UPDATE 2-US FDA extends review of Pfizer osteoporosis drug

(Adds Pfizer comment, updates shares)

WASHINGTON, Oct 21 (Reuters) - U.S. regulators have extended by three months their review of a proposed osteoporosis drug from Pfizer Inc (PFE.N) to study the pill in more depth, a company spokeswoman said on Tuesday.

The Food and Drug Administration extended the time period through January 2009 "to allow the agency to fully review complete five-year data from the pivotal Pearl trial" of the drug, called Fablyn, Pfizer spokeswoman Kristen Neese said in an e-mail.

Pfizer developed the drug with technology from Ligand Pharmaceuticals Inc (LGND.O). The drug's generic name is lasofoxifene.

The once-a-day pill is under review for treating post- menopausal women with osteoporosis, which weakens bones. The FDA rejected the drug three years ago.

In September, an FDA advisory panel voted 9-3, with one abstention, that Fablyn offered benefits that outweighed risks such as blood clots.

FDA reviewers at that meeting raised concerns about a higher rate of death among women who took a low dose of Fablyn compared with a placebo. Most members of the advisory panel said they could not tell if the deaths represented a true mortality increase with the drug. Pfizer had argued they were not tied to the drug.

At the time of the meeting, the FDA had received the five-year data from the Pearl study but had not reviewed it "in depth," Neese said. The panel heard safety findings from the study through five years but no effectiveness data for that time period, she said.

Pfizer shares closed unchanged at $17.34 on the New York Stock Exchange. (Reporting by Lisa Richwine; Editing by Maureen Bavdek, Andre Grenon and Susan Kelly)