Intra- and interethnic variability in reduced haloperidol to haloperidol ratios

Effectiveness and safety of olanzapine in the treatment of schizophrenia among Asian patients switching from conventional antipsychotics

OBJECTIVES

The objectives of this study were: (1) to investigate, in a clinical practice setting, the effectiveness of olanzapine in the treatment of schizophrenia among partially-responding, symptomatic Asian patients who switch from conventional antipsychotic treatment, (2) to assess the safety of olanzapine and (3) to assess the change in quality of life in Asian patients with schizophrenia who switch to olanzapine.

METHODS

Effectiveness, safety and quality of life were assessed in outpatients with schizophrenia (n=1267) who lacked symptomatic control with conventional antipsychotics and were switched to olanzapine therapy. Data for this prospective, observational study were collected for 12 months from Asian patients in China, Hong Kong, the Philippines, South Korea and Taiwan.

RESULTS

Significant clinical improvements (P<0.05) were observed following 12 months of olanzapine treatment and 87.3% of the subjects responded to treatment at endpoint (i.e. Brief Psychiatric Rating Scale Total score reduced by > or =30% relative to baseline; last observation carried forward). Abnormal involuntary movements (mean change in Abnormal Involuntary Movement Scale: -3.20, P<0.001) and quality of life were significantly improved in patients treated with olanzapine. However, some patients experienced significant weight gain (3.60+/-4.50 kg, P<0.001) with olanzapine treatment, relative to baseline.

CONCLUSIONS

This study shows that switching to olanzapine may be effective in improving symptoms, may be well-tolerated and may improve the quality of life in Asian patients who are only partially responsive to treatment with conventional antipsychotics. The pragmatic design and naturalistic setting of this large study make the findings relevant for treating patients from some Asian countries in routine clinical practice.

Treatment with olanzapine, risperidone or typical antipsychotic drugs in Asian patients with schizophrenia

OBJECTIVE

To examine clinical outcomes in Asian patients with schizophrenia receiving monotherapy with olanzapine, risperidone or typical antipsychotics in naturalistic settings.

METHOD

In this report, data from the first 12 months of the prospective, observational, 3-year Intercontinental Schizophrenia Outpatient Health Outcomes study are presented for patients from participating Asian countries (Korea, Taiwan and Malaysia) who were started on, or switched to, monotherapy with olanzapine (n = 484), risperidone (n = 287) or a typical antipsychotic drug (n = 127) at baseline.

RESULTS

At 12 months, overall reduction in the score of Clinical Global Impressions-Severity of Illness rating scale was greatest with olanzapine (p < 0.001 vs typical agents), followed by risperidone (p = 0.007 vs typical agents) treatment. Olanzapine treatment was found to have significantly better effects than typical agents on negative and depressive symptom scores, and significantly greater improvements than risperidone on negative and cognitive symptoms. The occurrence of extrapyramidal symptoms was least likely with olanzapine (p < 0.001 vs typical agents, and p = 0.012 vs risperidone), while the estimated odds of tardive dyskinesia were greatest in the typical treatment group (p = 0.046 vs olanzapine, and p = 0.082 vs risperidone). Mean weight increase was greater for olanzapine-treated patients compared with the other agents (p = 0.030 vs typical agents and p < 0.001 vs risperidone). The risk of menstrual disturbance was relatively high with risperidone when compared with olanzapine treatment (p < 0.001).

CONCLUSIONS

The results of this observational study indicate that, in Asian patients with schizophrenia, olanzapine may offer benefits when compared with typical agents or risperidone. However, the significantly greater odds of weight gain should be considered in the clinical management of olanzapine-treated patients.

Higher activity of polymorphic NAD(P)H:quinone oxidoreductase in liver cytosols from blacks compared to whites

In human liver, the two-electron reduction of quinone compounds, such as menadione is catalyzed by cytosolic carbonyl reductase (CBR) and NAD(P)H:quinone oxidoreductase (NQO1) activities. We assessed the relative contributions of CBR and NQO1 activities to the total menadione reducing capacity in liver cytosols from black (n=31) and white donors (n=63). Maximal menadione reductase activities did not differ between black (13.0+/-5.0 nmol/min mg), and white donors (11.4+/-6.6 nmol/min mg; p=0.208). In addition, both groups presented similar levels of CBR activities (CBR(blacks)=10.9+/-4.1 nmol/min mg) versus CBR(whites)=10.5+/-5.8 nmol/min mg; p=0.708). In contrast, blacks showed higher NQO1 activities (two-fold) than whites (NQO1(blacks)=2.1+/-3.0 nmol/min mg versus NQO1(whites)=0.9+/-1.6 nmol/min mg, p<0.01). To further explore this disparity, we tested whether NQO1 activity was associated with the common NQO1(*)2 genetic polymorphism by using paired DNA samples for genotyping. Cytosolic NQO1 activities differed significantly by NQO1 genotype status in whites (NQO1(whites[NQO1*1/*1])=1.3+/-1.7 nmol/min mg versus NQO1(whites[NQO1*1/*2+NQO1*2/*2])=0.5+/-0.7 nmol/min mg, p<0.01), but not in blacks (NQO1(blacks[NQO1*1/*1])=2.6+/-3.4 nmol/min mg versus NQO1(blacks[NQO1*1/*2])=1.1+/-1.2 nmol/min mg, p=0.134). Our findings pinpoint the presence of significant interethnic differences in polymorphic hepatic NQO1 activity.

Block of Delayed-Rectifier Potassium Channels by Reduced Haloperidol and Related Compounds in Mouse Cortical Neurons

Haloperidol is known as an antagonist of dopamine D2 receptors. However, it also blocks a variety of ion channels at concentrations above the therapeutic range. Reduced haloperidol (R-haloperidol), one of the main metabolites of haloperidol, has been reported to accumulate in certain tissues, particularly in brain cortex, and it may produce the pharmacological effects associated with haloperidol treatment. In this study, we assessed the effect of R-haloperidol and other related compounds on native delayed-rectifier potassium channels (K(DR)) in mouse cortical neurons by using the whole-cell patch-clamp technique. Although R-haloperidol has much lower affinity to D2 receptors than haloperidol, the IC50 of R-haloperidol to block K(DR) currents was 4.4 microM, similar to its parent compound. The binding site of R-haloperidol is on the cytoplasmic side of the channel because its quaternary derivative preferentially inhibited the currents from intracellular side. 4-Chlorophenyl-4-hydroxypiperidine (4C4HP) is the active fragment of haloperidol because other compounds containing this moiety, including L-741,626 (3-[4-(4-chlorophenyl)-4-hydroxypiperidin-L-yl]-methyl-1H-indole) and loperamide, also blocked K(DR) channels. The potency of the 4C4HP fragment positively correlated with the hydrophobicity index (clogP) of the compounds tested. We conclude that R-haloperidol is a K(DR) channel blocker, although it does not interfere with the normal channel function at a clinically relevant concentration.

Development of an HPLC method for determination of diphenidol in plasma and its application in an oral multi-dose bioequivalence study in a healthy female mexican population

Diphenidol was determined by an HPLC method developed in our laboratory. It was validated and proved to be linear in the 40-400 ng/ml range. Accuracy for quality-control samples for intra and inter day assays ranged from 96.1-98.9% and 98.8-101.4%, respectively. This method was applied to a multi-dose bioequivalence study. No serious side effects were observed in the multi-dose design. Pharmacokinetic parameters (mean+/-standard error [S.E.]) of Cavg (ng/ml) and AUCtau (ngh/ml) for reference and test products were 139.54+/-12.66 versus 148.60+/-16.51 and 551.07+/-53.53 versus 588.78+/-69.02, respectively. Log-transformed values were compared by analysis of variance (ANOVA) followed by the classical 90% confidence interval (CI 90%) test and Schuirmann's test. Confidence limits ranged from 91.48-116.18% for Cmax and from 91.24-117.65% for AUCtau. These results suggest that the analytical method was linear, precise, and accurate for our purpose, and that both assayed formulations were bioequivalent.

Carbonyl reduction of naltrexone and dolasetron by oxidoreductases isolated from human liver cytosol

The opioid receptor antagonist naltrexone and the antiemetic 5-HT(3) receptor antagonist dolasetron are ketonic drugs that are efficiently reduced to their corresponding alcohols in-vivo. These experiments aimed at characterizing the role in these reactions of individual oxidoreductases present in human liver cytosol. Aldo-keto reductases (AKRs) and carbonyl reductase (CR, EC 1.1.1.184) purified from human liver cytosol were incubated with varying substrate concentrations and 6beta-naltrexol or reduced dolasetron were analysed by HPLC. AKR1C1, AKR1C2, and AKR1C4 were able to reduce both substrates. On the basis of k(cat)/K(m) values, AKR1C4 was nearly 1000-fold more efficient in reducing naltrexone than was AKR1C1, while AKR1C2 was of intermediate efficiency. Substrate inhibition was observed on incubating AKR1C2 or AKR1C4 with naltrexone. In contrast, dolasetron was also a substrate of CR. AKR1C1 and AKR1C4 were the most efficient enzymes in producing reduced dolasetron. We concluded that the efficient reduction of naltrexone by AKR1C4 probably causes the high 6beta-naltrexol/naltrexone ratio in man. The rapid disappearance from human plasma of dolasetron given intravenously and its virtual absence after oral dosage are explained by its liability to reduction by several enzymes, including CR which shows widespread expression in human tissues.

Extrapyramidal symptom profiles assessed with the Drug-Induced Extrapyramidal Symptom Scale: comparison with Western scales in the clinical double-blind studies of schizophrenic patients treated with either olanzapine or haloperidol

The superiority of olanzapine to haloperidol with respect to a decreased incidence of treatment-emergent extrapyramidal syndromes (EPS) in patients with schizophrenia was demonstrated in studies conducted in both Japan and Western countries. EPS measurements used in Western countries included the Simpson-Angus, Barnes akathisia and the Abnormal Involuntary Movement Scale, while the Drug-Induced Extrapyramidal Symptom Scale (DIEPSS) was used in Japan. The aim of this study was to clarify how the DIEPSS captures EPS profiles. The baseline prevalence and treatment-emergent incidence of EPS in Japanese schizophrenic patients treated with olanzapine or haloperidol were retrospectively compared as assessed by the DIEPSS to the prevalence and incidence of EPS in primarily Caucasian schizophrenic patients who were treated with olanzapine or haloperidol. Specifically, the prevalence and incidence of dyskinesia, akathisia and parkinsonism were compared between the Japanese trial and an international trial to examine if appropriate definitions using the DIEPSS can be derived assuming that a comparable prevalence and incidence of the syndromes would be observed when any differences in residual antipsychotic exposure at the initiation of study treatment were accounted for. For the incidence of all EPS syndromes, odds ratios were observed to be similar between the two studies, indicating that appropriate criteria for the clinical diagnosis of the EPS syndromes could be established based on the DIEPSS. This preliminary and retrospective work suggests that the DIEPSS can be used to operationally define the presence or absence, and make the clinical diagnosis, of specific EPS syndromes.

The pharmacological treatment of schizophrenia in Chinese patients: a comparison of prescription patterns between 1996 and 1999

AIMS

In 1996 a Hong Kong-wide survey of pharmacological treatment for schizophrenic inpatients identified a few idiosyncratic prescription patterns that were not consistent with international standards. In the context of continuous quality assurance, the survey was repeated in 1999 to monitor trends in the pharmacotherapy of schizophrenia and to identify changes in prescribing practices. It was expected that the widespread introduction of novel antipsychotic drugs coupled with continuing medical education would favourably alter prescribing habits.

METHODS

A representative random sample of 1202 schizophrenic patients from all inpatient settings in Hong Kong was surveyed. Basic socio-demographic data and a list of all psychotropic medication taken on census day were collected.

RESULTS

Significant improvement was noted in several aspects of pharmacotherapy in 1999 as compared with 1996. The total daily dosage of antipsychotic drugs decreased from 716 +/- 627 to 561 +/- 482 mg day-1 chlorpromazine equivalent (CPZeq) (Mann-Whitney U-test; Z = -3.906, P = 0.000; 95% confidence interval [CI] = 70.0, 240.2 mg day-1 CPZeq), for acute patients and from 923 +/- 809 to 675 +/- 545 mg day-1 CPZeq (Mann-Whitney U-test; Z=-5.036, P = 0.000; 95% CI = 175.57, 321.63 mg day-1 CPZeq) for chronic patients. The percentage of patients coprescribed antipsychotics and an antiparkinson drug significantly decreased from 67.8% to 61.8% (chi2 = 5.558, P = 0.018) in the chronic schizophrenia group. The percentage of patients taking atypical antipsychotic medication increased significantly from 5.5% to 19.2% (chi2 = 27.14, P = 0.000) in the acute and 3.4% to 12.3% (chi2 = 27.76, P = 0.000) in the chronic group.

CONCLUSIONS

Repeated surveys of prescribing practices proved to be a useful tool to demonstrate trends in the pharmacological treatment of schizophrenia. Our findings suggest that the widespread use of atypical antipsychotic drugs contributed to a more rational pharmacotherapy for schizophrenia.

Olanzapine versus haloperidol in the treatment of patients with chronic schizophrenia: results of the Japan multicenter, double-blind olanzapine trial

This randomized double-blind trial was conducted to test the efficacy and safety of olanzapine in Japanese patients with schizophrenia. Importantly, this study also represents the first large clinical trial of olanzapine conducted in an Asian population. Patients (n = 182) were randomly assigned to treatment with olanzapine or haloperidol over a period of 8 weeks. The primary analyses included: (i) a test of non-inferiority of olanzapine compared with haloperidol in efficacy using the Final Global Improvement Rating (FGIR); and (ii) comparison between the treatment groups in extrapyramidal symptom severity using the Drug-Induced Extrapyramidal Symptoms Scale (DIEPSS). Olanzapine was comparable to haloperidol in efficacy in treating positive symptoms and significantly superior in treating negative symptoms. Extrapyramidal symptom severity was significantly improved for olanzapine-treated patients versus haloperidol-treated patients. Olanzapine was shown to be more effective and better tolerated than haloperidol in the treatment of Japanese patients suffering from chronic schizophrenia.

Factors affecting serum haloperidol level assessed by longitudinal therapeutic monitoring

1. Since the development of a kit for the assay of haloperidol by enzyme immunoassay, therapeutic monitoring of haloperidol has been utilised in Japan for several years. By retrospectively analysing the accumulated data, this study was carried out to investigate the factors affecting haloperidol level. Especially, the effects of enzyme-inducing comedications were analysed in relation to serum gamma-glutamyltransferase (GGT) level, which has been measured simultaneously with haloperidol. Serum haloperidol level measurements were obtained from medical records of inpatients on multiple medications (n = 102). For each subject, average haloperidol level was computed during the same prescription and doses. The effects of age, sex, smoking status, and the coadministration of carbamazepine and barbiturates (including phenobarbital, amobarbital and pentobarbital) were analysed using correlation, between-group comparison and multiple regression analysis. Separately, the effect of comedications on haloperidol and GGT levels were analysed individually in a small number of patients (n = 5) who had received those comedications intermittently. Significant lowering of serum haloperidol level by the coadministration of carbamazepine and/or barbiturates was observed. The coadministration was also correlated with the elevated GGT level in between-group comparison. In the separate analysis, the change in haloperidol level was correlated with the change in GGT level in some individuals but not in others. None of the other clinical factors investigated in the study showed significant effect on haloperidol level. This study suggests that the lowering of haloperidol level and the elevation of GGT level may often occur coincidentally by the coadministration of enzyme-inducing drugs. However, whether there is a causal relationship between these phenomena and whether elevated serum GGT level could serve as a clinically useful marker need to be clarified by further basic pharmacological research.

Pharmacokinetics of haloperidol: an update

Haloperidol is commonly used in the therapy of patients with acute and chronic schizophrenia. The enzymes involved in the biotransformation of haloperidol include cytochrome P450 (CYP), carbonyl reductase and uridine diphosphoglucose glucuronosyltransferase. The greatest proportion of the intrinsic hepatic clearance of haloperidol is by glucuronidation, followed by the reduction of haloperidol to reduced haloperidol and by CYP-mediated oxidation. In studies of CYP-mediated disposition in vitro, CYP3A4 appears to be the major isoform responsible for the metabolism of haloperidol in humans. The intrinsic clearances of the back-oxidation of reduced haloperidol to the parent compound, oxidative N-dealkylation and pyridinium formation are of the same order of magnitude, suggesting that the same enzyme system is responsible for the 3 reactions. Large variation in the catalytic activity was observed in the CYP-mediated reactions, whereas there appeared to be only small variations in the glucuronidation and carbonyl reduction pathways. Haloperidol is a substrate of CYP3A4 and an inhibitor, as well as a stimulator, of CYP2D6. Reduced haloperidol is also a substrate of CYP3A4 and inhibitor of CYP2D6. Pharmacokinetic interactions occur between haloperidol and various drugs given concomitantly, for example, carbamazepine, phenytoin, phenobarbital, fluoxetine, fluvoxamine, nefazodone, venlafaxine, buspirone, alprazolam, rifampicin (rifampin), quinidine and carteolol. Overall, drug interaction studies have suggested that CYP3A4 is involved in the biotransformation of haloperidol in humans. Interactions of haloperidol with most drugs lead to only small changes in plasma haloperidol concentrations, suggesting that the interactions have little clinical significance. On the other hand, the coadministration of carbamazepine, phenytoin, phenobarbital, rifampicin or quinidine affects the pharmacokinetics of haloperidol to an extent that alterations in clinical consequences would be expected. In vivo pharmacogenetic studies have indicated that the metabolism and disposition of haloperidol may be regulated by genetically determined polymorphic CYP2D6 activity. However, these findings appear to contradict those from studies in vitro with human liver microsomes and from studies of drug interactions in vivo. Interethnic and pharmacogenetic differences in haloperidol metabolism may explain these observations.

Effects of smoking, CYP2D6 genotype, and concomitant drug intake on the steady state plasma concentrations of haloperidol and reduced haloperidol in schizophrenic inpatients

The effects of smoking, CYP2D6 genotype, and concomitant use of enzyme inducers or inhibitors on the steady state plasma concentrations of haloperidol (HAL) and reduced haloperidol (RHAL) were evaluated in 92 schizophrenic inpatients. All but three of these patients received concomitant medication, in many cases with drugs potentially interacting with HAL. Of the 92 patients, 63 were treated orally with HAL in a daily dose of 0.4 to 50 mg; 29 patients were treated intramuscularly with a daily equivalent dose of HAL decanoate (expressed as HAL) of 1.8 to 17.9 mg. A wide interindividual variation in HAL dose and in steady state plasma concentrations of HAL and RHAL was observed. In the patients treated orally, the daily oral dose was about 4 times higher and the dose-normalized HAL (but not RHAL) plasma concentrations were significantly lower in smokers (n = 40) than in nonsmokers (n = 23) (p < 0.01). The dose-normalized RHAL (but not HAL) plasma concentrations and the RHAL/HAL ratio were significantly higher in poor metabolizers (PMs) than in extensive metabolizers (EMs). There was a trend toward an effect of potentially interacting drugs (inducers or inhibitors) on dose, dose-normalized HAL and RHAL plasma concentrations, and the RHAL/HAL ratio. In the patients treated intramuscularly, the dose-normalized HAL (but not RHAL) plasma concentrations were significantly lower in smokers than in nonsmokers, but no differences in doses were observed. This naturalistic study of modest sample size in a polymedicated population shows an effect of smoking and CYP2D6 genotype (and to a lesser extent, of interacting drugs) on the kinetics of HAL.

Fosinopril: pharmacokinetics and pharmacodynamics in Chinese subjects

This study examined thepharmacokinetics and pharmacodynamics of fosinopril (IVand oral) in Chinese subjects to determine whether they were different from a group of somewhat heavier and older Western control subjects previously published using the same methods. It was an open-label, randomized, balanced, two-way crossover study comparing oral and IV pharmacokinetics in 12 healthy Chinese subjects in a clinic in Taiwan. Each subject received 10 mg of oral fosinopril or 7.5 mg of IV fosinoprilatin a randomized sequence with sampling for fosinoprilat concentrations over 48 hours. Standard pharmacokinetics, including AUC, Cmax Tmax, T 1/2, Vss, bioavailability, total clearance, and renal and nonrenal clearance, were determined as well as pharmacodynamic effects on angiotensin-converting enzyme (ACE) activity. Following oral administration of 10 mg fosinopril, AUC0-T and AUCinf were 1,556 +/- 586 ng x hr/mL and 1,636 +/- 620 ng x hr/mL, respectively; T 1/2 was 17.4 +/- 11.4 hr; Cmax was 183.4 +/- 59.4 ng/mL; and median Tmax was 4.0 hr, with > 99% protein binding. Following IV administration of 7.5 mg fosinoprilat, AUC0-T and AUCinf were 7,727 +/- 2,638 ng x hr/mL and 7,816 +/- 2,693 ng x hr/mL, respectively; T 1/2 was 13.0 +/- 5.2 hr; and median Tmax was 4.0 hr, with 99.5% +/- 0.22% protein binding and a Vss of 5,850 +/- 2,780 mL. Bioavailability was 22.3% +/- 7.9%. Percent urinary excretion was 7.6% +/- 2.6% after oral dosing and 42.6% +/- 6.1% after IV dosing. After IV, dosing total clearance was 1,088 +/- 439 mL/hr, renal clearance was 472 +/- 213 mL/hr, and nonrenal clearance was 617 +/- 246 mL/hr. ACE inhibition was essentially complete through 12 hours and markedly reduced through 24 hours. Compared to a somewhat heavier and older previously reported control group, pharmacokinetic values were similar except for a slightly lower AUC and total clearance in Chinese and a statistically significantly lower nonrenal clearance. Pharmacodynamic effects on ACE activity were essentially identical. There is no reason to expect significant differences in fosinopril dosing or effect in a Chinese population compared to a Western population.

Clinical pharmacokinetic considerations in the elderly. An update

There are a number of areas in which advances have been made over the last few years in the area of pharmacokinetics in the elderly. There is increasing understanding of the diversity of cytochrome P450s (CYP) and the variability of the age-related decline in CYP activity. This has helped to explain some of the interindividual variability in drug metabolism with age. The importance of ethnic differences has emerged, but specific work is needed in this area in the elderly. Differences in the handling of chiral compounds has been reported but as yet no clinically important findings that may lead to a change in clinical practice have emerged. The emerging importance of extrahepatic drug metabolism, especially in the intestine, has added a new complexity to our understanding of pharmacokinetics. The issue of frailty is also discussed in this article. Whether it will be of value at the bedside has yet to emerge. Nonetheless, as a concept, recent data has supported its potential use to define those more at risk of clinically meaningful pharmacokinetic alterations. Other advances have included the appreciation that selectivity in induction and inhibition in the elderly are due to the existence of multiple CYP forms. Similarly, the role of these various enzymes in disease is also improving our clinical understanding, as exemplified in Parkinson's disease.

Dose-dependent reduced haloperidol/haloperidol ratios: influence of patient-related variables

Plasma reduced haloperidol (RH) concentrations or RH to haloperidol (HL) ratios have been suggested to be important in determining the clinical efficacy and extrapyramidal side effects of HL. In this study, we measured the steady-state plasma HL and RH levels by high performance liquid chromatography and analyzed the effects of various variables (dose, gender, age, and body weight) on RH/HL ratios in four dose groups of Chinese schizophrenic inpatients: 10 mg/day (n = 84), 20 (n = 111), 30 (n = 29), and 60 (n = 55). In addition, the polymorphic distribution of RH/HL ratios, suggested by previous investigators, was further tested in each dosage group (for controlling the potential dosage effect on RH/HL ratios). As a result, both age and body weight could influence RH/HL ratios. Each year increase in age (after adjusting the effects of gender, body weight, and dosage) would elevate the RH/HL ratio by 0.0067 (P < 0.0001). On the other hand, after adjusting gender, age, and dosage effects, each kg increment in body weight would decrease the RH/HL ratio by 0.0044 (P < 0.01). Gender did not influence the ratio. Furthermore, the high dosage groups had higher RH/HL ratios (even with other variables being controlled). In comparison with the 10 mg group, the 60 mg group exhibited a higher mean RH/HL ratio by 0.84 (P < 0.0001) and the 30 mg group did by 0.31 (P < 0.0001). The 20 mg group was almost equal to the 10 mg group in RH/HL ratios. Besides, at each dosage group, the frequency distribution of RH/HL ratios seemed to be predominantly unimodal with a small proportion of extreme outliers. The results of this study clearly indicate that aging or a high dose (> or = 30 mg/day) of HL could raise the plasma RH/HL ratio, while an increasing body weight would reduce that. In contrast, gender does not affect the ratios.

Clinical evaluation and plasma clozapine concentrations in Chinese patients with schizophrenia

The relationships between clozapine dosages, plasma concentrations, and clinical responses in Chinese schizophrenics were studied. Fourteen treatment-refractory schizophrenic patients were treated with clozapine for 12 weeks. Patients were assessed before and after 6 and 12 weeks of treatment using the Brief Psychiatric Rating Scale (BPRS), the Clinical Global Impression (CGI), and the Simpson-Angus Scale for Extrapyramidal Side Effect. Plasma clozapine concentrations were determined by high-performance liquid chromatography. Ten patients (71.4%) responded after 12 weeks of treatment. Although the mean daily dosage at week 12 (373 +/- 90 mg/day) was lower than that reported in American trials (444 mg/day), the mean plasma clozapine concentration attained (1,078 +/- 385 ng/ml) was higher. This higher concentration may be due to the lower body wight and the preponderance of women among our patients, absence of smoking and alcohol use, and/or ethnic difference between Chinese and non-Chinese. There was wide interindividual variation in the plasma clozapine concentrations. Compared with other studies, the plasma clozapine concentrations and the response rate were higher. Although the sample size was small, the findings are suggestive of pharmacokinetic and pharmacodynamic ethnic differences in Chinese with clozapine therapy.