Analysts have been left stunned by the news that advisors to the US Food and Drug Administration have given a positive recommendation to GlaxoSmithKline and Ligand’s Promacta, days after agency staff had aired their concerns about the drug which is designed to treat a rare blood disorder.

The FDA’s Oncology Drugs Advisory Committee has unanimously voted (16-0), that Promacta (eltrombopag) should be approved for the short-term treatment of patients with chronic idiopathic thrombocytopenic purpura. The go-ahead was based on studies evaluating the safety and efficacy of the drug in the short-term setting, ie six weeks and this showed that Promacta increased platelet counts and reduced bleeding.

However the committee did not consider whether Promacta, an oral, non-peptide thrombopoietin receptor agonist, should be used long-term due to a lack of data. Fears over long-term risks include liver toxicity, bleeding following drug discontinuation and potential marrow fibrosis were what prompted the concerns expressed by the FDA staffers earlier in the week and led to a 36% slump in Ligand shares.

Panel members said that a comprehensive risk-management programme needs to be put in place to combat these risks and that some of the burden of the cost associated with restricting Promacta should be borne by GSK. The UK drugmaker is expected to consider that proposal while it gathers additional long-term data which should be presented by the end of the year.

If Promacta gets approval, the number of patients able to get access to the drug will be limited due to the short-term restrictions. As a result, it will be some time before GSK manages to get up to the peak sales levels suggested by analysts which vary from $1 billion to $2.5 billion. There are estimated to be approximately 60,000 people diagnosed with chronic ITP in the USA.

The concerns the FDA has about Promacta are also governing its approach to Amgen’s rival product Nplate (romiplostim). In April, the agency delayed a decision until July 23 on whether to approve the US biotechnology firm’s injectable chronic ITP compound for long-term use so it can consider Amgen’s own risk management programme.

The FDA has still to make the final decision on Promacta but investors seem to have made their minds up. From the stock sinking last week, the panel’s decision led to a 49.4% hike in Ligand shares to $3.51.