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The first generation of CB(1) positive allosteric modulators (e.g., ZCZ011) featured a 3-nitroalkyl-2-phenyl-indole structure. Although a small number of drugs include the nitro group, it is generally not regarded as being “drug-like”, and this is particularly true for aliphatic nitro groups. There are very few case studies where an appropriate bioisostere replaced a nitro group that had a direct role in binding. This may be indicative of the difficulty of replicating its binding interactions. Herein, we report the design and synthesis of ligands targeting the allosteric binding site on the CB(1) cannabinoid receptor, in which a CF(3) group successfully replaced the aliphatic NO(2). In general, the CF(3)-bearing compounds were more potent than their NO(2) equivalents and also showed improved in vitro metabolic stability. The CF(3) analogue (1) with the best balance of properties was selected for further pharmacological evaluation. Pilot in vivo studies showed that (±)-1 has similar activity to (±)-ZCZ011, with both showing promising efficacy in a mouse model of neuropathic pain.

This research was funded by the Scottish Universities Life Science Alliance (SULSA) assay development fund. This research was also kindly supported by The Rosetrees Trust and The Alzheimer’s Society, specifically The Barcopel Foundation, and partly funded by the MSD Scottish Life Sciences fund. As part of an ongoing contribution to Scottish life sciences, MSD Limited, a global health care leader, has given substantial monetary funding to the Scottish Funding Council for distribution via SULSA to develop and deliver a high-quality drug discovery research and training program. A major hallmark of Alzheimer’s disease (AD) is the formation of neurotoxic aggregates composed of the amyloid-β peptide (Aβ). Aβ has been recognized to interact with numerous proteins, resulting in pathological changes to the metabolism of patients with AD. One such mitochondrial metabolic enzyme is amyloid-binding alcohol dehydrogenase (ABAD), where altered enzyme function caused by the Aβ-ABAD interaction is known to cause mitochondrial distress and cytotoxic effects, providing a feasible therapeutic target for AD drug development. Here we have established a high-throughput screening platform for the identification of modulators to the ABAD enzyme. A pilot screen with a total of 6759 compounds from the NIH Clinical Collections (NCC) and SelleckChem libraries and a selection of compounds from the BioAscent diversity collection have allowed validation and robustness to be optimized. The pilot screen revealed 16 potential inhibitors in the low µM range against ABAD with favorable physicochemical properties for blood-brain barrier penetration. Postprint

Kallikrein‐related peptidase 6 (KLK6) is a secreted serine protease that belongs to the family of tissue kallikreins. Aberrant expression of KLK6 has been found in different cancers and neurodegenerative diseases, and KLK6 is currently studied as a potential target in these pathologies. We report a novel series of KLK6 inhibitors discovered in a high‐throughput screen within the European Lead Factory program. Structure‐guided design based on docking studies enabled rapid progression of a hit cluster to inhibitors with improved potency, selectivity and pharmacokinetic properties. In particular, inhibitors 32 ((5R)‐3‐(4‐carbamimidoylphenyl)‐N‐((S)‐1‐(naphthalen‐1‐yl)propyl)‐2‐oxooxazolidine‐5‐carboxamide) and 34 ((5R)‐3‐(6‐carbamimidoylpyridin‐3‐yl)‐N‐((1S)‐1‐(naphthalen‐1‐yl)propyl)‐2‐oxooxazolidine‐5‐carboxamide) have single‐digit nanomolar potency against KLK6, with over 25‐fold and 100‐fold selectivities against the closely related enzyme trypsin, respectively. The most potent compound, 32, effectively reduces KLK6‐dependent invasion of HCT116 cells. The high potency in combination with good solubility and low clearance of 32 make it a good chemical probe for KLK6 target validation in vitro and potentially in vivo., A successful screen performed within the European Lead Factory program, followed by docking studies, has resulted in compound 32, a potent and selective N‐(4‐benzamidino)‐oxazolidinone inhibitor of kallikrein‐related peptidase 6 (KLK6). Aberrant levels of this serine protease have been found in different malignancies and in neurodegenerative diseases. Good solubility and low clearance of 32 make it a good probe for KLK6 target validation.

8-Chloro-1-(2?,4?-dichlorophenyl)-N-piperidin-1-yl-1,4,5,6-tetrahydrobenzo[6,7]cyclohepta[1,2-c]pyrazole-3-carboxamide 9a was discovered as potent and selective CB1 antagonist by part of our group few years ago. In particular it was reported to have an affinity towards the CB1 cannabinoid receptor (CB1R), expressed as Ki, of 0.00035 nM. Nevertheless significantly divergent data were reported for the same compound from other laboratories. To unequivocally define the receptor profile of 9a, we have critically reviewed both its synthesis approach and binding data. Here we report that, in contrast to our previously reported data, 9a showed a Ki value for CB1R in the order of nanomolar rather than of fentomolar range. The new determined receptor profile of 9a was also ascertained for analogue derivatives 9b-i, as well as for 12. Moreover, the structural features of the synthesized compounds necessary for CB1R were investigated. Amongst the novel series, effects on CB1R intrinsic activity was highlighted due to the substituents at the position 3 of the pyrazole ring of the 1,4,5,6-tetrahydrobenzo[6,7]cyclohepta[1,2-c]pyrazole scaffold. Although the cannabinoid receptor profile of 9a was reviewed in this work, the relevance of this compound in CB1R antagonist based drug discovery is confirmed.

Existing CB1 negative allosteric modulators (NAMs) fall into a limited range of structural classes. In spite of the theoretical potential of CB1 NAMs, published in vivo studies have generally not been able to demonstrate the expected therapeutically-relevant CB1-mediated effects. Thus, a greater range of molecular tools are required to allow definitive elucidation of the effects of CB1 allosteric modulation. In this study, we show a novel series of indole sulfonamides. Compounds 5e and 6c (ABD1075) had potencies of 4 and 3 nM respectively, and showed good oral exposure and CNS penetration, making them highly versatile tools for investigating the therapeutic potential of allosteric modulation of the cannabinoid system.

Recent studies have demonstrated that the actin binding protein, ezrin, and the cAMP-sensor, EPAC1, cooperate to induce cell spreading in response to elevations in intracellular cAMP. To investigate the mechanisms underlying these effects we generated a model of EPAC1-dependent cell spreading based on the stable transfection of EPAC1 into HEK293T (HEK293T–EPAC1) cells. We found that direct activation of EPAC1 with the EPAC-selective analogue, 8-pCPT-2′-O-Me-cAMP (007), promoted cell spreading in these cells. In addition, co-activation of EPAC1 and PKA, with a combination of the adenylate cyclase activator, forskolin, and the cAMP phosphodiesterase inhibitor, rolipram, was found to synergistically enhance cell spreading, in association with cortical actin bundling and mobilisation of ezrin to the plasma membrane. PKA activation was also associated with phosphorylation of ezrin on Thr567, as detected by an electrophoretic band mobility shift during SDS-PAGE. Inhibition of PKA activity blocked ezrin phosphorylation and reduced the cell spreading response to cAMP elevation to levels induced by EPAC1-activation alone. Transfection of HEK293T–EPAC1 cells with inhibitory ezrin mutants lacking the key PKA phosphorylation site, ezrin-Thr567Ala, or the ability to associate with actin, ezrin-Arg579Ala, promoted cell arborisation and blocked the ability of EPAC1 and PKA to further promote cell spreading. The PKA phospho-mimetic mutants of ezrin, ezrin-Thr567Asp had no effect on EPAC1-driven cell spreading. Our results indicate that association of ezrin with the actin cytoskeleton and phosphorylation on Thr567 are required, but not sufficient, for PKA and EPAC1 to synergistically promote cell spreading following elevations in intracellular cAMP., Graphical abstract, Highlights • EPAC1 is absolutely required for cAMP-induced cell spreading in HEK293T cells. • EPAC1 facilitates further PKA-dependent cell spreading in HEK293T cells. • PKA-dependent cell spreading is associated with cortical actin bundling. • Ezrin is required for EPAC1- and PKA-dependent cell spreading. • Phosphorylated ezrin (pThr567) stabilises cortical actin and prevents membrane projections.

The cannabinoid 1 (CB1) allosteric modulator, 5-chloro-3-ethyl-1H-indole-2-carboxylic acid [2-(4-piperidin-1-yl-phenyl)-ethyl]-amide) (ORG27569), has the paradoxical effect of increasing the equilibrium binding of [3H](−)-3-[2-hydroxyl-4-(1,1-dimethylheptyl)phenyl]-4-[3-hydroxylpropyl]cyclohexan-1-ol (CP55,940, an orthosteric agonist) while at the same time decreasing its efficacy (in G protein-mediated signaling). ORG27569 also decreases basal signaling, acting as an inverse agonist for the G protein-mediated signaling pathway. In ligand displacement assays, ORG27569 can displace the CB1 antagonist/inverse agonist, N-(piperidiny-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide(SR141716A). The goal of this work was to identify the binding site of ORG27569 at CB1. To this end, we used computation, synthesis, mutation, and functional studies to identify the ORG27569-binding site in the CB1 TMH3-6-7 region. This site is consistent with the results of K3.28192A, F3.36200A, W5.43279A, W6.48356A, and F3.25189A mutation studies, which revealed the ORG27569-binding site overlaps with our previously determined binding site of SR141716A but extends extracellularly. Additionally, we identified a key electrostatic interaction between the ORG27569 piperidine ring nitrogen and K3.28192 that is important for ORG27569 to act as an inverse agonist. At this allosteric site, ORG27569 promotes an intermediate conformation of the CB1 receptor, explaining ORG27569's ability to increase equilibrium binding of CP55,940. This site also explains ORG27569's ability to antagonize the efficacy of CP55,940 in three complementary ways. 1) ORG27569 sterically blocks movements of the second extracellular loop that have been linked to receptor activation. 2) ORG27569 sterically blocks a key electrostatic interaction between the third extracellular loop residue Lys-373 and D2.63176. 3) ORG27569 packs against TMH6, sterically hindering movements of this helix that have been shown to be important for receptor activation.

An array of cannabinoid ligands, bearing meta- and para-substituted pentafluorosulfanyl (SF5) aniline groups in position 3 of the pyrazole ring, was efficiently synthesised and compared with the exact trifluoromethyl and tert-butyl analogues. In general, the SF5 substituted ligands showed higher lipophilicity (i.e. log P values) than the CF3 counterparts and lower lipophilicity than the tert-butyl ones. In terms of pharmacological activity, SF5 pyrazoles generally showed slightly higher or equivalent CB1 receptor affinity (Ki), always in the nanomolar range, and selectivity towards the CB2 relative to both CF3 and tert-butyl analogues. Functional β-arrestin recruitment assays were used to determine equilibrium dissociation constants (Kb) and showed that all of the tested SF5 and CF3 compounds are CB1 neutral antagonists. These results confirm the possibility of successfully using an aromatic SF5 group as a stable, synthetically accessible and effective bioisosteric analogue of the electron-withdrawing CF3 group, and possibly also of bulky aliphatic groups, for drug discovery and development applications.

A novel class of cannabinoid ligands was synthesized in good overall yields by means of oxime-bio-conjugation between hydroxylamine-functionalized Rimonabant-type pyrazoles and fluoro-deoxy-carbohydrates (D-2-fluoro-deoxy-glucose, FDG, and D-5-fluoro-5-deoxy-ribose, FDR). FDR proved to be superior to FDG for bio-conjugation, as it occurred in milder conditions and at faster rate (rt, 20 min vs. 100 degrees C, 30 min). All of the title compounds showed relatively modest affinity for the CBI receptor (high nanomolar range) and selectivities vs. the CB2. (C) 2013 Elsevier B.V. All rights reserved.

Screening of a carefully selected library of 5,195 small molecules identified 34 hit compounds that interact with the regulatory cyclic nucleotide-binding domain (CNB) of the cAMP sensor, EPAC1. Two of these hits (I942 and I178) were selected for their robust and reproducible inhibitory effects within the primary screening assay. Follow-up characterisation by ligand observed nuclear magnetic resonance (NMR) revealed direct interaction of I942 and I178 with EPAC1 and EPAC2-CNBs in vitro. Moreover, in vitro guanine nucleotide exchange factor (GEF) assays revealed that I942 and, to a lesser extent, I178 had partial agonist properties towards EPAC1, leading to activation of EPAC1, in the absence of cAMP, and inhibition of GEF activity in the presence of cAMP. In contrast, there was very little agonist action of I942 towards EPAC2 or protein kinase A (PKA). To our knowledge, this is the first observation of non-cyclic-nucleotide small molecules with agonist properties towards EPAC1. Furthermore, the isoform selective agonist nature of these compounds highlights the potential for the development of small molecule tools that selectively up-regulate EPAC1 activity.

We have previously identified allosteric modulators of the cannabinoid CB(1) receptor (Org 27569, PSNCBAM-1) that display a contradictory pharmacological profile: increasing the specific binding of the CB(1) receptor agonist [(3)H]CP55940 but producing a decrease in CB(1) receptor agonist efficacy. Here we investigated the effect one or both compounds in a broad range of signaling endpoints linked to CB(1) receptor activation. We assessed the effect of these compounds on CB(1) receptor agonist-induced [(35)S]GTPγS binding, inhibition, and stimulation of forskolin-stimulated cAMP production, phosphorylation of extracellular signal-regulated kinases (ERK), and β-arrestin recruitment. We also investigated the effect of these allosteric modulators on CB(1) agonist binding kinetics. Both compounds display ligand dependence, being significantly more potent as modulators of CP55940 signaling as compared with WIN55212 and having little effect on [(3)H]WIN55212 binding. Org 27569 displays biased antagonism whereby it inhibits: agonist-induced guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding, simulation (Gα(s)-mediated), and inhibition (Gα(i)-mediated) of cAMP production and β-arrestin recruitment. In contrast, it acts as an enhancer of agonist-induced ERK phosphorylation. Alone, the compound can act also as an allosteric agonist, increasing cAMP production and ERK phosphorylation. We find that in both saturation and kinetic-binding experiments, the Org 27569 and PSNCBAM-1 appeared to influence only orthosteric ligand maximum occupancy rather than affinity. The data indicate that the allosteric modulators share a common mechanism whereby they increase available high-affinity CB(1) agonist binding sites. The receptor conformation stabilized by the allosterics appears to induce signaling and also selectively traffics orthosteric agonist signaling via the ERK phosphorylation pathway.

Background and purpose: A nonpsychoactive constituent of the cannabis plant, cannabidiol has been demonstrated to have low affinity for both cannabinoid CB1 and CB2 receptors. We have shown previously that cannabidiol can enhance electrically evoked contractions of the mouse vas deferens, suggestive of inverse agonism. We have also shown that cannabidiol can antagonize cannabinoid receptor agonists in this tissue with a greater potency than we would expect from its poor affinity for cannabinoid receptors. This study aimed to investigate whether these properties of cannabidiol extend to CB1 receptors expressed in mouse brain and to human CB2 receptors that have been transfected into CHO cells.

The CB1 receptor represents a promising target for the treatment of several disorders including pain-related disease states. However, therapeutic applications of Δ(9)-tetrahydrocannabinol and other CB1 orthosteric receptor agonists remain limited because of psychoactive side effects. Positive allosteric modulators (PAMs) offer an alternative approach to enhance CB1 receptor function for therapeutic gain with the promise of reduced side effects. Here we describe the development of the novel synthetic CB1 PAM, 6-methyl-3-(2-nitro-1-(thiophen-2-yl)ethyl)-2-phenyl-1H-indole (ZCZ011), which augments the in vitro and in vivo pharmacological actions of the CB1 orthosteric agonists CP55,940 and N-arachidonoylethanolamine (AEA). ZCZ011 potentiated binding of [(3)H]CP55,940 to the CB1 receptor as well as enhancing AEA-stimulated [(35)S]GTPγS binding in mouse brain membranes and β-arrestin recruitment and ERK phosphorylation in hCB1 cells. In the whole animal, ZCZ011 is brain penetrant, increased the potency of these orthosteric agonists in mouse behavioral assays indicative of cannabimimetic activity, including antinociception, hypothermia, catalepsy, locomotor activity, and in the drug discrimination paradigm. Administration of ZCZ011 alone was devoid of activity in these assays and did not produce a conditioned place preference or aversion, but elicited CB1 receptor-mediated antinociceptive effects in the chronic constriction nerve injury model of neuropathic pain and carrageenan model of inflammatory pain. These data suggest that ZCZ011 acts as a CB1 PAM and provide the first proof of principle that CB1 PAMs offer a promising strategy to treat neuropathic and inflammatory pain with minimal or no cannabimimetic side effects.

Pyrazoles bearing a 5-thiophenyl and a 4-cyano group were synthesised and tested for their affinity to the cannabinoid CB1 receptor showing in many cases single digit nanomolar K-i values and moderate to good selectivity towards the CB2 receptor. Some of these pyrazole ligands, such as 8g, displayed relatively low lipophilicity (experimental log P < 4) and high calculated Topological Polar Surface Area (TPSA) (>90) suggesting that these compounds may behave as peripherally restricted CB1 ligands. Furthermore, 2-fluoroethyl carboxamides 8d, 8h and 8l are interesting candidates for further development into PET tracers.

Δ9-tetrahydrocannabivarin (THCV) displaced [3H]CP55940 from specific binding sites on mouse brain and CHO-hCB2 cell membranes (Ki=75.4 and 62.8 nM, respectively). THCV (1 μM) also antagonized CP55940-induced stimulation of [35S]GTPγS binding to these membranes (apparent KB=93.1 and 10.1 nM, respectively). In the mouse vas deferens, the ability of Δ9-tetrahydrocannabinol (THC) to inhibit electrically evoked contractions was antagonized by THCV, its apparent KB-value (96.7 nM) approximating the apparent KB-values for its antagonism of CP55940- and R-(+)-WIN55212-induced stimulation of [35S]GTPγS binding to mouse brain membranes. THCV also antagonized R-(+)-WIN55212, anandamide, methanandamide and CP55940 in the vas deferens, but with lower apparent KB-values (1.5, 1.2, 4.6 and 10.3 nM, respectively). THCV (100 nM) did not oppose clonidine, capsaicin or (−)-7-hydroxy-cannabidiol-dimethylheptyl-induced inhibition of electrically evoked contractions of the vas deferens. Contractile responses of the vas deferens to phenylephrine hydrochloride or β,γ-methylene-ATP were not reduced by 1 μM THCV or R-(+)-WIN55212, suggesting that THCV interacts with R-(+)-WIN55212 at prejunctional sites. At 32 μM, THCV did reduce contractile responses to phenylephrine hydrochloride and β,γ-methylene-ATP, and above 3 μM it inhibited electrically evoked contractions of the vas deferens in an SR141716A-independent manner. In conclusion, THCV behaves as a competitive CB1 and CB2 receptor antagonist. In the vas deferens, it antagonized several cannabinoids more potently than THC and was also more potent against CP55940 and R-(+)-WIN55212 in this tissue than in brain membranes. The bases of these agonist- and tissue-dependent effects remain to be established. Keywords: Δ9-Tetrahydrocannabivarin, Δ9-tetrahydrocannabinol, R-(+)-WIN55212, anandamide, methanandamide, CP55940, cannabinoids, mouse vas deferens, CB1 receptor antagonist, CB2 receptor antagonist Introduction Cannabis sativa is the natural source of a set of at least 66 oxygen-containing aromatic hydrocarbon compounds that are known collectively as phytocannabinoids (reviewed in ElSohly, 2002). This study focused on a little-investigated phytocannabinoid, the n-propyl analogue of Δ9-tetrahydrocannabinol (THC) (Figure 1), which was first detected in cannabis by Gill et al. (1970) and named Δ9-tetrahydrocannabivarin (THCV) by Merkus (1971). The initial objective of this research was to establish whether THCV can activate or block cannabinoid CB1 or CB2 receptors. Some of our experiments were performed with membranes prepared from healthy brain tissue, which is densely populated with CB1 but not CB2 receptors (reviewed in Howlett et al., 2002), or from Chinese hamster ovary (CHO) cells transfected with hCB2 receptors. These membranes were used to investigate the ability of THCV to displace [3H]CP55940 from CB1- and CB2-binding sites and to determine whether it behaves as a CB1 or CB2 receptor agonist or antagonist. Experiments were also carried out with the mouse isolated vas deferens, a tissue in which cannabinoid receptor agonists such as R-(+)-WIN55212, CP55940, THC and 2-arachidonoyl ethanolamide (anandamide) can inhibit electrically evoked contractions (Devane et al., 1992; Pertwee et al., 1995b). This they are thought to do by acting on prejunctional neuronal cannabinoid CB1 receptors to inhibit release of the contractile neurotransmitters, ATP, acting on postjunctional P2X purinoceptors, and noradrenaline, acting on postjunctional α1-adrenoceptors (von Kugelgen & Starke, 1991; Trendelenburg et al., 2000; see also Pertwee, 1997; Schlicker & Kathman, 2001). Experiments were also performed with (−)-7-hydroxy-cannabidiol-dimethylheptyl, a synthetic analogue of the phytocannabinoid, (−)-cannabidiol, that inhibits electrically evoked contractions of the mouse vas deferens through a mechanism that appears to operate prejunctionally and to be at least partly CB1 receptor-independent (Pertwee et al., 2005). Some of the results described in this paper have been presented to the British Pharmacological Society (Pertwee et al., 2004). Figure 1 Structures of THC and THCV. Methods The methods used comply with the U.K. Animals (Scientific Procedures) Act, 1986 and Associated Guidelines for the Use of Experimental Animals. Drugs and chemicals THCV was supplied by GW Pharmaceuticals (Porton Down, Wiltshire, U.K.), THC by the National Institute on Drug Abuse (Bethesda, MD, U.S.A.) and (−)-7-hydroxy-cannabidiol-dimethylheptyl by Professor R. Mechoulam (Hebrew University of Jerusalem, Israel). SR141716A and SR144528 were obtained from Sanofi-Aventis (Montpellier, France). Phenylephrine hydrochloride, β,γ-methyleneadenosine 5′-triphosphate (β,γ-methylene-ATP), anandamide and clonidine hydrochloride were purchased from Sigma-Aldrich (Poole, Dorset, U.K.), R-(+)-WIN55212 and CP55940 from Tocris (Bristol, U.K.) and capsaicin from Research Biochemicals International (Natick, MA, U.S.A.). Phenylephrine hydrochloride, β,γ-methylene-ATP and clonidine were dissolved in a 0.9% aqueous solution of NaCl (saline). R-(+)-WIN55212 was dissolved in a 50% (v v−1) solution of dimethyl sulphoxide (DMSO) in saline and all other drugs were dissolved in pure DMSO. Drugs were added to organ baths in a volume of 10 μl. For the binding experiments, [3H]CP55940 (168 Ci mmol−1), [3H]R-(+)-WIN55212 (40 Ci mmol−1) and [35S]GTPγS (1250 Ci mmol−1) were obtained from Perkin-Elmer Life Sciences Inc. (Boston, MA, U.S.A.). [3H]SR141716A (44 Ci mmol−1) was obtained from Amersham Biosciences U.K. Ltd (Little Chalfont, Buckinghamshire, U.K.), GTPγS and adenosine deaminase from Roche Diagnostic (Indianapolis, IN, U.S.A.) and GDP from Sigma-Aldrich.

Pregnenolone is considered the inactive precursor of all steroid hormones, and its potential functional effects have been largely uninvestigated. The administration of the main active principle of Cannabis sativa (marijuana), Δ(9)-tetrahydrocannabinol (THC), substantially increases the synthesis of pregnenolone in the brain via activation of the type-1 cannabinoid (CB1) receptor. Pregnenolone then, acting as a signaling-specific inhibitor of the CB1 receptor, reduces several effects of THC. This negative feedback mediated by pregnenolone reveals a previously unknown paracrine/autocrine loop protecting the brain from CB1 receptor overactivation that could open an unforeseen approach for the treatment of cannabis intoxication and addiction. Supported by the EU-FP7 (REPROBESITY, HEALTH-F2-2008-223713, G.M.), the European Research Council (ENDOFOOD, ERC-2010-StG-260515, G.M.), the Agence Nationale pour la Recherche (contracts HICOMET and TIMMS, P.V.P.), the U.S. National Institutes of Health (NIH) (grants DA-03672, DA-003934, and DA-09789, R.A.R. and G.L.B., and RO1 DA003934 and KO5 DA021358, P.H.R.

GPR55 is activated by l-α-lysophosphatidylinositol (LPI) but also by certain cannabinoids. In this study, we investigated the GPR55 pharmacology of various cannabinoids, including analogues of the CB1 receptor antagonist Rimonabant®, CB2 receptor agonists, and Cannabis sativa constituents. To test ERK1/2 phosphorylation, a primary downstream signaling pathway that conveys LPI-induced activation of GPR55, a high throughput system, was established using the AlphaScreen® SureFire® assay. Here, we show that CB1 receptor antagonists can act both as agonists alone and as inhibitors of LPI signaling under the same assay conditions. This study clarifies the controversy surrounding the GPR55-mediated actions of SR141716A; some reports indicate the compound to be an agonist and some report antagonism. In contrast, we report that the CB2 ligand GW405833 behaves as a partial agonist of GPR55 alone and enhances LPI signaling. GPR55 has been implicated in pain transmission, and thus our results suggest that this receptor may be responsible for some of the antinociceptive actions of certain CB2 receptor ligands. The phytocannabinoids Δ9-tetrahydrocannabivarin, cannabidivarin, and cannabigerovarin are also potent inhibitors of LPI. These Cannabis sativa constituents may represent novel therapeutics targeting GPR55.

We investigated the pharmacology of three novel compounds, Org 27569 (5-chloro-3-ethyl-1H-indole-2-carboxylic acid [2-(4-piperidin-1-yl-phenyl)-ethyl]-amide), Org 27759 (3-ethyl-5-fluoro-1H-indole-2-carboxylic acid [2-94-dimethylamino-phenyl)-ethyl]-amide), and Org 29647 (5-chloro-3-ethyl-1H-indole-2-carboxylic acid (1-benzyl-pyrrolidin-3-yl)-amide, 2-enedioic acid salt), at the cannabinoid CB1 receptor. In equilibrium binding assays, the Org compounds significantly increased the binding of the CB1 receptor agonist [3H]CP 55,940 [(1R,3R,4R)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl)cyclohexan-1-ol], indicative of a positively cooperative allosteric effect. The same compounds caused a significant, but incomplete, decrease in the specific binding of the CB1 receptor inverse agonist [3H]SR 141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride], indicative of a limited negative binding cooperativity. Analysis of the data according to an allosteric ternary complex model revealed that the estimated affinity of each Org compound was not significantly different when the radioligand was [3H]CP 55,940 or [3H]SR 141716A. However, the estimated cooperatively factor for the interaction between modulator and radioligand was greater than 1 when determined against [3H]CP 55,940 and less than 1 when determined against [3H]SR 141716A. [3H]CP 55,940 dissociation kinetic studies also validated the allosteric nature of the Org compounds, because they all significantly decreased radioligand dissociation. These data suggest that the Org compounds bind allosterically to the CB1 receptor and elicit a conformational change that increases agonist affinity for the orthosteric binding site. In contrast to the binding assays, however, the Org compounds behaved as insurmountable antagonists of receptor function; in the reporter gene assay, the guanosine 5′-O-(3-[35S]thio)triphosphate binding assay and the mouse vas deferens assay they elicited a significant reduction in the Emax value for CB1 receptor agonists. The data presented clearly demonstrate, for the first time, that the cannabinoid CB1 receptor contains an allosteric binding site that can be recognized by synthetic small molecule ligands.

The CB1 allosteric modulator, ORG27569, has the paradoxical effect of increasing the equilibrium binding of CP55,940 (an orthosteric agonist), while at the same time decreasing its efficacy. ORG27569 also acts as an inverse agonist. We have previously used computational methods, synthesis, mutation, and functional studies to identify ORG27569's binding site in the THM3/TMH6/TMH7 region (Shore et al., ICRS, 2012). In this site, ORG27569 promotes an active-like conformation of the CB1 receptor, explaining ORG27569's ability to increase CP55,940's equilibrium binding. This site explains ORG27569's ability to antagonize CP55,940's efficacy in three complimentary ways: 1) ORG27569 sterically blocks movements of the second extracellular loop that have been linked to receptor activation, 2) ORG27569 sterically blocks a key electrostatic interaction between the third extracellular loop residue K373 and D2.63(176), and 3) ORG27569 packs against TMH6, sterically hindering movements of TMH6 that the Farrens lab have shown to be important to receptor activation. Additionally, we identified a key interaction between ORG27569's piperidine ring nitrogen and K3.28(192) that is required for ORG27569 to act as an inverse agonist.Using our model of ORG27569 docked in our active state model (in the presence of CP55,940), we designed, synthesized, and functionally characterized 4 analogs of ORG27569 that were designed to test our model and have improved interactions with the receptor. The analogs were functionalized with 3 different goals: 1) to form electrostatic interactions with D6.58(366), 2) to form an aromatic stack with F3.25(189), or 3) to test packing with TMH6-7. Our strategy to form new interactions with D6.58(366) was the most successful, resulting in an analog that is more potent than ORG27569. Interestingly, none of the analogs acted as inverse agonists, suggesting the potential therapeutic promise of CB1's allosteric site.