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. Dihydropyrimidine dehydrogenase (DPYD) is an enzyme that has the ability to break down a class of medicines called fluoropyrimidines. Fluoropyrimidines include 5-fluorouracil (5-FU, Adrucil®) and capecitabine (Xeloda®).
These medicines are important chemotherapy agents used to treat certain tumors. Most commonly, they are used to treat breast cancer and cancers of the upper (mouth, throat, esophagus, etc.) and lower (colon, rectum) digestive tract. Like many medicines, how well they work and their side effects can be different from person to person.
Most people have no problem breaking down fluoropyrimidines. However, a small percentage of people (about 4 in 100) have a lower ability to break down these medicines. For these people, toxic levels of the medicine build up in the body. They are at higher risk of serious side effects such as infection, diarrhea, bleeding, painful sores on the hands, feet, or mouth, and altered mental state if they receive normal doses of fluoropyrimidine medicines. A very small number of people (2 in 1,000) have no DPYD enzyme, and they may have fatal side effects if given normal doses of fluoropyrimidines.
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 DPYD enzymes will work is called the DPYD gene. The study of how genes like DPYD affect the way you break down medicines is called pharmacogenetics (FAR mah coh je NEH tiks).
Differences in your DNA that make up the DPYD gene can change how well you are able to break down fluoropyrimidine medicines (such as 5-fluorouracil and capecitabine). By testing your DNA (with a pharmacogenetic test), we may find differences that can allow us to predict how well your DPYD enzyme will work. The results of this test will help your doctor choose the correct dose or type of medicine to give you. The results of your DPYD pharmacogenetic test will likely place you into one of three groups:
- Normal DPYD function – Patients in this group have normal working DPYD enzymes. Doses of fluoropyrimidines do not need to be changed based upon this DPYD genetic test result. About 95 in 100 people fall in this category.
- Low DPYD function – This means there is one (1) normal activity gene and one (1) copy of a no-activity DPYD gene. Patients in this group have 30 to 70 percent lower DPYD enzyme function as compared to patients with normal DPYD function. Doses of fluoropyrimidines may need to be reduced, or other medicine may need to be chosen to avoid side effects. About 4 in 100 people fall in this category.
- Deficient DPYD function – This means there are 2 copies of the no-activity gene, and there is no normal DPYD enzyme. These patients are at very high risk of having severe side effects from fluoropyrimidine medicines. These patients should not receive medicines like fluorouracil or capecitabine. About 2 in 1,000 people fall in this category.
If you have questions 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 of 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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