AstraZeneca Pharmaceuticals and its lawyers at Covington & Burling have suffered another, and possibly final, setback in their attempt to stop generic versions of the company’s blockbuster drug Seroquel from being sold before December.

On Thursday, U.S. District Judge Beryl Howell of the District of Columbia held in this 49-page summary judgment ruling that the Food and Drug Administration had not improperly approved 11 generic versions of the drug in March. Sales of Seroquel–which is used to treat schizophrenia and bipolar disorder–topped $3 billion in the U.S. last year. Generic version are already on the market.

In its complaint, which you can read here, AstraZeneca argued that it was entitled to an additional period of exclusivity that would expire in December because it submitted supplemental data about clinical trials that led to labeling changes. Judge Howell ruled that the statute that governed the exclusivity period was ambiguous on this issue, and, as a result, the court must defer to the FDA, absent a finding that its action was arbitrary, capricious, or contrary to law. The government’s action was not any of those things, she held, noting that the FDA’s interpretation of the statute was consistent with its past practices. “The FDA has emphasized that generally applicable safety information in labeling should not be subject to exclusivity,” she wrote. “Notably, the FDA has also denied exclusivity for similar labeling changes to other drugs in Seroquel’s class of antipsychotics.”

Previously, AstraZeneca had filed so-called “citizen’s petitions” in September objecting to the approval of any generic drugs that would compete with Seroquel. When the FDA denied those petitions in March, the company then filed a lawsuit that was thrown out as premature because the FDA hadn’t yet approved any generic drugs. Shortly after the agency approved the 11 drugs later in March, AstraZeneca filed this suit.

Judge Howell noted that Seroquel has been marketed without generic competition for fourteen years. She agreed with the FDA’s argument that AstraZeneca’s interpretation of the statute would result in an “unwarranted evergreening of exclusivity.” AstraZeneca could keep a monopoly over the drug by periodically updating safety information in its labeling, she pointed out. “AstraZeneca’s interpretation would create a perverse incentive for pharmaceutical companies to drag out their presentation of vital safety data to the FDA in order to bar generic competition beyond the periods determined acceptable by Congress,” she wrote.

Covington & Burling’s Timothy Hester, who led the litigation effort for AstraZeneca, declined to comment. Company spokesperson Lynn Shepherd gave us this statement: “We are disappointed and we disagree with the ruling and are considering our options.”