Martine Rothblatt, who founded United Therapeutics and also SiriusXM, founded the drugmaker after her daughter was diagnosed with pulmonary arterial hypertension. (Andre Chung/For The Washington Post)

One of Maryland’s largest drug manufacturers is suing the federal government to fend off competition on one of its key drugs, saying its right to exclusively sell the medication should continue an additional three years. The suit comes as the company, United Therapeutics, faces new competition from other firms trying to sell similar treatments on the generic market.

Silver Spring-based United Therapeutics filed suit Aug. 4 in U.S. District Court for the District of Columbia, alleging the firm is entitled to a seven-year period of exclusive sale of the drug’s oral formulation.

Media representatives from the Food and Drug Administration and the Department of Health and Human Services declined to comment, citing policies against speaking about on pending litigation.

The case is based on the Orphan Drug Act, a 1983 law meant to give pharmaceutical companies a stronger financial incentive to pursue cures for rare diseases. The law gives drug manufacturers a seven-year period to sell the drug free of competition, with the hope that the period of monopoly status will make the market for these rare drugs more attractive to drugmakers.

United Therapeutics’ drug is an orally administered treatment called Orenitram, which treats the rare blood-pressure disease pulmonary arterial hypertension. The often-fatal condition is estimated to affect less than 0.01 percent of the population.

The disease holds personal significance for United Therapeutics chief executive Martine Rothblatt, who left her job at SiriusXM radio, which she also founded, to start United Therapeutics in 1996.

Rothblatt’s daughter was one of the few who suffered from the rare disease, for which there was no known treatment at the time. Rothblatt entered the pharmaceutical industry in large part out of a desire to find a treatment, which ultimately led to Orenitram.

United Therapeutics also has approvals for injectable and inhalable treatments that are based on the same underlying ingredients, at least one version of which has been approved since 2002.

The FDA approved the pill-based version in 2013 but did not award the company the full seven-year period of exclusivity, saying it had not proven that the oral formulation of the drug is clinically superior to its predecessors. United Therapeutics’ suit alleges that 2013 decision was unlawful, saying Orenitram should be protected by the Orphan Drug Act.

The firm is arguing that the oral formulation is a distinct treatment from its injectable and inhalable predecessors, a quality that would entitle it to a long runway of exclusivity.

The company is seeing new competition from other firms trying to sell the drug on the generic market, where therapies tend to be cheaper. A Florida biotech firm, Actavis Laboratories, applied for the rights to sell a competing drug last year, according to a news release from United Therapeutics.

In the release, the company vowed to stand up for its exclusive right to sell the drug.

“United Therapeutics intends to vigorously enforce its intellectual property rights relating to Orenitram,” the company said.