Initiation of warfarin therapy using trial-and-error dosing is problematic. our goal was to develop and validate a pharmacogenetic algorithm. in the derivation cohort of 1,015 participants, the independent predictors of therapeutic dose were: VKORC1 polymorphism −1639/3673 g>a (−28% per allele), body surface area (Bsa) (+11% per 0.25 m 2 ), CYP2C9*3 (−33% per allele), CYP2C9*2 (−19% per allele), age (−7% per decade), target international normalized ratio (inr) (+11% per 0.5 unit increase), amiodarone use (−22%), smoker status (+10%), race (−9%), and current thrombosis (+7%). This pharmacogenetic equation explained 53−54% of the variability in the warfarin dose in the derivation and validation (N = 292) cohorts. For comparison, a clinical equation explained only 17−22% of the dose variability (P < 0.001). in the validation cohort, we prospectively used the pharmacogenetic-dosing algorithm in patients initiating warfarin therapy, two of whom had a major hemorrhage. To facilitate use of these pharmacogenetic and clinical algorithms, we developed a nonprofit website, http://www.WarfarinDosing.org.Correspondence: BF Gage (E-mail: bgage@im.wustl.edu). CONFLICT OF INTEREST Dr Gage has consulted for Bristol-Myers Squibb on work unrelated to this article. Drs Rieder and Rettie report having applied for a patent (application serial no. 10/967,879) on the use of VKORC1 haplotypes and SNPs. The other authors declared no conflict of interest. NIH Public Access RESULTSIn the derivation cohort (N = 1,015), the daily therapeutic warfarin dose ranged from 1 to 18 mg/day. The mean age was 65 (range of 18−93); 83% were Caucasian, and 64% were male. The (geometric) mean daily warfarin dose was 4.8 mg ( Table 1). The most common indications for warfarin therapy were atrial fibrillation (N = 392) and prior venous thromboembolism (N = 376; 13 of whom also had atrial fibrillation). Patients in the validation cohort (N = 292) were younger, more often female, and had more often (77%) undergone joint replacement as their indication for warfarin therapy (Table 1).VKORC1 alleles were highly heterogeneous (Table 2), reflecting their original selection as common (>5% allele frequency), informative tagging SNPs (Table 2). 12 VKORC1 3673G>A was in high linkage disequilibrium with VKORC1 6853G>C (D' = 0.97). In both cohorts, all alleles were in Hardy-Weinberg equilibrium. Genotype data from all participants at Washington University and University of Florida have been submitted to the PharmGKB (accession numbers: PS207479 and PS207480 pending). Pharmacogenetic model developmentThe VKORC1 3673G>A SNP was the first variable to enter the stepwise regression model (Table 3); each VKORC1 3673A allele was associated with a 28% reduction (95% confidence interval 25−30%) in the therapeutic warfarin dose. Once VKORC1 3673G>A entered the model, none of the other VKORC1 SNPs was an independent predictor of warfarin dose. Body surface area (BSA) was the second variable to enter the model, and each 0.25 m 2 increase in BSA was associated with an 11% ...

Background Initiation of warfarin therapy using trial-and-error dosing can cause bleeding. Clinical factors explain only 20%–30% of the variability in the therapeutic dose of warfarin. Single nucleotide polymorphisms (SNPs) in the cytochrome P450 2C9 (CYP2C9) gene correlate with the clearance of S-warfarin and SNPs in the vitamin K epoxide reductase (VKORC1) gene predict warfarin sensitivity. We test the hypothesis that the combination of clinical and pharmacogenetic information can predict the therapeutic warfarin dose. Methods We collected DNA, demographic variables, laboratory values, and medication histories from patients taking warfarin. Subjects either attended an outpatient anticoagulation clinic or participated in the PREVENT (prevention of venous thromboembolism) study. After PCR amplification, we used Pyrosequencing® to genotype DNA regions for 2 coding CYP2C9 SNPs, *2 (C430T) and *3 (A1075C), and for 4 noncoding VKORC1 SNPs: C861A, A5808C, G6853C, and G9041A. Using multiple regression, we quantified the association between therapeutic warfarin dose and clinical and genetic factors in a derivation cohort of 900 participants and a validation cohort of 100 participants. Results The VKORC1 G6853C SNP was the first variable to enter the stepwise regression equation and was associated with a 27% decrease in the warfarin dose per allele in Caucasian patients. The VKORC1 A5808C SNP was associated with a 33% decrease per allele in warfarin dose in African-American patients. Other significant (p &lt; 0.05) predictors of the therapeutic warfarin dose, in order of entry into the regression equation and their effect on warfarin dose were: body surface area (+12% per SD increase), CYP2C9*3 (−33% per allele), CYP2C9*2 (−20% per allele), age (−7% per decade), target INR (+8% per 0.5 unit increase), amiodarone use (−24%), African-American race (+12%), smoker (+9%), and simvastatin or fluvastatin use (−5%). A dosing equation that included these pharmacogenetic and clinical factors explained 52% of the dose variability in derivation cohort and 55% of the variability in the validation cohort. Conclusions The therapeutic warfarin dose can be estimated from clinical and pharmacogenetic factors that can be obtained when warfarin is started. Use of this dosing equation has potential to aid in the prediction of an optimal warfarin dose, which may decrease the risk of bleeding during the initiation of warfarin therapy.