Structure-based design, synthesis, and biochemical and pharmacological characterization of novel salvinorin A analogues as active state probes of the kappa-opioid receptor

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".

Synthesis and evaluation of 3,4,5-trisubstituted triazoles as G protein-biased kappa opioid receptor agonists

Kappa opioid receptor (KOR) agonists represent promising therapeutics for pain relief due to their analgesic properties along with lower abuse potential than opioids that act at the mu opioid receptor. However, typical KOR agonists produce sedation and dysphoria. Previous studies have shown that G protein signaling-biased KOR agonists may present a means to untangle the desired analgesic properties from undesired side effects. In this paper, we report a new series of G protein signaling-biased KOR agonists entailing -S- → -CH2- replacement in a previously reported KOR agonist, triazole 1.1. With an optimized carbon linker in hand, further development of the scaffold was undertaken to investigate the appendages of the triazole core. The structure-activity relationship study of this series is described, including several analogues that display enhanced potency while maintaining G protein-signaling bias compared to triazole 1.1.

Discovery of N'-benzyl-3-chloro-N-((1S,3R,4R)-3-((dimethylamino)methyl)-4-hydroxy-4-(3-methoxyphenyl)cyclohexyl)benzenesulfonamide as a novel selective KOR ligand

The effective management of moderate to severe pain often relies on the use of analgesic agents. However, the widespread utility of these medications is hindered by the occurrence of several undesirable side effects. In light of this challenge, there is growing interest in the development of κ opioid receptor (KOR) agonists, which have shown promise in mitigating these adverse effects. In this study, leveraging the structural scaffold of compound D (our previous study), we embarked on the design, synthesis, and evaluation of a series of N'-benzyl-3-chloro-N- ((1S,3R,4R)-3-((dimethylamino)methyl)-4-hydroxy-4-(3-methoxyphenyl)cyclohexyl)benzenesulfonamide derivatives. These compounds were subjected to comprehensive in vitro and in vivo test. Through systematic structure-activity relationship (SAR) exploration, we successfully identified compound 23p (Ki(KOR):1.9 nM) as a highly selective KOR ligand of new chemotype. 23p showed high clearance in vitro PK test, and abdominal contraction test showed potent antinociceptive effect. 23p and its O-demethyl metabolite 25 were both found in the plasma of mouse, 25 also showed potent affinity toward KOR (Ki(KOR): 3.1 nM), both they contribute to the analgesic effect. Moreover, 23p exhibited potent antinociceptive activity in abdominal constriction test, which was effectively abolished by pre-treatment of nor-BNI, a selective KOR antagonist.

Discovery of Novel, Selective, and Nonbasic Agonists for the Kappa-Opioid Receptor Determined by Salvinorin A-Based Virtual Screening

Modulating the kappa-opioid receptor (KOR) is a promising strategy for treating various human diseases. KOR agonists show potential for treating pain, pruritus, and epilepsy, while KOR antagonists show potential for treating depression, anxiety, and addiction. The diterpenoid Salvinorin A (SalA), a secondary metabolite of Salvia divinorum, is a potent and selective KOR agonist. Unlike typical opioids, SalA lacks a basic nitrogen, which encouraged us to search for nonbasic KOR ligands. Through structure-based virtual screening using 3D pharmacophore models based on the binding mode of SalA, we identified novel, nonbasic, potent, and selective KOR agonists. In vitro studies confirmed two virtual hits, SalA-VS-07 and SalA-VS-08, as highly selective for the KOR and showing G protein-biased KOR agonist activity. Both KOR ligands share a novel spiro-moiety and a nonbasic scaffold. Our findings provide novel starting points for developing therapeutics aimed at treating pain and other conditions in which KOR is a central player.

The expanding repertoire of covalent warheads for drug discovery

The reactive functionalities of drugs that engage in covalent interactions with the enzyme/receptor residue in either a reversible or an irreversible manner are called 'warheads'. Covalent warheads that were previously neglected because of safety concerns have recently gained center stage as a result of their various advantages over noncovalent drugs, including increased selectivity, increased residence time, and higher potency. With the approval of several covalent inhibitors over the past decade, research in this area has accelerated. Various strategies are being continuously developed to tune the characteristics of warheads to improve their potency and mitigate toxicity. Here, we review research progress in warhead discovery over the past 5 years to provide valuable insights for future drug discovery.

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 (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.

Signaling underlying kappa opioid receptor-mediated behaviors in rodents

Kappa opioid receptor (KOR) agonists are potentially useful as analgesic and anti-pruritic agents, for prevention and treatment of substance use disorders, and for treatment of demyelinating diseases. However, side effects of KOR agonists, including psychotomimesis, dysphoria, and sedation, have caused early termination of clinical trials. Understanding the signaling mechanisms underlying the beneficial therapeutic effects and the adverse side effects may help in the development of KOR agonist compounds. In this review, we summarize the current knowledge in this regard in five sections. First, studies conducted on mutant mouse lines (GRK3-/-, p38alpha MAPK-/-, β-arrestin2-/-, phosphorylation-deficient KOR) are summarized. In addition, the abilities of four distinct KOR agonists, which have analgesic and anti-pruritic effects with different side effect profiles, to cause KOR phosphorylation are discussed. Second, investigations on the KOR agonist nalfurafine, both in vitro and in vivo are reviewed. Nalfurafine was the first KOR full agonist approved for clinical use and in the therapeutic dose range it did not produce significant side effects associated with typical KOR agonists. Third, large-scale high-throughput phosphoproteomic studies without a priori hypotheses are described. These studies have revealed that KOR-mediated side effects are associated with many signaling pathways. Fourth, several novel G protein-biased KOR agonists that have been characterized for in vitro biochemical properties and agonist biases and in vivo behavior effects are described. Lastly, possible mechanisms underlying KOR-mediated CPA, hypolocomotion and motor incoordination are discussed. Overall, it is agreed upon that the analgesic and anti-pruritic effects of KOR agonists are mediated via G protein signaling. However, there is no consensus on the mechanisms underlying their side effects. GRK3, p38 MAPK, β-arrestin2, mTOR pathway, CB1 cannabinoid receptor and protein kinase C have been implicated in one side effect or another. For drug discovery, after initial in vitro characterization, in vivo pharmacological characterizations in various behavior tests are still the most crucial steps and dose separation between beneficial therapeutic effects and adverse side effects are the critical determinant for the compounds to be moved forward for clinical development.

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.

Development of disulfide-functionalized peptides covalently binding G protein-coupled receptors

A broadly applicable synthesis of peptides incorporating mixed disulfides between cysteine and homocysteine and cysteamine was developed. The method was established using pharmacologically relevant G protein-coupled receptor (GPCR) ligands including the μ-receptor agonist Dmt-DALDA and extended to the orexin derivative Oxa(17-33) and NT(8-13), the C-terminal hexapeptide of neurotensin. The newly developed NT(8-13) analog 6b incorporating an S-functionalized homocysteine revealed covalent binding of the neurotensin receptor 1 (NTSR1) in a radioligand depletion study.

Analgesic Opioid Ligand Discovery Based on Nonmorphinan Scaffolds Derived from Natural Sources

Strong opioid analgesics, including morphine, are the mainstays for treating moderate to severe acute pain and alleviating chronic cancer pain. However, opioid-related adverse effects, including nausea or vomiting, sedation, respiratory depression, constipation, pruritus (itch), analgesic tolerance, and addiction and abuse liability, are problematic. In addition, the use of opioids to relieve chronic noncancer pain is controversial due to the "opioid crisis" characterized by opioid misuse or abuse and escalating unintentional death rates due to respiratory depression. Hence, considerable research internationally has been aimed at the "Holy Grail" of the opioid analgesic field, namely the discovery of novel and safer opioid analgesics with improved opioid-related adverse effects. In this Perspective, medicinal chemistry strategies are addressed, where structurally diverse nonmorphinan-based opioid ligands derived from natural sources were deployed as lead molecules. The current state of play, clinical or experimental status, and novel opioid ligand discovery approaches are elaborated in the context of retaining analgesia with improved safety and reduced adverse effects, especially addiction liability.

Advances in the molecular understanding of G protein-coupled receptors and their future therapeutic opportunities

Background

Understanding the mechanisms, activated and inhibited pathways as well as other molecular targets involved in existing and emerging disease conditions provides useful insights into their proper diagnosis and treatment and aids drug discovery, development and production. G protein-coupled receptors (GPCRs) are one of the most important classes of targets for small-molecule drug discovery. Of all drug targets, GPCRs are the most studied, undoubtedly because of their pharmacological tractability and role in the pathophysiology as well as the pathogenesis of human diseases.

Main body of the abstract

GPCRs are regarded as the largest target class of the “druggable genome” representing approximately 19% of the currently available drug targets. They have long played a prominent role in drug discovery, such that as of this writing, 481 drugs (about 34% of all FDA-approved drugs) act on GPCRs. More than 320 therapeutic agents are currently under clinical trials, of which a significant percentage targets novel GPCRs. GPCRs are implicated in a wide variety of diseases including CNS disorders, inflammatory diseases such as rheumatoid arthritis and Crohn’s disease, as well as metabolic disease and cancer. The non-olfactory human GPCRs yet to be clinically explored or tried are endowed with perhaps a huge untapped potential drug discovery especially in the field of immunology and genetics.

Short conclusion

This review discusses the recent advances in the molecular pharmacology and future opportunities for targeting GPCRs with a view to drug development.

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

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

In Silico Identification of Tripeptides as Lead Compounds for the Design of KOR Ligands

The kappa opioid receptor (KOR) represents an attractive target for the development of drugs as potential antidepressants, anxiolytics and analgesics. A robust computational approach may guarantee a reduction in costs in the initial stages of drug discovery, novelty and accurate results. In this work, a virtual screening workflow of a library consisting of ~6 million molecules was set up, with the aim to find potential lead compounds that could manifest activity on the KOR. This in silico study provides a significant contribution in the identification of compounds capable of interacting with a specific molecular target. The main computational techniques adopted in this experimental work include: (i) virtual screening; (ii) drug design and leads optimization; (iii) molecular dynamics. The best hits are tripeptides prepared via solution phase peptide synthesis. These were tested in vivo, revealing a good antinociceptive effect after subcutaneous administration. However, further work is due to delineate their full pharmacological profile, in order to verify the features predicted by the in silico outcomes.

Comprehensive overview of biased pharmacology at the opioid receptors: biased ligands and bias factors

One of the main challenges in contemporary medicinal chemistry is the development of safer analgesics, used in the treatment of pain. Currently, moderate to severe pain is still treated with the "gold standard" opioids whose long-term often leads to severe side effects. With the discovery of biased agonism, the importance of this area of pharmacology has grown exponentially over the past decade. Of these side effects, tolerance, opioid misuse, physical dependence and substance use disorder (SUD) stand out, since these have led to many deaths over the past decades in both USA and Europe. New therapeutic molecules that induce a biased response at the opioid receptors (MOR, DOR, KOR and NOP receptor) are able to circumvent these side effects and, consequently, serve as more advantageous therapies with great promise. The concept of biased signaling extends far beyond the already sizeable field of GPCR pharmacology and covering everything would be vastly outside the scope of this review which consequently covers the biased ligands acting at the opioid family of receptors. The limitation of quantifying bias, however, makes this a controversial subject, where it is dependent on the reference ligand, the equation or the assay used for the quantification. Hence, the major issue in the field of biased ligands remains the translation of the in vitro profiles of biased signaling, with corresponding bias factors to in vivo profiles showing the presence or the lack of specific side effects. This review comprises a comprehensive overview of biased ligands in addition to their bias factors at individual members of the opioid family of receptors, as well as bifunctional ligands.

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.

Design and Characterization of an Intracellular Covalent Ligand for CC Chemokine Receptor 2

Covalently acting inhibitors constitute a large and growing fraction of approved small-molecule therapeutics as well as useful tools for a variety of in vitro and in vivo applications. Here, we aimed to develop a covalent antagonist of CC chemokine receptor 2 (CCR2), a class A GPCR that has been pursued as a therapeutic target in inflammation and immuno-oncology. Based on a known intracellularly binding CCR2 antagonist, several covalent derivatives were synthesized and characterized by radioligand binding and functional assays. These studies revealed compound 14 as an intracellular covalent ligand for CCR2. In silico modeling followed by site-directed mutagenesis confirmed that 14 forms a covalent bond with one of three proximal cysteine residues, which can be engaged interchangeably. To our knowledge, compound 14 represents the first covalent ligand reported for CCR2. Due to its unique properties, it may represent a promising tool for ongoing and future studies of CCR2 pharmacology.

The Role of Dynorphin and the Kappa Opioid Receptor in Schizophrenia and Major Depressive Disorder: A Translational Approach

The kappa opioid receptor (KOR) and its endogenous ligands dynorphins (DYN) have been implicated in the development or symptomatology of a variety of neuropsychiatric disorders. This review covers a brief history of the development of KOR agonists and antagonists, their effects in healthy volunteers, and the potential role of DYN/KOR dysfunction in schizophrenia and major depressive disorder from a translational perspective. The potential role of DYN/KOR dysfunction in schizophrenia is based on several lines of evidence. Selective KOR agonists induce affective states in healthy volunteers with similarities to the symptoms of schizophrenia. Studies have shown increased DYN in patients with schizophrenia, although the data have been mixed. Finally, meta-analytic data have shown that opioid antagonists are associated with reductions in the symptoms of schizophrenia. The potential role of DYN/KOR dysfunction in major depressive disorder is also based on a combination of preclinical and clinical data. Selective KOR agonists have shown pro-depressive effects in human volunteers, while selective KOR antagonists have shown robust efficacy in several preclinical models of antidepressant activity. Small studies have shown that nonselective KOR antagonists may have efficacy in treatment-resistant depression. Additionally, recent clinical data have shown that the KOR may be an effective target for treating anhedonia, a finding relevant to both schizophrenia and depression. Finally, recommendations are provided for translating preclinical models for schizophrenia and major depressive disorder into the clinic.

Kappa Opioid Receptor Ligands and Pharmacology: Diphenethylamines, a Class of Structurally Distinct, Selective Kappa Opioid Ligands

The kappa opioid receptor (KOR), a G protein-coupled receptor, and its endogenous ligands, the dynorphins, are prominent members of the opioid neuromodulatory system. The endogenous kappa opioid system is expressed in the central and peripheral nervous systems, and has a key role in modulating pain in central and peripheral neuronal circuits and a wide array of physiological functions and neuropsychiatric behaviors (e.g., stress, reward, emotion, motivation, cognition, epileptic seizures, itch, and diuresis). We review the latest advances in pharmacology of the KOR, chemical developments on KOR ligands with advances and challenges, and therapeutic and potential applications of KOR ligands. Diverse discovery strategies of KOR ligands targeting natural, naturally derived, and synthetic compounds with different scaffolds, as small molecules or peptides, with short or long-acting pharmacokinetics, and central or peripheral site of action, are discussed. These research efforts led to ligands with distinct pharmacological properties, as agonists, partial agonists, biased agonists, and antagonists. Differential modulation of KOR signaling represents a promising strategy for developing pharmacotherapies for several human diseases, either by activating (treatment of pain, pruritus, and epilepsy) or blocking (treatment of depression, anxiety, and addiction) the receptor. We focus on the recent chemical and pharmacological advances on diphenethylamines, a new class of structurally distinct, selective KOR ligands. Design strategies and investigations to define structure-activity relationships together with in vivo pharmacology of diphenethylamines as agonists, biased agonists, and antagonists and their potential use as therapeutics are discussed.

Chemical syntheses of the salvinorin chemotype of KOR agonist

Covering: 2000 to 2020 The hallucinogenic diterpene salvinorin A potently and selectively agonizes the human kappa-opioid receptor (KOR). Its unique attributes-lack of a basic nitrogen, rapid brain penetrance, short half-life-combined with the potential of KOR as an emerging target for analgesics have stimulated extensive medicinal chemistry based on semi-synthesis from extracts of Salvia divinorum. Total synthesis efforts have delivered multiple, orthogonal routes to salvinorin A, its congeners and related analogs with the goal of optimizing its activity towards multiple functional endpoints. Here we review total syntheses of the salvinorin chemotype and discuss outstanding problems that synthesis can address in the future.

Covalent allosteric modulation: An emerging strategy for GPCRs drug discovery

Designing covalent allosteric modulators brings new opportunities to the field of drug discovery towards G-protein-coupled receptors (GPCRs). Targeting an allosteric binding pocket can allow a modulator to have protein subtype selectivity and low drug resistance. Utilizing covalent warheads further enables the modulator to increase the binding potency and extend the duration of action. This review starts with GPCR allosteric modulation to discuss the structural biology of allosteric binding pockets, the different types of allosteric modulators, as well as the advantages of employing allosteric modulation. This is followed by a discussion on covalent modulators to clarify how covalent ligands can benefit the receptor modulation and to illustrate moieties that can commonly be used as covalent warheads. Finally, case studies are presented on designing class A, B, and C GPCR covalent allosteric modulators to demonstrate successful stories on combining allosteric modulation and covalent binding. Limitations and future perspectives are also covered.

Development of Diphenethylamines as Selective Kappa Opioid Receptor Ligands and Their Pharmacological Activities

Among the opioid receptors, the kappa opioid receptor (KOR) has been gaining substantial attention as a promising molecular target for the treatment of numerous human disorders, including pain, pruritus, affective disorders (i.e., depression and anxiety), drug addiction, and neurological diseases (i.e., epilepsy). Particularly, the knowledge that activation of the KOR, opposite to the mu opioid receptor (MOR), does not produce euphoria or leads to respiratory depression or overdose, has stimulated the interest in discovering ligands targeting the KOR as novel pharmacotherapeutics. However, the KOR mediates the negative side effects of dysphoria/aversion, sedation, and psychotomimesis, with the therapeutic promise of biased agonism (i.e., selective activation of beneficial over deleterious signaling pathways) for designing safer KOR therapeutics without the liabilities of conventional KOR agonists. In this review, the development of new KOR ligands from the class of diphenethylamines is presented. Specifically, we describe the design strategies, synthesis, and pharmacological activities of differently substituted diphenethylamines, where structure–activity relationships have been extensively studied. Ligands with distinct profiles as potent and selective agonists, G protein-biased agonists, and selective antagonists, and their potential use as therapeutic agents (i.e., pain treatment) and research tools are described.

The wonderful and masterful G protein-coupled receptor (GPCR): A focus on signaling mechanisms and the neuroendocrine control of fertility

Human GnRH deficiency, both clinically and genetically, is a heterogeneous disorder comprising of congenital GnRH deficiency with anosmia (Kallmann syndrome), or with normal olfaction [normosmic idiopathic hypogonadotropic hypogonadism (IHH)], and adult-onset hypogonadotropic hypogonadism. Our understanding of the neural mechanisms underlying GnRH secretion and GnRH signaling continues to increase at a rapid rate and strikingly, the heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) continue to emerge as essential players in these processes. GPCRs were once viewed as binary on-off switches, where in the "on" state they are bound to their Gα protein, but now we understand that view is overly simplistic and does not adequately characterize GPCRs. Instead, GPCRs have emerged as masterful signaling molecules exploiting different physical conformational states of itself to elicit an array of downstream signaling events via their G proteins and the β-arrestins. The "one receptor-multiple signaling conformations" model is likely an evolved strategy that can be used to our advantage as researchers have shown that targeting specific receptor conformations via biased ligands is proving to be a powerful tool in the effective treatment of human diseases. Can biased ligands be used to selectively modulate signaling by GPCR regulators of the neuroendocrine axis in the treatment of IHH? As discussed in this review, the grand possibility exists. However, while we are still very far from developing these treatments, this exciting likelihood can happen through a much greater mechanistic understanding of how GPCRs signal within the cell.

Strategies for Developing κ Opioid Receptor Agonists for the Treatment of Pain with Fewer Side Effects

There is significant need to find effective, nonaddictive pain medications. κ Opioid receptor (KOPr) agonists have been studied for decades but have recently received increased attention because of their analgesic effects and lack of abuse potential. However, a range of side effects have limited the clinical development of these drugs. There are several strategies currently used to develop safer and more effective KOPr agonists. These strategies include identifying G-protein-biased agonists, developing peripherally restricted KOPr agonists without centrally mediated side effects, and developing mixed opioid agonists, which target multiple receptors at specific ratios to balance side-effect profiles and reduce tolerance. Here, we review the latest developments in research related to KOPr agonists for the treatment of pain. SIGNIFICANCE STATEMENT: This review discusses strategies for developing safer κ opioid receptor (KOPr) agonists with therapeutic potential for the treatment of pain. Although one strategy is to modify selective KOPr agonists to create peripherally restricted or G-protein-biased structures, another approach is to combine KOPr agonists with μ, δ, or nociceptin opioid receptor activation to obtain mixed opioid receptor agonists, therefore negating the adverse effects and retaining the therapeutic effect.

Kappa opioid receptors internalization is protective against oxygen-glucose deprivation through β-arrestin activation and Akt-mediated signaling pathway

Hypoxia induces reversible κ-opioid receptor (KOR) internalization similar to the internalization that is induced by KOR agonists. In the current study, we demonstrate that this KOR internalization is a protective mechanism via the β-arrestin specific pathway in an oxygen-glucose deprivation (OGD) model. Mouse neuroblastoma Neuro2A cells were stably transfected with mouse KOR-tdTomato fusion protein (N2A-mKOR-tdT cells). Various concentrations of salvinorin A (SA), a highly selective KOR agonist, were given in the presence and absence of norbinaltorphimine (norBNI), which is a KOR antagonist, or Dyngo-4a (internalization inhibitor) or API-2 (Akt/Protein kinase B signaling inhibitor-2). Various concentrations of SA and RB-64 (22-thiocyanatosalvinorin A, selective for the G protein signaling pathway) were administered both in normoxic and hypoxic conditions. Autophagosomes and ultrastructural components of cells were observed using transmission electron microscopy (TEM). Cell viability, severity of cell injury, and levels of proteins related to the Akt signaling pathway were evaluated using live cell counting (by Cell Counting Kit-8), the lactic acid dehydrogenase (LDH) release rate, and Western blot analysis, respectively. SA promoted cell survival and attenuated OGD-induced cell injury. The Akt signaling pathway is activated by SA. KOR internalization, when blocked by norBNI or Dyngo-4a, increased LDH release and decreased cell viability under OGD. Treatment with SA significantly inhibited autophagy, and the effects of SA on autophagy were reversed by API-2 pretreatment. RB-64 in a low concentration without β-arrestin recruitment did not reduce LDH release and increase cell viability as observed with SA. KOR internalization through β-arrestin activation is a protective mechanism against OGD. The Akt pathway might play a critical role in modulating these protective effects by inhibiting autophagy.

Progress in the development of more effective and safer analgesics for pain management

Opioid analgesics have been used for thousands of years in the treatment of pain and related disorders, and have become among the most widely prescribed medications. Among opioid analgesics, mu opioid receptor (MOR) agonists are the most commonly used and are indicated for acute and chronic pain management. However, their use results in a plethora of well-described side-effects. From selective delta opioid receptor (DOR) and kappa opioid receptor (KOR) agonists to multitarget MOR/DOR and MOR/KOR ligands, medicinal chemistry provided different approaches aimed at the development of opioid analgesics with an improved pharmacological and tolerability fingerprint. The emergent medicinal chemistry strategy to develop ameliorated opioid analgesics is based upon the concept that functional selectivity for G-protein signalling is necessary for the therapeutic effect, whether β-arrestin recruitment is mainly responsible for the manifestation of side effects, including the development of tolerance after repeated administrations. This review summarises most relevant biased MOR, DOR, KOR and multitarget MOR/DOR ligands synthesised in the last decade and their pharmacological profile in "in vitro" and "in vivo" studies. Such biased ligands could have a significant impact on modern drug discovery and represent a new strategy for the development of better-tolerated drug candidates.

A Review of the Therapeutic Potential of Recently Developed G Protein-Biased Kappa Agonists

Between 2000 and 2005 several studies revealed that morphine is more potent and exhibits fewer side effects in beta-arrestin 2 knockout mice. These findings spurred efforts to develop opioids that signal primarily via G protein activation and do not, or only very weakly, recruit beta-arrestin. Development of such molecules targeting the mu opioid receptor initially outpaced those targeting the kappa, delta and nociceptin opioid receptors, with the G protein-biased mu opioid agonist oliceridine/TRV130 having completed phase III clinical trials with improved therapeutic window to treat moderate-to-severe acute pain. Recently however, there has been a sharp increase in the development of novel G protein-biased kappa agonists. It is hypothesized that G protein-biased kappa agonists can reduce pain and itch, but exhibit fewer side effects, such as anhedonia and psychosis, that have thus far limited the clinical development of unbiased kappa opioid agonists. Here we summarize recently discovered G protein-biased kappa agonists, comparing structures, degree of signal bias and preclinical effects. We specifically reviewed nalfurafine, 22-thiocyanatosalvinorin A (RB-64), mesyl-salvinorin B, 2-(4-(furan-2-ylmethyl)-5-((4-methyl-3-(trifluoromethyl)benzyl)thio)-4H-1,2,4-triazol-3-yl)pyridine (triazole 1.1), 3-(2-((cyclopropylmethyl)(phenethyl)amino)ethyl)phenol (HS666), N-n-butyl-N-phenylethyl-N-3-hydroxyphenylethyl-amine (compound 5/BPHA), 6-guanidinonaltrindole (6′GNTI), and collybolide. These agonists encompass a variety of chemical scaffolds and range in both their potency and efficacy in terms of G protein signaling and beta-arrestin recruitment. Thus unsurprisingly, the behavioral responses reported for these agonists are not uniform. Yet, it is our conclusion that the kappa opioid field will benefit tremendously from future studies that compare several biased agonists and correlate the degree of signaling bias to a particular pharmacological response.

Development and In Vivo Evaluation of a κ-Opioid Receptor Agonist as a PET Radiotracer with Superior Imaging Characteristics

Studies have shown κ-opioid receptor (KOR) abnormalities in addictive disorders, other central nervous system diseases, and Alzheimer's disease. We have developed the first set of agonist ¹¹C-GR103545 and antagonist ¹¹C-LY2795050 radiotracers for PET imaging of KOR in humans. Nonetheless, ¹¹C-GR103545 displays protracted uptake kinetics and is not an optimal radiotracer. Here, we report the development and evaluation of ¹¹C-methyl-(R)-4-(2-(3,4-dichlorophenyl)acetyl)-3-((diethylamino)methyl)piperazine-1-carboxylate (¹¹C-EKAP) and its comparison with ¹¹C-GR103545. Methods: EKAP was synthesized and assayed for in vitro binding affinities and then radiolabeled. PET studies were performed on rhesus monkeys. Blocking studies were performed with naloxone and the selective KOR antagonists LY2795050 and LY2456302. Arterial input functions were generated for use in kinetic modeling. Brain TACs were analyzed with multilinear analysis 1 to derive binding parameters. Results: EKAP has high KOR affinity (inhibition constant, 0.28 nM) and good selectivity in vitro. ¹¹C-EKAP was prepared in good radiochemical purity. ¹¹C-EKAP rapidly metabolized in plasma and displayed fast and reversible kinetics in brain, with peak uptake at less than 20 min after injection. Preblocking with naloxone (1 mg/kg) or LY2795050 (0.2 mg/kg) produced 84%-89% receptor occupancy, whereas LY2456302 (0.05 and 0.3 mg/kg) dose-dependently reduced ¹¹C-EKAP-specific binding, thus demonstrating its binding specificity and selectivity in vivo. Mean multilinear analysis 1-derived nondisplaceable binding potential values were 1.74, 1.79, 1.46, 0.80, and 0.77 for cingulate cortex, globus pallidus, insula, striatum, and frontal cortex, respectively, consistent with the known KOR distribution in primate brains. Conclusion: We have successfully developed ¹¹C-EKAP as a KOR agonist tracer with dual attractive imaging properties of fast uptake kinetics and high specific binding in vivo.

In vivo brain GPCR signaling elucidated by phosphoproteomics

A systems view of G protein-coupled receptor (GPCR) signaling in its native environment is central to the development of GPCR therapeutics with fewer side effects. Using the kappa opioid receptor (KOR) as a model, we employed high-throughput phosphoproteomics to investigate signaling induced by structurally diverse agonists in five mouse brain regions. Quantification of 50,000 different phosphosites provided a systems view of KOR in vivo signaling, revealing novel mechanisms of drug action. Thus, we discovered enrichment of the mechanistic target of rapamycin (mTOR) pathway by U-50,488H, an agonist causing aversion, which is a typical KOR-mediated side effect. Consequently, mTOR inhibition during KOR activation abolished aversion while preserving beneficial antinociceptive and anticonvulsant effects. Our results establish high-throughput phosphoproteomics as a general strategy to investigate GPCR in vivo signaling, enabling prediction and modulation of behavioral outcomes.

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.

Structure of the Nanobody-Stabilized Active State of the Kappa Opioid Receptor

The κ-opioid receptor (KOP) mediates the actions of opioids with hallucinogenic, dysphoric, and analgesic activities. The design of KOP analgesics devoid of hallucinatory and dysphoric effects has been hindered by an incomplete structural and mechanistic understanding of KOP agonist actions. Here, we provide a crystal structure of human KOP in complex with the potent epoxymorphinan opioid agonist MP1104 and an active-state-stabilizing nanobody. Comparisons between inactive- and active-state opioid receptor structures reveal substantial conformational changes in the binding pocket and intracellular and extracellular regions. Extensive structural analysis and experimental validation illuminate key residues that propagate larger-scale structural rearrangements and transducer binding that, collectively, elucidate the structural determinants of KOP pharmacology, function, and biased signaling. These molecular insights promise to accelerate the structure-guided design of safer and more effective κ-opioid receptor therapeutics.

Tryptophan-Containing Non-Cationizable Opioid Peptides - a new chemotype with unusual structure and in vivo activity

Recently, a new family of opioid peptides containing tryptophan came to the spotlight for the absence of the fundamental protonable tyramine 'message' pharmacophore. Structure-activity relationship investigations led to diverse compounds, characterized by different selectivity profiles and agonist or antagonist effects. Substitution at the indole of Trp clearly impacted peripheral/central antinociceptivity. These peculiarities prompted to gather all the compounds in a new class, and to coin the definition 'Tryptophan-Containing Non-Cationizable Opioid Peptides', in short 'TryCoNCOPs'. Molecular docking analysis suggested that the TryCoNCOPs can still interact with the receptors in an agonist-like fashion. However, most TryCoNCOPs showed significant differences between the in vitro and in vivo activities, suggesting that opioid activity may be elicited also via alternative mechanisms.

Modular Approach to pseudo-Neoclerodanes as Designer κ-Opioid Ligands

Informed by previous semisynthetic work on salvinorin A, a modular total synthesis has been developed capable of producing novel compounds targeting the κ-opioid receptor. The strategy has permitted the deliberate simplification and introduction of functionality about the target molecule to provide access to molecular features on salvinorin A otherwise unattainable by semisynthesis. Using this approach, a potent pseudo-neoclerodane κ-opioid receptor ligand (2) has been realized.

The C-2 derivatives of salvinorin A, ethoxymethyl ether Sal B and β-tetrahydropyran Sal B, have anti-cocaine properties with minimal side effects

RATIONALE

Kappa-opioid receptor (KOPr) agonists have pre-clinical anti-cocaine and analgesic effects. However, side effects including sedation, dysphoria, aversion, anxiety and depression limit their therapeutic development. The unique structure of salvinorin A has been used to develop longer acting KOPr agonists.

OBJECTIVES

We evaluate two novel C-2 analogues of salvinorin A, ethoxymethyl ether Sal B (EOM Sal B) and β-tetrahydropyran Sal B (β-THP Sal B) alongside U50,488 for their ability to modulate cocaine-induced behaviours and side effects, pre-clinically.

METHODS

Anti-cocaine properties of EOM Sal B were evaluated using the reinstatement model of drug seeking in self-administering rats. EOM Sal B and β-THP Sal B were evaluated for effects on cocaine-induced hyperactivity, spontaneous locomotor activity and sucrose self-administration. EOM Sal B and β-THP Sal B were evaluated for aversive, anxiogenic and depressive-like effects using conditioned place aversion (CPA), elevated plus maze (EPM) and forced swim tests (FSTs), respectively.

RESULTS

EOM Sal B (0.1, 0.3 mg/kg, intraperitoneally (i.p.)) dose dependently attenuated drug seeking, and EOM Sal B (0.1 mg/kg, i.p.) and β-THP Sal B (1 mg/kg, i.p.) attenuated cocaine-induced hyperactivity. No effects on locomotor activity, open arm times (EPM) or swimming behaviours (FST) were seen with EOM (0.1 or 0.3 mg/kg, i.p.) or β-THP Sal B (1 or 2 mg/kg, i.p.). However, β-THP Sal B decreased time spent in the drug-paired chamber.

CONCLUSION

EOM Sal B is more potent than Sal A and β-THP Sal B in reducing drug-seeking behaviour with fewer side effects. EOM Sal B showed no effects on sucrose self-administration (0.1 mg/kg), locomotor, depressive-like, aversive-like or anxiolytic effects.

Recent Advances in Structure-Based Drug Design Targeting Class A G Protein-Coupled Receptors Utilizing Crystal Structures and Computational Simulations

G protein-coupled receptors (GPCRs) represent the largest and most physiologically important integral membrane protein family, and these receptors respond to a wide variety of physiological and environmental stimuli. GPCRs are among the most critical therapeutic targets for numerous human diseases, and approximately one-third of the currently marketed drugs target this receptor family. The recent breakthroughs in GPCR structural biology have significantly contributed to our understanding of GPCR function, ligand binding, and pharmacological action as well as to the design of new drugs. This perspective highlights the latest advances in GPCR structures with a focus on the receptor-ligand interactions of each receptor family in class A nonrhodopsin GPCRs as well as the structural features for their activation, biased signaling, and allosteric mechanisms. The current state-of-the-art methodologies of structure-based drug design (SBDD) approaches in the GPCR research field are also discussed.

An immunochromatographic assay for rapid etection of salvinorin A

We developed an immunochromatographic assay (ICA) that enables rapid analysis of salvinorin A (Sal A) in Salvia divinorum within 10 min. The result shows that no Sal A in other samples of Lamiaceae plants was detected, but it could recognize Sal A among other substances in complex samples. The main advantage of the ICA is its high performance in combination with low cost, simplicity, and speed. Our newly developed combined ICA/indirect competitive ELISA(icELISA) system enables analysis of large numbers of samples over short periods of time without cumbersome pretreatments in complex mixtures. This method can complement other instrumental analyses for salvinorins, and could be used to deter S. divinorum abuse.

Allosteric modulation model of the mu opioid receptor by herkinorin, a potent not alkaloidal agonist

Modulation of opioid receptors is the primary choice for pain management and structural information studies have gained new horizons with the recently available X-ray crystal structures. Herkinorin is one of the most remarkable salvinorin A derivative with high affinity for the mu opioid receptor, moderate selectivity and lack of nitrogen atoms on its structure. Surprisingly, binding models for herkinorin are lacking. In this work, we explore binding models of herkinorin using automated docking, molecular dynamics simulations, free energy calculations and available experimental information. Our herkinorin D-ICM-1 binding model predicted a binding free energy of -11.52 ± 1.14 kcal mol^-1 by alchemical free energy estimations, which is close to the experimental values -10.91 ± 0.2 and -10.80 ± 0.05 kcal mol^-1 and is in agreement with experimental structural information. Specifically, D-ICM-1 molecular dynamics simulations showed a water-mediated interaction between D-ICM-1 and the amino acid H297^6.52, this interaction coincides with the co-crystallized ligands. Another relevant interaction, with N127^2.63, allowed to rationalize herkinorin's selectivity to mu over delta opioid receptors. Our suggested binding model for herkinorin is in agreement with this and additional experimental data. The most remarkable observation derived from our D-ICM-1 model is that herkinorin reaches an allosteric sodium ion binding site near N150^3.35. Key interactions in that region appear relevant for the lack of β-arrestin recruitment by herkinorin. This interaction is key for downstream signaling pathways involved in the development of side effects, such as tolerance. Future SAR studies and medicinal chemistry efforts will benefit from the structural information presented in this work.

Recent Advances and Applications of Molecular Docking to G Protein-Coupled Receptors

The growing number of studies on G protein-coupled receptors (GPCRs) family are a source of noticeable improvement in our understanding of the functioning of these proteins. GPCRs are responsible for a vast part of signaling in vertebrates and, as such, invariably remain in the spotlight of medicinal chemistry. A deeper insight into the underlying mechanisms of interesting phenomena observed in GPCRs, such as biased signaling or allosteric modulation, can be gained with experimental and computational studies. The latter play an important role in this process, since they allow for observations on scales inaccessible for most other methods. One of the key steps in such studies is proper computational reconstruction of actual ligand-receptor or protein-protein interactions, a process called molecular docking. A number of improvements and innovative applications of this method were documented recently. In this review, we focus particularly on innovations in docking to GPCRs.

G Protein-coupled Receptor Biased Agonism

G protein-coupled receptors are the largest family of targets for current therapeutics. The classic model of their activation was binary, where agonist binding induced an active conformation and subsequent downstream signaling. Subsequently, the revised concept of biased agonism emerged, where different ligands at the same G protein-coupled receptor selectively activate one downstream pathway versus another. Advances in understanding the mechanism of biased agonism have led to the development of novel ligands, which have the potential for improved therapeutic and safety profiles. In this review, we summarize the theory and most recent breakthroughs in understanding biased signaling, examine recent laboratory investigations concerning biased ligands across different organ systems, and discuss the promising clinical applications of biased agonism.

Clerodane diterpenes: sources, structures, and biological activities

Covering: 1990 to 2015The clerodane diterpenoids are a widespread class of secondary metabolites and have been found in several hundreds of plant species from various families and in organisms from other taxonomic groups. These substances have attracted interest in recent years due to their notable biological activities, particularly insect antifeedant properties. In addition, the major active clerodanes of Salvia divinorum can be used as novel opioid receptor probes, allowing greater insight into opioid receptor-mediated phenomena, as well as opening additional areas for chemical investigation. This article provides extensive coverage of naturally occurring clerodane diterpenes discovered from 1990 until 2015, and follows up on the 1992 review by Merritt and Ley in this same journal. The distribution, chemotaxonomic significance, chemical structures, and biological activities of clerodane diterpenes are summarized. In the cases where sufficient information is available, structure activity relationship (SAR) correlations and mode of action of active clerodanes have been presented.

Guidelines for the Synthesis of Small-Molecule Irreversible Probes Targeting G Protein-Coupled Receptors

Irreversible probes have been proven to be useful pharmacological tools in the study of structural and functional features in drug receptor pharmacology. They have been demonstrated to be particularly valuable for the isolation and purification of receptors. Furthermore, irreversible probes are helpful tools for the identification and characterization of binding sites, thereby supporting the advancement of rational drug design. In this Minireview, we provide insight into universal strategies and guidelines to successfully synthesize irreversible probes that target G protein-coupled receptors (GPCRs). We provide an overview of commonly used chemoreactive and photoreactive groups, and make a comparison of their properties and potential applications. Furthermore, there is a particular focus on synthetic approaches to introduce these reactive groups based on commercially available reagents.

New Technologies for Elucidating Opioid Receptor Function

Recent advances in technology, including high resolution crystal structures of opioid receptors, novel chemical tools, and new genetic approaches have provided an unparalleled palette of tools for deconstructing opioid receptor actions in vitro and in vivo. Here we provide a brief description of our understanding of opioid receptor function from both molecular and atomic perspectives, as well as their role in neural circuits in vivo. We then show how insights into the molecular details of opioid actions can facilitate the creation of functionally selective (biased) and photoswitchable opioid ligands. Finally, we describe how newly engineered opioid receptor-based chemogenetic and optogenetic tools, and new mouse lines, are expanding and transforming our understanding of opioid function and, perhaps, paving the way for new therapeutics.

Ligand-Independent Adenosine A2B Receptor Constitutive Activity as a Promoter of Prostate Cancer Cell Proliferation

Aberrant ligand-independent G protein-coupled receptor constitutive activity has been implicated in the pathophysiology of a number of cancers. The adenosine A2B receptor (A2BAR) is dynamically upregulated under pathologic conditions associated with a hypoxic microenvironment, including solid tumors. This, in turn, may amplify ligand-independent A2BAR signal transduction. The contribution of A2BAR constitutive activity to disease progression is currently unknown yet of fundamental importance, as the preferred therapeutic modality for drugs designed to reduce A2BAR constitutive activity would be inverse agonism as opposed to neutral antagonism. The current study investigated A2BAR constitutive activity in a heterologous expression system and a native 22Rv1 human prostate cancer cell line exposed to hypoxic conditions (2% O2). The A2BAR inverse agonists, ZM241385 [4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol] or PSB-603 (8-(4-(4-(4-chlorophenyl)piperazide-1-sulfonyl)phenyl)-1-propylxanthine), mediated a concentration-dependent decrease in baseline cAMP levels in both cellular systems. Proliferation of multiple prostate cancer cell lines was also attenuated in the presence of PSB-603. Importantly, both the decrease in baseline cAMP accumulation and the reduction of proliferation were not influenced by the addition of adenosine deaminase, demonstrating that these effects are not dependent on stimulation of A2BARs by the endogenous agonist adenosine. Our study is the first to reveal that wild-type human A2BARs have high constitutive activity in both model and native cells. Furthermore, our findings demonstrate that this ligand-independent A2BAR constitutive activity is sufficient to promote prostate cancer cell proliferation in vitro. More broadly, A2BAR constitutive activity may have wider, currently unappreciated implications in pathologic conditions associated with a hypoxic microenvironment.

Biased agonism at kappa opioid receptors: Implication in pain and mood disorders

The kappa opioid receptor (k receptor) and its endogenous ligand dynorphin have received significant attention due to their involvement in pathophysiology of mood disorders, drug addiction, psychotic disorders and pain. Multiple lines of evidences suggest that the k receptor modulates overlapping neurocircuits connecting brainstem monoaminergic nuclei with forebrain limbic structures and thereby regulates neurobiological effects of stress and psychostimulants. The emerging concept of "biased agonism" (also known as functional selectivity) for G Protein Coupled Receptor (GPCR) ligands have provided new insights into overall response generated by a ligand, which could be exploited for drug discovery. According to this concept, every ligand possesses the unique ability (coded in its structure) that dictates distinct signalling pattern, and consequently beneficial or adverse response. Although still a long way to comprehend the clinical potential of biased GPCR ligands, such ligand could be vital pharmacological probes. This article highlights various lines of evidence, which indicates different ligands of k receptor as "biased", and their potential implications in mood and pain disorders.

Covalent molecular probes for class A G protein-coupled receptors: advances and applications

Covalent modification of G protein-coupled receptors (GPCRs) by employing specific molecular probes has for decades provided a successful strategy to facilitate the elucidation of the structure and function of this pharmacologically important class of membrane proteins. The ligands typically comprise a pharmacophore that generates affinity for a given GPCR and contain a reactive functionality that may form a covalent bond with a suitably positioned amino acid residue. Covalent ligands have been successfully applied to circumvent poor affinity of compounds when stable labeling of receptor populations was required, and they have been used in the isolation, purification, and pharmacological characterization of specific subtypes of GPCRs. Recently, structural studies have demonstrated that covalent molecular probes are effective at stabilizing GPCRs to obtain X-ray crystal structures, thus providing valuable insights for the development of novel therapeutics. Herein, we review covalently binding molecular probes for class A GPCRs with a focus on ligands comprising cross-linking groups that do not require photoactivation and further highlight their significant and diverse applications.

The G protein-biased κ-opioid receptor agonist RB-64 is analgesic with a unique spectrum of activities in vivo

The hypothesis that functionally selective G protein-coupled receptor (GPCR) agonists may have enhanced therapeutic benefits has revitalized interest for many GPCR targets. In particular, although κ-opioid receptor (KOR) agonists are analgesic with a low risk of dependence and abuse, their use is limited by a propensity to induce sedation, motor incoordination, hallucinations, and dysphoria-like states. Several laboratories have produced a body of work suggesting that G protein-biased KOR agonists might be analgesic with fewer side effects. Although that has been an intriguing hypothesis, suitable KOR-selective and G protein-biased agonists have not been available to test this idea. Here we provide data using a G protein-biased agonist, RB-64 (22-thiocyanatosalvinorin A), which suggests that KOR-mediated G protein signaling induces analgesia and aversion, whereas β-arrestin-2 signaling may be associated with motor incoordination. Additionally, unlike unbiased KOR agonists, the G protein-biased ligand RB-64 does not induce sedation and does not have anhedonia-like actions, suggesting that a mechanism other than G protein signaling mediates these effects. Our findings provide the first evidence for a highly selective and G protein-biased tool compound for which many, but not all, of the negative side effects of KOR agonists can be minimized by creating G protein-biased KOR agonists.

Michael acceptor approach to the design of new salvinorin A-based high affinity ligands for the kappa-opioid receptor

The neoclerodane diterpenoid salvinorin A is a major secondary metabolite isolated from the psychoactive plant Salvia divinorum. Salvinorin A has been shown to have high affinity and selectivity for the κ-opioid receptor (KOR). To study the ligand-receptor interactions that occur between salvinorin A and the KOR, a new series of salvinorin A derivatives bearing potentially reactive Michael acceptor functional groups at C-2 was synthesized and used to probe the salvinorin A binding site. The κ-, δ-, and μ-opioid receptor (KOR, DOR and MOR, respectively) binding affinities and KOR efficacies were measured for the new compounds. Although none showed wash-resistant irreversible binding, most of them showed high affinity for the KOR, and some exhibited dual affinity to KOR and MOR. Molecular modeling techniques based on the recently-determined crystal structure of the KOR combined with results from mutagenesis studies, competitive binding, functional assays and structure-activity relationships, and previous salvinorin A-KOR interaction models were used to identify putative interaction modes of the new compounds with the KOR and MOR.