Zanamivir: Mechanism of Action, Indications and Important Notes

Zanamivir is an antiviral active substance from the group of neuraminidase inhibitors used for the treatment and prevention of influenza infections. The active substance specifically intervenes in the replication cycle of the influenza virus by inhibiting an enzyme essential for viral spread. Zanamivir was one of the first neuraminidase inhibitors approved for clinical use and has been in use since the late 1990s. Unlike the equally well-known neuraminidase inhibitor oseltamivir (Tamiflu), zanamivir is not administered orally but by inhalation.

Mechanism of Action

The influenza virus multiplies in the mucosal cells of the respiratory tract. After replication inside the cell, new virus particles must be released from the host cell and transmitted to further cells. The viral enzyme neuraminidase plays a decisive role in this process: it cleaves connections between newly formed virus particles and the cell surface, and between the virus particles themselves. Without this cleavage, new virus particles would accumulate on the cell surface and would be unable to infect neighbouring cells.

Zanamivir competitively inhibits neuraminidase by binding to the active centre of the enzyme. This inhibition blocks the release of new viruses from infected cells and significantly slows the spread of the virus in the respiratory system. Zanamivir is active against neuraminidase types N1 and N2 and is therefore effective against both influenza A and influenza B.

Because the neuraminidase binding site is highly conserved, meaning it remains very similar across different influenza strains, zanamivir retains its efficacy against various influenza strains. Resistance to zanamivir is possible, but has so far occurred less frequently than resistance to other antiviral agents for influenza. This is partly because resistance mutations in neuraminidase often also impair enzyme function and thereby weaken the virus.

Indications

Treatment of Influenza

Zanamivir is approved for the treatment of influenza in adults and children above a certain age (generally from 5 years) who present with typical influenza symptoms. For optimal efficacy, zanamivir should be used as early as possible after symptom onset, ideally within the first 48 hours. When treatment is started in a timely manner, zanamivir can reduce the duration of illness by an average of about one to two days. Whether antiviral therapy with zanamivir is appropriate in a specific situation is assessed by the physician on the basis of the clinical situation and the infection context.

Prophylaxis of Influenza

Zanamivir is also approved for the prevention of influenza infection, for example after close contact with an infected person or in outbreak situations. Prophylactic use generally involves once-daily inhalation over a defined period. It does not replace influenza vaccination, but can be used as a supplementary measure in certain situations, for example in persons with contraindications to vaccination or in acute outbreak situations.

Special Patient Groups

In high-risk patients with severe disease or in immunosuppressed patients with influenza who do not respond to other antiviral therapies, zanamivir is also available as an intravenous formulation (within compassionate use or emergency authorisation frameworks). This parenteral form of administration is reserved for special clinical situations and is administered under intensive care supervision.

Dosage Form and Dosing

Zanamivir is administered by inhalation as a dry powder using a special inhalation device (Diskhaler). The patient inhales the powder deep into the airways, where it reaches its target concentration directly at the site of action in the respiratory tract. Systemic absorption is low, meaning that most effects occur locally in the airways and systemic exposure is limited. This explains the more favourable systemic side-effect profile of zanamivir compared to orally administered antiviral agents.

For the treatment of influenza, zanamivir is administered twice daily as an inhalation, two inhalations each time (corresponding to 2 x 5 mg, i.e. 10 mg per dose). The duration of treatment is generally five days. For prophylaxis, one inhalation daily is given over the recommended prophylaxis period.

Important Notes

Correct inhalation technique is essential for the efficacy of zanamivir. The powder must be inhaled deeply and evenly. Patients generally receive instructions on how to use the Diskhaler inhalation device. In case of uncertainty, the pharmacy team or medical staff should be asked to provide a demonstration.

Zanamivir should be used with particular caution in patients with respiratory conditions such as asthma or chronic obstructive pulmonary disease (COPD), as bronchospasm has occurred after inhalation. The powder can irritate sensitive airways. For patients with asthma or COPD, oseltamivir is generally the preferred antiviral option. If zanamivir is nevertheless used, a fast-acting bronchodilator should be at hand and inhaled beforehand.

Adverse Effects

Zanamivir is generally well tolerated. Possible adverse effects include:

• Bronchospasm (spasm of the airway muscles) with breathing difficulties, particularly in patients with pre-existing respiratory conditions. This is the most clinically important side effect.
• Cough and irritation of the airways after inhalation
• Headache
• Dizziness
• Nausea (rare, as systemic absorption is low)
• Allergic reactions including angioedema have been reported very rarely
• Neuropsychiatric events such as delirium or confusion have been reported very rarely, similar to other neuraminidase inhibitors. The causal relationship has not yet been conclusively evaluated, so patients and caregivers should watch for unusual behaviour.

Comparison with Oseltamivir

Both zanamivir and oseltamivir are neuraminidase inhibitors with similar principles of action. The key differences are:

• Dosage form: zanamivir is inhaled, oseltamivir is taken orally as a capsule or suspension
• Patient groups: oseltamivir is approved from birth; zanamivir generally from 5 years
• Respiratory conditions: oseltamivir is preferred in asthma or COPD, as zanamivir can trigger bronchospasm
• Resistance profile: resistance to oseltamivir, particularly through mutations at certain positions of neuraminidase, may preserve the efficacy of zanamivir. Cross-resistance between the two agents is possible but not fully predictable.
• Gastrointestinal side effects: oseltamivir more frequently causes nausea and vomiting, while zanamivir causes fewer gastrointestinal side effects due to low systemic absorption

Zanamivir on Sanoliste

On Sanoliste you will find general practitioners, internists and pulmonologists who offer competent advice and treatment for influenza infections and other respiratory diseases. Medical assessment is important to evaluate whether antiviral therapy with zanamivir or another active substance is appropriate and medically suitable.

The information on this page is for general information purposes only and does not replace consultation with a physician. Zanamivir is a prescription-only medicine and may only be used following medical prescription.

Frequently Asked Questions about Zanamivir

How quickly must zanamivir be used to be effective?

Zanamivir should be used as early as possible after the onset of first influenza symptoms. The greatest benefit is achieved when treatment begins within the first 48 hours after symptom onset. Within this time window, viral replication is still active and inhibition of neuraminidase can effectively slow the spread of the virus. With later treatment, efficacy decreases. In special clinical situations, for example in immunosuppressed patients or with a severe course, starting treatment after 48 hours may still be appropriate. This decision rests with the treating physician.

Does zanamivir also protect against other respiratory conditions such as a cold?

No, zanamivir acts specifically against influenza viruses by inhibiting neuraminidase. Other pathogens causing respiratory infections such as rhinoviruses (common cold pathogens), coronaviruses, RSV or other viruses do not possess neuraminidase in the same sense. Zanamivir has no activity against these pathogens. Since influenza can sometimes be clinically difficult to distinguish from other flu-like infections, laboratory diagnostics can help confirm the pathogen. Antiviral therapy with zanamivir is only appropriate in cases of confirmed or highly probable influenza infection.

Can zanamivir replace influenza vaccination?

Zanamivir does not replace vaccination against influenza. Vaccination offers broader and more durable protection through activation of the immune system and is considered the most important measure for influenza prevention, particularly for risk groups such as older people, persons with chronic conditions and pregnant women. Zanamivir can be used as a supplementary measure for prophylaxis in certain situations, for example when vaccination is not possible or in acute outbreak situations, but it is not a full substitute for vaccine protection.