Kohsuke Mamiya

The Effects of Genetic Polymorphisms of CYP2C9 and CYP2C 19 on Phenytoin Metabolism in Japanese Adult Patients with Epilepsy: Studies in Stereoselective Hydroxylation and Population Pharmacokinetics

Summary: Purpose: The aim of this study was to clarify the effects of genetic polymorphisms of cytochrome P450 (CYP) 2C9 and 2C19 on the metabolism of phenytoin (PHT). In addition, a population pharmacokinetic analysis was performed.

Genetic polymorphism of the CYP2C subfamily and excessive serum phenytoin concentration with central nervous system intoxication

The authors report on a Japanese adult male patient with a long history of partial seizures that were poorly controlled by conventional doses of phenytoin and other drugs. His treatment was complicated by toxic symptoms and an excessive serum phenytoin concentration, 32.6 microg/mL at a dose of 187.5 mg/day. Polymerase chain reaction-restriction fragment length polymorphism analysis disclosed heterozygosity involving cytochrome P450 subfamilies 2C9 (*1/*3) and 2C19 (*1/*3). Currently, it is generally accepted that the former mutation is responsible for the CYP2C9 poor metabolizer phenotype. Pharmacokinetic parameters were estimated by a kinetic analysis, MULTI, using 17 observed dose-concentration data sets: a lower Vmax (5.6 mg/kg/day) and a higher Km (11.5 microg/mL) were observed. Although phenytoin is metabolized predominantly by CYP2C9 with a minor contribution of CYP2C19, patients with the Leu359 variant should be monitored closely when treated with a moderate to high daily dose of phenytoin.

Genetic polymorphisms and functional characterization of the 5′‐flanking region of the human CYP2C9 gene: In vitro and in vivo studies

Genetic polymorphisms were identified in the 5′‐flanking region of the human CYP2C9 gene, and their effects on the phenotype were evaluated on the basis of the luciferase reporter gene assay and the in vivo pharmacokinetics of phenytoin.

CYP2C19 polymorphism effect on phenobarbitone

AbstractObjective: The aim of this study was to clarify the effect of genetic polymorphisms of CYP2C19 on the pharmacokinetics of phenobarbitone (PB) using a nonlinear mixed-effects model (NONMEM) analysis in Japanese adults with epilepsy. Methods: A total of 144 serum PB concentrations were obtained from 74 subjects treated with both PB and phenytoin but without valproic acid. All patients were classified into three groups by CYP2C19 genotyping: G1, G2 and G3 were homozygous for the wild type of CYP2C19 (*1/*1), heterozygous extensive metabolizers (EMs), (*1/*2 or *1/*3), and poor metabolizers (PMs), (*2/*2, *2/*3), respectively. All data were analyzed using NONMEM to estimate pharmacokinetic parameters of PB with respect to the CYP2C19 genotype. Results: Thirty-three patients belonged to G1 (44.6%), 35 to G2 (47.3%), and 6 to G3 (8.1%). The total clearance (CL) of PB significantly decreased by 18.8% in PMs (G3) relative to EMs (G1 and G2). The CL tended to be lower in G2 than in G1. Conclusion: In this study, we first demonstrated the effect of the CYP2C19 polymorphism on pharmacokinetics of PB by genotyping. The contribution of other metabolic enzymes in the metabolism of PB in humans remains to be elucidated; however, it appears that the disposition of PB is mediated in part by this enzyme. The estimated population clearance values in the three genotype groups can be used to predict the PB dose required to achieve an appropriate serum concentration in an individual patient.

P-hydroxylation of phenobarbital: relationship to (S)-mephenytoin hydroxylation (CYP2C19) polymorphism.

The aim of the current study was to compare the pharmacokinetics of phenobarbital (PB) in extensive metabolizers (EMs) and poor metabolizers (PMs) of S-mephenytoin. Ten healthy volunteers (5 EMs and 5 PMs) were given 30 mg PB daily for 14 days. PB and p-hydroxyphenobarbital (p-OHPB) in serum and urine were measured by high-performance liquid chromatography (HPLC). Urinary excretion (12.5% versus 7.7%) and formation clearance (29.8 versus 21.1 mL/h) of p-OHPB, one of the main metabolites of PB, were significantly lower (p < .05) in PMs than in EMs. However, area under the serum concentration–time curve (153.3 in the EMs versus 122.9 &mgr;g · h/mL in the PMs) , total (210.8 versus 254.9 mL/h) and renal clearance (53.1 versus 66.1 mL/h) of PB were identical between the two groups. To compare the inducibility of CYP2C19, mephenytoin was also given prior to and on the last day of PB treatment. The urinary level of 4´-hydroxymephenytoin was analyzed by a validated gas chromatograpy/mass spectrometry (GC/MS) method. The mephenytoin hydroxylation index did not change in either EMs (1.42 versus 1.42) or PMs (341.4 versus 403.5), showing that CYP2C19 was not induced by treatment with PB. These results indicated that the p-hydroxylation pathway of PB co-segregates with the CYP2C19 metabolic polymorphism. However, the overall disposition kinetics of PB were not different between EMs and PMs, and therefore polymorphic CYP2C19 seems have no major clinical implications.

Phenytoin intoxication induced by fluvoxamine.

A patient had phenytoin intoxication after administration of fluvoxamine, a selective serotonin reuptake inhibitor. The serum concentration of phenytoin increased dramatically from 16.6 to 49.1 microg/mL when fluvoxamine was coadministered, although the daily dosage of phenytoin and other drugs had not changed. During phenytoin and fluvoxamine treatment, ataxia, a typical side effect of phenytoin, was observed. The genotypes of CYP2C9 and 2C19, the enzymes responsible for phenytoin metabolism, were homozygous for the wild-type alleles (CYP2C9*1/*1 and 2C19*1/ *1). The interaction may be a result of inhibition of both CYP2C9 and 2C19 by fluvoxamine.

Synergistic Effect of Valproate Coadministration and Hypoalbuminemia on the Serum-free Phenytoin Concentration in Patients With Severe Motor and Intellectual Disabilities

We investigated whether a combination of risk factors affects the free phenytoin (PHT) fraction by multiple regression analyses in 30 patients with severe motor and intellectual disabilities (SMID) with epilepsy. The risk factors analyzed were gender, age, total PHT concentration, albumin concentration, aspartate aminotransferase, alanin aminotransferase, serum creatinine, blood urea nitrogen, and antiepileptic drug concentrations. Serum levels of total and free PHT were measured by fluorescence polarization immunoassay. Free PHT fractions were between 7.2% and 17.3% (average 10.9%). Two factors, hypoalbuminemia and valproate (VPA) coadministratation with PHT, increased free PHT fraction, and a combination of these two markedly increased free PHT fraction. Patients with these double risk factors have a high risk of exceeding the therapeutic range of serum-free PHT concentration even if their total PHT concentration does not. Therefore, we should monitor free PHT concentration, especially in SMID patients with epilepsy, because they may have hypoalbuminemia and are treated with antiepileptic drug polytherapy and, moreover, cannot report adverse effects of the drugs.

Hydroxylation of Phenytoin (PHT) and the Cytochrome P450 (CYP) 2C Subfamily

Purpose: Four members of the CYP2C subfamily, CYP2C8, CYP2C9, CYP2C18, and CYP2C19, have been identified in humans, and a number of allelic variants of the CYP2C9, CYP2C18. and CYP2C19 genes associated with metabolic polymorphisms have been reported (Pharmacogenetics) 1994;4:285–99). CYP2C9 is a major enzyme responsible for the formation of 5–(4‐hydroxyphenyl)‐5‐phenylhydantoin (p‐HPPH), a major hydroxylation metabolite of PHT. However, we recently reported that CYP2C19 contributes to the stereoselective hydroxylation of PHT (Br J Clin Pharmacol 1997;43:431–5). To clarify the relation between hydroxylation of PHT and the CYP2C subfamily, we examined their stereoselective para‐hydroxylation properties by using cDNAs expressing CYP2C8, 9, 18, or 19. In addition, the allelic linkage among members of the CYP2C subfamily was evaluated.