WASHINGTON – Amgen Inc drug Nplate is safe and effective for treating a rare clotting disorder that can cause dangerous bleeding, the Food and Drug Administration said Friday in approving the product for U.S. sales.

The injectable drug helps stimulate bone marrow into producing blood platelets in patients with chronic immune thrombocytopenic purpura (ITP), the FDA said. The agency approved its use for patients only after other available treatments fail to help.

“This product is important in that it offers a new approach to the treatment of patients with an uncommon blood disorder who are often very ill,” said Dr. Janet Woodcock, head of the FDA's Center for Drug Evaluation and Research.

Nplate, also known as romiplostim, is not expected to be a big seller and also faces competition.

Some analysts expect it to bring in about $117 million in 2009 sales – a small amount compared with the giant biotech's 2007 revenue of $14.7 billion.

GlaxoSmithKline Plc's rival drug, Promacta, or eltrombopag, faces an FDA approval decision by Sept. 19.

Amgen and Glaxo shares were near flat in midday trading on the Nasdaq and the New York Stock Exchange, respectively.

Doctors who want to prescribe Nplate will have to enroll themselves and their patients into a special registry that will be used to track the drug's long-term safety, the FDA said.

Other approved ITP treatments include steroids, immune- suppressing drugs or surgery to remove the spleen.

The condition occurs when the immune system destroys platelet cells that help the blood clot and low platelet levels can trigger life-threatening bleeding.

About 60,000 U.S. adults have chronic ITP, Amgen has said.

(Editing by Andre Grenon)