Examining concentration-dependent toxicity of clozapine: role of therapeutic drug monitoring

Overcoming clozapine's adverse events: a narrative review of systematic reviews and meta-analyses

INTRODUCTION

Clozapine is the gold standard treatment for treatment-resistant schizophrenia, however adverse events remain a clinical challenge.

AREAS COVERED

This review presents a narrative synthesis of systematic reviews and meta-analyses that have reported the onset, incidence, prevalence, and management of clozapine's adverse events. We conducted a systematic literature search using PubMed, Embase, PsycINFO, OvidMEDLINE, CINAHL, and the Cochrane Database of Systematic Reviews from inception to April 2024.

EXPERT OPINION

Effective management of clozapine's adverse events necessitates multi-faceted, individualized, and shared-decision strategies. Despite a lack of high-quality systematic evidence, expert inter-disciplinary solutions are provided to help address a critical need for clinical guidance. This 35-year update offers an evidence-based framework to assist clinicians, patients, and caregivers navigate the adverse events associated with clozapine therapy.

Effect of propranolol on pharmacokinetics of clozapine in schizophrenic patients: a meta-analysis

PURPOSE

Clozapine is the effective therapy for treatment-refractory schizophrenia. However, the use of clozapine is limited by its adverse effects. As propranolol is frequently used for the prevention and treatment of clozapine-induced tachycardia, we performed a meta-analysis to evaluate the effects of propranolol on steady state pharmacokinetics of clozapine in schizophrenic patients.

METHODS

We included 16 retrospective studies on the effects of propranolol on steady state pharmacokinetics of clozapine in schizophrenic patients, with data from both generic and brand name treatment phases in eight clozapine bioequivalence studies conducted in a single center in China from 2018 to 2022. Review Manager 5.4 was used for meta-analysis of the included studies.

RESULTS

The SMDs with 95% CIs of AUC0-12, Cmax,ss, C, and C were calculated to be 0.44 (0.23, 0.64), 0.40 (0.20, 0.61), 0.43 (0.22, 0.63), and 0.44 (0.23, 0.64), respectively. These findings proved that combination with propranolol would increase the systemic exposure of clozapine. T1/2 of clozapine was significantly longer in the presence of propranolol than in the absence of propranolol (SMD = 0.32, 95% CI [0.12, 0.52], p = 0.002). There was no statistically significant difference for T of clozapine in the presence or absence of propranolol (SMD =  - 0.05, 95% CI [- 0.25, 0.15], p = 0.63).

CONCLUSION

The combination with propranolol could significantly increase systemic exposure and extended T1/2 of clozapine, and thus need to be considered in prescribing decisions.

Evidence for Therapeutic Drug Monitoring of Atypical Antipsychotics

Second-generation antipsychotics (SGAs), also known as atypical antipsychotics, are a newer class of antipsychotic drugs used to treat schizophrenia, bipolar disorder, and related psychiatric conditions. The plasma concentration of antipsychotic drugs is a valid measure of the drug at its primary target structure in the brain, and therefore determines the efficacy and safety of these drugs. However, despite the well-known high variability in pharmacokinetics of these substances, psychiatric medication is usually administered in uniform dosage schedules. Therapeutic drug monitoring (TDM), as the specific method that can help personalised medicine in dose adjustment according to the characteristics of the individual patient, minimizing the risk of toxicity, monitoring adherence, and increasing cost-effectiveness in the treatment, thus seems to be an elegant tool to solve this problem. Non-response to therapeutic doses, uncertain adherence to medication, suboptimal tolerability, or pharmacokinetic drug-drug interactions are typical indications for TDM of SGAs. This review aims to summarize an overview of the current knowledge and evidence of the possibilities to tailor the dosage of selected SGAs using TDM, including the necessary pharmacokinetic parameters for personalised pharmacotherapy.

Negative symptoms and cognitive impairment are associated with distinct motivational deficits in treatment resistant schizophrenia

BACKGROUND

Motivational deficits are a central feature of the negative syndrome in schizophrenia. They have consistently been associated with reduced willingness to expend physical effort in return for monetary rewards on effort based decision making (EBDM) paradigms. Nevertheless, the mechanisms underlying such altered performance are not well characterised, and it remains unclear if they are driven purely by negative symptoms, or also in part by cognitive impairment, antipsychotic treatment or even positive symptoms. Here we investigated the impact of all these factors using a paradigm that has not previously been used to measure EBDM in schizophrenia.

METHODS

Forty treatment resistant schizophrenia (TRS) patients on clozapine and matched controls (N = 80) completed a well validated EBDM task which offers monetary rewards in return for physical effort. Choice and reaction time data was analysed using logistic regressions, as well as Bayesian hierarchical drift diffusion modelling (HDDM). Behavioural parameters were compared between groups and their association with negative symptoms, cognitive function and serum clozapine levels were assessed.

RESULTS

Overall, TRS patients accepted significantly less offers than controls during effort-based decision making, suggesting they were less motivated. They demonstrated reduced sensitivity to increasing rewards, but surprisingly were also less averse to increasing effort. Despite a positive correlation between negative symptoms and cognitive function in TRS, reward sensitivity was associated only with cognitive performance. In contrast, reduced effort aversion correlated with negative symptom severity. Clozapine levels and positive symptoms were not associated with either behavioural parameter.

CONCLUSION

Motivational deficits in TRS are characterised by both diminished reward sensitivity and reduced effort aversion during EBDM. Cognitive dysfunction and negative symptom severity account for distinct aspects of these behavioural changes, despite positive associations between themselves. Overall, these findings demonstrate that negative symptoms and cognitive impairment have significant independent contributions to EBDM in TRS, thereby opening the possibility of individualised treatment targeting these mechanisms to improve motivation.

[Therapeutic drug monitoring of clozapine]

Clozapine is a prototypical atypical antipsychotic used to treat severe schizophrenia and for which a therapeutic drug monitoring (TDM) is quite commonly proposed. Clozapine is rapidly absorbed (maximum concentration reached within 1 to 4hours), and is extensively metabolized in the liver by CYP1A2 to an active metabolite (and to a lesser extent, to inactive metabolites via other enzymes). Its half-life is 8 to 16h. A therapeutic range has been proposed for clozapine as some studies have reported both a relationship between low plasmatic concentrations and resistance to treatment (threshold level is likely between 250 and 400μg/L), and a relationship between high plasmatic concentrations and an increase in the occurrence of toxicity (alert level=1000μg/L). Given the data obtained in different studies, the TDM was evaluated for this molecule, to recommended.

ABC Transporters and CYP3A4 Mediate Drug Interactions between Enrofloxacin and Salinomycin Leading to Increased Risk of Drug Residues and Resistance

Enrofloxacin (ENR) is one of the most common drugs used in poultry production to treat bacterial diseases, and there is a high risk of drug interactions (DDIs) between polyether anticoccidial drugs added to poultry feed over time. This may affect the efficacy of antibiotics or lead to toxicity, posing a potential risk to the environment and food safety. This study aimed to investigate the DDI of ENR and salinomycin (SAL) in broilers and the mechanism of their DDI. We found that SAL increased the area under the curve and elimination half-life of ENR and ciprofloxacin (CIP) by 1.3 and 2.4 times, 1.2 and 2.5 times, respectively. Cytochrome 3A4 (CYP3A4), p-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) were important factors for the DDI between ENR and SAL in broilers. ENR and SAL are substrates of CYP3A4, P-gp and BCRP in broilers; ENR and SAL inhibited the expression of CYP3A4 activity in a time- and concentration-dependent. Meanwhile, ENR downregulated the expression of P-gp and BCRP in a time- and concentration-dependent manner. A single oral administration of SAL inhibited CYP3A4, P-gp, and BCRP, but long-term mixed feeding upregulated the expression of CYP3A4, P-gp, and BCRP. Molecular docking revealed that ENR and SAL compete with each other for CYP3A4 to affect hepatic metabolism, and compete with ATP for P-gp and BCRP binding sites to inhibit efflux. ENR and SAL in broilers can lead to severe DDI. Drug residues and resistance following co-administration of ENR and SAL and other SAL-based drug-feed interactions warrant further study.

Adverse Drug Reactions in Relation to Clozapine Plasma Levels: A Systematic Review

Clozapine is the gold standard for treatment-resistant schizophrenia. Serious and even life-threatening adverse effects, mostly granulocytopenia, myocarditis, and constipation, are of great clinical concern and constitute a barrier to prescribing clozapine, thus depriving many eligible patients of a lifesaving treatment option. Interestingly, clozapine presents variable pharmacokinetics affected by numerous parameters, leading to significant inter- and intra-individual variation. Therefore, therapeutic drug monitoring of plasma clozapine levels confers a significant benefit in everyday clinical practice by increasing the confidence of the prescribing doctor to the drug and the adherence of the patient to the treatment, mainly by ensuring effective treatment and limited dose-related side effects. In the present systematic review, we aimed at identifying how a full range of adverse effects relates to plasma clozapine levels, using the Jadad grading system for assessing the quality of the available clinical evidence. Our findings indicate that EEG slowing, obsessive-compulsive symptoms, heart rate variability, hyperinsulinemia, metabolic syndrome, and constipation correlate to plasma clozapine levels, whereas QTc, myocarditis, sudden death, leucopenia, neutropenia, sialorrhea, are rather unrelated. Rapid dose escalation at the initiation of treatment might contribute to the emergence of myocarditis, or leucopenia. Strategies for managing adverse effects are different in these conditions and are discussed accordingly.

Recognition and management of clozapine adverse effects: A systematic review and qualitative synthesis

OBJECTIVE

Clozapine is substantially underutilized in most countries and clinician factors including lack of knowledge and concerns about adverse drug effects (ADEs) contribute strongly to treatment reluctance. The aim of this systematic review was to provide clinicians with a comprehensive information source regarding clozapine ADEs.

METHODS

PubMed and Embase databases were searched for English language reviews concerned with clozapine ADEs; publications identified by the automated search were manually searched for additional relevant citations. Following exclusion of redundant and irrelevant reports, pertinent information was summarized in evidence tables corresponding to each of six major ADE domains; two authors reviewed all citations for each ADE domain and summarized their content by consensus in the corresponding evidence table. This study was conducted in accordance with PRISMA principles.

RESULTS

Primary and secondary searches identified a total of 305 unique reports, of which 152 were included in the qualitative synthesis. Most clozapine ADEs emerge within 3 months, and almost all appear within 6 months, after initiation. Notable exceptions are weight gain, diabetic ketoacidosis (DKA), severe clozapine-induced gastrointestinal hypomotility (CIGH), clozapine-induced cardiomyopathy (CICM), seizures, and clozapine-induced neutropenia (CIN). Most clozapine ADEs subside gradually or respond to dose reduction; those that prompt discontinuation generally do not preclude rechallenge. Rechallenge is generally inadvisable for clozapine-induced myocarditis (CIM), CICM, and clozapine-induced agranulocytosis (CIA). Clozapine plasma levels >600-1000 μg/L appear more likely to cause certain ADEs (e.g., seizures) and, although there is no clear toxicity threshold, risk/benefit ratios are generally unfavorable above 1000 μg/L.

CONCLUSION

Clozapine ADEs rarely require discontinuation.

Population pharmacokinetic model and limited sampling strategy for clozapine using plasma and dried blood spot samples

BACKGROUND

To improve efficacy, therapeutic drug monitoring is often used in clozapine therapy. Trough level monitoring is regular, but trough levels provide limited information about the pharmacokinetics of clozapine and exposure in time. The area under the concentration time curve (AUC) is generally valued as better marker of drug exposure in time but calculating AUC needs multiple sampling. An alternative approach is a limited sampling scheme in combination with a population pharmacokinetic model meant for Bayesian forecasting. Furthermore, multiple venepunctions can be a burden for the patient, whereas collecting samples by means of dried blood spot (DBS) sampling can facilitate AUC-monitoring, making it more patient friendly.

OBJECTIVE

Development of a population pharmacokinetic model and limited sampling strategy for estimating AUC_0-12h (a twice-daily dosage regimen) and AUC_0-24h (a once-daily dosage regimen) of clozapine, using a combination of results from venepunctions and DBS sampling.

METHOD

From 15 schizophrenia patients, plasma and DBS samples were obtained before administration and 2, 4, 6, and 8 h after clozapine intake. MwPharm^® pharmacokinetic software was used to parameterize a population pharmacokinetic model and calculate limited sampling schemes.

RESULTS

A three-point sampling strategy with samples at 2, 6, and 8 h after clozapine intake gave the best estimation of the clozapine AUC_0-12h and at 4, 10, and 11 h for the AUC_0-24h. For clinical practice, however, a two-point sampling strategy with sampling points at 2 and 6 h was sufficient to estimate AUC_0-12h and at 4 and 11 h for AUC_0-24h.

CONCLUSION

A pharmacokinetic model with a two-time point limited sampling strategy meant for Bayesian forecasting using DBS sampling gives a better prediction of the clozapine exposure in time, expressed as AUC, compared to trough level monitoring. This limited sampling strategy might therefore provide a more accurate prediction of effectiveness and occurrence of side effects compared to trough level monitoring. The use of DBS samples also makes the collection of clozapine samples easier and wider applicable.

Combining Therapeutic Drug Monitoring and Pharmacokinetic Modelling Deconvolutes Physiological and Environmental Sources of Variability in Clozapine Exposure

Background: Clozapine is a key antipsychotic drug for treatment-resistant schizophrenia but exhibits highly variable pharmacokinetics and a propensity for serious adverse effects. Currently, these challenges are addressed using therapeutic drug monitoring (TDM). This study primarily sought to (i) verify the importance of covariates identified in a prior clozapine population pharmacokinetic (popPK) model in the absence of environmental covariates using physiologically based pharmacokinetic (PBPK) modelling, and then to (ii) evaluate the performance of the popPK model as an adjunct or alternative to TDM-guided dosing in an active TDM population. Methods: A popPK model incorporating age, metabolic activity, sex, smoking status and weight was applied to predict clozapine trough concentrations (C_min) in a PBPK-simulated population and an active TDM population comprising 142 patients dosed to steady state at Flinders Medical Centre in Adelaide, South Australia. Post hoc analyses were performed to deconvolute the impact of physiological and environmental covariates in the TDM population. Results: Analysis of PBPK simulations confirmed age, cytochrome P450 1A2 activity, sex and weight as physiological covariates associated with variability in clozapine C_min (R² = 0.7698; p = 0.0002). Prediction of clozapine C_min using a popPK model based on these covariates accounted for <5% of inter-individual variability in the TDM population. Post hoc analyses confirmed that environmental covariates accounted for a greater proportion of the variability in clozapine C_min in the TDM population. Conclusions: Variability in clozapine exposure was primarily driven by environmental covariates in an active TDM population. Pharmacokinetic modelling can be used as an adjunct to TDM to deconvolute sources of variability in clozapine exposure.

Selective Serotonin Reuptake Inhibitors and Clozapine: Clinically Relevant Interactions and Considerations

The monoamine hypothesis of depression attributes the symptoms of major depressive disorders to imbalances of serotonin, noradrenaline, and dopamine in the limbic areas of the brain. The preferential targeting of serotonin receptor (SERT) by selective serotonin reuptake inhibitors (SSRIs) has offered an opportunity to reduce the range of these side effects and improve patient adherence to pharmacotherapy. Clozapine remains an effective drug against treatment-resistant schizophrenia, defined as failing treatment with at least two different antipsychotic medications. Patients with schizophrenia who display a constellation of negative symptoms respond poorly to antipsychotic monotherapy. Negative symptoms include the diminution of motivation, interest, or expression. Conversely to the depressive symptomology of interest presently, supplementation of antipsychotics with SSRIs in schizophrenic patients with negative symptoms lead to synergistic improvements in the function of these patients. Fluvoxamine is one of the most potent inhibitors of CYP1A2 and can lead to an increase in clozapine levels. Similar increases in serum clozapine were detected in two patients taking sertraline. However, studies have been contradictory as well, showing no such increases, which are worrying. Clinicians should be aware that clozapine levels should be monitored with any coadministration with SSRIs.

Clozapine and Norclozapine Plasma Levels in Patients Switched Between Different Liquid Formulations

BACKGROUND AND OBJECTIVE

Clozapine is the drug of choice for treatment-resistant schizophrenia. The primary objective of this study was to compare plasma clozapine and N-desmethylclozapine levels in patients switched between 2 liquid formulations [Denzapine suspension and clozapine oral solution (St George's ZTAS)]. Secondary objectives included comparison of safety, tolerability, and patient acceptability.

METHODS

This was a noninterventional, observational, prospective follow-up of patients consecutively switched between formulations of clozapine liquid in a large inner-city NHS mental health trust. The authors also performed retrospective analysis of outcomes from patient case notes.

RESULTS

The authors identified 43 patients receiving Denzapine suspension in the trust. Data were available for 43 patients switched from Denzapine to clozapine oral solution (St George's ZTAS), among whom, 15 (32%) were excluded from the analysis. Of the 28 patients for whom data were available, the 90% confidence interval for the ratio of mean values for corrected Cmin 91.5 (85.2%-98.4%) and uncorrected Cmin 91.2 (84.4%-98.6%) were within the guideline range of bioequivalence (80%-125%). Safety and tolerability profiles were comparable between the 2 formulations (P = 0.10). Patient acceptability was also similar between the brands in most domains. However, there was a taste preference for Denzapine suspension.

CONCLUSIONS

No significant difference in clozapine plasma levels was observed after switching from Denzapine suspension to a recently introduced clozapine solution. This study also highlights the significance of medicinal characteristics such as taste for patient acceptability and compliance.

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.

Association between electroencephalogram changes and plasma clozapine levels in clozapine-treated patients

This retrospective observational study was performed to investigate electroencephalogram abnormalities in clozapine-treated patients with refractory schizophrenia or bipolar disorder. The electroencephalogram and plasma clozapine and norclozapine levels in 71 patients were measured on the same day. Fifty-nine patients (85.9%) had a diagnosis of schizophrenia, and 12 patients (14.1%) had a diagnosis of bipolar disorder. The mean daily clozapine dose was 242.9 ± 105.5 mg (range 25-500 mg), and the mean plasma clozapine and norclozapine levels were 429.4 ± 264.1 and 197.8 ± 132.6 ng/ml, respectively. Twenty-five patients (35.2%) were taking valproate in combination with clozapine. electroencephalogram abnormalities were found in 51 (71.8%) patients. No patient reported clinical seizures. Plasma clozapine level was significantly associated with electroencephalogram abnormalities and was identified as a significant predictor of electroencephalogram abnormalities in a logistic regression analysis. The plasma norclozapine levels of patients taking both clozapine and valproic acid were significantly lower than those of patients treated with clozapine alone. These results demonstrate that electroencephalogram abnormalities are closely correlated with plasma clozapine levels. Valproate reduced plasma norclozapine levels. Simultaneous monitoring of electroencephalogram and plasma clozapine levels was useful for adjusting clozapine doses, improving clinical efficacy, and preventing the side effects of clozapine treatment.

Relative toxicity of mood stabilisers and antipsychotics: case fatality and fatal toxicity associated with self-poisoning

BACKGROUND

Bipolar and other psychiatric disorders are associated with considerably increased risk of suicidal behaviour, which may include self-poisoning with medication used to treat the disorder. Therefore, choice of medication for treatment should include consideration of toxicity, especially for patients at risk. The aim of this study was to estimate the relative toxicity of specific drugs within two drug categories, antipsychotics and mood stabilizers, using large-scale databases to provide evidence that could assist clinicians in making decisions about prescribing, especially for patients at risk of suicidal behaviour.

METHOD

Two indices were used to assess relative toxicity of mood stabilisers and antipsychotics: case fatality (the ratio between rates of fatal and non-fatal self-poisoning) and fatal toxicity (the ratio between rates of fatal self-poisoning and prescription). Mood stabilisers assessed included lithium [reference], sodium valproate, carbamazepine, and lamotrigine, while antipsychotics included chlorpromazine [reference], clozapine, olanzapine, quetiapine and risperidone. Fatal self-poisoning (suicide) data were provided by the Office for National Statistics (ONS), non-fatal self-poisoning data by the Multicentre Study of Self-harm in England, and information on prescriptions by the Clinical Practice Research Datalink. The primary analysis focussed on deaths due to a single drug. Cases where the drug of interest was listed as the likely primary toxic agent in multiple drug overdoses were also analysed. The study period was 2005-2012.

RESULTS

There appeared to be little difference in toxicity between the mood stabilisers, except that based on case fatality where multiple drug poisonings were considered, carbamazepine was over twice as likely to result in death relative to lithium (OR 2.37 95% CI 1.16-4.85). Of the antipsychotics, clozapine was approximately18 times more likely to result in death when taken in overdose than chlorpromazine (single drug case fatality: OR 18.53 95% CI 8.69-39.52). Otherwise, only risperidone differed from chlorpromazine, being less toxic (OR 0.06 95% CI 0.01-0.47).

CONCLUSIONS

There was little difference in toxicity of the individual mood stabilisers. Clozapine was far more toxic than the other antipsychotics. The findings are relevant to prescribing policy, especially for patients at particular risk of suicidal behaviour.

Influence of dose, gender, and cigarette smoking on clozapine plasma concentrations

INTRODUCTION

Therapeutic drug monitoring (TDM) of clozapine is a very useful method for verifying both the correct intake and the interindividual variability of its metabolism, thereby avoiding the risk of toxicity. The purposes of this paper were to discover how many patients using clozapine in common clinical practice have clozapine plasma concentration (PC) levels in the proposed reference range and to identify factors that influence clozapine PC levels.

METHODS

Our study included 100 inpatients (diagnosed with schizophrenia or schizoaffective disorder) taking standard doses of clozapine (100-700 mg/day). Clozapine concentration was measured by high-performance liquid chromatography. Correlations between doses and PC levels and the influence of smoking and gender on clozapine PC levels were calculated.

RESULTS

A large number of the patients (67%) had PC levels outside the proposed reference range. The clozapine PC levels were influenced by dose, gender, and cigarette smoking.

CONCLUSION

The correlations between dose, gender, and cigarette smoking and clozapine PC levels highlighted by our study overlap other research. It was surprising to find such a large number of patients with clozapine PC levels outside the therapeutic range. This result suggests the importance of clozapine TDM due to misunderstood inter- and/or intraindividual variability or misestimated partial therapeutic compliance.

The significance of sampling time in therapeutic drug monitoring of clozapine

OBJECTIVE

Therapeutic drug monitoring (TDM) of clozapine is standardized to 12-h postdose samplings. In clinical settings, sampling time often deviates from this time point, although the importance of the deviation is unknown. To this end, serum concentrations (s-) of clozapine and its metabolite N-desmethyl-clozapine (norclozapine) were measured at 12 ± 1 and 2 h postdose.

METHOD

Forty-six patients with a diagnosis of schizophrenia, and on stable clozapine treatment, were enrolled for hourly, venous blood sampling at 10-14 h postdose.

RESULTS

Minor changes in median percentage values were observed for both s-clozapine (-8.4%) and s-norclozapine (+1.2%) across the 4-h time span. Maximum individual differences were 42.8% for s-clozapine and 38.4% for s-norclozapine. Compared to 12-h values, maximum median differences were 8.4% for s-clozapine and 7.3% for s-norclozapine at deviations of ±2 h. Maximum individual differences were 52.6% for s-clozapine and 105.0% for s-norclozapine. The magnitude of s-clozapine differences was significantly associated with age, body mass index and the presence of chronic basophilia or monocytosis.

CONCLUSION

The impact of deviations in clozapine TDM sampling time, within the time span of 10-14 h postdose, seems of minor importance when looking at median percentage differences. However, substantial individual differences were observed, which implies a need to adhere to a fixed sampling time.

Prevalence of Therapeutic Drug Monitoring for Antidepressants and Antipsychotics in Stockholm, Sweden: A Longitudinal Analysis

Background:

Although therapeutic drug monitoring (TDM) is considered an underused tool in psychiatric care, the prevalence of TDM is largely unknown. The aim of this study was to analyze the prevalence of TDM for antidepressants and antipsychotics during 2006–2013.

Methods:

The study population consisted of individuals ≥5 years of age residing in Stockholm County. The prevalence of TDM for each study year was calculated with the number of individuals in whom TDM had been performed as nominator (extracted from the TDM database at Karolinska University Laboratory) and the number of treated individuals as denominator (extracted from the Swedish Prescribed Drug Register). All data were obtained at the third and the fifth level of the anatomical therapeutic chemical classification system (pharmacological subgroup and chemical substance, respectively). The prevalence of TDM was compared between substances according to the level of TDM recommendation by guidelines.

Results:

For antidepressants, the prevalence of TDM decreased from 0.48% (95% confidence interval, 0.45%–0.52%) in 2006 to 0.36% (0.33%–0.39%) in 2013 (among 133,275 and 162,998 treated individuals, respectively). For antipsychotics, the prevalence of TDM increased from 2.3% (2.2%–2.5%) to 4.1% (3.9%–4.3%) (31,463 and 32,534 treated individuals). For both drug groups, TDM was more common in men than in women. The most frequently analyzed drugs were clozapine, perphenazine, zuclopenthixol, nortriptyline, and flupentixol. Although not reaching statistical significance, the TDM prevalence was greater for substances strongly recommended for TDM than for substances with a lower level of recommendation, median (interquartile range): 5.6% (2.8%–22%) versus 1.1% (0.2%–2.2%), P = 0.063.

Conclusions:

The prevalence of TDM is generally low, more frequent, and increasing for antipsychotics, and more frequent for men and substances where TDM is strongly recommended.

Ultrarapid clozapine metabolism and CYP2D6 gene duplication in a patient with schizophrenia

An optimal clozapine serum level is required for both optimization of clinical response and minimization of troublesome or some of the life-threatening side effects. Serum levels can be influenced by comedication that can cause phenoconversion. When norclozapine/clozapine serum levels and ratios are consistently and significantly lower/higher than expected and there are no concomitant drugs that could account for these findings, further investigation of the genetic variants of CYP1A2, 2D6 and 3A4 are warranted.

An observational study of clozapine induced sedation and its pharmacological management

Clozapine induced sedation is common but its management is unclear. We analyzed the factors associated with clozapine-induced sedation and the efficacy of common pharmacological strategies. We conducted a naturalistic observational study using two years electronic records of a cohort patients and three analyses: a cross sectional analysis of factors associated with total number of hours slept (as an objective proxy of sedation), and two prospective analyses of which factors were associated with changes in hours slept and the efficacy of two pharmacological strategies. 133 patients were included, of which 64.7% slept at least 9h daily. Among monotherapy patients (n=30), only norclozapine levels (r=.367, p=.03) correlated with hours slept. Using the prospective cohort (n=107), 42 patients decreased the number of hours slept, due to decreasing clozapine (40%) or augmenting with aripiprazole (36%). These two strategies were recommended to 22 (20.6%) and 23 (21.5%) subjects respectively but the majority (81.8% and 73.9%) did not reduce number of hours slept. Thus, pharmacological and non-pharmacological factors are involved in sedation. Norclozapine plasma levels correlated with total sleeping hours. Reducing clozapine and aripiprazole augmentation were associated to amelioration of sedation, although both strategies were effective only in a limited numbers of subjects.

Bioequivalence of Generic and Brand Name Clozapine in Korean Schizophrenic Patients: A Randomized, Two-Period, Crossover Study

OBJECTIVE

Clozapine is the treatment of choice for refractory schizophrenia. The aim of this study was to compare the pharmacokinetics of the brand name (Clozaril) formulation and a generic formulation (Clzapine) of clozapine in Korean schizophrenic patients.

METHODS

A prospective, randomized, crossover study was conducted to evaluate the steady-state pharmacokinetic profiles of Clozaril and Clzapine. Schizophrenic patients were randomized to receive either the brand name or generic formulation (100 mg twice daily) for 10 days, followed by the other formulation for 10 days. Plasma samples were collected on the last day of each treatment period.

RESULTS

Twenty-two of 28 patients (78.6%) completed the study. The mean Cmax,ss values for Clzapine and Clozaril were 524.62 and 551.18 ng/mL, and the mean AUC0-12 values were 4479.90 hr·ng/mL and 4724.56 hr·ng/mL, respectively. The 90% CI values for the natural logarithmically transformed Cmax,ss and AUC0-12 ratios (Clzapine to Clozaril) after a single oral dose (100 mg) were 0.934 (0.849-1.028) and 0.936 (0.869-1.008), respectively. Five patients (20.8%) among 24 patients who took Clzapine reported 11 adverse events and six adverse events were reported by four patients (15.4%) among 26 who took Clozaril; there were no significant differences on physical examination or in vital signs, ECG, and laboratory tests between groups.

CONCLUSION

Generic clozapine (Clzapine) appears to be bioequivalent to brand name clozapine (Clozaril).

Effects of notoginsenoside R1 on CYP1A2, CYP2C11, CYP2D1, and CYP3A1/2 activities in rats by cocktail probe drugs

CONTEXT

Notoginsenoside R1 (NGR1) is the main component with cardiovascular activity in Panax notoginseng (Burk.) F. H. Chen, an herbal medicine that is widely used to enhance blood circulation and dissipate blood stasis.

OBJECTIVE

The objective of this study is to investigate NGR1's effects on CYP1A2, CYP2C11, CYP2D1, and CYP3A1/2 activities in rats in vivo through the use of the Cytochrome P450 (CYP450) probe drugs.

MATERIALS AND METHODS

After pretreatment with NGR1 or physiological saline, the rats were administered intraperitoneally with a mixture solution of cocktail probe drugs containing caffeine (10 mg/kg), tolbutamide (15 mg/kg), metoprolol (20 mg/kg), and dapsone (10 mg/kg). The bloods were then collected at a set of time-points for the ultra-performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS) analysis.

RESULTS

NGR1 was shown to exhibit an inhibitory effect on CYP1A2 by increased caffeine Cmax (43.13%, p < 0.01) and AUC0 - ∞ (40.57%, p < 0.01), and decreased CL/F (62.16%, p < 0.01) in the NGR1-treated group compared with those of the control group, but no significant changes in pharmacokinetic parameters of tolbutamide, metoprolol, and dapsone were observed between the two groups, indicating that NGR1 had no effects on rat CYP2C11, CYP2D1, and CYP3A1/2.

DISCUSSION AND CONCLUSION

When NGR1 is co-administered with drugs that are metabolized by CYP1A2, the pertinent potential herb-drug interactions should be monitored.

Seizure associated with clozapine: incidence, etiology, and management

Seizures are a known adverse effect of clozapine therapy. The literature varies on incidence rates of seizures, secondary to varying time frames in which each seizure occurred. Tonic-clonic seizures comprise the majority of seizures experienced secondary to clozapine use, but it is imperative to recognize the potential variety of seizure presentation. The exact etiology of clozapine-induced seizure is unknown. Conflicting reports regarding total oral dose, serum concentration, dose titration, and concomitant medications make it difficult to identify a single cause contributing to seizure risk. Following seizure occurrence, it may be in the best interests of the patient to continue clozapine treatment. In this clinical situation, the use of an antiepileptic drug (AED) for seizure prophylaxis may be required. The AED of choice appears to be valproate, but several successful case reports also support the use of lamotrigine, gabapentin and topiramate. Well-designed clinical trials regarding clozapine seizure prophylaxis are lacking. Given clozapine's strong evidence for efficacy in the treatment of schizophrenia and schizoaffective disorder, every attempt to manage side effects, including seizure, should be implemented to allow for therapeutic continuation.

Use of atypical antipsychotics in the elderly: a clinical review

The use of atypical antipsychotic drugs in the elderly has become wider and wider in recent years; in fact, these agents have novel receptor binding profiles, good efficacy with regard to negative symptoms, and reduced extrapyramidal symptoms. However, in recent years, the use of both conventional and atypical antipsychotics has been widely debated for concerns about their safety in elderly patients affected with dementia and the possible risks for stroke and sudden death. A MEDLINE search was made using the words elderly, atypical antipsychotics, use, schizophrenia, psychosis, mood disorders, dementia, behavioral disorders, and adverse events. Some personal studies were also considered. This paper reports the receptor binding profiles and the main mechanism of action of these drugs, together with their main use in psychiatry and the possible adverse events in elderly people.

Antipsychotic dosing: found in translation

In the field of schizophrenia research, as in other areas of psychiatry, there is a sense of frustration that greater advances have not been made over the years, calling into question existing research strategies. Arguably, many purported gains claimed by research have been "lost in translation," resulting in limited impact on diagnosis and treatment in the clinical setting. There are exceptions; for example, we would argue that different lines of preclinical and clinical research have substantially altered how we look at antipsychotic dosing. While this story remains a work in progress, advances "found in translation" have played an important role. Detailing these changes, the present paper speaks to a body of evidence that has already shifted clinical practice and raises questions that may further alter the manner in which antipsychotics have been administered over the last 6 decades.

The CYP1A2 -163C>A polymorphism is associated with clozapine-induced generalized tonic-clonic seizures in Brazilian schizophrenia patients

We evaluated two polymorphisms at CYP1A2 (*1C and *1F) in a sample of 108 European-derived patients with schizophrenia and their influence on the pro-convulsive effect of clozapine. We found the *1F/*1F genotype to be significantly associated with seizures, and no relationship was observed with combinations of *1F and *1C alleles.

Plasma clozapine and norclozapine in relation to prescribed dose and other factors in patients aged <18 years: data from a therapeutic drug monitoring service, 1994-2010

AIM

Clozapine is used in children and adolescents to treat early onset schizophrenia, but data on efficacy and on the plasma clozapine concentrations attained are limited.

METHODS

We studied data from a clozapine therapeutic drug monitoring (TDM) service, patients in the UK and Eire aged <18 years, 1994-2010. Multiple linear regression analysis was performed to investigate the relationship between plasma clozapine concentration and dose, age, sex, body weight, plasma clozapine:norclozapine ratio (clozapine metabolic ratio (MR)) and smoking habit.

RESULTS

There were 1408 samples from 454 patients, 267 (59%) males aged at time of first sample (median = 17; range = 8-17 years) and 187 (41%) females aged 16 (10-17) years. The plasma clozapine concentration was <0.35 mg L(-1) in 36%, and ≥0.60 mg L(-1) in 31% of samples (6.4% samples ≥1.0 mg L(-1) ). Although plasma clozapine was broadly related to prescribed dose, there was much variation: 10% of samples had plasma clozapine >0.60 mg L(-1) at prescribed clozapine doses of 50-150 mg d(-1) (66% <0.35 mg L(-1) ), while 12% of samples had plasma clozapine <0.35 mg L(-1) at doses ≥650 mg d(-1) (62% >0.6 mg L(-1) ). The covariates studied in the 16-17-year-olds had proportionately similar influences to those observed in adults. Together they explained 48% of the variance observed in plasma clozapine, with dose, smoking habit, MR and sex being major influences. In the younger patients, there were very few smokers, and the influence of sex did not reach statistical significance.

CONCLUSIONS

As in adults, clozapine TDM may help in assessing adherence and in dose adjustment, for example if smoking habit changes.

Clozapine and therapeutic drug monitoring: is there sufficient evidence for an upper threshold?

RATIONALE

Clozapine levels are advocated in the monitoring of patients on this drug and have now been used for a number of years. A safety-related threshold has also been proposed, as well as therapeutic lower and upper thresholds. While there has been reasonable consensus regarding a lower therapeutic threshold, this is not the case for the upper thresholds.

OBJECTIVES

Our aim was to review available evidence related to upper thresholds.

METHODS

We carried out an electronic search of different databases and a manual search of articles between 1960 and 2011, cross-referencing the following terms with clozapine-interactions, monitoring, pharmacokinetics, plasma levels, serum levels, and toxicity.

RESULTS

Sixty-nine articles met our search criteria and these could be divided into reviews (11), studies (24), and case reports (35). Study quality was evaluated, and none met criteria for a prospective, randomized controlled trial specifically addressing higher plasma levels, e.g., >500 ng/ml. Case reports emphasize in particular the impact of interactions, e.g., antidepressants and smoking. There is clear evidence indicating a dose-related increased risk of seizures, at least to 500-600 mg/day, but a lack of data to suggest such a relationship between plasma levels, dose, and side effects linked to safety, e.g., seizures, myocarditis, and agranulocytosis. The very limited evidence addressing an upper threshold related to clinical response suggests a "ceiling effect" in the range of 600-838 ng/ml.

CONCLUSIONS

It appears that the current safety-related threshold is not supported by evidence. There may be an upper threshold for clinical response, beyond which chance of response falls off, although further studies are warranted.

Association between CYP1A2 polymorphisms and clozapine-induced adverse reactions in patients with schizophrenia

We investigated the relationships between common polymorphisms in CYP1A2 (CYP1A2(⁎)1C and (⁎)1F), CYP1A2-mRNA levels in circulating lymphocytes and clozapine(CLZ)-induced adverse drug reactions (ADRs) in 34 patients. Patients with ADRs had a higher frequency of CYP1A2 low activity allele combinations (8/12; 67%) and lower CYP1A2-mRNA levels than patients without ADRs (6/22; 27%, P=0.019).

The stability of amitriptyline N-oxide and clozapine N-oxide on treated and untreated dry blood spot cards

Procedures for drug monitoring based on Dried Blood Spot (DBS) sampling are gaining acceptance for an increasing number of clinical and preclinical applications, where ease of use, small sample requirement, and improved sample stability have been shown to offer advantages over blood tube sampling. However, to-date, the vast majority of this work has described the analysis of well characterized drugs. Using amitriptyline, clozapine, and their potentially labile N-oxide metabolites as model compounds, we consider the merits of using DBS for discovery pharmacokinetic (PK) studies where the metabolic fate of test compounds are often unknown. Both N-oxide metabolites reverted to parent compound under standard drying (2hr) and extraction conditions. Card type significantly affected the outcome, with 14% and 22% degradation occurring for clozapine-N-oxide and amitriptyline-N-oxide on a brand of untreated DBS cards, compared to 59 and 88% on a brand of treated DBS cards. Enrichment of the parent compound ex vivo leads to overestimation of circulating blood concentration and inaccurate determination of the PK profile.

Population pharmacokinetics of clozapine and its primary metabolite norclozapine in Chinese patients with schizophrenia

AIM

To develop a combined population pharmacokinetic model (PPK) to assess the magnitude and variability of exposure to both clozapine and its primary metabolite norclozapine in Chinese patients with refractory schizophrenia via sparse sampling with a focus on the effects of covariates on the pharmacokinetic parameters.

METHODS

Relevant patient concentration data (eg, demographic data, medication history, dosage regimen, time of last dose, sampling time, concentrations of clozapine and norclozapine, etc) were collected using a standardized data collection form. The demographic characteristics of the patients, including sex, age, weight, body surface area, smoking status, and information on concomitant medications as well as biochemical and hematological test results were recorded. Persons who had smoked 5 or more cigarettes per day within the last week were defined as smokers. The concentrations of clozapine and norclozapine were measured using a HPLC system equipped with a UV detector. PPK analysis was performed using NONMEM. Age, weight, sex, and smoking status were evaluated as main covariates. The model was internally validated using normalized prediction distribution errors.

RESULTS

A total of 809 clozapine concentration data sets and 808 norclozapine concentration data sets from 162 inpatients (74 males, 88 females) at multiple mental health sites in China were included. The one-compartment pharmacokinetic model with mixture error could best describe the concentration-time profiles of clozapine and norclozapine. The population-predicted clearance of clozapine and norclozapine in female nonsmokers were 21.9 and 32.7 L/h, respectively. The population-predicted volumes of distribution for clozapine and norclozapine were 526 and 624 L, respectively. Smoking was significantly associated with increases in the clearance (clozapine by 45%; norclozapine by 54.3%). The clearance was significantly greater in males than in females (clozapine by 20.8%; norclozapine by 24.2%). The clearance of clozapine and norclozapine did not differ significantly between Chinese patients and American patients.

CONCLUSION

Smoking and male were significantly associated with a lower exposure to clozapine and norclozapine due to higher clearance. This model can be used in individualized drug dosing and therapeutic drug monitoring.

Age and sex impact clozapine plasma concentrations in inpatients and outpatients with schizophrenia

BACKGROUND

Although clozapine is primarily used in a younger to mid-life population of patients with psychosis, there are limited data on the clinical pharmacology of clozapine later in life. The objective of this study was to assess the magnitude and variability of plasma concentrations of clozapine and norclozapine across the lifespan in a real-world clinical setting.

DESIGN

A population pharmacokinetic study using nonlinear mixed effect modeling (NONMEM). Age, sex, height, weight, and dosage formulation were covariates.

SETTING

Inpatients and outpatients at the Centre for Addiction and Mental Health, Toronto, from 2001 to 2007.

PARTICIPANTS

Patients ranging in ages from 11 to 79 with schizophrenia spectrum disorders and prescribed clozapine (Clozaril).

MEASUREMENTS

A total of 1142 plasma clozapine and norclozapine concentrations (2,284 concentration measurements) from 391 patients with schizophrenia spectrum disorder.

RESULTS

A one-compartment model with first-order absorption and elimination best described the data. The population predicted clearance of clozapine for females was 27.1 L/h (SE 11.1%) and 36.7 L/h (SE 9.7%) for males. For norclozapine, clearance in females was 48.6 L/h (SE 10.8%) and 63.1 L/h (SE 9.3%) in males. The only covariates with a significant effect on clearance were age and sex: clearance for both parent and metabolite decreased exponentially with age at least 39 years.

CONCLUSIONS

Decreased clearance of clozapine and norclozapine with age results in increased blood concentrations and, hence, the potential for adverse drug reactions. These findings have particular clinical relevance for the dosing and safety monitoring of clozapine in older adults, highlighting a need for increased vigilance.

Development of a method of clozapine dosage by selective electrode to the iodides

Clozapine (Leponex(®)), the main neuroleptic indicated in the treatment of resistant schizophrenia, requires therapeutic monitoring because of its side effects and the individual variability in metabolism. In addition, several cases of intoxication by this drug were described in the literature. In this work, we studied the indirect dosage of clozapine by selective electrode to the iodides for the optimization of an analytical protocol allowing therapeutic monitoring and the diagnosis of intoxication and/or overdose. Our results showed that the developed method is linear between 0.05 and 12.5 µg/mL (r = 0.980), with a limit of detection of 0.645.10(-3) µg/mL. It presents good precision (coefficient of variation less than 4%) and accuracy (coefficient less than 10%) for all the studied concentrations. With a domain of linearity covering a wide margin of concentrations, this method can be applicable to the dosage of clozapine in tablets and in different biological matrices, such as plasma, urines, and postmortem samples.

Optimizing clozapine treatment

OBJECTIVE

Clozapine treatment remains the gold standard for treatment-resistant schizophrenia, but treatment with clozapine is associated with several side-effects that complicate the use of the drug. This clinical overview aims to provide psychiatrists with knowledge about how to optimize clozapine treatment. Relevant strategies for reducing side-effects and increasing the likelihood of response are discussed.

METHOD

Studies of clozapine available in MEDLINE were reviewed.

RESULTS

A slow up-titration of clozapine is recommended in order to reach the optimal dosage of clozapine and diminish the risk of dose-dependent side-effects. Particularly, in case of partial response or non-response, the use of therapeutic drug monitoring of clozapine is recommended. Plasma levels above the therapeutic threshold of 350-420 ng/ml are necessary to determine non-response to clozapine. To ease the burden of dose-dependent side-effects, dose reduction of clozapine should be tried and combination with another antipsychotic drug may facilitate further dose reduction. For most side-effects, counteracting medication exists. Augmentation with lamotrigine, antipsychotics, or electroconvulsive therapy may be beneficial in case of partial response to clozapine.

CONCLUSION

Treatment with clozapine should be optimized in order to increase the rate of response and to minimize side-effects, thus diminishing the risk of discontinuation and psychotic relapse.

Effect of sodium tanshinone II A sulfonate on the activity of CYP1A2 in healthy volunteers

Sodium tanshinone IIA sulfonate (STS) is a water-soluble derivative of tanshinone IIA, a famous Chinese medicine which has been used in the treatment of cardiovascular disorders for many years. Using caffeine as a probe drug, this project was designed to investigate the effect of STS on the activity of CYP1A2 in humans. Sixteen unrelated healthy volunteers were recruited for this two-phase, randomized and crossover study. The volunteers received either placebo or 60 mg day(-1) of STS injections through vein for 13 days. Pharmacokinetics of caffeine and the metabolite paraxanthine was determined by high-performance liquid chromatography. CYP1A2 activity was monitored by the ratio of paraxanthine to caffeine at 6 h in plasma. Enzyme activity analysis showed that STS significantly increased the activity of CYP1A2 by 41.1% [90% confidence interval (CI), 17.4-64.8%] (p = 0.036). The area under the curve [AUC((0-24h))] of caffeine significantly decreased by 13.3% [90% CI = 7.0-19.6%] (p = 0.005) with 13 days of treatment of STS. AUC((0-24h)) of paraxanthine significantly increased by 17.4% [90% CI = 4.3-30.5%] (p = 0.035). No significant difference was found for other parameters of caffeine and paraxanthine between two phases. STS has significantly induced the activity of CYP1A2 in vivo. Simultaneously, AUC((0-24h)) of caffeine and paraxanthine were significantly affected by STS. The findings have provided some useful information for safe and effective usage of STS in clinic.

Clozapine exposure and the impact of smoking and gender: a population pharmacokinetic study

The primary objective of this study was to evaluate the magnitude and variability of concentration exposure to clozapine and norclozapine in a real-world clinical setting, with a focus on smoking status, using population pharmacokinetic methodologies. A retrospective review of plasma clozapine and norclozapine concentrations taken from inpatients at the Centre for Addiction and Mental Health, Toronto, from 2001 to 2007 was conducted. A nonlinear mixed-effects model was developed using NONMEM, including age, gender, weight, smoking status, and dosage formulation as covariates. Pharmacokinetic parameters and interindividual and residual variabilities were estimated with 1- and 2-compartment models. A total of 519 plasma clozapine concentrations from 197 patients (138 males; mean +/- SD age, 38 +/- 13 years; schizophrenia spectrum disorder 98.2%) were included for the analysis. A 1-compartment model with first-order absorption and elimination best described the data. Apparent volume of distribution was fixed to a previously reported value in the literature of 7 L/kg.The population-predicted oral clearance of clozapine and norclozapine was 18.0 and 39.0 L/h, respectively; both the predicted clearance values vary nearly 6-fold (range, 9.18-59.06 and 16.29-97.84 L/h, respectively). For clozapine, smokers and males showed increased oral clearance by 6.0 and 4.5 L/h, respectively. For norclozapine, smokers and male gender were associated with an increased oral clearance of 11.3 and 7.6 L/h, respectively. The formulation of clozapine administered had an impact on the absorption rate with a Ka of 0.14/h for tablet and 10.3/h for the suspension form.The data suggest that smoking and male gender are associated with lower exposure to clozapine and norclozapine due to the higher oral clearance. These findings may account for some of the variability in clozapine exposure and have important implications for individualized drug dosing and therapeutic drug monitoring.

Clozapine toxicity: A discussion of pharmacokinetic factors

This report seeks to analyze and discuss different pharmacokinetic factors that might be responsible for a case of clozapine toxicity on a conventional clozapine dose. A 41-year-old Caucasian male with schizoaffective disorder was cross-titrated to 400mg/day of clozapine to manage inadequate response on 6mg/day of risperidone. A week later the patient became gradually confused and disoriented and eventually lost consciousness. The combined clozapine and norclozapine levels were elevated at 2500ng/mL. Patient's symptoms resolved after clozapine was reduced to 75mg/day with a reduction in clozapine and norclozapine levels to 420ng/mL. Toxic clozapine levels may result from abnormal drug absorption, distribution, metabolism or elimination. Changes in absorption and/or distribution are unlikely to explain the toxic levels as clozapine has relatively high oral bioavailability at steady state and a large volume of distribution. In terms of metabolism, clozapine is primarily metabolized by CYP1A2, which biotransforms clozapine to norclozapine. However, it is unlikely that CYP1A2 was responsible, as any reduction in CYP1A2 activity would have likely altered clozapine and norclozapine ratio, which was not observed in this patient. Involvement of other CYP enzymes in the development of clozapine toxicity was ruled out through genotyping. Since liver and renal function tests were also within normal limit, it is difficult to pinpoint a single pharmacokinetic factor responsible for unusually high clozapine and norclozapine levels in this patient. However, a combination of various pharmacokinetic factors may provide an explanation for clozapine toxicity in this patient.

CONCLUSION

Some patients can develop unusually high levels of clozapine and/or its metabolites on routine clozapine dosages resulting in clinically serious adverse effects as observed in our patient.