Synthetic studies of neoclerodane diterpenes from Salvia divinorum: role of the furan in affinity for opioid receptors

From Plant to Chemistry: Sources of Active Opioid Antinociceptive Principles for Medicinal Chemistry and Drug Design

Pain continues to be an enormous global health challenge, with millions of new untreated or inadequately treated patients reported annually. With respect to current clinical applications, opioids remain the mainstay for the treatment of pain, although they are often associated with serious side effects. To optimize their tolerability profiles, medicinal chemistry continues to study novel ligands and innovative approaches. Among them, natural products are known to be a rich source of lead compounds for drug discovery, and they hold potential for pain management. Traditional medicine has had a long history in clinical practice due to the fact that nature provides a rich source of active principles. For instance, opium had been used for pain management until the 19th century when its individual components, such as morphine, were purified and identified. In this review article, we conducted a literature survey aimed at identifying natural products interacting either directly with opioid receptors or indirectly through other mechanisms controlling opioid receptor signaling, whose structures could be interesting from a drug design perspective.

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 (C210^ECL2, Y312^7.35, Y313^7.36) and hydrophobic contacts (V118^2.63, I139^3.33, I294^6.55, I316^7.39). 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.

Erxian Decoction Attenuates TNF-α Induced Osteoblast Apoptosis by Modulating the Akt/Nrf2/HO-1 Signaling Pathway

Erxian decoction (EXD), a traditional Chinese medicine formula, has been used for treatment of osteoporosis for many years. The purpose of this study was to investigate the pharmacological effect of EXD in preventing osteoblast apoptosis and the underlying mechanism of prevention. Putative targets of EXD were predicted by network pharmacology, and functional and pathway enrichment analyses were also performed. Evaluations of bone mineral density, serum estradiol level, trabecular area fraction, serum calcium levels, and tumor necrosis factor (TNF)-α levels in ovariectomized rats, as well as cell proliferation assays, apoptosis assays, and western blotting in MC3T3-E1 osteoblasts were performed for further experimental validation. Ninety-three active ingredients in the EXD formula and 259 potential targets were identified. Functional and pathway enrichment analyses indicated that EXD significantly influenced the PI3K-Akt signaling pathway. In vivo experiments indicated that EXD treatment attenuated bone loss and decreased TNF-α levels in rats with osteoporosis. In vitro experiments showed that EXD treatment increased cell viability markedly and decreased levels of caspase-3 and the rate of apoptosis. It also promoted phosphorylation of Akt, nuclear translocation of transcription factor NF-erythroid 2-related factor (Nrf2), and hemeoxygenase-1 (HO-1) expression in TNF-α-induced MC3T3-E1 cells. Our results suggest that EXD exerted profound anti-osteoporosis effects, at least partially by reducing production of TNF-α and attenuating osteoblast apoptosis via Akt/Nrf2/HO-1 signaling pathway.

A review of salvinorin analogs and their kappa-opioid receptor activity

The plant metabolite salvinorin A potently and selectively agonizes the human kappa-opioid receptor, an emerging target for next-generation analgesics. Here we review analogs of the salvinorin chemotype and their effects on selectivity, affinity and potency. Extensive peripheral modifications using isolated salvinorin A have delivered a trove of SAR information. More deep-seated changes are now possible by advances in chemical synthesis.

Total Synthesis of (-)-Salvinorin A

Salvinorin A (1) is natural hallucinogen that binds the human κ-opioid receptor. A total synthesis has been developed that parlays the stereochemistry of l-(+)-tartaric acid into that of (-)-1 via an unprecedented allylic dithiane intramolecular Diels-Alder reaction to obtain the trans-decalin scaffold. Tsuji allylation set the C9 quaternary center and a late-stage stereoselective chiral ligand-assisted addition of a 3-titanium furan upon a C12 aldehyde/C17 methyl ester established the furanyl lactone moiety. The tartrate diol was finally converted into the C1,C2 keto-acetate.

The Association of Salvia divinorum and Psychotic Disorders: A Review of the Literature and Case Series

The association of substance abuse and psychotic disorders is of interest to clinicians, academics, and lawmakers. Commonly abused substances, such as cannabis, cocaine, amphetamines, and alcohol, have all been associated with substance-induced psychosis. Hallucinogens can induce desired psychedelic effects and undesirable psychomimetic reactions. These are usually transient and resolve once the duration of action is over. Sometimes, these effects persist, causing distress and requiring intervention. This article focuses on the hallucinogenic substance Salvia divinorum, the use of which has been observed, particularly among youth worldwide. We present background information based on a review of the literature and on our own clinical encounters, as highlighted by two original case reports. We hypothesize that consumption of Salvia divinorum could be associated with the development of psychotic disorders. We propose that clinicians routinely inquire about the use of Salvia in patients with substance use disorders or psychotic illnesses. More research is required to assess any relationship between Salvia divinorum and psychosis. Additionally, we advocate increased public and medical awareness of this substance and other emerging drugs of abuse.

Semisynthetic neoclerodanes as kappa opioid receptor probes

Modification of the furan ring of salvinorin A (1), the main active component of Salvia divinorum, has resulted in novel neoclerodane diterpenes with opioid receptor affinity and activity. Conversion of the furan ring to an aldehyde at the C-12 position (5) has allowed for the synthesis of analogues with new carbon-carbon bonds at that position. Previous methods for forming these bonds, such as Grignard and Stille conditions, have met with limited success. We report a palladium catalyzed Liebeskind-Srogl cross-coupling reaction of a thioester and a boronic acid that occurs at neutral pH and ambient temperature to produce ketone analogs at C-12. To the best of our knowledge, this is the first reported usage of the Liebeskind-Srogl reaction to diversify a natural product scaffold. We also describe a one-step protocol for the conversion of 1 to 12-epi-1 (3) through microwave irradiation. Previously, this synthetically challenging process has required multiple steps. Additionally, we report in this study that alkene 9 and aromatic analogues 12, 19, 23, 25, and 26 were discovered to retain affinity and selectivity at kappa opioid receptors (KOP). Finally, we report that the furan-2-yl analog of 1 (31) has similar affinity to 1. Collectively, these findings suggest that different aromatic groups appended directly to the decalin core may be well tolerated by KOP receptors, and may generate further ligands with affinity and activity at KOP receptors.

Potential Drug Abuse Therapeutics Derived from the Hallucinogenic Natural Product Salvinorin A

Previous structure-activity relationship studies of salvinorin A have shown that modification of the acetate functionality off the C-2 position to a methoxy methyl or methoxy ethyl ether moiety leads to increased potency at KOP receptors. However, the reason for this increase remains unclear. Here we report our efforts towards the synthesis and evaluation of C-2 constrained analogs of salvinorin A. These analogs were evaluated at opioid receptors in radioligand binding experiments as well as in the GTP-γ-S functional assay. One compound, 5, was found to have affinity and potency at κ opioid (KOP) receptors comparable to salvinorin A. In further studies, 5 was found to attenuate cocaine-induced drug seeking behavior in rats comparably to salvinorin A. This finding represents the first example of a salvinorin A analog that has demonstrated anti-addictive capabilities.

Synthesis of neoclerodane diterpenes and their pharmacological effects

Salvinorin A is a neoclerodane diterpene that has been shown to be an agonist at kappa opioid receptors. Its unique structure makes it an attractive target for synthetic organic chemists due to its seven chiral centers and diterpene scaffold. This molecule is also interesting to pharmacologists because it is a non-serotonergic hallucinogen, and the first opioid ligand discovered that lacks a basic nitrogen. There have been several total synthesis approaches to salvinorin A, and these will be detailed within this chapter. Additionally, research efforts have concentrated on structure modification of the salvinorin A scaffold through semi-synthetic methods. Most modifications have focused on the manipulation of the acetate at C-2 and the furan ring. However, chemistry has also been developed to generate analogs at the C-1 ketone, the C-4 methyl ester, and the C-17 lactone. The synthetic methodologies developed for the salvinorin A scaffold will be described, as well as specific analogs with interesting biological activities.

Opioid receptor probes derived from cycloaddition of the hallucinogen natural product salvinorin A

As part of our continuing efforts toward more fully understanding the structure-activity relationships of the neoclerodane diterpene salvinorin A, we report the synthesis and biological characterization of unique cycloadducts through [4+2] Diels-Alder cycloaddition. Microwave-assisted methods were developed and successfully employed, aiding in functionalizing the chemically sensitive salvinorin A scaffold. This demonstrates the first reported results for both cycloaddition of the furan ring and functionalization via microwave-assisted methodology of the salvinorin A skeleton. The cycloadducts yielded herein introduce electron-withdrawing substituents and bulky aromatic groups into the C-12 position. Kappa opioid (KOP) receptor space was explored through aromatization of the bent oxanorbornadiene system possessed by the cycloadducts to a planar phenyl ring system. Although dimethyl- and diethylcarboxylate analogues 5 and 6 retain some affinity and selectivity for KOP receptors and are full agonists, their aromatized counterparts 13 and 14 have reduced affinity for KOP receptors. The methods developed herein signify a novel approach toward rapidly probing the structure-activity relationships of furan-containing natural products.

Neuropharmacology of the naturally occurring kappa-opioid hallucinogen salvinorin A

Salvia divinorum is a perennial sage native to Oaxaca, Mexico, that has been used traditionally in divination rituals and as a treatment for the "semimagical" disease panzón de borrego. Because of the intense "out-of-body" experiences reported after inhalation of the pyrolized smoke, S. divinorum has been gaining popularity as a recreational hallucinogen, and the United States and several other countries have regulated its use. Early studies isolated the neoclerodane diterpene salvinorin A as the principal psychoactive constituent responsible for these hallucinogenic effects. Since the finding that salvinorin A exerts its potent psychotropic actions through the activation of KOP receptors, there has been much interest in elucidating the underlying mechanisms behind its effects. These effects are particularly remarkable, because 1) salvinorin A is the first reported non-nitrogenous opioid receptor agonist, and 2) its effects are not mediated by the 5-HT(2A) receptor, the classic target of hallucinogens such as lysergic acid diethylamide and mescaline. Rigorous investigation into the structural features of salvinorin A responsible for opioid receptor affinity and selectivity has produced numerous receptor probes, affinity labels, and tools for evaluating the biological processes responsible for its observed psychological effects. Salvinorin A has therapeutic potential as a treatment for pain, mood and personality disorders, substance abuse, and gastrointestinal disturbances, and suggests that nonalkaloids are potential scaffolds for drug development for aminergic G-protein coupled receptors.