Swiss drug firm Roche has cited data from a study from the WorldHealth Organization which confirms a low frequency of resistance to thecompany’s antiviral drug Tamiflu® (oseltamivir) over the last threeinfluenza seasons.
The firm said the data, published by the Neuraminidase Inhibitor Susceptibility Network in the WHO’s Weekly Epidemiological Record,shows that resistance of around 0.3% to Tamiflu was seen during theinfluenza seasons (2003–6), in which there had been substantial use by35 million patients in Japan, the highest use in any market. Thecompany noted that this level of resistance is extremely low comparedwith rates of 65% seen in Japan with the much older antiviral,amantadine.
Roche pandemic task force leader David Reddysaid: “These results confirm that the potential for the development ofresistance to Tamiflu is very low, even when used extensively in themanagement of seasonal influenza. This provides reassurance to thescientific community that since the introduction of Tamiflu in 1999,the levels of resistance have remained similar to those seen in theclinical development programme.”
Roche has repeatedlyattempted to counter speculation that resistance to Tamiflu is on therise, especially in patients infected with H5N1 avian flu. The firmacknowledged that possible development of antiviral resistance “is ofconcern for pandemic planning and preparedness”, but stated that todate there have only been three documented cases of Tamiflu resistanceto bird flu.
In one case, the prophylactic (75mg daily)rather than the treatment dose (75mg twice daily) was given to apatient already exhibiting clinical symptoms, “thus underdosing thepatient and increasing the risk of resistance”; once the twice-dailydose was provided, the patient recovered. In the other two cases therecommended dose and duration of oseltamivir was indeed followed, butwhile one patient received treatment on the second day of illness, theother patient started treatment late, on the sixth day.
Thecompany added that two further possible cases of resistance of H5N1 toTamiflu, in Egypt, have been identified and are under investigation. Itwas the news of these cases, in January, that led to the WHO issuing awarning about the possibility of such a scenario, but the organisationalso noted that it had no plans to change its recommendation to treatbird flu patients with Tamiflu.