Comparison of 5-HT1A and dopamine D2 pharmacophores. X-ray structures and affinities of conformationally constrained ligands

Conformational and molecular mechanics studies of a new series of tricyclic ligands with affinity for either the dopamine D2 receptor or the 5-HT1A receptor, or both, has enabled us to elaborate considerably on previous pharmacophore models for these receptors. The new tricyclic ligands are either angular, 2,3,3a,4,5,9b-hexahydro-1H-benz[e]indole derivatives, or linear, 2,3,3a,4,5,9a-hexahydro-1H-benz[f]indole derivatives; they have either cis or trans ring junctions, and many of the ligands are resolved. In order to have X-ray crystal coordinates for every structural type, two additional crystal structures were determined: 14a, the trans-(+-)-6-hydroxy-3-(n-propyl) angular derivative as the hydrochloride, and (+-)-1,2,2a,3,4,8b-hexahydro-8-methoxy-2-(2-propenyl)-naphth[2,1- b]azetidine hydrochloride (16d). Several recently reported imidazoquinolinones with dopaminergic and serotonergic activities were also used in developing the models as were other known ligands which are conformationally constrained. A new method for determining intrinsic activity at the D2 receptor made consistent and reliable estimates of dopamine agonist, partial agonist, and antagonist activities available. The models explain these activities in terms of the 3-dimensional structural features of the ligands and their probable orientations at the D2 receptor site. They also explain why allyl and propyl analogs of some structures have very different affinities while affinities are quite similar for allyl and propyl analogs of other structures; at both receptors a particular orientation of the amine substituent in the binding site correlates with preference for allyl over propyl derivatives. Suggestions are made for enhancing selectivity at the 5-HT1A receptor or at the dopamine D2 receptor. An angular, cis, (3aR,9bS), 2-propyl, 9-hydroxy, 3-(n-propyl) analog should be selective for the 5-HT1A receptor. A linear, trans, (3aR,9aS), 7-hydroxy, 1-(2-propenyl) analog should be selective for the dopamine D2 receptor, and would be predicted to be an antagonist.