Gualberto Ruaño, M.D., Ph.D., Greg Makowski, Ph.D., Bruce Bower, M.D.

Warfarin management presents a clinical conundrum. There is a broad range of warfarin maintenance doses within the population with individual patients requiring from 1 mg/day to 10 mg/day to obtain the same benefit. Despite early dose titration and subsequent periodic monitoring by Prothrombin Time (PT) and International Normalized Ratio (INR) hemorrhagic complications are frequent and can be fatal. They lead to protracted management and great risk to patient safety. The warfarin label now carries a “Black Box” warning for “major or fatal bleeding” caused by the drug. So far, most standard of care protocols for warfarin therapy consider height, weight, gender, co-medications and diet for dose adjustments, but not DNA. These clinical factors account for at most ~20% of the variability in dosing.

DNA-GUIDED WARFARIN DOSE

Warfarin is metabolized to inactive metabolites by the CYP2C9 isoenzyme of the Cytochrome P450 system. About 25% of the population is a carrier of one functionally deficient CYP2C9 DNA variant and 3% are carriers of two. Functionally deficient CYP2C9 variants lead to higher incidence of supra-therapeutic INR values (>4), delays in achieving a stable maintenance dose, and increased bleeding complications. CYP2C9 polymorphism accounts for ~25% of the overall variability in warfarin dosing needs.

Warfarin exerts its anticoagulant effect through its inhibition of Vitamin K Epoxide Reductase (VKOR). VKOR reduces Vitamin K, which is necessary for activation of Clotting Factors II, VII, IX and X. VKOR is coded by the Vitamin K Epoxide Reductase Complex, subunit 1 (VKORC1) gene. DNA variants of the VKORC1 gene decrease the sensitivity of VKOR to warfarin. About 75% of the population carries a DNA variant which reduces expression of the VKORC1 gene and accounts for an additional ~25% of clinical variance in warfarin dosage.

CYP2C9 and VKORC1 are independently segregating genes whose protein products act at two different sites of warfarin biochemistry. Their individual contributions when added yield at least ~50% prediction of warfarin variability by DNA Typing. DNA adds significant predictive power to warfarin therapeutic algorithms: the combined predictive power of clinical factors and DNA is ~70%.

TECHNICAL INFORMATION

The HILOmet WARFARIN system performs high-resolution DNA Typing to individualize warfarin management. It is revolutionary for clinical practice because by means of DNA Typing, the innate metabolic capacity and drug sensitivity of the patient relevant to warfarin can be predicted and diagnosed simply from a blood sample. With the HILOmet WARFARIN system, patients with greatly compromised metabolism of warfarin or with reduced sensitivity to the drug can be identified to avoid clinical complications and side effects. The System determines an individual’s warfarin metabolic capacity and sensitivity by DNA Typing simultaneously the CYP2C9 and VKORC1 genes, at 12 variable sites. DNA Typing results and derived dosage guidelines are reported.

DNA Typing is performed at the Laboratory of Personalized Health (LPH), a division of Genomas Inc. (Hartford CT). LPH is a high-complexity clinical DNA testing center licensed by the Connecticut Department of Health (CL-0644) and certified by the Centers for Medicare and Medicaid Services (ID# 07D1036625) under CLIA (Clinical Laboratory Improvement Amendments). The HILOmet WARFARIN DNA Typing system is available through the Clinical Laboratory Partners network throughout Connecticut.

DNA FOR WARFARIN PER FDA

The FDA Clinical Pharmacology Advisory Subcommittee in November 2005 recommended revising the warfarin label to include DNA Typing of both CYP2C9 and VKORC1 based on evidence that lower doses are needed for patients with certain gene variants. A label revision with both genes is expected in 2007. Healthcare will be revolutionized by DNA-guided medicine in clinical practice.

ORDERING HILOmet WARFARIN

Test name: HILOmet WARFARIN,

CLP Test Code: 70004

Specimen Requirements:

3 ml whole blood in a EDTA lavender topped tube. Fasting is not necessary.

Order forms are available from the website of the Laboratory of Personalized Health (LPH) at www.genomas.net/LPH . The test is available through Clinical Laboratory Partners.  A listing of the Clinical Laboratory Partners Patient Service Centers is on the LPH website or at www.clpct.com. Clinical Laboratory Partners can be reached at 1-800-286-9800.

REFERENCES

  1. Hampton T. Researchers Draft Guidelines for Clinical Use of Pharmacogenomics. Journal of the American Medical Association 296: 1453-4, 2006
  2. Higashi MK, Veenstra DL, Kondo LM, et al. Association between CYP2C9 genetic variants and anti-coagulation-related outcomes during warfarin therapy. Journal of the American Medical Association 287: 1690–8, 2002
  3. Ruaño G, Thompson PD, et al. High carrier prevalence of deficient and null alleles of CYP2 genes in a major USA hospital: Implications for personalized drug safety. Personalized Medicine 3: 131-7, 2006
  4. Ruaño G, Bower B. The Pharmacogenomics of Warfarin (Coumadin) Administration. Connecticut Medicine, 70 (4), 251-252, 2006
  5. Sconce EA, Kahn TI, Wynne HA, et al. The Impact of CYP2C9 and VKORC1 genetic polymorphism and patient characteristics upon warfarin dose requirements: Proposal for a new dosing regimen. Blood 106: 2329–33, 2005.