Antivirals and the avian flu

While a number of antiviral medications are used to combat typical types of influenza, only 2 are active against the new avian influenza virus, known as H5N1, according to Joseph Guglielmo, PharmD, UCSF School of Pharmacy professor and clinical pharmacist specializing in medications to treat infectious diseases.

The 4 current, standard antiviral medications are from 2 classes of drugs. Oseltamivir (brand name Tamiflu) and zanamivir (brand name Relenza) are in a class of drugs called neuraminidase inhibitors. They shorten the severity and duration of symptoms from typical influenza infections. Amantadine (brand name Symmetrel) and rimantadine (brand name Flumadine) are in a second class of drugs called M2 inhibitors. They stop or prevent the common influenza viruses from replicating.

"The avian influenza virus is readily inhibited solely by the neuraminidase inhibitors, such as Tamiflu and Relenza," explains Guglielmo.

The strategy of using these drugs to contain a large flu outbreak has never been tested in any controlled clinical trial. However, at least one US research study has shown that people who take courses of Tamiflu treatment for several weeks at the beginning of flu season are several times less likely to catch the flu.

Rimantadine is approved by the US Food and Drug Administration (FDA) for use in adults only. Approvals for the use of the other 3 antiviral drugs vary by the patient's age. No antiviral drugs are approved by the FDA for use in children who are less than one year of age.

Shortage of antiviral medications worldwide heightens the fear of a potential avian flu pandemic. "We are talking about a matter of years before even developed countries will have sufficient numbers of doses of Tamiflu to protect the populace," says Guglielmo.

Independent of the need for adequate prophylactic antiviral medications, there is urgent need for a vaccine proven to be effective against avian influenza. Similar to the past prevention of "typical" influenza, vaccination should be the mainstay prophylactic strategy. Currently, there are ongoing world-wide efforts to develop such a vaccine. However, treatment of non-immunized, actively infected patients with avian flu will require the use of antiviral medications such as oseltamavir (Tamiflu) and perhaps zanamavir (Relenza)."

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