UPDATE 3-US FDA staff raises concerns on Geron's blood disorder drug

(Adds shares in paragraph 2, details in paragraphs 9 & 11)

By Christy Santhosh and Pratik Jain

March 12 (Reuters) - The U.S. Food and Drug Administration's staff reviewers said on Tuesday they were unclear if Geron's blood disorder drug provided a clear benefit to patients in a late-stage trial and raised multiple safety concerns with the treatment.

Shares of the California-based company fell 11.5% following the release of the briefing documents, ahead of a meeting of the FDA's independent advisers scheduled for Thursday.

Geron is seeking approval of the injectable drug, called imetelstat, for treating transfusion-dependent anemia in patients with a group of blood cancers called myelodysplastic syndromes (MDS).

The drug, if approved, will compete with Bristol Myers Squibb's Reblozyl, which was greenlit by the FDA last year for the same disease indication.

While imetelstat met the main goal of increasing independence from transfusion at eight weeks in a late-stage study, the FDA's staff said the "clinical meaningfulness" of the data was unclear.

"The general consensus among MDS experts has been that only a 16-week or longer period of transfusion independence is clinically meaningful," the agency's staff said.

Last year in January, Geron's CEO John Scarlett told Reuters that the company anticipates a peak market potential of $1.2 billion for the treatment in the United States and some key EU countries by 2030.

The FDA staff on Tuesday said the late-stage study on imetelstat did not show a "disease-modifying effect" in either extending survival or helping drive disease remission. It also highlighted a high rate of low red blood cell count.

Geron earlier said the adverse effects were not uncommon, adding "the side effects are short lived and reversible".

Wedbush analyst Robert Driscoll said it was expected that the briefing documents would lean negative, and believes Geron will be able to provide a strong case against the points made by the FDA.

The regulator, which usually follows the advice of its panel but is not bound to do so, is expected to make a decision on the drug by June 16. (Reporting by Christy Santhosh and Pratik Jain in Bengaluru; Editing by Shailesh Kuber)