The European Patent Office said Thursday said that it is revoking a patent for AstraZeneca PLC's (AZN) asthma treatment Symbicort.
The office, Europe's patent-granting authority, said it will explain the reasons for its decision at a later stage.
The ruling follows an appeal by several generic drug makers challenging a March 2005 ruling in favor of AstraZeneca.
A spokeswoman for AstraZeneca said that, despite this setback, Symbicort remains protected by data exclusivity until August 2010 in major European markets, and by other patents on the device until 2019.
No generic manufacturer can launch an equivalent product until AstraZeneca's data exclusivity expires or the generic company applies for marketing approval utilizing its own clinical data. AstraZeneca shares closed down 0.1% at 2,554 pence in London.