The impact of VKORC1-1639 G>A polymorphism on the maintenance dose of oral anticoagulants for thromboembolic prophylaxis in North India: A pilot study

BACKGROUND: The dose requirements for oral anticoagulants in thromboembolic events are infl uenced by promoter polymorphism in the VKORC1 gene. However, limited data are available on the infl uence of the polymorphism in various Indian populations. The present study aimed at determining the relationship between the VKORC1-1639 G>A genotypes and maintenance doses of oral anticoagulants for therapeutically stable INR values in patients taking Acitrom after valve replacement surgery. MATERIALS AND METHODS: Fifty patients from the northern Indian region were genotyped for VKORC1-1639 G>A by polymerase chain reaction and restriction fragment length polymorphism. Means of the weight-normalized daily Acitrom dose were calculated for every patient. RESULTS AND DISCUSSION: The VKORC1-1639G>A minor allele frequency in the study population (n = 50) was found to be 22%. The patients with a wild type genotype required the maximum drug dose as suggested for full functionality of the enzyme. Heterozygous patients were found to have an intermediate drug dose and the patients with a variant homozygous genotype had the minimum maintenance drug dose requirement. These fi ndings are in concurrence with the effect of the promoter polymorphism on vitamin K epoxide reductase activity. CONCLUSION: The VKORC1-1639 G>A status can be indicative of establishing the therapeutic dose of oral anticoagulants in Indian patients.


Introduction
The use of oral anticoagulants has become one of the most common therapies to treat patients suffering from thromboembolic disorders. Warfarin/Acitrom and related coumarins are widely used in thromboembolic prophylaxis. The subtherapeutic and supratherapeutic use of these drugs is associated with an increased risk of clot formation and bleeding, respectively. This is because of the narrow therapeutic index of these anticoagulants.
There is much concern to ensure improvement in the safety of oral anticoagulant therapy, and many efforts have been made for this purpose. However, still, the risks for clotting and bleeding subsequent to oral anticoagulant therapy remain signifi cantly high. [1,2] Warfarin and Acitrom are two widespread oral anticoagulant drugs that are used to maintain the International Normalized Ratio (INR) within the therapeutic range. However, the dose requirements of these anticoagulants vary between individuals as well as within an individual, depending on the dietary intake or as a result of variations in pharmacokinetics and pharmacodynamics, compliance, etc. [3] In recent years, common genetic variations in vitamin K epoxide reductase (VKOR) have been discovered to signifi cantly influence the oral anticoagulant maintenance dose requirements. Acitrom (or Warfarin) inhibits vitamin K epoxide reductase, [4] specifi cally the VKORC1 subunit, [5,6] thereby diminishing the available vitamin K and vitamin K hydroquinone in the tissues. Interindividual variations in the VKORC1-1639 G>A gene polymorphism has been  [7] and reported the oral anticoagulant dose requirement in Indians to be higher than that in the Chinese. In this study, most Indians belonged to South India. The Indian population comprises multiple ethnic groups, and there is a need to obtain data from other regions. Therefore, we carried out a pilot study in the North Indian population to fi nd out the relationship between the VKORC1-1639 G>A genotypes and maintenance doses of oral anticoagulants for therapeutically stable INR values in patients taking Acitrom after valve replacement surgery.

Patients and DNA extraction
The present study was carried out on the patients who had undergone heart valve replacement surgery and were being followed-up for oral-anticoagulant therapy. anti-infl ammatory agents; allopurinol; cyclosporine (INN, ciclosporin); barbiturates; and oral contraceptives. [8] In the course of warfarin/acitrom therapy, all patients were given dietary advice to avoid foods that may interfere with drug pharmacokinetics. Clinical data, including age, gender, ethnicity, body weight, medical indication for drug use and average maintenance dose, were recorded. The average maintenance dose was the mean dose during the period when two consecutive stable INR values were documented. The north Indian ethnicity was based on place of residence in the last three generations, food habits and the mother tongue (Hindi or related languages). Blood samples were collected in ethylenediaminetetraacetic acid and genomic DNA was extracted from peripheral blood leukocyte pellets using the standard salting-out method. [9] The quality and quantity of DNA was checked by gel electrophoresis and spectrophotometry using the NanoDrop Analyzer

Statistical analysis
The 95% confi dence intervals were calculated using the Confi dence Interval Analysis software version 1.0.
The level of statistical signifi cance was set at P <0.05. All

Discussion
The VKORC1-1639G>A minor allele frequency in the study patient group was 22%, which was signifi cantly different from the East Asians as well as the Caucasian populations. [11] The role of genetics in oral anticoagulant dose requirement, in patients with thromboembolic disorders, has been investigated in many previous studies. [12] The daily dose requirement is the highest in the African population, while the white population required intermediate doses and the Asian population required the lowest doses. Lee et al. [7] reported that the mean weight-normalized warfarin dose was lower for the Chinese and the Malays than for the Indians.