The condition, which usually develops in adults, is known as chronic immune thrombocytopenic purpura (ITP), a disease that results in a low number of platelets, the blood components that help with clotting. In patients with chronic ITP, the immune system is believed to destroy platelets and the patient's bone marrow is often unable to compensate for this loss.

"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 Janet Woodcock, M.D., director, Center for Drug Evaluation and Research, FDA.

The estimated 140,000 people with chronic ITP are prone to bruising and at risk for life-threatening bleeding. Current medical treatment includes corticosteroids and immunoglobulin. Surgery to remove the spleen, a procedure known as a splenectomy, may help some patients. Nplate is approved only for patients with chronic ITP who do not respond sufficiently to current treatments.

FDA based its approval on two randomized clinical trials of about 125 patients who had received at least one prior ITP treatment. One study enrolled patients who still had their spleen, the other enrolled patients who did not.

During six months of treatment, patients who received Nplate had significantly higher platelet counts and maintained those higher counts compared to those who did not receive the drug. The response to Nplate was higher in those patients who still had their spleen than in those patients who had undergone a splenectomy. In those patients who did not receive Nplate, only one experienced a sustained increase in platelet counts.

Safety concerns with Nplate include fibrous deposits in the bone marrow and the possibility that once Nplate is stopped, platelet counts could drop below what they were before beginning treatment.

Additional risks include blood clots due to excessive increases in platelets and, if Nplate were given to patients with an abnormal blood condition known as myelodysplasia, a risk for a form of blood cancer known as acute leukemia. Myelodysplasia, which is associated with low platelet counts, predisposes some patients to leukemia. In a study of 44 patients who had myelodysplasia and received Nplate, four patients developed leukemia. Further clinical trials in patients with predisposing conditions for leukemia will be needed to determine whether the development of leukemia may relate to the use of Nplate. Nplate is approved only for use among patients with chronic ITP.

A Risk Evaluation and Mitigation Strategy (REMS) has been developed to address the risks of Nplate therapy. Under the Food and Drug Administration Amendments Act of 2007, FDA has determined that a REMS is necessary for the benefits of Nplate to outweigh the risks of the product. The REMS will include a Medication Guide for patients and requires that all prescribers and patients enroll in a special program to track the long term safety of Nplate therapy.

Nplate is manufactured by Amgen, Inc. of Thousand Oaks, Calif.

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