* Single dose treats and protects

* Drug is longer acting than Tamiflu or Relenza

By Maggie Fox, Health and Science Editor

WASHINGTON, Feb 25 (Reuters) - A single dose of an experimental influenza drug saves more mice from H5N1 avian influenza than the preferred drug Tamiflu, researchers reported on Thursday, and can also protect against infection.

The tests of Daiichi Sankyo Co Ltd"e;s <4568.T> CS 8958 or laninamivir show one inhaled dose worked better than Tamiflu to keep mice alive when infected with a normally deadly dose.

The report in the Public Library of Science Journal PLoS Pathogens covers one of the dozens of ongoing studies of a new batch of influenza drugs being developed by a variety of companies.

"Importantly, a single dose of CS-8958 conferred a more potent and long-lasting protective effect to mice against H5N1 influenza viruses than that of oseltamivir phosphate," Yoshihiro Kawaoka of the University of Wisconsin and colleagues wrote in their report.

They gave a single dose to mice two hours after infecting them with H5N1, which experts fear could cause a pandemic, and also used it to prevent infection. Their report is available at http://dx.plos.org/10.1371/journal.ppat.1000768 .

"CS-8958 is, therefore, a promising candidate for a new neuraminidase inhibitor to prevent and treat influenza patients infected with H5N1 and other subtype viruses," they wrote.

The study was funded by Daiichi Sanyo and the Japanese government, neither of which had a say in the study"e;s design or publication.

The now-waning pandemic of H1N1 influenza sparked new interest in the development of better drugs to fight flu, which kills 250,000 to 500,000 people globally in a normal year and far more in a severe pandemic.

The World Health Organization said on Tuesday it was premature to declare the H1N1 swine flu pandemic had passed its peak, although the agency noted the virus had not been as severe as was feared.

H5N1 is still circulating and experts say it could also change at any time into a form easily passed from one person to another, and it has been far more deadly than H1N1.

SEEKING TO BRING DRUG TO MARKET NEXT YEAR

Drugs that work against one form of influenza usually work against others and Daiichi Sankyo has said it plans to apply for approval of the drug by next month. It aims to bring it to market by March 2011.

Four drugs are on the market for treating flu but the two older ones, amantadine and rimantadine, are not used because most flu strains resist their effects.

Roche AG <ROG.VX> and Gilead Sciences Inc"e;s <GILD.O> oseltamivir, a pill sold under the brand name Tamiflu, is the favored drug but must be taken quickly to be useful. Flu viruses develop resistance to it quickly.

GlaxoSmithKline"e;s <GSK.L> <GSK.N> and Biota Inc"e;s <BTA.AX> zanamivir or Relenza also works against the known circulating flu strains but must be inhaled.

Laninamivir is a long-acting neuraminadase inhibitor. It is in the same class of drugs as Relenza and Tamiflu but persists longer in the body.

Other flu drugs in the works:

* BioCryst <BCRX.O> Inc"e;s infusable drug peramivir is approved for emergency use in the United States and Japanese drugmaker Shionogi & Co <4507.T> has applied for wider use in Japan.

* Fujifilm Holdings <4901.T> pharmaceuticals unit Toyama Chemical has started Phase II clinical testing of the experimental influenza drug T-705 or favipiravir in the United States. Tests in mice show it may work even when taken three days after flu symptoms begin.

* NexBio Inc"e;s FluDase is also in Phase II trials, the second step of human testing before seeking government licenses. (Editing by Peter Cooney)