United Therapeutics Corp., a Silver Spring biotechnology firm, reported its first profitable quarter yesterday.

The company reported $18.3 million in revenue for the quarter, up 31 percent from $14 million in the second quarter last year.

Profit for the quarter was $4.1 million (18 cents per diluted share), up sharply from a loss of $2.4 million (11 cents) in the second quarter last year.

The company's shares rose 17 percent, closing at $28.05, up $4.01.

United Therapeutics' chief drug on the market so far is Remodulin, which is used to treat abnormally high blood pressure in arteries between the heart and lungs. The condition is rare.

During the second quarter, United Therapeutics said, the number of people using Remodulin increased by about 75 people, to 700.

More than 200 doctors prescribe Remodulin, according to Martine Rothblatt, the company's chairman and chief executive.

Juan F. Sanchez, an analyst with Punk, Ziegel & Co., a New York investment bank, said the number of doctors prescribing Remodulin and increased patient use reflected growing comfort with the medicine despite the pain it causes patients.

The company is trying to develop a less intrusive way to deliver the drug, which is administered through a catheter under the skin.

United Therapeutics is awaiting Food and Drug Administration approval to market the drug for less intrusive intravenous use, and it expects to do so early next year, Rothblatt said.

Sanchez forecasts $67 million in Remodulin sales this year. He expects sales eventually will surpass $250 million a year.