The study, published in the New England Journal of Medicine, is the first scientifically rigorous clinical trial of Pfizer's antibiotic linezolid, or Zyvox, in patients with extensively drug-resistant TB (XDR-TB), which is resistant to at least four of the drugs most often used to treat the lung infection.

Led by Clifton Barry of the National Institute of Allergy and Infectious Diseases and Sang-Nae Cho, professor of infectious diseases at Yonsei University, South Korea, the study showed the drug was effective when added to patients' current treatments.

"The data that were reported are rather impressive," Dr. Anthony Fauci, director of the institute, a part of the National Institutes of Health, said in a telephone interview.

However, most of the patients - 82 percent - experienced side effects while on the treatment, which tempered the findings, the team reported.

"That is the bad news," Fauci said. "The somewhat encouraging news is that despite the toxicity, when the drug dose was decreased or was temporarily discontinued, the toxic side effects diminished dramatically."

Researchers are desperately looking for new treatments for drug-resistant forms of TB, which threatens to derail progress in the global fight to eradicate the disease.

"To have a study that has a positive result, despite the fact that there are caveats, is an important advance in the right direction," Fauci said.

Treating typical TB cases is a long process, with patients needing to take a cocktail of powerful antibiotics for six months. Many patients fail to complete their treatment, a factor which has fueled a rise in the drug-resistant forms.

Although rare, 77 countries worldwide reported at least one case of XDR-TB by the end of 2011, according to the World Health Organization.

In the United States, at least 57 cases of XDR-TB were reported between 1993 and 2010. Patients infected with XDR-TB typically have very poor clinical outcomes, and with no effective drugs available, they often die.

The study enrolled 39 patients with XDR-TB at South Korean hospitals who had failed to respond to any treatment during the six months before enrolling in the study. Most of the patients at the time were taking 11 drugs on average.

Patients were divided into two groups. One group was treated with 600 milligrams of linezolid immediately, while the other received the medication two months later.

After patients no longer tested positive for TB, or after four months on treatment, study volunteers were randomly assigned to take either a 600 or 300 milligram dose of the drug for another 18 months.

In the patients who took the drug immediately, the addition of linezolid offered a significant benefit in helping patients clear TB bacteria, compared with the delayed treatment group.

Side effects, which occurred in 82 percent of patients, included a decrease in the ability of the bone marrow to produce blood cells, resulting in low red and white blood cell counts, and damage to the optic and peripheral nerves.

Dr. Mel Spigelman, president and chief executive of the TB Alliance, a nonprofit research group, said the findings highlight the potential of this new class of TB drug candidates, known as oxazolidinones, and the treatment challenges posed by XDR-TB.

"Clinicians are currently forced to combat XDR-TB with an incomplete arsenal of inadequate weapons, and the fact that linezolid may be an addition to that arsenal is good news," said Spigelman, who was not involved in the research.

"On the other hand, like many of the existing drugs used to treat XDR-TB, linezolid presents toxicity concerns that may suggest that other, newer oxazolidinones could be more attractive for TB treatment," he said in an e-mail.

Spigelman said additional drugs from this class are in earlier-stage development, and the hope is that they may be as effective and safer than linezolid.

(Reporting by Julie Steenhuysen; Editing by Stacey Joyce)

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