Extrapyramidal side effects with risperidone and haloperidol at comparable D2 receptor occupancy levels

Risperidone is an antipsychotic drug with high affinity at dopamine D2 and serotonin 5-HT2 receptors. Previous clinical studies have proposed that risperidone's pharmacologic profile may produce improved efficacy for negative psychotic symptoms and decreased propensity for extrapyramidal side effects; features shared by so-called 'atypical' neuroleptics. To determine if routine risperidone treatment is associated with a unique degree of D2 receptor occupancy and pattern of clinical effects we used [¹²³I]IBZM SPECT to determine D2 occupancy in subjects treated with routine clinical doses of risperidone (n = 12) or haloperidol (n = 7). Both risperidone and haloperidol produced D2 occupancy levels between approximately 60 and 90% at standard clinical doses. There was no significant difference between occupancy levels obtained with haloperidol or risperidone. Drug-induced parkinsonism was observed in subjects treated with risperidone (42%) and haloperidol (29%) and was observed at occupancy levels above 60%. Based on these observations, it is concluded that 5-HT2 blockade obtained with risperidone at D2 occupancy rates of 60% and above does not appear to protect against the risk for extrapyramidal side effects.