Inhibition of Epoxide Hydrolase by Valproic Acid in Epileptic Patients Receiving Carbamazepine

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) Study design with carbamazepine (CBZ) has been found to increase the concentration of CBZ-10,11-epoxide Five epileptic patients who required VPA addition (epoxide) relative to that of CBZ in patients to their therapy and who had no history of (McKauge et al., 1981; Brodie et al., 1983; Levy haematological disease, renal or hepatic et al., 1984). The epoxide, a stable metabolite of dysfunction participated in this study. The CBZ, is converted by epoxide hydrolase to a investigation was approved by the University terminal trans-dihvdrodiol product (Tomson et Investigational Review Board, and written al., 1983). Thus, a decrease in the elimination of informed consent was obtained from the subjects epoxide and the consequent increase in the and parents prior to enrollment into the study. epoxide/CBZ ratio has been attributed to a VPA Patient demographics are shown in Table 1. associated decrease in epoxide hydrolase activity. The study was divided into two phases, with Only limited information, however, is available the first phase performed before the addition of on epoxide disposition in epileptic patients VPA to the CBZ regimen and the second phase (Eichelbaum et al., 1985), and no direct evidence 3-4 weeks after the initiation of combined VPA- has been forthcoming which demonstrates that CBZ therapy. During each phase, the patient VPA elevates the epoxide/CBZ ratio by inhibiting was admitted to the University of Kentucky epoxide elimination during CBZ and VPA Medical Center the evening before the study and coadministration. fasted over night. Just prior to ingesting their The purpose of this investigation was to factor regular morning dose of CBZ subjects were out the cause(s) for the elevated epoxide/CBZ asked to void their bladders of urine and breakfast steady-state ratio in epileptic patients receiving was withheld for 2 h. Blood samples (5 ml each) VPA. To delineate the possible mechanism(s) were collected through an indwelling catheter at involved, an examination of the effect of VPA 0, 1, 2, 3, 4, 5, 6, 7 and 8 h after the CBZ dose on epoxide formation, epoxide elimination, and and the plasma was harvested. Throughout the on the binding of CBZ and epoxide to plasma study and upon its completion, subjects were proteins was carried out in patients on chronic asked to collect all their urine in a container CBZ therapy. provided. The urine volume was measured and 759 760 D. K. Robbins et al.

Table 1 Patient demographics and dosing Patients 1 2 3 4 5 Age (years) 17 20 12 12 16 Sex F F M F M Weight (kg) 62 56 82 49 99 CBZ dose day-' (mg) 2100 1600 1800 1000 2600 VPA dose day-' (mg) 750 1000 750 750 1250 *Average VPA concentration(.gmMl') 42.9 54.3 34.2 66.5 51.1 Other medications Oral contra- Propranolol ceptives (80 mg day-) Dibenzyline (30 mg day-) *Average concentration determined from equation Cs = AUC where Tr dosage interval.

the plasma and urine samples stored at -20° C 2. Formation clearance of unbound epoxide until assayed. (CLuint):

Assays = [Ae(T)+Ae(E)] F CLui,t AUC-fu (2) The analysis of CBZ and epoxide was performed according to the method of Mendez-Alvarez et where Ae(T) and Ae(E) = the total urinary al. (1986). The measurement of the trans- molar amounts of the transdiol and epoxide dihydrodiol metabolite in urine was performed metabolite in the 8 h urine, respectively; fu = by the method ofRobbins et al. (1987). VPA was fraction of CBZ unbound in plasma. F, the oral assayed by gas chromatography: following acid availability of CBZ, was assumed to be constant extraction of the plasma sample, a 1 RI aliquot of between the two phases. the organic layer (chloroform) was injected onto of a 30 m capillary column (Supelcowax 1000) 3. Apparent activity epoxide hydrolase: heated at 1420 C and analyzed with a flame ionization detector. The plasma protein binding CLui,0(m) = Ae(T)-F (3) of CBZ and epoxide was measured in each AUC(m)-fu(m) plasma sample by equilibrium dialysis at 370 C. where fu(m) = fraction of epoxide unbound in plasma. Theoretical considerations Statistical significance was determined for each The plasma and urinary data for each subject parameter using Student's paired t-test with and were substituted into the following equations in without logarithmic transformation to minimize order to estimate each parameter in the absence potential effects associated with nongaussian and presence of VPA. distribution of the data. The significance level was set at 0.05 in all instances. 1. Epoxide-CBZ steady-state plasma concentration ratio: Results and discussion C(m),, AUC(m) (1) css AUC Previous attempts to define the effect of VPA and its amide analogue (valpromide) on epoxide Areas under the curves were determined using elimination have relied upon the administration the trapezoidal rule from the plasma concentra- of a single oral dose of epoxide in healthy tions ofCBZ (AUC) and epoxide AUC(m) over volunteers (Kerr et al., 1989; Pisani et al., 1988). an 8 h period following CBZ administration. However, the epoxide is a unique metabolite of Short report 761

a I]b - -

| ~ ~~~~.~i I 4 :.u Cl I S. 'h.e',J l.I [. ~i 2-?; *ss' ! ,$I @A i4 ,1*f ' Eti Ext j~~~s;;r_it $ tE; ii E;>zSf /' 'i .-;rJ'n - S rX;2rwa 3|, 8 t* 8itd [)gfft; X r.;.,pa ~ .. St t.1.t IW is~~~~~~~~~~~~~W~t- '' {rts'. SiRhWi_8.,£Tt {= vM E; f <.y ',} \,,'4. $l 'It}4 3b5ry ,1t iS ~ VPAi_'f ..t..8 s.'.... -w. ....)' ..._ ji 415 .tA' ..$. .IV.TI .

o.L 'ti ti t .j M*AA .

Figure 1 The effect of VPA therapy (sVPA = without VPA; cVPA = with VPA) on the a) epoxide/CBZ steady-state concentration ratio, b) formation clearance and c) clearance of elimination of the unbound CBZ-10,11-epoxide.

CBZ which need not be administered per se in I; 0.52 ± 0.02 phase II), this increase diminished order to characterize its disposition. The extensive rather than magnified the 'real' decrease in metabolism of epoxide to its trans-dihydrodiol unbound epoxide elimination produced by VPA. metabolite and subsequent elimination of this As epoxide appears to be eliminated primarily terminal metabolite in the urine suggest that by epoxide hydrolase (Tomson et al., 1983; epoxide formation and elimination can be Tybring et al., 1981), any decrease in the elimina- characterized from plasma and urinary data tion of epoxide should be a direct reflection of (Eichelbaum et al., 1985). diminished epoxide hydrolase activity. This The effect of VPA coadministration on CBZ hypothesis has been strengthened by a recent and epoxide disposition is illustrated in Figure 1. report (Kerr et al., 1989) where an in vivo and in The epoxide/CBZ steady-state ratio was elevated vitro correlation of human microsomal epoxide significantly in all patients (mean 0.23 ± 0.04 hydrolase inhibition by valproic acid and valpro- phase I; 0.30 ± 0.05 phase II). While the fraction mide was established. As demonstrated in this of CBZ unbound in plasma was increased by study, it appears possible to measure changes in VPA (mean 0.23 ± 0.01 phase I; 0.25 ± 0.02 the activity of this enzyme using CBZ as probe phase II) this effect was modest and did not and employing metabolite kinetics to define the result in a significant change in either the total or effect(s) of various agents on epoxide formation the unbound formation of epoxide. However, and elimination. This study suggests that VPA the clearance of unbound epoxide (i.e. the therapy results in an inhibition of epoxide apparent activity of epoxide hydrolase) was hydrolase activity, but does not reduce signifi- diminished significantly in all subjects by con- cantly the activity of those isozymes responsible current VPA therapy (mean 18.7 ± 3.4 1 h-1 for the formation of CBZ-10,11-epoxide. phase I; 14.2 ± 4.31 h-1 phase II). Although the fraction ofunbound epoxide was slightly increased This work was supported by the Epilepsy Foundation in the presence of VPA (mean 0.48 ± 0.02 phase of America and the University of Kentucky CRC Unit. 762 D. K. Robbins et al. References

Brodie, M. J., Forrest, G. & Rapeport, W. G. (1983). on the stability of carbamazepine and carbamaze- Carbamazepine-10,11-epoxide concentrations in pine-10, 11-epoxide in plasma. Clin. Chim. Acta, epileptics on carbamazepine alone and in combina- 154, 243-246. tion with other anticonvulsants. Br. J. clin. Pisani, F., Fazio, A., Oteri, G., Spina, E., Perucca, E. Pharmac., 16, 747-750. & Bertilsson, L. (1988). Effect of valpromide on Eichelbaum, M., Tomson, T., Tybring, G. & Bertilsson the pharmacokinetics of carbamazepine-10,11- L. (1985). Carbamazepine metabolism in man: epoxide. Br. J. clin. Pharmac., 25, 611-613. induction and pharmacogenetic aspects. Clin. Robbins, D. K., Chang, S-L., Baumann, R. J. & Pharmacokin., 10, 80-90. Wedlund, P. J. (1987). Quantitation trans-10,11- Kerr, B. M., Rettie, A. E., Eddy, A. C., Loiseau, P., dihydroxy-10,11-dihydrocarbamazepine in human Guyot, M., Wilensky, A. J. & Levy, R. H. (1989). urine by high-performance liquid chromatography. Inhibition of human liver microsomal epoxide J. Chromatogr., 415, 208-213. hydrolase by valproate and valpromide: In vivo/in Tomson, T., Tybring, G. & Bertilsson, L. (1983). vitro correlation. Clin. Pharmac. Ther., 46, 82-93. Single-dose kinetics and metabolism of carba- Levy, R. H., Moreland, T. A., Morselli, P. L., Guyot, mazepine-10,11-epoxide. Clin. Pharmac. Ther., 33, M., Brachet-Liermain, A. & Loiseau, P. (1984). 58-65. Carbamazepine/valproic acid interaction in man Tybring, G., von Bahr, C., Bertilsson, L., Collste, H., and rhesus monkey. Epilepsia, 25, 338-345. Glaumann, H. & Solbrand, M. (1981). Metabolism McKauge, L., Tyrer, J. H. & Eadie, M. J. (1981). of carbamazepine and its epoxide metabolite in Factors influencing simultaneous concentrations human and rat liver in vitro. Drug Metab. Disp., 9, of carbamazepine and its epoxide in plasma. Ther. 561-564. Drug Monit., 3, 63-70. Mendez-Alvarez, E., Soto-Otero, R. & Sierra- (Received 19 June 1989, Marcuno, G. (1986). The effect of storage conditions accepted 23 January 1990)