Warfarin Sensitivity

Indications for Testing

Not enough evidence to recommend widespread adoption of genotyping
Most useful in warfarin-naive patients but may be useful for patients with previous history of difficulty with anticoagulation
- May also be useful with challenging patients, particularly those who might be placed on warfarin prophylactically after surgery (eg, those requiring <21 mg or >49 mg per week to maintain therapeutic INR)
  - Includes nearly half of patients

Laboratory Testing

CYP2C9 and VKORC1 genotyping – for clinical dosing

Many algorithms and models for dosing are available
- Algorithms predict maintenance dose but do not necessarily consider pharmacokinetic differences that may influence the time required to achieve a steady state
- Algorithms typically offer no guidance regarding dosing intervals or indicate when interpretation of an international normalized ratio (INR) result is appropriate
- Dose revision algorithms after INR response is observed are also available (Lenzini, 2010)
See warfarin dosing calculator for therapeutic dose estimates – other models for dosing are also available
Dosing equation example from Sconce, 2005
- Dose (mg/d) = [0.628 – 0.0135 (age in years) – 0.240 (a) – 0.370 (b) – 0.241 (c) + 0.0162 (height in cm)]²
- a = CYP2C9*2, input 0, 1, 2 based on number of alleles
- b = CYP2C9*3, input 0, 1, 2 based on number of alleles
- c = VKORC1, input 1 for GG, 2 for GA, 3 for AA
2010 revision of the Coumadin^® label includes expected maintenance dosing based on genotype

Range of expected therapeutic warfarin doses based on CYP2C9 and VKORC1 genotypes

*Range of Expected Therapeutic Warfarin Doses Based on CYP2C9* and VKORC1 Genotypes^†**
VKORC1	CYP2C9
VKORC1	1/1	1/2	1/3	2/2	2/3	3/3
GG	5-7 mg	5-7 mg	3-4 mg	3-4 mg	3-4 mg	0.5-2 mg
AG	5-7 mg	3-4 mg	3-4 mg	3-4 mg	0.5-2 mg	0.5-2 mg
AA	3-4 mg	3-4 mg	0.5-2 mg	0.5-2 mg	0.5-2 mg	0.5-2 mg
^†Ranges are derived from multiple published clinical studies. Other clinical factors (eg, age, race, body weight, sex, concomitant medications, and comorbidities) are generally accounted for along with genotype in the ranges expressed in the table. VKORC1 -1639 G→A (rs9923231) variant is used in this table. Other co-inherited VKORC1 variants may also be important determinants of warfarin dose. Patients with CYP2C9 1/3, 2/2, 2/3 and 3/3 may require more prolonged time (>2-4 weeks) to achieve maximum INR effect for a given dosage regimen.

When dosing, also consider warfarin drug interactions
- Cytochrome P450 drug interaction table
Pharmacogenetic testing does not replace the need to monitor warfarin therapy through INR testing

Warfarin (Coumadin^®) is one of the most widely used anticoagulants worldwide, with potentially severe hemorrhagic or thrombotic consequences if dosed incorrectly. The labeling for a popular formulation of warfarin was revised in August 2007 and February 2010 to include information about how the three most widely studied pharmacogenetic mutations (CYP2C9*2, CYP2C9*3, and VKORC1 -1639G>A) affect dose requirements (NDA 9-218/S-105). These mutations account for as much as 45% of variation in warfarin response in Caucasians and 30% in African Americans.

Epidemiology

Incidence
- 2 million new anticoagulant-naive patients placed on warfarin annually in U.S.
- CYP2C9*2 – Caucasians 8-19%, Asians <0-4%, African Americans 0-12%
- CYP2C9*3 – Caucasians 5-16%, Asians 1-8%, African Americans 0-6%
- VKORC1 sensitivity haplotypes, predicted by the -1639G>A mutation – Caucasians 37%, Asians 89%, African Americans 14%

Risk Factors

CYP2C9 mutations reduce warfarin clearance
- *2 allele (430C>T)
  - Reduces metabolism of S-warfarin by ~30%
  - Extends half-life – requires a longer time to achieve steady state
  - Average daily warfarin requirement reduced (see table in Diagnosis tab)
- *3 allele (1075A>C)
  - Reduces metabolism of S-warfarin by ~80%
  - Extends half-life – requires a longer time to achieve steady state
  - Average daily warfarin requirement reduced more than with the CYP2C9*2 allele (see table in Diagnosis tab)
VKORC1 mutations affect response to warfarin
- Many mutations are known and exist in linkage disequilibrium
- Genotyping to detect a common promoter mutation (-1639G>A) predicts warfarin sensitivity haplotypes
- Average daily warfarin requirement reduced (see table in Diagnosis tab)
Combinations of mutations from one or more genes further reduce the average daily warfarin requirement

Pathophysiology

Warfarin prescribed for the following
- Atrial fibrillation
  - Patients in therapeutic range ~55% of the time
- Venous thromboembolism
- Mechanical heart valves
- Postoperative prophylaxis
Warfarin inhibits production of vitamin K-dependent coagulation factors through inhibition of vitamin K epoxide reductase (VKOR)
- Response to warfarin therapy routinely assessed through prothrombin time/international normalized ratio (INR) testing
Warfarin is a racemic mixture of R and S enantiomers
- S is 3-5 times more potent than R
- S-warfarin is metabolized primarily by CYP2C9
- Warfarin has a narrow therapeutic index that is influenced by a variety of drugs and illnesses
Pharmacogenetics affect dosing
- CYP2C9 genotype accounts for as much as 18% of the variability in warfarin dosing
- VKORC1 genotype accounts for as much as 29% of the variability in warfarin dosing
- Combining genotypes with clinical factors may account for 50-70% of variability in warfarin dosing

Tests generally appear in the order most useful for common clinical situations

Test name: Warfarin Sensitivity (CYP2C9 and VKORC1 ) 3 Mutations

ARUP #: 0051370

Methodology: Polymerase Chain Reaction/DNA Hybridization/Electrochemical Detection

Use:

Consider ordering in warfarin-naive patients or patients with a history of difficulty with anticoagulation in order to confirm genotype affecting warfarin metabolism

Clinical sensitivity - 90% in Caucasians; detection rate in other ethnicities is reduced

Limitations:

Only the two common CYP2C9 and one common VKORC1 mutations are detected

Other mutations that may impact warfarin sensitivity or resistance in these genes or other genes are not detected

Rare mutations within primer or probe regions may interfere with the assay

Test name: Cytochrome P450 2C9 (CYP2C9) 2 Mutations

ARUP #: 0051103

Methodology: Polymerase Chain Reaction/DNA Hybridization/Electrochemical Detection

Use:

Consider ordering in warfarin-naive patients or patients with a history of difficulty with anti-coagulation in order to confirm genotype affecting warfarin metabolism

Clinical sensitivity - 90% in Caucasians; not well characterized in other ethnicities

Limitations:

Only the two common CYP2C9 mutations are detected

Additional mutations in this or other genes are not detected

Mutation detection is not a substitute for therapeutic drug or other clinical monitoring

Nongenetic factors may also affect drug metabolism

Test name: Prothrombin Time/International Normalized Ratio

ARUP #: 0030224

Methodology: Electromagnetic Mechanical Clot Detection

Use:

Monitor warfarin therapy

Test name: Warfarin Genotyping Plus

ARUP #: 2004358

Methodology: Polymerase Chain Reaction/DNA Hybridization/Electrochemical Detection

Use:

Consider ordering in warfarin-naive patients or patients with a history of difficulty with anticoagulation in order to confirm genotype affecting warfarin metabolism

Test algorithm generates customized patient report that provides clinician with estimated daily and weekly maintenance warfarin doses

Additional Tests Available

Test name: Warfarin, Urine

ARUP #: 0091230

Methodology: High Performance Liquid Chromatography

Comments:

Test name: Warfarin, Serum or Plasma

ARUP #: 0090805

Methodology: High Performance Liquid Chromatography

Comments: