US FDA approves use of genetic testing to estimate warfarin dose
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US FDA approves use of genetic testing to estimate warfarin dose The US FDA has approved the labeling changes for warfarin [Coumadin] that provide information on how people’s genetic makeup may influence their response to the drug;1 manufacturers of the generic versions of Coumadin must make similar labeling changes. Healthcare providers can now use genetic tests to estimate the initial dose of warfarin for individual patients. This may optimise drug use and may lower the risk of warfarin-related haemorrhages, says the agency. According to the FDA, warfarin is being metabolised differently in one-third of the drug’s recipients, and it has been shown that variations in the genes CYP2C9 and VKORC1 play an important role in the unexpected responses observed during warfarin treatment.1 Patients receiving higher warfarin doses are at increased risk of life-threatening haemorrhages, whereas those who receive too low a dose can develop serious blood clots. Hence, the personalised approach of using genetic tests to decide the initial warfarin dose may improve the drug’s safety and efficacy, says the agency. The updated prescribing information for warfarin includes results from a meta-analysis of nine studies, involving 2775 patients;2 the meta-analysis examined the clinical outcomes related to CYP2C9 gene variants in warfarin recipients. Patients with either the CYP2C9*2 or CYP2C9*3 alleles were found to be at an increased risk for haemorrhage. Compared with patients homozygous for the CYP2C9*1 allele, those with at least one copy of CYP2C9*2 or CYP2C9*3 alleles required 17% and 37% less mean daily warfarin doses, respectively. Another study found the risk of overanticoagulation being almost doubled (INR > 3) during the first 2 weeks of warfarin therapy for patients with CYP2C9*2 or CYP2C9*3 alleles, compared with those homozygous for *1 allele. The prescribing information also includes results from an another study showing that variations in the VKORC1 gene were associated with reduced warfarin doses. Variations in the VKORC1 gene alone were associated with about 30% of the variance in warfarin dose, and variations in the VKORC1 and CYP2C9 genes combined were associated with about 40% of the variability in warfarin dose. "Strokes and bleeds are expensive to treat", commented a senior member of FDA’s economic staff, Clark Nardinelli, who added that the estimated "annual net healthcare savings of integrating genetic testing into the decision-making process for administering warfarin therapy could be as much as $1 billion per year".3 1. FDA. FDA Approves Updated Warfarin (Coumadin) Prescribing Information. Media Release : 16 Aug 2007. Available from: URL: http://www.fda.gov. 2. Bristol-Myers Squibb Company. COUMADIN (Rm) TABLETS (warfarin sodium tablets, USP) crystalline. COUMADIN (Rm) FOR INJECTION (warfarin sodium for injection, USP). Prescribing Information : [34 pages], 16 Aug 2007. Available from: URL: http://www.fda.gov. 3. FDA. Critical path initiative: warfarin dosing. Internet Document : [4 pages], 16 Aug 2007. A
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