Some of the views expressed in the following notes on newly approved products should be regarded as preliminary, as there may have been limited published data at the time of publication, and little experience in Australia of their safety or efficacy. However, the Editorial Executive Committee believes that comments made in good faith at an early stage may still be of value. Before new drugs are prescribed, the Committee believes it is important that more detailed information is obtained from the manufacturer's approved product information, a drug information centre or some other appropriate source.

Relenza (Glaxo Wellcome)
blister packs containing 5 mg powder for inhalation
Approved indication: influenza
Australian Medicines Handbook Section 5.3

Zanamivir was developed in Australia by Biota Holdings as a treatment for influenza. The celebration which followed its approval by the Australian Drug Evaluation Committee in February 1999 was short-lived. Within a few days, the share price of Biota Holdings slumped when the Antiviral Advisory Committee of the Food and Drug Administration (FDA) in the U.S.A. voted against recommending the drug for approval by the FDA.

That Committee's decision may have been influenced by an unpublished study which did not show the benefit seen in other trials. Two multi centre studies, in North America and Europe, found that zanamivir reduced the symptoms of influenza if treatment began early enough in the illness. Patients with fever got better 3 days sooner if they were treated with zanamivir rather than a placebo.1

Zanamivir acts by inhibiting the neuraminidase of the influenza virus. This surface enzyme normally allows the release of the virus from infected cells, so inhibition reduces viral propagation. When volunteers were given zanamivir and then inoculated with the virus, only 13% developed influenza, compared with 70-83% of those given a placebo. Viral shedding was also significantly reduced.2

The powdered drug is inhaled using the Diskhaler device. Patients inhale 10mg twice daily for 5 days. Most of the dose is deposited in the oropharynx and lungs. The 10-20% of the dose which is absorbed into the systemic circulation is excreted unchanged in the urine.

Zanamivir was well tolerated by patients in the clinical trials. No dose adjustments are recommended for the elderly or patients with liver or kidney disease. Caution is required when prescribing for patients with severe asthma in case inhaling zanamivir affects lung function.

While epidemics of influenza can increase morbidity and mortality, high-risk groups, such as the elderly, can be protected by immunisation. Amantadine is already available for people who have been exposed to influenza, so what is the role of zanamivir? An advantage of zanamivir is that, unlike amantadine, it is effective against the less common influenza B.

When a patient presents, the doctor does not know if the patient has influenza or another infection. Even if the infection is confirmed as influenza, zanamivir will only alleviate the symptoms one day faster than a placebo.1 As influenza is a self-limiting condition for most otherwise healthy people, zanamivir is more likely to be a niche drug than a treatment for everyone with flu-like symptoms.