10/7 PG of Warfarin

Explain how genetic variation in CYP2C9 and VKORC1 affects warfarin response and dose requirements.

CYP2C9 gene: codes for a major enzyme that clears S-warfarin (PK)

• Reduced CYP2C9 slows clearance, increasing warfarin exposure and the risk of excessive anticoagulation and bleeding → lower dose is generally needed

VKORC1 gene: codes for the drug target of warfarin (PD)

• Reduced VKORC1 expression means less target enzyme and greater sensitivity to warfarin → lower dose is generally needed

Identify the major CYP2C9 and VKORC1 alleles and describe their effects on enzyme activity or VKORC1 expression.

CYP2C9*1: normal, wild-type activity

CYP2C9*2: reduced activity

CYP2C9*3: more markedly reduced activity

VKORC1 −1639G>A (rs9923231): the A variant reduces gene transcription and VKORC1 protein expression

• GA and AA genotypes typically require less warfarin than GG

Explain how CYP2C9 genotype affects the response to warfarin and other clinically important drugs.

For warfarin: reduced CYP2C9 activity means slower S-warfarin metabolism → higher exposure and increased risk of over-anticoagulation at a standard dose.

CYP2C9 also metabolizes clinically important drugs including many NSAIDs, some oral hypoglycemic agents, ARBs, and phenytoin → altered enzyme activity can affect how these drugs are handled

• PG especially for drugs with narrow therapeutic windows.

Describe how CYP2C9 genotype/phenotype frequencies vary among populations.

CYP2C9 poor-metabolizer phenotypes: 35% of Caucasians and less than 5% of Asians.

*2 and *3 are major inherited variants in Caucasian populations.

Explain the concept of polygenic drug response using CYP2C9 + VKORC1 as an example.

With warfarin, CYP2C9 affects PK (how much active S-warfarin remains in the body) while, VKORC1 affects PD (how sensitive the drug’s target is).

Together, these genes help explain dose differences better than either gene alone.

Integrate genetic and non-genetic factors to predict an individual's warfarin dose.

Dose prediction should combine CYP2C9 and VKORC1 genotypes with non-genetic factors such as age, sex, height, weight, smoking, dietary vitamin K, interacting drugs, and health or liver function.

Can using genotype-informed warfarin dosing algorithm when available, but INR monitoring is still required (target INR = 2-3)

Interpret a warfarin pharmacogenomic test result

Identify the patient’s CYP2C9 genotype and VKORC1 −1639G>A genotype

Reduced-function CYP2C9 alleles (*2 or *3) = slower S-warfarin clearance.

VKORC1 A allele = lower target expression and greater sensitivity.

CYP2C9 *1/*1 and VKORC1 GG will have standard S-warfarin clearance and standard amount of drug target. Can give standard dose.

CYP2C9 (*2/*2, *2/*3, or *3/*3) and VKORC1 (AG or AA) will have reduced S-warfarin clearance and fewer drug target (more sensitive to warfarin). Should lower dose.