Modulatory effect of clozapine on levodopa response in Parkinson's disease: a preliminary study.

Clozapine has been shown not only to be effective in ameliorating dopaminomimetic psychosis but to improve parkinsonian symptomatology. Six parkinsonian patients with motor fluctuations under levodopa treatment and severe interdose "off" periods (believed to be mediated by an inhibitory effect of subthreshold levels of levodopa) underwent a trial of clozapine. The effects of this drug on levodopa response were measured by means of an acute levodopa test both before and after receiving clozapine. After 1 month of treatment, clozapine 25 mg/day reduced parkinsonian scores at all stages of the evaluation (pre-levodopa "off," "on," and interdose "off"). The effect was consistently more significant for the interdose "off." Clozapine could be exerting its beneficial effects through the inhibition of an inhibitory effect mediated by low-level dopaminergic stimulation, thus behaving as an apparent anti-parkinsonian drug.