Genetic association between sensitivity to warfarin and expression of CYP2C9*3

Area of Science:

• Pharmacogenomics
• Drug Metabolism
• Enzyme Kinetics

Background:

• Cytochrome P450 2C9 (CYP2C9) is crucial for warfarin metabolism, primarily metabolizing the active S-enantiomer.
• Genetic mutations in CYP2C9 lead to allelic variants (CYP2C9*1, *2, *3) with altered enzyme activity.
• CYP2C9*2 and CYP2C9*3 variants exhibit reduced catalytic efficiency compared to wild-type.

Purpose of the Study:

• Investigate the impact of CYP2C9*3 homozygosity on warfarin sensitivity in a patient.
• Evaluate the diagnostic utility of plasma and urinary warfarin enantiomer ratios as alternatives to genetic testing.

Main Methods:

• Genotyping of a warfarin-sensitive patient for CYP2C9 gene mutations (exons 3 and 7).
• PCR-amplification, restriction digest, and sequence analysis for genotyping.
• Chiral-phase high-performance liquid chromatography (HPLC) to determine plasma and urinary warfarin enantiomer ratios.

Main Results:

• The patient was homozygous for the CYP2C9*3 variant, explaining extreme warfarin sensitivity (0.5 mg/day tolerance).
• Patient's plasma S:R warfarin ratio was significantly elevated (3.9:1) compared to controls (0.50±0.25:1).
• Urinary 7-hydroxywarfarin S:R ratio in the patient mirrored control ratios, indicating similar stereoselectivity.

Conclusions:

• CYP2C9*3 expression is linked to impaired S-warfarin clearance and exaggerated response to warfarin.
• Elevated plasma S:R warfarin ratio can serve as a valuable diagnostic indicator for the CYP2C9*3 genetic defect.
• Plasma enantiomer ratio analysis offers a potential alternative to genotyping for identifying patients at risk of warfarin toxicity.