New approaches to influenza chemotherapy. Neuraminidase inhibitors

Epidemic influenza continues to be associated with significant morbidity in the general population, and mortality in the elderly and other high risk patients. Although the case fatality rate averages less than 0.01%, tens of thousands of deaths occur each year. Control through immunisation programmes has not been possible due to incomplete protective efficacy and antigenic variations that occur frequently. Currently available anti-influenza medications (amantadine and rimantadine) have had limited success due to underutilisation, lack of activity against influenza B, the rapid development of viral resistance to the drugs, and adverse effects. A new class of antiviral agents designed to inhibit influenza neuraminidase, an important surface glycoprotein, is currently under active development for use in the prophylaxis and treatment of influenza A and B infections. Two of these compounds, zanamivir (GG167) and GS4104 (the ethyl ester prodrug of GS4071) have reached clinical trials. Most studies of zanamivir have involved topical administration by inhalation of dry powder aerosols and/or intranasal doses of aqueous solutions. These routes rapidly provide high local concentrations at the sites of delivery. GS4104 is administered orally, which allows for greater ease of administration, and probably more uniform distribution of the parent compound GS4071 in the respiratory tract. Both have shown potent inhibitory activity against influenza in animal models and experimental human influenza with excellent tolerability profiles. Zanamivir treatment has been shown to reduce the severity and duration of naturally occurring, uncomplicated influenza illness in adults. Clinical resistance to these drugs has not been recognised as a significant problem to date, although strains resistant to each agent have been produced in the laboratory. This class of agents shows considerable promise as a novel approach to prophylaxis and treatment of influenza infections. Ongoing studies should provide the data needed to allow the addition of 1 or more of the neuraminidase inhibitors to the clinician's anti-influenza armamentarium.