Clinical effect of CYP2C9*5/*6 genotype on a patient's warfarin dose requirement

Warfarin Dosing in Patients with CYP2C9*5 Variant Alleles

Pharmacogenetic dosing improves the accuracy of warfarin dosing, but current pharmacogenetic dosing algorithms are less accurate in populations of African ancestry. The cytochrome P450 2C9*5 (CYP2C9*5) allele is found almost exclusively in populations of African ancestry, and in-vitro studies suggest CYP2C9*5 is associated with reduced clearance of warfarin. The clinical relevance of this SNP is uncertain. In this multi-centered study of 2298 patients (49% female, 35% Black) taking warfarin, we quantified the association between the CYP2C9*5 allele and warfarin requirements. The CYP2C9*5 SNP was present in 2.3% of Black and 0.07% of White patients. Without taking CYP2C9*5 into account, pharmacogenetic algorithms that include other SNPs overestimated the warfarin dose by 30% (95% CI [19%-40%], p<0.001), an average of 1.87 mg/d (SD 1.64) in heterozygotes (p < 0.001). Non-carriers required a slightly (0.23 mg/d, SD 2.09) higher than predicted dose. Genotyping for CYP2C9*5 corrected the potential overdose and halved overall dosing error in heterozygotes. Patients carrying CYP2C9*5 require a clinically relevant reduction in warfarin dose. Given the potential to improve the accuracy and safety of warfarin dosing in populations of African ancestry, we have incorporated this SNP into a non-profit website to assist warfarin initiation (www.WarfarinDosing.org).

Pharmacogenetic determinants of warfarin in the Indian population

BACKGROUND

Several studies optimized the warfarin dose based on CYP2C9*2, CYP2C9*3, VKORC1 -1639 G > A, CYP4F2 V433M. But, the information on the rare variants is lacking. In this study, we have explored the prevalence of common and rare pharmacogenetic determinants of warfarin and determined their damaging nature.

METHODS

We have analyzed 2000 healthy adults using the Infinium global screening array (GSA) for 15 pharmacogenetic determinants of warfarin. In addition, we have elucidated the impact of these variants on protein function, stability, dynamics, evolutionary preservation, and ligand binding propensity.

RESULTS

The GSA Analysis has revealed that CYP4F2 V433M (MAF: 39.425%), VKORC1 -1639 G > A (MAF: 20.5%), CYP2C9*3 (MAF:9.925%), and CYP2C9*2 (MAF:4.575%) are common, while CYP2C9*14 (MAF: 1.475%), CYP2C9*4 (0.175%), CYP2C9*5 (0.125%), and CYP2C9*11 (0.125%) are rare. Position-specific evolutionary preservation (PSEP) analysis has revealed that CYP2C9*4 is possibly damaging, while CYP2C9*5, CYP2C9*11, and CYP2C9*14 are probably damaging. CYP2C9*4 has high thermolability (-10.14 kcal/mol). Among the rare CYP2C9 variants, CYP2C9*4 and CYP2C9*11 exert destabilizing effects and may have increased molecular flexibility, while CYP2C9*5 and CYP2C9*14 exert stabilizing effects and may have decreased molecular flexibility. DNase I footprint analysis has revealed the loss of the E-box consensus sequence due to VKORC1 -1639 G > A polymorphism.

CONCLUSION

CYP2C9*2, CYP2C9*3, VKORC1 -1639 G > A and CYP4F2 V433M are common; CYP2C9*4, CYP2C9*5, CYP2C9*11, and CYP2C9*14 variants are rare in Indian subjects. All the CYP2C9 variants are found to be damaging. DNase I footprint analysis provided the mechanistic rationale for the association of VKORC1 -1639 G > A with warfarin sensitivity.

Effect of CYP2C9 *11/*11 genotype on initial and long-term warfarin dose requirement and therapeutic response

The warfarin dose requirement and therapeutic response of a 42-year-old African-American male with genotype CYP2C9 *11/*11, VKORC1 -1639GG and CYP4F2 433Val/Val anticoagulated for ischemic stroke is described herein. Warfarin was dosed according to the institution's personalized medicine program recommendations of a 10 mg mini-load dose, followed by dose decreases to 4-6 mg/day through discharge. Stable international normalized ratio was achieved after eight doses, with good overall long-term maintenance of therapeutic international normalized ratio over several years with warfarin doses of 3.1-4.3 mg/day. This case report sheds further light on the clinical impact of CYP2C9 *11/*11 on warfarin dose requirements, short- and long-term treatment response and practical considerations for warfarin management in suspected carriers of rare variant CYP2C9 alleles.

Effects of rare CYP2C9 alleles on stable warfarin doses in Chinese Han patients with atrial fibrillation

Aim: Gene polymorphisms are critical in warfarin dosing variation. Here, the role of rare CYP2C9 alleles on warfarin doses in Chinese Han patients was investigated. Methods: A retrospective study recruited 681 warfarin treated atrial fibrillation patients. The genetic and clinical data were collected. Dose-related variables were selected by univariate analyses and the warfarin-dosing algorithm was derived by multivariate regression analysis. Results: Three rare CYP2C9 alleles (CYP2C9*13, *16 and *60) were associated with lower stable doses. Inclusion of the rare CYP2C9 alleles in the prediction model added an extra 3.7% warfarin dose predictive power. Conclusion: CYP2C9*13, *16 and *60 was associated with lower stable warfarin doses in Chinese patients. The algorithm including rare CYP2C9 alleles tends to more accurately predict stable warfarin doses.

Recommendations for Clinical CYP2C9 Genotyping Allele Selection: A Joint Recommendation of the Association for Molecular Pathology and College of American Pathologists

The goals of the Association for Molecular Pathology Pharmacogenomics (PGx) Working Group of the Association for Molecular Pathology Clinical Practice Committee are to define the key attributes of PGx alleles recommended for clinical testing and a minimum set of variants that should be included in clinical PGx genotyping assays. This document provides recommendations for a minimum panel of variant alleles (Tier 1) and an extended panel of variant alleles (Tier 2) that will aid clinical laboratories when designing assays for CYP2C9 testing. The Working Group considered the functional impact of the variants, allele frequencies in different populations and ethnicities, the availability of reference materials, and other technical considerations for PGx testing when developing these recommendations. Our goal is to promote standardization of testing PGx genes and alleles across clinical laboratories. These recommendations are not to be interpreted as restrictive but to provide a reference guide. The current document will focus on CYP2C9 testing that can be applied to all CYP2C9-related medications. A separate recommendation on warfarin PGx testing is being developed to include recommendations on CYP2C9 alleles and additional warfarin sensitivity-associated genes and alleles.

Creating and validating a warfarin pharmacogenetic dosing algorithm for Colombian patients

Purpose

Warfarin is an oral anticoagulant associated with adverse reaction to drugs due to wide inter- and intra-individual dosage variability. Warfarin dosage has been related to non-genetic and genetic factors. CYP2C9 and VKORC1 gene polymorphisms affect warfarin metabolism and dosage. Due to the central role of populations’ ethnical and genetic origin on warfarin dosage variability, novel algorithms for Latin American subgroups are necessary to establish safe anticoagulation therapy.

Patients and methods

We genotyped CYP2C9*2 (c.430C > T), CYP2C9*3 (c.1075A > C), CYP4F2 (c.1297G > A), and VKORC1 (−1639 G > A) polymorphisms in 152 Colombian patients who received warfarin. We evaluated the impact on the variability of patients’ warfarin dose requirements. Multiple linear regression analysis, using genetic and non-genetic variables, was used for creating an algorithm for optimal warfarin maintenance dose.

Results

Median weekly prescribed warfarin dosage was significantly lower in patients having the VKORC1-1639 AA genotype and poor CYP2C9*2/*2,*2/*3 metabolizers than their wild-type counterparts. We found a 2.3-fold increase in mean dose for normal sensitivity patients (wild-type VKORC1/CYP2C9 genotypes) compared to the other groups (moderate and high sensitivity); 31.5% of the patients in our study group had warfarin sensitivity-related genotypes. The estimated regression equation accounted for 44.4% of overall variability in regard to warfarin maintenance dose. The algorithm was validated, giving 45.9% correlation (R²=0.459).

Conclusion

Our results describe and validate the first algorithm for predicting warfarin maintenance in a Colombian mestizo population and have contributed toward the understanding of pharmacogenetics in a Latin American population subgroup.