Zanamivir

Zanamivir is an inhaled neuraminidase inhibitor with activity against influenza A and B, including most oseltamivir-resistant strains. It is delivered by dry-powder inhaler because oral absorption is negligible. It can cause bronchospasm and is avoided in asthma and chronic obstructive pulmonary disease. Intravenous formulations are available in some countries for severe disease.

Class. Neuraminidase inhibitor antiviral.

Mechanism. Blocks viral neuraminidase, preventing release of virions from infected cells.

Pharmacokinetics. About 10% to 20% of an inhaled dose is absorbed through the lung; high concentrations at the site of infection. Half life 2.5 to 5 hours. Excreted unchanged in urine.

Influenza treatment (adults and children 7 years and over). 10 mg (two inhalations of 5 mg) twice daily for 5 days, with the first two doses 2 hours apart on the first day if needed; start within 48 hours of onset.

Post-exposure prophylaxis (age 5 years and over). 10 mg once daily for 10 days; for up to 28 days during community outbreaks.

Intravenous zanamivir (where available, severe disease). 600 mg every 12 hours for 5 to 10 days; dose reduction for kidney function.

Loading dose, maintenance dose and titration. Not applicable. Maximum: 20 mg daily by inhalation.

Main alternatives. Oseltamivir, baloxavir marboxil.

Contraindications. Hypersensitivity to zanamivir or lactose (milk protein) in the powder.

Serious adverse effects. Bronchospasm and acute deterioration in lung function, particularly in asthma and chronic obstructive pulmonary disease; anaphylaxis and severe skin reactions.

Common adverse effects. Cough, throat irritation, headache and nausea.

Interactions. None of clinical importance. Do not give with live attenuated influenza vaccine within 2 weeks after or 48 hours before.

Common prescribing errors. Use in asthma or chronic obstructive pulmonary disease without a bronchodilator available, nebulising the powder, and giving to patients who cannot inhale properly.

Special groups. - Pregnancy: acceptable when oseltamivir is unsuitable, although oseltamivir has more data. - Breastfeeding: compatible. - Children: from 7 years for treatment, from 5 years for prophylaxis. - Older adults: usual dose; check inhaler technique. - Kidney impairment: no adjustment for inhaled use. - Liver impairment: no adjustment.

Formulations. Dry powder for inhalation 5 mg blisters (with a device); intravenous solution 10 mg/mL where available.

Administration. Use the supplied device only; inhale deeply and hold the breath. Do not nebulise or mix the powder. Patients with airway disease carry a rapid-acting bronchodilator and use it before zanamivir.

Baseline. Assess airway disease and ability to use the inhaler.

Monitoring. Respiratory symptoms and peak flow during treatment in patients with airway disease.

Stopping thresholds. Stop for bronchospasm, wheeze or breathlessness after dosing.

Referral and escalation. Admission for respiratory distress or hypoxia; specialist review for oseltamivir-resistant or severe influenza.