An experimental drug from GlaxoSmithKline does not significantly lower bleeding in patients with a rare blood disorder, according to the Food and Drug Administration.

British drugmaker Glaxo is seeking approval for Promacta to treat a disorder that prevents patients’ blood from clotting, causing excessive bruising and bleeding.

FDA reviewers said Wednesday that company studies “do not provide robust evidence to support” Glaxo’s claim that the drug helps control the severity and frequency of bleeding. The review was posted to the agency’s Web site. Two studies submitted to FDA showed no significant difference between patients taking Promacta and those taking a placebo.

On Friday, the FDA will seek opinions from its outside panel of experts on the benefits and risks of the platelet-boosting Promacta. The agency is not required to follow the panel’s advice, though it often does.

About 60,000 people in the U.S. have chronic idiopathic thrombocytopenic purpura, an immune system disorder that causes the body to destroy its own platelets, components of blood that help with clotting. In worst-case scenarios, the condition can cause fatal hemorrhaging in the brain.

Shares of GlaxoSmithKline PLC rose 34 cents Wednesday to $44.23 in midday trading.