POTENTIAL INTERACTION BETWEEN CARBAMAZEPINE AND LOXAPINE - CASE-REPORT AND RETROSPECTIVE REVIEW

OBJECTIVE: To report a potential interaction between carbamazepine (CB Z), an anticonvulsant medication, and loxapine, an antipsychotic. CBZ is being increasingly used to treat a variety of psychiatric disorders . Because of this, the potential for multiple-drug therapy and subsequ ent drug interactions is increased. METHODS: We prospectively monitore d plasma CBZ and carbamazepine epoxide (CBZE) concentrations in a sing le patient during and after withdrawal of loxapine comedication. Addit ionally, we retrospectively evaluated four patients who had received c oncomitant therapy with CBZ and loxapine. RESULTS: Plasma CBZE/CBZ rat ios decreased from 0.76 to 0.18 following discontinuation of loxapine. Absolute CBZE plasma concentrations also decreased from 1.7 to 0.6 mu g/ml after loxapine therapy was stopped. Retrospectively screened pati ents who were receiving concomitant loxapine-CBZ therapy all had unusu ally elevated CBZE/CBZ plasma concentration ratios (0.75-1.91). CONCLU SIONs: These cases suggest that an interaction occurs between loxapine and CBZ. Possible mechanisms include either induction of CBZ metaboli sm to its epoxide metabolite, or inhibition of the enzymatic metabolis m of CBZE (epoxide hydrolase).