United Therapeutics Corp. on Thursday said it is suing Sandoz Inc., which wants to sell a generic version of United Therapeutics' blood pressure drug Remodulin.

United Therapeutics maintained Sandoz, a unit of Novartis AG, is infringing on three patents relating to Remodulin. The drug is used to treat pulmonary arterial hypertension, or high blood pressure in the arteries of the lungs.

United Therapeutics gets more than half its revenue from sales of Remodulin. Sales of the drug, also called treprostinil, rose 7 percent to $430.1 million in 2011. The drug was approved in May 2002, and the first patents supporting it expire in October 2014.

Silver Spring, Md.-based United Therapeutics said it filed its suit in response to Sandoz's recent application for the Food and Drug Administration to approve its generic. As a result of the suit, the FDA won't approve the generic for 30 months or until the lawsuit is resolved.

Sandoz declined to comment on the filing and the lawsuit.

The suit was filed in U.S. District Court for the District of New Jersey.

Shares of United Therapeutics rose 78 cents to $47.34 in morning trading. U.S.-traded shares of Novartis added 4 cents to $54.31.