Low dose requirement for warfarin treatment in a patient with CYP2C9*3/*13 genotype

Cytochrome P450 2C9 (CYP2C9) is the major isoform of the CYP2C subfamily, and is involved in the metabolism of many clinically important therapeutic agents. Here we describe a patient who was intolerant of warfarin treatment because of impaired drug metabolism related to the CYP2C9*3/*13 genotype and high warfarin sensitivity associated with the VKORC1 1173TT genotype.

A 64-year-old Korean man had taken warfarin for treatment of an occluded left subclavian artery and atrial fibrillation. Although the warfarin doses prescribed were not high (14 mg/week) during the induction of anticoagulation, the prothrombin time (PT) was elevated to over 120 s after two weeks of warfarin therapy. For this patient, a very low dose of warfarin (4 mg/week) was required in order to achieve target INR values.

To the best of our knowledge, this is the first report of a patient with CYP2C9*3/*13 and VKORC1 1173TT genotypes who had slower than normal warfarin metabolism, resulting in the need to administer an extremely low dose of warfarin in order to achieve the target INR value. Our report reinforces the relevance of pharmacogenetic testing to explain warfarin dose variability and to enable individualized dosage adjustments for improved warfarin treatment outcomes.