Arena Pharmaceuticals suffered a major setback on Thursday when US advisers rejected its proposed weight-loss pill amid concerns that tumors in rats might indicate a risk of cancer in humans.

Lorcaserin, Arena's most advanced drug candidate, is the second diet drug to stumble in a race among three small companies to launch the first new prescription diet drug in a decade.

The advisory committee voted 9-5 that potential risks from long-term use of Arena's lorcaserin outweighed possible benefits from modest weight loss. Panelists said they were unsure if tumors in lab rats meant the drug would increase brain or breast cancers in people.

'I do think (lorcaserin) is promising, but there's too much uncertainty at this time,' said Jessica Henderson, the panel's consumer representative.   

An FDA panel in July urged against approval of a diet drug from Vivus after voicing concern about depression, memory loss and potential birth defects.

Orexigen's drug, Contrave, is expected to face FDA advisers in December.

The expert panel called on Thursday for more study of Arena's lorcaserin in people with diabetes, high blood pressure and other health problems, to better reflect patients who would use the drug if it was approved.

The FDA usually follows panel recommendations. A final ruling is due Oct. 22. - Reuters