Warfarin Metabolism, continued

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The page below is a sample from the LabCE course Pharmacology in the Clinical Lab: Therapeutic Drug Monitoring and Pharmacogenomics. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Warfarin Metabolism, continued

The genes involved in warfarin metabolism are CYP2C9 and vitamin K epoxide reductase complex subunit 1 (VKOR).
Warfarin owes its anticoagulant action to its inhibition of VKOR. This enzyme recycles vitamin K, a critical element for the clotting factors II, VII, IX, and X, as well as for proteins C, S, and Z.
There are six CYP2C9 alleles that are known to cause prolonged metabolism of warfarin: CYP2C9 *2, *3, *4, *5, *6, and *11. (Polymorphisms in CYP450 genes are denoted with asterisks.)
One-third of the patients who receive warfarin metabolize it differently than expected and experience a higher risk of bleeding.
Genetic testing for the two most common polymorphisms (CYP2C9*2 and *3) as well as for VKOR may be able to reduce the variability associated with warfarin dosing response.
Labs performing PGx testing can provide general warfarin dosing recommendations based on the patient's genotype analysis. The lab report will indicate whether a patient has a normal, mild, moderate, high, or very high sensitivity to warfarin. Online calculators or handy paper charts can then help physicians calculate an appropriate starting and maintenance dose given the phenotype of a particular person.
For example, a patient who has one CYP2C9 normal wild-type allele (CYP2C9 *1), one polymorphism (CYP2C9*3), and also a VKOR polymorphism is predicted to have a moderate sensitivity to warfarin. This patient should have frequent INR monitoring and possible warfarin dose reduction.
It is important to recognize that knowing a genotype does not necessarily guarantee accurate dose prediction; other drugs and/or environmental or disease factors can also alter CYP2C9 activity. Therefore, monitoring the INR is still very important.