For several reasons, warfarin is the ideal drug to test the hypothesis that pharmacogenetics can reduce drug toxicity: it is commonly prescribed, has a narrow therapeutic/toxic ratio, and is affected by common genetic polymorphisms. Using clinical factors alone we explained 17−21% of the variability in the therapeutic warfarin dose, similar to other clinical algorithms.18
However, by including CYP2C9
genotypes, we could explain 53−54% of this variability. These encouraging results are similar to conclusions of smaller, retrospective studies of homogenous populations that found that the addition of genetic information substantially reduces the unknown intersubject variability in warfarin dose.13,19,20
These previous studies did not provide prospectively validated algorithms for individualized warfarin dosing, but made it clear that this issue had to be addressed.
In the present study, the most important factor in predicting the warfarin dose was VKORC1 3673G>A
(located 1,639 bases upstream of the translation start site of VKORC1). This polymorphism alters a VKORC1
transcription factor binding site15
and haplotypes associated with the A allele result in lower VKORC1
mRNA expression in human liver.12
This change in gene expression likely lowers the steady-state concentration of tissue vitamin K epoxide reductase, the rate-limiting enzyme in the vitamin K cycle.21
The clinical effect was that individuals with this allele were more susceptible to inhibition by warfarin, resulting in a 28% decrease in the therapeutic warfarin dose per allele. Because of this decrease and the prevalence of the VKORC1 3673G>A
(), this SNP was the most important predictor of dose at the time of warfarin initiation. We confirmed that this SNP is present at a low frequency in African-American patients and intermediate frequency in Caucasian patients, consistent with previous work.12,16,22
Genotyping of this single informative VKORC1
SNP should simplify clinical genetic testing and implementation of pharmacogenetics-based therapy.
allele predicted 19 and 33% reductions (respectively) in the therapeutic dose of warfarin. These alleles correlate with slower clearance of the S-warfarin enantiomer,19,23
a longer drug half-life, and a delay until stable dosing.1,22
To ameliorate this delay, we gave a slightly greater first dose in poor warfarin metabolizers; we did not reduce their initial dose despite the presence of CYP2C9*2 and/or CYP2C9*3.
In contrast to previous single-center studies of Caucasian patients,13,19,20
we genotyped a diverse, multicenter cohort. To further enhance generalizability, we included patients from multiple anticoagulation centers regardless of their comorbid conditions, target INR, and concomitant therapy. To improve clinical usefulness, we focused on the effects of SNPs that were suspected to be clinically relevant. One interesting result was the inclusion of the VKORC1 3673
SNP as the only significant VKORC1
SNP in predicting warfarin dose. An important strength of the current study is that we prospectively validated pharmacogenetics-based warfarin therapy, thereby demonstrating its feasibility and safety.
All patients in the validation cohort received pharmacogenetic therapy without comparison to patient management vianongenetic algorithms. In the future, other genetic variants may strengthen the predictive power of pharmacogenetic warfarin therapy. For example, one study showed significant associations between SNPs in factor VII and in γ−glutamyl carboxylase and warfarin dose,24
but this association has not been replicated in a subset of our cohort (B.F. Gage and C. Eby unpublished data) or in other cohorts.20
The factor II Thr165Met SNP, previously associated with lower warfarin dose in studies of 147 Italian24
and 45 Japanese patients,25
was of borderline significance in our univariate analysis and not an independent predictor of warfarin dose. In keeping with our clinical focus, we did not consider additional SNPs26,27
that are too rare to significantly improve dosing accuracy in the general population.
This study demonstrates the feasibility of pharmacogenetic warfarin therapy and supports the need for a multicenter, randomized control trial quantifying its effect on laboratory and clinical outcomes. Several companies have developed genotyping platforms to facilitate pharmacogenetic dosing, and at least two have received Food and Drug Administration approval. The Food and Drug Administration has also changed the Coumadin/warfarin labeling to encourage lower initial doses in patients who have the VKORC1 3673G>A, CYP2C9*2, or CYP2C9*3 allele.
Two recent trials have randomized patients to pharmacogenetic vs. clinically-based warfarin initiation but had conflicting findings. Caraco and colleagues at Hadassah University found that CYP2C9 genotyping improved INR control and reduced minor (but not major) bleeding compared to dosing by a computerized clinical algorithm.28
In the other randomized trial, Anderson and colleagues in Utah found no significant benefit of pharmacogenetic vs. clinically dosed warfarin.29
The studies were similarly sized—each having ~100 patients per treatment arm. The Anderson-dosing algorithm had an R2
of 47%; Caraco did not report the accuracy of their algorithm, but a similar CYP2C9
-dosing algorithm had an R2
Given these findings, a multicenter randomized and controlled trial is needed to quantify how pharmacogenetic therapy affects INR control and clinical adverse events.
To facilitate a trial and the use of the dosing algorithms in this report, we have developed a nonprofit website, http://www.WarfarinDosing.org
. While the genotyping is pending, WarfarinDosing.org estimates the warfarin dose using the clinical algorithm in this report. Once genotype is available, WarfarinDosing.org uses the more accurate pharmacogenetic algorithm (). If an INR is available after three doses, WarfarinDosing.org uses a pharmacogenetic algorithm and the INR response to estimate the therapeutic dose (R2
A randomized controlled trial should test the hypothesis that pharmacogenetic dosing improves laboratory and clinical outcomes in patients beginning warfarin for a variety of indications. The National Institutes of Health has announced their intention to fund such a trial via RFP-NHLBI-HV-08−03.