UPDATE 3-US FDA staff questions data on leukemia drug

* FDA staff asks if drug benefits enough to back approval

* Glaxo says response rate better than other options

* Genmab shares fall 17.5 pct, Glaxo shares up (Adds analyst, Glaxo comments; updates shares)

By Lisa Richwine

WASHINGTON, May 27 (Reuters) - U.S. drug reviewers questioned whether a proposed leukemia drug from GlaxoSmithKline PLC (GSK.L: Quote, Profile, Research) and Genmab (GEN.CO: Quote, Profile, Research) provides enough benefit to warrant approval, documents released on Wednesday said.

Shares of Danish biotech company Genmab fell 17.5 percent in Copenhagen, while shares of much larger Glaxo gained 0.9 percent on the New York Stock Exchange.

Food and Drug Administration staff said the magnitude of anti-cancer activity from the drug, Arzerra, was "difficult to quantify" in tests of patients with chronic lymphocytic leukemia, a common type of blood cancer in adults.

"The major issue regarding this application is whether the effect sizes ... are reasonably likely to predict clinical benefit," FDA staff said in a memo prepared for an FDA advisory panel that meets Friday.

The FDA will ask the panel for input on the companies' data. The agency usually follows panel recommendations when deciding whether to approve a drug but does not have to. An FDA ruling is due by Aug. 1.