Genzyme Corp should run a new study before the company can win approval to promote a pediatric leukemia drug for older adults with an aggressive blood cancer, a U.S. advisory panel said on Tuesday.

In a 9-3 vote, the committee urged regulators to require a new study of adults randomly assigned to the Genzyme drug, Clolar, or another treatment for comparison. Genzyme has applied for approval of adult use based on a study that did not include a comparison group.

Several panelists said the drug appeared to have cancer-fighting ability but Genzyme's study was not sufficient to establish its level of safety and effectiveness and which types of patients would benefit.

In the absence of a randomized trial, we don't have an evidence-based justification that alternative standard therapy wouldn't be better, said panel member Thomas Fleming, a statistician at the University of Washington in Seattle.

Genzyme shares fell 0.2 percent to $55.61 on Nasdaq late Tuesday afternoon.

The panel recommendation makes it unlikely the FDA will approve Genzyme's current application for use in previously untreated adults aged 60 and older with acute myeloid leukemia (AML). The agency usually follows advice from its panels of outside experts, and FDA reviewers said they had urged Genzyme in 2007 to run a trial with a comparison group.

Clolar is approved for fighting a different type of leukemia in children, and doctors already use it to treat adults with AML. Genzyme needs FDA clearance, however, to market the drug specifically for the adults. The drug's generic name is clofarabine.

JP Morgan analyst Geoffrey Meacham said the panel vote was an incremental negative as it appears unlikely Clolar will be a meaningful revenue contributor in the near-term.

Meacham, in a research note, said he was not adjusting his estimates for Clolar sales. He forecast the drug would make up less than 20 percent of the $550 million expected from Genzyme's hematologic oncology sales in 2010.

Company officials told the panel they had consulted with AML experts and could not find an appropriate comparison group for their clinical trial. They said current data supported the expanded use of Clolar as 46 percent of 112 patients had their disease go into remission for the median time of one year.

These data are compelling, Dr. Michael Vasconcelles, a Genzyme vice president, told the panel, adding that potential side effects were acceptable.

After the panel meeting, Genzyme Vice President Mark Hayes said the company had further opportunity to engage the FDA in discussions about the application as the agency had pushed back its deadline to rule on Clolar to December.