Transcript Association of polymorphisms in the cytochrome P450 CYP2C9

Association of polymorphisms in the
cytochrome P450 CYP2C9 with
warfarin dose requirement and risk of
bleeding complications
Mark Bleackley
MEDG 505
March 10 2005
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 Ca2+
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 55-88
(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 33-94
(median 70.5)
• Control 100 individuals (58 male, 42 female)
aged 38-91 (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 2.48-15.6) odds ratio
• When compared with general population
odds ratio of only one variant allele 2.68
(CI1.22-5.86) with two variant alleles 7.8
(CI1.90-32.1)
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 10.0
• 20 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 132.8 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, 311.1 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?