In some patients, achieving a therapeutic serum clozapine concentration requires a high dose entailing a prolonged series of dose increases. This may be more common among smokers, and people with schizophrenia are more likely to smoke and to smoke more cigarettes per day than the population at large. One short cut to achieving a therapeutic level is adding fluvoxamine, a serotonin reuptake inhibitor used to treat anxiety and depression. Clozapine is converted by cytochrome enzyme 1A2 to the metabolite N-desmethylclozapine, known as norclozapine in clinical settings. Fluvoxamine inhibits this process which shifts the ratio of clozapine to norclozapine upward and prolongs the half life of clozapine. Fluvoxamine also inhibits CYP2C19 and in some people, CYP3A4 as well. The clinical effect is to permit a lower total dose of clozapine, and it may make once-daily dosing more tolerable. Furthermore, fluvoxamine can be used to treat concurrent anxiety and depression while maximizing clozapine serum levels. The evidence for the safety and benefits of these uses of fluvoxamine was reviewed in a recently published article.
The authors identified 24 case reports and series comprising 29 patients, and 9 prospective studies comprising 212 patients; 2 of these were randomized trials. Most patients had a primary diagnosis of schizophrenia, and the rationales for the various studies were diverse.
- Increasing clozapine plasma level
- Treating negative symptoms
- Treating positive symptoms
- Treating depressive symptoms
- Treating obsessive-compulsive symptoms
- Reducing metabolic adverse effects
The available evidence for most of these indications is mediocre or poor except for increasing plasma levels; according to the authors, fluvoxamine increases clozapine, norclozapine, and clozapine N-oxide plasma levels in a dose-dependent manner. The data among smokers is supportive but surprisingly limited. One point they raise is that the effects of changing the ratios of metabolites other than norclozapine is not understood. The evidence for reducing metabolic adverse effects is relatively good as it comes from a 12-week RCT, but long-term efficacy is unknown. As for treating depression or obsessive-compulsive symptoms, the authors conclude that it is safer to use an appropriate antidepressant without the pharmacokinetic complications of fluvoxamine given the risk of toxicity if clozapine levels rise abruptly.
Safety concerns addressed in studies include the risk of agranulocytosis and seizures for which there is no evidence of a protective or facilitative effect; the available evidence correlates increasing clozapine dose and not plasma level with risk of seizures. (2) The reports reviewed mention frequent occurrence of common adverse effects including sedation, sialorrhea with drooling, and constipation in fluvoxamine-treated patients. Clinicians and patients should be aware that prescribing fluvoxamine to enhance clozapine effects is not approved by Health Canada or the US Food and Drug Administration.
Polcwiartek C, Nielsen J. The clinical potentials of adjunctive fluvoxamine to clozapine treatment: a systematic review. Psychopharmacology. Published online Dec 2, 2015. Abstract
Remington G, Agid O, Foussias G, et al. Clozapine and therapeutic drug monitoring: is there sufficient evidence for an upper threshold? Psychopharmacology. 2013;225:505–518. Abstract