When you take a medicine (drug), your body has to have a way to handle the medicine. One way is for enzymes to metabolize (break down) the medicine. A family of enzymes called cytochrome P450s have the ability to break down certain medicines. By metabolizing a medicine, cytochrome P450 enzymes make the medicine either more or less active, depending upon the medicine. Cytochrome P450 2C19 (CYP2C19) is part of the cytochrome P450 family of proteins in the body. It is responsible for breaking down several medicines that are commonly used.
DNA is like a set of instructions for your body that can help decide how well your enzymes will work. Each person differs from another at the DNA (gene) level. This means that each person has small differences in the genes that code for enzymes. The part of DNA that instructs how well the CYP2C19 enzyme will work is called the CYP2C19 gene. The study of how genes like CYP2C19 affect the way you break down medicines is called pharmacogenetics (FAR mah coh je NEH tiks).
Differences in your DNA that make up the CYP2C19 gene can change how well you are able to break down certain medicines. If you break down a medicine too fast or too slowly, this may cause a bad reaction to the medicine. By testing your DNA (with a pharmacogenetic test), we may find DNA differences that can allow us to predict how well your CYP2C19 enzyme will work. The result of this test will guide your doctor to choose the correct dose of medicine to give you. The results of your CYP2C19 test will place you into one of four groups:
- Poor metabolizers – People in this group have little or no active CYP2C19 enzyme. People who are poor metabolizers break down some medicines slowly and are likely to need altered doses or even a different medicine in some cases. About 10 percent of people are poor metabolizers of CYP2C19.
- Intermediate metabolizers – People in this group break down some medicines at a rate in between the poor and extensive metabolizers. About 30 percent of people are intermediate metabolizers of CYP2C19.
- Extensive metabolizer – People in this group have normal working CYP2C19 enzymes. About 43 percent of people are extensive metabolizers of CYP2C19.
- Ultra-rapid metabolizers – People in this group have very high activity of CYP2C19 enzymes. People who are ultra-rapid metabolizers break down some medicines rapidly and are likely to need different doses or even a different medicine in some cases. About 17 percent of people are ultra-rapid metabolizers of CYP2C19.
Medicines that may be affected
CYP2C19 enzymes break down several commonly used medicines, including: clopidogrel (used to prevent platelets from forming a clot), voriconazole (used for fungal infections), some tricyclic antidepressants, benzodiazepines (used for anxiety), and some proton pump inhibitors (used to treat stomach ulcers and reduce stomach acid). The result of your CYP2C19 genotype test will place you into one of the four groups above. Knowing what group you are in may help your doctor to pick the right medicine and right dose for you.
For example, for clopidogrel:
- Poor metabolizers are not likely to get adequate blood thinning effect from clopidogrel and are at a higher risk for adverse events like blood clots.
- Intermediate metabolizers may have less blood thinning effect from clopidogrel than extensive metabolizers.
- Extensive metabolizers don’t normally need dosage changes of clopidogrel based on CYP2C19 genotype test result.
- Ultra-rapid metabolizers don’t normally need dosage changes of clopidogrel based on CYP2C19 genotype test result.
Scientists continue to find new information about which medicines are affected by gene test results. For details about which medicines are broken down by CYP2C19, please go to www.stjude.org/pg4kds or www.pharmgkb.org.
If you have questions or concerns about pharmacogenetic testing at St. Jude, call one of the Pharmaceutical Sciences research nurses at 901-595-2482 or email email@example.com. If you are calling from outside the Memphis area, dial toll-free 1-866-2ST-JUDE (1-866-278-5833), then dial extension 2482.
This document is not intended to take the place of the care and attention of your personal physician or other professional medical services. Our aim is to promote active participation in your care and treatment by providing information and education. Questions about individual health concerns or specific treatment options should be discussed with your physician.
St. Jude complies with health care-related federal civil rights laws and does not discriminate on the basis of race, color, national origin, age, disability, or sex.
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