Abstracts

RATIONALE: Unexplained reports of clinically relevant elevations of phenyoin (PHT) plasma levels following the addition of carbamazepine (CBZ) as well as decreases in the formation clearances of its major metabolite 5-(4-hydroxyphenyl)-5-phenyl hydantoin (HPPH) have been observed in several studies (Leppik et al., 1984, Effect of carbamazepine on the Michaelis-Menten parameters of phenytoin in Metabolism of Antiepileptic Drugs (Levy, R. H., Pitlick, W. H., Eichelbaum, M., Meijer, J., eds), Raven Press, New York. pp 217-222; Zielinski et al., 1985, Ther Drug Monit 7, 51-53; Zielinski et al., 1987, Ther Drug Monit 9, 21-23; Browne et al., 1988, Neurology 38, 1146-1150). However, CBZ is a well known inducer of cytochromes P450 (CYP) such as CYP3A4 and CYP2C9. PHT is metabolized to HPPH by CYP2C9. The identification of CYP2C19 as a second enzyme contributing to HPPH formation has provided a rationale for a number of PHT interactions with inhibitors of that enzyme (ticlopidine, felbamate, and topiramate). These interactions exhibit a significant degree of intersubject variability associated with the variable contribution of CYP2C19 and its pharmacogenetic behavior. Aim. The present study was designed to test the hypothesis that CBZ (and/or carbamazepine 10,11 epoxide; CBZE) inhibit PHT metabolism through inhibition of CYP2C19. Parallel studies were also undertaken with oxcarbazepine (OXC), and its 10-monohydroxy metabolite (MHD).
METHODS: The inhibition potency of CBZ, CBZE, OXC or MHD on PHT metabolism to HPPH was evaluated in incubations with cDNA-expressed CYP2C19 with 20 [mu]M and 40 [mu]M PHT. To further support and quantify the inhibitory effects of CBZ, CBZE, OXC, and MHD, the CYP2C19 probe (S)-mephenytoin was used (25-225 [mu]M) and inhibition constants were determined in two systems: cDNA-expressed CYP2C19 and human liver microsomes.
RESULTS: Incubation of PHT with CBZ (both at therapeutic concentrations) showed that CBZ is a potent inhibitor of HPPH formation. The degree of inhibition was dependent on CBZ concentration (33-64% at 30-90 [mu]M CBZ.) CBZE was a less potent inhibitor (7-33% at 15-60 [mu]M). Relevant concentrations of OXC (4-12 [mu]M) and of MHD (10-40 [mu]M) also inhibited the metabolism of PHT, and the degree of inhibition was dependent on inhibitor concentrations. MHD was a more potent PHT inhibitor than OXC. The inhibitory effects of CBZ and OXC and their metabolites on CYP2C19 (S)-mephenytoin metabolism were confirmed in cDNA-expressed CYP2C19 and human liver microsomal preparations. CBZ and MHD exhibited Ki values within their respective therapeutic ranges (8.3 [plusminus] 1.8 [mu]M and 15 [plusminus] 10 [mu]M, respectively) indicating that they will behave as inhibitors under normal conditions of use. The Ki values obtained for CBZE (53.5 [plusminus] 22.2 [mu]M) and OXC (17.9 [plusminus] 2.7 [mu]M) were higher than their respective therapeutic concentrations indicating that these species could only contribute to a small extent to the inhibition produced by CBZ and MHD, respectively.
CONCLUSIONS: CBZ, OXC, and their metabolites have the potential to inhibit the metabolism of CYP2C19 substrates in vivo. These observations can explain the reported elevations in PHT concentration following the addition of CBZ or OXC and the associated intersubject variability.
[Supported by: This work was supported by grants from the National Institutes of Health (GM32165).]; (Disclosure: Grant - The laboratory of Dr. Levy received a research grant from Warner Lambert-Parke Davis)