Warfarin is a narrow therapeutic index drug where CYP2C9 genotype significantly affects dose requirement. Compared to CYP2C9*1/*1 (wild-type extensive metabolisers), CYP2C9*2/*3 patients require approximately 50–70% lower warfarin doses. What additional pharmacogenomic factor is co-considered in warfarin dosing algorithms (e.g., IWPC algorithm)?

• A ABCB1 (P-glycoprotein) polymorphisms that alter warfarin absorption from the intestine
• B VKORC1 (vitamin K epoxide reductase complex 1) gene promoter polymorphisms that alter VKOR enzyme expression and target sensitivity to warfarin's pharmacodynamic effect ✓
• C CYP3A4 polymorphisms that determine conversion of warfarin to its active hydroxy-warfarin metabolites
• D Factor V Leiden mutation that renders factor V resistant to warfarin anticoagulant effect

Explanation

Warfarin pharmacogenomics requires considering both pharmacokinetic and pharmacodynamic genetic variants. CYP2C9 polymorphisms (*2, *3 alleles) reduce metabolism of S-warfarin (the more potent enantiomer), increasing its plasma levels and requiring lower doses. VKORC1 polymorphisms (especially the A-allele promoter haplotype, frequency ~40% in Europeans) reduce VKOR enzyme expression, increasing pharmacodynamic sensitivity to warfarin's target inhibition. VKORC1 1173C>T (rs9934438) or -1639G>A polymorphisms identify patients who need ~30–40% dose reduction independent of CYP2C9 status. The IWPC dosing algorithm incorporates both CYP2C9 and VKORC1 genotypes plus clinical factors to predict stable warfarin dose.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.