GlaxoSmithKline and partner Adolor Corp have reported encouraging data with Entereg, a new drug designed to block the side effects suffered by people who are taking opioid analgesics, such as morphine, to manage chronic pain. By Phil Taylor

The Phase IIb trial of Entereg (alvimopan), the first in a new class of peripheral mu opioid receptor antagonists, found that the drug was significantly more effective than placebo in combating side effects - such as constipation, abdominal pain and bloating - that affect around half of all patients taking opioids to tackle moderate to severe pain.

Entereg caused a doubling in spontaneous bowel movements (i.e. without resorting to laxatives) compared to placebo, and patients on the active drug also reported improvements on quality-of-life scales.

GSK and Adolor have concentrated initially 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. While rarely a serious problem for patients, it is one of the most common causes of prolonged hospital stay after surgery, so a simple drug treatment to tackle it could prove popular and cost-effective.

But GSK and Adolor suffered a setback in their bid to bring Entereg to market last July, when the US Food and Drug Administration turned down their marketing application and asked for more evidence of efficacy. Since then, the companies have completed an additional Phase III study in this patient population, and plan to submit the results to the FDA later this year.

While the potential market for POI is likely fairly small – in the region of $150 million – approval for managing opioid side effects could be significantly more lucrative, bringing in $500 million-plus by 2010, according to analysts .

Last month, GSK said it had completed enrollment in a Phase III study of Entereg in the prevention of opioid side effects in patients with non-cancer pain. A filing for the follow-up indication is expected in 2007.