Thursday, October 06, 2005

Tamiflu resistance: maybe not

When widespread reports of Tamiflu resistance in H5N1 surfaced last week we tried to find the source, one of the reasons we didn't post on it. It turns out we weren't the only ones. The scientific flu community also wanted to know the basis for the reports and so did the drug's manufacturer, Roche. It now turns out it was based on old data of one isolate from Vietnam that seemed to show partial resistance.
The professor of pharmacology from Hong Kong University quoted as warning of an emerging resistant strain of the virus says he was citing old data, not new evidence, when he gave an interview last week.

He was trying to urge GlaxoSmithKline to reintroduce an injectable form of their rival flu drug, Relenza. The resulting report suggested Tamiflu was becoming less useful - a claim that was widely repeated.

"My point is to emphasize on the introduction of injectable drugs. But they use a headline 'Resistant H5N1 appears in Vietnam,' " Dr. William Chui, who is also chief of the pharmacy service of Hong Kong's Queen Mary Hospital, said in an interview.


When the reports quoting Chui started to circulate, flu experts around the world sent out urgent e-mails trying to find out who had found new evidence of resistance. The flu community keeps close tabs on the efficacy of these important drugs, known as neuraminidase inhibitors.
They came up with a puzzling blank.

Except for that one partially resistant H5N1 isolate from Vietnam, no researchers have reported new discoveries of Tamiflu-resistant viruses isolated from human cases of H5N1, both the WHO and Tamiflu's manufacturer, Hoffman-La Roche confirm.

"There is a network of laboratories that has been set up to follow antiviral resistance among influenza strains," says Michael Perdue, a scientist in the WHO's global influenza program.

"One of the first things they look at (when they get new viral isolates) is the antiviral sensitivity and resistance. And the papers that have been published thus far have shown all the strains to be sensitive." (Canadian Press)
There were some biological reasons to believe Tamiflu would not exhibit the kind of resistance that amantadine does, so the latest reports were a surprise. It may still turn out that oseltamivir (Tamiflu) develops significant resistance and remains virulent, but the evidence so far is scant.

On the other hand, this drug is not likely to be much use except on the margins and for some people. It needs to be taken continuously for prophylactic effect when exposure is prevalent in the community so compliance will be an issue as will cost. The dose required for whatever virus becomes the pandemic strain is also uncertain. Its main use is likely to be in health care workers and other critical personnel and possibly in household members of those who are ill.

We will keep following this story.