The largest prospective warfarin-treated cohort supports genetic forecasting

Abstract

Genetic variants of cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase (VKORC1) are known to influence warfarin dose, but the effect of other genes has not been fully elucidated. We genotyped 183 polymorphisms in 29 candidate genes in 1496 Swedish patients starting warfarin treatment, and tested for association with response. CYP2C9*2 and *3 explained 12% (P = 6.63 x 10(-34)) of the variation in warfarin dose, while a single VKORC1 SNP explained 30% (P = 9.82 x 10(-100)). No SNP outside the CYP2C gene cluster and VKORC1 regions was significantly associated with dose after correction for multiple testing. During initiation of therapy, homozygosity for CYP2C9 and VKORC1 variant alleles increased the risk of over-anticoagulation, hazard ratios 21.84 (95% CI 9.46; 50.42) and 4.56 (95% CI 2.85; 7.30), respectively. One of 8 patients with CYP2C9*3/*3 (12.5%) experienced severe bleeding during the first month compared with 0.27% of other patients (P = .066). A multiple regression model using the predictors CYP2C9, VKORC1, age, sex, and druginteractions explained 59% of the variance in warfarin dose, and 53% in an independent sample of 181 Swedish individuals. In conclusion, CYP2C9 and VKORC1 significantly influenced warfarin dose and predicted individuals predisposed to unstable anticoagulation. Our results strongly support that initiation of warfarin guided by pharmacogenetics would improve clinical outcome.

Figure 1

Validation of the multiple regression model. The solid line illustrates perfect prediction in the pilot cohort and the dashed reference lines show deviation from the prediction of plus or minus 1.25 mg (½, pill) and plus or minus 2.5 mg (1 pill) per day. Predictors: VKORC1 rs9923231, CYP2C9 *2 and *3, age, sex, and drug interactions. rs9923231 is used in this algorithm since it is possibly functional.

Figure 2

The dose algorithm illustrated by nomograms. The nomograms show predicted maintenance dose for VKORC1 and CYP2C9 genotypes, sexes, and ages with the number of interacting drugs increasing warfarin effect set as 0, hence they do not account for drug interactions (Table 3).

Figure 3

Time to stable anticoagulation. (A) Lowess smoothed plot of INR values over time as a function of VKORC1 rs9923231 (−1639 G>A) genotype. (B) Lowess smoothed plot of INR values over time as a function of CYP2C9 *2 and *3 genotype.

Figure 4

Survival (Kaplan-Meier) curve showing time to first INR peak greater than 4. VKORC1 rs9923231 (−1639 G>A) and CYP2C9*3 are presented. *3 indicates a carrier of at least 1 CYP2C9*3 allele, while *1 in this case means that *3 is not present. The table beneath the graph shows the number of individuals that had not reached INR > 4 at certain time points.