MedGen

Warfarin is an oral anti-coagulant used world-wide to treat and prevent thrombotic disorders. While it is highly effective, it has a very narrow therapeutic index making it difficult to dose correctly. Genetic variants in cytochrome P450-2C9 (CYP2C9), vitamin K-epoxide reductase complex (VKORC1), cytochrome P450-CYP4F2 (CYP4F2) and the CYP2C cluster (eg. rs12777823), along with non-genetic factors, are known to affect warfarin dose variability. Patients with specific variants in the gene CYP2C9 (the primary warfarin-metabolizing enzyme) may require a lower dose of warfarin as compared to patients without these variants. Patients with specific variants in VKORC1 (the target enzyme of warfarin) may require a lower warfarin dose as compared to patients who do not have these variants. The combination of CYP2C9/VKORC1/CYP4F2/CYP2C genetic variants, along with clinical factors, can put some patients at risk for therapeutic failure or adverse events such as bleeding. Guidelines regarding the use of pharmacogenomic tests in dosing for warfarin have been published in Clinical Pharmacology and Therapeutics by the Clinical Pharmacogenetics Implementation Consortium (CPIC) and are available on the CPIC and PharmGKB websites. [from PharmGKB]