Low-dose fluvoxamine as an adjunct to reduce olanzapine therapeutic dose requirements: a prospective dose-adjusted drug interaction strategy.

Despite the advances in antipsychotic pharmacotherapy over the past decade, many atypical antipsychotic agents are not readily accessible by patients with major psychosis or in developing countries where the acquisition costs may be prohibitive. Olanzapine is an efficacious and widely prescribed atypical antipsychotic agent. In theory, olanzapine therapeutic dose requirement may be reduced during concurrent treatment with inhibitors of drug metabolism. In vitro studies suggest that smoking-inducible cytochrome P450 (CYP) 1A2 contributes to formation of the metabolite 4'-N-desmethylolanzapine. The present prospective study tested the hypothesis that olanzapine steady-state doses can be significantly decreased by coadministration of a low subclinical dose of fluvoxamine, a potent inhibitor of cytochrome P450 1A2. The study design followed a targeted "at-risk" population approach with a focus on smokers who were likely to exhibit increased cytochrome P450 1A2 expression. Patients with stable psychotic illness (N = 10 men, all smokers) and receiving chronic olanzapine treatment were evaluated for steady-state plasma concentrations of olanzapine and 4'-N-desmethylolanzapine. Subsequently, olanzapine dose was reduced from 17.5 +/- 4.2 mg/d (mean +/- SD) to 13.0 +/- 3.3 mg/d, and a nontherapeutic dose of fluvoxamine (25 mg/d, PO) was added to regimen. Patients were reevaluated at 2, 4, and 6 weeks during olanzapine-fluvoxamine cotreatment. There was no significant change in olanzapine plasma concentration, antipsychotic response, or metabolic indices (eg, serum glucose and lipids) after dose reduction in the presence of fluvoxamine (P > 0.05). 4'-N-desmethylolanzapine/olanzapine metabolic ratio decreased from 0.45 +/- 0.20 at baseline to 0.25 +/- 0.11 at week 6, suggesting inhibition of the cytochrome P450 1A2-mediated olanzapine 4'-N-demethylation by fluvoxamine (P < 0.05). In conclusion, this prospective pilot study suggests that a 26% reduction in olanzapine therapeutic dose requirement may be achieved by coadministration of a nontherapeutic oral dose of fluvoxamine.