Psychopharmacology (1996) 127 : 311–314 © Springer-Verlag 1996

ORIGINAL INVESTIGATION

Tsuyoshi Kondo · Osamu Tanaka · Koichi OtaniKazuo Mihara · Noboru Tokinaga · Sunao KanekoKan Chiba · Takashi Ishizaki

Possible inhibitory effect of diazepam on the metabolism of zotepine,an antipsychotic drug

Received: 25 March 1996/Final version: 28 May 1996

Abstract Effects of smoking and cytochrome P4502C19 (CYP2C19) status on the single dose kinetics ofzotepine and pharmacokinetic interaction betweenzotepine and diazepam were investigated. In 14 healthyvolunteers, the pharmacokinetics of zotepine after a sin-gle oral 25 mg dose were compared between eight smok-ers and six non-smokers, or between seven extensivemetabolizers (EMs) and seven poor metabolizers (PMs)of S-mephenytoin. There was no significant differencein any pharmacokinetic parameters between smokersand non-smokers, or between the EM and PM groups.In 17 patients treated with zotepine 80–340 mg/day,intra-individual changes in plasma concentrations ofzotepine caused by coadministration of diazepam10 mg/day for 2 weeks were examined. Plasma con-centrations of zotepine were significantly increased aftercoadministration of diazepam (P < 0.05). Con-sequently, it is suggested that neither smoking norCYP2C19 status affects the metabolism of zotepine. Theelevation in plasma concentrations of zotepine aftercoadministration of diazepam may be a result of com-petitive inhibition of zotepine metabolism by diazepamvia other isoenzyme than CYP2C19, e.g., CYP3A4.

Key words Zotepine · Metabolism · Smoking ·CYP2C19 · Diazepam

Introduction

Zotepine is a dibenzothiepine derivative antipsychoticdrug developed in Japan. This drug possesses a potent

antiserotonergic property (Yamawaki 1987) as well asantidopaminergic effects (Uchida et al. 1979). Thus,zotepine has a broad therapeutic spectrum in acuteexacerbation of schizophrenia; it is effective in reducinganxiety-depression and negative symptoms as well aspositive symptoms (Kondo et al. 1993). Also, zotepinehas fewer extrapyramidal side effects than haloperidol(Fleischhacker et al. 1989; Barnas et al. 1992), and theadverse effects of this drug are mild (Kondo et al. 1994).

Although it has been suggested that N-desmethyla-tion and hydroxylation are important metabolic pathways (Noda et al. 1993), the isoenzymes involvedin the metabolism of zotepine are virtually unknown.Our previous study (Otani et al. 1993) showed that thepatients coadministered with benzodiazepines hadhigher serum concentrations of zotepine than the others, suggesting inhibitory effects of benzodiazepineson the metabolism of zotepine. Meanwhile, it has beensuggested that the metabolism of diazepam is at leastpartly catalyzed by cytochrome P450 2C19 (CYP2C19)designated as S-mephenytoin 4-hydroxylase, whoseactivity is genetically determined (Bertilsson et al. 1989;Sohn et al. 1992). Therefore, it is possible that CYP2C19is involved in zotepine metabolism.

To confirm this pharmacokinetic finding, we stud-ied the effect of CYP2C19 status on the single dosekinetics of zotepine in healthy volunteers, together withthe pharmacokinetic interaction between zotepine anddiazepam in psychiatric patients. We also examined theeffect of smoking on the single dose kinetics of zotepine,since smokers had lower serum concentrations ofzotepine than non-smokers in our previous study(Otani et al. 1993).

Subjects and methods

Single dose kinetic study in volunteers

The single dose kinetics of zotepine were investigated in 14 healthymale volunteers, who gave their written informed consent to

T. Kondo (*) · O. Tanaka · K. Otani · K. Mihara N. Tokinaga · S. KanekoDepartment of Neuropsychiatry, Hirosaki University School ofMedicine, Hirosaki 036, Japan

K. Chiba · T. IshizakiDepartment of Clinical Pharmacology, Research Institute,International Medical Center of Japan, Tokyo 162, Japan

participate in the study. The mean age (and range) was 31.8 (24–41)years and body weight 61.7 (52–70) kg. According to the methodreported from our laboratory (Horai et al. 1989; Sohn et al. 1992),each of the subjects had previously been phenotyped for their indi-vidual capacity to 4-hydroxylate S-mephenytoin using the amountof 4-hydroxymephenytoin excreted in the 8-h urine after taking anoral dose of 100 mg racemic mephenytoin. Seven of 14 subjects wereidentified as extensive and poor metabolizers (EMs and PMs, respec-tively) by using an antimode of 0.3 in the log10 percentage of thepostdose 8-h urinary excretion of 4-hydroxymephenytoin per thedose administered as racemic mephenytoin. Eight cases were smok-ers (≥10 cigarettes/day), and the remaining six were non-smokers.

A single oral 25 mg dose of zotepine was administered to eachvolunteers. Blood samplings were performed before and 0.5, 1, 1.5,2, 3, 4, 6, 8, 10, 12, 16, 24 and 36 h after administration of the drug.Side effects were monitored using the UKU Side Effect Rating ScaleLingjaerde et al. 1987). Plasma concentrations of zotepine weredetermined in duplicate by the gas chromatography-mass spec-trometry method of Tanaka et al. (1996).

The elimination constant (k) of zotepine estimated from the non-linear least squares regression analysis of the terminal log-linearconcentration data, and the elimination half-life (T1/2) was calcu-lated as 0.693/k. The area under the plasma concentration-timecurve from 0 to 36 h, AUC (0–36), was calculated by the trape-zoidal rule. The AUC from 0 h to infinity, total AUC, was calcu-lated as AUC (0–48) + C36/k, where C36 is the plasma concentrationat 36 h postdose. The apparent oral clearance (CLoral ) and volumeof distribution (Vd) were estimated from CLoral = dose/total AUCand Vd = CLoral/k, respectively. Statistical analyses were performedby Wilcoxon rank-sum test; a P value of 0.05 or less was regardedas statistically significant.

Drug-drug interaction between zotepineand diazepam in patients

Seventeen psychiatric inpatients (seven males, ten females) treatedwith zotepine alone who gave written informed consent to the studyparticipated in the zotepine-diazepam interaction study. The meanvalue (and ranges) of age, body weight and the daily dose of zotepinewere 38.4 (17–72) years, 63.0 (35–105) kg and 199 (80–340) mg/day,respectively. After the doses of zotepine were fixed at least for 2weeks, the first blood sampling for the measurement of the steady-state plasma concentrations of zotepine was performed 12 h afterthe last dose and before the next morning dose. Then, diazepam10 mg/day was coadministered in each patient for 2 weeks, and thesecond blood sampling was conducted in the same way as the firstone. Intraindividual changes in zotepine concentrations caused bycoadministration of diazepam were examined. Statistical analysiswas performed by Wilcoxon signed-rank test, and a P value of 0.05or less was regarded as statistically significant.

These studies were approved by the Ethics Committee of HirosakiUniversity Hospital.

Results

In the volunteer study, there was no significantdifference in any pharmacokinetic parameters ofzotepine between smokers and non-smokers (Table 1),or between the EM and PM groups (Table 2). TotalUKU side effect scores did not differ significantlybetween smokers and non-smokers, nor between theEM and PM groups at any time point during the study.

In the 17 patients treated with zotepine, the steady-state plasma concentrations of zotepine weresignificantly (P < 0.05) elevated after coadministration

of diazepam (Fig. 1). The plasma concentrations ofzotepine (mean ± SD) before and after coadministra-tion of diazepam were 13.8 ± 11.3 ng/ml and17.5 ± 13.8 ng/ml, respectively.

Discussion

Regarding the effects of smoking on the metabolismof antipsychotic drugs, it has been suggested that

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Table 1 Pharmacokinetic parameters of zotepine in smokers andnon-smokers

Parameters Smokers Non-smokers(n = 8) (n = 6)

Cmax (ng/ml) 4.4 ± 1.5 7.9 ± 3.2Tmax (h) 3.6 ± 0.5 3.9 ± 1.6T1/2 (h) 24.0 ± 8.8 16.9 ± 7.2Vd (l/kg) 131.5 ± 60.9 79.0 ± 40.0CLoral (l/h/kg) 5.0 ± 5.0 4.1 ± 2.6

Table 2 Pharmacokinetic parameters of zotepine in extensive andpoor metabolizers of S-mephenytoin. EMs, PMs extensive and poormetabolizers of S-mephenytoin

Parameters EMs PMs(n = 7) (n = 7)

Cmax (ng/ml) 6.7 ± 3.5 5.0 ± 1.9Tmax (h) 3.6 ± 1.5 3.6 ± 0.5T1/2 (h) 21.4 ± 8.8 20.5 ± 9.0Vd (l/kg) 110.0 ± 74.2 108.0 ± 59.0CLoral (l/h/kg) 5.2 ± 5.6 4.1 ± 1.5

Fig. 1 Intraindividual changes in plasma concentrations of zotepinecaused by coadministration of diazepam 10 mg/day for 2 weeks in17 inpatients

smoking enhances the metabolism of fluphenazine(Ereshefsky et al. 1985) and haloperidol Miller et al.1990). Recently, we reported that the mean steady-stateserum concentration of zotepine was lower in 37 smok-ers than in 22 non-smokers, suggesting the enhancedmetabolism of zotepine by smoking (Otani et al. 1993).We re-examined this possibility in a single dose kineticstudy in volunteers. However, there was no differencein any pharmacokinetic parameters of zotepinebetween eight smokers and six non-smokers in the pre-sent study. This discrepancy may be partly explainedby the small number of subjects in the present studyand a large interindividual variation in the pharmaco-kinetics and metabolism of zotepine. However, the pos-sibility that the effect of smoking on the metabolismof zotepine is marginal should also be considered.

The involvement of CYP2D6 in the metabolism ofantipsychotic drugs such as haloperidol (LLerena et al.1992), perphenazine (Dahl-Puustinen et al. 1989) andthioridazine (von Bahr et al. 1991) was demonstratedin several studies. On the other hand, our previousstudy (Otani et al. 1993) showed that patients coad-ministered with benzodiazepines had significantlyhigher steady-state serum concentrations of zotepinethan the others, suggesting the involvement ofCYP2C19 in the metabolism of zotepine. In the pre-sent volunteer study, the involvement of CYP2C19 inzotepine metabolism was directly examined by com-paring the single dose kinetics of zotepine in EMs andPMs of S-mephenytoin. The lack of difference in anypharmacokinetic parameters of zotepine between theEM and PM groups clearly indicates that the metabo-lism of zotepine is not mediated by CYP2C19.

Meanwhile, the increased zotepine concentrationscaused by coadministration of benzodiazepines in ourprevious study (Otani et al. 1993) were replicated inthe drug-drug interaction between zotepine anddiazepam in the present study. Since previous resultswere based on the comparison between the two differentgroups, both of which had large interindividual varia-tions, we investigated the intra-individual changes inthe plasma concentrations of zotepine caused by coad-ministration of diazepam. As a result, coadministra-tion of diazepam significantly elevated the steady-stateplasma concentrations of zotepine. Therefore, the effectof coadministration of benzodiazepines on zotepineconcentrations is regarded as highly reproducible.

It has been suggested that in addition to CYP2C19(Bertilsson et al. 1989) such isoenzymes as CYP1A2(Louis et al. 1989) and CYP3A4 (Andersson et al. 1994)are involved in the metabolism of diazepam. In the pre-sent study, diazepam significantly elevated the plasmaconcentrations of zotepine. Thus, these isoenzymes fordiazepam metabolism may also be involved in themetabolism of zotepine. However, CYP1A2 does notappear to play an important role in the metabolism ofzotepine, since in the present study the pharmacoki-netics of zotepine were not significantly affected by

smoking, which clearly induces CYP1A2-catalyzedmetabolism (Bock et al. 1994). Also, the involvementof CYP2C19 in the metabolism of zotepine can be ruledout from the present data. Consequently, CYP1A2 andCYP2C19 are unlikely to be associated with the inter-action between zotepine and diazepam.

The in vitro study using human liver microsomes ondiazepam metabolism (Andersson et al. 1994) showedthat the hydroxylation was mediated by CYP3A4, andthat the desmethylation was mediated by both CYP3A4and CYP2C19. Likewise, the main metabolic pathwaysof zotepine were hydroxylation and desmethylation(Noda et al. 1979). It is possible that the metabolismof both zotepine and diazepam is mediated byCYP3A4. Thus, the pharmacokinetic interactionbetween these two drugs may result from competitiveinhibition via CYP3A4.

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