Synthesis and κ-opioid receptor activity of furan-substituted salvinorin A analogues

Limitations and potential of κOR biased agonists for pain and itch management

The concept of ligand bias is based on the premise that different agonists can elicit distinct responses by selectively activating the same receptor. These responses often determine whether an agonist has therapeutic or undesirable effects. Therefore, it would be highly advantageous to have agonists that specifically trigger the therapeutic response. The last two decades have seen a growing trend towards the consideration of ligand bias in the development of ligands to target the κ-opioid receptor (κOR). Most of these ligands selectively favor G-protein signaling over β-arrestin signaling to potentially provide effective pain and itch relief without adverse side effects associated with κOR activation. Importantly, the specific role of β-arrestin 2 in mediating κOR agonist-induced side effects remains unknown, and similarly the therapeutic and side-effect profiles of G-protein-biased κOR agonists have not been established. Furthermore, some drugs previously labeled as G-protein-biased may not exhibit true bias but may instead be either low-intrinsic-efficacy or partial agonists. In this review, we discuss the established methods to test ligand bias, their limitations in measuring bias factors for κOR agonists, as well as recommend the consideration of other systematic factors to correlate the degree of bias signaling and pharmacological effects. This article is part of the Special Issue on "Ligand Bias".

Targeting α1- and α2-adrenergic receptors as a countermeasure for fentanyl-induced locomotor and ventilatory depression

This study assessed the ability of α1 and α2-adrenergic drugs to decrease fentanyl-induced locomotor and ventilatory depression. Rats were given saline or fentanyl, followed by: (1) naltrexone, (2) naloxone, (3) nalmefene, (4) α1 agonist phenylephrine, (5) α1 antagonist prazosin, (6) α1D antagonist BMY-7378, (7) α2 agonist clonidine, (8) α2 antagonist yohimbine or (9) vehicle. All µ-opioid antagonists dose-dependently reversed fentanyl-induced locomotor and ventilatory depression. While the α1 drugs did not alter the effects of fentanyl, clonidine dose-dependently decreased locomotion and respiration with and without fentanyl. Conversely, yohimbine given at a low dose (0.3-1 mg/kg) stimulated ventilation when given alone and higher doses (>1 mg/kg) partially reversed (∼50 %) fentanyl-induced ventilatory depression, but not locomotor depression. High doses of yohimbine in combination with a suboptimal dose of naltrexone reversed fentanyl-induced ventilatory depression, suggestive of additivity. Yohimbine may serve as an effective adjunctive countermeasure agent combined with naltrexone to rescue fentanyl-induced ventilatory depression.

Searching for Synthetic Opioid Rescue Agents: Identification of a Potent Opioid Agonist with Reduced Respiratory Depression

While in the process of designing more effective synthetic opioid rescue agents, we serendipitously identified a new chemotype of potent synthetic opioid. Here, we report that conformational constraint of a piperazine ring converts a mu opioid receptor (MOR) antagonist into a potent MOR agonist. The prototype of the series, which we have termed atoxifent (2), possesses potent in vitro agonist activity. In mice, atoxifent displayed long-lasting antinociception that was reversible with naltrexone. Repeated dosing of atoxifent produced antinociceptive tolerance and a level of withdrawal like that of fentanyl. In rats, while atoxifent produced complete loss of locomotor activity like fentanyl, it failed to produce deep respiratory depression associated with fentanyl-induced lethality. Assessment of brain biodistribution demonstrated ample distribution of atoxifent into the brain with a Tmax of approximately 0.25 h. These results indicate enhanced safety for atoxifent-like molecules compared to fentanyl.

Towards tetrasubstituted furans through rearrangement and cyclodimerization of acetylenic ketones

Cyclodimerization of readily accessible acetylenic ketones facilitated by a phosphane-borane complex under basic conditions is achieved. This methodology allows one-pot synthesis of phosphorus-involved tetrasubstituted furans via the construction of a C-P bond and a furan ring within a single procedure. A plausible reaction mechanism is proposed.

Gold(III)-Catalyzed Propargylic Substitution Reaction Followed by Cycloisomerization for Synthesis of Poly-Substituted Furans from N-Tosylpropargyl Amines with 1,3-Dicarbonyl Compounds

The treatment of N-tosylpropargyl amines 1 with 1,3-dicarbonyl compounds 2 in the presence of AuBr3 (5 mol%) and AgOTf (15 mol%) afforded poly-substituted furans 3 in good-to-high yields via the gold-catalyzed cleavage of the sp3 carbon-nitrogen bond.

Cu(ii)-catalyzed aerobic oxidative coupling of furans with indoles enables expeditious synthesis of indolyl-furans with blue fluorescence

With the purpose of incorporating sustainability in chemical processes, there has been a renewed focus on utilizing earth-abundant metal catalysts to expand the repertoire of organic reactions and processes. In this work, we have explored the atom-economic oxidative coupling between two important electron-rich heterocycles - indoles and furans - using commonly available, inexpensive metal catalyst CuCl2·2H2O (<0.25$ per g) to develop an expeditious synthesis of indolyl-furans. Moreover, the reaction proceeded well in the presence of the so-called 'ultimate oxidant' - air, without the need for any external ligand or additive. The reaction was found to be scalable and to work even under partially aqueous conditions. This makes the methodology highly economical, practical, operationally simple and sustainable. In addition, the methodology provides direct access to novel indole-furan-thiophene (IFT)-based electron-rich π-conjugated systems, which show green-yellow fluorescence with large Stokes shift and high quantum yields. Mechanistic investigations reveal that the reaction proceeds through chemoselective oxidation of indole by the metal catalyst followed by the nucleophilic attack by furan.

Potent MOR Agonists from 2'-Hydroxy-5,9-dimethyl-N-phenethyl Substituted-6,7-benzomorphans and from C8-Hydroxy, Methylene and Methyl Derivatives of N-Phenethylnormetazocine

(-)-5,9-Dimethyl-6,7-benzomorphan (normetazocine) derivatives with a para-OH or ortho-F substituent in the aromatic ring of the N-phenethyl moiety were synthesized and found to have subnanomolar potency at MOR, and both were fully efficacious in vitro. These new compounds, (1R,5R,9R)-6,11-dimethyl-3-(2-fluorophenethyl)-1,2,3,4,5,6-hexahydro-2,6-methanobenzo[d]azocin-8-ol and (1R,5R,9R)-6,11-dimethyl-3-(4-hydroxyphenethyl)-1,2,3,4,5,6-hexahydro-2,6-methanobenzo[d]azocin-8-ol, were more potent than the unsubstituted compound N-phenethylnormetazocine and about 30 or 40 times more potent than morphine, respectively. A variety of substituents in the ortho, meta, or para position in the aromatic ring of the N-phenethyl moiety were synthesized, 25 of these compounds, and found to have varying effects on potency and efficacy as determined by the forskolin-induced cAMP accumulation assay. The N-phenethyl moiety was also modified by increasing chain length to form a N-phenylpropyl side chain with and without a para-nitro moiety, and by an N-cinnamyl side chain. Also, an indole ethylamine normetazocine was synthesized to replace the N-phenethylamine side chain in normetazocine. The phenylpropylamine, propenylamine (cinnamyl) and the para-nitropropylamine had little or no MOR potency. The indole-ethylamine on the normetazocine nucleus, however, had moderate potency (MOR EC50 = 12 nM), and was fully efficacious (%Emax = 102%) in the cAMP assay. Retention of the N-phenethyl moiety and the addition of alkyl and alkenyl moieties on C8 in (-)-N-phenethylnormetazocine gave a C8-methylene derivative that had subnanomolar potency at MOR and a C8-methyl analog that had nanomolar potency. Five C8-substituted compounds were synthesized.

Antinociceptive effect of cyclic and linear diterpenoids as new atypical agonists of κ-opioid receptors obtained from four species of the Baccharis genus, and vehiculated in nanometric niosomes

New natural analgesic compounds that act in KORs are important alternatives for potential therapeutical use in medicine. In this work, we report and compare here the antinociceptive activity displayed by cyclic and linear diterpenes, obtained from the genus Baccharis. The antinociceptive activities determined were relatively strong, in comparison whit morphine. The antinociceptive mechanism of action was made through naloxone administration (a non-selective antagonist of opioid receptors). The more active compounds were vehiculized successfully in niosomes at nanometric scale. The observed antinociceptive activity for Bartemidiolide oxide (BARTO), obtain from Baccharis artemisioides, was greater than Flabeloic acid dimer (DACD), the first compound isolated from Baccharis flabellata that was reported possessing antinociceptive effects. We also conducted docking calculations and molecular dynamics simulations, which suggested that the newly identified diterpenes might share the molecular action mechanism reported for Salvinorin A (SalA). Molecular simulations have allowed us to appreciate some subtle differences between molecular interactions of these ligands stabilizing their respective complexes; such information might be useful for designing and searching for new inhibitors of KORs.

The Kappa Opioid Receptor Agonist 16-Bromo Salvinorin A Has Anti-Cocaine Effects without Significant Effects on Locomotion, Food Reward, Learning and Memory, or Anxiety and Depressive-like Behaviors

Kappa opioid receptor (KOR) agonists have preclinical antipsychostimulant effects; however, adverse side effects have limited their therapeutic development. In this preclinical study, conducted in Sprague Dawley rats, B6-SJL mice, and non-human primates (NHPs), we evaluated the G-protein-biased analogue of salvinorin A (SalA), 16-bromo salvinorin A (16-BrSalA), for its anticocaine effects, side effects, and activation of cellular signaling pathways. 16-BrSalA dose-dependently decreased the cocaine-primed reinstatement of drug-seeking behavior in a KOR-dependent manner. It also decreased cocaine-induced hyperactivity, but had no effect on responding for cocaine on a progressive ratio schedule. Compared to SalA, 16-BrSalA had an improved side effect profile, with no significant effects in the elevated plus maze, light-dark test, forced swim test, sucrose self-administration, or novel object recognition; however, it did exhibit conditioned aversive effects. 16-BrSalA increased dopamine transporter (DAT) activity in HEK-293 cells coexpressing DAT and KOR, as well as in rat nucleus accumbens and dorsal striatal tissue. 16-BrSalA also increased the early phase activation of extracellular-signal-regulated kinases 1 and 2, as well as p38 in a KOR-dependent manner. In NHPs, 16-BrSalA caused dose-dependent increases in the neuroendocrine biomarker prolactin, similar to other KOR agonists, at doses without robust sedative effects. These findings highlight that G-protein-biased structural analogues of SalA can have improved pharmacokinetic profiles and fewer side effects while maintaining their anticocaine effects.

Discovery of a Potent Highly Biased MOR Partial Agonist among Diastereomeric C9-Hydroxyalkyl-5-phenylmorphans

All possible diastereomeric C9-hydroxymethyl-, hydroxyethyl-, and hydroxypropyl-substituted 5-phenylmorphans were synthesized to explore the three-dimensional space around the C9 substituent in our search for potent MOR partial agonists. These compounds were designed to lessen the lipophilicity observed with their C9-alkenyl substituted relatives. Many of the 12 diastereomers that were obtained were found to have nanomolar or subnanomolar potency in the forskolin-induced cAMP accumulation assay. Almost all these potent compounds were fully efficacious, and three of those chosen for in vivo evaluation, 15, 21, and 36, were all extremely G-protein biased; none of the three compounds recruited beta-arrestin2. Only one of the 12 diastereomers, 21 (3-((1S,5R,9R)-9-(2-hydroxyethyl)-2-phenethyl-2-azabicyclo[3.3.1]nonan-5-yl)phenol), was a MOR partial agonist with good, but not full, efficacy (E_max = 85%) and subnanomolar potency (EC₅₀ = 0.91 nM) in the cAMP assay. It did not have any KOR agonist activity. This compound was unlike morphine in that it had a limited ventilatory effect in vivo. The activity of 21 could be related to one or more of three well-known theories that attempt to predict a dissociation of the desired analgesia from the undesirable opioid-like side-effects associated with clinically used opioids. In accordance with the theories, 21 was a potent MOR partial agonist, it was highly G-protein biased and did not attract beta-arrestin2, and it was found to have both MOR and DOR agonist activity. All the other diastereomers that were synthesized were either much less potent than 21 or had either too little or too much efficacy for our purposes. It was also noted that a C9-methoxymethyl compound with 1R,5S,9R stereochemistry (41) was more potent than the comparable C9-hydroxymethyl compound 11 (EC₅₀ = 0.65 nM for 41 vs. 2.05 nM for 11). Both 41 and 11 were fully efficacious.

Rhodium(II)-Catalyzed Desaturative [3+2] Tandem Cyclization of Arylcycloalkanes with β-Dicarbonyls

Synthetically important scaffolds, fused tricyclic frameworks containing a 2,3-cyclo[b]dihydrofuran unit, play a crucial role in drug discovery. In this study, we demonstrate that rhodium(II)/N-fluorobenzenesulfonimide can catalyze the in situ generation of highly reactive alkene intermediates from commonly accessible alkanes, which undergo intermolecular [3+2] tandem cyclization with the simultaneously generated β-dicarbonyl radical to synthesize a series of fused tricyclic frameworks containing a 2,3-cyclo[b]dihydrofuran unit with a quaternary carbon center.

Solving an Old Puzzle: Elucidation and Evaluation of the Binding Mode of Salvinorin A at the Kappa Opioid Receptor

The natural product Salvinorin A (SalA) was the first nitrogen-lacking agonist discovered for the opioid receptors and exhibits high selectivity for the kappa opioid receptor (KOR) turning SalA into a promising analgesic to overcome the current opioid crisis. Since SalA's suffers from poor pharmacokinetic properties, particularly the absence of gastrointestinal bioavailability, fast metabolic inactivation, and subsequent short duration of action, the rational design of new tailored analogs with improved clinical usability is highly desired. Despite being known for decades, the binding mode of SalA within the KOR remains elusive as several conflicting binding modes of SalA were proposed hindering the rational design of new analgesics. In this study, we rationally determined the binding mode of SalA to the active state KOR by in silico experiments (docking, molecular dynamics simulations, dynophores) in the context of all available mutagenesis studies and structure-activity relationship (SAR) data. To the best of our knowledge, this is the first comprehensive evaluation of SalA's binding mode since the determination of the active state KOR crystal structure. SalA binds above the morphinan binding site with its furan pointing toward the intracellular core while the C2-acetoxy group is oriented toward the extracellular loop 2 (ECL2). SalA is solely stabilized within the binding pocket by hydrogen bonds (C210ECL2, Y3127.35, Y3137.36) and hydrophobic contacts (V1182.63, I1393.33, I2946.55, I3167.39). With the disruption of this interaction pattern or the establishment of additional interactions within the binding site, we were able to rationalize the experimental data for selected analogs. We surmise the C2-substituent interactions as important for SalA and its analogs to be experimentally active, albeit with moderate frequency within MD simulations of SalA. We further identified the non-conserved residues 2.63, 7.35, and 7.36 responsible for the KOR subtype selectivity of SalA. We are confident that the elucidation of the SalA binding mode will promote the understanding of KOR activation and facilitate the development of novel analgesics that are urgently needed.

Synthesis and Pharmacological Evaluation of Enantiopure N-Substituted Ortho-c Oxide-Bridged 5-Phenylmorphans

The design of enantiopure stereoisomers of N-2-phenylcyclopropylmethyl-substituted ortho-c oxide-bridged phenylmorphans, the E and Z isomers of an N-cinnamyl moiety, and N-propyl enantiomers were based on combining the most potent oxide-bridged phenylmorphan (the ortho-c isomer) with the most potent N-substituent that we previously found with a 5-(3-hydroxy)phenylmorphan (i.e., N-2-phenylcyclopropyl methyl moieties, N-cinnamyl, and N-propyl substituents). The synthesis of the eight enantiopure N-2-phenylcyclopropylmethyl ortho-c oxide-bridged phenylmorphans and six additional enantiomers of the N-substituted ortho-c oxide-bridged phenylmorphans (N-E and Z-cinnamyl compounds, and N-propyl compounds) was accomplished. The synthesis started from common intermediates (3R,6aS,11aS)-10-methoxy-1,3,4,5,6,11a-hexahydro-2H-3,6a-methano-benzofuro[2,3-c]azocine (+)-6 and its enantiomer, (3S, 6aR, 11aR)-(-)-6, respectively. The enantiomers of ±-6 were obtained through salt formation with (S)-(+)- and (R)-(-)-p-methylmandelic acid, and the absolute configuration of the (R)-(-)-p-methylmandelate salt of (3S, 6aR, 11aR)-(-)-6 was determined by single-crystal X-ray analysis. The enantiomeric secondary amines were reacted with N-(2-phenylcyclopropyl)methyl derivatives, 2-(E)-cinnamyl bromide, and (Z)-3-phenylacrylic acid. These products led to all of the desired N-derivatives of the ortho-c oxide-bridged phenylmorphans. Their opioid receptor binding affinity was measured. The compounds with MOR affinity < 50 nM were examined for their functional activity in the forskolin-induced cAMP accumulation assay. Only the enantiomer of the N-phenethyl ortho-c oxide-bridged phenylmorphan ((-)-1), and only the (3S,6aR,11aR)-2-(((1S,2S)-2-phenylcyclopropyl)methyl)-1,3,4,5,6,11a-hexahydro-2H-3,6a-methanobenzofuro[2,3-c]azocin-10-ol isomer ((+)-17), and the N-phenylpropyl derivative ((-)-25) had opioid binding affinity < 50 nM. Both (-)-1 and (-)-25 were partial agonists in the cAMP assay, with the former showing high potency and low efficacy, and the latter with lower potency and less efficacy. Most interesting was the N-2-phenylcyclopropylmethyl (3S,6aR,11aR)-2-(1S,2S)-enantiomer ((+)-17). That compound had good MOR binding affinity (Ki = 11.9 nM) and was found to have naltrexone-like potency as a MOR antagonist (IC50 = 6.92 nM).

A systematic review on the kappa opioid receptor and its ligands: New directions for the treatment of pain, anxiety, depression, and drug abuse

Kappa opioid receptor (KOR) is a member of the opioid receptor system, the G protein-coupled receptors that are expressed throughout the peripheral and central nervous systems and play crucial roles in the modulation of antinociception and a variety of behavioral states like anxiety, depression, and drug abuse. KOR agonists are known to produce potent analgesic effects and have been used clinically for the treatment of pain, while KOR antagonists have shown efficacy in the treatment of anxiety and depression. This review summarizes the history, design strategy, discovery, and development of KOR ligands. KOR agonists are classified as non-biased, G protein-biased, and β-arrestin recruitment-biased, according to their degrees of bias. The mechanisms and associated effects of the G protein signaling pathway and β-arrestin recruitment signaling pathway are also discussed. Meanwhile, KOR antagonists are classified as long-acting and short-acting, based on their half-lives. In addition, we have special sections for mixed KOR agonists and selective peripheral KOR agonists. The mechanisms of action and pharmacokinetic, pharmacodynamic, and behavioral studies for each of these categories are also discussed in this review.

Synthesis of Furans via Rhodium(III)-Catalyzed Cyclization of Acrylic Acids with α-Diazocarbonyl Compounds

An efficient protocol for the synthesis of furans through Rh(III)-catalyzed vinyl C-H activation from acrylic acids and α-diazocarbonyl compounds has been developed. The reaction features broad functional group tolerance and affords a series of furans in moderate to good yields. Moreover, no additives such as copper or silver salts are required. Some control experiments are performed to give insight into the mechanism of this cascade transformation and the decarbonylation process is involved in the formation of the furan product.

An Approach toward 17-Arylsubstituted Marginatafuran-Type Isospongian Diterpenoids via a Palladium-Catalyzed Heck-Suzuki Cascade Reaction of 16-Bromolambertianic Acid

Isospongian diterpenes are a small but growing family of natural tetracyclic secondary metabolites isolated from marine organisms, primarily sponges and nudibranchs. A palladium-catalyzed domino Heck–Suzuki reaction sequence for the synthesis of the tetracyclic skeleton of marginatafuran-type isospongian diterpenoids with a wide variety of substituents in the C-17 position is reported. The proposed approach was based on selective transformations of the accessible plant diterpenoid lambertianic acid and includes an intramolecular Heck reaction of 16-bromolambertianic and arylation of the palladium intermediate with arylboronic acid. The influence of the nature of the substituent both in arylboronic acids and in the furan ring of 16-bromolambertianic acid on the direction and chemoselectivity of the reaction has been studied. The described derivatization of natural furanolabdanoid lambertianic acid produced new functionalized molecules for biological study and gave novel insights into the reactivity of complex molecular structures.

Iron(II)-Catalyzed Bisphosphorylation Cascade Cycloisomerization of γ-Hydroxyl Ynones and Diphenylphosphine Oxides: Synthesis of Highly Substituted Bisphosphorylated Dihydrofuran Derivatives

An iron(II)-catalyzed bisphosphorylation cascade cycloisomerization of readily accessible γ-hydroxyl ynones and diphenylphosphine oxides is described. This strategy provides a variety of valuable polysubstituted bisphosphorylated dihydrofuran scaffolds via the construction of two C-P bonds and one C-O bond within a single procedure. This developed reaction system demonstrates good functional group compatibilities with considerably low catalyst consumption (as low as 1%), which could be further scaled up to gram quantities in satisfactory yields.

An updated assessment of the translational promise of G-protein-biased kappa opioid receptor agonists to treat pain and other indications without debilitating adverse effects

Kappa opioid receptor (κOR) agonists lack the abuse liability and respiratory depression effects of clinically used mu opioid receptor (μOR) analgesics and are hypothesized to be safer alternatives. However, κOR agonists have limiting adverse effects of their own, including aversion, sedation, and mood effects, that have hampered their clinical translation. Studies performed over the last 15 years have suggested that these adverse effects could result from activation of distinct intracellular signaling pathways that are dependent on β-arrestin, whereas signaling downstream of G protein activation produces antinociception. This led to the hypothesis that agonists biased away from β-arrestin signaling would have improved therapeutic windows over traditional unbiased agonists and allow for clinical development of analgesic G-protein-biased κOR agonists. Given a recent controversy regarding the benefits of G-protein-biased μOR agonists, it is timely to reassess the therapeutic promise of G-protein-biased κOR agonists. Here we review recent discoveries from preclinical κOR studies and critically evaluate the therapeutic windows of G-protein-biased κOR agonists in each of the adverse effects above. Overall, we find that G-protein-biased κOR agonists generally have improved therapeutic window relative to unbiased agonists, although frequently study design limits strong conclusions in this regard. However, a steady flow of newly developed biased κOR agonists paired with recently engineered behavioral and molecular tools puts the κOR field in a prime position to make major advances in our understanding of κOR function and fulfill the promise of translating a new generation of biased κOR agonists to the clinic.

Sex Differences in Kappa Opioid Receptor Agonist Mediated Attenuation of Chemotherapy-Induced Neuropathic Pain in Mice

Chemotherapy-induced neuropathic pain is a common side effect for cancer patients which has limited effective treatment options. Kappa opioid receptor (KOR) agonists are a promising alternative to currently available opioid drugs due to their low abuse potential. In the current study, we have investigated the effects of Salvinorin A (SalA) analogues, 16-Ethynyl SalA, 16-Bromo SalA and ethyoxymethyl ether (EOM) SalB, and in a preclinical model of paclitaxel-induced neuropathic pain in male and female C57BL/6J mice. Using an acute dose-response procedure, we showed that compared to morphine, 16-Ethynyl SalA was more potent at reducing mechanical allodynia; and SalA, 16-Ethynyl SalA, and EOM SalB were more potent at reducing cold allodynia. In the mechanical allodynia testing, U50,488 was more potent in males and SalA was more potent in females. There were no sex differences in the acute cold allodynia testing. In the chronic administration model, treatment with U50,488 (10 mg/kg) reduced the mechanical and cold allodynia responses to healthy levels over 23 days of treatment. Overall, we have shown that KOR agonists are effective in a model of chemotherapy-induced neuropathic pain, indicating that KOR agonists could be further developed to treat this debilitating condition.

Lewis-Acid-Catalyzed Regioselective Construction of Diversely Functionalized Polycyclic Fused Furans

A novel, facile, and efficient Lewis-acid-catalyzed [4 + 1] annulation protocol for the construction of functionalized polycyclic-fused furans is developed. This methodology is free of transition metals and ligands and provides a rapid synthetic route to divergently orientated polycyclic furans in good yields. In addition, this protocol can also be used to synthesize multisubstituted furans.

Ready Access to Densely Substituted Furans Using Tsuji-Wacker-Type Cyclization

A competent method for the construction of highly substituted furans catalyzed by Pd(II) and Cu(II) chloride has been developed. The method provides easy access to di-, tri-, and tetrasubstituted furans from corresponding diols with relatively mild conditions in a unified strategy. The developed method has been successfully tested with more than 25 substrates, which resulted in furans of multiple substitution patterns with up to 84% isolated yields.

The Salvinorin Analogue, Ethoxymethyl Ether Salvinorin B, Promotes Remyelination in Preclinical Models of Multiple Sclerosis

Multiple sclerosis is a neurodegenerative disease associated with demyelination and neuroinflammation in the central nervous system. There is an urgent need to develop remyelinating therapies to better treat multiple sclerosis and other demyelinating diseases. The kappa opioid receptor (KOR) has been identified as a potential target for the development of remyelinating therapies; however, prototypical KOR agonists, such as U50,488 have side effects, which limit clinical use. In the current study, we investigated a Salvinorin A analog, ethoxymethyl ether Salvinorin B (EOM SalB) in two preclinical models of demyelination in C57BL/6J mice. We showed that in cellular assays EOM SalB was G-protein biased, an effect often correlated with fewer KOR-mediated side effects. In the experimental autoimmune encephalomyelitis model, we found that EOM SalB (0.1-0.3 mg/kg) effectively decreased disease severity in a KOR-dependent manner and led to a greater number of animals in recovery compared to U50,488 treatment. Furthermore, EOM SalB treatment decreased immune cell infiltration and increased myelin levels in the central nervous system. In the cuprizone-induced demyelination model, we showed that EOM SalB (0.3 mg/kg) administration led to an increase in the number of mature oligodendrocytes, the number of myelinated axons and the myelin thickness in the corpus callosum. Overall, EOM SalB was effective in two preclinical models of multiple sclerosis and demyelination, adding further evidence to show KOR agonists are a promising target for remyelinating therapies.

High Value-Added Application of Natural Products in Crop Protection: Semisynthesis and Acaricidal Activity of Limonoid-Type Derivatives and Investigation of Their Biocompatible O/W Nanoemulsions as Agronanopesticide Candidates

The increasingly serious resistance of Tetranychus cinnabarinus Boisduval to a wide range of insecticides/acaricides poses a major challenge to their control. The citrus processing industry generates a huge quantity of various wastes that contain many limonoids. To effectively utilize these byproducts and discover more potent green acaricidal molecules as sustainable alternatives for traditional resistant pesticides, various limonoid-type derivatives (halogenated/seven-membered lactam derivatives of obacunone and halogenated/oxime esters/oxime ethers/seven-membered lactam derivatives of limonin) were synthesized based on a diversity-oriented synthetic strategy. The key steric configurations of 10 derivatives were further confirmed by X-ray crystallography. Compound 9m, which displayed greater than 9.7-fold potent acaricidal activity of limonin, was of preeminence. In addition, some interesting structure-activity relationships were observed. Moreover, a biocompatible O/W nanoemulsion delivery system was used to prepare the limonin-based agronanoacaricide, which exhibited pronounced control efficiency against T. cinnabarinus Boisduval in the greenhouse. This systematic investigation will provide valuable information and guidance for future value-added applications of novel eco-friendly natural product-based nanopesticides.

A computer-aided method for controlling chemical resistance of drugs using RRKM theory in the liquid phase

The chemical resistance of drugs against any change in their composition and studying the rate of multiwell-multichannel reactions in the liquid phase, respectively, are the important challenges of pharmacology and chemistry. In this article, we investigate two challenges together through studying drug stability against its unimolecular reactions in the liquid phase. Accordingly, multiwell-multichannel reactions based on 1,4-H shifts are designed for simplified drugs such as 3-hydroxyl-1H-pyrrol-2(5H)-one, 3-hydroxyfuran-2(5H)-one, and 3-hydroxythiophen-2(5H)-one. After that, the reverse and forward rate constants are calculated by using the Rice Ramsperger Kassel Marcus theory (RRKM) and Eckart tunneling correction over the 298-360 K temperature range. Eventually, using the obtained rate constants, we can judge drug resistance versus structural changes. To attain the goals, the potential energy surfaces of all reactions are computed by the complete basis set-quadratic Becke3 composite method, CBS-QB3, and the high-performance meta hybrid density functional method, M06-2X, along with the universal Solvation Model based on solute electron Density, SMD, due to providing more precise and efficient results for the barrier heights and thermodynamic studies. To find the main reaction pathway of the intramolecular 1,4-H shifts in the target molecules, all possible reaction pathways are considered mechanistically in the liquid phase. Also, the direct dynamics calculations that carry out by RRKM theory on the modeled pathways are used to distinguish the main reaction pathway. As the main finding of this research, the results of quantum chemical calculations accompanied by the RRKM/Eckart rate constants are used to predict the stability of drugs. This study proposes a new way to examine drug stability by the computer-aided reaction design of target drugs. Our results show that 3-hydroxyfuran-2(5H)-one based drugs are the most stable and 3-hydroxythiophen-2(5H)-one based drugs are more stable than 3-hydroxy-1H-pyrrol-2 (5H)-one based drugs in water solution.

Synthesis of Polysubstituted Furans through Electrochemical Selenocyclization of Homopropargylic Alcohols

The current method represents an electrochemically driven synthetic route to access polysubstituted selenofuran derivatives through the diselenide-promoted cyclization of homopropargyl alcohols. The tandem electro-oxidative transformation operates at ambient temperature and in the absence of an external oxidant. This mild and efficient methodology exhibits good functional group compatibility, providing a broad range of substrate scopes up to 84% isolated yield. Further conversion of the seleno-functionality afforded other valuable furan derivatives.

Design, synthesis, and preliminary evaluation of a potential synthetic opioid rescue agent

Background

One of the most prominent opioid analgesics in the United States is the high potency agonist fentanyl. It is used in the treatment of acute and chronic pain and as an anesthetic adjuvant. When used inappropriately, however, ingestion of just a few milligrams of fentanyl or other synthetic opioid can cause opioid-induced respiratory depression (OIRD), often leading to death. Currently, the treatment of choice for OIRD is the opioid receptor antagonist naloxone. Recent reports, however, suggest that higher doses or repeated dosing of naloxone (due to recurrence of respiratory depression) may be required to reverse fully fentanyl-induced respiratory depression, rendering this treatment inadequate. To combat this synthetic opioid overdose crisis, this research aims at identifying a novel opioid reversal agent with enhanced efficacy towards fentanyl and other synthetic opioids.

Methods

A series of naltrexone analogues were characterized for their ability to antagonize the effects of fentanyl in vitro utilizing a modified forskolin-induced cAMP accumulation assay. Lead analogue 29 was chosen to undergo further PK studies, followed by in vivo pharmacological analysis to determine its ability to antagonize opioid-induced antinociception in the hot plate assay.

Results

A series of potent MOR antagonists were identified, including the highly potent analogue 29 (IC₅₀ = 2.06 nM). Follow-up PK studies revealed 29 to possess near 100% bioavailability following IP administration. Brain concentrations of 29 surpassed plasma concentrations, with an apparent terminal half-life of ~ 80 min in mice. In the hot plate assay, 29 dose-dependently (0.01–0.1 mg/kg; IP) and fully antagonized the antinociception induced by oxycodone (5.6 mg/kg; IP). Furthermore, the dose of 29 that is fully effective in preventing oxycodone-induced antinociception (0.1 mg/kg) was ineffective against locomotor deficits caused by the KOR agonist U50,488.

Conclusions

Methods have been developed that have utility to identify enhanced rescue agents for the treatment of OIRD. Analogue 29, possessing potent MOR antagonist activity in vitro and in vivo, provides a promising lead in our search for an enhanced synthetic opioid rescue agent.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-021-00758-y.

Gold-Catalyzed Oxidative Cyclization Involving Nucleophilic Attack to the Keto Group of α,α'-Dioxo Gold Carbene and 1,2-Alkynyl Migration: Synthesis of Furan-3-carboxylates

A multicomponent strategy for the synthesis of functionalized furan-3-carboxylates based on gold-catalyzed oxidative cyclization of diynones with alcohols or water has been developed. Mechanistic studies revealed that a rare nucleophilic attack to the carbonyl group of the α,α'-dioxo gold carbene instead of the carbene center and 1,2-alkynyl group migration were involved in this transformation. This method offers several advantages such as mild conditions, high regio- and chemoselectivity, and wide functional group compatibility.

Copper-Catalyzed 1,1-Alkylmonofluoroalkylation of Terminal Alkynes with Diazo Compounds and 2-Fluoro-1,3-dicarbonyl Compounds: Access toward (E)-β-Monofluoroalkyl-β,γ-unsaturated Esters or Ketones

An efficient copper-catalyzed three-component 1,1-alkylmonofluoroalkylation of terminal alkynes, diazo compounds, and 2-fluoro-1,3-dicarbonyl compounds for the synthesis of (E)-β-monofluoroalkyl-β,γ-unsaturated esters or ketones has been developed. The methodology features a broad substrate scope, an inexpensive and easily available catalytic system, and excellent selectivity with good yields. The mechanism of the tandem Cu-catalyzed cross-coupling and nucleophilic addition of allenes has been investigated.

Natural Products for the Treatment of Pain: Chemistry and Pharmacology of Salvinorin A, Mitragynine, and Collybolide

Pain remains a very pervasive problem throughout medicine. Classical pain management is achieved through the use of opiates belonging to the mu opioid receptor (MOR) class, which have significant side effects that hinder their utility. Pharmacologists have been trying to develop opioids devoid of side effects since the isolation of morphine from papaver somniferum, more commonly known as opium by Sertürner in 1804. The natural products salvinorin A, mitragynine, and collybolide represent three nonmorphinan natural product-based targets, which are potent selective agonists of opioid receptors, and emerging next-generation analgesics. In this work, we review the phytochemistry and medicinal chemistry efforts on these templates and their effects on affinity, selectivity, analgesic actions, and a myriad of other opioid-receptor-related behavioral effects.

A neoclerodane orthoester and other new neoclerodane diterpenoids from Teucrium yemense chemistry and effect on secretion of insulin

Teucrium yemense, a medicinal plant commonly grown in Saudi Arabia and Yemen, is traditionally used to treat infections, kidney diseases, rheumatism, and diabetes. Extraction of the dried aerial parts of the plant with methanol, followed by further extraction with butanol and chromatography, gave twenty novel neoclerodanes. Their structures, relative configurations and some conformations were determined by MS and 1-D and 2-D NMR techniques. Most were fairly conventional but one contained an unusual stable orthoester, one had its (C-16)-(C-13)-(C-14)-(C-15) (tetrahydro)furan unit present as a succinic anhydride and one had a rearranged carbon skeleton resulting from ring-contraction to give a central octahydroindene bicyclic core, rather than the usual decalin. Mechanisms are proposed for the biosynthetic formation of the orthoester and for the ring-contraction. Four novel neoclerodanes increased the glucose-triggered release of insulin from isolated murine pancreatic islets by more than the standard drug tolbutamide, showing that they are potential leads for the development of new anti-diabetic drugs.

Ti(OiPr)4-Facilitated Formal Deoxygenative Annulation of Alkynyl 1,2-Diketones for the Synthesis of Highly Functionalized Furans

A unique deoxygenative cyclodimerization of alkynyl 1,2-diketones facilitated by Ti(OⁱPr)₄ is achieved, affording a series of highly functionalized furan products. An unusual C-C bond and C═O bond cleavage of the substrates is observed, and Ti(OⁱPr)₄ plays triplicate roles in the reaction. Furthermore, the products show uncommon fluorescent emission in the solid state, indicating the potential practical applications of this work.

Tetra-substituted furans by a gold-catalysed tandem C(sp3)–H alkynylation/oxy-alkynylation reaction

A gold-catalysed cascade C(sp3)–H alkynylation/oxy-alkynylation of acceptor-substituted carbonyl compounds with hypervalent iodine(iii) reagents for the synthesis of tetra-substituted furans, offering distinct advantages over previous methods.

Evaluation of Biased and Balanced Salvinorin A Analogs in Preclinical Models of Pain

In the search for safer, non-addictive analgesics, kappa opioid receptor (KOPr) agonists are a potential target, as unlike mu-opioid analgesics, they do not have abuse potential. Salvinorin A (SalA) is a potent and selective KOPr agonist, however, clinical utility is limited by the short duration of action and aversive side effects. Biasing KOPr signaling toward G-protein activation has been highlighted as a key cellular mechanism to reduce the side effects of KOPr agonists. The present study investigated KOPr signaling bias and the acute antinociceptive effects and side effects of two novel analogs of SalA, 16-Bromo SalA and 16-Ethynyl SalA. 16-Bromo SalA showed G-protein signaling bias, whereas 16-Ethynyl SalA displayed balanced signaling properties. In the dose-response tail-withdrawal assay, SalA, 16-Ethynyl SalA and 16-Bromo SalA were more potent than the traditional KOPr agonist U50,488, and 16-Ethynyl SalA was more efficacious. 16-Ethynyl SalA and 16-Bromo SalA both had a longer duration of action in the warm water tail-withdrawal assay, and 16-Ethynyl had greater antinociceptive effect in the hot-plate assay, compared to SalA. In the intraplantar 2% formaldehyde test, 16-Ethynyl SalA and 16-Bromo SalA significantly reduced both nociceptive and inflammatory pain-related behaviors. Moreover, 16-Ethynyl SalA and 16-Bromo SalA had no anxiogenic effects in the marble burying task, and 16-Bromo SalA did not alter behavior in the elevated zero maze. Overall, 16-Ethynyl SalA significantly attenuated acute pain-related behaviors in multiple preclinical models, while the biased KOPr agonist, 16-Bromo SalA, displayed modest antinociceptive effects, and lacked anxiogenic effects.

Synthetic Studies of Neoclerodane Diterpenes from Salvia divinorum: Design, Synthesis, and Evaluation of Analogues with Improved Potency and G-protein Activation Bias at the μ-Opioid Receptor

Previous structure-activity relationship (SAR) studies identified the first centrally acting, non-nitrogenous μ-opioid receptor (MOR) agonist, kurkinorin (1), derived from salvinorin A. In an effort to further probe the physiological effects induced upon activation of MORs with this nonmorphine scaffold, a variety of analogues were synthesized and evaluated in vitro for their ability to activate G-proteins and recruit β-arrestin-2 upon MOR activation. Through these studies, compounds that are potent agonists at MORs and either biased toward β-arrestin-2 recruitment or biased toward G-protein activation have been identified. One such compound, 25, has potent activity and selectivity at the MOR over KOR with bias for G-protein activation. Impressively, 25 is over 100× more potent than morphine and over 5× more potent than fentanyl in vitro and elicits antinociception with limited tolerance development in vivo. This is especially significant given that 25 lacks a basic nitrogen and other ionizable groups present in other opioid ligand classes.

The Intriguing Effects of Substituents in the N-Phenethyl Moiety of Norhydromorphone: A Bifunctional Opioid from a Set of "Tail Wags Dog" Experiments

(−)-N-Phenethyl analogs of optically pure N-norhydromorphone were synthesized and pharmacologically evaluated in several in vitro assays (opioid receptor binding, stimulation of [³⁵S]GTPγS binding, forskolin-induced cAMP accumulation assay, and MOR-mediated β-arrestin recruitment assays). “Body” and “tail” interactions with opioid receptors (a subset of Portoghese’s message-address theory) were used for molecular modeling and simulations, where the “address” can be considered the “body” of the hydromorphone molecule and the “message” delivered by the substituent (tail) on the aromatic ring of the N-phenethyl moiety. One compound, N-p-chloro-phenethynorhydromorphone ((7aR,12bS)-3-(4-chlorophenethyl)-9-hydroxy-2,3,4,4a,5,6-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7(7aH)-one, 2i), was found to have nanomolar binding affinity at MOR and DOR. It was a potent partial agonist at MOR and a full potent agonist at DOR with a δ/μ potency ratio of 1.2 in the ([³⁵S]GTPγS) assay. Bifunctional opioids that interact with MOR and DOR, the latter as agonists or antagonists, have been reported to have fewer side-effects than MOR agonists. The p-chlorophenethyl compound 2i was evaluated for its effect on respiration in both mice and squirrel monkeys. Compound 2i did not depress respiration (using normal air) in mice or squirrel monkeys. However, under conditions of hypercapnia (using air mixed with 5% CO₂), respiration was depressed in squirrel monkeys.

Synthesis of Tetrasubstituted Furans through One-Pot Formal [3 + 2] Cycloaddition Utilizing [1,2]-Phospha-Brook Rearrangement

An efficient method for the synthesis of tetrasubstituted furans was developed by utilizing the [1,2]-phospha-Brook rearrangement under Brønsted base catalysis. The two-step one-pot formal [3 + 2] cycloaddition involves the nucleophilic addition of a propargyl anion, which is catalytically generated through the [1,2]-phospha-Brook rearrangement, to an aldehyde and the subsequent intramolecular cyclization mediated by N-iodosuccinimide to provide 2,4,5-trisubstituted-3-iodofurans. The present method with readily available substrates provides new access to a wide range of well-organized tetrasubstituted furans.

Semi-Synthesis of C-Ring Cyclopropyl Analogues of Fraxinellone and Their Insecticidal Activity Against Mythimna separata Walker

Fraxinellone (1) is a naturally occurring degraded limonoid isolated from Meliaceae and Rutaceae plants. As a potential natural-product-based insecticidal agent, fraxinellone has been structurally modified to improve its activity. Furan ring of fraxinellone is critical in exhibiting its insecticidal activity, but with few modifications. Herein, C-ring-modified cyclopropyl analogues were semi-synthesized by Rh(II)-catalyzed cyclopropanation. The structures of the target compounds were well characterized by NMR and HRMS. The precise three-dimensional structural information of 3a was established by X-ray crystallography. Their insecticidal activity was evaluated against Mythimna separata Walker by a leaf-dipping method. Compound 3c exhibited stronger insecticidal activity than 1 and toosendanin against M. separata with teratogenic symptoms during the different periods, implying that cyclopropanation of the furan ring could strengthen the insecticidal activity of fraxinellone.

Syntheses of Z-Iodovinylfurans and 2-Acyl Furans via Controllable Cyclization of Ynenones

A divergent synthesis of Z-iodovinylfurans and 2-acyl furans promoted by NIS via controllable cyclization of ynenones is reported. The reaction proceeded by sequential 5-exo-dig electrophilic cyclization to intermediate 2-(iodomethylene)-2H-furanium cation D, providing a range of synthetically valuable and useful trisubstituted furan derivatives 2 and 3 in moderate to excellent yields. This approach is metal-free, mild, and atom-economic, with good selectivity and high stereoselectivity.

Direct synthesis of highly functionalized furans from donor-acceptor cyclopropanes via DBU-mediated ring expansion reactions

A DBU-mediated, unprecedented formal ring expansion reaction of 2-acyl-3-arylcyclopropane-1,1-dicarbonitriles for the synthesis of multisubstituted furan derivatives is reported. This transformation represents the regioselective ring-opening reaction of cyclopropane-1,1-dicarbonitriles and annulation using an intramolecular addition cascade reaction protocol for the synthesis of fully substituted furans includes use of readily available starting materials, mild reaction conditions, and it is transition-metal catalyst free, has good functional tolerance, and broad substrate scope.

Total Syntheses of Clerodane Diterpenoids—A Review

Total syntheses of clerodane diterpenoids have been reviewed from the literature since 2000.

Functional characterization of the cytochrome P450 monooxygenase CYP71AU87 indicates a role in marrubiin biosynthesis in the medicinal plant Marrubium vulgare

BACKGROUND

Horehound (Marrubium vulgare) is a medicinal plant whose signature bioactive compounds, marrubiin and related furanoid diterpenoid lactones, have potential applications for the treatment of cardiovascular diseases and type II diabetes. Lack of scalable plant cultivation and the complex metabolite profile of M. vulgare limit access to marrubiin via extraction from plant biomass. Knowledge of the marrubiin-biosynthetic enzymes can enable the development of metabolic engineering platforms for marrubiin production. We previously identified two diterpene synthases, MvCPS1 and MvELS, that act sequentially to form 9,13-epoxy-labd-14-ene. Conversion of 9,13-epoxy-labd-14-ene by cytochrome P450 monooxygenase (P450) enzymes can be hypothesized to facilitate key functional modification reactions in the formation of marrubiin and related compounds.

RESULTS

Mining a M. vulgare leaf transcriptome database identified 95 full-length P450 candidates. Cloning and functional analysis of select P450 candidates showing high transcript abundance revealed a member of the CYP71 family, CYP71AU87, that catalyzed the hydroxylation of 9,13-epoxy-labd-14-ene to yield two isomeric products, 9,13-epoxy labd-14-ene-18-ol and 9,13-epoxy labd-14-ene-19-ol, as verified by GC-MS and NMR analysis. Additional transient Nicotiana benthamiana co-expression assays of CYP71AU87 with different diterpene synthase pairs suggested that CYP71AU87 is specific to the sequential MvCPS1 and MvELS product 9,13-epoxy-labd-14-ene. Although the P450 products were not detectable in planta, high levels of CYP71AU87 gene expression in marrubiin-accumulating tissues supported a role in the formation of marrubiin and related diterpenoids in M. vulgare.

CONCLUSIONS

In a sequential reaction with the diterpene synthase pair MvCPS1 and MvELS, CYP71AU87 forms the isomeric products 9,13-epoxy labd-14-ene-18/19-ol as probable intermediates in marrubiin biosynthesis. Although its metabolic relevance in planta will necessitate further genetic studies, identification of the CYP71AU87 catalytic activity expands our knowledge of the functional landscape of plant P450 enzymes involved in specialized diterpenoid metabolism and can provide a resource for the formulation of marrubiin and related bioactive natural products.