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Drug Interaction Can Render Tamiflu Ineffective

November 1, 2006

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Health officials are counting on the anti-viral drug Tamiflu to be a critical weapon in the event of an influenza pandemic, but it may provide only limited protection for some patients.

A new study at the University of Rhode Island's College of Pharmacy indicates that the drug can be rendered ineffective in patients also taking the anti-clotting drug Plavix.

"Concurrent use of both drugs would inhibit the activation of the drug, thus making this anti-viral agent therapeutically inactive." said URI Pharmacy Professor Bingfang Yan, head of a URI research team that focuses on why people respond to medications differently.

Yan has notified the U.S. Food and Drug Administration and the National Institutes of Health about the effect of Plavix on Tamiflu. The Journal of Pharmacology and Experimental Therapeutics will publish the results of his team's research in its December issue.

Yan's team completed laboratory analysis of the two drugs. "Now we need to study the effects of the combination in human trials," Yan said.

Tamiflu must be hydrolyzed in the body to be effective. Hydrolysis is the decomposition of a chemical compound through a reaction with water. But in the presence of clopidogrel (the generic, scientific name of Plavix), the hydrolysis of Tamiflu was inhibited as much as 90 percent, Yan said.

"This is epidemiologicially significant because people who receive Tamiflu and Plavix simultaneously may maintain susceptibility to influenza or a source that spreads influenza if they are already affected," Yan said.

Yan said that because patients who have suffered from stroke, heart attack or peripheral artery disease would be among the high-risk individuals for contracting influenza of any strain, they could be among those getting Tamiflu and Plavix at the same time.

The Tamiflu website says it is the number one flu medication in the country, with industry data showing that more than 2 million prescriptions being written in 2005. Since the government has made Tamiflu one of its main weapons in the fight against a flu pandemic, that number would increase considerably during an outbreak. Industry data also show that more than 20 million Plavix prescriptions were written in 2005.

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