updated 3/10/2008 6:01:47 PM ET 2008-03-10T22:01:47

Federal health regulators on Monday raised safety concerns about an experimental biotech drug from Amgen designed to treat a blood-clotting disorder.

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The Food and Drug Administration said studies of Amgen's drug Nplate showed a range of adverse reactions, from bone-marrow abnormalities to dangerous blood clots. On Wednesday FDA will ask a panel of outside experts whether the drug should be approved and, if so, under what conditions. The agency is not required to follow its panel's recommendations.

Thousand Oaks, Calif.-based Amgen has asked the government to approve Nplate for patients with a disorder that causes the body to attack its own platelets, which are red blood cells that help blood clot. The condition, which causes bruising and bleeding after minor injuries, affects about 200,000 people in the U.S., according to the Platelet Disorder Support Association.

FDA said patients treated with the drug showed significantly higher platelet levels than those taking placebo in two, six-month studies conducted by Amgen. Patients received weekly injections of Nplate, which is a genetically engineered version of the protein that encourages red blood cell production.

However, FDA reviewers noted several adverse reactions seen in small numbers of patients, according to briefing documents posted online Monday.

FDA noted that 14 of 204 patients had dangerous blood clots after beginning treatment with Nplate. Agency reviewers said the risk of such clots could increase after patients stop taking the drug, though Amgen's studies have not yet addressed the issue.

Agency staff also expressed concern that eight percent of patients taking Nplate developed a resistance to the drug. Only four percent of patients taking placebo showed the similar results.

FDA also expressed concerns about abnormal bone marrow growth and malignant tumors seen in some patients.

FDA will ask its experts on Wednesday whether the drug's benefits outweigh its risks, and whether it should be approved on a limited basis. For example, panelists will be asked if the drug should only be approved for patients who have not responded to other drugs.

Lehman Bros. analyst Jim Birchenough said he believes FDA will eventually approve the drug because there are few options for patients who have failed other treatments, which include removal of the spleen. Birchenough projects peak sales of $500 million for Nplate.

In its application to FDA, Amgen laid out a plan for managing Nplate's risks through drug-warning labels, educational outreach and follow-up studies.

Under the proposal, Nplate's label would warn of safety problems seen in company studies. The company would distribute additional guides on the risks to patients. For doctors, Amgen would hold workshops on proper drug use.

Amgen proposed studying irregular bone marrow growth and other adverse reactions in four follow-up studies.

The company also said it would not advertise the drug directly to consumers.

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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