Monitoring of clozapine blood levels in the real world

A review of the use of clozapine levels to guide treatment and determine cause of death

OBJECTIVE

To review the literature to examine the use of clozapine levels to (i) guide therapy and prevent toxicity in clinical care and (ii) determine cause of death in post-mortem examination of patients who were treated with clozapine.

METHODS

MEDLINE was searched in December 2010 using the following keywords: 'clozapine levels', 'clozapine and toxicity', 'clozapine and death', 'clozapine and mortality' and 'post-mortem redistribution'. Data was also collected from the 2010 MIMS Annual.

RESULTS

The literature reported significant variation in clozapine levels attained with any given dose, and considerable variability in the clinical response achieved at any given clozapine level. The lowest effective clozapine levels ranged from 250 to 550 µg/L, while the recommended upper limit to prevent toxicity varied from 600 to 2000 µg/L. There was minimal correlation between clozapine levels and side effects, with the exception of sedation, hypotension and seizure activity. The risk of seizures increased with plasma clozapine levels greater than 600 µg/L or rapid upward titration. In addition to prescribed dose, there are many factors that influence plasma clozapine levels. After death, the process of post-mortem drug redistribution resulted in 3.00 to 4.89 times increases in clozapine levels in central blood vessels and 1.5 fold increases in peripheral vessels compared to ante-mortem levels.

CONCLUSIONS

The exact range of clozapine levels that corresponds to toxicity remains unclear. However, levels between 350 µg/L and 1000 µg/L achieved with gradual upward titration are more likely to be effective and less likely to cause toxicity. Ongoing clozapine level monitoring is indicated, especially when (i) prescribing higher doses (> 600 mg/day) of clozapine, (ii) there has been a change in a patient's concomitant pharmacotherapy or cigarette use and (iii) there has been a suboptimal response to treatment. The use of post-mortem clozapine levels to determine clozapine toxicity as a cause of death is unreliable.

Atypical antipsychotics as noncompetitive inhibitors of alpha4beta2 and alpha7 neuronal nicotinic receptors

It has been suggested that the interaction of antipsychotic medications with neuronal nicotinic receptors may increase the cognitive dysfunction associated with schizophrenia and may explain why current therapies only partially address this core feature of the illness. In the present studies we compared the effects of the atypical antipsychotics quetiapine, clozapine and N-desmethylclozapine to those of the typical antipsychotics haloperidol and chlorpromazine on the alpha4beta2 and alpha7 nicotinic receptor subtypes. The binding of [(3)H]-nicotine to rat cortical alpha4beta2 receptors and [(3)H]-methyllycaconitine to rat hippocampal alpha7 receptors was not affected by any of the compounds tested. However, Rb(+) efflux evoked either by nicotine or the selective alpha4beta2 agonist TC-1827 from alpha4beta2 receptors expressed in SH-EP1 cells and nicotine-evoked [(3)H]-dopamine release from rat striatal synaptosomes were non-competitively inhibited by all of the antipsychotics. Similarly, alpha-bungarotoxin-sensitive epibatidine-evoked [(3)H]-norepinephrine release from rat hippocampal slices and acetylcholine-activated currents of alpha7 nicotinic receptors expressed in oocytes were inhibited by haloperidol, chlorpromazine, clozapine and N-desmethylclozapine. The inhibitory effects on nicotinic receptor function produced by the antipsychotics tested occurred at concentrations similar to plasma levels achieved in schizophrenia patients, suggesting that they may lead to clinically relevant effects on cognition.

Smoking and therapeutic response to clozapine in patients with schizophrenia

BACKGROUND

Of patients with schizophrenia, 70 to 80% smoke. Nicotine corrects certain information processing and cognitive psychomotor deficits seen in many patients with schizophrenia. Clozapine, but not conventional antipsychotics, has been shown to correct some of these deficits.

METHODS

We assessed psychopathology and smoking in 70 patients with treatment refractory schizophrenia (55 smokers and 15 nonsmokers) at baseline when they were receiving conventional antipsychotics and again after they were switched to clozapine.

RESULTS

Smokers showed significantly greater therapeutic response to clozapine than nonsmokers. Smokers smoked less when treated with clozapine than when treated with conventional antipsychotics.

CONCLUSIONS

Certain patients with schizophrenia have contributing pathophysiologic mechanisms that respond favorably to either nicotine or clozapine.

The clinical use of plasma clozapine levels

OBJECTIVE

This review examines the evidence supporting the proposition that a threshold clozapine plasma level can predict clinical response. In addition, it provides a brief overview of the pharmacokinetics, side effects, drug interactions and assay methodology of clozapine.

METHOD

A comprehensive search of relevant literature was made with respect to the above criteria. The findings were collated and analysed to produce an overview of the usefulness of using clozapine levels in clinical practice.

RESULTS

Most researchers find that, although the correlation between dose of clozapine and clinical effect is not high, a threshold plasma level of 350-420 ng mL-1 of clozapine is associated with an increased probability of a good clinical response to the drug. Results vary, however, with the study design.

CONCLUSIONS

The data reviewed present a case for increasing the dose of clozapine in non-responsive patients to achieve a plasma level of at least 350-420 ng mL-1. Non-response at these levels, however, should not preclude a further upward titration of dose. This should occur unless (i) clinical response is obtained at a lower dose, (ii) intolerable side effects occur, or (iii) a daily dose of 900 mg is reached.