Revised Pharmacophore Model for 5-HT_2A Receptor Antagonists Derived from the Atypical Antipsychotic Agent Risperidone

Pharmacophore models for 5-HT_2A receptor antagonists consist of two aromatic/hydrophobic regions at a given distance from a basic amine. We have previously shown that both aromatic/hydrophobic moieties are unnecessary for binding or antagonist action. Here, we deconstructed the 5-HT_2A receptor antagonist/serotonin-dopamine antipsychotic agent risperidone into smaller structural segments that were tested for 5-HT_2A receptor affinity and function. We show, again, that the entire risperidone structure is unnecessary for retention of affinity or antagonist action. Replacement of the 6-fluoro-3-(4-piperidinyl)-1,2-benz[d]isoxazole moiety by isosteric tryptamines resulted in retention of affinity and antagonist action. Additionally, 3-(4-piperidinyl)-1,2-benz[d]isoxazole (10), which represents less than half the structural features of risperidone, retains both affinity and antagonist actions. 5-HT_2A receptor homology modeling/docking studies suggest that 10 binds in a manner similar to risperidone and that there is a large cavity to accept various N₄-substituted analogues of 10 such as risperidone and related agents. Alterations of this "extended" moiety improve receptor binding and functional potency. We propose a new risperidone-based pharmacophore for 5-HT_2A receptor antagonist action.