Vivus has suffered a setback with the news that advisors to the US Food and Drug Admininstration have voted not to recommend Qnexa, which the company is still hoping will be the first weight loss treatment to be approved across the pond in over a decade.

The Endocrinologic and Metabolic Drugs Advisory Committee of the US Food and Drug Administration voted 10-6 against backing Qnexa (phentermine/topiramate) for the treatment of obesity. Although the effectiveness of the drug was not disputed, potential side effects such as depression, memory loss, increased heart rate and birth defects cancelled out the benefits in the view of the committee.

The panel were particularly concerned that the data presented by Vivus is not long-term enough (ie just over 12 months) and potential patients would be taking the combo for life. Qnexa consists of the appetite suppressant phentermine, a component of the controversial ‘fen-phen’ diet pill (along with fenfluramine) which was withdrawn in 1997 after it was linked to potentially fatal heart valve complications. The other ingredient, topiramate, is an epilepsy drug sold before it recently went off-patent by Johnson & Johnson’s Topamax and has been linked to serious adverse effects.

The rejection, though narrow, came as a bit of a surprise, and Eric Colman, deputy director of FDA's deputy director of the FDA's metabolic and endocrinology products division, said at a press conference that “when you listen to even the ‘no’ votes, you got the sense that a lot of people, they weren't strongly against the drug but had lingering concerns”.

Leland Wilson, Vivus chief executive, said “we are disappointed with the advisory committee's vote”. However, he noted that the final decision was close, “and we are encouraged that the committee recognised the efficacy demonstrated in the Qnexa clinical trials”.

Mr Leland added that the company will work closely with the FDA leading up to the October 28 final decision date to address the labelling and safety questions raised during the proceedings. Vivus also hopes to unveil data from a two-year extension study of the drug to be available in the current quarter.

Analysts believe Qnexa could eventually gain approval, but it may have to run a large-scale clinical trial, perhaps in as many as 10,000 patients. This will clearly be expensive and Vivus will need a partner to finance the studies.

Investors are worried, however, and shares in Vivus, which climbed almost 17% earlier this week on the back of promising notes from FDA staffers, slumped over 65% in after-hours trading.