A U.S. district court found the formulation patent for AstraZeneca PLC's (AZN, AZN.LN) top-selling antipsychotic drug Seroquel XR was valid, the pharmaceutical company said Thursday, offering it ammunition in its fight against generic versions of the drug.

The U.S. District Court for New Jersey also found Par Pharmaceutical Cos.'s (PRX) Anchen Pharmaceuticals, Osmotica Pharmaceutical Corp., Torrent Pharmaceuticals Ltd. (500420.BY) and Mylan Pharmaceuticals Inc. had infringed the drug's extended-release formulation patent, which expires in 2017, AstraZeneca said.

The ruling comes after the U.K.'s second-biggest drug company failed this month to prevent the U.S. Food and Drug Administration from giving final marketing approval to generic forms of Seroquel. A federal judge Friday refused to grant AstraZeneca an injunction against the FDA, ruling that the company had failed to make a clear case that it was entitled to one.

AstraZeneca faces one of the highest exposures to drug-patent expirations within the pharmaceuticals sector. Seroquel, AstraZeneca's second best-selling product, generated $5.83 billion in revenue last year for the company and is the first of its best-selling drugs to lose U.S. patent protection.

A U.K. court ruled against AstraZeneca's formulation patent following challenges from companies such as Teva Pharmaceutical Industries Ltd. (TEVA, TEVA.TV).

AstraZeneca's American depositary shares Thursday closed at $44.50 and were inactive after-hours.

 
   -By Drew FitzGerald, Dow Jones Newswires; 212-416-2909; Andrew.FitzGerald@dowjones.com