Some of the views expressed in the following notes on newly approved products should be regarded as tentative, as there may have been limited published data 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. As a result of fuller experience, initial comments may need to be modified. The Committee is prepared to do this. Before new drugs are prescribed, the Committee believes it is important that full information is obtained either from the manufacturer's approved product information, a drug information centre or some other appropriate source.
Tamiflu (Roche)
75 mg capsules
Approved indication: influenza
Australian Medicines Handbook Section 5.3.2
Oseltamivir is the second neuraminidase inhibitor to receive Australian approval for the treatment of influenza. Unlike its predecessor, zanamivir, oseltamivir is taken by mouth.
After absorption oseltamivir is converted to its active form, oseltamivircarboxylate. The plasma concentrations of this metabolite reach a peak within three hours and then decline with a half-life of 6-10 hours. Oseltamivir carboxylate is excreted in the urine, so the dose should be reduced if renal function is impaired.
In a clinical trial, twice-daily doses of 75 mg or 150 mg were compared with a placebo. The 629 adults involved in the trial had presented within 36 hours of developing a febrile respiratory illness. Laboratory testing confirmed the presence of influenza virus (mostly influenza A) in 374 cases. In these cases oseltamivir reduced the duration of illness by more than 30%.1
Patients given oseltamivir are more likely to experience gastrointestinal upsets than those given a placebo. In clinical trials 12% of patients suffered vomiting and a further 11% complained of nausea. There is potential for the influenza virus to develop resistance.
Although patients treated with oseltamivir recover significantly faster than those given a placebo, the difference is only about one day. Treating 1000patients reduces illness by 254 hours. However, patients given 75 mg twice a day were able to resume their normal activities 2-3 days sooner than those given a placebo. Giving a higher dose does not make patients recover more quickly.1 As few elderly or debilitated people were included in the clinical trials, the best strategy for those at risk is still immunisation to prevent influenza. Little information is available about the efficacy of oseltamivir in influenzaB.
Reference
1. Treanor JJ, Hayden FG, Vrooman PS, Barbarash R, BettisR, Riff D, et al. Efficacy and safety of the oral neuraminidase inhibitor oseltamivirin treating acute influenza. JAMA 2000;283:1016-24.
