WASHINGTON (Reuters) – U.S. regulators have approved emergency use of one experimental new antiviral drug against pandemic flu and researchers say several more in the pipeline may provide better ways to fight the virus.

Late on Friday the Food and Drug Administration issued an authorization for peramivir, made by Biocryst Pharmaceuticals Inc and licensed to Shionogi & Co Ltd.

The authorization allows the intravenous drug to be used in seriously ill patients who cannot take pills or inhale Relenza or when Tamiflu or Relenza do not seem to be helping.

So far, H1N1 swine flu seems mild enough so that most patients recover with no treatment, and those who get seriously ill are often helped by Tamiflu, Roche AG and Gilead Sciences Inc’s popular pill.

But flu experts fully expect the virus will evolve resistance to Tamiflu, just as at least one strain of seasonal flu has. And it is spreading fast, ahead of efforts to get vaccines out, so drugs are vital for serious cases.

Some of the new drugs in the works could be used if and when that happens, and some hold better potential for using to protect people against the virus before they become infected.

They include Daiichi Sankyo Co Ltd’s CS 8958 or laninamivir and another drug called T-705 or favipiravir, made by Fujifilm Holdings Corp unit Toyama Chemical Co.

If these drugs could be readied and if they work as well as hoped, they can fill in during the months it takes to make flu vaccine — especially if the H1N1 virus mutates and a new vaccine is needed or if some new strain of influenza emerges.

PROMISING FIELD

“In general, I think that the field is looking much more promising than it has in some years and obviously this is in part due to the threats of H5N1 (avian influenza) and pandemic H1N1 disease,” said Dr. Frederick Hayden of the University of Virginia and Britain’s Wellcome Trust.

“It’s good there are alternative drugs and approaches, including combinations, that are being developed.”

The few instances of Tamiflu-resistant swine flu viruses that have been seen all succumb to Relenza, GlaxoSmithKline‘s and Biota Inc’s inhaled flu drug. But that could change, too — the H1N1 virus is resistant to both of the older flu drugs known as amantadine and rimantadine.

Privately held NexBio Inc. has a drug in the works called FluDase, which the company hopes can overcome the problem of resistance. FluDase blocks the molecular doorways, or receptors, that flu viruses use to infect cells.

Because it works on the host — the patient — instead of on the virus, the company thinks resistance may not be a problem.

But researchers like Hayden say this also makes them cautious. “That approach, although it is the most novel one, it is one that is going to take a bit longer in terms of getting the full safety data necessary to advance it,” Hayden said in a telephone interview.

Officials at NexBio say the company is preparing to present studies in the coming weeks that will demonstrate that safety.

“You could envision that it could be used in conjunction with other drugs once its own safety and efficacy have been clarified,” Hayden said. “I think combinations are the way to go.”

Favipiravir is being tested in Japan and Hayden said the U.S. government is working to design good trials that could demonstrate how safe it is, perhaps among hospitalized flu patients this year.

LONG-ACTING DRUGS

Laninamivir, invented by Australia’s Biota, is further along in testing. A phase III clinical trial result released in August showed a single inhaled dose worked as well as taking five days of Tamiflu pills.

It also appeared to be working slightly faster than Tamifu, which can reduce the severity of illness and cut about a day off recovery time.

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.

Hayden said that means fewer doses can be taken and it might be possible to take a dose maybe once a week to prevent infection, an approach that would be used only for people at very high risk of infection such as healthcare workers or people with other diseases with an infected relative.

Tamiflu and Relenza can be used this way but global health experts do not recommend it except to protect the most vulnerable because of the fear the virus could become resistant more quickly with widespread use.

“Biota also has something that they call FluNet,” Hayden added. This drug is a longer-acting version of Relenza, known generically as zanamivir. The National Institute of Allergy and Infectious Diseases has given Biota $2.4 million, with a promise of up to $8.5 million, to develop this one.

In addition, Glaxo is developing an intravenous form of Relenza for the sickest patients.