Plasma haloperidol and clinical response: a role for reduced haloperidol in antipsychotic activity?

Seventeen hospitalized psychotic patients were treated with a fixed oral dose of haloperidol, 5 mg twice daily for 28 days. Most were chronic schizophrenics with an acute exacerbation of their illness. All patients also received benztropine 3 mg twice daily during the last 11 days of the study, regardless of the presence or absence of extrapyramidal side effects. No significant linear or curvilinear relationship was found between steady state plasma levels of haloperidol and clinical response measured by total Brief Psychiatric Rating Scale score. At 28 days, the correlations between plasma levels and percent improvement were rs = 0.187 (not significant) for haloperidol and rs = 0.582 (p = 0.04) for the hydroxymetabolite, reduced haloperidol. The correlation for the sum was rs = 0.511 (p = 0.078). Metabolite levels were substantially higher than plasma haloperidol, on the average 2.7 times greater. Eleven days of benztropine treatment had no significant effect on haloperidol or metabolite plasma levels or on clinical status. At the end of the study, nine of 17 patients (53%) had recovered as judged by discharge readiness. Within the limitations implied by the small number of patients, these data suggest that reduced haloperidol is an important component of plasma antipsychotic activity and cannot be neglected in correlative studies, and that a substantial proportion of patients--about half--can be successfully treated with only 10 mg of haloperidol. The routine use of large doses is thus not necessary for many patients.