Arena Pharmaceuticals and partner Eisai have presented data on lorcaserin which may help the firms persuade regulators in the USA about the benefits of the diet pill which was rejected last year.

Last October, the US Food and Drug Administration issued a complete response letter regarding lorcaserin, citing a number of reasons for its decision. The agency was particularly concerned about tumours found in animal studies.

The firms have begun their response and completed a clinical study conducted to provide additional data for determining the human relevance of the observation of brain astrocytoma in male rats. The results suggest that the mean exposure of the human brain to lorcaserin (10 mg dosed twice daily (BID)) is approximately 1.7 times the exposure in the human plasma while in contrast, the exposure of the male rat brain to the drug at the dose at which no brain astrocytoma was observed (10 mg/kg/day) is 24 times the exposure in the rat plasma.

Arena's chief scientific officer William Shanahan said "we estimate that humans concentrate lorcaserin in the brain to a much lower extent than do rats". The company added that activities intended to address the observation of mammary adenocarcinoma in female rats and other issues identified by the FDA are ongoing.

However, Arena stressed that it is important to note its estimates "are based on certain assumptions and extrapolations" and the FDA may use different ones in analysing the data.

How impressed the agency will be remains to be seen and no new weight loss drug has been approved since Roche's Xenical (orlistat) in 1999. As well as turning down lorcaserin, the FDA has also recently turned down Vivus' Qnexa (phentermine/topiramate) and Orexigen Therapeutics' Contrave (sustained-release naltrexone/bupropion), partnered with Takeda.