Early ReportAssociation of polymorphisms in the cytochrome P450 CYP2C9 with warfarin dose requirement and risk of bleeding complications
Introduction
Warfarin is the oral anticoagulant of choice in the UK and many other countries and is being prescribed to an increasing number of patients. The risk of serious haemorrhage during warfarin therapy ranges from 1·3 to 4·2 per 100 patient years of exposure.1, 2 Haemorrhage incidence is associated with the intensity of anticoagulation, with the deviation in prothrombin time ratio shown to be the strongest risk factor for bleeding complications.3
Widespread interindividual variation in the response to a given dose of warfarin makes the prediction of an accurate maintenance dose difficult, with an effective daily dose ranging from 0·5 mg to 60 mg.4, 5 Standardised induction regimens, with monitoring of International Normalised Ratio (INR) over the first 4 days, have only a 69% success rate in predicting the correct maintenance dose,6 and, although complex normograms and computer programs requiring protein C and S estimations have been shown to improve the prediction they are not suitable for routine use.7 An understanding of the genetics of warfarin metabolism and the identification of genetic factors that contribute to the individuality of the warfarin dose response may help to ameliorate clinical difficulties associated with warfarin therapy.
The asymmetric carbon of warfarin (C9) gives rise to two enantiomeric forms, R-warfarin and S-warfarin, which are differentially metabolised. When administered as a racemate, S-warfarin is about three times as potent as R-warfarin.8 The cytochrome P450 CYP2C9 is the principal enzyme that catalyses the conversion of S-warfarin to inactive 6-hydroxy and 7-hydroxy metabolites whereas the oxidative metabolism of R-warfarin is mainly catalysed by CYP1A2 and CYP3A4.9 The existence of genetic polymorphisms in CYP2C9 giving rise to functionally significant effects on enzyme activity is now well established. In addition to the wild-type (CYP2C9*1) allele, point mutations in the CYP2C9 gene result in two allelic variants—CYP2C9*2, where cysteine substitutes for arginine at aminoacid 144, and CYP2C9*3, where leucine substitutes for isoleucine at residue 359.10 Both allelic variants have impaired hydroxylation of S-warfarin when expressed in vitro.11, 12 The CYP2C9*3 variant is less than 5% as efficient as the wild-type enzyme, while CYP2C9*2 shows about 12% of wild-type activity, apparently as a result of the aminoacid substitution altering the interaction of the enzyme with cytochrome P450 oxidoreductase.11, 12, 13 Because these in-vitro data and some preliminary in-vivo data7, 14 have suggested that the known polymorphisms in CYP2C9 significantly impair S-warfarin metabolism, we have now investigated whether either CYP2C9 allelic variant was more common in patients with a lower than average warfarin dose requirement and have also examined whether the incidence of bleeding complications is associated with CYP2C9 genotype.
Section snippets
Methods
The investigation was approved by the Newcastle upon Tyne Joint Ethics Committee and all the participants gave informed consent.
Participants
Patients whose warfarin dose requirement was 1·5 mg per day or less, had a stable warfarin dose requirement for at least three consecutive clinic visits with a target INR of 2·0 to 3·0, and no apparent cause for low-dose requirement (eg, drug interactions or liver disease) were recruited from an anticoagulation clinic in north-east England. Of a total of
Results
CYP2C9 genotype distributions in the low-dose warfarin group compared with the clinic controls and the community controls are summarised in table 1. In the low-dose warfarin group, 29 (81%) of the 36 patients had one or more of the variant alleles present compared with 40 (40%) of 100 in the control group. The odds ratio for individuals with a low warfarin dose requirement having one or more CYP2C9 variant alleles compared with the normal population was 6·21 (95% CI 2·48–15·6). When possession
Discussion
In this study we show that there is a strong association between CYP2C9 genotype and warfarin sensitivity. An individual requiring a low warfarin dose is six times more likely to be positive for one or more of the variant alleles associated with impaired S-warfarin metabolism (CYP2C9*2 and CYP2C9*3) compared with the general population. CYP2C9 genotyping appears to have the potential to identify a subgroup of individuals who are poor metabolisers of warfarin, hence they require a very low dose
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