Seizure following addition of erythromycin to clozapine treatment

Effect of lemborexant on pharmacokinetics of clozapine: A potential drug-drug interaction mediated by time-dependent inhibition of CYP3A4

Clozapine (CLZ) is extensively used for treatment-resistant schizophrenia (TRS) with caution to avoid serious adverse events such as agranulocytosis and drug-drug interactions (DDIs). In the current report, we present a case of a 35-year-old male non-smoking TRS patient whose steady-state plasma trough concentrations (Ctrough ) of CLZ and its active metabolite, N-desmethylclozapine (NDMC), were significantly increased after initiating oral administration of lemborexant (LEM), a dual orexin receptor antagonist, for the treatment of insomnia. The patient experienced oversedation with sleepiness and fatigue while maintaining high levels of Ctrough of CLZ. The increased concentrations of CLZ returned to normal ranges after the discontinuation of LEM dosing, implying a pharmacokinetic DDI between CLZ and LEM. To gain insight into possible mechanisms, we performed in vitro assays of CYP1A2- and CYP3A4-mediated CLZ metabolism by measuring the formations of NDMC and clozapine N-oxide (CNO). In accordance with previous studies, the incubation of CLZ with each enzyme resulted in the production of both metabolites. LEM had only a weak inhibitory effect on CYP1A2- and CYP3A4-mediated CLZ metabolism. However, the preincubation of LEM with CYP3A4 in the presence of NADPH showed a significant enhancement of inhibitory effects on CLZ metabolism with IC50 values for the formations of CNO and NDMC of 2.8 μM and 4.1 μM, respectively, suggesting that LEM exerts as a potent time-dependent inhibitor for CYP3A4. Taken together, the results of the current study indicate that co-medication of CLZ with LEM may lead to increase in exposure to CLZ and risks of CLZ-related adverse events.

Clinically Relevant Interactions between Atypical Antipsychotics and Anti-Infective Agents

This is a comprehensive review of the literature on drug interactions (DIs) between atypical antipsychotics and anti-infective agents that focuses on those DIs with the potential to be clinically relevant and classifies them as pharmacokinetic (PK) or pharmacodynamic (PD) DIs. PubMed searches were conducted for each of the atypical antipsychotics and most commonly used anti-infective agents (13 atypical antipsychotics by 61 anti-infective agents/classes leading to 793 individual searches). Additional relevant articles were obtained from citations and from prior review articles written by the authors. Based on prior DI articles and our current understanding of PK and PD mechanism, we developed tables with practical recommendations for clinicians for: antibiotic DIs, antitubercular DIs, antifungal DIs, antiviral DIs, and other anti-infective DIs. Another table reflects that in clinical practice, DIs between atypical antipsychotics and anti-infective agents occur in patients also suffering an infection that may also influence the PK and PD mechanisms of both drugs (the atypical antipsychotic and the anti-infective agent(s)). These tables reflect the currently available literature and our current knowledge of the field and will need to be updated as new DI information becomes available.

Elevated clozapine levels associated with infection: A systematic review

Clozapine is the most effective anti-psychotic medication for treatment refractory schizophrenia. A growing number of case reports have linked infection to high clozapine levels and associated adverse outcomes. We present a systematic review of published cases to clarify the relationship between infection and elevated clozapine levels. The case reports were located through PubMed and Embase. In addition, 8 new cases from two Australian states were included. Demographics, psychiatric diagnoses and medical morbidities, medications, clinical symptoms, clozapine levels, inflammatory markers and final clinical outcome were extracted. 40 cases were identified in 23 publications that demonstrated elevated clozapine levels associated with infection. Infections were commonly respiratory in origin. Adverse events, typically sedation, were associated with raised clozapine levels during infection. In many cases the signs of infection such as fever and white blood cell count were reduced. Severe adverse effects were uncommon, with one case each of seizure, myocarditis and neutropenia. The relationship between infection, clozapine levels and adverse events is complex and multi-factorial. Monitoring of clozapine levels is essential during hospitalisation for infection and consideration should be given to gradual dose reduction to minimise dose related side effects.

Variation in the Response of Clozapine Biotransformation Pathways in Human Hepatic Microsomes to CYP1A2- and CYP3A4-selective Inhibitors

The atypical antipsychotic agent clozapine (CLZ) is effective in many patients who are resistant to conventional antipsychotic drugs. Cytochromes P450 (CYPs) 1A2 and 3A4 oxidize CLZ to norCLZ and CLZ N-oxide in human liver. Concurrent treatment with inducers and inhibitors of CYP1A2 modulates CLZ elimination that disrupts therapy. Drug-drug interactions involving CYP3A4 are also significant but less predictable. To further characterize the factors underlying these interactions, we used samples from a cohort of human livers to assess variation in CLZ oxidation pathways in relation to intrinsic CYP3A4 and CYP1A2 activities and the effects of the corresponding selective inhibitors ketoconazole (0.2 and 2 μM) and fluvoxamine (1 and 10 μM). The CYP3A4-selective inhibitor ketoconazole (2 μM) impaired CLZ N-oxide formation in all 14 of the livers used in inhibition studies (≥50% inhibition) while the CYP1A2-selective inhibitor fluvoxamine (10 μM) decreased norCLZ formation in nine. Ketoconazole effectively inhibited CLZ metabolism in five of seven livers that catalysed CYP3A4-dependent testosterone 6β-hydroxylation at or above the median rate and in four other livers with lower intrinsic CYP3A4 activity. Similarly, fluvoxamine (10 μM) readily inhibited CLZ oxidation in seven livers with high CYP1A2-mediated 7-ethoxyresorufin O-deethylation activity (at or above the median) and three livers with lower intrinsic CYP1A2 activity. In three livers, CLZ biotransformation was impaired by both ketoconazole and fluvoxamine, consistent with a major role for both CYPs. These findings suggest that the intrinsic activities of CYPs 1A2 and 3A4 are unrelated to the response to CYP-selective inhibitors and that assessment of the activities in vivo may not assist the prediction of drug-drug interactions.

Clozapine Monitoring in Clinical Practice: Beyond the Mandatory Requirement

Clozapine is effective in treatment resistant schizophrenia; however, it is underutilised probably because of its side effects. The side effects are also the potential reasons for clozapine discontinuation. A mandatory requirement for its use is regular monitoring of white blood cell count and absolute neutrophil count. However there are many side effects that need monitoring in clinical practice considering their seriousness. This article tries to summarise the clinical concerns surrounding the serious side effects of clozapine some of which are associated with fatalities and presents a comprehensive way to monitor patients on clozapine in clinical practice. It emphasizes the need to broaden the monitoring beyond the mandatory investigations. This may help in improving the safety in clinical practice and increasing clinician confidence for greater and appropriate use of this effective intervention.

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 anticonvulsants as prophylaxis for seizures in patients on clozapine

OBJECTIVE

The aim of this study is to conduct a critical review of the literature regarding the use of anticonvulsants in the prophylaxis of clozapine-induced seizures, to examine the relationship of the latter with clozapine daily dose, serum concentration and other factors than dosage that effect clozapine blood concentration, and to make recommendations for the management of clozapine-induced seizures.

METHOD

A systematic review of English-language MEDLINE articles was undertaken.

CONCLUSIONS

Clozapine-induced seizures may occur at any dose; the risk increases with dose and goes up to 4% at ≥ 600 mg/day. Some authors have advocated that patients on that dose regimen have anticonvulsant added as a primary prophylactic measure. The author discusses the pitfalls of this recommendation and highlights that seizures are better predicted from serum concentration (1300 ng/ml) rather than dose alone, and that serum concentration is strongly influenced by sex, age, smoking habit, drug-drug interactions and variations in the 1A2, 2D6 and 3A4 genotypes. Anticonvulsants are not recommended as a primary prophylaxis for clozapine-induced seizures. When deemed necessary as secondary prophylaxis, the clinician's choice should consider drug-drug interactions that may increase/decrease clozapine serum concentration and lead to more side effects, including neutropenia/agranulocytosis and seizures, or compromise therapeutic response. Recommendations for primary and secondary prophylaxis of clozapine related-seizures are provided.

Clinically significant drug interactions with atypical antipsychotics

Atypical antipsychotics [also known as second-generation antipsychotics (SGAs)] have become a mainstay therapeutic treatment intervention for patients with schizophrenia, bipolar disorders and other psychotic conditions. These agents are commonly used with other medications--most notably, antidepressants and antiepileptic drugs. Drug interactions can take place by various pharmacokinetic, pharmacodynamic and pharmaceutical mechanisms. The pharmacokinetic profile of each SGA, especially with phase I and phase II metabolism, can allow for potentially significant drug interactions. Pharmacodynamic interactions arise when agents have comparable receptor site activity, which can lead to additive or competitive effects without alterations in measured plasma drug concentrations. Additionally, the role of drug transporters in drug interactions continues to evolve and may effect both pharmacokinetic and pharmacodynamic interactions. Pharmaceutical interactions occur when physical incompatibilities take place between agents prior to drug absorption. Approximate therapeutic plasma concentration ranges have been suggested for a number of SGAs. Drug interactions that markedly increase or decrease the concentrations of these agents beyond their ranges can lead to adverse events or diminished clinical efficacy. Most clinically significant drug interactions with SGAs occur via the cytochrome P450 (CYP) system. Many but not all drug interactions with SGAs are identified during drug discovery and pre-clinical development by employing a series of standardized in vitro and in vivo studies with known CYP inducers and inhibitors. Later therapeutic drug monitoring programmes, clinical studies and case reports offer methods to identify additional clinically significant drug interactions. Some commonly co-administered drugs with a significant potential for drug-drug interactions with selected SGAs include some SSRIs. Antiepileptic mood stabilizers such as carbamazepine and valproate, as well as other antiepileptic drugs such as phenobarbital and phenytoin, may decrease plasma SGA concentrations. Some anti-infective agents such as protease inhibitors and fluoroquinolones are of concern as well. Two additional important factors that influence drug interactions with SGAs are dose and time dependence. Smoking is very common among psychiatric patients and can induce CYP1A2 enzymes, thereby lowering expected plasma levels of certain SGAs. It is recommended that ziprasidone and lurasidone are taken with food to promote drug absorption, otherwise their bioavailability can be reduced. Clinicians must be aware of the variety of factors that can increase the likelihood of clinically significant drug interactions with SGAs, and must carefully monitor patients to maximize treatment efficacy while minimizing adverse events.

Clozapine-related EEG changes and seizures: dose and plasma-level relationships

Clozapine is a widely used atypical antipsychotic with a unique effectiveness in treatment-resistant schizophrenia. An important adverse effect is seizures, which have been observed at all stages of clozapine treatment. Valproate has traditionally been considered the drug of choice for the prophylaxis of clozapine seizures, however it may not be the most suitable choice for all patients. There is disagreement as to the best point to prescribe valproate or a suitable antiepileptic: as seizure prophylaxis at a certain clozapine dose or level, or only as remedial treatment. In this review, we examine the relevant literature with an aim to evaluate the following relationships: clozapine dose and electroencephalogram (EEG) abnormalities, plasma levels and EEG abnormalities, dose and occurrence of seizures and plasma levels and occurrence of seizures. Weighted linear regression models were fitted to investigate these relationships. There was a strong relationship between clozapine dose and plasma level and occurrence of clozapine-induced EEG abnormalities. However, a statistically significant relationship between dose and occurrence of seizures was not found. A relationship between clozapine plasma level and occurrence of seizures was not established because of the scarcity of useful data although our review found three case reports which suggested that there is a very substantial risk of seizures with clozapine plasma levels exceeding 1300 μg/l. Seizures are more common during the initiation phase of clozapine treatment, suggesting a slow titration to target plasma levels is desirable. An antiepileptic drug should be considered when the clozapine plasma level exceeds 500 μg/l, if the EEG shows clear epileptiform discharges, if seizures, myoclonic jerks or speech difficulties occur and when there is concurrent use of epileptogenic medication. The antiepileptics of choice for the treatment and prophylaxis of clozapine-induced seizures are valproate (particularly where there is mood disturbance) and lamotrigine (where there is resistance to clozapine).

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.

Clozapine-induced seizures: recognition and treatment

OBJECTIVES

To inform clinicians about the types of seizures that can be induced by clozapine and to provide recommendations for treatment.

METHODS

We identified articles on clozapine-induced seizures from a MEDLINE search of the English-language literature from 1978 to July 2006. The frequency of each type of seizure and the dosages of clozapine associated with seizures were compiled. In addition to this review, we report a new case illustrating the challenge of diagnosing subtle seizure activity.

RESULTS

The tonic-clonic variety is the most frequently described clozapine-induced seizure. Myoclonic and atonic seizures together constitute about one-quarter of the reported seizures. The mean dosage of clozapine associated with seizures is not high (less than 600 mg daily).

CONCLUSIONS

It may be difficult for clinicians to recognize subtle types of clozapine-induced seizures, such as myoclonic, atonic, or partial seizures. Clinicians should not place excessive reliance on the plasma level of clozapine or electroencephalogram findings to predict the occurrence of seizures. When a first seizure occurs, it is recommended that the dosage of clozapine be reduced or an alternative antipsychotic agent be employed. If a second seizure occurs, an anticonvulsant drug should be started. Special attention should be paid when commencing or discontinuing concurrent medication that may affect the plasma level of clozapine.

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.

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.

Cytochrome P450 and therapeutic drug monitoring with respect to clozapine

Clozapine is an atypical antipsychotic drug that is mainly used for the treatment of refractory schizophrenia. Clozapine is eliminated by oxidation in the liver, predominantly by cytochrome P4501A2 (CYP1A2). Due to the influence of inhibitors, inducers and genetic factors on CYP1A2-activity, several studies have reported a very large interindividual variability in clozapine plasma concentrations at a fixed dose. A number of methods have been published for the measurement of clozapine and metabolites in plasma. Plasma concentrations are most frequently measured by high-performance liquid chromatography. Most methods measure clozapine and the main metabolite, norclozapine, whereas two methods measure clozapine and two metabolites. Several studies suggest that a minimum effective clozapine plasma concentration of >350 microg/l must be achieved in order to ensure acceptable clinical response, whereas the upper limit of the therapeutic interval not yet has been clearly defined. The occurrence of agranulocytosis, the most serious side-effect of clozapine treatment does not seem to be dose-related and it is not possible to predict which patients are at risk of developing agranulocytosis. The risk of central nervous system side-effects seems to increase with concentrations above 1300 microg/l. Monitoring of clozapine plasma concentrations is recommended during concomitant use of other drugs that are known to interact with the oxidation of clozapine, such as carbamazepine (inducer) or fluvoxamine (inhibitor). Overall, it is concluded that therapeutic drug monitoring may be of value in the clinical management of clozapine.

Drug metabolism and atypical antipsychotics

The introduction of the atypical antipsychotics clozapine, risperidone, olanzapine, quetiapine and sertindole for the treatment of schizophrenia has coincided with an increased awareness of the potential of drug-drug interactions, particularly involving the cytochrome P450 (CYP) enzymes. The current literature describing the pharmacokinetics of the metabolism of these agents, including their potential to influence the metabolism of other medications, is reviewed. Clozapine appears to be metabolized primarily by CYP1A2 and CYP3A4, with additional contributions by CYP2C19 and CYP2D6. In addition, clozapine may inhibit the activity of CYP2C9 and CYP2C19, and induce CYP1A, CYP2B and CYP3A. Risperidone is metabolized by CYP2D6, and possibly CYP3A4. In vitro data indicate that olanzapine is metabolized by CYP1A2 and CYP2D6. Quetiapine is metabolised by CYP3A4 and sertindole by CYP2D6. There is, however, a general paucity of in vivo data regarding the metabolism of the atypical antipsychotics, indicating a need for further research in this area.

Characterization of the human hepatic cytochromes P450 involved in the in vitro oxidation of clozapine

It was aimed to identify the cytochrome(s) P450 (CYPs) involved in the N-demethylation and N-oxidation of clozapine (CLZ) by various approaches using human liver microsomes or microsomes from human B-lymphoblastoid cell lines. The maximum rates of formation were measured in the microsomal fraction of human livers and the Michaelis-Menten kinetics one enzyme model was found to best fit the data with mean K(M) for CLZ N-oxide and N-desmethyl-CLZ of 336 and 120 microM, respectively. Significant correlations were observed between the maximum rates of formation (Vmax) for CLZ N-oxide and N-desmethyl-CLZ with the microsomal immunoreactive contents of CYP1A2 (r = 0.92, P < 0.009 and r = 0.77, P < 0.077; respectively) and CYP3A (r = 0.89, P < 0.02 and r = 0.82, P < 0.05; respectively). Antibodies directed against CYP1A2 and CYP3A inhibited formation of CLZ N-oxide in human liver microsomes by 10.7+/-6.1%) and 37.2+/-6.9% of control, respectively, whereas CLZ N-demethylation was inhibited by 32.2+/-15.4% and 33.6+/-7.4%, respectively. Troleandomycin (CYP3A inhibitor) and furafylline (CYP1A2 inhibitor) inhibited CLZ N-oxidation in human liver microsomes by 23.2+/-12.1% and 7.8+4.3%, respectively, whereas CLZ N-demethylation was inhibited by 17.5+/-13.9% and 25.6+/-16.5%, respectively. While ketoconazole did not inhibit N-oxidation of CLZ, the N-demethylation pathway was inhibited by 34.1+/-10.0%. Formation in stable expressed enzymes indicated involvement of CYP3A and CYP1A2 in CLZ N-oxide formation and CYP2D6, CYP1A2 and CYP3A4 in CLZ N-demethylation. This apparent involvement of CYP2D6 in the N-demethylation of CLZ did not corroborate with the findings of other experiments. In conclusion, these data indicate that while both CYP isoforms readily catalyze both metabolic routes in vitro, CYP1A2 and CYP3A4 are more important in N-demethylation and N-oxidation, respectively.

Drug-drug interactions in pediatric psychopharmacology

Serious consequences caused by drug-drug interactions continue to plague contemporary pharmacotherapy. The possibility of a drug-drug interaction should be suspected anytime a new or unexpected effect occurs that complicates the clinical management of a patient in the setting where the patient is receiving more than one drug. In this article, the authors address the mechanisms of pharmacokinetic-based drug-drug interactions focusing on important interactions that may occur with the common medications a pediatrician may prescribe to the child receiving psychoactive medication(s) prescribed by a child psychiatrist.

The clinical use of plasma clozapine levels

OBJECTIVE

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

METHOD

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

RESULTS

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

CONCLUSIONS

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

Whole saliva and plasma levels of clozapine and desmethylclozapine

BACKGROUND

Therapeutic drug monitoring of clozapine as an aid in the treatment of schizophrenic states is commonly used in our hospital.

OBJECTIVE

Development of a high-performance liquid chromatographic method for the determination of clozapine (CLZ) and its major metabolite desmethylclozapine (DMCLZ) in plasma and saliva, and investigation of the relationship between plasma concentrations of CLZ and DMCLZ and concentrations in saliva in patients treated with clozapine.

METHODS

Subjects were either inpatients or outpatients with a DSM IV diagnosis of schizophrenia (n=34). Determination of CLZ and DMCLZ saliva concentrations appeared to be a satisfactory method to check compliance to treatment, particularly in outpatients.

RESULTS

Mean CLZ and DMCLZ plasma concentrations were 432+/-264 ng/ml (+/-SD) (range 90-1310 ng/ml) and 257+/-144 ng/ml (range 55-580 ng/ ml), respectively. The CLZ/DMCLZ plasma ratio was equal to 1.7+/-0-5 (daily dosage 7.2+/-2.3 mg/kg, n=34). Mean CLZ plasma and saliva levels were 336+/-157 ng/ ml (range 90-580 ng/ml) and 159+/-86 ng/ml (range 40-364ng/ml), respectively (r=0.56, n=14). Mean DMCLZ plasma and saliva levels were 196+/-112 ng/ ml (range 55-481 ng/ml) and 109+/-67ng/ml (range 40-250ng/ml), respectively (r=0.73, n=14). Mean CLZ/DMCLZ ratios determined in plasma and saliva were 1.9+/-0.6 (range 1.0-3.4) and 1.7+/-0.6 (range 1.0-3.2), respectively (r=0.85, n=14). CLZ and DMCLZ saliva concentrations appear to be useful for checking compliance to treatment, in particular among outpatients.

Pharmacokinetic interactions involving clozapine

BACKGROUND

Metabolism of clozapine is complex and not fully understood. Pharmacokinetic interactions with other drugs have been described but, in some cases, their mechanism is unknown.

METHOD

Published trials and case reports relevant to the human metabolism of clozapine and to suspected pharmacokinetic interactions were reviewed.

RESULTS

Metabolism of clozapine appears to be largely controlled by the function of the hepatic cytochrome p450IA2 (CYPIA2). Compounds which induce CYPIA2 activity (carbamazepine, tobacco smoke) may reduce plasma clozapine levels. Inhibitors of CYPIA2 (caffeine, erythromycin) have the opposite effect. Drugs which inhibit the hepatic cytochrome p4502D6 (CYP2D6) have also been reported to elevate plasma clozapine levels. The mechanism of this interaction is unclear.

CONCLUSIONS

The co-administration of clozapine and compounds reported to alter its metabolism should be avoided where possible. A host of other interactions can be predicted and so caution should be exercised when co-administering drugs which affect the function of CYPIA2 and CYP2D6. The pharmacokinetics of clozapine require further investigation so that its safe use can be assured.

Will routine therapeutic drug monitoring have a place in clozapine therapy?

Clozapine is an atypical antipsychotic medication with proven efficacy in the management of refractory schizophrenia. It is also recommended for patients who do not tolerate the extrapyramidal adverse effects of traditional antipsychotic medications. However, the therapeutic promise of clozapine has been limited by a higher incidence of agranulocytosis. Currently, plasma clozapine concentrations are not routinely used in clinical management. Therapeutic effects are monitored empirically during a 6 to 8 week titration period in which the dosage is raised to 300 to 450 mg/day. Clozapine nevertheless fulfils a number of criteria which make it a candidate for therapeutic monitoring. These include an identifiable therapeutic range, an unpredictable dose-concentration relationship between patients, a potential for clinically relevant pharmacokinetic interaction with other drugs and a high probability of patient noncompliance. The therapeutic threshold plasma concentration appears to be about 400 micrograms/L. Concentrations above 1000 micrograms/L increase the risk of adverse effects on the central nervous system (confusion, delirium and generalised seizures). There is no evidence to link increased concentrations of clozapine or its metabolite to the development of agranulocytosis. We conclude that therapeutic drug monitoring can play a useful role in the clinical management of patients treated with clozapine. The clinician is advised to primarily use clinical judgement during dosage escalation, but intermittent monitoring is recommended to quickly optimise a therapeutic dosage for each patient. At steady state, occasional measurements could be made when clinical signs indicate possible toxicity or lack of effect (possibly caused by a lack of compliance or drug interaction). Long term monitoring would, in our view, not be necessary.

Macrolide antibacterials. Drug interactions of clinical significance

Macrolide antibiotics can interact adversely with commonly used drugs, usually by altering metabolism due to complex formation and inhibition of cytochrome P-450 IIIA4 (CYP3A4) in the liver and enterocytes. In addition, pharmacokinetic drug interactions with macrolides can result from their antibiotic effect on microorganisms of the enteric flora, and through enhanced gastric emptying due to a motilin-like effect. Macrolides may be classified into 3 different groups according to their affinity for CYP3A4, and thus their propensity to cause pharmacokinetic drug interactions. Troleandomycin, erythromycin and its prodrugs decrease drug metabolism and may produce drug interactions (group 1). Others, including clarithromycin, flurithromycin, midecamycin, midecamycin acetate (miocamycin; ponsinomycin), josamycin and roxithromycin (group 2) rarely cause interactions. Azithromycin, dirithromycin, rikamycin and spiramycin (group 3) do not inactivate CYP3A4 and do not engender these adverse effects. Drug interactions with carbamazepine, cyclosporin, terfenadine, astemizole and theophylline represent the most frequently encountered interactions with macrolide antibiotics. If the combination of a macrolide and one of these compounds cannot be avoided, serum concentrations of concurrently administered drugs should be monitored and patients observed for signs of toxicity. Rare interactions and those of dubious clinical importance are those with alfentanil and sufentanil, antacids and cimetidine, oral anticoagulants, bromocriptine, clozapine, oral contraceptive steroids, digoxin, disopyramide, ergot alkaloids, felodipine, glibenclamide (glyburide), levodopa/carbidopa, lovastatin, methylprednisolone, phenazone (antipyrine), phenytoin, rifabutin and rifampicin (rifampin), triazolam and midazolam, valproic acid (sodium valproate) and zidovudine.