Gilead Sciences Inc has received a complete response letter from the U.S. Food and Drug Administration for a new drug application for aztreonam lysine for inhalation, an investigational treatment for cystic fibrosis.

The US FDA informed the company that it cannot approve the new drug application in its current form and that an additional clinical study will be required.Gilead plans to continue talks with the FDA to determine if further analyses of existing data could lead to approval, or if it needs to conduct the additional study as suggested in FDA letter.

John Martin, Gilead's chairman and chief executive, said the FDA has not raised any significant concerns regarding the safety of aztreonam lysine, and that the company will continue offering it in its expanded access program for patients who have limited treatment options.

Gilead submitted its drug application on Nov. 16, 2007. Marketing applications for aztreonam lysine for inhalation are under review by regulatory agencies in the European Union, Australia, Switzerland, Turkey and Canada.