Discovery Laboratories Inc said U.S. health regulators have approved its drug to prevent a breathing disorder in premature infants, sending its shares up 35 percent in extended trade.

The company expects to launch its drug, Surfaxin, the first synthetic surfactant approved for the prevention of respiratory distress syndrome (RDS), later this year in the United States.

RDS is a condition in which lungs of premature infants are not able to make enough surfactant, a liquid that coats the inside of the lungs and helps to keep them open. Without enough surfactant, the lungs collapse.

Currently, infants with RDS are often treated with animal-derived surfactant replacement therapy along with mechanical ventilation to survive.

Discovery Lab's spokesman Michael Parks told Reuters that Surfaxin will be targeting a market worth about $50-$70 million. However, he declined to give any sales estimate for the drug.

Surfaxin, which had been rejected four times by the U.S. Food and Drug Administration, becomes the fifth approved drug in the United States to treat RDS in premature infants.

The other approved surfactants are Survanta, Curosurf, Infasurf, and Exosurf, which is no longer marketed.

"The approval of Surfaxin is an important medical advancement for the neonatology community and parents of preterm infants who will soon have an effective alternative to animal-derived surfactants to prevent the development of RDS," Chief Executive Thomas Amick said.

Discovery Lab's shares, which have more than doubled in value since the company resubmitted its marketing application in September last year, were up at $5.05 in aftermarket trade. They closed at $3.75 on Tuesday on the Nasdaq.