The Bioactive Conformation of Aminoalkylindoles at the Cannabinoid CB1 and CB2 Receptors:  Insights Gained from (E)- and (Z)-Naphthylidene Indenes

Abstract

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...

Functional Role of Tryptophan Residues in the Fourth Transmembrane Domainof the CB2 Cannabinoid Receptor

“…For example, it was suggested that the aminoalkylindole WIN55,212-2 interacts with both CB 1 and CB 2 by aromatic stacking (Reggio et al, 1998). The replacement of the pentyl group with a dimethylheptyl group at position 3 of the cyclic and bicyclic cannabinoids was found to increase ligand affinity to CB 1 and CB 2 .…”

Functional Activity and Switching of Novel Cannabinergic Ligands

“…One such extrapolation lies in WIN-55,212-2's biologically assumed conformation to be s-trans-conformer. 260 With the s-cis-conformer, there are suggested steric interactions with V113(3.32) (Figure 3.22). 261 By extension, WIN-55,212-3, the S-(-) isomer of WIN-55,212-2 and antagonist, may also orient the morpholino side chain such to prevent critical amino acids interactions in the CB2 LBP as discussed below.…”

“…260,280 In the s-trans (Figure 4.13A) conformation, which predominates when the C-2 substitution is hydrogen, the aryl group is nearest C-2, while the carbonyl oxygen locates near C-4. In the s-cis (Figure 4.13B) conformation, which predominates when the C-2 substituent is a methyl group, the conformational preference shows the aryl ring to be located near C-4, and the carbonyl oxygen near C-2.…”

“…Later, it was shown that the CB1 and CB2 affinities and pharmacological potencies were higher for the E-isomer (144) compared to the Z-isomer. 260 By removing the carbonyl oxygen of the C-3 aryl group in AAIs, having an unsubstituted C-2, one gets a moderate reduction in affinity for CB1 relative to their carbonyl precursors. 295 This loss of affinity is larger in the 2-methyl substituted analogs (145, Figure 4.12).…”

“…Both observations support the hypothesis that the s-trans conformation of AAIs, such as WIN-55,212-2, is the preferred conformation for interaction at both CB1 and CB2 receptors, and that aromatic stacking of the ligands with aromatic residues in helices 3, 4, and 5 of both receptors may be an important interaction for AAIs at these receptors. 260,295,296 The spatial and electronic requirements of the C-3 substituent were further explored by Bristol-Myers-Squibb, introducing a C-3 amide group. The AAI C-3 amide ligand 146 (Figure 4.12) with a methoxy group at C-7, exhibited high CB2 affinity (Ki = 8 nM) and selectivity (CB1/CB2 = 500).…”