July 25 (Reuters) - Celgene Corp said on Monday that its flagship drug Revlimid failed to extend survival as a maintenance therapy for a type of blood cancer after patients had responded to prior treatment.

As a result, the U.S. biotechnology company said it would not seek an additional approval for Revlimid for that use, and its shares fell more than 2 percent.

Revlimid, a multiple myeloma treatment with annual sales of about $6 billion, was being tested versus a placebo in patients whose diffuse large B-cell lymphoma had responded to initial treatment with Roche's Rituxan and chemotherapy.

Approval as a maintenance therapy, used to prevent recurrence and improve survival, can significantly add to sales as the drugs are taken long term.

While the Revlimid study delayed progression of the disease, no survival benefit was seen, the company said. The trial was conducted along with the Lymphoma Study Association.

Celgene said it will continue to study Revlimid in a variety of lymphomas.

(Reporting by Bill Berkrot; Editing by Sandra Maler)