Amgen Inc drug Nplate is safe and effective for treating a rare clotting disorder that can cause dangerous bleeding, the Food and Drug Administration 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.

Other approved ITP treatments include steroids, immune-suppressing drugs or surgery to remove the spleen. The total cost of care for Nplate patients is expected to be "less than or comparable to the total costs of care with standard treatment regimens," the Thousand Oaks, California-based biotech company said.

Nplate was approved based on two studies of 125 patients who took the drug for six months, according to the FDA. Doctors who want to prescribe Nplate will have to enroll themselves and their patients into a special registry that will track long-term safety.

Those who received Nplate showed "significantly higher platelet counts and maintained those higher counts compared to those who did not receive the drug," the FDA said.

Patients who still had their spleen fared better than those who did not, it added.

ITP occurs when the immune system destroys platelet cells that help the blood clot and low platelet levels can trigger life-threatening bleeding. Patients with the condition are also more likely to experience bruising.

Nplate, also known as romiplostim, is not expected to be a big seller and also faces competition. About 60,000 U.S. adults have chronic ITP, Amgen has said.Some analysts expect it to earn anywhere from $66 million to $117 million in 2009 sales and peak around $200 million — small compared with Amgen's 2007 revenue of $14.7 billion.

Amgen's shares were largely unaffected by the FDA's decision, in part because the ruling had initially been expected in July before the agency postponed it.Analysts have instead been buoyed by hopes for its experimental osteoporosis drug called denosumab, which is still in development.