Case report: dose adjustment of warfarin using genetic information and plasma concentration monitoring.

J Clin Pharm Ther

Center for Pharmacy Practice, Faculty of Pharmacy, Keio University, Tokyo, Japan; Department of Pharmacy, Gunma University Hospital, Maebashi, Japan; Department of Clinical Pharmacology, Gunma University Graduate School of Medicine, Maebashi, Japan.

Published: June 2014

What Is Known And Objective: Carbamazepine is known to interact with warfarin. We report on a case of this interaction and on its management using the patient's genetic information.

Case Summary: The case concerns a 74-year-old Japanese woman with a mood disorder and a central retinal vein occlusion. She was on therapy that included carbamazepine and had started to take warfarin. However, the patient's prothrombin time expressed as the international normalized ratio (PT-INR) was 1·40 despite taking a dose three times higher than the average. The patient's S-warfarin concentration was 0·15 μg/mL and R-warfarin was 0·52 μg/mL. Her cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase complex, subunit 1 (VKORC1), genotypes were *1/*1 and -1639GA, respectively. The VKORC1 genotype indicated that she would require an even higher dose. We proposed a further increase in dose and the patient's PT-INR rose to 1·99.

What Is New And Conclusion: The patient required a high warfarin dose because of the VKORC1 genotype, and induction of CYP2C9 by carbamazepine. We improved the patient's pharmacotherapy based on her genetic information.

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Source
http://dx.doi.org/10.1111/jcpt.12135DOI Listing

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