The likelihood of GlaxoSmithKline and Ligand Pharmaceuticals getting approval for Promacta, an experimental drug designed to treat a rare blood disorder, looks gloomier this morning after US regulatory staff declared their doubts about the data presented.

Ahead of a US Food and Drug Administration's Oncologic Drugs Advisory Committee meeting tomorrow, agency staff have posted documents doubting the potential of Promacta (eltrombopag). Specifically, they say that the data presented by the firms do not "provide robust evidence to support" the claim that the drug decreases the frequency and severity of bleeding in patients with chronic idiopathic thrombocytopenic purpura.

The FDA staff notes said that although a trend toward reduced incidence of bleeding was observed with Promacta, the data was not statistically significant. Rather, the FDA's analysis found that “the decrease in bleeding events from baseline is generally similar” between eltrombopag and placebo.

GSK stock was not particularly affected by the news but the same cannot be said for Ligand. Its shares collapsed almost 36% to close at $2.30. The UK drugmaker plans to submit a marketing application for eltrombopag later this year in Europe, where it will be sold as Revolade.