Effects of caffeine withdrawal from the diet on the metabolism of clozapine in schizophrenic patients

Effect of Coffee and Chocolate Ingestion on Clozapine Dose and on Plasma Clozapine and Norclozapine Concentrations in Clinical Practice

BACKGROUND

Some reports point to dietary caffeine intake as a cause of increased plasma clozapine concentrations in certain patients.

METHODS

We compared clozapine dose and plasma clozapine and N-desmethylclozapine (norclozapine) concentrations in male and female smokers and nonsmokers in relation to reported (i) coffee (caffeine) and (ii) chocolate (caffeine and theobromine) intake in samples submitted for clozapine therapeutic drug monitoring, 1993-2017.

RESULTS

There was information on coffee ingestion for 16,558 samples (8833 patients) from males and 5886 samples (3433 patients) from females and on chocolate ingestion for 12,616 samples (7568 patients) from males and 4677 samples (2939 patients) from females. When smoking was considered, there was no discernible effect of either coffee or chocolate ingestion either on the median dose of clozapine or on the median plasma clozapine and norclozapine concentrations in men and in women. However, cigarette smoking was associated with higher coffee and chocolate consumption. Although male nonsmokers who reported drinking 3 or more cups of coffee daily had significantly higher median plasma clozapine and norclozapine concentrations than those who drank less coffee, they were also prescribed a significantly higher clozapine dose. There was no clear effect of coffee ingestion on plasma clozapine and norclozapine in female nonsmokers.

IMPLICATIONS

Inhibition of clozapine metabolism by caffeine at the doses of caffeine normally encountered in those treated with clozapine is unlikely even in male nonsmokers. Measurement of plasma caffeine in an appropriate sample should be considered in any future investigation into a presumed clozapine-caffeine interaction.

[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.

Exploring low clozapine C/D ratios, inverted clozapine-norclozapine ratios and undetectable concentrations as measures of non-adherence in clozapine patients: A literature review and a case series of 17 patients from 3 studies

BACKGROUND

Up to 1/2 of outpatients prescribed clozapine may be partially/fully non-adherent, based on therapeutic drug monitoring (TDM). Three indices for measuring partial/full non-adherence are proposed a: 1) clozapine concentration/dose (C/D) ratio which drops to half or more of what is expected in the patient; 2) clozapine/norclozapine ratio that becomes inverted; and 3) clozapine concentration that becomes non-detectable.

METHODS

These 3 proposed indices are based on a literature review and 17 cases of possible non-adherence from 3 samples: 1) an inpatient study in a Chinese hospital, 2) an inpatient randomized clinical trial in a United States hospital, and 3) and a Uruguayan outpatient study.

RESULTS

The first index of non-adherence is a clozapine C/D ratio which is less than half the ratio corresponding to the patient's specific ancestry group and sex-smoking subgroup. Knowing the minimum therapeutic dose of the patient based on repeated TDM makes it much easier to establish non-adherence. The second index is inverted clozapine/norclozapine ratios in the absence of alternative explanations. The third index is undetectable concentrations. By using half-lives, the chronology of the 3 indices of non-adherence was modeled in two patients: 1) the clozapine C/D ratio dropped to ≥1/2 of what is expected from the patient (around day 2); 2) the clozapine/norclozapine ratio became inverted (around day 3); and 3) the clozapine concentration became undetectable by the laboratory (around days 9-11).

CONCLUSION

Prospective studies should further explore these proposed clozapine indices in average patients, poor metabolizers (3 presented) and ultrarapid metabolizers (2 presented).

Therapeutic Drug Monitoring of Second- and Third-Generation Antipsychotic Drugs-Influence of Smoking Behavior and Inflammation on Pharmacokinetics

Both inflammation and smoking can influence a drug’s pharmacokinetic properties, i.e., its liberation, absorption, distribution, metabolism, and elimination. Depending on, e.g., pharmacogenetics, these changes may alter treatment response or cause serious adverse drug reactions and are thus of clinical relevance. Antipsychotic drugs, used in the treatment of psychosis and schizophrenia, should be closely monitored due to multiple factors (e.g., the narrow therapeutic window of certain psychotropic drugs, the chronicity of most mental illnesses, and the common occurrence of polypharmacotherapy in psychiatry). Therapeutic drug monitoring (TDM) aids with drug titration by enabling the quantification of patients’ drug levels. Recommendations on the use of TDM during treatment with psychotropic drugs are presented in the Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology; however, data on antipsychotic drug levels during inflammation or after changes in smoking behavior—both clinically relevant in psychiatry—that can aid clinical decision making are sparse. The following narrative review provides an overview of relevant literature regarding TDM in psychiatry, particularly in the context of second- and third-generation antipsychotic drugs, inflammation, and smoking behavior. It aims to spread awareness regarding TDM (most pronouncedly of clozapine and olanzapine) as a tool to optimize drug safety and provide patient-tailored treatment.

Therapeutic Drug Monitoring of Second- and Third-Generation Antipsychotic Drugs—Influence of Smoking Behavior and Inflammation on Pharmacokinetics

Both inflammation and smoking can influence a drug’s pharmacokinetic properties, i.e., its liberation, absorption, distribution, metabolism, and elimination. Depending on, e.g., pharmacogenetics, these changes may alter treatment response or cause serious adverse drug reactions and are thus of clinical relevance. Antipsychotic drugs, used in the treatment of psychosis and schizophrenia, should be closely monitored due to multiple factors (e.g., the narrow therapeutic window of certain psychotropic drugs, the chronicity of most mental illnesses, and the common occurrence of polypharmacotherapy in psychiatry). Therapeutic drug monitoring (TDM) aids with drug titration by enabling the quantification of patients’ drug levels. Recommendations on the use of TDM during treatment with psychotropic drugs are presented in the Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology; however, data on antipsychotic drug levels during inflammation or after changes in smoking behavior—both clinically relevant in psychiatry—that can aid clinical decision making are sparse. The following narrative review provides an overview of relevant literature regarding TDM in psychiatry, particularly in the context of second- and third-generation antipsychotic drugs, inflammation, and smoking behavior. It aims to spread awareness regarding TDM (most pronouncedly of clozapine and olanzapine) as a tool to optimize drug safety and provide patient-tailored treatment.

Therapeutic Drug Monitoring of Second- and Third-Generation Antipsychotic Drugs-Influence of Smoking Behavior and Inflammation on Pharmacokinetics

Both inflammation and smoking can influence a drug's pharmacokinetic properties, i.e., its liberation, absorption, distribution, metabolism, and elimination. Depending on, e.g., pharmacogenetics, these changes may alter treatment response or cause serious adverse drug reactions and are thus of clinical relevance. Antipsychotic drugs, used in the treatment of psychosis and schizophrenia, should be closely monitored due to multiple factors (e.g., the narrow therapeutic window of certain psychotropic drugs, the chronicity of most mental illnesses, and the common occurrence of polypharmacotherapy in psychiatry). Therapeutic drug monitoring (TDM) aids with drug titration by enabling the quantification of patients' drug levels. Recommendations on the use of TDM during treatment with psychotropic drugs are presented in the Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology; however, data on antipsychotic drug levels during inflammation or after changes in smoking behavior-both clinically relevant in psychiatry-that can aid clinical decision making are sparse. The following narrative review provides an overview of relevant literature regarding TDM in psychiatry, particularly in the context of second- and third-generation antipsychotic drugs, inflammation, and smoking behavior. It aims to spread awareness regarding TDM (most pronouncedly of clozapine and olanzapine) as a tool to optimize drug safety and provide patient-tailored treatment.

The Medicinal Chemistry of Caffeine

The purine alkaloid caffeine is the most widely consumed psychostimulant drug in the world and has multiple beneficial pharmacological activities, for example, in neurodegenerative diseases. However, despite being an extensively studied bioactive natural product, the mechanistic understanding of caffeine's pharmacological effects is incomplete. While several molecular targets of caffeine such as adenosine receptors and phosphodiesterases have been known for decades and inspired numerous medicinal chemistry programs, new protein interactions of the xanthine are continuously discovered providing potentially improved pharmacological understanding and a molecular basis for future medicinal chemistry. In this Perspective, we gather knowledge on the confirmed protein interactions, structure activity relationship, and chemical biology of caffeine on well-known and upcoming targets. The diversity of caffeine's molecular activities on receptors and enzymes, many of which are abundant in the CNS, indicates a complex interplay of several mechanisms contributing to neuroprotective effects and highlights new targets as attractive subjects for drug discovery.

Caffeine-clozapine interaction associated with severe toxicity and multiorgan system failure: a case report

BACKGROUND

Caffeine is a known inhibitor of Clozapine metabolism mediated by inhibition of CYP1A2. Hitherto, the effects of caffeine on Clozapine levels have always been modest, as have the clinical manifestations of toxicity resulting from their interaction. We present a case of severe toxicity associated with the co-consumption of caffeine and Clozapine culminating in life-threatening complications requiring management in Intensive Care.

CASE PRESENTATION

A 34 year old male with a history of chronic schizophrenia, who had been managed stably on 400 mg Clozapine for the previous 5 years, changed his dietary behaviour and began consuming caffeine-containing energy drinks over the course of 3 weeks. The total daily dose of caffeine was estimated as 600 mg/day (four cans of Red Bull). He subsequently presented to the Emergency Department with life-threatening Clozapine toxicity, resulting in a decreased level of consciousness, severe metabolic acidosis, acute respiratory failure, raised inflammatory markers and acute renal failure attributed to interstitial nephritis. Maximum recorded Clozapine level was 1796 ng/ml.

CONCLUSIONS

This case describes the interaction between a common caffeine-containing beverage and a commonly prescribed antipsychotic medication, associated with severe adverse effects. We call for clinical and scientific attention to the possible interaction between these substances and draw attention to the implications for prescribing practices and patient counselling.

The clozapine to norclozapine ratio: a narrative review of the clinical utility to minimize metabolic risk and enhance clozapine efficacy

Introduction: Clozapine remains the most effective antipsychotic for treatment-refractory schizophrenia. However, ~40% of the patients respond insufficiently to clozapine. Clozapine's effects, both beneficial and adverse, have been proposed to be partially attributable to its main metabolite, N-desmethylclozapine (NDMC). However, the relation of the clozapine to norclozapine ratio (CLZ:NDMC; optimally defined as ~2) to clinical response and metabolic outcomes is not clear.Areas covered: This narrative review comprehensively examines the clinical utility of the CLZ:NDMC ratio to reduce metabolic risk and increase treatment efficacy. The association of the CLZ:NDMC ratio with changes in psychopathology, cognitive functioning, and cardiometabolic burden will be explored, as well as adjunctive treatments and their effects.Expert opinion: The literature suggests a positive association between the CLZ:NDMC ratio and better cardiometabolic outcomes. Conversely, the CLZ:NDMC ratio appears inversely associated with better cognitive functioning but less consistently with other psychiatric domains. The CLZ:NDMC ratio may be useful for predicting and monitoring cardiometabolic adverse effects and optimizing potential cognitive benefits of clozapine. Future studies are required to replicate these findings, which if substantiated, would encourage examination of adjunctive treatments aiming to alter the CLZ:NDMC ratio to best meet the needs of the individual patient, thereby broadening clozapine's clinical utility.

Caffeine Restrictions in Inpatient Psychiatric Settings: Precipitating Withdrawal?

: Caffeine is often restricted in locked inpatient psychiatric facilities based on concern that it may exacerbate clinical symptoms. However, psychiatric inpatients may be at particularly high risk of caffeine withdrawal during psychiatric hospitalization, which may cause undue discomfort or distress, limit their engagement in inpatient treatment, and confound the management of psychiatric illness. Psychiatric patients who regularly consume caffeine also possess a degree of caffeine tolerance which may reduce the risk of adverse effects associated with continued inpatient caffeine intake. For these reasons, it appears reasonable to allow caffeine in inpatient psychiatric settings.

Relationship between Clozapine-Induced Electroencephalogram Abnormalities and Serum Concentration of Clozapine in Japanese Patients with Treatment-Resistant Schizophrenia

OBJECTIVE

The objective of this study was to investigate the relationship between the serum concentration of clozapine (C-CLZ), Ndesmethylclozapine (N-CLZ) and the daily dose of CLZ (D-CLZ), and the relationships among CLZ and electroencephalogram (EEG) abnormalities.

METHODS

Twenty-eight patients were recruited to this study, but 8 patients were excluded because clozapine was discontinued before the post-treatment measurement of EEG or C-CLZ. Ultimately, 20 patients (6 men, 14 women) with an average age of 36 years were enrolled. The subjects were divided into EEG normal and abnormal groups. C-CLZ and N-CLZ were measured at 4, 12, 26, and 52 weeks after initiating CLZ administration.

RESULTS

All patients had normal baseline EEG signals, and 8 patients showed EEG abnormalities later. There were significant correlations between C-CLZ and D-CLZ, and between N-CLZ and D-CLZ. The C-CLZ/D-CLZ, N-CLZ/D-CLZ, and C-CLZ/N-CLZ ratio were not significantly different between the EEG normal and EEG abnormal groups. The EEG abnormal group had significant higher proportion of patients with high intra-individual variability in their C-CLZ/D-CLZ ratio.

CONCLUSION

There is no relationship between C-CLZ and EEG abnormalities. However, patients with high intra-individual variability in their C-CLZ/D-CLZ ratio had greater possibility of exhibiting EEG abnormalities.

A review of the nutritional challenges experienced by people living with severe mental illness: a role for dietitians in addressing physical health gaps

People experiencing a severe mental illness (SMI), such as schizophrenia, schizoaffective disorder, bipolar affective disorder or depression with psychotic features, have a 20-year mortality gap compared to the general population. This 'scandal of premature mortality' is primarily driven by preventable cardiometabolic disease, and recent research suggests that the mortality gap is widening. Multidisciplinary mental health teams often include psychiatrists, clinical psychologists, specialist mental health nurses, social workers and occupational therapists, offering a range of pharmacological and nonpharmacological treatments to enhance the recovery of clients who have experienced, or are experiencing a SMI. Until recently, lifestyle and life skills interventions targeting the poor physical health experienced by people living with SMI have not been offered in most routine clinical settings. Furthermore, there are calls to include dietary intervention as mainstream in psychiatry to enhance mental health recovery. With the integration of dietitians being a relatively new approach, it is important to review and assess the literature to inform practice. This review assesses the dietary challenges experienced by people with a SMI and discusses potential strategies for improving mental and physical health.

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.

Optimising plasma levels of clozapine during metabolic interactions: a review and case report with adjunct rifampicin treatment

BACKGROUND

Clozapine is the only licensed medication for treatment-resistant schizophrenia. The metabolism of clozapine is affected by multiple pharmacokinetic interactions, so the co-administration of adjunct medications can have a significant clinical effect. The anti- tuberculosis medication rifampicin is a potent inducer of the cytochrome P450 system and therefore can cause a reduction in the plasma concentration of clozapine. There is limited clinical evidence regarding co-administration of these medications; in particular there is a lack of data regarding the effect on plasma clozapine levels, which is the key factor determining clinical efficacy. This is clinically relevant given evidence of an increased risk of tuberculosis in patients with schizophrenia.

CASE PRESENTATION

We present a case of a 28 year old British man with a diagnosis of schizoaffective disorder who presented with persistent psychotic symptoms. He developed a systemic inflammatory condition, diagnosed as tuberculosis, and was commenced on a six month course of treatment that included rifampicin. This case presents comprehensive data to illustrate the effect on clozapine plasma levels of a complete course of tuberculosis therapy.

CONCLUSION

This case report provides guidance to clinicians in managing drug interactions between clozapine and rifampicin to enable safe and effective treatment. The co-administration of these medications is likely to increase as the existing underuse of clozapine is recognised whilst the incidence of tuberculosis increases.

A review of the clinical utility of serum clozapine and norclozapine levels

Treatment refractory schizophrenia is a serious issue affecting at least 30% of all patients with schizophrenia despite the continued emergence of new agents aimed at treating this disease. Clozapine therapy remains the most efficacious treatment for patients with schizophrenia who have failed two prior antipsychotics or those deemed an imminent harm to themselves or others. Because data are lacking on how to proceed if a patient should prove nonresponsive to clozapine therapy, the utmost care should be taken to ensure the optimization of clozapine. Therapeutic drug monitoring (TDM) is used with many other psychoactive agents to ensure the optimal therapeutic efficacy while minimizing adverse effects. The unique pharmacology of clozapine and the inter- and intraindividual variations in its pharmacokinetics make it a difficult agent with which to use TDM. The consensus is that 350 ng/mL is the lower threshold of therapeutic efficacy to define an adequate trial of clozapine. As of this writing, no clearly defined threshold exists for the upper limit of therapeutic efficacy or toxicity. TDM of clozapine can be useful in the following circumstances: when a clozapine-induced central nervous system toxicity is suspected, a medication that can inhibit or induce the metabolism of clozapine is being added or withdrawn, a change in smoking status has occurred, concerns for medication nonadherence are present, or decompensation while on a previously effective clozapine dosage is observed. The psychiatric pharmacist may play a crucial role in the interpretation and effective utilization of serum clozapine and norclozapine levels. This review will examine the current evidence for the clinical utility of monitoring serum levels of clozapine and its metabolites.

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.

Comparative cytochrome p450 in vitro inhibition by atypical antipsychotic drugs

The goal of this study was to assess in human liver microsomes the inhibitory capacity of commonly used antipsychotics on the most prominent CYP450 drug metabolizing enzymes (CYP1A2, CYP2C9, CYP2D6, and CYP3A). Chlorpromazine was the only antipsychotic that inhibited CYP1A2 activity (IC₅₀ = 9.5 μM), whilst levomepromazine, chlorpromazine, and thioridazine significantly decreased CYP2D6-mediated formation of 1′-hydroxybufuralol (IC₅₀ range, 3.5–25.5 μM). Olanzapine inhibited CYP3A-catalyzed production of 1′, and 4′-hydroxymidazolam (IC₅₀ = 14.65 and 42.20 μM, resp.). In contrast, risperidone (IC₅₀ = 20.7 μM) and levomepromazine (IC₅₀ = 30 μM) showed selectivity towards the inhibition of midazolam 1′-hydroxylation reaction, and haloperidol did so towards 4′-hydroxylation (IC₅₀ of 2.76 μM). Thioridazine displayed a K_i of 1.75 μM and an inhibitory potency of 1.57 on CYP2D6, suggesting a potential to induce in vivo interactions. However, with this exception, and given the observed K_i values, the potential of the assayed antipsychotics to produce clinically significant inhibitions of CYP450 isoforms in vivo seems limited.

Factors affecting interindividual differences in clozapine response: a review and case report

OBJECTIVE

Clozapine is the most powerful new‐generation antipsychotic. Although this drug leads to great therapeutic benefits, two types of undesirable conditions frequently occur with its use: side effects and resistance to treatment. Therapeutic drug monitoring of clozapine would be very useful to avoid both these situations. The necessity of monitoring the therapy is the result of a wide interindividual variability in the metabolism of clozapine. In this review, we highlight all the conditions underlying this variability, analyzing them one by one.

METHODS

Relevant literature was identified through a search of MEDLINE and PubMed. In addition, the case of a treatment‐resistant patient with accelerated metabolism of clozapine is reported as representative of utility of therapeutic drug monitoring in terms of clozapine dose adjustment.

RESULTS

Genetic polymorphisms of cytochrome P450 enzymes and of neurotransmitter receptors; drug interactions; interactions of clozapine with other substances such as food and drink; smoking; and nonmodifiable variables such as age, ethnicity, and gender have been examined in relation to the existing scientific literature. The laboratory techniques that clinicians could use to identify these variables and adequate therapies are also reviewed.

Pharmacogenetic testing and therapeutic drug monitoring are complementary tools for optimal individualization of drug therapy

Genetic factors contribute to the phenotype of drug response, but the translation of pharmacogenetic outcomes into drug discovery, drug development or clinical practice has proved to be surprisingly disappointing. Despite significant progress in pharmacogenetic research, only a few drugs, such as cetuximab, dasatinib, maraviroc and trastuzumab, require a pharmacogenetic test before being prescribed. There are several gaps that limit the application of pharmacogenetics based upon the complex nature of the drug response itself. First, pharmacogenetic tests could be more clinically applicable if they included a comprehensive survey of variation in the human genome and took into account the multigenic nature of many phenotypes of drug disposition and response. Unfortunately, much of the existing research in this area has been hampered by limitations in study designs and the nonoptimal selection of gene variants. Secondly, although responses to drugs can be influenced by the environment, only fragmentary information is currently available on how the interplay between genetics and environment affects drug response. Third, the use of a pharmacogenetic test as a standard of care for drug therapy has to overcome significant scientific, economic, commercial, political and educational barriers, among others, in order for clinically useful information to be effectively communicated to practitioners and patients. Meanwhile, the lack of efficacy is in this process is quite as costly as drug toxicity, especially for very expensive drugs, and there is a widespread need for clinically and commercially robust pharmacogenetic testing to be applied. In this complex scenario, therapeutic drug monitoring of parent drugs and/or metabolites, alone or combined with available pharmacogenetic tests, may be an alternative or complementary approach when attempts are made to individualize dosing regimen, maximize drug efficacy and enhance drug safety with certain drugs and populations (e.g. antidepressants in older people).

Role of CYP pharmacogenetics and drug-drug interactions in the efficacy and safety of atypical and other antipsychotic agents

Cytochrome P450 (CYP) drug oxidases play a pivotal role in the elimination of antipsychotic agents, and therefore influence the toxicity and efficacy of these drugs. Factors that affect CYP function and expression have a major impact on treatment outcomes with antipsychotic agents. In particular, aspects of CYP pharmacogenetics, and the processes of CYP induction and inhibition all influence in-vivo rates of drug elimination. Certain CYPs that mediate the oxidation of antipsychotic drugs exhibit genetic variants that may influence in-vivo activity. Thus, single nucleotide polymorphisms (SNPs) in CYP genes have been shown to encode enzymes that have decreased drug oxidation capacity. Additionally, psychopharmacotherapy has the potential for drug-drug inhibitory interactions involving CYPs, as well as drug-mediated CYP induction. Literature evidence supports a role for CYP1A2 in the clearance of the atypical antipsychotics clozapine and olanzapine; CYP1A2 is inducible by certain drugs and environmental chemicals. Recent studies have suggested that specific CYP1A2 variants possessing individual SNPs, and possibly also SNP combinations (haplotypes), in the 5′-regulatory regions may respond differently to inducing chemicals. CYP2D6 is an important catalyst of the oxidation of chlorpromazine, thioridazine, risperidone and haloperidol. Certain CYP2D6 allelic variants that encode enzymes with decreased drug oxidation capacity are more common in particular ethnic groups, which may lead to adverse effects with standard doses of psychoactive drugs. Thus, genotyping may be useful for dose optimization with certain psychoactive drugs that are substrates for CYP2D6. However, genotyping for inducible CYPs is unlikely to be sufficient to direct therapy with all antipsychotic agents. In-vivo CYP phenotyping with cocktails of drug substrates may assist at the commencement of therapy, but this approach could be complicated by pharmacokinetic interactions if applied when an antipsychotic drug regimen is ongoing.

Effects of green tea extract administration on the pharmacokinetics of clozapine in rats

The pharmacokinetic interaction between clozapine, an atypical antipsychotic with metabolic complications, including weight gain, and green tea consumption has not been evaluated, although green tea is responsible for beneficial effects, including weight reduction, and is widely consumed in the world. Commercial green tea extract (175 mg kg−1) or saline was administered orally for 4 days before the oral administration of clozapine (20 mg kg−1) to rats. Plasma concentrations of clozapine were measured up to 5 h after clozapine administration, and then hepatic CYP1A2 expression and activity were determined. There was no significant difference in the elimination half-life of clozapine between the green tea extract and saline groups. However, the time to reach peak concentration (Tmax) was significantly increased by green tea extract. The mean total area under the plasma concentration-time curve (AUC0-∞) and maximal peak plasma concentration (Cmax) of clozapine in the green tea extract group were significantly lower than those of controls. Green tea extract induced a ∼2-fold increase in hepatic CYP1A2 levels, while the activity increased slightly (by 10% of control). Because of this reduction in AUC and Tmax of clozapine by green tea extract pretreatment, we suggest that both the rate and amount of absorption of clozapine may be reduced by green tea extract, although the hepatic elimination phase may not be significantly altered. Therefore, the clinical implications of the effects of green tea on the bioavailability of clozapine in patients should be further evaluated.

Does level of care, sex, age, or choice of drug influence adherence to treatment with antipsychotics?

Rates of nonadherence during treatment with antipsychotics have been found to vary in a wide range from 20% to 90%. The aim of the present study was to investigate the influence of inpatient versus outpatient status on the adherence to treatment with olanzapine and clozapine. In the period from 1999 to 2007, olanzapine and clozapine were the 2 most frequently analyzed antipsychotics at the Department of Clinical Pharmacology at St. Olavs University Hospital, Trondheim, Norway, with more than 24,000 and more than 18,000 samples, respectively. In total, 111 patients on olanzapine and 95 patients on clozapine had provided samples in both the inpatient and outpatient settings and were included in the study. The primary outcome variable was the serum concentration-to-dose ratio (C/D ratio), that is, the serum drug concentration per milligram of drug given. For olanzapine, the C/D ratio in the outpatient setting was 10.7% lower than in the inpatient setting (P = 0.013). No such difference was found for clozapine. The difference in the olanzapine group was exclusively attributed to a lower outpatient ratio in females. For clozapine, no sex influence was found. No effect of age on the C/D ratios was found either for olanzapine or for clozapine. The lower C/D ratio in females using olanzapine in the outpatient setting might imply that they, in contrast to males, are less adherent to their medication when outside hospital. For clozapine, there were no indications of differences in adherence between inpatients and outpatients.

ABCB1 and cytochrome P450 polymorphisms: clinical pharmacogenetics of clozapine

To examine the genetic factors influencing clozapine kinetics in vivo, 75 patients treated with clozapine were genotyped for CYPs and ABCB1 polymorphisms and phenotyped for CYP1A2 and CYP3A activity. CYP1A2 activity and dose-corrected trough steady-state plasma concentrations of clozapine correlated significantly (r = -0.61; P = 1 x 10), with no influence of the CYP1A2*1F genotype (P = 0.38). CYP2C19 poor metabolizers (*2/*2 genotype) had 2.3-fold higher (P = 0.036) clozapine concentrations than the extensive metabolizers (non-*2/*2). In patients comedicated with fluvoxamine, a strong CYP1A2 inhibitor, clozapine and norclozapine concentrations correlate with CYP3A activity (r = 0.44, P = 0.075; r = 0.63, P = 0.007, respectively). Carriers of the ABCB1 3435TT genotype had a 1.6-fold higher clozapine plasma concentrations than noncarriers (P = 0.046). In conclusion, this study has shown for the first time a significant in vivo role of CYP2C19 and the P-gp transporter in the pharmacokinetics of clozapine. CYP1A2 is the main CYP isoform involved in clozapine metabolism, with CYP2C19 contributing moderately, and CYP3A4 contributing only in patients with reduced CYP1A2 activity. In addition, ABCB1, but not CYP2B6, CYP2C9, CYP2D6, CYP3A5, nor CYP3A7 polymorphisms, influence clozapine pharmacokinetics.

Variability of the in vivo metabolism of clozapine

OBJECTIVES

Clozapine, the gold standard of antipsychotic treatment in treatment-refractory patients with schizophrenia, is metabolized in vivo to clozapine-N-oxide and N-desmethylclozapine (NDMC = norclozapine). N-desmethylclozapine is an active metabolite of clozapine and combines unique pharmacological properties. Because little is known about the rate of metabolic conversion of clozapine in vivo, we assessed the association between clozapine dose and plasma levels for clozapine and NDMC.

METHODS

Plasma levels of clozapine and NDMC were measured in 485 blood samples from 108 patients with schizophrenia treated with clozapine. %NDMC, the ratio of NDMC to total clozapine (NDMC + clozapine), was used as a measure of the in vivo metabolism of clozapine.

RESULTS

Daily clozapine doses correlated significantly with clozapine levels and NDMC levels, whereas %NDMC showed a weaker negative correlation with clozapine dose. The mean %NDMC value was 37.0% +/- 16.8%, with high variability between subjects. Repeated measurements in subjects treated with the same dose of clozapine showed a high within-subject variability of %NDMC.

CONCLUSIONS

Our results suggest a high degree of between-subject and within-subject variability in the metabolism of clozapine in vivo. Direct administration of NDMC may be preferable to reliably achieve sufficient plasma levels of this compound.

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.

Metabolism of antidepressant and neuroleptic drugs by cytochrome p450s: clinical and interethnic aspects

Early after the introduction of the classical tricyclic antidepressants and neuroleptics, it was shown that the plasma concentrations of these drugs varied between patients given the same dose. This variation is to a major extent due to the variation in the activity of cytochrome P450 (CYP) enzymes (cf. review by Bertilsson et al.1) During recent year(s), the different CYP enzymes catalyzing the metabolism of these drugs have been identified and the clinical relevance has also been identified. This brief review highlights the clinical importance and ethnic differences in the metabolism of these drugs.

Metabolic drug interactions with newer antipsychotics: a comparative review

Newer antipsychotics introduced in clinical practice in recent years include clozapine, risperidone, olanzapine, quetiapine, sertindole, ziprasidone, aripiprazole and amisulpride. These agents are subject to drug-drug interactions with other psychotropic agents or with medications used in the treatment of concomitant physical illnesses. Most pharmacokinetic interactions with newer antipsychotics occur at the metabolic level and usually involve changes in the activity of the major drug-metabolizing enzymes involved in their biotransformation, i.e. the cytochrome P450 (CYP) monooxygenases and/or uridine diphosphate-glucuronosyltransferases (UGT). Clozapine is metabolized primarily by CYP1A2, with additional contribution by other CYP isoforms. Risperidone is metabolized primarily by CYP2D6 and, to a lesser extent, CYP3A4. Olanzapine undergoes both direct conjugation and CYP1A2-mediated oxidation. Quetiapine is metabolized by CYP3A4, while sertindole and aripiprazole are metabolized by CYP2D6 and CYP3A4. Ziprasidone pathways include aldehyde oxidase-mediated reduction and CYP3A4-mediated oxidation. Amisulpride is primarily excreted in the urine and undergoes relatively little metabolism. While novel antipsychotics are unlikely to interfere with the elimination of other drugs, co-administration of inhibitors or inducers of the major enzymes responsible for their metabolism may modify their plasma concentrations, leading to potentially significant effects. Most documented metabolic interactions involve antidepressant and anti-epileptic drugs. Of a particular clinical significance is the interaction between fluvoxamine, a potent CYP1A2 inhibitor, and clozapine. Differences in the interaction potential among the novel antipsychotics currently available may be predicted based on their metabolic pathways. The clinical relevance of these interactions should be interpreted in relation to the relative width of their therapeutic index. Avoidance of unnecessary polypharmacy, knowledge of the interaction profiles of individual agents, and careful individualization of dosage based on close evaluation of clinical response and, possibly, plasma drug concentrations are essential to prevent and minimize potentially adverse drug interactions in patients receiving newer antipsychotics.

Effects of Caffeine on the Kinetics of Fluvoxamine and its Major Metabolite in Plasma After a Single Oral Dose of the Drug

The effects of caffeine on the kinetics of fluvoxamine (FLV) and its major metabolite fluvoxamino acid (FLA) in plasma, after a single oral dose of the drug, were studied in 12 healthy male volunteers. The subjects received caffeine 300 mg/d or placebo for 11 days in a double-blind randomized crossover manner, and on the eighth day they received a single oral 50-mg dose of FLV. Blood sampling and pharmacodynamic evaluation were conducted up to 72 hours after FLV dosing. Plasma concentrations of FLV and FLA were measured by high-performance liquid chromatography. Caffeine significantly decreased the plasma concentrations at 6 time points (P<0.05) and total area under the plasma concentration-time curve (156.5+/-51.7 vs. 118.9+/-38.2 ng/h/mL, P<0.01) of FLV. Plasma concentration and pharmacokinetic parameters of FLA were not affected by caffeine. Caffeine induced no significant change in the pharmacodynamic effects of FLV. The present study suggests that caffeine slightly induces the metabolism of FLV, probably mediated by CYP1A2.

Plasma clozapine levels and clinical response in treatment-refractory Chinese schizophrenic patients

PURPOSE

To evaluate clinical efficacy of clozapine in relation with its plasma level in a group of Chinese patients with treatment-resistant schizophrenia. In addition, the relationship between plasma level and side effects were examined.

METHOD

Fifty-one patients with treatment-resistant schizophrenia were put on a fixed dose of clozapine at 300 mg/day for 6 weeks. Non-responders to week 6 received 500 mg/day in subsequent 6 weeks. Responders to week 6 continued to receive 300 mg/day. Clozapine plasma levels were checked at weeks 6 and 12.

FINDINGS

No association was found between clozapine plasma level, response and side effects. Sodium valproate was found to elevate clozapine plasma level while lowering norclozapine/clozapine ratio.

CONCLUSION

Clozapine plasma level was not found to be associated with response and side effect in Chinese treatment-resistant schizophrenic patients. Various explanations were postulated for the lack of relationship observed between clozapine plasma level and response in this population.

Therapeutic Drug Monitoring and Pharmacogenetic Tests as Tools in Pharmacovigilance

Therapeutic drug monitoring (TDM) and pharmacogenetic tests play a major role in minimising adverse drug reactions and enhancing optimal therapeutic response. The response to medication varies greatly between individuals, according to genetic constitution, age, sex, co-morbidities, environmental factors including diet and lifestyle (e.g. smoking and alcohol intake), and drug-related factors such as pharmacokinetic or pharmacodynamic drug-drug interactions. Most adverse drug reactions are type A reactions, i.e. plasma-level dependent, and represent one of the major causes of hospitalisation, in some cases leading to death. However, they may be avoidable to some extent if pharmacokinetic and pharmacogenetic factors are taken into consideration. This article provides a review of the literature and describes how to apply and interpret TDM and certain pharmacogenetic tests and is illustrated by case reports. An algorithm on the use of TDM and pharmacogenetic tests to help characterise adverse drug reactions is also presented. Although, in the scientific community, differences in drug response are increasingly recognised, there is an urgent need to translate this knowledge into clinical recommendations. Databases on drug-drug interactions and the impact of pharmacogenetic polymorphisms and adverse drug reaction information systems will be helpful to guide clinicians in individualised treatment choices.

An overview of psychotropic drug-drug interactions

The psychotropic drug-drug interactions most likely to be relevant to psychiatrists' practices are examined. The metabolism and the enzymatic and P-glycoprotein inhibition/induction profiles of all antidepressants, antipsychotics, and mood stabilizers are described; all clinically meaningful drug-drug interactions between agents in these psychotropic classes, as well as with frequently encountered nonpsychotropic agents, are detailed; and information on the pharmacokinetic/pharmacodynamic results, mechanisms, and clinical consequences of these interactions is presented. Although the range of drug-drug interactions involving psychotropic agents is large, it is a finite and manageable subset of the much larger domain of all possible drug-drug interactions. Sophisticated computer programs will ultimately provide the best means of avoiding drug-drug interactions. Until these programs are developed, the best defense against drug-drug interactions is awareness and focused attention to this issue.

An inter-ethnic comparison study of clozapine dosage, clinical response and plasma levels

The present study investigated clozapine dosage, plasma clozapine and metabolite levels, clinical and side-effect profiles in Asian versus Caucasian patients with chronic schizophrenia who were on stable maintenance treatment. Twenty Asian patients from Singapore and 20 Caucasian patients from Australia were systematically evaluated with the following rating scales: Positive and Negative Syndrome Scale for Schizophrenia, drug attitude scale (DAI-10), drug adverse reaction profile (Liverpool University Neuroleptic Side-effect Rating Scale), extrapyramidal side-effects scales (Abnormal Involuntary Movement Scale, Simpson and Angus Scale). Cigarette and caffeine consumption were recorded and steady-state plasma clozapine and metabolites levels were measured. Although Asian patients received a significantly lower mean clozapine dose (176 mg/day) than the Caucasian group (433 mg/day, P<0.001), plasma clozapine levels were similar between the groups. As a result, Asian patients had more than twice the effective clozapine concentration to dose ratio (P<0.001). The findings remained significant even after controlling for gender, body mass index, cigarette, alcohol and caffeine use. Conversely, the plasma metabolites (desmethylclozapine and clozapine N-oxide) to clozapine ratios were higher in the Caucasian patients (P<0.01). Compared to Caucasian patients, Asian patients appeared to have a lower dosage requirement for clinical efficacy. Hence, appropriate dose adjustment should be considered in Asian patients receiving maintenance clozapine therapy in clinical practice.

CYP1A2 activity is an important determinant of clozapine dosage in schizophrenic patients

Clozapine is an effective atypical antipsychotic drug applied in the treatment of resistant schizophrenia. The drug is mainly metabolized by cytochrome P-450 (CYP) enzymes especially the isozyme CYP1A2. Remarkably, the effective dosage varies widely among patients, making it necessary to individualize drug therapy with clozapine. The explanation for dosage variation may be differences in drug metabolism, and more specifically of CYP1A2 activity. This study is aimed at determining to what extent variability in clozapine dose can be explained by pharmacokinetic (PK) factors and more specifically by CYP1A2 activity in effectively treated psychiatric patients. In 22 evaluable patients with a schizophrenic disorder chronically using clozapine, the CYP1A2 activity and the clozapine clearance were estimated. For calculation of the pharmacokinetic parameters of clozapine, population PK software based upon Bayesian analysis was used. Caffeine clearance was estimated with the paraxanthine/caffeine ratio and served as estimate of CYP1A2 activity.A significant linear relationship was found between the clozapine dose and clozapine clearance (R: 0.71; P<0.05), whereas no relationship was found between clozapine dosage and clozapine serum trough concentration. Moreover, individual caffeine and clozapine clearances were found to be significantly related (R: 0.62; P<0.05) as were clozapine dose per kg body weight and P/C mol ratio (R: 0.44; P<0.05). We conclude that CYP1A2 activity is an important determinant of the variability of effective clozapine doses in psychiatric patients.

Influence of dose, cigarette smoking, age, sex, and metabolic activity on plasma clozapine concentrations: a predictive model and nomograms to aid clozapine dose adjustment and to assess compliance in individual patients

The measurement of plasma clozapine concentrations is useful in assessing compliance, optimizing therapy, and minimizing toxicity. We measured plasma clozapine and norclozapine (N-desmethylclozapine) concentrations in samples from 3782 patients (2648 male, 1127 female). No clozapine was detected in 291 samples (227 patients, median prescribed dose 300 mg/d). In 4963 (50.2 %) samples (2222 patients); plasma clozapine concentration ranged from 10 to 350 ng/mL.Step-wise backward multiple regression analysis (37 % of the total samples) of log10 plasma clozapine concentration against log10 clozapine dose (mg/d), age (year), sex (male = 0, female = 1), cigarette smoking habit (nonsmokers = 0; smokers = 1), body weight (kg), and plasma clozapine/norclozapine ratio (clozapine metabolic ratio, MR) showed that these covariates explained 48% of the observed variation in plasma clozapine concentration (C = ng/mL x 10-3) (P < 0.001) according to the following equation: log 10 (C) = 0.811 log 10 (dose) + 0.332 (MR) + 69.42 X 10 (-3) (sex) + 2.263 x 10 (-3) (age) + 1.976 x 10(-3) (weight) - 0.171 (smoking habit) - 3.180. This model and its associated confidence intervals were used to develop nomograms of plasma clozapine concentration versus dose for male and female smokers and nonsmokers. Predicted plasma clozapine changes by +48% in nonsmokers, +17% in females, +/-8 % for every 0.1 change in MR (reference 1.32), +/-4% for every 5 years (reference 40 years), and +/-5 % for every 10 kg body weight (reference 80 kg). The nomograms can be used (i) to individualize dosage to achieve a given target plasma clozapine concentration, and (ii) for quantitative evaluation of adherence by estimating the likelihood of an observed concentration being achieved by a given dosage regimen. The model has been validated against published data.

Toxic rise of clozapine plasma concentrations in relation to inflammation

Recently a small number of patients were observed in two psychiatric hospitals in the Netherlands with clozapine intoxications that complicate or mimic infections. These patients were on chronic medication and normally had stable clozapine blood plasma levels. This article presents four of these cases. Medline was searched for reports of similar cases. A hypothesis was formulated and tested by literature study. Immune modulatory and toxic effects of clozapine protein reactive metabolites or haptens, may play a role in the development of inflammation. Clozapine has a direct influence on different cytokines resembling an inflammatory reaction. Infection or inflammation could induce bioactivation of clozapine into its nitrenium ion that can exert a toxic reaction that induces apoptosis and gives rise to elevated cytokine levels. Clozapine can function as a hapten and induce an IgG, IgM or IgE mediated hypersensitivity reaction. The cytokines released during infection or inflammation downregulate the clozapine metabolism in the P450 system through CYP 1A2. Clozapine plasma levels should be monitored closely if an inflammatory or infectious process is suspected.

Metabolic drug interactions with new psychotropic agents

New psychotropic drugs introduced in clinical practice in recent years include new antidepressants, such as selective serotonin reuptake inhibitors (SSRI) and 'third generation' antidepressants, and atypical antipsychotics, i.e. clozapine, risperidone, olanzapine, quetiapine, ziprasidone and amisulpride. These agents are extensively metabolized in the liver by cytochrome P450 (CYP) enzymes and are therefore susceptible to metabolically based drug interactions with other psychotropic medications or with compounds used for the treatment of concomitant somatic illnesses. New antidepressants differ in their potential for metabolic drug interactions. Fluoxetine and paroxetine are potent inhibitors of CYP2D6, fluvoxamine markedly inhibits CYP1A2 and CYP2C19, while nefazodone is a potent inhibitor of CYP3A4. These antidepressants may be involved in clinically significant interactions when coadministered with substrates of these isoforms, especially those with a narrow therapeutic index. Other new antidepressants including sertraline, citalopram, venlafaxine, mirtazapine and reboxetine are weak in vitro inhibitors of the different CYP isoforms and appear to have less propensity for important metabolic interactions. The new atypical antipsychotics do not affect significantly the activity of CYP isoenzymes and are not expected to impair the elimination of other medications. Conversely, coadministration of inhibitors or inducers of the CYP isoenzymes involved in metabolism of the various antipsychotic compounds may alter their plasma concentrations, possibly leading to clinically significant effects. The potential for metabolically based drug interactions of any new psychotropic agent may be anticipated on the basis of knowledge about the CYP enzymes responsible for its metabolism and about its effect on the activity of these enzymes. This information is essential for rational prescribing and may guide selection of an appropriate compound which is less likely to interact with already taken medication(s).

Serious respiratory infections can increase clozapine levels and contribute to side effects: a case report

Clozapine is mainly metabolized by the cytochrome P450 1A2 (CYP1A2), which may be inhibited by serious respiratory infections. This case report supports that a serious respiratory infection may increase clozapine levels and contribute to side effects. Plasma clozapine and norclozapine levels were monitored 17 times during 1 year. The concentration-to-dose ratio (C/D), an index of metabolic activity, was obtained by dividing the sum of plasma clozapine and norclozapine concentration (total clozapine concentration) by clozapine dose. The coefficient of variation (CV) of the total clozapine concentrations was calculated at different doses to provide a measure of the noise associated with determining clozapine concentrations in clinical practice. During a respiratory infection, the patient was taking 600 mg/day of clozapine. Clozapine levels were 1245 ng/ml (norclozapine 472 ng/ml), reflecting a decrease in clozapine metabolism by approximately a factor of 2. The high clozapine levels were associated with side effects (myoclonus and increased sedation). The C/D during the infection was 2.9, while the rest of C/Ds ranged between 1.0 and 1.6. CVs before and after the infection, at different doses, were always lower than 20%. When the level during the infection was included to calculate the CV on 600 mg/day, the CV increased to 54%. The theophylline literature, a prior case report and this case all suggest that if a clozapine patient develops a severe respiratory infection with fever, the psychiatrist must pay particular attention to any signs suggestive of major clozapine toxicity associated to a decrease in clozapine metabolism. If any of these signs appear, the psychiatrist may need to consider cutting the clozapine dose in half until the patient has recovered from the infection.

Serum clozapine levels: a review of their clinical utility

Therapeutic drug monitoring (TDM) is frequently utilized in the treatment of psychiatric conditions, but its clinical application concerning the use of clozapine is unclear. We present three case reports of patients taking clozapine, review the relevant literature, and propose guidelines to aid the clinical use of TDM of clozapine. Due to its complex metabolism, there are significant inter- and intra-individual variations in clozapine serum levels, for a given dose. However, the range of serum levels that corresponds with toxicity remains unclear. Although central nervous system side-effects may correlate with serum level, many adverse effects of clozapine appear to be unrelated, including haematological and cardiac events. There are numerous clinically significant interactions between clozapine and other substances, including prescribed medications, nicotine and caffeine. TDM of clozapine may be of clinical value in certain situations, such as poor clinical response; signs of toxicity; onset of seizures; changes in concurrent medication, caffeine or nicotine; liver disease; and suspected non-compliance. The current literature does not support the routine testing of serum clozapine levels in everyday clinical practice.

Clozapine pharmacokinetics in children and adolescents with childhood-onset schizophrenia

Clozapine (CLZ) dose-related adverse effects may be more common in children than adults, perhaps reflecting developmental pharmacokinetic (PK) differences. However, no pediatric CLZ PK data are available. Accordingly, we studied CLZ and its metabolites, norclozapine (NOR), and clozapine-N-oxide (NOX) in six youth, ages 9-16 years, with childhood onset schizophrenia (COS). At the time of the PK study, mean CLZ dose was 200 mg (3.4 mg/kg). Serum was collected during week 6 on CLZ before and 0.5-8 h after a morning dose. Serum concentrations were assayed by liquid chromatography/UV-detection. Mean concentration, area-under-the-curve (AUC), and clearance were calculated. CLZ clearance averaged 1.7 L/kg-h. NOR concentrations (410) exceeded CLZ (289) and NOX (63 ng/ml) and AUC(0-8h) of NOR (3,356) > CLZ (2,359) > NOX (559 ng/ml-h) [53, 38, and 9% of total analytes, respectively]. In adults, NOR serum concentrations on average are 10-25% < CLZ, differing significantly from our sample. Dose normalized concentrations of CLZ (mg/kg-d) did not vary with age and were similar to reported adult values. Clinical improvement seen in 5/6 patients correlated with serum CLZ concentrations. In addition, clinical response and total number of side effects correlated with NOR concentrations. NOR (a neuropharmacologically active metabolite) and free CLZ may contribute to the effectiveness and adverse effects in youth.

Evidence from a population pharmacokinetics analysis for a major effect of CYP1A2 activity on inter- and intraindividual variations of clozapine clearance

Interindividual variations of clozapine clearance could be related to individual CYP1A2 activity. A population approach was used to investigate clozapine pharmacokinetics of multiple doses of clozapine in patients. Clozapine plasma concentrations were obtained in 23 patients from therapeutic drug monitoring (83 samples). CYP1A2 activity was estimated by the norclozapine/clozapine plasma levels ratio and data were processed by a nonlinear mixed-effect modelling method. Different covariates (age, body weight, height, CYP1A2 activity, daily dose of clozapine) were tested but CYP1A2 activity was the single parameter that improved significantly the predictive model. The best fit was obtained by integration of a linear relationship between clozapine clearance and CYP1A2 activity. The findings suggest that (i) CYP1A2 activity is a major factor that determines clozapine clearance and (ii) the norclozapine/clozapine ratio could constitute a valuable measure of the CYP1A2 activity. This ratio can be simply determined in the context of therapeutic drug monitoring and could explain the inter- and intraindividual variation of clozapine plasma levels.

Pharmacogenetics of bipolar disorder

To review the pharmacogenetics of bipolar disorders, the authors searched databases for genetic association and linkage studies involving response to long-term prophylactic lithium treatment, as well as treatment with antidepressants or clozapine. Significant ethnic variations in the metabolism and efficacy of antidepressants, as well as clozapine, have been reported by several groups. Systematic studies suggest that that genetic factors affect the response to prophylactic lithium treatment. Numerous associations between the three traits of interest and candidate gene polymorphisms have been proposed. Among these, an association between the serotonin transporter gene and response to serotonin reuptake inhibitors appears robust. Considerable interest has also focused on serotonergic gene polymorphisms and response to clozapine. Response to pharmacotherapy in bipolar disorders may be mediated by genetic factors, but the role played by heritability is unknown.

Unanticipated plasma concentrations in two clozapine-treated patients

OBJECTIVE

To report two cases of lower than anticipated clozapine plasma concentrations despite near maximum recommended doses of clozapine 800-900 mg/d in two medication-compliant schizophrenic inpatients.

CASE SUMMARIES

Clozapine therapy was initiated in two male schizophrenic inpatients for treatment of psychotic symptoms refractory to other typical and atypical antipsychotics. Despite receiving adequate doses of clozapine for at least two months, these patients remained symptomatic. Therapeutic drug monitoring was used to target a clozapine plasma concentration of > or =250 ng/mL, the minimum value reported in the literature to be associated with increased clinical response. Clozapine plasma concentrations remained at 200 ng/mL in one patient despite dosage increases from 600 to 800 mg/d. In the second patient, administration of the maximum recommended dose resulted in concentrations between 200 and 250 ng/mL. Increasing the clozapine dosage to 1000 mg/d did not increase the clozapine plasma concentration. Evaluation of the ratio of clozapine plasma concentration clozapine to dose yielded lower than expected values compared with those reported in the literature.

DISCUSSION

These two patients exhibited lower than anticipated clozapine plasma concentrations despite receiving high doses of clozapine. Several studies evaluating clozapine serum concentrations and clinical response have suggested threshold concentrations of > or =350 ng/mL, > or =370 ng/mL, or > or =420 ng/mL. The only study that randomized patients to three concentration ranges found that patients who achieved a clozapine serum concentration in a medium range (mean 251 ng/mL) responded better than patients in a low range (mean 91 ng/mL) and similar to patients in a high range (mean 396 ng/mL). However, attaining plasma concentrations in this range for these patients proved difficult. Reasons for the low concentrations are unclear and may be related to increased metabolic activity at several cytochrome P450 isoenzymes involved in the metabolism of clozapine.

CONCLUSIONS

These cases illustrate lower than anticipated clozapine plasma concentrations despite high-dose clozapine therapy. Strategies to increase clozapine plasma concentrations in such patients might include adding a drug to partially inhibit the metabolism of clozapine. If those strategies are unacceptable based on risk assessment, patient compliance, or other reasons, clinicians may consider addition of a low-dose typical or other atypical antipsychotic drug to augment clozapine response.