree native to Southeast Asia with dose-dependent stimulant and opioid-like effects. Dried, powdered leaf material is among the kratom products most commonly consumed in the US and Europe, but other formulations also exist including enriched extracts, resins, tinctures, and edibles. Its prevalence in the US remains debated and the use pattern includes self-treatment of mood disorders, pain, and substance use disorders. Most of the adverse effects of kratom and its alkaloid mitragynine have been reported in the literature as case reports or part of surveys necessitating confirmation by clinical trials. Toxicities associated with kratom consumption have focused on hepatic, cardiac, and CNS effects with the potential to cause fatalities primarily as part of polydrug exposures. Kratom may also present with drug-drug interactions primarily through CYP 3A4 and 2D6 inhibition, although the clinical significance remains unknown to date. The variability in composition of commercially available kratom products complicates generalization of findings and requires further investigation by employing clinical trials. Healthcare professionals should remain cautious in counseling patients on the use of kratom in a therapeutic setting.

Agonist and antagonist TRUPATH assays for G protein-coupled receptors

TRUPATH is a bioluminescence resonance energy transfer-based platform for quantifying G protein-coupled receptor activity via dissociation of heterotrimeric G protein biosensors. Here, we present protocols for agonist and antagonist TRUPATH assays in the 384-well plate format, thereby providing an opportunity for higher throughput. We also provide both data analysis and quality control analyses for these assays, along with considerations for assay optimization and solutions for troubleshooting needs that may be encountered. For complete details on the use and execution of this protocol, please refer to Olsen et al. (2020).

Attenuated G protein signaling and minimal receptor phosphorylation as a biochemical signature of low side-effect opioid analgesics

Multi-receptor targeting has been proposed as a promising strategy for the development of opioid analgesics with fewer side effects. Cebranopadol and AT-121 are prototypical bifunctional ligands targeting the nociceptin/orphanin FQ peptide receptor (NOP) and µ-opioid receptor (MOP) that elicit potent analgesia in humans and nonhuman primates, respectively. Cebranopadol was reported to produce typical MOP-related side effects such as respiratory depression and reward, whereas AT-121 appeared to be devoid of these liabilities. However, the molecular basis underlying different side effect profiles in opioid analgesics remains unknown. Here, we examine agonist-induced receptor phosphorylation and G protein signaling profiles of a series of chemically diverse mixed MOP/NOP agonists, including cebranopadol and AT-121. We found that these compounds produce strikingly different MOP phosphorylation profiles. Cebranopadol, AT-034 and AT-324 stimulated extensive MOP phosphorylation, whereas AT-201 induced selective phosphorylation at S375 only. AT-121, on the other hand, did not promote any detectable MOP phosphorylation. Conversely, none of these compounds was able to elicit strong NOP phosphorylation and low NOP receptor phosphorylation correlated with partial agonism in a GIRK-channel assay. Our results suggest a close correlation between MOP receptor phosphorylation and side effect profile. Thus, bifunctional MOP/NOP opioid ligands combining low efficacy G protein signaling at both NOP and MOP with no detectable receptor phosphorylation appear to be devoid of side-effects such as respiratory depression, abuse liability or tolerance development, as with AT-121.

Role of Efficacy as a Determinant of Locomotor Activation by Mu Opioid Receptor Ligands in Female and Male Mice

Mu opioid receptor (MOR) agonists produce locomotor hyperactivity in mice as one sign of opioid-induced motor disruption. The goal of this study was to evaluate the degree of MOR efficacy required to produce this hyperactivity. Full dose-effect curves were determined for locomotor activation produced in male and female ICR mice by (1) eight different single-molecule opioids with high to low MOR efficacy, and (2) a series of fixed-proportion fentanyl/naltrexone mixtures with high to low fentanyl proportions. Data from the mixtures were used to quantify the efficacy requirement for MOR agonist-induced hyperactivity relative to efficacy requirements determined previously for other MOR agonist effects. Specifically, efficacy requirement was quantified as the EP₅₀ value, which is the "Effective Proportion" of fentanyl in a fentanyl/naltrexone mixture that produces a maximal effect equal to 50% of the maximal effect of fentanyl alone. Maximal hyperactivity produced by each drug and mixture in the present study correlated with previously published data for maximal stimulation of GTPɣS binding in MOR-expressing Chinese hamster ovary cells as an in vitro measure of relative efficacy. Additionally, the EP₅₀ value for hyperactivity induced by fentanyl/naltrexone mixtures indicated that opioid-induced hyperactivity in mice has a relatively high efficacy requirement in comparison to some other MOR agonist effects, and in particular is higher than the efficacy requirement for thermal antinociception in mice or fentanyl discrimination in rats. Taken together, these data show that MOR agonist-induced hyperactivity in mice is efficacy dependent and requires relatively high levels of MOR agonist efficacy for its full expression. Significance Statement Mu opioid receptor (MOR) agonist-induced hyperlocomotion in mice is dependent on the MOR efficacy of the agonist and requires a relatively high degree of efficacy for its full expression.

The respiratory depressant effects of mitragynine are limited by its conversion to 7-OH mitragynine

Background and Purpose

Mitragynine, the major alkaloid in Mitragyna speciosa (kratom), is a partial agonist at the μ opioid receptor. CYP3A‐dependent oxidation of mitragynine yields the metabolite 7‐OH mitragynine, a more efficacious μ receptor agonist. While both mitragynine and 7‐OH mitragynine can induce anti‐nociception in mice, recent evidence suggests that 7‐OH mitragynine formed as a metabolite is sufficient to explain the anti‐nociceptive effects of mitragynine. However, the ability of 7‐OH mitragynine to induce μ receptor‐dependent respiratory depression has not yet been studied.

Experimental Approach

Respiration was measured in awake, freely moving, male CD‐1 mice, using whole body plethysmography. Anti‐nociception was measured using the hot plate assay. Morphine, mitragynine, 7‐OH mitragynine and the CYP3A inhibitor ketoconazole were administered orally.

Key Results

The respiratory depressant effects of mitragynine showed a ceiling effect, whereby doses higher than 10 mg·kg⁻¹ produced the same level of effect. In contrast, 7‐OH mitragynine induced a dose‐dependent effect on mouse respiration. At equi‐depressant doses, both mitragynine and 7‐OH mitragynine induced prolonged anti‐nociception. Inhibition of CYP3A reduced mitragynine‐induced respiratory depression and anti‐nociception without affecting the effects of 7‐OH mitragynine.

Conclusions and Implications

Both the anti‐nociceptive effects and the respiratory depressant effects of mitragynine are partly due to its metabolic conversion to 7‐OH mitragynine. The limiting rate of conversion of mitragynine into its active metabolite results in a built‐in ceiling effect of the mitragynine‐induced respiratory depression. These data suggest that such ‘metabolic saturation’ at high doses may underlie the improved safety profile of mitragynine as an opioid analgesic.

The Chemical and Pharmacological Properties of Mitragynine and Its Diastereomers: An Insight Review

Mitragynine, is a naturally occurring indole alkaloid that can be isolated from the leaves of a psychoactive medicinal plant. Mitragyna speciosa, also known as kratom, is found to possess promising analgesic effects on mediating the opioid receptors such as µ (MOR), δ (DOR), and κ (KOR). This alkaloid has therapeutic potential for pain management as it has limited adverse effect compared to a classical opioid, morphine. Mitragynine is frequently regarded to behave like an opioid but possesses milder withdrawal symptoms. The use of this alkaloid as the source of an analgesic candidate has been proven through comprehensive preclinical and clinical studies. The present data have shown that mitragynine is able to bind to opioid receptors, particularly MOR, to exhibit the analgesic effect. Moreover, the chemical and pharmacological aspects of mitragynine and its diastereomers, speciogynine, speciociliatine, and mitraciliatine, are discussed. It is interesting to know how the difference in stereochemical configuration could lead to the difference in the bioactivity of the respective compounds. Hence, in this review, the updated pharmacological and toxicological properties of mitragynine and its diastereomers are discussed to render a comprehensive understanding of the pharmacological properties of mitragynine and its diastereomers based on their structure-activity relationship study.

A diencephalic circuit in rats for opioid analgesia but not positive reinforcement

Mu opioid receptor (MOR) agonists are potent analgesics, but also cause sedation, respiratory depression, and addiction risk. The epithalamic lateral habenula (LHb) signals aversive states including pain, and here we found that it is a potent site for MOR-agonist analgesia-like responses in rats. Importantly, LHb MOR activation is not reinforcing in the absence of noxious input. The LHb receives excitatory inputs from multiple sites including the ventral tegmental area, lateral hypothalamus, entopeduncular nucleus, and the lateral preoptic area of the hypothalamus (LPO). Here we report that LHb-projecting glutamatergic LPO neurons are excited by noxious stimulation and are preferentially inhibited by MOR selective agonists. Critically, optogenetic stimulation of LHb-projecting LPO neurons produces an aversive state that is relieved by LHb MOR activation, and optogenetic inhibition of LHb-projecting LPO neurons relieves the aversiveness of ongoing pain.

Opioids are potent analgesics but also have addiction risk. Here a lateral preoptic area to lateral habenula connection is identified by which opioids relieve ongoing pain but do not produce reward in animals that do not have ongoing pain.

Kratom Abuse Potential 2021: An Updated Eight Factor Analysis

Drugs are regulated in the United States (US) by the Controlled Substances Act (CSA) if assessment of their abuse potential, including public health risks, show such control is warranted. An evaluation via the 8 factors of the CSA provides the comprehensive assessment required for permanent listing of new chemical entities and previously uncontrolled substances. Such an assessment was published for two kratom alkaloids in 2018 that the Food and Drug Administration (FDA) have identified as candidates for CSA listing: mitragynine (MG) and 7-hydroxymitragynine (7-OH-MG) (Henningfield et al., 2018a). That assessment concluded the abuse potential of MG was within the range of many other uncontrolled substances, that there was not evidence of an imminent risk to public health, and that a Schedule I listing (the only option for substances that are not FDA approved for therapeutic use such as kratom) carried public health risks including drug overdoses by people using kratom to abstain from opioids. The purpose of this review is to provide an updated abuse potential assessment reviewing greater than 100 studies published since January 1, 2018. These include studies of abuse potential and physical dependence/withdrawal in animals; in-vitro receptor binding; assessments of potential efficacy treating pain and substance use disorders; pharmacokinetic/pharmacodynamic studies with safety-related findings; clinical studies of long-term users with various physiological endpoints; and surveys of patterns and reasons for use and associated effects including dependence and withdrawal. Findings from these studies suggest that public health is better served by assuring continued access to kratom products by consumers and researchers. Currently, Kratom alkaloids and derivatives are in development as safer and/or more effective medicines for treating pain, substances use disorders, and mood disorders. Placing kratom in the CSA via scheduling would criminalize consumers and possession, seriously impede research, and can be predicted to have serious adverse public health consequences, including potentially thousands of drug overdose deaths. Therefore, CSA listing is not recommended. Regulation to minimize risks of contaminated, adulterated, and inappropriately marketed products is recommended.

Respiratory effects of oral mitragynine and oxycodone in a rodent model

RATIONALE

Kratom derives from Mitragyna speciosa (Korth.), a tropical tree in the genus Mitragyna (Rubiaceae) that also includes the coffee tree. Kratom leaf powders, tea-like decoctions, and commercial extracts are taken orally, primarily for health and well-being by millions of people globally. Others take kratom to eliminate opioid use for analgesia and manage opioid withdrawal and use disorder. There is debate over the possible respiratory depressant overdose risk of the primary active alkaloid, mitragynine, a partial μ-opioid receptor agonist, that does not signal through ß-arrestin, the primary opioid respiratory depressant pathway.

OBJECTIVES

Compare the respiratory effects of oral mitragynine to oral oxycodone in rats with the study design previously published by US Food and Drug Administration (FDA) scientists for evaluating the respiratory effects of opioids (Xu et al., Toxicol Rep 7:188-197, 2020).

METHODS

Blood gases, observable signs, and mitragynine pharmacokinetics were assessed for 12 h after 20, 40, 80, 240, and 400 mg/kg oral mitragynine isolate and 6.75, 60, and 150 mg/kg oral oxycodone hydrochloride.

FINDINGS

Oxycodone administration produced significant dose-related respiratory depressant effects and pronounced sedation with one death each at 60 and 150 mg/kg. Mitragynine did not yield significant dose-related respiratory depressant or life-threatening effects. Sedative-like effects, milder than produced by oxycodone, were evident at the highest mitragynine dose. Maximum oxycodone and mitragynine plasma concentrations were dose related.

CONCLUSIONS

Consistent with mitragynine's pharmacology that includes partial µ-opioid receptor agonism with little recruitment of the respiratory depressant activating β-arrestin pathway, mitragynine produced no evidence of respiratory depression at doses many times higher than known to be taken by humans.

TRUPATH: An Open-Source Biosensor Platform for Interrogating the GPCR Transducerome

G protein-coupled receptors (GPCRs) are the most highly targeted protein family by United States Food and Drug Administration-approved drugs. Despite their historic and continued importance as drug targets, their therapeutic potential remains underexplored and underexploited. While it has been known for some time that GPCRs are able to engage multiple signaling pathways, the majority of drug research and development has followed the older dogma of a single primary pathway for each receptor. This has been due in part to historical reasons, or to a lack of appreciation of the potential to exploit specific pathways over others as a therapeutic modality. Additionally, only recently have technologies been developed to discern selective GPCR-G protein interactions. In this chapter, we introduce TRUPATH, a bioluminescence resonance energy transfer (BRET)-based platform that allows the unambiguous measurement of receptor-catalyzed dissociation or rearrangement of 14 Gα subunits from their respective Gβ and Gγ subunits. Specifically, we provide a detailed protocol for TRUPATH plasmid transfection, microplate preparation, assay implementation, and data analysis. In doing so, we create a template for using TRUPATH to answer basic biological questions, such as "To which G proteins does a given GPCR couple?", and facilitate drug discovery efforts to identify ligands with intra- and inter-G protein family pathway selectivity.

Evaluation of Kratom Opioid Derivatives as Potential Treatment Option for Alcohol Use Disorder

Background and Purpose:Mitragyna speciosa extract and kratom alkaloids decrease alcohol consumption in mice at least in part through actions at the δ-opioid receptor (δOR). However, the most potent opioidergic kratom alkaloid, 7-hydroxymitragynine, exhibits rewarding properties and hyperlocomotion presumably due to preferred affinity for the mu opioid receptor (µOR). We hypothesized that opioidergic kratom alkaloids like paynantheine and speciogynine with reduced µOR potency could provide a starting point for developing opioids with an improved therapeutic window to treat alcohol use disorder.

Experimental Approach: We characterized paynantheine, speciociliatine, and four novel kratom-derived analogs for their ability to bind and activate δOR, µOR, and κOR. Select opioids were assessed in behavioral assays in male C57BL/6N WT and δOR knockout mice.

Key Results: Paynantheine (10 mg∙kg⁻¹, i.p.) produced aversion in a limited conditioned place preference (CPP) paradigm but did not produce CPP with additional conditioning sessions. Paynantheine did not produce robust antinociception but did block morphine-induced antinociception and hyperlocomotion. Yet, at 10 and 30 mg∙kg⁻¹ doses (i.p.), paynantheine did not counteract morphine CPP. 7-hydroxypaynantheine and 7-hydroxyspeciogynine displayed potency at δOR but limited µOR potency relative to 7-hydroxymitragynine in vitro, and dose-dependently decreased voluntary alcohol consumption in WT but not δOR in KO mice. 7-hydroxyspeciogynine has a maximally tolerated dose of at least 10 mg∙kg⁻¹ (s.c.) at which it did not produce significant CPP neither alter general locomotion nor induce noticeable seizures.

Conclusion and Implications: Derivatizing kratom alkaloids with the goal of enhancing δOR potency and reducing off-target effects could provide a pathway to develop novel lead compounds to treat alcohol use disorder with an improved therapeutic window.