In advance of an advisory committee meeting on Friday, an FDA review finds that Promacta, an experimental drug for boosting platelets, failed to control bleeding any better than a placebo. Glaxo wants approval to sell the drug as a short-term treatment for chronic ITP, an autoimmune disease that destroys blood platelets need for clotting and affects about 60,000 Americans.

“The data provided…do not demonstrate statistically significant, robust…treatment effect in decreasing bleeding events,” according to agency documents, which went on to say that “this trend is not statistically significant and does not provide robust evidence to support the proposed labeling claim.” Here are is the FDA review.

Glaxo and its partner, Ligand Pharmaceuticals, hope Promacta can become a blockbuster and some analysts have predicted annual sales could top $1 billion, according to Reuters. In its own documents, the drugmaker aruge Promacta “can address the unmet medical need for the short-term treatment of patients with chronic ITP.” Glaxo maintains its studies show more patients taking the med saw higher platelet counts than those given a placebo. (Glaxo documents).

But FDA staffers also raised an alarm about Glaxo’s decision to seek approval for short-term use of a drug that may be used on a chronic basis. According to the documents, the FDA “repeatedly stressed the critical need to evaluate (Promacta’s) longer term safety and efficacy data given the likelihood of chronic use regardless if data are only available for short-term use.”

Long-term studies are still under way. The FDA, meanwhile, will ask the panel whether approving short-term use is appropriate for the chronic disease.