Flu virus shows surprising resistance to oseltamivir

Aug 31, 2004 (CIDRAP News) – Influenza viruses in 18% of a group of Japanese children who were treated with oseltamivir (Tamiflu) developed resistance to the antiviral drug, which is viewed as one of the key defenses against pandemic influenza, according to a new report in The Lancet.

Oseltamivir and zanamivir block neuraminidase, an enzyme that enables the influenza virus to escape from an infected cell and spread to other cells. Previous clinical trials have shown low rates of viral resistance to the two drugs, called neuraminidase inhibitors. Because of this, and because they are effective against all strains of influenza, they are considered an important bulwark against new flu strains that could cause a pandemic. (Two older antiviral drugs used against influenza, amantadine and rimantadine, are effective only for influenza A and are associated with higher rates of viral resistance.)

The new study suggests that viral resistance to oseltamivir may be a greater problem than previously believed. "In our study, about a fifth of children developed resistance by day 4 or later during treatment with oseltamivir," says the report by Maki Kiso of the University of Tokyo and several Japanense and American colleagues. "Furthermore, just over a quarter of children who shed virus for 3 days or more had drug-resistant influenza viruses."

The researchers analyzed influenza A (H3N2) viruses collected from 50 children before and during treatment with oseltamivir in 2002 and 2003. The children ranged from 2 months to 15.8 years of age, but 80% were younger than 5 years; 29 of them were hospitalized for treatment.

The authors extracted viral RNA from patient specimens, used polymerase chain reaction to amplify the hemagglutinin and neuraminidase genes, and then analyzed the gene sequences. To identify neuraminidase mutations, viruses from samples taken during patients' first clinical visits were compared with those from samples obtained later. The investigators also isolated viruses from clinical samples, grew them in cell culture, and, after identifying those with particular mutations, tested them for sensitivity to oseltamivir.

Neuraminidase mutations were found in viruses from 9 (18%) of the 50 children, the report says. In eight cases the mutation was known to confer resistance to neuraminidase inhibitors. The mutation in the ninth case had not previously been shown to cause resistance, but sensitivity testing by the authors showed that it did. The testing showed that, depending on the particular mutation, mutant neuraminidase was anywhere from 300 to 100,000 times more resistant to oseltamivir than pretreatment neuraminidase was.

Drug resistance was most frequent in viruses from 1-year-old children (4 of 12 patients) and was less common in those younger than 1 year (1 of 9) and from 2 to 6 years old (4 of 22). No resistance was found in the seven children aged 7 years or older.

Resistance showed up in the viruses as early as 4 days after the beginning of treatment and persisted through day 7, according to the report. Children with resistant variants generally shed virus longer than the rest, but some of the patients with nonresistant virus were still infectious after 5 days of oseltamivir treatment. The study design did not allow the researchers to assess the relationship between drug resistance and the patients' clinical course.

The researchers write that the increased resistance they detected might be partly explained by their rigorous techniques. They also note that, depending on a patient's weight, oseltamivir dosages used in Japan may be lower than in other countries and may be suboptimal, which could contribute to viral resistance.

In an accompanying commentary, Dr. Anne Moscona of Mt. Sinai School of Medicine in New York City calls the study "a timely wake-up call." She writes that neuraminidase inhibitors have been shown to be effective against neuraminidase from the virus that caused the 1918 flu pandemic, the avian flu viruses that infected some humans in Hong Kong in 1997 and 1999, and this year's H5N1 avian flu strain.

"Stockpiles of these drugs are a key piece of preparedness and would be critical to an effective response," Moscona states.

The question now, she says, is whether oseltamivir-resistant viruses are transmissible and pathogenic. "If we are very lucky, they may have a growth disadvantage, or, for other reasons, be less virulent or transmissible." She adds that although there are no reports of transmission of oseltamivir-resistant viruses in humans, the Japanese findings suggest that it "is only a matter of time" until this happens.

Moscona concludes that the findings should be taken as "an energizing mandate to learn more about the incidence and mechanisms of resistance to the neuraminidase inhibitors, so that appropriate strategies can be developed for their use during the next pandemic."

Kiso M, Mitamura K, Sakai-Tagawa Y, et al. Resistant influenza A viruses in children treated with oseltamivir: descriptive study. Lancet 2004;364(9436):759-65 [Abstract—access requires free registration]

Moscana A. Oseltamivir-resistant influenza? (Commentary) Lancet 2004;364(9436):733-4 [Preview]

See also:

Aug 26 CIDRAP News story, "HHS releases plan for coping with pandemic flu"

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