Acenocoumarol pharmacokinetics in relation to cytochrome P4502C9 genotype

Background and Objectives. Cytochrome P450 (CYP) 2C9 is one of the major CYP enzymes involved in the biotransformation of drugs, among others, the oral anticoagulant acenocoumarol. The enzyme has several polymorphisms, with the CYP2C9*2 and CYP2C9*3 variants most commonly present in white patients. Patients with the CYP2C9*3 variant are known to require a lower maintenance dose of racemic acenocoumarol. We investigated the impact of the polymorphisms CYP2C9*2 and CYP2C9*3 on the pharmacokinetics of R- and S-acenocoumarol. Methods and Results: In the first study 26 healthy volunteers with the genotype *1/*1(n = 9), *1/*2 (n = 7), *1/*3 (n = 6), *2/*3 (n = 3), and *2/*2 (n = 1) were given 8 mg of racemic acenocoumarol as a single oral dose. Plasma R- and S-acenocoumarol concentrations were assayed at 4, 7, and 24 hours. Mean plasma S-acenocoumarol concentrations at 7 hours were higher in subjects with a variant allele; the differences were significant (P =.01) for the *1/*3 and *2/*3 genotypes. In the second study, the oral pharmacokinetics of acenocoumarol was investigated in 6 subjects (*1/*1 [n = 3] and *1/*3 [n = 3]). The mean oral clearance of S-acenocoumarol was 45% lower in the CYP2C9*1/*3 genotypes (10.9 +/- 3.0 L/h versus 19.8 +/- 3.1 L/h, P =.02). Plasma half-life was prolonged from 1.0 +/- 0.2 hours to 2.0 +/- 0.7 hours (P=.09). R-acenocoumarol pharmacokinetics did not differ between the genotypes. There was no difference in mean international normalized ratio at 24 hours, which was 1.2 in both groups. In vitro enzyme kinetics showed reduced (85%) intrinsic activity of the *3 enzyme to catalyze the hydroxylations of S-acenocoumarol. The lower activity resulted from higher Michaelis-Menten constant (2-fold) and lower maximum rate of metabolism by an enzyme-mediated reaction (by 70%). The activity of the *2 enzyme was 50% of the wild-type one. Conclusion: The results show S-acenocoumarol pharmacokinetics to be dependent on CYP2C9 polymorphism. In particular, the presence of the CYP2C9*3 allele impairs oral clearance of the coumarin.