Objective: To determine whether genetic polymorphism of cytochrome P450 (CYP) 2C9 affects the in vivo metabolism of warfarin enantiomers.
Methods: Eighty-six Japanese patients heart disease who were given warfarin participated in the study. Plasma unbound concentrations of warfarin enantiomers and urinary (S)-7-hydroxywarfarin concentrations were measured by means of a chiral HPLC and ultrafiltration technique to calculate the unbound oral clearance (CLpo,u) for the enantiomers and the formation clearance (CLm) for (S)-warfarin 7-hydroxylation. Genotyping for CYP2C9 (the wild type [wt], Arg144/Cys, and I1e359/Leu) and for CYP2C19 (wt, ml, and m2) was performed with a polymerase chain reaction method.
Results: Three patients were heterozygous for the CYP2C9 Leu359 mutation but none were homozygous for the mutation (the allele frequency of 0.017). None had a CYP2C9 Cys144 allele. The medians for (S)-warfarin CLpo,u and its 7-hydroxylation CLm obtained from heterozygotes of CYP2C9 Leu359 were significantly less than those obtained from homozygotes of the wt allele, as follows: 234 ml/min (range, 156 to 269 ml/min) versus 632 ml/min (range, 180 to 2070 ml/min) (p < 0.001) and 0.20 ml/min (range, 0.05 to 0.77 ml/min) versus 0.80 ml/min (range, 0.05 to 14.9 ml/min) (p < 0.05), respectively. In contrast, no difference was observed in (R)-warfarin CLpo,u between the groups. The allele frequencies for CYP2C19 m1 and CYP2C19 m2 were 0.26 and 0.14, respectively, indicating 15% of patients were genotypically poor metabolizers of CYP2C19. No difference in CLpo,u for warfarin enantiomers was observed between the assumed CYP2C19 phenotypes.
Conclusion: Heterozygotes for CYP2C9 I1e359/Leu allele have reduced in vivo metabolism of (S)-warfarin but not (R)-warfarin. Because (S)-warfarin has a greater anticoagulant potency than its (R)-congener, the genetic polymorphism of CYP2C9 may partly account for the large interpatient variability in therapeutic dosages of warfarin.