Drug makers generally don’t complain when regulators widen the market for their medicines. But AstraZeneca filed a lawsuit on Monday claiming the US Food and Drug Administration is on the verge of illegally broadening the indication for its best-selling Crestor cholesterol pill, and the move would unfairly allow generic competition.

The argument, which the company also made late last month in a citizen’s petition, hinges on the interpretation of federal law governing product labeling. Depending upon the outcome, AstraZeneca may either maintain a monopoly on Crestor for another seven years or face lower-cost rivals to a key revenue stream when the Crestor patent expires on July 8.

Here’s the background: Last month, the drug maker won FDA approval to sell Crestor to treat children with a rare genetic disorder called homozygous familial hypercholesterolemia or HoFH, which causes very high cholesterol. Under the Orphan Drug Act, the company was awarded an additional seven years of marketing exclusivity for Crestor, but only for treating this particular rare, or orphan, disease.

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This is where things get tricky.

Several generic companies are lined up to sell a version of Crestor. AstraZeneca argues that a generic must include all pediatric labeling information approved for the corresponding brand-name drug. The company filed its lawsuit over concerns that the FDA will, instead, rely on a decision it made last year allowing generic companies to exclude certain information, so long as a safety risk is not created. The case involved Otsuka Pharmaceuticals, which was trying to prevent generic versions of the Abilify antipsychotic.

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In both its petition and its lawsuit, AstraZeneca maintained that the FDA’s interpretation in that case was “unlawful” and would, in fact, pose a safety risk if applied to Crestor. How so? The drug maker contended that any generic label could be dangerous, because a doctor may still prescribe a generic for HoFH, but choose an incorrect dose since the generic labeling would not contain the same information.

By allowing generic drug makers to market their lower-cost versions without the corresponding product labeling information, “there are substantial risks that doctors would over- or undertreat pediatric HoFH patients if [a] generic version omitted” the Crestor labeling information, according to the citizen’s petition the company filed with the agency.

Moreover, AstraZeneca maintained that it faces “an imminent risk” of harm to revenues and market share if generic versions can be sold without all of the Crestor labeling. Crestor has been the company’s biggest-selling product, generating $5 billion in sales last year out of a total of $23.6 billion, according to its annual report, and more than half of Crestor sales are made in the United States

Even though the FDA has not yet responded to the citizen’s petition, AstraZeneca probably filed the lawsuit now “because they want to try to avoid having to put the genie back into the bottle if FDA approves [generic versions of Crestor] on July 8 and manufacturers launch” their pills,” said Kurt Karst of Hyman Phelps & McNamara, who quarterbacks the FDA Law Blog. An AstraZeneca spokeswoman confirmed this is the strategy.

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Some see the legal filings, however, mostly as a maneuver to protect sales, especially since the company only began testing Crestor for children with HoFH in 2014, according to its citizen’s petition. Critics contend AstraZeneca viewed the rare disease as an opportunity to reap the benefits of the marketing exclusivity conferred by the Orphan Drug Act.

“It’s not about finding a way to treat children who have rare diseases, it’s a way to extend [the] life of a brand-name drug,” said Walid Gellad, an associate professor of medicine and health policy at the University of Pittsburgh. “AstraZeneca — and society — would be better off putting the money being spent on the lawsuit into researching new drugs.”

The AstraZeneca spokeswoman wrote us that the drug maker “believes federal law entitles the company to an additional exclusivity period of seven years for Crestor in the US.”

In a paper issued last fall in the American Journal of Clinical Oncology, a team of researchers argued that drug makers are exploiting loopholes in the Orphan Drug Act that allow them to widen the market for such drugs and distorting the original purpose of the law.

Gellad also noted that existing Crestor patients may suffer if lower-cost generics are prevented from becoming available. He pointed to a new study in JAMA Internal Medicine showing that a six-month delay in the availability of generic versions of the Lipitor cholesterol pill prevented consumers from benefiting from lower out-of-pocket payments.