An investigation into the structural determinants of cannabinoid receptor ligand efficacy

Review of delta-8-tetrahydrocannabinol (Δ8 -THC): Comparative pharmacology with Δ9 -THC

The use of the intoxicating cannabinoid delta-8-tetrahydrocannabinol (Δ⁸ -THC) has grown rapidly over the last several years. There have been dozens of Δ⁸ -THC studies dating back over many decades, yet no review articles have comprehensively covered these findings. In this review, we summarize the pharmacological studies of Δ⁸ -THC, including receptor binding, cell signalling, in vivo cannabimimetic activity, clinical activity and pharmacokinetics. We give special focus to studies that directly compared Δ⁸ -THC to its more commonly studied isomer, Δ⁹ -THC. Overall, the pharmacokinetics and pharmacodynamics of Δ⁸ -THC and Δ⁹ -THC are very similar. Δ⁸ -THC is a partial agonist of the cannabinoid CB₁ receptor and has cannabimimetic activity in both animals and humans. The reduced potency of Δ⁸ -THC in clinical studies compared with Δ⁹ -THC can be explained by weaker cannabinoid CB₁ receptor affinity, although there are other plausible mechanisms that may contribute. We highlight the gaps in our knowledge of Δ⁸ -THC pharmacology where further studies are needed, particularly in humans.

Emergence of Delta-8 Tetrahydrocannabinol (THC) in DUID Investigation Casework: Method Development, Validation and Application

Cannabinoids is the most frequently reported illicit drug class in Driving Under the Influence of Drugs (DUID) investigation casework. In recent years, our laboratory observed an increasing rate of overlapping peaks for the cannabinoids confirmation performed using two-dimensional (2D)-High Performance Liquid Chromatography/Tandem Mass Spectrometry (LC-MS/MS). Starting in early 2018, the incidence of unresolved interfering substances increased, contributing to a higher rate of canceled testing that peaked at 3.7% in February 2019. The observed interference demonstrates a distinctive pattern affecting identifications and quantification of both Delta-9 THC and Delta-9 carboxy THC. An improved quantitative method was developed and validated to separate Delta-8 and -9 isomers and their metabolites in blood. All acceptance criteria were met with identical measurement ranges from the original method (lower limit of quantitation: 0.5 ng/mL for Delta-9 THC, 1.0 ng/mL for 11-Hydroxy Delta-9 THC, and 5.0 ng/mL for Delta-9 carboxy THC). Cannabinoids were extracted from whole blood using liquid-liquid extraction, separated in a 2D liquid chromatography system over a run-time of 10 min and detected by a tandem mass spectrometry system equipped with ESI source operating in positive ionization mode with scheduled multiple reaction mass spectrometric monitoring (MRM). The LC system consisted of a pair of Phenomenex® SecurityGuard™ C6 Phenyl (4x2 mm) cartridges for extracting the compounds with 5 mM ammonium formate buffer in deionized (DI) water and 0.1% formic acid in methanol as mobile phase, and a Phenomenex® Kinetex C18 column (100x3 mm) with 0.1% formic acid in DI water and 0.1% formic acid in methanol for LC separation at 45°C. Each set of isomers was fully resolved by the longer run-time method. To the authors' knowledge, this is the first report of a method that successfully quantitates these primary cannabinoids in blood specimens where significant concentrations of both Delta-9 and Delta-8 isomers are present.

Dissecting AI-2-mediated quorum sensing through C5-analogue synthesis and biochemical analysis

Autoinducer-2 (AI-2)-mediated quorum sensing (QS) is utilised for both intra- and inter-species communication by a wide variety of bacteria. An understanding of the mechanism of this communication has the potential to elucidate new targets for antibacterial therapeutics. Herein, we report the synthesis of DPD analogues with modified dynamic equilibria and the evaluation of their behaviour in Gram-negative bacteria. None of the compounds showed modulation of QS in S. Typhimurium, and although no antagonism of V. harveyi was observed, chloro-analogue C5-Cl-DPD showed modest agonism in this marine bacterium. This raises the possibility that access to a cyclic form of DPD may not be required for AI-2-mediated QS in V. harveyi.

Cannabinoids in disguise: Δ9-tetrahydrocannabinol-like effects of tetramethylcyclopropyl ketone indoles

Synthetic indole-derived cannabinoids have become commonly used recreational drugs and continue to be abused despite their adverse consequences. As compounds that were identified early in the epidemic (e.g., naphthoylindoles) have become legally banned, new compounds have appeared on the drug market. Two tetramethylcyclopropyl ketone indoles, UR-144 [(1-pentyl-1H-indol-3-yl)-(2,2,3,3-tetramethylcyclopropyl)methanone] and XLR-11 [(1-(5-fluoropentyl)-1H-indol-3-yl)-(2,2,3,3-tetramethylcyclopropyl)methanone], recently have been identified in confiscated products. These compounds are structurally related to a series of CB2-selective compounds explored by Abbott Labs. The purpose of the present study was to evaluate the extent to which UR-144 and XLR-11 shared cannabinoid effects with Δ9-tetrahydrocannabinol (Δ9-THC). Indices of in vitro and in vivo activity at cannabinoid receptors were assessed. Similar to other psychoactive cannabinoid agonists, XLR-11 and UR-144 showed low nanomolar (<30) affinity for CB1 and CB2 receptors, activated these receptors as full agonists, and produced dose-dependent effects that were blocked by rimonabant in mice, including antinociception, hypothermia, catalepsy and suppression of locomotor activity. The potency of both compounds was several-fold greater than Δ9-THC. XLR-11 and UR-144 also substituted for Δ9-THC in a Δ9-THC discrimination procedure in mice, effects that were attenuated by rimonabant. Analysis of urine from mice treated with the compounds revealed that both were extensively metabolized, with predominant urinary excretion as glucuronide conjugates. Together, these results demonstrate that UR-144 and XLR-11 share a pharmacological profile of in vitro and in vivo effects with Δ9-THC and other abused indole-derived cannabinoids and would be predicted to produce Δ9-THC-like subjective effects in humans.

Structural and pharmacological analysis of O-2050, a putative neutral cannabinoid CB(1) receptor antagonist

Rimonabant, the prototypic antagonist of cannabinoid CB(1) receptors, has been reported to have inverse agonist properties at higher concentrations, which may complicate its use as a tool for mechanistic evaluation of cannabinoid pharmacology. Consequently, recent synthesis efforts have concentrated on discovery of a neutral antagonist using a variety of structural templates. The purpose of this study was to evaluate the pharmacological properties of the putative neutral cannabinoid CB(1) receptor antagonist O-2050, a sulfonamide side chain analog of Δ(8)-tetrahydrocannabinol. O-2050 and related sulfonamide cannabinoids exhibited good affinity for both cannabinoid CB(1) and CB(2) receptors. While the other sulfonamide analogs produced cannabinoid agonist effects in vivo (e.g., activity suppression, antinociception, and hypothermia), O-2050 stimulated activity and was inactive in the other two tests. O-2050 also decreased food intake in mice, an effect that was reminiscent of that produced by rimonabant. Unlike rimonabant, however, O-2050 did not block the effects of cannabinoid agonists in vivo, even when administered i.c.v. In contrast, O-2050 antagonized the in vitro effects of cannabinoid agonists in [(35)S]GTPγS and mouse vas deferens assays without having activity on its own in either assay. Further evaluation revealed that O-2050 fully and dose-dependently substituted for Δ(9)-tetrahydrocannabinol in a mouse drug discrimination procedure (a cannabinoid agonist effect) and that it inhibited forskolin-stimulated cyclic AMP signaling with a maximum efficacy of approximately half that of the full agonist CP55,940 [(-)-cis-3-[2-hydroxy-4(1,1-dimethyl-heptyl)phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol]. Together, these results suggest that O-2050 is not a viable candidate for classification as a neutral cannabinoid CB(1) receptor antagonist.

Ligand Binding Sensitivity of the Extracellular Loop Two of the Cannabinoid Receptor 1

The cannabinoid receptor one (CB1) is a class A G-protein-coupled receptor thought to bind ligands primarily within its helical bundle. Evidence suggests, however, that the extracellular domain may also play a role. We have previously shown that the C-terminus of the extracellular loop 2 of CB1 is important in binding some compounds; receptors with mutations in this region (F268W, P269A, H270A, and I271A) bound some agonists with severely reduced affinity relative to the wild-type receptor. In the present work, we examine the impact of these mutations on binding a chemically diverse set of ligands. The receptors, F268W and I271A, exhibited a greater sensitivity to binding the inverse agonists/antagonists SLV319, AVE1625, NESS0327 relative to P269A and H270A, suggesting that the Pro and His are not involved in binding those compounds. In contrast, binding of the agonists, BAY593074 and WIN55212-2, was diminished in all four receptors, suggesting the conformational unit contributed by all four residues is important. A more marked loss in binding was observed for agonists of the nonclassical (CP55940) and classical (HU-210, JWH061, JWH179) cannabinoid classes and for a silent antagonist derivative (O-2050), pointing to the critical nature of this region for binding both the bicyclic/tricyclic core and the alkyl chain of these derivatives. However, moving the location of the alkyl chain on a series of pyrazole analogues shows it can be better accommodated in certain locations (O-1255) than others (O-1302, O-1690) and underscores the involvement of residues F268 and I271.

Antidepressant-like effect of delta9-tetrahydrocannabinol and other cannabinoids isolated from Cannabis sativa L

The antidepressant action of cannabis as well as the interaction between antidepressants and the endocannabinoid system has been reported. This study was conducted to assess the antidepressant-like activity of Delta(9)-THC and other cannabinoids. Cannabinoids were initially evaluated in the mouse tetrad assay to determine doses that do not induce hypothermia or catalepsy. The automated mouse forced swim (FST) and tail suspension (TST) tests were used to determine antidepressant action. At doses lacking hypothermic and cataleptic effects (1.25, 2.5, and 5 mg/kg, i.p.), both Delta(9)-THC and Delta(8)-THC showed a U-shaped dose response with only Delta(9)-THC showing significant antidepressant-like effects at 2.5 mg/kg (p<0.05) in the FST. The cannabinoids cannabigerol (CBG) and cannabinol (CBN) did not produce antidepressant-like actions up to 80 mg/kg in the mouse FST, while cannabichromene (CBC) and cannabidiol (CBD) exhibited significant effect at 20 and 200mg/kg, respectively (p<0.01). The antidepressant-like action of Delta(9)-THC and CBC was further confirmed in the TST. Delta(9)-THC exhibited the same U-shaped dose response with significant antidepressant-like action at 2.5 mg/kg (p<0.05) while CBC resulted in a significant dose-dependent decrease in immobility at 40 and 80 mg/kg doses (p<0.01). Results of this study show that Delta(9)-THC and other cannabinoids exert antidepressant-like actions, and thus may contribute to the overall mood-elevating properties of cannabis.

Some effects of CB1 antagonists with inverse agonist and neutral biochemical properties

The CB1 inverse agonist/antagonist SR141716A recently has been introduced for the management of obesity (rimonabant; Acomplia) and appears to have beneficial effects. However, its utility may be hampered in some individuals by adverse effects including nausea or emesis or by mood depression. The recent development of biochemically 'neutral' antagonists such as AM4113 (Sink et al., 2007) has allowed an initial evaluation of the proposition that adverse effects of SR141716A are associated with its inverse agonist activity. Thus far, data comparing SR141716A and AM4113 across several species indicate that both drugs produce dose-related direct effects on operant behavior within the same range of doses that serve to antagonize the behavioral and hypothermic effects of a CB1 agonist. However, initial observations suggest that AM4113 may not produce preclinical indications of nausea or emesis. Further studies with AM4113 and other novel CB1 antagonists differing in efficacy should amplify our understanding of the relationship between the pharmacological activity of CB1 antagonists and their behavioral effects.

Medicinal Chemistry Endeavors around the Phytocannabinoids

Over the past 50 years, a considerable research in medicinal chemistry has been carried out around the natural constituents of Cannabis sativa L. Following the identification of Delta9-tetrahydrocannabinol (Delta9-THC) in 1964, critical chemical modifications, e.g., variation of the side chain at C3 and the opening of the tricyclic scaffold, have led to the characterization of potent and cannabinoid receptor subtype-selective ligands. Those ligands that demonstrate high affinity for the cannabinoid receptors and good biological efficacy are still used as powerful pharmacological tools. This review summarizes past as well as recent developments in the structure-activity relationships of phytocannabinoids.

3D-QSAR Studies on Cannabinoid CB1 Receptor Agonists: G-Protein Activation as Biological Data

G-protein activation via the CB1 receptor was determined for a group of various CB1 ligands and utilized as biological activity data in subsequent CoMFA and CoMSIA studies. Both manual techniques and automated docking at CB1 receptor models were used to obtain a common alignment of endocannabinoid and classical cannabinoid derivatives. In the final alignment models, the endocannabinoid headgroup occupies a unique region distinct from the classical cannabinoid structures, supporting the hypothesis that these structurally diverse molecules overlap only partially within the receptor binding site. Both CoMFA and CoMSIA produce statistically significant models based on the manual alignment and a docking alignment at one receptor conformer. Leave-half-out cross-validation and progressive scrambling were successfully used in assessing the predictivity of the QSAR models.

Separation of cannabinoid receptor affinity and efficacy in delta-8-tetrahydrocannabinol side-chain analogues

1. The activities of a number of side-chain analogues of delta-8-tetrahydrocannabinol (Delta(8)-THC) in rat cerebellar membrane preparations were tested. 2. The affinities of each compound for the CB(1) receptor were compared by their respective abilities to displace [(3)H]-SR141716A and their efficacies compared by stimulation of [(35)S]-GTPgammaS binding. 3. It was found that the affinities varied from 0.19+/-0.03 nM for 3-norpentyl-3-[6'-cyano,1',1'-dimethyl]hexyl-Delta(8)-THC to 395+/-66.3 nM for 5'-[N-(4-chlorophenyl)]-1',1'-dimethyl-carboxamido-Delta(8)-THC. 4. The efficacies of these compounds varied greatly, ranging from the very low efficacy exhibited to acetylenic compounds such as 1'-heptyn-Delta(8)-THC and 4'-octyn-Delta(8)-THC to higher efficacy compounds such as 5'-(4-cyanophenoxy)-1',1'-dimethyl-Delta(8)-THC and 5'-[N-(4-aminosulphonylphenyl)]-1',1' dimethyl-carboxamido Delta(8)-THC. All agonist activities were antagonized by the CB(1)-selective antagonist SR141716A. 5. It was found that a ligand's CB(1) affinity and efficacy are differentially altered by modifications in the side-chain. Decreasing the flexibility of the side-chain reduced efficacy but largely did not alter affinity. Additionally, the positioning of electrostatic moieties, such as cyano groups, within the side-chain also has contrasting effects on these two properties. 6. In summary, this report details the characterization of a number of novel Delta(8)-THC analogues in rat cerebellar membranes. It provides the first detailed pharmacological analysis of how the inclusion of electrostatic moieties in the side-chain and also how alteration of the side-chain's flexibility may differentially affect a CB(1) cannabinoid receptor ligand's affinity and efficacy.

Effects of unsaturated side-chain analogs of tetrahydrocannabinol on cytochromes P450

The ability of unsaturated side-chain analogs of Delta(8)-tetrahydrocannabinol (THC) to selectively inactivate mouse hepatic cytochromes P450 3A11 and 2C29 was examined. THC side-chain analogs were preincubated with mouse hepatic microsomes and NADPH for various times before dilution and determination of Delta(9)-THC metabolism specific for P450s 3A11 and 2C29. THC-enyl analogs had little or no effect on P450 3A11 but inactivated P450 2C29 in a time-dependent manner, with approximately 50% inactivation observed after a 30-min preincubation. THC-ynyl analogs were less selective in their P450 inactivation but appeared to be more effective than their corresponding enyl analogs. THC-ynyl analogs inactivated P450s 3A11 and 2C29 in a time-dependent manner and could inactive 40-80% of their activities after a 30-min preincubation. The THC-ynyl analogs were nearly as effective as cannabidiol, a well-characterized inactivator of these mouse P450s. Despite their ability to inactivate P450 in vitro, neither the THC-enyl nor the THC-ynyl analogs were very effective after in vivo administration. Unsaturated side-chain THC analogs may be useful in the development of specific P450 inactivators.

Cannabinoid receptor ligands: clinical and neuropharmacological considerations, relevant to future drug discovery and development

This review highlights some important advances that have taken place in cannabinoid research over the last four years. It focuses on novel ligands that are of interest either as experimental tools or as lead compounds for therapeutic agents and possible clinical applications for some of these ligands. The molecular targets for these compounds are various components of the system of endogenous cannabinoids (endocannabinoids) and receptors that together constitute the 'endocannabinoid system'. These are CB(1) cannabinoid receptors that are present mainly on central and peripheral neurones, CB(2) cannabinoid receptors that are expressed predominantly by immune cells, the biochemical mechanisms responsible for the tissue uptake or metabolism of endocannabinoids and vanilloid receptors. Other cannabinoid receptor types may also exist. Recently developed ligands include potent and selective agonists for CB(1) and CB(2) receptors, a potent CB(2)-selective antagonist/inverse agonist and inhibitors of endocannabinoid uptake or metabolism. Future research should be directed at characterising the endocannabinoid system more completely and at obtaining more conclusive clinical data about the possible beneficial effects of cannabinoids as well as their adverse effects. There is also a need for improved cannabinoid formulations/modes of administration in the clinic and advances in this area should be facilitated by the recent development of a potent water-soluble CB(1)/CB(2) receptor agonist. A growing number of strategies for separating sought-after therapeutic effects of cannabinoid receptor agonists from the unwanted consequences of CB(1) receptor activation are now emerging and these are discussed at the end of this review.

Comparison of novel cannabinoid partial agonists and SR141716A in the guinea-pig small intestine

The controversial nature of the CB(1) receptor antagonist, SR141716A, in the guinea-pig small intestine was investigated by comparing it with four analogues of Delta(8)-tetrahydrocannabinol (Delta(8)-THC): O-1184, O-1238, O-584 and O-1315. These compounds (10 - 1000 nM) inhibited the electrically-evoked contractions with a rank order of potency of O-1238>O-1184>O-584>O-1315. Log concentration-response curves for O-1238, O-1184 and O-1315 were significantly shifted to the right by SR141716A and the maxima were significantly less than that of the CB(1) agonist, WIN55212-2, an indication of partial agonism. Partial saturation of the triple bond in O-1184 to a cis double bond (O-1238) increased its potency as an agonist (pEC(50) from 6.42 to 7.63) and as an antagonist of WIN55212-2, (pK(B), from 8.36 to 9.49). Substitution of the terminal azide group by an ethyl group (O-584) or removal of the phenolic hydroxyl group (O-1315) had no significant effect on the agonist or antagonist potency. None of these analogues increased the twitch response in a manner resembling that of SR141716A. O-1184 (10 and 100 nM) shifted the log concentration-response curve of WIN55212-2 for inhibition of the twitch responses to the right with pK(B) values of 8.29 and 8.38, respectively. We conclude that these Delta(8)-THC analogues behave as partial agonists rather than silent antagonists at CB(1) binding sites in this tissue. There was no evidence of antagonism of endocannabinoids thus supporting the hypothesis that, in this tissue, SR141716A is an inverse agonist of constitutively active CB(1) receptors.

Structural determinants of the partial agonist-inverse agonist properties of 6'-azidohex-2'-yne-delta8-tetrahydrocannabinol at cannabinoid receptors

1. We have extended previous investigations of four analogues of Delta8-tetrahydrocannabinol (Delta8-THC): 6'-azidohex-2'-yne-Delta8-THC (O-1184), 6'-azidohex-cis-2'-ene-Delta8-THC (O-1238) and octyl-2'-yne-Delta8-THC (O-584) and its 1-deoxy-analogue (O-1315). 2. O-1184, O-1238 and O-584 displaced [3H]-CP55940 from specific binding sites on Chinese hamster ovary (CHO) cell membranes expressing CB1 or CB2 cannabinoid receptors, with pKi values of 8.28 to 8.45 (CB1) and 8.03 to 8.13 (CB2). The pKi values of O-1315 were significantly less, 7.63 (CB1) and 7.01 (CB2). 3. All the analogues inhibited forskolin-stimulated cyclic AMP production by CB1-transfected CHO cells (pEC50=9.16 to 9.72). Only O-1238 behaved as a full agonist in this cell line. 4. In mouse vasa deferentia, O-1238 inhibited electrically-evoked contractions (pEC50=10.18 and Emax=70.5%). Corresponding values for O-1184 were 9.08 and 21.1% respectively. At 1 nM, O-1184 produced surmountable antagonism of the cannabinoid receptor agonist, CP55940. However, at 0.1 nM, O-1184 did not attenuate CP55940-induced inhibition of cyclic AMP production by CB1-transfected CHO cells. 5. In CB2-transfected CHO cells, cyclic AMP production was inhibited by CP55940 (pEC50=8.59), enhanced by O-1184 and O-584 (pEC50=8.20 and 6.86 respectively) and not significantly affected by O-1238 or O-1315. 6. At 100 nM, O-1184 and O-1238 produced surmountable antagonism of CP55940 in CB2 cells, decreasing the pEC50 of CP55940 from 8.61 to 7.42 (O-1184) or from 8. 54 to 7.44 (O-1238). 7. These data support the hypothesis that increasing the degree of unsaturation of the aliphatic side-chain of Delta8-THC analogues has little effect on CB1 or CB2 receptor affinity but can reduce CB1 receptor efficacy and reverse the direction of responses elicited at CB2 receptors.

Cannabinoid agonists and antagonists discriminated by receptor binding in rat cerebellum

1. The effect of allosteric regulators on the binding affinity of a number of cannabinoid receptor ligands of varying efficacy in the rat cerebellum was investigated. 2. Radioligand ([3H]-SR141716A) competition curves were constructed in the presence or absence of sodium ions, magnesium ions and guanine nucleotides. 3. It was found that the presence of these allosteric regulators did not affect the affinity of the two antagonists used but did cause a significant decrease in the affinity of full and partial agonists. 4. This reduction in affinity ranged from a 3.67 fold rightward shift of the displacement curve of a mixed agonist/antagonist (3-(6-cyano-2-hexynyl)-delta-8-tetrahydrocannabinol-O-823) to a 38 fold rightward shift for 3-(1, 1-dimethyl-6-dimethylcarboxamide)-delta-8-tetrahydrocannabinol (O-1125), a full agonist. 5. In summary, the results of this study suggest a simple method for the inference of functional data using the classical radioligand binding assay.