New Flu Remedy: Zanamivir
The New
Flu Drugs: Neuraminidase Inhibitors
In 1999, the Food and Drug Administration (FDA) approved
two new drugs to fight the flu: zanamivir (Relenza®) and
oseltamivir (Tamiflu®), the first of a new class of antiviral
drugs called neuraminidase inhibitors. ^
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How do
these flu remedies work?
The surfaces of influenza viruses are dotted with neuraminidase
proteins. Neuraminidase, an enzyme, breaks the bonds that
hold new virus particles to the outside of an infected
cell. Once the enzyme breaks these bonds, this sets free
new viruses that can infect other cells and spread infection.
Neuraminidase inhibitors block the enzyme's activity and
prevent new virus particles from being released, thereby
limiting the spread of infection. ^
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Are these flu drugs
an effective flu remedy against any kind of influenza virus?
Although zanamivir and oseltamivir inhibit both influenza
A and B viruses, more information is available on the
effectiveness of these drugs against influenza A viruses.
(Note - the previously approved antiviral flu drugs, rimantadine
and amantidine, are effective only against type A influenza
- see below.) ^
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Who should
consider using this flu remedy?
Zanamivir is approved only for treating uncomplicated
influenza virus infection in people 7 years of age and
older who have not had flu symptoms for more than two days.
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Oseltamivir is approved for treating
uncomplicated influenza virus infection in people 18 years
of age or older who have not had symptoms for more than
two days. A liquid suspension of oseltamivir is approved
for treating acute illness in children who are 1 year
of age and older who have been symptomatic for no more
than two days. Oseltamivir also is approved for preventing
influenza A and B in people 13 years and older.
Currently, oseltamivir is the only neuraminidase inhibitor
approved to prevent the flu. ^
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How are
these drugs administered?
You orally inhale zanamivir, available as a dry powder,
using a device known as a "Diskhaler." The recommended
dosage is two inhalations twice a day, morning and night,
for five days. Oseltamivir, available as a pill, is taken
twice daily for five days. A liquid suspension of oseltamivir
can be taken by children or adults who cannot swallow
a capsule. ^
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Does This Flu Remedy help?
Studies have shown that both oseltamivir and zanamivir
can reduce the duration of flu symptoms by one day if
taken within two days of the onset of the illness. There
is no information about how effective these drugs are
if treatment is started more than two days after onset
of flu symptoms. When taken as directed to prevent the
flu, oseltamivir can significantly reduce your chance
of getting the disease if there is a flu outbreak in your
family or community. ^
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What
other benefits might this Flu Remedy have?
Neither zanamivir nor oseltamivir has been shown to effectively
prevent serious influenza-related complications such as
bacterial or viral pneumonia. However, studies of the
use of zanamivir in families and in nursing homes at risk
for influenza infection resulted in the reduced use of
antibiotics, which are frequently prescribed to treat
these serious complications. ^
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What
about side effects?
The neuraminidase inhibitors generally cause fewer side
effects than the older flu drugs. The most common side
effects seen with oseltamivir are nausea and vomiting.
In some people, zanamivir can cause wheezing or serious
breathing problems that need prompt treatment (see below).
The other most common side effects seen with zanamivir
include headache and diarrhea. Your health care provider
or pharmacist can discuss with you a more complete list
of possible side effects. ^
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Should
certain people not take these flu drugs?
People allergic to these drugs or their ingredients should
not take them.
Zanamivir generally is not recommended
for people with chronic respiratory diseases such as asthma
or chronic obstructive pulmonary disease. In clinical
studies, some patients with mild or moderate asthma or
chronic obstructive pulmonary disease had bronchospasm
(wheezing) after taking zanamivir. If you have an underlying
respiratory disease and have been prescribed zanamivir,
your health care provider should instruct you to have
a fast-acting inhaled bronchodilator available for use
when taking the drug.
The dosage of oseltamivir may
need to be adjusted if you have any type of kidney disease.
Neither drug is generally recommended
for use during pregnancy or nursing, as no information
is available about the effects of these drugs on the unborn
child because these drugs have not been evaluated in pregnant
women.
In laboratory and in limited
clinical studies, there have been no reported interactions
of the neuraminidase inhibitors with other drugs.
For complete safety information
about these drugs, talk with your pharmacist or health
care provider. ^
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Can
influenza viruses develop resistance to these drugs?
Laboratory studies have shown that influenza A and B viruses
can develop resistance to zanamivir and oseltamivir. Surveillance
for neuraminidase inhibitor-resistance has been initiated.
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What
other neuraminidase inhibitors are in the pipeline?
A third neuramindase inhibitor, known only as RWJ-270201,
is being developed by R.W. Johnson Pharmaceutical Research
Institute (a Johnson and Johnson company) and BioCryst
Pharmaceuticals (Birmingham, Alabama). It is currently
being evaluated in Phase I and II clinical trials. ^
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What
other drugs are approved for influenza?
In 1976, amantadine was approved by the FDA to both treat
and prevent influenza type A in adults. Rimantadine-a
derivative of amantadine-was similarly approved in 1993.
These two drugs are active against influenza A viruses
but not against influenza B viruses. These compounds inhibit
the activity of the influenza virus M2 protein, which
forms a channel in the virus membrane. As a result, the
virus cannot replicate after it enters a cell. The FDA
does recommended daily doses for amantadine and rimantadine
treatment and prophylaxis in different age groups; however,
the use of amantadine and rimantadine among children younger
than one year old has not been adequately evaluated. ^
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So which
anti-flu drug should I take?
No study to date has directly compared the effectiveness
of zanamivir, oseltamivir, amantadine, and rimantadine
for the treatment of influenza A. The available information
suggests that these four drugs are similarly effective
in reducing the duration of uncomplicated acute illness
due to influenza A. Zanamivir and oseltamivir differ from
amantadine and rimantadine in terms of their side effects
and cost. Use of amantadine, and to a lesser extent rimantadine,
has been associated with side effects including tremors,
excitability, insomnia, dizziness, mood change, muscle
movement problems, psychosis, and fatigue. These side
effects have not been associated with zanamivir or oseltamivir.
On the other hand, zanamivir and oseltamivir are currently
significantly more expensive than either rimantadine or
amantadine. All four drugs are available by prescription
only, and it is best to consult with your health care
provider to determine what drug might be best for you.
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Since
these flu remedies are available, do I still need a flu shot?
Neither the neuraminidase inhibitors nor the older flu
drugs are a substitute for the flu shot. The flu shot
can provide season-long protection against influenza A
and B and more effectively prevent you from spreading
the virus to others.^
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Source: NIAID is a component
of the National Institutes of Health (NIH).
