Warfarin Dosing in Patients With CYP2C9*5 Variant Alleles

Area of Science:

• Pharmacogenomics
• Clinical Pharmacology
• Genetics

Background:

• Pharmacogenetic dosing algorithms enhance warfarin accuracy but show limitations in African ancestry populations.
• The cytochrome P450 2C9*5 (CYP2C9*5) allele, prevalent in African ancestry, is linked to reduced warfarin clearance.
• The clinical significance of the CYP2C9*5 single-nucleotide variation (SNV) in warfarin dosing remains unclear.

Purpose of the Study:

• To quantify the association between the CYP2C9*5 allele and warfarin dose requirements.
• To assess the impact of CYP2C9*5 on the accuracy of existing pharmacogenetic warfarin dosing algorithms.
• To determine if genotyping for CYP2C9*5 can improve warfarin dosing precision in diverse populations.

Main Methods:

• A multicentered study involving 2,298 patients on warfarin.
• Quantification of CYP2C9*5 allele presence in Black (2.3%) and White (0.07%) patients.
• Comparison of predicted warfarin doses from existing algorithms versus actual requirements in CYP2C9*5 carriers and non-carriers.

Main Results:

• Existing pharmacogenetic algorithms overestimated warfarin dose by 30% in heterozygotes without accounting for CYP2C9*5.
• CYP2C9*5 heterozygotes required an average reduction of 1.87 mg/day.
• Genotyping for CYP2C9*5 corrected predicted overdoses and halved dosing errors in heterozygotes.

Conclusions:

• Patients with the CYP2C9*5 allele necessitate a clinically significant reduction in warfarin dosage.
• Incorporating CYP2C9*5 genotyping enhances the accuracy and safety of warfarin dosing, particularly for individuals of African ancestry.
• The findings support the integration of CYP2C9*5 into warfarin dosing strategies and resources like WarfarinDosing.org.