Characterization of structurally novel G protein biased CB1 agonists: Implications for drug development

Hexahydrocannabinol (HHC) and Δ9-tetrahydrocannabinol (Δ9-THC) driven activation of cannabinoid receptor 1 results in biased intracellular signaling

The Cannabis sativa plant has been used for centuries as a recreational drug and more recently in the treatment of patients with neurological or psychiatric disorders. In many instances, treatment goals include relief from posttraumatic disorders, anxiety, or to support treatment of chronic pain. Ligands acting on cannabinoid receptor 1 (CB1R) are also potential targets for the treatment of other health conditions. Using an evidence-based approach, pharmacological investigation of CB1R agonists is timely, with the aim to provide chronically ill patients relief using well-defined and characterized compounds from cannabis. Hexahydrocannabinol (HHC), currently available over the counter in many countries to adults and even children, is of great interests to policy makers, legal administrators, and healthcare regulators, as well as pharmacologists. Herein, we studied the pharmacodynamics of HHC epimers, which activate CB1R. We compared their key CB1R-mediated signaling pathway activities and compared them to the pathways activated by Δ9-tetrahydrocannabinol (Δ9-THC). We provide evidence that activation of CB1R by HHC ligands is only broadly comparable to those mediated by Δ9-THC, and that both HHC epimers have unique properties. Together with the greater chemical stability of HHC compared to Δ9-THC, these molecules have a potential to become a part of modern medicine.

Snapshot of the cannabinoid receptor 1-arrestin complex unravels the biased signaling mechanism

Cannabis activates the cannabinoid receptor 1 (CB1), which elicits analgesic and emotion regulation benefits, along with adverse effects, via Gi and β-arrestin signaling pathways. However, the lack of understanding of the mechanism of β-arrestin-1 (βarr1) coupling and signaling bias has hindered drug development targeting CB1. Here, we present the high-resolution cryo-electron microscopy structure of CB1-βarr1 complex bound to the synthetic cannabinoid MDMB-Fubinaca (FUB), revealing notable differences in the transducer pocket and ligand-binding site compared with the Gi protein complex. βarr1 occupies a wider transducer pocket promoting substantial outward movement of the TM6 and distinctive twin toggle switch rearrangements, whereas FUB adopts a different pose, inserting more deeply than the Gi-coupled state, suggesting the allosteric correlation between the orthosteric binding pocket and the partner protein site. Taken together, our findings unravel the molecular mechanism of signaling bias toward CB1, facilitating the development of CB1 agonists.

Allosteric crosstalk in modular proteins: Function fine-tuning and drug design

Modular proteins are regulatory proteins that carry out more than one function. These proteins upregulate or downregulate a biochemical cascade to establish homeostasis in cells. To switch the function or alter the efficiency (based on cellular needs), these proteins require different facilitators that bind to a site different from the catalytic (active/orthosteric) site, aka 'allosteric site', and fine-tune their function. These facilitators (or effectors) are allosteric modulators. In this Review, we have discussed the allostery, characterized them based on their mechanisms, and discussed how allostery plays an important role in the activity modulation and function fine-tuning of proteins. Recently there is an emergence in the discovery of allosteric drugs. We have also emphasized the role, significance, and future of allostery in therapeutic applications.

Cannabinoid Tolerance in S426A/S430A x β-Arrestin 2 Knockout Double-Mutant Mice

Tolerance to compounds that target G protein-coupled receptors (GPCRs), such as the cannabinoid type-1 receptor (CB1R), is in part facilitated by receptor desensitization. Processes that mediate CB1R desensitization include phosphorylation of CB1R residues S426 and S430 by a GPCR kinase and subsequent recruitment of the β-arrestin2 scaffolding protein. Tolerance to cannabinoid drugs is reduced in S426A/S430A mutant mice and β-arrestin2 knockout (KO) mice according to previous work in vivo. However, the presence of additional phosphorylatable residues on the CB1R C-terminus made it unclear as to whether recruitment to S426 and S430 accounted for all desensitization and tolerance by β-arrestin2. Therefore, we assessed acute response and tolerance to the cannabinoids delta-9-tetrahydrocannabinol (Δ9-THC) and CP55,940 in S426A/S430A x β-arrestin2 KO double-mutant mice. We observed both delayed tolerance and increased sensitivity to the antinociceptive and hypothermic effects of CP55,940 in male S426A/S430A single- and double-mutant mice compared with wild-type littermates, but not with Δ9-THC. Female S426A/S430A single- and double-mutant mice were more sensitive to acute antinociception (CP55,940 and Δ9-THC) and hypothermia (CP55,940 only) exclusively after chronic dosing and did not differ in the development of tolerance. These results indicate that phosphorylation of S426 and S430 are likely responsible for β-arrestin2-mediated desensitization as double-mutant mice did not differ from the S426A/S430A single-mutant model in respect to cannabinoid tolerance and sensitivity. We also found antinociceptive and hypothermic effects from cannabinoid treatment demonstrated by sex-, agonist-, and duration-dependent features. SIGNIFICANCE STATEMENT: A better understanding of the molecular mechanisms involved in tolerance will improve the therapeutic potential of cannabinoid drugs. This study determined that further deletion of β-arrestin2 does not enhance the delay in cannabinoid tolerance observed in CB1R S426A/S430A mutant mice.

Cannabimimetic effects of abused indazole-carboxamide synthetic cannabinoid receptor agonists AB-PINACA, 5F-AB-PINACA and 5F-ADB-PINACA in mice: Tolerance, dependence and withdrawal

BACKGROUND

Chronic abuse of synthetic cannabinoid receptor agonists (SCRAs), known as "K2″ or "Spice", threatens public health and safety. Recently, SCRAs of the indazole-carboxamide structural class have become more prevalent. Preclinical studies investigating the tolerance and dependence potentially involved in chronic SCRA abuse is limited. The present study determined the in vivo effects of chronic exposure to indazole-carboxamide SCRAs, AB-PINACA, 5F-AB-PINACA and 5F-ADB-PINACA compared to the first-generation SCRA, JWH-018.

METHODS

Adult male C57Bl/6 mice were used for dose-effect determinations of hypothermic effects. Adult male NIH Swiss mice were used in biotelemetry studies to assess tolerance to hypothermic effects following repeated SCRA administration over 5 consecutive days, and to determine the role of Phase I drug metabolism via acute CYP450 inhibition in the presence of 1-ABT, a nonspecific CYP450 inhibitor. SCRA dependence was determined in adult male NIH Swiss mice via assessment of rimonabant-precipitated observable sign of withdrawal (i.e., front paw tremors).

RESULTS

All SCRAs elicited dose-dependent hypothermia mediated through cannabinoid CB1 receptors (CB1Rs). 1-ABT increased duration of hypothermia for all SCRAs tested, and increased the magnitude of hypothermia for all SCRAs except 5F-ADB-PINACA. Upon repeated administration, tolerance to hypothermic effects of AB-PINACA, 5F-AB-PINACA and 5F-ADB-PINACA was much less than that of JWH-018. Similarly, rimonabant-precipitated front paw tremors were much less frequent in mice treated with 5F-AB-PINACA and 5F-ADB-PINACA than in mice treated with JWH-018.

CONCLUSIONS

These findings suggest a decreased potential for tolerance and withdrawal among indazole-carboxamide SCRAs, and may imply structural class-dependent profiles of in vivo effects among SCRAs.

Metabolites of Synthetic Cannabinoid 5F-MDMB-PINACA Retain Affinity, Act as High Efficacy Agonists and Exhibit Atypical Pharmacodynamic Properties at CB1 Receptors

Synthetic cannabinoid receptor agonists (SCRAs) are a large group of abused psychoactive compounds that elicit numerous toxic effects not observed with cannabis, including death. Abuse of third-generation SCRA 5F-MDMB-PINACA (also known as 5F-ADB) has been associated with over 40 fatalities. This SCRA is metabolized to several active phase I metabolites, including excessively high post-mortem serum concentrations of an ester hydrolysis metabolite, 5F-MDMB-PINACA-M7 (M7). Although high serum concentrations of M7 (and other active metabolites) have been suggested to contribute to 5F-MDMB-PINACA toxicity, the affinity of M7 for CB1 receptors is unknown and more complete pharmacodynamic characterization of 5F-MDMB-PINACA and its active metabolites is needed. Competition binding and G-protein modulation studies presented here confirm reports that 5F-MDMB-PINACA and a second N-5-hydroxypentyl metabolite (M2) exhibit nM affinity and act as high efficacy agonists at CB1 receptors. Also as previously published, M7 exhibits high efficacy at CB1 receptors; however, demonstrated here for the first time, M7 retains only low μΜ affinity. Empirically derived Kb values indicate rimonabant differentially antagonizes G-protein activation produced by 5F-MDMB-PINACA, relative to Δ9-THC (THC) or its metabolites. Chronic administration of 5F-MDMB-PINACA and metabolites results in CB1 down-regulation, but only 5F-MDMB-PINACA produces desensitization. Although low CB1 affinity/potency of M7 precluded in vivo studies, both M2 and THC produce locomotor suppression and CB1-mediated dose-dependent hypothermia and analgesia in mice. Collectively, these data confirm and extend previous studies suggesting that 5F-MDMB-PINACA is metabolized to active compounds exhibiting atypical pharmacodynamic properties at CB1 receptors, that may accumulate with parent drug to produce severe toxicity.

Molecular basis for ligand modulation of the cannabinoid CB1 receptor

The cannabinoid CB₁ receptor is the most abundant G protein coupled receptor (GPCR) in the central nervous system, which mediates the functional response to endocannabinoids and Cannabis compounds. A variety of ligands for CB₁ receptors have been developed as promising drug candidates for the treatment of neurological disorders. New high-resolution structures of CB₁ receptor in different functional states have significantly improved our molecular understanding of CB₁ ligand interactions, selectivity, receptor activation and allosteric modulation. These advances have paved the way for development of novel ligands for different therapeutic applications. In this review, we describe the structural determinants for modulation of CB₁ receptors by different types of ligands, as well as the differences between CB₁ and its homologous, the CB₂ receptor.

The endocannabinoid system - current implications for drug development

In this review, the state of the art for compounds affecting the endocannabinoid (eCB) system is described with a focus on the treatment of pain. Amongst directly acting CB receptor ligands, clinical experience with ∆⁹ -tetrahydracannabinol and medical cannabis in chronic non-cancer pain indicates that there are differences between the benefits perceived by patients and the at best modest effect seen in meta-analyses of randomized controlled trials. The reason for this difference is not known but may involve differences in the type of patients that are recruited, the study conditions that are chosen and the degree to which biases such as reporting bias are operative. Other directly acting CB receptor ligands such as biased agonists and allosteric receptor modulators have not yet reached the clinic. Amongst indirectly acting compounds targeting the enzymes responsible for the synthesis and catabolism of the eCBs anandamide and 2-arachidonoylglycerol, fatty acid amide hydrolase (FAAH) inhibitors have been investigated clinically but were per se not useful for the treatment of pain, although they may be useful for the treatment of post-traumatic stress disorder and cannabis use disorder. Dual-acting compounds targeting this enzyme and other targets such as cyclooxygenase-2 or transient potential vanilloid receptor 1 may be a way forward for the treatment of pain.

Systematic evaluation of a panel of 30 synthetic cannabinoid receptor agonists structurally related to MMB-4en-PICA, MDMB-4en-PINACA, ADB-4en-PINACA, and MMB-4CN-BUTINACA using a combination of binding and different CB1 receptor activation assays. Part III: The G protein pathway and critical comparison of different assays

The present work is the last of a three-part study investigating a panel of 30 systematically designed synthetic cannabinoid receptor agonists (SCRAs) including features such as the 4-pentenyl tail and varying head groups including amides and esters of l-valine (MMB, AB), l-tert-leucine (ADB), and l-phenylalanine (APP), as well as adamantyl (A) and cumyl moieties (CUMYL). Here, we evaluated these SCRAs for their capacity to activate the human cannabinoid receptor 1 (CB₁ ) via indirect measurement of G protein recruitment. Furthermore, we comparatively evaluated the results obtained from three in vitro assays, based on the recruitment of β-arrestin 2 (βarr2 assay) or Gα_i protein (mini-Gα_i assay), or binding of [³⁵ S]-GTPγS. The observed efficacies (E_max ) varied depending on the conducted assay. Statistical analysis suggests that the population means of the relative intrinsic activity (RA_i ) significantly differ for the [³⁵ S]-GTPγS assay and the other two assays, but the population means of the βarr2 and mini-Gα_i assays were not statistically different. Our data suggest that differences observed between the βarr2 and mini-Gα_i assays are the best predictor for 'biased agonism' towards βarr or G protein recruitment in our study. SCRAs carrying an ADB or MPP moiety as a head group tended to produce elevated E_max values in the βarr2 assay, which might result in a tendency of these compounds to cause pronounced tolerance in users-a hypothesis that should be evaluated further by future studies. In general, a comparison of efficacies derived from different assays is difficult and should only be conducted very cautiously.

Systematic evaluation of a panel of 30 synthetic cannabinoid receptor agonists structurally related to MMB-4en-PICA, MDMB-4en-PINACA, ADB-4en-PINACA, and MMB-4CN-BUTINACA using a combination of binding and different CB1 receptor activation assays-Part II: Structure activity relationship assessment via a β-arrestin recruitment assay

Synthetic cannabinoid receptor agonists (SCRAs) are the second largest class of new psychoactive substances (NPS) and are associated with serious adverse effects and even death. Despite this, little pharmacological data are available for many of the most recent SCRAs. This study consists of three different parts, aiming to systematically evaluate a panel of 30 SCRAs using binding and different in vitro human cannabinoid 1 receptor (CB₁ ) activation assays. The present Part II investigated the SCRA analogs for their CB₁ activation via a β-arrestin recruitment assay. The panel was systematically designed to include key structural sub-features of recent SCRAs. Thus, the 4-pentenyl tail of MMB-4en-PICA and MDMB-4en-PINACA was retained while incorporating varying head groups from other prevalent SCRAs, including amides and esters of L-valine, L-tert-leucine, and L-phenylalanine, and adamantyl and cumyl moieties. All 30 SCRAs activated CB₁ , with indazoles generally showing the greatest potency (EC₅₀  = 1.88-281 nM), followed by indoles (EC₅₀  = 11.5-2293 nM), and the corresponding 7-azaindoles (EC₅₀  = 62.4-9251 nM). Several subunit-linked structure-activity relationships were identified: (i) tert-leucine-functionalized SCRAs were more potent than the corresponding valine derivatives; (ii) no major difference in potency or efficacy was observed between tert-leucine/valine-derived amides and the corresponding methyl esters; however, phenylalanine analogs were affected by this change; and (iii) minor structural changes to the 4-pentenyl substituent had little influence on activity. These findings elucidate structural features that modulate the CB₁ activation potential of currently prevalent SCRAs and a systematic panel of analogs, some of which may appear in NPS markets in future.

Therapeutic potential and safety considerations for the clinical use of synthetic cannabinoids

The phytocannabinoid Δ9-tetrahydrocannabinol (THC) was isolated and synthesized in the 1960s. Since then, two synthetic cannabinoids (SCBs) targeting the cannabinoid 1 (CB1R) and 2 (CB2R) receptors were approved for medical use based on clinical safety and efficacy data: dronabinol (synthetic THC) and nabilone (synthetic THC analog). To probe the function of the endocannabinoid system further, hundreds of investigational compounds were developed; in particular, agonists with (1) greater CB1/2R affinity relative to THC and (2) full CB1/2R agonist activity. This pharmacological profile may pose greater risks for misuse and adverse effects relative to THC, and these SCBs proliferated in retail markets as legal alternatives to cannabis (e.g., novel psychoactive substances [NPS], "Spice," "K2"). These SCBs were largely outlawed in the U.S., but blanket policies that placed all SCB chemicals into restrictive control categories impeded research progress into novel mechanisms for SCB therapeutic development. There is a concerted effort to develop new, therapeutically useful SCBs that target novel pharmacological mechanisms. This review highlights the potential therapeutic efficacy and safety considerations for unique SCBs, including CB1R partial and full agonists, peripherally-restricted CB1R agonists, selective CB2R agonists, selective CB1R antagonists/inverse agonists, CB1R allosteric modulators, endocannabinoid-degrading enzyme inhibitors, and cannabidiol. We propose promising directions for SCB research that may optimize therapeutic efficacy and diminish potential for adverse events, for example, peripherally-restricted CB1R antagonists/inverse agonists and biased CB1/2R agonists. Together, these strategies could lead to the discovery of new, therapeutically useful SCBs with reduced negative public health impact.

Advances in the Treatment of Chronic Pain by Targeting GPCRs

Pain is an essential protective mechanism that the body uses to alert or prevent further damage. Pain sensation is a complex event involving perception, transmission, processing, and response. Neurons at different levels (peripheral, spinal cord, and brain) are responsible for these pro- or antinociceptive activities to ensure an appropriate response to external stimuli. The terminals of these neurons, both in the peripheral endings and in the synapses, are equipped with G protein-coupled receptors (GPCRs), voltage- and ligand-gated ion channels that sense structurally diverse stimuli and inhibitors of neuronal activity. This review will focus on the largest class of sensory proteins, the GPCRs, as they are distributed throughout ascending and descending neurons and regulate activity at each step during pain transmission. GPCR activation also directly or indirectly controls the function of co-localized ion channels. The levels and types of some GPCRs are significantly altered in different pain models, especially chronic pain states, emphasizing that these molecules could be new targets for therapeutic intervention in chronic pain.

Binding Modes and Selectivity of Cannabinoid 1 (CB1) and Cannabinoid 2 (CB2) Receptor Ligands

The cannabinoid (CB) receptors (CB₁R and CB₂R) represent a promising therapeutic target for several indications such as nociception and obesity. The ligands with nonselectivity can be traced to the high similarity in the binding sites of both cannabinoid receptors. Therefore, the need for selectivity, potency, and G-protein coupling bias has further complicated the design of desired compounds. The bias of currently studied cannabinoid agonists is seldom investigated, and agonists that do exhibit bias are typically nonselective. However, certain long-chain endocannabinoids represent a class of selective and potent CB₁R agonists. The binding mode for this class of compounds has remained elusive, limiting the implementation of its binding features to currently studied agonists. Hence, in the present study, the binding poses for these long-chain cannabinoids, along with other interesting ligands, with the receptors have been determined, by using a combination of molecular docking and molecular dynamics (MD) simulations along with molecular mechanics-Poisson-Boltzmann surface area (MM-PBSA) binding free energy calculations. The binding poses for the long-chain cannabinoids implicate that a site surrounded by the transmembrane (TM)2, TM7, and extracellular loop (ECL)2 is vital for providing the long-chain ligands with the selectivity for CB₁R, especially I267 of CB₁R (corresponding to L182 of CB₂R). Based on the obtained binding modes, the calculated relative binding free energies and selectivity are all in good agreement with the corresponding experimental data, suggesting that the determined binding poses are reasonable. The computational strategy used in this study may also prove fruitful in applications with other GPCRs or membrane-bound proteins.

Molecular Pharmacology of Synthetic Cannabinoids: Delineating CB1 Receptor-Mediated Cell Signaling

Synthetic cannabinoids (SCs) are a class of new psychoactive substances (NPSs) that exhibit high affinity binding to the cannabinoid CB1 and CB2 receptors and display a pharmacological profile similar to the phytocannabinoid (-)-trans-Δ9-tetrahydrocannabinol (THC). SCs are marketed under brand names such as K2 and Spice and are popular drugs of abuse among male teenagers and young adults. Since their introduction in the early 2000s, SCs have grown in number and evolved in structural diversity to evade forensic detection and drug scheduling. In addition to their desirable euphoric and antinociceptive effects, SCs can cause severe toxicity including seizures, respiratory depression, cardiac arrhythmias, stroke and psychosis. Binding of SCs to the CB1 receptor, expressed in the central and peripheral nervous systems, stimulates pertussis toxin-sensitive G proteins (Gi/Go) resulting in the inhibition of adenylyl cyclase, a decreased opening of N-type Ca2+ channels and the activation of G protein-gated inward rectifier (GIRK) channels. This combination of signaling effects dampens neuronal activity in both CNS excitatory and inhibitory pathways by decreasing action potential formation and neurotransmitter release. Despite this knowledge, the relationship between the chemical structure of the SCs and their CB1 receptor-mediated molecular actions is not well understood. In addition, the potency and efficacy of newer SC structural groups has not been determined. To address these limitations, various cell-based assay technologies are being utilized to develop structure versus activity relationships (SAR) for the SCs and to explore the effects of these compounds on noncannabinoid receptor targets. This review focuses on describing and evaluating these assays and summarizes our current knowledge of SC molecular pharmacology.

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.

New Insights in Cannabinoid Receptor Structure and Signaling

BACKGROUND

Cannabinoid has long been used for medicinal purposes. Cannabinoid signaling has been considered the therapeutic target for treating pain, addiction, obesity, inflammation, and other diseases. Recent studies have suggested that in addition to CB1 and CB2, there are non-CB1 and non-CB2 cannabinoid-related orphan GPCRs including GPR18, GPR55, and GPR119. In addition, CB1 and CB2 display allosteric binding and biased signaling, revealing correlations between biased signaling and functional outcomes. Interestingly, new investigations have indicated that CB1 is functionally present within the mitochondria of striated and heart muscles directly regulating intramitochondrial signaling and respiration.

CONCLUSION

In this review, we summarize the recent progress in cannabinoid-related orphan GPCRs, CB1/CB2 structure, Gi/Gs coupling, allosteric ligands and biased signaling, and mitochondria-localized CB1, and discuss the future promise of this research.

In vitro determination of the efficacy of illicit synthetic cannabinoids at CB1 receptors

BACKGROUND AND PURPOSE

The morbidity and mortality associated with recreational use of synthetic cannabinoid receptor agonists (SCRAs) may reflect strong activation of CB₁ receptors and is a major health concern. The properties of SCRA at CB₁ receptors are not well defined. Here we have developed an assay to determine acute CB₁ receptor efficacy using receptor depletion with the irreversible CB₁ receptor antagonist AM6544, with application of the Black and Leff operational model to calculate efficacy.

EXPERIMENTAL APPROACH

Receptor depletion in mouse AtT-20 pituitary adenoma cells stably expressing human CB₁ receptors was achieved by pretreatment of cells with AM6544 (10 μM, 60 min). The CB₁ receptor-mediated hyperpolarisation of AtT-20 cells was measured using fluorescence-based membrane potential dye. From data fit to the operational model, the efficacy (τ) and affinity (K_A ) parameters were obtained for each drug.

KEY RESULTS

AM6544 did not affect the potency or maximal effect of native somatostatin receptor-induced hyperpolarization. The τ value of ∆⁹ -THC was 80-fold less than the reference CB receptor agonist CP55940 and 260-fold less than the highest efficacy SCRA, 5F-MDMB-PICA. The operational efficacy of SCRAs ranged from 233 (5F-MDMB-PICA) to 28 (AB-PINACA), with CP55940 in the middle of the efficacy rank order. There was no correlation between the τ and K_A values.

CONCLUSIONS AND IMPLICATIONS

All SCRAs tested showed substantially higher efficacy at CB₁ receptors than ∆⁹ -THC, which may contribute to the adverse effects seen with these drugs but not ∆⁹ -THC.

Assessment of Biased Agonism among Distinct Synthetic Cannabinoid Receptor Agonist Scaffolds

Cannabinoid receptor 1 (CB₁) is a key drug target for a number of diseases, including metabolic syndromes and neuropathic pain. Most of the typical cannabinoid ligands provoke psychotropic side effects that impair their therapeutic utility. As of today, it is not yet clearly known which structural features of cannabinoid ligands determine a preference toward specific signaling pathways. Distinct bioassays are typically used to elucidate signaling preferences. However, these are often based on different cell lines and use different principles and/or read-outs, which makes straightforward assessment of "ligand bias" difficult. Within this context, this study is the first to investigate ligand bias among synthetic cannabinoid receptor agonists (SCRAs) in as closely analogous conditions as possible, by applying a new functional complementation-based assay panel to assess the recruitment of Gα_i protein or β-arrestin2 to CB₁. In a panel of 21 SCRAs, chosen to cover a broad diversity in chemical structures, distinct, although often subtle, preferences toward specific signaling pathways were observed. Relative to CP55940, here considered as a "balanced" reference agonist, most of the selected SCRAs (e.g., 5F-APINACA, CUMYL-PEGACLONE, among others) displayed preferred signaling through the β-arrestin2 pathway, whereas MMB-CHMICA could serve as a potential "balanced" agonist. Interestingly, EG-018 was the only SCRA showing a significant (10-fold) preference toward G protein over β-arrestin2 recruitment. While it is currently unclear what this exactly means in terms of abuse potential and/or toxicity, the approach proposed here may allow construction of a knowledge base that in the end may allow better insight into the structure-"functional" activity relationship of these compounds. This may aid the development of new therapeutics with less unwanted psychoactive effects.

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.

Cannabinoid Signaling in the Skin: Therapeutic Potential of the "C(ut)annabinoid" System

The endocannabinoid system (ECS) has lately been proven to be an important, multifaceted homeostatic regulator, which influences a wide-variety of physiological processes all over the body. Its members, the endocannabinoids (eCBs; e.g., anandamide), the eCB-responsive receptors (e.g., CB₁, CB₂), as well as the complex enzyme and transporter apparatus involved in the metabolism of the ligands were shown to be expressed in several tissues, including the skin. Although the best studied functions over the ECS are related to the central nervous system and to immune processes, experimental efforts over the last two decades have unambiguously confirmed that cutaneous cannabinoid ("c[ut]annabinoid") signaling is deeply involved in the maintenance of skin homeostasis, barrier formation and regeneration, and its dysregulation was implicated to contribute to several highly prevalent diseases and disorders, e.g., atopic dermatitis, psoriasis, scleroderma, acne, hair growth and pigmentation disorders, keratin diseases, various tumors, and itch. The current review aims to give an overview of the available skin-relevant endo- and phytocannabinoid literature with a special emphasis on the putative translational potential, and to highlight promising future research directions as well as existing challenges.

Reduced Tolerance and Asymmetrical Crosstolerance to Effects of the Indole Quinuclidinone Analog PNR-4-20, a G Protein-Biased Cannabinoid 1 Receptor Agonist in Mice: Comparisons with Δ9-Tetrahydrocannabinol and JWH-018

Most cannabinoid 1 receptor (CB₁R) agonists will signal through both G protein-dependent and -independent pathways in an unbiased manner. Recruitment of β-arrestin 2 desensitizes and internalizes receptors, producing tolerance that limits therapeutic utility of cannabinoids for chronic conditions. We developed the indole quinuclidinone (IQD) analog (Z)-2-((1-(4-fluorobenzyl)-1H-indol-3-yl)methylene)quinuclidin-3-one (PNR-4-20) as a novel G protein-biased agonist at CB₁Rs, and the present studies determine if repeated administration of PNR-4-20 produces lesser tolerance to in vivo effects compared with unbiased CB₁R agonists Δ⁹-tetrahydrocannabinol (Δ⁹-THC) and 1-pentyl-3-(1-naphthoyl)indole (JWH-018). Adult male National Institutes of Health Swiss mice were administered comparable doses of PNR-4-20 (100 mg/kg), Δ⁹-THC (30 mg/kg), or JWH-018 (3 mg/kg) once per day for five consecutive days to determine tolerance development to hypothermic, antinociceptive, and cataleptic effects. Persistence of tolerance was then determined after a drug abstinence period. We found that unbiased CB₁R agonists Δ⁹-THC and JWH-018 produced similar tolerance to these effects, but lesser tolerance was observed with PNR-4-20 for hypothermic and cataleptic effects. Tolerance to the effects of PNR-4-20 completely recovered after drug abstinence, while residual tolerance was always observed with unbiased CB₁R agonists. Repeated treatment with PNR-4-20 and Δ⁹-THC produced asymmetric crosstolerance to hypothermic effects. Importantly, binding studies suggest PNR-4-20 produced significantly less downregulation of CB₁Rs relative to Δ⁹-THC in hypothalamus and thalamus of chronically treated mice. These studies suggest that the G protein-biased CB₁R agonist PNR-4-20 produces significantly less tolerance than unbiased cannabinoid agonists, and that the IQD analogs should be investigated further as a novel molecular scaffold for development of new therapeutics.

Assessment of rimonabant-like adverse effects of purported CB1R neutral antagonist / CB2R agonist aminoalkylindole derivatives in mice

BACKGROUND

Cannabinoids may be useful in the treatment of CNS disorders including drug abuse and addiction, where both CB1R antagonists / inverse agonists and CB2R agonists have shown preclinical efficacy. TV-5-249 and TV-6-41, two novel aminoalkylindoles with dual action as neutral CB1R antagonists and CB2R agonists, previously attenuated abuse-related effects of ethanol in mice.

PURPOSE

To further characterize these drugs, TV-5-249 and TV-6-41 were compared with the CB1R antagonist / inverse agonist rimonabant in assays relevant to adverse effects and cannabinoid withdrawal.

PROCEDURES AND FINDINGS

The cannabinoid tetrad confirmed that TV-5-249 and TV-6-41 were devoid of CB1R agonist effects at behaviorally-relevant doses, and neither of the novel drugs induced rimonabant-like scratching. Generalized aversive effects were assessed, and rimonabant and TV-5-249 induced taste aversion, but TV-6-41 did not. Schedule-controlled responding and observation of somatic signs were used to assess withdrawal-like effects precipitated by rimonabant or TV-6-41 in mice previously treated with the high-efficacy CB1R agonist JWH-018 or vehicle. Rimonabant and TV-6-41 dose-dependently suppressed response rates in all subjects, but TV-6-41 did so more potently in JWH-018-treated mice than in vehicle-treated mice, while rimonabant equally suppressed responding in both groups. Importantly, rimonabant elicited dramatic withdrawal signs, but TV-6-41 did not.

CONCLUSIONS

These findings suggest differences in both direct adverse effects and withdrawal-related effects elicited by rimonabant, TV-5-249, and TV-6-41, which could relate to neutral CB1R antagonism, CB2R agonism, or a combination of both. Both mechanisms should be explored and exploited in future drug design efforts to develop pharmacotherapies for drug dependence.

Atypical Pharmacodynamic Properties and Metabolic Profile of the Abused Synthetic Cannabinoid AB-PINACA: Potential Contribution to Pronounced Adverse Effects Relative to Δ9-THC

Recreational use of marijuana is associated with few adverse effects, but abuse of synthetic cannabinoids (SCBs) can result in anxiety, psychosis, chest pain, seizures and death. To potentially explain higher toxicity associated with SCB use, we hypothesized that AB-PINACA, a common second generation SCB, exhibits atypical pharmacodynamic properties at CB1 cannabinoid receptors (CB1Rs) and/or a distinct metabolic profile when compared to Δ⁹-tetrahydrocannabinol (Δ⁹-THC), the principal psychoactive cannabinoid present in marijuana. Liquid chromatography tandem mass spectrometry (LC/MS) identified AB-PINACA and monohydroxy metabolite(s) as primary phase I metabolites (4OH-AB-PINACA and/or 5OH-AB-PINACA) in human urine and serum obtained from forensic samples. In vitro experiments demonstrated that when compared to Δ⁹-THC, AB-PINACA exhibits similar affinity for CB1Rs, but greater efficacy for G-protein activation and higher potency for adenylyl cyclase inhibition. Chronic treatment with AB-PINACA also results in greater desensitization of CB1Rs (e.g., tolerance) than Δ⁹-THC. Importantly, monohydroxy metabolites of AB-PINACA retain affinity and full agonist activity at CB1Rs. Incubation of 4OH-AB-PINACA and 5OH-AB-PINACA with human liver microsomes (HLMs) results in limited glucuronide formation when compared to that of JWH-018-M2, a major monohydroxylated metabolite of the first generation SCB JWH-018. Finally, AB-PINACA and 4OH-AB-PINACA are active in vivo, producing CB1R-mediated hypothermia in mice. Taken collectively, the atypical pharmacodynamic properties of AB-PINACA at CB1Rs relative to Δ⁹-THC (e.g., higher potency/efficacy and greater production of desensitization), coupled with an unusual metabolic profile (e.g., production of metabolically stable active phase I metabolites) may contribute to the pronounced adverse effects observed with abuse of this SCB compared to marijuana.

Targeting Cannabinoid Signaling in the Immune System: "High"-ly Exciting Questions, Possibilities, and Challenges

It is well known that certain active ingredients of the plants of Cannabis genus, i.e., the "phytocannabinoids" [pCBs; e.g., (-)-trans-Δ⁹-tetrahydrocannabinol (THC), (-)-cannabidiol, etc.] can influence a wide array of biological processes, and the human body is able to produce endogenous analogs of these substances ["endocannabinoids" (eCB), e.g., arachidonoylethanolamine (anandamide, AEA), 2-arachidonoylglycerol (2-AG), etc.]. These ligands, together with multiple receptors (e.g., CB₁ and CB₂ cannabinoid receptors, etc.), and a complex enzyme and transporter apparatus involved in the synthesis and degradation of the ligands constitute the endocannabinoid system (ECS), a recently emerging regulator of several physiological processes. The ECS is widely expressed in the human body, including several members of the innate and adaptive immune system, where eCBs, as well as several pCBs were shown to deeply influence immune functions thereby regulating inflammation, autoimmunity, antitumor, as well as antipathogen immune responses, etc. Based on this knowledge, many in vitro and in vivo studies aimed at exploiting the putative therapeutic potential of cannabinoid signaling in inflammation-accompanied diseases (e.g., multiple sclerosis) or in organ transplantation, and to dissect the complex immunological effects of medical and "recreational" marijuana consumption. Thus, the objective of the current article is (i) to summarize the most recent findings of the field; (ii) to highlight the putative therapeutic potential of targeting cannabinoid signaling; (iii) to identify open questions and key challenges; and (iv) to suggest promising future directions for cannabinoid-based drug development.

Effect fingerprinting of new psychoactive substances (NPS): What can we learn from in vitro data?

The use of new psychoactive substances (NPS) is increasing and currently >600 NPS have been reported. However, limited information on neuropharmacological and toxicological effects of NPS is available, hampering risk characterization. We reviewed the literature on the in vitro neuronal modes of action to obtain effect fingerprints of different classes of illicit drugs and NPS. The most frequently reported NPS were selected for review: cathinones (MDPV, α-PVP, mephedrone, 4-MEC, pentedrone, methylone), cannabinoids (JWH-018), (hallucinogenic) phenethylamines (4-fluoroamphetamine, benzofurans (5-APB, 6-APB), 2C-B, NBOMes (25B-NBOMe, 25C-NBOMe, 25I-NBOMe)), arylcyclohexylamines (methoxetamine) and piperazine derivatives (mCPP, TFMPP, BZP). Our effect fingerprints highlight the main modes of action for the different NPS studied, including inhibition and/or reversal of monoamine reuptake transporters (cathinones and non-hallucinogenic phenethylamines), activation of 5-HT₂receptors (hallucinogenic phenethylamines and piperazines), activation of cannabinoid receptors (cannabinoids) and inhibition of NDMA receptors (arylcyclohexylamines). Importantly, we identified additional targets by relating reported effect concentrations to the estimated human brain concentrations during recreational use. These additional targets include dopamine receptors, α- and β-adrenergic receptors, GABA_Areceptors and acetylcholine receptors, which may all contribute to the observed clinical symptoms following exposure. Additional data is needed as the number of NPS continues to increase. Also, the effect fingerprints we have obtained are still incomplete and suffer from a large variation in the reported effects and effect sizes. Dedicated in vitro screening batteries will aid in complementing specific effect fingerprints of NPS. These fingerprints can be implemented in the risk assessments of NPS that are necessary for eventual control measures to reduce Public Health risks.