(Updates with FDA comments, adds background)
* Warning advises of tests for poor responders -- FDA
* Doctors told to consider alternatives for those patients
WASHINGTON, March 12 (Reuters) - The blockbuster anti-clotting drug Plavix will carry a new warning about patients who have a poor response to the drug because they do not break it down well, U.S. regulators said on Friday.
The new language will “warn about reduced effectiveness in patients who are poor metabolizers of Plavix,” the FDA said in a notice on its website.
Poor metabolizers are people whose bodies do not effectively convert Plavix to its active form.
“Because the patient makes less of the active form there is less anti-platelet effect in the blood and the patient may not receive the full benefit of Plavix treatment,” Mary Ross Southworth of the FDA’s Center for Drug Evaluation and Research told reporters in a teleconference.
An estimated 2 percent to 14 percent of the population are poor metabolizers of Plavix, the FDA said.
Bristol said published studies suggested the percentage of poor metabolizers is approximately 3 percent. “Patients should continue taking Plavix unless told to do otherwise by their healthcare professional,” the company said in a statement.
The new warning will advise doctors of genetic tests that can identify those patients and to consider alternatives for them.
The length of time it takes to get results of the genetic tests and costs will vary depending on the laboratories used, but the tests are expected be priced under $500, FDA officials said.
Plavix is the second-best selling drug in the world. It loses patent protection in 2012.
Researchers have been studying an alternative drug for patients who cannot benefit from Plavix and details of their findings are expected to be released at a meeting of heart specialists next week.
Plavix reduces the risk of heart attack, unstable angina, stroke, and cardiovascular death in patients with heart disease by making platelets less likely to form blood clots.
Plavix does not have its anti-platelet effects until it is metabolized into its active form by the liver enzyme, CYP2C19.
Information about poor metabolizers and diminished effectiveness was first included in the Plavix label in May, 2009, officials said.
“Since that label was approved, further evidence has accumulated to support evidence of an association between CYPC219 type, Plavix exposure and poor treatment outcomes,” Southworth said.
“After reviewing all of that data, we felt like we probably needed to strengthen the warning to include that the provider should consider alternative treatment strategies.” (Reporting by Lisa Richwine and JoAnne Allen; editing by Andre Grenon, Gary Hill)