The bioavailability of zanamivir is 2%. After inhalation, zanamivir is concentrated in the lungs and oropharynx, where up to 15% of the dose is absorbed and excreted in urine.
Dosing is limited to the inhaled route. This restricts its usage, as treating asthmatics could induce bronchospasm. The U.S. Food and Drug Administration (FDA) has issued a Public Health Advisory warning that it has received some reports of respiratory problems following inhalation of zanamivir by patients with underlying asthma or chronic obstructive pulmonary disease. The zanamivir package insert contains precautionary information regarding risk of bronchospasm in patients with respiratory disease.
Zanamivir has not been known to cause toxic effects, does not spread around through the body's systemic circulation and shows no signs of viral resistance from any flu.
GlaxoSmithKline (GSK) and FDA notified healthcare professionals of a report of the death of a patient with influenza who received Relenza (zanamivir) Inhalation Powder which was solubilized and administered by mechanical ventilation.
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