Less expensive versions of the Gemzar cancer drug, manufactured by Eli Lilly, may appear in the US in November.
The firm has tried to argue that its product should be protected from generic copying because a patent of the method by which the drug is administered lasts until the middle of 2013.
But a US federal appeal court ruled Eli Lilly’s claim invalid.
The drug’s basic chemical patent lapses in November, which means the firm will lose market exclusivity.
The panel of appeal judges on the federal circuit said on Wednesday that it agreed with a Michigan federal district court which ruled last August that the method-of-use patent is invalid.
In a statement, the drug firm said: “We strongly disagree with the ruling.” It added that it will be looking at “all possible legal options”, which may mean it will ask all members of the Washington DC-based appeals court to review the matter again.
Lilly filed the lawsuit in Michigan against Sun Pharmaceuticals Industries of India which plans to sell a generic form of Gemzar. The drug is used to treat people with ovarian, lung, breast and pancreatic cancer.