The bioactive conformation of aminoalkylindoles at the cannabinoid CB1 andCB2 receptors: Insights gained from (E)- and (Z)-naphthylidene indenes

The aminoalkylindoles (AAIs) are agonists at both the cannabinoid CB1 and C B2 receptors. To determine whether the s-trans or s-cis form of AAIs is the ir receptor-appropriate conformation, two pairs of rigid AAI analogues were studied. These rigid analogues are naphthylidene-substituted aminoalkylind enes that lack the carbonyl oxygen of the AAIs. Two pairs of (E)- and (Z)-n aphthylidene 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 confo rmational analyses of two AAIs, pravadoline (2) and WIN-55,212-2 (1), and o f each indene were performed using the semiempirical method AM1. S-trans an d s-cis conformations of 1 and 2 were identified. AM1 single-point energy c alculations revealed that when 1 and each indene were overlayed at their co rresponding indole/indene rings, the (E)- and (Z)-indenes were able to over lay naphthyl rings with the corresponding s-trans or s-cis conformer of 1 w ith 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 hypot hesis 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 syn thesis of the naphthylidene indenes was developed using Horner-Wittig chemi stry that afforded the Z isomer in the C-2 H series, which was not produced in significant amounts from an earlier reported indene/aldehyde condensati on reaction. This approach was extended to the C-2 Me series as well. Photo chemical interconversions in both the C-2 H and C-2 Me series were also suc cessful in obtaining the less favored isomer. Thus, the photochemical process can be used to provide quantities of the mi nor isomers C-2 H/Z and C-2 Me/E. The CB1 and. CB2 affinities as well as th e 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 th e higher affinity for both the CB1 and CB2 receptors. In the rat brain memb rane assay versus [H-3]CP-55,940, the K-i's for the C-2 H/C-2 Me series wer e 2.72/2.89 nM (E isomer) and 148/1945 nM (Z isomer). In membrane assays ve rsus [H-3]SR141716A, a two-site model was indicated for the C-2 H/C-2 Me (E isomers) with K-i's of 10.8/9.44 nM for the higher-affinity site and 611/6 02 nM for the lower-affinity site. For the Z isomers, a one-site model was indicated with K-i'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 K-i'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 greater-than C-2 Me E greater-than C-2 H Z greater-than C-2 Me Z. The C-2 H E isomer was fou nd to be equipotent with 1, while the C-2 Me Z isomer was inactive at conce ntrations up to 3.16 mu M. Thus, results indicate that the E geometric isom er in each: pair of analogues is the isomer with the higher CB1 and CB2 aff inities and the higher pharmacological potency. Taken together, results rep orted here support the hypothesis that the s-trans conformation of AAIs suc h as 1 is the preferred conformation for interaction at both the CB1 and CB 2 receptors and that aromatic stacking may be an important interaction for AAIs at these receptors.