CYP2C9 acts on 15% of drugs in current clinical use. About 35% of Caucasians have a slow acting form of this enzyme. CYP2C9 is an important drug-metabolizing enzyme that catalyzes the biotransformation of many other clinically useful drugs including angiotensin II blockers, non-steroidal anti-inflammatory drugs, the alkylating anticancer prodrugs, sulfonylureas, and many others. ^[29]

Of special interest are those drugs with narrow therapeutic index, such as warfarin (Coumadin®)^[73], tolbutamide^[74], and phenytoin (Dilantin®)^[75], where impairment in CYP2C9 metabolic activity might cause difficulties in dose adjustment as well as toxicity. Indications for testing include lack of therapeutic effect or difficulties with side effects to any of the drugs metabolized by CYP2C9.

CYP2C9 and VKORC1 variation greatly affect the half-life of warfarin (Coumadin®) and the time it takes to reach a stable dose. VKORC1 is the site of action of warfarin.^[30] The level of the enzyme is under genetic control according to the DNA sequence present in the control region of the gene. Inherited differences in VKORC1 increase or decrease the amount of warfarin needed to inhibit the formation of the clotting factors. When the amount of warfarin exceeds what is needed, the risk of bleeding is increased. Indications for testing include lack of therapeutic effect or difficulties with side effects of warfarin.

This test identifies the nine most common alleles of CYP2C9 (*2-*6,*8,*11,*13,*15) and a VKORC1 polymorphism (-1639G>A) which influences warfarin maintenance dose. Analytical specificity and sensitivity for detection of these mutations are >99%.