Although psychiatrists know that long-term disability in schizophrenia is related to negative and cognitive symptoms, our treatment goals often focus on positive symptoms. The unfortunate reason is that we lack good treatments for cognitive and behavioral deficits. Many antipsychotics have anticholinergic properties which we know can dull cognition or even cause delirium in sensitive patients, but we accept this risk in exchange for the benefit of eliminating psychosis.
Clozapine is highly anticholinergic but its metabolite, N-desmethylclozapine or norclozapine, has partial cholinergic agonist activity at muscarinic receptors. Although psychopharmacologists have suspected that this property of norclozapine may have cognitive benefits, most psychiatrists focus on optimizing clozapine serum level which is sometimes achieved by blocking clozapine metabolism with cytochrome enzyme inhibitors such as the antidepressant fluvoxamine. This increases the clozapine/norclozapine ratio; some evidence suggests this may reduce the metabolic burden of clozapine therapy (1). However, new evidence suggests that this may come at the price of worse cognitive performance, in particular working memory.
A team of researchers at the Centre for Mental Health and Addictions in Toronto examined the hypothesis that a greater ratio of clozapine to norclozapine would correlate with worse cognitive performance as measured by the Measurement and Treatment Research to Improve Cognition in Schizophrenia Consensus Cognitive Battery (MATRICS battery) (2). They recruited 30 adults, mean age 38.6 years, on clozapine monotherapy for schizophrenia or schizoaffective disorder whose dose had been unchanged for at least 4 weeks. The subjects had blood taken for clozapine and norclozapine concentrations on the same day that they underwent PANSS rating and the MATRICS battery, which comprises 10 tests in 7 cognitive domains such as verbal learning, visual learning, working memory and processing speed.
In multiple regression analyses, the clozapine/norclozapine ratio was negatively correlated with performance on working memory but not with the other 6 cognitive domains. Working memory performance was not correlated with demographic variables or PANSS score, nor with either clozapine or norclozapine serum concentrations independently of each other.
The investigators suggest that the significance of the clozapine/norclozapine ratio reflects the opposing effects of the two molecules on muscarinic type 1 receptors, given that the cholinergic system is crucial for complex attentional processes as tested by working memory measures. They acknowledge that the MATRICS battery may not be sensitive enough to detect effects on other domains, and also that this is a cross-sectional study that cannot establish causation. Serial measurements in individuals as the clozapine/norclozapine ratio changes would be more conclusive.
1. Lu ML, Lane HY, Lin SK, Chen KP, Chang WH. Adjunctive fluvoxamine inhibits clozapine-related weight gain and metabolic disturbances. J Clin Psychiatry. 2004;65:766-771. Abstract
2. Rajji TK, Mulsant BH, Davies S, et al. Prediction of working memory performance in schizophrenia by plasma ratio of clozapine to N-desmethylclozapine. Am J Psychiatry. 2015;172:579-585. Abstract