Pharmacology Journal

The enzyme (CYP2C19) that 4′-hydroxylates S- mephenytoin (S-MP) is a genetically polymorphic cytochrome P450. Based on the activity of CYP2C19, individuals are phenotyped as extensive metabolizers (EM) and poor metabolizers (PM). S-MP is extensively metabolized in EMs; this results in a lower S/R ratio measured in EMs than that in PMs. The PM phenotype is inherited as a Mendelian autosomal recessive trait involving two alleles at a single gene locus. Recently, two genetic mutations (m₁ and  m₂ ) have been found to result in PM of S-MP. The m₁ mutation, caused by a single base pair (G→A) mutation in exon 5 corresponding to base pair 681 of the cDNA, creates an aberrant splice site. This defect accounts for 75–85% of Caucasian and Japanese PMs. The  m₂ mutation consisting of a G→A transition at base 636 has been found only in Oriental populations. It is well known that some of the enzymes in P4502C subfamily (including CYP2C19) can be induced in both animals and humans; also, chronic use of mephenytoin causes autoinduction. Treating EMs and PMs of S-MP with rifampicin, using mephenytoin as a probe, we previously reported that CYP2C19 activity was inducible in EMs but not in PMs. Recently we found that some of the PMs of S-MP whose genotypes were defined as m₁/m₁ or m₁/m₂ excreted as much as 8% of the dose as 4′-OH-MP. In addition, the S/R mephenytoin ratio of those m₁ mutation heterozygous was lower than that of  m₂ mutation heterozygou. These data suggest that the activity of 4′-hydroxylase of those heterozygous with m₁ mutation is different from that of heterozygous with  m₂ mutation. Moreover, the mephenytoin S/R ratio of the heterozygous (wt/m₁ and wt/m₂ ) is slightly higher than that of homozygous (wt/wt), indicating that genetics affects the 4′-hydroxylation of S-MP. Therefore, we hypothesize that the activity of 4′-hydroxylase in PMs with m₁ mutation may not be lost completely and consequently its activity in these PMs should be inducible and the induction may be related to gene dose. The study was designed to test this hypothesis.

Seven male Chinese EMs of S-MP and five PMs with m₁ mutation whose genotype was defined in previous studies, aged 19 to 22 (21±1 years, mean±s.d.), weighing 54 to 75  kg (58.9±6.2  kg) were enrolled in the study. No subject had had recent illness, and none had taken drugs for at least 2 weeks prior to or during the study. No subject had any abnormalities on physical examination or any biochemical evidence of renal or hepatic disorders. Four of the EMs were genotyped as homozygous (wt/wt), three as heterozygous (two as wt/m₁ and one as wt/m₂ ); four of the PMs were genotyped as m₁/m₁, one as m₁/m₂. The study protocol was approved by the Ethics Committee of Hunan Medical University and written consent was obtained from the subjects.

After an overnight fast, all the subjects took 100  mg racemic mephenytoin (Mesantoin^®, Sandoz Pharmaceuticals, Inc.). 0–8  h and 8–24  h urine were collected and the volume was measured. Urine samples were stored at −15°  C until assayed. From the second day on, every subject received 300  mg rifampicin (Rifampicin Capsule, 150  mg/capsule, pitch number: 950106, Xinyang Pharmaceuticals of Henan Province, PRC) daily for 22 days and racemic mephenytoin was again administered along with the final dose of rifampicin, urine samples were collected and stored in the same way as mentioned above.

The 0–8  h urinary mephenytoin S/R ratio was measured by gas chromatography. The amount of 4′-OH-MP excreted in the 0–8  h and 8–24  h urine was determined by high performance liquid chromatography.

The data were analyzed by paired t-test and rank sum test with P<0.05 as the minimal level of statistical significance.

Filed under: Clinical Pharmacology