Association of polymorphisms in the cytochrome P450 CYP2C9 with warfarin dose requirement and risk of bleeding complications Mark Bleackley MEDG 505 March

Overview of paper Study of randomly selected patients from anticoagulant clinic in north-east England Found that individuals with low warfarin dose requirement had higher odds of having one or more CYP2C9 variant alleles compared to a normal population Patients in low dose group have increased risk of major bleeding complications

What is warfarin? Drug used to prevent the formation of blood clots or to prevent blood clots from becoming larger Often prescribed to patients with irregular heart beats and after heart attacks or heart valve replacement surgery Acts as a vitamin K antagonist

How does warfarin work Interacts with KO reductase enzyme to prevent reduction of oxidized vitamin K Reduced vitamin K is required for carboxylation of Glu residues to Gla residues in a number of blood coagulation proteases and other proteins Gla residues give the proteins Ca 2+ binding properties that are essential for activity

Blood coagulation

CYP2C9 Responsible for metabolism of warfarin Two know allelic variants, CYP2C9*2 and CYP2C9*3, differ from wild type by one amino acid each Associated with impaired hydroxylation of warfarin in inactivation due to an alteration of the interaction with cytochrome p450 oxidoreductase CYP2C9*2 ~12% efficiency, CYP2C9*3<5% efficiency of wildtype

Why is this important? Standardized induction regimens with monitoring of International Normalized Ratio (INR) over the first four days only 69% successful in predicting correct dosage Understanding genetics of warfarin dose response will minimize clinical difficulties associated with warfarin therapy

Methods 36 patients with low warfarin dose requirement selected (17 male, 19 female) aged (median 73) No apparent cause for low dose requirement (eg drug interaction, disease) 52 patients with wide range of warfarin dose requirement (26 male, 26 female) aged (median 70.5) Control 100 individuals (58 male, 42 female) aged (median 69) not on warfarin

Genotyping Take blood from each patient, extract DNA analyzed by PCR CYP2C9*2 detected by AvaII digestion CYP2C9*3 detected by NsiI digestion

Bleeding Complications Review history of low dose group Determine difficulties during induction of anticoagulation Categorized bleeding complications associated with a raised INR (>4) as mild serious or life threatening

Results Found that 81% of low dose group had one or more of the variant alleles, 40% in the control group (wide range of doses) 6.21 (CI ) odds ratio When compared with general population odds ratio of only one variant allele 2.68 (CI ) with two variant alleles 7.8 (CI )

What does this mean? There is a strong association between CYP2C9 genotype and warfarin sensitivity Individuals with low warfarin doses are six times as likely to possess one of the variant alleles

Genotype and allele frequencies Test whether CYP2C9 variant genotypes were associated with increased risk of requiring anticoagulant therapy Found no significant difference between clinic control and general control

Response to treatment All patients received an initial dose of 10mg warfarin Peak INR during the first week in the low dose group ranged from 2.0 to of the 36 had a peak INR above the therapeutic range of greater than 4.0 Two of these were homozygous wild type 9 of the 20 cases resulted in a prolonged inpatient stay while optimum anticoagulation was achieved, none of the clinic control group had prolonged stays due to poor anticoagulation

Bleeding episodes During patient years of warfarin treatment 7 minor, 5 serious and 6 life threatening bleed episodes occurred in 11 patients in the low dose group In the clinic control, patient years had 6 minor, 5 serious and 2 life threatening episodes in 11 patients Significantly higher number of serious and life threatening episodes in low dose group

Risks associated with CYP2C9 variants Significant proportion of individuals with variant alleles have difficulty at the onset of treatment and are more likely to have a serious or life threatening bleeding event while on warfarin

What can we take from this paper? Variant alleles that have no apparent phenotype with regards to disease or susceptibility to disease can have a significant influence on response to specific treatments Can use genotypic information to design better methods of treatment for individuals as well as more effective methods that are less of a burden on the medical system

Questions Can this type of information be used to develop drugs that are specific to variant alleles? How quickly does the complexity of “personalized” medicine escalate as more of these situations are realized? Social aspect: Will the public allow themselves to be genotyped?