Documents have been released ahead of a keenly-anticipated US regulatory panel meeting on Tuesday for Human Genome Science's investigational lupus treatment Benlysta.

Papers have been published on the website of the US Food and Drug Administration ahead of a meeting of its Arthritis Advisory Committee meeting which will evaluate Benlysta (belimumab). The company is hoping the treatment, which is partnered with GlaxoSmithKline, will get the green light for the reduction of disease activity in adults with active, autoantibody-positive, systemic lupus erythematosus (SLE) who are receiving standard therapy.

If the drug gets approval, it will transform HGS and observers believe it will make the biotech a takeover target. However, FDA staffers said that "various analyses of the data raise questions about the robustness of the efficacy findings".

They wrote of their concerns about the potential increased risk of death, infection or adverse neuropsychiatric effects, including suicide, for patients on Benlysta. The staffers also noted that the drug may not be effective in patients of African-American or African heritage, who "are known to have more aggressive SLE", and they added that the data demonstrated less effectiveness in the USA and Canada, compared with other regions.

Arguing that the data package demonstrated "somewhat marginal efficacy", the staffers said "the relative safety profile of the product must be weighed". The reaction from investors to all this was a nervous one and HGS shares fell 10.9% to $23.60.

However the market's reaction to the FDA papers seems excessive according to analysts, the majority of whom expect a positive vote tomorrow. Christopher Raymond at Robert W Baird issued a research note saying that "as is customary with such documents, the FDA puts the risk/benefit of Benlysta under a microscope with the aim of encouraging vigourous debate".

He added that "we would remind investors as they read these documents that Benlysta's two Phase III trials...met their primary endpoints convincingly."__ HGS seems confident too, and in a document posted on the FDA website, said Benlysta has been shown to "reduce the overall signs and symptoms of lupus disease activity and can be safely administered in combination with currently available therapies."