Amgen has announced interim results from an open-label extension study showing that long-term administration (up to 48 weeks) of its investigational therapy AMG 531 stimulated platelet production and was generally well-tolerated in adult patients with immune thrombocytopenic purpura (ITP).

These updated interim data were presented in an oral session at the American Society of Haematology (ASH) 48th Annual Meeting in Orlando, Florida.

ITP is a chronic and potentially serious bleeding disorder caused by an immune system malfunction that mistakenly recognizes the body's own platelets as foreign and destroys them, as well as a decrease in platelet production, which results in low platelet counts. Platelets are specialized blood cells that help prevent and stop bleeding by participating in clotting. The risk of a bleeding event increases when platelet counts drop to less than 30,000 platelets per microliter.

"Unlike most current ITP treatments, which interfere with platelet destruction, AMG 531 is designed to increase the production of platelets at a rate that outpaces their destruction by the immune system," said David J. Kuter, MD, DPhil., director of Centre for Haematology, Massachusetts General Hospital, Boston. "This study now includes two years of follow-up data and the interim results at 48 weeks are encouraging. Individualized dosing of AMG 531 may provide a new option for patients with ITP, potentially allowing tapering off of steroid therapy."

The long-term follow-up study has been ongoing for more than two years and is open to patients who have completed a previous AMG 531 study. To date, 104 patients have been enrolled. This planned interim analysis at 48 weeks includes 36 patients, who previously completed a phase 2 trials, with AMG 531. Overall, 86 per cent of patients achieved a platelet response, defined as achieving platelet count of at least 50,000 platelets per microliter of blood and doubling of their baseline.

At 48 weeks, more than 57 per cent of patients still on study had maintained this platelet response. The median time to first response was three weeks and the mean dose at first response was 3.4 ug/kg.

Thirty patients had undergone splenectomy prior to study enrolment and nine were receiving concurrent corticosteroids, which could be tapered when platelet counts reached greater than 50,000 platelets per microliter of blood.