Br. J. clin. Pharmac. (1990), 29, 759-762

Inhibition of epoxide hydrolase by valproic acid in epileptic patients receiving carbamazepine D. K. ROBBINS1, P. J. WEDLUND', R. KUHN', R. J. BAUMANN2, R. H. LEVY4 & S.-L. CHANG3 'College of Pharmacy, 2College of Medicine, 3College of Agricultural Sciences, University of Kentucky, Lexington, KY 40536 and 4College of Pharmacy, University of Washington, Seattle, WA 98195, USA

The effect of valproic acid (VPA) on the disposition of carbamazepine-10,11-epoxide (epoxide) was studied in five epileptic patients on chronic carbamazepine (CBZ) therapy. The individual pharmacokinetic parameters influencing epoxide disposition were determined in the presence and absence of VPA. VPA significantly decreased the clearance of unbound epoxide (an in vivo index of epoxide hydrolase activity), but did not appear to affect epoxide formation. VPA also increased the free concentrations of both CBZ and epoxide.

Keywords epoxide hydrolase valproic acid carbamazepine metabolite kinetics steady-state Introduction

Methods

The coadministration of valproic acid (VPA) with carbamazepine (CBZ) has been found to increase the concentration of CBZ-10,11-epoxide (epoxide) relative to that of CBZ in patients (McKauge et al., 1981; Brodie et al., 1983; Levy et al., 1984). The epoxide, a stable metabolite of CBZ, is converted by epoxide hydrolase to a terminal trans-dihvdrodiol product (Tomson et al., 1983). Thus, a decrease in the elimination of epoxide and the consequent increase in the epoxide/CBZ ratio has been attributed to a VPA associated decrease in epoxide hydrolase activity. Only limited information, however, is available on epoxide disposition in epileptic patients (Eichelbaum et al., 1985), and no direct evidence has been forthcoming which demonstrates that VPA elevates the epoxide/CBZ ratio by inhibiting epoxide elimination during CBZ and VPA coadministration. The purpose of this investigation was to factor out the cause(s) for the elevated epoxide/CBZ steady-state ratio in epileptic patients receiving VPA. To delineate the possible mechanism(s) involved, an examination of the effect of VPA on epoxide formation, epoxide elimination, and on the binding of CBZ and epoxide to plasma proteins was carried out in patients on chronic CBZ therapy.

Study design Five epileptic patients who required VPA addition to their therapy and who had no history of haematological disease, renal or hepatic dysfunction participated in this study. The investigation was approved by the University Investigational Review Board, and written informed consent was obtained from the subjects and parents prior to enrollment into the study. Patient demographics are shown in Table 1. The study was divided into two phases, with the first phase performed before the addition of VPA to the CBZ regimen and the second phase 3-4 weeks after the initiation of combined VPACBZ therapy. During each phase, the patient was admitted to the University of Kentucky Medical Center the evening before the study and fasted over night. Just prior to ingesting their regular morning dose of CBZ subjects were asked to void their bladders of urine and breakfast was withheld for 2 h. Blood samples (5 ml each) were collected through an indwelling catheter at 0, 1, 2, 3, 4, 5, 6, 7 and 8 h after the CBZ dose and the plasma was harvested. Throughout the study and upon its completion, subjects were asked to collect all their urine in a container provided. The urine volume was measured and

759

760 D. K. Robbins et al. Table 1 Patient demographics and dosing 1

2

3

4

5

Age (years) Sex Weight (kg) CBZ dose day-' (mg) VPA dose day-' (mg) *Average VPA

17 F 62 2100 750

20 F 56 1600 1000

12 M 82 1800 750

12 F 49 1000 750

16 M 99 2600 1250

concentration(.gmMl')

42.9

54.3 Oral contraceptives

34.2

66.5

51.1 Propranolol (80 mg day-) Dibenzyline (30 mg day-)

Patients

Other medications

*Average concentration determined from equation Cs = AUC where Tr dosage interval.

the plasma and urine samples stored at -20° C until assayed. Assays The analysis of CBZ and epoxide was performed according to the method of Mendez-Alvarez et al. (1986). The measurement of the transdihydrodiol metabolite in urine was performed by the method of Robbins et al. (1987). VPA was assayed by gas chromatography: following acid extraction of the plasma sample, a 1 RI aliquot of the organic layer (chloroform) was injected onto a 30 m capillary column (Supelcowax 1000) heated at 1420 C and analyzed with a flame ionization detector. The plasma protein binding of CBZ and epoxide was measured in each plasma sample by equilibrium dialysis at 370 C.

Theoretical considerations The plasma and urinary data for each subject were substituted into the following equations in order to estimate each parameter in the absence and presence of VPA. 1. Epoxide-CBZ steady-state plasma concentration ratio:

2. Formation clearance of unbound epoxide

(CLuint): F CLui,t = [Ae(T)+Ae(E)] AUC-fu

(2)

where Ae(T) and Ae(E) = the total urinary molar amounts of the transdiol and epoxide metabolite in the 8 h urine, respectively; fu = fraction of CBZ unbound in plasma. F, the oral availability of CBZ, was assumed to be constant between the two phases. 3. Apparent activity of epoxide hydrolase:

CLui,0(m)

=

Ae(T)-F

AUC(m)-fu(m)

(3)

where fu(m) = fraction of epoxide unbound in plasma. Statistical significance was determined for each parameter using Student's paired t-test with and without logarithmic transformation to minimize potential effects associated with nongaussian distribution of the data. The significance level was set at 0.05 in all instances. Results and discussion

C(m),, css

AUC(m) AUC

(1)

Areas under the curves were determined using the trapezoidal rule from the plasma concentrations ofCBZ (AUC) and epoxide AUC(m) over an 8 h period following CBZ administration.

Previous attempts to define the effect of VPA and its amide analogue (valpromide) on epoxide elimination have relied upon the administration of a single oral dose of epoxide in healthy volunteers (Kerr et al., 1989; Pisani et al., 1988). However, the epoxide is a unique metabolite of

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