BEIJING, Jan. 16 (Xinhuanet) -- Canada's public health agency , following in the footsteps of U.S., instructs doctors not to prescribe a flu drug called amantadine. This drug has been proved ineffective in killing flu viruses that circulate in both countries.
The director of the Public Health Agency of Canada's laboratory said Sunday that testing showed 91 per cent of H3N2 flu viruses the lab has received so far this year carry a mutation that makes them resistant to amantadine.
"This was a real surprise," said Dr. Frank Plummer, noting that only one per cent of H3N2 viruses ¡ª the currently predominant human influenza A strain ¡ª tested from the 2003-2004 influenza season were resistant to the drug.
That resistance figure from Canada mirrors precisely the situation in the United States, which officials of the U.S. Centers for Disease Control announced Saturday in a hastily called press conference.
"That is a startling correlation and confirmation of our work," Rick Bright, a CDC flu researcher who has charted the rise of resistance to amantadine and its sister drug, rimantadine over the past decade.
Amantadine and rimantadine belong to the adamantane class of drugs. Both are sold in the U.S. but rimantadine was not brought to market in Canada.
In North America, the drugs have mainly been used to control outbreaks of influenza A in hospitals and long-term care facilities. The adamantane drugs are not effective against influenza B viruses.
Amantadine and rimantadine have been off patent for years and are inexpensive, especially in comparison to the only other flu drugs on the market, the neuraminidase inhibitors ¡ª oseltamivir (sold as Tamiflu) and zanamivir (sold as Relenza). Neuraminidase inhibitors work against both influenza A and B viruses.
Despite their increased cost, the neuraminidase inhibitors have been supplanting the adamantane drugs in many jurisdictions because they cause fewer drug interactions and fewer side-effects and because they work against a broader range of influenza viruses.
In instructing doctors not to prescribe adamantane drugs, both public health agencies are advocating increased use of neuraminidase inhibitors.
In the face of the threat of a possible flu pandemic, some worry that broader use now of these precious drugs may give rise to increased resistance against them later.
The resistance problem also highlights the need to invest in vaccine development, Gerberding said, noting constantly evolving flu viruses will always challenge the efficacy of antiviral drugs. Enditem
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