Differential effects of clozapine versus other antipsychotics on clinical outcome and dopamine release in the brain.

The first atypical antipsychotic, clozapine (Clozaril), dramatically improved the outcome of treatment of patients with schizophrenia in two ways. First, it reduced psychotic symptoms without eliciting significant neurological side effects. Second, it was effective in approximately 30% of individuals who were previously refractory to treatment. Efforts to develop similar drugs have been partially successful in that newer antipsychotics are also less likely to produce neurological side effects. However, it has not yet been established that the newest antipsychotics are more effective than conventional agents in individuals who are refractory to treatment. In the first part of this review, the results of studies that evaluated the new antipsychotics and provided an outcome measure of response rate (regardless of how this index was defined) are summarized. Even with this broad criterion, the evidence suggests that the newer antipsychotics do not share the clinical advantages of clozapine. To explore the possible mechanisms for the clinical advantage of clozapine, evidence of antipsychotic-induced dopamine release in the brain is discussed in the second half of this article. This analysis indicates that acute clozapine administration induces the release of more dopamine in the cortex than in the striatum or limbic system. With conventional antipsychotics, this relationship is reversed. The newest antipsychotics do not show a preference among these sites. Moreover, after long-term treatment, tolerance develops to haloperidol, but not to clozapine, with regard to the amount of dopamine released in the brain. No data are available on the newest antipsychotics. Although more studies need to be done-especially studies of the effects of long-term administration of various conventional and atypical antipsychotics-this distinction might be relevant to the unique clinical advantage of clozapine.