Eur J Clin Pharmacol (1990) 39:51-54

@ e c Gce eg @ Springer-Verlag 1990

Effect of omeprazole and cimetidine on plasma diazepam levels T. A n d e r s s o n 1, K. A n d r 6 n 1, C. Cederberg 1, G. Edvardsson ~, A. Heggelund 2, and R L u n d b o r g 1 1 Research Laboratories, AB H~issle,MOlndaland 2 Department of Medicine II, Sahlgrenska Hospital, G6teborg, Sweden Received: October 20, 1989/Accepted in revised form: January 23, 1990

Summary. The effects of steady state dosing with omeprazole and cimetidine on plasma diazepam levels have been studied in 12 healthy males. Single doses of diazepam (0.1 mg.kg -1 i.v.) were administered after one week of treatment with omeprazole 20 mg once daily, cimetidine 400 mg b. d. or placebo, and the treatment was continued for a further 5 days. Blood was collected for 120 h after the dose of diazepam for the measurement of diazepam and its major metabolite desmethyl diazepam. The mean clearance of diazepam was decreased by 27% and 38% and its half-life was increased by 36% and 39% after omeprazole and cimetidine, respectively. Neither drug had any apparent effect on the volume of distribution of diazepam. Desmethyldiazepam appeared more slowly after both omeprazole and cimetidine. It is concluded that the decrease in diazepam clearance was associated with inhibition of hepatic metabolism both by omeprazole and cimetidine. However, since diazepam has a wide therapeutic range, it is unlikely that concomitant treatment with therapeutically recommended doses of either omeprazole or cimetidine will result in a clinically significant interaction with diazepam.

Key words: Diazepam, omeprazole, cimetidine, drug interaction, healthy volunteers, pharmacokinetics Omeprazole is a substituted benzimidazole, which is a potent inhibitor of gastric acid secretion [1, 2] and which is likely to be prescribed to a considerable number of patients who are concurrently taking other medications. Imidazole and benzimidazole derivatives are known to inhibit hepatic microsomal oxidation [3], and alteration in the disposition of other drugs mainly metabolised by the liver might be anticipated. The H2-receptor antagonist cimetidine, which is a substituted imidazole, decreases the hepatic clearance of the majority of concomitantly given drugs [4]. Omeprazole, on the other hand, decreases the clearance of phenytoin, diazepam and to a small extent that of one of the warfarin enantiomers [5, 6] but not that

of other drugs, such as theophylline and propranolol [7, 8], which are extensively metabolised by hepatic oxidation. The effect of administration for 1 week of omeprazole 40 mg once daily on the kinetics of diazepam in 8 healthy young males has previously been studied [5]. The mean clearance of diazepam was decreased by 54% and its halflife was prolonged by 130% following omeprazole therapy, while the degree of protein binding and volume of distribution were not affected. It was concluded from these results, as well as from in vitro studies of omeprazole and cimetidine [5], that omeprazole inhibits the metabolism of diazepam, perhaps more markedly than does cimetidine. However, in that study, blood samples for the determination of diazepam were taken for only 72 h, and there was a considerable contribution of the extrapolated area under the plasma concentration-time curve to the total A U C in some experiments. The dose of omeprazole used was twice that currently recommended for the treatment of peptic ulcer. Thus, no firm conclusion could be drawn as to the actual degree of the interaction of omeprazole with diazepam in the therapeutic situation. Comparative data on the effects of omeprazole and cimetidine on the metabolism of diazepam in man are not available. Consequently, the present study was initiated to compare the effects of therapeutic doses of omeprazole and cimetidine, at steady state on the kinetics of single doses of diazepam.

Subjects and methods Twelve healthy (based on physical examination, ECG and a laboratory screen) males of median age 26 y (range 22-32 y) and median weight 77 kg (range 71-86 kg) completed the study. The study was approved by the Ethics Committee of the University of GOteborg, Sweden and by the Swedish National Board of Health and Welfare, and was carried out in accordance with the guidelines of the Declaration of Helsinki. Written consent was obtained from each subject prior to his participation. The study was of the double blind, double dummy,cross-over design, and consisted of three ll-day experiments, each separated by a washout period of at least 10 days. The 12 subjects were allocated to

T. Andersson et al.: Diazepam clearance, omeprazole and cimetidine

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Fig.1. Mean plasma concentrations of diazepam following a single i.v. dose of 0.1 mg/kg during oral treatment with placebo, omeprazole 20 mg o. d. or cimetidine 400 mg b. d. of 12 healthy, male volunteers four different groups, and a Latin square design was used for each group, with persons as rows and time periods as columns. Omeprazole was given once daily as a hard gelatine capsule containing 20 nag omeprazole as enteric coated granules. Identical capsules, containing sugar granules of the same size as the omeprazole granules, were used as the omeprazoleplacebo. Cimetidinewas given twice daily as two capsules, each containing 200 mg cimetidine (800 mg/day). The capsules were filled with a granulated mixture of cimetidine (Traeco, Milan, Italy) and lactose, and were prepared by AB Hfissle. identical capsules, containing lactose granules of the same size, were used as the cimetidineplacebo. All oral doses were taken with 200 ml water. Diazepam (Diazemuls| KabiVitrum) was given as an intravenous infusion. The dose, 0.1 mg. kg -1, was diluted with Intralipid| (KabiVitrum) to give a total volume of 10 ml and was infused over 10 min. Each experiment consisted of administration for t l days of omeprazole once daily, or cimetidine twice daily, or placebo, with the injection of the diazepam on Day 7. The 7th morning dose of omeprazole, cimetidine or placebo was taken in the laboratory after an overnight fast, and the diazepam was administered 1 h later. Blood samples were collected from an antecubital vein in the arm contralateral to that used for the infusion before and 30 min after the dose of omeprazole, cimetidine or placebo, and 5, 15, 30, 45 rain and 1.0, 1.5, 2.0, 2.5, 3, 4, 6, 8, 9,10, 24, 36, 48, 72, 96 and 120 h after the end of the diazepam infusion. The blood was taken by an indwelling cannula up to 10 h and thereafter by separate venepunctures. Heparin was used as the anticoagulant. The samples were centrifuged and the

plasma stored at - 20 ~ until analysis. The subjects received standardised meals at 1.5, 3.5, 6 and 9 h after the dose of diazepam. Alcohol and all medication, including OTC drugs, were prohibited during the two days prior to and during each of the three experiments. A routine laboratory screen was repeated at the end of the study. Diazepam and desmethyldiazepam were extracted from plasma with n-butyl acetate and determined by gas chromatography with electron capture detection. To a volume of 250 gl plasma was added 500 gl n-butyl acetate containing the internal standard (propyldesmethyldiazepam 0.1 gmo1.1 1). The mixture was shaken for 15 min and centrifuged. Extraction recovery was 96% for diazepam and 97% for desmethyldiazepam. Organic phase 3 gl was injected onto the gas chromatograph (VARIAN Vista 6000) at a split ratio of approximately 1:10 and an injector temperature of 300~ The fused silica capillary column was coated with a 20% phenyl substituted methyl silicone stationary phase (length 30 m, i.d. 0.32 ram, film thickness 0.25 btm), The helium carrier gas had an inlet pressure of 1.5 bar. The oven was temperature programmed from 120~ to 280 ~ giving an approximate retention time of 14 rain for diazepam and 16 min for desmethyldiazepam. The limit of quantification was 10 nmol/1 in plasma with relative standard deviations (n = 10) of 3.5% at 311 nmol/1 and 6.9% at 11.2 nmol/1. The linear range for diazepam was 10 to 1500 nmol/1 and for desmethyldiazepam 10 to 700 nmol/1. Plasma was analysed for omeprazole by liquid chromatography [9], and for cimetidine by liquid chromatography following extraction with methylene chloride containing heptafluorobutanol 1, and back-extraction into sulphuric acid (modified from reference [10]). The limit of quantification of cimetidine was 0.3 gmol. 1-1 (rel. standard deviation < 10%). The relative standard deviation (n = 10) at 3 gmol. 1- i was 3%. The area under the plasma concentration-time curve (AUC) of diazepam was calculated using the linear and log trapezoidal rules. The A U C was extrapolated to infinity using the last measured plasma concentration and the rate constant determined by log-linear regression analysis of the terminal concentration-time points. Standard pharmacokinetic parameters of diazepam were calculated: terminal half-life, tl/2,systemic clearance, CL, volume of distribution during the terminal phase, Vz, the mean residence time, MRT [11], and the volume of distribution at steady state, V~s. The A U C of desmethyldiazepam was calculated from 0 to 24 h by the linear trapezoidal rule. This AUC-value is an approximation of the formation rate of the metabolite, since at 24 h all individual plasma profiles of desmethyldiazepam were still ascending, i.e. the elimination phase was not reached until after 24 h. The AUCs of omeprazole and cimetidine were calculated using the linear and log trapezoidal rules. The A U C of omeprazole was calculated up to the last measured plasma concentration, and the A U C of cimetidine was calculated for the time period representing approximately one dosing interval, i. e., from the time just before the morning dose of cimetidine to 11 h after it. An initial analysis to test for a possible period and carry-over effect from prior experiments was based on analysis of variance for a three-period cross-over study. It showed no sign of such an effect and so the final analysis of diazepam kinetics was based on a simpler model for analysis of variance of repeated measurements [12];

Table 1. Single dose kinetics of i.v. diazepam (0.1 m g - k g - ) after 7 days of oral treatment with placebo (P), omeprazole 20 mg o.d. (O) or

cimetidine 400 mg b. d. (C) AUC (pg-min. ml-1) mean (SD) min max all treatment effects: O vs C:

tla (h)

CL (ml. min- 1)

P

O

C

P

O

238 (68) 165 379

332 (105) 187 582

365 (58) 298 477

33.2 (8.3) 22.4 49.5

45.0 46.3 (15.5) (9.0) 20.4 34.6 69.3 57,8

P < 0,001 P > 0.20

C

P = 0,001 P > 0.20

V~s (1- kg -i)

P

O

C

34.4 (8.4) 22.2 46.2

25.0 (7.7) 14.4 41.2

21.5 (2.9) 17.0 26.7

P < 0.001 P = 0.12

MRT (h)

P O 1.03 1.02 (0.12) (0.28) 0.84 0.45 1.20 1.50 P > 0.20 P > 0.20

C 0.99 (0.13) 0.83 1.22

P 41.1 (12.6) 25.6 64.1

O C 57.4 60.5 (21.4) (11.8) 27.2 43.9 101 77.8 P < 0.001 P > 0.20

53

T. Andersson et al.: Diazepam clearance, omeprazole and cimetidine 1000

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Fig.2. Mean plasma concentrations of desmethyldiazepam following a single i.v. dose of diazepam 0.1 mg/kg during oral treatment with placebo, omeprazole 20 mg o.d. or cimetidine 400 mg b.d. of 12 healthy, male volunteers

almost twice as large a decrease (P = 0.003) in the A U C than omeprazole; the mean A U C of desmethyldiazepam was decreased by 29% (P = 0.0001) and 16% (P = 0.0003) following cimetidine and omeprazole treatment, respectively, from a mean value of 25.2 gg. min. ml -~ during placebo. The mean plasma profiles also appeared to suggest a slower elimination rate of this metabolite during omeprazole and cimetidine treatment. However, the tl/2 of desmethyldiazepam was too long to be calculated accurately within the blood sampling period employed. The results of the omeprazole and cimetidine analyses are given in Tables 2 and 3, respectively. There was a 15-fold intersubject variability in the maximum plasma concentration (C~x) and A U C of omeprazole. Cimetidine exhibited less intersubject variability, with only a 2fold range in AUC. The time to reach Cmax(tm~) was approximately 2 h for both drugs. There was no correlation, based on linear regression analysis, between the A U C or C .... either of omeprazole or cimetidine and the decrease in the clearance of diazepam after the various treatment periods.

Discussion

P < 0.05 was regarded as significant. Comparisons of AUC(0-24 h) for desmethyldiazepam were performed by Student's paired t-test and to compensate for the three comparisonsperformed the P-value had to be < 0.05/3 to be regarded as statisticallysignificant. Linear least squares regression analysis was used to examine potential relationships between the AUC-values of the drugs and their effects on diazepam clearance.

Results

No serious adverse experiences were reported by any subject at any time. Adverse reactions were reported by only two subjects: mild dizziness following the diazepam infusion during omeprazole treatment in Subject 3, and moderate stomach ache for two consecutive days during placebo treatment in Subject 4. The mean plasma diazepam concentration-time profiles during the omeprazole, cimetidine and placebo treatment periods are presented in Fig 1. The mean plasma concentrations were lowest during the control period and highest during the cimetidine period. The mean tl/2 of diazepam was increased by 36% after omeprazole treatment and by 39% after cimetidine treatment (Table 1). The mean clearance of diazepam was decreased by 27% after omeprazole and by 38% after cimetidine, whereas the volume of distribution did not appear to be affected by either drug. Thus, the prolonged ti/2 of diazepam may be dependent on decreased clearance. The mean residence time (MRT) of diazepam, representing the average time that diazepam molecules persist in the body, was similarly prolonged by omeprazole and cimetidine (40-50%). The mean plasma concentrations of desmethyldiazepam (Fig 2) showed a slower rate of formation during omeprazole and cimetidine treatment than during placebo treatment, as demonstrated by the decreased A U C during the first 24 h. Cimetidine actually resulted in

The results of the present study show that continuous treatment with cimetidine or omeprazole in therapeutically recommended doses resulted in an equal decrease in systemic clearance and prolongation of the elimination half-life of a single i.v. dose of diazepam (0.1 mg/kg). Further, the plasma profiles of desmethyldiazepam indicated a decrease both in its formation and elimination rates during cimetidine and omeprazole treatment. Previous studies have shown that cimetidine [4,13,14] and omeprazole [5] decrease the clearance of diazepam, an effect attributed to hepatic enzyme inhibition. Studies in vitro have suggested omeprazole to be an even more potent enzyme inhibitor than cimetidine [5]. However, the present results show that the therapeutically recommended doses of omeprazole (20 mg o. d.) and cimetidine (400 mg b.d.) have similar effects on the metabolism of diazepam. A previous study of omeprazole 40 mg o. d. re-

Table 2. Kinetics of omeprazole after one week on 20 mg o. d.

mean (SD) min max

Cmax (gmol. 1-i) 0.658 (0.391) 0.102 1.419

AUC (~tmol.h. 1- 1) 1.170 (0.871) 0.204 2.983

Table 3. Kinetics of eimetidine after one week on 400 mg b. d.

mean (SD) rain max

G .... (gmol. 1 i) 3.77 (0.63) 2.91 5.30

AUC (gmol 9h. 1-1) 17.2 (4.6) 12.5 29.8

54

T. Andersson et al.: Diazepam clearance, omeprazole and cimetidine

vealed a 54% decrease in diazepam clearance in comparison to the 27% decrease observed here after a 20 mg dose, which indicates that the interaction of omeprazole with the metabolism of diazepam, i. e., the degree of enzyme inhibition by omeprazole, is dose-dependent. This would agree with the results of earlier studies with antipyrine and aminopyrine, which indicated dose-dependent oxidative metabolic interaction of omeprazole [15]. However, differences in design, and the fact that the two studies were performed in different subjects in two different laboratories might have contributed to the apparent difference in the effect of omeprazole on diazepam metabolism, The decreased formation rate of the metabolite, desmethyldiazepam, during omeprazole and cimetidine treatment is a logical consequence of the inhibition of the metabolism of diazepam. The further metabolism of desmethyldiazepam m a y be somewhat slower, too, during omeprazole or cimetidine treatment, which might indicate that this metabolic pathway was also somewhat inhibited. Since the first-pass elimination of an oral dose of diazepam is negligible, the interaction following oral dosing will probably not differ from that after intravenous dosing. Therefore, it can be concluded that since diazepare has a wide therapeutic range, it is unlikely that concomitant treatment either with omeprazole or cimetidine in therapeutically r e c o m m e n d e d doses will result in any clinically significant interaction with diazepam.

cytochrome P-450 and the inhibition of monooxygenation. Biochem Pharmaco131:1315-1320 4. Greene W (1984) Drug interactions involving cimetidine mechanisms, documentation, implications. Drug Metab Drug Interact 5:25-51 5. Gugler R, Jensen JC (1985) Omeprazole inhibits oxidative drug metabolism - studies with diazepam and phenytoin in vivo and 7ethoxycoumarin in vitro. Gastroenterology 89:1235-1241 6. Sutfin T, Balmdr K, Bostr6m H, Eriksson S, H0glund R Paulsen O (1989) Stereoselective interaction of omeprazole with warfarin in healthy men. Ther Drug Monit 11:176-184 7. Gugler R, Jensen JC (1987) Drugs other than H2-receptor antagonists as clinically important inhibitors of drug metabolism in vivo. Pharmacol Ther 33:133-137 8. HenryD, Brent E Whyte I, Mihaly G, Devenish-Meares S (1987) Propranolol steady-state pharmacokinetics are unaltered by omeprazole. Eur J Clin Pharmaco133: 369-373 9. Lagerstr6m P-O, Persson B-A (1984) Determination of omeprazole and metabolites in plasma and urine by liquid chromatography. J Chromatogr 309:347-356 10. Larsson R, Erlandsson R Bodemar G, Norlander B, Fransson L, Strouth L (1982) Pharmacokinetics of cimetidine and its sulphoxide metabolite during haemodialys. Eur J Clin Pharmacol 21:325-330 11. Yamaoka K, Nakagawa T, Uno T (1978) Statistical moments in pharmacokinetics. J Pharm Biopharm 6:547-558 12. Fleiss J (1986) The design and analysis of clinical experiments. Wiley, New York 220-223 13. Klotz U, Reimann I (1980) Delayed clearance of diazepam due to cimetidine. N Engl J Med 302:1012-1014 14. Greenblatt D, Abernethy D, Morse D, Harmatz J, Shader R (1984) Clinical importance of the interaction of diazepam and cimetidine. N Engl J Med 310:1639-1643 15. Henry DA, Somerville KW, Kitchingman GK, Langman MJS (1984) Omeprazole: effects on oxidative drug metabolism. Br J Clin Pharmacol 18:195-200

References 1. Fellenius E, Berglindh T, Sachs G (1981) Substituted benzimidazoles inhibit gastric acid secretion by blocking (H +K § Nature 290:159-161 2. Wallmark B, Lorentzon R Larsson H (1985) The mechanism of action of omeprazole - a survey of its inhibitory actions in vitro. Scand J Gastroentero120 [Suppl 108]:37-51 3. Dickins M, Bridges JW (1982) The relationship between the binding of 2-n-alkylbenzimidazoles to rat hepatic microsomal

T. Andersson Gastrointestinal Research Department of Clinical Pharmacology and Medicine AB H~issle S-43183 M61ndal Sweden