UPDATE 1-Discovery Labs to file Surfaxin response in 4 weeks

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Sept 24 (Reuters) - Discovery Laboratories Inc (DSCO.O) said it expects to submit a complete response to the U.S. Food and Drug Administration's concerns on its experimental lung drug for infants within the next four weeks.

The company earlier expected to file the response in September.

In May, the biotechnology company had received an "approvable letter" from the FDA for the drug, Surfaxin. Approvable letters generally mean that a company must satisfy certain conditions before the FDA approves its drug.

Discovery Labs said it achieved "technical success" in addressing remaining requirements by the FDA to get marketing approval for Surfaxin, including additional preclinical studies.

Surfaxin treats respiratory distress syndrome, in which prematurely born infants have difficulty breathing. It has orphan drug designation from the FDA -- a status given to therapies being developed for rare conditions.

Shares of the Warrington, Pennsylvania-based company were trading flat at $1.97 in morning trade on Nasdaq. (Reporting by Suzannah Benjamin in Bangalore; Editing by Varsha Tickoo)