WASHINGTON (Reuters) - The Food and Drug Administration will not rule on Amgen Inc's (AMGN.O: Quote, Profile, Research) experimental clotting disorder drug Nplate by Wednesday as expected, an Amgen spokeswoman said.

"We remain optimistic that a final decision will be made soon, but Amgen cannot speculate on the timing of the FDA's response," Amgen spokeswoman Ashleigh Koss said said via e-mail.

"Amgen continues to work with the FDA to assist in the completion of the review process," she added.

The company is seeking approval to sell Nplate for adults who have not responded to other treatments for chronic immune, or idiopathic, thrombocytopenia purpura (ITP).

Patients with chronic ITP have low platelet levels that can put them at risk for life-threatening bleeding.

Analyst Michael King of Rodman & Renshaw has estimated Nplate sales in 2009 of $117 million, which would make it a relatively small drug for Amgen. The company's 2007 revenue was $14.7 billion.

In March, an FDA advisory panel unanimously urged approval of Nplate, known generically as romiplostim. The FDA usually follows panel recommendations.

GlaxoSmithKline Plc (GSK.L: Quote, Profile, Research) is developing a rival drug called Promacta, or eltrombopag. A decision on that drug is expected by Sept. 19.

(Editing by Dave Zimmerman)