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Risperidone is an ‘atypical’ antipsychotic and is approved by the USFDA mainly for the treatment of schizophrenia and symptoms of bipolar disorder. Risperidone (an SDA or serotonin-dopamine antagonist) has ~20-fold higher affinity at 5-HT2A receptors over dopamine D2 receptors, which makes it more efficacious against the negative symptoms of schizophrenia and less liable to causing extrapyramidal side effects than ‘typical’ antipsychotics. The major goal of the current investigation was to study the structure of risperidone and to identify the minimum structural features required for 5-HT2A receptor affinity that retain antagonist action. The structure of risperidone was systematically deconstructed, and functional activity studies using calcium imaging in HEK293 cells and a two-electrode voltage clamp (TEVC) assay in a Xenopus laevis heterologous system were coupled with radioligand binding affinity studies to achieve this goal. The biological studies showed that the entire structure of risperidone was not required for activity or affinity at the receptor, as 6-fluoro-[3-(1-methylpiperidin-4-yl)]benz[d]isoxazole was comparable to risperidone in both affinity and activity. Next, the structure of risperidone was elaborated to determine the importance of its left and right “halves” in its actions. The left and the right halves of risperidone were substituted with those of another antagonist, ketanserin, to give structural hybrids. Biological studies suggested that the right half of risperidone [i.e., the 6-fluoro-(3-piperidin-4-yl)benz[d]isoxazole moiety] might be important for affinity. In order to assess how the biologically-active compounds interact at the receptor, homology models of the human 5-HT2A receptor were developed, and docking and Hydropathic INTeraction studies were conducted. Risperidone seemed to form a bifurcated hydrogen bond with S159 (TM3), which ketanserin was unable to form. This interaction might account for high binding affinity at the receptor as it is common to both, risperidone and 3-[2-(4-(6-fluorobenz[d]isoxazol-3-yl)piperidin-1-yl)ethyl]-2,4-(1H,3H)quinazolinedione. With the data currently in hand, we can conclude that the entire structure of risperidone is not required for activity or affinity, and that the right “half” (i.e. the benzisoxazolyl portion) of risperidone might be influencing activity and affinity at 5-HT2A receptors. |
