HONG KONG, June 3 (Reuters) - Scientists in Hong Kong have used a cocktail of three drugs which appeared to raise the survival rates of mice infected with lethal doses of the H5N1 avian flu virus.

Writing in the Proceedings of the National Academy of Sciences, the scientists said the drugs suppressed the virus and toned down an over-reaction of the immune system.

Some experts believe H5N1 triggers a "cytokine storm" -- a reaction in the immune system so severe it ends up killing the patient. The H5N1 is associated with a mortality rate of between 60 and 80 percent in people.

Professor Yuen Kwokyung, a leading microbiologist at the University of Hong Kong, said the team used the antiviral drug zanamivir and two non-steroidal anti-inflammatory agents.

Commenting on the latter, Yuen said: "They suppress the cytokine storm without suppressing the good protective response, which steroids do. Steroids suppress everything."

"So the patient has the chance of mounting ... an antibody response," he told Reuters.

In the experiment, the scientists infected mice with lethal doses of H5N1 virus and did not start treating them until 48 hours after they were infected.

"In the last 10 years, no regimen has been shown to work after 48 hours," Yuen said.

But with the combination of three drugs, more of the mice survived.

"We find that by giving an antiviral (zanamivir) systemically combined with two non-steroidal anti-inflammatory agents, they can increase the survival of the infected mice from 13.3 percent (zanamivir alone) to 53.3 percent (combined treatment). That's increasing the survival fourfold," Yuen said.

The two non-steroidal anti-inflammatory agents were celecoxib and mesalazine, which is useful for inflammatory bowel disease.

"We should consider doing clinical trials with these three drugs. They may work in humans, you can decrease theh mortality rate to 20 percent, theoretically," Yuen said. (Reporting by Tan Ee Lyn; Editing by Jerry Norton)

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