Genes influence how you respond to medications through two distinct routes: they can change how your body processes a drug, or they can change how the drug acts once it arrives. The first route is called pharmacokinetics—what the body does to the drug. The second is pharmacodynamics—what the drug does to the body. The blood thinner warfarin shows how both can operate at once, with variants in the metabolizing enzyme CYP2C9 and the drug target VKORC1 combining to shape the dose a patient actually needs.
Pharmacokinetics: How Your Body Processes Drugs
Before a drug can exert its effect, your body must absorb, distribute, metabolize, and eventually eliminate it. Pharmacokinetics is the study of that journey. Writing in Circulation in 2011, Dan M. Roden and colleagues at Vanderbilt University School of Medicine, with Heyo K. Kroemer of Ernst Moritz Arndt University, noted that many important examples in pharmacogenetics relate to variable drug uptake, metabolism, or elimination.
Why does metabolism matter so much? Enzymes—mostly in the liver—chemically transform drugs, often converting them into forms that are easier to excrete. The genes that encode these enzymes come in variant forms, and a variant enzyme may work faster, slower, or not at all. The practical consequence is a change in drug concentration: too little enzyme activity and a standard dose can build up to toxic levels; too much and the drug may be cleared before it can act.
Consider warfarin. The drug is administered as a racemate—a mixture of two mirror-image molecules—and most of its anticoagulant action is mediated by the more potent S-enantiomer, as Roden and colleagues describe. S-warfarin is bioinactivated primarily by CYP2C9, an enzyme encoded by the CYP2C9 gene. If a patient carries CYP2C9 variants that reduce the enzyme's activity, S-warfarin is broken down more slowly. The active drug lingers, and the same dose that is safe for one person can over-anticoagulate another.
This is the pharmacokinetic pathway in its clearest form: the gene affects the enzyme, the enzyme affects the drug's concentration, and the concentration determines the response. Nothing about the drug's target has changed—only how long the drug stays in the body.
