Overview: What is pharmacogenetics?
Some of us respond differently than others to the same medications that we take, or we may experience different side effects from drugs. The way we respond can be due to the genes we have inherited. With respect to drugs, our unique genetic make-up and our individual response may mean that a drug that is effective for one person may be less effective for another or that a drug that is safe for one person may be less safe for another person—even at the same dosage. Photo source: NHGRI, Ernesto del Aguila III
Most drugs are broken down (metabolized) in the body by various enzymes. In some cases, an active drug is made inactive (or less active) through metabolism. In other cases, an inactive (or less active) drug is made more active through metabolism. The challenge in drug therapy is to make sure that the active form of a drug stays around long enough to do its job. However, some people have variable enzyme action so that they may metabolize the drug too quickly or too slowly or not at all — meaning that the drug may not produce its intended effect or it may remain in a person's system too long and may lead to side effects.
Individual response to a drug may also be related to variability in the drug target, for example a protein that the drug binds to in order to produce its specific effect. Furthermore, individuals may experience side effects (known as hypersensitivity reactions) from certain medications due to variability in proteins involved in the immune response.
Pharmacogenetics is the study of genetic variability that causes individual responses to medications. By analyzing the genes that produce the specific drug targets or enzymes that metabolize a medication or are associated with immune response, a healthcare practitioner may decide to raise or lower the dose or even change to a different drug. The decision about which drug to prescribe may also be influenced by other drugs the person is taking in order to avoid drug-drug interactions.
The terms "pharmacogenetics" and "pharmacogenomics" are sometimes used interchangeably. There are subtle differences in the meaning of the two terms and there is no consensus on the exact definitions. In general, pharmacogenomics refers to the overall study of the many various genes that contribute to drug response. Pharmacogenetics is the study and evaluation of the inherited differences (genetic variations) that affect drug metabolism and an individual's response to medications. For the purposes of this article, the term pharmacogenetics will be used.
Why is pharmacogenetics important?
When initiating drug therapy to treat a particular condition, healthcare practitioners typically prescribe one of several appropriate drugs. Dosages and timing of drugs are usually based upon the anticipated rate of metabolism and clearance from the body in the average person. They prescribe a "standard" dose based on factors such as weight, sex, and age. Clinically, however, each person responds uniquely to treatment and healthcare practitioners must make adjustments. For example, the healthcare practitioner may adjust the drug dose or switch to a different therapy, depending on whether the person's condition is responding to the medication and whether the individual is experiencing unpleasant or dangerous side effects. Sometimes a person may find that a treatment that has been working well suddenly causes symptoms when that person starts taking an additional drug.
The concentrations or effects of some drugs are monitored with blood tests and the drug dosages may be increased or decreased to maintain the drug level in an established therapeutic range. Follow-up of drug concentration is called therapeutic drug monitoring. If changing the drug dose is not effective in treating or controlling the person's condition, or the person still has side effects, then the person may be given a different drug.
In contrast, pharmacogenetics offers healthcare providers the opportunity to individualize drug therapy for people based on their genetic make-up. Testing people prior to initiating drug therapy to determine their likely response to different classes of drugs is a key emerging area of testing. Such genetic information could prove useful to both the healthcare practitioner and patient when choosing current and future drug therapies and drug doses. For certain medications, pharmacogenetics is already helping healthcare providers predetermine proper therapies and dosages to have a better chance of achieving the desired therapeutic effect while reducing the likelihood of adverse effects.