Background: Interindividual variability in responses to warfarin is attributed to dietary vita-min K, drug interactions, age, or genetic polymorphism in the cytochrome P4502C9 enzyme (CY22C9) (allelic variants 2C9*2 and 2C9*3) linked with impaired metabolism of the potent enantiomere S-warfarin. Patients and Methods: We quantified the relative effects of age and of simultaneously determined CYP2C9 genotype, plasma warfarin and vitamin K concentrations, and concurrent medications on warfarin maintenance doses in 156 patients at optimized stable anticoagulation. Results: Allele frequencies for CYP2C9*1, CYP2C9*2, and CYP2C9*3 were 0.84, 0.10, and 0.06. Warfarin doses were 6.5 +/- 3.2, 5.2 +/- 2.4, and 3.3 +/- 2.0 mg/d in the 3 genotype groups (P < .0001). Warfarin doses decreased with age as follows: 7.7 +/- 3.7 versus 4.9 +/- 2.9 mg/d at < 50 years and > 66 years (P < .001), mainly as a result of decreased plasma warfarin,clearance (2.8 +/- 1.4 mL/min versus 1.9 +/- 0.8 mL/min; P < .001). Vitamin K (1.6 +/- 1.1 ng/mL) did not differ among the age or genotype groups. Z' Patients greater than or equal to 66 years old with the CYP2C9*3 allele required only 2.2 +/- 1.2 mg/d compared with 7.9 +/- 3.7 mg/d in those less than or equal to 65 years old bearing the CYP2C9*1 allele (P < .001). On multiple regression, warfarin maintenance doses were independently associated with plasma warfarin (reflecting its metabolic clearance) (r(2) = 0.26), age (possibly reflecting increased intrinsic sensitivity) (r(2) = 0.12), and genotype (reflecting S-warfarin levels) (r(2) = 0.10) but not with plasma vitamin K. Conclusions: At optimized steady state, individual sensitivity to warfarin is determined by CYP2C9 genotype and age with no effect of vitamin K. Prospective studies will determine the impact of these findings in clinical practice.
