Journal of Pharmacokinetics and Biopharmaceutics, Vol. 5, No. 5, 1977

Pharmacokinetics of Diazepam Following Multiple- Dose Oral Administration to Healthy Human Subjects

F. B. Eatman, a W. A. Colburn, 2 H. G. Boxenbaum, 1 H. N. Posmanter, 1 R. E. Weinfeld, 1 R. Ronfeld, 3'4 L. Weissman, s J. D. Moore, 6 M. Gibaldi, 2 and S. A. Kaplan 1'7

Received December 23, 1976--Final March 18, 1977

Six healthy subjects between the ages of 21 and 31 years received diazepam tablets orally at a dose of 5 mg t.i.d, at O, 5, and 10 hr on days 1-13. On day 14, the dose was 5 mg at 0 and 5 hr and 15 mg at 10 hr. Subsequently, the dose was 15 mg once daily on days 15-24. Numerous plasma samples were obtained during the multiple -dose regimen, and appropriate equations were fitted to all the multiple-dose data. Diazepam absorption was satisfactorily described by a first-order process, with disposition characterized by a linear two-compartment open model. The harmonic mean absorption half-life was 32 min, and the harmonic mean terminal exponential half-life was 57hr. The mean apparent oral total drug plasma clearance was 22.7 mff hr/ kg. Steady-state plasma levels of the primary metabolite, desmethyldiazepam, were reached after 5-8 days of dosing. Steady-state diazepam plasma concentration-time profiles suggested that once daily administration of the total daily dose at bedtime might be a satisfactory dosing regimen.

KEY WORDS: diazepam; multiple-dose pharmacokinetics; two-compartment model; ben- zodiazepine.

I N T R O D U C T I O N

D i a z e p a m is a b e n z o d i a z e p i n e de r iva t ive useful in the t r e a t m e n t of anx ie ty s tates , a l coho l wi thdrawal , ske le ta l musc le spasm, and convuls ive

1Department of Biochemistry and Drug Metabolism, Hoffmann-La Roche, Inc., Nutley, New Jersey 07110.

2Department of Pharmaceutics, School of Pharmacy, State University of New York at Buffalo, Buffalo, New York 14214.

3Department of Pharmacy, School of Pharmacy, University of Connecticut, Storrs, Connec- ticut 06268.

4Present address: Astra Pharmaceutical Products, Inc., Framingham, Massachusetts 01701. ~Department of Clinical Pharmacology, Hoffmann-La Roche, Inc., Nutley, New Jersey 07110. 6Deer Lodge Research Unit, P.O. Box 149, Deer Lodge, Montana 59722. 7Address correspondence to S.A.K. Raw data are available on request.

481 This journal is copyrighted by Plenum. Each article is available for $7.50 from Plenum Publishing Corporation, 227 West 17th Street, New York, N.Y. 10011.

482 Eatman, et al.

disorders. In the treatment of anxiety states, the usual oral adult dose is 2-10 mg two to four times daily (1). In humans, diazepam is completely metabolized (2) and the major metabolite found in blood is desmethyl- diazepam. Two hydroxylated metabolites, oxazepam and 3- hydroxydiazepam, have also been detected (2). Single- and multiple-dose blood or plasma concentration-time data have been reported and discussed by several groups (2-17). The disposition of diazepam has been described by Kaplan et al. (7) in terms of a three-compartment open model, whereas other groups (14-17) have utilized a two-compartment model. The model utilized seems to have been dependent on experimental design as well as the criteria selected for determining goodness of fit. In particular, the frequency of blood sampling shortly following the intravenous injection may deter- mine whether or not a third exponential term is needed. In the opinion of the authors, either model may be satisfactory. The best guidance one has is that the selection of the appropriate equation and model should be consistent with the data and with the goals of the investigation.

Following single-dose intravenous administration, terminal exponen- tial half-lives of diazepam exhibit a striking age dependence; as a rough rule of thumb, age in years for adults is approximately equal to terminal disposition half-life in hours (14). It appears that these changes in half-life with age are not due to changes in clearance but rather are primarily dependent on changes in the initial distribution volume of the drug, i.e., the volume of the central compartment. Blood clearance of diazepam is approx- imately 45 ml/min, with a blood/plasma concentration ratio of approxi- mately 0.58; the percentage in plasma bound to plasma proteins is approxi- mately 98% (14). Assuming the liver to be the sole metabolizing organ, the mean blood extraction ratio for diazepam is approximately 3%.

Oral multiple-dose blood level data for diazepam and desmethyl- diazepam have been reported from several laboratories (2,4,5,7,9,10,15); however, only one investigation (7) attempted to fit pharmacokinetic equa- tions to the data. It was the purpose of the present investigation to determine diazepam and desmethyldiazepam blood levels following multiple-dose oral administration employing two dosing regimens, and to subsequently fit appropriate equations to the data. Since prescribing practices today (1) call for daily oral doses to be divided into two to four administrations, it seemed desirable to evaluate plasma concentration-time profiles following a typical regimen in which 5-mg doses were administered three times daily. Addition- ally, since the long elimination half-life of diazepam suggests that the daily dose of the drug could conceivably be administered on a once daily basis, plasma levels following once daily administration were also investigated. Therefore, six healthy subjects received diazepam at a dose of 5 mg three times daily (0, 5, and 10 hr) on days 1-13. On day 14, the dose was 5 mg at 0 and 5 hr, and 15 mg at 10 hr. Subsequently, the dose was 15 mg once daily

Pharmacokinetics of Diazepam Following Multiple-Dose Oral Admlnis~ration 483

on days 15-24. Numerous blood samples were obtained during the study, and appropriate equations were fitted to the data. It shall be reported herein that, following multiple-dose oral administration of diazepam, absorption may be satisfactorily described by a first-order process with disposition charcterized by a two-compartment open model.

EXPERIMENTAL

Protocol

Six healthy male volunteers between the ages of 21 and 31 years participated in this investigation. Each subject initially received diazepam tablets (Valium 5-mg tablets, Hoffmann-La Roche, Inc., Nutley, New Jersey) at a dose of 5 mg administered three times daily at 6 A.M., 11 A.M., and 4 P.M. on days 1-13. On day 14, the regimen was altered to 5 mg at 6 A.M. and 11 A.M., with a 15-mg dose (3-5 mg tablets) administered at 4 P.M. Thereafter, on days 15-24, a single 15 mg dose (3-5 mg tablets) was administered at 4 P.M. All doses were ingested with 200 ml tap water, at least 1 hr prior to a meal. All subjects were in good general health, were taking no other medication, had no history of alcoholism or drug addiction, and had no known sensitivity to benzodiazepines.

Eight milliliters of oxalated whole blood was obtained at 0, 0.25, 0.5, 1, 1.5, 2, 3, 4, 5, 6,5, 8, 10, and 11.5 hr on day i subsequent to the initial dose. On days 2, 3, 5, and 8-13, during three times daily administration, blood was obtained at 0 and 1.5 hr after both the 6 A.M. and 4 P.M. doses. On day 14, bloods were obtained at 0 and 1.5 hr after all three doses. On days i5-18 and 20-23, bloods were obtained 0 and 1.5 hr after the 4 P.M. dose. On day 19, samples were obtained at 0, 1.5, 3, 6, and 8 hr after the 4 P.M. dose. On day 24 (last dose), samples were obtained at 0, 0.25, 0.5, 1, 1 5, 2, 3, 4, 5, 6.5, 8, 10, 11.5, 15, 18, 24, 40, 48, 72, 96, 120, 144, 168, 192, 216, and 240hr after this 4 P.M. dose. Blood samples were immediately centrifuged, and the plasma was separated and frozen.

Analytical Methodology

Diazepam and desmethyldiazepam were determined in plasma speci- mens by the method of Weinfeld et al. (18). This procedure is accurate and specific and has a lower limit of sensitivity of 10 ng/ml for each component using a 0.5-ml plasma specimen.

Mathematical, Statistical, and Computer Methods

Equations for plasma concentration-time relationships were obtained from Wagner (19) and were appropriately adapted to fit multiple-dose data.

484 Eatman, et al.

Two fitting procedures were used; the first has been described by Colburn et al. (20) and utilized the 1969 version of NONLIN (21). The second has been described by Ronfeld and Boxenbaum (22) and utilized the 1974 version of NONLIN (23). Both procedures, which use all data, gave essentially the same results; consequently, only the results of the latter method are reported. Initial parameter estimates were derived by standard graphical procedures (24). Weighting factors of 1 (unweighted), l /y , and 1/y 2 were employed. Since 1/y resulted in the best randomness of scatter, this factor was employed in obtaining the reported results. When data were weighted, a normalization factor was generally applied (25) such that the sum of the weights equaled the number of data points. In the fitting procedures, iteration of either macroscopic (hybrid) or microscopic (pure) parameters gave essentially the same results. Additionally, it seemed to make little difference in the parameter estimates whether differential or integrated equations were used. Statistical parameters and tests relating to the data have been described previously (25).

It is appropriate to use an arithmetic mean in the averaging of rate constants, but this procedure is not correct for half-lives. Therefore, har- monic mean half-lives were calculated. Consequently, the In 2 divided by a harmonic mean half-life equals the arithmetic mean of the individual rate constants.

R E S U L T S A N D D I S C U S S I O N

Visual inspection of semilogarithmic plasma concentration-time plots suggested that a triexponential function would satisfactorily describe the data. Consequently, a two-compartment open model was used to charac- terize disposition, with absorption taken to be a first-order process. The equation characterizing single-dose data (19) was appropriately program- med into NONLIN (21,23) so as to fit multiple-dose data. Examination of weighted residuals indicated that a weighting factor of 1/y provided the best randomness of scatter, and consequently this factor was used to obtain parameter estimates. With the exception of subject 5, the appropriate F test (25) indicated that inclusion of a lag time did not significantly reduce the weighted sums of squared deviations (p > 0.05). With regards to subject 5, goodness of fit was also evaluated according to the criterion described by Wagner et al. (16). Addition of a lag time did not sufficiently improve goodness of fit so as to justify its inclusion. Therefore, a lag time was not considered necessary. Table I lists several pharmacokinetic parameters, and Figs. 1-3 illustrate the fits of the theoretical curves to the data. Visual inspection of Figs. 1-3 indicates that thediazepam plasma concentration- time data are generally well predicted by the theoretical curves. The data

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Pharmacokinetics of Diazepam Following Multiple-Dose Oral Administration 489

from the first dose are an exception, and this will be discussed subsequently. There does exist some nonrandomness of scatter, probably resulting from day-to-day intrasubject variability of parameters. This problem has been discussed previously (26). Correlation coefficients between observed and theoretical plasma levels always exceeded 0.9. The parameters k12, k21, ke~, V~, and ka were iterated in the computer analysis. The parameters k12, k2~, and ke~ have their usual meaning in a two-compartment open model, with elimination occurring solely from the central compartment. The parameter k~ is the first-order absorption rate constant, and V~ is the volume of the central compartment divided by the bioavailability.

Wagner (27) has recently suggested that in most instances there is no need to assume or assign a specific pharmacokinetic model when dealing with linear data. The authors are in general agreement with this approach. Although a specific model was used in this analysis, its sole purpose was to facilitate the computer analysis. The model utilized herein is acknowledged to be nonunique, and any other linear two-compartment open model is also consistent with the data.

The apparent oral total drug plasma clearance, C10, is the most mean- ingful parameter obtained, and this is equal to (27)

C10 = Do/AUC0 (1)

where AUC0 is the area under the curve following a single oral dose Do. The average steady-state plasma concentration (C) resulting from fixed dose administration at a constant interval, ~-, is given by

0 = Do/rClo (2)

It is immediately apparent from equation 2 that during multiple-dose oral administration, steady-state levels are very much dependent on Clo.

The average steady-state plasma concentrations of desmethyldiazepam are presented in Table I, and the plasma concentration-time profiles ar~ illustrated for two representative subjects in Fig. 4. The average steady-state plasma levels were determined by dividing the area under the 0-24 hr plasma level-time curve on day 24 by the dosing interval. As is required in a linear system, crossing over from t.i.d, to once daily administration of diazepam does not appear to influence average steady-state levels of desmethyldiazepam. There was no significant correlation (p>0 .05) between apparent oral total drug plasma clearance of diazepam (uncor- rected for body weight) and C for desmethyldiazepam (r = 0.322).

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Pharmacokinetics of Diazepam Following Multiple-Dose Oral Administration 491

multiple-dose study, there did not exist significant correlations (p > 0.05) between age and either (tl/2)~ (r = 0.493) or V~ (r = 0.246). However, this may result from the fact that only six subjects within a narrow age range were investigated in this study and that the parameters were obtained from oral multiple-dose data rather than intravenous single-dose data. Another apparent discrepancy between the findings of Klotz et al. (15) and those from this study is that on subchronic dosing with diazepam Klotz et aL reported that the accumulated desmethyldiazepam may inhibit the rate of metabol- ism of diazepam, i.e., reduce diazepam clearance and increase (tl/2)~. In this regard, it may be noted that the plasma levels following the first dose are

�9 generally higher than is predicted by the theoretical curves. This finding is somewhat at odds with the data of Klotz et al. (15), who reported that subchronic administration of diazepam produces levels of desmethyl- diazepam that reduce diazepam clearance. In the present studies, the parameters were primarily determined by diazepam levels obtained during chronic dosing, since more data points existed at steady-state than during the accumulation phase. One might have expected these diazepam parame- ters to be influenced by the reported inhibitory effect of desmethyldiazepam on diazepam disposition. However, if this were the case, clearance would be underestimated during the first dose, since desmethyldiazepam levels would not yet have accumulated. The net effect would therefore be for the theoretical curves to overestimate levels following the first dose. However, quite the opposite effect is observed. One possible explanation for this observation relates to the potential influence of diazepam on gastrointesti- nal motility. Huck et al. (29) reported that local application of diazepam noncompetitively inhibited carbachol and barium chloride induced contrac- tions of the isolated guinea pig ileum. In addition, data on file demonstrate that intravenous diazepam can protect mice against stress-induced ulcera- tion of the gastrointestinal tract. Therefore, it is conceivable that diazepam reduces gastrointestinal motility in man. If this effect were more prevalent on multiple dosing, one might expect absorption of the first oral dose not to be much influenced. However, with subsequent dosing, motility and hence absorption rate might be diminished. Hence one could observe higher than predicted levels following the first dose when the prediction is based on multiple-dose rather than single-dose data. In any event, it is apparent from Figs. 1-3 that, with the possible exception of the first dose, a single set of pharmacokinetic parameters can reasonably well predict diazepam plasma levels. The large values of (tl/2)a determined in these studies are consistent with the finding of Klotz et al. (15) that upon multiple dosing diazepam clearance is reduced by desmethyldiazepam.

Figure 5 illustrates diazepam steady-state profiles during the t.i.d, and once daily administrations utilized in these studies. These curves were

492 Eatman, et al.

700

~ 600

~ 500

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~ 300 bJ o 250 0

20( I 0 5 I 0

I I i I I I 15 20 25 30 35 40

TIME ( H o u r s )

Fig. 5. Steady-state diazepam plasma concentration-time profiles in a 70-kg subject receiving either 5 mg at 0, 5, and 10 hr (t.i.d.) (solid line) or 15 mg at 10 hr once daily (dashed line). Curves were generated from mean values in Table I.

constructed using the mean parameters in Table I, assuming a body weight of 70 kg. It is interesting to note that if the 15-mg once daily dose was administered at bedtime the plasma level maximum would occur during the sleeping hours. On the subject's arising approximately 8 hr later, plasma concentrations would have declined to levels comparable with those obtained during steady-state dosing at 0, 5, and 10 hr (t.i.d.). Therefore, once daily administration of the total daily dose of diazepam at bedtime might be a satisfactory dosing regimen.

In conclusion, these results indicate that oral multiple-dose diazepam plasma concentration-time profiles may be reasonably well described in terms of a linear pharmacokinetic system, with reproducible absorption and disposition from dose to dose.

ACKNOWLEDGMENTS

The authors wish to acknowledge the secretarial work of Mrs. A. Szilagyi and Mrs. A. Ott and the drawings of Mr. T. Daniels and Mr. R. McGlynn.

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