A new study casts doubt on the usefulness of a highly touted genetic test for patients taking the popular anti-clotting drug Plavix.
It also calls into question whether the US Food and Drug Administration (FDA) was premature in issuing a warning for the medicine last year based on the test, according to the study authors and other prominent physicians.
The test, which helps determine how well patients can process the drug, is meant to help doctors decide whether patients should take Plavix, marketed by Bristol-Myers Squibb and Sanofi, or an alternative treatment such as Eli Lilly & Co.'s Effient.
Plavix, or clopidogrel, is used to prevent blood clots in people at risk for heart attacks and strokes, including patients who have had artery-opening stents implanted. The blood thinner, which had $6.7 billion in global sales last year and is taken by some 40 million patients, is the second biggest selling drug after the statin Lipitor.
But the drug must be activated by enzymes in the body in order to have a therapeutic effect. Patients with a certain variant of a gene that makes the CYP2C19 enzyme have been found in a number of studies to process the drug more poorly, prompting concern that these patients were not receiving an adequate dose.
The FDA considered the evidence strong enough to slap its strongest warning, the black box on Plavix's label in March 2010, cautioning patients and physicians about reduced effectiveness of the medication in patients with a particular CYP2C19 variant. About 30 percent of the US population has the variant.
The findings, which come from a review of 32 published studies involving some 42,000 patients, confirmed that patients with the gene variant indeed differed in their ability to activate the drug. But those patients did not have a substantially higher rate of heart attacks and other cardiac events, according to Michael Holmes, the primary author of the study and a researcher in the genetic epidemiology department at University College London.
The results suggest that "there's insufficient evidence" to support use of genetic testing to guide Plavix use, said Aroon Hingorani, another author and director of University College London's Institute for Cardiovascular Science.
What this means for patients is that some Plavix users who got the genetic test may have been switched to more expensive drugs, when they did not need to be. And some doctors may have increased weak-responders' medication dose.
The study was published Tuesday in the Journal of the American Medical Association and was funded by the UK government and a private foundation.