The US Food and Drug Administration has asked for more information before it will approve Entereg, a drug intended for use as a treatment for bowel dysfunction caused by treatment with opioid drugs developed by Adolor and multinational partner GlaxoSmithKline.

The setback is the latest delay for a drug that has already languished at the FDA for more than two years, and shares in Adolor plummeted as a result, down 44% to $7.69 on the Nasdaq and the day’s biggest faller.

Entereg (alvimopan), the first in a new class of peripheral mu opioid receptor antagonists designed to block symptoms such as constipation, bloating and abdominal pain in patients treated with narcotic painkillers, has suffered from conflicting clinical results as well as a worrying trend towards an increase in cardiovascular side effects compared to placebo.

GSK and Adolor initially concentrated on developing Entereg for a condition known as post-operative ileus - a transient interruption in bowel motility that is an almost inevitable consequence of abdominal surgery, but suffered a setback in July 2005 when US regulators issued an approvable letter requesting more evidence of efficacy.

In its ‘approvable’ letter, the FDA has asked for data from a 12-month safety analysis that would specifically look at the cardiovascular risks of the drug compared to placebo.

GSK is already carrying out a Phase III trial of Entereg , study 014, that is looking at a lower dose (0.5mg) than the 12mg used in the earlier post-operative ileus studies and is evaluating Entereg in the treatment of opioid-induced bowel dysfunction in patients with chronic, non-cancer-related pain. Results from this study should be available in the first half of 2007.