UPDATE 2-US FDA advisers back Glaxo, Genmab leukemia drug

* Advisory panel recommends approval of Arzerra

* Would treat chronic lymphocytic leukemia

(Adds company comments, details)

By Deena Beasley

ORLANDO, Fla., May 29 (Reuters) - A small trial of Arzerra, being developed by GlaxoSmithKline Plc (GSK.L) and Genmab (GEN.CO) for treating chronic lymphocytic leukemia, indicated benefits sufficient for U.S. approval, an advisory committee to regulators said on Friday.

The panel voted 10-3 the trial results were likely to predict clinical benefit in patients with CLL, a common type of blood cancer in adults.

The Food and Drug Administration, which usually, but not always, carries out the recommendations of its advisors, is expected to decide on the application by Aug. 1, according to Glaxo.

Initially, the companies are seeking approval for patients who have failed two other treatments -- fludarabine and alemtuzumab -- and others who have failed fludarabine and for whom alemtuzumab, sold by Genzyme (GENZ.O) as Campath, was inappropriate because of their bulky tumors.

"I am impressed by the resolution of symptoms," said Dr. Margaret Tempero, deputy director at the University of California at San Francisco's cancer center and a member of the advisory committee. "I also thought the overall survival benefit looked encouraging."

Glaxo and Genmab are seeking accelerated approval of Arzerra, also known as ofatumumab, based on results from a small trial that did not include a comparison arm.

"While current initial treatments for CLL can provide prolonged remissions, some patients will progress rapidly and relapse, which highlights the need for new therapies," Dr. Debasish Roychowdhury, head of oncology medicines development at Glaxo, said in a statement.

Much of the discussion at the panel hearing centered on whether new criteria requiring that tumors be measured with CT scans should be applied to the Arzerra trial.

Analysts believe annual sales of Arzerra could eventually top $2 billion, although the drug is unlikely to reach its full potential until the middle of the next decade. The makers are studying larger populations beyond CLL.

A clinical trial showed a 58 percent response rate to Arzerra in patients who failed the two prior chemotherapy regimens, a higher level than the normal 25 percent level, Glaxo spokeswoman Lisa Behrens said.

The median duration of response for patients in the trial was 6.5 months.

Infections, including fatal ones, were seen frequently in the study, but it was not possible to determine how much infection risk was due to the drug, FDA staff said. Heavily treated CLL patients normally have high infection rates.