The likelihood of Arena Pharmaceuticals and Eisai's lorcaserin getting approval any time soon has been reduced by a rejection of the investigational weight loss treatment by advisors to the US Food and Drug Administration.

The agency's Endocrinologic and Metabolic Drugs Advisory Committee (EMDAC) has voted 9 to 5 that the available data do not adequately demonstrate that the potential benefits of lorcaserin outweigh the potential risks, when used long-term in overweight and obese individuals. Arena, which does not have any products on the market, had argued that the drug was safe at recommended doses but the panel was not convinced.

They noted concerns that had been raised earlier this week by FDA staff who claimed results from Arena's Phase III BLOOM trial did not meet "mean efficacy criterion," where the difference in weight loss between patients on a drug compared to patients on placebo differs by at least 5%. In the aforementioned study, patients on lorcaserin lost an average of 5.8% of body weight compared with 2.2% in the placebo arm.

The FDA committee also noted that studies of lorcaserin in rodents revealed breast tumours among those dosed at seven times higher than the proposed clinical dose for humans, but acknowledged that no such increase was seen in the 7,000 patients treated with the drug.

Jack Lief, Arena's chief executive, was not particularly downbeat, saying that "we believe that lorcaserin has a positive benefit-risk profile and represents a potential advance in the treatment of obesity". He added that "we will work with the FDA as the agency completes its review".

The regulator is expected to deliver its verdict by October 22 but things do not look good for Arena, which signed up Eisai as its US marketing partner in July. Since the FDA staffers voiced their concerns, the San Diego-based firm's stock has collapsed over 45% and will likely fall again when the markets open later today.

Arena is battling with Vivus and Orexigen Therapeutics to be the first firms to get a new weight loss drug approved by the FDA since Roche's Xenical (orlistat) in 1999. In July, the EMDAC voted 10-6 against backing Vivus' Qnexa (phentermine/topiramate) and Orexigen's Contrave (sustained-release naltrexone/bupropion), partnered with Takeda, is scheduled to go in front of the panel on December 7.