The FDA review of Novartis's tobramycin inhaled powder comes ahead of an advisory meeting of outside experts next Wednesday to discuss the drug.

The FDA will make a final decision later, taking into account the advisers' recommendations.

Novartis is seeking approval of the inhaled antibiotic powder as a more convenient alternative to its older, nebulized version of tobramycin, called Tobi. The company says a powder taken with an inhaler is faster to use, and more convenient for patients.

The antibiotic treats a lung infection which often afflicts patients with cystic fibrosis, a genetic disease that affects about 30,000 people in the United States.

The FDA reviewers said it was unclear whether the inhaled powder helped patients, or was as effective and safe as the nebulized version. Only one of two clinical trials showed the tobramycin powder worked better than a placebo, they said.

They also questioned whether Novartis's drug worked as well over time, which may limit how effective it is for cystic fibrosis patients who usually develop chronic lung infections.

"The sustainability of improvements in (lung function) found ... may raise concern regarding the clinical significance of these findings," FDA staff wrote.

The tobramycin powder, administered with an inhaler, has already been approved for use in Europe and Canada. Overall sales of the "Tobi" inhaled antibiotic - which includes the inhaled powder and the nebulized solution - were $296 million in 2011, Novartis said.

Cystic fibrosis causes the thin layer of mucus that helps keep lungs free of germs to thicken, clogging airways and damaging the lungs. The average life expectancy for the disease is 37 years as damage to the lungs progresses and limits the ability to breathe.

The thickening of mucus results in life-threatening infections that must be treated with antibiotics. Novartis's drug treats an infection called pseudomonas aeruginosa.

Novartis shares were little changed at 56.45 francs on the Swiss stock exchange.

(Reporting by Anna Yukhananov; Editing by Bernadette Baum)

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