Inducing effect of phenobarbital on clozapine metabolism in patients with chronic schizophrenia.

The steady state plasma concentrations of clozapine and its two major metabolites, norclozapine and clozapine N-oxide, were compared in patients with schizophrenia treated with clozapine in combination with phenobarbital (n=7), and in control patients treated with clozapine alone (n=15). Patients were matched for sex, age, body weight, and antipsychotic dosage. Patients comedicated with phenobarbital had significantly lower plasma clozapine levels than those of the controls (232+/-104 versus 356+/-138 ng/ml; mean, SD, p < 0.05). Plasma norclozapine levels did not differ between the two groups (195+/-91 versus 172+/-61 ng/ml, NS), whereas clozapine N-oxide levels were significantly higher in the phenobarbital group (115+/-49 versus 53+/-31 ng/ml, p < 0.01). Norclozapine/clozapine and clozapine N-oxide/ clozapine ratios were also significantly higher (p < 0.001) in patients comedicated with phenobarbital. These findings suggest that phenobarbital stimulates the metabolism of clozapine, probably by inducing its N-oxidation and demethylation pathways.