Relenza (Zanamivir) Pharmocology

Zanamivir is the pharmaceutically active component of the antiviral medication Relenza. It stops the replication of the influenza virus and can lead to a shorter duration of symptoms, if taken shortly after the infection presents.

Pathophysiology of Influenza Virus

Influenza is caused by a viral infection of the respiratory tract. Once inside the host cell, the influenza virus replicates rapidly and spreads throughout the cell.  After a short time – approximately an hour – the host cell is exhausted of its resources and is destroyed by the virus, which then goes on to find another host cell to continue the replication process.

As a result of the replication of the influenza virus, the cells in the respiratory tract become swollen and inflamed and causing symptoms typical of the flu. These effects occur in varying severity according to the immune function of the individual and the extent of the influenza infection. In some people, the complications of the flu can be particularly severe and may even be fatal.

Pharmacology of Zanamivir

Zanamivir is type of drug called a neuroaminidase inhibitor, which is beneficial in the treatment and prophylaxis of both influenza A and influenza B viruses.

Its mechanism of action is by binding to the active site of the neuraminidase protein. The activation of this protein is essential for the influenza virus to escape the host cell prior to its death to invade a subsequent host cell. The antagonistic effect of zanamivir blocks the neuraminidase protein and inhibits the ability of the virus to move on and affect other cells.

As a result of this effect, the infection is unable to progress and it is halted by the zanamivir. This causes the symptoms that present with the viral infection last for less time and studies have found that taking this medication soon after the onset of symptoms is able to reduce the duration of the infection by approximately half a day.

Administration of Zanamivir

Zanamivir is formulated into a powder that is designed for inhalation, providing direct access to the respiratory tract where the influenza virus usually resides. This is particularly useful as the bioavailability of the drug is only 2%, whereas with administration via inhalation as much as 15% of the drug is absorbed and excreted in the urine.

The oral inhalation can lead to specific side effects that relate to this method of administration. For example, the risk of bronchospasm in patients with co-existing respiratory conditions such as asthma is much higher than an orally administered medication of the same drug class.

Drug Interactions and Side Effects

Zanamivir has a low potential to interact with other substances administered at the same time for several reasons. One of the prominent explanations is its low volume of distribution and greater local effect on the respiratory tract, reducing possible interactions in systemic circulation.

From a theoretical perspective, there are no drug interaction expected to occur and this has been seen in studies that test the drug in vivo. Additionally, the effect of zanamivir remains unaltered, regardless of other drugs administered simultaneously.

If zanamivir needs to be taken in conjunction with other inhaled medications, the other medication should be taken first, followed by administration of zanamivir.

References

• http://www.medicines.org.uk/emc/medicine/2608
• http://www.gsk.ca/english/docs-pdf/product-monographs/Relenza.pdf
• http://www.ncbi.nlm.nih.gov/pubmed/10429839
• http://www.nejm.org/doi/full/10.1056/NEJMra050740