Cannabinoid receptors and their ligands: ligand-ligand and ligand-receptor modeling approaches

The cannabinoid CB1 and CB2 receptors belong to the class A, rhodopsin-like family of GPCRs. Antagonists for each receptor sub-type, as well as four structural classes of agonists that bind to both receptors, have been identified. An extensive amount of structure-activity relationship information (SAR) has been developed for agonists and antagonists that bind at CB1, while the SAR of CB2 ligands is only now emerging in the literature. This chapter focuses both on recent CB1 and CB2 SAR and on the pharmacophores for ligand recognition at the CB1 receptor that have been developed using ligand-ligand or ligand-receptor approaches. In a ligand-ligand approach, the structure of the binding site of the ligand is not directly considered. This approach is an attempt to infer information about the macromolecular binding site, and/or modes of binding interactions from a correlation between experimentally determined biological activities and the structural and electronic features of a series of small molecules. In a ligand-receptor approach, cannabinoid (CB) receptor models are probed for ligand binding sites and binding sites can be screened using energetic criteria, as well as ligand SAR and the CB mutation literature. This chapter discusses the factors that control the quality of the results emanating from each of these approaches and identifies areas of agreement and of disagreement in the existing CB literature. Challenges for future SAR and pharmacophore development are also identified.

Activation of the cannabinoid CB1 receptor may involve a W6 48/F3 36 rotamer toggle switch

The cannabinoid CB1 receptor, a member of the Rhodopsin (Rho) family of G protein coupled receptors (GPCRs), exhibits high levels of constitutive activity. In contrast, Rho exhibits an exquisite lack of constitutive activity. In Rho, W6.48(265) on transmembrane helix 6 (TMH6) is flanked by aromatic residues at positions i-4 (F6.44) and i + 3 (Y6.51), while in CB1 the residues i-4 and i + 3 to W6.48 are leucines (L6.44 and L6.51). Based upon spectroscopic evidence, W6.48 has been proposed to undergo a rotamer switch (chi1 g+ -->trans) upon activation of Rho. In the work reported here, the biased Monte Carlo method, Conformational Memories (CM) was used to test the hypothesis that the high constitutive activity exhibited by CB1 may be due, in part, to the lack of aromatic residues i-4 and i + 3 from W6.48. In this work, the W6.48 rotamer shift (chi1 g+ -->trans) was used as the criterion for activation. Conformational Memories (CM) calculations on WT CB1 TMH6 and L6.44F and L6.51Y mutant TMH6s revealed that an aromatic residue at 6.44 tends to disfavor the W6.48 chi1 g+ -->trans transition and an aromatic residue at 6.51 would require a concomitant movement of the Y6.51 chi1 from trans-->g+ when the W6.48 chi1 undergoes a g+ -->trans shift. In contrast, CM calculations on WT CB1 TMH6 revealed that the presence of leucines at 6.44 and 6.51 provide W6.48 with greater conformational mobility, with a W6.48 transchi1 preferred. Conformational Memories calculations also revealed that the W6.48 chi1 g+ -->trans transition in WT CB1 TMH6 is correlated with the degree of kinking in TMH6. The average proline kink angles for TMH6 were higher for helices with a W6.48 g+ chi1 than for those with a W6.48 transchi1. These results are consistent with experimental evidence that TMH6 straightens during activation. Transmembrane helix (TMH) bundle models of the inactive (R) and active (R*) states of CB1 were then probed for interactions that may constrain W6.48 in the inactive state of CB1. These studies revealed that F3.36 (transchi1) helps to constrain W6.48 in a g+ chi1 in the inactive (R) state of CB1. In the R* state, these studies suggest that F3.36 must assume a g+ chi1 in order to allow W6.48 to shift to a transchi1. These results suggest that the W6.48/F3.36 interaction may act as the 'toggle switch' for CB1 activation, with W6.48 chi1 g+/F3.36 chi1 trans representing the inactive (R) and W6.48 chi1 trans/F3.36 chi1 g+ representing the active (R*) state of CB1.

Conformational requirements for endocannabinoid interaction with the cannabinoid receptors, the anandamide transporter and fatty acid amidohydrolase

Anandamide (N-arachidonoylethanolamine) has been identified as an endogenous ligand of the G-protein coupled cannabinoid CB(1) receptor. Recent studies have postulated the existence of carrier-mediated anandamide transport which is involved in the termination of the biological effects of anandamide. A membrane bound amidohydrolase (fatty acid amide hydrolase, FAAH), located intracellulary, hydrolyzes and inactivates anandamide and other endogenous cannabinoids such as 2-arachidonoylglycerol (2-AG). Structure-activity relationships (SARs) for endocannabinoid interaction with the CB receptors, the anandamide transporter and FAAH are currently emerging in the literature. This review considers the divergences between these SARs and focuses upon the conformational implications for endocannabinoid recognition at each of these biological targets.

The difference between the CB(1) and CB(2) cannabinoid receptors at position 5.46 is crucial for the selectivity of WIN55212-2 for CB(2)

It has been reported that WIN55212-2, a prototypic aminoalkylindole, has higher affinity for CB(2) than for CB(1). To explain the selectivity of WIN55212-2 for CB(2), molecular modeling studies were performed to probe the interacting sites between WIN55212-2 and cannabinoid receptors. In TMH5 the position 5.46 is a Phe in CB(2) versus a Val in CB(1). Docking of WIN55212-2 into the models of CB(1) and CB(2) predicts that F5.46 will result in a greater aromatic stacking of CB(2) with WIN55212-2. Using site-directed mutagenesis, this hypothesis was tested by exchanging the amino acids at position 5.46 between CB(1) and CB(2). Two mutations, including a Phe to Val mutation at the position 5.46 in CB(2) (CB2F5. 46V), and a corresponding Val to Phe mutation at the position 5.46 in CB(1) (CB(1)V5.46F), were made. The mutant receptors were transfected into 293 cells, and stable cell lines expressing similar numbers of receptors as wild-type receptors were chosen for additional ligand binding and cAMP accumulation studies. In ligand- binding assays, the CB(2)F5.46V mutation decreased the affinity of WIN55212-2 for CB(2) by 14-fold. In contrast, the CB(1)V5.46F mutation increased the affinity of WIN55212-2 for CB(1) by 12-fold. However, these mutations did not change the affinity of HU-210, CP-55940, and anandamide for CB(1) and CB(2). In cAMP accumulation assays, the changes in EC(50) values of WIN55212-2 were consistent with the changes in its binding affinity caused by the mutations. These results strongly support the hypothesis that the selectivity of WIN55212-2 for CB(2) over CB(1) is attributable to the change from Val in CB(1) at position 5.46 to Phe in CB(2).

Ligand-ligand and ligand-receptor approaches to modeling the cannabinoid CB1 and CB2 receptors: achievements and challenges

The cannabinoid CB1 and CB2 receptors belong to the super family of G protein-coupled receptors. Antagonists for each receptor sub-type, as well as four structural classes of agonists that bind to both receptors, have been identified. In the absence of an experimentally determined structure for each of these two receptors, computational molecular modeling approaches have been employed to begin to probe structure-function relationships. Molecular modeling studies have been approached from two perspectives: calculations involving only ligands (Ligand-Ligand Approach) or calculations of the interaction of a ligand with its receptor macromolecule (Ligand-Receptor Approach) [49]. The Ligand-Ligand Approach does not directly consider the structure of the ligand binding site, but attempts to infer information about this site from a correlation between experimentally determined biological activities and the structural and electronic features of a series of small molecules. Ligand-Ligand Approaches result in development of pharmacophore models. Although closer to the event of interest, the study of the binding of a ligand to its receptor is less common because it requires a working knowledge of the receptor structure [54]. Mutation/ chimera studies, as well as structure activity relationships can be used to test models developed in a Ligand-Receptor Approach. This review considers both Ligand-Ligand and Ligand-Receptor computational studies of the CB1 and CB2 receptors. Challenges for further modeling studies are also identified.

Role of a conserved lysine residue in the peripheral cannabinoid receptor (CB2): evidence for subtype specificity

The human cannabinoid receptors, central cannabinoid receptor (CB1) and peripheral cannabinoid receptor (CB2), share only 44% amino acid identity overall, yet most ligands do not discriminate between receptor subtypes. Site-directed mutagenesis was employed as a means of mapping the ligand recognition site for the human CB2 cannabinoid receptor. A lysine residue in the third transmembrane domain of the CB2 receptor (K109), which is conserved between the CB1 and CB2 receptors, was mutated to alanine or arginine to determine the role of this charged amino acid in receptor function. The analogous mutation in the CB1 receptor (K192A) was found to be crucial for recognition of several cannabinoid compounds excluding (R)-(+)-[2, 3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1, 4-benzoxazin-6-yl](1-naphthalenyl)methanone (WIN 55,212-2). In contrast, in human embryonic kidney (HEK)-293 cells expressing the mutant or wild-type CB2 receptors, we found no significant differences in either the binding profile of several cannabinoid ligands nor in inhibition of cAMP accumulation. We identified a high-affinity site for (-)-3-[2-hydroxyl-4-(1, 1-dimethylheptyl)phenyl]-4-[3-hydroxyl propyl] cyclohexan-1-ol (CP-55,940) in the region of helices 3, 6, and 7, with S3.31(112), T3.35(116), and N7.49(295) in the K109A mutant using molecular modeling. The serine residue, unique to the CB2 receptor, was then mutated to glycine in the K109A mutant. This double mutant, K109AS112G, retains the ability to bind aminoalkylindoles but loses affinity for classical cannabinoids, as predicted by the molecular model. Distinct cellular localization of the mutant receptors observed with immunofluorescence also suggests differences in receptor function. In summary, we identified amino acid residues in the CB2 receptor that could lead to subtype specificity.

The bioactive conformation of aminoalkylindoles at the cannabinoid CB1 and CB2 receptors: insights gained from (E)- and (Z)-naphthylidene indenes

The aminoalkylindoles (AAIs) are agonists at both the cannabinoid CB1 and CB2 receptors. To determine whether the s-trans or s-cis form of AAIs is their receptor-appropriate conformation, two pairs of rigid AAI analogues were studied. These rigid analogues are naphthylidene-substituted aminoalkylindenes that lack the carbonyl oxygen of the AAIs. Two pairs of (E)- and (Z)-naphthylidene indenes (C-2 H and C-2 Me) were considered. In each pair, the E geometric isomer is intended to mimic the s-trans form of the AAIs, while the Z geometric isomer is intended to mimic the s-cis form. Complete conformational analyses of two AAIs, pravadoline (2) and WIN-55, 212-2 (1), and of each indene were performed using the semiempirical method AM1. S-trans and s-cis conformations of 1 and 2 were identified. AM1 single-point energy calculations revealed that when 1 and each indene were overlayed at their corresponding indole/indene rings, the (E)- and (Z)-indenes were able to overlay naphthyl rings with the corresponding s-trans or s-cis conformer of 1 with an energy expense of 1.13/0.69 kcal/mol for the C-2 H (E/Z)-indenes and 0.82/0.74 kcal/mol for the C-2 Me (E/Z)-indenes. On the basis of the hypothesis that aromatic stacking is the predominant interaction of AAIs such as 1 at the CB receptors and on the demonstration that the C-2 H (E/Z)- and C-2 Me (E/Z)-indene isomers can mimic the positions of the aromatic systems in the s-trans and s-cis conformers of 1, the modeling results support the previously established use of indenes as rigid analogues of the AAIs. A synthesis of the naphthylidene indenes was developed using Horner-Wittig chemistry that afforded the Z isomer in the C-2 H series, which was not produced in significant amounts from an earlier reported indene/aldehyde condensation reaction. This approach was extended to the C-2 Me series as well. Photochemical interconversions in both the C-2 H and C-2 Me series were also successful in obtaining the less favored isomer. Thus, the photochemical process can be used to provide quantities of the minor isomers C-2 H/Z and C-2 Me/E. The CB1 and CB2 affinities as well as the activity of each compound in the twitch response of the guinea pig ileum (GPI) assay were assessed. The E isomer in each series was found to have the higher affinity for both the CB1 and CB2 receptors. In the rat brain membrane assay versus [3H]CP-55,940, the Ki's for the C-2 H/C-2 Me series were 2.72/2.89 nM (E isomer) and 148/1945 nM (Z isomer). In membrane assays versus [3H]SR141716A, a two-site model was indicated for the C-2 H/C-2 Me (E isomers) with Ki's of 10. 8/9.44 nM for the higher-affinity site and 611/602 nM for the lower-affinity site. For the Z isomers, a one-site model was indicated with Ki's of 928/2178 nM obtained for the C2 H/C-2 Me analogues, respectively. For the C-2 H/C-2 Me series, the CB2 Ki's obtained using a cloned cell line were 2.72/2.05 nM (E isomer) and 132/658 nM (Z isomer). In the GPI assay, the relative order of potency was C-2 H E > C-2 Me E > C-2 H Z > C-2 Me Z. The C-2 H E isomer was found to be equipotent with 1, while the C-2 Me Z isomer was inactive at concentrations up to 3.16 microM. Thus, results indicate that the E geometric isomer in each pair of analogues is the isomer with the higher CB1 and CB2 affinities and the higher pharmacological potency. Taken together, results reported here support the hypothesis that the s-trans conformation of AAIs such as 1 is the preferred conformation for interaction at both the CB1 and CB2 receptors and that aromatic stacking may be an important interaction for AAIs at these receptors.

Exploration of biologically relevant conformations of anandamide, 2-arachidonylglycerol, and their analogues using conformational memories

The endogenous cannabinoid anandamide (N-arachidonoylethanolamide) has been shown to possess higher affinity for the cannabinoid CB1 receptor than for the CB2 receptor. Carrier-mediated transport has been proposed to be essential for the termination of the biological effects of anandamide, while hydrolysis of anandamide is performed by a membrane-bound amidohydrolase, fatty acid amidohydrolase (FAAH). As interaction of anandamide with each of these targets occurs in different environments, the conformations of anandamide for interaction with each target may differ. To ascertain what conformations of anandamide, a highly flexible molecule, are favored in polar and nonpolar environments, the new method of Conformational Memories (CM) was used. CM has been shown to achieve complete conformational sampling of highly flexible ligands, to converge in a very practical number of steps, and to be capable of overcoming energy barriers very efficiently (Guarnieri et al. J. Am. Chem. Soc. 1996, 118, 5580). The generalized Born/surface area (GB/SA) continuum solvation models for chloroform and for water were used in the CM calculations. As a means of validation, CM was first applied to arachidonic acid because both experimental and theoretical conformational studies of arachidonic acid have been reported. CM was also applied to sn-2-arachidonylglycerol (2-AG), another endogenous CB ligand; to a 1,1-dimethylheptyl derivative of anandamide, an analogue with higher CB1 affinity than anandamide; and to N-(2-hydroxyethyl)prostaglandin-B2-ethanolamide (PGB2-EA), a prostanoid ligand which does not bind to CB1. Consistent with the literature, arachidonic acid was found to exist in an extended, angle-iron shape and in back-folded conformations which were U, J, or helical in shape. The angle-iron and U-shapes were both highly populated conformations with the angle-iron preferred in CHCl3 and the U-shape preferred in H2O. Results for anandamide and 2-AG paralleled those for arachidonic acid with the exception that anandamide in water does not adopt a pure extended conformation but, rather, favors a hybrid-extended/U-shape. For the dimethyl-heptyl derivative of anandamide, the U-shape was found to be predominant in both environments (42% in CHCl3, 38% in H2O), but the population of the angle-iron shape was still significant (25% in CHCl3, 29% in H2O). For all of these ligands, J-shaped conformers constituted from 7% to 17% of the conformer population, while the helical shape was less than 5%. In both CHCl3 and H2O, the presence of the five-membered ring attenuates the ability of PGB2-EA to adopt an extended conformation. PGB2-EA was found instead to exist predominantly in an L-shape (i.e., distorted U-shape). The low probability of PGB2-EA adopting an extended conformation may be why PGB2-EA shows such low affinity for the CB1 receptor. The conformational information obtained here for anandamide and 2-AG may be useful in the design of rigid analogues which mimic the preferred molecular conformations (shapes) of these ligands. Such rigid analogues may be useful in deducing the bioactive conformation of these endogenous cannabinoids, not only at the CB receptors but also at the FAAH enzyme active site and possibly at the binding site(s) on the newly proposed anandamide transporter.

Importance of the C-1 substituent in classical cannabinoids to CB2 receptor selectivity: synthesis and characterization of a series of O,2-propano-delta 8-tetrahydrocannabinol analogs

The separation of the mood-altering effects of cannabinoids from their therapeutic effects has been long sought. Results reported here for a series of C-9 analogs of the cyclic ether O,2-propano-delta 8-tetrahydrocannabinol (O,2-propano-delta 8-THC) point to the C-1 position in classical cannabinoids as a position for which CB2 subtype selectivity occurs within the cannabinoid receptors. O,2-Propano-11-delta 8-THC, O,2-propano delta 9,11-THC, O,2-propano-9-oxo-11-nor-hexahydrocannabinol (O,2-propano-9-oxo-11-nor-HHC), and O,2-propano-9 alpha- and O,2-propano-9 beta-OH-11-nor-HHC were synthesized and evaluated in radioligand displacement assays for affinity at the CB1 and CB2 receptors and in the mouse vas deferens in vitro assay and the mouse tetrad in vivo assay for cannabinoid activity. Evaluation of binding affinity at the CB1 and CB2 receptors revealed that each compound possesses a modest increased affinity for the CB2 receptor. Analogs which contained an oxygen attached to C-9 (i.e., oxo and hydroxy derivatives) showed the highest affinity and selectivity for CB2 (for O,2-propano-9-oxo-11-nor-HHC, Ki(CB1) = 90 nM, Ki(CB2) = 23 nM, selectivity ratio 3.9; for O,2-propano-9 beta-OH-11-nor-HHC, Ki(CB1) = 26 nM, Ki(CB2) = 5.8 nM, selectivity ratio 4.5). Each compound was found to produce a dose-dependent inhibition of electrically-evoked contractions of the mouse isolated vas deferens when administered at submicromolar concentrations. This inhibition could readily be prevented by the selective CB1 cannabinoid receptor antagonist SR-141716A. The analogs exhibited unique in vivo profiles with O,2-propano-delta 9,11-THC exhibiting antinociception with reduced activity in three other in vivo measures and O,2-propano-9 beta-OH-HHC exhibiting lack of dose responsiveness in all measures. The CB2 selectivities in the O,2-propano analogs may be due to differences in solvation/desolvation that occur when the ligands enter the CB1 vs CB2 binding site. Alternatively, the CB2 selectivities may be a results of an amino acid change from a hydrogen bond-accepting residue in CB1 to a hydrogen bond-donating residue in CB2.

Evidence against cannabinoid receptor involvement in intraocular pressure effects of cannabinoids in rabbits

Cannabinoid receptor involvement in the intraocular-pressure (IOP)-lowering effect of the cannabinoids has not been determined. These receptors bind four structural classes of agonists: the classical and the nonclassical cannabinoids, the aminoalkylindoles and the arachidonylethanolamides (the endogenous ligands). We examined the IOP effect in normotensive rabbits after systemic administration of an aminoalkylindole, WIN-55,212-2, and of (R)-(+)-methanandamine, a hydrolysis-resistant endogenous ligand derivative. The decrease in IOP for 6 animals treated with WIN-55,212-2 was marginal and for 6 animals treated with (R)-(+)-methanandamide was negligible compared to animals treated with vehicle alone. These studies suggest that the IOP-lowering effects of cannabinoids are not mediated by a cannabinoid receptor in rabbits.

Construction of a 3D model of the cannabinoid CB1 receptor: determination of helix ends and helix orientation

The goal of this study was to determine the ends and orientations of the seven transmembrane helices of the cannabinoid (CB1) receptor, a G-protein coupled receptor (GPCR). After initial sequence alignment, Fourier transform methods were used with the nPRIFT hydrophobicity scale and with a variability profile to calculate the alpha-helical periodicity (AP) in the primary amino acid sequence of the human CB1 receptor and of its alignment. AP plots were used to identify the amino acids which comprise each of the seven CB1 transmembrane helices. An intracellular alpha helix extension of Helix 7 was characterized by analyzing the relative direction of variability and hydrophobic moment vectors. Variability moment vectors were then used to delineate the orientation of each helix in the membrane. Based upon these vector calculations, a tentative helix bundle arrangement was obtained. This arrangement is largely consistent with the proposed transmembrane helix bundle arrangement in rhodopsin, a GPCR.

The design, synthesis and testing of desoxy-CBD: further evidence for a region of steric interference at the cannabinoid receptor

Cannabidiol CBD, a non-psychoactive constituent of marihuana, has been reported to possess essentially no affinity for cannabinoid CB1 receptor binding sites in the brain. Our hypothesis concerning CBD's lack of affinity for the cannabinoid CB1 receptor is that CBD is not capable of clearing a region of steric interference at the CB1 receptor and thereby not able to bind to this receptor. We have previously characterized this region of steric interference at the CB1 receptor [P.H. Reggio, A.M. Panu, S. Miles J. Med. Chem. 36, 1761-1771 (1993)] in three dimensions using the Active Analog Approach. We report here a conformational analysis of CBD which, in turn, led to the design of a new analog, desoxy-CBD. Modeling results for desoxy-CBD predict that this compound is capable of clearing the region of steric interference by expending 3.64 kcal/mol of energy in contrast to the 12.39 kcal/mol expenditure required by CBD. Desoxy-CBD was synthesized by condensation of 3-pentylphenol with p-mentha-2,8-dien-1-ol mediated by DMF-dineopental acetal. Desoxy-CBD was found to behave as a partial agonist in the mouse vas deferens assay, an assay which is reported to detect the presence of cannabinoid receptors. The compound produced a concentration related inhibition of electrically-evoked contractions of the mouse vas deferens, possessing an IC50 of 30.9 nM in this assay. Taken together, these results support the hypothesis of the existence of a region of steric interference at the CB1 receptor. While the energy expenditure to clear this region was too high for the parent compound, CBD, the removal of the C6' hydroxyl of CBD produced a molecule (desoxy-CBD) able to clear this region and produce activity, albeit at a reduced level.

Construction of a steric map of the binding pocket for cannabinoids at the cannabinoid receptor

In order to gain information about the topology of the brain cannabinoid receptor (CB1), a Receptor Steric (RS) Map for cannabinoids at this receptor was calculated. The classical cannabinoids (-)-11-hydroxy-delta-9-tetrahydrocannabinol (K1 = 210 +/- 56 nM), (-)-9-nor-9-beta-hydroxy-hexahydrocannabinol (K1 = 124 +/- 17 nM), nabilone (K1 = 120 +/- 13 nM), and the non-classical cannabinoid, CP-55,244 (K1 = 1.4 +/- .3 nM) were used as template molecules. The RS map was obtained as the union of the van der Waals' volumes of only those accessible conformers identified by MMP2 calculations that were able to clear a region of steric interference at the CB1 receptor previously characterized by us [Reggio, P.H., Panu, A.M. and Miles, S. (1993), J. Med. Chem., 36, 1761-1771]. The utility of the RS Map was explored by screening the accessible conformers of the classical cannabinoid, cannabinol (CBN), (K1 = 3200 +/- 450 nM), for its ability to fit within the RS map. Only the global minimum energy conformer of CBN (53.2% abundance at 298K) was able to fit within the RS map. These results imply that one reason for the reduced affinity of CBN may be that only 53.2% of CBN molecules are shaped properly to fit in the binding pocket for cannabinoids at the CB1 receptor.

Characterization of a region of steric interference at the cannabinoid receptor using the active analog approach

In this paper, it is hypothesized that the distinction between certain active and inactive cannabinoids is that the inactive analogs possess extra volume associated with their carbocyclic rings that may be responsible for an unfavorable interaction at the cannabinoid receptor. Using the active analog approach, a model is developed of a region of steric interference at this receptor using the active cannabinoids (-)-trans-delta 9-tetrahydrocannabinol, (-)-trans-delta 8-tetrahydrocannabinol, (-)-11-hydroxy-beta-hexahydrocannabinol, and a (-)-trans-11-hydroxy-delta 8-tetrahydrocannabinol dimethylheptyl derivative and the inactive cannabinoids (9S,6aR)-trans-delta 10,10a-tetrahydrocannabinol and a (+)-trans-11-hydroxy-delta 8-tetrahydrocannabinol dimethylheptyl derivative. Each of these molecules satisfy the cannabinoid pharmacophoric requirements, i.e., a phenolic oxygen at C1 and a side chain of acceptable length at C3. Accessible conformers of each molecule were identified by using the method of molecular mechanics as encoded in the MMP2(85) program. The MAP facility within the Chem-X molecular modeling program was then used to calculate the region of steric interference (termed the receptor essential volume, REV) from these accessible conformers. The calculations revealed an REV region located near the top of the carbocyclic ring in the bottom face of the molecule. In order to explore the use of this REV to account for the activities of other cannabinoids, the minimally active classical cannabinoid (-)-11-hydroxy-alpha-hexahydrocannabinol, an active benzofuran cannabinoid, and the active nonclassical cannabinoid CP-47,497 were then studied. In each case, the activity or minimal activity of each compound can be explained on the basis of the ability of one or more accessible conformer of each molecule to clear the REV calculated here. The results of this study provide an explanation at the molecular level for observed activity differences between cannabinoids that exhibit shape differences associated with their carbocyclic rings.

A rational search for the separation of psychoactivity and analgesia in cannabinoids

The compound 9-beta-hydroxy-hexahydrocannabinol [(-)-9 beta-OH-HHC] was designed to fit a combined theoretical profile of an analgesic cannabinoid (equatorial alcohol at C-9, phenol at C-1 and a C-3 side chain) with reduced psychoactivity (axial C-9 substituent which protrudes into the alpha face). (-)-9 beta-OH-HHC was synthesized by the addition of methyl Grignard to 9-oxo-11-nor-HHC. Its alpha epimer was obtained by the regiospecific epoxide ring opening of 9 alpha, 10 alpha-epoxy-HHC acetate. (-)-9 beta-OH-HHC and (-)-9 alpha-OH-HHC were each evaluated in a battery of tests in mice and were found to be 10-25 times less potent than (-)-trans-delta 9-tetrahydrocannabinol (delta 9-THC) in all tests including the tail flick test for antinociception (analgesia). Molecular mechanics calculations [MMP2(85)] revealed that, in the global minimum energy conformation of (-)-9 beta-OH-HHC, the axial methyl at C-9 protrudes into the alpha face of the molecule, while the axial hydroxyl at C-9 in (-)-9 alpha-OH-HHC protrudes into this same face. These calculations also identified a higher energy carbocyclic ring (twist) conformer of each in which there is no protrusion of a C-9 substituent of the carbocyclic ring into the alpha face. The minimal activity of both compounds is attributed to these higher energy forms.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigation of the role of the phenolic hydroxyl in cannabinoid activity

Structure-activity relationship studies have suggested that the phenolic hydroxyl group is essential for the pharmacological activity of the cannabinoids. However, it remains to be established whether it is the hydrogen of the phenolic hydroxyl that is important (possibly because this hydrogen can participate in a hydrogen bonding interaction) or whether it is the oxygen of the phenolic hydroxyl that is important (possibly because one of the lone pairs of electrons in this oxygen can serve as a hydrogen bond acceptor). Two new etherified cannabinoids were prepared in which the phenolic hydroxyl oxygen is incorporated into a fourth ring. These new compounds were designed to test the importance both of the phenolic hydroxyl oxygen and of the orientation of its lone pairs of electrons for cannabinoid pharmacological activity. O,2-Propano-delta 8-tetrahydrocannabinol (0,2-Propano-delta 8-THC) was designed to mimic delta 9-THC in its phenol conformation I (C2-C1-O-H = 7 degrees). O,10-Methano-delta 9-tetrahydro-cannabinol (0,10-Methano-delta 9-THC) was designed to mimic delta 9-THC in its phenol conformation II (C2-C1-O-H = 167 degrees). Molecular mechanics calculations revealed that 1) there are two accessible minimum energy conformers for O,2-propano-delta 8-THC, which differ principally in the conformation of the new fourth ring, and 2) there are three accessible minimum energy conformers for O,10-methano-delta 9-THC, the first two of which differ mainly in the conformation of the new fourth ring, whereas the third possesses an alternate pyran ring conformation. Wave functions and molecular electrostatic potential (MEP) maps were calculated for each accessible conformer of O,2-propano-delta 8-THC and of O,10-methano-delta 9-THC. The resultant MEP maps compared well with the corresponding MEP maps generated for delta 9-THC in each of its two minimum energy conformations (two phenolic hydroxyl positions). These results imply that 1) O,2-propano-delta 8-THC should be capable of being recognized at a site that would recognize delta 9-THC in its phenol conformation 1 and 2) O,10-methano-delta 9-THC should be capable of being recognized at a site that would recognize delta 9-THC in its phenol conformation II. Pharmacological evaluation of the analogs revealed that O,10-methano-delta 9-THC was inactive in all mouse tests, as well as the rat drug discrimination model. O,2-Propano-delta 8-THC was similar to delta 8-THC in that it depressed rectal temperature and produced antinociception and ring immobility in mice.(ABSTRACT TRUNCATED AT 250 WORDS)

The importance of the orientation of the C9 substituent to cannabinoid activity

We have found a correlation between cannabinoid psychopharmacological activity and the orientation of the C9 substituent in one class of cannabinoid derivatives. We report here a study of the active cannabinoids delta 9-tetra-hydrocannabinol (delta 9-THC), delta 8-tetrahydrocannabinol (delta 8-THC), and 11 beta-hexahydrocannabinol (11 beta-HHC); the minimally active cannabinoid 11 alpha-hexahydrocannabinol (11 alpha-HHC); and the inactive cannabinoids delta 7-tetrahydrocannabinol (delta 7-THC) and delta 9,11-tetrahydrocannabinol (delta 9,11-THC). Our working hypothesis is that there are two components of cannabinoid structure which confer upon these compounds reactivity characteristics crucial to activity: the directionality of the lone pairs of electrons of the phenyl group hydroxyl oxygen and the orientation of the carbocyclic ring relative to this oxygen. The structures of these six molecules were optimized by using the method of molecular mechanics as encoded in the MMP2(85) program. Other possible minimum-energy conformations of the carbocyclic ring were calculated by driving one torsion angle in this ring by use of the dihedral driver option in MMP2(85). The rotational energy behavior of the phenyl group hydroxyl in each molecule was studied also by using the dihedral driver option in MMP2(85). We found that the carbocyclic ring in 11 alpha-HHC can exist in either a chair or a twist conformation. The carbocyclic ring in delta 9-THC, in delta 8-THC, and in delta 7-THC was found to exist only in a half-chair conformation, while the carbocyclic ring in 11 beta-HHC and in delta 9,11-THC was found to exist only in a chair form. The results of the rotational energy profiles indicated that the minimum-energy positions of the phenyl group hydroxyls are nearly identical in all molecules. These molecules, then, were found to differ only in the conformation of the carbocyclic ring in each. This conformation, in turn, determines the orientation of this ring and its C9 substituent relative to the oxygen of the phenyl group hydroxyl. In order to assess the orientation of the carbocyclic ring with respect to the phenyl group hydroxyl oxygen in each optimized structure, the following nonbonded torsion angles were measured: C10-C10a-C1-O, C8-C7-C1-O, C11-C9-C1-O, and C9-Q-C1-O (where Q is a dummy atom placed midway between C8 and C10).(ABSTRACT TRUNCATED AT 400 WORDS)