Effect of CYP2C19 genetic polymorphism on pharmacokinetics of phenytoin and phenobarbital in Japanese epileptic patients using Non-linear Mixed Effects Model approach.

OBJECTIVE: To clarify the effect of genetic polymorphism of CYP2C19 on pharmacokinetics of phenytoin and phenobarbital using a Non-linear Mixed Effects Modelling analysis in Japanese epileptic patients.

METHOD: A total of 326 serum phenytoin concentrations were collected from 132 patients, and a total of 144 serum phenobarbital concentrations were collected from 74 patients during their clinical routine care.

RESULT: The maximal elimination rate of phenytoin decreased by 10.2% in patients with CYP2C19*1/*2 compared with patients with normal CYP2C19. The Michaelis-Menten constants in the patients with CYP2C19*1/*3 and the poor metabolizers of (CYP2C19*2/*2 or *2/*3 or *3/*3) were 27% and 54% higher than those for the patients with normal CYP2C19, respectively. The total body clearance of phenobarbital decreased by 19.3% in patients with CYP2C19*1/*3 or the poor metabolizers of CYP2C19 compared with patients with normal CYP2C19 or with CYP2C19*1/*2.

CONCLUSION: These findings indicated that the genetic polymorphisms of CYP2C19 contribute to the pharmacokinetic variability of phenytoin and phenobarbital, the poor metabolizers of CYP2C19, which are relatively common in Asian groups.