Download Pharmacogenetic Testing Prior to Initiation of Warfarin

Pharmacogenetic Testing
Prior to Initiation of Warfarin Therapy
Introduction
Warfarin’s anticoagulant effect is mediated through its ability to
block hepatic synthesis of functional vitamin K-dependent clotting factors, specifically factors II, VII, IX, and X.2 Warfarin
inhibits the intrahepatic vitamin K cycle and thus impairs vitamin
K-dependent gamma-carboxylation.2 Recently, the gene encoding the vitamin K epoxide cycle has been identified (VKORC1).8
Alterations in the VKORC1 gene can affect the ability of warfarin
to impair the activity of the vitamin K-dependent factors and,
therefore, alter response to therapy.9 VKORC1 variants (polymorphisms) are thought to ac­count for as much as 25% of the interindividual variation in re­quired warfarin dose while about 10% of
this variation is due to CYP2C9 variants.4,9 The VKORC1 genetic
variant that is a marker for warfarin hypersensitivity is designated
(-)1639 G>A.7,10 Carriers of this variant require significantly
lower maintenance doses of warfarin than noncarriers.7 The frequency of (-)1639 G>A varies with ethnicity; it is more common
in Chinese,10 less common in Caucasians, and even less common
among those of African American ancestry.11
Variability in an individual’s response to a given drug may be
due to a number of different genetic factors. Genetic variants
may influence drug response by altering (1) drug metabolizing
enzymes, (2) drug target(s) or receptor(s), and/or (3) drug transport protein(s) or a combination thereof.1 Pharmacogenetics is
the investigation of the genetic basis of drug response. Unlike
traditional therapeutic drug monitoring, a significant advantage
of pharmacogenetic testing is that investigation can be undertaken prior to initiation of drug therapy to enable more effective
initial dose stratification or identify specific situations in which a
specific therapy will not be effective.1 Warfarin is an ideal drug
target for the application of pharmacogenetic testing due to its
narrow therapeutic index, large interindividual variation in dose
requirements and high frequency of identified genetic variants
that have an impact on both its effect and metabolism.2,3
Daily maintenance doses of warfarin in study subjects range
widely, from 1.25 mg to 34 mg.4 Given this significant variation
in required dose and warfarin’s narrow therapeutic index, the anticoagulant effect of warfarin must be regularly monitored. This is
accomplished using a mathematical conversion of the prothrombin
time (PT).2 The PT is converted to the International Normalized
Ratio (INR) based on the mean PT of the population and responsiveness of the test reagent (INR = [patient PT/mean PT]ISI).2 The
recommended therapeutic range for patients on warfarin therapy is
an INR value between 2.0 and 3.0.2 Below this range, patients are
at risk of thrombosis while supratherapeutic INR values increase
the potential for bleeding.4–7 There is a sharp increase in bleeding
risk when the INR is above the therapeutic range such that the risk
of hemorrhage doubles with each one point rise in INR.5
Warfarin Metabolism
Warfarin is metabolized in the liver through the cytochrome P450
(CYP) system.2 The CYP drug metabolizing enzyme 2C9 is
re­spon­sible for the metabolism of the biologically active isomer of
warfarin (S-warfarin), as well as other agents including certain antiepileptics (phenytoin), antidiabetic drugs (glipizide, tolbutamide),
nonsteroidal anti-inflammatory agents and antihypertensive agents
(losartan).1,12,13 Genetic variants in the CYP2C9 system can impair
the degree to which warfarin is metabolized, thereby altering drug
response.13 Common genetic variants (polymorphisms) of CYP2C9,
designated CYP2C9*2 and CYP2C9*3, have been shown to decrease
the in vivo clearance of warfarin therapy.14 In CYP2C9*2 homozygotes, metabolism of warfarin is reduced by 40% and in those who
are homozygous for CYP2C9*3, warfarin metabolism is reduced by
almost 90%.12,14 During initiation of warfarin therapy, individuals
with the CYP2C9*2 and/or CYP2C9*3 genotype are more likely
than noncarriers to have INR values greater than 4 and suffer a twofold to threefold elevated bleeding rate.14 Carriers of these variants,
furthermore, require more frequent dose adjustments during warfarin initiation and take longer to reach a stable maintenance dose.15
The CYP2C9*2 and CYP2C9*3 polymorphisms are common,
occurring in up to 30% of Caucasian populations, while both of
these variants are less common in African and Asian populations.12
It has been esti­mated that 18% of Caucasians have both VKORC1
and one of the CYP2C9 variants16 and these individuals therefore
require significantly lower doses of warfarin to achieve a therapeutic
response and prevent overdose.4,10,16
Bleeding Complications
Bleeding is the most clinically significant complication associated with warfarin therapy.4–6 Risk for bleeding is highest during
warfarin initiation as required dose is not easily predicted in the
individual patient.4–6 Commonly occurring genetic variants in
warfarin’s target enzyme (or the enzyme system largely responsible for its metabolism) account for much of the observed variation in warfarin’s anticoagulant effect.7 Individuals who carry
these common genetic variants require lower maintenance doses
of warfarin.4,7 The application of pharmacogenetic testing prior
to the start of warfarin therapy to identify these genetic variants,
therefore, can identify a population that is hypersensitive to warfarin therapy, avoiding the potential for serious and life-threatening
bleeding complications.3,4,7
1
Predicting warfarin dose at therapy initiation is enhanced by genotyping for CYP2C9*2 or *3 and (-)1639 G>A and consideration
of other factors.4,7,10,15 For each CYP2C9*2 variant, required war­
farin dose is reduced by 19% and reduction in dose is 29% with
CYP2C9*3 alleles.4 In determination of optimum warfarin dose,
other factors such as patient age, intercurrent illness(es), nutrition,
drug-drug interactions and body surface area should be taken into
account.4,7 Certain drugs such as amiodorone, isoniazid, and phenylbutazone have been shown to decrease warfarin clearance and,
therefore, reduce the required dose while rifampin and barbiturates
have been shown to enhance warfarin metabolism, increasing the
required amount of drug.17
Warfarin (P450 2C9 and VKORC1) . . . . . . . . . . . 511460
Synonyms DME Genotyping; Warfarin Genotyping
Specimen Whole blood or LabCorp buccal swab kit (buccal swab
collection kit contains instructions for use of a buccal swab.)
Volume 7 mL whole blood or LabCorp buccal swab kit
Minimum Volume 3 mL whole blood or two buccal swabs
Container Lavender-top (EDTA) tube, yellow-top (ACD) tube, or
LabCorp buccal swab kit
Storage Instructions Maintain specimen ambient, or refrigerate at
4°C.
Causes for Rejection Frozen specimen; hemolysis; quantity not sufficient for analysis; improper container; one buccal swab
Use Warfarin, the coumarin derivative, is the most widely prescribed
anticoagulant for thromboembolic disorders. This test detects variants within two genes, CYP2C9 and VKORC1, that influence the
response to warfarin therapy. Understanding different rates of warfarin metabolism can help define optimal patient-specific warfarin
dosage.
Limitations The metabolism of drugs is also influenced by ethnicity,
diet, and other medications. All factors should be considered prior
to initiating new therapy.
Methodology Polymerase chain reaction (PCR); gel electrophoresis
Pharmacogenetic testing for warfarin de­
tects the presence of
CYP2C9*2, *3, and VKCORC1 (-)1639 G>A variants. CYP2C9
DNA analysis is performed by allele-specific PCR followed by
electrophoresis to detect the *2 and *3 alleles. VKORC1 DNA
analysis is performed by PCR followed by restriction enzyme digestion to detect (-)1639 G>A. Individuals with one or more of these
variant alleles are hypersensitive to warfarin therapy and have an
increased risk for bleeding if warfarin therapy is initiated at standard
doses. The dose of warfarin can be individually determined based
on CYP2C9 and VKORC1 genotype, patient age, body surface area,
and concurrent drug therapies.4
For the most current information regarding test options, including
specimen requirements and CPT codes, please consult the online Test
Menu at www.LabCorp.com.
References
9. Rieder MJ, Reiner AP, Gage BF, et al. Effect of VKORC1 haplotypes on transcriptional regulation and warfarin dose. N Engl J Med. 2005 Jun 2; 352(22):2285-2293.
10. Yuan HY, Chen J-J, Lee MTM, et al. A novel functional VKORC1 promoter
polymorphism is associated with inter-individual and inter-ethnic differences in warfarin
sensitivity. Hum Mol Genet. 2005; 14(13):1745-1751.
11. Geisen C, Watzka M, Sittinger K, et al. VKORC1 haplotypes and their impact
on the inter-individual and inter-ethnical variability of oral anticoagulation. Thromb
Haemost. 2005; 94:773-779.
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15. Higashi MK, Veenstra DL, Kondo LM, et al. Association between CYP2C9
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17. Wells PS, Holbrook AM, Crowther NR, Hirsh J. Interactions of warfarin with
drugs and food. Ann Intern Med. 1994 Nov 1; 121(9):676-683.
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Am J Med. 1989 Aug; 87(2):153-159.
6. White RH, Beyth RJ, Zhou H, Romano PS. Major bleeding after hospitalization for
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