Allosteric modulation of a cannabinoid G protein-coupled receptor: binding site elucidation and relationship to G protein signaling

Negative allosteric modulation of CB1 cannabinoid receptor signaling suppresses opioid-mediated tolerance and withdrawal without blocking opioid antinociception

The direct blockade of CB1 cannabinoid receptors produces therapeutic effects as well as adverse side-effects that limit their clinical potential. CB1 negative allosteric modulators (NAMs) represent an indirect approach to decrease the affinity and/or efficacy of orthosteric cannabinoid ligands or endocannabinoids at CB1. We recently reported that GAT358, a CB1-NAM, blocked opioid-induced mesocorticolimbic dopamine release and reward via a CB1-allosteric mechanism of action. Whether a CB1-NAM dampens opioid-mediated therapeutic effects such as analgesia or alters other unwanted opioid side-effects remain unknown. Here, we characterized the effects of GAT358 on nociceptive behaviors in the presence and absence of morphine in male rats. We examined the impact of GAT358 on formalin-evoked pain behavior and Fos protein expression, a marker of neuronal activation, in the lumbar spinal cord. We also assessed the impact of GAT358 on morphine-induced slowing of colonic transit, tolerance, and withdrawal behaviors in male mice. GAT358 attenuated morphine antinociceptive tolerance without blocking acute antinociception and reduced morphine-induced slowing of colonic motility without impacting fecal boli production. GAT358 also produced antinociception in the presence and absence of morphine in the formalin model of inflammatory nociception and reduced the number of formalin-evoked Fos protein-like immunoreactive cells in the lumbar spinal cord. Finally, GAT358 mitigated the somatic signs of naloxone-precipitated, but not spontaneous, opioid withdrawal following chronic morphine dosing. Our results support the therapeutic potential of CB1-NAMs as novel drug candidates aimed at preserving opioid-mediated analgesia while preventing their unwanted side-effects. Our studies also uncover previously unrecognized antinociceptive properties associated with an arrestin-biased CB1-NAM.

Negative allosteric modulation of cannabinoid CB 1 receptor signaling suppresses opioid-mediated tolerance and withdrawal without blocking opioid antinociception

The direct blockade of CB 1 cannabinoid receptors produces therapeutic effects as well as adverse side-effects that limit their clinical potential. CB 1 negative allosteric modulators (NAMs) represent an indirect approach to decrease the affinity and/or efficacy of orthosteric cannabinoid ligands or endocannabinoids at CB 1 . We recently reported that GAT358, a CB 1 -NAM, blocked opioid-induced mesocorticolimbic dopamine release and reward via a CB 1 -allosteric mechanism of action. Whether a CB 1 -NAM dampens opioid-mediated therapeutic effects such as analgesia or alters other unwanted side-effects of opioids remain unknown. Here, we characterized the effects of GAT358 on nociceptive behaviors in the presence and absence of morphine. We examined the impact of GAT358 on formalin-evoked pain behavior and Fos protein expression, a marker of neuronal activation, in the lumbar dorsal horn. We also assessed the impact of GAT358 on morphine-induced slowing of colonic transit, tolerance, and withdrawal behaviors. GAT358 attenuated morphine antinociceptive tolerance without blocking acute antinociception. GAT358 also reduced morphine-induced slowing of colonic motility without impacting fecal boli production. GAT358 produced antinociception in the presence and absence of morphine in the formalin model of inflammatory nociception and reduced the number of formalin-evoked Fos protein-like immunoreactive cells in the lumbar spinal dorsal horn. Finally, GAT358 mitigated the somatic signs of naloxone-precipitated, but not spontaneous, opioid withdrawal following chronic morphine dosing in mice. Our results support the therapeutic potential of CB 1 -NAMs as novel drug candidates aimed at preserving opioid-mediated analgesia while preventing their unwanted side-effects. Our studies also uncover previously unrecognized antinociceptive properties associated with an arrestin-biased CB 1 -NAMs.

Highlights

CB 1 negative allosteric modulator (NAM) GAT358 attenuated morphine tolerance GAT358 reduced morphine-induced slowing of colonic motility but not fecal productionGAT358 was antinociceptive for formalin pain alone and when combined with morphineGAT358 reduced formalin-evoked Fos protein expression in the lumbar spinal cordGAT358 mitigated naloxone precipitated withdrawal after chronic morphine dosing.

Should oncologists trust cannabinoids?

Cannabis enjoyed a "golden age" as a medicinal product in the late 19th, early 20th century, but the increased risk of overdose and abuse led to its criminalization. However, the 21st century have witnessed a resurgence of interest and a large body of literature regarding the benefits of cannabinoids have emerged. As legalization and decriminalization have spread around the world, cancer patients are increasingly interested in the potential utility of cannabinoids. Although eager to discuss cannabis use with their oncologist, patients often find them to be reluctant, mainly because clinicians are still not convinced by the existing evidence-based data to guide their treatment plans. Physicians should prescribe cannabis only if a careful explanation can be provided and follow up response evaluation ensured, making it mandatory for them to be up to date with the positive and also negative aspects of the cannabis in the case of cancer patients. Consequently, this article aims to bring some clarifications to clinicians regarding the sometimes-confusing various nomenclature under which this plant is mentioned, current legislation and the existing evidence (both preclinical and clinical) for the utility of cannabinoids in cancer patients, for either palliation of the associated symptoms or even the potential antitumor effects that cannabinoids may have.

Endocannabinoid signaling in the central nervous system

It is hard to overestimate the influence of the endocannabinoid signaling (ECS) system on central nervous system (CNS) function. In the 40 years since cannabinoids were found to trigger specific cell signaling cascades, studies of the ECS system continue to cause amazement, surprise, and confusion! CB1 cannabinoid receptors are expressed widely in the CNS and regulate cell-cell communication via effects on the release of both neurotransmitters and gliotransmitters. CB2 cannabinoid receptors are difficult to detect in the CNS but seem to "punch above their weight" as compounds targeting these receptors have significant effects on inflammatory state and behavior. Positive and negative allosteric modulators for both receptors have been identified and examined in preclinical studies. Concentrations of the endocannabinoid ligands, N-arachidonoylethanolamine and 2-arachidonoylglycerol (2-AG), are regulated by a combination of enzymatic synthesis and degradation and inhibitors of these processes are available and making their way into clinical trials. Importantly, ECS regulates many essential brain functions, including regulation of reward, anxiety, inflammation, motor control, and cellular development. While the field is on the cusp of preclinical discoveries providing impactful clinical and therapeutic insights into many CNS disorders, there is still much to be learned about this remarkable and versatile modulatory system.

Negative allosteric modulation of CB1 cannabinoid receptor signaling suppresses opioid-mediated reward

Blockade of cannabinoid type 1 (CB₁)-receptor signaling decreases the rewarding properties of many drugs of abuse and has been proposed as an anti-addiction strategy. However, psychiatric side-effects limit the clinical potential of orthosteric CB₁ antagonists. Negative allosteric modulators (NAMs) represent a novel and indirect approach to attenuate CB₁ signaling by decreasing affinity and/or efficacy of CB₁ ligands. We hypothesized that a CB₁-NAM would block opioid reward while avoiding the unwanted effects of orthosteric CB₁ antagonists. GAT358, a CB₁-NAM, failed to elicit cardinal signs of direct CB₁ activation or inactivation when administered by itself. GAT358 decreased catalepsy and hypothermia but not antinociception produced by the orthosteric CB₁ agonist CP55,940, suggesting that a CB₁-NAM blocked cardinal signs of CB₁ activation. Next, GAT358 was evaluated using in vivo assays of opioid-induced dopamine release and reward in male rodents. In the nucleus accumbens shell, a key component of the mesocorticolimbic reward pathway, morphine increased electrically-evoked dopamine efflux and this effect was blocked by a dose of GAT358 that lacked intrinsic effects on evoked dopamine efflux. Moreover, GAT358 blocked morphine-induced reward in a conditioned place preference (CPP) assay without producing reward or aversion alone. GAT358-induced blockade of morphine CPP was also occluded by GAT229, a CB₁ positive allosteric modulator (CB₁-PAM), and absent in CB₁-knockout mice. Finally, GAT358 also reduced oral oxycodone (but not water) consumption in a two-bottle choice paradigm. Our results support the therapeutic potential of CB₁-NAMs as novel drug candidates aimed at preventing opioid reward and treating opioid abuse while avoiding unwanted side-effects.

Covalent cannabinoid receptor ligands - structural insight and selectivity challenges

X-ray crystallography and cryogenic electronic microscopy have provided significant advancement in the knowledge of GPCR structure and have allowed the rational design of GPCR ligands. The class A GPCRs cannabinoid receptor type 1 and type 2 are implicated in many pathophysiological processes and thus rational design of drug and tool compounds is of great interest. Recent structural insight into cannabinoid receptors has already led to a greater understanding of ligand binding sites and receptor residues that likely contribute to ligand selectivity. Herein, classes of heterocyclic covalent cannabinoid receptor ligands are reviewed in light of the recent advances in structural knowledge of cannabinoid receptors, with particular discussion regarding covalent ligand selectivity and rationale design.

CB1 allosteric modulators and their therapeutic potential in CNS disorders

CB1 is the most abundant GPCR found in the mammalian brain. It has garnered considerable attention as a potential therapeutic drug target. CB1 is involved in a wide range of physiological and psychiatric processes and has the potential to be targeted in a wide range of disease states. However, most of the selective and non-selective synthetic CB1 agonists and antagonists/inverse agonists developed to date are primarily used as research tools. No novel synthetic cannabinoids are currently in the clinic for use in psychiatric illness; synthetic analogues of the phytocannabinoid THC are on the market to treat nausea and vomiting caused by cancer chemotherapy, along with off-label use for pain. Novel strategies are being explored to target CB1, but with emphasis on the elimination or mitigation of the potential psychiatric adverse effects that are observed by central agonism/antagonism of CB1. New pharmacological options are being pursued that may avoid these adverse effects while preserving the potential therapeutic benefits of CB1 modulation. Allosteric modulation of CB1 is one such approach. In this review, we will summarize and critically analyze both the in vitro characterization and in vivo validation of CB1 allosteric modulators developed to date, with a focus on CNS therapeutic effects.

Hemopressin as a breakthrough for the cannabinoid field

Hemopressin (PVNFKFLSH in rats, and PVNFKLLSH in humans and mice), a fragment derived from the α-chain of hemoglobin, was the first peptide described to have type 1 cannabinoid receptor activity. While hemopressin was shown to have inverse agonist/antagonistic activity, extended forms of hemopressin (i.e. RVD-hemopressin, also called pepcan-12) exhibit type 1 and type 2 cannabinoid receptor agonistic/allosteric activity, and recent studies suggest that they can activate intracellular mitochondrial cannabinoid receptors. Therefore, hemopressin and hemopressin-related peptides could have location-specific and biased pharmacological action, which would increase the possibilities for fine-tunning and broadening cannabinoid receptor signal transduction. Consistent with this, hemopressins were shown to play a role in a number of physiological processes including antinociceptive and anti-inflammatory activity, regulation of food intake, learning and memory. The shortest active hemopressin fragment, NFKF, delays the first seizure induced by pilocarpine, and prevents neurodegeneration in an experimental model of autoimmune encephalomyelitis. These functions of hemopressins could be due to engagement of both cannabinoid and non-cannabinoid receptor systems. Self-assembled nanofibrils of hemopressin have pH-sensitive switchable surface-active properties, and show potential as inflammation and cancer targeted drug-delivery systems. Upon disruption of the self-assembled hemopressin nanofibril emulsion, the intrinsic analgesic and anti-inflammatory properties of hemopressin could help bolster the therapeutic effect of anti-inflammatory or anti-cancer formulations. In this article, we briefly review the molecular and behavioral pharmacological properties of hemopressins, and summarize studies on the intricate and unique mode of generation and binding of these peptides to cannabinoid receptors. Thus, the review provides a window into the current status of hemopressins in expanding the repertoire of signaling and activity by the endocannabinoid system, in addition to their new potential for pharmaceutic formulations.

Modern approaches to the development of synthetic cannabinoid receptor probes

The endocannabinoid system, which spans the central and peripheral nervous systems and regulates many biologic processes, is an important target for probe discovery and medications development. Whereas the earliest endocannabinoid receptor probes were derivatives of the non-selective phytocannabinoids isolated from Cannabis species, modern drug discovery techniques have expanded the definitions of what constitutes a CB1R or CB2R cannabinoid receptor ligand. This review highlights recent advances in synthetic cannabinoid receptor chemistry and pharmacology. We provide examples of new CB1R- and CB2R-selective probes, and discuss rational approaches to the design of peripherally-restricted agents. We also describe structural classes of positive- and negative allosteric modulators (PAMs and NAMs) of CB1R and CB2R. Finally, we introduce new opportunities for cannabinoid receptor probe development that have emerged in recent years, including biased agonists that may lead to medications lacking adverse effects.

Negative allosteric modulators of cannabinoid receptor 1: Ternary complexes including CB1, orthosteric CP55940 and allosteric ORG27569

In October 2019, the first X-ray crystal structure of a ternary cannabinoid receptor 1 (CB1) complex (PDB ID: 6KQI) was published, including the well-known orthosteric agonist, CP55940, and the well-studied negative allosteric modulator, ORG27569. Prior to the release of 6KQI, we applied binding pocket analysis and molecular docking to carefully prepared computational models of the ternary CB1 complex, in order to predict the binding site for ORG27569 with the CP55940-bound CB1 receptor. We carefully studied the binding pose of agonist ligands in the CB1 orthosteric pocket, including CP55940. Our computational studies identified the most favorable binding site for ORG27569, in the CP55940-CB1 complex, to be at the intracellular end of the receptor. However, in the 6KQI structure, ORG27569 was found at an extrahelical, intramembrane site on the complex, a site that partially overlaps with the site predicted in our calculations to be second-best. We performed molecular dynamics simulations of the CP55940-bound CB1 complex with ORG27569 at different binding sites. Our analysis of the simulations indicated that ORG27569 bound favorably and stably in each simulation, but, as in the earlier calculations, bound best at the intracellular site, which is different than that found in the crystal structure. These results suggest that the intracellular site might serve as an alternative binding site in CB1. Our studies show that the computational techniques we used are valuable in identifying ligand-binding pockets in proteins, and could be useful for the study of the interaction mode of other allosteric modulators.

G protein-coupled receptors: structure- and function-based drug discovery

As one of the most successful therapeutic target families, G protein-coupled receptors (GPCRs) have experienced a transformation from random ligand screening to knowledge-driven drug design. We are eye-witnessing tremendous progresses made recently in the understanding of their structure-function relationships that facilitated drug development at an unprecedented pace. This article intends to provide a comprehensive overview of this important field to a broader readership that shares some common interests in drug discovery.

Focused structure-activity relationship profiling around the 2-phenylindole scaffold of a cannabinoid type-1 receptor agonist-positive allosteric modulator: site-III aromatic-ring congeners with enhanced activity and solubility

Specific tuning of cannabinoid 1 receptor (CB1R) activity by small-molecule allosteric modulators is a therapeutic modality with multiple properties inherently advantageous to therapeutic applications. We previously generated a library of unique CB1R positive allosteric modulators (PAMs) derived from GAT211, which has three pharmacophoric sites critical to its ago-PAM activity. To elaborate our CB1R PAM library, we report the rational design and molecular-pharmacology profiling of several 2-phenylindole analogs modified at the "site-III" aromatic ring. The comprehensive structure-activity relationship (SAR) investigation demonstrates that attaching small lipophilic functional groups on the ortho-position of the GAT211 site-III phenyl ring could markedly enhance CB1R ago-PAM activity. Select site-III modifications also improved GAT211's water solubility. The SAR reported both extends the structural diversity of this compound class and demonstrates the utility of GAT211's site-III for improving the parent compound's drug-like properties of potency and/or aqueous solubility.

Allosteric modulators targeting cannabinoid cb1 and cb2 receptors: implications for drug discovery

Allosteric modulators of cannabinoid receptors hold great therapeutic potential, as they do not possess intrinsic efficacy, but instead enhance or diminish the receptor's response of orthosteric ligands allowing for the tempering of cannabinoid receptor signaling without the desensitization, tolerance and dependence. Allosteric modulators of cannabinoid receptors have numerous advantages over the orthosteric ligands such as higher receptor type selectivity, probe dependence and biased signaling, so they have a great potential to separate the therapeutic benefits from side effects own of orthosteric ligands. This review aims to give an overview of the CB1 and CB2 receptor allosteric modulators highlighting the structure-activity relationship and pharmacological profile of each classes, and their future promise.

A Guide to Targeting the Endocannabinoid System in Drug Design

The endocannabinoid system (ECS) is one of the most crucial systems in the human organism, exhibiting multi-purpose regulatory character. It is engaged in a vast array of physiological processes, including nociception, mood regulation, cognitive functions, neurogenesis and neuroprotection, appetite, lipid metabolism, as well as cell growth and proliferation. Thus, ECS proteins, including cannabinoid receptors and their endogenous ligands’ synthesizing and degrading enzymes, are promising therapeutic targets. Their modulation has been employed in or extensively studied as a treatment of multiple diseases. However, due to a complex nature of ECS and its crosstalk with other biological systems, the development of novel drugs turned out to be a challenging task. In this review, we summarize potential therapeutic applications for ECS-targeting drugs, especially focusing on promising synthetic compounds and preclinical studies. We put emphasis on modulation of specific proteins of ECS in different pathophysiological areas. In addition, we stress possible difficulties and risks and highlight proposed solutions. By presenting this review, we point out information pivotal in the spotlight of ECS-targeting drug design, as well as provide an overview of the current state of knowledge on ECS-related pharmacodynamics and show possible directions for needed research.

Recent Advances in the Drug Discovery and Development of Dualsteric/ Bitopic Activators of G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) represent the largest family of proteins targeted by drug design and discovery efforts. Of these efforts, the development of GPCR agonists is highly desirable, due to their therapeutic robust utility in treating diseases caused by deficient receptor signaling. One of the challenges in designing potent and selective GPCR agonists lies in the inability to achieve combined high binding affinity and subtype selectivity, due to the high homology between orthosteric sites among GPCR subtypes. To combat this difficulty, researchers have begun to explore the utility of targeting topographically distinct and less conserved binding sites, namely "allosteric" sites. Pursuing these sites offers the benefit of achieving high subtype selectivity, however, it also can result in a decreased binding affinity and potency as compared to orthosteric agonists. Therefore, bitopic ligands comprised of an orthosteric agonist and an allosteric modulator connected by a spacer and allowing binding with both the orthosteric and allosteric sites within one receptor, have been developed. It may combine the high subtype selectivity of an allosteric modulator with the high binding affinity of an orthosteric agonist and provides desired advantages over orthosteric agonists or allosteric modulators alone. Herein, we review the recent advances in the development of bitopic agonists/activators for various GPCR targets and their novel therapeutic potentials.

Missing Pieces to the Endocannabinoid Puzzle

The most bioactive ingredient of cannabis (Cannabis sativa or indica) extracts, Δ⁹-tetrahydrocannabinol (THC), was identified in the 1960s as one of more than 110 phytocannabinoids. It activates receptors of chemically different endogenous ligands (endocannabinoids) that, unlike THC, are metabolized by several enzymes of the endocannabinoid system. Here, the complexity of the plant-derived and endogenous cannabinoids (eCBs) is discussed, to better appreciate the challenge of: (i) dissecting their mutual interactions; (ii) understanding their impact on human pathophysiology; and (iii) exploiting them for human disease. To this aim, missing pieces to the eCB puzzle must be urgently found, by solving the 3D structures of key components, and interrogating noncanonical modes of regulation and trafficking of these lipid signals.

Allosteric Modulation of Cannabinoid Receptor 1-Current Challenges and Future Opportunities

The cannabinoid receptor type 1 (CB1R), a G protein-coupled receptor (GPCR), plays an essential role in the control of many physiological processes such as hunger, memory loss, gastrointestinal activity, catalepsy, fear, depression, and chronic pain. Therefore, it is an attractive target for drug discovery to manage pain, neurodegenerative disorders, obesity, and substance abuse. However, the psychoactive adverse effects, generated by CB1R activation in the brain, limit the use of the orthosteric CB1R ligands as drugs. The discovery of CB1R allosteric modulators during the last decade provided new tools to target the CB1R. Moreover, application of the site-directed mutagenesis in combination with advanced physical methods, especially X-ray crystallography and computational modeling, has opened new horizons for understanding the complexity of the structure, function, and activity of cannabinoid receptors. In this paper, we present the latest advances in research on the CB1R, its allosteric modulation and allosteric ligands, and their translational potential. We focused on structural essentials of the cannabinoid 1 receptor- ligand (drug) interactions, as well as modes of CB1R signaling regulation.

Structure of an allosteric modulator bound to the CB1 cannabinoid receptor

The CB1 receptor mediates the central nervous system response to cannabinoids, and is a drug target for pain, anxiety and seizures. CB1 also responds to allosteric modulators, which influence cannabinoid binding and efficacy. To understand the mechanism of these compounds, we solved the crystal structure of CB1 with the negative allosteric modulator (NAM) ORG27569 and the agonist CP55940. The structure reveals that the NAM binds to an extrahelical site within the inner leaflet of the membrane, which overlaps with a conserved site of cholesterol interaction in many G protein-coupled receptors (GPCRs). The ternary structure with ORG27569 and CP55940 captures an intermediate state of the receptor, in which aromatic residues at the base of the agonist-binding pocket adopt an inactive conformation despite the large contraction of the orthosteric pocket. The structure illustrates a potential strategy for drug modulation of CB1 and other class A GPCRs.

Indomethacin Enhances Type 1 Cannabinoid Receptor Signaling

In addition to its known actions as a non-selective cyclooxygenase (COX) 1 and 2 inhibitor, we hypothesized that indomethacin can act as an allosteric modulator of the type 1 cannabinoid receptor (CB1R) because of its shared structural features with the known allosteric modulators of CB1R. Indomethacin enhanced the binding of [³H]CP55940 to hCB1R and enhanced AEA-dependent [³⁵S]GTPγS binding to hCB1R in Chinese hamster ovary (CHO) cell membranes. Indomethacin (1 μM) also enhanced CP55940-dependent βarrestin1 recruitment, cAMP inhibition, ERK1/2 and PLCβ3 phosphorylation in HEK293A cells expressing hCB1R, but not in cells expressing hCB2R. Finally, indomethacin enhanced the magnitude and duration of CP55940-induced hypolocomotion, immobility, hypothermia, and anti-nociception in C57BL/6J mice. Together, these data support the hypothesis that indomethacin acted as a positive allosteric modulator of hCB1R. The identification of structural and functional features shared amongst allosteric modulators of CB1R may lead to the development of novel compounds designed for greater CB1R or COX selectivity and compounds designed to modulate both the prostaglandin and endocannabinoid systems.

Cannabidiol binding and negative allosteric modulation at the cannabinoid type 1 receptor in the presence of delta-9-tetrahydrocannabinol: An In Silico study

Recent evidence has raised in discussion the possibility that cannabidiol can act as a negative allosteric modulator of the cannabinoid type 1 receptor. Here we have used computational methods to study the modulation exerted by cannabidiol on the effects of delta-9-tetrahydrocannabinol in the cannabinoid receptor type 1 and the possibility of direct receptor blockade. We propose a putative allosteric binding site that is located in the N-terminal region of receptor, partially overlapping the orthosteric binding site. Molecular dynamics simulations reveled a coordinated movement involving the outward rotation of helixes 1 and 2 and subsequent expansion of the orthosteric binding site upon cannabidiol binding. Finally, changes in the cytoplasmic region and high helix 8 mobility were related to impaired receptor internalization. Together, these results offer a possible explanation to how cannabidiol can directly modulate effects of delta-9-tetrahydrocannabinol on the cannabinoid receptor type 1.

Structural Insights into CB1 Receptor Biased Signaling

The endocannabinoid system has emerged as a promising target for the treatment of numerous diseases, including cancer, neurodegenerative disorders, and metabolic syndromes. Thus far, two cannabinoid receptors, CB1 and CB2, have been discovered, which are found predominantly in the central nervous system (CB1) or the immune system (CB2), among other organs and tissues. CB1 receptor ligands have been shown to induce a complex pattern of intracellular effects. The binding of a ligand induces distinct conformational changes in the receptor, which will eventually translate into distinct intracellular signaling pathways through coupling to specific intracellular effector proteins. These proteins can mediate receptor desensitization, trafficking, or signaling. Ligand specificity and selectivity, complex cellular components, and the concomitant expression of other proteins (which either regulate the CB1 receptor or are regulated by the CB1 receptor) will affect the therapeutic outcome of its targeting. With an increased interest in G protein-coupled receptors (GPCR) research, in-depth studies using mutations, biological assays, and spectroscopic techniques (such as NMR, EPR, MS, FRET, and X-ray crystallography), as well as computational modelling, have begun to reveal a set of concerted structural features in Class A GPCRs which relate to signaling pathways and the mechanisms of ligand-induced activation, deactivation, or activity modulation. This review will focus on the structural features of the CB1 receptor, mutations known to bias its signaling, and reported studies of CB1 receptor ligands to control its specific signaling.

The Endocannabinoid System as a Target in Cancer Diseases: Are We There Yet?

The endocannabinoid system (ECS) has been placed in the anti-cancer spotlight in the last decade. The immense data load published on its dual role in both tumorigenesis and inhibition of tumor growth and metastatic spread has transformed the cannabinoid receptors CB1 (CB1R) and CB2 (CB2R), and other members of the endocannabinoid-like system, into attractive new targets for the treatment of various cancer subtypes. Although the clinical use of cannabinoids has been extensively documented in the palliative setting, clinical trials on their application as anti-cancer drugs are still ongoing. As drug repurposing is significantly faster and more economical than de novo introduction of a new drug into the clinic, there is hope that the existing pharmacokinetic and safety data on the ECS ligands will contribute to their successful translation into oncological healthcare. CB1R and CB2R are members of a large family of membrane proteins called G protein-coupled receptors (GPCR). GPCRs can form homodimers, heterodimers and higher order oligomers with other GPCRs or non-GPCRs. Currently, several CB1R and CB2R-containing heteromers have been reported and, in cancer cells, CB2R form heteromers with the G protein-coupled chemokine receptor CXCR4, the G protein-coupled receptor 55 (GPR55) and the tyrosine kinase receptor (TKR) human V-Erb-B2 Avian Erythroblastic Leukemia Viral Oncogene Homolog 2 (HER2). These protein complexes possess unique pharmacological and signaling properties, and their modulation might affect the antitumoral activity of the ECS. This review will explore the potential of the endocannabinoid network in the anti-cancer setting as well as the clinical and ethical pitfalls behind it, and will develop on the value of cannabinoid receptor heteromers as potential new targets for anti-cancer therapies and as prognostic biomarkers.

Diarylureas Containing 5-Membered Heterocycles as CB1 Receptor Allosteric Modulators: Design, Synthesis, and Pharmacological Evaluation

Allosteric modulators have attracted significant interest as an alternate strategy to modulate CB₁ receptor signaling for therapeutic benefits that may avoid the adverse effects associated with orthosteric ligands. Here we extended our previous structure-activity relationship studies on the diarylurea-based CB₁ negative allosteric modulators (NAMs) by introducing five-membered heterocycles to replace the 5-pyrrolidinylpyridinyl group in PSNCBAM-1 (1), one of the first generation CB₁ allosteric modulators. Many of these compounds had comparable potency to 1 in blocking the CB₁ agonist CP55,940 stimulated calcium mobilization and [³⁵S]GTP-γ-S binding. Similar to 1, most compounds showed positive cooperativity by increasing [³H]CP55,940 binding, consistent with the positive allosteric modulator (PAM)-antagonist mechanism. Interestingly, these compounds exhibited differences in ability to increase specific binding of [³H]CP55,940 and decrease binding of the antagonist [³H]SR141716. In saturation binding studies, only increases in [³H]CP55,940 B_max, but not K_d, were observed, suggesting that these compounds stabilize low affinity receptors into a high affinity state. Among the series, the 2-pyrrolyl analogue (13) exhibited greater potency than 1 in the [³⁵S]GTP-γ-S binding assay and significantly enhanced the maximum binding level in the [³H]CP5,5940 binding assay, indicating greater CB₁ receptor affinity and/or cooperativity.

GPCR Allosteric Modulator Discovery

G protein-coupled receptors (GPCRs) influence virtually every aspect of human physiology; about one-third of all marketed drugs target members of this family. GPCR allosteric ligands hold the promise of improved subtype selectivity, spatiotemporal sensitivity, and possible biased property over typical orthosteric ligands. However, only a small number of GPCR allosteric ligands have been approved as drugs or in clinical trials since the discovery process is very challenging. The rapid development of GPCR structural biology leads to the discovery of several allosteric sites and sheds light on understanding the mechanism of GPCR allosteric ligands, which is critical for discovering novel therapeutics. This book chapter summarized different GPCR allosteric modulating mechanisms and discussed validated mechanisms based on allosteric modulator-GPCR complex structures.

Potential for endocannabinoid system modulation in ocular pain and inflammation: filling the gaps in current pharmacological options

Challenges in the management of ocular pain are an underappreciated topic. Currently available therapeutics lack both efficacy and clear guidelines for their use, with many also possessing unacceptable side effects. Promising novel agents would offer analgesic, anti-inflammatory, and possibly neuroprotective actions; have favorable ocular safety profiles; and show potential in managing neuropathic pain. Growing evidence supports a link between the endocannabinoid system (ECS) and a range of physiological and disease processes, notably those involving inflammation and pain. Both preclinical and clinical data suggest analgesic and anti-inflammatory actions of cannabinoids and ECS-modifying drugs in chronic pain conditions, including those of neuropathic origin. This review will examine existing evidence for the anatomical and physiological basis of ocular pain, specifically, ocular surface disease and the development of chronic ocular pain. The mechanism of action, efficacy, and limitations of currently available treatments will be discussed, and current knowledge related to ECS-modulation of ocular pain and inflammatory disease will be summarized. A perspective will be provided on the future directions of ECS research in terms of developing cannabinoid therapeutics for ocular pain.

Disordered Peptides Looking for Their Native Environment: Structural Basis of CB1 Endocannabinoid Receptor Binding to Pepcans

Endocannabinoid peptides, or “pepcans,” are endogenous ligands of the CB1 cannabinoid receptor. Depending on their length, they display diverse activity: For instance, the nona-peptide Pepcan-9, also known as hemopressin, is a powerful inhibitor of CB1, whereas the longer variant Pepcan-12, which extends by only three amino acid residues at the N-terminus, acts on both CB1 and CB2 as an allosteric modulator, although with diverse effects. Despite active research on their pharmacological applications, very little is known about structure-activity relationships of pepcans. Different structures have been proposed for the nona-peptide, which has also been reported to form fibrillar aggregates. This might have affected the outcome and reproducibility of bioactivity studies. In an attempt of elucidating the determinants of both biological activity and aggregation propensity of Pepcan-9 and Pepcan-12, we have performed their structure characterization in solvent systems characterized by different polarity and pH. We have found that, while disordered in aqueous environment, both peptides display helical structure in less polar environment, mimicking the proteic receptor milieu. In the case of Pepcan-9, this structure is fully consistent with the observed modulation of the CB1. For Pepcan-12, whose allosteric binding site is still unknown, the presented structure is compatible with the binding at one of the previously proposed allosteric sites on CB1. These findings open the way to structure-driven design of selective peptide modulators of CB1.

Positive Allosteric Modulation of Cannabinoid Receptor Type 1 Suppresses Pathological Pain Without Producing Tolerance or Dependence

BACKGROUND

Activation of cannabinoid CB₁ receptors suppresses pathological pain but also produces unwanted central side effects. We hypothesized that a positive allosteric modulator of CB₁ signaling would suppress inflammatory and neuropathic pain without producing cannabimimetic effects or physical dependence. We also asked whether a CB₁ positive allosteric modulator would synergize with inhibitors of endocannabinoid deactivation and/or an orthosteric cannabinoid agonist.

METHODS

GAT211, a novel CB₁ positive allosteric modulator, was evaluated for antinociceptive efficacy and tolerance in models of neuropathic and/or inflammatory pain. Cardinal signs of direct CB₁-receptor activation were evaluated together with the propensity to induce reward or aversion and physical dependence. Comparisons were made with inhibitors of endocannabinoid deactivation (JZL184, URB597) or an orthosteric cannabinoid agonist (WIN55,212-2). All studies used 4 to 11 subjects per group.

RESULTS

GAT211 suppressed allodynia induced by complete Freund's adjuvant and the chemotherapeutic agent paclitaxel in wild-type but not CB₁ knockout mice. GAT211 did not impede paclitaxel-induced tumor cell line toxicity. GAT211 did not produce cardinal signs of direct CB₁-receptor activation in the presence or absence of pathological pain. GAT211 produced synergistic antiallodynic effects with fatty acid amide hydrolase and monoacylglycerol lipase inhibitors in paclitaxel-treated mice. Therapeutic efficacy was preserved over 19 days of chronic dosing with GAT211, but it was not preserved with the monoacylglycerol lipase inhibitor JZL184. The CB₁ antagonist rimonabant precipitated withdrawal in mice treated chronically with WIN55,212-2 but not in mice treated with GAT211. GAT211 did not induce conditioned place preference or aversion.

CONCLUSIONS

Positive allosteric modulation of CB₁-receptor signaling shows promise as a safe and effective analgesic strategy that lacks tolerance, dependence, and abuse liability.

Allosteric and orthosteric pharmacology of cannabidiol and cannabidiol-dimethylheptyl at the type 1 and type 2 cannabinoid receptors

BACKGROUND AND PURPOSE

We sought to understand why (-)-cannabidiol (CBD) and (-)-cannabidiol-dimethylheptyl (CBD-DMH) exhibit distinct pharmacology, despite near identical structures.

EXPERIMENTAL APPROACH

HEK293A cells expressing either human type 1 cannabinoid (CB₁ ) receptors or CB₂ receptors were treated with CBD or CBD-DMH with or without the CB₁ and CB₂ receptor agonist CP55,940, CB₁ receptor allosteric modulator Org27569 or CB₂ receptor inverse agonist SR144528. Ligand binding, cAMP levels and βarrestin1 recruitment were measured. CBD and CBD-DMH binding was simulated with models of human CB₁ or CB₂ receptors, based on the recently published crystal structures of agonist-bound (5XRA) or antagonist-bound (5TGZ) human CB₁ receptors.

KEY RESULTS

At CB₁ receptors, CBD was a negative allosteric modulator (NAM), and CBD-DMH was a mixed agonist/positive allosteric modulator. CBD and Org27569 shared multiple interacting residues in the antagonist-bound model of CB₁ receptors (5TGZ) but shared a binding site with CP55,940 in the agonist-bound model of CB₁ receptors (5XRA). The binding site for CBD-DMH in the CB₁ receptor models overlapped with CP55,940 and Org27569. At CB₂ receptors, CBD was a partial agonist, and CBD-DMH was a positive allosteric modulator of cAMP modulation but a NAM of βarrestin1 recruitment. CBD, CP55,940 and SR144528 shared a binding site in the CB₂ receptor models that was separate from CBD-DMH.

CONCLUSION AND IMPLICATIONS

The pharmacological activity of CBD and CBD-DMH in HEK293A cells and their modelled binding sites at CB₁ and CB₂ receptors may explain their in vivo effects and illuminates the difficulties associated with the development of allosteric modulators for CB₁ and CB₂ receptors.

LINKED ARTICLES

This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Emerging strategies targeting CB2 cannabinoid receptor: Biased agonism and allosterism

During these last years, the CB₂ cannabinoid receptor has emerged as a potential anti-inflammatory target in diseases such as multiple sclerosis, amyotrophic lateral sclerosis, Huntington's disease, ischemic stroke, autoimmune diseases, osteoporosis, and cancer. However, the development of clinically useful CB₂ agonists reveals to be very challenging. Allosterism and biased-signaling mechanisms at CB₂ receptor may offer new avenues for the development of improved CB₂ receptor-targeted therapies. Although there has been some exploration of CB₁ receptor activation by new CB₁ allosteric or biased-signaling ligands, the CB₂ receptor is still at initial stages in this domain. In an effort to understand the molecular basis behind these pharmacological approaches, we have analyzed and summarized the structural data reported so far at CB₂ receptor.

Identification of the First Synthetic Allosteric Modulator of the CB2 Receptors and Evidence of Its Efficacy for Neuropathic Pain Relief

The direct activation of cannabinoid receptors (CBRs) results in several beneficial effects; therefore several CBRs ligands have been synthesized and tested in vitro and in vivo. However, none of them reached an advanced phase of clinical development due mainly to side effects on the CNS. Medicinal chemistry approaches are now engaged to develop allosteric modulators that might offer a novel therapeutic approach to achieve potential therapeutic benefits avoiding inherent side effects of orthosteric ligands. Here we identify the first ever synthesized positive allosteric modulator (PAM) that targets CB₂Rs. The evidence for this was obtained using [³H]CP55940 and [³⁵S]GTPγS binding assays. This finding will be useful for the characterization of allosteric binding site(s) on CB₂Rs which will be essential for the further development of CB₂R allosteric modulators. Moreover, the new CB₂R PAM displayed antinociceptive activity in vivo in an experimental mouse model of neuropathic pain, raising the possibility that it might be a good candidate for clinical development.

Allosteric modulators of cannabinoid receptor 1: developing compounds for improved specificity

The cannabinoid receptor 1 (CB1) is a G protein-coupled receptor (GPCR) that is located primarily in the central nervous system. CB1 is a therapeutic target which may impact pathways to mediate pain, neurodegenerative disorders, hunger, and drug-seeking behavior. Despite these benefits, development of orthosteric therapeutic compounds, which target the endogenous ligand-binding site of CB1, has been challenging due to detrimental side effects including psychoactivity, depression, and suicidal thoughts. However, CB1 also has an allosteric binding site(s), which is topographically distinct from the orthosteric site. Allosteric modulation of CB1 has a number of potential advantages including providing a mechanism for more precise control of downstream pathways and circumventing these side effects. In this review, we summarize the concept of allosteric modulation and focus on the structure-activity relationship studies of the well-characterized allosteric modulators, ORG27569 and PSNCBAM-1 and their derivatives, and a few other recent modulators. We review studies on the properties of these modulators on CB1 signaling in cells and their effects in vivo. While many current allosteric modulators also produce complex outcomes, they provide new advances for the design of CB1 centered therapeutics.

The future of type 1 cannabinoid receptor allosteric ligands

Allosteric modulation of the type 1 cannabinoid receptor (CB1R) holds great therapeutic potential. This is because allosteric modulators do not possess intrinsic efficacy, but instead augment (positive allosteric modulation) or diminish (negative allosteric modulation) the receptor's response to endogenous ligand. Consequently, CB1R allosteric modulators have an effect ceiling which allows for the tempering of CB1R signaling without the desensitization, tolerance, dependence, and psychoactivity associated with orthosteric compounds. Pain, movement disorders, epilepsy, obesity are all potential therapeutic targets for CB1R allosteric modulation. Several challenges exist for the development of CB1R allosteric modulators, such as receptor subtype specificity, translation to in vivo systems, and mixed allosteric/agonist/inverse agonist activity. Despite these challenges, elucidation of crystal structures of CB1R and compound design based on structure-activity relationships will advance the field. In this review, we will cover recent progress for CB1R allosteric modulators and discuss the future promise of this research.

In silico mapping of allosteric ligand binding sites in type-1 cannabinoid receptor

The recent resolution of the crystal structure of type-1 cannabinoid receptor (CB₁ ) and the discovery of novel modulators for this target open the way to the possibility of elucidating the structural requirements for CB₁ binding, and thereby facilitate a rational drug design. Compounds that target the orthosteric site of CB₁ in some cases have shown side effects. Allosteric modulators could potentially avoid these side effects by influencing binding and/or efficacy of orthosteric ligands. Here, we summarize and compare previous data on different putative allosteric binding sites observed in CB₁ homology models with an in silico docking study of the recently published crystal structure of the same receptor on endogenous and natural hydrophobic ligands that act as positive allosteric modulators and negative allosteric modulators of CB₁ . In particular, a lipid-exposed pocket targeted by most of the tested molecules is reported and discussed.

Preclinical Studies of Cannabinoid Reward, Treatments for Cannabis Use Disorder, and Addiction-Related Effects of Cannabinoid Exposure

Cannabis use has become increasingly accepted socially and legally, for both recreational and medicinal purposes. Without reliable information about the effects of cannabis, people cannot make informed decisions regarding its use. Like alcohol and tobacco, cannabis can have serious adverse effects on health, and some people have difficulty discontinuing their use of the drug. Many cannabis users progress to using and becoming addicted to other drugs, but the reasons for this progression are unclear. The natural cannabinoid system of the brain is complex and involved in many functions, including brain development, reward, emotion, and cognition. Animal research provides an objective and controlled means of obtaining information about: (1) how cannabis affects the brain and behavior, (2) whether medications can be developed to treat cannabis use disorder, and (3) whether cannabis might produce lasting changes in the brain that increase the likelihood of becoming addicted to other drugs. This review explains the tactics used to address these issues, evaluates the progress that has been made, and offers some directions for future research.

Modulation of CB1 cannabinoid receptor by allosteric ligands: Pharmacology and therapeutic opportunities

Cannabinoid pharmacology has been intensely studied because of cannabis' pervasive medicinal and non-medicinal uses as well as for the therapeutic potential of cannabinoid-based drugs for the treatment of pain, anxiety, substance abuse, obesity, cancer and neurodegenerative disorders. The identification of allosteric modulators of the cannabinoid receptor 1 (CB1) has given a new direction to the development of cannabinoid-based therapeutics due to the many advantages offered by targeting allosteric site(s). Allosteric receptor modulators hold potential to develop subtype-specific and pathway-specific therapeutics. Here we briefly discuss the first-generation of allosteric modulators of CB1 receptor, their structure-activity relationships, signaling pathways and the allosteric binding site(s) on the CB1 receptor. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

The great divide: Separation between in vitro and in vivo effects of PSNCBAM-based CB1 receptor allosteric modulators

While allosteric modulators of the cannabinoid type-1 receptor (CB₁) continue to be developed and characterized, the gap between the in vitro and in vivo data is widening, raising questions regarding translatability of their effects and biological relevance. Among the CB₁ allosteric modulators, PSNCBAM-1 has received little attention regarding its effects in vivo. Recently, pregnenolone was reported to act as an allosteric modulator of CB₁, blocking THC's effects in vitro and in vivo, highlighting the potential of CB₁ allosteric modulators for treatment of cannabis intoxication. We investigated the pharmacological effects of PSNCBAM-1 and two structural analogs, RTICBM-15 and -28, as well as pregnenolone, in both signaling and behavioral assays including [³⁵S]GTPγS binding, the cannabinoid tetrad and drug discrimination. While the CB₁ allosteric modulator PSNCBAM-1 attenuated THC-induced anti-nociception and its structural analog RTICBM-28 reduced THC's potency in drug discrimination, most cannabinoid effects in mice were unaffected. In contrast to the mouse studies, PSNCBAM-1 and analogs insurmountably antagonized CP55,940- and THC-stimulated [³⁵S]GTPγS binding and exhibited negative binding cooperativity with [³H]SR141716 with similar apparent affinities. Notably, RTICBM-28, which contains a cyano substitution at the 4-chlorophenyl position of PSNCBAM-1, exhibited enhanced binding cooperativity with CP55,940. In contrast to previous findings, pregnenolone did not block THC's effects in drug discrimination or [³⁵S]GTPγS. These data further highlight the difficulty in translating pharmacological effects of CB₁ allosteric modulators in vivo but confirm the established pharmacology of PSNCBAM-1 and analogs in molecular assays of CB₁ receptor function.

Assessing Allosteric Modulation of CB1 at the Receptor and Cellular Levels

The cannabinoid CB1 receptor is abundant in the central nervous system and regulates neuronal transmission and other key physiological processes including those leading to pain, inflammation, memory, and feeding behavior. CB1 is activated by the endogenous ligands, arachidonoyl ethanolamine and 2-arachidonoyl glycerol, by various synthetic ligands (e.g., CP55940), and by Δ9-tetrahydrocannabinol, the psychoactive component of Cannabis sativa. These CB1 ligands are orthosteric and transduce downstream signals by binding CB1 and primarily inducing Gi coupling, but Gs and β-arrestin coupling are also possible. Recently, allosteric modulators for CB1 were discovered that bind to topographically distinct sites and can noncompetitively impact the potency and efficacy of orthosteric compounds. These offer the exciting potential for mechanistic analyses and for developing therapeutics. Yet, it is critical to elucidate whether a compound is a positive allosteric modulator or a negative allosteric modulator of orthosteric ligand-induced CB1 profiles to understand pathway specificity and ameliorate diseases. In this chapter, we present equilibrium and kinetic binding analysis to reveal the impact of allosteric modulators on CB1. Also described are activities consistent with CB1 activation (or inactivation) and include cellular internalization of CB1 and downstream signaling patterns. Since many CB1 allosteric modulators do not enhance G protein coupling, it is critical to distinguish CB1 activation and biased signaling patterns via β-arrestin from CB1 inactivation. These strategies can illuminate pathway specificity and are valuable for the fine-tuning of CB1 function.

Molecular Dynamics Methodologies for Probing Cannabinoid Ligand/Receptor Interaction

The cannabinoid type 1 and 2 G-protein-coupled receptors are currently important pharmacological targets with significant drug discovery potential. These receptors have been shown to display functional selectivity or biased agonism, a property currently thought to have substantial therapeutic potential. Although recent advances in crystallization techniques have provided a wealth of structural information about this important class of membrane-embedded proteins, these structures lack dynamical information. In order to fully understand the interplay of structure and function for this important class of proteins, complementary techniques that address the dynamical aspects of their function are required such as NMR as well as a variety of other spectroscopies. Complimentary to these experimental approaches is molecular dynamics, which has been effectively used to help unravel, at the atomic level, the dynamics of ligand binding and activation of these membrane-bound receptors. Here, we discuss and present several representative examples of the application of molecular dynamics simulations to the understanding of the signatures of ligand-binding and -biased signaling at the cannabinoid type 1 and 2 receptors.

Enantiospecific Allosteric Modulation of Cannabinoid 1 Receptor

The cannabinoid 1 receptor (CB1R) is one of the most widely expressed metabotropic G protein-coupled receptors in brain, and its participation in various (patho)physiological processes has made CB1R activation a viable therapeutic modality. Adverse psychotropic effects limit the clinical utility of CB1R orthosteric agonists and have promoted the search for CB1R positive allosteric modulators (PAMs) with the promise of improved drug-like pharmacology and enhanced safety over typical CB1R agonists. In this study, we describe the synthesis and in vitro and ex vivo pharmacology of the novel allosteric CB1R modulator GAT211 (racemic) and its resolved enantiomers, GAT228 (R) and GAT229 (S). GAT211 engages CB1R allosteric site(s), enhances the binding of the orthosteric full agonist [³H]CP55,490, and reduces the binding of the orthosteric antagonist/inverse agonist [³H]SR141716A. GAT211 displayed both PAM and agonist activity in HEK293A and Neuro2a cells expressing human recombinant CB1R (hCB1R) and in mouse-brain membranes rich in native CB1R. GAT211 also exhibited a strong PAM effect in isolated vas deferens endogenously expressing CB1R. Each resolved and crystallized GAT211 enantiomer showed a markedly distinctive pharmacology as a CB1R allosteric modulator. In all biological systems examined, GAT211's allosteric agonist activity resided with the R-(+)-enantiomer (GAT228), whereas its PAM activity resided with the S-(-)-enantiomer (GAT229), which lacked intrinsic activity. These results constitute the first demonstration of enantiomer-selective CB1R positive allosteric modulation and set a precedent whereby enantiomeric resolution can decisively define the molecular pharmacology of a CB1R allosteric ligand.

Human Cannabinoid Receptor 2 Ligand-Interaction Motif: Transmembrane Helix 2 Cysteine, C2.59(89), as Determinant of Classical Cannabinoid Agonist Activity and Binding Pose

Cannabinoid receptor 2 (CB2R)-dependent signaling is implicated in neuronal physiology and immune surveillance by brain microglia. Selective CB2R agonists hold therapeutic promise for inflammatory and other neurological disorders. Information on human CB2R (hCB2R) ligand-binding and functional domains is needed to inform the rational design and optimization of candidate druglike hCB2R agonists. Prior demonstration that hCB2R transmembrane helix 2 (TMH2) cysteine C2.59(89) reacts with small-molecule methanethiosulfonates showed that this cysteine residue is accessible to sulfhydryl derivatization reagents. We now report the design and application of two novel, pharmacologically active, high-affinity molecular probes, AM4073 and AM4099, as chemical reporters to interrogate directly the interaction of classical cannabinoid agonists with hCB2R cysteine residues. AM4073 has one electrophilic isothiocyanate (NCS) functionality at the C9 position of its cyclohexenyl C-ring, whereas AM4099 has NCS groups at that position and at the terminus of its aromatic A-ring C3 side chain. Pretreatment of wild-type hCB2R with either probe reduced subsequent [³H]CP55,940 specific binding by ∼60%. Conservative serine substitution of any hCB2R TMH cysteine residue except C2.59(89) did not affect the reduction of [³H]CP55,940 specific binding by either probe, suggesting that AM4073 and AM4099 interact irreversibly with this TMH2 cysteine. In contrast, AM841, an exceptionally potent hCB2R megagonist and direct AM4073/4099 congener bearing a single electrophilic NCS group at the terminus of its C3 side chain, had been demonstrated to bind covalently to TMH6 cysteine C6.47(257) and not C2.59(89). Molecular modeling indicates that the AM4073-hCB2R* interaction at C2.59(89) orients this classical cannabinoid away from TMH6 and toward the TMH2-TMH3 interface in the receptor's hydrophobic binding pocket, whereas the AM841-hCB2R* interaction at C6.47(257) favors agonist orientation toward TMH6/7. These data constitute initial evidence that TMH2 cysteine C2.59(89) is a component of the hCB2R binding pocket for classical cannabinoids. The results further demonstrate how interactions between classical cannabinoids and specific amino acids within the hCB2R* ligand-binding domain act as determinants of agonist pharmacological properties and the architecture of the agonist-hCB2R* conformational ensemble, allowing the receptor to adopt distinct activity states, such that interaction of classical cannabinoids with TMH6 cysteine C6.47(257) favors a binding pose more advantageous for agonist potency than does their interaction with TMH2 cysteine C2.59(89).

The role of CB1 in intestinal permeability and inflammation

The endocannabinoid system has previously been shown to play a role in the permeability and inflammatory response of the human gut. The goal of our study was to determine the effects of endogenous anandamide (AEA) and 2-arachidonoyl glycerol (2-AG) on the permeability and inflammatory response of intestinal epithelium under normal, inflammatory, and hypoxic conditions. Human intestinal mucosa was modeled using Caco-2 cells. Human tissue was collected from planned colorectal resections. Accumulation of AEA and 2-AG was achieved by inhibiting their metabolizing enzymes URB597 (a fatty acid amide hydrolase inhibitor) and JZL184 (a monoacylglycerol lipase inhibitor). Inflammation and ischemia were simulated with TNF-α and IFN-γ and oxygen deprivation. Permeability changes were measured by transepithelial electrical resistance. The role of the CB₁ receptor was explored using CB₁-knockdown (CB₁Kd) intestinal epithelial cells. Endocannabinoid levels were measured using liquid chromatography-mass spectrometry. Cytokine secretion was measured using multiplex and ELISA. URB597 and JZL184 caused a concentration-dependent increase in permeability via CB₁ (P < 0.0001) and decreased cytokine production. Basolateral application of JZL184 decreased permeability via CB₁ (P < 0.0001). URB597 and JZL184 increased the enhanced (worsened) permeability caused by inflammation and hypoxia (P < 0.0001 and P < 0.05). CB₁Kd cells showed reduced permeability response to inflammation (P < 0.01) but not hypoxia. 2-AG levels were increased in response to inflammation and hypoxia in Caco-2 cells. In human mucosal tissue, inflammation increased the secretion of granulocyte macrophage-colony stimulating factor, IL-12, -13, and -15, which was prevented with ex vivo treatment with URB597 and JZL184, and was inhibited by a CB₁ antagonist. The results of this study show that endogenous AEA and 2-AG production and CB₁ activation play a key modulatory roles in normal intestinal mucosa permeability and in inflammatory and hypoxic conditions.-Karwad, M. A., Couch, D. G., Theophilidou, E., Sarmad, S., Barrett, D. A., Larvin, M., Wright, K. L., Lund, J. N., O'Sullivan, S. E. The role of CB₁ in intestinal permeability and inflammation.

CB1 allosteric modulator Org27569 is an antagonist/inverse agonist of ERK1/2 signaling

Introduction: Allosteric modulation of cannabinoid type-1 receptors (CB₁) is a novel means through which signaling bias may be exerted. Org27569 remains the most-characterized CB₁ allosteric modulator, yet there are conflicting reports regarding its effects on extracellular signal-regulated kinase (ERK) signaling. We conducted a systematic evaluation of Org27569's effects on cannabinoid agonists and ERK signaling.

Materials and Methods: HEK293 cells transfected with the human cannabinoid type-1 receptor (hCB1) were treated with Org27569 alone or in combination with the endocannabinoid 2-arachidonoylglycerol (2-AG), the synthetic cannabinoid CP55,940, or the phytocannabinoid delta-9-tetrahydrocannabinol (THC) and ERK activation was measured by western blot. Overnight treatment with pertussis toxin (PTX) was used to determine the role of G_i/o in Org27569's inverse agonist effects. HEK293 cells transfected with green fluorescent protein tagged rat CB₁ receptor were used to assess effects of Org27569 on CP55,940-induced receptor internalization. Subcellular fractionation was used to determine effects of Org27569 on ERK phosphorylation in both nuclear and cytosolic compartments.

Results: We found that Org27569 is an antagonist of hCB₁-mediated ERK signaling in HEK293 cells where it fully blocks CP55,940-but does not completely inhibit THC- and 2-AG-stimulated ERK1/2 activation following 5 min treatment. In rat CB₁ HEK293 cells, CP55,940 (1 μM) treatment produced a significant increase in puncta at 20, 40, 60, and 120 min, consistent with receptor internalization. Org27569 (10 μM) co-treatment prevented internalization at each time point and alone had no effect. Org27569 reduced basal ERK phosphorylation in hCB₁ HEK293 cells but not in untransfected cells following 20 min treatment. Overnight treatment with PTX abated this response. Following subcellular fractionation, Org27569 produced a significant decrease in ERK phosphorylation in the nuclear-enriched and cytosolic fractions.

Conclusions: These data are consistent with previous studies demonstrating that CB₁-mediated ERK1/2 activation is G_i/o-dependent and that Org27569 is an inverse agonist of CB₁ receptors. Abrogation of Org27569's ability to reduce basal ERK phosphorylation following treatment with PTX and lack of inverse agonist effects in untransfected HEK293 cells demonstrates that Org27569 acts via CB₁-G_i/o to produce this effect. To our knowledge, this is the first reported demonstration of inverse agonism of ERK signaling by Org27569.

Allosteric Modulation: An Alternate Approach Targeting the Cannabinoid CB1 Receptor

The cannabinoid CB1 receptor is a G protein coupled receptor and plays an important role in many biological processes and physiological functions. A variety of CB1 receptor agonists and antagonists, including endocannabinoids, phytocannabinoids, and synthetic cannabinoids, have been discovered or developed over the past 20 years. In 2005, it was discovered that the CB1 receptor contains allosteric site(s) that can be recognized by small molecules or allosteric modulators. A number of CB1 receptor allosteric modulators, both positive and negative, have since been reported and importantly, they display pharmacological characteristics that are distinct from those of orthosteric agonists and antagonists. Given the psychoactive effects commonly associated with CB1 receptor agonists and antagonists/inverse agonists, allosteric modulation may offer an alternate approach to attain potential therapeutic benefits while avoiding inherent side effects of orthosteric ligands. This review details the complex pharmacological profiles of these allosteric modulators, their structure-activity relationships, and efforts in elucidating binding modes and mechanisms of actions of reported CB1 allosteric modulators. The ultimate development of CB1 receptor allosteric ligands could potentially lead to improved therapies for CB1-mediated neurological disorders.

Leveraging allostery to improve G protein-coupled receptor (GPCR)-directed therapeutics: cannabinoid receptor 1 as discovery target

INTRODUCTION

Allosteric modulators of G-protein coupled receptors (GPCRs) hold the promise of improved pharmacology and safety over typical orthosteric GPCR ligands. These features are particularly relevant to the cannabinoid receptor 1 (CB1R) GPCR, since typical orthosteric CB1R ligands are associated with adverse events that limit their translational potential. Areas covered: The contextual basis for applying allostery to CB1R is considered from pharmacological, drug-discovery, and medicinal standpoints. Rational design of small-molecule CB1R allosteric modulators as potential pharmacotherapeutics would be greatly facilitated by direct experimental characterization of structure-function correlates underlying the biological activity of chemically-diverse CB1R allosteric modulators, CB1R allosteric ligand-binding binding pockets, and amino acid contact residues critical to allosteric ligand engagement and activity. In these regards, designer covalent probes exhibiting well-characterized molecular pharmacology as CB1R allosteric modulators are emerging as valuable molecular reporters enabling experimental interrogation of CB1R allosteric site(s) and informing the design of new CB1R agents as drugs. Expert opinion: Synthesis and pharmacological profiling of CB1R allosteric ligands will continue to provide valuable insights into CB1R structure-function correlates. The resulting data should expand the repertoire of novel agents capable of exerting therapeutic benefit by modulating CB1R-dependent signaling.

Mapping Cannabinoid 1 Receptor Allosteric Site(s): Critical Molecular Determinant and Signaling Profile of GAT100, a Novel, Potent, and Irreversibly Binding Probe

One of the most abundant G-protein coupled receptors (GPCRs) in brain, the cannabinoid 1 receptor (CB1R), is a tractable therapeutic target for treating diverse psychobehavioral and somatic disorders. Adverse on-target effects associated with small-molecule CB1R orthosteric agonists and inverse agonists/antagonists have plagued their translational potential. Allosteric CB1R modulators offer a potentially safer modality through which CB1R signaling may be directed for therapeutic benefit. Rational design of candidate, druglike CB1R allosteric modulators requires greater understanding of the architecture of the CB1R allosteric endodomain(s) and the capacity of CB1R allosteric ligands to tune the receptor's information output. We have recently reported the synthesis of a focused library of rationally designed, covalent analogues of Org27569 and PSNCBAM-1, two prototypic CB1R negative allosteric modulators (NAMs). Among the novel, pharmacologically active CB1R NAMs reported, the isothiocyanate GAT100 emerged as the lead by virtue of its exceptional potency in the [(35)S]GTPγS and β-arrestin signaling assays and its ability to label CB1R as a covalent allosteric probe with significantly reduced inverse agonism in the [(35)S]GTPγS assay as compared to Org27569. We report here a comprehensive functional profiling of GAT100 across an array of important downstream cell-signaling pathways and analysis of its potential orthosteric probe-dependence and signaling bias. The results demonstrate that GAT100 is a NAM of the orthosteric CB1R agonist CP55,940 and the endocannabinoids 2-arachidonoylglycerol and anandamide for β-arrestin1 recruitment, PLCβ3 and ERK1/2 phosphorylation, cAMP accumulation, and CB1R internalization in HEK293A cells overexpressing CB1R and in Neuro2a and STHdh(Q7/Q7) cells endogenously expressing CB1R. Distinctively, GAT100 was a more potent and efficacious CB1R NAM than Org27569 and PSNCBAM-1 in all signaling assays and did not exhibit the inverse agonism associated with Org27569 and PSNCBAM-1. Computational docking studies implicate C7.38(382) as a key feature of GAT100 ligand-binding motif. These data help inform the engineering of newer-generation, druggable CB1R allosteric modulators and demonstrate the utility of GAT100 as a covalent probe for mapping structure-function correlates characteristic of the druggable CB1R allosteric space.

Synthesis and biological evaluation of indole-2-carboxamides bearing photoactivatable functionalities as novel allosteric modulators for the cannabinoid CB1 receptor

5-Chloro-3-ethyl-N-(4-(piperidin-1-yl)phenethyl)-1H-indole-2-carboxamide (ORG27569, 1) is a prototypical allosteric modulator for the cannabinoid CB1 receptor. Based on this indole-2-carboxamide scaffold, we designed and synthesized novel CB1 allosteric modulators that possess photoactivatable functionalities, which include benzophenone, phenyl azide, aliphatic azide and phenyltrifluoromethyldiazrine. To assess their allosteric effects, the dissociation constant (KB) and allosteric binding cooperativity factor (α) were determined and compared to their parent compounds. Within this series, benzophenone-containing compounds 26 and 27, phenylazide-containing compound 28, and the aliphatic azide containing compound 36b showed allosteric binding parameters (KB and α) comparable to their parent compound 1, 7, 8, and 9, respectively. We further assessed these modulators for their impact on G-protein coupling activity. Interestingly, these compounds exhibited negative allosteric modulator properties in a manner similar to their parent compounds, which antagonize agonist-induced G-protein coupling. These novel CB1 allosteric modulators, possessing photoactivatable functionalities, provide valuable tools for future photo-affinity labeling and mapping the CB1 allosteric binding site(s).

New paradigms in chemokine receptor signal transduction: Moving beyond the two-site model

Chemokine receptor (CKR) signaling forms the basis of essential immune cellular functions, and dysregulated CKR signaling underpins numerous disease processes of the immune system and beyond. CKRs, which belong to the seven transmembrane domain receptor (7TMR) superfamily, initiate signaling upon binding of endogenous, secreted chemokine ligands. Chemokine-CKR interactions are traditionally described by a two-step/two-site mechanism, in which the CKR N-terminus recognizes the chemokine globular core (i.e. site 1 interaction), followed by activation when the unstructured chemokine N-terminus is inserted into the receptor TM bundle (i.e. site 2 interaction). Several recent studies challenge the structural independence of sites 1 and 2 by demonstrating physical and allosteric links between these supposedly separate sites. Others contest the functional independence of these sites, identifying nuanced roles for site 1 and other interactions in CKR activation. These developments emerge within a rapidly changing landscape in which CKR signaling is influenced by receptor PTMs, chemokine and CKR dimerization, and endogenous non-chemokine ligands. Simultaneous advances in the structural and functional characterization of 7TMR biased signaling have altered how we understand promiscuous chemokine-CKR interactions. In this review, we explore new paradigms in CKR signal transduction by considering studies that depict a more intricate architecture governing the consequences of chemokine-CKR interactions.

The endocannabinoid system in guarding against fear, anxiety and stress

The endocannabinoid (eCB) system has emerged as a central integrator linking the perception of external and internal stimuli to distinct neurophysiological and behavioural outcomes (such as fear reaction, anxiety and stress-coping), thus allowing an organism to adapt to its changing environment. eCB signalling seems to determine the value of fear-evoking stimuli and to tune appropriate behavioural responses, which are essential for the organism's long-term viability, homeostasis and stress resilience; and dysregulation of eCB signalling can lead to psychiatric disorders. An understanding of the underlying neural cell populations and cellular processes enables the development of therapeutic strategies to mitigate behavioural maladaptation.