By Julie Steenhuysen

CHICAGO, Dec 17 (Reuters) - An older antibiotic once used against tuberculosis but since abandoned may work to treat the most common and actively contagious form of the disease more quickly, U.S. researchers said on Monday.

By substituting a high-dose version of the drug rifapentine for another antibiotic in the standard TB treatment cocktail, researchers cured mice with the disease two to three times faster.

If confirmed in people, using the drug could cut the average treatment time from six months to three months or less, said Dr. Eric Nuermberger, an infectious disease specialist at Johns Hopkins University, whose study appears in the Public Library of Science journal PLoS Medicine.

"This drug could well be the new backbone for treating drug-susceptible disease," Nuermberger said in a telephone interview.

Daily doses of rifapentine, made by Sanofi-Aventis <SASY.PA> and previously marketed under the brand name Priftin, were so effective in mice that the drug is being moved into human trials in eight countries starting next year, he said.

Shorter treatment time could help people stick to the prescribed therapy better and reduce the development of resistant strains, he said in a telephone interview.

"Shortening the duration of treatment has become the most important goal of TB drug devolvement," he said.

Tuberculosis infects about a third of the world's population. Most cases are latent, meaning people have no symptoms and are not infectious to others.

But the infection can become active when people's immune systems are suppressed -- most notably by the AIDS virus.

HARD TO COME BY

Nuermberger wanted to test a high-dose version of rifapentine because it was in the same class as rifampin, which is part of the standard antibiotic cocktail that also includes pyrazinamide and isoniazid.

But the Sanofi drug, which had been approved for sale in the United States in 1998 as a long-acting weekly treatment, was out of production and hard to come by.

"When we tried to get some samples of the drug, we couldn't even find anyone in the company who remembered if they still made it," Nuermberger recalled.

They persevered and began testing the drug in 2006 in mice using different dosing strategies, including daily doses and a higher dose given three times a week.

"In both cases it dramatically shortened treatment in the mouse model. It cuts it in half or more," Nuermberger said.

He said rifapentine stayed in the blood in three times higher concentrations throughout the treatment than rifampin.

Johns Hopkins researchers plan to conduct two clinical trials in humans in eight countries starting next year. "The only thing we have to show is that people can tolerate a little more exposure to the drug," Nuermberger said.

Meanwhile, Sanofi is gearing up for studies of its own, Nuermberger said. "It is kind of a resurrection story."

Nuermberger said the drug would probably not work against multi-drug resistant or MDR tuberculosis, a strain that can survive the effects of most antibiotics, because it is so similar to rifampin -- one of the drugs useless against MDR TB.

About 1.6 million people died from TB in 2005, according to the World Health Organization. (Editing by Eric Walsh)

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