Effects of CYP2C9 and VKORC1 on INR variations and dose requirements during initial phase of anticoagulant therapy.

INTRODUCTION: Anticoagulants of the coumarin type are effective drugs for the treatment and prevention of thromboembolic diseases. However, they have a narrow therapeutic range and show inter- and intra-individual variability in dose requirement, largely conditioned by both environmental and genetic factors.
METHODS: This prospective study investigated, during the initial phase of acenocoumarol therapy, the effect of CYP2C9 variant alleles and VKORC1 haplotypes, single and in combination, in 220 Italians.
RESULTS: CYP2C9*3 was associated with a 25% dose reduction and an increased risk of over-anticoagulation (International Normalized Ratio [INR] > 6) on day 4. Two copies of the VKORC1*2 haplotype were associated with a 45% dose reduction and an increased risk of over-anticoagulation. Homozygosity for VKORC1*3 and VKORC1*4 was associated with an increased dose requirement and a reduced risk of over-anticoagulation. The VKORC1*3 or *4 plus CYP2C9*1 genotype combination was associated with the highest dose requirement and the lowest INR on day 4; VKORC1*2 plus CYP2C9*3 was associated with the lowest dose requirement, the highest INR and an increased risk of over-anticoagulation. Even though they spent approximately 50% of the time within the target therapeutic range, VKORC1*3 or *4 plus CYP2C9*1 carriers spent a large percentage of the remaining time below and carriers of VKORC1*2 plus CYP2C9*3 above the target range. DISCUSSION: The determination of VKORC1*3 and VKORC1*4 haplotypes may be an important addition to CYP2C9 and VKORC1*2 genotyping to identify patients at risk of being outside the target range during initial anticoagulation with acenocoumarol.