UPDATE 3-US FDA staff question Genzyme leukemia drug study

* Agency reviewers: Clolar results hard to interpret

* Genzyme says drug provides benefits, risks manageable

* FDA panel reviews drug on Tuesday

* Shares fall 2.4 pct (Adds analyst comment, shares)

By Lisa Richwine

WASHINGTON, Aug 28 (Reuters) - U.S. drug reviewers questioned a Genzyme Corp GENZ.O study meant to back wider use of leukemia drug Clolar, according to documents released on Friday ahead of an advisory panel review.

Genzyme shares fell 2.4 percent to $55.53 on Nasdaq.

Clolar is approved for treating a type of leukemia in children. Genzyme is seeking permission to promote the drug for adults with acute myeloid leukemia (AML), a blood cancer.

“The lack of a randomized study combined with the heterogeneous patient population regarding AML prognostic factors makes interpretation of the study results difficult,” Food and Drug Administration staff said in an analysis prepared for the panel, which meets Tuesday.

The company’s main study tested Clolar in elderly patients with previously untreated AML and at least one other factor that gave them a poor prognosis. The study did not include a comparison group of people who were not treated with the drug.

FDA staff said they recommended to the company at a 2007 meeting that it conduct a randomized controlled study that would compare patients who received Clolar with another group.

The reviewers said they would ask the advisory panel, a group of outside experts, if that type of trial was needed to establish the safety and effectiveness of Clolar for adults.

Sanford Bernstein analyst Geoffrey Porges, in a research note, said 60 percent of Clolar’s U.S. sales already are generated from adult AML use and a rejection by the panel, which he said was possible, “would not be particularly harmful to current revenue for Clolar, although it may dampen the product’s future growth potential.”

Porges estimated Clolar sales would grow to $150 million in 2013 and $240 million in 2015.

Genzyme defended its data and said current treatments for adults with AML were insufficient. No first-choice therapies for newly diagnosed cases have been approved since 1990.

An estimated 12,810 people will be diagnosed with AML in the United States in 2009, and about 70 percent will die from the disease, the company said.

In a summary prepared for the panel, Genzyme said its data on Clolar “demonstrate clinically meaningful benefits and manageable risks.” Forty-five percent of patients had their leukemia go into remission, the company said. The drug, known generically as clofarabine, is given intravenously.

Dr. Beth Trehu, Genzyme’s product general manager for Clolar, said the company designed the study with input from a group of AML experts. The experts recommended a single-arm study as “there was no accepted standard of care” that they could agree to use as a comparison, she said.

The FDA will consider the advisory panel’s input before deciding whether to approve Clolar for adult use. The agency usually follows panel recommendations.

The deadline for an FDA ruling on Clolar has been extended to December, Genzyme said in a statement.