Atypical antipsychotics may be defined as drugs that produce antipsychotic action with minimal extrapyramidal symptoms, although their effects on negative symptoms and in refractory patients are also of interest. All the currently available atypical antipsychotics (clozapine, risperidone, olanzapine, and sertindole) share only 1 pharmacological property that distinguishes them from typical antipsychotics in humans: they block a high number of serotonin2 receptors (>80%) and a low to modest number of D2 receptors (eg, 20%-70% for clozapine and 60%-80% for risperidone and olanzapine).1,2 This contrasts with the typical antipsychotics that block mainly the dopamine D2 receptors and have minimal, if any, serotonin2 receptor occupancy.
Kapur S, Zipursky RB, Remington G. Do Loxapine Plus Cyproheptadine Make an Atypical Antipsychotic? PET Analysis of Their Dopamine D2 and Serotonin2 Receptor Occupancy. Arch Gen Psychiatry. 1998;55(7):666-667. doi: