The risk of overanticoagulation in patients with cytochrome P450CYP2C9*2 or CYP2C9*3 alleles on acenocoumarol or phenprocoumon

Cytochrome P4502C9 (CYP2C9) is the main enzyme implicated in coumarin anticoagulant metabolism. The variant alleles CYP2C9*2 and CYP2C9*3 are associated with an increased response to warfarin. However, an effect on acenocoumarol dose requirements appears to be absent for the CYP2C9*2 allele and the consequences for the metabolism of phenprocoumon have not yet been established. We investigated CYP2C9 polymorphisms in relation to the international normalized ratio (INR) during the first 6 weeks of treatment and its effect on the maintenance dose in a cohort of 1124 patients from the Rotterdam Study who were treated with acenocoumarol or phenprocoumon. There was a statistically significant difference in first INR between patients with variant genotypes and those with the wild-type. Almost all acenocoumarol-treated patients with a variant genotype had a significantly higher mean INR and had a higher risk of an INR greater than or equal to 6.0 during the first 6 weeks of treatment. A clear genotype-dose relationship was found for acenocoumarol-treated patients. For patients on phenprocournon, no significant differences were found between variant genotypes and the wild-type genotype. Individuals with one or more CYP2C9*2 or CYP2C9*3 acenocoumarol compared to wild-type patients. Phenprocoumon appears to be a clinically useful alternative in patients carrying the CYP2C9*2 and *3 alleles. Pharmacogenetics 14:27-33 (C) 2004 Lippincott Williams Wilkins.