Single- vs multiple-dose pharmacokinetics of clozapine in psychiatric patients

A population pharmacokinetic model to guide clozapine dose selection, based on age, sex, ethnicity, body weight and smoking status

AIMS

Guidance on clozapine dosing in treatment-resistant schizophrenia is based largely on data from White young adult males. This study aimed to investigate the pharmacokinetic profiles of clozapine and N-desmethylclozapine (norclozapine) across the age range, accounting for sex, ethnicity, smoking status and body weight.

METHODS

A population pharmacokinetic model, implemented in Monolix, that linked plasma clozapine and norclozapine via a metabolic rate constant, was used to analyse data from a clozapine therapeutic drug monitoring service, 1993-2017.

RESULTS

There were 17 787 measurements from 5960 patients (4315 male) aged 18-86 years. The estimated clozapine plasma clearance was reduced from 20.2 to 12.0 L h-1 between 20 and 80 years. Model-based dose predictions to attain a predose plasma clozapine concentration of 0.35 mg L-1 was 275 (90% prediction interval 125, 625) mg day-1 in nonsmoking, White males weighing 70 kg and aged 40 years. The corresponding predicted dose was increased by 30% in smokers, decreased by 18% in females, and was 10% higher and 14% lower in otherwise analogous Afro-Caribbean and Asian patients, respectively. Overall, the predicted dose decreased by 56% between 20 and 80 years.

CONCLUSION

The large sample size and wide age range of the patients studied allowed precise estimation of dose requirements to attain predose clozapine concentration of 0.35 mg L-1 . The analysis was, however, limited by the absence of data on clinical outcome and future studies are required to determine optimal predose concentrations specifically in those aged over 65 years.

[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.

Drug Development for New Psychiatric Drug Therapies

Drug development is an expensive, high risk, and highly regulated process. Only about 6.2% of new molecules tested for mental disorders eventually achieve Food and Drug Administration (FDA) approval. New molecular entities are produced, and extensive in vitro animal testing is performed before they are evaluated in humans. The compound is used in animals to predict clinical effects in humans, and studies addressing pharmacodynamics, pharmacokinetics, toxicology, and mutagenicity are conducted. Human research proceeds in three stages with the ultimate goal of proving that a new agent is efficacious and safe for a treatment of a specific disease in humans. If efficacy and safety are demonstrated in two Phase III studies, then the sponsor can submit a new drug application (NDA) to the FDA. The FDA oversees each step of the process to assure that good research practices are followed, data integrity is assured, and human research subjects are protected.

Dose-Related Reference Range as a Tool in Therapeutic Drug Monitoring

BACKGROUND

Therapeutic drug monitoring (TDM) aims to individualize drug therapy. This systematic review provides a state-of-the-art overview of the benefits of adding the dose-related reference range (DRR) as a second reference range to the set of tools used by TDM for measurement and evaluation. It discusses alternative pharmacokinetic approaches for individualization of drug therapy.

METHODS

Literature was searched in PubMed. Textbooks provided Bateman transformations for calculating expected drug concentrations at various times after drug application in "normal patients," that is, the population of phase II clinical trials. The review compiles conditions and prerequisites for these transformations to be valid.

RESULTS

Relating a measured drug concentration to the orienting therapeutic reference range provides pharmacodynamic information for improving the benefit-to-risk ratio of desired drug effects versus adverse drug effects. The discriminating DRR considers a patient's individual pharmacokinetic situation. DRR is statistically based on the pharmacokinetic parameters total clearance, time to reach maximal concentrations, and elimination half-life. Relating the measured drug concentration to a range rather than a particular value, DRR determines if individual patients do or do not belong to the population of "normal patients." Once a patient is identified to be outside the population of "normal patients," the clinical-pharmacological TDM report elaborates the cause. It consists of the measured value, the TDM 9-field-board, the elimination pathways table, and a medication recommendation taking into account clinical information. The internet-based platform KONBEST supports editing of the clinical-pharmacological TDM report. It is personally signed and send to the therapist.

CONCLUSIONS

The DRR embedded into a clinical-pharmacological TDM report allows adjusting a patient's medication to the patient's individual needs (individualization of drug therapy).

Alternative Routes of Administration of Clozapine

Clozapine is the only antipsychotic with proven effectiveness in treatment-resistant schizophrenia. It is usually administered using commercially available oral tablets, but not all patients are willing or able to take medicines in this way. Orodispersible clozapine tablets are available from several manufacturers and may be useful where swallowing solid dosage forms is difficult, or as an aid to observe compliance. Liquid formulations of clozapine can be prepared extemporaneously or purchased commercially, but most preparations are suspensions (clozapine is poorly soluble) and patients may find them unpalatable. The administration of clozapine (suspension or crushed tablets) via enteral feeding tubes (predominantly nasogastric) has been reported both in medically unwell patients and in patients refusing clozapine. Enteral administration is likely to be superseded by intramuscular clozapine, which has recently been re-introduced and is being widely used in some countries. Successful use of this formulation in enforced treatment strategies has been described by several authors with good long-term outcomes when switched to oral treatment. Intramuscular clozapine has also been used in physically ill patients who are unable to take any form of enteral medication. Other methods of delivery (transdermal, nasal) are not yet commercially available, but offer promise of further treatment options for this group of seriously ill patients.

Understanding variability in the pharmacokinetics of atypical antipsychotics - focus on clozapine, olanzapine and aripiprazole population models

Antipsychotic medicines are widely used for the management of psychotic symptoms regardless of the underlying diagnosis. Most atypical antipsychotics undergo extensive metabolism prior to excretion. Various factors may influence their pharmacokinetics, particularly elimination, leading to highly variable drug concentrations between individual patients following the same dosing regimen. Population pharmacokinetic approach, based on nonlinear mixed effects modeling, is a useful tool to identify covariates explaining pharmacokinetic variability, as well as to characterize and distinguish unexplained residual and between-subject (interindividual) variability. In addition, this approach allows the use of both sparsely and intensively sampled data. In this paper, we reviewed the pharmacokinetic characteristics of clozapine, olanzapine and aripiprazole, focusing on a population modeling approach. In particular, models based on a nonlinear mixed effects approach performed by NONMEM^® software in order to identify and quantify sources of pharmacokinetic variability are presented. Population models were identified through systematic searches of PubMed and sixteen studies were selected. Some of the factors identified that significantly contribute to variability in elimination among clozapine, olanzapine, and aripiprazole are demographic characteristics, body weight, genetic polymorphism, smoking and in some cases drug interactions. Scientific research based on pharmacometric modeling is useful to further characterize sources of variability and their combined effect.

Population Pharmacokinetics of Clozapine and Norclozapine and Switchability Assessment between Brands in Uruguayan Patients with Schizophrenia

Clozapine (CZP) is an atypical antipsychotic agent commonly used in the treatment of schizophrenia. It is metabolized primarily by CYP1A2 enzyme, yielding a pharmacologically active metabolite, norclozapine (NCZP). Significant intra- and interindividual pharmacokinetic (PK) variability for CZP and NCZP has been observed in routine therapeutic drug monitoring. So the goal of this study was to evaluate the magnitude and variability of concentration exposure to CZP and its active metabolite NCZP on pharmacokinetic parameters in Uruguayan patients with schizophrenia with a focus on covariates such as cigarette smoking, age, sex, caffeine consumption, brands available of CZP, and comedication using population PK (PPK) modeling methodologies. Patients with a diagnosis of schizophrenia treated with brand-name CZP (Leponex®) for more than a year were included in the study. Then these patients were switched to the similar brand of CZP (Luverina®). Morning predose blood samples for determination of CZP and NCZP using a HPLC system equipped with a UV detector were withdrawn on both occasions at steady state and under the same comedication. Ninety-eight patients, 22 women and 76 men, took part in the study. Mean ± standard deviation for CZP and NCZP concentration was 421 ± 262 ng/mL and 275 ± 180 ng/mL, respectively. After covariate evaluation, only smoking status remained significant in CZP apparent clearance, inducing a mean increment of 32% but with no clinical impact. The results obtained with the two brands of CZP should ensure comparable efficacy and tolerability with the clinical use of either product. Smoking was significantly associated with a lower exposure to CZP due to higher clearance. The results obtained with the two brands commercialized in our country hint a bioequivalence scenario in the clinical setting.

Clinical pharmacology of dopamine-modulating agents in Tourette's syndrome

Forty years of research and clinical practice have proved dopamine (DA) receptor antagonists to be effective agents in the treatment of Tourette's syndrome (TS), allowing a significant tic reduction of about 70%. Their main effect seems to be mediated by the blockade of the striatal DA-D2 receptors. Various typical and atypical agents are available and there is still discord between experts about which of them should be considered as first choice. In addition, there are suggestions to use DA receptor agonists such as pergolide or non-DA-modulating agents. The present chapter is focusing on the clinical pharmacology of DA-modulating agents in the treatment of TS. The introduction outlines their clinical relevance and touches on the hypotheses of the role of DA in the pathophysiology of TS. Subsequently, general information about the mechanisms of action and adverse effects are provided. The central part of the chapter forms a systematic review of all DA-modulating agents used in the treatment of TS, including an overview of studies on their effectiveness, and a critical discussion of their specific adverse effects. The present chapter closes with a summary of the body of evidence and a description of the resulting recommendations for the pharmacological treatment of TS.

A Kinetic Model for Simultaneous Fit of Clozapine and Norclozapine Concentrations in Chronic Schizophrenic Patients during Long-Term Treatment

OBJECTIVE

The pharmacokinetic profiles of clozapine and its main metabolite, norclozapine, were investigated in 18 chronic schizophrenic inpatients during long-term treatment.

PATIENTS

Patients received stable daily doses (between 300 and 900mg) for at least 1 month. Plasma drug concentrations were determined by high performance liquid chromatography. The pharmacokinetic parameters were calculated from both noncompartmental and compartmental approaches with zero-order input rate using a kinetic model for simultaneous fit of clozapine and norclozapine (active metabolite) concentrations.

RESULTS

Large interpatient variations in pharmacokinetic parameters of the two drugs were observed. Plasma clozapine concentration peaked on average at 2 hours. The mean elimination rate constants from compartments 1 (k(10)) and 2 (k(20 ), elimination rate constant of norclozapine) were 0.087 and 0.156h(-1), respectively. The rate of formation of norclozapine, k(12), averaged 1.25h(-1). The mean fraction of the administered dose converted to norclozapine was estimated to be 66%. The apparent clearance of clozapine (CL/F) averaged 44.7 L/h and the volume of distribution (V(c)/F) was 7.00 L/kg. The pharmacokinetics of clozapine after multiple doses were linear over the range of clozapine plasma concentrations of 145 to 1411 microg/L.

CONCLUSION

This is the first study assessing the pharmacokinetic profile of clozapine plus norclozapine in plasma during long-term treatment. This pharmacokinetic model can be used to determine the population pharmacokinetic parameters of clozapine and norclozapine in order to optimise individual dosage regimens using a Bayesian methodology.

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.

Clozapine versus typical neuroleptic medication for schizophrenia

BACKGROUND

Long-term drug treatment of schizophrenia with typical antipsychotics has limitations: 25 to 33% of patients have illnesses that are treatment-resistant. Clozapine is an antipsychotic drug, which is claimed to have superior efficacy and to cause fewer motor adverse effects than typical drugs for people with treatment-resistant illnesses. Clozapine carries a significant risk of serious blood disorders, which necessitates mandatory weekly blood monitoring at least during the first months of treatment.

OBJECTIVES

To evaluate the effects of clozapine compared with typical antipsychotic drugs in people with schizophrenia.

SEARCH STRATEGY

For the current update of this review (March 2006) we searched the Cochrane Schizophrenia Group Trials Register.

SELECTION CRITERIA

All relevant randomised clinical trials (RCTs).

DATA COLLECTION AND ANALYSIS

We extracted data independently. For dichotomous data we calculated relative risks (RR) and their 95% confidence intervals (CI) on an intention-to-treat basis, based on a fixed-effect model. We calculated numbers needed to treat/harm (NNT/NNH) where appropriate. For continuous data, we calculated weighted mean differences (WMD) again based on a fixed-effect model.

MAIN RESULTS

We have included 42 trials (3950 participants) in this review. Twenty-eight of the included studies are less than 13 weeks in duration, and, overall, trials were at significant risk of bias. We found no significant difference in the effects of clozapine and typical neuroleptic drugs for broad outcomes such as mortality, ability to work or suitability for discharge at the end of the study. Clinical improvements were seen more frequently in those taking clozapine (n=1119, 14 RCTs, RR 0.72 CI 0.7 to 0.8, NNT 6 CI 5 to 8). Also, participants given clozapine had fewer relapses than those on typical antipsychotic drugs (n=1303, RR 0.62 CI 0.5 to 0.8, NNT 21 CI 15 to 49). BPRS scores showed a greater reduction of symptoms in clozapine-treated patients, (n=1145, 16 RCTs, WMD -4.22 CI -5.4 to -3.1), although the data were heterogeneous (Chi(2) 0.0001, I(2) 66%). Short-term data from the SANS negative symptom scores favoured clozapine (n=196, 5 RCTs, WMD -5.92 CI -7.8 to -4.1). We found clozapine to be more acceptable in long-term treatment than conventional antipsychotic drugs (n=982, 16 RCTs, RR 0.60 CI 0.5 to 0.7, NNT 15 CI 12 to 20). Blood problems occurred more frequently in participants receiving clozapine (3.2%) compared with those given typical antipsychotics (0%) (n=1031, 13 RCTs, RR 7.09 CI 2.0 to 25.6). Clozapine participants experienced more drowsiness, hypersalivation, or temperature increase, than those given conventional neuroleptics. However, clozapine patients experienced fewer motor adverse effects (n=1433, 18 RCTs, RR 0.58 CI 0.5 to 0.7, NNT 5 CI 4 to 6).The clinical effects of clozapine were more pronounced in participants resistant to typical neuroleptics in terms of clinical improvement (n=370, 4 RCTs, RR 0.71 CI 0.6 to 0.8, NNT 4 CI 3 to 6) and symptom reduction. Thirty-four per cent of treatment-resistant participants had a clinical improvement with clozapine treatment.

AUTHORS' CONCLUSIONS

Clozapine may be more effective in reducing symptoms of schizophrenia, producing clinically meaningful improvements and postponing relapse, than typical antipsychotic drugs - but data are weak and prone to bias. Participants were more satisfied with clozapine treatment than with typical neuroleptic treatment. The clinical effect of clozapine, however, is, at least in the short term, not reflected in measures of global functioning such as ability to leave the hospital and maintain an occupation. The short-term benefits of clozapine have to be weighed against the risk of adverse effects. Within the context of trials, the potentially dangerous white blood cell decline seems to be more frequent in children and adolescents and in the elderly than in young adults or people of middle-age.The existing trials have largely neglected to assess the views of participants and their families on clozapine. More community-based long-term randomised trials are needed to evaluate the efficacy of clozapine on global and social functioning as trials in special groups such as people with learning disabilities.

[Expanding therapeutic reference ranges using dose-related reference ranges]

Evidence-based therapeutic drug monitoring (TDM), which may be successfully employed to guide drug therapy in clinical routine, supplies all the information from laboratory determination of a drug concentration in a patient's blood specimen. This value is interpreted first of all in relation to a therapeutic reference range that must be established according to the same rules that are generally accepted for clinical studies aimed to license a new drug. The drug concentration may be furthermore interpreted in reference to a dose-related reference range. Thereby a signal is created to alert for individual abnormalities such as drug/drug interactions, gene polymorphisms that give rise to slow/rapid metabolizers, altered function of the excretion organs liver and kidneys by age and/or disease, compliance problems, a missing pharmacokinetic steady state, and even signal overlay in the laboratory analysis. We return all information available and clinical pharmacological comments to physicians who send specimens to our laboratory.

Electrochemical Behavior and Determination of Clozapine on a Glassy Carbon Electrode Modified by Electrochemical Oxidation

The adsorptive and electrochemical behaviors of clozapine (CLZ) were investigated on a glassy carbon electrode that was electrochemically treated by anodic oxidation at +1.8 V, following potential cycling in the potential range from -0.8 to 1.0 V vs. Ag/AgCl reference electrode. Based on the obtained electrochemical results, an electrochemical-chemical (EC) mechanism was proposed to explain the electrochemical oxidation of CLZ. The resulting electrochemically pretreated glassy carbon electrode (EPGCE) showed good activity to improve the electrochemical response of the drug. CLZ was accumulated in a phosphate buffer (pH 6) at a certain time, and then determined by differential pulse voltammetry. The anodic and cathodic peak currents showed a linear function in the concentration ranges of 0.1 - 1, 1 - 10 and 10 - 100 microM with various accumulation times. The proposed method was successfully used for the determination of CLZ in pharmaceutical preparations. The preconcentration medium-exchange approach was utilized for the selective determination of the drug in spiked urine samples with satisfactory results. The recovery levels of the method reached 96% (RSD, 1.8%) and 90% (RSD, 2.8%) for urine and plasma samples, respectively.

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.

Contributions of five human cytochrome P450 isoforms to the N-demethylation of clozapine in vitro at low and high concentrations

The authors assessed the in vitro contribution of cytochrome P450 (CYP) isoforms 1A2, 3A4, 2C9, 2C19, and 2D6 to the N-demethylation of clozapine mediated by human liver microsomal preparations (HLM). In contrast to previous studies, the authors focused on a relatively low hepatic concentration level, 5 microM, to assess the conditions at a therapeutically relevant hepatic concentration level of clozapine. The optimal concentrations of specific inhibitors were initially established using cDNA-expressed CYP isoforms. The mean contributions of CYPs 1A2, 2C19, 3A4, 2C9, and 2D6 amounted to 30%, 24%, 22%, 12%, and 6%, respectively, with regard to the total HLM-mediated N-demethylation. Thus, the present in vitro study on clozapine N-demethylation suggests that CYP1A2 is the most important form at low concentrations, which is in agreement with clinical findings. CYP2C19 is also of considerable importance, while the roles of CYP2C9 and 2D6 are more modest. CYP3A4 attained a dominating role with an average contribution of 37% at a high clozapine concentration (50 microM). The rate of other metabolic routes mediated by CYP2D6 only corresponded to about one fifth of the CYP2D6 catalyzed N-demethylation rate.

CYP1A2 activity as measured by a caffeine test predicts clozapine and active metabolite steady-state concentrationin patients with schizophrenia

Clozapine is an atypical antipsychotic drug and displays efficacy in 30% to 60% of patients with schizophrenia who do not respond to traditional antipsychotics. A clozapine concentration greater than 1,150 nmol/L increases the probability of antipsychotic efficacy. However, plasma clozapine concentration can vary more than 45-fold during long-term treatment. The aim of this study was to assess the contribution of CYP1A2 to variability in steady-state concentration of clozapine and its active metabolite norclozapine. Patients with schizophrenia or schizoaffective disorder were prospectively monitored during clozapine treatment (N = 18). The in vivo CYP1A2 activity was measured using the caffeine metabolic ratio (CMR) in overnight urine. Trough plasma samples were drawn after at least 5 days of treatment with a constant regimen of clozapine. A significant negative association was found between the CMR and the dose-corrected clozapine (r(s) = -0.87,p < 0.01) and norclozapine (r(s) = -0.76,p < 0.01) concentrations. Nonsmokers displayed a higher clozapine (3.2-fold) and norclozapine (2.3-fold) concentration than smokers (p < 0.05). Furthermore, there was marked person-to-person variation in CYP1A2 activity during multiple-dose clozapine treatment (coefficient of variation = 60%). Age, weight, serum creatinine, and grapefruit juice consumption did not significantly contribute to variability in clozapine and norclozapine concentration (p > 0.05). In conclusion, CYP1A2 is one of the important contributors to disposition of clozapine during multiple-dose treatment. Although further in vitro experiments are necessary, the precise metabolic pathways catalyzed by CYP1A2 seem to be subsequent to the formation of norclozapine, hitherto less recognized quantitatively important alternate disposition routes, or both. From a clinical perspective, an environmentally induced or constitutively high CYP1A2 expression can lead to a decrease in steady-state concentration of clozapine as well as its active metabolite norclozapine. Thus, interindividual variability in CYP1A2 activity may potentially explain treatment resistance to clozapine in some patients. CYP1A2 phenotyping with a simple caffeine test may contribute to individualization of clozapine dosage and differentiate between treat ment noncompliance and high CYP1A2 activity.

Monitoring of clozapine and norclozapine plasma concentration-time curves in acute overdose

CASE REPORT

A 40-year-old schizophrenic man was found unconscious, with constricted pupils, sinus tachycardia, and twitching of the limbs. There were signs of lung infection, which was treated with antibiotics, and mild rhabdomyolysis. He regained consciousness over 8 hours, and reported taking 3-4 g clozapine. Recovery was uneventful. Measured peak clozapine and norclozapine concentrations were 3.53 mg/L and 0.70 mg/L, respectively. The concentration-time curves were biphasic, with secondary peaks at approximately 36 hours postadmission. Terminal elimination half-lives were 16.9 hours and 22.5 hours for clozapine and norclozapine, respectively.

CONCLUSION

Clozapine and its metabolite norclozapine can show biphasic plasma concentration-time curves after overdosage.

Intravenous and oral clozapine pharmacokinetics, pharmacodynamics, and concentration-effect relations: acute tolerance

We examined the pharmacokinetics and pharmacodynamics of intravenous (1-5 mg/kg) and oral clozapine (2.5-10 mg/kg) in rats (terminal half-life=81.8 min; oral bioavailability=5.32%). Both dose- and concentration-effect relations of clozapine were characterized. Clozapine's effects were similar to those of benzodiazepines because of the similarity in effect-time profiles between the two classes of drugs. The IC(50) value increased as a function of dose; consequently, clozapine's relative potency decreased linearly with the logarithm of AUC((0-infinity)), or bioavailable dose regardless of route of administration. The IC(50) is an index for the sensitivity of behavioral performance to clozapine; relative potency provides an index for estimating the extent of acute tolerance. As IC(50) increases, relative potency decreases, and consequently, acute tolerance increases. Our results demonstrated that greater acute tolerance was observed for i.v. clozapine than for p.o. clozapine; however, clozapine exhibited a single concentration-effect relation across dose and route of administration after correcting for relative potencies.

Metabolism of clozapine by rat brain: the role of flavin-containing monooxygenase (FMO) and cytochrome P450 enzymes

The atypical antipsychotic clozapine has been reported to be metabolised mainly to its N-oxide and N-demethylated products. Brain, the target organ of clozapine, is known to contain numerous drug-metabolising enzymes which could alter the local concentrations of the drug. The metabolism of clozapine was, therefore, studied in rat brain preparations. Clozapine N-oxide was the major metabolic pathway in rat brain. We characterised the N-oxygenation of clozapine by rat brain preparations. The Km and Vmax values were found to be 319.6 microM and 28.1 pmol/min/mg protein, respectively. The formation of clozapine N-oxide was shown to be inhibited by thiourea (a flavin-containing monooxygenase inhibitor) but not by ketoconazole, quinidine or furafylline. This finding suggests prominent involvement of FMO in the N-oxygenation of clozapine in rat brain. This conclusion was further confirmed by the observation that the formation of clozapine N-oxide is sensitive to heat treatment of the brain preparation and can be partially protected from thermal degeneration by the presence of an NADPH generating system. It was further observed that the rate of clozapine N-oxygenation was much higher at pH 8.5 than at pH 7.4. Taken together, the data suggest that N-oxygenation is the major metabolic pathway catalysed by rat brain and this reaction is catalysed mainly by FMO. As significant interindividual differences have been observed in brain FMO activities, these differences may contribute to the interindividual differences in patient response to clozapine.

Pharmacogenetics of antipsychotic treatment: lessons learned from clozapine

The reintroduction of clozapine, the prototype of atypical antipsychotics, in the late 1980s has led to significant advances in the pharmacological management of schizophrenia. Since then, there has been a rapid development of novel "atypical" antipsychotic agents that have been pharmacologically modeled, to a certain extent, after their predecessor clozapine. As with all antipsychotics, there is variability among individuals in their response to these "atypical" drugs. Pharmacogenetics can provide a foundation for understanding this interindividual variability in antipsychotic response. This review first provides a rationale for the pharmacogenetic investigation of this variable trait. Studies of pharmacokinetic and pharmacodynamic factors of antipsychotic therapy are considered in the development of this rationale. Next, the molecular genetic techniques used to study this interindividual variation in response are described. This is followed by a review and discussion of the published studies examining genetic factors involved in clozapine response. From this, several recommendations for future pharmacogenetic investigations of antipsychotic response are proposed. Although still in its early stages, psychiatric pharmacogenetics should provide a basis for individualized pharmacotherapy of schizophrenia, and may also lead to the development of newer, more efficacious antipsychotic agents.

Clozapine versus typical neuroleptic medication for schizophrenia

BACKGROUND

Long-term drug treatment of schizophrenia with conventional antipsychotics has limitations: 25-33% of patients have illnesses that are treatment-resistant. Clozapine is an atypical antipsychotic drug, which is claimed to have superior efficacy and to cause fewer motor adverse effects than typical drugs for people with treatment-resistant illnesses. Clozapine carries a significant risk of serious blood disorders, which necessitates mandatory weekly blood monitoring at least during the first months of treatment.

OBJECTIVES

To evaluate the effects of clozapine for schizophrenia in comparison to typical antipsychotic drugs.

SEARCH STRATEGY

Publications in all languages were searched from the following databases: Biological Abstracts (1982-1999), The Cochrane Library CENTRAL (Issue 2, 1999), Cochrane Schizophrenia Group's Specialised Register (1999), EMbase (1980-1999), ISI Citation Index, LILACS (1982-1999), MEDLINE (1966-1999), and PsycLIT (1974-1999). Reference list screening of included papers was performed. Authors of recent trials and the manufacturer of clozapine contacted.

SELECTION CRITERIA

All randomised controlled trials comparing clozapine with typical antipsychotic drugs were included by independent assessment by at least two reviewers.

DATA COLLECTION AND ANALYSIS

Data were extracted independently by at least two reviewers. Authors of trials published since 1980 were contacted for additional and missing data. Odds ratios (OR) and 95% confidence intervals (CI) of homogeneous dichotomous data were calculated with the Peto method. A random effects model was used for heterogeneous dichotomous data. Where possible the numbers needed to treat (NNT) or needed to harm (NNH) were also calculated. Weighted or standardised means were calculated for continuous data.

MAIN RESULTS

Currently the review includes 31 studies, 26 of which are less than 13 weeks in duration. These studies include 2589 participants, most of whom were men (74%). The average age was 38 years. There was no difference in the effects of clozapine and typical neuroleptic drugs for broad outcomes such as mortality, ability to work or suitability for discharge at end of the study. Clinical improvement was seen more frequently in those taking clozapine (random effects OR 0.4 CI 0.2-0.6, NNT 6) both in the short and the long term. Also, in the short term, participants on clozapine had fewer relapses than those on typical antipsychotic drugs (OR 0.6 CI 0.4-0.8, NNT 20 CI 17-38), and this may be true for long-term treatment as well. Symptom assessment scales showed a greater reduction of symptoms in clozapine-treated patients. Clozapine treatment was more acceptable than low-potency antipsychotics such as chlorpromazine (OR 0.6 CI 0.4-0.9) but did not differ from acceptability of high-potency neuroleptics such as haloperidol (random effects OR 0.8 CI 0.4-1.5). Clozapine was more acceptable in long-term treatment than conventional antipsychotic drugs (random effects OR 0.4 CI 0.2-0.7, NNT 6 CI 3-111). Patients were more satisfied with clozapine treatment (OR 0.5 CI 0.3-0.8, NNT 12 CI 7-37), but they experienced more hypersalivation, temperature increase, and drowsiness than those given conventional neuroleptics. However, clozapine patients experience fewer motor side effects and less dry mouth. The clinical efficacy of clozapine was more pronounced in participants resistant to typical neuroleptics in terms of clinical improvement (random effects OR 0.2 CI 0.1-0.5, NNT 5 CI 4-7) and symptom reduction. Thirty-two percent of treatment resistant people had a clinical improvement with clozapine treatment.

REVIEWER'S CONCLUSIONS

This systematic review confirms that clozapine is convincingly more effective than typical antipsychotic drugs in reducing symptoms of schizophrenia, producing clinically meaningful improvements and postponing relapse. Patients were more satisfied with clozapine treatment than with typical neuroleptic treatment. (ABSTRACT TRUNCATED

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.

Metabolism, pharmacogenetics, and metabolic drug-drug interactions of antipsychotic drugs

1. Antipsychotic drugs are extensively metabolised by cytochrome P450 (CYP) enzymes. 2. Dispositions of a number of antipsychotic drugs have been shown to cosegregate with polymorphism of CYP2D6. 3. Metabolic drug-drug interactions have frequently been observed when antipsychotics are coadministered with other drugs. 4. Many antipsychotic drugs are converted to active metabolites which can contribute to the therapeutic or side effects of the parent drug. 5. Information concerning the individual CYP isoenzymes involved in the metabolism of antipsychotic drugs is important for the safe clinical use of this group of drugs.

Clozapine and metabolite concentrations during treatment of patients with chronic schizophrenia

Results presented in this article are focused on the variability in pharmacokinetics. The purpose of this study was (1) to investigate intra- and interindividual variabilities of pharmacokinetic parameters of clozapine and its two main metabolites in plasma after multiple oral administration in 8 chronic schizophrenic patients (Study 1) and (2) to gain more information regarding plasma concentrations of these drugs after multiple doses in a group of 25 treatment-responsive patients (Study 2). Patients were treated with clozapine in fixed daily doses (given every 8-12 hours) between 200 and 900 mg. Plasma drug concentrations were determined by high-performance liquid chromatography. The mean volume of distribution and the total plasma clearance of clozapine, uncorrected for bioavailability, were 7 L/kg and 40.5 L/h, respectively. The terminal elimination half-lives averaged 10.5 hours for clozapine, 19.2 hours for norclozapine, and 8.6 hours for the N-oxide metabolite. Significant relationships were observed between clozapine and norclozapine (or clozapine N-oxide) plasma concentrations. Large inter- and intrapatient variations in pharmacokinetics were observed. Clozapine was generally well tolerated by the patients, with sedation, hypersialorrhea, and tiredness as the most common side effects encountered.

Distribution of clozapine and desmethylclozapine between blood and brain in rats

Desmethylclozapine is the major metabolite of clozapine in serum. Although the metabolite is pharmacologically active in vitro, the occurrence of desmethylclozapine in brain under steady-state conditions and its role for clinical actions of clozapine are unclear. In this study 20 male Sprague-Dawley rats received five oral doses of clozapine 20 mg/kg at 1.5-h intervals. At 0.5, 1, 2 and 5 h after the last administration, at a time four animals were killed for analysis of clozapine and desmethylclozapine concentrations in serum and brain. The treatment yielded steady-state serum concentrations of clozapine that are considered as therapeutically effective in man. Desmethylclozapine concentrations exceeded those of clozapine at 2-5 h after drug application. In brain, drug concentrations were 15.8-fold higher for clozapine than in serum, but only 2.7-fold higher for desmethylclozapine. The brain clozapine concentrations exceeded those of desmethylclozapine by about 3 times. These data indicate that desmethylclozapine is unlikely to play a role for CNS-mediated effects.

A clozapine overdose with markedly elevated serum levels

Clozapine (Clozaril) is an atypical antipsychotic agent used to treat schizophrenia refractory to other pharmacological agents. This report describes an accidental clozapine overdose. The half-life of clozapine in this patient was determined from two blood levels, one obtained on admission and the second 10.5 hours later. The calculated half-life of 8.11 hours is consistent with published levels for single doses and suggests an apparent stability in clozapine elimination half-life in the face of overdose. The maximum clozapine blood level attained was probably among the highest nonfatal levels reported. The patient recovered fully following hospital admission for monitoring and supportive care. This case report illustrates the usefulness of following the time course of the changes in blood level at selected time intervals, as well as the importance of entertaining a diagnosis of drug overdose in patients presenting with acute mental status changes.

Multiple-dose pharmacokinetics of clozapine in patients with chronic schizophrenia

The pharmacokinetic parameters of clozapine and its two main metabolites, N-desmethylclozapine (norclozapine, active metabolite) and clozapine N-oxide, were evaluated, after oral administration, in 19 patients with chronic schizophrenia. Plasma and red blood cell (RBC) drug concentrations were determined by high-performance liquid chromatography. Large interpatient variations in pharmacokinetic parameters of clozapine and its two metabolites were observed. Plasma clozapine concentration peaked, on average, at 2.3 hours. The mean volume of distribution and the total plasma clearance, uncorrected for bioavailability, were 6 L/kg and 38 L/hr, respectively. The terminal elimination half-lives averaged 7.6 hours for clozapine, 13 hours for norclozapine, and 7 hours for the N-oxide metabolite. The mean RBC/plasma concentration ratios were 23, 61, and 81% for clozapine, N-desmethylclozapine, and clozapine N-oxide, respectively. From RBC concentration data, the mean elimination half-lives were 7.6 hours for clozapine, 16 hours for N-desmethylclozapine, and 8 hours for the N-oxide metabolite. The average value for blood clearance of clozapine was 54.7 L/hr. Significant correlations were observed between dose and maximum plasma concentrations and between dose and area under the curve concentrations; these results suggested linear steady-state pharmacokinetics over the range of concentrations studied.

Effects of clozapine on the delta- and kappa-opioid receptors and the G-protein-activated K+ (GIRK) channel expressed in Xenopus oocytes

1. To investigate the effects of clozapine, an atypical antipsychotic, on the cloned mu-, delta- and kappa-opioid receptors and G-protein-activated inwardly rectifying K+ (GIRK) channel, we performed the Xenopus oocyte functional assay with each of the three opioid receptor mRNAs and/or the GIRK1 mRNA. 2. In the oocytes co-injected with either the delta- or kappa-opioid receptor mRNA and the GIRK1 mRNA, application of clozapine induced inward currents which were attenuated by naloxone, an opioid-receptor antagonist, and blocked by Ba2+, which blocks the GIRK channel. Since the opioid receptors functionally couple to the GIRK channel, these results indicate that clozapine activates the delta- and kappa-opioid receptors and that the inward-current responses are mediated by the GIRK channel. The action of clozapine at the delta-opioid receptor was more potent and efficacious than that at the kappa-opioid receptor. In the oocytes co-injected with the mu-opioid receptor and GIRK1 mRNAs, application of clozapine (100 microM) did not induce an inward current, suggesting that clozapine could not activate the mu-opioid receptor. 3. Application of clozapine caused a reduction of the basal inward current in the oocytes injected with the GIRK1 mRNA alone, but caused no current response in the uninjected oocytes. These results indicate that clozapine blocks the GIRK channel. 4. To test the antagonism of clozapine for the mu- and kappa-opioid receptors, we applied clozapine together with each selective opioid agonist to the oocytes co-injected with either the mu- or kappa-opioid receptor mRNA and the GIRK1 mRNA. Each of the peak currents induced by each selective opioid agonist together with clozapine was almost equal to the responses to a selective opioid agonist alone. These results indicate that clozapine has no significant antagonist effect on the mu- and kappa-opioid receptors. 5. We conclude that clozapine acts as a delta- and kappa-agonist and as a GIRK channel blocker. Our results suggest that the efficacy and side effects of clozapine under clinical conditions may be partly due to activation of the delta-opioid receptor and blockade of the GIRK channel.

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.

Characterization of metabolites of clozapine N-oxide in the rat by micro-column high performance liquid chromatography/mass spectrometry with electrospray interface

The metabolism of clozapine N-oxide was investigated in the rat (n = 6) after a single oral dose of 20 mg kg-1. The organic extracts of rat urine were separated by conventional high performance liquid chromatography (HPLC) and individual collected fractions were analyzed by micro-column electrospray HPLC/mass spectroscopy. The compounds identified in rat urine were clozapine N-oxide, clozapine, N-desmethylclozapine, 8-deschloro-8-hydroxyclozapine, 8-deschloro-8-thiomethylclozapine, N-desmethylclozapine, 8-deschloro-8-hydroxyclozapine, 8-deschloro-8-thiomethylclozapine, N-desmethyl-8-deschloro-8-thiomethylclozapine and 8-deschloro-8-methylsulfinylclozapine. With the exception of the unchanged clozapine N-oxide, no other metabolite containing a N-oxide functional group could be found, the concentrations of clozapine N-oxide, clozapine and N-desmethylclozapine excreted from rat urine were determined utilizing a conventional HPLC procedure with UV detection. The recoveries of these three analytes reported as the percentage of the dosage from the 0.24 h urine are 0.93 +/- 0.54%, 0.06 +/- 0.03% and 0.01 +/- 0.006% respectively.

Pharmacokinetics of clozapine and risperidone: a review of recent literature

The current literature describing the pharmacokinetics of the atypical antipsychotics clozapine and risperidone is reviewed, and discussion on the clinical significance of these data is presented. These drugs are well absorbed when taken orally but are poorly bioavailable because of presystemic elimination. They are highly cleared by hepatic metabolism involving specific P450 isozymes. Risperidone elimination produces a potent active metabolite. Neither of the drugs has received extensive study related to drug-drug interactions, but several are potentially important because a purported therapeutic plasma concentration range is proposed for clozapine and a possible curvilinear dose response relationship has been reported for risperidone. The current clinical pharmacokinetic database for these atypical antipsychotics suggests that much can be learned with additional study that would be of value in individualizing their dosage regimens.

Clozapine plasma level monitoring: current status

Plasma level monitoring of clozapine and metabolites may prove beneficial in treating patients who show unusual drug metabolic activity. A threshold plasma level for patients who will respond to this medication is suggested. The interaction of gender, age, smoking, other medication and side effects with plasma clozapine and metabolites are discussed. Plasma level monitoring of clozapine and/or metabolites is recommended in patients who do not respond at usual therapeutic dose, who show untoward side effects at low dose or who are treated with other medications. Finally monitoring of patients who require more than 600 mg/day should be implemented because there is evidence that the incidence of seizures increases significantly above this dosage level. There is some evidence that high plasma clozapine levels are associated with seizures.

Disposition of clozapine and desmethylclozapine in schizophrenic patients

The disposition of the atypical clozapine and its desmethyl metabolite were evaluated in fourteen male chronic patients. A single 100 mg dose of clozapine was administered and blood sampling performed over the following 72 hours. The mean (SD) oral clearance and half-life of clozapine were 55.4 (29.7) L/hr and 13.7 (9.9) hours, respectively. The mean (SD) AUC for clozapine and desmethylclozapine was 2389.9 (1406) and 751.1 (622.9) ng.hr/mL, respectively. The elimination of the metabolite is rate limited by its formation from cloza-pine. A wide interpatient variability in clozapine and desmethylclozapine pharmacokinetics was observed.

Pharmacokinetics and pharmacodynamics of clozapine

The introduction of clozapine has given clinicians a unique agent for treating patients with schizophrenia that is refractory to other neuroleptics. Despite its efficacy, the drug continues to be prescribed with trepidation due to the incidence of agranulocytosis. This article reviews the pharmacokinetic and pharmacological properties of clozapine and the clinical implications for monitoring plasma concentrations. Various assays have been developed for clozapine that include gas-liquid chromatography, radioimmunoassay and high performance liquid chromatography. Only a few studies have examined the pharmacokinetics of clozapine in patients with schizophrenia. These studies have revealed a wide interpatient variability in pharmacokinetic parameters that include: time to reach peak plasma concentrations 1.1 to 3.6h; elimination half-life 9.1 to 17.4h; clearance 8.7 to 53.3 L/h; and a volume of distribution of 1.6 to 7.3 L/kg. Clozapine is metabolised via the hepatic microsomal enzyme system into 2 principle metabolites: demethyl-clozapine and clozapine N-oxide. Urine samples have reported the ratio of clozapine:demethyl:N-oxide to be 1:1:2. The clozapine N-oxide binding affinity with 3H-haloperidol was 4 times lower than clozapine and its conversion back to clozapine is hypothesised. Although the exact pharmacological mechanism of action of clozapine is not fully understood, the drug does possess significant binding affinity for different dopamine receptors, with recent evidence supporting binding to the D4 receptor subtype. Clozapine transiently increases serum prolactin levels with minimal changes in homovanillic acid plasma levels. Limited studies investigating the relationship between clinical response and plasma clozapine concentrations have investigated the range between 100 and 800 micrograms/L. In the treatment of patients with refractory schizophrenia, a minimum concentration of 350 micrograms/L was suggested as needed. The occurrence of agranulocytosis could have a genetic basis and patients should be rigorously monitored during treatment. The incidence of tardive dyskinesia and extrapyramidal side effects is minimal. Clozapine can lower the seizure threshold in a dose- and time-dependent manner. Careful patient selection and monitoring are required when clozapine therapy is used in patients with schizophrenia.

Differentiation of haloperidol and clozapine using a complex operant schedule in the dog

This study aimed to differentiate chronically administered typical (haloperidol) and atypical (clozapine) neuroleptics in the dog using a complex temporal regulation schedule combining operant, voluntary, and involuntary motor parameters. Although clozapine and haloperidol showed some characteristics of neuroleptics, justifying their adherence to the same class of compounds, differences have also been highlighted and compared to the clinical observations. Haloperidol induced catalepsy, tremor, dystony, hyperkinesia, and stereotypy. Subjects produced anticipated responses before any stimulus. Incomplete and delayed responses were also produced. An interpretation in terms of akathisia and anhedonia has been suggested. Clozapine induced tremor, exploration, dystony, and hypersalivation. Subjects produced disinhibitory responses to the negative stimulus and incomplete responses but these latter were submitted to tolerance. The simultaneous presence of tranquilizing and disinhibitory effects has been reported on the clinical potential of clozapine both in cases of positive and negative schizophrenic symptomatologies.