Three-dimensional quantitative structure-activity relationship study of the cannabimimetic (aminoalkyl)indoles using comparative molecular field analysis.

The present study describes the implementation of comparative molecular field analysis (CoMFA) to develop two 3D-QSAR (quantitative structure-activity relationship) models (CoMFA models 1 and 2) of the cannabimimetic (aminoalkyl)indoles (AAIs) for CB1 cannabinoid receptor binding affinity, based on pKi values measured using radioligand binding assays that displace two different agonist ligands, [3H]CP-55940 and [3H]WIN-55212-2. Both models exhibited a strong correlation between the calculated steric-electrostatic fields and the observed biological activity for the respective training set compounds. In light of the basicity of the morpholine nitrogen in the AAIs, separate CoMFA models were built for the AAIs as unprotonated and protonated species. Comparison of the statistical parameters resulting from these CoMFA models failed to provide unequivocal evidence as to whether the AAIs are protonated or neutral as receptor-bound species. Although the training sets of CoMFA model 1 and CoMFA model 2 differed with respect to composition and to the choice of displacement radioligand in each biological assay, their CoMFA StDevCoeff contour plots reveal similarities in terms of identifying those regions around the AAIs that are important for CB1 cannabinoid receptor binding such as the sterically favored region around the C3 aroyl group and the sterically forbidden region around the indole ring. When the experimental pKi values for the training set compounds to displace the AAI radioligand [3H]WIN-55212-2 were plotted against the pKi values as predicted for the same compounds to displace the cannabinoid radioligand [3H]CP-55940, the correlation was moderately strong (r = 0.73). However, the degree of correlation may have been lowered by the structural differences in the compounds comprising the training sets for CoMFA model 1 and CoMFA model 2. Taken together, the results of this study suggest that the binding site region within the CB1 cannabinoid receptor can accommodate a wide range of structurally diverse cannabimimetic analogues including the AAIs.