Download Aithal, 1999

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?