UPDATE 1-Amgen study shows platelets response with Nplate

(Adds detail on drug name, extension study data, analyst comment)

By Lisa Baertlein

LOS ANGELES, Dec 10 (Reuters) - Amgen Inc (AMGN.O) said on Monday its experimental drug AMG 531 had the desired effect of increasing platelet production in adults with a bleeding disorder known as chronic immune thrombocytopenic purpura.

The Phase 3 trial of the drug, also known as Nplate or romiplostim, studied 62 ITP patients who had failed previous treatments. The results were presented at the American Society of Hematology's annual meeting in Atlanta.

Data from the Amgen-sponsored study showed that the trial met its primary endpoint with 61 percent of the 41 Nplate-treated patients achieving target levels of durable platelet response, compared with just 5 percent of the 21 patients in the placebo arm.

Amgen's injectable drug works by stimulating platelet growth, while current ITP treatments work to decrease platelet destruction and can cause serious side effects.

Nplate's main competitor, from GlaxoSmithKline (GSK.L), is oral Promacta, or eltrombopag, which also stimulates platelet response.

Sanford Bernstein analyst Geoff Porges said he found "limited enthusiasm" for Nplate at the ASH meeting.

"With their weekly injected regimen, Amgen faces a challenge going head-to-head against GSK's oral agent eltrombopag, which appears to have a similar profile other than mode of administration," Porges said.

There are an estimated 15,050 to 30,100 new cases of ITP in U.S. adults and children each year, based on a U.S. government population estimate and a disease prevalence estimate from the Annual Review of Medicine.

Amgen has asked regulators in the United States, European Union, Australia and Canada to approve Nplate as a treatment for adults with chronic ITP. Regulatory authorities in the United States, Australia and Canada have granted priority review of Amgen's application.

ITP is a disorder in which the immune system mistakenly targets blood platelets as foreign objects and destroys them. The platelets, which are needed for clotting, are eliminated by the spleen.

The disease can result in dangerously low platelet counts that may result in spontaneous bleeding or bruising. In the study, overall response -- defined as either a durable response or a transient response of at least four weeks

-- was nearly 88 percent in Nplate-treated patients, compared -- was nearly 88 percent in Nplate-treated patients, compared with 14 percent of patients in the placebo group.

Researchers said the mean number of weeks with a platelet response was significantly greater in Nplate-treated patients than in the placebo group at 15 weeks versus 1 week.