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FDA news:Genetic Lab Test for Warfarin Sensitivity

9/17/07 press release: FDA cleared for marketing a new genetic test that will help physicians assess whether a patient may be especially sensitive to the blood-thinning drug warfarin which is used to prevent potentially fatal clots in blood vessels.

Daniel Schultz, M.D., director, FDA’s Center for Devices and Radiological Health said,

"Today’s action offers physicians the first FDA cleared genetic test for warfarin sensitivity, which is another step in our commitment to personalized medicine. With this test, physicians may be able to use genetic information along with other clinical information to treat their patients.”

Warfarin is the second most common drug, after insulin, implicated in emergency room visits for adverse drug events. FDA reports that it can be a difficult drug to use because the optimal dose varies depending on many risk factors, including a patient's diet, age, and the use of other medications. Rapidly achieving the correct dose is important. Patients who receive doses that are higher than needed to correctly thin the blood are at risk of life-threatening bleeding. Those who receive doses that are too low may remain at risk of life-threatening blood clots.

One-third of patients receiving warfarin metabolize it quite differently than expected and experience a higher risk of bleeding.
Research has shown that some of the unexpected response to warfarin depends on variants of two genes, (CYP2C9 and VKORC1). The Nanosphere Verigene Warfarin Metabolism Nucleic Acid Test detects some variants of both genes. In August, FDA approved updated labeling for Coumadin, the brand name version of warfarin, explaining that people with variations of the genes may respond differently to the drug. Manufacturers of generic warfarin are adding similar information to their products' labeling. Read the 8/16 transcript of FDA's press conference at http://www.fda.gov/bbs/transcripts/transcript081607.pdf
The Nanosphere test is not intended to be a stand-alone tool to determine optimum drug dosage, but should be used along with clinical evaluation and other tools, including INR, to determine the best treatment for patients. (health care professionals who prescribe warfarin regularly check to see if the drug is working properly by ordering a test that evaluates the blood's ability to clot properly called the prothrombin time, PT. Results are measured in seconds and compared with the expected value in healthy people, known as the International Normalized Ratio,INR.)

FDA cleared the test based on results of a study conducted by the manufacturer of hundreds of DNA samples as well as on a broad range of published literature. In a three site study, the test was accurate in all cases where the test yielded a result; 8 percent of the tests could not identify which genetic variants were present.

Pharmacogenomic information is contained in about 10% of labels for drugs approved by the FDA. In order to provide a reference for genomic biomarkers in labels of FDA-approved drug products, FDA created a table. The table provides a comprehensive list of these markers and links to pharmacogenomic data, taking into account multiple regulatory contexts in which these biomarkers were approved. Most drug labels in the table provide pharmacogenomic information with no immediate recommendation for a specific action (i.e. genetic testing); however a few labels recommend or require genetic testing thereby specifying the use of these markers for reaching a therapeutic decision. The classification of biomarkers is context specific. Reference is made to the requirement of testing for the biomarker:
1 = test required;
2 = test recommended;
3 = information only
See http://www.fda.gov/cder/genomics/genomic_biomarkers_table.htm#_msocom_7
Prescribing the right dose of the right medicine the first time for everyone should speed recovery and reduce adverse drug reactions -- estimated at 100,000 deaths and 2 million hospitalizations annually in the US.

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