To the Editor: Clozapine is effective in treating patients with schizophrenia who are unresponsive to, or intolerant of, classic antipsychotics (1). However, a substantial proportion of patients fail to respond to clozapine or respond only partially. Durson et al. (2) reported substantial improvement in six patients when lamotrigine, an anticonvulsant and mood stabilizer, was added to clozapine therapy in a dose of 125–250 mg/day.

How can we explain the tripling of the patient’s clozapine plasma level during his use of lamotrigine? It is highly unlikely that the patient had taken an overdose; he was certainly not suicidal. We searched for a pharmacokinetic explanation. Drugs that are highly protein bound (e.g., warfarin) can influence clozapine plasma levels, but lamotrigine is not. The metabolism of clozapine occurs mainly in the liver and is dependent on cytochrome P-450. The P-450 isoenzymes responsible for the metabolism of clozapine have not yet been fully characterized, but CYP1A2 appears to contribute to a major extent (3). Some drugs (e.g., fluoxetine and fluvoxamine) can interfere with the metabolism of clozapine by inhibiting CYP1A2 (4). Lamotrigine, however, is believed to be free of these enzyme interactions. It is metabolized in the liver by glucuronidation and eliminated almost entirely by the kidneys (5).

Consequently, we have no explanation for the great increase in the patient’s plasma level of clozapine during concomitant use of lamotrigine. When lamotrigine is added to clozapine, clinicians should be aware of this possibility.