Point-of-care measurement of clozapine concentration using a finger-stick blood sample

Background: The use of clozapine demands regular monitoring of clozapine plasma concentrations and of white blood cell parameters. The delay between sending blood samples for analysis and receiving the results hinders clinical care. Point-of-care testing (POCT) can provide drug assay results within a few minutes. Aim: This study aimed to investigate the utility of a novel point-of-care device that can measure clozapine concentrations using capillary blood samples collected via a finger stick. Method: During a five-week period starting in June 2019 eligible patients were asked to provide a finger-stick capillary sample in addition to their usual venous blood sample. Samples were analysed by the novel point-of-care device and by the standard laboratory method. Capillary blood samples were tested by the MyCare™ Insite POCT analyser, and a quantitative measurement of clozapine concentration was provided within six minutes. Results: A total of 309 patients agreed to measurements by the two methods. Analysis revealed clozapine concentrations in venous blood as determined by the laboratory method ranged from 20 to 1310 ng/mL and by POCT from 7 to 1425 ng/mL. There was a strong positive correlation ( R = 0.89) between the results from the venous and the capillary sample methods. The slope of the association between standard assay and MyCare™ Insite was 1.0 with an intercept of –21 ng/mL, indicating minimal bias. Conclusion: Clozapine concentrations can be accurately measured at the point of care using capillary blood samples collected via a finger stick. This approach may be more acceptable than venous sampling to patients and, with almost instant results available, more useful to clinicians.

Clozapine metabolism is associated with Absolute Neutrophil Count in individuals with treatment-resistant schizophrenia

AIMTo investigate the relationship between clozapine concentration and neutrophils in a European cohort of long-term clozapine users.METHODSPearson’s Correlation and Linear Regression analyses were applied to a subset of the CLOZUK2 dataset (N = 208) to assess the association between Absolute Neutrophil Count (ANC) and plasma clozapine concentration. Norclozapine and the metabolic ratio between clozapine and norclozapine were also investigated, along with SNPs associated with clozapine metabolismRESULTSAssociation between ANC and plasma clozapine concentration was found to be significant in a linear regression model (β = −1.41, p = 0.009), with a decrease in ANC of approximately 141 cells/mm3 for every 0.1 mg/litre increase in clozapine concentration. This association was attenuated by the addition of the metabolic ratio, which was significantly negatively correlated with ANC (β=-0.69, p=0.021). In a further regression model, three SNPs previously associated with norclozapine plasma concentrations and clozapine/norclozapine ratio were also found to be significantly associated with ANC: rs61750900 (β=-0.410, p=0.048), rs2011425 (β=0.450, p=0.026) and rs1126545 (β=0.330, p=0.039)CONCLUSIONANC was found to be significantly negatively associated with plasma clozapine concentration. Further investigation has suggested that the relationship is mediated by the clozapine/norclozapine ratio, and potentially moderated by genetic variants with effects on clozapine metabolism

Association of clozapine-related metabolic disturbances with CYP3A4 expression in patients with schizophrenia

Clozapine is effective in treatment-resistant schizophrenia; however, adverse effects often result in discontinuation of clozapine therapy. Many of the side-effects are associated with pharmacokinetic variations; therefore, the expression of major clozapine-metabolizing enzymes (CYP1A2, CYP3A4) in patients may predict development of adverse effects. In patients with schizophrenia (N = 96), development of clozapine concentration-dependent metabolic side-effects was found to be associated with pharmacokinetic variability related to CYP3A4 but not to CYP1A2 expression. In low CYP3A4 expressers, significant correlation was detected between fasting glucose level and clozapine concentration; moreover, the incidence of abnormal glucose level was associated with exaggerated clozapine concentrations (> 600 ng/ml). In low CYP3A4 expressers, exaggerated concentrations were more frequently observed than in normal/high expressers. Moderate/high risk obesity (BMI ≥ 35) more frequently occurred in low CYP3A4 expresser patients than in normal/high expressers. In patients with normal/high CYP3A4 expression and consequently with extensive clozapine-metabolizing capacity, norclozapine/clozapine ratio correlated with fasting glucose levels, triglyceride concentrations and BMI. Low CYP3A4 expression often resulting in exaggerated clozapine concentrations was considered to be as an important risk factor for some concentration-dependent adverse effects as normal/high CYP3A4 expression evoking high norclozapine/clozapine ratios. CYP3A4-status can identify patients with increased risk for metabolic side-effects and prevent their development by careful therapeutic strategy.

Correction: Identification of a novel polymorphism associated with reduced clozapine concentration in schizophrenia patients—a genome-wide association study adjusting for smoking habits

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Change in plasma concentration of clozapine and norclozapine following a switch of oral formulation

Background: Clozapine formulation has been shown to affect plasma concentrations of clozapine and norclozapine. Changes in formulation might result in toxicity or treatment failure. Methods: We identified, from electronic records, patients who switched from clozapine tablets to oral liquid or vice versa and compared plasma concentrations before and after the switch. Results: We identified 13 patients with 85 blood samples who changed formulation of clozapine. Overall mean standardized clozapine plasma level was 0.67 mg/l daily dose on liquid and 0.87 mg/l daily dose on tablets ( p = 0.035). Conclusion: Use of clozapine liquid results in lower plasma clozapine concentration than the same dose of tablets.

Changes in smoking status, mental state and plasma clozapine concentration: retrospective cohort evaluation

Aims and methodTo investigate the percentage of patients who commenced smoking after transferring out of a non-smoking forensic psychiatric unit, the corresponding clozapine dose adjustments, the effects on plasma clozapine/norclozapine concentrations and observed changes in mental state. We reviewed the notes and plasma clozapine/norclozapine concentrations of 46 patients transferred to medium secure units between July 2008 and December 2013.ResultsThirty-five patients commenced smoking. Their median clozapine dose was increased by 50 mg/d. In the non-smokers, the median clozapine dose remained unchanged. Plasma clozapine/norclozapine concentrations were significantly reduced in smokers despite dosage adjustment. Eighteen patients experienced deterioration in mental state after transfer; almost all these patients were smokers.Clinical implicationsApproximately three-quarters of patients who were non-smokers by virtue of being in a secure non-smoking environment commenced smoking after transfer. Monitoring of clozapine serum levels and assessment of mental state in the immediate period after a change in smoking status is indicated.