Gender differences in plasma clozapine levels and its metabolites in schizophrenic patients

Forty refractory schizophrenic patients (21 females and 19 males) participated in a fixed-dose study with clozapine. After a 6-week trial of haloperidol and a 1-week washout time period, non-responding patients were placed on clozapine and the dosage titrated up to 400 mg/day for the next 5 weeks. Plasma clozapine levels and its two metabolites desmethylclozapine (DCLOZ) and clozapine N-oxide (CNO) were measured at weeks 2, 4 and 6. Blood samples were obtained 10-12 h post-evening dose and prior to the morning dose. Clozapine and its metabolites were assayed by HPLC with UV detection. Patients were assessed for clinical response with the Brief Psychiatric Rating Scale (BPRS) at baseline and at weeks 2, 4 and 6. BPRS scores were also divided into positive (+) and negative (-) symptoms subscales. Plasma clozapine and DCLOZ levels were significantly lower in males. Plasma CNO levels were slightly lower in males but it was not statistically significant. Decreased total BPRS, (+) and (-) symptoms subscale scores occurred during the study for both gender groups. A greater magnitude of change for the (-) symptom subscale score was observed in the male group. Gender was not a significant factor in the incidence or severity of side-effects.