Warfarin Therapy and the Genotypes CYP2C9 and VKORC1

Dean L.

Publication Details

Introduction

Warfarin is an anticoagulant that acts by reducing the activity of vitamin K-dependent clotting factors. It is used in the prevention and treatment of thrombotic disorders. The dose of warfarin must be tailored for each patient according to the patient’s INR response and the condition being treated.

A patient’s CYP2C9 and VKORC1 genotype can be used to help determine the optimal starting dose of warfarin. The CYP2C9 gene encodes one of the main enzymes involved in the metabolism of warfarin. Several variant CYP2C9 alleles are associated with reduced enzyme activity and lower clearance rates of warfarin. Patients who carry at least one copy of such a variant allele (such as CYP2C9*2 and CYP2C9*3) have reduced metabolism leading to higher warfarin concentrations. On average, they require a lower daily warfarin dose than patients who are homozygous for the wild-type CYP2C9*1 allele.

The VKORC1 gene encodes the vitamin K epoxide reductase enzyme, the target of warfarin. Patients who carry the -1639G>A polymorphism in the promoter region of the VKORC1 gene are more sensitive to warfarin and require lower doses.

The FDA-approved warfarin drug label provides a dosing table based on CYP2C9 and VKORC1 genotypes (Table 1). The label states if the patient’s CYP2C9 and/or VKORC1 genotype are known, to consider these ranges in choosing the initial doses, but whether this strategy reduces warfarin-related adverse events is controversial. The label also states that patients with CYP2C9 *1/*3, *2/*2, *2/*3, and *3/*3 may require more time (longer than 2 to 4 weeks) to achieve maximum INR effect for a given dosage regimen than patients without these CYP variants (1).

However, the Clinical Pharmacogenetics Implementation Consortium (CPIC) recommends that this dosing table should only be used when electronic access is not possible. Instead, CPIC recommends that whenever possible, the pharmacogenetic algorithms available on http://www.warfarindosing.org should be used to predict the optimal warfarin dose (2). Although one randomized trial found that genotype-guided dosing might improve INR control after warfarin initiation (3), the largest completed trial found no benefit. (4). The largest trial of pharmacogenetic dosing of warfarin (ClinicalTrials.gov Identifier: NCT01006733) is expected to have results in December 2016.

Table 1. . Three Ranges of Expected Maintenance Warfarin Doses based on CYP2C9 and VKORC1 Genotypes, adapted from the FDA drug label.

Table 1.

Three Ranges of Expected Maintenance Warfarin Doses based on CYP2C9 and VKORC1 Genotypes, adapted from the FDA drug label.

Drug: Warfarin

Warfarin is an anticoagulant used in the prevention and treatment of venous thrombosis, pulmonary embolism, and the complications associated with atrial fibrillation and/or cardiac valve replacement. Warfarin is sometimes prescribed to reduce the risk of stroke after a myocardial infarction (MI).

Warfarin has no direct effect on an established thrombus. However, once a thrombus has occurred (e.g., deep venous thrombosis), the goal of warfarin therapy is to prevent further extension of the formed clot and to prevent secondary thromboembolic complications that may be fatal (e.g., pulmonary embolism).

Warfarin exerts its anticoagulant effect by inhibiting the enzyme encoded by VKORC1, which catalyzes the conversion of vitamin K epoxide to the active reduced form of vitamin K, vitamin K hydroquinone. Vitamin K hydroquinone is an essential cofactor in the synthesis of several clotting factors—it promotes the synthesis of γ-carboxyglutamic acid residues in the proteins essential for biological activity. The decreased availability of vitamin K hydroquinone leads to decreased activity of the clotting factors II, VII, IX, and X, and the anticoagulant proteins C and S (5).

Warfarin is administered as a racemic mixture of the R and S stereoisomers. (S)-warfarin is two to five times more potent than (R)-warfarin, and is mainly metabolized by CYP2C9. (R)-warfarin is mainly metabolized via CYP3A4, with involvement of several other cytochrome P450 enzymes (6).

The initial and maintenance dosing of warfarin must be individualized for each patient. The goal of warfarin therapy is to achieve an international normalized ratio (INR) in a target range for the condition being treated (most commonly 2-3). This involves selecting an initial starting dose, followed by regular testing of the INR so that the dose of warfarin can be adjusted until the appropriate daily maintenance dose is determined. In general, the duration of anticoagulant therapy varies by clinical indication and should be continued until the danger of thrombosis and embolism has passed.

Selecting the initial dose of warfarin should be based on the expected maintenance dose, having taken into account the factors known to influence warfarin dose. Using an optimal starting dose for an individual may reduce the time taken to reach a stable INR, and reduce the risk of having either a high INR (with a risk of bleeding) or a low INR (with a risk of thrombosis) (2). Appropriate dosing of warfarin varies widely between individuals, and not all factors responsible for the variability in warfarin dose are known or easily quantified.

Known factors that influence an individual’s response to the first dose of warfarin include clinical factors (e.g., age, race, body weight, sex, concomitant medications—including those that compete for binding to albumin, comorbidities, diet, nutritional status) and genetic factors (e.g., CYP2C9 and VKORC1 genotypes). Therefore, the initial dose should be modified to take into account these and any additional patient-specific factors that may influence warfarin response.

The FDA-approved drug label for warfarin suggests considering a lower initial and maintenance dose of warfarin for elderly and/or debilitated patients, and in Asian patients. The drug label recommends against the routine use of loading doses because this practice may increase hemorrhagic and other complications and does not offer more rapid protection against clot formation.

Warfarin can cause major or fatal bleeding. Bleeding is more likely to occur within the first month, and the risk factors include a high intensity of anticoagulation (INR greater than 4), age greater than or equal to 65, and a history of highly variable INRs. Other serious adverse events associated with warfarin therapy include necrosis of the skin and other tissues, particularly when used prematurely to manage thrombosis associated with heparin-induced thrombocytopenia (HIT).

Gene: CYP2C9

The cytochrome P450 superfamily (CYP450) is a large and diverse group of enzymes that form the major system for metabolizing lipids, hormones, toxins, and drugs in the liver. The CYP450 genes are very polymorphic and can result in reduced, absent, or increased enzyme activity.

CYP450 isozymes involved in the metabolism of warfarin include CYP2C9 and CYP3A4. The more potent warfarin S-enantiomer is metabolized by CYP2C9 while the R-enantiomer is metabolized by CYP1A2 and CYP3A4. The FDA-drug label for warfarin states that drugs that inhibit or induce CYP2C9, CYP1A2, and/or CYP3A4 have the potential to alter the effect (INR) of warfarin by altering the exposure of warfarin.

CYP2C9*1 is the wild-type allele and is associated with normal enzyme activity and the normal metabolizer phenotype.

Two common allelic variants associated with reduced enzyme activity are CYP2C9*2 (Arg144Cys) and CYP2C9*3 (Ile359Leu). Compared to normal metabolizers, patients who inherit one or two copies of *2 or *3 are more sensitive to warfarin—they require lower doses and are at a greater risk of bleeding during warfarin initiation (7-10).

The frequencies of the CYP2C9 alleles vary between different ethnic groups (11-13). The *2 allele is more common in Caucasian (10-20%) than Asian (1-3%) or African (0-6%) populations (14). The *3 allele is less common (<10% in most populations) and extremely rare in African populations (15). In African Americans, it is likely that other CYP2C9 variants such as CYP2C9*5, *6, *8, and *11 contribute to the variability in patient response to warfarin (2).

Gene: VKORC1

The VKORC1 gene encodes the vitamin K epoxide reductase enzyme. It catalyzes the rate-limiting step in vitamin K recycling, and it is the target of the drug warfarin.

A common non-coding variant, -1639G>A, is associated with an increased sensitivity to warfarin (16). The polymorphism occurs in the promoter region of VKORC1 and is thought to alter a transcription factor binding site, leading to lower protein expression. As a result, patients starting warfarin therapy who are −1639A carriers require lower initial and maintenance doses of the drug than −1639G carriers.

The −1639G>A allele frequency varies among different ethnic groups. It is the major allele (around 90%) in Asian populations, and may be a contributing factor for lower warfarin dosing requirements often observed in patients of Asian descent. It is also common in Caucasians (around 40%) and African Americans (around 14%) (17-19).

Less commonly, missense mutations in VKORC1 can lead to warfarin resistance (20, 21).

Genetic Testing

VKORC1 and CYP2C9 genotypes are the most important genetic determinants of warfarin dosing. The contribution of VKORC1 to the variation in dose requirement is larger (approximately 30%) than the contribution of CYP2C9 (usually less than 10%) (22).

Individuals who are most likely to benefit from genetic testing are those who have yet to start warfarin therapy. However, genotype-guided warfarin dosing is not the standard of care in most healthcare systems, and most (but not all) recent studies have reported that, in general, the use of genotype-guided dosing algorithms did not improve anticoagulation control in the first few weeks of warfarin therapy (4, 23-27).

Genetic testing is available for CYP2C9 and VKORC1. The variants that are routinely tested for are CYP2C9*2, CYP2C9*3, and −1639G>A. These variants are used in the FDA table to guide therapy, and also in the International Warfarin Pharmacogenomics Consortium (IWPC) algorithm.

Other variants that are not routinely tested for include the CYP2C9*6 and *8, alleles, the genes CYP4F2, EPHX1, and GGX (which all have a role in the vitamin-K cycle), and the gene CALU (a cofactor in the VKOR complex) (2, 28). Including these additional genotypes in an expanded dosing algorithm improves warfarin dose prediction in African-Americans, while maintaining high performance in European-Americans (29).

Therapeutic Recommendations based on Genotype

This section contains excerpted1 information on gene-based dosing recommendations. Neither this section nor other parts of this review contain the complete recommendations from the sources.

2015 Statement from the US Food and Drug Administration (FDA):

Dosing Recommendations without Consideration of Genotype

If the patient’s CYP2C9 and VKORC1 genotypes are not known, the initial dose of warfarin is usually 2 to 5 mg once daily. Determine each patient’s dosing needs by close monitoring of the INR response and consideration of the indication being treated. Typical maintenance doses are 2 to 10 mg once daily.

Dosing Recommendations with Consideration of Genotype

Table 1 displays three ranges of expected maintenance COUMADIN doses observed in subgroups of patients having different combinations of CYP2C9 and VKORC1 gene variants […]. If the patient’s CYP2C9 and/or VKORC1 genotype are known, consider these ranges in choosing the initial dose. Patients with CYP2C9 *1/*3, *2/*2, *2/*3, and *3/*3 may require more prolonged time (>2 to 4 weeks) to achieve maximum INR effect for a given dosage regimen than patients without these CYP variants.

Please review the complete therapeutic recommendations that are located here: (1)

2014 Statement from the Clinical Pharmacogenetics Implementation Consortium (CPIC): The pharmacogenetic algorithms available on http://www.warfarindosing.org should be used whenever possible to determine the dose of warfarin required. Such algorithms have been derived from large studies across different ethnic populations, and they take into account both the genetic and non-genetic factors that influence the variability in warfarin response. The existence of rare genetic variants may be responsible for individuals whose warfarin dosing is not well predicted. However, overall the dosing equations are well validated and fairly precise. Only if electronic access to a pharmacogenetic algorithm is not possible should the table-based dosing approach be used, which is preferable to a fixed-dose approach.

Please review the complete therapeutic recommendations that are located here: (2, 30).

Table 2.

Table 2.

Recommended daily warfarin doses (mg/day) to achieve a therapeutic INR based on CYP2C9 and VKORC1 genotype using the warfarin product insert approved by the US Food and Drug Administration

Nomenclature

Guidelines for the description and nomenclature of gene variations are available from the Human Genome Variation Society (HGVS): http://www.hgvs.org/content/guidelines

Nomenclature for Cytochrome P450 enzymes is available from the Human Cytochrome P450 (CYP) Allele Nomenclature Database: http://www.cypalleles.ki.se/

Acknowledgments

The author would like to thank Brian F. Gage, MD, MSC, Professor of Medicine, Washington University, St. Louis; and Sol Schulman, MD, Clinical Fellow in Medicine, Division of Hemostasis and Thrombosis, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston; for reviewing this summary.

First edition:

The Pharmacogenomics Knowledgebase: http://www.pharmgkb.org

The Clinical Pharmacogenetics Implementation Consortium: http://www.pharmgkb.org/page/cpic

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Footnotes

1

The FDA labels specific drug formulations. We have substituted the generic names for any drug labels in this excerpt. The FDA may not have labelled all formulations containing the generic drug.